Metal Oxide Nanostructures Synthesis Characterizations and Applications (1)

Metal Oxide Nanostructures; Synthesis, Characterizations and Applications *1S.C. Singh, 2D.P. Singh, 3J. Singh, 3P.K. Dubey, 3R.S. Tiwari and 3O.N.Srivastava 1 National Centre for Plasma Science and Technology (NCPST), School of Physical Sciences, Dublin City University, Dublin-9, Ireland 2 Thin Film Nanotechnology Laboratory, Department of Physics, Southern Illinois University, Carbondale, USA 3 Condensed Matter Physics & Hydrogen Lab., Department of Physics, Banaras Hindu University, Varanasi - 221005, INDIA *[email protected]; Phone Number: +353-1700-7787 Table of Contents 1. Metal Oxide Nanostructures and their applications 2. Zinc Oxide Nanostructures; Synthesis, Characterizations and applications 2.1: Introduction 2.2: Crystal Structure and Physical Properties of Zinc Oxide 2.3: Synthesis of ZnO Nanostructures 2.3.1: Synthesis of ZnO Nanostructures by Solution routes 2.3.1.1: Precipitation Method 2.3.1.2: Hydrothermal Method 2.3.1.3: Solvothermal Method 2.3.1.4: Sol-gel Method 2.3.1.5: Micro emulsion Method 2.3.1.6: Combustion Synthesis 2.3.1.7: Electrochemical Synthesis 2.3.1.8 Sonochemical method 2.3.1.9 Laser ablation on solid liquid interface 2.3.2: Gas phase methods 2.3.2.1: Chemical Vapor Deposition (CVD) 2.3.2.2: Physical Vapor Deposition 2.3.2.3: Spray Pyrolysis Deposition 2.4: Applications of Zinc Oxide 2.4.1: Semiconductor nanolasers 2.4.2: Light emitting diodes (LEDs) 2.4.3: Solar Cells and light detectors 2.4.4: Electronic device fabrication 2.4.5: Hydrogen generation and storage 2.4.6: Sensors 2.4.7: Water and Air Purification 2.4.5: Cancer Treatment 2.4.6: Generation of PV Electricity 2.4.7: Air Purification 2.4.8: Biological and medical Application 2.4.9: Other Applications 2.5: Summary 3. Cuprous Oxide (Cu2O) Nanostructures; Synthesis, Characterizations and applications 3.1 Introduction 3.2 Crystal Structure and Physical Properties of Cuprous Oxide 3.3 Synthesis of Cu2O Nanostructures 3.3.1 Synthesis of Cu2O Nanostructures by Electrodeposition 3.3.2 Synthesis of Cu2O Nanostructures by Anodic Oxidation 3.3.3 Synthesis of Cu2O Nanostructures by Chemical Methods 3.3.4 Synthesis of Cu2O Nanostructures by Hydrothermal Process 3.4 Applications of bulk Cuprous Oxide 3.5 Application of different Cuprous Oxide Nanostructures 2 3.6 Summary 4. Titanium Dioxide Nanostructures; Synthesis, Characterizations and applications 4.1: Introduction 4.2: Crystal Structure and Physical Properties of Titanium Dioxide 4.3: Synthesis of TiO2 Nanostructures 4.3.1: Synthesis of TiO2 Nanostructures by Solution routes 4.3.1.1: Precipitation Method 4.3.1.2: Solvothermal Method 4.3.1.3: Hydrothermal Method 4.3.1.4: Sol-gel Method 4.3.1.5: Microemulsion Method 4.3.1.6: Combustion Synthesis 4.3.1.7: Electrochemical Synthesis 4.3.1.8: Laser ablation on solid liquid interface 4.3.2: Gas phase methods 4.4.1: Chemical Vapor Deposition (CVD) 4.4.2: Physical Vapor Deposition 4.4.3: Spray Pyrolysis Deposition 4.4.4: Other Gas Phase Methods 4.4: Applications of Titanium Dioxide 4.4.1: Photoelectrochemical generation of Hydrogen (solar Hydrogen) 4.4.2: Water Purification 4.4.3: Self Cleaning Surfaces 4.4.4: Sensors 4.4.5: Cancer Treatment 4.4.6: Generation of PV Electricity 4.4.7: Air Purification 4.4.8: Other Applications 4.5: Summary 5. Over all conclusion 6. Future prospects 7. References Table Captions Tables Figure captions Figures 3 Now a days. safe. tapes. crystalline as well as amorphous. cuboids. cubes. nanogenerators to convert mechanical energy into electrical and LEDs then titanium dioxide have largest potential into solar cells and environmental purifications.Recently. circular and hexagonal discs. This review chapter is devoted to the various physical and chemical routes of the fabrication of zinc. Metal oxides specially. nano-lasers. nano-generators. medical and fabrication of electronic devices. copper and titanium can be fabricated in various morphologies such as nanoparticles. which requires specific and costly substrates. belts. nanowires. tetra pods. polymers. environmental friendly. nanorods. They can be fabricated on any type of substrates such as metals. 4 . light detectors. cheap synthesis procedure and technological applications in the fabrication of devices for energy harvesting and storage. copper and titanium oxide nanostructures and their applications. and flexible plastics unlike other III-V semiconductor and silicon. semiconductors. oxide nanostructures of zinc. colloid as well as nanostructures films phase. while cuprous oxide has a lot of potential in the biological. these metal oxide nanostructures are widely used in the fabrication of cheap and efficient solar cells. flowers using various cheap physical and chemical routes in powder. Due to the availability of cheap and versatile routes of fabrication of metal oxide nanostructures. photonics. sensors and electronic devices such as transistors and FETs. sensors as well as medical and biological applications. they may be treated as cheap replacement of silicon and gallium nitride based costly devices. If zinc oxide nanostructures have highest potential as nanolasers. scientific and research community have shown their great interest on metal oxide nanostructures and their applications due to their easy. write heads and data storage devices [157-166].1. TiO2. SnO2. have distinctive properties and are now widely used as transparent conducting oxide materials [187. cancer treatments. TiO2. In2O3. CuO. Nd. Oxides of transition metals have strong ferromagnetism with high Curie temperature and are used as magnetic read. such as ZnO.138] and photodetectors [139-148]. hydrogen production by water photolysis and its storage [65-83]. UV-screening [137. Light emitting devices (LEDs) [49-64]. transistors/FETs [38-48]. water and air purification by degradation and adsorption of organic/inorganic pollutants and toxic gases [84. Tb doped metal oxides are usually used as phosphor materials for fabrication of LEDs. Sm. Recently oxide based nanomaterials such as ZnO. [167-177] are called diluted magnetic semiconductors (DMS) and are applicable in the fabrication of spin based electronic devices i. SnO2 nanomaterials are regarded as one of the most important sensor materials for detecting leakage of several inflammable gases owing to their high sensitivity to low gas concentrations [190]. and CdO. Metal Oxide Nanostructures and their applications Nanostructures of metal Oxides have shown their revival of interest in the fabrication of energy saving and harvesting devices such as Lithium ion batteries [1-5]. fluorescent imaging. Binary semiconducting oxides. Instead of these they have also fabulous applications in biological and medical sciences such as drug delivery. Such as fluorine doped SnO2 film have potential application in architectural glass applications due to its low emissivity for thermal infrared heat [189]. environmental monitoring by their applications in the fabrication of gas. Spintronics. CuO2 and so on have revolutionized the nanomaterials research because of the availability/possibility of soft chemical synthesis besides tremendous application potential.e.104]. displays and laser materials [178-187]. bio labeling and bio tagging etc [149-156]. Metal oxides are expected replacement and alternative of silicon and metal nitride based expensive electronic devices and ICs.188] and sensors [105-136]. Transition metal doped active oxides such as ZnO. CuO2/Cu2O. solar cells [11-37]. humidity and temperature sensors [105. Indium-doped tin oxide (In:SnO2.136]. fuel cells [6-10]. One of the salient features of these oxide nanomaterials is the bio compatibility which opens an avenue for interdisciplinary research to have better bridge up between physicist and biotechnologist. Similarly rare earth elements such as Eu. Al2O3 etc. TiO2. ITO) film is an ideal material for flat panel displays because of its high electrical conductivity and 5 . copper and titanium metals. it also crystallizes into zincblende [Figure 1(b)] and rocksalt structures [Figure 1 (c)] at different experimental conditions. It is the most promising inorganic oxide. It has direct band gap energy of 3. which is the hexagonal lattice with space group P63mc [Figure 1 (a)]. which makes its transparency in the visible region and most of the activity in the UV/blue region. Characterizations and applications 2.1: Introduction: Recently scientific community has shown their revival of interest in Zinc oxide as a cheap replacement of Si and GaN and regarded it as a “future promising material”.high optical transparency [191.in such a manner that each of the zinc atom is surrounded by four 6 . facial powders. and as a transparent conducting electrodes. low threshold value and bio compatibility. Synthesis. Lattice structure of zinc oxide is combination of two interconnected sublattices of Zn2+ and O2. Zinc oxide in its bulk polycrystalline form has been commonly used in a wide range of applications such as sunscreen. Zinc Oxide Nanostructures. piezoelectric transducers. 2.40 eV. 2. 143-148]. In spite of higher exciton binding energy zinc oxide has a lot of other virtues over GaN including its ability to easily grow on the single crystal substrate. high quality of zinc oxide nanostructures are greatly demanded. spintronics [169-177] and sensor applications [117-136]. Instead of wurtzite. This chapter deals with different methods of synthesis. Higher exciton binding energy of zinc oxide (∼60 meV) as compared to GaN (∼24 meV) enhances its luminescence efficiency. dye sensitized. lubricant additives.193].2: Crystal Structure and Physical Properties of Zinc Oxide: At normal temperature and pressure zinc oxide exhibit wurtzite crystal structure. Zinc oxide is widely used for the fabrication of transistors and FETs [42-48]. Zinc oxide is an n. light emitting diodes [52-64]. hybrid and quantum dot solar cells [15-35] and nanogenerators [197-203]. ointments.type. which is widely used for fabrication of devices and other applications. direct wide band gap semiconductor material having several applications in UV/blue optoelectronics [52-64. catalyst. which induces world wide research and development on the synthesis and application of zinc oxide nanostructures. characterization and applications of oxides nanostructures of zinc. paint pigments. transparent electronics [42-48]. varistors. Due to the advanced technological applications. and ZnO is regarded as an ideal alternative material for ITO because of its lower cost and easier etchability [194]. In addition. perpendicular to the c-axis. solid powder and film. In the single crystal of wurtzite zinc oxide there are four atoms per unit cell causes 12 modes of vibrations with 6 transverse optical (TO) 3 longitudinal optical (LO). which is responsible for its application as nanogenerators. Wurtzite zinc oxide has four common face terminations having Zn2+ (0001) and O2− (000 1) polar surfaces and − (112 0) and − (10 1 0) non-polar surfaces containing equal number of zinc and oxygen atoms. rods. Understanding the fundamental physical properties is crucial to the rational design of functional devices. Investigation of the properties of individual ZnO nanostructures is essential for developing their potential as a building block for future nanoscale devices. Physical properties of zinc oxide are tabulated in table 1. while that of the oxygen at opposite − (000 1) surfaces of the wurtzite symmetry. ±[ ī 2 ī 0]. the carrier concentration in 1D system can be significantly affected by the surface states. ±[ ī ī 20]). The two E2 modes are only raman active. as suggested from nanowire chemical sensing studies. <01 ī 0> (±[01 ī 0]. The physical properties of zinc oxide change with the dimension of the nanostructures. ±[1 ī 00]). X-ray absorption spectroscopy and scanning photoelectron microscopy reveals the enhancement of surface states with the downsizing of ZnO nanorods [206]. which induces a normal dipole moment and spontaneous polarization along the c-axis as well as a divergence in surface energy.3: Synthesis of ZnO Nanostructures Various physical and chemical routes are investigated for the synthesis of zinc oxide nanostructures in the form of stable colloid. spiral.oxygen atoms at the tetrahedral corners and vice versa. and ±[0001]. The tetrahedral arrangement of zinc and oxygen atoms in zinc oxide makes zinc atoms at (0001). wires. 2. ±[10 ī 0]. Due to the polar nature of zinc oxide it exhibits a variety of novel properties such as piezoelectricity. while the rest are raman as well as IR active. 2 transverse acoustical (TA) and 1 longitudinal acoustical (LA). quantum confinement increases the band gap energy of one-dimensional (1D) ZnO. For example. 7 . helical. For detail please see references [207]. It has three-types of fast growth directions as <2ī ī 0> (± [2 ī ī 0]. which has been confirmed by photoluminescence [204] band gap of ZnO nanoparticles also demonstrates such size dependence [205]. Depending on experimental conditions different type of ZnO nanostructures such as particles. These properties changes very rapidly when size reduces below the 10 nm dimension called “quantum confinement”. while that the zinc oxide synthesized at 70 °C has seems that the growth pattern of the particles have a preferential direction and appears as leaves sticking around the nucleus. stoichiometric solution of sodium or ammonium hydroxide is added immediately under vigorous stirring. tetrapod etc. of zinc with the other solution containing some reducing agent such as sodium or ammonium hydroxide/nitrates/carbonates in the presence of stabilizing agents. sol-gel. Morphologies of synthesized zinc oxide nanostructures change with the reaction temperature [Figure 2 (a)]. are observed in both physical as well as chemical routes. When temperature of the precursor solution get stabilizes.1: Precipitation Method In the precipitation method particular concentration of zinc precursor (nitrate or acetate or carbonate of zinc) placed at particular reaction temperature. Zinc oxide precipitate is separated from the reaction mixture by centrifugation and washed with the deionized water. Guzman et al. nitrate. solvothermal. sono-chemical etc. Stirring and heating of the reaction mixture is continued for 3-4 hours after addition of hydroxide solution. 70 and 80 degrees of reaction temperature [208].05 M concentration is filled in the pore of template by placing 8 . Zn(NO3)2 solution of 0. electrochemical.1: Synthesis of ZnO Nanostructures by Solution routes It is the widely used method for the synthesis of zinc oxide nanostructures.3. combustion.3. 2. have obtained snowflake and flower like morphologies of zinc oxide microstructures using aqueous precipitation method at 60. These routes are separately discussed in the following subsections in details. Particular routes should be employed for the synthesis of zinc oxide nanostructures for particular applications. Xiao and coworkers [209] have prepared zinc oxide nanowires [Figure 3] inside the pore of anodized alumina template using precipitation method. are available for the synthesis of zinc oxide nanoparticles. It deals with the reaction of one solution containing zinc source such as acetate. 2. chloride etc. The sample synthesized at 60 °C appear as that the flakes are grown around a common nucleus. Several solution based routes such as precipitation. hydrothermal. The microstructure obtained at 80 °C temperatures has same morphology as 70 °C with more defined structures.flower. Temperature of reaction mixture is varied in order to synthesize zinc oxide nanostructures of different size.1. shape and morphologies. Here we describe various physical and chemical routes for the synthesis of various types of zinc oxide nanostructures. micro-emulsion. There are some advantages and shortcomings of each of the routes. The reaction mixture at 90 °C for 90 minutes and cooled by ice bath. Zinc acetate.3. Variations are brought in terms of concentrations of zinc acetate and PVP and reported that concentration of zinc acetate does not affect the size of zinc oxide particles but the its shape changes from plate like to sphere with the increase precursor concentrations. Nature and concentration of zinc precursor. reaction duration.1. 2. Taubert and co-workers [212] have synthesized zinc dumbbell shaped zinc oxide nanostructure [Figure 4] using precipitation route. The white precipitate was separated by centrifugation washed with water and ethanol and dried in vacuum at 60°C. different concentrations of PVP to control the growth and coagulation process of synthesized intermediate product and sodium hydroxide as reducing agent. nature and concentration of surfactant of polymers used. Solution of 210 mg of hexamethylene tetramine in 2 ml of deionized water is added to the continuously stirred solution.the template in the solution and placed horizontally on the outlet of conical flask containing ammonia solution. triethanolamine solution in ethanol and n-propylamine solution in ethanol is also used to prepare zinc oxide nanoparticles by precipitation method [213]. which causes synthesis of zinc hydroxy carbonate precursor [211]. [210] have synthesized zinc oxide nanostructures using zinc acetate as precursor solution. The template is kept into the tube furnace at 150 °C for 1 hour and finally cooled down to room temperature. Annealing of thus obtained precursor at 300°C for different times produces zinc oxide particles of various grain sizes. In another study solutions of hydrated zinc chloride and anhydrous ammonium carbonate was mixed under intensive stirring. Mostly Teflon lined stainless steel sealed chamber named as “hydrothermal bomb” is used as thermal reactor.2: Hydrothermal Method Hydrothermal is an aqueous solution base wet chemistry method for the synthesis of zinc oxide nanoparticles. In a typical synthesis process 446 mg of Zn(NO3)2.6H2O and 12 mg of polymers P(EO-b-MAA) or P(EO-bSSH) are dissolved in 100 ml of deionized water and subjected to heat at 90 °C. Suwanboon et al. In a typical reaction process aqueous solution of any of the zinc precursor is placed in thermal reactor. Crystallite size of the zinc oxide particles decreases with the increase of PVP concentrations. solvent used. reaction temperature. pH of the 9 . The temperature of reaction mixture and time of the hydrothermal treatment is the key variable parameters to get various types of zinc oxide nanostructures. which causes growth of mesoporous zinc oxide nanowires inside the pore of alumina template. Rod shaped zinc oxide nanoparticles are collected by using the upstream inline filter.87 g of zinc nitrate [Zn(NO3)2.4%PEG to synthesize zinc oxide nanowires using hydrothermal route. rinsed with distilled water and dispersed into 70 ml of PEG solution. washed several times with ethanol and distilled water and dried at 60°C in vacuum oven [Figure 5]. Similarly Greene et al [216] have also added aqueous solution of zinc acetate (4 mmol) to 15 ml of triocetylamine and 3g of oleic acid (12 mmol) at room temperature and resultant mixture is heat treated at 286°C to get zinc oxide nanorods and particles. 10 ml solution of zinc nitrate was added into 20 ml solution of zinc carbonate with vigorous stirring and the precipitate was filtered. [214] used a special reactor in which the reaction solution is used to flow through the tube furnace. while zinc oxide nanoparticles are precipitated by adding ethanol after the cooling of reaction mixture at room temperature. On the completion of reaction process zinc oxide nanorods of 50 nm diameters are obtained by centrifugation. In another experimental procedure [217] 14. The fluid was rapidly quenched by the addition of KOH solution at the rate of 10 cm3/min and cooling with external water jacket. shape. 3 ml of above solution was mixed with 0-5 ml of distilled water and 25-30 ml of ethanol. They used aqueous solution of zinc acetate and added solution of sodium hydroxide with stirring. zinc carbonate (1M) and 0. Li et al.6H2O] and 40 g of NaOH was dissloved in 100 ml of distilled water. which was washed with deionized water prior to characterization. Wang and Gao [218] used aqueous solution of zinc nitrate (1M). The aqueous solution of 3 zinc (a)nitrate was fed into the reactor by high pressure pump at a flow rate of 2 cm /min and mixed (a) with supercritical water system placed at 450°C at the flow rate of 10 cm3/min and the reaction mixture was heated at 400°C under 30 MPa pressure for about 10 sec.reaction mixture are some of the parameters to control size. Ohara et al. The resultant suspension was transferred in the teflon lined stainless steel autoclave of 10 . [215] have synthesized zinc oxide nanorods and nanowires by PEG assisted hydrothermal route. and was being sealed and maintained at 140°C for 24 hours and allowed to cool to room temperature. When the temperature is maintained at 286°C for 1 hour under N2 flow zinc oxide nanorods are produced. The solution was transferred into teflon lined autocalve and heat treated 180°C for 20 hours. The precipitate was filtered off. 2 ml solution of obtained mixture with 5 ml PEG and absolute ethanol (upto 90% of the total volume) was loaded in the Teflon lined autoclave. followed by the addition of 5-6 ml of ethylenediamine and the resultant solution was homogenized with ultrasonic bath treatment for 20-40 min. morphology and crystallinity of nanoparticles synthesized by hydrothermal route. 250 ml) and resultant mixture was transferred into teflon lined stainless steel autoclave placed and heated at 100°C for 1h with mechanical stirring at the rate of 300 rpm.5 M aqueous media of zinc nitrate and obtained precipitate was filtered. Spherical ZnO particles are produced with trietahnolamine [Figure 6 a-d). crystallinity as well as aspect ratio increases [Figure 6 eh]. They employed zinc acetate as precursor and distilled water. shape and morphology of hydrothermally synthesized zinc oxide nanopowders.20 mol/lit. They used addition of ammonia solution into 0. With the addition of NH4OH in dietahnolamine. ZnO sample synthesized with ammonium hydroxide has c-oriented growth of the nanorods.25-2 mol/lit.) and ammonia solution (0.025-0. [221].and tri. They reported synthesis of pencil like zinc oxide powder in pure water.1M. tri ethanolamine (EA) and NH4OH were added separately into aqueous solution of zinc nitrate (0. pH of the mixture is varied in the range of 9-12 with ammonia solution. In the procedure. 1M) with vigorous stirring. They concluded that reaction time does not make any effect. Effect of nature of the solvent. Molar solutions of different alkaline source such as mono. reaction temperature and pH of the mixture on the size. while raising the reaction temperature above 100°C slightly reduces the particle size and yield.) as solvent for the synthesis of zinc oxide nanopowder with hydrothermal process. washing and drying. while using KOH solution as solvent in different concentrations produced different morphologies of zinc oxide nanostructures such as 11 . washed and filled into teflon lined stainless steel autoclave with the addition of extra 250 ml distilled water. di. when it is replaced by mono. In another set of experiments Lu and Yeh [220] have reported effect of reaction time. The autoclave was maintained at 200°C for 2h and cooled to room temperature prior to filtering. used in the hydrothermal process.5 ml.100 ml and heated at 200° for 10 hours.ethanolamine crystallinity as well as aspect ratio decreases from mono to tri. 26 ml of solvent was added into teflon lined autoclave of 40 ml volume containing zinc acetate solution (6. After completion of reaction the products was separated by centrifugation. on the morphology of synthesized zinc oxide nanopowders is investigated by Xu et al. di. KOH (0. hydrothermal temperature was 100-200 °C and time of heat treatment was varied from 0. After the reaction the zinc oxide was separated by centrifugation and washed. washed several times and dried at 60°C temperatures.5 to 2h. while increase of pH from 9 to 12 changes shape of zinc oxide from ellipsoidal to rod like structures with the increase in crystallinity and particle size but decrease in the yield. Lu et al [219] studied influence of alkaline sources on the structural and morphological properties of hydrothermally synthesized zinc oxide powders [Figure 6]. The aspect ratio of synthesize zinc oxide nanostructure increases with the increase of length of carbon chain in the alcohol solvent and reported linear relationship of aspect ratio with the boiling point of solvent. sheet and prismatic like etc. So solvothermal synthesis allows for the precise control over size. 1-hexanol. Precipitate was filtered and washed with ethanol and distilled water. 2. With the 12 . absolute alcohol (4 ml) and ethylenediamine (6 ml) in 20 ml of stainless steel autocalve under solvothermal conditions (300 °C for 20 h). [225] have synthesized zinc oxide nanostructures by putting zinc oxide foils in the teflon lined stainless steel autoclave containing different ratios of water and ethylenediamine (EN) and placed in furnace at 150-230 °C temperatures for 3-12 h. the only difference is in the precursor solution which is usually non-aqueous or mixture of aqueous and non-aqueous. morphology and optical properties of zinc oxide nanostructures were demonstrated. shape. Reaction temperature.1. 7] shapes. Dev et al. Crystallinity aspect ratio and quality of zinc oxide nanorods increases with the concentration of hydrazenehydrate added in the autoclave containing pure zinc acetate powder [224] in the solution of zinc acetate.3. solvent type. Making use of the solvothermal route. 