Titanium Dioxide Nano Materials Synthesis, Properties, Modifications, And

Chem. Rev. 2007, 107, 2891−2959

2891

Titanium Titan ium Dioxi Dioxide de Nanom Nanomateri aterials: als: Synth Synthesis, esis, Properties, Properties, Modifications, Modifications, and Applications Xiaobo Chen* and Samuel S. Mao† Lawrencee Berkele Lawrenc Berkeleyy Nationa Nationall Laborat Laboratory, ory, and Univers University ity of Califor California, nia, Berkeley, California 94720 Received March 27, 2006

Contents 1. Introduction 2. Synth Synthetic etic Methods Methods for TiO2 Nanostructures 2.1.. Sol−Gel Method 2.1 2.22. Mic 2. Miceellllee an andd In Inve vers rsee Mi Mice cellllee Me Meth thoods 2.3. Sol Method 2.4. Hydrothermal Method 2.5. Solvothermal Method 2.6. Direct Oxidation Method 2.7. Chemical Vapor Deposition 2.8. Physical Vapor Deposition 2.9. Electrodeposition 2.10. Sonochemical Method 2.11. Microwave Method 2.12. 2.1 2. TiO2 Me Meso sopo poro rous us/N /Nan anop opor orou ouss Ma Mate teririal alss 2.13. 2.1 3. TiO2 Aerogels 2.14. 2.1 4. TiO2 Opal and Photonic Materials 2.15.. Prepa 2.15 Preparatio rationn of TiO 2 Nanosheets 3. Prope Properties rties of of TiO2 Nanomaterials 3.1. Struct Structural ural Propertie Propertiess of TiO 2 Na Nano noma mate teria rials ls 3.2. Therm Thermodyna odynamic mic Properties Properties of TiO 2 Nanomaterials 3.3. X-ray Diffraction Diffraction Propertie Propertiess of TiO 2 Nanomaterials 3.4. Raman Vibratio Vibrationn Properties Properties of TiO 2 Nanomaterials 3.5. Electr Electronic onic Properties Properties of TiO2 Na Nano noma mate teririal alss 3.6. Optica Opticall Properties Properties of TiO2 Nan anom omat ater eria ials ls 3.7. Photo Photon-Ind n-Induced uced Electron Electron and Hole Properties Properties of TiO2 Nanomaterials 4. Modifi Modificatio cations ns of TiO2 Nanomaterials 4.11. Bu 4. Bulk lk Che hemi mica call Mo Modi dififica catition on:: Do Dopi ping ng 4.1.1.. Synth 4.1.1 Synthesis esis of Doped Doped TiO 2 Na Nano noma mate teria rials ls 4.1.2.. Prope 4.1.2 Properties rties of Doped Doped TiO 2 Nan Nanoma omater terial ialss 4.2. Surface Chemical Modifications 4.2.1. Inorganic Sensitization 5. Applic Application ationss of TiO2 Nanomaterials 5.1. Photocatalytic Applications 5.1.1.. Pure TiO2 Nano 5.1.1 Nanomateri materials: als: First Generation 5.1.2.. Metal 5.1.2 Metal-Dope -Dopedd TiO 2 Nanomaterials: Second Generation 5.1.3.. Nonme 5.1.3 Nonmetal-Do tal-Doped ped TiO 2 Nanomaterials: Third Generation * Corresponding Corresponding author. E-mail: [email protected]. † E-mail: [email protected].

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5.2. Photovoltaic Applications 5.2.1. 5.2 .1. The TiO TiO2 Nanocrystalline Electrode in DSSCs 5.2.2. Metal/Semiconductor Junction Schottky Schottky Diode Solar Cell 5.2.3.. Dope 5.2.3 Dopedd TiO2 Nanomaterials-Based Solar Cell 5.3. Photocatalytic Water Splitting 5.3.1.. Funda 5.3.1 Fundamenta mentals ls of Photocatalytic Photocatalytic Water Splitting 5.3. 5. 3.2. 2. Use Use of Re Reve versi rsibl blee Re Redo doxx Me Medi diat ator orss 5.3.3. 5.3 .3. Use of of TiO2 Nanotubes 5.3. 5. 3.4. 4. Wat Water er Sp Splilitt ttin ingg un unde derr Vi Visi sibl blee Li Ligh ghtt 5.3.5. Coupled/Composite Water-Splitting System 5.4. Electrochromic Devices 5.4.1.. Funda 5.4.1 Fundamenta mentals ls of Electr Electrochro ochromic mic Devic Devices es 5.4.2.. Electr 5.4.2 Electrochro ochromopho mophore re for an Electr Electrochro ochromic mic Device 5.4.3.. Coun 5.4.3 Counterele terelectrod ctrodee for an Electr Electrochro ochromic mic Device 5.4.4. Photoele lecctrochromic Devic icees 5.5. Hydrogen Storage 5.6. Sensing Applications 6. Summary 7. Acknowledgment 8. References

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1. Introduction Introduction Since its commercial production in the early twentieth century, titanium dioxide (TiO 2) has been widely used as a pigment1 and in sunscreens, 2,3 paints,4 ointments, toothpaste, 5 etc. In 1972, Fujishima and Honda discovered the phenomenon of photocatalytic splitting splitting of water on a TiO 2 electrode under ultraviolet (UV) light. 6-8 Since then, enormous efforts have been devoted to the research of TiO 2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors.9-12 These applications can be roughly divided into “energy” and “environmental” categories, many of which depend not only on the properties of the TiO 2 material itself but also on the modifications of the TiO 2 material host (e.g., with inorganic and organic dyes) and on the interactions of TiO2 materials with the environment. An exponential growth of research activities has been seen in nanoscience and nanotechnology in the past decades. 13-17 New physical and chemical properties emerge when the size of the material becomes smaller and smaller, and down to

10.1021/cr050 10.102 1/cr0500535 0535 C CCC: CC: $65.00 $65.00 © 2007 Ameri American can Chemical Chemical Society Published on Web 06/23/2007

2892 Chemical Reviews, 2007, Vol. 107, No. 7

Dr. Xiaobo Chen is a research engineer at The University University of California California at Berkeley Berkeley and a Lawrence Lawrence Berkeley Berkeley National National Laboratory Laboratory scientist. He obtained his Ph.D. Degree in Chemistry from Case Western Reserve University. His research interests include photocatalysis, photovoltaics, hydrogen storage, fuel cells, environmental pollution control, and the related materials materials and devices development. development.

Dr. Samuel S. Mao is a career staff scientist at Lawrence Berkeley National Laboratory and an adjunct faculty at The University of California at Berkeley. He obtained his Ph.D. degree in Engineering from The University of California at Berkeley in 2000. His current research involves the development of nanostructured materials and devices, as well as ultrafast laser technologies. Dr. Mao is the team leader of a high throughput materials processing program supported by the U.S. Department of Energy.

the nanometer scale. Properties also vary as the shapes of the shrinking nanomaterials change. Many excellent reviews and reports on the preparation and properties of nanomaterials have been published recently. 6-44 Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials. 13-16 The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases. 13,21 The high surface area brought about by small particle size is beneficial to many TiO2-bas -based ed devic devices, es, as it facil facilitat itates es reac reaction/ tion/inte interacti raction on between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. Thus, the performance of TiO2-based devices is largely influenced by the sizes of the TiO2 building units, apparently at the nanometer scale. As the most promising photocatalyst, 7,11,12,33 TiO2 materials are expected to play an important role in helping solve

Chen and Mao

many serious environmental and pollution challenges. TiO 2 also bears tremendous hope in helping ease the energy crisis throug thr oughh eff effect ective ive uti utiliz lizati ation on of sol solar ar ene energy rgy bas based ed on 9,31,32 photovoltaic and water-splitting devices. As continued breakthroughs breakth roughs have been made in the prepara preparation, tion, modification, and applications of TiO 2 nanomaterial nanomaterialss in recent years, especially after a series of great reviews of the subject in the 1990s.7,8,10-12,33,45 we believe that a new and comprehensive review of TiO 2 nanomaterials would further promote TiO2-based research and development efforts to tackle the environmental and energy challenges we are currently facing. Here, we focus on recent progress in the synthesis, properties, modifications, and applications of TiO 2 nanomaterial nanomaterials. s. The syntheses of TiO2 nanomaterials, including nanoparticles, nanorods, nanowires, and nanotubes are primarily categorized with the preparation method. The preparations of mesoporous/nanoporous TiO 2, TiO2 aerogels, opals, and photonic materials are summarized separately. In reviewing nanomaterial teri al synth synthesis esis,, we pres present ent a typi typical cal procedure and repre repre-sentative sent ative tran transmis smission sion or scann scanning ing elect electron ron micr microsco oscopy py images to give a direct impression of how these nanomaterials are obtained and how they normally appear. For detailed instructions on each synthesis, the readers are referred to the corresponding literature. The structural, thermal, electronic, and optical properties of TiO2 nanomaterials are reviewed in the second section. As the size, shape, and crystal structure of TiO 2 nanomaterials vary, not only does surface stability change but also the tra transi nsitio tions ns bet betwee weenn dif differ ferent ent pha phases ses of TiO 2 under pressure or heat become size dependent. The dependence of X-ray diffraction patterns and Raman vibrational spectra on the size of TiO 2 nanomaterials is also summarized, as they could help to determine the size to some extent, although correlation of the spectra with the size of TiO 2 nanomaterials is not straightforward. The review of modifications of TiO 2 nanomaterials is mainly limited to the research related to the modifications of the optical properties of TiO 2 nanomaterials, since many applications of TiO 2 nanomaterials are closely related to their optical properties. TiO 2 nanomaterials normally are transparent in the visible light region. By doping or sensitization, it is possible to improve the optical sensitivity and activity of TiO 2 nanomaterials in the visible light region. regi on. Envir Environmen onmental tal (pho (photocat tocatalysi alysiss and sens sensing) ing) and energy (photovoltaics, water splitting, photo-/electrochromics, photo-/electrochromics, and hydrogen storage) applications are reviewed with an emphasis on clean and sustainable energy, since the increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. The fundamentals and working principles of the TiO 2 nanomaterials-based devices are discussed to facilitate the understanding and further improvement of current and practical TiO2 nanotechnology.

2. Synthetic Methods for TiO 2 2 Nanostructures 2.1. Sol−Gel Method The sol-gel method is a versatile process used in making various ceramic materials. 46-50 In a typical sol -gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase. Thin films can be produced on a piece of


Dr. Xiaobo Chen is a research engineer at The University University of California California at Berkeley Berkeley and a Lawrence Lawrence Berkeley Berkeley National National Laboratory Laboratory scientist. He obtained his Ph.D. Degree in Chemistry from Case Western Reserve University. His research interests include photocatalysis, photovoltaics, hydrogen storage, fuel cells, environmental pollution control, and the related materials materials and devices development. development.

Dr. Samuel S. Mao is a career staff scientist at Lawrence Berkeley National Laboratory and an adjunct faculty at The University of California at Berkeley. He obtained his Ph.D. degree in Engineering from The University of California at Berkeley in 2000. His current research involves the development of nanostructured materials and devices, as well as ultrafast laser technologies. Dr. Mao is the team leader of a high throughput materials processing program supported by the U.S. Department of Energy.

the nanometer scale. Properties also vary as the shapes of the shrinking nanomaterials change. Many excellent reviews and reports on the preparation and properties of nanomaterials have been published recently. 6-44 Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials. 13-16 The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases. 13,21 The high surface area brought about by small particle size is beneficial to many TiO2-bas -based ed devic devices, es, as it facil facilitat itates es reac reaction/ tion/inte interacti raction on between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. Thus, the performance of TiO2-based devices is largely influenced by the sizes of the TiO2 building units, apparently at the nanometer scale. As the most promising photocatalyst, 7,11,12,33 TiO2 materials are expected to play an important role in helping solve

Chen and Mao

many serious environmental and pollution challenges. TiO 2 also bears tremendous hope in helping ease the energy crisis throug thr oughh eff effect ective ive uti utiliz lizati ation on of sol solar ar ene energy rgy bas based ed on 9,31,32 photovoltaic and water-splitting devices. As continued breakthroughs breakth roughs have been made in the prepara preparation, tion, modification, and applications of TiO 2 nanomaterial nanomaterialss in recent years, especially after a series of great reviews of the subject in the 1990s.7,8,10-12,33,45 we believe that a new and comprehensive review of TiO 2 nanomaterials would further promote TiO2-based research and development efforts to tackle the environmental and energy challenges we are currently facing. Here, we focus on recent progress in the synthesis, properties, modifications, and applications of TiO 2 nanomaterial nanomaterials. s. The syntheses of TiO2 nanomaterials, including nanoparticles, nanorods, nanowires, and nanotubes are primarily categorized with the preparation method. The preparations of mesoporous/nanoporous TiO 2, TiO2 aerogels, opals, and photonic materials are summarized separately. In reviewing nanomaterial teri al synth synthesis esis,, we pres present ent a typi typical cal procedure and repre repre-sentative sent ative tran transmis smission sion or scann scanning ing elect electron ron micr microsco oscopy py images to give a direct impression of how these nanomaterials are obtained and how they normally appear. For detailed instructions on each synthesis, the readers are referred to the corresponding literature. The structural, thermal, electronic, and optical properties of TiO2 nanomaterials are reviewed in the second section. As the size, shape, and crystal structure of TiO 2 nanomaterials vary, not only does surface stability change but also the tra transi nsitio tions ns bet betwee weenn dif differ ferent ent pha phases ses of TiO 2 under pressure or heat become size dependent. The dependence of X-ray diffraction patterns and Raman vibrational spectra on the size of TiO 2 nanomaterials is also summarized, as they could help to determine the size to some extent, although correlation of the spectra with the size of TiO 2 nanomaterials is not straightforward. The review of modifications of TiO 2 nanomaterials is mainly limited to the research related to the modifications of the optical properties of TiO 2 nanomaterials, since many applications of TiO 2 nanomaterials are closely related to their optical properties. TiO 2 nanomaterials normally are transparent in the visible light region. By doping or sensitization, it is possible to improve the optical sensitivity and activity of TiO 2 nanomaterials in the visible light region. regi on. Envir Environmen onmental tal (pho (photocat tocatalysi alysiss and sens sensing) ing) and energy (photovoltaics, water splitting, photo-/electrochromics, photo-/electrochromics, and hydrogen storage) applications are reviewed with an emphasis on clean and sustainable energy, since the increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. The fundamentals and working principles of the TiO 2 nanomaterials-based devices are discussed to facilitate the understanding and further improvement of current and practical TiO2 nanotechnology.

2. Synthetic Methods for TiO 2 2 Nanostructures 2.1. Sol−Gel Method The sol-gel method is a versatile process used in making various ceramic materials. 46-50 In a typical sol -gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase. Thin films can be produced on a piece of

Titanium Dioxide Nanomaterials

substrate by spin-coat substrate spin-coating ing or dip-coating. A wet gel will form when the sol is cast into a mold, and the wet gel is converted into a dense ceramic with further drying and heat treatment. A highly porous and extremely low-density material called an aerogel is obtained if the solvent in a wet gel is removed under a supercritical condition. Ceramic fibers can be drawn from the sol when the viscosity of a sol is adjusted into a proper pro per vis viscos cosity ity ran range. ge. Ult Ultraf rafine ine and uni unifor form m cer ceram amic ic powders are formed by precipitation, spray pyrolysis, or emulsion techniques. Under proper conditions, nanomaterials can be obtained. TiO2 nanomaterials have been synthesized with the sol gel method from hydrolysis of a titanium precusor. 51-78 This process normally proceeds via an acid-catalyzed hydrolysis stepp of tit ste titani anium( um(IV) IV) alk alkoxi oxide de fol follow lowed ed by con conden densasation.51,63,66,79-91 The development of Ti-O-Ti chains is favored with low content of water, low hydrolysis rates, and excess titanium alkoxide in the reaction mixture. Threedimensional polymeric skeletons with close packing result from the development of Ti -O-Ti chains. The formation of Ti(OH Ti(OH))4 is favored with high hydrolysis rates for a medium amount of water. The presence of a large quantity of Ti-OH and insufficient development of three-dimensional polyme pol ymeric ric ske skelet letons ons lea leadd to loo loosel selyy pac packed ked fir firstst-ord order er particles. Polymeric Ti -O-Ti chains are developed in the presence of a large excess of water. Closely packed firstorder particles are yielded via a three-dimensionally developed gel skeleton. 51,63,66,79-91 From the study on the growth kinetics of TiO 2 nanoparticles in aqueous solution using titanium tetraisopropoxide (TTIP) as precursor, it is found that the rate constant for coarsening increases with temperature due to the temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO 2.63 Secondary particles are formed by epitaxial self-assembly of primary particles at longer times and higher temperatures, and the number of primary particles per secondary particle increases with time. The average TiO 2 nanoparticle radius increases linearly with time, in agreement with the Lifshitz -SlyozovWagner model for coarsening. 63 Highly crystalline anatase TiO 2 nanoparticles with different sizes and shapes could be obtained with the polycondensation of titanium alkoxide in the presence of tetramethylammonium tetramethylammonium hydroxide.52,62 In a typical procedure, titanium alkoxide is added to the base at 2 °C in alcoholic solvents in a threeneck flask and is heated at 50 -60 °C for 13 days or at 90 100 °C for 6 h. A secondary treatment involving autoclave heating at 175 and 200 °C is performed to improve the crystallinityy of the TiO 2 nanoparticles. Representative TEM crystallinit images are shown in Figure 1 from the study of Chemseddine et al.52 A series of thorough studies have been conducted by Sugimoto et al. using the sol -gel method on the formation of TiO2 nanoparticles of different sizes and shapes by tuning the reaction parameters. 67-71 Typically, a stock solution of a 0.50 M Ti source is prepared by mixing TTIP with triethanolamine triethanolam ine (TEOA) ([TTIP]/[TEOA] ) 1:2), followed by addition of water. The stock solution is diluted with a shape controller solution and then aged at 100 °C for 1 day and at 140 °C for 3 days. The pH of the solution can be tuned by adding HClO4 or NaOH solution. Amines are used as the shape controllers of the TiO 2 nanomaterials and act as surfactants. These amines include TEOA, diethylenetriamine, ethylenediamine, trimethylenediamine, and triethylenetetramine. The morphology of the TiO 2 nanoparticles

Chemical Reviews, 2007, Vol. 107, No. 7 2893

changes from cuboidal to ellipsoidal at pH above 11 with TEOA. The TiO2 nanopartic nanoparticle le shape evolves into ellipsoid ellipsoidal al above pH 9.5 with diethylenetriamine with a higher aspect ratio than that with TEOA. Figure 2 shows representative TEM images of the TiO2 nanoparticles under different initial pH conditions with the shape control of TEOA at [TEOA]/ [TIPO] ) 2.0. Secondary amines, such as diethylamin diethylamine, e, and tertiary amines, such as trimethylamine and triethylamine, act as complexing agents of Ti(IV) ions to promote the growth of ellipsoidal particles with lower aspect ratios. The shape of the TiO2 nanoparticle can also be tuned from roundcornered cubes to sharp-edged cubes with sodium oleate and sodium stearate. 70 The shape control is attributed to the tuning of the growth rate of the different crystal planes of TiO 2 nanoparticles by the specific adsorption of shape controllers to these planes under different pH conditions. 70 A prolonged heating time below 100 °C for the as-prepared gel can be used to avoid the agglomeration of the TiO 2 nanoparticles during the crystallization process. 58,72 By heati heating ng amorphous TiO2 in air, large quantities of single-phase anatase TiO2 nanoparticles with average particle sizes between 7 and 50 nm can be obtained, as reported by Zhang and Banfield.73-77 Much effort has been exerted to achieve highly crystallized and narrowly dispersed TiO 2 nanoparticles using the sol-gel method with other modifications, such as a semicontinuous reaction method by Znaidi et al. 78 and a twostage mixed method and a continuous reaction method by Kim et al. 53,54 By a combination of the sol -gel method and an anodic alumina membrane (AAM) template, TiO 2 nanorods have been successfully synthesized by dipping porous AAMs into a boiled TiO 2 sol followed by drying and heating processes.92,93 In a typical experiment, a TiO 2 sol solution is prepared by mixing TTIP dissolved in ethanol with a solution containing water, acetyl acetone, and ethanol. An AAM is immersed into the sol solution for 10 min after being boiled in ethanol; then it is dried in air and calcined at 400 °C for 10 h. The AAM template is removed in a 10 wt % H 3PO4 aqueous solution. The calcination temperature can be used to control the crystal phase of the TiO 2 nanorods. At low temperature, anatase nanorods can be obtained, while at high temperature rutile nanorods can be obtained. The pore size of the AAM template can be used to control the size of these TiO2 nanorods, which typically range from 100 to 300 nm in diameter and several micrometers in length. Apparently, the size distribution of the final TiO 2 nanorods is largely controlled by the size distribution of the pores of the AAM template. In order to obtain smaller and monosized TiO2 nanorods, it is necessary to fabricate high-quality AAM templates. Figure 3 shows a typical TEM for TiO 2 nanorods fabricated with this method. Normally, the TiO 2 nanorods are composed of small TiO 2 nanoparticles or nanograins. By electrophoretic deposition of TiO 2 colloidal suspensions into the pores of an AAM, ordered TiO 2 nanowire arrays can be obtained. 94 In a typical procedure, TTIP is dissolved in ethanol at room temperature, and glacial acetic acid mixed with deionized water and ethanol is added under pH ) 2-3 with nitric acid. Platinum is used as the anode, and an AAM with an Au substrate attached to Cu foil is used as the cathode. A TiO2 sol is deposited into the pores of the AMM under a voltage of 2 -5 V and annealed at 500 °C for 24 h. After dissolving the AAM template in a 5 wt % NaOH solution, isolated TiO 2 nanowires are obtained. In order to


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TEM images of TiO2 nanoparticles prepared by hydrolysis of Ti(OR)4 in the presence of tetramethylammonium hydroxide. Reprinted with permission from Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. Copyright 1999 Wiley-VCH. Figure 1.

Figure 2. TEM images of uniform anatase TiO2 nanoparticles. Reprinted from Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 53, Copyright 2003, with permission from Elsevier.

fabricate TiO2 nanowires instead of nanorods, an AAM with long pores is a must. TiO2 nanotubes can also be obtained using the sol -gel method by templating with an AAM 95-98 and other organic compounds.99,100 For example, when an AAM is used as the template, a thin layer of TiO 2 sol on the wall of the pores of

the AAM is first prepared by sucking TiO 2 sol into the pores of the AAM and removing it under vacuum; TiO 2 nanowires are obtained after the sol is fully developed and the AAM is removed. In the procedure by Lee and co-workers, 96 a TTIP solution was prepared by mixing TTIP with 2-propanol and 2,4-pentanedione. After the AAM was dipped into this



SEM of a TiO2 nanotube array; the inset shows the ZnO nanorod array template. Reprinted with permission from Qiu, J. J.; Yu, W. D.; Gao, X. D.; Li, X. M. Nanotechnology 2006, 17 , 4695. Copyright 2006 IOP Publishing Ltd. Figure 5.

TEM image of anatase nanorods and a single nanorod composed of small TiO2 nanoparticles or nanograins (inset). Reprinted from Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. J. Cryst. Growth 2004, 264, 246, Copyright 2004, with permission from Elsevier. Figure 3.

SEM image of TiO2 nanotubes prepared from the AAO template. Reprinted with permission from Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14, 1391. Copyright 2002 American Chemical Society. Figure 4.

solution, it was removed from the solution and placed under vacuum until the entire volume of the solution was pulled through the AAM. The AAM was hydrolyzed by water vapor over a HCl solution for 24 h, air-dried at room temperature, and then calcined in a furnace at 673 K for 2 h and cooled to room temperature with a temperature ramp of 2 °C/h. Pure TiO2 nanotubes were obtained after the AAM was dissolved in a 6 M NaOH solution for several minutes. 96 Alternatively, TiO2 nanotubes could be obtained by coating the AAM membranes at 60 °C for a certain period of time (12 -48 h) with dilute TiF 4 under pH ) 2.1 and removing the AAM after TiO2 nanotubes were fully developed. 97 Figure 4 shows a typical SEM image of the TiO 2 nanotube array from the AAM template.97 In another scheme, a ZnO nanorod array on a glass substrate can be used as a template to fabricate TiO 2 nanotubes with the sol -gel method. 101 Briefly, TiO2 sol is

deposited on a ZnO nanorod template by dip-coating with a slow withdrawing speed, then dried at 100 °C for 10 min, and heated at 550 °C for 1 h in air to obtain ZnO/TiO 2 nanorod arrays. The ZnO nanorod template is etched-up by immersing the ZnO/TiO 2 nanorod arrays in a dilute hydrochloric acid aqueous solution to obtain TiO 2 nanotube arrays. Figure 5 shows a typical SEM image of the TiO 2 nanotube array with the ZnO nanorod array template. The TiO 2 nanotubes inherit the uniform hexagonal cross-sectional shape and the length of 1.5 µm and inner diameter of 100 120 nm of the ZnO nanorod template. As the concentration of the TiO2 sol is constant, well-aligned TiO 2 nanotube arrays can only be obtained from an optimal dip-coating cycle number in the range of 2 -3 cycles. A dense porous TiO 2 thick film with holes is obtained instead if the dip-coating number further increases. The heating rate is critical to the formation of TiO 2 nanotube arrays. When the heating rate is extra rapid, e.g., above 6 °C min-1, the TiO2 coat will easily crack and flake off from the ZnO nanorods due to great tensile stress between the TiO 2 coat and the ZnO template, and a TiO 2 film with loose, porous nanostructure is obtained.

2.2. Micelle and Inverse Micelle Methods Aggregates of surfactant molecules dispersed in a liquid colloid are called micelles when the surfactant concentration exceeds the critical micelle concentration (CMC). The CMC is the concentration of surfactants in free solution in equilibrium with surfactants in aggregated form. In micelles, the hydrophobic hydrocarbon chains of the surfactants are oriented toward the interior of the micelle, and the hydrophilic groups of the surfactants are oriented toward the surrounding aqueous medium. The concentration of the lipid present in solution determines the self-organization of the molecules of surfactants and lipids. The lipids form a single layer on the liquid surface and are dispersed in solution below the CMC. The lipids organize in spherical micelles at the first CMC (CMC-I), into elongated pipes at the second CMC (CMC-II), and into stacked lamellae of pipes at the lamellar point (LM or CMC-III). The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length. Reverse micelles are formed in nonaqueous media, and the hydrophilic headgroups are directed toward the core of the micelles while the hydrophobic groups are


directed outward toward the nonaqueous media. There is no obvious CMC for reverse micelles, because the number of aggregates is usually small and they are not sensitive to the surfactant concentration. Micelles are often globular and roughly spherical in shape, but ellipsoids, cylinders, and bilayers are also possible. The shape of a micelle is a function of the molecular geometry of its surfactant molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic strength. Micelles and inverse micelles are commonly employed to synthesize TiO 2 nanomaterials.102-110 A statistical experimental design method was conducted by Kim et al. to optimize experimental conditions for the preparation of TiO 2 nanoparticles. 103 The values of H2O/surfactant, H2O/titanium precursor, ammonia concentration, feed rate, and reaction temperature were significant parameters in controlling TiO 2 nanoparticle size and size distribution. Amorphous TiO 2 nanoparticles with diameters of 10 -20 nm were synthesized and converted to the anatase phase at 600 °C and to the more thermodynamically stable rutile phase at 900 °C. Li et al. developed TiO2 nanoparticles with the chemical reactions between TiCl4 solution and ammonia in a reversed microemulsion system consisting of cyclohexane, poly(oxyethylene)5 nonyle phenol ether, and poly(oxyethylene) 9 nonyle phenol ether.104 The produced amorphous TiO2 nanoparticles transformed into anatase when heated at temperatures from 200 to 750 °C and into rutile at temperatures higher than 750 °C. Agglomeration and growth also occurred at elevated temperatures. Shuttle-like crystalline TiO 2 nanoparticles were synthesized by Zhang et al. with hydrolysis of titanium tetrabutoxide in the presence of acids (hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid) in NP-5 (Igepal CO-520) cyclohexane reverse micelles at room temperature. 110 The crystal structure, morphology, and particle size of the TiO 2 nanoparticles were largely controlled by the reaction conditions, and the key factors affecting the formation of rutile at room temperature included the acidity, the type of acid used, and the microenvironment of the reverse micelles. Agglomeration of the particles occurred with prolonged reaction times and increasing the [H 2O]/[NP-5] and [H2O]/[Ti(OC4H9)4] ratios. When suitable acid was applied, round TiO 2 nanoparticles could also be obtained. Representative TEM images of the shuttle-like and round-shaped TiO 2 nanoparticles are shown in Figure 6. In the study carried out by Lim et al., TiO2 nanoparticles were prepared by the controlled hydrolysis of TTIP in reverse micelles formed in CO 2 with the surfactants ammonium carboxylate perfluoropolyether (PFPECOO-NH4+) (MW 587) and poly(dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl methacrylate) (PDMAEMA-b-PFOMA). 106 It was found that the crystallite size prepared in the presence of reverse micelles increased as either the molar ratio of water to surfactant or the precursor to surfactant ratio increased. The TiO2 nanomaterials prepared with the above micelle and reverse micelle methods normally have amorphous structure, and calcination is usually necessary in order to induce high crystallinity. However, this process usually leads to the growth and agglomeration of TiO 2 nanoparticles. The crystallinity of TiO 2 nanoparticles initially (synthesized by controlled hydrolysis of titanium alkoxide in reverse micelles in a hydrocarbon solvent) could be improved by annealing in the presence of the micelles at temperatures considerably lower than those required for the traditional calcination

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TEM images of the shuttle-like and round-shaped (inset) TiO2 nanoparticles. From: Zhang, D., Qi, L., Ma, J., Cheng, H. J. Mater. Chem. 2002, 12, 3677 (http://dx.doi.org/10.1039/b206996b). s Reproduced by permission of The Royal Society of Chemistry. Figure 6.

HRTEM images of a TiO2 nanoparticle after annealing. Reprinted with permission from Lin, J.; Lin, Y.; Liu, P.; Meziani, M. J.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2002, 124, 11514. Copyright 2002 American Chemical Society. Figure 7.

treatment in the solid state. 108 This procedure could produce crystalline TiO2 nanoparticles with unchanged physical dimensions and minimal agglomeration and allows the preparation of highly crystalline TiO 2 nanoparticles, as shown in Figure 7, from the study of Lin et al. 108

2.3. Sol Method The sol method here refers to the nonhydrolytic sol -gel processes and usually involves the reaction of titanium chloride with a variety of different oxygen donor molecules, e.g., a metal alkoxide or an organic ether. 111-119

TiX4 + Ti(OR)4 f 2TiO2 + 4RX

(1)

TiX4 + 2ROR f TiO2 + 4RX

(2)



TEM image of TiO2 nanoparticles derived from reaction of TiCl4 and TTIP in TOPO/heptadecane at 300 °C. The inset shows a HRTEM image of a single particle. Reprinted with permission from Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. Copyright 1999 American Chemical Society. Figure 8.

