Electrochromism and Electrochromic Devices

This page intentionally left blankELECTROCHROMISM AND ELECTROCHROMIC DEVICES Electrochromism has advanced greatly over the past decade with electrochromic substances – organic and/or inorganic materials and polymers – providing widespread applications in light-attenuation, displays and analysis. Using reader-friendly electrochemistry, this book leads fromelectrochromic scope and history to newand searching presentations of optical quantification and theoretical mechanistic models. Non-electrode electrochromism and photo-electrochromismare summarised, with updated comprehensive reviews of electrochromic oxides (tungsten trioxide particularly), metal coordination complexes and metal cyanometallates, viologens and other organics; and more recent exotics such as fullerenes, hydrides and conjugated electroactive polymers are also covered. The book concludes by examining device construction and durability. Examples of real-world applications are provided, including minimal-power electrochromic building fenestration, an eco-friendly application that could replace air conditioning; moderately sized electrochromic vehicle mirrors; large electrochromic windows for aircraft; and reflective displays such as quasi-electrochromic sensors for analysis, and electrochromic strips for monitoring of frozen-food refrigeration. With an extensive bibliography, and step-by-step development from simple examples to sophisticated theories, this book is ideal for researchers in materials science, polymer science, electrical engineering, physics, chemistry, bioscience and (applied) optoelectronics. P. M. S. MONK is a Senior Lecturer in physical chemistry at the Manchester Metropolitan University in Manchester, UK. R. J . MORTI MER is a Professor of physical chemistry at Loughborough University, Loughborough, UK. D. R. ROS S EI NS KY, erstwhile physical chemist (Reader) at the University of Exeter, UK, is an Hon. University Fellow in physics, and a Research Associate of the Department of Chemistry at Rhodes University inGrahamstown, South Africa. ELECTROCHROMISM AND ELECTROCHROMIC DEVICES P. M. S. MONK, Manchester Metropolitan University R. J. MORTIMER Loughborough University AND D. R. ROSSEINSKY University of Exeter CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK First published in print format ISBN-13 978-0-521-82269-5 ISBN-13 978-0-511-50806-6 © P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky 2007 2007 Information on this title: www.cambridge.org/9780521822695 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Published in the United States of America by Cambridge University Press, New York www.cambridge.org eBook (NetLibrary) hardback Contents Preface page ix Acknowledgements xii List of symbols and units xiv List of abbreviations and acronyms xvii 1 Introduction to electrochromism 1 1.1 Electrode reactions and colour: electrochromism 1 1.2 Non-redox electrochromism 3 1.3 Previous reviews of electrochromism and electrochromic work 6 1.4 Criteria and terminology for ECD operation 7 1.5 Multiple-colour systems: electropolychromism 17 References 18 2 A brief history of electrochromism 25 2.1 Bibliography; and ‘electrochromism’ 25 2.2 Early redox-coloration chemistry 25 2.3 Prussian blue evocation in historic redox-coloration processes 25 2.4 Twentieth century: developments up to 1980 27 References 30 3 Electrochemical background 33 3.1 Introduction 33 3.2 Equilibrium and thermodynamic considerations 34 3.3 Rates of charge and mass transport through a cell 41 3.4 Dynamic electrochemistry 46 References 51 4 Optical effects and quantification of colour 52 4.1 Amount of colour formed: extrinsic colour 52 4.2 The electrochromic memory effect 53 4.3 Intrinsic colour: coloration efficiency 54 4.4 Optical charge transfer (CT) 60 4.5 Colour analysis of electrochromes 62 References 71 v 5 Kinetics of electrochromic operation 75 5.1 Kinetic considerations for type-I and type-II electrochromes: transport of electrochrome through liquid solutions 75 5.2 Kinetics and mechanisms of coloration in type-II bipyridiliums 79 5.3 Kinetic considerations for bleaching type-II electrochromes and bleaching and coloration of type-III electrochromes: transport of counter ions through solid electrochromes 79 5.4 Concluding summary 115 References 115 6 Metal oxides 125 6.1 Introduction to metal-oxide electrochromes 125 6.2 Metal oxides: primary electrochromes 139 6.3 Metal oxides: secondary electrochromes 165 6.4 Metal oxides: dual-metal electrochromes 190 References 206 7 Electrochromism within metal coordination complexes 253 7.1 Redox coloration and the underlying electronic transitions 253 7.2 Electrochromism of polypyridyl complexes 254 7.3 Electrochromism in metallophthalocyanines and porphyrins 258 7.4 Near-infrared region electrochromic systems 265 References 274 8 Electrochromism by intervalence charge-transfer coloration: metal hexacyanometallates 282 8.1 Prussian blue systems: history and bulk properties 282 8.2 Preparation of Prussian blue thin films 283 8.3 Electrochemistry, in situ spectroscopy and characterisation of Prussian blue thin films 285 8.4 Prussian blue electrochromic devices 289 8.5 Prussian blue analogues 291 References 296 9 Miscellaneous inorganic electrochromes 303 9.1 Fullerene-based electrochromes 303 9.2 Other carbon-based electrochromes 304 9.3 Reversible electrodeposition of metals 305 9.4 Reflecting metal hydrides 307 9.5 Other miscellaneous inorganic electrochromes 309 References 309 10 Conjugated conducting polymers 312 10.1 Introduction to conjugated conducting polymers 312 10.2 Poly(thiophene)s as electrochromes 318 vi Contents 10.3 Poly(pyrrole)s and dioxypyrroles as electrochromes 327 10.4 Poly(aniline)s as electrochromes 328 10.5 Directed assembly of electrochromic electroactive conducting polymers 331 10.6 Electrochromes based on electroactive conducting polymer composites 332 10.7 ECDs using both electroactive conducting polymers and inorganic electrochromes 333 10.8 Conclusions and outlook 334 References 335 11 The viologens 341 11.1 Introduction 341 11.2 Bipyridilium redox chemistry 342 11.3 Bipyridilium species for inclusion within ECDs 346 11.4 Recent elaborations 360 References 366 12 Miscellaneous organic electrochromes 374 12.1 Monomeric electrochromes 374 12.2 Tethered electrochromic species 387 12.3 Electrochromes immobilised within viscous solvents 391 References 391 13 Applications of electrochromic devices 395 13.1 Introduction 395 13.2 Reflective electrochromic devices: electrochromic car mirrors 395 13.3 Transmissive ECD windows for buildings and aircraft 397 13.4 Electrochromic displays for displaying images and data 401 13.5 ECD light modulators and shutters in message-laser applications 404 13.6 Electrochromic paper 405 13.7 Electrochromes applied in quasi-electrochromic or non- electrochromic processes: sensors and analysis 406 13.8 Miscellaneous electrochromic applications 407 13.9 Combinatorial monitoring of multiples of varied electrode materials 409 References 410 14 Fundamentals of device construction 417 14.1 Fundamentals of ECD construction 417 14.2 Electrolyte layers for ECDs 419 14.3 Electrodes for ECD construction 422 Contents vii 14.4 Device encapsulation 424 References 425 15 Photoelectrochromism 433 15.1 Introduction 433 15.2 Direction of beam 433 15.3 Device types 434 15.4 Photochromic–electrochromic systems 438 References 440 16 Device durability 443 16.1 Introduction 443 16.2 Durability of transparent electrodes 444 16.3 Durability of the electrolyte layers 445 16.4 Enhancing the durability of electrochrome layers 445 16.5 Durability of electrochromic devices after assembly 446 References 449 Index 452 viii Contents Preface While the topic of electrochromism – the evocation or alteration of colour by passing a current or applying a potential – has a history dating back to the nineteenth century, only in the last quarter of the twentieth century has its study gained a real impetus. So, applications have hitherto been limited, apart from one astonishing success, that of the Gentex Corporation’s self-darkening rear-view mirrors now operating on several million cars. Now they have achieved a telling next step, a contract with Boeing to supply adjustably darkening windows in a new passenger aircraft. The ultimate goal of contemporary studies is the provision of large-scale electrochromic windows for buildings at modest expenditure which, applied widely in the USA, would save billions of dollars in air-conditioning costs. In tropical and equatorial climes, savings would be proportionally greater: Singapore for example spends one quarter of its GDP (gross domestic product) on air conditioning, a sine qua non for tolerable living conditions there. Another application, to display systems, is a further goal, but universally used liquid crystal displays present formidable rivalry. However, large-scale screens do offer an attractive scope where liquid crystals might struggle, and electrochromics should almost certainly be much more economical than plasma screens. Numerous other applications have been contemplated. There is thus at present a huge flurry of activity to hit the jackpot, attested by the thousands of patents on likely winners. However, as a patent is sui generis, and we wish to present a scientific overview, we have not scanned in detail the patent record, which would have at least doubled the work without in our view commensurate advantages. There are thousands of chemical systems that are intrinsically electrochromic, and while including explanatory examples, we incorporate here mostly those that have at least a promise of being useful. Our approach has been to concentrate on systems that colorise or change colour by electron transfer (‘redox’) processes, without totally neglecting other, electric-potential ix dependent, systems now particularly useful in applications to bioscience. The latter especially seem set to shine. Several international gatherings have been convened to discuss electrochromism for devices. Probably the first was The Electrochemical Society meeting in 1989 (in Hollywood, Fl). 1 Soon afterwards was ‘Fundamentals of Electrochromic Devices’ organised by The American Institute of Chemical Engineers at their Annual Meeting in Chicago, 11–16 November 1990. 2 The following year, the authors of this present volume called a Solid-State Group (Royal Society of Chemisty) meeting in London. At the Electrochemical Society meeting in NewOrleans (in 1994), 3 it was decided to host the first of the so-called International Meetings on Electrochromism, ‘IME’. The first such meeting ‘IME-1’ met in Murano, Venice in 1994, 4 IME-2 in San Diego in 1996, 5 IME- 3 was in London in 1998, 6 IME-4 in Uppsala in 2000, 7 IME-5 in Colorado in 2002 and IME-6 in Brno, Czech Republic in 2004. 8 Further electrochromics symposia occurred at Electrochemical Society meetings that took place at San Antonio, TX, in 1996 9 and Paris in 2003. 10 The basis of the processes on which we concentrate is electrochemical, as is outlined in the first chapter. A historical outline is given in Chapter 2, and any reader not familiar with the electrochemistry presented here may find this explained sufficiently in Chapter 3. A fairly extensive presentation of twentieth- century electrochemistry in Chapter 3 seems necessary also to followsome later details of the exposition, and those familiar with this arcane science may choose to flip through a chapter largely comprising ‘elderly electrochemistry’, to quote from ref. 18 of Chapter 1. Details of assessing coloration follow in Chapter 4, and in Chapter 5 attempts at theoretically modelling the electrochromic process in the most popular electrochromic material to date, tungsten trioxide, are outlined. In subsequent chapters, the work that has been conducted on a wide variety of materials follow, from metal oxides through complexed metals and metalorganic complexes to conjugated conductive polymers. Applications and tests finish the account. In order hopefully to make each chapter almost freestanding, we do quite frequently repeat the gist of some previous chapter(s). A comment about the citations which end each chapter: early during our discussions of the book’s contents, we decided to reproduce the full titles of each paper cited. Each title is cited as it appeared when first published. We have systematised capitalisation throughout (and corrected spelling errors in two papers). In our account we have probably not succeeded in conveying all the aesthetic pleasure of studying aspects of colour and its creation, or the profound science-and-technology interest of understanding the reactions and of mastering the associated processes: this book does represent an attempt to spread x Preface these interests. However, further at stake is the prospect of controlling an important part of personal environments while economising on air-conditioning costs, thereby cutting down fuel consumption and lessening the human ‘carbon footprint’, to cite the mode words. There are the other perhaps lesser applications that are also promisingly useful. So, to a more controlled-colour future, read on. DISCLAIMER: Superscripted reference citations in the text are, unusually, listed in full e.g. 1, 2, 3, 4 rather than the customary 1–4. The need arises from the parallel publication of this monograph as an e-book. In this version, ‘each reference citation is hyper-linked to the reference itself, which requires that they be cited separately.’ References 1. Proceedings volume was Electrochromic Materials, Carpenter, M. K. and Corrigan, D. A. (eds.), 90–2, Pennington, NJ, Electrochemical Society, 1990. 2. Proceedings of the Annual Meeting of the American Institute of Chemical Engineers, published in Sol. Energy Mater. Sol. Cells, 25, 1992, 195–381. 3. Proceedings volume was Electrochromic Materials II, Ho, K.-C. and MacArthur, D. A. (eds.), 94–2, Pennington, NJ, Electrochemical Society, 1994. 4. Proceedings volume was Sol. Energy Mater. Sol. Cells, 1995, 39, issue 2–4. 5. Proceedings volume was Sol. Energy Mater. Sol. Cells, 1998, 54, issue 1–4. 6. Proceedings volume was Sol. Energy Mater. Sol. Cells, 1999, 56, issue 3–4. 7. Proceedings volume was Electrochim. Acta, 2001, 46, issue 13–14. 8. Proceedings volume was Sol. Energy Mater. Sol. Cells, 2006, 90, issue 4. 9. Proceedings volume was Electrochromic Materials III, Ho, K. C., Greenberg, C. B. and MacArthur, D. M. (eds.), 96–24, Pennington, NJ, Electrochemical Society, 1996. 10. Proceedings volume was Electrochromic Materials and Applications, Rougier, A., Rauh, D. and Nazri, G. A. (eds.), 2003–17, Pennington, NJ, Electrochemical Society, 2003. Preface xi Acknowledgements We are indebted to numerous colleagues and correspondents who have collaborated in research or in providing information. PMSM wishes to thank: Professor Claus-G ¨ oren Granqvist of Uppsala University, Professor Susana de Co´ rdoba, Universidade de Sa˜ o Paulo, Brazil, Professor L. M. Loew of the University of Connecticut Health Center and Dr Yoshinori Nishikitani of the Nippon Mitsubishi Oil Corporation. Also, those on the computer helpdesk at MMU who helped with the scanning of figures. RJM wishes to thank: Dr Joanne L. Dillingham, (Ph.D, Loughborough University), Dr Steve J. Vickers erstwhile of the Universities of Birmingham and Sheffield, Dr Natalie M. Rowley of the University of Birmingham; Dr Frank Marken, University of Bath; Professor Paul D. Beer, University of Oxford; Professor John R. Reynolds, University of Florida; Aubrey L. Dyer, University of Florida; Dr Barry C. Thompson, University of California, Berkeley – erstwhile of the Reynolds group at the University of Florida; Professor Mike D. Ward, University of Sheffield, and Professor Steve Fletcher of Loughborough University. DRRwishes to thank: Bill Freeman Esq. then of Finisar Corp., nowheading Thermal Transfer (Singapore), Dr Tom Guarr of Gentex, Dr Andrew Glidle nowof GlasgowUniversity, Dr Richard Hann of ICI, the late Dr Brian Jackson of Cookson Ltd, Professor Hassan Kellawi of Damascus University, Mr (now Captain) Hanyong Limof Singapore, graduate of Carnegie-Mellon University, Professor Paul O’Shea of NottinghamUniversity, Ms Julie Slocombe, erstwhile of Exeter University, and Dr AndrewSoutar and Dr Zhang Xiao of SIMTech in Singapore. We also wish to thank the following for permission to reproduce the figures (in alphabetical order): The American Chemical Society, The American Institute of Physics, The Electrochemical Society, Elsevier Science, The Royal xii Society of Chemistry (RSC), The Japanese Society of Physics, The Society of Applied Spectroscopy, and the Society for Photo and Information Engineering. In collecting the artwork for figures, we also acknowledge the kind help of the following: Dr Charles Dubois, formerly of the University of Florida. From the staff of Cambridge University Press, we wish to thank Dr Tim Fishlock (now at the RSC in Cambridge, UK), who first commissioned the book, and his successor Dr Michelle Carey, and Assistant Editor, Anna Littlewood, together with Jo Bottrill of the production team for their help; and a particularly big thank you to the copy-editor Zoe¨ Lewin for her consistent good humour and professionalism. We owe much to our families, who have enabled us to undertake this project. We apologise if we have been preoccupied or merely absent when you needed us. We also thank the numbers of kindly reviewers of our earlier book (and even the two who commented adversely) and much appreciate passing comment in a paper by Dr J. P. Collman and colleagues. Though obvious newleaders exploring different avenues are currently emerging, if one individual is to be singled out in the general field, Claes-Goran Granqvist of the A ˚ ngstrom Laboratory, Uppsala, has to be acknowledged for the huge input into electrochromism that he has sustained over decades. We alone are responsible for the contents of the book including the errors. Acknowledgements xiii Symbols and units A ampere, area Abs optical absorbance c(y,t) time-dependent concentration of charge at a distance of y into a solid thin film c m maximum concentration of charge in a thin film c 0 initial concentration of charge in a thin film D diffusion coefficient D chemical diffusion coefficient d thickness of a thin film e charge on an electron e À electron E energy E potential E a activation energy E (appl) applied potential E (eq) equilibrium potential E pa potential of anodic peak E pc potential of cathodic peak E F standard electrode potential eV electron volt F Faraday constant Hz hertz i current density i subscripted, represents component 1 or 2 . . . i b bleaching current density i c coloration current density i o exchange current density J imaginary part of impedance J o charge flux (rate of passage of electrons or ionic species) K equilibrium constant K a equilibrium constant of acid ionisation xiv K sp equilibrium constant of ionic solubility (‘solubility product’) l(t) time-dependent thickness of a narrow layer of the WO 3 film adjacent to the electrolyte (during electro-bleaching) M mol dm À3 n number in part of iterative calculation n number of electrons in a redox reaction p volume charge density of protons in the H 0 WO 3 p the operator –log 10 Pa pascal q charge per unit volume Q charge R gas constant R real component of impedance r radius of sphere (e.g. of a solid, spherical grain) S Seebeck coefficient s second T thermodynamic temperature t time v scan rate V volt V volume V a applied potential W Wagner enhancement factor (‘thermodynamic enhancement factor’) x insertion coefficient x (critical) insertion coefficient at a percolation threshold x 1 constant (of value %0.1) x o proton density in a solid thin film x, y, z, w or c subscripted, non-integral composition indicators, in non- stoichiometric materials Z impedance g gamma photon " extinction coefficient (‘molar absorptivity’) coloration efficiency o coloration efficiency of an electrochromic device p coloration efficiency of primary electrochrome s coloration efficiency of secondary electrochrome overpotential List of symbols and units xv wavelength max wavelength maximum L ionic molar conductivity mobility, chemical potential (ion) mobility of ions (electron) mobility of electrons frequency of light density of atoms in a thin film 0 constant equal to (2 e d i 0 ) s electronic conductivity D ‘characteristic time’ for diffusion j s membrane surface potential kinematic viscosity velocity of solution flow o frequency of ac signal xvi List of symbols and units Abbreviations and acronyms a amorphous ac alternating current AEIROF anodically electrodeposited iridium oxide film AES atomic emission spectroscopy AFM atomic force microscopy AIROF anodically formed iridium oxide film AMPS 2-acrylamido-2-methylpropanesulfonic acid ANEPPS 3-{4-[2-(6-dibutylamino)-2-naphthyl]-trans-ethenyl pyridinium} propane sulfonate aq aqueous AR anti reflectance ASSD all-solid-state device ATO antimony–tin oxide BEDOT 2,2 0 -bis(3,4-ethylenedioxythiophene) BEDOT-NMeCz 3,6-bis[2-(3,4-ethylenedioxythiophene)]- N-alkylcarbazole bipy 2,2 0 -bipyridine bipm 4,4 0 -bipyridilium c crystalline CAT catecholate CCE composite coloration efficiency CE counter electrode ChLCs cholesteric liquid crystals CIE Commission Internationale de l’Eclairage cmc critical micelle concentration CPQ cyanophenyl paraquat [1,1 0 -bis(p-cyanophenyl)- 4,4 0 -bipyridilium] CRT cathode-ray tube CT charge transfer xvii CTEM conventional transmission electron microscopy CuHCF copper hexacyanoferrate CVD chemical vapour deposition dc direct current DDTP 2,3-di(thien-3-yl)-5,7-di(thien-2-yl)thieno[3,4-b] pyrazine DEG diethyleneglycol DMF dimethylformamide DMSO dimethyl sulfoxide EC electrochromic EC electrode reaction followed by a chemical reaction ECB electrochromic battery ECD electrochromic device ECM electrochromic material ECW electrochromic window EDAX energy dispersive analysis of X-rays EDOT 3,4-(ethylenedioxy)thiophene EIS electrochemical impedance spectroscopy EQCM electrochemical quartz-crystal microbalance FPE fluoresceinphosphatidyl-ethanolamine FTIR Fourier-transform infrared FTO fluorine[-doped] tin oxide GC glassy carbon HCF hexacyanoferrate HOMO highest occupied molecular orbital HRTEM high-resolution transmission electron microscopy HTB hexagonal tungsten bronze HV heptyl viologen (1,1 0 -di-n-heptyl-4,4 0 -bipyridilium) IBM Independent Business Machines ICI Imperial Chemical Industries IR infrared ITO indium–tin oxide IUPAC International Union of Pure and Applied Chemistry IVCT intervalence charge transfer LB Langmuir–Blodgett LBL layer-by-layer [deposition] LCD liquid crystal display LED light-emitting diode LFER linear free-energy relationships xviii List of abbreviations and acronyms LPCVD liquid-phase chemical vapour deposition LPEI linear poly(ethylene imine) LUMO lowest unoccupied molecular orbital MB Methylene Blue MLCT metal-to-ligand charge transfer MOCVD metal-oxide chemical vapour deposition MV methyl viologen (1,1 0 -dimethyl-4,4 0 -bipyridilium) nc naphthalocyanine NCD nanochromic display Ni HCF nickel hexacyanoferrate NMP N-methylpyrrolidone NRA nuclear reaction analysis NREL National Renewable Energy Laboratory, USA NVS # Night Vision System # OD optical density OEP octaethyl porphyrin OLED organic light-emitting diode OTE optically transparent electrode OTTLE optically transparent thin-layer electrode pa peak anodic PAA poly(acrylic acid) PAH poly(allylamine hydrochloride) PANI poly(aniline) PB Prussian blue PBEDOT-B(OC 12 ) 2 poly{1,4-bis[2-(3,4-ethylenedioxy)thienyl]- 2,5-didodecyloxybenzene} PBEDOT-N-MeCz poly{3,6-bis[2-(3,4-ethylenedioxy)thienyl]- N-methylcarbazole} PBEDOT-Pyr poly{3,6-bis[2-(3,4-ethylenedioxy)thienyl] pyridine} PBEDOT-PyrPyr(Ph) 2 poly{5,8-bis(3-dihydro-thieno[3,4-b]dioxin-5-yl)- 2,3-diphenyl-pyrido[3,4-b]pyrazine} PBuDOP poly[3,4-(butylenes dioxy)pyrrole] pc peak cathodic Pc dianion of phthalocyanine PC propylene carbonate PCNFBS poly{cyclopenta[2,1-b;4,3-b 0 ]dithiophen-4- (cyanononafluorobutylsulfonyl)methylidene} PdHCF palladium hexacyanoferrate List of abbreviations and acronyms xix PDLC phase-dispersed liquid crystals PEDOP poly[3,4-(ethylenedioxy)pyrrole] PEDOT poly[3,4-(ethylenedioxy)thiophene] PEDOT-S poly{4-(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl- methoxy}-1-butanesulfonic acid, sodium salt PEO poly(ethylene oxide) PET poly(ethylene terephthalate) PG Prussian green PITT potentiostatic intermittence titration technique PMMA poly(methyl methacrylate) PMT polaromicrotribometric PP plasma polymerised PP poly(1,3,5-phenylene) PProDOP poly[3,4-(propylenedioxy)pyrrole] PProDOT poly(3,4-propylenedioxythiophene) PSS poly(styrene sulfonate) PTPA poly(triphenylamine) PVA poly(vinyl acrylate) PVC poly(vinyl chloride) PVD physical vapour deposition PW Prussian white PX Prussian brown Pyr pyridine Q Quinone RE reference electrode rf radio frequency RP ruthenium purple: iron(III) hexacyanoruthenate(II) RRDE rotated ring-disc electrode s solid s. soln solid solution SA sacrificial anode SCE saturated calomel electrode SQ semi quinone SEM scanning electron microscopy SHE standard hydrogen electrode SI Syste` me internationale SIMS secondary ion mass spectroscopy SIROF sputtered iridium oxide film soln solution xx List of abbreviations and acronyms SPD suspended particle device SPM solid paper matrix STM scanning tunnelling microscopy TA thiazine TCNQ tetracyanoquinodimethane TGA thermogravimetric analysis THF tetrahydrofuran TMPD tetramethylphenylenediamine Tp* hydrotris(3,5-dimethylpyrazolyl)borate TTF tetrathiafulvalene UCPC user-controllable photochromic [material] UPS ultraviolet photoelectron spectroscopy VDU visual display unit VHCF vanadium hexacyanoferrate WE working electrode WPA tungsten phosphoric acid XAS X-ray absorption spectroscopy XPS X-ray photoelectron spectroscopy XRD X-ray diffraction XRG xerogel List of abbreviations and acronyms xxi 1 Introduction to electrochromism 1.1 Electrode reactions and colour: electrochromism The terminology and basis of the phenomenon that we address are briefly outlined in this chapter. Although there are several usages of the term ‘electrochromism’, several being summarised later in this chapter, ‘electrochromes’ later in the present text are always ‘electroactive’, as follows. An electroactive species can undergo an electron uptake, i.e. ‘reduction’, Eq. (1.1), or electron release, i.e. ‘oxidation’, the reverse of Eq. (1.1) in a ‘redox’ reaction that takes place at an electrode. An electrode basically comprises a metal or other conductor, with external connections, in contact with forms O and R of an ‘electroactive’ material, and can be viewed as a ‘half-cell’: oxidised form, Oþelectron(s) ! reduced form, R. (1.1) Though in strict electrochemical parlance all the components, Oand Rand the metallic or quasi-metallic conductor, comprise ‘the electrode’, we and others often depart from this complete definition when we imply that ‘the electrode’ comprises the just-italicised component, which conforms with the following definition: ‘An electrode basically comprises a metal or metallic conductor or, especially in electrochromism, an adequately conductive semiconductor often as a thin filmon glass.’ We thus usually refer to the ‘electrode substrate’ for the metal or metal-like component to make the distinction clear. Furthermore, in Chapter 3 it is emphasised that any electrode in a working system must be accompanied by a second electrode, with intervening electrolyte, in order to make up a cell allowing passage of current, in part comprising the flow of just those electrons depicted in Eq. (1.1). An electroactive material may be an atom or ion, a molecule or radical, sometimes multiply bonded in a solid film, and must be in contact with the electrode substrate prior to successful electron transfer. It may be in 1 solution – solvated and/or complexed – in which case it must approach sufficiently closely to the electrode substrate and undergo the adjustments that contribute to the (sometimes low) activation energy accompanying electron transfer. In other systems, the electroactive material may be a solid or dispersed within a solid matrix, in which case that proportion of the electrochrome physically in contact with the electrode substrate undergoes the redox reaction most rapidly, the remainder of the electroactive material less so. The underlying theory of electrochemical electron-transfer reactions is treated elsewhere. 1 That part of a molecular system having or imparting a colour is termed a chromophore. White light comprises the wavelengths of all the colours, and colour becomes evident when photons from part of the spectrum are absorbed by chromophores; then the colour seen is in fact the colour complementary to that absorbed. Thus, for example, a blue colour is reflected (hence seen) if, on illumination with white light, the material absorbs red. Light absorption enables electrons to be promoted between quantised (i.e. wave-mechanically allowed) energy levels, such as the ground and first excited states. The wavelength of light absorbed, , is related to the magnitude of the energy gap E between these levels according to the Planck relation, Eq. (1.2): E ¼ h ¼ hc ; (1:2) where is the frequency, h is the Planck constant and c the speed of light in vacuo. The magnitude of E thus relates to the colour since, when is the wavelength at the maximum (usually denoted as max ) of the absorption band observed in the spectrumof a chromophore, its position in the spectrumclearly governs the observed colour. (To repeat, the colour arises from the nonabsorbed wavelengths.) Most electrochromes colourise by reflection, as in displays; transmission-effective systems, as in windows, follow a corresponding mechanism. Electroactive species comprise different numbers of electrons before and after the electron-transfer reaction (Eq. (1.1) or its reverse), so different redox states will necessarily exhibit different spectroscopic transitions, and hence will require different energies E for electron promotion between the ground and excited states. Hence all materials will undergo change of spectra on redox change. However, the colours of electroactive species only may be different before and after electron transfer because often the changes are not visible (except by suitable spectrometry) when the wavelengths involved fall outside the visible range. In other words, the spectral change accompanying a redox reaction is visually indiscernible if the optical absorptions by the two redox states fall in 2 Introduction to electrochromism the ultraviolet (UV) or near infrared (NIR). When the change is in the visible region, then a pragmatic definition of electrochromism may be formulated as follows. ‘Electrochromism is a change, evocation, or bleaching, of colour as effected either by an electron-transfer (redox) process or by a sufficient electric potential. In many applications it is required to be reversible.’ However, regarding intensity-modulation filters for, say, IR message-laser pulses in optical fibres, such terms as ‘electrochromic switching or modulation’ are increasingly being used for such invisible effects. Visible electrochromism is of course only ever useful for display purposes if one of the colours is markedly different from the other, as for example when the absorption band of one redox state is in the visible region while the other is in the UV. If the colours are sufficiently intense and different, then the material is said to be electrochromic and the species undergoing change is usefully termed an ‘electrochrome’. 2 Simple laboratory demonstrations of electrochromism are legion. 3,4 The website in ref. 5 contains a video sequence clearly demonstrating electrochromic coloration, here of a highly conjugated poly(thiophene) derivative. Many organic and inorganic materials are electrochromic; and even some biological species exhibit the phenomenon: 6 Bacteriorhodopsin is said to exhibit very strong electrochromism with a colour change from bright blue to pale yellow. 6 The applications of electrochromism are outlined in Chapter 13 and the general criteria of device fabrication are outlined in Chapter 14. 1.2 Non-redox electrochromism The word ‘electrochromism’ is applied to several, disparate, phenomena. Many are not electrochromic in the redox sense defined above. Firstly, charged species such as 3-{4-[2-(6-dibutylamino-2-naphthyl)-trans- ethenyl] pyridinium} propane sulfonate (‘di-4-ANEPPS’) (I), called ‘electrochromic probes’, are employed in studies of biological membrane potentials. 7 (A similar-looking but intrinsically different mechanism involving deprotonation is outlined below.) For a strongly localised system, such as a protein system where electron-donor and -acceptor sites are separated by large distances, the potential surfaces involved in optical electron excitation (see Eq. (1.2)) become highly asymmetrical. 7 For this reason, the electronic spectrum of (I) is extraordinarily sensitive to its environment, demonstrating large solvent-dependent ‘solvatochromic’ shifts, 8 so much information can be gained by quantitative analysis of its UV-vis spectra. In effect, it is possible to image the electrical activity of a cell membrane. 7 Loew et al. first suggested this use of such 1.2 Non-redox electrochromism 3 electrochromism in 9 1979; they pointed out how the best species for this type of work are compounds like (I), its 8-isomer, or nitroaminostilbene, 10 both of which have large non-linear second-harmonic effects. 9 In consequence, significant changes are induced by the environment in the dipole moment so on excitation from the ground to the excited states, different colours result. I N S O O O − + N This application is not electrochromismas effected by redox processes of the kind we concentrate on in the present work, but can alternatively be viewed as a molecular Stark effect 11 in which some of the UV-vis bands of polarisable molecules evince a spectroscopic shift in the presence of a strong electric field. Vredenberg 12 reviewed this aspect of electrochromism in 1997. Such a Stark effect was the original sense implied by ‘electrochromism’ when the word was coined in 1961. 13 While many biological and biochemical references to ‘electrochromism’ mean a Stark effect of this type, some are electrochromic in the redox sense. For example, the (electrochromic) colours of quinone reduction products have been used to resolve the respective influences of electron and proton transfer processes in bacterial reactions. 14,15,16 In some instances, however, this electrochromic effect is unreliable. 17 A valuable electrochromic application has been employed by O’Shea 18 to probe local potentials on surfaces of biological cell membranes. The effect of electric potential on acidity constants is employed: weak acids in solution are partly ionised into proton and (‘base’) residue to an extent governed ordinarily by the equilibrium constant particular to that acid, its acidity constant K a . However, if the weak acid experiences an extraneous electric potential, the extent of ionisation is enhanced by further molecular scission (i.e. proton release) resulting from the increased stabilisation of the free-proton charge. With ‘p’ representing (negative) decadic logarithms, the outcome may be represented by the equation pK a (j s ) ¼pK a (0) ÀFj s /RT (ln 10) where j s is the membrane surface potential. 18 This result (a close parallel of the observed ‘second Wien effect’ in high-field conductimetry on weak acids) arises from combining the Boltzmann equation with the Henderson–Hasselbalch equation. The application proceeds as follows. A fluorescent molecule is chosen, that is a proton-bearing acid of suitable K a , with only its deprotonated moiety 4 Introduction to electrochromism showing visible fluorescence, and then only when the potential experienced is high enough. The probe molecules are inserted by suitable chemistry into the surface of the cell membrane. Then it will fluoresce, in areas of sufficiently high electric potential, thus illuminating such areas of j s , and monitoring even rapid rates of change as can result from say cation acquisition by the surface. Suitable probe molecules are 18,19 fluoresceinphosphatidyl-ethanolamine (FPE) and 20,21,22 1-(3-sulfonatopropyl)-4-[p[2-(di-n-octylamino)-6-naphthyl]- vinyl]-pyridinium betaine. To quote, 23 ‘Probe molecules such as FPE have proved to be particularly versatile indicators of the electrostatic nature of the membrane surface in both artificial and cellular membrane systems.’ This ingenious probe of electrical interactions underlying biological cell function thus relies unusually not on electron transfer but on proton transfer as effected by electric potential changes. Secondly, the adjective ‘electrochromic’ is often applied to a widely differing variety of fenestrative and device applications. For example, a routine web search using the phrase ‘electrochromic window’ yielded many pages describing a suspended-particle-device (SPD) window. Some SPD windows are also termed ‘Smart Glass’ 24 – a term that, until now, has related to genuine electrochromic systems. On occasion (as occurs also in some patents) a lack of scientific detail indicates that the claims of some manufacturers’ websites are perhaps excessively ambitious – a practice that may damage the reputation of electrochromic products should a device fail to respond to its advertised specifications. Also to be noted, ‘gasochromic windows’ (also called gasochromic smart- glass windows) are generally not electrochromic, although sometimes described as such, because the change in colour is wholly attributable to a direct chemical gas þsolid redox reaction, with no externally applied potential, and no measurable current flow. The huge complication of the requisite gaseous plumbing is rarely addressed, while electrochromic devices require only cables. (The most studied gasochromic material is, perhaps confusingly, tungsten oxide, which is also a favoured electrochrome.) The gasochromic devices in refs. 25,26,27,28,29,30,31,32 are not electrochromic in the sense adopted by this book. Thirdly, several new products are described as ‘electrochromic’ but are in fact electrokinetic–colloidal systems, somewhat like SPDs with micro-encapsulation of the active particles. A good example is Gyricon ‘electrochromic paper’, 33 developed by Xerox. Lucent and Philips are developing similar products. Such paper is now being marketed as ‘SmartPaper TM ’. Gyricon is intended for products like electronic books, electronic newspapers, portable signs, and foldable, rollable displays. It comprises two plastic sheets, each of thickness ca. 140 mm, between which are millions of ‘bichromal’ (i.e. two colour) highly 1.2 Non-redox electrochromism 5 dipolar spheres of diameter 0.1 mm, and are suspended within minute oil-filled pockets. The spheres rotate following exposure to an electric field, as from a ‘pencil’ tip attached to a battery also connected to a metallically conductive backing sheet; 34 the spheres rotate fully to display either black or white, or partially (in response to weaker electrical pulses), to display a range of grey shades. 33 Similar mechanisms operate in embedded sacs of sol in which charged black particles are ‘suspended’ (when in the colourless state) but on application of a potential by an ‘electric pencil’, black particles visibly deposit on the upper surface of the sacs. Some of these systems being deletable and re-usable promise substantial saving of paper. Note that the NanoChromics TM paper described on page 347, marketed by NTera of Eire, is genuinely electrochromic in the redox sense. 1.3 Previous reviews of electrochromism and electrochromic work The broadest overview of all aspects of redox electrochromism is Electrochromism: Fundamentals and Applications, by Monk, Mortimer and Rosseinsky. 2 It includes criteria for electrochromic application, the preparation of electrochromes and devices, and encompasses all types of electrochromic materials considered in the present book, both organic and inorganic. A major review of redox electrochromism appears in Handbook of Inorganic Electrochromic Materials by Granqvist, 35 a thorough and detailed treatise covering solely inorganic materials. Other reviews of electrochromism appearing within the last fifteen years include (in alphabetical order) those of: Agnihotry 36 in 1996, Bange et al. 37 in 1995, Granqvist (sometimes with co-workers) in 1992, 38 1993, 39,40 1997, 41,42 1998, 43,44,45 2000, 46 2003, 47,48 and 2004, 49 Green 50 in 1996, Greenberg in 1991 51 and 1994, 52 Lampert in 1998, 53 2001 54 and 2004, 55 Monk in 2001 56 and 2003, 57 Mortimer 58 in 1997, Mortimer and Rosseinsky 59 in 2001, Mortimer and Rowley 60 in 2002, Mortimer, Dyer and Reynolds 61 in 2006, Scrosati, Passerini and Pileggi 62 and Scrosati, 63 1992, Somani and Radhakrishnan 64 in 2003 and Yamamoto and Hayashida 65 in 1998. Bamfield’s book 8 Chromic Phenomena, published in 2001, includes a substantial reviewof electrochromism. Non-English reviews include that by Volke and Volkeova 66 (in Czech: 1996). McGourty (in 1991), 67 Hadfield (in 1993), 68 Hunkin (in 1993) 69 and Monk, Mortimer and Rosseinsky (in 1995) 70 have all written ‘popular-science’ articles on electrochromism. Bowonder et al.’s 1994 review 71 helps frame electrochromic displays within the wider corpus of display technology. Lampert’s 55 2004 review ‘Chromogenic materials’ similarly helps place electrochromism within the wider scope of 6 Introduction to electrochromism other forms of driven colour change, such as thermochromism. Lampert’s review, shorter, crammed with acronyms but more up-to-date, includes other forms of display device, such as liquid crystal displays (LCDs), phase- dispersed liquid crystals (PDLCs), cholesteric liquid crystals (ChLCs) and suspended particle devices (SPDs). There are also many dozen reviews concerning specific electrochromes, electrochromic-device applications and preparative methodologies, which we cite in relevant chapters. The now huge numbers of patents on materials, processes or devices are usually excluded, the reliability – often just the plausibility – of patents being judged by different, not always scientific, criteria. 1.4 Criteria and terminology for ECD operation The jargon used in discussions of the operation of electrochromic devices (ECD) is complicated, hence the criteria and terminology cited below, necessarily abridged, might aid clarification. The terms comply with the 1997 IUPAC recommended list of terms on chemically modified electrodes (CMEs). A CME is 72 an electrode made up of a conducting or semi conducting material that is coated with a selected monomolecular, multimolecular, ionic or polymeric film of a chemical modifier and that, by means of faradaic . . . reactions or interfacial potential differences . . . exhibits chemical, electrochemical and/or optical properties of the film. Chemically modified electrodes are often referred to as being derivatised, especially but not necessarily when the modifier is organic or polymeric. All electrochromic electrodes comprise some element of modification, but are rarely referred to as CMEs; this is simply to be understood. 1.4.1 Electrochrome type In the early days of ECD development, the kinetics of electrochromic coloration were discussed in terms of ‘types’ as in the seminal work of Chang, Sun and Gilbert 73 in 1975. Such types are classified in terms of the phases, present initially and thence formed electrochemically, which dictate the precise form of the current–time relationships evinced during coloration, and thus affect the coloration–time relationships. While the original classifications are somewhat dated, they remain useful and are followed here throughout. A type-I electrochrome is soluble, and remains in solution at all times during electrochromic usage. A good example is aqueous methyl viologen 1.4 Criteria and terminology for ECD operation 7 (1,1 0 -dimethyl-4,4 0 -bipyridilium – II), which colours during a reductive electrode reaction, Eq. (1.3): MV 2þ (aq) þe À !MV þ (aq). (1.3) colourless intense blue N N CH 3 H 3 C + + 2X – II X À can be a halide or complex anion such as BF 4 À . The cation is abbreviated to MV 2 þ . Other type-I electrochromes include any viologen often soluble in aqueous solution, or a phenathiazine (such as Methylene Blue), in non- aqueous solutions. Type-II electrochromes are soluble in their colourless forms but form a coloured solid on the surface of the electrode following electron transfer. This phase change increases the write–erase efficiency and speeds the response time of the electrochromic bleaching. A suitable example of a type-II system is cyanophenyl paraquat (III), again in water, 74,75,76 Eq. (1.4): CPQ 2þ ðaqÞ þ e À þ X À ðaqÞ ! ½CPQ þ X À ðsolidÞ: colourless olive green (1:4) N NC CN N III The solid material here is a salt of the radical cation product 74 (the incorporation of the anionic charge X À ensures electro-neutrality within the solid product). Other type-II electrochromes commonly encountered include aqueous viologen systems such as heptyl or benzyl viologens, 77 or methoxyfluorene compounds in acetonitrile solution. 78 Inorganic examples include the solid products of electrodeposited metals such as bismuth (often deposited as a finely divided solid), or a mirror of metallic lead or silver (Section 9.3), in which the electrode reaction is generally reduction of an aquo ion or of a cation in a complex with attached organic or inorganic moieties (‘ligands’). 8 Introduction to electrochromism Type-III electrochromes remain solid at all times. Most inorganic electrochromes are type III, e.g. for metal oxides, Eq. (1.5), MO y ðsÞ þ xðH þ ðsoln:Þ þ e À Þ ! H x MO y ðsÞ; colourless intense colour (1:5) where the metal M is most commonly a d-block element such as Mo, Ni or W, and the mobile counter ion (arbitrarily cited here as the proton) could also be lithium; y ¼3 is commonly found, and WO 3 has been the most studied. The parameter x, the ‘insertion coefficient’, indicates the proportion of metal sites that have been electro-reduced. The value of x usually lies in the approximate range 0 x <0.3. Other inorganic type-III electrochromes include phthalocyanine complexes and metal hexacyanometallates such as Prussian blue. Organic type-III systems are typified by electroactive conducting polymers. The three groups of polymer encountered most often in the literature of electrochromism are generically termed poly(pyrrole), poly(thiophene) or poly(aniline) and relate to the parenthesised monomer from which the electrochromic solid is formed by electro-polymerisation, as discussed below. 1.4.2 Contrast ratio CR The contrast ratio CR is a commonly employed measure denoting the intensity of colour formed electrochemically, as seen by eye, Eq. (1.6): CR ¼ R o R x ; (1:6) where R x is the intensity of light reflected diffusely though the coloured state of a display, and R o is the intensity reflected similarly but from a non-shiny white card. 79 The ratio CR is best quoted at a specific wavelength – usually at max of the coloured state. As in practice, a CRof less than about 3 is almost impossible to see by eye. As high a value as possible is desirable. The CRis commonly expressed as a ratio such as 7:1. ACRof 25:1 is cited for a type-II display involving electrodeposited bismuth metal, 80 and as high 81 as 60:1 for a system based on heptyl viologen radical cation, electrodeposited from aqueous solution with a charge 82 of 1 mC cm À2 , and 10:1 for the cell WO 3 jelectrolytejNiO. 83 More elaborate measures of coloration are outlined in Chapter 4. 1.4 Criteria and terminology for ECD operation 9 1.4.3 Response time t The response time is the time required for an ECD to change from its bleached to its coloured state (or vice versa). It is generally unlikely that (coloration) ¼ (bleach) . At present, there are few reliable response times in the literature since there is no consistency in the reporting and determination of cited data, and especially in the way different kinetic criteria are involved when determining . For example, may represent the time required for some fraction of the colour (defined or arbitrary) to form, or it may relate to the time required for an amount of charge (again defined or arbitrary) to be consumed in forming colour at the electrode of interest. While most applications do not require a rapid colour change, some such as for electrochromic office windows actually require a very slow response, as workers can feel ill when the colour changes too rapidly. 84 For example, a film of WO 3 (formed by spray pyrolysis of a solution generated by dissolving W powder in H 2 O 2 ) became coloured in 15 min, and bleached in 3 min, 85 but the choice of both potential and preparative method was made to engender such slowness. In contrast, a film of sol–gel-derived titanium dioxide is coloured by reductive insertion of Li þ ions at a potential of about –2 Vwith a response time of about 40 s. 86 However, applications such as display devices require a more rapid response. To this end, Sato 87 reports an anodically formed film of iridium oxide with a response time of 50 ms; Canon 88 made electrochromic oxide mixtures that undergo absorbance changes of 0.4 in 300 ms. Reynolds et al. 89 prepared a series of polymers based on poly(3,4-alkylenedioxythiophene) ‘PEDOT’ (IV); multiple switching studies, monitoring the electrochromic contrast, showed that films of polymer of thickness ca. 300 nm could be fully switched between reduced and oxidised forms in 0.8–2.2 s with a modest transmittance change of 44–63%. Similarly, a recently fabricated electrochromic device was described as ‘ultra fast’, with a claimed 90 of 250 ms; the viologen bis(2-phosphonoethyl)-4,4 0 -bipyridilium (V), with a coloration efficiency of 270 cm 2 C À1 was employed as chromophore. IV S O n O V N N P O OH OH P O HO OH + 2Cl – + 10 Introduction to electrochromism Furthermore, the electrochrome–electrolyte interface has a capacitance C. Such capacitances are well known in electrochemistry to arise from ionic ‘double layer’ effects in which the field at (or charge on) the electrode attracts a ‘layer’ – really just an excess – of oppositely charged electrolyte ions from the bulk solution. The so-called ‘rise time’ of any electrochemical system denotes the time needed to set up (i.e. fully charge) this interfacial capacitance prior to successful transfer of electronic charge across the interface. Coloration will not commence between instigation of the colouring potential and completion of the rise time, a time that may be tens of milliseconds. Applying a pulsed potential has been shown 91,92,93,94,95,96,97,98 to enhance significantly the rate at which electrochromic colour is generated, relative to potential-jump (or linear potential-increase) coloration. Although a quantitative explanation is not readily formulated, in essence the pulsing modifies the mass transport of electrochrome, eliminating kinetic ‘bottle-necks’, as outlined in Chapter 5. Pulsing is reported to speed up the response of viologen-based displays, enhancing the rate of electrochromic colour formation for ‘viologens’, 91 methyl, 92 heptyl 93 and aryl-substituted viologens; 94 pulsing also enhances the rates of electro-coloration ECDs based on TiO 2 , 95 WO 3 96,97 and ‘oxides’. 98 The Donnelly mirror in ref. 97 operates with a pulse sequence of frequency 10–20Hz. Substrate resistance The indium–tin oxide (ITO) electrode substrate in an ECD has an appreciable electrical resistance R, although its effects will be ignored here. References 98 and 99 present a detailed discussion of the implications. In many chemical systems, the uncoloured form of the electrochrome also has a high resistance R: poly(thiophene), poly(aniline), WO 3 and MoO 3 are a few examples. Sudden decreases in R during electro-coloration can cause unusual effects in the current time profiles. 100,101 1.4.4 Write–erase efficiency The write–erase efficiency is the fraction (percentage) of the originally formed coloration that can be subsequently electro-bleached. The efficiency must approach 100% for a successful display, which is a stringent test of design and construction. The write–erase efficiency of an ECD of aqueous methyl viologen MV 2þ as the electrochrome will always be low on a realistic time scale owing to the slowness of diffusion to and from the electrode through solution. The kinetics of electrochrome diffusion here are complicated since this electrochrome is 1.4 Criteria and terminology for ECD operation 11 extremely soluble in all applicable solvents for both its dicationic (uncoloured) and radical-cation (coloured) forms. Electrochrome diffusion is discussed in Chapters 4 and 5. The simplest means of increasing the write–erase efficiency is to employ a type-II or type-III electrochrome, since between the write and erase parts of the coloration cycle the coloured formof the electrochrome is not lost fromthe electrode by diffusion. The write–erase efficiency of a type-I ECD may be improved by retarding the rate at which the solution-phase electrochrome can diffuse away from the electrode and into the solution bulk. Such retardation is achieved either by tethering the species to the surface of an electrode (then termed a ‘derivatised’ electrode), with, e.g., chemical bonding of viologens to the surface of particulate 102 TiO 2 , or by immobilising the viologen species within a semi-solid electrolyte such as poly(AMPS). This is amplified in Section 14.2. Such modified type-I systems are effectively ‘quasi type-III’ electrochromes. While embedding in this way engenders an excellent long- term write–erase efficiency and a good electrochromic memory, it will also cause all response times to be extremely slow, perhaps unusably so. 1.4.5 Cycle life An adjunct to the write–erase efficiency is the electrochromic device’s cycle life which represents the number of write–erase cycles that can be performed by the ECD before any significant extent of degradation has occurred. (Such a write–erase cycle is sometimes termed a ‘double potential step’.) The cycle life is therefore an experimental measure of the ECD durability. Figure 1.1 shows such a series of double potential steps, describing the response of hydrous nickel oxide immersed in KOH solution (0.1 mol dm À3 ). The effect of film degradation over an extended time is clear. However, a 50% deterioration is often tolerable in a display. Since ECDs are usually intended for use in windows or data display units, deterioration is best gauged by eye and with the same illumination, environment and cell driving conditions, that would be employed during normal cell operation. While it may seem obvious that the cycle life should be cited this way, many tests of cell durability in the literature of electrochromism involve cycles of much shorter duration than the ECD response time . Such partial tests are clearly of dubious value, but studies of cycle life are legion. Some workers have attempted to address this problem of variation in severity of the cycle test by borrowing terminology devised for the technology of battery discharge and describing a write–erase cycle as ‘deep’ or ‘shallow’ (i.e. the cycle length being greater than or less than , respectively). 12 Introduction to electrochromism The maximising of the cycle life is an obvious aim of device fabrication. A working minimum of about 10 5 is often stipulated. There are several common reasons why devices fail: the conducting electrodes fail, the electrolyte fails, or one or both of the electrochromic layers fail. The electrolyte layers are discussed in Section 14.2, and overall device stability is discussed in Chapter 16. An individual device may fail for any or all of these reasons. Briefly, the most common causes of low cycle life are photodegradation of organic components within a device, either of the solvent or the electrochrome itself; and also the repeated recrystallisations within solid electrochromes associated with the ionic ingress and egress 99 that necessarily accompany redox processes of type-II and -III electrochromes. 1.4.6 Power consumption An electrochromic display consumes no power between write or erase cycles, this retention of coloration being called the ‘memory effect’. The intense colour of a sample of viologen radical cation remains undimmed for many months in the absence of chemical oxidising agents, such as molecular oxygen. 10 s On Time R e l a t i v e t r a n s m i t t a n c e 0 Off On Off Off Off On On Fresh Aged Figure 1.1 Optical switching behaviour of a fresh and an aged film of NiO electrodeposited onto ITO. The potential was stepped between 0 V (representing ‘off’) and 0.6 V (as ‘on’). The aged film had undergone about 500 write–erase cycles. (Figure reproduced from Carpenter, M. K., Conell, R. S. and Corrigan, D. A. ‘The electrochromic properties of hydrous nickel oxide’. Sol. Energy Mater. 16, 1987, 333–46, by permission of Elsevier Science.) 1.4 Criteria and terminology for ECD operation 13 However, no-one has ever invented a perfect battery of infinite shelf life, and any ECD(all of which follow battery operation) will eventually fade unless the colour is renewed by further charging. The charge consumed during one write–erase cycle is a function of the amount of colour formed (and removed) at an electrode during coloration (and decoloration). Schoot et al. 103 state that a contrast ratio of 20:1 may be achieved with a device employing heptyl viologen (1,1 0 -di-n-heptyl-4,4 0 - bipyridilium dibromide, VI) with a charge of 2 mC cm À2 , yielding an optical reflectance of 20%. Figure 1.2 shows a plot of response time for electrochromic coloration for HV 2þ 2Br À in water as a function of electrochemical driving voltage. 2 5 Reflectance 20% 40% 60% 80% 2 10 10 2 5 2 1 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 W r i t i n g t i m e ( m s ) Writing voltage/V a Figure 1.2 Calibration curve of electrochromic response time against the potentiostatically applied ‘writing’ potential V a (cited against SCE) for heptyl viologen dibromide (VI) (0.1 mol dm À3 ) in aqueous KBr (0.3 mol dm À3 ). It is assumed that (bleaching) ¼ (coloration) . (Figure reproduced from Schoot, C. J., Ponjee´ , J. J., van Dam, H. T., van Doorn, R. A. and Bolwijn, P. J. ‘New electrochromic memory device.’Appl. Phys. Lett., 23, 1973, 64, by permission of the American Institute of Physics.) 14 Introduction to electrochromism VI N N C 7 H 15 H 15 C 7 + 2Br – + Displays operating via cathode ray tubes (CRTs) and mechanical devices consume proportionately much more power than do ECDs. The amount of power consumed is so small that a solar-powered ECD has recently been reported, 104 the driving power coming from a single small cell of amorphous silicon. Such photoelectrochromic systems are discussed further in Chapter 15. The power consumption of light-emitting diodes (LEDs) is relatively low, usually less than that of an ECD. Furthermore, ECDs consume considerably more power than liquid crystal displays, although a LCD-based display requires an applied field at all times if an image is to be permanent, i.e. it has no ‘memory effect.’ For this reason, Cohen asserts that ECD power consumption rivals that of LCDs; 105 he cites 7 or 8 mC cm À2 during the short periods of coloration or bleaching, and a zero consumption of charge during the longer periods when the optical density remains constant. This last criterion is overstated: a miniscule current is usually necessary to maintain the coloured state against the ‘self-bleaching’ processes mentioned earlier, comparable to battery deterioration (see p. 54). 1.4.7 Coloration efficiency h The amount of electrochromic colour formed by the charge consumed is characteristic of the electrochrome. Its value depends on the wavelength chosen for study. The optimumvalue is the absorbance formed per unit charge density measured at max of the optical absorption band. The coloration efficiency is defined according to Eq. (1.7): Abs ¼ Q; (1:7) where Abs is the absorbance formed by passing a charge density of Q. A graph of Abs against Q accurately gives as the gradient. For a detailed discussion of the way such optical data may be determined; see Section 4.3. The majority of values cited in the literature relate to metal oxides; few are for organic electrochromes. A comprehensive list of coloration efficiencies is included in Section 4.3; additional values are sometimes cited in discussions of individual electrochromes. 1.4 Criteria and terminology for ECD operation 15 1.4.8 Primary and secondary electrochromism To repeat the definition, a cell comprises two half-cells. Each half-cell comprises a redox couple, needing the second electrode to allow the passage of charge through cell and electrodes. As ECDs are electrochemical cells, so each ECD requires a minimum of two electrodes. The simplest electrochromic light modulators have two electrodes directly in the path of the light beam. Solid- state electrochromic displays are, in practice, multi-layer devices (often called ‘sandwiches’; see Chapter 14). If both electrodes bear an electrochromic layer, then the colour formation within the two should operate in a complementary sense, as illustrated belowusing the example of tungsten and nickel oxides. The WO 3 becomes strongly blue-coloured during reduction, while being effectively colourless when oxidised. However, sub-stoichiometric nickel oxide is dark brown-black when oxidised and effectively colourless when reduced. When an ECD is constructed with these two oxides – each as a thin film (see Chapter 15) – one electrochrome film is initially reduced while the other is oxidised; accordingly, the operation of the device is that portrayed in Eq. (1.8): WO 3 þ M x NiO ð1ÀyÞ zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{ bleached ! M x WO 3 þ NiO ð1ÀyÞ zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{ coloured : pale yellow colourless dark blue brown-black (1:8) The tungsten oxide in this example is the more strongly coloured material, so is termed the primary electrochrome, and the NiO (1–y) acts as the secondary (or counter) electrode layer. Ideally, the secondary electrochrome is chosen in order to complement the primary electrochrome, one colouring on insertion of counter ions while the other loses that ionic charge (or gains an oppositely charged ion) concurrently with its own coloration reaction, i.e. their respective values of are of different sign. Note the way that charge passes through the cell from left to right and back again during electrochromic operation – ‘electro- chromismvia the rocking chair mechanism’, an uninformative phrase coined by Goldner et al. in 1984. 106 Nuclear-reaction analysis (NRA) is said to confirm this mechanistic mode, 107 but it is difficult to conceive of any other mechanism. In perhaps a majority of recent investigations, tungsten trioxide has been the primary electrochrome chosen owing to its high coloration efficiency, while the secondary layer has been an oxide of, e.g., iridium, nickel or vanadium. The second electrode need not acquire colour at all. So-called ‘optically passive’ materials (where ‘passive’ here implies visibly non-electrochromic) are often the choice of counter electrode for an ECD. Examples of optically 16 Introduction to electrochromism passive oxide layers include indium–tin oxide and niobium pentoxide. In an unusual design, if the counter electrode is a mirror-finish metal that is very thin and porous to ions, then ECDs can be made with one electroactive layer behind this electrode. In such a case, the layer behind the mirror electrode can be either strongly (but ineffectively) coloured or quite optically passive. Chapter 14 cites examples of such counter electrodes. In devices operating in a complementary sense, both electrodes form their colour concurrently, although it is often impossible to deconvolute the optical response of a whole device into those of the two constituent electrochromic couples. When the electrochrome is a permanently solid in both forms (that is, type III), an approximate deconvolution is possible. This requires sophisticated apparatus such as in situ ellipsometry 108 and accompanying mathematical transformations. Recently, however, the group of Hagen and Jelle 109,110,111,112,113,114,115,116 have devised an ingenious and valuable means of overcoming this fundamental problemof distinguishing the optical contributions of each electrode. Devices were fabricated in which each constituent film had a narrow ‘hole’ (a bare area) of diameter ca. 5 mm, the hole in each film being positioned at a different portion of each film. By careful positioning of a narrow spectrometer beam through the ECD, the optical response of each individual layer is obtainable, while simultaneously the electrochemical response of the overall ECD is obtained concurrently via chronoamperometry in real time. This simple yet powerful ‘hole’ method has led to otherwise irresolvable analyses of these complicated, multi layer systems. For optimal results, the holes should not exceed about one hundredth of the overall active electrode area. 1.5 Multiple-colour systems: electropolychromism While single-colour electrochromic transformations are usually considered elsewhere in this book, applications may be envisaged in which one electrochrome, or more together, evince a whole series of different colours, each coloured state generated at a characteristic applied potential. For a singlespecies electrochrome, a series of oxidation states, or charge states – each with its own colour – could be produced. Each state forms at a particular potential if each such state can be sustained, that is, if the species is ‘multivalent’ in chemical parlance. Such systems should be called electropolychromic (but ‘polyelectrochromic’ prevails). A suitable example is methyl viologen, which is colourless as a dication, MV 2þ (II), blue as a radical cation, and red–brown as a di-reduced neutral species, as described in Chapter 11. Electrochromic viologens with as many as six colours have been synthesised. 117 1.5 Multiple-colour systems: electropolychromism 17 Other systems that are electropolychromic are actually mixtures of several electrochromes. An example is Yasuda and Seto’s 118 trichromic device comprising individual pixels addressed independently, each encapsulated to contain a different electrochrome. For example, the red electrochrome was 2,4,5,7- tetranitro-9-fluorenone (VII); a product from 2,4,7-trinitro-9-fluorenylidene malononitrile (VIII) is green, and reduction of TCNQ (tetracyanoquinodimethane, IX) yields the blue radical anion TCNQ – * . The chromophores in this system always remained in solution, i.e. were type I. 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Electrochemical studies of molecular electrochromism. Sol. Energy Mater. Sol. Cells, 25, 1992, 257–68. 24 Introduction to electrochromism 2 A brief history of electrochromism 2.1 Bibliography; and ‘electrochromism’ Brief histories of electrochromism have been delineated by Chang 1 (in 1976), Faughnan and Crandall 2 (in 1980), Byker 3 (in 1994) and Granqvist 4 (in 1995). Other published histories rely very heavily on these sources. The additional histories of Agnihotry and Chandra 5 (in 1994) and Granqvist et al. 6 (in 1998) chronicle further advances in making electrochromic devices for windows. The first books on electrochromism were those of Granqvist, 4 and Monk, Mortimer and Rosseinsky, 7 which were both published in 1995. Platt 8 coinedthe term‘electrochromism’ in 1961 to indicate a colour generated via a molecular Stark effect (see page 4) in which orbital energies are shifted by an electric field. His work follows earlier studies by Franz and Keldysh in 1958, 9,10 who applied huge electric fields to a film of solid oxide causing spectral bands to shift. These effects are not the main content of this book. 2.2 Early redox-coloration chemistry In fact, redox generation of colour is not new À twentieth-century redox titration indicators come to the chemist’s mind (‘redox’, Section 1.1, implying electron transfer). However, as early as 1815 Berzelius showed that pure WO 3 (which is pale yellow) changed colour on reduction when warmed under a flow of dry hydrogen gas, 11 and in 1824, W ¨ ohler 12 effected a similar chemical reduction with sodium metal. Section 1.4 and Eq. (1.5), and Eq. (2.5) below, indicate the extensive role of WO 3 in electrochromism, amplified further in Section 6.2.1. 2.3 Prussian blue evocation in historic redox-coloration processes An early form of photography devised in 1842 by Sir John Frederick William Herschel 13 is a ubiquitous example of a photochromic colour 25 change involving electron transfer, devised for a technological application. Its inventor was a friend of Fox Talbot, who is credited with inventing silver-based photography, of like mechanism, in 1839. Herschel’s method produced photographs and diagrams by generating Prussian blue KFe III [Fe II (CN) 6 ](s) from moist paper pre-impregnated with ferric ammonium citrate and potassium ferricyanide, forming yellow Prussian brown Fe 3þ [Fe(CN) 6 ] 3À or Fe III [Fe III (CN) 6 ] (for Prussian blue details see reaction (3.12) and p. 282 ff.; for oxidation-state representation by Roman numerals; see p. 35). Wherever light struck the photographic plate, photo reduction of Fe III yielded Fe II in the complex, hence Prussian blue formation; see eq. (2.1): Fe 3þ ½Fe III ðCNÞ 6 3À ðsÞ þ K þ ðaqÞ þ e À ðhÞ ! KFe III ½Fe II ðCNÞ 6 ðsÞ; (2:1) where e À (h) represents an electron photolysed from water or other ambient donor, a process often oversimplified as resulting from reduction of Fe 3þ by the photolysed e À : ½H 2 O þ h ! e À þ fH 2 O þ g ; Fe 3þ ðaqÞ þ e À ! Fe 2þ ðaqÞ; (2:2) followed by K þ ðaqÞ þ Fe 2þ ðaqÞ þ ½Fe III ðCNÞ 6 3À ðaqÞ ! KFe III ½Fe II ðCNÞ 6 ðsÞ; (2:3) where {H 2 O þ } represents water-breakdown species. Herschel called his process ‘cyanotype’. By the 1880s, so-called ‘blueprint’ paper was manufactured on a large scale as engineers and architects required copies of architectural drawings and mechanical plans. This widespread availability revived cyanotype, as a photographic process for large reproductions, to late in the twentieth century, under the common name of ‘blueprint’. This word has become an English synonym for ‘plan’. Soon after Herschel, in 1843 Bain patented a primitive formof fax transmission that again relied on the generation of a Prussian blue compound. 14,15 It involved a stylus of pure soft iron resting on damp paper pre-impregnated with potassium ferrocyanide. In an electrical circuit, electro-oxidation of the (positive) iron tip formed ferric ion from the metal, which consumes the iron as it combines with ferrocyanide ion to produce a very dark form of insoluble Prussian blue. Thus the iron electrode generates a track of darkly-coloured deposit wherever the positive stylus touches the paper. 26 A brief history of electrochromism 2.4 Twentieth century: developments up to 1980 Probably the first suggestion of an electrochromic device involving electrochemical formation of colour is presented in a London patent of 1929, 16 which concerns the electrogeneration of molecular iodine from iodide ion. Such molecular I 2 then effects the chemical oxidation of a dye precursor, thus forming a bright colour. This example again represents an electrochromic reaction. However, the proneness of iodide to photo-oxidation is discouraging to any further development. In 1962, Zaromb published now-neglected studies of electrodepositing silver in desired formats from aqueous solutions of Ag þ 17,18 or complexes thereof. 19 Electro-reduction of Ag(I) ion yields a thin layer of metallic silver that reflects incident light if continuous, or is optically absorbent if the silver is particulate. Zaromb called his system an ‘electroplating light modulator’, and explicitly said it represented a ‘viable basis for a display’. His work was not followed up until the mid 1970s, e.g. by the groups of Camlibel 20 and of Ziegler, who deposited metallic bismuth. 21,22 The first recorded colour change following electrochemical reduction of a solid, tungsten trioxide, was that of Kobosew and Nekrassow 23 in 1930. The colour generation reaction (cf. Section 9.2.1) followed Eq. (2.4): WO 3 ðsÞ þ xðH þ þ e À Þ ! H x WO 3 ðsÞ: (2:4) Their WO 3 was coated on an electrode, itself immersed in aqueous acid. The electrode substrate is unknown, but presumably inert. By 1942 Talmay 24,25 had a patent for electrochromic printing – he called it ‘electrolytic writing paper’ – in which paper was pre-impregnated with particulate MoO 3 and/or WO 3 . A blue–grey image forms following an electron- transfer reaction: in effect, the electrode acted as a stylus, forming colour wherever the electrode traversed the paper. The electrochromic coloration reaction followed Eq. (2.4) above, and the proton counter ion came from the ionisation of the water in the paper. In 1951, Brimm et al. 26 extended the work of Kobosew and Nekrassow to effect reversible colour changes, for Na x WO 3 immersed in aqueous acid (sulfuric acid of concentration 1 mol dm À3 ). Alittle later, in 1953, Kraus of Balzers in Lichtenstein 27 advocated the reversible colour–bleach behaviour of WO 3 (again immersed in aqueous H 2 SO 4 ) as a basis for a display: this work was regrettably never published. Probably the first company to seek commercial exploitation of an electrochromic product was the Dutch division of Philips, again in the early 1960s. 2.4 Twentieth century: developments up to 1980 27 Their prototype device utilised an aqueous organic viologen (see Chapter 11), heptyl viologen (HV: 1,1 0 -n-heptyl-4,4 0 -bipyridilium) as the bromide salt. Their first patent dates from 1971, 28 and their first academic paper from 1973. 29 At much the same time, Imperial Chemical Industries (ICI ) in Britain initiated a far-reaching program to develop an electrochromic device. Like Philips, they first analysed the response of heptyl viologen in water but quickly decided its coloration efficiency was too low, and changed to the larger viologen cyanophenyl paraquat [CPQ: 1,1 0 -bis(1-cyanophenyl)-4,4 0 -bipyridilium] as the sulfate salt. Their first patent dates from May 1969. 30 By early 1970, ICI was seeking tenders to commercialise a CPQ-based device. 31,32 Other devices based on heptyl viologen were being investigated by Barclay’s group at Independent Business Machines (IBM), 33 and by Texas Instruments in Dallas, although their work was not published until after their programme was discontinued. 34 As none of these studies attracted much attention, probably most workers now attribute the first widely accepted suggestion of an electrochromic device to Deb (then at Cyanamid in the USA) in 1969, 35 following a technical report from the previous year. 36 Deb formed electrochromic colour by applying an electric field of 10 4 V cm À1 across a thin film of dry tungsten trioxide vacuum deposited on quartz: he termed the effect ‘electrophotography’. (This wording may reflect his earlier work dating from 1966, when he analysed thin-film vacuum-deposited MoO 3 on quartz, which acquired colour following UV irradiation. 37 ) Figure 2.1 shows a schematic representation of his cell. In fact, Deb’s film of WO 3 was open to the air rather than immersed in ion- containing electrolyte solutions, suggesting that the mobile counter cations might have come from simultaneous ionisation of interstitial and/or adsorbed water. At the time, Deb suggested the colour arose from F-centres, much like the colour formed by heating or irradiating crystals of metal halides in a field. The background to Deb’s work was recounted much later, 38 in 1995. In 1971, Blanc and Staebler 39 produced an electrochromic effect superior to most previously published. They applied electrodes to the opposing faces of doped, crystalline SrTiO 3 and observed an electrochromic colour move into the crystal from the two electrodes. The charge carriers are (apparently) oxide ions, which migrate through the crystal in response to redox changes at the electrodes. Their work has not been followed, probably because no viable device was likely to ensue as their crystal had to be heated to ca. 200 8C. In 1972, Beegle developed a display of WO 3 having identical counter and working electrodes, with an intervening opaque layer. 40 28 A brief history of electrochromism Nowadays most workers cite Deb’s later paper, 41 which dates from 1973, as the true birth of electrochromic technology. It is often said that this seminal paper describes the first ‘true’ electrochromic device, with a film of WO 3 immersed in an ion-containing electrolyte. In fact ref. 41 does not mention aqueous electrolytes at all but rather, another filmof WO 3 vacuumevaporated onto a substrate of quartz. Deb does correctly identify the ionisation of water as the source of the protons necessary for Eq. (2.4), but suggests oxide ions extracted from the WO 3 lattice, rather than proton insertion, for the coloration mechanism. 42 Within a year of Deb’s 1973 paper, Green and Richman 43 in London proposed a system based on WO 3 in which the mobile ion was Ag þ . In 1975, Faughnan et al. of the RCA Laboratories in Princeton, New Jersey, in a pivotal review, 44 reported WO 3 undergoing reversible electrochromic colour changes while immersed in aqueous sulfuric acid. Faughnan et al. analysed the speed of colour change in terms of Butler–Volmer electrode dynamics, establishing a pioneering model of electro-bleaching 45 and electro-coloration 46 that is still relevant now. Mohapatra of the Bell Laboratories in New Jersey published the first description of the reversible electro-insertion of lithiumion, Eq. (2.5), in 1978: 47 WO 3 ðsÞ þ xðLi þ ðaqÞ þ e À Þ ! Li x WO 3 ðsÞ: (2:5) Figure 2.1 Electrocoloration of thin-film WO 3 film using a surface electrode geometry. (Figure reproduced from Deb, S. K. ‘Reminiscences on the discovery of electrochromic phenomena in transition-metal oxides’. Sol. Energy Mater. Sol. Cells, 39, 1995, 191–201.) 2.4 Twentieth century: developments up to 1980 29 Meanwhile, the electrochromism of organic materials also developed momentum. In 1974, Parker et al. 48 prepared methoxybiphenyl species, the electrogenerated radical cations of which are intensely coloured (see p. 379). While he nowhere employs the word ‘electrochromism’ or its cognates, his paper, displaying acute awareness of the technological scope of such colour changes, cited values of max for the several radical cations. Later, Kaufman et al. of IBM in New York published the first report of an electrochromic polymer comprising an alkyl-chain backbone with pendant electroactive species 49,50 (see Section 10.2). The details in his preliminary report 51 are as indistinct as are many patents, but his later work reveals that his electrochromes were based on tetrathiafulvalene and quinone moieties. 49 In 1979 came the first account of an electrochromic conducting polymer, when Diaz et al. 52 (also of IBM in San Jose, California), announced the electrosynthesis of thin-film poly(pyrrole); see Section 10.3. The electrochemical literature of the twentieth century will undoubtedly provide further early reports of electrochromism. References 1. Chang, I. F. Electrochromic and electrochemichromic materials and phenomena. In Kmetz, A. R. and von Willisen, F. K. (eds.), Non-emissive Electrooptical Displays, New York, Plenum Press, 1976, pp. 155–96. 2. Faughnan, B. W. and Crandall, R. S. Electrochromic displays based on WO 3 . In Pankove, J. I. (ed.), Display Devices, Berlin, Springer Verlag, 1980, ch. 5, pp. 181–211. 3. Byker, H. J. Commercial developments in electrochromics. Proc. Electrochem. Soc., 94–2, 1994, 1–13. 4. Granqvist, G. C. Handbook of Inorganic Electrochromic Materials, Amsterdam, Elsevier, 1995. 5. Agnihotry, S. A. and Chandra, S. Electrochromic devices: present and forthcoming technology, Indian J. Eng. Mater. Sci., 1, 1994, 320–34. 6. Granqvist, C. G., Azens, A., Hjelm, A., Kullman, L., Niklasson, G. A., R ¨ onnow, D., Strømme Mattson, M., Veszelei, M. and Vaivers, G. Recent advances in electrochromics for smart window applications, Sol. Energy, 63, 1998, 199–216. 7. Monk, P. M. S., Mortimer, R. J. and Rosseinsky, D. R. Electrochromism: Fundamentals and Applications, Weinheim, VCH, 1995. 8. Platt, J. R. Electrochromism, a possible change of color producible in dyes by an electric field. J. Chem. Phys., 34, 1961, 862–3. 9. Franz, W. Z. Naturforsch, 13A, 1958, 44, as cited in ref. 7. 10. Keldysh, L. V. Zh. Eksp. Teor. Fiz., 34, 1958, 1138, as cited in ref. 7. 11. Berzelius, J. J. Afhandlingar i fysik. Kemi Och Mineralogie, 4, 1815, 293, as cited in ref. 4. 12. F. W ¨ ohler. Ann. Phys., 2, 1824, 350, as cited in ref. 4. 13. See the article ‘True Blue (cyanotype) part 2: blue history’ by Peter Marshall, available [online] at photography.about.com/library/weekly/aa061801b.htm (accessed 26 January 2006). 30 A brief history of electrochromism 14. Bain, A., UK Patent, 27 May 1843, as cited in ref. 4. 15. Hunkin, T. Just give me the fax. New Scientist, 13 February 1993, 33–7. 16. Smith, F. H., British Patent, 328,017, 1929, as cited in ref. 4. 17. Zaromb, S. Theory and design principles of the reversible electroplating light modulator. J. Electrochem. Soc., 109, 1962, 903–12. 18. Zaromb, S. Geometric requirements for uniform current densities at surfaceconductive insulators of resistive electrodes. J. Electrochem. Soc., 109, 1962, 912–18. 19. Mantell, J. and Zaromb, S. Inert electrode behaviour of tin oxide-coated glass on repeated plating–deplating cycling in concentrated NaI–AgI solutions. J. Electrochem. Soc., 109, 1962, 992–3. 20. Camlibel, I., Singh, S., Stocker, H. J., Van Ultert, L. G. and Zydzik, G. I. An experimental display structure based on reversible electrodeposition, Appl. Phys. Lett., 33, 1978, 793–4. 21. Howard, B. M. and Ziegler, J. P. Optical properties of reversible electrodeposition electrochromic materials, Sol. Energy Mater. Sol. Cells, 39, 1995, 309–16. 22. Ziegler, J. P. Status of reversible electrodeposition electrochromic devices, Sol. Energy Mater. Sol. Cells, 56, 1995, 477–93. 23. Kobosew, N. and Nekrassow, N. I. Z. Electrochem., 36, 1930, 529, as cited in ref. 4. 24. Talmay, P. US Patent 2,281,013, 1942, as cited in ref. 4. 25. Talmay, P. US Patent 2,319,765, 1943, as cited in ref. 4. 26. Brimm, E. O., Brantley, J. C., Lorenz, J. H. and Jellinek, M. H. J. Am. Chem. Soc., 73, 1951, 5427, as cited in ref. 4. 27. Kraus, T. Laboratory report: Balzers AG, Lichtenstein, entry date 30 July 1953, as cited in ref. 4. 28. Philips Electronic and Associated Industries Ltd. Image display apparatus. British Patent 1,302,000, 4 Jan 1973. [The patent was first filed on 24 June 1971.] 29. Schoot, C. J., Ponje´ e, J. J., van Dam, H. T., van Doorn, R. A. and Bolwijn, P. T. Newelectrochromic memory display. Appl. Phys. Lett., 23, 1973, 64–5. [The paper was first submitted in April 1973.] 30. Short, G. D. and Thomas, L. Radiation sensitive materials containing nitrogenous cationic materials, British Patent 1,310,813, published 21 March 1973. [The patent was first filed on 28 May 1969.] 31. J. G. Allen, ICI Ltd. Personal communication, 1987. 32. Kenworthy, J. G., ICI Ltd. Variable light transmission device. British Patent 1,314,049, 18 April 1973. [The patent was first filed on 8 Dec 1970.] 33. Barclay, D. J., Bird, C. L., Kirkman, D. K., Martin, D. H. and Moth, F. T. An integrated electrochromic data display, SID 80 Digest, 1980, abstract 12.2, 124. 34. For example, see Jasinski, R. J. N-Heptylviologen radical cation films on transparent oxide electrodes. J. Electrochem. Soc., 125, 1978, 1619–23. 35. Deb, S. K. Anovel electrophotographic system, Appl. Opti., Suppl. 3, 1969, 192–5. 36. Van Ruyven, L. J. The role of water in vacuum deposited electrochromic structures. Cyanamid Technical Report, 14, 1968, 187. As cited in Giglia, R. D. and Haake, G. Performance achievements in WO 3 based electrochromic displays. Proc. SID, 12, 1981, 76–81. 37. Deb, S. K. and Chopoorian, J. A. Optical properties and color-center formation in thin films of molybdenum trioxide. J. Appl. Phys., 37, 1966, 4818–25. 38. Deb, S. K. Reminiscences on the discovery of electrochromic phenomena in transition-metal oxides, Sol. Energy Mater. Sol. Cells, 39, 1995, 191–201. References 31 39. Blanc, J. and Staebler, D. L. Electrocoloration in SrTiO 3 : vacancy drift and oxidation–reduction of transition metals. Phys. Rev. B., 4, 1971, 3548–57. 40. Beegle, L. C. Electrochromic device having identical display and counter electrodes. US Patent 3,704,057, 28 November 1972. 41. Deb, S. K. Optical and photoelectric properties and colour centres in thin films of tungsten oxide. Philos. Mag., 27, 1973, 801–22. [The paper was submitted in November 1972.] 42. Deb, S. K. Some aspects of electrochromic phenomena in transition metal oxides. Proc. Electrochem. Soc., 90–92, 1990, 3–13. 43. Green, M. and Richman, D. A solid state electrochromic cell – the RbAg 4 I 5 /WO 3 system. Thin Solid Films, 24, 1974, S45–6. 44. Faughnan, B. W., Crandall, R. S. and Heyman, P. M. Electrochromism in WO 3 amorphous films. RCA Rev., 36, 1975, 177–97. 45. Faughnan, B. W., Crandall, R. S. and Lampert, M. A. Model for the bleaching of WO 3 electrochromic films by an electric field. Appl. Phys. Lett., 27, 1975, 275–7. 46. Crandall, R. S. and Faughnan, B. W. Dynamics of coloration of amorphous electrochromic films of WO 3 at low voltages. Appl. Phys. Lett., 1976, 28, 95–7. 47. Mohapatra, S. K. Electrochromism in WO 3 . J. Electrochem. Soc., 285, 1978, 284–8. [The paper was submitted for publication in April 1977.] 48. Ronla´ n, A., Coleman, J., Hammerich, O. and Parker, V. D. Anodic oxidation of methoxybiphenyls: the effect of the biphenyl linkage on aromatic cation radical and dication stability. J. Am. Chem. Soc., 96, 1974, 845–9. 49. Kaufman, F. B., Schroeder, A. H., Engler, E. M. and Patel, V. V. Polymer- modified electrodes: a newclass of electrochromic materials. Appl. Phys. Lett., 36, 1980, 422–5. 50. Kaufman, F. B. and Engler, E. M. Solid-state spectroelectrochemistry of crosslinked donor bound polymer films. J. Am. Chem. Soc., 101, 1979, 547–9. 51. Kaufman, F. B. New organic materials for use as transducers in electrochromic display devices. Conference Record., 1978 Biennial Display Research Conference, Publ. IEEE, 23–4. 52. Kanazawa, K. K., Diaz, A. F., Geiss, R. H., Gill, W. D., Kwak, J. F., Logan, J. A., Rabolt, J. F. and Street, G. B. ‘Organic metals’: polypyrrole, a stable synthetic ‘metallic’ polymer. J. Chem. Soc., Chem. Commun., 1979, 854–5. 32 A brief history of electrochromism 3 Electrochemical background 3.1 Introduction This chapter introduces the basic elements of the electrochemistry encompassing the redox processes that are the main subject of this monograph. Section 3.2 describes the fundamentals, starting with the origin of the cell emf (the electric potential across it), introducing the use of electrode potentials, and their determination in equilibrium conditions within simple electrochemical cells. In the first example (with electroactive species that resemble type-I electrochromes), the reactants are all ions in solution. In the second example, the cell assembly comprises two electrodes, each a metal in contact with a solution of its own ions, somewhat resembling type-II electrochromes. Though electrochromic electrodes are intrinsically more complicated than the two examples cited here, they follow just the principles established. Details of fabrication for electrochromic devices (ECDs) appear in Chapter 14. Section 3.3 exemplifies the kinetic features underlying electrochromic coloration. In it, the rates of mass transport and those of electron transfer, the three rate-limiting (thus current-limiting) processes encountered during the electrochemistry, are described. Diffusion of both electrochrome and counter ions is discussed more fully in Chapter 5, to illustrate the way charge-carrier movement limits the rate of the coloration/bleaching redox processes within ECDs. Section 3.4 covers electrochemical methods involving dynamic electrochemistry, particularly cyclic voltammetry, which is important in studying electrochromism; three-electrode systems are required here. More comprehensive treatments of electrochemical theory will be found elsewhere. 1,2,3 33 3.2 Equilibrium and thermodynamic considerations 3.2.1 A cell with dissolved ions as reactants: the Gibbs energy and electromotive force The fundamental origin of an electrochemical emf (‘electromotive force’) in a cell sometimes seems obscure. Basically it arises from the energy of a chemical reaction involving electron transfer (exactly, the Gibbs free energy change for unit amount of reaction). The simplest example involves solely ions in water, such as the reaction that occurs on mixing the ions: Fe 2þ þMn 3þ ÐFe 3þ þMn 2þ . (3.1) This electron transfer reaction is known to proceed from left to right spontaneously, effectively to completion, and quite rapidly. If Fe 2þ and Fe 3þ were contained in one solution and Mn 2þ and Mn 3þ in another, and the two solutions were connected via a tube containing a salt solution, there would be no way for the reaction to proceed, although a ‘cell’ would have been partly created. If however two inert wires, of say Pt, were inserted into each of the metal-ion solutions, then on connecting the wires, Fe 2þ would transfer electrons e À to the Pt so becoming Fe 3þ , while at the other Pt, Mn 3þ would gain e À becoming Mn 2þ . Thus the reaction proceeds as it would on directly mixing the reactants, but now via the electrode processes, each at its own rate, with rate constants k et . The flow of electrons in the wire is accompanied by net ionic motion through the solutions: a current flows through the cell and in the wire. If, instead of connecting the Pt wires, a meter, or opposing voltage, were connected, so frustrating the electrode processes, these would indicate the voltage (the cell emf, E (cell) ) evoked by the tendency of the reactions of the ions to proceed as stated, owing to the Gibbs free energy change DGthat would accompany direct reaction. The connection of E (cell) with the thermodynamics of the cell reaction then follows from the identification ÁG ¼ ÀnFE ðcellÞ (3.2) as a charge nF traverses a potential E (cell) in a (virtual) occurrence of the cell reaction, Eq. (3.1). Here n is the number of electrons transferred in the written reaction (1 in this example), and F is the Faraday constant, the charge on 6.022 Â10 23 electrons, i.e. the charge involved in unit-quantity (a mole) of a complete reaction where n ¼1. In general, an electrochemical cell comprises a minimum of two electrodes, each made up of two different ‘charge states’ of a particular chemical. For 34 Electrochemical background inorganic species, the charge state is more properly the oxidation state or (colloquially) redox state, which is shown in superscripted Roman numerals by the element symbol, thus Fe II , Fe III and Mn II , Mn III in the initial examples, sometimes as Fe(II), Fe(III), and so on. This is a widely used ‘chemicalaccountancy’ abbreviation ploy based on summarily assigning a charge of 2 À to the oxide ion, e.g. as in W VI O 3 . Here the precise charge distribution will differ considerably from the conventional, assigned, oxidation state. (The use of Roman numerals for oxidation states in chemistry differs fromthat used for gaseous species by spectroscopists, who write an atom as MI, a singly charged ion M þ as MII, M 2þ as MIII, etc., the numerals here being on par and unparenthesised.) 3.2.2 Individual electrode processes Consider what happens at the electrodes individually. At an electrode, the two states stay in equilibrium (i.e. constant in composition) at only one potential, the ‘equilibrium potential’, applied to this electrode. A comparable statement is also true for the other electrode. ‘Applying a potential’ always requires the presence in the cell of the second electrode also connected to the source, say a battery, of the external potential. If the potential applied to the electrode, when in contact with both redox states, is different from this equilibrium electrode potential, then one of two ‘redox’ reactions (or ‘half-reactions’) can occur: electron gain – reduction, Eq. (3.3): Oþn e À !R. (3.3) or electron loss – oxidation, the reverse of Eq. (3.3) – which will alter compositions at the electrode. O and R, like Mn 3þ , Mn 2þ or Fe 3þ , Fe 2þ are called a ‘redox couple’. (Equation (3.3), itself abbreviated to ‘O,R’, is sometimes loosely referred to as ‘the O,R electrode’.) 3.2.3 Electrode potentials defined and illustrated The potential of an unreactive metal in contact, and in equilibrium, with the two redox states, is termed the electrode potential E O,R or, colloquially, the ‘redox potential’. When just this value of potential is applied from a battery or voltage source, no overall composition change occurs via Eq. (3.3), but electron transfer does persist because in these conditions the forward and reverse processes in Eq. (3.3) are conduced to proceed at the same rate. If no external potential is applied, the O,R species at their particular concentrations control the energy (and hence the potential) of the electrons in 3.2 Equilibrium and thermodynamic considerations 35 the metal contact, thereby allowing electrical communication to a meter. (Measurement of this energy in a single electrode can be contemplated in principle but is difficult in practice and will henceforth be viewed as impossible.) While a value of E O,R for the (O,R) half cell cannot be determined independently, only differences in electric potential between two sites being ordinarily accessible by communication to a meter, the usual cell construction comprising two electrodes intrinsically avoids this problem. Assigning an arbitrary value to E O,R for one O,R couple (the H þ /H 2 couple) then establishes, for all other couples, values of their electrode potentials as appear in tabulations. This is amplified below. Only redox couples (i.e. ‘electroactive materials’) that can transfer electrons with reasonable rapidity can set up stable redox potentials for measurement. The application of a potential greater or less than the equilibrium value (see ‘Overpotentials’ on p. 42 below) can effect desired composition changes in either direction by driving the electron process in either direction, to the required extent. Only with fast redox couples can the composition be rapidly governed by an applied voltage. For rapidly reacting redox couples, the (equilibrium) electrode potential E O,R is governed by the ratio of the respective O,R concentrations (which are related to their ‘activities’ – a thermodynamic concept, see next paragraph) by a form of the Nernst equation: E O.R ¼ E F O.R þ RT nF ln aðOÞ aðRÞ . (3.4) where E F is the standard electrode potential (see below), the terms a are the activities, R is the gas constant, F the Faraday constant, T the thermodynamic temperature and n is the number of electrons in the electron-transfer reaction in Eq. (3.3). The two oxidation states O and R can be solid, liquid, gaseous or dissolved. Dissolved states can comprise either liquids or solids as solvent. Activity may be described as the ‘thermodynamically perceived concentration’. The relationship between concentration c and activity a is: a ¼(c/c std ) Âg, where g is the dimensionless activity coefficient representing interactions with ambient ions, and c std is best set at unity in the chosen concentration units. Observed values of g for ions are somewhat less than 1 in moderately dilute aqueous solutions. Here just for illustration we take activities of ions (or other solutes) in liquid solution as being the ionic concentrations (which is empirically true if always in a maintained excess of inert salt). Activities of gases are closely enough their pressures, while activities of pure solids – those that remain unaffected in composition by possible redox reactions, thus being always 36 Electrochemical background constant in composition – are assigned the value unity. However, when solid electrode material undergoes a redox reaction where the product forms a solid solution within the reactant, the respective activities are represented by mole fractions x; but if the result of redox reaction is a mixture of two pure bulk solids, then each is represented as being of unit activity. The term E F O.R is the standard electrode potential defined as the electrode potential E O,R measured at a standard pressure of 0.1013 MPa and designated temperature, with both O and R (and any other ion species in the redox reaction) present at unit activity. Fundamentally, the value of E F O.R is determined by the effective condensed-phase electron affinity of O(or, equivalently, the effective condensed-phase ionisation potential of R) on a relative scale. This scale of E F O.R values was established by assigning a particular value to one selected redox system, by convention zero for H þ /H 2 , as detailed below. 3.2.4 A cell with metal electrodes in contact with ions of those metals Figure 3.1 shows an electrochemical cell that comprises our second simple example. The left-hand electrode is a zinc rod immersed in an aqueous solution containing Zn 2þ ; the two redox states Zn 2þ , Zn comprise the redox couple and, when connected to an external wire, make up the redox electrode. As in Fig. 3.1, one of the redox species in Eq. (3.3) also functions as the contact electrode by which E may be monitored, since zinc metal is a good conductor, Glass sleeves Salt bridge Solution containing zinc ion Solution containing copper ion Rodof zinc metal Rodof copper metal Voltmeter to read E (cell) V Figure 3.1 Schematic of the primitive cell Zn(s)jZn 2þ (aq)jjCu 2þ (aq)jCu(s) for equilibrium measurements. Each metal rod is immersed in a solution of its own ions: the two half cells are Zn 2þ , Zn and Cu 2þ , Cu. 3.2 Equilibrium and thermodynamic considerations 37 as is copper. Both electrodes, however, need to be connected to the same ‘inert’ conducting material in connections between the cell and meter; Pt is often used, as in the introductory example. For other redox couples the inert metal is not written but taken as understood. The ‘inert electrodes’ – better, inert contacts – do not contribute to the electrode reaction. They comprise an inert metal such as platinum or gold in contact with two oxidation states O,R of a chemical species dissolvedeither inwater or other solvent, or in solid solution, or otherwise from gaseous, insoluble-salt, or pure-liquid components. The spontaneous reaction in the cell depicted in Figure 3.1 is the following: Cu 2þ ðaqÞ þZnðsÞ !CuðsÞ þZn 2þ ðaqÞ. (3.5) where (s) denotes ‘solid’ and (aq) is aqueous (alternatively here, one uses (soln) for general solvent, or specifies which solvent by suitable abbreviation). The suffixes (l) and (g) are for ‘liquid’ and ‘gas,’ and there is a need for (s. soln) meaning ‘solid solution’, that of one species within another, forming a solid. The Nernst equation for the whole cell is: E ðcellÞ ¼ E F ðcellÞ À RT nF ln ½CuðsÞ½Zn 2þ ½Cu 2þ ½ZnðsÞ ¼ E F ðcellÞ À RT nF ln ½Zn 2þ ½Cu 2þ . (3.6) where the square brackets [ ] represent concentrations (better, activities), but the fictional values for the metals are conventionally represented by unit activity as in the right-hand form of the equation here. Comparably with our first example, the cell depicted would therefore spontaneously produce current if the electrodes were connected externally with a conducting wire, the ‘applied potential’ then obviously being zero and not E (cell) . This reaction proceeds via the two reactions, Cu 2þ þ2e À !Cu and Zn!Zn 2þ þ2e À at the two respective electrodes. The resultant flow of electrons e À is discernible as an external current I in the wire. Concomitant ion motion occurs within the solution phase in attempting to maintain electrical neutrality throughout the cell. The direction of the reaction is reflected in the relative values of the two electrode potentials E, evaluated as outlined below. The magnitude of I depends on the net rate of reaction (3.3) or its reverse, when applicable, at the more slowly operating electrode. The electrode reactions are shown above as simple processes though in detail comprising a complicated series of steps. To exemplify, aqueous Cu 2þ has hexacoordinated water molecules, two on longer ‘polar’ bonds than the other four ‘equatorial’ waters. All these have to be shed, in obscure steps; meanwhile Cu 2þ becomes Cu þ then Cu 0 atoms, then metal-lattice components. So in such an apparently simple process, appreciable mechanistic 38 Electrochemical background complexity underlies the simplified reaction cited. Thus even greater complexity can be expected in the chemically more intricate electrochromic systems dealt with later. 3.2.5 The cell emf and the electrode potentials: the hydrogen scale The amount of Zn 2þ in solution will remain constant, that is, at equilibrium, only when the potential applied to the Zn equals the electrode potential E Zn 2þ .Zn and, simultaneously, the copper redox couple (right-hand side of the cell) is only at equilibrium when the potential applied to the copper is E Cu 2þ .Cu . Neither electrode potential as explained above is known as an absolute or independent value: only the difference between the two, that is, E (cell) , is the measurable quantity. Then E ðcellÞ ¼ E ðright-hand sideÞ ÀE ðleft-hand sideÞ þE j ¼ E Cu 2þ . Cu ÀE Zn 2þ . Zn þE j . (3.7) where E j is a junction potential at the contact between the solutions about the two electrodes, usually minimised. Further detail concerning cell notation is set out in ref. 1. (E j is usually of unknown magnitude but approaches zero when the two solutions are nearly similar in composition. Alternatively, precautions can be taken to minimise the value of E j via, e.g., a ‘salt bridge’, a tube containing suitable electrolyte, between the two solutions. Often an inert electrolyte uniformly distributed throughout the cell suffices.) E (cell) is then the observed electrical potential difference to be applied across the cell to effect zero current flow, i.e. to prevent thereby any redox reaction at either electrode, so ‘preserving equilibrium’, and is simply the difference between the electrode potentials: E ðcellÞ ¼ E ðright-hand sideÞ ÀE ðleft-hand sideÞ . (3.8) This statement is obviously applicable to all electrochemical cells operating ‘reversibly’ (i.e. rapidly). E (cell) may be measured on a voltmeter by allowing a negligibly small (essentially zero) current to flow through the voltmeter, but applying a measured potential from an external source that exactly opposes E (cell) is the precision choice. At zero current, E (cell) is the electromotive force (‘emf’) of the cell. When we wish to emphasise that the electrodes are being kept at equilibrium by an externally applied potential, we shall write E (eq) instead of E (cell) . For many redox couples, an electrode-potential scale has been devised. After measurement of E (cell) , if one of the electrode potentials which comprise 3.2 Equilibrium and thermodynamic considerations 39 E (cell) is summarily assigned a value, then the other is predetermined, following Eq. (3.7). In order to establish this formal scale, the half cell Pt j H 2 ðgÞð1 atmÞ. H þ (aq, unit activity) is assigned an electrode potential E F of zero for all temperatures. This is the standard hydrogen electrode (SHE), in which the electrode reaction is H þ ðaqÞ þe À ¼ 1 , 2 H 2 ðgÞ. (3.9) It is the standard reference electrode: from comparisons made with cells in which one of the electrodes is a SHE, all standard electrode potentials are cited with respect to it. (Since no single ionic species like H þ can make up a solution, to emulate the extreme dilutions that approximate to single-ion conditions, Nernst-equation extrapolation procedures can correct for finite-concentration effects. These considerations apply also to Eq. (3.4). This ‘activity-coefficient’ factor is henceforth supererogatory for our purposes.) Unless stated otherwise, the solvent is water. Any change of solvent changes the values of E F and, in general, alters the sequence of E F values somewhat. Note that in tabulations, 2,3,4,5 the half reactions (putatively taking place in ‘half cells’) to which these E F refer, are formally written as reduction reactions with the electron e À on the left-hand side. The SHE is the primary reference electrode, but is thought cumbersome and care is needed handling H 2 . Thus other, ‘secondary’, reference electrodes are preferred. The most common are the saturated calomel electrode (SCE) and the silver–silver chloride electrode. Quasi-reference electrodes are also admissible, the most common being a bare silver wire, presumably bearing traces of silver oxide to complete the redox couple. Potentials cited in this text have been converted to the saturated calomel electrode (SCE) potential scale, when aqueous electrolyte solution was used. (This attempt at uniformity will have involved cumbersomely reversing the procedures followed by some authors, of citing potentials with respect to zero for a SHE, for values measured with respect to an SCE, then ‘corrected’ to the hydrogen scale. We have used the value of 0.242 V for the SCE on the hydrogen scale. 6 ) 3.2.6 Electrochromic electrodes To link the introductory electrochemical examples above with electrochromic systems, we cite the widely studied tungsten trioxide electrode: W VI O 3 ðsÞ þe À !W V O 3 ðsÞ. (3.10) 40 Electrochemical background This is an idealisation of the reaction that in practice proceeds only fractionally to the extent of the insertion coefficient x (x <1 and in many cases <<1): W VI O 3 ðsÞ þxe À þxM þ ðsolnÞ !M x ðW V Þ x ðW VI Þ 1Àx O 3 ðs. solnÞ. (3.11) where the product is a solid solution with mole fractions x incorporating an unreactive electrolyte cation M þ , often Li þ , but sometimes H þ . The counter cations may not always be unreactive. Further detail follows in Section 6.4. Another oft-studied electrochrome is Prussian blue (PB) that undergoes the half-reaction, here represented in the reductive bleaching process in Eq. (3.12), the blue pigment PB on the left being decolourised: M þ Fe 3þ ½Fe II ðCNÞ 6 4À ðsÞ þe À þM þ ðsolnÞ !ðM þ Þ 2 Fe 2þ ½Fe II ðCNÞ 6 4À ðsÞ. blue white ðclearÞ (3.12) In the formulae, each CN is actually CN À and M þ is usually K þ . The oxidation-state notation allows a shorthand version of the essential reaction, Fe III ½ðFe II ðCNÞ 6 þe À !Fe II ½ðFe II ðCNÞ 6 . (3.13) where only the actual chromophore segment can thus be shown. 3.3 Rates of charge and mass transport through a cell: overpotentials To reiterate, an electrochromic device is fundamentally an electrochemical cell. Applying a potential V a 6¼E (cell) across the cell causes charge to flow, and hence effects electrochromic operation. As just outlined, these charges enforce the consumption and generation of redox materials within the cell. Above a particular applied potential V a , the reaction in the cell will proceed oxidatively at one electrode and reductively at the other and below it the electrochemical reactions at the electrodes are the reverse of these. At only one applied potential is the current through the cell zero: we call this potential the equilibrium potential E (eq) ¼E (cell) . A steady state exists at E (eq) and no charge is consumed at either electrode To elaborate, considering the electrodes separately, above a certain potential applied to a particular electrode, the reaction there within the cell is an oxidation reaction, and below it the electrode reaction is reduction. Complementary processes must occur at the partner electrode. Considering both electrodes, at only one potential applied across the cell is the current through the cell zero: at this equilibrium potential, E (eq) ¼E (cell) . 3.3 Rates of charge and mass transport through a cell 41 As before, we concentrate attention on one electrode. The charge that flows is measured per unit time as current I, which is clearly proportional to the rate at which electronic charge Q at an electrode is consumed by the electroactive species, or generated from it, by reduction or oxidation, respectively, I ¼ dQ dt . (3.14) If the redox (electroactive) species are in solution, the magnitude of an electrochemical current is a function of three rates at that electrode: (i) the rate of electron transport through the materials comprising the electrode; (ii) the rate of electron movement across the electrode–solution interface, and (iii) the rate at which the electroactive material (ion, atom or molecule) moves through solution prior to a successful electron-transfer reaction (also, in the case of solid electroactive materials, involving the movement of non-electroactive ions if they are taken up or lost by electroactive solids). Processes (i) and (ii) are termed charge transfer (or charge transport); process (iii) involves mass transfer or transport. When net (observable) current flows, the slowest of the three rates is ‘(overall) rate limiting’, governing the overall rate of charge movement in a device or electrode process. Rate (i) is determined by the magnitude of the electronic conductivity s of the material from which the electrode is constructed, when one or both components of the redox couple are solid. Electrodes comprising platinum, gold or glassy carbon contacts possess high electronic conductivities s so rate (i) is rarely rate limiting with such substrates. For transparent electrode systems fluoride-doped tin oxide or ITO act the role, in the place of metals, of the ‘inert contact’ to the redox species. Their conductivities are both low relative to true metals, so rate (i) can apply in such systems. The magnitude of rate (ii) is ‘activated’, that is, the systemmust surmount an energy barrier prior to electron transfer. The magnitude of rate (ii) is governed by the rate constant of the electron-transfer process k et , and is dictated by the overpotential j of the electrode, defined by Eq. (3.15): j ¼ V a ÀE ðeqÞ . (3.15) The rate constants k et are potential dependent, the ‘constancy’ appellation referring to concentration dependences at a predetermined potential. Thus k et is a curious rate constant dependent on the overpotential j, a complication dealt with below in the Butler–Volmer treatment. (In the literature, overpotential and coloration efficiency are unfortunately represented by the same symbol j. In later chapters, overpotential will be spelt out, and the symbol j alone will mean only coloration efficiency.) 42 Electrochemical background Overpotential has sign as well as magnitude. More usefully, it is applied to just one electrode. By definition, an overpotential of zero indicates equilibrium, and hence zero current, i.e. no conversion of electrochrome to form its coloured state, and hence no electrochromic operation. Provided the overpotential applied is sufficiently large, k et will be high and therefore rate (ii) will not be rate limiting. Applying an overpotential (i.e. forcing the potential of the electrode away from E (eq) ) causes a current I to flow, which is related to overpotential j by Eq. (3.16), a form of Tafel’s law: 7,8 j ¼ a þb ln I; that is, I / exponentialðj,bÞ. (3.16) where a and b are constants particular to the system (see ‘Butler–Volmer kinetics’ towards the end of the chapter, p. 46). Occasionally, the overpotential j needs to be relatively small to prevent electrolytic side reactions, in which case rate (ii) may be rate limiting. Rate (iii) is rate limiting in a number of electrochromic devices; but while electrons may be intuitively adjudged the fast movers in the processes with rates (i) and (ii), this is by no means always so. In type-I systems, the electrochrome must come into contact with the electrode before a successful electron- transfer reaction can occur. Since a type-I electrochrome is evenly distributed throughout the solution before the device is switched on, most of the electrochrome is distributed in the solution bulk, and must move toward the electrode interphase until sufficiently close for the electron transfer to take place. (The term interphase here is preferred to ‘interface’ to emphasise the number and diverse nature of the many layers between bulk electrochrome and bulk solvent, including, on the liquid side, potential-distributed ions, oriented molecules and adsorbed species, as well as the outermost solid surface, that always differs from bulk solid.) 3.3.1 Mass transport mechanisms The process by which the electroactive material moves from the solution bulk toward the electrode, mass transport, proceeds via three separate mechanisms: migration, convection and diffusion. Mass transport is formally defined as the flux J i of electroactive species i, that is, the number of i reaching the solution–electrode interphase per unit time, as defined in the Nernst–Planck equation, Eq. (3.17): 9 3.3 Rates of charge and mass transport through a cell 43 J i ¼ À z i F RT c i 0cðxÞ 0x þc i · i ðxÞ ÀD i 0c i ðxÞ 0x . migration convection diffusion (3.17) where c(x) is the strength of the electric field along the x-axis, · i is the velocity of solution (as a vector, where applicable), and D i and c i are respectively the diffusion coefficient and concentration of species i in solution. (Strictly, the equation describes one-dimensional mass transfer along the x-axis.) The three transport modes operate in an additive sense. Convection is the physical movement of the solution. Deliberate stirring of the solution is termed ‘forced’ convection; density differences of the solution adjacent to the electrode cause ‘natural’ convection. Both forms of convection can be assumed absent in electrochromic cells, or at least of a negligible extent. Convection will not be discussed in any further detail since it is irrelevant for solid electrolytes and otherwise uncontrolled in other ECDs. 3.3.2 Migration Migration represents the movement of ions in response to an electric field in accord with Ohm’s Law: positive electrodes obviously attract negatively charged anions, negatively charged electrodes attracting cations. Migration may be neglected for liquid electrolytes containing ‘swamping’ excess of unreactive ionic salt (often termed a ‘supporting electrolyte’), as excess concentrations of inert cations or anions that accumulate about their respective electrodes effectively inhibit continued migration. However, solid polymer electrolytes or solid-state electrochromic layers experience a significant extent of migration since the transport numbers of (i.e. fractions of total current borne by) the electroactive species or of mobile counter ions become appreciable. In the absence of both convection and migration, diffusion becomes the sole means of mass transport, delivering electroactive species to the electrode. Migration is still important in liquid-phase systems such as that in the Gentex mirror, described in Sections 11.1 (Fig. 11.3) and 13.2. 3.3.3 Diffusion The most important mode of mass transport in electrochromism is usually diffusion, which ideally follows Fick’s laws. The first law defining the flux J i (the amount of diffusant traversing unit area of a cross-section in the solution normal to the direction of motion per unit time) is: 44 Electrochemical background J i ¼ ÀD i 0c i 0x . (3.18) where D i is the diffusion coefficient of the species i, and (0c i /0x) is the change in concentration c of species i per unit distance x (i.e. the concentration gradient). The concentration gradient (0c i /0x) arises in any electrochemical process with current flow because some of the electroactive species is consumed around the electrode, this depletion causing the concentration gradient. Diffusion results froma natural minimising of the magnitude of internal concentration gradients. Fick’s second law describes the time dependence (rate) of such diffusion, Eq. (3.19): 0c i 0t ¼ D i 0 2 c i 0x 2 . (3.19) where t is time and i denotes the ith species in solution. The required integration of this second-order differential equation often leads to difficulty in accurately modelling a diffusive system. A rough-and-ready but useful version gives the approximate relation, Eq (3.20): l % ðDtÞ 1 , 2 . (3.20) where l is the distance travelled by species with diffusion coefficient Din time t. The implications of diffusive control are discussed below. Movement of type-I and type-II electrochromes toward an electrode during coloration (see Sections 3.4 and 3.5 below) represents true diffusion of electrochrome. By contrast, electro-bleaching of a type-II electrochrome and coloration and bleaching of type-III electrochromes are all processes involving solids. Such diffusional movement is complicated by concomitant migration. For this reason, the ‘diffusion’ of a charged species through a solid is characterised by the so-called ‘chemical diffusion coefficient’ D. The kinetics of bleaching in a type-II system, and either coloration or bleaching kinetics for a type-III electrochrome, will be characterised by the chemical, rather than the normal, diffusion coefficient D. The implications for electrochromic coloration of straightforward diffusion are discussed in Section 5.1, and the kinetic distinctions between D andD are discussed in depth in Section 5.2. Faradaic and non-faradaic currents The contribution to any current that results in a redox (electron-transfer) reaction is termed ‘faradaic’ – that is, it obeys Faraday’s laws – whereas that part arising solely from ionic motion without such accompanying redox, such 3.3 Rates of charge and mass transport through a cell 45 as in the formation of the ionic double layer, is ‘non-faradaic’. Faraday’s laws specifically relate to material deposition or dissolution effected by redox reactions, and, by extension, to redox transformation of dissolved species. 3.4 Dynamic electrochemistry 3.4.1 Butler–Volmer kinetics of electrode reactions It is noted in Section 3.2 (page 39 above) that the (net) zero current at an electrode, when an external applied potential is equal to the electrode potential E, is the resultant of two opposing currents I cath (cathodic, when electrons e À are relinquished fromthe electrode) and I an (anodic, the e À are acquired by the electrode). At E these are equal in magnitude. We write that at E I ¼ I cath þI an ¼ 0. (3.21) where implied signs attach to the individual currents. (In this outline the OandR species are both in solution, as with type-I electrochromes. Minor elaborations are needed for type-II systems and major ones for type-III, but the underlying physics is identical throughout.) Details are in ref. 3 and works cited therein. When, from Eq. (3.15), the applied potential differs by j (the overpotential) from E, I is non-zero and one or other of the individual currents dominates, depending on whether the electrode is positive or negative of E. The (net) rate of the electrode reaction is defined as: rate ¼ I nFA ¼ i nF . (3.22) where n is the number of electrons involved in the reaction, F is the Faraday constant and A is the area of the electrode. Rate constants covering concentration dependences on c O and c R for the reactions at a particular potential are defined in Eq. (3.23): I cath ¼ ÀnFA k cath c O and I an ¼ nFA k an c R . (3.23) As Tafel’s law states, Eq. (3.16), log I is linear with j, but this holds only when one of the individual (‘cath’ or ‘an’) currents dominates to the exclusion of the other; it therefore fails ever more seriously for decreasing j because when near to or approaching the electrode potential E (j small), I becomes small (both ‘cath’ and ‘an’ currents are appreciable), then I !0. (The lawalso fails for very large values of j, when the at-electrode concentrations of reactant decreases from the bulk values owing to the high consumption rates prevailing as follows, and replenishment by diffusion controls the current.) 46 Electrochemical background The rate constants k cath and k an (for general reference we call either k et ) are both dependent on j. A zero value of j implies an applied potential equal to E, and a net current of zero. To obtain the current values I cath and I an applicable at E, these need to be obtained from extrapolation back to E of observed (ln I) values vs. j, from linear Tafel’s-law regions of j (Eqs. (3.15) and (3.16)). Here, at j ¼0, the extrapolated values of each of the opposing currents I cath and I an pertain (and cancel) at E. When E ¼E F (that is, with c O ¼c R ), this procedure results in the requisite values of the electrode rate parameters. These are |I cath | ¼|I an | ¼ I 0 , the standard exchange current; i 0 ¼ I 0 /A is the standard exchange current density; k cath ðE F Þ ¼ k an ðE F Þ ¼ k F where the parenthesised ‘(E F )’ denotes ‘pertaining at E F ’, and k F is the standard electron transfer rate constant for the electrode reaction. Nowk F , along with other rate constants, includes an exponential activation- energy term for the activation barrier to be surmounted in the electron transfer, which is intrinsic to the particular reaction involved. Then that activation energy is diminished by the energy supplied via j, some of which favours one direction of reaction, some the reverse; how much depends on the detail of the energy barrier, which if symmetrical results in a fraction c¼½ of the supplied energy for each direction. When not equal to ½, c is usually found experimentally to be between 0.4 and 0.6, from Tafel-law slopes. The value ½ is reasonably assumed in straightforward cases when not otherwise readily available. The activation energy term exp(ÀE a /RT), that arises from the barrier to electron transfer, is implicit within k F , hence the counter (driving) energy deriving from j will likewise comprise an exponential factor in the k et expressions, with overpotential contributions in straightforward cases weighted as c and 1 Àc for opposing directions: k cath ¼ k F exp À cnFj RT and k an ¼ k F exp ð1 ÀcÞnFj RT . (3.24) This equation leads to the final Butler–Volmer form, holding until j is made so large that reactant consumption becomes great (from the high prevailing k et values), this depletion therefore bringing in diffusion control. Hence, Eq. (3.25) is obtained: i ¼ i 0 exp À cnFj RT Àexp ð1 ÀcÞnFj RT & ' . (3.25) 3.4 Dynamic electrochemistry 47 where the overpotential j is negative when the electrode is made cathodic but positive with electrode anodic. Wider expositions follow different sign conventions and include special cases, but the essence of the kinetics is as outlined here. Advanced theories, besides indicating probable cvalues, showthat the linearity of the Tafel region is not necessarily general, but it is certainly found to hold for the vast majority of reactions examined. 3.4.2 Cyclic voltammetry Current flow through a cell alters the potentials at both electrodes, in accord with Eq. (3.16) which holds with different intrinsic parameters for each electrode. In order to isolate the processes at one electrode, the effects at the other are ignored (and this ‘counter electrode’ can then be chosen merely for convenience: Pt, electrolysing solvent water, for example; any unwanted byproducts are segregated within a sinter-separated compartment). The potential at the ‘working electrode’ (WE) is then measured not via the potential applied across the cell, but by measuring the potential between the WE and a closely juxtaposed reference electrode (RE) like the SCE (see Section 3.3). No net current flows through the SCE so its potential may be regarded as constant, while the WE bears a variable current and shows a true, measurable, potential. The cyclic-voltammetry experiment involves applying a potential smoothly varying with time t, over a range including the electrode potential E O,R of the WE and observing the resultant current, which will peak (with value I p ) near E O,R . At the end of the chosen range the potential is reversed, to change at the same rate as for the forward ‘potential sweep’. The control device (a potentiostat) in fact drives a current across the cell of such (changing) magnitude as to effect the desired steady potential change at a desired rate; at any instant of time the potential is in fact constant and known, hence the name of the control device. A record of the potential with time will show a saw-tooth trace of this ‘potential ramping’. The so-called scan- or ‘sweep’-rate (the rate of potential variation) i can be varied to give desiderata like diffusion coefficients (see Chapter 5). Alternative procedures employ potentials varying as sine waves, rather than the saw-tooth mode described. Each voltammetric scan of an electrochromic electrode thus represents an on/off switching cycle and can be used to estimate survival times of such electrodes if allowed to run for a sufficiently long time. Figure 3.2 (a) depicts a schematic circuit for cyclic voltammetric analyses, indicating the nature of the connections between the three electrodes. Figure 3.2 (b) shows a schematic cyclic voltammogram (CV). 48 Electrochemical background The controlling device can (or should be able to) measure total charge passed at each stage of the sweep, and with prolonged examination any loss or decomposition of electrochrome becomes apparent from observable diminution of cycle charge. Optical/spectroscopic examination of the electrode can be undertaken concomitantly. Other modifications of measurement are used, such as continuous pulses of potential, which trace versus time a series of square-well potentials above and below an average. A widely used application involves the Randles–Sevcˇ ik equation linking the peak current I p with concentration c, v and the diffusion A V WE RE (a) C u r r e n t l 0.2 0.1 0.0 I pa I pc E pa (E – E )/ V E pc E λ –0.1 –0.2 (b) CE sinter WE =Working electrode RE = Reference electrode CE = Counter electrode Figure 3.2 (a) Schematic cell (depicted within a circular vessel) for obtaining a cyclic voltammogram, showing connections between the three electrodes. The sinter prevents the products of electrode reactions at the counter electrode diffusing into the studied solution. (b) Schematic cyclic voltammogram for a simple, reversible, one-electron redox couple, in which all species remain in solution. 3.4 Dynamic electrochemistry 49 coefficient D, from a solution of Fick’s laws. D is dealt with in further detail in Chapter 5: I ðlim.tÞ ¼ À0.4463 nF A nF RT 1 2 D 1 2 c v 1 2 . (3.26) The other symbols have already been defined. 3.4.3 Impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is summarised here, in an outline employing the familiar concepts of resistance and capacitance. Thus, one can measure the resistance of a circuit element, such as a redox electrode, and its apparent dependence on the frequency of the potential applied, together with the capacitance and its frequency dependence, and directly convert these data into the real R and imaginary J parts of the impedance Z. Plots of J against R or of either against applied frequency, or of other functions against either quantity, can yield useful rate parameters for electrode processes. 10 There are in fact four ways in which what can be thought of as basically resistance and capacitance measurements can be represented, each providing different weightings with respect to frequency. For example, the inverse of impedance is the quantity called admittance. All such treatments are called immitance measurements. 3.4.4 Ellipsometry Ellipsometry is an optical technique that employs polarised light to study thin films. In this context, ‘thin’ means films ranging fromessentially zero thickness to several thousand A ˚ ngstroms, although this upper limit can sometimes be extended. The technique has been known for almost a century, and today has many standard applications, including the measurement of film thicknesses and probing dielectric properties. It is mainly used in semiconductor research and fabrication to determine properties of layer stacks of thin films and the interfaces between the layers. In the ellipsometry technique, linearly polarised light of known orientation strikes on the surface of a sample at an oblique angle of incidence. The reflected light is then polarised elliptically (hence ‘ellipsometry’). The shape and orientation of this ellipse depends on the angle of incidence, the direction of the polarisation of the incident light, and the reflective properties of the surface. An ellipsometer quantifies the changes in the polarisation state of light as it reflects from a sample, as a function of these variables. 50 Electrochemical background If the thin-filmsample undergoes changes, for example its thickness alters, then its reflection properties will also change. More importantly to electrochromism, applying a potential across an electroactive film changes the optical properties of the film, and hence the polarisation of the reflected light. Therefore, by monitoring the polarisation of the reflected light while changing the applied potential (‘in-situ electrochemical ellipsometry’) and subsequently manipulating the resultant optical data, it is possible to deduce much concerning the electrochromic layers, such as any changes in film thickness with potential (called ‘electrostriction’) and the formation of concentration gradients within the film. 11,12 References 1. Bard, A. J. and Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd edn, New York, Wiley, 2001, pp. 2–3, 48–9, 51–2. 2. A. J. Bard (ed.). Encyclopedia of Electrochemistry of the Elements, New York, Marcel Dekker, 1973–1986. 3. Antelman, M. S. Encyclopedia of Chemical Electrode Potentials, New York, Plenum, 1982. 4. David R. Lide (ed.). The CRC Handbook of Chemistry & Physics, 86th edn, Boca Raton, FL, CRC Press, 2005. 5. Bard, A. J. and Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd edn, New York, Wiley, 2001, pp. 808–12. 6. Hitchcock, D. I. and Taylor, A. C. The standardization of hydrogen ion determinations, I: hydrogen electrode measurements with a liquid junction. J. Am. Chem. Soc., 59, 1937, 1813–18. 7. Tafel, J. Z. Physik. Chem., 50A, 1905, 641, as cited in Bard and Faulkner. 8. Bard, A. J. and Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd edn, New York, Wiley, 2001, pp. 102 ff. 9. Bard, A. J. and Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd edn, New York, Wiley, 2001, p. 29. 10. Macdonald, D. D. Transient Techniques in Electrochemistry, New York, Plenum, 1977. 11. Tompkins, H. G. and McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry: A User’s Guide, New York, Wiley, 1999. 12. [Online] at www.beaglehole.com/elli_intro/elli_intro.html (accessed 15 November 2005). References 51 4 Optical effects and quantification of colour 4.1 Amount of colour formed: extrinsic colour The coloured form of the electrochrome is produced by electrochemical reaction(s) at the electrode, Eq. (4.1) and its reverse (see Section 3.2). At the electrode, each redox centre of the electroactive species can accept or donate electrons from or to an external metal connection, one centre being formed per n electrons, where n is usually one or two according to the balanced redox reaction, Eq. (4.1): oxidisedform; Oþelectronfsg ! reduced form; R: (4:1) In the simplest cases, the number of colour centres formed by the electrode reaction, and hence the change in absorbance D(Abs), is in direct proportion to the electrochemical charge passed Q, following Faraday’s first law, ‘The amount of (new) material formed at an electrode is proportional to the electrochemical charge passed’: DðAbsÞ / Q: (4:2) The term‘electrochemical’ charge here implies that no unwanted side reactions involving electron transfer occur at the electrode during electrochromic colour change, i.e. that the relevant reaction is 100% efficient. The component of the total charge passed that is directly involved in forming the desired product is termed the faradaic charge for that process (but redox side reactions involving unwanted electrochemical products also involve faradaic charge). If the total charge passed is greater than the faradaic charge, then the difference is termed ‘non-faradaic’. This represents processes like ‘parasitic’ current leakage possibly resulting from undesirable electronic current such as through the intra-electrode cell materials (electrolyte), or double-layer charging in the electrolyte adjacent to the electrode, an effect emulating the charging of an electrolytic capacitor. 52 The magnitude of the optical absorbance change obviously follows the (ideal) faradaic charge Q governing the amount of coloured material formed. The Beer–Lambert law – Eq. (4.3) – relates the optical absorbance Abs proportionally to the concentration of a chromophore: Abs ¼ "lc; (4:3) where " is the extinction coefficient or molar absorptivity, c is the concentration of the coloured species and l the spectroscopic path length in the sample; l could be the thickness of a thin solid film of electrochrome, or the thickness of a liquid layer containing a dissolved chromophore. In the case of electrochemically generated colour, D(Abs) is the change in the optical absorbance and, from Eq. (4.3), is related by Eq. (4.4) to Dc, the change in the concentration of chromophore generated by the electrochemical charge passed: DðAbsÞ ¼ "lDc: (4:4) Even when the electrode efficiency is 100%, the relationship DAbs /Q in Eq. (4.2) will only hold if the absorbance is determined at fixed wavelength. However, many solid-state electrochromic systems do not follow the relation D(Abs) /Q because both the shape of the major absorption band and the wavelength maximum can change somewhat with the extent of charge insertion (i.e. of electrochemical change as gauged by the insertion coefficient x) and hence, of course, with concentration of coloured species. This deviation can result from changes in the molecular environment about the colorant with amount of colorant produced. 4.2 The electrochromic memory effect A liquid-crystal display (LCD) is field-responsive, while electrochromic devices are potential-responsive. As colour generated in an ECD results from the application of a voltage across it that causes charge to flow, in an ECD therefore the colour intensity can be readily modulated between ‘negligible’ at the one extreme (all electroactive sites being in a non- or weakly absorbing redox state), and ‘intense’ at the other (all electroactive sites being in the coloured redox state). In brief, light intensity is modulated by varying the amount of charge passed. Exemplifying, Figure 4.1 shows an electrochromic figure ‘3’, the image being formed at those separate and insulated electrodes to which a suitable potential is applied, where charge therefore flows and coloration ensues. The electrochromic colour is removed (bleached) by applying a potential now with the 4.2 The electrochromic memory effect 53 polarity reversed, thereby reversing the electron-transfer process in Eq. (4.1). The ECD on/off operation thus relies on the reversible redox reaction at an electrochromic electrode, oxidation þne À ! reductant, (4.1) as discussed more fully in connection with Eq. (3.1) and elaborated in Section 3.2. With a second electrode plus interposed electrolyte, ECDs behave just like rechargeable (i.e. ‘secondary’) batteries, but in thin-film form; the similarities are explored by Heckner and Kraft. 1 Since the perception of ECD colour arises from formation of a coloured chemical, rather than from a light-emitting or interference effect, the colour in solid-state electrochromes – i.e. type III – will persist after the current has ceased to flow. This persistence of colour leads to the useful property of ECDs, the so-called ‘memory effect’. Such memory is occasionally referred to as being ‘non-volatile’. However, since nearly all redox states are somewhat reactive, unwanted redox reactions can occur within devices after colour formation, thus, in the sense that no storage battery is ever perfect, most ECDs do not retain their colour indefinitely. Furthermore, type-I all-solution electrochromes diffuse from the solid contact, then being decolorised by reactions in mid-solution, and a maintaining current is necessary for colour persistence. In practice then, the memory is never permanent. Organic electrochromes in particular can also photodegrade. Such colour loss is often termed ‘self bleaching’; see p. 15. Device durability is addressed in Chapter 16. 4.3 Intrinsic colour: coloration efficiency h Although the number of colour centres formed is a function of the electrochemical charge passed, the observed intensity of colour will also depend on the specific electrochrome, some electrochromes being intensely coloured, others only feebly so. The optical absorption of an electrochrome is related a c r a =anode c =cathode r =reference electrode Figure 4.1 Schematic representation of an electrochromic alphanumeric character comprising seven separate electrodes. 54 Optical effects and quantification of colour to the inserted charge per unit area Q (the ‘charge density’) by an expression akin to the Beer–Lambert law (Eq. (4.3) above), since Q is proportional to the number of colour centres formed, Eq. (4.5): Abs ¼ log I o I ¼ Q: (4:5) Here the proportionality factor , the ‘coloration efficiency’, is a quantitative measure of the electrochemically formed colour. For an ECD in transmission mode, is measured as the change in optical absorbance D(Abs) evoked by the electrochemical charge density Q passed, Eq. (4.6): ¼ DðAbsÞ Q : (4:6) The proportionality factor is clearly independent of the optical pathlength l within the sample. The coloration efficiency can be thought of as an electrochemical equivalent of the more familiar extinction coefficient " (cf. Eq. (4.3) above), which characterises a chromophore in solution (in a particular solvent); thus represents the area of electrochrome on which colour is intensified, in absorbance units per coulomb of charge passed. In many electrochromic studies it is (erroneously) expressed in cm 2 , rather than area per unit charge, for example cm 2 C À1 . Needless to say, values of should thus be maximised for most efficient device operation. A compendium of for metal oxide electrochromes is given in Table 4.1, and for organic species in Table 4.2. Many additional values are available in refs. 2 and 3; and many other values are cited elsewhere in this work. The obviously larger values of for organic species owes largely to enhanced quantum-mechanical properties governing the probability of electronic transitions responsible for coloration (see p. 60 ff.). Since the optical absorbance Abs depends on the wavelength of observation, must be determined at a fixed, cited, wavelength; is defined as positive if colour is generated cathodically, but negative if colour is generated anodically (in accordance with the IUPAC definitions: anodic currents are deemed negative, cathodic currents positive). Anegative for anodic coloration is not always stated, however, so care is needed here. Values of are clearly smaller for metal oxides than for all other classes of electrochrome, but this has not deterred most investigators from studying the electrochromic properties of oxides (see Chapter 6). 4.3 Intrinsic colour: coloration efficiency 55 Table 4.1. Coloration efficiencies for thin films of metal-oxide electrochromes. Positive values denote cathodically formed colour, negative values denote anodic coloration. Oxide Morphology Preparative method a b /cm 2 C À1 Ref. Anodically colouring oxides FeO Polycrystalline CVD À6.0 4 FeO Polycrystalline Sol–gel À28 5 FeO Polycrystalline Electrodeposition À30 6 IrO x Polycrystalline rf sputtering À15 (633) 7 IrO x Amorphous Anodic deposition À30 8 NiO Polycrystalline dc sputtering À41 À25 9 NiO Amorphous Dipping technique À35 10 NiO Amorphous Electrodeposition À20 11 NiO Polycrystalline rf sputtering À36 (640) 12 NiO Polycrystalline Spray pyrolysis À37 13 NiO Amorphous Vacuum evaporation À32 (670) 14 Rh 2 O 5 Amorphous Anodic deposition À20 (546) 8 V 2 O 5 Polycrystalline rf sputtering À35 (1300) 15 Cathodically colouring oxides Bi 2 O 3 Amorphous Sputtering 3.7 (650) 16 CoO Polycrystalline CVD 21.5 17 CoO Amorphous Electrodeposited 24 18,19 CoO Polycrystalline Sol–gel 25 20 CoO Polycrystalline Spray pyrolysis 12 (633) 21 CoO Amorphous Thermal evaporation 20–27 22 MoO 3 Amorphous Ther. evap. of Mo(s) 19.5 (700) 23 MoO 3 Polycrystalline Oxidation of MoS 3 35 (634) 24 MoO 3 Amorphous Thermal evaporation 77 (700) 12 Mo 0.008 W 0.992 O 3 Amorphous Thermal evaporation 110 (700) 25 Nb 2 O 5 Polycrystalline rf sputtering 12 (800) 26 Nb 2 O 5 Polycrystalline Sol–gel 38 (700) 27 Ta 2 O 5 Polycrystalline rf sputtering 5 (540) 26 TiO 2 Amorphous Thermal evaporation 7.6 28 TiO 2 Polycrystalline rf sputtering 8 (546) 29 TiO 2 Amorphous Thermal evaporation 8 (646) 30 TiO 2 Polycrystalline Sol–gel 50 31 WO 3 Amorphous Thermal evaporation 115 (633) 32 WO 3 Amorphous Electrodeposition 118 (633) 33 WO 3 Amorphous Electrodeposition 62–66 (633) 34 WO 3 Amorphous Thermal evaporation 79 (800) 35 WO 3 Polycrystalline rf sputtering 21 36 WO 3 Polycrystalline Spin-coated gel 64 (650) 37 WO 3 Amorphous Dip-coating c 52 38 WO 3 Polycrystalline Spray pyrolysis 42 39 WO 3 Polycrystalline Sol–gel 36 (630) 40 WO 3 Polycrystalline CVD 38–41 41 WO 3 Polycrystalline dc sputtering 109 (1400) 26 a ‘CVD’ ¼ chemical vapour deposition; ‘dc sputtering’ ¼ dc magnetron sputtering. b Wavelength (/nm) used for measurement in parentheses. 56 Optical effects and quantification of colour 4.3.1 Intrinsic colour: composite coloration efficiency (CCE) Although measuring values of is important for assessing the power requirements of an electrochrome, Reynolds et al. 44 emphasise that the methods chosen for measurement often vary between research groups which causes difficulty in comparing values for different electrochromes. A general method for effectively and consistently measuring composite coloration efficiencies (CCEs) (see below) has been proposed, 44 and applied to measurements on electrochromic films of conductive polymers 44,45,46 and the mixed-valence inorganic complex, Prussian blue – PB, iron(III) hexacyanoferrate(II): 47 PB is reducible to the clear Prussian white – PW, iron(II) hexacyanoferrate(II). Such measurements have also been applied to conductive polymers 48 but performed with reflected light as opposed to the usual transmitted light. Atandemchronocoulometry–chronoabsorptometry method is employed to measure composite coloration efficiencies, with CCEs being calculated at Table 4.2. Coloration efficiencies for organic electrochromes. Positive values denote cathodically formed colour, negative values denote anodic coloration. (Table reproduced from Rauh, R. D., Wang, F., Reynolds, J. R. and Meeker, D. L. ‘High coloration efficiency electrochromics and their application to multi-color devices’. Electrochim. Acta, 46, 2001, 2023–2029, by permission of Elsevier Science.) Electrochrome (max) / nm a / cm 2 C À1 Monomeric organic redox dyes Indigo Blue 608 À158 Toluylene Red 540 À150 Safranin O 530 À274 Azure A 633 À231 Azure B 648 À356 Methylene Blue 661 À417 Basic Blue 3 654 À398 Nile Blue 633 À634 Resazurin 598 À229 Resorufin 573 À324 Methyl viologen 604 176 Conducting polymers Poly(3,4-ethylenedioxythiophenedidodecyloxybenzene) 552 À1240 b 730 650 c Poly(3,4-propylenedioxypyrrole) 480 À520 Poly(3,4-propylenedioxythiophene), PProDOT 551 À275 a Values were calculated from data published in ref. 43; b reduced form; c oxidised form. 4.3 Intrinsic colour: coloration efficiency 57 specific percentage transmittance changes, at the max of the appropriate absorbance band. To illustrate this approach, Figure 4.2(a) shows the absorbance during the dynamic measurement of a film of Prussian blue (PB) at 686 nm, to effect the electrochromic transition. A square wave pulse was switched between þ0.50 V (PB, of high absorbance) and – 0.20 V (PW, of low absorbance); these potentials are cited against a AgjAgCl wire in KCl solution (0.2 mol dm À3 ). For the PB ! PW transition, the electrochromic 0 5 10 15 20 –6 –5 –4 –3 –2 –1 0 1 (b) Q /m C c m –2 t / s 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 (a) A Figure 4.2 Tandem chronoabsorptometric (a) and chronocoulometric (b) data for a PB|ITO|glass electrode in aqueous KCl (0.2 mol dm À3 ) supporting electrolyte, on square-wave switching between þ0.50 V (PB, high absorbance) and À0.20 V (PW, low absorbance) vs. Ag|AgCl. (Figure reproduced fromMortimer, R. J. and Reynolds, J. R. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. J. Mater. Chem., 15, 2005, 2226–33, with permission from The Royal Society of Chemistry.) 58 Optical effects and quantification of colour contrast at 686 nm was 60% of the total transmittance (D%T), calculated from the maximum and minimum absorbance values. The charge measurements recorded simultaneously with the absorbance data are given in Figure 4.2(b). In the composite coloration efficiency method, to provide points of reference with which to compare the CCE values of various electrochromes, values of are calculated at a specific transmittance change, as a percentage of the total D(%T). Table 4.3 shows data for 90, 95 and 98% changes, for both reduction of PB to form PW, and the reverse process, oxidation of PW to re-form PB. Although the chronocoulometric data in Table 4.3 were corrected for background charging, as were the measurements with conducting polymer films, 44 the values for the reduction process are seen to decrease slightly with increases in optical change. This decrease demonstrates the importance of measuring the charge passed at a very specific transmittance value and not simply to divide the total absorbance change by the maximumcharge passed. This practice is important in considering the reduction of PBto PW, because PWis a good catalyst for the reduction of oxygen: molecular O 2 may diffuse into the cuvette during long measurement times, resulting in an erroneously high charge measurement. It should be noted that in the original publication 44 that introduced composite coloration efficiency measurements, the calculated values of were described as being at 90, 95 and 98% of the total optical density change [DOD (¼ DAbs)], at Table 4.3. Optical and electrochemical data collected for coloration efficiency measurements. Prussian blue is reviewed in Chapter 7, and PEDOT in Chapter 10. (Table reproduced with permission of The Royal Society of Chemistry, from: Mortimer, R. J. and Reynolds, J. R. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue.’ J. Mater. Chem., 15, 2005, 2226–33.) Transition % of full switch D(%T) DA Q/ mCcm À2 / cm 2 C À1 t/s Ref. PB!PW 90 53.8 0.673 4.49 À150 3.4 PB!PW 95 56.6 0.691 4.85 À143 4.4 47 PB!PW 98 58.3 0.701 5.18 À135 6.0 PW!PB 90 52.9 0.564 3.85 À147 1.9 PW!PB 95 55.9 0.632 4.21 À150 2.2 47 PW!PB 98 57.5 0.675 4.54 À149 2.6 PEDOT 90 48 0.48 2.49 192 0.33 PEDOT 95 51 0.49 2.68 183 0.36 44 PEDOT 98 53 0.50 3.04 165 0.45 The bold figures represent the authors’ preferred reference percentage. 4.3 Intrinsic colour: coloration efficiency 59 max . In view of the fundamental definition of DOD, this choice of variable represents a mis-statement and all composite coloration efficiencies recorded in Table 4.3, and previously, 44 were determined using the DODat 90, 95 and 98%of D(%T). Although as observed above, inorganic materials typically exhibit lower values than conducting polymers, it is interesting to note from Table 4.3 how the carefully measured values calculated here are comparable to those for films of poly(3,4-ethylenedioxythiophene) – PEDOT – (at a film thickness of 150nm), although switching times are longer for the PB–PW transition. The values are similar for both the reduction of PB to PW, and for the re-oxidation of PW back to PB, although the switching times for the latter process are slightly shorter. To preserve the electroneutrality of the solid electrochrome, uptake or loss of potassium ions must accompany the colour-transforming electron transfer; see Chapter 8. The difference in switching times probably arises from different rates of ingress or egress of potassium ions in these films. 4.4 Optical charge transfer (CT) Films of solid electrochrome are comparatively thin, usually sub-micron in thickness, and thus comprise very little material; and solution-phase electrochromes are enclosed in ECDs within small volumes of solvent, typically of maximum optical path length 1 mm. An electrochromic colour that is intense enough to observe under normal illumination will therefore require a spectroscopic transition that is very intense, i.e. having a very high extinction coefficient ". Of the organic electrochromes, the most intense absorptions are encountered with systems having an extended conjugation system, such as cyanines and conductive polymers, or a large extent of internal conjugation such as radicals of the viologens (see Chapter 11). As an example, the radical cation of CPQ, cyanophenyl paraquat (I) (formally 1,1 0 -bis(p-cyanophenyl)-4,4 0 -bipyridilium) in acetonitrile has an intense green colour: 49 at (max) ¼674 nmits " is 83 300 dm 3 mol À1 cm À1 , cf. " for the aqueous MnO 4 À ion (which is generally thought to be intensely coloured) of only 50 2400 dm 3 mol À1 cm À1 . I N N NC CN The metal-oxide system to have received the most attention for electrochromic purposes is tungsten trioxide, WO 3 (Section 6.2). The bulk trioxide is pale 60 Optical effects and quantification of colour yellow in colour and transparent as a thin film, but forms a blue colour on reduction. In metal-oxide systems, the source of the required intense electrochromic colour is usually an intervalence optical charge-transfer (CT) transition, 51,52 where the term ‘intervalence’ implies here that the two atoms or ions are of the same element. In colourless WO 3 , all tungsten sites have a common oxidation state of þVI. Reductive electron transfer to a W VI site forms W V , and the blue form of the electrochrome becomes evident from the optical CT. This blue form is commonly called a ‘bronze’ (see Chapter 6), although strictly, tungsten bronzes are characterised by metallic conductivity, and have compositions M x WO 3 where x is typically greater than about 0.3. A WO 3 -based electrochrome (rather than a bronze), as used in an ECD, must be restricted to a lower value of x in order to preserve switchability, and is thus a semiconductor. The optical intervalence CT of this sort is usually regarded as the major cause of the electrochromic colour in many inorganic systems. Other mechanisms such as the Stark effect are briefly dealt with in Chapter 1. In a CT-based system, following photon absorption an electron is optically excited from an orbital on the donor species in the ground-state (pre-transfer) electronic configuration of the system, to a vacant electronic orbital on an adjacent ion or atom, producing an excited state. The blue colour is caused by red light being absorbed to effect the intervalence transition between adjacent (‘A’ and ‘B’) W VI and W V centres, Eq. (4.7): W VI ðAÞ þW V ðBÞ þh !W V ðAÞ þW VI ðBÞ : (4:7) The product species, which are hence in an excited state, subsequently lose the excess energy acquired from the absorbed photon by thermal dissipation to surrounding structures. (Close examination of the PB–PW structures shows that the photo-effected product distribution unusually involves an intrinsic chemical change absent in the Eq. (4.7) transition for W VI/V , ferric ferrocyanide being chemically different from photo-product ferrous ferricyanide, in contrast with the transition depicted in Eq. (4.7).) These intervalence transitions are characterised by broad, intense and relatively featureless absorption bands in the UV, visible or near IR, with molar absorptivities (extinction coefficients) of useful magnitudes. As an example, " for the W V,VI oxide system in Eq. (4.7) lies in the range 53 1400–5600 dm 3 mol À1 cm À1 , the value decreasing with increasing insertion coefficient x. (The optical properties of WO 3 are discussed in Chapter 6, Section 6.4 on p. 140 ff.) 4.4 Optical charge transfer (CT) 61 4.5 Colour analysis of electrochromes Colour is a very subjective phenomenon, causing its description or, for example, the comparison of two colours, to be quite difficult. However, a new method of colour analysis, in situ colorimetric analysis has recently been developed. 54 It is based on the CIE (Commission Internationale de l’Eclairage (the ‘International Commission on Illumination’)) system of colorimetry, which is elaborated below. The CIE method has been applied 44,45,54,55,56,57,58,59,60,61 to the quantitative colour measurement of conducting electroactive polymer and other electrochromic films on optically transparent electrodes (OTEs) under electrochemical potential control in a spectroelectrochemical cell. Experimentally, the method is straightforward in operation: a spectroelectrochemical cell is assembled within a light box, and a commercial portable colorimeter (such as the Minolta CS-100 Chroma Meter), mounted on a tripod, measures changes in the electrochromic filmduring transformations performed under potentiostatic control. This method allows the quantitative colour description of electrochromes, as perceived by the human eye, in terms of hue, saturation and luminance (that is, relative transmissivity). Such colour analyses provide a more precise way to define colour 62,63 than more familiar forms of spectrophotometry. Rather than simply measuring spectral absorption bands, in colour analysis the human eye’s sensitivity to light across the whole visible spectral region is measured and a numerical description of a particular colour is given. This approach, which has been applied to electrochromic conducting electroactive polymer films and, more recently, to Prussian blue films, 47 is likely to be applicable to a wide range of both organic and inorganic electrochromes. There are three main advantages to in situ colorimetric analysis. First, by acquiring a quantitative measure of the colour, it is possible to report accurately the colour of new materials. Second, by utilising colorimetric analysis, it is possible to represent graphically the path of an electrochrome’s colour change. Third, the method can ultimately function as a valuable tool in the construction of electrochromic devices. Beyond these practical considerations, colorimetric analyses can also provide valuable information about the optical and electrochemical processes in electrochromes. The approach is exemplified in Figures 4.5 and 4.6 for PB, and elaborated below. 4.5.1 A brief synopsis of colorimetric theory Colour is described by three attributes. The first identifies a colour by its location in the spectral sequence, i.e. the wavelength associated with the colour. This is known as the hue, dominant wavelength or chromatic colour, 62 Optical effects and quantification of colour and is the wavelength where maximum contrast occurs. It is this aspect which is commonly, but mistakenly, referred to as colour. The second attribute relates to the relative levels of white and/or black, and is known as saturation, chroma, tone, intensity or purity. The third attribute is the brightness of the colour, and is also referred to as value, lightness or luminance. Luminance provides information about the perceived transparency of a sample over the entire visible range. Using the three attributes of hue, saturation and luminance, any colour can be both described and actually quantified. In order to assign a quantitative scale to colour measurement, the hue, saturation and luminance must be defined numerically in a given colour system. The most well known and most frequently used colour system is that developed by the Commission Internationale de l’Eclairage, commonly known as the CIE system of colorimetry. It was first devised in 1931, and is based on a so-called ‘28 Standard Observer’, that is, a system characterised by the result of tests in which people had to visually match colours in a 28 field of vision. 64 Thus the CIE system is based on how the ‘average’ person subjectively sees colours, and thus simulates mathematically how people perceive colours. The original CIE experiments resulted in the formulation of colour-matching functions, which were based on the individual’s response to various colour stimuli. There are three modes by which the eye is stimulated when viewing a colour, hence the CIE system is expressed in terms of a ‘tristimulus’. These colour matching functions are used to calculate such tristimulus values (symbolised as X, Y and Z), which define the CIE system of colorimetry. Once obtained, values of X, Y and Z allow the definition of all the CIE recommended colour spaces, where the phrase ‘colour space’ implies a method for expressing the colour of an object or a light source using some kind of notation, such as numbers. The concept for the XYZ tristimulus values is based on the three-component theory of colour vision, which states that the eye possesses three types of cone photoreceptors for three primary colours (red, green and blue) and that all colours are seen as mixtures of these three primary colours. Colour spaces are usually defined as imaginary geometric constructs, containing all possible colour perceptions, and represented in a systematic manner according to the three attributes. Colour spaces are the means by which the information of the X, Yand Ztristimulus values is represented graphically, either in two- or three-dimensional space. Actually the tristimulus values themselves constitute a colour space, although the three-dimensional vectoral nature of the comprehensive system makes it quite unwieldy for presenting data. Colour is a three-dimensional phenomenon, so it is not easily represented quantitatively. 4.5 Colour analysis of electrochromes 63 Colour quantification is more easily visualised if separated into the two attributes, lightness and chromaticity. The ‘lightness’ describes how light or dark a colour is, and ‘chromaticity’ (representing hue and chroma) can be shown two-dimensionally. The CIE has defined numerous colour spaces based on various criteria. The three most commonly used are the CIE 1931 Yxy colour space, the CIE 1976 L*u*v* colour space, and the CIE 1976 L*a*b* colour space. The latter is also referred to as CIELAB. The evolution of the CIE criteria is now outlined. The colour sensitivity of the eye changes according to the angle of view. In 1931, the CIE proposed its first recommended colour space based on the X, Y and Z tristimulus values and a 28 field of view, hence the name ‘28 Standard Observer’. In this system, the tristimulus value Yis retained as a direct measure of the brightness or luminance of the colour. The two-dimensional graph obtained with such data is Cartesian – an xy graph – and known as the ‘xy chromaticity diagram’. From this diagram, respective values of x and y are calculated from the X, Y and Z tristimulus values via Eq. (4.8) and Eq. (4.9): x ¼ X X þY þZ ; (4:8) y ¼ Y X þY þZ : (4:9) On the graph represented in Figure 4.3, the line surrounding the horse- shoe-shaped area is called the ‘spectral locus’, which shows the wavelengths of light in the visible region. Colour Plate 1 shows a colour representation of this figure. The line connecting the longest and shortest wavelengths contains the non- spectral purples, and is therefore known as the ‘purple line’. Surrounded by the spectral locus and the purple line is the region known as the ‘colour locus’, which contains every colour that can exist. The point (labelled as W in Figure 4.3) within this locus is known as the white point and its location is dependent on the light source. The CIEhas several recommended light sources (so-called ‘illuminants’), such as the D 50 (5000 K) constant-temperature daylight simulating light source. The location of a point in the xy diagram then gives the hue and chroma of the colour. The hue is determined by drawing a straight line through the point representing ‘white’ and the point of interest to the spectral locus thus obtaining the dominant wavelength of the colour. To exemplify, Figure 4.4 shows the determination of the dominant wavelength ($550 nm) for ‘sample B’; and to reiterate terminology, the spectral locus refers only to the horse-shoe-shaped curve and not the purple line which 64 Optical effects and quantification of colour is defined by non-spectral purples. For placing a wavelength dependence on samples such as ‘sample A’ that are found along the purple line, a complementary wavelength can be expressed by drawing a straight line from the sample coordinate through the white point to the spectral locus. Indeed a complementary wavelength can be expressed for any sample with which this procedure can be applied. The purity (or saturation) as expressed by the relation in the figure is a measure of the intensity of specific hue, with the most intense (or saturated) colours lying closest to the spectral locus. The most saturated colours lie along the spectral locus. It is important, however, to realise that the CIE does not associate any given colour with any point on the diagram: if colours are ever included on a diagram, they are only an artist’s representation of what colour a region is most likely to represent. The reason that colours cannot be specifically associated with a given pair of 520 510 500 490 480 470 450 460 440 420 0.0 0.2 0.4 y 0.6 0.8 0.2 0.4 x 0.6 0.8 –380 530 550 560 570 580 590 600 610 630 650 700–780 640 620 nm 540 W Figure 4.3 CIE 1931 xy chromaticity diagram with labelled white point (W). 4.5 Colour analysis of electrochromes 65 xy coordinates is because the third dimension of colour, lightness, is not included in the diagram. The relative lightness or darkness of a colour is very important in how it is perceived. The brightness is usually presented as a percentage, as expressed in Eq. (4.10): %Y ¼ Y Y 0 Â 100; (4:10) in which Y 0 is the background luminance and Y is the luminance measured for the sample. In the corresponding dome-shaped three-dimensional diagram, it is recognised that the highest purity or saturation can only be achieved when the luminance or lightness of the colour is at a low value. 63 In 1976, the CIE proposed two new colour spaces, L*u*v* and L*a*b*, in order to correct flaws in the earlier proposed systems. Both were defined as uniform colour spaces, which are geometrical constructs containing all possible colour sensations. This new system is formulated in such a way that equal distances correspond to colours that are perceptually equidistant. The main reason for designing such systems was to provide an accurate means of representing and calculating colour difference. 0.9 525 500 475 450 380 550 Purity = 575 625 600 650 780 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 x y 0.6 0.7 0.8 0.9 λ d λ c b Sample B Sample A Illuminant source a a a + b Figure 4.4 CIE 1931 xy chromaticity diagram showing the determination of the complementary wavelength of a sample with xy coordinates of arbitrary sample A, and the dominant wavelength and purity of a sample with xy coordinates of arbitrary sample B. (Figure reproduced from DuBois, Jr, C. J. ‘Donor-acceptor methods for band gap control in conjugated polymers’. Ph.D. Thesis, Department of Chemistry, University of Florida, 2003, p. 21, by permission of the author.) 66 Optical effects and quantification of colour The CIE L*u*v* colour space is a uniform colour space based on the X, Y and Z tristimulus values defined in 1931. The L* value measures the lightness; chroma and hue are defined in terms of u* and v*. The CIE L*u*v* system has a corresponding two-dimensional chromaticity diagram known as the u’v’ UCS (‘uniform colour space’), which is very similar to the 1931 xy chromaticity diagram. The L*u*v* colour space is now used as a standard in television, video and the display industries. In a further development the L*a*b* colour space is also a uniform colour space defined by the CIEin 1976. The L* value represents the same quantity as in CIE L*u*v* and hue and saturation bear similar relationships to a* and b*. The CIE L*a*b* space is a standard commonly used in the paint, plastic and textile industries. The values of L*, a* and b* are defined as in Equations (4.11)–(4.13): L* ¼ 116 Â Y Y n 1=3 À16; (4:11) a* ¼ 500 Â X X n 1=3 À Y Y n 1=3 " # ; (4:12) b* ¼ 200 Â Y Y n 1=3 À Z Z n 1=3 " # ; (4:13) where X n , Y n and Z n are the tristimulus values of a perfect reflecting diffuser (as calculated from the background measurement). In the L*a*b* chromaticity diagram, þa* relates to the red direction, Àa* is the green direction, þb* is the yellow direction, and Àb* is the blue direction. The centre of the chromaticity diagram (0, 0) is achromatic; as the values of a* and b* increase, the saturation of the colour increases. None of the systems is perfect, but the 1931 xy chromaticity diagram is probably the best known and most widely recognised way to represent a colour. The diagram conveys information in a straightforward manner and hence is very easy to use and understand. In addition, the CIE 1931 system is useful in that it can be used to analyse colour in many different ways; notably, the systemcan be used to predict the outcome of mixing colour. The result of mixing two colours is known to lie along the straight line on the xy chromaticity diagram connecting the points representing the colours of the pure components in the mixture. The position on this line representing the actual chromicity depends on the ratio of the amounts of the two mixed colours. 4.5 Colour analysis of electrochromes 67 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 (b ) λ d =488 nm 475 Illumination source 600 575 550 500 525 780 y x 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.34 0.35 0.36 0.37 0.38 0.39 x (a) (+0.50 V) (–0.20 V) y Figure 4.5 CIE 1931 xy chromaticity diagrams for a Prussian blue (PB)|ITO|glass electrode in aqueous KCl (0.2 mol dm À3 ) supporting electrolyte. (a) The potential (vs. Ag/AgCl) was decreased, in the steps indicated in Table 4.4, from the coloured PB (þ0.50 V) to the transparent Prussian white (PW) (À0.20 V) redox states. (b) The xy coordinates are plotted onto a diagram that shows the locus coordinates, with labelled hue wavelengths, and the evaluation of the dominant wavelength (488 nm) of the PB redox state. (Figure reproduced from Mortimer, R. J. and Reynolds, J. R. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. J. Mater. Chem., 15, 2005, 2226–33, with permission from The Royal Society of Chemistry.) 68 Optical effects and quantification of colour The advantage of the CIE L*u*v* and CIE L*a*b* colour spaces is that they are ‘uniform’, i.e. equal distances on the graph represent equal perceived colour differences; the L*u*v* and L*a*b* systems therefore resolve a major drawback of the earlier 1931 system, correcting a defect of the latter which was that equal distances on the graph did not represent equal perceived colour differences. As uniform colour spaces, CIE L*u*v* and CIE L*a*b* allow the accurate representation and calculation of colour differences. In addition, calculations can be performed to conclude whether differences in colour are due to differences in lightness, hue or saturation. The only difference between the L*u*v* 40 50 60 70 80 90 100 E/Vvs. A g/A gCl (b) R e l a t i ve l u m i n a n c e % –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 40 50 60 70 80 90 100 (a) Figure 4.6 Relative luminance (%), vs. applied potential (E/V vs. Ag/AgCl), for a PB|ITO|glass electrode in aqueous KCl (0.2 mol dm À3 ) as supporting electrolyte. The potential was decreased (a) and then increased (b), in the same steps as used for Figure 4.5, between the coloured PB (þ0.50 V) and the transparent PW (À0.20 V) redox states. (Figure reproduced from Mortimer, R. J. and Reynolds, J. R. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. J. Mater. Chem., 15, 2005, 2226–33, with permission from The Royal Society of Chemistry.) 4.5 Colour analysis of electrochromes 69 and L*a*b* colour spaces is that the L*a*b* lacks a two-dimensional diagram, which is probably its only major drawback. The u’v’ uniform colour space diagram only functions as a uniform colour space when the plotted points lie in a plane of constant luminance. Therefore, the graphical representation of colour for materials with widely varying luminance, causes the u’v’ chromaticity diagram to lose all advantage over the 1931 xy chromaticity diagram. Considering all the assets and drawbacks of these three different colour spaces, generally in situ colorimetric results are expressed graphically in the CIE 1931 Yxy colour space system. (In addition, due to the common use of the L*a*b* system, values of L*a*b* are also often reported.) By way of illustration, Figures 4.5 and 4.6 show sample colour coordinates and luminance data on switching between the (oxidised) blue and (reduced) colourless (‘bleached’) states of the electrochrome Prussian blue. Inthis example, sharpchanges inhue, saturationandluminance take place, with an exact coincidence of data in the reverse (colourless to blue) direction. Table 4.4 Table 4.4. Coordinates for reduction of Prussian blue to Prussian white as a film on an ITOjglass substrate in aqueous KCl (0.2 mol dm À3 ) supporting electrolyte. Data come from ref. 47. (Table reproduced with permission of The Royal Society of Chemistry, from: Mortimer, R. J. and Reynolds, J. R. In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue. J. Mater. Chem., 15, 2005, 2226–33.) E/V vs. Ag/AgCl %Y x y L* a* b* 0.500 44.9 0.255 0.340 73 À26 À33 0.400 45.0 0.255 0.340 73 À26 À33 0.300 45.9 0.257 0.342 73 À26 À32 0.275 46.6 0.259 0.344 74 À26 À31 0.250 47.7 0.261 0.347 75 À27 À30 0.225 49.3 0.265 0.350 76 À26 À29 0.200 51.4 0.270 0.354 77 À26 À27 0.175 54.7 0.278 0.360 79 À25 À24 0.150 60.3 0.292 0.368 82 À22 À19 0.125 77.5 0.334 0.384 91 À10 À6 0.100 82.1 0.343 0.386 93 À7 À3 0.075 84.1 0.347 0.386 93 À5 À2 0.050 85.4 0.349 0.387 94 À5 À1 0.025 86.1 0.352 0.387 94 À3 À1 0.000 87.4 0.353 0.387 95 À3 0 À0.050 89.4 0.356 0.387 96 À2 0 À0.100 90.7 0.357 0.387 96 À1 1 À0.200 91.4 0.359 0.386 97 0 1 70 Optical effects and quantification of colour shows the Yxy coordinates, together with the calculated L*a*b* coordinates. Comparing the PB L*a*b* coordinates with those of the blue states for a range of different electrochromic conducting polymer films 54 shows the distinct nature of the blue colour provided by PB. For example, the L*a*b* coordinates for the (deep blue) neutral formof PEDOTare 20, 15, and –43 respectively, 54 while for PB they are 73, À26 and À33. References 1. Heckner, K.-H. and Kraft, A. Similarities between electrochromic windows and thin film batteries. Solid State Ionics, 152–153, 2002, 899–905. 2. Granqvist, G. C. Handbook of Inorganic Electrochromic Materials, Amsterdam, Elsevier, 1995. 3. Lev, O., Wu, Z., Bharathi, S., Glezer, V., Modestov, A., Gun, J., Rabinovich, L. and Sampath, S. Sol–gel materials in electrochemistry. Chem. Mater., 9, 1997, 2354–75. 4. Maruyama, T. and Kanagawa, T. Electrochromic properties of iron oxide thin films prepared by chemical vapor deposition. J. Electrochem. Soc., 143, 1996, 1675–8. 5. ¨ Ozer, N. and Tepehan, F. Optical and electrochemical characterisation of sol–gel deposited iron oxide films. Sol. Energy Mater. Sol. Cells, 56, 1999, 141–52. 6. Zotti, G., Schiavon, G., Zecchin, S. and Casellato, U. Electrodeposition of amorphous Fe 2 O 3 films by reduction of iron perchlorate in acetonitrile. J. Electrochem. Soc., 145, 1998, 385–9. 7. Sato, Y., Ono, K., Kobayashi, T., Watanabe, H. and Yamanoka, H. Electrochromism in iridium oxide films prepared by thermal oxidation of iridium–carbon composite films. J. Electrochem. Soc., 134, 1987, 570–5. 8. Dautremont-Smith, W. C. Transition metal oxide electrochromic materials and displays, a review. Part 2: oxides with anodic coloration. Displays, 3, 1982, 67–80. 9. Scarminio, J., Gorenstein, A., Decker, F., Passerini, S., Pileggi, R. and Scrosati, B. Cation insertion in electrochromic NiO x films. Proc. SPIE, 1536, 1991, 70–80. 10. Fantini, M. C. A., Bezerra, G. H., Carvalho, C. R. C. and Gorenstein, A. Electrochromic properties and temperature dependence of chemically deposited Ni(OH) x thin films. Proc., SPIE, 1536, 1991, 81–92. 11. Carpenter, M. K., Conell, R. S. and Corrigan, D. A. The electrochromic properties of hydrous nickel oxide. Sol. Energy Mater. Sol. Cells, 16, 1987, 333–46. 12. Kitao, M. and Yamada, S. Electrochromic properties of transition metal oxides and their complementary cells. In Chowdari, B. V. R. and Radharkrishna, S. (eds.), Proceedings of the International Seminar on Solid State Ionic Devices, Singapore, World Scientific Publishing Co., 1988, 359–78. 13. Kadam, L. D. and Patil, P. S. Studies on electrochromic properties of nickel oxide thin films prepared by spray pyrolysis technique. Sol. Energy Mater. Sol. Cells, 69, 2001, 361–9. 14. Velevska, J. and Ristova, M. Electrochromic properties of NiO x prepared by low vacuum evaporation. Sol. Energy Mater. Sol. Cells, 73, 2002, 131–9. 15. Cogan, S. F., Nguyen, N. M., Perrotti, S. J. and Rauh, R. D. Optical properties of electrochromic vanadium pentoxide, J. Appl. Phys., 66, 1989, 1333–7. 16. Shimanoe, K., Suetsugu, M., Miura, N. and Yamazoe, N. Bismuth oxide thin film as new electrochromic material. Solid State Ionics, 113–115, 1998, 415–19. References 71 17. Maruyama, T. and Arai, S. Electrochromic properties of cobalt oxide thin films prepared by chemical vapour deposition. J. Electrochem. Soc., 143, 1996, 1383–6. 18. Polo da Fonseca, C. N., De Paoli, M.-A. and Gorenstein, A. The electrochromic effect in cobalt oxide thin films. Adv. Mater., 3, 1991, 553–5. 19. Polo da Fonseca, C. N., De Paoli, M.-A. and Gorenstein, A. Electrochromism in cobalt oxide thin films grown by anodic electroprecipitation, Sol. Energy Mater. Sol. Cells, 33, 1994, 73–81. 20. Svegl, F., Orel, B., Hutchins, M. G. and Kalcher, K. Structural and spectroelectrochemical investigations of sol–gel derived electrochromic spinel Co 3 O 4 films. J. Electrochem. Soc., 143, 1996, 1532–9. 21. Kadam, L. D., Pawar, S. H. and Patil, P. S. Studies on ionic intercalation properties of cobalt oxide thin films prepared by spray pyrolysis technique. Mater. Chem. Phys., 68, 2001, 280–2. 22. Svegl, F., Orel, B., Bukovec, P., Kalcher, K. and Hutchins, M. G. Spectroelectrochemical and structural properties of electrochromic Co(Al)-oxide and Co(Al, Si)-oxide films prepared by the sol–gel route. J. Electroanal. Chem., 418, 1996, 53–66. 23. Bica De Moraes, M. A., Transferetti, B. C., Rouxinol, F. P., Landers, R., Durant, S. F., Scarminio, J. and Urbano, B. Molybdenum oxide thin films obtained by hot-filament metal oxide deposition technique. Chem. Mater., 163, 2004, 513–20. 24. Laperriere, G., Lavoie, M. A. and Belenger, D. Electrochromic behavior of molybdenum trioxide thin films, prepared by thermal oxidation of electrodeposited molybdenum trisulfide, in mixtures of nonaqueous and aqueous electrolytes. J. Electrochem. Soc., 143, 1996, 3109–17. 25. Faughnan, B. W. and Crandall, R. S. Optical properties of mixed-oxide WO 3 / MoO 3 electrochromic films. Appl. Phys. Lett., 31, 1977, 834–6. 26. Cogan, S. F., Anderson, E. J., Plante, T. D. and Rauh, R. D. Materials and devices in electrochromic window development, Proc. SPIE, 562, 1985, 23–31. 27. Ohtani, B., Masuoka, M., Atsui, T., Nishimoto, S. andKagiya, N. Electrochromism of tungsten oxide film prepared from tungstic acid. Chem. Express, 3, 1988, 319–22. 28. Yonghong, Y., Jiayu, Z., Peifu, G., Xu, L. and Jinfa, T. Electrochromism of titanium oxide thin films. Thin Solid Films, 298, 1997, 197–9. 29. Dyer, C. K. and Leach, J. S. Reversible optical changes within anodic oxide films of titanium and niobium. J. Electrochem. Soc., 125, 1978, 23–9. 30. Bange, K. and Gambke, T. Electrochromic materials for optical switching devices. Adv. Mater., 2, 1992, 10–16. 31. Lee, G. R. and Crayston, J. A. Sol-gel processing of transition-metal alkoxides for electronics. Adv. Mater., 5, 1993, 434–42. 32. Faughnan, B. W., Crandall, R. S. and Heyman, P. M. Electrochromism in WO 3 amorphous films. RCA Rev., 36, 1975, 177–97. 33. Deepa, M., Srivastava, A. K., Singh, S. and Agnihotry, S. A. Structure–property correlation of nanostructured WO 3 thin films produced by electrodeposition. J. Mater. Res., 19, 2004, 2576–85. 34. Pauporte´ , T. A simplified method for WO 3 electrodeposition. J. Electrochem. Soc., 149, 2002, C539–45. 35. Cogan, S. F., Plante, T. D., Anderson, E. J. and Rauh, R. D. Materials and devices in electrochromic window development. Proc. SPIE, 562, 1985, 23–31. 36. Hutchins, M., Kamel, N. and Abdel-Hady, K. Effect of oxygen content on the electrochromic properties of sputtered tungsten oxide films with Li þ insertion. Vacuum, 51, 1998, 433–9. 72 Optical effects and quantification of colour 37. ¨ Ozkan, E., Lee, S.-H., Liu, P., Tracy, C. E., Tepehan, F. Z., Pitts, J. R. and Deb, S. K. Electrochromic and optical properties of mesoporous tungsten oxide films. Solid State Ionics, 149, 2002, 139–46. 38. Park, N.-G., Kim, M. W., Poquet, A., Campet, G., Portier, J., Choy, J. H. and Kim, Y. I. New and simple method for manufacturing electrochromic tungsten oxide films. Active and Passive Electronic Components, 20, 1998, 125–33. 39. Arakaki, J., Reyes, R., Horn, M. and Estrada, W. Electrochromism in NiO x and WO x obtained by spray pyrolysis. Sol. Energy Mater. Sol. Cells, 37, 1995, 33–41. 40. Bessie` re, A., Badot, J.-C., Certiat, M.-C., Livage, J., Lucas, V. and Baffier, N. Sol–gel deposition of electrochromic WO 3 thin film on flexible ITO/PET substrate. Electrochim. Acta, 46, 2001, 2251–6. 41. Davazoglou, D., Donnadieu, A. and Bohnke, O. Electrochromic effect in WO 3 thin films prepared by CVD. Sol. Energy Mater., 16, 1987, 55–65. 42. Rauh, R. D., Wang, F., Reynolds, J. R. and Meeker, D. L. High coloration efficiency electrochromics and their application to multi-color devices. Electrochim. Acta, 46, 2001, 2023–9. 43. Green, F. J. The Sigma–Aldrich Handbook of Stains, Dyes and Indicators, Milwaukee, WI, Aldrich Chemical Company, Inc., 1990. 44. Gaupp, C. L., Welsh, D. M., Rauh, R. D. and Reynolds, J. R. Composite coloration efficiency measurements of electrochromic polymers based on 3,4-alkylenedioxythiophenes. Chem. Mater., 14, 2002, 3964–70. 45. Cirpan, A., Argun, A. A., Grenier, C. R. G., Reeves, B. D. and Reynolds, J. R. Electrochromic devices based on soluble and processable dioxythiophene polymers. J. Mater. Chem., 13, 2003, 2422–8. 46. Reeves, B. D., Grenier, C. R. G., Argun, A. A., Cirpan, A., McCarley, T. D. and Reynolds, J. R. Spray coatable electrochromic dioxythiophene polymers with high coloration efficiencies. Macromolecules, 37, 2004, 7559–69. 47. Mortimer, R. J. and Reynolds, J. R. In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue. J. Mater. Chem., 15, 2005, 2226–33. 48. Aubert, P.-H., Argun, A. A., Cirpan, A., Tanner, D. B. and Reynolds, J. R. Microporous patterned electrodes for color-matched electrochromic polymer displays. Chem. Mater., 16, 2004, 2386–93. 49. Compton, R. G., Waller, A. M., Monk, P. M. S. and Rosseinsky, D. R. Electron paramagnetic resonance spectroscopy of electrodeposited species from solutions of 1,1 0 -bis(p-cyanophenyl)-4,4 0 -bipyridilium (cyanophenyl paraquat, CPQ). J. Chem. Soc., Faraday Trans., 86, 1990, 2583–6. 50. Duffy, J. A. Bonding, Energy Levels and Inorganic Solids, London, Longmans, 1990. 51. Robin, M. B. and Day, P. Mixed valence chemistry – a survey and classification. Adv. Inorg. Chem. Radiochem., 10, 1967, 247–422. 52. Brown, D. B. (ed.), Mixed Valence Compounds (NATO Conference), 1980, London, D. Reidel. 53. Baucke, F. G. K., Duffy, J. A. and Smith, R. I. Optical absorption of tungsten bronze thin films for electrochromic applications. Thin Solid Films, 186, 1990, 47–51. 54. Thompson, B. C., Schottland, P., Zong, K. and Reynolds, J. R. In situ colorimetric analysis of electrochromic polymers and devices. Chem. Mater., 12, 2000, 1563–71. 55. Thompson, B. C., Schottland, P., S ¨ onmez, G. and Reynolds, J. R. In situ colorimetric analysis of electrochromic polymer films and devices. Synth. Met., 119, 2001, 333–4. References 73 56. Schwendeman, I., Hickman, R., S ¨ onmez, G., Schottland, P., Zong, K., Welsh, D. M. and Reynolds, J. R. Enhanced contrast dual polymer electrochromic devices. Chem. Mater., 14, 2002, 3118–22. 57. S ¨ onmez, G., Schwendeman, I., Schottland, P., Zong, K. and Reynolds, J. R. N-Substituted poly(3,4-propylenedioxypyrrole)s: high gap and low redox potential switching electroactive and electrochromic polymers. Macromolecules, 36, 2003, 639–47. 58. S ¨ onmez, G., Meng, H. and Wudl, F. Organic polymeric electrochromic devices: polychromism with very high coloration efficiency. Chem. Mater., 16, 2004, 574–80. 59. Thomas, C. A., Zong, K., Abboud, K. A., Steel, P. J. and Reynolds, J. R. Donor- mediated band gap reduction in a homologous series of conjugated polymers. J. Am. Chem. Soc., 126, 2004, 16440–50. 60. S ¨ onmez, G., Shen, C. K. F., Rubin, Y. and Wudl, F. Ared, green, and blue (RGB) polymeric electrochromic device (PECD): the dawning of the PECD era. Angew. Chem., Int. Ed. Engl., 43, 2004, 1498–502. 61. S ¨ onmez, G. and Wudl, F. Completion of the three primary colours: the final step towards plastic displays. J. Mater. Chem., 15, 2005, 20–2. 62. Berns, R. S. Billmeyer and Saltzman’s Principles of Color Technology, 3rd edn, New York, J. Wiley & Sons, 2000. 63. Wyszecki, G. and Stiles, W. S. Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd edn, New York, J. Wiley & Sons, 1982. 64. [online] at www.efg2.com/Lab/Graphics/Colors/Chromaticity.htm (accessed 4 January 2006). 74 Optical effects and quantification of colour 5 Kinetics of electrochromic operation 5.1 Kinetic considerations for type-I and type-II electrochromes: transport of electrochrome through liquid solutions Type-I and type-II electrochromes are dissolved in solution prior to the electron-transfer reaction that results in colour. Such electron-transfer reactions are said to be ‘nernstian’ or ‘reversible’ when uncomplicated and fast and in accord with the Nernst equation (Eq. (3.1), Chapter 3). When two conditions regarding the motions of electroactive species (or indeed other participant species) are met, there is a particular means, that needs definition, whereby the key electroactive species arrives at the electrode. These conditions are: the absence both of convection (i.e. the solution unstirred, ‘still’), and also of electroactive-species migration. a Then ‘mass transport’ (directional motion) of any electroactive species is constrained to occur wholly by diffusion. On the one hand, the rate of forming coloured product can be dictated by the rate of electron transfer with rate constant k et , which if low may render the electrode response non-nernstian (the electrode potential E O,R diverges from the Nernst equation (3.1) in terms of bulk electroactive concentrations), and furthermore, the rate of the process governed by k et largely determines the current. On the other hand, if k et is high, then electroactive/electrode electron transfer is not the rate- and current-controlling bottleneck, and the overall rate of colour formation is dictated by the rate of mass transport of electroactive species toward the electrode. a To recapitulate Section 3.3, ‘migration’ here means charge motion resulting in ohmic conduction of current. This migration is subtly prevented when the solution contains an excess of inert (‘swamping’) electrolyte ions that themselves cannot conduct, because, being inert (i.e. redox-unreactive), such ions, on contact with the appropriate electrode, cannot undergo the electron transfer required to complete the conduction process. Excess ionic charge of these species accumulates up to a potential-determined limit. Huge applied potentials can in some cases subvert ‘inertness’. 75 The experimental context of these considerations arises as follows. An electrochromic cell is primed for use (‘polarised’) by applying an overpotential (Section 3.3, Chapter 3). Polarising the cell ensures that, if in solution, some of the electrochrome impinging on the electrode will undergo an electron-transfer reaction. However, all of the electrochrome reaching the electrode is electromodified if the overpotential is sufficiently large, in which case the current becomes directly proportional to the concentration of electrochrome, a result that arises from Fick’s laws of diffusion 1 (Chapter 3). The current is then said to have its limiting value I (lim) , i.e. increases in the applied overpotential will not increase the magnitude of the current. The value of I (lim) decreases slowly with time (with electrode and solution motionless), as outlined below. A large positive value of overpotential generates a limiting anodic (oxidative) current, while a large negative value of overpotential results in a limiting cathodic (reductive) current. Because the amount of colour formed in a given time is by definition proportional to the rate of charge passage at the electrode, as high a current as possible is desirable for rapid device operation, i.e. if possible, a limiting current is enforced. (If the current I is made too high, however, deleterious side reactions may occur at the electrode, as discussed below. The current that yields electrochemical reaction is termed ‘faradaic’, but current otherwise utilised say in solely ionic movement is ‘non-faradaic’– Section 3.4, Chapter 3.) The current is thus best increased by enhancing the rates of mass transport to the electrode. In a laboratory cell, stirring the solution will maximise the current since convection (Section 3.3) is the most efficient form of mass transport. However, in a practicable ECD this expedient will always be impossible, and natural convection, as e.g. caused by localised heating of the solution at the electrode, can also be dismissed. If migration is also minimised because an excess of inert ‘swamping’ electrolyte has been added to the solution (Section 3.3 and footnote to previous page), then the time-dependence of the limiting current, I (lim,t) owing to electrode reaction of the ion i is given by the Cottrell equation, Eq. (5.1): I ðlim. tÞ ¼ n FAc i ffiffiffiffiffiffi D i pt _ . (5.1) where F is the Faraday constant, c i is the concentration of the electroactive species i, n is the number of electrons involved in the electron-transfer reaction, Eq. (1.1), and A is the electrode area. The derivation of the Cottrell equation presupposes semi-infinite linear diffusion toward a planar electrode, and more complicated forms of the Cottrell equation have been derived for the thin-layer 76 Kinetics of electrochromic operation cells 2 that are used for type-I ECDs. Table 5.1 lists a few values of diffusion coefficient D obtained from Cottrell analyses. Equation (5.1) predicts that the magnitude of the current – and hence the rate at which charge is consumed in forming the coloured formof the electrochrome – is not constant, but decreases monotonically with a t À½ dependence in a diffusion-controlled electrochemical system. This kinetic result is indeed found until quite long times (10 s after the current flowcommences). Figure 5.1 shows such a plot of current I against time t À½ during the electro-oxidation of aqueous o-tolidine (3,3 0 -dimethyl-4,4 0 -diamino-1,1 0 -biphenyl) (I), which, being a kinetically straightforward (‘nernstian’) system, 5 conforms with the analysis. I H 2 N NH 2 CH 3 H 3 C The rate of colorationis obviously a linear functionof the rate of electronuptake, I ¼dQ/dt. Accordingly, for optical absorbance Abs (which is / Q), the rate of colour formation d(Abs)/dt (which is / I, Eq. (1.7)) ought also to have the time dependence of t À½ according to the Cottrell relation, Eq. (5.1). Integration hence predicts Abs / t þ½ and for (I) in water; the test plot, Figure 5.2, is satisfactorily linear. 5 Support for a diffusion-controlled mechanism is thus demonstrated. The slope of Figure 5.2 should be independent of the concentrations of the electroactive species, as is shown in Figure 5.3. Here, slopes of Abs versus t ½ plots at various concentrations and currents are plotted against I for the electro-oxidation of o-tolidine (I) in water, 5 and they superimpose regardless of concentration, as expected. However, the plot Figure 5.3 should not be linear, as d(Abs)/dt ½ is clearly not linear with I, which can be inferred from Eq. (1.7), and the spurious straight line shown results largely from employing restricted ranges of the variables. Absorbance–time relationships like these have seldom been used as tests (presumably discouraged by confusion arising from the apparent irrationality Table 5.1. Diffusion coefficients D of solvated cations moving through solution prior to reductive electron transfer. Diffusing entity D/cm 2 s À1 Diffusion medium Ref. Fe 3þ 5 Â10 À6 Water 2 Methyl viologen 8.6 Â10 À6 Water 3 Cyanophenyl paraquat 2.1 Â10 À6 Propylene carbonate 4 5.1 Transport of electrochromes through solutions 77 of the Figure 5.3-type plots) but in 1995 Tsutsumi et al. 6 emulated the tests of these relations for electrogenerating the aromatic radical anion of p-diacetylbenzene (II) with similar success. II COCH 3 H 3 COC Such diffusion control is expected during coloration for all type-I electrochromes, while type-II electrochromes should evince the same behaviour at very short times. Deviations must occur at longer times because the transferring 9 8 7 6 5 3 2 1 0 0.1 0.2 0.3 0.4 0.5 4 (t /s) –½ I /m A Figure 5.1 Cottrell plot of limiting current I against t À½ during the electro- oxidation of o-tolidine (3,3 0 -dimethyl-4,4 0 -diamino-1,1 0 -biphenyl) in aqueous solution at a ITO electrode polarised to 1.5 V vs. SCE. (Figure reproduced from Hansen, W. N., Kuwana, T. and Osteryoung, R. A. ‘Observation of electrode–solution interface by means of internal reflection spectrometry’. Anal. Chem., 38, 1966, 1810–21, by permission of The American Chemical Society.) 78 Kinetics of electrochromic operation electron needs to traverse a layer of solid coloured product, with concomitant complication of the analysis. 5.2 Kinetics and mechanisms of coloration in type-II bipyridiliums As the details of the coloration mechanisms are, exceptionally, so specific to the chemistry of this group of type-II electrochromes, where the uncoloured reactant is dissolved but the coloured form becomes deposited as a solid film, the complications of the chemistry are dealt with in Chapter 11, on the bipyridiliums. Sections 11.2 and 11.3 specifically are devoted to these aspects. 5.3 Kinetic considerations for bleaching type-II electrochromes and bleaching and coloration of type-III electrochromes: transport of counter ions through solid electrochromes Type-II electrochromes such as heptyl viologen (see Chapter 11) are solid prior to bleaching. Type-III electrochromes remain solid during oxidation and reduction reactions. The majority of studies relating to the kinetic aspects of electrochromic operation of solid materials relate to tungsten oxide as a thin film. With suitable and probably slight modification, the theories below relating to solid WO 3 will equally apply to many other solid electrochromes, such 0.04 0.03 0.02 0.01 Δ ( Abs o r ba n c e ) 0 1 2 3 4 5 6 7 (t /s) ½ Figure 5.2 Plot of the change of optical absorbance Abs against t ½ during the electro-oxidation of o-tolidine (3,3 0 -dimethyl-4,4 0 -diamino-1,1 0 -biphenyl) in aqueous solution at a ITO. (Figure reproduced from Hansen, W. N., Kuwana, T. and Osteryoung, R. A. ‘Observation of electrode–solution interface by means of internal reflection spectrometry’. Anal. Chem., 38, 1966, 1810–21, by permission of The American Chemical Society.) 5.3 Transport of counter ions through solid systems 79 as the other metal oxides in Chapter 6. Some of the results may also apply straightforwardly to the inherently conducting polymers in Chapter 10. Even a brief survey of the literature on tungsten trioxide shows a number of competing models in circulation for the coloration and decoloration processes. As already noted, the most by far of reported kinetic studies of electrochromism relate to solid tungsten trioxide. Its coloration reaction is summarised in Eq. (5.2) (which is actually ‘a gross over-simplification’, 7 since the initial solid almost invariably also involves water and hydroxyl ions): WO 3 þxðM þ þe À Þ !M x WO 3 . (5.2) Thus in the discussion below WO 3 is the paradigm, with the M þ as an inert, i.e. electro-inactive ion, usually designated ‘counter ion’, that is entrained to d( A b s ) /d( t /s ) ½ 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0 2 4 6 I/ mA 8 10 [o-tolidine] × 10 3 2.5 5 10 Figure 5.3 Plot of d(Abs)/dt ½ against current I for the electro-oxidation of o- tolidine (3,3 0 -dimethyl-4,4 0 -diamino-1,1 0 -biphenyl) in aqueous solutions at a ITO electrode polarised to 1.5 V vs. SCE. The concentrations of electrochrome are o 2.5 Â10 À3 mol dm À3 , * 5 mmol dm –3 , and & 10 mmol dm À3 . The straight line is spurious – see text. (Figure reproduced from Hansen, W. N., Kuwana, T. and Osteryoung, R. A. ‘Observation of electrode-solution interface by means of internal reflection spectrometry’. Anal. Chem., 38, 1966, 1810–21, by permission of The American Chemical Society.) 80 Kinetics of electrochromic operation preserve or maximise electroneutrality within the solid oxide film. (Systems generally adjust, subject to electromagnetic, electrostatic and quantal laws, to minimise concentrations of charge and high potentials.) Other electrochromes, organic as well as inorganic, are mentioned here if data are available. 5.3.1 Kinetic background: preliminary assumptions (i) Initial state: mass balance Prior to the application of the coloration potential V a , solid films of WO 3 are assumed to contain no electro-inserted counter ions. However, an ellipsometric study by Ord et al. 8 apparently disproves this assumption. His thin-film WO 3 , formed anodically on W metal immersed in acetic acid, was shown to contain protonic charge, but this charge had no optical effect: presumably acid had been unreactively absorbed by the solid. Another source of charge inside a film is the ionisation of water: H 2 O ! H þ þOH À (or with sufficient H 2 Oabout, 2H 2 O!H 3 O þ þOH À ). Such water may be replenished during coloration and bleaching since there is evidence for movement of molecular water through transition-metal oxides during redox cycling, e.g. H 2 O will be inserted into electrodeposited cobalt oxyhydroxide 9 or into vacuum-evaporated 10 WO 3 when the impressed potential is cathodic; and water will also move through polymers of organic viologen in response to redox cycling. (ii) Electronic motion As we assume a particulate electron, the niceties of quantum-mechanical tunneling associated with wave properties will be glossed over. At low extent of reduction x, electron conduction probably occurs via activated site-to-site hopping rather than through occupied conduction bands, since most of these metal oxides when fully oxidised are, at best, poorly conducting semiconductors. 11 In accord, the electrical conductivity of fully oxidised WO 3 is extremely low, both as a solid and as a thin film. In contrast, the electronic conductivity of M x WO 3 (where M¼H þ , Li þ or Na þ ) is metallic for so-called ‘bronzes’ b of x greater than ca. 0.3. Figure 5.4 shows a plot of electronic conductivity in WO 3 as a function of insertion coefficient x. The WO 3 was prepared either by vacuumevaporation, that produces an amorphous oxide, denoted a-WO 3 , or by sputtering, that produces a crystalline oxide, b In this context, a ‘bronze’ is a solid with metallic or near-metallic conductivity. Belowa metal-to-insulator transition, WO 3 is a semiconductor, but above it near-free electrons impart reflectivity. ‘Free’ here implies ‘akin to conduction electrons in true metals’. 5.3 Transport of counter ions through solid systems 81 denoted c-WO 3 . It should be noted that c-WO 3 is less electronically conductive than a-WO 3 . Circumscribing the use of WO 3 in ECDs, the formation of the high-x bronzes M x WO 3 (x0.3) is not reversible, so e.g. Li 0.4 WO 3 cannot be electro- oxidised back to 12 WO 3 . At high x values the transferred electrons, acquired in the electrochemical coloration process, are stabilised in an accessible conduction band largely comprising the tungsten d orbitals. Electrons from interphase redox reactions by external electroactive species, via a dissipating conduction through this band, may thwart the re-oxidative extraction of electrons from W V by the electrode substrate. (Interphase rather than ‘interface’ is defined in Chapter 3, p. 43) (iii) Motion of ions The solid electrochromic oxide, as a film on its electrode substrate, can be immersed in a solution containing a salt of the counter ions, such as H 2 SO 4 for mobile protons, or LiClO 4 for Li þ ion. During electro- coloration, electrons enter the filmvia the electrode substrate and, concurrently, x Evaporatedla yer Sputteredla yer T = 123 K 0.05 0.1 0.15 0.2 0.25 5 4 3 2 1 l o g ( σ /c m –1 ) Figure 5.4 Plot of electronic conductivity s of H x WO 3 as a function of insertion coefficient x. Data determined at 123 K. (Figure reproduced from Wittwer, V., Schirmer, O. F. and Schlotter, P. ‘Disorder dependence and optical detection of the Anderson transition in amorphous H x WO 3 bronzes’. Solid State Commun., 25, 1978, 977–80, copyright (1978) with permission from Elsevier Science.) 82 Kinetics of electrochromic operation counter ions enter the film through the electrolyte-facing interphase of the WO 3 cathode. Bleaching entails a reversal of these steps. So coloration or bleaching proceed with associated movements of both electrons and cations. 13 When the kinetics of electrochemical redox change are dictated by the motion of a species within the film, it is the slower, hence rate limiting, of the two charge carriers that is the determinant. The slower charge carrier is usually the ion because of its relatively large size. Indeed, the transport number t (¼fraction of current borne) of ions can approach zero, then correspondingly the electron transport number t (electron) !1. Such dual motion is the cause of the curiously named ‘thermodynamic enhancement’ described by Weppner and Huggins, 14 as mentioned below. A good gauge of rapidity of ion motion is its diffusion coefficient D. However, the movement of counter ions through solid WO 3 proceeds by both diffusion and migration. The two modes of mass transport operate additively, but the separate extents are usually not known. Exemplifying, Bell and Matthews 15 cite activation energies E a for diffusion, varying in the range 56–70 kJ mol À1 (values that denote an appreciable temperature dependence): the spread of values arises from the pronounced curvature of an Arrhenius plot. True diffusion is an activated process and normally obeys the Arrhenius equation that gives a linear graph of ln D against 1/T. In contrast, the temperature dependence of migration is relatively modest. As dual mechanisms with different activation energies often show curved 1/T plots of the rate-parameter logarithm, the non-linearity of Bell and Matthews’ graph accordingly points to a significant extent of migration in the measured ‘diffusion coefficient’. The latter is therefore unlikely to be a true diffusion coefficient but a combined-mechanism quantityD, as defined below. Diffusion coefficients are obtained from several measurements: impedance spectra, chronoamperometry, analysis of cyclic-voltammetric peak heights as a function of scan rate via the Randles–Sevcˇ ik equation, Eq. (3.12), and radiotracer methods. 16 Compendia from the literature of D values for mobile ions moving through WO 3 in refs. 12,17,18,19,20 provide the representative selection in Table 5.2, together with preparation method and insertion coefficient, x. For comparative purposes, values for mobile ions moving through other type-III electrochromes are listed in Table 5.3. The variations in diffusion coefficient could reflect the disparity in rate between electrons and ions as they move through the solid. To minimise the charge imbalance during ion insertion or egress, the slower ions move faster and the fast electrons are slowed. 14 In this way, the overall rate is altered, 36 causing D to change by a factor of W, an enhancement factor. The factor W quantifies the extent of the so-called ‘thermodynamic enhancement’, and the resultant 5.3 Transport of counter ions through solid systems 83 diffusion coefficient is the ‘chemical diffusion coefficient’; W is also termed the ‘Wagner factor’. The two diffusion coefficients D and - D are related as: 14 - D ¼ WD. (5.3) In consequence, probably most of the ‘diffusion coefficients’ in the literature of solid-state electrochromism are chemical diffusion coefficients. The factor W was derived as: 14 W¼ t ðelectronÞ 0 ln a ðionÞ 0 ln c ðionÞ þz ðionÞ 0 ln a ðelectronÞ 0 ln c ðionÞ _ _ . (5.4) Here the letters c and a are respectively concentration and activity, (see Chapter 3, p. 36); z (ion) is the charge on the mobile ion. The enhancement factor W can be 14 as great as 10 5 , but is said to be ‘about 10’ for the motion of H þ through WO 3 . 12 In addition to morphological differences born of preparative Table 5.2. Chemical diffusion coefficientsD representing movement of lithium ions through tungsten trioxide: effect of preparative methodology and insertion coefficient. Measurements as in text, on three-electrode cells avoiding ECD complications. Morphology x in Li x WO 3 D /cm 2 s À1 Ref. (a) Effect of preparative methodology WO 3 *ab – 5 Â10 À9 21 WO 3 *bd – 1.6 Â10 À12 22 WO 3 *cd – 1.3 Â10 À11 23 WO 3 *e – 2 Â10 À11 24 WO 3 f – 5 Â10 À13 25 (b) Effect of insertion coefficient x a-WO 3 *bc 0.097 2.5 Â10 À12 21 a-WO 3 *bc 0.138 4.9 Â10 À12 21 a-WO 3 *bc 0.170 1.5 Â10 À11 21 a-WO 3 *bc 0.201 2.6 Â10 À11 21 a-WO 3 *bc 0.260 2.8 Â10 À11 21 Li 0.1 WO 3 f 0.1 1.7 Â10 À9 26 Li 0.37 WO 3 f 0.37 5.6 Â10 À10 26 *Thin film. a Sputtered film. b Impedance measurement. c Thermally evaporated sample. d Chronoamperometric measurement. e Electrodeposited film. f Film prepared from sol–gel intermediate. 84 Kinetics of electrochromic operation routes, variations in Ware a likely reason for the wide differences in the - Dvalues listed in Tables 5.2 and 5.3. Being fast, the transport number of the electron t (electron) ! 1, hence the observed rate of transport through WO 3 is determined by the slower ions. Thus the expression for W can be simplified, Eq. (5.4) becoming: W¼ 0 ln a ðionÞ 0 ln c ðionÞ _ _ . (5.5) Substituting for W from Eq. (5.3) into Eq. (5.5) yields the so-called Darken relation: 14 - D ¼ D 0 ln a ðionÞ 0 ln c ðionÞ _ _ . (5.6) It is assumed in Eqs. (5.3)–(5.6) above that only the counter ion is mobile since all other ions (e.g. oxide ions O 2– that are, more likely, 37 in the oxygen bridges –O–) are covalently bound or otherwise immobile. This tenable assumption has been verified in part by impedance spectroscopy. 38 Table 5.3. Chemical diffusion coefficientsD of mobile ions through permanent, solid films of type-III electrochromes; diffusion of counter ion through the electrochromic layer. Methods as for Table 5.2. Compound Ion : Solvent D/cm 2 s À1 Ref. Cerium(IV) oxide Li þ :PC 5.2 Â10 À13 27 (F 16 -pc)Zn a TBAT:DMF 1.6–8.0 Â10 À12 28 Lutetium bis(phthalocyanine) Cl À : H 2 O 10 À7b 29 Nickel hydroxide H þ :H 2 O 2 Â10 À7 to 2 Â10 À9 30,31 H 0.042 Nb 2 O 5 H þ 3.6 Â10 À8 32 H 0.08 Nb 2 O 5 H þ 5.2 Â10 À7 32 Poly(carbazole) ClO À 4 :H 2 O 10 À11 33 Poly(isothianaphthene) BF À 4 :PC 10 À14 34 Tungsten(VI) trioxide c H þ :HCl(aq) 2 Â10 À8d 35 Tungsten(VI) trioxide e Li þ :PC 2.1 Â10 À11f 27 Vanadium(V) trioxide Li þ :PC 3.9 Â10 À11 27 PC ¼ Propylene carbonate. F 16 -pc ¼ perfluorinated phthalocyanine. a Value from analysis of a Randles–Sevcˇ ik graph. b Apparently calculated from chloride ion mobility. c Thermally-evaporated sample. d Chronoamperometric measurement. e Sputtered film. f Value determined from impedance measurement. 5.3 Transport of counter ions through solid systems 85 (iv) Energetic assumptions A relatively crude model of insertion has the counter ion entering or leaving the oxide layer after surmounting an activation barrier E a associated with the WO 3 –electrolyte interphase. For example, a recent Raman-scattering investigation of H x WO 3 electro-bleached in aqueous H 2 SO 4 is said to indicate, by analysis of the WO 3 vibrational modes, that the rate of electro-bleaching is dominated by proton expulsion fromthe H x WO 3 as the H þ traverses the electrochrome–solution interphase. 10 There is also an activation barrier to electron insertion/egress from or to the electrode substrate, the barrier often being represented as the resistance to charge transfer, R (CT) . Many of the measured values of ‘R (CT) ’ may be composites of terms containing the interphase activation energy E a (in an exponential) for ion insertion together with R (CT) for the electron transfer at the electrode substrate, with the former E a effect being the larger. The motion of counter ions within the film may also contribute, and certainly play a role in the interpretative models considered below. The motion of a (bare thus minute) proton will be the most rapid of all the cations, in moving within the oxide layer following insertion during coloration. Protons come to rest when the external potential is removed and when, in addition, they attain sites of lowest potential energy. On equilibration inside the oxide layer, the inserted ion is assumed in most models to be uniformly distributed throughout the film, perhaps with slight deviations in concentration at interphases due to the interactions born of surface states. 39 The discussion below indicates how this last assumption probably understates the role of interphases. 5.3.2 Kinetic complications The complications caused by the innate resistance of the ITO, called ‘terminal effects’, can be largely bypassed (but see refs. 40, 41) by including an ultra-thin layer of metallic nickel between the electrochrome and ITO, 42 or an ultrathin layer of precious metal on the outer, electrolyte-facing, side of the electrochrome. Both apparently improve the response time t. 43,44,45,46 The effect is elaborated in refs. 40 and 41. (i) Crystal structure There are several distinct crystallographic phases notably monoclinic discernible in reduced crystalline tungsten oxide (c-WO 3 ) at low insertion coefficients (0 <x 0.03). 47 Slight spatial rearrangement of atoms (i.e. local phase transitions from the predominantly monoclinic) in c-WO 3 are said to occur during reduction, 48 which may affect the electrochromic response 86 Kinetics of electrochromic operation time of WO 3 for colouring or for bleaching. Such structural changes are sometimes believed to be the rate-limiting process during ion insertion into WO 3 . 49,50 The value of - D increases slightly with increasing insertion coefficient x, as exemplified by the data of Ho et al. 21 in Table 5.2; Avellaneda and Bulho˜ es find the same effect. 26 Green 24 has stated that WO 3 expands by ca. 6% on reductive ion insertion; and Ord et al. 51 show by ellipsometry that V 2 O 5 on reduction in acetic acid electrolyte also expands by 6%, despite the thicknesses of electrochromic oxides being somewhat diminished when a field is applied owing to electrostriction. 52,53 Similarly, samples of c-WO 3 , when injected with Li þ ion at a continuous rate, were found to have a higher capacity for lithium ion than do otherwise identical samples that are charged fitfully. 54 It was argued 54 that this result demonstrates that the Li x WO 3 product has sufficient time to change structure on a microscopic scale during the slower, stepwise, charging, thus impeding subsequent scope for reduction. (ii) The effect of the size of the mobile ion Questions arise as to what counter ion is taken up during reduction, and which one provides the charge motion within the film, but the picture is not clear-cut. A general picture does emerge from envisaging the constraints on ionic motion and the experimental observations, but it is not always intrinsically consistent in detail. As ions that move through solid oxide experience obstruction within the channels, ionic size is expected to govern the values of D for different ions. A model for this process from which activation energies can be estimated is outlined later, on p. 112. For rapid ECD coloration, ion size should be minimised, so protons are favoured for WO 3 . Deuterons 55,56,57 are found to be somewhat slower than protons; and lithiumions are slower still (see Table 5.2). Though some workers have reversibly inserted Na þ , 58,59,60 and even reversible incorporation of Ag þ has been reported, 61 most other cations are too slow to act in ECDs. (The sequence of cations follows the indications of the activation-energy model referred to.) The only anion small and mobile enough to be inserted into anodically colouring electrochromes is OH À . Scarminio 62 reported that the stress induced in a film is approximately proportional to the insertion coefficient, x. The film capacitance also increases linearly with x. 63 Scrosati et al. 48 used a laser-beamdeflection method toassess the stresses from electro-inserting Li þ and Na þ , finding that phase transitions were induced. Counter-ion swapping can occur since WO 3 does entrain indeterminate amounts of water, even if prepared as an anhydrous film. Variable water 5.3 Transport of counter ions through solid systems 87 content may be the cause of the great discrepancies between reported values of - D. Some chemical diffusion coefficients for the (nominally) slow lithium ion appear to be fairly high for motion through WO 3 . This suggests diffusion of the more mobile proton (presumably taken up interstitially, or formed by ionisation of interstitial water), followed at longer times by exchange of Li þ for H þ as charge-carrier, which is illustrated in the electrochemical quartz-crystal microbalance (EQCM) study by Bohnke et al. 64,65,66,67 Such unexpected swapping is considered thermodynamically (specifically entropy) driven. In common with Bohnke et al., Babinec’s EQCM study 68 also suggested swapping of Li þ for the more mobile H þ , but also suggested egress of hydroxide ions from the film during coloration (from water within the film ionising to OH À and H þ ). A dual-cation mechanism is suggested by Plinchon et al.’s 69 mirage-effect experiment that implied dual insertion of H þ and Li þ during reduction of WO 3 . Kim et al., 20,70 studying the dual injection of H þ and Li þ by impedance spectroscopy, report the process to be ‘extremely complicated’. For a chemically different WO 3 , the diffusion coefficient of lithium ions inserted into rf- sputtered WO 3 was found to decrease as the extent of oxygen deficiency increased. 71 (iii) The effect of electrochrome morphology Diffusion through amorphous oxides is significantly faster than through those same oxides when crystalline. 24 Kubo and Nishikitani, 72 in a Raman spectral study of WO 3 , cite polaron– polaron interactions within clusters of c-WO 3 embedded in amorphous material, as a function of cluster size, concluding that the coloration efficiency j increases as the cluster becomes larger. Also, since electrochromic films commonly comprise both amorphous and crystalline WO 3 , the mobile ions tend to move through the amorphous material as a kinetic ‘fast-track’. Indeed, diffusion through c-WO 3 is so slow by comparison with diffusion through amorphous tungsten oxide (a-WO 3 ) that the c-WO 3 need not even be considered during kinetic modelling of films comprising both amorphous and crystalline oxide; 24 see page 98. In similar vein, the value of D for Li þ motion through a-WO 3 that is thermally annealed decreases by about 5% over annealing temperatures ranging from 300–400 8C; the decrease in D is ascribed to increased crystallinity. 73 Similarly, diffusion through the amorphous grain boundaries within polycrystalline NiO is faster than through the NiO crystallites. 74 An additional means of increasing the electrochromic coloration rate is to increase the size of the channels through the WO 3 by introducing heteroatoms into the lattice. The incorporation of other atoms like Mo, to form e.g. W y Mo (1 – y) O 3 , causes strains in the lattice which are relieved by increases in all the lattice constants. 88 Kinetics of electrochromic operation (iv) The effect of water The presence of water can greatly complicate kinetic analyses intrinsically, and additionally, adsorption of water at the electrochrome– electrolyte interface can make some optical analyses quite difficult 75 since specular effects are altered. Even the coloration efficiency can change following such adsorption. 76 Hurditch 77 has stated that electrochromic colour of H x WO 3 will formonly if films contain moisture and, similarly, Arnoldussen 78 states that MoO 3 is not electrochromic if its moisture content drops below a minimum level. Curiously, he also states that his MoO 3 was electro-coloured as a dry film in a vacuum. One concludes that water, presumably adsorbed initially, is essential in effecting the reductive coloration, either by ionising to H þ and OH À so providing the conductive protons, or by being reduced to H 2 (also with OH À ) which itself can effect chemical reduction. Hygroscopicity Thin films of metal oxide are often somewhat hygroscopic, 79 although it has been concluded that the cubic phase of WO 3 prefers two H þ to one water molecule. 80 Adsorbed water can be removed by heating 81 above ca. 1908C (but extensive film crystallisation will also occur at such temperatures; see p. 140). References 82 and 83 describe the depth-profile of H 2 O in WO 3 , as shown in Figure 5.5. Proton conductivity through solid-state materials, and its measurement, have been reviewed by Kreuer. 84 Aquatic degradation Excess moisture inside films (especially evaporated films) will cause much structural damage, 85 perhaps following the formation of soluble tungstate ions. 81 Faughnan and Crandall, 86 Arnoldussen 87 and Randin 88 have all discussed dissolution effects. Furthermore, the rate of WO 3 dissolution is promoted by aqueous chloride ion. 89 Energetics The effect on stabilities resulting from the incorporation of water needs consideration. The forces exerted on an atom, ion or molecule during its movement through an oxide interior are determined by the microscopic environment through which it moves, and on the physical size of the channels through which it must pass. Ions undergo some or total desolvation during ion insertion from solution, i.e. when traversing the solution–electrochrome interphase into the lattice. The loss of solvation stabilisation can be partly compensated by interaction with lattice oxides or indeed occluded H 2 O, but the former – in addition to lattice-penetration obstacles – could retard motion (EQCM studies 67 however show Li þ to be unsolvated as it moves through 5.3 Transport of counter ions through solid systems 89 WO 3 ). Proton motion through hydrated films is accordingly found to be much faster than through dry films, 90 the retarding proton/oxide interactions possibly being weaker than in dry oxides. Alternatively a Gr ¨ otthus-type conduction process could be facilitating rapid proton conduction through hydrated oxide interiors. Bohnke et al. 67 used data from EQCM studies to explain non-adherence to Nernst-type relations, postulating that adsorbed, unsolvated, anions are expelled from the surface of the WO 3 as cathodic coloration commences. The effects of interactions between inserted Li þ and the lattice were also mentioned. (iv) The effect of insertion coefficient on - D Values of - D can be obtained from the gradient of a graph of impedance vs. o À½ as by Huggins and co-workers. 21 Three independent groups found that - D decreased as the insertion coefficient of Li þ in WO 3 increased; 60,71,91 Masetti et al. 60 found that - D for Li þ and Na þ decreased by thirty-fold in WO 3 over the insertion coefficient range H / W 1.2 0.8 0.4 0 0 0.25 0.5 7000 8000 Energy [k eV] 9000 10 000 11 000 H /S i Glass IT O WO 3 WO 3 SiO 2 SiO 2 Rh Rh Figure 5.5 Hydrogen profile within the electrochromic cell at an applied voltage of 0 V: RhjWO 3 jSiO 2 jRhjSiO 2 jWO 3 jITOjglass. The rhodium layers act as both a mirror and an ion-permeable layer. (Figure reproduced from Wagner, W., Rauch, F., Ottermann, C. and Bange, K. ‘Hydrogen dynamics in electrochromic multilayer systems investigated by the 15 N technique’. Nuc. Instr. Meth. Phys. Res. Sect. B, 50, 1990, 27–30, copyright (1990) with permission from Elsevier Science.) 90 Kinetics of electrochromic operation 0 <x<0.05; see Figure 5.6. By contrast, Huggins’ results froman independent ac technique show the opposite trend, with - D of Li þ in WO 3 increasing as x increases. The sensitivity of motion parameters to preparative method has already been remarked on: fluctuations in - Dwith x appear highly complicated, possibly too complicated to model at present. 5.3.3 Kinetic modelling of the electrochromic coloration process For the electrochromic coloration reaction of WO 3 given in Eq. (5.2), each of the models belowwill be discussed, identifying M þ as a proton unless specified otherwise. The distinctive features of the models discussed in the following sections are summarised in Table 5.4 overleaf. Model of Faughnan and Crandall: potentiostatic coloration Assumptions Faughnan and Crandall 86,92,93,94,95 provided a semi-empirical model for WO 3 coloration and bleaching, semi-empirical because they used data from measured values of the electrode potential E to provide empirical parameters used in their formulation. The main assumptions at the heart of the model 86,92,93,94,95 are the following. 0.01 0 –8.5 –9.0 –9.5 –10.0 –10.5 0 1 2 Charge inserted/mC cm – 2 3 4 5 0.02 0.03 0.04 0.05 Appro ximate composition / x l o g ( D /c m 2 s –1 ) log D (Li) log D (Na) Figure 5.6 Plot of chemical diffusion coefficientD for Li þ and Na þ through WO 3 as a function of insertion coefficient. (Figure reproduced from Masetti, E., Dini, D. and Decker, F. ‘The electrochromic response of tungsten bronzes M x WO 3 with different ions and insertion rates’. Sol. Energy Mater. Sol. Cells, 39, 301–7, copyright (1995) with permission from Elsevier Science.) 5.3 Transport of counter ions through solid systems 91 (i) The rate-limiting motion is always that of the proton as it enters the WO 3 from the electrolyte, in traversing the electrochrome–electrolyte interphase. The proton motion (intercalation) is rate limiting also because of assumption (ii). (ii) A ‘back potential’ (Faughnan et al., always call this potential a ‘back emf ’) forms across the WO 3 –electrolyte interphase during coloration, the potential increasing as the extent of insertion x increases. From assumption (i), it is argued that, having entered the WO 3 , the proton motion is relatively unhindered, apart from the restraint arising from the back Table 5.4. Summary of the coloration models described on pages 91–104. Principal authors Distinctive features Refs. Faughnan and Crandall * No concentration gradients form within the film. * There is an H þ injection barrier at the electrolyte–WO 3 interphase. * An empirically characterised back- potential acts at that interphase. * The back potential dominates the rate of coloration. 86,93 Green * Concentration gradients of counter cations within the M x WO 3 films were computed by analogy with heat flow through metal slabs. * The diffusing entity is uncharged so there are no effects owing to the electric field. * Hence the kinetic effects of cations and electrons are indistinguishable. * The H x WO 3 adjacent to the inert electrode substrate remains H !0 WO 3 . 100 Ingram, Duffy and Monk * Apercolation threshold sets in at x¼0.03. * When x < 0.03, rate-limiting species are electrons; at x 0.03 counter-ion motions are rate limiting. 96 Bohnke * Electrons and proton counter ions in the film form a neutral species [H þ e À ]. * Reduction of WO 3 is a chemical reaction, effected by atomic hydrogen arising from this neutral. 101,102,103 Various * W IV species participate in addition to W V and W VI . * Reduction of WO 3 may be a two- electron process. 116,117,118,119, 120,121,122, 123, 124,125,126,127, 128,129,130,131,132 92 Kinetics of electrochromic operation potential. Because the central kinetic determinant is the energy barrier to motion of protons into and out of the WO 3 layer via the WO 3 –electrolyte interphase, a further assumption (iii) may be inferred. (iii) The absence of concentration gradients of H þ within the H x WO 3 is implied, hence diffusion never directly controls current. Only the back potential – assumption (ii) – restrains proton motion and hence also the current flow 86 and the rate of increase in proton concentration. (iv) The WO 3 film initially is free of W V and hence of any initial complication from separate counter-cation charge (but this initial-state assumption – essentially a clarification – lacks mechanistic implication, and thus has no further role). The unusual back potential – assumption (ii) – opposes the expected current flow. 86 It is invoked because the chemical potential of the inserted cation is increased (i.e. it is increasingly energetically disfavoured) as the proton concentration within the oxide c increases. The back potential then corresponds to the change in chemical potential of the proton that accompanies coloration. In essence, the developing back potential within the solid smooths out the usual requisite applied potentials (i.e. those sufficiently exceeding the electrode potential E so as to drive the coloration process) that would ordinarily result in a current ‘jump’ or peak associated with (i.e. effecting) oxidation-state change (Chapter 3). The involvement of the back potential is clearly seen in cyclic voltammograms (CVs) of WO 3 , where there is no current peak directly associated with the reductive formation of colour. However, by contrast, CVs do show a peak associated with the (oxidative) electrochemical bleaching: see Figure 5.7. Clearly the back potential will oppose ion insertion during coloration but will aid ion egress (proton removal) during bleaching. 90,96 The kinetics of the model Electro-coloration commences as soon as the potential is stepped froman initial value E in at which reduction just starts to a second potential V a . Since an equilibrium electrode potential E associated with the W VI /W V couple is set up following any reduction, the applied potential V a is, in fact, an overpotential, so V a is cited with respect to E (that is, V a ¼applied potential minus E, where E changes with increase of W V ). Note that we now retain the symbolism of the original authors, 86,92,93,94,95 especially regarding V a , which here, rather than the j of Chapter 3, denotes overpotential and not simply the applied voltage. Apart from this difference in meaning, the main c Whether the increase in the protonic chemical potential with increase of its concentration is sufficient to produce an effective back potential could find independent support from a sufficiently detailed lattice- energy calculation, as has proved invaluable for comparable situations in other electrochromes: see ref. 41 of Chapter 8. 