1. while addition of NH3 produces N doped zinc oxide nanorods. Tonto et al. precursor type are some of the experimental parameters. The relative intensity of (002) peaks in the synthesized nanorods increases with the increase of EN in the 60 ml of water-EN system and reaches at maximum for 10 ml water and 50 ml EN. one gains the benefits of both the sol-gel and hydrothermal routes. temperature and filling factor inside the autoclave on the size. [223] have synthesized zinc oxide nanorods by reaction of zinc acetate in various alcohols (1-butanol. Ammonia solution of different concentrations causes zinc oxide particles of ellipsoidal and long prismatic [Figure. surfactant type. reaction time.3: Solvothermal Method Solvothermal synthesis route is very similar to the hydrothermal route. There are number of reports on the synthesis of zinc oxide nanostructures with solvothermal route employing different zinc precursors. The effect of reaction time.octanol and 1-decanol) under solvothermal conditions. Zinc oxide nanorods of 80-800 nm diameter are synthesized by Varghese et al [222] with the reaction between zinc acetate (300 mg). shortened prismatic.twinned pyramidal. shape distribution and high crystallinity of zinc oxide nanoparticles or nanostructure. different organic solvents and reaction temperatures [222-228]. Addition of Triton X-100 into the reaction mixture produces zinc oxide nanorods of uniform 300 nm diameter. which undergoes various forms of hydrolysis and poly-condensation reactions.6H2O (0. They have also investigated morphological evolution of zinc oxide nanostructures on the reaction time [Figure 10] and ratio of water to EN. Thus. The zinc precursor and methenamine solution were used in 13 . Zinc oxide monoliths were synthesized by Gao et al [229] by sol-gel route using alcoholic zinc nitrate solution with propylene oxide as the gelatin initiator. The formation of zinc oxide involves connecting the zinc centers with oxo (M-O-M) or hydroxo (MOH-M) bridges. Zhang et al [227] used ethylene glycol (EG) as a solvent for the synthesis of zinc oxide microspheres consisting of orderly and redical nanorods. sodium hydroxide. Zinc chloride (1mmol) was dissolved into the 120 ml of EG and sodium acetate (3. absolute alcohol and sodium chloride by solvothermal route [228].6 g) was added in the solution after 30 min. 4. therefore generating zinc-oxo or al-zinchydroxo polymers in solution. They have also reported that average diameter and length of the nanorods increases with the reaction time. Zinc oxide nanostructures on the silicon substrate are grown by Li et al. deionized water as solvent and methenamine as stabilizing agent. The mixture was transferred into autoclave at 200°C for 20hours.8 mmol) was dissolved in the solvent and stirred to get the clear solution to which 8mmol of propylene oxide was added with stirring. The precipitate was filtered and washed with deionized water and ethanol [Figure 11].1. The sample produced in 10 ml water and 50 ml EN system is almost perfectly aligned on the surface of zinc foil with 75-150 nm diameter and 2µm length. In a typical reaction process Zn(NO3)2. In the similar study Lu et al.3. [230] using sol-gel approach employing Zn(NO3)2.6H2O as zinc precursor. of stirring. the sol evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. [226] have synthesized zinc oxide hierarchical nanostructurures by placing zinc foils in the Water/EN system with 1:7 volume ratios with a small ratio of NaOH at 160 °C temperature [Figure 9]. Typical precursors are zinc alkoxides or zinc chloride. This solution was placed undisturbed.addition of EN decreases the diameter and increases orientation of zinc oxide nanorods significantly [Figure 8].4: Sol-gel Method Another solution based wet chemical route for the synthesis of zinc oxide nanostructures is sol-gel method evolving a solution which acts as precursor for an integrated network (or gel) of either discrete particles or network of polymers. Zinc oxide tubular nanostructures are synthesized by Li and coworkers using zinc nitrate hexahydrate. equi-molar (0.01 mol/L) concentrations. After making homogeneous precursor solution using magnetic stirring at 60 °C for 2 h it was placed in air for 24h to get homogeneous clear sol. Neutral (pH=7) and acidic solution (pH=6, by adding HNO3) were used to get different zinc oxide nanostructures. The cleaned and etched silicon substrate was immersed in the solution and heated in the oven at 90°C for 2h. Thereafter Si was removed from the solution and baked in the oven at 108°C followed by annealing in the quartz tube at 500 °C for 4h under O2 flow. Neutral solution produces rods, while acidic generates rods as well as plate like structures [Figure 12]. In another studies zinc oxide nanostructures are obtained by sol-gel route using 20 ml, 1M aqueous solution of NaOH and 0.1 M aqueous solution of zinc acetate, which were mixed with magnetic stirring at 50-60 °C temperature for 1h and kept at room temperature for 7days [231]. 4.3.1.5: Microemulsion Method Micro-emulsion is another important solution based method for zinc oxide synthesis. Microemulsions are usually clear, stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a co-surfactant. The aqueous phase may contain salt/salts of zinc and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins. For the preparation of zinc oxide nanoparticles, particular concentration of zinc salt [e.g. Zn (NO3)2] as the aqueous phase, has been commonly used to mix another microemulsion containing the precipitation agent [e.g. NH4CO3] [232-236]. Continuous collision of these micro-droplets leads to their coalescence and subsequent formation of insoluble precipitate of zinc compound (in this case Zinc carbonate) in the droplet. The surfactant prevents the growth and coagulation process of the carbonate particles. The synthesized zinc carbonate particles undergoes washing with 1:1 solution of methanol and chloroform and heated to get zinc oxide nanoparticles. Size, shape, distribution, morphology and hence properties of zinc oxide nanoparticles synthesized by microemulsion method depends on the concentration and nature of zinc salt, nature and concentration of reducing agent, type and concentrations of surfactant and oil used to form microemulsion. Therefore choosing different type and concentrations of zinc precursors, reducing agent, surfactant and oils one can synthesize zinc oxide nanostructures of different size, shape and morphologies. Zhang et al [232] have synthesized 1D single crystalline zinc oxide nanostructures using facile microemulsion method. In the typical reaction procedure zinc acetate and sodium hydroxide was mixed in stochiometric ratio to get Zn(OH)4-2 precursor solution. Thus obtained resultant precursor solution (1.2 ml), CTAB surfactant (1g), n-hexanol 14 cosurfactant (2 ml) and n-heptane (11.2 ml) solvent were mixed with various molar under vigorous stirring ratio to get microemulsion based system, which was transferred to 25 ml of Teflon lined stainless steel autoclave at a given temperature for certain time. Precipitate was filtrated and washed with water and alcohol after cool down naturally upto room temperature and dried in vacuum oven at 50-60 °C. They have reported growth and evolution of zinc oxide nanostructures with reaction time [Figure 13]. Zinc oxide nanostructures with various morphologies were prepared by Li et al [233] using microemulsion process utilizing heptane and hexanol (mol ratio 3:1) as oil phase and Triton X-100 as a non-ionic surfactant. Calculated amount of triton X-100 was added to the oil phase under stirring to get 0.2mol/L solution (ME). Two microemulsions ME-1 and ME-2 containing different reactants was prepared. ME-1 was obtained by adding 3 ml of 0.25 mol/L aqueous solution of Zn(NO3)2 containing different concentrations of PEG 400 additives to 30 ml of ME. Addition of 3 ml of 0.5 mol/L aqueous solution of NaOH resulted ME-2 reactant. ME-1 was slowly added to ME-2 under stirring and resultant mixture was transferred into 100 ml of teflon lined stainless steel autoclave placed at 140°C for 14h. After completion of reaction autoclave was cooled naturally to room temperature, precipitate was filtered, washed with water and ethanol and dried in air at 60°C. They have synthesized zinc oxide nanostructures of different size (mean diameter 54.2, 70, 65, and 46.2 nm) and shapes (needle and spherical, column, column, and spherical) by varying the concentrations (0, 12.5, 25, and 50 weight %) of PEG. 4.3.1.6: Combustion Synthesis Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. Most fuels of interest are organic compounds (especially hydrocarbon) in the gas, liquid or solid phase. According to the phase of the fuel there are three types of combustion synthesis of nanomaterials (a) solid phase combustion (b) solution combustion synthesis and (c) gas phase combustion synthesis. Solution phase combustion out of the three is mostly used, while the gas phase combustion is least. Jayalaxmi et al [237] have synthesized ZnO nanopowder using solution combustion synthesis and employed 10g of zinc nitrate and 3.6 g of dextrose solution into 25 ml of water. Glass vessel containing aqueous solution was placed on the hotplate for 15min. to form a gel and placed into muffle furnace at 400°C for 5 min. The formed powder was highly amorphous in nature. Zinc oxide nanostructures are synthesized by Alvarado-Ibarra et al. [238] 15 using solid as well as solution combustion methods. In a typical reaction process 0.2 g of Zn(NO3)2.6H2O and 0.4g of urea was mixed and suspended in 1ml of distilled water. For solid combustion the mixture was heated until all the water evaporated before placing it in the muffle furnace, while for solution phase synthesis aqueous mixture was placed in the furnace operating at 800 °C. SEM images of the zinc oxide nanostructures obtained by direct calcination of zinc nitrate, solid combustion and solution phase combustion methods are illustrated [Figure 14]. Zinc oxide nano-tetrapod like structure is synthesized by Chen et al. [239] using melting combustion method. The stainless steel container having a nozzle at its centre carrying bulk metallic zinc was charged into an electric furnace and got melted into liquid, which flowed down on the flame of O2 and C2H2 gas and burned to produce huge zinc fumes. The zinc fume was carried by fan to the cooling collector and deposited as zinc oxide nano-tetrapod powder [Figure 15]. 2.3.1.7: Electrochemical Synthesis Electrochemistry, a method that employs deposition of a layer of metal/metal oxide on the conducting electrode, was invented by Italian Chemist Luigi V. Brugnatelli [240]. First metal oxide that was deposited electrochemically were thallium oxide [241] and zirconium oxide [242] while electrochemical synthesis of zinc oxide was first time reported by Izaki and Omi [243] and Peulon and Lincot [244]. Cathode potential, current density, deposition temperature, electrolyte composition and concentration are some key parameters to control the size, shape, composition and morphology of synthesized nanostructures. Number of papers on the electrochemical deposition of zinc oxide is reported with variety of conducting electrodes such as transparent semiconductors ITO [245], FTO [246], Metals electrodes such as Au [247], Sn [248], Pt [249], Zn [250] and Cu [251], AAO [252] and Silicon [253]. Deposition of doped zinc oxides such as Mn/Co;ZnO [254], Co:ZnO[255], Al:ZnO [256], In:ZnO [257], Bi:ZnO [258], and Eu:ZnO [259] by adding source of dopant materials into electrolytic solution are also reported. In the first report of fabrication of zinc oxide by electrochemical method Izaki and Omi [243] used 0.1M solution of zinc nitrate as electrolyte, tin oxide as cathode and substrate for deposition. Zinc foil was utilized as anode material and nanostructures of zinc oxide with various morphologies were deposited using variation in potential difference [Figure 16]. Deposition thickness increases with the increase of applied voltage. At the same time Peulon and Lincot [244] deposited zinc oxide by electrochemical method employing ZnCl2 salt (10-3 to 10-1M) with KCl (0.1M) supporting 16 06 M KCl with 0.06 M EDA and 0.8: Sonochemical method: Sonochemical method is another solution based method for zinc oxide nanostructured material. zinc acetate. Zinc oxide dendritic nanostructures are fabricated by Li et al [260] using electrochemical method in three electrode cell. 0.01 M EDA . platinum electrode. was deposited on the ITO/glass substrate from the 0. A zinc oxide nanosheets. Electrodeposited zinc oxide on the ITO/glass substrate was washed with water and used as working electrode for second step of electro-deposition. A graphite electrode of 4 cm2 area as supplementary electrode. counter electrode and reference electrode respectively.05 M zinc nitrate mixed with 0. 90 °C cathode temperature and -1.05 M zinc nitrate with drop wise addition of ammonia produces highly oriented ZnO nanorod arrays on the surface of primarily grown ZnO nanosheets [Figure 18]. respectively.3. 100 nm of thickness and 10 µm of diameter.10 V potential in 0.1. and saturated calomel electrode as working electrode.05 M of zinc nitrate + 0. They have also studied time dependent morpholgical evolution of zinc oxide nanostructures at -1. For the second step of electrodeposition ammonia solution was added drop wise in the 0.05M solution of zinc nitrate solution at 70 °C and stirred until clear solution obtained and used as electrolyte. Various morphologies of zinc oxide nanostructures are obtained by varying the ratio of composition of zinc chloride with citric acid [Figure 17].5 h in 0. Secondary deposition for 1.05 M solution of zinc nitrate [Figure 19] 2.5 V electrostatic potential difference was used to deposit zinc oxide dendritic nanostructures. In the synthesis procedure aqueous solution of zinc precursor such as zinc nitrate hexahydrate. and hydroxide anion precursor such as 17 .06 M KCl. zinc chloride etc.05 M zinc nitrate/EDA solution in which doses of EDA was varied are also used as electrolyte for second step of deposition.electrolyte on tin oxide coated glass substrate as cathode. Deposition was carried out in three electrode shell having ITO coated glass substrate. Xu et al [261] used two step of electrodeposition to grow hierarchical zinc oxide nanostructures on the surface of zinc oxide nanostructures grown by first step of electro-deposition. nanorods and nanoneedles of zinc oxide were electrodeposited in 0. Nanosheets. a saturated calomel electrode connected by cell using double salt bridge system as reference electrode and copper foil as working electrode with aqueous solution of ZnCl2 + citric acid as electrolyte.05 M KCl electrolytic solution in the first deposition step. The 0. 5H2O (2. The stock solution Na2 [Zn(OH)4] was prepared by 18 . washed with water and alcohol before characterization. For zinc oxide nanoflowers a solution containing 90 ml of zinc acetate dihydrate (0. The resultant reaction mixture was ultrasonically treated by ultrasonochemical apparatus (39. Zinc oxide/ Bi2O3 nanocomposite materials are synthesized by Wu et al. while for zinc oxide nanospheres 0. Xiong et al. Two different concentrations (0. 50 ml solution of zinc nitrate hexahydrate with equi-molar and equal volume of HMT. [262] have synthesized zinc oxide nanorods. power of ultrasonic wave and time of ultrasonic treatments are key parameters available to control size.05 and 0.57 M) was undergone sonochemical treatment for 30 min. A100 ml aqueous solution containing 0. shape and morphology of zinc oxide nanostructures. hydroxide anion precursor.5 mmol of zinc nitrate hexahydrate into 30 ml of deionized water and 25 weight % ammonia solution was added drop wise under vigorous stirring until the solution became clear and subjected to ultrasonic treatment for 2h at 90°C. with same power. For the synthesis of zinc oxide nanoflowers and nanospheres zinc acetate dihydrate and ammonia/water system was used as zinc and hydroxide anion precursors respectively.01 M zinc nitrate hexahydrate.01 M) and 10 ml ammonia/water (1. The obtained product was filtered.01M solution of triethyl citrate was added in the above solution before ultrasonic treatment for same power and same time [Figure 20]. In the similar experimental procedure Pu et al [263] have obtained several morphologies of zinc oxide nanostructures with sonochemical method using solution of 1.10) of Zn2+ ion and time (before and at the beginning of ultrasonic treatment) of addition of ammonia solution in the reaction mixture were used to control the morphology of zinc oxide nanostructures. In a typical reaction procedure Bi (NO3)2.1 M triethylcitrate was prepared at room temperature and subjected to ultrasonic treatment (39. Concentration of zinc precursor. 20 kHz) for 30 min. nanocups.01M HMT and 0. 0. Jung et al.2 M zinc precursor and 50ml. nanoflowers.02 M.2 M of hydroxide precursor was ultrsonochemically treated with same power for 2h to get zinc oxide nanocups.hexmethylenetetramine (HMT) is taken as starting materials. The solution is being placed in ultasonochemical apparatus for different time. have synthesized Mg doped highly luminescent zinc oxide quantum dots with different doping concentrations using sonochemical method [264]. 0. nanodiscs and various nanoarchitectures employing sonochemical approach employing 0. [265] using sonochemical approach.5 W/cm2. 20kHz) for 30 min. surfactant nature and concentration. to prepare ZnO nanorods and 50ml. to get ZnO nanodiscs.5 W/cm2. 0.456g) was dissolved into 50 ml of EG to obtain a transparent solution. simple. washed and dried at 100°C for 12 h. shape and morphology.0 to 13 with the addition of 1.9 Laser ablation on solid liquid interface Laser ablation in the liquid media is a solution based physical route for the synthesis of nanomaterials without of any chemical except some surfactants to prevent aggregation and agglomeration. zinc sulphate heptahydrate and zinc chloride as zinc precursors.3. The pH of the reaction mixture was adjusted in between 9. Laser ablation in liquid media was first carried by Partil and coworkers [271] for the synthesis of iron oxide nanoparticles using 694 nm light of ruby laser.5M aqueous solution of zinc compounds were dissolved with 50 ml 1M aqueous solution of NaOH under vigorous stirring. Xiao et al. surface of synthesized nanostructures are free from chemical contamination..19g) in concentrated sodium hydrate solution. All the zinc compounds with NaOH was dissolved in distilled water by adjusting Zn2+ and NaOH concentrations 0. Bismuth nitrate solution was added into stock solution during sonochemical treatment. efficient and fast route of the synthesis of ultra fine nanostructures. 2.5 and 1M respectively.0M aqueous solution of sodium hydroxide. After the completion of reaction precipitate was filtered. It is a clean and green approach for the synthesis of metal as well as metal oxide nanostructures with advantage of having large number of available parameters to control size. [266] have produced zinc oxide nanosheets by sonochemistry using zinc nitrtae hexahydrate. Some others but not all the reports of the preparation of zinc oxide nanostructures of various morphologies by sonochemical method with different precursors and parameters are given in the references [267-270].1. 20 kHz) ultrasonic system at room temperature for 2 h. 21 & 22]. Morphology of the zinc oxide nanostructures are found dependent on the pH value of the reaction mixture and nature of zinc precursor [Figure. The reaction mixture was ultrasonically irradiated using high intensity (600W. For example 50 ml of 0.dissolving zinc nitrate hexahydrate (9. 19 . Synthesis of zinc oxide nanostructures by laser ablation in liquid media has been already discussed by Singh et al. zinc acetate dihydrate. therefore readers are requested to consult the reference [272]. and sodium hydroxide as hydroxide anion precursor. Rod shaped zinc oxide nanostructures were obtained after ultrasonic treatment for 5-20 minutes. nature and molecular weight of carrier gas are some key parameters to control the morphology of zinc oxide nanostructures in CVD approach. electron beam. volatile by-products are also produced. while Ar was used as a carrier gas. laser ablation. molecular beam etc.3. nature of substrate.2. they prepared star shaped [Figure 23 (B)] zinc oxide nanostructures [274]. Zinc vapor is produced by any means such as vaporizing sold zinc metal under oxygen environment by thermal. which gets deposit on the substrate to form zinc oxide nanostructures and film. ion beam. [273] have synthesized flower shaped [Figure 23 (A)] zinc oxide nanostructures on the silicon substrate utilizing cyclic feeding CVD approach. Depending on the source of producing zinc vapor there are several deposition process such as chemical vapor deposition in which zinc vapor is produced by evaporating and dissociation chemical zinc precursors.2: Gas phase methods Gas phase methods are usually used for the fabrication of zinc oxide thin films or nanostructures on the particular substrate applicable for devices.2. or evaporation ion beam. which react and/or decompose on the substrate surface to produce the zinc oxide nanostructures. 20 . Using same metal organic zinc precursor. physical vapor deposition consists production of zinc vapor by physical means such as laser ablation. which are removed by gas flow through the reaction chamber. molecular beam or by vaporizing and dissociating any zinc chemical precursor. This process is often used in the semiconductor industry to produce thin films. thermal evaporation. The substrate placed at 500°C was allowed to expose alternatively with DEZn and oxygen. Umar et al. Chamber pressure.3. The zinc vapors thus produced react with oxygen to form zinc oxide vapors. In a typical CVD process. the wafer (substrate) is exposed to one or more volatile zinc precursors. temperature of vapor as well as substrate. O2 and Ar as carrier gas Kim et al [275] controlled the morphology of zinc oxide nanostructures by varying substrate temperature. Frequently.1: Chemical Vapor Deposition (CVD) Chemical vapor deposition (CVD) is a chemical process used to produce high-purity and high-performance solid materials. electron beam. In the similar experimental procedure except coating Si (100) substrate with 10 nm thin film of Au. These are discussed separately as follows 2. Diethyl zinc (DEZn) and high purity oxygen was used as precursor of zinc and oxygen respectively. There are number of reports for the synthesis of zinc oxide nanostructures by CVD method [273-279]. 2. Various PVD methods for the deposition or synthesis of zinc oxide thin films or nanostructured materials include thermal evaporation or evaporation deposition. These deposition methods for the fabrication of zinc oxide are discussed in brief as follows. The source temperature selection mainly depends on volatility of the source material. Oxygen influences not only the volatility of the source material and the stoichiometry of the vapor phase. Zn and oxygen or oxygen mixture vapor are transported and react with each other. The coating method involves purely physical processes such as high temperature vacuum evaporation or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition. It is also noted that the thermal evaporation process is very sensitive to the concentration of oxygen in the growth system. it is slightly lower than the melting point of the source material. The local temperature determines the type of the product to be received. atmosphere. The substrate temperature usually drops with increasing distance from the position of source material. Usually. It is a simple process in which condensed or powder phase of source material is vaporized at elevating temperature. (a) Evaporative deposition: In which the material to be deposited is heated to a high vapor pressure by electrically resistive heating in "low" vacuum. sputtering (magnetron and RF sputtering). Various type of zinc oxide nanostructures are synthesized by thermal evaporation technique. but also the formation of product. The most common method to synthesize ZnO nanostructures utilizes a vapor transport process. pressure.2. cathodic arc deposition and pulsed laser deposition etc. In such a process. forming ZnO nanostructures. pressure.3. There are several processing parameters such as temperature. Decomposition of ZnO 21 . carrier gas (including gas species and its flow rate). substrate.2: Physical Vapor Deposition Physical vapor deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the material onto surface of various substrates. etc. There are several ways to generate Zn and oxygen vapor. substrate and evaporation time period that can be controlled and need to be selected properly before or during the thermal evaporation.) to form the desired product. and then the resultant vapor phase condenses at certain conditions (temperature. electron beam physical vapor deposition. The pressure is determined according to the evaporation rate or vapor pressure of source material. 12 cm separation between the target and substrate. it is limited to a very high temperatures (~1200-1400 °C) [280-282]. Electron beam energy. nanoflowers and various zinc oxide nanostructures by thermal evaporation method is discussed by us in the reference [272]. Si (100) substrate at 500°C temperature and 8×10-5 torr oxygen pressure. At about 800-1100 °C. Another direct method is to heat up Zn powder under oxygen flow [283-285]. In the widely used carbothermal method which is widely used. ZnO powder is mixed with graphite powder as source material [287]. The growth rate of the film is of the order of 3-5 A°/sec depending on the key parameters used. diethyl-zinc for example.is a direct and simple method. [288] have deposited zinc oxide nanostructured thin film using electron beam evaporation. This method facilitates relative low growth temperature (500~900 °C). 5keV energy of electron beam. ∼3×10-3Pa background pressure of NH3/H2 gas. Zn and CO/CO2 later react and result in ZnO nanocrystals. graphite reduces ZnO to form Zn and CO/CO2 vapors. It has been observed that the change of this ratio contributes to a large variation on the morphology of nanostructures. Polycrystalline zinc oxide pellet or zinc metal are used as target for the electron beam bombardment and deposition of nanostructured thin film on the substrate. They employed polycrystalline zinc oxide target placed in the deposition chamber with 10-6 torr pressure. Asmar et al. In another report Qiu et al. is used under appropriate oxygen or N2O flow [286]. Synthesis of zinc oxide nanobelts. order of vacuum are some key parameters to control the morphology of zinc oxide nanostructures by electron beam evaporation method. distance between target and substrate. however. (b) Electron beam physical vapor deposition: In which the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum of the order of 10-610-9 torr. in which organometallic Zn compound. The diameter and length of the zinc oxide nanocolumns 22 . but the ratio between the Zn vapor pressure and oxygen pressure needs to be carefully controlled in order to obtain desired ZnO nanostructures. Si (100) wafer as substrate. [289] have fabricated well aligned zinc oxide Nanocolumns on Si (100) wafers using electron beam evaporation. Electronic as well as target material collision with O2 ionize it and improve the crystallinity and stochiometry of zinc oxide thin film. substrate temperature and orientation. nanonails. Zinc oxide polycrystalline pellet as target. Usually O2 gas at low pressure of the order of 10-5 torr is introduced in the deposition chamber. The indirect methods to provide Zn vapor include metal-organic vapor phase epitaxy. The advantages of this method lie in that the existence of graphite significantly lowers the decomposition temperature of ZnO. nanotapes. Since the arc is basically a 23 . In a typical experimental procedure. 50 mm substrate to target distance. [294]. Giri et al [290] have fabricated nanocrystalline zinc oxide thin film on alumina silicon and glass substrates at various substrate temperatures using 6kW electron beam evaporator.increase with the deposition time [Figure 24]. Saw el al.3Pa ZnO vapor pressure. and then it self-extinguishes and re-ignites in a new area close to the previous crater. [291] have fabricated zinc oxide thin film of 1µm thickness on saphire (0001) substrate using sputtering of a pure sintered zinc oxide bulk target in argon atmosphere using a 200 W direct current magnetron source. highly energetic emitting area known as a cathode spot. After sputtering the film was annealed at 80°C temperature in nitrogen atmosphere for 2h. The cathode spot is only active for a short period of time. (c) Sputter deposition: In which a glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as a vapor. This behavior causes the apparent motion of the arc. 0.2. zinc oxide polycrystalline target placed in the deposition chamber with 1×10-5 torr vacuum is subjected to 300W of RF power for 60 minute to deposit zinc oxide thin film on the copper substrate at room temperature. Tetrapod like nanostructures of zinc oxide are grown after the annealing of as synthesized zinc oxide thin film [Figure 25]. (d) Cathodic Arc Deposition or Arc-PVD: It is a physical vapor deposition technique in which an electric arc is used to vaporize material from a cathode target (zinc for the fabrication of zinc oxide). The O2/Ar flow ratio was 0. leaving a crater behind on the cathode surface. low voltage arc on the surface of a cathode (known as the target) that gives rise to a small (usually few micrometers wide). which results in a high velocity (10 km/s) jet of vaporized cathode material. The localized temperature at the cathode spot is extremely high (around 15000 °C). 