The condensation between Ti -Cl and Ti-OR leads to the formation of Ti -O-Ti bridges. The alkoxide groups can be provided by titanium alkoxides or can be formed in situ by reaction of the titanium chloride with alcohols or ethers. In the method by Trentler and Colvin, 119 a metal alkoxide was rapidly injected into the hot solution of titanium halide mixed with trioctylphosphine oxide (TOPO) in heptadecane at 300 °C under dry inert gas protection, and reactions were completed within 5 min. For a series of alkyl substituents including methyl, ethyl, isopropyl, and tert -butyl, the reaction rate dramatically increased with greater branching of R, while average particle sizes were relatively unaffected. Variation of X yielded a clear trend in average particle size, but without a discernible trend in reaction rate. Increased nucleophilicity (or size) of the halide resulted in smaller anatase nanocrystals. Average sizes ranged from 9.2 nm for TiF 4 to 3.8 nm for TiI4. The amount of passivating agent (TOPO) influenced the chemistry. Reaction in pure TOPO was slower and resulted in smaller particles, while reactions without TOPO were much quicker and yielded mixtures of brookite, rutile, and anatase with average particle sizes greater than 10 nm. Figure 8 shows typical TEM images of TiO 2 nanocrystals developed by Trentler et al. 119 In the method used by Niederberger and Stucky, 111 TiCl4 was slowly added to anhydrous benzyl alcohol under vigorous stirring at room temperature and was kept at 40 150 °C for 1-21 days in the reaction vessel. The precipitate was calcinated at 450 °C for 5 h after thoroughly washing. The reaction between TiCl 4 and benzyl alcohol was found suitable for the synthesis of highly crystalline anatase phase TiO2 nanoparticles with nearly uniform size and shape at very low temperatures, such as 40 °C. The particle size could be selectively adjusted in the range of 4 -8 nm with the appropriate thermal conditions and a proper choice of the relative amounts of benzyl alcohol and titanium tetrachloride. The particle growth depended strongly on temperature, and lowering the titanium tetrachloride concentration led to a considerable decrease of particle size. 111 Surfactants have been widely used in the preparation of a variety of nanoparticles with good size distribution and dispersity.15,16 Adding different surfactants as capping agents, such as acetic acid and acetylacetone, into the reaction matrix

TEM of TiO2 nanorods. The inset shows a HRTEM of a TiO2 nanorod. Reprinted with permission from Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. Copyright 2003 American Chemical Society. Figure 9.

can help synthesize monodispersed TiO 2 nanoparticles. 120,121 For example, Scolan and Sanchez found that monodisperse nonaggregated TiO2 nanoparticles in the 1 -5 nm range were obtained through hydrolysis of titanium butoxide in the presence of acetylacetone and p-toluenesulfonic acid at 60 °C.120 The resulting nanoparticle xerosols could be dispersed in water-alcohol or alcohol solutions at concentrations higher than 1 M without aggregation, which is attributed to the complexation of the surface by acetylacetonato ligands and through an adsorbed hybrid organic -inorganic layer made with acetylacetone, p-toluenesulfonic acid, and water. 120 With the aid of surfactants, different sized and shaped TiO 2 nanorods can be synthesized.122-130 For example, the growth of high-aspect-ratio anatase TiO 2 nanorods has been reported by Cozzoli and co-workers by controlling the hydrolysis process of TTIP in oleic acid (OA). 122-126,130 Typically, TTIP was added into dried OA at 80 -100 °C under inert gas protection (nitrogen flow) and stirred for 5 min. A 0.1 -2 M aqueous base solution was then rapidly injected and kept at 80-100 °C for 6-12 h with stirring. The bases employed included organic amines, such as trimethylamino-N-oxide, trimethylamine, tetramethylammonium hydroxide, tetrabutylammonium hydroxyde, triethylamine, and tributylamine. In this reaction, by chemical modification of the titanium precursor with the carboxylic acid, the hydrolysis rate of titanium alkoxide was controlled. Fast (in 4 -6 h) crystallization in mild conditions was promoted with the use of suitable catalysts (tertiary amines or quaternary ammonium hydroxides). A kinetically overdriven growth mechanism led to the growth of TiO 2 nanorods instead of nanoparticles. 123 Typical TEM images of the TiO 2 nanorods are shown in Figure 9.123 Recently, Joo et al. 127 and Zhang et al. 129 reported similar procedures in obtaining TiO 2 nanorods without the use of catalyst. Briefly, a mixture of TTIP and OA was used to generate OA complexes of titanium at 80 °C in 1-octadecene.


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TEM images of TiO2 nanorods with lengths of (A) 12 nm, (B) 30 nm, and (C) 16 nm. (D) 2.3 nm TiO 2 nanoparticles. Inset in parts C and D: HR-TEM image of a single TiO2 nanorod and nanoparticle. Reprinted with permission from Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright 2005 Wiley-VCH. Figure 10.

The injection of a predetermined amount of oleylamine at 260 °C led to various sized TiO 2 nanorods.129 Figure 10 shows TEM images of TiO 2 nanorods with various lengths, and 2.3 nm TiO2 nanoparticles prepared with this method. 129 In the surfactant-mediated shape evolution of TiO 2 nanocrystals in nonaqueous media conducted by Jun et al., 128 it was found that the shape of TiO 2 nanocrystals could be modified by changing the surfactant concentration. The synthesis was accomplished by an alkyl halide elimination reaction between titanium chloride and titanium isopropoxide. Briefly, a dioctyl ether solution containing TOPO and lauric acid was heated to 300 °C followed by addition of titanium chloride under vigorous stirring. The reaction was initiated by the rapid injection of TTIP and quenched with cold toluene. At low lauric acid concentrations, bulletand diamond-shaped nanocrystals were obtained; at higher concentrations, rod-shaped nanocrystals or a mixture of nanorods and branched nanorods was observed. The bulletand diamond-shaped nanocrystals and nanorods were elongated along the [001] directions. The TiO 2 nanorods were found to simultaneously convert to small nanoparticles as a function of the growth time, as shown in Figure 11, due to the minimization of the overall surface energy via dissolution and regrowth of monomers during an Ostwald ripening.

2.4. Hydrothermal Method Hydrothermal synthesis is normally conducted in steel pressure vessels called autoclaves with or without Teflon

liners under controlled temperature and/or pressure with the reaction in aqueous solutions. The temperature can be elevated above the boiling point of water, reaching the pressure of vapor saturation. The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced. It is a method that is widely used for the production of small particles in the ceramics industry. Many groups have used the hydrothermal method to prepare TiO2 nanoparticles.131-140 For example, TiO2 nanoparticles can be obtained by hydrothermal treatment of peptized precipitates of a titanium precursor with water. 134 The precipitates were prepared by adding a 0.5 M isopropanol solution of titanium butoxide into deionized water ([H 2O]/ [Ti] ) 150), and then they were peptized at 70 °C for 1 h in the presence of tetraalkylammonium hydroxides (peptizer). After filtration and treatment at 240 °C for 2 h, the as-obtained powders were washed with deionized water and absolute ethanol and then dried at 60 °C. Under the same concentration of peptizer, the particle size decreased with increasing alkyl chain length. The peptizers and their concentrations influenced the morphology of the particles. Typical TEM images of TiO 2 nanoparticles made with the hydrothermal method are shown in Figure 12. 134 In another example, TiO 2 nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic ethanol-water solution.132 Briefly, TTIP was added dropwise to a mixed ethanol and water solution at pH 0.7 with nitric acid, and reacted at 240 °C for 4 h. The TiO 2 nanoparticles



TEM images of TiO2 nanoparticles prepared by the hydrothermal method. Reprinted from Yang, J.; Mei, S.; Ferreira, J. M. F. Mater. Sci. Eng. C 2001, 15, 183, Copyright 2001, with permission from Elsevier. Figure 12.

TEM image of TiO2 nanorods prepared with the hydrothermal method. Reprinted with permission from Zhang, Q.; Gao, L. Langmuir 2003, 19, 967. Copyright 2003 American Chemical Society. Figure 13.

Time dependent shape evolution of TiO2 nanorods: (a) 0.25 h; (b) 24 h; (c) 48 h. Scale bar ) 50 nm. Reprinted with permission from Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. Copyright 2003 American Chemical Society. Figure 11.

synthesized under this acidic ethanol -water environment were mainly primary structure in the anatase phase without secondary structure. The sizes of the particles were controlled to the range of 7 -25 nm by adjusting the concentration of Ti precursor and the composition of the solvent system.

Besides TiO2 nanoparticles, TiO2 nanorods have also been synthesized with the hydrothermal method. 141-146 Zhang et al. obtained TiO2 nanorods by treating a dilute TiCl 4 solution at 333-423 K for 12 h in the presence of acid or inorganic salts.141,143-146 Figure 13 shows a typical TEM image of the TiO2 nanorods prepared with the hydrothermal method. 141 The morphology of the resulting nanorods can be tuned with different surfactants 146 or by changing the solvent compositions.145 A film of assembled TiO 2 nanorods deposited on a glass wafer was reported by Feng et al. 142 These TiO2 nanorods were prepared at 160 °C for 2 h by hydrothermal treatment of a titanium trichloride aqueous solution supersaturated with NaCl. TiO2 nanowires have also been successfully obtained with the hydrothermal method by various groups. 147-151 Typically, TiO2 nanowires are obtained by treating TiO 2 white powders in a 10-15 M NaOH aqueous solution at 150 -200 °C for 24-72 h without stirring within an autoclave. Figure 14 shows the SEM images of TiO 2 nanowires and a TEM image of a single nanowire prepared by Zhang and co-workers. 150 TiO2 nanowires can also be prepared from layered titanate particles using the hydrothermal method as reported by Wei


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TEM images of TiO2 nanowires made from the layered Na2Ti3O7 particles, with the HRTEM image shown in the inset. Reprinted from Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.; Arakawa, H. Chem. Phys. Lett. 2004, 400, 231, Copyright 2004, with permission from Elsevier. Figure 15.

SEM images of TiO2 nanowires with the inset showing a TEM image of a single TiO2 nanowire with a [010] selected area electron diffraction (SAED) recorded perpendicular to the long axis of the wire. Reprinted from Zhang, Y. X.; Li, G. H.; Jin, Y. X.; Zhang, Y.; Zhang, J.; Zhang, L. D. Chem. Phys. Lett. 2002, 365, 300, Copyright 2002, with permission from Elsevier. Figure 14.

et al.152 In their experiment, layer-structured Na 2Ti3O7 was dispersed into a 0.05 -0.1 M HCl solution and kept at 140 170 °C for 3-7 days in an autoclave. TiO 2 nanowires were obtained after the product was washed with H 2O and finally dried. In the formation of a TiO 2 nanowire from layered H2Ti3O7, there are three steps: (i) the exfoliation of layered Na2Ti3O7; (ii) the nanosheets formation; and (iii) the nanowires formation. 152 In Na2Ti3O7, [TiO6] octahedral layers are held by the strong static interaction between the Na + cations between the [TiO 6] octahedral layers and the [TiO 6] unit. When the larger H 3+O cations replace the Na + cations in the interlayer space of [TiO 6] sheets, this static interaction is weakened because the interlayer distance is enlarged. As a result, the layered compounds Na 2Ti3O7 are gradually exfoliated. When Na+ is exchanged by H + in the dilute HCl solution, numerous H2Ti3O7 sheet-shaped products are formed. Since the nanosheet does not have inversion symmetry, an intrinsic tension exists. The nanosheets split to form nanowires in order to release the strong stress and lower the total energy.152 A representative TEM image of TiO 2 nanowires from Na2Ti3O7 is shown in Figure 15. 152 The hydrothermal method has been widely used to prepare TiO2 nanotubes since it was introduced by Kasuga et al. in 1998.153-175 Briefly, TiO2 powders are put into a 2.5 -20 M NaOH aqueous solution and held at 20 -110 °C for 20 h in an autoclave. TiO 2 nanotubes are obtained after the products are washed with a dilute HCl aqueous solution and distilled water. They proposed the following formation process of TiO2 nanotubes.154 When the raw TiO2 material was treated with NaOH aqueous solution, some of the Ti -O-Ti bonds were broken and Ti-O-Na and Ti-OH bonds were formed. New Ti-O-Ti bonds were formed after the Ti -O-Na and Ti-OH bonds reacted with acid and water when the material was treated with an aqueous HCl solution and distilled water. The Ti-OH bond could form a sheet. Through the dehydration of Ti-OH bonds by HCl aqueous solution, Ti -O-Ti bonds or Ti-O-H-O-Ti hydrogen bonds were generated. The bond distance from one Ti to the next Ti on the surface decreased. This resulted in the folding of the sheets and the

TEM image of TiO2 nanotubes. Reprinted with permission from Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. Copyright 1998 American Chemical Society. Figure 16.

connection between the ends of the sheets, resulting in the formation of a tube structure. In this mechanism, the TiO 2 nanotubes were formed in the stage of the acid treatment following the alkali treatment. Figure 16 shows typical TEM images of TiO2 nanotubes made by Kasuga et al. 153 However, Du and co-workers found that the nanotubes were formed during the treatment of TiO 2 in NaOH aqueous solution. 161 A 3D f 2D f 1D formation mechanism of the TiO 2 nanotubes was proposed by Wang and co-workers. 171 It stated that the raw TiO2 was first transformed into lamellar structures and then bent and rolled to form the nanotubes. For the formation of the TiO 2 nanotubes, the two-dimensional lamellar TiO2 was essential. Yao and co-workers further suggested, based on their HRTEM study as shown in Figure



TEM micrographs of TiO2 nanoparticles prepared with the solvothermal method. Reprinted with permission from Li, X. L.; Peng, Q.; Yi, J. X.; Wang, X.; Li, Y. D. Chem.s Eur. J. 2006, 12, 2383. Copyright 2006 Wiley-VCH. Figure 18.

(a) HRTEM images of TiO2 nanotubes. (b) Crosssectional view of TiO2 nanotubes. Reused with permission from B. D. Yao, Y. F. Chan, X. Y. Zhang, W. F. Zhang, Z. Y. Yang, N. Wang, Applied Physics Letters 82 , 281 (2003). Copyright 2003, American Institute of Physics. Figure 17.

17, that TiO2 nanotubes were formed by rolling up the singlelayer TiO2 sheets with a rolling-up vector of [001] and attracting other sheets to surround the tubes. 172 Bavykin and co-workers suggested that the mechanism of nanotube formation involved the wrapping of multilayered nanosheets rather than scrolling or wrapping of single layer nanosheets followed by crystallization of successive layers. 156 In the mechanism proposed by Wang et al., the formation of TiO 2 nanotubes involved several steps. 176 During the reaction with NaOH, the Ti-O-Ti bonding between the basic building blocks of the anatase phase, the octahedra, was broken and a zigzag structure was formed when the free octahedras shared edges between the Ti ions with the formation of hydroxy bridges, leading to the growth along the [100] direction of the anatase phase. Two-dimensional crystalline sheets formed from the lateral growth of the formation of oxo bridges between the Ti centers (Ti -O-Ti bonds) in the [001] direction and rolled up in order to saturate these dangling bonds from the surface and lower the total energy, resulting in the formation of TiO 2 nanotubes.176

2.5. Solvothermal Method The solvothermal method is almost identical to the hydrothermal method except that the solvent used here is nonaqueous. However, the temperature can be elevated much higher than that in hydrothermal method, since a variety of organic solvents with high boiling points can be chosen. The solvothermal method normally has better control than hydrothermal methods of the size and shape distributions and the crystallinity of the TiO 2 nanoparticles. The solvothermal method has been found to be a versatile method for the

synthesis of a variety of nanoparticles with narrow size distribution and dispersity. 177-179 The solvothermal method has been employed to synthesize TiO 2 nanoparticles and nanorods with/without the aid of surfactants. 177-185 For example, in a typical procedure by Kim and co-workers, 184 TTIP was mixed with toluene at the weight ratio of 1 -3:10 and kept at 250 °C for 3 h. The average particle size of TiO 2 powders tended to increase as the composition of TTIP in the solution increased in the range of weight ratio of 1 -3: 10, while the pale crystalline phase of TiO 2 was not produced at 1:20 and 2:5 weight ratios. 184 By controlling the hydrolyzation reaction of Ti(OC 4H9)4 and linoleic acid, redispersible TiO2 nanoparticles and nanorods could be synthesized, as found by Li et al. recently.177 The decomposition of NH 4HCO3 could provide H 2O for the hydrolyzation reaction, and linoleic acid could act as the solvent/reagent and coordination surfactant in the synthesis of nanoparticles. Triethylamine could act as a catalyst for the polycondensation of the Ti O-Ti inorganic network to achieve a crystalline product and had little influence on the products’ morphology. The chain lengths of the carboxylic acids had a great influence on the formation of TiO2, and long-chain organic acids were important and necessary in the formation of TiO 2.177 Figure 18 shows a representative TEM image of TiO 2 nanoparticles from their study. 177 TiO2 nanorods with narrow size distributions can also be developed with the solvothermal method. 177,183 For example, in a typical synthesis from Kim et al., TTIP was dissolved in anhydrous toluene with OA as a surfactant and kept at 250 °C for 20 h in an autoclave without stirring. 183 Long dumbbell-shaped nanorods were formed when a sufficient amount of TTIP or surfactant was added to the solution, due to the oriented growth of particles along the [001] axis. At a fixed precursor to surfactant weight ratio of 1:3, the concentration of rods in the nanoparticle assembly increased as the concentration of the titanium precursor in the solution increased. The average particle size was smaller and the size distribution was narrower than is the case for particles synthesized without surfactant. The crystalline phase, diameter, and length of these nanorods are largely influenced by the precursor/surfactant/solvent weight ratio. Anatase nano-


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TEM images of TiO2 nanowires synthesized by the solvothermal method. From: Wen, B.; Liu, C.; Liu, Y. New J. Chem. 2005, 29, 969 (http://dx.doi.org/10.1039/b502604k) s Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS). Figure 20.

TEM micrographs and electron diffraction patterns of products prepared from solutions at the weight ratio of precursor/ solvent/surfactant ) 1:5:3. Reprinted from Kim, C. S.; Moon, B. K.; Park, J. H.; Choi, B. C.; Seo, H. J. J. Cryst. Growth 2003, 257 , 309, Copyright 2003, with permission from Elsevier. Figure 19.

rods were obtained from the solution with a precursor/ surfactant weight ratio of more than 1:3 for a precursor/ solvent weight ratio of 1:10 or from the solution with a precursor/solvent weight ratio of more than 1:5 for a precursor/surfactant weight ratio of 1:3. The diameter and length of these nanorods were in the ranges of 3 -5 nm and 18-25 nm, respectively. Figure 19 shows a typical TEM image of TiO 2 nanorods prepared from the solutions with the weight ratio of precursor/solvent/surfactant ) 1:5:3.183 Similar to the hydrothermal method, the solvothermal method has also been used for the preparation of TiO 2 nanowires. 180-182 Typically, a TiO2 powder suspension in an 5 M NaOH water-ethanol solution is kept in an autoclave at 170-200 °C for 24 h and then cooled to room temperature naturally. TiO 2 nanowires are obtained after the obtained sample is washed with a dilute HCl aqueous solution and dried at 60 °C for 12 h in air. 181 The solvent plays an important role in determining the crystal morphology. Solvents with different physical and chemical properties can influence the solubility, reactivity, and diffusion behavior of the reactants; in particular, the polarity and coordinating ability of the solvent can influence the morphology and the crystallization behavior of the final products. The presence of ethanol at a high concentration not only can cause the polarity of the solvent to change but also strongly affects the ζ potential values of the reactant particles and the increases solution viscosity. For example, in the absence of ethanol, short and wide flakelike structures of TiO 2 were obtained instead of nanowires. When chloroform is used, TiO2 nanorods were obtained. 181 Figure 20 shows representative TEM images of the TiO 2 nanowires prepared from the solvothermal method. 181 Alternatively, bamboo-shaped Agdoped TiO2 nanowires were developed with titanium butoxide as precursor and AgNO 3 as catalyst.180 Through the electron diffraction (ED) pattern and HRTEM study, the Ag

SEM morphology of TiO2 nanorods by directly oxidizing a Ti plate with a H2O2 solution. Reprinted from Wu, J. M. J. Cryst. Growth 2004, 269, 347, Copyright 2004, with permission from Elesevier. Figure 21.

phase only existed in heterojunctions between single-crystal TiO2 nanowires.180

2.6. Direct Oxidation Method TiO2 nanomaterials can be obtained by oxidation of titanium metal using oxidants or under anodization. Crystalline TiO2 nanorods have been obtained by direct oxidation of a titanium metal plate with hydrogen peroxide. 186-191 Typically, TiO2 nanorods on a Ti plate are obtained when a cleaned Ti plate is put in 50 mL of a 30 wt % H 2O2 solution at 353 K for 72 h. The formation of crystalline TiO 2 occurs through a dissolution precipitation mechanism. By the addition of inorganic salts of NaX (X ) F-, Cl-, and SO42-), the crystalline phase of TiO 2 nanorods can be controlled. The addition of F - and SO42- helps the formation of pure anatase, while the addition of Cl - favors the formation of rutile.189 Figure 21 shows a typical SEM image of TiO 2 nanorods prepared with this method. 186 At high temperature, acetone can be used as a good oxygen source and for the preparation of TiO 2 nanorods by oxidizing



SEM images of TiO2 nanotubes prepared with anodic oxidation. Reprinted with permission from Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Ad V. Mater. 2003, 15, 624. Copyright 2003 Wiley-VCH. Figure 23.

SEM images of large-scale nanorod arrays prepared by oxidizing a titanium with acetone at 850 °C for 90 min. From: Peng, X.; Chen, A. J. Mater. Chem. 2004, 14, 2542 (http:// dx.doi.org/10.1039/b404750h) s Reproduced by permission of The Royal Society of Chemistry. Figure 22.

a Ti plate with acetone as reported by Peng and Chen. 192 The oxygen source was found to play an important role. Highly dense and well-aligned TiO 2 nanorod arrays were formed when acetone was used as the oxygen source, and only crystal grain films or grains with random nanofibers growing from the edges were obtained with pure oxygen or argon mixed with oxygen. The competition of the oxygen and titanium diffusion involved in the titanium oxidation process largely controlled the morphology of the TiO 2. With pure oxygen, the oxidation occurred at the Ti metal and the TiO2 interface, since oxygen diffusion predominated because of the high oxygen concentration. When acetone was used as the oxygen source, Ti cations diffused to the oxide surface and reacted with the adsorbed acetone species. Figure 22 shows aligned TiO2 nanorod arrays obtained by oxidizing a titanium substrate with acetone at 850 °C for 90 min. 192 As extensively studied, TiO 2 nanotubes can be obtained by anodic oxidation of titanium foil. 193-228 In a typical experiment, a clean Ti plate is anodized in a 0.5% HF solution under 10 -20 V for 10 -30 min. Platinum is used as counterelectrode. Crystallized TiO 2 nanotubes are obtained after the anodized Ti plate is annealed at 500 °C for 6 h in oxygen.210 The length and diameter of the TiO 2 nanotubes could be controlled over a wide range (diameter, 15 -120 nm; length, 20 nm to 10 µm) with the applied potential between 1 and 25 V in optimized phosphate/HF electrolytes. 229 Figure 23 shows SEM images of TiO 2 nanotubes created with this method. 208

2.7. Chemical Vapor Deposition Vapor deposition refers to any process in which materials in a vapor state are condensed to form a solid-phase material. These processes are normally used to form coatings to alter the mechanical, electrical, thermal, optical, corrosion resistance, and wear resistance properties of various substrates. They are also used to form free-standing bodies, films, and fibers and to infiltrate fabric to form composite materials. Recently, they have been widely explored to fabricate various nanomaterials. Vapor deposition processes usually take place within a vacuum chamber. If no chemical reaction occurs, this process is called physical vapor deposition (PVD);

SEM images of TiO2 nanorods grown at 560 °C. Reprinted with permission from Wu, J. J.; Yu, C. C. J. Phys. Chem. B 2004, 108, 3377. Copyright 2004 American Chemical Society. Figure 24.

otherwise, it is called chemical vapor deposition (CVD). In CVD processes, thermal energy heats the gases in the coating chamber and drives the deposition reaction. Thick crystalline TiO 2 films with grain sizes below 30 nm as well as TiO 2 nanoparticles with sizes below 10 nm can be prepared by pyrolysis of TTIP in a mixed helium/oxygen atmosphere, using liquid precursor delivery. 230 When deposited on the cold areas of the reactor at temperatures below 90 °C with plasma enhanced CVD, amorphous TiO 2 nanoparticles can be obtained and crystallize with a relatively high surface area after being annealed at high temperatures. 231 TiO2 nanorod arrays with a diameter of about 50 -100 nm and a length of 0.5 -2 µm can be synthesized by metal organic CVD (MOCVD) on a WC-Co substrate using TTIP as the precursor. 232 Figure 24 shows the TiO 2 nanorods grown on fused silica substrates with a template- and catalyst-free MOCVD method.233 In a typical procedure, titanium acetylacetonate (Ti(C10H14O5)) vaporizing in the low-temperature zone of a furnace at 200-230 °C is carried by a N 2 /O2 flow into the high-temperature zone of 500 -700 °C, and TiO2 nanostructures are grown directly on the substrates. The phase and


SEM images of the TiO2 nanowire arrays prepared by the PVD method. Reprinted from Wu, J. M.; Shih, H. C.; Wu, W. T. Chem. Phys. Lett. 2005, 413, 490, Copyright 2005, with permission from Elsevier.

Chen and Mao

Figure 25.

morphology of the TiO 2 nanostructures can be tuned with the reaction conditions. For example, at 630 and 560 °C under a pressure of 5 Torr, single-crystalline rutile and anatase TiO2 nanorods were formed respectively, while, at 535 °C under 3.6 Torr, anatase TiO 2 nanowalls composed of well-aligned nanorods were formed. 233 In addition to the above CVD approaches in preparing TiO2 nanomaterials, other CVD approaches are also used, such as electrostatic spray hydrolysis, 234 diffusion flame pyrolysis,235-239 thermal plasma pyrolysis, 240-246 ultrasonic spray pyrolysis, 247 laser-induced pyrolysis,248,249 and ultronsicassisted hydrolysis, 250,251 among others.

2.8. Physical Vapor Deposition In PVD, materials are first evaporated and then condensed to form a solid material. The primary PVD methods include thermal deposition, ion plating, ion implantation, sputtering, laser vaporization, and laser surface alloying. TiO 2 nanowire arrays have been fabricated by a simple PVD method or thermal deposition. 252-254 Typically, pure Ti metal powder is on a quartz boat in a tube furnace about 0.5 mm away from the substrate. Then the furnace chamber is pumped down to ∼300 Torr and the temperature is increased to 850 °C under an argon gas flow with a rate of 100 sccm and held for 3 h. After the reaction, a layer of TiO 2 nanowires can be obtained.254 A layer of Ti nanopowders can be deposited on the substrate before the growth of TiO 2 nanowires,252,253 and Au can be employed as catalyst. 252 A typical SEM image of TiO 2 nanowires made with the PVD method is shown in Figure 25. 252

2.9. Electrodeposition Electrodeposition is commonly employed to produce a coating, usually metallic, on a surface by the action of reduction at the cathode. The substrate to be coated is used as cathode and immersed into a solution which contains a salt of the metal to be deposited. The metallic ions are attracted to the cathode and reduced to metallic form. With the use of the template of an AAM, TiO 2 nanowires can be obtained by electrodeposition. 255,256 In a typical process, the electrodeposition is carried out in 0.2 M TiCl 3 solution with

Cross-sectional SEM image of TiO2 nanowires electrodeposited in AAM pores. Reprinted from Liu, S.; Huang, K. Sol. Energy Mater. Sol. Cells 2004, 85, 125, Copyright 2004, with permission from Elsevier. Figure 26.

pH ) 2 with a pulsed electrodeposition approach, and titanium and/or its compound are deposited into the pores of the AAM. By heating the above deposited template at 500 °C for 4 h and removing the template, pure anatase TiO 2 nanowires can be obtained. Figure 26 shows a representative SEM image of TiO2 nanowires.256

2.10. Sonochemical Method Ultrasound has been very useful in the synthesis of a wide range of nanostructured materials, including high-surfacearea transition metals, alloys, carbides, oxides, and colloids. The chemical effects of ultrasound do not come from a direct interaction with molecular species. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating ( ∼5000 K), high pressures (∼1000 atm), and enormous heating and cooling rates (>109 K/s). The sonochemical method has been applied to prepare various TiO2 nanomaterials by different groups. 257-269 Yu et al. applied the sonochemical method in preparing highly photoactive TiO 2 nanoparticle photocatalysts with anatase and brookite phases using the hydrolysis of titanium tetraisoproproxide in pure water or in a 1:1 EtOH -H2O solution under ultrasonic radiation. 109 Huang et al. found that anatase and rutile TiO 2 nanoparticles as well as their mixtures could be selectively synthesized with various precursors using ultrasound irradiation, depending on the reaction temperature and the precursor used. 259 Zhu et al. developed titania whiskers and nanotubes with the assistance of sonication as shown in Figure 27. 269 They found that arrays of TiO2 nanowhiskers with a diameter of 5 nm and nanotubes with a diameter of ∼5 nm and a length of 200 -300 nm could be obtained by sonicating TiO 2 particles in NaOH aqueous solution followed by washing with deionized water and a dilute HNO3 aqueous solution. 2.11. Microwave Method A dielectric material can be processed with energy in the form of high-frequency electromagnetic waves. The principal



SEM image of the mesoporous TiO2 film synthesized from the acetic acid-modified precursor and autoclaved at 230 °C. Reprinted with permission from Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. Copyright 1997 Blackwell Publishing. Figure 28.