5.3 Transport of counter ions through solid systems 93 further change from the j of Chapter 3 is that now the overpotential V a has simply a value without sign. The chemical potential of H þ was obtained from a statistical entropy-ofmixing term, together with empirical constants, as j H þ ¼ A þ2Bx þnRTln x 1 Àx _ _ . (5.7) where n=1 and the Aand Bterms were derived froma plot of the observed emf E values versus x. 0.05 mA –0.4 –0.2 0.0 Potential/Vvs . Ag 0.4 V Figure 5.7 Typical cyclic voltammogramof an amorphous thin filmof tungsten trioxide evaporated on ITO and immersed in PC–LiClO 4 (1 mol dm À3 ) at 500 mV s À1 (solid line) and 50 mV s À1 (dotted line). (Figure reproduced from Kim, J. J., Tryk, D. A., Amemiya, T., Hashimoto, K. and Fujishima, F. ‘Color impedance and electrochemical impedance studies of WO 3 thin films: H þ and Li þ transport’. J. Electroanal. Chem., 435, 31–8, copyright (1997) with permission from Elsevier Science.) 94 Kinetics of electrochromic operation Taking into account the back potential induced within the WO 3 , the magnitude of the current is governed by two energy barriers, each showing an exponential dependence on the applied potential. The first is influenced by the insertion coefficient x within the H x WO 3 , while the other is influenced by the barrier to ionic charge-transfer current flow across the WO 3 –electrolyte interphase, owing to proton desolvation and the accompanying difficulty of intercalating into the lattice. The basis to the development of the theory is to treat the proton uptake at the interphase as a conventional ion-uptake electrode process following the Tafel law (Eq. (3.16), Chapter 3). The kinetics that ensue then follow the Butler–Volmer development, where the effect of the driving potential V a overcomes the intrinsic energy barrier by an extent cV a where c is variously viewed as the symmetry or transmission coefficient and so represents the effectiveness of V a . The c values found for various systems usually fall between 0.4 and 0.6, and ½ is often summarily assigned to it faute de mieux, as here. From this simplified Butler–Volmer viewpoint the observed current is hence expected to be proportional to exp(V a e/2RT); the positive sign in the exponential arises because V a opposes activation energies. As colour forms with increasing x, so the current during coloration, i c , decreases, from the back-potential influence. This current is a function of time t, decreasing because the back potential increases with time: 86,93 i c ¼ i o 1 Àx x _ _ exp À x x 1 _ _ exp V a e 2RT _ _ . (5.8) Faughnan et al.’s x 1 ¼0.1 appears to be the extent of intercalation at which both assumptions (i) and (ii) are taken to be fulfilled. The term e is the electronic charge and i o is the exchange current, itself a function of the coloration current and the extent of coloration, that needs to be established from the primed system at onset of operating: i o ¼ i e 0.53 e x o RT x o 1 Àx o _ _ . (5.9) Here x o is the mole fraction of protons within the film prior to the application of the voltage V a and i e is the current immediately on applying V a . The numeral is an empirical value 0.53 V from a plot of emf E against x so relating to the back-potential effect invoked earlier. Faughnan and Crandall introduce a ‘characteristic time’ (t D ) for diffusion into the film, from an approximate solution of Fick’s second law, akin to 5.3 Transport of counter ions through solid systems 95 Eq. (3.12), depending on the film thickness d and the proton diffusion coefficient D: t D ¼ d 2 4 D . (5.10) i.e. time needed for the proton to penetrate to a representative point mid-film. (Note that the diffusion coefficient here was chosen to be D rather than the chemical diffusion coefficient - D.) This value is employed in arriving at a time- dependence for the effect of the back potential on current. Combining these considerations 91 led to the equation: i c ¼ i o t o t _ _1 , 2 exp V a e 4 RT _ _ . (5.11) where t o is a constant equal to (, d e/ 2i o ) in which , is the density of W sites within the film; incorporation of [1/t exp(V a e/2RT)] ½ (unsquaring d 2 ) underlies the form of the exponential factor in the equation. The coloration current predicted in Eq. (5.11) thus depends strongly on the applied voltage (overpotential) V a . Furthermore, if diffusion through the film were alone responsible for the observed i–t behaviour, any potential dependence (above the redox-effecting value of V a ) would be absent. Equation (5.11) has been verified experimentally for films of WO 3 on Pt immersed in liquid electrolytes and with either a proton 91 or a lithium ion 54 as the mobile counter ion. The equation has also been shown to apply to WO 3 on ITO in contact with solid electrolytes, the mobile cation being lithium 97,98 or the proton. 91 Equation (5.11) is obeyed only for limited ranges of x if the counter ion is the proton. The kinetic treatment by Luo et al. 99 is somewhat similar to that above. Their principal divergence from Faughnan and Crandall is to suggest the magnitude of the bleaching voltage is unimportant below a certain critical value. Model of Green: galvanostatic coloration Assumptions An altogether different treatment is that of Green and co-workers. 100 In his model, coloration is effected galvanostatically, with the charge passed at low electric fields. The a priori conditions are that dQ/dt ¼i ¼constant, therefore, from Faraday’s laws, dx/dt ¼constant, where x is the average insertion coefficient throughout the entire film of H x WO 3 . Green assumes the following. (i) All activity coefficients are the same. (ii) The diffusing entity is uncharged so there are no effects owing to the electric field, i.e. migration is wholly absent, itself implying assumption (iii). 96 Kinetics of electrochromic operation (iii) All diffusion coefficients are D rather than - D. (iv) The WO 3 contains no mobile protons prior to the application of current. (v) The film may or may not contain interstitial water. Assumption (i) contradicts the derivation of the Weppner and Huggins’ relations in Eqs. (5.3)–(5.6). Assumption (ii) can be classed as consistent with the model of Bohnke et al., 101,102,103 as described below. The application of assumption (iv) is unlikely to affect significantly the utility of Green’s model. The kinetic features of the model The filmof WO 3 has a thickness d. Aconstant ionic flux of J o from the electrolyte layer reaches the solid WO 3 and thence penetrates to a distance y, where 0 < y <d. The distance y ¼d denotes the WO 3 –electrolyte interphase. There is no ionic flux at the back-electrode (at y ¼0). By analogy with the conduction of heat through a solid slab positioned between two parallel planes, 104 Green obtained Eq. (5.12): cðy. tÞ Àc o ¼ J o t d þ J o d - D 3y 2 Àd 2 6d 2 À 2 p 2 1 n¼1 ðÀ1Þ n n 2 exp Àp 2 n 2 - Dt d 2 _ _ cos npy d _ _ _ _ . (5.12) or, in abbreviated form: cðy. tÞ Àc o ¼ J o t d þ J o d - D Fðy. tÞ. (5.13) where c(y,t) is the concentration of H þ (possibly partially solvated) at a distance of y into the WO 3 film at the time t. Green omits specifying migration effects but does cite diffusion coefficients as - D. All the diffusion coefficients pertain to solid phase(s). In Green’s notation, quantities c are number densities, and currents i represent numbers of ionic or electronic charges passing per unit time rather than, say, Ampe` res per unit area. If - D is large, then c(y,t) is independent of y and so c(y,t) increases linearly with J o t/d, causing the concentration of H þ throughout the film to be even, in agreement with assumption (iv) in the model of Faughnan and Crandall above. The second termon the right-hand side of Eq. (5.13) acts as a correction term to account for diffusion-limited processes in the solid. Green has plotted curves of F (y,t) against y/d for various values of - Dt/d 2 ; see Figure 5.8. These show, at short times, that only the WO 3 adjacent to the electrolyte will contain any protonic charge, but the proton concentration gradient flattens out at longer times. 5.3 Transport of counter ions through solid systems 97 In a later development, Green 24 added into his model the effects on the concentration gradients of incorporating grain boundaries into his model. For simplicity the grains of c-WO 3 are assumed to be spherical. When such boundaries are considered, and assumed to be regions within the film acting as pathways for ‘fast-track’ diffusion, the second term on the right-hand side of Eq. (5.13) is simplified to (J o r 2 )/(15 d - D), where the sphere radius is r. Green 24 concluded that for a response time of t, the relationship - Dt c m n _ _ 2 ! 1 (5.14) should be followed, where c m is the maximum concentration of H þ that arises (the number of H þ equalling that of W V ), and n is the number of optically absorbing colour centres per unit area required to produce the required absorbance, equal to the number of H þ per cm 2 . The parenthesised term thus roughly represents the inverse of the average distance separating colour centres. None of the concentration gradients predicted by Green’s model have been measured. The kinetic treatment of Seman and Wolden, 105 closely similar to that of Green, departs from Green’s model in incorporating the back potential of Faughnan and Crandall. 0.3 F ( y , t ) y /d 0.2 0.1 0.5 1.0 0.005 0.06 0.03 0.01 0.1 0.15 0.3 00 0 –0.1 Figure 5.8 Green’s model of coloration: values of F(y, t) for a film of thickness d with no mass flow at y ¼0 and constant flux J o at y ¼d. The numbers on the curves are the values of Dt/d 2 . (Figure reproduced from Green, M., Smith, W. C. and Weiner, J. A. ‘A thin film electrochromic display based on the tungsten bronzes’. Thin Solid Films, 38, 89–100, copyright (1976) permission from Elsevier Science.) 98 Kinetics of electrochromic operation The model of Ingram, Duffy and Monk: 96 an electronic percolation threshold A percolation threshold is attained when previous directed electronic motions, proceeding by individual ‘hops’ from a small number of sites, during a steady increase in the number of occupied sites to a critical value, suddenly become profuse, because of the onset of multiple pathways through the increased number of occupied sites. In ordinary site-wise conductive systems this occurs when occupied sites become $15% of the maximum. 106 Assumptions (i) The central assumption underlying the model of Ingram et al. 96 is that the motion of the electron is rate limiting below a percolation threshold, at x (critical) , but electron movement is rapid when xx (critical) . Such a transition is documented 107,108 for WO 3 . (ii) Most of the assumptions and hence the theoretical elaboration of Faughnan and Crandall’s model (see p. 91 ff.) are obeyed when xx (critical) . The model It is already clear from Figure 5.4 and the discussions above that the electronic conductivity s of pure WO 3 is negligibly low. The conductivity s increases as x increases until, at ca. x %0.3, the conductivity becomes metallic. The onset of metallicity is an example of a semiconductor-to-metal transition, an Anderson transition. 107 Then if the mobility j (ion) of ions is approximately constant, but the mobility of the electron j (electron) increases dramatically over the compositional range 0 x< 0.3 then, at a critical composition x (critical) , the ionic and electronic mobilities will be equal: j (ion) ¼j (electron) . It follows then that j (ion) <j (electron) when x x (critical) . Hence, at low x, the motion of the electron is rate limiting; and only above x (critical) will electron movement be the more rapid. It is shown in ref. 96 that Faughnan and Crandall’s model (page 91 ff.) is obeyed extremely well when x x (critical) but not at low values of x, below x (critical) . Ingram et al. 96 analysed the potentiostatic coloration of evaporated a-WO 3 on ITO, which involved obtaining transients of current i against time t during electroreduction. Such plots showed a peculiar current ‘peak’, see Figure 5.9, which was rationalised in terms of attaining a percolation threshold, with the electron velocity rising dramatically at x %0.03. d d The low value 0.03–0.04 claimed for the electron-conduction percolation limit may be understood as arising from a restricted electron delocalisation about a few neighbouring W VI , that in effect extends the size of the ‘sites’ involved in allowing the onset of the critical percolation, which would hence lower the numbers of sites needed for criticality. The onset of metallicity at x $0.3 results fromthe wave-mechanical overlap of conduction sites or bands with the valence bands, and the approximate correspondence here with the customary percolation value $0.15 is then probably fortuitous. 5.3 Transport of counter ions through solid systems 99 Similar chronoamperometric plots of i against t which include a current peak have also been found by Armand and co-workers 97 and by Craig and Grant. 109 The value of x at the peak is also ca. 0.03 in ref. 97, as can be gauged by manual integration of the peak in the traces published. The percolation phenomenon was not seen by Ingram et al. when electro-colouring WO 3 with a small field, by applying a very small cathodic driving potential, perhaps because the transition was too slow to be noted. Armand and co-workers 97 explain the peak in terms of the nucleation of hydrogen gas (via the electroreduction of H þ ), possibly with the surface of the incipient H x WO 3 acting as a catalyst: 2H þ þ2 e À !H 2 . (5.15) While such nucleation phenomena can certainly cause strange current peaks in chronoamperometric traces, Armand’s explanation may not be correct here since Craig and Grant, 109 who found a similar current peak, had inserted lithium ion into WO 3 from a super-dry PC-based electrolyte, i.e. an electrolyte free of mobile H þ : in this case Li 0 would have to be the corresponding reactant. 440 420 400 380 360 340 320 0.0 0.2 0.4 0.6 Time/s C u r r e n t / μ A 0.8 1.0 1.2 Figure 5.9 Chronoamperometric trace of current vs. time during the electro- coloration (reduction) of the cell ITOjWO 3 jPEO–H 3 PO 4 j(H)ITO. The potential was stepped from a rest potential of about 0.0 V to –0.6 V at t ¼0. Note the current peak at ca. 0.2 s. (Figure reproduced from Ingram, M. D., Duffy, J. A. and Monk, P. M. S. ‘Chronoamperometric response of the cell ITOjH x WO 3 jPEO–H 3 PO 4 (MeCN)jITO’. J. Electroanal. Chem., 380, 1995, 77–82, with permission from Elsevier Science.) 100 Kinetics of electrochromic operation Also, in a different system, a current peak has been observed by Aoki and Tezuka 110 during the anodic electro-doping of poly(pyrrole), that was successfully modelled in terms of a percolation threshold. There was no mention of such a threshold in the study by Torresi and co-workers, 111 but their model of ‘relaxation processes’ in thin-film poly(aniline) does, again, suggest a sudden change in electronic conductivity with composition change. In summary, Ingram, Duffy and Monk suggest that the kinetic behaviour of WO 3 in the insertion-coefficient range 0 x<0.03–0.04 is dictated by slow electron motion; only after a percolation threshold at the upper coefficient limit here does ion motion become rate limiting. In contrast with the assumptions implicit in deriving Eq. (5.5), t (electron) does not tend toward 1, so values of - D alter dramatically as the percolation threshold is reached. In studies claiming free electronic motion, by Goldner et al., 112 and Rauh and co-workers, 113 both groups employ Drude-type models (see p. 142) to describe the free-electron behaviour, but following Ingram et al., electrons are ‘free’ only above the percolation threshold. Model of Bohnke: reduction of W V via neutral inserted species Assumptions The requirement of a new interpretation of the WO 3 coloration process was indicated by the need to explain the temporal relationships governing the optical data obtained during electrochromic coloration. Accordingly, the bases of most of the theories in the electrochemical models above are still regarded as valid (see discussion, below). The major divergence from the models above is the following. (i) The rate-limiting process during electrochromism is the diffusion of an electron– ion pair (such as [H þ e À ]), which may be atomic (as H * ). Because the [H þ e À ] pair has no overall charge, the diffusion coefficient evinced by the system is D rather than - D. The meeting of H þ and e À is outlined below. (ii) The rate of electrochromic colour formation is thus a chemical rather than an electrochemical reaction: W VI þ½H þ e À 0 !W V þH þ . (5.16) The proton product of Eq. (5.16) resides as a counter ion adjacent to the site of the chemical reduction reaction, i.e. to the W V . (iii) The chemical reduction in Eq. (5.16) occurs ‘spontaneously’ on the time scale for diffusion of the [H þ e À ] pair. From Eq (3.16), d $ (D t) ½ , inserting a reasonable assumed D of ca. 10 À12 cm 2 s À1 indicates that the mobile species would traverse a 5.3 Transport of counter ions through solid systems 101 typical filmin many seconds, lending reality to these suppositions, providing k et is high enough (see point (iv), following). (iv) The observed current is thus a function of the rate of forming [H þ e À ] pairs, but does not represent the formation rate of colour centres. The rate of forming colour is thus either a function of the rate of diffusion of the [H þ e À ] pair to available W VI sites prior to ‘instantaneous’ electron transfer, Eq. (5.16), or, if the appropriate rate constant k et is quite low, it is a function of the rate of the electron-transfer reaction itself, W VI þe À !W V . (The electrochromic colour in this model is still due to intervalence optical transitions between W VI and W V .) The Model In contrast to the models above of Faughnan and Crandall, and of Green in which the motion of H þ is rate limiting, or the model of Ingram et al. in which first the motion of the electron and then the motion of the proton is rate limiting, in Bohnke’s model 101,102,103 the mobile diffusing species is suggested to be an electron–ion pair. Indeed, it is even possible that electron transfer has occurred within the pair, resulting in the formation of atomic hydrogen or lithiumprior to coloration. On entering the WO 3 , the inserted H þ ion moves through the WO 3 , probably moving only a very short distance within the WO 3 before encountering the faster electron from the electrode substrate. The charged species within the encounter pair then diffuse together as a neutral entity, or they react to form atomic hydrogen. Furthermore, the model implies that the kinetics-controlling mobility, in moving through WO 3 , of the [H þ e À ] pair that provides a quasi counter ion to W V , will be simplified since migration effects, born of coulombic attractions, can be wholly neglected and, accordingly, the measured diffusion coefficient is better considered as D than as - D. In common with Faughnan and Crandall, and Ingram et al., Bohnke acknowledges that the observed current–time behaviour is governed by the formation of a back potential, but parts from Faughnan and Crandall in asserting that concentration gradients are formed within the incipient H x WO 3 during coloration. Bohnke’s model is said 101,102,103 to be satisfactory in simulating the observed absorbance–time data except at short times, but is not applied in any detail to data for bleaching. In support of the model, the rate of diffusion through Nb 2 O 5 is similarly said to be dominated by ‘redox pairs’. 114,115 Recent developments: intervalence between W VI and W IV Assumption A new view of the key tungsten species has emerged in the last decade. While broadly agreeing with the model of Faughnan and Crandall (above), Deb and co-workers 116 suggested in 1997 that the coloured form of the electrochrome is not H x W V,VI O 3 but H x W VI (1 – y) W IV y O (3 – y) , and hence 102 Kinetics of electrochromic operation that the optical intervalence transition is W VI W IV rather than the hitherto widely accepted W VI W V . The fully oxidised form of the trioxide (MoO 3 or WO 3 ) is confirmed to contain only the þVI oxidation state by studies with XPS 117,118,119 and ESCA. 120,121 Reduction during the coloration reaction MO 3 þx(H þ þe À ) ! H x MO 3 is expected to yield the þV oxidation state: but XPS shows that some of the þIV state is also formed during the reduction of Mo, 118,122,123,124 and of W. 117,118,119,125 Rutherford backscattering studies furthermore suggest that the amount of W IV in nominal ‘WO 3 ’ is a function of the extent of oxygen deficiency. 126 Infrared 127 and Raman studies 128,129 also indicate the presence of W IV . Indeed, Lee et al. 128 say that even as-deposited films contain appreciable amounts of W IV . Additionally, it is notable that Sun and Holloway 130 (in 1983) and Bohnke and co-workers 131 (in 1991) both suggest that reduction of WO 3 is a two-electron process. Similarly, the electrochromic and photochromic properties of O-deficient WO 3 have also been found to depend on similar W IV participation in both mechanisms. 132 Possibly the observed W V is formed by comproportionation, as in Eq. (5.17): W VI þW IV !2W V . (5.17) Siokou et al. 118 suggest that the W IV state ‘plays a dominant role in deep coloration’. Finally, de Wijs and de Groot deliberately omitted the involvement of W IV in their recent wave-mechanical calculations. 133 Rather, from densityfunctional computations, they argue for W V –W V dimers rather than W IV and W VI . The on-going growth of views on the roles played by the several W species, and their ultimate resolution, promises intriguing physicochemical developments for the near future. Additional experimental results (i) Coloration of non-stoichiometric ‘bronzes’ A non-stoichiometric reduced oxide has a non-integral ratio of oxygen and metal ions, e.g. WO (3 – y) , where y is likely to be small. Such materials are also called ‘sub-stoichiometric’. Zhang and Goto 71 found that Dincreased as the extent of sub-stoichiometry increased, i.e. as y in Li x WO (3–y) increased; WO (3–y) is then in reality, W VI (1–y) W IV y O (3–y) . Other materials of the type WO (3–y) are indeed also electrochromic, but trapping of electrons at shear planes and defect sites can be problematic for rapid, reversible electrochromic coloration. 134 For this reason, non-stoichiometry is best avoided, although note that 135 MoO ð3ÀyÞ 5.3 Transport of counter ions through solid systems 103 apparently electro-colours at a faster rate than does MoO 3 alone, and also has a superior contrast ratio CR. Nevertheless, such materials will not be considered further here because the additional complexities encountered with these systems, comparable to (but different from) those of the tungsten systems, do not yet lead to a clearer or general view of the mechanisms in electrochromic oxides. (ii) Electrochemical titration In a brief study of galvanostatically injected lithium ion in 47 c-WO 3 , the electrode potential E of the lithiated oxide was monitored as a function of x while a continuous (and constant) current was passed. It was found that dE/dx decreased suddenly at x ¼0.04–0.05, close to the values of x (critical) noted above on page 99. In plots of emf against x, obtained during injection of Li þ into, and removal from, c-WO 3 , there is a considerable hysteresis between the E for reductive charge injection and that for oxidative Li þ egress. This is a mobility-controlled kinetic phenomenon: on the time scales involved, there is a higher concentration of lithium on the surface of the particles than in the particle bulk. (iii) Use of an interrupted current (from a ‘pulsed’ potential) The rate of electrochromic coloration of tungsten oxide-based ECDs may be enhanced considerably by applying a progression of potentiostatically controlled current pulses rather than enforcing a continuous current. 136 The rate of coloration depends strongly on the pulse length employed, the optimum pulse duration for a high d(Abs)/dt also depending strongly on the pulse amplitude. However, according to the final paragraph of ‘Kinetic complications: (i) crystal structure’ above, p. 86, steady reduction does effect a greater capacity for Li þ before bulk metallicity intervenes. The effects of interrupting the current, by applying current pulses, is attributed to the formation of a thin layer of high-x bronze on the electrolyte-facing side of the WO 3 . By interspersing the coloration currents with short periods of zero current, the steep concentration gradient associated with a high-x layer is allowed to dissipate into the film. The amount of charge that can be inserted per current pulse is thus greatly increased, as evidenced by increased peak currents. An additional advantage of pulsing is to enhance the durability of electrochromic devices by decreasing the occurrence of undesirable electrolytic side reactions such as the formation of molecular hydrogen gas: it is likely that the catalytic properties of H x WO 3 for H 2 generation are impaired. Several groups 104 Kinetics of electrochromic operation have found that a pulsed potential enhances the rate of coloration and bleaching, and suppresses the extent of side reactions. 136,137,138,139,140,141,142,143 Kinetic modelling of the electrochromic bleaching process The process of film bleaching, Eq. (5.18), represents the reverse of Eq. (5.2) above: H x WO 3 À!xðH þ þe À Þ þWO 3 . (5.18) Bleaching is somewhat simpler than is coloration since the back potential contributes to, rather than acts against, the movement of the mobile counter ions. Table 5.5 above summarises the various bleaching models cited in this section, citing the distinctive features of each. Model of Faughnan and Crandall: potentiostatic bleaching The potentiostatic removal of charge (i.e. bleaching of the electrochromic colour) of the WO 3 bronze has been modelled by Faughnan and Crandall. 35 Assumptions (i) The bleaching time of H x WO 3 is primarily governed by a field-driven space-charge limited current of protons in the H x WO 3 next to the electrolyte. (ii) The resistance to charge transfer at the electrochrome–electrolyte interphase does not limit the magnitude of the bleaching current. Table 5.5. Summary of the bleaching models described on pages 105–109. Principal authors Distinctive features Refs. Faughnan and Crandall * The bleaching current is primarily governed by a field- driven space-charge limited current of protons in the H x WO 3 next to the electrolyte. * The activation energy to proton expulsion is slight. * No concentration gradients form within the film. 35,86 Green * Concentration gradients of counter cations in M x WO 3 films were computed from analogy with heat flow through metal slabs. * The kinetic effects of cations and electrons are indistinguishable. 100 5.3 Transport of counter ions through solid systems 105 (iii) Ionic charge leaves the H x WO 3 film during electro-bleaching, resulting in a layer of proton-depleted WO 3 at the electrolyte-facing side of the electrochrome. All the voltage applied across the electrochrome layer film drops across this narrow layer of WO 3 . The layer has a time-dependent thickness termed l(t). (iv) There is a clear interface between H x WO 3 and WO 3 layers withinthe electrochrome, the position of this interface moving into the oxide film from the electrolyte as the bleaching progresses, with l (t) becoming thicker with time. Since the back potential contributes toward the movement of the mobile charged species, rather than against it, the time-dependent bleaching current i b shows a different response to the applied voltage V a fromthat during coloration, according to Eq. (5.11): i b now depends on the proton mobility j H þ: i b ðtÞ ¼ · j H þV 2 a lðtÞ 3 . (5.19) where · is the proper permittivity, and l (t) is the time-dependent thickness of a narrow layer of the WO 3 film adjacent to the electrolyte. (Faughnan denotes this length x I rather than l (t) as here. Note that · is not the molar absorptivity of Chapter 1.) The thickness l (t) is proportional to time, and is related to the initial proton concentration (number density) within the filmc o , such that 35,86 l (t) 3 ¼J o t/c o e. All the voltage applied to the ECD is assumed to occur across this thin layer, hence the observed i–V a square law. Solution of the differential equations for time-dependent diffusion across l (t) during bleaching leads to an additional relationship: i b ðtÞ ¼ ðp 3 · j H þÞ 1 , 4 V 1 , 2 a ð4 tÞ 3 , 4 . (5.20) where p is the volume charge density of protons in the H !0 WO 3 . The result in Eq. (5.20) assumes that bleaching occurs potentiostatically implying a fixed V a across the whole of the WO 3 layer. The current i b decreases as l (t) grows thicker, incurring a time dependence of i b /t À 3 , 4 . This i–t relationship has been verified often for WO 3 in contact with liquid electrolytes 35,54,98 and for WO 3 in contact with semi-solid polymeric electrolytes. 96,98 Figure 5.10 shows the logarithmic current–time response of H x WO 3 bleached in LiClO 4 –PC, clearly showing the expected gradient of – 3 , 4 at intermediate times. A superior fit between experiment and theory is seen if the electrolyte is aqueous, as in Fig. 5.10 (a); Fig. 5.10 (b) is the analogous plot but for propylene carbonate as solvent. 106 Kinetics of electrochromic operation –1 VH x WO 3 : 10 N H 2 SO 4 : In H x WO 3 : 10 N H 2 SO 4 : WO 3 –0.8 V –0.6 V –0.4 V 1 2 3 4 1 2 3 4 10 –1 10 – 1 10 0 10 1 10 1 10 0 10 2 Time/s C u r r e n t de n s i t y i /m A c m –2 Slope(–3/4) (a) (b) Time/s C u r r e n t de n s i t y i /m A c m –2 Slope(–3/4) 10 1 10 1 10 – 2 10 2 10 – 1 10 – 1 10 0 10 0 –1.6 V Li x WO 3 : LiCIO 4 : WO 3 Li x WO 3 : LiCIO 4 : In –1.8 V –1.8 V (PC) (PC) – Figure 5.10 Current–time characteristics of H x WO 3 during electrochemical bleaching as a function of potential: (a) H x WO 3 in H 2 SO 4 (15 mol dm À3 ) and (b) H x WO 3 in PC–LiClO 4 (1 mol dm À3 ). The gradient of – 3 , 4 predicted from Faughnan and Crandall’s theory is indicated. (Figure reproduced from Mohapatra, S. K. ‘Electrochromism in Li x WO 3 ’. J. Electrochem. Soc., 125, 1978, 284–8, by permission of The Electrochemical Society, Inc.) 5.3 Transport of counter ions through solid systems 107 The time for complete bleaching to occur t b (i.e. the time required for l (t) to become the film thickness) is a function of film thickness d, proton mobility j, permittivity · of the film and the insertion coefficient: t b ¼ e , d 4 x 4jV 2 a · . (5.21) where x is the insertion coefficient at the commencement of bleaching and , is the corresponding density of W atoms. Equation (5.21) fulfils expectation in indicating the longer time needed for a film to bleach if the sample is thick or is strongly coloured prior to bleaching. Model of Green: potentiostatic bleaching The potentiostatic bleaching of thin film WO 3 has also been modelled by Green. 100 In common with his model for coloration, the film thickness is d and the distance of a proton fromthe back inert-metal electrode is y. The time- dependent proton concentration is c(y,t), and the initial concentration of H þ c o , both actually number densities. Assumptions (i) All H þ ions reaching the electrochrome–electrolyte interphase are instantly removed, implying assumption (ii) below. (ii) The activation energy for charge electron transfer across the interphase (ions and electrons) is slight. The best means of ensuring assumption (i) is to potentiostatically control the rates of charge movement, i.e. ensuring that assumption (ii) holds by applying a sufficiently large positive potential. (iii) Accordingly, c(y ¼d,t) ¼0 for all time t 0. The time- and thickness-dependent concentration of H þ [c(y,t)] is then obtained as: cðy.tÞ ¼ 4c o p 1 n¼0 1 2n þ1 exp ÀDð2n þ1Þ 2 p 2 t d 2 _ _ sin ð2n þ1Þpy d _ _ . (5.22) Green 100 has again computed theoretical curves, in this instance of c(t)/c o against y/d, where c(t) is the concentration of H þ in the film at time t at a distance of 0 <y <d. Such curves drawn for various Dt/d 2 are reproduced in Figure 5.11. As was the case during coloration, the condition for a rapidly responding ECD is - Dt(c m /n) 2 !1; cf. Eq. (5.14). The computed concentration gradients await experimental verification. 108 Kinetics of electrochromic operation Additional experimental evidence for concentration gradients Ellipsometry While the exact form of Green’s computed concentration gradients need confirmation, other data suggest steep concentration gradients are likely. For example, in situ electrochemical ellipsometry – a non-destructive technique – has demonstrated a clear interface between the oxidised (colourless) and reduced (coloured) regions of thin films of vanadium oxide 51 or molybdenum oxide, 52,144 cf. assumption (iii) on page 108 above, but ellipsometry has so far failed to detect such an interface within films of WO 3 during reduction, 8 so weakening Faughnan and Crandall’s assumption (iv). Within thin-film V 2 O 5 , this boundary separates the reduced (hydrogen-free) and oxidised (proton-containing) forms of the oxide. The interface was detected both during reduction and oxidation reactions of V 2 O 5 . By contrast, the ellipsometric study by Duffy and co-workers 145 did find evidence that implied a surface layer of bronze does form during reduction, although note that in this latter study dry WO 3 was employed, then reduced chemically by gaseous H 2 þN 2 . The surface of the bronze had a sufficiently high insertion coefficient to be metallic (implying that x ! 0.3). Ingram and co-workers 96,136 also found evidence for surface layers of H x WO 3 at very short, sub-millisecond, times; these latter studies involved electrochemical reduction. Results from inserting the relatively large Na þ ion into WO 3 also 0 0.2 0.4 0.4 0.1 0.04 0.01 1.0 1.0 0.8 0.6 0.4 0.2 0.6 0.8 1.0 y/d C / C o Figure 5.11 Green’s model of bleaching: concentration c in the film0 <y <d; c o is the initial concentration; at t 0, c(d) ¼0. The numbers on the curves are values of Dt/d 2 . Note that c(y ¼d, t) is 0 for all t 0. (Figure reproduced from Green, M., Smith, W. C. and Weiner, J. A. ‘Athin filmelectrochromic display based on the tungsten bronzes.’ Thin Solid Films, 38, 1976, 89–100, with permission from Elsevier Science.) 5.3 Transport of counter ions through solid systems 109 suggest the formation of a high-x layer of Na x WO 3 on the electrolyte-facing side of the WO 3 during reduction; 59,60 the slow motion of the entering Na þ cation could accentuate incipient concentration gradients. Nuclear reaction analysis While in some of the simpler models a constant concentration of inserted cation is assumed throughout the electrochromic film, several investigations afford compelling evidence of steep concentration gradients forming during electro-coloration and bleaching. For example, Bange and co-workers 82,83 measured proton densities with the 15 N technique (the ‘nuclear reaction analysis’, NRA): 15 Nþ 1 H ! 14 C þ 4 He þg. (5.23) in which, prior to analysis, a sample of WO 3 is electro-coloured normally with proton counter ion, and then bombarded with ‘hot’ 15 N atoms. The depth to which the 15 N atoms are inserted is varied by controlling their kinetic energy during bombardment. The emitted gamma rays are monitored as a function of energy, thus as a function of depth: the g-ray count is taken to be directly proportional to the proton concentration. It has been thereby shown that a concentration gradient forms in a film of electrochromic oxide during coloration. SIMS Secondary-ion mass spectroscopy (SIMS) was the technique of choice to study cation concentrations as a function of film thickness, exemplified by the work by Porqueras et al., 146 Zhong et al., 147 Deroo and co-workers 22 and Wittwer et al. 107 In each case, the surface of the film was slowly etched away, and the ablated material analysed. The last study 107 showed %50% change in cation across the WO 3 film. Again, a concentration gradient was clearly shown to form during electrochromic coloration. However, both the NRA and SIMS techniques destroy the sample during measurement, allowing possible movement of mobile ions during measurement, so the results are not without qualification. Discussion – coloration and bleaching The back potential The theory of Faughnan and Crandall onp. 91 ff. is the most widely used in describing the coloration kinetics of thin-film electrochromic tungstentrioxide. It is nowalmost universally agreedthat a back potential forms during coloration. Also, the relationships (from Eq. (5.11)) of i c / t À½ and i c / exp(V a ) have often been verified experimentally during electro-coloration; 110 Kinetics of electrochromic operation and the relationship i b / t À 3 , 4 , Eq. (5.20), has also been verified during the electro-bleaching of H x WO 3 , albeit over limited time scales in each case. Concentration gradients The second area of consensus concerns concentration gradients: these are inferred from the ellipsometry, NRA, and SIMS analyses outlined above, that concentration gradients of H þ form within the H x WO 3 both during coloration (with a higher x at the electrolyte-facing side of the electrochrome) and during bleaching (with higher x at the inert-electrodefacing side of the electrochrome). The existence of a concentration gradient in the electrochrome cannot be established directly, and can only be inferred. If they exist, they additionally contradict one of the few explicit assumptions of the model of Faughnan and Crandall, since in their theory the protonic charge is assumed effectively to be evenly distributed within the film at all times t t D (where t D is the ‘characteristic time’ describing the temporal requirements for diffusion within the film, as defined in Eq. (5.10) above) thus implying t ! milliseconds. However, even in Faughnan’s treatment, diffusion within the film arises at t <t D , hence implying that concentration gradients enforcing Fick’s laws do form within the film. With the wide acceptance of the i–t–V a characteristics predicted by Faughnan and Crandall’s model (except where x <0.03 in WO 3 reduction), Faughnan’s central kinetic assumption of the interphase energy barrier that dictates the proton-insertion rate does appear tenable. The finding that concentration gradients are formed within the incipient H x WO 3 does not contradict the model, but merely indicates that any contribution to an observed activation energy is small: the activation energy for diffusion are often not excessive (Table 5.6), thus any concentration gradients do not dominate Table 5.6. Activation energies for diffusion of mobile ions through a solid metal-oxide host. Host Mobile ion E a /kJ mol À1 Ref. WO 3 Li þ 20–40 91 a-WO 3 Li þ 50 148 WO 3 Li þ 64 149 WO 3 Li þ 20 150 NiOH H þ 7.0 30 1 The value of E a depends sensitively on x. 2 Converted from the original eV. 5.3 Transport of counter ions through solid systems 111 the observed kinetic laws. They might, however, influence the numerical magnitudes of the rates determined experimentally. The activation energy E a for ionic movement has been modelled by Anderson and Stuart; 151 M z þ (of charge z þ and radius r þ ) is transferred over a distance d from an oxygen ligand (of radius r À ), to a vacancy near a similar oxygen (each bearing a charge z À ). The activation energy E a is then given in Eq. (5.24) as: 152,153 E a ¼ Bz þ z À e 2 ·ðr þ þr À Þ À 2z þ z À e 2 1 2 d · þ Gp lðr þ Àr d Þ 2 2 . (5.24) where z þ and z À are the respective charge numbers on the cation and the non- bridging oxygen, and r þ and r À are the corresponding radii. The symbol · here is the relative permittivity of the material, and B/· is a form of effective Madelung constant, this term representing the loss of lattice stabilisation at onset of the ionic ‘jump’ from its initially stable lattice site. The second term is the coulombic stabilisation acquired by interaction with two oxygens at the midpoint of the jump, i.e. to ½d, mid-way between these oxygens, the numerator ‘2’ denoting interaction with both. The final term covers mechanical stress: G is the shear modulus, l is the jump length and r d is half the distance between bridging-oxygen surfaces that form the ‘doorway’ needing enlargement to r þ to enable M zþ to pass. The values of E a calculated from Eq. (5.24) are ‘about satisfactory’ for Li þ and Na þ . 154 Energetics of diffusion Since concentration gradients in ECDs can only be inferred, they are too limited for precise measurement. In such attempts, on etching away the surface of a solid by SIMS, the energy required to remove the surface is sufficient to perturb the H þ or Li þ ions, not only volatilising many of the ions but also driving others into the remaining WO 3 during measurement. Diffusion of neutral species The more recent novel model of Bohnke et al., 101,102,103 encompassing ion–electron pairs, could have serious implications for many solid-state ionic devices in addition to those involving electrochromism: the good fit between her data and the model does invite attention. In contrast, in the thermodynamic-enhancement model of Huggins and Weppner, 14 differing rates are presupposed of ionic and electronic motion in the film, which appears impossible except prior to the meeting of an ion and electron, thereby forming a neutral pair. If Bohnke’s model holds, all values of diffusion coefficient observed will be those of D rather than - D. 112 Kinetics of electrochromic operation While none of the other authors’ models comprise the concept of ion–electron pairs, Green 100 notably states that the kinetic behaviour of electrons and ions can be separated only under the influence of a high electric field, implying that the kinetics both of counter ions and electrons moving separately and in pairs could be identical for electrochromic coloration effected by applying a small electric potential. It may be that ion–electron pairs are present but not noted in other studies. Complementarily, perhaps the need to invoke their existence can be dismissed in studies employing higher electric fields. However, several results contradict the Bohnke model: Hall-effect measurements on preprepared Li x WO 3 and Na x WO 3 showdiffusion coefficients that are roughly proportional to the number of alkali-metal cations inserted and are independent of temperature, 155,156 thus demonstrating the complete dissociation of electrons and cations. (Conductivity s as a function of x has also been measured by Bohnke et al. by microwave results coupled with electrochemical measurements, on an electrochemical cell containing lithium electrolyte. 157 The conductivity of a-WO 3 increased during insertion and decreased during extraction of Li þ ions.) Similarly, the Seebeck coefficient S is proportional to x À 2 , 3 (x being the insertion coefficient), which is consistent 158,159 ‘with a free electron’ moving through preprepared reduced oxide; and magnetic susceptibility data appear to show the same results. 160 Insertion coefficients 0 x 0.03 As mentioned on p. 99 ff. above, Faughnan and Crandall’s i–t–V a characteristics are poorly followed when x <0.03. The near ubiquity of this value of insertion coefficient, in demarcating discontinuities in the physicochemical behaviour of H x WO 3 , has been ascribed by Ingram et al. 96 to reaching thence surpassing a percolation threshold. The invocation of a percolation threshold does answer a number of questions, such as the cause of the peculiar current peak in potential-step traces, and possibly the deviating at very low x of the Bohnke model. The relationship between the extent of localisation and x may also be discerned from the electronic conductivity s, which only becomes significant at x ¼0.05 or so (Figure 5.4). Furthermore, the relationship between the extent of localisation and x may also be discerned optically since the molar absorptivity (extinction coefficient) · is not constant but decreases as x increases, 161 summarised in Figure 5.12. Following the reduction of the WO 3 , four separate absorbance–insertion coefficient domains may be discerned: 0 < x <0.04 (of extinction coefficient · 1 ¼5600 dm 3 mol À1 cm À1 ); 0.04 <x <0.28 (of · 2 ¼2800 dm 3 mol À1 cm À1 ); 0.28 <x <0.44 (of · 3 ¼1400 dm 3 mol À1 cm À1 ), and x 0.4 (of · 4 ¼0 dm 3 mol À1 cm À1 ). The value of · 4 probably means the current did not effect reduction of further tungsten sites. All data were obtained at 5.3 Transport of counter ions through solid systems 113 constant wavelength, `. Similarly, Mohapatra 54 shows a plot of absorbance vs. inserted charge, where some traces are linear only in the range 0 <x<0.033, and Scrosati and co-workers 48 found · of Li x WO 3 and Na x WO 3 differed significantly over the insertion coefficient range 0 <x1. Values of x characterised by · 1 were postulated 96 to represent single W V species belowthe percolation threshold and, similarly, values of x for · 4 represent metallic H x WO 3 in which charge ‘inserted’ is conducted without any valence trapping (i.e. without reduction). The values of x represented by · 2 and · 3 are, in all probability, representations of different extents of electron delocalisation. Notably, in the studies by Monk et al. 136 and by Siddle and co-workers, 162,163 the optical absorbance of the incipiently reduced oxide was observed to increase for a short time after the driving potential was removed. Since the extinction coefficient · for H x WO 3 is a function of insertion coefficient x, these observations can be taken as direct evidence for flattening-out of a concentration gradient in the absence of an applied field. Toward a consensus model The evidence for each model seems quite convincing if taken in isolation and, as discussed above, some elements of the theories Inser tedchar ge/mC O p t i c a l a bs o r ba n c e 0 1.0 2.0 3.0 500 1000 1500 (a) (b) (c) Figure 5.12 Increase in the absorbance of the intervalence charge-transfer band of H x WO 3 as a function of charge passed: (a) at the wavenumber maximum of 9000cm À1 (the points O were calculated from the curves in (b) and (c); (b) absorbances at 20000cm À1 and (c) absorbances at 16 000cm À1 . (Figure reproduced fromBaucke, F. G. K., Duffy, J. A. and Smith, R. I. ‘Optical absorption of tungsten bronze thin film for electrochromic applications’. Thin Solid Films, 186, 1990, 47–51, with permission from Elsevier Science.) 114 Kinetics of electrochromic operation seem to fit all models: the positing of a back potential is a case in point. Only Faughnan and Crandall 92 dismiss the idea of concentration gradients within the incipient H x WO 3 . A combined model describing the electro-coloration of thin-film tungsten trioxide would suggest that the kinetics are dominated by the formation of a back potential. Initially, when insertion coefficients are small, in the range 0 <x<0.03–0.05, the motion of electrons is rate limiting, but as the upper value of this x limit is passed, so the mobility of electrons increases and ionic motion becomes rate limiting. The percolation threshold x of 0.03 is sufficiently small that many workers may have missed anomalous properties at small x. Also, the mobility j (electron) is usually said to be higher in hydrated WO 3 – and perhaps also for reduced oxides immersed in electrolyte solutions – thus further masking the effects of low x. The percolation phenomenon was not seen by Ingram et al. when electro-colouring WO 3 with a very small field, where no current peak was observed. Green’s observation, that the behaviour of electrons and ions cannot be separated except at high fields, may be sufficient explanation. Bohnke’s 101,102,103 assumption that [H þ e À ] pairs form during coloration has received no support from subsequent workers; but many of these studies may have been incapable of discerning such pairs. 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Most of the electrochromic colours derive from intervalence charge-transfer optical transitions, as described in Section 4.4. The intervalence coloured forms of most transition-metal oxide electrochromes are in the range blue or grey through to black; it is much less common for transition-metal oxides to form other colours by intervalence transitions (see Table 6.1). The oxides of tungsten, molybdenum, iridium and nickel show the most intense electrochromic colour changes. Other metal oxides of lesser colourability are therefore more useful as optically passive, or nearly passive, counter electrodes; see Section 1.4 on ‘secondary electrochromism’. At least one redox state of each of the oxides IrO 2 , MoO 3 , Nb 2 O 5 , TiO 2 , NiO, RhO 2 and WO 3 can be prepared as an essentially colourless thin film, so allowing the electrochromic transition colourless (clear) ! coloured. This property finds application in on–off or light-intensity modulation roles. Other oxides in Section 6.2 demonstrate electrochromism differently by showing two colours, i.e. switching as colour 1 ! colour 2, one of these colours often being much more intense than the other. Display-device applications can be envisaged for the latter group of electrochromes. Granqvist 2 describes how the solid-state crystals of all of the well-known electrochromic metal oxides Ce, Co, Cr, Cu, Ir, Ni, Mo, Nb, Ni, Mo, Ta, Ti, V, 125 Table 6.1. Summary of the colours of metal-oxide electrochromes. Metal Oxidised form a of oxide Reduced form a of oxide Balanced redox reaction for electrochromic operation Bismuth Bi 2 O 3 Transparent Li x Bi 2 O 3 Dark brown (6.16) Cerium CeO 2 Colourless M x CeO 2 Colourless (6.17) Cobalt CoO Pale yellow Co 3 O 4 Dark brown (6.19) LiCoO 2 Pale yellow–brown M x LiCoO 2 (M6¼ Li) Dark brown (6.20) Copper CuO Black Cu 2 O Red–brown (6.22) Iridium Ir(OH) 3 Colourless IrO 2 Á H 2 O Blue–grey (6.11) or (6.12) Iron FeOÁ OH Yellow–green Fe(OH) 2 Transparent (6.24) Fe 2 O 3 Brown Fe 3 O 4 Black (6.25) Fe 3 O 4 Black FeO Colourless (6.26) Fe 2 O 3 Brown M x Fe 2 O 3 Black (6.27) FeO Colourless Fe 2 O 3 Brown (6.28) Manganese MnO 2 Dark brown Mn 2 O 3 Pale yellow (6.29) MnO 2 Brown MnO (2–x) (OH) x Yellow (6.30) MnO 2 Brown M x MnO 2 Yellow (6.31) Molybdenum MoO 3 Colourless M x MoO 3 Intense blue (6.9) Nickel Ni II O (1 Ày) H z Brown–black Ni II (1Àx) Ni III x O (1Ày) H (zÀx) Colourless (6.13) Niobium Nb 2 O 5 Colourless M x Nb 2 O 5 Blue (6.33) Praseodymium PrO (2–y) Dark orange M x PrO (2–y) Colourless (6.34) Rhodium Rh 2 O 3 Yellow RhO 2 Dark green (6.35) Ruthenium RuO 2 Blue–brown Ru 2 O 3 Black (6.36) Tantalum Ta 2 O 5 Colourless TaO 2 Very pale blue (6.37) 126 Metal oxides W, are composed of MO 6 octahedra arranged in a variety of corner-sharing and edge-sharing arrangements, and emphasises that these structural units persist in electrochromic films. Furthermore, he explains howthe coordination of the ions leads to electronic bands that are able to explain the presence or absence of cathodic and anodic electrochromism in the numerous defect perovskites, rutiles and layer structures adopted by these oxides. Solid-state electrochromism as in metal oxides requires the following. (i) Bonding in structures whose electron orbital energies (or where applicable, band energies) allow of electron uptake or loss from an inert contact, i.e. ‘redox switchability’; (ii) During the redox coloration process, a uniformity-conferring charge dispersibility via electron hopping or conduction bands, and complementary ion motion; (iii) Subsequent photon-effected electronic transitions involving the redox-altered species, that are responsible for colour evocation or colour change. The electron-hopping in (ii) is sometimes deemed to be small-polaron motion. That transition energies in (iii) comprise a spread around a most probable value is shown in spectroscopy by absorption bands having an appreciable width. The optical charge transfers in (iii) can either involve discrete sites of the same element in different charge states, (different ‘oxidation states’), in homonuclear intervalence charge transfer (‘IVCT’), or between sites occupied by different elements, in heteronuclear IVCT. The former often (perhaps usually) holds in single-metal oxides, though optical charge transfer between a metal and an oxide ion is also a possibility. In binary-metal oxides, homonuclear Table 6.1.(cont.) Metal Oxidised form a of oxide Reduced form a of oxide Balanced redox reaction for electrochromic operation Tin SnO 2 Colourless Li x SnO 2 Blue–grey (6.38) Titanium TiO 2 Colourless M x TiO 2 Blue–grey (6.39) Tungsten WO 3 Very pale yellow M x WO 3 Intense blue (6.8) Vanadium V 2 O 5 Brown–yellow M x V 2 O 5 Very pale blue (6.40) a The counter cation M is lithium unless stated otherwise. 6.1 Introduction to metal-oxide electrochromes 127 or heteronuclear transfer between the metals, or metal/oxide-ion electron transfer, are possible. (All of the several possibilities here could in principle occur together but no corresponding totality of discrete bands has been so assigned). Intra-atomic or inter-band transitions (resulting from the redox-effected changes) can also – perhaps less usually – confer some colour, the former rarely being intense. Most of the electrochromic oxides above are compounds of d-block metals. Some oxides of p-block elements – bismuth oxide, tin oxide, or mixed-cation such as indium–tin oxide (ITO) – likewise show a new colour (i.e. absorption band) on electro-reduction. 6.1.1 Bibliography The literature describing the electrochromism of metal oxides is extensive. Granqvist’s 3 1995 book Handbook of Inorganic Electrochromic Materials provides the standard text. There is also a chapter on ‘metal oxides’ in Electrochromism: Fundamentals and Applications (1995). 4 Early reviews on cathodic coloration 5 and on anodic coloration 6 (both 1982) are still informative, as are those on WO 3 amorphous films 7 (1975) and WO 3 displays 7 (1980). ‘Tungsten bronzes, vanadium bronzes and related compounds’ 8 is the most thorough survey, despite its date (1973), of the electronic and structural properties of compounds of interest such as M 0 x MO 3 where M is W, V or Mo, and M 0 represents a wide range of metal cations. The description ‘bronzes’ should strictly apply to metallically reflective, quite highly reduced, oxides, but the term is widely used in the literature for the moderately reduced nonmetallic regimes also. 6.1.2 Stability and durability of oxide electrochromes Metal-oxide electrochromes are studied for their relative photolytic stability, and ease of deposition in thin, even films over large-area electrodes (Section 6.1.3, below). However, four main disadvantages are detailed below. Firstly, the metal oxides can be somewhat unstable chemically, particularly to the presence of moisture. Secondly, while more photostable than organic electrochromes, many do evince some photoactivity. Thirdly, the metal oxides are inherently brittle. And finally, many oxides achieve only low coloration efficiencies. Reaction with moisture and chemical degradation Most studies of ECDs suggest that chemical degradation is the principal cause of poor durability. 128 Metal oxides Thus, some workers believe that the thin-film ITO used to manufacture optically transparent electrodes (OTEs) is so moisture sensitive, particularly in its partially reduced form M x ITO, that all traces of moisture should be excluded from ITO-containing ECDs. 9,10 Similarly, the avoidance of water is sometimes advised 11 if ECDs contain either Ni(OH) 2 or NiOÁ OH. Tungsten oxide is said to be particularly prone to dissolution in water and aqueous acid, 12,13,14 particularly if the film is prepared by evaporation in vacuo; 15 see p. 150. Photochemical stability The photochemical stability of metal oxides surpasses that of organic systems like polymers and viologens, or metallo-organic systems such as the phthalocyanines. Nevertheless, the metal oxides are not wholly photo-inert. For example, titanium dioxide is notably photoactive, particularly in its anatase allotrope, although in different applications like catalysing the photodecomposition of organic materials, such a high photoactivity is extremely desirable. Irradiating TiO 2 generates large numbers of positively charged holes, which are particularly reactive toward organic materials. Hence no electrochromic device should comprise thin-film TiO 2 in intimate contact with an organic electrolyte. Other metal oxides show photoactivity such as photochromism in a few cases. Photo-electrochromism is discussed in Chapter 15. The following electrochromic oxides show photoactivity such as photochromism or photovoltaism in thin-film form: iridium (in its reduced state), 16 nickel, 17,18 molybdenum, 19,20,21,22,23 titanium, 24,25,26 andtungsten. 27,28,29,30,31,32,33,34 Mechanical stability Like most solid-state crystalline structures, thin films of metal oxide are fragile. Bending or mechanical shock can readily cause insulating cracks and dislocations. Cracking is particularly problematic if the electrolyte layer(s) also comprise metal oxide, like Ta 2 O 5 . Some recent electrochromic devices have been developed in which the substrate is ITO deposited on PET or other polyester (see Section 14.3) in the fabrication of flexible ECDs, although their life expectancy is unlikely to be high because of fragility to bending. Mechanical breakdown also occurs because the films swell and contract with the chemical changes taking place during electrochromic coloration and bleaching. Stresses arise from changes in the lattice constants, that adjust to the insertion and egress of charged counter ions, and also to the change of charge on the central metal cations. Green 35 and Ord et al. 36 show that WO 3 35 and V 2 O 5 36 expand by about 6% during ion insertion. The oxide film cracks then disintegrates after repeated write–erase cycles if no accommodation or compensation is allowed for these stresses; see below. 6.1 Introduction to metal-oxide electrochromes 129 Amongst many probes, stresses from electrochromic cycling can be sensitively monitored by the laser-deflection method: a laser beam impinges on the outer surface of the electrochrome, and analysis of the way its trajectory is deflected during redox cycling provides data that allow quantification of these mechanical stresses. In this way Scrosati and co-workers 37 found a linear dependence between the amount of charge inserted into WO 3 and the induced stress, when the inserted ions were H þ , Li þ and Na þ . The linearity held only for small 37 amounts of inserted charge. Their correlation also suggests this induced stress is relieved in direct proportion to the extent of ion egress. Above certain values of x, though, new (unnamed) crystal phases were formed, particularly when the inserted ions were Li þ or Na þ , that caused the loss of reversibility. Laser-beam deflection has been used to monitor electrochromic transitions in the oxides of iridium, 38 nickel 39 and tungsten. 30,40,41 Alternative methods of analysing electrochromically induced stresses include electrochemical quartz-crystal microbalance (EQCM) studies, as described in Section 3.4. The stresses in oxide films of nickel, 11,42,43 titanium, 44 and tungsten 45,46,47,48 have been analysed thus. Information on electrochemically induced stresses can also be inferred from X-ray diffraction, e.g. in oxides of nickel 49 and vanadium, 50 while those in molybdenum oxide have been studied by Raman vibrational spectroscopy. 51 Employing an elastomeric polymer electrolyte largely accommodates the ion volume changes occurring during redox cycling: Goldner et al. 52 says ‘nearly complete stress-change compensation’ can be achieved by this method, for switching electrochromic windows. Other methods include adding small amounts of other metal oxides to the film: these minor built-in distortions introduce some mechanical ‘slack’ into the crystal lattices. For example, adding about 95% nickel oxide to WO 3 greatly enhances its cycle life. 53 Chapter 16 contains an assessment of the durability of assembled electrochromic devices, and how such durability is tested. 6.1.3 The preparation of thin-film oxide electrochromes In ECDs the metal-oxide electrochrome must be deposited on an electrode substrate as a thin, even film of sub-micron thickness, typically in the range 0.2–0.5 mm. Such thin films are either amorphous or polycrystalline, sometimes both admixed, the morphology depending strongly on the mode of film preparation. (i) Amorphous layers result from electrodeposition or thermal evaporation in vacuo. (ii) Other methods, sputtering for example, tend to form layers that are polycrystalline (microcrystalline or ‘nanocrystalline’). 130 Metal oxides Methods such as CVD or sol–gel generally proceed in two stages: the firstformed amorphous layer needs to be subsequently annealed (‘curing,’ ‘sintering’ or ‘high-temperature heating’). Annealing assists the phase transition amorphous !polycrystalline, which greatly extends the growth of crystalline material within the amorphous. 54 Such crystallisation is sometimes called 55 a ‘history effect’, thereby alluding to the extent of crystallinity, which depends largely on whether the sample was previously warmed or not. The crystallites formed can remain embedded in amorphous material, which could have serious implications for the speed of electrochromic operation; see p. 98. The number and size distribution of the crystallites depends on the temperature and duration of the annealing process. 56 There are no reviews dedicated solely to the deposition of metal oxides, although many authors have reviewed one or more specific deposition methods: Granqvist’s book 3 gives extensive detail on the preparation of metal-oxide films. Granqvist’s review 57 ‘Electrochromic tungsten oxide films: review of progress 1993–1998’ provides further detail, as does Kullman’s book, Components of Smart Windows: Investigations of Electrochromic Films, Transparent Counter Electrodes and Sputtering Techniques 58 (published in 1999). Finally, Venables’ 59 book Introduction to Surface and Thin Film Processes (published in 2000) contains some useful comments about these preparations. Deposition methods are outlined below, in alphabetical order. Chemical vapour deposition (CVD) In the CVD technique, a volatile precursor is introduced into the vacuum deposition chamber, and decomposes on contact with a heated substrate. Such volatiles commonly include metal hexacarbonyls or alkoxides and hexafluorides. For example, W(CO) 6 decomposes according to Eq. (6.1): W(CO) 6 (g) !W(s) þ6CO(g). (6.1) The carbon monoxide waste byproduct is extracted by the vacuum system. The solid tungsten product is finely divided, approaching the atomic level. Annealing at high temperature in an oxidising atmosphere yields the required oxide. The films are made polycrystalline by the annealing process. Chemical vapour deposition with carbonyl precursors has provided thin oxide films of both molybdenum 60,61,62,63 and tungsten. 62,63,64,65,66,67,68,69,70,71 An alternative precursor, a metal alkoxide such as Ta(OC 2 H 5 ) 5 , 72 is allowed into the deposition chamber at a lowpartial pressure. Decomposition occurs at the surface of a heated substrate (in this example 72 the temperature was 620 8C) to effect the reaction in Eq. (6.2): 6.1 Introduction to metal-oxide electrochromes 131 2Ta(OC 2 H 5 ) 5 (g) þ5O 2 (g) !Ta 2 O 5 (s) þproducts (g). (6.2) The resulting oxide filmis heated for a further hour at 750 8Cin an oxygen-rich atmosphere. 72 Vanadium oxide can similarly be prepared from the volatile alkoxide, VO(O i Pr) 3 . 71 If the CVD precursor does not decompose completely, the resultant films may contain carbon and hydrogen impurities, or other elements if different precursors are employed. The impurities either form gas-filled insulating voids in the oxide film, or their trace contamination adversely affects the electronic and optical properties of the electrochrome. Other metallo-organic precursors have been used, e.g. Watanabe et al. 73 employed the two volatile materials tris(acetylacetonato)indium and di(pivaloylmethanato)tin to make ITO. Furthermore, precursors can be wholly inorganic, such as TaCl 5 . 74 Electrodeposition Virtually all electrochromic films made by electrodeposition are amorphous prior to annealing. 75 Transition-metal oxides other than W or Mo are easily electrodeposited from aqueous solutions of metal nitrates, the lowest metal oxidation state usually being employed if there is a choice. Electrochemical reduction of aqueous nitrate ion generates hydroxide ion 76,77,78 according to Eq. (6.3): NO 3 À (aq) þ7 H 2 Oþ8 e À !NH 4 þ (aq) þ10 OH À (aq). (6.3) The electrogenerated hydroxide ions diffusing away from the electrode associate with metal ions in solution. Subsequent precipitation then forms an insoluble layer of metal oxide as in Eq. (6.4): M nþ (aq) þnOH À (aq) ![M(OH) n ] (s), (6.4) followed by dehydration during heating according to Eq. (6.5): [M(OH) n ] (s) !½ [M 2 O n ] (s) þn/ 2 H 2 O. (6.5) Dehydration as in Eq. (6.5) is usually incomplete, so the electrochrome comprises both oxide and hydroxide, often termed ‘oxyhydroxide’ and given the formulae MOÁ OH or MOÁ (OH) x . Hence most electrogenerated films of ‘oxide’ are oxyhydroxide of indeterminate composition unless sufficient annealing followed the electrodeposition. Electrodeposition from nitratecontaining solutions has produced oxide (and oxyhydroxide) films of cobalt 79,80,81 and nickel. 77,78,79,82,83,84,85,86,87 The mechanism of WO 3 electrodeposition is discussed at length by Meulenkamp. 75 Tungsten- or molybdenum-containing films can be electrodeposited fromaqueous solutions of tungstate or molybdate ions, but good-quality 132 Metal oxides oxide films are prepared from a solute obtained by oxidative dissolution of powdered metal in H 2 O 2 . This generates a peroxometallate species of uncertain composition, but the dissolution may proceed according to Eq. (6.6), as depicted for tungsten: 2W(s) þ6H 2 O 2 !2H þ [(O 2 ) 2 (O)W–O–W(O)(O 2 ) 2 ] 2À (aq) þH 2 Oþ4H 2 (g). (6.6) Such peroxo species are also employed in the sol–gel deposition method described below. The counter cations in Eq. (6.6) are either protons (as shown here), or they could be uncomplexed metal cations. 88 Excess peroxide is removed when the reactive dissolution is complete, usually by catalytic decomposition at an immersed surface coated with Pt-black. While still relatively unstable, dilution with an H 2 O–EtOH mixture (volume ratio 1:1) confers greater long-term stability until used. 89 Marginal ethanol incorporation in the electroformation of WO 3 89 and NiO 90 films has been investigated. Oxide films of cobalt, 80,81 molybdenum, 91,92 tantalum, 93 tungsten 56,89,94,95,96,97 and vanadium 98 have been made by electrodeposition from similar solutions. It is difficult to tailor the composition of films comprising mixtures of metal oxide since the ratio of metals in the resultant film is not always determined by the cation ratio in the precursor solution. This divergence in composition arises from thermodynamic speciation. When the deposition solution contains more than one cation, the electrogenerated hydroxide must partition between all the metal cations in solution, each involving the consumption of hydroxide ions as governed by both the kinetics and/or equilibria associated with the formation of each particular hydroxo complex. As the mixing of the precursor cations in solution occurs on the molecular level, the final mixed-metal oxide can be homogeneous and even. The mole fractions x of each metal oxyhydroxide in the deposit can be tailored by using both predetermined compositions and potentiostatically applied voltages V a . 81,91,96,99 Alternatively, applying a limiting current by imposing a large electrodeposition overpotential (Section 3.3) yields a film with a composition approximating that of the deposition solution. 79,80,81,91,97,100,101,102,103,104,105 Computer-based speciation analyses have been demonstrated that describe the product distribution during the electrodeposition of such mixed-metal depositions. 105,106 In a modification, electrochromes derived from Ni(OH) 2 and Co(OH) 2 are electrodeposited while the precursor solution is sonicated. 107 The main difference from conventional electrodeposition is the way sonication causes the formation, growth and subsequent collapse of microscopic bubbles. The 6.1 Introduction to metal-oxide electrochromes 133 bubble collapse takes place in less than 1 ns when the size is maximal, at which time the local temperature can be as high as 5000–25 000 K. After collapse, the local rate of cooling is about 10 11 Ks À1 , leading to crystallisation and reorganisation of the solute. 108 The reasons for the differences in the nanoproducts formed using this method are somewhat controversial; Gedanken and co-workers 108 suggest it obviates the need for particles to grow at finite rates. 107 The method has been used to make thin films of Ni(OH) 2 107,109 and Co(OH) 2 . 107,110 Co´ rdoba de Torresi and co-workers report 107 that the method yields electrochromes with significantly higher coloration efficiencies j. Sol–gel techniques Regarding present terminology, ‘colloid’ is a general term denoting any more- or-less subdivided phase determined by its surface properties, ‘sol’ denotes sub-micron or nano particles visible only by the scattering of a parallel visible light beam (the so-called Tyndall effect); while ‘gel’ denotes linked species forming a three-dimensional network, sometimes including a second species within the minute enclosures. 111 The sol–gel method involves decomposing a precursor (one chosen from often several candidates) in a liquid, to form a sol, which, on being allowed to stand, is further spontaneously transformed into a gel. The sol–gel method is an attractive route to preparing large-area films, as outlined in the review(2001) by Bell et al. 112 Many reviews of sol–gel chemistry include electrochromism: for example, Lakeman and Payne: 113 ‘Sol–gel processing of electrical and magnetic ceramics’ (1994); ‘The hydrothermal synthesis of new oxide materials’ (1995) by Whittingham et al.; 114 Alber and Cox, 115 (1997) ‘Electrochemistry in solids prepared by sol–gel processes’; Lev et al., 116 ‘Sol–gel materials in electrochemistry’ (1997), ‘Electrochemical synthesis of metal oxides and hydroxides’ (2000) by Therese and Kamath; 117 ‘Electrochromic thin films prepared by sol–gel process’ (2001) by Nishio and Tsuchiya; 118 ‘Anti-reflection coatings made by sol–gel processes: a review’ (2001) by Chen; 119 ‘Sol–gel electrochromic coatings and devices: a review’ (2001) by Livage and Ganguli, 120 and ‘Electrochromic sol–gel coatings’ by Klein (2002). 121 As indicated by the number of literature citations, the preferred sol–gel precursors are metal alkoxides such as M(OEt) 3 . 122 Many alkoxides react with water, so adding water to, say, Nb(OEt) 5 yields colloidal (sol) Nb 2 O 5 123 according to Eq. (6.7): 2Nb(OEt) 5 (l) þ5H 2 O(l) !Nb 2 O 5 (sol) þ10EtOH(aq), (6.7) which, on standing, expands to form the gel. 134 Metal oxides The other favoured sol–gel precursor is the peroxometallate species formed by oxidative dissolution of the respective metal in hydrogen peroxide (Eq. (6.6) above). Thus appropriate peroxo precursors have yielded electrochromic oxide films of cobalt, 80,81,99 molybdenum, 91,124,125 nickel, 126 titanium, 127,128 tungsten 99,123,127,129,130,131 and vanadium. 124,132,133 A similar peroxo species is formed by dissolving a titanium alkoxide Ti(OBu) 4 in H 2 O 2 . 128,134 Whatever the preparative method, the gel is then applied to an electrode substrate, as below. Spray pyrolysis The simplest method of applying a gelled sol involves spraying it onto the hot substrate, often in a relatively dilute ‘suspension’. 135,136 This method, sometimes called ‘spray pyrolysis’, has been used to make electrochromic oxides of cerium, 137 cobalt, 138,139 nickel 140,141,142 and tungsten. 143,144,145,146 It is especially suitable for making mixtures, since the stoichiometry of the product accurately reproduces that of the precursor solution. The coated electrode is annealed at high temperature in an oxidising atmosphere, as for CVD-derived films, to give a polycrystalline electrochrome. Burning away the organic components is more problematic than for CVD since the proportion of carbon and other elements in the gel is usually higher, with concomitant increases in impurity levels. Dip coating ‘Dip coating’ is comparable to spraying: the conductive substrate (inert metal; ITO on glass, etc.) is fully immersed in the gel then removed slowly to leave a thin adherent film. The process may be repeated many times when thicker layers are desired. The film is then annealed in an oxidising atmosphere. The method has produced oxide films of cerium, 147 nickel, 148,149,150,151,152 iridium, 153 iron, 154 niobium, 147,155,156,157,158,159,160,161, 162,163,164,165,166,167,168,169,170,171,172,173 titanium, 174 tungsten 29,129,130,131,175,176, 177,178,179,180,181 and vanadium. 182 Being particularly well suited to making mixed oxides, it has been used extensively for mixtures of precisely defined compositions such as indium tin oxide (ITO). 183 Spin coating A further modification of dip coating is the ‘spin coating’ method: the solution or gel is applied to a spinning substrate, and excess is flung away by centrifugal motion. Film thickness is controlled by altering solution viscosity, temperature and spinning rate. Many oxide films have been made this way: cerium, 184 cobalt, 185 ITO, 186,187 iron, 188 molybdenum, 189,190 niobium, 191,192 tantalum, 193 titanium, 128 tungsten 129,190,194,195,196,197,198 and vanadium. 132,133,199,200 Once formed, such films are annealed in an oxidising atmosphere. 6.1 Introduction to metal-oxide electrochromes 135 Spin coating is one of the preferred ways of forming thin-film metal-oxide mixtures, again producing precisely defined final compositions. 124,201,202,203,204,205 Other methods: sputtering in vacuo Sputtering techniques detailed below generally yield polycrystalline material 206 since the high temperatures within the deposition chamber effectively anneals the incipient film, thereby facilitating the crystallisation process amorphous !polycrystalline. Thin films of sputtered electrochrome are formed by three comparable techniques: dc magnetron sputtering, electron- beam sputtering and rf sputtering. In dc magnetron sputtering, a target of the respective metal is bombarded by energetic ions from an ion gun aimed at it at an oblique angle. The ion of choice is Ar þ , which is both ionised and accelerated by a high potential comprising the ‘magnetron’. The high-energy ions smash into the target in inelastic collisions that cause small particles of target to be dislodged by ablation. The atmosphere within the deposition chamber contains a small partial pressure of oxygen, so the ablated particles are oxidised: ablated tungsten becomes WO 3 . The substrate is positioned on the far side of the target. The oxidised, ablated material impinges on it and condenses, releasing much energy. The substrate thus has to be water-cooled to prevent its melting, especially if it is ITO on glass. Granqvist’s 1995 book 3 and 2000 review 57 describe in detail how the experimental conditions, such as the partial pressures, substrate composition, sputtering energiser and impact angle, affect the properties of deposited films. As an example, Azens et al. 207 made films of W–Ce oxide and Ti–Ce oxide by co-sputtering from two separate targets of the respective metals. Such targets are typically 5 cm in diameter and have a purity of 99.9%. The deposition chamber contained a precisely controlled mixture of Ar and O 2 , each of purity 99.998%. This sputter-gas pressure was maintained at 5–40 mTorr, the operating power varying between 100 and 250 W. The ratio of gaseous O 2 to Ar was adjusted from 1, to produce pure WO 3 and TiO 2 oxides, to 0.05 when pure Ce oxide was required. The deposition substrates were positioned 13 cm from the target. Deposition rates (from sputter time and ensuing film thicknesses as recorded by surface profilometry) were typically 0.4 nms À1 . Such reactive dc magnetron sputtering has been used to make oxide films of ITO, 208 molybdenum, 209 nickel, 210,211,212,213,214,215,216 niobium, 192,217,218 praseodymium, 219 tantalum, 220 tungsten 221,222 and vanadium. 50,223,224,225,226 Electron-beam sputtering Here an impinging electron beam generates a vapour stream from the target for condensation on the substrate. This 136 Metal oxides technique, also called ‘reactive electron-beam evaporation,’ has been used to prepare thin films of ITO, 227,228,229,230 MnO 2 , 231 MoO 3 232 and V 2 O 5 . 233 Radio-frequency (rf) sputtering Like dc sputtering, a target of the respective metal is bombarded with reactive atoms (argon or oxygen) at low pressure. The required thin film of metal oxide forms by heating the ablated material in an oxidising atmosphere. In the rf variant, the target-vaporising energy is derived from a beam of reactive atoms, generated at an rf frequency. The rf-sputtering technique is often employed for making metal oxides, and yields good-quality films which are flat and even. No post-deposition treatment is needed, since the high temperatures within the deposition chamber yield samples that are already polycrystalline. The technique has been used to make oxide films of: iridium, 234,235 lithium cobalt oxide, 236,237,238 ITO, 239,240,241,242,243,244,245 manganese, 246,247 nickel, 86,248,249,250,251,252,253,254,255,256 tantalum, 257,258,259,260 titanium, 261 tungsten 262 and vanadium. 206,263,264,265,266,267 Thermal deposition in vacuo The oxides of tungsten, molybdenum and vanadium are highly cohesive solids with extensive intra-lattice bonding, which require high temperatures for vaporisation when heated in vacuo. The vapour consists of molecular species (oligomers) such as the tungsten oxide trimer (WO 3 ) 3 . 268 (Arnoldussen suggests that these trimers persist in the solid state. 14 ) A pressure of about 10 À5 Torr is maintained during the deposition process. Thin films of metal oxide form when the sublimed vapour condenses on a cooled substrate. In practice, a small quantity of powdered oxide is placed in an electrically heated boat, typically of sheet molybdenum. Molybdenumor tungsten oxides can be prepared thus, although small amounts of elemental molybdenum can sublime and contaminate the electrochromic film. 269 The electrochromic properties of films deposited in vacuo are usually highly dependent on the method and conditions employed. Higher temperatures may cause slight decomposition in transit between the evaporation boat and substrate. Hence, evaporated tungsten oxide is often oxygen deficient to an extent y, in WO (3Ày) . Deb 270 suggests y ¼0.03 but Bohnke and Bohnke 271 quote 0.3. The extent of oxygen deficiency will depend on the temperature of the evaporation boat and/or of the substrate target. 272 Nickel oxide formed by thermal deposition is generally of poor quality, since the high temperatures needed for sublimation cause loss of oxygen, resulting in sub-stoichiometric films NiO (1Ày) , where the extent of oxygen deficiency y (1, so good-quality NiO is best made by sputtering methods. Thermal evaporation is often used to make the oxides of molybdenum, 23,273,274,275,276 tantalum, 220 tungsten 15,277,278,279 and vanadium. 233,277 6.1 Introduction to metal-oxide electrochromes 137 Vacuum Deposition of Thin Films 280 by Holland (1956), though old, remains a valued text on thermal evaporation in the preparation of thin films. Langmuir–Blodgett deposition Langmuir–Blodgett methodology for preparing films of metal-oxide electrochromes was reviewed in 1994 by Goldenberg. 281 In essence, by the arcane methods of Langmuir–Blodgettry employing an appropriately constructed bath, an electrochrome precursor in a solvent is laid down on the surface of another, non-dissolving, liquid in monolayer form. This can then be drawn onto the (say metal or ITO-glass) substrate by slow immersion then emersion of the latter, suitably repeated for multi-layers. Conversion to the required oxide follows one of the routes described above. 6.1.4 Electrochemistry in electrochromic films of metal oxides To add detail to the electrochemistry outlined in Chapter 3, electrochromic coloration of metal-oxide systems proceeds via the dual insertion of electrons (that effect redox change) and ions (that ensure the ultimate overall charge neutrality of the film). The dual charge injection is shown in Figure 6.1: the thin film of electrochrome concurrently accepts or loses electrons through the electrochrome–metal-electrode interface while ions enter or exit through the outer, electrochrome–electrolyte, interface. Thus a considerable electric field is set up initially across the film before these separate charges reach their Cations Electrons Electrons Anions (a) (b) Solid electrochrome Solid electrochrome Figure 6.1 Schematic representation of ‘double charge injection’, depicted for a reduction reaction: (a) cations as mobile ion, and (b) anions as mobile ion. The charge carriers move in their opposite directions during oxidation. Note the way that equal amounts of ionic and electronic charge move into or out from the film in order to maintain charge neutrality within the solid layer of electrochrome, though separation can occur, causing potential gradients. 138 Metal oxides ultimate, equilibrium, distributions. An important aspect of mechanistic studies concerns whether the ionic motion or the electronic is the slower, because the outcome often decides what determines the rate of coloration (cf. Chapter 5). A simple but not unexceptionable surmise would impute faster electronic motion to predominant crystallinity, but ionic rapidity to predominant amorphism (for the same material). The conductive electrode substrate can be either a metal or semiconductor; a highly-doped ITO or FTO film on glass usually acts as a transparent inert quasi-metal. The solid electrode assembly is in contact with a solution (solid or liquid) containing mobile counter ions (the ion source being termed ‘electrolyte’ hereafter). The mobile ion we imply to be lithium unless otherwise stated, though the proton also is often used thus. Anions are only occasionally employed as mobile ion, usually being the hydroxide ion OH À . While the following sections inevitably represent but an excerpt from the huge literature available, Granqvist’s monograph 3 (1995) is comprehensive to that date. Tungsten trioxide is treated first in Section 6.2 because it has been investigated more fully than the other highly colourant metal-oxide electrochromes. Other oxide electrochromes are reviewed subsequently in Section 6.3. Finally, mixtures of oxide electrochromes are discussed in Section 6.4, including metal-oxide mixtures with noble metals and films of metal oxyfluoride. For ECD usage, amorphous films are generally preferred for superior coloration efficiency j and response times. Polycrystalline films, by contrast, generally are more chemically durable. For this reason, studies have employed both amorphous and polycrystalline materials. 6.2 Metal oxides: primary electrochromes 6.2.1 Tungsten trioxide Selected biblography There are many reviews in the literature. The most comprehensive is: ‘Case study on tungsten oxide’ in Granqvist’s 1995 book. 3 Also by Granqvist is: ‘Electrochromic tungsten-oxide based thin films: physics, chemistry and technology’, 282 (1993); ‘Progress in electrochromism: tungsten oxide revisited’ 283 (1999); and ‘Electrochromic tungsten oxide films: reviewof progress 1993–1998’ (2000). 57 Also useful are the reviews by Azens et al.: 284 ‘Electrochromism of W-oxide-based thin films: recent advances’(1995); and by Monk: 285 ‘Charge movement through electrochromic thin-film tungsten oxide’ (1999). 6.2 Metal oxides: primary electrochromes 139 Finally in this section the reader is referred to reviews by Bange: 286 ‘Colouration of tungsten oxide films: a model for optically active coatings’ (1999) and Faughnan and Crandall: 7 ‘Electrochromic devices based on WO 3 ’ (1980). Morphology The structure of WO 3 is based on a defect perovskite. 2,287,288,289,290 An XRD crystallographic study of thick and thin films from screen-printed WO 3 established that WO 3 nanopowder has two monoclinic phases of space groups P2 1 /n and Pc. 291 Metal dopants (see Section 6.4) such as In, Bi and Ag have different influences on the phase ratio P2 1 /n to Pc. Cell parameters and crystallite sizes (about 50 nm) were marginally affected by these inclusions and, in detail, depended on the dopant. Tungsten trioxide as a thin filmcan be amorphous or microcrystalline, a-WO 3 or c-WO 3 , or indeed a mixture of phases and crystal forms. The preparative method dictates the morphology, the amorphous form resulting from thermal evaporation in vacuo and electrodeposition, the microcrystalline from sputtering or from thermal annealing of a-WO 3 . X-Ray diffraction showed Deb’s 270 evaporated WO 3 to be amorphous, but WO 3 films prepared by rf sputtering are partially crystalline. 292 The spacegroup of crystalline D 0.52 WO 3 is Im3. 287 Annealing WO 3 results in enhanced response times, 271 caused by the increased proportion of crystalline WO 3 . The temperature at which the (endothermic 12 ) amorphous-to-crystalline transition occurs is ca. 90 8 C, as determined by thermal gravimetric analysis (TGA). 293 By contrast, for crystallinity Deepa et al. 56 and Bohnke and Bohnke 271 both annealed samples at 250 8 C, and in the study by Deb and co-workers 294 of thermally evaporated WO 3 the crystallisation process is said to start at 390 8 C and is complete at 450 8 C, while Antonaia et al. 295 maintain that annealing commences at 400 8C. As the physical (and optical) properties of WO 3 , and its reduced forms, are highly preparation-sensitive, the apparent contradictions noted here and elsewhere in this text are almost certainly ascribable to intrinsic variability in (sometimes marked, sometimes minute) structural aspects of the solids. Preparation of tungsten oxide electrochromes Thermal evaporation Pure bulk tungsten trioxide is pale yellow. The colour of the WO 3 deposited depends on the preparative method, thin films sometimes showing a pale-blue aspect owing to oxygen deficiency in a sub- stoichiometric oxide WO (3Ày) , y lying between 0.03 270 and 0.3 271 (see p. 137). 140 Metal oxides The extent of oxygen deficiency depends principally on the temperature of the evaporation boat. 272 Sun and Holloway employ a modification of this method in which evaporation occurs in a relatively high partial pressure of oxygen. They call it ‘oxygen backfilling’, 296,297 which partly remedies the non- stoichiometry. Chemical vapour deposition, CVD (see p. 131). The volatile carbonyl CVD precursor W(CO) 6 is the most widely used. Pyrolysis in a stream of gaseous oxygen generates finely divided tungsten, and then thin-filmWO 3 after annealing in an oxygen-rich atmosphere. 62,64,65,66,67,68,69,70 Other organometallic precursors include tungsten(pentacarbonyl-1-methylbutylisonitrile) 298,299 and tungsten tetrakis(allyl), W(j 3 -C 3 H 5 ) 4 . 300 Sputtering (Section 6.1.4, p. 136). Many studies 221,292,301,302,303,304,305,306,307,308, 309,310 involve sputtered WO 3 films which are chemically more robust than evaporated films. Pilkington plc employed rf sputtering, bombarding a tungsten target with reactive argon ions in a low-pressure oxygen to sputter WO 3 onto ITO. 27,311,312 Direct-current magnetron sputtering is less often employed. 221,222 Electrodeposition WO 3 films electrodeposited onto ITO or Pt from a solution of the peroxotungstate anion, 56,88,89,94,95,96,97,99,198,313,314,315 (putatively [(O 2 ) 2 –(O)–W–(O)–(O 2 ) 2 )] 2 À , formed by oxidative dissolution of powdered tungsten metal in hydrogen peroxide) sometimes appear gelatinous, and are essentially amorphous in XRD. The tungsten carboxylates represent a different class of precursor for electrodeposition, yielding products that are amorphous. 314 Sol–gel The sol–gel technique is widely used, 46,55,99,118,123,127,129,130,131,175,176, 180,181,196,197,239,316,317,318,319,320,321,322,323,324,325,326,327,328 applying the sol–gel precursor by spin coating, 129,190,194,195,196,197,198 dip-coating 29,129,130,131,175, 176,177,178,179,180,181 and spray pyrolysis. 143,144,145,146 Livage et al. 129,176,180,329, 330,331,332 often made their WO 3 films from a gel of colloidal hydrogen tungstate applied to an OTE and annealed. Other sol–gel precursors include WOCl 4 in iso-butanol, 176 ethanolic WCl 6 , 197 tungsten alkoxides 333,334,335 and phosphotungstic acids. 140,336 The sol–gel method is often deemed particularly suited to producing large- area ECDs, for example for fabricating electrochromic windows. 310 Response times of 40 s are reported, 329 together with an open-circuit memory in excess of six months. 331,337 6.2 Metal oxides: primary electrochromes 141 Redox properties of WO 3 electrochromes On applying a reductive potential, electrons enter the WO 3 film via the conductive electrode substrate, while cationic counter charges enter concurrently through the other (electrolyte-facing) side of the WO 3 film, Eq. (6.8), W VI O 3 (s) þx(M þ (soln)) þxe À !M x (W VI ) (1Àx) (W V ) x O 3 (s), (6.8) very pale yellow intense blue (where M¼Li usually). For convenience, we abbreviate M x (W VI ) (1Àx) (W V ) x O 3 to M x WO 3 . The speed of ion insertion is slower for larger cations. Babinec, 338 studying the coloration reaction with an EQCM (see p. 88), found the insertion reaction to be complicated, depending strongly on the deposition rate employed in forming the electrochromic layer. Cation diffusion through WO 3 has received particular study with the cations of hydrogen ions, 339,340,341,342,343 deuterium cation, 344,345,346 Li þ , 271,339,347,348 Na þ , 40,349,350,351 K þ , 352 or even Ag þ . 339,353 The overwhelming majority of these cations cannot be inserted reversibly into WO 3 , as only H þ and Li þ can be expelled readily following electro-insertion. In a further EQCM study, the coloration usually attributable to Li þ is suggested to result rather from proton insertion, the proton then swapping with Li þ at longer times. 354 Consequences of electron localisation/delocalisation The non-metal-to-metal transition in H x WO 3 occurs at a critical composition x c ¼0.32, determined for an amorphous H x WO 3 by conductimetry 355 (the precise value cited no doubt applies exactly only to that type of product). Below x c , the bronze is a mixed-valence species 356 in the Robin–Day 348 Group II (involving moderate electron delocalisation of the ‘extra’ W V electron acquired by injection, that conducts by the sitewise hopping mechanism, or ‘polaron hopping’). H x WO 3 with x x c is metallic with completely delocalised transferable electrons (the Robin and Day 347 Group IIIB). It is this unbound electron plasma in metallic WO 3 bronzes that confers reflectivity, as in Drude-type delocalisation, 302,340,357,358,359,360 an essentially free-electron model (but dismissed by Schirmer et al. 361,362 for amorphous WO 3 ). Dickens et al. analysed the reflectance spectra of Na x WO 3 in terms of modified Drude–Zener theory that includes lattice interactions. 363 Kinetic dependences on x The rates of charge transport in electrochromic WO 3 films are reviewed by Monk 285 and Goldner, 364 and salient details from Chapter 5 are reiterated here. 142 Metal oxides Considerable evidence now suggests that the value of the insertion coefficient x influences the rates of electrochromic coloration, because the electronic conductivity 362 s follows x. At very lowx, the charge mobility j of the inserted electron is low, 362 hence rate-limiting, owing to the minimal delocalisation of conduction electrons which conduct by polaron-hopping. The electronic conductivity of evaporated WO 3 , subsequently reduced, has been determined as a function of x. 362,365,366 Figure 5.4 shows H x WO 3 to be effectively an insulator at x¼0, but s increases rapidly until at about x%0.3 the electronic conductivity becomes metallic following the delocalisation at this and higher x values. Most properties of the proton tungsten bronzes H x WO 3 depend on the insertion coefficient x, such as the emf, 367 the reflectance spectra, 363 and the dielectric- 368 and ferroelectric properties. 369 (It is notable that the alignment of spins in the ferroelectric states differs in proton-containing bronzes compared with that in Na x WO 3 , owing to the occupation of different crystallographic sites by the minute protons. 35 ) The ellipsometric studies by Ord and co-workers 370,371 of thin-film WO 3 (grown anodically) show little optical hysteresis associated with coloration, provided the reductive current is only applied for a limited duration: films then return to their original thicknesses and refractive indices. Colour cycles of longer duration, however, reach a point at which further coloration is accompanied by film dissolution (cf. comments in Section 1.4 and above, concerning cycle lives). The optical data for WO 3 grown anodically on Wmetal best fit a model in which the colouring process takes place by a progressive change throughout the film, rather than by the movement of a clear interface that separates coloured and uncoloured regions of the material. The former therefore represents a diffuse interface between regions of the film, the latter a ‘colour front’. Furthermore, Ord et al. conclude that a ‘substantial’ fraction of the H þ inserted during coloration cycles is still retained within the film when bleaching is complete. 371 The different mechanisms of colouring and bleaching discussed in Chapter 5 may be sufficient to explain the significant extent of optical hysteresis observed. 7,372 Figure 6.2 demonstrates such hysteresis for coloration and bleaching. Structural changes occurring during redox cycling In Whittingham’s 1988 review ‘The formation of tungsten bronzes and their electrochromic properties’ 373 the structures and thermodynamics of phases formed during the electro-reduction of WO 3 are discussed. Other studies of structure changes during redox change are cited in references 37 and 374–376. The effects of structural change are discussed in greater depth in Section 5.2 on p. 86. 6.2 Metal oxides: primary electrochromes 143 Some authors, such as Kitaoet al., 377 say that when the mobile ionin Eq. (6.8) is the proton, it forms a hydrogen bond with bridging oxygen atoms. However, the X-ray and neutron study by Wiseman and Dickens 287 of D 0.53 WO 3 shows the O–D and O–D–O distances are almost certainly too large for hydrogen bonding to occur. Similarly, Georg et al. 378 suggest the proton resides at the centres of the hexagons created by WO 6 octahedra. Whatever its position, X-ray results 379 suggest that extensive write–erase (on–off) electrochromic cycling generates non-bridging oxygen, i.e. causes fragmentation of the lattice structure. Optical properties of tungsten oxide electrochromes Optical effects: absorption The intense blue colour of reduced films gives a UV-visible spectrumexhibiting a broad, structureless band peaking in the near infra-red. Figure 6.3 shows this (electronic) spectrum of H x WO 3 . In transmission, the electrochromic transition is effectively colourless-to-blue at low x (0.2). At higher values of x, insertion irreversibly forms a reflecting, metallic (now properly named) ‘bronze’, red or golden in colour. 0.20 0.15 0 2 4 6 8 10 0.25 0.30 0.35 0.40 0.45 0.50 Amor phous Abs o r ba n c e Polycrystalline Charge density (mC cm –2 ) Figure 6.2 Optical density vs. intercalated charge density obtained for polycrystalline and amorphous WO 3 films during dynamic coloration and bleaching. (Figure reproduced from: Scarminio, J., Urbano, A. and Gardes, B. ‘The Beer–Lambert law for electrochromic tungsten oxide thin films’. Mater. Chem. Phys., 61, 1999, 143–146, by permission of Elsevier Science.) 144 Metal oxides The origin of the blue colour of low-x tungsten oxides is contentious. The absorption is often attributed to an F-centre-like phenomenon, localised at oxygen vacancies within the WO 3 sub-lattice. 270 Elsewhere the blue colour is attributed to the electrochemical extraction of oxygen, forming the coloured sub-stoichiometric product WO (3Ày) . 272,380 Faughnan et al. 381 and Krasnov et al. 382 proposed that injected electrons are predominantly localised on W V ions, the electron localisation and the accompanying lattice distortion around the W V being treated as a bound small polaron. 270,276,364,382,383,384,385 The colour was attributed 381 to the intervalence transition W V A þW VI B !W VI A þW V B (sub- scripts A and B being just site labels). While this is now widely accepted, among critics Pifer and Sichel, 386 studying the ESR spectrum of H x WO 3 at lowx, could find no evidence for unpaired electrons on the W V sites. Could the ground-state electrons formpaired rather than single spins, at adjacent loosely interacting W V sites? 384,385 Provisionally we assign the blue colour to an intervalence charge-transfer transition. While the wavelength maximum ` max of a particular H x WO 3 is essentially independent of the insertion coefficient x, the value of ` max does vary considerably with the preparative method (see p. 146): ` max depends crucially on morphology and occluded impurities such as water, electrolyte, and also the extent and nature of the electronic surface states (i.e. vacant electronic orbitals on the surface). Thus the value of ` max shifts from 900 nm in amorphous and hydrated reduced films of H x WO 3 , 5,361,387,388,389 to longer wavelengths in polycrystalline 389 materials, where ` max can reach 1300 nm for 1.5 Visib le range of spectrum 1.2 0.9 Abs o r ba n c e 0.6 0.3 500 1000 1500 2000 2500 λ (nm) Figure 6.3 UV-visible spectrum of thin-film H 0.17 WO 3 deposited by sputtering on ITO. The visible region of the spectrum is indicated. (Figure reproduced from Baucke, F. G. K., Bange, K. and Gambke, T. ‘Reflecting electrochromic devices’. Displays, 9, 1988, 179–187, by permission of Elsevier Science.) 6.2 Metal oxides: primary electrochromes 145 average grain sizes of 250 A ˚ . 339,361 Intervalence optical transitions are known to be neighbour-sensitive. As outlined in Chapter 4, a graph of absorbance Abs against the charge density consumed in forming a bronze M x WO 3 is akin to a Beer’s-law plot of absorbance versus concentration, since each electron acquired generates a colour centre. The gradient of such a graph is the coloration efficiency j (see Equations (4.5) and (4.6)). Most authors 5,355 believe the colour of the bronze is independent of the cation used during reduction, be it M¼H þ , Li þ , Na þ , K þ , Cs þ , Ag þ or Mg 2þ (here M¼½Mg 2þ ). However, Dini et al. 349 state that the coloration efficiency j does depend on the counter ion, and, for `¼700 nm, give values of j(H x WO 3 ) ¼63, j(Li x WO 3 ) ¼36 and j(Na x WO 3 ) ¼27 cm 2 C À1 . While sputtered films are more robust chemically than evaporated films, their electrochromic colour formed per unit charge density is generally weaker, i.e. j is smaller, although one sputtered film 390 had a contrast ratio CR of 1000:1, which is high enough to implicate reflection effects (as below, possibly even specular reflection). The higher absorbances of evaporated samples arise because the W species will be on average closer within (amorphous) grain boundaries, as discussed in ref. 285. Close proximities increase the probability of the optical intervalence transition in the electron-excitation colour-forming process, Eq. (6.8). This could explain why films sputtered from a target of W metal show different Beer’s-law behaviour from sputtered films made from targets of WO 3 . 312 The role of defects, and their influence on electrochromic properties, turns out to be far from clear, but the amorphous material (of course) contains a very high proportion of (what are from a crystal viewpoint) defects. The forms of defect in polycrystalline and amorphous WO 3 influence the optical spectra of WO 3 and its coloured reduction products. 391 Chadwick and co-workers 392 analysed the interdependence of defects and electronic structure, using WO 3 as a case study. They show how structural defects exert a strong influence upon electronic structure and hence on chemical properties. For example, while little is known about how the chemical activity at the interface is affected by interaction of liquid, their results suggest that any liquid suppresses water dissociation at the surface and the formation of OH 3 þ structures near to it. As expected, the overall absorbance Abs of any particular WO 3 film always increases as the insertion coefficient x increases, although Abs is never a simple function of the electrochemical charge Q passed over all (especially high) values of x. Beer’s law is therefore not followed except over limited ranges of Q and hence of x; see Figure 5.12. 393 146 Metal oxides Probably reflecting the preparation-dependence of film properties, there are considerable discrepancies in such graphs. At one extreme, the coloration efficiency for Li þ insertion is asserted to be essentially independent of x, so a Beer’s-law plot is linear until x is quite large. 394 Contrarily, for H þ or Na þ , the gradient of a Beer’s-law plot is claimed to decrease with increasing x, i.e. for coloration efficiency j decreasing as x increases. The non-linearity in such Beer’s-law graphs seems not to be due to competing electrochemical sidereactions 5 but is, rather, attributed to either a decrease in the oscillator strength per electron, 393,a or a broadening of the envelope of the absorption band owing to differing neighbour-interactions. In the middle ground, workers such as Batchelor et al., 311 who used sputtered WO 3 to form Li x WO 3 , found only two distinct regions, · in the range 0 <x<0.2 being higher than when x 0.2. At the other extreme, other workers suggest that Beer’s-law plots for thin-film WO 3 are only linear for small x values (0 <x 0.03) 5,381 or (0 <x 0.04). 393 This result applies both for the insertion of protons 5,381,393 and sodium ions 394 in evaporated (amorphous) WO 3 films. Beer’s-law plots are linear to larger x values from data for the insertion of Li þ into evaporated therefore amorphous WO 3 . Such graphs have a smaller gradient, so j is smaller. 387 The most intense coloration per electron (that is, the highest values of j) is seen when x is very small (<0.04). 393 The higher intensities follow since, at low x, the electron is localised within a very deep potential well described as a W V polaron or, possibly, as a spin-paired (diamagnetic) W V –W V dimeric ‘bipolaron’, located at defect sites. 395 Only at higher values of x, as the extent of electronic delocalisation increases, will conduction bands start to form as polaron distortions extend and coalesce (as mentioned under Kinetic dependences on x on p. 142). The existence of polarons may explain the finding that oxygen deficiency improves the coloration efficiency. 396 Duffy andco-workers 393 conductedextensive studies of suchBeer’s-lawgraphs on a range of H x WO 3 films made by immersing evaporated (hence amorphous) WO 3 on ITOin dilute acid. Beer’s-lawplots showed four linear regions, each with a different apparent extinction coefficient ·. Structural changes accompanying electro-reduction were inferred, that resulted in stepwise alteration of oscillator strength or optical bandwidth. These accord somewhat with views of Tritthart et al. 397 who proposed three definite types of colour centre in H x WO 3 . a The oscillator strength f ij is defined by IUPAC as a measure for integrated intensity of electronic transitions and related to the Einstein transition probability coefficient A ij : f ij ¼ 1.4992 Â 10 À14 ðA ij ,s À1 Þð`,nmÞ 2 . where ` is the transition wavelength. 6.2 Metal oxides: primary electrochromes 147 A wholly different behaviour is exhibited by films of polycrystalline WO 3 , prepared, e.g., by rf sputtering or by high-temperature annealing of amorphous WO 3 . At low x, the Beer’s-law plot is linear (but of low gradient) but j increases with an increase in x 387,398 possibly due to specular reflection, clearly not a wholly absorptive phenomenon. For thin films of WO 3 prepared by CVD, 62,66,67,68,69 Beer’s-law plots are said to be linear for H þ or Li þ only when the insertion coefficient x is low. Coloration efficiency j decreases at higher x, but the x value at the onset of curvature was not reported. Table 6.2 cites some coloration efficiencies j. Other Beer’s-law plots appear in refs. 393 and 399. The wide variations in j are no doubt caused in part by monitoring the optical absorbance at different wavelengths, but also result from morphological and other differences arising from the preparative methods. Optical effects by reflection As recorded in Table 6.3, the colour of crystalline M x WO 3 , when viewed by reflected light, shows a colour that depends on x, where x is proportional to charge injected. 363,373,407 For x values at and beyond the insulator/metal transition – i.e. those exceeding ca. x ¼0.2 or 0.3 Table 6.2. Sample values of coloration efficiency j for WO 3 electrochromes. Preparative route Morphology j/cm 2 C À1 (` (obs) in nm) Ref. Electrodeposition Amorphous 118 (633) 400 Thermal evaporation Amorphous 115 (633) 206 Thermal evaporation Amorphous 115 (633) 401 Thermal evaporation Amorphous 79 (800) 206 rf sputtering Polycrystalline 21 307 Sputtering Polycrystalline 42 (650) 401 Dip coating Amorphous 52 402 Sol–gel a PAA composite 38 403 Sol–gel Crystalline 70 (685) 404 Sol–gel Crystalline 167 (800) 405 Sol–gel Crystalline 36 (630) 406 Spin-coated gel Crystalline 64 (650) 197 Effect of counter cation – all samples prepared by thermal evaporation H x WO 3 Amorphous 63 (700) 349 Li x WO 3 Amorphous 36 (700) 349 Na x WO 3 Amorphous 27 (700) 349 PAA¼poly(acrylic acid); a alternate layers of PAA and WO 3 . 148 Metal oxides depending on preparation – the reflections become ever more metallic in origin. In consequence, crystalline WO 3 is both optically absorbent and also partially reflective. Amorphous M x WO 3 does not showthe same clear changes in reflected colour, probably because its insulator–metal transition is much less distinct. Devices containing tungsten trioxide electrochrome Much device-led research into solid-state ECDs concentrates on the tungsten trioxide electrochrome in, for example, ‘smart windows’, 408,409 alphanumeric watch-display characters, 410 electrochromic mirrors 393,411,412,413,414,415,416,417, 418,419 and display devices. 387,420,421,422,423,424,425,426 When the second electrochrome is a metal oxide, the WO 3 will be the primary electrochrome owing to the greater intensity of its optical absorption. Electrochromic devices of WO 3 have been fabricated with the oxides of iridium, 427 nickel, 428,429,430,431,432 niobium 433 and vanadium (as pentoxide) 242,277,434,435 as the secondary electrochrome. Thin-film WO 3 has also been used in ECDs in conjunction with the hexacyanoferrates of indium 436,437 or iron (i.e. Prussian blue), 438,439,440,441,442 and the organic polymers poly(aniline), 443,444,445,446,447,448,449,450,451,452,453 the thiophene-based polymer PEDOT 454 and poly(pyrrole). 455,456,457 A response time of 40 s is reported for a WO 3 film prepared by a sol–gel technique, 329 together with an open-circuit memory in excess of six months. 331,337 Following Deb’s 1969 electrochromic experiments on solid WO 3 (p. 29) significant progress ensued in 1975 when Faughnan et al. 381 published the construction of a device with WO 3 in contact with liquid electrolyte (see Chapter 2). This ECD worked well at short times, but failed rapidly owing to film dissolution in the H 2 SO 4 solution employed. The effect of steadily drying the electrolyte has been studied often. 7,12,13,14,15,354,458,459,460 To summarise, the rate and extent of film dissolution decreases as the water content decreases, but the rate of coloration also decreases. Table 6.3. Colours of light reflected from tungsten oxides of varying insertion extents of reduction x. x Colour 0.1 Grey 0.2–0.4 Blue 0.6 Purple 0.7 Brick red 0.8–1.0 Golden bronze To repeat: x 0.3 prevents electrochromic reversal. 6.2 Metal oxides: primary electrochromes 149 Reichman and Bard 461 showed, for the electrochromic processes of WO 3 on samples prepared by either anodic oxidation of tungsten metal or by vacuum evaporation onto ITO, that the electrochromic response time t was faster with the anodically grown material because it is microscopically porous. Furthermore, the value of t was an incremental function of the water content and film porosity, both properties unfortunately producing films susceptible to dissolution, which is accelerated by aqueous Cl À . 460 WO 3 films, in aqueous sulfuric acid as ECD electrolyte, b form crystalline hydrates such as WO 3 Á m(SO 4 ) Á n(H 2 O) which decrease the electrochromic efficiency considerably. 463 Film dissolution can be prevented by two means; the use of non-aqueous acidic solutions, for example, anhydrous perchloric acid in DMSO (dimethyl sulfoxide), 464 or, rather than the use of acid, a non-protonic (alkali-metal) cation, usually lithium, is employed as insertion ion. Examples include films of WO 3 immersed in lithium-containing electrolytes such as LiClO 4 , lithium triflate (LiCF 3 CH 2 CO 2 ), or occasionally LiAlF 6 or LiAsF 5 , in dry propylene carbonate. Alternatively, WO 3 ECDs have been constructed which incorporate solid inorganic electrolytes such as Ta 2 O 5 , or organic polymers such as poly(acrylic acid), poly(AMPS) or poly(ethylene oxide) – PEO, each containing a suitable ionic electrolyte; see Section 14.2 for further detail. Such cells have slower response times and also a poorer open-circuit memory, although Tell 465,466 has made such a solid-state ECD from phosphotungstic acid, claiming a t of 10 ms (but for an unspecified change in absorbance). Such liquid-free devices are preferred for their chemical and mechanical robustness. Tungsten trioxide in aqueous acidic electrolytes is more durable if the electrochrome–electrolyte interface is protected with a very thin over-layer of Nafion TM , 467 Ta 2 O 5 , 468 or tungsten oxyfluoride, 469 although charge transport through such layers will be slower. Other layers used to protect WO 3 are described on p. 446. Other over-layers can speed up the electrochromic response. For example, a layer of gold enhances the response time t and also protects against chemical degradation. 470,471 Clearly, the layer needs to be ion-permeable, hence very thin or porous. In solid-state WO 3 devices, the stability of the electrochromic colour is generally good, despite some loss of absorbance with time. This ‘self bleaching’ or ‘spontaneous hydrogen deintercalation’, 472 has been studied often: 15,295, 473,474 in one study, CVD-prepared WO 3 returned to its initial transparency b The reaction of acid with WO 3 prepared by anodising W metal is found to be kinetically first order with respect to acid, 460 and zeroth order with respect to film thickness. 462 150 Metal oxides after only three minutes. 475 Deb and co-workers 474 have also investigated the chemistry underlying the self bleaching of evaporated WO 3 on ITO, suggesting that adsorbed water in the films reacts with the coloured Li x WO 3 to form LiOH and molecular hydrogen. 6.2.2 Molybdenum oxide Preparation of molybdenum oxide electrochromes Molybdenumtrioxide films may be formed with amorphous or polycrystalline morphologies. Amorphous material can be formed by vacuum evaporation of solid, powdered MoO 3 , 21,23,273,274,275,476,477 by anodic oxidation of molybdenum metal immersed in e.g. acetic acid, 478 or deposited electrochemically; a widely used precursor is prepared by oxidative dissolution of molybdenum metal in hydrogen peroxide solution. 91,92,313,479 Sputtering yields polycrystalline material. The product of dc magnetron sputtering is of good quality and colourless. 209 In rf sputtering, however, over-rapid rates of deposition can yield oxygen-deficient material, which is blue, 20,209,480 and clearly different from the desired ‘bronze’, M x MoO 3 , being in fact substoichiometric 23,275,481,482 with composition Mo VI c Mo V (1–c) O (3Àc/2) , where c can be as high as 0.3. Granqvist and co-workers 209 show that sub- stoichiometric blue ‘MoO 3 ’ forms at deposition rates up to 1.5 nms À1 , whereas clear MoO 3 requires a deposition rate of about 0.85 and 0.1 nms À1 for films made with dual-target and single-target sputtering, respectively. (Dual- target sputtering is twenty times faster than single-target deposition. 480 ) Nevertheless, the electrochromic properties, particularly in bleaching, of sub-stoichiometric films improve after about five colour/bleach cycles in a LiClO 4 /PC electrolyte. 209 Gorenstein and co-workers found 481 that blue sputtered ‘MoO 3 ’ forms particularly at low fluxes of ionised Ar þ , which could be a result of differing conditions such as the sputtering geometries. In rf sputtering a target of metallic molybdenum and low-pressure Ar þO 2 20,483 are employed. Controlling the flow and composition of the atmosphere dictates the composition and structure of the final electrochrome. 484 The flow rate and hence the exact composition have a profound effect on the optical properties of the film. 20 The best films were made with a low rate of oxygen flow that gave a sub-stoichiometric oxide, although the relationship(s) between optical, electrochemical and mechanical properties and flow rate are complex. 20,481,485 Chemical vapour deposition also yields polycrystalline material from an initial deposit of usually finely divided metal. This needs to be roasted in an oxidising atmosphere, that causes amorphous material to crystallise. Chemical 6.2 Metal oxides: primary electrochromes 151 vapour deposition precursors include gaseous molybdenumhexacarbonyl 62 or organometallics like the pentacarbonyl-1-methylbutylisonitrile compound. 486 Molybdenum trioxide films derived from sol–gel precursors are also polycrystalline as a consequence of high-temperature annealing after deposition. The most common precursor is a spin-coated gel of peroxopolymolybdate 189,487 resulting from oxidative dissolution of metallic molybdenum in hydrogen peroxide. Such films are claimed to show a superior memory effect to sputtered films of MoO 3 . 488 Other sol–gel precursors include alkoxide species such as 190 MoO(OEt) 4 . Films have also been made by spray pyrolysis, spraying aqueous lithium molybdate at low pH onto ITO, itself deposited on a copper substrate 489 by electron-beam evaporation. 232 Thermal oxidation of thin-film MoS 3 also yields electrochromic MoO 3 . 490 Finally, solid phosphomolybdic acid is also found to be electrochromic. 466 Redox chemistry of molybdenum oxide electrochromes The electrochromism of molybdenum oxide is similar to that of WO 3 , above, so little detail will be given here. There is a considerable literature on the electrochemistry of thin-film MoO 3 , but smaller than for WO 3 . As with WO 3 , annealing amorphous MoO 3 causes crystallisation. The electrochromic behaviour of the films depend on the extent of crystallinity, and therefore on the annealing. McEvoy et al. 491 suggest that electrodeposited films of MoO 3 on ITO are completely amorphous if not heated beyond about 100 8C. Films heated to 250 8Ccomprise a disordered mixture of orthorhombic a-MoO 3 and monoclinic b-MoO 3 phases, giving voltammetry which is ‘complicated’. Crystallisation to form the thermodynamically stable a phase occurs at temperatures above 350 8C. The dark-blue coloured form of the electrochrome is generated by simultaneous electron and proton injection into the MoO 3 , in the electrochromic reaction Eq. (6.9): Mo VI O 3 þx(H þ þe À ) !H x Mo V,VI O 3 . (6.9) colourless intense blue Whittingham 492 considers H þ mobility in layered H x MoO 3 (H 2 O) n but many workers prefer to insert lithium ions Li þ , from anhydrous solutions of salts such as LiAlF 6 or LiClO 4 in PC, 20,51,209,480,488 while Sian and Reddy preferred Mg 2þ as the mobile counter cation. 275,477 Equation (6.9) is over-simplified because Mo IV appears in the XPS of the coloured bronze, as well as the expected valence states of Mo V and Mo VI . 19,275 152 Metal oxides Some oxygen deficiency can complicate spectroscopic analyses: 275 evaporated MoO 3 films, colourless when deposited, nevertheless give an ESR signal characteristic of Mo V at 23 g ¼1.924. Molybdenum bronzes H x MoO 3 show an improved open-circuit memory compared with the tungsten bronzes H x WO 3 , since H x MoO 3 films oxidise more slowly than do films of H x WO 3 having the same value of x. 5 Also, protons enter the molybdenum films at potentials more cathodic than þ0.4 V (against the SHE), leaving a coloration range of about 0.4 V prior to formation of molecular hydrogen; the gas possibly forms catalytically on the surface of the bronze, as in Eq. (6.10): 2H þ (aq) þ2e À !H 2 (g). (6.10) The corresponding range for H x WO 3 is larger, about 0.5 V. 5 Additionally beneficial, the chemical diffusion coefficients D of H þ through MoO 3 are faster than through the otherwise similar WO 3 , implying faster electrochromic operation. 5 Ord and DeSmet 478,493 interpret their ellipsometric study of the proton injection into MoO 3 as showing two distinct insertion sites for the mobile hydrogen ion within the reduced film. There is a readily observed, well-defined boundary between the oxidised and reduced regions within the oxide, perhaps in contrast to WO 3 , implying a somewhat different mechanism for electroreduction. The XRD study by Crouch-Baker and Dickens 494 suggests that hydrogen insertion proceeds without the occurrence of major structural rearrangement in the bulk of the oxide film. The electrochromism of molybdenum oxide is enhanced when coated with a thin, 20 nm, transparent film of Au or Pt, 495 presumably because the precious metal helps minimise the effects of IR drop caused by the poor electronic conductivity across the surface of the MoO 3 . Coating the MoO 3 with precious metal also decreases the extent of oxide corrosion, 495 perhaps similarly to protecting WO 3 with a thin film of gold 470 or tungsten oxyfluoride. 288,496 Optical properties of molybdenum oxide electrochromes An XPS study 476 shows that the colour in the reduced state of the film arises from an intervalence transition between Mo V and Mo VI in the partially reduced oxide, cf. WO 3 . In appearance, the optical absorption spectrum of H x MoO 3 is very similar to that of H x WO 3 (e.g. see Figure 6.4) except that the wavelength maximum of H x MoO 3 falls at shorter wavelengths than does ` max for H x WO 3 . The wavelength maximum of the partly reduced oxide is centred at 23 770 nm. This band 6.2 Metal oxides: primary electrochromes 153 is clearly not of simple origin, 23 but comprises a collection of discrete bands having maxima at around 500 nm, 625 nm, and 770 nm. The absorption edge of MoO 3 occurs at 476 385 nm, but shifts to %390 nm for the coloured reduced film. 476 The ‘apparent coloration efficiency’ for partly reduced molybdenum oxide is therefore slightly greater than for partly reduced tungsten trioxide since the absorption envelope coincides more closely with the visible region of the spectrum. The optical constants n and k of thermally annealed MoO 3 (i.e. amorphous MoO 3 that was formed by thermal evaporation but then roasted) depend quite strongly on the annealing temperature. 275 Unlike H x WO 3 , the value of ` max for H x MoO 3 is not independent of x, 292 but moves to shorter wavelengths as x increases; see Figure 6.5. Table 6.4 contains a few representative values of coloration efficiency j. Devices containing molybdenum oxide electrochromes Devices containing MoO 3 are comparatively rare. For example, Kuwabara et al. 497,498 made several cells of the form WO 3 j tin phosphate jH x MoO 3 . The solid electrolyte layer is opaque: otherwise, no discernible change in absorbance would occur during device operation. The response times of ECDs may be enhanced by depositing an ultra-thin layer of platinum or gold on the 0 0.5 1.0 1.5 Photon energy (eV) Abs o r ba n c e 0.0 1.0 2.0 3.0 1 2 3 4 5 Figure 6.4 UV-visible spectrum of thin-film molybdenum oxide for various amounts of inserted charge: (1) 0; (2) 490; (3) 1600; (4) 2200 and (5) 3800 mCcm À3 . (Figure reproduced from Hiruta, Y., Kitao, M. and Yamada, M. ‘Absorption bands of electrochemically-colored films of WO 3 , MoO 3 and Mo c W 1Àc O 3 .’ Jpn. J. Appl. Phys., 23, 1984, 1624–7, with permission of The Institute of Pure and Applied Physics.) 154 Metal oxides electrolyte-facing side of the electrochrome. 495 As with WO 3 , clearly the layer of precious metal must be permeable to ions. 6.2.3 Iridium oxide Preparation of iridium oxide electrochromes There are now two commonly employed methods of film preparation: firstly, electrochemical deposition to form an ‘anodic iridium oxide film’ (‘AIROF’ in a jargon abbreviation). The second major class are ‘sputtered iridium oxide films’ (‘SIROFs’). The anodically grown films 464,499,500,501,502,503,504,505,506,507,508,509 are made by the potentiostatic cycling between À0.25 Vand þ1.25 V(against SCE) of an Table 6.4. Sample values of coloration efficiency j for molybdenum oxide electrochromes. Preparative route j/cm 2 C À1 (` (obs) /nm) Ref. Thermal evaporation of MoO 3 77 7 Evaporation of Mo metal in vacuo 19.5 (700) 482 Oxidation of thin-film MoS 3 35 (634) 490 Q i (mC cm –2 ) 2000 1000 0 E p ( e V) 1.0 1.5 2.0 Figure 6.5 Plot of E (as E¼hi, where i is the frequency maximum of the intervalence band) for the reduced oxides H x MoO 3 as a function of the electrochemical charge inserted, Q i , which is proportional to the hydrogen content, x. (Figure redrawn from Fig. 4 of Hurita, Y., Kitao, M. and Yamada, W. ‘Absorption bands of electrochemically coloured films of WO 3 , MoO 3 and Mo c W (1–c) O 3 .’ Jpn. J. Appl. Phys., 23, 1984, 1624–162, by permission of The Japanese Physics Society.) 6.2 Metal oxides: primary electrochromes 155 iridium electrode immersed in a suitable aqueous solution. Such AIROFs are largely amorphous. 510 They have a CR as high as 70:1 which forms within t ¼20 to 40 ms; 504 such response times are considerably faster than for WO 3 or V 2 O 5 films of similar thickness and morphology. Anodic iridium oxide films degrade badly under intense illumination, 16 sometimes a serious disadvantage. Anodic iridium oxide films can also be generated by immersing a suitable electrode (e.g. ITO) into an aqueous solution of iridium trichloride. 499,511,512 The solution must also contain hydrogen peroxide and oxalic acid. (Following the usual desire for acronyms, such films are nowdesignated as ‘AEIROFs’ i.e. anodically electrodeposited iridium oxide films.) Once formed and dried, the electrochromic activity of an AEIROF increases as the proportion of water in the electrolyte increases. Conversely, if annealed, the electrochromic activity decreases as the anneal temperature increases. The second method of forming films is reactive sputtering in an oxygen– argon atmosphere (the respective partial pressures being 1:4). 506 Hydrogen may also be added. 513 Such films are grey–blue in the coloured state with ` max ¼610 nm. A denser SIROF, that forms a black electrochromic colour, can be made with oxygen alone as the flow-gas during the sputtering process. Sputtered iridium oxide films have a complicated structure which, unlike AIROFs, is not macroscopically porous, i.e. decreased response times are observed since ionic insertion is slowed. These black SIROFs are deposited as coloured films which can be decolorised by up to 85%on cycling, while blue SIROFs give superior films which may be transformed to a truly colourless state. In fact, blue SIROFs are very similar to AIROFs in being totally decolorisable. Furthermore, in terms of write–erase response times and absorbance spectra, blue SIROFs and AIROFs are again similar, cyclic voltammetry confirming the similarity. 506 Blue SIROFs have superior response times to black SIROFs, and a longer open-circuit memory. Beni and Shay 506 view the blue SIROFs as aesthetically the more pleasing. The reliability of AIROFs is apparently variable. 511 Extremely porous films of iridium oxide can be prepared by thermally oxidising vacuum-deposited iridium–carbon composites. 514 The electrochromically generated colour of a SIROF is only moderately stable, and decreases by about 8% per day. 509 Sol–gel methods also yield polycrystalline iridiumoxide, and start froma sol formed from iridium trichloride solution in an ethanol–acetic acid mixture, 118,153,239 and iridium oxide films have been prepared by sputtering metallic iridium onto an OTE in an oxygen atmosphere. 505 Finally, electrochromic films are formed on ITO when g-rays irradiate solutions of iridium chloride in ethanol. 118,515 156 Metal oxides The redox chemistry of iridium oxide electrochromes In aqueous solution, the mechanism of coloration is still uncertain, 234 so two different reactions are current. The first is described in terms of proton loss, 502,503 Eq. (6.11): Ir(OH) 3 !IrO 2 Á H 2 OþH þ (soln) þe À , (6.11) colourless blue–grey which is confirmed by probe-beamdeflection methods. 516 The second involves anion insertion, 507 Eq. (6.12): Ir(OH) 3 (s) þOH À (soln) !IrO 2 Á H 2 O(s) þH 2 Oþe À . (6.12) While XPS measurements 500 seem to confirm Eq. (6.11), AIROFs do not colour in anhydrous acid solutions, e.g. HClO 4 in anhydrous DMSO, 464 so the reaction (6.11) probably applies only to aqueous electrolytes. While protons are ejected from AIROFs during oxidation, 516 their electrochromic behaviour is independent of the pH of the electrolyte solution, 501 suggesting that both protons and hydroxide ions are involved in the electrochromic process. Equation (6.12) is not without question: some workers 507 assert that AIROFs will colour when oxidised while immersed in solutions containing the counter ions of 507 F À or CN À ; others disagree. 517 Regardless of whether the mechanism is hydroxide insertion or proton extraction, Ir(OH) 3 is the bleached form of the oxide and the coloured form is IrO 2 . Ellipsometric data 518 suggest little hysteresis during redox cycling, the optical constants during reduction retracing the path followed during oxidation. Unlike other metal oxides, neither the coloration nor bleaching reactions proceed by movement of an interface between oxidised and reduced material traversing a ‘duplex’ film. Nor does redox conversion proceed with a singlestage conversion of a homogeneous film. In fact, the optical and electrochemical data both suggest that conversion occurs in two distinct stages: Rice 519 suggests that a satisfactory model requires the recognition that AIROFs act as a conductor of both electrons and anions during the electrochromic reaction, which helps explain the relatively low faradaic efficiency in dilute acid. 520 The participation of the electrons, and the sudden change in electrochromic rate, may correlate with the occurrence of a non-metal-to-metal transition between 0 and 0.12 V (vs. SCE). 521 Phase changes in iridium oxide are discussed by Hackwood and Beni. 522 Gutierrez et al. 523 have investigated AIROFs using potential-modulated reflectance tentatively to assign the peaks in the cyclic voltammetry of anodic films of iridium oxide to the various redox processes occurring. 6.2 Metal oxides: primary electrochromes 157 Optical properties of iridium oxide electrochromes Figure 6.6 shows an absorbance spectrum of thin-film iridium oxide sputtered onto quartz. 524 The change in transmittance of crystalline Ir 2 O 3 films made by sol–gel techniques is larger than that of the amorphous Ir 2 O 3 under the same experimental conditions. 118,153 There are relatively few coloration efficiencies j in the literature: j for an oxide film made by thermal oxidation of an iridium–carbon composite 525,526 is quite low at À(15 to 20) cm 2 C À1 at a ` max of 633 nm. The AIEROF film 511 is characterised by j of À22 cm 2 C À1 at 400 nm, À38 cm 2 C À1 at 500 nm and À65.5 cm 2 C À1 at 600 nm; j for spray-deposited oxide depends strongly on the annealing temperature, 512 varying from À10 cm 2 C À1 at 630 nm for films annealed at 400 K to À26 cm 2 C À1 for films annealed at 250 K. Optical study of the electrochromic transition of AIROFs is greatly complicated by anion adsorption at the electrochrome–solution interphase. 527 Colouredstate 26 mC cm –2 Bleachedstate 0 0.3 0.5 0.7 0.9 1.1 λ ( μ m) 1.3 1.5 1.7 1 2 3 Abs o r ba n c e 4 5 6 Figure 6.6 UV-visible spectrum of thin-film iridium oxide sputtered onto quartz. The broken line is the reduced (uncoloured) form of the film and the continuous line is the spectrum following oxidative electro-coloration with 26 mCcm À2 . (Figure reproduced from Kang, K. S. and Shay, J. L. ‘Blue sputtered iridium oxide films (blue SIROF’s)’. J. Electrochem. Soc., 130, 1983, 766–769, by permission of The Electrochemical Society, Inc.). 158 Metal oxides Electrochromic devices containing iridium oxide electrochromes Thin-film iridium oxide was one of the first metal-oxide electrochromes to be investigated for ECD use. Electrochromic cells containing iridium oxide generate colour rapidly: the cell SnO 2 jAIROFjfluoridejAu develops colour in 0.1 second (where ‘fluoride’ represents PbF 2 on PbSnF 4 ). 528 Another ECD was prepared with two iridium oxide films in different oxidation states, ‘ox-AIROF’ being one oxide film in its oxidised form while ‘red-AIROF’ is the second film in its reduced form. 508 The cell fabricated was ‘ox-AIROFjNafion 1 jred-AIROF’, the Nafion 1 containing an opaque whitener against which the coloration was observed; otherwise, the electrochromic colour of the two AIROF layers would change in a complementary sense, with the overall result of almost negligible modulation. When a voltage of 1.5 Vwas applied across the cell, the maximum colour formed in about 1 second. 509 Clearly, the device can only operate when initially one iridium layer is oxidised and the other reduced. This cell is described in detail in ref. 508. Solid-state AIROFs have been made with polymer electrolyte, but these have slower response times. 509 Ishihara 529 used iridium oxide in a solid-state device in which reduced chromium oxyhydroxide was the source of protons migrating into the electrochrome layer. Anodic iridium oxide films are superior to WO 3 -based electrochromes since they do not degrade in water but retain a high cycle life (of about 10 5 ) even in solutions of low pH, 507 provided the temperature remains low: 507 the bleached form of iridium oxide decomposes thermally above about 100 8C. 530 Acomposite device based on iridiumoxide and poly(p-phenylene terephthalate) on ITO shows different electrochromic colours: blue–green when oxidised, but colourless when reduced. 531 The reaction at the counter electrode is unidentified. Other ECDs have been made with sputtered IrO x as the secondary electrochrome and WO 3 as the primary layer. On fabrication, one layer contains ionic charge; both layers colour in a complemetary sense as charge is decanted from one electrochrome layer to the other. 427,532 6.2.4 Nickel oxide Much of the nickel oxide prepared in thin-film form is oxygen deficient. The extent of deficiency varies according to the choice of preparative route and deposition parameters. For this reason, ‘nickel oxide’ is often written as NiO x or NiO y where the symbols x or y indicate oxygen non-stoichiometry. We prefer 6.2 Metal oxides: primary electrochromes 159 an alternative notation and denote oxygen non-stoichiometry by NiO (1þy) H z when hydroxyl is a ligand, otherwise for hydroxyl free species, by NiO (1þy) . Preparation of nickel oxide electrochromes There is a large literature on making thick films of nickel oxide owing to its use in secondary batteries. 247,533 One of the principal difficulties in making thin-film nickel oxide is its thermal instability: heating an oxide film can cause degradation or outright decomposition. The thermal stability of thin-film nickel oxide is the subject of several investigations: by Cerc Korosˇ ec and co-workers 148,534,535 on electrochromes made via sol–gel methods; by Jiang et al. 251 studying the effects of annealing rf-sputtered NiO (1Ày) ; by Natarajan et al. 536 probing the stability of electrodeposited samples; and by Kamal et al., 141 examining samples made by spray pyrolysis. Thin films of nickel oxide electrochromes are usually made by sputtering in vacuo, by the dc-magnetron 211,212,213,214,215,216,537 or rf-beamtechniques. 86,248, 249,250,251,252,253,254,255,256,538,539 The target is usually a block of solid nickel oxide, 211,214,215,540 but a nickel target and a relatively high partial pressure of oxygen is also common. 211,214,215,253,254,255,256 Rutherford backscattering c suggests that rf-sputtered NiO is rich in oxygen, i.e. nickel oxide of composition NiO (1þy) . 256 Excess oxygen at grain boundaries enhances the extent of electrochromic colour. 539 A target of solid LiNiO 2 generates a pre-lithiated film. 252,541 Addition of gaseous hydrogen to the sputtering chamber has profound effects on the optical properties of the resultant films. 542 Other films of NiO (1Ày) are reported via electron-beam sputtering, 543,544 or pulsed laser ablation, 545,546,547,548,549 e.g. from a target of compacted LiNiO 2 powder. 546,548 A cathodic-arc technique also yields NiO (1Ày) if metallic nickel is sputtered in vacuo in an oxidising atmosphere. 550 Thermal vacuum evaporation seems a poor way of making NiO (1þy) films since the electrochrome readily decomposes in vacuo to yield a material with little oxygen. Nevertheless, this technique is reported to generate NiO (1þy) films satisfactorily. 544,551,552 Electrodeposition of thin-film nickel oxide is more widely used, e.g. from solutions of aqueous nickel nitrate. 82,212,553 Equations (6.3) and (6.4) describe c In the backscattering experiment, alpha particles typically possessing energies of several MeVare fired at a thin sample. The majority of alpha particles remain embedded in the sample, but a small proportion scatter from the atomic nuclei in the near surface (1 to 2 mm) of the sample. The energy with which they backscatter relates to the mass of the target element. For heavy target atoms such as tungsten, the backscattered energy is high – almost as high as the incident energy, but for lighter target atoms such as oxygen, the backscattered energy is low. Analysis of the backscattering pattern enables Rutherford backscattering (RBS) to measure the stoichiometry of thin films. 160 Metal oxides the reactions that form the immediate oxyhydroxide product NiO(OH) z , which can be dehydrated according to Eq. (6.5) by annealing. Other aqueous electrodeposition solutions include an alkaline nickel–urea complex, 554 nickel diammine 554,555,556,557 nickel diacetate, 558 [Ni(NH 3 ) 2 ] 2þ or nickel sulfate, 17, 536,559,560 albeit by an unknown deposition mechanism. Electrodeposition from a part-colloidal slurry has also been achieved. 561 Fewer sol–gel films of nickel oxide electrochrome have been made, in part because the necessary annealing can damage the films. Electrochromic films have been made via sol–gels derived from NiSO 4 with formamide and PVA, 152 or nickel diacetate dimethylaminoethanol, although the resulting solid film is not durable. 149 Precursors of nickel bis(2-ethylhexanoate) 562 or NiCl 2 in butanol and ethylene glycol 151 have been employed in spin coating prior to thermal treatment to effect dehydration and crystallisation. Dip-coating has also been used: electrodes are immersed repeatedly into a nickel-containing solution, like buffered NiF 2 , 563 NiSO 4 in water 564 or polyvinyl alcohol, 152 or NiCl 2 in butanol and ethylene glycol. 151 Again, sol–gels have been made by adding LiOH drop-wise to NiSO 4 solution until quite alkaline, 148,534,535 then peptising (i.e making colloidal) the resulting green precipitate with glacial acetic acid. Such precursors are often termed a ‘xerogel’, although the sols are not completely desiccated. d Additional water is added to ensure an appropriate viscosity prior to dipping. Conversely, an (uncharged) conducting electrode may be dipped alternately in solutions of aqueous NiSO 4 and either NaOH 563 or NH 4 OH. 560 In all cases, the precursor film on the electrode is heated to effect dehydration, chemical oxidation and crystallisation. Electrochromes are also reported 141,565,566,567 to have been made by spray pyrolysis, e.g. from a precursor of aqueous nickel chloride solution. 567 Chemical vapour deposition is not a popular route to forming NiO (1þy) , perhaps again owing to the need for annealing. Precursors include nickel acetylacetonate. 568 Finally, NiO films have been made by plasma oxidation of Ni–C composite films, previously deposited by co-evaporation of Ni and C from two different sources. 569 Redox electrochemistry ‘Hydrated nickel oxide’ (also called nickel ‘hydroxide’) is an anodically colouring electrochrome, the redox now differing in direction from that with the d A xerogel is defined by IUPAC as, ‘the dried out open structures which have passed a gel stage during preparation (e.g. silica gel).’ 6.2 Metal oxides: primary electrochromes 161 preceding metals. In acidic media, the electrode reaction for nickel oxide follows Eq. (6.13): Ni II O ð1ÀyÞ H z !½Ni II ð1ÀxÞ Ni III x O ð1ÀyÞ H ðzÀxÞ þxðH þ þe À Þ colourless brownÀblack Á (6.13) Nakaoka et al. 17 believe the coloured form is blue. Equation (6.13) is an amended formof the reaction in ref. 570. Furthermore, the sub-stoichiometric ‘NiO (1Ày) H z ’ is, in reality, Ni II (1Àx) Ni III O (1þy) H z . The values of y and z in Eq. (6.13) are unknown and likely to depend on the pH of the electrolyte solution. Proton egress from rf-sputtered NiO (1Ày) H z is more difficult than entry to the oxide. 571 The mechanism is different in alkaline solution: Murphy and Hutchins 572 cite the simplified reaction in Eq. (6.14), NiðOHÞ 2 ðsÞ þOH À ðaqÞ !NiOÁ OHðsÞ þe À þH 2 O. (6.14) Granqvist and Svensson believe that 15 N nuclear reaction analysis (see page 110) shows that coloration is accompanied by proton extraction. 253 Furthermore, Murphy and Hutchins 572 suggest that the following nickel species: Ni 3 O 4 , Ni 2 O 4 , Ni 2 O 3 and NiO 2 are all involved. In this analysis, the bleached state is Ni 3 O 4 and the coloured form is Ni 2 O 3 . Additionally, anodic coloration occurs in two distinct stages. 572 Chigane et al. 555 cite the involvement of: a-Ni(OH) 2 , g 2 -2NiO 2 –NiOÁ OH, b-Ni(OH) 2 and b-NiOÁ OH; Bouessay et al. 573 suggest that conversion of NiO into Ni(OH) 2 is a major cause of device degradation. The complex structures and phase changes occurring during the redox cycling of ‘nickel oxide’ were reviewed by Oliva et al. 574 in 1992. The problem of mass balance in thin-film ‘nickel oxide’ has been described in great detail by Bange and co-workers, 575 Co´ rdoba-Torresi et al., 11 Giron and Lampert, 39 Lampert, 576 Gorenstein and co-workers 577 and Granqvist and co-workers. 216,253 Svensson and Granqvist 253 conclude that the bleached state in a nickel oxide based display is b-NiOÁ OH, and the coloured state is b-Ni(OH) 2 . Conell et al. concur in this assignment. 86 They also suggest that only a minority of the film participates in the electrochromic reaction. Furthermore, the reduced form of the oxide contains a small amount of Ni III : a startling result. Some workers have detected Ni IV in the oxidised form of electrochromic NiO (1Ày) films. 85,572 Gorenstein, Scrosati and co-workers 578 suggest the electronic conductivities of the coloured and bleached states (which are said to differ dramatically) play a major role in the electrochromic process, although the rate of ion movement dictates the overall kinetic behaviour of nickel oxide based films. The kinetic 162 Metal oxides behaviour is described further by MacArthur 579 and by Arvia´ and co-workers 580 The mechanism is, not unusually, quite sensitive to the method of film preparation. As Granqvist et al. 216 say, incontrovertibly, Electrochromic nickel-oxide based films produced by different types of sputtering, evaporation, anodic oxidation and cathodic deposition, [and] thermal conversion all can have different optical, electrochemical and durability related properties, and therefore be more or less well suited for technical applications. Water trapped preferentially at defect and grain boundaries (which are numerous in NiO (1þy) 250 ) plays a crucial role in the electrochromic reaction. Water is formed as a product of NiO (1þy) H z degradation, the amount of water in the solid film increasing with cycle life. Its role is not beneficial, though, for it promotes chemical degradation. The efficiency of this electrochromic oxide, as prepared by rf sputtering, has been analysed in terms of microstructure, morphology and stoichiometry by Gorenstein and co-workers; 581 and Cordo´ ba-Torresi et al. 11 in support say that the presence of lattice defects is a prerequisite for electrochromic activity. Furthermore, they believe that neither Ni(OH) 2 nor NiOÁ OH are beneficial to device operation because of their solubility in water. 11 The tendency for water to cause deterioration is such that many workers now avoid water and hydroxide ions altogether, and prefer non-aqueous electrolytes. The reaction cited for electrochromic activity is then Eq. (6.15): NiO ð1þyÞ þxðLi þ þe À Þ !Li x NiO ð1þyÞ . brownÀblack colourless (6.15) the mobile Li þ ion most commonly coming from LiClO 4 dissolved in a polymeric electrolyte. 49 Even at quite low potentials, the rate of electrochromic coloration and bleaching is dictated by the rates of ionic movement. 578 Detailed measurements with the electrochemical EQCM suggest cation swapping, e.g. H þ being the first counter cation to enter the lattice, with subsequent insertion of Li þ . 43 Optical properties of nickel oxide electrochromes The electrochromic colour in NiO (1þy) undoubtedly derives from an Ni III / Ni II intervalence transition. Figure 6.7 shows absorption spectra of nickel oxide. 18 There are wide variations reported in the values of coloration efficiency j. For example, although j is said to be À36 cm 2 C À1 at 640 nm for nickel oxide made by rf sputtering, 542 the value depends strongly on the sputtering 6.2 Metal oxides: primary electrochromes 163 conditions. This value of j was cited for a film obtained at a total pressure of 8 Pa, of which gaseous hydrogen accounted for 40%. Other values of j are cited in Table 6.5; a value of À10 cm 2 C À1 is cited for thin-film lithium nickel oxide deposited by rf sputtering from a stoichiometric LiNiO 2 target. 252 Electrochromic devices containing nickel oxide electrochromes Films made by rf sputtering are significantly more durable than those made by electrodeposition: Conell cites 2500 and 500 write–erase cycles for the respective preparations. 86 Xu et al. suggest that 10 5 cycles are possible for dc magnetron sputtered samples. 211 Corrigan 82 reports that the durability can be improved to thousands of cycles by incorporating cobalt or lanthanum, but nevertheless, Ushio et al. 540 show that such sputtered NiO x degrades relatively easily. Coloration/bleaching times of electrodeposited films range between 20 and 40 s, and depend on the applied potential. 584 The speed of electrochromic operation often depends on so-called ‘terminal effects’ that arise because optically transparent conductive layers such as ITO have only modest electronic conductivities. Depositing an ultra-thin layer of metallic nickel between the ITO and NiO layers significantly improves the response time t. 585 200 0 0.5 1.0 1.5 400 Wavelength (nm) Abs o r ba n c e 600 800 Figure 6.7 UV-visible spectrum of reduced (Á Á Á Á) and oxidised (–––) forms of thin-film nickel oxide on ITO. The film was electrodeposited onto ITO with a thickness of about 1 mm. Electro-coloration was performed with the film immersed in 0.1 mol dm À3 KOH solution. (Figure reproduced from Carpenter, M. K. and Corrigan, D. A. ‘Photoelectrochemistry of nickel hydroxide thin films’. J. Electrochem. Soc., 136, 1989,1022–6, by permission of The Electrochemical Society, Inc.) 164 Metal oxides At present, much of the interest in nickel oxide electrochromes is focussed on their use as secondary electrochrome (i.e. not the main colourant) on the counter electrode, i.e. as redox reagent on the second electrode in an ECD cell where a primary electrochrome is redox reagent on the other electrode. Primary electrochromes so partnered could be WO 3 , 42,49,149,248, 254,310,431,544,548,586,587,588 or poly(pyrrole), 589 poly(thiophene) 590 or poly (methylthiophene). 590 However, in some prototype ECDs, NiO was the primary electrochrome on the one electrode while on the other, CuO, 591 MnO 592 or SnO 2 560 acted as secondary electrochrome. 6.3 Metal oxides: secondary electrochromes 6.3.1 Introduction As outlined in Section 1.1, while for the usual two-electrode ECD it would generally be advantageous that both electrodes bear strongly colourant electrochromes, final conditions may dictate that one electrode provides the major colourant (hence, bears the primary electrochrome). The counter electrode would bear a feebly colouring secondary electrochrome, or even a non-colouring (passive) redox couple, either of the latter being chosen simply for superior electrochemical properties, stability and durability. This chapter covers the latter classes of ‘electrochrome’. (‘‘Electrochrome’’ here is not a misnomer because as has been established in Section 1.1, even invisible _ IRand/or UV _ changes, that attend all redox reactions, are nowadays being deemed ‘electrochromic’.) Table 6.5. Sample values of coloration efficiency j for nickel oxide electrochromes. Preparative route j/cm 2 C À1 (`/nm) Ref. CVD (from a nickel acetylacetonate precursor) À44 568 dc sputtering À25 to 41 537 Dipping technique À35 564 Electrodeposition À20 78 Electrodeposition %À50 (450) 582 Electrodeposition À24 (670) 560 rf sputtering À36 542 Sol–gel (NiSO 4 , PVA and formamide) À35 to 40 (450) 152 Sol–gel (NiSO 4 , glycerol, PVA and formamide) À23.5 (450) 583 Sonicated solution À80.3 (457) 107 Spray pyrolysis À37 565 Spray pyrolysis À30 566 Vacuum evaporation À32 (670) 551 6.3 Metal oxides: secondary electrochromes 165 However, this chapter covers only visible-wavelength ECD applications, so the materials encompassed are chosen largely just to complete the electrochemical cell that operates as an ECD by depending on the primary-electrochrome process. Secondary electrochromes Bismuth oxide An electrochromic bismuth oxide formed by sputtering or vacuum evaporation was studied by Shimanoe et al. 593 The best electrochromic performance was observed for a sputtered oxide annealed at 300–4008C in air for 30min. Films showed an electrochromic transition when immersed in LiClO 4 –propylene carbonate electrolyte, Eq. (6.16): Bi 2 O 3 þxðLi þ þe À Þ !Li x Bi 2 O 3 . transparent darkbrown (6.16) Bleaching ocurred at þ1.2 V and coloration at À2.0 V vs. SCE. The response time either way was about 10 s, with coloration efficiency j of 3.7 cm 2 C À1 . Bismuth oxide has also been co-deposited with other oxides. 594 Cerium oxide Preparation of cerium oxide Thin-film CeO 2 can be prepared by spray pyrolysis via spraying aqueous cerium chloride (CeCl 3 Á 7H 2 O) onto ITO. 137 Films prepared at temperatures below about 300 8C were amorphous, while those prepared at higher temperatures have a cubic (‘cerianite’) crystal structure. ¨ Ozer et al. 184,595 made ceriumoxide films on fluoride-doped SnO 2 electrodes using a sol–gel procedure. The precursor derived from cerium ammonium nitrate in ethanol, with diethanolamine as a complexing agent. They recommend annealing at 450 8C or higher. Spectroelectrochemistry showed that these films were optically passive, and therefore ideal as counter electrodes in transmissive ECDs. Porqueras et al. 596 deposited the oxide by electron-beam PVD (physical vapour deposition) on various substrates, such as glass, ITO-coated glass, Si wafers and fused silica. The substrate temperature was maintained at 125 8C. In contrast, ion-bombarded films show a denser structure and a different layer growth. 597 The utility of cerium oxide derives from its near optical passivity. The redox reaction follows Eq. (6.17): 166 Metal oxides CeO 2 þxðLi þ þe À Þ !Li x CeO 2 . (6.17) Both redox states are essentially colourless in the visible region. Porqueras claims that films on ITO remain ‘fully transparent after’ Li þ insertion and egress. 137 Cerium oxide is therefore not electrochromic, but is a widely-used choice of counter electrode material. 137,184,595,596,597 It is also widely used as a matrix in which other, electrochromic, oxides are dispersed. These mixed- metal oxide electrochromes are described in Section 6.4. Chromium oxide The electrochromism of chromium oxide has received little attention. The properties of a sputtered oxide are described as ‘only slightly inferior to those of Ni oxide and with good stability in acidic electrolytes’. 598 The composition of the material is nowhere mentioned; the sputtered materials made by Cogan et al. 599 are said to be similar, and are called ‘lithium chromate’. In a fundamental study, Azens, Granqvist and co-workers 598 immersed films made by rf sputtering in aqueous H 3 PO 4 . The electrochromic colour did not vary by more than 10% during redox cycling, making it almost optically ‘passive’. Alternatively, thin films of chromium oxide, identified only as ‘CrO y ’, can be formed by electron-beam evaporation of Cr 2 O 3 . 231 The electrochromic operation was studied with films immersed in g-butyrolactone containing LiClO 4 . Chromium oxide has been studied extensively for battery applications, 600,601,602 with the redox reaction Eq. (6.18): Cr 2 O 3 ðsÞ þxðLi þ þe À Þ !Li x Cr 2 O 3 ðsÞ. (6.18) Chromium oxide allows device operation with a lower voltage than do most other electrochromic oxides. 603 The only coloration efficiency available is that for vacuum-evaporated material, for which j is À4 cm 2 C À1 . 231 Cobalt oxide Preparation of cobalt oxide electrochromes Thin-film LiCoO 2 is made by rf sputtering from a target of LiCoO 2 , and is polycrystalline. Because the as-deposited films are lithium deficient, 236,237,238 such nominal ‘LiCoO 2 ’ shows significant absorption at `<600 nm; Goldner et al. 238 state that films can be coloured electrochemically, but will not decolour completely. Controlling the 6.3 Metal oxides: secondary electrochromes 167 amount of lithiumwithin films of rf-sputtered lithiumcobalt oxide is, however, difficult. 238 Other vacuum methods such as CVD generate thin films of metallic cobalt as initial layer, which is converted to CoO by being annealed in an oxidising atmosphere. Chemical vapour deposition precursors include Co(acetylacetonate) 2 . 604 Electrochemical studies of anodically generated layers of oxide on metallic cobalt, 605,606 for example, of pure cobalt metal anodised in a solution of aqueous 1 molar NaOH or a solution buffered to pH 7, show the films to be blue, 605 but the colour soon changes to brown on standing, 607 owing to atmospheric oxidation. Electrodeposited oxyhydroxide, CoOÁ OH, 84,608 may be electrodeposited on Pt or ITO from an aqueous solution of Co(NO 3 ) 2 via Eqs. (6.3) and (6.4). Subsequent thermal annealing converts most of the oxyhydroxide to oxide CoO, but some CoOÁ OH persists. 103,608 For this reason, such ‘cobalt oxide’ is sometimes written as CoO x or, better, CoO (1þy) . This CoO (1þy) has a pale green colour owing to a slight stoichiometric excess of oxide ion, causing a weak charge-transfer transition from O 2À to the Co 2þ ion. 609 Gorenstein et al. 608 suggest the as-grown film may be Co(OH) 3 , unlikely in our view owing to the strongly oxidising nature of Co III . As with W and Mo, Co metal can be dissolved oxidatively in H 2 O 2 , 80,81 to form the peroxo anion for use in sol–gel or electrodeposition procedures. Cobalt oxide can also be deposited from a Co II –(tartrate) complex via Co II (OH) 2 in aqueous sodium carbonate. 100 Thin-film cobalt oxide can be made by spray pyrolysis in oxygen of aqueous CoCl 2 solutions 139 onto e.g. fluorine-doped tin oxide (FTO) coatings on glass substrates. These films change electrochemically fromgrey to pale yellow, with a response time of 2 to 4 s. Alternatively, sols of Co 3 O 4 have been applied to an electrode substrate by both dipping and spraying. 610 Redox chemistry of cobalt oxide electrochromes Equation (6.19) is the supposed electrochromic reaction of cobalt oxide grown anodically in aqueous electrolytes on cobalt metal: 605,606,607 3Co II OðsÞ þ2OH À ðsoln.Þ !Co II.III 3 O 4 ðsÞ þ2e À þH 2 O. pale yellow dark brown (6.19) The Co 3 O 4 product would formally be Co II OþCo 2 III O 3 (cf. magnetite, the iron equivalent). The colour of the brown form is probably due to a mixed-valence charge-transfer transition in the Co 3 O 4 , although the identity of the Co III 168 Metal oxides oxide(s) formed by oxidation of Co(OH) 2 could not be assigned conclusively by FTIR. 611 Reference 611 cites IR data for all the known oxides of cobalt including those above, together with CoO and CoOÁ OH. In non-aqueous solutions, e.g. LiClO 4 in propylene carbonate, oxidation of sputtered LiCoO 2 electrochrome results in an electrochromic colour change from effectively transparent to dark brown. The electrochromic reaction is Eq. (6.20): LiCoO 2 þxðM þ þe À Þ !M x LiCoO 2 . pale yellowÀbrown dark brown (6.20) where M þ is generally Li þ , when the rate-limiting process during coloration and bleaching is the movement of the Li þ counter ion. 612 The study by Pyun et al. 613 clearly demonstrates the complexity of the charge-transfer process(es) across the oxide–electrolyte interphase. For the novel green product formed by reductive electrolysis of nitrate ion, the electrochromic transition is green ! brown, in the electrochromic reaction 80,81 in Eq. (6.21): 3CoOþ2OH À !Co 3 O 4 þ2e À þH 2 O. pale green brown (6.21) Optical properties of cobalt oxide electrochromes Figure 6.8 shows UV-visible spectra of electrodeposited CoO (pale green) and Co 3 O 4 (dark brown), 81 and Figure 6.9 shows a coloration-efficiency plot of absorbance against charge passed 614 Q. This figure demonstrates how absorbance is generally not proportional to Q, since the graph is only linear for addition of small-to-medium amounts of inserted charge. Table 6.6 cites representative values of coloration coefficient j. Behl and Toni 618 find that many electrochromic colours may be achieved in films generated on metallic cobalt, presumably from varying oxide–hydroxide compositions, accompanied by composition-dependent CT or intervalence absorptions. Colours include white, pink, brown and black, confirming Benson et al.’s views. 619 Below 1.47 V vs. SCE, films are orange (or yellow– brown) but above this potential the films become dark brown (or even black if films are thick). The orange form of the oxide may also contain hydrated Co(OH) 2 following H 2 O uptake; on Co metal anodised in NaOH (0.1 or 1.0 mol dm À3 ) this oxide is predominantly the low-valence product, as demonstrated by FTIR. 611 6.3 Metal oxides: secondary electrochromes 169 Films made by spray pyrolysis from CoCl 2 solution exhibited anodic electrochromism, changing colour from grey to pale yellow. 139 Electrochromic devices containing cobalt oxide electrochromes Cobalt oxide is usually employed as a secondary electrochrome (on the counter electrode) against a more strongly colouring primary electrochrome on the major colourant electrode comprising e.g. WO 3 . 248 Copper oxide Preparation of copper oxide electrochromes ¨ Ozer and Tepehan 620,621 prepared a copper oxide electrochrome from sol–gel precursors, hydrolysing copper ethoxide, then annealing in an oxidising atmosphere. Ray 622 prepared a different sol–gel precursor via copper chloride in methanol, yielding films of either CuO or Cu 2 O, the product depending on the annealing conditions. 0 20 40 60 80 100 300 500 700 900 1100 % T Q = 0 (as grown film) 3.3 4.0 5.3 5.9 6.8 mC cm –2 λ /nm Figure 6.8 UV-visible spectra of thin-film cobalt oxide electrodeposited onto ITO. The figures above each trace represent the charge passed in mCcm À2 , beginning with the most coloured state at the bottom of the figure, and progressively bleaching. (Figure reproduced from Polo da Fontescu, C. N., De Paoli, M.-A. and Gorenstein, A. ‘The electrochromic effect in cobalt oxide thin films’. Adv. Mater., 3, 1991, 553–5, with permission of Wiley–VCH.) 170 Metal oxides Richardson et al. 591,623 have made transparent films of Cu 2 O on conductive SnO 2 :F (FTO) substrates by anodic oxidation of sputtered copper films, or by electrodeposition. The electrochromic transition is colourless to pale brown but, apparently, neither redox state has yet been identified. ¨ Ozer and Tepehan 620 call their 0.0 5 0 0.1 0.2 0.3 (a) Electrochromic efficiency = 24 cm 2 C –1 Charge density /mC cm –2 10 15 + + + + + + + + + + + + + + x x x x x x x x x x j=–0.08 mAcm –2 j=–0.38 mAcm –2 j=–0.76 mAcm –2 j=–1.14 mAcm –2 + x – Δ O D (b) Electrochromic efficiency =27 cm 2 C –1 24 cm 2 mC –1 j=–0.08 mA cm –2 j=–0.38 mA cm –2 j=–0.76 mA cm –2 j=–1.14 mA cm –2 + + + + + + + + + + + + + + + + + + x x x x x x x x x x x x x – Δ O D 0.0 5 0 0.1 0.2 0.3 Charge density /mC cm –2 10 15 Figure 6.9 Coloration-efficiency plot of absorbance (ÀDOD) against charge passed Q for thin-film CoO electrodeposited onto ITO, and immersed in NaOH solution (0.1 mol dm À3 ): (a) during coloration and (b) during bleaching. The current density i during coloration was * ¼0.08 mAcm À2 ; x ¼0.38 mAcm À2 ; þ¼0.76 mAcm À2 ; o ¼1.14 mAcm À2 . The wavelength at which Abs was determined is not known. (Figure reproduced from Polo da Fontescu, C. N., De Paoli, M.-A. and Gorenstein, A. ‘The electrochromic effect in cobalt oxide thin films’. Adv. Mater., 3, 1991, 553–5, with permission of Wiley–VCH.) 6.3 Metal oxides: secondary electrochromes 171 electrochrome ‘Cu w O’. The response time and optical properties of this electrochrome depend markedly on the temperature and duration of post- deposition annealing. Redox chemistry of copper oxide electrochromes The Cu 2 O films transform reversibly to black CuO at more anodic potentials. 622 In alkaline solution, a suggested redox reaction is Eq. (6.22): 2CuOðsÞ þ2e À þ2H 2 O !Cu 2 OðsÞ þ2OH À . black red-brown (6.22) In acidic electrolytes, 622 Cu 2 O is transformed reversibly to opaque and highly reflective copper metal, according to Eq. (6.23): Cu 2 OðsÞ þ2e À þ2H þ !2CuðsÞ þH 2 O. (6.23) The cycle life of such electrochromic materials is said to be poor at ca. 20–100 cycles 591 owing to the large increase in molar volume of about 65% during conversion fromCu to Cu II , Eq. (6.23). Alarge change in optical transmittance is claimed, from 85 to 10% transmittance. The coloration efficiency is about 32 cm 2 C À1 . 591 However, the usefulness in ECDs is virtually zero unless display applications are found, and this entry merely records an EC electrochemistry. Iron oxide Yellow–green films of iron oxide form on the surface of an iron electrode anodised in 0.1 M NaOH. 624,625,626 Such films display significant electrochromism. For successful film growth, the pH must exceed 9, and the temperature Table 6.6. Sample values of coloration efficiency j for cobalt oxide electrochromes Preparative route j/cm 2 C À1 (` (obs) /nm) Ref. CVD a 21.5 604 Electrodeposited 24 614, 615 Sol–gel 25 616 Sonicated solution 130 107 Spray pyrolysis 12 (633 nm) b 139 Thermal evaporation 20–27 617 a The precursor was Co(acetylacetonate) 2 . b Figure in parenthesis is ` max . 172 Metal oxides lower than 80 8C. This coloured material may be hydrated Fe III OÁ OH; the film becomes transparent at cathodic potentials as hydrated Fe(OH) 2 is formed, so the electrochromic reaction is Eq. (6.24): Fe III OÁ OHðsÞ þe À þH 2 O !Fe II ðOHÞ 2 ðsÞ þOH À ðsolnÞ. yellowÀgreen transparent (6.24) Gutie´ rrez and Beden use differential reflectance spectroscopy to show that iron oxyhydroxide underlies the electrochromic effect. 624 These films are prone to slight electrochemical irreversibility owing to a surface layer of anhydrous FeO or Fe(OH) 2 , which may preclude their use as ECD electrochromes. 625 The oxides g-Fe 2 O 3 (maghemite) and a-Fe 2 O 3 (hematite) are also formed in the passivating layer. 624 Thin films of Fe 2 O 3 may be formed by electro-oxidation of Fe(ClO 4 ) 2 in MeCN solution. 104 This oxide is amorphous, the polycrystalline analogue being formed by annealing at high-temperature; polycrystalline Fe 2 O 3 is essentially electro-inert. During electrochromic reactions, the first reduction product is Fe 3 O 4 , according to Eq. (6.25): 3Fe 2 O 3 ðsÞ þ2H þ ðsolnÞ þe À !2Fe 3 O 4 ðsÞ þH 2 O. brown black (6.25) This black Fe 3 O 4 contains the mixed-valence oxide formally FeOÁ Fe 2 O 3 , magnetite. Such Fe 3 O 4 can be further reduced to form a colourless oxide, FeO – Eq. (6.26): Fe 3 O 4 ðsÞ þ2H þ ðsolnÞ þ2e À !3FeOðsÞ þH 2 O. black colourless (6.26) Electrochromic Fe 2 O 3 was made by ¨ Ozer and Tepehan 627 from a sol of the iron alkoxide Fe(O i Pr) 3 . After annealing, the Fe 2 O 3 was immersed in LiClO 4 –PC solution. 627 The electrochromic reaction, formally Eq. (6.27), showed good electro-reversibility. Fe 2 O 3 ðsÞ þxðLi þ þe À Þ !Li x Fe 2 O 3 ðsÞ. pale brown black (6.27) The product may be thought of as mixed-valence Fe 3 O 4 , the lithium counter ion being incorporated for charge balancing during reaction. The source of the lithium ion is LiClO 4 in PC; the lithium insertion reaction here is wholly reversible. 627 Such Li x Fe 2 O 3 is of good optical quality, although the coloured 6.3 Metal oxides: secondary electrochromes 173 films were insufficiently intense to consider their use as a primary electrochrome, but counter-electrode use is suggested. Other sol–gel precursors have yielded electrochromic iron oxide films. Electrochromic films were made from a gel prepared by raising the pH of aqueous ferric chloride during addition of ammonium hydroxide, then homogenising the resultant precipitate with ethanoic acid to form a sol. 154 A dip- coating procedure, repeatedly immersing an electrode in the precursor solution and then annealing, yields Fe 2 O 3 which bleaches cathodically and colours anodically in lithium-containing electrolytes of aqueous 10 À3 mol dm À3 LiOH. Similar but inferior electrochromic activity was seen when the film was immersed in NaOH or KOH of the same concentration: 154 Na þ and K þ cations were presumably too large to enter the lattice readily. Spin coating a further sol–gel film, based on iron pentoxide in propanol, yields Fe 2 O 3 after firing at 180 8C; 628 Li þ insertion into this oxide is fully reversible. Figure 6.10 shows the electronic spectra. Iron(acetylacetonate) 2 is a suitable CVD precursor for iron oxide electrochromes. A thin film of metallic iron is formed first, which yields an 100 80 60 40 20 300 400 500 Wavelength (nm) T r a n s m i t t a n c e ( % ) 600 700 Bleached Coloured 800 Figure 6.10 Transmittance spectrumof thin-filmiron oxide Fe 2 O 3 formed by spin-coated sol–gel onto an ITO electrode. The coloured form was generated at À2.0 V, and the bleached form at þ0.5 V. (Figure reproduced from ¨ Ozer, N. and Tepehan, F. ‘Optical and electrochemical characteristics of sol–gel deposited iron oxide films’. Sol. Energy Mater. Sol. Cells, 56, 1999, 141–52, by permission of Elsevier Science.) 174 Metal oxides electrochromic oxide after annealing. 629 The coloured form is Fe 2 O 3 , so the redox reaction is that given in Eq. (6.28): 2 FeOðsÞ þH 2 O !Fe 2 O 3 ðsÞ þ2e À þ2 H þ ðsolnÞ. colourless brown (6.28) Optical properties of iron oxide electrochromes Values of j are relatively rare for this electrochrome; see Table 6.7. Manganese oxide Preparation of manganese oxide electrochromes Anodising metallic manganese in base (alkali) yields a thin surface film of electrochromic oxide. 605 Films of electrochromic MnO 2 can also be formed by reductive electrodeposition from aqueous MnSO 4 , 630,631,632 the oxide originating from H 2 O. A sol–gel precursor, prepared by adding fumaric acid to sodium permanganate, can yield MnO 2 films. This electrochrome contains some immobile sodium ions, and has been formulated as Na c MnO 2 .nH 2 O. 633 Films can also be formed by rf sputtering, 246,247 while electron-beam evaporation yields an electron-deficient oxide, denoted here as MnO (2Ày) . 231 Redox chemistry of manganese oxide electrochromes The electrochromic mechanismof MnO 2 grown on Mn metal is complicated. In aqueous solutions, electrochromic coloration involves hydroxide expulsion when solutions are alkaline, 634 according to Eq. (6.29): 2MnO 2 ðsÞ þH 2 Oþe À !Mn 2 O 3 ðsÞ þ2OH À ðaqÞ. dark brown pale yellow (6.29) The colours stated are for thin films; the electrochrome is black in thick films. The colourless form may comprise some hydrated hydroxide Mn(OH) 3 or oxyhydroxide, MnOÁ OH. The couple responsible for the electrochromic Table 6.7. Sample values of coloration efficiency j for iron oxide electrochromes. Preparative route j/cm 2 C À1 Ref. CVD À6.0 to 6.5 629 Sol–gel À28 627 Electrodeposition À30 104 6.3 Metal oxides: secondary electrochromes 175 transition is probably MnO 2 –MnOÁ OH, 634 which is confirmed by XPS spectroscopy. 635 If the pH is low, coloration proceeds in accompaniment with proton uptake according to Eq. (6.30): Mn IV O 2 ðsÞ þxðH þ þe À Þ !Mn III.IV O ð2ÀxÞ ðOHÞ x ðsÞ. (6.30) The redox reactions of manganese dioxide in non-aqueous electrolytes are straightforward, and generally involve the insertion and extraction of Li þ , e.g. from LiClO 4 in PC via Eq. (6.31): MnO 2 ðsÞ þxðLi þ þe À Þ !Li x MnO 2 ðsÞ. brown yellow (6.31) X-Ray photoelectron spectroscopy suggested that hydrated MnO 2 represents the composition in the oxidised state. 592 The redox process in Eq. (6.31) is better understood than for many other electrochromes, since MnO 2 is the vital component in many rechargeable and alkaline batteries. 247 The electrochromic operation of MnO 2 films made from sol–gel precursors is saidto performbest when immersedin aqueous base. 633 The films are very stable and are said to showhigh write–erase efficiencies in this electrolyte. Lithium ion can also be inserted from aqueous solution into sputtered MnO 2 . 246 Optical properties of manganese oxide electrochromes Figure 6.11 shows the spectrum of sputter-deposited MnO 2 . Sol–gel drived electrochromic MnO 2 follows Beer’s law fairly closely 633 on electro-inserting Li þ from LiClO 4 –PC solution. A plot of Abs against x for Eq. (6.31) is linear, with a coloration efficiency of 12 to 14 cm 2 C À1 , depending slightly on preparation conditions. 633 The value of j for thin-filmLi x MnO (2Ày) made by electron-beam evaporation is 7.2 cm 2 C À1 . 231 Electrochromic efficiencies as high as 130 cm 2 C À1 have been reported for MnO y films in aqueous borate buffer solution. 631 Electrochromic devices containing manganese oxide electrochromes Manganese oxide has been suggested as a counter electrode (or secondary electrochrome) since its coloration efficiency j is relatively low. 592 A device has been made by Ma et al. 636 in which the primary electrochrome was nickel oxide. Niobium oxide Preparation of niobium oxide electrochromes Sol–gel methods are now the most widely used procedure for forming electrochromic Nb 2 O 5 films, for 176 Metal oxides example by hydrolysing niobium alkoxides. 637,638 Precursors include ethoxide, 191 butoxide 155 or pentachloride 639,640,641 salts. Chloralkoxide sols of the type NbCl x (OEt) 5Àx , formed by mixing NbCl 5 and anhydrous ethanol, 123,170,642 are also used. Hydrolysis yields the solid oxide, Eq. (6.32): 2NbCl x ðOEtÞ 5Àx ðaqÞ þ5H 2 O !Nb 2 O 5 ðsÞ þ2ð5 ÀxÞEtOHþ2xHCl ðaqÞ. (6.32) The gel is then spin coated. Such films are ‘slightly crystalline’ 192 since they require high-temperature annealing, between 560 and 600 8C. 168 Niobium pentoxide films annealed at temperatures below 450 8C are said to be still amorphous. 643 Films of Nb 2 O 5 have also been prepared by anodising Nb metal, for example by redox cycling Nb metal in dilute aqueous acid. 644,645,646 An electrochromic layer of Nb 2 O 5 can also be prepared on niobium metal by thermal oxidation. 647,648 Direct-current (dc) magnetron sputtering is only occasionally used in preparations of Nb 2 O 5 . 192,217,218 Lampert and co-workers, 192 comparing the properties of films prepared by dc-magnetron sputtering and by the spin coating of gels subsequently annealed, found that the films were electrochromically essentially equivalent. Redox electrochemistry of niobium pentoxide electrochromes The accepted redox reaction describing the process of Nb 2 O 5 coloration is Eq. (6.33): 0 1 2 SnO 2 /MnO 2 /Bor ate 0.1M 0.8 V 0.8 0.0 E t 0.4 V 0.2 V 0.0 V O p t i c a l de n s i t y Wavelength/nm 300 400 500 600 700 800 Figure 6.11 UV-visible spectrum of sputter-deposited thin-film manganese oxide at a variety of potentials (vs. SCE, as indicated on the figure). The oxide film was electrodeposited onto a SnO 2 -coated optical electrode, and analysed while immersed in a borate electrolyte at pH¼9.2. (Figure reproduced from Co´ rdoba de Torresi, S. I. and Gorenstein, A. ‘Electrochromic behaviour of manganese dioxide electrodes in slightly alkaline solutions.’ Electrochim. Acta, 37, 1992, 2015–19, with permission of Elsevier Science.) 6.3 Metal oxides: secondary electrochromes 177 Nb 2 O 5 ðsÞ þxðM þ þe À Þ !M x Nb 2 O 5 ðsÞ. colourless blue (6.33) where M þ is generally Li þ . The response time of Nb 2 O 5 grown on Nb metal in aqueous 1 M H 2 SO 4 is said to be less than 1 s. 644 The cycle life of crystalline sol–gel-derived films is cited variously as ‘up to 2000 voltammetry cycles between 2 and À1.8 V’ 168 and ‘beyond 1200 cycles without change in performance’. 191 Films of sol–gel-derived Nb 2 O 5 are superior if they are made to contain up to about 20 mole per cent of lithium oxide. 637 Firstly they can accommodate a larger charge (see the cyclic voltammograms in Figure 6.12); secondly, they do not degrade so fast, and thirdly, they can be decoloured completely, whereas sputtered Nb 2 O 5 films retain some slight residual coloration. Optical properties of niobium pentoxide electrochromes Thin films of niobium oxide are transparent and essentially colourless when fully oxidised, and present a deep blue colour on Li þ ion insertion. 168 Some sol–gel-derived films of Nb 2 O 5 also form a brown colour between the tonal extremes of colourless and blue. 170 Figure 6.13 depicts spectra of Nb 2 O 5 and Li x Nb 2 O 5 . The coloration efficiencies of niobium oxide electrochromes are listed in Table 6.8. Use of niobium oxide electrochromes in devices Owing to its low coloration efficiency, Nb 2 O 5 has been used as a ‘passive’ counter electrode, generally with WO 3 433 as primary electrochrome. Table 6.8. Coloration efficiencies j of niobium oxide electrochromes. Preparative procedure j/cm 2 C À1 (` (obs) /nm) Ref. rf sputtering 5 258 rf sputtering 10 649 rf sputtering 100 401 Sol–gel 22 (600) 170, 172, 650 Sol–gel 28 (550) 171 Sol–gel 38 (700) 405 Spraying a 6 (800) 641 a NbCl 5 in ethanol 178 Metal oxides Palladium oxide Amongst the few studies of electrochromic PdO 2 , the most extensive, by Bolza´ n and Arvia, 651 concerns hydrated PdO 2 (prepared by anodising Pd metal in acidic solution), revealing some redox complexity. The coloured (black) form is hydrated PdO, hydrated PdO 2 is yellow, while anhydrous PdO 2 is reddish brown. This electrochemical complexity, coupled with high cost, means that palladium electrochromes are unlikely to be viable. Praseodymium oxide Electrochromic praseodymium oxide was studied by Granqvist and co-workers 219 who made thin-film PrO 2 by dc-magnetron sputtering, varying the ratio of O 2 to argon from 0.025 to 0.005. Thomas and Owen 652 used CVD from a metallo-organic precursor. The electrochromic reaction is 652 Eq. (6.34): PrO ð2ÀyÞ ðsÞ þxðLi þ þe À Þ !Li x PrO ð2ÀyÞ ðsÞ. dark orange colourless (6.34) Films of electrochromic oxide switch in colour from dark orange (presumably PrO 2 -like) to transparent. X-Ray diffraction of the CVD-derived samples suggest that the first lithiuminsertion cycle was accompanied by an irreversible (a) (b) (i) (i) (iii) (iii) (ii) (ii) 0.0 0.5 –0.5 –1.0 –1500 –1000 –500 500 0 (i) Undoped E/mVvs . Ag 0.0 0.5 –0.5 –1.0 –1500 –1000 –500 500 0 E/mVvs . Ag i /m A c m –2 i /m A c m –2 (ii) 10 mol% Li doped (iii) 20 mol% Li doped (i) Undoped (ii) 10 mol% Li doped (iii) 20 mol% Li doped Figure 6.12 The effect of cycle number on the cyclic voltammogram of thin- film Nb 2 O 5 , deposited onto ITO by a sol–gel process. (a) The first cycle and (b) the twenty-first cycle. During redox cycling, the film was immersed in propylene carbonate solution itself comprising LiClO 4 (0.1 mol dm À3 ). Note also the higher charge capacity of the lithium-containing films. (Figure reproduced from Bueno, P. R., Avellaneda, C. O., Faria, R. C. and Bulho˜ es, L. O. S. ‘Electrochromic properties of undoped and lithium doped Nb 2 O 5 films prepared by the sol–gel method’. Electrochimica Acta, 46, 2001, 2113–18, by permission of Elsevier Science.) 6.3 Metal oxides: secondary electrochromes 179 phase change. 652 Thereafter, provided the switching was relatively fast and that the film was not left in the reduced state for long periods, the charge insertion was reversible over 500 cycles. The charge capacity ranged from comparability with that of WO 3 , for oxygen-rich films, to virtually zero for oxygen-depleted films. 652 The initially dark films made by sputtering showed strong anodic electrochromism. In a device incorporating WO 3 as the primary electrochrome, the use of PrO 2 as the secondary layer made the colour more ‘neutral’ (i.e. more grey). Praseodymiumfilms do not promise wide usage, but PrO 2 has been added to films of cerium oxide; 653 see p. 193. (b) (a) 0.6 0.5 0.4 0.3 Abs o r ba n c e ( r e l . u n i t s ) 0.2 0.1 0.0 400 450 500 550 600 λ / nm 650 700 750 800 Figure 6.13 UV-visible spectrum of thin-film niobium pentoxide on ITO. The spectrum(a) of the reduced format À0.875 Vand (b) of the oxidised form was obtained at 0 V against SCE. The film was prepared by a sol–gel method and had a thickness of ca. 5 mm. The electro-coloration was performed in 1.0 mol dm À3 H 2 SO 4 solution. (Figure reproduced from Lee, G. R. and Crayston, J. A. ‘Electrochromic Nb 2 O 5 and Nb 2 O 5 /silicone composite thin films prepared by sol–gel processing’. J. Mater. Chem., 1, 1991, 381–6, by permission of The Royal Society of Chemistry.) 180 Metal oxides Rhodium oxide Electrochromic rhodiumoxide has been little studied. Films may be formed on Rh metal by anodising metallic rhodium immersed in concentrated solution of alkali. 654,655 It can also be made from sol–gel precursors. 656 In an early study, Gottesfeld 657 cites the electrochromic reaction Eq. (6.35): Rh 2 O 3 ðsÞ þ2OH À ðaqÞ !2 RhO 2 ðsÞ þH 2 Oþ2e À . yellow dark green (6.35) Both the rhodium oxides in Eq. (6.35) are hydrated, Rh 2 O 3 probably more so than RhO 2 . Dark-green RhO 2 appears black if films are sufficiently thick. A fully colourless state is not attainable. The oxide RhO 2 is unusual in being green; the only other inorganic electrochromes evincing this colour are Prussian green (a mixed-valence species of partly oxidised Prussian blue), and electrodeposited cobalt oxide; see p. 168. Rhodium oxide made by a sol–gel procedure switched from bright yellow to olive green. 656 Such films are polycrystalline, owing to annealing after deposition. The coloration efficiency at 700 nm was 29 cm 2 C À1 . Figure 6.14 shows a cyclic voltammogram of rhodium oxide; 657 reflectance and charge insertion are also shown as a function of potential. Ruthenium oxide Thin films of hydrous ruthenium oxide can be prepared by repeated cyclic voltammetry on ITO-coated glass substrates immersed in an aqueous solution of ruthenium chloride. 658 Films may also be generated by anodising metallic Ru in alkaline solution. 659 The oxide changes colour electrochemically 659 according to Eq. (6.36): RuO 2 Á 2H 2 OðsÞ þH 2 Oþe À ! 1 , 2 ðRu 2 O 3 Á 5H 2 OÞ ðsÞ þOH À . blueÀbrown black (6.36) The electrogenerated colour is not intense. The ruthenium oxide electrode exhibits a 50% modulation of optical transmittance at 670 nm wavelength. 658 Tantalum oxide Preparation of tantalum oxide electrochromes Few electrochromism studies have been performed on tantalumoxide Ta 2 O 5 , but it has been used sometimes as a layer of ion-conductive electrolyte. 74,173,220,257,259,260,468,660,661,662,663, 664,665,666 6.3 Metal oxides: secondary electrochromes 181 Thin films may be prepared by anodising Ta metal in sulfuric acid 662,667,668 or thermal oxidation of sputtered Ta metal. 666 Other films have been made by rf sputtering from a target of Ta 2 O 5 , 257,258,259,260 reactive dc sputtering 220 or thermal evaporation. 220 The most widely used tantalum CVD precursors of Ta 2 O 5 are Ta(OEt) 5 , 72,74,173,663 TaCl 5 74 or TaI 5 , 74 each volatilised in an oxygen-rich atmosphere. Carbon or halide impurities are however incorporated into the resultant films. Otherwise, solutions of the supposed peroxypolytantalate may be spin coated onto ITO, and then sintered; this solute is prepared by reactive dissolution in H 2 O 2 of either Ta 93 or Ta(OEt) 5 . 193 Thin-film Ta 2 O 5 can also be formed by dip coating using a liquor rich in Ta(OEt) 5 as the precursor. An electrode substrate is repeatedly dipped into the liquor, or slowly immersed and withdrawn at a predetermined rate. 12 8 4 0 i ( m A c m –2 ) R / R o ( % ) Q ( m C c m –2 ) –4 –8 – 12 0.6 25 100 75 50 25 20 15 10 5 0.8 1.0 E (Vvs . RHE) 1.2 1.4 Rhodium,1 M KOH 150 mVs –1 Figure 6.14 Cyclic voltammogram of rhodium oxide grown on an electrode of metallic rhodium, immersed in hydroxide solution (5 mol dm À3 KOH). Also included on the figure are the reflectance at 546 nm (– . – . ) and charge inserted (– – –) as a function of potential. The scan rate was 150 mV s À1 . (Figure reproduced from Gottesfeld, S. ‘The anodic rhodium oxide film: a two-colour electrochromic system’. J. Electrochem. Soc., 127, 1980, 272–7, by permission of The Electrochemical Society, Inc.). 182 Metal oxides Redox chemistry of tantalum oxide electrochromes The electrochromic reaction of thin-film Ta 2 O 5 in aqueous alkali is Eq. (6.37): Ta V 2 O 5 ðsÞ þH 2 Oþe À !2 Ta IV O 2 ðsÞ þ2OH À ðaqÞ. colourless very pale blue (6.37) While the kinetics here have been little studied, the kinetics of charge transport are dominated by movement of polaron species. 3 Garikepati and Xue, 665 studying the dynamics of charge movement (comprising proton conductance) across the Ta 2 O 5 –WO 3 interphase, found the rate of proton movement was dictated by water adsorbed within the interphase. While in the studies of Ahn et al. 664 on the interface comprising Ta 2 O 5 and NiO or Ni(OH) 2 , the authors do mention the effects of such adsorbed water on the rate of ionic movement across the interphase. However, they conclude that the rate is dictated by the extent to which the crystal structures of the oxides making the interface are complementary, i.e. how well structurally the oxides join. The conductivity of protons through Ta 2 O 5 is so fast that it is often classed as a ‘fast ion conductor’. 669 Accordingly, workers are increasingly choosing to employ thin-film Ta 2 O 5 as the ion-conductive electrolyte layer between the solid layers of primary and secondary electrochrome in all-solid-state devices. 220,257,259,260,660,663,670,671,672,673 Optical properties of tantalum oxide electrochromes The value of ` max for Ta 2 O 5 made by anodised tantalum metal is 541 nm, 674 but the electrochromic effect is weak. For example, films made by rf sputtering have j values as low as 5 cm 2 C À1 , 206 while material made by laser ablation has j of 10 cm 2 C À1 . 675 The Ta 2 O 5 films exhibit high transmittance except in the UV, where the films absorb strongly. Tin oxide In the few studies on the electrochromism of tin oxide, Eq. 6.38: SnO 2 ðsÞ þxðLi þ þe À Þ !Li x SnO 2 ðsÞ. colourless blueÀgrey (6.38) the tin(IV) oxide films were made by reactive rf-magnetron sputtering. 676 The films are conductive, by both electrons and ions. The wavelength maximum of Li x SnO 2 lies in the infrared. 676 Granqvist and co-workers 677 assign the peak to intervalency transitions as in other cathodically colouring electrochromic oxides. The peak occurs in the near infrared. 676 6.3 Metal oxides: secondary electrochromes 183 At low insertion coefficients (0 <x<$0.1), the electro-inserted lithium ions appear to be located in internal double layers within the film. 677 Increasing the insertion coefficient x from $0.1 to $0.2 yielded significant transmittance drops, and M ¨ ossbauer spectra unambiguously show the conversion Sn IV ! Sn II . Electrocrystallisation appears to dominate the electrochemistry at x 0.2. 677 The electronic spectrum of tin-oxide films remains relatively unchanged following electro-insertion of lithium ion, but optical constants such as the refractive index increase with increasing insertion coefficient. Titanium oxide Thin-film TiO 2 can be made in vacuo by thermal evaporation of TiO 2 , 678 reactive rf sputtering from a titanium target, 261 or pulsed laser ablation. 679 Alternatively, non-vacuum techniques involve alkoxides or the peroxo precursor made by dissolving a titanium alkoxide Ti(OBu) 4 in H 2 O 2 . 128,134 Methods involve sol–gel, 127,128,319,680 spin coating 128 and dip-coating procedures. 158,174 The electrochromic reaction of TiO 2 is usually written as Eq. (6.39): TiO 2 ðsÞ þxðLi þ þe À Þ !Li x TiO 2 ðsÞ. colourless blueÀgrey (6.39) Ord et al. 681 have studied the electrochromism of titanium oxide grown anodically on metallic titanium via in situ ellipsometry. Both reduction and oxidation processes occur via movement of a phase boundary which separates the reduced and oxidised regions within the TiO 2 . The rate of TiO 2 reduction is controlled by the rate of counter-ion diffusion through the solid: 682,683 ionic insertion into the crystal form of anatase (the Li þ deriving from a LiClO 4 –propylene carbonate electrolyte) is characterised by a diffusion coefficient of 682 10 À10 cm 2 s À1 . To accelerate diffusion, Scrosati and co-workers 682 drove the electrochromic process with potentiostatic pulses. Titanium oxide-based electrochromes show two optical bands at 420 and 650 nm. 679 The coloration efficiency is low, hence TiO 2 is used as a secondary electrochrome or even as an ‘optically passive’ counter electrode, with WO 3 as the primary electrochrome. 158,319,678,684 Values of coloration efficiency j for thin-film TiO 2 are low; see Table 6.9. Nevertheless, Yoshimura et al. 685 claim to have modulated an incident beam by between 14% and 18%. Thin-film titanium oxynitride is also electrochromic. 686 184 Metal oxides Vanadium oxide Preparation of vanadium oxide electrochromes Thin-film V 2 O 5 is commonly made by reactive rf sputtering, 206,263,264,265,266,267 with a high pressure of oxygen and a target of vanadium metal. Direct-current sputtering is also used. 50,223,224,225,226 Other vacuum methods employed include pulsed laser ablation, 687,688,689 cathodic arc deposition 550 and electron-beam sputtering. 233,265 Thermal evaporation in vacuo 476,526,690 affords a different class of preparative method, and includes flash evaporation. 691 Films of V 2 O 5 deposited by thermal evaporation in vacuo are amorphous, 476 sputtered samples are more crystalline, 264,267,692 although X-ray diffraction suggests the extent of crystallinity is low. 264 Annealing a sample of thermally evaporated V 2 O 5 to above 180 8C improves the electrochemical performance, 693 presumably by increasing the extent of crystallinity in the amorphous material. Thin films of V 2 O 5 from vanadium metal anodised in acetic acid 36,694 are essentially amorphous. Electrochromic thin films have often been prepared using xerogels of V 2 O 5 , the precursor of choice generally being an alkoxide species such as VO(O i Pr) 3 . 199,200 Subsequent annealing yields the desired electrochrome, which is always hydrated. 695 The preparation and use of such gels has been reviewed extensively by Livage 696,697 (in 1991 and 1996 respectively). A more general review was published in 2001. 120 Livage made VO 2 films by sol–gel methods, generally via alkoxide precursors. 698 Alkoxide precursors are also used in preparing films by CVD, like VO(O i Pr) 3 in 2-propanol; 71,699,700 bis(acetylacetonato)vanadyl has also been employed. 73 The deposition product is immediately annealed in an oxidising atmosphere, ensuring polycrystallinity. Spin coating has also been used to prepare films of V 2 O 5 . Coating solutions include the liquor made by dissolving V 2 O 5 powder in a mixed solution of benzyl alcohol and iso-butanol, 701,702 or that produced by oxidative Table 6.9. Sample values of coloration efficiency j for titanium oxide electrochromes. Preparative procedure j/cm 2 C À1 (` (obs) /nm) Ref. Reactive thermal evaporation 7.6 678 Thermal evaporation 8 (646) 401 rf sputtering 14 261 Sol–gel 50 641 6.3 Metal oxides: secondary electrochromes 185 dissolution of powdered vanadium in hydrogen peroxide. 132,133 The liquor made by dissolving metallic vanadium in H 2 O 2 can also be spin coated e.g. onto ITO substrates. 132 Deb and co-workers 703 prepared thin films of mesoporous vanadium oxide by electrochemical deposition from a water–ethanol solution of vanadyl sulfate and a non-ionic polymer surfactant. Aggregates of the polymer surfactant appeared to act as a form of template during deposition. Electrochemical methods of making V 2 O 5 electrochrome are rarely used, no doubt owing to their sensitivity to water. Nevertheless, thin-film V 2 O 5 has been grown anodically on vanadium metal immersed in dilute acetic acid. 36,694,704 Redox chemistry of vanadium oxide electrochromes The electrochromism of thin-film V 2 O 5 was apparently first mentioned in 1977 by Gavrilyuk and Chudnovski, 705 who prepared samples by thermal evaporation in vacuo. Since thin-film V 2 O 5 dissolves readily in dilute acid, alternative electrolytes have been used, for example, distilled water, 705 LiCl in anhydrous methanol 706 or LiClO 4 in propylene carbonate 263,264,266 or g-butyrolactone. 707 The electrochromic reaction in non-aqueous solution follows Eq. (6.40): V V 2 O 5 ðsÞ þxðM þ þe À Þ !M x V IV.V 2 O 5 ðsÞ. brownÀyellow very pale blue (6.40) where M þ is almost universally Li þ owing to appreciable solubility of V 2 O 5 in aqueous acid. The rates of ion insertion and egress are so much slower for Na þ than for Li þ that the sodium ions in Na 0.33 V 2 O 5 may be regarded as immobile. In aqueous solution, 708 an alternative reaction is Eq. (6.41): V V 2 O 5 þ2H þ þ2e À !V IV 2 O 4 þH 2 O. (6.41) The relationships between the structure of V 2 O 5 films (prepared by sol–gel) and their redox state has been described at length by Meulenkamp et al: 709 a transition occurs from a-V 2 O 5 at x¼0.0 to ·-Li x V 2 O 5 at x¼0.4. These phases are nearly identical. For larger insertion coefficients, however, the structure undergoes significant changes: firstly, the phase for x¼0.8 shows an elongated c-axis relative to ·-Li x V 2 O 5 , which may represent a monoclinic structure. Secondly, at x¼1.0 the structure distorts further and shows features in common with d-LiV 2 O 5 . Thirdly, at x¼1.4, the structure bears further resemblance to d-LiV 2 O 5 . (Here, · and d are but phase labels.) Granqvist et al. 226 describe the structure of Li x V 2 O 5 as orthorhombic, later with additional details. 49 186 Metal oxides Cyclic voltammetry of sputtered V 2 O 5 , as a thin film supported on an OTE immersed in a lithium-containing PC electrolyte, shows two well-defined quasi-reversible redox couples 263 with anodic peaks at 3.26 and 3.45 V, and cathodic peaks at 3.14 and 3.36 V relative to the Li þ , Li couple in propylene carbonate; see Figure 6.15. Benmoussa et al. 710 produced V 2 O 5 films by rf sputtering, obtaining ‘excellent cyclic voltammograms’, again with a two- step electrochromism: they cite yellow to green, and then green to blue during reduction. These two pairs of peaks may correspond to the two phases of Li x V 2 O 5 identified by Dickens and Reynolds. 711 Ord et al. 36,694 grew thin anodic films of V 2 O 5 on vanadium metal immersed in acetic acid, and studied the redox processes using the in situ technique of ellipsometry, in tandemwith more traditional electrochemical methods such as cyclic voltammetry. As soon as the film is made cathodic, the outer surface is converted to H 4 V 2 O 5 . Thereafter, their results clearly suggest how, in common with MoO 3 (but unlike WO 3 ), a well-defined boundary forms between the coloured and bleached phases during redox cycling: this boundary sweeps inward toward the substrate from the film–electrolyte interface during the bleaching and coloration processes. (Higher fields are required for bleaching E pa(1) E pc(1) E pc(2) E pa(2) 50 mV s –1 10 mV s –1 2.3 1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5 2.5 2.7 2.9 3.1 Potential / V(vs . Li + ,Li couple) C u r r e n t de n s i t y /m A c m –2 3.3 3.5 3.7 3.9 4.1 4.3 Figure 6.15 Cyclic voltammogram of thin film of V 2 O 5 sputtered on an OTE, and immersed in propylene carbonate containing LiClO 4 (1.0 mol dm À3 ). (Figure reproduced from Cogan, S. F., Nguyen, N. M., Perrotti, S. J. and Rauh, R. D. ‘Electrochromism in sputtered vanadium pentoxide’. Proc. SPIE, 1016, 1988, 57–62, with permission of the International Society for Optical Engineering.) 6.3 Metal oxides: secondary electrochromes 187 than for coloration.) The rates of coloration and bleaching are both dictated by the rate of proton movement. 694 The bleaching process is complicated, and proceeds in three stages. 694 A study by Scarminio et al. 225 monitored the stresses induced in V 2 O 5 during redox cycling, this time with Li þ as the mobile ion; their thin-film V 2 O 5 was immersed in a solution of LiClO 4 in PC. Their results suggest the crystal structure within the film is determined by the sputter conditions employed during film fabrication. Deep charge–discharge cycles (performed under constant current density) allow correlations to be drawn between the stress changes in the crystalline film and the electrode potential steps. The authors say this behaviour is typical of the lithium insertion mechanisms in bulk V 2 O 5 prepared as a cathode material for secondary lithium batteries. They also suggest the redox cycling is somewhat irreversible, implying a poor write–erase efficiency. The crystal structure of vanadium pentoxide is complicated, with the nominally octahedral vanadium being almost tetragonal bipyramidal, with one distant oxygen. 712 Reductive injection of lithium ion into V 2 O 5 forms Li x V 2 O 5 . The Li x V 2 O 5 (of x<0.2) prepared by sputtering is the a-phase, which is not readily distinguishable from the starting pentoxide. 263 At higher injection levels (0.3 <x<0.7), the crystalline formof the oxide is ·-Li x V 2 O 5 , 263 as identified by the groups of Hub et al. 706 and Murphy et al. 713 The generation of the ·-phase of Li x V 2 O 5 in V 2 O 5 thin films accompanies the electrochromic colour change. Also, a-Li x V 2 O 5 from the un-lithiated oxide is formed, and contributes an additional, slight change in absorbance. 263 Since several species participate in the spectrum of the partially reduced oxide, spectral regions following the Beer–Lambert law cannot be identified readily. 266 Films of mesoporous V 2 O 5 colour faster than evaporated films, 703 attributable to enhanced ion mobility. Such vanadium oxide also exhibits a higher lithium storage capacity and greatly enhanced charge-discharge rate. Na 0.33 V 2 O 5 made by a sol–gel process is also electrochromic; 714 see Eq. (6.42): Na 0.33 V 2 O 5 ðsÞ þxðLi þ þe À Þ !Li x Na 0.33 V 2 O 5 ðsÞ. (6.42) The sodium ions are essentially immobile. Optical properties of vanadium oxide electrochromes The absorption bands formed on reduction are generally considered to be too weak to imply the formation of any intervalence optical parameters (although there are other arguments formulated by Nabavi et al. 715 ). Wu et al. 716 suggest the anodic electrochromism of V 2 O 5 is due to a blue shift of the absorption edge, and the 188 Metal oxides near-infrared electrochromism arises from absorption by small polarons in the V 2 O 5 . From X-ray photoelectron spectroscopy, Fujita et al. 690 assign the colour change in evaporated films incorporating lithium to the formation of VO 2 (which is blue) in the V 2 O 5 . Colten et al., 476 using the same technique, did infer a weak charge-transfer transition between the oxygen 2p and vanadium 3d states, but only for an entirely V IV solid. Vanadium pentoxide films have a characteristic yellow–brown colour, attributable to the tail of an intense optical UV band appearing in the visible region; 224 see Figure 6.16. The electrogenerated colour is blue–green for evaporated films 717 at low insertion levels, going via dark blue to black at higher insertion levels. 705 The colour changes from purple to grey if films are sputtered. 5 Rauh and co-workers 263 state that certain film thicknesses of V 2 O 5 yield colourless films between the brown and pale-blue conditions. The value of ` max of the yellow–brown form lies in the range 1100–1250 nm. The loss of the yellow colour is attributed to the shift of the band edge from about 450 to 250 nm during reductive bleaching from V 2 O 5 to Li 0.782 V 2 O 5 . 266 A few representative values of j are listed in Table 6.10. Electrochromic devices containing vanadium pentoxide Since the electrochromic colours of V 2 O 5 films are yellow and very pale blue, the CR values T r a n s m i t t a n c e ( % ) 100 80 60 40 20 0 Wavelength (mm) 0 0.21 x =0.85 p – X Li x V 2 0 5 0 0.5 1 1.5 2 2.5 Figure 6.16 UV-visible spectrum of thin-film vanadium pentoxide on ITO. The polycrystalline V 2 O 5 was sputter deposited to a thickness of 0.25 mm. The numbers refer to values of insertion coefficient x. (Figure reproduced in slightly altered form from Talledo, A., Andersson, A. M. and Granqvist, C. G. ‘Structure and optical absorption of Li x V 2 O 5 thin films’. J. Appl. Phys., 69, 1991, 3261–5, by permission of Professor Granqvist and The American Institute of Physics.) 6.3 Metal oxides: secondary electrochromes 189 for such films are not great, hence the system is generally investigated for possible ECD use as a secondary electrochrome, i.e. in counter-electrode use. 263,266,526,718,719,720,721 For example, cells have often been constructed with V 2 O 5 as the secondary material to WO 3 as the primary, e.g. ITOj Li x WO 3 j electrolyte j V 2 O 5 j ITO. 266,277,722 Gustaffson et al. 723 made similar cells but with the conducting polymer PEDOT as the primary electrochrome. Thin-film vanadium dioxide VO 2 is electrochromic, 724,725 and lithium vanadate (LiVO 2 ) is not only electrochromic but also thermo-chromic, 726 and can be prepared by reactive sputtering; 723 LiVO 2 doped with titanium oxide is also thermochromic. 727 Finally, composites of V 2 O 5 in poly(aniline) and a ‘melanin-like’ polymer have been reported. 728 6.4 Metal oxides: dual-metal electrochromes 6.4.1 Introduction Preparing mixtures of metal oxide has been a major research goal during the past few years, for two reasons. Firstly, mixing these oxides can modify the solid-state structure through which the mobile ion moves, and thus increase the chemical diffusion coefficient - D that results in superior response times t. Secondly, mixtures are capable of providing different colours. In particular, there is a desire for so-called ‘neutral’ electrochromic colours; see p. 399. Varying the energies of the optical bands by altering the mix allows the colour to be adjusted to that desired. Thus the choice of constituent oxides and their relative mole fractions allows a wide array of options. There are several models to correlate these variables with the electrochromic colour. One of the most successful is the so-called ‘site-saturation’ model. Here, all inserted electrons are considered to be localised, and optical absorptions are Table 6.10. Coloration efficiencies j of thin-film vanadium oxide electrochromes. Deposition method j/cm 2 C À1 (` (obs) /nm) Ref. rf magnetron sputtering À35 263 rf magnetron sputtering À15 (600–1600) 264 Sol–gel À50 641 CVD À34 699 190 Metal oxides considered to occur simply by photo-excitation of an electron to an empty redox site. To a good first approximation, the optical absorption intensity is proportional to the number of vacant redox sites surrounding the reductant site. As the insertion coefficient x increases, so the proportion of vacant sites neighbouring a given electron on a reductant’s site decreases, with the effect of decreasing the oscillator strength. The treatment by Denesuk and Uhlman 729 has been tested with data for Li x WO 3 on ITO – a system displaying a curious dependence of ` max and j on the insertion coefficient x. Their model only applies to situations in which a dominant fraction of the electrons associated with the intercalating species are appreciably localised. The computed and experimental data correlated well, with published traces showing only slight deviation between respective values. The earlier work of Hurita et al. 730 relates to mixtures of MoO 3 and WO 3 . Again, the computed and experimental data correlate well, although published traces show somewhat more scatter. Several other reports 24,283,731 have discussed electrochromic colours in terms of this model. van Driel et al. 731 again studied ` max and j for the Li x WO 3 system. Published traces show significant divergences between calculated results and experiment, which are explained in terms of partial irreversibility during coloration. In the discussion below, tungsten-based systems are considered first, as exemplar systems, since they were among the first mixed-metal oxides to receive attention for electrochromic applications. The chemistry of tungsten– molybdenum oxides has been reviewed briefly by Ge´ rand and Seguin 732 (1996). Other host oxides are listed alphabetically. 6.4.2 Electrochromic mixtures of metal oxide A full, systematic evaluation of the data below is not yet possible because the electrochromic properties of films depend so strongly on the modes of preparation, as has been copiously illustrated above, and such a wide range of preparative techniques has been employed. Clearly, oxide mixtures of the type X–Y can be incorporated into either a section on oxides of X or of Y, so some slight duplication is inevitable. Tungsten oxide as electrochromic host Tungsten trioxide has been employed as a host or ‘matrix’ for a series of electrochromic oxides, containing the following oxides: (in alphabetical order) Ba, 733 Ce, 734 Co, 5,80,94,99,735,736 Mo, 61,62,91,292,361,475,730,737,738,739, 740,741,742,743,744,745,746,747 Nb, 29,317,748,749,750 Ni, 53,80,81,89,94,99,735,736,751,752 Re, 753 Ta, 29,661,754 Si, 316,333 Ti 127,129,134,203,316,333,335,404,755,756,757,758,759,760 6.4 Metal oxides: dual-metal electrochromes 191 or V. 204,254,325,327,687,761 Thin-film WO 3 can also be co-electrodeposited with phosphomolybdic acid to yield an electrochrome having a colour change described as ‘light yellow !bluish brown’, although the transition is reported not to be particularly intense. 745,762 Kitao et al. 292 prepared a range of films of molybdenum–tungsten oxide of the formula Mo c W (1Àc) O 3 , and analysed the shift in wavelength maxima as a function of the mole fractions of either constituent oxide (see Figure 6.17) and found a complicated relationship, sometimes described within the ‘site saturation model’; see p. 190. Here, it is recognised that electrons are captured (that is, they effect reduction) first at the sites of lowest energy. In practice, it is found that Mo sites are of lower energy than W, thereby explaining why Mo–Mo and Mo–Wintervalence bands are formed at lower insertion coefficients x than are any W–W bands. The value of ` max for mixed films of WO 3 –MoO 3 shifts to higher energy (lower `) relative to the pure oxides. Since the wavelength of the shifted ` max corresponds more closely to the sensitive range of the human eye, mixing the oxides effectively enhances the coloration efficiency j in the visible region. Faughnan and Crandall found the highest value of j occurs with a mole fraction of 0.05 of MoO 3 . 738 Deb and Witzke 763 say the range of j is 30–40%. Additionally, the films of W–Mo oxide become darker because they can accommodate more charge, i.e. have a larger maximal insertion coefficient. 61 Disadvantageously, the electron mobility is decreased in thin- film WO 3 –MoO 3 relative to the pure oxides. 738 1.0 1.5 E p ( e V) 2.0 0 1000 2000 Q i (C cm –3 ) Figure 6.17 Photon energy of the absorption peak E p as a function of the inserted charge: thin-film samples of partially reduced oxides of composition Mo c W (1Àc) O 3 . o¼WO 3 , * ¼MoO 3 , & ¼mixed film with c ¼0.008, D¼c ¼0.13, and x ¼0.80. (Figure reproduced from Hiruta, Y., Kitao, M. and Yamada, M. Absorption bands of electrochemically-colored films of WO 3 , MoO 3 and Mo c W 1Àc O 3 . Jpn. J. Appl. Phys., 23, 1984, 1624–7, with permission of The Institute of Pure and Applied Physics.) 192 Metal oxides In the study by Hiruta et al., 746 the optical band for W–Mo oxide is said to comprise two bands. The first is the intervalence band, and the second is a new band at higher energies, which is thought to relate to the Mo ions. Fur- thermore, the energy of the absorption band depends on the concentration of Mo in the film and the insertion coefficient x. 747 Ge´ rand and Seguin 732 suggest that ion insertion into W–Mo oxide occurs readily, but ion removal is usually somewhat difficult, thus precluding all but the slowest of electrochromic applications. The slowness is ascribed to induced ‘amorphisation’ of the mixed-metal oxide at high insertion coefficients. Such a result, if confirmed, would contradict the usual assumption that values of - D for ion movement through amorphous material are higher than through polycrystalline material. The cause of such ‘amorphisation’ is as yet not clear. The temperature dependence of the electrochromic response of sol–gel deposited titanium–tungsten mixed oxide was shown by Bell and Matthews 335 to be highly complicated, implicating multiple competing processes. The W–Ce film consisted of a self-assembly structure based on the poly(oxotungsceriumate) cluster K 17 [Ce III (P 2 W 17 O 61 ) 2 ] Á 30H 2 O[Ce(P 2 W 17 ) 2 ] and poly(allylamine) hydrochloride. 734 Comparatively long response times of 108 and 350 s were found for coloration and bleaching respectively. Adding about 5% of nickel oxide significantly improves the cyle life of WO 3 . 53 Finally, a hybrid of WO 3 and Perspex (polymethylmethacrylate) has a relatively low j of 38 cm 2 C À1 . 403 Antimony oxide as an electrochromic host Thin-film antimony–tin oxide (ATO), grown by pulsed laser deposition, colours cathodically. 764 Its electrochromic properties are ‘poor’; the electrochemical and optical properties were found to be extremely sensitive to their morphology. Naghavi et al. 765 suggested that the best electrochromic films were obtained by depositing at 200 8C in an oxygen atmosphere at a pressure of 10 À2 mbar, followed by annealing at 550 8C. This last condition is described as ‘critical’. Cerium oxide as electrochromic host Electrochromic mixtures have been prepared of cerium oxide together with the oxides of Co, 766 Hf, 767 Mo, 768 Nb, 769 Pr, 653 Si, 768,770 Sn, 755,771 Ti, 122,205,323,324,755, 767,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787 V, 788,789,790,791,792,793 W, 734 and Zr. 122,783,786,794,795,796 The relative amount of the second oxide varies from a trace to a molar majority. 6.4 Metal oxides: dual-metal electrochromes 193 Thin-film Ce–Ti oxide is more stable than CeO 2 alone 797 although, interestingly, evidence from EXAFS suggests that the electrons inserted into Ce–Ti oxide reside preferentially at cerium sites; 774 the oxide layer was prepared by dc magnetron sputtering. The charge movement necessary for electrochromic operation involves insertion and/or extraction of electrons via the Ce 4f states, 779,780 which are located in the gap between the valence and conduction bands of the CeO 2 . The chemical diffusion coefficient - D of mobile Li þ ions through thin-film Ce–Ti oxide increases as the mole fraction of cerium oxide decreases: 779,780 a plot of ln - D against mole fraction of CeO 2 (see Figure 6.18) is almost linear: - D ðLi þ Þ increases from 10 À16 cm 2 s À1 for pure CeO 2 to 10 À10 cm 2 s À1 for pure TiO 2 . These values of - Dsuggest the extent of electron trapping is slighter (or at least the depths of such traps are shallower) for TiO 2 than for CeO 2 . Clearly, then, electrochromes having as high a proportion of TiO 2 as possible are desirable to achieve rapid ECD operation. Conversely, adding CeO 2 to TiO 2 increases the cycle life, the cycle life of pure TiO 2 (as prepared by sol–gel techniques) being relatively low. 319 Addition of cerium oxide to TiO 2 also decreases 779,780 the coloration efficiency j until, as the ratio Ce:Ti (call it g) exceeds 0.3, the electrochromic ‘absorbance’ is essentially independent of the insertion coefficient, i.e. films of Ce–Ti oxide (with Li þ as the counter ion) are optically passive and can 10 –10 10 –12 10 –14 10 –16 D /c m 2 s –1 – Ce/Ti Ratio 0.1 0 0.3 0.5 ∞ Figure 6.18 Graph of chemical diffusion coefficient - D of Li þ ion moving through films of CeO 2 –TiO 2 : the effect of varying the composition. (Figure reproduced in slightly altered form from Kullman, L., Azens, A. and Granqvist, C. G. ‘Decreased electrochromism in Li-intercalated Ti oxide films containing La, Ce, and Pr’. J. Appl. Phys., 81, 1997, 8002–10, by permission of Professor Granqvist and The American Institute of Physics.) 194 Metal oxides function as ECD counter electrodes. Films prepared by magnetron sputtering with g 0.6 are not chemically stable. 781 Clearly, optimising the electrochromic response of Ce–Ti oxide will require that all three of the parameters t, j and cycle life are considered. Cobalt oxide as electrochromic host Many electrochromic mixtures of cobalt oxide have been prepared, e.g. with oxides of Al, 617,798 Ce, 766 Cr, 80 Fe, 80,81 Ir, 799 Mo, 80,81 Ni, 79,80,83,140,158,799, 800,801 W 80,81,736 or Zn. 80,81 Diffusion through films of cobalt oxide mixed with other d-block oxides can be considerably faster than through CoO alone: the value of - D for the OH À ion is 2.3 Â10 À8 cm 2 s À1 through CoO, 5.5 Â10 À8 cm 2 s À1 through WO 3 , but 48.7 Â10 À8 cm 2 s À1 through Co–W oxide. 80 All these films were electrodeposited. The value of - D relates to H þ as the mobile ion through WO 3 , and to OH À ions for CoO and Co–W oxide. The larger value of - D probably reflects a more open, porous structure. These values of - D are summarised in Table 6.11. Thin-film Co–Al oxide 617 prepared by dip coating has a coloration efficiency of 22 cm 2 C À1 , which compares with j for CoO alone of 21.5 cm 2 C À1 (as prepared by CVD 604 ) or 25 cm 2 C À1 (the CoO having been prepared by a sol–gel method 616 ). Cobalt–aluminium oxide has a coloration efficiency of 25 cm 2 C À1 , and Co–Al–Si oxide has j of 22 cm 2 C À1 . 401 Since thin-film Al 2 O 3 is rarely electroactive let alone electrochromic, the similarity between these j values probably indicates that the alumina component acts simply as a kind of matrix or ‘filler’, allowing of a more open structure; but any increase in the rate of electro-coloration follows from enhancements of - D rather than from increases in j. Since effective intervalence relies on juxtaposition of Co sites, and admixture would inevitably increase the mean Co–Co distance within this solid-state Table 6.11. Comparative speeds of hydroxide-ion movement through electrodeposited cobalt, tungsten and Co–W mixed oxides. The - D data come from ref. 80. Oxide film D/cm 2 s À1 CoO 2.3 Â10 À8 Co–WO 3 48.7 Â10 À8 WO 3 5.5 Â10 À8 6.4 Metal oxides: dual-metal electrochromes 195 mixture, possibly the ‘Co–Al oxide’ here in reality comprises aggregated clusters of the two constituent oxides, each as a pure oxide. Indium oxide as an electrochromic host The most commonly encountered mixed-metal oxide is indium–tin oxide (ITO), which is widely used in the construction of ECDs, and typically comprises about 9 mol% SnO 2 . 802 Some of the tin oxide dopant has the composition of Sn 2 O 3 . 803 The most common alternative to ITO as an optically transparent electrode is tin oxide doped with fluoride (abbreviated to FTO), although the oxides of Ni 804 and Sb 764,765 have also been incorporated into In 2 O 3 . While old, a review in 1983 by Chopra et al. 805 still contains information of interest, although the majority concerns ITO acting as a conductive electrode rather than a redox-active insertion electrode. The more recent review (2001) by Nagai 806 discusses the electrochemical properties of ITO films; however, the most recent review was in 2002 by Granqvist and Hulta˚ ker. 807 Preparation of ITO electrochromes Electrochromic ITO is generally made by rf sputtering, 239,240,241,242,243,244,245 or reactive dc sputtering. 208 Room- temperature pulsed-laser deposition can also yield ITO. 808 Reactive electron- beam deposition onto heated glass also yields good-quality ITO, 229 but is not employed often since the resultant film is oxygen deficient and has a poorer transparency than material of complete stoichiometry. When preparing ITO films by electron-beam evaporation, 229,809 the precursor is In 2 O 3 þ9 mol% of SnO 2 , evaporated directly onto a glass substrate in an oxygen atmosphere of pressure of $5 Â10 À4 Torr. The ITO made by these routes is largely amorphous. Other electrochromic ITO layers have been made via sol–gel, 183 and spin coating a dispersion of tin-doped indium oxide ‘nanoparticle’. 186,187 Redox electrochemistry of ITO When a thin-film ITO immersed in a solution of electroactive reactants has a negative potential applied, it will conduct charge to and/or from the redox species in solution. It behaves as a typical electrode substrate (see, for example, Section 14.3). By contrast, if the surrounding electrolyte solution contains no redox couple, then some of the metal centres within the film are themselves electroreduced. 810 Curiously, doubt persists whether it is the tin or the indium species of ITO which are reduced: the majority view is that all redox chemistry in such ITO occurs at the tin sites, the product being a solid solution; Eq. (6.43): 196 Metal oxides ITOðsÞ þxðM þ þe À Þ !M x ITOðsÞ. colourless pale brown (6.43) where M is usually Li þ , e.g. from LiClO 4 electrolyte in PC, but it may be H þ . The resultant partially reduced oxide M x ITO may be symbolised as M x Sn IV,II O 2 (In 2 O 3 ), where the indium is inert. The reduced form of ITO is chemically unstable, as outlined in Section 16.2. Ion insertion into ITO is extremely slow, with most of the cited values of chemical diffusion coefficient - D lying in the range 10 À13 to 10 À16 cm 2 s À1 , 809 although Yu et al. 811 cite 1 Â10 À11 cm 2 s À1 . These low values may also be the cause of hysteresis in coulometric titration curves. 811 Electroreversibility is problematic if Li þ rather than H þ is the mobile ion inserted, so redox cycles ought to be shallow (i.e. with x in Eq. (6.43) kept relatively small). Contrarily, reductive incorporation of Li þ increases the electronic conductivity of the ITO. 241,243 Few cycle lives are cited in the literature: Golden and Steele 244 and Corradini et al. 812 are probably the only authors to cite a high write–erase efficiency (of 10 4 and 2 Â10 4 cycles, respectively). Optical properties of ITO Some ITO has no visible electrochromism, 809 and is therefore a perfect choice for a ‘passive’ counter electrode. The colour of reduced ITO of different origin is pale brown (possibly owing to Sn II ); see Figure 6.19. The coloration efficiency j is 2.8 cm 2 C À1 at 600 nm; 244 M x ITO is too pale to adopt as a primary electrochrome since its maximal CR is only 1:1.2. 241,242,243,802,809,812,813,814,815,816 A recent report suggesting a yellow–blue colour was formed during electroreduction of ITO is intriguing since the source of the blue is, as yet, quite unknown. 817 Perhaps similar is the mixed-valent behaviour recently inferred for 677 Li x SnO 2 , as determined by M ¨ ossbauer measurements. The Li x SnO 2 in that study was made by Li þ insertion into sputtered SnO 2 . 676 Devices containing ITO counter electrodes When considered for use as an electrochrome, ITO is always the secondary ‘optically passive’ ion- insertion layer, e.g. with WO 3 243,277,811,818,819 or poly(3-methylthiophene) 812 as the primary electrochrome. Bressers and Meulenkamp 820 consider that ITO‘probably cannot be used as a combined ion-storage layer and transparent conductor for all-solid-state . . . switching device in viewof its [poor] long-termstability’. X-Ray photoelectron spectroscopy studies seem to support this conclusion. 821 6.4 Metal oxides: dual-metal electrochromes 197 Iridium oxide as electrochromic host Iridium oxide has been doped with the oxides of magnesium 822 and with tantalum. 823 Films of composition IrMg y O z (2.5 <y <3) are superior to iridium oxide alone, for the electrochromic modulation is wider, and the bleached state is more transparent. 822 Such a high proportion of magnesium is surprising, considering the electro-inactive nature of MgO. Addition of Ta 2 O 5 decreases the coloration efficiency j but increases chemical diffusion coefficient D. The changes are thought to be the result of diluting the colouring IrO 2 with Ta 2 O 5 , which supports a superior ionic conductivity. Iridium oxide has also been incorporated into aramid resin, poly(p- phenylene terephthalamide). 531 Iron oxide as electrochromic host Iron oxide has been host to the oxides of Si and Ti, as prepared by sol–gel methods. 824 The films investigated are able reversibly to take up Li þ , Na þ and K þ ions. The coloration efficiencies j of this mixed oxide lie in the range 824 6–14 cm 2 C À1 at ` max of 450 nm (the authors do not say which compositions 0.5 1.0 1.5 2.0 2.5 0.0 0.1 0.2 0.3 0.4 T r a n s m i s s i vi t y Wavelength (μ m) 0.5 0.6 0.7 0.8 0.9 1.0 Figure 6.19 UV-visible spectrum of thin-film ITO in its oxidised (–– clear) and partially reduced ( Á Á Á Á pale brown) forms. (Figure reproduced from Goldner, R. B. et al. ‘Electrochromic behaviour in ITO and related oxides’. Appl. Opt., 24, 1985, 2283–4, by permission of The Optical Society of America.) 198 Metal oxides relate to these values of j except ‘the largest extent of colouring and bleaching was for pure iron oxide’). Molybdenum oxide as electrochromic host Thin-film molybdenum oxide has also been made as a mixture with the oxides of Co, 80,81,91 Cr, 91 Fe, 91 Nb, 165,171,750,825 Ni, 91 Sn, 826,827 Ti, 22,826,828 V 124,202, 829 or W. 61,62,91,292,361,475,730,737,738,739,740,741,742,743,744,745,746,747 Most thin films of Mo–W oxide were prepared from reactive sputtering, but others have been prepared by sol–gel techniques, e.g. from a solution of peroxopolymolybdotungstate, 125 itself made by oxidative dissolution of both metallic molybdenum and tungsten in hydrogen peroxide (see p. 133 ff.). Molybdenum–vanadiumoxide is also made by dissolving the respective metals in H 2 O 2 . 124,202 The electrochromic transition for the resultant film is ‘green– yellow ! violet’ when cycled in LiClO 4 –PC solution as the ion-providing electrolyte. An additional benefit of incorporating molybdenum into an electrochromic mixture is its ability to extend the overpotential for hydrogen evolution (a nuisance if occurring at lower potentials) when in contact with a protonic acid. As an example, H 2 is first formed at the surface of the MoO 3 layer at À0.85 V (vs. SCE), cf. À0.75 V for electrodeposited Mo–W oxide (electrodeposited together on gold). More impressive still, no gas whatsoever forms when a gold electrode is coated with similarly formed Mo–Cr and Mo–Fe oxides. 830 The coloration efficiency j for MoO 3 –SnO 2 films 826 is low, being in the range 2–10 cm 2 C À1 , cf. 77 cm 2 C À1 for MoO 3 alone 7 and 3 cm 2 C À1 for ITO alone 244 (although some ITO is completely passive optically 809 ). These data are summarised in Table 6.12, which clearly shows how the optical behaviour of Mo–Sn oxide is more akin to SnO 2 than to MoO 3 . Possibly the tin sites are electroactive while the Mo sites are not. The value of j for Mo–Ti oxide lies in the range 10–50 cm 2 C À1 , the value increasing as the mole fraction of molybdenum increases. 22 Table 6.12. Effect on the coloration efficiency j of mixing molybdenum and tin oxides. Films j/cm 2 C À1 Ref. MoO 3 77 7 ITO 3 244 ITO 0 809 MoO 3 –SnO 2 2–10 826 6.4 Metal oxides: dual-metal electrochromes 199 Nickel oxide as electrochromic host Several electrochromic mixtures have been prepared of nickel oxide, e.g. with oxides of Ag, 77,831 Al, 831,832,833,834 Cd, 77,83 Ce, 77 Co, 77,79,80,81,82,83,140,801 Cr, 77,835 Cu, 77 Fe, 77,836 La, 77,82,84,837,838 Mg, 77,831,832,833,839 Mn, 636,833,840 Nb, 636,831 Pb, 77 Si, 167,831 Sn, 841 Ta, 831 V, 761,831,832,833,834 W, 53,80,81,89,94, 99,735,736,751,752 Y 77 and Zn. 83 Nickel oxide has also been mixed with particles of various alloys, such as Ni–Au alloy, 842 to yield films with markedly different spectra. Traces of ferrocyanide have been incorporated, 82 and films containing gold are also made readily. 161 Nickel tungstate is also electrochromic. 843 Nickel oxide often shows a residual absorption, an unwanted brown tint, but incorporating Al or Mg in the film virtually eliminates this colour. 832 Thus for applications requiring a highly bleached transmittance, such as architectural windows, the Al- and Mg-containing oxides are superior to conventional nickel oxide, for their greatly enhanced transparency. 832,834 Such films also show superior charge capacity. 832 Incorporation of Ce, Cr or La into NiO improves the rates of electro- coloration, while adding Ce, Cr or Pb retards the rates of bleaching. 77 Addition of yttrium oxide severely impedes the rate of NiO electrocoloration, for reasons not yet clear. 77 An important observation for ECD construction is that electrodeposited Ni–La and Ni–Ce oxides are significantly more durable than NiO alone, 77 as evidenced by longer cycle life. Tungsten trioxide is cathodically colouring while NiO is anodically colouring, so it is interesting that electrodeposited Ni–W oxide has a rather low coloration efficiency of 99 4.4 cm 2 C À1 while j for sol–gel-derived NiO is 152 À(35 to 40) cm 2 C À1 . The complex [Ru 3 O(acetate) 6 -m-{pyrazine} 3 -[Fe(CN) 5 ] 3 ] nÀ has also been incorporated into NiO x . 844 Niobium oxide as electrochromic host Electrochromic films have been prepared that are doped with the oxides of Ce, 769 Fe, 845 Mo, 171,750,825 Ni, 831 Sn, 171 Ti, 171,846 W 749,750 and Zn. 165,171 Lee and Crayston have also made a Nb–silicone composite. 642 In a recent study of sol–gel deposited Nb 2 O 5 , Schmitt and Aegerter 171 prepared a variety of films that were doped with a variety of d-block oxides. The coloration efficiencies of such films were not particularly sensitive to the other metals, the highest being for Nb 2 O 5 containing 20% TiO 2 , which has a coloration efficiency of 27 cm 2 C À1 . The maximum change in transmittance was observed for films comprising 20% Mo. 200 Metal oxides It is clear that Nb–W oxide behaves more like WO 3 than Nb 2 O 5 ; 749 and Nb–Fe oxide behaves more like Nb 2 O 5 than either FeOor Fe 2 O 3 . 845 Hydrated HNbWO 6 also has a superior chemical stability to that of WO 3 alone, 748 and doped niobium oxide is also more electrochemically stable. 171 The coloration efficiencies j for such mixed Nb–metal oxide films are all low. Representative values are summarised in Table 6.13. Hydrated HNbWO 6 has a similar coloration efficiency (54 cm 2 C À1 ) 748,847 to that of WO 3 ; cf. 48 cm 2 C À1 for WO 3 prepared by the same procedures. Tin oxide as electrochromic host Electrochromic mixtures have been prepared of tin oxide, with the oxides of Ce, 755,771 Mo, 826,827 Ni, 841 Sb 827 or V. 848 The film of Ce–Sn oxide was wholly optically inactive, with a transparency higher than 90%. Titanium oxide as electrochromic host Electrochromic mixtures of titaniumare at present much used. Electrochromic mixtures have been prepared of TiO 2 with oxides of Ce, 122,205,323,324,755,771,772, 773,774,775,776,777,778,779,780,781,782,783,784,785,786,787 Fe, 849,850 La, 779,780 Mo, 22,826, 828 Nb, 165,171,825 Ni, 150,841 Pr, 779,780 Ta, 754 V, 721,851,852,853,854 W 127,129,134,203, 316,335,404,755,756,757,758,759,760,855 and Zn. 563 A mixture of TiO 2 and phosphotungstic acid has been made via sol–gel techniques, 768 and TiO 2 containing hexacyanoferrate has also been produced. 856 Most of these electrochrome mixtures were made by sol–gel or sputtering techniques. For example, Ni–Ti oxide is made from NiCl 2 and Ti alkoxide, 150 and Ti–Fe oxide was prepared by a dip-coating procedure 850 via a liquor comprising alcoholic ferric nitrate and Ti(O i Pr) 4 ), followed by Table 6.13. Effect on the coloration efficiency j of mixing niobium oxides with iron or titanium oxide: the effect of mixing and preparation method. Components Preparation route j/cm 2 C À1 Ref. Nb 2 O 5 rf sputtering 22 170 Nb 2 O 5 rf sputtering <12 258 Nb 2 O 5 Sol–gel 16 171 Nb 2 O 5 Sol–gel 25–30 748 FeO CVD À6 to À6.5 629 Fe 2 O 3 Electrodeposition À30 102 Nb 2 O 5 –FeO CVD 20 845 Nb 2 O 5 þ20% TiO 2 Sol–gel 27 171 HNbWO 6 (hydrated) Sol–gel 54 748 WO 3 Sol–gel 48 748 6.4 Metal oxides: dual-metal electrochromes 201 annealing in air. In ref. 680, however, the layer of W–Ti oxide was made by pulsed cathodic electrodeposition. The value of ` max for the Ni–Ti oxide 150 is 633 nm, and j lies in the range À(10–42) cm 2 C À1 . The optical charge-transfer transition in the Ti–Fe systemis responsible for the blue colour of naturally occurring sapphire; 857 but thin-film Ti–Fe oxide (prepared in this case by a dip-coating procedure 850 ) did not possess the same colour as sapphire, probably having a different structure. Vanadium oxide as electrochromic host Electrochromic mixtures have been prepared of vanadium oxide, with the oxides of Bi, 594 Ce, 788,789,790,791,792,793 Dy, 858 Fe, 159 In, 859 Mo, 124,202,829 Nd, 858 Ni, 761,831,832 Pa, 703 Pr, 858,860 Sm, 858 Sn, 848 Ti 166,687,721,851,852,853,854 or W. 204,254, 325,327,687,761 Thin films of composition (V 2 O 5 ) 3 –(TiO 2 ) 7 oxide form a reddish brown colour at anodic potentials which Nagase et al. 854 attribute to the vanadium component, implying the majority TiO 2 component is optically passive. When doped with the rare-earth oxides of Nd, Sm, Dy, 858 films of V 2 O 5 show a considerably enhanced cycle life. X-Ray diffraction results suggest the formation of the respective orthovanadate species SmVO 4 and DyVO 4 . The V–Sm oxide film showed a very small coloration efficiency j of only 0.6 cm 2 C À1 , so the authors suggest counter-electrode use. Similarly, a film of Ni–V oxide is ‘virtually [optically] passive’, although no values of j are cited. 