150W RF power. ceramic.4 and deposition time was 15-50 minute under 2. Nanostructured zinc oxide thin films are grown on p-type Si (100) substrate using RF sputtering by Youn et al. and composite film. Metallic zinc was used as magnetron target for the fabrication of zinc oxide thin film on the soda lime glass substrate by Kim et al. They have employed 3. Single crystalline zinc oxide nanobelts are synthesized by Choopun et al. The technique can be used to deposit a metallic. [292]. [293] using radio frequency (RF) sputtering. Oxygen/Ar ratios in carrier gas and deposition time were used for controlling the morphology of zinc oxide nanostructured thin films [Figure 26]. The vaporized material then condenses on a substrate.8×10-3 Pa vacuum. forming a thin film.0. and 0. The arc evaporation process begins with the striking of a high current. Laser irradiance. so that the total surface is eroded over time. Aluminum doped zinc oxide thin films were also deposited by W. If a reactive gas is introduced during the evaporation process. distance of substrate from target. [297] using cathodic arc deposition with activated anode (CADAA). dissociation. 24 . The arc has an extremely high power density resulting in a high level of ionization (30-100%). neutral particles. aluminum precursor powder as a dopant placed into a crucible acting as anode. pressure inside the chamber. nanoparticle assisted pulsed laser deposition (NAPLD) in gas chamber [302-305]. H. All the deposited thin films had strong ZnO (200) peak. [272]. Vaporized materials get deposit on seeded/ unseeded. laser vaporization controlled condensation (LVCC) [306-309] and NAPLD in quartz tube furnace [310-313] are discussed in detail by us in the ref. obtaining high quality coating at high deposition rates. Kakikawa et al. Takikawa et al. laser wavelength. clusters and macroparticles (droplets). The rate of deposition increases with pressure upto 1Pa with weak as well as strong magnets. nature and orientation of substrate are some key parameters to control morphology of laser produced zinc oxide thin films or nanostructures. substrate temperature. The anodic plume plasma appears on the crucible anode is composed of cathodic material zinc. which in practice is used to rapidly move the arc over the entire surface of the target. [296] have deposited zinc oxide thin film on glass substrate using steered and shielded reactive vacuum arc deposition. (e) Pulsed laser ablation deposition: High energy laser is used to vaporize or ablated depending on laser irradiance zinc/zinc oxide bulk target in O2 or/and inert gas at particular pressure. They have used a strong permanent magnet behind the cathode to drive cathode spot on the cathode surface and employed 30A DC and 0. crystalline/ non crystalline substrate placed perpendicular to the direction of plume flow at particular distance from the target substrate. They employed zinc cathodic arc with O2 flow at the pressure of 1 Pa. Fabrication of zinc oxide thin films and nanostructured materials uing pulsed laser deposition (PLD) [298-301].current carrying conductor it can be influenced by the application of an electromagnetic field. highly ionized and energetic plasma beam onto substrates [295]. Filtered vacuum cathodic arc deposition (FVAD) of zinc oxide thin film at low substrate temperature (50-400°C) is employed by magnetically directing vacuum arc produced. revealing c-axis orientation [Figure 27].1 to 5 Pa in process pressure. multiply charged ions. anode material Al as well as reactive gas of oxygen. ionization and excitation can occur during interaction with the ion flux and a compound film will be deposited. ratio of oxygen with inert gas. and electrostatic (Electric field is used for atomization) are used recent days in spray pyrolysis. Zinc oxide thin film was prepared by Ashour et al. They have also reported that the growth rate of zinc oxide is unaffected by adding (0-5 at %) InCl3 in the 25 .2. It does not require high quality of substrates and chemicals and is used to produce dense film. porous film and powders. Several types of atomizer such as air blast (liquid is exposed to a stream of air).02-0. [315] by spraying 0. In a similar experimental procedure Krunks and Mellikov [316] fabricated zinc oxide nanostructured thin film utilizing solution of zinc acetate dihydrate in a mixture of 2:3 volume ratios of deionized water and isopropyl alcohol with addition of some drops of acetic acid to prevent zinc hydroxide precipitation. 2.3: Spray Pyrolysis Deposition: Spray pyrolysis is a simple CVD processing technique used to prepare thin and thick films. and offers easy technique to prepare thin film of any composition. It is simple. solvent. pH of the solution and type of atomizer are some key parameters some key parameters to control the morphology of thin film in spray pyrolysis method. In a typical experimental procedure they utilized 0.2M aqueous solution of zinc acetate mixed with methanol in the ration of 1:3 with 5ml/min spray rate on a glass substrate at 420°C substrate temperature. Deposition temperature.2 mol/lit aqueous solution of zinc chloride as zinc precursor. For the fabrication of zinc thin films and nanostructured materials any zinc precursor can be used. Polycrystalline zinc oxide thin film having wurtzite crystal structure with 20 nm grain size was produced. ceramic coating and powders. and temperature controller. 400-560°C substrate temperature. substrate heater. ultrasonic atomizer (ultrasonic wave is used for atomic ionization). Nanostructured zinc oxide layers having well shaped hexagonal zinc oxide nanorods were deposited on ITO coated glass sheets using spray pyrolysis by Krunks et al. [314].5ml/min spray rate and compressed air as carrier gas for the fabrication of zinc oxide nanostructured film [Figure 28]. Typically spray pyrolysis equipment consist of an atomizer. precursor solution. The stock solution thus obtained was subjected to spraying on the 475-700°K heated glass substrate using air as carrier gas. precursors solution properties such as precursor. They have reported that the thickness of deposited zinc oxide thin film decreases with the increase of substrate temperature from 475-700°K. relatively cheap. which may be due to the diminished mass transport to the substrate at higher temperatures.2.3. In this technique precursor of the material to be deposited is in solution and sprayed onto a heated substrate using air as a carrier gas. while change in the shape from planner at low pH to round shape at high pH was observed. There is no any report on the electrically pumped lasing action in the zinc oxide nanostructures besides availability of n-ZnO as well as p-ZnO nanostructures. strong room-temperature luminescence. therefore excitonic emission may also be realized for efficient stimulated emission and lasing. Quantana et al. and electronic applications of ZnO as thin-film transistor and light-emitting diode. while optically [195-204] and electron beam [205-207] pumped stimulated emission and lasing have been reported by several workers in zinc oxide nanostructures including nanorods. The potential applications of zinc oxide are discussed under the following sections. wide bandgap. These properties are already used in emerging applications for transparent electrodes in liquid crystal displays and in energy-saving or heat-protecting windows. [317] have studied influence of pH of precursor solution and substrate temperature on the growth and morphology of nanostructured zinc oxide thin films by spray pyrolysis method and reported that increasing the deposition temperature increase the particle size until transform it in round nodules.exciton scattering due to its occurrence at a threshold smaller than that require for 26 . As zinc oxide has higher excitonic binding energy at room temperature as compared to other wide band gap semiconductor. energy storing and harvesting device fabrications etc. Efficient stimulated emission and lasing may be obtained from nanostructures because band edge transfer integral of nanostructures is larger than the bulk semiconductor. Instead of high transparency it has several other favorable properties such as high electron mobility. etc.4: Applications of Zinc Oxide nanostructures Zinc oxide nanostructured material has numerous potential applications in photonics. used as window and sunscreen material.1: Semiconductor nanolasers Semiconductor nanostructures have potential application as a lasing material to produce intense. nanowires as well as epitaxial layers synthesized by various routes. Zinc oxide has excellent transparency in the visible. monochromatic and coherent light sources due to the decrease in threshold potential for lasing with decrease in size. Low threshold stimulated emission and lasing action may be induced in zinc oxide nanostructures by exciton . while good absorbance in the UV region therefore. 2. optoelectronics. 2.precursor solution for the fabrication of indium doped zinc oxide nanostructured film. electronics.4. sensors. NWs grown on any substrates emits monochromatic light in the direction of the length of rods or wires and the phenomenon is called as ordered lasing. In this case light is non monochromatic and not unidirectional. The physical 27 . nanowires and thin films achieve the condition of population inversion for lasing. rod or wires have random orientation and condition of population inversion is achieved by scattering among the surfaces of several nanoparticles. Optically pumped ordered array of zinc oxide NRs. 3-D nanostructures [320]. ZnO also promises very high quantum efficiencies. and UV detectors based on this material have produced external quantum efficiencies (EQE) of 90%.4. These LEDs have capability to outperform their GaN-based cousins (which offer a narrower spectral range) due to its three key characteristics such as superior material quality. and zinc oxide nanorod array embedded into zinc oxide epilayers [321]. They have stimulated emission from vertical zinc oxide cavity using pulsed 266 nm optical excitation at grazing incidence and collected signal using microscope objective. [322] have observed lasing from a single zinc oxide nanowire using scanning confocal microscopy. There is large number of reports on the random lasing from zinc oxide nanoneedles [319]. Gargas et al.electron hole plasma recombination. an effective dopant and the availability of better alloys. three times that of equivalent GaN-based detectors. fiber coupled spectrometer and recorded with CCD [Figure 30]. The p-type dopant has provided hole-conducting layers for ZnO-based devices and growth of BeZnO layers has shown that it is possible to fabricate ZnO-based highquality heterostructures ("The advantages of ZnO over GaN"). 2. Stimulated emission spectra from zinc oxide nanowire array below and above the lasing threshold are investigated by Huang et al [318]. Zinc oxide spherical nanoparticles or films containing particles. Tanemura et al. have observed random lasing action in N-doped ZnO nanoneedles [319] and studied pumping power dependent lasing action [Figure 29]. The superior material quality is seen in the low defect densities of ZnO layers.2: Light emitting diodes (LEDs) The attractiveness of zinc oxide LEDs stems from the potential for phosphor-free spectral coverage from the deep ultraviolet (UV) to the red. coupled with a quantum efficiency that could approach 90% and a compatibility with high-yield low-cost volume production. Opposite facets of zinc oxide nanorods and nanowires act as two mirrors of laser cavity and zinc oxide acts as active media. Multiple oscillations between the opposite facets of zinc oxide nanorods. Stimulated emission combined with the excitonic transition makes zinc oxide nanostructures the most important candidate for the development of UV/blue semiconductor lasers. lasers and transistors that have less disorder than the structure produced using the AlGaN/GaN material system. 40 nm of Zn0. a value typically associated with the best GaN films. In comparison. due to our recent development of high quality BeZnO films. on zinc oxide based LEDs illustrates recent advances of zinc oxide based LEDs on crystalline as well as amorphous substrates [55]. 40 nm i-ZnO. They have investigated passivation effects of PECVD SiO2 and SiNx on ZnO based p-i-n LEDs.9Mg0. The LED structure consisted 450 nm Ga:ZnO on sapphire. Kim et al. The reduced disorder is a consequence of the large difference in band gap between ZnO and BeO and enables only small changes in the alloy’s composition to produce relatively large changes in band gap. There are three main advantages of zinc oxide based LEDs over that of GaN based as follows (i) Superior material quality. A topical review by Willander et al. which is less than that of 215 meV for magnesium doped p type GaN. They have measured and analyzed I-V curve in order to insure that the fabricated LED behavior is the consequence of p-n junction between n type ZnO and p-type SiC epilayer. a much larger shift in aluminum composition is required. 40 nm of Zn0.9Mg0. (ii) Improve doping performance. which results from the arsenic p-type dopant that has activation energy of 119 meV in ZnO films. So it is plausible that ZnO LEDs will have an EQE upper limit that is three times higher than that of GaN-based devices. have used p-type zinc oxide as a hole injection layer to enhance the output power of GaN LED [54]. which has been demonstrated by growth of high purity ZnO with defect densities below 105cm-2.1O. [53] have fabricated zinc oxide hetrojunction LEDs with plasma enhanced chemical vapor deposited (PECVD) SiO2 and SiNx . (iii) The availability of better alloys.processes associated with detection suggest that similarly high efficiency values should be possible for the conversion of electrical carriers to photons. This lower activation energy produces a ten fold increase in the proportion of the activated accepter atoms that are needed for electrical conduction and also reduces the number of defects for a given hole carrier density. have grown n type zinc oxide nanorods using different approaches on the p type SiC and observed expected electroluminescence from them [56]. Willander et al. These layers have driven the fabrication of LEDs.1O and 40 nm of phosphor p doped ZnO as shown [Figure 31]. Wang et al. The fabrication of n-ZnO nanorods on p-SiC LEDs by VLS approach illustrated visible emission at 28 . which shows strong white light emission with color rendering index 92.63]. Zinc oxide thin films [60] as well as nanorods [61. Epitaxial zinc oxide thin films or nanostructures grown on GaN crystalline substrates have potential application as LEDs or nanolasers due to the very small lattice mismatch (only 2 %). polymers. All of these reports on ZnO nanorods/GaN substrate LEDs exhibit enhanced EL emission and improved injection current as compared to the conventional ZnO thin film/GaN substrate LEDs due to the reduced interfacial defects from nanosized junctions. combination of UV and green emission from zinc oxide excitonic and defect level. There are several reports on the growth of zinc oxide nanostructures on GaN crystalline substrates and their LED demonstrations [57-59]. plastics. The fabrication of n-zinc oxide nanostructures as n-electrode on the ptype organic semiconductor substrate as p-electrode called as hybrid inorganic/organic LEDs and have potential for industrialization due to the numerous advantages including possibility to fabricate UV to red LEDs varying the type and nature of p-type substrate. Zinc oxide/polymer nanocompites are synthesized for the fabrication of inorganic-organic hybrid white light LEDs separately by Zhang el al. In spite of these they have same fundamental band gap of 3. Fabrication of n-ZnO/p-Si LEDs are also reported besides indirect and large bandgap difference of Si from zinc oxide. respectively with the emission from the hybrid structure makes the white light of better CI values. Inorganic zinc oxide nanorods are grown on the organic PEDOT: PSS [poly (3.4 eV and same wurtzite crystal structure. It is the property of zinc oxide NRs or NWs that it can be also easily grown on any type of substrates such as glass. [64]. 62] are grown on Si substrates and demonstrated for visible hetrojunction n-ZnO/p-Si LEDs. [63] and Uthirakumar et al. Large numbers of reports on zinc oxide nanostructures on organic substrates to fabricated hybrid LEDs are available in literatures [62. For LED applications zinc oxide nanostructures are usually grown on ITO or FTO coated glass substrates due to their transparent and conducting nature. higher than that of commercially available white light LEDs [56]. which insures a lower defect density in zinc oxide nanostructures on GaN substrate as compared to others.almost 30 V [Figure 32]. and metal oxide bulk surfaces.4ethylenedioxythiophene) poly(styrenesulfonate)] on glass substrates with some other organic hole injection layer for the fabrication of inorganic/organic hetrostructure LEDs by Willander et 29 . They used CVD grown ptype epitaxial thin layer on n-type SiC epitaxial layer as a substrate to grow n-ZnO ordered nanorods to fabricate n-ZnO/p-SiC hetrojunction LED to operate at lower operating potential (18V) compared to that mentioned earlier (30V) [56]. while it is only 3 for the second device. 2. followed by deposition of hole transport and polymer emitting layer [Figure 33]. array of NRs.9-dioctyl-fuorene-co-N(4.phenyl-amino] biphenyl] and PFO [Poly (9. nanotubes on substrates) with dye molecules. In the first case PFO was added with the NPD in multilayered configuration. and hence improves quality of film. It also increases the viscosity of blended solution. Therefore. Solar cells are p-n junction diodes in which photocarriers are generated by sunlight at the p-n junction and are migrated through the external circuit before the recombination. A material having efficient photocarrier generation efficiency.[55]. and acts as an electron blocker. or 30 . or organic molecules/polymers. Investigations in the fabrication of new and improvement in the existing solar cells is a most effective effort in this direction.al. For hole transport and polymer emitting layer they have selected two different combinations NPD [44’-bis[N-(1-naphthyl)-N. Gold was deposited on the top of zinc oxide nanorods as top contact layer.4. Crystalline silicon solar cell with 20% practical efficiency is one of the photovoltaic cells and currently occupies 94% of the market. Low turn on voltage of second device as compared to first is due to less number of layers hence less series resistance. but it is too costly to commercialize. low electron/hole trapping level is required to fabricate efficient solar cells. Similarly TFB enhances the hole transport. other options of cheap fabrication of efficient solar cells are developed. One of the options is excitonic solar cells having metal oxide thin films or nanostructures (Film of nanoparticles or other nanostructures. NPD enhances the hole transport divides the hole energy barrier into two separate barriers to increase the probability of exciton recombination.butylphenyl)diphenylamine] with PVK [poly(N-vinylcarbazol] as the later case [Figure 34 b]. On the surface of polymer emitting layer zinc oxide nanorods are grown as electron transport and light emitting layer.3: Solar Cells and light detectors Research and development in the direction of renewable energy sources are highly demanded due to the limited availability and pollution creating nature of conventional fossil fuels. They have reported turn on and breakdown voltages for first device are 4V and -15V respectively. Two layers of PEDOT:PSS were deposited by them on glass substrate as bottom contact layer. high charge mobility. while PVK was blended with TFB for the second case. Rectification factor for the first device is 10.9-dioctylfuorene)] for the first case [Figure 34 a] and TFB [poly(9. while these are 3V and -6 V for second device. provide a wider emission range. due to the porous nature of titanium dioxide thin film (Provide larger surface area for the loading of dye molecules) and lower energy of its conduction band edge as compared to the LUMO of the dye molecule. Titanium dioxide thin film is most efficient DSCs with 10. spherical nanoparticles and other nanostructures have different light 31 . which causes carrier loss due to the recombination process. while the holes are released by redox couples in the electrolyte. nanotetrapods. nanoflowers. Electrons are injected through the metal oxide nanostructures. absorbs most of the portion of solar radiation and generates photocarriers. Monolayer of the dye molecule absorbs photon and creates excitons. which determines how long sunlight falls on the photo-sensitizer loaded on the nanostructured surface. a platinized fluorine or indium doped tin oxide as a counter electrode. Depending on the type of sensitizer there are three types of semiconductor solar cells (a) with dye as sensitizer called Dye Sensitized Solar Cells (DSCs) (b) with polymer/organic molecule called Hybrid Solar Cells (HSCs) and with quantum dots as sensitizer called Quantum Dot Solar Cells (QSCs). which makes it suitable for the future alternative of costly Si based solar cells.redox couple to electrically connect the two electrodes. Not only the large surface area.organic complex dyes open an emerging area of fabrication of cost effective and efficient photovoltaic solar cells. which rapidly get split at the surface of metal oxide nanostructures. (a) Zinc oxide based Dye Sensitized Solar Cells (DSCs) DSCs based on oxide semiconductor film or nanostructures and organic or metal. Therefore. and a liquid electrolyte usually contain I-/I3. But conversion efficiency of titanium dioxide based DSCs are limited due to the absence of depletion layer on its surface.4% conversion efficiency [15-18]. nanowires. therefore reduces photo carrier loss due to recombination when used in DSCs. Dye or organic molecule/polymers or quantum dots acts as sensitizer. The DScs are the photoelectrochemical systems having porous metal oxide semiconductor thin film or nanostructures with adsorbed dyes as photoanode. Instead of these properties zinc oxide can be easily grown in various anisotropic nanostructures on cheap substrates with low electron trap level. higher carrier mobility and good crystallinity is required for fabrication of efficient solar cells but also geometry such as orientation and morphology are equally important.semiconductor quantum dots as photoanode and a counter metallic photocathode. DSCs fabricated using various zinc oxide nanostructures such as ordered and disordered nanorods. As zinc oxide is wide band gap semiconductor and have physical and electronic band structure same as that of the titanium dioxide with higher electron mobility. They used atomic layer deposition (ALD) of amorphous Al2O3 or anatase TiO2 on the surface of zinc oxide NRs and reported that alumina shells of all the thickness act as insulating barriers between the NRs and enhance VOC with large decrease in the ISC. which suggest that morphology of zinc oxide used in the fabrication of working electrode plays an important role in the electron transport properties and hence conversion efficiency. Umar [23] has fabricated DSCs from thermal evaporation synthesized zinc oxide nanocomb like structure grown directly on the FTO substrates and reported 0. It is also reported by Niinobe et al. Zinc oxide double layer structured film with mono-dispersed aggregates as under layer and submicron sized platelike structure as upper layer was used for DSC fabrication by Zeng et al. They have reported 32 . Effect of doping of zinc oxide nanorods of different sizes in the TiO2 photoanode on the performance of DSCs is investigated by Pang et al.conversion efficiency. [24] that VOC of tin oxide DSCs enhances by the addition of zinc oxide nanostructures. [26] and reported that electron transport in ZnO NRs is two times faster than that of the zinc oxide colloids.671V open circuit voltage (VOC). [22] and reported that the 3. Gao et al.68% conversion efficiency (η). which is 47 % higher compared to single monodispersed aggregate and far larger than that obtained by micron sized platelike structures (only 0. Effect of dye loading conditions on the η value of DSCs fabricated from zinc oxide film as photoanode is investigated by Chou and coworkers and reported that higher and lower dye concentration requires a shorter and longer immersion time. [20] have fabricated DSCs from the solution derived zinc oxide nanowire array films and studied effect of aspect ratio of nanowires on the conversion efficiency. 