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Titanium Dioxide Nanomaterials: Synthesis,Properties, Modifications, and Applications

Xiaobo Chen, and Samuel S. MaoChem. Rev. , 2007 , 107 (7), 2891-2959• DOI: 10.1021/cr0500535 • Publication Date (Web): 23 June 2007

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Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, andApplications

Xiaobo Chen* and Samuel S. Mao†

Lawrence Berkeley National Laboratory, and University of California, Berkeley, California 94720

Received March 27, 2006

Contents 1. Introduction 28912. Synthetic Methods for TiO2 Nanostructures 2892

2.1. Sol−Gel Method 28922.2. Micelle and Inverse Micelle Methods 28952.3. Sol Method 28962.4. Hydrothermal Method 28982.5. Solvothermal Method 29012.6. Direct Oxidation Method 29022.7. Chemical Vapor Deposition 29032.8. Physical Vapor Deposition 29042.9. Electrodeposition 2904

2.10. Sonochemical Method 29042.11. Microwave Method 29042.12. TiO2 Mesoporous/Nanoporous Materials 29052.13. TiO2 Aerogels 29062.14. TiO2 Opal and Photonic Materials 29072.15. Preparation of TiO2 Nanosheets 2908

3. Properties of TiO2 Nanomaterials 29093.1. Structural Properties of TiO2 Nanomaterials 29093.2. Thermodynamic Properties of TiO2

Nanomaterials2911

3.3. X-ray Diffraction Properties of TiO2Nanomaterials 29123.4. Raman Vibration Properties of TiO2

Nanomaterials2912

3.5. Electronic Properties of TiO2 Nanomaterials 29133.6. Optical Properties of TiO2 Nanomaterials 29153.7. Photon-Induced Electron and Hole Properties

of TiO2 Nanomaterials2918

4. Modifications of TiO2 Nanomaterials 29204.1. Bulk Chemical Modification: Doping 2921

4.1.1. Synthesis of Doped TiO2 Nanomaterials 29214.1.2. Properties of Doped TiO2 Nanomaterials 2921

4.2. Surface Chemical Modifications 29264.2.1. Inorganic Sensitization 2926

5. Applications of TiO2 Nanomaterials 29295.1. Photocatalytic Applications 2929

5.1.1. Pure TiO2 Nanomaterials: FirstGeneration

2930

5.1.2. Metal-Doped TiO2 Nanomaterials:Second Generation

2930

5.1.3. Nonmetal-Doped TiO2 Nanomaterials:Third Generation

2931

5.2. Photovoltaic Applications 29325.2.1. The TiO2 Nanocrystalline Electrode in

DSSCs2932

5.2.2. Metal/Semiconductor Junction SchottkyDiode Solar Cell

2938

5.2.3. Doped TiO2 Nanomaterials-Based SolarCell

2938

5.3. Photocatalytic Water Splitting 29395.3.1. Fundamentals of Photocatalytic Water

Splitting2939

5.3.2. Use of Reversible Redox Mediators 29395.3.3. Use of TiO2 Nanotubes 29405.3.4. Water Splitting under Visible Light 2945.3.5. Coupled/Composite Water-Splitting

System2942

5.4. Electrochromic Devices 29425.4.1. Fundamentals of Electrochromic Devices 2945.4.2. Electrochromophore for an Electrochromic

Device2943

5.4.3. Counterelectrode for an ElectrochromicDevice

2944

5.4.4. Photoelectrochromic Devices 29455.5. Hydrogen Storage 29455.6. Sensing Applications 2947

6. Summary 29487. Acknowledgment 29498. References 2949

1. Introduction Since its commercial production in the early twenti

century, titanium dioxide (TiO2) has been widely used as apigment1 and in sunscreens,2,3paints,4 ointments, toothpaste,5

etc. In 1972, Fujishima and Honda discovered the phenoenon of photocatalytic splitting of water on a TiO2 electrodeunder ultraviolet (UV) light.6- 8 Since then, enormous effortshave been devoted to the research of TiO2 material, whichhas led to many promising applications in areas ranging fphotovoltaics and photocatalysis to photo-/electrochromand sensors.9- 12 These applications can be roughly divideinto “energy” and “environmental” categories, many of whdepend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e.g.,with inorganic and organic dyes) and on the interactionsTiO2 materials with the environment.