761 Other electrochromic vanadates include FeVO 4 159 and CeVO 4 . 861 The electrochromic behaviour of V–Ti oxide films is complicated: 853 in the best explanatory model, the inserted electrons are supposed to be localised, residing preferentially at vanadium sites. The V–Ti films have a larger charge capacity if the mole fraction of vanadium is relatively high. 862 Oxide electrochromes having a grey hue, rather than blue, are said to be ‘neutral’ in colour; see p. 399. Such neutral colours have been made with V–Ti oxide (with brown–blue electrochromism); 687 and for V–W oxide which has a coloration efficiency in the range 7 to 30 cm 2 C À1 , the value depending on the composition, with j decreasing as the mole % of vanadium increases. 863 Composites of vanadium oxide have been formed by reacting a xerogel (see p. 161) with organic materials such as the nanocomposite [poly(aniline N-propanesulfonic acid) 0.3 V 2 O 5 ]. 864 This material has a superior electronic conductivity to the precursor V 2 O 5 xerogel alone and exhibits shorter ionic diffusion pathways, both properties implying a fast electrochromic transition. 864 The second V 2 O 5 –organic composite is a ‘melanine like’ material formed by reacting 3,4-dihydroxyphenylalanine with a V 2 O 5 xerogel. This latter material generates a dark blue metallic electrochromic colour. 728,865 202 Metal oxides Zirconium oxide as electrochromic host Pure zirconium oxide is not electrochromic and has practically zero charge capacity, 796 but has been host to a large number of other oxides. It is now a popular choice of optically passive electrochromic layer when mixed with cerium oxide. 122,767,783,786,794,795,796 For example, in Granqvist et al.’s 1998 review of devices, 797 they cite Zr–Ce as the optically passive secondary layer, referring to material in the compositional range Zr 0.4 Ce 0.6 O 2 to Zr 0.25 Ce 0.75 O 2 . The charge capacity of Zr–Ce oxide increases with increasing ceriumcontent. 796 Miscellaneous electrochromic hosts Tantalum–zirconium oxide is electrochromic. 866 Its electrochromic qualities are said to be superior to either constituent oxide, suggesting a new phase rather than a mixture. Its coloration efficiency j is estimated to be 47 cm 2 C À1 at 650 nm. Electrochromic iridium–ruthenium oxide in the molar ratio 40:50% is said to be 300 times more stable than either constituent oxide. 867 Ternary and higher oxides A few multiple-metal oxides have been made: for example, electrodeposition can be employed to produce mixtures of tungsten oxide together with three or even four additional metal oxides. 96 A notable mixture is W–Cr–Mo–Ni oxide, 96 which forms a green electrochromic colour – a colour not often seen in the field of inorganic electrochromism, and, though insufficiently analysed, possibly not caused here by charge transfer. Most of these mixtures were prepared to ‘tweak’ the optical properties of a host oxide. For example, thin films of oxides based on Ni–V–Mg (made by reactive dc magnetron sputtering) show pronounced anodic electrochromism. The addition of magnesium significantly enhances the optical transparency of the films in their bleached state, 839 over the wavelength range 400 <`<500 nm. With counter-electrode use in mind, Orel and co-workers 166 made V–Ti–Zr and V–Ti–Ce oxides, and Avandano et al. made CeO 2 –TiO 2 –ZrO 2 , 156 NiV 0.08 Mg 0.5 oxides, 832 and CeO 2 –TiO 2 –ZrO 2 . 156 Samples of NiOÁ WO x P y were obtained from a polytungsten gel in which H 3 PO 4 was added. The electrochromism was optimised when the P:W ratio was 100:8.3. 140 Several other ternary oxides comprising three transition-metal oxides have received attention: the oxides of Co–Ni–Ir 868 and Cr–Fe–Ni (this latter oxide being grown anodically on the metallic alloy Inconel-600) 869 and W–V–Ti 6.4 Metal oxides: dual-metal electrochromes 203 oxide. 727 Ternary oxides comprising p-block metals include Co–Al–Si 617,798 and Ce–Mo–Si. 768 The electrochromic behaviour of the materials (WO 3 ) x (Li 2 O) y (MO) z where M¼Ce, Fe, Mn, Nb, Sb or V has also been studied. 870 Finally, Lian and Birss 871 have studied the electrochromism of the hydrous oxide layer formed on the alloy Ni 51 Co 23 Cr 10 Mo 7 Fe 5.5 B 3.5 . Its electrochromic behaviour is, apparently, similar to that of NiO x . 6.4.3 Electrochromic oxides incorporating precious metals Several workers have incorporated particulate precious metal in an oxide host. Table 6.14 lists a few such studies. Such composites can be made in various ways: dual-target sputtering, mixed sputtering and sol–gel, or all sol–gel. 161 In the study of Au–NiO films by Fantini et al., 874 the Au mole fraction of gold varied between from 0.0 to 0.05. The films reflected the different colours blue, green, yellow and orange– red, depending on mole fraction. The electrochromic ceramic metal (‘cermet’) Au–WO 3 prepared by Sichel and Gittleman 879 comprised a matrix of amorphous WO 3 containing grains of Au of approximate diameter 20–120 A ˚ . The cermet is blue as prepared, but is redor pink when electrochemically coloured – a relatively rare colour for an electrochromic oxide. The matrix must be amorphous in order for the red colour to develop. In the study in ref. 877, Yano et al. also incorporated particulate gold (and V 2 O 5 ) in an aramid resin. Table 6.14. Electrochromic mixtures of metal oxide incorporating precious metal. Precious metal Host Ref. Ag ITO 800, 872 Ag V 2 O 5 873 Ag WO 3 830 Au CoO 874 Au IrO 2 531 Au NiO 161, 874, 875 Au MoO 3 495, 876 Au V 2 O 5 531, 877, 878 Au WO 3 201, 830, 879, 880, 881 Pt MoO 3 495 Pt RuO 2 882 Pt Ta 2 O 5 883 Pt WO 3 879, 884 204 Metal oxides 6.4.4 Metal oxyfluorides Many thin-film metal oxyfluorides are electrochromic. In the literature, the exact stoichiometry is often indefinite or unknown. In effect, they represent fluorinated analogues of the respective metal oxide. For this reason, we term the oxides, ‘F:MO x ’. Tin Films of F:SnO 2 were made by reactive rf sputtering in Ar þO 2 þCF 4 atmosphere. Rutherford backscattering (RBS) suggests the film composition is SnO 2.1 F 0.6 C 0.3 . 885 When such films are immersed in PC containing LiClO 4 , the electrochromic effect is weak. The redox reaction causing the colour is: F:SnO 2 þxðLi þ þe À Þ !Li x F:SnO 2 . (6.44) It is easier to electro-insert Li þ into SnO 2 electrodes than into fluorinated F:SnO 2 . 885 For this reason, fluorinated tin oxide is superior as an optically transparent electrode, but is a poor electrochromic oxide. Titanium Thin-film titanium oxyfluoride is made by reactive dc sputtering in an Ar þO 2 þCF 4 atmosphere. The amount of fluorine incorporated in the film is quite small: results from RBS suggest a composition of TiO 1.95 F 0.1 . 886 When such films are immersed in PC containing LiClO 4 , the electrochromic effect is ‘pronounced’. The redox reaction causing the colour is: F:TiO 2 þxðLi þ þe À Þ !Li x F:TiO 2 . (6.45) The coloration efficiency is 37 cm 2 C À1 at 700 nm, the colour said to derive from photo-effected polaron interaction. The cycle life is as high as 2 Â10 4 cycles. 887 As expected, the diffusion of Na þ or K þ through F:TiO 2 is too slow to countenance inclusion within devices. In fact, structural changes accompany the incorporation of K þ . 888 Tungsten Granqvist and co-workers 889 made thin-film tungsten oxyfluoride by reactive dc magnetron sputtering in plasmas containing O 2 þCF 4 . Elevated target temperatures yielded strongly enhanced rates of electrochromic coloration. The coloration efficiency j is 60 cm 2 C À1 , and the wavelength maximum occurs at $780 nm. 890 The redox reaction causing the colour is: F:WO 3 þxðLi þ þe À Þ !Li x F:WO 3 . (6.46) 6.4 Metal oxides: dual-metal electrochromes 205 The durability of such films with extensive Li þ intercalation and egress was said to be poor, but the electrochromic colour–bleach dynamics are faster than for films of WO 3 . Covering the film with a thin, protective layer of electron- bombarded WO 3 yields an electrochrome with rapid dynamics and good durability. 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Phys., 78, 1995, 1968–74. 252 Metal oxides 7 Electrochromism within metal coordination complexes 7.1 Redox coloration and the underlying electronic transitions Metal coordination complexes show promise as electrochromic materials because of their intense coloration and redox reactivity. 1 Chromophore properties arise from low-energy metal-to-ligand charge-transfer (MLCT), intervalence charge-transfer (IVCT), intra-ligand excitation, and related visible-region electronic transitions. Because these transitions involve valence electrons, chromophoric characteristics are altered or eliminated upon oxidation or reduction of the complex, as touched on in Chapter 1. A familiar example used in titrations is the redox indicator ferroin, [Fe II (phen) 3 ] 2þ (phen ¼1,10-phenanthroline), which has been employed in a solid-state ECD, the deep red colour of which is transformed to pale blue on oxidation to the iron(III) form. 2 Often more markedly than other chemical groups, a coloured metal coordination complex susceptible to a redox change will in general undergo an accompanying colour change, and will therefore be electrochromic to some extent. The redox change – electron loss or gain – can be assigned to either the central coordinating cation or the bound ligand(s); often it is clear which, but not always. If it is the central cation that undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals, while the less clear convention for ligands is usually to indicate the extra charge lost or gained by a superscripted þ or À. As mentioned in Chapter 1, whilst the term ‘coloured’ generally implies absorption in the visible region, metal coordination complexes that switch between a colourless state and a state with strong absorption in the near infra red (NIR) region are now being intensively studied. 3 While these spectroscopic and redox properties alone would be sufficient for direct use of metal coordination complexes in solution-phase ECDs, in addition, polymeric systems based on metal coordination-complex monomer units, which have prospective use in all-solid-state systems, have also been investigated. Following usage in the field, in this chapter an arrow between two species can indicate the direction of transfer of an electron. 253 7.2 Electrochromism of polypyridyl complexes 7.2.1 Polypyridyl complexes in solution The complexes [M II (bipy) 3 ] 2 þ (M ¼ Fe, Ru, Os; bipy ¼2,2 0 -bipyridine) are respectively red, orange and green, due to the presence of an intense MLCT absorption band. 4 Electrochromismresults fromloss of the MLCTabsorption band on switching to the M III redox state. Such complexes also exhibit a series of ligand-based redox processes, the first three of which are accessible in solvents such as acetonitrile and dimethylformamide (DMF). 4 Attachment of electron-withdrawing substituents to the 2,2 0 -bipyridine ligands allows additional ligand-based redox processes to be observed, due to the anodic shift of the redox potentials induced by these substituents. Thus Elliott and co-workers have shown that a series of colours is available with [M(bipy) 3 ] 2 þ derivatives when the 2,2 0 -bipyridine ligands have electron-withdrawing substituents at the 5,5 0 positions (see below). 5 The electrochromic colours established by bulk electrochemical reactions in acetonitrile are given in Table 7.1. N N R R L 1 R = CO 2 Et L 2 R = CONEt 2 L 3 R = CON(Me)Ph L 4 R = CN L 5 R = C(O) n Bu A surface-modified polymeric system can be obtained by spin coating or heating [Ru(L 6 ) 3 ] 2 þ as its p-tosylate salt. 6 The resulting film shows sevencolour electrochromism with colours covering the full visible region spectral range, which can be scanned in 250 ms. N N O O O O O O O L 6 Spectral modulation in the NIR region has been reported for the related complex [Ru(L 7 ) 3 ] 2 þ which undergoes six ligand-centred reductions, two per ligand. 7 The complex initially shows no absorption between 700 and 2100 nm; however, upon reduction by one electron a very broad pair of overlapping peaks appear with maxima at 1210 nm (" ¼2600dm 3 mol À1 cm À1 ) and 1460 nm (" ¼3400dm 3 mol À1 cm À1 ). Following the second one-electron reduction, the peaks shift to slightly lower energy (1290 and 1510 nm) and increase in 254 Electrochromism within metal coordination complexes intensity (" ¼6000 and 7300dm 3 mol À1 cm À1 respectively). Following the third one-electron reduction, the two peaks coalesce into a broad absorption at 1560nm, which is again enhanced in intensity (" ¼12000 dm 3 mol À1 cm À1 ). Upon reduction by the fourth and subsequent electrons the peak intensity diminishes continuously to approximately zero for the six-electron reduction product. These NIR transitions are almost exclusively ligand-based. An optically transparent thin-layer electrode (OTTLE) study 8 revealed that the visible spectra of the reduced forms of [Ru(bipy) 3 ] 2þ derivatives can be separated into two classes. Type-A complexes, such as [Ru(bipy) 3 ] 2þ , [Ru(L 7 ) 3 ] 2þ and [Ru(L 1 ) 3 ] 2þ show spectra on reduction which contain lowintensity (" <2500dm 3 mol À1 cm À1 ) bands; these spectra are similar to those of the reduced free ligand and are clearly associated with ligand radical anions. In contrast, type-B complexes such as [Ru(L 8 ) 3 ] 2þ and [Ru(L 9 ) 3 ] 2þ on reduction exhibit spectra containing broad bands of greater intensity (1000<" <15000dm 3 mol À1 cm À1 ). N N R R L 7 R = Me L 8 R = COOEt L 9 R = CONEt 2 7.2.2 Reductive electropolymerisation of polypyridyl complexes The reductive electropolymerisation technique relies on the ligand-centred nature of the three sequential reductions of complexes such as [Ru(L 10 ) 3 ] 2 þ (L 10 ¼4-vinyl-4 0 -methyl-2,2 0 -bipyridine), combined with the anionic polymerisability of suitable ligands. 9 Vinyl-substituted pyridyl ligands such as L 10 –L 12 are generally employed, although metallopolymers have also been formed Table 7.1. Colours (established by bulk electrolysis in acetonitrile) of the ruthenium(II) tris-bipyridyl complexes of the ligands L 1 –L 5 , in all accessible oxidation states (from ref. 5). Charge on RuL 3 unit L 1 L 2 L 3 L 4 L 5 þ2 Orange Orange Orange Red–orange Red–orange þ1 Purple Wine red Grey–blue Purple Red–brown 0 Blue Purple Turquoise Blue Purple–brown À1 Green Blue Green Turquoise Grey–blue À2 Brown Aquamarine Green À3 Red Brown–green Purple 7.2 Electrochromism of polypyridyl complexes 255 from chloro-substituted pyridyl ligands, via electrochemically initiated carbon– halide bond cleavage. In either case, electrochemical reduction of their metal complexes generates radicals leading to carbon–carbon bond formation and oligomerisation. Oligomers above a critical size are insoluble and thus thin films of the electroactive metallopolymer are deposited on the electrode surface. N N N N N N L 11 L 10 L 12 7.2.3 Oxidative electropolymerisation of polypyridyl complexes Oxidative electropolymerisation has been described for iron(II) and ruthenium(II) complexes containing amino- 10 and pendant aniline-substituted 11 2,2 0 -bipyridyl ligands, and amino- and hydroxy- substituted 2,2 0 :6 0 ,2 00 -terpyridinyl ligands. 12 Analysis of IR spectra suggests that the electropolymerisation of [Ru(L 13 ) 2 ] 2þ , via the pendant aminophenyl substituent, proceeds by a reaction mechanism similar to that of aniline. 12 The resulting modified electrode reversibly switched frompurple to pale pink on oxidation of Fe II to Fe III . For polymeric films formed from [Ru(L 14 ) 2 ] 2þ , via polymerisation of the pendant hydroxyphenyl group, the colour switch was from brown to dark yellow. The dark yellow was attributed to an absorption band at 455nm, probably due to quinone moieties in the polymer formed during electropolymerisation. Infrared spectra confirmed the absence of hydroxyl groups in the initially deposited brown films. Metallopolymer films have also been prepared by oxidative polymerisation of complexes of the type [M(phen) 2 (4,4 0 -bipy) 2 ] 2þ (M ¼ Fe, Ru or Os; 4,4 0 -bipy ¼ 4,4 0 -bipyridine). 13 Such films are both oxidatively and reductively electrochromic; reversible film-based reduction at potentials below À1 V results in dark purple films, 13 the colour and potential region being consistent with the viologen-dication/radical-cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L 15 ) 3 ] 2 þ . 14 Electropolymerised films prepared from the complexes [Ru(L 16 )(bipy) 2 ] [PF 6 ] 2 15 and [Ru(L 17 ) 3 ] [PF 6 ] 2 16,17 exhibit reversible orange– transparent electrochromic behaviour associated with the Ru II /Ru III interconversion. 256 Electrochromism within metal coordination complexes N N N NH 2 N N N OH L 13 L 14 N N O O N O O L 15 N N Fe Fe L 16 N N OMe OMe L 17 7.2.4 Spatial electrochromism of polymeric polypyridyl complexes Spatial electrochromism has been demonstrated in metallopolymeric films. 18 Photolysis of poly[Ru II (L 10 ) 2 (py) 2 ]Cl 2 thin films on tin-doped indium oxide- coated (ITO) glass in the presence of chloride ions leads to photochemical loss of the photolabile pyridine ligands, and sequential formation of poly[Ru II (L 10 ) 2 (py)Cl]Cl and poly[Ru II (L 10 ) 2 Cl 2 ] (see Scheme 7.1). poly[Ru II (L 10 ) 2 (py) 2 ]Cl 2 (orange) E f (Ru III/II ) = + 1.27 Vvs. SCE poly[Ru II (L 10 ) 2 (py)Cl]Cl (red) E f (Ru III/II ) = + 0.77 Vvs. SCE poly[Ru II (L 10 ) 2 Cl 2 ] (purple) E f (Ru III/II ) = + 0.35 Vvs. SCE hν –py hν –py Scheme 7.1 Spatial electrochromism in metallopolymeric films using photolabile pyridine ligands. (Scheme reproduced from Leasure, R. M., Ou, W., Moss, J. A., Linton, R. W. and Meyer, T. J. ‘Spatial electrochromism in metallo-polymeric films of ruthenium polypyridyl complexes.’ Chem. Mater., 8, 1996, 264–73, with permission of The American Chemical Society.) 7.2 Electrochromism of polypyridyl complexes 257 Contact lithography can be used to spatially control the photosubstitution process to form laterally resolved bicomponent films with image resolution below 10 mm. Dramatic changes occur in the colours and redox potentials of such ruthenium(II) complexes upon substitution of chloride for the pyridine ligands (Scheme 7.1). Striped patterns of variable colours are observed on addressing such films with a sequence of potentials. 7.3 Electrochromism in metallophthalocyanines and porphyrins 7.3.1 Introduction to metal phthalocyanines and porphyrins The porphyrins are a group of highly coloured, naturally occurring pigments containing a tetrapyrrole porphine nucleus (see below) with substituents at the eight b-positions of the pyrroles, and/or the four meso-positions between the pyrrole rings. 19 The natural pigments themselves are metal chelate complexes of the porphyrins. Phthalocyanines are tetraazatetrabenzo derivatives of porphyrins with highly delocalised p-electron systems. Metallophthalocyanines are 21H,23H-Porphine N HN NH N Tetraphenyl porphyrin (H 2 TPP) N HN NH N 29H,31H-Phthalocyanine N N HN N NH N N N 1:1 Metallophthalocyanine complex N N N N N N N N M N N N N N N N N N N N N N N N N M A'sandwich'-type metallophthalocyanine complex N HN NH N Et Et Et Et Et Et Et Et Octaethyl porphyrin (H 2 OEP) 258 Electrochromism within metal coordination complexes important industrial pigments, blue to green in colour, used primarily in inks and for colouring plastics and metal surfaces. 19,20,21 The water-soluble sulfonate derivatives are used as dyestuffs for clothing. In addition to these uses, the metallophthalocyanines have been extensively investigated in many fields including catalysis, liquidcrystals, gas sensors, electronic conductivity, photosensitisers, non-linear optics and electrochromism. 20 The purity and depth of the colour of metallophthalocyanines arise from the unique property of having an isolated, single band located in the far-red end of the visible spectrum(near 670 nm), with " often exceeding 10 5 dm 3 mol À1 cm À1 . The next, more energetic, set of transitions is generally much less intense, near 340nm. Charge transfer transitions between a chosen metal and the phthalocyanine ring introduce additional bands around 500nm that allow tuning of the hue. 20 The metal ion in metallophthalocyanines lies either at the centre of a single phthalocyanine (Pc ¼ dianion of phthalocyanine), or between two rings in a sandwich-type complex. 20 Phthalocyanine complexes of transition metals usually contain only a single Pc ring while lanthanide-containing species usually form bis(phthalocyanines), where the p-systems interact strongly with each other, resulting in characteristic features such as the semiconducting (¼5Â10 À5 O À1 cm À1 ) properties of thin films of bis- (phthalocyaninato)lutetium(III) [Lu(Pc) 2 ]. 22 7.3.2 Sublimed bis(phthalocyaninato)lutetium(III) films The electrochromism of the phthalocyanine ring-based redox processes of vacuum-sublimed thin films of [Lu(Pc) 2 ] was first reported in 1970, 23 and since that time this complex has received most attention, although many other (mainly lanthanide) metallophthalocyanines have been investigated for their electrochromic properties. The complex Lu(Pc) 2 has been studied extensively by Collins and Schiffrin 24,25 and by Nicholson and Pizzarello. 26,27,28,29,30,31 It was initially studiedas a filmimmersedin aqueous electrolyte, but hydroxide ion from water causes gradual film destruction, attacking nitrogens of the Pc ring. 24 Acidic solution allows a greater number of stable write–erase cycles, up to 5 Â10 6 cycles in sulfuric acid, 24 approaching exploitable device requirements. Films of [Lu(Pc) 2 ] inethylene glycol solutionwere found tobe even more stable. 25 Fresh [Lu(Pc) 2 ] films (likely to be singly protonated, 31 although this issue is contentious 24,32 ), which are brilliant green in colour ( max ¼605 nm), are electro-oxidised to a yellow–tan form, Eq. (7.1): 26,29,32 ½Pc 2 LuH þ ðsÞ ! ½Pc 2 Lu þ ðsÞ þ H þ þ e À : green yellow-tan (7:1) 7.3 Metallophthalocyanines and porphyrins 259 A further oxidation product is red, 26,29,32 yet of unknown composition. Electroreduction of [Lu(Pc) 2 ] films gives a blue-coloured film, Eq. (7.2): 33 ½Pc 2 LuH þ ðsÞ þ e À ! ½Pc 2 LuH ðsÞ; green blue (7:2) with further reduction yielding a violet–blue product, Eqs. (7.3) and (7.4): 29 ½Pc 2 LuH ðsÞ þ e À ! ½Pc 2 LuH À ðsÞ; blue violet (7:3) ½Pc 2 LuH À ðsÞ þ e À ! ½Pc 2 LuH 2À ðsÞ: (7:4) The lutetium bis(phthalocyanine) system is a truly electropolychromic one, 23 but usually only the blue-to-green transition is used in ECDs. Although prototypes have been constructed, 34 no ECD incorporating [Lu(Pc) 2 ] has yet been marketed, owing to experimental difficulties such as film disintegration caused by constant counter-anion ingress/egress on colour switching. 24 For this reason, larger anions are best avoided to minimise the mechanical stresses. A second, related, handicap of metallophthalocyanine electrochromic devices is their relatively long response times. Nicholson and Pizzarello 30 investigated the kinetics of colour reversal and found that small anions like chloride and bromide allow faster colour switching. Sammells and Pujare overcame the problem of slow penetration of anions into solid lattices by using an ECD containing an electrochrome suspension in semi-solid poly(AMPS) – AMPS¼2-acrylamido-2- methyl propane sulfonic acid) electrolyte. 34 While the response times are still somewhat long, the open-circuit life times (‘memory’ times) of all colours were found to be very good. 30 Films in chloride, bromide, iodide and sulfatecontaining solutions were found to be especially stable in this respect. 7.3.3 Other metal phthalocyanines Moskalev et al. prepared the phthalocyanine complexes of neodymium, americium, europium, thorium and gallium (the latter as the half acetate). 35 Collins and Schiffrin 24 have reported the electrochromic behaviour of the phthalocyanine complexes CoPc, SnCl 2 Pc, SnPc 2 , MoPc, CuPc and the metal-free H 2 Pc. No electrochromism was observed for either the metal-free or for the copper phthalocyanines in the potential ranges employed; all of the other complexes showed limited electrochromism. Both SnCl 2 Pc and SnPc 2 260 Electrochromism within metal coordination complexes could be readily reduced, but showed no anodic electrochromism. Other molecular phthalocyanine electrochromes studied include complexes of aluminium, 36 copper, 37 chromium, 36,38 erbium, 39 europium, 40 iron, 41 magnesium, 42 manganese, 38,42 titanium, 43 uranium, 44 vanadium, 43 ytterbium, 45,46 zinc 47 and zirconium. 36,40,48 Mixed phthalocyanine systems have also been prepared by reacting mixed-metal precursors comprising the lanthanide metals dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium and small amounts of others; 49 the response times for such mixtures are reportedly superior to those for single-component films. Walton et al. have compared the electrochemistry of lutetium and ytterbium bis(phthalocyanines), finding them to be essentially identical. 50 Both chromium and manganese mono-phthalocyanine complexes undergo metal-centred oxidation and reduction processes. 38 In contrast, the redox reactions of LuPc 2 occur on the ligand; electron transfer to the central lutetium causes molecular dissociation. 51 Lever and co-workers have studied cobalt phthalocyanine systems in which two or four Co(Pc) units are connected via chemical links. 52,53,54,55 This group has also studied tetrasulfonated cobalt and iron phthalocyanines. 56 Finally, polymeric ytterbium bis(phthalocyanine) has been investigated 57,58,59 using a plasma to effect the polymerisation. 7.3.4 Electrochemical routes to metallophthalocyanine electrochromic films For complexes with pendant aniline and hydroxy-substituted ligands, oxidative electropolymerisation is an alternative route to metallophthalocyanine electrochromic films. Although polymer films prepared from[Lu(L 18 ) 2 ] monomer show loss of electroactivity on being cycled to positive potentials, in dimethyl sulfoxide (DMSO) the electrochemical response at negative potentials is stable, with the observation of two broad quasi-reversible one-electron redox couples. 60 Spectroelectrochemical measurements revealed switching times of <2 s for the observed green–grey–blue colour transitions in this region. Oxidative electropolymerisation using pendant aniline substituents has also been applied to monophthalocyaninato transition-metal complexes; 61 the redox reactions and colour changes of two of the examples studied are given in Eqs. (7.5)–(7.8). poly½Co II ðL 18 Þ þ ne À ! poly½Co I ðL 18 Þ À ; blue-green yellow-brown (7:5) poly½Co I ðL 18 Þ À þ ne À ! poly½Co I ðL 18 Þ 2À ; yellow-brown red-brownðthickfilmsÞ; deeppinkðthin filmsÞ (7:6) 7.3 Metallophthalocyanines and porphyrins 261 poly½Ni II ðL 18 Þ þ ne À ! poly½Ni II ðL 18 Þ À ; green blue (7:7) poly½Ni II ðL 18 Þ À þ ne À ! poly½Ni II ðL 18 Þ 2À : blue purple (7:8) The first reduction in the cobalt-based polymer is metal-centred, resulting in the appearance of a new MLCT transition, the second reduction being ligand- centred. By contrast, for the nickel-based polymer both redox processes are ligand-based. N N N N N N N N R R R R L 18 R = NH 2 L 19 R = O–(2-C 6 H 4 OH) L 20 R = CO 2 –CH 2 CH 2 CMe 3 NB These complexes exist as a mixture ofisomers with the substituents attachedat either ofthe positions labelled • on the benzyl rings. Electrochromic polymer films have been prepared by oxidative electropolymerisation of the monomer [Co(L 19 )]. 62 The technique involved voltammetric cycling from À0.2 to þ1.2 V vs. SCE at 100 mV s À1 in dry acetonitrile, resulting in the formation of a fine green polymer. Cyclic voltammograms during polymer growth showed the irreversible phenol oxidation peak at þ0.58 V and a reversible phthalocyanine-ring oxidation peak at þ0.70 V. Polymer-modified electrodes gave two distinct redox processes with half-wave potentials at À0.35 [from Co II ! Co I ] and À0.87V (from ring reduction). The coloration switched from transparent light green [Co II state] to yellowish green [Co I state] to dark yellow (reduced ring). 7.3.5 Langmuir–Blodgett metallophthalocyanine electrochromic films The electrochemical properties of a variety of metallophthalocyanines have been studied as multilayer Langmuir–Blodgett (LB) films. For example, LB films of alkyloxy-substituted [Lu(Pc) 2 ] exhibited a one-electron reversible reduction and a one-electron reversible oxidation corresponding to a transition from green to orange and blue forms respectively, with the electron transport through the multilayers being at least in part diffusion controlled. 63 262 Electrochromism within metal coordination complexes An explanation of the relatively facile redox reaction in such multilayers is that the Pc ring is large compared with the alkyl tail, and there is enough space and channels present in the LB films to allow the necessary charge-compensating ion transport. More recently, the structure, electrical conductivity and electrochromism in thin films of substituted and unsubstituted lanthanide bis-phthalocyanine derivatives have been investigated with particular reference to the differences between unsubstituted and butoxy-substituted [Lu(Pc) 2 ] materials. 64 Scanning tunnelling microscopy (STM) images on graphite reveal the differences in the two structures, giving molecular dimensions of 1.5 Â1.0 nm and 2.8 Â1.1nmrespectively. The in-plane dc conductivity was studied as a function of film thickness and temperature, with unsubstituted [Lu(Pc) 2 ] being approximately 10 6 times more conductive than the substituted material. The green–red oxidative step is seen for both cases but the green–blue reductive step is absent in the butoxy-substituted material. High-quality LB films of [M(L 20 )] (M ¼ Cu, Ni) have also been reported. 65 Ellipsometric and polarised optical absorption measurements suggest that the Pc rings are oriented with their large faces perpendicular to the immersion direction and to the substrate plane. The LB technique may be used for the fabrication of ECDs: an LB thin- film display based on bis(phthalocyaninato)praseodymium(III) has been reported. 66 The electrochromic electrode was fabricated by deposition of multilayers (10–20 layers, % 100–200 A ˚ ) of the complex onto ITO-coated glass (7 Â4 cm 2 ) slides. The display exhibited blue–green–yellow–red electropolychromism over a potential range of À2 to þ2 V. After 10 5 cycles no significant changes are observed in the spectra of these colour states, again approaching exploitability. The high stability of the device was ascribed to the preparation, by the LB technique, of well-ordered mono layers that allow better diffusion of the counter ions into the film, which improves reversibility. Unless these structures provide ion channels, ordered structures might be considered to favour electronic rather than ionic motion, as the latter could benefit more from defects arising from disorder. 7.3.6 Species related to metallophthalocyanines Naphthalocyanine (nc) species are structurally similar to the simpler phthalocyanines described above and have two isomers, denoted here (2,3-nc) and (1,2-nc). Naphthalocyanines show an intense optical absorption at long wavelengths (700 <<900 nm) owing to electronic processes within the extended conjugated system of the ligand. 67,68 Thin-film [Co(2,3-nc) 2 ] is green and is readily oxidised to forma violet-coloured species. Thin-film[Zn(2,3-nc) 2 ] is also green 7.3 Metallophthalocyanines and porphyrins 263 when neutral. A ‘triple-decker’ naphthalocyanine compound [(1,2-nc)Lu(1,2- nc)Lu(1,2-nc)] has been reported. 69 Electrochromism in the pyridinoporphyrazine system and its cobalt complex has also received some attention. 70 Here, the ligand is similar to a phthalocyanine but with quaternised pyridyl residues replacing all four fused benzo groups. It is not only homoleptic (i.e. all ligands similar) phthalocyanine complexes that can form sandwich structures; recently a substantial number of heteroleptic sandwich-type metal complexes, with mixed phthalocyaninato and/or porphyrinato ligands, have been synthesised and are likely to show interesting electrochromic properties. 71 Although considerable progress has been made in this field, there is clearly much room for further investigation. By attaching functional groups or special (donor or acceptor) moieties to these compounds, it may be possible to tune their electronic properties without altering the ring- to-ring separation. The properties associated with these units may also be imparted to the parent sandwich compounds. The electrochromic properties of some silicon–phthalocyanine thin films, in which a redox active ferrocenecarboxylato unit is appended to the electrochromic centre, have been studied. 72 7.3.7 Electrochromic properties of porphyrins Early results suggest that an investigation into porphyrin electrochromism is warranted, although there has been little systematic study to date. Thus, the spectra of the chemical reduction products of Zn(TPP) have been N N HN N NH N N N 2,3-Naphthalocyanine N N HN N NH N N N 1,2-Naphthalocyanine 264 Electrochromism within metal coordination complexes reported, 73,74 with colours changing between a pink (parent complex), green (mono-negative ion), and amber (di-negative ion). 73 Felton and Linschitz 75 reported that the electrochemically produced monoanion spectrum is similar to that produced chemically. Fajer et al. 76 showed that Zn(TPP) changes colour to green upon one-electron oxidation by controlled potential electrolysis. Felton et al. 77 reported that the electrolysis of Mg(OEP) yielded a blue– green solution. The recently reported 78 green–pink colour change of a porphyrin monomer appears to be a pH-change-induced transformation of the J-aggregate (ordered molecular arrangement, excited state spread over N molecules in one dimension) to the monomer, and is therefore electrochromic only indirectly, from the redox viewpoint. Recently it has been found that oxidative electropolymerisation of substituted porphyrins could be useful towards the development of electrochromic porphyrin devices. 79 7.4 Near-infrared region electrochromic systems 7.4.1 Significance of the near-infrared region The metal complexes described so far in this chapter have been of interest for their electrochromism in the visible region of the spectrum, a property which is of obvious interest for use in display devices and windows. Electrochromismin the near-infrared (NIR) region of the spectrum (ca. 800–2000 nm) is an area which has also attracted much recent interest 3 because of the considerable technological importance of this region of the spectrum. Near-infrared radiation finds use in applications as diverse as optical data storage, 1 in medicine, where photodynamic therapy exploits the relative transparency of living tissue to NIR radiation around 800 nm, 80 and in telecommunications, where fibre- optic signal transmission through silica fibres exploits the ‘windows of transparency’ of silica in the 1300–1550 nm region. Near-infrared radiation is also felt as radiant heat, so NIR-absorbing or reflecting materials could have use in smart windows that allow control of the environment inside buildings; and the fact that much of the solar emission spectrum is in the NIR region means that effective light-harvesting compounds for use in solar cells need to capture NIR as well as visible light. 81 Many molecules with strong NIR absorptions have been investigated, often with a view to examining their performance as dyes in optical data-storage media. 82,83,84 The majority of these are highly conjugated organic molecules that are not redox active. A minority however are based on transition-metal complexes and it is generally these which have the redox activity necessary for 7.4 Near-infrared region electrochromic systems 265 electrochromic behaviour and which are discussed in the following sections. One such set of complexes has already been discussed in this chapter: spectral modulation in the NIR region has been reported for a variety of [Ru(bipy) 3 ] 2 þ derivatives, whose reduced forms contain ligand radical anions that show intense, low-energy electronic transitions. Since these are also electrochromic in the visible region, they were discussed earlier in Section 7.2.1. 7 Near-infrared electrochromic materials based on doped metal oxides 85 (see Chapter 6) and conducting polymeric films 86 (see Chapter 10) are also extensively studied. 7.4.2 Planar dithiolene complexes of Ni, Pd and Pt One of the earliest series of metal complexes which showed strong, redoxdependent NIRabsorptions is the well-studied set of square-planar bis-dithiolene complexes of Ni, Pd and Pt (see below). Extensive delocalisation between metal and ligand orbitals in these ‘non-innocent’ systems means that assignment of oxidation states is problematic, but it does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic and dianionic species. The structures and redox properties of these complexes have been extensively reviewed; 87,88 of interest here is the presence of an intense NIRtransition in the neutral and monoanionic forms, but not the dianionic forms, i.e. the complexes are electropolychromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. Alarge number of substituted dithiolene ligands have been prepared and used to prepare complexes of Ni, Pd and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1400 nm and extinction coefficients up to ca. 40 000 dm 3 mol À1 cm À1 (see refs. 87 and 88 for an extensive listing). The main application of the strong NIR absorbance of these complexes, pioneered by M ¨ uller-Westerhoff and co-workers, 88,89 is for use in the neutral state as dyes to induce Q-switching of NIR lasers such as the Nd-YAG S M S R R S R R n– Generic structure ofplanar bis-dithiolene complexes: M = Ni,Pd,Pt; n = 0,1,2 S S M S S S N N N N S S R R R R Complexes ofdialkyl-substituted imidazolidine-2,4,5-trithiones (M = Ni,Pd) (refs. 90,91,92,93,94,95,96) 266 Electrochromism within metal coordination complexes (1064 nm), iodine (1310 nm) and erbium (1540 nm) lasers. This relies on a combination of very high absorbance at the laser wavelengths, an appropriate excited-state lifetime following excitation, and good long-term thermal and photochemical stability. The use of a range of metal dithiolene complexes in this respect has been reviewed. 88,89 The strong NIR absorptions of these complexes have continued to attract attention since these reviews appeared. A new series of neutral, planar dithiolenes of Ni and Pd has been prepared based on the ligands [R 2 timdt] À which contain the dialkyl-substituted imidazolidine-2,4,5-trithione core (see above). 90,91,92,93,94,95,96 In these ligands the peripheral ring system ensures that the electron-donating N substituents are coplanar with the dithiolene unit, maximising the electronic effect. This shifts the NIR absorptions of the [M(R 2 timdt) 2 ] complexes to lower energy than found in the ‘parent’ dithiolene complexes. The result is that the NIR absorption maximum occurs at around 1000nm and has a remarkably high extinction coefficient (up to 80000 dm 3 mol À1 cm À1 ). The high thermal and photochemical stabilities of these complexes make them excellent candidates for Q-switching of the 1064 nmNd-YAG laser. In addition, one-electron reduction to the monoanionic species [M(R 2 timdt) 2 ] À results in a shift of the NIR absorption maximum to ca. 1400nm, indicating possible exploitation of their electrochromism. 96 7.4.3 Mixed-valence dinuclear complexes of ruthenium Another well-known class of metal complexes showing NIR electrochromism is the extensive series of dinuclear mixed-valence complexes based principally on ruthenium–ammine or ruthenium–polypyridine components, in which a strong electronic coupling between the metal centres makes a stable Ru II –Ru III mixed-valence state possible. Such complexes generally show a Ru II !Ru III IVCT transition which is absent in both the Ru II –Ru II and Ru III –Ru III forms. These complexes have primarily been of interest because the characteristics of the IVCT transition provide quantitative information on the magnitude of the electronic coupling between the metal centres, and is accordingly an excellent diagnostic tool. Nevertheless, the position and intensity of the IVCTtransition in some cases mean that complexes of this sort could be exploited for their optical properties. Table 7.2 shows a small, representative selection of recent examples which show electrochromic behaviour (in terms of the intensity of the IVCT transitions) typical of this class of complex. 97,98,99,100 The main purpose of this selection is to draw the reader’s attention to the fact that these complexes which, as a class, are so familiar, in a different context could be equally valuable for their electrochromic properties. Of course the field 7.4 Near-infrared region electrochromic systems 267 is not limited to ruthenium complexes, although these have been the most extensively studied because of their synthetic convenience and ideal electrochemical properties; analogous complexes of other metals have also been prepared and could be equally effective NIR electrochromic dyes. Very recently, a trinuclear Ru II complex has been reported which shows a typical IVCT transition at 1550 nm in the mixed-valence Ru II –Ru III form. The complex has pendant hydroxyl groups which react with a tri-isocyanate to give a crosslinked polymer which was deposited on an ITO substrate. Good electrochromic switching of 1550 nm radiation was maintained, with fast switching times (of the order of 1 second), over several thousand redox cycles. 101 Table 7.2. Examples of mixed-valence dinuclear complexes showing NIR electrochromism. [3+] 2000 14 000 N N O O Ru(bipy) 2 (bipy) 2 Ru 97 [–] 1210 3 900 N N N Fe III (CN) 5 (H 3 N) 5 Ru II 99 [3+] 1920 10 000 N C N N N N N C Ru(terpy)(bpy) (bpy)(terpy)Ru 100 [3+] 1600 11 700 N N O O Ru(bipy) 2 98 (bipy) 2 Ru Complex Ref. λ /nm (ε/dm 3 mol –1 cm –1 ) 268 Electrochromism within metal coordination complexes 7.4.4 Tris(pyrazolyl)borato-molybdenum complexes In the last few years McCleverty, Ward and co-workers have reported the NIR electrochromic behaviour of a series of mononuclear and dinuclear complexes containing the oxo-Mo V core unit [Mo(Tp*)(O)Cl(OAr)], where ‘Ar’ denotes a phenyl or naphthyl ring system and [Tp* ¼hydrotris(3,5-dimethylpyrazolyl) borate]. 102,103,104,105,106,107 Mononuclear complexes of this type undergo reversible Mo IV –Mo V and Mo V –Mo VI redox processes with all three oxidation states accessible at modest potentials. Whilst reduction to the Mo IV state results in unremarkable changes in the electronic spectrum, oxidation to Mo VI results in the appearance of a low-energy phenolate- (or naphtholate)-to-Mo VI LMCT process. 102,103 In mononuclear complexes these transitions are at the low-energy end of the visible region and of moderate intensity: for [Mo(Tp*)(O)Cl(OPh)] for example the LMCTtransition is at 681 nmwith " ¼13 000 dm 3 mol À1 cm À1 . 103 However in many dinuclear complexes of the type [{Mo(Tp*)(O)Cl} 2 (m-OC 6 H 4 EC 6 H 4 O)], in which two oxo-Mo(V) fragments are connected by a bis- phenolate bridging ligand in which a conjugated spacer ‘E’ separates the two phenyl rings, the NIR electrochromism is much stronger. In these complexes an electronic interaction between the two metals results in a separation of the two Mo V –Mo VI couples, such that the complexes can be oxidised from the Mo V –Mo V state to Mo V –Mo VI and then Mo VI –Mo VI in two distinct steps. The important point here is that in the oxidised forms, containing one or two Mo VI centres, the LMCT transitions are at lower energy and of much higher intensity than in the mononuclear complexes (Figure 7.1 gives a representative example). 103,104,105 Depending on the nature of the group E in the bridging ligand, the absorption maxima can span the range 800–1500 nm, with extinction coefficients of up to 50 000 dm 3 mol À1 cm À1 (see Table 7.3) 103,104,105 A prototypical device to illustrate the possible use of these complexes for modulation of NIR radiation has been described. 106 A thin-film cell was prepared containing a solution of an oxo-Mo V dinuclear complex and base electrolyte between transparent, conducting-glass slides. The complex used has the spacer E ¼ bithienyl between the two phenolate termini (sixth entry in Table 7.3); this complex develops an LMCT transition (centred at 1360 nm, with " ¼30 000 dm 3 mol À1 cm À1 ) on one-electron oxidation to the Mo V –Mo VI state which is completely absent in the Mo V –Mo V state. Application of an alternating potential, stepping between þ1.5 V and 0 V for a few seconds each, resulted in fast switching on/off of the NIR absorbance reversibly over several thousand cycles. A larger cell was used to show how a steady increase in the potential applied to the solution, which resulted in a larger proportion of the 7.4 Near-infrared region electrochromic systems 269 material being oxidised, allowed the intensity of a 1300 nm laser to be attenuated reversibly and controllably over a dynamic range of 50 dB (a factor of ca. 10 5 ): the cell accordingly acts as a NIR variable optical attenuator. 106 The disadvantage of this prototype is that, being solution-based, switching is relatively slow compared to thin films or solid-state devices, but the optical properties of these complexes show great promise for further development. Some nitrosyl–Mo I complexes of the form [Mo(Tp*)(NO)Cl(py-R)] (where py-R is a substituted pyridine) also undergo moderate NIRelectrochromismon reversible reduction to the Mo 0 state. In these complexes reduction of the metal centre results in appearance of a Mo 0 ! py(p*) MLCT transition at the red end of the spectrum(for R¼4-CH( n Bu) 2 , max ¼830nmwith " ¼12000dm 3 mol À1 cm À1 ). However, when the pyridyl ligand contains an electron-withdrawing substituent meta to the N atom (R¼3-acetyl or 3-benzoyl) an additional MLCT transition at much longer wavelength develops ( max ¼1274 and 1514 nm, respectively, with " ca. 2400 dm 3 mol À1 cm À1 in each case). 107 7.4.5 Ruthenium and osmium dioxolene complexes Lever and co-workers described in 1986 how the mononuclear complex [Ru(bipy) 2 (CAT)], which has no NIR absorptions, undergoes two reversible λ /nm 400 0 60 0 +1 +2 CI(O)(Tp*)Mo Mo(Tp*)(O)CI O O n+ 800 1200 1600 2000 1 0 –3 ε /dm 3 m o l –1 c m –1 Figure 7.1 Electrochromic behaviour of [{Mo(Tp*)(O)Cl} 2 (m-OC 6 H 4 C 6 H 4 C 6 H 4 O)] nþ in the oxidation states Mo V –Mo V (n ¼0), Mo V –Mo VI (n ¼1), Mo VI –Mo VI (n¼2). Spectra were measured at 243K in CH 2 Cl 2 . (Figure reproduced from Harden, N. C., Humphrey, E. R., Jeffrey, J. C. et al. ‘Dinuclear oxomolybdenum(V) complexes which show strong electrochemical interactions across bis-phenolate bridging ligands: a combined spectroelectrochemical and computational study.’ J. Chem. Soc., Dalton Trans. 1999, 2417–26, with permission of The Royal Society of Chemistry.) 270 Electrochromism within metal coordination complexes oxidations whichare ligand-centredCAT–SQandSQ–Qcouples (where CAT, SQ andQare catecholate, 1,2-benzosemiquinone monoanion, and1,2-benzoquinone, respectively; see Scheme 7.2). 108 In the two oxidised forms the presence of a ‘hole’ in the dioxolene ligand results in the appearance of Ru II ! SQ and Ru II !Q MLCT transitions, the former at 890nm and the latter at 640nm with intensities of ca. 10 4 dm 3 mol À1 cm À1 . The CAT–SQand SQ–Qcouples accordingly result in modest NIR electrochromic behaviour (see structures L 21 –L 23 ). Table 7.3. Principal low-energy absorption maxima of dinuclear complexes [{Mo(Tp*)(O)Cl} 2 (L)] nþ in their oxidised forms (n ¼1, 2). O O O O O O S O O O S S O O O S O O 1096 (50) 1245 (19) 1131 (25) 1047 (24) 1197 (35) (Not stable) 1360 (30) 900 (10) Bridging ligand L λ max /nm (10 –3 ε/dm 3 mol –1 cm –1 ) O O O N N O 1210 (41) (Not stable) 1268 (35) 409 (38) O O 1554 (23) 978 (37) Mo(V)–Mo(VI) Mo(VI)–Mo(VI) 1017 (48) 832 (32) 1016 (62) 1033 (50) 684 (54) 900 (20) 7.4 Near-infrared region electrochromic systems 271 HO HO OH OH H 4 L 21 HO HO OH OH HO OH H 6 L 22 O HO HO O OH H 3 L 23 As with the oxo-Mo V complexes mentioned in the previous section, the NIR transitions become far more impressive when two or more of these chromophores are linked by a conjugated bridging ligand, as in [{Ru(bipy) 2 } 2 (m–L 21 )] nþ (n ¼0–4), which exhibits a five-membered redox chain, with reversible conversions between the fully reduced (bis-catecholate) and fully oxidised (bis-quinone) states all centred on the bridging ligand (Scheme 7.2). In the state n ¼2, the NIR absorption is at 1080 nm with " ¼37000dm 3 mol À1 cm À1 ; this disappears in the fully reduced form and moves into the visible region in the fully oxidised form. 109 Likewise, the trinuclear complex [{Ru(bipy) 2 } 3 (m–L 22 )] nþ (n ¼3–6) exists in four stable redox O O O O O O − e + e − e + e (CAT) 2– (SQ) – Q (a) O O O O − e + e O O O O − e + e O O O O (b) CAT–CAT CAT–SQ SQ–SQ − e + e O O O O − e + e O O O O SQ–Q Q–Q Scheme 7.2 Ligand-based redox activity of (a) the CAT–SQ–Q series; (b) [L 21 ] nÀ (n ¼4–0). 272 Electrochromism within metal coordination complexes Ru Ru Ru Ru Ru Ru Ru 100 Ru Ru Ru Ru Ru O O O O O O O O O O O O O O O 3+ 4+ 6+ 5+ +0.34 V +0.73 V +1.03 V O O SQ–SQ–SQ SQ–Q–Q Q–Q–Q SQ–SQ–Q SQ–SQ–Q SQ–SQ–SQ SQ–Q–Q Q–Q–Q 400 0 800 1200 1600 λ = 1083 nm λ = 1170 nm λ = 909 nm λ / nm λ = 759 nm ε = 50000 ε = 74000 ε = 40000 ε = 32000 O O O O O O – O 1 0 –3 ε /dm 3 m o l –1 c m –1 Figure 7.2 Ligand-centred redox interconversions of [{Ru(bipy) 2 } 3 (m–L 22 )] n þ (n ¼3–6) (potentials vs. SCE), and the resulting electrochromic behaviour. Spectra were measured at 243 K in MeCN. (Figure reproduced from Barthram, A. M., Cleary, R. L., Kowallick, R. and Ward, M. D. ‘A new redox-tunable near-IR dye based on a trinuclear ruthenium(II) complex of hexahydroxy-triphenylene.’ Chem. Commun. 1998, 2695–6, with permission of The Royal Society of Chemistry.) 7.4 Near-infrared region electrochromic systems 273 states based on redox interconversions of the bridging ligand (from SQ–SQ–SQ to Q–Q–Q; Figure 7.2). 110 Thus the complexes are electropolychromic, with a large number of stable oxidation states accessible in which the intense NIR MLCTtransitions involving the oxidised forms of the bridging ligand are redoxdependent. In this (typical) example, the NIR transitions vary in wavelength between 759 and 1170 nmover these four oxidation states, with intensities of up to 70 000 dm 3 mol À1 cm À1 . Other polydioxolene bridging ligands such as [L 23 ] 3 À have been investigated and their {Ru(bipy) 2 } 2 þ complexes show comparable electropolychromic behaviour in the NIR region. 111,112 The analogous complexes with osmiumhave also been characterised and, despite the differences in formal oxidation state assignment of the components (e.g. Os III –catecholate instead of Ru II –semiquinone), also show similar NIR electrochromic behaviour over several oxidation states. 113 Incorporation of these complexes into films or conducting solids, for faster switching, has yet to be described. Recently, a mononuclear [Ru(bipy) 2 (cat)] derivative bearing carboxylate substituents that anchor it to a nanocrystalline Sb-doped tin oxide surface has been reported. 114 Redox cycling of the catecholate–semiquinone couple results in fast electrochromic switching (of the order of one second) of the filmat 940 nmas the Ru II ! SQ MLCT transition appears and disappears. 114 References 1. Mortimer, R. 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Iodized polymeric Yb-diphthalocyanine films prepared by plasma polymerization method. Jpn. J. Appl. Phys., 31, 1992, 1892–6. References 277 60. Moore, D. J. and Guarr, T. F. Electrochromic properties of electrodeposited lutetium diphthalocyanine thin-films. J. Electroanal. Chem., 314, 1991, 313–21. 61. Li, H. F. and Guarr, T. F. Reversible electrochromism in polymeric metal phthalocyanine thin-films. J. Electroanal. Chem., 297, 1991, 169–83. 62. Kimura, M., Horai, T., Hanabusa, K. and Shirai, H. Electrochromic polymer derived from oxidized tetrakis(2-hydroxyphenoxy) phthalocyaninatocobalt(II) complex. Chem. Lett., 7, 1997, 653–4. 63. Besbes, S., Plichon, V., Simon, J. and Vaxiviere, J. Electrochromism of octaalkoxymethyl-substituted lutetium diphthalocyanine. J. Electroanal. Chem., 237, 1987, 61–8. 64. Jones, R., Krier, A. and Davidson, K. Structure, electrical conductivity and electrochromism in thin films of substituted and unsubstituted lanthanide bisphthalocyanines. 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References 281 8 Electrochromism by intervalence charge-transfer coloration: metal hexacyanometallates 8.1 Prussian blue systems: history and bulk properties Prussian blue – PB; ferric ferrocyanide, or iron(III) hexacyanoferrate(II) – first made by Diesbach in Berlin in 1704, 1 is extensively used as a pigment in the formulation of paints, lacquers and printing inks. 2,3 Since the first report 4 in 1978 of the electrochemistry of PB films, numerous studies concerning the electrochemistry of PB and related analogues have been made, 5,6,7 with, in addition to electrochromism, proposed applications in electroanalysis and electrocatalysis. 8,9,10,11 Fundamental studies 12,13,14 on basic PB properties (electronic structure, spectra and conductimetry) underlie the elaborations that follow. Prussian blue is the prototype of numerous polynuclear transition-metal hexacyanometallates, which form an important class of insoluble mixed- valence compounds. 15,16,17 They have the general formula M 0 k [M 00 (CN) 6 ] l (k, l integral) where M 0 and M 00 are transition metals with different formal oxidation numbers. These materials can contain ions of other metals and varying amounts of water. In PB the two transition metals in the formula are the two common oxidation states of iron, Fe III and Fe II . Prussian blue is readily prepared by mixing aqueous solutions of a hexacyanoferrate(III) salt with iron(II), the preferred industrial-production route (rather than iron(III) with a hexacyanoferrate(II) salt). In the PBchromophore, the distribution of oxidation states is Fe III –Fe II respectively; i.e. it contains Fe 3þ and [Fe II (CN) 6 ] 4À , as established by the CN stretching frequency in the IR spectrum and confirmed by M ¨ ossbauer spectroscopy. 18 The chromophore alone thus has a negative charge, therefore in the solid a counter cation is to be incorporated. The Fe III is usually high spin with H 2 O coordinated, whereas the Fe II is low spin. While the precise composition of any PB solid is extraordinarily preparation-sensitive, the major classification of extreme cases delineates ‘insoluble’ PB (abbreviated 282 to i-PB) which is Fe 3þ [Fe 3þ {Fe II (CN) 6 } 4À ] 3 , and ‘soluble’ PB (s-PB), in full K þ Fe 3þ [Fe II (CN) 6 ] 4À , i.e. dependent on the counter cation. All forms of PBare in fact highly insoluble in water (K sp $10 À40 ), 19 the ‘solubility’ attributed to the latter form being an illusion caused by its easy dispersion as colloidal particles, forming a blue sol in water that looks like a true solution. The Fe 3þ [Fe II (CN) 6 ] 4À chromophore falls into Group II of the Robin–Day mixed-valence classification, the blue IVCT band on analysis of the intensity indicating $1% delocalisation of the transferable electron in the ground state (i.e., before any optical CT). 20 X-Ray powder diffraction patterns for s-PB indicate a face-centred cubic lattice, with the high-spin Fe III and low-spin Fe II ions coordinated octahedrally by the N or C of the cyanide ligands, with K þ ions occupying interstitial sites. 21 In i-PB, M ¨ ossbauer spectroscopy confirms the interstitial ions to be the Fe 3þ counter cation. 18 Single-crystal X-ray diffraction patterns of i-PB indicate however a primitive cubic lattice, where one quarter of the Fe II sites are vacant. 22 This proposed structure contains no interstitial ions, with one quarter of the Fe III centres being coordinated by six N-bound cyanide ligands, the remainder by four N-bound cyanides, and every Fe II centre surrounded by six C-bound cyanides ligands. The Fe II vacancies are randomly distributed, and occupied by water molecules, which complete the octahedral coordination about Fe III . The widespread assumption of Ludi et al.’s model 22 for i-PB is highly questionable 23 in view of the substantial differences between the (very slowly grown) single crystals 22 and the more usual polycrystalline forms arising from relatively rapid growth, as in the electrodeposition for electrochromic use. Other (bivalent) counter cations also appear to be interstitial. 24 8.2 Preparation of Prussian blue thin films Prussian blue thin films are generally prepared by the original method based on electrochemical deposition, 4 although electroless deposition, 25 sacrificialanode (SA) methods, 26,27 the extensive redox cycling of hexacyanoferrate(II)- containing solutions, 28 the embedding of micrometre-sized crystals directly into electrode surfaces using powder abrasion, 29 and a method using catalytic silver paint 30,31 have all been described. Thus PB films can be electrochemically deposited onto a variety of inert electrode substrates by electroreduction of solutions containing iron(III) and hexacyanoferrate(III) ions as the adduct Fe 3þ [Fe III (CN) 6 ] 3À , Eq. (8.1). Prussian blue electrodeposition has been studied by numerous techniques. Voltammetry 32,33,34 and galvanostatic studies 35 have indicated that reduction of iron(III) hexacyanoferrate(III) is the principal electron-transfer process in PB electrodeposition. This 8.2 Preparation of Prussian blue thin films 283 brown–yellow soluble complex dominates in solutions containing iron(III) and hexacyanoferrate(III) ions as a result of the equilibrium in Eq. (8.1): Fe 3þ þ Fe III ðCNÞ 6 Â Ã 3À ¼ Fe III Fe III ðCNÞ 6 Â Ã 0 : (8:1) Chronoabsorptiometric studies 36 for galvanostatic PB electrodeposition onto ITO electrodes have shown that the absorbance due to the IVCT band of the growing PB film is proportional to the charge passed. Electrochemical quartz-crystal microbalance (EQCM) measurements for potentiostatic PB electrodeposition onto gold have revealed that the mass gain per unit area is proportional to the charge passed. 37 Ellipsometric measurements for potentiostatic PB electrodeposition onto platinum indicated that the level of hydration was around 34 H 2 O per PB unit cell. 38 Hydration is in fact variable and, for bulk PB taken out of solution, depends on ambient humidity. 39 Changes in the ellipsometric parameters during PB electrodeposition revealed initial growth of a single homogeneous film for the first 80 seconds, followed by growth of a second, outer, more porous film on top of the relatively compact inner film. 38 Chronoamperometric measurements (over a scale of several seconds) supported by scanning electron microscopy (SEM) for the electrodeposition of PB onto ITO and platinum by electroreduction from solutions of iron(III) hexacyanoferrate(III) have been performed. 40 In earlier preparations in the ‘zeroth’ step the deposition electrode was first made positive during addition of solutions in order to preclude spontaneous or uncontrolled deposits of PB, but this was later shown to cause initial deposition of the solid Fe III Fe III complex, which, when the electrode was made cathodic, persisted briefly before being incorporated into the growing PB. 41 A solubility of the Fe III Fe III complex was estimated 41 as ca. 10 À3 mol dm À3 . Variation of electrode potential, supporting electrolyte and concentrations of electroactive species have established a subsequent three-stage electrodeposition mechanism. In the early growth phase 40 the surface becomes uniformly covered as small PB nuclei form and grow on electrode substrate sites. In the second growth phase there is an increase in rate towards maximal roughness, as the electroactive area increases by formation and three-dimensional growth of PBnuclei attached to the PBinterface formed in the initial stage. In the final growth phase, diffusion of locally depleted electroactive species to the now three-dimensional PB interface plays an increasingly dominant role and limits electron transfer, resulting in a fall in growth rate. (If through-film electron transfer to the film–electrolyte interface wanes with growth, the seeping in of reactant solution between the PB film and electrode substrate for later growth phases is not precluded.) 284 Electrochromism of metal hexacyanometallates More recently, a new method of assembling multilayers of PB on surfaces has been described. 42,43 In contrast to the familiar process of self-assembly, which is spontaneous and can lead to single monolayers, ‘directed assembly’ is driven by the experimenter and leads to extended multilayers. In a proof-ofconcept experiment, the generation of multilayers of Prussian blue (and the mixed Fe III –Ru II analogue ‘Ruthenium purple’) on gold surfaces, by exposing them alternately to positively charged iron(III) cations and [Fe II (CN) 6 ] 4À or [Ru II (CN) 6 ] 4À anions, has been demonstrated. 42 Tieke and co-workers 43,44,45 have investigated the optical, electrochemical, structural and morphological properties of such multilayer systems, and have also demonstrated their application as ion-sieving membranes. They take care to note that ‘because metal hexacyanoferrate salts are known to organise in a cubic crystal lattice structure, a normal layering of metal cations and hexacyanoferrate anions is highly unlikely’. They avoid the term ‘layer-by-layer’ deposition and instead use ‘multiple sequential deposition’. 8.3 Electrochemistry, in situ spectroscopy and characterisation of Prussian blue thin films Electrodeposited PB films may be partially oxidised 32,33,34 to Prussian green (PG), a species historically also known as Berlin green and assigned the fractional composition shown, Eq. (8.2): Fe III Fe II ðCNÞ 6 Â Ã À ! Fe III fFe III ðCNÞ 6 g 2=3 fFe II ðCNÞ 6 g 1=3 h i 1=3À þ 2 = 3 e À : PB PG (8:2) The fractions 2 ⁄ 3 and 1 ⁄ 3 are illustrative rather than precise. Thus, although in bulk form PG is believed to have a fixed composition with the anion composition shown above, it has been inferred (but with reservations, below) that there is a continuous composition range in thin films from PB, via the partially oxidised PG form, to the fully oxidised all-Fe III form Prussian brown (PX). 34 Prussian brown appears brown as a bulk solid, brown–yellow in solution, and golden yellow as a particularly pure form that is prepared on electro-oxidation of thin-film PB – Eq. (8.3): 33,34 Fe III Fe II ðCNÞ 6 Â Ã À ! Fe III Fe III ðCNÞ 6 Â Ã 0 þe À : PB PX (8:3) 8.3 Electrochemistry, spectroscopy and characterisation 285 Redox in the other direction, that is, reduction of PB, yields Prussian white (PW), also known as Everitt’s salt, which appears colourless as a thin film – Eq. (8.4). Fe III Fe II ðCNÞ 6 Â Ã À þe À ! Fe II Fe II ðCNÞ 6 Â Ã 2À : PB PW (8:4) Figure 8.1 shows a cyclic voltammogram of the PB–PW transition. For all redox reactions above there is concomitant counter-ion movement into or out of the films to maintain overall electroneutrality. The electron transfer occurs at the electrode-substrate–film interface, while counter-ion egress or ingress occurs at the film–electrolyte interface; it is not established which through-filmtransport, that of electron or ion, determines the rate of coloration. Whilst s-PB, i-PB, PG and PW are all insoluble in water, PX is slightly soluble in its pure (golden-yellow) form (indeed the electrodeposition technique depends on the solubility of the [Fe III Fe III (CN) 6 ] 0 complex). This implies a positive potential limit of about þ0.9 V for a high write-erase efficiency in –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 –0.2 –0.1 0.0 0.1 0.2 0.3 i / m A c m –2 E(V) vs. Ag/AgCl Figure 8.1 Cyclic voltammogram at 5 mV s À1 scan rate for a PBjITOjglass electrode in aqueous KCl supporting electrolyte (0.2 mol dm À3 ), showing the voltammetric wave for the PB–PW redox switch. The initial potential was þ0.50 V vs. AgjAgCl. The arrows indicate the direction of potential scan. (Figure reproduced from Mortimer, R. J. and Reynolds, J. R. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. J. Mater. Chem., 15, 2005, 2226–33, with permission from The Royal Society of Chemistry.) 286 Electrochromism of metal hexacyanometallates contact with water. Although practical electrochromic devices based on PB have primarily exploited the PB–PW transition, this does not rule out the prospect of four-colour PB electropolychromic ECDs, as other solvent systems might not dissolve PX. The spectra of the yellow, green, blue and clear (‘white’) forms of PB and its redox variants are shown in Figure 8.2, together with spectra of possibly two intermediate states between the blue and the yellow forms. The yellow absorption band corresponds with that of [Fe III Fe III (CN) 6 ] 0 in solution, both maxima being at 425 nm and coinciding with the (weaker) [Fe III (CN) 6 ] 3À absorption maximum. On increase from þ0.50 V to more oxidising potentials, the original 690 nm PB peak continuously shifts to longer wavelengths with diminishing absorption, while the peak at 425 nm steadily increases, owing to the increasing [Fe III Fe III (CN) 6 ] 0 absorption. The reduction of PB to PW is by contrast abrupt, with transformation to all PW or all PB without pause, depending on the applied potential. One broad voltammetric peak usually seen for PB ! PX, in contrast with the sharply peaked PB ! PW transition, apparently indicates a range of compositions to be involved. The contrast (broad vs. sharp) behaviour, supported by ellipsometric measurements, 38 could imply continuous mixed-valence compositions over the blue- 1.0 0.8 0.6 0.4 0.2 0 Abs o r ba n c e 400 600 800 1000 1200 (i) (iii) (iv) (v) (vi) (ii) Figure 8.2 Spectra of iron hexacyanoferrate films on ITO-coated glass at various potentials [(i) þ0.50 (PB, blue), (ii) À0.20 (PW, transparent), (iii) þ0.80 (PG, green), (iv) þ0.85 (PG, green), (v) þ0.90 (PG, green) and (vi) þ1.20 V (PX, yellow) (potentials vs. SCE)] with KCl 0.2 mol dm À3 þ HCl 0.01 mol dm À3 as supporting electrolyte. Wavelengths (abscissa) are in nm. (Figure reproduced from Mortimer, R. J. and Rosseinsky, D. R. ‘Iron hexacyanoferrate films: spectroelectrochemical distinction and electrodeposition sequence of ‘soluble’ (K þ -containing) and ‘insoluble’ (K þ -free) Prussian blue and composition changes in polyelectrochromic switching’. J. Chem. Soc., Dalton Trans., 1984, 2059–61, by permission of the Royal Society of Chemistry.) 8.3 Electrochemistry, spectroscopy and characterisation 287 to-yellow range in contrast with the (presumably immiscible) PB and PW, which clearly transform, one into the other, without intermediacy of composition. However, two-peak PB ! PX voltammetry pointing to a specific intermediate composition has also been seen, first attributed to the absence of traces of Cl À from those samples, 41 but later also observed in slow voltammetry on PB from KCl-containing preparations. 46 Thus the intermediate green colour observed in PB–PX voltammetry could be a true compound PG rather than either a continuously changing mixed-valence phenomenon, a PB þ PX series of solid solutions, or varying PB þ PX physical mixtures of microcrystals. The identity as s-PB or i-PB of the initially electrodeposited PB has been debated in the literature. 34,47,48,49,50,51 Based on changes that take place in the IVCT band on redox cycling, it has been postulated that i-PB is first formed, followed by a transformation to s-PB on potential cycling. 34 Further evidence for this is provided by the difference in the voltammetric response for the PB–PW transition between the first cycle and all succeeding cycles, suggesting structural reorganisation of the film during the first cycle. 33 On soaking s-PB films in saturated FeCl 3 solutions partial reversion of the absorbance maximum and broadening of the spectrum, approaching the values observed for i-PB, is found. 34 Itaya and Uchida, 47 however, claimed that the film is always i-PB. Their argument is based on the ratio of charge passed on oxidation to PX to that passed on reduction to PW, which was 0.708 rather than 1.00; that is, Eqs. (8.5) and (8.6) are applicable: Fe 3þ Fe III Fe II ðCNÞ 6 Â Ã 3 þ4e À þ 4K þ ! K 4 Fe 2þ Fe II Fe II ðCNÞ 6 Â Ã 3 ; i-PB PW (8:5) Fe 3þ Fe III Fe II ðCNÞ 6 Â Ã 3 À3e À þ 3X À ! Fe 3þ Fe III Fe III ðCNÞ 6 Â Ã 3 X 3 : i-PB PX (8:6) In refutation, Emrich et al. 48 using X-ray photoelectron spectroscopy (XPS) data, and Lundgren and Murray 49 using cyclic voltammetry (CV), energy dispersive analysis of X-rays (EDAX), XPS, elemental analytical and spectroelectrochemical measurements, both confirmed i-PB as the initially-deposited form with a ‘gradual’ transformation to s-PBon potential cycling. Later work 41 established a major (approximately one-third) conversion in the first cycle, but thereafter a much slower introduction of K þ . Other support for the i-PBto s-PB transformation comes from an ellipsometric study by Beckstead et al. 50 who 288 Electrochromism of metal hexacyanometallates found that the PB film, after the first and subsequent cycles for the PB–PW transition, developed optical properties that differed from the original PB film. Results fromin situ Fourier-transforminfrared spectroscopy also demonstrated an i-PB to s-PB transformation on repeated reductive cycling. 51 The EQCM mass-change measurements on voltammetrically scanned PB films reinforce the theory of lattice reorganisation during the initial film reduction. 37 Only about one third of the three K þ ions, expected to replace the counter- cationic Fe 3þ , are found to be incorporated in the first substitutive voltammetric cycle. It has been suggested that this follows from reduction in PB ! PW of the counter-cationic Fe 3þ to Fe 2þ which is retained on re-oxidation of the PW to the PB, so requiring Fe 2þ K þ as counter cations; the now somewhat dispersed counter-cation population does not subsequently drive K þ incorporation as strongly as happens with solely Fe 3þ as (charge-concentrated) counter cation. 41 Lattice-energy calculations support most of this argument. 52 In detail, a further EQCMstudy 53 shows mass changes, following one PB! PW!PB cycle of KCl-prepared PB in different M þ Cl À solutions, in the sequence of counter cations Na þ >K þ <Rb þ <Cs þ . This sequence correlates with the wavelengths of the maximum in each case of the PB absorption in the region of 700 nm. Together with related observations on PB samples that contained sundry M 2þ counter cations, varied M þ or M 2 þ lattice interactions with the chromophore were concluded to affect the optical absorptions commensurately. 53,54 Whilst PB film stability is frequently discussed in the preceding papers, Stilwell et al. 55 have studied in detail the factors that influence the cycle stability of PB films. They found that electrolyte pH was the overwhelming factor in film stability; cycle numbers in excess of 100 000 were easily achieved in solutions of pH2–3, though other conclusions regarding stabilisation by pH have since been reached. 41 Concurrently with this increase in stability at lower pH was a considerable increase in switching rate. Furthermore, films grown from chloride-containing solutions were said to be slightly more stable, in terms of cycle life, compared to those grown from chloride-free solutions, 55 but again with contrary conclusions. 41 8.4 Prussian blue electrochromic devices Early PB-based ECDs employed PB as the sole electrochromic material. Examples include a seven-segment display using PB-modified SnO 2 working and counter electrodes at 1 mm separation, 56 and an ITO j PB–Nafion 1 j ITO solid-state device. 57,58 For the solid-state system, device fabrication involved chemical (rather than electrochemical) formation of the PB, by 8.4 Prussian blue electrochromic devices 289 immersion of a membrane of the solid polymer electrolyte Nafion 1 (a sulfonated poly(tetrafluoroethane)polymer) in aqueous solutions of FeCl 2 , then K 3 Fe(CN) 6 . The resulting PB-containing Nafion 1 composite film was sandwiched between the two ITO plates. The construction and optical behaviour of an ECD utilising a single film of PB, without addition of a conventional electrolyte, has also been described. 59 In this design, a film of PB is sandwiched between two optically transparent electrodes (OTEs). Upon application of an appropriate potential across the film, oxidation occurs near the positive electrode and reduction near the negative electrode to yield PX and PW respectively. The conversion of the outer portions of the film results in a net half-bleaching of the device. The functioning of the device relies on the fact that PB can be bleached both anodically – to the yellow state, Eq. (8.3) – and cathodically – to a transparent state, Eq. (8.4) – and that it is a mixed conductor through which potassium cations can move to provide charge compensation required for the electrochromic redox reactions. However, at the conjunction of the (II)(II) state with the (III)(III) state, their comproportionation reaction results in half the material remaining in the device centre as the (III)(II) form, PB. Since PB and WO 3 (see Chapter 6) are respectively anodically and cathodically colouring electrochromic materials, they can be used together in a single device 60,61,62,63,64 so that their electrochromic reactions are complementary, Eqs. (8.7) and (8.8): Fe III Fe II ðCNÞ 6 Â Ã À þe À ! Fe II Fe II ðCNÞ 6 Â Ã 2À ; blue transparent (8:7) WO 3 þ xðM þ þ e À Þ ! M x W VI ð1ÀxÞ W V x O 3 : transparent blue (8:8) In an example of the construction of such a device, thin films of these materials are deposited on OTEs that are separated by a layer of a transparent ionic conductor such as KCF 3 SO 3 in poly(ethylene oxide). 64 The films can be coloured simultaneously (giving deep blue) when a sufficient voltage is applied between themsuch that the WO 3 electrode is the cathode and the PBelectrode the anode. On appropriate switching, the coloured films can be bleached to transparency when the polarity is reversed, returning the ECD to a transparent state. Numerous workers 65,66,67,68,69,70,71 have combined PB with the conducting polymer poly(aniline) in complementary ECDs that exhibit deep blue-to-light 290 Electrochromism of metal hexacyanometallates green electrochromism. Electrochromic compatibility is obtained by combining the coloured oxidised state of the polymer (see Chapter 10) with the blue PB, and the bleached reduced state of the polymer with PG, Eq. (8.9): OxidisedpolyðanilineÞ þ PB ! Emeraldine polyðanilineÞ þ PG: coloured bleached (8:9) Jelle and Hagen 68,69,71,72 have developed an electrochromic window for solar modulation using PB, poly(aniline) and WO 3 . They took advantage of the symbiotic relationship between poly(aniline) and PB, and incorporated PB together with poly(aniline), and WO 3 , in a complete solid-state electrochromic window. The total device comprised Glass j ITO j poly(aniline) j PB j poly(AMPS) j WO 3 j ITOj Glass. Compared with their earlier results with a poly(aniline)–WO 3 window, Jelle and Hagen were able to block off much more of the light by inclusion of PB within the poly(aniline) matrix, while still regaining about the same transparency during the bleaching of the window. As noted in Chapter 10, a new complementary ECD has recently been described, 73 based on the assembly of the cathodically colouring conducting polymer, poly[3,4-(ethylenedioxy)thiophene] – PEDOT– on ITOglass and PB on ITO glass substrates with a poly(methyl methacrylate) – PMMA-based gel polymer electrolyte. The colour states of the PEDOT (blue-to-colourless) and PB (colourless-to-blue) films fulfil the requirement of complementarity. The ECD exhibited deep blue–violet at À2.1 V and light blue at 0.6 V. Kashiwazaki 74 has fabricateda complementary ECDusing plasma-polymerised ytterbium bis(phthalocyanine) (pp–Yb(Pc) 2 ) and PB films on ITO with an aqueous solution of KCl (4 mol dm À3 ) as electrolyte. Blue-to-green electrochromicity was achieved in a two-electrode cell by complementing the green-to-blue colour transition (on reduction) of the pp–Yb(Pc) 2 film with the blue (PB)-to-colourless (PW) transition (oxidation) of the PB. A threecolour display (blue, green and red) was fabricated in a three-electrode cell in which a third electrode (ITO) was electrically connected to the PB electrode. A reduction reaction at the third electrode, as an additional counter electrode, provides adequate oxidation of the pp–Yb(Pc) 2 electrode, resulting in the red colouration of the pp–Yb(Pc) 2 film. 8.5 Prussian blue analogues Prussian blue analogues, comprising other polynuclear transition-metal hexacyanometallates, 12,13,14 which have been prepared and investigated as thin 8.5 Prussian blue analogues 291 films, are surveyed in this section. The majority are expected to be electrochromic, although this property has only been studied in any depth in a few cases. The field therefore appears to be open for further investigation and exploitation, although it is to be noted that from the qualitative description of colour states, contrast ratios are likely to be low. 8.5.1 Ruthenium purple Bulk ruthenium purple – RP; ferric ruthenocyanide, iron(III) hexacyanoruthenate(II) – is synthesised via precipitation from solutions of the appropriate iron and hexacyanoruthenate salts. The visible absorption spectrum of a colloidal suspension of bulk synthesised RP with potassium as counter cation confirms the Fe 3þ [Ru II (CN) 6 ] 4À combination as the chromophore. 12 The X-ray powder pattern with iron(III) as counter cation gives a lattice constant of 10.42 A ˚ as compared to 10.19 A ˚ for the PB analogue. 12 However, although no single-crystal studies have been made, RP could have a disordered structure similar to that reported for single-crystal PB. 13 The potassium and ammonium salts give cubic powder patterns similar to their PB analogues. 14 Ruthenium purple films have been prepared by electroreduction of the soluble iron(III) hexacyanoruthenate(III) complex potentiostatically, galvanostatically or by using a copper wire as sacrificial anode. 75,76 The visible absorption spectrum of RP prepared in the presence of excess of potassium ion showed a broad CT band, as for bulk synthesised RP, with a maximum at approximately 550 nm. 75 Ruthenium purple films can be reversibly reduced to the colourless iron(II) hexacyanoruthenate(II) form, but no partial electro- oxidation to the Prussian green analogue is observed. The large background oxidation current observed in chloride-containing electrolyte suggests electrocatalytic activity of RP for either oxygen or chlorine evolution. 76 8.5.2 Vanadium hexacyanoferrate Vanadium hexacyanoferrate (VHCF) films have been prepared on Pt or fluorine-doped tin oxide (FTO) electrodes by potential cycling from a solution containing Na 3 VO 4 and K 3 Fe(CN) 6 in H 2 SO 4 (3.6 mol dm À3 ). 77,78 Carpenter et al., 77 by correlation with CVs for solutions containing only one of the individual electroactive ions, have proposed that electrodeposition involves the reduction of the dioxovanadium ion VO 2 þ (the stable form of vanadium(V) in these acidic conditions), followed by precipitation with hexacyanoferrate(III) ion. While the reduction of the hexacyanoferrate(III) 292 Electrochromism of metal hexacyanometallates ion in solution probably also occurs when the electrode is swept to more negative potentials, this reduction does not appear to be critical to film formation, since VHCF films can be successfully deposited by potential cycling over a range positive of that required for hexacyanoferrate(III) reduction. No evidence was obtained for the formation of a vanadium(V)– hexacyanoferrate(III) type complex analogous to iron(III) hexacyano-ferrate(III), the visible absorption spectrum of the mixed solution being a simple summation of spectra of the single-component solutions. While VHCF films are visually electrochromic, switching from green in the oxidised state to yellow in the reduced state, Carpenter et al. show that most of the electrochromic modulation occurs in the ultraviolet (UV) region. 77 From electrochemical data and XPS they conclude that the electrochromism involves only the iron centres in the film. The vanadium ions, found to be present predominantly in the þIV oxidation state, are not redox active under these conditions. 8.5.3 Nickel hexacyanoferrate Nickel hexacyanoferrate (NiHCF) films can be prepared by electrochemical oxidation of nickel electrodes in the presence of hexacyanoferrate(III) ions, 79 or by voltammetric cycling of inert substrate electrodes in solutions containing nickel(II) and hexacyanoferrate(III) ions. 80 The NiHCF films do not show low-energy IVCT bands, but when deposited on ITO they are observed to switch reversibly from yellow to colourless on electroreduction. 81 Amore dramatic colour change can be observed by substitution of two ironbound cyanides by a suitable bidentate ligand. 82 Thus, 2,2 0 -bipyridine can be indirectly attached to nickel metal via a cyano–iron complex to form a derivatised electrode. When 2,2 0 -bipyridine is employed as the chelating agent, the complex [Fe II (CN) 4 (bipy)] 2À is formed which takes on an intense red colour associated with a MLCT absorption band centred at 480 nm. This optical transition is sensitive to both the iron oxidation state, only arising in the Fe II formof the complex, and to the environment of the cyanide-nitrogen lone pair. Reaction of the complex with Ni 2þ either under bulk conditions or at a nickel electrode surface generates a bright red material. By analogy with the parent iron complex this red colour is associated with the (dp)Fe II !(p*)bipy CT transition. For bulk samples, chemical oxidation to the Fe III state yields a light-orange material, while modified electrodes can be reversibly cycled between the intensely red and transparent forms, a process which correlates well with the observed CV response. 82 In principle, orange–transparent and 8.5 Prussian blue analogues 293 green–transparent electrochromism could be available, using the complexes [Ru II (CN) 4 (bipy)] 2À and [Os II (CN) 4 (bipy)] 2À respectively. 8.5.4 Copper hexacyanoferrate Copper hexacyanoferrate (CuHCF) films can be prepared voltammetrically by electroplating a thin filmof copper on glassy carbon (GC) or ITOelectrodes in the presence of hexacyanoferrate(II) ions. 83,84,85,86 Films are deposited by first cycling between þ0.40 and þ0.05 V in a solution of cupric nitrate in aqueous KClO 4 . Copper is then deposited on the electrode by stepping the potential from þ0.03 to À0.50 V, and subsequently removed (stripped) by linearly scanning the potential from À0.50 to þ0.50 V. The deposition and removal sequence was repeated until a reproducible CV was obtained during the stripping procedure. The CuHCF film was then formed by stepping the electrode potential in the presence of cupric ion from þ0.03 to À0.50 V followed by injection of an aliquot of K 4 Fe(CN) 6 solution (a red–brown hexacyanoferrate(II) sol formed immediately) into the cell. The CuHCF film formation mechanism has not been elucidated but the co-deposition of copper is important in the formation of stable films. Films formed by galvanostatic or potentiostatic methods from solutions of cupric ion and hexacyanoferrate(III) ion showed noticeable deterioration within a few CV scans. The co-deposition procedure provides a fresh copper surface for film adhesion and the resulting films are able to withstand $1000 voltammetric cycles. Such scanning of a CuHCF film in K 2 SO 4 (0.5 mol dm À3 ) gave a well-defined reversible couple at þ0.69 V, characteristic of an adsorbed species. Copper hexacyanoferrate films exhibit red-brown to yellow electrochromicity. 86 For the reduced film, a broad visible absorption band associated with the iron-to-copper CT in cupric hexacyanoferrate(II) was observed( max ¼490nm, " ¼2 Â10 3 dm 3 mol À1 cm À1 ). This band was absent in the spectrum of the oxidised film, the yellow colour arising from the CN À !Fe III CT band at 420nm for the hexacyanoferrate(III) species (arrow denoting electron transfer). 8.5.5 Palladium hexacyanoferrate The preparation of electrochromic palladium hexacyanoferrate (PdHCF) films by simple immersion of the electrode substrate for at least one hour, or potential cycling of conducting substrates (Ir, Pd, Au, Pt, GC), in a mixed solution of PdCl 2 and K 3 Fe(CN) 6 has been reported. 87 The resulting modified electrodes gave broad CV responses, assigned to Fe III (CN) 6 –Fe II (CN) 6 , the Pd II sites being electro-inactive. Films were orange at >1.0 V and yellow-green 294 Electrochromism of metal hexacyanometallates at <0.2V. More recently, potentiodynamically grown PdHCF films have been studied using cyclic voltammetry, in situ infrared and UV-visible spectroelectrochemistry. 88 UV-visible reflectance spectra of films on platinum demonstrated the reversible progressive conversion of PdHCF between its reduced (light yellow) and oxidised (yellow green) states. 8.5.6 Indium hexacyanoferrate and gallium hexacyanoferrate Indium hexacyanoferrate films 89,90,91,92 have been grown by potential cycling in a mixed solution containing InCl 3 and K 3 Fe(CN) 6 . The electrodeposition occurs during the negative scans as sparingly soluble deposits of In 3þ with [Fe(CN) 6 ] 4À were formed. 89 The resulting films are electrochromic, being white when reduced and yellow when oxidised. 92 Solid films of gallium hexacyanoferrate have been prepared by direct modification of a gallium electrode surface in an aqueous solution of 5 mmol dm À3 potassium hexacyanoferrate(III) in KCl (0.1 mol dm À3 ). 93 This one-step electroless deposition proceeds via a chemical oxidation reaction of the metallic gallium to Ga 3þ in the aqueous solution, followed by reaction with the hexacyanoferrate(III) ions. To date, the electrochromic properties of the films have not been investigated. 8.5.7 Miscellaneous Prussian blue analogues Prussian blue analogues investigated include thin films of cadmium hexacyanoferrate 94 (reversibly white to colourless on reduction 81 ), chromium hexacyanoferrate 95 (reversibly blue to pale blue-grey on reduction 81 ), cobalt hexacyanoferrate 96 (reversibly green-brown to dark green on reduction 81 ), manganese hexacyanoferrate 97 (reversibly pale yellow to colourless on reduction 81 ), molybdenum hexacyanoferrate 98 (pink to red on reduction 81 ), osmium hexacyanoferrate, 99 osmium(IV) hexacyanoruthenate, 100 platinum hexacyanoferrate 101 (pale blue to colourless on reduction 81 ), rhenium hexacyanoferrate 81 (pale yellow to colourless on reduction 81 ), rhodium hexacyanoferrate 81 (pale yellow to colourless on reduction 81 ), ruthenium oxide–hexacyanoruthenate, 102 mixed films of ruthenium oxide–hexacyanoferrate and ruthenium hexacyanoferrate, 103 silver hexacyanoferrate, 5 silver–‘crosslinked’ nickel hexacyanoferrate 104 (reversibly yellow to white on reduction 81 ), titanium hexacyanoferrate 105 (reversibly brown to pale yellow on reduction 81 ), zinc hexacyanoferrate 106 and zirconium hexacyanoferrate. 107 8.5 Prussian blue analogues 295 Mixed-ligand Prussian blue analogues reported as redox-active thin films include copper heptacyanonitrosylferrate, 108 iron(III) carbonylpentacyanoferrate, 5 and iron(III) pentacyanonitroferrate. 5 Of the lanthanoids and actinoids, lanthanum hexacyanoferrate, 109 samarium hexacyanoferrate 110 and uranium hexacyanoferrate, 111 as thin redox- active films have been studied. 8.5.8 Mixed-metal hexacyanoferrates Glassy carbon electrodes have been modified with films of mixed metal hexacyanoferrates. 97 Cyclic voltammograms of PB–nickel hexacyanoferrate and PB–manganese hexacyanoferrate films show electroactivity of both metal hexacyanoferrate components in each mixture. It is suggested that the mixed-metal hexacyanoferrates have a structure in which some of the outer sphere iron centres in the PB lattice are replaced by Ni 2þ or Mn 2þ , rather than being a co-deposited mixture of PB and nickel or manganese hexacyanoferrate. 97 Although film colours are not reported, it seems likely that variation of metal hexacyanoferrate and compositions of electrodeposition solution could allow colour choice in the anticipated electropolychromic systems. The approach seems general, with PB–metal hexacyanoferrate (metal ¼ Co, Cu, In, Cr, Ru) modified electrodes also being successfully prepared. Thin films of mixed nickel–palladium hexacyanoferrates have been prepared and characterised, and spectral measurements show them to be electrochromic, although colours have not been reported. 112 References 1. Diesbach (1704), cited in Gmelin, Handbuch der Anorganischen Chemie, Frankfurt am Main, Deutsche Chemische Gesellschaft, 1930, vol. 59, Eisen B, p. 671. 2. Fukuda, K. 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Chem., 487, 2000, 57–65. 302 Electrochromism of metal hexacyanometallates 9 Miscellaneous inorganic electrochromes 9.1 Fullerene-based electrochromes The electrochromism of thin films of Buckminsterfullerene C 60 was first demonstrated in 1993 by Rauh and co-workers. 1 The electro-coloration occurs during reduction to form lithium fulleride, Li x C 60 : C 60 þxðLi þ þe À Þ !Li x C 60 : light brown dark brown (9:1) The reduced form develops a band maximum in the near infrared, in the range 1060–1080 nm. A band also forms in the UV. Figure 9.1 shows the spectrum of C 60 as a function of applied potential. Electrochemically formed Li x C 60 is identical with the fulleride salt formed by exposing C 60 to alkali-metal vapour. Konesky 2 has shown that Ag þ , Cr 3þ , Cu 2þ , Mg 2þ and Ba 2þ ions, in addition to Li þ , can be electro-intercalated into such fulleride films during coloration as counter ions from solvents g-butyrolactone or water. As fullerene and fulleride films are partially soluble in the polar organic electrolytes used, the cycle life is depleted by prolonged exposure to such electrolytes. 3 The solubility increases with higher insertion coefficient, x. 4 Furthermore, the higher-x outer layers of the film can peel away from the electrode. 4 The electrochromismis reversible with electrochemically intercalated alkali- metal or alkaline-earth ions, although the extent of reversibility depends on the insertion coefficient x: reversibility is lost if x is too high, 3 as found with tungsten oxide (cf. p. 114). Steep concentration gradients form in the fulleride films during electrochromic operation. Analysis is complicated since ionic mobilities are a function of insertion coefficient. 4,5 Applying pulsed potentials improves both the durability of the film and the extent of electro-reversibility, presumably by allowing such gradients to dissipate during the ‘off’ period between pulses. 3 303 de Torresi et al. 6 suggest the coloured form of the electrochrome, Li x C 60 , is not stable: electrochromic stability is degraded by residual oxygen in both the electrolyte system and the fullerene film. This rapid reaction yields C 60 , Li þ and oxide ion. 5 Reaction with water is also rapid. Additionally Li x C 60 catalyses the electro-decomposition of solvent, which may explain why the coloration efficiency is 20 cm 2 C À1 during coloration but 35 cm 2 C À1 during bleaching. Goldenberg 7 has also prepared thin electrochromic films of fullerene via Langmuir–Blodgett techniques. 9.2 Other carbon-based electrochromes Pfluger et al. 8 have reported an ECD with graphite as a solid-solution intercalation electrode. Many alkali-metal cations may be inserted into graphite sheets from aprotic solutions, lithium apparently giving the best speed and electro reversibility. This ECD is electropolychromic switching from brassy black !deep blue !light green !golden yellow within the potential range 3–5 V. When the potential was reversed, the ECD reverted back to the brassy 1.5 1.0 –2.7 –0.2 0.5 0.0 350 550 750 –0 . 2 –0 . 2 –1 . 2 –1 . 2 –2 . 2 –2 . 2 E / V λ /n m O p t i c a l de n s i t y E / V t Figure 9.1 UV-visible spectrum of immobilised fullerene on an electrode surface as a function of applied potential E: the C 60 was on a SnO 2 -coated glass electrode immersed in PC containing LiClO 4 (1 mol dm À3 ). (Figure reproduced from de Torresi, S. I. C., Torresi, R. M., Ciampi, G. and Luengo, C. A. ‘Electrochromic phenomena in fullerene thin films’. J. Electroanal. Chem., 377, 1994, 283–5, by permission of Elsevier Science.) 304 Miscellaneous inorganic electrochromes black colour, with of about 0.2 s. Kuwabara and Noda 9 and White and co-workers 10 have also used graphite as counter-electrode layer in an ECD. Diamond, 11 electrodeposited by the oxidation of lithiumacetylide, is yellow, but becomes brown following reductive ion insertion, showing a new band in the UV. Other forms of carbon have been used as counter electrodes: screen-printed carbon black, 12 ‘carbon’ 9,13,14,15 and ‘carbon-based’ electrodes. 16,17 No colour change is mentioned regarding these materials. 9.3 Reversible electrodeposition of metals Comparatively few inorganic type-II electrochromes have been reported. Of these few, the only viable systems are those in which finely divided metal is electrodeposited onto an OTE, as reviewed by Ziegler (in 1999). 18 In all these systems, reduction of a dissolved metal cation results in the deposition of finely divided metal, so the ‘electrochromism’ results not from photon absorption but rather fromthe filmbecoming opaque or even optically reflective (by specular reflection). The three systems studied for electrochromism are listed below. Bismuth In recent work on the electrodeposition of metallic bismuth from aqueous solution, 19,20,21,22,23 the deposition/coloration reaction is cited 22 as Eq. (9.2): 2 Bi 3þ ðsolnÞ þ9 Br À ðsolnÞ !2 Bi 0 ðsÞ þ3 Br À 3 ðsolnÞ: colourless opaque (9:2) The deposition of particulate bismuth, rather than a continuous metal film, is achieved by underpotential deposition, the solution containing traces of copper to act as an electron mediator. The reaction sequence has not yet been detailed. Gelling an aqueous–organic electrolyte makes the image less patchy. 24 The pH of the deposition solution must be relatively low in order to maintain high solubility of the bismuth cation precursor, but not so lowas to cause deterioration of the ITO layer of the transparent electrode (the OTE). Despite experimental problems, however, electrodeposited particulate bismuth exhibiting opacity has shown 18 a cycle-life of 5 Â10 7 . Thus the ‘spectrum’ of such bismuth on an OTE is invariant with wavelength, lacking absorption peaks, appearing as an almost horizontal line that increases in height with thickness of electrodeposited bismuth; see Figure 9.2. Accordingly, 9.3 Reversible electrodeposition of metals 305 the ‘coloration efficiency’ for such systems is also little dependent on , varying only between 73 cm 2 C À1 at 550 nmand 77 cm 2 C À1 at 700 nm, with a fairly high contrast ratio of 25:1, 18 reflecting as much as 60% of all incident visible light. 20 A bismuth-based ECD has been marketed commercially by the Polyvision Corporation. 20,23 Lead Metallic lead may be electrodeposited 25,26 onto ITOfromaqueous solutions of Pb(NO 3 ) 2 ; see Eq. (9.3): Pb 2þ ðaqÞ þ2e À !Pb 0 ðsÞ: colourless opaque (9:3) Similarly to bismuth, traces of copper are added to the colourless precursor solution as a mediator. 26 However, the Cu 2þ is not merely a mediator, it also affects the morphology of the deposit, effecting increased transmittance changes by up to 60%. Copper(II) chloride in the electrolyte also leads to a 1.0 0.8 0.6 0.4 Δ T 0.2 0.0 400 450 500 550 λ (nm) 600 650 700 Figure 9.2 UV-visible spectra of electrodeposited bismuth on ITO. The bismuth was deposited reductively from a solution initially comprising aqueous Bi 3 þ (0.02 mol dm À3 ). This is not a true ‘spectrum’ because the bismuth is reflective, rather than optically absorbing. (Figure reproduced from Ziegler, J. P. and Howard, B. M. ‘Spectroelectrochemistry of reversible electrodeposition electrochromic materials’. Proc. Electrochem. Soc., 94(2), 1994, 158–69, by permission of The Electrochemical Society, Inc.) 306 Miscellaneous inorganic electrochromes more homogeneous deposit on the ITO surface. The use of bromide ion to mediate the underpotential deposition of Pb has also been investigated. 27 Silver Thin films of silver have also been prepared by electrodeposition fromAg þ ion onto OTEs, 28 Eq. (9.4). Ag þ ðaqÞ þe À !Ag 0 ðsÞ: (9:4) A thin film of non-particulate, continuous metallic plate is formed. (‘Electro- chromism’ was not referred to in this 1962 work, done prior to Deb’s use of the term in 1969.) 9.4 Reflecting metal hydrides An impressive example of electrochromes showing specular reflectance are the lanthanide hydride devices, sometimes called ‘switchable mirrors’. 29 The reflective properties are those of the electrochrome, not any underlying substrate. Thin-film LaH 2 exhibits specular reflection of this sort, but chemical oxidation to form LaH 3 results in a loss of the metallicity and hence the reflectivity. Chemical reaction therefore causes switching between reflective and non-reflective states, Eq. (9.5): LaH 2 ðsÞ þH À ðsoln:Þ !LaH 3 ðsÞ þe À : reflective non-reflective (9:5) The cause of the change in reflectivity is a metal–insulator transition. Although dramatic changes in optical and electrical properties accompany such transitions, their interpretation is complicated by attendant changes in crystallographic structure; such changes are expected as such electronic transitions require changes in nuclear spin. For these reasons, Eq. (9.5) is not a mechanistically comprehensive representation of the redox reaction. Yttrium, lanthanum and the trivalent rare-earth elements all form hydrides that exhibit such transitions. The transition time scale is about a few seconds. The transition from a metallic state (YH 2 or LaH 2 ) to a semiconducting state (YH 3 or LaH 3 ) occurs during the continuous absorption of hydrogen, accompanied by profound changes in their optical properties. The extreme reactivity and fragility of these materials preclude their ready utilisation. To overcome these problems, thin films of hydride are coated with a thin layer of palladium, through which hydrogen can diffuse, presumably 9.4 Reflecting metal hydrides 307 forming atomic hydrogen. While the palladiumlayer also catalyses the adsorption and desorption of hydrogen, 30,31 it also limits the maximum visible transmittance of the hydride layer to about 35–40%. 32 Alloys of lanthanum also show this reflective transition. For example, magnesium–lanthanide alloys can pass through three different optical states: a colour-neutral, transparent state at high pressures of hydrogen; a dark, nontransparent state at intermediate pressures of hydrogen; and a highly reflective metallic state at lowpressures of hydrogen. The optical properties of alloys are also preferred because their colours contrast with the red–yellow colour of the transparent lanthanide states, 33 thereby lending them a ‘neutral hue’. 29,34 Furthermore, the La–Mg alloy has virtually no transmittance at high pressures of hydrogen. von Rottkay suggests the change in reflectivity is about 50% for Mg–La hydride. 32 The coloration efficiency of thin-film Sm 0.3 Mg 0.7 H x is slightly lower than for H x WO 3 . 35 The use of hydrogen gas effects a very rapid optical transition, but elemental H 2 is neither safe nor an attractive proposal for a viable device. Notten et al. 36 have more recently shown how the same effect can be observed with the lanthanum film immersed in aqueous KOH (1 mol dm À3 ), depicted in Eq. (9.6) for lanthanum hydride via an electrochemical reaction: LaH x ðsÞ þyOH À ðaqÞ !LaH ðxÀyÞ ðsÞ þyH 2 Oþye À : (9:6) In this way, more typical ECDs can be fabricated in which a clear, solid electrolyte layer allows the transport of hydrogen. 29 The main technological drawbacks at present are the formation of an oxide layer between the lanthanum and the palladium top-coat (cf. the operation of palladium oxides electrochromes on p. 178) and slower colouration kinetics than with H 2 gas. 29,37 Alternatively, van der Sluis et al. 38 show that thin films of lanthanide hydride can be switched from absorbing to transparent with aqueous NaBH 4 solution. The reverse reaction can be accomplished with an aqueous H 2 O 2 solution. The optical properties of these films are similar to those of films switched electrochemically or exposed to hydrogen gas. No yttrium-based reflective devices are ready for marketing, but rapid technological advances are likely. Janner et al. 37 have examined the durability of lanthanide hydride films immersed in aqueous KOHsolution. Typically, the macroscopic effects of degeneration upon cycling of the switchable mirror include slower rates of coloration and bleaching, irreversible oxidation of the metal hydride films, and delamination as the films peel from their substrates. Of the various attempts to improve the cycle lifetime, the best results were obtained with switchable mirrors pre-loaded with hydrogen during deposition. 308 Miscellaneous inorganic electrochromes 9.5 Other miscellaneous inorganic electrochromes Electrochromism has also been reported for the other miscellaneous inorganic materials such as nickel-doped strontium titanate, SrTiO 3 ; 39 indium nitride, 40 ruthenium dithiolene, 41 phosphotungstic acid, 42,43,44 organic ruthenium complexes, 45 and ferrocene–naphthalimides dyads. 46 References 1. Klein, J. D., Yen, A., Rauh, R. D. and Causon, S. L. Near-infrared electrochromism in Li x C 60 films. Appl. Phys. Lett., 63, 1993, 599–601. 2. Konesky, G. A. Reversible electrochromic effect in fullerene thin films utilizing alkali and transition metals. Mater. Res. Soc. Symp. Proc., 417, 1996, 407–13. 3. Konesky, G. A. Pulse-width modulation effects on fullerene electrochromism. Proc. SPIE, 3788, 1999, 14–21. 4. Konesky, G. Fullerene electrochromism under high pulsed fields. Proceedings of the Annual Technical Conference: Society of Vacuum Coaters, Boston, MA, 18–23 April 1998, pp. 144–6. 5. Konesky, G. Stability and reversibility of the electrochromic effect in fullerene thin films. Proc. SPIE, 3142, 1997, 205–15. 6. de Torresi, S. I. C., Torresi, R. M., Ciampi, G. and Luengo, C. A. Electrochromic phenomena in fullerene thin films. J. Electroanal. Chem., 377, 1994, 283–5. 7. Goldenberg, L. M. Electrochemical properties of Langmuir–Blodgett films. J. Electroanal. Chem., 379, 1994, 3–19. 8. Pfluger, P., K ¨ unzi, H. U. and G ¨ untherodt, H. J. Discovery of a new reversible electrochromic effect. Appl. Phys. Lett, 35, 1979, 771–2. 9. Kuwabara, K. and Noda, Y. Potential wave-form measurements of an electrochromic device, WO 3 /Sb 2 O 5 /C, at coloration–bleaching processes using a new quasi-reference electrode. Solid State Ionics, 61, 1993, 303–8. 10. Yu, P., Popov, B. N., Ritter, J. A. and White, R. E. Determination of the lithium ion diffusion coefficient in graphite. J. Electrochem. Soc., 146, 1999, 8–14. 11. Kulak, A. I., Kokorin, A. I., Meissner, D., Ralcherko, V. G., Vlasou, I. I., Kondratyuk, A. V. and Kulak, T. I. Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide. Electrochem. Commun., 5, 2003, 301–5. 12. Wang, J., Tian, B. M., Nascomento, V. B. and Angnes, L. Performance of screen- printed carbon electrodes fabricated from different carbon inks. Electrochim. Acta, 43, 1998, 3459–65. 13. Edwards, M. O. M., Andersson, M., Gruszecki, T., Pettersson, H., Thunman, R., Thuraisingham, G., Vestling, L. and Hagfeldt, A. Charge–discharge kinetics of electric-paint displays. J. Electroanal. Chem., 565, 2004, 175–84. 14. Edwards, M. O. M., Boschloo, G., Gruszecki, T., Pettersson, H., Sohlberg, R. and Hagfeldt, A. ‘Electric-paint displays’ with carbon counter electrodes. Electrochim. Acta, 46, 2001, 2187–93. 15. Edwards, M. O. M., Gruszecki, T., Pettersson, H., Thuraisingham, G. and Hagfeldt, A. A semi-empirical model for the charging and discharging of electric- paint displays. Electrochem. Commun., 4, 2002, 963–7. References 309 16. Asano, T., Kubo, T. and Nishikitani, Y. Durability of electrochromic windows fabricated with carbon-based counterelectrode. Proc. SPIE, 3788, 1999, 84–92. 17. Nishikitani, Y., Asano, T., Uchida, S. and Kubo, T. Thermal and optical behavior of electrochromic windows fabricated with carbon-based counterelectrode. Electrochim. Acta, 44, 1999, 3211–17. 18. Ziegler, J. P. Status of reversible electrodeposition electrochromic devices. Sol. Energy Mater. Sol. Cells, 56, 1999, 477–93. 19. Ziegler, J. P. and Howard, B. M. Applications of reversible electrodeposition electrochromic devices. Sol. Energy Mater. Sol. Cells, 39, 1995, 317–31. 20. Howard, B. M. and Ziegler, J. P. Optical properties of reversible electrodeposition electrochromic materials. Sol. Energy Mater. Sol. Cells, 39, 1995, 309–16. 21. de Torresi, S. I. C. and Carlos, I. A. Optical characterization of bismuth reversible electrodeposition. J. Electroanal. Chem., 414, 1996, 11–16. 22. Ziegler, J. P. and Howard, B. M. Spectroelectrochemistry of reversible electrodeposition electrochromic materials. Proc. Electrochem. Soc., 94–2, 1994, 158–69. 23. Richards, T. C. and Brzezinski, M. R. Oxidation mechanism for reversibly electrodeposited bismuth in electrochromic devices. 121st Electrochemical Society Meeting, Montreal, Canada, 6 May 1997, abstract 945. 24. de Oliveira, S. C., de Morais, L. C., da Silva Curvelo, A. A. and Torresi, R. M. Improvement of thermal stability of an organic–aqueous gel electrolyte for bismuth electrodeposition devices. Sol. Energy Mater. Sol. Cells, 85, 2005, 489–97. 25. Mascaro, L. H., Kaibara, E. K. and Bulhoˆ es, L. A. An electrochromic system based on redox reactions. Proc. Electrochem. Soc., 96–24, 1996, 96–105. 26. Mascaro, L. H., Kaibara, E. K. and Bulhoˆ es, L. A. An electrochromic systembased on the reversible electrodeposition of lead. J. Electrochem. Soc., 144, 1997, L273–4. 27. Markovic´ , N. M., Grgur, B. N., Lucas, C. A., and Ross, jr, P. N. Underpotential deposition of lead on Pt(111) in the presence of bromide: RRD Pt(111) E and X-ray scattering studies. J. Electroanal. Chem., 448, 1998, 183–8. 28. Mantell, J. and Zaromb, S. Inert electrode behaviour of tin oxide-coated glass on repeated plating–deplating cycling in concentrated NaI–AgI solutions. J. Electrochem. Soc., 109, 1962, 992–3. 29. van der Sluis, P. and Mercier, V. M. M. Solid state Gd–Mg electrochromic devices with ZrO 2 H x electrolyte. Electrochim. Acta, 46, 2001, 2167–71. 30. Huiberts, J. N., Griessen, R., Rector, J. H., Wijngaarden. R. J., Decker, J. P., de Groot, D. G. and Koeman, N. J. Yttrium and lanthanum hydride films with switchable optical properties. Nature (London), 380, 1996, 231–4. 31. Huiberts, J. N., Rector, J. H., Wijngaarden, R. J., Jetten, S., de Groot, D. G., Dan, B., Koeman, N. J., Griessen, R., Hj ¨ orvarsson, B., Olafsson, S. and Cho, Y. S. Synthesis of yttrium trihydride films for ex-situ measurements. J. Alloys Compd., 239, 1996, 158–71. 32. von Rottkay, K., Rubin, M., Michalak, F., Armitage, R., Richardson, T., Slack, J. and Duine, P. A. Effect of hydrogen insertion on the optical properties of Pd-coated magnesium lanthanides. Electrochim. Acta, 44, 1999, 3093–100. 33. Kooij, E. S., van Gogh, A. T. M. and Griessen, R. In situ resistivity measurements and optical transmission and reflection spectroscopy of electrochemically loaded switchable YH x films. J. Electrochem. Soc., 146, 1999, 2990–4. 34. van der Sluis, P., Ouwerkerk, M. and Duine, P. A. Optical switches based on magnesium lanthanide alloy hydrides. Appl. Phys. Lett., 70, 1997, 3356–8. 310 Miscellaneous inorganic electrochromes 35. Ouwerkerk, M. Electrochemically induced optical switching of Sm 0.3 Mg 0.7 H x thin layers. Solid State Ionics, 113–15, 1998, 431–7. 36. Notten, P. L. H., Kremers, M. and Griessen, T. R. Optical switching of Y-hydride thin film electrodes: a remarkable electrochromic phenomenon. J. Electrochem. Soc., 143, 1996, 3348–53. 37. Janner, A.-M., van der Sluis, P. and Mercier, V. Cycling durability of switchable mirrors. Electrochim. Acta, 46, 2001, 2173–8. 38. van der Sluis, P. Chemochromic optical switches based on metal hydrides. Electrochim. Acta, 44, 1999, 3063–6. 39. Mohapatra, S. K. and Wagner, S. Electrochromism in nickel-doped strontium titanate. J. Appl. Phys., 50, 1979, 5001–6. 40. Ohkubo, M., Nonomura, S., Watanabe, H., Gotoh, T., Yamamoto, K. and Nitta, S. Optical properties of amorphous indium nitride films and their electrochromic and photodarkening effects. Appl. Surf. Sci., 1130–14, 1997, 476–9. 41. Garcı´ a-Cana˜ das, J., Meacham, A. P., Peter, L. M. and Ward, M. D. Electrochromic switching in the visible and near IR with a Ru–dioxolene complex adsorbed on a nanocrystalline SnO 2 electrode. Electrochem. Commun., 5, 2003, 416–20. 42. Tell, B. Electrochromism in solid phosphotungstic acid. J. Electrochem. Soc., 127, 1980, 2451–4. 43. Medina, A., Solis, J. L., Rodriguez, J. and Estrada, W. Synthesis and characterization of rough electrochromic phosphotungstic acid films obtained by spray-gel process. Sol. Energy Mater. Sol. Cells, 80, 2003, 473–81. 44. Tell, B. and Wudl, F. Electrochromic effects in solid phosphotungstic acid and phosphomolybdic acid. J. Appl. Phys., 50, 1979, 5944–6. 45. Qi, Y. H., Desjardins, P., Meng, X. S. and Wang, Z. Y. Electrochromic ruthenium complex materials for optical attenuation. Opt. Mater., 21, 2003, 255–63. 46. Gan, J., Tian, H., Wang, Z., Chen, K., Hill, J., Lane, P. A., Rahn, M. D., Fox, A. M. and Bradley, D. D. C. Synthesis and luminescence properties of novel ferrocene–naphthalimides dyads. J. Organometallic Chem., 645, 2002, 168–75. References 311 10 Conjugated conducting polymers 10.1 Introduction to conjugated conducting polymers 10.1.1 Historical background and applications The history of conjugated conducting polymers or ‘synthetic metals’ can be traced back to 1862, when Letheby, a professor of chemistry in the College of London Hospital, reported the electrochemical synthesis of a ‘thick layer of dirty bluish-green pigment’ (presumably a form of ‘aniline black’ or poly(aniline)) by oxidation of aniline in sulfuric acid at a platinum electrode. 1 However, widespread interest in these fascinating materials did not take place until after 1977, following the discovery 2,3,4 of the metallic properties of poly(acetylene), which led to the award of the 2000 Nobel Prize in Chemistry to Shirakawa, Heeger and MacDiarmid. 5,6 Since 1977, electroactive conducting polymers have been intensively investigated for their conducting, semiconducting and electrochemical properties. Numerous electronic applications have been proposed and some realised, including electrochromic devices (ECDs), electroluminescent organic light-emitting diodes (OLEDs), 7,8 photovoltaic elements for solar-energy conversion, 9 sensors 10 and thin-film field-effect transistors. 11 10.1.2 Types of electroactive conducting polymers Poly(acetylene), (CH) x , is the simplest form of conjugated conducting polymer, with a conjugated p system extending over the polymer chain. Its electrical conductivity exhibits a twelve order of magnitude increase when doped with iodine. 2 However, due to its intractability and air sensitivity, poly(acetylene) has seen few applications and most research on conjugated conductive polymers has been carried out with materials derived from aromatic and heterocyclic aromatic structures. Thus, chemical or electrochemical oxidation 312 of numerous resonance-stabilised aromatic molecules, such as pyrrole, thiophene, 3,4-(ethylenedioxy)thiophene (EDOT), aniline, furan, carbazole, azulene, indole (see structures below), and others, produces electroactive conducting polymers. 12,13,14,15,16,17,18,19 N S O N H H NH 2 N H Pyrrole Thiophene Aniline Furan Carbazole Azulene Indole S O O EDOT Of the resulting polymers, the poly(thiophene)s, poly(pyrrole)s and poly(aniline)s have received the most attention in regard to their electrochromic properties, and will be discussed in this chapter. Note that ‘electroactive’ denotes the capability of interfacial electron transfer in one or other direction (oxidation and/or reduction, i.e. a redox capability that allows of colour change). On the other hand, the enhanced conductivity of a charged state (oxidised or reduced) relative to an uncharged state is an accompaniment that is useful in assisting towards rapid redox change, hence rapid colour change. However, the relation between redox properties and conductivity is not necessarily straightforward and varies from polymer to polymer. 10.1.3 Mechanism of oxidative polymerisation of resonance-stabilised aromatic molecules Polymerisation begins with the formation of an oxidatively generated monomer radical cation. The succeeding mechanism is believed to involve either coupling between radical cations, or reaction of a radical cation with a neutral monomer. As an example, the electropolymerisation mechanism for the five- membered heterocycle, pyrrole, showing radical cation–radical cation coupling is given in Scheme 10.1. After the loss of two protons and re-aromatisation, the pyrrole dimer forms from the corresponding dihydro dimer dication. The dimer (and succeeding oligomers) are more easily oxidised than the monomer and the resulting dimer radical cation undergoes further coupling reactions, proton loss and 10.1 Introduction 313 re-aromatisation. Electropolymerisation proceeds through successive electrochemical and chemical steps according to a general E(CE) n scheme, 20 until the oligomers become insoluble in the electrolyte solution and precipitate (like a salt) as the electroactive conducting polymer. Films of high-quality oxidised polymer can be formed directly onto electrode surfaces. 16 10.1.4 Conductivity and optical properties Electronic conductivity in electroactive polymers results from the extended conjugation within the polymer, longer chains promoting high conductivity. The average number of linked monomer units within a conducting polymer is often termed the ‘conjugation length’. X-Ray diffraction of pyrrole oligomers suggests the poly(pyrrole) rings to be coplanar 21 but substitution at nitrogen and the b-carbon introduces a significant twist in the polymer backbone, imposing a non-zero dihedral angle . Note that 6¼0 if R 1 6¼H and R 2 6¼H. N N N N 2H + 2H + –e – –e – . + 2 + + + etc. + + H H H N H N H H N H N H H H + + N H N H N H N H . + N H N H H . + . N H N H + H H N + H H H N H N H N H . Scheme 10.1 Proposed mechanism of the electropolymerisation of pyrrole. The case of radical cation–radical cation coupling is shown. 314 Conjugated conducting polymers N R 2 R 1 N R 1 R 2 ∅ n In the conducting oxidised state with positive charge carriers, electroactive conducting polymers are charge-balanced (doped) with counter anions (‘p-doping’) and have delocalised p-electron band structures, 16 with typical conductivity values in the range 10 1 –10 5 Scm À1 . Figure 10.1 shows illustrative conductivity ranges for poly(acetylene), poly(thiophene) and poly(pyrrole). Values of s are compared with those for common metals, semiconductors and insulators. Reduction of such p-doped conducting polymers, with concurrent SIL VER COPPER IR ON BISMUTH InSb (SN) x POL Y - ACETYLENE σ max > 2 × 10 4 S cm –1 POL Y - THIOPHENE σ max = 2000 S cm –1 POL Y - PYRR OLE σ max = S cm –1 TTF.TCNQ DOPED NMP.TCNQ KCP TRANS (CH) x CIS (CH) x MOST MOLECULAR CRYST ALS UNDOPED H X X ( ) ( ) 10 6 10 4 10 2 10 –2 10 –4 10 –6 10 –8 10 –10 10 –12 10 –14 10 –16 10 –18 Ω –1 cm –1 1 GERMANIUM METALS SEMICONDUCT ORS INSULA TORS SILICON SILICON BROMIDE GLASS DNA DIAMOND SULFUR QUAR TZ N S Figure 10.1 The conductivity range available with electroactive conducting polymers spans those common for metals through to insulators. (Figure reproduced from Thomas, C. A. ‘Donor–Acceptor methods for band gap reduction in conjugated polymers: the role of electron rich donor heterocycles’. Ph.D. Thesis, Department of Chemistry, University of Florida, 2002, p. 17, by permission of the author, who adapted it from the Handbook of Conducting Polymers. 18 ) 10.1 Introduction 315 counter-anion egress to, or cation ingress from, the electrolyte, removes the electronic conjugation, that results in the undoped (that is to say, electrically neutral) insulating form. The magnitude of the conductivity change depends on the extent of doping, which, when under electrochemical control, can be adjusted by the applied potential. The energy gap E g , the electronic bandgap between the highest-occupied p-electron band (the valence band) and the lowest-unoccupied band (the conduction band), determines the intrinsic optical properties of these materials. This is illustrated in Scheme 10.2, which gives the electrochromic colour states in thin films of poly(pyrrole): the non-conjugation of the oxidised form, that allows visibly evident photo-excitation, provides the coloured structure, as explained in detail towards the end of this section. In the reduced form, such neutral polymers are typically semiconductors and exhibit an aromatic form with alternating double and single bonds in the polymer backbone. On oxidative doping, radical cation charge carriers (polarons) are generated, and the polymer assumes a quinoidal bonding state that facilitates charge transfer along the backbone. Further oxidation results in the formation of dication charge carriers (bipolarons). In some instances, the undoped (electrically neutral) state of electroactive conducting polymers can undergo reductive cathodic doping or n-doping, with accompanying cation insertion to balance the injected charge. This doping has been exploited in the development of a model ECD using poly{cyclopenta[2,1-b;4,3-b 0 ]dithiophen-4-(cyanononafluorobutylsulfonyl)methylidene} (PCNFBS), a low-bandgap conducting polymer that is both p- and n-dopable, as both the anode and the cathode material. 22 The polymer PCNFBS is one of a series of fused bithiophene polymers whose E g values can be controlled by N H N N H H n +nX – Yellow–green (insulating) N H N N H H n + + X – X – +ne – Blue–violet (conductive) p-doping undoping Scheme 10.2 Electrochromism in poly(pyrrole) thin films. The yellow-green (undoped) form undergoes reversible oxidation to the blue-violet (conductive) form, with insertion of charge-compensating anions. 316 Conjugated conducting polymers inclusion (initially in the precursor monomers) of electron-withdrawing substituents. Electrochemically polymerised films of the polymer switch from red in the neutral state to purple in both the p- and n-doped states. 22 The spectral changes observed in an electrochemical cell assembled from two polymercoated transparent electrodes were a combination of those seen in the separate p- and n-doped films. 22 Although this is a fascinating example, the stability of negatively charged polymer states is generally limited, and n-doping is difficult to achieve. It is to be noted that the ‘p-doping’ and ‘n-doping’ nomenclature comes from classical semiconductor theory. The supposed similarity between conducting polymers and doped semiconductors arises from the manner in which the redox changes in the polymer alter its optoelectronic properties. In fact, the suitability of the terms ‘doping’ and ‘dopant’ has been criticised 23 when they refer to the movement of counter ions and electronic charge through these polymers, because in its initial sense doping involved minute (classically, below ppm) amounts of dopant. However, ‘doping’ and similar terms are now so widely used in connection with conjugated conducting polymers that attempts to change the terminology could cause confusion. As already noted in the case of poly(pyrrole), in fact all thin films of electroactive conducting polymers have electrochromic possibilities, since redox switching involving ingress or egress of counter ions gives rise to new optical absorption bands and allows transport of electronic charge in the polymer matrix. Electroactive conducting polymers are type-III electrochromes since they are permanently solid. Oxidative p-doping shifts the optical absorption band towards the lower energy part of the spectrum. The colour change or contrast between doped and undoped forms of the polymer depends on the magnitude of the bandgap of the undoped polymer. Thin films of conducting polymers with E g greater than 3 eV, a which gives a corresponding spectroscopic value of max of $400 nm, are colourless and transparent in the undoped form, while in the doped form they generally absorb in the visible region. Those with E g equal to or less than 1.5 eV ($800 nm) are highly absorbing in the undoped form but, after doping, the free carrier absorption is relatively weak in the visible region as it is transferred to the near infrared (NIR) part of the spectrum. Polymers with a bandgap of intermediate magnitude have distinct optical changes throughout the visible region, and can be made to induce many colour changes. a 1 eV¼1.602 Â10 À19 J. 10.1 Introduction 317 10.1.5 Previous reviews of electroactive conducting polymer electrochromes Avast literature encompasses the electrochromismof electroactive conducting polymers, and many reviews are available, including ‘Application of polyheterocycles to electrochromic display devices’ by Gazard 24 (in 1986), ‘Electrochromic devices’ by Mastragostino 25 (in 1993), ‘Electrochromism of conducting polymers’ by Hyodo 26 (in 1994), Chapter 9 of Electrochromism: Fundamentals and Applications by Monk, Mortimer and Rosseinsky 12 (in 1995), ‘Organic electrochromic materials’ by Mortimer (in 1999), 27 ‘Electrochromic polymers’ by Mortimer (in 2004), 28 ‘Polymeric electrochromics’ by Sonmez (in 2005) 29 and ‘Electrochromic organic and polymeric materials for display applications’ by Mortimer et al. (in 2006). 30 10.2 Poly(thiophene)s as electrochromes 10.2.1 Introduction to poly(thiophene)s Poly(thiophene)s 16,19,31 are of interest as electrochromes due to their relative ease of chemical and electrochemical synthesis, environmental stability, and processability. 31 A vast number of substituted thiophenes has been synthesised, which has led to the study of numerous novel poly(thiophene)s, with particular emphasis on poly(3-substituted thiophene)s and poly(3,4-disubstituted thiophene)s. 16 Thin polymeric films of the parent poly(thiophene) are blue ( max ¼730nm) in the doped (oxidised) state and red ( max ¼470nm) in the undoped form. However, due to its lower oxidation potential, b the electropolymerisation and switching of b-methylthiophene has been more intensively studied than the unsubstituted parent thiophene. Furthermore, the introduction of a methyl group at the 3-position of the thiophene ring leads to a significant increase of the polymer conjugation length and hence electronic conductivity. 16 This effect has been attributed to the statistical decrease in the number of insulative a–b 0 couplings and also to the decrease of the oxidation potential caused by the inductive (electron-donating) effect of the methyl group. 16 Poly(3-methylthiophene) is purple when neutral with an absorption maximum at 530nm (2.34eV), and turns pale blue upon oxidation. 32 b When oxidation processes predominate in discussion, it is convenient to cite oxidation potentials, which are for processes that are the reverse of the conventional half reactions (i.e. reductions) of Chapter 3. In the present chapter, positive values are implied: the greater the value, the more positive (and the more oxidising) is the potential that is applied to the electrode under consideration. 318 Conjugated conducting polymers The evolution of the electronic band structure during electrochemical p-doping of electrochromic polymers can be followed by recording in situ visible and NIR spectra as a function of applied electrode potential. Figure 10.2 shows the spectroelectrochemical series for an alkylenedioxy-substituted thiophene polymer, poly[3,4-(ethylenedioxy)thiophene] – PEDOT, which exhibits a deep blue colour in its neutral state and a light blue transmissive state upon oxidation. 33 The strong absorption band of the undoped polymer, with a maximum at 621 nm (2.0 eV), is characteristic of a p–p* interband transition. Upon doping, the interband transition decreases, and two new optical transitions (at $1.25 and $0.80eV) appear at lower energy, corresponding to the presence of a polaronic charge carrier (a single charge of spin ½). Further oxidation leads Figure 10.2 Spectroelectrochemistry for a PEDOT film on an ITO–glass substrate. The film had been deposited from EDOT (0.3 mol dm À3 ) in propylene carbonate solution containing tetrabutylammonium perchlorate (0.1 mol dm À3 ) and spectra are shown on switching in tetrabutylammonium perchlorate (0.1 mol dm À3 ) in acetonitrile. The inset shows absorbance vs. potential. The bandgap is determined by extrapolating the onset of the p to p* absorbance to the background absorbance. The E b1 transition is allowed and is visible at intermediate doping levels. (Figure reproduced from Thomas, C. A. ‘Donor–Acceptor methods for band gap reduction in conjugated polymers: the role of electron rich donor heterocycles’. Ph.D. Thesis, Department of Chemistry, University of Florida, 2002, p. 41, by permission of the author.) 10.2 Poly(thiophene)s as electrochromes 319 to formation of a bipolaron and the absorption is enhanced at lower energies, i.e. the colour shifts towards the characteristic absorption band of the free carrier of the metallic-like state, which appears when the bipolaron bands finally merge with the valence and conduction bands. In such electroactive conducting polymers, the optical and structural changes are often reversible through repeated doping and de-doping over many thousands of redox cycles. 10.2.2 Poly(thiophene)s derived from substituted thiophenes and oligothiophenes As already noted above in the comparison of poly(thiophene) and poly(3- methylthiophene), tuning of colour states can be achieved by suitable choice of thiophene monomer. This tuning represents a major advantage of using conducting polymers for electrochromic applications. Subtle modifications to the thiophene monomer can significantly alter spectral properties. A recent example is provided by cast films of chemically polymerised thiophene-3-acetic acid, which reversibly switch from red to black on oxidation. 34 There has been much interest in polymer films derived from electrochemical oxidation of thiophene-based monomers that comprise more than one thiophene heterocyclic unit. The species containing two thiophene units (joined at the a-carbon, i.e. that next to S) is called bithiophene, while compounds containing three or more thiophene units have the general name of ‘oligothiophene’. It has been shown 35 that the wavelength maxima of undoped poly(oligothiophene) films decrease as the length of the oligothiophene monomer increases, Table 10.1. The oxidation potentials included in this table do not vary much with oligothiophene. The colours available with polymer films prepared from3-methylthiophenebased oligomers are strongly dependent on the relative positions of methyl groups on the polymer backbone. 32,36 As listed in Table 10.2, these include pale blue, blue and violet in the oxidised form, and purple, yellow, red and orange in the reduced form. The colour variations have been ascribed to changes in the effective conjugation length of the polymer chain. To investigate the effect of the dihedral angle between thiophene planes, oligothiophenes containing alkyl groups at the b-carbon have been synthesised. 35 Groups at the b-carbon cause steric hindrance, whereas bridged species (exemplified in Scheme 10.3 below) are linear. The results in Table 10.3 showthat those polymers withthe smallest dihedral angle generally have the highest wavelength maxima. Oxidation potentials are generally unaffected by variations in . Further study of the effects of steric factors is provided by the electronic properties of poly(thiophene)s with 3,4-dialkyl substituents. In principle, 320 Conjugated conducting polymers disubstitution at the b, b 0 positions should provide the synthetic basis to perfectly stereoregular polymers. However, this approach is severely limited by the steric interactions between substituents, which lead to a decrease in polymer conjugation length. In fact, poly(3,4-dialkylthiophene)s have higher oxidation potentials, higher optical bandgaps, and lower conductivities than poly(3-alkylthiophene)s. 16 Alternation between the 3 and 4 positions relieves steric hindrance in thiophenes, but many are harder to electropolymerise than, say, 3-methylthiophene. The electron-donating effect of alkoxy groups offers an answer here, and alkoxy-substituted poly(thiophene)s are being intensively investigated for their electrochromic properties. 37,38 10.2.3 Poly(thiophene)s derived from 3,4-(ethylenedioxy)thiophenes Materials based on PEDOThave a bandgap lower than either poly(thiophene) or alkyl-substituted poly(thiophene)s, owing to the presence of the two electron- donating oxygen atoms adjacent to the thiophene unit. Scheme 10.3 shows the Table 10.1. Wavelength maxima and oxidation potentials of polymers derived from oligothiophenes (based on ref. 35). Monomer a λ max /nm b (undoped) E ox /V S 519 0.95 S S 484 1.00 S S S 356 1.04 S S S S 340 0.93 a Note that these structures do not represent the molecular stereochemistry. b Wavelength maximum refers to the reduced (undoped) redox state of the polymer. 10.2 Poly(thiophene)s as electrochromes 321 structural changes of PEDOT upon reproducible electrochemical oxidation and reduction. The attributes of ethylenedioxy substitution are also pointed out in the figure. As shown above, the bandgap of PEDOT (E g ¼1.6À1.7 eV) itself is 0.5 eV lower than poly(thiophene), which results in an absorbance maximum in the red region of the electromagnetic spectrum. Compared with other substituted poly(thiophene)s, these materials exhibit excellent stability in the doped state, which has a high electronic conductivity. The polymer PEDOT was first Table 10.2. Colours of polymers derived from oligomers based on 3-methylthiophene (based on ref. 15). Monomer S S S S S S S S S S S S S S S S S S S S S S S λ max /nm (undoped) 530 415 505 450 425 405 410 425 Polymer colour (reduced form) Purple Yellow Red Orange Yellow Yellow Yellow Yellow–orange Polymer colour (oxidised form) Pale blue Violet Blue Blue Blue Violet Blue–violet Blue 322 Conjugated conducting polymers developed by Bayer AG research laboratories in Germany in an attempt to produce an easily oxidised, soluble and stable conducting polymer. 39,40 Bayer AG now produce the EDOT monomer, 3, 4-(ethylenedioxy)thiophene, 41 on a multi-ton scale and it is available commercially as BAYTRON M. To aid processing, the insolubility of PEDOT can be overcome by the use of a water- soluble polyelectrolyte – poly(styrene sulfonate), PSS – as the counter ion in the doped state, to yield the commercially available product PEDOT:PSS BAYTRON P by Bayer AG and ORGATRON by AGFA Gevaert, which forms a dispersion in water. Table 10.3. Effect of the dihedral angle : Spectroscopic and electrochemical characteristics of poly(oligothiophene)s (based on ref. 35). Monomer λ max /nm (undoped) E ox /V S S 484 1.00 S S 475 0.96 S S 420 0.99 S S 413 0.88 S S 550 0.90 S S S 356 1.04 S S S 375 0.94 10.2 Poly(thiophene)s as electrochromes 323 PEDOT : PSS SO 3 – SO 3 – SO 3 H SO 3 H SO 3 H SO 3 H S O O S O O S O O S O O S O O S O O n . + . + n As PEDOT and its alkyl derivatives are cathodically colouring electrochromic materials, they can be used with anodically colouring conducting polymers S O O S O O S O O S O O S O O S O O S O O S O O S O O S + X – + X – O O S O O S O O S O O S O O S O O S O O Ethylene br idge ‘ties bac k’ substituents ,relie ving steric interactions betw een adjacent monomer units. Alk oxy groups provide electron donation,leading to lo wer monomer oxidation potentials anddecreasedbandgaps ,while blocking the β positions so only α α ’ coupling may occur. Neutral state (blue) Oxidiz edstate (transmissive sky b lue) Scheme 10.3 Structural changes of poly[3,4-ethylenedioxythiophene] – PEDOT – upon reproducible electrochemical oxidation and reduction. Attributes of ethylenedioxy substitution are also pointed out. (Figure reproduced from Gaupp, C. L. ‘Structure–property relationships of electrochromic 3,4-alkylenedioxyheterocycle-based polymers and co-polymers’. Ph.D. Thesis, Department of Chemistry, University of Florida, 2002, p. 28, by permission of the author.) 324 Conjugated conducting polymers as the other electrode in the construction of dual-polymer ECDs. 42 Changes in the size of the alkylenedioxy ring in general poly[3,4-(alkylenedioxy)thiophene] – PXDOT – materials, and the nature of the substituents on the alkyl bridge, have led to polymers with faster electrochromic switching times, 43,44,45 higher optical contrasts 43,44,45,46 and better processability through increased solubility. 47,48,49,50 As for thiophene, numerous substituted EDOT monomers have been synthesised, which has led to the study of a range of variable-bandgap PEDOT-based materials. 37,38 The bandgap of such conjugated polymers is controlled by varying the extent of p-overlap along the backbone via steric interactions, and by controlling the electronic character of the p-system with electron-donating or -accepting substituents. The latter is accomplished by using substituents and co-repeat units that adjust the energies of the highest- occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of the p-systems. 37,38 An interesting set of materials is the family of EDOT-based polymers which have been prepared with higher energy gaps than the parent PEDOT. From a series of oxidatively polymerisable bis- arylene EDOT monomers (see structures below), polymers with bandgaps in the range 1.4–2.5 eV have been prepared, which exhibit two to three distinct coloured states. 37,38,51,52,53 S O O Ar S O O R R N R′ C 10 H 21 C 10 H 21 S O Ar = alkyl alkoxy oligoether F CN NO 2 R = R′ = H alkyl oligoether In the neutral polymers, a full ‘rainbow’ of colours is available, from blue through purple, red, orange, green and yellow as seen in Colour Plate 2. A few examples include bis-arylene EDOT-based polymers, with spacers of vinylene (E g ¼1.4 eV) that has a deep-purple neutral state, biphenyl (E g ¼2.3 eV) that is orange, p-phenylene (E g ¼1.8 eV) that is red, and carbazole (E g ¼2.5 eV) that is yellow. 51,52 10.2 Poly(thiophene)s as electrochromes 325 Another approach to extend colour choice is electrochemical co-polymerisation from a solution containing two monomers. For example, the ability to adjust the colour of the neutral polymer by electrochemical copolymerisation has been demonstrated using co-monomer solutions of 2,2 0 -bis 3,4-ethylenedioxythiophene) – BEDOT – and 3,6-bis[2-(3,4-ethylenedioxythiophene)]-N-alkylcarbazole – BEDOT-NMeCz. 54 As shown in Colour Plate 3, by varying the ratios of co-monomer concentrations, colours ranging from yellow via red to blue can be evoked in the neutral polymer film. 54 In all co-polymer compositions, the films pass through a green intermediate state to a blue fully oxidised state. 54 As mentioned previously, some electrochromic conducting polymers also undergo n-type doping. Although n-type doping of most of these polymers results in inherent instability to water and oxygen, the introduction of donor– acceptor units has been shown to increase the stability of this n-type redox state. While incorporation of an electron-rich donor unit allows oxidation for p-doping, the inclusion of an electron-poor acceptor unit allows reduction. This has been shown with EDOT acting as the donor unit and both pyridine (Pyr) and pyrido[3,4-b]pyrazine, i.e. PyrPyr(Ph) 2 , as the acceptor unit. 55,56 The polymer PBEDOT-Pyr is red in the neutral state. It changes with p-doping to a light-blue colour. Furthermore, it shows a marked blue with n-doping. 55,56 The polymer PBEDOT-PyrPyr(Ph) 2 is green when neutral, grey upon p-doping, and magenta upon n-doping. 55,56 More recently, 57,58,59 a study has been carried out on the development of an electroactive conducting polymer which is green in the neutral state and virtually transparent (very pale brown) in the oxidised state. To achieve this, it was proposed that a polymer backbone be synthesised that contains two well-defined, isolated, conjugated systems which absorb red and blue light. Thus, a 2,3-di(thien-3-yl)-5,7-di(thien-2-yl)thieno[3,4-b]pyrazine (DDTP) monomer that would afford two conjugated chains was designed and synthesised. 57 One chain has electron donor and acceptor groups to decrease the bandgap, which results in absorption of the red light at wavelengths longer than 600 nm; while the other chain absorbs in the blue at wavelengths below 500 nm. Films of poly(DDTP) were synthesised electrochemically on platinum and ITO-coated glass, to obtain the desired green electrochrome in the neutral state. On electrochemical oxidation of the film, the p–p* transitions of both bands are depleted at the expense of an intense absorption band centred in the NIR, which corresponds to low-energy charge carriers. The depletion upon oxidation makes the polymer film more transparent, but, unfortunately, residual absorptions remain in the visible region, giving a transmissive brown colour. The processibility of the poly(DDTP) systemhas been enhanced by the 326 Conjugated conducting polymers electrochemical and chemical synthesis of a soluble form of the polymer, using dioctyl-substituted DDTP. 60 10.2.4 ‘Star’ polymers based on poly(thiophene)s Star-shaped electroactive conducting polymers, which have a central core with multiple branching points and linear conjugated polymeric arms radiating outward, are now being investigated for electrochromic applications. 61,62,63,64 Examples include star conducting polymers in which the centrosymmetric cores include hyper-branched poly(1,3,5-phenylene) (PP) and poly(triphenylamine) (PTPA), and the radiating arms are regioregular poly(3-hexylthiophene), poly[3,4-(ethylenedioxy)thiophene didodecyloxybenzene] and poly[dibutyl-3,4-(propylenedioxy)thiophene]. 61,62,63,64 These polymers have the advantage that they can be spin coated from a carrier solvent such as tetrahydrofuran (THF), and several can be doped in solution, so that thin films of both doped and undoped forms can be prepared. Despite the branched structure, star polymers self-assemble into thin films with morphological, electrical, and optical properties that reveal a surprisingly high degree of structural order. The polymers, which are smooth and reflecting, all have spectral features that produce a strong band in the visible region for the reduced state and a broad band extending into the NIR for the oxidised state. The colour of the polymers ranges from red via violet to deep blue in the reduced state, and blue to very pale blue in the oxidised state. 10.3 Poly(pyrrole)s and dioxypyrroles as electrochromes As outlined for poly(thiophene)s, poly(pyrrole)s are also extensively studied for their electrochromic properties, and can easily be chemically or electrochemically synthesised. Again, a wide range of optoelectronic properties are available through alkyl and alkoxy substitution. As noted in Scheme 10.2 above, thin films of the parent poly(pyrrole) are yellow-to-green (E g $2.7 eV) in the undoped insulating state and blue-to-violet in the doped conductive state. 65 Poly(pyrrole)s exhibit lower oxidation potentials than their thiophene analogues, 66 and their enhanced compatibility in aqueous electrolytes has led to interest in their use in biological systems. 67 As for dialkoxy-substituted thiophenes, addition of oxygen at the b- positions lowers the bandgap of the resulting polymer by raising the HOMO level. This fact, combined with the already relatively low oxidation potential for poly(pyrrole), gives the poly(alkylenedioxypyrrole)s the lowest oxidation 10.3 Poly(pyrrole)s and dioxypyrroles as electrochromes 327 potential for p-type doping in conducting electrochromic polymers. 68 Poly[3,4-(ethylenedioxy)pyrrole] – PEDOP – exhibits a bright-red colour in its neutral state and a light-blue transmissive state upon oxidation, with a bandgap of 2.05 eV, 0.65 eV lower than that of the parent pyrrole. 69 Furthermore, increasing the ring size of the alkyl bridge has the effect of generating another coloured state at low doping levels. 68 For poly[3,4-(propylenedioxy)pyrrole] – PProDOP – the neutral state is orange, and on intermediate doping passes through brown, and finally to light grey–blue upon full oxidation. 68 Such polychromism is also seen in the substituted PProDOPs and poly[3,4-(butylenedioxy)pyrrole] – PBuDOP. 68 By effecting substitution at the nitrogen in poly(3,4-alkylenedioxypyrrole)s (i.e. PXDOPs), higher bandgap polymers can be created, which retain their low oxidation potentials. 70 Substitution induces a twist in the polymer backbone, which results in a decrease of the effective p-conjugation, and an increase in the bandgap of the polymer. This bandgap increase results in a blue shift in the p–p* transition absorbance, with the intragap polaron and bipolaron transitions occurring in the visible region. The nature of the substituent has an effect on the extent to which the p–p* transition is shifted. For N-methyl-PProDOP the bandgap occurs at 3.0 eV, compared to 2.2 eV for PProDOP, and has a purple colour in the neutral state becoming blue when fully oxidised passing through a dark green colour at intermediate extents of oxidation. 70 Both N-[2-(2-ethoxy-ethoxy)ethyl] PProDOP (N-Gly PProDOP) and N-propanesulfonate PProDOP (N-PrS PProDOP) are colourless when fully reduced but coloured upon full oxidation. 70 Both polymers also exhibit multiple coloured states at intermediate extents of oxidation. 70 These two polymers are thus anodically colouring polymers, in that they change from a colourless state to a coloured one upon oxidation, in contrast with cathodically colouring polymers that are coloured in their reduced state and become colourless upon oxidation. These N-substituted polymers have been shown to work effectively in dual-polymer high-contrast absorptive/transmissive ECDs as the anodically colouring material, due to their electrochemical and optical compatibility with various PXDOT polymers. 71 10.4 Poly(aniline)s as electrochromes Poly(aniline) films 72 are generally prepared from aqueous solutions of aniline in strong mineral acids. 73 Several redox mechanisms involving protonation– deprotonation and/or anion ingress/egress have been proposed. 74,75,76 Scheme 10.4 gives the composition of the various poly(aniline) redox states. 328 Conjugated conducting polymers Leucoemeraldine is an insulator since all rings are benzenoid in form and separated by –NH– or (in strong acid solution) –NH 2 þ – groups, thus preventing conjugation between rings. Emeraldine, as either base or salt, has a ratio of three benzenoid rings to one quinoidal ring, and is electrically conductive. Pernigraniline has equal proportions of quinoidal and benzenoid moieties and shows metallic conductivity. The aniline units within the poly(aniline) backbone are not coplanar, as has been shown by solid-state 13 C-NMR spectroscopy. 77 Electrodes bearing such poly(aniline) films are electropolychromic and exhibit the following reversible colour changes as the potential is varied: transparent leucoemeraldine to yellow-green emeraldine to dark blue-black N N N N H H H H N N N N H H H H X – X – N N N N H H n N N N N Yellow(leucoemeraldine) Green (emeraldine salt - conductor) Blue (emeraldine base) Black(perni graniline) n n n Scheme 10.4 Proposed composition of some of the redox states of poly(aniline), from the fully reduced (leucoemeraldine) through to the fully oxidised (pernigraniline) forms; X À is a charge-balancing anion. 10.4 Poly(aniline)s as electrochromes 329 pernigraniline, in the potential range À0.2 to þ1.0 V vs. SCE. 73 The yellow ! green transition is especially durable to repetitive colour switching. Pernigraniline is an intense blue colour, but appears black at very positive potentials if the film is thick. The yellow form of poly(aniline) has an absorbance maximum at 305 nm, but no appreciable absorbance in the visible region. The electrochemistry of poly(aniline) has been shown to involve a two-step oxidation with radical cations as intermediates. At lower applied potentials, the absorbances of poly(aniline) films at 430 and 810 nm are enhanced as the applied potential is made more positive. 78 At higher applied potentials, the absorbance at 430 nm begins to decrease while the wavelength of maximum absorbance shifts from 810 nm to wavelengths of higher energies. 78 Of the numerous conducting polymers based on substituted anilines that have been hitherto investigated, those with alkyl substituents have drawn much attention. Poly(o-toluidine) and poly(m-toluidine) films have been found to offer enhanced stability of electropolychromic response in comparison with poly(aniline). 79 Absorption maxima and redox potentials shift from values found for poly(aniline) due to the lower conjugation length in poly(toluidine)s. The response times for the yellow–green electrochromic transition in the films correlate with the likely differences in the conjugation length implied from the spectroelectrochemical data. The values for poly(aniline) are found to be lower than for poly(o-toluidine), which in turn has lower values than poly(m-toluidine). As found for poly(aniline), response times indicate that the reduction process is faster than the oxidation. Electrochemical quartz crystal microbalance (EQCM) studies have demonstrated the complexity of redox switching in poly(o-toluidine) films in aqueous perchloric acid solutions, which occurs in two stages and is accompanied by non-monotonic mass changes that are the result of perchlorate counter ion, proton co-ion, and solvent transfers. 80 The relative extents and rates of each of these transfers depend on electrolyte concentration, experimental time scale, and the switching potential, so that observations in a single electrolyte on a fixed time scale cannot be unambiguously interpreted. Poly(aniline)-based ECDs include a device that exhibits electrochromism using electropolymerised 1,1 0 -bis{[p-phenylamino(phenyl)]amido}ferrocene. 81 The monomer consists of a ferrocene group and two flanking polymerisable diphenylamine endgroups linked to the ferrocene by an amide bond. A solid- state aqueous-based ECD was constructed utilising this polymer as the electrochromic material in which the polymer switched from a yellow neutral state to blue upon oxidation. 81 330 Conjugated conducting polymers 10.5 Directed assembly of electrochromic electroactive conducting polymers 10.5.1 Layer-by-layer deposition of electrochromes Following earlier work 82 with poly(viologen) systems, the ‘directed-assembly’ layer-by-layer deposition of PEDOT:PSS (as the polyanion) with linear poly(ethylene imine) (LPEI) (as the polycation) has been reported. 83 The cathodically colouring PEDOT:PSS/LPEI electrode was then combined with a poly(aniline)–poly(AMPS) anodically colouring layered system to give a blue-green to yellow ECD. More recently, Reynolds et al. 84 have studied the redox and electrochromic properties of films prepared by the ‘layer-by-layer’ deposition of fully water-soluble, self-doped poly{4-(2,3-dihydrothieno[3,4-b]- [1,4]dioxin-2-yl-methoxy}-1-butanesulfonic acid, sodium salt (PEDOT-S) and poly(allylamine hydrochloride) – PAH – onto unmodified ITO-coated glass. The polymer PEDOT-S is self-doping where oxidation and reduction of the polymer backbone are coupled with cation movement out of, and back into, the polymer film, in its oxidised and reduced forms respectively. Both the film preparation and redox switching of this system are carried out in an aqueous medium. The PEDOT-S/PAH film was found to switch from light blue in the oxidised form to pink-purple in the reduced form. 10.5.2 All-polymer ECDs The studies outlined in this chapter led to the construction of the first truly all- polymer ECD, where the film of ITO has been replaced by PEDOT:PSS as the conducting electrode material, with the glass substrate replaced by plastic. 85 In the construction of this device, electrodes were first prepared by spin coating an aqueous dispersion of PEDOT:PSS (mixed with 5 wt.%N-methylpyrrolidone (NMP) or diethylene glycol (DEG)) onto commercial plastic transparency films for overhead projection. Multiple layers of PEDOT:PSS were achieved by drying the films with hot-air drafts between coatings and subsequent air drying in an oven of the multilayer film. After three coatings, the surface resistivity of the electrodes had decreased to 600 Oper square (at 300 nm thickness) while remaining highly transmissive throughout the visible region. Following the heat treatment, the PEDOT:PSS multiple-layer film did not return to the non-conducting form over the voltage ranges of the ECD operation. Two ECDs were reported 85 that employed different complementary pairs of electrochromic polymers. In the first device, poly(3,4-propylenedioxythiophene) – PProDOT-Me 2 – and poly{3,6-bis[2-(3,4-ethylenedioxy)thienyl]- N-methylcarbazole} – PBEDOT-N-MeCz – were used respectively as the 10.5 Directed assembly of conducting polymers 331 cathodically and anodically colouring polymers, in a sandwich device, with a polymer-gel electrolyte interposed. In the initial ECD state, PProDOT-Me 2 is in its oxidised (sky-blue) form and PBEDOT-N-MeCz is in its neutral (paleyellow) form, hence the overall colour is an acceptably transmissive green. Application of a voltage (negative bias to PProDOT-Me 2 ) switches the oxidation states of both polymers, causing the device to become blue. In a second all-polymer ECD, two cathodically colouring electrochromic polymers were selected to demonstrate switching between two absorptive colour states (blue and red), with a transmissive intermediate state. The polymer PProDOT-Me 2 was again used, together with, as second electrochromic electrode, poly{1,4- bis[2-(3,4-ethylenedioxy)thienyl]-2,5-didodecyloxy-benzene) – PBEDOT- B(OC 12 ) 2 – showing red to sky-blue electrochromism. Following this work, an all-plastic ECDhas been reported, 86 where PEDOT layers act simultaneously on both electrodes as electrochromes and current collectors, thereby simplifying the construction of electrochromic sandwich devices from seven to five layers. In this research, PEDOT-covered poly(ethylene terephthalate) – PET – foils, commercialised by AGFA under the trademark of ORGACON EL-350, were simply sandwiched together with a poly(ethylene oxide) random co-polymer/lithium triflate polymer electrolyte layer. The contrast ratio for this type of ECD was, however, found to be relatively low, not surprisingly because, as has been noted earlier, both oxidised and reduced forms of a PEDOT are unlikely to be effective electrochromes, but there is clearly scope for improvement. (Several different ORGACON films are available that differ in conductivity, as indicated by the associated numerals.) 10.6 Electrochromes based on electroactive conducting polymer composites The oxidative polymerisation of monomers in the presence of selected additives has been a popular approach to the preparation of electroactive conducting polymers with tailored properties. 12 10.6.1 Novel routes to castable poly(aniline) films While electropolymerisation is a suitable method for preparing relatively lowsurface-area electrochromic conducting polymer films, it may not be suitable for fabricating large-area coatings. As noted above for PEDOT materials, significant effort has gone into synthesising soluble poly(aniline) conducting polymers, such as poly(o-methoxyaniline), which can then be deposited as a thin film by casting from solution. In a novel approach, large-area 332 Conjugated conducting polymers electrochromic coatings have been prepared by incorporating poly(aniline) into poly(acrylate)–silica hybrid sol–gel networks generated from suspended particles or solutions, and then spraying or brush coating onto ITO surfaces. 87 Silane functional groups on the poly(acrylate) chain act as coupling and crosslinking agents to improve surface adhesion and mechanical properties of the resulting composite coatings. A water-soluble poly(styrenesulfonic acid)-doped poly(aniline) has been prepared both by persulfate oxidative coupling and by anodic oxidation of aniline in aqueous dialysed poly(styrene sulfonic acid) solution. 88 Com- posites of poly(aniline) and cellulose acetate have been prepared both by casting of films from a suspension of poly(aniline) in a cellulose acetate solution, and by depositing cellulose acetate films onto electrochemically prepared poly(aniline) films. 89 The electrochromic properties of the latter films were studied by in situ spectroelectrochemistry, where the presence of the cellulose acetate was found not to impede the redox processes of the poly(aniline). The electroactivity and electrochromism of the graft copolymer of poly(aniline) and nitrilic rubber have been studied using stress–strain measurements, cyclic voltammetry, frequency response analysis (i.e. impedance spectroscopy) and visible-range spectroelectrochemistry. 90 The results indicated that the graft co-polymer exhibits mechanical properties similar to a cross-linked elastomer having the electrochromic and electrochemical properties typical of poly(aniline). 10.6.2 Encapsulation of dyes into electroactive conducting polymers An example of a case where the additive itself is electrochromic is the encapsulation of the redox indicator dye Indigo Carmine within a poly(pyrrole) matrix. 91,92 The enhancement and modulation of the colour change on Indigo Carmine insertion into polypyrrole or poly(pyrrole)–dodecylsulfonate films was established. 93 As expected, the use of Indigo Carmine as dopant improves the electrochromic contrast ratio of the film. 10.7 ECDs using both electroactive conducting polymers and inorganic electrochromes As noted in Chapter 8, numerous workers 94,95,96,97,98,99,100,101 have combined a poly(aniline) electrode with an electrode covered with the inorganic mixed valence complex, Prussian blue – PB, iron(III) hexacyanoferrate(II) – or with WO 3 , in complementary ECDs that exhibit deep-blue to light-green electrochromism. Electrochromic compatibility is obtained by combining the 10.7 Electrochromic devices 333 coloured oxidised state of the polymer with the blue of PB, versus the (bleached) reduced state of the polymer coincident with the lightly coloured Prussian green (PG). An electrochromic window for solar modulation using PB, poly(aniline) and WO 3 has been developed, 97,98,100,101 where the symbiotic relationship between poly(aniline) and PB was exploited in a complete solid- state electrochromic ‘window’. Compared to earlier results with a poly(aniline)–WO 3 window, much more light was blocked off by including PB within the poly(aniline) as matrix, while still retaining approximately the same transparency in the bleached state of the window. A new complementary ECD has recently been described, 102 based on the assembly of PEDOT on ITO glass and PB on ITO glass substrates with a poly(methyl methacrylate) – PMMA-based gel polymer electrolyte. The colour states of the PEDOT(blue-to-colourless) and PB(colourless-to-blue) films fulfil the requirement of complementarity. 10.8 Conclusions and outlook Intense interest continues to drive the highly novel research into the electrochromic properties of electroactive conducting polymers outlined here. Through the skills of organic chemists in the synthesis of novel monomers and soluble polymers, the possibilities in colour choice and performance characteristics seem endless and await further exploitation, particularly in the field of display applications. Tailoring the colour of electroactive conducting polymers remains a particularly active research area. Although not described in this chapter, in addition to the synthesis of novel functionalised monomers and use of composites, other chemical and physical methods are investigated for the control of the perceived colour of electrochromic polymers. 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In situ colorimetric analysis of electrochromic polymer films and devices. Synth. Met., 119, 2001, 333–4. 340 Conjugated conducting polymers 11 The viologens 11.1 Introduction The next major group of electrochromes are the bipyridiliumspecies formed by the diquaternisation of 4,4 0 -bipyridyl to form 1,1 0 -disubstituted-4,4 0 - bipyridilium salts (Scheme 11.1). The positive charge shown localised on N is better viewed as being delocalised over the rings. The compounds are formally named as 1,1 0 -di-substituent-4,4 0 -bipyridilium if the two substituents at nitrogen are the same, and as 1-substitituent-1 0 -substituent 0 -4,4 0 -bipyridilium should they differ. The anion X À in Scheme 11.1 need not be monovalent and can be part of a polymer. The molecules are zwitterionic (i.e. bearing plus and minus charge concentrations at different molecular regions or sites) when a substituent at one nitrogen bears a negative charge. 1,2 Aconvenient abbreviation for any bipyridyl unit regardless of its redox state is ‘bipm’, with its charge indicated. The literature of these compounds contains several trivial names. The most common is ‘viologen’ following Michaelis, 3,4 who noted the violet colour formed when 1,1 0 -dimethyl-4,4 0 -bipyridilium undergoes a one-electron reduction to form a radical cation. 1,1 0 -Dimethyl- 4,4 0 -bipyridilium is therefore called ‘methyl viologen’ (MV) in this nomenclature. Another extensively used name is ‘paraquat’, PQ, after the ICI brand name for methyl viologen, which they developed for herbicidal use. In this latter style, bipyridilium species other than the dimethyl are called ‘substituent paraquat’. There are several reviews of this field extant. The most substantial is The Viologens: Physicochemical Properties, Synthesis, and Applications of the Salts of 4,4 0 -Bipyridine (1998) by Monk. 5 Other works are dated, but some still incorporate valuable bibliographic data, including ‘Bipyridilium systems’ (1995) by Monk et al.; 6 ‘The bipyridines’ (1984), by Summers, 7 deals at length with syntheses and properties of 4,4 0 -bipyridine, and Summers’ 1980 book The 341 BipyridiniumHerbicides 8 comprises copious detail. Although dated, the review entitled ‘The Electrochemistry of the viologens’ (1981) by Bird and Kuhn 9 is particularly relevant to this chapter. ‘Formation, properties and reactions of cation radicals in solution’ (1976) by Bard et al. 10 has a section on bipyridilium radical cations. Finally, the review, ‘Chemistry of viologens’ (1991) by Sliwa et al. 11 also alludes to electrochromism. 11.2 Bipyridilium redox chemistry There are three common bipyridilium redox states: a dication (bipm 2 þ ), a radical cation (bipm þ* ) and a di-reduced neutral compound (bipm 0 ). The dicationic salt is the most stable of the three and is the species purchased or first prepared in the laboratory. It is colourless when pure unless exhibiting optical charge transfer with the counter anion, or other charge-donating species. Such absorbances are feeble for anions like chloride, but are stronger for CT-interactive anions like iodide; 12 MV 2 þ 2I À is brilliant scarlet. Reductive electron transfer to the dication forms a radical cation: bipm 2 þ þe À !bipm þ* . (11.1) colourless intense colour Bipyridilium radical cations are amongst the most stable organic radicals, and may be prepared as air-stable solid salts. 13,14 In solution the colour of the radical will persist almost indefinitely 15 in the absence of oxidising agents like periodate or ferricyanide; a its reaction with molecular oxygen is particularly rapid. 16 The stability of the radical cation is attributable to the delocalisation N N R 1 R 2 N N R 1 R 2 N N R 1 R 2 + + + 2X – X – Scheme 11.1 The three common bipyridyl redox states. Different substituents as R 1 and R 2 may be attached to form unsymmetrical species. X À is a singly charged anion. a ‘Ferricyanide’ is better termed hexacyanoferrate(III), but we stick to the usage in this field. Likewise, ‘ferrocyanide’ is properly hexacyanoferrate(II). 342 The viologens of the radical electron throughout the p-framework of the bipyridyl nucleus, the 1-and 1 0 -substituents commonly bearing some of the charge. The potential needed to effect the reduction reaction in Eq. (11.1) depends on both the substituents at nitrogen and on the bipyridyl core – so-called ‘nuclear substituted’ compounds. For example, H ¨ unig and co-workers have correlated the polarographic value of E ½ , values of max from electronic spectra, and the results of theoretical calculations, with informative parameters like s and s* 17,18,19 that relate empirically to electron densities and electronic shifts, as derived from the widely used linear free-energy relationships of physical organic chemistry. Electrochromism occurs in bipyridilium species because, in contrast to the bipyridilium dications, radical cations are intensely coloured owing to optical charge transfer between the (formally) þ1 and (formally) zero-charge nitrogens, in a simplified view of the phenomenon; however, because of the delocalisation already mentioned, the source of the colour is probably better viewed as an intramolecular photo-effected electronic excitation. The colours of radical cations depend on the substituents on the nitrogen. 5 Simple alkyl groups, for example, promote a blue-violet colour whereas aryl groups generally impart a variety of colours to the radical cation, the exact choice depending on the substituents. Manipulation of the substituents at N or the bipyridyl ‘nucleus’ to attain the appropriate molecular-orbital energy levels can also, in principle, tailor the colour as desired. The colour will also depend on the solvent. b Figure 11.1 shows the UV-visible spectrum of methyl viologen. The molar absorptivity " for the methyl viologen radical cation is large; for example, in water " ¼13 700 dm 3 mol À1 cm À1 when extrapolated to zero concentration. 21 The value of " is usually somewhat solvent dependent. 22 A few values of wavelength maxima and " are listed in Table 11.1. The data refer to monomeric radical-cation species unless stated otherwise. Comparatively little is known about the third redox formof the bipyridilium series, the di-reduced or so-called ‘di-hydro’ 32 compounds formed by one- electron reduction of the respective radical cation, Eq. (11.2): bipm þ* þe À !bipm 0 . (11.2) intense colour weak colour b Kosower’s solvent Zvalues (optical CTenergies for the denoted solute with a variety of solvents) in ref. 20 were determined using the different but related system comprising 4-carboethoxy-1-methylpyridinium iodide. The Z values correlate well with many solvent–solute interactions. Other, comparable, CT scales have also been set up. 11.2 Bipyridilium redox chemistry 343 Table 11.1. Optical data for some bipyridilium radical cations. R Anion Solvent max /nm "/dm 3 mol À1 cm À1 Ref. Methyl Cl À H 2 O 605 13 700 22 Methyl I À H 2 O–MeCN 605 a 10 060 23,24 Methyl Cl À H 2 O 606 13 700 21 Methyl Cl À MeCN 607 13 900 22 Methyl Cl À MeOH 609 13 800 22 Methyl Cl À EtOH 611 13 800 22 Methyl Cl À H 2 O 604 16 900 25 Ethyl ClO À 4 DMF 603 12 200 26 Heptyl Br À H 2 O 545 b, c 26 000 27 Octyl Br À H 2 O 543 c 28 900 28 Benzyl Cl À H 2 O 604 17 200 29 p-CN-Ph BF À 4 PC 674 83 300 30 p-CN-Ph Cl À H 2 O 535 b, c – 31 a Estimated from reported spectra. b Solid on OTE. c Solution-phase radical-cation dimer. 2.0 1.5 1.0 0.5 0.0 400 600 800 1000 1200 1400 Wavelength (nm) A b s o r b a n c e (a) (b) Figure 11.1 UV-visible spectra of the methyl viologen radical cation in aqueous solution. (a) ––––––– Monomeric (blue) radical cation and (b) – – – Red radical-cation dimer, the sample also containing a trace of monomer. (Figure reproduced from Monk, P. M. S., Fairweather, R. D., Duffy, J. A. and Ingram, M. D. ‘Evidence for the product of viologen comproportionation being a spin-paired radical cation dimer’. J. Chem. Soc., Perkin Trans. II, 1992, 2039–41, by permission of The Royal Society of Chemistry.) 344 The viologens This product may also be formed by direct two-electron reduction of the dication: bipm 2 þ þ2 e À !bipm 0 . (11.3) Di-reduced compounds are often termed ‘bi-radicals’ 33 because of their extreme reactivity, but magnetic susceptibility measurements have shown such species to be diamagnetic 34 in the solid state, indicating that spins are paired. In fact, di-reduced bipm 0 compounds are simply reactive amines. 35 The intensity of the colour exhibited by bipm 0 species is often low since no obvious optical charge transfer or internal transition corresponding to visible wavelengths is accessible. Figure 11.2 shows cyclic voltammograms depicting these processes. 0 –0.2 –0.4 –0.6 A′ –0.8 –1.0 –1.2 Volts vs. Ag/AgCl B′ A C B Figure 11.2 Cyclic voltammograms on glassy carbon of aqueous methyl viologen dichloride (1 mmol dm À3 ) in KCl (0.1 mol dm À3 ). Scan-rate dependence. Note the evidence of comproportionation – Eq. (11.7): the oxidation peak for spin-paired radical-cation dimer (C) is prominent while the peak for re-oxidation of bipm 0 (B 0 ) is greatly diminished at slow scan rates. The outermost trace is fastest. (Figure reproduced from Datta, M., Jansson, R. E. and Freeman, J. J. ‘In situ resonance Raman spectroscopic characterisation of electrogenerated methyl viologen radical cation on carbon electrode’. Appl. Spectrosc., 40, 1986, 251–8, with permission of the Society of Applied Spectroscopy.) 11.2 Bipyridilium redox chemistry 345 11.3 Bipyridilium species for inclusion within ECDs The most extensive literature on a bipyridilium compound is that for 1,1 0 -dimethyl-4,4 0 -bipyridilium. The write–erase efficiency of an ECD with aqueous MV as electrochrome is low on a moderate time scale, its being type I as both dication and radical cation states are very soluble in polar solvents. The write–erase efficiency of such ECDs may be improved by retarding the rate at which the radical-cation product of electron transfer diffuses away from the electrode and into the solution bulk either by tethering the dication to the surface of an electrode, so forming a chemically modified (‘derivatised’) electrode (Section 1.4), or by immobilising the viologen species within a semi-solid electrolyte. These approaches, with the methyl viologen behaving as a pseudo-solid electrochrome, are described in Section 11.3.1. The solubility–diffusion problemcan also be avoided by the use of viologens having long alkyl-chain substituents at nitrogen, for which the coloured radical- cation product of Eq. (11.1) is insoluble, so here the viologen is a solution-tosolid type II electrochrome, as discussed in Section 11.3.3. Effecting a large improvement in CR (60:1) and response times ( colour ¼1 ms, bleach ¼10 ms) while employing light-scattering by a limited amount of HV 2 þ (deposited by 1 mCcm À2 ), a complex optical systemhas been devised for display applications. 36,37 11.3.1 Electrodes derivatised with viologens for ECD inclusion Wrighton and co-workers 38,39 have often derivatised electrodes with bipyridilium species, initially using substituents at N consisting of a short alkyl chain terminating in the trimethoxysilyl group, which can bond to the oxide lattice on the surface of an optically transparent electrode (OTE). With chemical tethering of this type, Wrighton and co-workers attached the viologen (I) 38 and a benzyl viologen 40 species to electrode surfaces. Wrighton and co-workers also diquaternised a bipyridilium nucleus with a short alkyl chain terminating in pyrrole (which was bonded to the alkyl chain at nitrogen 39 ) – see II; anodic polymerisation of the pyrrole allowed an I N (CH 2 ) 3 Si(OMe) 3 N (MeO) 3 Si(CH 2 ) 3 + + 346 The viologens adherent film of the linked poly(pyrrole) to derivatise the electrode surface, 39 thereby attaching the bipyridilium units. An identical analogue has been prepared with thiophene as the polymerisable heterocycle. 41 The electroactivity of the poly(thiophene) backbone in this latter polymer degraded rapidly after only a few doping/de-doping cycles, but the electroactivity of the viologen moiety remained high. Itaya and co-workers 42 used polymeric electrolytes, but with an electrochromic salt bonded electrostatically to a poly(styrene sulfonate) electrolyte. A bipyridilium salt of poly(p- or m-xylyl)-4,4 0 -bipyridilium bromide (III, shown here as the p form) was employed in this manner: the interaction between the cationic bipyridiliumnucleus and the sulfonyl group is coulombic. The electrode was prepared by dipping the conducting substrate into solutions of electrochrome-containing polymer which, after drying, is insoluble in aqueous solution. 42 Polymeric bipyridiliumsalts have also been prepared by Berlin et al., 43 Factor and Heisolm, 44 Leider and Schlapfer, 45 Sato and Tamamura 46 and Willman and Murray. 47 More recently, NTera of Eire have devised a so-called NanoChromics TM device in which the viologen (IV) is bonded via a strong chemisorptive interaction to a metal-oxide surface. The oxide of choice was nanostructured titanium dioxide, which can be deposited as a thin film of high surface area. The amount of IV adsorbed was therefore high, leading to a good contrast ratio. Fitzmaurice and co-workers 48 in 1994 were probably the first to use a viologen adsorbed onto such layers. III C H 2 C H 2 N N n SO 3 – n + + II N N CH 3 (CH 2 ) 6 + + 11.3 Bipyridilium species for inclusion within ECDs 347 The electrochromismof IVis discussed in Section 11.4 below. Corr et al. 49 of NTera also studied the electrochromic properties of an analogue of IV, in which the phosphonate substituent is replaced with a simple alkyl chain. 11.3.2 Immobilised viologen electrochromes for ECD inclusion A different method of ensuring a high ‘write–erase efficiency’ is to embed the bipyridilium salt within a polymeric electrolyte. Thus, Sammells and Pujare 50,51 suspended heptyl viologen in poly(2-acrylamido-2-methylpropanesulfonic acid) – ‘poly(AMPS)’ – while Calvert et al. 52 used methyl viologen also in poly(AMPS). Both groups report an excellent long-term write–erase efficiency, and a good electrochromic memory. The response times of such devices are, as expected, intrinsically extremely slow. Another means to a similar end is to employ a normally liquid solvent containing a gelling agent (silica, for example 53 ) which is just as effective in immobilising viologens, the concentration of which can be 53 as high as 4 mol dm À3 . 11.3.3 Soluble-to-insoluble viologen electrochromes for ECD inclusion As noted above, in aqueous solution, it is usual for the final product of reduction of a type-II dicationic viologen to be a solid film of radical-cation salt. The process of forming such a salt is usually termed ‘electrodeposition’. Strictly, the term ‘electrodeposition’ implies that the solid product is the immediate product of electron transfer. Most workers now consider the formation of viologen radical-cation salts to be a three-step process, radical cation being formed at the electrode, Eq. (11.1), followed by acquisition of an anion X À in solution and thence precipitation of the salt from solution: bipm þ* (aq) þX À (aq) ![bipm þ* X À ] (s). (11.4) Equation (11.4) represents the chemical step of an ‘EC’ type process in which the product of electron transfer – ‘E’ – undergoes a chemical reaction – ‘C’, Eq. (11.4). Such an overall EC reaction is strictly ‘electroprecipitation’ but commonly termed ‘electrodeposition’. (If electroprecipitation occurs by two IV N N P O OH OH P O HO OH + 2Cl – + 348 The viologens steps that are in effect instantaneously sequential, ‘electrodeposition’ is an adequate description.) 11.3.4 Applications of bipyridilium systems in electrochromic devices The first ECD using bipyridilium salts was reported by Schoot et al. 54 (of Philips in the Netherlands) in 1973. Philips submitted Dutch patents in 1970 55 for heptyl viologen (HV ¼ 1,1 0 -diheptyl-4,4 0 -bipyridilium) as the dibromide salt. The HV 2þ dication is soluble in water, but forms an insoluble film of crimson-coloured radical-cation salt that adheres strongly to the electrode surface following a one-electron reduction, as in Eq. (11.4). The Philips ECD had a contrast ratio of 20:1, an erase time of 10 to 50 ms, 54 and cycle life of more than 10 5 cycles. Philips chose heptyl viologen for their ECDrather than a viologen with a shorter-chain, because reduction of the HV 2þ dication formed a durable film on the electrode, whereas shorter alkyl chains yield somewhat soluble radical-cation salts. The Philips device was never marketed. In 1971, ICI first submitted a patent for the use of the aryl-substituted viologen 1,1 0 -bis(p-cyanophenyl)-4,4 0 -bipyridilium (‘cyanophenyl paraquat’ or ‘CPQ’), 56 which electroprecipitates according to Eq. (11.4) to form a green electrochrome with a superior colour and resistance to aerial oxidation. ICI preferred CPQ to HV owing to its greater extinction coefficient (and hence higher ) and therefore its faster response time per inserted charge. Figure 11.3 shows a schematic of the ECD cell, which is extremely simple. The conducting layer of the ITO(of fairly high resistance, $80 Oper square) acts as the working electrode that displays the colour. Astrip of insulating cellulose acetate is placed near opposing edges of the base, and a stripe of conducting silver paint is applied to its upper surface to facilitate an ohmic contact with the electrode surface. The electrolyte layer was gelled with agar (5%) to improve its stability, and containing the electrochrome in a concentration of 10 À3 mol dm À3 in sulfuric acid or potassium chloride (either of concentration 0.1 mol dm À3 ). The layer is applied over the platinum-wire counter electrode, itself positioned over the insulating layer. The device is completed by encapsulating the electrolyte layer, so a sheet of plain non-conducting glass covers the device. Electrical connection is made to the counter electrode and the exposed end of the silver paint. A potential of À0.2 V (relative to a small, internal silverjsilver chloride electrode) is applied to the silver paint to effect electrochromic coloration – cf. Eq. (1.1) – to form a thin, even layer of insoluble, green radical-cation salt, Eq. (11.5): CPQ 2þ (soln.) þe À þX À ![CPQ þ* X À ](s). (11.5) 11.3 Bipyridilium species for inclusion within ECDs 349 It is best to prevent the formation of a further reduction product, the pale-red species CPQ o (oxidation of which is slow), so the reducing potential should not exceed À0.4 V. The intense green colour of the CPQ þ* radical is stable on open circuit, the colour persisting for many tens of hours. Reversing the polarity and applying a potential of þ1.0 V (measured vs. silver–silver chloride electrode) oxidatively removes the electrogenerated colour in a bleaching time of ca. 1 minute. The Pt counter electrode in Figure 11.3 is pre-coated with solid CPQ þ* and undergoes the reverse of Eq. (11.5) during coloration on the ITO. Then for bleaching at the ITO – i.e. the reverse of reaction (11.5) – the reaction (11.5) takes place at the Pt counter electrode in a confined, invisible volume. This precoating procedure represents an ingenious resolution of the often problematic choice of counter electrode. (In demonstration devices, often electrolysis of solvent is allowed to take place at the counter electrode, which in progressively destroying solvent will of course not serve in long-term use.) A less-anodic potential of þ0.4 V (vs. AgCl–Ag) can be used if the electrolyte is gelled and also contains sodium ferrocyanide (0.1 mol dm À3 ) as an electron mediator to facilitate electro-bleaching; see p. 358. Top elevation Electrical leadto working electrode,touching silver paint Electrical contact, heldin place with conducting silver paint Gelledelectrolyte containing electrochrom e Platinum contact Platinum contact to counter electrode Insulating strip ofcellulose acetate over working electrode Conducting silver-paint contact Optically-transparent electrode,conducting sides innermost Side elevation Cellulose layer Glass sheet (not conductive) Figure 11.3 Schematic representation of an ECD operating by a type-II electrocoloration mechanism, with colourless CPQ 2þ in solution being electroreduced to form a coloured film of radical cation salt. (Figure reproduced from J. G. Kenworthy, ICI Ltd. British Patent, 1,314,049, 1973, with permission of ICI.) 350 The viologens Following extensive and successful field trials, this ECD was first marketed in the early 1970s as a data display device, but liquid-crystal displays (LCDs) entered the market at about the same time, and had faster response times ; LCDs rapidly captured an unassailable market share. The slow kinetics of the ICI type-II cell were the result of including agar to gel the electrochrome- containing electrolyte. Removal of the agar allows for considerable improvements in device response times (to seconds or tenths of seconds), but the electrochromic image is usually streaky and uneven. Ultimately, a yellow- brown oil stains the electrode surface. Yasuda et al. 57 (of Sony Corporation) added encapsulating sugars such as b-cyclodextrin to aqueous heptyl viologen to circumvent the problem of ‘oil’ formation; but ICI believed that this molecular encapsulant would not improve the long-term write–erase efficiency of the different viologen CPQ. 58 The origin of ‘oiling’ as a result of dimer formation by the radicals is considered in more detail in Section 11.3.10 below. 11.3.5 The effect of the bipm N substituents van Dam and Ponjee´ 59 examined the effect that variations in the length of the alkyl chain have on the film-forming properties of the radical cation as the bromide salt (Table 11.2), and redox potentials have been added to this table from ref. 9. As the length of the alkyl chain is increased, the pentyl chain produces the first truly insoluble viologen radical-cation salt. The heptyl is the first salt for which the solubility product is small enough for realistic device usage. Table 11.2 shows that an effective chain length in excess of four CH 2 units is necessary for stable solid films to form. The radical-cation salt of cyanophenyl paraquat (CPQ) is more insoluble in water than is HV þ* , yet the dicationic salt is very soluble. The solubility product K sp of HV þ* Br À in water is 59 3.9 Â10 À7 mol 2 dm À6 . The radical cations of viologen species containing short alkyl chains have a blue colour becoming blue–purple when concentrated. 24 The colour of the radical cation tends towards crimson as the length of the alkyl chain increases, largely owing to increasing incidence of radical-cation dimerisation; the dimer of alkyl-substituted radical cations is red. 24 By comparison, aryl-substituted viologens generally form green or dark-red radical-cation salts. Also, dication solubility and radical-cation stability (in thin films) are both greatly improved by using aryl substituents. This underlay ICI’s use, presented in detail above, of the aryl-substituted viologens, particularly p-cyanophenyl CPQ in their ECD since the electrochromic colour of the heptyl 11.3 Bipyridilium species for inclusion within ECDs 351 viologen radical cation was deemed insufficiently intense: the molar absorptivity (and therefore the CR) of aryl-substituted viologens is always greater than that of alkyl-substituted viologens (Table 11.1). Furthermore, the green radical cation of CPQ apparently 56 is more stable than the other aryl viologen radical cations. 11.3.6 The effect of the counter anion The counter anion in the viologen salt may crucially affect the ECD performance. Different counter ions yield solid radical-cation products of electrodeposition having a wide range of solubilities and chemical stabilities. 30 For example, CPQ þ* is oxidised chemically by the nitrate ion via a rapid but complicated mechanism. 30 Studies of counter-ion effects may be performed using cyclic voltammetry (e.g. ref. 60) or by observing the time dependence of an ESRtrace, which demonstrates the bipm þ* concentration. 30 The ICI group used the SO 2À 4 salt of CPQ 2 þ in their prototype ECDs. 56 The properties of heptyl viologen radical-cation films also depend on the anion as shown by van Dam and Ponjee´ . 59 Jasinski 60 (Texas Instruments) found the optimum anion in water to be dihydrogen phosphate. Anions found Table 11.2. Symmetrical viologens: the effect of varying the alkyl chain length on radical-cation film stability (refs. 9 and 59). The E F values are quoted against the SCE, and refer to viologen salts with the parenthesised anion. Substituent R Effective length (units of CH 2 ) Solid bromide salt film on Pt? Colour E F =mV Methyl 1 No Blue À688 (Cl À ) Ethyl 2 No Blue À691 (Cl À ) Propyl 3 No Blue À690 (Br À ) Butyl 4 No Blue À686 (Br À ) Pentyl 5 Yes Purple À686 (Br À ) Hexyl 6 Yes Purple À710 (Br À ) Heptyl 7 Yes Mauve À600 (Br À ) Octyl 8 Yes Crimson À705 (Br À ) iso-Pentyl 4 Yes Purple À696 (Br À ) Benzyl 4–5 Yes Mauve À573 (Cl À ) CH 3 ðClÞCH 2 OCH 2 À 4 No – CH 3 ÀCH¼CHÀCH 2 À 4 No – HÀCH¼CHÀðCH 2 Þ 3 À 4–5 No – NCÀC 3 H 6 À 4–5 No – À362 a (Cl À ) a Polarographic E ½ value. 352 The viologens useful for ECDs were dihydrogen phosphate, sulfate, fluoride, formate and acetate. Bromide, chloride, tetrafluoroborate and perchlorate also proved satisfactory (as also concluded by van Dam and Ponjee´ 59 ). Heptyl viologen salts of bicarbonate (at pH 5.5), thiocyanate, tetrahydroborate, hexafluorophosphate, tetrafluoroantimonate and tetrafluoroarsenate are all water insoluble. Like CPQ þ* , HV þ* is also oxidised by the nitrate ion, 60 presumably by a similar mechanism. Jasinski’s values 60 of reduction potentials for aqueous HV 2 þ on various metals as electrode substrate, with a variety of anions, are given in Table 11.3. Many other redox potentials for mono-reduction of bipyridilium salts are quoted in the reviews by Monk 5 and by Bird and Kuhn. 9 The choice of anion in the viologen-containing solution can be important since it often participates in charge-transfer type interactions with the viologen. Recent evidence suggests the CT complex must dissociate prior to reductive electron transfer. 61 Reduction is therefore a two-step process: ionpair dissociation !reduction. Electron transfer may be thought of as a special type of second-order nucleophilic substitution (‘S N 2’) reaction in which the ‘nucleophile’ is the electron and the leaving group is an anion. The rates at which the CT complexes of methyl viologen dissociate vary: the complex with iodide dissociates at a rate of 8.7 Â10 5 s À1 , while that with Table 11.3. The effect of supporting electrolyte anion, and of electrode substrate, on the reduction potentials a of heptyl viologen. Values of peak potential E pc are cited against the SCE. (Table reproduced from Jasinski, R. J. ‘The electrochemistry of some n-heptyl viologen salt solutions’. J. Electrochem. Soc., 124, 1977, 637–41, with permission of The Electrochemical Society, Inc.) Anion E pcð1Þ =V E pcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Au E pcð1Þ =V E pcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Pt E pcð1Þ =V E pcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Ag Bromide (0.3 mol dm À3 ) À0.698 À1.008 À0.708 (<À0.818) À0.708 À0.978 H 2 PO À 4 ð2 mol dm À3 Þ À0.668 À1.048 À0.668 (<À0.818) À0.668 Formate (0.4 mol dm À3 ) À0.848 À0.928 À0.828 À0.948 HCO À 3 ð1 mol dm À3 Þ À0.768 À0.958 À0.778 À0.948 Acetate (0.5 mol dm À3 ) À0.828 À0.928 Fluoride (1 mol dm À3 ) À0.818 c À0.878 c À0.868 À0.808 Sulfate (0.3 mol dm À3 ) À0.818 c À0.928 c À0.848 b À0.898 À0.798 À0.918 a Reduction potentials determined at pH 5.5. b Millimolar viologen dication employed for measurement. c No colour formed. 11.3 Bipyridilium species for inclusion within ECDs 353 chloride dissociates at 26.3 Â10 5 s À1 . The anion may thus also influence the speed of electrochromic coloration, as discussed more fully below. 61 11.3.7 The kinetics and mechanism of viologen electrocoloration Kinetic aspects of electro-coloration of type-I electrochromes are discussed in Section 5.1 on p. 75ff. Exemplar viologen systems include viologens in non- aqueous solutions, or short-chain-length viologens in water. The coloration of type-II systems is considerably more complicated, as follows. Bruinink and van Zanten 62 (of Philips) and Jasinski 63 studied the kinetics of HV 2þ dibromide reduction in response to a potential step; both groups found the kinetics of mono-reduction to depend on the electrode history and mode of preparation. For the HV 2 þ –(H 2 PO 4 À ) 2 system, the data obtained do not allow any distinction between two possible but different reduction mechanisms. Jasinski prefers a two-stage process of electroprecipitation (also favoured by van Dam 59 and Schoot et al. 54 ) in which solution- phase HV 2 þ is reduced to formthe radical cation, Eq (11.1), followed by anion acquisition and precipitation of the salt, Eq (11.6) for the bromide anion: HV þ* (aq) þBr À (aq) ![HV þ* Br À ] (s). (11.6) Bruinink and Kregting 64 (cf. Jasinski 65 ), while citing the two-step electroprecipitation mechanism, found the reduction process to be compatible also with a theoretical model of metal deposition derived by Berzins and Delahay. 66 The rate of film growth is controlled by instantaneous three-dimensional nucleation, as seen by a current–time relationship of I vs. t ½ . Ultimately, these nuclei overlap, the commencement of which is shown by a transition in the current–time domain to the expected Cottrell relationship of I dependence on t À½ . The number of nucleation sites available are suggested 66 to depend on the potential. Fletcher et al. 67 reduced HV 2þ in solutions of bromide or biphthalate at a disc of SnO 2 on glass, and agree that the reduction process proceeds via a nucleation step. At low overpotentials of mono-reduction, the rate of reduction was controlled by electron transfer and, at high overpotentials, the nucleation process, once initiated, was sufficiently fast for the crystal-growth process to be controlled by mass transport. Hemispherical diffusion was inferred, creating diffusion zones that could overlap, after formation, and lead to semi-infinite planar diffusion. In summary, the process may be written as electron transfer ! nucleation ! hemispherical diffusion ! linear diffusion, but the process is too complicated to allow precise mathematical models of deposition to be used. 354 The viologens By way of confirmation, in cyclic voltammetry, the current–potential curve (cyclic voltammogram, CV) associated with electro-coloration is unusually steep, which usually implies a catalytic or nucleation process; the usual shape of the CV before the current peak is exponential. (The presence of radical- cation dimer on an electrode also causes a steep CV peak, 68 which may imply that nucleation sites are comprised of dimer.) The morphology of HV þ* films has been addressed by Barna 69 (Texas Instruments). Deposited films are partially crystalline but largely amorphous, acquiring a greater degree of crystallinity with time; this acquisition of crystallinity is probably associated with additional sharp peaks observed during cyclic voltammetry of heptyl viologen films. 9,59,60,63 The time-dependent change within deposits of HV þ* on an OTE has been observed by Goddard et al. 70 using UV-visible spectroscopy with a novel potential cycling technique (but now rendered obsolete by use of diode-array spectrophotometry). Bewick et al. 71,72 have investigated HV 2þ dibromide and many asymmetric bipyridilium salts (that is, with substituents at N and N 0 being different), by diode-array spectroscopy. The initial solid product of mono-reduction was considered to be HV þ* radical cation in a salt which incorporates some unreduced HV 2þ dication. 72 Subsequent aging effects and previously inexplicable additional CV peaks are explained in terms of this composite form of solid deposit. Asimilar explanationfor the complicatedcyclic voltammetric behaviour observed during the formation of solid CPQ þ* radical-cation salt has also been advanced. 13,73 11.3.8 Micellar species Association of bipyridilium species to form p-dimers is a well-documented phenomenon 23,24 for the viologen radical cation, but is not so well attested for the dication (although see references 13, 37, 74 and 75). A particular problem with aqueous solutions of viologen can be the formation of micelles of dication, particularly if the substituents are large aryl groups or long alkyl chains that are hydrophobic. In this latter case, the analogy between such viologens and quaternary ammonium cationic surfactants is clear. 76 Barclay et al. 37 (of IBM) quote a critical micelle concentration cmc for the HV 2þ dication of 10 À2 mol dm À3 in aqueous bromide solution. Electrochemistry at these micelles is envisaged to proceed in discrete steps, with dication on the micelle periphery being reduced preferentially. 77 If the concentration of HV 2þ lies above the cmc, interaction of such a micelle with a cathodically biased electrode causes reduction of the outside of the micelle to form bipm þ* , yet the inside of the micelle remains fully oxidised dication. 13 11.3 Bipyridilium species for inclusion within ECDs 355 A similar explanation for the complicated cyclic voltammetric behaviour observed during the formation of solid CPQ þ* radical-cation salt has also been advanced. 13,73 Heyrovsky´ has also postulated the existence of solution- phase mixed-valence species of methyl viologen in water. 78,79,80 Although such mixed-valence viologens are not involved often, and comprise very small amounts of material, they are capable of greatly complicating the electrochemistry of solutions containing them. In an effort to mimic the properties of bipyridilium species within micelle environments, Kaifer and Bard 74 investigated the electrochemistry of methyl viologen in the presence of various surfactants (anionic, cationic and nonionic), finding that the properties of methyl viologen were largely unaffected by the presence of surfactant when below the cmc, but above it, the properties of the methyl viologen were markedly different, e.g. the EPR spectrum of MV þ* lost all hyperfine coupling due to rapid inter-radical spin swapping. Engleman and Evans 81,82 also investigated the electrochemical reduction of MV 2 þ in the presence of micellar anions. In the presence of anionic surfactant, the position of the monomer–dimer equilibrium was displaced significantly in favour of the monomeric formwhen above the cmc, whereas cationic and nonionic surfactant did not affect the equilibrium either way. Cyclic voltammetry also differed above and below the cmc. Given the complexity of the overall electroprecipitation mechanism, and the different speciations in solution during each of the steps during electroprecipitation, it is again worth remarking that comparison of results from different authors is difficult since each step is defined by the number and nature of each anion in solution, and different experimental conditions (where known) have been employed by each author. 11.3.9 The write–erase efficiency The write–erase efficiency of a viologen electrochrome is very high if the solvent is non-aqueous and rigorously dried. For example, the archetypal type-I Gentex mirror described in Section 12.1 contains a viologen (which is probably zwitterionic) as the primary electrochrome. Byker, formerly of Gentex, 83 cites a cycle life of ‘>4 Â10 4 cycles’ for the related system benzyl viologen (as the BF À 4 salt) in propylene carbonate (PC). Ho et al. 84,85 have modelled the electrochemical behaviour and cycle life of similar electrochromic devices, particularly one with heptyl viologen as the primary electrochrome and tetramethylphenylenediamine (TMPD) as the secondary. The cycle lives are high, but the response times are quite slow, for the reasons discussed above. 356 The viologens The mechanism of deposition was examined at length since here the nature of the solid deposit is important. For example, firstly a fresh film of HV þ* is amorphous, even 60 and smooth, 86 yet soon after deposition (<10 s 70 ) the film appears patchy as an aging process occurs, which probably involves ordering (crystallisation) of radical moieties. Re-oxidation of the film to bleach the colour is rapidfor freshHV þ* films, but patchyfilms that showsigns of agingare more difficult to oxidise, requiring a higher potential or a longer re-oxidation time. Dimerisation of radicals could participate in this complication. Secondly, after prolonged cycling between the coloured and bleached states, bipyridilium ECD devices form an unsightly yellow–brown stain on the electrode. Some evidence now suggests that this stain is a form of crystalline radical-cation salt 30 containing spin-paired radical-cation dimer, or intervalence species comprising both bipm 2 þ and bipm þ* . Spectroscopic studies 87 (a surface-enhanced resonance Raman analysis of a disulfide-containing dimeric viologen adsorbed on rough silver) strongly suggest the presence of a ‘liquid-like’ environment at the electrode surface following reduction to form dimerised radical bipm ð Þ 2þ 2 . It is also likely that reordering of radical species (‘recrystallisation’) occurs within the electroprecipitated viologen deposit soon after it forms. In order to understand these processes, thin films of HV þ* salt have been studied by many techniques including UV-visible spectroelectrochemistry, 72,88,89,90 EPR, 91 Raman spectroscopy, 92,93,94 photoacoustic spectroscopy, 95,96 photothermal spectroscopy 97 and the electrochemical quartz-crystal microbalance (EQCM). 86 Scharifker and Wehrmann 98 investigated phase changes within radical- cation salt deposits of HV þ* and benzyl viologen radical cation, and Gołden and Przyłuski 99 looked at HV þ* . Both groups found the aging effect to be due, in part, to the dimerisation of radical cation in solution. Belinko 33 suggested that device failure is also due to production of di-reduced bipyridilium (bipm 0 ) as a minor electrode product. The formation of diamagnetic HV 0 (at large negative potentials) should be avoided since it is only electrochemically quasi- reversible electrochemically, i.e. slow, in aqueous solution. 9 Belinko 33 investigated the write–erase efficiency of HV þ* films by cyclic voltammetry, making the lower scanning limit progressively more negative deliberately to generate bipm 0 . After bipm 0 is formed, it may react with bipm 2 þ from the solution in the comproportionation: bipm 2þ þbipm 0 ! bipm þ À Á 2 ! 2 bipm þ : (11:7) The immediate product of Eq. (11.7) is the radical-cation dimer. In solution, subsequent dimer dissociation yields monomeric radical cation 100 but often 11.3 Bipyridilium species for inclusion within ECDs 357 solid deposits of ‘bipm þ* ’ exhibit spectroscopic IR bands attributable to the spin-paired (bipm þ* ) 2 dimer. 101 In effect, spin pairing is ‘locked into’ solid deposits of viologen radical cation. Recent work has shown that the radical-cation dimer is electrochemically only quasi-reversible, that is, its electro-oxidation is slow, 100,102 hence the observed failure of ECDs containing traces of dimer. The 1998 review by Monk 5 demonstrates how widely comproportionations occur in viologen redox chemistry. Its fast rate constant and moderate equilibrium constant make it almost certain that comproportionation processes always occur whenever bipm 0 is formed electrochemically. Invoking the participation of Eq. (11.7) can greatly simplify Belinko’s otherwise complicated mechanistic observations. (In this context, see also the way comproportionation can simplify mechanistic observations in ref. 103.) Engelmann and Evans have also published 104 studies of potentiostatic deposited MV 0 , EtV 0 , BzV 0 and HV 0 ; each was formed at a glassy-carbon rotated ring-disc electrode (RRDE) from solutions containing the respective dications. The reductions are concerted two-electron reactions. To summarise their findings, deposition is initiated by nucleation of supersaturated bipm 0 close to the electrode; the rate of deposition decreases as the bulk of the deposit increases, i.e. as the surface of the disc becomes blocked. Comproportionation of bipm 2 þ (aq) from the solution with bipm 0 (s) on the disc becomes increasingly important with time, so that the total amount of bipm 0 on the disc decreases until the amount reaches a steady state. That comproportionation occurs in the solid state has been confirmed for CPQ 0 and CPQ 2 þ from aqueous electrolytes. 30,105 The mechanism of comproportionation differs when ferrocyanide is involved (see footnote a on p. 342). This result may be important because this ion is a popular choice of electron mediator. 106 Generally, the bipm 2 þ and bipm 0 species approach and thence form a sandwich-like structure with their p-orbitals overlapping. Comproportionation occurs when the electron transfers through these orbitals. However, when ferrocyanide is involved, the ferrocyanide ion is believed to lie between the two viologen species, in a structure reminiscent of a metallocene. Equation (11.7) could thus occur by electron transfer through the ferrocyanide possibly by a concerted doubleexchange mechanism. 106 Hence the two radical-cation moieties produced by reaction (11.7) are never in contact; after reaction, they separate from the ferrocyanide to form individual ions. Benzyl viologen has also been extensively investigated since it will also form an insoluble film of radical-cation salt following one-electron reduction. 70,89,98,107 358 The viologens To summarise, the speciation of the viologen dication is complicated prior to the transfer of an electron: the rate of anion–dication separation prior to (or during) electron transfer follows 61 the rate k et and may in fact dictate its magnitude; the rates of anion–radical cation association following the electron transfer is completely unknown; the way the length of the substituents at nitrogen dictates the solubility constants of radical cation–anion pairs is fairly well understood; and the way the solubility index dictates the rate of precipitation has been investigated extensively. 11.3.10 Attempts to improve the write–erase efficiency The first and most effective method of improving the write–erase efficiency is to employ non-aqueous solutions, although the coloration time will necessarily be slow, and the bleaching time slower still. The second method used to prevent the non-erasure of films of HV þ* salt is to add an auxiliary redox couple (that is, an electron mediator) to the dicationcontaining electrolyte solution. The mediators used include hydroquinones, 55 ferrous ion, 56 ferrocyanide, 56,57,92 or cerous ion, 50,51 and ferrocene in acetonitrile has been used in a type-II device. 108 During electro-coloration, bipm 2 þ ion is reduced to bipm þ* but, during re-oxidation at a positive potential, it is the mediator (e.g. ferrocyanide) that is oxidised at the electrode. The oxidised form of the mediator – in this example, ferricyanide, i.e. hexacyanoferrate(III) – allows for chemical oxidation of the radical-cation film, to reform the dication. Such oxidation is very rapid. 15 Mediators facilitate the electro-oxidation of the radical cations of type-II species, such as heptyl viologen. For aryl viologens in aqueous solution, a mediator is always necessary to ensure complete colour removal on re-oxidation. 5 As ferrocyanide is known to form a charge-transfer complex with methyl viologen dication 109,110 and also with the dications of CPQ 5,12,111 and HV, 57 it will be the free-anion equilibrium fraction of the species that can be assumed to act. The unsightly yellow–brown stains still persist, however, even with the HV 2 þ and CPQ 2 þ systems that contain K 4 Fe(CN) 6 . 56,92 In a notable advance, addition of the sugar b-cyclodextrin to the voltammetry solution has been found to impede the formation of yellow–brown stains, 57,91 probably by encapsulating the dication within the cavity of the cyclodextrin in a guest–host relationship. Because close contact between bipyridilium dications is greatly impeded in such a guest–host relationship, association of bipm 2 þ cations in solution 57 is largely thereby prevented, so alignment of bipm þ* species in the solid deposit is impossible. However, such ‘oiling’ is claimed still to occur ‘ultimately’ with CPQ þ* . 58 11.3 Bipyridilium species for inclusion within ECDs 359 Other attempts to stop the ageing phenomenon have used different, modified bipyridilium compounds. 112,113,114 For example, Bruinink et al. 112 prepared the compound V in which the two pyridinium rings are separated by methylene linkages. To a similar end, Barna and Fish 113 prepared asymmetric bipyridiliumsalts, that is species in which R 1 6¼R 2 (Scheme 11.1), thereby inhibiting the crystallisation process: for example, a compound was made having R 1 ¼C 7 H 15 and R 2 ¼C 18 H 37 . Barltrop and Jackson 114 have prepared similar asymmetric viologens, and a diquaternised (that is, made cationic by alkyl or aryl addition) 3,8-phenanthroline salt (VI), together with a series of nuclear-substituted bipyridyls (species in which substituents are directly bonded to carbon in the pyridine rings). Again, films with superior write–erase properties were formed. Despite the many drawbacks recounted above, a large number of prototype viologen ECD devices have been made. 36,37,55,56,115 For example, an impressive device from the IBM laboratories utilised a 64 Â64 pixel integrated ECD device with eight levels of grey tone of heptyl viologen 115 on a 1 inch square silicon chip, to give quite detailed images (Figure 11.4). These devices were not exploited further owing to competition from LCD systems, though they may still have a size advantage in large devices. 11.4 Recent elaborations The majority of the newdevelopments reported here aimto enhance the rate of coloration in bipyridilium-based ECDs. 11.4.1 Displays based on viologens adsorbed on nanostructured titania Nanostructured electrodes are easily prepared by spreading a concentrated colloidal suspension on a conducting substrate and firing the resulting gel film V CH 3 CH 2 (CH 2 ) 4 CH 2 CH 3 N N + + VI N N C 6 H 13 C 6 H 13 + + 360 The viologens at 450 8C. 116 Such electrodes have been widely investigated for use in dye- sensitised photoelectrochemical cells. 116,117 The rough surface of the porous titanium dioxide film consists of a network of interconnected semi-conducting metal oxide nanocrystals. Because the oxide crystals are so small, such films have an extraordinarily high internal surface area. The ratio between the internal surface area and the smooth geometrical area of the electrode (the ‘roughness factor’) approaches 1000 for a film that is only 4 mm thick. 117 This means that a high number of electrochromic viologen molecules can occupy a relatively small area, leading to a high coloration efficiency . Furthermore, as they are surface-confined, the viologen molecules need not diffuse to the electrode surface, which leads to shorter switching times. Nanostructured titaniumdioxide in its anatase formcan be deposited as a thin film of high surface area. Viologens are strongly adsorbed on its surface Figure 11.4 Reproduction of an IBM electrochromic image displayed on a 64 Â64 pixel integrated ECD device with eight levels of ‘grey tone’ of heptyl viologen. The original is clearer. (Figure reproduced from Barclay, D. J. and Martin, D. H. ‘Electrochromic displays’. In Howells, E. R. (ed.), Technology of Chemicals and Materials for the Electronics Industry, Chichester, Ellis Horwood, 1984, 266–76, by permission of Ellis Horwood.) 11.4 Recent elaborations 361 owing to their electron deficiency. Such systems have long been investigated in research on dye-sensitised solar cells, for example Gra¨ tzel’s work on his photoelectrochemical cell. 117,118 Originally developing a spin-off from the Gra¨ tzel cell, Fitzmaurice and co-workers at the Dublin-based NTera Ltd 119 (founded in 1997, having manufacturing facilities in Ireland and Taiwan) have developed a ‘next generation display technology’ called NanoChromics TM displays that are based on these principles. 49,120,121,122 NTera also describe their ECD as a ‘paper quality’ electrochromic display, that is, an ECD of very high definition. An assembled NanoChromics TM electrochromic device uses two metal-oxide films – one at the negative electrode and, unusually, one at the positive electrode. In a typical device 122 (borrowed fromGra¨ tzel 123 ) the negative F-doped tin oxide conducting glass electrode (the cathode on coloration) is coated with the wide bandgap titanium dioxide film 4 mm thick, followed by a monolayer of self-assembled, chemisorbed phosphonated viologen molecules. The positive F-doped tin oxide conducting glass counter electrode (i.e. the anode on coloration) carries a film of heavily doped antimony tin oxide (SnO 2 :Sb) 3 mm thick, followed by a monolayer of self-assembled, chemisorbed phosphonated phenothiazine molecules. The TiO 2 film is further modified with an adsorbed monolayer of viologen (IV), bis(2-phosphonoethyl)-4,4 0 -bipyridilium dichloride. The electrolyte was g-butyrolactone containing LiClO 4 (0.2 mol dm À3 ) and ferrocene (0.05mol dm À3 ). 120,121 In trials, their device had a coloration efficiency of 170cm 2 C À1 at 608nm, 121 and was said to be stable over 10 000 ‘standard’ test cycles. The counter electrode is viewed as having a high capacitance, which assists charge storage during coloration. The ECD is sealed with a thermoplastic gasket and a UV-curable epoxy resin. Application of a potential of 1.2 V reduces the dicationic viologen to its blue radical cation, and oxidises the phenothiazine from its weak yellow colour to red. The overall colour change is therefore from virtually colourless to a blue-red purple. Placing a diffuse reflector between the electrodes, e.g. a layer of an ion- permeable nanostructured solid film of titanium dioxide, gives on coloration the visual effect of ink on pure white paper. Without the intermediate TiO 2 layer the display is transparent while retaining readability. Different colours can be achieved in ECDs depending on the nature of the substituent(s) on the viologen molecule. 120 In such devices, many thousands of switches are possible before there is significant degradation of performance. Some open-circuit memory persists, the colour remaining for more than 10 min after the voltage is switched off, but readily regenerated. Electrodes can be micro-patterned for display applications. 362 The viologens Fitzmaurice’s display is said to be ‘ultra fast’, 122 although the criterion for this claim is unclear, since the switching time is 1 s for a change in absorbance of 0.60. 120 However, this is certainly faster than most of the other viologen-based devices, since the anchored viologen electrochrome avoids the diffusion delay before electron transfer. Fitzmaurice notes that charge compensation within the viologen layer is also fast because many counter ions are also adsorbed on the TiO 2 layer. If the counter electrode is covered with a secondary electrochrome such as a phenothiazine, the value of increases to about 270 cm 2 C À1 and the response time is decreased to 250 ms. 122 Published spectra suggest an optical density (OD) change of about 0.55, again at 608 nm. The NTera group state that they are working with a number of marketleading strategic partners for access to the market. Recently, NTera have demonstrated a NanoChromics TM display operating in a converted iPod (the portable digital audio players from Apple Computer Corporation). 124 The NTera website 119 provides ‘consumer product reference designs’ for digital clocks and an eight-digit calculator. That NanoChromics TM displays can be manufactured by existing LCD manufacturing processes will clearly enhance the likely success in the commercial development of this technology. NTera also state that their flexible display prototype can, in principle, be applied to all the product types: displays, windows and mirrors, giving rise to products such as ‘smart card’ displays, dimmable window laminates, applications in toys and games and ultimately flexible electronic paper displays. The company notes that signs using NanoChromics TM display technology are ideal for sports player-substitution boards. They claim that the current LED boards can become bleached out and difficult to read in bright daylight in sports stadia, and that NanoChromics TM display signs are perfect for this application as they are easy to read in bright daylight and at all angles. Several workers have adapted these ideas. Gra¨ tzel et al., 125,126 for example, have prepared such devices with a series of viologens, with aryl as well as alkyl substituents. In each case, the anchored group attaching the viologen to the titania was benzoate, salicylate or phosphonate (as in IV). Electrochromic devices they have constructed include shutters and displays. The cell OTEjTiO 2 - poly(viologen)jglutaronitrile–LiN(SO 2 CF 3 ) 2 jPrussian-bluejOTE exhibited an optical density change of about 2; the colour changes on reduction were transparent to blue, or yellowish to green, and (at higher potentials) to red– brown. They report switching times in the range of 1–3 s. Higher optical density changes are possible if the switching times are slower. 126 Gra¨ tzel and co-workers also made a variety of cell geometries for ECDs operating on 11.4 Recent elaborations 363 reflectors. The viologens in such devices were generally oligomers rather than polymers. In a similar way, Boehlen et al. 127 prepared a salt of 2,2 0 -bipyridine (VII) calling it a ‘viologen’; they generated a pink colour on reduction (which could indicate that a proportion of the viologen exists as radical-cation dimer). Edwards et al. 128,129,130,131,132 have prepared many similar systems for devices, with viologen electrochromes adsorbed on titania, naming such devices ‘electric paint’. They generally employed the viologen IV to produce amazing clarity. For example, Figure 11.5 shows a prototype, demonstrating clarity capable of high-definition patterning. The response time is about 0.5 s. VII N N P O HO HO +. Figure 11.5 Prototype electrochromic display showing an ‘electric paint’ display: the primary electrochrome was viologen (IV) adsorbed on nanocrystalline TiO 2 . (Figure reproduced from Pettersson, H., Gruszecki, T., Johansson, L.-H., Edwards, M. O. M., Hagfeldt, A. and Matuszczyk, T. ‘Direct-driven electrochromic displays based on nanocrystalline electrodes’. Displays, 25 2004, 223–30, with permission of Elsevier Science Ltd.) 364 The viologens 11.4.2 The use of pulsed potentials Pulses of current have been shown to enhance the rate at which electrochromic colour is formed, relative to coloration with a continuous potential. 133 The procedure relies on the solution-phase redox reaction between bipm 2þ (from the bulk solution) and bipm 0 electrogenerated during the current pulse. The reaction is comproportionation, Eq. (11.7), so a sufficiently cathodic potential must be applied at the working electrode. The amounts of bipm 2þ and bipm 0 at the electrode and in the region around the electrode depleted of bipm 2þ will govern the rate of comproportionation and hence the rate of product colour formation. Thus for a given concentration of bipm 2þ and bipm 0 in such a region, the most intense colour will ensue when the two species are in equal concentration. It is envisaged that the pulse procedure possibly favours this equality. 11.4.3 Electropolychromism Bipyridilium salts may typically possess three colours, one for each oxidation state in Scheme 11.1, although the dication in solution is essentially colourless. Viologen electrochromes comprising n bipyridilium units could thus, in principle, exhibit 2n þ1 colours. This maximal number is not achieved however when delocalisation allows simultaneous coloration of two or more of the bipyridiliums. 134 Several approaches have employed a number of bipyridilium units connected either with alkyl linkages 135,136 or benzylic moieties. 134 A different, highly promising, combination, the complementary use of a bipyridilium with a Prussian blue electrochrome, allows the fabrication of a five-colour ECD. 137,143 11.4.4 Viologens incorporated within paper Viologen electrochromes have been incorporated within paper, to effect electrochromic writing. These include methyl viologen, 138,139,140,141 heptyl viologen, 141 and the asymmetric system, methyl–benzyl paraquat (VIII). 139 The adsorption of methyl viologen onto the carbohydrate structures of paper follows Langmuir adsorption isotherms that imply chemisorptive behaviour. 138 VIII N N H 3 C + + 2X – 11.4 Recent elaborations 365 While methyl viologen in paper is electrochromic, 138,140 its response time is prohibitively slow. The speed is faster if the paper is layered with the polyelectrolyte poly(AMPS), presumably because it provides an additional source of ions. With MV 2þ , the speed of response depends critically on the paper’s relative moisture. The results can be summarised as showing that in paper of marginal moistness, the solution-phase electrochemistry of both Prussian blue and viologens can be reproduced as though in a standard electrochemical cell. Alternatively, incorporation within Nafion TM has been shown to produce good results. 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Most standard texts on quantitative and qualitative analytical chemistry cite many examples of redox indicators, like the compendia in ref. 1. There is a severe lack of any systematic survey of the electrochromic properties of such species for ECD application. Most of the aromatic species in this chapter form either a molecular radical cation or radical anion following electron transfer. All the organic species in this chapter, and the viologen species in the previous chapter, are ‘violenes’ – a conceptual classification pioneered by H ¨ unig. 2 A violene is a conjugated molecular fragment of the form X ( CH¼CH ) n CH–X, where X¼O, N or S. The conjugated ( CH¼CH ) n portion is normally part of an aromatic ring or series of rings. As a direct consequence of their structure, all violenes typically possess three stable redox states: an uncharged species, a species with a double charge, and a species of intermediate redox state that is either a radical cation or radical anion. The conjugation within the violene that allows extensive delocalisation is the ultimate cause of the extraordinary stability of many such radicals. 12.1.1 Aromatic amine electrochromes Aromatic amines are generally colourless unless they undergo some form of charge-transfer interaction with an electron-deficient acceptor species. By contrast, the product of one-electron oxidation yields a radical cation which, in organic solution, possesses a brilliant colour. Aromatic amines are thus candidate electrochromes. 374 If both the neutral and radical-cation redox states are soluble, such amines show type-I electrochromism. In relatively non-polar solvents, the radical cation together with an electrolyte anion may deposit as a salt. 3,4 In such solvent systems, the aromatic amines are type-II electrochromes. These species fall into two categories: 2 (i) the nitrogen is incorporated into an aromatic ring and is a derivative of the pyridine ring C 5 H 5 N, for example; (ii) the amine is attached to an aromatic (e.g. C 6 H 5 À) ring, like the NH 2 group of aniline, C 6 H 5 ÀNH 2 . The nitrogens of all the amine groups need to be fully substituted, to preclude polymerisation reactions: the radical cations of aromatic secondary amines retaining an N–H functionality readily form an inherently conducting polymer, for example poly(aniline), as described in Section 10.4. The monomeric aromatic amine that has probably received the most attention for its electrochromic prospects is tetramethylphenylenediamine (TMPD) as the p- (I) and o-isomers. 5,6,7,8,9,10,11,12 The radical cation of I is stable and is brilliant blue-green. Ho et al. 10,11 have modelled the electrochemical behaviour and cycle lives of electrochromic devices in which I was the secondary electrochrome, against heptyl viologen (see Chapter 11) as the primary. The cycle lives of such devices are high, but the response times are slowowing to the requirement for all solution-phase electrochromes to diffuse toward the electrode–solution interphase prior to electron transfer; bleaching times likewise are also long. I N N H 3 C H 3 C CH 3 CH 3 A more bulky electrochrome is the triphenylamine derivative II. When a solution of II is injected between two ITO-coated electrodes (with n-tetrabutylammonium perchlorate as an inert electrolyte) and a voltage of 2.2 V applied, the initially colourless neutral compound forms a brilliant bluish-red radical with l max of 530 nm. Changes to the substituent causes a blue shift in the colour of the radical. II N N 12.1 Monomeric electrochromes 375 Several aromatic amines show electrochromic activity in the near infrared (NIR). 13 Table 12.1 contains data for several such species, as prepared and studied by the US Gentex Corporation. In each case, the amine was the secondary electrochrome, and an aryl-substituted viologen was the primary. All amines colour anodically and are envisaged for use within solution-phase (type-I) devices. Some of the changes in optical transmission are marked. For example, most compounds in Table 12.1 show a contrast ratio of 5:1 on coloration. 13 Thus, compound III has a visible transmission of 75% in its clear state and only 9% in its coloured state; compound VIII (‘Crystal violet’) has a NIR transmission of 46% in its clear state and 14% in its coloured state. 12.1.2 Carbazole electrochromes The monomeric, substituted carbazole species X readily undergoes a one- electron oxidation to form a radical-cation salt. In their neutral form, carbazoles are soluble and essentially colourless, whereas films of radical cation generated oxidatively according to Eq. (12.1) form a highly coloured, solid precipitate on the electrode: carbazole ðsolnÞ þ X À ! ½carbazole þ : X À 0 ðsÞ þ e À : colourless strongly coloured (12:1) The carbazoles therefore represent an example of type-II electrochromism. Table 12.2 summarises results obtained by Dubois and co-workers for a few carbazole electrochromes. 14 N R X 12.1.3 Cyanine electrochromes Spiropyrans 15,16,17 such as XI are both electrochromic and photoelectrochromic. The initial product of electroreducing XI is a radical anion, while further reduction yields a ring-opened merocyanine species. In the absence of an electrode, photolysis of XI yields the merocyanine directly. A plot of 376 Miscellaneous organic electrochromes Table 12.1. Aromatic amine electrochromes that modulate NIR radiation. (Table reproduced with permission from Theiste, D., Baumann, K. and Giri, P. ‘Solution phase electrochromic devices with near infrared attenuation’. Proc. Electrochem. Soc., 2003–17, 2003, 199–207, with permission of The Electrochemical Society.) Electrochrome λ max /nm ε/dm 3 mol –1 cm –1 727 1500 N N CH 3 CH 3 III 19 000 912 N O CH 3 N CH 3 H 3 C V 15 000 1694 N N N CH 3 CH 3 CH 3 CH 3 H 3 C H 2 C IV 1198 38 500 S S N N CH 3 CH 3 CH 3 CH 3 VI 12.1 Monomeric electrochromes 377 absorbance Abs against Q for species XI is essentially linear, and allows ¼21 cm 2 C À1 to be calculated. 17 N O NO 2 CH 3 H 3 C CH 3 XI Table 12.1. (cont.) 954 47 000 S O N N VII Electrochrome λ max /nm ε / dm 3 mol –1 cm –1 968 46 000 N N H 3 C H 3 C N N H 3 C CH 3 CH 3 H 3 C VIII 907 N N N N CH 3 CH 3 CH 3 CH 3 IX 378 Miscellaneous organic electrochromes Another recently discovered series of electrochromes are the squarylium dyes, 18 such as XII, which have some structural elements in common with XI. The reduced form of the dyes are blue, while the radical species formed on oxidation are green. N N C H X H 3 C CH 3 O – O – C H H 3 C CH 3 X (X = H,Me or Et) XII 12.1.4 Methoxybiphenyl electrochromes The next class of compounds are the violenes based on a core of polymethoxybiphenyl. The uncharged parent compounds are essentially colourless, while electro-oxidation yields a thin, solid filmof brilliantly coloured radical-cation salt. Methoxybiphenyl compounds have been studied by the groups of Parker 19,20 and Grant, 21 although Parker mentions neither electrochromism nor electrochemical applications. Many of these species should more correctly be called ‘biphenyls’, ‘fluorenes’ or ‘phenanthrenes’ according to the nature of the bridging group (if any) connecting the two aromatic rings. The stability of the radical cation formed by one-electron oxidation of the neutral species is a function of molecular planarity, as demonstrated by the stability series XIII << XIV< XV: compound XIII is forced out of planarity Table 12.2. Colours and electrode potentials of oligomers derived from various carbazole electrochromes in MeCN solution. (Table reproduced from: Desbe`ne- Monvernay, A., Lacaze, P.-C. and Dubois, J.-E. ‘Polaromicrotribometric (PMT) and IR, ESCA, EPRspectroscopic study of colored radical films formed by the electrochemical oxidation of carbazoles, part I: carbazole and N-ethyl, N-phenyl and N-carbazyl derivatives’. J. Electroanal. Chem., 129, 1981, 229–41, with permission of Elsevier Science Ltd.) Monomer Colour of radical cation E Õ /V Carbazole Dark green þ0.9 N-ethylcarbazole Green þ1.3 N-phenylcarbazole ‘Iridescent’ þ1.2 N-carbazylcarbazole Yellow–brown þ1.1 12.1 Monomeric electrochromes 379 by the steric repulsion induced by the two o-methoxy groups, whereas XV, by necessity, is always planar owing to the methylene bridge. H 3 CO OCH 3 OCH 3 H 3 CO XIII H 3 CO OCH 3 XIV H 3 CO OCH 3 XV H 3 CO OCH 3 H 3 CO OCH 3 XVI XVII H 3 CO OCH 3 H 3 CO OCH 3 OCH 3 OCH 3 OCH 3 H 3 CO XVIII OCH 3 H 3 CO OCH 3 H 3 CO XIX As a crude generalisation, 21 fluorenes with a single methoxy group are oxidised irreversibly, but the electrochemistry of compounds with two or more methoxy groups is much more reversible, i.e. monomethoxy species are not truly violenes. Ortho- and meta-methoxy substituents engender lower redox potentials than para groups. 21 The fluorene compounds that appear most suitable for ECD inclusion, that is, those yielding the most stable films of radical-cation salt, are listed in Table 12.3. Other fluorene compounds investigated did not form radical cations of sufficient stability for viable use as electrochromes, or evinced irreversible electrochemistry. For example, Table 12.4 lists some biphenyl compounds of interest, within which compound XVIII aromatises slowly by deprotonating to form 2,7-dimethoxyphenanthrene. 380 Miscellaneous organic electrochromes 12.1.5 Quinone electrochromes Many quinone species are soluble, stable, and only moderately coloured as neutral molecules but on one-electron reduction form brightly coloured, stable, solid films of radical anion on the electrode surface. 