34% fill factor (FF). [28]. A recent review summarizes light conversion efficiencies of DSCs fabricated from various zinc oxide nanostructures and thin films grown by different methods [19].44% efficiency for double layer zinc oxide DSCs.14mA/cm2 short circuit current (ISC) and 0. ZnO/Al2O3 and ZnO/TiO2 core/shell NRs were used for the fabrication of DSCs by Law and coworkers [27].81%) alone. Tornow and Schwarzburg [26] transient electrical response of zinc oxide NRs DSCs and reported intrinsic resistance and capacitance of as synthesized and annealed zinc oxide nanorods of different length used in the DSCs. respectively for the optimal sensitization of zinc oxide to obtain maximum sensitization. while titania shells of 10-25 nm thickness cause high increase in the VOC and fill factor with slight fall in the ISC. Comparative studies between DSCs fabricated from MOCVD grown zinc oxide nanorods (NRs) array and a mesoporous film of the same thickness prepared from zinc oxide colloids was done by Galopinni et al. 3. Single crystalline zinc oxide NWs having 30-100 nm dia. Incorporation of lithium ions during the sol-gel processing of metal oxide layer is done by Lloyd and coworkers to enhance conversion efficiency of ZnO/P3HT HSCs by a factor of 2.that conversion efficiency enhances 15% with the addition of 1w% zinc oxide.9 [30]. [31] have studied effect of zinc oxide processing conditions i. roll by roll technology. DSC fabricated with branched zinc oxide NRs shows twice conversion efficiency as compared to bare ones. Olson et al. which was higher than that in which ZnO was treated with UV/ozone. therefore several parameters have to be optimize to get good conversion efficiency. As most of the polymers have high absorption coefficient (105cm-1). Single crystalline zinc oxide NRs/NWs array are used in the HSCs to speed electron conduction and hence improving conversion efficiency. (b) Zinc oxide based Hybrid Solar Cells (HSCs) Organic –inorganic hybrid solar cells have ability to provide cheap. however. while carrier diffusion rate increases 1-3 orders of magnitude depending on size of NRs and suggested that addition of ZnO NRs enhances charge carrier transport. inkjet printing. smaller and thinner photovoltaic devices in large scale. it is beneficial due to the cheapest and easiest fabrication of excitonic solar cells. The HSCs are not as more advanced as DSCs due to their low conversion efficiencies. they reported that the atomic layer deposited TiO2 thin shell on zinc oxide surface enhances voltage and fill factor relative to that without shell and performance of the cell increases with its exposure to air. ZnO-TiO2 coreshell/P3HT HSC was fabricated by Greene et al [32]. flexible.e morphology on the photovoltage of ZnO/P3HT HSCs and reported that ZnO film/NRs annealed in air at 150°C temperatures granted ∼ 200mV VOC. screen printing and spray methods. They have reported enhancement in the VOC and ISC upto an optimum Li concentration between 15 and 20 atomic percentage. and micron sized lengths on quartz substrate obtained by solution and vapor phase methods are used for the fabrication of ZnO/ poly(3-hexylthiophene) (P3HT) and ZnO/ didodecylquaterthiophene (QT) hybrid solar cells [29]. improved VOC value and hence improve in the overall conversion efficiency. short exciton length due to the less than 10 nm diffusion length and very low hole mobility (10-1-10-7 cm2V-1 s-1) as compared to silicon (500 cm2V-1 s-). An elaborative work on HSCs having crystalline zinc oxide nanoparticles (nc-ZnO) as electron acceptor and blends of 33 . As most of the organic molecules and polymers are soluble in the organic solvent and can be easily coated on the surface of semiconductor film or nanostructures by any one of the solution processing such as spin coating. decreases rate of recombination. Other reports for the application of zinc oxide nanorods. As zinc oxide is wide band gap semiconductor. The working principle of solar cells is similar to that of the light detector. they can have potential ability to enhance efficiency of excitonic solar cells. They have investigated conversion efficiency as a function of zinc oxide concentration and thickness of the layer. [33] and reported that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub picosecond time scale and produces a long lived charge separated state. shape and surface modification of zinc oxide and degree and type of mixing of the two components. An array of vertically grown zinc oxide nanowires on the conducting FTO substrate along with mercaptopropionic acid capped CdSe QDs as sensitizer was used for the fabrication of QDSC by Leschkies and coworkers [34]. According to Maxtronics Inc.. QDs when illuminated with light eject electrons across the quantum dot nanowire interface. Choosing suitable semiconductor QDs of particular size in such a way that its conduction band lies above the conduction band of zinc oxide NRs/NWs or thin films or other nanostructures used as electron acceptor in QSCs. Lead selenide quantum dot was used as photosensitizer with zinc oxide thin film to fabricated zinc oxide based QDSSCs with higher conversion efficiency as compared to the Schottky diode made of the similar zinc oxide film [34]. while the liquid electrolyte provide the path for the transportation of holes to the counter electrode.conjugated polymer poly[2-methoxy-5-(3’-7’-dimethyloctyloxy)-1. thin films in the fabrication of QDSSCs excitonic solar cells are also available [35-37]. which are transported to the photoanode through the path provided by the morphology of zinc oxide nanorods. high exciton binding energy and higher carrier mobility therefore it is widely used for the fabrication of zinc oxide based UV detector. size. (b) Zinc oxide based Quantum Dot Sensitized Solar Cells (QDSSCs) In this case semiconductors or metallic quantum dots (QDs) are used as photo-sensitizer to create photoelectron and metal oxide thin films or NRs/NWs arrays are used as electron acceptor. absorbs UV light.4-phenylenevinylene] (MDMO-PPV) as electron donor has been done by Beek et al. which is fabricating zinc oxide based optoelectronic 34 . Both require high number excitons generated by single photon on the p-n junction. Due to the size dependent tunable optical and electronic properties of semiconductor QDs. their dissociation into electron and holes at the interface and transportation of charge carriers towards the electrodes with minimum number of recombination to fabricate efficient solar cells and photo detector. However. 2. 2. Fei et al. Doping of zinc oxide with various atoms can easily change its electronic properties such as carrier concentrations and mobility for the fabrication of efficient transistors and FETs. More recently.83 wt% H2 at room temperature and ~30 atm hydrogen pressure.4. Zinc oxide transparent thin film transistors are very recent development in this area [42]. César et al have observed that ZnO/TiO2 is a promising system for H2 production [142].6 wt % (71 %) of the stored hydrogen has been released during desorption. the 35 .4: Electronic device fabrication Electronic devices such as transistors and FETs are fabricated from zinc oxide single nanorod or nanowire. Wan et al [146] have first time reported that ZnO nanowires absorbs 0. zinc oxide photodetectors for UV as well as visible range of high efficiencies and fast response are fabricated [144-148]. Tuning the zinc oxide bandgap by variation in its size or by doping it with various atoms. have fabricated external force triggered FET based on a free standing piezoelectric zinc oxide wire utilizing Ag and Au source and drain electrodes at the end of the zinc oxide nanowire channel [45]. LEDs and light detector. Mg doping in place of Zn does not change the wurzite structure of ZnO. Deborah V. ZnO with TiO2 has also been observed to be a good composite for producing hydrogen by partial oxidation of methanol [143]. Some other but not all reports on the fabrication of zinc oxide transistors/FETs are given in the literatures [46-48]. Water is the cheapest source of hydrogen and can be break into its molecular constituents by photolysis. Self assembly of colloidal zinc oxide nanorods are used for the fabrication thin film field effect transistor [44]. Zinc oxide nanostructures can be used for the electrochemical photolysis of water. detector fabricated by zinc oxide nanostructures are three times more efficient compared to any other solid state detector [143]. H.devices such as solar cells. Pan et al [145] have investigated the difference in hydrogen storage characteristics of ZnO nanowires with Mg doping.4. However. 147]. Recent studies reveal that ZnO nanostructures can able to reversibly absorbs/desorbs hydrogen at ambient temperature and pressure [146. it is observed that only ~0. as well as using its assembly. Zinc oxide also stores hydrogen by its absorption/adsorption. Using of organic molecules and/or polymers with zinc oxide has also potential for the fabrication of hybrid electronic devices [43].5: Hydrogen generation and storage Hydrogen is an efficient source of renewable energy and has potential for cheap replacement of the fossil fuels. thin films. 2. chemical [128. which can measure blood flow rate and generated pressure in the veins [122]. even though bulk zinc oxide cannot absorb arsenic. pressure.maximum hydrogen uptake in Mg doped ZnO nanowire and undoped ZnO nanowire were found to be 2. This phenomenon is utilized to fabricate small pressure sensors. which can be measured with calibrated devices. NO etc. Therefore.129]. zinc oxide nanostructures have tremendous applications in the water filter and waste water treatment. respectively. temperature [121]. US. All types of sensors such as gas [117-120]. 324]. molecules in the solution (glucose. which is an important step to detect the presence of biothreat agents.4. CO . which can detect any change such as temperature. humidity [123127]. alcohol and other industrial chemical from waste water on its surface and photo catalytically [323.7: Water and Air Purification Researchers at Oklahoma State University. optical and vibrational properties of zinc oxide. The simplest and thus most popular way is to pass electrical current through the zinc oxide nanorods and observe its changes upon gas exposure. Zinc oxide nanostructures have the property to adsorb most of the toxic gases such as CO2. hence are being used as air purifier. 36 . As zinc oxide nanowires are ultra sensitive to the tiny forces in the range of nano to pico newton. glucose [133-135]. They produced the zinc oxide in a porous aggregate form that was suitable for water treatment. One cannot imagine that sequences of bases in the DNA can also be detected using zinc oxide NR based sensor [136]. and other biomolecules are available.8 wt% and 2. 2. or presence of any gas or liquid molecule in its ambient atmosphere. have used nanoparticles of zinc oxide to remove arsenic from water.57 wt%. It also adsorbs organic molecules such as dyes [323. humidity etc. on its surface.6: Sensors Zinc oxide nanomaterials sensor is an electronic device. Under ambient conditions these materials show faster hydrogen uptake and release kinetics. Electrical charges get accumulated on the nanowire surface and decrease the current flowing through it when such a small forces try to deform it. pressure [122]. Zinc oxide based biological sensors to detect urea [130-132]. urea etc) are fabricated from zinc oxide nanostructures [130-135]. Any change in environmental conditions or presence of any atoms/molecules highly affects electronic.4. 324] degrade these to purify the water. 5% Fe2O3 known as calamine is used in calamine lotion. which are also known as calamine when mixed with eugenol. 2. adhesive. the mixture is called zinc oxide eugenol and has restorative and prosthodontic applications in dentistry.4. high thermal conductivity. rubber. sealants. Its industrial applications are given as follows (a) Rubber Industry About 50% of the total production of ZnO is used in rubber industry. it has also several industrial applications. ZnO2 . Its ability to absorb ultraviolet light makes zinc oxide an active ingredient of choice in suntan lotions. some toothpaste formulations and in dental cements. Zinc oxide based pressure sensors can measure pressure inside a single blood veins. fire retardants. ointments. Due to its high refractive index. It is also used in medical tapes and plasters. which otherwise may not occur at all. Zinc oxide is well known for its ability to neutralize acid and for its mild bactericidal properties. There are also two minerals. Zinc oxide and stearic acid are ingredients in the commercial manufacture of rubber goods. cement. glass. lubricants. ferrites. Zinc oxide along with stearic acid activates vulcanization.2. A mixture of these two compounds allows a quicker and more controllable rubber cure.½ H2O. batteries. And.4. Mixture of Zinc oxide alongwith about 0. foods. etc. including plastics. last but not least.9: Other Applications: In addition to above mentioned potential applications of zinc oxide. ceramics. is a white to yellow powder that is used in antiseptic ointments. Zinc peroxide. it is added into various materials and products. antibacterial and UV-protection properties. zinc oxide is incorporated in dietary supplements and vitamin tablets as source of the essential micronutrient zinc for the human body. making it an ideal component in body cream/antiseptic healing cream to help reduce soreness and redness. paints. It has potential to replace conventional dyes and toxic quantum dots in biomedical applications owing its high photo-stability and low toxicity. pigments. while nano-generator fabricated from these nanostructures can power heart pacemakers. zincite and hemimorphite. Zinc oxide has found use in a wide range of medical and cosmetic applications.8: Biological and medical Application Zinc oxide nanostructures have wide range of biological and medical applications due to its high level of bio and haemo-compatibility. binding. ZnO is also an 37 . It is also a main ingredient of mineral makeup. which is crucial to dissipate the heat produced by the deformation when the tyre rolls. (e) Pigment Zinc white is used as a pigment in paints and is more opaque than lithopone. Addition of ZnO improves the processing time and the resistance of concrete against water. (c) Cigarette filters Zinc oxide is a constituent of cigarette filters for removal of selected components from tobacco smoke. Some prepackaged foods also include trace amounts of ZnO even if it is not intended as a nutrient. As ZnO highly n-type doped with Al. They are especially effective for galvanised Iron. A filter consisting of charcoal impregnated with zinc oxide and iron oxide removes significant amounts of HCN and H2S from tobacco smoke without affecting its flavor. It is the broadest spectrum UVA and UVB absorber that is approved for use as a sunscreen by the FDA. a necessary nutrient. zinc oxide can be used in ointments. (d) Food additive Zinc oxide is added to many food products. and it considerably improves the thermal conductivity. creams. Because it absorbs both UVA (320-400 nm) and UVB (280-320 nm) rays of ultraviolet light. and lotions to protect against sunburn and other damage to the skin caused by ultraviolet lights. Ga or In is transparent and conductive (transparency ~90%. (f) Coatings Paints containing zinc oxide powder have long been utilized as anticorrosive coatings for various metals.important additive to the rubber of car tyres. (b) Concrete industry Zinc oxide is widely used for concrete manufacturing. lowest resistivity ~10−4 Ωcm ). e. and is completely photostable.. Zinc oxide paints however. breakfast cereals. The latter is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. ZnO:Al coatings are being used for energy-saving or heatprotecting windows. retain their flexibility and adherence on such surfaces for many years. ZnO additive also protect rubber from fungi and UV light. as a source of zinc. Chinese white is a special grade of zinc white used in artists' pigments. It is also used in coatings for paper. but less opaque than titanium dioxide. The coating lets the visible part of the spectrum in but either reflects the 38 . Vulcanization catalysts are derived from zinc oxide.g. Instead of technological and biological applications it has also tremendous industrial applications. sensors (Physical. Solar cells for energy harvesting. As the highest conversion efficiency of zinc oxide based DSCs have reached upto 4-5% yet. The coating reduces the diffusion of oxygen with PEN. hydrogen generation and its storage. biological as well as chemical). Fe. which is too lower than that based on the titanium dioxide base DSCs. spintronic and optical properties. etc. Zinc oxide layers can also be used on polycarbonate (PC) in outdoor applications. Synthesis. which makes it more popular amongst the researchers. white as well as colored LEDs. 2.) and becomes ferromagnetic. Characterizations and applications 3. environmental pollution monitoring and biological/medical applications. ZnO has also been considered for spintronics applications if it is doped with 1-10% of magnetic ions (Mn. depending on which side of the window has the coating. light emitting diodes. 3: Cuprous Oxide (Cu2O) Nanostructures. even at room temperature. but it is expected that zinc oxide may be used to replace Si based costly solar cells with high efficiency. Lithium ion and fuel cells for energy storage. such as poly(ethylene-naphthalate) (PEN). Various plastics. Co. transistors/FETs. solution and vapor phase routes for the synthesis of wide morphology of zinc oxide nanostructures with great optoelectronic. V.1 Introduction Synthesis of inorganic nanostructures with reliable low cost and well defined morphology have attracted considerable attentions for the dimensional and structural characteristics of these 39 . There is large number of cheap and simple available physical and chemical. can be protected by applying zinc oxide coating. electronic. The coating protects PC form solar radiation and decreases the oxidation rate and photo-yellowing of PC .infrared (IR) radiation back into the room (energy saving) or does not let the IR radiation into the room (heat protection). It provides material for laser diodes with broad spectral coverage from deep UV to near IR. The piezoelectricity in textile fibers coated with ZnO has been shown capable of fabricating "self-powered nanosystems" with everyday mechanical stress from wind or body movements.5: Summary Zinc oxide is one of the best semiconductors materials with advanced technological applications in the fabrication of semiconductor laser diodes. 3 Synthesis of Cu2O Nanostructures The Cu2O nanostructures have been prepared by several different methods such as electrodeposition. It is insoluble in water and organic solvents. Copper(I) oxide dissolves in concentrated ammonia solution to form the colorless complex [Cu(NH3)2]+. The unique properties of semiconductor and metal oxides could be harnessed for the design and fabrication of nanosensors [325]. Basically these methods can be divided in to two methods ie template based methods and template free methods. When it is exposed to oxygen. Copper (I) oxide or Cu2O is an oxide of copper and belongs to the BCC Cubic crystal system with lattice parameters a = 0. Cuprous oxide forms on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged. 40 . copper(I) oxide will form copper(II) oxide. copper will naturally oxidize to copper (I) oxide. With further heating. this kind of corrosion is known as red plague. Artificial formation is usually accomplished at high temperature or at high oxygen pressure. switches [326]. Copper(I) oxide is found as the mineral cuprite in some redcolored rocks. Copper containing complexes are indispensable in biological processes for their ability to act as oxygen carriers in oxidation reactions [329]. in reactions involving DNA hydroxocomplexes [330] and in biological enzymes such astyrosinase [331] and oxyhemocyanin [332]. Among different metal oxide materials copper based nanomaterials (nanowires and nanobelts etc) are of great interest because of their applications as inter connectors for microelectronics. but this takes extensive time. while dilute sulfuric acid and nitric acid produce copper(II) sulfate and copper(II) nitrate. nanolasers [327] and transistors [328]. liquid phase reduction. 3.4269 nm. respectively. thermal relaxation. which easily oxidizes in air to the blue [Cu(NH3)4(H2O)2]2+.materials endowed with wide range of potential applications in electronic magnetic and photonic devices. 3. sonochemical method. Various approaches for the synthesis of different nanostructures have been discussed below in detail.2 Crystal structures and Physical Properties of Cuprous Oxide The physical properties of the cuprous oxide (Cu2O) are given in the table 2. complex precursor surfactant-assisted (CPSA) route and vacuum evaporation. It dissolves in hydrochloric acid to form HCuCl2 (a complex of CuCl). Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars which reduce an alkaline solution of a copper(II) salt and give a precipitate of Cu2O. had a strong influence on the morphology of the oxide and on the deposition kinetics (Figure 35). Larger crystals were grown at higher temperatures. Resistivity measurements suggested improved alignment of the reverse hexagonal liquid crystalline phase under electric field during electrodeposition.1: Synthesis of Cu2O nanostructures by electrodeposition Well-defined. while a granular structure was observed at higher temperatures. R. ranging from 10 to 65 °C. The growth kinetics was controlled by the pH of the deposition solution and led to a pyramidal morphology at pH 9 and a cubelike morphology at pH 12. The temperature of the deposition solution. Cuprite (Cu2O) nanowires with diameter of 25-100 nm were electrodeposited from anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate reverse hexagonal liquid crystalline phase by L. Leopold et al. Jongh et al [333]. [336] from the self-oscillating Cu(II)-lactate system using etched ion track polycarbonate membranes as templates. After removal of the polymer. The influence of the applied current density and the deposition temperature on the oscillation pattern and quality of the deposited wires were studied for pores having a diameter of about 1000 nm. [335] deposited epitaxial bulk Cu2O films onto InP(001) with tunable morphologies and epitaxial chiral CuO films onto Au single crystals. Cylindrical microstructures of Cu/Cu2O were electrochemically produced by S. The widest current density range for oscillations was found at 25 0C. At and below room temperature. The nanowires were grown up to tens of micrometers in length by simply changing the electrodeposition time. The enhanced alignment of the liquid crystalline phase was essential for the growth of nanowires with high aspect ratio. They electrodeposited the epitaxial Cu2O nanocrystals onto InP(001). a steady decrease in potential occurred during the deposition.3.3. The pH of the solution strongly influenced the nucleation process and the morphology of the layers. limiting the thickness of the layers. E. arrays of free-standing cylinders were obtained. N. The as-deposited Cu2O was stable under ambient conditions. Hexagonal liquid crystal phase with improved alignment played an important role in the formation of the nanowires. Under these conditions the deposited wires were of equal length and showed smooth contours. Potentiostatic deposition of Cu2O 41 . Liu et al. polycrystalline Cu2O was electrodeposited on TFO conducting substrates by de P. Huang et al [334]. They showed that Cu2O nanowires with high aspect ratio can be readily electrodeposited from lyotropic reverse hexagonal liquid crystalline phases. et al [338]. H. The applied voltage was varied from 2 to10 V. 3. No special electrolytes. R. [337]. Guo et al [341] reported a electrochemical route for the controlled synthesis of a Cu2O microcrystal from perfect octahedra to monodisperse colloid spheres via control of the electrodeposition potential without the introduction of any template or surfactant. The Cu2O nanowires had uniform dimension of approximately 100 nm and lengths up to 16µm. Template-mediated electroplating was used to fabricate Cu2O nanorod arrays by Y. [342] synthesized different cuprous oxide (Cu2O) nanostructures (Figure 39) by anodic oxidation of copper through a simple electrolysis process employing plain water (with ionic conductivity~6µS/m) as an electrolyte. and surfactants were used.3. Perfect Cu2O octahedra and monodisperse colloid spheres were obtained in high yield (~100%) (Figure 37). [339]. One type was delaminated from copper anode and collected from the bottom of the electrochemical cell and the other was located 42 . Singh et al.7 V. E. Platinum was taken as cathode and copper as anode. chemicals. The morphology of Cu2O was changed to monodisperse colloid spheres when the electrodeposition potential was changed to -0. mainly consisting of (100) and (200) with a length of 4 µm were prepared by electrochemical deposition using a porous alumina template by K.nanowires (Figure 36) in polycarbonate membrane by cathodic reduction of alkaline cupric lactate solution was studied by A. L. A morphological study showed that the nanorods of 60 nm diameter and 450 nm in length were perpendicular to the substrate. The optimum anodization time of about 1 h was employed for each case. Daltin et al. Two different types of Cu2O nanostructures were formed. Lee. S. the electrodeposition potential. The optimized electrochemical conditions to prepare Cu2O nanowires were different from those for the formation of a bulk thin Cu2O layer since different pH values were found between the tip of the pores and the bulk. and the concentration of Cu(OH)42-. Eunseong et al. P.2: Synthesis of Cu2O nanostructures by anodic Oxidation D. Brown et al [340] demonstrated a facile electrochemical procedure that can produce highly transparent Cu2O films using a DMSO medium. The resulting films were composed of ~ 10 nm sized nanocrystals. The size of octahedral Cu2O were easily controlled via simple control of the deposition time. Pure cuprous oxide nanowires were obtained by the potentiostatic technique even if high current intensities were reached in the first moments of the deposition process. Cu2O nanowires. K. J. The nanostructures collected from the bottom of the cell were either nanothreads embodying beads of different lengths and diameter ~10-40 nm or nanowires (length ~600-1000 nm and diameter ~10-25 nm). The shell thickness of these hollow spheres were adjusted through the choice of the bromide source used for the formation of intermediate templates. Those present on the copper anode were nanoblocks (Figure 39) with a preponderance of nanocubes (nanocube edge ~400 nm). Gao et al [343] synthesized uniform hollow spheres of Cu2O and CuS by chemical transformation of in situ formed sacrificial templates containing Cu(I) in aqueous solutions. Both anodization potential and time influenced the morphology of nanostructures of Cu2O. Specifically. pure Cu nanowires were deposited regardless of the anodic potential. thick-shell hollow spheres (about 130-180 nm in shell thickness) were obtained by using CuBr solid spheres as the templates. The copper electrode served as a sacrificial anode for the synthesis of different nanostructures. and (C4H9)4N+ ions as the templates. The content of Cu2O in the copper nanowires was controlled by varying the anodic potential of the pulse-reverse electrolysis and the pH of the electrolyte within a range of 2.0. Shin et al. which were formed by the reduction of CuBr2 with ascorbic acid. Figure 40 a and b show the SEM and TEM images of the thick shell hollow Cu2O nanostructure after reaction with NaOH and CuBr solid spheres. Nanothreads were formed at 6 V during 15-30 min. on the other hand. When the anodic potential became higher than the cathodic one.0–3.9. thin-shell hollow spheres (about 20-25 nm in shell thickness) were obtained by using spherical aggregates consisting of the Cu+. pure Cu2O nanowires were produced at a pH of 3. Uniform Cu2O hollow spheres (Figure 40 c and d) with a diameter of about 510 nm were obtained when NaOH was added to the reaction solution at room temperature. Br-. For the pH of 2. as well as the capacitive barrier layer of the template. 43 . Nanowires of Cu2O as well as Cu were synthesized by H. whereas nanowires resulted when anodization time was extended to 45-60 min. [344] within the anodic aluminum oxide templates in an aqueous acidic electrochemical cell. The growth of Cu2O nanowires in the acidic electrolyte was ascribed to the local increase of the pH at the pore base. which were formed by the reduction of CuCl2 with ascorbic acid in the presence of (C4H9)4NBr.9.on the copper anode itself. S. in the presence of polyethylene glycol (PEG) and sodium hydroxide. A series of shape evolutions of Cu2O particles from the transient species such as multi-pod and star-shaped particles to cubic crystals were formed. The average edge length of the cubes were controlled from 25 to 200 nm by changing the order of addition of reagents. Copper (II) salts in water were reduced with sodium ascorbate in air. in the presence of a surfactant. The effect of concentration of CTAB during the synthesis on the size and shape of the Cu2O nanoparticles was also studied. The average edge length of the cubes varied from 200 to 450 nm. As an example. L.3. These cubes were composed of small nanoparticles and appeared to be hollow.3: Synthesis of Cu2O Nanostructures by Chemical Methods W. The experimental results suggested that the building blocks with desired architecture can be selectively synthesized by programming the growth parameters in the initial synthetic scheme. at room temperature. and the 44 .and microcubes. et al [346] developed a strategy that small complexes with several linear-aligned metal cations can provide precursors for the growth of metal oxide nanowires. [348] in high yield by reducing the copper-citrate complex solution with glucose. if they can be linearly connected with each other by bridging anions with rod like micelles confined. Copper (II) salts in water were reduced with ascorbic acid in air. Y. Figure 42 (a-b) shows the TEM image of the flowery Cu2O nanoparticles synthesized by using ascorbic acid as the reductant. The higher growth rate along {111} induces the shrinking of the eight {111} faces. Gou et al. Gou et al [347] reported the solution-phase synthesis of highly uniform and monodisperse cubic Cu2O nano. L. in which linear alignment of copper cations in Cu3(dmg)2Cl4 as precursors provided orientation for the growth of Cu2O nanowires while rodlike SDS micelles drove the linear units of Cu3(dmg)2Cl4 to connect with each other by Cl anions to form [Cu3(dmg)2Cl2]n2n+ and confined the diameter of nanowires. while six {100} faces remained to form Cu2O cubes because of their lower growth rate.3. Wang et al. Xiong. Cu2O monocrystalline nanowires were prepared via a novel complex-precursor surfactantassisted (CPSA) route. Figure 41 (a-f) shows the TEM of the samples obtained by using an increasing amount of the CTAB as the protecting agent. as a function of surfactant concentration. Uniform crystalline Cu2O cubes were synthesized by D. polyethylene glycol. [362] developed highly uniform Cu2O nanocubes (Figure 43) by using a simple solution approach. Wang et al [345] reported a novel reduction route for preparing crystalline Cu2O nanowires in the presence of a suitable surfactant. One-dimensional (1D) cuprite (Cu2O) nano-whiskers with diameter of 15–30nm were synthesized by Yu et al. Figure 44 (g-i) are the crystal assemblies of type (iii) ie 12 pod branching along <100> directions. three-dimensional microcrystals were organized into simple cubic or face centered cubic lattices according to space instruction of the formed frameworks (Figure 44). and their sizes were controlled by the sequence of addition of reagents and stabilizer concentrations. The average edge length of the octahedron-shaped nanocrystals varied from 45 to 95 nm as a function of the dose rate. Cu2O nanocrystals at different stages of formation were explored by the absorption spectra. glucose and sodium dodecylbenzenesulfonate (SDS) were used as templates. cetyl trimethyl ammonium bromide (CTAB). respectively.