TEM images of TiO2 nanotubes (A) and nanowhiskers (B) prepared with the sonochemical method. From: Zhu, Y.; Li, H.; Koltypin, Y.; Hacohen, Y. R.; Gedanken, A. Chem. Commun. 2001, 2616 (http://dx.doi.org/10.1039/b108968b) s Reproduced by permission of The Royal Society of Chemistry. Figure 27.

frequencies of microwave heating are between 900 and 2450 MHz. At lower microwave frequencies, conductive currents flowing within the material due to the movement of ionic constituents can transfer energy from the microwave field to the material. At higher frequencies, the energy absorption is primarily due to molecules with a permanent dipole which tend to reorientate under the influence of a microwave electric field. This reorientation loss mechanism originates from the inability of the polarization to follow extremely rapid reversals of the electric field, so the polarization phasor lags the applied electric field. This ensures that the resulting current density has a component in phase with the field, and therefore power is dissipated in the dielectric material. The major advantages of using microwaves for industrial processing are rapid heat transfer, and volumetric and selective heating. Microwave radiation is applied to prepare various TiO 2 nanomaterials.270-276 Corradi et al. found that colloidal titania nanoparticle suspensions could be prepared within 5 min to 1 h with microwave radiation, while 1 to 32 h was needed for the conventional synthesis method of forced hydrolysis at 195 °C.270 Ma et al. developed high-quality rutile TiO 2 nanorods with a microwave hydrothermal method and found that they aggregated radially into spherical secondary nanoparticles.272 Wu et al. synthesized TiO 2 nanotubes by microwave radiation via the reaction of TiO 2 crystals of anatase, rutile, or mixed phase and NaOH aqueous solution under a certain microwave power.275 Normally, the TiO 2 nanotubes had the central hollow, open-ended, and multiwall structure with diameters of 8 -12 nm and lengths up to 200 -1000 nm.275

2.12. TiO2 Mesoporous/Nanoporous Materials In the past decade, mesoporous/nanoporous TiO 2 materials have been well studied with or without the use of organic

surfactant templates. 28,80,264,265,277 -312 Barbe et al. reported the preparation of a mesoporous TiO 2 film by the hydrothermal method as shown Figure 28. 80 In a typical experiment, TTIP was added dropwise to a 0.1 M nitric acid solution under vigorous stirring and at room temperature. A white precipitate formed instantaneously. Immediately after the hydrolysis, the solution was heated to 80 °C and stirred vigorously for 8 h for peptization. The solution was then filtered on a glass frit to remove agglomerates. Water was added to the filtrate to adjust the final solids concentration to ∼5 wt %. The solution was put in a titanium autoclave for 12 h at 200 -250 °C. After sonication, the colloidal suspension was put in a rotary evaporator and evaporated to a final TiO 2 concentration of 11 wt %. The precipitation pH, hydrolysis rate, autoclaving pH, and precursor chemistry were found to influence the morphology of the final TiO 2 nanoparticles. Alternative procedures without the use of hydrothermal processes have been reported by Liu et al. 292 and Zhang et al.311 In the report by Liu et al., 24.0 g of titanium(IV) n-butoxide ethanol solution (weight ratio of 1:7) was prehydrolyzed in the presence of 0.32 mL of a 0.28 M HNO 3 aqueous solution (TBT/HNO 3 ∼ 100:1) at room temperature for 3 h. 0.32 mL of deionized water was added to the prehydrolyzed solution under vigorous stirring and stirred for an additional 2 h. The sol solution in a closed vessel was kept at room temperature without stirring to gel and age. After aging for 14 days, the gel was dried at room temperature, ground into a fine powder, washed thoroughly with water and ethanol, and dried to produce porous TiO 2. Upon calcination at 450 °C for 4 h under air, crystallized mesoporous TiO2 material was obtained. 292 Yu et al. prepared three-dimensional and thermally stable mesoporous TiO2 without the use of any surfactants. 265 Briefly, monodispersed TiO2 nanoparticles were formed initially by ultrasound-assisted hydrolysis of acetic acidmodified titanium isopropoxide. Mesoporous spherical or globular particles were then produced by controlled conden-


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sation and agglomeration of these sol nanoparticles under high-intensity ultrasound radiation. The mesoporous TiO 2 had a wormhole-like structure consisting of TiO 2 nanoparticles and a lack of long-range order. 265 In the template method used by the Stucky group278-280,287,295,302,306-307,313 and other groups, 264,293,297,303,309 structure-directing agents were used for organizing networkforming metal oxide species in nonaqueous solutions. These structure-directing agents were also called organic templates. The most commonly used organic templates were amphiphilic poly(alkylene oxide) block copolymers, such as HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designated EO20PO70EO20, called Pluronic P-123) and HO(CH 2CH2O)106(CH2CH(CH3)O)70(CH2CH2O)106H (designated EO106PO70EO106, called Pluronic F-127). In a typical synthesis, poly(alkylene oxide) block copolymer was dissolved in ethanol. Then TiCl4 precursor was added with vigorous stirring. The resulting sol solution was gelled in an open Petri dish at 40 °C in air for 1 -7 days. Mesoporous TiO 2 was obtained after removing the surfactant species by calcining the as-made sample at 400 °C for 5 h in air. 306 Figure 29 shows typical TEM images of the mesoporous TiO 2. Besides triblock copolymers as structure-directing agents, diblock polymers were also used such as [CnH2n-1(OCH2CH2) yOH, Brij 56 (B56, n / y y ) 16/20)] by Sanchez et al. 285 ) 16/10) or Brij 58 (B58, n / Other surfactants employed to direct the formation of mesoporous TiO2 include tetradecyl phosphate (a 14-carbon chain) by Antonelli and Ying 277 and commercially available dodecyl phosphate by Putnam and co-workers, 298 cetyltrimethylammonium bromide (CTAB) (a cationic surfactant), 281,283,296 the recent Gemini surfactant, 294 and dodecylamine (a neutral surfactant). 304 Carbon nanotubes310 and mesoporous SBA-15286 have also been used as the skeleton for mesoporous TiO 2.

2.13. TiO2 Aerogels The study of TiO2 aerogels is worthy of special mention.314-326 The combination of sol -gel processing with supercritical drying offers the synthesis of TiO 2 aerogels with morphological and chemical properties that are not easily achieved by other preparation methods, i.e., with high surface area. Campbell et al. prepared TiO 2 aerogels by sol-gel synthesis from titanium n-butoxide in methanol with the subsequent removal of solvent by supercritical CO 2.315 For a typical synthesis process, titanium n-butoxide was added to 40 mL of methanol in a dry glovebox. This solution was combined with another solution containing 10 mL of methanol, nitric acid, and deionized water. The concentration of the titanium n-butoxide was kept at 0.625 M, and the molar ratio of water/HNO 3 /titanium n-butoxide was 4:0.1: 1. The gel was allowed to age for 2 h and then extracted in a standard autoclave with supercritical CO 2 at a flow rate of 24.6 L/h, at 343 K under 2.07 × 107 Pa for 2-3 h, resulting in complete removal of solvent. After extraction, the sample was heated in a vacuum oven at 3.4 kPa and 383 K for 3 h to remove the residual solvent and at 3.4 kPa and 483 K for 3 h to remove any residual organics. The pretreated sample had a brown color and turned white after calcination at 773 K or above. The resulting TiO 2 aerogel, after calcination at 773 K for 2 h, had a BET surface area of >200 m2 /g, contained mesopores in the range 2 -10 nm, and was of the pure anatase form. Dagan et al. found the TiO 2 aerogels obtanied by using a Ti/ethanol/H 2O/nitric acid ratio of 1:20: 3:0.08 could have a porosity of 90% and surface areas of

TEM micrographs of two-dimensional hexagonal mesoporous TiO2 recorded along the (a) [110] and (b) [001] zone axes, respectively. The inset in part a is selected-area electron diffraction patterns obtained on the image area. (c) TEM image of cubic mesoporous TiO2 accompanied by the corresponding (inset) EDX spectrum. Reprinted with permission from Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. Copyright 1999 American Chemical Society. Figure 29.

600 m2 /g, as compared to a surface area of 50 m 2 /g for TiO 2 P25.316,317 Figure 30 shows a typical SEM image of a TiO 2 aerogel with a surface area of 447 m 2 /g and an interpore structure constructed by near uniform grains of elliptical shapes with 30 nm × 50 nm axes.326



SEM image of a TiO2 aerogel. Reprinted with permission from Zhu, Z.; Tsung, L. Y.; Tomkiewicz, M. J. Phys. Chem. 1995, 99, 15945. Copyright 1995 American Chemical Society. Figure 30.

2.14. TiO2 Opal and Photonic Materials The syntheses of TiO 2 opal and photonic materials have been well studied by various groups. 327-358 Holland et al. reported the preparation of TiO 2 inverse opal from the corresponding metal alkoxides, using latex spheres as templates.334,335 Millimeter-thick layers of latex spheres were deposited on filter paper in a Buchner funnel under vacuum and soaked with ethanol. Titanium ethoxide was added dropwise to cover the latex spheres completely while suction was applied. Typical mass ratios of alkoxide to latex were between 1.4 and 3. After drying the composite in a vacuum desiccator for 3 to 24 h, the latex spheres were removed by calcination in flowing air at 575 °C for 7 to 12 h, leaving hard and brittle powder particles with 320- to 360-nm voids. The carbon content of the calcined samples varied from 0.4 to 1.0 wt %, indicating that most of the latex templates had been removed from the 3D host. Figure 31 shows an illustration of the simple synthesis of TiO 2 inverse opal and an SEM image of TiO2 inverse opals. Similar studies have also been carried out by other researchers. 327,356 Dong and Marlow prepared TiO 2 inversed opals with a skeleton-like structure of TiO 2 rods by a template-directed method using monodispersed polystyrene particles of size 270 nm.328-330,345 Infiltration of a titania precursor (Ti(i-OPr) 4 in EtOH) was followed by a drying and calcination procedure. The precursor concentration was varied from 30% to 100%, and the calcination temperature was tuned from 300 to 700 °C. A SEM picture of the TiO2 inversed opal is shown in Figure 32. 329 The skeleton structure consists of rhombohedral windows and TiO2 cylinders forming a highly regular network. The cylinders connect the centers of the former octahedral and tetrahedral voids of the opal. These voids form a CaF2 lattice which is filled with cylindrical bonds connecting the Ca and F sites. Wang et al. reported their study on the large-scale fabrication of ordered TiO 2 nanobowl arrays.354 The process starts with a self-assembled monolayer of polystyrene (PS) spheres, which is used as a template for atomic layer deposition of a TiO 2 layer. After ion-milling, toluene-etching, and annealing of the TiO 2-coated spheres, ordered arrays of nanostructured TiO 2 nanobowls can be fabricated as shown in Figure 33. Wang et al. fabricated a 2D photonic crystal by coating patterned and aligned ZnO nanorod arrays with TiO 2.355 PS spheres were self-assembled to make a monolayer mask on

(A) Schematic illustration of the synthesis of a TiO2 inversed opal. (B) SEM image of the TiO2 inversed opal. Reprinted with permission from Holland, B. T.; Blanford, C.; Stein, A. Science 1998, 281, 538 (http://www.sciencemag.org). Copyright 1998 AAAS. Figure 31.

a sapphire substrate, which was then covered with a layer of gold. After removing the PS spheres with toluene, ZnO nanorods were grown using a vapor -liquid-solid process. Finally, a TiO2 layer was deposited on the ZnO nanorods by introducing TiCl4 and water vapors into the atomic layer deposition chamber at 100 °C. Figure 34 shows SEM images of a ZnO nanorod array and the TiO 2-coated ZnO nanorod array. Li et al. reported the preparation of ordered arrays of TiO 2 opals using opal gel templates under uniaxial compression at ambient temperature during the TiO 2 sol/gel process. 337 The aspect ratio was controllable by the compression degree, R. Polystyrene inverse opal was template synthesized using silica opals as template. The silica was removed with 40 wt % aqueous hydrofluoric acid. Monomer solutions consisting of dimethylacrylamide, acrylic acid, and methylenebisacrylamide in 1:1:0.02 weight ratios were dissolved in a water/


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SEM picture of a TiO2 skeleton with a cylinder radius of about 0.06a. a is the lattice constant of the cubic unit cell. Reprinted from Dong, W.; Marlow, F. Physica E 2003, 17 , 431, Copyright 2003, with permission from Elsevier. Figure 32.

(A) SEM images of short and densely aligned ZnO nanorod array on a sapphire substrate. Inset: An optical image of the aligned ZnO nanorods over a large area. (B) SEM image of the TiO2-coated ZnO nanorod array. Reprinted with permission from Wang, X.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger, L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J. Ad V. Mater. 2005, 17 , 2103. Copyright 2005 Wiley-VCH. Figure 34.

gels with correspondingly different properties can be produced. Water was completely removed from the opal hydrogel by repeatedly rinsing it with a large amount of ethanol. Afterward, the opal gel was put into a large amount of tetrabutyl titanate (TBT) at ambient temperature for 24 h. The TBT-swollen opal gel was then immersed in a water/ ethanol (1:1 wt/wt) mixture for 5 h to let the TiO 2 sol/gel process proceed. Figure 35A shows the opal structure of the gel/titania composite spheres formed. After calcination, TiO 2 opal with distinctive spherical contours could be found. The compression degree, R, was adjusted by the spacer height when the substrates were compressed. When the substrates were slightly compressed against each other to the extent of producing a 20% reduction in the thickness of the composition opal, the deformation of the template-synthesized titania spheres was not substantial (Figure 35B). When the compression degree was increased to the point of reaching 35% deformation in the opal gel, noticeably deformed titania opals could be obtained (Figure 35C and D). (A) Experimental procedure for fabricating TiO2 nanobowl arrays. (B) Low- and high- (inset) magnification SEM image of TiO2 nanobowl arrays. Reprinted with permission from Wang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers, C. J. Nano Lett. 2004, 4, 2223. Copyright 2004 American Chemical Society. Figure 33.

ethanol mixture (4:7 wt/wt) with total monomer content 30 wt %. Ethanol was used to facilitate diffusion of the monomer solution into the inverse opal polystyrene. After the inverse opal was infiltrated by the monomer solution containing 1 wt % of the initiator AIBN and a subsequent free radical polymerization at 60 °C for 3 h, a solid composite resulted. The initial inverse opal polystyrene template was then removed with chloroform in a Soxhlet extractor for 12 h, whereupon the opal gel was formed. By using different compositions of the monomer solution, hole sizes, and stacking structures of the starting inverse opal templates, opal

2.15. Preparation of TiO2 Nanosheets The preparation of TiO 2 nanosheets has also been explored recently. 359-368 Typically, TiO2 nanosheets were synthesized by delaminating layered protonic titanate into colloidal single layers. A stoichiometric mixture of Cs 2CO3 and TiO2 was calcined at 800 °C for 20 h to produce a precursor, cesium titanate, Cs0.7Ti1.82500.175O4 (0: vacancy), about 70 g of which was treated with 2 L of a 1 M HCl solution at room temperature. This acid leaching was repeated three times by renewing the acid solution every 24 h. The resulting acidexchanged product was filtered, washed with water, and airdried. The obtained protonic titanate, H 0.7Ti1.82500.175O4‚H2O, was shaken vigorously with a 0.017 M tetrabutylammonium hydroxide solution at ambient temperature for 10 days. The solution-to-solid ratio was adjusted to 250 cm 3 g-1. This procedure yielded a stable colloidal suspension with an



SEM of the TiO2 opals. (A) A gel/titania composite opal fabricated without compressing the opal gel template during the sol/gel process. (Inset) Image of the sample after calcination at 450 °C for 3 h. (B-D) (Main panel) Oblate titania opal materials after calcination at 450 °C for 3 h, subject to compression degree R of (B) 20%, (C) 35%, and (D) 50%. The images were taken for the fractured surfaces containing the direction of applied compression. (Inset) Image of the same sample, but with the fracture surface perpendicular to the direction of applied compression. From: Ji, L.; Rong, J.; Yang, Z. Chem. Commun. 2003, 1080 (http://dx.doi.org/10.1039/b300825h) s Reproduced by permission of The Royal Society of Chemistry. Figure 35.

opalescent appearance. Figure 36 shows TEM and AFM images of TiO 2 nanosheets with thicknesses of 1.2 -1.3 nm, which is the height of the TiO 2 nanosheet with a monolayer of water molecules on both sides (0.70 + 0.25 × 2) thick.366

3. Properties of TiO 2 Nanomaterials 3.1. Structural Properties of TiO2 Nanomaterials Figure 37 shows the unit cell structures of the rutile and anatase TiO2.11 These two structures can be described in terms of chains of TiO 6 octahedra, where each Ti 4+ ion is surrounded by an octahedron of six O 2- ions. The two crystal structures differ in the distortion of each octahedron and by the assembly pattern of the octahedra chains. In rutile, the

octahedron shows a slight orthorhombic distortion; in anatase, the octahedron is significantly distorted so that its symmetry is lower than orthorhombic. The Ti -Ti distances in anatase are larger, whereas the Ti -O distances are shorter than those in rutile. In the rutile structure, each octahedron is in contact with 10 neighbor octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while, in the anatase structure, each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO2. Hamad et al. performed a theoretical calculation on Ti nO2n clusters (n ) 1-15) with a combination of simulated


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(A) TEM of Ti1-δO24δ- nanosheets. (B and C) AFM image and height scan of the TiO2 nanosheets deposited on a Si wafer. (D) Structural model for a hydrated TiO2 nanosheet. Closed, open, and shaded circles represent Ti atom, O atom, and H2O molecules, respectively. All the water sites are assumed to be half occupied. Reprinted with permission from Sasaki, T.; Ebina, Y.; Kitami, Y.; Watanabe, M.; Oikawa, T. J. Phys. Chem. B 2001, 105, 6116. Copyright 2001 American Chemical Society. Figure 36.

Lattice structure of rutile and anatase TiO2. Reprinted with permission from Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. Copyright 1995 American Chemical Society. Figure 37.

annealing, Monte Carlo basin hopping simulation, and genetic algorithms methods. 369 They found that the calculated global minima consisted of compact structures, with titanium atoms reaching high coordination rapidly as n increased. For n g 11, the particles had at least a central octahedron surrounded by a shell of surface tetrahedra, trigonal bipyramids, and square base pyramids. Swamy et al. found the metastability of anatase as a function of pressure was size dependent, with smaller crystallites preserving the structure to higher pressures. 370 Three size regimes were recognized for the pressure-induced phase transition of anatase at room temperature: an anatase -

amorphous transition regime at the smallest crystallite sizes, an anatase-baddeleyite transition regime at intermediate crystallite sizes, and an anatase -R-PbO2 transition regime comprising large nanocrystals to macroscopic single crystals. Barnard et al. performed a series of theoretical studies on the phase stability of TiO 2 nanoparticles in different environments by a thermodynamic model. 371-375 They found that surface passivation had an important impact on nanocrystal morphology and phase stability. The results showed that surface hydrogenation induced significant changes in the shape of rutile nanocrystals, but not in anatase, and that the size at which the phase transition might be expected increased dramatically when the undercoordinated surface titanium atoms were H-terminated. For spherical particles, the crossover point was about 2.6 nm. For a clean and faceted surface, at low temperatures (a phase transition pointed at an average diameter of approximately 9.3 -9.4 nm for anatase nanocrystals), the transition size decreased slightly to 8.9 nm when the surface bridging oxygens were H-terminated, and the size increased significantly to 23.1 nm when both the bridging oxygens and the undercoordinated titanium atoms of the surface trilayer were H-terminated. Below the cross point, the anatase phase was more stable than the rutile phase. 371 In their study on TiO 2 nanoparticles in vacuum or water environments, they found that the phase transition size in water (15.1 nm) was larger than that under vacuum (9.6 nm).373 In their predictions on the transition enthalpy of nanocrystalline anatase and rutile, they found that thermochemical results could differ for various faceted or spherical


Morphology predicted for anatase (top), with (a) hydrogenated surfaces, (b) hydrogen-rich surface adsorbates, (c) hydrated surfaces, (d) hydrogen-poor adsorbates, and (e) oxygenated surfaces, and for rutile (bottom), with (f) hydrogenated surfaces, (g) hydrogen-rich surface adsorbates, (h) hydrated surfaces, (i) hydrogen-poor adsorbates, and (j) oxygenated surfaces. Reprinted with permission from Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 5, 1261. Copyright 2005 American Chemical Society. Figure 38.

nanoparticles as a function of shape, size, and degree of surface passivation. 372 Their study on anatase and rutile titanium dioxide polymorphs passivated with complete monolayers of adsorbates by varying the hydrogen to oxygen ratio with respect to a neutral, water-terminated surface showed that termination with water consistently resulted in the lowest values of surface free energy when hydrated or with a higher fraction of H on the surface on both anatase and rutile surfaces, but conversely, the surfaces generally had a higher surface free energy when they had an equal ratio of H and O in the adsorbates or were O-terminated. 375 They demonstrated that, under different pH conditions from acid to basic, the phase transition size of a TiO 2 nanoparticle varied from 6.9 to 22.7 nm, accompanied with shape changes of the TiO2 nanoparticles as shown in Figure 38. 374 Enyashin and Seifert conducted a theoretical study on the structural stability of TiO 2 layer modifications (anatase and lepidocrocite) using the density-functional-based tight binding method (DFTB). 376 They found that anatase nanotubes were the most stable modifications in a comparison of singlewalled nanotubes, nanostrips, and nanorolls. Their stability increased as their radii grew. The energies for all TiO 2 nanostructures relative to the infinite monolayer followed a 1/ R2 curve. Chen et al. found that severe distortions existed in Ti site environments in the structures of 1.9 nm TiO 2 nanoparticles compared to those octahedral Ti sites in bulk anatase Ti using K-edge XANES.377 The distorted Ti sites were likely to adopt a pentacoordinate square pyramidal geometry due to the truncation of the lattice. The distortions in the TiO 2 lattice were mainly located on the surface of the nanoparticles and were responsible for binding with other small molecules. Qian et al. found that the density of the surface states on TiO2 nanoparticles was likely dependent upon the details of the preparation methods. 378 The TiO2 nanoparticles prepared from basic sol were found to have more surface states than those prepared from acidic sol based on a surface photovoltage spectroscopy study.

3.2. Thermodynamic Properties of TiO2 Nanomaterials Rutile is the stable phase at high temperatures, but anatase and brookite are common in fine grained (nanoscale) natural


Enthalpy of nanocrystalline TiO2. Reprinted with permission from Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6476. Copyright 2002 National Academy of Sciences, U.S.A. Figure 39.

and synthetic samples. On heating concomitant with coarsening, the following transformations are all seen: anatase to brookite to rutile, brookite to anatase to rutile, anatase to rutile, and brookite to rutile. These transformation sequences imply very closely balanced energetics as a function of particle size. The surface enthalpies of the three polymorphs are sufficiently different that crossover in thermodynamic stability can occur under conditions that preclude coarsening, with anatase and/or brookite stable at small particle size. 73,74 However, abnormal behaviors and inconsistent results are occasionally observed. Hwu et al. found the crystal structure of TiO 2 nanoparticles depended largely on the preparation method. 379 For small TiO2 nanoparticles ( <50 nm), anatase seemed more stable and transformed to rutile at >973 K. Banfield et al. found that the prepared TiO 2 nanoparticles had anatase and/or brookite structures, which transformed to rutile after reaching a certain particle size. 73,380 Once rutile was formed, it grew much faster than anatase. They found that rutile became more stable than anatase for particle size > 14 nm. Ye et al. observed a slow brookite to anatase phase transition below 1053 K along with grain growth, rapid brookite to anatase and anatase to rutile transformations between 1053 K and 1123 K, and rapid grain growth of rutile above 1123 K as the dominant phase. 381 They concluded that brookite could not transform directly to rutile but had to transform to anatase first. However, direct transformation of brookite nanocrystals to rutile was observed above 973 K by Kominami et al. 382 In a later study, Zhang and Banfield found that the transformation sequence and thermodynamic phase stability depended on the initial particle sizes of anatase and brookite in their study on the phase transformation behavior of nanocrystalline aggregates during their growth for isothermal and isochronal reactions. 74 They concluded that, for equally sized nanoparticles, anatase was thermodynamically stable for sizes < 11 nm, brookite was stable for sizes between 11 and 35 nm, and rutile was stable for sizes > 35 nm. Ranade et al. investigated the energetics of the TiO 2 polymorphs (rutile, anatase, and brookite) by high-temperature oxide melt drop solution calorimetry, and they found the energetic stability crossed over between the three phases as shown in Figure 39. 383 The dark solid line represents the phases of lowest enthalpy as a function of surface area. Rutile was energetically stable for surface area < 592 m2 /mol (7 m2 /g or >200 nm), brookite was energetically stable from


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592 to 3174 m2 /mol (7-40 m2 /g or 200-40 nm), and anatase was energetically stable for greater surface areas or smaller sizes (<40 nm). The anatase and rutile energetics cross at 1452 m2 /mol (18 m2 /g or 66 nm). Assuming spherical particles, the calculated average diameters of rutile and brookite for a 7 m 2 /g surface area were 201 and 206 nm, and those of brookite and anatase for a 40 m 2 /g surface area are 36 and 39 nm. These differences in particle size at the same surface area existed because of the differences in density. If the phase transformation took place without further coarsening, the particle size should be smaller after the transformation. Phase stability in a thermodynamic sense is governed by the Gibbs free energy ( ∆G ) ∆ H - T ∆S) rather than the enthalpy. Rutile and anatase have the same entropy. Thus, the T ∆S will not significantly perturb the sequence of stability seen from the enthalpies. For nanocrystalline TiO 2, if the initially formed brookite had surface area > 40 m 2 /g, it was metastable with respect to both anatase and rutile, and the sequence brookite to anatase to rutile during coarsening was energetically downhill. If anatase formed initially, it could coarsen and transform first to brookite (at 40 m 2 /g) and then to rutile. The energetic driving force for the latter reaction (brookite to rutile) was very small, explaining the natural persistence of coarse brookite. In contrast, the absence of coarse-grained anatase was consistent with the much larger driving force for its transformation to rutile.383 Li et al. found that only anatase to rutile phase transformation occurred in the temperature range of 973 -1073 K.384 Both anatase and rutile particle sizes increased with the increase of temperature, but the growth rate was different, as shown in Figure 40. Rutile had a much higher growth rate than anatase. The growth rate of anatase leveled off at 800 °C. Rutile particles, after nucleation, grew rapidly, whereas anatase particle size remained practically unchanged. With the decrease of initial particle size, the onset transition temperature was decreased. An increased lattice compression of anatase with increasing temperature was observed. Larger distortions existed in samples with smaller particle size. The values for the activation energies obtained were 299, 236, and 180 kJ/mol for 23, 17, and 12 nm TiO 2 nanoparticles, respectively. The decreased thermal stability in finer nanoparticles was primarily due to the reduced activation energy as the size-related surface enthalpy and stress energy increased.