22,23,24,25,26,27 For example, the electrochromism of several benzoquinones has been studied such as the ortho (XX) and para (XXI) isomers. The most comprehensive study of (XXI) involved the electrochrome dissolved in a solution of propylene carbonate containing LiClO 4 as supporting electrolyte. 24 O O XX O O XXI O O Cl Cl Cl Cl XXII Table 12.4. Colours, CV peak potentials and spectral properties for methoxybiphenyl species forming only a soluble radical cation on reduction in dichloromethane–TFA (5:1 v:v) solution. (Table reproduced from Ronlań, A., Coleman, J., Hammerich, O. and Parker, V. D. ‘Anodic oxidation of methoxybiphenyls: effect of the biphenyl linkage on aromatic cation radical and dication stability’. J. Am. Chem. Soc., 96, 1974, 845–9, with permission of The American Chemical Society.) Compound Colour of radical E pa(1) /V E pc(1) /V l max /nm "/dm 3 mol À1 cm À1 XIV – þ1.28 þ1.22 417 29 512 XVIII Green þ1.14 þ1.07 386 20 420 XIX Green þ0.94 þ0.88 Table 12.3. Colours, CV peak potentials, and spectral properties for methoxybiphenyl species forming a solid radical-cation filmon reduction in MeCN solutions. (Table reproduced from Grant, B., Clecak, N. J. and Oxsen, M. ‘Study of the electrochromism of methoxyfluorene compounds’. J. Org. Chem., 45, 1980, 702–5, with permission of The American Chemical Society.) Compound Colour of radical E pa /V E pc /V l max /nm "/dm 3 mol À1 cm À1 XV Blue þ0.91 þ0.79 411 40 400 XVI – þ0.96 þ0.84 385 32 800 XVII Blue þ0.87 þ0.81 415 44 300 12.1 Monomeric electrochromes 381 The quinone to have received the most attention is probably p-2,3,5,6- tetrachlorobenzoquinone (‘p-chloranil’ XXII), 22 which forms a pink radical cation; see Eq. (12.2): M þ þ ðXXIIÞ 0 ðsolnÞ þ e À ! ½M þ ðXXIIÞ À ðsÞ; (12:2) where the alkali or alkaline-earth cation M is needed to co-deposit with the radical anion when forming an insoluble salt. Desbe` ne-Monvernay et al. 27 say the best results are obtained if the cation forms a ‘visible light-forming charge transfer complex between [the] o-chloranil *À and the counter ion M þ .’ This is doubtful for they also say the best results are obtained when M¼Na, but as the sodium cation does not undergo colour-forming charge-transfer interactions, merely undergoing co-deposition with the quinone radical cation, the source of the quinone radical-cation colour is best conceived as an internal charge-transfer transition modified by M þ . The colour of the radical cation depends on the substituents around the quinone: the tetrafluoro analogue of (XXII) ‘fluoranil’ forms a yellow radical anion, and the radical anion of p-2,3-dicyano-5,6-dichloroquinone is pink. Table 12.5 lists a fewsample quinone species together with electrochemical and optical data. Figure 12.1 shows the absorbance spectrumof a filmof p-chloranil radical anion on ITO polarised to À0.6 V vs. SCE. In CH 3 CNsolution, only p-benzoquinone, o-chloranil (XXII) and o-bromoanil form films that are both stable and adherent. 22 Desbe` ne-Monvernay and 300 400 500 Wavelength/nm Abs o r ba n c e A b s Figure 12.1. Spectrum of a thin, solid film of p-chloranil (XXII) as the radical anion salt on an ITO electrode polarised to À0.6 V. The spectrum baseline was that of the uncharged, colourless p-chloranil prior to charge passage. (Figure reproduced from Desbe` ne-Monvernay, A., Lacaze, P. C. and Cherigui, A. ‘UV-visible spectroelectrochemical study of some para- and ortho- benzoquinoid compounds: comparative evaluation of their electrochromic properties’. J. Electroanal. Chem., 260, 1989, 75–90, by permission of Elsevier Science.) 382 Miscellaneous organic electrochromes co-workers 22 say that o-bromanil forms a superior radical-cation filmto any of the other para-substituted quinones, from its low solubility product and good adherence. In general, electrochromes based on ortho quinones are superior to the para analogues: they are more electrochemically stable, 22 and the solubility constants K s are lower. The values of K s for the para isomers are generally too high, sometimes allowing soluble radical cation to diffuse back into the solution bulk, which therefore represents type-I response rather than the perhaps more desirable type-II electrochromism. 22 The quinone evincing the highest electrochemical stability is o-chloranil (the ortho analogue of XXII). Its electrochromic properties are ‘outstanding’, 22,28 with a cycle life exceeding 10 5 write–erase cycles. 22 While the electrochromism of most quinones requires the formation of radical species, i.e. a transition from pale to intense colour, a recent example Table 12.5. Quinone systems: film-forming properties, colours, wavelength maxima, and reduction potentials. Values of E pc were obtained from CVs, or standard electrode potentials E F ; all solutions in MeCN with tetraethylammonium perchlorate (0.1 mol dm À3 ). Quinone (R–Q) Solid film? Colour of R–Q À* l max /nm E pc(1) /V E pc(2) /V Ref. o-3,4,5,6-tetrachlorobenzoquinone Yes Intense blue À0.170 þ0.210 22,23 o-3,4,5,6-tetrabromobenzoquinone Yes Blue À0.190 þ0.140 22 p-benzoquinone Yes Light blue À0.720 À0.430 22,24 p-2,3,5,6-tetrafluorobenzoquinone No Yellow À0.430 À0.100 22 p-2,3,5,6-tetrachlorobenzoquinone No Yellow À0.420 À0.060 22 p-2,3-dicyano-5,6- dichlorobenzoquinone No Pink þ0.070 þ0.330 22 5-aminonaphthoquinone Yes Purple– blue 410 E F ð1Þ ¼ À0:83 25 1-aminoanthraquinone Yes – – E F ð1Þ ¼ À1:03 25 2-aminoanthraquinone Yes – – E F ð1Þ ¼ À0:99 25 1,5-diaminoanthraquinone Yes Purple 570 E F ð1Þ ¼ À1:10 25 12.1 Monomeric electrochromes 383 operates differently: the red quinone species 1-amino-4-bromoanthraquinone- 2-sulfonate may be electroreduced in aqueous solution to form a colourless dihydroxy compound, rather than a coloured quinone radical, 29 cf. the so-called ‘quinhydrone electrode’, a 1:1 compound of p-benzoquinone XX and dihydroxybenzene (both depicted in Eq. (12.3)). O O OH OH 2e − + 2 H + (12.3) Molecular naphthaquinone and anthraquinone species are also type-I electrochromes. Exemplar species include 1,4-naphthaquinone (XXIII) and anthra-9,10-quinone (XXIV). Aminoanthraquinones show a more complicated electrochemical behaviour than the naphthaquinone: at moderate potentials, two redox couples are exhibited during cyclic voltammetry, representing first Eq. (12.4): quinone 0 þ e À ÀÀÀ!quinone À ; (12:4) followed at more negative potentials by a second reduction reaction, Eq. (12.5): quinone À þ e À ÀÀÀ!quinone 2À : (12:5) In addition to this behaviour, polymerisation of the amine moiety occurs when the electrode is made very positive, cf. the formation of poly(aniline) in Section 10.4. O O XXIII O O XXIV More advanced again is a trichromic ECD 30 with the capacity to form the colours red, green and green-blue, which has been developed using 2-ethylanthraquinone in PC together with 4,4 0 -bis(dimethylamino)diphenylamine. The electrolyte is gelled with a white ‘filler’ to enhance the contrast ratio. In this way, the anthraquinone compound produces the red colour when reduced (CR¼2:1 at l max ¼545 nm), while the other colours derive from the diphenylamine, which yields two different oxidation states: its first oxidation product is a green radical cation (CR¼2:1) and a subsequent oxidation product is a 384 Miscellaneous organic electrochromes green-blue dication (CR¼3.5:1 at l max %500 nm). Because the electrochromes are not encapsulated in separate pixels, the various redox states formed will diffuse back into the solution bulk and undergo radical-annihilation reactions. Furthermore, being violenes, it is also likely that the 2þ and 0 redox states will undergo comproportionation thus: (2þ) þ(0) ! 2(þ * ). 12.1.6 Thiazine electrochromes Thiazine compounds contain a heterocyclic ring comprising both nitrogen and sulfur moieties. Methylene Blue (XXV), the common dye and biological stain, is the archetypal thiazine. The Greek descriptor Leucos (‘white’) is used in organic chemistry and in the dyestuffs industry to describe the colourless form of a redox dye, so XXV is blue when oxidised and colourless following reduction to form the neutral radical, so called leuco-Methylene Blue. S N N N CH 3 CH 3 CH 3 H 3 C + XXV The thiazine XXV is soluble in a wide range of solvents, but has been occasionally considered for ECD usage when immobilised in a semi-solid polymer matrix, as described in Section 12.3. The world’s best-selling electrochromic device is undoubtedly the Night Vision System (NVS # ) produced by the US Gentex Corporation, 31,32 a self- darkening rear-view mirror that is a standard feature in many millions of expensive high-performance cars. 33 The Gentex device comprises two electrochromes, a viologen species (see Chapter 11) and a phenothiazine. 31,34,35 In MeCN solution, thiazines such as XXV are used. At heart, each NVS # mirror incorporates a front electrode of ITO-coated glass and a metallic rear electrode having a highly reflective surface. These two parallel electrodes separated by a sub-millimetre gap form the basis of the cell. (In a similar device containing heptyl viologen and tetramethylphenylenediamine in PC, the cell would only function when the gap was narrower than 0.28 mm. 12 ) The dual-electrochrome solution is injected into the cavity between the electrodes. The exact composition of the Gentex NVS # mirror is obscured within densely worded patents, but it is possible to infer some details of the operation: a substituted viologen species ‘bipm’ (see Section 11.3), undoubtedly cationic as ‘bipm 2þ ’, serves as the cathodic electrochrome. When the mirror is switched on, mass transport occurs as the positive charge of the uncoloured precursor 12.1 Monomeric electrochromes 385 propels it toward the cathode in response to ohmic migration (the electrolyte in the Gentex mirror is free of additional swamping electrolyte). Reductive coloration then occurs at the cathode, Eq. (12.6): bipm 2þ ðsolnÞ ! bipm þ ðsolnÞ: (12:6) The other electrochrome (which is initially in its reduced form) is probably a molecular thiazine ‘TA’ (or perhaps a phenylenediamine species, see p. 375 above). The TA is uncharged and depletion by oxidation at the anode ensures that mass transport of TA ensues by diffusion alone. Oxidation of TA evokes colour, Eq. (12.7): TA ðsolnÞ ! TA þ ðsolnÞ þ e À : (12:7) In operation, the colour in a commercially-available NVS # mirror is an intense blue–green. The colour-forming reduction process, bipm 2þ þ e À ! bipm þ* , and the complementary oxidation reaction, TA!TA þ* þe À occur in dual electro-coloration processes in tandem. The coloured species diffuse away from the respective electrodes and meet in the intervening solution where their mutual reaction (‘radical annihilation’) ensues, Eq. (12.8) TA þ ðsolnÞ þ bipm þ ðsolnÞ ! TA ðsolnÞ þ bipm 2þ ðsolnÞ; (12:8) that regenerates the original uncoloured species. These reactions are depicted schematically in Figure 12.2. Reflective back electrode Transparent front electrode Positive Negative bipm +. bipm 2+ −e − TA +. TA 0 −e − +e − +e − Figure 12.2. Schematic representation of the redox cycles occurring within the Gentex Night Vision System # . Coloration occurs electrochemically at both electrodes; bleaching occurs chemically at the centre of the cell by radical annihilation. 386 Miscellaneous organic electrochromes The radical annihilation in Eq. (12.7) represents a divergence from one of the benefits of electrochromism since the ‘memory effect’ is lost. Thus, maintenance of coloration requires the passage of a continuous (albeit minute) current to replenish the coloured electrochromes lost by the annihilation. Reaction (12.7) obviates any need to electro-bleach the Gentex NVS # mirror, since colour fades spontaneously on switch-off. For this reason, the Gentex NVS # is sometimes termed the self-erasing mirror. United States law requires the ‘failure mode’, on loss of current, to be the clear condition, with which these mirrors comply. 12.1.7 Miscellaneous monomeric electrochromes Atrichromic ECDhas been fabricated including 2,4,5,7-tetranitro-9-fluorenone (XXVI) as the red-forming material, 2,4,7-trinitro-9-fluorenylidene malononitrile (XXVII) as the green and tetracyanoquinodimethane (TCNQ) as the blue electrochrome. 36 O O 2 N NO 2 NO 2 NO 2 XXVI O 2 N NO 2 NO 2 CN NC XXVII Finally, a Japanese group has prepared a TCNQ derivative and studied its spectra as a function of applied potential. 37 While their study was not concerned with electrochromic activity, their results may facilitate the preparation of new electrochromes. 12.2 Tethered electrochromic species 12.2.1 Pyrazoline electrochromes Atethered organic systemthat has received some attention is that based on the oxidation of the pyrazolines XXVIII and XXIX, spectral details for which are listed in Table 12.6. Kaufman et al. 38,39 have published most of the current work on tethered pyrazolines. Such species are more intensely absorbing than the tetrathiafulvalene (TTF) species below, and have faster response times . 39 Pure pyrazoline monomers are readily prepared, and are soluble in many solvents prior to polymerisation. 39 12.2 Tethered electrochromic species 387 A solid-state ECD which incorporates such polymeric pyrazolines has been constructed, 40 and has a response time of 10 ms and a CR of 10:1. N N OCH 3 O z OCH 3 N O XXVIII Z = XXIX Z = n 12.2.2 Tetracyanoquinodimethane (TCNQ) electrochromes Neutrally charged TCNQ is a stable, colourless molecule that forms a blue- green coloured radical anion following one-electron reduction. 36,37 The stability of the tetracyanoquinonedimethanide radical is ascribed to appreciable delocalisation of the single negative charge over the four CN groups. Since TCNQ and its radical anion are both soluble in most common solvents, Chambers et al. 41,42,43 have improved the electrochromic write–erase efficiency by chemically tethering the TCNQ species XXX to an electrode surface by means of polymerisation. The oligomer XXX is estimated 41 to have a molecular weight of about 2200 g mol À1 , i.e. a chain comprising an average chain length of 6.3 electrochrome units. Table 12.6. Half-wave potentials E 1/2 , colours, and response times t for tethered pyrazoline species bound covalently to an electrode substrate, immersed in MeCN solution containing TEAP electrolyte (0.1 mol dm À3 ). (Table reproduced from Kaufman, F. B. and Engler, E. M. Solid-state spectroelectrochemistry of crosslinked donor bound polymer films. J. Am. Chem. Soc., 101, 1979, 547–9, with permission of The American Chemical Society.) Compound E ½ /V Colour change l max /nm /ms XXVII þ0.55 Yellow-to-green 510 50 XXVIII þ0.45 Yellow-to-red 554 100 388 Miscellaneous organic electrochromes O O O O CN NC NC CN XXX O O n Electrodes modified with XXX are electrochemically reversible. 41 Spectroscopic data for TCNQ and TCNQ À* are listed in Table 12.7. In solution, additional species to those in Table 12.7 have also been identified, including a dianion (TCNQ) 2À , and (in aqueous solution only) a dianion (TCNQ) 2 2À dimer. 42 12.2.3 Tetrathiafulvalene (TTF) electrochromes Like TCNQ, TTF has been used in ECDs chemically tethered to an electrode surface. In this way Kaufman and co-workers 44,45 used the two species XXXI and XXXII to modify electrodes. In early trials, a TTF device underwent >10 4 cycles without visible deterioration. 44 The electrochromic TTF colouration accompanies oxidation of neutral TTF to form a radical cation. Spectral characteristics of XXXI and XXXII are listed in Table 12.8. Table 12.7. Spectroscopic data for a modified electrode bearing a thin film of the TCNQ-based polymer XXX, immersed in MeCN solution. (Reproduced from Inzelt, G., Day, R. W., Kinstle, J. F. and Chambers, J. Q. ‘Spectroelectrochemistry of tetracyanoquinodimethane modified electrodes’. J. Electroanal. Chem., 161, 1984, 147–61 with permission of Elsevier Science.) Species l max /nm ln("/dm 3 mol À1 cm À1 ) TCNQ 0 408 5.06 430 5.06 TCNQ À* 445 4.30 660 3.38 728 3.92 812 4.20 12.2 Tethered electrochromic species 389 X S S S S O O C O n XXXI X= XXXII X= Electrochemical studies show the rate-determining step during coloration is ion movement into and through the film; 39,46 furthermore, electron transport through the film proceeds via hopping or tunnelling between TTF sites. In addition to TTF þ* , the other TTF species listed in Table 12.9 will also form in the layer around the electrode; their spectral characteristics are reproduced in Table 12.9. Although the minor species in Table 12.9 do not contribute much to the colouration of a TTF device, they greatly complicate any electrochemical interpretation. Recent TTFdisplays comprise solid-state devices with polymeric electrolytes. 40 Table 12.9. Spectroscopic data for TTF redox species in MeCN solution. (Data reproduced from Kaufman, F. B. ‘New organic materials for use as transducers in electrochromic display devices’, Conference Record of the IEEE, Biennial Display Research Conference, 1978, New York, p. 23–5, with permission of The IEEE.) Species l max /nm TTF þ* 393, 653 (TTF þ* ) 2 1800 (TTF) þ* 2 820 TTF 2 þ 533 Table 12.8. Half-wave potentials E 1/2 , colours, wavelength maxima and response times t for tethered TTF species.(Data reproduced from Kaufman, F. B., Schroeder, A. H., Engler, E. M. and Patel, V. V. ‘Polymer-modified electrodes: a newclass of electrochromic materials’. Appl. Phys. Lett., 36, 1980, 422–5, with permission of American Institute of Physics.) Compound E ½ /V Colour change l max /nm /ms a XXXI þ0.45 Orange-to-brown 515 200 XXXII þ0.35 Yellow-to-green 650 150 a Time required for a charge injection of 1 mC cm À2 into a film of thickness 5 mm. 390 Miscellaneous organic electrochromes 12.3 Electrochromes immobilised within viscous solvents The write–erase efficiency can be enhanced by dissolving or dispersing an electrochrome in a semi-solid electrolyte of high viscosity. Such immobilised species are essentially type-III electrochromes. The usual matrix for entrapment is an electrolyte gel of high viscosity, 47 such as the polyelectrolytes or polymeric electrolytes described in Chapter 14. In this context, the host polymers of choice are semi-solid poly(AMPS), 48 poly(aniline), 49 and poly(1-vinyl- 2-pyrrolidinone-co-N,N 0 -methylenebisacrylamide) PVPD. 50 Table 12.10 lists a few electrochromes which have been immobilised in this way. Clearly, only a small proportion of the electrochrome dispersed in viscous electrolyte will ever be juxtaposed with the electrode, or can reach the electrode within a tolerable time lag. For this reason, the majority of the electrochrome must be considered to be ‘passive’, with most remaining in its colourless form. Methylene Blue (XXV) is thus unpromising as an electrochrome as its colourless leuco formreverts back to the coloured formquite rapidly, especially when exposed to oxygen. In the studies by Tsutseumi et al., 50,51 the electrochromes dispersed in PVPD were all ester-based. In each case, the colour formed after the potential had been applied for a few seconds, but a rapid self-bleaching process occurred under open circuit. Such gel films therefore lack any optical memory effect. Carbazoles (cf. Section 12.1 above) have similarly been immobilised in a ‘matrix’ of poly(siloxane) to yield viable ECDs. 52,53,54,55,56,57 References 1. Kodama, K. 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J., Gentex Corporation. Single-compartment, self-erasing, solution- phase electrochromic devices, solutions for use therein and uses thereof. US Patent No. 4,902,108, 1990. 32. [online] at www.gentex.com/auto_how_nvs_work.html (accessed 6 September 2005). 33. [online] at http://uk.cars.yahoo.com/010419/4/749k.html (accessed 25 July 2003). 34. Byker, H. J. Electrochromics and polymers. Electrochim. Acta, 46, 2001, 2015–22. 35. Byker, H. J. Commercial developments in electrochromics. Proc. Electrochem. Soc., 94–2, 1994, 1–13. 36. Yasuda, A. and Seto, J. Electrochromic properties of vacuum-evaporated organic thin films, part 4: the case of 2,4,7-trinitro-9-fluorenylidene malononitrile. J. Electroanal. Chem., 303, 1991, 161–9. 37. Higuchi, H., Ichioka, K., Kawai, H., Fujiwara, K., Ohkita, M., Tsuji, T. and Suzuki, T. Three-way-output response system by electric potential: UV-vis, CD, and fluorescence spectral changes upon electrolysis of the chiral ester of tetracyanoanthraquinodimethane. Tetrahedron Lett., 45, 2004, 3027–30. 38. Kaufman, F. B. and Engler, E. M. Solid-state spectroelectrochemistry of crosslinked donor bound polymer films. J. Am. Chem. Soc., 101, 1979, 547–9. 39. Kaufman, F. B., Schroeder, A. H., Engler, E. M. and Patel, V. V. Polymer- modified electrodes: a newclass of electrochromic materials. Appl. Phys. Lett., 36, 1980, 422–5. 40. Hirai, Y. and Tani, C. Electrochromism for organic materials in polymeric all- solid-state systems. Appl. Phys. Lett, 43, 1983, 704–5. 41. Day, R. W., Inzelt, G., Kinstle, J. F. and Chambers, J. Q. Tetracyanoquinodimethane-modified electrodes. J. Am. Chem. Soc., 104, 1982, 6804–5. 42. Inzelt, G., Day, R. W., Kinstle, J. F. and Chambers, J. Q. Spectroelectrochemistry of tetracyanoquinodimethane modified electrodes. J. Electroanal. Chem., 161, 1984, 147–61. References 393 43. Inzelt, G., Day, R. W., Kinstle, J. F. and Chambers, J. Q. Electrochemistry and electron spin resonance of tetracyanoquinodimethane modified electrodes: evidence for mixed-valence radical anions in the reduction process. J. Phys. Chem., 87, 1983, 4592–8. 44. Kaufman, F. B. New organic materials for use as transducers in electrochromic display devices. Conference Record of the IEEE, Biennial Display Research Conference, New York, 1978, p. 23–5. 45. Torrance, J. B., Scott, B. A., Welber, B., Kaufman, F. B. and Seiden, P. E. Optical properties of the radical cation tetrathiafulvalenium (TTF þ ) in its mixed-valence and monovalence halide salts. Phys. Rev., B19, 1979, 730–41. 46. Kaufman, F. B., Schroeder, A. H., Engler, E. M., Kramer, S. R. and Chambers, J. Q. Ion and electron transport in stable, electroactive tetrathiafulvalene polymer coated electrodes. J. Am. Chem. Soc., 102, 1980, 483–8. 47. Tsutsumi, H., Nakagawa, Y., Miyazaki, K., Morita, M. and Matsuda, Y. Polymer gel films with simple organic electrochromics for single-film electrochromic devices. J. Polym. Chem., 30, 1992, 1725–9. 48. Calvert, J. M., Manuccia, T. J. and Nowak, R. J. A polymeric solid-state electrochromic cell. J. Electrochem. Soc., 133, 1986, 951–3. 49. Kuwabata, S., Mitsui, K. and Yoneyama, H. Preparation of polyaniline films doped with methylene blue-bound Nafion and the electrochromic properties of the resulting films. J. Electroanal. Chem., 281, 1990, 97–107. 50. Tsutsumi, H., Nakagawa, Y. and Tamura, K. Single-film electrochromic devices with polymer gel films containing aromatic electrochromics. Sol. Energy Mater. Sol. Cells, 39, 1995, 341–8. 51. Tsutsumi, H., Nakagawa, Y., Miyazaki, K., Morita, M. and Matsuda, Y. Single polymer gel film electrochromic device. Electrochim. Acta, 37, 1992, 369–70. 52. Goldie, D. M., Hepburn, A. R., Maud, J. M. and Marshall, J. M. Carrier mobility studies of carbazole modified polysiloxanes. Mol. Cryst. Liq. 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Met., 43, 1991, 2935–8. 394 Miscellaneous organic electrochromes 13 Applications of electrochromic devices 13.1 Introduction While the applications of electrochromism are ever growing, all devices utilising electrochromic colour modulation fall within two broad, overlapping categories according to the mode of operation: electrochromic devices (ECDs) operating by transmission (see schematic in Figure 13.1) or by reflection (see the schematic representation in Figure 13.2). Several thousand patents have been filed to describe various electrochromic species and devices deemed worthy of commercial exploitation, so the field is vast. Much duplication is certain in such patents, but it is clear how large scale are the investments directed toward implementing electrochromism as viable in displays or light modulation. In this field, vital details of compositions are often well hidden, as these comprise the valued intellectual property rights on which substantial financial considerations rest. The most common applications are electrochromic mirrors and windows, as below. These and other applications are reviewed at length by Lampert 1 (1998), who cites all the principal manufacturers of electrochromic goods worldwide, and also several novel applications. 13.2 Reflective electrochromic devices: electrochromic car mirrors Mirrors, which obviously operate in a reflectance mode, illustrate the first application of electrochromism (cf. Figure 13.2). Self-darkening electrochromic mirrors, for automotive use at night, disallow the lights of following vehicles to dazzle by reflection from the driver’s or the door mirror. Here an optically absorbing electrochromic colour is evoked over the reflecting surface, reducing reflection intensity and thereby alleviating driver discomfort. However, total opacity is to be avoided as muted reflection must persist in the darkened state. The back electrode is a reflective material 395 allowing customary mirror reflection in the bleached state. Reference 2 has some nice graphics that illustrate the necessary components. The best-selling electrochromic mirror is the Gentex Night-Vision System 3,4 (NVS # ), of which many millions have been sold, probably 90 to 95% of all self-darkening mirror sales. 5 Its operation employing type-I electrochromes is described in detail in Chapter 12, Section 12.1 under ‘Thiazine electrochromes’, and the mechanism is illustrated in Figure 12.2 (None of the accounts available reveal the dramatic events at the onset of Gentex’s first big auto contract, when a small adventurous inventive company that had some impressive demo devices had suddenly to tool up for mass production. The Gentex Corporation we refer to here are based in Zeeland, Michigan, and are not to be confused with an identically named but independent firm in Pennsylvania that supplies amongst other things protective clothing, fireproofing and the like for aeronauts and astronauts.) ECD Incident beam Emergent beam Figure 13.1 Schematic diagram of an ECD operating in transmittance mode. Both the front and back electrodes are optically transparent. The respective widths of the arrows indicate the relative magnitudes of the light intensities. ECD Incident beam Emergent beam Reflective surface Figure 13.2 Schematic diagram of an ECD operating in reflectance mode. The front electrode is optically transparent and the back electrode is made of polished platinum or platinum-based alloy. The respective widths of the arrows indicate the relative magnitudes of the light intensities. 396 Applications of electrochromic devices An example of all-solid-state mirror is the SchottDonnelly 6 solid polymer matrix (SPM TM ) mirror for lorries and trucks, which relies on WO 3 and NiO, and is thus a type-III system. A different solid-state electrochromic mirror is based on WO 3 . 7,8,9,10,11 Electrochromic mirrors are fitted on luxury cars made by, among others, Audi, Bentley, BMW, Daewoo, DaimlerChrysler, Fiat, Ford, General Motors, Hyundai, Infiniti, Kia Motors, Lexus, Mitsubishi, Nissan, Opel, Porsche, Rolls Royce and Toyota. 12 The likely thermal and other stresses resulting from mounting ECDs in or on cars require particularly stringent tests of ECD design and fabrication. The durability of ECDs is discussed in Chapter 16. Gesheva et al. 13 have developed an electrochromic mirror that is, apparently, reflective to X-rays. Films of WO 3 , MoO 3 or mixed W–Mo oxide, were deposited on wafers of silica by plasma-enhanced CVD from a metal-carbonyl precursor. Electrochromic modulation changes the X-reflectivity of the underlying silica. 13.3 Transmissive ECD windows for buildings and aircraft 13.3.1 Buildings Svensson and Granqvist coined the term ‘smart window’ in 1985 to describe windows that electrochromically change in transmittance. 14 The termhas since been augmented with ‘smart windows’ and ‘self-darkening windows’ to describe novel fenestrative applications. The British Fenestration Rating Council describes electrochromic windows as ‘Chromogenic glazing’. 15 However, the terms ‘electrochromic window’ and ‘smart glass’ are now widespread and attract attention, particularly in popular-science articles. 16 The construction of electrochromic windows has often been reviewed, for example, ‘Toward the smart window: progress in electrochromics’ by Granqvist et al. (in 1999), 17 ‘Electrochromic windows: an overview’ by Rauh (also in 1999), 18 ‘Windows’ by Bell et al. (2002), 19 and ‘Electrochromic smart windows: energy efficiency’ by Azens and Granqvist (in 2003). 20 Smart windows for automotive usage have not been reviewed so often: one of the few reviews to mention this application explicitly is ‘Angular selective window coatings: theory and experiments’ by Granqvist et al. 21 in 1997. Although dated (1991), ‘A review on electrochromic devices for automotive glazing’ by Demiryont 22 is still relevant. The appeal of smart windows is both economic and environmental: if successful, they preclude much solar radiation from a room or a car. The 13.3 Transmissive electrochromic devices 397 exact cost of air conditioning in summer is unknown, but is surely greater than losses through windows in winter, which in 2003 cost $25 billion in the USA alone. 23 Smart windows might thus both improve working environments and alleviate costs. Nevertheless, Lee and DiBartolomeo 24 suggest that electrochromic windows ‘may not be able to fulfil both energy-efficiency and visual comfort objectives when low winter direct sun is present’. The rush to develop smart windows is also a response to pressure from environmental campaigners of the ‘Green’ lobby. 25,26 Many ‘green’ considerations are assessed by Griffiths et al. 27 and Syrrakou et al. 28 Architectural applications are at present the subject of intense research activity: the web page from the National Renewable Energy Laboratories (NREL) in ref. 29 aims to cite all the present-day producers of electrochromic windows; the number of manufacturers appears to be expanding rapidly. However, many of these products are poorly described in the associated publicity, so the identities of the electrochromes are unclear. This is scarcely informative, or a boost for electrochromic applications. For example, from their the web site SAGE Electrochromics Inc. 30 of Minnesota clearly produce two products, one of which is said to be ‘organic’ and the other ‘inorganic’, without identifying either electrochrome. Many websites show video clips of electrochromic windows: the short sequences available in ref. 31 show dramatic colour changes of organic films of PEDOT-based polymers (see Section 10.2). Reference 32 contains a short video clip of an electrochromic window measuring 3 ft Â6 ft, made by Research Frontiers Inc. (though it is not clear whether this is an ECD or an SPD 33 ), and ref. 34 contains several longer .mpeg clips of varying clarity. Colour Plate 4 shows a window made by Gentex. In the smart-window application, individual panes of glass or whole windows can be coloured electrochromically to darken sunlight intensity in rooms or offices. Similar electrochromic applications are planned for car sunroofs, 22,35 and the motorcycle helmets and ski goggles developed by the Granqvist group in Sweden; 36,37,38 see Colour Plate 5. Recently, Zinzi 39 published a full study describing the preferences of office workers, as follows. He made a full mock-up room, illuminated internally with conventional fluorescent and incandescent bulbs, and externally by solar radiation that entered the room through electrochromic windows. The results were interesting and not always as expected. Most workers preferred to control the external lighting via electrochromic windows rather than blinds or other mechanical forms of shutter. When the transparency of the windows was changed automatically, via a photocell connected to a microprocessor, the alteration of the visual environment was sufficiently smooth and slow that few workers actually 398 Applications of electrochromic devices perceived the changes; those who did were not unhappy with its effects. Nevertheless, most preferred a manually operated transparency control, presumably to ‘personalise’ their own working space. Some workers wanted electrochromic windows to adjust more rapidly, to accommodate fluctuating ambient illumination. Many manufacturers of electrochromic windows prefer so-called ‘neutral’ colours, i.e. shades of grey, to the richer blue colour of (for example) H x WO 3 alone. Office workers are said to favour such grey hues, because other colours can induce nausea. 40 Thus there is now a considerable research effort to optimise the hue for the working environment, with many researchers seeking to effect subtle changes in optical bands, for example by mixing various metal oxides in precise relative amounts. In this regard, a promising electrochrome is a mixture of vanadium and tungsten oxides, which evinces an electrochromic colour that is more grey than that of either constituent oxide alone, because the absorption spectrum comprises several broad and overlapping optical bands. 41,42,43 The quest for ‘neutral colour’ is presented in refs. 42,43,44,45, 46,47,48, while mixtures of metal oxide are considered in greater detail in Section 6.5. Electrochromes commonly show colour in the visible spectrum. While most WO 3 -based ECDs develop a band peaking in the near infrared (NIR), it is sufficiently broad for much visible light also to be absorbed. Some smart windows, however, develop a band almost wholly in the NIR. While not altering the perceived colour of an electrochromic window, such electrochromic windows do further regulate the transmission of the thermal components of sunlight. This desirable property is found also in two unexpected electrochromes, namely the fullerene 49 Li x C 60 , the reduced form having a band maximum at 1060–80 nm, and electrodeposited diamond, 50 which is yellow but becomes brown following reduction due to a band with a maximum in the UV (see Section 9.2). While light transmittances may be modulated between, say, 85% when bleached, to 15% when coloured, complete blocking of sunlight would need the dissipation of much absorbed heat, unless the solar radiation could be reflected metallically by the electrochrome, requiring a material with metallic, specular, reflectivity. Some few electrochromes show specular reflectivity, the most remarkable being yttrium hydride, which can be cycled between highly transparent and mirror-like conditions (see Section 9.4). Other electrochromes indicating specular reflectance are the inorganic systems copper oxide, 51 iridium oxide, 52 lithium pnictide, 51 tungsten oxyfluoride 53 and tungsten trioxide; 54 and organic systems such as poly(pyrrole) composites, 55 poly(diphenylamine) 56 and PEDOT. 56 13.3 Transmissive electrochromic devices 399 The reflectance of iridium oxide in ref. 52 arose from a thin layer of electrochromic IrO 2 deposited on opaque Ir metal, so the reflectance may be that of the metallic under-layer. Other systems in which a metallic layer is electrodeposited are outlined in Section 9.3. While in theory there is no absolute upper limit to the contrast ratio CR in ordinary electrochromism, in practice the values are never particularly high. Thus, unusually large values of CR are assumed to indicate reflective effects, and a sputtered film of WO 3 , with a reported 57 CR of 1000:1, probably comprises WO 3 particles that reflect some of the incident light. Schott Glass has shown demonstration models of their ‘Ucolite’ roomillumination system at Schott Glass Singapore (June 2000). The 40 or 50 cm diameter circular electrochromic window was no doubt WO 3 -based, but darker, so possibly comprising a nickel hydroxy-oxide counter electrode. Designed to be fitted into the ceiling of an interior room, sunlight was to be funnelled to it down a tube from the outer roof, with a clear glass roof-lid, the intensity to be electrochromically controlled from within. The inside placing would protect fromsolar photodegradation, but it could be of use only for topfloor or single-storey illumination. Furthermore it would have found use only in tropical or equatorial sunshine intensities as a light source, when other windows could be permanently darkened against solar heating. Apart from the undisclosed cost of the window itself, the funnel-tube and roof-installation expense has probably vitiated any commercial appeal. The Stadtsparkasse Bank in Dresden however has operating electrochromic external windows supplied by Flabeg Gmbh, who acquired Pilkington Glass’s ECD technology based on WO 3 with FTO substrates. 58 Asahi Glass in Japan have an electrochromic window of small panes (ca. 30 Â30 cm), also WO 3 , which they claim to have been operating in a building for some years. The stage is poised for a wider use of ECD windows in buildings, but the period between ‘possible’ (it works) and ‘commercial’ (it will pay its way) can be appreciable. 13.3.2 Aircraft – the first ubiquitous ECD window application: Gentex and Boeing In December 2005 Gentex Corporation, PPG Aerospace and Boeing signed agreements to install electrochromic windows in the new long-range Boeing (B787) aircraft, the ‘Dreamliner’ (an artistic name). 59,60 The windows are said to be 25% greater in area than the usual. Several hundred of the B787 aircraft, due to operate in 2008, are already on order. The Gentex–PPG systems will allow passengers to set the windows from clear to five increasing 400 Applications of electrochromic devices levels of darkening up to virtual opacity. The electrochromic system employed has not been revealed. The screen is said to be sited between the external cabin windowand the plastic dust shield; whether the outermost glass layer is part of the ECD system is not disclosed. The company PPG Aerospace is an experienced aircraft-window manufacturer and an ideally imaginative collaboration with Gentex has been created, to effect the first mass-produced application of ECDs of appreciable size. Clearly a specialist, niche, application is involved here, but it represents a substantial advance on the only other mass-produced device, the car mirror, that has thoroughly proved its worth on the smaller scale. The costs to Boeing are reported as being $50 million (of which the larger part goes to Gentex). The B787 has 100 windows, for the 221 seats. One can do a little simple arithmetic to arrive at a pricey sumper window, but perhaps only about 10 times that of an ECD car-mirror installation. As at present constructed, an avenue to mass-produced architectural applications is not yet open, but this substantial growth in window production can only lead to advances towards accommodating the requirements of buildings. Airbus are reported to be considering electrochromic windows for the A380, their new aircraft undergoing development, as are no doubt other aircraft manufacturers. 13.3.3 Capital screening: sunglasses and visors The Swedish invention of motorcyclists’ ECD visors is referred to below (see p. 422). Electrochromic sunglasses, also necessarily operating in a transmittance mode (cf. Figure 13.1), have been produced that may be darkened at will, contrasting with the automatic operation of the now widely available photochromic lenses that darken automatically. Nikon were the first to market electrochromic sunglasses in 1981, calling them a ‘variable-opacity lens filter’. 61 Subsequently Nikon marketed WO 3 -based sunglasses in 1993, but these are no longer available. Donnelly have also produced electrochromic sunglasses that apparently operate via a different mechanism. 62 13.4 Electrochromic displays for displaying images and data Electrochromic devices operating as displays can act in either reflectance or transmissive modes, the majority being of the reflectance type. The two reviews in 1986 by Agnihotry et al. covering both physicochemical properties 63 and device technology 64 delineate the historical development of such devices. Additionally, Faughnan and Crandall’s still useful 1980 review 65 13.4 Displaying images and data 401 ‘Electrochromic devices based on WO 3 ’ helps justify the claim that these workers introduced the concept of electrochromic displays. Although dated, the extensive review by Bowonder et al. 66 in 1994 helps establish the place of electrochromism within the wide varieties of display device. Byker’s two reviews of ECDs, ‘Commercial developments in electrochromics’ (in 1994) 5 and ‘Electrochromics and polymers’ (in 2001), 67 provide much detail. Electrochromic devices are often termed ‘passive’ since they do not emit light and hence require external illumination, a possible disadvantage: light- emitting diodes (LEDs) and cathode-ray tubes (CRTs) are emissive, but liquid crystal displays (LCDs) and almost all mechanical displays are also nonemissive. A newer emissive competitor is the ‘plasma’ screen, in televisions and large advertising displays, which comprise individual pixels of three (for tri-colour emission) minute, gas-filled, fluorescent light-emitting units. The construction costs are high, which, however, users seem prepared to bear. The global display market is expanding rapidly. For example, the total global display market was $11.6 billion in 1994 and will top $100 billion in 2007. 66 The market for ‘flat-panel’ information displays was worth approximately $18 billion in 2003, and is growing very fast. Since 1996, LCD devices have formed a larger proportion than CRTs. They now dominate with about 90% of the market share, and are superseding CRTs in applications such as television screens and visual-display units (VDUs) for computers and instruments requiring monitors. Electrochromic devices have been proposed for flat-panel displays for applications such as television and VDU screens (but note possible disqualifications spelt out below), data boards at transport terminuses, advertising boards, 68 and even as an electrochromic ‘indicator’ (based on WO 3 ) on a cash card. 69 The range of applications for flat-panel displays increases rapidly, and are incorporated into a wide array of electronic devices both large and small, from calculators and watches to, perhaps, mobile phones and screens on lap-, palm- or desk-top computers. For example, at the ‘DEMO 2005’ show, NTera of Eire demonstrated an iPod with a Nanochromics TM screen (as below). One commentator thought the new electrochromic screen ‘definitely exceeded the original iPod [screen] in crisp- and brightness’. 70 There are many other nascent applications of ECDs, so when technological barriers are overcome, these materials are likely to play an increasing role in such uses. The first application suggested for ECDs was in watch faces. 71 A modern variant is the face of the so-called Moonwatch; 72 here the face does not tell the time but represents a display with fourteen separate areas, which darken progressively to indicate the phases of the moon. Other specialist ECDs designed for use as watch faces are cited in refs. 73,74,75. 402 Applications of electrochromic devices Liquid crystal displays can be fabricated extremely cheaply, sometimes so cheaply as to be disposable. The main reason for their cheapness is the sheer volume of production worldwide, which decreases the capital costs. Electrochromic devices must compete with LCDs for commercial viability, and therefore possess economic advantages over them. The claimed advantages are as follows: firstly, ECDs consume little power in producing images which, once formed, remain with little or no additional input of power – the so-called ‘memory effect’ outlined onp. 53. Secondly, in principle, there is no limit to the size an ECD can take, so a device may be constructed having a larger electrode expanse or a greater number of small electrodes. Electrochromic devices may be either flat or curved for wide-angle viewing. By contrast, large- area LCDs are expensive, and large CRTs require a huge electron ‘gun’ behind the screen, which is both bulky and prohibitively expensive. Realistically, however, ECDs have insufficiently fast response times to be considered for applications such as television and (most) VDU screens, and cycle lives are probably also somewhat low (see Section 16.1). Indicative response times can be roughly estimated from Eq. (3.16), l %(Dt) ½ , for type-I and type-III electrochromes. Typical distances l to be traversed by a key species in a coloration step are between 10 and 100 nm, say $50 nm intermediately. With D for type-I (solution-phase) species about 10 À7 cm 2 s À1 , a response time of less than a millisecond is obtained, but the type-I coloration in solution will be mobile. For immobile coloration, as obtained with type III, D is typically 10 À12 cm 2 s À1 , giving a response time of $20 s. For televisions and VDUs, the image must be coloured at fixed points, so requiring responses from a type-III system, which our order-of-magnitude arithmetic shows to be slow. Displays of digits and alphanumeric displays could however comprise liquid-containing elements or solids with faster diffusion coefficients D%10 À10 cm 2 s À1 , so responding within a range of a few milliseconds to a second or so. (Note that these estimates, while of illustrative value, ride roughshod over the detail of the mechanisms summarised in Chapter 5. Furthermore, tethered monolayer systems, with l but a few nm – see Section 11.3.1 – could be 10 2 to 10 3 times faster than these ‘guesstimates’.) Accordingly, the most suitable roles envisaged at this stage involve displaying information more slowly, for long-term perusal, e.g. at transport terminuses as mentioned above, for re-useable price labels, or on advertising boards and frozen-food monitors. Toproduce such an image, multiple electrodes – ‘picture elements’ or ‘pixels’ – allow text or images to be displayed rather than mere blocks of colour. The electrochromic ‘3’ shown in Figure 4.1 is achieved with seven relatively large electrodes; the IBM Laboratories made an ECD with a 64 Â64 pixel image on 13.4 Displaying images and data 403 a one inch square silicon chip 76 and the NTera NanoChromics TM display (see further detail in Section 11.4, p. 361) comprises an array of transparent electrodes, each about 0.25 mm square, or about 100 dots per inch. 77 Colour Plate 6 shows a reflective cell with nine pixels. In such multi-pixel ECDs, tonal variation is achieved by stippling with dots as with LCD displays; alternatively, the image may be intensified by passing more charge into specified areas where more of the coloured substance is to be formed. There is however the technical problem with any large-area ECD. Areas of patchy colour may form when the current distribution is uneven across the electrode surface, since the electric field can be larger at the edges of the electrode substrate nearest the metallic leads, if the electrode substrate is semiconductive (like ITO). This allows a potential drop with distance towards the centre of the conducting area. Increasing the viscosity of the electrolyte, and subtle choice of potentials and dimensions, can more-or-less obviate this problem. 67 13.5 ECD light modulators and shutters in message-laser applications In addition to displays and windows, electrochromic systems find a novel application as optical shutters or light modulators where the ECD operates in a transmissive mode (Figure 13.1). It is often the case that in fibre-optic message-laser applications the transmitting front-end puts out too high an intensity for the fibre. This is best remedied by a permanent filter, which could be a once-for-all photochromically evoked colour filter for the particular laser wavelength (the photochemistry of this coloration being effected by a pulse from a laser of different wavelength from that of the message laser). However, at the receiving end a variety of detectors are in use, with an associated variety of sensitivities, not always commensurate with the incoming signal. To match the output laser intensity to the detector sensitivity, an adjustable ECD is inserted in the optical path before the detector, that needs particular circuitry to evoke the most fitting coloration intensity. This task requires that the ECD remains almost constant for any one transmission. As this is a preliminary setting preceding message reception, instant (i.e. nanosecond) responses are not required. As receivers get messages from a number of sources with varying intensities, automatic adjustment preceding reception is desirable; this takes place during the communicationlinking protocol. A patent describes the circuitry detail required for this purpose. 78 However, for operation of fibre-optic message transmissions (or in optical computer action), a response time of sub-nanoseconds is necessary, so no 404 Applications of electrochromic devices redox ECDs are sufficiently fast to act in this particular role as on–off shutters. Possibly for more leisurely optical data storage, pixels need only represent either ‘off’ or ‘on’, as in Figure 13.1 when coloured or bleached respectively, which thus totally interrupts (or not) a light beam, without regard to gradations of intensity. Electrochromic data storage is thus not precluded. 13.6 Electrochromic paper The impetus behind developing electrochromic paper is environmental: electrochromes embedded within a sheet of paper can in principle be switched reversibly between coloured and bleached, thereby allowing the paper to be re-used, rather than recycled. Relatively little work has yet been done on electrochromic materials impregnated into paper. Talmay 79,80 patented an idea for electrochromic printing in 1942 with ‘electrolytic writing paper’ consisting of paper pre-impregnated with particulate MoO 3 and WO 3 that formed an image following reduction at an inert-metal electrode acting as a pen. The electroformation of Prussian blue within the fibres of the paper has also been suggested: cf. the comments in Chapter 2 concerning ‘blue prints’. Several recent patents have been issued for elaborations of electrochromic printing systems usually based on organic electrochromic dyes, as cited in ref. 5. In 1989 Rosseinsky and Monk 82 investigated whether voltammetry in paper was possible, revealing marginal problems associated with IR drop across the paper and variations in its internal humidity. Moist paper was impregnated with a variety of viologens or Prussian blue precursors, together with an ionic electrolyte in sufficient concentration. In paper of marginal moistness, the electrochemistry of both Prussian blue and viologen electrochromes was quite well reproduced as though in a laboratory electrochemical cell, establishing electrochromic reactions to occur within the paper, as described in Section 11.4. Details of the study have been improved on. 81 In 1989, IBM 83 prepared a form of electrochromic paper capable of multiple coloration, but the complexity of their system precluded economic commercialisation. Investigations on electrochromes impregnated into paper included viologens, 82,84,85 Prussian blue 82,84 and the metal oxides MoO 3 and WO 3 . 84 Incorporation of an electrochrome within a thin layer of Nafion 1 as a host matrix has also been shown to produce good results: the electrochromes included viologen, 86 Methylene Blue 87 and phenolsafranine dyes. 87 Printable electrochromic paper has not been further pursued. However, NTera of Eire have developed a product called ‘electrochromic paper’ (and 13.6 Electrochromic paper 405 marketed as NanoChromics TM ), which is based on the viologen I. 88 The display, not based on paper, uses phosphonate groups bound by chemisorption to a metal-oxide surface such as a titanium dioxide film deposited on FTO. The oxidised form of I is colourless while the reduced form is blue–mauve. N N P O OH OH P O HO OH + + 2Cl – I NTera call their ECD a nanochromic display (NCD), claiming their technology has more than four times the reflectivity and contrast of a liquid crystal display (LCD). 13.7 Electrochromes applied in quasi-electrochromic or non-electrochromic processes: sensors and analysis It is of interest to consider the substances that can be electrochromic when they are used in another, analytical, context. ‘Gasochromic’ coloration outlined in Section 1.2 involves a mechanism of the kind further contemplated here. Only the first example of Co 3 O 4 that we cite below has some electrochromic basis to its operation; we then suggest an extension of this principle. In the solely analytical applications presented below, these normally electrochromic substances acquire or lose electrons from (or to) solution or gaseous species, rather than from (or to) electrode substrates when in electrochromic mode. The analytical relevance arises from the ensuing colour changes: a direct relationship exists between the absorbance of an electrochrome and the amount of charge passed, and thus the amount of test substance present. In a quasi-electrochromic application, Shimizu et al. 89,90 made a sensor based on cobalt oxide Co 3 O 4 , which, electrically polarised, colorises in the presence of phosphate ion: a thin Co 3 O 4 film changed transmittance T in the range 550–800 nm, becoming coloured when polarised to 0.4 V vs. SCE, but only in the presence of sufficient phosphate ion. No colour ensued in the absence of [HPO 4 ] 2À . The sensor transmittance T depends on the logarithm of [HPO 4 ] 2À concentration in the range 10 À6 to 10 À2 mol dm À3 , via a mechanism depending on the redox reaction of Eq. (6.19) in Chapter 6, but with the electrons now coming from a chemical reductant. (So it is not truly electrochromic.) Other electrochromic sensors have been fabricated in which the optical absorbance relates to pH, 91 or NO 3 À , or Cl À concentrations. 89 406 Applications of electrochromic devices The term ‘gasochromic’, Section 1.2, describes devices that operate with a gas-phase reductant or oxidant providing or accepting the electrons that would be necessary were these electrochromic redox processes. Thus, in a non- electrochromic analytical application, Cook and co-workers used a variety of phthalocyanines, e.g. in the form of a Langmuir–Blodgett film, to test for such diverse gases as NO 2 and toluene. 92,93,94,95,96,97,98 Many examples of gasochromic sensing devices are listed in Table 13.1, all electrochromes remaining in the solid state during coloration. While not strictly electrochromic, they are cited here because the chemical compositions and device geometry could readily be transformed into reversibly electrochromic systems, with the possibility of re-use for testing. In several cases, the device changes transmittance chemically following contact with gaseous analysis sample, but can be refreshed electrochemically for re-use. In an interesting gasochromic–analytical application, Khatko et al. show that doping a solid layer of WO 3 with different metals increases the sensitivity and selectivity to different gases. 118 Thin films of tungsten trioxide respond readily and rapidly to gaseous hydrogen. Many of the WO 3 -based gasochromic devices cited in Table 13.1 incorporate tungsten trioxide bearing a thin layer of platinum coated on the outer surface. In such cases, the WO 3 is responding to atomic hydrogen formed by a ‘spillover’ process catalysed by Pt, as described by Wittwer et al. 109 13.8 Miscellaneous electrochromic applications Portable identification cards for membership or security purposes can all bear an electrochromic fragment. Obvious applications include cash-point Table 13.1. Electrochromes utilised in gasochromic sensing devices, responding to gaseous analyte. Electrochrome Analyte Refs. Chromium oxide Ozone 99 Metalloporphyrins Chlorine 100 Nickel oxide Ozone 99 Phthalocyanine Chlorine 101 Phthalocyanine NO 2 , toluene 92,93,94,95,96,97,98 Tungsten trioxide Hydrogen 102,103,104,105,106,107,108,109 Tungsten trioxide Oxygen 109 Tungsten trioxide CH 4 /NH 3 /CO 110 Tungsten trioxide H 2 S 111,112,113,114 Tungsten trioxide NO 115,116,117 13.8 Miscellaneous electrochromic applications 407 machines and credit cards, etc., for which patents have already been filed. 69 Other security-related applications possible with an electrochrome impregnated into (solid) paper include security devices such as vouchers, tokens and tickets – even bank notes – where fraudulent copying is likely. The only extant review of electrochromic printing is ours 119 in 1995. Some applications rely on a thermo-electrochromic system, in which the speed of electrochromic coloration depends on temperature. In such applications, the device is usually so slow when cold that it is effectively switched ‘off’, even when a suitable potential is applied. As the temperature rises, so the speed of operation increases until a threshold is attained, above which the device will colour and bleach quite normally. The temperature dependence of a thermoelectrochromic device is best achieved by incorporating an ionic electrolyte for which the movement of counter ions has a high activation energy, E a . The magnitude of E a ensures that a relatively small change in temperature causes a substantial increase in ionic conductivity, and hence in device operation. Scrosati et al. 120 were probably the first to make such a device: the electrochrome was WO 3 and the electrolyte comprised poly(ethylene oxide) containing dissolved LiClO 4 . More recently, Owen and co-workers 121 developed a thermo-electrochromic device for displaying the safety of food, and is to be positioned above shop refrigerators. The electrolyte is again poly(ethylene oxide) containing dissolved LiClO 4 ,. The rate of coloration followed an Arrhenius-type expression at temperatures in the range 30 to –25 8C, provided the electrolytes remained amorphous (achieved by adding a high concentration of LiClO 4 and also a small amount of ZnI 2 ). So long as the rate of electro-coloration is essentially the same as the rate at which harmful bacteria multiply in the food, then the food is safe to eat while the device has not formed any colour. Conversely, the refrigerated food may be unsafe when the thermoelectrochromic ECD has changed its colour, because bacteria in the food will have had time to multiply. The Eveready Battery Company have produced a long, narrow electrochromic strip to indicate the state of charge, for use with dry-cell batteries. 122 During use, the two ends of the ‘charge indicator’ strip are attached to the two termini of a battery: the level of charge within the battery is indicated via the intensity of the strip’s colour and the proportion of the strip’s length that has become coloured. The identity of the electrochrome is obscured by the prose of the patent. (The strip on Duracell batteries is based on liquid-crystal technology, and is not electrochromic.) Kojimo and Terao 123 have developed an electrochromic system as a component within a DVD. Here an electrochromic layer serves as the 408 Applications of electrochromic devices multi-information-layer for an optical disk system. The active electrochrome is PEDOT(see Section 10.2). The claimed advantages of the electrochromic layer disk are in its large capacity, high sensitivity in recording, and the relative simplicity of the attendant hardware. The military in the USA are investigating fitting electrochromic panels as camouflage. The organic electrochromes are being developed by EIC Laboratories in conjunction with the Reynolds group in Florida. 124 13.9 Combinatorial monitoring of multiples of varied electrode materials A hugely ingenious application of electrochromism, a major aid to multiple monitorings of electrode processes, has just been announced. 125 It matches the ‘combinatorial’ methods of organic chemistry in which mixtures of products from concurrently occurring organic reactions in one pot are simultaneously analysed at the conclusion of reaction. As illustration, using a sheet of WO 3 deposited onto a FTO on glass of surface resistance 50 ohm per square, the electro-oxidation of methanol by a variety of Pt catalysts was employed. The 56 electrodes undergoing tests comprised various masses (groups of 6, 12, 18 or 24 mg) of Pt-containing electrode catalysts, each of similar diameter, 3 mm. These were deposited on vitreous carbon electrodes mounted on a non-conducting poly(tetrafluoroethylene (PTFE) planar support in a 7 Â8 matrix. The counter electrode, placed only 1 mm apart from the matrix, was the single WO 3 -coated sheet. The methanol reactant was at 1 mol dm À3 while the electrolyte was very dilute (H 2 SO 4 , 1 mmol dm À3 ), but the otherwise high resistance engendered is totally mitigated by the closeness of the two electrode sheets. The several millimetre lateral spacing between the Pt ‘dots’ confers high inter-dot resistances and thereby ‘focusses’ currents onto WO 3 areas directly opposite the Pt electrodes. For a suitable fixed duration, with the same potential simultaneously applied versus the WO 3 electrode to all the Pt electrodes, the relative effectiveness of each Pt electrode, as measured by the current or charge passed by each, is recorded as a small disc of blue coloration on the WO 3 , in a matrix corresponding to the geometry of the Pt electrodes. The intensity of coloration of each dot is directly proportional to the charge or current passed by each Pt catalyst. The simple photometric measurement of the colour intensity of each, from say a CCD camera image, bypasses separate or seriatim monitorings by voltammetry or galvanometry of each Pt electrode, by this simple and convenient quantitative method. For rapid comparative purposes, viewing by eye provides an instant estimate, if the quantity or quality of the catalyst in the monitored electrodes are arranged in sequence in the electrode mountings. 13.9 Multiple monitoring with electrode materials 409 A filter paper interposed between the electrodes acted both as a cell separator and a diffuse reflector aiding the optical monitoring by CCDcamera. In the experiments reported in the paper, but not essential in application, separate currents were individually monitored for comparison with the optical imprints on the WO 3 , providing very satisfactory evidence of the quantitative precision of the method. (This current monitoring, being expensive of apparatus or time, would not of course be needed except perhaps introductorily once-off in actual test applications.) Several tests on smaller groups of electrodes confirmed the satisfactory operation. 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Georg, A., Graf, W., Neumann, R. and Wittwer, V. Stability of gasochromic WO 3 films. Sol. Energy Mater. Sol. Cells, 63, 2000, 165–76. 105. Opara Krasˇ ovec, U., Orel, B., Georg, A. and Wittwer, V. The gasochromic properties of sol–gel WO 3 films with sputtered Pt catalyst. Sol. Energy, 68, 2000, 541–51. 106. Shanak, H., Schmitt, H., Nowoczin, J. and Ziebert, C. Effect of Pt-catalyst on gasochromic WO 3 films: optical, electrical and AFM investigations. Solid State Ionics, 171, 2004, 99–106. 107. Georg, A., Graf, W., Neumann, R. and Wittwer, V. Mechanism of the gasochromic coloration of porous WO 3 films. Solid State Ionics, 127, 2000, 319–28. 108. Salinga, C., Weis, H. and Wuttig, M. Gasochromic switching of tungsten oxide films: a correlation between film properties and coloration kinetics. Thin Solid Films, 414, 2002, 288–95. 109. Wittwer, V., Datz, M., Ell, J., Georg, A., Graf, W. and Walze, G. Gasochromic windows. Sol. Energy Mater. Sol. Cells, 84, 2004, 305–14. 110. Shaver, P. Activated tungsten oxide gas detectors. Appl. Phys. Lett, 11, 1967, 255–7. 111. Dwyer, D. G. Surface chemistry of gas sensors: H 2 S on WO 3 films. Sens. Actuators, B5, 1991, 155–9. 112. Solis, J. L., Saukko, S., Kish, L., Granqvist, C. G. and Lantto, V. Semiconductor gas sensors based on nanostructured tungsten oxide. Thin Solid Films, 391, 2001, 255–60. 113. Solis, J. L., Saukko, S., Kish, L. B., Granqvist, C. G. and Lantto, V. Nanocrystalline tungsten oxide thick-films with high sensitivity to H 2 S at room temperature. Sens. Actuators, B77, 2001, 316–21. 114. Heszler, P., Reyes, L. F., Hoel, A., Landstrome, L., Lantto, V. and Granqvist, C. G. Nanoparticle films made by gas phase synthesis: comparison of various techniques and sensor applications. Proc. SPIE, 5055, 2003, 106–19. 115. Tomchenko, A. A., Emelianov, I. L. and Khatko, V. V. Tungsten trioxide-based thick-filmNOsensor: designand investigation. Sens. Actuators, B57, 1999, 166–70. 116. Tomchenko, A. A., Khatko, V. V. and Emelianov, I. L. WO 3 thick-film gas sensors. Sens. Actuators, B46, 1998, 8–14. 117. Ho, J.-J. Novel nitrogen monoxide (NO) gas sensors integrated with tungsten trioxide (WO 3 )/pin structure for room temperature operation. Solid State Electronics, 47, 2003, 827–30. References 415 118. Khatko, V., Guirado, F., Hubalek, J., Llobet, E. and Correig, Z. X-Ray investigation of nanopowder WO 3 thick films. Physica Status Solidi, 202, 2005, 1973–9. 119. Monk, P. M. S., Mortimer, R. J. and Rosseinsky, D. R. Electrochromism: Fundamentals and Applications, Weinheim, VCH, 1995. 120. Pantaloni, S., Passerini, S. and Scrosati, B. Solid state thermoelectrochromic device. J. Electrochem. Soc., 134, 1987, 753–75. 121. Colley, R. A., Budd, P. M., Owen, J. R. and Balderson, S. Poly[oxymethyleneoligo(oxyethylene)] for use in subambient temperature electrochromic devices. Polym. Int., 49, 2000, 371–6. 122. Bailey, J. C. Eveready Battery Company. Electrochromic thin film state-ofcharge detector for on-the-cell application. US Patent 05458992, 1995. 123. Kojima, K. and Terao, M. Proposal of a multi-information-layer electrically selectable optical disk (ESD) using the same optics as DVD. Proc. SPIE, 5069, 2003, 300–5. 124. [Online] at www.nttc.edu/resources/funding/awards/dod/1998sbir/982army.asp (accessed 6 September 2005). 125. Brace, K., Hayden, B. E., Russell, K. E. and Owen, J. R. Aparallel optical screen for the rapid combinatorial analysis of electrochemical materials. Adv. Mater., 18, 2006, 3253–70. 416 Applications of electrochromic devices 14 Fundamentals of device construction 14.1 Fundamentals of ECD construction All electrochromic devices are electrochemical cells, so each contains a minimum of two electrodes separated by an ion-containing electrolyte. Since the colour and optical-intensity changes occurring withinthe electrochromic cell define its utility, the compositional changes within the ECDmust be readily seen under workplace illumination. In practice, high visibility is usually achieved by fabricating the cell with one or more optically transparent electrodes (OTEs), as below. Electrochromic operation of the ECD is effected via an external power supply, either by manipulation of current or potential. Applying a constant potential in ‘potentiostatic coloration’ is referred to in Chapter 3, while imposing a constant current is said to be ‘galvanostatic’. Galvanostatic coloration requires only two electrodes, but a true potentiostatic measurement requires three electrodes (Chapter 3), so an approximation to potentiostatic control, with two electrodes, is common. The electrolyte between the electrodes is normally of high ionic conductivity (although see p. 386). In ECDs of types I and II, the electrolyte viscosity can be minimised to aid a rapid response. For example, a liquid electrolyte (that actually comprises the electrochromes) is employed in the world’s best-selling ECD, the Gentex rear-viewmirror described in Section 13.2. The electrolyte in a type-III cell is normally solid or at least viscoelastic, e.g. a semi-solid or polymer, as below. In fact, virtually all the type-III cells in the literature are designed to remain solid during operation, as ‘all-solid-state devices’, or ‘ASSDs’. Such solid-state ECDs have multilayer structures, and a wide range of device geometries has been contemplated, 1,2,3,4,5,6,7,8,9,10 involving variations in the positions of the counter and working electrodes. Figure 14.1 shows schematically one such solid-state device. Layer (i) is an optical electrode comprising a glass slide coated with ITO, 417 the conductive side innermost. The second electrode (ii) could be another inward-facing OTE if the device is to operate in a transmittance mode. Alternatively, devices operating in a reflectance mode generally require the second electrode to be made of polished metal, the metal being chosen both for its electronic conductivity and its aesthetic qualities, including its ability to act as a reflector, as described in Section 14.3 below. However, the colour and reflectivity of the second electrode are unimportant if it is positioned behind a layer of electrolyte containing an opaque filler; see p. 421. The other layers of an all-solid-state ECD lie parallel and between the two electrodes. At least one of the ‘ion-insertion layers’ will be electrochromic. The primary electrochromic layer (iii) is juxtaposed with the front OTE; the secondary electrochrome (iv) is deposited on the rear, counter electrode. Finally, an electrolyte layer (v) separates the two ion-insertion layers, as described in Section 14.2. Since the primary electrochrome is oxidised concurrently with reduction of the secondary (and vice versa when switching off), it is sometimes necessary to construct an all-solid-state ECDwith one of the layers precharged with mobile ions. In practice, this is rarely a simple procedure. To effect this with say WO 3 , lithium metal can be evaporated in vacuo onto the surface of one electrochrome film before device assembly – so-called ‘dry lithiation’. 11,12,13,14,15,16 Elemental lithium is a powerful reducing agent, so gaseous lithium diffuses into the solid layer to effect chemical reduction, as in Eq. (14.1): WO 3 (s) þx Li 0 (g) !Li x WO 3 (s); (14.1) Wire connection Wire connection (i) Optically transparent electrode (iii) Primary electrochromic layer (v) Electrolyte (iv) Secondary electrochrome (ii) Electrode Seal Seal Figure 14.1 Schematic of a typical all-solid-state, multi layer electrochromic cell. Layer (i) is an optically transparent electrode, OTE. The second electrode (ii) could be another inward-facing OTE. Layer (iii) is the primary electrochrome and layer, (iv) is the secondary. Layer (v) is the electrolyte. 418 Fundamentals of device construction x should not exceed about 0.3, since subsequent electrochemical extraction of Li þ in attempted re-oxidation is irreversible (see p. 142). Somewhat similarly, nickel oxide, in some commercial prototypes, is precharged using ozone; 17,18 in practice, films were irradiated with UV light in the presence of gaseous oxygen. 14.2 Electrolyte layers for ECDs Reviews of electrolyte layers for ECD usage include ‘Electrical and electrochemical properties of ion conducting polymers’ by Linford 19 (in 1993), ‘Sol–gel electrochromic coatings and devices: a review’ by Livage and Ganguli 20 (in 2001) and ‘Electrochromics and polymers’ by Byker 21 (in 2001). The layer of electrolyte between the two electrodes must be ionically conductive but electronically an insulator. In type-I and type-II ECDs, the electrochrome is dissolved in a liquid electrolyte, which can be either aqueous or a polar organic solvent such as acetonitrile or a variety of other nitriles, dimethylformamide, propylene carbonate or g-butyrolactone. The electrochrome approaches the working electrode through this milieu during electrochromic coloration. Solutions may also contain a dissolved supporting electrolyte in high concentration to suppress migration effects (see Sections 3.3.2 and 3.3.3). A thickener, such as acrylic polymer, poly(vinylbutyral) or colloidal silica, 21 may be added to the solution to increase its viscosity. This practice improves the appearance of an ECD because the coloration develops at different rates in different areas in a fast device (see end of Section 13.4), hence artificially slowing the rate of coloration helps ensure an even coloration intensity. Thickening also improves the safety of a device should breakage occur, and helps minimise mass transport by convection (Section 3.3). Gelling the electrolyte, e.g. by adding a polyether such as PEO, is claimed to enhance the electrochemical stability. 22 In type-III systems, while the electrolyte holds no soluble electrochrome, it now enacts two roles (see Chapter 3). Firstly, during coloration and bleaching, for electroneutrality it supplies the mobile counter ions that enter and leave the facing solid-electrochrome layers. However, secondly, the electrolyte still effects the accompanying conduction between the electrodes. Quite neglecting the latter, however, the electrolyte layer is called by some an ‘ion-storage (IS) layer’, which represents only the former action. Thus an ‘ion-storage layer’ and an ‘electrolyte layer’ are by no means equivalent terms. Better (but possibly too late and too long) is an inclusive term such as ‘ionogenic electrolyte layer’; or – shorter – ‘ion-supplying layer’, which at least allows of both roles. 14.2 Electrolyte layers for ECDs 419 Type-III ECDs operating with protons as the mobile ions can contain aqueous acids. In Deb’s ECD, 23 for example, the electrolyte was aqueous sulfuric acid of concentration 0.1 mol dm À3 . Liquid acids are rarely used today owing to their tendency to degrade or dissolve electrochromes, and from safety considerations should the device leak. A majority of type-III ECDs now employ inorganic solids or viscoelastic organic polymers, the latter being flexible and resistant to mechanical shock. Solid organic acids of amorphous structure might serve similarly, although considerably higher potentials would be needed to drive any such ECD. They are apparently untested in this role, their electrical connectivity with electrochromes being critical. Ionic liquids somewhat below their solidification temperature might also serve but their ion-insertion capability could be questionable. 14.2.1 Inorganic and mixed-composition electrolytes Many ECDs contain as electrolyte a thin layer of solid inorganic oxide; thin- film Ta 2 O 5 is becoming widely used. Such layers are generally evaporated or sputtered. However, they are mechanically weak and cannot endure bending or mechanical shock. There may be a role here for mixed organic/inorganic solids like tetraalkylammonium salts with small inorganic anions, or alkali- metal salts containing large organic anions (provided that insertions only of the smaller ion are required); these might evince greater mechanical robustness. Like organic acids Àprevious paragraphÀthese also appear not to have been tried. 14.2.2 Organic electrolytes Semi-solid organic electrolytes fall within two general categories: polyelectrolytes and polymer electrolytes, as described below. Polyelectrolytes Polyelectrolytes are polymers containing ion-labile moieties at regular intervals along the backbone. A popular example is poly(2-acrylamido-2- methylpropanesulfonic acid), ‘poly(AMPS)’, in which the proton-donor moiety is an acid. The molar ionic conductivity L of polymers such as poly(AMPS) depends critically on the extent of water incorporation; wholly dehydrated poly(AMPS) is not conductive, but L increases rapidly as the water content increases. Table 14.1 lists some polyelectrolytes used in solid- state ECDs. 420 Fundamentals of device construction Polymer electrolytes Polymer electrolytes contain, as solvent, neutral macromolecules such as poly(ethylene oxide) – PEO, poly(propylene glycol) – PPG, or poly(vinyl alcohol) – PVA. Added inert salt acts to form an inorganic electrolyte layer. Common examples include LiClO 4 , triflic acid CF 3 SO 3 H, or H 3 PO 4 . The viscosity of such polymers increases with increasing molecular weight, so polymers range from liquid, at low molecular weight, through to longer polymers which behave as rigid solids. Table 14.1 lists a selection of polymer electrolytes and polyelectrolytes used in solid-state ECDs. It is quite common for polymeric electrolytes to have an opaque white ‘filler’ powder added, such as TiO 2 to enhance the contrast ratio in displays. A white layer also dispenses with any need to tailor the optical properties of the secondary layer. Thus, Duffy and co-workers 9 have described a device in which WO 3 forms both the primary and secondary electrodes, a device which could not show any observable change in colour unless the rear electrode was screened from view by incorporating such an opaque filler in the intervening electrolyte. The inclusion of particulate TiO 2 does not seem to affect the response times of such ‘filled’ ECDs, but the photocatalytic activity Table 14.1. Solid ion-conducting electrolytes for use in ECDs. Electrolyte Refs. Inorganic electrolytes LiAlF 4 24 LiNbO 3 25,26,27,28 Sb 2 O 5 (inc. HSbO 3 ) 29,30,31 HSbO 3 based polymer 32 Ta 2 O 5 (including ‘TaO x ’) 33,34,35,36,37,38,39,40 TiO 2 (including ‘TiO x ’) 40 H 3 UO 3 (PO 4 ) Á3H 2 O (‘HUP’) 41 ZrO 2 42,43,44,45,46 Organic polymers Nafion TM 47,48,49 Poly(acrylic acid) 50,51,52 Poly(AMPS) 47,53,54,55 Poly(methyl methacrylate), PMMA (‘Perspex’) 56,57,58,59,60,61,62,63,64, 65,66,67,68,69,70 Poly(2-hydroxyethyl methacrylate) 42,56,71,72 Poly(ethylene oxide), PEO 73,74,75,76,77,78,79,80,81,82,83 Poly(vinyl chloride), PVC 84,85 14.2 Electrolyte layers for ECDs 421 of TiO 2 may accelerate photolytic deterioration of organic materials such as the electrolyte. The stability of electrolyte layers is discussed in Section 16.3. 14.3 Electrodes for ECD construction All ECD devices require at least one transparent electrode. Devices operating in a transmissive mode, such as spectacles, goggles, visors or whole windows, must of course operate with a second OTE as the rear electrode, whereas devices operating in a reflective sense, as in information displays, do not. It is common but expensive for polished platinum to act as both mirror and supporting electrode in a reflecting ECD. Otherwise, the electrolyte-with-filler ploy (previous paragraph) is used. Reviews of materials for OTE construction for electrochromic devices include ‘Transparent conductors: a status review’ by Chopra et al. 86 (in 1983), ‘Transparent electronic conductors’ by Lynam 87 (in 1990), ‘Transparent conductive electrodes for electrochromic devices – a review’ by Granqvist 88 (in 1993), ‘Transparent and conducting ITO films: new developments and applications’ by Granqvist and Hulta˚ ker 89 (in 2002), and ‘Frontier of transparent oxide semiconductors’ by Ohta et al. 90 (in 2003). 14.3.1 Transparent conductors The most common choice of OTE is indium–tin oxide as a thin film sputtered onto glass. Another common choice is fluorine-doped tin oxide (FTO), an example being so-called ‘K-glass TM ’ from Pilkington, which comprises FTO on glass. 57,71,91,92,93 Its UV-visible absorption is less than 2% and its thermal infrared reflectance exceeds 90%. Indium–tin oxide is electrically semiconducting rather than metallic. The relatively high innate resistance of semiconducting ITO (or other OTEs) can cause complications such as IR drop 94 and the so-called ‘terminal effect’. As a consequence of IR drop, a gradient of potential forms across the electrode surface: the potential near the external contact is higher than elsewhere, so the electrochromic coloration or image formed during coloration is generated at different speeds across the electrode surface, and the intensity of colour will often be more intense near the external electrical contact, leading to a non-uniform image. Ho et al. 95 discuss such ‘terminal effects’ in ECDs. The best conductivity of ITO is about 20 O per square; substrates of higher electronic conductivity are attainable, but are slightly yellow. Thus the 422 Fundamentals of device construction conductivity of OTEs is relatively poor, considerably affecting ECD response times; 96 see p. 349. Ways of combating IR drop and terminal effects involve increasing the electronic conductivity. Methods adopted include incorporating an ultra-thin layer of metallic nickel between the electrochrome and ITO, 97 or depositing an ultra-thin layer of precious metal on the electrolyte-facing side of the electrochrome. 98,99,100,101 Thin films of Cr 2 O 3 102 or MgF 2 103,104 can also fulfil this goal. The idea of flexible ECDs is attractive for lightweight, temporary electrochromic window coverings and the like. 105,106,107,108,109,110,111,112,113 Azens et al. 114 describe the fabrication and applications of such electrochromic ‘foils’. Clearly any such device will need to be enclosed within thin sheets of an appropriate polymer. Furthermore, all the layers, including the conductive ITO and both electrochromes, must be durable, since any cracks formed by bending cause irreversible insulating discontinuities that lead to certain device failure. A review (1995) that addressed the use of polymeric substrates for electrochromic purposes is the short work by Antinucci et al. 78 The deposition conditions must be milder when ITO is to be deposited onto polymeric substrates rather than on glass. Such deposition is now relatively easy but, nevertheless, the differing deposition conditions result in ITO layers with poorer electrical conductivity to that made on glass. Bertran et al. 115 overcame this problem by incorporating small amounts of silver within their ITO films, which is known to lower the electrical resistivity 116 albeit with a slight decrease in optical transmittance. The highest electrical conductivities were achieved in depositions using lowAr pressures of 0.4 Pa (without oxygen) and the relatively high power density of 2 Â10 4 W m À2 . Glass and polyester substrates exhibited different growth rates and samples deposited onto glass substrates showed better film-to-polymer adhesion. Nevertheless, ITO for counter-electrode use has been deposited on sheet plastics such as Mylar, 117 poly(ethyleneterephthalate) or PET 78,105,106,107,112,118 and polyester. 111,115,119 (Such flexible displays could also be photo-electrochromic. 110 ) Several all- polymer ECDs have also been fabricated; see Section 10.5. The stability of ITO electrodes is discussed in Section 16.2. 14.3.2 Opaque and metallic conductors The most common choice of rear electrode is platinum or Pt-based alloys. 3,5,6,10 Other materials have also been advocated: Liu and Richardson 120 suggest an alloy of antimony and copper. The second electrode need not bear a separate layer of electrochrome: redox-active counter electrodes can themselves ‘absorb charge’ with the 14.3 Electrodes for ECD construction 423 accompaniment of counter-ion intercalation. For example, ECDs have been constructed in which charge is intercalated into a counter electrode of carbon: examples of such counter electrodes include ‘carbon’ 29,121,122,123 or ‘carbon- based’ materials, 79,124 screen-printed carbon black, 125 and graphite. 126 All these counter electrodes remain black during electrochromic operation, and need therefore to remain hidden behind a layer of electrolyte containing an opaque white filler. 14.3.3 ECDs requiring no transparent conductor Transparent conductors are not always needed. A novel design by Liu and Coleman 113 has recently been described which employs a ‘side-by-side’ structure. Ultrafine electrodes are screen-printed onto a non-conductive glass substrate, with electrochrome deposited above and between them; see Figure 14.2. 14.4 Device encapsulation The process of assembling the components of a commercial device, and the mounting materials, are clearly as important as (in some views more important than) the operation of the parts taken individually. In devices containing a liquid or semi-solid electrolyte, the separation between the two electrodes can be maintained by introducing flat or spherical Polymeric substrate andsupport Transparent film Gel electrolyte Layer ofelectrochrome Dispersion ofconductive metal oxide Carbon ink Silver-carbon ink Working electrode Insulator Counter electrode Insulator Counter electrode Figure 14.2 ‘Side-by-side’ design of a screen-printed electrochromic display device: schematic representation illustrating the arrangement of the electrodes. (Figure redrawn from Liu, J. and Coleman, J. P. ‘Nanostructured metal oxides for printed electrochromic displays’. Mater. Sci. Eng. A, 286, 2000, 144–8, by permission of Elsevier Science.) 424 Fundamentals of device construction ‘spacers’, acting in a similar manner to the minute spherical beads of constant diameter employed in fabricating an LCD, to maintain the precisely defined distance between the two parallel electrodes. For example, PPG Industries used this approach. 127,128 Finally, the device must be sealed. In fact, the fabrication of a robust, leakproof seal to encapsulate a type-I or -II ECDis not a trivial problem: Byker (at that time, of Gentex Corporation) recently stated, ‘polymer sealant materials are often crucial to the life of an EC device, and may represent as big a R&D challenge as the EC system itself’, 21 in bringing a device to commercial viability. One of the principal problems is chemical durability; a second is the hydrostatic pressures that form in large devices containing liquid electrolytes, since the weight of liquid causes the bottom of the device to swell, yet can push the top of the panes together till they break. Byker believes that all-solid-state systems also require an elastomeric polymer seal. 21 He discusses the use of polymers as electrolytes within ECDs in ref. 21. To these ends, PPG employed an adhesive layer to coat the edges of their devices, 127,128 and Gentex designed a complicated type of clip, 129 to withstand hydrostatic pressures. The sealant around a device must be chemically stable. It is regrettable – but perhaps inevitable in view of industrial competitiveness – how many reports of actual devices (prototype and in production) fail to divulge details of device encapsulation. 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A., Gentex Corporation. Clip for use with transparent conductive electrodes in electrochromic devices. US Patent 06064509, 2000. 130. Pettersson, H., Gruszecki, T., Johansson, L.-H., Edwards, M. O. M., Hagfeldt, A. and Matuszczyk, T. Direct-driven electrochromic displays based on nanocrystalline electrodes. Displays, 25, 2004, 223–30. 432 Fundamentals of device construction 15 Photoelectrochromism 15.1 Introduction Systems that change colour electrochemically, but only on being illuminated, are termed photoelectrochromic (cf. electrochromic or photochromic when only one of these stimuli is applied). Relatively few photoelectrochromic systems have been examined as such, although in some studies of photoelectrochemistry, colour changes are mentioned; see refs. 1,2,3. One study calls such devices ‘user controllable photochromic devices’. 4 Fewreviews of the topic are extant: the chapter on photoelectrochromismin our 1995 book 5 is dated, but still the most comprehensive. Others include ‘Photoelectrochromic cells and their applications’ by Gregg (of NREL in Colorado) 6 in 1997, and ‘All-polymeric electrochromic and photoelectrochemical devices: new advances’ by De Paoli et al. 7 in 2001. Two bases of photoelectrochromic operation are available. In the first, the potential required to evoke electrochromismis already applied but can act only through a photo-activated switch, filter or trigger. A separate photoconductor or other photocell serves as a switch, or the actual electrochromic electrode surface itself could be a photoconductor, or sandwiched together with a photoconductor. Such photo-activated systems contrast with photo-driven devices, in which illumination of one or other part of the circuit produces the photovoltaic potential required to drive the electrochromic current. 15.2 Direction of beam The direction of illumination during cell operation is important. If the incident beam traverses a (minimum) distance in the cell prior to striking the photoactive layer, then illumination is said to be ‘front-wall’, 8 as shown by arrow (a) in Figure 15.1. Conversely ‘back-wall’ illumination, arrow (b), Figure 15.1, 433 operates with the beam directed from behind the cell, so traversing more cell material before reaching the photosensitive layer. Front-wall illumination generally yields superior results since additional absorptions by other layers within the ECD are minimised. Back-wall illumination is used only if undesirable photolytic processes occur with front-wall illumination of the cell. 15.3 Device types 15.3.1 Devices acting in tandem with a photocell The simplest circuits for photoelectrochromic device operation comprise a conventional electrically driven ECD together with a photo-operated switch. The switch operates by illumination of a suitable photocell, be it photovoltaic or photoconductive, which triggers a microprocessor or similar element which in turn switches on the already ‘poised’ cell. Such an arrangement is not intrinsically photoelectrochromic but is switched on by photocontrolled circuitry: the cell itself could be any straightforward electrochromic system. 15.3.2 Photoconductive layers Photoconductive materials are insulators in the absence of light but become conductive when illuminated. Such photoconductors were traditionally semiconductors like amorphous silicon but, in recent years, many organic photoconductors have become candidates, as below. The mechanism of photoconduction involves the photo-excitation of charge carriers (electrons or holes) from localised sites, or from bonds in the valence band, into the delocalised energy levels L igh t-sensitiv e layer ECD (a) hν (b ) hν Optically transparent layer Figure 15.1 Schematic representation of a photoelectrochromic cell. Illu- mination from direction (a) represents ‘front-wall’ illumination and (b) ‘back- wall’ illumination. 434 Photoelectrochromism forming the conduction band. The mobilised charges can be driven by an externally applied potential, 9 yielding a current that can effect electrochromism. Electrochromic cells may employ a layer of photoconductive material in one of two ways. 10,11 In the first, a photoconductive component is positioned outside the ECDand acts as a photocell switch: illumination of the photoconductor completes the circuit, allowing for electrochromic coloration. Current ceases in the dark, so coloration stops. In the second arrangement, a photoconductive layer is incorporated within the electrochromic cell. Figure 15.2 shows an ECD with a photoconductor (light-sensitive layer) positioned between an optically conducting substrate and a film of electrochrome. During electrochromic coloration or bleaching, ions from the electrolyte enter the electrochromic layer as in normal operation (see Section 1.4 on page 11), but electrons enter via the photoconductor. This arrangement has the difficulty that, since most photoconductors are somewhat opaque, ECDs operating with a photoconductor will probably have to operate in a reflective mode. Back-wall illumination of the ECD in Figure 15.2 would allow for strong, metallic electrodes to be employed as the photoconductor support. A few photoelectrochromic devices have been fabricated with semi-transparent photoconductors. 12,13 In a variation of this latter arrangement, the photoconductor might conceivably be located between the electrochrome and the electrolyte layers (Figure 15.3). 10,14 Here the photoconductor would need to be completely ion-permeable, although note that the attendant physical stresses of continual ion movement through the photoconductor could lead to its eventual disintegration. Accordingly, the arrangement in Figure 15.2 is preferred. Several workers 12,15,16,17 of the NREL laboratories in Colorado, made a photoelectrochromic device in which the photoconductor was a thin, hν Light-sensitive layer Primary electrochrome Secondary electrochrome Electrolyte layer Transparent conductor Conductor (Pt or otherwise) Figure 15.2 Schematic representation of a photoelectrochromic cell: front- wall illumination of an ECD containing a photoconductive layer between the transparent conductor and the primary electrochrome layer. 15.3 Device types 435 semi-transparent layer of hydrogenated amorphous silicon. It yielded a photocurrent of 3.9 mAcm À2 , and an open-circuit potential V oc of 0.92 V, which is deemed adequate to colour a lithium-based device with a response time of less than one minute. Their window covering could be produced on a flexible polymer substrate, allowing it to be affixed to the inside surface of a window, i.e. this represents a photo-electrochromic ‘smart-glass’ window(cf. Section 13.3); NREL called the device a ‘stand-alone photovoltaic-powered electrochromic window’. The primary electrochrome was WO 3 . Photoelectrochromic ‘writing’ has been suggested by several authors; NREL made a photoelectrochromic prototype that could be bleached with a light pen: 18 they envisaged use in light-on-dark viewgraph projection or possibly within children’s toys. The writing appeared light yellow on a black background. The photoconductor within the display was hydrogenated amorphous silicon carbide. The primary electrochrome was WO 3 , with ion- conducting LiAlF 4 as the electrolyte, and Ni–Woxide as the counter electrode. Similarly, Yoneyama 19 labelled his device ‘a photo-rewritable . . . image’. In fact, many intrinsically conducting polymers are photoconductive: 20 photoelectrochromic devices employing poly(aniline) as a photoconductor have been made by Fitzmaurice, 21 Hagen, 22 Ileperuma, 23 Kobayashi 24,25,26,27 and their co-workers. The electrochrome in Kobayashi’s cell was methyl viologen 27 (cf. Chapter 11), with a variant of ruthenium tris(bipyridyl) as a photosensitiser. Other polymer electrochromes than poly(aniline) have been used as photoconductive layers within photoelectrochromic devices: poly(pyrrole), 28 poly (o-methoxyaniline) 7 and the thiophene-based polymers poly(3-methylthiophene) 29 and PEDOT. 7 hν Light-sensitive layer Primary electrochrome Secondary electrochrome Electrolyte layer Transparent conductor Conductor (Pt or otherwise) Figure 15.3 Schematic representation of a photoelectrochromic cell: front- wall illumination of an ECD containing a photoconductive layer between the primary electrochrome layer and the electrolyte. 436 Photoelectrochromism Titanium dioxide (in its anatase allotrope) is one of the most intensely studied photo-active materials, and has been incorporated into many photoelectrochromic devices. For example, Hagen’s et al.’s 22 photoelectrochromic device employed a nanocrystalline layer of TiO 2 as a photoconductor, in addition to poly(aniline), as above. The coloration process was photosensitised using a dye based on rutheniumtris(2,2 0 -bipyridine). Their ‘self-powered’ cell was able to modulate its transmission over the whole visible spectral region. (The illuminating lamps simulated solar spectral intensities.) The photoactive TiO 2 need not be a continuous layer: in the device fabricated by Liao and Ho, 30 particulate titanium dioxide was the photoactive material; a ruthenium complex acted as a photosensitiser, and the I À / I 3 À redox couple was incorporated as the electron mediator. The electrochrome was a thin layer of PEDOT polymer, yielding a device with an overall coloration efficiency of 280 cm 2 C À1 . 15.3.3 Photovoltaic materials Aphotovoltaic material produces a potential when illuminated, froma process similar to the excitation of electrons within a photoconductor but with an internal rectifying field to provide a driving force on the electrons. The ionic charges needed to accompany the electrochromic transition enter the filmfrom juxtaposed electrolyte or an electron mediator. The photovoltaic layer is not consumed in this process. The photovoltage produced need not be large; indeed, its actual magnitude is not a problem because an external bias can be applied until the cell is ‘poised’. Illumination of such a poised cell generates a photovoltage which, when supplementing the external bias, is sufficient to enable the coloration process to proceed, even if the photovoltage is itself too small to effect the required redox chemistry. For example, a cell comprising tungsten trioxide deposited on TiO 2 requires a bias 31 since the photovoltage generated is insufficient. Prussian blue (PB) has also been used as the electrochrome in photoelectrochromic devices, with a photovoltage coming from polycrystalline n-type SrTiO 3 , 32,33 TiO 2 34,35 or CdS 36 as the photolayer. (Indeed, PB has been used with WO 3 to make a photorechargeable battery. 36 ) Other photoelectrochromic cells operating via photovoltaism include WO 3 on CdS, 14,37 GaAs, 38 GaP, 39 or on TiO 2 , 40,41 Films of indium hexacyanometallate grown in a bath containing colloidal TiO 2 are also photoelectrochromic. 42,43 Few monomeric organic systems claim photoelectrochromism, perhaps owing to their tendency to photodegrade. Among the fewin the literature are Methylene 15.3 Device types 437 Blue 44 (I) and the spirobenzopyran 45 (II), both of which undergo reversible photoelectrochromic transitions at TiO 2 electrodes. S N N N CH 3 H 3 C CH 3 CH 3 CH 3 CH 3 CH 3 Cl – + NO 2 N O I II 15.3.4 Photogalvanic materials Photogalvanic materials generate current when illuminated. The photogalvanic material is generally consumed during the photoreaction 14 which inevitably causes the (photo-operated) write–erase efficiency to be poor. Photoelectrochromism in the cell WO 3 jPEO, H 3 PO 4 (MeCN)jV 2 O 5 is believed to operate in a photogalvanic sense 14 since the brown colour of the V 2 O 5 layer disappears gradually during illumination. Curiously, the cell is still photoelectrochromic even after the colour of the V 2 O 5 has gone and an alternative cathodic reaction (possibly catalysed consumption of oxygen, or reduction of VO 2 ?) must be envisaged. 15.4 Photochromic–electrochromic systems Some systems are not photoelectrochromic in the sense defined above, yet do not function as electrochromic or photochromic alone. For example, De Filpo and co-workers devised ‘photoelectrochromic systems’ comprising either ethyl viologen 46 or Methylene Blue (I) in solution, 46,47 together with a suitable electron donor such as an amine. Irradiation e.g. with a He–Ne laser induces an electron-transfer process with concomitant formation of colour. The colour-forming process is straightforwardly photochromic. The colour may be erased electro chromically. We adopt the compound adjective ‘photochromic– electrochromic’ for those systems that colour and bleach via the alternate use of photochromism and electrochromism. Yoneyama et al. 19,48 developed a photochromic–electrochromic cell functioning in the opposite sense to that of De Filpo’s, so the colour bleached photo chromically and was regenerated electro chromically. Yoneyama’s photo chromic–electrochromic device employed poly(aniline) as the colour- changing material. The polymer film contained entrapped particles of TiO 2 , enabling the poly(aniline) to act as both photoconductor and colour-changing 438 Photoelectrochromism material. The device was assembled with the polymer as one layer in a multilayer ‘sandwich’. Illumination effected photoreduction of the poly(aniline) with concomitant bleaching of the polymer’s dark-blue colour. During illumination, the film was immersed in aqueous methanol, the methanol acting as a sacrificial electron donor. In this example, the dark blue colour of the poly(aniline) was subsequently recoloured electro chromically. The colour of the poly(aniline) did not bleach completely during illumination, presumably because the photoconducting properties of poly(aniline) decrease in proportion to the extent of the bleaching; it is the oxidised form of the polymer that photoconducts. The poly(aniline) film can only photoconduct through those areas that are illuminated, so images, rather than uniform blocks of tone, may be formed if the light source passes through a patterned mask or photographic negative. To this end, Yoneyama et al. 19 illuminated their photochromic–electrochromic poly(aniline) film through a photographic negative to form the notable image in Figure 15.4. Kobayashi et al. 49 have also generated impressive images by illuminating a film of poly(aniline) through a photographic negative. Figure 15.4 Photoelectrochromic image generated on a thin film of poly(aniline)–TiO 2 : the film was immersed in a solution of phosphate buffer (0.5 mol dm À3 at pH 7) containing 20 wt% methanol as a sacrificial electron donor. The filmwas illuminated through a photographic negative with a 500 W xenon lamp for 1 min. (Figure reproduced from Yoneyama, H., Takahashi, N. and Kuwabata, S. Formation of a light image in a polyaniline film containing titanium(IV) oxide particles. J. Chem. Soc., Chem. Commun., 1992, 716–17, with permission of The Royal Society of Chemistry.) 15.4 Photochromic–electrochromic systems 439 References 1. Hirochi, K., Kitabatake, M. and Yamazaki, O. Electrochromic effects of Li–W–O films under ultraviolet light exposure. J. Electrochem. Soc., 133, 1986, 1973–4. 2. Buttner, W., Rieke, P. and Armstrong, N. R. Photoelectrochemical response of GaPc-Cl thin film electrode using two photon sources and two illumination devices. J. Electrochem. Soc., 131, 1984, 226–8. 3. Stilkans, M. P., Purans, Y. Y. and Klyavin, Y. K. Integral photoelectrical properties of thin-film systems based on photosensitive conductor and photochromiummaterial. Zh. Tekh. Fiz., 61, 1991, 91–7 [in Russian]. The title and abstract are cited in Curr. Contents, 31, 1991, 1969. 4. Teowee, G., Gudgel, T., McCarthy, K., Agrawal, A., Allemand, P. and Cronin, J. User controllable photochromic (UCPC) devices. Electrochim. Acta, 44, 1999, 3017–26. 5. Monk, P. M. S., Mortimer, R. J. and Rosseinsky, D. R. Electrochromism: Fundamentals and Applications, Weinheim, VCH, 1995, ch. 12. 6. Gregg, B. A. Photoelectrochromic cells and their applications. Endeavour, 21, 1997, 52–5. 7. De Paoli, M.-A., Nogueira, A. F., Machado, D. A. and Longo, C. All-polymeric electrochromic and photoelectrochemical devices: new advances. Electrochim. Acta, 46, 2001, 4243–9. 8. Rauh, R. D. Cadmium chalcogenides. Stud. Phys. Chem., 55, 1988, 277–327. 9. Duffy, J. A. Bonding, Energy Levels and Inorganic Solids, London, Longmans, 1990. 10. Shizukuishi, M., Shimizu, S. and Enoue, E. Application of amorphous silicon to WO 3 photoelectrochromic device. Jpn. J. Appl. Phys., 20, 1981, 2359–63. 11. Yoneyama, H., Wakamoto, K. and Tamura, H. Photoelectrochromic properties of polypyrrole-coated silicon electrodes. J. Electrochem. Soc., 132, 1985, 2414–17. 12. Bullock, J. N., Bechinger, C., Benson, D. K. and Branz, H. M. Semi-transparent a-SiC:H solar cells for self-powered photovoltaic-electrochromic devices. J. Non- Cryst. Solids, 198–200, 1996, 1163–7. 13. Bechinger, C. and Gregg, B. A. Development of a new self-powered electrochromic device for light modulation without external power supply. Sol. Energy Mater. Sol. Cells, 54, 1998, 405–10. 14. Monk, P. M. S., Duffy, J. A. and Ingram, M. D. Electrochromic display devices of tungstic oxide containing vanadium oxide or cadmium sulphide as a lightsensitive layer. Electrochim. Acta, 38, 1993, 2759–64. 15. Deb, S. K., Lee, S.-H., Tracy, C. E., Pitts, J. R., Gregg, B. A. and Branz, H. M. Stand-alone photovoltaic-powered electrochromic smart window. Electrochim. Acta, 46, 2001, 2125–30. 16. Benson, D. K. and Branz, H. M. Design goals for a photovoltaic-powered electrochromic window covering. Sol. Energy Mater. Sol. Cells, 39, 1995, 203–11. 17. Deb, S. K. Recent developments in high efficiency photovoltaic cells. Renewable Energy, 15, 1998, 467–72. 18. Gao, W., Lee, S.-H., Benson, D. K. and Branz, H. M. Novel electrochromic projection and writing device incorporating an amorphous silicon carbide photodiode. J. Non-Cryst. Solids, 266–9, 2000, 1233–7. 19. Yoneyama, H., Takahashi, N. and Kuwabata, S. Formation of a light image in a polyaniline film containing titanium(IV) oxide particles. J. Chem. Soc., Chem. Commun., 1992, 716–17. 440 Photoelectrochromism 20. Ingana¨ s, O., Carlberg, C. and Yohannes, T. Polymer electrolytes in optical devices. Electrochim. Acta, 43, 1998, 1615–21. 21. Sotomayor, J., Will, G. and Fitzmaurice, D. Photoelectrochromic heterosupramolecular assemblies. J. Mater. Chem., 10, 2000, 685–92. 22. Li, Y., Hagen, J. and Haarer, D. Novel photoelectrochromic cells containing a polyaniline layer and a dye-sensitized nanocrystalline TiO 2 photovoltaic cell. Synth. Met., 94, 1998, 273–7. 23. Ileperuma, O., Dissanayake, M., Somusunseram, S. and Bandara, L. Photoelectrochemical solar cells with polyacrylonitrile-based and polyethylene oxide-based polymer electrolytes. Sol. Energy Mater. Sol. Cells, 84, 2004, 117–24. 24. Kobayashi, N., Yano, T., Teshima, K. and Hirohashi, R. Photoelectrochromism of poly(aniline) derivatives in a Ru complex–methylviologen system containing a polymer electrolyte. Electrochim. Acta, 43, 1998, 1645–9. 25. Kobayashi, N., Hirohashi, R., Kim, Y. and Teshima, K. Photorewritable conducting polyaniline image formation with photoinduced electron transfer. Synth. Met., 101, 1999, 699–700. 26. Kim, Y., Teshima, K. and Kobayashi, N. Improvement of reversible photoelectrochromic reaction of polyaniline in polyelectrolyte composite film with the dichloroethane solution system. Electrochim. Acta, 45, 2000, 1549–53. 27. Kobayashi, N., Fukuda, N. and Kim, Y. Photoelectrochromism and photohydrolysis of sulfonated polyaniline containing Ru(bpy) 3 2þ film for negative and positive image formation. J. Electroanal. Chem., 498, 2001, 216–22. 28. Ingana¨ s, O. and Lundstr ¨ om, I. Some potential applications for conducting polymers. Synth. Met., 21, 1987, 13–19. 29. De Saja, J. A. and Tanaka, K. Photoelectrochemical cells with p-type poly(3-methylthiophene). Phys. Stat. Sol. A, 108, 1988, K109–14. 30. Liao, J. and Ho, K.-C. A photoelectrochromic device using a PEDOT thin film. J. New Mater. Electrochem. Syst., 8, 2005, 37–47. 31. Ohtani, B., Atsumi, T., Nishimoto, S. and Kagiya, T. Multiple responsive device: photo- and electrochromic composite thin film of tungsten trioxide with titanium oxide. Chem. Lett., 1988, 295–8. 32. Ziegler, J. P., Lesniewski, E. K. and Hemminger, J. C. Polycrystalline n-SrTiO 3 as an electrode for the photoelectrochromic switching of Prussian blue films. J. Appl. Phys., 61, 1987, 3099–104. 33. Ziegler, J. P. and Hemminger, J. C. Spectroscopic and electrochemical characterization of the photochromic behaviour of Prussian blue on n-SrTiO 3 . J. Electrochem. Soc., 134, 1987, 358–63. 34. DeBerry, D. W. and Viehbeck, A. Photoelectrochromic behaviour of Prussian blue-modified TiO 2 electrodes. J. Electrochem. Soc., 130, 1983, 249–51. 35. Itaya, K., Uchida, I., Toshima, S. and De La Rue, R. M. Photoelectrochemical studies of Prussian blue on n-type semiconductor (n-TiO 2 ). J. Electrochem. Soc., 131, 1984, 2086–91. 36. Kaneko, M., Okada, T., Minoura, H., Sugiura, T. and Ueno, Y. Photochargeable multilayer membrane device composed of CdS film and Prussian blue battery. J. Electrochem. Soc., 35, 1990, 291–3. 37. Stikans, M., Kleparis, J. and Klevins, E. J. Photoelectric characterization of solid- state photochromic system. Latv. P. S. R. Zinat. Akad. Vestis. Fiz. Tekh. Zinat. Ser., 4, 1988, 43. References 441 38. Reichman, B., Fan, F.-R. F. and Bard, A. J. Semiconductor electrodes, XXV: the p-GaAs / heptyl viologen system: photoelectrochemical cells and photoelectrochromic cells. J. Electrochem. Soc., 127, 1980, 333–8. 39. Butler, M. A. Photoelectrochemical imaging. J. Electrochem. Soc., 131, 1984, 2185–90. 40. Opara Krasˇ ovec, U., Georg, A., Georg, A., Wittwer, V., Luther, J. and Topic, M. Performance of a solid-state photoelectrochromic device. Sol. Energy Mater. Sol. Cells, 84, 2004, 369–80. 41. Hauch, A., Georg, A., Abumga¨ rtner, S., Opara Krasˇ ovec, U. and Orel, B. New photoelectrochromic device. Electrochim. Acta, 46, 2001, 2131–6. 42. de Tacconi, N. R., Rajeshwar, K. and Lezna, R. O. Photoelectrochemistry of indium hexacyanoferrate–titania composite films. J. Electroanal. Chem., 500, 2001, 270–8. 43. de Tacconi, N. R., Rajeshwar, K. and Lezna, R. O. Preparation, photoelectrochemical characterization, and photoelectrochromic behavior of metal hexacyanoferrate–titanium dioxide composite films. Electrochim. Acta, 45, 2000, 3403–11. 44. de Tacconi, N. R., Carmona, J. and Rajeshwar, K. Reversibility of photoelectrochromismat the TiO 2 /methylene blue interface. J. Electrochem. Soc., 144, 1997, 2486–90. 45. Zhi, J. F., Baba, R., Hashimoto, K. and Fujishima, A. Photoelectrochromic properties of spirobenzopyran derivative. J. Photochem. Photobiol., A92, 1995, 91–7. 46. Macchione, M., De Filpo, G., Nicoletta, F. and Chidichimo, G. Improvement of response times in photoelectrochromic organic film. Chem. Mater., 16, 2004, 1400–1. 47. Macchione, M., De Filpo, G., Mashin, A., Nicoletta, F. and Chidichimo, G. Laser-writable electrically erasable photoelectrochromic organic film. Adv. Mater., 15, 2003, 327–9. 48. Yoneyama, H. Writing with light on polyaniline films. Adv. Mater., 5, 1993, 394–6. 49. Kobayashi, M., Hashimoto, K. and Kim, Y. Photoinduced electrochromism of conducting polymers and its application. Proc. Electrochem. Soc., 2003–17, 2003, 157–65. 442 Photoelectrochromism 16 Device durability 16.1 Introduction Like all other types of display device, mechanical or electronic, no electrochromic device will continue to function indefinitely. For this reason, cycle lives are reported. The definition of cycle life has not been conclusively settled. Even by the definition in Section 1.4, reported lives vary enormously: some workers suggest their devices will degrade and thereby preclude realistic use after a few cycles while others claim a device surviving several million cycles. Table 16.1 contains a few examples; in each case the cycle life cited represents ‘deep’ cycles, as defined on p. 12. Some of these longer cycle lives were obtained via methods of accelerated testing, as outlined below. It is important to appreciate that results obtained with a typical three- electrode cell in conjunction with a potentiostat can yield profoundly different results from the same components assembled as a device: most devices operate with only two electrodes. The results of Biswas et al., 9 who potentiostatically cycled a thin film of WO 3 immersed in electrolyte, are typical insofar as the electrochemical reversibility of the cycle remained quite good with little deterioration. Their films retained their physical integrity, but the intensity of the coloration decreased with the number of cycles. Some devices are intended for once-only use, such as the freezer indicator of Owen and co-workers; 10 other applications envisage at most a few cycles, like the Eveready battery-charge indicator. 11 Clearly, degradation can be allowed to occur after no more than a few cycles with applications like the latter. Conversely, applications such as a watch display will need to withstand many billions of cycles without significant deterioration – a stringent requirement. Devices can fail for one or more of three related reasons: failure of the conductive electrodes; failure of the electrolyte layer; and failure of the 443 electrochromes. The durability of individual electrochromes is discussed in their respective chapters. Here the durability of transparent electrodes is discussed in Section 16.2; that of electrolyte layers is discussed in Section 16.3; and general methods of enhancing electrochrome durability are outlined in Section 16.4. Finally, Section 16.5 contains details of how cycle lives are assessed for complete, assembled, devices. 16.2 Durability of transparent electrodes The first reason for device failure is breakdown of an optically transparent electrode, OTE. The most common cause of OTE degradation is decomposition of ITO, which occurs readily in acidic solutions: the oxides within the ITO layer are themselves reduced when not in contact with solution. Such reduction both decreases its chemical stability and increases its electrical resistance: 12,13,14 while the oxidised form of ITO is chemically stable, reduced ITO is unstable and rarely bears the strains of repeated redox cycling because it dissolves readily in aqueous acids. 15 Indeed, in aqueous solution, the subsequent reaction of over-reduction to form metallic tin is difficult to stop. 16,17,18 For this reason, some workers tentatively suggest that all moisture must be excluded rigorously from the electrolyte of an ECD. 19,20,21 In the study by Bressers and Meulenkamp 22 it was shown that a thin layer of metallic indium forms on the surface of the ITO during reduction, possibly facilitating the observed dissolution in water-containing electrolytes, which is faster if the ITO is partially reduced. 14,23 Even ITO in contact with semi-solid poly(ethylene) oxide (PEO) electrolyte can deteriorate: Radhakrishnan et al. 15 show how ITO electrodes in contact with PEOdeteriorate after repeated cycling, both in terms of their conductivity Table 16.1. A selection of cycle lives of electrochromic devices, reported as number of cycles survived. Primary electrochrome Secondary electrochrome Cycle life Ref. WO 3 Poly(aniline) 20 000 1 WO 3 Prussian blue 20 000 2,3 WO 3 ‘VO x H y ’ 30 000 4 WO 3 (CeO 2 ) x (TiO 2 ) 1Àx 50 000 5 WO 3 Nickel oxide 100 000 6 WO 3 Iridium oxide 10 000 000 7 Electrodeposited bismuth Prussian blue 50 000 000 8 444 Device durability and transparency. Their XPS studies of ITO electrodes clearly show the metallic impurities being expelled into the PEO. The change of composition leads to eventual diminution of the ITO conductivity, with concomitant decrease in ECD cycle life. 16.3 Durability of the electrolyte layers The second reason for device failure is electrolyte breakdown. Most organic polymers have relatively poor photolytic stability, particularly when in solution or intimately mixed with an ionic salt, as is typical for ECD usage. 24 Hence long-termsolar irradiation will inevitably cause ECDbreakdown. In an ECD operating in a reflectance mode, such as a mirror, a particularly photounstable primary electrochrome can be placed adjacent to the reflective back electrode rather than situated on the front OTE, i.e. behind the electrolyte and secondary electrochrome layers (provided both have a high optical transparency for all wavelengths). It is quite common for polymeric electrolytes to be ‘filled’ with an opaque white powder such as TiO 2 , to enhance the contrast ratio of the primary electrochrome. While the inclusion of particulate TiO 2 does not affect the response times of an ECD, its photo-activity (particularly if the TiO 2 is in its anatase form) will significantly accelerate photolytic deterioration of organic polymers. 25,26,27 A further danger associated with devices operating via proton conduction is underpotential (catalysed) generation of molecular hydrogen gas, formed according to Eq. (16.1), which both removes protonic charges and also forms insulating bubbles of gas inside the ECD: 2H þ (soln) þ2e À ! H 2 (g). (16.1) Areas of the electrode adjacent to such a bubble are insulated, thereby disabling the device. 16.4 Enhancing the durability of electrochrome layers Great care is needed when the electrolyte in an ECDis a layer of rigid inorganic solid, since most type-III electrochromes change volume during redox changes, owing to chemical volume changes and the volume decrease of a dielectric in a field, electrostriction. Thin-layer WO 3 , for example, expands by about 6% during reduction 28 from H !0 WO 3 to H 1 WO 3 (see pp. 87, 129). Most type-III devices comprise two solid layers of electrochrome. Therefore the extent of chemical-volume change in either layer could be approximately the same, 16.4 Enhancing the durability of electrochrome layers 445 changing in a complementary sense with one expanding while the other contracts; however, electrostriction acts only by contracting. Placing an elastomeric (semi-solid polymer) electrolyte between the two ECD electrochromic layers considerably cushions the strains engendered by expansion and contraction in a two-layer ECD. To confirmthe scope for cushioning the effects of electrostriction, Scrosati and co-workers 29 note how the stresses engendered by ion insertion/egress within the cell WO 3 jelectrolytejNiO are similar in both the WO 3 and NiO layers. (Methods of quantifying the stresses induced during electrochromic activity are discussed on p. 130). Many electrochromes dissolve in, or are damaged by prolonged contact with, the electrolyte layer. To protect the interphase between the electrochrome and electrolyte, several studies suggest depositing a thin, protective film over the electrochrome film. Enhancement of chemical stability will obviously extend the cycle life of an ECD. There are a number of examples of this practice. Thus for example, Haranahalli and Dove 30 deposited a thin semi-transparent layer of gold on their WO 3 , so protecting it from chemical attack, and incidentally also accelerating the speed of device operation. Similarly, in one study, Granqvist and co-workers 31 deposited a thin film of tungsten oxyfluoride on solid WO 3 , and in another, deposited a thin protective layer of electron-bombarded WO 3 onto a layer of metal oxyfluoride. 32,33 Yoo et al. 34 coated WO 3 with lithium phosphorus oxynitride. Deb and co-workers 35 coated V 2 O 5 with a protective layer of LiAlF 4 , which exhibited improved durability and electrochemical charge capacity during 800 write– erase cycles. Long et al. 36 electrodeposited poly(o-phenylenediamine) onto porous MnO 2 . He et al. 37 accelerated the operation of a WO 3 -based device with a surface layer of gold nanoparticles. In the same way, the perfluorinated polymer Nafion 1 has been coated on Prussian blue, 38 tantalum pentoxide 39 and tungsten oxide, 40,41 in each case improving the stability and enhancing the electrochromic characteristics of these electrochromes. While such barrier films protect the electrochrome from chemical degradation, they also hinder the motion of the counter ions needed for charge balance. Movement across the electrolyte–electrochrome interphase will therefore increase the ECD response time. However, the acceleration noted by Haranahalli and Dove 30 and He et al. 37 follows because a potential was applied to the gold layer, itself conductive, covering the respective electrode surfaces. 16.5 Durability of electrochromic devices after assembly Studies describing the durability of assembled electrochromic devices are to be found in the following reports: ‘Durability evaluation of electrochromic 446 Device durability devices – an industry perspective’ by Lampert et al. 42 (in 1999), ‘Failure modes of sol–gel deposited electrochromic devices’ by Bell and Skryabin 43 (in 1999) and ‘A feasibility study of electrochromic windows in vehicles’ by Jaksic and Salahifar 44 (in 2003). Many individual studies of device durability are extant. For example, Nishikitani and co-workers 45 of the Japanese Nippon Mitsubishi Oil Cor- poration employed a variety of weathering tests on electrochromic windows designed for automotive applications. Their two-year outdoor weathering tests suggest their ECDs are highly durable, but as expected, outdoor exposure ultimately causes device degradation. Many workers consider it impractical to wait for results from such trials in real time, so considerable effort has been expended in the use of accelerated testing methods. A few exemplar studies below will suffice. Asahi Glass failed to detect deterioration in their lithium-based ECD windows stored during 1000 hours of testing at 70 8C and 90% humidity. Similarly, Deb and co-workers 46 of the National Renewable Energy Laboratory (NREL) in Colorado, USA, used accelerated testing conditions on several prototype ECDs. Deb and co-workers 47 have also described the way such devices were illuminated with a high-intensity UV lamp to mimic the effects of long-term exposure to solar light, and concluded that the effects of long-term exposure can indeed be mimicked readily within a considerably shortened time – even a few days – with concomitant savings in overheads. However, the applicability of the NREL results is limited since all devices were fabricated by anonymous US companies. Sbar et al. 48 of SAGE Electrochromics in New Jersey, USA, tested electrochromic architectural windows during external exposure at test sites in New Jersey and the Arizona desert. Their accelerated testing methods included electrochemical cycling over a range of temperatures, with changes in illumination and/or humidity. They concluded that their windows showed ‘good switching performance’. Colour Plate 7 shows similar testing of a Gentex window. Skryabin et al. 49 present a more fundamental, partly theoretical, assessment of testing and quality control criteria for large devices. The durability of electrochromic devices was assessed from three perspectives: mimicking the device behaviour with an equivalent circuit; arranging the external electrical connections; and optimizing the switching procedure. Their principal conclusion was that mimicking is difficult: ECDs are ‘inherently complicated devices’. Nagai et al., 6 also using a programme of accelerated testing, concluded that their device, GlassjITOjNiOjTa 2 O 5 (electrolyte)jWO 3 jITOjadhesive-filmjGlass was capable of 10 5 cycles at 608C. 16.5 Durability after assembly 447 Mathew et al. 50 of The Optical Coating Laboratory, in Santa Rosa, USA, consider electrochromic devices for large-area architectural applications, viability requirements for minimumacceptable performance encompassing depth of colour, switching time, chromatism and durability. Within these criteria, windows were deemed acceptable if they coloured to a contrast ratio of 10:1 and were capable of 20 000 cycles. The brief list above demonstrates the way criteria for study can differ considerably: many studies do not even state the criteria chosen. The report ‘Evaluation criteria and test methods for electrochromic windows’ (1990, but made widely available in 1999) by Czanderna and Lampert 51 was compiled to address this problem, and goes some way toward generating a template for reproducible testing of electrochromic devices. These authors elaborate the requirements in a subsequent paper, 47 but excessive use of unexplained abbreviations detracts from clarity. Device durability is then defined in terms of the following five criteria: 47,51 1. The environment for a specific application, which clearly dictates the speed at which the device must operate. 2. The upper and lower temperatures of operation (they chose À40 8C to 50 8C). Device operation was discussed in terms of likely variations in temperature in the USA; rather wider variations around the globe are to be expected. 3. Stresses induced in a device by ‘thermal shock’ as it cools and warms rapidly. In the authors’ Californian climate, no drastic temperature changes occurred during electrochromic operation. They conclude that no major stresses born of thermal shock occur during clear, sunny days, nor when the sky is continually overcast; rapid temperature changes were only observed when the sun appeared from behind a cloud, or was partially obscured; and during thunderstorms. Such variations scarcely cover conditions in other countries, let alone other US states. Holidaymakers in Skegness, UK, for example, would need more assurance of ECD robustness against the weather. 4. The effect of deterioration owing to solar exposure, especially by UV light. The UV was provided by a xenon light source of output 0.55 Wm À2 at 340 nm, a severe test in view of the peak daylight intensity in Miami of 0.8 Wm À2 . 5. The effect of additional stresses such as changes in humidity, and mechanical shock. Devices operating via proton movement may need water; the concentration of water needed for optimum performance needs to remain within a narrow, desirable range, so such devices are sealed to minimise changes in internal humidity levels. A robust seal also protects against oxygen ingress. Device encapsulation is described on p. 424. Strong frames are required for rough handling or percussive incidents. Having noted that variations on the above test methodology will depend on many factors (the choice of electrochrome, device construction, customer specifications, the intended application, and so on), they conclude: 47 448 Device durability Our major conclusions are that substantial R&D is [still] necessary to understand the factors that limit electrochromic windows [ECWs] durability, . . .[but] that it is possible to predict the service lifetime of ECWs. They add, ‘The accelerated tests are reasonable for the evaluation of the lifetime of EC glazing but have not been verified with real time testing.’ 47 References 1. Bernard, M.-C., Hugot-Le Goff, A. and Zeng, W. Elaboration and study of a PANI/PAMPS/WO 3 all solid-state electrochromic device. Electrochim. Acta, 44, 1998, 781–96. 2. Ho, K.-C. Cycling and at-rest stabilities of a complementary electrochromic device based on tungsten oxide and Prussian blue thin films. Electrochim. Acta, 44, 1999, 3227–35. 3. Ho, K.-C. 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Synthesis, characterization and electrochromic properties of NiO x H y thin film prepared by a sol–gel method. Solid State Ionics, 113–15, 1998, 457–63. 21. Shiyanovskaya, I. V. Structure rearrangement and electrochromic properties of amorphous tungsten trioxide films. J. Non-Cryst. Solids, 187, 1995, 420–4. 22. Bressers, P. M. M. C. and Meulenkamp, E. A. Electrochromic behavior of indium tin oxide in propylene carbonate. J. Electrochem. Soc., 145, 1998, 2225–30. 23. Monk, P. M. S. and Man, C. M. Reductive ion insertion into thin-film indium tin oxide (ITO) in aqueous acidic solutions: the effect of leaching of indium from the ITO. J. Mater. Sci., Electron. Mater., 10, 1999, 101–7. 24. Halim Hamid, S. (ed). Handbook of Polymer Degradation, 2nd edn, New York, Marcel Dekker, 2000. 25. Jin, C., Christensen, P. A., Egerton, T. A., Lawson, E. J. and White, J. R. Rapid measurement of polymer photo-degradation by FTIR spectrometry of evolved carbon dioxide. Polymer Degradation and Stability, 91, 2006, 1086–96. 26. Gesenhues, U., Influence of titanium dioxide pigments on the photodegradation of poly(vinyl chloride), Polym. Degrad. Stab., 68, 2000, 185–96. 27. Su, C., Hong, B.-Y. and Tseng, C.-M. Sol–gel preparation and photocatalysis of titanium dioxide. Catal. Today, 96, 2004, 119–26. 28. Green, M. Atom motion in tungsten bronze thin films. Thin Solid Films, 50, 1978, 148–50. 29. Passerini, S., Scrosati, B., Hermann, V., Holmblad, C. and Bartlett, T. Laminated electrochromic windows based on nickel oxide, tungsten oxide, and gel electrolytes. J. Electrochem. Soc., 141, 1994, 1025–8. 30. Haranahalli, A. R. and Dove, D. B. Influence of a thin gold surface layer on the electrochromic behavior of WO 3 films. Appl. Phys. Lett., 36, 1980, 791–3. 31. Azens, A., Hjelm, A., Le Bellac, D., Granqvist, C. G., Barczynska, J., Pentjuss, E., Gabrusenoks, J. and Wills, J. M. Electrochromism of W-oxide-based films: some theoretical and experimental results. Proc. SPIE, 2531, 1995, 92–104. 32. Azens, A. and Granqvist, C. G. Electrochromic films of tungsten oxyfluoride and electron bombarded tungsten oxide. Sol. Energy Mater. Sol. Cells, 44, 1996, 333–40. 33. Azens, A., Granqvist, C. G., Pentjuss, E., Gabrusenoks, J. and Barczynska, J. Electrochromism of fluorinated and electron-bombarded tungsten oxide. J. Appl. Phys., 78, 1995, 1968–74. 34. Yoo, S. J., Lim, J. W. and Sung, Y.-E. Improved electrochromic devices with an inorganic solid electrolyte protective layer. Sol. Energy Mater. Sol. Cells, 90, 2006, 477–84. 35. Lee, S.-H., Cheong, H. M., Liu, P., Tracy, C. E., Pitts, J. R. and Deb, S. K. Improving the durability of ion insertion materials in a liquid electrolyte. Solid State Ionics, 165, 2003, 81–7. 450 Device durability 36. Long, J. W., Rhodes, C. P., Young, A. L. and Rolison, D. R. Ultrathin, protective coatings of poly(o-phenylenediamine) as electrochemical porous gates: making mesoporous MnO 2 nanoarchitectures stable in acid electrolytes. Nano. Lett., 3, 2003, 1155–61. 37. He, T., Ma, Y., Cao, Y., Yang, W. and Yao, J. Enhanced electrochromism of WO 3 thin film by gold nanoparticles. J. Electroanal. Chem., 514, 2001, 129–32. 38. John, S. A. and Ramaraj, R. Role of acidity on the electrochemistry of Prussian blue at plain and Nafion film-coated electrodes. Proc. Ind. Acad. Sci., 107, 1995, 371–83. 39. Sone, Y., Kishimoto, A., Kudo, T. and Ikeda, K. Reversible electrochromic performance of Prussian blue coated with proton conductive Ta 2 O 5 .nH 2 O film. Solid State Ionics, 83, 1996, 135–43. 40. Chen, K. Y. and Tseung, A. C. C. Effect of Nafion dispersion on the stability of Pt/WO 3 electrodes. J. Electrochem. Soc., 143, 1996, 2703–8. 41. Shen, P. K., Huang, H. and Tseung, A. C. C. Improvements in the life of WO 3 electrochromic films. J. Mater. Chem., 2, 1992, 497–9. 42. Lampert, C. M., Agrawal, A., Baertlien, C. and Nagai, J. Durability evaluation of electrochromic devices – an industry perspective. Sol. Energy Mater. Sol. Cells, 56, 1999, 449–63. 43. Bell, J. M. and Skryabin, I. L. Failure modes of sol–gel deposited electrochromic devices. Sol. Energy Mater. Sol. Cells, 56, 1999, 437–48. 44. Jaksic, N. I. and Salahifar, C. A feasibility study of electrochromic windows in vehicles. Sol. Energy Mater. Sol. Cells, 79, 2003, 409–23. 45. Kubo, T., Shinada, T., Kobayashi, Y., Imafuku, H., Toya, T., Akita, S., Nishikitani, Y. and Watanabe, H. Current state of the art for NOC–AGC electrochromic windows for architectural and automotive applications. Solid State Ionics, 165, 2003, 209–16. 46. Tracy, C. E., Zhang, J.-G., Benson, D. K., Czanderna, A. W. and Deb, S. K. Accelerated durability testing of electrochromic windows. Electrochim. Acta, 44, 1999, 3195–202. 47. Czanderna, A. W., Benson, D. K., Jorgensen, G. J., Zhang, J.-G., Tracy, C. E. and Dep, S. K. Durability issues and service lifetime prediction of electrochromic windows for buildings applications. Sol. Energy Mater. Sol. Cells, 56, 1999, 419–36. 48. Sbar, N., Badding, M., Budziak, R., Cortez, K., Laby, L., Michalski, L., Ngo, T., Schulz, S. and Urbanik, K. Progress toward durable, cost effective electrochromic window glazings. Sol. Energy Mater. Sol. Cells, 56, 1999, 321–41. 49. Skryabin, I. L., Evans, G., Frost, D., Vogelman, G. and Bell, J. M. Testing and control issues in large area electrochromic films and devices. Electrochim. Acta, 44, 1999, 3203–9. 50. Mathew, J. G. H., Sapers, S. P., Cumbo, M. J., O’Brien, N. A., Sargent, R. B., Raksha, V. P., Lahaderne, R. B. and Hichwa, B. P. Large area electrochromics for architectural applications. J. Non-Cryst. Solids, 218, 1997, 342–6. 51. Czanderna, A. W. and Lampert, C. M. 1990: SERI/TP-255-3537, as cited in ref. 47. References 451 Index absorbance, optical, 43, 52 change in, 53 shape of bands, 53 accelerated ECD testing humidity, 447, 448 weathering, 447 xenon arc, 448 acetate silicone, ECD encapsulation, 425 acetonitrile, ECD electrolyte, 254, 262, 359, 385, 390T, 419, 438 achromatic, centre of colour diagram, 67 acidity constants K a , 4 acrylic powder, ECD electrolyte thickener, 419 activation energy, 86, 87, 108, 111, 112 in colouring metal oxides, 93 in nickel oxide, 111T in tungsten trioxide, 111T to bacterial growth, 408 to counter-ion movement, 408 to diffusion, 83, 86 to electron transfer, 47 activity, 36–7, 38, 84 coefficient, 36, 40, 96 of pure solids, 38 admittance, 50 AEIROF, 156, 158 agar, as gelling agent, 349, 351 AGFA, 332 air conditioning, ix, 398 Airbus, ECD, windows, 401 AIROF, 155, 156 degradation of, 156 alkoxides CVD precursors, 131 forming molybdenum trioxide, 152 forming titanium dioxide, 184 alloy Inconel-600, oxide mixture on, 203 nickel–aluminium, 200 all-polymer devices, 332 all-solid-state-devices, 417 alpha particles, 160 aluminium–cobalt oxide, 195, 196 aluminium–nickel alloy, 200 aluminium–nickel oxide, 200 aluminium–silicon–cobalt oxide, 204 amino-4-bromoanthaquinone-2-sulfonate, 384 aminonaphthaquinone, 384 amorphisation, tungsten–molybdenum oxide, 193 amorphous, oxides, 88 made by vacuum evaporation, 81 amorphous silicon, 15 as photoconductor, 436 anatase, see titanium dioxide Anderson transition, 81, 99, 142, 149, 307 ANEEPS, 3 aniline, 313 aniline black, 312 aniline–polypyridyl complexes, 256 annealing endothermic process, 140 to effect crystallisation, 88, 89 cerium oxide, 166 cobalt oxide, 168 CVD product, 131 iridium oxide, 158 iron oxide, 173, 175 molybdenum trioxide, 152, 154 nickel oxide, 161 niobium pentoxide, 177 rhodium oxide, 181 spin-coated products, 135 spray pyrolysis product, 135 tungsten trioxide, 88, 140, 141, 148 vanadium pentoxide, 185 anodic coloration, coloration efficiency negative, 55 anodic reactions, definition, 46 antimony pentoxide as electrochromic host, 193 as ECD electrolyte, 421T antimony–copper alloy, substrate, 423 antimony-doped tin oxide, 193–5, 196, 274, 362 optically passive, 193 applications, ECD battery charge indicator, 408, 443 camouflage, 409 452 displays, 401–4 advertising boards, 402 bank notes, 408 cash- and credit cards, 402, 408 display, cashpoint machines, 407 computer screens, 363, 402 data display, 149, 265, 363 electric paint, 364 games, 363 iPod, 363, 402 laptop computer screens, 402 mobile phone screens, 402 NanoChromics, 347, 362, 363, 402, 406 optical data storage, 265 palmtop computer screens, 402 smart cards, 363 tickets, 408 tokens, 408 toys, 363, 436 transport terminus screens, 402 vouchers, 408 watch faces, 149, 402, 443 electrochromic paper, 363, 405–6 eye wear, goggles, 398, 422 motorcycle helmets, 398 sunglasses, 401, 422 visors, 401, 422 fibre-optics, 265 light modulation, 404–5 medicine, 265 mirrors, ix, 11, 44, 149, 307, 356, 363, 385–7, 395–7 optical attenuator, 270 shutters, 363, 404–5 solar-energy storage, 265, 266 temperature management, 265 windows, 149, 200, 363, 422 Airbus, 401 aircraft, 400–1 Asahi Glass, 400 Boeing ‘Dreamliner’, 400 car sun roof, 398 chromogenic glazing, 397 dimmable laminates, 363 Flabeg Gmbh, 400 Gentex, 396, 398, 400 optical attenuator, 270 Pilkington Glass, 400 PPG Aerospace, 400 Schott Glass, 400 shutters, 363 Stadsparkasse Bank, 400 X-ray reflector, 397 Aramid resin, 198 aromatic amines, 374–6, 377T–378T charge transfer, 374 contrast ratio, 376 near infrared absorption, 376 response time, 375 type-I electrochromism, 375 type-II electrochromism, 375 Arrhenius equation, 83, 408 aryl viologens, 11, 28 Asahi Glass, 400, 447 asymmetric viologens, 355, 360 automotive mirrors, see Applications, ECD mirror azulene, 313 Azure A, coloration efficiency, 57T Azure B, coloration efficiency, 57T back potential, 92, 93, 98, 102, 105, 106, 110–11, 115 bacteria growth, activation energy, 408 reactions, 4 Bacteriorhodopsin, 3 band conduction, 81 band structure, poly(thiophene)s, 152 bandgap, 316 of PEDOT, 322 Basic Blue 3, coloration efficiency, 57T batteries, 14, 54, 167 dry cell, 408 ECD charge indicator, 408 ECD like a secondary, 54 photo-chargeable, of Prussian blue and tungsten trioxide, 437 rechargeable, manganese oxide, 176 Bayer AG, 323 Baytron M, 323 Baytron P, 323 beam direction, photoelectrochromism, 433 BEDOT, 326 BEDOT-NMeCz, 326 Beer–Lambert law, 53, 55, 146, 147, 148, 150, 151, 176 Bell Laboratories, 29 benzoquinones benzoquinone, o-, 381 benzoquinone, p-, 381, 382 benzyl viologen, 8, 344T, 346, 352T, 356, 358 di-reduced, 358 radical, recrystallisation, 357 Berlin green, see Prussian green betaines, 5 biological membrane potentials, 3 biphenyls, 379, 380 bipolaron in poly(thiophene)s, 320 in tungsten trioxide, 147 bis(dimethylamino)diphenylamine, 4,4 0 -, 384 bismuth oxide, 166 coloration efficiency, 56T, 166 formation via evaporation, 166; rf sputtering, 166 response time, 166 bismuth as secondary electrochrome, 444T electrodeposition of, 8, 27, 304, 305–6 coloration efficiency, 306 cycle life, 305 ECD, 306 Index 453 bismuth (cont.) electrochemistry, 304, 305 electron mediation, 305 bithiophenes, 320 conducting polymers, 316 bleaching chemical, viologens, 359 models, 105–8 Faughnan and Crandall, 105–8 Green, 108, 109 rate, 33 electrochromes, for nickel oxide, 164; for vanadium pentoxide, 188 potentiostatic, 105–8 self, 15, 54, 150, 153 types type-II electrochromes, 79–115 type-III electrochromes, 79–115 blueprints, Prussian blue, 26, 405 Boeing ‘Dreamliner’, ECD, windows, 400 brightness, and colour analysis, 64 British Fenestration Rating Council, 397 bromoanil, o-, 382, 383 solubility product, 383 bronze, 82, 103 of lithiumtungsten trioxide, electro-irreversibility of, 82 of metal oxide, 61, 81, 82, 103 of molybdenum trioxide, 103, 151 of sodium tungsten trioxide, 27 of tungsten trioxide, 81, 113, 144 Butler–Volmer equation, 29, 42, 46–8, 95 butyl viologen, 352T g-butyrolactone, ECD electrolyte, 167, 186, 303, 304, 362, 419 cadmium sulfide, 437 calomel reference electrode, see saturated calomel electrode camouflage, ECD application, 409 capacitance effects, 11, 50 electrolytic capacitors, 52 car mirrors, see applications, ECD, mirrors car sun roof, ECD application, 398 carbazoles, 313, 376, 379T immobilised, 391 N-carbazylcarbazole, 379T N-ethylcarbazole, 379T N-phenylcarbazole, 379T, 381T type-II electrochromes, 376 carbon electrochromes, ‘carbon based’, 303, 305 screen printed carbon, 303, 305 see also diamond, fullerene and graphite carbon, substrate, 424 castable films, poly(aniline), 332–3 catalytic silver paint, depositing Prussian blue, 283 catechole, 271, 272 cathode ray tube power consumption, 15 television, 402, 403 cathodic coloration, coloration efficiency positive, 55 cathodic-arc deposition, of vanadium pentoxide, 185 cathodic, definition, 46 CE, see coloration efficiency cells, 34 aqueous, 37–9 electrochemical, 417 electroneutrality in, 38 cellulose acetate, composite with poly(aniline), 333 cerianite, 166 cerium oxide, 166–7, 194 annealing of, 166 chemical diffusion coefficient, 85T electrochemistry of, 167 electrochromic host, 193–5 formation via dip coating, 135 physical vapour deposition, 166 spin coating, 135 spray pyrolysis, 135, 166 optical properties, 166 optically passive, 166 cerium vanadate, 202 cerium–nickel oxide, 200 cerium–praseodymium oxide, 179 cerium–tin oxide, 201 cerium–titanium oxide, 194 as secondary electrochrome, 444T chemical diffusion coefficient, 194 coloration efficiency, 194 EXAFS, of, 194 optically passive, 194 via dc magnetron sputtering, 194, 195 cerium–titanium–titanium oxide, 203 cerium–titanium–zirconium oxide, 203 cerium–tungsten oxide, see tungsten–cerium oxide cerium–vanadium–titanium oxide, 203 cerium–zirconium oxide, 203 cerous ion, as electron mediator, 359 characteristic time, in Faughnan and Crandall model of coloration, 95, 111 charge electronic, 42 faradaic, 52 charge capacity, 200 charge density, 55 charge dispersibility, 127 charge transfer, 42 aromatic amines, 374 complexation of cyanophenyl paraquat, 60, 359 ferrocyanide, 359 heptyl viologen, 359 methyl viologen, 359 viologens, 342–5, 353, 359 intervalence, 60–1, 127, 145 orbitals, 61 oxides and cobalt ion, 169 in iron–titanium oxide, 202 454 Index in oxide ion, 168 in permanganate, 60 in tungsten trioxide, 60 rate of, 95 resistance to, 105 charging, double-layer, 52 chemical diffusion coefficient, 46, 84T, 87, 88, 90, 96, 101, 102, 112, 190, 195T and diffusion coefficient, 84 and insertion coefficient, 90–1 definition of, 84 electrode reactions when, 47 ions through oxides, 85T cobalt oxide, 195T ions through WO 3 , 87, 88 Li þ in Li x WO 3 , 91 molybdenum trioxide, 153 nickel oxide, 85T niobium pentoxide, 85T titanium dioxide, 184 tungsten trioxide, 84T, 85T, 101, 195T vanadium pentoxide, 85T ions through oxide mixtures cerium–titanium oxide, 194 cobalt–tungsten oxide, 195, 195T indium–tin oxide, 197 tungsten–cobalt oxide, 195T ions through phthalocyanines lutetium phthalocyanine, 85T zinc phthalocyanine, 85T ions through conducting polymers poly(carbazole), 85T poly(isothianaphene), 85T chemical potential, of H þ in WO 3 , 93, 94 chemical tethering, write–erase efficiency, 346 chemical vapour deposition annealing needed, 131 of metal oxides, 131–2 iron oxide, 174 molybdenum trioxide, 151, 397 nickel oxide, 161 praseodymium oxide, 178, 179 tantalum oxide, 182 tungsten trioxide, 141, 148, 150, 397 of mixtures of metal oxide, tungsten–molybdenum oxide, 397 precursors alkoxides, 131 hexacarbonyls, 131, 135–6, 397 products impure, 132 process is two-step, 131 chemically modified electrode, see derivatised electrodes chloranil, 382 o-, 382, 383 cycle life, 383 p-, 382 chloride ion, gasochromic, sensor for, 406 Chroma meter, 62 and colour analysis, 63, 64–71 chromatic colour, and colour analysis, 62, 64 chromium oxide, 167 and batteries, 167 coloration efficiency, 167 electrochemistry, 167 formation via electron-beam evaporation, 167 rf sputtering, 167 gasochromic, 407T terminal effect suppressor, 423 chromium phthalocyanine, 261 chromium–iron–nickel oxide, 203 chromium–molybdenum oxide, 199 chromium–nickel oxide, 200 chromogenic glazing, 397; see also ECD, windows chromophore, definition, 2 chronoabsorptometry, 57 chronoamperometry, 83 peaks, 99–101 chronocoulometry, 57, 59 CIE, see Commission internationale de l’eclairage circuit element, 50 clusters, c-WO 3 in a-WO 3 , 88 cobalt acetylacetonate complex, 168 cobalt hydroxide, 169 cobalt oxide, 167–70, 195 annealing, 168 charge transfer in, 169 chemical diffusion coefficient, 195T coloration efficiency, 56T, 169, 172T ECDs of, 170 electrochemistry of, 168–9 electrochromic host, 195–6 incorporating gold, 204T formation via CVD, 172T dip coating, 168 electrodeposition, 132, 172T evaporation, 172T oxidation of cobalt, 168, 169 peroxo species, 168 rf sputtering, 167 sol–gel, 135, 168, 172T, 195 sonication, 133, 134 spin coating, 135 spray pyrolysis, 135, 168, 169, 172T gasochromic applications, 406 lithium deficient, 167 optical properties, 168, 169–70 secondary electrochrome, 170 cobalt oxyhydroxide, 81, 168 formation via electrodeposition, 168 cobalt phthalocyanine, 261 cobalt tartrate complex, 168 cobalt–aluminium oxide, 195, 196 coloration efficiency, 195 via sol–gel, 195 cobalt–aluminium–silicon oxide, 195, 204 cobalt–nickel–iridium oxide, 203 Index 455 cobalt–tungsten oxide, 195 chemical diffusion coefficient, 195, 195T colloid, via sol–gel, 134 coloration, 2 and colour analysis, 66 extrinsic, 52–3 after potential stopped, 114 chemical, 101–2 galvanostatic 96–8, 104, 417 iridium oxide, 157 phase changes in, 157 metal oxides involves counter ions, 80 involves ionisation of water, 81 potentiostatic, 99, 104, 358, 417 three-electrode, 443 potential step, 354–5 pulsed, 87, 365 pulsed current, titanium dioxide, 184 tailoring, 334 tungsten trioxide, 80 hysteresis, 143 involves water, 80 two-electron process, 103 type-II electrochromes, 79–115 coloration efficiency, 10, 15, 16, 42, 54–60, 88, 139 and conjugation length, 60 and extinction coefficient, 55 anodic coloration, Z is negative, 55 cathodic coloration, Z is positive, 55 composite CCE, 55, 57–60 definition, 15 intrinsic, 54–60 metal hexacyanoferrates Prussian blue, 59T Prussian white, 59T metal hydrides magnesium–samarium hydride, 308 samarium–magnesium hydride, 308 metal oxides, 56T bismuth oxide, 56T, 166 chromium oxide, 167 cobalt oxide, 56T, 169, 172T copper oxide, 172 iridium oxide, 56T, 70, 158 iron oxide, 56T, 70, 175, 175T, 201T manganese oxide, 176 molybdenum trioxide, 56T, 154, 155T, 199, 199T nickel oxide, 56T, 70, 165T niobium pentoxide, 56T, 178, 181T, 199T–201T, 200 niobium pentoxide, mixtures, 201 rhodium oxide, 56T, 181 tantalum oxide, 56T, 183 titanium dioxide, 56T, 184, 185T tungsten trioxide, 56T, 146, 147, 148, 148T, 191, 193, 201 vanadium pentoxide, 56T, 189, 190T metal oxyfluorides titanium oxyfluoride, 205 tungsten oxyfluoride, 205 metals, bismuth, 306 mixtures of metal oxide cerium–titanium oxide, 194 cobalt–aluminium oxide, 195 indium–tin oxide, 197, 199, 199T iron oxide, mixtures, 198 iron–niobium oxide, 201T molybdenum–tin oxide, 199T–201T nickel–titanium oxide, 202 nickel–tungsten oxide, 200 niobium–iron oxide, 201T niobium–tungsten oxide, 201 samarium–vanadium oxide, 202 titanium–molybdenum oxide, 199 tungsten–niobium oxide, 201 tungsten–molybdenum oxide, 56T, 192 tungsten–vanadium oxide, 202 vanadium–samarium oxide, 202 zirconium–tantalum oxide, 203 organic dyes Azure A, 57T Azure B, 57T Basic Blue ix, 57T Indigo Blue, 57T Methylene Blue, 57T Nile Blue, 57T Resazurin, 57T Resorufin, 57T Safranin O, 57T Toluylene Red, 57T organic electrochromes, 57T cyanines, 378 fullerene, 304, 305–6 organic polymers PEDOT, 59T, 437 poly(3,4-ethylenedioxy thiophenedidodecyloxybenzene), 57T poly(3,4-propylenedioxypyrrole), 57T poly(3,4-propylenedioxythiophene), 57T phthalocyanines lutetium phthalocyanine, 260 quantum mechanical, 55 sign of, 55 viologens, 349, 361, 362, 363; methyl viologen, 57T coloration models Bohnke, 101–2, 113, 115 Faughnan and Crandall, 91–6, 99, 102, 110, 111, 113, 115 Green, 96–8, 102, 113, 115 Ingram, Duffy, Monk, 99–101, 102, 113, 115 W IV and W V , 102–3 coloration rate, 33, 139, 149 and flux, 75 mixing oxides, enhances rate, 200 nickel oxide, 163 vanadium pentoxide, 188 colorimetric theory, 62 colour analysis, 62–71 and light sources, 64 456 Index conducting polymers, 62 Prussian blue, 62, 70 colour diagram, achromatic centre, 67 colour formed, amount of, 53 colour manipulation, metal-oxide mixtures, 190 colour space, 63, 64–71 colour tailoring, 399 combinatorial chemistry, 409 Commission internationale de l’eclairage (CIE), 62, 63, 334 complementarity, during cell operation, 41 complementary electrochromism, 290 complexes, see, charge-transfer complexation; coordination complexes composite coloration efficiency CCE, 55, 57–60 determined at wavelength maximum, 59 determined with reflected light, 57 composites, conducting polymer, 332–3 comproportionation tungsten trioxide, 103 viologens, 357–8, 365 computer screen, ECD applications, 363 concentration gradient, 45, 51, 93, 97, 98, 104, 110, 112, 114, 115, 303, 305 conducting polymers, 9, 62, 80, 312–34 and electroluminescent organic light-emitting diodes, 312 and field-effect transistors, 312 and sensors, 312 and solar-energy conversion, 312 colour analysis of, 62 composites, 332–3 electrochromic, 57, 60 high resistance, 11 history, 312 oxidative polymerisation, 313–14 p-doping, 315 type-III electrochromes, 317 conductivity, electronic, 113 phthalocyanine complexes, 263 silver paint, 349 through bands, 81, 127, 147 conductivity indium–tin oxide, 422 ionic, metal oxides, 89 M x WO 3 , 81, 113, 142 protons in tantalum oxide, 181, 183 of amorphous and polycrystalline WO 3 , 82 conjugation length, 314 and coloration efficiency, 60 construction, ECD, 417 contact lithography, 258 contrast ratio, 9, 14, 104, 146, 156, 189, 197, 333, 346, 348, 349, 352, 376, 384, 385, 388, 400 all-polymer ECD, 332 and electrolyte fillers, 445 convection, 43, 44, 75, 76 absent in solid-state ECDs, 44 coordination complexes, 253 intervalence charge transfer, 253 metal-to-ligand charge transfer, 253 copper ethoxide, 170 copper hexacyanoferrate, 294 copper oxide, 170–2 as secondary electrochrome, 165 coloration efficiency, 172 electrochemistry, 172 formation via copper ethoxide, 170 electrodeposition, 171 electron mediator, 305, 306 sol–gel, 170 specular reflectance, 407T Cottrell equation, 76, 77, 354 Coulomb’s law, 102 counter electrode, 41, 48 electrochromic, see secondary electrochrome counter ion activation energy, 408 movement, 82–5, 188 during coloration of metal oxides, 80 rate of, 33 size, 87–8 swapping of, 87 through solid film, 86 viologens, effect of, 352–4 Ag þ through tungsten trioxide, 142, 146 CN – through iridium oxide, 157 Cs þ through tungsten trioxide, 146 deuterons through tungsten trioxide, 87 F – through iridium oxide, 157 K þ through iron oxide, 174 iron oxide mixtures, 198 tin oxide, 205 tungsten trioxide, 142, 146 Li þ through cerium oxide, 167 cerium–titanium oxide, 194 cobalt oxide, 169 fullerene, 303, 304–5 graphite, 303, 304 iron oxide, 173, 174 iron oxide mixtures, 198 ITO, 197 manganese oxide, 176 molybdenum trioxide, 152 nickel oxide, 163 niobium pentoxide, 178 praseodymium oxide, 179 tin oxide, 197, 205 titanium dioxide, 184 tungsten trioxide, 88, 89, 90, 96, 113, 130, 142, 146, 148, 150, 151, 410, 419 vanadium pentoxide, 186, 188 Mg 2 þ through molybdenum oxide, 152 tungsten trioxide, 146 Na þ through iron oxide, 174 iron oxide mixtures, 198 tin oxide, 205 Index 457 counter ion (cont.) tungsten trioxide, 87, 90, 109, 113, 130, 142, 146, 147 vanadium pentoxide, 186 OH – through anodic oxides, 87 cobalt oxide, 195, 195T lutetium phthalocyanine, 259 CR, see contrast ratio critical micelle concentration, heptyl viologen, 355 CRT, see cathode ray tube crystal lattice changes during coloration, 86–7 motion of is rate limiting, 87 stresses in, 130 crystal violet, 376 crystallisation, by annealing, 89 CT, see charge transfer current, 38, 41 as rate, 38 coloration, 93 definition, 42, 77 depends on rates, 75 faradaic, 45, 76 leakage, 52 limiting, 76 non-faradaic, 76 parasitic, 52 CVD, see chemical vapour deposition cyanines, 376 coloration efficiency, 378 electrochromic, 60 merocyanines, 376 spiropyrans, 376 squarylium, 379 cyanophenyl paraquat, 8, 28, 60, 344T, 349, 350, 351, 352, 356, 358 charge transfer complexation, 359 diffusion coefficient, 77T optical charge transfer in, 60 cyanotype photography, of Prussian blue, 26 cycle life, 12–13, 172, 178, 179, 188, 197, 205, 269, 294, 303, 304, 305, 308, 362, 383, 389, 443, 444T, 447 and kinetics, 11 deep and shallow cycles, 12, 443 enhanced by mixing oxides, 200 measurement of, 12 cyclic voltammetry, 48–50, 83 of conducting polymers, poly(aniline), 333 of metal hexacyanoferrates copper hexacyanoferrate, 294 Prussian blue, 286, 287 of metal oxides iridium oxide, 156 niobium pentoxide, 178 rhodium oxide, 181 tungsten trioxide, 93 vanadium pentoxide, 187 of viologens, 352, 355, 356, 357, 359 schematic, 48 cyclodextrin, beta, 351, 359 Darken relation, 85 data display, ECD applications, 149, 363 dc magnetron sputtering, 136 of mixtures of metal oxide cerium–titanium oxide, 194, 195 indium–tin oxide, 136 titanium–cerium oxide, 136 tungsten–cerium oxide, 136 of metal oxides molybdenum trioxide, 136, 141, 151 nickel oxide, 136 niobium pentoxide, 136, 177 praseodymium oxide, 136, 178 tantalum oxide, 136, 182 tungsten trioxide, 136 vanadium pentoxide, 136, 185 of metal oxyfluorides titanium oxyfluoride, 205 tungsten oxyfluoride, 205 onto ITO, 136 DDTP, 326 decomposition, of electrochrome, 49 deep cycles, cycle life, 443 defect sites, 103, 127, 146 DEG, see diethylene glycol degradation, 443 acid, sulfuric, 420 aquatic, 89 mechanical stresses, 397 caused by ion movement, 13 fullerene electrochromes, 303, 305 indium–tin oxide, 423, 444–5 lutetium phthalocyanine, 260 metal oxides, photolytic, 54, 125 molybdenum trioxide, 153 nickel oxide, 163 tungsten trioxide, 149, 150 via Cl – ion, 150; yields tungstate, 89 vanadium pentoxide in acid, 186 viologens, 351, 357 DEMO 2005 show, 402 deposition in vacuo, 137–8 depth profiling, 89 derivatised electrodes, 7 definition, 12 pyrazolines, 387–8 contrast ratio, 388 ECD, 388 response times, 387 TCNQ species, 388–9 reversibility, 389 write–erase efficiency, 388 TTF species, 387, 389–90 cycle life, 389 ion hopping, 390 ion tunnelling, 390 viologen ECDs, 346–8, 361 desolvation, during ion insertion, 89 458 Index deuteron, motion through WO 3 , 87 diacetylbenzene, p-, 77, 78 immobilised, 390T, 391T diamond electrochromes, 303, 305 absorption in near infrared, 399 dielectric properties, 50 diethyl terephthalate, immobilised, 391T diethylene glycol, 331 diffusion, 43, 111, 386 activation energy for, 83 energetics of, 112 fast track, 98 length, 45, 101, 403 linear, 76 of electrochromes, 12 diffusion and migration, concurrent, 83 diffusion coefficient, 44, 45, 48, 77, 77T, 83, 90–1, 96, 97, 101, 102, 112, 403, 407T and chemical diffusion coefficient, 84 and oxygen deficiency, 103 includes migration effects, 83 solution-phase species cyanophenyl paraquat, 77T ferric ion, 77T methyl viologen, 77T diffusion rate, 33 digital video disc, 408 dihedral angle, 314, 320 poly(thiophene)s, 323T dihydro viologen, see viologen, doubly reduced dimer, of W V –W V , 103, 145, 147 dimethoxyphenanthrene, 2,7-, 380 dimethylterephthalate, immobilised, 391T dimmable window laminates, ECD applications, 363 dinuclear ruthenium complexes, mixed-valency, 268T near infrared electrochromism, 268T diode-array spectroscopy, 355 dioxypyrrole, 327–8 dip coating of metal oxides, 135 cerium oxide, 135 cobalt oxide, 168 iridium oxide, 135 iron oxide, 135 nickel oxide, 135, 161 niobium pentoxide, 135 tantalum oxide, 182 titanium dioxide, 135, 184 tungsten trioxide, 135, 141 vanadium pentoxide, 135 of mixed metal oxides, 135 iron–titanium oxide, 202 titanium–iron oxide, 201, 202 substrates, ITO, 135 directed assembly, ECD, 157 of Prussian blue, 285 di-reduced, viologens, 343, 357, 358 ethyl viologen, 358 heptyl viologen, 358 methyl viologen, 358 displays, see applications, ECD dissolution, of WO 3 , 89 dithiolene complexes, 266–7 DMF, as ECD electrolyte, 254, 419 DMSO, as ECD electrolyte, 150, 157, 261 dodecylsulfonate, within poly(pyrrole), 333 dominant wavelength, and colour analysis, 62 Donnelly mirror, 11 as sunglasses, 401 double insertion, of ions and electrons, 138 double potential step and cycle life, 12 double-layer, charging, 11, 52 Dreamliner, Boeing, windows, 400 Drude theory, 101, 142 Drude–Zener theory, 142 dry-cell, battery, 408 dry lithiation, 418 of tungsten trioxide, 418 dual insertion, of ions and electrons, 138 during coloration and bleaching, 83 DuPont, 425 durability, 443–9 accelerated tests humidity, 447, 448 weathering, 447 xenon arc, 448 of ECD electrolyte, 445 of ECDs during pulsing, 104 of substrates, 444–5 Duracell, 408 DVD, see digital video disc dyes, encapsulated within poly(aniline), 333 dynamic electrochemistry, 46–8 dysprosium–vanadium pentoxide, 202 E (cell) , see emf ECD, 53, 60, 76, 106, 108, 112 all polymer, 330, 331–2 applications, see applications, ECD assembly, 157, 417 directed assembly, 157 dual organic–inorganic, 333–4 durability, 443–9 electrodeposited bismuth, 306 electrodes, 52, 419–24 electrochromes conducting polymers, PEDOT, 409 poly(aniline)s, 330, 331 poly(pyrrole)s, 328 inorganic electrochromes, oxo-molybdenum complexes, 269 metal hexacyanoferrates, Prussian blue, 289–91 metal hydrides, lanthanide hydride, 308 metal oxides cobalt oxide, 170 iridium oxide, 159 manganese oxide, 176 molybdenum trioxide, 154–5, 397 nickel oxide, 164–5, 397 Index 459 ECD (cont.) niobium pentoxide, 178 of tungsten trioxide, 61, 82, 87, 104, 139, 397, 399, 402, 408, 409, 410 vanadium pentoxide, 189–90 mixtures of metal oxide indium–tin oxide, 197 tungsten–molybdenum oxide, 397 phthalocyanine complexes, 263 lutetium phthalocyanine, 259, 260 organic pyrazolines, 388 quinones, 384 thiazines, 385 viologens, 346–8, 349, 352, 357, 362, 385 heptyl viologen, 360 viologens, paper quality, 362 electrolytes acetonitrile, 254, 262, 359, 385, 390T, 419, 438 antimony pentoxide, 421T DMF, 254, 419 DMSO, 150, 157, 261 ethylene glycol, 259 fillers (titanium dioxide), 421 g-butyrolactone, 167, 186, 303, 304, 362, 419 gelled, 106, 305, 350, 384 hydrogen uranyl phosphate, 421T inorganic, 420 lead fluoride, 159 lead tetrafluorostannate, 159 lithium niobate, 421T lithium pentafluoroarsenate, 150 lithium perchlorate, 82, 150, 151, 152, 163, 166, 167, 169, 173, 176, 184, 186, 188, 197, 199, 205, 362, 408, 421 lithium phosphorous oxynitride, 363 lithium tetrafluoroaluminate, 150, 152, 421T, 436 Nafion, 421T organic, 420–2 perchloric acid, 150, 157 phosphoric acid, 167, 421, 438 polyelectrolytes, 420–2 Nafion, 421T poly(AMPS), 150, 260, 348, 366, 391, 391T, 395–410, 420 polymer electrolytes, 421–2 poly(acrylic acid), 150, 421T poly(ethylene oxide), 150, 290, 408, 421, 438 poly(methyl methacrylate), 291, 334 poly(propylene glycol), 421 poly(vinyl alcohol), 421 poly(1-vinyl-2-pyrrolidone-co-N,N 0 - methylenebisacrylamide), 391 potassium chloride, 291, 349 potassium hydroxide, 308 potassium triflate, 290 propylene carbonate, 151, 152, 166, 169, 173, 176, 184, 186, 187, 188, 197, 199, 205, 356, 384, 419 solid, 96 stibdic acid polymer, 421T sulfuric acid, 82, 86, 149, 178, 259, 349, 409, 420 tantalum oxide, 150, 420, 421T thickeners, 419 acrylic powder, 419 poly(ethylene oxide), 419 poly(vinylbutyral), 419 silica, 419 tin phosphate, 154 titanium dioxide, 421T triflic acid, 150, 421 viscosity, 417 whiteners, 159, 384, 418, 422, 424 zinc iodide, 408 zirconium dioxide, 421T encapsulation, 424–5, 448 Surlyn, 425 first patents, 27 flexible, 129, 423 illumination of, 417 large-area, 141, 332, 447 memory effect, 15, 53–4, 152, 153, 403 sealing, 362 self bleaching, 15, 54, 150, 153 and memory effect, 15, 53–4, 152, 153, 403 radical annihilation, 386 type-I electrochromes, 77 substrates, 422–4 trichromic, 384 ultra fast, viologens, 363 EDAX, Prussian blue, 288 EDOT, 323, 325, 326 polymers of, 325 EIC laboratories, 409 Einstein transition probability, 147 electric field, 43, 44, 138 electric paint, ECD application, 364 electroactive material, definition, 1 electroactive polymers, 9 electrochemical cells, 417 electrochemical formation of colour, 52–3 electrochemical impedance spectroscopy (EIS), see impedance electrochemical quartz-crystal microbalance (EQCM), 88, 89, 90, 130, 142, 163, 284, 288, 289, 330, 331 electrochemical titration, 104 electrochemistry, 11 dynamic, 46–8 equilibrium, 34–9 electrochromes electrodeposition, bismuth, 304, 305 hexacyanoferrates, Prussian blue, 285–9 metal oxides, 138 cerium oxide, 167 chromium oxide, 167 cobalt oxide, 168–9 copper oxide, 172 iridium oxide, 157 iron oxide, 173 460 Index manganese oxide, 175–6 molybdenum trioxide, 152–3 nickel oxide, 161–3 niobium pentoxide, 177–8 palladium oxide, 178–9 praseodymium oxide, 178 rhodium oxide, 180 ruthenium oxide, 181 tantalum oxide, 183 titanium dioxide, 184 tungsten trioxide, 142 vanadium pentoxide, 186–8 metal oxyfluorides titanium oxyfluoride, 205 tungsten oxyfluoride, 206 mixtures of metal oxide indium–tin oxide, 196–7 phthalocyanines, 262 lutetium phthalocyanine, 260 polymers, 60 poly(aniline), 329–30, 331 thermodynamics of, 34–9 viologens, 342, 353, 354–5 electrochromes changes in film thickness, 51 colours of, 2 decomposition, 49 laboratory examples of, 3 memory effect, 15, 53–4, 152, 153, 403 metal-oxide systems and insertion coefficient, 61 metal-oxide systems, intervalence of, 61 photodegradation of, 54 type, 7–9 electrochromic colours counter electrode, see secondary electrochrome device, see ECD electrodes, 40–1 extrinsic intensity of, 52–3 intensity of, 3 electrochromic hosts metal oxides, 190–206 antimony oxide, 193 cerium oxide, 193–5 cobalt oxide, 195–6 indium oxides, 196–7 iridium oxide, 198 iron oxide, 198–9 molybdenum oxide, 199 nickel oxide, 200 niobium pentoxide, 200–1 titanium dioxide, 201–2 tungsten trioxide, 191–3, 407 vanadium pentoxide, 202 zirconium oxide, 203 polymers, Nafion as, 405 electrochromic modulation, 3, 53 electrochromic paper, ECD application, 405–6 electrochromic probes, 3 electrochromic–photochromic systems, 438–9 electrochromism chemical, 3 complementary, 290 definitions, x, 1, 3 fax transmissions, 26 first use of term, 25 history of, 25–30 ligand-based, 255 near infrared, 165, 183, 253, 254, 265–74 electrode as conductor, 37 ECD, 422–4 interphase, 43 kinetics, 46–8 potential, 35, 39, 48, 75, 91, 93, 104 reactions, 52 reactions, under diffusion control, 47 substrate, see entries listed under substrate electrodeposition of metals, type-II electrochromes, 303, 305 electrodeposition forming hexacyanoferrates Prussian blue, 283, 284 forming metals, 303, 305–7 bismuth, 27, 304, 305–6 lead, 306–7 silver, 27, 307 forming metal oxides, 132–4 cobalt oxide, 132 copper oxide, 171 iron oxide, 173 manganese oxide, 175 molybdenum trioxide, 151, 152 nickel oxide, 132, 160–1 oxide mixtures, 133 ruthenium oxide, 181 tungsten trioxide, 140, 141 vanadium pentoxide, 186 forming mixtures of metal oxide molybdenum–tungsten oxide, 199 nickel–titanium oxide, 201 titanium–tungsten oxide, 202 tungsten–molybdenum oxide, 199 forming oxyhydroxides cobalt oxyhydroxide, 168 nickel oxyhydroxide, 161 nitrate forming metal hydroxide, 132 forming viologen radicals, 354 potentiostatic, 133 precursors, peroxo species, 133 yields oxyhydroxide, 132 electrokinetic colloids, 5 electroless deposition, of Prussian blue, 283 electroluminescent organic light-emitting diodes, conducting polymers, 312 electrolyte fillers to enhance contrast ratio, 445 electrolyte, ECD chemical systems, see ECD, electrolyte dissolves ITO, 444 durability, 445 failure of, 443 Index 461 electrolyte, ECD (cont.) fillers, 445 organic polymers, 445 photochemical stability, 445 semi-solid, 446 electrolytic capacitor, 52 electrolytic side reactions, 43 electrolytic writing paper, 405 electron conduction, through bands, 81 electron donors, photochromism, 438 electron hopping, 81, 99 electron mediation bismuth electrodeposition, 305 mediators cerous ion as, 359 copper as, 305, 306 ferrocene as, 359 ferrocyanide as, 342, 350, 358, 359 ferrous ion as, 359 hydroquinone as, 359 electron mobility, tungsten–molybdenum oxide, 192 electron transfer energy barrier to, 42–3, 47 fast, 102 rate of, 33, 34, 42–3, 46, 75 standard rate constant of, 47 electron-beamevaporation, of chromiumoxide, 167 electron-beam sputtering forming metal oxides manganese oxide, 137, 175 molybdenum trioxide, 137 vanadium pentoxide, 138–206 forming metal oxide mixtures indium–tin oxide, 137, 196 electroneutrality, need for, 8 electronic bands, 127 electronic charge, 42 electronic conductivity, 42, 113 in metal oxides nickel oxide, 162 tungsten trioxide, 99 in metal oxide mixtures indium–tin oxide, 445 in phthalocyanine complexes, 259 in polymers poly(acetylene), 312 poly(aniline), 101 rate of, 42 electronic motion, 81–2 electronic paper, ECD applications, 363 electron–ion pair, see redox pair electron-transfer rate, viologens, 359 electron-transfer reaction, 75 electrophotography, of tungsten trioxide, 28 electropolychromism, 17–18 graphite electrochromes, 303, 304 poly(aniline), 329, 331 Prussian blue, 287 seven-colours, 254 polypyridyl complexes, 255–6 quinones, 384 viologens, 365 electroreduction, of ITO, 444 electroreversibility poor, indium–tin oxide, 197 electrostriction, 51, 129, 445 definition, 87 of iridium oxide, 130 of nickel oxide, 130 of tungsten trioxide, 87, 129, 445 of vanadium pentoxide, 87, 129 element, circuit, 50 ellipsometry, 17, 50–1, 81, 109–10 and film thickness, 50 and interfaces, 50 in situ, 51 of iridium oxide, 157 of molybdenum trioxide, 109, 153 of phthalocyanine complexes, 263 of Prussian blue, 284, 287, 288, 289 of titanium dioxide, 184 of tungsten trioxide, 81, 109, 143 of vanadium pentoxide, 109, 187 emeraldine, 329, 331 emf, 33, 34, 39–40, 94, 95, 104, 143 encapsulation, ECD, 424–5, 448 energetics, 86 of ion movement through solid oxides, 89–90 energy barrier, 93, 95 to electron transfer, 47 enhancement factor W, 83, 84 entropy, 88 environmentalism, 398 epoxy resin, 362 equilibrium potential, 35, 41 equivalent circuit, 447 equivalent circuit, impedance, 447 erbium laser, 267 ESCA, 103 ESR of methyl viologen, 356 of molybdenum trioxide, 153 of tungsten trioxide, 145 of viologens, 352, 356 ethyl viologen, 344T, 352T, 438 di-reduced, 358 ethylanthraquinone, 2-, 384 N-ethylcarbazole, carbazoles, 379T, 381T ethylene glycol, ECD electrolyte, 259 ethylenedioxythiophene, 3,4-, 313, 321–7 evaporated metal-oxide films, water in, 89 evaporation, vacuum of bismuth oxide, 166 of metal-oxide films, 89 of molybdenum trioxide, 151 of nickel oxide, 160 of tantalum oxide, 182 of tungsten trioxide, 140–1, 147, 150 of vanadium pentoxide, 185, 186 Eveready, battery charge indicator, 408, 443 Everitt’s salt, see Prussian white 462 Index EXAFS, of cerium–titanium oxide, 194 exchange current, 47, 95 extinction coefficient, 53, 55, 60, 61, 113, 269, 274, 294, 343, 344T, 349 and coloration efficiency, 55 extrinsic colour, 52–3 eye, human, see human eye eye wear, see applications, ECD, eye wear Faradaic current, 45, 52, 76 Faraday constant, 34 Faraday’s laws, 46, 52 fax transmission, using electrochromism, 26 F-centres, 28 ferric ion, diffusion coefficient, 77T ferricyanide, 342 as oxidant, 342 ferrocene derivatives, 330, 331 electron mediator, 359 ferrocene–naphthalimide dyads, 309 ferrocyanide charge transfer complexation, 359 electron mediator, 342, 350, 358, 359 incorporation into nickel oxide, 200 titanium dioxide, 201 mediating viologen comproportionation, 358 ferroin, 253 ferrous ion, as electron mediator, 359 fibre-optics, ECD, applications, 265, 404 Fick’s laws, 44, 45, 50, 76, 111 approximation, 45 first law, 44 second law, 45, 95 field-effect transistors, conducting polymers, and 312 fillers, ECD electrolyte, 445 film thickness, and ellipsometry, 50 Flabeg Gmbh, ECD, windows, 400 flash evaporation, of vanadium pentoxide, 185 flat-panel screens, TV, 402 flexible ECD, 129, 423 on indium–tin oxide, 423 fluoreneones, 18, 379, 380, 387 quasi reversibility of, 380 2,4,5,7-tetranitro-9-fluorenone, 387 2,4,7-trinitro-9-fluorenylidene malononitrile, 387 fluorescence, 5 fluorine-doped tin oxide, as substrate, 139, 166, 168, 171, 196, 205, 292, 362, 400, 406, 409, 422 fluoroanil, p-, 382 flux, 44, 97 and colour formation, 75 formation of colour, electrochemical, 52 Fox Talbot, 26 frequency, and impedance, 50 frequency response analysis FRA, see impedance spectroscopy fullerene electrochromes, 303 coloration efficiency, 304, 305–6 degradation of, 303, 305 formation via Langmuir–Blodgett, 304, 305 quasi-reversiblity, 303, 305 near-infrared absorbance, 399 furan, 313 fused bithiophenes, conducting polymers of, 316 gallium hexacyanoferrate, 295 galvanostatic coloration, 96–8, 104, 417 games, ECD applications, 363 gamma rays, 110 gasochromism, 5, 406–7, 407T materials chromium oxide, 407T cobalt oxide, 406 metalloporphyrin, 407T nickel oxide, 407T phthalocyanine, 407T tungsten trioxide, 407T sensors for chloride ion, 406 for nitrate ion, 406 nitric oxide, 407 phosphate ion, 406 toluene, 407 gelled ECD electrolyte, 305, 349, 350, 351, 384 using agar, 349, 351 using silica, 348 Gentex Corporation, 376, 385, 396, 398, 417, 425, 447 aircraft windows, 400 mirrors (Night-Vision System), ix, 44, 356, 385–7 cycle life, 356 memory effect, 387 radical annihilation, 386 type-I electrochrome, 396 Gibbs energy, 34–9 and emf, 34 glassy carbon, substrate, 294, 358 gold, 150, 153, 159 additive in cobalt oxide, 204T in iridium oxide, 204T in molybdenum trioxide, 204T in nickel oxide, 200, 204, 204T in tungsten trioxide, 204, 204T in vanadium pentoxide, 204, 204T as substrate, 285 overlayer of, 446–7 gold nanoparticles, overlayer of, 446 graft copolymer, poly(aniline), 333 grain boundaries, 88, 98, 146 graphite electrochromes, 303, 304–5 electropolychromic, 303, 304 substrate, 424 Gr ¨ otthus, conduction in metal oxides, 90 Gyridon ‘electrochromic paper’, 5 Index 463 half reaction, 35 Hall effect, 113 hematite, 173 Henderson–Hasselbalch equation, 4 He–Ne laser, 438 heptyl viologen, 8, 9, 11, 14, 28, 190, 344T, 346, 348, 349, 351, 352T, 352–3, 354–5, 356, 357, 359 anion effects, 353T as primary electrochrome, 356, 375, 385 charge transfer complexation, 359 critical micelle concentration, 355 di-reduced, 358 ECDs of, 360 incorporated in paper, 365 morphology of, 355 power consumption, 14 radical of, 357, 359 aging effects, 357 recrystallisation of, 357 reduction potentials, 353T solubility constant, 351 hexacarbonyl, as CVD precursor, 131, 135–6, 397 hexacyanoferrate(II), see ferrocyanide hexacyanoferrate(III), see ferricyanide hexacyanoferrate of copper, 294 gallium, 295 indium, 295, 437 iron, see Prussian blue miscellaneous, 295–6 mixed-metal, 296 nickel, 293–4 palladium, 294–5 vanadium, 292–3 hexyl viologen, 352T history of conducting polymers, 312 of electrochromism, 25–30 of Prussian blue, 282 history effect, 131 hopping electron, 81, 99, 127 polarons, 143 hue, and colour analysis, 56T, 62, 64, 70 human eye, spectral response, 62 hydride, electrochromic, 307–8 Anderson transition in, 307 cycle life, 308 durability, 307, 308 ECD, 308 electrochromic alloys, 308 lanthanum–magnesium, 308 samarium–magnesium, 308 electrochromic metal lanthanum, 307 yttrium, 307 mirrors, 307 palladium overlayer on, 307 response time, 307 switchable mirrors, 307 hydrogen electrode, 36, 37, 40 evolution at molybdenum oxide, 199 evolution at tungsten oxide, 89, 102, 104, 445 uranyl phosphate, ECD electrolyte, 421T hydrogen peroxide, 135, 308 hydroquinone, electron mediator, 359 hygroscopicity, of metal oxides, 89 hysteresis, 104, 157 IBM Laboratories, 28, 30, 355, 360, 403, 405 ICI Plc, 28, 341, 349–51, 352 illumination back-wall, 433, 435 front-wall, 433 light sources, 64 of ECDs, 417 imaginary, impedance 50 immitance, 50 immobilised viologens, see derivatised electrodes impedance spectroscopy, 50, 83, 85, 333 and frequency, 50 equivalent circuit, 447 imaginary, 50 real, 50 incident light, 50 Inconel-600, oxide on, 203 Indigo Blue, coloration efficiency, 57T Indigo Carmine, within poly(pyrrole), 333 indium hexacyanoferrate, 295, 437 indium nitride, 309 indium oxide, as electrochromic host, 196–7 indium–tin oxide chemical stability, 423 cycle life, 197 degradation of, 444–5 composition of, 196 containing silver, 204T ECDs of, 197 electrochemistry of, 196–7 electroreduction of, 444 electro-reversibility poor, 197 formation via CVD, 132 dc magnetron sputtering, 136 electron-beam deposition, 137, 196 laser ablation, 196 rf sputtering, 196 sol–gel, 196 spin coating, 135, 196 kinetics of, chemical diffusion coefficient, 197 optical properties as secondary electrochrome, 197 coloration efficiency, 197, 199, 199T contrast ratio, 197 optical properties, 197 optically passive, 17, 197, 199 flexible ECDs, 423 on Mylar, 423 on PET, 129, 423 on polyester, 423 464 Index mechanical stability, 129 resistance, effect of, 349 electronic conductivity, 422, 445 substrate, 17, 70T, 86, 96, 128, 129, 135, 138, 139, 141, 150, 151, 152, 156, 158, 159, 164, 166, 167, 181, 182, 191, 257, 284, 293, 294, 305, 306, 326, 330, 331, 333, 349, 375, 382, 385, 404, 417, 422–3, 444–5, 447 water sensitivity, 444 XPS of, 197, 445 indole, 313 inert electrode, 38 infra red spectroscopy, 103, 358 inorganic–organic, dual ECD, 333–4 insertion coefficient, 9, 41, 53, 61, 81, 83, 87, 90–1, 92, 95, 96, 101, 104, 108, 113–14, 143, 146, 186, 188, 191, 192, 193, 303, 305 effect on diffusion coefficient, 90–1 effect on electroreversibility, 82 effect on wavelength maximum, 53 high at grain boundaries, 104 metal-oxide systems, 61 intensity and colour analysis, 63 of electrochromic colours, 3 interactions, counter ion with water, 89 interfaces, between films, 50 international meetings on electrochromism (IME), x interphase, electrode, 43 intervalence charge transfer, 61, 102, 125, 127, 145, 153, 188, 192, 253, 267, 284 heteronuclear, 127 homonuclear, 127 intrinsic coloration efficiency, 54–60 iodine, 27, 437 iodine laser, 266 ion-conductive electrolyte, tantalum oxide, 181, 183 ion–electron pair, see redox pairs ionic interactions, 36 ionic mobility, 99, 303, 305–7 ionisation, of water, 89 during coloration, 81 iPod screen, ECD application, 363, 402 IR drop, 153, 405, 422–3 iridium oxide, 10, 125, 155–9, 198 annealing of, 158 as secondary electrochrome, 16, 149, 444T coloration efficiency, 56T, 70, 158 reflectance of, 400 coloration mechanism, 157 containing aramid resin, 198 gold, 204T water, 156 electrostriction of, 130 ECDs, 159 electrochemistry, 157 electrochromic host, 198 ellipsometry, 157 formation via anodically grown on Ir, 155–6 dip coating, 135 IrCl 3 , 156 iridium–carbon composite, 156, 158 peroxo species, 156 sol–gel, 156 spray deposition, 158 sputtering, 155, 156 hysteresis of, 157 mechanical stability, 129 optical properties, 158 phase changes, 157 response time, 156, 159 specular reflectance, 407T water content, 156 write–erase efficiency, 156 XPS of, 157 iridium trichloride, 156 iridium trihydroxide, 157 iridium–carbon composite, 156, 158 iridium–cobalt–nickel oxide, 203 iridium–magnesium oxide, 198 iridium–ruthenium oxide, 203 iridium–silicon oxide, 198 formation via sol–gel, 198 iridium–tantalum oxide, chemical diffusion coefficient, 198 iridium–titanium oxide, 198 formation via sol–gel, 198 iron acetylacetonate, 174 iron hexacyanoferrate, see Prussian blue iron oxide, 172–5, 201 annealing, 173, 175 electrochemistry, 173 formation via CVD, 174, 175T dip coating, 135 electrodeposition, 173, 175T oxidised film on Fe metal, 172 sol–gel, 173, 174, 175T spin coating, 135, 174 mixtures, coloration efficiency, 198 as electrochromic host, 198–9 optical properties, 175 coloration efficiency, 56T, 70, 175, 175T, 201T secondary electrochrome, 174 iron oxyhydroxide, 173 iron perchlorate, 173 iron phthalocyanine, 261 iron polypyridyl complexes, 256 iron vanadate, 202 iron–molybdenum oxide, 199 iron–nickel–chromium oxide, 203 iron–niobium oxide, 200, 201 coloration efficiency, 201T formation via sol–gel, 200 iron–titanium oxide charge transfer of, 202 formation via dip coating, 202 irreversibility, when oxidising Li x WO 3 , 82 iso-pentyl viologen, 352T Index 465 IUPAC, 55, 147, 161 IVCT, see intervalence charge transfer J-aggregates, 265 junction potential, 39 K-glass, substrate, 422 kinetics bleaching models Faughnan and Crandall, 105–8 Green, 108, 109 of metal oxides nickel oxide, 164 vanadium pentoxide, 188 potentiostatic, 105–8 type-II, 79–115 type-III, 79–115 coloration, 75–115 of amorphous oxides, 88 electron as rate limiting, 99 faster in damp films, 90 of lutetium phthalocyanine, 260 type-I, 75–9 type-II, 75–9 type-III, 91–115 effect of counter-ion size on, 87–8 effect of morphology on, 88 effect of high resistance of polymers, 11 effect of water on, 89 electrochrome transport, 75 electron transfer, 33, 34 rate-limiting process, 33, 83, 87, 92 write–erase efficiency and, 11 Kosower, solvent Z-scale, 343 L*a*b* colour space, 64–71 data for Prussian blue !Prussian white, 70T L*u*v* colour space, 64–71 laboratory examples, of electrochromes, 3 Langmuir–Blodgett deposition, 138–206 forming fullerene electrochromes, 304, 305 forming phthalocyanine, 262–3, 407 lanthanide hydride, see hydride, lanthanide lanthanum–nickel oxide, 200 large-area ECDs, 141, 332, 447 laser ablation, forming indium–tin oxide, 196 tantalum oxide, 183 titanium dioxide, 184 vanadium pentoxide, 185 laser, 404 Q-switching, 267 types erbium, 267 He–Ne, 438 iodine, 266 YAG, 266, 267 laser-beam deflection, 87, 130, 157 lattice constants, 129 lattice energy, 93 Prussian blue, 289 lattice defects, nickel oxide, 163 lattice stabilisation, 112 layer-by-layer deposition of PEDOT:PSS, 329, 331 of poly(aniline), 329–30, 331 of poly(viologen), 328–9, 331 LCD, see liquid crystal display lead, electrodeposition of, 8, 306–7 lead fluoride, as ECD electrolyte, 159 lead tetrafluorostannate, as ECD electrolyte, 159 leakage current, 52 leucoemeraldine, 329, 331 LFER, see linear free-energy relationships ligand based, electrochromism, 255 ligand-to-metal charge transfer, 269 light modulation, ECD application, 404–5 light-emitting diodes, 363, 402 lightness, and colour analysis, 63, 64, 66 limiting current, 76 linear diffusion, 76 linear free-energy relationships, 343 liquid electrolytes, transport through, 75 liquid-crystal display, ix, 53, 351, 360, 363, 402, 403, 404, 406, 408, 425 power consumption of, 15 lithiation, dry, 418 lithium chromate, 167 lithium deficient, cobalt oxide, 167 lithium niobate, ECD electrolyte, 421T lithium pentafluoroantimonate, ECD electrolyte, 150 lithium perchlorate, ECD electrolyte, 82, 106, 150, 151, 152, 163, 166, 167, 169, 173, 176, 184, 186, 188, 197, 199, 205, 362, 408, 421 lithium phosphorous oxynitride, ECD electrolyte, 363 overlayer of, 446 lithium pnictide, specular reflectance of, 407T lithium tetrafluoroaluminate, electrolyte ECD, 150, 152, 421T, 436 overlayer of, 446 lithium tin oxide, 197 lithium tungsten bronze, 191 electro-irreversibility of, 82 lithium vanadate, 190 vanadate, thermochromic, 190 Lucent, 5 luminance, and colour analysis, 56T, 63, 64, 66, 70 lutetium phthalocyanine, 259–60, 261 cation-free not electrochromic, 260 chemical diffusion coefficient, 85T coloration kinetics, 260 degradation, 260 ECDs, 259, 260 electrochemistry, 260 protonated, 259 response times, 260, 261 formation via sublimation, 259 write–erase cycles, 259 466 Index Madelung constant, 112 maghemite, 173 magnesium fluoride, terminal effect suppressor, 423 magnesium OEP, 265 magnesium phthalocyanine, 260 magnesium–iridium oxide, 198 magnesium–nickel, 200 magnesium–nickel–vanadium oxide, 203 magnetic susceptibility, 113, 345 magnetite, 168, 173 manganese oxide, 175–6, 446 as secondary electrochrome, 165, 176 ECDs, 176 electrochemistry, 175–6 optical properties, 176 coloration efficiency, 176 rechargeable batteries, 176 formation via anodising Mn metal, 175 electrodeposition, 175 electron-beam sputtering, 137, 175 rf sputtering, 175 sol–gel, 175, 176 XPS, 176 manganese phthalocyanine, 261 mass balance, 81 nickel oxide, 162 mass transport, 33, 43–5, 75 mechanical stability, metal oxides, 129–30 medicine, ECD, applications, 265 melamine, plus vanadium pentoxide, 190, 202 membrane potentials, 4 biological, 3 memory, 15, 53–4, 152, 153, 403 and Gentex ECD, 387 and molybdenum trioxide ECD, 152, 153 and tungsten trioxide ECD, 149, 150 and viologens ECD, 348, 362 ECD self-erasure, 54, 387 metal hexacyanomellates, 282–96 metal oxidation to form oxide cobalt, 168, 169 iron, 172 manganese, 175 niobium, 177 rhodium, 179 ruthenium, 181 tantalum 182 titanium, 184 tungsten, 81, 150 vanadium, 185, 186, 187 metal oxide, 125–206 amorphous, 132, 139 bronzes of, 61, 81, 82, 103 coloration efficiency, 56T insertion coefficient and, 61 doped, 266 effect of moisture on, 128–9 electrochemistry of, 138 intervalence of, 61 metal oxide optical properties as primary electrochromes, 139–65 optical passivity, 125 neutral colours, mixtures, 399 photochemical stability, 125 preparation, 130–8 oxide formed by chemical vapour deposition, 131–2 oxide formed by dip coating, 135 oxide formed by electrodeposition, 132–4 oxide formed by evaporation, 89 oxide formed by Langmuir–Blodgett deposition, 138–206 oxide formed by oxidising alloy, Inconel-600, 203 oxide formed by oxidising metal cobalt, 168, 169 iron, 172 manganese, 175 niobium, 177 rhodium, 179 ruthenium, 181 tantalum, 182 titanium, 184 tungsten, 81, 150 vanadium, 185, 186, 187 oxide formed by sol–gel deposition, 134–6 oxide formed by spin coating, 131, 135–6 oxide formed by spray pyrolysis, 135 stability, 128–30 mechanical, 129–30 photochemical, 128, 129 metal-oxide mixtures, 190–206 colour manipulation, 190 containing precious metal, 204 formation via dip coating, 135 rf sputtering, 204 sol–gel, 204 neutral colour, 190 site-saturation model, 190, 192 metal oxyfluorides, 203 metal–insulator transition, see Anderson transition metallic substrates, 423–4 metalloporphyrin, gasochromic, 407T metals, electrodeposition, 303, 305–7 metal-to-ligand charge transfer, 262, 293 coordination complexes, 253 methanol, electro-oxidation, 409 methoxybiphenyls, 30, 379–80 electrode potentials, 379T, 381T, 381T optical properties, 379T, 379T, 381T, 381T steric effects, 380 methoxyfluorene, 8 methyl viologen, 7, 11, 17, 341, 344T, 346, 352T, 353, 436 charge-transfer complexation, 359 coloration efficiency, 57T diffusion coefficient, 77T di-reduced, 358 electropolychromic, 17 Index 467 methyl viologen (cont.) ESR, 356 in paper, 365 follows Langmuir adsorption isotherm, 365 mixed-valence salt, 356 methyl–benzyl viologen, in paper, 365 Methylene Blue, 8, 437, 438 coloration efficiency, 57T immobilised, 391, 391T in Nafion, 405 methylthiophene, 3-, 318 oligomers, of 320 micellar, viologens, 355–6 microbalance, see electrochemical quartz crystal microbalance migration, 43, 44, 75, 96–7 diffusion concurrent, 83 temperature dependence of, 83 mirror, ECD, see applications, ECD, mirrors mixed valency, methyl viologen, 356 dinuclear ruthenium complexes, 268T Robin–Day classification, 142, 283 tungsten trioxide, 142 viologens, 356 mixtures, of metal oxide, see metal-oxide mixtures MLCT, see metal-to-ligand charge transfer mobility ionic, 99, 104, 115, 303, 305–7 proton, 106, 108 modulation, electrochromic, 3, 53 molar absorptivity, 53 mole fraction x, 37 molybdenum ethoxide, forming molybdenum trioxide, 152 molybdenum sulfide, 152 molybdenum trioxide, 11, 27, 28, 103, 109, 125, 130, 151–5, 187 annealing of, 152, 154 bronze, 103, 151 chemical diffusion coefficient, 153 coloration in vacuo, 89 requires water, 89 containing gold, 204T platinum, 204T crystal phases a phase, 152 monoclinic, 152 orthorhombic, 152 ECD, 154–5, 397 effect of water on, 89 electrochromic host, 199 ellipsometry of, 153 ESR of, 153 formation via alkoxides, 152 CVD, 131, 151, 397 dc magnetron sputtering, 136, 151 electrodeposition, 132, 151, 152 electron-beam sputtering, 137 evaporation, 137–8, 151, 155T Mo(CO) 6 , 397 molybdenum ethoxide, 152 molybdenum sulphide, 152, 155T organometallic precursors, 152 oxidation of Mo metal, 151 peroxo species, 151 rf sputtering, 151 sol–gel, 135, 152 spin coating, 135 spray pyrolysis, 152 hydrogen evolution at, 199 in paper, 27, 405 memory effect, 152, 153 optical properties, 153–4 coloration efficiency, 56T, 154, 155T, 199, 199T oxygen deficient, 103, 151, 153 response time, 154 self bleaching of, 153 stability, mechanical, 129 UV irradiation of, 28 XPS, 152, 153 XRD, 153 molybdenum–chromium oxide, 199 molybdenum–iron oxide, 199 molybdenum–niobium oxide, 200 formation by sol–gel, 200 molybdenum–tin oxide, 199 coloration efficiency, 199, 199T–201T molybdenum–titanium oxide, 199 coloration efficiency, 199 molybdenum–tungsten oxide, see tungsten–molybdenum oxide molybdenum–vanadium oxide, 199 formation via peroxo species, 199 Moonwatch, ECD display, 402, 403 M ¨ ossbauer, 197 Prussian blue, 282, 283 tin oxide, 184 motorcycle helmet, ECD application, 398 Mylar, indium–tin oxide substrate, 423 Nafion, 159, 290, 366, 385 as electrochromic host, 405 ECD electrolyte, 421T incorporating Methylene Blue, 405 phenolsafranine dye, 405 viologen, 405 overlayer of, 150, 446 Nanochromic (NTera) displays, 347, 362, 363, 402, 406 naphthalimide–ferrocene dyads, 309 naphthalocyanine complexes, 263–4 1, 4-naphthaquinone, 384 cyclic voltammetry, 384 N-carbazylcarbazole, carbazoles, 379T NCD, see Nanochromic display near infrared, electrochromism, 165, 253, 254, 265–74, 303–4, 317, 319, 327, 377T–378T, 399 of aromatic amines, 376 468 Index of diamond, 399 of dinuclear ruthenium complexes, 268T of fullerene, 399 neodymium–vanadium pentoxide, 202 Nernst equation, 36, 38, 40, 75, 90 Nernst–Planck equation, 43 Nerstian systems, 77 neutral colour, 399 metal-oxide mixtures, 190, 399 tungsten–vanadium oxide, 399 neutron diffraction, 144 nickel, underlayer of, 86, 164 nickel dithiolene, 266 nickel hexacyanoferrate, 293–4 nickel hydroxide, 129, 161 formation via sonication, 133, 134 nickel oxide, 9, 125, 130, 159–65, 167, 200 activation energy, 111T annealing of, 161 as primary electrochrome, 16, 149, 165, 176 as secondary electrochromes, 444T, 446, 447 bleaching of, 164 chemical diffusion coefficient, 85T degradation of, 163 ECDs, 164–5, 397 electrochemical quartz microbalance, 163 electrochemistry of, 161–3 electrochromic host, 200 containing cobalt metal, 164 ferrocyanide, 200 gold, 200, 204, 204T lanthanum, 164 organometallics, 200 electronic conductivity, 162 electrostriction of, 130 formation via CVD, 36, 161 dc magnetron sputtering, 136, 165T dip coating, 135, 161, 165T electrodeposition, 132, 160–1, 165T evaporation, 160, 165T plasma oxidation of Ni–C, 161 rf sputtering, 160, 162, 163, 164 sol–gel, 135, 161–3, 165T sonication, 133, 134, 165T spray pyrolysis, 135, 160, 161, 165T gasochromic, 407T ionic movement rate, 162 mass balance, 162 defects lattice, 163 oxygen deficiency, 16, 159–60 mechanical stability, 129 optical properties, 163–4 coloration efficiency, 56T, 70, 165T phases, 162 crystallites in amorphous NiO, 88 response times, 12, 164 thermal instability, 160 water occluded, 163 write–erase cycles, 164 nickel oxyhydroxide, 129 as secondary electrochrome, 400 formed via electrodeposition, 161 nickel tungstate, 200 nickel–aluminium alloy, 200 nickel–aluminium oxide, 200 nickel–cerium oxide, 200 nickel–chromium oxide, 200 nickel–chromium–iron oxide, 203 nickel-doped tin oxide, 196 nickel–iridium–cobalt oxide, 203 nickel–lanthanum oxide, 200 nickel–magnesium, 200 nickel–titanium oxide, 201 coloration efficiency, 202 formation via electrodeposition, 201 nickel–tungsten oxide, 200, 436 coloration efficiency, 200 formed via sol–gel, 200 nickel–vanadium pentoxide, 202 nickel–vanadium–magnesium oxide, 203 nickel–yttrium oxide, 200 Nikon, ECD sunglasses, 401 Nile Blue, coloration efficiency, 57T niobium ethoxide, sol–gel precursor, 134 niobium pentoxide, 17, 125, 176, 201 annealing, 177 as secondary electrochrome, 149, 178 chemical diffusion coefficient, 85T cycle life, 178 cyclic voltammetry, 178 ECDs, 178 electrochemistry, 177–8 electrochromic host, 200–1 coloration efficiency of mixtures, 201 optical properties, 178 coloration efficiency, 56T, 178, 181T, 199T–201T, 200 optically passive, 17, 178 redox pairs, 102 formation via dc magnetron sputtering, 136, 177 dip coating, 135 oxidising Nb metal, 177 rf sputtering, 181T sol–gel, 134, 176, 178, 181T, 200 spin coating, 135, 177 spray pyrolysis, 181T niobium–iron oxide, 200, 201 coloration efficiency, 201T formation via sol–gel, 200 niobium–molybdenum oxide, 200 via sol–gel, 200 niobium–silicone oxide, 200 niobium–titanium oxide, 200 niobium–tungsten oxide, 201 coloration efficiency, 201 Nippon Mitsubishi Oil Corporation, 446–7 NIR, see near infrared nitrate ion, gasochromic, sensor for, 406 nitric oxide, gasochromic, sensor for, 407 Index 469 nitroaminostilbene, 4 nitrogen-15, see nuclear reaction analysis nitrosylmolybdenum complexes, 270 N-methyl PProDOP, 328 N-methylpyrrolidone, 330, 331 NMP, see N-methylpyrrolidone non-faradaic current, 45, 52, 76 non-linear optical effects, 4 non-redox electrochromism, 3 non-volatile memory, see memory effect N-phenylcarbazole, carbazoles, 379T, 381T N-PrS PProDOP, 328 NREL Laboratories, 398, 433, 436, 447 NTera, ECD, 6, 10, 347, 348, 362, 363, 402, 404, 405, 406; see also NanoChromic (NTera)displays phosphonated viologen, 10, 362, 406 as primary electrochrome, 363, 365 coloration efficiency, 362, 363 cycle life, 362 ECD, response time, 363, 364 nuclear reaction analysis, 16, 110, 111, 162 nucleation, of hydrogen gas, 100 nucleation, of viologen reduction, 354 NVS, see Gentex Corporation occlusion, of water during deposition, 89 octyl viologen, 344T, 352T Ohm’s law, 44 Ohmic migration, 386 oligomers of 3-methylthiophene, 320 of thiophene, 320, 321 of viologens, 364 opaque substrates, 423–4 optical absorbance, 52 optical analyses, water effect of, 89 optical attenuator, ECD, application, 270 optical charge transfer, see charge transfer Optical Coating Laboratory, Santa Rosa, 448 optical data storage, ECD applications, 265 optical path length, 55 optical properties metal oxides cerium oxide, 166 cobalt oxide, 168, 169–70 iridium oxide, 158 iron oxide, 175 manganese oxide, 176 molybdenum trioxide, 153–4 nickel oxide, 163–4 niobium pentoxide, 178 tantalum oxide, 183 tin oxide, 184 titanium dioxide, 184 tungsten trioxide, 144–9 vanadium pentoxide, 188–9 mixtures of metal oxide, indium–tin oxide, 197 methoxybiphenyls, 379T, 381T oligothiophenes, 321T pyrazolines, 388T quinones, 383T tetracyanoquinonedimethanide, 389T tetrathiafulvalene, 390T viologens, 344T optical response, deconvolution of, 17 optically passive, 16, 125 metal oxides, 125 cerium oxide, 166 nickel–vanadium oxide, 202 niobium pentoxide, 178 titanium dioxide, 184 mixtures of metal oxide antimony–tin oxide, 193 cerium–titanium oxide, 194 indium–tin oxide, 197, 199 titanium–vanadium oxide, 202 vanadium–nickel oxide, 202 vanadium–titanium oxide, 202 optically transparent electrode OTE, 62, 129–30, 141, 156, 417, 444, see also substrates optically transparent thin-layer electrode OTTLE, 255 orbitals, and charge transfer, 61 Orgacon EL-350, 332 organic, ECD electrolytes, 420–2 organic electrochromes, coloration efficiency, 57T organic–inorganic, dual ECD, 333–4 organic, polymers, ECD electrolyte, 445 organometallic precursors, of molybdenum trioxide, 152 in nickel oxide, 200 Orgatron, 323 oscillator strength, 147, 191, 206 osmium dithiolene complexes, 270–4 OTTLE, see optically transparent thin-layer electrode overlayers gold, 446–7 gold nanoparticles, 446 lithium phosphorus oxynitride, 446 lithium tetrafluoroaluminate, 446 Nafion, 446 palladium, 307 poly(o-phenylenediamine), 446 tantalum oxide, 150 tungsten oxyfluoride, 446 tungsten trioxide, 446 overpotential, 36, 42, 46, 76, 93, 96, 199 oxidation, chemical ferricyanide, 342 oxygen gas, 342 periodate, 342 oxidation number, 35 oxidation potential, 318, 320 oxidative polymerisation, conducting polymers, 313–14 of pyrrole, 313 oxide ions, charge transfer with, 85 oxide mixtures, coloration rate enhancement, 200 oxidising metal, to form metal oxide film cobalt, 168, 169 470 Index iron, 172 manganese, 175 niobium, 177 rhodium, 179 ruthenium, 181 tantalum, 182 titanium, 184 tungsten, 81, 150 vanadium, 185, 186, 187 oxyfluoride, metal, see metal oxyfluoride oxygen as oxidant, 342 molecular, 59 oxygen backfilling, 141 oxygen bridges, in solid metal oxides, 85 oxygen deficiency in molybdenum trioxide, 103, 151, 153 in nickel oxide, 159–60 in praseodymium oxide, 179 in tungsten trioxide, 102, 103, 140, 147 oxyhydroxide, via electrodeposition, 132, 133 PAH, see poly(allylamine hydrochloride) paints and pigments of, Prussian blue, 282 palladium, overlayer of, 307 palladium dithiolene, 266 palladium hexacyanoferrate, 294–5 palladium oxide, 150, 178 electrochemistry, 178–9 paper containing hexacyanoferrates, 405 containing metal oxides molybdenum trioxide, 27, 405 tungsten trioxide, 27, 405 containing viologens, 365, 366, 405 heptyl viologen 365 methyl viologen, 365 methyl–benzyl viologen, 365 paraquat, 341 parasitic currents, 52 passive, optical, see optically passive patents, 395 PBEDOT-Pyr, 326 PBEDOT-PyrPyr(Ph) 2 , 326 PBuDOP, 328 PEDOP, 328 PEDOT, 10, 60, 319, 332, 437 as photoconductor, 436 as primary electrochrome, 190, 291, 334 as secondary electrochromes, 149 band gap of, 322 coloration efficiency, 437 colour analysis of, 70, 71 ECDs of, 409 specular reflectance, 407T PEDOT:PSS, 323, 329, 330, 331 formed via layer-by-layer deposition, 329, 331 PEDOT-S, 330, 331 formed via spin coating, 330, 331 self-doped polymers, 330, 331 pentyl viologen, 351, 352T perchloric acid, as ECD electrolyte, 150, 157 percolation threshold, 99, 100, 101, 113, 114, 115 periodate, as oxidant, 342 permanganate, 60 permittivity, 106, 108, 112 Pernigraniline, 329, 331–2 Perovskite, tungsten trioxide, 127, 140 peroxo species, 10, 133, 135, 141, 151, 156, 168, 184, 186 electrodeposition with, 133 forming cobalt oxide, 168 iridium oxide, 156 molybdenum oxide, 151, 269 molybdenum–tungsten oxide, 199 molybdenum–vanadium pentoxide, 199 titanium dioxide, 184 tungsten–molybdenum oxide, 199 tungsten trioxide, 10, 133, 135, 141 vanadium–molybdenum–oxide, 199 vanadium pentoxide, 186 Perspex, plus tungsten oxide, 193 PET, indium–tin oxide substrate, 423 phenanthrenes, 379 phenanthroline, 3,8-, pseudo viologen, 360 Phenolsafranine dye, in Nafion, 405 phenothiazines, as secondary electrochromes, 362, 363 phenylenediamine, 386 Philips, 5, 27, 349, 354 phosphate ion, gasochromic sensor for, 406 phosphomolybdic acid, 152 phosphonated viologen, see NTera phosphoric acid, ECD electrolyte, 167, 421, 438 phosphotungstic acid, 150, 192, 309 in titanium dioxide, formed via sol–gel, 201 photo-activated ECD cells, 433 photo-activity, 129 titanium dioxide, 445 photocells, photoelectrochromism, 433, 434 photochemistry, metal-oxide stability, 125, 128, 129 photochromic–electrochromic systems, 438–9 photochromism, 28, 404 electron donors, 438 of MoO 3 , 28 of SrTiO 3 , 28 of WO 3 , 103 photoconductors, 433, 434–7 amorphous silicon, 436 PEDOT, 436 poly(3-methylthiophene), 436 poly(aniline), 436, 439 poly(o-methoxyaniline), 436 poly(pyrrole), 436 silicon carbide, 436 titanium dioxide, 437, 438 photodegradation, 54 photo-driven ECD cells, photoelectrochromism, 433 photoelectrochemistry, 361, 362 viologens, 362 Index 471 photoelectrochromism, 129, 421T, 423, 433–9 beam direction, 433 back-wall illumination, 433, 435 front-wall illumination, 433 of Prussian blue, 267, 437 photo-activated ECD cells, 433 photocells, 433, 434 photoconductors, 433, 434–7 amorphous silicon, 436 PEDOT, 436 poly(aniline), 436, 439 poly(o-methoxyaniline), 436 poly(3-methylthiophene), 436 poly(pyrrole), 436 silicon carbide, 436 titanium dioxide, 437, 438 photo-driven ECD cells, 433 poised cells, 434 response time, 436 photogalvanic, 438 vanadium pentoxide, 438 photography, and Prussian blue, 25 photosensitising, ruthenium tris(2,2 0 -bipyridyl), 436, 437 photovoltaic, 437–8 cadmium sulfide, 437 strontium titanate, 437 titanium dioxide, 437 phthalocyanine complexes, 9, 258 conductivity electronic, 263 ECDs of, 263 electrochemistry, 262 electro quasi-reversibility, 261 electronic conductivity, 259 ellipsometry, 263 formation, via electrochemistry, 261–2 Langmuir–Blodgett, 262–3, 407 gasochromic, 407T including aniline moieties, 261 mixed cation, 261 requires central cation, 260 response times, 261 tetrasulfonated, 261 physical vapour deposition, of cerium oxide 166 pigments, industrial, 259 Pilkington Glass, 141, 400 pixels, 360, 385, 402, 403 plasma oxidation, of Ni–C, forming nickel oxide, 161 plasma screens, television, 402 platinum as substrate, 153, 284, 326, 409, 422, 423 black, 133 incorporated into molybdenum trioxide, 204T ruthenium dioxide, 204T tantalum pentoxide, 204T tungsten trioxide, 204T platinum dithiolene, 266 poised cells, photoelectrochromism, 434 polarisation of electrode, 76 of light, 50, 51 polaron, 145, 183, 316 hopping, 127, 143 in tungsten trioxide, 147 polaron–polaron interactions, in WO 3 , 88 polished metal, substrates, 418 poly(acetylene), 312, 315 air sensitive, 312 electronic conductivity, 312 poly(acrylate), composite formed via spin-coating, 333 with poly(aniline), 333; composite with silica and poly(aniline), via sol–gel, 333 poly(acrylic acid), as ECD electrolyte, 150, 421T poly(alkeneldioxypyrrole)s, 327 poly(allylamine hydrochloride), 330, 331 poly(AMPS), as ECD electrolyte, 12, 150, 157, 260, 330, 331, 348, 366, 391, 391T, 395–410, 420 immobilising electrochromes, 391 poly(aniline), 9, 11, 30, 101, 313, 329–30, 331, 333, 384, 438, 439 as photoconductor, 436, 439 as secondary electrochrome, 149, 290–1, 333, 334, 444T castable films, 332–3 composites with cellulose acetate, 333 with poly(acrylate), 333 with poly(styrene sulfonic acid), 333 containing vanadium pentoxide, 190, 202 cyclic voltammetry, 333 electrochemistry of, 329–30, 331 electropolychromic, 329, 331 encapsulating dyes, 333 formation via electropolymerisation, 329–30, 331; layer-by-layer deposition, 329–30, 331 graft copolymer of, 333 immobilising electrochromes, 391 poly(acrylate)–silica composite, formed via sol–gel, 333 redox states, 329, 331 poly(aniline)s, 328–30 ECDs, 330, 331 protonation reactions, 328–9, 331 response times, 330, 331 spectroelectrochemistry of, 333 poly[3,4-(butylenedioxy)pyrrole], 328 poly(carbazole), chemical diffusion coefficient, 85T poly(CNFBS), 316 polycrystalline, metal oxides, made by sputtering, 81 poly(DDTP), 326 poly(diphenylamine), specular reflectance, 407T polyelectrochromism, see electropolychromism polyelectrolytes, ECD electrolyte, 420–2 polyester, indium–tin oxide substrate, 423 poly(ethylene imine), 329, 331 poly(ethylene oxide) as ECD electrolyte, 150, 290, 408, 421, 438, 444 472 Index as thickener in ECD electrolyte, 419 poly(3,4-ethylenedioxy thiophenedidodecyloxybenzene), coloration efficiency, 57T poly(ethylene terephthalate), 332 ITO on, 27 poly(iso-thianaphthene), chemical diffusion coefficient, 85T polymer electrolytes, ECD electrolytes, 421–2 conducting, 9, 11 electrolyte, 44 of EDOT, 325 polypyridyl complex, via spin-coating, 254–6 TTF species, ion movement rate limiting, 390 viologens, 347 photostability,151 poly(m-toluidine), 330, 331–2 poly(methyl methacrylate) blend, as ECD electrolyte, 291, 334 poly(3-methylthiophene) 320, 322T as photoconductor, 436 as primary electrochrome, 197 poly(o-methoxyaniline), 332 as photoconductor, 436 poly(o-phenylenediamine), overlayer of, 446 poly(o-toluidine), 159, 330, 331 poly(oligothiophene)s, 321T, 323T poly(1,3,5-phenylene), 327 poly(p-phenylene terephthalate), 198 as secondary electrochrome, 159 poly[3,4-(propylenedioxy)pyrrole], 328 coloration efficiency, 57T poly(3,4-propylenedioxythiophene), coloration efficiency, 57T poly(propylene glycol), ECD electrolyte, 421 poly(pyrrole), 101, 313, 314, 315, 316, 317, 327 as photoconductor, 436 as primary electrochrome, 165 as secondary electrochrome, 149 containing dodecylsulfonate, 333 containing Indigo Carmine, 333 electro-synthesis of, 30 specular reflectance, 407T viologens of, 346 poly(pyrrole)s, 327–8 ECDs of, 328 N-Gly PProDOP, 328; PBuDOP, 328 PEDOP, 328 PProDOP, 328 poly(siloxane), immobilising electrochromes, 391 poly(styrene sulfonic acid), 333, 347 composite with poly(aniline), 333 poly(thiophene), 9, 11, 313, 315, 321 as primary electrochrome, 165 star polymers, 327 viologens of, 347 PBEDOT-Pyr, 326 PBEDOT-PyrPyr(Ph) 2 , 326 PEDOT, 10, 60, 319, 332, 437 as photoconductor, 436 as primary electrochrome, 190, 291, 334 as secondary electrochrome, 149 bandgap of, 322 coloration efficiency, 437 colour analysis of, 70, 71 ECDs of, 409 specular reflectance, 407T PEDOT:PSS, 323, 329, 330, 331 formed via layer-by-layer deposition, 329, 331 PEDOT-S, 330, 331 formed via spin coating, 330, 331 self-doped polymers, 330, 331 poly(thiophene)s, 318–27 band structure, 152 Baytron M, 323 Baytron P, 323 BEDOT, 326 BEDOT-N-MeCz, 326 bipolarons in, 320 DDTP, 326 dihedral angle, 323T formation via spin coating, 327 PBEDOT PBEDOT-B(OC 12 ) 2 , 332 PBEDOT-N-MeCz, 331 PBEDOT-Pyr, 326 PBEDOT-PyrPyr(Ph) 2 , 326 PProDOT-Me 2 , 331, 332 response time, 325 substituted, 320–1 poly(toluidine)s, 330, 331 poly(triphenylamine), 327 poly(vinyl alcohol), ECD electrolyte, 421 poly(vinyl butyral), ECD electrolyte thickener, 419 poly(1-vinyl-2-pyrrolidone-co- N,N 0 -methylenebisacrylamide), 391, 391T, 395–410 ECD electrolyte, 391 poly(viologen), 328–9, 331 formation via layer-by-layer deposition, 328–9, 331 Polyvision, 306 porphyrin complexes, 258, 264–5 potassium chloride, ECD electrolyte, 291, 349 potassium triflate, ECD electrolyte, 290 potential, equilibrium, 41 potential, sweep, 48 potential step and cycle life, 12 and coloration, 354–5 potentiostat, 48, 62 potentiostatic coloration, 99, 417 electrodeposition, 133 interrupted coloration, see pulsed potentials three-electrode, 443 powder abrasion, of Prussian blue, 283 power consumption, 13–15 different types of display, 15 poly(methylthiophene), 165 PPG Aerospace, ECD, windows, 400 PPG Industries, 425 Index 473 praseodymium oxide, 178–9 cycle life, 179 electrochemistry of, 178 oxygen deficiency, 179 containing cerium oxide, 179 as secondary electrochrome, 179 formation via, CVD, 178, 179 dc magnetron sputtering, 136, 178 XRD, 179 praseodymium phthalocyanine, 263 precious metal, in metal oxide, 204 formation via rf-sputtering, 204 sol–gel, 204 preparation, of metal oxides, chemical vapour deposition, 131–2 primary and secondary electrochromism, 16–17; see also complementary electrochromism primary electrochromism, 45, 165, 418, 421, 445 hexacyanoferrates as, Prussian blue, 333 metal oxides as, 139–65 nickel oxide, 165, 176 tungsten trioxide, 149, 154, 165, 170, 178, 179, 184, 190, 197, 290, 291, 333, 334, 400, 421, 436, 438, 444T, 446, 447 polymers as PEDOT, 190, 291, 334 poly(3-methylthiophene), 165, 197 poly(pyrrole), 165 poly(thiophene), 165 viologens as heptyl viologen, 356, 375, 385 NTera viologen, 363, 365 primary reference electrode, see standard hydrogen electrode probe molecules, 5 propyl viologen, 352T propylene carbonate, as ECD electrolyte, 100, 106, 151, 152, 166, 169, 173, 176, 184, 186, 187, 188, 197, 199, 205, 356, 384, 419 proton, conductivity in metal oxides, 89 in tantalum oxide, 183 mobility, 4, 86, 106, 108 proton transfer, across solution–oxide interface, 86 protonation reactions, poly(aniline)s, 328–9, 331 Prussian blue, 9, 25–6, 41, 57, 61, 446 and blueprints, 26, 405 and cyanotype photography, 26 and drawing, 26 and photography, 25 as secondary electrochromes, 149, 290, 333, 334, 363, 365, 444T bulk properties, 282–3 chronoamperometry, 284 colour analysis of, 62, 70 cyclic voltammetry, 286, 287 ECD, 289–91 comprising single film of, 290 EDAX of, 288 electrochemistry of, 58–60, 285–9 electropolychromism of, 287 ellipsometry of, 284, 287, 288, 289 formation via, 283–5 catalytic silver paint, 283 directed assembly, 285 electrodeposition, 283, 284 electroless deposition, 283 photolysis, 26 powder abrasion, 283 redox cycling, 283 sacrificial anode methods, 283 history, 282 in paper, 405 ‘insoluble’, 282 lattice energy, 289 M ¨ ossbauer of, 282, 283 paints and pigments of, 282 pH effect of, 289 photochargeable battery of, 437 photoelectrochromism of, 267, 437 preparation ‘soluble’, 283 write–erase efficiency, 286 XPS, 288 XRD, 283 Prussian brown, 26, 285 Prussian green, 285, 334 Prussian white, 57, 286 pseudo viologen, see viologen, pseudo pulsed potential, 104–5, 303, 305 coloration, 87 viologens, 365 enhanced ECD durability, 104 response time acceleration, 11 purity, and colour analysis, 63, 65, 66 purple line, and colour analysis, 64 PVPD, immobilising electrochromes, 391, 391T, 395–410 PVPD, see poly(1-vinyl-2-pyrrolidone-co- N,N-methylenebisacrylamide) PXDOP, 328, 356 PXDOT, 328 pyrazolines, 387–8 optical properties, 388T response times, 388T pyridinoporphyrazine complexes, 264 pyrrole, 313; for polymers of pyrrole, see poly(pyrrole) oxidative polymerisation of, 313 Q-switching, of lasers, 267 quantum-mechanical effects, tunnelling, 81 quartz-crystal microbalance, see electrochemical quartz-crystal microbalance quasi-electrochromism, 406–7 quasi-reference electrodes, 40 quasi-reversibility fullerene electrochromes, 303, 305 phthalocyanine electrochromes, 261 viologen electrochromes, 358 quaternary oxides, 203 474 Index quinhydrone, 384 quinones, 4, 30, 256, 381–5 amino-4-bromoanthaquinone-2-sulfonate, 384 aminonaphthaquinone, 384 benzoquinones o-, 381; p-, 381, 382 bis(dimethylamino)diphenylamine, 4,4 0 -, 384 bromoanil, o-, 382, 383 solubility product, 383 catechole, 270–4 chloranil o-, 382, 383 p-, 382 type-II electrochrome, 382 contrast ratio, 348, 384, 385 ECDs of, 384 electrode potentials, 383T electropolymerisation, 384 2-ethylanthraquinone, 384 fluoroanil, p-, 382 naphthaquinone, 1,4-, 384 cyclic voltammetry, 384 type-I electrochromes, 384 optical properties, 383T quinhydrone, 384 radical annihilation, and ECD self-erasure, 386 Gentex mirror, 386 radical, viologen, see viologen, radical radii, ionic, 112 Raman spectroscopy, 86, 88, 103, 130, 357 Randles–Sevcˇ ik equation, 49, 83 rate of cell operation, 41–6 of coloration, 33, 139 of charge transfer, 95 of electron transfer, 42–3, 75 of electronic conduction, 42 of mass transport, 42 rate constant, electron transfer, 34, 46, 102 rate limiting kinetics, 83 crystal structure changes, 87 electronic motion, 99, 101, 115, 143 ionic motion, 92, 163, 188, 390 diffusion, 97 RBS, see Rutherford backscattering RCA Laboratories, 29 real, impedance, 50 rear-view mirrors, see applications, ECD, mirrors rechargeable batteries, manganese oxide, 176 redox couple, 35, 37 redox cycling, of Prussian blue, 283 redox electrode, 50 redox indicators, 374 redox pairs, 101, 112–13, 115 niobium pentoxide, 102 redox potential, see electrode potential redox reaction, 35, 54 redox states, of poly(aniline), 329, 331 reference electrode 40, 48, 58, 70T, 149, 155, 157 primary standard, 40 quasi, 40 saturated calomel electrode, 40, 48, 149, 155, 157, 169, 199, 262, 382, 406, 410 secondary, 40 silver–silver chloride, 58, 70T, 349, 350 silver–silver oxide, 40 reflective, 81, 146, 148, 149T, 303, 305, 399, 407T metal oxides copper oxide, 407T iridium oxide, 400, 407T rhodium oxide, 188 tungsten trioxide, 148–9, 149T, 400, 407T miscellaneous lithium pnictide, 407T tungsten oxyfluoride, 407T polymers PEDOT, 407T poly(diphenylamine), 407T poly(pyrrole), 407T Resazurin, coloration efficiency, 57T Research Frontiers, 398 resistance, 50 of electrode substrate, 11 ITO, effect of, 349 to charge transfer, 86, 105 Resorufin, coloration efficiency, 57T response time, 10–11, 86, 98, 139, 141, 150, 268, 274 metal oxides bismuth oxide, 166 iridium oxide, 156, 159 molybdenum trioxide, 154 nickel oxide, 164 tungsten trioxide, 149, 150 mixtures of metal oxide, tungsten–cerium oxide, 193 organic monomers aromatic amines, 375 pyrazolines, 387, 388T photoelectrochromism, 436 phthalocyanine complexes, 261 lutetium phthalocyanine, 260, 261 polymers poly(aniline)s, 330, 331 poly(thiophene)s, 325 pulsed potentials acceleration, 11 tetrathiafulvalenes, 390T viologens, 346, 349, 351, 361, 363 NTera viologen, 363, 364 reversibility, 39 rf sputtering, 137 metal oxides bismuth oxide, 166 chromium oxide, 167 cobalt oxide, 167 manganese oxide, 175 molybdenum trioxide, 151 nickel oxide, 160, 162, 163, 164 tantalum oxide, 182, 183–4 tin oxide, 183 titanium dioxide, 184 tungsten trioxide, 140, 141, 148 Index 475 rf sputtering (cont.) vanadium pentoxide, 185, 187, 188 mixtures of metal oxide indium–tin oxide, 196 molybdenum–tungsten oxide, 199 titanium dioxide mixtures, 201 tungsten–molybdenum oxide, 199 precious metal incorporation, 204 rhodium oxide, 125, 179–81 annealing of, 181 coloration efficiency, 56T, 181 cyclic voltammetry of, 181 electrochemistry of, 180 formation via anodising Rh metal, 179 sol–gel, 180, 181 hydrated, 180 reflective, 188 Robin–Day classification, 142, 283 rocking-chair mechanism, 16 rotated ring-disc electrode, 358 ruthenium complexes, 309 dinuclear, 267–8 trinuclear, 268 ruthenium dioxide, 181 electrochemistry, 181 formation via electrodeposition, 181; oxidising Ru metal, 181 incorporating platinum, 204T ruthenium dithiolene complexes, 270–4, 309 ruthenium hexacyanoferrate, see ruthenium purple ruthenium polypyridyl complexes, 256 ruthenium purple, 285, 292 XRD, 292 ruthenium tris(2,2 0 -bipyridyl), 265, 266 photosensitiser, 436, 437 effects of ligands, 255T ruthenium–iridium oxide, 203 Rutherford backscattering, 103, 160, 205 rutiles, 127 sacrificial anode methods, of Prussian blue, 283 Safranin O, coloration efficiency, 57T SAGE Incorporated, 398, 447 salt bridge, 39 salvation stabilisation, 89 samarium–vanadium oxide, 202 coloration efficiency, 202 sapphire, 202 saturated calomel electrode, 40, 48, 149, 155, 157, 169, 199, 262, 382, 406, 410 saturation, and colour analysis, 56T, 63, 65, 66, 70 scan rate, 48 scanning tunnelling microscope, 263, 284 SCE, see saturated calomel electrode Schott Glass, ECD, 400 SchottDonnelly mirror, 397 screen printing carbon black, 424 carbon ink, 303, 305 tungsten trioxide, 140 sealing, ECD, see encapsulation, ECD secondary battery, ECD like, 54 secondary electrochromism, 16–17, 165–90, 418, 421 hexacyanoferrates, Prussian blue, 149, 290, 334, 363, 365, 444T metals, bismuth, 444T metal oxides cobalt oxide, 170 copper oxide, 165 iridium oxide, 149, 444T iron oxide, 174 manganese oxide, 165, 176 nickel oxide, 149, 165, 444T, 446, 447 niobium pentoxide, 149, 178 praseodymium oxide, 179 tin oxide, 165 titanium dioxide, 184 tungsten trioxide, 421 vanadium pentoxide, 149, 190, 438, 444T mixtures of metal oxide cerium–titanium oxide, 444T indium–tin oxide, 197 titanium–cerium oxide, 444T organic monomers phenothiazines, 362, 363 tetramethyl phenylenediamine, as, 356, 375, 385 oxyhydroxides, nickel, 400 polymers PEDOT, 149 poly(aniline), 149, 290–1, 333, 334, 444T poly(p-phenylene terephthalate), 159 secondary reference electrodes, 40 second-harmonic effects, 4 Seebeck coefficient, 113 self bleaching, of ECD, 15, 54, 150, 153 self-doped polymers, PEDOT-S, 330, 331 self-erasing ECD mirrors, 387 semiconductor theory, 317 semi-solid, ECD electrolyte, 446 sensors, conducting polymers, and, 312 SERS, see surface-enhanced Raman spectroscopy SHE, see standard hydrogen electrode shear planes, 103 shutters, ECD applications, 363, 404–5 side reactions, 43, 54, 76, 199 hydrogen evolution at MoO 3 , 199 silica, ECD electrolyte thickener, 348, 419 silicon carbide, as photoconductor, 436 silicon phthalocyanine, 264 silicon–cobalt–aluminium oxide, 204 silicon–iridium oxide, 198 silicone–niobium oxide, 200 silver conductive paint, 349 electrodeposition of, 8, 27, 307 476 Index incorporated into indium–tin oxide, 204T tungsten trioxide, 204T vanadium pentoxide, 204T silver oxide, 40 silver–silver chloride, reference electrode, 58, 70T, 349, 350 silver–silver oxide, reference electrode, 40 SIMS, 110, 111–12 SIROF, 155 site-saturation model, metal-oxide mixtures, 190, 192 ski goggles, ECD application, 398 smart cards, ECD applications, 363 smart glass, 397; see also ECD, windows non-electrochromic, 5 smart windows, see ECD applications, windows SmartPaper, 5 sodium tungsten bronze, 27 solar energy storage, ECD applications, 265, 266 solar-energy conversion, conducting polymers, and, 312 solar-powered cells, 15 sol–gel formation of phosphotungstic acid, in titanium dioxide, 201 formation of poly(acrylate)–silica composite with poly(aniline), 333 forming metal oxides, 134–6 cobalt oxide, 135, 168, 195 copper oxide, 170 iridium oxide, 156 iron oxide, 173, 174 manganese oxide, 175, 176 molybdenum trioxide, 135, 152 nickel oxide, 135, 161–3 niobium pentoxide, 134, 176, 178, 200 rhodium oxide, 180, 181 titanium dioxide, 135, 184 tungsten trioxide, 135, 141, 149 vanadium pentoxide, 135, 185 forming mixtures of metal oxides cobalt–aluminium oxide, 195 indium–tin oxide, 196 iridium–titanium oxide, 198 iridium–silicon oxide, 198 iron–niobium pentoxide, 200 molybdenum–niobium oxide, 200 molybdenum–tungsten oxide, 199 nickel–tungsten oxide, 200 niobium–iron oxide, 200 niobium–molybdenum oxide, 200 titanium dioxide mixtures, 201 titanium dioxide, plus phosphotungstic acid, 201 tungsten–molybdenum oxide, 199 with precious metals, 204 with titanium butoxide, 135 solid polymer matrix, mirror of, 397 solid solution electrodes, 41 solubility product bromoanil, o-, 383 viologens, 351, 359 solvatochromism, 3 sonication, 133–4 Sony Corporation, 351 space charges, 105 SPD, see suspended-particle device speciation analyses, 133 spectral locus, 64, 65 spectroelectrochemistry, poly(aniline)s, 333 spectroscopy, impedance, see impedance specular reflectance, see reflective spillover, 407 spin coating annealing of product, 135 formation of metal oxides, 131, 135–6 cerium oxide, 135 cobalt oxide, 135 iron oxide, 135, 174 molybdenum trioxide, 135 niobium pentoxide, 135, 177 tantalum oxide, 135 titanium dioxide, 135, 184 tungsten trioxide, 135, 141 vanadium pentoxide, 135, 185, 186 formation of mixtures of metal oxide, 136 indium–tin oxide, 135, 196 formation of polymers PEDOT-S, 330, 331 poly(acrylate)–poly(aniline) composite, 333 polymeric polypyridyl complex, 254 poly(thiophene)s, 327 spirobenzopyran, 438 spiropyrans, 376 SPM, see solid polymer matrix spray pyrolysis annealing of product, 135 formation of metal oxides, 135 cerium oxide, 135, 166 cobalt oxide, 135, 168, 169 iridium oxide, 158 molybdenum trioxide, 152 nickel oxide, 135, 160 tungsten trioxide, 135, 141 sputtering product oxide is polycrystalline, 81 sputtering in vacuo, 136–8; see also dc magnetron sputtering, electron-beam sputtering, evaporation and rf sputtering stability metal oxide, 128–30 photochemical, 125, 128, 129 electrolyte, 445 tungsten trioxide, 143 Stadsparkasse Bank, ECD, windows, 400 standard electrode potential, 36, 37, 40 of hydrogen electrode, 40 standard exchange current, 47 standard exchange current density, 47 standard hydrogen electrode (SHE), 40 standard observer, in colour analysis, 63, 64 standard rate constant, of electron transfer, 47 Index 477 star polymers, of poly(thiophene)s, 327 Stark effects, 4, 25, 61 stibdic acid polymer, ECD electrolyte, 421T STM, see scanning tunnelling microscope stress, in crystal lattice, 130 strontium titanate, 28 nickel doped, 309 photovoltaic, 437 sublimation, of lutetium phthalocyanine, 259 substituted poly(thiophene)s, 320–1 sub-stoichiometry, see oxygen deficient substrates antimony–copper alloy, 423 antimony-doped tin oxide, 362 carbon, 424 glassy carbon, 358 graphite, 424 durability of, 444–5 ECD, 422–4 fluorine-doped tin oxide, 139, 166, 168, 171, 196, 205, 292, 362, 400, 406, 409, 422 gold, 285 indium–tin oxide, 17, 70T, 86, 96, 128, 129, 135, 138, 139, 141, 150, 151, 152, 156, 158, 159, 164, 166, 167, 181, 182, 191, 257, 284, 293, 294, 305, 306, 326, 330, 331, 333, 349, 375, 382, 385, 404, 417, 422–3, 444–5, 447 K-glass, 422 metallic, 423–4 opaque, 423–4 platinum, 153, 284, 312, 326, 409, 418, 422, 423 resistance of, 11 tin oxide, 289, 354 titanium dioxide, 406 viologens and effect of, 353 sulfuric acid, ECD electrolyte, 82, 86, 149, 178, 259, 349, 409, 420 degradation by, 420 sunglasses, ECD application, 401 supporting electrolyte, 44 surface enhanced Raman spectroscopy viologens, 357 surface potentials, 4 surface states, 86 surfactants, voltammetry and, 356 Surlyn, ECD encapsulation, 425 suspended-particle device, SPD, 5 swamping electrolyte, 75, 76 swapping, of counter ions, 87 sweep rate, see scan rate switchable mirrors, metal hydrides, 307 symmetry factor, 95 Tafel region, 47, 48 Tafel’s law, 43, 46, 47 deviations from, 46 tailoring, of colours, 334 tantalum oxide, 181–3, 198, 446 as ECD electrolyte, 150, 420, 421T, 447 electrochemistry, 183 ion-conductive electrolyte, 181, 183 mechanical stability, 129 optical properties, 183 tantalum oxide, coloration efficiency, 56T, 183 overlayer of, 150 plus platinum, 204T protonic motion, 183 formed via anodising a metal, 182 CVD, 132, 182 dc magnetron sputtering, 136, 182 dip-coating, 182 evaporation, 182 laser ablation, 183 rf sputtering, 182, 183–4 spin coating, 135 water adsorbed on, 183 tantalum–zirconium oxide, 203 coloration efficiency, 203 TCNQ, see tetracyanoquinodimethanide Teflon, 409 television flat-panel screens, 402 pixels, 402, 403 plasma screen, 402 temperature management, ECD, applications, 265 terminal effects, 86, 164, 423 suppressors chromium oxide, 423 magnesium fluoride, 423 tethered electrochromes, see derivatised electrodes tetracyanoquinonedimethanide species, 387, 388–9 optical properties, 389T reversibility, 389 write–erase efficiency, 388 tetrahydrofuran, 327 tetramethylphenylenediamine, 356 as secondary electrochrome, 356, 375, 385 tetrathiafulvalene species, 30, 387, 389–90 ion hopping, 390 ion tunnelling, 390 optical properties, 390T response times, 390T Texas Instruments, 28, 352 thermal evaporation, see evaporation thermal instability, nickel oxide, 160 thermoelectrochromism, 408 lithium vanadate, 190 thermodynamic enhancement, 83–5, 112 enhancement factor W, 83, 84 thiazines, 385–7 ECDs, 385 Methylene Blue, see Methylene Blue thickener, see electrolyte thickener thickness, changes in electrochrome, 51; see also electrostriction thiophene, 313 oligomers, 321T thiophene acetic acid, 3-, 320 three-electrode, potentiostatic coloration, 443 tin oxide, 183 as ionic conductor, 159 478 Index as secondary electrochrome, 165 doped antimony-doped, see antimony-doped tin oxide fluorine-doped, see fluorine-doped tin oxide nickel-doped, 196 electrochromic host, 201 formation via rf sputtering, 183 M ¨ ossbauer spectroscopy, 184 optical properties, 184 infrared max , 183 substrate, 289, 354 tin oxyfluoride, 205 tin phosphate, as ECD electrolyte, 154 tin–cerium oxide, 201 tin–molybdenum oxide, 199 coloration efficiency, 199, 199T–201T titanium alkoxides, 184 titanium butoxide, sol–gel precursor, 135 titanium dioxide, 10, 11, 12, 125, 130, 184, 194 anatase, 437 as secondary electrochrome, 184 coloured with pulsed current, 184 diffusion coefficient, 184 ECD electrolyte, 421T, 445 electrolyte filler, 421 electrochemistry, 184 electrochromic host, 201–2 formed via sol–gel, 201 sputtering, 201 plus ferrocyanide, 201 plus phosphotungstic acid, 201 mechanical stability, 129 ellipsometry, 184 formation via alkoxides, 184 dip coating, 135, 184 evaporation, 8, 185T laser ablation, 184 oxidation of Ti, 184 peroxo species, 184 rf sputtering, 184, 185T sol–gel, 135, 184, 185T spin coating, 135, 184 thermal evaporation, 184 nanostructured, 360–4 optical properties, 184 coloration efficiency, 56T, 184, 185T optically passive, 184 photo-activity, 445 photoconductor, 437, 438 photostability, 1153 photovoltaic, 437 substrate, 406 titanium oxyfluoride, 205 coloration efficiency, 205 cycle life, 205 electrochemistry, 205 formation via dc-sputtering, 205 titanium oxynitride, 184 titanium propoxide, 201 titanium–cerium oxide as secondary electrochrome, 444T formed via dc magnetron sputtering, 136 titanium–cerium–titanium oxide, 203 titanium–cerium–vanadium oxide, 203 titanium–iridium oxide, 198 titanium–iron oxide charge transfer, 202 formed via dip-coating, 201, 202 titanium–molybdenum oxide, 199 coloration efficiency, 199 titanium–nickel oxide, 201 formed via electrodeposition, 201 titanium–niobium oxide, 200 titanium–tungsten oxide, 202 titanium–zirconium–cerium oxide, 203 formed via electrodeposition, 202 titanium–tungsten–vanadium oxide, 203 titanium–vanadium oxide, 202 optically passive, 202 titanium–zirconium–cerium oxide, 203 titanium–zirconium–vanadium oxide, 203 titration, electrochemical, 104 tolidine, o-, 77, 78 toluene, gasochromic, sensor for, 407 Toluylene Red, coloration efficiency, 57T tone, and colour analysis, 63 toys, as ECD application, 363 transfer coefficient, 47 transmittivity, 62 transport number, 44, 83 transport, through liquid electrolytes, 75 triflic acid, as ECD electrolyte, 150, 421 trimethoxysilyl viologen, 346 tris(pyrazolyl)borato-molybdenum complexes, 269–70 tris-isocyanate complexes, 268 tristimulus, and colour analysis, 63, 67 TTF, see tetrathiafulvalene tungstate ion, from degradation of WO 3 , 89 tungsten hexacarbonyl, 131, 141 forming tungsten trioxide, 397 tungsten oxyfluoride, 153, 205–6, 446 coloration efficiency, 205 electrochemistry, 206 overlayer of, 150, 446 specular reflectance, 407T formed via dc magnetron sputtering, 205 tungsten trioxide, 9, 10, 11, 25, 27, 28, 35, 40, 79, 81, 103, 109, 110, 111, 115, 125, 130, 139–51, 156, 187, 191, 200, 201, 206, 303, 305, 308, 399, 410, 419, 436, 437, 438, 443, 446 activation energy, 111T amorphous, 81, 88, 113 Anderson transition, 81, 99, 142, 149 annealing of, 88, 140, 148 as primary electrochrome, 16, 149, 154, 159, 165, 170, 178, 179, 184, 190, 197, 290, 291, 333, 334, 400, 421, 436, 438, 444T, 446, 447 as secondary electrochromes, 421 bleaching, 106 Index 479 tungsten trioxide (cont.) bronze, 81, 113, 144 charge transport through, 60, 85 chemical reduction of, 25, 89, 109 chemical degradation, 149 dissolution in acid, 89 chemical diffusion coefficient, 83, 84T, 85T, 195T colour, source of F-centres, 145 intervalence, 145 oxygen extraction, 145 polarons, 145 coloration mechanism a two-electron process, 103 ‘complicated’, 80 involves W IV , 103 coloration, without electrolyte, 28 conductivity, 142 dielectric properties, 143 electron localisation, 142 electronic, 99 electrons are rate limiting, 143 insulator at x ¼ 143 ionic, 83 low conductivity of, 81 metallic at high x, 143 polaron–polaron interactions in, 88 dry lithiation of, 418 ECD, first, 27 in paper, 27, 405 ECDs of, 28, 29, 61, 104, 139, 149, 397, 399, 402, 408, 409, 410 ECD applications display devices, 149 mirrors, 149 sunglasses, 401 watch displays, 149 windows, 149, 400 formation via, 16 colloidal tungstate, 141 CVD, 131, 141, 148, 150, 397 dc magnetron sputtering, 136, 141 deposition in vacuo, 129 dip coating, 135, 141, 148T electrodeposition, 132, 140, 141, 148T evaporation, 81, 99, 140–1, 147, 148T, 150 organometallic precursors, 141 oxidising W metal, 81, 150 peroxo species, 10, 133, 135, 141 rf sputtering, 140, 141, 146, 147, 148, 148T screen printing, 140 sol–gel, 135, 141, 148T, 149 spin coating, 135, 141, 148T spray pyrolysis, 135, 141 electrochemistry, 142 electrophotography, 28 electrostriction, 87, 129, 130, 445 ellipsometry of, 81, 143 ferroelectric properties, 143 gasochromic, 407T mechanical stability, 129 memory effect, 149, 150 mixtures of, 88, 191–3, 407 plus bismuth, 140; gold, 204T; indium, 140; Perspex, 193; platinum, 204T; silver, 140, 204T neutron diffraction, 144 optical effects, 144–9 colour, by reflection, 149T coloration efficiency, 56T, 148T, 191, 193, 201 spectrum, 144 overlayer of, 446 photo-chargeable battery, 437 photochromism, 103 proton-free layers while bleaching, 106 reflective effects, 143, 148–9, 400, 407T response time, 149, 150 structure crystal phases, 86 crystalline, 98, 104 cubic phase, 89 morphology, 140 oxygen deficiency, 102, 103, 140, 147 perovskite, 140 polycrystalline, 81 structural changes, 143–4 stability, 129, 143 water and, 89, 145, 150 hydrated, 115, 150 tungsten–cerium oxide, 193, 195 formation via dc magnetron sputtering, 136 response time, 193 tungsten–cobalt oxide, chemical diffusion coefficient, 195T tungsten–molybdenum oxide, 88, 191, 192, 199 amorphisation, 193 coloration efficiency, 56T, 192 ECD, 397 electron mobility, 192 formation via CVD, 397 electrodeposition, 199 peroxo species, 199 rf sputtering, 199 sol–gel, 199 intervalence, 192 tungsten–nickel oxide, 130, 200, 436 coloration efficiency, 200 formation via sol–gel, 200 tungsten–niobium oxide, 201 coloration efficiency, 201 tungsten–titanium oxide, 202 tungsten–vanadium oxide, 202 neutral colour, 399 tungsten–vanadium–titanium oxide, 203 tunnelling, 81 Tyndall effect, 134 type-I electrochromes, 33, 43, 45, 46, 54, 328, 346, 354, 356, 359, 396, 403, 417, 419, 425 aromatic amines, 375 coloration kinetics, 75–9, 403 Gentex mirror, 396, 398, 400 480 Index naphthaquinones, 384 type-II electrochromes, 33, 45, 46, 78, 79–115, 417, 419, 425 aromatic amines, 375 bleaching, 79–115 carbazoles, 376 chloranil, 382 coloration kinetics, 75–9 electrodeposition of metals, 303, 305 viologens, 346, 348–9, 351, 354 type-III electrochromes, 45, 46, 54, 79, 397, 403, 407, 417, 445 bleaching of, 79–115 coloration, 79–115, 403 concentration gradients, 303, 305 diffusion coefficients through, 83 formation via chemical tethering, 346, 361 kinetic modelling, 91–115; see also coloration models viologens, 346, 361 viscous solvents immobilising, 391 types, of electrochrome, 7–9 u 0 v 0 uniform colour space, 67, 70, 71 Ucolite, ECD, 400 underlayers, nickel, 86, 164 uniform colour space, 66, 67, 70, 71 UV electrochromism, 165 vacuum evaporation, product oxide is amorphous, 81 value, and colour analysis, 63 vanadium dioxide, 190 vanadium ethoxide, 131 vanadium hexacyanoferrate, 292–3 cyclic voltammetry, 292 XPS, 293 vanadium pentoxide, 16, 56T, 87, 109, 130, 156, 185–90, 399, 446 annealing, 185 anodising vanadium metal, 185, 186, 187 as secondary electrochrome, 16, 149, 190, 438, 444T bleaching rate, 188 chemical diffusion coefficient, 85T coloration rate, 188 cycle life, 188 cyclic voltammetry, 187 dissolution in acid, 186 ECDs of, 189–90 electrochemistry, 186–8 quasi-reversible, 188 electrostriction of, 87, 129 ellipsometry, 187 formation via cathodic arc deposition, 185 CVD, 132, 190T dc sputtering, 136, 185 dip coating, 135 electrodeposition, 186 electron-beam sputtering, 138–206 evaporation, 185, 186 flash evaporation, 185 laser ablation, 185 peroxo species, 186 rf sputtering, 185, 187, 188, 190T sol–gel, 135, 185, 190T spin coating, 135, 185, 186 vanadium propoxide, 185 xerogel, 185 intervalence effects, 188 mixtures as electrochromic host, 202 composites with gold, 204, 204T with melamine, 190, 202 with poly(aniline), 190, 202 with silver, 204T optical properties, 188–9 coloration efficiency, 56T, 189, 190T structure, 186, 188 monoclinic, 186 write–erase efficiency, 188 xerogel, 202 XPS, 189 XRD, 185, 202 vanadium propoxide, forming vanadium pentoxide, 185 vanadium–dysprosium oxide, 202 vanadium–magnesium–nickel oxide, 203 vanadium–molybdenum oxide, 199 vanadium–neodymium oxide, 202 vanadium–nickel oxide, 202 optically passive, 202 vanadium–samarium oxide, 202 coloration efficiency, 202 vanadium–titanium oxide, 202 optically passive, 202 vanadium–titanium–cerium oxide, 203 vanadium–titanium–tungsten oxide, 203 vanadium–titanium–zirconium oxide, 203 vanadium–tungsten oxide, 202 coloration efficiency, 202 neutral colour, 399 video display units, 402, 403 violenes, 374 viologens, 12, 17, 341–66, 385 asymmetric, 355, 360 bleaching, chemical, 359 chain length, see viologens, substituent charge movement through solid layers of, 81 charge transfer complexation, 342–5, 353, 359 potentiostatic, 358 via pulsed potentials, 365 comproportionation of, 357–8, 365 contrast ratio, 346, 349, 352 counter ions, effect of, 352–4 covalently tethered, 10, 12 cycle life, 362 cyclic voltammetry, 352, 355, 356, 357, 359 degradation of, 357 oiling, 351 Index 481 viologens (cont.) derivatised electrodes, 348 di-reduced, 343, 357, 358 ECDs, 346–8, 349, 352, 357 five-colour, 385 memory, 348, 362 paper quality, 362 see also cyanophenyl paraquat, heptyl viologen, Nanochromics and NTera ultra fast, 363 electrochemistry, 342, 353, 354–5 electrochemistry, quasi-reversibility, 358 electrodeposition, 354 electron transfer rate, 359 electropolychromic, 365 ESR, 352, 356 in Nafion, 405 in paper, 365, 366, 405 infrared spectroscopy of, 358 memory effect, 348, 362 micellar, 355–6 critical micelle concentration, 355, 356 mixed valency of, 356 modified, 360 optical properties, 344T coloration efficiency, 349, 361, 362, 363 colours of, 343, 351 extinction coefficient, 343, 344T, 349 polymers of, 328–9, 331, 347 poly(pyrrole), 346 poly(thiophene), 347 oligomers, 364 photoelectrochemistry, 362 photostability, 129 pseudo bipyridine, 2,2 0 -, 364 phenanthroline, 3,8-, 360 radicals of aging effects, 355, 357; see also recrystallisation chemical oxidation of, 352, 359 dimerisation, 351, 355, 357, 358 radical, nucleation, 358 radical, recrystallisation, 357–8, 359 radical, stability, 352T reduction, multi-step, 353, 354–5 occurs via nucleation, 354 response time, 346, 349, 351, 361, 363 solubility product, 351, 359 substituent, 349, 351–2 alkyl, 359 aryl, cyanophenyl, see cyanophenyl paraquat benzyl, 8, 344T, 346, 352T, 356, 358 butyl, 352T ethyl, 344T, 352T, 438 heptyl, see heptyl viologen hexyl, 352T methyl, see methyl viologen pentyl, 351, 352T propyl, 352T octyl, 344T, 352T substrates effect of, 353 on nanostructured titania, 360–4 tethered, 361 type type-I electrochrome, 328, 346, 354, 356, 359 type-II electrochrome, 346, 348–9, 351, 354 type-III electrochrome, 346, 361 write–erase efficiency, 348, 351, 356–60 viscous solvents forming type-III electrochromes, 391 immobilised electrochromes carbazoles in, 391 diacetylbenzene, p-, 390T, 391T diethyl terephthalate, 391T dimethyl terephthalate, 391T Methylene Blue, 391, 391T thickeners poly(AMPS), 391 poly(aniline), 391 poly(siloxane), 391 PVPD, 391T, 395–410 visors, ECD application, 401 volatile memory, 54 voltammetry, cyclic, see cyclic voltammetry voltmeters, 39 watch face, application, ECD, 443 of tungsten trioxide, 149 water adsorbed, 89, 183 and molybdenum trioxide, 89 and tungsten trioxide, 145, 150 coloration, acceleration, 90 counter-ion interaction, 89 degrades metal-oxide films, 89, 128–9 dissolves ITO, 444 ionisation of, 89 occluded, 87, 89, 96–7, 156, 163 solid oxide films, effect on, 89 wavelength maximum, 53 change with insertion coefficient for WO 3 , 53 Wien effect, 4 white point, and colour analysis, 64 whitener, in ECD electrolyte, 159, 384, 418, 422 windows, ECD, see applications, ECD windows working electrode, 48 write–erase efficiency, 11–12, 129, 144–9, 156, 164, 259, 286, 348, 351, 356–60 and tethered electrochromes, 12, 346 xerogel, 161, 185 vanadium pentoxide, 202 Xerox, 5 XPS of indium–tin oxide, 197, 445 iridium oxide, 26 manganese oxide, 176 molybdenum trioxide,1529, 153 Prussian blue, 288 tungsten trioxide, 103 vanadium pentoxide, 189 482 Index X-ray reflector, ECD application, 397 XRD of molybdenum trioxide, 153 praseodymium oxide, 179 Prussian blue, 283 ruthenium purple, 292 tungsten trioxide, 140, 141 vanadium pentoxide, 185, 202 XYZ-tristimulus, and colour analysis, 63 YAG laser, 266, 267 ytterbium phthalocyanine, 261, 291 colour source, 261 formed via plasma polymerisation, 291 yttrium–nickel oxide, 200 zinc iodide, ECD electrolyte, 408 zinc phthalocyanine, chemical diffusion coefficient, 85T zinc TPP, 264 zirconium dioxide, ECD electrolyte, 421T electrochromic host, 203 electro-inert, 203 zirconium–cerium oxide, 203 zirconium–cerium–titanium oxide, 203 zirconium– titanium–vanadium oxide, 203 zirconium–tantalum oxide, 203 coloration efficiency, 203 Z-scale, Kosower, 343 Index 483 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 y x W Plate 1 Colour CIE 1931 xy chromaticity diagram with labelled white point (W). Plate 2 A series of neutral EDOT and BEDOT-arylene variable colour electrochromic polymer films on ITO–glass illustrating range of colours available. (Original figure as used for published black and white photo from Sapp, S., Sotzing, G. A. and Reynolds, J. R. ‘High contrast ratio and fast-switching dual polymer electrochromic devices’. Chem. Mater., 10, 1998, 2101–8, by permission of The American Chemical Society.) Comonomer Solution Composition O O O O O O O O O O O O O O O O O S S S y + BiEDOT BEDOT-NMeCz electropolymerization x S N CH 3 N S O O O y O S S O O O S S S S S N CH 3 CH 3 x 100% BiEDOT 577 559 530 464 434 431 429 420 420 90:10 80:20 70:30 50:50 30:70 20:80 10:90 100% BEDOT-NMeCz Neutral Polymer λ max (nm) Neutral Electrochromic Response (Photograph) Plate 3 Representative structures and electrochromic properties of electrochemically prepared copolymers of varied compositions. (Figure reproduced from Gaupp, C. L. and Reynolds, J. R. ‘Multichromic copolymers based on 3,6-bis[2-(3,4-ethylenedioxythiophene)]-N-alkylcarbazole derivatives’. Macromolecules, 36, 2003, 6305–15, by permission of The American Chemical Society.) Plate 4 Gentex window of area 1 Â2 m 2 . The top right pane has been electro- coloured. The other three panes are bleached. (Reproduced with permission from Rosseinsky, D. R. and Mortimer, R. J. ‘Electrochromic systems and the prospects for devices’. Adv. Mater., 13, 2001, 783–93, with permission of VCH–Wiley.) Plate 5 All-solid-state electrochromic motorcycle helmet manufactured in Sweden by Chromogenics AB. The primary electrochrome layer is WO 3 , and the secondary layer is NiO x . (Reproduced with permission of Professor C. G. Granqvist, of Uppsala University.) Plate 6 Pixel array showing no cross-talk between close picture elements (‘pixels’), with solution-phase electrochromes. The unconnected pixels experience insufficient potential for coloration spread to ensue, even though the electrochromes (TMPD and heptyl viologen) are always in solution. The pixels can be made virtually microscopic in size. (Reproduced with permission from Leventis, N., Chen, M., Liapis, A. I., Johnson, J. W. and Jain, A. ‘Characterization of 3 Â3 matrix arrays of solution-phase electrochromic cells’. J. Electrochem. Soc., 145, 1998, L55–8, with permission of The Electrochemical Society.) Plate 7 Gentex windows being tested in Florida. A man is just visible beneath the nearest. (Reproduced with permission from Rosseinsky, D. R. and Mortimer, R. J. ‘Electrochromic systems and the prospects for devices’. Adv. Mater., 13, 2001, 783–93, with permission of VCH–Wiley.) This page intentionally left blank ELECTROCHROMISM AND ELECTROCHROMIC DEVICES Electrochromism has advanced greatly over the past decade with electrochromic substances – organic and/or inorganic materials and polymers – providing widespread applications in light-attenuation, displays and analysis. Using reader-friendly electrochemistry, this book leads from electrochromic scope and history to new and searching presentations of optical quantification and theoretical mechanistic models. Non-electrode electrochromism and photo-electrochromism are summarised, with updated comprehensive reviews of electrochromic oxides (tungsten trioxide particularly), metal coordination complexes and metal cyanometallates, viologens and other organics; and more recent exotics such as fullerenes, hydrides and conjugated electroactive polymers are also covered. The book concludes by examining device construction and durability. Examples of real-world applications are provided, including minimal-power electrochromic building fenestration, an eco-friendly application that could replace air conditioning; moderately sized electrochromic vehicle mirrors; large electrochromic windows for aircraft; and reflective displays such as quasi-electrochromic sensors for analysis, and electrochromic strips for monitoring of frozen-food refrigeration. With an extensive bibliography, and step-by-step development from simple examples to sophisticated theories, this book is ideal for researchers in materials science, polymer science, electrical engineering, physics, chemistry, bioscience and (applied) optoelectronics. P . M . S . M O N K is a Senior Lecturer in physical chemistry at the Manchester Metropolitan University in Manchester, UK. R . J . M O R T I M E R is a Professor of physical chemistry at Loughborough University, Loughborough, UK. D . R . R O S S E I N S K Y , erstwhile physical chemist (Reader) at the University of Exeter, UK, is an Hon. University Fellow in physics, and a Research Associate of the Department of Chemistry at Rhodes University in Grahamstown, South Africa. ELECTROCHROMISM AND ELECTROCHROMIC DEVICES P. M. S. MONK, Manchester Metropolitan University R. J. MORTIMER Loughborough University AND D. R. ROSSEINSKY University of Exeter CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521822695 © P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky 2007 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2007 ISBN-13 ISBN-13 978-0-511-50806-6 978-0-521-82269-5 eBook (NetLibrary) hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Contents 1 2 3 4 Preface page ix Acknowledgements xii List of symbols and units xiv List of abbreviations and acronyms xvii Introduction to electrochromism 1 1.1 Electrode reactions and colour: electrochromism 1 1.2 Non-redox electrochromism 3 1.3 Previous reviews of electrochromism and electrochromic work 6 7 1.4 Criteria and terminology for ECD operation 17 1.5 Multiple-colour systems: electropolychromism 18 References A brief history of electrochromism 25 2.1 Bibliography; and ‘electrochromism’ 25 2.2 Early redox-coloration chemistry 25 2.3 Prussian blue evocation in historic redox-coloration processes 25 2.4 Twentieth century: developments up to 1980 27 References 30 Electrochemical background 33 3.1 Introduction 33 3.2 Equilibrium and thermodynamic considerations 34 3.3 Rates of charge and mass transport through a cell 41 3.4 Dynamic electrochemistry 46 References 51 Optical effects and quantification of colour 52 4.1 Amount of colour formed: extrinsic colour 52 4.2 The electrochromic memory effect 53 4.3 Intrinsic colour: coloration efficiency 54 4.4 Optical charge transfer (CT) 60 4.5 Colour analysis of electrochromes 62 References 71 v vi Contents 5 Kinetics of electrochromic operation 75 5.1 Kinetic considerations for type-I and type-II electrochromes: transport of electrochrome through liquid solutions 75 5.2 Kinetics and mechanisms of coloration in type-II bipyridiliums 79 5.3 Kinetic considerations for bleaching type-II electrochromes and bleaching and coloration of type-III electrochromes: transport of counter ions through solid electrochromes 79 5.4 Concluding summary 115 References 115 125 6 Metal oxides 125 6.1 Introduction to metal-oxide electrochromes 6.2 Metal oxides: primary electrochromes 139 6.3 Metal oxides: secondary electrochromes 165 6.4 Metal oxides: dual-metal electrochromes 190 References 206 7 Electrochromism within metal coordination complexes 253 7.1 Redox coloration and the underlying electronic transitions 253 7.2 Electrochromism of polypyridyl complexes 254 7.3 Electrochromism in metallophthalocyanines and porphyrins 258 7.4 Near-infrared region electrochromic systems 265 References 274 8 Electrochromism by intervalence charge-transfer coloration: metal hexacyanometallates 282 8.1 Prussian blue systems: history and bulk properties 282 8.2 Preparation of Prussian blue thin films 283 8.3 Electrochemistry, in situ spectroscopy and characterisation of Prussian blue thin films 285 8.4 Prussian blue electrochromic devices 289 8.5 Prussian blue analogues 291 References 296 9 Miscellaneous inorganic electrochromes 303 9.1 Fullerene-based electrochromes 303 9.2 Other carbon-based electrochromes 304 9.3 Reversible electrodeposition of metals 305 9.4 Reflecting metal hydrides 307 9.5 Other miscellaneous inorganic electrochromes 309 References 309 10 Conjugated conducting polymers 312 10.1 Introduction to conjugated conducting polymers 312 10.2 Poly(thiophene)s as electrochromes 318 Contents vii 11 12 13 14 10.3 Poly(pyrrole)s and dioxypyrroles as electrochromes 10.4 Poly(aniline)s as electrochromes 10.5 Directed assembly of electrochromic electroactive conducting polymers 10.6 Electrochromes based on electroactive conducting polymer composites 10.7 ECDs using both electroactive conducting polymers and inorganic electrochromes 10.8 Conclusions and outlook References The viologens 11.1 Introduction 11.2 Bipyridilium redox chemistry 11.3 Bipyridilium species for inclusion within ECDs 11.4 Recent elaborations References Miscellaneous organic electrochromes 12.1 Monomeric electrochromes 12.2 Tethered electrochromic species 12.3 Electrochromes immobilised within viscous solvents References Applications of electrochromic devices 13.1 Introduction 13.2 Reflective electrochromic devices: electrochromic car mirrors 13.3 Transmissive ECD windows for buildings and aircraft 13.4 Electrochromic displays for displaying images and data 13.5 ECD light modulators and shutters in message-laser applications 13.6 Electrochromic paper 13.7 Electrochromes applied in quasi-electrochromic or nonelectrochromic processes: sensors and analysis 13.8 Miscellaneous electrochromic applications 13.9 Combinatorial monitoring of multiples of varied electrode materials References Fundamentals of device construction 14.1 Fundamentals of ECD construction 14.2 Electrolyte layers for ECDs 14.3 Electrodes for ECD construction 327 328 331 332 333 334 335 341 341 342 346 360 366 374 374 387 391 391 395 395 395 397 401 404 405 406 407 409 410 417 417 419 422 viii Contents 14.4 Device encapsulation References 15 Photoelectrochromism 15.1 Introduction 15.2 Direction of beam 15.3 Device types 15.4 Photochromic–electrochromic systems References 16 Device durability 16.1 Introduction 16.2 Durability of transparent electrodes 16.3 Durability of the electrolyte layers 16.4 Enhancing the durability of electrochrome layers 16.5 Durability of electrochromic devices after assembly References Index 424 425 433 433 433 434 438 440 443 443 444 445 445 446 449 452 There are thousands of chemical systems that are intrinsically electrochromic. However. and we wish to present a scientific overview. a contract with Boeing to supply adjustably darkening windows in a new passenger aircraft. Numerous other applications have been contemplated. attested by the thousands of patents on likely winners. that of the Gentex Corporation’s self-darkening rear-view mirrors now operating on several million cars. would save billions of dollars in air-conditioning costs. applied widely in the USA. So. a sine qua non for tolerable living conditions there. applications have hitherto been limited. only in the last quarter of the twentieth century has its study gained a real impetus. electric-potential ix . as a patent is sui generis. However. we incorporate here mostly those that have at least a promise of being useful. to display systems.Preface While the topic of electrochromism – the evocation or alteration of colour by passing a current or applying a potential – has a history dating back to the nineteenth century. apart from one astonishing success. but universally used liquid crystal displays present formidable rivalry. In tropical and equatorial climes. large-scale screens do offer an attractive scope where liquid crystals might struggle. Now they have achieved a telling next step. Another application. and while including explanatory examples. Our approach has been to concentrate on systems that colorise or change colour by electron transfer (‘redox’) processes. without totally neglecting other. savings would be proportionally greater: Singapore for example spends one quarter of its GDP (gross domestic product) on air conditioning. and electrochromics should almost certainly be much more economical than plasma screens. is a further goal. which would have at least doubled the work without in our view commensurate advantages. The ultimate goal of contemporary studies is the provision of large-scale electrochromic windows for buildings at modest expenditure which. There is thus at present a huge flurry of activity to hit the jackpot. we have not scanned in detail the patent record. as is outlined in the first chapter. ‘IME’. the work that has been conducted on a wide variety of materials follow. systems now particularly useful in applications to bioscience. The latter especially seem set to shine. and any reader not familiar with the electrochemistry presented here may find this explained sufficiently in Chapter 3. Each title is cited as it appeared when first published. tungsten trioxide. TX. 18 of Chapter 1.10 The basis of the processes on which we concentrate is electrochemical. we decided to reproduce the full titles of each paper cited. the authors of this present volume called a Solid-State Group (Royal Society of Chemisty) meeting in London. are outlined. we do quite frequently repeat the gist of some previous chapter(s). We have systematised capitalisation throughout (and corrected spelling errors in two papers).8 Further electrochromics symposia occurred at Electrochemical Society meetings that took place at San Antonio. In our account we have probably not succeeded in conveying all the aesthetic pleasure of studying aspects of colour and its creation. Fl). Applications and tests finish the account.7 IME-5 in Colorado in 2002 and IME-6 in Brno. and those familiar with this arcane science may choose to flip through a chapter largely comprising ‘elderly electrochemistry’. In subsequent chapters.5 IME3 was in London in 1998.4 IME-2 in San Diego in 1996. Venice in 1994.1 Soon afterwards was ‘Fundamentals of Electrochromic Devices’ organised by The American Institute of Chemical Engineers at their Annual Meeting in Chicago.2 The following year. A historical outline is given in Chapter 2. Probably the first was The Electrochemical Society meeting in 1989 (in Hollywood.x Preface dependent. Details of assessing coloration follow in Chapter 4. The first such meeting ‘IME-1’ met in Murano. A comment about the citations which end each chapter: early during our discussions of the book’s contents. 