[350] from liquid deposition method at 25°C by adding a surfactant. The resultant microcrystal stacks (step (ii)) also provided a base for generation of intracrystal porosity and crystal selfamplification. The nanocrystals formed mostly had an octahedral shape. resulted that the role of CTAB was to interact with tiny Cu(OH)2. Ping et al. The Cu2O microcrystals grew into cubical. 1D structures were not obtained.PEG concentration. reagent concentrations. The nanowhiskers exhibited a wellcrystallized 1D structure of more than 200nm in length. Various crystal morphologies of Cu2O microcrystals were well correlated to their respective multipod frameworks. to disperse the tiny Cu(OH)2 solid and to induce the growth of Cu2O along the 1D direction. More importantly. which can adsorb OH¯ and become negative charged. reaction time and temperature. Figure 44 (a-f) are the SEM image of the multipods frameworks and crystal assemblies of the type (ii) ie 12 pod branching along <110>. Chang et al [352] prepared a variety of multipod frameworks of Cu2O microcrystals through careful control of synthetic parameters such as water content. and (ii) attachment of microcrystal building units (space occupation). [351] prepared size-controlled mono dispersed cuprous oxide octahedron nanocrystals by the reduction of copper nitrate in Triton X-100 water-in-oil (W/O) microemulsions by gamma-irradiation method. Nanocubes were very uniform and monodisperse. Y. as a template. and grew mainly along the [111] direction. Figure 44 (j-m) are the SEM images of the type (iv) 45 . including higher ordered hierarchical organizations of crystals. cuboctahedral and octahedral morphologies. TEM images at different stages during the growth of the Cu2O nano-whiskers. H. When polyethylene glycol (PEG). with an increase in water content in synthesis. This self-organizing scheme were viewed in the following steps: (i) fractal growth of multipod frameworks from a nucleation center (space expansion). The prepared Cu2O nanoparticles were capped by a thin CuO shell. and (iv) crystal aging and hollowing of Cu2O nanospheres. In low CTAB concentration. Huairuo et al. the diameter. and monodispersity of Cu2O nanospheres was kinetically controlled. [356].405-2. nanocubic Cu2O were formed with a <001> orientation and highly uniform size. In this process. Ostwald ripening was operative in (iv) for controlling crystallite size in shell structures. with an increase of CTAB concentration to 0.03 M in the presence of NaOH. When prepared without CTAB. crystallization. (iii) reductive conversion of CuO to Cu2O. A low-temperature solution phase method for monodisperse Cu2O and CuO nanospheres was developed by Z. A detailed process mechanism revealed: (i) formation of CuO nanocrystallites. By modulating the concentration of reactant H2O. well-defined hollow Cu2O irregular polyhedral nanoparticles formed with a preferred <111> orientation. The nanostructures were obtained when the cetyltrimethylammonium (CTAB) concentration ranged from 0 to 0. Higher order multipod frameworks and crystal assemblies were also formed as shown in Figure 44 (n-q). Chang et al [353] demonstrated that cuprous oxide Cu2O nanospheres (Figure 45) with hollow interiors can be fabricated from a reductive conversion of aggregated CuO nanocrystallites without using templates. were fabricated via this chemical route. porous Cu2O polycrystalline nanocubes formed with an edge length ranging from 20 to 100 nm. Longshan et al. [357] with glucose as the reducing agent and gelatin as a soft template. which was formed by the adsorbed oxygen modifying the Cu2O surface layer and can enhance the stability of the Cu2O nanoparticles. Jiatao et al. Y. Superfine single-crystal hollow cuprous oxide (Cu2O) spheres with nanoholes were prepared by X. [354]. Cuprous oxide (Cu2O) nanocrystals were synthesized by solution-phase reduction using nonionic surfactant octylphenyl ether (Triton X-100) as solvent by L. Hollow polyhedra and cubes of nanostructured Cu2O particles were synthesized by Z.170 eV. with variable Eg in the range of 2. Fang et al.ie 6 pod branching along <100> directions. Huaming et al [358]. [355] by the reduction of CuSO4 with ascorbate acid in the solution phase. Cu2O nanoparticles of 35 nm in crystal size were synthesized via electrochemical method in alkali NaCl solutions with copper as electrodes and K2Cr2O7 as additive by Y.03M. The colorful Cu2O hollow nanospheres (outer diameters in 100-200 nm). The rapid production of supersaturated Cu2O nanocrystals and their regular spherical aggregation led to the monodispersity of Cu2O nanospheres. 46 . (ii) spherical aggregation of primary CuO crystallites. on {111} planes of Cu2O nanoparticles induced the selective crystal growth of metastable platelike structures with {111} faces as the basal planes. On aging. Haolan et al. C. 150. NH3 to OH.. These particles were synthesized at molar ratio of 1:7:2 of Cu2+. they demonstrated that. In general. with edge length of 600nm were produced. 30. [359]. Figure 46 (c) is the corresponding TEM image of an octahedron. the growth process appeared to shift into the thermodynamic regime and the thermodynamically stable octahedral shape was obtained. J. Preferential adsorption of I. and 3 days. Using the small optical band-gap cuprous oxide Cu2O as a model case. 90.Octahedral Cu2O crystals with tunable edge length were synthesized by reducing copper hydroxide with hydrazine by X. The shape of the obtained nanocrystals evolved from hexagonal to triangular to octahedral. Ng Bernard et al. the growth patterns were governed by kinetically and thermodynamically controlled growth. hexagonal and triangular plate like morphologies. cube like. The slow oxidation process and use of crystallographic selective surfactants were essential for the appearance of anisotropic metastable shapes. primary nanocrystalline particles were first self-aggregated into porous organized solids with a welldefined polyhedral shape according to the oriented attachment mechanism. The molar ratios of the reagents (NH3:Cu2+ and OH-:Cu2+) detected the morphology and size of the corresponding products via affecting the coordination between NH3 and Cu2+. [360] reported the shape evolution process of Cu2O nanocrystals upon slow oxidation of Cu under ambient conditions. Figure 46 (a) and (b) are the FESEM image of the regular octahedral Cu2O particles with narrow size 130-150 nm distribution. J. yielding. Figure 48 (a-f) shows the SEM images taken at various stage of the growth process of Cu2O nanostructures after the solution was exposed in air for 0. and octahedra were obtained. surface energy control by surfactant molecules might provide a convenient channel for tailoring nanocrystal shapes of metal oxides. It was demonstrated that the ratio of growth rate along (111) vs. H. instead of normally known spherical aggregates. The edge lengths of octahedra were easily tuned from 130 to 600 nm by adjusting the molar ratio of OH. thus Cu2O crystals with different morphologies such as spheres.to Cu2+. In contrast to the 47 . At this ratio of R1:R2~8 much larger Cu2O octahedral. 210min. Figure 47 (a-c) are the SEM and TEM images after varying OH. Teo et al [361] described a template-free synthetic approach for generating singlecrystalline hollow nanostructures. (100) was varied by adjusting the molar ratio of NH3 to Cu2+.concentration when the molar ratio of NH3 to Cu2+ (R1) was kept constant ie 7:1. Due to the presence of intercrystallite space. The growth of the hierarchical nanostructures included three procedures. hollowing and chemical conversion could also be carried out in order to create central space and change the chemical phase of nanostructured polyhedrons. they prepared single-crystal-like Cu2O nanocubes and polycrystalline Cu nanocubes with hollow interiors. where the nanocrystallites were randomly joined together. The effect of reaction temperature and the concentration of NaOH solution on the formation of hierarchical nanostructures were also observed as shown in Figure 50 (a-b) and 51 (a-b).spherical aggregates. maintaining a definite geometric shape and global crystal symmetry. pure Cu nanoparticles were formed instead of Cu2O nanowires. The whole lengths of the Cu2O stems from one end to the other end were ~2µm and diameter around ~150nm (Figure 49b). Z. 48 . It was found that the microemulsion system was a prerequisite for the formation of Cu2O nanostructures with hierarchical double tower-tip-like morphology. the crystal habit of Cu2O also played an important role in the formation of Cu2O hierarchical double tower-tip-like nanostructures. Zhang et al [362] by optimizing the reaction parameters in w/o microemulsion. They showed that spherical particles may also disassemble into cone shaped bundles made of nanowires (Figure 52 d). Cu2O nanowires with a diameter of approximately 20 nm and a length up to 5 µm were synthesized at the reaction temperature (190°C). the precursor concentration (0. Using this synthetic strategy. Cu2O hierarchical double tower-tip-like nanostructures were prepared by H. C. and the next growth procedure started even though the last one did not end.01-0.1 mol/L.1 mol/L). the Cu2O nanocrystallites in this case were well organized.01-0. The reaction temperature and time were also found to play an important role in obtaining Cu2O nanowires: at temperatures above 190°C and times longer than 8 h. This type of structure was obtained only with CuAc2 in the concentration range 0. The whole growth was along the direction of <001>. Figure 52 (a) shows the spherically organized particles of about 10 µm consisting of selfassembled individual Cu2O nanowires (Figure 52 b). In addition. Orel et al [363] prepared the Cuprous Oxide Nanowires by an Additive-FreePolyol Process by using a precursor of Cu(II) acetate monohydrate and diethylene glycol (DEG). It was also found that the final morphology of the Cu2O nanowires was highly dependent on the concentration of the starting CuAc2. and the reaction time (6hrs). It was revealed that Ostwald ripening played a key role in the solid evacuation process. Figure 49 (a) and (b) are the FE-SEM images of the Cu2O hierarchical double tower-tip-like nanostructures. respectively. these nanocrystals were converted (based on the Kirkendall effect) into hollow Cu2-xSe nanocages that kept their corresponding original morphologies. [366] when different reducing agents were applied. the octahedral Cu2O nanocrystals were obtained as shown in Figure 53 (c) and (d). and molar ratios of fructose/copper (II). M. from cupric nitrate in alkaline aqueous solutions containing fructose and ascorbic acid at room temperature. When the reducing agent was replaced by hydroxylamine. Figure 53 shows the FESEM and SEM images after addition of different reducing agent and under different synthetic conditions. H. cubic. However. Kim et al [365] employed the polyol method to synthesize copper(I) oxide (Cu2O) nanostructures with well-defined shapes and in large quantities. Zhu. After adding selenium sources at room temperature. different Cu2O nanostructures were prepared. concentrations of fructose. When a small amount of sodium chloride was introduced. The as-prepared Cu2O nanocubes possessed size-dependence absorption and luminescence characteristics. octahedral and spherical Cu2O nanocrystals were obtained in aqueous media by H. Figure 53 (a) and (b) are the FESEM and TEM of cubic Cu2O particles of the size~200nm prepared by reducing with ascorbic acid. Cao et al. et al [368] synthesized well dispersed Cu2O hollow microspheres (Figure 56) consisted of Cu2O nanoparticles. By this strategy high-quality Cu2O nanocubes (yield > 95%) with sizes smaller than 30 nm could be prepared. [364]. J. Hun et al. only spherical particles with porous structure Figure 53 (e) and (f) were prepared by different adding procedure.The bistable effects of cuprous oxide (Cu2O) nanoparticles embedded in a polyimide (PI) matrix were investigated by Jung. Cu2O nanocrystals were formed inside the polyimide layer. In this case. single-crystal nanocubes were obtained. Hollow and filled Cu2O nanocubes (Figure 54) of about 28 ± 5 nm in edge length with a band gap ~2. By controlling several important experimental parameters such as pH. The polycrystalline colloidal spheres were prepared in high yields by simply reducing copper nitrate with ethylene glycol heated to 1400C in the presence of poly(vinyl pyrrolidone). Yang et al. chloride played a pivotal role in controlling the formation of seeds and the growth rates of various crystallographic planes to shape the Cu2O nanostructures into nanocubes. H. They suggested that the Cu2O nanocubes were formed from hollow to filled structures by conducting time-evolution TEM measurements (Figure 55).[367]. which were quickly synthesized in aqueous solution at room 49 . With the use of PVP (polyvinylpyrrolidone) as capping reagent.42 eV were prepared by Z. nanocubes and nanospheres of cuprous oxides were readily synthesized by L. They studied the influences of the reaction time. Cu2O nanosheets were synthesized by Y. by reducing Cu(CH3COO)2 _H2O with ethylene glycol at different concentrations of poly(vinyl pyrrolidone). Water dispersible Cu2O nanocubes were prepared at room temperature by using a simple two steps procedure: the reduction of copper(II) sulfate by sodium borohydride generates the copper(0) nanoclusters stabilized by Hydrogen phosphate towards the aggregation in aqueous solution. [373] prepared highly uniform porous cuprous oxide (Cu2O) octahedra with an average size of 1 mm with high yield by one-step seed-mediated approach. Zahmakıran et al. and citric acid as the assistant vesicant. Zhang et al. [371] developed the method for the organic and polymer free preparation of Cu2O nanocubes. M.temperature (25 0C) with polyvinylpyrrolidone (PVP) as surfactant. The crucial influence of citric acid and poly(vinylpyrrolidone) (PVP) on the morphology of porous octahedron in the synthesis were also observed. Cu2O nanoparticles aggregated to form loose aggregations and then quickly transform to hollow spheres through Ostwald ripening. The solvent agent of ethanol played key roles in the formation of the as-synthesized nanosheets. and unique morphology. Formation of loose aggregations was the key to the fast synthesis of hollow spheres at low temperature. Huang [372]. They also investigated the application of Cu2O hollow microspheres in DNA biosensor. X. X. By choosing the different solvent agent to limit the oxidized processes. The Cu2O nanowires were of ~30nm in diameter. 50 . with the modification and steric effect of PVP molecules. Nanoboxes. The explained formation mechanism of Cu2O hollow spheres was that. The hydrogen phosphate stabilized copper (0) nanoclusters initially formed were slowly oxidized by dissolved oxygen forming the Cu2O nanocubes which were isolated from the solution. PVP amount and pH value of NaOH solution (Figure 57). Cu2O nanospheres and nanocubes were selectively synthesized accordingly. Luo et al [369] by using simple wet chemical route. Liu et al [370] synthesized well-aligned arrays of Cu2O nanowires through the reduction of Cu(CH3COO)2 by ethylene glycol (EG) without the assistance of externally introduced template. employing cupreous acetate and sodium sulfite as the reactants. hierarchical. These nanostructures showed very porous. reaction time. almost down to the speed of sound. allowing 51 . Another extraordinary feature of the ground state excitons is that all primary scattering mechanisms are known quantitatively. [376] prepared different morphologies of Cu2O nano/microstructures on copper foil via a mild hydrothermal process in the presence of mixed cationic/anionic surfactants. and effects like BoseEinstein condensation. Zhao et al [375] synthesized cuprous oxide (Cu2O) nanostructures with controlled morphology (Figure 58) via a hydrothermal method by reducing copper nitrate with formic acid. This results in high polariton densities. a fungicide. and phonoritons have been demonstrated. Tan et al [374] described solution-phase syntheses of single crystalline Cu2O nanowires under hydrothermal conditions. Temperature.4: Synthesis of Cu2O Nanostructures by Hydrothermal Process Y. Lv et al. That means light moves almost as slow as sound in this medium. S. 3. The reaction system of mixed cationic/anionic surfactants and the reaction temperature played key roles in the formation of different morphologies of Cu2O nano/microstructures. Copper nitrate concentration was the key factor on the morphology development by affecting the growth rate in the <100> and <111> directions.5-dimethoxyaniline) core/ sheath nanowires are fabricated. and concentration of source materials showed strong effects on the phase purity and morphology development of the products. Y. H. Moreover. addiction reagent ammonia hydroxide. Rectifier diodes based on this material were used industrially as early as 1924.3.4 Application of Bulk Cuprous Oxide (Cu2O) (i) General applications Cuprous oxide is commonly used as a pigment. the dynamical Stark effect. the unique Cu2O/poly(2. The associated polaritons are also well understood. The diameter and morphology of Cu2O nanowires were easily tuned by the choice of reductant type and synthetic temperature. a main ingredient in "Astroglide" and an antifouling agent for marine paints. (ii) Applications as semiconductor Copper(I) oxide was the first substance known to behave as a semiconductor. Cu2O was the first substance where an entirely parameterfree model of absorption line-width broadening by temperature could be established. their group velocity turns out to be very low. by using copper foil to serve as both copper source and substrate. water content in the mixed solvent. long before silicon became the standard. Copper(I) oxide shows four well-understood series of excitons with resonance widths in the range of neV.3. cubooctahedras and octahedral). As methyl orange (MeO) has been usually used as simulating dye contaminating in many catalytic experiments. (5) submicron Cu2O hollow spheres [385. 3.5 Application of different cuprous oxide nanostructures Cuprous oxide (Cu2O) nanostructures have attracted significant attention as it is one of the first known p-type direct band gap semiconductor [377] with a band gap of 2. Now a days textile dyes and other industrial pollutions have attracted attention as they are typical organic compounds. Different nanostructures of Cu2O like nanowires.6: Summary Cu2O clearly exhibits a fascinating class of material which exhibits a variety of novel properties as we go to its nano-versions. nano/micro cubes. Several catalysts have been used for decolorization or decomposition of the organic compounds. nontoxic and can be prepared in large quantities due to natural abundance of the base material copper. nanothreads hollow spheres. Therefore the synthesis of different nanomaterials of cuprous oxide by simple. multipod frameworks(cubical. It can be shown using Cu2O that the Kramers–Kronig relations do not apply to polaritons. perfect octahedral. 386] can be used as the negative electrode materials for lithium ion batteries [287] and (6) Cu2O has been reported to act as a stable catalyst for water splitting under visible light irradiation [388. hierarchical double tower tip like nanostructures. which would seriously pollute the environment. 3. low cost and high yield is greatly required. nanospheres with hollow interiors. truncated nanoprism. This makes it promising material for the conversion of solar energy into electrical or chemical energy [378].the corresponding absorption coefficient to be deduced. nanowires assembled into a spherical or cone shaped bundles. Some of these are (1) Cu2O is a potential photovoltaic material which is low cost. [382-384] (3) Cu2O is a basic compound for superconducting material. 52 . (4) Cu2O nanostructures can be used as high performance gas sensors. 389]. hexagonal nanoplates.17 eV. but could not be easily degraded. triangular nanoplates. The Cu2O has been shown to lead to photocatalytic degradation of this dye. [379-381] (2) excitons created in Cu2O have been shown as suitable candidate for Bose Einstein Condensate because of the large exciton binding energy of 150 meV. Various nanostructures of Cu2O have been synthesized by employing different techniques. The growing interest in Cu2O nanostructures is due to several reasons. It is interesting to note that they had already compared the photocatalytic activities of various TiO2 powders using twelve types of commercial anatase and three types of rutile. there were a series of reports by Mashio et al. it is active under UV light irradiation. paints. Cuprous oxide has potential to exist in variety of nanostructures by simple tuning of the the physical and chemical parameters at the time of synthesis. there was a report on the photobleaching of dyes by TiO2 both in vacuum and in oxygen in 1938 [391]. and the study of TiO2 photocatalysis had not developed widely in either academic or industrial society. It was also known that TiO2 itself does not change through the photoreaction. Since its commercial production in the early twentieth century. They dispersed TiO2 powders into various organic solvents such as alcohols and hydrocarbons followed by the UV irradiation with an Hg lamp. It is equivocal when and who started utilizing first such a photochemical power of TiO2 to induce chemical reactions actively. titanium dioxide (TiO2) has been widely used in sunscreens. chemical or hydrothermal method. 53 . They observed the autooxidation of solvents and the simultaneous formation of H2O2 under ambient conditions. from 1956. 4: Titanium Dioxide Nanostructures. causing the photobleaching of dyes. but called a photosensitizer.. but at least in Japan. inducing some chemical reactions. The chemical stability of TiO2 holds only in the dark. Characterizations and applications 4. suggesting a fairly high degree of progress of the research [393]. In those days. and concluded that the anatase activity of the auto oxidation is much higher than that of rutile. It was reported that UV absorption produces active oxygen species on the TiO2 surface. although the “photocatalyst” terminology was not used for TiO2 in the report. Such activity under sunlight was known from the flaking of paints and the degradation of fabrics incorporating TiO2 [390].hollow cages etc have synthesized by electrodeposition. Scientific studies on such photoactivity of TiO2 have been reported since the early part of the 20th century. ointments. Synthesis. toothpaste. however. etc. Instead. For example. entitled “Autooxidation by TiO2 as a photocatalyst” [392]. the photocatalytic power of TiO2 might have attracted only partially limited scientists’s attention in the field of either catalysis or photochemistry.1: Introduction Titanium dioxide (TiO2) powders have been commonly used as white pigments from ancient times. Thus. Many applications of TiO2 do not depend only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e. Among the unique properties of nanomaterials. apparently at the nanometer scale. this report attracted the attention not only of electrochemists but also of many scientists in a broad area. this electrochemical photolysis of water was reported in Nature by analogy with a natural photosynthesis in 1972 [395].” Therefore. The possibility of solar photoelectrolysis was demonstrated for the first time in 1969 [394]. crude oil prices ballooned suddenly. the performance of TiO2-based devices is largely influenced by the sizes of the TiO2 building units.Then. because it has a sufficiently positive valence band edge to oxidize water to oxygen. and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials. this became known as the time of “oil crisis. An exponential growth of research activities has been observed in nanoscience and nanotechnology in the past few decades. In those days.g. using a single crystal n-type TiO2 (rutile) semiconductor electrode. Thus. TiO2 materials are expected to play an important role in solving many serious 54 . which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. These applications can roughly be divided into two main “energy” and “environmental” categories. Properties of the materials also changes with the morphology of the nanomaterials. As one of the most promising photocatalysts. and the future lack of crude oil was a matter of serious concern. with inorganic and organic dyes) and on the interactions of TiO2 materials with the environment.. It is also an extremely stable material even in the presence of aqueous electrolyte solutions. Physical and chemical properties of the materials changes dramatically when the size of the material becomes smaller and smaller down to the nanometer scale. as it facilitates reaction/interaction between the devices and the interacting media. much more so than other types of semiconductors that has been tried so far. The high surface area induced by the small particle size is beneficial to many TiO2-based devices. The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases. the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement. and numerous related studies have been done on TiO2 which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors.In the late 1960s. Akira Fujishima began to investigate the photoelectrolysis of water. The structures of rutile. they have a white color. 