3.3. X-ray Diffraction Properties of TiO2 Nanomaterials XRD is essential in the determination of the crystal structure and the crystallinity, and in the estimate of the crystal grain size according to the Scherrer equation D )

K λ

β cos θ

(3)

where K is a dimensionless constant, 2θ is the diffraction angle, λ is the wavelength of the X-ray radiation, and β is the full width at half-maximum (fwhm) of the diffraction peak.385 Crystallite size is determined by measuring the broadening of a particular peak in a diffraction pattern associated with a particular planar reflection from within the crystal unit cell. It is inversely related to the fwhm of an individual peaksthe narrower the peak, the larger the crystallite size. The periodicity of the individual crystallite

(A) Changes in particle sizes of anatase and rutile phases as a function of the annealing temperatures. (B) Arrenhius plot of ln( AR / A0) vs 1/ T for activation energy calculations as a function of the size of the TiO2 nanoparticles. AR and A0 are the integrated diffraction peak intensity from rutile (110), and the total integrated anatase (101) and rutile (110) peak intensity, respectively. Reused with permission from W. Li, C. Ni, H. Lin, C. P. Huang, and S. Ismat Shah, Journal of Applied Physics , 96 , 6663 (2004). Copyright 2004, American Institute of Physics. Figure 40.

domains reinforces the diffraction of the X-ray beam, resulting in a tall narrow peak. If the crystals are randomly arranged or have low degrees of periodicity, the result is a broader peak. This is normally the case for nanomaterial assemblies. Thus, it is apparent that the fwhm of the diffraction peak is related to the size of the nanomaterials. Figure 41 shows the XRD patterns for TiO 2 nanoparticles of different sizes 111 and for TiO 2 nanorods of different lengths. 129 As the nanoparticle size increased, the diffraction peaks became narrower. In the anatase nanoparticle and nanorods developed by Zhang et al., the diameters of the TiO2 nanoparticles and nanorods were both around 2.3 nm. The nanorods were elongated along the [001] direction with preferred anisotropic growth along the c-axis of the anatase lattice, which was indicated by the strong peak intensity and narrow width of the (004) reflection and relatively lower intensity and broader width for the other reflections. With an increase in length of the nanorods, the (004) diffraction peak became much stronger and sharper, whereas other peaks remained similar in shape and intensity. 129 Similar results have been observed by other groups. 123,127,177,183

3.4. Raman Vibration Properties of TiO2 Nanomaterials As the size of TiO 2 nanomaterials decreases, the featured Raman scattering peaks become broader. 255,318,370,386-395 The size effect on the Raman scattering in nanocrystalline TiO2 is interpreted as originating from phonon confine-



selection rule for the excitation of Raman active optical phonons with long-range order and crystallite size. 318,370 In a perfect “infinite” crystal, conservation of phonon momentum requires that only optic phonons near the Brillouin zone (BZ) center (q ≈ 0) are involved in first-order Raman scattering. In an amorphous material lacking long-range order, the q vector selection rule breaks down and the Raman spectrum resembles the phonon density of states. For nanocrystals, the strict “infinite” crystal selection rule is replaced by a relaxed version. This results in a range of accessible q vectors (as large as ∆q ≈ 1/ L ( L diameter)) due to the uncertainty principle. The anatase TiO2 has six Raman-active fundamentals in the vibrational spectrum: three E g modes centered around 144, 197, and 639 cm-1 (designated here Eg(1), Eg(2), and Eg(3), respectively), two B 1g modes at 399 and 519 cm -1 (designated B1g(1) and B1g(2d)), and an A1g mode at 513 cm -1.370 As the particle size decreases, the Raman peaks show increased broadening and systematic frequency shifts (Figure 42).370 The most intense Eg(1) mode shows the maximum blue shift and significant broadening with decreasing crystallite size. A small blue shift is seen for the E g(2) mode, while the B1g(1) mode and the B 1g(2)+A1g modes show very small blue shifts and red shifts (the latter peak represents a combined effect of two individual modes), respectively. Whereas the frequency shifts for the A 1g and B1g modes are not pronounced, increased broadening with decreasing crystallite size is clearly seen for these modes. The E g(3) mode shows significant broadening and a red shift with decreasing crystallite size. Choi et al. found a volume contraction effect in anatase TiO2 nanoparticles due to increasing radial pressure as particle size decreases, and they suggested that the effects of decreasing particle size on the force constants and vibrational amplitudes of the nearest neighbor bonds contributed to both broadening and shifts of the Raman bands with decreasing particle diameter. 388

(A) Powder XRD patterns of TiO 2 samples of different diameters: (a) 5 nm; (b) 7 nm; (c) 13 nm. Reprinted with permission from Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater. 2002, 14, 4364. Copyright 2002 American Chemical Society. (B) Powder XRD patterns of TiO2 samples of diameter 2.3 nm: (a) spherical particles; (b) 16-nm nanorods; (c) 30-nm nanorods. Reprinted with permission from Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright 2005 Wiley-VCH. Figure 41.

ment,255,318,370,386,387,395 nonstoichiometry,391,392 or internal stress/surface tension effects. 390 Among these theories, the most convincing is the three-dimensional confinement of phonons in nanocrystals.255,318,370,386,387,394,395 The phonon confinement model is also referred to as the spatial correlation model or q vector relaxation model. It links the q vector

3.5. Electronic Properties of TiO2 Nanomaterials The DOS of TiO2 is composed of Ti e g, Ti t2g (d yz, d zx, and d xy), O pσ (in the Ti3O cluster plane), and O p π (out of the Ti3O cluster plane), as shown in Figure 43A. 396 The upper valence bands can be decomposed into three main regions: the σ bonding in the lower energy region mainly due to O pσ bonding; the π bonding in the middle energy region; and O pπ states in the higher energy region due to O p π nonbonding states at the top of the valence bands where the hybridization with d states is almost negligible. The contribution of the π bonding is much weaker than that of the σ bonding. The conduction bands are decomposed into Ti e g (>5 eV) and t2g bands (<5 eV). The d xy states are dominantly located at the bottom of the conduction bands (the vertical dashed line in Figure 43A). The rest of the t 2g bands are antibonding with p states. The main peak of the t 2g bands is identified to be mostly d yz and d zx states. In the molecular-orbital bonding diagram in Figure 43B, a noticeable feature can be found in the nonbonding states near the band gap: the nonbonding O pp orbital at the top of the valence bands and the nonbonding d xy states at the bottom of the conduction bands. A similar feature can be seen in rutile; however, it is less significant than in anatase. 397 In rutile, each octahedron shares corners with eight neighbors and shares edges with two other neighbors, forming a linear chain. In anatase, each octahedron shares corners with four


Chen and Mao

(A) Ambient pressure Raman spectra of anatase with an average crystallite size of 4 ( 1 nm (A), 8 ( 2 nm (B), 20 ( 8 nm (C), and 34 ( 5 nm (D). The spectrum marked “E” is from a bulk anatase. (B) The Raman line width (fwhm) of the E g(1) mode versus crystallite size. Reprinted with permission from Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle, B. C. Phys. ReV. B 2005, 71, 184302/1 (http://link.aps.org/abstract/PRB/v71/p184302). Copyright 2005 by the American Physical Society. Figure 42.

neighbors and shares edges with four other neighbors, forming a zigzag chain with a screw axis. Thus, anatase is less dense than rutile. Also, anatase has a large metal -metal distance of 5.35 Å. As a consequence, the Ti d xy orbitals at the bottom of the conduction band are quite isolated, while the t2g orbitals at the bottom of the conduction band in rutile provide the metal -metal interaction with a smaller distance of 2.96 Å. The electronic structure of TiO 2 has been studied with various experimental techniques, i.e., with X-ray photoelectron and X-ray absorption and emission spectroscopies.379,398-405 Figure 44 shows a schematic energy level diagram of the lowest unoccupied MOs of a [TiO 6]8- cluster with Oh , D2h (rutile), and D2d (anatase) symmetry and the Ti K-edge XANES and O K-edge ELNES spectra for rutile and anatase.398 The anatase structure is a tetragonally distorted octahedral structure in which every titanium cation is

surrounded by six oxygen atoms in an elongated octahedral geometry ( D2d ). The further splitting of the 3d levels of Ti 3+ due to the asymmetric crystals is shown for rutile and anatase structures. The fine electronic structure of TiO 2 can be directly probed by Ti K-edge X-ray-absorption near-edge structure (XANES), and the right panel of Figure 44B contains O K-edge experimental electron-energy-loss nearedge structure (ELNES) spectra. 398 Hwu et al. found that the crystal field splitting of nanocrystal TiO2 was approximately 2.1 eV, slightly smaller than that of bulk TiO 2, as shown in Figure 45A. 379 Luca et al. found that 1s f np transitions broadened as particle size (increased or decreased) in the postedge region in the X-ray absorption spectroscopy for TiO 2 nanoparticles.403 Also, a clear trend in the X-ray absorption spectroscopy for different sized TiO2 nanoparticles was observed, as shown in Figure 45B from the study by Choi et al. 401



nanoparticle falls below the Bohr radius of the first excitation state or becomes comparable to the de Broglie wavelength of the charge carriers, the charge carriers begin to behave quantum mechanically and the charge confinement leads to a series of discrete electronic states. 408 However, there is a discrepancy in this critical size below which quantization effects are observed for TiO 2 nanomaterials with indirect band gaps. The estimated critical diameter depends critically on the effective masses of the charge carriers. 409 Kormann et al. estimated the excitation radii for titania particles to be between 7.5 and 19 Å. 84 Quantum confinement size effects were observed for TiO 2 nanoparticles with a small apparent band gap blue shift ( <0.1-0.2 eV) caused by quantum size effects for spherical particles sizes down to 2 nm. 58,60 Such small effects are mainly due to the relatively high effective mass of carriers in TiO2 and an exciton radius in the approximate range 0.75 -1.90 nm.84 On the other hand, Serpone et al. suggested that the blue shifts in the effective band gap of TiO2 with particle sizes of 21, 133, and 267 Å may in fact not be a quantum confinement effect. 410 Monticone et al. did an excellent study on the quatum size effects in anatase nanoparticles and found no quantum size effect in anatase TiO 2 nanoparticles for sizes 2 R g 1.5 nm, but they did find unusual variation of the oscillator strength of the first allowed direct transition with particle size. 411

3.6. Optical Properties of TiO2 Nanomaterials

(A) Total and projected densities of states (DOSs) of the anatase TiO2 structure. The DOS is decomposed into Ti eg, Ti t2g (d yz, d zx, and d xy), O pσ (in the Ti3O cluster plane), and O pπ (out of the Ti3O cluster plane) components. The top of the valence band (the vertical solid line) is taken as the zero of energy. The vertical dashed line indicates the conduction-band minimum as a guide to the eye. (B) Molecular-orbital bonding structure for anatase TiO2: (a) atomic levels; (b) crystal-field split levels; (c) final interaction states. The thin-solid and dashed lines represent large and small contributions, respectively. Reprinted with permission from Asahi, R.; Taga, Y.; Mannstadt, W.; Freeman, A. J. Phys. ReV. B 2000, 61, 7459 (http://link.aps.org/abstract/PRB/v61/p7459). Copyright 2000 by the American Physical Society. Figure 43.

It is well-known that for nanoparticles the band gap energy increases and the energy band becomes more discrete with decreasing size.84,406,407 As the size of a semiconductor

The main mechanism of light absorption in pure semiconductors is direct interband electron transitions. This absorption is especially small in indirect semiconductors, e.g., TiO2, where the direct electron transitions between the band centers are prohibited by the crystal symmetry. Braginsky and Shklover have shown the enhancement of light absorption in small TiO 2 crystallites due to indirect electron transitions with momentum nonconservation at the interface.412 This effect increases at a rough interface when the share of the interface atoms is larger. The indirect transitions are allowed due to a large dipole matrix element and a large density of states for the electron in the valence band. Considerable enhancement of the absorption is expected in small TiO2 nanocrystals, as well as in porous and microcrystalline semiconductors, when the share of the interface atoms is sufficiently large. A rapid increase in the absorption takes place at low ( hν < E g + W c, where W c is the width of the conduction band) photon energies. Electron transitions to any point in the conduction band become possible when hν ) E g + W c. Further enhancement of the absorption occurs due to an increase of the electron density of states in only the valence band. The interface absorption becomes the main mechanism of light absorption for the crystallites that are smaller than 20 nm. 412 Sato and Sakai et al. showed through calculation and measurement that the band gap of TiO 2 nanosheets was larger than the band gap of bulk TiO 2, due to lower dimensionality, i.e., a 3D to 2D transition, as shown in Figure 46. 360,413 From the measurement, it was found that the lower edge of the conduction band for the TiO 2 nanosheet was approximately 0.1 V higher, while the upper edge of the valence band was 0.5 V lower than that of anatase TiO 2.360 The absorption of the TiO2 nanosheet colloid blue shifted ( >1.4 eV) relative to that of bulk TiO 2 crystals (3.0-3.2 eV), due to a sizequantization effect, accompanied with a strong photoluminescence of well-developed fine structures extending into the visible light regime. 362,363 The band gap energy shift, ∆ E g,


Chen and Mao

(A) Schematic energy level diagram of the lowest unoccupied MOs of a [TiO6]8- cluster with Oh, D2h (rutile), and D2d (anatase) symmetry. (B) Ti K-edge XANES and O K-edge ELNES spectra for rutile (a) and anatase (b). Reprinted with permission from Wu, Z. Y.; Ouvrared, G.; Gressier, P.; Natoli, C. R. Phys. ReV. B 1997, 55, 10382 (http://link.aps.org/abstract/PRB/v55/p10382). Copyright 1997 by the American Physical Society. Figure 44.

by exciton confinement in anisotropic two-dimensional crystallites is formulated as follows: 2

∆ E g )

2

1 1 h h + + 2 2 8 µ xz L x L z 8 µ y L y2

(

)

(4)

where h is Plank’s constant, µ xz and µ y are the reduced effective masses of the excitons, and L x, L y, and L z are the

crystallite dimensions in the parallel and perpendicular directions with respect to the sheet, respectively. Since the first term can be ignored, the blue shift is predominantly governed by the sheet thickness. The onset of a 270 nm peak in the photoluminescence of TiO 2 nanosheets was assigned to resonant luminescence. The series of peaks extending into a longer wavelength region were attributed to interband levels generated by the intrinsic Ti site vacancies. The contrasting



sharp peaks were also attributed to the subnanometer thickness and its uniformity. 362 Bavykin et al. studied the optical absorption and photoluminescence of colloidal TiO 2 nanotubes with internal diameter in the range of 2.5 -5 nm, and they found that, in spite of the different diameters, all the TiO 2 nanotubes had similar optical properties. 158 They attributed this to the complete smearing of all 1-dimensional effects due to the large effective mass of charge carriers in TiO 2, which resulted in an apparent 2D behavior of TiO 2 nanotubes. Figure 47 shows the absorption, photoluminescence, and luminescence excitation spectra of TiO 2 nanotubes of different mean diameters. 158 Within the effective mass model, the energy spectrum of 2D TiO2 nanosheets can be described by eq 10, where the “plus” and “minus” signs correspond to the conduction and valence bands, respectively, E G is the energy gap, p is Planck’s constant, and me and mh are the effective masses of the electrons and holes, respectively. (

E 2D ) (

E G

2

(

2 2 p k

(5)

2me ,h

The electronic band structure of a TiO 2 nanotube can be obtained from this relation by zone-folding and is given by a series of quasi-1D sub-bands with different indices n (Figure 48b): (

E n1D ) (

E G

2

(

p

2

2me ,h [

2

k | +

2n 2

( d ) ]

(6)

This transition from the 2D to the quasi-1D energy spectrum has a dramatic effect on the energy density of states. In the 2D case, the density of states, G2D ) mc.h / πp 2, has a constant value for energies outside the energy gap (see Figure 48c). In the quasi-1D case, however, the density of states of each sub-band

{2

Gn,1D( E ) ) (

me ,h

π 2p2[ E - E n(0)]

1/2

}

(7)

diverges at the band edge E n(0), leading to van Hove singularities. The resulting density of state is formed by a series of sharp peaks with long overlapping tails (Figure 48c). The energy gap between the valence and conductance bands in the quasi-1D case is larger than that in the parental 2D material, and the difference increases with decreasing diameter of the nanotube. The change in the energy gaps between a nanosheet and a nanotube is

2p2 1 1 1D 2D ∆ E G ) E G - E G ) + 2 m mh d e

(

)

(A) Ti L2.3 absorption of nanocrystal and bulk TiO2. Reprinted from Hwu, Y.; Yao, Y. D.; Cheng, N. F.; Tung, C. Y.; Lin, H. M. Nanostruct. Mater. 1997, 9, 355, Copyright 1997, with permission from Elsevier. (B) Ti L2.3 absorption of TiO2 nanocrystals with different sizes. Reprinted with permission from Choi, H. C.; Ahn, H. J.; Jung, Y. M.; Lee, M. K.; Shin, H. J.; Kim, S. B.; Sung, Y. E. Appl. Spectrosc. 2004, 58, 598. Copyright 2004 Society for Applied Spectroscopy. Figure 45.

(8)

In TiO2, the effective masses of electrons me can vary between 5m0 and 30m0, and the mass of holes mh is more than 3m0. With me ) 9m0 and mh ) 3m0, the difference between energy gaps of nanotubes with diameters 2.5 and 5 nm is 8 meV. The energy difference between the two first peaks in the density of states G1D( E ) (Figure 48) is less than 24 meV for d ) 2.5 nm and 6 meV for d ) 5 nm, which are too small to be resolved in room-temperature experiments due to the thermal fluctuations of kT ) 26 meV.158 In the theoretical study conducted by Enyashin and Seifert recently, the band structures for anatase nanotubes, nanostrips, and nanorolls were similar to the DOS of the

corresponding bulk phase. 376 The valence band of both bulk TiO2 and their nanostructures was composed of 3d Ti -2p O states, and the lower part of the conduction band was formed by 3d Ti states. The differences between these nanostructures were insignificant. All anatase systems were semiconductors with a wide direct band gap ( ∼4.2 eV), while the lepidocrocite nanotubes were semiconductors with an indirect band gap ( ∼4.5 eV). Independent from the specific topology of the titania nanostructures, the band gap approached the band gap of the corresponding nanocrystals with radii of about 25 Å. 376 In addition to the above investigation on the bulk electronic structures for various TiO 2 nanomaterials, Mora-Sero´ and Bisquert investigated the Fermi level of surface states in TiO 2 nanoparticles by the nonequilibrium steady-state statistics of electrons. 414 They found that the electrons trapped in surface states did not generally equilibrate to the free electrons’ Fermi level, E Fn, and a distinct Fermi level for surface states, E Fs, could be defined consistent with Fermi -Dirac statistics, determining the surface states’ occupancy far from equilibrium. The difference between the free electrons’ Fermi level


Chen and Mao

(A) Total and partial densities of states for (a) stacked TiO2 sheets, (b) a single-layered TiO2, (c) rutile, and (d) anatase. Reprinted with permission from Sato, H.; Ono, K.; Sasaki, T.; Yamagishi, A. J. Phys. Chem. B 2003, 107 , 9824. Copyright 2003 American Chemical Society. (B) Schematic illustration of electronic band structure: (a) TiO2 nanosheets; (b) anatase. Reprinted with permission from Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126 , 5851. Copyright 2004 American Chemical Society. (C) UV-visible spectra of (a) TiO2 sheets and (b) a film of nanosheets on a SiO 2 glass substrate. The data for the colloidal suspension is denoted by a dashed trace. Reprinted with permission from Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. Copyright 1997 American Chemical Society. Figure 46.

and the surface Fermi level ( ∆ E Fn - E Fs) was found to depend on the rate constants for charge transfer and detrapping and could reach several hundred millielectronvolts.414

3.7. Photon-Induced Electron and Hole Properties of TiO2 Nanomaterials After TiO2 nanoparticles absorb, impinging photons with energies equal to or higher than its band gap ( >3.0 eV), electrons are excited from the valence band into the unoccupied conduction band, leading to excited electrons in the conduction band and positive holes in the valence band. These charge carriers can recombine, nonradiatively or radiatively (dissipating the input energy as heat), or get trapped and react with electron donors or acceptors adsorbed on the surface of the photocatalyst. The competition between these processes determines the overall efficiency for various

applications of TiO2 nanoparticles. These fundamental processes can be expressed as follows: 415

TiO2 + hυ 98e- + h+

(9)

e- + Ti(IV)O-H f Ti(III)O-H-(X)

(10)

h+ + Ti(IV)O-H f Ti(IV)O•-H+(Y)

(11)

1 1 h+ + O2-lattice T O2(g) + vacancy 2 4

(12)

e-| + O2 ,s f O2 ,s-

(13)



O2 ,s- + H+ T HO2 ,s

(14)

h+ + Ti(III)O-H- f Ti(IV)O-H

(15)

e- + Ti(IV)O•-H+ f Ti(IV)O-H

(16)

O2 ,s + Ti(IV)O•-H+ f Ti(IV)O-H + O2 ,s (17) Reaction 8 is the photon absorption process. Reactions 10 14 are photocatalytic redox pathways, whereas reactions 15 17 represent the recombination channels. Reactions 11 and 12 are the competition pathways for holes, leading to bound OH radicals and O vacancies, respectively. The reverse of reaction 12 generates O adatom intermediates upon exposing defective surfaces to O 2-(g).415 Electrons and holes generated in TiO2 nanoparticles are localized at different defect sites on the surface and in the bulk. Electron paramagnetic resonance (EPR) results showed that electrons were trapped as two Ti(III) centers, while the holes were trapped as oxygen-centered radicals covalently linked to surface titanium atoms. 416-419 Howe and Gra¨tzel found that irradiation at 4.2 K in vacuo produced electrons trapped at Ti 4+ sites within the bulk and holes trapped at lattice oxide ions immediately below the surface, which decayed rapidly in the dark at 4.2 K. In the presence of O 2, trapped electrons were removed and the trapped holes were stable to 77 K. Warming to room-temperature caused loss of trapped holes and formation of O 2- at the surface. 416,417 Hurum et al. found that, upon band gap illumination, holes appeared at the surface and preferentially recombined with electrons in surface trapping sites for mixed-phase TiO 2, such as Degussa P25, and recombination reactions were dominated by surface reactions that followed charge migration. 419 Colombo and Bowman studied the charge carrier dynamics of TiO2 nanoparticles with femtosecond time-resolved diffuse reflectance spectroscopy and found a dramatic increase in the population of trapped charge carriers within the first few picoseconds. 420,421 Skinner et al. found that the trapping time for photogenerated electrons on 2 nm TiO 2 nanoparticles in acetonitrile by ultrafast transient absorption was about 180 fs.422 Serpone et al. found that localization (trapping) of the electron as a Ti 3+ species occurred with a time scale of about 30 ps and about 90% or more of the photogenerated electron/ hole pairs recombined within 10 ns. 409 They suggested that photoredox chemistry occurring at the particle surface emanated from trapped electrons and trapped holes rather than from free valence band holes and conduction band electrons. Bahnemann et al. found that, in 2.4 nm TiO 2 nanoparticles, electrons were instantaneously trapped within the duration of the laser flash (20 ns). Deeply trapped holes were rather long-lived and unreactive, and shallowly trapped holes were in a thermally activated equilibrium with free holes which exhibited a very high oxidation potential. 423 Szczepankiewicz and Hoffmann et al. found that O 2 was an efficient scavenger of conduction band electrons at the gas/solid interface and the buildup of trapped carriers eventually resulted in extended surface reconstruction involving Ti-OH functionalities. 415 They found that photogenerated free conduction band electrons were coupled with acoustic phonons in the lattice and their lifetimes were lengthened when dehydrated. 424 The photoexcited charge carriers in TiO2 nanoparticles produced Stark effect intensity and wavelength shifts for surface TiO -H stretching vibrations. Although deep electron-trapping states affected certain

(A) (a) Absorption spectrum and (b) luminescence excitation spectrum (wavelength of emission light is 400 nm) of colloidal TiO2 nanotubes of different mean diameters: (1) 2.5 nm; (2) 3.1 nm; (3) 3.5 nm; (4) 5 nm. The curves are shifted vertically for clarity. (B) Photoluminescence spectra of colloidal TiO2 nanotubes of different mean diameters: (1) 2.5 nm; (2) 3.1 nm; (3) 3.5 nm; (4) 5 nm. Room temperature, excitation wavelength 237 nm, slits width 5 nm. The range of wavelengths, 455-490 nm, in the spectra is omitted due to the high signal of the second harmonic from scattered excitation light. The curves are shifted vertically for clarity. Vertical lines (5) show the positions of the peaks in the PL spectrum of the nanosheets. Reprinted with permission from Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A. V.; Lapkin, A. A.; Walsh, F. C. J. Phys. Chem. B 2005, 109, 8565. Copyright 2005 American Chemical Society. Figure 47.

types of TiO -H stretch, shallow electron-trapping states produced a homogeneous electric field and were suggested not to be associated with localized structures, but rather delocalized across the TiO 2 surface.424 Berger et al. studied UV light-induced electron -hole pair excitations in anatase TiO 2 nanoparticles by electron paramagnetic resonance (EPR) and IR spectroscopy.425 The localized states such as holes trapped at oxygen anions (O -)


Chen and Mao

Schematic presentation of the transformation of the electron band structure of the nanosheet semiconductor accompanying the formation of nanotubes: (a) band diagram of a 2-dimensional nanosheet; (b) band diagram of quasi-1-D nanotubes; (c) energy density of states for nanosheets (G2D) and nanotubes (G1D). E G1D and E G2D are the band gaps of the 1D and 2D structures, respectively. k x and k y are the wave vectors. Reprinted with permission from Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A. V.; Lapkin, A. A.; Walsh, F. C. J. Phys. Chem. B 2005, 109, 8565. Copyright 2005 American Chemical Society. Figure 48.

trapped at localized sites, giving paramagnetic Ti 3+ centers, or remained in the conduction band as EPR silent species which may be observed by their IR absorption and that the EPR-detected holes produced by photoexcitation were O species, produced from lattice O 2- ions. It was also found that, under high-vacuum conditions, the majority of photoexcited electrons remained in the conduction band. At 298 K, all stable hole and electron states were lost.

4. Modifications of TiO 2 Nanomaterials

Scheme of UV-induced charge separation in TiO2. Electrons from the valence band can either be trapped (a) by defect states, which are located close to the conduction band (shallow traps), or (b) in the conduction band, where they produce absorption in the IR region. Electron paramagnetic resonance spectroscopy detects both electrons in shallow traps, Ti3+, and hole centers, O-. Reprinted with permission from Berger, T.; Sterrer, M.; Diwald, O.; Knoezinger, E.; Panayotov, D.; Thompson, T. L.; Yates, J. T., Jr. J. Phys. Chem. B 2005, 109, 6061. Copyright 2005 American Chemical Society. Figure 49.

and electrons trapped at coordinatively unsaturated cations (Ti3+ formation) were accessible to EPR spectroscopy. Delocalized and EPR silent electrons in the conduction band may be traced by their IR absorption, which results from their electronic excitation within the conduction band in the infrared region (Figure 49). They found that, during continuous UV irradiation, photogenerated electrons were either

Many applications of TiO2 nanomaterials are closely related to its optical properties. However, the highly efficient use of TiO2 nanomaterials is sometimes prevented by its wide band gap. The band gap of bulk TiO 2 lies in the UV regime (3.0 eV for the rutile phase and 3.2 eV for the anatase phase), which is only a small fraction of the sun’s energy ( <10%), as shown in Figure 50. 11 Thus, one of the goals for improvement of the performance of TiO2 nanomaterials is to increase their optical activity by shifting the onset of the response from the UV to the visible region.21,426-428 There are several ways to achieve this goal. First, doping TiO 2 nanomaterials with other elements can narrow the electronic properties and, thus, alter the optical properties of TiO 2 nanomaterials. Second, sensitizing TiO 2 with other colorful inorganic or organic compounds can improve its optical activity in the visible light region. Third, coupling collective oscillations of the electrons in the conduction band of metal nanoparticle surfaces to those in the conduction band of TiO 2 nanomaterials in metal -TiO2 nanocomposites can improve the performance. In addition, the modification of the TiO 2 nanomaterials surface with other semiconductors can alter the charge-transfer properties between TiO2 and the surrounding environment, thus im-


Solar spectrum at sea level with the sun at its zenith. Reprinted with permission from Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. Copyright 1995 American Chemical Society. Figure 50.

proving the performance of TiO 2 nanomaterials-based devices.

4.1. Bulk Chemical Modification: Doping The optical response of any material is largely determined by its underlying electronic structure. The electronic properties of a material are closely related to its chemical composition (chemical nature of the bonds between the atoms or ions), its atomic arrangement, and its physical dimension (confinement of carriers) for nanometer-sized materials. The chemical composition of TiO 2 can be altered by doping. Specifically, the metal (titanium) or the nonmetal (oxygen) component can be replaced in order to alter the material’s optical properties. It is desirable to maintain the integrity of the crystal structure of the photocatalytic host material and to produce favorable changes in electronic structure. It appears easier to substitute the Ti 4+ cation in TiO 2 with other transition metals, and it is more difficult to replace the O 2anion with other anions due to differences in charge states and ionic radii. The small size of the nanoparticle is beneficial for the modification of the chemical composition of TiO 2 due to the higher tolerance of the structural distortion than that of bulk materials induced by the inherent lattice strain in nanomaterials. 426,429 4.1.1. Synthesis of Doped TiO 2 Nanomaterials

Different metals have been doped into TiO 2 nanomaterials.313,430-465 The preparation methods of non-metal-doped TiO 2 nanomaterials can be divided into three types: wet chemistry, hightemperature treatment, and ion implantation on TiO 2 nanomaterials. Wet chemistry methods usually involve hydrolysis of a titanium precursor in a mixture of water and other reagents, followed by heating. Choi et al. performed a systematic study of TiO 2 nanoparticles doped with 21 metal ions by the sol -gel method and found the presence of metal ion dopants significantly influenced the photoreactivity, charge carrier recombination rates, and interfacial electrontransfer rates. 434 Li et al. developed La 3+-doped TiO2 by the sol-gel process and found that the lanthanum doping could inhibit the phase transformation of TiO 2, enhance the thermal stability of the TiO 2, reduce the crystallite size, and increase the Ti3+ content on the surface. 442 Nagaveni et al. prepared W, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO 2 nanoparticles by a solution combustion method and found that the solid solution formation was limited to a narrow range of concentrations of the dopant ions. 448 Wang et al. prepared 4.1.1.1. Metal-Doped TiO2 Nanomaterials.


Nd3+-doped and Fe(III)-doped TiO 2 nanoparticles with a hydrothermal method and found that anatase, brookite, and a trace of hematite coexisted at lower pH (1.8 and 3.6) when the Fe(III) content was as low as 0.5% and the distribution of iron ions was nonuniform between particles, but at higher pH (6.0), the uniform solid solution of iron -titanium oxide formed.460,463 Anpo et al. prepared TiO 2 nanoparticles doped with Cr and V ions with an ion-implantation method. 466-471 Bessekhouad et al. investigated alkaline (Li, Na, K)-doped TiO 2 nanoparticles prepared by sol -gel and impregnation technology and found that the crystallinity level of the products was largely dependent on both the nature and the concentration of the alkaline, with the best crystallinity obtained for Lidoped TiO2 and the lowest for K-doped TiO 2.430 Cao et al. prepared Sn4+-doped TiO2 nanoparticle films by the plasmaenhanced CVD method and found that, after doping by Sn, more surface defects were present on the surface. 433 Gracia et al. synthesized M (Cr, V, Fe, Co)-doped TiO 2 by ion beam induced CVD and found that TiO 2 crystallized into the anatase or rutile structures depending on the type and amount of cations present with partial segregation of the cations in the form of M2On after annealing. 438 Wang et al. synthesized Fe(III)-doped TiO 2 nanoparticles using oxidative pyrolysis of liquid-feed organometallic precursors in a radiationfrequency (RF) thermal plasma and found that the formation of rutile was strongly promoted with iron doping compared to the anatase phase being prevalent in the undoped TiO 2.246 4.1.1.2. Nonmetal-Doped TiO2 Nanomaterials. Various nonmetal elements, such as B, C, N, F, S, Cl, and Br, have been successfully doped into TiO 2 nanomaterials. C-doped TiO2 nanomateirals have been obtained by heating titanium carbide472-474 or by annealing TiO 2 under CO gas flow at high temperatures (500 -800 °C)475 or by direct burning of a titanium metal sheet in a natural gas flame. 476 N-doped TiO2 nanomaterials have been synthesized by hydrolysis of TTIP in a water/amine mixture and the posttreatment of the TiO 2 sol with amines426,428,477-482 or directly from a Ti-bipyridine complex 483 or by ball milling of TiO 2 in a NH3 water solution.484 N-doped TiO2 nanomaterials were also obtained by heating TiO 2 under NH3 flux at 500-600 °C485,486 or by calcination of the hydrolysis product of Ti(SO4)2 with ammonia as precipitator487 or by decomposition of gas-phase TiCl 4 with an atmosphere microwave plasma torch488 or by sputtering/ion-implanting techniques with nitrogen489,490 or N2+ gas flux.491 S-doped TiO2 nanomaterials were synthesized by mixing TTIP with ethanol containing thiourea 492-494 or by heating sulfide powder495,496 or by using sputtering or ion-implanting techniques with S + ion flux.497-499 Different doping methods can induce the different valence states of the dopants. For example, the incorporated S from thiourea had S 4+ or S6+ state,492-494 while direct heating of TiS2 or sputtering with S+ induced the S2- anion.496-499 F-doped TiO2 nanomaterials were synthesized by mixing TTIP with ethanol containing H 2O-NH4F,500-502 or by heating TiO2 under hydrogen fluoride503,504 or by spray pyrolysis from an aqueous solution of H 2TiF6505,506 or using ion-implanting techniques with F + ion flux.507 Cl- and Brco-doped nanomaterials were synthesized by adding TiCl 4 to ethanol containing HBr. 508 4.1.2. Properties of Doped TiO 2 Nanomaterials 4.1.2.1. Electronic Properties of Doped TiO 2 Nanomaterials. 4.1.2.1.1. Metal-Doped TiO2 Nanomaterials. Ac-


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(A) Bonding diagram of TiO2. (B) DOS of the metal-doped TiO2 (Ti1- xA xO2: A ) V, Cr, Mn, Fe, Co, or Ni). Gray solid lines: total DOS. Black solid lines: dopant’s DOS. The states are labeled (a) to (j). Reprinted from Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2002, 63, 1909, Copyright 2002, with permission from Elsevier. Figure 51.

cording to Soratin and Schwarz’s study, the electronic states of TiO2 can be decomposed into three parts: the σ bonding of the O pσ and Ti eg states in the lower energy region; the π bonding of the O p π and Ti eg states in the middle energy region; and the O p π states in the higher energy region (Figure 51A).397,509 The bottom of the lower conduction band (CB) consisting of the Ti d xy orbital contributes to the metal metal interactions due to the σ bonding of the Ti t 2g-Ti t 2g states. At the top of the lower CB, the rest of the Ti 2g states are antibonding with the O p π states. The upper CB consists of the σ antibonding orbitals between the O p σ and Ti eg states. The electronic structures, i.e., the densities of states (DOSs), of V-, Cr-, Mn-, Fe-, Co-, and Ni-doped TiO 2 were analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized augmented plane wave (FLAPW) method by Umebayashi et al. (Figure 51B).509 They found that when TiO 2 was doped with V, Cr, Mn, Fe, or Co, an electron occupied level formed and the electrons were localized around each dopant. As the atomic number of the dopant increased, the localized level shifted to lower energy. The energy of the localized level due to Co doping was low enough that it lay at the top of the valence band while the other metals produced midgap states. The electrons from the Ni dopant were somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. The states due to the 3d dopants shifted to a lower energy as the atomic number of the dopant increased. For Ti 1- xV xO2: two localized levels occurred at 1.5 eV above the VB (a) and between the lower and upper CBs (b). Level a was occupied by one electron consisting of the V t 2g and O pπ states and was localized around V. Level b consisted of the V e g and O pσ states forming the σ antibonding orbital. For Cr- and Mn-doped TiO2, state c was localized at 1.0 eV (0.5 eV for Mn) above the VB due to Cr (Mn) t 2g and O pπ , the former of which was occupied by 2 (3) electrons. The σ antibonding orbital

formed by the Cr (Mn) e g and O pσ states occurred within the lower CB. For Fe- and Co-doped TiO 2, the localized level (e) was situated 0.2 eV above the VB (or at the top of the VB for Co) due to the π antibonding of the Fe e g and O pπ states. This level was occupied by four (or five for Co) electrons. The Fe (Co) e g state was split into d z2 (f) and d x2- y2 (g) orbitals in the band gap. For Ni-doped TiO 2, the π antibonding of the Ni t 2g and O pπ states was somewhat delocalized and appeared within the VB (h) due to the Ni e g states from the d z2 and d x2- y2 orbitials situated in the band gap. The electron densities around the dopant were large in the VB and small in the CB compared to the case of pure TiO2. The metal-O interaction strengthened, and the metal metal interaction became weak as a result of the 3d metal doping. Li et al. found that 1.5 at % Nd 3+-doped TiO2 nanoparticles reduced the band gap by as much as 0.55 eV and that the band gap narrowing was primarily attributed to the substitutional Nd3+ ions, which introduced electron states into the band gap of TiO 2 to form the new lowest unoccupied molecular orbital (LUMO). 444 Wang and Doren found that Nd 4f electrons changed the electronic structure of Nd-doped TiO2 into the half-metallic or the insulating ground state 510 and that V 3d states were located at the bottom of the conduction band of the TiO 2 host in V-doped TiO 2, which was shown to be a half-metal or an insulator from their theoretical studies. 511 4.1.2.1.2. Nonmetal-Doped TiO2 Nanomaterials. Recent theoretical and experimental studies have shown that the desired band gap narrowing of TiO 2 can also be achieved by using nonmetal dopants (refs 385, 428, 444, 489, 481, 482, 484, 503, 504, and 512-547). Asahi and co-workers calculated the electronic band structures of anatase TiO 2 with different substitutional dopants, including C, N, F, P, or S, using the FLAPW method in the framework of the local density approximation (LDA) as shown in Figure 52. 489 In this study, C dopant introduced deep states in the gap. 489


(A) Total DOSs of doped TiO2 and (B) the projected DOSs into the doped anion sites, calculated by FLAPW, for the dopants F, N, C, S, and P located at a substitutional site for an O atom in the anatase TiO2 crystal (eight TiO2 units per cell). Nidoped stands for N doping at an interstitial site, and Ni+s-doped stands for doping at both substitutional and interstitial sites. Reprinted with permission from Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269 (http://www.sciencemag.org). Copyright 2001 AAAS. Figure 52.