11–16 November 1990. in 19969 and Paris in 2003. or the profound science-and-technology interest of understanding the reactions and of mastering the associated processes: this book does represent an attempt to spread . and in Chapter 5 attempts at theoretically modelling the electrochromic process in the most popular electrochromic material to date. to quote from ref. At the Electrochemical Society meeting in New Orleans (in 1994). A fairly extensive presentation of twentiethcentury electrochemistry in Chapter 3 seems necessary also to follow some later details of the exposition. Several international gatherings have been convened to discuss electrochromism for devices.6 IME-4 in Uppsala in 2000. Czech Republic in 2004. In order hopefully to make each chapter almost freestanding. from metal oxides through complexed metals and metalorganic complexes to conjugated conductive polymers.3 it was decided to host the first of the so-called International Meetings on Electrochromism. issue 4. Proceedings of the Annual Meeting of the American Institute of Chemical Engineers. published in Sol. Sol.).). 25.g. Proceedings volume was Sol. D.’ References 1. Proceedings volume was Sol. Energy Mater. Energy Mater. 4 rather than the customary 1–4. 10. issue 1–4. 96–24. Cells. Sol. 54. and Nazri. 1990. 2001. 46. 3. C. 2006. Energy Mater. However. and MacArthur. In this version. 2. NJ. Rougier. K. (eds. (eds. issue 2–4. Pennington. D. further at stake is the prospect of controlling an important part of personal environments while economising on air-conditioning costs. 6. issue 3–4. 90. .. 9. 1994. 5. 4.). thereby cutting down fuel consumption and lessening the human ‘carbon footprint’. 2003–17. and MacArthur. which requires that they be cited separately. Proceedings volume was Electrochromic Materials III. 2003. Proceedings volume was Electrochim. 1998. G.). NJ. to cite the mode words. Proceedings volume was Sol. 1992. NJ. Cells. Pennington. Cells. B. (eds. Electrochemical Society. Electrochemical Society. C. Sol. D. Pennington. 94–2. Pennington. read on. Carpenter. 195–381. M.. Electrochemical Society. 1999. Electrochemical Society. There are the other perhaps lesser applications that are also promisingly useful. Proceedings volume was Electrochromic Materials II. A. Greenberg. M. 39. to a more controlled-colour future. A. (eds. Proceedings volume was Electrochromic Materials. D. Sol. Acta. Ho. Cells. Cells. 2. 8. K. 7. 1996. Sol. Proceedings volume was Electrochromic Materials and Applications. Ho. 3. and Corrigan. Energy Mater. listed in full e. issue 13–14.Preface xi these interests. 56. So. K. 90–2. A. ‘each reference citation is hyper-linked to the reference itself. A.-C. 1. 1995. The need arises from the parallel publication of this monograph as an e-book. unusually. DISCLAIMER: Superscripted reference citations in the text are. Energy Mater. Rauh. NJ. Proceedings volume was Sol. Acknowledgements We are indebted to numerous colleagues and correspondents who have collaborated in research or in providing information. Professor Paul O’Shea of Nottingham University. and Professor Steve Fletcher of Loughborough University. graduate of Carnegie-Mellon University. those on the computer helpdesk at MMU who helped with the scanning of figures. Universidade de Sao Paulo.D. Elsevier Science. M. University of Oxford. Berkeley – erstwhile of the Reynolds group at the University of Florida. Professor Hassan Kellawi of Damascus University. ˜ Brazil. We also wish to thank the following for permission to reproduce the figures (in alphabetical order): The American Chemical Society. Dr Steve J. RJM wishes to thank: Dr Joanne L. University of Sheffield. Professor Mike D. University of Florida. Ms Julie Slocombe. University of California. Professor John R. The Electrochemical Society. Loughborough University). Dr Tom Guarr of Gentex. Vickers erstwhile of the Universities of Birmingham and Sheffield. Dr Barry C. Dr Natalie M. and Dr Andrew Soutar and Dr Zhang Xiao of SIMTech in Singapore. Dr Frank Marken. Rowley of the University of Birmingham. PMSM wishes to thank: Professor Claus-Goren Granqvist of Uppsala ¨ ´ University. now heading Thermal Transfer (Singapore). The Royal xii . Dillingham. Beer. Also. Reynolds. Dr Richard Hann of ICI. the late Dr Brian Jackson of Cookson Ltd. Ward. erstwhile of Exeter University. University of Florida. Mr (now Captain) Hanyong Lim of Singapore. (Ph. DRR wishes to thank: Bill Freeman Esq. Dyer.. then of Finisar Corp. Loew of the University of Connecticut Health Center and Dr Yoshinori Nishikitani of the Nippon Mitsubishi Oil Corporation. Professor Susana de Cordoba. Thompson. University of Bath. The American Institute of Physics. Professor Paul D. Aubrey L. Dr Andrew Glidle now of Glasgow University. Professor L. and Assistant Editor. P. who have enabled us to undertake this project. We alone are responsible for the contents of the book including the errors. we also acknowledge the kind help of the following: Dr Charles Dubois. and a particularly big thank you to the copy-editor Zoe Lewin for her consistent ¨ good humour and professionalism. The Japanese Society of Physics. Anna Littlewood. . we wish to thank Dr Tim Fishlock (now at the RSC in Cambridge. Collman and colleagues. who first commissioned the book. if one individual is to be singled out in the general field. UK). Claes-Goran ˚ Granqvist of the Angstrom Laboratory. together with Jo Bottrill of the production team for their help. We apologise if we have been preoccupied or merely absent when you needed us. Though obvious new leaders exploring different avenues are currently emerging. formerly of the University of Florida. and his successor Dr Michelle Carey. The Society of Applied Spectroscopy.Acknowledgements xiii Society of Chemistry (RSC). has to be acknowledged for the huge input into electrochromism that he has sustained over decades. and the Society for Photo and Information Engineering. In collecting the artwork for figures. We owe much to our families. We also thank the numbers of kindly reviewers of our earlier book (and even the two who commented adversely) and much appreciate passing comment in a paper by Dr J. From the staff of Cambridge University Press. Uppsala. Symbols and units A Abs c(y. area optical absorbance time-dependent concentration of charge at a distance of y into a solid thin film maximum concentration of charge in a thin film initial concentration of charge in a thin film diffusion coefficient chemical diffusion coefficient thickness of a thin film charge on an electron electron energy potential activation energy applied potential equilibrium potential potential of anodic peak potential of cathodic peak standard electrode potential electron volt Faraday constant hertz current density subscripted.t) cm c0 D D d e eÀ E E Ea E(appl) E(eq) Epa Epc E eV F Hz i i ib ic io J Jo K Ka F ampere. . represents component 1 or 2 . . bleaching current density coloration current density exchange current density imaginary part of impedance charge flux (rate of passage of electrons or ionic species) equilibrium constant equilibrium constant of acid ionisation xiv . g. y. z.1) x1 proton density in a solid thin film xo x. of a solid. spherical grain) S Seebeck coefficient s second T thermodynamic temperature t time v scan rate V volt V volume applied potential Va W Wagner enhancement factor (‘thermodynamic enhancement factor’) x insertion coefficient insertion coefficient at a percolation threshold x(critical) constant (of value %0. w or c subscripted.List of symbols and units xv equilibrium constant of ionic solubility (‘solubility product’) time-dependent thickness of a narrow layer of the WO3 film adjacent to the electrolyte (during electro-bleaching) M mol dmÀ3 n number in part of iterative calculation n number of electrons in a redox reaction p volume charge density of protons in the H0WO3 p the operator –log10 Pa pascal q charge per unit volume Q charge R gas constant R real component of impedance r radius of sphere (e. non-integral composition indicators. in nonstoichiometric materials Z impedance Ksp l(t) g " o p s gamma photon extinction coefficient (‘molar absorptivity’) coloration efficiency coloration efficiency of an electrochromic device coloration efficiency of primary electrochrome coloration efficiency of secondary electrochrome overpotential . chemical potential mobility of ions mobility of electrons frequency of light density of atoms in a thin film constant equal to (2 e d i0) electronic conductivity ‘characteristic time’ for diffusion membrane surface potential kinematic viscosity velocity of solution flow frequency of ac signal .xvi List of symbols and units max L (ion) (electron) 0 s D js o wavelength wavelength maximum ionic molar conductivity mobility. 20 -bipyridine 4.40 -bipyridilium] cathode-ray tube charge transfer xvii .10 -bis(p-cyanophenyl)4.6-bis[2-(3.20 -bis(3.4-ethylenedioxythiophene)]N-alkylcarbazole 2.4-ethylenedioxythiophene) 3.40 -bipyridilium crystalline catecholate composite coloration efficiency counter electrode cholesteric liquid crystals Commission Internationale de l’Eclairage critical micelle concentration cyanophenyl paraquat [1.Abbreviations and acronyms a ac AEIROF AES AFM AIROF AMPS ANEPPS aq AR ASSD ATO BEDOT BEDOT-NMeCz bipy bipm c CAT CCE CE ChLCs CIE cmc CPQ CRT CT amorphous alternating current anodically electrodeposited iridium oxide film atomic emission spectroscopy atomic force microscopy anodically formed iridium oxide film 2-acrylamido-2-methylpropanesulfonic acid 3-{4-[2-(6-dibutylamino)-2-naphthyl]-trans-ethenyl pyridinium} propane sulfonate aqueous anti reflectance all-solid-state device antimony–tin oxide 2. 3-di(thien-3-yl)-5.xviii List of abbreviations and acronyms CTEM CuHCF CVD dc DDTP DEG DMF DMSO EC EC ECB ECD ECM ECW EDAX EDOT EIS EQCM FPE FTIR FTO GC HCF HOMO HRTEM HTB HV IBM ICI IR ITO IUPAC IVCT LB LBL LCD LED LFER conventional transmission electron microscopy copper hexacyanoferrate chemical vapour deposition direct current 2.10 -di-n-heptyl-4.4-(ethylenedioxy)thiophene electrochemical impedance spectroscopy electrochemical quartz-crystal microbalance fluoresceinphosphatidyl-ethanolamine Fourier-transform infrared fluorine[-doped] tin oxide glassy carbon hexacyanoferrate highest occupied molecular orbital high-resolution transmission electron microscopy hexagonal tungsten bronze heptyl viologen (1.7-di(thien-2-yl)thieno[3.4-b] pyrazine diethyleneglycol dimethylformamide dimethyl sulfoxide electrochromic electrode reaction followed by a chemical reaction electrochromic battery electrochromic device electrochromic material electrochromic window energy dispersive analysis of X-rays 3.40 -bipyridilium) Independent Business Machines Imperial Chemical Industries infrared indium–tin oxide International Union of Pure and Applied Chemistry intervalence charge transfer Langmuir–Blodgett layer-by-layer [deposition] liquid crystal display light-emitting diode linear free-energy relationships . 6-bis[2-(3.List of abbreviations and acronyms xix LPCVD LPEI LUMO MB MLCT MOCVD MV nc NCD Ni HCF NMP NRA NREL NVS# OD OEP OLED OTE OTTLE pa PAA PAH PANI PB PBEDOT-B(OC12)2 PBEDOT-N-MeCz PBEDOT-Pyr PBEDOT-PyrPyr(Ph)2 PBuDOP pc Pc PC PCNFBS PdHCF liquid-phase chemical vapour deposition linear poly(ethylene imine) lowest unoccupied molecular orbital Methylene Blue metal-to-ligand charge transfer metal-oxide chemical vapour deposition methyl viologen (1.6-bis[2-(3.10 -dimethyl-4. USA Night Vision System# optical density octaethyl porphyrin organic light-emitting diode optically transparent electrode optically transparent thin-layer electrode peak anodic poly(acrylic acid) poly(allylamine hydrochloride) poly(aniline) Prussian blue poly{1.4-ethylenedioxy)thienyl]N-methylcarbazole} poly{3.4-b]pyrazine} poly[3.8-bis(3-dihydro-thieno[3.4-b]dioxin-5-yl)2.5-didodecyloxybenzene} poly{3.3-b0 ]dithiophen-4(cyanononafluorobutylsulfonyl)methylidene} palladium hexacyanoferrate .4-(butylenes dioxy)pyrrole] peak cathodic dianion of phthalocyanine propylene carbonate poly{cyclopenta[2.3-diphenyl-pyrido[3.4-bis[2-(3.40 -bipyridilium) naphthalocyanine nanochromic display nickel hexacyanoferrate N-methylpyrrolidone nuclear reaction analysis National Renewable Energy Laboratory.1-b.4.4-ethylenedioxy)thienyl]2.4-ethylenedioxy)thienyl] pyridine} poly{5. 4-b]-[1. sodium salt poly(ethylene oxide) poly(ethylene terephthalate) Prussian green potentiostatic intermittence titration technique poly(methyl methacrylate) polaromicrotribometric plasma polymerised poly(1.4-propylenedioxythiophene) poly(styrene sulfonate) poly(triphenylamine) poly(vinyl acrylate) poly(vinyl chloride) physical vapour deposition Prussian white Prussian brown pyridine Quinone reference electrode radio frequency ruthenium purple: iron(III) hexacyanoruthenate(II) rotated ring-disc electrode solid solid solution sacrificial anode saturated calomel electrode semi quinone scanning electron microscopy standard hydrogen electrode ` Systeme internationale secondary ion mass spectroscopy sputtered iridium oxide film solution .4-(ethylenedioxy)thiophene] poly{4-(2.4-(ethylenedioxy)pyrrole] poly[3.4]dioxin-2-ylmethoxy}-1-butanesulfonic acid.3-dihydrothieno[3.3.5-phenylene) poly[3.xx List of abbreviations and acronyms PDLC PEDOP PEDOT PEDOT-S PEO PET PG PITT PMMA PMT PP PP PProDOP PProDOT PSS PTPA PVA PVC PVD PW PX Pyr Q RE rf RP RRDE s s. soln SA SCE SQ SEM SHE SI SIMS SIROF soln phase-dispersed liquid crystals poly[3.4-(propylenedioxy)pyrrole] poly(3. List of abbreviations and acronyms xxi SPD SPM STM TA TCNQ TGA THF TMPD Tp* TTF UCPC UPS VDU VHCF WE WPA XAS XPS XRD XRG suspended particle device solid paper matrix scanning tunnelling microscopy thiazine tetracyanoquinodimethane thermogravimetric analysis tetrahydrofuran tetramethylphenylenediamine hydrotris(3.5-dimethylpyrazolyl)borate tetrathiafulvalene user-controllable photochromic [material] ultraviolet photoelectron spectroscopy visual display unit vanadium hexacyanoferrate working electrode tungsten phosphoric acid X-ray absorption spectroscopy X-ray photoelectron spectroscopy X-ray diffraction xerogel . . Furthermore. R.1) in a ‘redox’ reaction that takes place at an electrode. i. with external connections.1 Electrode reactions and colour: electrochromism The terminology and basis of the phenomenon that we address are briefly outlined in this chapter. comprise ‘the electrode’. ‘oxidation’.1). we and others often depart from this complete definition when we imply that ‘the electrode’ comprises the just-italicised component. a molecule or radical. in order to make up a cell allowing passage of current. Eq. An electroactive material may be an atom or ion. and must be in contact with the electrode substrate prior to successful electron transfer. and can be viewed as a ‘half-cell’: oxidised form.’ We thus usually refer to the ‘electrode substrate’ for the metal or metal-like component to make the distinction clear. in Chapter 3 it is emphasised that any electrode in a working system must be accompanied by a second electrode. sometimes multiply bonded in a solid film.1) Though in strict electrochemical parlance all the components.1 Introduction to electrochromism 1. in contact with forms O and R of an ‘electroactive’ material. i. O and R and the metallic or quasi-metallic conductor.e. the reverse of Eq. An electrode basically comprises a metal or other conductor.e. especially in electrochromism. An electroactive species can undergo an electron uptake. in part comprising the flow of just those electrons depicted in Eq. (1. (1. It may be in 1 . which conforms with the following definition: ‘An electrode basically comprises a metal or metallic conductor or. with intervening electrolyte. several being summarised later in this chapter. ‘electrochromes’ later in the present text are always ‘electroactive’. an adequately conductive semiconductor often as a thin film on glass. or electron release. (1. ‘reduction’.1). Although there are several usages of the term ‘electrochromism’. O þ electron(s) ! reduced form. as follows. (1. the colour arises from the nonabsorbed wavelengths. Thus.e. (1.) Most electrochromes colourise by reflection. the spectral change accompanying a redox reaction is visually indiscernible if the optical absorptions by the two redox states fall in . for example. in which case that proportion of the electrochrome physically in contact with the electrode substrate undergoes the redox reaction most rapidly.1 That part of a molecular system having or imparting a colour is termed a chromophore. on illumination with white light. then the colour seen is in fact the colour complementary to that absorbed. as in displays. The underlying theory of electrochemical electron-transfer reactions is treated elsewhere. (1:2) where is the frequency. transmission-effective systems. follow a corresponding mechanism. the colours of electroactive species only may be different before and after electron transfer because often the changes are not visible (except by suitable spectrometry) when the wavelengths involved fall outside the visible range. as in windows. . wave-mechanically allowed) energy levels. In other systems.2): E ¼ h ¼ hc . (To repeat. when is the wavelength at the maximum (usually denoted as max) of the absorption band observed in the spectrum of a chromophore.1) or its reverse). the material absorbs red. the electroactive material may be a solid or dispersed within a solid matrix. is related to the magnitude of the energy gap E between these levels according to the Planck relation. Light absorption enables electrons to be promoted between quantised (i. Eq. the remainder of the electroactive material less so.2 Introduction to electrochromism solution – solvated and/or complexed – in which case it must approach sufficiently closely to the electrode substrate and undergo the adjustments that contribute to the (sometimes low) activation energy accompanying electron transfer. and colour becomes evident when photons from part of the spectrum are absorbed by chromophores. The magnitude of E thus relates to the colour since. Hence all materials will undergo change of spectra on redox change. h is the Planck constant and c the speed of light in vacuo. (1. and hence will require different energies E for electron promotion between the ground and excited states. so different redox states will necessarily exhibit different spectroscopic transitions. a blue colour is reflected (hence seen) if. However. White light comprises the wavelengths of all the colours. The wavelength of light absorbed. In other words. its position in the spectrum clearly governs the observed colour. Electroactive species comprise different numbers of electrons before and after the electron-transfer reaction (Eq. such as the ground and first excited states. When the change is in the visible region. called ‘electrochromic probes’.1.3. are employed in studies of biological membrane potentials. regarding intensity-modulation filters for. Many are not electrochromic in the redox sense defined above.’ However. evocation.7 For this reason. demonstrating large solvent-dependent ‘solvatochromic’ shifts. If the colours are sufficiently intense and different. as for example when the absorption band of one redox state is in the visible region while the other is in the UV. (1. then the material is said to be electrochromic and the species undergoing change is usefully termed an ‘electrochrome’. such terms as ‘electrochromic switching or modulation’ are increasingly being used for such invisible effects.6 The applications of electrochromism are outlined in Chapter 13 and the general criteria of device fabrication are outlined in Chapter 14. 5 contains a video sequence clearly demonstrating electrochromic coloration. and even some biological species exhibit the phenomenon:6 Bacteriorhodopsin is said to exhibit very strong electrochromism with a colour change from bright blue to pale yellow.7 Loew et al. first suggested this use of such . it is possible to image the electrical activity of a cell membrane.7 (A similar-looking but intrinsically different mechanism involving deprotonation is outlined below. In many applications it is required to be reversible. the electronic spectrum of (I) is extraordinarily sensitive to its environment. In effect. of colour as effected either by an electron-transfer (redox) process or by a sufficient electric potential.2 Non-redox electrochromism The word ‘electrochromism’ is applied to several. ‘Electrochromism is a change. Firstly. charged species such as 3-{4-[2-(6-dibutylamino-2-naphthyl)-transethenyl] pyridinium} propane sulfonate (‘di-4-ANEPPS’) (I).8 so much information can be gained by quantitative analysis of its UV-vis spectra.2)) become highly asymmetrical. phenomena.4 The website in ref. 1. Visible electrochromism is of course only ever useful for display purposes if one of the colours is markedly different from the other. disparate.2 Simple laboratory demonstrations of electrochromism are legion. then a pragmatic definition of electrochromism may be formulated as follows. IR message-laser pulses in optical fibres. say. the potential surfaces involved in optical electron excitation (see Eq. Many organic and inorganic materials are electrochromic.) For a strongly localised system. here of a highly conjugated poly(thiophene) derivative.2 Non-redox electrochromism 3 the ultraviolet (UV) or near infrared (NIR). such as a protein system where electron-donor and -acceptor sites are separated by large distances. or bleaching. its 8-isomer.14.17 A valuable electrochromic application has been employed by O’Shea18 to probe local potentials on surfaces of biological cell membranes.13 While many biological and biochemical references to ‘electrochromism’ mean a Stark effect of this type.15. if the weak acid experiences an extraneous electric potential. the extent of ionisation is enhanced by further molecular scission (i. O− O S O + N N I This application is not electrochromism as effected by redox processes of the kind we concentrate on in the present work. Vredenberg12 reviewed this aspect of electrochromism in 1997. different colours result. its acidity constant Ka.9 In consequence. With ‘p’ representing (negative) decadic logarithms. but can alternatively be viewed as a molecular Stark effect11 in which some of the UV-vis bands of polarisable molecules evince a spectroscopic shift in the presence of a strong electric field. For example. some are electrochromic in the redox sense. proton release) resulting from the increased stabilisation of the free-proton charge. the outcome may be represented by the equation pKa (js) ¼ pKa (0) À Fjs/RT (ln 10) where js is the membrane surface potential. that is a proton-bearing acid of suitable Ka. with only its deprotonated moiety .10 both of which have large non-linear second-harmonic effects.16 In some instances. the (electrochromic) colours of quinone reduction products have been used to resolve the respective influences of electron and proton transfer processes in bacterial reactions. The effect of electric potential on acidity constants is employed: weak acids in solution are partly ionised into proton and (‘base’) residue to an extent governed ordinarily by the equilibrium constant particular to that acid. or nitroaminostilbene.18 This result (a close parallel of the observed ‘second Wien effect’ in high-field conductimetry on weak acids) arises from combining the Boltzmann equation with the Henderson–Hasselbalch equation.e. A fluorescent molecule is chosen. Such a Stark effect was the original sense implied by ‘electrochromism’ when the word was coined in 1961. The application proceeds as follows. however. this electrochromic effect is unreliable. they pointed out how the best species for this type of work are compounds like (I). However. significant changes are induced by the environment in the dipole moment so on excitation from the ground to the excited states.4 Introduction to electrochromism electrochromism in9 1979. several new products are described as ‘electrochromic’ but are in fact electrokinetic–colloidal systems. Also to be noted. until now. and monitoring even rapid rates of change as can result from say cation acquisition by the surface. although sometimes described as such. and then only when the potential experienced is high enough. The probe molecules are inserted by suitable chemistry into the surface of the cell membrane. tungsten oxide.27.) The gasochromic devices in refs. It comprises two plastic sheets. while electrochromic devices require only cables.’ This ingenious probe of electrical interactions underlying biological cell function thus relies unusually not on electron transfer but on proton transfer as effected by electric potential changes.28. A good example is Gyricon ‘electrochromic paper’. in areas of sufficiently high electric potential. somewhat like SPDs with micro-encapsulation of the active particles. and no measurable current flow. rollable displays. has related to genuine electrochromic systems.23 ‘Probe molecules such as FPE have proved to be particularly versatile indicators of the electrostatic nature of the membrane surface in both artificial and cellular membrane systems. two colour) highly . thus illuminating such areas of js. ‘gasochromic windows’ (also called gasochromic smartglass windows) are generally not electrochromic. 140 mm.31. The huge complication of the requisite gaseous plumbing is rarely addressed. which is also a favoured electrochrome. each of thickness ca. Secondly.29. the adjective ‘electrochromic’ is often applied to a widely differing variety of fenestrative and device applications.e.2 Non-redox electrochromism 5 showing visible fluorescence. For example. Thirdly. To quote. a routine web search using the phrase ‘electrochromic window’ yielded many pages describing a suspended-particle-device (SPD) window.1. Such paper is now being marketed as ‘SmartPaperTM’.32 are not electrochromic in the sense adopted by this book. 25. and foldable.21. (The most studied gasochromic material is. portable signs. electronic newspapers. Then it will fluoresce.26. Lucent and Philips are developing similar products.30. On occasion (as occurs also in some patents) a lack of scientific detail indicates that the claims of some manufacturers’ websites are perhaps excessively ambitious – a practice that may damage the reputation of electrochromic products should a device fail to respond to its advertised specifications. Some SPD windows are also termed ‘Smart Glass’24 – a term that. with no externally applied potential. between which are millions of ‘bichromal’ (i.22 1-(3-sulfonatopropyl)-4-[p[2-(di-n-octylamino)-6-naphthyl]vinyl]-pyridinium betaine. perhaps confusingly. Suitable probe molecules are18. Gyricon is intended for products like electronic books. because the change in colour is wholly attributable to a direct chemical gas þ solid redox reaction.19 fluoresceinphosphatidyl-ethanolamine (FPE) and20.33 developed by Xerox. 2 It includes criteria for electrochromic application. Lampert’s55 2004 review ‘Chromogenic materials’ similarly helps place electrochromism within the wider scope of .47.42 1998. includes a substantial review of electrochromism. The spheres rotate following exposure to an electric field. Some of these systems being deletable and re-usable promise substantial saving of paper. Mortimer and Rosseinsky (in 1995)70 have all written ‘popular-science’ articles on electrochromism.1 mm. Note that the NanoChromicsTM paper described on page 347. or partially (in response to weaker electrical pulses). Scrosati. the preparation of electrochromes and devices. Passerini and Pileggi62 and Scrosati.44. marketed by NTera of Eire. Other reviews of electrochromism appearing within the last fifteen years include (in alphabetical order) those of: Agnihotry36 in 1996. and encompasses all types of electrochromic materials considered in the present book. Dyer and Reynolds61 in 2006. published in 2001.’s 1994 review71 helps frame electrochromic displays within the wider corpus of display technology. Bange et al. Mortimer and Rosseinsky59 in 2001.49 Green50 in 1996. black particles visibly deposit on the upper surface of the sacs.39.68 Hunkin (in 1993)69 and Monk.52 Lampert in 1998. Mortimer. by Monk.48 and 2004. Mortimer and Rosseinsky. is genuinely electrochromic in the redox sense.55 Monk in 200156 and 2003.57 Mortimer58 in 1997.63 1992. Greenberg in 199151 and 1994.53 200154 and 2004. to display a range of grey shades. as from a ‘pencil’ tip attached to a battery also connected to a metallically conductive backing sheet. Bamfield’s book8 Chromic Phenomena. 1. Bowonder et al. Somani and Radhakrishnan64 in 2003 and Yamamoto and Hayashida65 in 1998. both organic and inorganic.37 in 1995.33 Similar mechanisms operate in embedded sacs of sol in which charged black particles are ‘suspended’ (when in the colourless state) but on application of a potential by an ‘electric pencil’. A major review of redox electrochromism appears in Handbook of Inorganic Electrochromic Materials by Granqvist.3 Previous reviews of electrochromism and electrochromic work The broadest overview of all aspects of redox electrochromism is Electrochromism: Fundamentals and Applications.41.34 the spheres rotate fully to display either black or white.35 a thorough and detailed treatise covering solely inorganic materials.45 2000. Non-English reviews include that by Volke and Volkeova66 (in Czech: 1996). and are suspended within minute oil-filled pockets.40 1997. Mortimer and Rowley60 in 2002. Granqvist (sometimes with co-workers) in 1992.38 1993.67 Hadfield (in 1993).6 Introduction to electrochromism dipolar spheres of diameter 0. McGourty (in 1991).46 2003.43. The now huge numbers of patents on materials. Lampert’s review. multimolecular. especially but not necessarily when the modifier is organic or polymeric. ionic or polymeric film of a chemical modifier and that. A CME is72 an electrode made up of a conducting or semi conducting material that is coated with a selected monomolecular. not always scientific. phasedispersed liquid crystals (PDLCs). might aid clarification. . criteria. exhibits chemical. and thus affect the coloration–time relationships. 1.1. reactions or interfacial potential differences . includes other forms of display device. All electrochromic electrodes comprise some element of modification. they remain useful and are followed here throughout. A good example is aqueous methyl viologen . A type-I electrochrome is soluble. present initially and thence formed electrochemically. . Sun and Gilbert73 in 1975. this is simply to be understood. which we cite in relevant chapters. cholesteric liquid crystals (ChLCs) and suspended particle devices (SPDs). processes or devices are usually excluded. electrochemical and/or optical properties of the film. While the original classifications are somewhat dated. which dictate the precise form of the current–time relationships evinced during coloration. Such types are classified in terms of the phases. necessarily abridged. crammed with acronyms but more up-to-date.1 Electrochrome type In the early days of ECD development. such as thermochromism. The terms comply with the 1997 IUPAC recommended list of terms on chemically modified electrodes (CMEs). the kinetics of electrochromic coloration were discussed in terms of ‘types’ as in the seminal work of Chang. but are rarely referred to as CMEs. .4 Criteria and terminology for ECD operation The jargon used in discussions of the operation of electrochromic devices (ECD) is complicated. There are also many dozen reviews concerning specific electrochromes. .4 Criteria and terminology for ECD operation 7 other forms of driven colour change. 1. and remains in solution at all times during electrochromic usage. such as liquid crystal displays (LCDs). hence the criteria and terminology cited below. by means of faradaic . shorter. Chemically modified electrodes are often referred to as being derivatised. the reliability – often just the plausibility – of patents being judged by different.4. electrochromic-device applications and preparative methodologies. Other type-II electrochromes commonly encountered include aqueous viologen systems such as heptyl or benzyl viologens. A suitable example of a type-II system is cyanophenyl paraquat (III). Other type-I electrochromes include any viologen often soluble in aqueous solution.74. (1. (1.75. The cation is abbreviated to MV2 þ.76 Eq. .78 Inorganic examples include the solid products of electrodeposited metals such as bismuth (often deposited as a finely divided solid). or a mirror of metallic lead or silver (Section 9.77 or methoxyfluorene compounds in acetonitrile solution. This phase change increases the write–erase efficiency and speeds the response time of the electrochromic bleaching. or a phenathiazine (such as Methylene Blue). Eq. again in water.8 Introduction to electrochromism (1.3): MV2þ(aq) þ eÀ ! MVþ (aq). Type-II electrochromes are soluble in their colourless forms but form a coloured solid on the surface of the electrode following electron transfer.4): CPQ2þ ðaqÞ þ eÀ þ XÀ ðaqÞ ! ½CPQþ XÀ ðsolidÞ: colourless olive green (1:4) NC N N CN III The solid material here is a salt of the radical cation product74 (the incorporation of the anionic charge XÀ ensures electro-neutrality within the solid product).3) H3C CH3 XÀ can be a halide or complex anion such as BF4À. which colours during a reductive electrode reaction. colourless intense blue + N 2X – II + N (1. in nonaqueous solutions.40 -bipyridilium – II). in which the electrode reaction is generally reduction of an aquo ion or of a cation in a complex with attached organic or inorganic moieties (‘ligands’).10 -dimethyl-4.3). Organic type-III systems are typified by electroactive conducting polymers. (1. as seen by eye.1.4 Criteria and terminology for ECD operation 9 Type-III electrochromes remain solid at all times. The value of x usually lies in the approximate range 0 x < 0. and the mobile counter ion (arbitrarily cited here as the proton) could also be lithium. and 10:1 for the cell WO3jelectrolytejNiO. for metal oxides. As high a value as possible is desirable. y ¼ 3 is commonly found. Most inorganic electrochromes are type III.2 Contrast ratio CR The contrast ratio CR is a commonly employed measure denoting the intensity of colour formed electrochemically. poly(thiophene) or poly(aniline) and relate to the parenthesised monomer from which the electrochromic solid is formed by electro-polymerisation.4. Eq. The three groups of polymer encountered most often in the literature of electrochromism are generically termed poly(pyrrole). (1:5) colourless intense colour where the metal M is most commonly a d-block element such as Mo.g. The CR is commonly expressed as a ratio such as 7:1. Eq. The parameter x. the ‘insertion coefficient’.83 More elaborate measures of coloration are outlined in Chapter 4.6): Ro . e.3.79 The ratio CR is best quoted at a specific wavelength – usually at max of the coloured state. 1.80 and as high81 as 60:1 for a system based on heptyl viologen radical cation. a CR of less than about 3 is almost impossible to see by eye. As in practice.5). and WO3 has been the most studied. Ni or W. indicates the proportion of metal sites that have been electro-reduced. (1:6) CR ¼ Rx where Rx is the intensity of light reflected diffusely though the coloured state of a display. A CR of 25:1 is cited for a type-II display involving electrodeposited bismuth metal. MOy ðsÞ þ xðHþ ðsoln:Þ þ eÀ Þ ! Hx MOy ðsÞ. Other inorganic type-III electrochromes include phthalocyanine complexes and metal hexacyanometallates such as Prussian blue. electrodeposited from aqueous solution with a charge82 of 1 mC cmÀ2. (1. . as discussed below. and Ro is the intensity reflected similarly but from a non-shiny white card. a recently fabricated electrochromic device was described as ‘ultra fast’.4-alkylenedioxythiophene) ‘PEDOT’ (IV). It is generally unlikely that (coloration) ¼ (bleach). the viologen bis(2-phosphonoethyl)-4. with a coloration efficiency of 270 cm2 CÀ1 was employed as chromophore. some such as for electrochromic office windows actually require a very slow response. there are few reliable response times in the literature since there is no consistency in the reporting and determination of cited data. At present. In contrast. a film of WO3 (formed by spray pyrolysis of a solution generated by dissolving W powder in H2O2) became coloured in 15 min. and especially in the way different kinetic criteria are involved when determining . Similarly. For example.89 prepared a series of polymers based on poly(3. and bleached in 3 min. showed that films of polymer of thickness ca.10 Introduction to electrochromism 1.8–2.2 s with a modest transmittance change of 44–63%.3 Response time t The response time is the time required for an ECD to change from its bleached to its coloured state (or vice versa). Canon88 made electrochromic oxide mixtures that undergo absorbance changes of 0.86 However. While most applications do not require a rapid colour change.40 -bipyridilium (V).4 in 300 ms. Sato87 reports an anodically formed film of iridium oxide with a response time of 50 ms. multiple switching studies.85 but the choice of both potential and preparative method was made to engender such slowness.4. 300 nm could be fully switched between reduced and oxidised forms in 0. applications such as display devices require a more rapid response.84 For example. Reynolds et al. a film of sol–gel-derived titanium dioxide is coloured by reductive insertion of Liþ ions at a potential of about –2 V with a response time of about 40 s. monitoring the electrochromic contrast. with a claimed90 of 250 ms. O O O HO P + N 2Cl – V + N OH O P OH OH S IV n . or it may relate to the time required for an amount of charge (again defined or arbitrary) to be consumed in forming colour at the electrode of interest. as workers can feel ill when the colour changes too rapidly. may represent the time required for some fraction of the colour (defined or arbitrary) to form. To this end. which is a stringent test of design and construction.98 The Donnelly mirror in ref.97 and ‘oxides’. relative to potential-jump (or linear potential-increase) coloration. enhancing the rate of electrochromic colour formation for ‘viologens’. the electrochrome–electrolyte interface has a capacitance C.95.94.98 to enhance significantly the rate at which electrochromic colour is generated.e.91 methyl.100.101 1.94 pulsing also enhances the rates of electro-coloration ECDs based on TiO2.93. The efficiency must approach 100% for a successful display.92 heptyl93 and aryl-substituted viologens.97. although its effects will be ignored here. WO3 and MoO3 are a few examples. In many chemical systems. The so-called ‘rise time’ of any electrochemical system denotes the time needed to set up (i. Sudden decreases in R during electro-coloration can cause unusual effects in the current time profiles.96. 97 operates with a pulse sequence of frequency 10–20 Hz. in essence the pulsing modifies the mass transport of electrochrome. the uncoloured form of the electrochrome also has a high resistance R: poly(thiophene). Applying a pulsed potential has been shown91. References 98 and 99 present a detailed discussion of the implications.1.4.92.4 Criteria and terminology for ECD operation 11 Furthermore. Although a quantitative explanation is not readily formulated.4 Write–erase efficiency The write–erase efficiency is the fraction (percentage) of the originally formed coloration that can be subsequently electro-bleached. poly(aniline). eliminating kinetic ‘bottle-necks’. Coloration will not commence between instigation of the colouring potential and completion of the rise time. The write–erase efficiency of an ECD of aqueous methyl viologen MV2þ as the electrochrome will always be low on a realistic time scale owing to the slowness of diffusion to and from the electrode through solution.95 WO396. Such capacitances are well known in electrochemistry to arise from ionic ‘double layer’ effects in which the field at (or charge on) the electrode attracts a ‘layer’ – really just an excess – of oppositely charged electrolyte ions from the bulk solution. The kinetics of electrochrome diffusion here are complicated since this electrochrome is . Pulsing is reported to speed up the response of viologen-based displays. fully charge) this interfacial capacitance prior to successful transfer of electronic charge across the interface. a time that may be tens of milliseconds. as outlined in Chapter 5. Substrate resistance The indium–tin oxide (ITO) electrode substrate in an ECD has an appreciable electrical resistance R. This is amplified in Section 14. The simplest means of increasing the write–erase efficiency is to employ a type-II or type-III electrochrome. with. The effect of film degradation over an extended time is clear. respectively). the cycle length being greater than or less than .. but studies of cycle life are legion.4. Such partial tests are clearly of dubious value.) The cycle life is therefore an experimental measure of the ECD durability.2. Since ECDs are usually intended for use in windows or data display units. many tests of cell durability in the literature of electrochromism involve cycles of much shorter duration than the ECD response time . perhaps unusably so.1 mol dmÀ3). environment and cell driving conditions. chemical bonding of viologens to the surface of particulate102 TiO2. Electrochrome diffusion is discussed in Chapters 4 and 5.5 Cycle life An adjunct to the write–erase efficiency is the electrochromic device’s cycle life which represents the number of write–erase cycles that can be performed by the ECD before any significant extent of degradation has occurred. While it may seem obvious that the cycle life should be cited this way. . While embedding in this way engenders an excellent longterm write–erase efficiency and a good electrochromic memory. describing the response of hydrous nickel oxide immersed in KOH solution (0. a 50% deterioration is often tolerable in a display. deterioration is best gauged by eye and with the same illumination. Such modified type-I systems are effectively ‘quasi type-III’ electrochromes.1 shows such a series of double potential steps. since between the write and erase parts of the coloration cycle the coloured form of the electrochrome is not lost from the electrode by diffusion. Some workers have attempted to address this problem of variation in severity of the cycle test by borrowing terminology devised for the technology of battery discharge and describing a write–erase cycle as ‘deep’ or ‘shallow’ (i. Such retardation is achieved either by tethering the species to the surface of an electrode (then termed a ‘derivatised’ electrode). e. 1. that would be employed during normal cell operation. The write–erase efficiency of a type-I ECD may be improved by retarding the rate at which the solution-phase electrochrome can diffuse away from the electrode and into the solution bulk. or by immobilising the viologen species within a semi-solid electrolyte such as poly(AMPS).12 Introduction to electrochromism extremely soluble in all applicable solvents for both its dicationic (uncoloured) and radical-cation (coloured) forms.g. it will also cause all response times to be extremely slow. However.e. (Such a write–erase cycle is sometimes termed a ‘double potential step’. Figure 1. The electrolyte layers are discussed in Section 14.6 V (as ‘on’). such as molecular oxygen. Energy Mater.6 Power consumption An electrochromic display consumes no power between write or erase cycles. A working minimum of about 105 is often stipulated. this retention of coloration being called the ‘memory effect’. and also the repeated recrystallisations within solid electrochromes associated with the ionic ingress and egress99 that necessarily accompany redox processes of type-II and -III electrochromes. R. The potential was stepped between 0 V (representing ‘off’) and 0. Conell. Briefly. (Figure reproduced from Carpenter. The intense colour of a sample of viologen radical cation remains undimmed for many months in the absence of chemical oxidising agents. the electrolyte fails. D. by permission of Elsevier Science. or one or both of the electrochromic layers fail. either of the solvent or the electrochrome itself. M. 16. K. S. Sol.2.1. the most common causes of low cycle life are photodegradation of organic components within a device. and Corrigan. The aged film had undergone about 500 write–erase cycles. There are several common reasons why devices fail: the conducting electrodes fail. ‘The electrochromic properties of hydrous nickel oxide’. 1987.) The maximising of the cycle life is an obvious aim of device fabrication. An individual device may fail for any or all of these reasons.. 1.4.1 Optical switching behaviour of a fresh and an aged film of NiO electrodeposited onto ITO. 333–46. A. and overall device stability is discussed in Chapter 16.4 Criteria and terminology for ECD operation 10 s 13 Relativ e transmittance On 0 Off On Off On Off On Off F resh Aged Time Figure 1. . VI) with a charge of 2 mC cmÀ2. A.4 Writing v oltage/V Figure 1.2 0. The charge consumed during one write–erase cycle is a function of the amount of colour formed (and removed) at an electrode during coloration (and decoloration).2 Calibration curve of electrochromic response time against the potentiostatically applied ‘writing’ potential Va (cited against SCE) for heptyl viologen dibromide (VI) (0.. Phys.. R.) However.103 state that a contrast ratio of 20:1 may be achieved with a device employing heptyl viologen (1. It is assumed that (bleaching) ¼ (coloration).10 -di-n-heptyl-4. J. T. P. ´ Ponjee. J. van Dam.4 0. ‘New electrochromic memory device. 64. yielding an optical reflectance of 20%.8 1. C.14 2 Introduction to electrochromism 102 5 Writing time (ms) Reflectance 2 20% 10 40% 5 60% 2 80% 1 0 0. by permission of the American Institute of Physics.1 mol dmÀ3) in aqueous KBr (0. Figure 1.40 bipyridilium dibromide. and Bolwijn.6 0. Schoot et al. H. 1973. and any ECD (all of which follow battery operation) will eventually fade unless the colour is renewed by further charging.0 1.. .3 mol dmÀ3).2 a 1. J.2 shows a plot of response time for electrochromic coloration for HV2þ 2BrÀ in water as a function of electrochemical driving voltage. no-one has ever invented a perfect battery of infinite shelf life. van Doorn. (Figure reproduced from Schoot. J.. Lett. 23.’Appl. Such photoelectrochromic systems are discussed further in Chapter 15. 1.e. 54). Cohen asserts that ECD power consumption rivals that of LCDs. Furthermore.3. i. (1. additional values are sometimes cited in discussions of individual electrochromes.’ For this reason.104 the driving power coming from a single small cell of amorphous silicon.4. A comprehensive list of coloration efficiencies is included in Section 4.4 Criteria and terminology for ECD operation H15C7 + N 2Br – VI + N C7H15 15 Displays operating via cathode ray tubes (CRTs) and mechanical devices consume proportionately much more power than do ECDs. (1:7) where Abs is the absorbance formed by passing a charge density of Q.105 he cites 7 or 8 mC cmÀ2 during the short periods of coloration or bleaching.7): Abs ¼ Q. A graph of Abs against Q accurately gives as the gradient. . The majority of values cited in the literature relate to metal oxides. and a zero consumption of charge during the longer periods when the optical density remains constant. The optimum value is the absorbance formed per unit charge density measured at max of the optical absorption band. few are for organic electrochromes. ECDs consume considerably more power than liquid crystal displays. although a LCD-based display requires an applied field at all times if an image is to be permanent. comparable to battery deterioration (see p. For a detailed discussion of the way such optical data may be determined. Its value depends on the wavelength chosen for study. it has no ‘memory effect.3. The power consumption of light-emitting diodes (LEDs) is relatively low. This last criterion is overstated: a miniscule current is usually necessary to maintain the coloured state against the ‘self-bleaching’ processes mentioned earlier. The amount of power consumed is so small that a solar-powered ECD has recently been reported.1. see Section 4. The coloration efficiency is defined according to Eq. usually less than that of an ECD.7 Coloration efficiency h The amount of electrochromic colour formed by the charge consumed is characteristic of the electrochrome. tungsten trioxide has been the primary electrochrome chosen owing to its high coloration efficiency.107 but it is difficult to conceive of any other mechanism. see Chapter 14). e. nickel or vanadium. The WO3 becomes strongly blue-coloured during reduction.106 Nuclear-reaction analysis (NRA) is said to confirm this mechanistic mode.g. When an ECD is constructed with these two oxides – each as a thin film (see Chapter 15) – one electrochrome film is initially reduced while the other is oxidised. Note the way that charge passes through the cell from left to right and back again during electrochromic operation – ‘electrochromism via the rocking chair mechanism’. (1. so is termed the primary electrochrome. as illustrated below using the example of tungsten and nickel oxides.. sub-stoichiometric nickel oxide is dark brown-black when oxidised and effectively colourless when reduced. the operation of the device is that portrayed in Eq. So-called ‘optically passive’ materials (where ‘passive’ here implies visibly non-electrochromic) are often the choice of counter electrode for an ECD. Ideally.16 Introduction to electrochromism 1. As ECDs are electrochemical cells. The second electrode need not acquire colour at all.4. an uninformative phrase coined by Goldner et al. one colouring on insertion of counter ions while the other loses that ionic charge (or gains an oppositely charged ion) concurrently with its own coloration reaction. accordingly.8 Primary and secondary electrochromism To repeat the definition. However. Solidstate electrochromic displays are. Examples of optically . Each half-cell comprises a redox couple. then the colour formation within the two should operate in a complementary sense. i. In perhaps a majority of recent investigations. in 1984. If both electrodes bear an electrochromic layer. a cell comprises two half-cells. The simplest electrochromic light modulators have two electrodes directly in the path of the light beam. multi-layer devices (often called ‘sandwiches’.8): bleached coloured zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{ zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{ þ Mx NiOð1ÀyÞ ! Mx WO3 þ NiOð1ÀyÞ : WO3 pale yellow colourless dark blue brown-black (1:8) The tungsten oxide in this example is the more strongly coloured material. in practice. and the NiO(1–y) acts as the secondary (or counter) electrode layer.e. needing the second electrode to allow the passage of charge through cell and electrodes. their respective values of are of different sign. while the secondary layer has been an oxide of. the secondary electrochrome is chosen in order to complement the primary electrochrome. so each ECD requires a minimum of two electrodes. iridium. while being effectively colourless when oxidised. the optical response of each individual layer is obtainable. multi layer systems. however. the holes should not exceed about one hundredth of the overall active electrode area. the layer behind the mirror electrode can be either strongly (but ineffectively) coloured or quite optically passive. Each state forms at a particular potential if each such state can be sustained. a series of oxidation states. the group of Hagen and Jelle109. When the electrochrome is a permanently solid in both forms (that is. type III). For a singlespecies electrochrome. A suitable example is methyl viologen.5 Multiple-colour systems: electropolychromism 17 passive oxide layers include indium–tin oxide and niobium pentoxide. both electrodes form their colour concurrently. as described in Chapter 11.114. if the counter electrode is a mirror-finish metal that is very thin and porous to ions. By careful positioning of a narrow spectrometer beam through the ECD. and red–brown as a di-reduced neutral species. then ECDs can be made with one electroactive layer behind this electrode. although it is often impossible to deconvolute the optical response of a whole device into those of the two constituent electrochromic couples. or charge states – each with its own colour – could be produced. or more together. For optimal results. Recently. 5 mm. Devices were fabricated in which each constituent film had a narrow ‘hole’ (a bare area) of diameter ca.5 Multiple-colour systems: electropolychromism While single-colour electrochromic transformations are usually considered elsewhere in this book. Chapter 14 cites examples of such counter electrodes. blue as a radical cation.115.110.112. 1. In devices operating in a complementary sense. the hole in each film being positioned at a different portion of each film. This simple yet powerful ‘hole’ method has led to otherwise irresolvable analyses of these complicated. Electrochromic viologens with as many as six colours have been synthesised. Such systems should be called electropolychromic (but ‘polyelectrochromic’ prevails). each coloured state generated at a characteristic applied potential. which is colourless as a dication.117 . if the species is ‘multivalent’ in chemical parlance.113.1. This requires sophisticated apparatus such as in situ ellipsometry108 and accompanying mathematical transformations. while simultaneously the electrochemical response of the overall ECD is obtained concurrently via chronoamperometry in real time. In an unusual design. evince a whole series of different colours. MV2þ (II). In such a case. applications may be envisaged in which one electrochrome.116 have devised an ingenious and valuable means of overcoming this fundamental problem of distinguishing the optical contributions of each electrode. an approximate deconvolution is possible.111. that is. ).ifm.4. Electrochromism: Fundamentals and Applications. A.7-trinitro-9-fluorenylidene malononitrile (VIII) is green. 2002.liu. M. . 962–3. P.5. 1997. a product from 2. IX) yields the blue radical anion TCNQ– .. Chem.. Bard. R. New York. Mortimer.7tetranitro-9-fluorenone (VII). i.html (accessed 27 January 2006). J.4. differs in E value: see Chapter 3) and there is no chemical interaction (that can be prevented by encapsulation). provided that each chromophore generates colour at a different potential (i. A simple laboratory demonstration of electrochromism. the red electrochrome was 2.chem. [Online] at jchemed.edu/Journal/Issues/1997/Aug/abs962. J. B. and Faulkner.e. R. [Online] at www. 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V. 1993. V. K. Sci. Goldner. Electrochem. K. J. Chem.. Electrochem. P. 106. C. Hesjevik. 380. and Ingram. Bechinger. Nuclear reaction analysis profiling as direct evidence for lithium ion mass transport in thin film ‘rocking chair’ structures. pp. 1994. Lett.310. 137. Haas. S.. A. 2000. G. New electrochromic memory device. 102.. 1998. 1997. Opt. Bader. F. 1993. J.. Benson. G.. Electrochromic windows based on viologen-modified nanostructured TiO2 films. Goldner. B. J. I. Solids. tungsten oxide and a solid polymer electrolyte. S. 108. S. M.. Cinnsealach. 104. Soc. H. 1989. 1981.. Lett. Protsenko. R. Digital electrochromic mirror system. 70. Y. 06089721. Electroanal.. L. Electrochromic display rivals liquid crystals for low-power needs.. and Lazzari.. Peculiarities of the mechanism of the electrochromic coloring of oxide films upon pulsed electrochemical polarization... 55–66. K.. J.. N. Panero. O. Solid State Ionics. A. O. P. A. G. I. P. C. ˚ 110. Kramarenko. C. P. 1981. 101. Ashrit. G. J. Wong. and Monk. and Bolwijn. Ottaviani. M. 70–71.. F. 1995. Appl. SPSTL 971. Cells. T. Electronics. 97. Sol. 64–5. . and Zerigian. E. Morizilli. S. G. Singleton. S.. Scrosati. P. F. Haas. D. Y. 98. US Patent. Performance of an electrochromic window based on polyaniline. B. J. 1993. Yasuda. S. Jelle. 1377–80. Studies of tetra-(bipyridilium) salts as possible polyelectrochromic materials. S. P. 58. Hagen. 1998. Electrochim.. M. Appl. 115. and Ødegard. and Monk. 114. tungsten oxide and a solid polymer electrolyte. 1998. 1213–21. P. 116. Appl. G. Jelle. and Hagen. Hagen. 315–20. 117. Sunde. 1061–65. Transmission properties for individual electrochromic layers in solid state devices based on polyaniline. P. O. Jelle. Sol. G. G. B. D. J. G. 28.. Cells. A. Met. Sol. Transmission spectra of an electrochromic window consisting of polyaniline. R. B. Transmission spectra of an electrochromic window based on polyaniline. Electrochem. 38. 54. Energy Mater. and Hagen. Prussian Blue and tungsten oxide. Dynamic light modulation in an electrochromic window consisting of polyaniline. 25. P. 257–68. Appl.. Hagen. J. P. and Birketveit. Synth.. Jelle. Jelle. Sol. 1992. 118. Prussian blue and tungsten oxide. S. G. Energy Mater. tungsten oxide and a solid polymer electrolyte.. Sol. 1999. 37. 1497–500. R. and Seto. 483–9. ˚ 112. Acta. R. 24. 28. B. Hagen. Electrochemical multilayer deposition of polyaniline and Prussian blue and their application in solid state electrochromic windows. Electrochem. ˚ 113. Electrochemical studies of molecular electrochromism. Electrochim. and Nodland. P. B. J.. Electrochem. prussian blue and tungsten oxide. 1994. Jelle. Cells.. 1992. 277–86. and Ødegard.24 Introduction to electrochromism 111. . B. Acta.. G. Rosseinsky. P. 1993.. However. Platt8 coined the term ‘electrochromism’ in 1961 to indicate a colour generated via a molecular Stark effect (see page 4) in which orbital energies are shifted by an electric field. amplified further in Section 6.5). and ‘electrochromism’ Brief histories of electrochromism have been delineated by Chang1 (in 1976). Faughnan and Crandall2 (in 1980).2 A brief history of electrochromism 2. indicate the extensive role of WO3 in electrochromism.1 Bibliography. (1. These effects are not the main content of this book.6 (in 1998) chronicle further advances in making electrochromic devices for windows. (2.1. The additional histories of Agnihotry and Chandra5 (in 1994) and Granqvist et al.9. The first books on electrochromism were those of Granqvist.10 who applied huge electric fields to a film of solid oxide causing spectral bands to shift. Byker3 (in 1994) and Granqvist4 (in 1995). 2. Section 1. Other published histories rely very heavily on these sources.5) below.2 Early redox-coloration chemistry In fact.4 and Monk. Section 1.2. His work follows earlier studies by Franz and Keldysh in 1958. implying electron transfer).7 which were both published in 1995. redox generation of colour is not new À twentieth-century redox titration indicators come to the chemist’s mind (‘redox’.3 Prussian blue evocation in historic redox-coloration processes An early form of photography devised in 1842 by Sir John Frederick William Herschel13 is a ubiquitous example of a photochromic colour 25 . Mortimer and Rosseinsky. as early as 1815 Berzelius showed that pure WO3 (which is pale yellow) changed colour on reduction when warmed under a flow of dry hydrogen gas. Wohler12 effected a similar chemical reduc¨ tion with sodium metal. and Eq. 2.4 and Eq.11 and in 1824.1. under the common name of ‘blueprint’. Herschel called his process ‘cyanotype’. 282 ff. to late in the twentieth century. (2:1) where eÀ(h) represents an electron photolysed from water or other ambient donor. Thus the iron electrode generates a track of darkly-coloured deposit wherever the positive stylus touches the paper. in 1839. as a photographic process for large reproductions.26 A brief history of electrochromism change involving electron transfer. 35).1): Fe3þ ½FeIII ðCNÞ6 3À ðsÞ þ Kþ ðaqÞ þ eÀ ðhÞ ! KFeIII ½FeII ðCNÞ6 ðsÞ. Herschel’s method produced photographs and diagrams by generating Prussian blue KFeIII[FeII(CN)6](s) from moist paper pre-impregnated with ferric ammonium citrate and potassium ferricyanide. for oxidation-state representation by Roman numerals. so-called ‘blueprint’ paper was manufactured on a large scale as engineers and architects required copies of architectural drawings and mechanical plans. see eq.15 It involved a stylus of pure soft iron resting on damp paper pre-impregnated with potassium ferrocyanide.14. followed by Kþ ðaqÞ þ Fe2þ ðaqÞ þ ½FeIII ðCNÞ6 3À ðaqÞ ! KFeIII ½FeII ðCNÞ6 ðsÞ. Wherever light struck the photographic plate. devised for a technological application. which consumes the iron as it combines with ferrocyanide ion to produce a very dark form of insoluble Prussian blue. in 1843 Bain patented a primitive form of fax transmission that again relied on the generation of a Prussian blue compound.12) and p. Fe3þ ðaqÞ þ eÀ ! Fe2þ ðaqÞ. By the 1880s. of like mechanism. In an electrical circuit. Its inventor was a friend of Fox Talbot. Soon after Herschel. who is credited with inventing silver-based photography. photo reduction of FeIII yielded FeII in the complex. hence Prussian blue formation. . forming yellow Prussian brown Fe3þ[Fe(CN)6]3À or FeIII[FeIII(CN)6] (for Prussian blue details see reaction (3. electro-oxidation of the (positive) iron tip formed ferric ion from the metal. (2. a process often oversimplified as resulting from reduction of Fe3þ by the photolysed eÀ: ½H2 O þ h ! eÀ þ fH2 Oþ g . see p.. This word has become an English synonym for ‘plan’. This widespread availability revived cyanotype. (2:3) (2:2) where {H2Oþ} represents water-breakdown species. Zaromb called his system an ‘electroplating light modulator’.25 had a patent for electrochromic printing – he called it ‘electrolytic writing paper’ – in which paper was pre-impregnated with particulate MoO3 and/or WO3. In 1962. and explicitly said it represented a ‘viable basis for a display’. (2. Brimm et al. e.2.4) above.4 Twentieth century: developments up to 1980 Probably the first suggestion of an electrochromic device involving electrochemical formation of colour is presented in a London patent of 1929.4 Twentieth century: developments up to 1980 27 2. but presumably inert. Such molecular I2 then effects the chemical oxidation of a dye precursor.21.16 which concerns the electrogeneration of molecular iodine from iodide ion. His work was not followed up until the mid 1970s.22 The first recorded colour change following electrochemical reduction of a solid. This example again represents an electrochromic reaction. itself immersed in aqueous acid. in 1953. tungsten trioxide. The colour generation reaction (cf. thus forming a bright colour. by the groups of Camlibel20 and of Ziegler. Zaromb published now-neglected studies of electrodepositing silver in desired formats from aqueous solutions of Agþ 17. was that of Kobosew and Nekrassow23 in 1930. A blue–grey image forms following an electrontransfer reaction: in effect. Kraus of Balzers in Lichtenstein27 advocated the reversible colour–bleach behaviour of WO3 (again immersed in aqueous H2SO4) as a basis for a display: this work was regrettably never published. In 1951. A little later. again in the early 1960s.4): WO3 ðsÞ þ xðHþ þ eÀ Þ ! Hx WO3 ðsÞ: (2:4) Their WO3 was coated on an electrode. the electrode acted as a stylus. for NaxWO3 immersed in aqueous acid (sulfuric acid of concentration 1 mol dmÀ3).19 Electro-reduction of Ag(I) ion yields a thin layer of metallic silver that reflects incident light if continuous. The electrode substrate is unknown.2. . or is optically absorbent if the silver is particulate. Section 9. (2.g. and the proton counter ion came from the ionisation of the water in the paper. the proneness of iodide to photo-oxidation is discouraging to any further development.26 extended the work of Kobosew and Nekrassow to effect reversible colour changes. Probably the first company to seek commercial exploitation of an electrochromic product was the Dutch division of Philips. who deposited metallic bismuth.18 or complexes thereof. The electrochromic coloration reaction followed Eq.1) followed Eq. However. By 1942 Talmay24. forming colour wherever the electrode traversed the paper. which migrate through the crystal in response to redox changes at the electrodes.31.40 -bipyridilium] as the sulfate salt. ICI was seeking tenders to commercialise a CPQ-based device. when he analysed thin-film vacuum-deposited MoO3 on quartz.40 . Imperial Chemical Industries (ICI ) in Britain initiated a far-reaching program to develop an electrochromic device. Beegle developed a display of WO3 having identical counter and working electrodes. and changed to the larger viologen cyanophenyl paraquat [CPQ: 1. (This wording may reflect his earlier work dating from 1966. In 1971. Like Philips.33 and by Texas Instruments in Dallas. probably because no viable device was likely to ensue as their crystal had to be heated to ca. 200 8C.36 Deb formed electrochromic colour by applying an electric field of 104 V cmÀ1 across a thin film of dry tungsten trioxide vacuum deposited on quartz: he termed the effect ‘electrophotography’.38 in 1995. The background to Deb’s work was recounted much later. much like the colour formed by heating or irradiating crystals of metal halides in a field.10 -bis(1-cyanophenyl)-4. which acquired colour following UV irradiation. suggesting that the mobile counter cations might have come from simultaneous ionisation of interstitial and/or adsorbed water.35 following a technical report from the previous year. The charge carriers are (apparently) oxide ions.28 and their first academic paper from 1973. Their first patent dates from May 1969.32 Other devices based on heptyl viologen were being investigated by Barclay’s group at Independent Business Machines (IBM).28 A brief history of electrochromism Their prototype device utilised an aqueous organic viologen (see Chapter 11).1 shows a schematic representation of his cell. They applied electrodes to the opposing faces of doped.29 At much the same time. Deb’s film of WO3 was open to the air rather than immersed in ioncontaining electrolyte solutions.30 By early 1970. In 1972. crystalline SrTiO3 and observed an electrochromic colour move into the crystal from the two electrodes. they first analysed the response of heptyl viologen in water but quickly decided its coloration efficiency was too low. heptyl viologen (HV: 1. Deb suggested the colour arose from F-centres. Blanc and Staebler39 produced an electrochromic effect superior to most previously published. In fact. At the time.10 -n-heptyl-4.37) Figure 2. Their work has not been followed. probably most workers now attribute the first widely accepted suggestion of an electrochromic device to Deb (then at Cyanamid in the USA) in 1969. although their work was not published until after their programme was discontinued.40 -bipyridilium) as the bromide salt. Their first patent dates from 1971. with an intervening opaque layer.34 As none of these studies attracted much attention. 5). but suggests oxide ions extracted from the WO3 lattice. (2. In fact ref. 39.4).) Nowadays most workers cite Deb’s later paper. Energy Mater.4 Twentieth century: developments up to 1980 29 Figure 2. 1995. Faughnan et al. in 1978:47 WO3 ðsÞ þ xðLiþ ðaqÞ þ eÀ Þ ! Lix WO3 ðsÞ: (2:5) . Green and Richman43 in London proposed a system based on WO3 in which the mobile ion was Agþ. In 1975. 191–201. (2.1 Electrocoloration of thin-film WO3 film using a surface electrode geometry. establishing a pioneering model of electro-bleaching45 and electro-coloration46 that is still relevant now. (Figure reproduced from Deb. New Jersey. Cells. K. rather than proton insertion.2. Faughnan et al. of the RCA Laboratories in Princeton. in a pivotal review. with a film of WO3 immersed in an ion-containing electrolyte. It is often said that this seminal paper describes the first ‘true’ electrochromic device. another film of WO3 vacuum evaporated onto a substrate of quartz.42 Within a year of Deb’s 1973 paper. Deb does correctly identify the ionisation of water as the source of the protons necessary for Eq. analysed the speed of colour change in terms of Butler–Volmer electrode dynamics.44 reported WO3 undergoing reversible electrochromic colour changes while immersed in aqueous sulfuric acid. Mohapatra of the Bell Laboratories in New Jersey published the first description of the reversible electro-insertion of lithium ion. for the coloration mechanism. Eq. Sol.41 which dates from 1973. ‘Reminiscences on the discovery of electrochromic phenomena in transition-metal oxides’. Sol. S. 41 does not mention aqueous electrolytes at all but rather. as the true birth of electrochromic technology. B. Later. Niklasson. K. 12. 181–211. 320–34. Indian J. S. 350. Elsevier. C. Ronnow. 8. Recent advances in electrochromics for smart window applications.. 94–2. Ann. Springer Verlag. 4. F. A. 7. Franz. V.. L. Chang. available [online] at photography.. but his later work reveals that his electrochromes were based on tetrathiafulvalene and quinone moieties.com/library/weekly/aa061801b. (eds. A. Byker.. Kullman. S. Faughnan.. Granqvist. 44. . 10. C. 1961. 1995. 34. Eksp. 4. the electrogenerated radical cations of which are intensely coloured (see p. Granqvist. 1. as cited in ref. Eng. and Chandra.. 7. 2.htm (accessed 26 January 2006). 1995. when Diaz et al. 1976. of IBM in New York published the first report of an electrochromic polymer comprising an alkyl-chain backbone with pendant electroactive species49. M. The electrochemical literature of the twentieth century will undoubtedly provide further early reports of electrochromism. Soc. 1138. pp. J. 6. announced the electrosynthesis of thin-film poly(pyrrole). 862–3. R. ch. Plenum Press. Sol. Keldysh. Naturforsch. R. G. see Section 10. Hjelm. ¨ Strømme Mattson. and Rosseinsky. Berzelius. 63. and Vaivers. 4. Platt. H. A. R. Agnihotry. cited values of max for the several radical cations. Proc. 199–216. D. as cited in ref. Mater. California).30 A brief history of electrochromism Meanwhile. Fiz. 7. D. In 1974. G. Parker et al.52 (also of IBM in San Jose. W. References 1. VCH. ¨ 13. I. J. R. 5. M.). Monk. G. 11. the electrochromism of organic materials also developed momentum. 379). 1958. Energy. Phys.. (ed. Berlin. Kaufman et al. J. I. 3. as cited in ref. S. 5. 1994. Amsterdam. Electrochromism: Fundamentals and Applications... Mortimer.48 prepared methoxybiphenyl species. 155–96. 1998. Azens. Zh. Sci. While he nowhere employs the word ‘electrochromism’ or its cognates.49 In 1979 came the first account of an electrochromic conducting polymer. In Pankove. Phys. 1980. Weinheim. Z. Electrochromic and electrochemichromic materials and phenomena. G. J. F. 13A. New York. 2. Electrochromic displays based on WO3. Display Devices. See the article ‘True Blue (cyanotype) part 2: blue history’ by Peter Marshall. Veszelei. Handbook of Inorganic Electrochromic Materials. 1958. Kemi Och Mineralogie.. M... Teor. L.). 34. a possible change of color producible in dyes by an electric field. as cited in ref.50 (see Section 10. pp. S. J. In Kmetz.3.about. 1815. 1994. his paper. 293. 1824. Electrochromism. 1–13. Commercial developments in electrochromics. 9.. Wohler. 4. J. The details in his preliminary report51 are as indistinct as are many patents. A. Electrochromic devices: present and forthcoming technology. W. Chem. Electrochem. Afhandlingar i fysik. and Crandall. P. R. F. A. Non-emissive Electrooptical Displays. J. displaying acute awareness of the technological scope of such colour changes. and von Willisen.2). Talmay.049. Sol. N-Heptylviologen radical cation films on transparent oxide electrodes. J. 1978. and Moth. 5427. P. 36. 4. 1962. Electrochem. J. 26. Soc. 76–81. Chem.. L. J. Appl. Cells. T. British Patent. D.. 109. as cited in ref. Appl. 477–93. Performance achievements in WO3 based electrochromic displays.000. and Thomas. 1930. 109. T. UK Patent. J. 1962. 328. 33–7. Stocker. J. S. L. 903–12. J. . Laboratory report: Balzers AG. as cited in ref. G. Reminiscences on the discovery of electrochromic phenomena in transition-metal oxides.. 1966. Kirkman. Phys. 14. New electrochromic memory display. ICI Ltd. Radiation sensitive materials containing nitrogenous cationic materials. 4818–25.. Variable light transmission device. J. Image display apparatus. Martin..] Short. 1981. Proc. Inert electrode behaviour of tin oxide-coated glass on repeated plating–deplating cycling in concentrated NaI–AgI solutions. 4. 38. 64–5.. as cited in ref. Electrochem. Phys. 16. Optical properties of reversible electrodeposition electrochromic materials. Electrochem. [The paper was first submitted in April 1973. 3. 1995. S. 27 May 1843. Lett.319. Soc. 1995. Kenworthy. 192–5. 1942. C. F.. G.. 18 April 1973. K. British Patent 1.281. Mantell.] ´ Schoot. Z. Appl. Camlibel. 34. Van Ruyven. D. A. Bird.302. 1943. published 21 March 1973. G. Sol. 912–18. H.. Zaromb. 1929. 1962. 1973. A. I. J. 37. F. J. T. Soc. The role of water in vacuum deposited electrochromic structures. 124. Deb. 1968. J. I. Soc. see Jasinski. S. Personal communication. Suppl. A. Energy Mater. Ponjee. 31. B. 109. 1951. J. N. Optical properties and color-center formation in thin films of molybdenum trioxide. R. Electrochem. Van Ultert. Sol. 793–4. and Zydzik. Electrochem. 33. 309–16. [The patent was first filed on 8 Dec 1970.. van Doorn. 25. Lorenz. Cyanamid Technical Report.017. H. [The patent was first filed on 24 June 1971. 21. 35. Kraus. Appl. K.. Kobosew. Sol. 18.. 27.. 22. J. 23. and Haake. J. E. van Dam. S. Soc. Lett. 12. Zaromb. S. T. 4. J. ICI Ltd.. 1978. 56. J. S. Theory and design principles of the reversible electroplating light modulator. as cited in ref. Brimm.813. O. C. L. J. T. Allen. C. N. P. Phys.. 187. British Patent 1.2.. Status of reversible electrodeposition electrochromic devices.. Deb. Ziegler. Philips Electronic and Associated Industries Ltd. 24.. D. and Nekrassow. SID. Sol. Howard. S. H. and Chopoorian. 1980.. J. H. US Patent 2. 36. 39. and Ziegler. Am. D. and Zaromb. and Bolwijn. 4 Jan 1973. K. J. 4.013. 33.. 17.] J. 1969.310. G. abstract 12. 15. Hunkin. 1619–23. An experimental display structure based on reversible electrodeposition. M.765. 1995.. Opti. 13 February 1993. R. 125. P. 73. 529. Talmay. 19. P. Geometric requirements for uniform current densities at surfaceconductive insulators of resistive electrodes. Singh. as cited in ref. H. 28. 4.314. G. as cited in ref. G. 30. Energy Mater. Deb. SID 80 Digest. L. A novel electrophotographic system. J. 29. Lichtenstein. British Patent 1. entry date 30 July 1953. Brantley.] Barclay. Just give me the fax. For example. and Jellinek. I. [The patent was first filed on 28 May 1969. P. Cells. 23. K. Bain.References 14. As cited in Giglia. An integrated electrochromic data display. US Patent 2. 4. Smith. J. 32. R. Energy Mater. D. 31 37. New Scientist. 4. as cited in ref.. 992–3.. 191–201. H. Sol.. 1987. Cells. 20. 39. M. R. 36. Deb. 52. 1971. F. Anodic oxidation of methoxybiphenyls: the effect of the biphenyl linkage on aromatic cation radical and dication stability. Kaufman. S45–6. Beegle. Proc. 96. Lett.. R. Chem. 284–8. Publ.. Soc. . F..057. Appl. New organic materials for use as transducers in electrochromic display devices.. [The paper was submitted in November 1972. H.. Rev. 1974. E. 44.] ´ 48. J. S. S. 801–22. Schroeder. Lett. Solid-state spectroelectrochemistry of crosslinked donor bound polymer films... 854–5. B. 1990. B. K.32 A brief history of electrochromism 39. 4. Electrochromic device having identical display and counter electrodes. Soc. 1974.. Some aspects of electrochromic phenomena in transition metal oxides. D. Soc. 1980. 1979. Ronlan. L. US Patent 3. 275–7. Philos. J. K. C. R. D. M. A. 43. Chem.. 1973. J. S. Am. 51.. B. V. M. Phys. 28 November 1972. 1975.. 1978. 24. and Heyman. M. J. and Lampert. E. J.. and Staebler.. A. B. and Engler. 1976.. J. 1978 Biennial Display Research Conference. B. S. Green. Hammerich. D. W. B. M. B. Electrochromism in WO3 amorphous films. 27. 28. V. 46. Thin Solid Films. F. ‘Organic metals’: polypyrrole. Polymermodified electrodes: a new class of electrochromic materials. Dynamics of coloration of amorphous electrochromic films of WO3 at low voltages. and Faughnan. H. K. Electrocoloration in SrTiO3: vacancy drift and oxidation–reduction of transition metals. Chem. 845–9... Am. 90–92. F. S. 45.. L. IEEE. Soc. F. K. 40. K. 95–7. V. Electrochem. J. RCA Rev. 50. 1975.. Conference Record. Electrochem. 49. Engler. Phys. Deb. 101. Logan. W. [The paper was submitted for publication in April 1977. J.704. F. 47. A. Faughnan. 1979. M. 285. B. and Parker. Kanazawa. A. Crandall. A solid state electrochromic cell – the RbAg4 I5/WO3 system. Mohapatra. Chem. Electrochromism in WO3. 547–9. O. Appl. Crandall. Appl. Mag. and Street... Optical and photoelectric properties and colour centres in thin films of tungsten oxide.. P. Geiss. Kaufman. Rabolt. Model for the bleaching of WO3 electrochromic films by an electric field. Faughnan. Crandall. Kwak. 36. 23–4.] 42. 422–5. and Patel. J. S. Blanc. G. D. 3–13. Phys. Gill.. W. Phys. W. 27. Lett. 3548–57. and Richman. Coleman. 177–97. a stable synthetic ‘metallic’ polymer.. Kaufman. Soc. Diaz.. Commun. A. R.. 41. the reactants are all ions in solution. Section 3. Section 3. In the second example. the rates of mass transport and those of electron transfer. Though electrochromic electrodes are intrinsically more complicated than the two examples cited here.3 33 . Section 3. and their determination in equilibrium conditions within simple electrochemical cells. In the first example (with electroactive species that resemble type-I electrochromes). introducing the use of electrode potentials.1. More comprehensive treatments of electrochemical theory will be found elsewhere.1 Introduction This chapter introduces the basic elements of the electrochemistry encompassing the redox processes that are the main subject of this monograph.4 covers electrochemical methods involving dynamic electrochemistry. to illustrate the way charge-carrier movement limits the rate of the coloration/bleaching redox processes within ECDs. Details of fabrication for electrochromic devices (ECDs) appear in Chapter 14. three-electrode systems are required here. somewhat resembling type-II electrochromes.3 Electrochemical background 3. they follow just the principles established. In it. starting with the origin of the cell emf (the electric potential across it). particularly cyclic voltammetry.2 describes the fundamentals. each a metal in contact with a solution of its own ions. the three rate-limiting (thus current-limiting) processes encountered during the electrochemistry.2. are described. Diffusion of both electrochrome and counter ions is discussed more fully in Chapter 5. the cell assembly comprises two electrodes. which is important in studying electrochromism.3 exemplifies the kinetic features underlying electrochromic coloration. The connection of E(cell) with the thermodynamics of the cell reaction then follows from the identification ÁG ¼ ÀnFEðcellÞ (3:2) as a charge nF traverses a potential E(cell) in a (virtual) occurrence of the cell reaction. instead of connecting the Pt wires. but now via the electrode processes. For . In general. Here n is the number of electrons transferred in the written reaction (1 in this example). Fe2þ would transfer electrons eÀ to the Pt so becoming Fe3þ. If Fe2þ and Fe3þ were contained in one solution and Mn2þ and Mn3þ in another. Basically it arises from the energy of a chemical reaction involving electron transfer (exactly. (3. a meter. were connected.34 Electrochemical background 3. Eq. E(cell)) evoked by the tendency of the reactions of the ions to proceed as stated. these would indicate the voltage (the cell emf. an electrochemical cell comprises a minimum of two electrodes. owing to the Gibbs free energy change DG that would accompany direct reaction. Thus the reaction proceeds as it would on directly mixing the reactants. The simplest example involves solely ions in water. and quite rapidly.1 A cell with dissolved ions as reactants: the Gibbs energy and electromotive force The fundamental origin of an electrochemical emf (‘electromotive force’) in a cell sometimes seems obscure. If. although a ‘cell’ would have been partly created. each made up of two different ‘charge states’ of a particular chemical. and the two solutions were connected via a tube containing a salt solution. i.e. there would be no way for the reaction to proceed. the charge on 6. each at its own rate.1).022 Â 1023 electrons. while at the other Pt. were inserted into each of the metal-ion solutions.2 Equilibrium and thermodynamic considerations 3. so frustrating the electrode processes. If however two inert wires. effectively to completion. Mn3þ would gain eÀ becoming Mn2þ.2. the Gibbs free energy change for unit amount of reaction). of say Pt. the charge involved in unit-quantity (a mole) of a complete reaction where n ¼ 1. or opposing voltage. and F is the Faraday constant. The flow of electrons in the wire is accompanied by net ionic motion through the solutions: a current flows through the cell and in the wire. then on connecting the wires. with rate constants ket. such as the reaction that occurs on mixing the ions: Fe2þ þ Mn3þ Ð Fe3þ þ Mn2þ : (3:1) This electron transfer reaction is known to proceed from left to right spontaneously. is different from this equilibrium electrode potential. say a battery. A comparable statement is also true for the other electrode.g. the reverse of Eq. At an electrode. which is shown in superscripted Roman numerals by the element symbol. Fe(III). e. (3. the two states stay in equilibrium (i. ‘Applying a potential’ always requires the presence in the cell of the second electrode also connected to the source. colloquially. If no external potential is applied. and in equilibrium. as in WVIO3.2 Equilibrium and thermodynamic considerations 35 inorganic species. then one of two ‘redox’ reactions (or ‘half-reactions’) can occur: electron gain – reduction. Eq.. constant in composition) at only one potential. with the two redox states. no overall composition change occurs via Eq. when in contact with both redox states. the numerals here being on par and unparenthesised.) 3. (The use of Roman numerals for oxidation states in chemistry differs from that used for gaseous species by spectroscopists. the ‘redox potential’. who write an atom as MI.R species at their particular concentrations control the energy (and hence the potential) of the electrons in . If the potential applied to the electrode. and so on. oxidation state.3) – which will alter compositions at the electrode.3). is sometimes loosely referred to as ‘the O.2. of the external potential. etc. the ‘equilibrium potential’. O and R. the O. (Equation (3.R or. When just this value of potential is applied from a battery or voltage source. Mn2þ or Fe3þ. assigned.3).e.2. FeIII and MnII.2 Individual electrode processes Consider what happens at the electrodes individually. (3:3) or electron loss – oxidation. Fe2þ are called a ‘redox couple’.R’. This is a widely used ‘chemicalaccountancy’ abbreviation ploy based on summarily assigning a charge of 2À to the oxide ion. M2þ as MIII. thus FeII.) 3.3 Electrode potentials defined and illustrated The potential of an unreactive metal in contact. (3. a singly charged ion Mþ as MII. but electron transfer does persist because in these conditions the forward and reverse processes in Eq.3) are conduced to proceed at the same rate. Here the precise charge distribution will differ considerably from the conventional. (3. sometimes as Fe(II). like Mn3þ.3. (3.3): O þ n eÀ ! R. itself abbreviated to ‘O. applied to this electrode.R electrode’. is termed the electrode potential EO. the charge state is more properly the oxidation state or (colloquially) redox state. MnIII in the initial examples. Activities of gases are closely enough their pressures.) While a value of EO. thereby allowing electrical communication to a meter. Activity may be described as the ‘thermodynamically perceived concentration’. Only with fast redox couples can the composition be rapidly governed by an applied voltage. (3:4) EO. The two oxidation states O and R can be solid. the terms a are the activities.36 Electrochemical background the metal contact.R for the (O.R) half cell cannot be determined independently. R is the gas constant. the (equilibrium) electrode potential EO. values of their electrode potentials as appear in tabulations. for all other couples.R for one O. (3. Assigning an arbitrary value to EO. to the required extent.R concentrations (which are related to their ‘activities’ – a thermodynamic concept.3). the usual cell construction comprising two electrodes intrinsically avoids this problem. Dissolved states can comprise either liquids or solids as solvent. (Measurement of this energy in a single electrode can be contemplated in principle but is difficult in practice and will henceforth be viewed as impossible. Observed values of g for ions are somewhat less than 1 in moderately dilute aqueous solutions. while activities of pure solids – those that remain unaffected in composition by possible redox reactions. see next paragraph) by a form of the Nernst equation: RT aðOÞ ln . liquid.R ¼ EO.R couple (the Hþ/H2 couple) then establishes. The application of a potential greater or less than the equilibrium value (see ‘Overpotentials’ on p. gaseous or dissolved.e. Only redox couples (i.R þ nF aðRÞ where E is the standard electrode potential (see below). ‘electroactive materials’) that can transfer electrons with reasonable rapidity can set up stable redox potentials for measurement. only differences in electric potential between two sites being ordinarily accessible by communication to a meter. The relationship between concentration c and activity a is: a ¼ (c/cstd) Â g. For rapidly reacting redox couples. 42 below) can effect desired composition changes in either direction by driving the electron process in either direction. Here just for illustration we take activities of ions (or other solutes) in liquid solution as being the ionic concentrations (which is empirically true if always in a maintained excess of inert salt). This is amplified below. and cstd is best set at unity in the chosen concentration units. T the thermodynamic temperature and n is the number of electrons in the electron-transfer reaction in Eq. where g is the dimensionless activity coefficient representing interactions with ambient ions. F the Faraday constant. thus being always F F .R is governed by the ratio of the respective O. Fundamentally.2. Zn comprise the redox couple and. Zn and Cu2þ. Voltmeter to read V Rod of zinc metal Glass sleeves Rod of copper metal E(cell) F Salt bridge Solution containing zinc ion Solution containing copper ion Figure 3. 3. since zinc metal is a good conductor.R is determined by the effective condensed-phase electron affinity of O (or. The term EO.1 Schematic of the primitive cell Zn(s)jZn2þ(aq)jjCu2þ(aq)jCu(s) for equilibrium measurements.R measured at a standard pressure of 0. one of the redox species in Eq. when solid electrode material undergoes a redox reaction where the product forms a solid solution within the reactant. Cu. (3.1 shows an electrochemical cell that comprises our second simple example. the value of EO. when connected to an external wire. F . This scale of EO.3) also functions as the contact electrode by which E may be monitored.3.1013 MPa and designated temperature. with both O and R (and any other ion species in the redox reaction) present at unit activity.2 Equilibrium and thermodynamic considerations 37 constant in composition – are assigned the value unity. then each is represented as being of unit activity. but if the result of redox reaction is a mixture of two pure bulk solids.R is the standard electrode potential defined as the electrode potential EO. F 3. The left-hand electrode is a zinc rod immersed in an aqueous solution containing Zn2þ. However.R values was established by assigning a particular value to one selected redox system. the two redox states Zn2þ. equivalently. Each metal rod is immersed in a solution of its own ions: the two half cells are Zn2þ. As in Fig. the respective activities are represented by mole fractions x.4 A cell with metal electrodes in contact with ions of those metals Figure 3. as detailed below. the effective condensed-phase ionisation potential of R) on a relative scale. by convention zero for Hþ/H2.1. make up the redox electrode. insoluble-salt. need to be connected to the same ‘inert’ conducting material in connections between the cell and meter.’ and there is a need for (s. So in such an apparently simple process. soln) meaning ‘solid solution’. the cell depicted would therefore spontaneously produce current if the electrodes were connected externally with a conducting wire. or specifies which solvent by suitable abbreviation). that of one species within another.3) or its reverse. The direction of the reaction is reflected in the relative values of the two electrode potentials E. but the fictional values for the metals are conventionally represented by unit activity as in the right-hand form of the equation here. the ‘applied potential’ then obviously being zero and not E(cell). The suffixes (l) and (g) are for ‘liquid’ and ‘gas. forming a solid. Cu2þ þ 2eÀ ! Cu and Zn ! Zn2þ þ 2eÀ at the two respective electrodes. at the more slowly operating electrode. All these have to be shed. evaluated as outlined below. The magnitude of I depends on the net rate of reaction (3. Concomitant ion motion occurs within the solution phase in attempting to maintain electrical neutrality throughout the cell. The Nernst equation for the whole cell is: EðcellÞ ¼ EðcellÞ À RT ½CuðsÞ½Zn2þ RT ½Zn2þ ¼ EðcellÞ À . appreciable mechanistic . activities). To exemplify. For other redox couples the inert metal is not written but taken as understood. The electrode reactions are shown above as simple processes though in detail comprising a complicated series of steps. then metal-lattice components.1 is the following: Cu2þ ðaqÞ þ ZnðsÞ ! CuðsÞ þ Zn2þ ðaqÞ. The spontaneous reaction in the cell depicted in Figure 3. or in solid solution. The resultant flow of electrons eÀ is discernible as an external current I in the wire. Pt is often used. Comparably with our first example. as in the introductory example. inert contacts – do not contribute to the electrode reaction. one uses (soln) for general solvent. meanwhile Cu2þ becomes Cuþ then Cu0 atoms.R of a chemical species dissolved either in water or other solvent. The ‘inert electrodes’ – better. or pure-liquid components. aqueous Cu2þ has hexacoordinated water molecules. They comprise an inert metal such as platinum or gold in contact with two oxidation states O.38 Electrochemical background as is copper. when applicable. two on longer ‘polar’ bonds than the other four ‘equatorial’ waters. however. This reaction proceeds via the two reactions. (3:5) where (s) denotes ‘solid’ and (aq) is aqueous (alternatively here. Both electrodes. or otherwise from gaseous. ln ln nF ½Cu2þ ½ZnðsÞ nF ½Cu2þ F F (3:6) where the square brackets [ ] represent concentrations (better. in obscure steps. Cu À EZn2þ . 3. When we wish to emphasise that the electrodes are being kept at equilibrium by an externally applied potential. Further detail concerning cell notation is set out in ref. the copper redox couple (right-hand side of the cell) is only at equilibrium when the potential applied to the copper is ECu2þ . that is. e. Neither electrode potential as explained above is known as an absolute or independent value: only the difference between the two. an electrode-potential scale has been devised. (3:7) where Ej is a junction potential at the contact between the solutions about the two electrodes.2 Equilibrium and thermodynamic considerations 39 complexity underlies the simplified reaction cited. a tube containing suitable electrolyte. At zero current. between the two solutions. we shall write E(eq) instead of E(cell). Thus even greater complexity can be expected in the chemically more intricate electrochromic systems dealt with later.Cu .2. (Ej is usually of unknown magnitude but approaches zero when the two solutions are nearly similar in composition. Then EðcellÞ ¼ Eðright-hand sideÞ À Eðleft-hand sideÞ þ Ej ¼ ECu2þ . After measurement of E(cell). precautions can be taken to minimise the value of Ej via.5 The cell emf and the electrode potentials: the hydrogen scale The amount of Zn2þ in solution will remain constant. 1.Zn and.) E(cell) is then the observed electrical potential difference to be applied across the cell to effect zero current flow. usually minimised. i. E(cell) is the electromotive force (‘emf’) of the cell.g. For many redox couples.e. so ‘preserving equilibrium’.. if one of the electrode potentials which comprise . Often an inert electrolyte uniformly distributed throughout the cell suffices. simultaneously. a ‘salt bridge’. and is simply the difference between the electrode potentials: EðcellÞ ¼ Eðright-hand sideÞ À Eðleft-hand sideÞ : (3:8) This statement is obviously applicable to all electrochemical cells operating ‘reversibly’ (i.3. Alternatively. but applying a measured potential from an external source that exactly opposes E(cell) is the precision choice. only when the potential applied to the Zn equals the electrode potential EZn2þ . E(cell). E(cell) may be measured on a voltmeter by allowing a negligibly small (essentially zero) current to flow through the voltmeter.e. to prevent thereby any redox reaction at either electrode. rapidly). at equilibrium. that is. is the measurable quantity. Zn þ Ej . 4. following Eq.242 V for the SCE on the hydrogen scale. In order to establish this formal scale.4). This is the standard hydrogen electrode (SHE). the half cell Pt j H2 ðgÞð1 atmÞ. of citing potentials with respect to zero for a SHE. This ‘activity-coefficient’ factor is henceforth supererogatory for our purposes.7). These considerations apply also to Eq. but is thought cumbersome and care is needed handling H2. We have used the value of 0. for values measured with respect to an SCE.2. presumably bearing traces of silver oxide to complete the redox couple. all standard electrode potentials are cited with respect to it. the solvent is water. unit activity) is assigned an electrode potential E of zero for all temperatures.) Unless stated otherwise.6 Electrochromic electrodes To link the introductory electrochemical examples above with electrochromic systems. (This attempt at uniformity will have involved cumbersomely reversing the procedures followed by some authors. The most common are the saturated calomel electrode (SCE) and the silver–silver chloride electrode. alters the sequence of E values somewhat. reference electrodes are preferred.3. to emulate the extreme dilutions that approximate to single-ion conditions. in general. the most common being a bare silver wire. The SHE is the primary reference electrode. in which the electrode reaction is Hþ ðaqÞ þ eÀ ¼ 1=2 H2 ðgÞ: (3:9) F It is the standard reference electrode: from comparisons made with cells in which one of the electrodes is a SHE. Potentials cited in this text have been converted to the saturated calomel electrode (SCE) potential scale. (3. ‘secondary’.5 the half reactions (putatively taking place in ‘half cells’) to which these E refer. are formally written as reduction reactions with the electron eÀ on the left-hand side.2. Nernst-equation extrapolation procedures can correct for finite-concentration effects. then ‘corrected’ to the hydrogen scale. Hþ (aq. then the other is predetermined. when aqueous electrolyte solution was used. we cite the widely studied tungsten trioxide electrode: WVI O3 ðsÞ þ eÀ ! WV O3 ðsÞ: (3:10) . Thus other. (Since no single ionic species like Hþ can make up a solution.6) F F F 3.40 Electrochemical background E(cell) is summarily assigned a value. (3. Any change of solvent changes the values of E and. Quasi-reference electrodes are also admissible. Note that in tabulations. above a certain potential applied to a particular electrode. E(eq) ¼ E(cell). the reaction there within the cell is an oxidation reaction. (3:11) where the product is a solid solution with mole fractions x incorporating an unreactive electrolyte cation Mþ. Further detail follows in Section 6. . but sometimes Hþ. (3.3 Rates of charge and mass transport through a cell 41 This is an idealisation of the reaction that in practice proceeds only fractionally to the extent of the insertion coefficient x (x < 1 and in many cases < 1): < WVI O3 ðsÞ þ xeÀ þ xMþ ðsolnÞ ! Mx ðWV Þx ðWVI Þ1Àx O3 ðs: solnÞ. considering the electrodes separately. The oxidation-state notation allows a shorthand version of the essential reaction.3. As just outlined. and below it the electrode reaction is reduction. often Liþ.4. here represented in the reductive bleaching process in Eq. where only the actual chromophore segment can thus be shown.3 Rates of charge and mass transport through a cell: overpotentials To reiterate. Considering both electrodes. At only one applied potential is the current through the cell zero: we call this potential the equilibrium potential E(eq) ¼ E(cell). FeIII ½ðFeII ðCNÞ6 þ eÀ ! FeII ½ðFeII ðCNÞ6 . (3:13) 3. Complementary processes must occur at the partner electrode. and hence effects electrochromic operation. the blue pigment PB on the left being decolourised: Mþ Fe3þ ½FeII ðCNÞ6 4À ðsÞ þ eÀ þ Mþ ðsolnÞ ! ðMþ Þ2 Fe2þ ½FeII ðCNÞ6 4À ðsÞ: blue white ðclearÞ (3:12) In the formulae. each CN is actually CNÀ and Mþ is usually Kþ. these charges enforce the consumption and generation of redox materials within the cell.12). the reaction in the cell will proceed oxidatively at one electrode and reductively at the other and below it the electrochemical reactions at the electrodes are the reverse of these. A steady state exists at E(eq) and no charge is consumed at either electrode To elaborate. The counter cations may not always be unreactive. an electrochromic device is fundamentally an electrochemical cell. Above a particular applied potential Va. at only one potential applied across the cell is the current through the cell zero: at this equilibrium potential. Another oft-studied electrochrome is Prussian blue (PB) that undergoes the half-reaction. Applying a potential Va 6¼ E(cell) across the cell causes charge to flow. Electrodes comprising platinum. respectively. When net (observable) current flows. we concentrate attention on one electrode. Rate (i) is determined by the magnitude of the electronic conductivity s of the material from which the electrode is constructed. overpotential will be spelt out. a complication dealt with below in the Butler–Volmer treatment. that is. the system must surmount an energy barrier prior to electron transfer. The magnitude of rate (ii) is ‘activated’.42 Electrochemical background As before. governing the overall rate of charge movement in a device or electrode process. the ‘constancy’ appellation referring to concentration dependences at a predetermined potential. defined by Eq. Their conductivities are both low relative to true metals. in the place of metals. involving the movement of non-electroactive ions if they are taken up or lost by electroactive solids). I¼ dQ : dt (3:14) If the redox (electroactive) species are in solution. by reduction or oxidation. or generated from it. For transparent electrode systems fluoride-doped tin oxide or ITO act the role. (3. and (iii) the rate at which the electroactive material (ion.) .15): ¼ Va À EðeqÞ : (3:15) The rate constants ket are potential dependent. when one or both components of the redox couple are solid. the slowest of the three rates is ‘(overall) rate limiting’. so rate (i) can apply in such systems. which is clearly proportional to the rate at which electronic charge Q at an electrode is consumed by the electroactive species. In later chapters. gold or glassy carbon contacts possess high electronic conductivities s so rate (i) is rarely rate limiting with such substrates. (ii) the rate of electron movement across the electrode–solution interface. Thus ket is a curious rate constant dependent on the overpotential . of the ‘inert contact’ to the redox species. atom or molecule) moves through solution prior to a successful electron-transfer reaction (also. the magnitude of an electrochemical current is a function of three rates at that electrode: (i) the rate of electron transport through the materials comprising the electrode. The magnitude of rate (ii) is governed by the rate constant of the electron-transfer process ket. in the case of solid electroactive materials. Processes (i) and (ii) are termed charge transfer (or charge transport). overpotential and coloration efficiency are unfortunately represented by the same symbol . and is dictated by the overpotential of the electrode. and the symbol alone will mean only coloration efficiency. The charge that flows is measured per unit time as current I. (In the literature. process (iii) involves mass transfer or transport. that always differs from bulk solid. proceeds via three separate mechanisms: migration. Occasionally. mass transport. an overpotential of zero indicates equilibrium.e. the overpotential needs to be relatively small to prevent electrolytic side reactions. i. In type-I systems. ket will be high and therefore rate (ii) will not be rate limiting. potential-distributed ions.16). Provided the overpotential applied is sufficiently large. (3:16) where a and b are constants particular to the system (see ‘Butler–Volmer kinetics’ towards the end of the chapter. it is applied to just one electrode. but while electrons may be intuitively adjudged the fast movers in the processes with rates (i) and (ii). as defined in the Nernst–Planck equation.3. oriented molecules and adsorbed species. that is. no conversion of electrochrome to form its coloured state. convection and diffusion. p. and hence no electrochromic operation. More usefully. that is. 46).1 Mass transport mechanisms The process by which the electroactive material moves from the solution bulk toward the electrode. By definition. Mass transport is formally defined as the flux Ji of electroactive species i. on the liquid side. the number of i reaching the solution–electrode interphase per unit time. as well as the outermost solid surface. and hence zero current. a form of Tafel’s law:7. most of the electrochrome is distributed in the solution bulk.) 3.17):9 . the electrochrome must come into contact with the electrode before a successful electrontransfer reaction can occur. (The term interphase here is preferred to ‘interface’ to emphasise the number and diverse nature of the many layers between bulk electrochrome and bulk solvent. Applying an overpotential (i.3 Rates of charge and mass transport through a cell 43 Overpotential has sign as well as magnitude. this is by no means always so. Eq.3. (3. forcing the potential of the electrode away from E(eq)) causes a current I to flow.e. Since a type-I electrochrome is evenly distributed throughout the solution before the device is switched on. and must move toward the electrode interphase until sufficiently close for the electron transfer to take place.8 ¼ a þ b ln I. including. (3. which is related to overpotential by Eq. in which case rate (ii) may be rate limiting. Rate (iii) is rate limiting in a number of electrochromic devices. I / exponentialð=bÞ. RT @x @x migration convection diffusion (3:17) where (x) is the strength of the electric field along the x-axis. the equation describes one-dimensional mass transfer along the x-axis. Both forms of convection can be assumed absent in electrochromic cells. delivering electroactive species to the electrode. i is the velocity of solution (as a vector. fractions of total current borne by) the electroactive species or of mobile counter ions become appreciable. Migration may be neglected for liquid electrolytes containing ‘swamping’ excess of unreactive ionic salt (often termed a ‘supporting electrolyte’). Convection is the physical movement of the solution.1 (Fig. where applicable).44 Electrochemical background zi F @ðxÞ @ci ðxÞ Ji ¼ À ci þ ci i ðxÞ À Di . which ideally follows Fick’s laws. (Strictly. However.) The three transport modes operate in an additive sense. 3. Migration is still important in liquid-phase systems such as that in the Gentex mirror. 3.e. density differences of the solution adjacent to the electrode cause ‘natural’ convection.3 Diffusion The most important mode of mass transport in electrochromism is usually diffusion.2. described in Sections 11. Convection will not be discussed in any further detail since it is irrelevant for solid electrolytes and otherwise uncontrolled in other ECDs. or at least of a negligible extent. The first law defining the flux Ji (the amount of diffusant traversing unit area of a cross-section in the solution normal to the direction of motion per unit time) is: . and Di and ci are respectively the diffusion coefficient and concentration of species i in solution. negatively charged electrodes attracting cations.3.3) and 13. as excess concentrations of inert cations or anions that accumulate about their respective electrodes effectively inhibit continued migration. diffusion becomes the sole means of mass transport. Deliberate stirring of the solution is termed ‘forced’ convection. solid polymer electrolytes or solid-state electrochromic layers experience a significant extent of migration since the transport numbers of (i. In the absence of both convection and migration.2 Migration Migration represents the movement of ions in response to an electric field in accord with Ohm’s Law: positive electrodes obviously attract negatively charged anions. 11.3. rather than the normal. the concentration gradient). such . and either coloration or bleaching kinetics for a type-III electrochrome. will be characterised by the chemical.19): 2 @ci @ ci . and (@ci/@x) is the change in concentration c of species i per unit distance x (i. Eq (3. A rough-and-ready but useful version gives the approximate relation. electro-bleaching of a type-II electrochrome and coloration and bleaching of type-III electrochromes are all processes involving solids.3. The implications of diffusive control are discussed below.4 and 3. Fick’s second law describes the time dependence (rate) of such diffusion. it obeys Faraday’s laws – whereas that part arising solely from ionic motion without such accompanying redox. (3. For this reason. diffusion coefficient D. and the kinetic distinctions between D and D are discussed in depth in Section 5. Faradaic and non-faradaic currents The contribution to any current that results in a redox (electron-transfer) reaction is termed ‘faradaic’ – that is.e. The required integration of this second-order differential equation often leads to difficulty in accurately modelling a diffusive system. ¼ Di @t @x2 (3:19) where t is time and i denotes the ith species in solution. Movement of type-I and type-II electrochromes toward an electrode during coloration (see Sections 3. the ‘diffusion’ of a charged species through a solid is characterised by the so-called ‘chemical diffusion coefficient’ D.1. The concentration gradient (@ci/@x) arises in any electrochemical process with current flow because some of the electroactive species is consumed around the electrode. Such diffusional movement is complicated by concomitant migration.2. 1 (3:20) where l is the distance travelled by species with diffusion coefficient D in time t. The kinetics of bleaching in a type-II system. Diffusion results from a natural minimising of the magnitude of internal concentration gradients.5 below) represents true diffusion of electrochrome. @x (3:18) where Di is the diffusion coefficient of the species i. The implications for electrochromic coloration of straightforward diffusion are discussed in Section 5. By contrast. this depletion causing the concentration gradient.20): l % ðDtÞ =2 . Eq.3 Rates of charge and mass transport through a cell 45 @ci Ji ¼ ÀDi . and replenishment by diffusion controls the current. when an external applied potential is equal to the electrode potential E. then I ! 0. I becomes small (both ‘cath’ and ‘an’ currents are appreciable).16). The (net) rate of the electrode reaction is defined as: rate ¼ I i ¼ . it therefore fails ever more seriously for decreasing because when near to or approaching the electrode potential E ( small). and.4. (The law also fails for very large values of . Minor elaborations are needed for type-II systems and major ones for type-III. 3 and works cited therein. when the at-electrode concentrations of reactant decreases from the bulk values owing to the high consumption rates prevailing as follows.) . F is the Faraday constant and A is the area of the electrode. nFA nF (3:22) where n is the number of electrons involved in the reaction.2 (page 39 above) that the (net) zero current at an electrode. Faraday’s laws specifically relate to material deposition or dissolution effected by redox reactions. but the underlying physics is identical throughout. Eq.4 Dynamic electrochemistry 3. 3.46 Electrochemical background as in the formation of the ionic double layer. depending on whether the electrode is positive or negative of E. At E these are equal in magnitude. I is non-zero and one or other of the individual currents dominates.15). log I is linear with . (3:21) where implied signs attach to the individual currents. the applied potential differs by (the overpotential) from E.1 Butler–Volmer kinetics of electrode reactions It is noted in Section 3. the eÀ are acquired by the electrode).) Details are in ref. (In this outline the O and R species are both in solution. from Eq. Rate constants covering concentration dependences on cO and cR for the reactions at a particular potential are defined in Eq. (3. when electrons eÀ are relinquished from the electrode) and Ian (anodic. We write that at E I ¼ Icath þ Ian ¼ 0.23): Icath ¼ ÀnFA kcath cO and Ian ¼ nFA kan cR : (3:23) As Tafel’s law states. When. to redox transformation of dissolved species. by extension. (3. is the resultant of two opposing currents Icath (cathodic. as with type-I electrochromes. but this holds only when one of the individual (‘cath’ or ‘an’) currents dominates to the exclusion of the other. (3. is ‘non-faradaic’. 3.15) and (3.4 Dynamic electrochemistry 47 The rate constants kcath and kan (for general reference we call either ket) are both dependent on . F F F kcath ðE Þ ¼ kan ðE Þ ¼ k F F F where the parenthesised ‘(E )’ denotes ‘pertaining at E ’. A zero value of implies an applied potential equal to E. the standard exchange current. When E ¼ E (that is. some of which favours one direction of reaction. Hence.4 and 0. (3:25) i ¼ i0 exp À RT RT F F . (3. along with other rate constants. Then that activation energy is diminished by the energy supplied via . from linear Tafel’s-law regions of (Eqs. this procedure results in the requisite values of the electrode rate parameters. holding until is made so large that reactant consumption becomes great (from the high prevailing ket values). i0 ¼ I0/A is the standard exchange current density. hence the counter (driving) energy deriving from will likewise comprise an exponential factor in the ket expressions. and k is the standard electron transfer rate constant for the electrode reaction. how much depends on the detail of the energy barrier. Eq. includes an exponential activationenergy term for the activation barrier to be surmounted in the electron transfer. the extrapolated values of each of the opposing currents Icath and Ian pertain (and cancel) at E.25) is obtained: & ' nF ð1 À ÞnF À exp . . is implicit within k . with cO ¼ cR). from Tafel-law slopes. that arises from the barrier to electron transfer. These are |Icath | ¼ |Ian | ¼ I0. When not equal to ½.6. Now k . and a net current of zero. at ¼ 0. which if symmetrical results in a fraction ¼ ½ of the supplied energy for each direction. with overpotential contributions in straightforward cases weighted as and 1 À for opposing directions: nF ð1 À ÞnF and kan ¼ k exp : (3:24) kcath ¼ k exp À RT RT F F F This equation leads to the final Butler–Volmer form. (3. these need to be obtained from extrapolation back to E of observed (ln I) values vs.16)). this depletion therefore bringing in diffusion control. is usually found experimentally to be between 0. some the reverse. The value ½ is reasonably assumed in straightforward cases when not otherwise readily available. The activation energy term exp(ÀEa/RT). To obtain the current values Icath and Ian applicable at E. which is intrinsic to the particular reaction involved. Here. 3. but the essence of the kinetics is as outlined here.48 Electrochemical background where the overpotential is negative when the electrode is made cathodic but positive with electrode anodic. In order to isolate the processes at one electrode. At the end of the chosen range the potential is reversed. the effects at the other are ignored (and this ‘counter electrode’ can then be chosen merely for convenience: Pt. The cyclic-voltammetry experiment involves applying a potential smoothly varying with time t. The so-called scan.16) which holds with different intrinsic parameters for each electrode. Each voltammetric scan of an electrochromic electrode thus represents an on/off switching cycle and can be used to estimate survival times of such electrodes if allowed to run for a sufficiently long time. at any instant of time the potential is in fact constant and known. but it is certainly found to hold for the vast majority of reactions examined. Figure 3. hence the name of the control device. besides indicating probable values. electrolysing solvent water. while the WE bears a variable current and shows a true. rather than the saw-tooth mode described. show that the linearity of the Tafel region is not necessarily general.3). Wider expositions follow different sign conventions and include special cases. Advanced theories.or ‘sweep’-rate (the rate of potential variation) can be varied to give desiderata like diffusion coefficients (see Chapter 5).2 (b) shows a schematic cyclic voltammogram (CV). any unwanted byproducts are segregated within a sinter-separated compartment). but by measuring the potential between the WE and a closely juxtaposed reference electrode (RE) like the SCE (see Section 3. which will peak (with value Ip) near EO. in accord with Eq. for example. . Alternative procedures employ potentials varying as sine waves. measurable. potential.4. Figure 3. over a range including the electrode potential EO. The potential at the ‘working electrode’ (WE) is then measured not via the potential applied across the cell. indicating the nature of the connections between the three electrodes. The control device (a potentiostat) in fact drives a current across the cell of such (changing) magnitude as to effect the desired steady potential change at a desired rate. A record of the potential with time will show a saw-tooth trace of this ‘potential ramping’.2 (a) depicts a schematic circuit for cyclic voltammetric analyses.R. No net current flows through the SCE so its potential may be regarded as constant. to change at the same rate as for the forward ‘potential sweep’.2 Cyclic voltammetry Current flow through a cell alters the potentials at both electrodes. (3.R of the WE and observing the resultant current. The controlling device can (or should be able to) measure total charge passed at each stage of the sweep. v and the diffusion .1 Ipa 0. ˇ A widely used application involves the Randles–Sevcik equation linking the peak current Ip with concentration c.2 Epa Figure 3. in which all species remain in solution. such as continuous pulses of potential. showing connections between the three electrodes. which trace versus time a series of square-well potentials above and below an average.3.1 (E – E )/ V –0.2 (a) Schematic cell (depicted within a circular vessel) for obtaining a cyclic voltammogram.4 Dynamic electrochemistry (a) WE = Working electrode RE = Reference electrode CE = Counter electrode A 49 V WE RE CE sinter (b) Current l Epc Ipc Eλ 0. one-electron redox couple. Other modifications of measurement are used.0 –0.2 0. and with prolonged examination any loss or decomposition of electrochrome becomes apparent from observable diminution of cycle charge. (b) Schematic cyclic voltammogram for a simple. The sinter prevents the products of electrode reactions at the counter electrode diffusing into the studied solution. reversible. Optical/spectroscopic examination of the electrode can be undertaken concomitantly. 4. 3. and directly convert these data into the real R and imaginary J parts of the impedance Z. linearly polarised light of known orientation strikes on the surface of a sample at an oblique angle of incidence. D is dealt with in further detail in Chapter 5: 1 nF 2 1 1 D2 c v2 : (3:26) Iðlim. and today has many standard applications. such as a redox electrode. and the reflective properties of the surface. An ellipsometer quantifies the changes in the polarisation state of light as it reflects from a sample. It is mainly used in semiconductor research and fabrication to determine properties of layer stacks of thin films and the interfaces between the layers.4.10 There are in fact four ways in which what can be thought of as basically resistance and capacitance measurements can be represented. The shape and orientation of this ellipse depends on the angle of incidence. although this upper limit can sometimes be extended. The technique has been known for almost a century. together with the capacitance and its frequency dependence.50 Electrochemical background coefficient D. All such treatments are called immitance measurements. ‘thin’ means films ranging from essentially zero thickness ˚ to several thousand Angstroms. from a solution of Fick’s laws. Plots of J against R or of either against applied frequency. one can measure the resistance of a circuit element. each providing different weightings with respect to frequency. In this context. . including the measurement of film thicknesses and probing dielectric properties. Thus.tÞ ¼ À0:4463 nF A RT The other symbols have already been defined.3 Impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is summarised here. In the ellipsometry technique. the direction of the polarisation of the incident light. or of other functions against either quantity. the inverse of impedance is the quantity called admittance. can yield useful rate parameters for electrode processes.4 Ellipsometry Ellipsometry is an optical technique that employs polarised light to study thin films. The reflected light is then polarised elliptically (hence ‘ellipsometry’). and its apparent dependence on the frequency of the potential applied. 3. For example. as a function of these variables. in an outline employing the familiar concepts of resistance and capacitance. Plenum. 86th edn. A. J. J. 11.. Transient Techniques in Electrochemistry. 6. A. L. I: hydrogen electrode measurements with a liquid junction. Bard. Bard. Spectroscopic Ellipsometry and Reflectometry: A User’s Guide. Wiley. and Faulkner. for example its thickness alters. Electrochemical Methods: Fundamentals and Applications. 50A. I. then its reflection properties will also change. [Online] at www.12 References 1. 2nd edn. Wiley. Wiley. 12. G. 2001. applying a potential across an electroactive film changes the optical properties of the film. Physik. New York. 2001. 1999. 4. 10. Electrochemical Methods: Fundamentals and Applications.beaglehole. New York. L. D. More importantly to electrochromism. pp. and Faulkner. Chem. 8. Encyclopedia of Chemical Electrode Potentials. R. C. 2001. 3. The CRC Handbook of Chemistry & Physics. New York. 102 ff. pp. A. 808–12.References 51 If the thin-film sample undergoes changes. Z. A. 2005. The standardization of hydrogen ion determinations. it is possible to deduce much concerning the electrochromic layers. 5. Tompkins. Therefore. M.). Am. H.. Encyclopedia of Electrochemistry of the Elements. New York. FL. Chem. Bard (ed. 48–9. 2nd edn. 1977. Hitchcock. by monitoring the polarisation of the reflected light while changing the applied potential (‘in-situ electrochemical ellipsometry’) and subsequently manipulating the resultant optical data. 1937. 2001. A. New York. Marcel Dekker. L. L.). Tafel. D. S. Bard. 1982. 2–3. R. W. A.html (accessed 15 November 2005). pp. J. J. and Taylor. Antelman. New York. and Faulkner. Wiley. 2nd edn. 1973–1986. 1813–18. 2. Soc. Boca Raton. . 7. 51–2. 9. 2nd edn.com/elli_intro/elli_intro. J. J. Wiley. as cited in Bard and Faulkner.11. Electrochemical Methods: Fundamentals and Applications. such as any changes in film thickness with potential (called ‘electrostriction’) and the formation of concentration gradients within the film. A. David R. and McGahan. 641. and Faulkner. and hence the polarisation of the reflected light. p. CRC Press. Macdonald. 59. R. J. Bard. Lide (ed. D. Electrochemical Methods: Fundamentals and Applications. Plenum. New York. New York. 1905. R. 29. is in direct proportion to the electrochemical charge passed Q. that the relevant reaction is 100% efficient. i. one centre being formed per n electrons. R: (4:1) In the simplest cases. and hence the change in absorbance D(Abs). (4. (4. then the difference is termed ‘non-faradaic’. If the total charge passed is greater than the faradaic charge. the number of colour centres formed by the electrode reaction.2). or double-layer charging in the electrolyte adjacent to the electrode.e. ‘The amount of (new) material formed at an electrode is proportional to the electrochemical charge passed’: DðAbsÞ / Q: (4:2) The term ‘electrochemical’ charge here implies that no unwanted side reactions involving electron transfer occur at the electrode during electrochromic colour change. following Faraday’s first law.1): oxidised form. 52 . At the electrode. Eq. Eq.4 Optical effects and quantification of colour 4. where n is usually one or two according to the balanced redox reaction.1) and its reverse (see Section 3. The component of the total charge passed that is directly involved in forming the desired product is termed the faradaic charge for that process (but redox side reactions involving unwanted electrochemical products also involve faradaic charge).1 Amount of colour formed: extrinsic colour The coloured form of the electrochrome is produced by electrochemical reaction(s) at the electrode. each redox centre of the electroactive species can accept or donate electrons from or to an external metal connection. O þ electronfsg ! reduced form. an effect emulating the charging of an electrolytic capacitor. This represents processes like ‘parasitic’ current leakage possibly resulting from undesirable electronic current such as through the intra-electrode cell materials (electrolyte). from Eq. the image being formed at those separate and insulated electrodes to which a suitable potential is applied. many solid-state electrochromic systems do not follow the relation D(Abs) / Q because both the shape of the major absorption band and the wavelength maximum can change somewhat with the extent of charge insertion (i. the relationship DAbs / Q in Eq. In the case of electrochemically generated colour. in an ECD therefore the colour intensity can be readily modulated between ‘negligible’ at the one extreme (all electroactive sites being in a non.3). or the thickness of a liquid layer containing a dissolved chromophore. light intensity is modulated by varying the amount of charge passed.2 The electrochromic memory effect A liquid-crystal display (LCD) is field-responsive.4. (4. (4. However. while electrochromic devices are potential-responsive. (4:3) where " is the extinction coefficient or molar absorptivity.e.3) – relates the optical absorbance Abs proportionally to the concentration of a chromophore: Abs ¼ "lc. and ‘intense’ at the other (all electroactive sites being in the coloured redox state). Figure 4. (4. This deviation can result from changes in the molecular environment about the colorant with amount of colorant produced. with concentration of coloured species. Exemplifying. c is the concentration of the coloured species and l the spectroscopic path length in the sample. of electrochemical change as gauged by the insertion coefficient x) and hence. is related by Eq. of course. where charge therefore flows and coloration ensues.2) will only hold if the absorbance is determined at fixed wavelength.or weakly absorbing redox state). l could be the thickness of a thin solid film of electrochrome. The Beer–Lambert law – Eq.1 shows an electrochromic figure ‘3’. As colour generated in an ECD results from the application of a voltage across it that causes charge to flow. D(Abs) is the change in the optical absorbance and. In brief.2 The electrochromic memory effect 53 The magnitude of the optical absorbance change obviously follows the (ideal) faradaic charge Q governing the amount of coloured material formed.4) to Dc. the change in the concentration of chromophore generated by the electrochemical charge passed: DðAbsÞ ¼ "lDc: (4:4) Even when the electrode efficiency is 100%. The electrochromic colour is removed (bleached) by applying a potential now with the . (4. 4. ECDs behave just like rechargeable (i. (3. others only feebly so. but in thin-film form. then being decolorised by reactions in mid-solution. 15. Furthermore. In practice then. This persistence of colour leads to the useful property of ECDs. type-I all-solution electrochromes diffuse from the solid contact. the observed intensity of colour will also depend on the specific electrochrome. The ECD on/off operation thus relies on the reversible redox reaction at an electrochromic electrode. some electrochromes being intensely coloured. Such colour loss is often termed ‘self bleaching’.1) and elaborated in Section 3. rather than from a light-emitting or interference effect. oxidation þ neÀ ! reductant. 4. (4. unwanted redox reactions can occur within devices after colour formation. most ECDs do not retain their colour indefinitely. and a maintaining current is necessary for colour persistence. polarity reversed. thus.3 Intrinsic colour: coloration efficiency h Although the number of colour centres formed is a function of the electrochemical charge passed. ‘secondary’) batteries. in the sense that no storage battery is ever perfect. With a second electrode plus interposed electrolyte.e. type III – will persist after the current has ceased to flow. thereby reversing the electron-transfer process in Eq.1 Since the perception of ECD colour arises from formation of a coloured chemical. Organic electrochromes in particular can also photodegrade.2.54 Optical effects and quantification of colour a = anode c = cathode r = reference electrode c r a Figure 4. Device durability is addressed in Chapter 16. the memory is never permanent. see p. the colour in solid-state electrochromes – i. the similarities are explored by Heckner and Kraft. The optical absorption of an electrochrome is related .1) as discussed more fully in connection with Eq. (4. Such memory is occasionally referred to as being ‘non-volatile’.1).e. However.1 Schematic representation of an electrochromic alphanumeric character comprising seven separate electrodes. since nearly all redox states are somewhat reactive. the so-called ‘memory effect’. The obviously larger values of for organic species owes largely to enhanced quantum-mechanical properties governing the probability of electronic transitions responsible for coloration (see p. (4.3) above). and for organic species in Table 4. . Eq. however. is defined as positive if colour is generated cathodically.3 Intrinsic colour: coloration efficiency 55 to the inserted charge per unit area Q (the ‘charge density’) by an expression akin to the Beer–Lambert law (Eq. Many additional values are available in refs. so care is needed here. Needless to say. for example cm2 CÀ1. since Q is proportional to the number of colour centres formed. is a quantitative measure of the electrochemically formed colour. Since the optical absorbance Abs depends on the wavelength of observation. (4.3) above). in absorbance units per coulomb of charge passed.5): Io ¼ Q: Abs ¼ log I (4:5) Here the proportionality factor . The coloration efficiency can be thought of as an electrochemical equivalent of the more familiar extinction coefficient " (cf. For an ECD in transmission mode. wavelength. A compendium of for metal oxide electrochromes is given in Table 4. values of should thus be maximised for most efficient device operation. and many other values are cited elsewhere in this work.4. the ‘coloration efficiency’.6): ¼ DðAbsÞ : Q (4:6) The proportionality factor is clearly independent of the optical pathlength l within the sample. which characterises a chromophore in solution (in a particular solvent). A negative for anodic coloration is not always stated. (4. cathodic currents positive).1. Values of are clearly smaller for metal oxides than for all other classes of electrochrome. (4. In many electrochromic studies it is (erroneously) expressed in cm2.). but negative if colour is generated anodically (in accordance with the IUPAC definitions: anodic currents are deemed negative. cited. 60 ff.2. Eq. must be determined at a fixed. rather than area per unit charge. thus represents the area of electrochrome on which colour is intensified. 2 and 3. but this has not deterred most investigators from studying the electrochromic properties of oxides (see Chapter 6). is measured as the change in optical absorbance D(Abs) evoked by the electrochemical charge density Q passed. Eq. Oxide Morphology Preparative method a CVD Sol–gel Electrodeposition rf sputtering Anodic deposition dc sputtering Dipping technique Electrodeposition rf sputtering Spray pyrolysis Vacuum evaporation Anodic deposition rf sputtering Sputtering CVD Electrodeposited Sol–gel Spray pyrolysis Thermal evaporation Ther. Positive values denote cathodically formed colour. 4 5 6 7 8 9 10 11 12 13 14 8 15 16 17 18.5 24 25 12 (633) 20–27 19.008W0.56 Optical effects and quantification of colour Table 4.0 À28 À30 À15 (633) À30 À41 À 25 À35 À20 À36 (640) À37 À32 (670) À20 (546) À35 (1300) 3. ‘dc sputtering’ ¼ dc magnetron sputtering. .992O3 Amorphous Polycrystalline Nb2O5 Nb2O5 Polycrystalline Ta2O5 Polycrystalline Amorphous TiO2 TiO2 Polycrystalline TiO2 Amorphous Polycrystalline TiO2 WO3 Amorphous WO3 Amorphous Amorphous WO3 WO3 Amorphous WO3 Polycrystalline WO3 Polycrystalline Amorphous WO3 WO3 Polycrystalline WO3 Polycrystalline Polycrystalline WO3 WO3 Polycrystalline a b À6. Wavelength (/nm) used for measurement in parentheses. evap.19 20 21 22 23 24 12 25 26 27 26 28 29 30 31 32 33 34 35 36 37 38 39 40 41 26 Anodically colouring oxides FeO Polycrystalline FeO Polycrystalline FeO Polycrystalline IrOx Polycrystalline IrOx Amorphous NiO Polycrystalline NiO Amorphous NiO Amorphous NiO Polycrystalline NiO Polycrystalline NiO Amorphous Rh2O5 Amorphous Polycrystalline V2O5 Cathodically colouring oxides Amorphous Bi2O3 CoO Polycrystalline CoO Amorphous CoO Polycrystalline CoO Polycrystalline Amorphous CoO MoO3 Amorphous MoO3 Polycrystalline MoO3 Amorphous Mo0. negative values denote anodic coloration. Coloration efficiencies for thin films of metal-oxide electrochromes.6 8 (546) 8 (646) 50 115 (633) 118 (633) 62–66 (633) 79 (800) 21 64 (650) 52 42 36 (630) 38–41 109 (1400) ‘CVD’ ¼ chemical vapour deposition.5 (700) 35 (634) 77 (700) 110 (700) 12 (800) 38 (700) 5 (540) 7.7 (650) 21. of Mo(s) Oxidation of MoS3 Thermal evaporation Thermal evaporation rf sputtering Sol–gel rf sputtering Thermal evaporation rf sputtering Thermal evaporation Sol–gel Thermal evaporation Electrodeposition Electrodeposition Thermal evaporation rf sputtering Spin-coated gel Dip-coating c Spray pyrolysis Sol–gel CVD dc sputtering b /cm2 CÀ1 Ref.1. ‘High coloration efficiency electrochromics and their application to multi-color devices’. Acta.44 emphasise that the methods chosen for measurement often vary between research groups which causes difficulty in comparing values for different electrochromes. 43.. Such measurements have also been applied to conductive polymers48 but performed with reflected light as opposed to the usual transmitted light. A general method for effectively and consistently measuring composite coloration efficiencies (CCEs) (see below) has been proposed. 2023–2029. 2001.) Electrochrome Monomeric organic redox dyes Indigo Blue Toluylene Red Safranin O Azure A Azure B Methylene Blue Basic Blue 3 Nile Blue Resazurin Resorufin Methyl viologen Conducting polymers Poly(3. 4. R.3. (Table reproduced from Rauh.4-ethylenedioxythiophenedidodecyloxybenzene) Poly(3.4-propylenedioxythiophene). b reduced form.1 Intrinsic colour: composite coloration efficiency (CCE) Although measuring values of is important for assessing the power requirements of an electrochrome. D. J. 46. c oxidised form. PProDOT a (max)/ nm 608 540 530 633 648 661 654 633 598 573 604 552 730 480 551 a / cm2 CÀ1 À158 À150 À274 À231 À356 À417 À398 À634 À229 À324 176 À1240 b 650 c À520 À275 Values were calculated from data published in ref. by permission of Elsevier Science. Reynolds et al. Coloration efficiencies for organic electrochromes. F. with CCEs being calculated at . D. and Meeker. Reynolds. Prussian blue – PB.3 Intrinsic colour: coloration efficiency 57 Table 4.4-propylenedioxypyrrole) Poly(3.4. Positive values denote cathodically formed colour. A tandem chronocoulometry–chronoabsorptometry method is employed to measure composite coloration efficiencies. negative values denote anodic coloration. iron(III) hexacyanoferrate(II):47 PB is reducible to the clear Prussian white – PW.45.2. L. Electrochim. iron(II) hexacyanoferrate(II). R.44 and applied to measurements on electrochromic films of conductive polymers44.46 and the mixed-valence inorganic complex. Wang.. Mater. of low absorbance). (Figure reproduced from Mortimer.2 Tandem chronoabsorptometric (a) and chronocoulometric (b) data for a PB|ITO|glass electrode in aqueous KCl (0.2(a) shows the absorbance during the dynamic measurement of a film of Prussian blue (PB) at 686 nm. to effect the electrochromic transition. A square wave pulse was switched between þ0. these potentials are cited against a AgjAgCl wire in KCl solution (0. J. 2005.50 V (PB.4 0. the electrochromic (a) 0. low absorbance) vs. R.2 mol dmÀ 3) supporting electrolyte.50 V (PB. with permission from The Royal Society of Chemistry. For the PB ! PW transition.8 0. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’..20 V (PW. R. Figure 4. high absorbance) and À0.6 A (b) Q /mC cm –2 0.2 0. at the max of the appropriate absorbance band. 2226–33. J.3 0.58 Optical effects and quantification of colour specific percentage transmittance changes. and Reynolds.) .7 0. 15.1 1 0 –1 –2 –3 –4 –5 –6 0 5 10 ts / 15 20 Figure 4. of high absorbance) and – 0.20 V (PW. Ag|AgCl. on square-wave switching between þ0.9 0.2 mol dmÀ3). To illustrate this approach. J. Chem.5 0. 2(b).04 À150 À143 À135 À147 À150 À149 192 183 165 t/s Ref. J.48 0. Prussian blue is reviewed in Chapter 7. The charge measurements recorded simultaneously with the absorbance data are given in Figure 4. 2005. 2226–33. Chem.2 47 2.) Transition % of full switch D(%T) PB ! PW PB ! PW PB ! PW PW ! PB PW ! PB PW ! PB PEDOT PEDOT PEDOT 90 95 98 90 95 98 90 95 98 53. 3.36 44 0. In the composite coloration efficiency method. for both reduction of PB to form PW. because PW is a good catalyst for the reduction of oxygen: molecular O2 may diffuse into the cuvette during long measurement times. 95 and 98% changes.54 2.9 55. resulting in an erroneously high charge measurement. Although the chronocoulometric data in Table 4. J. It should be noted that in the original publication44 that introduced composite coloration efficiency measurements.4 47 6.3 were corrected for background charging.68 3.632 0.5 48 51 53 DA 0.3. from: Mortimer.0 1. oxidation of PW to re-form PB. as a percentage of the total D(%T).3 shows data for 90. R. at .4.564 0.21 4. and the reverse process. and Reynolds.49 4. and PEDOT in Chapter 10. to provide points of reference with which to compare the CCE values of various electrochromes. Optical and electrochemical data collected for coloration efficiency measurements.. (Table reproduced with permission of The Royal Society of Chemistry.9 2.3 52. R. 95 and 98% of the total optical density change [DOD (¼ DAbs)].6 0.50 Q/ mC cmÀ2 / cm2 CÀ1 4.4 4.45 The bold figures represent the authors’ preferred reference percentage.85 5.6 58.85 4.44 the values for the reduction process are seen to decrease slightly with increases in optical change. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue.691 0.673 0.701 0.49 2.8 56. as were the measurements with conducting polymer films.18 3. contrast at 686 nm was 60% of the total transmittance (D%T). This decrease demonstrates the importance of measuring the charge passed at a very specific transmittance value and not simply to divide the total absorbance change by the maximum charge passed. values of are calculated at a specific transmittance change. 15. Mater.9 57.49 0.675 0.’ J. This practice is important in considering the reduction of PB to PW.33 0. calculated from the maximum and minimum absorbance values. the calculated values of were described as being at 90. Table 4.3 Intrinsic colour: coloration efficiency 59 Table 4. 4. WO3 (Section 6. NC N N CN I The metal-oxide system to have received the most attention for electrochromic purposes is tungsten trioxide. and previously. 95 and 98% of D(%T). " for the aqueous MnO4À ion (which is generally thought to be intensely coloured) of only50 2400 dm3 molÀ1 cmÀ1. although switching times are longer for the PB–PW transition.4 Optical charge transfer (CT) Films of solid electrochrome are comparatively thin. such as cyanines and conductive polymers.40 -bipyridilium) in acetonitrile has an intense green colour:49 at (max) ¼ 674 nm its " is 83 300 dm3 molÀ1cmÀ1. and solution-phase electrochromes are enclosed in ECDs within small volumes of solvent. the radical cation of CPQ.3 how the carefully measured values calculated here are comparable to those for films of poly(3. cyanophenyl paraquat (I) (formally 1. or a large extent of internal conjugation such as radicals of the viologens (see Chapter 11).4-ethylenedioxythiophene) – PEDOT – (at a film thickness of 150 nm). typically of maximum optical path length 1 mm. usually sub-micron in thickness. and for the re-oxidation of PW back to PB. see Chapter 8. inorganic materials typically exhibit lower values than conducting polymers. uptake or loss of potassium ions must accompany the colour-transforming electron transfer. having a very high extinction coefficient ".10 -bis(p-cyanophenyl)-4.60 Optical effects and quantification of colour max. this choice of variable represents a mis-statement and all composite coloration efficiencies recorded in Table 4. Although as observed above. To preserve the electroneutrality of the solid electrochrome. Of the organic electrochromes.3.44 were determined using the DOD at 90. it is interesting to note from Table 4. and thus comprise very little material. the most intense absorptions are encountered with systems having an extended conjugation system. The values are similar for both the reduction of PB to PW. i. In view of the fundamental definition of DOD. As an example. An electrochromic colour that is intense enough to observe under normal illumination will therefore require a spectroscopic transition that is very intense. The bulk trioxide is pale . cf. although the switching times for the latter process are slightly shorter.2).e. The difference in switching times probably arises from different rates of ingress or egress of potassium ions in these films. Other mechanisms such as the Stark effect are briefly dealt with in Chapter 1. The blue colour is caused by red light being absorbed to effect the intervalence transition between adjacent (‘A’ and ‘B’) WVI and WV centres.) . which are hence in an excited state. tungsten bronzes are characterised by metallic conductivity. In metal-oxide systems. ferric ferrocyanide being chemically different from photo-product ferrous ferricyanide. This blue form is commonly called a ‘bronze’ (see Chapter 6). As an example. 140 ff. A WO3-based electrochrome (rather than a bronze). (4. (4.3. and is thus a semiconductor.7) lies in the range53 1400–5600 dm3 molÀ1 cmÀ1. (The optical properties of WO3 are discussed in Chapter 6.) These intervalence transitions are characterised by broad. the source of the required intense electrochromic colour is usually an intervalence optical charge-transfer (CT) transition. although strictly. (Close examination of the PB–PW structures shows that the photo-effected product distribution unusually involves an intrinsic chemical change absent in the Eq.52 where the term ‘intervalence’ implies here that the two atoms or ions are of the same element.4. Section 6. and have compositions MxWO3 where x is typically greater than about 0. The optical intervalence CT of this sort is usually regarded as the major cause of the electrochromic colour in many inorganic systems. all tungsten sites have a common oxidation state of þVI. Reductive electron transfer to a WVI site forms WV. intense and relatively featureless absorption bands in the UV. following photon absorption an electron is optically excited from an orbital on the donor species in the ground-state (pre-transfer) electronic configuration of the system.7): WVI þ WV þ h ! WV þ WVI : ðAÞ ðBÞ ðAÞ ðBÞ (4:7) The product species. visible or near IR. but forms a blue colour on reduction. (4. as used in an ECD. In colourless WO3.VI oxide system in Eq. in contrast with the transition depicted in Eq.4 Optical charge transfer (CT) 61 yellow in colour and transparent as a thin film. Eq. the value decreasing with increasing insertion coefficient x. (4. and the blue form of the electrochrome becomes evident from the optical CT. must be restricted to a lower value of x in order to preserve switchability.7). with molar absorptivities (extinction coefficients) of useful magnitudes.4 on p. In a CT-based system.51.7) transition for WVI/V. " for the WV. to a vacant electronic orbital on an adjacent ion or atom. producing an excited state. subsequently lose the excess energy acquired from the absorbed photon by thermal dissipation to surrounding structures. and elaborated below.54 It is based on the CIE (Commission Internationale de l’Eclairage (the ‘International Commission on Illumination’)) system of colorimetry. saturation and luminance (that is. in terms of hue. it is possible to report accurately the colour of new materials. The first identifies a colour by its location in the spectral sequence.63 than more familiar forms of spectrophotometry. colorimetric analyses can also provide valuable information about the optical and electrochemical processes in electrochromes. Rather than simply measuring spectral absorption bands. which is elaborated below. . the method can ultimately function as a valuable tool in the construction of electrochromic devices. in situ colorimetric analysis has recently been developed. in colour analysis the human eye’s sensitivity to light across the whole visible spectral region is measured and a numerical description of a particular colour is given. and a commercial portable colorimeter (such as the Minolta CS-100 Chroma Meter). causing its description or.58. the comparison of two colours. as perceived by the human eye.1 A brief synopsis of colorimetric theory Colour is described by three attributes. more recently.e.54. a new method of colour analysis.5 Colour analysis of electrochromes Colour is a very subjective phenomenon.59.61 to the quantitative colour measurement of conducting electroactive polymer and other electrochromic films on optically transparent electrodes (OTEs) under electrochemical potential control in a spectroelectrochemical cell. i. The approach is exemplified in Figures 4. by acquiring a quantitative measure of the colour.60.57. Third. measures changes in the electrochromic film during transformations performed under potentiostatic control.56. to Prussian blue films.5. by utilising colorimetric analysis. Such colour analyses provide a more precise way to define colour62. relative transmissivity). mounted on a tripod.5 and 4. The CIE method has been applied44. for example.62 Optical effects and quantification of colour 4. There are three main advantages to in situ colorimetric analysis. it is possible to represent graphically the path of an electrochrome’s colour change. This is known as the hue.47 is likely to be applicable to a wide range of both organic and inorganic electrochromes. dominant wavelength or chromatic colour. 4. Beyond these practical considerations. which has been applied to electrochromic conducting electroactive polymer films and. Second. the method is straightforward in operation: a spectroelectrochemical cell is assembled within a light box.45. Experimentally. the wavelength associated with the colour.55. However. This approach.6 for PB. This method allows the quantitative colour description of electrochromes. to be quite difficult. First. Once obtained. Colour spaces are the means by which the information of the X. Actually the tristimulus values themselves constitute a colour space. the hue. a system characterised by the result of tests in which people had to visually match colours in a 28 field of vision.64 Thus the CIE system is based on how the ‘average’ person subjectively sees colours. any colour can be both described and actually quantified. saturation and luminance must be defined numerically in a given colour system. In order to assign a quantitative scale to colour measurement. and is also referred to as value. where the phrase ‘colour space’ implies a method for expressing the colour of an object or a light source using some kind of notation. that is. such as numbers. and thus simulates mathematically how people perceive colours. Y and Z tristimulus values is represented graphically. although the three-dimensional vectoral nature of the comprehensive system makes it quite unwieldy for presenting data. saturation and luminance. Colour is a three-dimensional phenomenon. lightness or luminance. Using the three attributes of hue. These colour matching functions are used to calculate such tristimulus values (symbolised as X. . and represented in a systematic manner according to the three attributes. so it is not easily represented quantitatively. which states that the eye possesses three types of cone photoreceptors for three primary colours (red. There are three modes by which the eye is stimulated when viewing a colour. which define the CIE system of colorimetry. referred to as colour. tone. The original CIE experiments resulted in the formulation of colour-matching functions. and is based on a so-called ‘28 Standard Observer’. Y and Z). either in two. chroma.5 Colour analysis of electrochromes 63 and is the wavelength where maximum contrast occurs.or three-dimensional space. and is known as saturation. hence the CIE system is expressed in terms of a ‘tristimulus’. Y and Z allow the definition of all the CIE recommended colour spaces. commonly known as the CIE system of colorimetry. It is this aspect which is commonly. values of X.4. containing all possible colour perceptions. which were based on the individual’s response to various colour stimuli. It was first devised in 1931. but mistakenly. Luminance provides information about the perceived transparency of a sample over the entire visible range. The most well known and most frequently used colour system is that developed by the Commission Internationale de l’Eclairage. intensity or purity. The concept for the XYZ tristimulus values is based on the three-component theory of colour vision. The third attribute is the brightness of the colour. The second attribute relates to the relative levels of white and/or black. Colour spaces are usually defined as imaginary geometric constructs. green and blue) and that all colours are seen as mixtures of these three primary colours. respective values of x and y are calculated from the X. XþYþZ Y : XþYþZ (4:8) y¼ (4:9) On the graph represented in Figure 4. hence the name ‘28 Standard Observer’.8) and Eq.3) within this locus is known as the white point and its location is dependent on the light source. the tristimulus value Y is retained as a direct measure of the brightness or luminance of the colour. In 1931. Y and Z tristimulus values via Eq. such as the D50 (5000 K) constant-temperature daylight simulating light source.3.9): x¼ X . The three most commonly used are the CIE 1931 Yxy colour space. (4. Figure 4. and ‘chromaticity’ (representing hue and chroma) can be shown two-dimensionally. The line connecting the longest and shortest wavelengths contains the nonspectral purples. which shows the wavelengths of light in the visible region. The latter is also referred to as CIELAB. The location of a point in the xy diagram then gives the hue and chroma of the colour. The point (labelled as W in Figure 4. The hue is determined by drawing a straight line through the point representing ‘white’ and the point of interest to the spectral locus thus obtaining the dominant wavelength of the colour. and the CIE 1976 L*a*b* colour space. (4. the line surrounding the horseshoe-shaped area is called the ‘spectral locus’. the CIE 1976 L*u*v* colour space.64 Optical effects and quantification of colour Colour quantification is more easily visualised if separated into the two attributes.4 shows the determination of the dominant wavelength ($550 nm) for ‘sample B’. and is therefore known as the ‘purple line’. From this diagram. The two-dimensional graph obtained with such data is Cartesian – an xy graph – and known as the ‘xy chromaticity diagram’. the CIE proposed its first recommended colour space based on the X. The evolution of the CIE criteria is now outlined. which contains every colour that can exist. The CIE has several recommended light sources (so-called ‘illuminants’). In this system. lightness and chromaticity. Colour Plate 1 shows a colour representation of this figure. the spectral locus refers only to the horse-shoe-shaped curve and not the purple line which . The CIE has defined numerous colour spaces based on various criteria. Y and Z tristimulus values and a 28 field of view. To exemplify. Surrounded by the spectral locus and the purple line is the region known as the ‘colour locus’. and to reiterate terminology. The colour sensitivity of the eye changes according to the angle of view. The ‘lightness’ describes how light or dark a colour is. 2 0.6 0. they are only an artist’s representation of what colour a region is most likely to represent.8 510 530 540 550 560 0.5 Colour analysis of electrochromes 520 0.6 570 500 580 y 590 0.8 Figure 4.3 CIE 1931 xy chromaticity diagram with labelled white point (W). however.4 x 0. It is important.2 65 480 470 460 450 440 –380 420 0. Indeed a complementary wavelength can be expressed for any sample with which this procedure can be applied. For placing a wavelength dependence on samples such as ‘sample A’ that are found along the purple line. The most saturated colours lie along the spectral locus. The reason that colours cannot be specifically associated with a given pair of .4 W 600 610 490 630 620 640 650 700–780 nm 0. with the most intense (or saturated) colours lying closest to the spectral locus. a complementary wavelength can be expressed by drawing a straight line from the sample coordinate through the white point to the spectral locus.0 0. The purity (or saturation) as expressed by the relation in the figure is a measure of the intensity of specific hue. is defined by non-spectral purples.4. to realise that the CIE does not associate any given colour with any point on the diagram: if colours are ever included on a diagram. 0 0. lightness. Thesis.63 In 1976.7 0. which are geometrical constructs containing all possible colour sensations.0 0.6 0. ‘Donor-acceptor methods for band gap control in conjugated polymers’. The relative lightness or darkness of a colour is very important in how it is perceived. p. Department of Chemistry.10): %Y ¼ Y Â 100.1 0. (Figure reproduced from DuBois.66 Optical effects and quantification of colour 0.5 0. (4. .4 0.3 0. This new system is formulated in such a way that equal distances correspond to colours that are perceptually equidistant. 2003.9 525 0.) xy coordinates is because the third dimension of colour.5 y 0. is not included in the diagram. In the corresponding dome-shaped three-dimensional diagram.9 Figure 4.3 Sample A Purity = λd b 550 a a+b 500 λc a 575 Sample B 600 Illuminant source 650 625 780 0. Both were defined as uniform colour spaces.2 475 380 0. 21.8 0.4 x 0. C.1 450 0. The brightness is usually presented as a percentage. by permission of the author. The main reason for designing such systems was to provide an accurate means of representing and calculating colour difference. Ph. Jr. and the dominant wavelength and purity of a sample with xy coordinates of arbitrary sample B.6 0. the CIE proposed two new colour spaces.D. it is recognised that the highest purity or saturation can only be achieved when the luminance or lightness of the colour is at a low value. Y0 (4:10) in which Y0 is the background luminance and Y is the luminance measured for the sample.4 CIE 1931 xy chromaticity diagram showing the determination of the complementary wavelength of a sample with xy coordinates of arbitrary sample A.8 0. J. in order to correct flaws in the earlier proposed systems. as expressed in Eq. L*u*v* and L*a*b*. University of Florida.7 0.2 0. Àa* is the green direction. the saturation of the colour increases. video and the display industries. but the 1931 xy chromaticity diagram is probably the best known and most widely recognised way to represent a colour. The L* value measures the lightness. In addition.4. chroma and hue are defined in terms of u* and v*. À Zn (4:12) " b* ¼ 200 Â (4:13) where Xn. The centre of the chromaticity diagram (0. (4:11) 1=3 1=3 # Y . In the L*a*b* chromaticity diagram. Y and Z tristimulus values defined in 1931. þb* is the yellow direction. 0) is achromatic. as the values of a* and b* increase. In a further development the L*a*b* colour space is also a uniform colour space defined by the CIE in 1976. The L*u*v* colour space is now used as a standard in television. a* and b* are defined as in Equations (4. The result of mixing two colours is known to lie along the straight line on the xy chromaticity diagram connecting the points representing the colours of the pure components in the mixture. The diagram conveys information in a straightforward manner and hence is very easy to use and understand. the system can be used to predict the outcome of mixing colour. notably.13): Y L* ¼ 116 Â Yn " a* ¼ 500 Â X Xn Y Yn 1=3 À 16. plastic and textile industries. which is very similar to the 1931 xy chromaticity diagram. The values of L*. The CIE L*u*v* system has a corresponding two-dimensional chromaticity diagram known as the u’v’ UCS (‘uniform colour space’). the CIE 1931 system is useful in that it can be used to analyse colour in many different ways. The CIE L*a*b* space is a standard commonly used in the paint. .5 Colour analysis of electrochromes 67 The CIE L*u*v* colour space is a uniform colour space based on the X. The L* value represents the same quantity as in CIE L*u*v* and hue and saturation bear similar relationships to a* and b*. The position on this line representing the actual chromicity depends on the ratio of the amounts of the two mixed colours. None of the systems is perfect. Yn and Zn are the tristimulus values of a perfect reflecting diffuser (as calculated from the background measurement).11)–(4. þa* relates to the red direction. and Àb* is the blue direction. À Yn 1=3 1=3 # Z . 68 (a) 0. 2005.6 500 0. (b) The xy coordinates are plotted onto a diagram that shows the locus coordinates.6 0. J.30 x 0.24 0.7 0.35 0.0 0. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’.4 0.5 CIE 1931 xy chromaticity diagrams for a Prussian blue (PB)|ITO|glass electrode in aqueous KCl (0. Chem. in the steps indicated in Table 4.32 0.) .8 0.0 0. and Reynolds.50 V) 0.3 0.5 550 575 y 0.28 0. 15. Ag/AgCl) was decreased.20 V) redox states.2 0. R. from the coloured PB (þ0. with labelled hue wavelengths. J.2 0. with permission from The Royal Society of Chemistry.5 0. and the evaluation of the dominant wavelength (488 nm) of the PB redox state.36 0.8 0.. (Figure reproduced from Mortimer.7 0.4.39 Optical effects and quantification of colour 0. (a) The potential (vs.9 525 0. J.1 0.50 V) to the transparent Prussian white (PW) (À0.34 0.3 0.37 (–0.20 V) y 0. R.2 mol dmÀ3) supporting electrolyte.38 0.36 () b 0. 2226–33.26 (+0.4 x 0.9 475 λ d = 488 nm Illumination source 600 780 Figure 4. Mater.1 0.34 0. (Figure reproduced from Mortimer.20 V) redox states. hue or saturation. In addition. correcting a defect of the latter which was that equal distances on the graph did not represent equal perceived colour differences. and Reynolds. 15. J.1 0. with permission from The Royal Society of Chemistry. i. J.2 0. The potential was decreased (a) and then increased (b). the L*u*v* and L*a*b* systems therefore resolve a major drawback of the earlier 1931 system.) . 2226–33. J.3 0. A g/A gCl Figure 4.1 0. Ag/AgCl). Chem. As uniform colour spaces. between the coloured PB (þ 0. CIE L*u*v* and CIE L*a*b* allow the accurate representation and calculation of colour differences. applied potential (E/V vs. 2005.50 V) and the transparent PW (À 0.5 0.6 Relative luminance (%).. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. R.0 0. in the same steps as used for Figure 4.2 mol dmÀ 3) as supporting electrolyte. for a PB|ITO|glass electrode in aqueous KCl (0.5. The only difference between the L*u*v* (a) 100 90 80 70 60 Relative luminance % 50 40 (b) 100 90 80 70 60 50 40 –0. equal distances on the graph represent equal perceived colour differences. R. calculations can be performed to conclude whether differences in colour are due to differences in lightness.e.4.6 E/V vs. Mater.5 Colour analysis of electrochromes 69 The advantage of the CIE L*u*v* and CIE L*a*b* colour spaces is that they are ‘uniform’.4 0.3 –0. vs.2 –0. 100 0.350 0.387 0.050 À0. R. from: Mortimer.200 %Y 44.384 0. values of L*a*b* are also often reported.352 0..255 0.7 91. J.360 0. 2005. with an exact coincidence of data in the reverse (colourless to blue) direction.265 0.353 0. and Reynolds.200 0.354 0.387 0.70 Optical effects and quantification of colour Table 4.357 0.9 45.4 90.5 and 4.075 0.500 0.255 0.368 0.344 0.275 0. J.150 0. the graphical representation of colour for materials with widely varying luminance. 15.000 À0. R.) E/V vs.6 show sample colour coordinates and luminance data on switching between the (oxidised) blue and (reduced) colourless (‘bleached’) states of the electrochrome Prussian blue.4 86.400 0.386 0. due to the common use of the L*a*b* system. Coordinates for reduction of Prussian blue to Prussian white as a film on an ITOjglass substrate in aqueous KCl (0. Therefore. Figures 4.050 0.3 51. (In addition.175 0.334 0. Table 4. J.1 85. In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue.100 À0.4 54.386 L* 73 73 73 74 75 76 77 79 82 91 93 93 94 94 95 96 96 97 a* À26 À26 À26 À26 À27 À26 À26 À25 À22 À10 À7 À5 À5 À3 À3 À2 À1 0 b* À33 À33 À32 À31 À30 À29 À27 À24 À19 À6 À3 À2 À1 À1 0 0 1 1 and L*a*b* colour spaces is that the L*a*b* lacks a two-dimensional diagram. Data come from ref. which is probably its only major drawback.4 .347 0. In this example.349 0.359 y 0.387 0.278 0. Chem. 47. Mater.347 0.225 0.5 82.4 x 0.1 84.125 0.1 87. The u’v’ uniform colour space diagram only functions as a uniform colour space when the plotted points lie in a plane of constant luminance.292 0.6 47.356 0.261 0.387 0.257 0. Considering all the assets and drawbacks of these three different colour spaces. causes the u’v’ chromaticity diagram to lose all advantage over the 1931 xy chromaticity diagram.7 49.250 0. generally in situ colorimetric results are expressed graphically in the CIE 1931 Yxy colour space system.0 45.025 0.340 0.270 0.387 0.7 60.342 0.343 0. Ag/AgCl 0.4. (Table reproduced with permission of The Royal Society of Chemistry. sharp changes in hue.386 0.300 0.259 0.2 mol dmÀ3) supporting electrolyte.3 77.9 46. 2226–33. saturation and luminance take place.4 89.340 0.) By way of illustration. Carpenter. Heckner. 1997. 1988. Comparing the PB L*a*b* coordinates with those of the blue states for a range of different electrochromic conducting polymer films54 shows the distinct nature of the blue colour provided by PB. À26 and À33. O. Y. and Corrigan. Soc. The electrochromic properties of hydrous nickel oxide.. and Tepehan. Energy Mater. Modestov. 145.. Chem. 1536. T. 1999. Electrochem. N. 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Excess ionic charge of these species accumulates up to a potential-determined limit.1) in terms of bulk electroactive concentrations). ‘still’). then electroactive/electrode electron transfer is not the rate. which if low may render the electrode response non-nernstian (the electrode potential EO. a To recapitulate Section 3. if ket is high. the rate of forming coloured product can be dictated by the rate of electron transfer with rate constant ket.5 Kinetics of electrochromic operation 5. redox-unreactive). on contact with the appropriate electrode. On the other hand. (3. whereby the key electroactive species arrives at the electrode. and the overall rate of colour formation is dictated by the rate of mass transport of electroactive species toward the electrode. the solution unstirred. 75 . there is a particular means. These conditions are: the absence both of convection (i. ‘migration’ here means charge motion resulting in ohmic conduction of current. being inert (i. the rate of the process governed by ket largely determines the current. Such electron-transfer reactions are said to be ‘nernstian’ or ‘reversible’ when uncomplicated and fast and in accord with the Nernst equation (Eq. cannot undergo the electron transfer required to complete the conduction process. such ions.1). Huge applied potentials can in some cases subvert ‘inertness’. On the one hand.and current-controlling bottleneck. because. This migration is subtly prevented when the solution contains an excess of inert (‘swamping’) electrolyte ions that themselves cannot conduct.e. and also of electroactive-species migration. and furthermore. Chapter 3). that needs definition.1 Kinetic considerations for type-I and type-II electrochromes: transport of electrochrome through liquid solutions Type-I and type-II electrochromes are dissolved in solution prior to the electron-transfer reaction that results in colour.a Then ‘mass transport’ (directional motion) of any electroactive species is constrained to occur wholly by diffusion. 76 Kinetics of electrochromic operation The experimental context of these considerations arises as follows. The current is then said to have its limiting value I(lim). pt (5:1) where F is the Faraday constant. i. (5. but current otherwise utilised say in solely ionic movement is ‘non-faradaic’– Section 3. However. increases in the applied overpotential will not increase the magnitude of the current. n is the number of electrons involved in the electron-transfer reaction.e.3) is the most efficient form of mass transport. and natural convection. some of the electrochrome impinging on the electrode will undergo an electron-transfer reaction. The current that yields electrochemical reaction is termed ‘faradaic’. then the time-dependence of the limiting current. deleterious side reactions may occur at the electrode. (1. I(lim. as discussed below. Eq. A large positive value of overpotential generates a limiting anodic (oxidative) current. while a large negative value of overpotential results in a limiting cathodic (reductive) current. however. if possible. i. a result that arises from Fick’s laws of diffusion1 (Chapter 3).) The current is thus best increased by enhancing the rates of mass transport to the electrode. stirring the solution will maximise the current since convection (Section 3. Eq.4. as e. as outlined below. An electrochromic cell is primed for use (‘polarised’) by applying an overpotential (Section 3. as high a current as possible is desirable for rapid device operation. Chapter 3.e. and A is the electrode area.1).3 and footnote to previous page). However. in which case the current becomes directly proportional to the concentration of electrochrome. and more complicated forms of the Cottrell equation have been derived for the thin-layer . a limiting current is enforced. caused by localised heating of the solution at the electrode. (If the current I is made too high. Because the amount of colour formed in a given time is by definition proportional to the rate of charge passage at the electrode. In a laboratory cell. can also be dismissed. The value of I(lim) decreases slowly with time (with electrode and solution motionless). Chapter 3).1): Iðlim.g.t) owing to electrode reaction of the ion i is given by the Cottrell equation.3. ci is the concentration of the electroactive species i. in a practicable ECD this expedient will always be impossible. If migration is also minimised because an excess of inert ‘swamping’ electrolyte has been added to the solution (Section 3. tÞ rffiffiffiffiffiffi Di ¼ n F A ci . Polarising the cell ensures that. if in solution. all of the electrochrome reaching the electrode is electromodified if the overpotential is sufficiently large. The derivation of the Cottrell equation presupposes semi-infinite linear diffusion toward a planar electrode. 1 Transport of electrochromes through solutions 77 Table 5. Table 5. and the spurious straight line shown results largely from employing restricted ranges of the variables. as d(Abs)/dt½ is clearly not linear with I.3. which can be inferred from Eq.5 and they superimpose regardless of concentration. (5.1 shows such a plot of current I against time tÀ½ during the electro-oxidation of aqueous o-tolidine (3. the test plot.1 Â 10À6 Diffusion medium Water Water Propylene carbonate Ref. H3C H2N CH3 NH2 I The rate of coloration is obviously a linear function of the rate of electron uptake. slopes of Abs versus t½ plots at various concentrations and currents are plotted against I for the electro-oxidation of o-tolidine (I) in water.10 -biphenyl) (I). which.2. but decreases monotonically with a tÀ½ dependence in a diffusion-controlled electrochemical system. Absorbance–time relationships like these have seldom been used as tests (presumably discouraged by confusion arising from the apparent irrationality . being a kinetically straightforward (‘nernstian’) system. Figure 5.1 lists a few values of diffusion coefficient D obtained from Cottrell analyses. The slope of Figure 5.40 -diamino-1. Here.1.2 should be independent of the concentrations of the electroactive species.1). I ¼ dQ/dt. as is shown in Figure 5.5 Support for a diffusion-controlled mechanism is thus demonstrated.5. is satisfactorily linear. the rate of colour formation d(Abs)/dt (which is / I. Equation (5.1) predicts that the magnitude of the current – and hence the rate at which charge is consumed in forming the coloured form of the electrochrome – is not constant. (1. Eq.5 conforms with the analysis. 2 3 4 cells2 that are used for type-I ECDs. Integration hence predicts Abs / t þ½ and for (I) in water. However. Eq. as expected. This kinetic result is indeed found until quite long times (>10 s after the current flow commences). Figure 5.6 Â 10À6 2.7)) ought also to have the time dependence of t À½ according to the Cottrell relation. Accordingly. (1.30 -dimethyl-4.7). for optical absorbance Abs (which is / Q). Diffusion coefficients D of solvated cations moving through solution prior to reductive electron transfer. Diffusing entity Fe3þ Methyl viologen Cyanophenyl paraquat D/cm2 sÀ1 5 Â 10À6 8. the plot Figure 5.3 should not be linear. H3COC II COCH3 Such diffusion control is expected during coloration for all type-I electrochromes.1 Cottrell plot of limiting current I against tÀ½ during the electrooxidation of o-tolidine (3. ‘Observation of electrode–solution interface by means of internal reflection spectrometry’.2 0. Deviations must occur at longer times because the transferring . while type-II electrochromes should evince the same behaviour at very short times.40 -diamino-1. 38.3 (t /s) – ½ 0. N. 1810–21. Anal. 1966. W. T.78 Kinetics of electrochromic operation 9 8 7 6 5 4 3 2 1 I /mA 0 0..5 Figure 5. by permission of The American Chemical Society.. Kuwana.6 emulated the tests of these relations for electrogenerating the aromatic radical anion of p-diacetylbenzene (II) with similar success.10 -biphenyl) in aqueous solution at a ITO electrode polarised to 1. (Figure reproduced from Hansen.1 0. SCE. Chem.4 0. A.) of the Figure 5. and Osteryoung.30 -dimethyl-4.3-type plots) but in 1995 Tsutsumi et al.5 V vs. R. 02 0. Anal.40 -diamino-1. Kuwana.5. and Osteryoung.04 0. The majority of studies relating to the kinetic aspects of electrochromic operation of solid materials relate to tungsten oxide as a thin film.2 and 11. A. W. by permission of The American Chemical Society.2 Plot of the change of optical absorbance Abs against t½ during the electro-oxidation of o-tolidine (3. 5.. so specific to the chemistry of this group of type-II electrochromes. T. (Figure reproduced from Hansen. on the bipyridiliums.3 Transport of counter ions through solid systems 79 0.) electron needs to traverse a layer of solid coloured product. 5. R.3 Kinetic considerations for bleaching type-II electrochromes and bleaching and coloration of type-III electrochromes: transport of counter ions through solid electrochromes Type-II electrochromes such as heptyl viologen (see Chapter 11) are solid prior to bleaching.3 specifically are devoted to these aspects. where the uncoloured reactant is dissolved but the coloured form becomes deposited as a solid film. Type-III electrochromes remain solid during oxidation and reduction reactions.01 0 Δ (Absorbance) 1 2 3 (t /s) 4 ½ 5 6 7 Figure 5. Sections 11. 1966.2 Kinetics and mechanisms of coloration in type-II bipyridiliums As the details of the coloration mechanisms are. exceptionally. with concomitant complication of the analysis. 38.03 0.. the complications of the chemistry are dealt with in Chapter 11. 1810–21.10 -biphenyl) in aqueous solution at a ITO. N. With suitable and probably slight modification. such .30 -dimethyl-4. ‘Observation of electrode–solution interface by means of internal reflection spectrometry’. the theories below relating to solid WO3 will equally apply to many other solid electrochromes. Chem. .e.04 0. (5.) as the other metal oxides in Chapter 6.5 5 10 × 103 0. 38. with the Mþ as an inert..08 0. A. electro-inactive ion. i.14 2. (Figure reproduced from Hansen. ‘Observation of electrode-solution interface by means of internal reflection spectrometry’. SCE.5 Â 10À3 mol dmÀ3. by permission of The American Chemical Society.80 Kinetics of electrochromic operation 0. W. the most by far of reported kinetic studies of electrochromism relate to solid tungsten trioxide.5 V vs. As already noted. Its coloration reaction is summarised in Eq. R.2) (which is actually ‘a gross over-simplification’. Even a brief survey of the literature on tungsten trioxide shows a number of competing models in circulation for the coloration and decoloration processes.7 since the initial solid almost invariably also involves water and hydroxyl ions): WO3 þ xðMþ þ eÀ Þ ! Mx WO3 : (5:2) Thus in the discussion below WO3 is the paradigm. N. and Osteryoung.06 0. usually designated ‘counter ion’.10 -biphenyl) in aqueous solutions at a ITO electrode polarised to 1. The straight line is spurious – see text. Some of the results may also apply straightforwardly to the inherently conducting polymers in Chapter 10. Chem.10 d( Abs)/d( t /s) ½ 0.02 0 2 4 I/mA 6 8 10 Figure 5.16 [o-tolidine] 0. * 5 mmol dm–3. 1810–21.40 -diamino-1. Anal. and & 10 mmol dmÀ3. 1966. The concentrations of electrochrome are o 2.30 -dimethyl-4. that is entrained to . Kuwana. T.12 0.3 Plot of d(Abs)/dt½ against current I for the electro-oxidation of otolidine (3. or by sputtering. Liþ or Naþ) is metallic for so-called ‘bronzes’b of x greater than ca. (ii) Electronic motion As we assume a particulate electron. and water will also move through polymers of organic viologen in response to redox cycling.g. that produces an amorphous oxide. the electronic conductivity of MxWO3 (where M ¼ Hþ.8 apparently disproves this assumption. since most of these metal oxides when fully oxidised are. electron conduction probably occurs via activated site-to-site hopping rather than through occupied conduction bands.4 shows a plot of electronic conductivity in WO3 as a function of insertion coefficient x. At low extent of reduction x. a ‘bronze’ is a solid with metallic or near-metallic conductivity. poorly conducting semiconductors. both as a solid and as a thin film. Below a metal-to-insulator transition. the niceties of quantum-mechanical tunneling associated with wave properties will be glossed over. (Systems generally adjust. denoted a-WO3. However.3. Another source of charge inside a film is the ionisation of water: H2O ! Hþ þ OHÀ (or with sufficient H2O about. the electrical conductivity of fully oxidised WO3 is extremely low. In contrast. H2O will be inserted into electrodeposited cobalt oxyhydroxide9 or into vacuum-evaporated10 WO3 when the impressed potential is cathodic.3 Transport of counter ions through solid systems 81 preserve or maximise electroneutrality within the solid oxide film. at best. solid films of WO3 are assumed to contain no electro-inserted counter ions. but above it near-free electrons impart reflectivity. WO3 is a semiconductor. . to minimise concentrations of charge and high potentials. 0. The WO3 was prepared either by vacuum evaporation. e. Such water may be replenished during coloration and bleaching since there is evidence for movement of molecular water through transition-metal oxides during redox cycling. was shown to contain protonic charge.11 In accord. 2H2O ! H3Oþ þ OHÀ). subject to electromagnetic. an ellipsometric study by Ord et al. b In this context. His thin-film WO3.3. but this charge had no optical effect: presumably acid had been unreactively absorbed by the solid.) Other electrochromes.5. formed anodically on W metal immersed in acetic acid. Figure 5. that produces a crystalline oxide. 5. electrostatic and quantal laws. organic as well as inorganic. ‘Free’ here implies ‘akin to conduction electrons in true metals’.1 Kinetic background: preliminary assumptions (i) Initial state: mass balance Prior to the application of the coloration potential Va. are mentioned here if data are available. the formation of the high-x bronzes MxWO3 (x > 0. 25. (Interphase rather than ‘interface’ is defined in Chapter 3. At high x values the transferred electrons. Schirmer.2 0. V. can be immersed in a solution containing a salt of the counter ions. Li0. may thwart the re-oxidative extraction of electrons from WV by the electrode substrate.4WO3 cannot be electrooxidised back to12 WO3. ‘Disorder dependence and optical detection of the Anderson transition in amorphous HxWO3 bronzes’.15 x 0. Data determined at 123 K. or LiClO4 for Liþ ion. . copyright (1978) with permission from Elsevier Science.1 0. Electrons from interphase redox reactions by external electroactive species.. Solid State Commun. 43) (iii) Motion of ions The solid electrochromic oxide. O. It should be noted that c-WO3 is less electronically conductive than a-WO3. and Schlotter.g.05 0. F.4 Plot of electronic conductivity s of HxWO3 as a function of insertion coefficient x.25 Figure 5. acquired in the electrochemical coloration process. During electrocoloration. electrons enter the film via the electrode substrate and. P. so e..82 5 Kinetics of electrochromic operation Ev aporated la yer 4 Sputtered la yer T = 123 K log(σ /cm –1 ) 3 2 1 0. such as H2SO4 for mobile protons. are stabilised in an accessible conduction band largely comprising the tungsten d orbitals. (Figure reproduced from Wittwer. 977–80.3) is not reversible. via a dissipating conduction through this band. Circumscribing the use of WO3 in ECDs.) denoted c-WO3. concurrently. p. 1978. as a film on its electrode substrate. 2. and radiotracer methods. analysis of cyclic-voltammetric peak heights as ˇ a function of scan rate via the Randles–Sevcik equation. the non-linearity of Bell and Matthews’ graph accordingly points to a significant extent of migration in the measured ‘diffusion coefficient’. and the resultant .13 When the kinetics of electrochemical redox change are dictated by the motion of a species within the film. The factor W quantifies the extent of the so-called ‘thermodynamic enhancement’. it is the slower. the transport number t (¼ fraction of current borne) of ions can approach zero. x.3. For comparative purposes. the overall rate is altered. However. In contrast. but the separate extents are usually not known. Diffusion coefficients are obtained from several measurements: impedance spectra.17.3 Transport of counter ions through solid systems 83 counter ions enter the film through the electrolyte-facing interphase of the WO3 cathode. Exemplifying. the movement of counter ions through solid WO3 proceeds by both diffusion and migration. Bleaching entails a reversal of these steps.14 as mentioned below. Indeed. together with preparation method and insertion coefficient.20 provide the representative selection in Table 5. The slower charge carrier is usually the ion because of its relatively large size. Such dual motion is the cause of the curiously named ‘thermodynamic enhancement’ described by Weppner and Huggins. then correspondingly the electron transport number t(electron) ! 1. The latter is therefore unlikely to be a true diffusion coefficient but a combined-mechanism quantityD.5. Bell and Matthews15 cite activation energies Ea for diffusion. (3. True diffusion is an activated process and normally obeys the Arrhenius equation that gives a linear graph of ln D against 1/T. To minimise the charge imbalance during ion insertion or egress. So coloration or bleaching proceed with associated movements of both electrons and cations.19. Eq. the temperature dependence of migration is relatively modest. of the two charge carriers that is the determinant. 12. as defined below. the slower ions move faster and the fast electrons are slowed. As dual mechanisms with different activation energies often show curved 1/T plots of the rate-parameter logarithm.14 In this way. The variations in diffusion coefficient could reflect the disparity in rate between electrons and ions as they move through the solid.16 Compendia from the literature of D values for mobile ions moving through WO3 in refs. an enhancement factor.36 causing D to change by a factor of W.12). chronoamperometry. varying in the range 56–70 kJ molÀ1 (values that denote an appreciable temperature dependence): the spread of values arises from the pronounced curvature of an Arrhenius plot. values for mobile ions moving through other type-III electrochromes are listed in Table 5. The two modes of mass transport operate additively.18. A good gauge of rapidity of ion motion is its diffusion coefficient D. hence rate limiting. d Chronoamperometric measurement. f Film prepared from sol–gel intermediate.84 Kinetics of electrochromic operation Table 5.5 Â 10À 11 2.12 In addition to morphological differences born of preparative .5 Â 10À 12 4. 36). e Electrodeposited film. b Impedance measurement. The two diffusion coefficients D and D are related as:14 D ¼ WD: (5:3) In consequence. probably most of the ‘diffusion coefficients’ in the literature of solid-state electrochromism are chemical diffusion coefficients. W is also termed the ‘Wagner factor’. diffusion coefficient is the ‘chemical diffusion coefficient’.170 a-WO3 * b c a-WO3 * b c 0.37 *Thin film.138 0.6 Â 10À 12 1.2. a Sputtered film. z(ion) is the charge on the mobile ion.6 Â 10À 11 2. but is said to be ‘about 10’ for the motion of Hþ through WO3.260 0.7 Â 10À 9 5. Chemical diffusion coefficientsD representing movement of lithium ions through tungsten trioxide: effect of preparative methodology and insertion coefficient. 21 22 23 24 25 21 21 21 21 21 26 26 (a) Effect of preparative methodology – WO3 * a b – WO3 * b d WO3 * c d – – WO3 * e WO3 f – (b) Effect of insertion coefficient x a-WO3 * b c 0.1WO3 f Li0.37WO3 f 0.3 Â 10À 11 2 Â 10À 11 5 Â 10À 13 2. on three-electrode cells avoiding ECD complications. c Thermally evaporated sample.9 Â 10À 12 1.201 a-WO3 * b c 0. The enhancement factor W can be14 as great as 105.8 Â 10À 11 1.1 Li0. (see Chapter 3. Morphology x in LixWO3 D /cm2 sÀ 1 5 Â 10À 9 1.097 a-WO3 * b c 0.6 Â 10À 10 Ref. The factor W was derived as:14 W ¼ tðelectronÞ ! @ ln aðionÞ @ ln aðelectronÞ : þ zðionÞ @ ln cðionÞ @ ln cðionÞ (5:4) Here the letters c and a are respectively concentration and activity. Measurements as in text. p. 37 in the oxygen bridges –O–) are covalently bound or otherwise immobile.31 32 32 33 34 35 27 27 PC ¼ Propylene carbonate.3. 27 28 29 30. the transport number of the electron t(electron) ! 1.08Nb2O5 Poly(carbazole) Poly(isothianaphthene) Tungsten(VI) trioxide c Tungsten(VI) trioxide e Vanadium(V) trioxide Ion : Solvent Liþ:PC TBAT:DMF ClÀ: H2O Hþ:H2O Hþ Hþ ClOÀ :H2O 4 BFÀ :PC 4 Hþ:HCl(aq) Liþ:PC Liþ:PC D/cm2 sÀ1 5. oxide ions O2– that are. more likely.6) above that only the counter ion is mobile since all other ions (e.2 Â 10 À7 10À11 10À14 2 Â 10À8 d 2. (5.6–8.3)–(5. solid films of type-III electrochromes. This tenable assumption has been verified in part by impedance spectroscopy.3) into Eq. diffusion of counter ion through the electrochromic layer. F16-pc ¼ perfluorinated phthalocyanine.g.042Nb2O5 H0. b Apparently calculated from chloride ion mobility.4) becoming: W¼ ! @ ln aðionÞ : @ ln cðionÞ (5:5) Substituting for W from Eq. d Chronoamperometric measurement.38 .2. Being fast.9 Â 10À11 Ref.1 Â 10À11 f 3. f Value determined from impedance measurement. (5. routes. Compound Cerium(IV) oxide (F16-pc)Zn a Lutetium bis(phthalocyanine) Nickel hydroxide H0. Chemical diffusion coefficientsD of mobile ions through permanent. (5. c Thermally-evaporated sample. Eq.5.2 and 5.3. Methods as for Table 5. hence the observed rate of transport through WO3 is determined by the slower ions. e Sputtered film.0 Â 10À12 10À7 b 2 Â10À7 to 2 Â10À9 3.2 Â 10À13 1. Thus the expression for W can be simplified. variations in W are a likely reason for the wide differences in the D values listed in Tables 5.3 Transport of counter ions through solid systems 85 Table 5.5) yields the so-called Darken relation:14 ! @ ln aðionÞ D¼D : @ ln cðionÞ (5:6) It is assumed in Eqs. a Value from ˇ analysis of a Randles–Sevcik graph.6 Â 10 À8 5. (5. they attain sites of lowest potential energy. in moving within the oxide layer following insertion during coloration.39 The discussion below indicates how this last assumption probably understates the role of interphases. R(CT). 5.e.47 Slight spatial rearrangement of atoms (i. perhaps with slight deviations in concentration at interphases due to the interactions born of surface states. local phase transitions from the predominantly monoclinic) in c-WO3 are said to occur during reduction. electrolyte-facing.43. with the former Ea effect being the larger. a recent Raman-scattering investigation of HxWO3 electro-bleached in aqueous H2SO4 is said to indicate. Protons come to rest when the external potential is removed and when. that the rate of electro-bleaching is dominated by proton expulsion from the HxWO3 as the Hþ traverses the electrochrome–solution interphase. 41) by including an ultra-thin layer of metallic nickel between the electrochrome and ITO. by analysis of the WO3 vibrational modes.3.48 which may affect the electrochromic response . side of the electrochrome. 40 and 41.03). called ‘terminal effects’. 40. Both apparently improve the response time . Many of the measured values of ‘R(CT)’ may be composites of terms containing the interphase activation energy Ea (in an exponential) for ion insertion together with R(CT) for the electron transfer at the electrode substrate.2 Kinetic complications The complications caused by the innate resistance of the ITO.46 The effect is elaborated in refs. the barrier often being represented as the resistance to charge transfer. can be largely bypassed (but see refs. The motion of counter ions within the film may also contribute. and certainly play a role in the interpretative models considered below. (i) Crystal structure There are several distinct crystallographic phases notably monoclinic discernible in reduced crystalline tungsten oxide (c-WO3) at low insertion coefficients (0 < x 0.42 or an ultrathin layer of precious metal on the outer.45.86 Kinetics of electrochromic operation (iv) Energetic assumptions A relatively crude model of insertion has the counter ion entering or leaving the oxide layer after surmounting an activation barrier Ea associated with the WO3–electrolyte interphase. For example.10 There is also an activation barrier to electron insertion/egress from or to the electrode substrate. On equilibration inside the oxide layer. The motion of a (bare thus minute) proton will be the most rapid of all the cations.44. the inserted ion is assumed in most models to be uniformly distributed throughout the film. in addition. Such structural changes are sometimes believed to be the rate-limiting process during ion insertion into WO3. As ions that move through solid oxide experience obstruction within the channels.2).54 It was argued54 that this result demonstrates that the LixWO3 product has sufficient time to change structure on a microscopic scale during the slower. Though some workers have reversibly inserted Naþ. 6% on reductive ion insertion.51 show by ellipsometry that V2O5 on reduction in acetic acid electrolyte also expands by 6%. (The sequence of cations follows the indications of the activation-energy model referred to.50 The value of D increases slightly with increasing insertion coefficient x. Counter-ion swapping can occur since WO3 does entrain indeterminate amounts of water. Scarminio62 reported that the stress induced in a film is approximately proportional to the insertion coefficient. finding that phase transitions were induced. stepwise. A general picture does emerge from envisaging the constraints on ionic motion and the experimental observations. as exemplified by the data of Ho et al.) The only anion small and mobile enough to be inserted into anodically colouring electrochromes is OHÀ.60 and even reversible incorporation of Agþ has been reported. when injected with Liþ ion at a continuous rate. even if prepared as an anhydrous film.3 Transport of counter ions through solid systems 87 time of WO3 for colouring or for bleaching. on p. (ii) The effect of the size of the mobile ion Questions arise as to what counter ion is taken up during reduction. Green24 has stated that WO3 expands by ca. A model for this process from which activation energies can be estimated is outlined later.21 in Table 5. samples of c-WO3. but it is not always intrinsically consistent in detail. were found to have a higher capacity for lithium ion than do otherwise identical samples that are charged fitfully. thus impeding subsequent scope for reduction.2. The film capacitance also increases linearly with x. despite the thicknesses of electrochromic oxides being somewhat diminished when a field is applied owing to electrostriction. so protons are favoured for WO3. ionic size is expected to govern the values of D for different ions. but the picture is not clear-cut. and which one provides the charge motion within the film.5. x. 112.57 are found to be somewhat slower than protons.52.59. Deuterons55.61 most other cations are too slow to act in ECDs. Variable water . For rapid ECD coloration.63 Scrosati et al.56. and lithium ions are slower still (see Table 5.58. charging. Avellaneda and Bulhoes ˜ 26 find the same effect. ion size should be minimised.48 used a laser-beam deflection method to assess the stresses from electro-inserting Liþ and Naþ.49.53 Similarly. and Ord et al. 73 Similarly.65. In similar vein. followed at longer times by exchange of Li þ for Hþ as charge-carrier. the mobile ions tend to move through the amorphous material as a kinetic ‘fast-track’. to form e. which is illustrated in the electrochemical quartz-crystal microbalance (EQCM) study by Bohnke et al. For a chemically different WO3.74 An additional means of increasing the electrochromic coloration rate is to increase the size of the channels through the WO3 by introducing heteroatoms into the lattice.24 Kubo and Nishikitani.24 see page 98. Babinec’s EQCM study68 also suggested swapping of Liþ for the more mobile Hþ.g. concluding that the coloration efficiency increases as the cluster becomes larger. report the process to be ‘extremely complicated’.20.64. diffusion through c-WO3 is so slow by comparison with diffusion through amorphous tungsten oxide (a-WO3) that the c-WO3 need not even be considered during kinetic modelling of films comprising both amorphous and crystalline oxide.’s69 mirage-effect experiment that implied dual insertion of Hþ and Liþ during reduction of WO3. diffusion through the amorphous grain boundaries within polycrystalline NiO is faster than through the NiO crystallites. Kim et al..66.88 Kinetics of electrochromic operation content may be the cause of the great discrepancies between reported values of D. Indeed. the diffusion coefficient of lithium ions inserted into rfsputtered WO3 was found to decrease as the extent of oxygen deficiency increased. causes strains in the lattice which are relieved by increases in all the lattice constants. A dual-cation mechanism is suggested by Plinchon et al. or formed by ionisation of interstitial water).. but also suggested egress of hydroxide ions from the film during coloration (from water within the film ionising to OHÀ and Hþ). cite polaron– polaron interactions within clusters of c-WO3 embedded in amorphous material. WyMo(1 – y)O3. The incorporation of other atoms like Mo.67 Such unexpected swapping is considered thermodynamically (specifically entropy) driven. the value of D for Liþ motion through a-WO3 that is thermally annealed decreases by about 5% over annealing temperatures ranging from 300–400 8C. the decrease in D is ascribed to increased crystallinity. since electrochromic films commonly comprise both amorphous and crystalline WO3. as a function of cluster size.72 in a Raman spectral study of WO3. Also. In common with Bohnke et al. Some chemical diffusion coefficients for the (nominally) slow lithium ion appear to be fairly high for motion through WO3. . This suggests diffusion of the more mobile proton (presumably taken up interstitially.70 studying the dual injection of Hþ and Liþ by impedance spectroscopy.71 (iii) The effect of electrochrome morphology Diffusion through amorphous oxides is significantly faster than through those same oxides when crystalline. have been reviewed by Kreuer. he also states that his MoO3 was electro-coloured as a dry film in a vacuum. presumably adsorbed initially.80 Adsorbed water can be removed by heating81 above ca. see p.76 Hurditch77 has stated that electrochromic colour of HxWO3 will form only if films contain moisture and.86 Arnoldussen87 and Randin88 have all discussed dissolution effects. References 82 and 83 describe the depth-profile of H2O in WO3. ion or molecule during its movement through an oxide interior are determined by the microscopic environment through which it moves. or by being reduced to H2 (also with OHÀ) which itself can effect chemical reduction. The loss of solvation stabilisation can be partly compensated by interaction with lattice oxides or indeed occluded H2O. i. but the former – in addition to lattice-penetration obstacles – could retard motion (EQCM studies67 however show Liþ to be unsolvated as it moves through . The forces exerted on an atom. and on the physical size of the channels through which it must pass. and its measurement. the rate of WO3 dissolution is promoted by aqueous chloride ion. Ions undergo some or total desolvation during ion insertion from solution. One concludes that water. similarly. Hygroscopicity Thin films of metal oxide are often somewhat hygroscopic. is essential in effecting the reductive coloration. 140).89 Energetics The effect on stabilities resulting from the incorporation of water needs consideration.3 Transport of counter ions through solid systems 89 (iv) The effect of water The presence of water can greatly complicate kinetic analyses intrinsically.5. 1908C (but extensive film crystallisation will also occur at such temperatures. Proton conductivity through solid-state materials.79 although it has been concluded that the cubic phase of WO3 prefers two Hþ to one water molecule.e. as shown in Figure 5. Furthermore. when traversing the solution–electrochrome interphase into the lattice. Arnoldussen78 states that MoO3 is not electrochromic if its moisture content drops below a minimum level. and additionally. Curiously.5. either by ionising to Hþ and OHÀ so providing the conductive protons.85 perhaps following the formation of soluble tungstate ions.84 Aquatic degradation Excess moisture inside films (especially evaporated films) will cause much structural damage.81 Faughnan and Crandall. Even the coloration efficiency can change following such adsorption. adsorption of water at the electrochrome– electrolyte interface can make some optical analyses quite difficult75 since specular effects are altered. and Bange.67 used data from EQCM studies to explain non-adherence to Nernst-type relations.4 0. W. F. Alternatively a Grotthus-type conduction ¨ process could be facilitating rapid proton conduction through hydrated oxide interiors. 50.60. 27–30.2 0.. Nuc. anions are expelled from the surface of the WO3 as cathodic coloration commences. Bohnke et al.5 0.25 0 7000 9000 8000 Energy [k eV] 10 000 11 000 0 Figure 5.90 Kinetics of electrochromic operation Rh WO3 SiO2 Rh SiO2 WO3 IT O Glass 1. Ottermann. Proton motion through hydrated films is accordingly found to be much faster than through dry films. unsolvated.) WO3). Res. C. The effects of interactions between inserted Liþ and the lattice were also mentioned. ‘Hydrogen dynamics in electrochromic multilayer systems investigated by the 15N technique’. The rhodium layers act as both a mirror and an ion-permeable layer.90 the retarding proton/oxide interactions possibly being weaker than in dry oxides. (iv) The effect of insertion coefficient on D Values of D can be obtained from the gradient of a graph of impedance vs. Sect. Meth. postulating that adsorbed. B.8 H/ W H/Si 0.. K. 1990. copyright (1990) with permission from Elsevier Science. oÀ½ as by Huggins and co-workers. Instr.91 Masetti et al.5 Hydrogen profile within the electrochromic cell at an applied voltage of 0 V: RhjWO3jSiO2jRhjSiO2jWO3jITOjglass.71. (Figure reproduced from Wagner.21 Three independent groups found that D decreased as the insertion coefficient of Liþ in WO3 increased. Rauch. Phys.60 found that D for Liþ and Naþ decreased by thirty-fold in WO3 over the insertion coefficient range . Sol.6 Plot of chemical diffusion coefficientD for Liþ and Naþ through WO3 as a function of insertion coefficient. E. Sol.04 0.01 0.95 are the following. Energy Mater. The distinctive features of the models discussed in the following sections are summarised in Table 5. 5. ‘The electrochromic response of tungsten bronzes MxWO3 with different ions and insertion rates’.3 Transport of counter ions through solid systems Appro ximate composition / x –8.93. possibly too complicated to model at present.94. semi-empirical because they used data from measured values of the electrode potential E to provide empirical parameters used in their formulation.93. By contrast. and Decker.02 0.) 0 < x < 0. copyright (1995) with permission from Elsevier Science.5 –10. Huggins’ results from an independent ac technique show the opposite trend.5 0 1 3 4 2 – Charge inserted/mC cm 2 5 Figure 5. The sensitivity of motion parameters to preparative method has already been remarked on: fluctuations in D with x appear highly complicated. (Figure reproduced from Masetti.95 provided a semi-empirical model for WO3 coloration and bleaching. Model of Faughnan and Crandall: potentiostatic coloration Assumptions Faughnan and Crandall86. 301–7.5 0 0.92..05. identifying Mþ as a proton unless specified otherwise.03 0.5.0 log(D/cm 2 s–1 ) log D(Li) log D(Na) –9. D. with D of Liþ in WO3 increasing as x increases. 39. see Figure 5.3.6.0 –10. Dini.92. Cells. each of the models below will be discussed. (5.3 Kinetic modelling of the electrochromic coloration process For the electrochromic coloration reaction of WO3 given in Eq. The main assumptions at the heart of the model86. .05 91 –9.4 overleaf.2). F.94. 92 Kinetics of electrochromic operation Table 5. Duffy and Monk * * Bohnke * * Various * * No concentration gradients form within the film.119. 86. An empirically characterised backpotential acts at that interphase. Reduction of WO3 is a chemical reaction..131. the proton motion is relatively unhindered.03 counter-ion motions are rate limiting. 124. Reduction of WO3 may be a twoelectron process. There is an Hþ injection barrier at the electrolyte–WO3 interphase. The back potential dominates the rate of coloration.126. 128. A percolation threshold sets in at x ¼ 0. Summary of the coloration models described on pages 91–104. From assumption (i). The HxWO3 adjacent to the inert electrode substrate remains H!0WO3.93 * * * Green * * * * Ingram.103 116.03. having entered the WO3. Electrons and proton counter ions in the film form a neutral species [H þ eÀ].117. apart from the restraint arising from the back . 120. it is argued that.127.121. The proton motion (intercalation) is rate limiting also because of assumption (ii). at x > 0.4. effected by atomic hydrogen arising from this neutral. Principal authors Faughnan and Crandall Distinctive features * Refs. rate-limiting species are electrons.132 (i) The rate-limiting motion is always that of the proton as it enters the WO3 from the electrolyte. 123. in traversing the electrochrome–electrolyte interphase.129. the potential increasing as the extent of insertion x increases. (ii) A ‘back potential’ (Faughnan et al.102. always call this potential a ‘back emf ’) forms across the WO3–electrolyte interphase during coloration. Concentration gradients of counter cations within the MxWO3 films were computed by analogy with heat flow through metal slabs. 100 96 101.130. WIV species participate in addition to WV and WVI.125. The diffusing entity is uncharged so there are no effects owing to the electric field. When x < 0.122.118. Hence the kinetic effects of cations and electrons are indistinguishable.03. 7.86 It is invoked because the chemical potential of the inserted cation is increased (i. by contrast. Apart from this difference in meaning. a further assumption (iii) may be inferred. The back potential then corresponds to the change in chemical potential of the proton that accompanies coloration. the applied potential Va is. In essence.e. Note that we now retain the symbolism of the original authors. the main c Whether the increase in the protonic chemical potential with increase of its concentration is sufficient to produce an effective back potential could find independent support from a sufficiently detailed latticeenergy calculation. as has proved invaluable for comparable situations in other electrochromes: see ref. Since an equilibrium electrode potential E associated with the WVI/WV couple is set up following any reduction. However.94.96 The kinetics of the model Electro-coloration commences as soon as the potential is stepped from an initial value Ein at which reduction just starts to a second potential Va. Va ¼ applied potential minus E.86. those sufficiently exceeding the electrode potential E so as to drive the coloration process) that would ordinarily result in a current ‘jump’ or peak associated with (i.92. in fact.5. and thus has no further role). hence diffusion never directly controls current. Only the back potential – assumption (ii) – restrains proton motion and hence also the current flow86 and the rate of increase in proton concentration. Because the central kinetic determinant is the energy barrier to motion of protons into and out of the WO3 layer via the WO3–electrolyte interphase. rather than the of Chapter 3. the developing back potential within the solid smooths out the usual requisite applied potentials (i.90.e. CVs do show a peak associated with the (oxidative) electrochemical bleaching: see Figure 5. The unusual back potential – assumption (ii) – opposes the expected current flow. effecting) oxidation-state change (Chapter 3). The involvement of the back potential is clearly seen in cyclic voltammograms (CVs) of WO3. 41 of Chapter 8. an overpotential.95 especially regarding Va. it is increasingly energetically disfavoured) as the proton concentration within the oxidec increases.93. . which here. (iii) The absence of concentration gradients of Hþ within the HxWO3 is implied. (iv) The WO3 film initially is free of WV and hence of any initial complication from separate counter-cation charge (but this initial-state assumption – essentially a clarification – lacks mechanistic implication.3 Transport of counter ions through solid systems 93 potential. Clearly the back potential will oppose ion insertion during coloration but will aid ion egress (proton removal) during bleaching.e. so Va is cited with respect to E (that is. where there is no current peak directly associated with the reductive formation of colour. where E changes with increase of WV). denotes overpotential and not simply the applied voltage. ‘Color impedance and electrochemical impedance studies of WO3 thin films: Hþ and Liþ transport’.. J. Electroanal. Hashimoto. copyright (1997) with permission from Elsevier Science. D.7 Typical cyclic voltammogram of an amorphous thin film of tungsten trioxide evaporated on ITO and immersed in PC–LiClO4 (1 mol dmÀ3) at 500 mV sÀ1 (solid line) and 50 mV sÀ1 (dotted line). J. The chemical potential of Hþ was obtained from a statistical entropy-ofmixing term.94 Kinetics of electrochromic operation 0. A. 31–8. together with empirical constants. .05 mA –0.. F. T. (5:7) Hþ ¼ A þ 2Bx þ nRT ln 1Àx where n=1 and the A and B terms were derived from a plot of the observed emf E values versus x. K. and Fujishima. Tryk. as x . J. (Figure reproduced from Kim..4 V Potential/V vs Figure 5.2 0.) further change from the of Chapter 3 is that now the overpotential Va has simply a value without sign.4 –0.A g 0.. 