398].2 eV) semiconductor. By doping or sensitization. and rutile is most stable at sizes greater than 35 nm [400]. shape. TiO2 nanomaterials normally are transparent in the visible light region. TiO2 also bears tremendous hope in solving the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices. rutile (tetragonal). Particle size experiments affirm that the relative phase stability may reverse when particle sizes decrease to sufficiently low values due to surface-energy effects (surface free energy and surface stress. Anatase and rutile. and in brookite.0 eV-3. Thermodynamic calculations based on calorimetric data predict that rutile is the most stable phase at all temperatures and pressures up to 60 kbar. Anatase can be regarded to be built up from octahedrals that are connected by their vertices.environmental and pollution challenges. The three crystal structures differ by the distortion of each octahedral and by the assembly patterns of the octahedral chains. chemically stable and harmless. which depend on particle size) [399].Apart from these three polymorphs. brookite (orthorhombic). in rutile. TiO2 is a wide band gap (3. Rutile can exist at any 55 . The enthalpy of the transformation from anatase to rutile phase transformation is low. few high temperature and high pressure polymorphs also exits [396]. anatase and brookite can be discussed in terms of (TiO26-) octahedrals.e. There are three polymorphs of TiO2 found in nature i. are more ordered than the orthorhombic brookite structure. the edges are connected. The small differences in the Gibbs free energy (4–20 kJ/mole) between the three phases suggest that the metastable polymorphs are almost as stable as rutile at normal pressures and temperatures [397. has empty channels along the a and b axex. anatase is most stable thermodynamically at sizes less than 11 nm. and has no absorption in the visible region due to its higher band gap therefore. it is possible to improve the optical sensitivity and activity of TiO2 nanomaterials in the visible light region.2 Crystal structures and Physical properties of Titanium Dioxide Titanium dioxide is inexpensive. 4. both vertices and edges are connected [Figure 59]. brookite is most stable between 11 and 35 nm. and crystal structure of TiO2 nanomaterials vary. The anatase which is the least dense structure. not only surface stability get change but also the transitions between different phases of TiO2 under pressure or heat become size dependent. which are tetragonal. If the particle sizes of the three crystalline phases are equal. anatase (tetragonal). As the size. Anatase phase is more suitable phase for the photocatalytic applications in spite of having large band gap in comparison to rutile and brookite. NH4OH and urea) to a raw material followed by calcination to crystallize the oxide. Few important methods for the preparation of nanostructured TiO2 are being described here. The most commonly used solution routes in the synthesis of nanostructured TiO2 are presented below. There are many novel methods which do not require any additional de-agglomeration step. 4.3 Synthesis of nanostructued TiO2 Nanostructured TiO2 having different morphologies such as nanoparticles. can be prepared in the form of powders. An increase in surface defects enhances the rutile transformation rate. Nanosized crystallites often tend to agglomerate therefore deagglomeration step is necessary when separate nanoparticles are desired. 4. there are several disadvantages among which can (but need not) be: expensive precursors. nanobelts. allowing formation of complex shapes.1. while for temperatures above 700°C the anatase structure changes to the rutile structure [401]. rutile 56 .temperature below 1800°C. producing homogeneous materials. nanofibres etc. convenient and utilizing methods for the synthesis of nanostructured TiO2 for some applications. thin films and nanocrystals. long processing times.e. nanofoams. Physical properties of titanium dioxide are mentioned in Table 3. In particular conditions. 4. as these defects acts as nucleation sites.3.1 Precipitation Method These involve precipitation of hydroxides by the addition of a basic solution (NaOH. nanotubes.3. nanorods.1 Synthesis of TiO2 Nanostructures by Solution routes Liquid-phase preparation method is one the most preferred. Powders and films can be built up from crystallites ranging from a few nanometers to several micrometers. and preparation of composite materials. especially the synthesis of thin films. at which titanium dioxide becomes liquid (i. melting point of TiO2). This method has the advantage of control over the stoichiometry. It usually produces anatase even though sulphate or chloride is used [404]. However. and the presence of carbon as an impurity. Concentration of lattice and surface defects mainly depend on the synthesis method [402] and presence of dopants [403]. and surface chemistry by regulating the solution composition. TiO2 nanoparticles 57 . 416-418]. and fluorides of alkali metals at different pH values [414. and Ti powder [421] are reported as examples. and ageing time. The disadvantage is the tedious control of particle size and size distribution. 406] are mainly used. As sources of TiO2. reaching the pressure of vapor saturation. a subsequent thermal treatment is required to crystallize the final material. For example. 4. powder of TiO2 nanoparticles was obtained. chlorides. The solvothermal treatment could be useful to control grain size. particle morphology. crystalline phase. A high level of attention is to be paid the hydrothermal treatment of TiO2. reaction temperature.3.2 Solvothermal Method These methods employ chemical reactions in aqueous [407] (hydrothermal method) or organic media (solvothermal method) such as methanol [407].may be obtained at room temperature [405]. Generally. TiOSO4 [413.25 H2O [407].3. H2Ti4O9. The precipitates were prepared by using solutiuon of isopropanol and titanium butoxide into deionized water. and then they were peptized at 70 °C for 1 h in the presence of tetraalkylammonium hydroxides (peptizer). H2TiO(C2O4)2 [419]. TiO2 nanoparticles can be obtained by hydrothermal treatment of peptized precipitates of a titanium precursor with water [422]. TiCl4 in acidic solution [420]. as fast (uncontrolled) precipitation often causes formation of larger particles instead of nanoparticles. such as hydroxides. but not always.0.1. 415]. The temperature can be elevated above the boiling point of water. As raw materials. Many groups have used the hydrothermal method to prepare TiO2 nanoparticles. toluene [409] under self-produced pressures at low temperatures (usually under 250 0C). TiCl3 [405] or TiCl4 [404.1. After filtration and heat treatment.3 Hydrothermal Method Hydrothermal synthesis is normally conducted in autoclaves with or without Teflon liners under controlled temperature and/or pressure with the reaction in aqueous solutions. 1. in hydrothermal synthesis. It is a method that is widely used for the production of small particles in the ceramics industry.4 butanol [408]. pressure. In another example. 4. solvent properties. additives.nH2O amorphous gels [410-413] either in pure distilled water or in the presence of different mineralizers. control over the composition. etc. TiO2 nanorods have also been synthesized with the hydrothermal method. in 1998 [435-454].3. TiO2 powders are put into a 2. 4. TiO2 nanowires are obtained by treating TiO2 white powders in a 10-15 M NaOH aqueous solution at 150-200 °C for 24-72 h without stirring within an autoclave. TiO2 nanowires have also successfully been obtained with the hydrothermal method by various groups [429-433]. chlorides. [434]. Zhang et al. TiO2 nanowires can also be prepared from layered Titanate particles using the hydrothermal method as reported by Wei et al.1.mainly with anatase phase were synthesized by using titanium alkoxide. The hydrothermal method has been widely used to prepare TiO2 nanotubes after Kasuga et al. namely non-alkoxide and the alkoxide. and reacted at 240 °C for 4 h [423]. a colloidal suspension. using titanium trichloride aquous solution supersaturated with NaCl [428]. which requires an additional removal of the inorganic anion. ease and flexibility in introducing dopants in large concentrations. nanobelts etc. while the alkoxide route (the most employed) uses metal alkoxides as starting 58 . Sol-gel method is mainly devided into two routes. which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. membranes. A film of assembled TiO2 nanorods deposited on a glass wafer was reported by Feng et al. stoichiometry control. homogeneity. Typically.). acetates. TiO2 nanotubes are obtained after the products were washed with a dilute HCl aqueous solution and distilled water. and the ability to coat large and complex areas compared to other fabrication techniques. Briefly.The non-alkoxide route uses inorganic salts [455-458] (such as nitrates. added drop wise to a mixed ethanol and water solution at pH 0. obtained TiO2 nanorods by treating a dilute TiCl4 solution at 333-423 K for 12 h in the presence of acid or inorganic salts [424-427].7 with nitric acid.4 Sol–gel Method Sol-gel methods are used for the synthesis of powders. or a sol. In a typical sol-gel process. Du and co-workers found that the nanotubes were formed during the treatment of TiO2 in NaOH aqueous solution [441]. carbonates. acetylacetonates. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase. and thin films of TiO2 nanoparticles. The sol–gel method has many advantages such as purity. nanotubes. is formed from the hydrolysis and polymerization reactions of the precursors.5-20 M NaOH aqueous solution and held at 20-110 °C for 20 h in an autoclave. we have prepared nanostructured TiO2 [Figure 60] film electrodes for the hydrogen production through controlled hydrolysis of Titanium-tetraisopropoxide Ti[OCH(CH3)2]4 [462].material [459-462]. Sol–gel methods coupled with hydrothermal routes for mesoporous structures [466] lead to large surface area even after heating at temperatures up to 500 0C. Sol–gel and templating synthetic methods were applied to prepare very large surface area titania phases [463]. In our studies on TiO2 nanostructures. As titanium sources. In alkoxide route a sol or gel of TiO2 is obtained by hydrolysis and condensation of Titanium alkoxides. Block copolymers can also be used as templates to direct formation of mesoporous TiO2 [465]. Metal ions such as Ca2+. For preparing sol–gel TiO2. The propanol. samples were annealed in argon atmosphere at 550°C for 4 h to improve the crystallanity. 59 . Deionized water was slowly added under vigorous stirring conditions for duration of 10 min. This process was repeated four to five times to increase film thickness. was allowed to evaporate during this time. Ionic and neutral surfactants have been successfully employed as templates to prepare mesoporous TiO2 [464]. This TiO2 solution was then concentrated by evaporation of water in vacuum at 25◦C. until a viscous liquid was obtained. have been introduced into nanostructured TiO2 and films by this method to improve its photocatalytic activity. Finally. Sr2+. then 1 ml of 70% HNO3 was added to the mixture. In this way. In addition. and titanium-tetra-butoxide are most commonly used alkoxides. Ti[OCH(CH3)2]4 solution was added slowly to propanol drop by drop. which exhibit a mesoporous structure. titanium-tetraisopropaxide. many non-surfactant organic compounds have been used as pore formers such as diolates [463] and glycerine [466]. a white precipitate was formed. together with some water. stable TiO2 colloidal solution was obtained. The mixture was then further stirred for 15 min at 80°C. Carbowax M-20000 (40% by weight of TiO2) was added and a viscous dispersion was obtained. titanium-tetra-ethoxide. The chemical process can be represented as Ti[OCH(CH3)2]4 Hydrolysis 80°C TiO2 (sol-gel) (1) The spin on technique using Photoresist Spinner was used for thin film deposition on titanium substrate. Ba2+ etc. During the addition. The film so obtained were dried in an air oven for 15 min at 80°C and then fired at 450°C for 30 min. Despite promising early studies. an improved method using carbon dioxide instead of oil has been applied in preparing nanosized TiO2 [470].3. the temperature reaches about 650 0 C for a short period of time (1–2 min) making the material crystalline.4. use of titanium inorganic salts in aqueous solutions is always accompanied by difficulties. superlattice. Since the time is so short.5 Microemulsion method Water in oil microemulsion has been successfully utilized for the synthesis of nanoparticles. optically isotopic solutions of two immiscible liquids consisting of microdomains of one or both stabilized by an interfacial film of surfactant. quantum dot and nanoporous ones. has synthesized highly ordered. due to the high tendency of the salts to hydrolyze. and (NH4)2TiO(C2O4)2 [474] is reported.H. Although electrodeposition of TiO2 films by various Ti compounds such as TiCl3 [472].1. 4. 4. TiO(SO4) [473]. densely packed and nearly oriented TiO2 nanotube arrays having different 60 .3. In addition to that nanoporous TiO2 thin films have been synthesized anodization of titanium sheet [475-477] in aqueous solution of fluorine containing compound. Recently. thereby considerably reducing the interfacial tension.7 Electrochemical synthesis Electrochemical synthesis may be used to prepare advanced thin films such as epitaxial.1. there have been only limited reports of controlled titania synthesis from these microemulsions [467]. Also. In particular.6 Combustion synthesis Combustion synthesis (hyperbolic reaction) leads to highly crystalline fine/large area particles [471]. Microemulsions may be defined as thermodynamically stable. Such molecules optimize their interactions by residing at the two-liquid interface.3. temperature and pH can easily control the characteristic states of the films.1. The synthetic process involves a rapid heating of a solution/compound containing redox mixtures/redox groups.U. hydrolysis of titanium alkoxides in microemulsions based on sol–gel methods has yielded uncontrolled aggregation and flocculation [468] except at very low concentrations [469]. The surfactant molecule generally has a polar (hydrophilic) head and a long-chained aliphatic (hydrophobic) tail. varying electrolysis parameters like potential. particle growth of TiO2 and phase transition to rutile is hindered. Recently our group at Nanoscience and Technology Unit at B. current density. During combustion. 87 eV. 481].0 M). The UV-Vis absorption spectra of the TiO2 nanotubes have shown that the band gap energy of TiO2 nanotubes synthesized at ~10 V is 3. ranging from metals to composite oxides.2. In contrast. Pore size (diameter) of TiO2 nanotubes has been found to increase from ~40–60 nm to ~100–125 nm with increasing anodization potential from ~10 V to ~20 V used for the synthesis of TiO2 nanotubes.2.3.3. are formed from a chemical reaction or decomposition of a precursor in the gas phase [480. and precursors. A tentative mechanism of the growth of TiO2 nanotubes in terms of controlled interaction of Ti4+ ions with O2− ions in the electrolyte and the rate of oxide growth at the metal/oxide interface and the rate of oxide dissolution at the pore bottom electrolyte interface has been proposed also.2 Physical Vapor deposition (PVD) Physical Vapor Deposition is another class of thin-film gas phase deposition techniques in which precursor and product do not go under chemical changes because of the stability of gas phase.3. few groups have reported the formation of TiO2 nanostructures in non fluorine containing solutions [479]. The family of CVD is extensive and split out according to differences in activation method. The electrolytes used correspond to H3PO4 and NaF. The main gas phase synthesis techniques are as follows 4.1 Chemical vapor deposition (CVD) Chemical Vapor Deposition is a widely used versatile technique to coat large surface areas in a short span of time. in which a 61 .2 Gas phase methods For the preparation of thin films gas phase method is preferred.lengths [Figure.5 to 1. Compounds.03 eV and at ~20 V is 2. The length of TiO2 nanotubes increases from ~350–450 to ~450–550 nm with increase in anodization time from ~1 to ~2 hrs. 4. The most commonly employed PVD technique is thermal evaporation. pressure. However it was found to decrease with increasing concentration of the electrolyte at constant anodization voltage (~20 V). 4. Powders can also be synthesized by this method. 61] grown through controlled specific anodization of Ti sheets [478]. These methods can involve chemical or physical reaction.The tube length decreases from ~450–550 to ~200–250 nm with the change of electrolyte (H3PO4 concentration from 0. This leads to shadow effects. 4. a broad spectrum of names for this class of techniques has evolved. It is. It has been used for preparation of (mixed) oxide powders/films and uses mostly metal-organic compounds or metal salts as precursors. Some of these parameters are mutually dependent on each other. Confusingly. the gaseous stream of material follows a straight line from source to substrate.3. which only becomes crystalline after an annealing step. PVD is a so-called line-ofsight technique. presence of contaminations. Sputtering (either using direct current (DC) [484] or radio frequency (RF) [485] currents) is used quite frequently to produce TiO2 films. composition and concentration of the precursor. In electron beam (Ebeam) evaporation.3. The material is deposited on the substrate in an argon/oxygen atmosphere or plasma. conductivity. have superior characteristics over CVD grown films where smoothness. There are several small derivatives of this technique. TiO2 films.4 Other methods There are several other methods based on vapour phase deposition for the synthesis of thin films.2.3 Spray pyrolysis deposition (SPD) SPD is a type of CVD in which aerosol deposition technique is used for the synthesis of nanostructured TiO2 thin films and powders [483]. Ion implantation is seldom used to synthesize TiO2 and is based on the transformation of precursor plasma to TiO2. but on the other hand. and substrate–nozzle distance. gas flow. a focused beam of electrons heats the selected material. 4.e. however. which are not present in CVD..2. production is slower and more laborious. mainly differing in the formation step of the aerosol and the character of the reaction at the substrate (gas-to-particle synthesis and droplet-to-particle synthesis). Molecular beam epitaxy [486] is a technique that uses a (pulsed) laser to ablate parts of a TiO2 ceramic target. i. The use of reduced TiO2 powder (heated at 900 0C in a hydrogen atmosphere) is necessary to make it conductive enough to focus the electron beam in the crucible [482].material is evaporated from a crucible and deposited onto a substrate. The size of the particles formed and the morphology of the resulting film are strongly dependent on deposition parameters like substrate temperature. These electrons in turn are thermally generated from a tungsten wire that is heated by current. deposited with E-beam evaporation. and crystallinity are concerned. frequently used to implant ions 62 . Fujishima and Honda [395] have demonstrated photoelectrolysis of water on n-type TiO2 single crystal electrode for solar energy conversion and storage in the form of hydrogen. In 1972. Schematics of the Dye Sensitized Solar Cells are illustrated in Figure 63. which can effectively substitute petroleum. 479]. In 1991 M. have higher efficiencies [492]. Sonochemical is another method in which ultrasound waves are used for the formation of nanostructured TiO2 [489. several groups have reported improved rate of hydrogen production using TiO2 nanotube photoelectrode in comparison to TiO2 nanoparticulate system [475-477. Numerous attempts have been made to shift the spectral response of the TiO2 into the visible range to increase the efficiencies of the Photoelectrochemical solar cells either by dye sensitization or doping with species that essentially reduce the band gap of the TiO2. Decades of R&D efforts have shown that hydrogen is the best substitute. Unfortunately because of its large band gap (3 -3.4. The production of hydrogen can reduce our dependence on imported oil and natural gas. 63 . Another unusual technique is dynamic ion beam mixing [488].4 Applications of titanium Dioxide 4. Gratzel et al have reported a new type of solar cells known as Dye Sensitized Solar Cells. There are a lot of reports available on the Hydrogen production using nanostructured TiO2 electrode [462.2 eV) TiO2 absorbs only the ultraviolet part of the solar emission. 4. Microwave method is also used for the synthesis of TiO2 nanomaterials [491]. Hydrogen can be produced through various routes particularly. in which the mesoporous nanocrystalline TiO2 film coated with monolayer of a charge transfer dye have been used to sensitized the film for light harvesting. 490].in TiO2 films (doping) to improve the photocatalytic activity [487]. consequently has low conversion efficiencies. As the length of the tube and doping of species (which can lower the band gap) play major role in band gap modification of TiO2 nanotube. Schematic view of the hydrogen production using titanium dioxide electrode is illustrated in Figure 62.1 Photoelectrochemical generation of hydrogen (solar hydrogen) There is a constant search of clean and renewable energy. Recently several groups have used thin films consisting of TiO2 nanotube as photoanode for the hydrogen production. 493-495]. most attractive routes are photoelectrochemical and photocatalytic decomposition of water. which uses high-energy O2+ and/or O+ beams and Ti vapour to deposit TiO2 films with high speed and control over the composition. The photocatalytic measurements were carried out in a reactor consisting of a quartz tube having its diameter. anastase: 3. These catalysts have been prepared through sol–gel technique using titanium tetra-isopropoxide as a raw material for synthesis. The optical characterization of this nanopowder shows its bandgap ~3.4. Commercially available TiO2 Degussa P-25. Visible light activated TiO2 nanoparticles modified by complex sensitizers and platinum (Pt) deposits drastically enhanced the rate of reductive dehalogenation of trichloroacetate and carbontetrachloride in aqueous solutions under visible light [497].U we have synthesized nanostructured TiO2 photocatalysts.H. TiO2 nanoparticle systems on irradiation with UV light degrade many water pollutants [496]. nanocrystalline TiO2 (particle sizes of ca. In our investigation at Nanoscience and Technology Unit at B.2 Water Purification TiO2 is the most commonly used photocatalyst for environmental applications. A suspension containing TiO2 nanocrystals showed complete conversion of As(III) to As(V) in the presence of sunlight and dissolved oxygen. However. The decrease in semiconductor particle size has not only increases the surface area but also fine tunes the band gap of the semiconductor. Degussa) 64 . Initially ~ 2. However. ~ 3 cm. Since UV light accounts for only a small fraction (less than 5%) of solar irradiation compared to visible light (45%). the wide technological usage of TiO2 is hampered by its wide band-gap (rutile: 3. with an inlet tube for oxygen purging during photocatalysis and another outlet for the collection of samples from the reactor at different time intervals. has emerged as promising photocatalysts for water remediation and purification.02 eV. consisting of 80% of anatase and 20% of rutile with an average particle size of 30 nm. through photocatalytic oxidation within 25 min [498]. is widely used in the treatment of contaminated wastewater. any shift in the optical response of TiO2 from the UV (λ ≤385 nm) to the visible spectral range (λ ≥420 nm) should have a beneficial effect on the photocatalytic efficiency of the material.4. The average particle sizes of the TiO2 nanopowders used in this study are ~ 5–10 nm and correspond to anatase phase. Several approaches have therefore been used to lower the band-gap energy of TiO2 photocatalysts. 6–8 nm). which have been used in the photocatalytic degradation of phenol (one of the most common water pollutants) [499].02eV.5 g of as synthesized nanostructured and the same amount of commercial TiO2 (P-25. which thus requires ultraviolet (UV) light irradiation for photocatalytic activation.2 eV). Variation of phenol concentration using TiO2 photocatalysts after irradiation as a function of time have been shown in Figure 65.g. UV-light was made to fall on the reactor through the tube walls. is only ~ 25%. solution was maintained at 25°C and ~ 3 cc of the samples was collected at given time intervals e.+ h+. the decrease in phenol concentration employing commercial TiO2 nanopowder (P-25. The schematic diagram of the fabricated photocatalytic reactor along with the other related accessories are given in Figure 64.→ H˙ +OH˙. Oxygen was bubbled through this solution at a rate of ~ 200 cc/min. 10 min. Thus the only effect of photocatalytic reaction is the dissociation of phenol. ~ 68 ppm) after ~ 1 h illumination. Phenol + H2O2 → Products. Degussa). It can be seen that the concentration of the phenol decreases from 100% (initial concentration of phenol. 65 . During the reaction.photocatalyst in ~ 400 cc of ~ 100 ppm phenol solution were taken. The total decrease in the phenol concentration using nanostructured TiO2 synthesized in this investigation is ~ 32%. oxygen gas was purged into the solution with the help of porous fused silica tube by an external cylinder. Thus the TiO2 nanopowder prepared in the investigation is more effective than the degradation of phenol through the commercial TiO2 nanopowder. which is nearly the safe limit of the phenol concentration in the solution. Possible mechanism for photocatalytic degradation of phenol using nanostructured TiO2 films as photocatalyst has been described as TiO2(ns) + hν → e. Phenol solution (~ 400 ml by volume) was prepared by dissolving phenol crystals in double distilled water. To determine the concentration of phenol in the solution. where the absorbance was measured at a fixed wavelength (e. After being sampled. H2O → H2O* → H2O2 + H2. Also analysis of water after photocatalytic degradation showed that no other compounds including any toxic components get formed. The concentration of phenol was determined by UV-visible absorption spectroscopic technique. During illumination. ~ 269 nm) for all the samples.g. 2H2O + h+ → H2O+ + H2O → ˙OH + H3O+. On the other hand. ~ 100 ppm) to ~ 68% (final concentration of phenol. OH˙ + ˙OH → H2O2. samples were collected at 10 min of interval upon UV-induced degradation of phenol. the suspension was centrifuged and the centrifugates were subjected to further analysis. H2O + e. Figure 68 shows the images of ordinary and TiO2 coated anti fogging mirrors respectively. TiO2 films are used to render surfaces self-cleaning. Beading of rainwater on automobile side-view mirrors can be a serious safety problem. no water drops are formed. Italy whose walls have been coated with TiO2. That’s why TiO2 coated glasses and mirrors remain transparent under rainwater or mist. On a highly hydrophilic surface.3 Self-cleaning Surfaces The properties which TiO2 possesses are hydrophilicity and hydrophobicity. 507]. have also been incorporated into ultrafiltration membranes and were shown to reduce the fouling burden and improve the permeate flux successfully [505]. Photocatalysts composed of nanostructured TiO2. Tubular arrays of meso and microporous molecular sieves composed of TiO2 nanoparticles.4. which attract drops of polar or nonpolar liquids. with the formation of many small water droplets. This property is due in part to the fact that irradiated TiO2 films are not just hydrophilic but ampiphilic: the surface contains both hydrophilic and hydrophobic microdomains. a uniform thin film of water is formed on the surface because of the lower contact angle between the surface and water [509]. 4. A schamatics of the tubular photocatalytic reactor for water purification is shown in Figure 66. The fogging of the surfaces of mirrors and glasses occurs when steam cools down or humid air condenses.Several groups have reported the improved degrading capability of the doped TiO2 nanoparticle systems for water purification [500-503]. The formation of the droplet depends on the contact angle between the surface and water. 