Nakano et al. found three deep levels located at approximately 0.86, 1.30, and 2.34 eV below the conduction band in C-doped TiO 2, which were attributed to the intrinsic nature of TiO2 for the first one and the two levels newly introduced by the C doping. 522 In particular, the pronounced 2.34 eV band contributed to band gap narrowing by mixing with the O 2p valence band. 522 Lee et al., in their firstprinciples density-functional LDA pseudopotential calcula-


tions of electronic properties of C-doped TiO 2, found that the bands originating from C 2p states appeared in the band gap of TiO2; however, the mixing of C with O 2p states was too weak to produce a significant band gap narrowing. 517 In Asahi’s study, the substitutional doping of N was the most effective in the band gap narrowing because its p states mixed with O 2p states, while the molecularly existing species, e.g., NO and N 2 dopants, gave rise to the bonding states below the O 2p valence bands and antibonding states deep in the band gap (N i and Ni+s), and were well screened and hardly interacted with the band states of TiO 2.489 Di Valentin et al. found that, for nitrogen doping in both anatase and rutile polymorphs, N 2p localized states were just above the top of the O 2p valence band.512,513 In anatase, these dopant states caused a red shift of the absorption band edge toward the visible region, while, in rutile, an overall blue shift was found by the N-induced contraction of the O 2p band.512 Experimental evidence supported the statement that nitrogen-doped TiO 2 formed nitrogen-induced midgap levels slightly above the oxygen 2p valence band. 486 Lee et al., in their first-principles density-functional LDA pseudopotential calculations of electronic properties of N-doped TiO 2, found that the bands originating from N 2p states appeared in the band gap of TiO2; however, the mixing of N with O 2p states was too weak to produce a significant band gap narrowing. 517 Wang and Doren found that N doping introduced some states at the valence band edge and thus made the original band gap of TiO2 smaller, and that a vacancy could induce some states in the band gap region, which acted as shallow donors.510 Nakano et al. found that, in N-doped TiO 2, deep levels located at approximately 1.18 and 2.48 eV below the conduction band were attributed to the O vacancy state as an efficient generation -recombination center and to the N doping which contributed to band gap narrowing by mixing with the O 2p valence band, respectively. 523 Okato et al. found that, at high doping levels, N was difficult to substitute for O to contribute to the band gap narrowing, instead giving rise to the undesirable deep-level defects. 524 S dopant induced a similar band gap narrowing as nitrogen,489 and the mixing of the sulfur 3p states with the valence band was found to contribute to the increased width of the valence band, leading to the narrowing of the band gap.495,497 When S existed as S 4+, replacing Ti4+, sulfur 3s states induced states just above the O 2p valence states, and S 3p states contributed to the conduction band of TiO 2 as shown in Figure 53A.494 When F replaced the O in the TiO 2 lattice, F 2p states were localized below the O 2p valence states without any mixing with the valence or conduction band as shown in Figure 53B, and additional states appeared just below the conduction edge, due to the electron occupied level composed of the t2g state of the Ti 3d orbital. 507 The electronic change induced by F dopant was considered to be similar to the O vacancy, thus reducing the effective band gap and improving visible light photoresponse. 507 Li et al. found that F doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity, and the increase of Ti 3+ ions, and doped N atoms formed a localized energy state above the valence band of TiO2, whereas doped F atoms themselves had no influence on the band structure in N -F-co-doped TiO2.519 4.1.2.2. Optical Properties of Doped TiO2 Nanomaterials. 4.1.2.2.1. Optical Properties of Metal-Doped TiO2 Nanomaterials. A red shift in the band gap transition or a


(A) Total DOS of S-doped TiO2. Reprinted with permission from Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Appl. Catal. A 2004, 265, 115, Copyright 2004, with permission from Elsevier. (B) Total DOSs of F-doped TiO2 calculated by FLAPW. E g indicates the (effective) band gap energy. The impurity states are labeled (I) and (II). Reprinted from Yamaki, T.; Umebayashi, T.; Sumita, T.; Yamamoto, S.; Maekawa, M.; Kawasuso, A.; Itoh, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 206 , 254, Copyright 2003, with permission from Elsevier. Figure 53.

visible light absorption was observed in metal-doped TiO 2 (refs 433-435, 438, 444, 445, 448, 449, 460-463, 465, 466, 470, 509, 548, and 549). For V-, Mn-, or Fe-doped TiO 2, the absorption spectra shifted to a lower energy region with an increase in the dopant concentration. 434,445,460 This red shift was attributed to the charge-transfer transition between the d electrons of the dopant and the CB (or VB) of TiO 2. Metalion doped TiO2 prepared by ion implantation with various transition-metal ions such as V, Cr, Mn, Fe, and Ni was found to have a large shift in the absorption band toward the visible light region, with the order of the effectiveness in the red shift being V > Cr > Mn > Fe > Ni.466-471 Anpo et al. found that the absorption band of Cr-ion-implanted TiO2 shifted smoothly toward the visible light region, with the extent of the red shift depending on the amount of metal ions implanted as shown in Figure 54A. 470 Impregnated or chemically Cr-ion-doped TiO2 showed no shift in the absorption edge of TiO2; however, a new absorption band appeared at around 420 nm as a shoulder peak due to the formation of an impurity energy level within the band gap, with its intensity increasing with the number of Cr ions (Figure 54B).470 In the study by Umebayashi et al., visible light absorption of V-doped TiO2 was due to the transition between the VB and the V t2g level.509 The holes in the VB produced an anodic photocurrent. The photoexcitation processes under visible light of V-, Cr-, and Mn-doped TiO 2 are illustrated in Figure 55. Photoexcitation for V-, Cr-, Mn-, and Fe-doped TiO2 occurred via the t 2g level of the dopant. The visible light absorption for Mn- and Fe-doped TiO 2 was due to the optical transitions from the impurity band tail into the CB. The Mn (Fe) t2g level was close to the VB and easily

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(A) The UV-vis absorption spectra of TiO2 (a) and Cr ion-implanted TiO2 photocatalysts (b-d). The amount of implanted Cr ions ( µmol / g) was (a) 0, (b) 0.22, (c) 0.66, or (d) 1.3. (B) The UV-vis absorption spectra of TiO2 (a) and Cr ion-doped TiO2 (b′-d′) photocatalysts prepared by an impregnation method. The amount of doped Cr ions (wt%) was (a) 0, (b′) 0.01, (c′) 0.1, (d′) 0.5, or (e′) 1. Reprinted from Anpo, M.; Takeuchi, M. J. Catal. 2003, 216 , 505, Copyright 2003, with permission from Elsevier. Figure 54.

overlapped in highly impure media. The visible light absorption for the Cr-doped TiO 2 can be attributed to a donor transition from the Cr t 2g level into the CB and the acceptor transition from the VB to the Cr t 2g level. Stucky et al. found that up to 8 mol % Eu 3+ ions could be doped into mesoporous anatase TiO 2, and excitation of the TiO2 electrons within their band gap led to nonradiative energy transfer to the Eu 3+ ions with a bright red luminescence.287 The mesoporous TiO2 acted as a sensitizer. 4.1.2.2.2. Optical Properties of Nonmetal-Doped TiO2 Nanomaterials. Nonmetal doped TiO2 normally has a color

from white to yellow or even light gray, and the onset of the absorption spectra red shifted to longer wavelengths (refs 385, 426, 478, 483, 489, 494, 495, 497, 498, 505, 506, 512, 516, 518, 519, 521, and 529). In N-doped TiO 2 nanomaterials, the band gap absorption onset shifted 600 nm from 380 nm for the undoped TiO 2, extending the absorption up to 600 nm, as shown in Figure 56. 426 The optical absorption of N-doped TiO2 in the visible light region was primarily located between 400 and 500 nm, while that of oxygendeficient TiO2 was mainly above 500 nm from their densityfunctional theory study. 520 N-F-co-doped TiO2 prepared by spray pyrolysis absorbs light up to 550 nm in the visible light spectrum. 518 The S-doped TiO2 also displayed strong absorption in the region from 400 to 600 nm. 494 The red shifts in the absorption spectra of doped TiO2 are generally attributed to the narrowing of the band gap in the electronic structure after doping. 489 C-doped TiO2 showed long-tail absorption spectra in the visible light region. 472,543 Cl-, Br-, and Cl-Br-doped TiO2 had increased optical response compared to the case of pure TiO 2 in the visible region. 508 Livraghi et al. recently found that N-doped TiO 2 contained single atom nitrogen impurity centers localized in the band gap of the oxide which were responsible for visible light



Reflectance spectra of N-doped TiO2 nanoparticles and pure TiO2 nanoparticles. Reprinted with permission from Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049. Copyright 2003 American Chemical Society. Figure 56.

IPCE λ and APCE λ curves for N-doped TiO2 and TiO2. SE stands for the substrate/electrode (SE) interface. The action spectra are recorded with light incident onto the SE interface. Reprinted from Lindgren, T.; Lu, J.; Hoel, A.; Granqvist, C. G.; Torres, G. R.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 2004, 84, 145, Copyright 2004, with permission from Elsevier. Figure 57.

4.1.2.3. Photoelectrical Properties of Doped TiO2 Nanomaterials. The photoelectrical properties of a material can

be measured with an “action spectrum” curve using a phototo-current conversion setup. 385,486,497,521 In this setup, light from a xenon lamp passing through a monochromator is radiated onto the electrode, and the photocurrents from the electrodes are measured as a function of wavelength. 385,486,497,521 The incident photo-to-current efficiency as a function of wavelength, IPCE λ, is called an “action spectrum”. IPCE λ can be calculated by

IPCE λ ) Schematic diagram to illustrate the photoexcitation process under visible light of metal-doped TiO2: (a) Ti1- xV xO2; (b) Ti1- xFe xO2; (c) Ti1- xCr xO2. Reprinted from Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2002, 63, 1909, Copyright 2002, with permission from Elsevier. Figure 55.

absorption with promotion of electrons from the band gap localized states to the conduction band. 547 Nick Serpone “proposed that the commonality in all...doped titanias rests with formation of oxygen vacancies and the advent of color centers...that absorb the visible light radiation, and he argued that the red shift of the absorption edge is in fact due to formation of the color centers. 546

hc I ph , λ e P λ λ

(18)

where I ph, λ is the photocurrent, P λ is the power intensity of the light at wavelength λ, and h, c, and e are Planck’s constant, the speed of light, and the elementary charge, respectively.385 The IPCE λ curve normally has a similar shape and trend as the absorption spectrum. When the IPCE λ is divided by the absorption, the absorbed photon-to-current efficiency (APCE λ; also called the quantum yield) is obtained.521 Figure 57 shows IPCE λ and APCE λ curves for N-doped TiO2 and TiO2.521 The photoelectrochemical onset for TiO2- xN x is shifted to around 550 nm into the visible region of the spectrum, and some ultraviolet (UV) efficiency for TiO2- xN x is lost compared to that of TiO 2, suggesting


the TiO2- xN x has a typical photoelectrochemical behavior of a material with states in the band gap which act as recombination centers for light-induced charge carriers. 521 In another study, the action spectrum of N-doped TiO 2 also displayed a higher response in the visible region than that of pure TiO2.486 The photocurrent spectra for the pure and S-doped crystals showed that the photocurrent spectrum edge shifted to the low-energy region below 2.9 eV for the S-doped crystal, compared to 3.0 eV for pure TiO 2, due to the transition of electrons across the narrowed band gap between the VB and the CB. 497

4.2. Surface Chemical Modifications When a photocurrent is generated with light energy less than that of the semiconductor band gap, the process is known as sensitization and the light-absorbing dyes are referred to as sensitizers. 9,10 TiO2 is a semiconductor with a wide band gap, with optical absorption in the UV region (<400 nm). Any materials with a narrower band gap or absorption in the visible or infrared regime can be used as a sensitizer for TiO 2 materials. These materials include inorganic semiconductors with narrow band gaps, metals, and organic dyes. How efficiently the sensitized TiO 2 can interact with the light depends largely on how efficiently the sensitizer interacts with the light. A common and key step in the photosensitization of TiO 2 is the efficient charge transfer from the excited sensitizer to TiO 2, and the resulting charge separation. The match between the electronic structures of the sensitizer and TiO 2 plays a large role in this process, as does the structure of the interface, including the grain boundaries and bonding between the sensitizer and TiO2. Careful design is needed to avoid the charge trapping and recombination which eventually harm the performance of sensitized TiO 2.9,10,550 4.2.1. Inorganic Sensitization 4.2.1.1. Sensitization by Narrow Band Gap Semiconductors. Narrow band gap semiconductors have been used

as sensitizers to improve the optical absorption properties of TiO2 nanomaterials in the visible light region by various groups.551-559 The preparation method for these inorganic semiconductor sensitized TiO2 nanomaterials systems is usually the sol -gel method.551-558 Hoyer et al. reported the sensitization of a nanocrystalline TiO 2 matrix by small PbS nanoparticles (<2.5 nm), and they found that the photogenerated excess electrons could be directly injected from the PbS to the TiO2, resulting in strong photoconductance in the visible region. 553 Fitzmaurice et al. found that excitation of the sensitizer AgI on TiO2 nanoparticles resulted in a stabilization of electron -hole pairs with a lifetime well beyond 100 µs and in electron migration from AgI to TiO 2.551 Vogel et al. studied the sensitization of nanoporous TiO 2 by CdS, PbS, Ag2S, Sb2S3, and Bi2S3 and found that the relative positions of the energetic levels at the interface between the quantum size particles and TiO 2 could be optimized for efficient charge separation by using the size quantization effect and that the photostability of the electrodes could be significantly enhanced by surface modification of the TiO 2 nanoparticles with CdS nanoparticles. 558 Qian et al. found from surface photovoltage spectra (SPS) measurements that the large surface state density present on the TiO2 nanoparticles could be efficiently decreased by sensitization using CdS nanoparticles and that the slow photocurrent response disappeared and the steady-state

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photocurrent increased drastically after the TiO 2 nanoparticulate thin film was sensitized using CdS nanoparticles.378,555 Sant and Kamat found that quantum size effects played an important role in interparticle electron transfer in the CdS-TiO2 semiconductor systems in that electron transfer from photoexcited CdS to TiO 2 was found to depend on the size of TiO 2 nanoparticles.560 Charge transfer occurred only when TiO2 nanoparticles were sufficiently large ( >1.2 nm) that the conduction band of the nanoparticles was located below that of CdS nanoparticles. 560 Shen et al. studied nanostructured TiO 2 electrodes with different nanocrystals sizes sensitized with CdSe nanoparticles and found that photoelectrochemical currents in the visible region in the CdSe-sensitized TiO 2 nanostructured electrodes were largely dependent on both the structure and electron diffusion coefficient of the TiO 2 electrodes.556 Zaban et al. studied nanocrystalline TiO2 electrodes sensitized with InP quantum dots, and found they exhibited strong photoconduction in the visible region and had a photocurrent action spectrum consistent with the absorption spectrum of the InP QDs, indicating electron transfers from InP QDs into TiO 2 nanoparticles under visible light illumination. 559 Kamat et al. recently reported the sensitization of mesoscopic TiO2 films using bifunctional surface modifiers (SHR-COOH) linked with CdSe nanoparticles. Upon visible light excitation, CdSe nanoparticles injected electrons into TiO 2 nanocrystallites.561 The TiO2-CdSe composite exhibited a photon-to-charge carrier generation efficiency of 12% when employed as a photoanode in a photoelectrochemical cell. 4.2.1.2. Sensitization by Metal Nanoparticles. Ohko et al. found that when the TiO2 nanoparticle films were sensitized with Ag nanoparticles, the color of the film could be reversely switched back and forth between brownish-gray under UV light and the color of illuminating visible light due to the oxidation of Ag by O 2 under visible light and reduction of Ag + under UV light.562 The color of the film under visible light could be tuned from green to red and white by changing the size of the Ag nanoparticles due to the plasmon-based absorption of Ag and the dielectric confinement of the TiO 2 nanoparticle film matrix. Figure 58 shows absorption spectra and photographs of Ag -TiO2 films. Naoi et al. found that the chromogenic properties of the Ag-TiO2 films could be improved by simultaneous irradiation during Ag deposition with UV and blue lights to suppress the formation of anisotropic Ag particles and that nonvolatilization of a color image could be achieved by removing Ag+ that was generated during the irradiation with a colored light. 563 The color of the film was further found to be affected by the resonance wavelengths of the Ag particles, the TiO2 film, and the nanopores in the TiO 2 film. They found that the photochromism and rewritability of Ag -TiO2 films could be deactivated by modification of Ag nanoparticles with thiols to make it possible to retain color images displayed on the films, and that the deactivated properties could be fully reactivated by UV irradiation (Figure 59A). 564 Kawahara et al. proposed the mechanism of charge separation at the interface between Ag and TiO 2 nanoparticles shown in Figure 59B. 565 They found that, in the multicolor photochromism of TiO2 nanoporous films loaded with photocatalytically deposited or electrodeposited and commercially available Ag nanoparticles, visible light-induced electron transfer from Ag to oxygen molecules played an essential role. Some of the photoexcited electrons on Ag were transferred to oxygen molecules via TiO 2 and nonexcited



(A) Schematic illustrations for photochromism of the Ag-TiO2 film (a, b) and deactivation (c) and reactivation (d) of the photochromism. From: Naoi, K.; Ohko, Y.; Tatsuma, T. Chem. Commun. 2005, 1288 (http://dx.doi.org/10.1039/b416139d) s Reproduced by permission of The Royal Society of Chemistry. (B) Proposed mechanism of the charge separation at the interface between Ag and TiO2 nanoparticles. From: Kawahara, K.; Suzuki, K.; Ohko, Y.; Tatsuma, T. Phys. Chem. Chem. Phys. 2005, 7 , 3851 (http://dx.doi.org/10.1039/b511489f) s Reproduced by permission of the PCCP Owner Societies. Figure 59.

(A) Absorption spectra of a Ag-TiO2 film by ultraviolet light irradiation and after visible light irradiation. Corresponding photographs are also shown. (B ) Photograph of multicolored spots on the Ag-TiO2 film on a glass substrate irradiated successively with monochromatic lights. A xenon lamp and an ultraviolet-cut filter (blocking light below 400 nm) was used with a 450 nm (blue), 530 nm (green), 560 nm (yellowish-green), 600 nm (orange), or 650 nm (red) bandpass filter (fwhm, 10 nm), or without any bandpass filter (white). Reprinted with permission from Ohko, Y.; Tatsuma, T.; Fujii, T.; Naoi, K.; Niwa, C.; Kubota, Y.; Fujishima, A. Nature Mater. 2003, 2, 29. Copyright 2003 Nature Publishing Group. Figure 58.

Ag, and replacement of the nonexcited Ag with Pt accelerated the electron transport from the photoexcited Ag to oxygen molecules and the photochromic behavior. Tian and Tatsuma found that nanoporous TiO 2 films loaded with Ag and Au nanoparticles exhibit negative potential changes and anodic currents in response to visible light irradiation, with potential applications for photovoltaic cells, photocatalysts, and plasmon sensors. 566 They found that for the Au-TiO2 system photoaction spectra for open-circuit potential and short-circuit current agreed with the absorption spectrum of Au nanoparticles in the TiO 2 film. After the Au nanoparticles were photoexcited due to plasmon resonance, charge separation occurred by the transfer of photoexcited electrons from the Au particle to the TiO 2 conduction band with the simultaneous transfer of compensating electrons from a donor in solution to the Au particles. 567 Cozzoli et al. found that, following UV illumination, TiO 2 nanorods sensitized with Ag or Au nanoparticles could sustain a higher degree of conduction band electron accumulation than pure TiO2.126 4.2.1.3. Organic Dye Sensitization. Organic dyes have been widely employed as sensitizers for TiO 2 nanomaterial to improve its optical properties, i.e., in dye-sensitized

nanocrystalline solar cells (DSSCs). 18,246,312,568-673Organic dyes are usually transition metal complexes with low lying excited states, such as polypyridine complexes, phthalocyanine, and metalloporphyrins.568-673 The metal centers for the dyes include Ru(II), Zn(II), Mg(II), Fe(II), and Al(III), while the ligands include nitrogen heterocyclics with delocalized π or aromatic ring systems. These organic dyes are normally linked to TiO 2 nanoparticle surfaces via functional groups by various interactions between the dyes and the TiO 2 nanoparticle substrate: (a) covalent attachment by directly linking groups of interest or via linking agents, (b) electrostatic interactions via ion exchange, ion-pairing, or donor -acceptor interactions, (c) hydrogen bonding, (d) van der Waals forces, etc. Most of the dyes of interest link in the first way. Groups such as silanyl (-O-Si-), amide (-NH-(CdO)-), carboxyl (-O(CdO)-), and phosphonato (-O-(HPO2)-) have been shown to from stable linkages with the surface hydroxyl groups on TiO2 substrates.610 Carboxylic and phosphonic acid derivatives react with the hydroxyl groups to form esters, while amide linkages are obtained via the reaction of amine derivatives and dicyclohexyl carbodiimide on TiO 2. The most common and successful functional groups are based on carboxylic acids. Qu and Meyer found spectroscopic evidence for ester linkages after carboxylic acids react with the surface titanol groups dehydratively. 674 Metal cyano compounds in acidic solutions were found to link to TiO 2 surfaces by a single cyanide ligand with a C 4V symmetry, i.e., TiIVNC-FeII(CN)5.668,669,675 The interfacial charge separation between the adsorbed dyes and TiO2 nanomaterials involves one of three mecha-


nisms, which differ by the nature of the donor that transfers the electron to the semiconductor: (1) excited state; (2) reduced state; or (3) molecule-to-particle charge-transfer complex.550 For complete knowledge of the charge transfer between the dye sensitizers and the TiO 2 nanomaterials, please refer to other excellent reviews. 10,18,19,550,676,677 Ultrafast electron transfer from metal-to-ligand charge transfer (MLCT) excited states to anatase TiO 2 is the most common category in dye-sensitized TiO 2.572,595,606,618,619,677-683 The mechanism of the dye sensitization of TiO 2 nanoparticles normally involves the excitation of the dye and the charge transfer from the dye to TiO 2 nanoparticles. The low-lying MLCT and ligand-centered ( π -π *) excited states of these complexes are fairly long-lived, allowing them to participate in electrontransfer processes. As an efficient photosensitizer, the dye has to meet several requirements. First, the dye should have high absorption efficiency and a wide spectral range of coverage of light absorption in the visible, near-IR, and IR regions. Second, the excited states of the dye should have a long lifetime and a high quantum yield. Third, the dye should have matched electronic structures for the ground and excited states with TiO2 nanoparticles to ensure the efficient charge transfer between them; that is, the energy level of the excitedstate should be well matched to the lower bound of the conduction band of TiO 2 to minimize energetic losses during the electron-transfer reaction. 550 The electron transfer from the dye to TiO 2 usually is very fast, in the range of tens of femtoseconds. Hannappel et al. found that electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate) into empty electronic states in a colloidal anatase TiO 2 film was on the time scale of <25 fs.595 Rehm et al. found the charge injection from a surface-bound coumarin 343 to the conduction band of TiO 2 occurred on a time scale of ∼200 fs due to strong electronic coupling between the dye and TiO 2 energy levels.684 The electron transport and recombination in dye-sensitized TiO 2 solar cells with different electrolytes had been investigated, including iodine-doped ionic liquids (diethylmethylsulfonium, dibutylmethylsulfonium, or 3-hexyl-1-methylimidazolium iodide) and an organic solvent (3-methoxypropionitrile with LiI, I 2, and 1-methylbenzimidazole).685 The most viscous electrolytes showed a clear limitation in photocurrent attributed to a low diffusion coefficient for the triiodide that transports positive charge to the counterelectrode. The electron transport of the solar cells appeared to be dominated by the properties of the nanostructured TiO 2 film, and the electron lifetime depended on the type of cation used in the ionic liquid. Bulky, less absorptive cations seem to give longer lifetimes. Schwarzburg et al. found a time constant of 13 fs for electron transfer from the excited singlet state of the chromophore perylene bonded to the surface via a carboxyl group into anatase TiO 2.686 The electron-transfer time of perylene became much longer (3.8 ps) at a distance of about 1.3 nm. Wenger et al. found that carefully controlled deposition of Ru(II) complex dye molecules onto nanocrystalline TiO 2 consistently yielded monophasic injection dynamics with a time constant shorter than 20 fs. 687 The anchoring of ruthenium dye {(C4H9)4N}[Ru(Htc-terpy)(NCS)3] (tc-terpy ) 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine), the so-called black dye, onto nanocrystalline TiO 2 films occurs by a bidentate binuclear coordination mode. 577 The electron injection process from the dye excited state into the TiO2 conduction band was biexponential with a fast com-