435.0 . Amemiya. Chem. As colour forms with increasing x. From this simplified Butler–Volmer viewpoint the observed current is hence expected to be proportional to exp(Vae/2RT).’s x1 ¼ 0. from an approximate solution of Fick’s second law. Chapter 3). Faughnan and Crandall introduce a ‘characteristic time’ ( D) for diffusion into the film.53 V from a plot of emf E against x so relating to the back-potential effect invoked earlier. This current is a function of time t.3 Transport of counter ions through solid systems 95 Taking into account the back potential induced within the WO3. The values found for various systems usually fall between 0. itself a function of the coloration current and the extent of coloration. so the current during coloration. as here. where the effect of the driving potential Va overcomes the intrinsic energy barrier by an extent Va where is variously viewed as the symmetry or transmission coefficient and so represents the effectiveness of Va. from the back-potential influence. owing to proton desolvation and the accompanying difficulty of intercalating into the lattice.6. decreasing because the back potential increases with time:86. (3.5.1 appears to be the extent of intercalation at which both assumptions (i) and (ii) are taken to be fulfilled.4 and 0. each showing an exponential dependence on the applied potential. The first is influenced by the insertion coefficient x within the HxWO3. The basis to the development of the theory is to treat the proton uptake at the interphase as a conventional ion-uptake electrode process following the Tafel law (Eq.93 ic ¼ io 1Àx x Va e exp exp À : x x1 2RT (5:8) Faughnan et al. while the other is influenced by the barrier to ionic charge-transfer current flow across the WO3–electrolyte interphase. The kinetics that ensue then follow the Butler–Volmer development. the positive sign in the exponential arises because Va opposes activation energies.16). akin to . that needs to be established from the primed system at onset of operating: 0:53 e xo xo : io ¼ ie RT 1 À xo (5:9) Here xo is the mole fraction of protons within the film prior to the application of the voltage Va and ie is the current immediately on applying Va. The numeral is an empirical value 0. ic. The term e is the electronic charge and io is the exchange current. the magnitude of the current is governed by two energy barriers. and ½ is often summarily assigned to it faute de mieux. decreases. the mobile cation being lithium97. depending on the film thickness d and the proton diffusion coefficient D: d2 .98 or the proton. with the charge passed at low electric fields.) This value is employed in arriving at a timedependence for the effect of the back potential on current.e. dx/dt ¼ constant. (Note that the diffusion coefficient here was chosen to be D rather than the chemical diffusion coefficient D. (5:11) t 4 RT where o is a constant equal to ( d e/ 2io) in which is the density of W sites within the film. . itself implying assumption (iii). migration is wholly absent. (5.91 Equation (5. Green assumes the following. coloration is effected galvanostatically. from Faraday’s laws. The a priori conditions are that dQ/dt ¼ i ¼ constant. The kinetic treatment by Luo et al. Combining these considerations91 led to the equation: =2 1 Va e o exp ic ¼ io . The equation has also been shown to apply to WO3 on ITO in contact with solid electrolytes. Their principal divergence from Faughnan and Crandall is to suggest the magnitude of the bleaching voltage is unimportant below a certain critical value. (3.11) thus depends strongly on the applied voltage (overpotential) Va. where x is the average insertion coefficient throughout the entire film of HxWO3. (ii) The diffusing entity is uncharged so there are no effects owing to the electric field. i.11) has been verified experimentally for films of WO3 on Pt immersed in liquid electrolytes and with either a proton91 or a lithium ion54 as the mobile counter ion.11) is obeyed only for limited ranges of x if the counter ion is the proton.12). The coloration current predicted in Eq. Furthermore.e. therefore.100 In his model. time needed for the proton to penetrate to a representative point mid-film. (5:10) D ¼ 4D i. any potential dependence (above the redox-effecting value of Va) would be absent. (i) All activity coefficients are the same. incorporation of [1/t exp(Vae/2RT)]½ (unsquaring d 2) underlies the form of the exponential factor in the equation. Model of Green: galvanostatic coloration Assumptions An altogether different treatment is that of Green and co-workers.99 is somewhat similar to that above. Equation (5.96 Kinetics of electrochromic operation Eq. if diffusion through the film were alone responsible for the observed i–t behaviour. . say. d 6d 2 d p n¼1 n2 d D (5:12) or. Amperes per unit area. but the proton concentration gradient flattens out at longer times. where 0 < y < d. These show. Assumption (ii) can be classed as consistent with the model of Bohnke et al. quantities c are number densities. (v) The film may or may not contain interstitial water. The distance y ¼ d denotes the WO3–electrolyte interphase. in agreement with assumption (iv) in the model of Faughnan and Crandall above. that only the WO3 adjacent to the electrolyte will contain any protonic charge.102.t) increases linearly with Jo t/d.103 as described below.101. d D (5:13) where c(y. in abbreviated form: cðy. The kinetic features of the model The film of WO3 has a thickness d. at short times. (5. .t) against y/d for various values of Dt/d 2.6). There is no ionic flux at the back-electrode (at y ¼ 0). If D is large.13) acts as a correction term to account for diffusion-limited processes in the solid.104 Green obtained Eq. A constant ionic flux of Jo from the electrolyte layer reaches the solid WO3 and thence penetrates to a distance y. tÞ À co ¼ Jo t Jo d þ . By analogy with the conduction of heat through a solid slab positioned between two parallel planes. causing the concentration of Hþ throughout the film to be even. see Figure 5. (5. tÞ.t) is independent of y and so c(y.8. Green omits specifying migration effects but does cite diffusion coefficients as D. (iv) The WO3 contains no mobile protons prior to the application of current. In Green’s notation. The second term on the right-hand side of Eq. then c(y.12): ( ) .t) is the concentration of Hþ (possibly partially solvated) at a distance of y into the WO3 film at the time t. 1 npy Dt Jo t Jo d 3y2 À d 2 2 X ðÀ1Þn À 2 exp Àp2 n2 2 cos cðy. The application of assumption (iv) is unlikely to affect significantly the utility of Green’s model.5. tÞ À co ¼ þ . (5.3)–(5. All the diffusion coefficients pertain to solid phase(s). Green has plotted curves of F (y. 97 Assumption (i) contradicts the derivation of the Weppner and Huggins’ relations in Eqs. and currents i represent numbers of ionic or electronic charges passing per ` unit time rather than.Fðy.3 Transport of counter ions through solid systems (iii) All diffusion coefficients are D rather than D. Smith. departs from Green’s model in incorporating the back potential of Faughnan and Crandall. .5 0. the second term on the right-hand side of Eq.1 Figure 5.3 0. (Figure reproduced from Green. When such boundaries are considered. where cm is the maximum concentration of Hþ that arises (the number of Hþ equalling that of WV).06 0. The parenthesised term thus roughly represents the inverse of the average distance separating colour centres.105 closely similar to that of Green. equal to the number of Hþ per cm2. Thin Solid Films. where the sphere radius is r. ‘A thin film electrochromic display based on the tungsten bronzes’..1 0.) In a later development. M. A. and n is the number of optically absorbing colour centres per unit area required to produce the required absorbance.005 1.13) is simplified to (Jo r2)/(15 d D).98 Kinetics of electrochromic operation 0.1 y/ d 0. J. the relationship . W. t ) 0. and assumed to be regions within the film acting as pathways for ‘fast-track’ diffusion. 89–100. For simplicity the grains of c-WO3 are assumed to be spherical.0 0.8 Green’s model of coloration: values of F(y. C. None of the concentration gradients predicted by Green’s model have been measured.15 0. (5. copyright (1976) permission from Elsevier Science.3 00 0.2 0 –0. Green24 concluded that for a response time of . Green24 added into his model the effects on the concentration gradients of incorporating grain boundaries into his model. The numbers on the curves are the values of Dt/d2.cm 2 D !1 (5:14) n should be followed.03 F(y.01 0. t) for a film of thickness d with no mass flow at y ¼ 0 and constant flux Jo at y ¼ d. 38. The kinetic treatment of Seman and Wolden. and Weiner. 3 results from the wave-mechanical overlap of conduction sites or bands with the valence bands. an Anderson transition. below x(critical). but electron movement is rapid when x > x(critical). proceeding by individual ‘hops’ from a small number of sites.03. see Figure 5. It follows then that (ion) < (electron) when x > x(critical). with the electron velocity rising dramatically at x % 0.5.) are obeyed when x > x(critical). but the mobility of the electron (electron) increases dramatically over the compositional range 0 x< 0. . Duffy and Monk:96an electronic percolation threshold A percolation threshold is attained when previous directed electronic motions. at x(critical).03–0.04 claimed for the electron-conduction percolation limit may be understood as arising from a restricted electron delocalisation about a few neighbouring WVI.) is obeyed extremely well when x > x(critical) but not at low values of x. because of the onset of multiple pathways through the increased number of occupied sites. The model It is already clear from Figure 5. which would hence lower the numbers of sites needed for criticality. at a critical composition x(critical). Such a transition is documented107. The onset of metallicity at x $ 0. 91 ff. 96 that Faughnan and Crandall’s model (page 91 ff. the ionic and electronic mobilities will be equal: (ion) ¼ (electron).107 Then if the mobility (ion) of ions is approximately constant. It is shown in ref.3. Ingram et al. that in effect extends the size of the ‘sites’ involved in allowing the onset of the critical percolation.3 Transport of counter ions through solid systems 99 The model of Ingram.4 and the discussions above that the electronic conductivity s of pure WO3 is negligibly low.106 Assumptions (i) The central assumption underlying the model of Ingram et al. which involved obtaining transients of current i against time t during electroreduction.15 is then probably fortuitous. the motion of the electron is rate limiting. which was rationalised in terms of attaining a percolation threshold.3 then. Such plots showed a peculiar current ‘peak’.96 is that the motion of the electron is rate limiting below a percolation threshold.d d The low value 0. the conductivity becomes metallic. at ca. suddenly become profuse. and only above x(critical) will electron movement be the more rapid. The onset of metallicity is an example of a semiconductor-to-metal transition. (ii) Most of the assumptions and hence the theoretical elaboration of Faughnan and Crandall’s model (see p. and the approximate correspondence here with the customary percolation value $0. during a steady increase in the number of occupied sites to a critical value.96 analysed the potentiostatic coloration of evaporated a-WO3 on ITO. at low x. x % 0. Hence. The conductivity s increases as x increases until. In ordinary site-wise conductive systems this occurs when occupied sites become $15% of the maximum.9.108 for WO3. Chem. . had inserted lithium ion into WO3 from a super-dry PC-based electrolyte.2 s.) Similar chronoamperometric plots of i against t which include a current peak have also been found by Armand and co-workers97 and by Craig and Grant. Armand’s explanation may not be correct here since Craig and Grant.0 Kinetics of electrochromic operation 0. i.4 0. P. 97.100 440 420 400 Current / μA 380 360 340 320 0. Note the current peak at ca. (Figure reproduced from Ingram.6 Time/s 0..0 1. D.0 V to –0. Duffy.. ‘Chronoamperometric response of the cell ITOjHxWO3jPEO–H3PO4 (MeCN)jITO’. 1995. The percolation phenomenon was not seen by Ingram et al.9 Chronoamperometric trace of current vs.03 in ref. time during the electrocoloration (reduction) of the cell ITOjWO3jPEO–H3PO4j(H)ITO. when electro-colouring WO3 with a small field. as can be gauged by manual integration of the peak in the traces published. by applying a very small cathodic driving potential. The potential was stepped from a rest potential of about 0.109 The value of x at the peak is also ca. 77–82. Electroanal. A. 0. M.e. an electrolyte free of mobile Hþ: in this case Li0 would have to be the corresponding reactant. with permission from Elsevier Science. S.2 Figure 5. J. M. J.2 0.6 V at t ¼ 0. perhaps because the transition was too slow to be noted. and Monk.8 1.109 who found a similar current peak. 0. Armand and co-workers97 explain the peak in terms of the nucleation of hydrogen gas (via the electroreduction of Hþ ). 380. possibly with the surface of the incipient HxWO3 acting as a catalyst: 2Hþ þ 2 eÀ ! H2 : (5:15) While such nucleation phenomena can certainly cause strange current peaks in chronoamperometric traces. that was successfully modelled in terms of a percolation threshold. Because the [Hþ eÀ] pair has no overall charge. to the WV. 10À12 cm2 sÀ1 indicates that the mobile species would traverse a . (iii) The chemical reduction in Eq. The major divergence from the models above is the following. (5. below). electrons are ‘free’ only above the percolation threshold.03–0. i. There was no mention of such a threshold in the study by Torresi and co-workers. by Goldner et al. d $ (D t)½. t(electron) does not tend toward 1. In contrast with the assumptions implicit in deriving Eq. a current peak has been observed by Aoki and Tezuka110 during the anodic electro-doping of poly(pyrrole).5).16). (ii) The rate of electrochromic colour formation is thus a chemical rather than an electrochemical reaction: * WVI þ ½Hþ eÀ 0 ! WV þ Hþ : (5:16) The proton product of Eq. In studies claiming free electronic motion. which may be atomic (as H ).5. but following Ingram et al. Duffy and Monk suggest that the kinetic behaviour of WO3 in the insertion-coefficient range 0 x < 0. (i) The rate-limiting process during electrochromism is the diffusion of an electron– ion pair (such as [Hþ eÀ]). so values of D alter dramatically as the percolation threshold is reached.113 both groups employ Drude-type models (see p.111 but their model of ‘relaxation processes’ in thin-film poly(aniline) does. the bases of most of the theories in the electrochemical models above are still regarded as valid (see discussion. (5. only after a percolation threshold at the upper coefficient limit here does ion motion become rate limiting. Model of Bohnke: reduction of WV via neutral inserted species Assumptions The requirement of a new interpretation of the WO3 coloration process was indicated by the need to explain the temporal relationships governing the optical data obtained during electrochromic coloration.e. The meeting of Hþ and eÀ is outlined below.. In summary.16) resides as a counter ion adjacent to the site of the chemical reduction reaction.04 is dictated by slow electron motion. in a different system. suggest a sudden change in electronic conductivity with composition change. again.16) occurs ‘spontaneously’ on the time scale for diffusion of the [Hþ eÀ] pair.112 and Rauh and co-workers. Ingram. Accordingly. (5. inserting a reasonable assumed D of ca. From Eq (3.. 142) to describe the free-electron behaviour. the diffusion coefficient evinced by the system is D rather than D.3 Transport of counter ions through solid systems 101 Also. 115 Recent developments: intervalence between WVI and WIV Assumption A new view of the key tungsten species has emerged in the last decade. lending reality to these suppositions. On entering the WO3. Deb and co-workers116 suggested in 1997 that the coloured form of the electrochrome is not HxWV. accordingly. The charged species within the encounter pair then diffuse together as a neutral entity. or they react to form atomic hydrogen. While broadly agreeing with the model of Faughnan and Crandall (above). Furthermore.102 Kinetics of electrochromic operation typical film in many seconds. The rate of forming colour is thus either a function of the rate of diffusion of the [Hþ eÀ] pair to available WVI sites prior to ‘instantaneous’ electron transfer.114. can be wholly neglected and.102. born of coulombic attractions. resulting in the formation of atomic hydrogen or lithium prior to coloration. in Bohnke’s model101.) The Model In contrast to the models above of Faughnan and Crandall. Bohnke acknowledges that the observed current–time behaviour is governed by the formation of a back potential. the inserted Hþ ion moves through the WO3. it is even possible that electron transfer has occurred within the pair.103 to be satisfactory in simulating the observed absorbance–time data except at short times. WVI þ eÀ ! WV. following). or the model of Ingram et al. or. the measured diffusion coefficient is better considered as D than as D. and of Green in which the motion of Hþ is rate limiting. but is not applied in any detail to data for bleaching. Bohnke’s model is said101. if the appropriate rate constant ket is quite low.VIO3 but HxWVI(1 – y) WIVy O(3 – y).. but parts from Faughnan and Crandall in asserting that concentration gradients are formed within the incipient HxWO3 during coloration. in moving through WO3. In common with Faughnan and Crandall. the rate of diffusion through Nb2O5 is similarly said to be dominated by ‘redox pairs’. and hence . Eq. providing ket is high enough (see point (iv). (The electrochromic colour in this model is still due to intervalence optical transitions between WVI and WV. in which first the motion of the electron and then the motion of the proton is rate limiting. but does not represent the formation rate of colour centres. the model implies that the kinetics-controlling mobility. probably moving only a very short distance within the WO3 before encountering the faster electron from the electrode substrate. In support of the model. Indeed.16).103 the mobile diffusing species is suggested to be an electron–ion pair. (iv) The observed current is thus a function of the rate of forming [Hþ eÀ] pairs. and Ingram et al. will be simplified since migration effects. (5.102. it is a function of the rate of the electron-transfer reaction itself. of the [Hþ eÀ] pair that provides a quasi counter ion to WV. as y in LixWO(3–y) increased.119. Lee et al. de Wijs and de Groot deliberately omitted the involvement of WIV in their recent wave-mechanical calculations.118 suggest that the WIV state ‘plays a dominant role in deep coloration’.122.3 Transport of counter ions through solid systems 103 that the optical intervalence transition is WVI WIV rather than the hitherto VI V W . they argue for WV–WV dimers rather than WIV and WVI. from densityfunctional computations. and their ultimate resolution. promises intriguing physicochemical developments for the near future. Zhang and Goto71 found that D increased as the extent of sub-stoichiometry increased.125 Rutherford backscattering studies furthermore suggest that the amount of WIV in nominal ‘WO3’ is a function of the extent of oxygen deficiency. but trapping of electrons at shear planes and defect sites can be problematic for rapid. as in Eq. Additional experimental results (i) Coloration of non-stoichiometric ‘bronzes’ A non-stoichiometric reduced oxide has a non-integral ratio of oxygen and metal ions.126 Infrared127 and Raman studies128. WO(3–y) is then in reality.e. non-stoichiometry is best avoided. Additionally. (5. Indeed. Similarly.134 For this reason. Such materials are also called ‘sub-stoichiometric’. the electrochromic and photochromic properties of O-deficient WO3 have also been found to depend on similar WIV participation in both mechanisms.17): WVI þ WIV ! 2WV : (5:17) Siokou et al. The on-going growth of views on the roles played by the several W species. where y is likely to be small.133 Rather. WO(3 – y).132 Possibly the observed WV is formed by comproportionation. widely accepted W The fully oxidised form of the trioxide (MoO3 or WO3) is confirmed to contain only the þVI oxidation state by studies with XPS117.118.117.121 Reduction during the coloration reaction MO3 þ x(Hþ þ eÀ) ! HxMO3 is expected to yield the þV oxidation state: but XPS shows that some of the þIV state is also formed during the reduction of Mo. although note that135 MoOð3ÀyÞ . WVI(1–y)WIVy O(3–y).5.118. Other materials of the type WO(3–y) are indeed also electrochromic. e. Finally.119 and ESCA.124 and of W. it is notable that Sun and Holloway130 (in 1983) and Bohnke and co-workers131 (in 1991) both suggest that reduction of WO3 is a two-electron process.g.128 say that even as-deposited films contain appreciable amounts of WIV.123.118.129 also indicate the presence of WIV. i.120. reversible electrochromic coloration. Nevertheless. By interspersing the coloration currents with short periods of zero current. In plots of emf against x. An additional advantage of pulsing is to enhance the durability of electrochromic devices by decreasing the occurrence of undesirable electrolytic side reactions such as the formation of molecular hydrogen gas: it is likely that the catalytic properties of HxWO3 for H2 generation are impaired. the steep concentration gradient associated with a high-x layer is allowed to dissipate into the film. c-WO3. and removal from. p. (iii) Use of an interrupted current (from a ‘pulsed’ potential) The rate of electrochromic coloration of tungsten oxide-based ECDs may be enhanced considerably by applying a progression of potentiostatically controlled current pulses rather than enforcing a continuous current. obtained during injection of Liþ into. there is a considerable hysteresis between the E for reductive charge injection and that for oxidative Liþ egress. is attributed to the formation of a thin layer of high-x bronze on the electrolyte-facing side of the WO3. do not yet lead to a clearer or general view of the mechanisms in electrochromic oxides. However. and also has a superior contrast ratio CR. steady reduction does effect a greater capacity for Liþ before bulk metallicity intervenes. 86. the optimum pulse duration for a high d(Abs)/dt also depending strongly on the pulse amplitude. according to the final paragraph of ‘Kinetic complications: (i) crystal structure’ above. close to the values of x(critical) noted above on page 99. Several groups .136 The rate of coloration depends strongly on the pulse length employed.104 Kinetics of electrochromic operation apparently electro-colours at a faster rate than does MoO3 alone. the electrode potential E of the lithiated oxide was monitored as a function of x while a continuous (and constant) current was passed. as evidenced by increased peak currents. by applying current pulses. The amount of charge that can be inserted per current pulse is thus greatly increased. This is a mobility-controlled kinetic phenomenon: on the time scales involved.04–0. comparable to (but different from) those of the tungsten systems. there is a higher concentration of lithium on the surface of the particles than in the particle bulk.05. (ii) Electrochemical titration In a brief study of galvanostatically injected lithium ion in47 c-WO3. It was found that dE/dx decreased suddenly at x ¼ 0. such materials will not be considered further here because the additional complexities encountered with these systems. The effects of interrupting the current. 5.139.86 * * The bleaching current is primarily governed by a fielddriven space-charge limited current of protons in the HxWO3 next to the electrolyte.137.e. 35. Summary of the bleaching models described on pages 105–109. rather than acts against.141.136.3 Transport of counter ions through solid systems 105 Table 5. citing the distinctive features of each. Table 5.138. The activation energy to proton expulsion is slight. (5. represents the reverse of Eq. The kinetic effects of cations and electrons are indistinguishable. Model of Faughnan and Crandall: potentiostatic bleaching The potentiostatic removal of charge (i. Green * 100 * have found that a pulsed potential enhances the rate of coloration and bleaching.5 above summarises the various bleaching models cited in this section. Principal authors Faughnan and Crandall Distinctive features * Refs. and suppresses the extent of side reactions.35 Assumptions (i) The bleaching time of HxWO3 is primarily governed by a field-driven space-charge limited current of protons in the HxWO3 next to the electrolyte. (5. Concentration gradients of counter cations in MxWO3 films were computed from analogy with heat flow through metal slabs.5. the movement of the mobile counter ions.140.143 Kinetic modelling of the electrochromic bleaching process The process of film bleaching. bleaching of the electrochromic colour) of the WO3 bronze has been modelled by Faughnan and Crandall.18). (ii) The resistance to charge transfer at the electrochrome–electrolyte interphase does not limit the magnitude of the bleaching current. No concentration gradients form within the film.2) above: !xðHþ þ eÀ Þ þ WO3 : Hx WO3 À (5:18) Bleaching is somewhat simpler than is coloration since the back potential contributes to. Eq. .142. Since the back potential contributes toward the movement of the mobile charged species.54. rather than against it. All the voltage applied to the ECD is assumed to occur across this thin layer. the time-dependent bleaching current ib shows a different response to the applied voltage Va from that during coloration.) The thickness l (t) is proportional to time.106 Kinetics of electrochromic operation (iii) Ionic charge leaves the HxWO3 film during electro-bleaching. (iv) There is a clear interface between HxWO3 and WO3 layers within the electrochrome. as in Fig. (5.11): ib now depends on the proton mobility Hþ : ib ðtÞ ¼ " Hþ V2 a lðtÞ3 : (5:19) where " is the proper permittivity. A superior fit between experiment and theory is seen if the electrolyte is aqueous. Fig. The result in Eq. The layer has a time-dependent thickness termed l(t). Solution of the differential equations for time-dependent diffusion across l (t) during bleaching leads to an additional relationship: ib ðtÞ ¼ ðp3 " Hþ Þ =4 Va 2 ð4 tÞ =4 3 1 1= . The current ib decreases as l (t) grows thicker. such that35.96. All the voltage applied across the electrochrome layer film drops across this narrow layer of WO3. clearly showing the expected gradient of –34 at intermediate times. This i–t relationship has been verified often for WO3 in contact with liquid electrolytes35.86 l (t)3 ¼ Jo t/co e. according to Eq. incurring a time dependence of 3 ib / tÀ 4 .98 and for WO3 in contact with semi-solid polymeric electrolytes. Note that " is not the molar absorptivity of Chapter 1. 5.10 (b) is the analogous plot but for propylene carbonate as solvent. (5:20) where p is the volume charge density of protons in the H!0WO3.20) assumes that bleaching occurs potentiostatically implying a fixed Va across the whole of the WO3 layer. = = . (5.98 Figure 5. resulting in a layer of proton-depleted WO3 at the electrolyte-facing side of the electrochrome. and is related to the initial proton concentration (number density) within the film co.10 (a).10 shows the logarithmic current–time response of HxWO3 bleached in LiClO4–PC. the position of this interface moving into the oxide film from the electrolyte as the bleaching progresses. (Faughnan denotes this length xI rather than l (t) as here. and l (t) is the time-dependent thickness of a narrow layer of the WO3 film adjacent to the electrolyte. hence the observed i–Va square law. with l (t) becoming thicker with time. 5. 5. J.8 V Li xWO3 : LiCIO (PC) 4 : WO3 4 : In – 1.6 V – 0. ‘Electrochromism in LixWO3’.8 V – Li xWO3 : LiCIO (PC) – 102 – 101 100 Time/s 101 102 Figure 5.4 V 1 2 3 4 xWO3 : 10 N H2SO4 : In HxWO3 : 10 N H2SO4 : WO3 Current density i /mA cm 101 Slope(–3/4) 100 – 101 10–1 100 Time/s 101 102 (b) 101 100 Slope(–3/4) Current density i /mA cm –2 – 101 – 1. by permission of The Electrochemical Society. S. K. (Figure reproduced from Mohapatra.3 Transport of counter ions through solid systems (a) 1 2 3 4 –2 107 –1 VH – 0.6 V – 1.. 1978.) = . 284–8.10 Current–time characteristics of HxWO3 during electrochemical bleaching as a function of potential: (a) HxWO3 in H2SO4 (15 mol dmÀ3) and (b) HxWO3 in PC–LiClO4 (1 mol dmÀ3). Electrochem. Inc. Soc. 125. The gradient of –34 predicted from Faughnan and Crandall’s theory is indicated.8 V – 0. t)] is then obtained as: ! & ' 1 4co X 1 ÀDð2n þ 1Þ2 p2 t ð2n þ 1Þpy sin exp : (5:22) cðy.e. Such curves drawn for various Dt/d2 are reproduced in Figure 5. . 4 V2 " a (5:21) where x is the insertion coefficient at the commencement of bleaching and is the corresponding density of W atoms. in this instance of c(t)/co against y/d. Model of Green: potentiostatic bleaching The potentiostatic bleaching of thin film WO3 has also been modelled by Green.11. The best means of ensuring assumption (i) is to potentiostatically control the rates of charge movement. As was the case during coloration. (iii) Accordingly. The time. where c(t) is the concentration of Hþ in the film at time t at a distance of 0 < y < d. Eq. The computed concentration gradients await experimental verification.21) fulfils expectation in indicating the longer time needed for a film to bleach if the sample is thick or is strongly coloured prior to bleaching. Assumptions (i) All Hþ ions reaching the electrochrome–electrolyte interphase are instantly removed. the time required for l (t) to become the film thickness) is a function of film thickness d. and the initial concentration of Hþ co.e.108 Kinetics of electrochromic operation The time for complete bleaching to occur tb (i.100 In common with his model for coloration. c(y ¼ d. implying assumption (ii) below.tÞ ¼ p n¼0 2n þ 1 d2 d Green100 has again computed theoretical curves. The timedependent proton concentration is c(y. i. the film thickness is d and the distance of a proton from the back inert-metal electrode is y.t) ¼ 0 for all time t > 0. (ii) The activation energy for charge electron transfer across the interphase (ions and electrons) is slight.t). both actually number densities. Equation (5. permittivity " of the film and the insertion coefficient: tb ¼ e d 4x .and thickness-dependent concentration of Hþ [c(y. proton mobility . (5. cf. ensuring that assumption (ii) holds by applying a sufficiently large positive potential. the condition for a rapidly responding ECD is D(cm/n)2 ! 1.14). Results from inserting the relatively large Naþ ion into WO3 also .8 0. other data suggest steep concentration gradients are likely. times.0 Figure 5. C.4 0.0 0. although note that in this latter study dry WO3 was employed. W.0 0 0.8 so weakening Faughnan and Crandall’s assumption (iv). Within thin-film V2O5. and Weiner.52. the ellipsometric study by Duffy and co-workers145 did find evidence that implied a surface layer of bronze does form during reduction. sub-millisecond. ‘A thin film electrochromic display based on the tungsten bronzes.4 y/ d 0. c(d) ¼ 0. these latter studies involved electrochemical reduction. For example. M. but ellipsometry has so far failed to detect such an interface within films of WO3 during reduction. 89–100.136 also found evidence for surface layers of HxWO3 at very short. in situ electrochemical ellipsometry – a non-destructive technique – has demonstrated a clear interface between the oxidised (colourless) and reduced (coloured) regions of thin films of vanadium oxide51 or molybdenum oxide.3 Transport of counter ions through solid systems 1.3). 1976.01 0. at t > 0. 38.) Additional experimental evidence for concentration gradients Ellipsometry While the exact form of Green’s computed concentration gradients need confirmation. A. J. Smith.6 0. assumption (iii) on page 108 above. then reduced chemically by gaseous H2 þ N2.04 0.6 C / Co 0.11 Green’s model of bleaching: concentration c in the film 0 < y < d.4 0. By contrast. (Figure reproduced from Green. The interface was detected both during reduction and oxidation reactions of V2O5. The numbers on the curves are values of Dt/d2. this boundary separates the reduced (hydrogen-free) and oxidised (proton-containing) forms of the oxide.144 cf.1 109 0. with permission from Elsevier Science. Note that c(y ¼ d.5. Ingram and co-workers96.8 1. co is the initial concentration.. The surface of the bronze had a sufficiently high insertion coefficient to be metallic (implying that x ! 0.2 1. t) is 0 for all t > 0.’ Thin Solid Films.2 0. 147 Deroo and co-workers22 and Wittwer et al. SIMS Secondary-ion mass spectroscopy (SIMS) was the technique of choice to study cation concentrations as a function of film thickness. a sample of WO3 is electro-coloured normally with proton counter ion. The last study107 showed %50% change in cation across the WO3 film. several investigations afford compelling evidence of steep concentration gradients forming during electro-coloration and bleaching.146 Zhong et al.83 measured proton densities with the 15N technique (the ‘nuclear reaction analysis’. Again. and the ablated material analysed. the surface of the film was slowly etched away.. Bange and co-workers82. Nuclear reaction analysis While in some of the simpler models a constant concentration of inserted cation is assumed throughout the electrochromic film. . allowing possible movement of mobile ions during measurement. NRA): 15 N þ 1 H ! 14 C þ 4 He þ g.110 Kinetics of electrochromic operation suggest the formation of a high-x layer of NaxWO3 on the electrolyte-facing side of the WO3 during reduction. (5.107 In each case. exemplified by the work by Porqueras et al. The emitted gamma rays are monitored as a function of energy.. is the most widely used in describing the coloration kinetics of thin-film electrochromic tungsten trioxide. and then bombarded with ‘hot’ 15N atoms. 91 ff. However.11)) of ic / tÀ½ and ic / exp(Va) have often been verified experimentally during electro-coloration. prior to analysis. so the results are not without qualification. the relationships (from Eq. It has been thereby shown that a concentration gradient forms in a film of electrochromic oxide during coloration. Also. thus as a function of depth: the g-ray count is taken to be directly proportional to the proton concentration. both the NRA and SIMS techniques destroy the sample during measurement. (5:23) in which. It is now almost universally agreed that a back potential forms during coloration.60 the slow motion of the entering Naþ cation could accentuate incipient concentration gradients.59. The depth to which the 15N atoms are inserted is varied by controlling their kinetic energy during bombardment. a concentration gradient was clearly shown to form during electrochromic coloration. For example. Discussion – coloration and bleaching The back potential The theory of Faughnan and Crandall on p. 3 Transport of counter ions through solid systems 3 111 and the relationship ib / tÀ =4. However. since in their theory the protonic charge is assumed effectively to be evenly distributed within the film at all times t > D (where D is the ‘characteristic time’ describing the temporal requirements for diffusion within the film. With the wide acceptance of the i–t–Va characteristics predicted by Faughnan and Crandall’s model (except where x < 0. even in Faughnan’s treatment. (5. albeit over limited time scales in each case.6. . Eq.5. Activation energies for diffusion of mobile ions through a solid metal-oxide host. diffusion within the film arises at t < D. 2 Converted from the original eV. The existence of a concentration gradient in the electrochrome cannot be established directly. (5.20). Host WO3 a-WO3 WO3 WO3 NiOH 1 Mobile ion Liþ Liþ Liþ Liþ Hþ Ea/kJ molÀ1 20–40 50 64 20 7. thus any concentration gradients do not dominate Table 5. If they exist. but merely indicates that any contribution to an observed activation energy is small: the activation energy for diffusion are often not excessive (Table 5. has also been verified during the electro-bleaching of HxWO3. and can only be inferred.03 in WO3 reduction). and SIMS analyses outlined above. 91 148 149 150 30 The value of Ea depends sensitively on x.0 Ref. that concentration gradients of Hþ form within the HxWO3 both during coloration (with a higher x at the electrolyte-facing side of the electrochrome) and during bleaching (with higher x at the inert-electrodefacing side of the electrochrome).6). as defined in Eq. NRA. Faughnan’s central kinetic assumption of the interphase energy barrier that dictates the proton-insertion rate does appear tenable. they additionally contradict one of the few explicit assumptions of the model of Faughnan and Crandall. hence implying that concentration gradients enforcing Fick’s laws do form within the film. Concentration gradients The second area of consensus concerns concentration gradients: these are inferred from the ellipsometry.10) above) thus implying t ! milliseconds. The finding that concentration gradients are formed within the incipient HxWO3 does not contradict the model. (5.103 encompassing ion–electron pairs. thereby forming a neutral pair. on etching away the surface of a solid by SIMS. could have serious implications for many solid-state ionic devices in addition to those involving electrochromism: the good fit between her data and the model does invite attention. (5. all values of diffusion coefficient observed will be those of D rather than D. l is the jump length and rd is half the distance between bridging-oxygen surfaces that form the ‘doorway’ needing enlargement to rþ to enable Mzþ to pass. mid-way between these oxygens.e. Diffusion of neutral species The more recent novel model of Bohnke et al. If Bohnke’s model holds.24) as:152.. . In such attempts.112 Kinetics of electrochromic operation the observed kinetic laws. The activation energy Ea for ionic movement has been modelled by Anderson and Stuart. which appears impossible except prior to the meeting of an ion and electron. and rþ and rÀ are the corresponding radii.154 Energetics of diffusion Since concentration gradients in ECDs can only be inferred. to ½d. The final term covers mechanical stress: G is the shear modulus. In contrast.101. The values of Ea calculated from Eq. however.102. they are too limited for precise measurement.153 Ea ¼ B zþ zÀ e2 2zþ zÀ e2 Gp lðrþ À rd Þ2 þ À 1 .24) are ‘about satisfactory’ for Liþ and Naþ. to a vacancy near a similar oxygen (each bearing a charge zÀ).151 Mz þ (of charge zþ and radius rþ ) is transferred over a distance d from an oxygen ligand (of radius rÀ). this term representing the loss of lattice stabilisation at onset of the ionic ‘jump’ from its initially stable lattice site. not only volatilising many of the ions but also driving others into the remaining WO3 during measurement. The symbol " here is the relative permittivity of the material. The activation energy Ea is then given in Eq. influence the numerical magnitudes of the rates determined experimentally. i. the energy required to remove the surface is sufficient to perturb the Hþ or Liþ ions. They might. in the thermodynamic-enhancement model of Huggins and Weppner. "ðrþ þ rÀ Þ 2 2 d" (5:24) where z þ and zÀ are the respective charge numbers on the cation and the nonbridging oxygen. and B/" is a form of effective Madelung constant. The second term is the coulombic stabilisation acquired by interaction with two oxygens at the midpoint of the jump. the numerator ‘2’ denoting interaction with both.14 differing rates are presupposed of ionic and electronic motion in the film. 04 (of extinction coefficient "1 ¼ 5600 dm3 molÀ1 cmÀ1).4).157 The conductivity of a-WO3 increased during insertion and decreased during extraction of Liþ ions. has been ascribed by Ingram et al. It may be that ion–electron pairs are present but not noted in other studies. Complementarily.12.05 or so (Figure 5. such as the cause of the peculiar current peak in potential-step traces. Green100 notably states that the kinetic behaviour of electrons and ions can be separated only under the influence of a high electric field.03 As mentioned on p. and possibly the deviating at very low x of the Bohnke model. The near ubiquity of this value of insertion coefficient. implying that the kinetics both of counter ions and electrons moving separately and in pairs could be identical for electrochromic coloration effected by applying a small electric potential. on an electrochemical cell containing lithium electrolyte. (Conductivity s as a function of x has also been measured by Bohnke et al.96 to reaching thence surpassing a percolation threshold. in demarcating discontinuities in the physicochemical behaviour of HxWO3. The relationship between the extent of localisation and x may also be discerned from the electronic conductivity s. which is consistent158. perhaps the need to invoke their existence can be dismissed in studies employing higher electric fields. However.155.5.161 summarised in Figure 5.44 (of "3 ¼ 1400 dm3 molÀ1 cmÀ1). by microwave results coupled with electrochemical measurements. which only becomes significant at x ¼ 0.159 ‘with a free electron’ moving through preprepared reduced oxide. The value of "4 probably means the current did not effect reduction of further tungsten sites. The invocation of a percolation threshold does answer a number of questions. 0.3 Transport of counter ions through solid systems 113 While none of the other authors’ models comprise the concept of ion–electron pairs.03. 99 ff. several results contradict the Bohnke model: Hall-effect measurements on preprepared LixWO3 and NaxWO3 show diffusion coefficients that are roughly proportional to the number of alkali-metal cations inserted and are independent of temperature. Faughnan and Crandall’s i–t–Va characteristics are poorly followed when x < 0. 0. Following the reduction of the WO3.28 (of "2 ¼ 2800 dm3 molÀ1 cmÀ1). above. All data were obtained at .156 thus demonstrating the complete dissociation of electrons and cations.28 < x < 0. Furthermore.) 2 Similarly. and magnetic susceptibility data appear to show the same results. four separate absorbance–insertion coefficient domains may be discerned: 0 < x < 0.4 (of "4 ¼ 0 dm3 molÀ1 cmÀ1). and x > 0.04 < x < 0. the relationship between the extent of localisation and x may also be discerned optically since the molar absorptivity (extinction coefficient) " is not constant but decreases as x increases. the Seebeck coefficient S is proportional to xÀ =3 (x being the insertion coefficient).160 Insertion coefficients 0 x 0. Values of x characterised by "1 were postulated96 to represent single WV species below the percolation threshold and. without reduction). some elements of the theories .12 Increase in the absorbance of the intervalence charge-transfer band of HxWO3 as a function of charge passed: (a) at the wavenumber maximum of 9000 cmÀ1 (the points O were calculated from the curves in (b) and (c).114 3. inserted charge. ‘Optical absorption of tungsten bronze thin film for electrochromic applications’. R. and Smith.033. in the studies by Monk et al.162. K. I. F. Notably. Duffy. A. Toward a consensus model The evidence for each model seems quite convincing if taken in isolation and. (b) absorbances at 20 000 cmÀ1 and (c) absorbances at 16 000 cmÀ1. 186. similarly. The values of x represented by "2 and "3 are. (Figure reproduced from Baucke. where some traces are linear only in the range 0 < x < 0.. Since the extinction coefficient " for HxWO3 is a function of insertion coefficient x. G. values of x for "4 represent metallic HxWO3 in which charge ‘inserted’ is conducted without any valence trapping (i. as discussed above.) constant wavelength. 47–51. Similarly. these observations can be taken as direct evidence for flattening-out of a concentration gradient in the absence of an applied field. Mohapatra54 shows a plot of absorbance vs.e. with permission from Elsevier Science. in all probability.0 (b) 1. 1990.163 the optical absorbance of the incipiently reduced oxide was observed to increase for a short time after the driving potential was removed.136 and by Siddle and co-workers.0 (c) 0 500 1000 Inser ted char ge/mC 1500 Figure 5. J. and Scrosati and co-workers48 found " of LixWO3 and NaxWO3 differed significantly over the insertion coefficient range 0 < x 1.0 Kinetics of electrochromic operation (a) Optical absorbance 2. Thin Solid Films. representations of different extents of electron delocalisation. . The percolation threshold x of 0. the mobility (electron) is usually said to be higher in hydrated WO3 – and perhaps also for reduced oxides immersed in electrolyte solutions – thus further masking the effects of low x.. Bard. and Faulkner. 253–9. Only Faughnan and Crandall92 dismiss the idea of concentration gradients within the incipient HxWO3. New York.96 and others suggest that in particular circumstances electrons and ions do indeed move autonomously. The data of Ingram et al.References 115 seem to fit all models: the positing of a back potential is a case in point. J. a complete mechanism describing the controlling redox processes and ionic motions in coloration and bleaching has not yet been established. Green’s observation. when insertion coefficients are small. Wiley. but many of these studies may have been incapable of discerning such pairs. . New York. 1974. Wiley. in the range 0 < x < 0. the motion of electrons is rate limiting. that the behaviour of electrons and ions cannot be separated except at high fields. and Kuwana. 2002. While several new studies provide general views of intercalation. when electro-colouring WO3 with a very small field. R. but as the upper value of this x limit is passed. pp. E. Steckhan. 164. 148 ff. Spectroelectrochemical study of mediators. where no current peak was observed. Bard. A combined model describing the electro-coloration of thin-film tungsten trioxide would suggest that the kinetics are dominated by the formation of a back potential. L. Initially. Bunsen-Ges. Ber. Chem. 78.103 assumption that [Hþ eÀ] pairs form during coloration has received no support from subsequent workers. 5. 3. I: bipyridilium salts and their electron transfer rates to Cytochrome c. 2.03 is sufficiently small that many workers may have missed anomalous properties at small x. 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Chem. 160. 31.. Bohnke. Phys. Soc. K. Effects of ordering on the transport properties of sodium tungsten bronze. Le Gorrec. Baucke. Burdis. 158. Garbaty. 1999. 163. 320–5. G. Phys. 190–2. E. H. M. Gire. Electrochem. J. Seebeck effect in sodium tungsten bronze. R. 159.. Muhlestein. 1994. 1996. and Sutcu.. Phys. copper. counter electrodes. Display-device applications can be envisaged for the latter group of electrochromes.e. This property finds application in on–off or light-intensity modulation roles. Other oxides in Section 6.2). nickel. Nb. Ir. rhodium. At least one redox state of each of the oxides IrO2. it is much less common for transition-metal oxides to form other colours by intervalence transitions (see Table 6. The intervalence coloured forms of most transition-metal oxide electrochromes are in the range blue or grey through to black. most.2 demonstrate electrochromism differently by showing two colours. titanium. see Section 1. Mo. switching as colour 1 ! colour 2. chromium. so allowing the electrochromic transition colourless (clear) ! coloured.6 Metal oxides 6. iridium. if not all. Mo. NiO. Ti. 125 . manganese. niobium. i. by contrast. or nearly passive.1 The oxides of the following transition metals are electrochromic: cerium. iridium and nickel show the most intense electrochromic colour changes.4 on ‘secondary electrochromism’. Ni. RhO2 and WO3 can be prepared as an essentially colourless thin film. as described in Section 4.1. Ni. tantalum. Most of the electrochromic colours derive from intervalence charge-transfer optical transitions. MoO3. The oxides of tungsten. cobalt. Cr. Cu. Other metal oxides of lesser colourability are therefore more useful as optically passive. Co. ruthenium. V. molybdenum. organic electrochromes may be susceptible to photochemical degradation. tungsten and vanadium.1 Introduction to metal-oxide electrochromes Metal oxides as thin films feature widely in the literature. molybdenum. Granqvist2 describes how the solid-state crystals of all of the well-known electrochromic metal oxides Ce. in large part owing to their photochemical stability (see Section 6.1). palladium. Ta.4. iron. one of these colours often being much more intense than the other. praseodymium. TiO2. Nb2O5. 19) (6. Balanced redox reaction for electrochromic operation (6.126 Metal oxides Table 6.28) (6.31) (6.33) (6.16) (6.27) (6.30) (6.13) (6.25) (6.34) (6.29) (6.9) (6.22) (6.37) Metal Bismuth Cerium Cobalt Oxidised form a of oxide Bi2O3 Transparent CeO2 Colourless CoO Pale yellow LiCoO2 Pale yellow–brown CuO Black Ir(OH)3 Colourless FeO Á OH Yellow–green Fe2O3 Brown Fe3O4 Black Fe2O3 Brown FeO Colourless MnO2 Dark brown MnO2 Brown MnO2 Brown MoO3 Colourless NiIIO(1 Ày)Hz Brown–black Nb2O5 Colourless PrO(2–y) Dark orange Rh2O3 Yellow RuO2 Blue–brown Ta2O5 Colourless Reduced form a of oxide LixBi2O3 Dark brown MxCeO2 Colourless Co3O4 Dark brown MxLiCoO2 (M 6¼ Li) Dark brown Cu2O Red–brown IrO2 Á H2O Blue–grey Fe(OH)2 Transparent Fe3O4 Black FeO Colourless MxFe2O3 Black Fe2O3 Brown Mn2O3 Pale yellow MnO(2–x)(OH)x Yellow MxMnO2 Yellow MxMoO3 Intense blue NiII NiIIIO(1Ày)H(zÀx) (1Àx) x Colourless MxNb2O5 Blue MxPrO(2–y) Colourless RhO2 Dark green Ru2O3 Black TaO2 Very pale blue Copper Iridium Iron Manganese Molybdenum Nickel Niobium Praseodymium Rhodium Ruthenium Tantalum .1.26) (6. Summary of the colours of metal-oxide electrochromes.24) (6.12) (6.35) (6.20) (6.17) (6.11) or (6.36) (6. (iii) Subsequent photon-effected electronic transitions involving the redox-altered species. and emphasises that these structural units persist in electrochromic films.1. In binary-metal oxides. and complementary ion motion.39) (6.e.8) (6. (ii) During the redox coloration process. ‘redox switchability’. (i) Bonding in structures whose electron orbital energies (or where applicable. That transition energies in (iii) comprise a spread around a most probable value is shown in spectroscopy by absorption bands having an appreciable width.38) (6. i. band energies) allow of electron uptake or loss from an inert contact. in heteronuclear IVCT. Solid-state electrochromism as in metal oxides requires the following. that are responsible for colour evocation or colour change. or between sites occupied by different elements. he explains how the coordination of the ions leads to electronic bands that are able to explain the presence or absence of cathodic and anodic electrochromism in the numerous defect perovskites.) Balanced redox reaction for electrochromic operation (6.6. a uniformity-conferring charge dispersibility via electron hopping or conduction bands.1 Introduction to metal-oxide electrochromes 127 Table 6. in homonuclear intervalence charge transfer (‘IVCT’). (different ‘oxidation states’). homonuclear . rutiles and layer structures adopted by these oxides. W.40) Metal Tin Titanium Tungsten Vanadium Oxidised form a of oxide SnO2 Colourless TiO2 Colourless WO3 Very pale yellow V2O5 Brown–yellow Reduced form a of oxide LixSnO2 Blue–grey MxTiO2 Blue–grey MxWO3 Intense blue MxV2O5 Very pale blue a The counter cation M is lithium unless stated otherwise. Furthermore. though optical charge transfer between a metal and an oxide ion is also a possibility. The electron-hopping in (ii) is sometimes deemed to be small-polaron motion. are composed of MO6 octahedra arranged in a variety of corner-sharing and edge-sharing arrangements.(cont. The former often (perhaps usually) holds in single-metal oxides. The optical charge transfers in (iii) can either involve discrete sites of the same element in different charge states. many do evince some photoactivity. There is also a chapter on ‘metal oxides’ in Electrochromism: Fundamentals and Applications (1995). quite highly reduced. the former rarely being intense.1. Most of the electrochromic oxides above are compounds of d-block metals. below). despite its date (1973).4 Early reviews on cathodic coloration5 and on anodic coloration6 (both 1982) are still informative. Secondly. the metal oxides can be somewhat unstable chemically. and M0 represents a wide range of metal cations.1. four main disadvantages are detailed below. many oxides achieve only low coloration efficiencies. as are those on WO3 amorphous films7 (1975) and WO3 displays7 (1980). the metal oxides are inherently brittle. The description ‘bronzes’ should strictly apply to metallically reflective. Firstly.128 Metal oxides or heteronuclear transfer between the metals.e.3.1 Bibliography The literature describing the electrochromism of metal oxides is extensive. or mixed-cation such as indium–tin oxide (ITO) – likewise show a new colour (i. and ease of deposition in thin. V or Mo. particularly to the presence of moisture. .1. Granqvist’s3 1995 book Handbook of Inorganic Electrochromic Materials provides the standard text. Intra-atomic or inter-band transitions (resulting from the redox-effected changes) can also – perhaps less usually – confer some colour. absorption band) on electro-reduction. 6. And finally. However. vanadium bronzes and related compounds’8 is the most thorough survey. 6. while more photostable than organic electrochromes. (All of the several possibilities here could in principle occur together but no corresponding totality of discrete bands has been so assigned). of the electronic and structural properties of compounds of interest such as M0 x MO3 where M is W. oxides. even films over large-area electrodes (Section 6. tin oxide.2 Stability and durability of oxide electrochromes Metal-oxide electrochromes are studied for their relative photolytic stability. Some oxides of p-block elements – bismuth oxide. Reaction with moisture and chemical degradation Most studies of ECDs suggest that chemical degradation is the principal cause of poor durability. ‘Tungsten bronzes. but the term is widely used in the literature for the moderately reduced nonmetallic regimes also. are possible. or metal/oxide-ion electron transfer. Thirdly. 16 nickel.12.6.30.23 titanium. Other metal oxides show photoactivity such as photochromism in a few cases. particularly in its anatase allotrope.14 particularly if the film is prepared by evaporation in vacuo. although in different applications like catalysing the photodecomposition of organic materials. although their life expectancy is unlikely to be high because of fragility to bending.28. the avoidance of water is sometimes advised11 if ECDs contain either Ni(OH)2 or NiO Á OH.27. the metal oxides are not wholly photo-inert. Some recent electrochromic devices have been developed in which the substrate is ITO deposited on PET or other polyester (see Section 14.3) in the fabrication of flexible ECDs. see below.26 and tungsten.34 Mechanical stability Like most solid-state crystalline structures. that all traces of moisture should be excluded from ITO-containing ECDs.21. 150.17.33. thin films of metal oxide are fragile. or metallo-organic systems such as the phthalocyanines. For example. which are particularly reactive toward organic materials. Photo-electrochromism is discussed in Chapter 15. Green35 and Ord et al.31. Stresses arise from changes in the lattice constants. Mechanical breakdown also occurs because the films swell and contract with the chemical changes taking place during electrochromic coloration and bleaching.20.18 molybdenum.19.25. titanium dioxide is notably photoactive. and also to the change of charge on the central metal cations. Hence no electrochromic device should comprise thin-film TiO2 in intimate contact with an organic electrolyte. The following electrochromic oxides show photoactivity such as photochromism or photovoltaism in thin-film form: iridium (in its reduced state). such a high photoactivity is extremely desirable.24. Cracking is particularly problematic if the electrolyte layer(s) also comprise metal oxide.15 see p. Photochemical stability The photochemical stability of metal oxides surpasses that of organic systems like polymers and viologens. Tungsten oxide is said to be particularly prone to dissolution in water and aqueous acid. Nevertheless. that adjust to the insertion and egress of charged counter ions.22.10 Similarly. . like Ta2O5.29.1 Introduction to metal-oxide electrochromes 129 Thus.36 show that WO335 and V2O536 expand by about 6% during ion insertion. Bending or mechanical shock can readily cause insulating cracks and dislocations.13. particularly in its partially reduced form MxITO. The oxide film cracks then disintegrates after repeated write–erase cycles if no accommodation or compensation is allowed for these stresses. some workers believe that the thin-film ITO used to manufacture optically transparent electrodes (OTEs) is so moisture sensitive. Irradiating TiO2 generates large numbers of positively charged holes.32.9. 47. (ii) Other methods. stresses from electrochromic cycling can be sensitively monitored by the laser-deflection method: a laser beam impinges on the outer surface of the electrochrome.30.130 Metal oxides Amongst many probes. (i) Amorphous layers result from electrodeposition or thermal evaporation in vacuo.1.44 and tungsten45.3 The preparation of thin-film oxide electrochromes In ECDs the metal-oxide electrochrome must be deposited on an electrode substrate as a thin. Laser-beam deflection has been used to monitor electrochromic transitions in the oxides of iridium. The stresses in oxide films of nickel.38 nickel39 and tungsten. tend to form layers that are polycrystalline (microcrystalline or ‘nanocrystalline’). when the inserted ions were Hþ. new (unnamed) crystal phases were formed. e.4.2–0. though.40. Information on electrochemically induced stresses can also be inferred from X-ray diffraction.48 have been analysed thus.51 Employing an elastomeric polymer electrolyte largely accommodates the ion volume changes occurring during redox cycling: Goldner et al.43 titanium.g. as described in Section 3. in oxides of nickel49 and vanadium. Such thin films are either amorphous or polycrystalline. Other methods include adding small amounts of other metal oxides to the film: these minor built-in distortions introduce some mechanical ‘slack’ into the crystal lattices.53 Chapter 16 contains an assessment of the durability of assembled electrochromic devices. for switching electrochromic windows.5 mm. adding about 95% nickel oxide to WO3 greatly enhances its cycle life.11. even film of sub-micron thickness. The linearity held only for small37 amounts of inserted charge.50 while those in molybdenum oxide have been studied by Raman vibrational spectroscopy. typically in the range 0.41 Alternative methods of analysing electrochromically induced stresses include electrochemical quartz-crystal microbalance (EQCM) studies. the morphology depending strongly on the mode of film preparation.46. sometimes both admixed. Their correlation also suggests this induced stress is relieved in direct proportion to the extent of ion egress. 6. For example. that caused the loss of reversibility. In this way Scrosati and co-workers37 found a linear dependence between the amount of charge inserted into WO3 and the induced stress. . Above certain values of x. sputtering for example.52 says ‘nearly complete stress-change compensation’ can be achieved by this method. particularly when the inserted ions were Liþ or Naþ. and analysis of the way its trajectory is deflected during redox cycling provides data that allow quantification of these mechanical stresses. Liþ and Naþ.42. and how such durability is tested. which depends largely on whether the sample was previously warmed or not. in alphabetical order.61.1 Introduction to metal-oxide electrochromes 131 Methods such as CVD or sol–gel generally proceed in two stages: the firstformed amorphous layer needs to be subsequently annealed (‘curing.63 and tungsten. W(CO)6 decomposes according to Eq.63.71 An alternative precursor. although many authors have reviewed one or more specific deposition methods: Granqvist’s book3 gives extensive detail on the preparation of metal-oxide films. which could have serious implications for the speed of electrochromic operation.64. Venables’59 book Introduction to Surface and Thin Film Processes (published in 2000) contains some useful comments about these preparations. 98. The number and size distribution of the crystallites depends on the temperature and duration of the annealing process. Deposition methods are outlined below. Transparent Counter Electrodes and Sputtering Techniques58 (published in 1999).72 is allowed into the deposition chamber at a low partial pressure.54 Such crystallisation is sometimes called 55 a ‘history effect’.68. see p. approaching the atomic level. The solid tungsten product is finely divided.62.70. and decomposes on contact with a heated substrate.66. (6. Finally. (6. Chemical vapour deposition with carbonyl precursors has provided thin oxide films of both molybdenum60. Decomposition occurs at the surface of a heated substrate (in this example72 the temperature was 620 8C) to effect the reaction in Eq.1) The carbon monoxide waste byproduct is extracted by the vacuum system.6. Annealing at high temperature in an oxidising atmosphere yields the required oxide.1): W(CO)6 (g) ! W (s) þ 6CO (g).69. a volatile precursor is introduced into the vacuum deposition chamber. Chemical vapour deposition (CVD) In the CVD technique. as does Kullman’s book. which greatly extends the growth of crystalline material within the amorphous. Granqvist’s review57 ‘Electrochromic tungsten oxide films: review of progress 1993–1998’ provides further detail.56 There are no reviews dedicated solely to the deposition of metal oxides. For example.’ ‘sintering’ or ‘high-temperature heating’).65. The crystallites formed can remain embedded in amorphous material. Such volatiles commonly include metal hexacarbonyls or alkoxides and hexafluorides. a metal alkoxide such as Ta(OC2H5)5. The films are made polycrystalline by the annealing process. thereby alluding to the extent of crystallinity. Annealing assists the phase transition amorphous ! polycrystalline. (6.62.67. Components of Smart Windows: Investigations of Electrochromic Films.2): . 79.86. or other elements if different precursors are employed. but good-quality . precursors can be wholly inorganic. e. VO(OiPr)3. Electrodeposition from nitratecontaining solutions has produced oxide (and oxyhydroxide) films of cobalt79. the resultant films may contain carbon and hydrogen impurities. Electrochemical reduction of aqueous nitrate ion generates hydroxide ion76.80.83. so the electrochrome comprises both oxide and hydroxide. Furthermore.71 If the CVD precursor does not decompose completely.4) Dehydration as in Eq.or molybdenum-containing films can be electrodeposited from aqueous solutions of tungstate or molybdate ions.3): NO3À (aq) þ 7 H2O þ 8 eÀ ! NHþ (aq) þ 10 OHÀ (aq).75 Tungsten. (6. such as TaCl5. (6. or their trace contamination adversely affects the electronic and optical properties of the electrochrome.72 Vanadium oxide can similarly be prepared from the volatile alkoxide.73 employed the two volatile materials tris(acetylacetonato)indium and di(pivaloylmethanato)tin to make ITO.77.74 Electrodeposition Virtually all electrochromic films made by electrodeposition are amorphous prior to annealing. Hence most electrogenerated films of ‘oxide’ are oxyhydroxide of indeterminate composition unless sufficient annealing followed the electrodeposition.78.77.78 according to Eq. followed by dehydration during heating according to Eq. the lowest metal oxidation state usually being employed if there is a choice.84.75 Transition-metal oxides other than W or Mo are easily electrodeposited from aqueous solutions of metal nitrates. often termed ‘oxyhydroxide’ and given the formulae MO Á OH or MO Á (OH)x. Subsequent precipitation then forms an insoluble layer of metal oxide as in Eq. (6.132 Metal oxides 2Ta(OC2H5)5 (g) þ 5O2 (g) ! Ta2O5 (s) þ products (g).87 The mechanism of WO3 electrodeposition is discussed at length by Meulenkamp.85. Watanabe et al. Other metallo-organic precursors have been used. The impurities either form gas-filled insulating voids in the oxide film. (6.82. (6.81 and nickel.2) The resulting oxide film is heated for a further hour at 750 8C in an oxygen-rich atmosphere.3) The electrogenerated hydroxide ions diffusing away from the electrode associate with metal ions in solution.5): [M(OH)n] (s) ! ½ [M2On] (s) þ n/2 H2O.4): Mnþ (aq) þ nOHÀ (aq) ! [M(OH)n] (s).5) (6.g. 4 (6.5) is usually incomplete. (6. the final mixed-metal oxide can be homogeneous and even. Oxide films of cobalt.79.88 Excess peroxide is removed when the reactive dissolution is complete. electrochromes derived from Ni(OH)2 and Co(OH)2 are electrodeposited while the precursor solution is sonicated.97 and vanadium98 have been made by electrodeposition from similar solutions.97. The . but the dissolution may proceed according to Eq.81.102.91. As the mixing of the precursor cations in solution occurs on the molecular level.96. (6. growth and subsequent collapse of microscopic bubbles. the electrogenerated hydroxide must partition between all the metal cations in solution.3) yields a film with a composition approximating that of the deposition solution. applying a limiting current by imposing a large electrodeposition overpotential (Section 3.105 Computer-based speciation analyses have been demonstrated that describe the product distribution during the electrodeposition of such mixed-metal depositions.6) are either protons (as shown here).91. When the deposition solution contains more than one cation.103.100. or they could be uncomplexed metal cations. as depicted for tungsten: 2W (s) þ 6H2O2 ! 2Hþ[(O2)2(O)W–O–W(O)(O2)2]2À (aq) þ H2O þ 4H2 (g).94.1 Introduction to metal-oxide electrochromes 133 oxide films are prepared from a solute obtained by oxidative dissolution of powdered metal in H2O2.93 tungsten56.106 In a modification.80.6.99 Alternatively. This generates a peroxometallate species of uncertain composition. The mole fractions x of each metal oxyhydroxide in the deposit can be tailored by using both predetermined compositions and potentiostatically applied voltages Va.89.89 Marginal ethanol incorporation in the electroformation of WO389 and NiO90 films has been investigated. The counter cations in Eq. each involving the consumption of hydroxide ions as governed by both the kinetics and/or equilibria associated with the formation of each particular hydroxo complex.6) Such peroxo species are also employed in the sol–gel deposition method described below. usually by catalytic decomposition at an immersed surface coated with Pt-black. dilution with an H2O–EtOH mixture (volume ratio 1:1) confers greater long-term stability until used. It is difficult to tailor the composition of films comprising mixtures of metal oxide since the ratio of metals in the resultant film is not always determined by the cation ratio in the precursor solution.80.92 tantalum.107 The main difference from conventional electrodeposition is the way sonication causes the formation.81.95.96.81 molybdenum.104.6). This divergence in composition arises from thermodynamic speciation.101. While still relatively unstable. (6.105.91. (6. . which. on being allowed to stand. ‘Electrochemical synthesis of metal oxides and hydroxides’ (2000) by Therese and Kamath. expands to form the gel. ‘sol’ denotes sub-micron or nano particles visible only by the scattering of a parallel visible light beam (the so-called Tyndall effect).119 ‘Sol–gel electrochromic coatings and devices: a review’ (2001) by Livage and Ganguli.112 Many reviews of sol–gel chemistry include electrochromism: for example. as outlined in the review (2001) by Bell et al.110 Cordoba de Torresi and co-workers report107 that the method yields electrochromes with significantly higher coloration efficiencies . ‘The hydrothermal synthesis of new oxide materials’ (1995) by Whittingham et al. while ‘gel’ denotes linked species forming a three-dimensional network.107 The method has been used to make thin films of Ni(OH)2107.107. at which time the local temperature can be as high as 5000–25 000 K. say. which. Lakeman and Payne:113 ‘Sol–gel processing of electrical and magnetic ceramics’ (1994). so adding water to. leading to crystallisation and reorganisation of the solute.121 As indicated by the number of literature citations. The sol–gel method is an attractive route to preparing large-area films.116 ‘Sol–gel materials in electrochemistry’ (1997).109 and ´ Co(OH)2. Sol–gel techniques Regarding present terminology..7): 2Nb(OEt)5 (l) þ 5H2O (l) ! Nb2O5 (sol) þ 10EtOH (aq).114 Alber and Cox.118 ‘Anti-reflection coatings made by sol–gel processes: a review’ (2001) by Chen.134 Metal oxides bubble collapse takes place in less than 1 ns when the size is maximal.115 (1997) ‘Electrochemistry in solids prepared by sol–gel processes’. After collapse. on standing.122 Many alkoxides react with water. (6.108 The reasons for the differences in the nanoproducts formed using this method are somewhat controversial. sometimes including a second species within the minute enclosures. Lev et al. Gedanken and co-workers108 suggest it obviates the need for particles to grow at finite rates. ‘colloid’ is a general term denoting any moreor-less subdivided phase determined by its surface properties. is further spontaneously transformed into a gel.120 and ‘Electrochromic sol–gel coatings’ by Klein (2002). (6.7) .111 The sol–gel method involves decomposing a precursor (one chosen from often several candidates) in a liquid. the preferred sol–gel precursors are metal alkoxides such as M(OEt)3. Nb(OEt)5 yields colloidal (sol) Nb2O5123 according to Eq. to form a sol. the local rate of cooling is about 1011 K sÀ1.117 ‘Electrochromic thin films prepared by sol–gel process’ (2001) by Nishio and Tsuchiya. 139 nickel140.91.192 tantalum.136 This method. The film is then annealed in an oxidising atmosphere.132. it has been used extensively for mixtures of precisely defined compositions such as indium tin oxide (ITO).129.144.149.200 Once formed.161.195. The process may be repeated many times when thicker layers are desired.128 tungsten99.131 and vanadium. with concomitant increases in impurity levels. ITO on glass.123.172.158.146 It is especially suitable for making mixtures.179.133.196. and excess is flung away by centrifugal motion.125 nickel.138.178.185 ITO. 162. as below. Spray pyrolysis The simplest method of applying a gelled sol involves spraying it onto the hot substrate. the gel is then applied to an electrode substrate. since the stoichiometry of the product accurately reproduces that of the precursor solution.190 niobium.124.173 titanium.180.190.157.142 and tungsten. as for CVD-derived films.133 A similar peroxo species is formed by dissolving a titanium alkoxide Ti(OBu)4 in H2O2. to give a polycrystalline electrochrome.159. (6.168.194.186. The method has produced oxide films of cerium.191.147 nickel.132.134 Whatever the preparative method.128.6.81. has been used to make electrochromic oxides of cerium.183 Spin coating A further modification of dip coating is the ‘spin coating’ method: the solution or gel is applied to a spinning substrate.154 niobium.1 Introduction to metal-oxide electrochromes 135 The other favoured sol–gel precursor is the peroxometallate species formed by oxidative dissolution of the respective metal in hydrogen peroxide (Eq.99 molybdenum.174 tungsten29.6) above).141. Thus appropriate peroxo precursors have yielded electrochromic oxide films of cobalt.127.184 cobalt.176.171. The coated electrode is annealed at high temperature in an oxidising atmosphere.129.188 molybdenum.80.) is fully immersed in the gel then removed slowly to leave a thin adherent film.147.137 cobalt. Dip coating ‘Dip coating’ is comparable to spraying: the conductive substrate (inert metal.127.166.167.199. Film thickness is controlled by altering solution viscosity.165.135. such films are annealed in an oxidising atmosphere. sometimes called ‘spray pyrolysis’.152 iridium. 177.163. .148. Many oxide films have been made this way: cerium.156. etc.128 tungsten129.175.164.170.189.126 titanium.131.187 iron.198 and vanadium.169. temperature and spinning rate.181 182 and vanadium.151.130. Being particularly well suited to making mixed oxides.155.130.150.197.145.153 iron.160.143. Burning away the organic components is more problematic than for CVD since the proportion of carbon and other elements in the gel is usually higher.193 titanium. often in a relatively dilute ‘suspension’.124. sputtering energiser and impact angle. electronbeam sputtering and rf sputtering.209 nickel. releasing much energy.214. the operating power varying between 100 and 250 W.205 Other methods: sputtering in vacuo Sputtering techniques detailed below generally yield polycrystalline material206 since the high temperatures within the deposition chamber effectively anneals the incipient film.225.9%. The deposition chamber contained a precisely controlled mixture of Ar and O2.215.50. which is both ionised and accelerated by a high potential comprising the ‘magnetron’. The atmosphere within the deposition chamber contains a small partial pressure of oxygen. again producing precisely defined final compositions. Azens et al.4 nm sÀ1.05 when pure Ce oxide was required. The ratio of gaseous O2 to Ar was adjusted from 1. This . to produce pure WO3 and TiO2 oxides. especially if it is ITO on glass.212. a target of the respective metal is bombarded by energetic ions from an ion gun aimed at it at an oblique angle. affect the properties of deposited films. Granqvist’s 1995 book3 and 2000 review57 describe in detail how the experimental conditions. As an example. The substrate is positioned on the far side of the target. This sputter-gas pressure was maintained at 5–40 mTorr.216 niobium.211. such as the partial pressures.224.201.223.998%. Such reactive dc magnetron sputtering has been used to make oxide films of ITO.217. The substrate thus has to be water-cooled to prevent its melting. ablated material impinges on it and condenses.208 molybdenum.192. The high-energy ions smash into the target in inelastic collisions that cause small particles of target to be dislodged by ablation.219 tantalum. The ion of choice is Arþ.213.202.210. Deposition rates (from sputter time and ensuing film thicknesses as recorded by surface profilometry) were typically 0. so the ablated particles are oxidised: ablated tungsten becomes WO3.207 made films of W–Ce oxide and Ti–Ce oxide by co-sputtering from two separate targets of the respective metals.124. substrate composition.220 tungsten221. each of purity 99. Thin films of sputtered electrochrome are formed by three comparable techniques: dc magnetron sputtering.222 and vanadium. thereby facilitating the crystallisation process amorphous ! polycrystalline.136 Metal oxides Spin coating is one of the preferred ways of forming thin-film metal-oxide mixtures. to 0. The oxidised.226 Electron-beam sputtering Here an impinging electron beam generates a vapour stream from the target for condensation on the substrate. In dc magnetron sputtering.204. Such targets are typically 5 cm in diameter and have a purity of 99.218 praseodymium. The deposition substrates were positioned 13 cm from the target.203. 14) A pressure of about 10À5 Torr is maintained during the deposition process.277.264. a small quantity of powdered oxide is placed in an electrically heated boat.272 Nickel oxide formed by thermal deposition is generally of poor quality.242.279 and vanadium.240.265. Deb270 suggests y ¼ 0.220 tungsten15. resulting in sub-stoichiometric films NiO(1Ày).230 MnO2.260 titanium.277 .273.243. The extent of oxygen deficiency will depend on the temperature of the evaporation boat and/or of the substrate target.254.267 Thermal deposition in vacuo The oxides of tungsten. The rf-sputtering technique is often employed for making metal oxides.241.231 MoO3232 and V2O5. Thin films of metal oxide form when the sublimed vapour condenses on a cooled substrate.266. also called ‘reactive electron-beam evaporation.274.235 lithium cobalt oxide.256 tantalum.247 nickel. Molybdenum or tungsten oxides can be prepared thus.248.239. the target-vaporising energy is derived from a beam of reactive atoms.246.3. evaporated tungsten oxide is often oxygen deficient to an extent y. Hence.’ has been used to prepare thin films of ITO.245 manganese.255.233. and yields good-quality films which are flat and even. No post-deposition treatment is needed.251.258.206. In practice.237.228. where the extent of oxygen deficiency y ( 1.268 (Arnoldussen suggests that these trimers persist in the solid state. typically of sheet molybdenum.236. Higher temperatures may cause slight decomposition in transit between the evaporation boat and substrate.263.250. generated at an rf frequency.6.238 ITO.229.244.269 The electrochromic properties of films deposited in vacuo are usually highly dependent on the method and conditions employed. although small amounts of elemental molybdenum can sublime and contaminate the electrochromic film. since the high temperatures within the deposition chamber yield samples that are already polycrystalline.86.278.252. In the rf variant.249.227.257.259.234.276 tantalum. The required thin film of metal oxide forms by heating the ablated material in an oxidising atmosphere.253. The technique has been used to make oxide films of: iridium.03 but Bohnke and Bohnke271 quote 0.1 Introduction to metal-oxide electrochromes 137 technique.261tungsten262 and vanadium. so good-quality NiO is best made by sputtering methods. molybdenum and vanadium are highly cohesive solids with extensive intra-lattice bonding. The vapour consists of molecular species (oligomers) such as the tungsten oxide trimer (WO3)3. since the high temperatures needed for sublimation cause loss of oxygen. in WO(3Ày). a target of the respective metal is bombarded with reactive atoms (argon or oxygen) at low pressure. which require high temperatures for vaporisation when heated in vacuo.233 Radio-frequency (rf) sputtering Like dc sputtering. Thermal evaporation is often used to make the oxides of molybdenum.275.23. . an electrochrome precursor in a solvent is laid down on the surface of another. Conversion to the required oxide follows one of the routes described above.281 In essence. and (b) anions as mobile ion. This can then be drawn onto the (say metal or ITO-glass) substrate by slow immersion then emersion of the latter. The dual charge injection is shown in Figure 6. depicted for a reduction reaction: (a) cations as mobile ion. electrochrome–electrolyte. causing potential gradients. electrochromic coloration of metal-oxide systems proceeds via the dual insertion of electrons (that effect redox change) and ions (that ensure the ultimate overall charge neutrality of the film).1.1 Schematic representation of ‘double charge injection’. liquid in monolayer form. Note the way that equal amounts of ionic and electronic charge move into or out from the film in order to maintain charge neutrality within the solid layer of electrochrome. Langmuir–Blodgett deposition Langmuir–Blodgett methodology for preparing films of metal-oxide electrochromes was reviewed in 1994 by Goldenberg. The charge carriers move in their opposite directions during oxidation.4 Electrochemistry in electrochromic films of metal oxides To add detail to the electrochemistry outlined in Chapter 3. Thus a considerable electric field is set up initially across the film before these separate charges reach their Solid electrochrome Solid electrochrome Electrons Cations Electrons Anions (a) (b) Figure 6. 6. interface.138 Metal oxides Vacuum Deposition of Thin Films280 by Holland (1956). by the arcane methods of Langmuir–Blodgettry employing an appropriately constructed bath. suitably repeated for multi-layers. remains a valued text on thermal evaporation in the preparation of thin films. though separation can occur. though old. non-dissolving.1: the thin film of electrochrome concurrently accepts or loses electrons through the electrochrome–metal-electrode interface while ions enter or exit through the outer. 57 Also useful are the reviews by Azens et al. The solid electrode assembly is in contact with a solution (solid or liquid) containing mobile counter ions (the ion source being termed ‘electrolyte’ hereafter).2 Metal oxides: primary electrochromes 139 ultimate. The most comprehensive is: ‘Case study on tungsten oxide’ in Granqvist’s 1995 book.1 Tungsten trioxide Selected biblography There are many reviews in the literature. Polycrystalline films. Chapter 5). studies have employed both amorphous and polycrystalline materials. For this reason. An important aspect of mechanistic studies concerns whether the ionic motion or the electronic is the slower. and ‘Electrochromic tungsten oxide films: review of progress 1993–1998’ (2000). by contrast. and by Monk:285 ‘Charge movement through electrochromic thin-film tungsten oxide’ (1999).:284 ‘Electrochromism of W-oxide-based thin films: recent advances’(1995). chemistry and technology’.3 Also by Granqvist is: ‘Electrochromic tungsten-oxide based thin films: physics. distributions. a highly-doped ITO or FTO film on glass usually acts as a transparent inert quasi-metal. though the proton also is often used thus. 6. . because the outcome often decides what determines the rate of coloration (cf. Anions are only occasionally employed as mobile ion. Other oxide electrochromes are reviewed subsequently in Section 6. The mobile ion we imply to be lithium unless otherwise stated.3. The conductive electrode substrate can be either a metal or semiconductor. generally are more chemically durable. but ionic rapidity to predominant amorphism (for the same material).2. While the following sections inevitably represent but an excerpt from the huge literature available. A simple but not unexceptionable surmise would impute faster electronic motion to predominant crystallinity. For ECD usage. Granqvist’s monograph3 (1995) is comprehensive to that date.6. including metal-oxide mixtures with noble metals and films of metal oxyfluoride. amorphous films are generally preferred for superior coloration efficiency and response times. Tungsten trioxide is treated first in Section 6. ‘Progress in electrochromism: tungsten oxide revisited’283 (1999).282 (1993). usually being the hydroxide ion OHÀ.2 Metal oxides: primary electrochromes 6.4. Finally. equilibrium. mixtures of oxide electrochromes are discussed in Section 6.2 because it has been investigated more fully than the other highly colourant metal-oxide electrochromes. in detail. 90 8C. . y lying between 0.271 caused by the increased proportion of crystalline WO3.56 and Bohnke and Bohnke271 both annealed samples at 250 8C. while Antonaia et al. are highly preparation-sensitive. and in the study by Deb and co-workers294 of thermally evaporated WO3 the crystallisation process is said to start at 390 8C and is complete at 450 8C. a-WO3 or c-WO3.289. sometimes minute) structural aspects of the solids. and its reduced forms. The colour of the WO3 deposited depends on the preparative method.293 By contrast.4) such as In.2.140 Metal oxides Finally in this section the reader is referred to reviews by Bange:286 ‘Colouration of tungsten oxide films: a model for optically active coatings’ (1999) and Faughnan and Crandall:7 ‘Electrochromic devices based on WO3’ (1980). for crystallinity Deepa et al. The temperature at which the (endothermic12) amorphous-to-crystalline transition occurs is ca. X-Ray diffraction showed Deb’s270 evaporated WO3 to be amorphous. depended on the dopant.291 Metal dopants (see Section 6. Bi and Ag have different influences on the phase ratio P21/n to Pc.287. as determined by thermal gravimetric analysis (TGA). the apparent contradictions noted here and elsewhere in this text are almost certainly ascribable to intrinsic variability in (sometimes marked. As the physical (and optical) properties of WO3. Preparation of tungsten oxide electrochromes Thermal evaporation Pure bulk tungsten trioxide is pale yellow. but WO3 films prepared by rf sputtering are partially crystalline.03270 and 0.287 Annealing WO3 results in enhanced response times. thin films sometimes showing a pale-blue aspect owing to oxygen deficiency in a substoichiometric oxide WO(3Ày). the microcrystalline from sputtering or from thermal annealing of a-WO3. Cell parameters and crystallite sizes (about 50 nm) were marginally affected by these inclusions and.3271 (see p. or indeed a mixture of phases and crystal forms.290 An XRD crystallographic study of thick and thin films from screen-printed WO3 established that WO3 nanopowder has two monoclinic phases of space groups P21/n and Pc. the amorphous form resulting from thermal evaporation in vacuo and electrodeposition.292 The spacegroup of crystalline D0.52WO3 is Im3. 137). Tungsten trioxide as a thin film can be amorphous or microcrystalline.288. Morphology The structure of WO3 is based on a defect perovskite. The preparative method dictates the morphology.295 maintain that annealing commences at 400 8C. Pilkington plc employed rf sputtering. 180.317.176.181.320.310 involve sputtered WO3 films which are chemically more robust than evaporated films. Pyrolysis in a stream of gaseous oxygen generates finely divided tungsten.67.239. and are essentially amorphous in XRD.315 (putatively [(O2)2–(O)–W–(O)–(O2)2)]2 À.66.177.311.196.55.70 Other organometallic precursors include tungsten(pentacarbonyl-1-methylbutylisonitrile)298. bombarding a tungsten target with reactive argon ions in a low-pressure oxygen to sputter WO3 onto ITO. 176. precursor by spin coating.318.179.65. They call it ‘oxygen backfilling’.272 Sun and Holloway employ a modification of this method in which evaporation occurs in a relatively high partial pressure of oxygen.68.180.176 ethanolic WCl6.4.302.127.308.331.326.99.181 and spray pyrolysis.143.6. Other sol–gel precursors include WOCl4 in iso-butanol.292.329. Many studies221.69.2 Metal oxides: primary electrochromes 141 The extent of oxygen deficiency depends principally on the temperature of the evaporation boat.146 Livage et al.336 The sol–gel method is often deemed particularly suited to producing largearea ECDs.97.175.190. 330.314. 136).196.299 and tungsten tetrakis(allyl).131.89.46.321.297 which partly remedies the nonstoichiometry. 309.313.324.296.195. The tungsten carboxylates represent a different class of precursor for electrodeposition.176.64. for example for fabricating electrochromic windows.221. 131).118. W(3-C3H5)4.314 Sol–gel The sol–gel technique is widely used.316.300 Sputtering (Section 6. Chemical vapour deposition.334.62.319.323.312 Direct-current magnetron sputtering is less often employed.337 .307.197.304.305. p.88.140.180.145.329 together with an open-circuit memory in excess of six months.178.335 and phosphotungstic acids.130.94.325.56.197 tungsten alkoxides333.131.306.129.95.144.175.303.332 often made their WO3 films from a gel of colloidal hydrogen tungstate applied to an OTE and annealed. formed by oxidative dissolution of powdered tungsten metal in hydrogen peroxide) sometimes appear gelatinous.99. yielding products that are amorphous.129.123.197. CVD (see p.322.194.198 dip-coating29.129.130.96.327.27.331. The volatile carbonyl CVD precursor W(CO)6 is the most widely used.198.310 Response times of 40 s are reported.301. and then thin-film WO3 after annealing in an oxygen-rich atmosphere.328 applying the sol–gel 129.222 Electrodeposition WO3 films electrodeposited onto ITO or Pt from a solution of the peroxotungstate anion.1. 361. It is this unbound electron plasma in metallic WO3 bronzes that confers reflectivity.40.339. The speed of ion insertion is slower for larger cations.360 an essentially free-electron model (but dismissed by Schirmer et al.364 and salient details from Chapter 5 are reiterated here. determined for an amorphous HxWO3 by conductimetry355 (the precise value cited no doubt applies exactly only to that type of product). that conducts by the sitewise hopping mechanism. Below xc.345.362 for amorphous WO3).302.343 deuterium cation. the coloration usually attributable to Liþ is suggested to result rather from proton insertion.352 or even Agþ . we abbreviate Mx(WVI)(1Àx)(WV)xO3 to MxWO3.347.339. depending strongly on the deposition rate employed in forming the electrochromic layer. Dickens et al. HxWO3 with x > xc is metallic with completely delocalised transferable electrons (the Robin and Day347 Group IIIB).357.350. In a further EQCM study. while cationic counter charges enter concurrently through the other (electrolyte-facing) side of the WO3 film. as only Hþ and Liþ can be expelled readily following electro-insertion.342.340.341. electrons enter the WO3 film via the conductive electrode substrate. WVIO3 (s) þ x(Mþ(soln)) þ xeÀ ! Mx (WVI)(1Àx) (WV)xO3 (s). For convenience. very pale yellow intense blue (6.8).32. (6. analysed the reflectance spectra of NaxWO3 in terms of modified Drude–Zener theory that includes lattice interactions.344.271. Babinec. or ‘polaron hopping’).340.346 Liþ .8) (where M ¼ Li usually).338 studying the coloration reaction with an EQCM (see p.142 Metal oxides Redox properties of WO3 electrochromes On applying a reductive potential.358.348 Naþ . as in Drude-type delocalisation. the bronze is a mixed-valence species356 in the Robin–Day348 Group II (involving moderate electron delocalisation of the ‘extra’ WV electron acquired by injection. .363 Kinetic dependences on x The rates of charge transport in electrochromic WO3 films are reviewed by Monk285 and Goldner. found the insertion reaction to be complicated. the proton then swapping with Liþ at longer times.359.349.351 Kþ .353 The overwhelming majority of these cations cannot be inserted reversibly into WO3.339.354 Consequences of electron localisation/delocalisation The non-metal-to-metal transition in HxWO3 occurs at a critical composition xc ¼ 0. Cation diffusion through WO3 has received particular study with the cations of hydrogen ions. Eq. 88). 363 and the dielectric-368 and ferroelectric properties.362.2 on p.371 of thin-film WO3 (grown anodically) show little optical hysteresis associated with coloration. comments in Section 1.371 The different mechanisms of colouring and bleaching discussed in Chapter 5 may be sufficient to explain the significant extent of optical hysteresis observed. owing to the minimal delocalisation of conduction electrons which conduct by polaron-hopping. rather than by the movement of a clear interface that separates coloured and uncoloured regions of the material. . the charge mobility of the inserted electron is low. The electronic conductivity of evaporated WO3. has been determined as a function of x. reach a point at which further coloration is accompanied by film dissolution (cf.4 shows HxWO3 to be effectively an insulator at x ¼ 0. Most properties of the proton tungsten bronzes HxWO3 depend on the insertion coefficient x. the latter a ‘colour front’.35) The ellipsometric studies by Ord and co-workers370. Structural changes occurring during redox cycling In Whittingham’s 1988 review ‘The formation of tungsten bronzes and their electrochromic properties’373 the structures and thermodynamics of phases formed during the electro-reduction of WO3 are discussed. 86.3 the electronic conductivity becomes metallic following the delocalisation at this and higher x values. The effects of structural change are discussed in greater depth in Section 5. Furthermore. At very low x. conclude that a ‘substantial’ fraction of the Hþ inserted during coloration cycles is still retained within the film when bleaching is complete. Colour cycles of longer duration. because the electronic conductivity362 s follows x.2 demonstrates such hysteresis for coloration and bleaching. The optical data for WO3 grown anodically on W metal best fit a model in which the colouring process takes place by a progressive change throughout the film. The former therefore represents a diffuse interface between regions of the film. subsequently reduced. Other studies of structure changes during redox change are cited in references 37 and 374–376.2 Metal oxides: primary electrochromes 143 Considerable evidence now suggests that the value of the insertion coefficient x influences the rates of electrochromic coloration.365. Ord et al.362 hence rate-limiting. but s increases rapidly until at about x % 0.7. owing to the occupation of different crystallographic sites by the minute protons. however.366 Figure 5. concerning cycle lives).4 and above.372 Figure 6. provided the reductive current is only applied for a limited duration: films then return to their original thicknesses and refractive indices.367 the reflectance spectra.369 (It is notable that the alignment of spins in the ferroelectric states differs in proton-containing bronzes compared with that in NaxWO3.6. such as the emf. Chem. i. the electrochromic transition is effectively colourless-to-blue at low x ( 0. In transmission. (Figure reproduced from: Scarminio. ‘The Beer–Lambert law for electrochromic tungsten oxide thin films’.144 0. Similarly. Georg et al. Mater. 143–146.378 suggest the proton resides at the centres of the hexagons created by WO6 octahedra. X-ray results379 suggest that extensive write–erase (on–off) electrochromic cycling generates non-bridging oxygen.40 0. (6. 61. At higher values of x.) Some authors. intercalated charge density obtained for polycrystalline and amorphous WO3 films during dynamic coloration and bleaching. causes fragmentation of the lattice structure.2 Optical density vs. structureless band peaking in the near infra-red. . Urbano. A.. by permission of Elsevier Science. such as Kitao et al.20 0. Figure 6. insertion irreversibly forms a reflecting.2).53WO3 shows the O–D and O–D–O distances are almost certainly too large for hydrogen bonding to occur.. it forms a hydrogen bond with bridging oxygen atoms. However. and Gardes.377 say that when the mobile ion in Eq. J. metallic (now properly named) ‘bronze’.45 Amor phous 0. Optical properties of tungsten oxide electrochromes Optical effects: absorption The intense blue colour of reduced films gives a UV-visible spectrum exhibiting a broad.3 shows this (electronic) spectrum of HxWO3.e. red or golden in colour. B. Phys.15 0 2 4 6 Charge density (mC cm–2 ) 8 10 Figure 6. Whatever its position.25 0.35 Absorbance Polycrystalline 0. the X-ray and neutron study by Wiseman and Dickens287 of D0..30 0. 1999.50 Metal oxides 0.8) is the proton. the value of max does vary considerably with the preparative method (see p.276.386 studying the ESR spectrum of HxWO3 at low x. F. 179–187. Bange. ‘Reflecting electrochromic devices’.17WO3 deposited by sputtering on ITO. The visible region of the spectrum is indicated. K.381 and Krasnov et al. G.2 Metal oxides: primary electrochromes 1.380 Faughnan et al. where max can reach 1300 nm for . Could the ground-state electrons form paired rather than single spins.385 Provisionally we assign the blue colour to an intervalence charge-transfer transition.2 0.e. by permission of Elsevier Science. and also the extent and nature of the electronic surface states (i.9 0.3 UV-visible spectrum of thin-film H0.270. electrolyte.388. the electron localisation and the accompanying lattice distortion around the WV being treated as a bound small polaron.389 to longer wavelengths in polycrystalline389 materials.272. T.387.3 Visib le range of spectrum 145 Absorbance 500 1000 1500 2000 2500 λ (nm) Figure 6. at adjacent loosely interacting WV sites?384.6 0.270 Elsewhere the blue colour is attributed to the electrochemical extraction of oxygen..382 proposed that injected electrons are predominantly localised on WV ions.382. forming the coloured sub-stoichiometric product WO(3Ày). could find no evidence for unpaired electrons on the WV sites. K.361.364. localised at oxygen vacancies within the WO3 sub-lattice. vacant electronic orbitals on the surface).383.384. While this is now widely accepted. 1988.) The origin of the blue colour of low-x tungsten oxides is contentious. The absorption is often attributed to an F-centre-like phenomenon. 146): max depends crucially on morphology and occluded impurities such as water. and Gambke. While the wavelength maximum max of a particular HxWO3 is essentially independent of the insertion coefficient x.5 1.5. (Figure reproduced from Baucke. 9.6.385 The colour was attributed381 to the intervalence transition WV þWVI ! WVI þWV (subA B A B scripts A and B being just site labels). among critics Pifer and Sichel. Displays. Thus the value of max shifts from 900 nm in amorphous and hydrated reduced films of HxWO3. Beer’s law is therefore not followed except over limited ranges of Q and hence of x. Close proximities increase the probability of the optical intervalence transition in the electron-excitation colour-forming process.391 Chadwick and co-workers392 analysed the interdependence of defects and electronic structure. be it M ¼ Hþ.339. The forms of defect in polycrystalline and amorphous WO3 influence the optical spectra of WO3 and its coloured reduction products. They show how structural defects exert a strong influence upon electronic structure and hence on chemical properties. give values of (HxWO3) ¼ 63.5) and (4. the overall absorbance Abs of any particular WO3 film always increases as the insertion coefficient x increases. Most authors5. and their influence on electrochromic properties. see Figure 5. although one sputtered film390 had a contrast ratio CR of 1000:1. and. For example. As expected. since each electron acquired generates a colour centre. The gradient of such a graph is the coloration efficiency (see Equations (4. This could explain why films sputtered from a target of W metal show different Beer’s-law behaviour from sputtered films made from targets of WO3.12.393 . possibly even specular reflection). Kþ. but the amorphous material (of course) contains a very high proportion of (what are from a crystal viewpoint) defects. Dini et al. their results suggest that any liquid suppresses water dissociation at the surface and the formation of OH3þ structures near to it.355 believe the colour of the bronze is independent of the cation used during reduction.6)).e. is smaller. (6. Liþ. Naþ. (LixWO3) ¼ 36 and (NaxWO3) ¼ 27 cm2 CÀ1. However. while little is known about how the chemical activity at the interface is affected by interaction of liquid.361 Intervalence optical transitions are known to be neighbour-sensitive. turns out to be far from clear. Csþ.146 Metal oxides ˚ average grain sizes of 250 A. using WO3 as a case study. which is high enough to implicate reflection effects (as below. a graph of absorbance Abs against the charge density consumed in forming a bronze MxWO3 is akin to a Beer’s-law plot of absorbance versus concentration.8). although Abs is never a simple function of the electrochemical charge Q passed over all (especially high) values of x. their electrochromic colour formed per unit charge density is generally weaker.312 The role of defects. 285. Agþ or Mg2þ (here M ¼ ½Mg2þ). As outlined in Chapter 4. i. for ¼ 700 nm. The higher absorbances of evaporated samples arise because the W species will be on average closer within (amorphous) grain boundaries. Eq. While sputtered films are more robust chemically than evaporated films.349 state that the coloration efficiency does depend on the counter ion. as discussed in ref. the electron is localised within a very deep potential well described as a WV polaron or. each with a different apparent extinction coefficient ". the highest values of ) is seen when x is very small (<0.393.397 who proposed three definite types of colour centre in HxWO3.396 Duffy and co-workers393 conducted extensive studies of such Beer’s-law graphs on a range of HxWO3 films made by immersing evaporated (hence amorphous) WO3 on ITO in dilute acid. there are considerable discrepancies in such graphs.393 The higher intensities follow since. 142). The existence of polarons may explain the finding that oxygen deficiency improves the coloration efficiency. possibly. In the middle ground. a The oscillator strength fij is defined by IUPAC as a measure for integrated intensity of electronic transitions and related to the Einstein transition probability coefficient Aij : fij ¼ 1:4992 Â 10À14 ðAij =sÀ1 Þð=nmÞ2 . that resulted in stepwise alteration of oscillator strength or optical bandwidth..e. found only two distinct regions. so a Beer’s-law plot is linear until x is quite large.381 or (0 < x 0. for Hþ or Naþ.03)5. the gradient of a Beer’s-law plot is claimed to decrease with increasing x. at low x.393 and sodium ions394 in evaporated (amorphous) WO3 films. The non-linearity in such Beer’s-law graphs seems not to be due to competing electrochemical sidereactions5 but is. as the extent of electronic delocalisation increases.311 who used sputtered WO3 to form LixWO3. located at defect sites.387 The most intense coloration per electron (that is. rather.6. Beer’s-law plots are linear to larger x values from data for the insertion of Liþ into evaporated therefore amorphous WO3.381.393 This result applies both for the insertion of protons5. Beer’s-law plots showed four linear regions. i. the coloration efficiency for Liþ insertion is asserted to be essentially independent of x. so is smaller.a or a broadening of the envelope of the absorption band owing to differing neighbour-interactions. Structural changes accompanying electro-reduction were inferred. workers such as Batchelor et al. These accord somewhat with views of Tritthart et al.04).395 Only at higher values of x. At one extreme. as a spin-paired (diamagnetic) WV–WV dimeric ‘bipolaron’. attributed to either a decrease in the oscillator strength per electron.2. where is the transition wavelength. for coloration efficiency decreasing as x increases. . Such graphs have a smaller gradient. " in the range 0 < x < 0.2 being higher than when x > 0.2 Metal oxides: primary electrochromes 147 Probably reflecting the preparation-dependence of film properties. will conduction bands start to form as polaron distortions extend and coalesce (as mentioned under Kinetic dependences on x on p. other workers suggest that Beer’s-law plots for thin-film WO3 are only linear for small x values (0 < x 0. At the other extreme.394 Contrarily.04). the Beer’s-law plot is linear (but of low gradient) but increases with an increase in x387.68.3 . 393 and 399. by rf sputtering or by high-temperature annealing of amorphous WO3. For thin films of WO3 prepared by CVD. clearly not a wholly absorptive phenomenon. Sample values of coloration efficiency for WO3 electrochromes.363.373.398 possibly due to specular reflection. x ¼ 0.2 cites some coloration efficiencies .e.g. prepared. Other Beer’s-law plots appear in refs.62.407 For x values at and beyond the insulator/metal transition – i. The wide variations in are no doubt caused in part by monitoring the optical absorbance at different wavelengths.69 Beer’s-law plots are said to be linear for Hþ or Liþ only when the insertion coefficient x is low. Table 6.2. e.66. Preparative route Electrodeposition Thermal evaporation Thermal evaporation Thermal evaporation rf sputtering Sputtering Dip coating Sol–gel a Sol–gel Sol–gel Sol–gel Spin-coated gel Morphology Amorphous Amorphous Amorphous Amorphous Polycrystalline Polycrystalline Amorphous PAA composite Crystalline Crystalline Crystalline Crystalline /cm2 CÀ1 ((obs) in nm) 118 (633) 115 (633) 115 (633) 79 (800) 21 42 (650) 52 38 70 (685) 167 (800) 36 (630) 64 (650) Ref. shows a colour that depends on x. Coloration efficiency decreases at higher x. where x is proportional to charge injected. Optical effects by reflection As recorded in Table 6. the colour of crystalline MxWO3.67. a alternate layers of PAA and WO3. when viewed by reflected light.3.. At low x.148 Metal oxides Table 6. but also result from morphological and other differences arising from the preparative methods.2 or 0. those exceeding ca. but the x value at the onset of curvature was not reported. A wholly different behaviour is exhibited by films of polycrystalline WO3. 400 206 401 206 307 401 402 403 404 405 406 197 349 349 349 Effect of counter cation – all samples prepared by thermal evaporation HxWO3 Amorphous 63 (700) LixWO3 Amorphous 36 (700) Amorphous 27 (700) NaxWO3 PAA ¼ poly(acrylic acid). Colours of light reflected from tungsten oxides of varying insertion extents of reduction x.460 To summarise.428.425.430. the rate and extent of film dissolution decreases as the water content decreases.443. x 0.444. Devices containing tungsten trioxide electrochrome Much device-led research into solid-state ECDs concentrates on the tungsten trioxide electrochrome in.414.449. but failed rapidly owing to film dissolution in the H2SO4 solution employed.432 niobium433 and vanadium (as pentoxide)242.441. 29) significant progress ensued in 1975 when Faughnan et al.331.427 nickel.456.3.419 and display devices.458. Prussian blue).424.6.7.277. .437 or iron (i.13.3 prevents electrochromic reversal.409 alphanumeric watch-display characters.1 0.442 and the organic polymers poly(aniline). probably because its insulator–metal transition is much less distinct.431. Amorphous MxWO3 does not show the same clear changes in reflected colour. the WO3 will be the primary electrochrome owing to the greater intensity of its optical absorption.417. This ECD worked well at short times.447.422.446.440.15. but the rate of coloration also decreases.453 the thiophene-based polymer PEDOT454 and poly(pyrrole). The effect of steadily drying the electrolyte has been studied often.408.4 0.0 Colour Grey Blue Purple Brick red Golden bronze To repeat: x > 0. crystalline WO3 is both optically absorbent and also partially reflective.434.415.455.410 electrochromic mirrors393.416.2–0.412.452.439.8–1.421.459.451.448.426 When the second electrochrome is a metal oxide.6 0.e. for example.354. Thin-film WO3 has also been used in ECDs in conjunction with the hexacyanoferrates of indium436.423.435 as the secondary electrochrome. 418.7 0. In consequence.12.2 Metal oxides: primary electrochromes 149 Table 6.420. depending on preparation – the reflections become ever more metallic in origin.381 published the construction of a device with WO3 in contact with liquid electrolyte (see Chapter 2).450.387.14.438.337 Following Deb’s 1969 electrochromic experiments on solid WO3 (p.413.457 A response time of 40 s is reported for a WO3 film prepared by a sol–gel technique.445.429.329 together with an open-circuit memory in excess of six months. Electrochromic devices of WO3 have been fabricated with the oxides of iridium.411. ‘smart windows’. 466 has made such a solid-state ECD from phosphotungstic acid.295. is employed as insertion ion. both properties unfortunately producing films susceptible to dissolution. This ‘self bleaching’ or ‘spontaneous hydrogen deintercalation’. Other layers used to protect WO3 are described on p.b form crystalline hydrates such as WO3 Á m(SO4) Á n(H2O) which decrease the electrochromic efficiency considerably. the value of was an incremental function of the water content and film porosity. rather than the use of acid. hence very thin or porous. the use of non-aqueous acidic solutions. in dry propylene carbonate. 446. Such liquid-free devices are preferred for their chemical and mechanical robustness. the layer needs to be ion-permeable. For example.460 WO3 films.468 or tungsten oxyfluoride.463 Film dissolution can be prevented by two means.470.467 Ta2O5. WO3 ECDs have been constructed which incorporate solid inorganic electrolytes such as Ta2O5. a non-protonic (alkali-metal) cation. each containing a suitable ionic electrolyte. which is accelerated by aqueous ClÀ. for example. or occasionally LiAlF6 or LiAsF5. anhydrous perchloric acid in DMSO (dimethyl sulfoxide).150 Metal oxides Reichman and Bard461 showed. despite some loss of absorbance with time. Alternatively. although Tell465.464 or. claiming a of 10 ms (but for an unspecified change in absorbance).472 has been studied often:15. CVD-prepared WO3 returned to its initial transparency b The reaction of acid with WO3 prepared by anodising W metal is found to be kinetically first order with respect to acid.474 in one study. in aqueous sulfuric acid as ECD electrolyte. that the electrochromic response time was faster with the anodically grown material because it is microscopically porous. In solid-state WO3 devices. Tungsten trioxide in aqueous acidic electrolytes is more durable if the electrochrome–electrolyte interface is protected with a very thin over-layer of NafionTM. Furthermore. see Section 14.469 although charge transport through such layers will be slower.471 Clearly. 473.2 for further detail. a layer of gold enhances the response time and also protects against chemical degradation. Such cells have slower response times and also a poorer open-circuit memory.462 . the stability of the electrochromic colour is generally good. for the electrochromic processes of WO3 on samples prepared by either anodic oxidation of tungsten metal or by vacuum evaporation onto ITO. poly(AMPS) or poly(ethylene oxide) – PEO.460 and zeroth order with respect to film thickness. Other over-layers can speed up the electrochromic response. Examples include films of WO3 immersed in lithium-containing electrolytes such as LiClO4. or organic polymers such as poly(acrylic acid). usually lithium. lithium triflate (LiCF3CH2CO2). Chemical . which is blue. powdered MoO3.480) Nevertheless. which could be a result of differing conditions such as the sputtering geometries.483 are employed.85 and 0.275. Controlling the flow and composition of the atmosphere dictates the composition and structure of the final electrochrome. c (1–c) where c can be as high as 0. electrochemical and mechanical properties and flow rate are complex.23.479 Sputtering yields polycrystalline material.480 and clearly different from the desired ‘bronze’. a widely used precursor is prepared by oxidative dissolution of molybdenum metal in hydrogen peroxide solution.209 In rf sputtering.275. although the relationship(s) between optical.3. of sub-stoichiometric films improve after about five colour/bleach cycles in a LiClO4/PC electrolyte.274.209 Gorenstein and co-workers found481 that blue sputtered ‘MoO3’ forms particularly at low fluxes of ionised Arþ. however. (Dualtarget sputtering is twenty times faster than single-target deposition.481. the electrochromic properties.476.477 by anodic oxidation of molybdenum metal immersed in e. suggesting that adsorbed water in the films reacts with the coloured LixWO3 to form LiOH and molecular hydrogen. In rf sputtering a target of metallic molybdenum and low-pressure Ar þ O220.20 The best films were made with a low rate of oxygen flow that gave a sub-stoichiometric oxide.21.20. Granqvist and co-workers209 show that substoichiometric blue ‘MoO3’ forms at deposition rates up to 1.5 nm sÀ1.92.2 Molybdenum oxide Preparation of molybdenum oxide electrochromes Molybdenum trioxide films may be formed with amorphous or polycrystalline morphologies. The product of dc magnetron sputtering is of good quality and colourless. respectively. particularly in bleaching. being in fact substoichiometric23.91. MxMoO3. 6. over-rapid rates of deposition can yield oxygen-deficient material.475 Deb and co-workers474 have also investigated the chemistry underlying the self bleaching of evaporated WO3 on ITO.482 with composition MoVIMoV O(3Àc/2). that causes amorphous material to crystallise.484 The flow rate and hence the exact composition have a profound effect on the optical properties of the film.273.2 Metal oxides: primary electrochromes 151 after only three minutes.20.313.478 or deposited electrochemically. Amorphous material can be formed by vacuum evaporation of solid.481.209. This needs to be roasted in an oxidising atmosphere.485 Chemical vapour deposition also yields polycrystalline material from an initial deposit of usually finely divided metal.g. whereas clear MoO3 requires a deposition rate of about 0. acetic acid.1 nm sÀ1 for films made with dual-target and single-target sputtering.2.6. above.19.9) Whittingham492 considers Hþ mobility in layered HxMoO3(H2O)n but many workers prefer to insert lithium ions Liþ.486 Molybdenum trioxide films derived from sol–gel precursors are also polycrystalline as a consequence of high-temperature annealing after deposition. solid phosphomolybdic acid is also found to be electrochromic. and therefore on the annealing. Films heated to 250 8C comprise a disordered mixture of orthorhombic a-MoO3 and monoclinic b-MoO3 phases.480.232 Thermal oxidation of thin-film MoS3 also yields electrochromic MoO3.490 Finally.477 Equation (6. (6. but smaller than for WO3. as well as the expected valence states of MoV and MoVI.491 suggest that electrodeposited films of MoO3 on ITO are completely amorphous if not heated beyond about 100 8C.487 resulting from oxidative dissolution of metallic molybdenum in hydrogen peroxide.9) is over-simplified because MoIV appears in the XPS of the coloured bronze. so little detail will be given here.275. McEvoy et al. annealing amorphous MoO3 causes crystallisation.VIO3. Crystallisation to form the thermodynamically stable a phase occurs at temperatures above 350 8C. in the electrochromic reaction Eq. spraying aqueous lithium molybdate at low pH onto ITO.20. giving voltammetry which is ‘complicated’. from anhydrous solutions of salts such as LiAlF6 or LiClO4 in PC. The most common precursor is a spin-coated gel of peroxopolymolybdate189. There is a considerable literature on the electrochemistry of thin-film MoO3. itself deposited on a copper substrate489 by electron-beam evaporation. The electrochromic behaviour of the films depend on the extent of crystallinity.9): MoVIO3 þ x(Hþ þ eÀ) ! HxMoV.51. Such films are claimed to show a superior memory effect to sputtered films of MoO3.488 Other sol–gel precursors include alkoxide species such as190 MoO(OEt)4.466 Redox chemistry of molybdenum oxide electrochromes The electrochromism of molybdenum oxide is similar to that of WO3.209. colourless intense blue (6.275 . As with WO3.488 while Sian and Reddy preferred Mg2þ as the mobile counter cation.152 Metal oxides vapour deposition precursors include gaseous molybdenum hexacarbonyl62 or organometallics like the pentacarbonyl-1-methylbutylisonitrile compound. The dark-blue coloured form of the electrochrome is generated by simultaneous electron and proton injection into the MoO3. Films have also been made by spray pyrolysis. 493 interpret their ellipsometric study of the proton injection into MoO3 as showing two distinct insertion sites for the mobile hydrogen ion within the reduced film. since HxMoO3 films oxidise more slowly than do films of HxWO3 having the same value of x. leaving a coloration range of about 0.495 presumably because the precious metal helps minimise the effects of IR drop caused by the poor electronic conductivity across the surface of the MoO3.5 Also. Coating the MoO3 with precious metal also decreases the extent of oxide corrosion.924.4 V prior to formation of molecular hydrogen. The electrochromism of molybdenum oxide is enhanced when coated with a thin. the optical absorption spectrum of HxMoO3 is very similar to that of HxWO3 (e.10): 2Hþ (aq) þ 2eÀ ! H2 (g).2 Metal oxides: primary electrochromes 153 Some oxygen deficiency can complicate spectroscopic analyses:275 evaporated MoO3 films. colourless when deposited.4 V (against the SHE). (6.4) except that the wavelength maximum of HxMoO3 falls at shorter wavelengths than does max for HxWO3. In appearance. protons enter the molybdenum films at potentials more cathodic than þ0. nevertheless give an ESR signal characteristic of MoV at23 g ¼ 1.495 perhaps similarly to protecting WO3 with a thin film of gold470 or tungsten oxyfluoride. as in Eq. implying faster electrochromic operation. the chemical diffusion coefficients D of Hþ through MoO3 are faster than through the otherwise similar WO3. This band . There is a readily observed. the gas possibly forms catalytically on the surface of the bronze.496 Optical properties of molybdenum oxide electrochromes An XPS study476 shows that the colour in the reduced state of the film arises from an intervalence transition between MoV and MoVI in the partially reduced oxide. Molybdenum bronzes HxMoO3 show an improved open-circuit memory compared with the tungsten bronzes HxWO3.g. WO3. (6. about 0. implying a somewhat different mechanism for electroreduction. cf.6. 20 nm.10) The corresponding range for HxWO3 is larger. perhaps in contrast to WO3. see Figure 6. well-defined boundary between the oxidised and reduced regions within the oxide. The wavelength maximum of the partly reduced oxide is centred at23 770 nm.5 Ord and DeSmet478.5 V.288. transparent film of Au or Pt.5 Additionally beneficial. The XRD study by Crouch-Baker and Dickens494 suggests that hydrogen insertion proceeds without the occurrence of major structural rearrangement in the bulk of the oxide film. but shifts to %390 nm for the coloured reduced film. J. Kitao.292 but moves to shorter wavelengths as x increases.e. M.0 3.476 The ‘apparent coloration efficiency’ for partly reduced molybdenum oxide is therefore slightly greater than for partly reduced tungsten trioxide since the absorption envelope coincides more closely with the visible region of the spectrum. Table 6. (2) 490. Kuwabara et al. 1984. 1624–7.’ Jpn. MoO3 and MocW1ÀcO3. M.154 Metal oxides 1..0 Absorbance 4 3 0. The absorption edge of MoO3 occurs at476 385 nm. and Yamada. with permission of The Institute of Pure and Applied Physics. see Figure 6.) is clearly not of simple origin. 23. the value of max for HxMoO3 is not independent of x.498 made several cells of the form WO3j tin phosphate jHxMoO3. Phys.5 5 1.0 Photon energy (eV) Figure 6.5. no discernible change in absorbance would occur during device operation. The optical constants n and k of thermally annealed MoO3 (i..4 UV-visible spectrum of thin-film molybdenum oxide for various amounts of inserted charge: (1) 0.5 2 1 0 0.0 1. (Figure reproduced from Hiruta. (4) 2200 and (5) 3800 mC cmÀ3. 625 nm.0 2.23 but comprises a collection of discrete bands having maxima at around 500 nm. amorphous MoO3 that was formed by thermal evaporation but then roasted) depend quite strongly on the annealing temperature. Y. (3) 1600. For example.4 contains a few representative values of coloration efficiency . and 770 nm. Appl.497.275 Unlike HxWO3. ‘Absorption bands of electrochemically-colored films of WO3. The solid electrolyte layer is opaque: otherwise. Devices containing molybdenum oxide electrochromes Devices containing MoO3 are comparatively rare. The response times of ECDs may be enhanced by depositing an ultra-thin layer of platinum or gold on the . 505. ‘Absorption bands of electrochemically coloured films of WO3.’ Jpn. by permission of The Japanese Physics Society.2 Metal oxides: primary electrochromes 155 Table 6.501. 1984.5 (700) 35 (634) Ref. M. which is proportional to the hydrogen content. 4 of Hurita.499. Preparative route Thermal evaporation of MoO3 Evaporation of Mo metal in vacuo Oxidation of thin-film MoS3 /cm2 CÀ1 ((obs)/nm) 77 19.) electrolyte-facing side of the electrochrome. J.500. x.503. 23. where is the frequency maximum of the intervalence band) for the reduced oxides HxMoO3 as a function of the electrochemical charge inserted.5 1.6. Kitao.5 Plot of E (as E ¼ h. The second major class are ‘sputtered iridium oxide films’ (‘SIROFs’).506.502.4. Sample values of coloration efficiency for molybdenum oxide electrochromes. 1624–162.0 0 1000 Q i (mC cm–2 ) 2000 Figure 6. Y. (Figure redrawn from Fig. The anodically grown films464.509 are made by the potentiostatic cycling between À0. and Yamada.0 Ep(eV) 1.2. W.495 As with WO3. Appl.504.25 V and þ1. Phys.507. Qi.. MoO3 and MocW(1–c)O3. 7 482 490 2. electrochemical deposition to form an ‘anodic iridium oxide film’ (‘AIROF’ in a jargon abbreviation). 6..3 Iridium oxide Preparation of iridium oxide electrochromes There are now two commonly employed methods of film preparation: firstly.25 V (against SCE) of an . clearly the layer of precious metal must be permeable to ions.508. 239 and iridium oxide films have been prepared by sputtering metallic iridium onto an OTE in an oxygen atmosphere.506 Hydrogen may also be added.118.499. Anodic iridium oxide films can also be generated by immersing a suitable electrode (e. while blue SIROFs give superior films which may be transformed to a truly colourless state. unlike AIROFs. and decreases by about 8% per day.e. decreased response times are observed since ionic insertion is slowed. the electrochromic activity of an AEIROF increases as the proportion of water in the electrolyte increases.e. anodically electrodeposited iridium oxide films. and a longer open-circuit memory. The reliability of AIROFs is apparently variable. the electrochromic activity decreases as the anneal temperature increases.156 Metal oxides iridium electrode immersed in a suitable aqueous solution.504 such response times are considerably faster than for WO3 or V2O5 films of similar thickness and morphology. The second method of forming films is reactive sputtering in an oxygen– argon atmosphere (the respective partial pressures being 1:4).506 Blue SIROFs have superior response times to black SIROFs.118.g. Conversely. In fact. can be made with oxygen alone as the flow-gas during the sputtering process.511. A denser SIROF.509 Sol–gel methods also yield polycrystalline iridium oxide.510 They have a CR as high as 70:1 which forms within ¼ 20 to 40 ms. Anodic iridium oxide films degrade badly under intense illumination.) Once formed and dried. and start from a sol formed from iridium trichloride solution in an ethanol–acetic acid mixture. that forms a black electrochromic colour. if annealed. blue SIROFs and AIROFs are again similar. These black SIROFs are deposited as coloured films which can be decolorised by up to 85% on cycling. i.514 The electrochromically generated colour of a SIROF is only moderately stable. Such AIROFs are largely amorphous.512 The solution must also contain hydrogen peroxide and oxalic acid.153. (Following the usual desire for acronyms. blue SIROFs are very similar to AIROFs in being totally decolorisable. ITO) into an aqueous solution of iridium trichloride. in terms of write–erase response times and absorbance spectra. electrochromic films are formed on ITO when g-rays irradiate solutions of iridium chloride in ethanol. Sputtered iridium oxide films have a complicated structure which.511 Extremely porous films of iridium oxide can be prepared by thermally oxidising vacuum-deposited iridium–carbon composites. is not macroscopically porous.515 .513 Such films are grey–blue in the coloured state with max ¼ 610 nm. such films are now designated as ‘AEIROFs’ i. Furthermore.505 Finally.16 sometimes a serious disadvantage. cyclic voltammetry confirming the similarity. Beni and Shay506 view the blue SIROFs as aesthetically the more pleasing. (6. the mechanism of coloration is still uncertain. .523 have investigated AIROFs using potential-modulated reflectance tentatively to assign the peaks in the cyclic voltammetry of anodic films of iridium oxide to the various redox processes occurring.517 Regardless of whether the mechanism is hydroxide insertion or proton extraction.507 Eq.11) which is confirmed by probe-beam deflection methods. the optical and electrochemical data both suggest that conversion occurs in two distinct stages: Rice519 suggests that a satisfactory model requires the recognition that AIROFs act as a conductor of both electrons and anions during the electrochromic reaction. e. (6. Ir(OH)3 is the bleached form of the oxide and the coloured form is IrO2. While protons are ejected from AIROFs during oxidation. Ellipsometric data518 suggest little hysteresis during redox cycling. Unlike other metal oxides.516 their electrochromic behaviour is independent of the pH of the electrolyte solution.503 Eq.520 The participation of the electrons. and the sudden change in electrochromic rate. In fact. Equation (6.11): Ir(OH)3 ! IrO2 Á H2O þ Hþ(soln) þ eÀ. (6.12) While XPS measurements500 seem to confirm Eq. SCE).516 The second involves anion insertion.11). neither the coloration nor bleaching reactions proceed by movement of an interface between oxidised and reduced material traversing a ‘duplex’ film. colourless blue–grey (6.g.2 Metal oxides: primary electrochromes 157 The redox chemistry of iridium oxide electrochromes In aqueous solution.11) probably applies only to aqueous electrolytes.522 Gutierrez et al.12) is not without question: some workers507 assert that AIROFs will colour when oxidised while immersed in solutions containing the counter ions of507 FÀ or CNÀ. (6.12): Ir(OH)3 (s) þ OHÀ(soln) ! IrO2 Á H2O (s) þ H2O þ eÀ.464 so the reaction (6. may correlate with the occurrence of a non-metal-to-metal transition between 0 and 0.501 suggesting that both protons and hydroxide ions are involved in the electrochromic process.6.234 so two different reactions are current. AIROFs do not colour in anhydrous acid solutions. which helps explain the relatively low faradaic efficiency in dilute acid. the optical constants during reduction retracing the path followed during oxidation.521 Phase changes in iridium oxide are discussed by Hackwood and Beni.502. The first is described in terms of proton loss.12 V (vs. Nor does redox conversion proceed with a singlestage conversion of a homogeneous film. others disagree. HClO4 in anhydrous DMSO. 9 λ (μm) 1. J.6 shows an absorbance spectrum of thin-film iridium oxide sputtered onto quartz.158 Metal oxides Optical properties of iridium oxide electrochromes Figure 6. by permission of The Electrochemical Society. .527 6 Coloured state 26 mC cm–2 5 Bleached state 4 Absorbance 3 2 1 0 0. The broken line is the reduced (uncoloured) form of the film and the continuous line is the spectrum following oxidative electro-coloration with 26 mC cmÀ2. K. for spray-deposited oxide depends strongly on the annealing temperature. L. 1983.153 There are relatively few coloration efficiencies in the literature: for an oxide film made by thermal oxidation of an iridium–carbon composite525.7 Figure 6. and Shay. S. J.3 0. Electrochem.524 The change in transmittance of crystalline Ir2O3 films made by sol–gel techniques is larger than that of the amorphous Ir2O3 under the same experimental conditions. Soc.1 1. À38 cm2 CÀ1 at 500 nm and À65. Optical study of the electrochromic transition of AIROFs is greatly complicated by anion adsorption at the electrochrome–solution interphase.5 cm2 CÀ1 at 600 nm. (Figure reproduced from Kang.3 1.). Inc. ‘Blue sputtered iridium oxide films (blue SIROF’s)’. 766–769. The AIEROF film511 is characterised by of À22 cm2 CÀ1 at 400 nm.118.5 1.5 0..526 is quite low at À(15 to 20) cm2 CÀ1 at a max of 633 nm.6 UV-visible spectrum of thin-film iridium oxide sputtered onto quartz.7 0. 130.512 varying from À10 cm2 CÀ1 at 630 nm for films annealed at 400 K to À26 cm2 CÀ1 for films annealed at 250 K. When a voltage of 1. Electrochromic cells containing iridium oxide generate colour rapidly: the cell SnO2jAIROFjfluoridejAu develops colour in 0. For this reason. the maximum colour formed in about 1 second.531 The reaction at the counter electrode is unidentified. Solid-state AIROFs have been made with polymer electrolyte. the electrochromic colour of the two AIROF layers would change in a complementary sense. but colourless when reduced. ‘ox-AIROF’ being one oxide film in its oxidised form while ‘red-AIROF’ is the second film in its reduced form.509 Clearly.4 Nickel oxide Much of the nickel oxide prepared in thin-film form is oxygen deficient. both layers colour in a complemetary sense as charge is decanted from one electrochrome layer to the other.530 A composite device based on iridium oxide and poly(p-phenylene terephthalate) on ITO shows different electrochromic colours: blue–green when oxidised. The extent of deficiency varies according to the choice of preparative route and deposition parameters. This cell is described in detail in ref. one layer contains ionic charge.2.1 second (where ‘fluoride’ represents PbF2 on PbSnF4).5 V was applied across the cell.509 Ishihara529 used iridium oxide in a solid-state device in which reduced chromium oxyhydroxide was the source of protons migrating into the electrochrome layer. 508.2 Metal oxides: primary electrochromes 159 Electrochromic devices containing iridium oxide electrochromes Thin-film iridium oxide was one of the first metal-oxide electrochromes to be investigated for ECD use. but these have slower response times. Anodic iridium oxide films are superior to WO3-based electrochromes since they do not degrade in water but retain a high cycle life (of about 105) even in solutions of low pH.507 provided the temperature remains low:507 the bleached form of iridium oxide decomposes thermally above about 100 8C. with the overall result of almost negligible modulation. the Nafion1 containing an opaque whitener against which the coloration was observed. On fabrication.528 Another ECD was prepared with two iridium oxide films in different oxidation states.427. the device can only operate when initially one iridium layer is oxidised and the other reduced.532 6. Other ECDs have been made with sputtered IrOx as the secondary electrochrome and WO3 as the primary layer. otherwise.508 The cell fabricated was ‘ox-AIROFjNafion1jred-AIROF’. ‘nickel oxide’ is often written as NiOx or NiOy where the symbols x or y indicate oxygen non-stoichiometry.6. We prefer . g.86.552 Electrodeposition of thin-film nickel oxide is more widely used.548 A cathodic-arc technique also yields NiO(1Ày) if metallic nickel is sputtered in vacuo in an oxidising atmosphere. Thin films of nickel oxide electrochromes are usually made by sputtering in vacuo.4) describe c In the backscattering experiment.215.253. by the dc-magnetron211.255.541 Addition of gaseous hydrogen to the sputtering chamber has profound effects on the optical properties of the resultant films. nickel oxide of composition NiO(1þy).550 Thermal vacuum evaporation seems a poor way of making NiO(1þy) films since the electrochrome readily decomposes in vacuo to yield a material with little oxygen.256 Excess oxygen at grain boundaries enhances the extent of electrochromic colour. by Natarajan et al.215. the backscattered energy is high – almost as high as the incident energy.251.535 on electrochromes made via sol–gel methods.160 Metal oxides an alternative notation and denote oxygen non-stoichiometry by NiO(1þy) Hz when hydroxyl is a ligand.141 examining samples made by spray pyrolysis.540 but a nickel target and a relatively high partial pressure of oxygen is also common.211. .e.215.212. by Jiang et al.214.82.214.256 Rutherford backscatteringc suggests that rf-sputtered NiO is rich in oxygen.536 probing the stability of electrodeposited samples.537 or rf-beam techniques.248.546.212. and by Kamal et al.254.544 or pulsed laser ablation. Preparation of nickel oxide electrochromes There is a large literature on making thick films of nickel oxide owing to its use in secondary batteries. The majority of alpha particles remain embedded in the sample.253. the backscattered energy is low.213. 249. i. from solutions of aqueous nickel nitrate. alpha particles typically possessing energies of several MeV are fired at a thin sample. Nevertheless.551.250.216.533 One of the principal difficulties in making thin-film nickel oxide is its thermal instability: heating an oxide film can cause degradation or outright decomposition.544.211.549 e.251 studying the effects of annealing rf-sputtered NiO(1Ày).548. For heavy target atoms such as tungsten. The energy with which they backscatter relates to the mass of the target element.545. otherwise for hydroxyl free species.538.553 Equations (6.g.547.534.3) and (6.247.542 Other films of NiO(1Ày) are reported via electron-beam sputtering.539 A target of solid LiNiO2 generates a pre-lithiated film. this technique is reported to generate NiO(1þy) films satisfactorily. from a target of compacted LiNiO2 powder.214.252.256.546.252. e.254.255.. but a small proportion scatter from the atomic nuclei in the near surface (1 to 2 mm) of the sample. The thermal stability of thin-film nickel oxide is the subject of several investigations: by Cerc Korosˇ ec and co-workers148.539 The target is usually a block of solid nickel oxide. but for lighter target atoms such as oxygen.543. by NiO(1þy). Analysis of the backscattering pattern enables Rutherford backscattering (RBS) to measure the stoichiometry of thin films. g. 536. Dip-coating has also been used: electrodes are immersed repeatedly into a nickel-containing solution.559. in part because the necessary annealing can damage the films. Other aqueous electrodeposition solutions include an alkaline nickel–urea complex. Such precursors are often termed a ‘xerogel’. the precursor film on the electrode is heated to effect dehydration.151 Again.5) by annealing.e making colloidal) the resulting green precipitate with glacial acetic acid. an (uncharged) conducting electrode may be dipped alternately in solutions of aqueous NiSO4 and either NaOH563 or NH4OH. e.148. NiO films have been made by plasma oxidation of Ni–C composite films.557 nickel diacetate.567 Chemical vapour deposition is not a popular route to forming NiO(1þy).554 nickel diammine554.569 Redox electrochemistry ‘Hydrated nickel oxide’ (also called nickel ‘hydroxide’) is an anodically colouring electrochrome.558 [Ni(NH3)2]2þ or nickel sulfate. (6.6.568 Finally.566. from a precursor of aqueous nickel chloride solution. silica gel). Conversely.555.2 Metal oxides: primary electrochromes 161 the reactions that form the immediate oxyhydroxide product NiO(OH)z.149 Precursors of nickel bis(2-ethylhexanoate)562 or NiCl2 in butanol and ethylene glycol151 have been employed in spin coating prior to thermal treatment to effect dehydration and crystallisation.565. Precursors include nickel acetylacetonate. although the sols are not completely desiccated.534. previously deposited by co-evaporation of Ni and C from two different sources.560 In all cases.g. although the resulting solid film is not durable. which can be dehydrated according to Eq. Electrochromic films have been made via sol–gels derived from NiSO4 with formamide and PVA.560 albeit by an unknown deposition mechanism. Electrodeposition from a part-colloidal slurry has also been achieved. like buffered NiF2.d Additional water is added to ensure an appropriate viscosity prior to dipping.152 or nickel diacetate dimethylaminoethanol. perhaps again owing to the need for annealing.567 to have been made by spray pyrolysis.17.563 NiSO4 in water564 or polyvinyl alcohol. the redox now differing in direction from that with the d A xerogel is defined by IUPAC as. ‘the dried out open structures which have passed a gel stage during preparation (e.152 or NiCl2 in butanol and ethylene glycol. chemical oxidation and crystallisation. sol–gels have been made by adding LiOH drop-wise to NiSO4 solution until quite alkaline.535 then peptising (i. Electrochromes are also reported141.’ .561 Fewer sol–gel films of nickel oxide electrochrome have been made.556. Ni2O4. NiII NiIIIO(1þy)Hz.39 Lampert.. Bouessay et al. Furthermore. Some workers have detected NiIV in the oxidised form of electrochromic NiO(1Ày) films. g2-2NiO2–NiOÁ OH.17 believe the coloured form is blue.86 They also suggest that only a minority of the film participates in the electrochromic reaction. (6. Murphy and Hutchins572 suggest that the following nickel species: Ni3O4.253 Svensson and Granqvist253 conclude that the bleached state in a nickel oxide based display is b-NiO Á OH.576 Gorenstein and co-workers577 and Granqvist and co-workers. Additionally. the bleached state is Ni3O4 and the coloured form is Ni2O3. and the coloured state is b-Ni(OH)2. b-Ni(OH)2 and b-NiOÁ OH. concur in this assignment. Scrosati and co-workers578 suggest the electronic conductivities of the coloured and bleached states (which are said to differ dramatically) play a major role in the electrochromic process. the sub-stoichiometric ‘NiO(1Ày)Hz’ is. anodic coloration occurs in two distinct stages. Proton egress from rf-sputtered NiO(1Ày)Hz is more difficult than entry to the oxide. The kinetic . Conell et al. in reality.253 Furthermore. 570.216. Equation (6.574 in 1992.572 Gorenstein.13) are unknown and likely to depend on the pH of the electrolyte solution.13): NiII Oð1ÀyÞ Hz ! ½NiII ð1ÀxÞ NiIII x Oð1ÀyÞ HðzÀxÞ þ xðHþ þ eÀ Þ Á colourless brownÀblack (6:13) Nakaoka et al.575 Cordoba-Torresi et al. (6.13) is an amended form of the reaction in ref. The complex structures and phase changes occurring during the redox cycling of ‘nickel oxide’ were reviewed by Oliva et al.571 The mechanism is different in alkaline solution: Murphy and Hutchins572 cite the simplified reaction in Eq. although the rate of ion movement dictates the overall kinetic behaviour of nickel oxide based films. The problem of mass balance in thin-film ‘nickel oxide’ has been described ´ in great detail by Bange and co-workers. In acidic media. the electrode reaction for nickel oxide follows Eq.572 Chigane et al.14). the reduced form of the oxide contains a small amount of NiIII: a startling result. In this analysis. NiðOHÞ2 ðsÞ þ OHÀ ðaqÞ ! NiO Á OHðsÞ þ eÀ þ H2 O: (6:14) Granqvist and Svensson believe that 15N nuclear reaction analysis (see page 110) shows that coloration is accompanied by proton extraction. Ni2O3 and NiO2 are all involved.85.11 Giron and Lampert. The (1Àx) values of y and z in Eq.555 cite the involvement of: a-Ni(OH)2.162 Metal oxides preceding metals. Furthermore.573 suggest that conversion of NiO into Ni(OH)2 is a major cause of device degradation. (6. 15): NiOð1þyÞ þ xðLiþ þ eÀ Þ ! Lix NiOð1þyÞ . has been analysed in terms of microstructure. and therefore be more or less well suited for technical applications. for it promotes chemical degradation. the amount of water in the solid film increasing with cycle life. quite sensitive to the method of film preparation.43 Optical properties of nickel oxide electrochromes The electrochromic colour in NiO(1þy) undoubtedly derives from an NiIII/ NiII intervalence transition. Furthermore. (6.542 the value depends strongly on the sputtering . Hþ being the first counter cation to enter the lattice. with subsequent insertion of Liþ. As Granqvist et al. they believe that neither Ni(OH)2 nor NiO Á OH are beneficial to device operation because of their solubility in water. morphology and stoichiometry by Gorenstein and co-workers. as prepared by rf sputtering.11 The tendency for water to cause deterioration is such that many workers now avoid water and hydroxide ions altogether. and prefer non-aqueous electrolytes. Electrochromic nickel-oxide based films produced by different types of sputtering.216 say. incontrovertibly.7 shows absorption spectra of nickel oxide. brownÀblack colourless (6:15) the mobile Liþ ion most commonly coming from LiClO4 dissolved in a polymeric electrolyte.581 and ´ Cordoba-Torresi et al. The efficiency of this electrochromic oxide. though. evaporation.578 Detailed measurements with the electrochemical EQCM suggest cation swapping. not unusually.g. Figure 6. Water trapped preferentially at defect and grain boundaries (which are numerous in NiO(1þy)250) plays a crucial role in the electrochromic reaction.49 Even at quite low potentials. For example.18 There are wide variations reported in the values of coloration efficiency .11 in support say that the presence of lattice defects is a prerequisite for electrochromic activity. although is said to be À36 cm2 CÀ1 at 640 nm for nickel oxide made by rf sputtering.2 Metal oxides: primary electrochromes 163 ´ behaviour is described further by MacArthur579 and by Arvia and co-work580 ers The mechanism is. e. The reaction cited for electrochromic activity is then Eq. electrochemical and durability related properties. anodic oxidation and cathodic deposition. Water is formed as a product of NiO(1þy)Hz degradation.6. [and] thermal conversion all can have different optical. Its role is not beneficial. the rate of electrochromic coloration and bleaching is dictated by the rates of ionic movement. M.584 The speed of electrochromic operation often depends on so-called ‘terminal effects’ that arise because optically transparent conductive layers such as ITO have only modest electronic conductivities. Coloration/bleaching times of electrodeposited films range between 20 and 40 s. Soc. but nevertheless. Other values of are cited in Table 6.5..585 . Depositing an ultra-thin layer of metallic nickel between the ITO and NiO layers significantly improves the response time .) conditions. (Figure reproduced from Carpenter. and depend on the applied potential. This value of was cited for a film obtained at a total pressure of 8 Pa. K.164 1. D. by permission of The Electrochemical Society.0 Absorbance 0. and Corrigan. J. 136.1022–6. A.252 Electrochromic devices containing nickel oxide electrochromes Films made by rf sputtering are significantly more durable than those made by electrodeposition: Conell cites 2500 and 500 write–erase cycles for the respective preparations.7 UV-visible spectrum of reduced (Á Á Á Á) and oxidised (–––) forms of thin-film nickel oxide on ITO.540 show that such sputtered NiOx degrades relatively easily. Electro-coloration was performed with the film immersed in 0.5 0 200 400 Wav elength (nm) 600 800 Figure 6.5 Metal oxides 1. of which gaseous hydrogen accounted for 40%. The film was electrodeposited onto ITO with a thickness of about 1 mm. 1989.211 Corrigan82 reports that the durability can be improved to thousands of cycles by incorporating cobalt or lanthanum.1 mol dmÀ3 KOH solution.86 Xu et al. a value of À10 cm2 CÀ1 is cited for thin-film lithium nickel oxide deposited by rf sputtering from a stoichiometric LiNiO2 target. Ushio et al. suggest that 105 cycles are possible for dc magnetron sputtered samples. Electrochem. Inc. ‘Photoelectrochemistry of nickel hydroxide thin films’. while for the usual two-electrode ECD it would generally be advantageous that both electrodes bear strongly colourant electrochromes.544. glycerol. The counter electrode would bear a feebly colouring secondary electrochrome.1. 6.587.310. much of the interest in nickel oxide electrochromes is focussed on their use as secondary electrochrome (i. bears the primary electrochrome). Preparative route CVD (from a nickel acetylacetonate precursor) dc sputtering Dipping technique Electrodeposition Electrodeposition Electrodeposition rf sputtering Sol–gel (NiSO4.431. not the main colourant) on the counter electrode.1. PVA and formamide) Sol–gel (NiSO4.588 or poly(pyrrole).42. 254.5 (450) À80. 568 537 564 78 582 560 542 152 583 107 565 566 551 At present. or even a non-colouring (passive) redox couple.248.589 poly(thiophene)590 or poly (methylthiophene).6. as redox reagent on the second electrode in an ECD cell where a primary electrochrome is redox reagent on the other electrode.3 Metal oxides: secondary electrochromes 165 Table 6. either of the latter being chosen simply for superior electrochemical properties.e.3 (457) À37 À30 À32 (670) Ref. in some prototype ECDs.3. PVA and formamide) Sonicated solution Spray pyrolysis Spray pyrolysis Vacuum evaporation /cm2 CÀ1 (/nm) À44 À25 to 41 À35 À20 % À50 (450) À24 (670) À36 À35 to 40 (450) À23.e.548.591 MnO592 or SnO2560 acted as secondary electrochrome.) .49. that attend all redox reactions. even invisible _ IR and/or UV _ changes.1 Introduction As outlined in Section 1. are nowadays being deemed ‘electrochromic’. i. (‘‘Electrochrome’’ here is not a misnomer because as has been established in Section 1. stability and durability. final conditions may dictate that one electrode provides the major colourant (hence. Sample values of coloration efficiency for nickel oxide electrochromes.3 Metal oxides: secondary electrochromes 6.590 However. NiO was the primary electrochrome on the one electrode while on the other.5.149. CuO. This chapter covers the latter classes of ‘electrochrome’. Primary electrochromes so partnered could be WO3.586. 166 Metal oxides However. such as glass.597 The utility of cerium oxide derives from its near optical passivity. The response time either way was about 10 s.595 made cerium oxide films on fluoride-doped SnO2 electrodes using a sol–gel procedure.137 Films prepared at temperatures below about 300 8C were amorphous. (6.596 deposited the oxide by electron-beam PVD (physical vapour deposition) on various substrates. and therefore ideal as counter electrodes in transmissive ECDs.7 cm2 CÀ1. (6. Films showed an electrochromic transition when immersed in LiClO4–propylene carbonate electrolyte. Si wafers and fused silica. Porqueras et al. The precursor derived from cerium ammonium nitrate in ethanol. ¨ Ozer et al. while those prepared at higher temperatures have a cubic (‘cerianite’) crystal structure. this chapter covers only visible-wavelength ECD applications. so the materials encompassed are chosen largely just to complete the electrochemical cell that operates as an ECD by depending on the primary-electrochrome process. The substrate temperature was maintained at 125 8C. with diethanolamine as a complexing agent.184.16): Bi2 O3 þ xðLiþ þ eÀ Þ ! Lix Bi2 O3 : transparent dark brown (6:16) Bleaching ocurred at þ1. SCE. ITO-coated glass. In contrast. ion-bombarded films show a denser structure and a different layer growth. Eq. Bismuth oxide has also been co-deposited with other oxides. They recommend annealing at 450 8C or higher. with coloration efficiency of 3.17): .594 Cerium oxide Preparation of cerium oxide Thin-film CeO2 can be prepared by spray pyrolysis via spraying aqueous cerium chloride (CeCl3 Á 7H2O) onto ITO.0 V vs.593 The best electrochromic performance was observed for a sputtered oxide annealed at 300–400 8C in air for 30 min. Spectroelectrochemistry showed that these films were optically passive.2 V and coloration at À2. Secondary electrochromes Bismuth oxide An electrochromic bismuth oxide formed by sputtering or vacuum evaporation was studied by Shimanoe et al. The redox reaction follows Eq. 6.598 The composition of the material is nowhere mentioned.237. The properties of a sputtered oxide are described as ‘only slightly inferior to those of Ni oxide and with good stability in acidic electrolytes’.601.236. Goldner et al.137. thin films of chromium oxide.231 Cobalt oxide Preparation of cobalt oxide electrochromes Thin-film LiCoO2 is made by rf sputtering from a target of LiCoO2. electrochromic.137 Cerium oxide is therefore not electrochromic. Chromium oxide The electrochromism of chromium oxide has received little attention. Porqueras claims that films on ITO remain ‘fully transparent after’ Liþ insertion and egress. and is polycrystalline.238 such nominal ‘LiCoO2’ shows significant absorption at < 600 nm.231 The electrochromic operation was studied with films immersed in g-butyrolactone containing LiClO4. Granqvist and co-workers598 immersed films made by rf sputtering in aqueous H3PO4. but is a widely-used choice of counter electrode material.599 are said to be similar.238 state that films can be coloured electrochemically.596. In a fundamental study.600. and are called ‘lithium chromate’. but will not decolour completely.184. can be formed by electron-beam evaporation of Cr2O3.18): Cr2 O3 ðsÞ þ xðLiþ þ eÀ Þ ! Lix Cr2 O3 ðsÞ: (6:18) Chromium oxide allows device operation with a lower voltage than do most other electrochromic oxides.602 with the redox reaction Eq. the sputtered materials made by Cogan et al.4. (6. The electrochromic colour did not vary by more than 10% during redox cycling. Chromium oxide has been studied extensively for battery applications.595. Because the as-deposited films are lithium deficient. identified only as ‘CrOy’. making it almost optically ‘passive’. Azens.603 The only coloration efficiency available is that for vacuum-evaporated material.3 Metal oxides: secondary electrochromes 167 CeO2 þ xðLiþ þ eÀ Þ ! Lix CeO2 : (6:17) Both redox states are essentially colourless in the visible region. oxides are dispersed. Controlling the . These mixedmetal oxide electrochromes are described in Section 6. Alternatively. for which is À4 cm2 CÀ1.597 It is also widely used as a matrix in which other. As with W and Mo.84.80.608 suggest the as-grown film may be Co(OH)3. causing a weak charge-transfer transition from O2À to the Co2þ ion.606. magnetite. CoO Á OH.168 Metal oxides amount of lithium within films of rf-sputtered lithium cobalt oxide is. although the identity of the CoIII . unlikely in our view owing to the strongly oxidising nature of CoIII. which is converted to CoO by being annealed in an oxidising atmosphere. of pure cobalt metal anodised in a solution of aqueous 1 molar NaOH or a solution buffered to pH 7. Electrodeposited oxyhydroxide. CoO(1þy).606 for example.103.604 Electrochemical studies of anodically generated layers of oxide on metallic cobalt. such ‘cobalt oxide’ is sometimes written as CoOx or.607 owing to atmospheric oxidation.608 may be electrodeposited on Pt or ITO from an aqueous solution of Co(NO3)2 via Eqs.81 to form the peroxo anion for use in sol–gel or electrodeposition procedures.19) is the supposed electrochromic reaction of cobalt oxide grown anodically in aqueous electrolytes on cobalt metal:605. the iron 2 equivalent).g.238 Other vacuum methods such as CVD generate thin films of metallic cobalt as initial layer. These films change electrochemically from grey to pale yellow. with a response time of 2 to 4 s.III O4 ðsÞ þ 2eÀ þ H2 O: 3 pale yellow dark brown (6:19) The Co3O4 product would formally be CoIIO þ CoIII O3 (cf.607 3CoII O ðsÞ þ 2OHÀ ðsoln:Þ ! CoII.100 Thin-film cobalt oxide can be made by spray pyrolysis in oxygen of aqueous CoCl2 solutions139 onto e. Subsequent thermal annealing converts most of the oxyhydroxide to oxide CoO. but some CoO Á OH persists. difficult. sols of Co3O4 have been applied to an electrode substrate by both dipping and spraying.608 For this reason. better.609 Gorenstein et al.605. The colour of the brown form is probably due to a mixed-valence charge-transfer transition in the Co3O4.610 Redox chemistry of cobalt oxide electrochromes Equation (6. fluorine-doped tin oxide (FTO) coatings on glass substrates.3) and (6. Alternatively. however. Co metal can be dissolved oxidatively in H2O2. show the films to be blue.4). Chemical vapour deposition precursors include Co(acetylacetonate)2. This CoO(1þy) has a pale green colour owing to a slight stoichiometric excess of oxide ion. (6.605 but the colour soon changes to brown on standing. Cobalt oxide can also be deposited from a CoII–(tartrate) complex via CoII(OH)2 in aqueous sodium carbonate. 3 Metal oxides: secondary electrochromes 169 oxide(s) formed by oxidation of Co(OH)2 could not be assigned conclusively by FTIR. e. as demonstrated by FTIR.6 cites representative values of coloration coefficient . the electrochromic transition is green ! brown.619 Below 1. Behl and Toni618 find that many electrochromic colours may be achieved in films generated on metallic cobalt.g. The orange form of the oxide may also contain hydrated Co(OH)2 following H2O uptake. oxidation of sputtered LiCoO2 electrochrome results in an electrochromic colour change from effectively transparent to dark brown.6. pink.9 shows a coloration-efficiency plot of absorbance against charge passed614 Q. when the rate-limiting process during coloration and bleaching is the movement of the Liþ counter ion. accompanied by composition-dependent CT or intervalence absorptions. This figure demonstrates how absorbance is generally not proportional to Q.21): 3CoO þ 2OHÀ ! Co3 O4 þ 2eÀ þ H2 O: pale green brown (6:21) Optical properties of cobalt oxide electrochromes Figure 6.1 or 1. films are orange (or yellow– brown) but above this potential the films become dark brown (or even black if films are thick). confirming Benson et al. In non-aqueous solutions. (6. (6. LiClO4 in propylene carbonate.611 Reference 611 cites IR data for all the known oxides of cobalt including those above.81 in Eq. brown and black. presumably from varying oxide–hydroxide compositions.612 The study by Pyun et al.81 and Figure 6.8 shows UV-visible spectra of electrodeposited CoO (pale green) and Co3O4 (dark brown). Table 6.611 . The electrochromic reaction is Eq. since the graph is only linear for addition of small-to-medium amounts of inserted charge. together with CoO and CoO Á OH.47 V vs. For the novel green product formed by reductive electrolysis of nitrate ion.0 mol dmÀ3) this oxide is predominantly the low-valence product. Colours include white.20): LiCoO2 þ xðMþ þ eÀ Þ ! Mx LiCoO2 .’s views. pale yellowÀbrown dark brown (6:20) where Mþ is generally Liþ. SCE.613 clearly demonstrates the complexity of the charge-transfer process(es) across the oxide–electrolyte interphase. in the electrochromic reaction80. on Co metal anodised in NaOH (0. ) Films made by spray pyrolysis from CoCl2 solution exhibited anodic electrochromism. Mater.-A. 1991. . (Figure reproduced from Polo da Fontescu. yielding films of either CuO or Cu2O. C. hydrolysing copper ethoxide.. 3. M.3 60 %T Q = 0 (as grown film) 40 20 0 300 500 700 λ /nm 900 1100 Figure 6. Adv. and Gorenstein. Ray622 prepared a different sol–gel precursor via copper chloride in methanol.8 mC cm–2 80 5. The figures above each trace represent the charge passed in mC cmÀ2. changing colour from grey to pale yellow. with permission of Wiley–VCH.139 Electrochromic devices containing cobalt oxide electrochromes Cobalt oxide is usually employed as a secondary electrochrome (on the counter electrode) against a more strongly colouring primary electrochrome on the major colourant electrode comprising e.248 Copper oxide ¨ Preparation of copper oxide electrochromes Ozer and Tepehan620. WO3. and progressively bleaching. the product depending on the annealing conditions. 553–5. De Paoli.0 3. then annealing in an oxidising atmosphere.621 prepared a copper oxide electrochrome from sol–gel precursors.170 100 Metal oxides 6.. ‘The electrochromic effect in cobalt oxide thin films’.g.9 5. N.3 4. A. beginning with the most coloured state at the bottom of the figure.8 UV-visible spectra of thin-film cobalt oxide electrodeposited onto ITO. apparently. M.76 mA j = –1. Adv. Ozer and Tepehan620 call their .0 Electrochromic efficiency = 24 cm2 C–1 0 (b) 0. 1991.1 + +x + + Electrochromic efficiency = 27 cm 2 C–1 j = –0.1 +x + x + x + x + + +x + x x 0.6.08 mA cmÀ2. þ ¼ 0.9 Coloration-efficiency plot of absorbance (ÀDOD) against charge passed Q for thin-film CoO electrodeposited onto ITO. C.14 mA cm + x + x+ x + + 0. The wavelength at which Abs was determined is not known. The electrochromic transition is colourless to pale brown but.08 mA cm–2 cm–2 cm–2 cm–2 x 24 cm2 mC–1 0.623 have made transparent films of Cu2O on conductive SnO2:F (FTO) substrates by anodic oxidation of sputtered copper films. The current density i during coloration was * ¼ 0.591.3 Metal oxides: secondary electrochromes (a) 0.76 mA cmÀ2..38 mA j = –0. and immersed in NaOH solution (0.38 mA cmÀ2.2 – ΔOD + j = –0. 3.08 mA cm + –2 –2 –2 –2 x 171 0. A.. (Figure reproduced from Polo da Fontescu. o ¼ 1. ‘The electrochromic effect in cobalt oxide thin films’. N.2 + x +x + + x +x + x x – ΔOD + 0.0 + j = –0.38 mA cm j = –0.14 mA cmÀ2. or by electrodeposition.3 j = –0.) Richardson et al. with permission of Wiley–VCH.14 mA + 0 5 10 15 Charge density / mC cm–2 Figure 6.1 mol dmÀ3): (a) during coloration and (b) during bleaching. De Paoli. 553–5. Mater. and Gorenstein.3 5 10 15 Charge density / mC cm–2 + + x + + x+ x x x 0. ¨ neither redox state has yet been identified.76 mA cm j = –1. x ¼ 0.-A. the usefulness in ECDs is virtually zero unless display applications are found. 604 614.591 However.23): Cu2 O ðsÞ þ 2eÀ þ 2Hþ ! 2Cu ðsÞ þ H2 O: (6:23) The cycle life of such electrochromic materials is said to be poor at ca.23). The response time and optical properties of this electrochrome depend markedly on the temperature and duration of postdeposition annealing.5 24 25 130 12 (633 nm) b 20–27 Ref. a suggested redox reaction is Eq. Eq. For successful film growth. highly reflective copper metal. the pH must exceed 9. A large change in optical transmittance is claimed. The coloration efficiency is about 32 cm2 CÀ1. and the temperature . Iron oxide Yellow–green films of iron oxide form on the surface of an iron electrode anodised in 0. 615 616 107 139 617 The precursor was Co(acetylacetonate)2.22): 2CuO ðsÞ þ 2eÀ þ 2H2 O ! Cu2 O ðsÞ þ 2OHÀ : black 622 (6:22) red-brown Cu2O is transformed reversibly to opaque and In acidic electrolytes.172 Metal oxides Table 6. Sample values of coloration efficiency for cobalt oxide electrochromes Preparative route CVD a Electrodeposited Sol–gel Sonicated solution Spray pyrolysis Thermal evaporation a /cm2 CÀ 1 (>(obs)/nm) 21. (6. from 85 to 10% transmittance.624.6. (6. 20–100 cycles591 owing to the large increase in molar volume of about 65% during conversion from Cu to CuII. (6. Redox chemistry of copper oxide electrochromes The Cu2O films transform reversibly to black CuO at more anodic potentials.1 M NaOH. and this entry merely records an EC electrochemistry. electrochrome ‘CuwO’. according to Eq.622 In alkaline solution. b Figure in parenthesis is max.626 Such films display significant electrochromism.625. (6. the film becomes transparent at cathodic potentials as hydrated Fe(OH)2 is formed. so the electrochromic reaction is Eq.625 The oxides g-Fe2O3 (maghemite) and a-Fe2O3 (hematite) are also formed in the passivating layer. according to Eq. Such Fe3O4 can be further reduced to form a colourless oxide. the polycrystalline analogue being formed by annealing at high-temperature. the first reduction product is Fe3O4. although the coloured . the Fe2O3 was immersed in LiClO4–PC solution. Fe2 O3 ðsÞ þ xðLiþ þ eÀ Þ ! Lix Fe2 O3 ðsÞ: pale brown black (6:27) The product may be thought of as mixed-valence Fe3O4. the lithium insertion reaction here is wholly reversible. (6. The source of the lithium ion is LiClO4 in PC.627 Such LixFe2O3 is of good optical quality. polycrystalline Fe2O3 is essentially electro-inert. This coloured material may be hydrated FeIIIO Á OH.25): 3Fe2 O3 ðsÞ þ 2Hþ ðsolnÞ þ eÀ ! 2Fe3 O4 ðsÞ þ H2 O: brown black (6:25) This black Fe3O4 contains the mixed-valence oxide formally FeO Á Fe2O3. During electrochromic reactions. the lithium counter ion being incorporated for charge balancing during reaction.624 Thin films of Fe2O3 may be formed by electro-oxidation of Fe(ClO4)2 in MeCN solution. FeO – Eq. magnetite. which may preclude their use as ECD electrochromes. After annealing.24): FeIII O Á OH ðsÞ þ eÀ þ H2 O ! FeII ðOHÞ2 ðsÞ þ OHÀ ðsolnÞ: yellowÀgreen transparent (6:24) ´ Gutierrez and Beden use differential reflectance spectroscopy to show that iron oxyhydroxide underlies the electrochromic effect.624 These films are prone to slight electrochemical irreversibility owing to a surface layer of anhydrous FeO or Fe(OH)2.3 Metal oxides: secondary electrochromes 173 lower than 80 8C.26): Fe3 O4 ðsÞ þ 2Hþ ðsolnÞ þ 2eÀ ! 3FeO ðsÞ þ H2 O: black colourless (6:26) ¨ Electrochromic Fe2O3 was made by Ozer and Tepehan627 from a sol of the i iron alkoxide Fe(O Pr)3. formally Eq.104 This oxide is amorphous. (6.27). showed good electro-reversibility. (6.6.627 The electrochromic reaction. (Figure reproduced from Ozer.10 shows the electronic spectra.154 A dipcoating procedure. which yields an . F. but counter-electrode use is suggested.10 Transmittance spectrum of thin-film iron oxide Fe2O3 formed by spin-coated sol–gel onto an ITO electrode. Energy Mater.628 Liþ insertion into this oxide is fully reversible. Other sol–gel precursors have yielded electrochromic iron oxide films. The coloured form was generated ¨ at À2. yields Fe2O3 after firing at 180 8C.0 V.174 100 Metal oxides 80 Bleached Transmittance (%) 60 Coloured 40 20 300 400 500 600 700 800 Wav elength (nm) Figure 6. ‘Optical and electrochemical characteristics of sol–gel deposited iron oxide films’. Similar but inferior electrochromic activity was seen when the film was immersed in NaOH or KOH of the same concentration:154 Naþ and Kþ cations were presumably too large to enter the lattice readily. Iron(acetylacetonate)2 is a suitable CVD precursor for iron oxide electrochromes. A thin film of metallic iron is formed first. 1999. Cells. and Tepehan. yields Fe2O3 which bleaches cathodically and colours anodically in lithium-containing electrolytes of aqueous 10À3 mol dmÀ3 LiOH. Electrochromic films were made from a gel prepared by raising the pH of aqueous ferric chloride during addition of ammonium hydroxide.5 V. 56. repeatedly immersing an electrode in the precursor solution and then annealing.) films were insufficiently intense to consider their use as a primary electrochrome. N. by permission of Elsevier Science. Sol. based on iron pentoxide in propanol. then homogenising the resultant precipitate with ethanoic acid to form a sol. Sol. and the bleached form at þ0. 141–52. Spin coating a further sol–gel film. Figure 6. This electrochrome contains some immobile sodium ions.247 while electron-beam evaporation yields an electron-deficient oxide.631.29): 2MnO2 ðsÞ þ H2 O þ eÀ ! Mn2 O3 ðsÞ þ 2OHÀ ðaqÞ: dark brown pale yellow (6:29) The colours stated are for thin films. prepared by adding fumaric acid to sodium permanganate.632 the oxide originating from H2O. 629 627 104 electrochromic oxide after annealing.28): 2 FeO ðsÞ þ H2 O ! Fe2 O3 ðsÞ þ 2eÀ þ 2 Hþ ðsolnÞ: colourless brown (6:28) Optical properties of iron oxide electrochromes Values of are relatively rare for this electrochrome. The couple responsible for the electrochromic .3 Metal oxides: secondary electrochromes 175 Table 6.634according to Eq.246.7. In aqueous solutions. A sol–gel precursor.633 Films can also be formed by rf sputtering.6. so the redox reaction is that given in Eq. Manganese oxide Preparation of manganese oxide electrochromes Anodising metallic manganese in base (alkali) yields a thin surface film of electrochromic oxide. Preparative route CVD Sol–gel Electrodeposition /cm2 CÀ1 À6.605 Films of electrochromic MnO2 can also be formed by reductive electrodeposition from aqueous MnSO4. can yield MnO2 films. MnO Á OH.0 to 6. The colourless form may comprise some hydrated hydroxide Mn(OH)3 or oxyhydroxide.231 Redox chemistry of manganese oxide electrochromes The electrochromic mechanism of MnO2 grown on Mn metal is complicated.629 The coloured form is Fe2O3. (6. Sample values of coloration efficiency for iron oxide electrochromes. the electrochrome is black in thick films.nH2O. and has been formulated as NaMnO2.7. (6. see Table 6. electrochromic coloration involves hydroxide expulsion when solutions are alkaline. denoted here as MnO(2Ày).630.5 À28 À30 Ref. 633 The value of for thin-film LixMnO(2Ày) made by electron-beam evaporation is 7. and generally involve the insertion and extraction of Liþ. (6.247 The electrochromic operation of MnO2 films made from sol–gel precursors is said to perform best when immersed in aqueous base.231 Electrochromic efficiencies as high as 130 cm2 CÀ1 have been reported for MnOy films in aqueous borate buffer solution.g.246 Optical properties of manganese oxide electrochromes Figure 6.636 in which the primary electrochrome was nickel oxide. Lithium ion can also be inserted from aqueous solution into sputtered MnO2.635 If the pH is low. with a coloration efficiency of 12 to 14 cm2 CÀ1.634 which is confirmed by XPS spectroscopy.31): MnO2 ðsÞ þ xðLiþ þ eÀ Þ ! Lix MnO2 ðsÞ: brown yellow (6:31) X-Ray photoelectron spectroscopy suggested that hydrated MnO2 represents the composition in the oxidised state. Sol–gel drived electrochromic MnO2 follows Beer’s law fairly closely633 on electro-inserting Liþ from LiClO4–PC solution.11 shows the spectrum of sputter-deposited MnO2.631 Electrochromic devices containing manganese oxide electrochromes Manganese oxide has been suggested as a counter electrode (or secondary electrochrome) since its coloration efficiency is relatively low. depending slightly on preparation conditions. A plot of Abs against x for Eq. from LiClO4 in PC via Eq. coloration proceeds in accompaniment with proton uptake according to Eq.31) is linear.633 The films are very stable and are said to show high write–erase efficiencies in this electrolyte. e.IV Oð2ÀxÞ ðOHÞx ðsÞ: (6:30) The redox reactions of manganese dioxide in non-aqueous electrolytes are straightforward. (6.176 Metal oxides transition is probably MnO2–MnO Á OH. for . Niobium oxide Preparation of niobium oxide electrochromes Sol–gel methods are now the most widely used procedure for forming electrochromic Nb2O5 films. (6.592 The redox process in Eq.31) is better understood than for many other electrochromes.2 cm2 CÀ1.30): MnIV O2 ðsÞ þ xðHþ þ eÀ Þ ! MnIII.592 A device has been made by Ma et al. (6. since MnO2 is the vital component in many rechargeable and alkaline batteries. formed by mixing NbCl5 and anhydrous ethanol. I.647. (6.1M 0. with permission of Elsevier Science. 37. as indicated on the figure). SCE. (Figure reproduced from ´ Cordoba de Torresi.648 Direct-current (dc) magnetron sputtering is only occasionally used in preparations of Nb2O5. Redox electrochemistry of niobium pentoxide electrochromes The accepted redox reaction describing the process of Nb2O5 coloration is Eq. and Gorenstein.11 UV-visible spectrum of sputter-deposited thin-film manganese oxide at a variety of potentials (vs. Acta.8 V 0. S.8 177 Optical density 1 0.642 are also used.191 butoxide155 or pentachloride639.638 Precursors include ethoxide.6. 2015–19. (6.2 V 0. 1992.’ Electrochim. ‘Electrochromic behaviour of manganese dioxide electrodes in slightly alkaline solutions.33): .192 comparing the properties of films prepared by dc-magnetron sputtering and by the spin coating of gels subsequently annealed.217.643 Films of Nb2O5 have also been prepared by anodising Nb metal.192. Such films are ‘slightly crystalline’192 since they require high-temperature annealing.168 Niobium pentoxide films annealed at temperatures below 450 8C are said to be still amorphous.637. The oxide film was electrodeposited onto a SnO2-coated optical electrode.123. between 560 and 600 8C.646 An electrochromic layer of Nb2O5 can also be prepared on niobium metal by thermal oxidation.645. A.4 V 0. found that the films were electrochromically essentially equivalent.641 salts. Hydrolysis yields the solid oxide.170.218 Lampert and co-workers.644.0 t 0 300 400 500 600 700 800 Wav elength/nm Figure 6.3 Metal oxides: secondary electrochromes 2 SnO2 / MnO 2 / Bor ate 0.640.) example by hydrolysing niobium alkoxides. Eq. for example by redox cycling Nb metal in dilute aqueous acid.2.0 V E 0. Chloralkoxide sols of the type NbClx(OEt)5Àx.32): 2NbClx ðOEtÞ5Àx ðaqÞ þ 5H2 O ! Nb2 O5 ðsÞ þ 2ð5 ÀxÞEtOH þ 2xHCl ðaqÞ: (6:32) The gel is then spin coated. and analysed while immersed in a borate electrolyte at pH ¼ 9. Nb2O5 has been used as a ‘passive’ counter electrode. 650 171 405 641 NbCl5 in ethanol . Optical properties of niobium pentoxide electrochromes Thin films of niobium oxide are transparent and essentially colourless when fully oxidised.178 Metal oxides Nb2 O5 ðsÞ þ xðMþ þ eÀ Þ ! Mx Nb2 O5 ðsÞ. they do not degrade so fast.637 Firstly they can accommodate a larger charge (see the cyclic voltammograms in Figure 6. they can be decoloured completely.170 Figure 6. 258 649 401 170. and thirdly. colourless blue (6:33) where Mþ is generally Liþ. 172. generally with WO3433 as primary electrochrome. Preparative procedure rf sputtering rf sputtering rf sputtering Sol–gel Sol–gel Sol–gel Spraying a a /cm2 CÀ1 ((obs)/nm) 5 10 100 22 (600) 28 (550) 38 (700) 6 (800) Ref. secondly.8. The coloration efficiencies of niobium oxide electrochromes are listed in Table 6.168 Some sol–gel-derived films of Nb2O5 also form a brown colour between the tonal extremes of colourless and blue. The response time of Nb2O5 grown on Nb metal in aqueous 1 M H2SO4 is said to be less than 1 s. whereas sputtered Nb2O5 films retain some slight residual coloration. Use of niobium oxide electrochromes in devices Owing to its low coloration efficiency.644 The cycle life of crystalline sol–gel-derived films is cited variously as ‘up to 2000 voltammetry cycles between 2 and À1. Table 6.191 Films of sol–gel-derived Nb2O5 are superior if they are made to contain up to about 20 mole per cent of lithium oxide.8.8 V’168 and ‘beyond 1200 cycles without change in performance’. and present a deep blue colour on Liþ ion insertion.12).13 depicts spectra of Nb2O5 and LixNb2O5. Coloration efficiencies of niobium oxide electrochromes. C.0 –1500 Figure 6. coupled with high cost. Praseodymium oxide Electrochromic praseodymium oxide was studied by Granqvist and co-workers219 who made thin-film PrO2 by dc-magnetron sputtering. by permission of Elsevier Science. 46.12 The effect of cycle number on the cyclic voltammogram of thinfilm Nb2O5. while anhydrous PdO2 is reddish brown. Ag 0 500 (i) Undoped (ii) 10 mol% Li doped (iii) 20 mol% Li doped (ii) (iii) (i) Undoped (ii) 10 mol% Li doped (iii) 20 mol% Li doped –1000 –500 E/mV vs .0 2 – 0. Avellaneda.) Palladium oxide Amongst the few studies of electrochromic PdO2. (6. ˜ L. R.1 mol dmÀ3).5 179 0. O. revealing some redox complexity.005. and Bulhoes.34): PrOð2ÀyÞ ðsÞ þ xðLiþ þ eÀ Þ ! Lix PrOð2ÀyÞ ðsÞ: dark orange colourless (6:34) Films of electrochromic oxide switch in colour from dark orange (presumably PrO2-like) to transparent. (Figure reproduced from Bueno. S.. hydrated PdO2 is yellow. means that palladium electrochromes are unlikely to be viable. Electrochimica Acta. Faria. This electrochemical complexity. varying the ratio of O2 to argon from 0. 2113–18. the film was immersed in propylene carbonate solution itself comprising LiClO4 (0. X-Ray diffraction of the CVD-derived samples suggest that the first lithium insertion cycle was accompanied by an irreversible .. R.5 (i) –0. The electrochromic reaction is652 Eq. C.0 i /mA cm 2 – i /mA cm (i) –0. 2001.5 (ii) (iii) –1. Note also the higher charge capacity of the lithium-containing films.5 (b) 0. ‘Electrochromic properties of undoped and lithium doped Nb2O5 films prepared by the sol–gel method’. Thomas and Owen652 used CVD from a metallo-organic precursor. by ´ Bolzan and Arvia. the most extensive. P. The coloured (black) form is hydrated PdO.025 to 0.6. During redox cycling. O.3 Metal oxides: secondary electrochromes (a) 0. Ag 0 500 –1.0 –1500 –1000 –500 E/mV vs . (a) The first cycle and (b) the twenty-first cycle. deposited onto ITO by a sol–gel process.651 concerns hydrated PdO2 (prepared by anodising Pd metal in acidic solution). by permission of The Royal Society of Chemistry.5 Absorbance (rel. (Figure reproduced from Lee. the charge insertion was reversible over 500 cycles. but PrO2 has been added to films of cerium oxide. The charge capacity ranged from comparability with that of WO3. 5 mm. .) phase change. Praseodymium films do not promise wide usage.e.0 mol dmÀ3 H2SO4 solution.652 The initially dark films made by sputtering showed strong anodic electrochromism. more grey). G.0 400 450 500 550 600 650 700 750 800 λ nm / Figure 6. R. J.. 1. Chem. In a device incorporating WO3 as the primary electrochrome. 193. 381–6. 1991.3 (b) 0.13 UV-visible spectrum of thin-film niobium pentoxide on ITO. to virtually zero for oxygen-depleted films.4 0.1 (a) 0. The film was prepared by a sol–gel method and had a thickness of ca. provided the switching was relatively fast and that the film was not left in the reduced state for long periods. units) 0. for oxygen-rich films.6 Metal oxides 0.875 V and (b) of the oxidised form was obtained at 0 V against SCE. the use of PrO2 as the secondary layer made the colour more ‘neutral’ (i. The spectrum (a) of the reduced form at À0. The electro-coloration was performed in 1. and Crayston.653 see p.180 0. ‘Electrochromic Nb2O5 and Nb2O5/silicone composite thin films prepared by sol–gel processing’. J. Mater. A.652 Thereafter.2 0. (6.660.74.661.257. Figure 6.654.220.662. (6.657 reflectance and charge insertion are also shown as a function of potential. 664.14 shows a cyclic voltammogram of rhodium oxide.659 The oxide changes colour electrochemically659 according to Eq.658 Films may also be generated by anodising metallic Ru in alkaline solution.468. Rh2O3 probably more so than RhO2. A fully colourless state is not attainable.666 . Ruthenium oxide Thin films of hydrous ruthenium oxide can be prepared by repeated cyclic voltammetry on ITO-coated glass substrates immersed in an aqueous solution of ruthenium chloride. (6.656 In an early study. The oxide RhO2 is unusual in being green. owing to annealing after deposition.6. the only other inorganic electrochromes evincing this colour are Prussian green (a mixed-valence species of partly oxidised Prussian blue).3 Metal oxides: secondary electrochromes 181 Rhodium oxide Electrochromic rhodium oxide has been little studied. Gottesfeld657 cites the electrochromic reaction Eq. see p. 168.663.658 Tantalum oxide Preparation of tantalum oxide electrochromes Few electrochromism studies have been performed on tantalum oxide Ta2O5.35) are hydrated.260. and electrodeposited cobalt oxide.665.656 Such films are polycrystalline. The coloration efficiency at 700 nm was 29 cm2 CÀ1.173.36): RuO2 Á 2H2 O ðsÞ þ H2 O þ eÀ ! 1=2ðRu2 O3 Á 5H2 OÞ ðsÞ þ OHÀ : blueÀbrown black (6:36) The electrogenerated colour is not intense. Films may be formed on Rh metal by anodising metallic rhodium immersed in concentrated solution of alkali.35): Rh2 O3 ðsÞ þ 2OHÀ ðaqÞ ! 2 RhO2 ðsÞ þ H2 O þ 2eÀ : yellow dark green (6:35) Both the rhodium oxides in Eq. Rhodium oxide made by a sol–gel procedure switched from bright yellow to olive green. but it has been used sometimes as a layer of ion-conductive electrolyte.655 It can also be made from sol–gel precursors. The ruthenium oxide electrode exhibits a 50% modulation of optical transmittance at 670 nm wavelength.259. Dark-green RhO2 appears black if films are sufficiently thick. 0 .2 1.) and charge inserted (– – –) as a function of potential. immersed in hydroxide solution (5 mol dmÀ3 KOH). Electrochem. RHE) 1.668 or thermal oxidation of sputtered Ta metal. J. Also included on the figure are the reflectance at 546 nm (–.74.259.667. 1980.663 TaCl574 or TaI5.220 The most widely used tantalum CVD precursors of Ta2O5 are Ta(OEt)5.). The scan rate was 150 mV sÀ1.74 each volatilised in an oxygen-rich atmosphere. solutions of the supposed peroxypolytantalate may be spin coated onto ITO. Otherwise.260 reactive dc sputtering220 or thermal evaporation.193 Thin-film Ta2O5 can also be formed by dip coating using a liquor rich in Ta(OEt)5 as the precursor. and then sintered. by permission of The Electrochemical Society.14 Cyclic voltammogram of rhodium oxide grown on an electrode of metallic rhodium. Thin films may be prepared by anodising Ta metal in sulfuric acid662.6 0.173.182 Metal oxides 12 100 25 8 Q (mC cm–2 ) 75 R/ Ro (%) 20 4 i (mA cm –2 15 0 50 10 25 5 12 – 0. this solute is prepared by reactive dissolution in H2O2 of either Ta93 or Ta(OEt)5. 272–7. Inc.. or slowly immersed and withdrawn at a predetermined rate. S. 1 M K OH 150 mV s –1 –4 –8 Figure 6. Carbon or halide impurities are however incorporated into the resultant films. An electrode substrate is repeatedly dipped into the liquor.72. ‘The anodic rhodium oxide film: a two-colour electrochromic system’.–. 127.258.257. ) .666 Other films have been made by rf sputtering from a target of Ta2O5. (Figure reproduced from Gottesfeld.8 E (V vs 1. Soc.4 Rhodium. found the rate of proton movement was dictated by water adsorbed within the interphase.665 studying the dynamics of charge movement (comprising proton conductance) across the Ta2O5–WO3 interphase. films made by rf sputtering have values as low as 5 cm2 CÀ1.6. the authors do mention the effects of such adsorbed water on the rate of ionic movement across the interphase.669 Accordingly.3 Metal oxides: secondary electrochromes 183 Redox chemistry of tantalum oxide electrochromes The electrochromic reaction of thin-film Ta2O5 in aqueous alkali is Eq. Eq. they conclude that the rate is dictated by the extent to which the crystal structures of the oxides making the interface are complementary.664 on the interface comprising Ta2O5 and NiO or Ni(OH)2.671.257. workers are increasingly choosing to employ thin-film Ta2O5 as the ion-conductive electrolyte layer between the solid layers of primary and secondary electrochrome in all-solid-state devices.259. (6.220.663.660.38: SnO2 ðsÞ þ xðLiþ þ eÀ Þ ! Lix SnO2 ðsÞ.672. how well structurally the oxides join.3 Garikepati and Xue.676 Granqvist and co-workers677 assign the peak to intervalency transitions as in other cathodically colouring electrochromic oxides.674 but the electrochromic effect is weak.676 .673 Optical properties of tantalum oxide electrochromes The value of max for Ta2O5 made by anodised tantalum metal is 541 nm. While in the studies of Ahn et al. The peak occurs in the near infrared.260. colourless blueÀgrey (6:38) the tin(IV) oxide films were made by reactive rf-magnetron sputtering. 6. the kinetics of charge transport are dominated by movement of polaron species. The conductivity of protons through Ta2O5 is so fast that it is often classed as a ‘fast ion conductor’.206 while material made by laser ablation has of 10 cm2 CÀ1. where the films absorb strongly.37): TaV O5 ðsÞ þ H2 O þ eÀ ! 2 TaIV O2 ðsÞ þ 2OHÀ ðaqÞ: 2 colourless very pale blue (6:37) While the kinetics here have been little studied.676 The films are conductive. However. i. For example.675 The Ta2O5 films exhibit high transmittance except in the UV.670. The wavelength maximum of LixSnO2 lies in the infrared. by both electrons and ions.e. Tin oxide In the few studies on the electrochromism of tin oxide. 678.174 The electrochromic reaction of TiO2 is usually written as Eq.128. The rate of TiO2 reduction is controlled by the rate of counter-ion diffusion through the solid:682.685 claim to have modulated an incident beam by between 14% and 18%.2.261 or pulsed laser ablation. Electrocrystallisation appears to dominate the electrochemistry at x > 0.9.677 The electronic spectrum of tin-oxide films remains relatively unchanged following electro-insertion of lithium ion.158. and Mossbauer spectra unambiguously show the conversion ¨ SnIV! SnII. Titanium oxide Thin-film TiO2 can be made in vacuo by thermal evaporation of TiO2. (6. hence TiO2 is used as a secondary electrochrome or even as an ‘optically passive’ counter electrode.681 have studied the electrochromism of titanium oxide grown anodically on metallic titanium via in situ ellipsometry. Scrosati and co-workers682 drove the electrochromic process with potentiostatic pulses. Yoshimura et al.184 Metal oxides At low insertion coefficients (0 < x < $0. the electro-inserted lithium ions appear to be located in internal double layers within the film.39): TiO2 ðsÞ þ xðLiþ þ eÀ Þ ! Lix TiO2 ðsÞ: colourless blueÀgrey (6:39) Ord et al.1).683 ionic insertion into the crystal form of anatase (the Liþ deriving from a LiClO4–propylene carbonate electrolyte) is characterised by a diffusion coefficient of68210À10 cm2 sÀ1.158.680 spin coating128 and dip-coating procedures. Titanium oxide-based electrochromes show two optical bands at 420 and 650 nm.677 Increasing the insertion coefficient x from $0.134 Methods involve sol–gel.678 reactive rf sputtering from a titanium target. but optical constants such as the refractive index increase with increasing insertion coefficient. see Table 6.679 The coloration efficiency is low.1 to $0. Nevertheless. non-vacuum techniques involve alkoxides or the peroxo precursor made by dissolving a titanium alkoxide Ti(OBu)4 in H2O2. with WO3 as the primary electrochrome.2 yielded significant transmittance drops.679 Alternatively.127.319.686 .684 Values of coloration efficiency for thin-film TiO2 are low. Thin-film titanium oxynitride is also electrochromic.128.319. To accelerate diffusion. Both reduction and oxidation processes occur via movement of a phase boundary which separates the reduced and oxidised regions within the TiO2. Direct-current sputtering is also used.690 affords a different class of preparative method.688.224.266.6.693 presumably by increasing the extent of crystallinity in the amorphous material. generally via alkoxide precursors.698 Alkoxide precursors are also used in preparing films by CVD.689 cathodic arc deposition550 and electron-beam sputtering.71.6 8 (646) 14 50 Ref.3 Metal oxides: secondary electrochromes 185 Table 6.264.233. Coating solutions include the liquor made by dissolving V2O5 powder in a mixed solution of benzyl alcohol and iso-butanol.265 Thermal evaporation in vacuo476.526.694 are essentially amorphous.701. A more general review was published in 2001. and includes flash evaporation.73 The deposition product is immediately annealed in an oxidising atmosphere.699.200 Subsequent annealing yields the desired electrochrome.700 bis(acetylacetonato)vanadyl has also been employed. ensuring polycrystallinity.264.697 (in 1991 and 1996 respectively). Sample values of coloration efficiency for titanium oxide electrochromes.225.199.264 Annealing a sample of thermally evaporated V2O5 to above 180 8C improves the electrochemical performance.267 with a high pressure of oxygen and a target of vanadium metal.263.691 Films of V2O5 deposited by thermal evaporation in vacuo are amorphous.695 The preparation and use of such gels has been reviewed extensively by Livage696. Electrochromic thin films have often been prepared using xerogels of V2O5.476 sputtered samples are more crystalline.267. which is always hydrated. Preparative procedure Reactive thermal evaporation Thermal evaporation rf sputtering Sol–gel /cm2 CÀ1 ((obs)/nm) 7.206.120 Livage made VO2 films by sol–gel methods.50.226 Other vacuum methods employed include pulsed laser ablation.687.9. Spin coating has also been used to prepare films of V2O5.702 or that produced by oxidative .692 although X-ray diffraction suggests the extent of crystallinity is low.265. like VO(OiPr)3 in 2-propanol.223. Thin films of V2O5 from vanadium metal anodised in acetic acid36. 678 401 261 641 Vanadium oxide Preparation of vanadium oxide electrochromes Thin-film V2O5 is commonly made by reactive rf sputtering. the precursor of choice generally being an alkoxide species such as VO(OiPr)3. 41): VV O5 þ 2Hþ þ 2eÀ ! VIV O4 þ H2 O: 2 2 (6:41) The relationships between the structure of V2O5 films (prepared by sol–gel) and their redox state has been described at length by Meulenkamp et al:709 a transition occurs from a-V2O5 at x ¼ 0.40): IV. (6. In aqueous solution. Secondly.4. later with additional details. Since thin-film V2O5 dissolves readily in dilute acid. distilled water. thin-film V2O5 has been grown anodically on vanadium metal immersed in dilute acetic acid.g. at x ¼ 1. These phases are nearly identical. Aggregates of the polymer surfactant appeared to act as a form of template during deposition.8 shows an elongated c-axis relative to "-LixV2O5.4.133 The liquor made by dissolving metallic vanadium in H2O2 can also be spin coated e. Nevertheless.V VV O5 ðsÞ þ xðMþ þ eÀ Þ ! Mx V2 O5 ðsÞ. for example.266 or g-butyrolactone.705 who prepared samples by thermal evaporation in vacuo. the structure bears further resemblance to d-LiV2O5. no doubt owing to their sensitivity to water. Electrochemical methods of making V2O5 electrochrome are rarely used. the structure undergoes significant changes: firstly. The rates of ion insertion and egress are so much slower for Naþ than for Liþ that the sodium ions in Na0. onto ITO substrates. 2 (6:40) brownÀyellow very pale blue where Mþ is almost universally Liþ owing to appreciable solubility of V2O5 in aqueous acid.705 LiCl in anhydrous methanol706 or LiClO4 in propylene carbonate263.36.226 describe the structure of LixV2O5 as orthorhombic.708 an alternative reaction is Eq. For larger insertion coefficients.704 Redox chemistry of vanadium oxide electrochromes The electrochromism of thin-film V2O5 was apparently first mentioned in 1977 by Gavrilyuk and Chudnovski.186 Metal oxides dissolution of powdered vanadium in hydrogen peroxide. which may represent a monoclinic structure. however.33V2O5 may be regarded as immobile.49 .132 Deb and co-workers703 prepared thin films of mesoporous vanadium oxide by electrochemical deposition from a water–ethanol solution of vanadyl sulfate and a non-ionic polymer surfactant.0 to "-LixV2O5 at x ¼ 0.132.0 the structure distorts further and shows features in common with d-LiV2O5. alternative electrolytes have been used.707 The electrochromic reaction in non-aqueous solution follows Eq. Thirdly.) Granqvist et al. " and d are but phase labels. (6.694.264. the phase for x ¼ 0. at x ¼ 1. (Here. 1 3. R. Nguyen.5 2. D. Proc.711 Ord et al.3 3. SPIE..0 Potential / V(vs .36 V relative to the Liþ. in common with MoO3 (but unlike WO3). Li couple) Figure 6. and immersed in propylene carbonate containing LiClO4 (1. in tandem with more traditional electrochemical methods such as cyclic voltammetry. and cathodic peaks at 3. N. the outer surface is converted to H4V2O5.3 2.9 4. S. (Figure reproduced from Cogan.5 0. As soon as the film is made cathodic. with permission of the International Society for Optical Engineering. (Higher fields are required for bleaching . S.9 3. Li couple in propylene carbonate.0 10 mVs –0.710 produced V2O5 films by rf sputtering. 1016. 1988.. J.5 Epa(1) 1. Benmoussa et al. Perrotti.15. Thereafter.) Cyclic voltammetry of sputtered V2O5.1 4.15 Cyclic voltammogram of thin film of V2O5 sputtered on an OTE.45 V.3 –1 –1. F.36. M. see Figure 6. and Rauh. a well-defined boundary forms between the coloured and bleached phases during redox cycling: this boundary sweeps inward toward the substrate from the film–electrolyte interface during the bleaching and coloration processes.7 2. obtaining ‘excellent cyclic voltammograms’.5 2. and studied the redox processes using the in situ technique of ellipsometry. their results clearly suggest how. as a thin film supported on an OTE immersed in a lithium-containing PC electrolyte.3 Metal oxides: secondary electrochromes 1.5 Epc(2) Epc(1) –1.26 and 3. shows two well-defined quasi-reversible redox couples263 with anodic peaks at 3.6. and then green to blue during reduction.14 and 3.7 3. 57–62. These two pairs of peaks may correspond to the two phases of LixV2O5 identified by Dickens and Reynolds.0 mol dmÀ3). Li +.5 3.0 –2 187 Epa(2) 50 mVs –1 Current density/mA cm 0.694 grew thin anodic films of V2O5 on vanadium metal immersed in acetic acid. again with a twostep electrochromism: they cite yellow to green. ‘Electrochromism in sputtered vanadium pentoxide’. 2) prepared by sputtering is the a-phase. They also suggest the redox cycling is somewhat irreversible.694 A study by Scarminio et al. with one distant oxygen.715).225 monitored the stresses induced in V2O5 during redox cycling.714 see Eq.42): Na0:33 V2 O5 ðsÞ þ xðLiþ þ eÀ Þ ! Lix Na0:33 V2 O5 ðsÞ: The sodium ions are essentially immobile. and contributes an additional. (6.694 The bleaching process is complicated. and proceeds in three stages.716 suggest the anodic electrochromism of V2O5 is due to a blue shift of the absorption edge. slight change in absorbance. spectral regions following the Beer–Lambert law cannot be identified readily. Such vanadium oxide also exhibits a higher lithium storage capacity and greatly enhanced charge-discharge rate.712 Reductive injection of lithium ion into V2O5 forms LixV2O5.3 < x < 0.713 The generation of the "-phase of LixV2O5 in V2O5 thin films accompanies the electrochromic colour change. which is not readily distinguishable from the starting pentoxide. Optical properties of vanadium oxide electrochromes The absorption bands formed on reduction are generally considered to be too weak to imply the formation of any intervalence optical parameters (although there are other arguments formulated by Nabavi et al.263 Since several species participate in the spectrum of the partially reduced oxide.7). this time with Liþ as the mobile ion. Their results suggest the crystal structure within the film is determined by the sputter conditions employed during film fabrication. Also. The authors say this behaviour is typical of the lithium insertion mechanisms in bulk V2O5 prepared as a cathode material for secondary lithium batteries. implying a poor write–erase efficiency.33V2O5 made by a sol–gel process is also electrochromic. The crystal structure of vanadium pentoxide is complicated. The LixV2O5 (of x < 0. Wu et al.706 and Murphy et al. the crystalline form of the oxide is "-LixV2O5. their thin-film V2O5 was immersed in a solution of LiClO4 in PC.263 At higher injection levels (0. Na0.188 Metal oxides than for coloration.263 as identified by the groups of Hub et al. with the nominally octahedral vanadium being almost tetragonal bipyramidal.703 attributable to enhanced ion mobility. a-LixV2O5 from the un-lithiated oxide is formed.266 Films of mesoporous V2O5 colour faster than evaporated films. and the (6:42) . Deep charge–discharge cycles (performed under constant current density) allow correlations to be drawn between the stress changes in the crystalline film and the electrode potential steps.) The rates of coloration and bleaching are both dictated by the rate of proton movement. Colten et al. by permission of Professor Granqvist and The American Institute of Physics.705 The colour changes from purple to grey if films are sputtered. From X-ray photoelectron spectroscopy. A. going via dark blue to black at higher insertion levels.16. The electrogenerated colour is blue–green for evaporated films717 at low insertion levels.782V2O5.85 0. 69. did infer a weak charge-transfer transition between the oxygen 2p and vanadium 3d states. 1991.5 Wav elength (mm) Figure 6. C. 3261–5.25 mm.266 A few representative values of are listed in Table 6. Electrochromic devices containing vanadium pentoxide Since the electrochromic colours of V2O5 films are yellow and very pale blue.21 40 0 Li x 189 V 2 05 60 20 0 0 0.690 assign the colour change in evaporated films incorporating lithium to the formation of VO2 (which is blue) in the V2O5. J..) near-infrared electrochromism arises from absorption by small polarons in the V2O5..6. The numbers refer to values of insertion coefficient x.3 Metal oxides: secondary electrochromes 100 p– X 80 Transmittance (%) x = 0.224 see Figure 6. Vanadium pentoxide films have a characteristic yellow–brown colour. The polycrystalline V2O5 was sputter deposited to a thickness of 0. Appl.16 UV-visible spectrum of thin-film vanadium pentoxide on ITO.5 1 1. M. G. 476 using the same technique. but only for an entirely VIV solid. The loss of the yellow colour is attributed to the shift of the band edge from about 450 to 250 nm during reductive bleaching from V2O5 to Li0. Phys.10. The value of max of the yellow–brown form lies in the range 1100–1250 nm. A.5 2 2. attributable to the tail of an intense optical UV band appearing in the visible region. ‘Structure and optical absorption of LixV2O5 thin films’. and Granqvist. (Figure reproduced in slightly altered form from Talledo.. Fujita et al.5 Rauh and co-workers263 state that certain film thicknesses of V2O5 yield colourless films between the brown and pale-blue conditions. Andersson. the CR values . 4. mixing these oxides can modify the solid-state structure through which the mobile ion moves. ITO j LixWO3 j electrolyte j V2O5 j ITO.190 Metal oxides Table 6.e.266. mixtures are capable of providing different colours. i. e. There are several models to correlate these variables with the electrochromic colour. Thin-film vanadium dioxide VO2 is electrochromic. all inserted electrons are considered to be localised.728 6.723 made similar cells but with the conducting polymer PEDOT as the primary electrochrome. composites of V2O5 in poly(aniline) and a ‘melanin-like’ polymer have been reported. in counter-electrode use. hence the system is generally investigated for possible ECD use as a secondary electrochrome. 399.721 For example.g. for two reasons.726 and can be prepared by reactive sputtering. Thus the choice of constituent oxides and their relative mole fractions allows a wide array of options.4 Metal oxides: dual-metal electrochromes 6. see p.277. In particular. 263 264 641 699 for such films are not great. Varying the energies of the optical bands by altering the mix allows the colour to be adjusted to that desired. Secondly.722 Gustaffson et al. Here. Deposition method rf magnetron sputtering rf magnetron sputtering Sol–gel CVD /cm2 CÀ1 ((obs)/nm) À35 À15 (600–1600) À50 À34 Ref.727 Finally.263. and optical absorptions are . and thus increase the chemical diffusion coefficient D that results in superior response times .266.718. Firstly. cells have often been constructed with V2O5 as the secondary material to WO3 as the primary.723 LiVO2 doped with titanium oxide is also thermochromic.720. Coloration efficiencies of thin-film vanadium oxide electrochromes.526. there is a desire for so-called ‘neutral’ electrochromic colours.10.725 and lithium vanadate (LiVO2) is not only electrochromic but also thermo-chromic.724.719. One of the most successful is the so-called ‘site-saturation’ model.1 Introduction Preparing mixtures of metal oxide has been a major research goal during the past few years. 53. Several other reports24. systematic evaluation of the data below is not yet possible because the electrochromic properties of films depend so strongly on the modes of preparation.731 again studied max and for the LixWO3 system.744. since they were among the first mixed-metal oxides to receive attention for electrochromic applications.753 Ta.361.736. Published traces show significant divergences between calculated results and experiment.99.760 . Other host oxides are listed alphabetically.80. van Driel et al.203.747 Nb. The computed and experimental data correlated well.6. Clearly. 740.754 Si.81.743. the optical absorption intensity is proportional to the number of vacant redox sites surrounding the reductant site.5.333 Ti127.29. the computed and experimental data correlate well.80. with the effect of decreasing the oscillator strength.737.94. The chemistry of tungsten– ´ molybdenum oxides has been reviewed briefly by Gerand and Seguin732 (1996).755.333. and such a wide range of preparative techniques has been employed.99. as exemplar systems. Their model only applies to situations in which a dominant fraction of the electrons associated with the intercalating species are appreciably localised.4. The earlier work of Hurita et al.283.756.2 Electrochromic mixtures of metal oxide A full.317.404.134. so the proportion of vacant sites neighbouring a given electron on a reductant’s site decreases.4 Metal oxides: dual-metal electrochromes 191 considered to occur simply by photo-excitation of an electron to an empty redox site. To a good first approximation.316.746.731 have discussed electrochromic colours in terms of this model.735.738.742.741.335.89. 6.759.91.749. Again.734 Co.745. As the insertion coefficient x increases.94. In the discussion below. so some slight duplication is inevitable.62.739.752 Re. with published traces showing only slight deviation between respective values.736 Mo. oxide mixtures of the type X–Y can be incorporated into either a section on oxides of X or of Y.748. although published traces show somewhat more scatter. Tungsten oxide as electrochromic host Tungsten trioxide has been employed as a host or ‘matrix’ for a series of electrochromic oxides.730. The treatment by Denesuk and Uhlman729 has been tested with data for LixWO3 on ITO – a system displaying a curious dependence of max and on the insertion coefficient x. containing the following oxides: (in alphabetical order) Ba. which are explained in terms of partial irreversibility during coloration.750 Ni.292.129. as has been copiously illustrated above.29.757.61.316.751.475. tungsten-based systems are considered first.733 Ce.735.661.758.730 relates to mixtures of MoO3 and WO3. see p. thereby explaining why Mo–Mo and Mo–W intervalence bands are formed at lower insertion coefficients x than are any W–W bands. with permission of The Institute of Pure and Applied Physics. MoO3 and MocW1ÀcO3.0 0 1000 Q i (C cm–3 ) 2000 Figure 6. and analysed the shift in wavelength maxima as a function of the mole fractions of either constituent oxide (see Figure 6. sometimes described within the ‘site saturation model’.008.80. Phys.762 Kitao et al. Since the wavelength of the shifted max corresponds more closely to the sensitive range of the human eye. the electron mobility is decreased in thinfilm WO3–MoO3 relative to the pure oxides. mixing the oxides effectively enhances the coloration efficiency in the visible region. & ¼ mixed film with c ¼ 0.192 Metal oxides 2. Y.5 1. o ¼ WO3. M.327.e.13.204. and x ¼ 0. D ¼ c ¼ 0. 23.0 Ep (eV) 1.745. and Yamada.05 of MoO3. 1624–7.292 prepared a range of films of molybdenum–tungsten oxide of the formula MocW(1Àc)O3. M. Jpn. have a larger maximal insertion coefficient.254.. * ¼ MoO3. although the transition is reported not to be particularly intense.) or V. Additionally. 190.738 Deb and Witzke763 say the range of is 30–40%. The value of max for mixed films of WO3–MoO3 shifts to higher energy (lower ) relative to the pure oxides.325. Faughnan and Crandall found the highest value of occurs with a mole fraction of 0.17) and found a complicated relationship.61 Disadvantageously. i. In practice. Here. Appl. the films of W–Mo oxide become darker because they can accommodate more charge.738 . 1984.761 Thin-film WO3 can also be co-electrodeposited with phosphomolybdic acid to yield an electrochrome having a colour change described as ‘light yellow ! bluish brown’. it is recognised that electrons are captured (that is. Kitao. Absorption bands of electrochemically-colored films of WO3. they effect reduction) first at the sites of lowest energy.17 Photon energy of the absorption peak E p as a function of the inserted charge: thin-film samples of partially reduced oxides of composition MocW(1Àc)O3. (Figure reproduced from Hiruta. it is found that Mo sites are of lower energy than W. J.687.. 770 Sn.769 Pr.747 ´ Gerand and Seguin732 suggest that ion insertion into W–Mo oxide occurs readily.734 Comparatively long response times of 108 and 350 s were found for coloration and bleaching respectively. but ion removal is usually somewhat difficult. The temperature dependence of the electrochromic response of sol–gel deposited titanium–tungsten mixed oxide was shown by Bell and Matthews335 to be highly complicated.780.768 Nb. Furthermore.795. This last condition is described as ‘critical’. colours cathodically.791.786.778.796 W. implicating multiple competing processes.781. 767.122.779.323. thus precluding all but the slowest of electrochromic applications.205. would contradict the usual assumption that values of D for ion movement through amorphous material are higher than through polycrystalline material.776. followed by annealing at 550 8C. which is thought to relate to the Mo ions. Cerium oxide as electrochromic host Electrochromic mixtures have been prepared of cerium oxide together with the oxides of Co.766 Hf.6.782.764 Its electrochromic properties are ‘poor’.755.767 Mo.53 Finally.792.790.772.403 Antimony oxide as an electrochromic host Thin-film antimony–tin oxide (ATO). The W–Ce film consisted of a self-assembly structure based on the poly(oxotungsceriumate) cluster K17[CeIII(P2W17O61)2] Á 30H2O[Ce(P2W17)2] and poly(allylamine) hydrochloride. Naghavi et al.787 V.777. The cause of such ‘amorphisation’ is as yet not clear. The first is the intervalence band.793 734 122.788. and Zr.746 the optical band for W–Mo oxide is said to comprise two bands.784..765 suggested that the best electrochromic films were obtained by depositing at 200 8C in an oxygen atmosphere at a pressure of 10À2 mbar.653 Si. a hybrid of WO3 and Perspex (polymethylmethacrylate) has a relatively low of 38 cm2 CÀ1.794.771.755. The relative amount of the second oxide varies from a trace to a molar majority.775. .768.771 Ti.773.783.785.786.774.789. the electrochemical and optical properties were found to be extremely sensitive to their morphology. Adding about 5% of nickel oxide significantly improves the cyle life of WO3. and the second is a new band at higher energies. The slowness is ascribed to induced ‘amorphisation’ of the mixed-metal oxide at high insertion coefficients. if confirmed. the energy of the absorption band depends on the concentration of Mo in the film and the insertion coefficient x. Such a result. grown by pulsed laser deposition.4 Metal oxides: dual-metal electrochromes 193 In the study by Hiruta et al.324.783. i. Phys..194 Metal oxides 10–10 – D/cm 2 s–1 10–12 10–14 10–16 0 0.780 which are located in the gap between the valence and conduction bands of the CeO2. and Granqvist. Ce. films of Ce–Ti oxide (with Liþ as the counter ion) are optically passive and can . J. by permission of Professor Granqvist and The American Institute of Physics. G.3. as the ratio Ce:Ti (call it g) exceeds 0. 81. A.5 ∞ Ce/Ti Ratio Figure 6. then. L. C.774 the oxide layer was prepared by dc magnetron sputtering. ‘Decreased electrochromism in Li-intercalated Ti oxide films containing La. The charge movement necessary for electrochromic operation involves insertion and/or extraction of electrons via the Ce 4f states. the cycle life of pure TiO2 (as prepared by sol–gel techniques) being relatively low. (Figure reproduced in slightly altered form from Kullman.319 Addition of cerium oxide to TiO2 also decreases779. the electrochromic ‘absorbance’ is essentially independent of the insertion coefficient.3 0. interestingly. 8002–10.1 0. The chemical diffusion coefficient D of mobile Liþ ions through thin-film Ce–Ti oxide increases as the mole fraction of cerium oxide decreases:779.e.780 a plot of ln D against mole fraction of CeO2 (see Figure 6. Conversely.18 Graph of chemical diffusion coefficient D of Liþ ion moving through films of CeO2–TiO2: the effect of varying the composition. These values of D suggest the extent of electron trapping is slighter (or at least the depths of such traps are shallower) for TiO2 than for CeO2. and Pr’..18) is almost linear: DðLiþ Þ increases from 10À16 cm2 sÀ1 for pure CeO2 to 10À10 cm2 sÀ1 for pure TiO2.780 the coloration efficiency until. evidence from EXAFS suggests that the electrons inserted into Ce–Ti oxide reside preferentially at cerium sites. Azens. Clearly.779. Appl. electrochromes having as high a proportion of TiO2 as possible are desirable to achieve rapid ECD operation. 1997. adding CeO2 to TiO2 increases the cycle life.) Thin-film Ce–Ti oxide is more stable than CeO2 alone797 although. Cobalt oxide as electrochromic host Many electrochromic mixtures of cobalt oxide have been prepared. These values of D are summarised in Table 6. Thin-film Co–Al oxide617 prepared by dip coating has a coloration efficiency of 22 cm2 CÀ1.617.83.401 Since thin-film Al2O3 is rarely electroactive let alone electrochromic. The D data come from ref.3 Â 10À8 48.5 Â 10À8 cm2 sÀ1 through WO3.799.798 Ce. Since effective intervalence relies on juxtaposition of Co sites. The value of D relates to Hþ as the mobile ion through WO3. and to OHÀ ions for CoO and Co–W oxide.4 Metal oxides: dual-metal electrochromes 195 function as ECD counter electrodes. Films prepared by magnetron sputtering with g > 0. tungsten and Co–W mixed oxides. 5.80. and admixture would inevitably increase the mean Co–Co distance within this solid-state Table 6. Oxide film CoO Co–WO3 WO3 D/cm2 sÀ1 2.80.79. 800.781 Clearly.11. with oxides of Al.80.799 Mo. but 48.140.81 Ir.801 W80.7 Â 10À8 5.11. but any increase in the rate of electro-coloration follows from enhancements of D rather than from increases in .5 cm2 CÀ1 (as prepared by CVD604) or 25 cm2 CÀ1 (the CoO having been prepared by a sol–gel method616). and cycle life are considered. Comparative speeds of hydroxide-ion movement through electrodeposited cobalt.81 Ni.5 Â 10À8 .80 All these films were electrodeposited.3 Â 10À 8 cm2 sÀ1 through CoO.g.81. The larger value of D probably reflects a more open.6 are not chemically stable. porous structure.766 Cr. allowing of a more open structure.158. Cobalt–aluminium oxide has a coloration efficiency of 25 cm2 CÀ1. optimising the electrochromic response of Ce–Ti oxide will require that all three of the parameters . and Co–Al–Si oxide has of 22 cm2 CÀ1.80.736 or Zn.6.81 Diffusion through films of cobalt oxide mixed with other d-block oxides can be considerably faster than through CoO alone: the value of D for the OHÀ ion is 2.7 Â 10À8 cm2 sÀ1 through Co–W oxide. which compares with for CoO alone of 21. 80. the similarity between these values probably indicates that the alumina component acts simply as a kind of matrix or ‘filler’.80 Fe. e. which is widely used in the construction of ECDs. if the surrounding electrolyte solution contains no redox couple.187 Redox electrochemistry of ITO When a thin-film ITO immersed in a solution of electroactive reactants has a negative potential applied. (6. doubt persists whether it is the tin or the indium species of ITO which are reduced: the majority view is that all redox chemistry in such ITO occurs at the tin sites. then some of the metal centres within the film are themselves electroreduced.229. It behaves as a typical electrode substrate (see.243.229 but is not employed often since the resultant film is oxygen deficient and has a poorer transparency than material of complete stoichiometry. evaporated directly onto a glass substrate in an oxygen atmosphere of pressure of $5 Â 10À 4 Torr. for example.803 The most common alternative to ITO as an optically transparent electrode is tin oxide doped with fluoride (abbreviated to FTO). The more recent review (2001) by Nagai806 discusses the electrochemical properties of ITO films.808 Reactive electronbeam deposition onto heated glass also yields good-quality ITO. Section 14.186.245 or reactive dc sputtering. By contrast. possibly the ‘Co–Al oxide’ here in reality comprises aggregated clusters of the two constituent oxides.809 the precursor is In2O3 þ 9 mol% of SnO2. a review in 1983 by Chopra et al.43): . the product being a solid solution. and typically comprises about 9 mol% SnO2. the most recent review was in 2002 by Granqvist and ˚ Hultaker.208 Roomtemperature pulsed-laser deposition can also yield ITO.242.239. When preparing ITO films by electron-beam evaporation.196 Metal oxides mixture.810 Curiously. although the majority concerns ITO acting as a conductive electrode rather than a redox-active insertion electrode. Other electrochromic ITO layers have been made via sol–gel. The ITO made by these routes is largely amorphous. While old.244.3).807 Preparation of ITO electrochromes Electrochromic ITO is generally made by rf sputtering.241. Eq. however.802 Some of the tin oxide dopant has the composition of Sn2O3. it will conduct charge to and/or from the redox species in solution.805 still contains information of interest. each as a pure oxide. although the oxides of Ni804 and Sb764.240.765 have also been incorporated into In2O3.183 and spin coating a dispersion of tin-doped indium oxide ‘nanoparticle’. Indium oxide as an electrochromic host The most commonly encountered mixed-metal oxide is indium–tin oxide (ITO). 809. where the indium is inert. Ion insertion into ITO is extremely slow. switching device in view of its [poor] long-term stability’.2.813.8 cm2 CÀ1 at 600 nm.811 Electroreversibility is problematic if Liþ rather than Hþ is the mobile ion inserted.243. The coloration efficiency is 2. .809 and is therefore a perfect choice for a ‘passive’ counter electrode. respectively).818. e. Optical properties of ITO Some ITO has no visible electrochromism. Bressers and Meulenkamp820 consider that ITO ‘probably cannot be used as a combined ion-storage layer and transparent conductor for all-solid-state .676 Devices containing ITO counter electrodes When considered for use as an electrochrome. Contrarily. . ITO is always the secondary ‘optically passive’ ioninsertion layer.817 Perhaps similar is the mixed-valent behaviour recently inferred for677 LixSnO2. so redox cycles ought to be shallow (i.819 or poly(3-methylthiophene)812 as the primary electrochrome.g.241. see Figure 6.821 .4 Metal oxides: dual-metal electrochromes 197 ITO ðsÞ þ xðMþ þ eÀ Þ ! Mx ITO ðsÞ.IIO2(In2O3).277.e. cite 1 Â 10À11 cm2 sÀ1.g. as yet.241. X-Ray photoelectron spectroscopy studies seem to support this conclusion. from LiClO4 electrolyte in PC. reductive incorporation of Liþ increases the electronic conductivity of the ITO. e. with most of the cited values of chemical diffusion coefficient D lying in the range 10À13 to 10À16 cm2 sÀ1. as outlined in Section 16.811.244 MxITO is too pale to adopt as a primary electrochrome since its maximal CR is only 1:1. as determined by Mossbauer measurements. The resultant partially reduced oxide MxITO may be symbolised as MxSnIV. quite unknown.809 811 although Yu et al. with WO3243. (6.43) kept relatively small).812.6.814. with x in Eq.243 Few cycle lives are cited in the literature: Golden and Steele244 and Corradini et al.802. but it may be Hþ.812 are probably the only authors to cite a high write–erase efficiency (of 104 and 2 Â 104 cycles.2. The colour of reduced ITO of different origin is pale brown (possibly owing to SnII).242. The LixSnO2 in ¨ that study was made by Liþ insertion into sputtered SnO2. colourless pale brown (6:43) where M is usually Liþ. The reduced form of ITO is chemically unstable. These low values may also be the cause of hysteresis in coulometric titration curves.816 A recent report suggesting a yellow–blue colour was formed during electroreduction of ITO is intriguing since the source of the blue is.815.19. 5 Wav elength (μm) 2. (Figure reproduced from Goldner.5 Figure 6. B. as prepared by sol–gel methods. by permission of The Optical Society of America. et al.5 0. and the bleached state is more transparent. considering the electro-inactive nature of MgO. Naþ and Kþ ions.0 0.0 Metal oxides 1.5 1. poly(pphenylene terephthalamide).198 1.4 0.5 < y < 3) are superior to iridium oxide alone. Addition of Ta2O5 decreases the coloration efficiency but increases chemical diffusion coefficient D.9 0.531 Iron oxide as electrochromic host Iron oxide has been host to the oxides of Si and Ti. ‘Electrochromic behaviour in ITO and related oxides’. The changes are thought to be the result of diluting the colouring IrO2 with Ta2O5. for the electrochromic modulation is wider.19 UV-visible spectrum of thin-film ITO in its oxidised (–– clear) and partially reduced ( Á Á Á Á pale brown) forms.2 0. Appl.1 0. Opt. The coloration efficiencies of this mixed oxide lie in the range824 6–14 cm2 CÀ1 at max of 450 nm (the authors do not say which compositions . 24. R.8 0. 1985. which supports a superior ionic conductivity.0 2.823 Films of composition IrMgyOz (2.824 The films investigated are able reversibly to take up Liþ.0 0.6 0.7 Transmissivity 0. Iridium oxide has also been incorporated into aramid resin.) Iridium oxide as electrochromic host Iridium oxide has been doped with the oxides of magnesium822 and with tantalum..822 Such a high proportion of magnesium is surprising.3 0. 2283–4. 12.737. no gas whatsoever forms when a gold electrode is coated with similarly formed Mo–Cr and Mo–Fe oxides.91 Cr. H2 is first formed at the surface of the MoO3 layer at À0.171.202.62. from a solution of peroxopolymolybdotungstate.124.12.745. 77 cm2 CÀ1 for MoO3 alone7 and 3 cm2 CÀ1 for ITO alone244 (although some ITO is completely passive optically809).91 Fe. 133 ff.22.825 Ni. e.91 Sn. but others have been prepared by sol–gel techniques. being in the range 2–10 cm2 CÀ1.730.).81. More impressive still. As an example.743. 829 or W.91 Nb. 7 244 809 826 .827 Ti. Films MoO3 ITO ITO MoO3–SnO2 /cm2 CÀ1 77 3 0 2–10 Ref. which clearly shows how the optical behaviour of Mo–Sn oxide is more akin to SnO2 than to MoO3.750.361.61. The value of for Mo–Ti oxide lies in the range 10–50 cm2 CÀ1.75 V for electrodeposited Mo–W oxide (electrodeposited together on gold). An additional benefit of incorporating molybdenum into an electrochromic mixture is its ability to extend the overpotential for hydrogen evolution (a nuisance if occurring at lower potentials) when in contact with a protonic acid. These data are summarised in Table 6.746.22 Table 6.741. SCE).744.475. Molybdenum oxide as electrochromic host Thin-film molybdenum oxide has also been made as a mixture with the oxides of Co.740.747 Most thin films of Mo–W oxide were prepared from reactive sputtering.202 The electrochromic transition for the resultant film is ‘green– yellow ! violet’ when cycled in LiClO4–PC solution as the ion-providing electrolyte.g.739.826. the value increasing as the mole fraction of molybdenum increases.742.85 V (vs. À0. cf.80. cf.125 itself made by oxidative dissolution of both metallic molybdenum and tungsten in hydrogen peroxide (see p.292. Molybdenum–vanadium oxide is also made by dissolving the respective metals in H2O2.4 Metal oxides: dual-metal electrochromes 199 relate to these values of except ‘the largest extent of colouring and bleaching was for pure iron oxide’).826. Possibly the tin sites are electroactive while the Mo sites are not.91.830 The coloration efficiency for MoO3–SnO2 films826 is low.6.165.828 V124. Effect on the coloration efficiency of mixing molybdenum and tin oxides.738. 842 to yield films with markedly different spectra. Cr or Pb retards the rates of bleaching. for reasons not yet clear.83 Ce.84.94. an unwanted brown tint.833. but incorporating Al or Mg in the film virtually eliminates this colour.840 Nb.832. The maximum change in transmittance was observed for films comprising 20% Mo.80.636.834 Such films also show superior charge capacity.77 Co. while adding Ce.831 Al.636. .735.77.77 Addition of yttrium oxide severely impedes the rate of NiO electrocoloration.736.832 Thus for applications requiring a highly bleached transmittance.167.837.831. Schmitt and Aegerter171 prepared a variety of films that were doped with a variety of d-block oxides.83.79.834 Cd. The complex [Ru3O(acetate)6-m-{pyrazine}3-[Fe(CN)5]3]nÀ has also been incorporated into NiOx. with oxides of Ag. such as architectural windows.77 Si.82.171. Cr or La into NiO improves the rates of electrocoloration.801 Cr.835 Cu.171.838 Mg.833. such as Ni–Au alloy.77.161 Nickel tungstate is also electrochromic.841 Ta.171 Lee and Crayston have also made a Nb–silicone composite.80.77.53.832.81. the Al. e.752 Y77 and Zn.831 V.77.77 An important observation for ECD construction is that electrodeposited Ni–La and Ni–Ce oxides are significantly more durable than NiO alone. the highest being for Nb2O5 containing 20% TiO2.834 W.171 Ti.831.761.750 and Zn.846 W749.82 and films containing gold are also made readily.77 Fe.845 Mo.843 Nickel oxide often shows a residual absorption.77.844 Niobium oxide as electrochromic host Electrochromic films have been prepared that are doped with the oxides of Ce.750.g.831 Sn.833.and Mg-containing oxides are superior to conventional nickel oxide. The coloration efficiencies of such films were not particularly sensitive to the other metals.836La.832.833.642 In a recent study of sol–gel deposited Nb2O5. so it is interesting that electrodeposited Ni–W oxide has a rather low coloration efficiency of 99 4.832 Incorporation of Ce.839 Mn.831.4 cm2 CÀ1 while for sol–gel-derived NiO is152 À(35 to 40) cm2 CÀ1. 99. Tungsten trioxide is cathodically colouring while NiO is anodically colouring.832. which has a coloration efficiency of 27 cm2 CÀ1.165.769 Fe.77.77 as evidenced by longer cycle life.89.831 Sn.831 Pb.81.82. for their greatly enhanced transparency.77.825 Ni.140.83 Nickel oxide has also been mixed with particles of various alloys. Traces of ferrocyanide have been incorporated.200 Metal oxides Nickel oxide as electrochromic host Several electrochromic mixtures have been prepared of nickel oxide.751. 852.848 The film of Ce–Sn oxide was wholly optically inactive. with a transparency higher than 90%.780 Mo. cf. Pr. Ta.755.784. Ni–Ti oxide is made from NiCl2 and Ti alkoxide.755. Electrochromic mixtures have been prepared of TiO2 with oxides of Ce.775. Representative values are summarised in Table 6.826.783.205.758.786.849.129.757.122. 316.13.404.826.324.841 Sb827 or V.22.776.787 Fe.777.841 779.760.323.4 Metal oxides: dual-metal electrochromes 201 Table 6.6.771Mo.856 Most of these electrochrome mixtures were made by sol–gel or sputtering techniques.13.782.134.781.825 150.171.768and TiO2 containing hexacyanoferrate has also been produced. V.851. Tin oxide as electrochromic host Electrochromic mixtures have been prepared of tin oxide.171 The coloration efficiencies for such mixed Nb–metal oxide films are all low. For example.335.759. with the oxides of Ce. 828 165.779.780.854 Nb.749 and Nb–Fe oxide behaves more like Nb2O5 than either FeO or Fe2O3.755.203.780 754 721.855 and Zn. Titanium oxide as electrochromic host Electrochromic mixtures of titanium are at present much used.850 La. 48 cm2 CÀ1 for WO3 prepared by the same procedures.150and Ti–Fe oxide was prepared by a dip-coating procedure850 via a liquor comprising alcoholic ferric nitrate and Ti(OiPr)4).563 A mixture of TiO2 and phosphotungstic acid has been made via sol–gel techniques.785. Ni.748 and doped niobium oxide is also more electrochemically stable. followed by .756.779.772. 170 258 171 748 629 102 845 171 748 748 It is clear that Nb–W oxide behaves more like WO3 than Nb2O5.774. Hydrated HNbWO6 has a similar coloration efficiency (54 cm2 CÀ1)748. Components Nb2O5 Nb2O5 Nb2O5 Nb2O5 FeO Fe2O3 Nb2O5–FeO Nb2O5 þ 20% TiO2 HNbWO6 (hydrated) WO3 Preparation route rf sputtering rf sputtering Sol–gel Sol–gel CVD Electrodeposition CVD Sol–gel Sol–gel Sol–gel /cm2 CÀ1 22 <12 16 25–30 À6 to À6. W127.5 À30 20 27 54 48 Ref.771.778.827 Ni. Effect on the coloration efficiency of mixing niobium oxides with iron or titanium oxide: the effect of mixing and preparation method. 773.847 to that of WO3.845 Hydrated HNbWO6 also has a superior chemical stability to that of WO3 alone.853. 790. are said to be ‘neutral’ in colour. When doped with the rare-earth oxides of Nd. Vanadium oxide as electrochromic host Electrochromic mixtures have been prepared of vanadium oxide.858 Ni. This latter material generates a dark blue metallic electrochromic colour.793 Dy.860 Sm.851. see p. The value of max for the Ni–Ti oxide150 is 633 nm. Such neutral colours have been made with V–Ti oxide (with brown–blue electrochromism).124.792.202 Metal oxides annealing in air. both properties implying a fast electrochromic transition. 325.861 The electrochromic behaviour of V–Ti oxide films is complicated:853 in the best explanatory model.791.858 films of V2O5 show a considerably enhanced cycle life.204. and lies in the range À (10–42) cm2 CÀ1. The V–Ti films have a larger charge capacity if the mole fraction of vanadium is relatively high. Sm. The optical charge-transfer transition in the Ti–Fe system is responsible for the blue colour of naturally occurring sapphire.858. 399.858 Sn. the inserted electrons are supposed to be localised.852.865 .4-dihydroxyphenylalanine with a V2O5 xerogel.829 Nd.854 attribute to the vanadium component.858 Fe.687. residing preferentially at vanadium sites.728.721.863 Composites of vanadium oxide have been formed by reacting a xerogel (see p.202. Dy.3V2O5]. a film of Ni–V oxide is ‘virtually [optically] passive’.853.703 Pr.862 Oxide electrochromes having a grey hue.831.327. with decreasing as the mole % of vanadium increases. probably having a different structure.857 but thin-film Ti–Fe oxide (prepared in this case by a dip-coating procedure850) did not possess the same colour as sapphire.6 cm2 CÀ1.761 Other electrochromic vanadates include FeVO4159 and CeVO4. 161) with organic materials such as the nanocomposite [poly(aniline N-propanesulfonic acid)0. X-Ray diffraction results suggest the formation of the respective orthovanadate species SmVO4 and DyVO4.687 and for V–W oxide which has a coloration efficiency in the range 7 to 30 cm2 CÀ1.159 In.864 The second V2O5–organic composite is a ‘melanine like’ material formed by reacting 3.761 Thin films of composition (V2O5)3–(TiO2)7 oxide form a reddish brown colour at anodic potentials which Nagase et al. with the oxides of Bi. however. the value depending on the composition.848 Ti166. Similarly.854 or W. 680. rather than blue.687.254. The V–Sm oxide film showed a very small coloration efficiency of only 0. although no values of are cited. the layer of W–Ti oxide was made by pulsed cathodic electrodeposition. so the authors suggest counter-electrode use.832 Pa.761. implying the majority TiO2 component is optically passive.864 This material has a superior electronic conductivity to the precursor V2O5 xerogel alone and exhibits shorter ionic diffusion pathways.859 Mo.594 Ce.788. In ref.789. 867 Ternary and higher oxides A few multiple-metal oxides have been made: for example.796 For example. Electrochromic iridium–ruthenium oxide in the molar ratio 40:50% is said to be 300 times more stable than either constituent oxide.156 NiV0.75O2. suggesting a new phase rather than a mixture. possibly not caused here by charge transfer. With counter-electrode use in mind. The addition of magnesium significantly enhances the optical transparency of the films in their bleached state. For example.794. referring to material in the compositional range Zr0.783.140 Several other ternary oxides comprising three transition-metal oxides have received attention: the oxides of Co–Ni–Ir868 and Cr–Fe–Ni (this latter oxide being grown anodically on the metallic alloy Inconel-600)869 and W–V–Ti .786. made CeO2–TiO2–ZrO2. electrodeposition can be employed to produce mixtures of tungsten oxide together with three or even four additional metal oxides.767.156 Samples of NiO Á WOxPy were obtained from a polytungsten gel in which H3PO4 was added. It is now a popular choice of optically passive electrochromic layer when mixed with cerium oxide.122.866 Its electrochromic qualities are said to be superior to either constituent oxide.839 over the wavelength range 400 < < 500 nm.96 A notable mixture is W–Cr–Mo–Ni oxide. thin films of oxides based on Ni–V–Mg (made by reactive dc magnetron sputtering) show pronounced anodic electrochromism.6.’s 1998 review of devices.5 oxides.4Ce0. The charge capacity of Zr–Ce oxide increases with increasing cerium content.4 Metal oxides: dual-metal electrochromes 203 Zirconium oxide as electrochromic host Pure zirconium oxide is not electrochromic and has practically zero charge capacity. Orel and co-workers166 made V–Ti–Zr and V–Ti–Ce oxides. The electrochromism was optimised when the P:W ratio was 100:8. in Granqvist et al. Most of these mixtures were prepared to ‘tweak’ the optical properties of a host oxide. and Avandano et al. Its coloration efficiency is estimated to be 47 cm2 CÀ1 at 650 nm.796 Miscellaneous electrochromic hosts Tantalum–zirconium oxide is electrochromic.08Mg0.796 but has been host to a large number of other oxides.795. and.3.96 which forms a green electrochromic colour – a colour not often seen in the field of inorganic electrochromism. though insufficiently analysed.797 they cite Zr–Ce as the optically passive secondary layer.6O2 to Zr0.25Ce0.832 and CeO2–TiO2–ZrO2. Electrochromic mixtures of metal oxide incorporating precious metal.4.870 Finally. 881 495 882 883 879.161 In the study of Au–NiO films by Fantini et al. The electrochromic ceramic metal (‘cermet’) Au–WO3 prepared by Sichel and Gittleman879 comprised a matrix of amorphous WO3 containing grains of Au of ˚ approximate diameter 20–120 A. Table 6. 