66 . Figure 67 shows the Misericordia Church in Rome.These properties are of great interest for a number of applications such as for example rear-view mirrors. supported by mesoporous silica have been used for water remediation of aromatic pollutants in the presence of UV light [504]. Highly ordered TiO2 nanotubes have also been used by several groups for the water remediation [506. This uniform water film prevents the fogging. This allows a water rinse to flush away an oily coating [510]. Instead. respectively. on these surfaces which scatter light due to which glasses and mirror becomes partially opaque. uniformly deposited onto Fe2O3. anti fogging glasses. self cleaning windows and buildings etc. TiO2 coated surfaces turns into hydrophilic surface under ultraviolet irradiation [508]. Oxygen sensors based on TiO2 nanomaterials include TiO2-x [518]. and chemotherapeutic treatments have been developed and are contributing to patient treatment yet the cancer has remained the cause of death in world. In their experiment they used polarized illuminated TiO2 film electrodes and TiO2 colloidal suspensions for effective in killing HeLe cells. Many types of TiO2 nanomaterial-based room-temperature hydrogen sensors based on Schottky barrier modulation of devices like Pd/TiO2 or Pt/TiO2 [512-515]. radiological.. found that TiO2 nanotubes were excellent room-temperature hydrogen sensors not only with a high sensitivity but also with ability to self clean photoactively after environmental contamination [511].5 cm. In their joint research with urologists they conducted animal experiments implanting cancer cells under the skin of mice to cause tumors to form. In addition. and short response time (<0.4 Sensors TiO2 nanocrystalline films have widely been studied as sensors for various gases. compared with those receiving TiO2 alone or UV irradiation alone. illuminated TiO2 microelectrode [533].Photoexcited TiO2 particles also significantly suppressed the growth of HeLa cells implanted in nude mice.4.1 s) [520]. including the effect of superoxide dismutase. TiO2 nanomaterials are also promising candidates for CO sensing and for methanol and ethanol sensing [521-527]. it was found possible to selectively kill a single cancer cell using a polarized. they injected a solution containing fine particles of titanium dioxide and after 2 or 3 days irradiated tumor and repeated it again after 13 days. However. In mid-1980s Fujishima and coworkers used the strong oxidizing power of illuminated TiO2 to kill tumor cells [529].5 Cancer treatment Cancer treatment is one of the most important topics that are associated with Titanium dioxide photocatalysis.4. due to the production of peroxide. CeO2-TiO2 [519] and doped TiO2 nanomaterial showed improved gas sensitivity. thermotherapeutic. SnO2-TiO2 [516] and undoped TiO2 nanotubes based sensor which exhibited 8. TiO2 nanomaterials are also used for humidity sensing [528]. this technique was not effective in 67 .7 orders of magnitude variation in electrical resistance at room temperature when exposed to hydrogen [517]. They examined a series of experimental conditions [530-532].4. When the size of the tumors grew to about 0. low operation temperature (350-800 °C). Grimes et al. immunological. which enhances the effect. 4. While surgical. and observed a marked antineoplastic effect [534]. The results of animal experiments have shown that near-UV rays. in urban and industrial areas. 4. 536]. Using hybrid TiO2 nanocrystalline electrode such as anatase-rutile TiO2 nanocrystalline electrode [535. Figure 69 shows the photograph of nude mouse just after initial and 4 weeks after treatment. with wavelengths of 300–400 nm.7 Air Purification Substances emitted into the atmosphere by human activities. and thus their transport become possible. Thus.4.stopping a cancer that had grown beyond a certain size. originating largely from industrial processes. attention for the application of this technology for air treatments increases. cause many environmental problems including air quality degradation. 4.4. 545] also enhances the efficiency of the cell. initially TiO2 photocatalysts were applied for water treatment. and stratospheric ozone depletion. core-shell structured nanocystalline TiO2 electrodes [538-543] and TiO2 nanocrystalline electrode coupled with photonic crystals [544. It 68 . including the utilization of pollutant air stripping from the liquid phase. reported the sensitized electrochemical photovoltaic device with a conversion efficiency of 7. Volatile organic compounds (VOCs) are major air pollutants. In 1991 Grätzel et. Although. al. it has been shown that the photocatalytic detoxification of volatile organic compounds is generally more efficient in the gas phase compared to the liquid phase. are safe and do not cause mutation to the cell. The mesoporosity and nanocrystallinity of the semiconductor are important not only because of the large amount of dye that can be adsorbed due to large surface area but also they allow the semiconductor small particles to become almost totally depleted upon immersion in the electrolyte and the proximity of the electrolyte to all particles makes screening of injected electrons. nanocrytalline TiO2 electrode with a buffer layer [537]. climate change.1% under solar illumination with polypyridyl ruthenium and osmium sensitizers [492] after that [Schematic view figure 63] several groups have reported the improvement into dye sensitized solar cells using different dyes and modifying the morphology of the nanostructured TiO2.6 Generation of PV electricity Photovoltaics based on TiO2 nanocrystalline electrodes have been widely studied for the generation of PV electricity. in recent years. global warming. which are used in photocatalytic reactions. has been reported that the use of illuminated TiO2 can result in the overall degradation of VOCs together with nitrogen oxides and sulfur oxides in air [546]. Photocatalytic oxidation (PCO) is shown to be more cost-effective than incineration, carbon adsorption, or bio-filtration for flow rates up to 20,000 cfm (ft3/min) for treating a 500 ppm VOC-laden stream gas phase reactions allow the direct application of analytical tools to monitor the composition, structure, and electronic state of the substrate and adsorbates and hence the reaction mechanisms can be directly elucidated [547]. Other Applications Apart from above mentioned applications TiO2 nanomaterials are used in various other applications.TiO2 nanomaterials are used in the fabrication of electrochromic devices such as electrochromic windows and displays and photoelectrochromic devices such as photoelectrochromic smart windows [548-550]. TiO2 nanomaterials are also used for the hydrogen storage [551, 552]. Recently films consisting of highly oriented TiO2 nanotubes have been used for the size dependent selective filtration by varing the diameter of the nanotubes [553]. Nanocrytalline Titanium Dioxide is also used in the memristor a new electronic circuit element which is used as the solid state memory device in various electronic devices [554]. 4.5 : Summary Due to extensive studies on the nanostructured TiO2 in recent past has resulted in new synthesis techniques which can control sizes and shapes of TiO2 nanomaterials. These new synthesis and modification techniques of TiO2 nanomaterials have brought new properties and new applications with improved performance. Apart from quantum-confinement effect, these nanostructured TiO2 demonstrate size-dependent as well as shape and structure dependent optical, electronic and thermal properties. TiO2 nanomaterials have been used in solar cells, photocatalysis, gas sensing, hydrogen storage, Cancer treatment, electro-chromic and photoelectrochromic devices, memory devices, water and air purification and in many new applications due to their some new and improved properties. TiO2 nanomaterials will play an important role in the search for new renewable and clean energy technologies and environmental protection. 69 5. Overall Conclusion Detailed investigation on the oxides of zinc, copper and titanium reveals that these oxide semiconductors have potential applications in the fabrication of electronic, photonic, sensing, energy storage and harvesting devices as well as in the biological and medical diagnosis. Zinc oxide is most efficient for the photonic application; titanium oxide has maximum IPCE for dye sensitized solar cells and copper oxide is well known for biological and electronic applications. Metal oxide nanostructures can be easily synthesized in various size, shape, morphologies, and architectures, simply get self assembled for the fabrication of devices, and can be functionalized or surface modified for various biological and chemical sensing applications without any complex and difficult processes, as compared to the mono-atomic and their sulphide, nitride and selenide counterparts. Bulk as well as nanomaterials of metal oxides is highly demandable by fabric, rubber, paint, cosmetic, pharmaceutical etc. industries. 6. Future Prospects Based on their performance, cheap and easy ways of synthesis and continuing research by highly interested and motivated scientist and technologist, and ever increasing interest of semiconductor industries, metal oxide nanostructures may be efficient future materials for the fabrication of most of the semiconductor devices and electronic chips. Semiconductor and microchip industries are continuously seeking alternative materials due to the high cost of the silicon wafers, and requirement of highly standard quality of clean rooms for their processing. Metal oxide nanostructures may comply with their need and create a new roadmap of the future semiconductor industry. Due to their continuously increasing biological, medical, and cheap device fabrication applications, it is expected that metal oxide nanostructures will be a reliable partner of mankind and society in the near future. Acknowledgement Dr. S.C. Singh is thankful to Irish Research Centre for Science Engineering and Technology (IRCSET), Ireland for providing EMPOWER Postdoctoral Fellowship Grant to carryout research in the field of photonics. 70 7. References 1. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.M. Tarascon, Nature, 407, 496 (2000). 2. A.C. Dillon , A.H. Mahan, R. Deshpande, P.A. Parilla, K.M. Jones, S-H. Lee, Thin Solid Films, 516, 794 (2008) 3. H. 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Physical Properties of Zinc Oxide Table No. Young Modulus (d) Epitax.7 GPa 5.44 eV mh =0.59 (film). Hardness (e) Bulk Modulus Lattice Vibrations (a) TO (E1) (b) LO (A1) (c) TO (A1) (d) E2 high (e) E2 low Thermal Properties (a) Specific heat (CP) (b) Thermal conductivity (c) Thermal exp. ne = 2.91(bulk) 3. (ε0) E ⊥C E C 5 Electrical properties (a) Exciton band energy (RT) (b) Band gap energy (c) Effective masses (d) Hall mobility (RT) Values 11.8 GPa 142. 3. µn=200 cm2s1 -1 V 4 99 . 1 S.66 (bulk) 60 meV 3.77( bulk) 8.3 Jmol-1K-1 ∼ 1.6 (bulk) 3.3. coeff.008. me =0.8.1 Wcm-1K-1 αa= 4.31×10-6 K-1 αc= 2. 1.24 m0 µp= 5-50.Table Captions.46(film). 1 Physical properties Mechanical Properties (a) Bulk Young Modulus (b) Bulk Hardness (c) Epitax. Physical Properties of Cuprous Oxide Table No.4 591 574 380 437 101 cm-1 cm-1 cm-1 cm-1 cm-1 2 3 40.7. 3. 143. S.09 g/mol 6. Formula 11. 6. 2255 °F 1800 °C. a=4. Molecular formula Appearance 4. 3272 °F Rutile Property 10.0 g/cm3 1235 °C. 7. Hydrochloric acid. Lattice parameters 6. ↓ Anatase Brookite 2.784 Å b = 5. Crystal structure Space group Molar mass Density Melting point Boiling point→ Polymorph Solubility in water Tab le 3: 9.515 Å c = 5. Molecules per unit cell 2 100 .184 Å 1.145 Å 4 8 mmm Pcab a = 9. 1. Dicopper oxide.959 Å c = 9.2696 Å . Point Band Group gap Space Group 5.Table 2: S. 1508 K. Molecular Solubility in acid Crystal System 12.594 Å 4/mmm I41/amd a = 3.137 4/mmm eV P42/mnm a = 4. Physical Properties Common Name Value IUPAC name: Copper(I) oxide Other names: Cuprous oxide. 8. No. Cuprite. Insoluble TiO2 TiO2 SolubleTiO2 Concentrated ammonia solution. depending on the size of the particles) Cubic. 3. 5. 2.447 Å c = 2. 4. Tetragonal Tetragonal Orthorhombic Dilute sulfuric acid and Nitric acid 2. 2073 K. No. Red copper oxide Cu2O Brownish-red solid (The compound may appear either yellow or red. 7.605 Å r(O2-)=1. Phys.. 13.25 3. 172.605 Å r(O2-)-=1.57 // to c axis 2. 42.[Reprinted with permission from Ref. Chem. B. 8. Copyright @ American Chemical Society] 101 . and (d) the schematic view for dumbbell shape. Inorganic Chem. 215 Li et al.60 // to c axis 2. Dielectric constant // to c axis 173 Cationic radius Anionic radius r (Ti4+)=0.12 3. 107. 208 Guzman et al.48 ⊥ to c axis 31 257.07 18.38 19.2: SEM images of zinc oxide nanostructures synthesized by precipitation method at (a) 60° (b) 70° and (c) 80° C temperatures [Reprinted with permission from Ref. Nanotech. Copyright @ Elsevier (2009)] Figure 3: SEM micrographs of mesoporous crystalline zinc oxide nanowires (a) in the PPA template and (b) released from PPA template.1 eV ⊥ to c axis 2.605 Å r(O2-)-=1. Phys. 2009. 8105.36 Å r (Ti4+)=0.69 78 12. Cell Volume Molar Volume Density Band gap Refraction index 62.156 3. 2660.36 Å // to c axis 48 r (Ti4+)=0.693 4. 2003 Copyright @ American Chemical Society (2003)] Figure 5: SEM images of hydrothermally synthesized zinc oxide nanomaterials (A) Nanowires [B] Nanorods [Reprinted with permission from Ref. 2005.55 // to c axis 2. 212 Taubert et al. 209 Xiao et al.2 eV ⊥ to c axis 2. 10.89 ⊥ to c axis 89 136. 2003.36 Å Figure Captions Figure 1: Crystal structure of zinc oxide (a) Wurtzite (b) zinc blende (c) rock salt Figure.377 4. 9. 14. Chem. 16. Matter. 11. J.89 3. Copyright @ Institute of Physics (2009)] Figure 4: SEM images of zinc oxide nanostructures produced with precipitation method with (a) the control sample having large prisms and small needles (b) sample precipitated with 120 mg/L of EO68-b-MAA8-C12.02 eV ⊥ to c axis 2. [Reprinted with permission from Ref. 671. 115.25 20.. .25 (b) 0.2M NH4OH + 1M DEA [Reprinted with permission from Ref. Cryst. 18. J. 2009. 227 Zhang et al.2004. 18. Copyright @ Elsevier ] Figure 8: SEM images of solvothermally produced zinc oxide nanostructures with different water/EN volume (ml) (a) 60/0 (b) 30/30 (c) 20/40 (d) 50/10 (e) cross sectional view of 50/10 and (e) 0/60 [Reprinted with permission from Ref. [Reprinted with permission from Ref. Growth Design 4.025 (f) 0. 523. 309.. 226 Lu et al..2M NH4OH + 1M diethanolamine (DEA) (f) 0. 2006. 2. 93. 1047. 599 (2008). Ceramic International.20 M solution of NH3.. 2007 Copyright @ Institute of Physics 2007 ] Figure 12: SEM images of Sol-gel derived zinc oxide nanostructures on the silicon substrates (ac) from neutral solutions (a) as-synthesized. 226 Lu et al. Adv. J. Func Mater. 17. 477.6M NH4OH + 1M DEA (h) 0. Alloy and Compounds.5 (c) 1. 30. 2004. 2008 Copyright @ Willey-VCH Verlag GmbH 2008 ] Figure 11: SEM images of 3D zinc oxide hollow micro-sphere synthesized by the solvothermal method in the EG solution at 200 °C temperature. 1047. 219 Lu et al.Figure 6: SEM images of hydrothermally synthesized zinc oxide powders using 1M aqueous solution of (a) NH4OH (b) mono (c) di. general view in the left...H2O as solvent. and thermal treatment at 500°C for (b) 4h and (c) 6h and (d) as obtained from acidic solution . Copyright @ Elsevier 2008 ] Figure 13: SEM images of one dimensional micro-emulsion derived zinc oxide nanostructures obtained after different reaction times (a) 10 min. 219 Xu et al. Copyright @ American Chemical Society 2004 ] 102 ... 225 Dev et al. Growth 310.and (d) tri-ethanolamine (e) 0. 18. Copyright @ Institute of Physics ] Figure 9: FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature for 12 hours with 1:7 v/v ratio of distilled water and EDA [Reprinted with permission from Ref. Cryst.0 (d) 2. 455604. at high magnification at right Fig.10 A: FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature for 12 hours with 1:7 v/v ratio of distilled water and EDA [Reprinted with permission from Ref. (b) 30 min. (c) 1h (d) 2h (e) 4h and (f) 8 h. Nanotech. Func Mater. 230 Li et al. 232 Zhang et al..4M NH4OH + 1M DEA (g) 0. 2008 Copyright @ Willey-VCH Verlag GmbH 2008 ] Figure 10: FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature and 1:7 v/v ratio of distilled water and EDA for different reaction times (A) 1h (B) 2h (C) 4h (D&E) 8h and (F) 16h [Reprinted with permission from Ref. Adv. Copyright @ Elsevier ] Figure 7: SEM images of hydrothermally prepared zinc oxide nanopowders at 200°C for 2h using (a) 0. [Reprinted with permission from Ref.05 (g) 0. [Reprinted with permission from Ref. 1533. Nanotech.0 M solution of KOH and (e) 0. [Reprinted with permission from Ref. 111. Lett.6H2O for 2h at 800°C (b) By combustion and (c) by solution combustion (d) solution combustion with 1. Aspects. Cryst. 11560. (e) 2. 11560. Chem. 266 Xiao et al. 6678.. 261 Xu et al.01M citric acid (f) 0.7 to -1. Copyright @ American Chemical Society 2007] Figure 18: SEM images of (a) primary ZnO nanosheets and (b-d) hierarchical ZnO nanorods on hexagonal nanosheets on ITO substrates... 61. 2007.5h. Copyright @ Elsevier 2008] Figure 15: SEM image of melting combustion synthesized zinc oxide nanostructure. 265.5M ZnCl2+0. C. 68.2g Zn(NO3).5M ZnCl2+0.0 ml of additional water. 260 Li et al. J. (c) 40min. 2007. 8. 2439. C. Copyright @ American Chemical Society 2007] Figure 20: SEM images of sonochemically synthesized zinc oxide (a) nanorods (b) nanoups (c) nanosheets (d) nanoflowers and (e) nanospheres [Reprinted with permission from Ref. J. Phys.. Alloy and Comp.25M ZnCl2+0. J. [Reprinted with permission from Ref. Eng. J.5. 111. 243 Izaki and Omi. Chem. Phys. (f)3.5(c) pH 11. Copyright @ American Institute of Physics 1996] Figure 17: SEM images of electrochemically synthesized ZnO nanostructures in the electrolytic solution of (a) 0. (b) 20min.05 M citric acid (d) 0.10 V in 0..25 M +0. 135.Figure 14: SEM images of zinc oxide nanostructures obtained by combustion synthesis method using (a) calcination of 0..05M [Zn(NH3)4-2 solution for different deposition times (a) 10min..4V vs Ag/AgCl [Reprinted with permission from Ref.. [Reprinted with permission from Ref. (d) 1. 2007. A: Physiochem.1MKCl [Reprinted with permission from Ref.5h [Reprinted with permission from Ref.0001 M citric acid (e) 0. Colloid Surf. 111.01 M citric acid (c) 0. 459.5M ZnCl2+0.5 and (d) pH 12. 261 Xu et al. 345. Copyright @ American Chemical Society 2007] Figure 19: SEM images of hierarchical ZnO nanostructures electrodeposited at a potential of 1.5 (b) pH 10.5h. Phys. Matter Lett. 239 Chen et al. [Reprinted with permission from Ref. 2007 Copyright @ Elsevier 2008] Figure 16: SEM images of the Eelctrochemically obtained zinc oxide films prepared at the cathode potential ranging from -0.5M ZnCl2+0. 238 Alvarado-Ibarra et al. C. 262 Jung et al. 2009. Chem. Appl. Phys.02 M citric acid (b) 0.01 M citric acid+0. 1996. 2008 Copyright @ Elsevier 2008] Figure 22: SEM images of as-made ZnO powders prepared by sonochemical method using the mixture of different zincs salt and NaOH as precursor at pH 12. .5 (a)Zn(NO3)2 (b)ZnCl2 103 . L18. Growth Design. 4603. 2008 Copyright @ American Chemical society 2008] Figure 21: SEM images of the ZnO samples prepared by sonochemical method using the mixture of Zn(NO3)2 and NaOH as precursor at different pH value:(a) pH 9. 274 Umar et al. 1157 . 4459. 2006 Copyright @ Japanese J. [Reprinted with permission from Ref. Surf. 479. Appl... 735. 134. 055506. J. 45.[Reprinted with permission from Ref. 2006 Copyright @ Elsevier 2006] Figure 29: (a) Emission spectra from N doped zinc oxide nanoneedle under different pumping powers (b) Output power vs pump energy curve [Reprinted with permission from Ref. J... 2006] Figure 27 : X-ray diffraction patterns of vacuum arc deposited zinc oxide thin films at (a) low (b) high magnetic fields [Reprinted with permission from Ref. Alloy and Comp. 2005 Copyright @ Elsevier 2005] Figure 24: FESEM images of zinc oxide nano columns grown by electron beam evaporation at 400°C on the Si(100) substrate for (a) 30min. 41.. Phys. 601. λB and 104 . Phys. [Reprinted with permission from Ref.1mol/lit. 266 Xiao et al. 294 Youn et al.. 289 Qiu et al. 319.. 74 . 2007 Copyright @ Elsevier 2007] Figure 30: (a) SEM image of zinc oxide vertical nanowire cavities grown on sapphire substrate (b) SEM iamge of a single verical NW with Fabry-Perot lasing modes as wavelengths λA. and (b) 50 min. 2000 Copyright @ Elsevier 2006] Figure 28: SEM images of Spray Pyrolysis deposited zinc oxide nanostructures on ITO/glass substrate using 0. J. Nanotech. Thin Solid Films 377. 273 Umar et al. Tanemura et al. 314 Krunks et al. 2005 Copyright @ Elsevier 2005] Figure 25: SEM images of magnetron sputtering derived [A] as synthesized zinc oxide thin film and [B] zinc oxide nanotetrapods grown on the surface of film after annealing . 2005 Copyright @ Institute of Physics 2005] Figure 23(B): FESEM images of multipod star shaped zinc oxide nanostructures grown by cyclic feeding chemical vapor deposited on Au coated Si (100) substrate [Reprinted with permission from Ref.(c)ZnSO4 and (d) Zn(C2H4O2)2 . L18. solution of zinc chloride at different (a) 400°C (b) 450°C (c) 490°C (d) 540°C and (e) 560°C substrate temperatures [Reprinted with permission from Ref. 2008 Copyright @ Institute of Physics 2008] Figure 26: SEM images of zinc oxide nanostructures grown by RF sputtering for (a) 15 min. 296 Takikawa et al.b) 100 and (c. D: Appl. Phys. Cryst... 2008 Copyright @ Elsevier 2008] Figure 23(A): FESEM images of cyclic feeding chemical vapor deposited zinc oxide flower shaped zinc oxide nanostructures on Si (a. Sci.d) 111 substrates. 8957. Phys. and (b) 50 min. J. 459. 2462. [Reprinted with permission from Ref. 16. Growth 277. [Reprinted with permission from Ref. Appl. Jap.. Solid State Comm. 291 Saw et al.. Thin Solid Films 515. (b) packaged LED (c) emission of white light and (d) EL spectra at 27 and 37V.1. 2009 Copyright @ Institute of Physics 2009] Figure 33: Cross sectional SEM image of a ZnO/polymer multi layer LED fabricated on glass substrate. Wang et al. Copyright @ American Chemical Society (2007)] Figure 38: (a) Typical transmission electron micrographs of Cu2O nanothreads embodying beads. Inorganic Chemistry 46. Chem. 3512. Soc. 2008 Copyright @ American Institute of Physics 2008] Figure 32: Optical photographs of the n-zinc oxide nanorods/p-SiC: (a) before measurements. 10°C). 0. Jongh et al.is 25 mM). Willander et al. 9537. [Reprinted with permission from Ref.. 414. 341 S. (b) Enlarged of (a). Gargas et al. 53. 55°C). [Reprinted with permission from Ref. 332001. 0.05 mA/cm2. 92.. 332001. E =-1:69V/SSE. 56.. (c) Representative TEM micrographs of the dense Cu2O 105 . Willander et al. Journal of Crystal Growth 282. 337 A. 20. Chem. 332001.. (b) Figure showing coalesced beads forming nanothreads. 2007. (b) Cu2O deposited at pH 9 (0.. Nanotech. 2125. the concentration of Cu(OH)42. Willander et al. 1999. Daltin et al. 20. [Reprinted with permission from Ref. [Reprinted with permission from Ref. 2005. Nanotech. [Reprinted with permission from Ref. Right inset: Dark field scattering images of ZnO vertical Nanowire cavity from white light excitation (top) and lasing induced by 266 nm pulsed excitation (d) PL imaging of a single zinc oxide vertical NW cavity (inset SEM image of ZnO) (e) Diagram of ZnO vertical nanowire cavity with corresponding PL images [Reprinted with permission from Ref. 11. Copyright @ American Chemical Society (1999)] Figure 36: (a) Scanning electron micrograph of electrodeposited Cu2O nanowires.05 mA/cm2. J..1 C/cm2.λc (c) lasing spectra of a single ZnO nanowire cavity (Left inset: Power dependence graph showing lasing threshold almost at 400 µJ/cm2. 0. 131. as collected at the bottom of the cell after electrolysis at 2 V for 1 h.. Mater. 55°C). Copyright @ Elsevier (2005)] Figure 37: Typical FE-SEM images of octahedral Cu2O located at the ITO substrate at different magnifications (the deposition time is 15 min.. [Reprinted with permission from Ref.. Phys. L. Guo et al. Bath temperature = 70 0C. 56. Lett. 56. 2009 Copyright @ American Chemical Society 2009] Figure 31: (Top) Schematic view of zinc oxide based hetrojunction p-n LED (Bottom) Optical microscope plan view of a zinc oxide based hetrojunction LED.10mA [Reprinted with permission from Ref. 20. pH =9.13 C/cm2. 333 P.2 mA/cm2. Nanotech. 322. 2009 Copyright @ Institute of Physics 2009] Figure 35: (a) Cu2O deposited at pH 9 (0.1 /cm2. 2009 Copyright @ Institute of Physics 2009] Figure 34: (Top) Current density-voltage characteristic of (a) NPD-PFO structure and (b) PVKTFB structure (inset shows a schematic diagram of the corresponding energy band energy band diagram) and (Bottom) Coressponding EL spectra at 14V and 0. 112101.. Appl. Am. E. (c) Cu2O deposited at pH 11 (0. b: TEM micrograph (scale bar~100 nm) and histogram of the size distribution from sample B3. e) or 2 microns (d. 0. Copyright @ American Chemical Society (2003)] Figure 42: TEM images of cuprous oxide nanoparticles synthesized by using ascorbic acid as the reductant. 2008. Gao et al. Copyright @ American Chemical Society (2008)] Figure 41: TEM images of Cu2O nanoparticles prepared with different concentrations of CTAB. 2004. Gou et al. c: TEM micrograph (scale bar ~ 100 nm) and histogram of the size distribution from sample B4.5 mL of formic acid at 180°C (1.010 M Cu2+ solution (water at 22. C 111. and (e and f) prepared with 30 mL of 0. Copyright @ American Chemical Society (2003)] Figure 43: a: TEM micrograph (scale bar~100 nm) and histogram of the size distribution from sample B2.5 mL of formic acid at 180°C (1.5 mL of formic acid. 0. [Reprinted with permission from Ref.08.015MCu2+ solution (water at 5 vol %) and 4. 342 D. 273.. 3. C 111. 2003. type (iv)) (water at 30 vol %) and 4. SEM (c) and TEM (d) images of thin-shell hollow spheres of Cu2O. Gou et al. 1638. [Reprinted with permission from Ref. and 10 V.5 h). b. Singh et al. (b) 100 nm. Chem.5 vol %) and 4. Phys.5 mL of formic acid at 180°C (2 h). Chang et al. 2007. 0. 3. Copyright @ American Chemical Society (2004)] 106 .10 M. Mater. 1638. 735. Chem.. [Reprinted with permission from Ref. type (i)) (water at 5 vol %) and 4. 1903..010 M Cu2+ solution (water at 22. 347 L.02. J. (q. 347 L. [Reprinted with permission from Ref. 0.050 M Cu2+ solution and (n.5 mL of formic acid at 180°C (1. CCTAB equals 0.5 mL of formic acid at 185°C (2 h). The inset in (a) shows a broken hollow sphere. Singh et al.5 mL of formic acid at 180°C (2 h). 4.010 M Cu2+ solution (water at 15 vol %) and 1. (n-q) Higher-ordered multipod frameworks and crystal assemblies: prepared with 30 mL of 0. Chem. obtained after electrolysis at 6 and 10 V respectively for 1 h. (p... and (l and m) prepared with 30 mL of 0. 4. 6263. Gou et al. 342 D..5 h). (g-i) Type (iii) crystal assemblies prepared at 150°C (5 h) with 30 mL of 0. Copyright @ American Chemical Society (2007)] Figure 39: (a-d) SEM micrographs of the copper electrode after electrolysis at different voltage (2.5 h). Crystal Growth & Design. P. Insets indicate the cuboctahedral cages in type (ii) structures.5 mL of formic acid at 180°C (2 h). Nano Lett. The inset in (b) shows the ED pattern corresponding to a single hollow sphere. c. [Reprinted with permission from Ref. 8. J.5 vol %) and 1. 352 Y. type (ii)) (water at 5 vol %) and 4. [Reprinted with permission from Ref. f). SEM images were taken with increasing magnifications. 349 L.. and 0.015 M Cu2+ solution (water at 21 vol %) and 1. Nano Lett. From a-f. Scale bars are 500 nm (a.04.5 mL of formic acid at 180°C (2 h). 