An exponential growth of research activities has been sin nanoscience and nanotechnology in the past decades.13- 17

New physical and chemical properties emerge when the of the material becomes smaller and smaller, and down

* Corresponding author. E-mail: [email protected].† E-mail: [email protected].

2891Chem. Rev. 2007, 107, 2891−2959

10.1021/cr0500535 CCC: $65.00 © 2007 American Chemical SocietyPublished on Web 06/23/2007



the nanometer scale. Properties also vary as the shapes of the shrinking nanomaterials change. Many excellent reviewsand reports on the preparation and properties of nanomaterialshave been published recently.6- 44 Among the unique proper-ties of nanomaterials, the movement of electrons and holesin semiconductor nanomaterials is primarily governed by the

well-known quantum confinement, and the transport proper-ties related to phonons and photons are largely affected bythe size and geometry of the materials.13- 16 The specificsurface area and surface-to-volume ratio increase dramati-cally as the size of a material decreases.13,21The high surfacearea brought about by small particle size is beneficial to manyTiO2-based devices, as it facilitates reaction/interactionbetween the devices and the interacting media, which mainlyoccurs on the surface or at the interface and strongly dependson the surface area of the material. Thus, the performanceof TiO2-based devices is largely influenced by the sizes of the TiO2 building units, apparently at the nanometer scale.

As the most promising photocatalyst,7,11,12,33TiO2 mate-rials are expected to play an important role in helping solve

many serious environmental and pollution challenges. T2also bears tremendous hope in helping ease the energy crthrough effective utilization of solar energy based photovoltaic and water-splitting devices.9,31,32As continuedbreakthroughs have been made in the preparation, modiftion, and applications of TiO2 nanomaterials in recent years,especially after a series of great reviews of the subjectthe 1990s.7,8,10- 12,33,45we believe that a new and comprehensive review of TiO2 nanomaterials would further promot

TiO2-based research and development efforts to tackle tenvironmental and energy challenges we are currently facHere, we focus on recent progress in the synthesis, propermodifications, and applications of TiO2 nanomaterials. Thesyntheses of TiO2 nanomaterials, including nanoparticlesnanorods, nanowires, and nanotubes are primarily categorwith the preparation method. The preparations of mesorous/nanoporous TiO2, TiO2 aerogels, opals, and photonicmaterials are summarized separately. In reviewing nanomterial synthesis, we present a typical procedure and repsentative transmission or scanning electron microscoimages to give a direct impression of how these nanomrials are obtained and how they normally appear. For detainstructions on each synthesis, the readers are referred

the corresponding literature.The structural, thermal, electronic, and optical propertof TiO2 nanomaterials are reviewed in the second sectioAs the size, shape, and crystal structure of TiO2 nanomate-rials vary, not only does surface stability change but athe transitions between different phases of TiO2 underpressure or heat become size dependent. The dependencX-ray diffraction patterns and Raman vibrational spectrathe size of TiO2 nanomaterials is also summarized, as thecould help to determine the size to some extent, althoucorrelation of the spectra with the size of TiO2 nanomaterialsis not straightforward. The review of modifications of T2nanomaterials is mainly limited to the research relatedthe modifications of the optical properties of TiO2 nanoma-terials, since many applications of TiO2 nanomaterials areclosely related to their optical properties. TiO2 nanomaterialsnormally are transparent in the visible light region. By dopor sensitization, it is possible to improve the optical sensiity and activity of TiO2 nanomaterials in the visible lightregion. Environmental (photocatalysis and sensing) aenergy (photovoltaics, water splitting, photo-/electrochromand hydrogen storage) applications are reviewed withemphasis on clean and sustainable energy, since the increing energy demand and environmental pollution creatpressing need for clean and sustainable energy solutions. fundamentals and working principles of the TiO2 nanoma-terials-based devices are discussed to facilitate the undstanding and further improvement of current and practiTiO2 nanotechnology.