877. depending on mole fraction. Lian and Birss871 have studied the electrochromism of the hydrous oxide layer formed on the alloy Ni51Co23Cr10Mo7Fe5. 830.5. also incorporated particulate gold (and V2O5) in an aramid resin. 880. 872 873 830 874 531 161.14.204 Metal oxides oxide. The films reflected the different colours blue. Such composites can be made in various ways: dual-target sputtering.0 to 0. 876 531.727 Ternary oxides comprising p-block metals include Co–Al–Si617. yellow and orange– red.798 and Ce–Mo–Si. 879. Nb. Precious metal Ag Ag Ag Au Au Au Au Au Au Pt Pt Pt Pt Host ITO V2O5 WO3 CoO IrO2 NiO MoO3 V2O5 WO3 MoO3 RuO2 Ta2O5 WO3 Ref. 875 495. Its electrochromic behaviour is. 874. The cermet is blue as prepared. 878 201. or all sol–gel. mixed sputtering and sol–gel.14 lists a few such studies. 877. green.. Sb or V has also been studied. Table 6. similar to that of NiOx.3 Electrochromic oxides incorporating precious metals Several workers have incorporated particulate precious metal in an oxide host. The matrix must be amorphous in order for the red colour to develop. but is red or pink when electrochemically coloured – a relatively rare colour for an electrochromic oxide. 6. In the study in ref. apparently. 884 .768 The electrochromic behaviour of the materials (WO3)x(Li2O)y(MO)z where M ¼ Ce.874 the Au mole fraction of gold varied between from 0.05.5B3. 800. Yano et al. Fe. Mn. 885 For this reason. Rutherford backscattering (RBS) suggests the film composition is SnO2. the exact stoichiometry is often indefinite or unknown.4 Metal oxyfluorides Many thin-film metal oxyfluorides are electrochromic.6. Elevated target temperatures yielded strongly enhanced rates of electrochromic coloration.4. fluorinated tin oxide is superior as an optically transparent electrode.1F0. The cycle life is as high as 2 Â 104 cycles.886 When such films are immersed in PC containing LiClO4. but is a poor electrochromic oxide. In the literature. they represent fluorinated analogues of the respective metal oxide. The redox reaction causing the colour is: F:SnO2 þ xðLiþ þ eÀ Þ ! Lix F:SnO2 : (6:44) It is easier to electro-insert Liþ into SnO2 electrodes than into fluorinated F:SnO2. the electrochromic effect is ‘pronounced’. the diffusion of Naþ or Kþ through F:TiO2 is too slow to countenance inclusion within devices.6C0. we term the oxides.890 The redox reaction causing the colour is: F:WO3 þ xðLiþ þ eÀ Þ ! Lix F:WO3 : (6:46) . structural changes accompany the incorporation of Kþ.3. the colour said to derive from photo-effected polaron interaction. The redox reaction causing the colour is: F:TiO2 þ xðLiþ þ eÀ Þ ! Lix F:TiO2 : (6:45) The coloration efficiency is 37 cm2 CÀ1 at 700 nm. and the wavelength maximum occurs at $780 nm.885 When such films are immersed in PC containing LiClO4.95F0. the electrochromic effect is weak. For this reason. Tin Films of F:SnO2 were made by reactive rf sputtering in Ar þ O2 þ CF4 atmosphere. The amount of fluorine incorporated in the film is quite small: results from RBS suggest a composition of TiO1. ‘F:MOx’. The coloration efficiency is 60 cm2 CÀ1.887 As expected. In fact.888 Tungsten Granqvist and co-workers889 made thin-film tungsten oxyfluoride by reactive dc magnetron sputtering in plasmas containing O2 þ CF4.1.4 Metal oxides: dual-metal electrochromes 205 6. In effect. 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M. 5489–98. 151. Phys. 2877–84. L. Kaneko. F. 3555–62. M... 152–3. S. 175. M. Vuk. Rougier.. Fe-containing CeVO4 films as Li intercalation transparent counter-electrodes. Mater. Blyr. M. Lu. 2002.. F. Garcia. Acta. Electrochromic W–M–O (M ¼ V. Zanta. Electrochim. 46. S.. Acta. 1985. Bertolo. Decker.. T. 96–106. Atanasoska. ˇ 862. Electroanal. 1992. 2077–84. F. Sol–Gel Sci.. 1991.. and Niklasson. J. L.. Chem. S. Orel. K. 868. Fantini.. Coluzza. Granqvist. and Ganson. da Silva. 2001. F. Hartmann.. and Impey. Ferreira. Parvan. J. U. A. D. S. 2003. 867. R. 46. Picardi. Y.-Z. 875. 2002. Milun. 871.. Electrochem. 867–72..-Q. 28.. characterization and properties of a melanin-like/ ¸ vanadium pentoxide hybrid compound. 330. Solid State Ionics. H. Electrochemical and electrochromic characteristics of Ta2O5–ZnO composite films.. J. 371–5. O. M. G. and Qin. Energy Mater. ˚ 872. 2003. 160.. and de Torresi.References 251 858. 2004.797. Hydrous oxide growth on amorphous Ni–Co alloys. Chem. Huguenin. Mori. V. Dy) oxides.. and Milun... 343–57. Vukovic. Regis. H. Coustier. 83–6. M.. D. P. and Orel. A. Theoretical and experimental results on Au–NiO and Au–CoO electrochromic composite films. Marijan. and Rosa.-N. M. 1992. 71. R. G.. Ferreira. A. Decker. Nb) sol–gel thin films: a way to neutral colour. L. and Fantini. B. J. J. A. Solid State Ionics. A. Sci. Eng. 100.. Anodic stability and electrochromism of electrodeposited ruthenium–iridium coatings on titanium. Surface modification of Inconel-600 by growth of a hydrous oxide film. ˇ ˇ ˇ 861. C. Q... F. 864.. J. S. Buttry. Cells. Japanese Patent JP 6. Jarrendahl. Appl. 5. W. Surface analyses of In V oxide films aged electrochemically by Li insertion reactions. Hultaker.. 859. 1997. 873. and Yamanaka. Technol. Chu. G... F. Chu.. Y. C.. N. M. C. Investigations on counter electrode materials for solid state electrochromic systems. P. 2001. Canon. Solid State Ionics. K. Artuso. Bonino. V. R. C. 1991. 305–10. A. Passerini. J.. E. Electroanal. Cimino. U. 559.. B. and Atanasoski. P. F. Phys.. 13... NuLi. S. 874.-W. A. Zhang. 8. L. Stanger. 1998.. Galina. F. Sm.. 2002.. 863. Electrochemical and Raman studies on a hybrid organic–inorganic nanocomposite of vanadium oxide and a sulfonated polyaniline. 869. 865. A. Dip-coated silver-doped V2O5 xerogels as host materials for lithium intercalation. K. Surca. F. Jpn. D. J. Santo. C. R. D. Kokai Tokkyo Koho. Lian. D. Chem. 10. Oliveira. Electrical and optical properties of sputter deposited tin doped indium oxide thin films with silver additive. K. and Gorenstein. Chromogenic WPA/TiO2 hybrid gels and films. Q. 2001.. Leonhard. Multilayered composite Au–NiOx electrochromic films. Electrochromism of vanadium oxide films doped by rare-earth (Pr. 252 Metal oxides 876. He, T., Ma, Y., Cao, Y., Yin, Y., Yang, W. and Yao, J. Enhanced visible-light coloration and its mechanism of MoO3 thin films by Au nanoparticles. Appl. Surf. Sci., 180, 2001, 336–40. 877. Yano, J., Hirayama, T., Yamasaki, S., Yamazaki, S. and Kanno, Y. Stable freestanding aramid resin film containing vanadium pentoxide and new colour electrochromism of the film by electrodeposition of gold. Electrochem. Commun., 3, 2001, 263–6. 878. Nagase, K., Shimizu, Y., Miura, N. and Yamazoe, N. Electrochromism of gold–vanadium pentoxide composite thin films prepared by alternating thermal deposition. Appl. Phys. Lett., 9, 1994, 1059–61. 879. Sichel, E. K. and Gittleman, G. I. Characteristics of the electrochromic materials Au–WO3 and Pt–WO3. J. Electron. Mater., 8, 1979, 1–9. 880. Heszler, P., Reyes, L. F., Hoel, A., Landstrom, L., Lantto, V. and Granqvist, C. G. Nanoparticle films made by gas phase synthesis: comparison of various techniques and sensor applications. Proc. SPIE, 5055, 2003, 106–19. 881. Park, K.-W. Electrochromic properties of Au–WO3 nanocomposite thin-film electrode. Electrochim. Acta, 50, 2005, 4690–3. 882. Park, K.-W. and Sung, Y. E. Modulation of electrochromic performance and in situ observation of proton transport in Pt–RuO2 nanocomposite thin-film electrodes. J. Appl. Phys., 94, 2003, 7276–80. 883. Park, K.-W. and Toney, M. F. Electrochemical and electrochromic properties of nanoworm-shaped Ta2O5–Pt thin-films. Electrochem. Commun., 7, 2005, 151–5. 884. Chen, K. Y. and Tseung, A. C. C. Effect of Nafion dispersion on the stability of Pt/WO3 electrodes. J. Electrochem. Soc., 143, 1996, 2703–8. 885. Strømme, M., Isidorsson, J., Niklasson, G. A. and Granqvist, C. G. Impedance studies on Li insertion electrodes of Sn oxide and oxyfluoride. J. Appl. Phys., 80, 1996, 233–41. 886. Strømme, M., Gutarra, A., Niklasson, G. A. and Granqvist, C. G. Impedance spectroscopy on lithiated Ti oxide and Ti oxyfluoride thin films. J. Appl. Phys., 79, 1996, 3749–57. 887. Gutarra, A., Azens, A., Stjerna, B. and Granqvist, C. G. Electrochromism of sputtered fluorinated titanium oxide thin films. Appl. Phys. Lett., 64, 1994, 1604–6. 888. Strømme Mattson, M., Niklasson, G. A. and Granqvist, C. G. Diffusion of Li, Na, and K in fluorinated Ti dioxide films: applicability of the Anderson–Stuart model. J. Appl. Phys., 81, 1997, 2167–72. 889. Azens, A., Stjerna, B. and Granqvist, C. G. Chemically enhanced sputtering in fluorine-containing plasmas: application to tungsten oxyfluoride. Thin Solid Films, 254, 1995, 1–2. 890. Azens, A., Stjerna, B., Granqvist, C. G., Gabrusenoks, J. and Lusis, A. Electrochromism in tungsten oxyfluoride films made by chemically enhanced d. c. sputtering. Appl. Phys. Lett., 65, 1994, 1998–2000. 891. Azens, A., Granqvist, C. G., Pentjuss, E., Gabrusenoks, J. and Barczynska, J. Electrochromism of fluorinated and electron-bombarded tungsten oxide. J. Appl. Phys., 78, 1995, 1968–74. 7 Electrochromism within metal coordination complexes 7.1 Redox coloration and the underlying electronic transitions Metal coordination complexes show promise as electrochromic materials because of their intense coloration and redox reactivity.1 Chromophore properties arise from low-energy metal-to-ligand charge-transfer (MLCT), intervalence charge-transfer (IVCT), intra-ligand excitation, and related visible-region electronic transitions. Because these transitions involve valence electrons, chromophoric characteristics are altered or eliminated upon oxidation or reduction of the complex, as touched on in Chapter 1. A familiar example used in titrations is the redox indicator ferroin, [FeII(phen)3]2 þ (phen ¼ 1,10-phenanthroline), which has been employed in a solid-state ECD, the deep red colour of which is transformed to pale blue on oxidation to the iron(III) form.2 Often more markedly than other chemical groups, a coloured metal coordination complex susceptible to a redox change will in general undergo an accompanying colour change, and will therefore be electrochromic to some extent. The redox change – electron loss or gain – can be assigned to either the central coordinating cation or the bound ligand(s); often it is clear which, but not always. If it is the central cation that undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals, while the less clear convention for ligands is usually to indicate the extra charge lost or gained by a superscripted þ or À. As mentioned in Chapter 1, whilst the term ‘coloured’ generally implies absorption in the visible region, metal coordination complexes that switch between a colourless state and a state with strong absorption in the near infra red (NIR) region are now being intensively studied.3 While these spectroscopic and redox properties alone would be sufficient for direct use of metal coordination complexes in solution-phase ECDs, in addition, polymeric systems based on metal coordination-complex monomer units, which have prospective use in all-solid-state systems, have also been investigated. Following usage in the field, in this chapter an arrow between two species can indicate the direction of transfer of an electron. 253 254 Electrochromism within metal coordination complexes 7.2 Electrochromism of polypyridyl complexes 7.2.1 Polypyridyl complexes in solution The complexes [MII(bipy)3]2 þ (M ¼ Fe, Ru, Os; bipy ¼ 2,20 -bipyridine) are respectively red, orange and green, due to the presence of an intense MLCT absorption band.4 Electrochromism results from loss of the MLCT absorption band on switching to the MIII redox state. Such complexes also exhibit a series of ligand-based redox processes, the first three of which are accessible in solvents such as acetonitrile and dimethylformamide (DMF).4 Attachment of electron-withdrawing substituents to the 2,20 -bipyridine ligands allows additional ligand-based redox processes to be observed, due to the anodic shift of the redox potentials induced by these substituents. Thus Elliott and co-workers have shown that a series of colours is available with [M(bipy)3]2 þ derivatives when the 2,20 -bipyridine ligands have electron-withdrawing substituents at the 5,50 positions (see below).5 The electrochromic colours established by bulk electrochemical reactions in acetonitrile are given in Table 7.1. L L R N N L L R L 1 2 3 4 5 R= R= R= R= R= CO2Et CONEt2 CON(Me)Ph CN C(O)nBu A surface-modified polymeric system can be obtained by spin coating or heating [Ru(L6)3]2 þ as its p-tosylate salt.6 The resulting film shows sevencolour electrochromism with colours covering the full visible region spectral range, which can be scanned in 250 ms. O O O N L 6 O O N O O Spectral modulation in the NIR region has been reported for the related complex [Ru(L7)3]2 þ which undergoes six ligand-centred reductions, two per ligand.7 The complex initially shows no absorption between 700 and 2100 nm; however, upon reduction by one electron a very broad pair of overlapping peaks appear with maxima at 1210 nm (" ¼ 2600 dm3 molÀ1 cmÀ1) and 1460 nm (" ¼ 3400 dm3 molÀ1 cmÀ1). Following the second one-electron reduction, the peaks shift to slightly lower energy (1290 and 1510 nm) and increase in 7.2 Electrochromism of polypyridyl complexes 255 Table 7.1. Colours (established by bulk electrolysis in acetonitrile) of the ruthenium(II) tris-bipyridyl complexes of the ligands L1–L5, in all accessible oxidation states (from ref. 5). Charge on RuL3 unit L1 þ2 þ1 0 À1 À2 À3 Orange Purple Blue Green Brown Red L2 Orange Wine red Purple Blue L3 Orange Grey–blue Turquoise Green L4 Red–orange Purple Blue Turquoise Aquamarine Brown–green L5 Red–orange Red–brown Purple–brown Grey–blue Green Purple intensity (" ¼ 6000 and 7300 dm3 molÀ 1 cmÀ 1 respectively). Following the third one-electron reduction, the two peaks coalesce into a broad absorption at 1560 nm, which is again enhanced in intensity (" ¼ 12 000 dm3 molÀ 1 cmÀ 1). Upon reduction by the fourth and subsequent electrons the peak intensity diminishes continuously to approximately zero for the six-electron reduction product. These NIR transitions are almost exclusively ligand-based. An optically transparent thin-layer electrode (OTTLE) study8 revealed that the visible spectra of the reduced forms of [Ru(bipy)3]2 þ derivatives can be separated into two classes. Type-A complexes, such as [Ru(bipy)3]2 þ, [Ru(L7)3]2 þ and [Ru(L1)3]2 þ show spectra on reduction which contain lowintensity (" < 2500 dm3 molÀ1 cmÀ1) bands; these spectra are similar to those of the reduced free ligand and are clearly associated with ligand radical anions. In contrast, type-B complexes such as [Ru(L8)3]2 þ and [Ru(L9)3]2 þ on reduction exhibit spectra containing broad bands of greater intensity (1000 < " < 15 000 dm3molÀ1 cmÀ1). R R L L N N L 7 8 9 R = Me R = COOEt R = CONEt2 7.2.2 Reductive electropolymerisation of polypyridyl complexes The reductive electropolymerisation technique relies on the ligand-centred nature of the three sequential reductions of complexes such as [Ru(L10)3]2 þ (L10 ¼ 4-vinyl-40 -methyl-2,20 -bipyridine), combined with the anionic polymerisability of suitable ligands.9 Vinyl-substituted pyridyl ligands such as L10–L12 are generally employed, although metallopolymers have also been formed 256 Electrochromism within metal coordination complexes from chloro-substituted pyridyl ligands, via electrochemically initiated carbon– halide bond cleavage. In either case, electrochemical reduction of their metal complexes generates radicals leading to carbon–carbon bond formation and oligomerisation. Oligomers above a critical size are insoluble and thus thin films of the electroactive metallopolymer are deposited on the electrode surface. N N N N L 10 11 N N L L 12 7.2.3 Oxidative electropolymerisation of polypyridyl complexes Oxidative electropolymerisation has been described for iron(II) and ruthenium(II) complexes containing amino-10 and pendant aniline-substituted11 2,20 -bipyridyl ligands, and amino- and hydroxy- substituted 2,20 :60 ,200 -terpyridinyl ligands.12 Analysis of IR spectra suggests that the electropolymerisation of [Ru(L13)2]2 þ, via the pendant aminophenyl substituent, proceeds by a reaction mechanism similar to that of aniline.12 The resulting modified electrode reversibly switched from purple to pale pink on oxidation of FeII to FeIII. For polymeric films formed from [Ru(L14)2]2 þ, via polymerisation of the pendant hydroxyphenyl group, the colour switch was from brown to dark yellow. The dark yellow was attributed to an absorption band at 455 nm, probably due to quinone moieties in the polymer formed during electropolymerisation. Infrared spectra confirmed the absence of hydroxyl groups in the initially deposited brown films. Metallopolymer films have also been prepared by oxidative polymerisation of complexes of the type [M(phen)2(4,40 -bipy)2]2 þ (M ¼ Fe, Ru or Os; 4,40 -bipy ¼ 4,40 -bipyridine).13 Such films are both oxidatively and reductively electrochromic; reversible film-based reduction at potentials below À1 V results in dark purple films,13 the colour and potential region being consistent with the viologen-dication/radical-cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L15)3]2 þ.14 Electropolymerised films prepared from the complexes [Ru(L16)(bipy)2] [PF6]215 and [Ru(L17)3] [PF6]216,17 exhibit reversible orange– transparent electrochromic behaviour associated with the RuII/RuIII interconversion. 7.2 Electrochromism of polypyridyl complexes NH2 OH 257 N N N N N N L 13 L 14 O N O O O N N Fe OMe OMe N N N N L 15 Fe L 16 L 17 7.2.4 Spatial electrochromism of polymeric polypyridyl complexes Spatial electrochromism has been demonstrated in metallopolymeric films.18 Photolysis of poly[RuII(L10)2(py)2]Cl2 thin films on tin-doped indium oxidecoated (ITO) glass in the presence of chloride ions leads to photochemical loss of the photolabile pyridine ligands, and sequential formation of poly[RuII(L10)2(py)Cl]Cl and poly[RuII(L10)2Cl2] (see Scheme 7.1). poly[Ru I (L 10) 2(py)2]Cl 2 (orange) Ef (RuI/ ) = + 1.27 V vs. SCE hν –py poly[Ru I (L 10) 2(py)Cl]Cl (red) Ef (RuI/ ) = + 0.77 V vs. SCE hν –py poly[Ru I (L 10) 2Cl2] (purple) Ef (RuI/ ) = + 0.35 V vs. SCE Scheme 7.1 Spatial electrochromism in metallopolymeric films using photolabile pyridine ligands. (Scheme reproduced from Leasure, R. M., Ou, W., Moss, J. A., Linton, R. W. and Meyer, T. J. ‘Spatial electrochromism in metallo-polymeric films of ruthenium polypyridyl complexes.’ Chem. Mater., 8, 1996, 264–73, with permission of The American Chemical Society.) 258 Electrochromism within metal coordination complexes Contact lithography can be used to spatially control the photosubstitution process to form laterally resolved bicomponent films with image resolution below 10 mm. Dramatic changes occur in the colours and redox potentials of such ruthenium(II) complexes upon substitution of chloride for the pyridine ligands (Scheme 7.1). Striped patterns of variable colours are observed on addressing such films with a sequence of potentials. 7.3 Electrochromism in metallophthalocyanines and porphyrins 7.3.1 Introduction to metal phthalocyanines and porphyrins The porphyrins are a group of highly coloured, naturally occurring pigments containing a tetrapyrrole porphine nucleus (see below) with substituents at the eight b-positions of the pyrroles, and/or the four meso-positions between the pyrrole rings.19 The natural pigments themselves are metal chelate complexes of the porphyrins. Phthalocyanines are tetraazatetrabenzo derivatives of porphyrins with highly delocalised p-electron systems. Metallophthalocyanines are Et N HN Et N NH N HN Et NH N Et N NH N HN Et Et Et Et 21H, 23H-Porphine Tetraphenyl porphyrin (H2TPP) Octaethyl porphyrin (H2OEP) N N N N N N N NH N N HN N N N N N M N N N M N N N N N N N N N N N N N 29H, 31H-Phthalocyanine 1:1 Metallophthalocyanine complex A 'sandwich'-type metallophthalocyanine complex 7.3 Metallophthalocyanines and porphyrins 259 important industrial pigments, blue to green in colour, used primarily in inks and for colouring plastics and metal surfaces.19,20,21 The water-soluble sulfonate derivatives are used as dyestuffs for clothing. In addition to these uses, the metallophthalocyanines have been extensively investigated in many fields including catalysis, liquid crystals, gas sensors, electronic conductivity, photosensitisers, non-linear optics and electrochromism.20 The purity and depth of the colour of metallophthalocyanines arise from the unique property of having an isolated, single band located in the far-red end of the visible spectrum (near 670 nm), with " often exceeding 105 dm3 molÀ1 cmÀ1. The next, more energetic, set of transitions is generally much less intense, near 340 nm. Charge transfer transitions between a chosen metal and the phthalocyanine ring introduce additional bands around 500 nm that allow tuning of the hue.20 The metal ion in metallophthalocyanines lies either at the centre of a single phthalocyanine (Pc ¼ dianion of phthalocyanine), or between two rings in a sandwich-type complex.20 Phthalocyanine complexes of transition metals usually contain only a single Pc ring while lanthanide-containing species usually form bis(phthalocyanines), where the p-systems interact strongly with each other, resulting in characteristic features such as the semiconducting ( ¼ 5 Â 10À5 OÀ1 cmÀ1) properties of thin films of bis(phthalocyaninato)lutetium(III) [Lu(Pc)2].22 7.3.2 Sublimed bis(phthalocyaninato)lutetium(III) films The electrochromism of the phthalocyanine ring-based redox processes of vacuum-sublimed thin films of [Lu(Pc)2] was first reported in 1970,23 and since that time this complex has received most attention, although many other (mainly lanthanide) metallophthalocyanines have been investigated for their electrochromic properties. The complex Lu(Pc)2 has been studied extensively by Collins and Schiffrin24,25 and by Nicholson and Pizzarello.26,27,28,29,30,31 It was initially studied as a film immersed in aqueous electrolyte, but hydroxide ion from water causes gradual film destruction, attacking nitrogens of the Pc ring.24 Acidic solution allows a greater number of stable write–erase cycles, up to 5 Â 106 cycles in sulfuric acid,24 approaching exploitable device requirements. Films of [Lu(Pc)2] in ethylene glycol solution were found to be even more stable.25 Fresh [Lu(Pc)2] films (likely to be singly protonated,31 although this issue is contentious24,32), which are brilliant green in colour (max ¼ 605 nm), are electro-oxidised to a yellow–tan form, Eq. (7.1):26,29,32 ½Pc2 LuHþ ðsÞ ! ½Pc2 Luþ ðsÞ þ Hþ þ eÀ : green yellow-tan (7:1) 260 Electrochromism within metal coordination complexes A further oxidation product is red,26,29,32 yet of unknown composition. Electroreduction of [Lu(Pc)2] films gives a blue-coloured film, Eq. (7.2):33 ½Pc2 LuHþ ðsÞ þ eÀ ! ½Pc2 LuH ðsÞ; green blue (7:2) with further reduction yielding a violet–blue product, Eqs. (7.3) and (7.4):29 ½Pc2 LuH ðsÞ þ eÀ ! ½Pc2 LuHÀ ðsÞ; blue violet ½Pc2 LuHÀ ðsÞ þ eÀ ! ½Pc2 LuH2À ðsÞ: (7:3) (7:4) The lutetium bis(phthalocyanine) system is a truly electropolychromic one,23 but usually only the blue-to-green transition is used in ECDs. Although prototypes have been constructed,34 no ECD incorporating [Lu(Pc)2] has yet been marketed, owing to experimental difficulties such as film disintegration caused by constant counter-anion ingress/egress on colour switching.24 For this reason, larger anions are best avoided to minimise the mechanical stresses. A second, related, handicap of metallophthalocyanine electrochromic devices is their relatively long response times. Nicholson and Pizzarello30 investigated the kinetics of colour reversal and found that small anions like chloride and bromide allow faster colour switching. Sammells and Pujare overcame the problem of slow penetration of anions into solid lattices by using an ECD containing an electrochrome suspension in semi-solid poly(AMPS) – AMPS ¼ 2-acrylamido-2methyl propane sulfonic acid) electrolyte.34 While the response times are still somewhat long, the open-circuit life times (‘memory’ times) of all colours were found to be very good.30 Films in chloride, bromide, iodide and sulfatecontaining solutions were found to be especially stable in this respect. 7.3.3 Other metal phthalocyanines Moskalev et al. prepared the phthalocyanine complexes of neodymium, americium, europium, thorium and gallium (the latter as the half acetate).35 Collins and Schiffrin24 have reported the electrochromic behaviour of the phthalocyanine complexes CoPc, SnCl2Pc, SnPc2, MoPc, CuPc and the metal-free H2Pc. No electrochromism was observed for either the metal-free or for the copper phthalocyanines in the potential ranges employed; all of the other complexes showed limited electrochromism. Both SnCl2Pc and SnPc2 7.3 Metallophthalocyanines and porphyrins 261 could be readily reduced, but showed no anodic electrochromism. Other molecular phthalocyanine electrochromes studied include complexes of aluminium,36 copper,37 chromium,36,38 erbium,39 europium,40 iron,41 magnesium,42 manganese,38,42 titanium,43 uranium,44 vanadium,43 ytterbium,45,46 zinc47 and zirconium.36,40,48 Mixed phthalocyanine systems have also been prepared by reacting mixed-metal precursors comprising the lanthanide metals dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium and small amounts of others;49 the response times for such mixtures are reportedly superior to those for single-component films. Walton et al. have compared the electrochemistry of lutetium and ytterbium bis(phthalocyanines), finding them to be essentially identical.50 Both chromium and manganese mono-phthalocyanine complexes undergo metal-centred oxidation and reduction processes.38 In contrast, the redox reactions of LuPc2 occur on the ligand; electron transfer to the central lutetium causes molecular dissociation.51 Lever and co-workers have studied cobalt phthalocyanine systems in which two or four Co(Pc) units are connected via chemical links.52,53,54,55 This group has also studied tetrasulfonated cobalt and iron phthalocyanines.56 Finally, polymeric ytterbium bis(phthalocyanine) has been investigated57,58,59 using a plasma to effect the polymerisation. 7.3.4 Electrochemical routes to metallophthalocyanine electrochromic films For complexes with pendant aniline and hydroxy-substituted ligands, oxidative electropolymerisation is an alternative route to metallophthalocyanine electrochromic films. Although polymer films prepared from [Lu(L18)2] monomer show loss of electroactivity on being cycled to positive potentials, in dimethyl sulfoxide (DMSO) the electrochemical response at negative potentials is stable, with the observation of two broad quasi-reversible one-electron redox couples.60 Spectroelectrochemical measurements revealed switching times of < 2 s for the observed green–grey–blue colour transitions in this region. Oxidative electropolymerisation using pendant aniline substituents has also been applied to monophthalocyaninato transition-metal complexes;61 the redox reactions and colour changes of two of the examples studied are given in Eqs. (7.5)–(7.8). poly½CoII ðL18 Þ þ neÀ ! poly½CoI ðL18 ÞÀ ; blue-green yellow-brown poly½CoI ðL18 ÞÀ þ neÀ ! poly½CoI ðL18 Þ2À ; yellow-brown red-brownðthick filmsÞ; deep pinkðthin filmsÞ (7:5) (7:6) the second reduction being ligandcentred.63 .87 V (from ring reduction).35 [from CoII! CoI] and À0.2 to þ 1. 7.5 Langmuir–Blodgett metallophthalocyanine electrochromic films The electrochemical properties of a variety of metallophthalocyanines have been studied as multilayer Langmuir–Blodgett (LB) films. By contrast. L N N R L 18 R = 19 R = 20 R = NH2 O– (2-C6H4OH) CO2–CH 2CH2CMe3 Electrochromic polymer films have been prepared by oxidative electropolymerisation of the monomer [Co(L19)]. R N L N N N N R R N NB These complexes exist as a mixture of isomers with the substituents attached at either of the positions labelled • on the benzyl rings. for the nickel-based polymer both redox processes are ligand-based. resulting in the formation of a fine green polymer. resulting in the appearance of a new MLCT transition. with the electron transport through the multilayers being at least in part diffusion controlled.58 V and a reversible phthalocyanine-ring oxidation peak at þ 0.3. Polymer-modified electrodes gave two distinct redox processes with half-wave potentials at À0.70 V. LB films of alkyloxy-substituted [Lu(Pc)2] exhibited a one-electron reversible reduction and a one-electron reversible oxidation corresponding to a transition from green to orange and blue forms respectively. green blue (7:7) poly½NiII ðL18 ÞÀ þ neÀ ! poly½NiII ðL18 Þ2À : blue purple (7:8) The first reduction in the cobalt-based polymer is metal-centred. The coloration switched from transparent light green [CoII state] to yellowish green [CoI state] to dark yellow (reduced ring). For example. SCE at 100 mV sÀ1 in dry acetonitrile.2 V vs.262 Electrochromism within metal coordination complexes poly½NiII ðL18 Þ þ neÀ ! poly½NiII ðL18 ÞÀ .62 The technique involved voltammetric cycling from À0. Cyclic voltammograms during polymer growth showed the irreversible phenol oxidation peak at þ 0. The high stability of the device was ascribed to the preparation.3 Metallophthalocyanines and porphyrins 263 An explanation of the relatively facile redox reaction in such multilayers is that the Pc ring is large compared with the alkyl tail.65 Ellipsometric and polarised optical absorption measurements suggest that the Pc rings are oriented with their large faces perpendicular to the immersion direction and to the substrate plane.64 Scanning tunnelling microscopy (STM) images on graphite reveal the differences in the two structures. The LB technique may be used for the fabrication of ECDs: an LB thinfilm display based on bis(phthalocyaninato)praseodymium(III) has been reported. as the latter could benefit more from defects arising from disorder.1 nm respectively.68 Thin-film [Co(2. 7.67. The display exhibited blue–green–yellow–red electropolychromism over a potential range of À2 to þ 2 V.3-nc)2] is green and is readily oxidised to form a violet-coloured species.66 The electrochromic electrode was fabricated by deposition of ˚ multilayers (10–20 layers.2-nc). The in-plane dc conductivity was studied as a function of film thickness and temperature. More recently. Unless these structures provide ion channels. denoted here (2.7. electrical conductivity and electrochromism in thin films of substituted and unsubstituted lanthanide bis-phthalocyanine derivatives have been investigated with particular reference to the differences between unsubstituted and butoxy-substituted [Lu(Pc)2] materials. which improves reversibility. Thin-film [Zn(2.0 nm and 2. the structure. The green–red oxidative step is seen for both cases but the green–blue reductive step is absent in the butoxy-substituted material. ordered structures might be considered to favour electronic rather than ionic motion.8 Â 1.3-nc) and (1. with unsubstituted [Lu(Pc)2] being approximately 106 times more conductive than the substituted material.3. % 100–200 A) of the complex onto ITO-coated 2 glass (7 Â 4 cm ) slides.6 Species related to metallophthalocyanines Naphthalocyanine (nc) species are structurally similar to the simpler phthalocyanines described above and have two isomers. of well-ordered mono layers that allow better diffusion of the counter ions into the film. Ni) have also been reported. Naphthalocyanines show an intense optical absorption at long wavelengths (700 < < 900 nm) owing to electronic processes within the extended conjugated system of the ligand. by the LB technique. giving molecular dimensions of 1. High-quality LB films of [M(L20)] (M ¼ Cu. and there is enough space and channels present in the LB films to allow the necessary charge-compensating ion transport. again approaching exploitability.5 Â 1.3-nc)2] is also green . After 105 cycles no significant changes are observed in the spectra of these colour states. The properties associated with these units may also be imparted to the parent sandwich compounds. have been studied. with mixed phthalocyaninato and/or porphyrinato ligands. the ligand is similar to a phthalocyanine but with quaternised pyridyl residues replacing all four fused benzo groups. have been synthesised and are likely to show interesting electrochromic properties.3-Naphthalocyanine when neutral. there is clearly much room for further investigation.69 Electrochromism in the pyridinoporphyrazine system and its cobalt complex has also received some attention.2nc)Lu(1.72 7. By attaching functional groups or special (donor or acceptor) moieties to these compounds.264 Electrochromism within metal coordination complexes N N N NH N N HN N N NH N N HN N N N 1.e.2-Naphthalocyanine 2. although there has been little systematic study to date.2-nc)] has been reported.7 Electrochromic properties of porphyrins Early results suggest that an investigation into porphyrin electrochromism is warranted.70 Here. A ‘triple-decker’ naphthalocyanine compound [(1. in which a redox active ferrocenecarboxylato unit is appended to the electrochromic centre. Thus. It is not only homoleptic (i.3. all ligands similar) phthalocyanine complexes that can form sandwich structures. the spectra of the chemical reduction products of Zn(TPP) have been . The electrochromic properties of some silicon–phthalocyanine thin films. recently a substantial number of heteroleptic sandwich-type metal complexes.71 Although considerable progress has been made in this field. it may be possible to tune their electronic properties without altering the ringto-ring separation.2-nc)Lu(1. Recently it has been found that oxidative electropolymerisation of substituted porphyrins could be useful towards the development of electrochromic porphyrin devices. so NIR-absorbing or reflecting materials could have use in smart windows that allow control of the environment inside buildings. from the redox viewpoint.82. Fajer et al. a property which is of obvious interest for use in display devices and windows. where photodynamic therapy exploits the relative transparency of living tissue to NIR radiation around 800 nm.1 Significance of the near-infrared region The metal complexes described so far in this chapter have been of interest for their electrochromism in the visible region of the spectrum.80 and in telecommunications.76 showed that Zn(TPP) changes colour to green upon one-electron oxidation by controlled potential electrolysis. 800–2000 nm) is an area which has also attracted much recent interest3 because of the considerable technological importance of this region of the spectrum. green (mono-negative ion). and is therefore electrochromic only indirectly. and the fact that much of the solar emission spectrum is in the NIR region means that effective light-harvesting compounds for use in solar cells need to capture NIR as well as visible light.73. and amber (di-negative ion).4.77 reported that the electrolysis of Mg(OEP) yielded a blue– green solution.1 in medicine. Electrochromism in the near-infrared (NIR) region of the spectrum (ca. Felton et al. A minority however are based on transition-metal complexes and it is generally these which have the redox activity necessary for .81 Many molecules with strong NIR absorptions have been investigated.73 Felton and Linschitz75 reported that the electrochemically produced monoanion spectrum is similar to that produced chemically.4 Near-infrared region electrochromic systems 265 reported. excited state spread over N molecules in one dimension) to the monomer. The recently reported78 green–pink colour change of a porphyrin monomer appears to be a pH-change-induced transformation of the J-aggregate (ordered molecular arrangement.79 7. Near-infrared radiation is also felt as radiant heat.4 Near-infrared region electrochromic systems 7.74 with colours changing between a pink (parent complex).83. where fibreoptic signal transmission through silica fibres exploits the ‘windows of transparency’ of silica in the 1300–1550 nm region.84 The majority of these are highly conjugated organic molecules that are not redox active. often with a view to examining their performance as dyes in optical data-storage media.7. Near-infrared radiation finds use in applications as diverse as optical data storage. 93. 94. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. Pd and Pt (see below). 1. 87 and 88 for an extensive listing). 90. but not the dianionic forms. These complexes have two reversible redox processes connecting the neutral. whose reduced forms contain ligand radical anions that show intense.7 Near-infrared electrochromic materials based on doped metal oxides85 (see Chapter 6) and conducting polymeric films86 (see Chapter 10) are also extensively studied.4.266 Electrochromism within metal coordination complexes electrochromic behaviour and which are discussed in the following sections. 91. Pd) The structures and redox properties of these complexes have been extensively reviewed. 1400 nm and extinction coefficients up to ca. R N S R N R S S M S N R S R N S R R S M S S S R n– Generic structure of planar bis-dithiolene complexes: M = Ni. low-energy electronic transitions. A large number of substituted dithiolene ligands have been prepared and used to prepare complexes of Ni. pioneered by Muller-Westerhoff and co-workers. 95. i. 2 Complexes of dialkyl-substituted imidazolidine-2. n = 0. Pd and Pt One of the earliest series of metal complexes which showed strong.87.e. Extensive delocalisation between metal and ligand orbitals in these ‘non-innocent’ systems means that assignment of oxidation states is problematic. but it does result in intense electronic transitions. One such set of complexes has already been discussed in this chapter: spectral modulation in the NIR region has been reported for a variety of [Ru(bipy)3]2 þ derivatives. they were discussed earlier in Section 7. redoxdependent NIR absorptions is the well-studied set of square-planar bis-dithiolene complexes of Ni.4.89 is for use in the neutral ¨ state as dyes to induce Q-switching of NIR lasers such as the Nd-YAG .1.88. monoanionic and dianionic species.5-trithiones (M (refs. the complexes are electropolychromic. 96) = Ni. 92.2. Pd and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. Pt. 7. The main application of the strong NIR absorbance of these complexes.88 of interest here is the presence of an intense NIR transition in the neutral and monoanionic forms. Since these are also electrochromic in the visible region. 40 000 dm3 molÀ1 cmÀ1 (see refs.2 Planar dithiolene complexes of Ni. Pd. Nevertheless.3 Mixed-valence dinuclear complexes of ruthenium Another well-known class of metal complexes showing NIR electrochromism is the extensive series of dinuclear mixed-valence complexes based principally on ruthenium–ammine or ruthenium–polypyridine components.4. representative selection of recent examples which show electrochromic behaviour (in terms of the intensity of the IVCT transitions) typical of this class of complex. iodine (1310 nm) and erbium (1540 nm) lasers.7. These complexes have primarily been of interest because the characteristics of the IVCT transition provide quantitative information on the magnitude of the electronic coupling between the metal centres.92. The high thermal and photochemical stabilities of these complexes make them excellent candidates for Q-switching of the 1064 nm Nd-YAG laser. The result is that the NIR absorption maximum occurs at around 1000 nm and has a remarkably high extinction coefficient (up to 80 000 dm3 molÀ1 cmÀ1). in which a strong electronic coupling between the metal centres makes a stable RuII–RuIII mixed-valence state possible.91.2 shows a small. Of course the field . planar dithiolenes of Ni and Pd has been prepared based on the ligands [R2timdt]À which contain the dialkyl-substituted imidazolidine-2.100 The main purpose of this selection is to draw the reader’s attention to the fact that these complexes which.96 7. Table 7. an appropriate excited-state lifetime following excitation. 1400 nm.98. The use of a range of metal dithiolene complexes in this respect has been reviewed. and good long-term thermal and photochemical stability. the position and intensity of the IVCT transition in some cases mean that complexes of this sort could be exploited for their optical properties. one-electron reduction to the monoanionic species [M(R2timdt)2]À results in a shift of the NIR absorption maximum to ca.5-trithione core (see above). and is accordingly an excellent diagnostic tool. In addition.94. maximising the electronic effect. as a class.93.95.88.4.97. in a different context could be equally valuable for their electrochromic properties.99. Such complexes generally show a RuII!RuIII IVCT transition which is absent in both the RuII–RuII and RuIII–RuIII forms. indicating possible exploitation of their electrochromism.90. This relies on a combination of very high absorbance at the laser wavelengths.89 The strong NIR absorptions of these complexes have continued to attract attention since these reviews appeared.4 Near-infrared region electrochromic systems 267 (1064 nm).96 In these ligands the peripheral ring system ensures that the electron-donating N substituents are coplanar with the dithiolene unit. This shifts the NIR absorptions of the [M(R2timdt)2] complexes to lower energy than found in the ‘parent’ dithiolene complexes. A new series of neutral. are so familiar. 2. Examples of mixed-valence dinuclear complexes showing NIR electrochromism.268 Electrochromism within metal coordination complexes Table 7. although these have been the most extensively studied because of their synthetic convenience and ideal electrochemical properties. The complex has pendant hydroxyl groups which react with a tri-isocyanate to give a crosslinked polymer which was deposited on an ITO substrate. a trinuclear RuII complex has been reported which shows a typical IVCT transition at 1550 nm in the mixed-valence RuII–RuIII form. Complex [3+] 2000 14 000 Ru(bipy)2 97 λ /nm (ε/dm3 mol–1 cm–1) Ref. over several thousand redox cycles. Very recently.101 . with fast switching times (of the order of 1 second). N (bipy)2Ru O N O [3+] (bipy)2Ru O N N O Ru(bipy)2 [–] (H3N)5 RuII N N N FeIII(CN)5 1600 11 700 98 1210 3 900 99 [3+] N Ru(terpy)(bpy) N (bpy)(terpy)Ru C N N N C N 1920 10 000 100 is not limited to ruthenium complexes. analogous complexes of other metals have also been prepared and could be equally effective NIR electrochromic dyes. Good electrochromic switching of 1550 nm radiation was maintained. 102.104.1 gives a representative example).4.4 Near-infrared region electrochromic systems 269 7. The important point here is that in the oxidised forms.107 Mononuclear complexes of this type undergo reversible MoIV–MoV and MoV–MoVI redox processes with all three oxidation states accessible at modest potentials. Ward and co-workers have reported the NIR electrochromic behaviour of a series of mononuclear and dinuclear complexes containing the oxo-MoV core unit [Mo(Tp*)(O)Cl(OAr)]. containing one or two MoVI centres.103. such that the complexes can be oxidised from the MoV–MoV state to MoV–MoVI and then MoVI–MoVI in two distinct steps. The complex used has the spacer E ¼ bithienyl between the two phenolate termini (sixth entry in Table 7.103. A larger cell was used to show how a steady increase in the potential applied to the solution.105.105 Depending on the nature of the group E in the bridging ligand.102. the LMCT transitions are at lower energy and of much higher intensity than in the mononuclear complexes (Figure 7.105 A prototypical device to illustrate the possible use of these complexes for modulation of NIR radiation has been described. in which two oxo-Mo(V) fragments are connected by a bisphenolate bridging ligand in which a conjugated spacer ‘E’ separates the two phenyl rings.5-dimethylpyrazolyl) borate].106. the NIR electrochromism is much stronger.103 In mononuclear complexes these transitions are at the low-energy end of the visible region and of moderate intensity: for [Mo(Tp*)(O)Cl(OPh)] for example the LMCT transition is at 681 nm with " ¼ 13 000 dm3 molÀ1 cmÀ1. with extinction coefficients of up to 50 000 dm3 molÀ1 cmÀ1 (see Table 7.103 However in many dinuclear complexes of the type [{Mo(Tp*)(O)Cl}2(m-OC6 H4EC6H4O)]. conducting-glass slides.7. the absorption maxima can span the range 800–1500 nm.4 Tris(pyrazolyl)borato-molybdenum complexes In the last few years McCleverty.3). In these complexes an electronic interaction between the two metals results in a separation of the two MoV–MoVI couples.106 A thin-film cell was prepared containing a solution of an oxo-MoV dinuclear complex and base electrolyte between transparent.104. Application of an alternating potential.3)103. with " ¼ 30 000 dm3 molÀ1 cmÀ1) on one-electron oxidation to the MoV–MoVI state which is completely absent in the MoV–MoV state. oxidation to MoVI results in the appearance of a low-energy phenolate. Whilst reduction to the MoIV state results in unremarkable changes in the electronic spectrum. this complex develops an LMCT transition (centred at 1360 nm. stepping between þ1. which resulted in a larger proportion of the .5 V and 0 V for a few seconds each. where ‘Ar’ denotes a phenyl or naphthyl ring system and [Tp* ¼ hydrotris(3. resulted in fast switching on/off of the NIR absorbance reversibly over several thousand cycles.(or naphtholate)-to-MoVI LMCT process.104. C. Chem.106 The disadvantage of this prototype is that. C. Jeffrey. ‘Dinuclear oxomolybdenum(V) complexes which show strong electrochemical interactions across bis-phenolate bridging ligands: a combined spectroelectrochemical and computational study. 105): the cell accordingly acts as a NIR variable optical attenuator. undergoes two reversible . respectively. In these complexes reduction of the metal centre results in appearance of a Mo0! py(p*) MLCT transition at the red end of the spectrum (for R ¼ 4-CH(nBu)2.1 Electrochromic behaviour of [{Mo(Tp*)(O)Cl}2(m-OC6H4 C6H4 C6H4O)]nþ in the oxidation states MoV–MoV (n ¼ 0). when the pyridyl ligand contains an electron-withdrawing substituent meta to the N atom (R ¼ 3-acetyl or 3-benzoyl) an additional MLCT transition at much longer wavelength develops (max ¼ 1274 and 1514 nm.) material being oxidised.4.’ J. Spectra were measured at 243 K in CH2Cl2. max ¼ 830 nm with " ¼ 12 000 dm3 molÀ1 cmÀ1). but the optical properties of these complexes show great promise for further development. being solution-based. J. MoV–MoVI (n ¼ 1). E. Some nitrosyl–MoI complexes of the form [Mo(Tp*)(NO)Cl(py-R)] (where py-R is a substituted pyridine) also undergo moderate NIR electrochromism on reversible reduction to the Mo0 state.270 Electrochromism within metal coordination complexes O CI(O)(Tp*)Mo Mo(Tp*)(O)CI O n+ 60 mol–1 cm–1 +2 10–3 ε/dm 3 +1 0 400 0 800 1200 1600 2000 λ /nm Figure 7. However. switching is relatively slow compared to thin films or solid-state devices. N.5 Ruthenium and osmium dioxolene complexes Lever and co-workers described in 1986 how the mononuclear complex [Ru(bipy)2(CAT)]. Soc. et al. (Figure reproduced from Harden. MoVI–MoVI (n ¼ 2)... with permission of The Royal Society of Chemistry. with " ca.. Dalton Trans. R. 2417–26. allowed the intensity of a 1300 nm laser to be attenuated reversibly and controllably over a dynamic range of 50 dB (a factor of ca. 2400 dm3 molÀ1 cmÀ1 in each case). 1999. which has no NIR absorptions.107 7. Humphrey. The CAT–SQ and SQ–Q couples accordingly result in modest NIR electrochromic behaviour (see structures L21–L23).2-benzoquinone. respectively. . 1.4 Near-infrared region electrochromic systems 271 Table 7. 2).7. 104 dm3 molÀ1 cmÀ1. the former at 890 nm and the latter at 640 nm with intensities of ca. Principal low-energy absorption maxima of dinuclear complexes [{Mo(Tp*)(O)Cl}2 (L)]nþ in their oxidised forms (n ¼ 1.3.108 In the two oxidised forms the presence of a ‘hole’ in the dioxolene ligand results in the appearance of RuII! SQ and RuII!Q MLCT transitions. see Scheme 7. SQ and Q are catecholate.2-benzosemiquinone monoanion. and 1. λmax/nm (10–3 ε/dm3 mol–1cm–1) Bridging ligand L O O Mo(V)–Mo(VI) 1096 (50) Mo(VI)–Mo(VI) 1017 (48) O O 1245 (19) 832 (32) O O 1131 (25) 1016 (62) O O 1047 (24) 1033 (50) O S O 1197 (35) 684 (54) O S S S O O O 1360 (30) (Not stable) O 900 (10) 900 (20) O N N O 1210 (41) (Not stable) O 1268 (35) 409 (38) O O 1554 (23) 978 (37) oxidations which are ligand-centred CAT–SQ and SQ–Q couples (where CAT.2). 2 Ligand-based redox activity of (a) the CAT–SQ–Q series. the trinuclear complex [{Ru(bipy)2}3(m–L22)]nþ (n ¼ 3–6) exists in four stable redox . this disappears in the fully reduced form and moves into the visible region in the fully oxidised form. In the state n ¼ 2. as in [{Ru(bipy)2}2(m–L21)]nþ (n ¼ 0–4). HO OH HO HO H4L 21 HO HO OH OH HO H6L 22 OH O H3L 23 OH HO OH O As with the oxo-MoV complexes mentioned in the previous section.2). (b) [L21]nÀ (n ¼ 4–0). with reversible conversions between the fully reduced (bis-catecholate) and fully oxidised (bis-quinone) states all centred on the bridging ligand (Scheme 7.272 Electrochromism within metal coordination complexes O −e +e O O (SQ) – (a) O (CAT) 2– −e +e O O Q O O (b) O O O CAT–CAT CAT–SQ −e +e O O O −e +e O O O O SQ–SQ O −e +e O O O SQ–Q −e +e O O O O Q– Scheme 7. the NIR absorption is at 1080 nm with " ¼ 37 000 dm3 molÀ1 cmÀ1. the NIR transitions become far more impressive when two or more of these chromophores are linked by a conjugated bridging ligand.109 Likewise. which exhibits a five-membered redox chain. 73 V Ru O O Ru O O Ru O SQ–Q O Ru O Q– O +1.. with permission of The Royal Society of Chemistry.) . R.2 Ligand-centred redox interconversions of [{Ru(bipy)2}3(m–L22)]n þ (n ¼ 3–6) (potentials vs. Cleary.. and Ward.34 V O 3+ Ru O O Ru O 4+ 273 100 SQ–SQ– λ = 1083 nm ε = 74000 mol–1 cm–1 SQ–Q Q– λ = 909 nm ε = 32000 SQ–SQ–SQ λ = 1170 nm ε = 40000 10–3 ε/dm λ = 759 nm ε = 50000 3 0 400 800 1200 1600 λ nm / Figure 7.7. Kowallick.’ Chem. 2695–6. Spectra were measured at 243 K in MeCN. SCE). and the resulting electrochromic behaviour. M. ‘A new redox-tunable near-IR dye based on a trinuclear ruthenium(II) complex of hexahydroxy-triphenylene. A. D.03 V 5+ O Ru O O Ru 6+ O O Ru O– SQ–SQ– O +0. L. M.4 Near-infrared region electrochromic systems Ru O O Ru O O Ru O SQ–SQ–SQ +0. (Figure reproduced from Barthram. Commun. R. 1998. 111. J. W. Qui. Q. Juris. 1980. Liu. Chou. photochemistry. F. A..g. Electroanal. has yet to be described. S.. 1988. 5. Solid State Ionics..112 The analogous complexes with osmium have also been characterised and. 52. N. 2002. 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Polynuclear osmium–dioxolene complexes: comparison of electrochemical and spectroelectrochemical properties with those of their ruthenium analogues. A. In PB the two transition metals in the formula are the two common oxidation states of iron.8 Electrochromism by intervalence charge-transfer coloration: metal hexacyanometallates 8. l integral) where M0 and M00 are transition metals with different formal oxidation numbers.1 Prussian blue systems: history and bulk properties Prussian blue – PB.1 is extensively used as a pigment in the formulation of paints.8. the preferred industrial-production route (rather than iron(III) with a hexacyanoferrate(II) salt). the major classification of extreme cases delineates ‘insoluble’ PB (abbreviated 282 .9. Prussian blue is the prototype of numerous polynuclear transition-metal hexacyanometallates. i.7 with. ferric ferrocyanide. In the PB chromophore. it contains Fe3þ and [FeII(CN)6]4À.13.16.10. The FeIII is usually high spin with H2O coordinated. spectra and conductimetry) underlie the elaborations that follow. in addition to electrochromism.11 Fundamental studies12. as established by the CN stretching frequency in the IR spectrum and confirmed by Mossbauer spectroscopy. which form an important class of insoluble mixedvalence compounds. or iron(III) hexacyanoferrate(II) – first made by Diesbach in Berlin in 1704.17 They have the general formula M0 k[M00 (CN)6]l (k. Prussian blue is readily prepared by mixing aqueous solutions of a hexacyanoferrate(III) salt with iron(II).6. FeIII and FeII. lacquers and printing inks.3 Since the first report4 in 1978 of the electrochemistry of PB films. therefore in the solid a counter cation is to be incorporated. the distribution of oxidation states is FeIII–FeII respectively. While the precise composition of any PB solid is extraordinarily preparation-sensitive. proposed applications in electroanalysis and electrocatalysis. numerous studies concerning the electrochemistry of PB and related analogues have been made.5.e.15.18 The chromophore alone thus has a negative ¨ charge. whereas the FeII is low spin.14 on basic PB properties (electronic structure.2. These materials can contain ions of other metals and varying amounts of water. e. Prussian blue electrodeposition has been studied by numerous techniques. i.4 although electroless deposition.21 In i-PB.34 and galvanostatic studies35 have indicated that reduction of iron(III) hexacyanoferrate(III) is the principal electron-transfer process in PB electrodeposition.25 sacrificialanode (SA) methods.22 This proposed structure contains no interstitial ions.19 the ‘solubility’ attributed to the latter form being an illusion caused by its easy dispersion as colloidal particles. The FeII vacancies are randomly distributed. All forms of PB are in fact highly insoluble in water (Ksp $ 10À40). with the high-spin FeIII and low-spin FeII ions coordinated octahedrally by the N or C of the cyanide ligands. dependent on the counter cation. forming a blue sol in water that looks like a true solution.2 Preparation of Prussian blue thin films Prussian blue thin films are generally prepared by the original method based on electrochemical deposition.. in full KþFe3þ[FeII(CN)6]4À.33. (8.1). which complete the octahedral coordination about FeIII. the blue IVCT band on analysis of the intensity indicating $1% delocalisation of the transferable electron in the ground state (i. The widespread assumption of Ludi et al. as in the electrodeposition for electrochromic use.26. and occupied by water molecules. Voltammetry32. before any optical CT).e. Mossbauer spectroscopy confirms ¨ the interstitial ions to be the Fe3þ counter cation.31 have all been described. and ‘soluble’ PB (s-PB).18 Single-crystal X-ray diffraction patterns of i-PB indicate however a primitive cubic lattice.28 the embedding of micrometre-sized crystals directly into electrode surfaces using powder abrasion.2 Preparation of Prussian blue thin films 283 to i-PB) which is Fe3þ[Fe3þ{FeII(CN)6}4À]3.20 X-Ray powder diffraction patterns for s-PB indicate a face-centred cubic lattice. This .’s model22 for i-PB is highly questionable23 in view of the substantial differences between the (very slowly grown) single crystals22 and the more usual polycrystalline forms arising from relatively rapid growth. the remainder by four N-bound cyanides. Thus PB films can be electrochemically deposited onto a variety of inert electrode substrates by electroreduction of solutions containing iron(III) and hexacyanoferrate(III) ions as the adduct Fe3þ[FeIII(CN)6]3À.8. and every FeII centre surrounded by six C-bound cyanides ligands.24 8.29 and a method using catalytic silver paint30. The Fe3þ[FeII(CN)6]4À chromophore falls into Group II of the Robin–Day mixed-valence classification. Eq. Other (bivalent) counter cations also appear to be interstitial. with Kþ ions occupying interstitial sites.27 the extensive redox cycling of hexacyanoferrate(II)containing solutions. where one quarter of the FeII sites are vacant. with one quarter of the FeIII centres being coordinated by six N-bound cyanide ligands. diffusion of locally depleted electroactive species to the now three-dimensional PB interface plays an increasingly dominant role and limits electron transfer. (If through-film electron transfer to the film–electrolyte interface wanes with growth. resulting in a fall in growth rate.41 A solubility of the FeIII FeIII complex was estimated41 as ca. followed by growth of a second. Variation of electrode potential. for bulk PB taken out of solution.1): Â Ã3À Â III III Ã0 ¼ Fe Fe ðCNÞ6 : Fe3þ þ FeIII ðCNÞ6 (8:1) Chronoabsorptiometric studies36 for galvanostatic PB electrodeposition onto ITO electrodes have shown that the absorbance due to the IVCT band of the growing PB film is proportional to the charge passed. supporting electrolyte and concentrations of electroactive species have established a subsequent three-stage electrodeposition mechanism. Electrochemical quartz-crystal microbalance (EQCM) measurements for potentiostatic PB electrodeposition onto gold have revealed that the mass gain per unit area is proportional to the charge passed. but this was later shown to cause initial deposition of the solid FeIII FeIII complex. depends on ambient humidity.39 Changes in the ellipsometric parameters during PB electrodeposition revealed initial growth of a single homogeneous film for the first 80 seconds. (8. outer.40 In earlier preparations in the ‘zeroth’ step the deposition electrode was first made positive during addition of solutions in order to preclude spontaneous or uncontrolled deposits of PB. as the electroactive area increases by formation and three-dimensional growth of PB nuclei attached to the PB interface formed in the initial stage.38 Chronoamperometric measurements (over a scale of several seconds) supported by scanning electron microscopy (SEM) for the electrodeposition of PB onto ITO and platinum by electroreduction from solutions of iron(III) hexacyanoferrate(III) have been performed. persisted briefly before being incorporated into the growing PB. more porous film on top of the relatively compact inner film. 10À3 mol dmÀ3. In the second growth phase there is an increase in rate towards maximal roughness. In the final growth phase.) .37 Ellipsometric measurements for potentiostatic PB electrodeposition onto platinum indicated that the level of hydration was around 34 H2O per PB unit cell.284 Electrochromism of metal hexacyanometallates brown–yellow soluble complex dominates in solutions containing iron(III) and hexacyanoferrate(III) ions as a result of the equilibrium in Eq.38 Hydration is in fact variable and. when the electrode was made cathodic. In the early growth phase40 the surface becomes uniformly covered as small PB nuclei form and grow on electrode substrate sites. the seeping in of reactant solution between the PB film and electrode substrate for later growth phases is not precluded. which. 34 Prussian brown appears brown as a bulk solid.3 Electrochemistry. has been demonstrated.42. 8.44. structural and morphological properties of such multilayer systems. the generation of multilayers of Prussian blue (and the mixed FeIII–RuII analogue ‘Ruthenium purple’) on gold surfaces. which is spontaneous and can lead to single monolayers. it has been inferred (but with reservations.8. a normal layering of metal cations and hexacyanoferrate anions is highly unlikely’. in situ spectroscopy and characterisation of Prussian blue thin films Electrodeposited PB films may be partially oxidised32. Eq. (8.34 to Prussian green (PG). Thus. They take care to note that ‘because metal hexacyanoferrate salts are known to organise in a cubic crystal lattice structure.33. spectroscopy and characterisation 285 More recently. and have also demonstrated their application as ion-sieving membranes. a species historically also known as Berlin green and assigned the fractional composition shown. a new method of assembling multilayers of PB on surfaces has been described. to the fully oxidised all-FeIII form Prussian brown (PX). by exposing them alternately to positively charged iron(III) cations and [FeII(CN)6]4À or [RuII(CN)6]4À anions. (8.45 have investigated the optical. although in bulk form PG is believed to have a fixed composition with the anion composition shown above.42 Tieke and co-workers43.3):33. brown–yellow in solution.2): h i1=3À Â III II ÃÀ þ 2=3 eÀ : Fe Fe ðCNÞ6 ! FeIII fFeIII ðCNÞ6 g2=3 fFeII ðCNÞ6 g1=3 PB PG (8:2) The fractions 2⁄3 and 1⁄3 are illustrative rather than precise.43 In contrast to the familiar process of self-assembly. below) that there is a continuous composition range in thin films from PB. and golden yellow as a particularly pure form that is prepared on electro-oxidation of thin-film PB – Eq. ‘directed assembly’ is driven by the experimenter and leads to extended multilayers.3 Electrochemistry. electrochemical. In a proof-ofconcept experiment. They avoid the term ‘layer-by-layer’ deposition and instead use ‘multiple sequential deposition’.34 Â III II ÃÀ Â Ã0 Fe Fe ðCNÞ6 ! FeIII FeIII ðCNÞ6 þ eÀ : PB PX (8:3) . via the partially oxidised PG form. i-PB.) . R. (8.3 0. ‘In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue’. yields Prussian white (PW).4). showing the voltammetric wave for the PB–PW redox switch.9 V for a high write-erase efficiency in 0.1 shows a cyclic voltammogram of the PB–PW transition.2 –0.0 0.4 0. J. The electron transfer occurs at the electrode-substrate–film interface.286 Electrochromism of metal hexacyanometallates Redox in the other direction. PG and PW are all insoluble in water.2 mol dmÀ 3). Mater. reduction of PB. Whilst s-PB. while counter-ion egress or ingress occurs at the film–electrolyte interface.3 0. which appears colourless as a thin film – Eq.6 E(V) vs. Â III II ÃÀ Â Ã2À Fe Fe ðCNÞ6 þ eÀ ! FeII FeII ðCNÞ6 : PB PW (8:4) Figure 8.3 –0. This implies a positive potential limit of about þ0. 2226–33.50 V vs.1 Cyclic voltammogram at 5 mV sÀ 1 scan rate for a PBjITOjglass electrode in aqueous KCl supporting electrolyte (0. 15. J.1 0. with permission from The Royal Society of Chemistry. also known as Everitt’s salt.5 0. The arrows indicate the direction of potential scan.0 –0.1 i / mA cm –2 0. Chem.2 0.. R. that is.2 0. J. Ag/AgCl Figure 8. determines the rate of coloration. it is not established which through-film transport. 2005. The initial potential was þ 0.1 –0.1 0. PX is slightly soluble in its pure (golden-yellow) form (indeed the electrodeposition technique depends on the solubility of the [FeIIIFeIII(CN)6]0 complex). AgjAgCl. (Figure reproduced from Mortimer.2 –0. For all redox reactions above there is concomitant counter-ion movement into or out of the films to maintain overall electroneutrality. and Reynolds. that of electron or ion. by permission of the Royal Society of Chemistry. (iv) þ 0.20 V (PX. and Rosseinsky. owing to the increasing [FeIIIFeIII(CN)6]0 absorption. ‘Iron hexacyanoferrate films: spectroelectrochemical distinction and electrodeposition sequence of ‘soluble’ (Kþ-containing) and ‘insoluble’ (Kþ-free) Prussian blue and composition changes in polyelectrochromic switching’. together with spectra of possibly two intermediate states between the blue and the yellow forms. (Figure reproduced from Mortimer. Dalton Trans.85 (PG.. The contrast (broad vs. apparently indicates a range of compositions to be involved. this does not rule out the prospect of four-colour PB electropolychromic ECDs. in contrast with the sharply peaked PB ! PW transition.80 (PG. Wavelengths (abscissa) are in nm. (iii) þ 0.4 0.20 (PW. with transformation to all PW or all PB without pause.90 (PG.50 (PB. yellow) (potentials vs. One broad voltammetric peak usually seen for PB ! PX.2 0 (vi) (ii) 400 600 800 1000 1200 Figure 8. J. supported by ellipsometric measurements. The yellow absorption band corresponds with that of [FeIIIFeIII(CN)6]0 in solution.8.6 Absorbance 0. Soc.) . 2059–61. blue).3 Electrochemistry. R. SCE)] with KCl 0.01 mol dmÀ 3 as supporting electrolyte. (v) þ 0.50 V to more oxidising potentials. spectroscopy and characterisation 287 contact with water. as other solvent systems might not dissolve PX. while the peak at 425 nm steadily increases. sharp) behaviour. D. green).2 Spectra of iron hexacyanoferrate films on ITO-coated glass at various potentials [(i) þ 0.0 (i) (iii) (iv) (v) 0.38 could imply continuous mixed-valence compositions over the blue1. the original 690 nm PB peak continuously shifts to longer wavelengths with diminishing absorption. Although practical electrochromic devices based on PB have primarily exploited the PB–PW transition. both maxima being at 425 nm and coinciding with the (weaker) [FeIII(CN)6]3À absorption maximum. (ii) À 0. green) and (vi) þ 1.2. The reduction of PB to PW is by contrast abrupt. blue and clear (‘white’) forms of PB and its redox variants are shown in Figure 8. green. 1984.8 0. Chem. green). R. The spectra of the yellow. transparent). J. depending on the applied potential. On increase from þ0..2 mol dmÀ 3 þ HCl 0. one into the other. energy dispersive analysis of X-rays (EDAX). elemental analytical and spectroelectrochemical measurements.50. it has been postulated that i-PB is first formed.41 but later also observed in slow voltammetry on PB from KCl-containing preparations. and Lundgren and Murray49 using cyclic voltammetry (CV). However.34 Itaya and Uchida. Emrich et al.34 Further evidence for this is provided by the difference in the voltammetric response for the PB–PW transition between the first cycle and all succeeding cycles. XPS. but thereafter a much slower introduction of Kþ.00. (8. approaching the values observed for i-PB. claimed that the film is always i-PB. i-PB PW (8:6) (8:5) Â Ã Â Ã Fe3þ FeIII FeII ðCNÞ6 3 À 3eÀ þ 3XÀ ! Fe3þ FeIII FeIII ðCNÞ6 3 X3 : i-PB PX In refutation. which was 0.33 On soaking s-PB films in saturated FeCl3 solutions partial reversion of the absorbance maximum and broadening of the spectrum. both confirmed i-PB as the initially-deposited form with a ‘gradual’ transformation to s-PB on potential cycling. first attributed to the absence of traces of ClÀ from those samples.5) and (8. that is. suggesting structural reorganisation of the film during the first cycle.48 using X-ray photoelectron spectroscopy (XPS) data. Eqs. Other support for the i-PB to s-PB transformation comes from an ellipsometric study by Beckstead et al.51 Based on changes that take place in the IVCT band on redox cycling.47.288 Electrochromism of metal hexacyanometallates to-yellow range in contrast with the (presumably immiscible) PB and PW. Later work41 established a major (approximately one-third) conversion in the first cycle.34.6) are applicable: Â Ã Â Ã Fe3þ FeIII FeII ðCNÞ6 3 þ 4eÀ þ 4Kþ ! K4 Fe2þ FeII FeII ðCNÞ6 3 . is found. without intermediacy of composition. which clearly transform. a PB þ PX series of solid solutions.46 Thus the intermediate green colour observed in PB–PX voltammetry could be a true compound PG rather than either a continuously changing mixed-valence phenomenon.50 who .708 rather than 1.47 however. Their argument is based on the ratio of charge passed on oxidation to PX to that passed on reduction to PW. followed by a transformation to s-PB on potential cycling. The identity as s-PB or i-PB of the initially electrodeposited PB has been debated in the literature.48. or varying PB þ PX physical mixtures of microcrystals.49. two-peak PB ! PX voltammetry pointing to a specific intermediate composition has also been seen. 51 The EQCM mass-change measurements on voltammetrically scanned PB films reinforce the theory of lattice reorganisation during the initial film reduction. so requiring Fe2þ Kþ as counter cations. Examples include a seven-segment display using PB-modified SnO2 working and counter electrodes at 1 mm separation.54 Whilst PB film stability is frequently discussed in the preceding papers.41 Lattice-energy calculations support most of this argument.41 Concurrently with this increase in stability at lower pH was a considerable increase in switching rate. Stilwell et al.56 and an ITO j PB–Nafion1 j ITO solid-state device.58 For the solid-state system. films grown from chloride-containing solutions were said to be slightly more stable. compared to those grown from chloride-free solutions.8.55 but again with contrary conclusions. They found that electrolyte pH was the overwhelming factor in film stability. developed optical properties that differed from the original PB film.52 In detail.37 Only about one third of the three Kþ ions. in terms of cycle life. after the first and subsequent cycles for the PB–PW transition. Together with related observations on PB samples that contained sundry M2þ counter cations. expected to replace the countercationic Fe3þ.57. in the sequence of counter cations Naþ > Kþ < Rbþ < Csþ.55 have studied in detail the factors that influence the cycle stability of PB films. a further EQCM study53 shows mass changes.4 Prussian blue electrochromic devices Early PB-based ECDs employed PB as the sole electrochromic material. the now somewhat dispersed counter-cation population does not subsequently drive Kþ incorporation as strongly as happens with solely Fe3þ as (charge-concentrated) counter cation. This sequence correlates with the wavelengths of the maximum in each case of the PB absorption in the region of 700 nm.41 8.4 Prussian blue electrochromic devices 289 found that the PB film. It has been suggested that this follows from reduction in PB ! PW of the counter-cationic Fe3þ to Fe2þ which is retained on re-oxidation of the PW to the PB. Results from in situ Fourier-transform infrared spectroscopy also demonstrated an i-PB to s-PB transformation on repeated reductive cycling. though other conclusions regarding stabilisation by pH have since been reached. device fabrication involved chemical (rather than electrochemical) formation of the PB. by . varied Mþ or M2 þ lattice interactions with the chromophore were concluded to affect the optical absorptions commensurately.53. following one PB ! PW ! PB cycle of KCl-prepared PB in different MþClÀ solutions. Furthermore. cycle numbers in excess of 100 000 were easily achieved in solutions of pH 2–3. are found to be incorporated in the first substitutive voltammetric cycle. 59 In this design.70.66. (8. has also been described. at the conjunction of the (II)(II) state with the (III)(III) state. On appropriate switching.7) and (8.71 have combined PB with the conducting polymer poly(aniline) in complementary ECDs that exhibit deep blue-to-light .68.61.8): Â III II ÃÀ Â Ã2À Fe Fe ðCNÞ6 þ eÀ ! FeII FeII ðCNÞ6 . Numerous workers65. Eqs. Eq. then K3Fe(CN)6.290 Electrochromism of metal hexacyanometallates immersion of a membrane of the solid polymer electrolyte Nafion1 (a sulfonated poly(tetrafluoroethane)polymer) in aqueous solutions of FeCl2. returning the ECD to a transparent state. oxidation occurs near the positive electrode and reduction near the negative electrode to yield PX and PW respectively. they can be used together in a single device60. The conversion of the outer portions of the film results in a net half-bleaching of the device. PB.3) – and cathodically – to a transparent state. The resulting PB-containing Nafion1 composite film was sandwiched between the two ITO plates. However.69.4) – and that it is a mixed conductor through which potassium cations can move to provide charge compensation required for the electrochromic redox reactions. (8. blue transparent (8:8) (8:7) WO3 þ xðMþ þ eÀ Þ ! Mx WVI WV O3 : ð1ÀxÞ x transparent blue In an example of the construction of such a device. The construction and optical behaviour of an ECD utilising a single film of PB. without addition of a conventional electrolyte. (8.64 The films can be coloured simultaneously (giving deep blue) when a sufficient voltage is applied between them such that the WO3 electrode is the cathode and the PB electrode the anode.62. Since PB and WO3 (see Chapter 6) are respectively anodically and cathodically colouring electrochromic materials. a film of PB is sandwiched between two optically transparent electrodes (OTEs). their comproportionation reaction results in half the material remaining in the device centre as the (III)(II) form. the coloured films can be bleached to transparency when the polarity is reversed. thin films of these materials are deposited on OTEs that are separated by a layer of a transparent ionic conductor such as KCF3SO3 in poly(ethylene oxide).63. The functioning of the device relies on the fact that PB can be bleached both anodically – to the yellow state.67. Upon application of an appropriate potential across the film. Eq.64 so that their electrochromic reactions are complementary. comprising other polynuclear transition-metal hexacyanometallates. They took advantage of the symbiotic relationship between poly(aniline) and PB.72 have developed an electrochromic window for solar modulation using PB.69.9): Oxidised polyðanilineÞ þ PB ! Emeraldine polyðanilineÞ þ PG: coloured bleached (8:9) Jelle and Hagen68. Blue-to-green electrochromicity was achieved in a two-electrode cell by complementing the green-to-blue colour transition (on reduction) of the pp–Yb(Pc)2 film with the blue (PB)-to-colourless (PW) transition (oxidation) of the PB. (8. 8. Kashiwazaki74 has fabricated a complementary ECD using plasma-polymerised ytterbium bis(phthalocyanine) (pp–Yb(Pc)2) and PB films on ITO with an aqueous solution of KCl (4 mol dmÀ3) as electrolyte.71.6 V. Electrochromic compatibility is obtained by combining the coloured oxidised state of the polymer (see Chapter 10) with the blue PB. A threecolour display (blue.14 which have been prepared and investigated as thin .5 Prussian blue analogues 291 green electrochromism. The total device comprised Glass j ITO j poly(aniline) j PB j poly(AMPS) j WO3 j ITOj Glass. provides adequate oxidation of the pp–Yb(Pc)2 electrode.12. Compared with their earlier results with a poly(aniline)–WO3 window.5 Prussian blue analogues Prussian blue analogues. poly[3.4-(ethylenedioxy)thiophene] – PEDOT – on ITO glass and PB on ITO glass substrates with a poly(methyl methacrylate) – PMMA-based gel polymer electrolyte. as an additional counter electrode. A reduction reaction at the third electrode.13. and the bleached reduced state of the polymer with PG. Jelle and Hagen were able to block off much more of the light by inclusion of PB within the poly(aniline) matrix. The ECD exhibited deep blue–violet at À2. a new complementary ECD has recently been described.1 V and light blue at 0. and incorporated PB together with poly(aniline). The colour states of the PEDOT (blue-to-colourless) and PB (colourless-to-blue) films fulfil the requirement of complementarity. green and red) was fabricated in a three-electrode cell in which a third electrode (ITO) was electrically connected to the PB electrode. Eq. As noted in Chapter 10. and WO3. in a complete solid-state electrochromic window. poly(aniline) and WO3.8.73 based on the assembly of the cathodically colouring conducting polymer. while still regaining about the same transparency during the bleaching of the window. resulting in the red colouration of the pp–Yb(Pc)2 film. although it is to be noted that from the qualitative description of colour states.19 A for the PB analogue.14 Ruthenium purple films have been prepared by electroreduction of the soluble iron(III) hexacyanoruthenate(III) complex potentiostatically. iron(III) hexacyanoruthenate(II) – is synthesised via precipitation from solutions of the appropriate iron and hexacyanoruthenate salts.2 Vanadium hexacyanoferrate Vanadium hexacyanoferrate (VHCF) films have been prepared on Pt or fluorine-doped tin oxide (FTO) electrodes by potential cycling from a solution containing Na3VO4 and K3Fe(CN)6 in H2SO4 (3.1 Ruthenium purple Bulk ruthenium purple – RP. The visible absorption spectrum of a colloidal suspension of bulk synthesised RP with potassium as counter cation confirms the Fe3þ [RuII(CN)6]4À combination as the chromophore. galvanostatically or by using a copper wire as sacrificial anode. While the reduction of the hexacyanoferrate(III) . The majority are expected to be electrochromic. ferric ruthenocyanide. although this property has only been studied in any depth in a few cases.292 Electrochromism of metal hexacyanometallates films. although no single-crystal studies have been made.75 Ruthenium purple films can be reversibly reduced to the colourless iron(II) hexacyanoruthenate(II) form. RP could have a disordered structure similar to that reported for single-crystal PB. 8. are surveyed in this section.42 A as compared to 10.77. The large background oxidation current observed in chloride-containing electrolyte suggests electrocatalytic activity of RP for either oxygen or chlorine evolution. with a maximum at approximately 550 nm..78 Carpenter et al.77 by correlation with CVs for solutions containing only one of the individual electroactive ions. have proposed that electrodeposition involves the reduction of the dioxovanadium ion VO2þ (the stable form of vanadium(V) in these acidic conditions). but no partial electrooxidation to the Prussian green analogue is observed.12 However.76 The visible absorption spectrum of RP prepared in the presence of excess of potassium ion showed a broad CT band.75.5. as for bulk synthesised RP.13 The potassium and ammonium salts give cubic powder patterns similar to their PB analogues.12 The X-ray powder pattern with iron(III) as counter cation gives a ˚ ˚ lattice constant of 10. The field therefore appears to be open for further investigation and exploitation.76 8.6 mol dmÀ3). contrast ratios are likely to be low.5. followed by precipitation with hexacyanoferrate(III) ion. only arising in the FeII form of the complex.77 From electrochemical data and XPS they conclude that the electrochromism involves only the iron centres in the film. chemical oxidation to the FeIII state yields a light-orange material. No evidence was obtained for the formation of a vanadium(V)– hexacyanoferrate(III) type complex analogous to iron(III) hexacyano-ferrate(III). while modified electrodes can be reversibly cycled between the intensely red and transparent forms. 2. since VHCF films can be successfully deposited by potential cycling over a range positive of that required for hexacyanoferrate(III) reduction. Reaction of the complex with Ni2þ either under bulk conditions or at a nickel electrode surface generates a bright red material.81 A more dramatic colour change can be observed by substitution of two ironbound cyanides by a suitable bidentate ligand.80 The NiHCF films do not show low-energy IVCT bands.20 -bipyridine is employed as the chelating agent.8. When 2. are not redox active under these conditions. 8. but when deposited on ITO they are observed to switch reversibly from yellow to colourless on electroreduction. the visible absorption spectrum of the mixed solution being a simple summation of spectra of the single-component solutions. Carpenter et al. orange–transparent and .82 Thus.5 Prussian blue analogues 293 ion in solution probably also occurs when the electrode is swept to more negative potentials. and to the environment of the cyanide-nitrogen lone pair. For bulk samples.3 Nickel hexacyanoferrate Nickel hexacyanoferrate (NiHCF) films can be prepared by electrochemical oxidation of nickel electrodes in the presence of hexacyanoferrate(III) ions. While VHCF films are visually electrochromic. the complex [FeII(CN)4(bipy)]2À is formed which takes on an intense red colour associated with a MLCT absorption band centred at 480 nm. switching from green in the oxidised state to yellow in the reduced state.82 In principle. show that most of the electrochromic modulation occurs in the ultraviolet (UV) region. The vanadium ions.5. This optical transition is sensitive to both the iron oxidation state.79 or by voltammetric cycling of inert substrate electrodes in solutions containing nickel(II) and hexacyanoferrate(III) ions.20 -bipyridine can be indirectly attached to nickel metal via a cyano–iron complex to form a derivatised electrode. found to be present predominantly in the þIV oxidation state. By analogy with the parent iron complex this red colour is associated with the (dp)FeII!(p*)bipy CT transition. a process which correlates well with the observed CV response. this reduction does not appear to be critical to film formation. 03 to À0.0 V and yellow-green . The co-deposition procedure provides a fresh copper surface for film adhesion and the resulting films are able to withstand $1000 voltammetric cycles.87 The resulting modified electrodes gave broad CV responses.5 Palladium hexacyanoferrate The preparation of electrochromic palladium hexacyanoferrate (PdHCF) films by simple immersion of the electrode substrate for at least one hour. Au.40 and þ0.03 to À0.84. Pt.05 V in a solution of cupric nitrate in aqueous KClO4. The CuHCF film formation mechanism has not been elucidated but the co-deposition of copper is important in the formation of stable films. the yellow colour arising from the CNÀ! FeIII CT band at 420 nm for the hexacyanoferrate(III) species (arrow denoting electron transfer). Copper hexacyanoferrate films exhibit red-brown to yellow electrochromicity. Films were orange at >1.5.86 For the reduced film.294 Electrochromism of metal hexacyanometallates green–transparent electrochromism could be available.83. a broad visible absorption band associated with the iron-to-copper CT in cupric hexacyanoferrate(II) was observed (max ¼ 490 nm.50 V followed by injection of an aliquot of K4Fe(CN)6 solution (a red–brown hexacyanoferrate(II) sol formed immediately) into the cell.50 to þ0.50 V. in a mixed solution of PdCl2 and K3Fe(CN)6 has been reported. Films formed by galvanostatic or potentiostatic methods from solutions of cupric ion and hexacyanoferrate(III) ion showed noticeable deterioration within a few CV scans.5.50 V. or potential cycling of conducting substrates (Ir. characteristic of an adsorbed species. 8. The deposition and removal sequence was repeated until a reproducible CV was obtained during the stripping procedure. Copper is then deposited on the electrode by stepping the potential from þ0.69 V. assigned to FeIII(CN)6–FeII(CN)6.5 mol dmÀ3) gave a well-defined reversible couple at þ0. Pd. Such scanning of a CuHCF film in K2SO4 (0. 8. and subsequently removed (stripped) by linearly scanning the potential from À0. using the complexes [RuII(CN)4(bipy)]2À and [OsII(CN)4(bipy)]2À respectively. GC).86 Films are deposited by first cycling between þ0. the PdII sites being electro-inactive. " ¼ 2 Â 103 dm3 molÀ1 cmÀ1).4 Copper hexacyanoferrate Copper hexacyanoferrate (CuHCF) films can be prepared voltammetrically by electroplating a thin film of copper on glassy carbon (GC) or ITO electrodes in the presence of hexacyanoferrate(II) ions. The CuHCF film was then formed by stepping the electrode potential in the presence of cupric ion from þ0. This band was absent in the spectrum of the oxidised film.85. 102 mixed films of ruthenium oxide–hexacyanoferrate and ruthenium hexacyanoferrate.5 Prussian blue analogues 295 at <0. potentiodynamically grown PdHCF films have been studied using cyclic voltammetry.91.5.107 . titanium hexacyanoferrate105 (reversibly brown to pale yellow on reduction81). rhodium hexacyanoferrate81 (pale yellow to colourless on reduction81). being white when reduced and yellow when oxidised.5.1 mol dmÀ3).7 Miscellaneous Prussian blue analogues Prussian blue analogues investigated include thin films of cadmium hexacyanoferrate94 (reversibly white to colourless on reduction81). chromium hexacyanoferrate95 (reversibly blue to pale blue-grey on reduction81).103 silver hexacyanoferrate. in situ infrared and UV-visible spectroelectrochemistry. The electrodeposition occurs during the negative scans as sparingly soluble deposits of In3þ with [Fe(CN)6]4À were formed. the electrochromic properties of the films have not been investigated.92 Solid films of gallium hexacyanoferrate have been prepared by direct modification of a gallium electrode surface in an aqueous solution of 5 mmol dmÀ3 potassium hexacyanoferrate(III) in KCl (0. To date.5 silver–‘crosslinked’ nickel hexacyanoferrate104 (reversibly yellow to white on reduction81).89 The resulting films are electrochromic.92 have been grown by potential cycling in a mixed solution containing InCl3 and K3Fe(CN)6.93 This one-step electroless deposition proceeds via a chemical oxidation reaction of the metallic gallium to Ga3þ in the aqueous solution. zinc hexacyanoferrate106 and zirconium hexacyanoferrate.100 platinum hexacyanoferrate101 (pale blue to colourless on reduction81). 8.99 osmium(IV) hexacyanoruthenate.8. followed by reaction with the hexacyanoferrate(III) ions. osmium hexacyanoferrate. manganese hexacyanoferrate97 (reversibly pale yellow to colourless on reduction81).88 UV-visible reflectance spectra of films on platinum demonstrated the reversible progressive conversion of PdHCF between its reduced (light yellow) and oxidised (yellow green) states. cobalt hexacyanoferrate96 (reversibly green-brown to dark green on reduction81). 8.6 Indium hexacyanoferrate and gallium hexacyanoferrate Indium hexacyanoferrate films89. rhenium hexacyanoferrate81 (pale yellow to colourless on reduction81).90. molybdenum hexacyanoferrate98 (pink to red on reduction81). More recently. ruthenium oxide–hexacyanoruthenate.2V. The approach seems general. Wiley Interscience. and Neff.296 Electrochromism of metal hexacyanometallates Mixed-ligand Prussian blue analogues reported as redox-active thin films include copper heptacyanonitrosylferrate. lanthanum hexacyanoferrate. 6. in situ characterization.. Metal hexacyanoferrates: electrosynthesis. Bradford. 1986. K.5. Colour Index. and spectral measurements show them to be electrochromic. P. Neff. p. R. N. Society of Dyers and Colourists. Chem.5 Of the lanthanoids and actinoids. Soc. P. although colours have not been reported. 4673. 7. p. and Lezna. VCH. Lewis. V. Mater. R. K. and Rosseinsky. Electrochromism: Fundamentals and Applications. 15. Thin films of mixed nickel–palladium hexacyanoferrates have been prepared and characterised. Diesbach (1704). Itaya.. D. Uchida. ch.. In. 6. S.108 iron(III) carbonylpentacyanoferrate. R.. 2.111 as thin redoxactive films have been studied. 4.112 References 1. 162–168. 5. 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Konesky2 has shown that Agþ.3 The solubility increases with higher insertion coefficient.4 The electrochromism is reversible with electrochemically intercalated alkalimetal or alkaline-earth ions.3 303 .1 The electro-coloration occurs during reduction to form lithium fulleride. can be electro-intercalated into such fulleride films during coloration as counter ions from solvents g-butyrolactone or water. Electrochemically formed LixC60 is identical with the fulleride salt formed by exposing C60 to alkali-metal vapour. 114). Steep concentration gradients form in the fulleride films during electrochromic operation. presumably by allowing such gradients to dissipate during the ‘off’ period between pulses. As fullerene and fulleride films are partially soluble in the polar organic electrolytes used. Figure 9. although the extent of reversibility depends on the insertion coefficient x: reversibility is lost if x is too high.5 Applying pulsed potentials improves both the durability of the film and the extent of electro-reversibility.4 Furthermore. in the range 1060–1080 nm.4. in addition to Liþ. Additionally LixC60 catalyses the electro-decomposition of solvent. (Figure reproduced from de Torresi. Many alkali-metal cations may be inserted into graphite sheets from aprotic solutions.6 suggest the coloured form of the electrochrome. S. lithium apparently giving the best speed and electro reversibility. is not stable: electrochromic stability is degraded by residual oxygen in both the electrolyte system and the fullerene film. Liþ and oxide ion.2 –1.2 E/ V –1. G.7 Optical density 1. C. and Luengo.5 0.. Chem.2 –0. 9.5 –2. have reported an ECD with graphite as a solid-solution intercalation electrode. This rapid reaction yields C60. the ECD reverted back to the brassy 8 m λ /n . which may explain why the coloration efficiency is 20 cm2 CÀ1 during coloration but 35 cm2 CÀ1 during bleaching..2 –2.5 Reaction with water is also rapid. M.0 –0.2 Other carbon-based electrochromes Pfluger et al. Goldenberg7 has also prepared thin electrochromic films of fullerene via Langmuir–Blodgett techniques. This ECD is electropolychromic switching from brassy black ! deep blue ! light green ! golden yellow within the potential range 3–5 V.2 t 0. J. ‘Electrochromic phenomena in fullerene thin films’. by permission of Elsevier Science. Ciampi. I.) de Torresi et al. LixC60.2 Figure 9. 1994. R. C. A. When the potential was reversed. Electroanal.1 UV-visible spectrum of immobilised fullerene on an electrode surface as a function of applied potential E: the C60 was on a SnO2-coated glass electrode immersed in PC containing LiClO4 (1 mol dmÀ3)..2 –2. 283–5.0 350 550 750 –0. 377.304 Miscellaneous inorganic electrochromes E/ V 1. Torresi. 3 Reversible electrodeposition of metals Comparatively few inorganic type-II electrochromes have been reported. appearing as an almost horizontal line that increases in height with thickness of electrodeposited bismuth. The reaction sequence has not yet been detailed.19. Bismuth In recent work on the electrodeposition of metallic bismuth from aqueous solution. showing a new band in the UV. .18 In all these systems. rather than a continuous metal film.23 the deposition/coloration reaction is cited22 as Eq. reduction of a dissolved metal cation results in the deposition of finely divided metal.14. Gelling an aqueous–organic electrolyte makes the image less patchy.22. Accordingly. is yellow.9.2. but not so low as to cause deterioration of the ITO layer of the transparent electrode (the OTE).16. (9. so the ‘electrochromism’ results not from photon absorption but rather from the film becoming opaque or even optically reflective (by specular reflection). Kuwabara and Noda9 and White and co-workers10 have also used graphite as counter-electrode layer in an ECD.21. Despite experimental problems.24 The pH of the deposition solution must be relatively low in order to maintain high solubility of the bismuth cation precursor. see Figure 9.2 s. is achieved by underpotential deposition.12 ‘carbon’9. with of about 0.15 and ‘carbon-based’ electrodes. Other forms of carbon have been used as counter electrodes: screen-printed carbon black. Diamond. electrodeposited particulate bismuth exhibiting opacity has shown18 a cycle-life of 5 Â 107. but becomes brown following reductive ion insertion. the solution containing traces of copper to act as an electron mediator.2): 2 Bi3þ ðsolnÞ þ 9 BrÀ ðsolnÞ ! 2 Bi0 ðsÞ þ 3 BrÀ ðsolnÞ: 3 colourless opaque (9:2) The deposition of particulate bismuth. 9.3 Reversible electrodeposition of metals 305 black colour.11 electrodeposited by the oxidation of lithium acetylide. as reviewed by Ziegler (in 1999).13. lacking absorption peaks.17 No colour change is mentioned regarding these materials. The three systems studied for electrochromism are listed below. Thus the ‘spectrum’ of such bismuth on an OTE is invariant with wavelength. however. the only viable systems are those in which finely divided metal is electrodeposited onto an OTE.20. Of these few. 2 0.2 UV-visible spectra of electrodeposited bismuth on ITO.23 Lead Metallic lead may be electrodeposited25.02 mol dmÀ3).. it also affects the morphology of the deposit. Electrochem.306 1. 158–69. by permission of The Electrochemical Society. The bismuth was deposited reductively from a solution initially comprising aqueous Bi3 þ (0. M. ‘Spectroelectrochemistry of reversible electrodeposition electrochromic materials’.0 400 450 500 550 600 650 700 λ (nm) Figure 9. see Eq. This is not a true ‘spectrum’ because the bismuth is reflective.4 0. Soc. Proc.6 ΔT 0.) the ‘coloration efficiency’ for such systems is also little dependent on .26 However. Inc.8 0.18 reflecting as much as 60% of all incident visible light. and Howard. varying only between 73 cm2 CÀ1 at 550 nm and 77 cm2 CÀ1 at 700 nm.3): Pb2þ ðaqÞ þ 2eÀ ! Pb0 ðsÞ: colourless opaque (9:3) Similarly to bismuth. (9. 1994.20.0 Miscellaneous inorganic electrochromes 0.26 onto ITO from aqueous solutions of Pb(NO3)2. (Figure reproduced from Ziegler. P. the Cu2þ is not merely a mediator. J.20 A bismuth-based ECD has been marketed commercially by the Polyvision Corporation. with a fairly high contrast ratio of 25:1. B. 94(2). effecting increased transmittance changes by up to 60%. Copper(II) chloride in the electrolyte also leads to a . rather than optically absorbing. traces of copper are added to the colourless precursor solution as a mediator. done prior to Deb’s use of the term in 1969. continuous metallic plate is formed. thin films of hydride are coated with a thin layer of palladium.5) is not a mechanistically comprehensive representation of the redox reaction. To overcome these problems. sometimes called ‘switchable mirrors’. through which hydrogen can diffuse. not any underlying substrate. The transition from a metallic state (YH2 or LaH2) to a semiconducting state (YH3 or LaH3) occurs during the continuous absorption of hydrogen. (9.4). Thin-film LaH2 exhibits specular reflection of this sort. presumably . their interpretation is complicated by attendant changes in crystallographic structure. Chemical reaction therefore causes switching between reflective and non-reflective states. but chemical oxidation to form LaH3 results in a loss of the metallicity and hence the reflectivity. accompanied by profound changes in their optical properties. Eq. (‘Electrochromism’ was not referred to in this 1962 work. (9. Agþ ðaqÞ þ eÀ ! Ag0 ðsÞ: (9:4) A thin film of non-particulate.4 Reflecting metal hydrides An impressive example of electrochromes showing specular reflectance are the lanthanide hydride devices. Although dramatic changes in optical and electrical properties accompany such transitions. The use of bromide ion to mediate the underpotential deposition of Pb has also been investigated.27 Silver Thin films of silver have also been prepared by electrodeposition from Agþ ion onto OTEs.28 Eq.29 The reflective properties are those of the electrochrome. Eq. (9. The transition time scale is about a few seconds. lanthanum and the trivalent rare-earth elements all form hydrides that exhibit such transitions.9.4 Reflecting metal hydrides 307 more homogeneous deposit on the ITO surface.) 9. The extreme reactivity and fragility of these materials preclude their ready utilisation. For these reasons. such changes are expected as such electronic transitions require changes in nuclear spin. Yttrium.5): LaH2 ðsÞ þ HÀ ðsoln:Þ ! LaH3 ðsÞ þ eÀ : reflective non-reflective (9:5) The cause of the change in reflectivity is a metal–insulator transition. 30. but elemental H2 is neither safe nor an attractive proposal for a viable device.32 The coloration efficiency of thin-film Sm0. van der Sluis et al. the operation of palladium oxides electrochromes on p. While the palladium layer also catalyses the adsorption and desorption of hydrogen.32 Alloys of lanthanum also show this reflective transition.34 Furthermore.29 The main technological drawbacks at present are the formation of an oxide layer between the lanthanum and the palladium top-coat (cf. Of the various attempts to improve the cycle lifetime. the best results were obtained with switchable mirrors pre-loaded with hydrogen during deposition. irreversible oxidation of the metal hydride films. The optical properties of these films are similar to those of films switched electrochemically or exposed to hydrogen gas. The reverse reaction can be accomplished with an aqueous H2O2 solution. No yttrium-based reflective devices are ready for marketing. von Rottkay suggests the change in reflectivity is about 50% for Mg–La hydride.31 it also limits the maximum visible transmittance of the hydride layer to about 35–40%.38 show that thin films of lanthanide hydride can be switched from absorbing to transparent with aqueous NaBH4 solution.33 thereby lending them a ‘neutral hue’. For example.35 The use of hydrogen gas effects a very rapid optical transition. .29. Typically. 178) and slower colouration kinetics than with H2 gas. solid electrolyte layer allows the transport of hydrogen. nontransparent state at intermediate pressures of hydrogen.37 Alternatively. Notten et al.308 Miscellaneous inorganic electrochromes forming atomic hydrogen.36 have more recently shown how the same effect can be observed with the lanthanum film immersed in aqueous KOH (1 mol dmÀ3). and delamination as the films peel from their substrates.7Hx is slightly lower than for HxWO3. the La–Mg alloy has virtually no transmittance at high pressures of hydrogen. The optical properties of alloys are also preferred because their colours contrast with the red–yellow colour of the transparent lanthanide states.37 have examined the durability of lanthanide hydride films immersed in aqueous KOH solution.3Mg0.29. magnesium–lanthanide alloys can pass through three different optical states: a colour-neutral. more typical ECDs can be fabricated in which a clear. depicted in Eq. a dark. but rapid technological advances are likely. transparent state at high pressures of hydrogen. the macroscopic effects of degeneration upon cycling of the switchable mirror include slower rates of coloration and bleaching.6) for lanthanum hydride via an electrochemical reaction: LaHx ðsÞ þ yOHÀ ðaqÞ ! LaHðxÀyÞ ðsÞ þ yH2 O þ yeÀ : (9:6) In this way. and a highly reflective metallic state at low pressures of hydrogen. (9. Janner et al. Appl. 10. 205–15. 301–5. 14. M. Pettersson. Konesky. and Causon. M. Kondratyuk.. and Hagfeldt. R. Mater. 1993. 1998. and Kulak.. S. 2003. V. Edwards. I.. SrTiO3. D. B. K. Fullerene electrochromism under high pulsed fields. Lett. T. 5. S.41 phosphotungstic acid. M. 61. 9. 771–2.. MA. 417. J.44 organic ruthenium complexes. 377. Soc. Electrochem. Gruszecki. 46. 7. Solid State Ionics.. Electrochim. 4. Rauh. M.. Phys. 3788. Near-infrared electrochromism in LixC60 films. 144–6. Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide. 2. G.. Kuwabara.... A. G. and Luengo. 63. Boschloo. 407–13. G. Yu. D. 2004. SPIE. R. G. Electrochromic phenomena in fullerene thin films.5 Other miscellaneous inorganic electrochromes Electrochromism has also been reported for the other miscellaneous inorganic materials such as nickel-doped strontium titanate. 3142. Yen. H.40 ruthenium dithiolene. Acta. 11.. G. M. D.. Acta.. J. Electrochim. 175–84. M. P. 2002..42. Y. J. I. 963–7. 4.46 References 1. L. . Pettersson. and Noda. and Hagfeldt. SPIE. H. Commun. Vlasou. Electrochem. T. A. R.References 309 9. Thunman. Proceedings of the Annual Technical Conference: Society of Vacuum Coaters. 283–5. L. 146... Electrochemical properties of Langmuir–Blodgett films. A. M. A. Pfluger. de Torresi. at coloration–bleaching processes using a new quasi-reference electrode. Wang. Chem.. O. H. 3. L. Thuraisingham. M. Electrochem. Proc. Potential wave-form measurements of an electrochromic device. 599–601. J. Goldenberg. Chem. and Angnes. Reversible electrochromic effect in fullerene thin films utilizing alkali and transition metals. 6.. Boston. P. G. Symp. Kunzi. 3459–65. Meissner... Lett.. R. 35.. ‘Electric-paint displays’ with carbon counter electrodes. A. Electroanal. A. Electroanal. pp. R. and Hagfeldt. Appl.. Konesky. 43. Klein. H. Charge–discharge kinetics of electric-paint displays. 12. Nascomento. WO3/Sb2O5/C. 1999. V.. 18–23 April 1998. U. and Guntherodt.. G.. Pettersson.. 1979. A. Proc. Electroanal. 8. Phys. J. O. Determination of the lithium ion diffusion coefficient in graphite. Tian. M. Proc.. J. 565.. Ritter. A. I. and White. I. 14–21. Thuraisingham. Discovery of a new reversible ¨ ¨ electrochromic effect. B. I. 1999. 1993. Kokorin. J. Torresi. 1994. N. 15.39 indium nitride. A. Stability and reversibility of the electrochromic effect in fullerene thin films. T. Ciampi. L. Vestling. G. 303–8. G. 379. 5. A. 8–14. V. O. Gruszecki. 1997. Popov... Sohlberg. 3–19. A.. I. 2001. 2187–93. 1994. B. Konesky. T. C. Ralcherko. Performance of screenprinted carbon electrodes fabricated from different carbon inks. E. 1996. 13.. Chem. Kulak. M. Res. Andersson. Edwards. H.45 and ferrocene–naphthalimides dyads. C. Gruszecki.. J. Commun. Soc.43. Edwards.. Konesky. A semi-empirical model for the charging and discharging of electricpaint displays. Pulse-width modulation effects on fullerene electrochromism. Michalak..310 Miscellaneous inorganic electrochromes 16. A. T.. ¨ Synthesis of yttrium trihydride films for ex-situ measurements. Proc. C. Sol. M. Soc. J. de Groot. P. van der Sluis. 1999. Richardson. Grgur. G. H. A. Acta. Kaibara... 44. A.. C. V. T. Asano. Soc. Hjorvarsson. B. A. Markovic. Energy Mater.... Montreal. 1997.. 17. Dan. 1996. Durability of electrochromic windows fabricated with carbon-based counterelectrode. Y.. S. de Groot. Yttrium and lanthanum hydride films with switchable optical properties. T. B. 1998. J. Ziegler. C. and Duine. Soc. 29. P. van der Sluis.. 1997. M. 239. Jetten. An electrochromic system based ˆ on the reversible electrodeposition of lead. M. N. 2001. Status of reversible electrodeposition electrochromic devices. 31. and Mercier. Wijngaarden. Armitage. Sol. Howard. 317–31. 44. 1995. Acta. Cells. Electrochim... jr. 3093–100. and Cho. Electrochim. 1962. 24. 56. Sol. J. de Morais. P. 26. 109. L. Spectroelectrochemistry of reversible electrodeposition electrochromic materials. Rector. 1996. 158–69. Griessen. Uchida.. R. J. Solid state Gd–Mg electrochromic devices with ZrO2 Hx electrolyte. Alloys Compd. M. J. Lucas. C. L. 414. 6 May 1997. J. 33.. J.. and Bulhoes. Nishikitani. Applications of reversible electrodeposition electrochromic devices. . E. F. Inert electrode behaviour of tin oxide-coated glass on repeated plating–deplating cycling in concentrated NaI–AgI solutions. L. Lett. I. 992–3. K. M. Sol.. 96–105. Rector.. 34. J. Oxidation mechanism for reversibly electrodeposited bismuth in electrochromic devices. 489–97. I. 20. N. R. M. 85. An electrochromic system ˆ based on redox reactions. 183–8. E.. P. K. M. B. J. 18. L. J. Mascaro. 309–16. 448. D. 1996. N. P. J. 231–4.. Slack. T.. Cells.. ´ 27. R. N. Optical characterization of bismuth reversible electrodeposition. P. 32. 146. Electroanal. Kooij. Electrochem. and Griessen. B. Mantell. Sol. 22. 39. J.. J. L273–4. Cells. Electrochem. A. Decker. Kaibara. Sol. N. Improvement of thermal stability of an organic–aqueous gel electrolyte for bismuth electrodeposition devices. Electrochem. R. and Kubo.. Olafsson. and Carlos. Electrochem. S. H. SPIE. 84–92. J. P. 3788. 94–2. Electrochim. A. van Gogh. In situ resistivity measurements and optical transmission and reflection spectroscopy of electrochemically loaded switchable YHx films. T.. S. G. and Howard.. 11–16.. J. Sol. and Nishikitani. M. Y. 121st Electrochemical Society Meeting. and Bulhoes. H. R. J. J. A. Optical switches based on magnesium lanthanide alloy hydrides. 70.. 477–93. and Ross. N. 2167–71. P.. Nature (London). Ouwerkerk. 2990–4.. 23. J. J. and Ziegler. 1995.. D. S. P. Ziegler. Asano. de Oliveira. 19. von Rottkay. Cells. 25. Koeman.. 158–71. Huiberts. E. M. Appl. da Silva Curvelo.. Ziegler.. and Koeman. J. 1999. T. Mascaro. and Brzezinski. Electrochem. 39. Rubin. Energy Mater. H. 380. A. A. B. 1999. S. N. Richards. Phys. and Howard. R.. S. Energy Mater. R. 46.. 1994. Acta. Chem. L. and Duine. Energy Mater. 3211–17. 30. Sol. 21. 28. 2005. Kubo. M. 96–24. 1996. Soc. 1999. Chem. de Torresi. B. M... S. and Zaromb. Optical properties of reversible electrodeposition electrochromic materials. Effect of hydrogen insertion on the optical properties of Pd-coated magnesium lanthanides. Huiberts.. Proc. S. P. Griessen. 3356–8. 144. Soc. Wijngaarden. 1999. Y. K.. and Torresi. Thermal and optical behavior of electrochromic windows fabricated with carbon-based counterelectrode. Underpotential deposition of lead on Pt(111) in the presence of bromide: RRDPt(111) E and X-ray scattering studies. Electroanal. J. C. Proc. Canada. R. abstract 945. T. Cells.. B. and Ward.. Z. and Bradley. 2451–4. 44. M. Electrochem. A.. 41. C. J. R.. Cycling durability of switchable mirrors. X. 3348–53.. Opt.. Gan. 476–9. Peter. 255–63. 127. ˜ Electrochromic switching in the visible and near IR with a Ru–dioxolene complex adsorbed on a nanocrystalline SnO2 electrode. and Wagner. J. D. 40. T. Optical switching of Y-hydride thin film electrodes: a remarkable electrochromic phenomenon. P. L. L. J. S. Qi. 113–15. Commun. Mater. J.. 39. P. M. 46. Fox. P. Electrochem. and Wang. 143. 645. Janner. 46. Y. Tell. Electrochromic effects in solid phosphotungstic acid and phosphomolybdic acid. B. Wang. Solid State Ionics. Lane. M. A. H.. S. K.. F. M. Meng.. K. Y.. S. 5. L.. Electrochem. 3063–6. Desjardins. 42. 1999. Phys. J. 473–81. 80. Ohkubo. H. Watanabe. P. P. 5001–6. Sol. J. J.References 311 35. 431–7. 2001. V. Electrochromic ruthenium complex materials for optical attenuation.. and Estrada. 2173–8. van der Sluis. and Griessen.. Chen. H. Appl. J. 1979. J. 50. Electrochemically induced optical switching of Sm0. Sol. Meacham. Synthesis and characterization of rough electrochromic phosphotungstic acid films obtained by spray-gel process. M. 21. 44.. 1996. Acta.-M. Optical properties of amorphous indium nitride films and their electrochromic and photodarkening effects. and Mercier. A. A. Tell... Electrochim. J. Soc.. Energy Mater.. Ouwerkerk. Nonomura. Synthesis and luminescence properties of novel ferrocene–naphthalimides dyads. 1979. 416–20. 2002. 36.7Hx thin layers.. 50.3Mg0. . 5944–6. Electrochromism in nickel-doped strontium titanate. Appl. 1980. D. P. T. 37. M. W. 1998. K.. Electrochromism in solid phosphotungstic acid. Medina. D. and Wudl.. Kremers.. Electrochim. 2003. 45. Notten. Organometallic Chem. Acta. Solis. van der Sluis. S. H.. Gotoh. 38. 43.. Tian. Yamamoto. Z. Sci. A. and Nitta. Hill. Appl. Garcı´ a-Canadas.. Phys. 2003. S. D. Rodriguez. Soc. Mohapatra. 168–75.. Chemochromic optical switches based on metal hydrides. M. Rahn. 2003. 1130–14. Surf. 1997. 5. Heeger and MacDiarmid. electroluminescent organic light-emitting diodes (OLEDs). is the simplest form of conjugated conducting polymer.3. Its electrical conductivity exhibits a twelve order of magnitude increase when doped with iodine.9 sensors10 and thin-film field-effect transistors. a professor of chemistry in the College of London Hospital. chemical or electrochemical oxidation 312 .6 Since 1977.2 Types of electroactive conducting polymers Poly(acetylene).10 Conjugated conducting polymers 10. with a conjugated p system extending over the polymer chain. electroactive conducting polymers have been intensively investigated for their conducting.1 Historical background and applications The history of conjugated conducting polymers or ‘synthetic metals’ can be traced back to 1862. Numerous electronic applications have been proposed and some realised. Thus. due to its intractability and air sensitivity.8 photovoltaic elements for solar-energy conversion. reported the electrochemical synthesis of a ‘thick layer of dirty bluish-green pigment’ (presumably a form of ‘aniline black’ or poly(aniline)) by oxidation of aniline in sulfuric acid at a platinum electrode.2 However. semiconducting and electrochemical properties.7.1 Introduction to conjugated conducting polymers 10. (CH)x.1. which led to the award of the 2000 Nobel Prize in Chemistry to Shirakawa.1 However. following the discovery2.1. when Letheby. widespread interest in these fascinating materials did not take place until after 1977. including electrochromic devices (ECDs).11 10.4 of the metallic properties of poly(acetylene). poly(acetylene) has seen few applications and most research on conjugated conductive polymers has been carried out with materials derived from aromatic and heterocyclic aromatic structures. 3 Mechanism of oxidative polymerisation of resonance-stabilised aromatic molecules Polymerisation begins with the formation of an oxidatively generated monomer radical cation. After the loss of two protons and re-aromatisation.19 O N H Pyrrole S Thiophene S EDOT Aniline O NH2 O Furan N H Carbazole N H Azulene Indole Of the resulting polymers.14.1 Introduction 313 of numerous resonance-stabilised aromatic molecules. pyrrole. hence rapid colour change. However.18. the pyrrole dimer forms from the corresponding dihydro dimer dication.16.e. the electropolymerisation mechanism for the fivemembered heterocycle.10.1.13.1. and others. The succeeding mechanism is believed to involve either coupling between radical cations.12. azulene. the enhanced conductivity of a charged state (oxidised or reduced) relative to an uncharged state is an accompaniment that is useful in assisting towards rapid redox change. Note that ‘electroactive’ denotes the capability of interfacial electron transfer in one or other direction (oxidation and/or reduction. carbazole. indole (see structures below). proton loss and . produces electroactive conducting polymers. aniline. The dimer (and succeeding oligomers) are more easily oxidised than the monomer and the resulting dimer radical cation undergoes further coupling reactions. thiophene. a redox capability that allows of colour change). such as pyrrole. the relation between redox properties and conductivity is not necessarily straightforward and varies from polymer to polymer. On the other hand. or reaction of a radical cation with a neutral monomer. poly(pyrrole)s and poly(aniline)s have received the most attention in regard to their electrochromic properties. i. furan. and will be discussed in this chapter.4-(ethylenedioxy)thiophene (EDOT).15.17. As an example. 3. 10. the poly(thiophene)s. showing radical cation–radical cation coupling is given in Scheme 10. .16 10. Films of high-quality oxidised polymer can be formed directly onto electrode surfaces. longer chains promoting high conductivity. H N+ H H H etc.314 – e– Conjugated conducting polymers . N H N N H + 2H+ Scheme 10. The case of radical cation–radical cation coupling is shown. +N H H H H N+ N+ N H H N + 2H+ .20 until the oligomers become insoluble in the electrolyte solution and precipitate (like a salt) as the electroactive conducting polymer. H + +N H . H .1. Electropolymerisation proceeds through successive electrochemical and chemical steps according to a general E(CE)n scheme. N H +N H H 2 +N H H N N H H N+ N H – e– N H H H N+ N H . re-aromatisation.1 Proposed mechanism of the electropolymerisation of pyrrole. imposing a non-zero dihedral angle .4 Conductivity and optical properties Electronic conductivity in electroactive polymers results from the extended conjugation within the polymer. The average number of linked monomer units within a conducting polymer is often termed the ‘conjugation length’. Note that 6¼ 0 if R16¼ H and R26¼ H. X-Ray diffraction of pyrrole oligomers suggests the poly(pyrrole) rings to be coplanar21 but substitution at nitrogen and the b-carbon introduces a significant twist in the polymer backbone. 10.TCNQ KCP DOPED POLYTHIOPHENE σmax = 2000 S cm–1 InSb 1 GERMANIUM SEMICONDUCT ORS SILICON 10–6 SILICON BROMIDE GLASS INSULA TORS DNA DIAMOND SULFUR Q U AR TZ 10–8 10 –10 10–2 10–4 TRANS (CH) x CIS (CH) x UNDOPED POLYPYRR OLE σmax = S cm–1 H ( N ) X ( S )X 10–12 10–14 10–16 10–18 MOST MOLECULAR CRYST ALS Ω –1 cm–1 Figure 10. electroactive conducting polymers are charge-balanced (doped) with counter anions (‘p-doping’) and have delocalised p-electron band structures. Thesis. Figure 10.16 with typical conductivity values in the range 101–105 S cmÀ1. Reduction of such p-doped conducting polymers. 17. ‘Donor–Acceptor methods for band gap reduction in conjugated polymers: the role of electron rich donor heterocycles’. with concurrent METALS SIL VER COPPER IR ON BISMUTH 106 104 102 POLYA CETYLENE σmax > 2 × 104 S cm–1 (SN)x TTF . Department of Chemistry. Values of s are compared with those for common metals.1 shows illustrative conductivity ranges for poly(acetylene). who adapted it from the Handbook of Conducting Polymers. University of Florida. 2002. semiconductors and insulators. p.TCNQ NMP. (Figure reproduced from Thomas.18) . C.1 Introduction R1 N ∅ N R2 R1 n R2 315 In the conducting oxidised state with positive charge carriers. poly(thiophene) and poly(pyrrole).D. by permission of the author. A.1 The conductivity range available with electroactive conducting polymers spans those common for metals through to insulators. Ph. the electronic bandgap between the highest-occupied p-electron band (the valence band) and the lowest-unoccupied band (the conduction band). The energy gap Eg. the electrolyte. This is illustrated in Scheme 10.316 Conjugated conducting polymers H N N H N H n – + nX Yellow–green (insulating) undoping p-doping H N X – + ne– N+ n X – H +N H Blue–violet (conductive) Scheme 10. counter-anion egress to. the undoped (electrically neutral) state of electroactive conducting polymers can undergo reductive cathodic doping or n-doping. This doping has been exploited in the development of a model ECD using poly{cyclopenta[2. electrically neutral) insulating form. The magnitude of the conductivity change depends on the extent of doping. that results in the undoped (that is to say. or cation ingress from. radical cation charge carriers (polarons) are generated. such neutral polymers are typically semiconductors and exhibit an aromatic form with alternating double and single bonds in the polymer backbone. The yellow-green (undoped) form undergoes reversible oxidation to the blue-violet (conductive) form.22 The polymer PCNFBS is one of a series of fused bithiophene polymers whose Eg values can be controlled by . which gives the electrochromic colour states in thin films of poly(pyrrole): the non-conjugation of the oxidised form. On oxidative doping. as both the anode and the cathode material. provides the coloured structure. Further oxidation results in the formation of dication charge carriers (bipolarons). when under electrochemical control. with accompanying cation insertion to balance the injected charge. determines the intrinsic optical properties of these materials.and n-dopable. and the polymer assumes a quinoidal bonding state that facilitates charge transfer along the backbone. removes the electronic conjugation. with insertion of charge-compensating anions.2. a low-bandgap conducting polymer that is both p. which. can be adjusted by the applied potential. In the reduced form.3-b0 ]dithiophen-4-(cyanononafluorobutylsulfonyl)methylidene} (PCNFBS). In some instances.1-b.4.2 Electrochromism in poly(pyrrole) thin films. as explained in detail towards the end of this section. that allows visibly evident photo-excitation. . The supposed similarity between conducting polymers and doped semiconductors arises from the manner in which the redox changes in the polymer alter its optoelectronic properties. The colour change or contrast between doped and undoped forms of the polymer depends on the magnitude of the bandgap of the undoped polymer. Electrochemically polymerised films of the polymer switch from red in the neutral state to purple in both the p. Thin films of conducting polymers with Eg greater than 3 eV. in fact all thin films of electroactive conducting polymers have electrochromic possibilities. while in the doped form they generally absorb in the visible region.602 Â 10À19 J.10.and n-doped films. Those with Eg equal to or less than 1. because in its initial sense doping involved minute (classically.and n-doped states. the free carrier absorption is relatively weak in the visible region as it is transferred to the near infrared (NIR) part of the spectrum. In fact.22 The spectral changes observed in an electrochemical cell assembled from two polymercoated transparent electrodes were a combination of those seen in the separate p. below ppm) amounts of dopant. the stability of negatively charged polymer states is generally limited. since redox switching involving ingress or egress of counter ions gives rise to new optical absorption bands and allows transport of electronic charge in the polymer matrix. Polymers with a bandgap of intermediate magnitude have distinct optical changes throughout the visible region. ‘doping’ and similar terms are now so widely used in connection with conjugated conducting polymers that attempts to change the terminology could cause confusion. are colourless and transparent in the undoped form. a 1 eV ¼ 1. and n-doping is difficult to achieve. and can be made to induce many colour changes.a which gives a corresponding spectroscopic value of max of $400 nm. Oxidative p-doping shifts the optical absorption band towards the lower energy part of the spectrum.22 Although this is a fascinating example. after doping. Electroactive conducting polymers are type-III electrochromes since they are permanently solid. However. It is to be noted that the ‘p-doping’ and ‘n-doping’ nomenclature comes from classical semiconductor theory. the suitability of the terms ‘doping’ and ‘dopant’ has been criticised23 when they refer to the movement of counter ions and electronic charge through these polymers. As already noted in the case of poly(pyrrole).5 eV ($800 nm) are highly absorbing in the undoped form but.1 Introduction 317 inclusion (initially in the precursor monomers) of electron-withdrawing substituents. ‘Electrochromic devices’ by Mastragostino25 (in 1993). Chapter 9 of Electrochromism: Fundamentals and Applications by Monk. with particular emphasis on poly(3-substituted thiophene)s and poly(3.28 ‘Polymeric electrochromics’ by Sonmez (in 2005)29 and ‘Electrochromic organic and polymeric materials for display applications’ by Mortimer et al.5 Previous reviews of electroactive conducting polymer electrochromes A vast literature encompasses the electrochromism of electroactive conducting polymers.19. However. the more positive (and the more oxidising) is the potential that is applied to the electrode under consideration.b the electropolymerisation and switching of b-methylthiophene has been more intensively studied than the unsubstituted parent thiophene.1 Introduction to poly(thiophene)s are of interest as electrochromes due to their relative Poly(thiophene)s ease of chemical and electrochemical synthesis. including ‘Application of polyheterocycles to electrochromic display devices’ by Gazard24 (in 1986).1.32 16.31 b When oxidation processes predominate in discussion.30 10.318 Conjugated conducting polymers 10. ‘Organic electrochromic materials’ by Mortimer (in 1999). which are for processes that are the reverse of the conventional half reactions (i.16 Thin polymeric films of the parent poly(thiophene) are blue (max ¼ 730 nm) in the doped (oxidised) state and red (max ¼ 470 nm) in the undoped form. Furthermore.27 ‘Electrochromic polymers’ by Mortimer (in 2004). . the introduction of a methyl group at the 3-position of the thiophene ring leads to a significant increase of the polymer conjugation length and hence electronic conductivity. reductions) of Chapter 3. due to its lower oxidation potential. and many reviews are available. Mortimer and Rosseinsky12 (in 1995).2.16 This effect has been attributed to the statistical decrease in the number of insulative a–b0 couplings and also to the decrease of the oxidation potential caused by the inductive (electron-donating) effect of the methyl group. and turns pale blue upon oxidation. it is convenient to cite oxidation potentials. which has led to the study of numerous novel poly(thiophene)s.4-disubstituted thiophene)s.31 A vast number of substituted thiophenes has been synthesised.2 Poly(thiophene)s as electrochromes 10.34 eV).16 Poly(3-methylthiophene) is purple when neutral with an absorption maximum at 530 nm (2. and processability.e. environmental stability. (in 2006). positive values are implied: the greater the value. In the present chapter. ‘Electrochromism of conducting polymers’ by Hyodo26 (in 1994). which exhibits a deep blue colour in its neutral state and a light blue transmissive state upon oxidation. Department of Chemistry. Thesis. (Figure reproduced from Thomas. C. The bandgap is determined by extrapolating the onset of the p to p* absorbance to the background absorbance.80 eV) appear at lower energy. by permission of the author. Figure 10. Ph.25 and $0. The Eb1 transition is allowed and is visible at intermediate doping levels.4-(ethylenedioxy)thiophene] – PEDOT.2 Poly(thiophene)s as electrochromes 319 The evolution of the electronic band structure during electrochemical p-doping of electrochromic polymers can be followed by recording in situ visible and NIR spectra as a function of applied electrode potential. 41. University of Florida.3 mol dmÀ3) in propylene carbonate solution containing tetrabutylammonium perchlorate (0. potential. Upon doping. The film had been deposited from EDOT (0. A.D.1 mol dmÀ3) and spectra are shown on switching in tetrabutylammonium perchlorate (0. The inset shows absorbance vs. ‘Donor–Acceptor methods for band gap reduction in conjugated polymers: the role of electron rich donor heterocycles’.2 Spectroelectrochemistry for a PEDOT film on an ITO–glass substrate.0 eV). and two new optical transitions (at $1. corresponding to the presence of a polaronic charge carrier (a single charge of spin ½). Further oxidation leads Figure 10.) . p. 2002. poly[3.10. with a maximum at 621 nm (2.2 shows the spectroelectrochemical series for an alkylenedioxy-substituted thiophene polymer. the interband transition decreases. is characteristic of a p–p* interband transition.33 The strong absorption band of the undoped polymer.1 mol dmÀ3) in acetonitrile. 320 Conjugated conducting polymers to formation of a bipolaron and the absorption is enhanced at lower energies. i. The colours available with polymer films prepared from 3-methylthiophenebased oligomers are strongly dependent on the relative positions of methyl groups on the polymer backbone.34 There has been much interest in polymer films derived from electrochemical oxidation of thiophene-based monomers that comprise more than one thiophene heterocyclic unit. It has been shown35 that the wavelength maxima of undoped poly(oligothiophene) films decrease as the length of the oligothiophene monomer increases.36 As listed in Table 10. In principle. . which reversibly switch from red to black on oxidation. oligothiophenes containing alkyl groups at the b-carbon have been synthesised.2 Poly(thiophene)s derived from substituted thiophenes and oligothiophenes As already noted above in the comparison of poly(thiophene) and poly(3methylthiophene). This tuning represents a major advantage of using conducting polymers for electrochromic applications. red and orange in the reduced form. The oxidation potentials included in this table do not vary much with oligothiophene.1. i.32.3 show that those polymers with the smallest dihedral angle generally have the highest wavelength maxima.2. blue and violet in the oxidised form.35 Groups at the b-carbon cause steric hindrance.3 below) are linear.e. To investigate the effect of the dihedral angle between thiophene planes. yellow. whereas bridged species (exemplified in Scheme 10. Subtle modifications to the thiophene monomer can significantly alter spectral properties. while compounds containing three or more thiophene units have the general name of ‘oligothiophene’.2.e. A recent example is provided by cast films of chemically polymerised thiophene-3-acetic acid. that next to S) is called bithiophene. which appears when the bipolaron bands finally merge with the valence and conduction bands. the optical and structural changes are often reversible through repeated doping and de-doping over many thousands of redox cycles. Further study of the effects of steric factors is provided by the electronic properties of poly(thiophene)s with 3. 10. The colour variations have been ascribed to changes in the effective conjugation length of the polymer chain.4-dialkyl substituents. the colour shifts towards the characteristic absorption band of the free carrier of the metallic-like state. The results in Table 10. The species containing two thiophene units (joined at the a-carbon. these include pale blue. and purple. Table 10. In such electroactive conducting polymers. tuning of colour states can be achieved by suitable choice of thiophene monomer. Oxidation potentials are generally unaffected by variations in . higher optical bandgaps. and alkoxy-substituted poly(thiophene)s are being intensively investigated for their electrochromic properties. owing to the presence of the two electrondonating oxygen atoms adjacent to the thiophene unit. say.38 10.4-(ethylenedioxy)thiophenes Materials based on PEDOT have a bandgap lower than either poly(thiophene) or alkyl-substituted poly(thiophene)s.16 Alternation between the 3 and 4 positions relieves steric hindrance in thiophenes.37. However. b Wavelength maximum refers to the reduced (undoped) redox state of the polymer. The electron-donating effect of alkoxy groups offers an answer here. b0 positions should provide the synthetic basis to perfectly stereoregular polymers. Scheme 10.04 340 S a 0. Wavelength maxima and oxidation potentials of polymers derived from oligothiophenes (based on ref. disubstitution at the b. which lead to a decrease in polymer conjugation length.00 356 S S S 1.93 S S S Note that these structures do not represent the molecular stereochemistry.1. 35).95 484 S S 1.10. In fact. poly(3.3 Poly(thiophene)s derived from 3. this approach is severely limited by the steric interactions between substituents.3 shows the .4-dialkylthiophene)s have higher oxidation potentials.2 Poly(thiophene)s as electrochromes 321 Table 10. Monomera λmax/nmb (undoped) 519 S Eox/V 0. but many are harder to electropolymerise than.2. 3-methylthiophene. and lower conductivities than poly(3-alkylthiophene)s. 2. 15). these materials exhibit excellent stability in the doped state. The polymer PEDOT was first . Compared with other substituted poly(thiophene)s. The attributes of ethylenedioxy substitution are also pointed out in the figure.6À1. As shown above. which has a high electronic conductivity.7 eV) itself is 0. which results in an absorbance maximum in the red region of the electromagnetic spectrum. λmax/nm (undoped) Polymer colour (reduced form) Polymer colour (oxidised form) Monomer S 530 Purple Pale blue S S 415 Yellow Violet S S 505 Red Blue S S 450 Orange Blue S S S S 425 Yellow Blue S S S S 405 Yellow Violet S S S S 410 Yellow Blue–violet S S S S 425 Yellow–orange Blue structural changes of PEDOT upon reproducible electrochemical oxidation and reduction. Colours of polymers derived from oligomers based on 3-methylthiophene (based on ref.5 eV lower than poly(thiophene). the bandgap of PEDOT (Eg ¼ 1.322 Conjugated conducting polymers Table 10. Effect of the dihedral angle : Spectroscopic and electrochemical characteristics of poly(oligothiophene)s (based on ref. which forms a dispersion in water. 35).99 S S 413 0.40 Bayer AG now produce the EDOT monomer. PSS – as the counter ion in the doped state.00 S S 475 0. 3. to yield the commercially available product PEDOT:PSS BAYTRON P by Bayer AG and ORGATRON by AGFA Gevaert.3. . soluble and stable conducting polymer.94 developed by Bayer AG research laboratories in Germany in an attempt to produce an easily oxidised.41 on a multi-ton scale and it is available commercially as BAYTRON M. 4-(ethylenedioxy)thiophene.90 S S S 356 1.10.39.96 S S 420 0. To aid processing.88 S S 550 0.04 S S S 375 0. Monomer λmax/nm (undoped) Eox/V S S 484 1. the insolubility of PEDOT can be overcome by the use of a watersoluble polyelectrolyte – poly(styrene sulfonate).2 Poly(thiophene)s as electrochromes 323 Table 10. 4-ethylenedioxythiophene] – PEDOT – upon reproducible electrochemical oxidation and reduction. while b locking the β positions so only αα’ coupling may occur.3 Structural changes of poly[3. Ph. Department of Chemistry.) n SO3– SO3H SO3H SO3H SO3– SO3H O S S + O O O S O O S O O . p. C. leading to lo w er monomer oxidation potentials and decreased band gaps .4-alkylenedioxyheterocycle-based polymers and co-polymers’. ‘Structure–property relationships of electrochromic 3. relie v ing steric interactions betw een adjacent monomer units.D. 2002. S + O n PEDOT : PSS As PEDOT and its alkyl derivatives are cathodically colouring electrochromic materials. L. (Figure reproduced from Gaupp. Scheme 10.324 Conjugated conducting polymers Ethylene br idge ‘ties bac k ’ substituents . Attributes of ethylenedioxy substitution are also pointed out. by permission of the author. University of Florida. Thesis. 28. O S S O O O O O S S O O O O S S O O O O S S O O Neutral state (b lue) O S +S O O X – O S O O S S O O S +S O O S – Oxidiz ed state (transmissiv e sky b lue) O O O O X O O Alk oxy groups provide electron donation. S O O O . they can be used with anodically colouring conducting polymers . 49. The latter is accomplished by using substituents and co-repeat units that adjust the energies of the highestoccupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of the p-systems.3 eV) that is orange. orange.37.38.48. green and yellow as seen in Colour Plate 2.52 . and the nature of the substituents on the alkyl bridge.47.8 eV) that is red.42 Changes in the size of the alkylenedioxy ring in general poly[3.46 and better processability through increased solubility.4 eV) that has a deep-purple neutral state. biphenyl (Eg ¼ 2. polymers with bandgaps in the range 1.5 eV) that is yellow. p-phenylene (Eg ¼ 1.52.10.45. which has led to the study of a range of variable-bandgap PEDOT-based materials. From a series of oxidatively polymerisable bisarylene EDOT monomers (see structures below). from blue through purple. A few examples include bis-arylene EDOT-based polymers. a full ‘rainbow’ of colours is available.45 higher optical contrasts43. and by controlling the electronic character of the p-system with electron-donating or -accepting substituents.37. red.5 eV have been prepared.2 Poly(thiophene)s as electrochromes 325 as the other electrode in the construction of dual-polymer ECDs.4–2.37.44. have led to polymers with faster electrochromic switching times.50 As for thiophene.51. which exhibit two to three distinct coloured states.43.4-(alkylenedioxy)thiophene] – PXDOT – materials.38 An interesting set of materials is the family of EDOT-based polymers which have been prepared with higher energy gaps than the parent PEDOT.53 O R O O Ar O S Ar = S C10H21 C10H21 R R = alkyl alkoxy oligoether F CN NO2 S O N R′ R′ = H alkyl oligoether In the neutral polymers.44. with spacers of vinylene (Eg ¼ 1.51. numerous substituted EDOT monomers have been synthesised.38 The bandgap of such conjugated polymers is controlled by varying the extent of p-overlap along the backbone via steric interactions. and carbazole (Eg ¼ 2. residual absorptions remain in the visible region. to obtain the desired green electrochrome in the neutral state.57. While incorporation of an electron-rich donor unit allows oxidation for p-doping. a 2. Furthermore.58. This has been shown with EDOT acting as the donor unit and both pyridine (Pyr) and pyrido[3.4-b]pyrazine (DDTP) monomer that would afford two conjugated chains was designed and synthesised. The processibility of the poly(DDTP) system has been enhanced by the . For example. it was proposed that a polymer backbone be synthesised that contains two well-defined.55. The depletion upon oxidation makes the polymer film more transparent. but. To achieve this. Films of poly(DDTP) were synthesised electrochemically on platinum and ITO-coated glass.59 a study has been carried out on the development of an electroactive conducting polymer which is green in the neutral state and virtually transparent (very pale brown) in the oxidised state.54 As shown in Colour Plate 3. the films pass through a green intermediate state to a blue fully oxidised state. Thus. it shows a marked blue with n-doping. On electrochemical oxidation of the film. colours ranging from yellow via red to blue can be evoked in the neutral polymer film. isolated.4-b]pyrazine.3-di(thien-3-yl)-5. by varying the ratios of co-monomer concentrations. conjugated systems which absorb red and blue light.54 In all co-polymer compositions. the p–p* transitions of both bands are depleted at the expense of an intense absorption band centred in the NIR.56 The polymer PBEDOT-PyrPyr(Ph)2 is green when neutral. the inclusion of an electron-poor acceptor unit allows reduction. It changes with p-doping to a light-blue colour. which corresponds to low-energy charge carriers.4-ethylenedioxythiophene)]-N-alkylcarbazole – BEDOT-NMeCz. which results in absorption of the red light at wavelengths longer than 600 nm. while the other chain absorbs in the blue at wavelengths below 500 nm. the introduction of donor– acceptor units has been shown to increase the stability of this n-type redox state. as the acceptor unit. Although n-type doping of most of these polymers results in inherent instability to water and oxygen.57 One chain has electron donor and acceptor groups to decrease the bandgap. giving a transmissive brown colour. unfortunately.6-bis[2-(3.326 Conjugated conducting polymers Another approach to extend colour choice is electrochemical co-polymerisation from a solution containing two monomers.4-ethylenedioxythiophene) – BEDOT – and 3.56 More recently.7-di(thien-2-yl)thieno[3.20 -bis 3.55.e. and magenta upon n-doping. some electrochromic conducting polymers also undergo n-type doping. grey upon p-doping. i.54 As mentioned previously. the ability to adjust the colour of the neutral polymer by electrochemical copolymerisation has been demonstrated using co-monomer solutions of 2.56 The polymer PBEDOT-Pyr is red in the neutral state. PyrPyr(Ph)2.55. poly(pyrrole)s are also extensively studied for their electrochromic properties.66 and their enhanced compatibility in aqueous electrolytes has led to interest in their use in biological systems.62.2.5-phenylene) (PP) and poly(triphenylamine) (PTPA). which are smooth and reflecting.61.3 Poly(pyrrole)s and dioxypyrroles as electrochromes 327 electrochemical and chemical synthesis of a soluble form of the polymer. addition of oxygen at the bpositions lowers the bandgap of the resulting polymer by raising the HOMO level.10.7 eV) in the undoped insulating state and blue-to-violet in the doped conductive state. The polymers. As noted in Scheme 10.63. combined with the already relatively low oxidation potential for poly(pyrrole).64 These polymers have the advantage that they can be spin coated from a carrier solvent such as tetrahydrofuran (THF). and the radiating arms are regioregular poly(3-hexylthiophene). gives the poly(alkylenedioxypyrrole)s the lowest oxidation . using dioctyl-substituted DDTP. and optical properties that reveal a surprisingly high degree of structural order.64 Examples include star conducting polymers in which the centrosymmetric cores include hyper-branched poly(1. and blue to very pale blue in the oxidised state. so that thin films of both doped and undoped forms can be prepared. The colour of the polymers ranges from red via violet to deep blue in the reduced state. which have a central core with multiple branching points and linear conjugated polymeric arms radiating outward. Despite the branched structure. and several can be doped in solution.62.2 above.63. all have spectral features that produce a strong band in the visible region for the reduced state and a broad band extending into the NIR for the oxidised state. poly[3.60 10.67 As for dialkoxy-substituted thiophenes.4 ‘Star’ polymers based on poly(thiophene)s Star-shaped electroactive conducting polymers.4-(ethylenedioxy)thiophene didodecyloxybenzene] and poly[dibutyl-3. are now being investigated for electrochromic applications.3 Poly(pyrrole)s and dioxypyrroles as electrochromes As outlined for poly(thiophene)s. 10. Again. electrical. a wide range of optoelectronic properties are available through alkyl and alkoxy substitution.4-(propylenedioxy)thiophene].61. thin films of the parent poly(pyrrole) are yellow-to-green (Eg $ 2. This fact.65 Poly(pyrrole)s exhibit lower oxidation potentials than their thiophene analogues.3. and can easily be chemically or electrochemically synthesised. star polymers self-assemble into thin films with morphological. increasing the ring size of the alkyl bridge has the effect of generating another coloured state at low doping levels. For N-methyl-PProDOP the bandgap occurs at 3.4 gives the composition of the various poly(aniline) redox states.4 Poly(aniline)s as electrochromes Poly(aniline) films are generally prepared from aqueous solutions of aniline in strong mineral acids.69 Furthermore.68 Such polychromism is also seen in the substituted PProDOPs and poly[3. which results in a decrease of the effective p-conjugation.74.71 10.328 Conjugated conducting polymers potential for p-type doping in conducting electrochromic polymers.4-(propylenedioxy)pyrrole] – PProDOP – the neutral state is orange. These N-substituted polymers have been shown to work effectively in dual-polymer high-contrast absorptive/transmissive ECDs as the anodically colouring material. in contrast with cathodically colouring polymers that are coloured in their reduced state and become colourless upon oxidation. due to their electrochemical and optical compatibility with various PXDOT polymers.70 Both N-[2-(2-ethoxy-ethoxy)ethyl] PProDOP (N-Gly PProDOP) and N-propanesulfonate PProDOP (N-PrS PProDOP) are colourless when fully reduced but coloured upon full oxidation. with the intragap polaron and bipolaron transitions occurring in the visible region. which retain their low oxidation potentials. PXDOPs).4-(ethylenedioxy)pyrrole] – PEDOP – exhibits a bright-red colour in its neutral state and a light-blue transmissive state upon oxidation.76 Scheme 10.2 eV for PProDOP. higher bandgap polymers can be created. and on intermediate doping passes through brown.0 eV.4-alkylenedioxypyrrole)s (i. The nature of the substituent has an effect on the extent to which the p–p* transition is shifted. 72 .65 eV lower than that of the parent pyrrole.68 By effecting substitution at the nitrogen in poly(3.68 For poly[3.e.68 Poly[3. and finally to light grey–blue upon full oxidation.75.4-(butylenedioxy)pyrrole] – PBuDOP.70 Substitution induces a twist in the polymer backbone. This bandgap increase results in a blue shift in the p–p* transition absorbance. compared to 2.70 These two polymers are thus anodically colouring polymers.73 Several redox mechanisms involving protonation– deprotonation and/or anion ingress/egress have been proposed. 0.70 Both polymers also exhibit multiple coloured states at intermediate extents of oxidation. with a bandgap of 2. in that they change from a colourless state to a coloured one upon oxidation.05 eV. and an increase in the bandgap of the polymer. and has a purple colour in the neutral state becoming blue when fully oxidised passing through a dark green colour at intermediate extents of oxidation. conductor) H N N – N H n N H Blue (emeraldine base) N n N N N N n Black (perni graniline) Scheme 10. Pernigraniline has equal proportions of quinoidal and benzenoid moieties and shows metallic conductivity. as either base or salt. as has been shown by solid-state 13C-NMR spectroscopy. from the fully reduced (leucoemeraldine) through to the fully oxidised (pernigraniline) forms.4 Proposed composition of some of the redox states of poly(aniline).10. The aniline units within the poly(aniline) backbone are not coplanar. has a ratio of three benzenoid rings to one quinoidal ring. thus preventing conjugation between rings. XÀ is a charge-balancing anion. and is electrically conductive.77 Electrodes bearing such poly(aniline) films are electropolychromic and exhibit the following reversible colour changes as the potential is varied: transparent leucoemeraldine to yellow-green emeraldine to dark blue-black . Leucoemeraldine is an insulator since all rings are benzenoid in form and separated by –NH– or (in strong acid solution) –NH2þ– groups.4 Poly(aniline)s as electrochromes H N H N 329 N H Yellow (leucoemeraldine) H N H N X – N H n N X H Green (emeraldine salt . Emeraldine. 78 Of the numerous conducting polymers based on substituted anilines that have been hitherto investigated. The yellow form of poly(aniline) has an absorbance maximum at 305 nm. but no appreciable absorbance in the visible region.78 At higher applied potentials. the absorbances of poly(aniline) films at 430 and 810 nm are enhanced as the applied potential is made more positive. those with alkyl substituents have drawn much attention. The electrochemistry of poly(aniline) has been shown to involve a two-step oxidation with radical cations as intermediates. proton co-ion. but appears black at very positive potentials if the film is thick. Poly(aniline)-based ECDs include a device that exhibits electrochromism using electropolymerised 1. Electrochemical quartz crystal microbalance (EQCM) studies have demonstrated the complexity of redox switching in poly(o-toluidine) films in aqueous perchloric acid solutions.73 The yellow ! green transition is especially durable to repetitive colour switching. which in turn has lower values than poly(m-toluidine). which occurs in two stages and is accompanied by non-monotonic mass changes that are the result of perchlorate counter ion. so that observations in a single electrolyte on a fixed time scale cannot be unambiguously interpreted.81 . SCE.0 V vs. the absorbance at 430 nm begins to decrease while the wavelength of maximum absorbance shifts from 810 nm to wavelengths of higher energies. At lower applied potentials. The values for poly(aniline) are found to be lower than for poly(o-toluidine).2 to þ1. A solidstate aqueous-based ECD was constructed utilising this polymer as the electrochromic material in which the polymer switched from a yellow neutral state to blue upon oxidation.79 Absorption maxima and redox potentials shift from values found for poly(aniline) due to the lower conjugation length in poly(toluidine)s. and solvent transfers. Pernigraniline is an intense blue colour. As found for poly(aniline). response times indicate that the reduction process is faster than the oxidation.80 The relative extents and rates of each of these transfers depend on electrolyte concentration.81 The monomer consists of a ferrocene group and two flanking polymerisable diphenylamine endgroups linked to the ferrocene by an amide bond. in the potential range À0.10 -bis{[p-phenylamino(phenyl)]amido}ferrocene. The response times for the yellow–green electrochromic transition in the films correlate with the likely differences in the conjugation length implied from the spectroelectrochemical data. and the switching potential.330 Conjugated conducting polymers pernigraniline. experimental time scale. Poly(o-toluidine) and poly(m-toluidine) films have been found to offer enhanced stability of electropolychromic response in comparison with poly(aniline). the PEDOT:PSS multiple-layer film did not return to the non-conducting form over the voltage ranges of the ECD operation. In the first device.2 All-polymer ECDs The studies outlined in this chapter led to the construction of the first truly allpolymer ECD. Both the film preparation and redox switching of this system are carried out in an aqueous medium.% N-methylpyrrolidone (NMP) or diethylene glycol (DEG)) onto commercial plastic transparency films for overhead projection. the polymer film.5 Directed assembly of conducting polymers 331 10.4-b][1. and back into.4-propylenedioxythiophene) – PProDOT-Me2 – and poly{3.5 Directed assembly of electrochromic electroactive conducting polymers 10. the ‘directed-assembly’ layer-by-layer deposition of PEDOT:PSS (as the polyanion) with linear poly(ethylene imine) (LPEI) (as the polycation) has been reported.6-bis[2-(3. the surface resistivity of the electrodes had decreased to 600 O per square (at 300 nm thickness) while remaining highly transmissive throughout the visible region.84 have studied the redox and electrochromic properties of films prepared by the ‘layer-by-layer’ deposition of fully water-soluble. More recently.85 In the construction of this device.10. 10. poly(3.4-ethylenedioxy)thienyl]N-methylcarbazole} – PBEDOT-N-MeCz – were used respectively as the . After three coatings.4]dioxin-2-yl-methoxy}-1-butanesulfonic acid. Reynolds et al. Following the heat treatment. electrodes were first prepared by spin coating an aqueous dispersion of PEDOT:PSS (mixed with 5 wt.5. self-doped poly{4-(2.1 Layer-by-layer deposition of electrochromes Following earlier work82 with poly(viologen) systems.83 The cathodically colouring PEDOT:PSS/LPEI electrode was then combined with a poly(aniline)–poly(AMPS) anodically colouring layered system to give a blue-green to yellow ECD. sodium salt (PEDOT-S) and poly(allylamine hydrochloride) – PAH – onto unmodified ITO-coated glass. The PEDOT-S/PAH film was found to switch from light blue in the oxidised form to pink-purple in the reduced form. in its oxidised and reduced forms respectively. Multiple layers of PEDOT:PSS were achieved by drying the films with hot-air drafts between coatings and subsequent air drying in an oven of the multilayer film.3-dihydrothieno[3. where the film of ITO has been replaced by PEDOT:PSS as the conducting electrode material. with the glass substrate replaced by plastic.5. The polymer PEDOT-S is self-doping where oxidation and reduction of the polymer backbone are coupled with cation movement out of. Two ECDs were reported85 that employed different complementary pairs of electrochromic polymers. but there is clearly scope for improvement. hence the overall colour is an acceptably transmissive green. two cathodically colouring electrochromic polymers were selected to demonstrate switching between two absorptive colour states (blue and red). PProDOT-Me2 is in its oxidised (sky-blue) form and PBEDOT-N-MeCz is in its neutral (paleyellow) form. The contrast ratio for this type of ECD was. Following this work. In this research. as has been noted earlier. The polymer PProDOT-Me2 was again used. however. commercialised by AGFA under the trademark of ORGACON EL-350. in a sandwich device. not surprisingly because. which can then be deposited as a thin film by casting from solution. (Several different ORGACON films are available that differ in conductivity. In a novel approach. found to be relatively low.6 Electrochromes based on electroactive conducting polymer composites The oxidative polymerisation of monomers in the presence of selected additives has been a popular approach to the preparation of electroactive conducting polymers with tailored properties. as indicated by the associated numerals.12 10.1 Novel routes to castable poly(aniline) films While electropolymerisation is a suitable method for preparing relatively lowsurface-area electrochromic conducting polymer films. PEDOT-covered poly(ethylene terephthalate) – PET – foils. together with. large-area . As noted above for PEDOT materials. significant effort has gone into synthesising soluble poly(aniline) conducting polymers. as second electrochromic electrode. In a second all-polymer ECD.332 Conjugated conducting polymers cathodically and anodically colouring polymers. an all-plastic ECD has been reported. it may not be suitable for fabricating large-area coatings. were simply sandwiched together with a poly(ethylene oxide) random co-polymer/lithium triflate polymer electrolyte layer. such as poly(o-methoxyaniline). causing the device to become blue.5-didodecyloxy-benzene) – PBEDOTB(OC12)2 – showing red to sky-blue electrochromism. Application of a voltage (negative bias to PProDOT-Me2) switches the oxidation states of both polymers. with a polymer-gel electrolyte interposed.6. with a transmissive intermediate state. poly{1.4-ethylenedioxy)thienyl]-2.4bis[2-(3.86 where PEDOT layers act simultaneously on both electrodes as electrochromes and current collectors.) 10. In the initial ECD state. both oxidised and reduced forms of a PEDOT are unlikely to be effective electrochromes. thereby simplifying the construction of electrochromic sandwich devices from seven to five layers. 10.88 Composites of poly(aniline) and cellulose acetate have been prepared both by casting of films from a suspension of poly(aniline) in a cellulose acetate solution.10.99.98.7 Electrochromic devices 333 electrochromic coatings have been prepared by incorporating poly(aniline) into poly(acrylate)–silica hybrid sol–gel networks generated from suspended particles or solutions.97.95.90 The results indicated that the graft co-polymer exhibits mechanical properties similar to a cross-linked elastomer having the electrochromic and electrochemical properties typical of poly(aniline).93 As expected. in complementary ECDs that exhibit deep-blue to light-green electrochromism. Prussian blue – PB. where the presence of the cellulose acetate was found not to impede the redox processes of the poly(aniline). A water-soluble poly(styrenesulfonic acid)-doped poly(aniline) has been prepared both by persulfate oxidative coupling and by anodic oxidation of aniline in aqueous dialysed poly(styrene sulfonic acid) solution.92 The enhancement and modulation of the colour change on Indigo Carmine insertion into polypyrrole or poly(pyrrole)–dodecylsulfonate films was established.e.6.7 ECDs using both electroactive conducting polymers and inorganic electrochromes As noted in Chapter 8. the use of Indigo Carmine as dopant improves the electrochromic contrast ratio of the film.101 have combined a poly(aniline) electrode with an electrode covered with the inorganic mixed valence complex. The electroactivity and electrochromism of the graft copolymer of poly(aniline) and nitrilic rubber have been studied using stress–strain measurements. iron(III) hexacyanoferrate(II) – or with WO3.100.2 Encapsulation of dyes into electroactive conducting polymers An example of a case where the additive itself is electrochromic is the encapsulation of the redox indicator dye Indigo Carmine within a poly(pyrrole) matrix. numerous workers94. Electrochromic compatibility is obtained by combining the . and then spraying or brush coating onto ITO surfaces. frequency response analysis (i. 10. impedance spectroscopy) and visible-range spectroelectrochemistry.96.89 The electrochromic properties of the latter films were studied by in situ spectroelectrochemistry. cyclic voltammetry.87 Silane functional groups on the poly(acrylate) chain act as coupling and crosslinking agents to improve surface adhesion and mechanical properties of the resulting composite coatings. and by depositing cellulose acetate films onto electrochemically prepared poly(aniline) films.91. Through the skills of organic chemists in the synthesis of novel monomers and soluble polymers. while still retaining approximately the same transparency in the bleached state of the window. A new complementary ECD has recently been described. An electrochromic window for solar modulation using PB.105 As an example.103 Furthermore. particularly in the field of display applications. Methods include the use of polymer blends. by controlling the electron density and steric interactions along conjugated polymer backbones. 10. in addition to the synthesis of novel functionalised monomers and use of composites. poly(aniline) and WO3 has been developed.334 Conjugated conducting polymers coloured oxidised state of the polymer with the blue of PB. the possibilities in colour choice and performance characteristics seem endless and await further exploitation. versus the (bleached) reduced state of the polymer coincident with the lightly coloured Prussian green (PG). Although not described in this chapter.101 where the symbiotic relationship between poly(aniline) and PB was exploited in a complete solidstate electrochromic ‘window’. a set of electrochromic polymers that provide colours through the full range of colour space has been developed through the study of twelve electrochromic polymers.y)-chromaticity coordinates. This method is useful for the comparison of the electrochemical and optical properties of electroactive conducting polymers. as described in Chapter 4. laminates and patterning using screen and ink-jet printing. and for gaining control of the colour of dual-polymer electrochromic devices. much more light was blocked off by including PB within the poly(aniline) as matrix. using Commission Internationale de l’Eclairage (CIE) (x.8 Conclusions and outlook Intense interest continues to drive the highly novel research into the electrochromic properties of electroactive conducting polymers outlined here. The colour states of the PEDOT (blue-to-colourless) and PB (colourless-to-blue) films fulfil the requirement of complementarity.104.102 based on the assembly of PEDOT on ITO glass and PB on ITO glass substrates with a poly(methyl methacrylate) – PMMA-based gel polymer electrolyte.98. Tailoring the colour of electroactive conducting polymers remains a particularly active research area. other chemical and physical methods are investigated for the control of the perceived colour of electrochromic polymers.100. analysis of electrochrome and ECD colour changes are now routinely measured by in situ colour analysis.97.104 . Compared to earlier results with a poly(aniline)–WO3 window. Soc. A. (eds. 2004. 1978. J. Commun. R. K. 2001.. 1998. J.References 335 References 1. 1990. C. C. A. Rev. MacDiarmid. 1–74. Louis.. Conjugated poly(thiophenes): synthesis. Friend. 15–26. 15. Chem. 14. Grimsdale. E. J. H. Fully printed integrated circuits from solution processable polymers. 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Leventis.4-ethylenedioxythiophene and Prussian blue. 40 bipyridilium salts (Scheme 11.10 -dimethyl-4. after the ICI brand name for methyl viologen. which they developed for herbicidal use.2 A convenient abbreviation for any bipyridyl unit regardless of its redox state is ‘bipm’.1 Introduction The next major group of electrochromes are the bipyridilium species formed by the diquaternisation of 4. The positive charge shown localised on N is better viewed as being delocalised over the rings. Another extensively used name is ‘paraquat’. There are several reviews of this field extant. with its charge indicated.10 -Dimethyl4.1). The most common is ‘viologen’ following Michaelis. The compounds are formally named as 1.40 -bipyridilium undergoes a one-electron reduction to form a radical cation.10 -disubstituted-4. PQ.7 deals at length with syntheses and properties of 4. and Applications of the Salts of 4. The molecules are zwitterionic (i.11 The viologens 11.40 -bipyridilium should they differ.5 Other works are dated. and Summers’ 1980 book The 341 .4 who noted the violet colour formed when 1.10 -di-substituent-4. The most substantial is The Viologens: Physicochemical Properties.40 -bipyridilium is therefore called ‘methyl viologen’ (MV) in this nomenclature.40 -bipyridine. The anion X À in Scheme 11. bearing plus and minus charge concentrations at different molecular regions or sites) when a substituent at one nitrogen bears a negative charge. and as 1-substitituent-10 -substituent0 -4. The literature of these compounds contains several trivial names.1. 1.1 need not be monovalent and can be part of a polymer. including ‘Bipyridilium systems’ (1995) by Monk et al.3.6 ‘The bipyridines’ (1984).40 -Bipyridine (1998) by Monk. but some still incorporate valuable bibliographic data.40 -bipyridyl to form 1.e.40 -bipyridilium if the two substituents at nitrogen are the same.. by Summers. Synthesis. bipyridilium species other than the dimethyl are called ‘substituent paraquat’. In this latter style. 12 MV2 þ 2IÀ is brilliant scarlet.1 The three common bipyridyl redox states. or other charge-donating species.2 Bipyridilium redox chemistry There are three common bipyridilium redox states: a dication (bipm2 þ).10 has a section on bipyridilium radical cations. the review. Likewise.11 also alludes to electrochromism. It is colourless when pure unless exhibiting optical charge transfer with the counter anion. properties and reactions of cation radicals in solution’ (1976) by Bard et al. but we stick to the usage in this field. The dicationic salt is the most stable of the three and is the species purchased or first prepared in the laboratory. and may be prepared as air-stable solid salts. . the review entitled ‘The Electrochemistry of the viologens’ (1981) by Bird and Kuhn9 is particularly relevant to this chapter. ‘Formation. Reductive electron transfer to the dication forms a radical cation: * bipm2 þ þ eÀ ! bipmþ . Different substituents as R1 and R2 may be attached to form unsymmetrical species. 11. ‘ferrocyanide’ is properly hexacyanoferrate(II).a its reaction with molecular oxygen is particularly rapid.342 R1 + N The viologens + N 2X + N X – – R2 R1 N R2 R1 N N R2 Scheme 11. XÀ is a singly charged anion. Finally.1) Bipyridilium radical cations are amongst the most stable organic radicals. Although dated.13.16 The stability of the radical cation is attributable to the delocalisation a ‘Ferricyanide’ is better termed hexacyanoferrate(III). ‘Chemistry of viologens’ (1991) by Sliwa et al. but are stronger for CT-interactive anions like iodide. Bipyridinium Herbicides8 comprises copious detail. colourless intense colour * (11. a radical cation (bipmþ ) and a di-reduced neutral compound (bipm0). Such absorbances are feeble for anions like chloride.14 In solution the colour of the radical will persist almost indefinitely15 in the absence of oxidising agents like periodate or ferricyanide. The potential needed to effect the reduction reaction in Eq. with informative parameters like s and s*17. in contrast to the bipyridilium dications. Eq. The data refer to monomeric radical-cation species unless stated otherwise.22 A few values of wavelength maxima and " are listed in Table 11.2): bipmþ þ eÀ ! bipm0. as derived from the widely used linear free-energy relationships of physical organic chemistry. (11. values of max from electronic spectra. The Z values correlate well with many solvent–solute interactions. Comparatively little is known about the third redox form of the bipyridilium series. tailor the colour as desired. in a simplified view of the phenomenon.2) b Kosower’s solvent Z values (optical CT energies for the denoted solute with a variety of solvents) in ref. Manipulation of the substituents at N or the bipyridyl ‘nucleus’ to attain the appropriate molecular-orbital energy levels can also. The colour will also depend on the solvent.b Figure 11. because of the delocalisation already mentioned.2 Bipyridilium redox chemistry 343 of the radical electron throughout the p-framework of the bipyridyl nucleus. and the results of theoretical calculations. CT scales have also been set up. the exact choice depending on the substituents. in principle.18.19 that relate empirically to electron densities and electronic shifts.11.5 Simple alkyl groups. however.1) depends on both the substituents at nitrogen and on the bipyridyl core – so-called ‘nuclear substituted’ compounds. for example. the di-reduced or so-called ‘di-hydro’32 compounds formed by oneelectron reduction of the respective radical cation. the 1-and 10 -substituents commonly bearing some of the charge. The molar absorptivity " for the methyl viologen radical cation is large. (11. radical cations are intensely coloured owing to optical charge transfer between the (formally) þ1 and (formally) zero-charge nitrogens. Other. in water " ¼ 13 700 dm3 molÀ1 cmÀ1 when extrapolated to zero concentration. comparable. Hunig and co-workers have ¨ correlated the polarographic value of E½. intense colour weak colour * (11. For example.21 The value of " is usually somewhat solvent dependent. Electrochromism occurs in bipyridilium species because.1.1 shows the UV-visible spectrum of methyl viologen. the source of the colour is probably better viewed as an intramolecular photo-effected electronic excitation. . 20 were determined using the different but related system comprising 4-carboethoxy-1-methylpyridinium iodide. The colours of radical cations depend on the substituents on the nitrogen. promote a blue-violet colour whereas aryl groups generally impart a variety of colours to the radical cation. for example. the sample also containing a trace of monomer. Duffy. c "/dm3 molÀ1 cmÀ1 13 700 10 060 13 700 13 900 13 800 13 800 16 900 12 200 26 000 28 900 17 200 83 300 – Ref.1.5 (b) (a) 0. and Ingram. Chem. by permission of The Royal Society of Chemistry.344 The viologens Table 11. R. Soc.0 0. II. (a) ––––––– Monomeric (blue) radical cation and (b) – – – Red radical-cation dimer. M. 2039–41. c Solution-phase radical-cation dimer. R Methyl Methyl Methyl Methyl Methyl Methyl Methyl Ethyl Heptyl Octyl Benzyl p-CN-Ph p-CN-Ph a Anion ClÀ IÀ ClÀ ClÀ ClÀ ClÀ ClÀ ClOÀ 4 BrÀ À Br ClÀ BFÀ 4 ClÀ Solvent H2O H2O–MeCN H2O MeCN MeOH EtOH H2O DMF H2O H2O H2O PC H2O max/nm 605 605 a 606 607 609 611 604 603 545 b. P.0 400 600 800 1000 1200 1400 Wavelength (nm) Figure 11. S. J. Fairweather. 2. D. b Solid on OTE. 1992.0 1.24 21 22 22 22 25 26 27 28 29 30 31 Estimated from reported spectra.) . D. (Figure reproduced from Monk. M. J. c 543 c 604 674 535 b. A..5 Absorbance 1.. Perkin Trans. ‘Evidence for the product of viologen comproportionation being a spin-paired radical cation dimer’. 22 23. Optical data for some bipyridilium radical cations..1 UV-visible spectra of the methyl viologen radical cation in aqueous solution. J.11.2 Cyclic voltammograms on glassy carbon of aqueous methyl viologen dichloride (1 mmol dmÀ3) in KCl (0.2 shows cyclic voltammograms depicting these processes.2 –0.4 –0.2 Bipyridilium redox chemistry B A 345 C A ′ B′ 0 –0.7): the oxidation peak for spin-paired radical-cation dimer (C) is prominent while the peak for re-oxidation of bipm0 (B0 ) is greatly diminished at slow scan rates. M. 40. Scan-rate dependence.1 mol dmÀ3). Spectrosc. In fact.2 Figure 11.3) Di-reduced compounds are often termed ‘bi-radicals’33 because of their extreme reactivity. indicating that spins are paired. Appl. Ag/AgCl –0. Figure 11. with permission of the Society of Applied Spectroscopy. Jansson. E. R. and Freeman. 1986. di-reduced bipm0 compounds are simply reactive amines. but magnetic susceptibility measurements have shown such species to be diamagnetic34 in the solid state.0 –1.6 Volts vs.8 –1. The outermost trace is fastest..35 The intensity of the colour exhibited by bipm0 species is often low since no obvious optical charge transfer or internal transition corresponding to visible wavelengths is accessible. (11. 251–8. ‘In situ resonance Raman spectroscopic characterisation of electrogenerated methyl viologen radical cation on carbon electrode’. (11. J. . Note the evidence of comproportionation – Eq. (Figure reproduced from Datta..) This product may also be formed by direct two-electron reduction of the dication: bipm2 þ þ 2 eÀ ! bipm0. or by immobilising the viologen species within a semi-solid electrolyte.3. for which the coloured radicalcation product of Eq. bleach ¼ 10 ms) while employing light-scattering by a limited amount of HV2 þ (deposited by 1 mC cmÀ2).40 -bipyridilium. which can bond to the oxide lattice on the surface of an optically transparent electrode (OTE).36.4). These approaches.1.39 have often derivatised electrodes with bipyridilium species. Effecting a large improvement in CR (60:1) and response times ( colour ¼ 1 ms. The solubility–diffusion problem can also be avoided by the use of viologens having long alkyl-chain substituents at nitrogen.1 Electrodes derivatised with viologens for ECD inclusion Wrighton and co-workers38.346 The viologens 11. + N I (MeO)3Si(CH2)3 + N (CH2)3Si(OMe)3 Wrighton and co-workers also diquaternised a bipyridilium nucleus with a short alkyl chain terminating in pyrrole (which was bonded to the alkyl chain at nitrogen39) – see II.37 11. The write–erase efficiency of an ECD with aqueous MV as electrochrome is low on a moderate time scale.3. The write–erase efficiency of such ECDs may be improved by retarding the rate at which the radical-cation product of electron transfer diffuses away from the electrode and into the solution bulk either by tethering the dication to the surface of an electrode. with the methyl viologen behaving as a pseudo-solid electrochrome. a complex optical system has been devised for display applications. With chemical tethering of this type. are described in Section 11. its being type I as both dication and radical cation states are very soluble in polar solvents. so forming a chemically modified (‘derivatised’) electrode (Section 1. anodic polymerisation of the pyrrole allowed an . as discussed in Section 11. so here the viologen is a solution-tosolid type II electrochrome.3. (11.3 Bipyridilium species for inclusion within ECDs The most extensive literature on a bipyridilium compound is that for 1.3.10 -dimethyl-4. Wrighton and co-workers attached the viologen (I)38 and a benzyl viologen40 species to electrode surfaces. initially using substituents at N consisting of a short alkyl chain terminating in the trimethoxysilyl group.1) is insoluble. 47 n SO3– + N + N C H2 III C H2 n More recently.40 -bipyridilium bromide (III.45 Sato and Tamamura46 and Willman and Murray. Fitzmaurice and co-workers48 in 1994 were probably the first to use a viologen adsorbed onto such layers.3 Bipyridilium species for inclusion within ECDs 347 adherent film of the linked poly(pyrrole) to derivatise the electrode surface.or m-xylyl)-4.41 The electroactivity of the poly(thiophene) backbone in this latter polymer degraded rapidly after only a few doping/de-doping cycles.42 Polymeric bipyridilium salts have also been prepared by Berlin et al. shown here as the p form) was employed in this manner: the interaction between the cationic bipyridilium nucleus and the sulfonyl group is coulombic.44 Leider and Schlapfer. The oxide of choice was nanostructured titanium dioxide. is insoluble in aqueous solution. leading to a good contrast ratio. after drying. which can be deposited as a thin film of high surface area. but with an electrochromic salt bonded electrostatically to a poly(styrene sulfonate) electrolyte.43 Factor and Heisolm. NTera of Eire have devised a so-called NanoChromicsTM device in which the viologen (IV) is bonded via a strong chemisorptive interaction to a metal-oxide surface.39 thereby attaching the bipyridilium units. + (CH2)6 N II + N CH3 An identical analogue has been prepared with thiophene as the polymerisable heterocycle. but the electroactivity of the viologen moiety remained high. Itaya and co-workers42 used polymeric electrolytes. .11. The electrode was prepared by dipping the conducting substrate into solutions of electrochrome-containing polymer which.. The amount of IV adsorbed was therefore high. A bipyridilium salt of poly(p. 3 Soluble-to-insoluble viologen electrochromes for ECD inclusion As noted above. and a good electrochromic memory. intrinsically extremely slow. Strictly.348 O HO P OH + N The viologens O + N 2Cl – IV P OH OH The electrochromism of IV is discussed in Section 11.51 suspended heptyl viologen in poly(2-acrylamido-2-methylpropanesulfonic acid) – ‘poly(AMPS)’ – while Calvert et al. (11. (If electroprecipitation occurs by two .3. radical cation being formed at the electrode. as expected. 11. 11.4 below. Both groups report an excellent long-term write–erase efficiency. Eq.49 of NTera also studied the electrochromic properties of an analogue of IV.1). * * (11.4) represents the chemical step of an ‘EC’ type process in which the product of electron transfer – ‘E’ – undergoes a chemical reaction – ‘C’.4).4) Equation (11. Eq. Another means to a similar end is to employ a normally liquid solvent containing a gelling agent (silica. Such an overall EC reaction is strictly ‘electroprecipitation’ but commonly termed ‘electrodeposition’.3.2 Immobilised viologen electrochromes for ECD inclusion A different method of ensuring a high ‘write–erase efficiency’ is to embed the bipyridilium salt within a polymeric electrolyte. (11.52 used methyl viologen also in poly(AMPS). the term ‘electrodeposition’ implies that the solid product is the immediate product of electron transfer. Corr et al. for example53) which is just as effective in immobilising viologens. the concentration of which can be53 as high as 4 mol dmÀ3. The process of forming such a salt is usually termed ‘electrodeposition’. The response times of such devices are. in which the phosphonate substituent is replaced with a simple alkyl chain. Most workers now consider the formation of viologen radical-cation salts to be a three-step process. Sammells and Pujare50. in aqueous solution. followed by acquisition of an anion XÀ in solution and thence precipitation of the salt from solution: bipmþ (aq) þ XÀ(aq) ! [bipmþ XÀ] (s). it is usual for the final product of reduction of a type-II dicationic viologen to be a solid film of radical-cation salt. Thus. ‘electrodeposition’ is an adequate description.11. green radical-cation salt. (11. (1. (11. ICI preferred CPQ to HV owing to its greater extinction coefficient (and hence higher ) and therefore its faster response time per inserted charge. Figure 11. as in Eq. but forms an insoluble film of crimson-coloured radical-cation salt that adheres strongly to the electrode surface following a one-electron reduction. Electrical connection is made to the counter electrode and the exposed end of the silver paint. The Philips device was never marketed. even layer of insoluble.2 V (relative to a small.10 -diheptyl-4.1) – to form a thin. (11. ICI first submitted a patent for the use of the aryl-substituted viologen 1. The HV2þ dication is soluble in water. The conducting layer of the ITO (of fairly high resistance.3 shows a schematic of the ECD cell. internal silverjsilver chloride electrode) is applied to the silver paint to effect electrochromic coloration – cf. $80 O per square) acts as the working electrode that displays the colour. and containing the electrochrome in a concentration of 10À3 mol dmÀ3 in sulfuric acid or potassium chloride (either of concentration 0. Eq. A potential of À0. whereas shorter alkyl chains yield somewhat soluble radical-cation salts. * (11.3. In 1971.40 -bipyridilium) as the dibromide salt.54 (of Philips in the Netherlands) in 1973. and a stripe of conducting silver paint is applied to its upper surface to facilitate an ohmic contact with the electrode surface. Philips chose heptyl viologen for their ECD rather than a viologen with a shorter-chain. The device is completed by encapsulating the electrolyte layer.3 Bipyridilium species for inclusion within ECDs 349 steps that are in effect instantaneously sequential.5) . A strip of insulating cellulose acetate is placed near opposing edges of the base.1 mol dmÀ3).) 11.56 which electroprecipitates according to Eq. which is extremely simple.4). Philips submitted Dutch patents in 197055 for heptyl viologen (HV ¼ 1.4 Applications of bipyridilium systems in electrochromic devices The first ECD using bipyridilium salts was reported by Schoot et al. The layer is applied over the platinum-wire counter electrode. an erase time of 10 to 50 ms. The Philips ECD had a contrast ratio of 20:1.54 and cycle life of more than 105 cycles.40 -bipyridilium (‘cyanophenyl paraquat’ or ‘CPQ’). itself positioned over the insulating layer.4) to form a green electrochrome with a superior colour and resistance to aerial oxidation.) þ eÀ þ XÀ ! [CPQþ XÀ](s). so a sheet of plain non-conducting glass covers the device. Eq.5): CPQ2þ(soln. The electrolyte layer was gelled with agar (5%) to improve its stability.10 -bis(p-cyanophenyl)-4. because reduction of the HV2þ dication formed a durable film on the electrode. the colour persisting for many tens of hours. The intense green colour of the CPQþ radical is stable on open circuit.) A less-anodic potential of þ0.4 V (vs. held in place with conducting silver paint Gelled electrolyte containing electrochrom e Platinum contact Cellulose layer Glass sheet (not conductive) Optically-transparent electrode.0 V (measured vs. touching silver paint Side elevation Electrical contact.5) takes place at the Pt counter electrode in a confined. (Figure reproduced from J.4 V. the reverse of reaction (11.5) – the reaction (11. silver–silver chloride electrode) oxidatively removes the electrogenerated colour in a bleaching time of ca.350 The viologens Conducting silver-paint contact Insulating strip of cellulose acetate over working electrode Top elevation Platinum contact to counter electrode Electrical lead to working electrode. 358. invisible volume. British Patent. 1 minute.e. so the reducing potential should not exceed À0. 1. The Pt counter electrode in Figure 11.314. conducting sides innermost Figure 11. the pale-red species CPQo (oxidation of which is slow).3 Schematic representation of an ECD operating by a type-II electrocoloration mechanism. Then for bleaching at the ITO – i.5) during coloration on the ITO. G.1 mol dmÀ3) as an electron mediator to facilitate electro-bleaching. AgCl–Ag) can be used if the electrolyte is gelled and also contains sodium ferrocyanide (0. * * .049. (In demonstration devices. often electrolysis of solvent is allowed to take place at the counter electrode. 1973. which in progressively destroying solvent will of course not serve in long-term use. Reversing the polarity and applying a potential of þ1. ICI Ltd. see p.3 is pre-coated with solid CPQ þ and undergoes the reverse of Eq.) It is best to prevent the formation of a further reduction product. with colourless CPQ2þ in solution being electroreduced to form a coloured film of radical cation salt. (11. This precoating procedure represents an ingenious resolution of the often problematic choice of counter electrode. with permission of ICI. Kenworthy. but the electrochromic image is usually streaky and uneven. this ECD was first marketed in the early 1970s as a data display device.24 The colour of the radical cation tends towards crimson as the length of the alkyl chain increases. Yasuda et al.9 Â 10À7 mol2 dmÀ6.10 below. but liquid-crystal displays (LCDs) entered the market at about the same time.3. aryl-substituted viologens generally form green or dark-red radical-cation salts. The slow kinetics of the ICI type-II cell were the result of including agar to gel the electrochromecontaining electrolyte. 11.24 By comparison.58 The origin of ‘oiling’ as a result of dimer formation by the radicals is considered in more detail in Section 11.3. the dimer of alkyl-substituted radical cations is red. This underlay ICI’s use. and had faster response times .2 shows that an effective chain length in excess of four CH2 units is necessary for stable solid films to form.3 Bipyridilium species for inclusion within ECDs 351 Following extensive and successful field trials. largely owing to increasing incidence of radical-cation dimerisation.11. but ICI believed that this molecular encapsulant would not improve the long-term write–erase efficiency of the different viologen CPQ. dication solubility and radical-cation stability (in thin films) are both greatly improved by using aryl substituents. of the aryl-substituted viologens. The solubility product Ksp of HVþ BrÀ in water is59 3. presented in detail above.57 (of Sony Corporation) added encapsulating sugars such as b-cyclodextrin to aqueous heptyl viologen to circumvent the problem of ‘oil’ formation. As the length of the alkyl chain is increased. Table 11. LCDs rapidly captured an unassailable market share. the pentyl chain produces the first truly insoluble viologen radical-cation salt. The heptyl is the first salt for which the solubility product is small enough for realistic device usage. and redox potentials have been added to this table from ref. 9. particularly p-cyanophenyl CPQ in their ECD since the electrochromic colour of the heptyl * * .2). Ultimately. The radical cations of viologen species containing short alkyl chains have a blue colour becoming blue–purple when concentrated.5 The effect of the bipm N substituents ´ van Dam and Ponjee59 examined the effect that variations in the length of the alkyl chain have on the film-forming properties of the radical cation as the bromide salt (Table 11. a yellowbrown oil stains the electrode surface. Removal of the agar allows for considerable improvements in device response times (to seconds or tenths of seconds). The radical-cation salt of cyanophenyl paraquat (CPQ) is more insoluble in water than is HVþ . yet the dicationic salt is very soluble. Also. The E values are quoted against the SCE. the green radical cation of CPQ apparently56 is more stable than the other aryl viologen radical cations. 11.3.352 The viologens Table 11. Different counter ions yield solid radical-cation products of electrodeposition having a wide range of solubilities and chemical stabilities. viologen radical cation was deemed insufficiently intense: the molar absorptivity (and therefore the CR) of aryl-substituted viologens is always greater than that of alkyl-substituted viologens (Table 11. Anions found * * F .6 The effect of the counter anion The counter anion in the viologen salt may crucially affect the ECD performance. ref. CPQþ is oxidised chemically by the nitrate ion via a rapid but complicated mechanism. 9 and 59).2. Furthermore. which demonstrates the bipmþ concentration.1). Symmetrical viologens: the effect of varying the alkyl chain length on radical-cation film stability (refs. and refer to viologen salts with the parenthesised anion. 60) or by observing the time dependence of an ESR trace.30 For example.30 Studies of counter-ion effects may be performed using cyclic voltammetry (e.30 The ICI group used the SO2À salt of CPQ2 þ in their prototype ECDs.56 4 The properties of heptyl viologen radical-cation films also depend on the ´ anion as shown by van Dam and Ponjee.g.59 Jasinski60 (Texas Instruments) found the optimum anion in water to be dihydrogen phosphate. Effective length (units of CH2) 1 2 3 4 5 6 7 8 4 4–5 4 4 4–5 4–5 Solid bromide salt film on Pt? No No No No Yes Yes Yes Yes Yes Yes No No No No F Substituent R Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl iso-Pentyl Benzyl CH3 ðClÞCH2 OCH2 À CH3 ÀCH¼CHÀCH2 À HÀCH¼CHÀðCH2 Þ3 À NCÀC3 H6 À a Colour Blue Blue Blue Blue Purple Purple Mauve Crimson Purple Mauve – – – – E =mV À688 (ClÀ) À691 (ClÀ) À690 (BrÀ) À686 (BrÀ) À686 (BrÀ) À710 (BrÀ) À600 (BrÀ) À705 (BrÀ) À696 (BrÀ) À573 (ClÀ) À362 a (ClÀ) Polarographic E½ value. 3 mol dmÀ3) a (< À0.698 À0.828 À0. Many other redox potentials for mono-reduction of bipyridilium salts are quoted in the reviews by Monk5 and by Bird and Kuhn. The rates at which the CT complexes of methyl viologen dissociate vary: the complex with iodide dissociates at a rate of 8.008 À1. while that with * * . on the reduction potentials a of heptyl viologen.5).978 À0. Like CPQþ .798 Reduction potentials determined at pH 5..708 À0. and of electrode substrate. tetrafluoroantimonate and tetrafluoroarsenate are all water insoluble.3.048 À0.5 mol dmÀ3) Fluoride (1 mol dmÀ3) Sulfate (0.818) À0.4 mol dmÀ3) HCOÀ ð1 mol dmÀ3 Þ 3 Acetate (0. 124. ‘The electrochemistry of some n-heptyl viologen salt solutions’. Electrochem.848 À0.958 À0.3 mol dmÀ3) H2 POÀ ð2 mol dmÀ3 Þ 4 Formate (0. Inc.808 À0. are given in Table 11. The effect of supporting electrolyte anion.878 c À0.) Epcð1Þ =V Epcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Au À0. Jasinski’s values60 of reduction potentials for aqueous HV2 þ on various metals as electrode substrate.61 Reduction is therefore a two-step process: ionpair dissociation ! reduction.898 À0.768 À0.778 À0. b Millimolar viologen dication employed for measurement. c No colour formed. J.668 Epcð1Þ =V Epcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Ag À0.708 (< À0.60 presumably by a similar mechanism. Electron transfer may be thought of as a special type of second-order nucleophilic substitution (‘SN2’) reaction in which the ‘nucleophile’ is the electron and the leaving group is an anion.5. thiocyanate.668 À0. with a variety of anions.928 À0.928 c Epcð1Þ =V Epcð2Þ =V |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} on Pt À0. Heptyl viologen salts of bicarbonate (at pH 5.3.948 À0.828 À0. tetrafluoroborate and perchlorate also proved ´ satisfactory (as also concluded by van Dam and Ponjee59).818 c À0. (Table reproduced from Jasinski.868 À0.11. tetrahydroborate. Bromide. J. useful for ECDs were dihydrogen phosphate.3 Bipyridilium species for inclusion within ECDs 353 Table 11. chloride. with permission of The Electrochemical Society. fluoride. hexafluorophosphate.9 The choice of anion in the viologen-containing solution can be important since it often participates in charge-transfer type interactions with the viologen. Recent evidence suggests the CT complex must dissociate prior to reductive electron transfer.818) À0.818 c À1.918 Anion Bromide (0.848 b À0.7 Â 105 sÀ1.668 À0. 637–41. 1977. Soc. Values of peak potential Epc are cited against the SCE. formate and acetate.928 À0. sulfate.948 À0. HVþ is also oxidised by the nitrate ion. R. 1 on p. Ultimately. t½.1). creating diffusion zones that could overlap. * * (11. and lead to semi-infinite planar diffusion. The coloration of type-II systems is considerably more complicated. At low overpotentials of mono-reduction. Hemispherical diffusion was inferred.354 The viologens chloride dissociates at 26. as follows. these nuclei overlap. or short-chain-length viologens in water.67 reduced HV2þ in solutions of bromide or biphthalate at a disc of SnO2 on glass. but the process is too complicated to allow precise mathematical models of deposition to be used. Exemplar viologen systems include viologens in nonaqueous solutions. while citing the two-step electroprecipitation mechanism. The number of nucleation sites available are suggested66 to depend on the potential.7 The kinetics and mechanism of viologen electrocoloration Kinetic aspects of electro-coloration of type-I electrochromes are discussed in Section 5. followed by anion acquisition and precipitation of the salt.6) for the bromide anion: HVþ (aq) þ BrÀ (aq) ! [HVþ BrÀ] (s).66 The rate of film growth is controlled by instantaneous three-dimensional nucleation. The anion may thus also influence the speed of electrochromic coloration.3 Â 105 sÀ1. 75ff. In summary. For the HV2 þ–(H2POÀ)2 system. the rate of reduction was controlled by electron transfer and.61 11. Jasinski65). found the reduction process to be compatible also with a theoretical model of metal deposition derived by Berzins and Delahay. the process may be written as electron transfer ! nucleation ! hemispherical diffusion ! linear diffusion.6) Bruinink and Kregting64 (cf.3. as discussed more fully below. at high overpotentials. as seen by a current–time relationship of I vs. . was sufficiently fast for the crystal-growth process to be controlled by mass transport.54) in which solutionphase HV2 þ is reduced to form the radical cation. Jasinski prefers a two-stage process of electroprecipitation (also favoured by van Dam59 and Schoot et al. Eq (11. Fletcher et al. Bruinink and van Zanten62 (of Philips) and Jasinski63 studied the kinetics of HV2þ dibromide reduction in response to a potential step. once initiated. the data 4 obtained do not allow any distinction between two possible but different reduction mechanisms. the commencement of which is shown by a transition in the current–time domain to the expected Cottrell relationship of I dependence on tÀ½. after formation. and agree that the reduction process proceeds via a nucleation step. both groups found the kinetics of mono-reduction to depend on the electrode history and mode of preparation. the nucleation process. Eq (11. 13 * .63 The time-dependent change within deposits of HVþ on an OTE has been observed by Goddard et al. this acquisition of crystallinity is probably associated with additional sharp peaks observed during cyclic voltammetry of heptyl viologen films.73 * * * * 11.71. with substituents at N and N0 being different). CV) associated with electro-coloration is unusually steep. particularly if the substituents are large aryl groups or long alkyl chains that are hydrophobic.60. with dication on the micelle periphery being reduced preferentially. Deposited films are partially crystalline but largely amorphous. The initial solid product of mono-reduction was considered to be HVþ radical cation in a salt which incorporates some unreduced HV2þ dication. the usual shape of the CV before the current peak is exponential. Electrochemistry at these micelles is envisaged to proceed in discrete steps. (The presence of radicalcation dimer on an electrode also causes a steep CV peak.76 Barclay et al. A particular problem with aqueous solutions of viologen can be the formation of micelles of dication.72 Subsequent aging effects and previously inexplicable additional CV peaks are explained in terms of this composite form of solid deposit.8 Micellar species Association of bipyridilium species to form p-dimers is a well-documented phenomenon23. the current–potential curve (cyclic voltammogram. which usually implies a catalytic or nucleation process. acquiring a greater degree of crystallinity with time.37 (of IBM) quote a critical micelle concentration cmc for the HV2þ dication of 10À2 mol dmÀ3 in aqueous bromide solution. interaction of such a micelle with a cathodically biased electrode causes reduction of the outside of the micelle to form bipmþ .13.24 for the viologen radical cation. A similar explanation for the complicated cyclic voltammetric behaviour observed during the formation of solid CPQþ radical-cation salt has also been advanced. 74 and 75).70 using UV-visible spectroscopy with a novel potential cycling technique (but now rendered obsolete by use of diode-array spectrophotometry).3.77 If the concentration of HV2þ lies above the cmc. yet the inside of the micelle remains fully oxidised dication. In this latter case.3 Bipyridilium species for inclusion within ECDs 355 By way of confirmation. 37.72 have investigated HV2þ dibromide and many asymmetric bipyridilium salts (that is. Bewick et al. but is not so well attested for the dication (although see references 13.59. in cyclic voltammetry.9.) The morphology of HVþ films has been addressed by Barna69 (Texas Instruments).68 which may imply that nucleation sites are comprised of dimer. by diode-array spectroscopy. the analogy between such viologens and quaternary ammonium cationic surfactants is clear.11. 80 Although such mixed-valence viologens are not involved often.9 The write–erase efficiency The write–erase efficiency of a viologen electrochrome is very high if the solvent is non-aqueous and rigorously dried.1 contains a viologen (which is probably zwitterionic) as the primary electrochrome. the EPR spectrum of MVþ lost all hyperfine coupling due to rapid inter-radical spin swapping.85 have 4 modelled the electrochemical behaviour and cycle life of similar electrochromic devices. Engleman and Evans81. finding that the properties of methyl viologen were largely unaffected by the presence of surfactant when below the cmc. formerly of Gentex.83 cites a cycle life of ‘> 4 Â 104 cycles’ for the related system benzyl viologen (as the BFÀ salt) in propylene carbonate (PC).84. * * 11. whereas cationic and nonionic surfactant did not affect the equilibrium either way. .356 The viologens A similar explanation for the complicated cyclic voltammetric behaviour observed during the formation of solid CPQþ radical-cation salt has also ´ been advanced.78. it is again worth remarking that comparison of results from different authors is difficult since each step is defined by the number and nature of each anion in solution. e.73 Heyrovsky has also postulated the existence of solutionphase mixed-valence species of methyl viologen in water. For example. In an effort to mimic the properties of bipyridilium species within micelle environments. and the different speciations in solution during each of the steps during electroprecipitation. Ho et al. and comprise very small amounts of material. but above it.82 also investigated the electrochemical reduction of MV2 þ in the presence of micellar anions. Kaifer and Bard74 investigated the electrochemistry of methyl viologen in the presence of various surfactants (anionic. the properties of the methyl viologen were markedly different.3. Byker. The cycle lives are high.13. they are capable of greatly complicating the electrochemistry of solutions containing them. the archetypal type-I Gentex mirror described in Section 12. cationic and nonionic). Cyclic voltammetry also differed above and below the cmc. but the response times are quite slow. for the reasons discussed above. Given the complexity of the overall electroprecipitation mechanism. In the presence of anionic surfactant.79. particularly one with heptyl viologen as the primary electrochrome and tetramethylphenylenediamine (TMPD) as the secondary.g. and different experimental conditions (where known) have been employed by each author. the position of the monomer–dimer equilibrium was displaced significantly in favour of the monomeric form when above the cmc. In solution. even60 and smooth. Re-oxidation of the film to bleach the colour is rapid for fresh HVþ films. Both groups found the aging effect to be due.e.89. to the dimerisation of radical cation in solution. firstly a fresh film of HVþ is amorphous. Belinko33 suggested that device failure is also due to production of di-reduced bipyridilium (bipm0) as a minor electrode product.96 photothermal spectroscopy97 and the electrochemical quartz-crystal microbalance (EQCM).3 Bipyridilium species for inclusion within ECDs 357 The mechanism of deposition was examined at length since here the nature of the solid deposit is important. (11.86 Scharifker and Wehrmann98 investigated phase changes within radicalcation salt deposits of HVþ and benzyl viologen radical cation. i.90 EPR. slow. Spectroscopic studies87 (a surface-enhanced resonance Raman analysis of a disulfide-containing dimeric viologen adsorbed on rough silver) strongly suggest the presence of a ‘liquid-like’ environment at the electrode surface following reduction to form dimerised radical ðbipmÞ2þ .72. in part. which probably involves ordering (crystallisation) of radical moieties.88. subsequent dimer dissociation yields monomeric radical cation100 but often . In order to understand these processes. thin films of HVþ salt have been studied by many techniques including UV-visible spectroelectrochemistry.93.7) is the radical-cation dimer. bipyridilium ECD devices form an unsightly yellow–brown stain on the electrode. but patchy films that show signs of aging are more difficult to oxidise.9 Belinko33 investigated the write–erase efficiency of HVþ films by cyclic voltammetry.94 photoacoustic spectroscopy.11. Dimerisation of radicals could participate in this complication. and Gołden and Przyłuski99 looked at HVþ .91 Raman spectroscopy.86 yet soon after deposition (<10 s70) the film appears patchy as an aging process occurs. making the lower scanning limit progressively more negative deliberately to generate bipm0. Some evidence now suggests that this stain is a form of crystalline radical-cation salt30 containing spin-paired radical-cation dimer. it may react with bipm2 þ from the solution in the comproportionation: * * * * * * * À Á bipm2þ þbipm0 ! bipmþ 2 ! 2 bipmþ : (11:7) The immediate product of Eq. or intervalence species comprising both bipm2 þ and bipmþ . For example. after prolonged cycling between the coloured and bleached states. After bipm0 is formed.92. Secondly.95. in aqueous solution. requiring a higher potential or a longer re-oxidation time. The formation of diamagnetic HV0 (at large negative potentials) should be avoided since it is only electrochemically quasireversible electrochemically. 2 It is also likely that reordering of radical species (‘recrystallisation’) occurs within the electroprecipitated viologen deposit soon after it forms. 89. i.7) can greatly simplify Belinko’s otherwise complicated mechanistic observations. EtV0. Recent work has shown that the radical-cation dimer is electrochemically only quasi-reversible. The reductions are concerted two-electron reactions. To summarise their findings.105 The mechanism of comproportionation differs when ferrocyanide is involved (see footnote a on p. Comproportionation of bipm2 þ (aq) from the solution with bipm0 (s) on the disc becomes increasingly important with time. spin pairing is ‘locked into’ solid deposits of viologen radical cation. its electro-oxidation is slow.7) could thus occur by electron transfer through the ferrocyanide possibly by a concerted doubleexchange mechanism. that is.98.106 Generally.101 In effect. deposition is initiated by nucleation of supersaturated bipm0 close to the electrode. they separate from the ferrocyanide to form individual ions. the rate of deposition decreases as the bulk of the deposit increases. This result may be important because this ion is a popular choice of electron mediator.102 hence the observed failure of ECDs containing traces of dimer. when ferrocyanide is involved. Comproportionation occurs when the electron transfers through these orbitals. in a structure reminiscent of a metallocene.e.106 Hence the two radical-cation moieties produced by reaction (11.100. That comproportionation occurs in the solid state has been confirmed for CPQ0 and CPQ2 þ from aqueous electrolytes.107 * .70. see also the way comproportionation can simplify mechanistic observations in ref. 342). so that the total amount of bipm0 on the disc decreases until the amount reaches a steady state.) Engelmann and Evans have also published104 studies of potentiostatic deposited MV0. each was formed at a glassy-carbon rotated ring-disc electrode (RRDE) from solutions containing the respective dications. the bipm2 þ and bipm0 species approach and thence form a sandwich-like structure with their p-orbitals overlapping. 103. Its fast rate constant and moderate equilibrium constant make it almost certain that comproportionation processes always occur whenever bipm0 is formed electrochemically. (In this context. Equation (11.30.7) are never in contact. as the surface of the disc becomes blocked. The 1998 review by Monk5 demonstrates how widely comproportionations occur in viologen redox chemistry. BzV0 and HV0. However. the ferrocyanide ion is believed to lie between the two viologen species.358 * The viologens solid deposits of ‘bipmþ ’ exhibit spectroscopic IR bands attributable to the spin-paired (bipmþ )2 dimer. Invoking the participation of Eq. (11. Benzyl viologen has also been extensively investigated since it will also form an insoluble film of radical-cation salt following one-electron reduction. after reaction. Such oxidation is very rapid.3 Bipyridilium species for inclusion within ECDs 359 To summarise. The oxidised form of the mediator – in this example.50.92 or cerous ion. and the bleaching time slower still.57.3. ferrocyanide) that is oxidised at the electrode. Because close contact between bipyridilium dications is greatly impeded in such a guest–host relationship. However. addition of the sugar b-cyclodextrin to the voltammetry solution has been found to impede the formation of yellow–brown stains. The unsightly yellow–brown stains still persist. an electron mediator) to the dicationcontaining electrolyte solution.56. it is the mediator (e. For aryl viologens in aqueous solution.108 During electro-coloration. although the coloration time will necessarily be slow.57.5 As ferrocyanide is known to form a charge-transfer complex with methyl viologen dication109. i.15 Mediators facilitate the electro-oxidation of the radical cations of type-II species. bipm2 þ ion is reduced to bipm þ but.56.51and ferrocene in acetonitrile has been used in a type-II device. so alignment of bipmþ species in the solid deposit is impossible.e. such ‘oiling’ is claimed still to occur ‘ultimately’ with CPQþ . however. the rates of anion–radical cation association following the electron transfer is completely unknown. The mediators used include hydroquinones.56 ferrocyanide. during re-oxidation at a positive potential. even with the HV2 þ and CPQ2 þ systems that contain K4Fe(CN)6.111 and HV.57 it will be the free-anion equilibrium fraction of the species that can be assumed to act.110 and also with the dications of CPQ5.92 In a notable advance. a mediator is always necessary to ensure complete colour removal on re-oxidation.58 * * * * . the speciation of the viologen dication is complicated prior to the transfer of an electron: the rate of anion–dication separation prior to (or during) electron transfer follows61 the rate ket and may in fact dictate its magnitude.g. to reform the dication.12. such as heptyl viologen. association of bipm2 þ cations in solution57 is largely thereby prevented. ferricyanide. the way the length of the substituents at nitrogen dictates the solubility constants of radical cation–anion pairs is fairly well understood.55 ferrous ion. and the way the solubility index dictates the rate of precipitation has been investigated extensively.11. hexacyanoferrate(I I I ) – allows for chemical oxidation of the radical-cation film. 11.10 Attempts to improve the write–erase efficiency The first and most effective method of improving the write–erase efficiency is to employ non-aqueous solutions. The second method used to prevent the non-erasure of films of HVþ salt is to add an auxiliary redox couple (that is.91 probably by encapsulating the dication within the cavity of the cyclodextrin in a guest–host relationship. 112. a large number of prototype viologen ECD devices have been made. 11.56.115 For example. Barna and Fish113 prepared asymmetric bipyridilium salts. Barltrop and Jackson114 have prepared similar asymmetric viologens.55.112 prepared the compound V in which the two pyridinium rings are separated by methylene linkages.1 Displays based on viologens adsorbed on nanostructured titania Nanostructured electrodes are easily prepared by spreading a concentrated colloidal suspension on a conducting substrate and firing the resulting gel film . made cationic by alkyl or aryl addition) 3. Again.114 For example.8-phenanthroline salt (VI). films with superior write–erase properties were formed. a compound was made having R1 ¼ C7H15 and R2 ¼ C18H37.360 The viologens Other attempts to stop the ageing phenomenon have used different. CH3CH2 + N (CH2)4 V + N CH2CH3 To a similar end.4 Recent elaborations The majority of the new developments reported here aim to enhance the rate of coloration in bipyridilium-based ECDs. and a diquaternised (that is. that is species in which R1 6¼ R2 (Scheme 11.37. Bruinink et al. modified bipyridilium compounds. thereby inhibiting the crystallisation process: for example. together with a series of nuclear-substituted bipyridyls (species in which substituents are directly bonded to carbon in the pyridine rings). an impressive device from the IBM laboratories utilised a 64 Â 64 pixel integrated ECD device with eight levels of grey tone of heptyl viologen115 on a 1 inch square silicon chip. 11.36. + C6H13 N + N C6H13 VI Despite the many drawbacks recounted above.4).4. though they may still have a size advantage in large devices. to give quite detailed images (Figure 11. These devices were not exploited further owing to competition from LCD systems.1).113. Because the oxide crystals are so small. leading to a high coloration efficiency . Nanostructured titanium dioxide in its anatase form can be deposited as a thin film of high surface area. the viologen molecules need not diffuse to the electrode surface.4 Reproduction of an IBM electrochromic image displayed on a 64 Â 64 pixel integrated ECD device with eight levels of ‘grey tone’ of heptyl viologen. D. R.). ‘Electrochromic displays’. 266–76. Chichester. Viologens are strongly adsorbed on its surface .11.) at 450 8C. (ed. H. Ellis Horwood. such films have an extraordinarily high internal surface area. by permission of Ellis Horwood. The original is clearer.117 This means that a high number of electrochromic viologen molecules can occupy a relatively small area. The ratio between the internal surface area and the smooth geometrical area of the electrode (the ‘roughness factor’) approaches 1000 for a film that is only 4 mm thick. and Martin. In Howells. J. Technology of Chemicals and Materials for the Electronics Industry. Furthermore. (Figure reproduced from Barclay. which leads to shorter switching times.116.4 Recent elaborations 361 Figure 11.117 The rough surface of the porous titanium dioxide film consists of a network of interconnected semi-conducting metal oxide nanocrystals. 1984. D. E.116 Such electrodes have been widely investigated for use in dyesensitised photoelectrochemical cells. as they are surface-confined. e. unusually. Such systems have long been investigated in research on dye-sensitised solar cells.122 NTera also describe their ECD as a ‘paper quality’ electrochromic display. The counter electrode is viewed as having a high capacitance. .362 The viologens owing to their electron deficiency.49.120 In such devices.121. Electrodes can be micro-patterned for display applications.e. their device had a coloration efficiency of 170 cm2 CÀ1 at 608 nm. Placing a diffuse reflector between the electrodes. but readily regenerated. an ECD of very high definition.117. chemisorbed phosphonated viologen molecules.121 and was said to be stable over 10 000 ‘standard’ test cycles.40 -bipyridilium dichloride.g.118 Originally developing a spin-off from the Gratzel cell. Application of a potential of 1. followed by a monolayer of self-assembled. gives on coloration the visual effect of ink on pure white paper. Different colours can be achieved in ECDs depending on the nature of the substituent(s) on the viologen molecule. chemisorbed phosphonated phenothiazine molecules. The electrolyte was g-butyrolactone containing LiClO4 (0. The positive F-doped tin oxide conducting glass counter electrode (i. Fitzmaurice and ¨ co-workers at the Dublin-based NTera Ltd119 (founded in 1997. The ECD is sealed with a thermoplastic gasket and a UV-curable epoxy resin.121 In trials.2 V reduces the dicationic viologen to its blue radical cation.120. followed by a monolayer of self-assembled. one at the positive electrode. having manufacturing facilities in Ireland and Taiwan) have developed a ‘next generation display technology’ called NanoChromicsTM displays that are based on these principles. Without the intermediate TiO2 layer the display is transparent while retaining readability. An assembled NanoChromicsTM electrochromic device uses two metal-oxide films – one at the negative electrode and. the colour remaining for more than 10 min after the voltage is switched off. and oxidises the phenothiazine from its weak yellow colour to red.120. The overall colour change is therefore from virtually colourless to a blue-red purple. Some open-circuit memory persists. The TiO2 film is further modified with an adsorbed monolayer of viologen (IV). a layer of an ionpermeable nanostructured solid film of titanium dioxide. many thousands of switches are possible before there is significant degradation of performance.2 mol dmÀ3) and ferrocene (0. the anode on coloration) carries a film of heavily doped antimony tin oxide (SnO2:Sb) 3 mm thick.05 mol dmÀ3). bis(2-phosphonoethyl)-4. for example Gratzel’s work on his ¨ photoelectrochemical cell. In a typical device122 (borrowed from Gratzel123) the negative F-doped tin oxide conducting ¨ glass electrode (the cathode on coloration) is coated with the wide bandgap titanium dioxide film 4 mm thick. that is. which assists charge storage during coloration. 4 Recent elaborations 363 Fitzmaurice’s display is said to be ‘ultra fast’. The cell OTEjTiO2poly(viologen)jglutaronitrile–LiN(SO2CF3)2jPrussian-bluejOTE exhibited an optical density change of about 2. They claim that the current LED boards can become bleached out and difficult to read in bright daylight in sports stadia. with aryl as well as alkyl substituents. this is certainly faster than most of the other viologen-based devices. be applied to all the product types: displays.126 Gratzel and ¨ co-workers also made a variety of cell geometries for ECDs operating on . The company notes that signs using NanoChromicsTM display technology are ideal for sports player-substitution boards. In each case. salicylate or phosphonate (as in IV). applications in toys and games and ultimately flexible electronic paper displays. Higher optical density changes are possible if the switching times are slower. the colour changes on reduction were transparent to blue. the value of increases to about 270 cm2 CÀ1 and the response time is decreased to 250 ms. NTera also state that their flexible display prototype can. in principle. since the switching time is 1 s for a change in absorbance of 0. The NTera group state that they are working with a number of marketleading strategic partners for access to the market. That NanoChromicsTM displays can be manufactured by existing LCD manufacturing processes will clearly enhance the likely success in the commercial development of this technology. Gratzel et al.125. Recently. Fitzmaurice notes that charge compensation within the viologen layer is also fast because many counter ions are also adsorbed on the TiO2 layer. ¨ have prepared such devices with a series of viologens. again at 608 nm.60. or yellowish to green.122 Published spectra suggest an optical density (OD) change of about 0. windows and mirrors. Several workers have adapted these ideas.124 The NTera website119 provides ‘consumer product reference designs’ for digital clocks and an eight-digit calculator.55. and (at higher potentials) to red– brown. the anchored group attaching the viologen to the titania was benzoate. Electrochromic devices they have constructed include shutters and displays. They report switching times in the range of 1–3 s.11.126 for example. dimmable window laminates. NTera have demonstrated a NanoChromicsTM display operating in a converted iPod (the portable digital audio players from Apple Computer Corporation). and that NanoChromicsTM display signs are perfect for this application as they are easy to read in bright daylight and at all angles.. since the anchored viologen electrochrome avoids the diffusion delay before electron transfer.120 However.122 although the criterion for this claim is unclear. giving rise to products such as ‘smart card’ displays. If the counter electrode is covered with a secondary electrochrome such as a phenothiazine. T. M. HO O P HO +. demonstrating clarity capable of high-definition patterning.20 -bipyridine (VII) calling it a ‘viologen’.131.5 shows a prototype. In a similar way. Edwards.. M. For example.5 s. Boehlen et al. Johansson.-H. H. naming such devices ‘electric paint’..) . The viologens in such devices were generally oligomers rather than polymers. The response time is about 0.132 have prepared many similar systems for devices. L. and Matuszczyk. they generated a pink colour on reduction (which could indicate that a proportion of the viologen exists as radical-cation dimer). N VII N Edwards et al.. ‘Direct-driven electrochromic displays based on nanocrystalline electrodes’.127 prepared a salt of 2. 223–30.364 The viologens reflectors. A. (Figure reproduced from Pettersson. O. Hagfeldt.128. with permission of Elsevier Science Ltd..130. Figure 11. They generally employed the viologen IV to produce amazing clarity.5 Prototype electrochromic display showing an ‘electric paint’ display: the primary electrochrome was viologen (IV) adsorbed on nanocrystalline TiO2. with viologen electrochromes adsorbed on titania. Displays.129. T. Gruszecki. 25 2004. Figure 11. the complementary use of a bipyridilium with a Prussian blue electrochrome.133 The procedure relies on the solution-phase redox reaction between bipm2þ (from the bulk solution) and bipm0 electrogenerated during the current pulse. Thus for a given concentration of bipm2þ and bipm0 in such a region.3 Electropolychromism Bipyridilium salts may typically possess three colours.4 Recent elaborations 365 11.141 and the asymmetric system.140. allows the fabrication of a five-colour ECD.141 heptyl viologen.136 or benzylic moieties. This maximal number is not achieved however when delocalisation allows simultaneous coloration of two or more of the bipyridiliums.134 Several approaches have employed a number of bipyridilium units connected either with alkyl linkages135. to effect electrochromic writing.4. relative to coloration with a continuous potential.7). 11.138.4 Viologens incorporated within paper Viologen electrochromes have been incorporated within paper.134 A different. It is envisaged that the pulse procedure possibly favours this equality. one for each oxidation state in Scheme 11.137.11. These include methyl viologen.138 + H3C N 2X – VIII + N .4.139 The adsorption of methyl viologen onto the carbohydrate structures of paper follows Langmuir adsorption isotherms that imply chemisorptive behaviour. methyl–benzyl paraquat (VIII).1. in principle. (11. Viologen electrochromes comprising n bipyridilium units could thus.4.143 11.139. The reaction is comproportionation. The amounts of bipm2þ and bipm0 at the electrode and in the region around the electrode depleted of bipm2þ will govern the rate of comproportionation and hence the rate of product colour formation. so a sufficiently cathodic potential must be applied at the working electrode. the most intense colour will ensue when the two species are in equal concentration. exhibit 2n þ 1 colours. combination.2 The use of pulsed potentials Pulses of current have been shown to enhance the rate at which electrochromic colour is formed. Eq. highly promising. although the dication in solution is essentially colourless. T. N. A. Wiley. Academic Press.143 in which the viologen cation is immobilised by electrostatic interactions. 155–278. Formation. 1991. Synthesis and Applications of the Salts of 4. Sliwa. S. Electrochromism: Fundamentals and Applications. D. Chem. Comproportionation in propylene carbonate of substituted bipyridiliums. and Hill. Michaelis. 2. Bird. Ledwith. Chem.138. 1998. Summers. Faraday Trans. incorporation within NafionTM has been shown to produce good results. Alternatively. P.. R. and Neugebauer. Chem. Soc.366 The viologens While methyl viologen in paper is electrochromic. the intermediate steps of reversible organic oxidation–reduction. Adv. Chichester. Mehring. 10. and Kuhn. 16. P.. L. 32. 243–86. Chem. J. Rev. Chem. London. L. 8.. In situ studies on the structural mechanism of zwitter-viologen system during electrochemical charge-transfer reactions. Synth. J.. M. 1976. J. A. T. 6. 2971–4. Bard. The bipyridines. 49–82. Rev. properties and reactions of cation radicals in solution. Chem. Met. as colour printing in say newsprint is now commonplace. and Shine. 219–22. The speed is faster if the paper is layered with the polyelectrolyte poly(AMPS). Weinheim. The electrochemistry of the viologens. 8. 13. J. Adv. The Bipyridinium Herbicides.. Chem. H. N. W. and Zelichowicz. Monk. J. 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There is a severe lack of any systematic survey of the electrochromic properties of such species for ECD application. 1. and the viologen species in the previous chapter.1 Monomeric electrochromes A large number of organic compounds that are molecular aromatics form a coloured species on electron transfer. Most standard texts on quantitative and qualitative analytical chemistry cite many examples of redox indicators. a species with a double charge.12 Miscellaneous organic electrochromes 12. By contrast.1 Aromatic amine electrochromes Aromatic amines are generally colourless unless they undergo some form of charge-transfer interaction with an electron-deficient acceptor species. The conjugation within the violene that allows extensive delocalisation is the ultimate cause of the extraordinary stability of many such radicals. As a direct consequence of their structure. 12. 374 . most redox indicators are by definition electrochromic. in organic solution.1. possesses a brilliant colour. All the organic species in this chapter. and a species of intermediate redox state that is either a radical cation or radical anion. all violenes typically possess three stable redox states: an uncharged species. Aromatic amines are thus candidate electrochromes. Indeed. The conjugated ( CH ¼ CH )n portion is normally part of an aromatic ring or series of rings. the product of one-electron oxidation yields a radical cation which. where X ¼ O. are ‘violenes’ – a conceptual classification pioneered by Hunig. like the compendia in ref. The monomeric aromatic amine that has probably received the most attention for its electrochromic prospects is tetramethylphenylenediamine (TMPD) as the p. to preclude polymerisation reactions: the radical cations of aromatic secondary amines retaining an N–H functionality readily form an inherently conducting polymer. for example.8. the aromatic amines are type-II electrochromes.g.4 In such solvent systems. like the NH2 group of aniline.4.12 The radical cation of I is stable and is brilliant blue-green.2 V applied.11 have modelled the electrochemical behaviour and cycle lives of electrochromic devices in which I was the secondary electrochrome.1 Monomeric electrochromes 375 If both the neutral and radical-cation redox states are soluble. The nitrogens of all the amine groups need to be fully substituted.7.12.(I) and o-isomers.5. When a solution of II is injected between two ITO-coated electrodes (with n-tetrabutylammonium perchlorate as an inert electrolyte) and a voltage of 2. H3C N H3C N CH3 CH3 I A more bulky electrochrome is the triphenylamine derivative II. against heptyl viologen (see Chapter 11) as the primary.3. (ii) the amine is attached to an aromatic (e. for example poly(aniline). In relatively non-polar solvents. the initially colourless neutral compound forms a brilliant bluish-red radical with lmax of 530 nm. Ho et al.11. but the response times are slow owing to the requirement for all solution-phase electrochromes to diffuse toward the electrode–solution interphase prior to electron transfer.6.9. C6H5ÀNH2.10. as described in Section 10. The cycle lives of such devices are high. the radical cation together with an electrolyte anion may deposit as a salt. bleaching times likewise are also long. N N II . These species fall into two categories:2 (i) the nitrogen is incorporated into an aromatic ring and is a derivative of the pyridine ring C5H5N. Changes to the substituent causes a blue shift in the colour of the radical.10. such amines show type-I electrochromism. C6H5À) ring. and an aryl-substituted viologen was the primary. The initial product of electroreducing XI is a radical anion. In the absence of an electrode.2 summarises results obtained by Dubois and co-workers for a few carbazole electrochromes. compound III has a visible transmission of 75% in its clear state and only 9% in its coloured state.16. For example. In their neutral form. In each case.13 Thus. 12. the amine was the secondary electrochrome.3 Cyanine electrochromes Spiropyrans15.2 Carbazole electrochromes The monomeric.1 contains data for several such species.376 Miscellaneous organic electrochromes Several aromatic amines show electrochromic activity in the near infrared (NIR).1. whereas films of radical cation generated oxidatively according to Eq. compound VIII (‘Crystal violet’) has a NIR transmission of 46% in its clear state and 14% in its coloured state. (12.17 such as XI are both electrochromic and photoelectrochromic.14 N R X 12. while further reduction yields a ring-opened merocyanine species. All amines colour anodically and are envisaged for use within solution-phase (type-I) devices. A plot of . as prepared and studied by the US Gentex Corporation.1. carbazoles are soluble and essentially colourless. Some of the changes in optical transmission are marked.1) form a highly coloured. Table 12. solid precipitate on the electrode: carbazole ðsolnÞ þ XÀ ! ½carbazoleþ : XÀ 0 ðsÞ þ eÀ : colourless strongly coloured (12:1) The carbazoles therefore represent an example of type-II electrochromism. photolysis of XI yields the merocyanine directly.13 Table 12. most compounds in Table 12.1 show a contrast ratio of 5:1 on coloration. substituted carbazole species X readily undergoes a oneelectron oxidation to form a radical-cation salt. 1. D.1 Monomeric electrochromes 377 Table 12.. 2003. with permission of The Electrochemical Society. Electrochem.. 2003–17. K. ‘Solution phase electrochromic devices with near infrared attenuation’. 199–207. Baumann. (Table reproduced with permission from Theiste. Soc.12. Proc. P. Aromatic amine electrochromes that modulate NIR radiation. and Giri.) Electrochrome CH3 N λmax/nm ε/dm3 mol–1 cm–1 727 N CH3 1500 III H2C N CH3 H3C N CH3 N CH3 1694 15 000 CH3 IV CH3 N H3C 912 N CH3 19 000 O V CH3 N S CH3 S N CH3 CH3 1198 38 500 VI . 378 Miscellaneous organic electrochromes Table 12.1. and allows ¼ 21 cm2 CÀ1 to be calculated.) Electrochrome λmax/nm S N ε/dm3 mol–1 cm–1 N O 954 47 000 VII H3C N CH 3 H3C N H3C N CH3 H3C N 968 46 000 VIII CH3 CH3 N N N CH3 CH3 N 907 IX absorbance Abs against Q for species XI is essentially linear.17 H3C CH3 N CH3 O NO2 XI . (cont. Lacaze. The stability of the radical cation formed by one-electron oxidation of the neutral species is a function of molecular planarity.21 although Parker mentions neither electrochromism nor electrochemical applications. Many of these species should more correctly be called ‘biphenyls’.9 þ1. N-phenyl and N-carbazyl derivatives’. EPR spectroscopic study of colored radical films formed by the electrochemical oxidation of carbazoles. Chem.) Monomer Carbazole N-ethylcarbazole N-phenylcarbazole N-carbazylcarbazole Colour of radical cation Dark green Green ‘Iridescent’ Yellow–brown E Õ/V þ0.-C. A.1.1 Monomeric electrochromes 379 Table 12. J. Electroanal.2. Colours and electrode potentials of oligomers derived from various carbazole electrochromes in MeCN solution. as demonstrated by the stability series XIII < XIV < XV: compound XIII is forced out of planarity < .12. The reduced form of the dyes are blue. Me or Et) O– C H H3C CH3 N N XII 12.-E. ‘Polaromicrotribometric (PMT) and IR. ESCA. while the radical species formed on oxidation are green. (Table reproduced from: Desbe`neMonvernay. 129. which have some structural elements in common with XI. 1981.. 229–41. while electro-oxidation yields a thin. The uncharged parent compounds are essentially colourless.4 Methoxybiphenyl electrochromes The next class of compounds are the violenes based on a core of polymethoxybiphenyl.20 and Grant. J.3 þ1. Methoxybiphenyl compounds have been studied by the groups of Parker19.2 þ1. ‘fluorenes’ or ‘phenanthrenes’ according to the nature of the bridging group (if any) connecting the two aromatic rings.1 Another recently discovered series of electrochromes are the squarylium dyes. and Dubois. solid film of brilliantly coloured radical-cation salt. with permission of Elsevier Science Ltd. part I: carbazole and N-ethyl. H3C CH3 C H O– X X (X = H.. P.18 such as XII. 3.and meta-methoxy substituents engender lower redox potentials than para groups.e. Ortho. . are listed in Table 12.380 Miscellaneous organic electrochromes by the steric repulsion induced by the two o-methoxy groups.21 The fluorene compounds that appear most suitable for ECD inclusion. those yielding the most stable films of radical-cation salt. Table 12. by necessity. Other fluorene compounds investigated did not form radical cations of sufficient stability for viable use as electrochromes. OCH3 H3CO H3CO OCH3 H3CO OCH3 XIII H3CO XIV OCH3 H3CO OCH3 H3CO OCH3 XV XVI H3CO H3CO OCH3 OCH3 OCH3 OCH3 H3CO OCH3 XVII XVIII H3CO H3CO OCH3 OCH3 XIX As a crude generalisation. but the electrochemistry of compounds with two or more methoxy groups is much more reversible. whereas XV. that is.21 fluorenes with a single methoxy group are oxidised irreversibly.7-dimethoxyphenanthrene. is always planar owing to the methylene bridge. monomethoxy species are not truly violenes.4 lists some biphenyl compounds of interest. or evinced irreversible electrochemistry. For example. within which compound XVIII aromatises slowly by deprotonating to form 2. i. Colours. Org. Colours.26. 845–9. 96. and only moderately coloured as neutral molecules but on one-electron reduction form brightly coloured. stable. Am.4. O. D.5 Quinone electrochromes Many quinone species are soluble. Coleman.79 þ0. ‘Anodic oxidation of methoxybiphenyls: effect of the biphenyl linkage on aromatic cation radical and dication stability’.. stable.. 45.27 For example.91 þ0. Chem.23.24.. A. 1980. Soc. the electrochromism of several benzoquinones has been studied such as the ortho (XX) and para (XXI) isomers. with permission of The American Chemical Society. and Parker. M. J.28 þ1. J. Chem.. and spectral properties for methoxybiphenyl species forming a solid radical-cation film on reduction in MeCN solutions.96 þ0.) Compound XIV XVIII XIX Colour of radical – Green Green Epa(1)/V þ1.1.88 lmax/nm 417 386 "/dm3 molÀ1 cmÀ1 29 512 20 420 12. CV peak potentials.87 Epc/V þ0.) Compound XV XVI XVII Colour of radical Blue – Blue Epa/V þ0.22 þ1. 702–5.14 þ0. CV peak potentials and spectral properties for methoxybiphenyl species forming only a soluble radical cation on reduction in dichloromethane–TFA (5:1 v:v) solution. (Table reproduced from Ronlań. Hammerich.22. Clecak. 1974. J.. V. ‘Study of the electrochromism of methoxyfluorene compounds’. N. J.1 Monomeric electrochromes 381 Table 12. solid films of radical anion on the electrode surface.81 lmax/nm 411 385 415 "/dm3 molÀ1 cmÀ1 40 400 32 800 44 300 Table 12.25.07 þ0.12. with permission of The American Chemical Society.24 O O Cl Cl Cl O XXII O Cl O XX O XXI .84 þ0. (Table reproduced from Grant.94 Epc(1)/V þ1. The most comprehensive study of (XXI) involved the electrochrome dissolved in a solution of propylene carbonate containing LiClO4 as supporting electrolyte. B.3. and Oxsen. SCE. 75–90. solid film of p-chloranil (XXII) as the radical anion salt on an ITO electrode polarised to À0. by permission of Elsevier Science. In CH3CN solution. but as the sodium cation does not undergo colour-forming charge-transfer interactions. and the radical anion of p-2.5. o-chloranil (XXII) and o-bromoanil ` form films that are both stable and adherent.6tetrachlorobenzoquinone (‘p-chloranil’ XXII).3. 260.27 say the best results are obtained if the cation forms a ‘visible light-forming charge transfer complex between [the] o-chloranil À and the counter ion Mþ. Desbene-Monvernay et al.6 V vs.. 1989.’ This is doubtful for they also say the best results are obtained when M ¼ Na. The spectrum baseline was that of the uncharged.5 lists a few sample quinone species together with electrochemical and optical data. Table 12. The colour of the radical cation depends on the substituents around the quinone: the tetrafluoro analogue of (XXII) ‘fluoranil’ forms a yellow radical anion. (12:2) where the alkali or alkaline-earth cation M is needed to co-deposit with the ` radical anion when forming an insoluble salt. Spectrum of a thin. see Eq. Figure 12. merely undergoing co-deposition with the quinone radical cation. C..1 shows the absorbance spectrum of a film of p-chloranil radical anion on ITO polarised to À0. (12.) .22 which forms a pink radical cation.6 V.1. J. only p-benzoquinone. A. ‘UV-visible spectroelectrochemical study of some para.382 Miscellaneous organic electrochromes The quinone to have received the most attention is probably p-2. Lacaze.6-dichloroquinone is pink. A. (Figure ` reproduced from Desbene-Monvernay.2): Mþ þ ðXXIIÞ0 ðsolnÞ þ eÀ ! ½Mþ ðXXIIÞÀ ðsÞ.and orthobenzoquinoid compounds: comparative evaluation of their electrochromic properties’.3-dicyano-5. Electroanal. colourless p-chloranil prior to charge passage. P. Chem. and Cherigui. the source of the quinone radical-cation colour is best conceived as an internal charge-transfer transition modified by Mþ.22 Desbene-Monvernay and * Absorbance 300 Abs 400 500 Wav elength/nm Figure 12. The values of Ks for the para isomers are generally too high. Its electrochromic properties are ‘outstanding’.23 22 22.e. À0. i.170 À0. all solutions in MeCN with tetraethylammonium perchlorate (0.6dichlorobenzoquinone 5-aminonaphthoquinone 1-aminoanthraquinone 2-aminoanthraquinone 1.5.1 mol dmÀ3).720 À0.5.3-dicyano-5.5-diaminoanthraquinone Solid film? Yes Yes Yes No No No Yes Yes Yes Yes Colour of R–QÀ * lmax/nm Epc(1)/V Epc(2)/V Ref.22 While the electrochromism of most quinones requires the formation of radical species.4. or standard electrode potentials E .420 þ0. F Quinone (R–Q) o-3.5.24 22 22 22 25 25 25 25 Eð1Þ ¼ À0:83 Eð1Þ ¼ À1:03 Eð1Þ ¼ À0:99 Eð1Þ ¼ À1:10 F F F co-workers22 say that o-bromanil forms a superior radical-cation film to any of the other para-substituted quinones.22 and the solubility constants Ks are lower.22 The quinone evincing the highest electrochemical stability is o-chloranil (the ortho analogue of XXII).430 À0. a recent example .6-tetrabromobenzoquinone p-benzoquinone p-2. Quinone systems: film-forming properties. and reduction potentials.3.4. electrochromes based on ortho quinones are superior to the para analogues: they are more electrochemically stable. sometimes allowing soluble radical cation to diffuse back into the solution bulk.6-tetrachlorobenzoquinone p-2.12. which therefore represents type-I response rather than the perhaps more desirable type-II electrochromism.210 þ0.6-tetrachlorobenzoquinone o-3. a transition from pale to intense colour.22. colours.100 À0. from its low solubility product and good adherence.330 22.6-tetrafluorobenzoquinone p-2. wavelength maxima.5.1 Monomeric electrochromes 383 Table 12. Values of Epc were obtained from CVs.430 À0.5.28 with a cycle life exceeding 105 write–erase cycles.140 À0.190 À0. In general.060 þ0.3.070 410 – – 570 F Intense blue Blue Light blue Yellow Yellow Pink Purple– blue – – Purple þ0. (12. while the other colours derive from the diphenylamine. Eq.3) OH O Molecular naphthaquinone and anthraquinone species are also type-I electrochromes. The electrolyte is gelled with a white ‘filler’ to enhance the contrast ratio. a 1:1 compound of p-benzoquinone XX and dihydroxybenzene (both depicted in Eq. Exemplar species include 1.40 -bis(dimethylamino)diphenylamine. representing first Eq. rather than a coloured quinone radical. followed at more negative potentials by a second reduction reaction.4-naphthaquinone (XXIII) and anthra-9. cf.29 cf. Aminoanthraquinones show a more complicated electrochemical behaviour than the naphthaquinone: at moderate potentials. In this way. (12. two redox couples are exhibited during cyclic voltammetry. which yields two different oxidation states: its first oxidation product is a green radical cation (CR ¼ 2:1) and a subsequent oxidation product is a . O O O XXIII O XXIV More advanced again is a trichromic ECD30 with the capacity to form the colours red.5): À quinoneÀ þ eÀ ÀÀ! quinone2À : (12:5) In addition to this behaviour. the anthraquinone compound produces the red colour when reduced (CR ¼ 2:1 at lmax ¼ 545 nm).384 Miscellaneous organic electrochromes operates differently: the red quinone species 1-amino-4-bromoanthraquinone2-sulfonate may be electroreduced in aqueous solution to form a colourless dihydroxy compound.4): À (12:4) quinone0 þ eÀ ÀÀ! quinoneÀ . (12.10-quinone (XXIV). the so-called ‘quinhydrone electrode’. the formation of poly(aniline) in Section 10. which has been developed using 2-ethylanthraquinone in PC together with 4. O 2e− + 2 H+ OH (12. green and green-blue. polymerisation of the amine moiety occurs when the electrode is made very positive.4.3)). but has been occasionally considered for ECD usage when immobilised in a semi-solid polymer matrix. undoubtedly cationic as ‘bipm2þ ’. (In a similar device containing heptyl viologen and tetramethylphenylenediamine in PC. N H3C N CH3 S + N CH3 CH3 XXV The thiazine XXV is soluble in a wide range of solvents. When the mirror is switched on.1 Monomeric electrochromes 385 green-blue dication (CR ¼ 3.12) The dual-electrochrome solution is injected into the cavity between the electrodes.5:1 at lmax % 500 nm).31.3).28 mm.12. Furthermore. thiazines such as XXV are used.3. so XXV is blue when oxidised and colourless following reduction to form the neutral radical. the cell would only function when the gap was narrower than 0.33 The Gentex device comprises two electrochromes. the common dye and biological stain. each NVS# mirror incorporates a front electrode of ITO-coated glass and a metallic rear electrode having a highly reflective surface. a viologen species (see Chapter 11) and a phenothiazine. the various redox states formed will diffuse back into the solution bulk and undergo radical-annihilation reactions. These two parallel electrodes separated by a sub-millimetre gap form the basis of the cell. Methylene Blue (XXV). mass transport occurs as the positive charge of the uncoloured precursor .35 In MeCN solution. so called leuco-Methylene Blue. but it is possible to infer some details of the operation: a substituted viologen species ‘bipm’ (see Section 11. The world’s best-selling electrochromic device is undoubtedly the Night Vision System (NVS#) produced by the US Gentex Corporation. serves as the cathodic electrochrome.6 Thiazine electrochromes Thiazine compounds contain a heterocyclic ring comprising both nitrogen and sulfur moieties. The exact composition of the Gentex NVS# mirror is obscured within densely worded patents.1. The Greek descriptor Leucos (‘white’) is used in organic chemistry and in the dyestuffs industry to describe the colourless form of a redox dye. it is also likely that the 2þ and 0 redox states will undergo comproportionation thus: (2þ) þ (0) ! 2(þ *). At heart. as described in Section 12.32 a selfdarkening rear-view mirror that is a standard feature in many millions of expensive high-performance cars.31. 12. Because the electrochromes are not encapsulated in separate pixels. is the archetypal thiazine.34. being violenes. .8) * * TAþ ðsolnÞ þ bipmþ ðsolnÞ ! TA ðsolnÞ þ bipm2þ ðsolnÞ. Coloration occurs electrochemically at both electrodes.2. Schematic representation of the redox cycles occurring within the Gentex Night Vision System#. The coloured species diffuse away from the respective electrodes and meet in the intervening solution where their mutual reaction (‘radical annihilation’) ensues. bleaching occurs chemically at the centre of the cell by radical annihilation. The TA is uncharged and depletion by oxidation at the anode ensures that mass transport of TA ensues by diffusion alone. (12.386 Miscellaneous organic electrochromes propels it toward the cathode in response to ohmic migration (the electrolyte in the Gentex mirror is free of additional swamping electrolyte).2.7): TA ðsolnÞ ! TAþ ðsolnÞ þ eÀ : (12:7) In operation. Positive Negative TA0 Reflective back electrode bipm+. 375 above). bipm2+ Figure 12. the colour in a commercially-available NVS# mirror is an intense blue–green. Eq. see p. The colour-forming reduction process.6): bipm2þ ðsolnÞ ! bipmþ ðsolnÞ: (12:6) The other electrochrome (which is initially in its reduced form) is probably a molecular thiazine ‘TA’ (or perhaps a phenylenediamine species. (12. Eq. (12:8) that regenerates the original uncoloured species. Reductive coloration then occurs at the cathode. and the complementary oxidation reaction. These reactions are depicted schematically in Figure 12. Eq. Transparent front electrode −e− +e− −e− +e− TA+. bipm2þ þ eÀ ! bipmþ . (12. Oxidation of TA evokes colour. TA ! TAþ þ eÀ occur in dual electro-coloration processes in tandem. 7) represents a divergence from one of the benefits of electrochromism since the ‘memory effect’ is lost. For this reason.1 Pyrazoline electrochromes A tethered organic system that has received some attention is that based on the oxidation of the pyrazolines XXVIII and XXIX.7 Miscellaneous monomeric electrochromes A trichromic ECD has been fabricated including 2.39 .7) obviates any need to electro-bleach the Gentex NVS# mirror.5.2 Tethered electrochromic species 387 The radical annihilation in Eq.2. Reaction (12. Such species are more intensely absorbing than the tetrathiafulvalene (TTF) species below. with which these mirrors comply.4.7-tetranitro-9-fluorenone (XXVI) as the red-forming material. 12.39 have published most of the current work on tethered pyrazolines.6.2 Tethered electrochromic species 12.36 O NC CN O2N NO2 NO2 NO2 O 2N NO2 NO2 XXVI XXVII Finally. and are soluble in many solvents prior to polymerisation. their results may facilitate the preparation of new electrochromes. on loss of current. maintenance of coloration requires the passage of a continuous (albeit minute) current to replenish the coloured electrochromes lost by the annihilation.4.1. 2. and have faster response times . spectral details for which are listed in Table 12.39 Pure pyrazoline monomers are readily prepared. 12.12. since colour fades spontaneously on switch-off. to be the clear condition.7-trinitro-9-fluorenylidene malononitrile (XXVII) as the green and tetracyanoquinodimethane (TCNQ) as the blue electrochrome. (12. Thus. the Gentex NVS# is sometimes termed the self-erasing mirror. a Japanese group has prepared a TCNQ derivative and studied its spectra as a function of applied potential.38. United States law requires the ‘failure mode’. Kaufman et al.37 While their study was not concerned with electrochromic activity. 41. M. colours. 101. OCH3 XXVIII Z = OCH3 O N N XXIX Z = N O n z 12. The oligomer XXX is estimated41 to have a molecular weight of about 2200 g molÀ1. a chain comprising an average chain length of 6. E..6.43 have improved the electrochromic write–erase efficiency by chemically tethering the TCNQ species XXX to an electrode surface by means of polymerisation.55 þ0.45 Colour change Yellow-to-green Yellow-to-red lmax/nm 510 554 /ms 50 100 A solid-state ECD which incorporates such polymeric pyrazolines has been constructed. (Table reproduced from Kaufman.36. and response times t for tethered pyrazoline species bound covalently to an electrode substrate.2.) Compound XXVII XXVIII E½/V þ0. Chambers et al. Solid-state spectroelectrochemistry of crosslinked donor bound polymer films.37 The stability of the tetracyanoquinonedimethanide radical is ascribed to appreciable delocalisation of the single negative charge over the four CN groups. Chem.2 Tetracyanoquinodimethane (TCNQ) electrochromes Neutrally charged TCNQ is a stable.e.42. Since TCNQ and its radical anion are both soluble in most common solvents. Half-wave potentials E1/2.3 electrochrome units. Am. 547–9.388 Miscellaneous organic electrochromes Table 12. Soc. 1979.40 and has a response time of 10 ms and a CR of 10:1. i. immersed in MeCN solution containing TEAP electrolyte (0. F. J. . B. with permission of The American Chemical Society. and Engler. colourless molecule that forms a bluegreen coloured radical anion following one-electron reduction.1 mol dmÀ3). 92 4. In early trials.30 3..7. immersed in MeCN solution. In solution.38 3. 161.06 5. and (in aqueous solution only) a dianion (TCNQ)22À dimer.8.7. a TTF device underwent >104 cycles without visible deterioration.41 Spectroscopic data for TCNQ and TCNQÀ are listed in Table 12. G. R.7 have also been identified. J. (Reproduced from Inzelt. Day. additional species to those in Table 12. .20 NC CN O O O O n O O NC CN XXX Electrodes modified with XXX are electrochemically reversible.42 * 12.06 4. In this way Kaufman and co-workers44. ‘Spectroelectrochemistry of tetracyanoquinodimethane modified electrodes’.12.2 Tethered electrochromic species 389 Table 12. W..2.45 used the two species XXXI and XXXII to modify electrodes. Spectroscopic data for a modified electrode bearing a thin film of the TCNQ-based polymer XXX. and Chambers. Electroanal. Kinstle.. F. TTF has been used in ECDs chemically tethered to an electrode surface. 147–61 with permission of Elsevier Science.) Species TCNQ0 TCNQÀ * lmax/nm 408 430 445 660 728 812 ln("/dm3 molÀ1 cmÀ1) 5. Chem. Spectral characteristics of XXXI and XXXII are listed in Table 12. 1984. Q.3 Tetrathiafulvalene (TTF) electrochromes Like TCNQ. J. J. including a dianion (TCNQ)2À.44 The electrochromic TTF colouration accompanies oxidation of neutral TTF to form a radical cation. with permission of American Institute of Physics. V. A. E. and Patel. Biennial Display Research Conference.. Schroeder. 1978. 36.8. 1980. (Data reproduced from Kaufman. 23–5.9 will also form in the layer around the electrode. Table 12. Half-wave potentials E1/2. F. they greatly complicate any electrochemical interpretation.) Species TTFþ (TTFþ )2 (TTF)þ 2 TTF2 þ * * * lmax/nm 393. p. Phys.. electron transport through the film proceeds via hopping or tunnelling between TTF sites.39.35 Colour change Orange-to-brown Yellow-to-green lmax/nm 515 650 /ms a 200 150 Time required for a charge injection of 1 mC cmÀ2 into a film of thickness 5 mm. M. Recent TTF displays comprise solid-state devices with polymeric electrolytes. Spectroscopic data for TTF redox species in MeCN solution. Although the minor species in Table 12. the other TTF species listed in Table 12.(Data reproduced from Kaufman. V. colours.9. H. B.45 þ0. wavelength maxima and response times t for tethered TTF species.40 * . F. B. Conference Record of the IEEE.390 Miscellaneous organic electrochromes Table 12. ‘Polymer-modified electrodes: a new class of electrochromic materials’. Lett. In addition to TTFþ .46 furthermore. New York. Appl. 422–5.9 do not contribute much to the colouration of a TTF device..9. 653 1800 820 533 O S X S S S XXXI X = O C n XXXII X = O Electrochemical studies show the rate-determining step during coloration is ion movement into and through the film. with permission of The IEEE. Engler. ‘New organic materials for use as transducers in electrochromic display devices’.) Compound XXXI XXXII a E½ /V þ0. their spectral characteristics are reproduced in Table 12. the intermediate steps of reversible organic oxidation–reduction. Rev.54. with most remaining in its colourless form.. Pure Appl. . 50. but a rapid self-bleaching process occurred under open circuit. Schubert. New York.49 and poly(1-vinyl2-pyrrolidinone-co-N.51 the electrochromes dispersed in PVPD were all ester-based. 243–86. K. M. ¨ 3. Clearly. 16. 2. Methods of Quantitative Inorganic Analysis. For this reason.3 Electrochromes immobilised within viscous solvents The write–erase efficiency can be enhanced by dissolving or dispersing an electrochrome in a semi-solid electrolyte of high viscosity.. the colour formed after the potential had been applied for a few seconds.10. Such gel films therefore lack any optical memory effect.References 391 Table 12. only a small proportion of the electrochrome dispersed in viscous electrolyte will ever be juxtaposed with the electrode.1 above) have similarly been immobilised in a ‘matrix’ of poly(siloxane) to yield viable ECDs.49 12. 1981–92. In the studies by Tsutseumi et al. The free radicals of the type of Wurster’s salts.. L. 1939. Methylene Blue (XXV) is thus unpromising as an electrochrome as its colourless leuco form reverts back to the coloured form quite rapidly.48 poly(aniline). 15. Carbazoles (cf.. 61. 4. Hunig. Stable radical ions. Soc.10 lists a few electrochromes which have been immobilised in this way. especially when exposed to oxygen. 1963.47 such as the polyelectrolytes or polymeric electrolytes described in Chapter 14. Electrochrome p-Diacetylbenzene Diethyl terephthalate Dimethyl terephthalate Methylene Blue (XXV) Polarity to yield colour Cathodic Cathodic Cathodic Anodic Polymer PVPD PVPD PVPD poly(AMPS) Colour Green Red Red Blue Ref. Electrochromes dispersed within semi-solid polymer ‘matrices’. Am.N0 -methylenebisacrylamide) PVPD. Chem. 1935. Semiquinones. In each case. the host polymers of choice are semi-solid poly(AMPS). S.51 48.52. Section 12. Michaelis. Michaelis. L.51 51 50. J. S. or can reach the electrode within a tolerable time lag. In this context. the majority of the electrochrome must be considered to be ‘passive’. Interscience. Chem. P.55. Such immobilised species are essentially type-III electrochromes. 15. 1967. and Gramick. The usual matrix for entrapment is an electrolyte gel of high viscosity.56. Chem.50.53..57 References 1. Kodama. ch. 109–22.50 Table 12. Synthesis and electrochromic properties of poly-o-aminophenol. Ho. and Oxsen.-C. 16. O.. 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These and other applications are reviewed at length by Lampert1 (1998). reducing reflection intensity and thereby alleviating driver discomfort. Much duplication is certain in such patents. However. which obviously operate in a reflectance mode. all devices utilising electrochromic colour modulation fall within two broad.2 Reflective electrochromic devices: electrochromic car mirrors Mirrors. and also several novel applications. total opacity is to be avoided as muted reflection must persist in the darkened state. Here an optically absorbing electrochromic colour is evoked over the reflecting surface. who cites all the principal manufacturers of electrochromic goods worldwide. Self-darkening electrochromic mirrors. The most common applications are electrochromic mirrors and windows.1) or by reflection (see the schematic representation in Figure 13.2). but it is clear how large scale are the investments directed toward implementing electrochromism as viable in displays or light modulation.1 Introduction While the applications of electrochromism are ever growing. for automotive use at night. illustrate the first application of electrochromism (cf. as below.13 Applications of electrochromic devices 13. disallow the lights of following vehicles to dazzle by reflection from the driver’s or the door mirror. vital details of compositions are often well hidden. Several thousand patents have been filed to describe various electrochromic species and devices deemed worthy of commercial exploitation. 13.2). In this field. so the field is vast. overlapping categories according to the mode of operation: electrochromic devices (ECDs) operating by transmission (see schematic in Figure 13. Figure 13. The back electrode is a reflective material 395 . Both the front and back electrodes are optically transparent.2 (None of the accounts available reveal the dramatic events at the onset of Gentex’s first big auto contract. The respective widths of the arrows indicate the relative magnitudes of the light intensities. ECD Incident beam Emergent beam Reflective surface Figure 13.396 Applications of electrochromic devices ECD Incident beam Emergent beam Figure 13.2 Schematic diagram of an ECD operating in reflectance mode.1 under ‘Thiazine electrochromes’. Reference 2 has some nice graphics that illustrate the necessary components. The front electrode is optically transparent and the back electrode is made of polished platinum or platinum-based alloy.4 (NVS#).) . when a small adventurous inventive company that had some impressive demo devices had suddenly to tool up for mass production. and are not to be confused with an identically named but independent firm in Pennsylvania that supplies amongst other things protective clothing. Section 12. The best-selling electrochromic mirror is the Gentex Night-Vision System3. fireproofing and the like for aeronauts and astronauts.1 Schematic diagram of an ECD operating in transmittance mode. of which many millions have been sold. allowing customary mirror reflection in the bleached state. and the mechanism is illustrated in Figure 12. The respective widths of the arrows indicate the relative magnitudes of the light intensities. Michigan. The Gentex Corporation we refer to here are based in Zeeland.5 Its operation employing type-I electrochromes is described in detail in Chapter 12. probably 90 to 95% of all self-darkening mirror sales. The durability of ECDs is discussed in Chapter 16.8. The appeal of smart windows is both economic and environmental: if successful.20 Smart windows for automotive usage have not been reviewed so often: one of the few reviews to mention this application explicitly is ‘Angular selective window coatings: theory and experiments’ by Granqvist et al. were deposited on wafers of silica by plasma-enhanced CVD from a metal-carbonyl precursor.16 The construction of electrochromic windows has often been reviewed. the terms ‘electrochromic window’ and ‘smart glass’ are now widespread and attract attention.9. Mitsubishi. among others.19 and ‘Electrochromic smart windows: energy efficiency’ by Azens and Granqvist (in 2003). Porsche.13 have developed an electrochromic mirror that is. Rolls Royce and Toyota. (2002).18 ‘Windows’ by Bell et al. Gesheva et al. Lexus. Electrochromic modulation changes the X-reflectivity of the underlying silica. A different solid-state electrochromic mirror is based on WO3. Opel. ‘Toward the smart window: progress in electrochromics’ by Granqvist et al.10. Hyundai. Nissan. particularly in popular-science articles. reflective to X-rays. Fiat. and is thus a type-III system. Although dated (1991). Daewoo. (in 1999). 13.3.1 Buildings Svensson and Granqvist coined the term ‘smart window’ in 1985 to describe windows that electrochromically change in transmittance. Kia Motors. they preclude much solar radiation from a room or a car. apparently. MoO3 or mixed W–Mo oxide. BMW. Ford. General Motors.12 The likely thermal and other stresses resulting from mounting ECDs in or on cars require particularly stringent tests of ECD design and fabrication.17 ‘Electrochromic windows: an overview’ by Rauh (also in 1999).7. ‘A review on electrochromic devices for automotive glazing’ by Demiryont22 is still relevant. The .11 Electrochromic mirrors are fitted on luxury cars made by. which relies on WO3 and NiO. Bentley.13. Audi. for example.3 Transmissive electrochromic devices 397 An example of all-solid-state mirror is the SchottDonnelly6 solid polymer matrix (SPMTM) mirror for lorries and trucks.3 Transmissive ECD windows for buildings and aircraft 13.14 The term has since been augmented with ‘smart windows’ and ‘self-darkening windows’ to describe novel fenestrative applications. Films of WO3.21 in 1997.15 However. DaimlerChrysler. The British Fenestration Rating Council describes electrochromic windows as ‘Chromogenic glazing’. Infiniti. 31 show dramatic colour changes of organic films of PEDOT-based polymers (see Section 10. Many websites show video clips of electrochromic windows: the short sequences available in ref. Lee and DiBartolomeo24 suggest that electrochromic windows ‘may not be able to fulfil both energy-efficiency and visual comfort objectives when low winter direct sun is present’. Similar electrochromic applications are planned for car sunroofs. 34 contains several longer . via a photocell connected to a microprocessor. many of these products are poorly described in the associated publicity. Colour Plate 4 shows a window made by Gentex. and externally by solar radiation that entered the room through electrochromic windows. or a boost for electrochromic applications.398 Applications of electrochromic devices exact cost of air conditioning in summer is unknown. without identifying either electrochrome. but is surely greater than losses through windows in winter. as follows.26 Many ‘green’ considerations are assessed by Griffiths et al. Most workers preferred to control the external lighting via electrochromic windows rather than blinds or other mechanical forms of shutter.38 see Colour Plate 5. so the identities of the electrochromes are unclear. When the transparency of the windows was changed automatically. Reference 32 contains a short video clip of an electrochromic window measuring 3 ft Â 6 ft. and ref.23 Smart windows might thus both improve working environments and alleviate costs.35 and the motorcycle helmets and ski goggles developed by the Granqvist group in Sweden.mpeg clips of varying clarity. Nevertheless. (though it is not clear whether this is an ECD or an SPD33).22.28 Architectural applications are at present the subject of intense research activity: the web page from the National Renewable Energy Laboratories (NREL) in ref.37. the number of manufacturers appears to be expanding rapidly. Zinzi39 published a full study describing the preferences of office workers. one of which is said to be ‘organic’ and the other ‘inorganic’. made by Research Frontiers Inc.30 of Minnesota clearly produce two products. For example. In the smart-window application.2). He made a full mock-up room.36. However. Recently. This is scarcely informative.27 and Syrrakou et al. The rush to develop smart windows is also a response to pressure from environmental campaigners of the ‘Green’ lobby. 29 aims to cite all the present-day producers of electrochromic windows. the alteration of the visual environment was sufficiently smooth and slow that few workers actually . illuminated internally with conventional fluorescent and incandescent bulbs. individual panes of glass or whole windows can be coloured electrochromically to darken sunlight intensity in rooms or offices. which in 2003 cost $25 billion in the USA alone. from their the web site SAGE Electrochromics Inc.25. The results were interesting and not always as expected. 4).3 Transmissive electrochromic devices 399 perceived the changes. Many manufacturers of electrochromic windows prefer so-called ‘neutral’ colours.13. shades of grey. to 15% when coloured.43 The quest for ‘neutral colour’ is presented in refs. say. however. unless the solar radiation could be reflected metallically by the electrochrome. 42. Some smart windows. most preferred a manually operated transparency control. such electrochromic windows do further regulate the transmission of the thermal components of sunlight.48. While most WO3-based ECDs develop a band peaking in the near infrared (NIR). namely the fullerene49 LixC60.51 tungsten oxyfluoride53 and tungsten trioxide. because the absorption spectrum comprises several broad and overlapping optical bands. While not altering the perceived colour of an electrochromic window. which evinces an electrochromic colour that is more grey than that of either constituent oxide alone. which can be cycled between highly transparent and mirror-like conditions (see Section 9. complete blocking of sunlight would need the dissipation of much absorbed heat. reflectivity. to the richer blue colour of (for example) HxWO3 alone. 46.43. presumably to ‘personalise’ their own working space.55 poly(diphenylamine)56 and PEDOT. requiring a material with metallic. for example by mixing various metal oxides in precise relative amounts.54 and organic systems such as poly(pyrrole) composites. Some few electrochromes show specular reflectivity. a promising electrochrome is a mixture of vanadium and tungsten oxides.44. Nevertheless. While light transmittances may be modulated between. Other electrochromes indicating specular reflectance are the inorganic systems copper oxide. Electrochromes commonly show colour in the visible spectrum. those who did were not unhappy with its effects.2).50 which is yellow but becomes brown following reduction due to a band with a maximum in the UV (see Section 9. and electrodeposited diamond. Some workers wanted electrochromic windows to adjust more rapidly. because other colours can induce nausea. the most remarkable being yttrium hydride.e. develop a band almost wholly in the NIR. This desirable property is found also in two unexpected electrochromes. Office workers are said to favour such grey hues. i. specular. with many researchers seeking to effect subtle changes in optical bands.45. the reduced form having a band maximum at 1060–80 nm.56 . it is sufficiently broad for much visible light also to be absorbed. In this regard. while mixtures of metal oxide are considered in greater detail in Section 6.42. 85% when bleached.40 Thus there is now a considerable research effort to optimise the hue for the working environment.52 lithium pnictide.47.51 iridium oxide. to accommodate fluctuating ambient illumination.41.5. who acquired Pilkington Glass’s ECD technology based on WO3 with FTO substrates. which they claim to have been operating in a building for some years. The 40 or 50 cm diameter circular electrochromic window was no doubt WO3-based. but the period between ‘possible’ (it works) and ‘commercial’ (it will pay its way) can be appreciable. 52 arose from a thin layer of electrochromic IrO2 deposited on opaque Ir metal. the ‘Dreamliner’ (an artistic name). 13. the intensity to be electrochromically controlled from within. While in theory there is no absolute upper limit to the contrast ratio CR in ordinary electrochromism. so possibly comprising a nickel hydroxy-oxide counter electrode. PPG Aerospace and Boeing signed agreements to install electrochromic windows in the new long-range Boeing (B787) aircraft. Several hundred of the B787 aircraft. also WO3. 30 Â 30 cm). probably comprises WO3 particles that reflect some of the incident light. with a reported57 CR of 1000:1. due to operate in 2008. The stage is poised for a wider use of ECD windows in buildings.59. when other windows could be permanently darkened against solar heating. but it could be of use only for topfloor or single-storey illumination. Apart from the undisclosed cost of the window itself. with a clear glass roof-lid.60 The windows are said to be 25% greater in area than the usual. sunlight was to be funnelled to it down a tube from the outer roof. The inside placing would protect from solar photodegradation. in practice the values are never particularly high.58 Asahi Glass in Japan have an electrochromic window of small panes (ca. so the reflectance may be that of the metallic under-layer. Thus. the funnel-tube and roof-installation expense has probably vitiated any commercial appeal.3. The Stadtsparkasse Bank in Dresden however has operating electrochromic external windows supplied by Flabeg Gmbh. Schott Glass has shown demonstration models of their ‘Ucolite’ roomillumination system at Schott Glass Singapore (June 2000).400 Applications of electrochromic devices The reflectance of iridium oxide in ref. but darker.2 Aircraft – the first ubiquitous ECD window application: Gentex and Boeing In December 2005 Gentex Corporation. Furthermore it would have found use only in tropical or equatorial sunshine intensities as a light source. and a sputtered film of WO3. Designed to be fitted into the ceiling of an interior room.3. Other systems in which a metallic layer is electrodeposited are outlined in Section 9. are already on order. The Gentex–PPG systems will allow passengers to set the windows from clear to five increasing . unusually large values of CR are assumed to indicate reflective effects. whether the outermost glass layer is part of the ECD system is not disclosed. Electrochromic sunglasses. to effect the first mass-produced application of ECDs of appreciable size. contrasting with the automatic operation of the now widely available photochromic lenses that darken automatically. as are no doubt other aircraft manufacturers. Clearly a specialist.1). Nikon were the first to market electrochromic sunglasses in 1981. The costs to Boeing are reported as being $50 million (of which the larger part goes to Gentex). The two reviews in 1986 by Agnihotry et al. calling them a ‘variable-opacity lens filter’. The company PPG Aerospace is an experienced aircraft-window manufacturer and an ideally imaginative collaboration with Gentex has been created. 13.3 Capital screening: sunglasses and visors The Swedish invention of motorcyclists’ ECD visors is referred to below (see p. have been produced that may be darkened at will. Airbus are reported to be considering electrochromic windows for the A380. for the 221 seats. their new aircraft undergoing development.4 Electrochromic displays for displaying images and data Electrochromic devices operating as displays can act in either reflectance or transmissive modes.4 Displaying images and data 401 levels of darkening up to virtual opacity. The screen is said to be sited between the external cabin window and the plastic dust shield. but these are no longer available. the car mirror. As at present constructed. The B787 has 100 windows.62 13. but this substantial growth in window production can only lead to advances towards accommodating the requirements of buildings. application is involved here. the majority being of the reflectance type. niche. The electrochromic system employed has not been revealed. Additionally.61 Subsequently Nikon marketed WO3-based sunglasses in 1993. 422). an avenue to mass-produced architectural applications is not yet open. covering both physicochemical properties63 and device technology64 delineate the historical development of such devices.3. Donnelly have also produced electrochromic sunglasses that apparently operate via a different mechanism. that has thoroughly proved its worth on the smaller scale.13. Figure 13. One can do a little simple arithmetic to arrive at a pricey sum per window. also necessarily operating in a transmittance mode (cf. but it represents a substantial advance on the only other mass-produced device. Faughnan and Crandall’s still useful 1980 review65 . but perhaps only about 10 times that of an ECD car-mirror installation. The global display market is expanding rapidly. in televisions and large advertising displays. however. Other specialist ECDs designed for use as watch faces are cited in refs.6 billion in 1994 and will top $100 billion in 2007. ‘Commercial developments in electrochromics’ (in 1994)5 and ‘Electrochromics and polymers’ (in 2001).67 provide much detail. which comprise individual pixels of three (for tri-colour emission) minute.75.74. 73. gas-filled. so when technological barriers are overcome. Electrochromic devices have been proposed for flat-panel displays for applications such as television and VDU screens (but note possible disqualifications spelt out below).66 The market for ‘flat-panel’ information displays was worth approximately $18 billion in 2003. and are superseding CRTs in applications such as television screens and visual-display units (VDUs) for computers and instruments requiring monitors. at the ‘DEMO 2005’ show. perhaps.66 in 1994 helps establish the place of electrochromism within the wide varieties of display device. NTera of Eire demonstrated an iPod with a NanochromicsTM screen (as below). LCD devices have formed a larger proportion than CRTs. The first application suggested for ECDs was in watch faces.and brightness’. users seem prepared to bear. data boards at transport terminuses. For example. One commentator thought the new electrochromic screen ‘definitely exceeded the original iPod [screen] in crisp. Although dated. For example. these materials are likely to play an increasing role in such uses. mobile phones and screens on lap-. Electrochromic devices are often termed ‘passive’ since they do not emit light and hence require external illumination. A newer emissive competitor is the ‘plasma’ screen. Since 1996.72 here the face does not tell the time but represents a display with fourteen separate areas. They now dominate with about 90% of the market share.69 The range of applications for flat-panel displays increases rapidly. The construction costs are high. the extensive review by Bowonder et al. which darken progressively to indicate the phases of the moon. fluorescent light-emitting units.71 A modern variant is the face of the so-called Moonwatch.70 There are many other nascent applications of ECDs.68 and even as an electrochromic ‘indicator’ (based on WO3) on a cash card. from calculators and watches to. a possible disadvantage: lightemitting diodes (LEDs) and cathode-ray tubes (CRTs) are emissive. Byker’s two reviews of ECDs. palm. .or desk-top computers. and are incorporated into a wide array of electronic devices both large and small. advertising boards. which.402 Applications of electrochromic devices ‘Electrochromic devices based on WO3’ helps justify the claim that these workers introduced the concept of electrochromic displays. but liquid crystal displays (LCDs) and almost all mechanical displays are also nonemissive. and is growing very fast. the total global display market was $11. so responding within a range of a few milliseconds to a second or so. so a device may be constructed having a larger electrode expanse or a greater number of small electrodes. By contrast. or on advertising boards and frozen-food monitors. Electrochromic devices may be either flat or curved for wide-angle viewing. while of illustrative value. 53.13. (3. ECDs have insufficiently fast response times to be considered for applications such as television and (most) VDU screens. a response time of less than a millisecond is obtained. sometimes so cheaply as to be disposable. Realistically.16). Furthermore. for re-useable price labels. and therefore possess economic advantages over them.4 Displaying images and data 403 Liquid crystal displays can be fabricated extremely cheaply. l % (D t)½. largearea LCDs are expensive. at transport terminuses as mentioned above. multiple electrodes – ‘picture elements’ or ‘pixels’ – allow text or images to be displayed rather than mere blocks of colour. ECDs consume little power in producing images which. there is no limit to the size an ECD can take. in principle. (Note that these estimates.) Accordingly. remain with little or no additional input of power – the so-called ‘memory effect’ outlined on p. giving a response time of $20 s. Secondly. for type-I and type-III electrochromes. tethered monolayer systems. so requiring responses from a type-III system. which is both bulky and prohibitively expensive. Displays of digits and alphanumeric displays could however comprise liquid-containing elements or solids with faster diffusion coefficients D % 10À10 cm2 sÀ1. The main reason for their cheapness is the sheer volume of production worldwide.1 – could be 102 to 103 times faster than these ‘guesstimates’. the most suitable roles envisaged at this stage involve displaying information more slowly. the IBM Laboratories made an ECD with a 64 Â 64 pixel image on . once formed.1).3. Typical distances l to be traversed by a key species in a coloration step are between 10 and 100 nm. for long-term perusal. and large CRTs require a huge electron ‘gun’ behind the screen. e. For televisions and VDUs. Indicative response times can be roughly estimated from Eq. which our order-of-magnitude arithmetic shows to be slow. as obtained with type III. For immobile coloration. The claimed advantages are as follows: firstly. the image must be coloured at fixed points. say $50 nm intermediately. ride roughshod over the detail of the mechanisms summarised in Chapter 5. To produce such an image. with l but a few nm – see Section 11.g. With D for type-I (solution-phase) species about 10À7 cm2 sÀ1. D is typically 10À12 cm2 sÀ1. and cycle lives are probably also somewhat low (see Section 16. Electrochromic devices must compete with LCDs for commercial viability. which decreases the capital costs. but the type-I coloration in solution will be mobile. The electrochromic ‘3’ shown in Figure 4.1 is achieved with seven relatively large electrodes. however. 404 Applications of electrochromic devices a one inch square silicon chip76 and the NTera NanoChromicsTM display (see further detail in Section 11.4, p. 361) comprises an array of transparent electrodes, each about 0.25 mm square, or about 100 dots per inch.77 Colour Plate 6 shows a reflective cell with nine pixels. In such multi-pixel ECDs, tonal variation is achieved by stippling with dots as with LCD displays; alternatively, the image may be intensified by passing more charge into specified areas where more of the coloured substance is to be formed. There is however the technical problem with any large-area ECD. Areas of patchy colour may form when the current distribution is uneven across the electrode surface, since the electric field can be larger at the edges of the electrode substrate nearest the metallic leads, if the electrode substrate is semiconductive (like ITO). This allows a potential drop with distance towards the centre of the conducting area. Increasing the viscosity of the electrolyte, and subtle choice of potentials and dimensions, can more-or-less obviate this problem.67 13.5 ECD light modulators and shutters in message-laser applications In addition to displays and windows, electrochromic systems find a novel application as optical shutters or light modulators where the ECD operates in a transmissive mode (Figure 13.1). It is often the case that in fibre-optic message-laser applications the transmitting front-end puts out too high an intensity for the fibre. This is best remedied by a permanent filter, which could be a once-for-all photochromically evoked colour filter for the particular laser wavelength (the photochemistry of this coloration being effected by a pulse from a laser of different wavelength from that of the message laser). However, at the receiving end a variety of detectors are in use, with an associated variety of sensitivities, not always commensurate with the incoming signal. To match the output laser intensity to the detector sensitivity, an adjustable ECD is inserted in the optical path before the detector, that needs particular circuitry to evoke the most fitting coloration intensity. This task requires that the ECD remains almost constant for any one transmission. As this is a preliminary setting preceding message reception, instant (i.e. nanosecond) responses are not required. As receivers get messages from a number of sources with varying intensities, automatic adjustment preceding reception is desirable; this takes place during the communicationlinking protocol. A patent describes the circuitry detail required for this purpose.78 However, for operation of fibre-optic message transmissions (or in optical computer action), a response time of sub-nanoseconds is necessary, so no 13.6 Electrochromic paper 405 redox ECDs are sufficiently fast to act in this particular role as on–off shutters. Possibly for more leisurely optical data storage, pixels need only represent either ‘off’ or ‘on’, as in Figure 13.1 when coloured or bleached respectively, which thus totally interrupts (or not) a light beam, without regard to gradations of intensity. Electrochromic data storage is thus not precluded. 13.6 Electrochromic paper The impetus behind developing electrochromic paper is environmental: electrochromes embedded within a sheet of paper can in principle be switched reversibly between coloured and bleached, thereby allowing the paper to be re-used, rather than recycled. Relatively little work has yet been done on electrochromic materials impregnated into paper. Talmay79,80 patented an idea for electrochromic printing in 1942 with ‘electrolytic writing paper’ consisting of paper pre-impregnated with particulate MoO3 and WO3 that formed an image following reduction at an inert-metal electrode acting as a pen. The electroformation of Prussian blue within the fibres of the paper has also been suggested: cf. the comments in Chapter 2 concerning ‘blue prints’. Several recent patents have been issued for elaborations of electrochromic printing systems usually based on organic electrochromic dyes, as cited in ref. 5. In 1989 Rosseinsky and Monk82 investigated whether voltammetry in paper was possible, revealing marginal problems associated with IR drop across the paper and variations in its internal humidity. Moist paper was impregnated with a variety of viologens or Prussian blue precursors, together with an ionic electrolyte in sufficient concentration. In paper of marginal moistness, the electrochemistry of both Prussian blue and viologen electrochromes was quite well reproduced as though in a laboratory electrochemical cell, establishing electrochromic reactions to occur within the paper, as described in Section 11.4. Details of the study have been improved on.81 In 1989, IBM83 prepared a form of electrochromic paper capable of multiple coloration, but the complexity of their system precluded economic commercialisation. Investigations on electrochromes impregnated into paper included viologens,82,84,85 Prussian blue82,84 and the metal oxides MoO3 and WO3.84 Incorporation of an electrochrome within a thin layer of Nafion1 as a host matrix has also been shown to produce good results: the electrochromes included viologen,86 Methylene Blue87 and phenolsafranine dyes.87 Printable electrochromic paper has not been further pursued. However, NTera of Eire have developed a product called ‘electrochromic paper’ (and 406 Applications of electrochromic devices marketed as NanoChromicsTM), which is based on the viologen I.88 The display, not based on paper, uses phosphonate groups bound by chemisorption to a metal-oxide surface such as a titanium dioxide film deposited on FTO. The oxidised form of I is colourless while the reduced form is blue–mauve. O HO P OH + N 2Cl– I + N O P OH OH NTera call their ECD a nanochromic display (NCD), claiming their technology has more than four times the reflectivity and contrast of a liquid crystal display (LCD). 13.7 Electrochromes applied in quasi-electrochromic or non-electrochromic processes: sensors and analysis It is of interest to consider the substances that can be electrochromic when they are used in another, analytical, context. ‘Gasochromic’ coloration outlined in Section 1.2 involves a mechanism of the kind further contemplated here. Only the first example of Co3O4 that we cite below has some electrochromic basis to its operation; we then suggest an extension of this principle. In the solely analytical applications presented below, these normally electrochromic substances acquire or lose electrons from (or to) solution or gaseous species, rather than from (or to) electrode substrates when in electrochromic mode. The analytical relevance arises from the ensuing colour changes: a direct relationship exists between the absorbance of an electrochrome and the amount of charge passed, and thus the amount of test substance present. In a quasi-electrochromic application, Shimizu et al.89,90 made a sensor based on cobalt oxide Co3O4, which, electrically polarised, colorises in the presence of phosphate ion: a thin Co3O4 film changed transmittance T in the range 550–800 nm, becoming coloured when polarised to 0.4 V vs. SCE, but only in the presence of sufficient phosphate ion. No colour ensued in the absence of [HPO4]2À. The sensor transmittance T depends on the logarithm of [HPO4]2À concentration in the range 10À6 to 10À2 mol dmÀ3, via a mechanism depending on the redox reaction of Eq. (6.19) in Chapter 6, but with the electrons now coming from a chemical reductant. (So it is not truly electrochromic.) Other electrochromic sensors have been fabricated in which the optical absorbance relates to pH,91 or NO3À, or ClÀ concentrations.89 13.8 Miscellaneous electrochromic applications 407 Table 13.1. Electrochromes utilised in gasochromic sensing devices, responding to gaseous analyte. Electrochrome Chromium oxide Metalloporphyrins Nickel oxide Phthalocyanine Phthalocyanine Tungsten trioxide Tungsten trioxide Tungsten trioxide Tungsten trioxide Tungsten trioxide Analyte Ozone Chlorine Ozone Chlorine NO2, toluene Hydrogen Oxygen CH4/NH3/CO H 2S NO Refs. 99 100 99 101 92,93,94,95,96,97,98 102,103,104,105,106,107,108,109 109 110 111,112,113,114 115,116,117 The term ‘gasochromic’, Section 1.2, describes devices that operate with a gas-phase reductant or oxidant providing or accepting the electrons that would be necessary were these electrochromic redox processes. Thus, in a nonelectrochromic analytical application, Cook and co-workers used a variety of phthalocyanines, e.g. in the form of a Langmuir–Blodgett film, to test for such diverse gases as NO2 and toluene.92,93,94,95,96,97,98 Many examples of gasochromic sensing devices are listed in Table 13.1, all electrochromes remaining in the solid state during coloration. While not strictly electrochromic, they are cited here because the chemical compositions and device geometry could readily be transformed into reversibly electrochromic systems, with the possibility of re-use for testing. In several cases, the device changes transmittance chemically following contact with gaseous analysis sample, but can be refreshed electrochemically for re-use. In an interesting gasochromic–analytical application, Khatko et al. show that doping a solid layer of WO3 with different metals increases the sensitivity and selectivity to different gases.118 Thin films of tungsten trioxide respond readily and rapidly to gaseous hydrogen. Many of the WO3-based gasochromic devices cited in Table 13.1 incorporate tungsten trioxide bearing a thin layer of platinum coated on the outer surface. In such cases, the WO3 is responding to atomic hydrogen formed by a ‘spillover’ process catalysed by Pt, as described by Wittwer et al.109 13.8 Miscellaneous electrochromic applications Portable identification cards for membership or security purposes can all bear an electrochromic fragment. Obvious applications include cash-point 408 Applications of electrochromic devices machines and credit cards, etc., for which patents have already been filed.69 Other security-related applications possible with an electrochrome impregnated into (solid) paper include security devices such as vouchers, tokens and tickets – even bank notes – where fraudulent copying is likely. The only extant review of electrochromic printing is ours119 in 1995. Some applications rely on a thermo-electrochromic system, in which the speed of electrochromic coloration depends on temperature. In such applications, the device is usually so slow when cold that it is effectively switched ‘off’, even when a suitable potential is applied. As the temperature rises, so the speed of operation increases until a threshold is attained, above which the device will colour and bleach quite normally. The temperature dependence of a thermoelectrochromic device is best achieved by incorporating an ionic electrolyte for which the movement of counter ions has a high activation energy, Ea. The magnitude of Ea ensures that a relatively small change in temperature causes a substantial increase in ionic conductivity, and hence in device operation. Scrosati et al.120 were probably the first to make such a device: the electrochrome was WO3 and the electrolyte comprised poly(ethylene oxide) containing dissolved LiClO4. More recently, Owen and co-workers121 developed a thermo-electrochromic device for displaying the safety of food, and is to be positioned above shop refrigerators. The electrolyte is again poly(ethylene oxide) containing dissolved LiClO4,. The rate of coloration followed an Arrhenius-type expression at temperatures in the range 30 to –25 8C, provided the electrolytes remained amorphous (achieved by adding a high concentration of LiClO4 and also a small amount of ZnI2). So long as the rate of electro-coloration is essentially the same as the rate at which harmful bacteria multiply in the food, then the food is safe to eat while the device has not formed any colour. Conversely, the refrigerated food may be unsafe when the thermoelectrochromic ECD has changed its colour, because bacteria in the food will have had time to multiply. The Eveready Battery Company have produced a long, narrow electrochromic strip to indicate the state of charge, for use with dry-cell batteries.122 During use, the two ends of the ‘charge indicator’ strip are attached to the two termini of a battery: the level of charge within the battery is indicated via the intensity of the strip’s colour and the proportion of the strip’s length that has become coloured. The identity of the electrochrome is obscured by the prose of the patent. (The strip on Duracell batteries is based on liquid-crystal technology, and is not electrochromic.) Kojimo and Terao123 have developed an electrochromic system as a component within a DVD. Here an electrochromic layer serves as the 13.9 Multiple monitoring with electrode materials 409 multi-information-layer for an optical disk system. The active electrochrome is PEDOT (see Section 10.2). The claimed advantages of the electrochromic layer disk are in its large capacity, high sensitivity in recording, and the relative simplicity of the attendant hardware. The military in the USA are investigating fitting electrochromic panels as camouflage. The organic electrochromes are being developed by EIC Laboratories in conjunction with the Reynolds group in Florida.124 13.9 Combinatorial monitoring of multiples of varied electrode materials A hugely ingenious application of electrochromism, a major aid to multiple monitorings of electrode processes, has just been announced.125 It matches the ‘combinatorial’ methods of organic chemistry in which mixtures of products from concurrently occurring organic reactions in one pot are simultaneously analysed at the conclusion of reaction. As illustration, using a sheet of WO3 deposited onto a FTO on glass of surface resistance 50 ohm per square, the electro-oxidation of methanol by a variety of Pt catalysts was employed. The 56 electrodes undergoing tests comprised various masses (groups of 6, 12, 18 or 24 mg) of Pt-containing electrode catalysts, each of similar diameter, 3 mm. These were deposited on vitreous carbon electrodes mounted on a non-conducting poly(tetrafluoroethylene (PTFE) planar support in a 7 Â 8 matrix. The counter electrode, placed only 1 mm apart from the matrix, was the single WO3-coated sheet. The methanol reactant was at 1 mol dmÀ3 while the electrolyte was very dilute (H2SO4, 1 mmol dmÀ3), but the otherwise high resistance engendered is totally mitigated by the closeness of the two electrode sheets. The several millimetre lateral spacing between the Pt ‘dots’ confers high inter-dot resistances and thereby ‘focusses’ currents onto WO3 areas directly opposite the Pt electrodes. For a suitable fixed duration, with the same potential simultaneously applied versus the WO3 electrode to all the Pt electrodes, the relative effectiveness of each Pt electrode, as measured by the current or charge passed by each, is recorded as a small disc of blue coloration on the WO3, in a matrix corresponding to the geometry of the Pt electrodes. The intensity of coloration of each dot is directly proportional to the charge or current passed by each Pt catalyst. The simple photometric measurement of the colour intensity of each, from say a CCD camera image, bypasses separate or seriatim monitorings by voltammetry or galvanometry of each Pt electrode, by this simple and convenient quantitative method. For rapid comparative purposes, viewing by eye provides an instant estimate, if the quantity or quality of the catalyst in the monitored electrodes are arranged in sequence in the electrode mountings. 410 Applications of electrochromic devices A filter paper interposed between the electrodes acted both as a cell separator and a diffuse reflector aiding the optical monitoring by CCD camera. In the experiments reported in the paper, but not essential in application, separate currents were individually monitored for comparison with the optical imprints on the WO3, providing very satisfactory evidence of the quantitative precision of the method. (This current monitoring, being expensive of apparatus or time, would not of course be needed except perhaps introductorily once-off in actual test applications.) Several tests on smaller groups of electrodes confirmed the satisfactory operation. 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A. and Ramaraj, R. Electrochemical, in situ spectrocyclic voltammetric and electrochromic studies of phenosafranine in Nafion1 film. J. Electroanal. Chem., 424, 1997, 49–59. 87. Ganesan, V., John, S. A. and Ramaraj, R. Multielectrochromic properties of methylene blue and phenosafranine dyes incorporated into Nafion1 film. J. Electroanal. Chem., 502, 2001, 167–73. 88. [Online] at www.ntera.ie/nano.pdf (accessed 27 January 2006). 89. Shimizu, Y. and Furuta, Y. An opto-electrochemical phosphate-ion sensor using a cobalt-oxide thin-film electrode. Solid State Ionics, 113–15, 1998, 241–5. 90. Shimizu, Y., Furuta, Y. and Yamashita, T. Optical phosphate-ion sensor based on electrochromism of metal-oxide thin-film electrode. Trans. Inst. Elect. Eng. Jpn., 119, 1999, 285–9. 91. Talaie, A., Lee, J. Y., Lee, Y. K., Jang, J., Romagnoli, J. A., Taguchi, T. and Maeder, E. Dynamic sensing using intelligent composite: an investigation to development of new pH sensors and electrochromic devices. 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Galvanostatic coloration requires only two electrodes. a liquid electrolyte (that actually comprises the electrochromes) is employed in the world’s best-selling ECD.14 Fundamentals of device construction 14.7. so each contains a minimum of two electrodes separated by an ion-containing electrolyte. virtually all the type-III cells in the literature are designed to remain solid during operation. the electrolyte viscosity can be minimised to aid a rapid response. as ‘all-solid-state devices’. while imposing a constant current is said to be ‘galvanostatic’. or ‘ASSDs’.3. with two electrodes. Such solid-state ECDs have multilayer structures. so an approximation to potentiostatic control. In ECDs of types I and II.9. In practice. e. but a true potentiostatic measurement requires three electrodes (Chapter 3). as below. the compositional changes within the ECD must be readily seen under workplace illumination. The electrolyte between the electrodes is normally of high ionic conductivity (although see p.6. the Gentex rear-view mirror described in Section 13. is common. Layer (i) is an optical electrode comprising a glass slide coated with ITO.1. The electrolyte in a type-III cell is normally solid or at least viscoelastic.8. either by manipulation of current or potential.4.1 shows schematically one such solid-state device.2. 386).10 involving variations in the positions of the counter and working electrodes. high visibility is usually achieved by fabricating the cell with one or more optically transparent electrodes (OTEs). 417 . Since the colour and optical-intensity changes occurring within the electrochromic cell define its utility. and a wide range of device geometries has been contemplated.g.1 Fundamentals of ECD construction All electrochromic devices are electrochemical cells.5. Electrochromic operation of the ECD is effected via an external power supply. Applying a constant potential in ‘potentiostatic coloration’ is referred to in Chapter 3. as below. a semi-solid or polymer. 421. counter electrode. the metal being chosen both for its electronic conductivity and its aesthetic qualities.418 Fundamentals of device construction (i) Optically transparent electrode (iii) Primary electrochromic layer (v) Electrolyte Seal Wire connection Wire connection Seal (iv) Secondary electrochrome (ii) Electrode Figure 14. Layer (i) is an optically transparent electrode. it is sometimes necessary to construct an all-solid-state ECD with one of the layers precharged with mobile ions.3 below. see p. devices operating in a reflectance mode generally require the second electrode to be made of polished metal. The second electrode (ii) could be another inward-facing OTE if the device is to operate in a transmittance mode. the colour and reflectivity of the second electrode are unimportant if it is positioned behind a layer of electrolyte containing an opaque filler. (iv) is the secondary.12.14.2.11. including its ability to act as a reflector. the secondary electrochrome (iv) is deposited on the rear.13. as in Eq. Finally.1 Schematic of a typical all-solid-state. lithium metal can be evaporated in vacuo onto the surface of one electrochrome film before device assembly – so-called ‘dry lithiation’. To effect this with say WO3. this is rarely a simple procedure.1): WO3 (s) þ x Li0 (g) ! LixWO3 (s). At least one of the ‘ion-insertion layers’ will be electrochromic. so gaseous lithium diffuses into the solid layer to effect chemical reduction. Layer (v) is the electrolyte. OTE. Alternatively. The second electrode (ii) could be another inward-facing OTE. the conductive side innermost. However. an electrolyte layer (v) separates the two ion-insertion layers.16 Elemental lithium is a powerful reducing agent. as described in Section 14. multi layer electrochromic cell. Since the primary electrochrome is oxidised concurrently with reduction of the secondary (and vice versa when switching off). Layer (iii) is the primary electrochrome and layer.15.1) . (14. The other layers of an all-solid-state ECD lie parallel and between the two electrodes. In practice. as described in Section 14. The primary electrochromic layer (iii) is juxtaposed with the front OTE. (14. films were irradiated with UV light in the presence of gaseous oxygen.g.2 and 3. and helps minimise mass transport by convection (Section 3. however. . The layer of electrolyte between the two electrodes must be ionically conductive but electronically an insulator.14. the electrolyte layer is called by some an ‘ion-storage (IS) layer’. Solutions may also contain a dissolved supporting electrolyte in high concentration to suppress migration effects (see Sections 3. Quite neglecting the latter. ‘Sol–gel electrochromic coatings and devices: a review’ by Livage and Ganguli20 (in 2001) and ‘Electrochromics and polymers’ by Byker21 (in 2001). the electrochrome is dissolved in a liquid electrolyte. is claimed to enhance the electrochemical stability.17. 14.3). This practice improves the appearance of an ECD because the coloration develops at different rates in different areas in a fast device (see end of Section 13. The electrochrome approaches the working electrode through this milieu during electrochromic coloration. or – shorter – ‘ion-supplying layer’. since subsequent electrochemical extraction of Liþ in attempted re-oxidation is irreversible (see p. Somewhat similarly. Firstly. by adding a polyether such as PEO. secondly. which can be either aqueous or a polar organic solvent such as acetonitrile or a variety of other nitriles. propylene carbonate or g-butyrolactone. nickel oxide. A thickener.3. Gelling the electrolyte. while the electrolyte holds no soluble electrochrome. In type-I and type-II ECDs. it now enacts two roles (see Chapter 3).2 Electrolyte layers for ECDs Reviews of electrolyte layers for ECD usage include ‘Electrical and electrochemical properties of ion conducting polymers’ by Linford19 (in 1993).22 In type-III systems.3.21 may be added to the solution to increase its viscosity. However. is precharged using ozone. Thickening also improves the safety of a device should breakage occur.18 in practice.3. dimethylformamide. poly(vinylbutyral) or colloidal silica. the electrolyte still effects the accompanying conduction between the electrodes. Better (but possibly too late and too long) is an inclusive term such as ‘ionogenic electrolyte layer’. during coloration and bleaching.4). such as acrylic polymer. in some commercial prototypes. 142). hence artificially slowing the rate of coloration helps ensure an even coloration intensity. e. which at least allows of both roles. which represents only the former action.2 Electrolyte layers for ECDs 419 x should not exceed about 0. Thus an ‘ion-storage layer’ and an ‘electrolyte layer’ are by no means equivalent terms. for electroneutrality it supplies the mobile counter ions that enter and leave the facing solid-electrochrome layers.3). their electrical connectivity with electrochromes being critical. 14. the electrolyte was aqueous sulfuric acid of concentration 0. the latter being flexible and resistant to mechanical shock. as described below. They are apparently untested in this role. Polyelectrolytes Polyelectrolytes are polymers containing ion-labile moieties at regular intervals along the backbone. they are mechanically weak and cannot endure bending or mechanical shock.23 for example. Like organic acids À previous paragraph À these also appear not to have been tried. wholly dehydrated poly(AMPS) is not conductive. There may be a role here for mixed organic/inorganic solids like tetraalkylammonium salts with small inorganic anions. Ionic liquids somewhat below their solidification temperature might also serve but their ion-insertion capability could be questionable.2 Organic electrolytes Semi-solid organic electrolytes fall within two general categories: polyelectrolytes and polymer electrolytes. Liquid acids are rarely used today owing to their tendency to degrade or dissolve electrochromes.2. thinfilm Ta2O5 is becoming widely used. or alkalimetal salts containing large organic anions (provided that insertions only of the smaller ion are required). . ‘poly(AMPS)’. but L increases rapidly as the water content increases. and from safety considerations should the device leak.1 mol dmÀ3. In Deb’s ECD. although considerably higher potentials would be needed to drive any such ECD. However. A majority of type-III ECDs now employ inorganic solids or viscoelastic organic polymers. Such layers are generally evaporated or sputtered. Table 14.1 Inorganic and mixed-composition electrolytes Many ECDs contain as electrolyte a thin layer of solid inorganic oxide.2.420 Fundamentals of device construction Type-III ECDs operating with protons as the mobile ions can contain aqueous acids. in which the proton-donor moiety is an acid. these might evince greater mechanical robustness. 14. A popular example is poly(2-acrylamido-2methylpropanesulfonic acid).1 lists some polyelectrolytes used in solidstate ECDs. Solid organic acids of amorphous structure might serve similarly. The molar ionic conductivity L of polymers such as poly(AMPS) depends critically on the extent of water incorporation. 70 42.35. Added inert salt acts to form an inorganic electrolyte layer.48. Thus.1. 65.53.74. A white layer also dispenses with any need to tailor the optical properties of the secondary layer.64.55 56. triflic acid CF3SO3H.27. through to longer polymers which behave as rigid solids.61.69.51. 24 25.72 73.63.45.75.44.14.79. so polymers range from liquid. as solvent.68.81. such as TiO2 to enhance the contrast ratio in displays. PVC Refs.52 47.49 50.82.36.40 40 41 42.46 47.39.2 Electrolyte layers for ECDs 421 Table 14.28 29. a device which could not show any observable change in colour unless the rear electrode was screened from view by incorporating such an opaque filler in the intervening electrolyte. It is quite common for polymeric electrolytes to have an opaque white ‘filler’ powder added. poly(propylene glycol) – PPG.54.30.67.58.78.66.38. Duffy and co-workers9 have described a device in which WO3 forms both the primary and secondary electrodes.43. Electrolyte Inorganic electrolytes LiAlF4 LiNbO3 Sb2O5 (inc.71.83 84.80.57. Table 14. The inclusion of particulate TiO2 does not seem to affect the response times of such ‘filled’ ECDs.31 32 33. Solid ion-conducting electrolytes for use in ECDs.37.59. at low molecular weight. PEO Poly(vinyl chloride). or poly(vinyl alcohol) – PVA.76. PMMA (‘Perspex’) Poly(2-hydroxyethyl methacrylate) Poly(ethylene oxide).1 lists a selection of polymer electrolytes and polyelectrolytes used in solid-state ECDs.85 Polymer electrolytes Polymer electrolytes contain.56.26. The viscosity of such polymers increases with increasing molecular weight. neutral macromolecules such as poly(ethylene oxide) – PEO. but the photocatalytic activity .34.60. Common examples include LiClO4.77. HSbO3) HSbO3 based polymer Ta2O5 (including ‘TaOx’) TiO2 (including ‘TiOx’) H3UO3(PO4) Á3H2O (‘HUP’) ZrO2 Organic polymers NafionTM Poly(acrylic acid) Poly(AMPS) Poly(methyl methacrylate). or H3PO4.62. The stability of electrolyte layers is discussed in Section 16.95 discuss such ‘terminal effects’ in ECDs. such as spectacles.1 Transparent conductors The most common choice of OTE is indium–tin oxide as a thin film sputtered onto glass. Otherwise. substrates of higher electronic conductivity are attainable. the electrolyte-with-filler ploy (previous paragraph) is used. ‘Transparent and conducting ITO films: new developments and applica˚ tions’ by Granqvist and Hultaker89 (in 2002). Reviews of materials for OTE construction for electrochromic devices include ‘Transparent conductors: a status review’ by Chopra et al. It is common but expensive for polished platinum to act as both mirror and supporting electrode in a reflecting ECD. a gradient of potential forms across the electrode surface: the potential near the external contact is higher than elsewhere.92.90 (in 2003). as in information displays. leading to a non-uniform image. whereas devices operating in a reflective sense. but are slightly yellow.422 Fundamentals of device construction of TiO2 may accelerate photolytic deterioration of organic materials such as the electrolyte. and the intensity of colour will often be more intense near the external electrical contact.57.93 Its UV-visible absorption is less than 2% and its thermal infrared reflectance exceeds 90%. ‘Transparent electronic conductors’ by Lynam87 (in 1990). Thus the . As a consequence of IR drop. Another common choice is fluorine-doped tin oxide (FTO).71. ‘Transparent conductive electrodes for electrochromic devices – a review’ by Granqvist88 (in 1993). 14. The best conductivity of ITO is about 20 O per square. must of course operate with a second OTE as the rear electrode. Indium–tin oxide is electrically semiconducting rather than metallic. The relatively high innate resistance of semiconducting ITO (or other OTEs) can cause complications such as IR drop94 and the so-called ‘terminal effect’. which comprises FTO on glass. visors or whole windows. Devices operating in a transmissive mode. Ho et al. and ‘Frontier of transparent oxide semiconductors’ by Ohta et al. an example being so-called ‘K-glassTM’ from Pilkington. 14.3.3.3 Electrodes for ECD construction All ECD devices require at least one transparent electrode. so the electrochromic coloration or image formed during coloration is generated at different speeds across the electrode surface.86 (in 1983). do not.91. goggles. 104 can also fulfil this goal.111. Such deposition is now relatively easy but. including the conductive ITO and both electrochromes.4 Pa (without oxygen) and the relatively high power density of 2 Â 104 W mÀ2. considerably affecting ECD response times.100. 14. Glass and polyester substrates exhibited different growth rates and samples deposited onto glass substrates showed better film-to-polymer adhesion.107.3. nevertheless.2 Opaque and metallic conductors The most common choice of rear electrode is platinum or Pt-based alloys. The stability of ITO electrodes is discussed in Section 16. Bertran et al.112. Clearly any such device will need to be enclosed within thin sheets of an appropriate polymer. Nevertheless.110) Several allpolymer ECDs have also been fabricated.3 Electrodes for ECD construction 423 conductivity of OTEs is relatively poor. must be durable. The idea of flexible ECDs is attractive for lightweight.119 (Such flexible displays could also be photo-electrochromic. The highest electrical conductivities were achieved in depositions using low Ar pressures of 0.111.118 and polyester. temporary electrochromic window coverings and the like.106. The second electrode need not bear a separate layer of electrochrome: redox-active counter electrodes can themselves ‘absorb charge’ with the .5.107.6.105.112.10 Other materials have also been advocated: Liu and Richardson120 suggest an alloy of antimony and copper.105. ITO for counter-electrode use has been deposited on sheet plastics such as Mylar.114 describe the fabrication and applications of such electrochromic ‘foils’.113 Azens et al.117 poly(ethyleneterephthalate) or PET78.2. the differing deposition conditions result in ITO layers with poorer electrical conductivity to that made on glass. Furthermore.14.97 or depositing an ultra-thin layer of precious metal on the electrolyte-facing side of the electrochrome.109.110.101 Thin films of Cr2O3102 or MgF2103. all the layers.3. which is known to lower the electrical resistivity116 albeit with a slight decrease in optical transmittance. Methods adopted include incorporating an ultra-thin layer of metallic nickel between the electrochrome and ITO.106. see Section 10.98.96 see p.115 overcame this problem by incorporating small amounts of silver within their ITO films.99. A review (1995) that addressed the use of polymeric substrates for electrochromic purposes is the short work by Antinucci et al. since any cracks formed by bending cause irreversible insulating discontinuities that lead to certain device failure. 349.78 The deposition conditions must be milder when ITO is to be deposited onto polymeric substrates rather than on glass.115.5.108. Ways of combating IR drop and terminal effects involve increasing the electronic conductivity. Ultrafine electrodes are screen-printed onto a non-conductive glass substrate. A novel design by Liu and Coleman113 has recently been described which employs a ‘side-by-side’ structure. Eng. and need therefore to remain hidden behind a layer of electrolyte containing an opaque white filler. In devices containing a liquid or semi-solid electrolyte. Mater. with electrochrome deposited above and between them. 144–8. A. by permission of Elsevier Science.121.424 Fundamentals of device construction Transparent film Gel electrolyte Layer of electrochrome Dispersion of conductive metal oxide Carbon ink Silver-carbon ink Polymeric substrate and support Counter electrode Insulator Working electrode Insulator Counter electrode Figure 14.) accompaniment of counter-ion intercalation.3 ECDs requiring no transparent conductor Transparent conductors are not always needed.124 screen-printed carbon black. 286. and Coleman.3. J. the separation between the two electrodes can be maintained by introducing flat or spherical .2. see Figure 14. For example. 14. are clearly as important as (in some views more important than) the operation of the parts taken individually.79.126 All these counter electrodes remain black during electrochromic operation.123 or ‘carbonbased’ materials. ECDs have been constructed in which charge is intercalated into a counter electrode of carbon: examples of such counter electrodes include ‘carbon’29.122. and the mounting materials. J. P.4 Device encapsulation The process of assembling the components of a commercial device.2 ‘Side-by-side’ design of a screen-printed electrochromic display device: schematic representation illustrating the arrangement of the electrodes. Sci.125 and graphite. 2000. ‘Nanostructured metal oxides for printed electrochromic displays’. 14. (Figure redrawn from Liu. Baucke. Proc. of Gentex Corporation) recently stated. K. Baucke. the device must be sealed. 1. Rivista della Staz. Sci. Syrrakou et al. F. operation and application. 4. 21. 285–92. 11–13.128 and Gentex designed a complicated type of clip. 3. PPG employed an adhesive layer to coat the edges of their devices. G. 179–87. 16.21 He discusses the use of polymers as electrolytes within ECDs in ref. 119–22. Proc. For example. K. Sol. Bange. Electrochem. Reflectance control of automotive mirrors. F. 5. J. This latter polymer performs the role ‘reasonably well’. 1983. . K. K. F. 1991. F. F. yet can push the top of the panes together till they break. SPIE. It is regrettable – but perhaps inevitable in view of industrial competitiveness – how many reports of actual devices (prototype and in production) fail to divulge details of device encapsulation. to maintain the precisely defined distance between the two parallel electrodes. Schott Information. B. Of the few mentioned in the literature.. since the weight of liquid causes the bottom of the device to swell. Baucke. Displays. T. acting in a similar manner to the minute spherical beads of constant diameter employed in fabricating an LCD. G. Electrochromic mirrors with variable reflectance. Chem. Br. Baucke. leakproof seal to encapsulate a type-I or -II ECD is not a trivial problem: Byker (at that time.129 to withstand hydrostatic pressures. and Duffy. 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Conversely ‘back-wall’ illumination.1 Introduction Systems that change colour electrochemically.1. but only on being illuminated.8 as shown by arrow (a) in Figure 15. or sandwiched together with a photoconductor.2 Direction of beam The direction of illumination during cell operation is important. A separate photoconductor or other photocell serves as a switch. or the actual electrochromic electrode surface itself could be a photoconductor. arrow (b).1. In the first. 433 .4 Few reviews of the topic are extant: the chapter on photoelectrochromism in our 1995 book5 is dated. Two bases of photoelectrochromic operation are available. electrochromic or photochromic when only one of these stimuli is applied).7 in 2001. filter or trigger. see refs. Figure 15. 1. If the incident beam traverses a (minimum) distance in the cell prior to striking the photoactive layer.15 Photoelectrochromism 15. the potential required to evoke electrochromism is already applied but can act only through a photo-activated switch. colour changes are mentioned. are termed photoelectrochromic (cf.2. 15. Such photo-activated systems contrast with photo-driven devices. Relatively few photoelectrochromic systems have been examined as such. and ‘All-polymeric electrochromic and photoelectrochemical devices: new advances’ by De Paoli et al. Such photoconductors were traditionally semiconductors like amorphous silicon but.3 Device types 15. Back-wall illumination is used only if undesirable photolytic processes occur with front-wall illumination of the cell. many organic photoconductors have become candidates. into the delocalised energy levels .2 Photoconductive layers Photoconductive materials are insulators in the absence of light but become conductive when illuminated. be it photovoltaic or photoconductive. as below. The mechanism of photoconduction involves the photo-excitation of charge carriers (electrons or holes) from localised sites. which triggers a microprocessor or similar element which in turn switches on the already ‘poised’ cell. or from bonds in the valence band. operates with the beam directed from behind the cell. 15. so traversing more cell material before reaching the photosensitive layer.3. Front-wall illumination generally yields superior results since additional absorptions by other layers within the ECD are minimised. 15. Illumination from direction (a) represents ‘front-wall’ illumination and (b) ‘backwall’ illumination. Such an arrangement is not intrinsically photoelectrochromic but is switched on by photocontrolled circuitry: the cell itself could be any straightforward electrochromic system.434 Photoelectrochromism Optically transparent layer hν (a) hν () b igh t-sensitive L layer ECD Figure 15. The switch operates by illumination of a suitable photocell.3.1 Devices acting in tandem with a photocell The simplest circuits for photoelectrochromic device operation comprise a conventional electrically driven ECD together with a photo-operated switch. in recent years.1 Schematic representation of a photoelectrochromic cell. 15.10. but electrons enter via the photoconductor. a photoconductive layer is incorporated within the electrochromic cell. This arrangement has the difficulty that. so coloration stops. . During electrochromic coloration or bleaching.2 would allow for strong. the photoconductor might conceivably be located between the electrochrome and the electrolyte layers (Figure 15.3). allowing for electrochromic coloration. A few photoelectrochromic devices have been fabricated with semi-transparent photoconductors. metallic electrodes to be employed as the photoconductor support.11 In the first.2 shows an ECD with a photoconductor (light-sensitive layer) positioned between an optically conducting substrate and a film of electrochrome. ions from the electrolyte enter the electrochromic layer as in normal operation (see Section 1.12. Current ceases in the dark.3 Device types Transparent conductor Conductor (Pt or otherwise) 435 hν Light-sensitive layer Primary electrochrome Electrolyte layer Secondary electrochrome Figure 15. the arrangement in Figure 15.2 is preferred. Electrochromic cells may employ a layer of photoconductive material in one of two ways. Back-wall illumination of the ECD in Figure 15. a photoconductive component is positioned outside the ECD and acts as a photocell switch: illumination of the photoconductor completes the circuit.13 In a variation of this latter arrangement. ECDs operating with a photoconductor will probably have to operate in a reflective mode.15.10. although note that the attendant physical stresses of continual ion movement through the photoconductor could lead to its eventual disintegration. Accordingly. made a photoelectrochromic device in which the photoconductor was a thin. In the second arrangement. Several workers12.2 Schematic representation of a photoelectrochromic cell: frontwall illumination of an ECD containing a photoconductive layer between the transparent conductor and the primary electrochrome layer. Figure 15.4 on page 11). The mobilised charges can be driven by an externally applied potential. forming the conduction band.17 of the NREL laboratories in Colorado.14 Here the photoconductor would need to be completely ion-permeable.16.9 yielding a current that can effect electrochromism. since most photoconductors are somewhat opaque. 3). Section 13. The primary electrochrome was WO3. . this represents a photo-electrochromic ‘smart-glass’ window (cf. semi-transparent layer of hydrogenated amorphous silicon.9 mA cmÀ2.92 V.28 poly (o-methoxyaniline)7 and the thiophene-based polymers poly(3-methylthiophene)29 and PEDOT.21 Hagen. The electrochrome in Kobayashi’s cell was methyl viologen27 (cf.27 and their co-workers. which is deemed adequate to colour a lithium-based device with a response time of less than one minute. It yielded a photocurrent of 3.25. Other polymer electrochromes than poly(aniline) have been used as photoconductive layers within photoelectrochromic devices: poly(pyrrole). . Chapter 11). many intrinsically conducting polymers are photoconductive:20 photoelectrochromic devices employing poly(aniline) as a photoconductor have been made by Fitzmaurice. with ionconducting LiAlF4 as the electrolyte. The primary electrochrome was WO3. Yoneyama19 labelled his device ‘a photo-rewritable . Their window covering could be produced on a flexible polymer substrate.3 Schematic representation of a photoelectrochromic cell: frontwall illumination of an ECD containing a photoconductive layer between the primary electrochrome layer and the electrolyte. The photoconductor within the display was hydrogenated amorphous silicon carbide. with a variant of ruthenium tris(bipyridyl) as a photosensitiser. Photoelectrochromic ‘writing’ has been suggested by several authors.26.7 .23 Kobayashi24. allowing it to be affixed to the inside surface of a window.e. In fact. and Ni–W oxide as the counter electrode.22 Ileperuma. image’. NREL called the device a ‘stand-alone photovoltaic-powered electrochromic window’. i. The writing appeared light yellow on a black background. Similarly.436 Transparent conductor Photoelectrochromism Conductor (Pt or otherwise) hν Primary Light-sensitive electrochrome layer Electrolyte layer Secondary electrochrome Figure 15. NREL made a photoelectrochromic prototype that could be bleached with a light pen:18 they envisaged use in light-on-dark viewgraph projection or possibly within children’s toys. and an open-circuit potential Voc of 0. For example. perhaps owing to their tendency to photodegrade. its actual magnitude is not a problem because an external bias can be applied until the cell is ‘poised’. and has been incorporated into many photoelectrochromic devices.30 particulate titanium dioxide was the photoactive material.38 GaP. Prussian blue (PB) has also been used as the electrochrome in photoelectrochromic devices. indeed.33 TiO234.36) Other photoelectrochromic cells operating via photovoltaism include WO3 on CdS. The electrochrome was a thin layer of PEDOT polymer.15. Hagen’s et al.’s22 photoelectrochromic device employed a nanocrystalline layer of TiO2 as a photoconductor.14. PB has been used with WO3 to make a photorechargeable battery. (Indeed.42. For example. when supplementing the external bias. a cell comprising tungsten trioxide deposited on TiO2 requires a bias31 since the photovoltage generated is insufficient. 15. The photovoltaic layer is not consumed in this process.3 Photovoltaic materials A photovoltaic material produces a potential when illuminated. and the IÀ / I3À redox couple was incorporated as the electron mediator. The ionic charges needed to accompany the electrochromic transition enter the film from juxtaposed electrolyte or an electron mediator. Among the few in the literature are Methylene . in addition to poly(aniline).41 Films of indium hexacyanometallate grown in a bath containing colloidal TiO2 are also photoelectrochromic.20 -bipyridine). with a photovoltage coming from polycrystalline n-type SrTiO3.3 Device types 437 Titanium dioxide (in its anatase allotrope) is one of the most intensely studied photo-active materials. Illumination of such a poised cell generates a photovoltage which. from a process similar to the excitation of electrons within a photoconductor but with an internal rectifying field to provide a driving force on the electrons. is sufficient to enable the coloration process to proceed.43 Few monomeric organic systems claim photoelectrochromism.32. The photovoltage produced need not be large. The coloration process was photosensitised using a dye based on ruthenium tris(2.) The photoactive TiO2 need not be a continuous layer: in the device fabricated by Liao and Ho.40. yielding a device with an overall coloration efficiency of 280 cm2 CÀ1. even if the photovoltage is itself too small to effect the required redox chemistry.35 or CdS36 as the photolayer. a ruthenium complex acted as a photosensitiser. Their ‘self-powered’ cell was able to modulate its transmission over the whole visible spectral region.37 GaAs. as above.3.39 or on TiO2. (The illuminating lamps simulated solar spectral intensities. 438 Photoelectrochromism Blue44 (I) and the spirobenzopyran45 (II), both of which undergo reversible photoelectrochromic transitions at TiO2 electrodes. N H3C + N CH3 S Cl– I N CH3 N CH3 II O NO2 CH3 CH3 CH3 15.3.4 Photogalvanic materials Photogalvanic materials generate current when illuminated. The photogalvanic material is generally consumed during the photoreaction14 which inevitably causes the (photo-operated) write–erase efficiency to be poor. Photoelectrochromism in the cell WO3jPEO, H3PO4 (MeCN)jV2O5 is believed to operate in a photogalvanic sense14 since the brown colour of the V2O5 layer disappears gradually during illumination. Curiously, the cell is still photoelectrochromic even after the colour of the V2O5 has gone and an alternative cathodic reaction (possibly catalysed consumption of oxygen, or reduction of VO2?) must be envisaged. 15.4 Photochromic–electrochromic systems Some systems are not photoelectrochromic in the sense defined above, yet do not function as electrochromic or photochromic alone. For example, De Filpo and co-workers devised ‘photoelectrochromic systems’ comprising either ethyl viologen46 or Methylene Blue (I) in solution,46,47 together with a suitable electron donor such as an amine. Irradiation e.g. with a He–Ne laser induces an electron-transfer process with concomitant formation of colour. The colour-forming process is straightforwardly photochromic. The colour may be erased electro chromically. We adopt the compound adjective ‘photochromic– electrochromic’ for those systems that colour and bleach via the alternate use of photochromism and electrochromism. Yoneyama et al.19,48 developed a photochromic–electrochromic cell functioning in the opposite sense to that of De Filpo’s, so the colour bleached photo chromically and was regenerated electro chromically. Yoneyama’s photo chromic–electrochromic device employed poly(aniline) as the colourchanging material. The polymer film contained entrapped particles of TiO2, enabling the poly(aniline) to act as both photoconductor and colour-changing 15.4 Photochromic–electrochromic systems 439 material. The device was assembled with the polymer as one layer in a multilayer ‘sandwich’. Illumination effected photoreduction of the poly(aniline) with concomitant bleaching of the polymer’s dark-blue colour. During illumination, the film was immersed in aqueous methanol, the methanol acting as a sacrificial electron donor. In this example, the dark blue colour of the poly(aniline) was subsequently recoloured electro chromically. The colour of the poly(aniline) did not bleach completely during illumination, presumably because the photoconducting properties of poly(aniline) decrease in proportion to the extent of the bleaching; it is the oxidised form of the polymer that photoconducts. The poly(aniline) film can only photoconduct through those areas that are illuminated, so images, rather than uniform blocks of tone, may be formed if the light source passes through a patterned mask or photographic negative. To this end, Yoneyama et al.19 illuminated their photochromic–electrochromic poly(aniline) film through a photographic negative to form the notable image in Figure 15.4. Kobayashi et al.49 have also generated impressive images by illuminating a film of poly(aniline) through a photographic negative. Figure 15.4 Photoelectrochromic image generated on a thin film of poly(aniline)–TiO2: the film was immersed in a solution of phosphate buffer (0.5 mol dmÀ3 at pH 7) containing 20 wt% methanol as a sacrificial electron donor. The film was illuminated through a photographic negative with a 500 W xenon lamp for 1 min. (Figure reproduced from Yoneyama, H., Takahashi, N. and Kuwabata, S. Formation of a light image in a polyaniline film containing titanium(IV) oxide particles. J. Chem. Soc., Chem. Commun., 1992, 716–17, with permission of The Royal Society of Chemistry.) 440 Photoelectrochromism References 1. Hirochi, K., Kitabatake, M. and Yamazaki, O. Electrochromic effects of Li–W–O films under ultraviolet light exposure. J. Electrochem. Soc., 133, 1986, 1973–4. 2. Buttner, W., Rieke, P. and Armstrong, N. R. Photoelectrochemical response of GaPc-Cl thin film electrode using two photon sources and two illumination devices. J. Electrochem. Soc., 131, 1984, 226–8. 3. Stilkans, M. P., Purans, Y. Y. and Klyavin, Y. K. Integral photoelectrical properties of thin-film systems based on photosensitive conductor and photochromium material. Zh. Tekh. Fiz., 61, 1991, 91–7 [in Russian]. The title and abstract are cited in Curr. Contents, 31, 1991, 1969. 4. Teowee, G., Gudgel, T., McCarthy, K., Agrawal, A., Allemand, P. and Cronin, J. User controllable photochromic (UCPC) devices. Electrochim. Acta, 44, 1999, 3017–26. 5. Monk, P. M. S., Mortimer, R. J. and Rosseinsky, D. R. Electrochromism: Fundamentals and Applications, Weinheim, VCH, 1995, ch. 12. 6. Gregg, B. A. Photoelectrochromic cells and their applications. Endeavour, 21, 1997, 52–5. 7. De Paoli, M.-A., Nogueira, A. F., Machado, D. A. and Longo, C. All-polymeric electrochromic and photoelectrochemical devices: new advances. Electrochim. Acta, 46, 2001, 4243–9. 8. Rauh, R. D. Cadmium chalcogenides. Stud. Phys. Chem., 55, 1988, 277–327. 9. Duffy, J. A. Bonding, Energy Levels and Inorganic Solids, London, Longmans, 1990. 10. Shizukuishi, M., Shimizu, S. and Enoue, E. Application of amorphous silicon to WO3 photoelectrochromic device. Jpn. J. Appl. Phys., 20, 1981, 2359–63. 11. Yoneyama, H., Wakamoto, K. and Tamura, H. Photoelectrochromic properties of polypyrrole-coated silicon electrodes. J. Electrochem. Soc., 132, 1985, 2414–17. 12. Bullock, J. N., Bechinger, C., Benson, D. K. and Branz, H. M. Semi-transparent a-SiC:H solar cells for self-powered photovoltaic-electrochromic devices. J. NonCryst. Solids, 198–200, 1996, 1163–7. 13. Bechinger, C. and Gregg, B. A. Development of a new self-powered electrochromic device for light modulation without external power supply. Sol. Energy Mater. Sol. Cells, 54, 1998, 405–10. 14. Monk, P. M. S., Duffy, J. A. and Ingram, M. D. Electrochromic display devices of tungstic oxide containing vanadium oxide or cadmium sulphide as a lightsensitive layer. Electrochim. Acta, 38, 1993, 2759–64. 15. Deb, S. K., Lee, S.-H., Tracy, C. E., Pitts, J. R., Gregg, B. A. and Branz, H. M. Stand-alone photovoltaic-powered electrochromic smart window. Electrochim. Acta, 46, 2001, 2125–30. 16. Benson, D. K. and Branz, H. M. Design goals for a photovoltaic-powered electrochromic window covering. Sol. Energy Mater. Sol. Cells, 39, 1995, 203–11. 17. Deb, S. K. Recent developments in high efficiency photovoltaic cells. Renewable Energy, 15, 1998, 467–72. 18. Gao, W., Lee, S.-H., Benson, D. K. and Branz, H. M. Novel electrochromic projection and writing device incorporating an amorphous silicon carbide photodiode. J. Non-Cryst. Solids, 266–9, 2000, 1233–7. 19. Yoneyama, H., Takahashi, N. and Kuwabata, S. Formation of a light image in a polyaniline film containing titanium(IV) oxide particles. J. Chem. Soc., Chem. Commun., 1992, 716–17. References 441 20. Inganas, O., Carlberg, C. and Yohannes, T. Polymer electrolytes in optical ¨ devices. Electrochim. Acta, 43, 1998, 1615–21. 21. Sotomayor, J., Will, G. and Fitzmaurice, D. Photoelectrochromic heterosupramolecular assemblies. J. Mater. Chem., 10, 2000, 685–92. 22. Li, Y., Hagen, J. and Haarer, D. Novel photoelectrochromic cells containing a polyaniline layer and a dye-sensitized nanocrystalline TiO2 photovoltaic cell. Synth. Met., 94, 1998, 273–7. 23. Ileperuma, O., Dissanayake, M., Somusunseram, S. and Bandara, L. Photoelectrochemical solar cells with polyacrylonitrile-based and polyethylene oxide-based polymer electrolytes. Sol. Energy Mater. Sol. Cells, 84, 2004, 117–24. 24. Kobayashi, N., Yano, T., Teshima, K. and Hirohashi, R. Photoelectrochromism of poly(aniline) derivatives in a Ru complex–methylviologen system containing a polymer electrolyte. Electrochim. Acta, 43, 1998, 1645–9. 25. Kobayashi, N., Hirohashi, R., Kim, Y. and Teshima, K. Photorewritable conducting polyaniline image formation with photoinduced electron transfer. Synth. Met., 101, 1999, 699–700. 26. Kim, Y., Teshima, K. and Kobayashi, N. Improvement of reversible photoelectrochromic reaction of polyaniline in polyelectrolyte composite film with the dichloroethane solution system. Electrochim. Acta, 45, 2000, 1549–53. 27. Kobayashi, N., Fukuda, N. and Kim, Y. Photoelectrochromism and photohydrolysis of sulfonated polyaniline containing Ru(bpy)32þ film for negative and positive image formation. J. Electroanal. Chem., 498, 2001, 216–22. 28. Inganas, O. and Lundstrom, I. Some potential applications for conducting ¨ ¨ polymers. Synth. Met., 21, 1987, 13–19. 29. De Saja, J. A. and Tanaka, K. Photoelectrochemical cells with p-type poly(3-methylthiophene). Phys. Stat. Sol. A, 108, 1988, K109–14. 30. Liao, J. and Ho, K.-C. A photoelectrochromic device using a PEDOT thin film. J. New Mater. Electrochem. Syst., 8, 2005, 37–47. 31. Ohtani, B., Atsumi, T., Nishimoto, S. and Kagiya, T. Multiple responsive device: photo- and electrochromic composite thin film of tungsten trioxide with titanium oxide. Chem. Lett., 1988, 295–8. 32. Ziegler, J. P., Lesniewski, E. K. and Hemminger, J. C. Polycrystalline n-SrTiO3 as an electrode for the photoelectrochromic switching of Prussian blue films. J. Appl. Phys., 61, 1987, 3099–104. 33. Ziegler, J. P. and Hemminger, J. C. Spectroscopic and electrochemical characterization of the photochromic behaviour of Prussian blue on n-SrTiO3. J. Electrochem. Soc., 134, 1987, 358–63. 34. DeBerry, D. W. and Viehbeck, A. Photoelectrochromic behaviour of Prussian blue-modified TiO2 electrodes. J. Electrochem. Soc., 130, 1983, 249–51. 35. Itaya, K., Uchida, I., Toshima, S. and De La Rue, R. M. Photoelectrochemical studies of Prussian blue on n-type semiconductor (n-TiO2). J. Electrochem. Soc., 131, 1984, 2086–91. 36. Kaneko, M., Okada, T., Minoura, H., Sugiura, T. and Ueno, Y. Photochargeable multilayer membrane device composed of CdS film and Prussian blue battery. J. Electrochem. Soc., 35, 1990, 291–3. 37. Stikans, M., Kleparis, J. and Klevins, E. J. Photoelectric characterization of solidstate photochromic system. Latv. P. S. R. Zinat. Akad. Vestis. Fiz. Tekh. Zinat. Ser., 4, 1988, 43. 442 Photoelectrochromism 38. Reichman, B., Fan, F.-R. F. and Bard, A. J. Semiconductor electrodes, XXV: the p-GaAs / heptyl viologen system: photoelectrochemical cells and photoelectrochromic cells. J. Electrochem. Soc., 127, 1980, 333–8. 39. Butler, M. A. Photoelectrochemical imaging. J. Electrochem. Soc., 131, 1984, 2185–90. 40. Opara Krasˇ ovec, U., Georg, A., Georg, A., Wittwer, V., Luther, J. and Topic, M. Performance of a solid-state photoelectrochromic device. Sol. Energy Mater. Sol. Cells, 84, 2004, 369–80. 41. Hauch, A., Georg, A., Abumgartner, S., Opara Krasˇ ovec, U. and Orel, B. New ¨ photoelectrochromic device. Electrochim. Acta, 46, 2001, 2131–6. 42. de Tacconi, N. R., Rajeshwar, K. and Lezna, R. O. Photoelectrochemistry of indium hexacyanoferrate–titania composite films. J. Electroanal. Chem., 500, 2001, 270–8. 43. de Tacconi, N. R., Rajeshwar, K. and Lezna, R. O. Preparation, photoelectrochemical characterization, and photoelectrochromic behavior of metal hexacyanoferrate–titanium dioxide composite films. Electrochim. Acta, 45, 2000, 3403–11. 44. de Tacconi, N. R., Carmona, J. and Rajeshwar, K. Reversibility of photoelectrochromism at the TiO2/methylene blue interface. J. Electrochem. Soc., 144, 1997, 2486–90. 45. Zhi, J. F., Baba, R., Hashimoto, K. and Fujishima, A. Photoelectrochromic properties of spirobenzopyran derivative. J. Photochem. Photobiol., A92, 1995, 91–7. 46. Macchione, M., De Filpo, G., Nicoletta, F. and Chidichimo, G. Improvement of response times in photoelectrochromic organic film. Chem. Mater., 16, 2004, 1400–1. 47. Macchione, M., De Filpo, G., Mashin, A., Nicoletta, F. and Chidichimo, G. Laser-writable electrically erasable photoelectrochromic organic film. Adv. Mater., 15, 2003, 327–9. 48. Yoneyama, H. Writing with light on polyaniline films. Adv. Mater., 5, 1993, 394–6. 49. Kobayashi, M., Hashimoto, K. and Kim, Y. Photoinduced electrochromism of conducting polymers and its application. Proc. Electrochem. Soc., 2003–17, 2003, 157–65. 16 Device durability 16.1 Introduction Like all other types of display device, mechanical or electronic, no electrochromic device will continue to function indefinitely. For this reason, cycle lives are reported. The definition of cycle life has not been conclusively settled. Even by the definition in Section 1.4, reported lives vary enormously: some workers suggest their devices will degrade and thereby preclude realistic use after a few cycles while others claim a device surviving several million cycles. Table 16.1 contains a few examples; in each case the cycle life cited represents ‘deep’ cycles, as defined on p. 12. Some of these longer cycle lives were obtained via methods of accelerated testing, as outlined below. It is important to appreciate that results obtained with a typical threeelectrode cell in conjunction with a potentiostat can yield profoundly different results from the same components assembled as a device: most devices operate with only two electrodes. The results of Biswas et al.,9 who potentiostatically cycled a thin film of WO3 immersed in electrolyte, are typical insofar as the electrochemical reversibility of the cycle remained quite good with little deterioration. Their films retained their physical integrity, but the intensity of the coloration decreased with the number of cycles. Some devices are intended for once-only use, such as the freezer indicator of Owen and co-workers;10 other applications envisage at most a few cycles, like the Eveready battery-charge indicator.11 Clearly, degradation can be allowed to occur after no more than a few cycles with applications like the latter. Conversely, applications such as a watch display will need to withstand many billions of cycles without significant deterioration – a stringent requirement. Devices can fail for one or more of three related reasons: failure of the conductive electrodes; failure of the electrolyte layer; and failure of the 443 444 Device durability Table 16.1. A selection of cycle lives of electrochromic devices, reported as number of cycles survived. Primary electrochrome WO3 WO3 WO3 WO3 WO3 WO3 Electrodeposited bismuth Secondary electrochrome Poly(aniline) Prussian blue ‘VOxHy’ (CeO2)x(TiO2)1Àx Nickel oxide Iridium oxide Prussian blue Cycle life 20 000 20 000 30 000 50 000 100 000 10 000 000 50 000 000 Ref. 1 2,3 4 5 6 7 8 electrochromes. The durability of individual electrochromes is discussed in their respective chapters. Here the durability of transparent electrodes is discussed in Section 16.2; that of electrolyte layers is discussed in Section 16.3; and general methods of enhancing electrochrome durability are outlined in Section 16.4. Finally, Section 16.5 contains details of how cycle lives are assessed for complete, assembled, devices. 16.2 Durability of transparent electrodes The first reason for device failure is breakdown of an optically transparent electrode, OTE. The most common cause of OTE degradation is decomposition of ITO, which occurs readily in acidic solutions: the oxides within the ITO layer are themselves reduced when not in contact with solution. Such reduction both decreases its chemical stability and increases its electrical resistance:12,13,14 while the oxidised form of ITO is chemically stable, reduced ITO is unstable and rarely bears the strains of repeated redox cycling because it dissolves readily in aqueous acids.15 Indeed, in aqueous solution, the subsequent reaction of over-reduction to form metallic tin is difficult to stop.16,17,18 For this reason, some workers tentatively suggest that all moisture must be excluded rigorously from the electrolyte of an ECD.19,20,21 In the study by Bressers and Meulenkamp22 it was shown that a thin layer of metallic indium forms on the surface of the ITO during reduction, possibly facilitating the observed dissolution in water-containing electrolytes, which is faster if the ITO is partially reduced.14,23 Even ITO in contact with semi-solid poly(ethylene) oxide (PEO) electrolyte can deteriorate: Radhakrishnan et al.15 show how ITO electrodes in contact with PEO deteriorate after repeated cycling, both in terms of their conductivity 16.4 Enhancing the durability of electrochrome layers 445 and transparency. Their XPS studies of ITO electrodes clearly show the metallic impurities being expelled into the PEO. The change of composition leads to eventual diminution of the ITO conductivity, with concomitant decrease in ECD cycle life. 16.3 Durability of the electrolyte layers The second reason for device failure is electrolyte breakdown. Most organic polymers have relatively poor photolytic stability, particularly when in solution or intimately mixed with an ionic salt, as is typical for ECD usage.24 Hence long-term solar irradiation will inevitably cause ECD breakdown. In an ECD operating in a reflectance mode, such as a mirror, a particularly photounstable primary electrochrome can be placed adjacent to the reflective back electrode rather than situated on the front OTE, i.e. behind the electrolyte and secondary electrochrome layers (provided both have a high optical transparency for all wavelengths). It is quite common for polymeric electrolytes to be ‘filled’ with an opaque white powder such as TiO2, to enhance the contrast ratio of the primary electrochrome. While the inclusion of particulate TiO2 does not affect the response times of an ECD, its photo-activity (particularly if the TiO2 is in its anatase form) will significantly accelerate photolytic deterioration of organic polymers.25,26,27 A further danger associated with devices operating via proton conduction is underpotential (catalysed) generation of molecular hydrogen gas, formed according to Eq. (16.1), which both removes protonic charges and also forms insulating bubbles of gas inside the ECD: 2Hþ (soln) þ 2eÀ ! H2 (g). (16.1) Areas of the electrode adjacent to such a bubble are insulated, thereby disabling the device. 16.4 Enhancing the durability of electrochrome layers Great care is needed when the electrolyte in an ECD is a layer of rigid inorganic solid, since most type-III electrochromes change volume during redox changes, owing to chemical volume changes and the volume decrease of a dielectric in a field, electrostriction. Thin-layer WO3, for example, expands by about 6% during reduction28 from H!0WO3 to H1WO3 (see pp. 87, 129). Most type-III devices comprise two solid layers of electrochrome. Therefore the extent of chemical-volume change in either layer could be approximately the same, 446 Device durability changing in a complementary sense with one expanding while the other contracts; however, electrostriction acts only by contracting. Placing an elastomeric (semi-solid polymer) electrolyte between the two ECD electrochromic layers considerably cushions the strains engendered by expansion and contraction in a two-layer ECD. To confirm the scope for cushioning the effects of electrostriction, Scrosati and co-workers29 note how the stresses engendered by ion insertion/egress within the cell WO3jelectrolytejNiO are similar in both the WO3 and NiO layers. (Methods of quantifying the stresses induced during electrochromic activity are discussed on p. 130). Many electrochromes dissolve in, or are damaged by prolonged contact with, the electrolyte layer. To protect the interphase between the electrochrome and electrolyte, several studies suggest depositing a thin, protective film over the electrochrome film. Enhancement of chemical stability will obviously extend the cycle life of an ECD. There are a number of examples of this practice. Thus for example, Haranahalli and Dove30 deposited a thin semi-transparent layer of gold on their WO3, so protecting it from chemical attack, and incidentally also accelerating the speed of device operation. Similarly, in one study, Granqvist and co-workers31 deposited a thin film of tungsten oxyfluoride on solid WO3, and in another, deposited a thin protective layer of electron-bombarded WO3 onto a layer of metal oxyfluoride.32,33 Yoo et al.34 coated WO3 with lithium phosphorus oxynitride. Deb and co-workers35 coated V2O5 with a protective layer of LiAlF4, which exhibited improved durability and electrochemical charge capacity during 800 write– erase cycles. Long et al.36 electrodeposited poly(o-phenylenediamine) onto porous MnO2. He et al.37 accelerated the operation of a WO3-based device with a surface layer of gold nanoparticles. In the same way, the perfluorinated polymer Nafion1 has been coated on Prussian blue,38 tantalum pentoxide39 and tungsten oxide,40,41 in each case improving the stability and enhancing the electrochromic characteristics of these electrochromes. While such barrier films protect the electrochrome from chemical degradation, they also hinder the motion of the counter ions needed for charge balance. Movement across the electrolyte–electrochrome interphase will therefore increase the ECD response time. However, the acceleration noted by Haranahalli and Dove30 and He et al.37 follows because a potential was applied to the gold layer, itself conductive, covering the respective electrode surfaces. 16.5 Durability of electrochromic devices after assembly Studies describing the durability of assembled electrochromic devices are to be found in the following reports: ‘Durability evaluation of electrochromic 16.5 Durability after assembly 447 devices – an industry perspective’ by Lampert et al.42 (in 1999), ‘Failure modes of sol–gel deposited electrochromic devices’ by Bell and Skryabin43 (in 1999) and ‘A feasibility study of electrochromic windows in vehicles’ by Jaksic and Salahifar44 (in 2003). Many individual studies of device durability are extant. For example, Nishikitani and co-workers45 of the Japanese Nippon Mitsubishi Oil Corporation employed a variety of weathering tests on electrochromic windows designed for automotive applications. Their two-year outdoor weathering tests suggest their ECDs are highly durable, but as expected, outdoor exposure ultimately causes device degradation. Many workers consider it impractical to wait for results from such trials in real time, so considerable effort has been expended in the use of accelerated testing methods. A few exemplar studies below will suffice. Asahi Glass failed to detect deterioration in their lithium-based ECD windows stored during 1000 hours of testing at 70 8C and 90% humidity. Similarly, Deb and co-workers46 of the National Renewable Energy Laboratory (NREL) in Colorado, USA, used accelerated testing conditions on several prototype ECDs. Deb and co-workers47 have also described the way such devices were illuminated with a high-intensity UV lamp to mimic the effects of long-term exposure to solar light, and concluded that the effects of long-term exposure can indeed be mimicked readily within a considerably shortened time – even a few days – with concomitant savings in overheads. However, the applicability of the NREL results is limited since all devices were fabricated by anonymous US companies. Sbar et al.48 of SAGE Electrochromics in New Jersey, USA, tested electrochromic architectural windows during external exposure at test sites in New Jersey and the Arizona desert. Their accelerated testing methods included electrochemical cycling over a range of temperatures, with changes in illumination and/or humidity. They concluded that their windows showed ‘good switching performance’. Colour Plate 7 shows similar testing of a Gentex window. Skryabin et al.49 present a more fundamental, partly theoretical, assessment of testing and quality control criteria for large devices. The durability of electrochromic devices was assessed from three perspectives: mimicking the device behaviour with an equivalent circuit; arranging the external electrical connections; and optimizing the switching procedure. Their principal conclusion was that mimicking is difficult: ECDs are ‘inherently complicated devices’. Nagai et al.,6 also using a programme of accelerated testing, concluded that their device, GlassjITOjNiOjTa2O5 (electrolyte)jWO3jITOjadhesive-filmjGlass was capable of 105 cycles at 60 8C. The environment for a specific application. which clearly dictates the speed at which the device must operate. The report ‘Evaluation criteria and test methods for electrochromic windows’ (1990. 5. UK. no drastic temperature changes occurred during electrochromic operation.55 W mÀ2 at 340 nm. 424. they conclude:47 . viability requirements for minimum acceptable performance encompassing depth of colour. A robust seal also protects against oxygen ingress. These authors elaborate the requirements in a subsequent paper. rather wider variations around the globe are to be expected. Such variations scarcely cover conditions in other countries. nor when the sky is continually overcast. Holidaymakers in Skegness. USA. let alone other US states. Within these criteria. switching time. so such devices are sealed to minimise changes in internal humidity levels. and goes some way toward generating a template for reproducible testing of electrochromic devices. Device operation was discussed in terms of likely variations in temperature in the USA. 2. The brief list above demonstrates the way criteria for study can differ considerably: many studies do not even state the criteria chosen. and mechanical shock. the concentration of water needed for optimum performance needs to remain within a narrow. rapid temperature changes were only observed when the sun appeared from behind a cloud. especially by UV light. consider electrochromic devices for large-area architectural applications. would need more assurance of ECD robustness against the weather. a severe test in view of the peak daylight intensity in Miami of 0. in Santa Rosa. 4. windows were deemed acceptable if they coloured to a contrast ratio of 10:1 and were capable of 20 000 cycles. Device encapsulation is described on p. chromatism and durability. sunny days. The effect of deterioration owing to solar exposure. Device durability is then defined in terms of the following five criteria:47.50 of The Optical Coating Laboratory. The UV was provided by a xenon light source of output 0. and during thunderstorms. The upper and lower temperatures of operation (they chose À40 8C to 50 8C).47 but excessive use of unexplained abbreviations detracts from clarity. Stresses induced in a device by ‘thermal shock’ as it cools and warms rapidly. They conclude that no major stresses born of thermal shock occur during clear. and so on). but made widely available in 1999) by Czanderna and Lampert51 was compiled to address this problem.8 W mÀ2. for example.448 Device durability Mathew et al. Having noted that variations on the above test methodology will depend on many factors (the choice of electrochrome. desirable range. the intended application. or was partially obscured. 3. The effect of additional stresses such as changes in humidity. Devices operating via proton movement may need water.51 1. customer specifications. Strong frames are required for rough handling or percussive incidents. device construction. In the authors’ Californian climate. S. G. S. Mahapatra. 11. A. On the mechanism of ITO etching: the specificity of halogen acids. 4429–32. 781–96. Energy Mater. Sol. 10. 317–31. Nagai. K.[but] that it is possible to predict the service lifetime of ECWs. and Aegerter. Baarslag. Budd. Hugot-Le Goff. R.-C. 2321–6. Mater. K. Acta. M. 6. 157–63. B. Biswas. M. 1997. Bailey. Sun. Poly[oxymethylene]oligo(oxyethylene) for use in subambient temperature electrochromic devices.. Electrochem. 49. J. E. 268–71. Y. Cells. Ganguli. J. and Livage.. Puetz. 1993. K. 139–46. Solid State Ionics.-C. 2003.. Chem. A. and Kuwana.. Mater. 48. Ziegler. 165. 3. and Mandale. P. 56. Electrochem.. B. J. Scholten. 9. 14. Liu. P.References 449 Our major conclusions are that substantial R&D is [still] necessary to understand the factors that limit electrochromic windows [ECWs] durability. Anal. 1099–104. US Patent 05458992. 3227–35. J. 44. 54.. Cycling and at-rest stabilities of a complementary electrochromic device based on tungsten oxide and Prussian blue thin films. W. Soc. L. Solid State Ionics.. Electrochim. Bernard. A. A. N. 1992. 4. J.. Source of instability in solid state polymeric electrochromic cells: the deterioration of indium tin oxide electrodes. M. 1981. D. All solid state electrochromic devices on glass and polymeric foils. R. 1976. E. Sol. C. P. C... D. 139. 140.. Pramanik. Sol. 1999. M. W. . Polym. and Thomas. 2003. Soc. Ho. .. 1998. M. 741–50. . Soc. 7. and Balderson. Unde. Sol.-C. M.. A. 1995. 1995. 44.. and Scholten. 142. ‘The accelerated tests are reasonable for the evaluation of the lifetime of EC glazing but have not been verified with real time testing. Recent advances in inorganic electrochromics. J. 5. They add. L. Radhakrisnan. . 39. Colley. S. Chem. 181–9. Durability of electrochromic glazing.. M. P. Optical and electrochromic properties of sol–gel WO3 films on conducting glass. Energy Mater. M. and Howard. 48. R. 471–4. Elaboration and study of a PANI/PAMPS/WO3 all solid-state electrochromic device. C. G. A. A. 2000. Cells.. Cycling stability of an electrochromic system at room temperature. Int. Sol. Lett. M. Beni.. Lechner. Ho. 8. 309–19. Energy Mater. Eveready Battery Company. N. 2.. Influence of water on the electrochemical properties of ðCeO2 Þx ðTiO2 Þ1Àx and WO3 sol–gel coatings and electrochromic devices. Fujihira. 13. 3–4. McMeeking. K. J. 15. J. and Zeng. Electrochim.’47 References 1. Electrochemical and surface characterics of tin oxide and indium oxide electrodes. J. J. 57. M. 1999. Owen. J. On the mechanism of ITO etching in halogen acids: the influence of oxidizing agents. K. Electrochromic thin film state-ofcharge detector for on-the-cell application. Armstrong. Applications of reversible electrodeposition electrochromic devices. 1995. and van den Meerakker. Phys. 12.. and Saitoh. C. T. R. Heusing. Sol. Acta. Electrochem. Cells.. 371–6. S.. 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Influence of titanium dioxide pigments on the photodegradation of poly(vinyl chloride). and Chapman. 68. Solids. 185–96. and Tseng. 28. 2283–4. R. B. Optics. Solid State Ionics. Gesenhues. Sol. Thin-film tin-doped indium oxide counter electrode for electrochromic applications. Electrochromism of fluorinated and electron-bombarded tungsten oxide. 1025–8. A.. and Granqvist. Sci. J. 457–63. Polymer Degradation and Stability. 2000. 1980. A. M. J.. Appl. C. 17. 1984. 91. 2006. J. and Man. 145. E. 22. Shiyanovskaya. Marcel Dekker. Laminated electrochromic windows based on nickel oxide. 23. 25. J.. M. 1988. (ed).-M. 31. G. J.. 109–14. E. Egerton. E. characterization and electrochromic properties of NiOxHy thin film prepared by a sol–gel method. Improved electrochromic devices with an inorganic solid electrolyte protective layer. Handbook of Polymer Degradation. J. Passerini. B. Reductive ion insertion into thin-film indium tin oxide (ITO) in aqueous acidic solutions: the effect of leaching of indium from the ITO. Soc.. 81–7. Appl. R. G. A. 1086–96. I.. Scrosati.. New York. 24. A. Gabrusenoks. E. Electrochromic behaviour in ITO and related oxides. Lett. J. and gel electrolytes. J. Mater. Ingram.. S. Goldner. and Gorenstein. K. Solid State Ionics. T. 165. M. J. Barczynska. 19. and Dove. P. P. Hermann.. and Meulenkamp. Su. M. Le Bellac. A. Non-Cryst. J.. 2nd edn.. SPIE. B. 477–84. 2225–30.. Phys. A. J. . P. Granqvist. Y. Solid State Ionics... P. 1995. Electron. M. 2004. Azens. and Steele. Appl. Hjelm. 1994.. Seward. Electrochromism of W-oxide-based films: some theoretical and experimental results. 101–7. Soc. 1995. 1998. Polym. Monk. S. 33. and Bartlett. M. Goldner. 27. 21. E. M. Influence of a thin gold surface layer on the electrochromic behavior of WO3 films. P. Liu. C.-H. Duffy. Pentjuss. Yoo. The effect of moisture on tungsten oxide electrochromism in polymer electrolyte devices. G. and Wills. Lawson. 1996. A. P. C. M. 2006.450 Device durability 16. Cheong. G. and White. C. J. C. 1985. S. Energy Mater. Sol–gel preparation and photocatalysis of titanium dioxide. J. Proc. Cells. Solid State Ionics. 2003. 34. A. B. 119–26.. A. Lee. L. and Sung. 791–3. V... 18. Atom motion in tungsten bronze thin films... 141. Lim. Wong.. C. Christensen.. H. Electrochem. Svensson. A. Catal. and Granqvist. A. 113–15. 26. S. 1968–74... R. Degrad.. J... Stab. Thin Solid Films. 333–40. 2284–5. J. 28–30. 90. K. J. J. 148–50. 2000. Green. Halim Hamid. E.. 2531. G.. 32. 1978. Pentjuss. and Monk. Haas. Azens. D. 187. 1998. Synthesis. Tracy. U. 24. 29. K. Sol. 437–48. 56. 2003. W. L. Durability evaluation of electrochromic devices – an industry perspective. Solids.. Yang. S. K. L. S. Shinada. Toya. C. Cumbo. Skryabin.. 342–6. 2003. C. J. Durability issues and service lifetime prediction of electrochromic windows for buildings applications. Jorgensen. Badding.. G. Electrochim.. Jaksic. Testing and control issues in large area electrochromic films and devices. Current state of the art for NOC–AGC electrochromic windows for architectural and automotive applications. 47.. Accelerated durability testing of electrochromic windows.. Ngo. A.. Sol. C. M. A. 1999. Kudo. Huang. Y. J. 79. 143. S. Y. Agrawal.. Lampert. Sol. A. D. 37. Sol. Schulz. 42. Vogelman. Soc. P. Failure modes of sol–gel deposited electrochromic devices. C. D. 371–83. and Lampert. Cortez..-G. T.. Sbar. Sol... and Salahifar. and Tseung. 47.. Cells.-G. Chem. Kishimoto. 83. Nano. R. Proc. 165. K. and Ikeda.nH2O film. A. 50. A.. T. Ma. Energy Mater. Kubo. Benson. . D. 1996.. Sol. Sol.. Bell. 209–16. 449–63. H.. W. 38.. T.. and Urbanik. A feasibility study of electrochromic windows in vehicles. T. G. K. Frost.References 451 36.. 40. 2703–8. Sargent. Acta. 56.. O’Brien. S. S. G. and Hichwa. Kobayashi. M.. 129–32. Baertlien... Y. Sol. J. protective coatings of poly(o-phenylenediamine) as electrochemical porous gates: making mesoporous MnO2 nanoarchitectures stable in acid electrolytes.. Cao. P. N. M. 2. Lett. H.. R. C. Ind. Ultrathin. Sol. Solid State Ionics. Michalski. Chen. Czanderna. Cells. A. K. Sci. Sol.. 1155–61. Shen. 2003. 1999.. C. 2001. Mathew... 1996. and Nagai. I.. Energy Mater. I. 46. Benson. B. Imafuku. and Ramaraj. Long. R. J. N. Y. V.. and Deb. Czanderna.. Electrochim. Energy Mater. S. M. Sol. R. 1990: SERI/TP-255-3537. 514. L. and Skryabin. K. 44. Tracy. J. 1999. M. Solid State Ionics. and Bell. J. W. Large area electrochromics for architectural applications. 1997. C. 49. 3195–202. and Rolison. 218... Electrochem.. M. and Watanabe. 497–9. Rhodes. E. 48. L. Energy Mater. B. 45. Budziak. 51. J. W. K. Y. J. 1999. T. Raksha.. B. Cells. Reversible electrochromic performance of Prussian blue coated with proton conductive Ta2O5. Progress toward durable. Laby. A. Electroanal. 44. Y. 1995. Enhanced electrochromism of WO3 thin film by gold nanoparticles. Mater. Non-Cryst. G. P. 135–43. 3. Acta.. Improvements in the life of WO3 electrochromic films. J. 56... Cells. A. N. W. Role of acidity on the electrochemistry of Prussian blue at plain and Nafion film-coated electrodes. Zhang. A.. H. 43. J. 41. Czanderna. J. Acad. C. I. Sapers. 56. John. 1999. Nishikitani. and Tseung. Cells. Sone. K. J. Chem. Akita. 1992. Young. K. 321–41. 3203–9. C. C. 39. Effect of Nafion dispersion on the stability of Pt/WO3 electrodes. E. L. J. J. T. R... C. 419–36. 1999. 44. K. P. Lahaderne. 409–23. P. cost effective electrochromic window glazings. 107. Energy Mater. as cited in ref. Tracy. Evans. and Dep. and Yao. A.. D. H. Zhang. He. 148 vanadium pentoxide. ECD battery charge indicator. 154 nickel oxide. 425 acetonitrile. 200 aluminium–nickel oxide. 200 all-polymer devices. 86. 15 as photoconductor. 53 shape of bands. 152 forming titanium dioxide. 142. 111T in tungsten trioxide. 141. 50 AEIROF. 254. 193 applications. 112 in colouring metal oxides. 88. 96 of pure solids. coloration efficiency negative. 88 made by vacuum evaporation. 152. 421T antimony–copper alloy. 156 degradation of. 81. 359. 443 camouflage. 158 agar. 38 admittance. 38. 86 to electron transfer. 193–5. 36–7. 274. 436 anatase. 87. windows. 67 acidity constants Ka. 40. ECD electrolyte. 384 amorphisation. 55 anodic reactions. 423 antimony-doped tin oxide. 195. ECD electrolyte thickener. substrate. 173. 196. 203 nickel–aluminium. 84 coefficient.Index absorbance. 161 niobium pentoxide. 390T. 160 aluminium–cobalt oxide. 52 change in. 332 air conditioning. 409 452 . 181 spin-coated products. 140 to effect crystallisation. 184 alloy Inconel-600. optical. oxides. 156 alkoxides CVD precursors. 362 optically passive. 89 cerium oxide. 83. 131 forming molybdenum trioxide. 408 to diffusion. 88. 312 aniline–polypyridyl complexes. 262. oxide mixture on. 417 alpha particles. 177 rhodium oxide. 448 weathering. 196 aluminium–nickel alloy. 419. 401 AIROF. centre of colour diagram. ix. 408. 111T to bacterial growth. 53 accelerated ECD testing humidity. 168 CVD product. 135 tungsten trioxide. 438 achromatic. as gelling agent. 131 iridium oxide. 398 Airbus. 256 annealing endothermic process. 419 activation energy. 93 in nickel oxide. 200 aluminium–silicon–cobalt oxide. 36. 204 amino-4-bromoanthaquinone-2-sulfonate. 185 anodic coloration. 447 xenon arc. 175 molybdenum trioxide. see titanium dioxide Anderson transition. 313 aniline black. 4 acrylic powder. 156. 3 aniline. 349. 81 amorphous silicon. 332 all-solid-state-devices. 447. 158 iron oxide. 149. 448 acetate silicone. 385. 140. definition. 408 to counter-ion movement. 351 AGFA. 135 spray pyrolysis product. 43. 193 amorphous. ECD. 193 as ECD electrolyte. 166 cobalt oxide. 46 antimony pentoxide as electrochromic host. 307 ANEEPS. 111. 108. 384 aminonaphthaquinone. 47 activity. ECD encapsulation. 155. 99. tungsten–molybdenum oxide. 401–4 advertising boards. 363 electric paint. 398. 376 response time. 57T Azure B. 404–5 medicine. 83. 352T. 93. 408 ECD charge indicator. photoelectrochromism. 422 visors. 166 response time. 402. 375 type-II electrochromism. 147. 166 formation via evaporation. 265 light modulation. 166. 198 aromatic amines. 358 radical. 408 toys. 400. 200. 444T electrodeposition of. manganese oxide. 346. 44. 363. 166 coloration efficiency. 57T batteries. 402 mobile phone screens. 115 bacteria growth. 356. 374 contrast ratio. 105. 149. 408 watch faces. 380 bipolaron in poly(thiophene)s. coloration efficiency.Index displays. 8. ECD mirror azulene. 384 bismuth oxide. coloration efficiency. of Prussian blue and tungsten trioxide. poly(thiophene)s. 433 BEDOT. 402 smart cards. 404–5 solar-energy storage. 266 temperature management. 408 tokens. 55. 110–11. 8. 316 of PEDOT. 147 bis(dimethylamino)diphenylamine. 443 electrochromic paper. 382 benzyl viologen. 176 Bayer AG. 149. 400 X-ray reflector. 398. 408 reactions. 305–6 coloration efficiency.and credit cards. see Prussian green betaines. 323 Baytron M. 422 motorcycle helmets. coloration efficiency. o-. 400 shutters. 379. 151. 363 Stadsparkasse Bank. 323 Baytron P. 400 Schott Glass. 402 NanoChromics. 397 Aramid resin. recrystallisation. 265. 402 data display. 11. 401. 407 computer screens. 265 palmtop computer screens. 358 di-reduced. 150. 3 band conduction. 408 cash. 166 bismuth as secondary electrochrome. 54. 305 ECD. 56T. 402. 344T. 376 near infrared absorption. 364 games. cashpoint machines.40 -. 106. 5 biological membrane potentials. 398 sunglasses. 148. 363 tickets. 374–6. ix. 406 optical data storage. 381 benzoquinone. 401 aircraft. 405–6 eye wear. 395–7 optical attenuator. 4. 270 Pilkington Glass. 363. 400–1 Asahi Glass. 92. 176 Bell Laboratories. 363. 357 Berlin green. 28 Asahi Glass. 81 band structure. 363. 400 Gentex. 304. 98. 320 in tungsten trioxide. 149. p-. 377T–378T charge transfer. 265. 326 Beer–Lambert law. 265 mirrors. 347. 408 aryl viologens. 356. 27. 270 shutters. 408 ECD like a secondary. 400 Boeing ‘Dreamliner’. 57T back potential. 381. 402 laptop computer screens. 306 cycle life. rf sputtering. see Applications. 362. 408 display. 398 chromogenic glazing. 402 vouchers. 322 Basic Blue 3. 400 car sun roof. 360 automotive mirrors. 149. 146. 4 Bacteriorhodopsin. 265 windows. 54 photo-chargeable. 326 BEDOT-NMeCz. 313 Azure A. 306 . 422 Airbus. 363. 363. 355. 396. 400 PPG Aerospace. 363. 3 biphenyls. 447 asymmetric viologens. 397 dimmable laminates. 11. 363. 363 Flabeg Gmbh. 375 type-I electrochromism. 375 453 Arrhenius equation. 102. 402 bank notes. 402. 152 bandgap. 401. 167 dry cell. 53. 323 beam direction. 437 rechargeable. 436 transport terminus screens. 307. 422 fibre-optics. 14. goggles. 29 benzoquinones benzoquinone. 400 optical attenuator. activation energy. 363 iPod. 385–7. 33 electrochromes. 403 . 61 oxides and cobalt ion. 166 spin coating. 188 potentiostatic. 402. 376 carbon electrochromes. 405 Boeing ‘Dreamliner’. definition. 203 cerium–tungsten oxide. 103 of molybdenum trioxide. 55 charge dispersibility. substrate. 109 rate. 359 viologens. 391 N-carbazylcarbazole. 60. 108. 11. 353. 127. 305 electron mediation. Prussian blue. 95. 379T N-ethylcarbazole. 111 charge electronic. ECD electrolyte. 194 as secondary electrochrome. 398 carbazoles. 81. 332–3 catalytic silver paint. 424 castable films. 166 optically passive. 15. 166 cerium oxide. 359 methyl viologen. 185 cathodic. for nickel oxide. 29. 135 physical vapour deposition. ‘carbon based’. 195 cerium–titanium–titanium oxide. 359 ferrocyanide. 194. 194 via dc magnetron sputtering. 272 cathode ray tube power consumption. 383 bronze.454 Index cathodic coloration. 46–8. poly(aniline). 316 bleaching chemical. 179 cerium–tin oxide. see coloration efficiency cells. 103 of lithium tungsten trioxide. 153 types type-II electrochromes. 26. 46 CE. 417 electroneutrality in. 305 see also diamond. 95 butyl viologen. 194 coloration efficiency. 381T type-II electrochromes. 362. and colour analysis. 103. 303. 379T. 271. 376. 27 of tungsten trioxide. 305 bithiophenes. 64 British Fenestration Rating Council. 164. 342–5. 409 capacitance effects. 81. fullerene and graphite carbon. 60–1. 52 charge capacity. 379T N-phenylcarbazole. 52 car mirrors. 193–5 formation via dip coating. 186. 305 screen printed carbon. 352T g-butyrolactone. 383 solubility product. see applications. ECD. 34 aqueous. 105–8 Faughnan and Crandall. 194 optically passive. 166–7. o-. 42 aromatic amines.) electrochemistry. 79–115 type-III electrochromes. 444T chemical diffusion coefficient. 333 cerianite. 166 optical properties. ECD application. 38 cellulose acetate. 166 cerium vanadate. 150. 82. in Faughnan and Crandall model of coloration. 379T immobilised. 37–9 electrochemical. 144 Butler–Volmer equation. 82 of metal oxide. 105–8 Green. 359 heptyl viologen. 203 cerium–titanium–zirconium oxide. 400 brightness. 54. 313. 397 bromoanil. 167. 55 cathodic-arc deposition. 419 cadmium sulfide. 200 cerium–praseodymium oxide. 320 conducting polymers. 127 charge transfer. 50 electrolytic capacitors. 61. coloration efficiency positive. 82. 105–8 self. 304. windows. 374 complexation of cyanophenyl paraquat. 359 characteristic time. 283 catechole. 79–115 blueprints. 303. 145 orbitals. 166 chemical diffusion coefficient. 202 cerium–nickel oxide. see saturated calomel electrode camouflage. 113. 304. see tungsten–cerium oxide cerium–vanadium–titanium oxide. 15 television. 201 cerium–titanium oxide. 200 charge density. 303. of vanadium pentoxide. 42. as electron mediator. 359 intervalence. 135 spray pyrolysis. 167 electrochromic host. ECD. 202 bismuth (cont. 169 in iron–titanium oxide. electro-irreversibility of. of. mirrors car sun roof. 194 EXAFS. 203 cerous ion. 359 models. 382. viologens. 194 annealing of. 151 of sodium tungsten trioxide. 135. 437 calomel reference electrode. 203 cerium–zirconium oxide. for vanadium pentoxide. composite with poly(aniline). 42 faradaic. depositing Prussian blue. ECD application. 85T electrochemistry of. 167 gasochromic. 84T. 167 rf sputtering. 195 sonication. 135 spray pyrolysis. 397 precursors alkoxides. 63. 204 cobalt–nickel–iridium oxide. 105 charging. 167 and batteries. 169–70 secondary electrochrome. 397 products impure. 88 cobalt acetylacetonate complex. 47 ions through oxides. 101. 135–6. 195–6 incorporating gold. 85T zinc phthalocyanine. 102. and colour analysis. 133. write–erase efficiency. 423 chromium phthalocyanine. 346 chemical vapour deposition annealing needed. 167 optical properties. 195. 194 cobalt–tungsten oxide. 85T chemical potential. 195. see also ECD. 170 cobalt oxyhydroxide. 52 chemical diffusion coefficient. 168 electrodeposition. 182 tungsten trioxide. 85T ions through oxide mixtures cerium–titanium oxide. 172T ECDs of. 168 charge transfer in. 56T. see derivatised electrodes chloranil. 88 Liþ in LixWO3. 195T ions through WO3. 169 cobalt oxide. 168–9 electrochromic host. 174 molybdenum trioxide. 203 . 57 chronoamperometry. 50 clusters. 90. 196 coloration efficiency. 172T evaporation. 200 chromogenic glazing. sensor for. 85T poly(isothianaphene). 64–71 455 chromatic colour. 168 cobalt hydroxide. 184 tungsten trioxide. 135. 64 chromium oxide. 85T ions through conducting polymers poly(carbazole). 407T terminal effect suppressor. 168. 406 lithium deficient. 168. 85T cobalt oxide. 195 via sol–gel. 88. 167–70. double-layer. 85T titanium dioxide. 101. 93. 84T. 195T vanadium pentoxide. 84 electrode reactions when. 168. 141. windows chromophore. 169. 62. 190. 87. 135. 169 chemical diffusion coefficient. 161 praseodymium oxide. 60 in tungsten trioxide. 172T. 59 CIE. 148. 195 cobalt–aluminium–silicon oxide.Index in oxide ion. 169 peroxo species. 91 molybdenum trioxide. 382 chloride ion. 195T indium–tin oxide. 168 in permanganate. 383 p-. 195T ions through phthalocyanines lutetium phthalocyanine. 397 of mixtures of metal oxide. 167 electrochemistry. 57. 170 electrochemistry of. 62 and colour analysis. 197 tungsten–cobalt oxide. 131–2 iron oxide. 195. 168 formation via electrodeposition. c-WO3 in a-WO3. 169. 261 cobalt tartrate complex. 151. 203 chromium–molybdenum oxide. 134 spin coating. 172T gasochromic applications. tungsten–molybdenum oxide. 95 resistance to. 179 tantalum oxide. 153 nickel oxide. see Commission internationale de l’eclairage circuit element. 150. 96. 85T. 2 chronoabsorptometry. 199 chromium–nickel oxide. 168 cobalt phthalocyanine. 195 annealing. definition. 397 nickel oxide. 382 o-. 83 peaks. 132 process is two-step. 90–1 definition of. 167 sol–gel. 131 hexacarbonyls. 84 and insertion coefficient. 397. gasochromic. 167 formation via electron-beam evaporation. 85T niobium pentoxide. 168 cobalt–aluminium oxide. 167 coloration efficiency. 406 Chroma meter. 168 rf sputtering. 204T formation via CVD. 132. 195T and diffusion coefficient. 60 rate of. 195T coloration efficiency. 172T dip coating. 131 of metal oxides. 131 chemically modified electrode. 383 cycle life. 382. 99–101 chronocoulometry. 81. 178. 87. 131. 112. 168. 261 chromium–iron–nickel oxide. 46. 94 chemical tethering. 172T oxidation of cobalt. of Hþ in WO3. 99. 59T Prussian white. 195T colloid. 102. 205 . 198 iron–niobium oxide. 176 molybdenum trioxide. 304. 104. 101–2 galvanostatic 96–8. 194 cobalt–aluminium oxide. 56T. 57T poly(3. 200 niobium–iron oxide. 42. 56T. 88. 172 iridium oxide. bismuth. 158 iron oxide. 113. 113. 199. 178. 148T. 202 vanadium–samarium oxide. 172T copper oxide. 308 samarium–magnesium hydride. enhances rate. 358. 169. 57T Nile Blue. 202 zirconium–tantalum oxide. 57T organic electrochromes. 55 composite CCE. 103 type-II electrochromes. 199T–201T nickel–titanium oxide. 80 two-electron process. 70. 57T Safranin O. via sol–gel. 15. 52–3 after potential stopped. 305–6 organic polymers PEDOT. 192 tungsten–vanadium oxide. 62–71 and light sources. 15 intrinsic.4-propylenedioxypyrrole). 115 Green. titanium dioxide. 57T Basic Blue ix. 197. 57T Toluylene Red. 59T metal hydrides magnesium–samarium hydride. 57T Indigo Blue. mixtures. 66 extrinsic. 195 indium–tin oxide. 102. 102. 188 colorimetric theory. 139. 54–60 metal hexacyanoferrates Prussian blue. 56T. 56T. 181 tantalum oxide. 79–115 coloration efficiency. 189. 134 coloration. 362. 201 vanadium pentoxide. 203 organic dyes Azure A. 55 cathodic coloration. 113. 104. 185T tungsten trioxide. 102–3 coloration rate. 55. 167 cobalt oxide. 56T. methyl viologen. 184 tailoring. 417 three-electrode. 199 tungsten–niobium oxide. 195 chemical diffusion coefficient. 80 hysteresis. 96–8. 2 and colour analysis. 113. 193. 57T Resorufin. 115 Faughnan and Crandall. 354–5 pulsed. 201 samarium–vanadium oxide. 57T poly(3.4-propylenedioxythiophene). 205 metals. 202 titanium–molybdenum oxide. 54–60. Duffy. 80 involves ionisation of water. 56T bismuth oxide. 201T niobium–tungsten oxide. 202 nickel–tungsten oxide. 33. 139 and conjugation length. 70. 57T Azure B. 155T. 56T. 195. 111. 70.456 Index tungsten oxyfluoride. 55 anodic coloration. 184. 149 and flux. 59T. Z is negative. 56T. Z is positive. 115 WIV and WV. 57T coloration models Bohnke. 64 cobalt–tungsten oxide. 361. 183 titanium dioxide. 101–2. 115 Ingram. mixtures. 437 poly(3. 443 potential step. 56T. 99–101. 166 chromium oxide. 148. 378 fullerene. 260 quantum mechanical. 62 colour analysis. 75 mixing oxides.4-ethylenedioxy thiophenedidodecyloxybenzene). 56T. 199T nickel oxide. 365 pulsed current. 57T Resazurin. 60 and extinction coefficient. 175T. 55 sign of. 81 potentiostatic. 56T. 55 viologens. 201 rhodium oxide. 87. 190T metal oxyfluorides titanium oxyfluoride. 417 iridium oxide. 201T manganese oxide. 56T. 157 metal oxides involves counter ions. 349. 199. 199T–201T. 57T phthalocyanines lutetium phthalocyanine. 163 vanadium pentoxide. 146. 199T iron oxide. 99. 154. 56T. 147. 10. 200 nickel oxide. 57–60 definition. 181T. 157 phase changes in. 56T. 334 tungsten trioxide. 308 metal oxides. Monk. 143 involves water. 191. 57T cyanines. 363. 91–6. 201T molybdenum–tin oxide. 201 tungsten–molybdenum oxide. 200 niobium pentoxide. 175. 110. 57T Methylene Blue. 306 mixtures of metal oxide cerium–titanium oxide. 165T niobium pentoxide. 16. 114 chemical. 147 conductivity indium–tin oxide. 410. 385. 417 contact lithography. 188 2þ Mg through molybdenum oxide. 317 conductivity. 334 complementarity. 76 absent in solid-state ECDs. 102 counter electrode. 290 complexes. 263 silver paint. 169 fullerene. 142. coordination complexes composite coloration efficiency CCE. 81. 313–14 p-doping. 87–8 swapping of. 80. 9. 174 iron oxide mixtures. 181. 198 ITO. ECD applications. 346. 194 cobalt oxide. 348. 352. 170 electrodeposition. 303. 170–2 as secondary electrochrome. 146 deuterons through tungsten trioxide. 294 copper oxide. 173. 75. 333. 179 tin oxide. 87 F– through iridium oxide. 87 through solid film. metal oxides. 170 specular reflectance. 148. 312 oxidative polymerisation. 57 composites. 112. 113 phthalocyanine complexes. 303. 167 cerium–titanium oxide. see. 399 combinatorial chemistry. 205 tungsten trioxide. 314 and coloration efficiency. 408 movement. 82–5. 388. 188 during coloration of metal oxides. 197. 41 complementary electrochromism. 43. 57–60 determined at wavelength maximum. 152 nickel oxide. 67 colour formed. 305. 60 construction. effect of. 114. 409 Commission internationale de l’eclairage (CIE). metal-oxide mixtures. 305 conducting polymers. 332–3 electrochromic. 357–8. 363 concentration gradient. 190 colour space. 197. 205 titanium dioxide. 97. 11 history. 110. 163 niobium pentoxide. 41. 198 tin oxide. 53 colour manipulation. 376. 349. during cell operation. see secondary electrochrome counter ion activation energy. 156. 33 size. 365 computer screen. 258 contrast ratio. 9. 113. 142 protons in tantalum oxide. 59 determined with reflected light. 90. 445 convection. 113. 62 composites. 44. 104. 63. 174 iron oxide mixtures. 142. 104. 186. 157 Csþ through tungsten trioxide. 170 copper hexacyanoferrate. 44 coordination complexes. 349 through bands. 352–4 Agþ through tungsten trioxide. 151. 384. 150. 62. 315 type-III electrochromes. 127. 183 of amorphous and polycrystalline WO3. 253 metal-to-ligand charge transfer. 142. 304 iron oxide. 178 praseodymium oxide. 81. 60 high resistance. 130. 205 . electronic. 304–5 graphite. 306 sol–gel. 48 electrochromic. 303. 62. 70 colour diagram. 45. 98. ECD. 174 iron oxide mixtures. 51. 82 conjugation length. 80 rate of. 157 Kþ through iron oxide. 312 and sensors.Index conducting polymers. 312 colour analysis of. 146. achromatic centre. 332 and electrolyte fillers. 312 and field-effect transistors. 312–34 and electroluminescent organic light-emitting diodes. 14. 76. 146 Liþ through cerium oxide. 253 457 copper ethoxide. 184 tungsten trioxide. 253 intervalence charge transfer. 354 Coulomb’s law. 62 Prussian blue. 312 and solar-energy conversion. 103 viologens. 63. 422 ionic. 64–71 colour tailoring. 96. 55. 171 electron mediator. 62. 146 CN– through iridium oxide. 400 all-polymer ECD. 172 electrochemistry. 332–3 comproportionation tungsten trioxide. 146 Naþ through iron oxide. amount of. 115. 152 tungsten trioxide. 198 tin oxide. 146. 407T Cottrell equation. 77. 165 coloration efficiency. 172 formation via copper ethoxide. 197 manganese oxide. 88. conducting polymer. 89 MxWO3. 419 vanadium pentoxide. 86 viologens. 189. 57. 93. charge-transfer complexation. 176 molybdenum trioxide. 89. 77 depends on rates. 87. 205. 89 mechanical stresses. 344T. see diethylene glycol degradation. 136 tungsten–cerium oxide. 12 pyrazolines. 362. 136 niobium pentoxide. yields tungstate. see cathode ray tube crystal lattice changes during coloration. by annealing. 41 as rate. 38 coloration. 179. 60 cyanotype photography. 388 ECD. 26 cycle life. of Prussian blue. 294. 87 stresses in. 178 rhodium oxide. 308. 197. 52 limiting. 379 cyanophenyl paraquat. 45. 42. 87 cobalt oxide. 48–50. 150. 38. 387 TCNQ species. 376 crystallisation. 89 counter ion (cont. 93 definition. 178. see chemical vapour deposition cyanines. 389 ion hopping. 90. 137–8 depth profiling. 359 Darken relation. 358 charge transfer complexation. 350. 146. 8. 286. 75 faradaic. 11 deep and shallow cycles. 303. 389 write–erase efficiency. 355. 326 decomposition. 52 CVD. 142. 194. 188. 447 and kinetics. 136 of metal oxides molybdenum trioxide. 387. 388–9 reversibility. 351. cycle life. sulfuric. 85 data display. 186 viologens. 349. 356. beta. 13 fullerene electrochromes. 60. 397 caused by ion movement. 376 coloration efficiency. 12–13. 357 DEMO 2005 show. 352. 346–8.) tungsten trioxide. 444T. 376 spiropyrans. 356. 178 tantalum oxide. 388 response times. 48 cyclodextrin. 89 derivatised electrodes. 76 parasitic. 136 of mixtures of metal oxide cerium–titanium oxide. 151 nickel oxide. 147 vanadium pentoxide. 149. 260 metal oxides. 420 aquatic. 146 DEG. 195T lutetium phthalocyanine. 363 dc magnetron sputtering. 185 of metal oxyfluorides titanium oxyfluoride. 205 onto ITO. 136 DDTP. 355 CRT.458 Index schematic. see contrast ratio critical micelle concentration. 443 defect sites. 113. 402 deposition in vacuo. ECD applications. 77T optical charge transfer in. 443 acid. 351. 76 leakage. 304. 93 vanadium pentoxide. 305 indium–tin oxide. 357. see charge transfer current. 136. 86–7 motion of is rate limiting. 181 tungsten trioxide. 136. 76 non-faradaic. 153 nickel oxide. 182 tungsten trioxide. 136. 149. 163 tungsten trioxide. photolytic. 303. 352. 387–8 contrast ratio. 136 titanium–cerium oxide. 305. 83 of conducting polymers. 103. 12 cyclic voltammetry. 269. 49 deep cycles. poly(aniline). 89 vanadium pentoxide in acid. 60 merocyanines. of electrochrome. 172. 351. 186 – OH through anodic oxides. 359 diffusion coefficient. 141. 12. 333 of metal hexacyanoferrates copper hexacyanoferrate. 389. 361 desolvation. 125 molybdenum trioxide. 383. 376 squarylium. 156 niobium pentoxide. 127. during ion insertion. 136 vanadium pentoxide. 444–5 lutetium phthalocyanine. 136. heptyl viologen. 443. 130 crystal violet. 390 ion tunnelling. 200 measurement of. 150 via Cl– ion. 28. 130. 195 indium–tin oxide. 109. 259 CR. 136. 359 . 294 Prussian blue. 388 TTF species. 7 definition. 378 electrochromic. 195. 390 viologen ECDs. 187 of viologens. 177 praseodymium oxide. 54. 423. 389–90 cycle life. 205 tungsten oxyfluoride. 89 CT. 287 of metal oxides iridium oxide. 443 enhanced by mixing oxides. 403 linear. 343. 101. 447. 135 of mixed metal oxides. encapsulated within poly(aniline). 83 diffusion coefficient. 323T dihydro viologen. 101. 363 dinuclear ruthenium complexes. lanthanide hydride. and colour analysis. 96. 44. ECD. 62 Donnelly mirror. 138 during coloration and bleaching. 52. 83 solution-phase species cyanophenyl paraquat. ECD applications. 150. of WV–WV.Index deuteron. 89 dithiolene complexes. as ECD electrolyte. 176 molybdenum trioxide. motion through WO3. 135 directed assembly. 112. 101. mixed-valency. 77T. 266–7 DMF. 142 dry-cell. 97. 45. 448 weathering. 314. see applications. 328 inorganic electrochromes. ITO. 135 cobalt oxide. 78 immobilised. 333 dominant wavelength. 400 Drude theory. 135 tantalum oxide. 111. 145. 443–9 electrodeposited bismuth. 104 of substrates. 403. within poly(pyrrole). 106. 53. 333–4 durability. 33 digital video disc. 333 dynamic electrochemistry. 103 includes migration effects. 268T diode-array spectroscopy. 303. 108. 308 metal oxides cobalt oxide. 390T. ECD dissolution. PEDOT. doubly reduced dimer. of WO3. 135 iron–titanium oxide. 418 dual insertion. 11 as sunglasses. 407T and chemical diffusion coefficient. 330. 358 heptyl viologen. concurrent. 157. oxo-molybdenum complexes. 102. see applications. 401 double insertion. 355 dioxypyrrole. 380 dimethylterephthalate. 386 activation energy for. viologens. 444–5 Duracell. see viologen. as ECD electrolyte. 408 dihedral angle. 112 all polymer. 330. 182 titanium dioxide. 397 nickel oxide. 135 iron oxide. battery. 142 Drude–Zener theory. ECD assembly. 357. 261 dodecylsulfonate. 358 ethyl viologen. 46–8 dysprosium–vanadium pentoxide. 135. 103. 170 iridium oxide. 45. 12 double-layer. 77. 138 double potential step and cycle life. 419–24 electrochromes conducting polymers. 112 fast track. 168 iridium oxide. 77. 98 length. Prussian blue. 447 xenon arc. 408 dry lithiation. windows. 391T dimmable window laminates. 135. 77T ferric ion. 331 diffusion. 154–5. 164–5. 358 displays. 445 of ECDs during pulsing. charging. p-. 43. 83. 202 459 E(cell). 331–2 applications. 408 DVD. 159 manganese oxide. 141 vanadium pentoxide. 87 diacetylbenzene. 76 of electrochromes. 48. 399 dielectric properties. 320 poly(thiophene)s. see emf ECD. 83 DuPont. of ions and electrons. 50 diethyl terephthalate. 358 methyl viologen. 147 dimethoxyphenanthrene. 289–91 metal hydrides. 157 of Prussian blue. 184 tungsten trioxide. see digital video disc dyes. 135 nickel oxide. 331 poly(pyrrole)s. 157 dual organic–inorganic. 11. 84 and oxygen deficiency. 409 poly(aniline)s. 83 energetics of. 417 directed assembly. immobilised. of ions and electrons. 202 substrates. 77T diffusion rate. 327–8 dip coating of metal oxides. 425 durability. immobilised. 60. 306 electrodes. 90–1. 135. 77T methyl viologen. 305 absorption in near infrared. 76. 419 DMSO. 2. 268T near infrared electrochromism. 254. 201. Boeing. 391T diamond electrochromes. 52 Dreamliner. 391T diethylene glycol. 443–9 accelerated tests humidity. 269 metal hexacyanoferrates. 418 of tungsten trioxide. 157. 397 . 135 cerium oxide. 285 di-reduced. 202 titanium–iron oxide. 161 niobium pentoxide. 12 diffusion and migration.7-. 448 of ECD electrolyte. 408. 438 antimony pentoxide. 420–2 Nafion. 419 poly(ethylene oxide). 334 poly(propylene glycol). 77 substrates. 421T DMF. 141. 167. 167. 397. 291. 284. 421T sulfuric acid. 421T thickeners. 138 cerium oxide. 15. 106. 139. 363 EDAX. 421 poly(1-vinyl-2-pyrrolidone-co-N. 290 propylene carbonate. 187. 425 first patents. 150. 178. 150. 362. 308 potassium triflate. 288. 149. 259 fillers (titanium dioxide). 424–5. 326 polymers of. 421T poly(ethylene oxide). 325. 421 poly(vinyl alcohol). 362 electrolytes acetonitrile. 159. 384 ultra fast. 385. 197. 167 cobalt oxide. 348. 419 acrylic powder. 150. 391T. 385 heptyl viologen. 419 DMSO. 385 viologens. 391 potassium chloride. 438 polyelectrolytes. 409. 197. 363 lithium tetrafluoroaluminate. 386 type-I electrochromes. 82. 15. 331 electrochemical titration. viologens. 184. 150. 421T inorganic. 82. 305 hexacyanoferrates. 421T organic. 390T. 402. 421. 408 zirconium dioxide. 424 zinc iodide. 261 ethylene glycol. 87. 359. 197 tungsten–molybdenum oxide. 89. 11 dynamic. 53–4. 53–4. 420 lead fluoride. 150. 325 EIC laboratories. 61. 362. 349 potassium hydroxide. 262. paper quality. 360 viologens. 421T poly(AMPS). 410 vanadium pentoxide. 352. 362 self bleaching. 384 thiazines. 54. 403 sealing. 421–2 poly(acrylic acid). 304. 152. 150 lithium perchlorate. 260 organic pyrazolines. 151. 285–9 metal oxides. 421T triflic acid. 152. 356. 159 lithium niobate. 34–9 electrochromes electrodeposition. 259. 169. 88. 52–3 electrochemical impedance spectroscopy (EIS). 173. 419. 205.) niobium pentoxide. 150. 323. 384. 186. 205. 422. 330.460 Index solid. 419 silica. 150. 421 g-butyrolactone. 395–410. 153. 157 iron oxide. 150. 167 chromium oxide. 290. 130. 152. 448 Surlyn. 260. 288 EDOT. 420. Prussian blue. 178 of tungsten trioxide. 173. 364 electroactive material. 303. 9 electrochemical cells. 332. 169. see impedance electrochemical quartz-crystal microbalance (EQCM). 1 electroactive polymers. 421 viscosity. 151. 254. 157. 153 and memory effect. 291. 419 . 96 stibdic acid polymer. bismuth. 419 gelled. 150. 43. 189–90 mixtures of metal oxide indium–tin oxide. ECD application. 422–4 trichromic. 289. 420 tantalum oxide. 188. 176. 391. 350. 362. 199. Prussian blue. 142. 104 electrochemistry. 384 hydrogen uranyl phosphate. 419 tin phosphate. 166. 44. 90. 163. 138 electric paint. 408. 397 phthalocyanine complexes. 199. 366. 421. 259. 157 phosphoric acid. 421T lithium pentafluoroarsenate. 186. 166. 176. 172 iridium oxide. 150. 168–9 copper oxide. 152. 421T encapsulation. 305. 159 lead tetrafluorostannate. definition. 384. 409 Einstein transition probability. 129. 408. 104. 436 Nafion. 188. 349. 153. 27 flexible. 86. 417 electrochemical formation of colour. 420–2 perchloric acid. 154 titanium dioxide. 388 quinones. 254. 163. 184. 399. 409. 438 poly(methyl methacrylate). 167. 419 poly(vinylbutyral). 15. 147 electric field. 417 whiteners. 304. 447 memory effect. 263 lutetium phthalocyanine. 421T. 418. 349. 357. 421 lithium phosphorous oxynitride. 423 illumination of. 186. 420 polymer electrolytes. 346–8. 82. 417 large-area. 403 radical annihilation. 46–8 equilibrium. 173 ECD (cont. 152.N0 methylenebisacrylamide). 354–5 electrochromes changes in film thickness. 133 yields oxyhydroxide. 342. 175 molybdenum trioxide. 141 vanadium pentoxide. 75. 184 tungsten trioxide. 304. x. 3 461 complementary. 331 thermodynamics of. 303. 168 nickel oxyhydroxide. 306–7 silver. 305–6 lead. 54 type. 61 photodegradation of. 93. 53 electrochromic paper. 203 polymers. 403 metal-oxide systems and insertion coefficient. 3. see ECD. 193–5 cobalt oxide. 43 kinetics. 37 ECD. 132. 27. 405 electrochromic modulation. 199 nickel–titanium oxide. 205 tungsten oxyfluoride. 198 iron oxide. 283 electroluminescent organic light-emitting diodes. conducting polymers. 3 electrochromic–photochromic systems. 39. 265–74 electrode as conductor. 175–6 molybdenum trioxide. 312 electrolyte fillers to enhance contrast ratio. 3 electrochromic hosts metal oxides. 47 substrate. 161–3 niobium pentoxide. electrolyte dissolves ITO. 329–30. 160–1 oxide mixtures. 151. see ECD electrodes. 305 electrodeposition forming hexacyanoferrates Prussian blue. 133 ruthenium oxide.Index manganese oxide. 255 near infrared. 1. 262 lutetium phthalocyanine. 303. 61 metal-oxide systems. intervalence of. 438–9 electrochromism chemical. 181 tungsten trioxide. 186–8 metal oxyfluorides titanium oxyfluoride. 445 failure of. 307 forming metal oxides. 5 electroless deposition. 35. under diffusion control. 177–8 palladium oxide. 152 nickel oxide. 290 definitions. 200–1 titanium dioxide. 405–6 electrochromic probes. 200 niobium pentoxide. 186 forming mixtures of metal oxide molybdenum–tungsten oxide. 173 manganese oxide. 195–6 indium oxides. 253. 202 zirconium oxide. 26 first use of term. 40–1 extrinsic intensity of. 140. 353. 254. 7–9 electrochromic colours counter electrode. 201–2 tungsten trioxide. 443 . 190–206 antimony oxide. 196–7 phthalocyanines. 132–4 cobalt oxide. 152–3 nickel oxide. 422–4 interphase. 354 potentiostatic. of Prussian blue. 407 vanadium pentoxide. 49 laboratory examples of. 283. 444 durability. 183 titanium dioxide. 52 reactions. 445 electrolyte. 132 forming viologen radicals. see entries listed under substrate electrodeposition of metals. 142 vanadium pentoxide. 91. 165. 199 forming oxyhydroxides cobalt oxyhydroxide. 196–7 iridium oxide. 60 poly(aniline). type-II electrochromes. 3 fax transmissions. 53–4. 178 rhodium oxide. 178–9 praseodymium oxide. 132 electrokinetic colloids. ECD application. 25 history of. 202 tungsten–molybdenum oxide. 183. 48. 191–3. 153. 34–9 viologens. peroxo species. 15. 104 reactions. ECD chemical systems. 46–8 potential. 206 mixtures of metal oxide indium–tin oxide. 198–9 molybdenum oxide. Nafion as. 284 forming metals. see secondary electrochrome device. 51 colours of. 133 precursors. 132 copper oxide. 305–7 bismuth. 27. 201 titanium–tungsten oxide. 161 nitrate forming metal hydroxide. 181 tantalum oxide. 3 memory effect. 2 decomposition. 25–30 ligand-based. 193 cerium oxide. 260 polymers. 180 ruthenium oxide. 152. 199 nickel oxide. 52–3 intensity of. 171 iron oxide. see redox pair electron-transfer rate. 81. 359 electron-transfer reaction. 267 ESCA. 284. 344T. 162 tungsten trioxide. 321–7 evaporated metal-oxide films. ECD (cont. 384 N-ethylcarbazole. 50 ellipsometry. 47 fast. 3. 104. 87. 358 ethylanthraquinone. 145 of viologens. 130 of nickel oxide. 151 of nickel oxide. 75 electrophotography. 182 of tungsten trioxide. 448 energetics. 113 in metal oxides nickel oxide. 329. 186 Eveready. 359 ferrous ion as. of chromium oxide. ECD. 42 electronic motion. 109–10 and film thickness. 109. 43 electrolytic writing paper. 359 hydroquinone as. 445 photochemical stability. 263 of Prussian blue. 81–2 electronic paper. 51. of ITO. through bands. 331 Prussian blue. 17. 127 electronic charge. 17–18 graphite electrochromes. 362 equilibrium potential. 359 electron mobility. 187 emeraldine. viologens. 93. 352T. 34. battery charge indicator. 47 electron-beam evaporation. 138–206 forming metal oxide mixtures indium–tin oxide. 8 electronic bands. 365 electroreduction. 39–40. 167 electron-beam sputtering forming metal oxides manganese oxide. indium–tin oxide. 129 element. 137 vanadium pentoxide. 33. ECD electrolyte. 184 of tungsten trioxide. 81. 109. 305 mediators cerous ion as. 89 of molybdenum trioxide. 137. 444 electroreversibility poor. 359 copper as. 175 molybdenum trioxide. 95 to electron transfer. 306 ferrocene as. 352. tungsten–molybdenum oxide. 438 electron hopping. 88 environmentalism. 33. 35. 350. 447 erbium laser. 41 equivalent circuit. vacuum of bismuth oxide. 130 of tungsten trioxide. of tungsten trioxide. 363 electron–ion pair. water in. 197 electrostriction. 81 electron donors.462 Index quinones. photochromism. 153 of phthalocyanine complexes. 28 electropolychromism. 358. 147.) fillers. 153 of tungsten trioxide. 94. 438 di-reduced. 445 definition. 289 of titanium dioxide. 259 ethylenedioxythiophene. 89 evaporation. 255–6 . 34. 381T ethylene glycol. 89–90 energy barrier. 50–1. 50 in situ. 359 ferrocyanide as. 445 of vanadium pentoxide. 443 Everitt’s salt. carbazoles. 42–3. 342. need for. 83. see Prussian white electrolyte. 398 epoxy resin. 312 poly(aniline). 384 viologens. 424–5. 99 electron mediation bismuth electrodeposition. 75 standard rate constant of. 42–3. 143 encapsulation. 303. 445 semi-solid. 185. 304 poly(aniline). 99 in metal oxide mixtures indium–tin oxide. 103 ESR of methyl viologen. 287 seven-colours. 81. 140–1. 405 electron conduction. 192 electron transfer energy barrier to. 51 of iridium oxide. 42. 288. 254 polypyridyl complexes. 143 of vanadium pentoxide. 52 electrolytic side reactions. 87. 46. 408. 84 entropy. 445 in phthalocyanine complexes. 129. 160 of tantalum oxide. 150 of vanadium pentoxide. 157 of molybdenum trioxide. 287. 356 of molybdenum trioxide. 379T. 95. ECD applications. 101 rate of. 313. 329. 137. 47 enhancement factor W. 445 organic polymers. 166 of metal-oxide films. 305. 87 of iridium oxide. 109. 86 of ion movement through solid oxides. circuit. 331 emf. 102 rate of. 356 ethyl viologen. 446 electrolytic capacitor. 129. 447 equivalent circuit. impedance. 259 in polymers poly(acetylene). 50 and interfaces. 196 electroneutrality. 42 electronic conductivity. 2-.4-. 309 ferrocyanide charge transfer complexation.4. conducting polymers. 407 gelled ECD electrolyte. 396 Gibbs energy. 139. 76. ix. 330. 359 ferrocene–naphthalimide dyads. 400 flash evaporation. 445 film thickness. 313 fused bithiophenes. 351. 77T ferricyanide. 95 field-effect transistors. human. 196. 52. 104. 387 fluorescence. 204T as substrate. applications. 344T. 303. 399 furan. 204T in molybdenum trioxide. 34 glassy carbon. 95 extinction coefficient. 349. 305 near-infrared absorbance. 407T cobalt oxide. 407T materials chromium oxide. 417 games. 204. 53. 304 substrate. 407 phosphate ion. 398. 386 type-I electrochrome. 359 electron mediator. 350. 204T in iridium oxide. 129. 424 Grotthus. 343. 380 2. 159 additive in cobalt oxide. 382 flux. 396. 285 overlayer of. as substrate. 28 ferric ion. conducting polymers of. 303. windows. 342 ferrocene derivatives. 110 gasochromism. ECD electrolyte. see human eye eye wear. 168. 387 quasi reversibility of. 295 galvanostatic coloration. 400 mirrors (Night-Vision System). 90 ¨ Gyridon ‘electrochromic paper’. of vanadium pentoxide. 331 electron mediator. 45. 316 gallium hexacyanoferrate. 171. 384 using agar. 44. 425. see impedance spectroscopy fullerene electrochromes. 356. 52–3 eye. 50 frequency response analysis FRA. and 312 fillers. 446–7 gold nanoparticles.Index EXAFS. overlayer of. 60. 55. 61. 265. 5. 407T nickel oxide. 52 fax transmission. 166. 402 flexible ECD. ECD. 150. and impedance. 204T in tungsten trioxide. 5 . poly(aniline). 44. 362. 407T phthalocyanine. 349 and coloration efficiency. 423 fluoreneones. TV. 305. 50 Flabeg Gmbh. 34–9 and emf. 44. 205. 358. 348 Gentex Corporation. 76 Faraday constant. see applications. 305–6 degradation of. 406–7. 409. ECD. 113. 45. and ellipsometry. 333 grain boundaries. 75 formation of colour. 305 quasi-reversiblity. 349. 274. 253 ferrous ion. using electrochromism. 376. 26 frequency. 200 titanium dioxide. 304–5 electropolychromic. electrochemical. 363 gamma rays. 406 nitric oxide. 146 graphite electrochromes. 294. 351 using silica. 204. 97 and colour formation. 200. 303. 305 formation via Langmuir–Blodgett. 359 incorporation into nickel oxide. 406. p-. 303 463 coloration efficiency. eye wear Faradaic current. 379. 406 metalloporphyrin.5. 88. conduction in metal oxides. 342 as oxidant. 26 F-centres. 447 aircraft windows. 44 second law. 204T in vanadium pentoxide. 422 fluoroanil. 303. 387 2. 342. 356 memory effect. 358 ferroin. 407T sensors for chloride ion. 358 gold. 52 Fox Talbot. 385. 292. 96–8. 269. 204T in nickel oxide. 407T tungsten trioxide. 50.4. 204. 46. 304. ECD applications. 18. 201 mediating viologen comproportionation. 98. 400. 111 approximation. 380. 194 exchange current. 387 radical annihilation. 423 on indium–tin oxide. 359 fibre-optics. ECD. 294. diffusion coefficient. 45 first law. 406 for nitrate ion. 406 toluene. 350. 55 extrinsic colour. as electron mediator. 304. 446 graft copolymer. 185 flat-panel screens. 34 Faraday’s laws. substrate. 45. 385–7 cycle life. of cerium–titanium oxide. 153. 5 fluorine-doped tin oxide. 404 Fick’s laws. 47.7-tetranitro-9-fluorenone. 417.7-trinitro-9-fluorenylidene malononitrile. 14. 50 real. 352–3. 28. 196 spin coating. 17. 354–5. 308 lanthanum–magnesium. 308 samarium–magnesium. 307 yttrium. 308 electrochromic alloys. 196 kinetics of. 196–7 indium–tin oxide chemical stability. 64. electron mediator. chemical diffusion coefficient. 307 switchable mirrors. 375. 308 ECD. 197 optically passive. 197 optical properties as secondary electrochrome. 89. 405 ICI Plc. 355. and colour analysis. 196–7 electroreduction of. 204T ECDs of. 356. 197 optical properties. 353T as primary electrochrome. 352T history of conducting polymers. 37. of metal oxides. 437 iron. 197 coloration efficiency. 355 power consumption. 349. 309 indium oxide. 307–8 Anderson transition in. as CVD precursor. 129. 357. 433. 307 . spectral response. 127 polarons. 438 heptyl viologen. 351. 421T hydrogen peroxide. 296 nickel. 417 imaginary. 203 Indigo Blue. 104. 28. 157 IBM Laboratories. 104. 50 incident light. 102. 359 hygroscopicity. 307 response time. 352 illumination back-wall. 197. 135–6. see ferrocyanide hexacyanoferrate(III). 308 durability. within poly(pyrrole). 423 on Mylar. 143 hue. 295. 295. 50 equivalent circuit. 83. 131 hopping electron. 344T. 57T Indigo Carmine. impedance 50 immitance. 113 hematite. 308 electrochromic metal lanthanum.464 Index hydrogen electrode. 359 critical micelle concentration. 385 charge transfer complexation. 292–3 hexyl viologen. 357 reduction potentials. 349–51. 64 of ECDs. 403. 199. 4 He–Ne laser. 341. see derivatised electrodes impedance spectroscopy. 353T solubility constant. 294–5 vanadium. 197 degradation of. 282 history effect. 197 electrochemistry of. 199 evolution at tungsten oxide. 346. 433 light sources. 423 on polyester. 85. 36. electrochromic. 11. 136 electron-beam deposition. 56T. 423 on PET. 333 indium hexacyanoferrate. 190. 35 Hall effect. 357 recrystallisation of. 295 indium. 307 mirrors. 447 imaginary. 312 of electrochromism. 99. 307 cycle life. 50 Inconel-600. 445 uranyl phosphate. 360. see Prussian blue miscellaneous. 357. 196 sol–gel. 135. 307. 28. 196 rf sputtering. 131. 89 hysteresis. as electrochromic host. 294 gallium. 8. 359 aging effects. 423 half reaction. 307 palladium overlayer on. 197 formation via CVD. oxide on. 173 Henderson–Hasselbalch equation. 137. 25–30 of Prussian blue. 62. 358 ECDs of. 333 and frequency. 40 evolution at molybdenum oxide. 295–6 mixed-metal. 444 electro-reversibility poor. 50. 355 di-reduced. 199T contrast ratio. 135. 9. 81. 197. 365 morphology of. 293–4 palladium. 70 human eye. ECD electrolyte. 30. 444–5 composition of. 359 anion effects. 132 dc magnetron sputtering. 435 front-wall. see ferricyanide hexacyanoferrate of copper. 423 cycle life. 352T. 50 immobilised viologens. 199 flexible ECDs. 196 containing silver. 351 hexacarbonyl. 308 hydroquinone. 397 hexacyanoferrate(II). 360 incorporated in paper. 356. 348. 196 laser ablation. 14 radical of. 437 indium nitride. coloration efficiency. 62 hydride. 445 substrate. 193. 96. 151. 174 iron hexacyanoferrate. 175 electrochemistry. 90–1 effect on electroreversibility. 267. 198 annealing of. 306. 156. 9. 198 gold. 199 iron–nickel–chromium oxide. 331. 173 iron perchlorate. 82 effect on wavelength maximum. 349 electronic conductivity. 157 formation via 465 anodically grown on Ir. 200 iron–titanium oxide charge transfer of. 108. 198 ellipsometry. 158 iridium–cobalt–nickel oxide. 155–6 dip coating. 27. 182. of water. 156.Index mechanical stability. 158 peroxo species. 102. 156. 61. 173. 203 iridium–silicon oxide. 125. 363. 61. 202 irreversibility. 159 electrochemistry. 173. 41. 83. 158 as secondary electrochrome. 437 iodine laser. 135. 125. 188. 61 intensity and colour analysis. between films. 141. 143. 113–14. 253. 17. 181. 407T water content. 382. 266 ion-conductive electrolyte. 81. 358 inorganic–organic. 201 annealing. 158 reflectance of. 150. 175 coloration efficiency. 127 homonuclear. 153. 87. 175T. 89 during coloration. 159 specular reflectance. 70. 135 electrodeposition. 385. 99. 156 spray deposition. 188. 444T coloration efficiency. 204T water. 422–3. 198 iridium–titanium oxide. 43 intervalence charge transfer. 103. 86. 152. 191. 198 as electrochromic host. effect of. 96. 198 iridium–tantalum oxide. 129. when oxidising LixWO3. 159. ECD application. 198 formation via sol–gel. 157 electrochromic host. 156 electrostriction of. dual ECD. 198 iron acetylacetonate. 201 coloration efficiency. 53. 104. 135 IrCl3. 175T oxidised film on Fe metal. 172 sol–gel. 257. 157 response time. 139. 200. 402 IR drop. x interphase. 127. 284. 444 XPS of. 157 iridium trichloride. 444–5. 294. 192. 155. 129 optical properties. 157 mechanical stability. 313 inert electrode. 284 heteronuclear. 186. chemical diffusion coefficient. 128. 63 of electrochromic colours. 166. 127 intrinsic coloration efficiency. coloration efficiency. 95. 330. 303. 183 ion–electron pair. 56T. 417. 145. 175T spin coating. 422. 138. see redox pairs ionic interactions. 101. 156 hysteresis of. 352T . 153. 203 iron–niobium oxide. 82 iso-pentyl viologen. 167. 56T. 54–60 iodine. 175T dip coating. 333–4 insertion coefficient. 201T formation via sol–gel. 146. 181. 36 ionic mobility. 155–9. 202 formation via dip coating. 3 interactions. 174. 173 iron phthalocyanine. 50 international meetings on electrochromism (IME). 422–3 iridium oxide. 164. 256 iron vanadate. 349. 191. tantalum oxide. 305. 174 iron oxyhydroxide. 175. 89 interfaces. 400 coloration mechanism. 305–7 ionisation. 130 ECDs. 192. 70T. 173. 156 XPS of. 10. 129 resistance. 156 iridium–carbon composite. 404. 405. 174 mixtures. 293. 149. 157 iridium–carbon composite. 156 write–erase efficiency. 203 iridium–magnesium oxide. 92. 198–9 optical properties. 198 iridium–ruthenium oxide. 198 formation via sol–gel. 38 infra red spectroscopy. 172–5. 305 effect on diffusion coefficient. 135. 156 iridium trihydroxide. 445 indole. 261 iron polypyridyl complexes. 158. 156. see Prussian blue iron oxide. 158 phase changes. 201T secondary electrochrome. 90–1. 326. 447 water sensitivity. 70. 197. 16. counter ion with water. 81 iPod screen. 303. 333. 156 sol–gel. 174. 158 sputtering. 173 formation via CVD. 157 containing aramid resin. electrode. 53 high at grain boundaries. 202 iron–molybdenum oxide. 104 metal-oxide systems. 375. 93 Index Prussian blue. 404. 150. 261 cation-free not electrochromic. 446 lithium tin oxide. 331 LCD. dry. 87–8 effect of morphology on. 83. 147. 152. 261 formation via sublimation. 159 lead tetrafluorostannate. 260 chemical diffusion coefficient. 173. see liquid crystal display lead. 407 lanthanide hydride. 305 forming phthalocyanine. 105–8 type-II. 190 vanadate. of electrochromes. 82. 39 K-glass. 259. 360. 176. 408. 151. 90 of lutetium phthalocyanine. 163 lattice stabilisation. 185 laser. 164 vanadium pentoxide. 76 linear free-energy relationships. 163.466 IUPAC. 130. 166. 183 titanium dioxide. 269 light modulation. 76 linear diffusion. 184 vanadium pentoxide. 332. 259 response times. 363. 331 LFER. 265 junction potential. 75 electron transfer. lanthanide lanthanum–nickel oxide. ix. 138–206 forming fullerene electrochromes. as ECD electrolyte. 200 large-area ECDs. 407T lithium tetrafluoroaluminate. 363 overlayer of. 33. 55. 64–71 laboratory examples. 260 ECDs. 331 of poly(aniline). 266 YAG. 262–3. 3 Langmuir–Blodgett deposition. 199. electrolyte ECD. 447 laser ablation. 260. electrochromism. 99 faster in damp films. and colour analysis. ECD electrolyte. 404–5 light-emitting diodes. 64. 425 power consumption of. 66 limiting current. 259 . 260 degradation. 422 kinetics bleaching models Faughnan and Crandall. 56T. 260 electrochemistry. 406. 11 Kosower. 436 overlayer of. 152. 75–9 type-III. 196 tantalum oxide. 150. 362. 169. thermochromic. 421T. 403. as ECD electrolyte. electrodeposition of. 418 lithium chromate. 351. 408. 421 lithium phosphorous oxynitride. ECD electrolyte. solvent Z-scale. 402. 167. 5 luminance. 63. 92 write–erase efficiency and. 306–7 lead fluoride. 79–115 type-III. 331 of poly(viologen). specular reflectance of. 64–71 data for Prussian blue ! Prussian white. 329. 161 IVCT. 15 lithiation. 191 electro-irreversibility of. 363. 260 protonated. 33. 63. 150 lithium perchlorate. 87. 82 lithium vanadate. 91–115 effect of counter-ion size on. 197. 85T coloration kinetics. 402 lightness. substrate. 108. 289 lattice defects. 79–115 coloration. 129 lattice energy. 53. 184. 438 iodine. cobalt oxide. 109 of metal oxides nickel oxide. 141. and colour analysis. 105–8 Green. 267 laser-beam deflection. 75 liquid-crystal display. 205. transport through. 11 effect of water on. 75–9 type-II. nickel oxide. 329–30. 255 ligand-to-metal charge transfer. 188 potentiostatic. 186. 446 lithium pnictide. 112 layer-by-layer deposition of PEDOT:PSS. 66. 8. 329. ECD electrolyte. 197 lithium tungsten bronze. 87. 70T L*u*v* colour space. 266. 70 lutetium phthalocyanine. 260 type-I. 167 lithium deficient. ECD electrolyte. 328–9. see linear free-energy relationships ligand based. 267 types erbium. forming indium–tin oxide. 343 liquid electrolytes. 64. 343 L*a*b* colour space. 167 lithium niobate. 404 Q-switching. 34 rate-limiting process. 159 leakage current. see hydride. 190 Lucent. 157 lattice constants. 304. 259–60. 89 electrochrome transport. 88 electron as rate limiting. 267 He–Ne. 88 effect of high resistance of polymers. 188. ECD application. 75–115 of amorphous oxides. 106. 52 leucoemeraldine. 259 write–erase cycles. 421T lithium pentafluoroantimonate. see intervalence charge transfer J-aggregates. 387 metal hexacyanomellates. electro-oxidation. 128–30 mechanical. Inconel-600. 135 oxide formed by electrodeposition. 261 mass balance. 3 memory. 132–4 oxide formed by evaporation. 128–9 electrochemistry of. 303. 192 metal oxyfluorides. 409 methoxybiphenyls. 181 tantalum. 186. 33. 177 rhodium. 184 tungsten. 190 containing precious metal. 175 electron-beam sputtering.Index Madelung constant. 190–206 colour manipulation. 54. 381T. 56T insertion coefficient and. 15. 359 coloration efficiency. 348. 132. 135 stability. 162 mass transport. ECD. 282–96 metal oxidation to form oxide cobalt. 30. 138–206 oxide formed by oxidising alloy. 381T optical properties. 190 site-saturation model. 184 tungsten. metal oxides. 175–6 optical properties. 153 and tungsten trioxide ECD. 125 neutral colours. 381T steric effects. 379–80 electrode potentials. 265 melamine. 53–4. 253 methanol. 175 niobium. 77T di-reduced. 138 intervalence of. 81. 344T. 81 nickel oxide. 362 ECD self-erasure. 129–30 medicine. see Anderson transition metallic substrates. 75 mechanical stability. 128. 346. 172 manganese. 150 and viologens ECD. 380 methoxyfluorene. 190. 125–206 amorphous. 153. 176 XPS. 436 charge-transfer complexation. 175 sol–gel. 358 electropolychromic. 423–4 metalloporphyrin. 135–6 oxide formed by spray pyrolysis. 352T. 139 bronzes of. 345 magnetite. gasochromic. 129–30 photochemical. 57T diffusion coefficient. 175 niobium. 203 oxide formed by oxidising metal cobalt. 125 preparation. 185. 176 coloration efficiency. 81. 11. plus vanadium pentoxide. 379T. electrodeposition. 190. 168. 204 sol–gel. 175 rf sputtering. 175. 129 metal-oxide mixtures. 130–8 oxide formed by chemical vapour deposition. 423 magnesium OEP. 379T. 7. 81. 168. 293 coordination complexes. 139–65 optical passivity. 149. 182 titanium. 43–5. applications. 8 methyl viologen. 152. 165. 387 and molybdenum trioxide ECD. 131. 185. 112 maghemite. 198 magnesium–nickel. 82. 175–6. 176 ECDs. 341. 187 metal oxide. 262. 61 doped. 103 coloration efficiency. 446 as secondary electrochrome. 266 effect of moisture on. 176 formation via anodising Mn metal. 137. 89 oxide formed by Langmuir–Blodgett deposition. 176 electrochemistry. 204 neutral colour. 134–6 oxide formed by spin coating. 203 magnetic susceptibility. 399 photochemical stability. 203 metal–insulator transition. 176 manganese phthalocyanine. 179 ruthenium. 186. 17. 150 vanadium. 152. 260 magnesium–iridium oxide. 17 . 305–7 metal-to-ligand charge transfer. 407T metals. mixtures. terminal effect suppressor. 61 metal oxide 467 optical properties as primary electrochromes. 176 rechargeable batteries. 204 formation via dip coating. 150 vanadium. 169 iron. 381T. 169 iron. 265 magnesium phthalocyanine. 113. 135 rf sputtering. 177 rhodium. 179 ruthenium. 173 manganese oxide. 379T. 187 oxide formed by sol–gel deposition. 175 electrodeposition. 200 magnesium–nickel–vanadium oxide. 403 and Gentex ECD. 172 manganese. 353. 181 tantalum 182 titanium. 61. 202 membrane potentials. 173 magnesium fluoride. 168. 4 biological. 131–2 oxide formed by dip coating. ECD display. 8. 89 requires water. 155T. 204T crystal phases a phase. 153 molybdenum–chromium oxide. 391T in Nafion. 152 molybdenum sulphide. 199 formation via peroxo species. 89 containing gold. 152 molybdenum trioxide. 155T organometallic precursors. 305–7 proton. 125. of metal oxide.) ESR. see tungsten–molybdenum oxide molybdenum–vanadium oxide. 153 ESR of. 377T–378T. 4-naphthaquinone. 405 viologen. 152 oxidation of Mo metal. mechanical. 199 Moonwatch. 319. 384 N-carbazylcarbazole. ECD application. 136. 152 spin coating. 99. 303–4. 83 mirror. 199T–201T molybdenum–titanium oxide. 154 bronze. 56T. 142. 406 naphthalimide–ferrocene dyads. 108 modulation. 153 XRD. 199 coloration efficiency. 89 electrochromic host. 154 self bleaching of. 363. 43. see metal-to-ligand charge transfer mobility ionic. 376 methyl viologen (cont. of 320 micellar. 303. forming molybdenum trioxide. 184 motorcycle helmet. 290. 152. 152. 199 ellipsometry of. 199T oxygen deficient. 150. 137 evaporation. 282. 27. 27. 347. 365 Methylene Blue. 405 memory effect. 197 ¨ Prussian blue. 199 coloration efficiency. 365 mixed-valence salt. 75. 402. 283 tungsten trioxide. 151. 135 spray pyrolysis. 153 response time. 152 orthorhombic. 83 temperature dependence of. 96–7 diffusion concurrent. 103. viologens. 130. 421T incorporating Methylene Blue. 356 dinuclear ruthenium complexes. 398 Mylar. see electrochemical quartz crystal microbalance migration. 283 tin oxide. 437. 106. 151 sol–gel. 402. 391. in paper. 318 oligomers. 356 mixtures. 199 molybdenum–niobium oxide. 152 monoclinic. 399 of aromatic amines. 317. 11. 153 coloration in vacuo. ECD. 151–5. 199 molybdenum–tungsten oxide. 152 CVD. 135. 268T Robin–Day classification. 151. 199. 405 ECD electrolyte. 254. 159. 362. 154. 28. 355–6 microbalance. 37 molybdenum ethoxide. 28 XPS. 152. 151 chemical diffusion coefficient. 397 effect of water on. 265–74. 397 dc magnetron sputtering. 263–4 1. 187 annealing of. 379T NCD. 366. 142 viologens. 446 Nanochromic (NTera) displays. 152 molybdenum sulfide. 309 naphthalocyanine complexes. 129 UV irradiation of.468 Index Mo(CO)6. electrochromic. 165. 155T . 152 hydrogen evolution at. 327. 3. 152 ECD. 153 stability. 365 follows Langmuir adsorption isotherm. 151 peroxo species. 44. 200 formation by sol–gel. 103. 131. 109. 153 formation via alkoxides. 200 molybdenum–tin oxide. 103. 151. 423 Nafion. electrochromism. 104. 115. methyl viologen. 405 methylthiophene. ECD. 153 optical properties. 137–8. 53 mole fraction x. 154–5. 253. 384 cyclic voltammetry. 403 Mossbauer. see metal-oxide mixtures MLCT. carbazoles. 438 coloration efficiency. 199 in paper. 405 phenolsafranine dye. 199. 356 methyl–benzyl viologen. 204T platinum. 151 rf sputtering. indium–tin oxide substrate. 57T immobilised. 151 electrodeposition. 132. see applications. 53 molar absorptivity. mirrors mixed valency. 152. 397 molybdenum ethoxide. 3-. 199 molybdenum–iron oxide. 356 in paper. 151. 152 electron-beam sputtering. 153–4 coloration efficiency. see Nanochromic display near infrared. 385 as electrochromic host. 405 overlayer of. 90 Nernst–Planck equation. 162 defects lattice. 200 Nikon. 161. 200. 164 organometallics. 200. 134. 133. 399 metal-oxide mixtures. 149. 399 of dinuclear ruthenium complexes. 160 water occluded. 164 nickel dithiolene. 163. 176 as secondary electrochromes. 407 469 . 36. 134 niobium pentoxide. 178 chemical diffusion coefficient. 135 oxidising Nb metal. 130. 133. 200 nickel–titanium oxide. 399 neodymium–vanadium pentoxide. 202 nickel–vanadium–magnesium oxide. 164 chemical diffusion coefficient. 436 coloration efficiency. 201 nickel–tungsten oxide. 203 nickel-doped tin oxide. see near infrared nitrate ion. 16. 401 Nile Blue. 162 mass balance. 200 gold. 149. 17. 200 electronic conductivity. 136. 43 Nerstian systems. 268T of fullerene. 135. 102 formation via dc magnetron sputtering. 447 bleaching of. 177–8 electrochromic host. 181T sol–gel. 57T niobium ethoxide. underlayer of. 178 cyclic voltammetry. 162 crystallites in amorphous NiO. sol–gel precursor. 177 as secondary electrochrome. 125. 40. 16. 163 ECDs. 196 nickel–iridium–cobalt oxide. 161 as primary electrochrome. 129. 86. 177 spray pyrolysis. 162. 200 nickel–chromium oxide. 129 as secondary electrochrome. 159–65. 134. 167. 178. 135. 407T ionic movement rate. 177 dip coating. 85T cycle life. 161 dc magnetron sputtering. 165T gasochromic. 17. 176. 201 coloration efficiency. 177 rf sputtering. 56T. 200 nickel–vanadium pentoxide. 200 niobium–tungsten oxide. 160. 161 nickel tungstate. 399 tungsten–vanadium oxide. 190. 200 nickel–aluminium oxide. 201T formation via sol–gel. coloration efficiency. 165T electrodeposition. 200 activation energy. ECD sunglasses. 178 electrochemistry. 399 neutron diffraction. 132. 202 formation via electrodeposition. 397 electrochemical quartz microbalance. 56T. 9. 203 nickel–lanthanum oxide. 200 nickel–cerium oxide. 160. 178 coloration efficiency. 77 neutral colour. 400 formed via electrodeposition. 199T–201T. 176. 201 coloration efficiency. 165T spray pyrolysis. 202 Nernst equation. 200 via sol–gel.Index of diamond. 178 ECDs. 160. 200 optically passive. 165T phases. 85T degradation of. 161 rf sputtering. 200 niobium–molybdenum oxide. 203 nickel–yttrium oxide. 200 formed via sol–gel. 165T dip coating. 165T evaporation. 201 coloration efficiency. 70. 266 nickel hexacyanoferrate. 200 niobium–silicone oxide. 444T. 293–4 nickel hydroxide. 144 nickel. 161. 36. 163–4 coloration efficiency. 161–3. 200 niobium–titanium oxide. 200 containing cobalt metal. 178. 165T sonication. 200 nickel–chromium–iron oxide. 130 formation via CVD. 88 response times. 163 electrochemistry of. 159–60 mechanical stability. 181T niobium–iron oxide. 200 spin coating. 160–1. 406 nitric oxide. 164 sol–gel. 135. 136. 200–1 coloration efficiency of mixtures. 446–7 NIR. 165. 181T. 164 nickel oxyhydroxide. 161–3 electrochromic host. sensor for. 200 nickel–magnesium. 129 optical properties. 163 write–erase cycles. 201 annealing. 178 redox pairs. 12. 164 ferrocyanide. 201 optical properties. 125. 163 oxygen deficiency. 75. 181T. 200 nickel–aluminium alloy. 134 nickel oxide. 446. 162 electrostriction of. gasochromic. 111T annealing of. 135. 38. 204T lanthanum. gasochromic. sensor for. 200. 164 thermal instability. 201 Nippon Mitsubishi Oil Corporation. 164–5. 161 formation via sonication. 165T plasma oxidation of Ni–C. 204. 17 optically passive. 57T organic–inorganic. 199 titanium–vanadium oxide. 363. 344T. 406. 52 optical analyses. 446–7 gold nanoparticles. 52. application. 344T optical response. 352T Ohm’s law. 347. 35 oxidation potential. 16. 446 palladium. 433. 307 poly(o-phenylenediamine). 446 Nafion. ECD electrolytes. 156. 6. carbazoles. 363. 141. 152 in nickel oxide. 255 orbitals. 398. 202 vanadium–nickel oxide. 89 optical attenuator. ECD. 321 of viologens. 362. see N-methylpyrrolidone non-faradaic current. 62. to form metal oxide film cobalt. 144–9 vanadium pentoxide. 363 cycle life. 436. 4 non-redox electrochromism. dual ECD. see Gentex Corporation occlusion. 10. 381T N-PrS PProDOP. of water during deposition. 265 optical path length. 191. of hydrogen gas. 389T tetrathiafulvalene. see charge transfer Optical Coating Laboratory. 423–4 optical absorbance. 200 oxidising metal. 76 non-linear optical effects. 446 lithium tetrafluoroaluminate. 125 cerium oxide. 188–9 mixtures of metal oxide. 168. 444. 388T . indium–tin oxide. 342 periodate. 111. 85 oxide mixtures. 183 tin oxide. 169–70 iridium oxide. 318. 46. 363. 168. 197 methoxybiphenyls. 323 oscillator strength. 402. of viologen reduction. 354 NVS. 150 tungsten oxyfluoride. 342 oxygen gas. 364 nuclear reaction analysis. 76. 417. 178 tantalum oxide. 333–4 organic. 406 as primary electrochrome. 89 octyl viologen. 184 titanium dioxide. 404. ECD applications. 386 oligomers of 3-methylthiophene. 202 vanadium–titanium oxide. 194 indium–tin oxide. and charge transfer. 270 optical charge transfer. 348. 379T. 163–4 niobium pentoxide. 390T viologens. 446 tungsten trioxide. 200 Orgatron. 446 lithium phosphorus oxynitride. 362. 447 NTera. 321T pyrazolines. 365 coloration efficiency. 362. 328 NREL Laboratories. 158 iron oxide. 331 NMP. 313 oxide ions. 16. see optically transparent thin-layer electrode overlayers gold. 420–2 organic electrochromes. 175 manganese oxide. 110. 45. 270 N-methyl PProDOP.470 Index quinones. 166 nickel–vanadium oxide. 332 organic. 320 oxidative polymerisation. chemical ferricyanide. 206 osmium dithiolene complexes. Santa Rosa. 381T oligothiophenes. 330. 44 Ohmic migration. deconvolution of. 96. see also substrates optically transparent thin-layer electrode OTTLE. of molybdenum trioxide. 93. 166 cobalt oxide. 184 tungsten trioxide. see also NanoChromic (NTera)displays phosphonated viologen. 379T. 202 niobium pentoxide. 162 nucleation. 10. 100 nucleation. 61 Orgacon EL-350. water effect of. 320 of thiophene. 364 opaque substrates. see nuclear reaction analysis nitrosylmolybdenum complexes. see memory effect N-phenylcarbazole. 320. 448 optical data storage. 178 titanium dioxide. 176 molybdenum trioxide. 4 nitrogen-15. 197. 55 optical properties metal oxides cerium oxide. 169 nitroaminostilbene. conducting polymers. 313–14 of pyrrole. response time. coloration rate enhancement. coloration efficiency. 193 cerium–titanium oxide. 328 N-methylpyrrolidone. 199 oxidation. 270–4 OTTLE. 362 ECD. 147. 184 mixtures of metal oxide antimony–tin oxide. 36. 446 tantalum oxide. 445 organometallic precursors. 129–30. 405. 342 oxidation number. 383T tetracyanoquinonedimethanide. 42. 446 overpotential. 153–4 nickel oxide. ECD. 3 non-volatile memory. ECD electrolyte. 125 metal oxides. charge transfer with. 202 optically transparent electrode OTE. polymers. 404 electron donors. 151. 52 passive.Index iron. 365 methyl–benzyl viologen. 102. 354 phosphate ion. 323. 185. 322 coloration efficiency. 330. as ECD electrolyte. 201 photo-activated ECD cells. 127. 113. 436 silicon carbide. 405 heptyl viologen 365 methyl viologen. ECD electrolyte. 168 iridium oxide. indium–tin oxide substrate. 132. 379 phenanthroline. 103. 190. 125. 153 in nickel oxide. 192. 150 vanadium. 438 photodegradation. 329. 365 paraquat. 352T 471 perchloric acid. 436 poly(3-methylthiophene). 150. 193 PET. 27. 149 band gap of. 326 PBEDOT-PyrPyr(Ph)2. 54 photo-driven ECD cells. 437 colour analysis of. as secondary electrochromes. 152 phosphonated viologen. 5. photoelectrochromism. 135. 184 tungsten. 329. 81. 405 tungsten trioxide. 331 self-doped polymers. 199 vanadium pentoxide. Prussian blue. 103. 294–5 palladium oxide. 135. 60 permittivity. 28 of WO3. 199 tungsten trioxide. 365. 179 in tungsten trioxide. 70. 133. 330. 405 containing metal oxides molybdenum trioxide. 71 ECDs of. 330. 108. 184 tungsten–molybdenum oxide. 101. 27. 437. 27. 100. 114. 99. plus tungsten oxide. 10. 360 Phenolsafranine dye. 331–2 Perovskite. 331 formed via spin coating. 406 phosphomolybdic acid. 28. 438 of MoO3. 133. 159–60 in praseodymium oxide. 133 forming cobalt oxide. 175 niobium. photoelectrochromism. 140. via electrodeposition. 331 pentyl viologen. 112 Pernigraniline. 330. 438 phosphotungstic acid. 436 poly(pyrrole). 436 PEDOT.8-. 178–9 paper containing hexacyanoferrates. 351. 423 phenanthrenes. 362 . see metal oxyfluoride oxygen as oxidant. 150. 386 Philips. 326 PBuDOP. see NTera phosphoric acid. 407T PEDOT:PSS. 307 palladium dithiolene. 10. 437 as photoconductor. 436 as primary electrochrome. 199 molybdenum–vanadium pentoxide. 269 molybdenum–tungsten oxide. 291. 433 photo-activity. 178 electrochemistry. 181 tantalum. 186 Perspex. 186. 156. 405 containing viologens. 328 PEDOP. 151. 328 PEDOT. 147 oxyhydroxide. 363 phenylenediamine. 129 titanium dioxide. 436 poly(aniline). 434 photochemistry. 436. 438–9 photochromism. 168. formed via sol–gel. 331 formed via layer-by-layer deposition. optical. 167. 141 vanadium–molybdenum–oxide. 133 PAH. 150. overlayer of. 182 titanium. 106. tungsten trioxide. as oxidant. 332. 3. 187 oxyfluoride. 60. 10. 199 titanium dioxide. 366. 309 in titanium dioxide. 103 photoconductors. 128. 433 photoelectrochemistry. 362 viologens. 436 titanium dioxide. 433. 266 palladium hexacyanoferrate. 342 permanganate. 331 PEDOT-S. metal-oxide stability. 141. metal. 59 oxygen backfilling. in solid metal oxides. 28 of SrTiO3. 405 phenothiazines. 421. 334 as secondary electrochromes. see poly(allylamine hydrochloride) paints and pigments of. 141 oxygen bridges. 184. 329. in Nafion. pseudo viologen. 409 specular reflectance. 115 periodate. 395 PBEDOT-Pyr. 85 oxygen deficiency in molybdenum trioxide. 349. 433. 179 ruthenium. 342 molecular. 341 parasitic currents. 156 molybdenum oxide. 445 photocells. gasochromic sensor for. 140 peroxo species. 362. 186 electrodeposition with. 282 palladium. 319. 361. see optically passive patents. 439 poly(o-methoxyaniline). 172 manganese. 177 rhodium. 129 photochromic–electrochromic systems. 434–7 amorphous silicon. 151. 157 percolation threshold. 143 in tungsten trioxide. 312 electronic conductivity. 141. 329. 202 cyclic voltammetry. 258 conductivity electronic. 420 immobilising electrochromes. 328–9. 438. 437 strontium titanate. 423 black. as ECD electrolyte. 331. 266 poised cells. 331 spectroelectrochemistry of. 204T platinum dithiolene.20 -bipyridyl). 437 photo-activated ECD cells. 204T ruthenium dioxide. 436. 157. see electropolychromism polyelectrolytes. composite with silica and poly(aniline). 261–2 Langmuir–Blodgett. 433–9 beam direction. 433. 348. 81 poly(DDTP). 331. 436 photogalvanic. 331 poly(AMPS). 420–2 polyester. 423 poly(ethylene imine). 436 poly(3-methylthiophene). 437 titanium dioxide. 290–1. 327 poly(allylamine hydrochloride). 329–30. 312. 436 PEDOT. 204T tantalum pentoxide. 333 containing vanadium pentoxide. 30. 433 back-wall illumination. 328–30 ECDs. 312 poly(acrylate). and Prussian blue. 316 polycrystalline. 331 poly(aniline)s. 434–7 amorphous silicon. 407T polyelectrochromism. 333. 400 pixels. 330. 9. 12. 51 polaron. 330. 402 platinum as substrate. 261 mixed cation. 150.4-(butylenedioxy)pyrrole]. 260. 133 incorporated into molybdenum trioxide. via sol–gel. 438. 329–30. 439 as photoconductor. 422. 421T. 437 phthalocyanine complexes. 329–30. chemical diffusion coefficient. 423. 436. 101. 150. 385. 407T including aniline moieties. 444T castable films. 261 electronic conductivity. forming nickel oxide. photoelectrochromism. layer-by-layer deposition. 326. 434 polarisation of electrode. 333 with poly(styrene sulfonic acid). ECD electrolyte. 391. 334. ruthenium tris(2. 434 photoconductors. 331 protonation reactions. 391 poly(aniline). 25 photosensitising. 150. made by sputtering. 331 encapsulating dyes. 127. 333. 421. 333. of Ni–C. 262 electro quasi-reversibility. 436 titanium dioxide. 436 poly(pyrrole). 85T poly(CNFBS). 259 ellipsometry. 436. 290. 331. 329–30. 331 response times. composite formed via spin-coating. as ECD electrolyte. 435 front-wall illumination. 315 air sensitive. 329. 76 . 402. 331 graft copolymer of. 316 hopping. 331 poly(ethylene oxide) as ECD electrolyte. 261 requires central cation. 204T tungsten trioxide. 436 silicon carbide. 333 with poly(acrylate). 261 tetrasulfonated. 333 immobilising electrochromes. 88 polished metal. 333 formation via electropolymerisation. 330. 438 photo-driven ECD cells. in WO3. indium–tin oxide substrate. 153. 330. 409. 263 electrochemistry. television. 407 gasochromic. 333 with poly(aniline). formed via sol–gel. 391 poly(acrylate)–silica composite. via electrochemistry. 259 Pilkington Glass. 9. 145. 262–3. 403 plasma oxidation. 50. 437. substrates. 437 photovoltaic. 333 redox states. specular reflectance. 439 as secondary electrochrome. 366. 331 electropolychromic. 328 poly(carbazole). 11. 436 poly(aniline). 391T. 263 formation. 326 poly(diphenylamine). 333 electrochemistry of. 190. 438 photography. 332–3 composites with cellulose acetate. 267. 433. 408. 360. 434 response time. metal oxides. 384. 284. 261 physical vapour deposition. 444 photoelectrochromism. 129. 149. 433. 329. 418 poly(acetylene). 263 ECDs of. 433 poised cells. 421T poly(alkeneldioxypyrrole)s. 395–410. 437–8 cadmium sulfide. industrial. 438 vanadium pentoxide.472 Index of light. 439 poly(o-methoxyaniline). 147 polaron–polaron interactions. 433 photocells. 433 of Prussian blue. 333 poly(acrylic acid). 161 plasma screens. 183. 260 response times. 313. of cerium oxide 166 pigments. 333 poly[3. see pulsed potentials three-electrode. 331 self-doped polymers. 329. 165 as secondary electrochrome. 322 coloration efficiency. 323. 328–9. 331 poly(triphenylamine). 331. 328. 99. windows. 330. 349 potassium triflate. 290 potential.4-(propylenedioxy)pyrrole]. ECD electrolyte. 258. 326 PBEDOT-PyrPyr(Ph)2. 417 electrodeposition. 436 poly(o-phenylenediamine). 328 N-Gly PProDOP. 437 as photoconductor. 165 star polymers. ECD electrolyte. 326 PBEDOT-PyrPyr(Ph)2. 62 potentiostatic coloration. sweep. ECD electrolytes. 436 as primary electrochrome. 425 . 283 power consumption. 70. 407T viologens of. as ECD electrolyte. 331 Polyvision. 291. 13–15 different types of display. 391T. 254–6 TTF species. 30 specular reflectance.N0 -methylenebisacrylamide). 48. 407T PEDOT:PSS. 327 poly(p-phenylene terephthalate). 27 poly(iso-thianaphthene). ion movement rate limiting. 198 as secondary electrochrome. 333 containing Indigo Carmine. 331 PEDOT-S. 133 interrupted coloration. 313. 322T as photoconductor. 443 powder abrasion. 421 poly(vinyl butyral). 409 specular reflectance. 159 poly[3. 197 poly(o-methoxyaniline). 331 PBEDOT-Pyr. 190.3. 315. 331 formed via layer-by-layer deposition. 328 PEDOP. 321T.Index as thickener in ECD electrolyte. 264–5 potassium chloride. 419 poly(1-vinyl-2-pyrrolidone-coN. 327 viologens of. 419 poly(3. 12 and coloration. 436 as primary electrochrome. 149 containing dodecylsulfonate. 326 PEDOT. 60. 291. 354–5 potentiostat. 44 of EDOT. 347 PBEDOT-Pyr. 331–2 poly(methyl methacrylate) blend. 333 electro-synthesis of. 41 potential. 330. 328 coloration efficiency. 323T poly(1. coloration efficiency. overlayer of. 328 PProDOP. 421 poly(pyrrole).151 poly(m-toluidine). 327 poly(vinyl alcohol). 9. 330. ECD electrolyte thickener. 314. 329. 331 poly(thiophene)s. 327 as photoconductor. 57T poly(ethylene terephthalate). 331 poly(oligothiophene)s. 330. 11 electrolyte. 326 bipolarons in. 306 porphyrin complexes. 328 poly(siloxane). 48 potential step and cycle life. 85T polymer electrolytes. 446 poly(o-toluidine). ECD electrolyte. 331 formation via layer-by-layer deposition. 10. 346 poly(pyrrole)s. 333. 391. of Prussian blue. 332 as photoconductor. 165 PPG Aerospace. 333 poly(thiophene). 326 BEDOT-N-MeCz. chemical diffusion coefficient. 330. 332 PBEDOT-N-MeCz. 11. 332 response time. 332. 9. 327 PBEDOT PBEDOT-B(OC12)2. 71 ECDs of. 400 PPG Industries. 321 as primary electrochrome. coloration efficiency. 347 photostability. ECD electrolyte. 326 PProDOT-Me2. 15 poly(methylthiophene). ECD. 421–2 conducting. 331 formed via spin coating. 159. 316. equilibrium. 315.4-propylenedioxythiophene). 391 poly(styrene sulfonic acid). 320–1 poly(toluidine)s.4-ethylenedioxy thiophenedidodecyloxybenzene). 57T poly(3. PBuDOP. 436 as primary electrochrome. 318–27 band structure. 323 BEDOT. 317. 395–410 ECD electrolyte. 313. 325 substituted. 152 Baytron M. 320 DDTP. 347 composite with poly(aniline). 291. 327–8 ECDs of. 332 ITO on. via spin-coating. 330. 390 viologens. 325 polypyridyl complex. 391 poly(viologen). 57T poly(propylene glycol). 149 bandgap of. 330. 326 dihedral angle. 319. 334 473 as secondary electrochrome. 101. 334 poly(3-methylthiophene) 320. 328–9.5-phenylene). 323 Baytron P. immobilising electrochromes. 437 colour analysis of. 323T formation via spin coating. 405 ‘insoluble’. 287 ECD. 405 and cyanotype photography. 139–65 nickel oxide. 388T response times. 197. 437 preparation ‘soluble’. 136. 285–9 . 375. 289–91 comprising single film of. 436. 199. 57. 178 oxygen deficiency. see electrochemical quartz-crystal microbalance quasi-electrochromism. 179 as secondary electrochrome. 283 write–erase efficiency. 289 Mossbauer of. 365 primary reference electrode. 391. 86.474 Index electropolychromism of. 89 in tantalum oxide. 303. 26 and photography. 289 formation via. 87 viologens. 305 coloration. 447 polymers as PEDOT. 106. 283 sacrificial anode methods. 178. 184. 283 ¨ paints and pigments of. 165. 356 PXDOT. 66 purple line. 437 photoelectrochromism of. 313 Q-switching. 187. 333. 385 NTera viologen. 267 quantum-mechanical effects. 65. 26 and drawing. 149. 173. 286 pseudo viologen. 165. 165 viologens as heptyl viologen. 285. 204 formation via rf-sputtering. 282–3 chronoamperometry. 204 preparation. 328–9. 365. 152. 62. 261 viologen electrochromes. across solution–oxide interface. 179 electrochemistry of. 283. 285 electrodeposition. 334. for polymers of pyrrole. 176. 58–60. 384. of metal oxides. 283 history. 178. 283–5 catalytic silver paint. 290 EDAX of. 178–9 cycle life. and colour analysis. 26. 179 dc magnetron sputtering. 438. 334 poly(3-methylthiophene). as ECD electrolyte. 267. 5 propyl viologen. 4. 328 pyrazolines. 81 quartz-crystal microbalance. 288 electrochemistry of. 179. 108 proton transfer. 334 Prussian white. 286. 64 PVPD. see also complementary electrochromism primary electrochromism. Prussian blue. conductivity in metal oxides. 406–7 quasi-reference electrodes. 166. and colour analysis. 284 electroless deposition. 106. 358 quaternary oxides. 400. 204 sol–gel. 131–2 primary and secondary electrochromism. 197 poly(pyrrole). 190. 283 redox cycling. 333. 104 response time acceleration. 283 directed assembly. 170. 179 formation via. 16–17. 151. 186. 11 purity. 291. 149. 282 in paper. 356. 40 quasi-reversibility fullerene electrochromes. 365 enhanced ECD durability. see poly(pyrrole) oxidative polymerisation of. see standard hydrogen electrode probe molecules. 287 ellipsometry of. 290. 188. 446 and blueprints. 333 metal oxides as. 288. 356. immobilising electrochromes. 205. 418. 264 pyrrole. 290. 313. 444T bulk properties. 331 Prussian blue. 178 XRD. 284. 26. 387–8 optical properties. 169. 282 lattice energy. 303. 445 hexacyanoferrates as. 165. 263 precious metal. 100. 86 protonation reactions. 285 Prussian green. 421. 45. 446. 70 cyclic voltammetry. 183 mobility. CVD. 63. 328. 352T propylene carbonate. 288 XRD. 203 praseodymium oxide. 61. 391T. pseudo pulsed potential. 421. tunnelling. chemical vapour deposition. see poly(1-vinyl-2-pyrrolidone-coN. of lasers.N-methylenebisacrylamide) PXDOP. 284 colour analysis of. 179 praseodymium phthalocyanine. 334. 104–5. 9. 286 XPS. 57. poly(aniline)s. 176 tungsten trioxide. 190. 26 powder abrasion. 282. 165 poly(thiophene). 444T. 165. 282 pH effect of. 363. 41. 197. 283 Prussian brown. 291. 289 photochargeable battery of. 25 as secondary electrochromes. in metal oxide. 179 containing cerium oxide. 184. 154. 395–410 PVPD. 305 phthalocyanine electrochromes. 287. 388T pyridinoporphyrazine complexes. 25–6. 363. 419 proton. 283 photolysis. see viologen. 305. 400. 137 metal oxides bismuth oxide. 163. 149. 399. 149. 4. 167 cobalt oxide. 330. 140. 40 475 quasi. 97 RBS. 102 rate limiting kinetics. 384 bromoanil. 95 of electron transfer. 50 rear-view mirrors. 37 redox cycling. 149. 407T miscellaneous lithium pnictide. 81. 149T. 57T Research Frontiers.40 -. 381. see applications. 29 real. 262. see viologen. see electrode potential redox reaction. 40 reflective. 141. 4. 99. 349. 150 mixtures of metal oxide. 155. mirrors rechargeable batteries. 30. 382 contrast ratio. 115. 49. 159 molybdenum trioxide. 57T response time. 169. 98. 385 ECDs of. 167 manganese oxide. 139 of charge transfer. coloration efficiency. 363. 348. of Prussian blue. 157 primary standard. 92. 34. 40 saturated calomel electrode. p-. 58. 375 pyrazolines. 331 reference electrode 40. 199. 383 p-. 139. impedance. 364 reversibility. 398 resistance. 183–4 tin oxide. 148. 164 tantalum oxide. 384 optical properties. 155. 143 ionic motion. 157. effect of. 102 redox potential. 41–6 of coloration. 188. 176 redox couple. 382. 1. 83 rate of cell operation. 48. 86. 88. 383 solubility product. 160. 156. 11 ITO. 86. and ECD self-erasure. 86. 39 rf sputtering. 35. 83 crystal structure changes. see Rutherford backscattering RCA Laboratories. 407T polymers PEDOT. 33. coloration efficiency. 363 NTera viologen. 42–3. 256. 407T poly(diphenylamine).Index quinhydrone. 261 lutetium phthalocyanine. 75 of electronic conduction. viologen. 283 redox electrode. manganese oxide. 381–5 amino-4-bromoanthaquinone-2-sulfonate. 407T tungsten oxyfluoride. 11 tetrathiafulvalenes. 166 chromium oxide. electron transfer. 381. 130. 350 silver–silver oxide. 40 silver–silver chloride. 384 benzoquinones o-. 382. 390T viologens. 436 phthalocyanine complexes. ECD. 383T quinhydrone. o-. 163. 386 radical. 87 electronic motion. 303. 182. ionic. p-. 410 secondary. 35. 101. 112 Raman spectroscopy. 112–13. 388T photoelectrochromism. 10–11. 407T metal oxides copper oxide. 42 rate constant. 384 2-ethylanthraquinone. 384 type-I electrochromes. 148 . 387. 164 tungsten trioxide. 382 bis(dimethylamino)diphenylamine. 54 redox states. 382 type-II electrochrome. 148–9. 384 cyclic voltammetry. 384 electrode potentials. 162. 407T iridium oxide.4-. 166 iridium oxide. 175 molybdenum trioxide. 384 fluoroanil. 349 to charge transfer. 329. 115 niobium pentoxide. 149T. tungsten–cerium oxide. 70T. 260. 105 Resorufin. 274 metal oxides bismuth oxide. 407T rhodium oxide. 383 catechole. 42 of mass transport. 357 ˇ Randles–Sevcik equation. 50 of electrode substrate. 406. 407T Resazurin. 383T electropolymerisation. 48. 188 tungsten trioxide. 151 nickel oxide. of poly(aniline). 407T poly(pyrrole). 384 radical annihilation. 382. 268. 141. 384 quinones. 400. 270–4 chloranil o-. 184 tungsten trioxide. 390 diffusion. 361. 183 titanium dioxide. 101. 386 Gentex mirror. 374 redox pairs. 154 nickel oxide. 382 naphthaquinone. 346. 70T. 193 organic monomers aromatic amines. 58. 384. 150. 331 poly(thiophene)s. 146. 103. 351. 261 polymers poly(aniline)s. 50 redox indicators. 40. radical radii. 325 pulsed potentials acceleration. 384 aminonaphthaquinone. 349. 46. see ruthenium purple ruthenium polypyridyl complexes. 292 XRD. as photoconductor. 16 rotated ring-disc electrode. 199 titanium dioxide mixtures. conducting polymers. 283 Safranin O. 348. 57T SAGE Incorporated. 4 Seebeck coefficient.476 Index tungsten trioxide. 290–1. 178 praseodymium oxide. 436 silicon phthalocyanine. 149. 140 sealing. 263. 365. 363. 8. 268 ruthenium dioxide. 15. 333. of Prussian blue. 256 ruthenium purple. 382. ECD like. 199. 188 mixtures of metal oxide indium–tin oxide. 125. 40 second-harmonic effects. 309 ruthenium hexacyanoferrate. 89 samarium–vanadium oxide. 48 scanning tunnelling microscope. 385 oxyhydroxides. 444T poly(p-phenylene terephthalate). 169. 176 nickel oxide. 103. 199 hydrogen evolution at MoO3. 43. 181 hydrated. 185. 444T metals. 375. 159 secondary reference electrodes. 270–4. 180. 181 electrochemistry. 165. 317 semi-solid. 204T ruthenium dithiolene complexes. 203 Rutherford backscattering. 283 rocking-chair mechanism. 404–5 side reactions. 334. 27. 65. 184 tungsten trioxide. 179 sol–gel. 188 Robin–Day classification. 349 electrodeposition of. 179 tin oxide. 447 niobium pentoxide. 190. 255T ruthenium–iridium oxide. 303. 165–90. 181. 181 formation via electrodeposition. 149 poly(aniline). see standard hydrogen electrode shear planes. and. 400 SchottDonnelly mirror. 180 reflective. 181 electrochemistry of. 421 hexacyanoferrates. 160. 400 polymers PEDOT. 149. Prussian blue. 165 titanium dioxide. 157. 307 rf sputtering (cont. 334. 174 manganese oxide. coloration efficiency. 265. 170 copper oxide. 149. see saturated calomel electrode Schott Glass. as. 202 coloration efficiency. 179–81 annealing of. 424 carbon ink. 262. 127 sacrificial anode methods. 70 scan rate. 363 tetramethyl phenylenediamine. 39 salvation stabilisation. 153 self-doped polymers. 165 iridium oxide. 202 saturated calomel electrode. 358 ruthenium complexes. nickel. 181 cyclic voltammetry of. ECD. 180 formation via anodising Rh metal. 181 incorporating platinum. 284 SCE. ECD electrolyte. 150. 447 salt bridge. 205 rutiles. 410 saturation. 437 effects of ligands. 266 photosensitiser. 16–17.20 -bipyridyl). 181 coloration efficiency. 54 secondary electrochromism. 40. see surface-enhanced Raman spectroscopy SHE. 264 silicon–cobalt–aluminium oxide. 309 dinuclear. 149. 149. 54. ECD secondary battery. 149. 444T metal oxides cobalt oxide. 444T indium–tin oxide. ECD electrolyte thickener. 363. PEDOT-S. bismuth. 199 silica. 155. 54. ECD. 204 rhodium oxide. and colour analysis. 305 . 312 SERS. 292 ruthenium tris(2. 418. 113 self bleaching. 444T. 444T iron oxide. 438. 201 tungsten–molybdenum oxide. 419 silicon carbide. 76. 446 sensors. 165. 202 sapphire. 446. 290. 200 silver conductive paint. of ECD. 187. 63. 356. 103 shutters. 421 vanadium pentoxide.) vanadium pentoxide. 204 silicon–iridium oxide. 436. see encapsulation. oxidising Ru metal. 199 precious metal incorporation. 198 silicone–niobium oxide. 48. 406. 387 semiconductor theory. 444T mixtures of metal oxide cerium–titanium oxide. 331 self-erasing ECD mirrors. 142. 285. 56T. ECD applications. 196 molybdenum–tungsten oxide. 362. 66. 197 titanium–cerium oxide. 398. 330. 444T organic monomers phenothiazines. 397 screen printing carbon black. 267–8 trinuclear. 56T. 149. see also dc magnetron sputtering. 175. 135 formation of metal oxides. 135. 41 solubility product bromoanil. 168. 176. 200 molybdenum–niobium oxide. 327 spirobenzopyran. 312 solar-powered cells. 333 forming metal oxides. 141 sputtering product oxide is polycrystalline. 195 indium–tin oxide. 5 smart windows. 350 silver–silver oxide. 397 solid solution electrodes. 178. 136 indium–tin oxide. 58. plus phosphotungstic acid. 190. 266 solar-energy conversion. 27 solar energy storage. 185 forming mixtures of metal oxides cobalt–aluminium oxide. 135. 204T silver oxide. 135. 180. 152 nickel oxide. 135. 445 tungsten trioxide. see suspended-particle device speciation analyses. 184 tungsten trioxide. see reflective spillover. 135 titanium dioxide. 134–6 cobalt oxide. 196 iridium–titanium oxide. 198 iron–niobium pentoxide. 40 standard observer. 110. 149 vanadium pentoxide. 204T tungsten trioxide. mirror of. 192 ski goggles. 133–4 Sony Corporation. 174 manganese oxide. reference electrode. windows non-electrochromic. 5 sodium tungsten bronze. 47 standard hydrogen electrode (SHE). 199 with precious metals. 135 formation of metal oxides. 201 formation of poly(acrylate)–silica composite with poly(aniline). 128–30 photochemical. 70T. 161–3 niobium pentoxide. 363 smart glass. 135. 135–6 cerium oxide. 36. 141. 135. 198 iridium–silicon oxide. 201 tungsten–molybdenum oxide. 158 molybdenum trioxide. reference electrode. 169 iridium oxide. evaporation and rf sputtering stability metal oxide. see also ECD. 400 standard electrode potential. in titanium dioxide. 200 niobium–iron oxide. 398 smart cards. 181 titanium dioxide. 135. 136–8. 133 spectral locus. 204 with titanium butoxide. in colour analysis. 65 spectroelectrochemistry. 330. 200 titanium dioxide mixtures. 15 sol–gel formation of phosphotungstic acid. 37. 160 tungsten trioxide. 201 titanium dioxide. 407 spin coating annealing of product. ECD application. windows. 143 Stadsparkasse Bank. 152 nickel oxide. 135. 196 formation of polymers PEDOT-S. 359 solvatochromism. 195 copper oxide. 351 space charges. 105 SPD. 134. metal-oxide mixtures. 376 SPM. ECD.Index incorporated into indium–tin oxide. 135 cerium oxide. 200 rhodium oxide. conducting polymers. 81 sputtering in vacuo. 135 iron oxide. see impedance specular reflectance. 135. 40 SIMS. of electron transfer. 176 molybdenum trioxide. 200 niobium–molybdenum oxide. 135. and. 333 spectroscopy. 135. 438 spiropyrans. 170 iridium oxide. 156 iron oxide. ECD applications. 199 nickel–tungsten oxide. 135. 200 molybdenum–tungsten oxide. windows SmartPaper. 333 polymeric polypyridyl complex. 397. see ECD applications. 63. o-. 135. 383 477 viologens. see solid polymer matrix spray pyrolysis annealing of product. 135 cobalt oxide. 265. 186 formation of mixtures of metal oxide. 111–12 SIROF. 64. 125. electron-beam sputtering. 331 poly(acrylate)–poly(aniline) composite. ECD applications. 173. 3 sonication. 204T vanadium pentoxide. 185. impedance. 177 tantalum oxide. 166 cobalt oxide. 135. 135. 40 standard exchange current. 135 niobium pentoxide. 141 vanadium pentoxide. 47 . 64 standard rate constant. 349. 131. 351. 174 molybdenum trioxide. 135. 40 of hydrogen electrode. 129 electrolyte. 184 tungsten trioxide. 254 poly(thiophene)s. 40 silver–silver chloride. 168. 155 site-saturation model. poly(aniline)s. 135 solid polymer matrix. 128. 47 standard exchange current density. 334 tantalum oxide. 326. 204T protonic motion. 171. 423 resistance of. 183 tantalum oxide. 190 thermodynamic enhancement. 421T STM. 446 as ECD electrolyte. ECD encapsulation. metal hydrides. 390T response times. 203 coloration efficiency. 375. 402. 28 nickel doped. 390T Texas Instruments. 284. changes in electrochrome. 28. 183 overlayer of. 421T. 409. applications. coloration efficiency. 423 magnesium fluoride. 424 durability of. potentiostatic coloration. 203 TCNQ. 151. 385. 70T. 95 Tafel region. 159. 46 tailoring. ECD electrolyte. 164. 356. 159 star polymers. 257. see derivatised electrodes tetracyanoquinonedimethanide species. 183–4 spin coating. 86. 84 thiazines. 129 optical properties. 447 electrochemistry. 353 sulfuric acid. 112 enhancement factor W. 182. 333. in crystal lattice. 191. 182. 181. 166. 136. 402 pixels. 437 sublimation. 196. ECD electrolyte. 56T. of counter ions. see scanning tunnelling microscope stress. 423 tethered electrochromes. 389T reversibility. 402 temperature management. 182 dc magnetron sputtering. 139. 349. 406 viologens and effect of. 17. 390 optical properties. 183 formed via anodising a metal. 326. 135. 401 supporting electrolyte. 330. 312. 422. 128. 44 surface enhanced Raman spectroscopy viologens. 83–5. see oxygen deficient substrates antimony–copper alloy. 51. 385–7 ECDs. 182 evaporation. 294. 152. 86. 160 thermoelectrochromism. ECD. 354 titanium dioxide. 388 tetrahydrofuran. 182 dip-coating. 389 write–erase efficiency. 409 television flat-panel screens. 293. 168. see also electrostriction thiophene. 321T thiophene acetic acid. ECD application. 387. 409. 420 degradation by. 420. 306. 305. 156. 349. 403 plasma screen. 132. 352 thermal evaporation. 404. 356 as secondary electrochrome. 443 tin oxide. 183 . see electrolyte thickener thickness. 423–4 opaque. 183 tantalum–zirconium oxide. 182 CVD. 390 ion tunnelling. 408 lithium vanadate. 96. 358 graphite. 309 photovoltaic. 205. 167. 292. 259 substituted poly(thiophene)s. of colours. voltammetry and. see tetracyanoquinodimethanide Teflon. 307 symmetry factor. 30. 139. 424 glassy carbon. 181. 182 laser ablation. 375. 420 sunglasses. 43. 357 surface potentials. 83. 444–5. 47 deviations from. 130 strontium titanate. 75. 320 three-electrode. 181–3. 387. 166. 61 stibdic acid polymer. 183 rf sputtering. 422–3. 259. 4 surface states. 356 Surlyn. of poly(thiophene)s. 150 plus platinum. 164. 82. 285 indium–tin oxide. 418. 183 ion-conductive electrolyte. 149. 362 carbon. 284. of lutetium phthalocyanine. 265 terminal effects. 25. nickel oxide. 138. 331. 135 water adsorbed on. 48 Tafel’s law. 158. 422–4 fluorine-doped tin oxide. see scan rate switchable mirrors. 425 suspended-particle device. 422 gold. 3-. see Methylene Blue thickener.478 Index mechanical stability. 382. 4. 150. 385 Methylene Blue. 5 swamping electrolyte. 388–9 optical properties. 389–90 ion hopping. 417. 289. 46. 423 suppressors chromium oxide. 153. 178. 141. 327 Stark effects. 400. 76 swapping. 86. 447 K-glass. SPD. 444–5 ECD. 183 as ionic conductor. 198. 406. 327 tetramethylphenylenediamine. 423 antimony-doped tin oxide. 87 sweep rate. 86 surfactants. 150. 11 tin oxide. 423–4 platinum. 409. 313 oligomers. 129. 362. 385 tetrathiafulvalene species. 422 metallic. see evaporation thermal instability. 47. 320–1 sub-stoichiometry. 346 tris(pyrazolyl)borato-molybdenum complexes. 27. 47 transmittivity. 130. 198 titanium–iron oxide charge transfer. 178. 205 tin phosphate. 196 electrochromic host. 130. 201 titanium–cerium oxide as secondary electrochrome. 67 TTF. electrochemical. 79. 103. 106 . 363 transfer coefficient. 203 titanium–vanadium oxide. 190. 135. 199. 446 specular reflectance. 290. 446 coloration efficiency. 63 toys. 187. 444T. 150. o-. 437 as secondary electrochrome. 438 photostability. 399. 184 electrochromic host. 131. 202 titanium–zirconium–cerium oxide. 1153 photovoltaic. 109. 140. 88. 438. 185T spin coating. 205 coloration efficiency. 407 Toluylene Red. 184. 153. 113 Anderson transition. 407T formed via dc magnetron sputtering. 184 titanium butoxide. gasochromic. 443. 203 formed via electrodeposition. 201 mechanical stability. 205 cycle life. 206. 201. 142. 421T. 139–51. 135. 184 479 titanium propoxide. 184. 206 overlayer of. 81. 201 tin–molybdenum oxide. 40. 99. 135. 421 electrochemistry. 63. 104 tolidine. 202 titanium–molybdenum oxide. 150. 202 optically passive. 44. 201 plus ferrocyanide. 194 anatase. 400. 28. 184. see antimony-doped tin oxide fluorine-doped. 184 diffusion coefficient. 88. 444T formed via dc magnetron sputtering. 165. 184. 183 substrate. 111T amorphous. 81. 183 Mossbauer spectroscopy. 156. 205–6. 203 titanium–cerium–vanadium oxide. 62 transport number. 16. 159. 201 formation via rf sputtering. 89 tungsten hexacarbonyl. 334. 184 formation via alkoxides. 141 forming tungsten trioxide. 203 titanium–zirconium–vanadium oxide. 305. 184 coloration efficiency. 125. as ECD electrolyte. 303. 184 nanostructured. 202 titanium–zirconium–cerium oxide. 129 ellipsometry. 199 titanium–nickel oxide. 205 electrochemistry. 437 substrate. 135 titanium dioxide. 289. see tetrathiafulvalene tungstate ion. 421 bleaching. 184 infrared max. as ECD electrolyte. 170. 185T optically passive. 291. 35. 110. 445 photoconductor. 184 rf sputtering. 111. 438. 333. 445 electrolyte filler. 149. 202 formed via dip-coating. 184 oxidation of Ti. 184 ECD electrolyte. 446 activation energy. 203 titration. 184 evaporation. 115. 199 coloration efficiency. 446. 406 titanium oxyfluoride. see fluorine-doped tin oxide nickel-doped. 165 doped antimony-doped. 125. 268 tristimulus. and colour analysis. 397 tungsten oxyfluoride. 199T–201T titanium alkoxides. 201 formed via electrodeposition. 205 electrochemistry. 437. 184. 205 titanium oxynitride. 201 plus phosphotungstic acid. as ECD application. 9. 184 thermal evaporation. 8. sensor for. 269–70 tris-isocyanate complexes. 200 titanium–tungsten oxide. 184 peroxo species. 201. 202 titanium–tungsten–vanadium oxide. 57T tone. 179. 360–4 optical properties. 419. 437. 83 transport. 201 sputtering. 191. 136 titanium–cerium–titanium oxide. 184 ¨ optical properties. sol–gel precursor. 148 as primary electrochrome. 154. 201–2 formed via sol–gel. 421. 203 titanium–iridium oxide. 184 coloured with pulsed current. 184 dip coating. 197. from degradation of WO3. and colour analysis. 154 tin–cerium oxide. 447 as secondary electrochromes. 185T sol–gel. 354 tin oxyfluoride. 10. 199 coloration efficiency. 201 titanium–niobium oxide. 75 triflic acid. 12. 56T. 78 toluene. 185T laser ablation. 436. 205 tungsten trioxide. 77. 10. 205 formation via dc-sputtering. 200. through liquid electrolytes. 436. 11. 81. 184 photo-activity. 149 annealing of. 11. 25. coloration efficiency.Index as secondary electrochrome. 410. 308. 421 trimethoxysilyl viologen. 356. 141. 397 electron mobility. 401 watch displays. 437 photochromism. 148. 106 reflective effects. 150 tungsten–cerium oxide. 148. 142 electronic. 135. 191. 80 involves WIV. 446 photo-chargeable battery. 400. 85T. Perspex. 200 formation via sol–gel. 103 proton-free layers while bleaching. 141 rf sputtering. 89 morphology. 46. 141. 28 conductivity. 140–1. 16 colloidal tungstate. source of F-centres. 403. 81. 402. 81. 148T electrodeposition. 143 electron localisation. 104. 87. 28 electrostriction. 60.) bronze. indium. 143 gasochromic. 140. 203 tunnelling. 150 organometallic precursors. 405 ECDs of. 83 low conductivity of. 397. 140. 33. 140 polycrystalline. 54. 129. 398. 419. 408. 149 windows. 135. 149 dissolution in acid. 195T tungsten–molybdenum oxide. 140. 45. 143 insulator at x ¼ 143 ionic. 143–4 stability. 149. 145 intervalence. 150 structure crystal phases. 150 peroxo species. 88. 131. 88. 192 ECD. 396. 89. 346. 141 deposition in vacuo. 418 ECD. 199 intervalence. 81 Tyndall effect. 375 coloration kinetics. 134 type-I electrochromes. 397 dc magnetron sputtering. 135. 132. 142 electrophotography. 89. 354. 192. 145 coloration mechanism a two-electron process. 145. 43. 81 metallic at high x. platinum. 88 dry lithiation of. 129. 113. 143. 139. 445 ellipsometry of. 135. 141 oxidising W metal. 200. 200 tungsten–niobium oxide. 141. 149. 202 neutral colour. 192 tungsten–nickel oxide. 204T neutron diffraction. 148–9. 141. 191. 28. 149. 115. 397 electrodeposition. 27 in paper. without electrolyte. 149 sunglasses. 145 polarons. 201 spectrum. 202 tungsten–vanadium oxide. 149T coloration efficiency.480 Index memory effect. 136. 146. 83. 192 formation via CVD. 135. 103. 144 overlayer of. 144–9 colour. 204T. 99. 400 formation via. 147 perovskite. 141. 199 sol–gel. 102. 199 peroxo species. 81. 195 formation via dc magnetron sputtering. 199 rf sputtering. 140 sol–gel. 150 hydrated. 193 tungsten–cobalt oxide. 417. 204T. 143 ferroelectric properties. 130. 193. 147. gold. 56T. 149 spin coating. 399 tungsten–vanadium–titanium oxide. 150 mixtures of. 29. 407 plus bismuth. 56T. 141. first. 399. 140. 98. 201 coloration efficiency. 148T. 10. 109 chemical degradation. 133. 104 cubic phase. 359. 61. by reflection. 130. 150. 148T screen printing. 89 chemical diffusion coefficient. 103 coloration. 136 response time. 86 crystalline. silver. 85 chemical reduction of. 328. 410 ECD applications display devices. 148T evaporation. 195T colour. 199 amorphisation. 144 optical effects. 129 dip coating. 403 Gentex mirror. 148T spray pyrolysis. 81 structural changes. 409. 396. 400 tungsten trioxide (cont. 143 water and. 191–3. 144 charge transport through. 425 aromatic amines. chemical diffusion coefficient. 142 dielectric properties. 149 mirrors. 25. 103 ‘complicated’. 140. 141 electrochemistry. 193. 201 tungsten–titanium oxide. 75–9. 407T mechanical stability. 129 . 148T. 407T response time. 84T. 140 oxygen deficiency. 193. 81. 27. 99 electrons are rate limiting. 141 CVD. 436 coloration efficiency. 193 coloration efficiency. 140. 147. 145 oxygen extraction. 143 polaron–polaron interactions in. 148T. 149. 190. 342–5. 186. 45. 391 types. 189. 202 vanadium–magnesium–nickel oxide. 188 mixtures as electrochromic host. 349. 199 vanadium–neodymium oxide. 7–9 u0 v0 uniform colour space. 385 asymmetric. 204T with melamine. 109. 79–115. 185–90. 83 formation via chemical tethering. 292 XPS. 351 . 341–66. 67. ECD. 75–9 electrodeposition of metals. 185. 186 electron-beam sputtering. 190 vanadium ethoxide. 189–90 electrochemistry. 346. 346. of electrochrome. 87. 399 video display units. 188 cyclic voltammetry. 17. 149. 135 electrodeposition. 438. 361 viscous solvents immobilising. 185 vanadium–dysprosium oxide. 56T. 185 laser ablation. 445 bleaching of.Index naphthaquinones. 202 optically passive. 348–9. 187 as secondary electrochrome. 355. 352. 185. 46. 188–9 coloration efficiency. 186 vanadium propoxide. 305 viologens. 130. 189 XRD. 188 electrostriction of. 71 Ucolite. 382 coloration kinetics. 54. 185 peroxo species. 156. 446 annealing. see also coloration models viologens. 202 with silver. 354 type-III electrochromes. 185. 46. 12 cycle life. 417. 91–115. 204. 131 vanadium hexacyanoferrate. 10. 87. 71 UV electrochromism. 403 concentration gradients. 81 value. 293 vanadium pentoxide. 16. 402. 202 XPS. 66. 357. 188 xerogel. 190T dc sputtering. 400 underlayers. 138–206 481 evaporation. 359 chain length. see viologens. 185 xerogel. 203 vanadium–molybdenum oxide. 356. 164 uniform colour space. 79. 384 type-II electrochromes. 187 dissolution in acid. 407. 33. 132. 202 vanadium–titanium oxide. 186–8 quasi-reversible. 357–8. 375 bleaching. 202 vanadium–titanium–cerium oxide. 403. product oxide is amorphous. 86. nickel. 81 charge transfer complexation. 78. 351. 202 vanadium–nickel oxide. 353. 70. 346. 188 chemical diffusion coefficient. 352–4 covalently tethered. 135. 185 anodising vanadium metal. 419. 190T sol–gel. 67. 187 formation via cathodic arc deposition. 361 kinetic modelling. 190T spin coating. 305 diffusion coefficients through. 165 vacuum evaporation. 187. 185 intervalence effects. 16. 185. 303. 204T optical properties. 444T bleaching rate. 186 ECDs of. 203 vanadium–tungsten oxide. 190T structure. 374 viologens. 129 ellipsometry. 376 chloranil. effect of. 202 neutral colour. 186 write–erase efficiency. 202 coloration efficiency. 202 vanadium propoxide. 79–115 coloration. 79–115 carbazoles. 360 bleaching. 190. 202 composites with gold. forming vanadium pentoxide. 135. 190. 188 monoclinic. 85T coloration rate. 63 vanadium dioxide. 202 with poly(aniline). 185. 79–115. 186 rf sputtering. 292–3 cyclic voltammetry. 397. chemical. 188 cycle life. 346. 186 flash evaporation. 202 optically passive. 185. 136. 185 CVD. 188. 365 comproportionation of. 359 degradation of. 202 vanadium–samarium oxide. 352 counter ions. 202 coloration efficiency. 203 vanadium–titanium–zirconium oxide. 56T. 185 dip coating. 186. 70. 425 aromatic amines. 359 potentiostatic. substituent charge movement through solid layers of. and colour analysis. 45. 362 cyclic voltammetry. 399. 417. 403 violenes. 365 contrast ratio. 303. 203 vanadium–titanium–tungsten oxide. 12. 358 via pulsed potentials. 355. 357 oiling. 161. 89. 353.) derivatised electrodes. 391. 363 solubility product. 357–8. 352T. 145. 356–60 and tethered electrochromes. 348 di-reduced. 343. 150 coloration. 359 substituent. acceleration. 352. 353. 405 infrared spectroscopy of. recrystallisation. 352T methyl. 351. 354 electron transfer rate. 356–60 viscous solvents forming type-III electrochromes. 153 Prussian blue. 346. Nanochromics and NTera ultra fast. 53 change with insertion coefficient for WO3. 358 radical. 352T reduction. 53 Wien effect. 352T ethyl. 351. see also recrystallisation chemical oxidation of. 348. 103 vanadium pentoxide. 349. 344T. p-. 346 poly(thiophene). 405 in paper. 362 see also cyanophenyl paraquat. 64 whitener. 443 of tungsten trioxide. 354 type-III electrochrome. 348. 351 extinction coefficient. 286. 444 ionisation of. 351–2 alkyl. 89 degrades metal-oxide films. 385 memory. 361. 395–410 visors. 363 colours of. 354–5 electrochemistry. 348–9. 202 Xerox. 359 radical. 4 white point. 344T coloration efficiency. 391T diethyl terephthalate. 361 write–erase efficiency. 343. 54 voltammetry. 189 viologens (cont. 357. 3. 348. 346–8. 391T. 391T thickeners poly(AMPS). 342. 384. 361 type type-I electrochrome. 156. 358 radical. 357. 163 solid oxide films. 356. 328. 363 electrochemistry. 352T propyl. 349. 5 XPS of indium–tin oxide. ECD. 365. 364 phenanthroline.1529. 344T. 390T. 362. 391 PVPD. see heptyl viologen hexyl. 87. 360–4 tethered. 362 photostability. 360 radicals of aging effects. 351. 391T Methylene Blue. 89. 351. 331. 197. stability. 90 counter-ion interaction. 344T. 360 optical properties. 159. 359 dimerisation. 351. 391 poly(siloxane). 356 mixed valency of. 288 tungsten trioxide. 351. 422 windows. 355. 366. 353 on nanostructured titania. 48 write–erase efficiency.8-. see methyl viologen pentyl. 358 electrodeposition. 362 paper quality. 352T. heptyl viologen. 259. 354. 362 micellar. 144–9. 357 five-colour. 346 xerogel. 156. application. 89 occluded. 185 vanadium pentoxide. 8. 359 type-II electrochrome.482 Index effect of. in ECD electrolyte. 164. 351. 358 ECDs. 418. 355–6 critical micelle concentration. cyclic. cyanophenyl. 347 poly(pyrrole). 359 electropolychromic.20 -. 349 polymers of. 176 molybdenum trioxide. 358 butyl. 391T dimethyl terephthalate. 39 watch face. see cyanophenyl paraquat benzyl. 349. 96–7. 357. 89 and tungsten trioxide. 12. 89 wavelength maximum. ECD. ECD application. 129 pseudo bipyridine. 356 modified. nucleation. multi-step. 346. 328–9. 359 aryl. 344T. 354–5 occurs via nucleation. 346. ECD windows working electrode. 89. 129. 352. 361. 347 oligomers. 365 ESR. 11–12. 445 iridium oxide. see applications. 356 in Nafion. 346. 356. 358 memory effect. 348. 352. 438 heptyl. 352T substrates . quasi-reversibility. 391 diacetylbenzene. 343. 149 water adsorbed. 354 response time. 2. 391 immobilised electrochromes carbazoles in. effect on. 401 volatile memory. see cyclic voltammetry voltmeters. 355. and colour analysis. 352T octyl. 183 and molybdenum trioxide. 364 photoelectrochemistry. 349. 26 manganese oxide. 355. 128–9 dissolves ITO. 346. 391 poly(aniline). 140. 421T electrochromic host. 203 Z-scale. 203 zirconium– titanium–vanadium oxide. 283 ruthenium purple. Kosower. 291 yttrium–nickel oxide. 63 YAG laser. 185. 292 tungsten trioxide. 267 ytterbium phthalocyanine. 291 colour source. 343 483 . ECD electrolyte. 261. ECD application. 203 zirconium–cerium–titanium oxide. 203 coloration efficiency. 200 zinc iodide. 408 zinc phthalocyanine. chemical diffusion coefficient. 203 zirconium–tantalum oxide. ECD electrolyte. 141 vanadium pentoxide. 85T zinc TPP. 266. 397 XRD of molybdenum trioxide. 203 zirconium–cerium oxide. 203 electro-inert. 202 XYZ-tristimulus. 179 Prussian blue. 153 praseodymium oxide. 261 formed via plasma polymerisation. and colour analysis. 264 zirconium dioxide.Index X-ray reflector. . 4 x 0.5 y 0.8 W Plate 1 Colour CIE 1931 xy chromaticity diagram with labelled white point (W).6 0.3 0.4 0.1 0.1 0. Plate 2 A series of neutral EDOT and BEDOT-arylene variable colour electrochromic polymer films on ITO–glass illustrating range of colours available.6 0.3 0.0. Chem. by permission of The American Chemical Society. A. and Reynolds.2 0.0 0.5 0. G. (Original figure as used for published black and white photo from Sapp. S..7 0. ‘High contrast ratio and fast-switching dual polymer electrochromic devices’.) . 10.8 0. J.0 0. Mater.9 0. 1998.2 0. Sotzing.. 2101–8. R.7 0. C. ‘Multichromic copolymers based on 3.) . and Reynolds.4-ethylenedioxythiophene)]-N-alkylcarbazole derivatives’.O O x S O BiEDOT O O S + y S O O O S N CH3 BEDOT-N MeCz electropolymerization O S O O S O S O O O O S x O S O O O S S y CH3 N S O O O N CH3 O Comonomer Solution Composition 100% BiEDOT 90:10 80:20 70:30 50:50 30:70 20:80 10:90 100% BEDOT-NMeCz Neutral Polymer λmax (nm) 577 559 530 464 434 431 429 420 420 Neutral Electrochromic Response (Photograph) Plate 3 Representative structures and electrochromic properties of electrochemically prepared copolymers of varied compositions.6-bis[2-(3. by permission of The American Chemical Society. 6305–15. 36. (Figure reproduced from Gaupp. 2003. R. Macromolecules. L. J. 2001.. 13. The other three panes are bleached. The top right pane has been electrocoloured. Adv. with permission of VCH–Wiley. 783–93. Granqvist. G.) Plate 5 All-solid-state electrochromic motorcycle helmet manufactured in Sweden by Chromogenics AB. and the secondary layer is NiOx.) . R. ‘Electrochromic systems and the prospects for devices’. (Reproduced with permission from Rosseinsky. The primary electrochrome layer is WO3. and Mortimer. R.Plate 4 Gentex window of area 1 Â 2 m2. J. of Uppsala University. D. (Reproduced with permission of Professor C. Mater. Adv. L55–8. 145. A man is just visible beneath the nearest. J. and Jain. 1998.Plate 6 Pixel array showing no cross-talk between close picture elements (‘pixels’). Chen. Mater. J.) . with permission of VCH–Wiley. J. W.) Plate 7 Gentex windows being tested in Florida. R. Johnson. R.. A. I. M. 2001. N. The unconnected pixels experience insufficient potential for coloration spread to ensue. even though the electrochromes (TMPD and heptyl viologen) are always in solution. A. Soc... and Mortimer. with permission of The Electrochemical Society. with solution-phase electrochromes. (Reproduced with permission from Rosseinsky. Liapis. (Reproduced with permission from Leventis.. 13.. ‘Electrochromic systems and the prospects for devices’. The pixels can be made virtually microscopic in size. 783–93. ‘Characterization of 3 Â 3 matrix arrays of solution-phase electrochromic cells’. D. Electrochem.

Electrochromism and Electrochromic Devices

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