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ponent (200 ( 50 fs) and a slow component (20 ps), attributed to the electron injection from the initially formed and the relaxed dye excited states, respectively. 577 In the reduced sensitizer injection mechanism, the sensitizer excited state(s) is first quenched by an external donor, and subsequently the reduced state of the dye, S -, transfers an electron across the semiconductor interface. 550,688 A potential advantage of this mechanism is that the reduced sensitizer is a stronger reductant than the MLCT excited state, typically by 0.3 -0.5 eV. Thus, sensitizers that are weak photoreductants may sensitize TiO 2 efficiently after reductive quenching. This mechanism may be exploited to produce large open-circuit photovoltages or enhanced light harvesting in the near-IR regions. The observation of ultrafast electron injection coupled with the weak oxidizing power of the excited sensitizers currently in use strongly suggests that an excited-state injection mechanism is operative in regenerative solar cells based on these materials. The reduced sensitizer injection mechanism was reported by Thompson, 688 Haque,598 and Wang.659 The metal-to-particle charge-transfer mechanism involves interfacial chemistry between the compounds and the TiO 2 surface which produces color changes, observed by Gra¨tzel and identified as molecule-to-particle charge-transfer transitions.689 Metal cyanides, [M(CN) x]4- (M ) FeII, RuII, OsII, ReIII, MoIV, or WIV, x ) 6, 7, or 8), such as ferrocyanide, FeII(CN)64-, bind to TiO2 through ambidentate cyano ligands. For example, FeII(CN)64- does not absorb light above 380 nm, but a deep orange color with an absorption maximum centered at 420 nm was observed for Fe II(CN)64- /TiO2, due to a MPCT complex formed between Fe II(CN)64- and surface Ti4+ ions, Fe(II)fTi(IV).550 The metal-to-particle chargetransfer mechanism was consistent with the subpicosecond infrared spectroscopy study on the Fe II(CN)64- /TiO2 nanoparticle by Weng et al., where a mid-infrared absorption was assigned to TiO 2 electrons in the semiconductor. 690 The injection rate constant could not be time-resolved with a 50fs instrument response function. The MPCT was also found by Yang et al. in their study on Fe(bpy)(CN) 42--sensitized TiO2, where the absorption spectra were well modeled by a sum of MLCT (Fe f bpy) and metal-to-particle (Fe(II) f Ti(IV)) bands. The MLCT bands were solvatochromic, while the MPCT bands were not. 668,669 Benkoe et al. found that the larger the TiO 2 particle and the better its overall crystallinity, the faster the process of electron injection from the dye fluorescein 27 to the anatase TiO 2 film.578 Haque et al. found that a supramolecule dye with a remarkably longlived (4 s) charge-separation state could be obtained by controlling the spatial separation between the cation center of the dye and the electrode surface. 597 The dyes were Ru(II) complexes containing carboxylated polypyridyl chromophores and a bipyridyl ligand with aromatic amine-based electron donor substituents. 597 The kinetics and mechanisms of the injections, transport, recombination, and photovoltaic properties of electrons in nanostructured TiO2 solar cells have been thoroughly discussed in recent reviews 676,691-694 and will only be briefly mentioned below. Considerable effort has been devoted to the kinetics and energetics of transport and recombination in dye-sensitized solar cells with various techniques, such as intensity modulated photocurrent spectroscopy (IMPS), 640,695-702 intensity modulated photovoltage spectroscopy (IMVS), 639,700,703,704 electrical impedance spectroscopy (EIS), 700,705-710 transient photocurrent,695,706,711-718 and transient photovoltage.715,719 For


example, Searson and Cao studied the photocurrent response of dye-sensitized, porous nanocrystalline TiO 2 cells with photocurrent transient measurements and intensity-modulated photocurrent spectroscopy and found that the electron transport in the TiO 2 film can be fitted with a diffusion model where the diffusion coefficient for electrons in the particle network was a function of the light intensity. 695 Using intensity modulated photocurrent spectroscopy, Vanmaekelebergh et al. found that the electronic transport was controlled by trapping and detrapping of photogenerated electrons in interfacial band gap states, distributed in energy, and that the localization time of a trapped electron was controlled by the steady-state light intensity and interfacial kinetics. 696,697 Peter et al. recently found that the electron transport in dyesensitized nanocrystalline solar cells appeared to be a slow diffusion-controlled process, attributed to multiple trapping at energy levels distributed exponentially in the band gap of the nanocrystalline TiO 2.720 Frank et al. summarized that the electron motion is essentially ambipolarly diffusional and the morphology and defect structure of the TiO 2 film had a strong influence on electron transport. 676 The recombination predominates at the interface and depends on the spatial region of photoinjected charge buildup in the cell, the redox electrolyte, and the surface properties of both the TiO 2 nanoparticle film and the TCO substrate. For the recombination, two mechanisms assume either a dismutation reaction or an interfacial electron-transfer reaction as rate-limiting, while the third mechanism states electron transport limits recombination. 676 The spatial location of the traps limiting electron transport in nanocrystalline TiO 2 has been a long standing issue. These traps have been speculated to locate either at the particle surface,640,721,722 in the bulk of the particles, 723 or at interparticle grain boundaries.568 Kopidakis et al. recently investigated the dependence of the electron diffusion coefficient and the photoinduced electron density on the internal surface area of TiO 2 nanoparticle films in dye-sensitized solar cells by photocurrent transient measurements. 724 They found that the density of electron traps in the films changed in direct proportion with the internal surface area, which was varied by altering the average particle size of the films, and the scaling of the electron diffusion coefficient with the internal surface area. They suggested that the traps were located predominately at the surface of TiO 2 particles instead of in the bulk of the particles or at interparticle grain boundaries, and that surface traps limited transport in TiO 2 nanoparticle films. Kopidakis et al. found that the traps were located predominately at the surface of TiO 2 particles instead of in the bulk of the particles or at interparticle grain boundaries and that surface traps limited transport in TiO 2 nanoparticle films in the dye-sensitized TiO 2 solar cell.724

5. Applications of TiO 2 Nanomaterials The existing and promising applications of TiO 2 nanomaterials include paint, toothpaste, UV protection, photocatalysis, photovoltaics, sensing, and electrochromics as well as photochromics. TiO 2 nanomaterials normally have electronic band gaps larger than 3.0 eV and high absorption in the UV region. TiO 2 nanomaterials are very stable, nontoxic, and cheap. Their optical and biologically benign properties allow them to be suitable for UV protection applications. 725-730 A surface is defined as superhydrophilic or superhydrophobic if the water -surface contact angle is larger than 130 ° or less than 5 °, respectively.731 TiO2 nanomateirals can be


imparted with antifogging functions on various glass products, i.e., mirrors and eyeglasses, having superhydrophilic or superhydrophobic surfaces. 332,732-734 For example, Feng et al. found that reversible superhydrophilicity and superhydrophobicity could be switched back and forth for TiO 2 nanorod films.142 When the TiO2 nanorod films were irradiated with UV light, the photogenerated hole reacted with lattice oxygen to form surface oxygen vacancies. Water molecules kinetically coordinated to these oxygen vacancies, and the spherical water droplet filled the grooves along the nanorods and spread out on the film with a contact angle of about 0°, resulting in superhydrophilic TiO 2 films. After the hydroxy group adsorption, the surface transformed into an energetically metastable state. When the films were placed in the dark, the adsorbed hydroxy groups were gradually replaced by atmospheric oxygen, and the surface evolved back to its original state. The surface wettability converted from superhydrophilic to superhydrophobic. 142 Stain-proofing, self-cleaning properties can also be bestowed on many different types of surfaces due to the superhydrophilic or superhydrophobic surfaces. 735-744 TiO2 nanomaterials have also been used as sensors for various gases and humidity due to the electrical or optical properties which change upon adsorption.745-751 One of the most important research areas for future clean energy applications is to look for efficient materials for the production of electricity and/or hydrogen. When sensitized with organic dyes or inorganic narrow band gap semiconductors, TiO2 can absorb light into the visible light region and convert solar energy into electrical energy for solar cell applications.28,30,752 For example, an overall solar to current conversion efficiency of 10.6% has been reached by the group led by Gra¨tzel with DSSC technology.31 TiO2 nanomaterials have been widely studied for water splitting and hydrogen production due to their suitable electronic band structure given the redox potential of water. 198,475,476,753-770 Another application of TiO 2 nanomaterials when sensitized with dyes or metal nanoparticles is to build photochromic devices. 562,565,771-777 Of course, one of the many applications of TiO2 nanomaterials is the photocatalytic decomposition of various pollutants.

5.1. Photocatalytic Applications TiO2 is regarded as the most efficient and environmentally benign photocatalyst, and it has been most widely used for photodegradation of various pollutants. 121,127,132,430,442,778-822 TiO2 photocatalysts can also be used to kill bacteria, as has been carried out with E. coli suspensions.793,799 The strong oxidizing power of illuminated TiO 2 can be used to kill tumor cells in cancer treatment. 782,785,820,823-825 The photocatalytic reaction mechanisms are widely studied.7,12,20,33,406 The principle of the semiconductor photocatalytic reaction is straightforward. Upon absorption of photons with energy larger than the band gap of TiO 2, electrons are excited from the valence band to the conduction band, creating electron -hole pairs. These charge carriers migrate to the surface and react with the chemicals adsorbed on the surface to decompose these chemicals. This photodecomposition process usually involves one or more radicals or intermediate species such as •OH, O2-, H2O2, or O2, which play important roles in the photocatalytic reaction mechanisms. The photocatalytic activity of a semiconductor is largely controlled by (i) the light absorption properties, e.g., light absorption spectrum and coefficient, (ii) reduction and


oxidation rates on the surface by the electron and hole, (iii) and the electron -hole recombination rate. A large surface area with a constant surface density of adsorbents leads to faster surface photocatalytic reaction rates. In this sense, the larger the specific surface area, the higher the photocatalytic activity is. On the other hand, the surface is a defective site; therefore, the larger the surface area, the faster the recombination. The higher the crystallinity, the fewer the bulk defects, and the higher the photocatalytic activity is. Hightemperature treatment usually improves the crystallinity of TiO2 nanomaterials, which in turn can induce the aggregation of small nanoparticles and decrease the surface area. Judging from the above general conclusions, the relation between the physical properties and the photocatalytic activities is complicated. Optimal conditions are sought by taking these considerations into account and may vary from case to case. 20 5.1.1. Pure TiO 2 Nanomaterials: First Generation

As the size of the TiO 2 particles decreases, the fraction of atoms located at the surface increases with higher surface area to volume ratios, which can further enhance the catalytic activity. The increase in the band gap energy with decreasing nanoparticle size can potentially enhance the redox potential of the valence band holes and the conduction band electrons, allowing photoredox reactions, which might not otherwise proceed in bulk materials, to occur readily. One disadvantage of TiO2 nanoparticles is that they can only use a small percentage of sunlight for photocatalysis. Practically, there exists an optimal size for a specific photocatalytic reaction. Anpo et al. investigated the photocatalytic activity of TiO 2 nanoparticles on hydrogenation reactions of CH 3CCH with H2O, and they found the activity increased as the diameter of the TiO2 particles decreased, especially below 10 nm. 406 They suggested that the dependence of the yields on the particle size arose from the differences in the chemical reactivity and not from the physical properties of these catalysts. Wang et al. found that there was an optimal size for TiO 2 nanoparticles for maximum photocatalytic efficiency in the decomposition of chloroform. 815 They observed an improvement in activity when the particle size was decreased from 21 to 11 nm, but the activity decreased when the size was reduced further to 6 nm. They concluded that for this particular reaction the optimum particle size was about 10 nm. In large TiO 2 nanoparticles, bulk recombination of the charge carriers was the dominant process, which could be reduced by a decrease in particle size; as the particle size was lowered below a certain limit, surface recombination processes became dominant, since most of the electrons and holes were generated close to the surface and surface recombination was faster than interfacial charge carrier transfer processes. 826 Chae et al. studied the photocatalytic activity of four sizes of TiO2 nanoparticles on the decomposition of 2-propanol, and they found that 7-nm particles showed 1.6 times better photocatalytic activity than TiO 2 P25 and that 15- and 30nm particles showed lower photocatalytic efficiencies. 132 Mesoporous TiO2, TiO2 nanorods, and nanotubes have been demonstrated to have high photocatalytic performance under suitable conditions. 127,187,265,281,296,818 Peng et al. prepared mesoporous TiO 2 with a high specific surface area, which showed significant activity on the oxidation of Rhodamine B due to the large surface area, small crystal size, and well-crystallized anatase mesostructure. 296 Figure

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Photocatalytic properties of mesoporous TiO2 samples as prepared and calcined at different temperature as well as TiO2 P25 nanoparticles (RB, c0 ) 1.0 × 10-5 M, pH ) 6.0) under UVlight radiation. Reprinted with permission from Peng, T.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947. Copyright 2005 American Chemical Society. Figure 60.

60 shows the photocatalytic properties of mesoporous TiO 2 samples as prepared and calcined at different temperatures compared to those of TiO 2 P25 nanoparticles. All mesoporous TiO2 showed better activity than Deguessa P25 TiO 2. The optimum reactivity was obtained with the sample calcined at 400 °C, and the photoactivity gradually decreased with further increases in calcination temperature. Yang et al. found that TiO 2 nanotubes treated with H 2SO4 solutions showed photocatalytic activity on degradation of acid orange II in the following order: TiO 2 nanotubes treated with 1.0 mol/L H 2SO4 solution > TiO2 nanotubes treated with 0.2 mol/L H 2SO4 solution > untreated TiO2 nanotubes > TiO2 nanoparticles, since TiO2 nanotubes treated with H2SO4 were composed of smaller particles and had higher specific surface areas. 818 TiO2 aerogels were also suggested as promising candidates for photocatalysts.316,317,319 Degan et al. prepared TiO2 aerogels with a porosity of 90% and surface areas of 600 m2 /g, and they found that the photodegradation of salicylic acid on TiO2 aerogels, after 1 h of near-UV illumination, was about 10 times faster than that on the Degussa TiO 2.316,317 Figure 61 shows photodegradation profiles for the aerogel before and after annealing, as compared to the commercial Degussa P25 powder. 5.1.2. Metal-Doped TiO 2 Nanomaterials: Second Generation

Over the past decades, metal-doped TiO 2 nanomaterials have been widely studied for improved photocatalytic performance on the degradation of various organic pollutants, i.e., under visible light irradiation (refs 21, 430, 433 -435, 444, 446, 450-452, 455-457, 490, 515, 548, 810, 827836). Choi et al. conducted a systematic study on the photocatalytic activity of TiO 2 nanoparticles doped with 21 transition metal elements on the oxidation of CHCl 3 and the reduction of CCl4 and found that the photocatalytic activity was related to the electron configuration of the dopant ion in that dopant ions with closed electron shells had little or no effect on the activity. 434,435 Doping with Fe3+, Mo5+, Ru3+, Os3+, Re5+, V4+, and Rh3+ at 0.1-0.5 at % significantly increased the photoreactivity, while Co 3+ and Al3+ doping decreased the photoreactivity. The presence of metal ion dopants in the TiO 2 matrix significantly influenced the charge carrier recombination rates and interfacial electron-transfer



Photodegradation profiles of salicylic acid on annealed (Ela) and nonannealed (El) TiO2 aerogels as compared to a commercial Degussa P25. Reprinted with permission from Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1993, 97 , 12651. Copyright 1993 American Chemical Society. Figure 61.

rates. The photoreactivity of doped TiO 2 appeared to be a complex function of the dopant concentration, the energy level of dopants within the TiO 2 lattice, their d electronic configurations, the distribution of dopants, the electron donor concentrations, and the light intensity. Sn4+ ion-doped TiO2 nanoparticle films prepared by the plasma-enhanced CVD method displayed a higher photocatalytic activity for photodegradation of phenol than pure TiO2 under both UV and visible light, and the Sn 4+ dopant was found profitable to the separation of photogenerated carriers under both UV and visible light excitation. 433 Figure 62 shows the photocatalytic decomposition of phenol with reaction time under UV and visible light using Sn 4+-doped TiO2 nanoparticles as photocatalyst. 433 Fe-doped nanocrystalline TiO2 was shown to display higher photocatalytic activity with lower Fe content (optimal 0.05% mass fraction) than TiO 2 in the treatment of papermaking wastewater,837 and it was shown to be more efficient in the photoelectrocatalytic disinfection of E. coli than pure TiO2.827 V-doped TiO2 photocatalyst photooxidized ethanol under visible radiation and had comparable activity under UV radiation to that of pure TiO 2.548 Pt4+ ion-doped TiO2 nanoparticles exhibited higher visible light photocatalytic activities on the degradations of dichloroacetate and 4-chlorophenol,830 and Ag-TiO2 nanocatalysts displayed enhanced photocatalytic activity in the degradation of 2,4,6-trichlorophenol due to a better separation of photogenerated charge carriers and improved oxygen reduction inducing a higher extent of degradation of atoms. 809 Wei et al. synthesized La- and N-co-doped TiO 2 nanoparticles with superior catalytic activity under visible light, where N doping was responsible for the band gap narrowing of TiO2 and La3+ doping prevented the aggregation of nanoparticles.833 Chang et al. reported Cr- and N-co-doped TiO2 nanomaterials with visible light absorbance generally led to a reduction in photocatalytic efficacy in the decolorization of methylene blue, except at the low nitrogen doping concentration. 490 Bessekhouad et al. found that low concentration alkaline (Li, Na, K)-doped TiO 2 nanoparticles were promising materials for organic pollutants degradation. 430 Peng et al. found that in Be 2+-doped TiO2 nanomaterials,

Variation of phenol concentration with reaction time under (A) UV and (B) visible light: (a) pure TiO2 catalyst; (b) Sn4+-doped TiO2. From: Cao, Y.; Yang, W.; Zhang, W.; Liu, G.; Yue, P. New J. Chem. 2004, 28, 218 (http://dx.doi.org/10.1039/ b306845e) s Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS). Figure 62.

when the doping ions were in the shallow surface, the doping was beneficial, while, in the deep bulk, the doping was detrimental.451 However, not all the metal-doped TiO 2 nanomaterials showed higher photocatalytic activities than pure TiO 2 nanomaterials. Martin found V-doped TiO 2 nanoparticles had reduced photocatalytic activity on the photooxidation of 4-chlorophenol compared to pure TiO 2 nanoparticles. Vanadium appeared to reduce the photoreactivity of TiO 2 by promoting charge-carrier recombination with electron trapping at VO2+ centers or with hole trapping at V 4+ impurity centers, which shunted charge carriers away from the solid/ solution interface. 446 Hermann et al. found that although Crdoped (0.85 atomic %) TiO 2 absorbed in the visible region, its activity for oxidation of oxalic acid, propene, and 2-propanol and for O isotope exchange was null under visible illumination and was smaller under UV light than that of pure TiO2, due to an increase in electron -hole recombination at the Cr3+ ion sites.440 Luo et al. reported that the photoactivity of TiO 2 doped with 1.5 mol % Mo, 1 mol % V, 0.1 mol % V plus 1 mol % Al, or 0.1 mol % V plus 1 mol % Pb decreased, since the d electrons of Mo(4d) and V(3d), as majority carriers in TiO 2, could effectively quench the high-energy photogenerated holes at the impurity levels introduced by doping within the band gap of TiO 2.445 5.1.3. Nonmetal-Doped TiO 2 Nanomaterials: Third Generation

Nonmetal-doped TiO 2 nanomaterials have been regarded as the third generation photocatalyst. Various nonmetal-


Photocatalytic properties of TiO2- xN x and TiO2 based on decomposition rates [measuring the change in absorption of the reference light (∆abs)] of methylene blue as a function of the cutoff wavelength of the optical high-path filters under fluorescent light. The inset shows the decomposition rates of methylene blue in the aqueous solution under visible light as a function of the ratio of the decomposed area in the XPS spectra with the peak at 396 eV to the total area of N 1s. The total N concentrations were 1.0 atom % (a), 1.1 atom % (b), 1.4 atom % (c), 1.1 atom % (d), and 1.0 atom % (e). Reprinted with permission from Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269 (http:// www.sciencemag.org). Copyright 2001 AAAS. Figure 63.

doped TiO2 nanomaterials have been widely studied for their visible light photocatalytic activities (refs 21, 385, 426, 428, 452, 472-474, 481-487, 490, 492-494, 496, 505, 518, 520, 524, 525, 527, 532, 533, 802, 838 -847). Nonmetal-doped TiO2 nanomaterials have been demonstrated to have improved photocatalytic activities compared to those for pure TiO2 nanomaterials, especially in the visible light region.426,428,485,489,833,848 Figure 63 shows the decomposition of methylene blue using N-doped TiO2 as measured by Asahi and co-wokers. 489 It was found that N-doped TiO 2 had much higher photocatalytic activity than pure TiO 2 in the visible light region, while displaying lower activity in the UV-light region. A nitrogen concentration dependent performance of the photocatalytic activity of the nitrogen-doped TiO 2 was found in the visible region, and the active sites of N for photocatalysis under visible light were identified with the atomic β-N states peaking at 396 eV in the XPS spectra. 489 In the study of Irie and co-workers, the concentration dependent photocatalytic activity of the N-doped TiO 2 was attributed to the fact that the band structure of the N-doped TiO 2 with lower nitrogen concentration ( <2%) was different from that with higher concentration.483 It was found that the significant increase in photocatalytic activity in N-doped TiO 2 nanoparticles was due to the O -Ti-N bond formation as oxynitride during the substitutional doping process. 428,477 The photocatalytic oxidation of organic compounds by N-doped TiO 2 under visible illumination was mainly via reactions with surface intermediates of water oxidation or oxygen reduction, not by direct reactions with holes trapped at the N-induced midgap level.486 N-doped TiO2 nanotubes also exhibited high photocatalytic oxidation activity for decomposition of gaseous isopropanol into acetone and carbon dioxide when illuminated with visible light. 528 The photocatalytic activity of sulfur-doped TiO 2 has also been studied.492-494,496 The S-doped TiO2 was found to display a higher photocatalytic activity in the visible region but a lower photocatalytic activity in the UV region. 492-494 S-doped TiO2 prepared with different methods showed

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different photocatalytic activity under visible light due the different carrier behavior in these samples. 849 A noticeable photocatalytic activity on decompositions of methylene blue and isopropanal in the visible region was demonstrated for C-doped TiO 2 made from a TiC precursor.472,473 C-doped TiO2 made by pyrolyzing Ti metal in a natural gas flame displayed a much higher photoactivity in water splitting than pure TiO 2.476 C-doped TiO2 nanoparticles also displayed high photoactivity in degradation of trichloroacetic acid under visible light. 474 Yu et al. found that F-doped TiO 2 showed higher photocatalytic activity on the oxidation of acetone into CO 2 than did Degeussa P25 in the photodecomposition study of acetone under proper preparation conditions. 502 N/F-doped TiO2 nanomaterials had high visible light photocatalytic activities for decompositions of both acetaldehyde and trichloroethylene due to the creation of surface oxygen vacancies rather than the improvement of optical absorption properties.505,506,518,519 Luo et al. found that chlorine- and bromine-co-doped TiO 2 displayed a much higher photocatalytic activity than chlorine- or bromine-doped TiO 2.508

5.2. Photovoltaic Applications 5.2.1. The TiO 2 Nanocrystalline Electrode in DSSCs

Photovoltaics based on TiO 2 nanocrystalline electrodes have been widely studied. 9,28-32 A schematic presentation of the structure and operating principles of the DSSC is given in Figure 64. At the heart of the system is a nanocrystalline mesoporous TiO 2 film with a monolayer of the chargetransfer dye attached to its surface. The film is placed in contact with a redox electrolyte or an organic hole conductor. Photoexcitation of the dye injects an electron into the conduction band of TiO 2. The electron can be conducted to the outer circuit to drive the load and make electric power. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing a redox system, such as the iodide/ triiodide couple. The regeneration of the sensitizer by iodide prevents the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated in turn by the reduction of triiodide at the counterelectrode, with the circuit being completed via electron migration through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of TiO 2 and the redox potential of the electrolyte. Overall, the device generates electric power from light without suffering any permanent chemical transformation. 9,28-32 Cahen et al. explained the cause for the photocurrent and photovoltage in nanocrystalline mesoporous dye-sensitized solar cells in terms of the separation, recombination, and transport of electronic charge as well as in terms of electron energetics.721 The basic cause for the photovoltage is the change in the electron concentration in the nanocrystalline electron conductor that results from photoinduced charge injection from the dye. Pichot and Gregg found that the photovoltage was determined by photoinduced chemical potential gradients, not by equilibrium electric fields. 635 The maximum photovoltage is given by the difference in electron energies between the redox level and the bottom of the conduction band of the electron conductor, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs because of the enthalpic and entropic driving forces that exist at the dye/electron conductor



Figure 64.

under solar illumination with polypyridyl ruthenium and osmium sensitizers with the general structure ML 2(X)2, where L stands for 2,2 ′-bipyridyl-4,4′-dicarboxylic acid, M is Ru or Os, and X presents a halide, cyanide, thiocyanate, acetyl acetonate, thiacarbamate, or water substituent. 88 The dyesensitized solar cells with cis-dithiocyanatobis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) (N3) displayed absorption maxima at 518 and 380 nm and emission at 750 nm with a lifetime of 60 ns. 625,850 In 2001 the “black dye” tri(cyanato)-2,2′,2′′-terpyridyl-4,4′,4′′-tricarboxylate) ruthenium(II) was found to achieve 10.4% conversion efficiency in full sunlight. 631 Amphiphilic heteroleptic N3 equivalent dyes were recently applied to solar cells. 673 These amphiphilic heteroleptic sensitizers had several advantages compared to the N3 complex: (a) The ground-state p K a of the 4,4′dicarboxy-2,2′-bpy was higher to enhance the binding of the complex onto the TiO 2 surface. (b) The decreased charge on the sensitizer attenuated the electrostatic repulsion and increased the dye loading. (c) The presence of the hydrophobic moiety on the ligand increased the stability of solar cells toward water-induced desorption. (d) The oxidation potential of these complexes was cathodically shifted compared to that of the N3 sensitizer, which increased the reversibility of the ruthenium III/II couple, leading to enhanced stability. Combining the N3 dye with guanidinium thiocyanate brought a further increase in the open-circuit voltage of the solar cell. 30,31 Unlike the large amount of effort put forth to optimize the organic dyes in DSSCs in the past decades, attention has only recently been paid to the TiO 2 nanocrystalline electrode, and some important results have been obtained. In the following, various research efforts on the use of the TiO 2 nanocrystalline electrode for DSSCs are briefly summarized.

interface, with charge transport aided by such driving forces at the electron conductor/contact interface. The mesoporosity and nanocrystallinity of the semiconductor are important not only because of the large amount of dye that can be adsorbed on the very large surface but also for two additional reasons: (a) they allow the semiconductor small particles to become almost totally depleted upon immersion in the electrolyte (allowing for large photovoltages), and (b) the proximity of the electrolyte to all particles makes screening of injected electrons, and thus their transport, possible. 721 Many ruthenium complexes containing anchoring groups such as carboxylic acid, dihydroxy, and phosphonic acid on pyridyl ligands have been used as dyes in the DSSCs. Gra¨tzel et al. have been leading the research in this field since their breakthrough in the early 1990s. Tris(2,2 ′-bipyridyl-4,4′carboxylate) ruthenium(II) was used in DSSCs until the announcement in 1991 of a sensitized electrochemical photovoltaic device with a conversion efficiency of 7.1%

Zukalova et al. found that ordered mesoporous TiO 2 nanocrystalline films showed enhanced solar conversion efficiency by about 50% compared to traditional films of the same thickness made from randomly oriented anatase nanocrystals.312 The TiO2 nanocrystalline film was prepared via layerby-layer deposition with Pluronic P123 as template. The sensitizer used was cis-dithiocyanato(4,4′-dicarboxy-2,2′bipyridine)(4,4′-di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2′-bipyridine) ruthenium(II), N945. Figure 65 shows the photocurrent-voltage characteristics for solar cells based on ordered and nonordered TiO 2 films. When sensitized by N945, the 0.95- µm-thick nonorganized anatase film gave a conversion efficiency of only 2.21%, which increased to 2.74% with surface treatment by TiCl 4 prior to dye deposition. Under standard global AM 1.5 solar conditions, the cell with an ordered mesoporous TiO2 nanocrystallinne film gave a photocurrent density of I p ) 7 mA/cm2, an open circuit potential of U OC ) 0.799 V, and a fill factor of ff ) 0.72, yielding 4.04% conversion efficiency. This improvement resulted from a remarkable enhancement of the short circuit photocurrent, due to the huge surface area accessible to both the dye and the electrolyte. 312 5.2.1.2. TiO2 Nanotube Electrode. Adachi et al. found that dye-sensitized solar cells with electrodes made of disordered single-crystalline TiO 2 nanotubes (10-nm diameter, 30-300-nm length) displayed an efficiency of 4.88%, showing more than double the short-circuit current density compared to those made of TiO 2 nanoparticles of Deguessa P-25 in a similar thin-film thickness region. 569 Macak et al. found that, for Ru-dye (N3) sensitization of self-organized

(A) Structure and (B) principle of operation and energy level scheme of the dye-sensitized nanocrystalline solar cell. Photoexcitation of the sensitizer (S) is followed by electron injection into the conduction band of an oxide semiconductor film. The dye molecule is regenerated by the redox system, which itself is regenerated at the counterelectrode by electrons passed through the load. Potentials are referred to the normal hydrogen electrode (NHE). The open circuit voltage of the solar cell corresponds to the difference between the redox potential of the mediator and the Fermi level of the nanocrystalline film indicated with a dashed line. The energy levels drawn for the sensitizer and the redox mediator match the redox potentials of the doubly deprotonated N3 sensitizer ground state and the iodide/triiodide couple. Reprinted from Gra¨tzel, M. J. Photochem. Photobiol. A: Chem . 2004, 164, 3, Copyright 2004, with permission from Elsevier.

5.2.1.1. Mesoporous TiO2 Nanocrystalline Electrodes.


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Photocurrent-photovoltage characteristics of a TiO2 nanotube array DSSC under 100% AM-1.5 illumination. The inset shows an SEM image of TiO2 nanotubes. Reprinted with permission from Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6 , 215. Copyright 2006 American Chemical Society. Figure 66.

Photocurrent-voltage characteristics of a solar cell, based on TiO2 films sensitized by N945, cis-dithiocyanato(4,4′dicarboxy-2,2′-bipyridine)(4,4′-di-(2-(3,6-dimethoxyphenyl)ethenyl)2,2′-bipyridine) ruthenium(II). (1) Pluronic-templated three-layer film, 1.0- µm-thick; (2) nonorganized anatase treated by TiCl4, 0.95 µm-thick; (3) nonorganized anatase nontreated by TiCl4, 0.95- µmthick. The inset shows the SEM image of Pluronic-templated threelayer TiO2 films. Reprinted with permission from Zukalova, M.; Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gra¨tzel, M. Nano Lett. 2005, 5, 1789. Copyright 2005 American Chemical Society. Figure 65.

TiO2 nanotubes grown by Ti anodization, IPCE max values (at 540 nm) of 3.3% and 1.6% (at 530 nm) were obtained for 2.5- µm- and 500-nm-long nanotubes, respectively. 219 Ohsaki et al. found that the higher efficiency of solar cells with TiO2 nanotube-based electrodes resulted from an increase in electron density in nanotube electrodes compared to P25 electrodes. 851 Grimes et al. fabricated highly ordered nanotube arrays (46-nm pore diameter, 17-nm wall thickness, and 360-nm length) grown perpendicular to an F-doped SnO 2-coated glass substrate by anodic oxidization. 201 After crystallization by oxygen annealing and treatment with TiCl 4, the nanotube arrays were integrated into a DSC structure using a commercially available ruthenium-based dye N719. The cell generated a photocurrent of 7.87 mA/cm 2 with a photocurrent efficiency of 2.9%, using a 360-nm-thick electrode under AM 1.5 illumination. They found that the highly ordered TiO2 nanotube arrays had superior electron lifetimes and provided excellent pathways for electron percolation in comparison to nanoparticulate systems. Figure 66 shows the photocurrent-photovoltage characteristics of the TiO 2 nanotube DSSC.201 They also found that backside illuminated solar cells based on 6- µm-long highly ordered nanotubearray films sensitized by bis(tetrabutylammonium)- cis(dithiocyanato)- N , N -bis(4-carboxylato-4-carboxylic acid-2,2bipyridine)ruthenium(II) (commonly called “N719”) showed a power conversion efficiency of 4.24% under AM 1.5 illumination.203 5.2.1.3. Inversed TiO2 Opal. The relatively low efficiency obtained in solid-state DSSCs is attributed to the poor penetration of the material into pores of the thick TiO 2 films and the consequent noncontact of the hole transport layer with the titania electrode. A novel approach to increase the efficiency of solid-state Gra¨tzel solar cells was presented by Somani et al., using large-surface titania inverse opal films as electrodes in fabricating solid-state dye-sensitized organic inorganic hybrid Gra¨tzel solar cells. 352 Direct comparison

(A) Current-voltage ( I -V ) curves for an inversed opal cell obtained in the dark and under white light illumination using an AM 1.5 simulator ( I sc ) 1.8 × 10-7 A/cm2, V oc ) 0.78 V, FF ) 0.33). The inset shows an SEM image of an inverse opal TiO 2 film. (B) Current-voltage ( I -V ) characteristic of a nanocrystalline TiO2 cell in the dark and under white light illumination using an AM 1.5 simulator ( I sc ) 8.5 × 10-9 A/cm2, V oc ) 0.87 V, FF ) 0.40). Reprinted from Somani, P. R.; Dionigi, C.; Murgia, M.; Palles, D.; Nozar, P.; Ruani, G. Sol. Energy Mater. Sol. Cells 2005, 87 , 513, Copyright 2005, with permission from Elsevier. Figure 67.

indicated that light conversion efficiency increased by at least 1 order of magnitude by the usage of the inversed opal TiO 2 films rather than nanocrystalline TiO 2 films (Figure 67). The better performance of inversed opal cells was due to the wide and well-connected pores in mesoporous TiO 2 films that



Schematic band diagrams of working electrodes consisting of a TiO2-WO3 buffer layer between TCO and a P25 layer. From: Kang, T. S.; Moon, S. H.; Kim, K. J. J. Electrochem. Soc. 2002, 149, E155. Copyright 2002. Reproduced by permission of The Electrochemical Society, Inc. Figure 69.