14. type (iii)) (water at 22 vol %) and 1. Type (iv) multipod frameworks and crystal assemblies: (j and k) prepared with 30 mL of 0. Copyright @ RSC publishing (2004)] Figure 44: Type (ii) multipod frameworks and crystal assemblies: (a-d) prepared with 30 mL of 0.network of nanowires. 20. 2003.06. 2004. 2007. respectively). (c) The magnified TEM micrograph of the nanowires. [343] J. P. respectively. Phys. 1903. Chem. [Reprinted with permission from Ref. Mater.01. Scale bar is (a) 500 nm. J. Copyright @ American Chemical Society (2007)] Figure 40: SEM (a) and TEM (b) images of thick-shell Cu2O hollow spheres.. (o. C..333 molâL-1 NaOH. [Reprinted with permission from Ref. Copyright @ American Chemical Society (2006)] Figure 49: Typical FE-SEM images of the products prepared with 0. 20801. 2007... and 0. starting solution.635 molâL-1. (c) hexagonal nanoplates. or (b) cone-shaped bundles.010 M. B 110. D is the SAED pattern of the sphere shown in C). Zhang et al. Haolan et al. Copyright @ American Chemical Society (2006)] Figure 47: SEM images of octahedral Cu2O prepared when R2 ) 8: (a) the octahedra with longer edge length and (b) an octahedron with arched <111> surfaces and (c) its corresponding TEM image. 0. 2005.02 molâL-1 CuSO4.02 molâL-1 EDTA. (a) 0. 90. (c) insert of the organization of nanowires. 2006. 362 H. Copyright @ American Chemical Society (2007)] Figure 50: FE-SEM images of samples prepared with 0. Orel et al. Copyright @ American Chemical Society (2007)] 107 . 353 Y. (e) triangular nanoplates.02 molâL-1 EDTA. 30 mL. (a) Room temperature. 363 Z. 2007. Crystal Growth & Design 7.02 molâL-1 CuSO4. 0..028 molâL-1 C6H12O6 at 60 °C for 12 h (w ) 34). (b) 80 °C.is 1:7:2): (a. 13829. [Reprinted with permission from Ref. [Reprinted with permission from Ref. (a) Cu nanoparticles. Langmuir 21.02 molâL-1 EDTA. 13829. (d) truncated nanoprisms. (a) Low magnification image and (b) high magnification. NH3 to OH. 150. 453. Copyright @ American Chemical Society (2007)] Figure 52: SEM micrographs of Cu2O nanowires organized as (a) spherical structures. [Reprinted with permission from Ref. Journal of Physical Chemistry B 110. Copyright @ American Chemical Society (2007)] Figure 51: FE-SEM images of samples prepared with 0. Haolan et al. [Cu2+] ) 0.. [Reprinted with permission from Ref. Phys. 820. 359 X. 35 h. (d) conical structures of disassembled nanowires. 0. Zhang et al. Journal of Physical Chemistry B 110. 359 X. 820. 50 h. H. 362 H. and NaOH with various concentrations at 60 °C for 12 h (w ) 34). 2007. 2006. 360 C. [Reprinted with permission from Ref. 0.. Chem. [Reprinted with permission from Ref. 30. and 210 min and after an aging period of 3 days. Copyright @ American Chemical Society (2006)] Figure 48: TEM images of the sample after various times of oxidation: 0. b) SEM image of the Cu2O octahedra and (c) TEM image of a single octahedron. 0. and 0. C. 0. 820. Zhang et al. Crystal Growth & Design 7. Crystal Growth & Design 7. (b) faceted Cu2O nanocrystals. Bernard Ng et al. 2007. 362 H. and (f) octahedral nanocrystals.225 molâL-1 and (b) 0. Copyright @ American Chemical Society (2005)] Figure 46: SEM and TEM images of sample 1 (molar ratio of Cu2+.028 molâL-1 C6H12O6. 2006. J.028 molâL-1 C6H12O6 for 12 h at different reaction temperatures (w ) 34).333 molâL-1 NaOH.02 molâL-1 CuSO4. [Reprinted with permission from Ref. Chang et al.. Crystal Growth & Design 7. 1074.Figure 45: The core hollowing process in Cu2O nanospheres: TEM images of the samples prepared after different reactions times at 150°C (A and B. Int. Y. and brookite (c). 2008. Nanotechnology 19. the level bar on each column indicates the weighted average size of each sample. (c) High-magnification HR-TEM image of a Cu2O nanocube exhibited in (b). 28.. Copyright @ RSC Publishing (2006)] Figure 54: (a) TEM image of filled Cu2O nanocubes. (c) 30. [Reprinted with permission from Ref. Nanotechnology 19. (b) 15. (c) 0. 025604. Hyd. 368 H. 633. 2009..05 M.24 g. (b) 0. (C) and (D) octahedral Cu2O particles. respectively. Inset images of (B). Journal of Nanoscience and Nanotechnology.Figure 53: FESEM and TEM images of Cu2O particles with different shape. (A) and (B) cubiclike Cu2O particles. Copyright @ Institute of Physics (2008)] Figure 56: TEM images of Cu2O particles obtained with different PVP amount: (a) 0.. 1089 (2003). Copyright @ American Chemical Society (2009)] Figure 58: SEM images of Cu2O products prepared with different concentrations of copper nitrate: (a) 0. 375 H. Dubey et al. [Reprinted with permission from Ref. (c) size distributions of the Cu2O microcrystal versus copper nitrate concentration..189 P. Copyright @ Elsevier (2003)] Figure 61 (a) Scanning Electron Microscope Image of the TiO2 Nanotubes top view (b) Lateral View (c) Transmission Electron Microscope Image of the single TiO2 Nanotube and (d) top view of TiO2 Nanotubes. Zhao et al. et al. 10. et al. 366 Cao Hongliang et al. Mishra et al. [Reprinted with permission from Ref. K. 633. Figure 60 Transmission Electron Microscope image of ns-TiO2 [Reprinted with permission from Ref 173 P. anatase (b). (E) and (F) sphere-like Cu2O particles. Copyright @ American Chemical Society (2009)] Figure 57: (a) SEM image of Cu2O particles when the pH value of NaOH solution is 11 and (b) SEM and (c) TEM images and SAED of Cu2O particles when the pH value of NaOH solution is 12. Crystal Growth & Design 9. Zhu. Crystal Growth & Design 8. R. 367 Z Yang et al. 2009. [Reprinted with permission from Ref. [Reprinted with permission from Ref. (b) HR-TEM image of a representative filled Cu2O nanocube. Chem Commun. 4548.10 g. 2006. 2008. J. 9. and (d) 45 min. The inset in (c) is the FFT pattern of a Cu2O nanocube. respectively. 367 Z Yang et al. 5507 (2009) Copyright @ American Scientific Publisher (2009)] 108 . [Reprinted with permission from Ref. (D) and (F) are SAED patterns recorded from a single particle of a different shape. [Reprinted with permission from Ref.30 g. Crystal Growth & Design 9.. Zhu. 025604.025 M. 2008. Copyright @ American Chemical Society (2008)] Figure 59. Crystal structures of rutile (a). (b) 0. Copyright @ Institute of Physics (2008)] Figure 55: TEM images of hollow Cu2O nanocubes prepared at reactions times of (a) 1. Eng. 368 H. R. 31. 44. Zhang et al. Tokyo. BKC. 545 (2008). [Reprinted with permission from Ref. (The active photocatalytic principle) Figure 68: (a) Ordinary mirror (b)TiO2 nanoparticle coated anti-fogging mirror Reprinted with permission from K. Japanese Journal of Applied Physics... 545 (2008).210 P. 8269 (2005).. Italy.210 P. (The excitation wavelength is ~ 269 nm). Mater. Bull.Copyright @ ELSEVIER (2003)] Figure 67: Dives in Misericordia Church. [Reprinted with permission from A.338 (2001). Functionality of Molecular Systems. [Copyright @ Nature Publishing Group (2001)] Figure 64 Schematic diagram of the fabricated photocatalytic reactor. Sci. Mishra et al. Copyright @ BKC (1999)] 109 . 2. 1999. [Reprinted with permission from Ref. 414.. NATURE. 196 (1999). Separation and Purification Technology. Mishra et al. Degussa) photocatalyst.. Copyright @ Indian Academy of Sciences (2008)] Figure 65 Variation of concentration of phenol with irradiation time using as synthesized nanostructured TiO2 and commercial TiO2 (P-25. Sci. 31. Bull. Copyright @ Indian Academy of Sciences (2008)] Figure 66 : Tubular photocatalytic reactor for water purification [Reprinted with permission from Ref 208 L.© 2005 The Japan Society of Applied Physics. photograph of nude mouse just after initial treatment (A) and 4 weeks after treatment (B) [Reprinted with permission from A. 31. Figure 69: Animal test of photocatalytic cancer therapy.Copyright @ Springer 1999] Figure 63 : Schematics of Dye Sensitized Solar Cell Reprinted with permission from Michael Grätzel.. Rome.Figure 62 : Hydrogen generation through water photolysis using solar energy and Titanium Dioxide Photoelectrode.. R. Mater. 105 (2003). Fujishima. Hashimoto et al. Fujishima et al. (b) (a) (c) Figu re 1: Crys tal struc ture of zinc oxid e (a) Wurt zite (b) zinc blen de (c) rock salt 110 . Chem.. Matter. Copyright @ Elsevier (2009)] 111 . Phys.Figure.2: SEM images of zinc oxide nanostructures synthesized by precipitation method at (a) 60° (b) 70° and (c) 80° C temperatures [Reprinted with permission from Ref. 172. 208 Guzman et al. 115. 2009. Nanotech. 671. 2005. 16.[Reprinted with permission from Ref. 209 Xiao et al.Figure 3: SEM micrographs of mesoporous crystalline zinc oxide nanowires (a) in the PPA template and (b) released from PPA template. Copyright @ Institute of Physics (2009)] 112 . 2660.Figure 4: SEM images of zinc oxide nanostructures produced with precipitation method with (a) the control sample having large prisms and small needles (b) sample precipitated with 120 mg/L of EO68-b-MAA8-C12. Chem.. 212 Taubert et al. [Reprinted with permission from Ref. J. Phys. 2003 Copyright @ American Chemical Society (2003)] 113 . and (d) the schematic view for dumbbell shape. 107. B. 42. 2003.Figure 5: SEM images of hydrothermally synthesized zinc oxide nanomaterials (A) Nanowires [B] Nanorods [Reprinted with permission from Ref. Inorganic Chem. 215 Li et al. 8105. Copyright @ American Chemical Society] (a) (b) (e) (f) (c) (d) (g) (h) 114 . Copyright @ Elsevier ] (a) (b) (c) (d) (e) (f) (g) 115 .2M NH4OH + 1M diethanolamine (DEA) (f) 0. J.and (d) tri-ethanolamine (e) 0. Alloy and Compounds.4M NH4OH + 1M DEA (g) 0.. 477.2M NH4OH + 1M DEA [Reprinted with permission from Ref. 219 Lu et al. 523.6M NH4OH + 1M DEA (h) 0. 2009.Figure 6: SEM images of hydrothermally synthesized zinc oxide powders using 1M aqueous solution of (a) NH4OH (b) mono (c) di. 93. 30. 219 Xu et al. [Reprinted with permission from Ref.0 M solution of KOH and (e) 0.0 (d) 2.H2O as solvent.25 (b) 0..20 M solution of NH3. Copyright @ Elsevier ] 116 . Ceramic International.Figure 7: SEM images of hydrothermally prepared zinc oxide nanopowders at 200°C for 2h using (a) 0. 2004.5 (c) 1.05 (g) 0.025 (f) 0. .Figure 8: SEM images of solvothermally produced zinc oxide nanostructures with different water/EN volume (ml) (a) 60/0 (b) 30/30 (c) 20/40 (d) 50/10 (e) cross sectional view of 50/10 and (e) 0/60 [Reprinted with permission from Ref. 2006.. 1533. 225 Dev et al. Nanotech. 17. Copyright @ Institute of Physics ] 117 . 226 Lu et al. 2008 Copyright @ Willey-VCH Verlag GmbH 2008 ] 118 . 1047..Figure 9: FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature for 12 hours with 1:7 v/v ratio of distilled water and EDA [Reprinted with permission from Ref. 18. Func Mater. Adv. 1047. 226 Lu et al.. 18. 2008 Copyright @ Willey-VCH Verlag GmbH 2008 ] 119 .Figure 10 : FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature and 1:7 v/v ratio of distilled water and EDA for different reaction times (A) 1h (B) 2h (C) 4h (D&E) 8h and (F) 16h [Reprinted with permission from Ref. Func Mater. Adv. Figure 11: SEM images of 3D zinc oxide hollow micro-sphere synthesized by the solvothermal method in the EG solution at 200 °C temperature.. general view in the left. 18. at high magnification at right Fig. 2007 Copyright @ Institute of Physics 2007 ] (a) (b) (c) (d) 120 . 227 Zhang et al. 2. Nanotech.10 A: FESEM images of hierarchical zinc oxide micro/nanoarchitectures produced solvo thermally at 160° temperature for 12 hours with 1:7 v/v ratio of distilled water and EDA [Reprinted with permission from Ref. 455604. Figure 12: SEM images of Sol-gel derived zinc oxide nanostructures on the silicon substrates (ac) from neutral solutions (a) as-synthesized, and thermal treatment at 500°C for (b) 4h and (c) 6h and (d) as obtained from acidic solution . [Reprinted with permission from Ref. 230 Li et al., J. Cryst. Growth 310, 599 (2008). Copyright @ Elsevier 2008 ] 121 Figure 13: SEM images of one dimensional micro-emulsion derived zinc oxide nanostructures obtained after different reaction times (a) 10 min, (b) 30 min, (c) 1h (d) 2h (e) 4h and (f) 8 h. [Reprinted with permission from Ref. 232 Zhang et al., Cryst. Growth Design 4, 309,2004. Copyright @ American Chemical Society 2004 ] 122 Figure 14: SEM images of zinc oxide nanostructures obtained by combustion synthesis method using (a) calcination of 0.2g Zn(NO3).6H2O for 2h at 800°C (b) By combustion and (c) by solution combustion (d) solution combustion with 1.0 ml of additional water. [Reprinted with permission from Ref. 238 Alvarado-Ibarra et al., Colloid Surf. A: Physiochem. Eng. Aspects, 345, 135, 2009. Copyright @ Elsevier 2008] 123 61.Figure 15: SEM image of melting combustion synthesized zinc oxide nanostructure. Matter Lett.. 239 Chen et al. 2007 Copyright @ Elsevier 2008] 124 . [Reprinted with permission from Ref. 4603. Appl. 243 Izaki and Omi. Phys. 2439. Copyright @ American Institute of Physics 1996] (a) (b) (c) Figu re 17: SEM imag es of elect roch emic ally synt hesiz ed ZnO nano struc (d) (e) (f) 125 .Figure 16: SEM images of the Eelctrochemically obtained zinc oxide films prepared at the cathode potential ranging from -0. 1996.7 to -1.4V vs Ag/AgCl [Reprinted with permission from Ref. Lett. 68.. 01M citric acid (f) 0.25M ZnCl2+0.5M ZnCl2+0. 111.01 M citric acid+0.5M ZnCl2+0.. 2007.01 M citric acid (c) 0.0001 M citric acid (e) 0. J. 260 Li et al. Copyright @ American Chemical Society 2007] 126 .05 M citric acid (d) 0.5M ZnCl2+0. C. Phys. Chem.1MKCl [Reprinted with permission from Ref. 6678.02 M citric acid (b) 0.25 M +0.5M ZnCl2+0.tures in the electrolytic solution of (a) 0. 2007. Phys. 111. 261 Xu et al. Phys. 111. J. Chem. (b) 20min. 261 Xu et al. (f)3. (e) 2. J.. [Reprinted with permission from Ref. 11560.05M [Zn(NH3)4-2 solution for different deposition times (a) 10min.5h [Reprinted with permission from Ref.5h. Copyright @ American Chemical Society 2007] Figure 19: SEM images of hierarchical ZnO nanostructures electrodeposited at a potential of -1..10 V in 0.Figure 18: SEM images of (a) primary ZnO nanosheets and (b-d) hierarchical ZnO nanorods on hexagonal nanosheets on ITO substrates. (d) 1. C. 2007. (c) 40min. Copyright @ American Chemical Society 2007] 127 . C. Chem. 11560.5h. Growth Design.(a) (b) (c) (d) (e) Figure 20: SEM images of sonochemically synthesized zinc oxide (a) nanorods (b) nanoups (c) nanosheets (d) nanoflowers and (e) nanospheres [Reprinted with permission from Ref. 8. 262 Jung et al. 265. Cryst.. 2008 Copyright @ American Chemical society 2008] 128 . . J. 266 Xiao et al. 459. Alloy and Comp.5 (b) pH 10. 2008 Copyright @ Elsevier 2008] 129 .5(c) pH 11.5. L18.Fig. [Reprinted with permission from Ref.5 and (d) pH 12. 21: SEM images of the ZnO samples prepared by sonochemical method using the mixture of Zn(NO3)2 and NaOH as precursor at different pH value:(a) pH 9. 459. 2008 Copyright @ Elsevier 2008] 130 .. Alloy and Comp. 266 Xiao et al.5 (a)Zn(NO3)2 (b)ZnCl2 (c)ZnSO4 and (d) Zn(C2H4O2)2 . L18.[Reprinted with permission from Ref.Figure 22: SEM images of as-made ZnO powders prepared by sonochemical method using the mixture of different zincs salt and NaOH as precursor at pH 12. J. b) 100 and (c.. [Reprinted with permission from Ref.(a) (b) (c) (d) Figure 23[A]: FESEM images of cyclic feeding chemical vapor deposited zinc oxide flower shaped zinc oxide nanostructures on Si (a. Nanotech. 273 Umar et al. 2005 Copyright @ Institute of Physics 2005] 131 . 2462.d) 111 substrates. 16. Growth 277. Cryst. 274 Umar et al. 2005 Copyright @ Elsevier 2005] 132 .. 479. J.Figure 23[B]: FESEM images of multipod star shaped zinc oxide nanostructures grown by cyclic feeding chemical vapor deposited on Au coated Si (100) substrate [Reprinted with permission from Ref. and (b) 50 min.. Solid State Comm. 735. 2005 Copyright @ Elsevier 2005] 133 . [Reprinted with permission from Ref.Figure 24: FESEM images of zinc oxide nano columns grown by electron beam evaporation at 400°C on the Si(100) substrate for (a) 30min. 134. 289 Qiu et al. Phys. [Reprinted with permission from Ref. Phys.Figure 25: SEM images of magnetron sputtering derived [A] as synthesized zinc oxide thin film and [B] zinc oxide nanotetrapods grown on the surface of film after annealing . D: Appl. J. 291 Saw et al. 055506. 41. 2008 Copyright @ Institute of Physics 2008] 134 .. Figure 26: SEM images of zinc oxide nanostructures grown by RF sputtering for (a) 15 min. and (b) 50 min. [Reprinted with permission from Ref. 294 Youn et al., Jap. J. Appl. Phys. 45, 8957, 2006 Copyright @ Japanese J. Appl. Phys., 2006] 135 Figure 27 : X-ray diffraction patterns of vacuum arc deposited zinc oxide thin films at (a) low (b) high magnetic fields [Reprinted with permission from Ref. 296 Takikawa et al., Thin Solid Films 377, 74 , 2000 Copyright @ Elsevier 2006] 136 Figure 28: SEM images of Spray Pyrolysis deposited zinc oxide nanostructures on ITO/glass substrate using 0.1mol/lit. solution of zinc chloride at different (a) 400°C (b) 450°C (c) 490°C (d) 540°C and (e) 560°C substrate temperatures [Reprinted with permission from Ref. 314 Krunks et al., Thin Solid Films 515, 1157 , 2006 Copyright @ Elsevier 2006] 137 4459. 319. Tanemura et al.Figure 29: (a) Emission spectra from N doped zinc oxide nanoneedle under different pumping powers (b) Output power vs pump energy curve [Reprinted with permission from Ref. 601.. Sci.. 2007 Copyright @ Elsevier 2007] (a) (b) D (d) E (c) (e) 138 . Surf. Gargas et al.Figure 30: (a) SEM image of zinc oxide vertical nanowire cavities grown on sapphire substrate (b) SEM iamge of a single verical NW with Fabry-Perot lasing modes as wavelengths λA. Right inset: Dark field scattering images of ZnO vertical Nanowire cavity from white light excitation (top) and lasing induced by 266 nm pulsed excitation (d) PL imaging of a single zinc oxide vertical NW cavity (inset SEM image of ZnO) (E) Diagram of ZnO vertical nanowire cavity with corresponding PL images [Reprinted with permission from Ref. 322. Soc. 2009 Copyright @ American Chemical Society 2009] 139 .. Am. J. 131. λB and λc (c) lasing spectra of a single ZnO nanowire cavity (Left inset: Power dependence graph showing lasing threshold almost at 400 µJ/cm2. 2125.. Chem. 92. 53. Wang et al.Figure 31: (Top) Schematic view of zinc oxide based hetrojunction p-n LED (Bottom) Optical microscope plan view of a zinc oxide based hetrojunction LED. Lett. 2008 Copyright @ American Institute of Physics 2008] . Appl. [Reprinted with permission from Ref. (a) (b) (c) (d) 140 . 112101. Phys... . Nanotech. (b) packaged LED (c) emission of white light and (d) EL spectra at 27 and 37V. 56. Willander et al.Figure 32: Optical photographs of the n-zinc oxide nanorods/p-SiC: (a) before measurements. 20. 2009 Copyright @ Institute of Physics 2009] 141 . [Reprinted with permission from Ref. 332001. Figure 33: Cross sectional SEM image of a ZnO/polymer multi layer LED fabricated on glass substrate. [Reprinted with permission from Ref. 332001. Willander et al.. 2009 Copyright @ Institute of Physics 2009] (a) (b) 142 . 20. 56. Nanotech. .10mA [Reprinted with permission from Ref. 56. 55°C).1 /cm2.05 mA/cm2.1 C/cm2.Figure 34: (Top) Current density-voltage characteristic of (a) NPD-PFO structure and (b) PVKTFB structure (inset shows a schematic diagram of the corresponding energy band energy band diagram) and (Bottom) Coressponding EL spectra at 14V and 0. 3512. 0.2 mA/cm2. d. 332001. E. 333 P. Chem. 2009 Copyright @ Institute of Physics 2009] Figure 35: (a) Cu2O deposited at pH 9 (0.. 1999. (c) Cu2O deposited at pH 11 (0.05 mA/cm2. 20. [Reprinted with permission from Ref. Jongh et al. 10°C). 11. Copyright @ American Chemical Society (1999)] . 55°C). 143 Mater.13 C/cm2. Willander et al. (b) Cu2O deposited at pH 9 (0. 0. Nanotech. 0. Inorganic Chemistry 46. [Reprinted with permission from Ref. 2005. 341 S.. 414. pH =9.1. Guo et al. 337 A. 2007. E =-1:69V/SSE. Copyright @ American Chemical Society (2007)] 144 . [Reprinted with permission from Ref. Daltin et al. (b) Enlarged of (a). Journal of Crystal Growth 282. Bath temperature = 70 0C. the concentration of Cu(OH)42. L. Copyright @ Elsevier (2005)] Figure 37: Typical FE-SEM images of octahedral Cu2O located at the ITO substrate at different magnifications (the deposition time is 15 min. 9537.(a) (b) Figure 36: (a) Scanning electron micrograph of electrodeposited Cu2O nanowires..is 25 mM). P. Chem. 2007.. Singh et al. 4.(c) (d) Figure 39: (a-d) SEM micrographs of the copper electrode after electrolysis at different voltage (2. and 10 V. [Reprinted with permission from Ref. Phys. respectively). 342 D. J. Copyright @ American Chemical Society (2007)] a b 145 . 1638. 8. C 111. 347 L. 0. Scale bars are 500 nm (a. 0. e) or 2 microns (d. c. 0.08.06. Nano Lett. 1903. 0. and 0. respectively.01. [Reprinted with permission from Ref. Copyright @ American Chemical Society (2003)] 146 .10 M. Gou et al. CCTAB equals 0.. 3.02. 2003.Figure 41: TEM images of Cu2O nanoparticles prepared with different concentrations of CTAB. From a-f. b. f)..04. 14. 735. Mater. [Reprinted with permission from Ref. 2004. J. Copyright @ RSC publishing (2004)] (g) (j) (k) (n) (o) (h) (p) (l) (m) (q) 147 (i) . c: TEM micrograph (scale bar ~ 100 nm) and histogram of the size distribution from sample B4. b: TEMmicrograph (scale bar~100 nm) and histogram of the size distribution from sample B3.(a) (b) (c) Figure 43: a: TEM micrograph (scale bar~100 nm) and histogramof the size distribution fromsample B2. Gou et al.. 349 L. Chem. D is the SAED pattern of the sphere shown in C). 2005. 35 h. 30 mL. Copyright @ American Chemical Society (2005)] 148 . 1074. [Cu2+] ) 0. 353 Y..010 M.Figure 45: The core hollowing process in Cu2O nanospheres: TEM images of the samples prepared after different reactions times at 150°C (A and B. 50 h. starting solution. Langmuir 21. [Reprinted with permission from Ref. C. Chang et al. is 1:7:2): (a. b) SEM image of the Cu2O octahedra and (c) TEM image of a single octahedron. Journal of Physical Chemistry B 110. 2006.Figure 46: SEM and TEM images of sample 1 (molar ratio of Cu2+. Haolan et al. NH3 to OH. [Reprinted with permission from Ref. 359 X. 13829. Copyright @ American Chemical Society (2006)] 149 .. 2006. 30.. Chem. (d) truncated nanoprisms. and (f) octahedral nanocrystals. H. (c) hexagonal nanoplates. Bernard Ng et al. B 110. 20801. and 210 min and after an aging period of 3 days. (b) faceted Cu2O nanocrystals. (a) Cu nanoparticles. Phys.Figure 48: TEM images of the sample after various times of oxidation: 0. (e) triangular nanoplates. [Reprinted with permission from Ref. J. 360 C. Copyright @ American Chemical Society (2006)] 150 . 150. 90. Zhang et al. 0. 820. 0. (a) Room temperature. Copyright @ American Chemical Society (2007)] Figure 50: FE-SEM images of samples prepared with 0. 820. Zhang et al. 2007.028 molâL-1 C6H12O6 at 60 °C for 12 h (w ) 34).Figure 49: Typical FE-SEM images of the products prepared with 0.333 molâL-1 NaOH. Crystal Growth & Design 7. (b) 80 °C. and 0. Crystal Growth & Design 7. 362 H.02 molâL-1 CuSO4.. 2007. 0.. 362 H. [Reprinted with permission from Ref.028 molâL-1 C6H12O6 for 12 h at different reaction temperatures (w ) 34). (a) Low magnification image and (b) high magnification. Copyright @ American Chemical Society (2007)] 151 . [Reprinted with permission from Ref. 0.02 molâL-1 EDTA.02 molâL-1 CuSO4.02 molâL1 EDTA. and 0.333 molâL-1 NaOH. (d) conical structures of disassembled nanowires. Orel et al. 363 Z. [Reprinted with permission from Ref. Crystal Growth & Design 7. (c) insert of the organization of nanowires. 2007. Copyright @ American Chemical Society (2007)] 152 .Figure 52: SEM micrographs of Cu2O nanowires organized as (a) spherical structures. C. or (b) cone-shaped bundles. 453. 4548. [Reprinted with permission from Ref. Copyright @ RSC Publishing (2006)] 153 . (D) and (F) are SAED patterns recorded from a single particle of a different shape.Figure 53: FESEM and TEM images of Cu2O particles with different shape. 2006. 366 Cao Hongliang et al. (A) and (B) cubic-like Cu2O particles. (C) and (D) octahedral Cu2O particles. Inset images of (B). Chem Commun.. respectively. (E) and (F) sphere-like Cu2O particles. The inset in (c) is the FFT pattern of a Cu2O nanocube. [Reprinted with permission from Ref.Figure 54: (a) TEM image of filled Cu2O nanocubes. 2008.. Copyright @ Institute of Physics (2008)] 154 . 025604. (b) HR-TEM image of a representative filled Cu2O nanocube. (c) High-magnification HR-TEM image of a Cu2O nanocube exhibited in (b). 367 Z Yang et al. Nanotechnology 19. (b) 0. 2009. 2009. 633. Zhu. Copyright @ American Chemical Society (2009)] 155 . Crystal Growth & Design 9. (c) 0.24 g. 368 H. [Reprinted with permission from Ref. [Reprinted with permission from Ref. 633.30 g. 368 H. et al.Figure 56: TEM images of Cu2O particles obtained with different PVP amount: (a) 0. Zhu. et al.10 g. Crystal Growth & Design 9. Copyright @ American Chemical Society (2009)] Figure 57: (a) SEM image of Cu2O particles when the pH value of NaOH solution is 11 and (b) SEM and (c) TEM images and SAED of Cu2O particles when the pH value of NaOH solution is 12. the level bar on each column indicates the weighted average size of each sample. 375 H.025 M. 2008.. 10. Copyright @ American Chemical Society (2008)] 156 . Zhao et al. Y. (c) size distributions of the Cu2O microcrystal versus copper nitrate concentration. (b) 0.Figure 58: SEM images of Cu2O products prepared with different concentrations of copper nitrate: (a) 0. [Reprinted with permission from Ref.05 M. Crystal Growth & Design 8. (a) (b) (c) Figure 59. 157 . anatase (b). Crystal structures of rutile (a). and brookite (c). Hyd. Eng. J.Figure 60 Transmission Electron Microscope image of ns-TiO2 [Reprinted with permission from Ref 173 P. 1089 (2003). Mishra et al. Int. R. 28. Copyright @ Elsevier (2003)] 158 .. 5507 (2009) Copyright @ American Scientific Publisher (2009)] 159 . Journal of Nanoscience and Nanotechnology. K.189 P. 9.(a) (b) Figu re 61 (a) Scan ning (c) (d) Elect ron Micr osco pe Imag e of the TiO2 Nan otub es top view (b) Lateral View (c) Transmission Electron Microscope Image of the single TiO2 Nanotube and (d) top view of TiO2 Nanotubes. Dubey et al. [Reprinted with permission from Ref. Functionality of Molecular Systems. [Reprinted with permission from A. Fujishima et al. 2.. 196 (1999).Figure 62 : Hydrogen generation through water photolysis using solar energy and Titanium Dioxide Photoelectrode.Copyright @ Springer 1999] 160 . [Copyright @ Nature Publishing Group (2001)] 161 . NATURE.338 (2001).Figure 63 : Schematics of Dye Sensitized Solar Cell Reprinted with permission from Michael Grätzel. 414. Figure 64 Schematic diagram of the fabricated photocatalytic reactor. Copyright @ Indian Academy of Sciences (2008)] 162 . Bull.. 545 (2008). 31. Mater.. R. Sci. Mishra et al.210 P. [Reprinted with permission from Ref. 31.. Bull. R.. [Reprinted with permission from Ref. Mater. Copyright @ Indian Academy of Sciences (2008)] 163 . Degussa) photocatalyst. 545 (2008). Mishra et al.Figure 65 Variation of concentration of phenol with irradiation time using as synthesized nanostructured TiO2 and commercial TiO2 (P-25. Sci. (The excitation wavelength is ~ 269 nm).210 P. Figure 66 : Tubular photocatalytic reactor for water purification [Reprinted with permission from Ref 208 L.. Zhang et al.Copyright @ ELSEVIER (2003)] 164 . Separation and Purification Technology. 105 (2003). 31. Rome.Figure 67: Dives in Misericordia Church. Italy. (The active photocatalytic principle) 165 . (a) (b) Figure 68: (a) Ordinary mirror (b)TiO2 nanoparticle coated anti-fogging mirror Reprinted with permission from K. Japanese Journal of Applied Physics.. 166 . 8269 (2005). Hashimoto et al. 44.© 2005 The Japan Society of Applied Physics. Fujishima. Copyright @ BKC (1999)] 167 . Tokyo. BKC. 1999.Figure 69: Animal test of photocatalytic cancer therapy. photograph of nude mouse just after initial treatment (A) and 4 weeks after treatment (B) [Reprinted with permission from A.