Photocurrent density versus voltage for the photoelectrochemical cells based on the pure anatase (TiO2 II) and anataserutile (TiO2 I) nanocrystalline TiO2 electrodes sensitized by N3. The effective area for illumination is 0.5 cm2. The thicknesses of the sputter deposited layer and the nanocrystalline layer are 20 nm and 6 µm, respectively. Conditions: electrolyte, 0.5 M LiI + 0.04 M I2 in propylene carbonate (PC); room temperature; light intensity, 98 mW/cm2; AM1.5 spectral radiation. Inset: Performance parameters of solar cells. Reprinted Figure 2 from Han, H.; Zan, L.; Zhong, J.; Zhao, X. J. Mater. Sci. 2005, 40, 4921, Copyright 2005, with kind permission of Springer Science and Business Media. Figure 68.

allowed easy penetration of the hole transporting material, allowing good contact with the dye and hence the best efficiency of the cell. 5.2.1.4. Hybrid TiO2 Nanocrystalline Electrode. 5.2.1.4.1. Anatase- Rutile TiO2 Nanocrystalline Electrode. Han et al.

found that a hybrid TiO 2 electrode composed of a mixture of anatase and rutile phases showed a higher solar-to-electric energy conversion efficiency than one made of pure anatase.852,853 Figure 68 shows the performance of the photoelectrochemical cells built with pure anatase (TiO 2 II) and anatase-rutile (TiO2 I) nanocrystalline TiO2 electrodes sensitized by N3. These electrodes had the same crystalline sizes and surface areas (26 nm, BET 57 m 2 /g).852 TiO2 I had 71% anatase phase with 29% rutile phase, while TiO 2 II had pure anatase phase. The anatase-rutile-based DSSC showed higher performance (efficiency η ) 6.8%, short-circuit photocurrents density J sc ) 19.4 mA/cm2, open-circuit photovoltage V OC ) 652 mV, fill factor ff ) 0.53) than the pure TiO2 (η ) 5.3%, J sc ) 18.4 mA/cm2, V OC ) 582 mV, ff ) 0.51). 5.2.1.4.2. Nanocrystalline Electrode with a Buffer Layer.

In a standard nanoporous electrode during DSSC operation, two main problems are associated with the porous geometry: (a) the high-area cross section for recombination of photoinjected electrons with holes that are transferred to the electrochemical mediator and (b) the image field opposing the separation process that is distributed inside the TiO 2 nanoporous electrode. The conversion efficiency of a DSSC decreases due to recombination losses of photoinjected electrons with oxidized dye molecules or a redox couple at the surface of nanocrystalline TiO 2. Various methods have been adopted to prevent this loss. Kang et al. added a buffer layer of a TiO 2-WO3 composite material between a TCO substrate (Figure 69) and a TiO 2 layer and found that the buffer layer effectively isolated dye molecules and electrolytes from directly contacting the conducting substrate. 854 In the presence of the buffer layer having 15 -75 mol % WO 3,

both open-circuit photovoltage and short-circuit photocurrent were enhanced. In the case of the electrode having a buffer layer of less than about 10 mol % WO 3, due to the large negative V FB, a potential barrier to the conduction band electrons from TiO 2 emerged at the TiO 2-WO3 /TiO2 junction. This resulted in a drop in photoinjection efficiency and subsequently in the photocurrent. For electrodes having more than about 75 mol % WO 3, the conduction band edge of the buffer layer lay close to or lower than that of TCO, and the relative conduction band energy of the buffer layer was not particularly beneficial for the electron injection from the conduction band of TiO 2.854 5.2.1.4.3. Core-Shell Structured Nanocrystalline Electrode. Under the operating conditions of a DSSC, the

electrons need to diffuse several micrometers into the TiO 2 layer surrounded by electron acceptors at a distance of only several nanometers. The nanoporous structure of the TiO 2 layer provides a large surface area, allowing absorption of enough dye molecules to achieve significant optical density.10,855 However, the structure also enhances the recombination processes and decreases the total conversion efficiency of DSSC.856-858 The recombination processes are completely prohibited due to the lack of a significant electric field that could assist the separation of electrons from holes in the TiO2 layer, since small TiO 2 nanoparticles allow only limited band bending at the electrode surface. 721,856,857,859 Core-shell TiO2 electrodes consisting of a nanoporous TiO 2 covered with a shell of another metal oxide have been shown to slow the recombination processes by the formation of an energy barrier at the TiO 2 surface.560,648,649,860-865 The conduction band potential of the shell should be more negative than that of TiO 2 in order to generate an energy barrier for the reaction of the electrons present in TiO 2 with the oxidized dye or the redox mediator in solution. Two approaches are employed to fabricate the nanoporous core -shell electrodes. The first approach involves synthesis of core -shell nanoparticles that are applied onto the conducting substrate.560,648,649,863,866 An energy barrier forms not only at the electrode/electrolyte interface but also between the individual TiO2 nanoparticles. The second approach involves a nanoporous TiO2 electrode coated with the thin shell layer.670,860-862,864,865,867 The TiO2 nanoparticles are connected directly to each other allowing electron transport through TiO2. The approach involving nanoporous electrodes in a welldefined core-shell configuration is usually a TiO 2 core coated with Al2O3,596,865,868-870 MgO,871 SiO2,865 ZrO2,865 or Nb2O5.860,670 For example, Zaban et al. found that TiO 2 /Nb2O5


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Schematic view of the collector-shell electrode. This core shell electrode consists of a conductive nanoporous matrix that is coated with TiO2. Reprinted with permission from Chappel, S.; Grinis, L.; Ofir, A.; Zaban, A. J. Phys. Chem. B 2005, 109, 1643. Copyright 2005 American Chemical Society. Figure 71.

(A) Schematic view of the new bilayer nanoporous electrode which consists of a nanoporous TiO2 matrix covered with a thin layer of Nb2O5. The Nb2O5 coating forms an inherent energy barrier at the electrode/electrolyte interface, which reduces the recombination rate of the photoinjected electrons. From: Zaban, A.; Chen, S. G.; Chappel, S.; Gregg, B. A. Chem. Commun. 2000, 2231 (http://dx.doi.org/10.1039/b005921h) s Reproduced by permission of The Royal Society of Chemistry. (B) I -V curves of four DSSCs differing by the nanoporous electrodes used to fabricate them: the TiO2 reference electrode (a), and three bilayer electrodes (b-d). The Nb2O5 coating was made by a 30 s dipping of a 6 µm TiO2 matrix in a 5 mM solution of NbCl5 in dry ethanol (b), Nb(isopropoxide)5 in 2-propanol (c), and Nb(ethoxide)5 in ethanol (d). Reprinted with permission from Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. Copyright 2001 American Chemical Society. Figure 70.

nanoporous electrodes could improve the performance of dye-sensitized solar cells by >35%.670,860 Figure 70 shows a bilayer nanoporous electrode which consists of a nanoporous TiO2 matrix covered with a thin layer of Nb 2O5 and the performance of three TiO 2 electrodes coated with Nb 2O5. For the best coating condition, the photocurrent increased from 10.2 to 11.4 mA/cm2, the photovoltage from 661 to 730 mV, and the fill factor from 51.0 to 56.5%. As a result, the conversion efficiency of the solar cell increased by 35% from 3.62 to 4.97%. 860 They also found that sometimes the shell material shifted the conduction band potential of the core rather than forming an energy barrier. For example, coating of TiO2 with a SrTiO3 shell resulted in a shift of the TiO2 conduction band in the negative direction. 861,862 Consequently, introduction of a SrTiO 3-coated TiO2 electrode to a DSSC increased the open circuit photovoltage while reducing the short circuit photocurrent compared to that of the noncoated TiO 2 electrode. 861,862 Diamant et al. found that the mechanism by which the shell affected the electrode properties depended on the coating material. Coating materials included Nb 2O5, ZnO, SrTiO3, ZrO2, Al2O3, and SnO2.862 The coating Nb2O5 formed a surface energy barrier, which slowed the recombination reactions, while the other shell materials each formed a surface dipole layer that shifted the conduction band potential of the core TiO2. The shift direction and magnitude depended on the dipole parameters

which were induced by the properties of the two materials at the core/shell interface. 862 Palomares et al. found that the conformal growth of an overlayer of Al 2O3 on a nanocrystalline TiO 2 film resulted in a 4-fold retardation of interfacial charge recombination and a 30% improvement in photovoltaic device efficiency. 870 Fabregat-Santiago et al. found that the alumina barrier reduced the recombination of photoinjected electrons to both the dye cations and the oxidized redox couple, due to two effects: (a) almost complete passivation of surface trap states in TiO2 that were able to inject electrons to acceptor species and (b) slowing down by a factor of 3 -4 of the rate of interfacial charge transfer from conduction band states. 868 O’Regan found that the Al 2O3 layer acted as a tunnel barrier, thus increasing V oc and the fill factor. 869 Palomares et al. prepared SiO2, Al2O3, and ZrO2 overlayers by dipping mesoporous nanocrystalline TiO 2 films in organic solutions of their respective alkoxides, followed by sintering at 435 °C.865 The metal oxide overlayers acted as barrier layers for interfacial electron-transfer processes. The most basic overlayer coating, Al 2O3 (pzc ) 9.2), was optimal for retarding interfacial recombination losses under negative applied bias, with an increase in open-circuit voltage of up to 50 mV and a 35% improvement in overall device efficiency. Diamant et al. found that SrTiO 3-coated nanoporous TiO 2 electrodes increased the open circuit photovoltage while reducing the short circuit photocurrent and resulting in a 15% improvement of the overall conversion efficiency of the solar cell. 861 The SrTiO3 layer shifted the conduction band of the TiO 2 in the negative direction due to a surface dipole rather than forming an energy barrier at the TiO 2 /electrolyte interface.861,862 The shell having a more negative conduction band potential acted as an energy barrier that slowed recombination reactions. Photoexcitation of dye molecules anchored to ultrathin (e1 nm) outer shells of insulators or semiconductors on n-type semiconductor crystallites resulted in electron transfer to the inner core material. However, there is still considerable recombination that increases with the distance between the electron injection point and the current collector. In other words, the limited lifetime of the injected electron and the slow diffusion rate inside the porous structure limit the effective thickness of the nanoporous electrode. Chappel et al. proposed a electrode design, shown in Figure 71, with a core shell configuration based on a conductive ITO or Sb-doped SnO 2 matrix coated with TiO2.872 In principle, the conducting core extended the current collector into the nanoporous network and was



(a) SEM of a cross section of the bilayer photonic crystal-nano-TiO2 photoelectrode. The conductive glass is at the top of the image in part a. The photonic crystal layer and the nanocrystalline TiO2 layer are enlarged in parts b and c, respectively. Reprinted with permission from Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. Copyright 2003 American Chemical Society. Figure 72.

denoted the nanoporous “collector shell electrode”. Consequently, the distance between the injection spot and the current collector should decrease to several nanometers throughout the nanoporous electrode, in contrast to several micrometers with the standard electrode. All electrons injected into the electrode, including those generated several micrometers away from the substrate, had to travel a very short distance before reaching the current collector. As shown by several studies, transport shorter than 1 µm provides 100% collection efficiency. In addition, the new collector-shell electrode contained inherent screening capability due to the high doping level of the conducting matrix. Theoretically, the new design should enable efficient charge separation and collection for thick nanoporous layers and solid electrochemical mediators. They found that, unless the TiO 2 coating was thicker than 6 nm, the electrode performance was very low due to fast recombination. 872 5.2.1.4.4. Electrode Coupled with Photonic Crystals.

Development of photosensitizers with improved spectral response at the low-energy end of the solar spectrum has not proven so successful because dye molecules with high red absorbance have lower excited-state excess free energy, thus lowering the quantum yield for charge injection. Increasing the thickness of the film beyond 10 -12 µm in order to increase the absorbance in the red results in an increase in the electron transport length and the recombination rate, and a decrease in the photocurrent. An alternative approach to improving efficiency was to increase the path length of light by enhancing light scattering in the TiO 2 films.873-878 While the small size of TiO 2 nanoparticles (10 30 nm) employed to ensure a high surface area makes conventional nanocrystalline TiO 2 films poor light scatterers, mixing the nanoparticles with larger particles or applying a scattering layer to the nanocrystalline film has been shown to increase light harvesting by enhancing the scattering of light. 873-878

Nishimura347 and Halaoui333 reported an enhancement in the light conversion efficiency of dye-sensitized TiO 2 solar cells by coupling a conventional nanocrystalline TiO 2 film to a TiO2 inverse opal, with a 26% increase in the IPCE relative to that of a nanocrystalline film of the same overall thickness in the 550 -800 nm spectral range. They found that the bilayer architecture, rather than enhanced light harvesting within the inverse opal structures, was responsible for the bulk of the gain in the IPCE. 333 Figure 72 shows an SEM image of a cross section of the bilayer photonic crystal-nano-TiO2 photoelectrode.347 Figure 73 shows the sketch for the mechanism of the photonic crystal in enhancing absorption in certain regimes. 347 The fact that light waves were localized in different parts of the structure, depending on their energy, implied that an absorber in the high dielectric medium should interact more strongly with light at wavelengths to the red of the stop band, and less strongly to the blue. Effectively, the red part of the spectrum of this absorber would “borrow” intensity from the blue part. Figure 74A shows the effect of the TiO 2 photonic crystal as compared to a film of nanocrystalline TiO 2 on the absorption spectra when dye is adsorbed to the surface. 347 In a comparison of the spectrum of dye molecules adsorbed to the TiO2 photonic crystal film with that of a conventional nanocrystalline TiO2 film, there was a substantial enhancement absorbance on the red side of the stop band, as well as a slight attenuation of absorbance on the blue side of the stop band. The enhanced absorbance was most pronounced between 500 and 550 nm, but it persisted to a lesser degree at longer wavelengths. Figure 74B shows the enhancement of the performance of a bilayer electrode compared to a conventional nanocrystalline TiO 2 photoelectrode.347 Between 400 and 530 nm, there was little difference between the two kinds of electrodes. The close similarity in the maximum photocurrent from the two electrodes was consistent with


(A) Simplified optical band structure of a photonic crystal. Near the Brillouin zone center, light travels with velocity c0 / n, where c0 is the speed of light in a vacuum and n is the average refractive index. At photon energies approaching a full band gap or a stop band from the red side, the group velocity of light decreases and light can be increasingly described as a sinusoidal standing wave that has its highest amplitude in the high-refractiveindex part of the structure. At energies above the band gap or stop band, the standing wave is predominantly localized in the low index part of the photonic crystal, i.e., in the air voids. (B) Illustration of the effect of standing wave localization on dye absorbance. In an isotropic medium, the dye absorbs strongly in the blue but weakly in the red (heavy line). If the stop band is tuned to the position shown by the arrow, the blue absorbance is diminished and the red absorbance is increased when the dye is confined to the highrefractive-index part of the photonic crystal (dotted line). Reprinted with permission from Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. Copyright 2003 American Chemical Society. Figure 73.

the fact that both contain the same amount of dye. Between 540 and 750 nm, the short circuit photocurrent was substantially increased in the bilayer electrode. The overall gain, integrated over the visible spectrum (400 -750 nm), was about 30%. Localization of heavy photons at the edges of the photonic stop band 347,879,880 from Bragg diffraction in the periodic lattice and multiple scattering events at disordered regions in the photonic crystal or at disordered films led ultimately to enhanced backscattering. 333 This largely accounted for the enhanced light conversion efficiency in the red spectral range (600 -750 nm), where the sensitizer was a poor absorber. 333 5.2.2. Metal/Semiconductor Junction Schottky Diode Solar Cell

McFarland and Tang reported a multilayer photovoltaic device structure in which photon absorption occurred in photoreceptors deposited on the surface of an ultrathin metal/ semiconductor junction Schottky diode. 881 The device structure was a solid-state multilayer with a photoreceptor layer deposited on a 10 -50 nm Au film, which capped 200 nm of TiO2 on an ohmic metal back contact (Figure 75). The photon-to-electron conversion in this device occurred in four

Chen and Mao

(A) Absorption spectra of the TiO2 photonic crystal (a), the N719 dye adsorbed on the photonic crystal (b), and the dye adsorbed on a film of nanocrystalline TiO2 (c). The position of the stop band at 486 nm is indicated by the arrow. (B) Wavelength dependence of the short-circuit photocurrent in the bilayer electrode (a) and the conventional nanocrystalline TiO2 photoelectrode (b). The position of the stop band maximum in the bilayer electrode was 610 nm. Reprinted with permission from Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. Copyright 2003 American Chemical Society. Figure 74.

steps. First, light absorption occurred in the surface-absorbed photoreceptors, giving rise to energetic electrons. Second, electrons from the photoreceptor excited state were injected into the conduction levels of the adjacent conductor, where they travelled ballistically through the metal at an energy, 1e, above the Fermi energy, E f. Third, provided that 1e was greater than the Schottky barrier height, f , and the carrier mean-free path was long compared to the metal thickness, the electrons traversed the metal and entered the conduction levels of the semiconductor (internal electron emission). The absorbed photon energy was preserved in the remaining excess electron free energy when it was collected at the back ohmic contact, giving rise to the photovoltage, V . The photooxidized dye was reduced by transfer of thermalized electrons from states near E f in the adjacent metal. Devices fabricated by using a fluorescein photoreceptor on an Au/ TiO2 /Ti multilayer structure had typical open-circuit photovoltages of 600 -800 mV and short-circuit photocurrents of 10-18 mA cm-2 under 100 mW cm-2 visible light illumination: the internal quantum efficiency (electrons measured per photon absorbed) was 10%. This alternative approach to photovoltaic energy conversion might provide the basis for durable low-cost solar cells using a variety of materials. 5.2.3. Doped TiO 2 Nanomaterials-Based Solar Cell

Lindgren et al. found that N-doped TiO 2 nanocrystalline porous thin films showed visible light absorption in the wavelength range from 400 to 535 nm and generated an incident photon-to-current efficiency response in good agree-


Electron transfer in the operating photovoltaic device: (process A) photon absorption and electron excitation from the chromophore ground state, S, to the excited state, S*; (process B) energetic electron transfer from S* into and (ballistically) through the conducting surface layer and over the potential energy barrier into the semiconductor; (process C) conduction of electrons as majority carriers within the semiconductor to the ohmic back-contact and through the load; (process D) reduction of the oxidized chromophore, S, by a thermal electron from the conductor surface. Shown schematically are the relative energies of the electron levels within the device structures, the Schottky barrier, f , the Fermi energy, E f , and the semiconductor band gap, E g. Reprinted with permission from McFarland, E. W.; Tang, J. Nature 2003, 421, 616. Copyright Nature Publishing Group. Figure 75.

Reaction schemes for semiconductor photocatalysts. Reprinted Figure 2 from Kudo, A. Catal. Sur V. Asia 2003, 7 , 31, Copyright 2003, with kind permission of Springer Science and Business Media. Figure 76.

ment with the optical spectra. 385 For the best nitrogen-doped TiO2 electrodes, the photoinduced current due to visible light and at moderate bias increased around 200 times compared to the behavior of pure TiO 2 electrodes.

5.3. Photocatalytic Water Splitting 5.3.1. Fundamentals of Photocatalytic Water Splitting

An enormous research effort has been dedicated to the study of the properties and applications of TiO 2 under light illumination since the discovery of photocatalytic splitting of water on a TiO 2 electrode in 1972 (Fujishima and Honda).6-8 Photocatalytic splitting of water into H 2 and O2 using TiO2 nanomaterials continues to be a dream for clean and sustainable energy sources. 882 Figure 76 shows the principle of water splitting using a TiO2 photocatalyst.761 When TiO2 absorbs light with energy larger than the band gap, electrons and holes are generated in the conduction and valence bands, respectively. The photogenerated electrons and holes cause redox reactions. Water molecules are reduced by the electrons to form H 2


and oxidized by the holes to form O 2, leading to overall water splitting. 883-885 The width of the band gap and the potentials of the conduction and valence bands are important. The bottom level of the conduction band has to be more negative than the reduction potential of H + /H2 (0 V vs NHE), while the top level of the valence band has to be more positive than the oxidation potential of O 2 /H2O (1.23 V). The potential of the band structure of TiO 2 is just the thermodynamical requirement. Other factors such as charge separation, mobility, and lifetime of photogenerated electrons and holes also affect the photocatalytic properties of TiO 2. These factors are strongly affected by the bulk properties of the material such as crystallinity. Surface properties such as surface states, surface chemical groups, surface area, and active reaction sites are also important. 768 The water-splitting process in return affects the local pH environment and surface structures of the TiO2 electrode.769 Salvador conducted a thermodynamic and kinetic consideration of water-splitting and competitive reactions in the photoelectrochemical cell, and they found that the overvoltage for evolution of O must be minimized, which was on the order of 0.6 eV for n-TiO 2 electrodes loaded with RuO2.767 Cocatalysts such as Pt and NiO are often loaded on the surface in order to introduce active sites for H 2 evolution. Thus, suitable bulk and surface properties and energy structure are demanded for photocatalysts. Laser-induced photocatalytic oxidation/splitting of water over TiO2 catalysts was studied. 883,886,887 Sayama and Arakawa found that addition of carbonate salts to Pt-loaded TiO 2 suspensions led to highly efficient water splitting. 888 The carbonate ions affected both the Pt particles and the TiO 2 surface. The Pt was covered with some titanium hydroxide compounds and the rate of the back reaction on the Pt was suppressed effectively in the presence of carbonate ions. The carbonate species aided desorption of O 2 from the TiO 2 surface.888 Khan and Akikusa found that bare n-TiO 2 nanocrystalline film electrodes were unstable during watersplitting reactions under illumination of light and their stability could be significant improved when covered with Mn2O3.759 5.3.2. Use of Reversible Redox Mediators

It has been reported that pure TiO 2 could not easily split water into H2 and O2 in the simple aqueous suspension system.413,754,889 The main problem is the fast, undesired electron-hole recombination reaction.762 Therefore, it is important to prevent the electron -hole recombination process. The Pt-TiO2 system could be illustrated as a “shortcircuited” photoelectrochemical cell, where a TiO 2 semiconductor electrode and a platinum -metal counterelectrode are brought into contact. Well-dispersed metal particles act as miniphotocathodes, trapping electrons, which reduces water to hydrogen. The role of sacrificial reagents is shown in Figure 77. 761 When the photocatalytic reaction is carried out in aqueous solutions including easily oxidizable reducing reagents, photogenerated holes irreversibly oxidize the reducing reagents instead of water. This makes the photocatalyst electron-rich, and a H 2 evolution reaction is enhanced as shown in Figure 77a. On the other hand, in the presence of electron acceptors such as Ag + and Fe3+, the photogenerated electrons in the conduction band are consumed by them and an O2 evolution reaction is enhanced as shown in Figure 77b. These reactions using sacrificial reagents are regarded


Photocatalytic H2 (a) or O2 (b) evolution in the presence of sacrificial reagents. Reprinted Figure 5 from Kudo, A. Catal. Sur V. Asia 2003, 7 , 31, Copyright 2003, with kind permission of Springer Science and Business Media. Figure 77.

Proposed reaction mechanism for overall photocatalytic water splitting using a IO3- /I- redox mediator and a mixture of Pt-TiO2-antase and TiO2-rutile photocatalysts. Reprinted with permission from Abe, R.; Sayama, K.; Domen, K.; Arakawa, H. Chem. Phys. Lett. 2001, 344, 339. Copyright 2001 Elsevier. Figure 78.

as half reactions and are often employed for test reactions of photocatalytic H 2 or O 2 evolution. However, one should realize that the results do not guarantee a photocatalyst to be active for overall water splitting into H 2 and O2 in the absence of sacrificial reagents. A sacrificial reagent helps to control the electron -hole recombination process. The photoefficiency of the process can be improved by the addition of sacrificial reagents. 754,889,890 The sacrificial reagents help separation of the photoexcited electrons and holes. Various compounds such as methanol, ethanol, EDTA (an ethylenediaminetetraacetic derivative), Na2S, and Na2SO4 or ions such as I -, IO3-, CN-, and Fe3+ have been used as sacrificial reagents. 753-755,757,890,891 Abe et al. conducted a series of experiments on water splitting under sunlight. 753-755 They designed a new photocatalytic reaction that split water into H 2 and O2 by a twostep photoexcitation system composed of an IO 3- /I- shuttle redox mediator and two different TiO 2 photocatalysts: Ptloaded TiO2-anatase for H 2 evolution and TiO2-rutile for O 2 evolution (Figure 78). 753 Simultaneous gas evolution of H 2 (180 mmol/h) and O 2 (90 mmol/h) was observed from a basic (pH ) 11) NaI aqueous suspension of two different TiO 2 photocatalysts under UV radiation. The overall water splitting proceeded by the redox cycle between IO 3- and I- under basic conditions as follows: (a) water reduction to H 2 and

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I- oxidation to IO 3- over Pt-TiO2-anatase, and (b) IO 3reduction to I- and water oxidation to O 2 over TiO2-rutile. IO3- reduction to I - over Pt-TiO2-anatase is an undesirable reaction. If this reaction is suppressed, the total water-splitting reaction will take place more efficiently. The advantage of this system is that H 2 gas is evolved over the Pt -TiO2anatase photocatalyst only and that O 2 gas is evolved over the TiO2-rutile photocatalyst only, even from a mixture of IO3- and I- in a basic aqueous solution. Therefore, another undesirable backward reaction, H 2O formation from H 2 and O2 on Pt particles, was suppressed. 753 They found that addition of a small amount of iodide anion, I -, into the aqueous suspension of Pt -TiO2-anatase photocatalyst significantly improved the splitting into H 2 and O2 with a stoichiometric ratio. The iodide anion was adsorbed preferentially onto the Pt cocatalyst as iodine atom. This iodine layer effectively suppressed the backward reaction of water formation from H 2 and O2 to H2O over the Pt surface. 754 Fujihara et al. studied the photochemical splitting of water by combining the reduction of water to hydrogen using bromide ions and the oxidation of water to oxygen using FeIII ions.892 The bromide ions were oxidized to bromine on Pt-loaded TiO2 nanoparticles, and the Fe III ions were reduced to FeII ions on TiO2 nanoparticles. These two reactions were carried out in separated compartments and combined via platinum electrodes and cation-exchange membranes as shown in Figure 79. At the electrodes, Fe II ions were oxidized by bromine, and protons were transported through the membranes to maintain the electrical neutrality and pH of the solutions in the two compartments. As a result, water was continuously split into hydrogen and oxygen under radiation. The reversible reactions on photocatalysts which often suffered from the effects of back reactions were largely prevented due to the low concentration of the products in solution. Lee et al. found that a considerable amount of photocatalytic H2 was produced from water over NiO/TiO 2 in proportion to the hole scavenger CN -.890 Galinska and Walendziewski studied water splitting over a Pt -TiO2 catalyst with various sacrificial reagents, such as methanol, Na2S, EDTA, and I- and IO3- ions, and they found that the sacrificial reagents had a key role in hydrogen production via the photocatalyzed water-splitting reaction. 757 Photocatalytic water splitting was obtained when EDTA and Na 2S were used. They acted as effective hole scavengers, preventing oxygen formation and the recombination reaction of oxygen with hydrogen. 5.3.3. Use of TiO 2 Nanotubes

Mor et al. found that highly ordered TiO 2 nanotube arrays efficiently decomposed water under UV radiation. 198 The authors found that the nanotube wall thickness was a key parameter influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For TiO 2 nanotubes with 22-nm pore diameter and 34-nm wall thickness (Figure 80A), upon 320 -400 nm illumination at an intensity of 100 mW/cm 2, hydrogen gas was generated at the power -time normalized rate of 960 mmol/h W (24 mL/h W) at an overall conversion efficiency of 6.8% as shown in Figure 80B. 198,199 They also claimed that, for illumination at 320 -400 nm (98 mW/cm 2), the TiO2 nanotube-array photoanodes could generate H 2 by H2O photoelectrolysis with a photoconversion efficiency of 12.25%.212 Park et al. further found that, when doped with



(A) SEM images, top view, of 20 V TiO2 nanotube arrays anodized at 5 °C. (B) Photoconversion efficiency as a function of measured potential [vs Ag/AgCl] for 10 V samples anodized at four temperatures [i.e., 5, 25, 35, and 50 °C]. Reprinted with permission from Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. Copyright 2005 American Chemical Society. Figure 80.

(A) Schematic of the photocatalytic reaction cell for splitting water. (B) Energy diagram of splitting of water by combined photocatalytic reactions. From: Fujihara, K.; Ohno, T.; Matsumura, M. Faraday Trans. 1998, 94, 3705 (http://dx.doi.org/ 10.1039/a806398b) s Reproduced by permission of The Royal Society of Chemistry. Figure 79.

carbon, TiO2- xC x nanotube arrays showed more efficient water splitting under UV and visible light illumination ( >420 nm) than pure TiO 2 nanotube arrays. 475 5.3.4. Water Splitting under Visible Light 5.3.4.1. Water Splitting over Doped TiO2 Nanomaterials. In general, the conduction bands of stable oxide

semiconductor photocatalysts consisting of metal cations with a d0 and d10 configuration consist of empty orbitals (LUMO) of the metal cations. On the other hand, the valence bands consist of O 2p orbitals. The potential of this valence band (about +3 eV) is considerably more positive than the oxidation potential of H 2O to O2 ( E 0 ) 1.23 V). Therefore, the band gaps of oxide semiconductor photocatalysts with the potential for H 2 evolution inevitably become wide. Accordingly, a valence band or an electron donor level consisting of orbitals of some element, except for O 2p, has to be formed to make the band gaps or the energy gaps

Strategy of the development of photocatalysts with a visible light response. Reprinted Figure 6 from Kudo, A. Catal. Sur V. Asia 2003, 7 , 31, Copyright 2003, with kind permission of Springer Science and Business Media. Figure 81.

narrow. New photocatalysts having the band structure shown in Figure 81 are necessary in order to develop materials for splitting water into H 2 and O2 under visible light. 761 The created levels have to possess not only the thermodynamical potential for oxidation of H 2O but also the catalytic properties for the four-electron oxidation reaction. The following strategies can be considered for the development of visible light-driven photocatalysts: (i) forming a donor level above a valence band by doping some element into conventional photocatalysts with wide band gaps such as TiO 2; (ii) creating


a new valence band employing some element; and (iii) controling the band structure by making a solid solution. 761 Borgarello et al. found that water cleavage could be induced with visible light in colloidal solutions of Cr-doped TiO2 nanoparticles deposited with ultrafine Pt or RuO 2.431 A pronounced synergistic effect in catalytic activity was noted when both RuO 2 and Pt were co-deposited onto the particle. Jin and Lu found that Pt/B-doped TiO 2 was a good system for water splitting under a B 4O72- environment without sacrificial electron donor reagents. 893 Luo et al. found that Br-- and Cl--co-doped nanocrystalline TiO 2 with the absorption edge shifted to a lower energy region displayed higher efficiency for water splitting than pure TiO 2.508 Jing et al. found that a Ni-doped mesoporous TiO 2 photocatalyst with 0.2 wt % Pt accomplished hydrogen evolution at nearly 125.6 lmol/h compared to 81.2 lmol/h for TiO 2 P25.894 N-, B-doped TiO2 nanomaterials have displayed higher activity than pure TiO2 in water splitting, i.e., under visible light illumination.529,889 Khan et al. found that a C-doped TiO 2 nanocrystalline film with visible light response obtained by controlled combustion of Ti metal in a natural gas flame had a high water-splitting performance with a total conversion efficiency of 11% and a maximum photoconversion efficiency of 8.35% when illuminated at 40 mW/cm 2,476 although there were questions about its solar-to-hydrogen conversion efficiency by other researchers. 895-897 Matsuoka et al. developed visible light responsive TiO 2 nanocrystalline thin films by the radio frequency magnetron sputtering method, which decomposed water when Pt-loaded and in the presence of a sacrificial reagent such as methanol or silver nitrate under visible light. 763,764 5.3.4.2. Water Splitting over Dye-Sensitized TiO2.

Duonghong et al. found that TiO 2 loaded simultaneously with ultrafine Pt and RuO 2 displayed extremely high activity as an H2O decomposition catalyst under band gap excitation of the TiO2 and that, when Ru(bipy) 32+ or rhodamine B was used as a sensitizer, H 2O was decomposed under visible light. 898 Abe et al. investigated H 2 production over merocyanine or coumarin dye C343 or Ru complex dye N3 dye-sensitized Pt/TiO2 photocatalysts under visible light in a water acetonitrile solution containing iodide as an electron donor. 756 They found that the rates of H 2 evolution decreased with increasing proportion of water in the solutions because of a decrease in the energy gap between the redox potential of I3- /I- and the HOMO levels of the dyes, which decreases the efficiency of electron transfer from I - to dye. The energy diagram and the mechanism for the H 2 production from water over the dye-sensitized Pt/TiO 2 photocatalyst system are shown in Figure 82. The two key electron-transfer steps, electron injection from an excited state of the dye to the TiO 2 conduction band and oxidation of I - to I 3- (steps 2 and 5), occurred efficiently in acetonitrile solvent. The increased ratio of water hindered electron transfer from I - to the HOMO level of the oxidized dye (step 5). In addition, Park and Bard designed two different kinds of cells with bipolar dye-sensitized TiO 2 /Pt panels connected so that their photovoltages added to provide vectorial electron transfer for unassisted water splitting to yield the separated products H2 and O2.765 5.3.5. Coupled/Composite Water-Splitting System

Akikusa et al. found that a self-driven system for water splitting under illumination could be achieved with the

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Energy diagram of H2 production from water over dyesensitized Pt/TiO2 photocatalysts in the presence of I- or EDTA as an electron donor. Reprinted with permission from Abe, R.; Sayama, K.; Sugihara, H. J. Sol. Energy Eng. 2005, 127 , 413. Copyright 2005 by ASME. Figure 82.

combination of single-crystal p-SiC and nanocrystalline n-TiO2 photoelectrodes.899 Both photoelectrodes (p-SiC and n-TiO2) were placed side by side facing the light source and in contact with an electrolyte of 0.5 M H 2SO4. The open circuit potential was found to be 1.24 V between the n-TiO 2 and p-SiC photoelectrodes, with a maximum photocurrent density of 0.05 mA cm -2 under a closed circuit potential of 0.23 V, corresponding to an efficiency of 0.06%. The low cell photocurrent density and the photoconversion efficiency for the p-SiC/n-TiO 2 self-driven system for the water-splitting reaction were due to the high band gap energies of both semiconductors and high recombination of photogenerated carriers mainly in the covalently bonded p-SiC. Takabayashi et al. proposed a solar water-splitting system based on a composite polycrystalline-Si/doped TiO 2 thinfilm electrode for high-efficiency and low-cost by combining the advantages of Si and doped TiO 2: (1) an n-Si electrode with surface alkylation and a metal nanodot coating gave an efficient and stable photovoltaic characteristic, and (2) TiO2 doped with other elements, such as nitrogen and sulfur, could cause water photooxidation (oxygen photoevolution) under visible light illumination. 770 The structure and working mechanism of solar water splitting with this system is shown in Figure 83. Although a high solar-to-chemical conversion efficiency of more than 10% was calculated for this system, several major problems needed to be solved before the real device could show promising performance. 770

5.4. Electrochromic Devices TiO2 nanomaterials have been widely explored as electrochromic devices, such as electrochromic windows and displays.611,634,772,900-916 Electrochromism can be defined as the ability of a material to undergo color change upon oxidation or reduction. Electrochromic devices are able to vary their throughput of visible light and solar radiation upon electrical charging and discharging using a low voltage. A small voltage applied to the windows will cause them to darken; reversing the voltage causes them to lighten. Thus, one can regulate the amount of energy entering through a “smart window” so that the need for air conditioning in a cooled building decreases. The energy efficiency inherent in this technology can be large, provided that the control strategy is adequate. Additionally, the transmittance regulation can impart glare control as well as user control of the indoor environment. The absorbance, rather than the reflec-



The second type is the electrochromism of nanocrystalline TiO2 electrodes modified with viologens and/or anthrachinons equipped with a surface anchoring group.902-904,906-908,910,917-919 This category usually has fast switching times and considerable optical dynamics, due to the combination of good conductivity between the TiO 2 nanoparticles and the fast electron exchange between TiO 2 and the monolayer of the electrochromic compound covering each particle.904 Bach et al. demonstrated high-quality paperlike electrochromic displays based on nanostructured TiO 2 films modified with electrochromophores with excellent inkon-paper optical qualities, fast response times, and low power consumption.901 Moeller et al. demonstrated electrochromic pictures with unprecedented resolution (360 dpi) in transparent and reflective electrochromic displays (ECD) based on ink-jet printing technology and cascade-type crosslinking reactions of viologens in the mesopores of a TiO 2 electrode, with a completely transparent counterelectrode based on mesoporous antimony tin oxide coated with CeO 2.913 5.4.1. Fundamentals of Electrochromic Devices

(A) Schematic illustration and (B) the operation principle of solar water splitting with a composite polycrystallineSi/doped TiO2 semiconductor electrode. Reprinted from Takabayashi, S.; Nakamura, R.; Nakato, Y. J. Photochem. Photobiol., A: Chem. 2004, 166 , 107, Copyright 2004, with permission from Elsevier. Figure 83.

tance, is modulated so that the electrochromic devices tend to heat up in their low-transparent state. 752 Two types of electrochromism of nanocrystalline thin film TiO2 electrodes have been reported. The first type is the electrochromism of nanocrystalline TiO 2 electrodes in Licontaining electrolytes related to the reversible insertion of Li+ into the anatase lattice of the nanoparticles. 912 Hagfeldt et al. found that forward biasing of transparent nanocrystalline TiO2 films in lithium ion-containing organic electrolytes led to rapid and reversible coloration due to electron accumulation and Li + intercalation in the anatase lattice. 912 Absorption of >90% light throughout the visible and near IR could be switched on and off within a few seconds. The nanocrystalline morphology of the film played a role in enhancing the electrochromic process. Ottaviani et al. found that the rate of the electrochromic process was controlled by the diffusion of the Li + ions throughout the TiO 2 lattice.914 It was convenient to drive the electrochromic process with potentiostatic pulses, and under these conditions, many cycles with initially good color contrast and efficiencies which approached 100% were obtained with TiO2 thin film electrodes.

Figure 84A shows the principle of the electrochromism of a molecular monolayer adsorbed on TiO 2.902 A molecule, which functions as the electrochromophore and exhibits different colors in different oxidation states, must be chosen such that its redox potential lies above the conduction band edge of the TiO 2 nanocrystalline electrode at the liquid/solid interface. In this way, electrons can be transferred reversibly from the conduction band to the molecule. The TiO 2 electrode in fact behaves like a conductor for the adsorbed electroactive species. If the redox potential is situated below the conduction band edge, the reduction process is irreversible. Figure 84B shows the TiO 2 nanocrystalline electrochromic devices based on viologen (solvent: glutarodinitrile) with a counterelectrode made of Prussian blue. 902 The device could be switched back and forth between the colorless and the colored states within 1 s. The nanocrystalline structure of the TiO 2 film makes possible 100- to 1000-fold amplification compared to a flat surface as shown in Figure 85. 902 The combination of high conductivity of the nanocrystalline TiO 2 particles, fast electron exchange with the molecular monolayer, optical amplification by the porous structure, and fast charge compensation by ions in the contacting liquid makes the nanocrystalline electrodes highly attractive electrochromic elements. The principle of efficiency relies on fast interfacial electron transfer between the nanocrystalline TiO 2 and the adsorbed modifier as well as on the high surface area of the TiO2 support that amplifies optical phenomena by 2 or 3 orders of magnitude. 902 The investigated TiO 2 nanocrystalline electrodes include ordered 905 and disordered902-904 mesoporous films. Ordered mesoporous nanocrystalline TiO 2 electrodes were found to display enhanced color contrast yet have similar conduction band edge energy levels and electron percolation ability as electrodes made from nanocrystalline TiO2, attributed to the uniform and ordered mesopore architecture and the large accessible surface area for tethering viologen molecules. 905 5.4.2. Electrochromophore for an Electrochromic Device

The viologen group ( N , N ′-disubstituted-4,4 ′-bipyridinium) has been commonly chosen as an electrochromophore, for its remarkable stability in both the oxidized and the reduced (radical cation) states (Figure 86). 902,904,906,907 Oxidized vi-


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(A) Principle of the electrochromism of a molecular monolayer adsorbed on a semiconductor surface. Electrons are injected from the conducting substrate into the conduction band of the semiconductor and from there reduce the adsorbed electroactive molecule. Provided the redox potential of that molecule lies above the conduction band edge, the process is reversible by application of a positive potential to the conductive substrate. (B) Nanocrystalline electrochromic devices based on viologen (solvent: glutarodinitrile) with a counterelectrode made of Prussian blue, in the colorless and in the colored state. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.; Walder, L.; Gra¨tzel, M. Displays 1999, 20, 137, Copyright 1999, with permission from Elsevier. Figure 84.

ologen is colorless, while the radical cation can be blue, violet, purple, or green, depending on the substituents. The associated first reduction potential is between 0.2 and -0.6 V (vs NHE). The typical absorption spectrum of reduced N , N ′-dialkylviologen in an organic solvent has a maximum around 600 nm. With N , N ′-diarylviologens, the absorption band is shifted by about 50 nm to the red. In concentrated solution or in the solid state, viologen radical cations form dimers, with their blue-shifted absorption maximum in the 550 nm region. A second reduced state can be reached at potentials which are more negative by 0.2 -0.4 V. This state is neutral and almost colorless (yellowish). This second reduction is reversible in organic solvents like acetonitrile but not in water. The anchoring groups with strong affinity toward TiIV include carboxylates, salicylates, or phosphonates.902 Bonhote et al. examined phosphonated triarylamine as an electrochromophore due to its oxidation by the stable

triarylamminum radical cation, which is accompanied by a blue coloration with the absorption band at 730 nm. 903 Vayssieres et al. studied bis(phthalocyaninato)lutetium(III) complexes (Pc2Lu) as electrochromophores, and they found that the typical neutral green state of Pc 2Lu was reduced to a brown state at potentials < -0.3 V vs Ag/AgCl at neutral pH when Pc2Lu was adsorbed onto a nanostructured TiO 2 electrode.916 Ag-TiO2 films, prepared by loading nanoporous films with Ag nanoparticles by photocatalytic means, exhibited multicolor photochromism, which was related to the oxidation and reduction of Ag nanoparticles under UV and visible radiation.773 Please also see section 4.2.1.2 on Sensitization by Metal Nanoparticles. 5.4.3. Counterelectrode for an Electrochromic Device

Closed cells are built by combining a transparent nanocrystalline electrode with a counterelectrode able to provide



Principle of signal amplification by a TiO2 nanocrystalline film. Sintered 20-nm particles of TiO2 form a several millimeter thick film characterized by a very high surface area. Once derivatized with a molecular adsorbate, the structure contains the equivalent of hundreds of superposed monolayers. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.; Walder, L.; Gra¨tzel, M.Displays 1999, 20, 137, Copyright 1999, with permission from Elsevier. Figure 85.

ing glass and modified by the electrochromophore [ β-(10phenothiazyl)propoxy]phosphonic acid, which displayed cycles-switching times of <250 ms, a coloration efficiency of 270 cm2 C-1, and steady-state currents of <6 mA cm-2.908 Zinc can be used as a counterelectrode instead of PB for displays applications. 902 When the two electrodes are shortcircuited, the electrons flow from the zinc, which oxidizes to Zn2+ ions, to the viologens of the nanocrystalline electrode. The process can be reversed under a potential of 1 -2 V. Electrochromism mechanism of a viologen chromophore. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.; Walder, L.; Gra¨tzel, M. Displays 1999, 20, 137, Copyright 1999, with permission from Elsevier. Figure 86.

enough electrons to allow complete reduction of viologen. The simplest counterelectrode is conducting glass. Prussian blue (PB) is an inorganic polymeric material (iron phexacyanoferrate) that is commonly used on a conducting glass substrate as a counterelectrode. Being blue in the oxidized state and colorless in the reduced state, it is a suitable complementary electrochromic material to the nanocrystalline viologen electrode. When the latter electrode turns blue by reduction, the PB counterelectrode turns blue by oxidation.902,904 Bonhote et al. studied nanocrystalline WO3 films as counterelectrodes for electrochromic applications since they turn from colorless to blue by reduction and lithium ion insertion. 903 Fitzmaurice et al. constructed an electrochromic window based on a modified transparent nanostructured TiO 2 film supported on conducting glass and modified with the electrochromophore bis(2-phosphonoethyl)-4,4 ′-bipyridinium dichloride, the electrolyte LiClO 4, and ferrocene in γ-butyrolactone.906 They used a counterelectrode of conducting glass, which had excellent electrochromic performance with a coloration efficiency of 170 cm 2 C-1 at 608 nm, a switching time of 1 s, and stability over 10,000 steady test cycles. They upgraded this system with a counterelectrode based on a transparent nanostructured SnO 2 film supported on conduct-

5.4.4. Photoelectrochromic Devices

Pichot et al. demonstrated a photoelectrochromic smart window with flexible substrates and solid-state electrolytes based on a dye-sensitized TiO2 electrode spin-coated onto In-Sn oxide-coated polyester substrates coupled with a WO 3 electrochronic counterelectrode, separated by a cross-linked polymer electrolyte containing LiI (Figure 87). 634 The devices typically transmitted 75% of visible light in the bleached state. After a few minutes of exposure to white light, the windows turned dark blue, transmitting only 30% of visible light. They spontaneously bleached back to their initial noncolored state upon removal of the light source. The photoelectrochromic device ideally behaved like a capacitor: There was initially no mobile oxidized species (i.e., I 2) present in the electrolyte. A schematic representation of the components and the electron and ion transfers in the solidstate photoelectrochromic device is shown in Figure 87. The ultimate electron acceptor (WO 3) is localized as an insoluble material on the back electrode. Only the electron donor (I -), which serves as a regenerator to the oxidized dye, is initially present in the electrolyte introduced as LiI. Upon coloration of the device at short-circuiting under illumination, I 2 is generated at the TiO 2 electrode and Li + intercalates in WO3.

5.5. Hydrogen Storage Lim et al. found that TiO 2 nanotubes could reproducibly store up to approximately 2 wt % H 2 at room temperature


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Schematic representation of the components and the electron and ion transfers in a solid-state photoelectrochromic device. Upon illumination, electrons are injected into the TiO2 conduction band (CB), travel through the external circuit, and reduce the WO3 counterelectrode. Upon lithium ion intercalation into the reduced WO3 film, a “tungsten bronze” is formed that absorbs visible and near-IR radiation, presumably via an intervalence charge-transfer absorption. TCO stands for transparent conducting oxide, which is typically ITO or fluorine-doped tin oxide. From: Pichot, F.; Ferrere, S.; Pitts, R. J.; Gregg, B. A. J. Electrochem. Soc. 1999, 146 , 4324, Copyright 1999. Reproduced by permission of The Electrochemical Society, Inc. Figure 87.

and 6 MPa.165 About 75% of this stored hydrogen could be released when the hydrogen pressure was lowered to ambient conditions due to physisorption. Approximately 13% was weakly chemisorbed and could be released at 70 °C as H2, and approximately 12% was strongly bonded to oxide ions and released only at temperatures above 120 °C as H2O. The P-C isotherms of TiO 2 nanotubes are shown in Figure 88. At room temperature and a pressure of ∼900 psi (6 MPa), the atomic ratio H/TiO2 was ∼1.6, corresponding to ∼2.0 wt % H2 for TiO2 nanotubes, compared to a much lower hydrogen concentration of ∼0.8 wt % for bulk TiO 2. When the pressure was reduced, only ∼75% of the stored hydrogen could be released, whereas 25% of adsorbed hydrogen molecules were retained due to chemical adsorption. Bavykin et al. studied the sorption of hydrogen between the layers of the multilayered wall of nanotubular TiO 2 in the temperature range of -195 to 200 °C and at pressures of 0 to 6 bar. 157 Hydrogen could intercalate between layers in the walls of TiO 2 nanotubes forming host -guest compounds TiO2‚ xH2, where x e 1.5 and decreases at higher temperature. The rate of hydrogen uptake increased with temperature, and the characteristic time for hydrogen sorption in TiO2 nanotubes was several hours at 100 °C. The hydrogen adsorption isotherm for TiO 2 nanotubes at -195 °C is shown in Figure 89. Almost 1.5 hydrogen molecules per one Ti atom could be adsorbed at a hydrogen partial pressure of 2 bar. During the desorption of hydrogen, a large hysteresis was observed; even at 0 bar of pressure, the uptake of hydrogen achieved a 1.25 molar ratio (point B). The adsorption of hydrogen was a reversible process. Heating the sample in a vacuum to 200 °C led to a complete desorption of hydrogen, returning the weight of the sample to its initial value (point A). The author found that the diffusion of hydrogen molecules in the axial direction between the layers in multilayered walls of TiO 2 nanotubes was the rate-limiting step of the process of intercalation and the rate of hydrogen intercalation depended on the inverse

(a) Pressure-concentration isotherms of TiO2 nanotubes and bulk TiO2 at room temperature. (b) Pressure-concentration isotherms of TiO2 nanotubes at 24, 70, and 120 °C. Reprinted with permission from Lim, S. H.; Luo, J.; Zhong, Z.; Ji, W.; Lin, J. Inorg. Chem. 2005, 44, 4124. Copyright 2005 American Chemical Society. Figure 88.

Isotherm for (9) hydrogen sorption into and ( O) desorption out of the pores of TiO2 nanotubes at -196 °C. Reprinted with permission from Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Friedrich, J. M.; Walsh, F. C. J. Phys. Chem. B 2005, 109, 19422. Copyright 2005 American Chemical Society. Figure 89.

of the square of nanotube length from their proposed diffusion model. Recently, Xu et al. studied the hydrogen storage properties of a series of five pristine micro- and mesoporous Ti oxide materials, synthesized from C6, C8, C10, C12, and C14 amine templates possessing BET surface areas ranging from 643 to 1063 m 2 /g, and they found that at 77 K the isotherms for all materials gently rose sharply at low pressure and continued to rise in a linear fashion from 10 atm onward to 65 atm and then return on desorption without significant hysteresis. Extrapolation to 100 atm could yield total storage



Plot of real-time variation of resistance before, during, and after cleaning the contaminant, motor oil 10W-30, with UV exposure. The plot, broken into four parts for clarity, shows (a) the original sensor behavior from time 10 to 1000 s, (b) the behavior of the sensor over time 100-6000 s, during which the sensor is contaminated with oil, losing its hydrogen-sensing capabilities, and is initially exposed to UV light, and (c) the behavior of the sensor from time 5000 s to 45,000 s. At time 7000 s, the UV is turned off, with the sensor regaining its nominal starting resistance of approximately 100,000 Ω, at which point it is exposed to 1000 ppm hydrogen and its resistance changes by a factor of approximately 50. The sensor is then again exposed to UV, from roughly time 15,000 s to 29,000 s. After this second UV exposure, the sensor is again exposed to 1000 ppm hydrogen, showing an approximate factor of 500 change in electrical resistance. The sensor is once again exposed to UV, from time 36,000 s. (d) Sensor behavior from time 45,000 to 70,000 s continues with UV exposure of the sensor until time 52,000 s, after which the sensor is repeatedly cycled between air and 1000 ppm hydrogen, showing a relative change in impedance of approximately 1000×. Compared to the hydrogen sensitivity of a noncontaminated sensor, the relative response of the “recovered” sensor is within a factor of 2. Reprinted with permission from Mor, G. K.; Carvalho, M. A.; Varghese, O. K.; Pishko, M. V.; Grimes, C. A. J. Mater. Res. 2004, 19, 628. Copyright 2004 Materials Research Society. Figure 90.

values as high as 5.36 wt % and 29.37 kg/m 3, and surface Ti reduction by the appropriate organometallic reagent provided an increase in performance, possibly because of a Kubas-type interaction. 920

5.6. Sensing Applications TiO2 nanocrystalline films have been widely studied as sensors for various gases (refs 194, 196, 202, 206 -209, 211, 322, 745-747, 750, 751, and 921-961). Grimes et al. conducted a series of excellent studies on sensing using TiO 2 nanotubes.194,196,206-209,211 They found that TiO 2 nanotubes were excellent room-temperature hydrogen sensors not only with a high sensitivity of 10 4 but also with an ability to selfclean photoactively after environmental contamination. 196 At 24 °C, in response to 1000 ppm of hydrogen, the sensors showed a fully reversible change in electrical resistance of approximately 175,000%. The hydrogen-sensing capabilities of the sensors were largely recovered by ultraviolet (UV) light exposure after being completely extinguished by a rather extreme means of sensor contamination: immersion of the sensor in motor oil. Figure 90 shows a plot of real-time variation of resistance before, during, and after cleaning the contaminant, motor oil 10W-30, with UV exposure. Many types of TiO2 nanomaterial-based room-temperature hydrogen sensors are based on Schottky barrier modulation of devices like Pd/TiO 2 or Pt/TiO2.922,947,954 Elevated temperature hydrogen sensors examine the electrical resistance change with hydrogen concentration. Birkefeld et al. found

that the resistance of anatase TiO 2 varied in the presence of CO and H2 at temperatures above 500 °C, but on doping with 10% alumina it became selective for hydrogen. 922 Shimizu et al. reported that anodized nanoporous titania films with a Pd Schottky barrier were sensitive to hydrogen at 250 °C.951,952 Kobayashi et al. investigated the mechanism of hydrogen sensing by Pd/TiO 2 Schottky diodes, and they found that the formation of adsorbed water from adsorbed oxygen at the Pd/TiO 2 interface was the dominant reaction for the Pd/TiO2(001) diodes throughout the hydrogen concentration range of 0 -3000 ppm; for the Pd/TiO 2(100) diodes, this reaction was dominant only for hydrogen concentrations below 100 ppm and the hydrogen adsorption on bare Pd atoms became dominant for higher hydrogen concentrations.939,940 Carney et al. found that sensors based on SnO2-TiO2 with higher surface areas were more sensitive to H 2 in the presence of O 2 by measuring the change in the electrical resistance of the sensor upon exposure to different hydrogen concentrations under a constant hydrogen gas flow rate.923 Devi et al. found that ordered mesoporous TiO 2 exhibited higher H 2 and CO sensitivities than sensors made from common TiO 2 powders due to increased surface area, and the sensitivity could be further improved by loading the sensor with 0.5 mol % Nb 2O5.929 Gao et al. found that nanoscale TiO2 displayed higher performance in H 2 sensing than microscale TiO 2 due to larger surface area. 932 Oxygen sensors based on TiO 2 nanomaterials include TiO2- x,961 TiO2-Nb2O5,928 CeO2-TiO2,957 and Ta-,935


(A) Conductance response of TiO2 nanoparticle films to methanol in concentrations going from zero (baseline) to 50 µmol/mol in 10 µmol/mol steps, sequentially, sequentially, at a sensor temperature of 450 °C. Between the steps, the analyte concentration was returned returned to 0 µmol/mol. The thickness of the films was controlled by the number of drops of the nanoparticle dispersion used to deposit the film. (B) Comparison of the sensitivity of a TiO2 thin film deposited by CVD (solid line) and that of a TiO 2 thin film composed composed of 15-nm-diameter 15-nm-diameter anatase nanoparticles (dotted line). Reprinted from Benkstein, K. D.; Semancik, S. Sens. Actuators, B 2006, 113, 445, Copyright 2006, with permission from Elsevier. Figure 91.

Nb-,949 Cr-,949 and Pt-doped TiO 2.950 Pt-doped TiO2 sensors showed improved gas sensitivit sensitivity, y, low operation temperature (350-800 °C), and short response time ( <0.1 s).941,950,958 The oxygen-sensing mechanism was the combination of Pt/TiO 2 interfaces in a Schottky-barrier mechanism and an oxygenvacancy bulk effect mechanism. 959 At high temperatures, TiO2 devices can be used as thermodynamically controlled bulk defect sensors to detect oxygen over a large range of partial pressures; at low temperatures, Pt/TiO 2 Schottky diodes make extremely sensitive oxygen detection possible. 938 In Ta-doped TiO 2 sensors, oxygen vacancies formed by photoirradiation acted as oxygen-sensing sites. 935 Sotter et al. found Nb-do Nb-doped ped TiO 2 nanomaterials to be good sensor materials for O 2.953 Nb- and Cr-doped TiO 2 nanocrystalline films displayed higher O 2 sensitivity than pure TiO 2 films 949 in that Nb 5+-doped TiO2 showed 65 times enhancement in the sensitivity compared to undoped material at a lower operating temperature. 474 TiO2 nanom nanomater aterials ials are prom promising ising candidates candidates for CO sensing927,945 and for methanol and ethanol sensing. 933,946,955,956

Chen and Mao

Ruiz et al. found that La-doped TiO 2 nanoparticles were good sensing materials for ethanol based on electrical resistance, 746 whil wh ilee Cu Cu-- or Co Co-d -dop oped ed Ti TiO O 2 nano nanoparti particles cles were good 747 candidates for CO sensing. Garzella et al. found W-doped TiO2 displayed better performance for ethanol sensing than pure TiO2.934 The addition of Ta and Nb to TiO 2 was beneficial for stabilization of the nanophase, resulting in selectiv sele ctivity ity enha enhanceme ncement nt towar towardd CO 930,931 and NO2.930,960 Comini et al. found that the sensitivity enhancement toward ethanol and methanol of TiO 2 films could be improved when doped with Pt and Nb. 926 Benkstein and Semancik found that mesoporous TiO 2 nanoparticle thin films prepared on MEMS micro-hot-plate platforms could be used as high-sensitivity conductometric gas sensor materials. 921 The nanoparticle films were deposited onto selected micro-hot-plates in a multielement array via microcapillary pipet and were sintered using the micro-hotplate. Figure 91A shows the conductometric response of four TiO2 nanoparticle films. The relative thickness of the films was varied by using one, two, three, or four drops of 6% mass fraction TiO 2 to cast the film. Sensitivity was defined as the ratio of the film conductance in the presence of an analyte to the baseline conductance measured in dry air ( S thicke ckerr fil films ms sho showed wed a hig higher her bas baseli eline ne ) G / G0). The thi conductancee and a higher overall sensitivity to methanol ( G / conductanc G0(1 drop) ) 4.1, G / G0(4 drops) ) 7.5). Shown in Figure 91B are sensitivity responses to µmol/mol levels of methanol of a mesoporous TiO 2 nanoparticle film and a CVD TiO 2 film. The nanoparticle films were found to demonstrate higher sensitivity to target analytes, attributed to the high internal surface area of the porous nanoparticle films. Montesperelli et al. found K-doped TiO 2 nanocrystalline films showed high sensitivity of magnitude of 10 7 with great stability over time. 943 Yadav et al. fabricated TiO 2 nanocrystalline films as optical humidity sensors based on the variations in the intensity of light with in humidity changes. 750 The sensor element consisted of a thin U-shaped borosil glass rod with a film of TiO 2 deposited on it. Both the ends of the glass rod were coupled to optical fibers. Light from a He Ne laser was launched into the sensing element through one of them. Light received from the other fiber was fed into an optical power meter.

6. Summary Over the past decades, the tremendous effort put into TiO 2 nanoma nan omater terial ialss has res result ulted ed in a ric richh dat databa abase se for the their ir synthesis, properties, modifications, and applications. The continuing breakthroughs breakthroughs in the synthesis and modificati modifications ons of TiO2 nanomater nanomaterials ials have brought new properties and new applications with improved performance. Accompanied by the progress in the synthesis of TiO 2 nanoparticles are new findin fin dings gs in the syn synthe thesis sis of TiO2 nanorods, nanotubes, nanowires, as well as mesoporous and photonic structures. Besides the well-know quantum-confinement effect, these new nanomaterials demonstrate size-dependent as well as shape- and structure-dependent optical, electronic, thermal, and structural properties. TiO 2 nanomaterials have continued to be hig highly hly act active ive in pho photoc tocata atalyt lytic ic and pho photov tovolt oltaic aic applications, and they also demonstrate new applications including electrochromics, sensing, and hydrogen storage. This steady progress has demonstrated that TiO 2 nanomaterials are playing and will continue to play an important role in the protections of the environment and in the search for renewable and clean energy technologies.


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