2. Synthetic Methods for TiO 2 Nanostructures

2.1. Sol−Gel MethodThe sol- gel method is a versatile process used in maki

various ceramic materials.46- 50 In a typical sol- gel process,a colloidal suspension, or a sol, is formed from the hydroland polymerization reactions of the precursors, which usually inorganic metal salts or metal organic compounsuch as metal alkoxides. Complete polymerization and lof solvent leads to the transition from the liquid sol intsolid gel phase. Thin films can be produced on a piece

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

Dr. Samuel S. Mao is a career staff scientist at Lawrence Berkeley NationalLaboratory and an adjunct faculty at The University of California atBerkeley. He obtained his Ph.D. degree in Engineering from The Universityof California at Berkeley in 2000. His current research involves thedevelopment of nanostructured materials and devices, as well as ultrafastlaser technologies. Dr. Mao is the team leader of a high throughputmaterials processing program supported by the U.S. Department of Ener-gy.

2892 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and M



substrate by spin-coating or dip-coating. A wet gel will formwhen the sol is cast into a mold, and the wet gel is convertedinto a dense ceramic with further drying and heat treatment.A highly porous and extremely low-density material calledan aerogel is obtained if the solvent in a wet gel is removedunder a supercritical condition. Ceramic fibers can be drawnfrom the sol when the viscosity of a sol is adjusted into aproper viscosity range. Ultrafine and uniform ceramicpowders are formed by precipitation, spray pyrolysis, or

emulsion techniques. Under proper conditions, nanomaterialscan be obtained.TiO2 nanomaterials have been synthesized with the sol-

gel method from hydrolysis of a titanium precusor.51- 78 Thisprocess normally proceeds via an acid-catalyzed hydrolysisstep of titanium(IV) alkoxide followed by condensa-tion.51,63,66,79- 91 The development of Ti- O- Ti chains isfavored with low content of water, low hydrolysis rates, andexcess titanium alkoxide in the reaction mixture. Three-dimensional polymeric skeletons with close packing resultfrom the development of Ti- O- Ti chains. The formationof Ti(OH)4 is favored with high hydrolysis rates for amedium amount of water. The presence of a large quantityof Ti- OH and insufficient development of three-dimensionalpolymeric skeletons lead to loosely packed first-orderparticles. Polymeric Ti- O- Ti chains are developed in thepresence of a large excess of water. Closely packed first-order particles are yielded via a three-dimensionally devel-oped gel skeleton.51,63,66,79- 91 From the study on the growthkinetics of TiO2 nanoparticles in aqueous solution usingtitanium tetraisopropoxide (TTIP) as precursor, it is foundthat the rate constant for coarsening increases with temper-ature due to the temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO2.63 Second-ary particles are formed by epitaxial self-assembly of primaryparticles at longer times and higher temperatures, and thenumber of primary particles per secondary particle increaseswith time. The average TiO2 nanoparticle radius increaseslinearly with time, in agreement with the Lifshitz- Slyozov-Wagner model for coarsening.63

Highly crystalline anatase TiO2 nanoparticles with differentsizes and shapes could be obtained with the polycondensationof titanium alkoxide in the presence of tetramethylammoniumhydroxide.52,62 In a typical procedure, titanium alkoxide isadded to the base at 2 °C in alcoholic solvents in a three-neck flask and is heated at 50- 60 °C for 13 days or at 90-

100 °C for 6 h. A secondary treatment involving autoclaveheating at 175 and 200 °C is performed to improve thecrystallinity of the TiO2 nanoparticles. Representative TEMimages are shown in Figure 1 from the study of Chemseddineet al.52

A series of thorough studies have been conducted bySugimoto et al. using the sol- gel method on the formationof TiO2 nanoparticles of different sizes and shapes by tuningthe reaction parameters.67- 71 Typically, a stock solution of a 0.50 M Ti source is prepared by mixing TTIP withtriethanolamine (TEOA) ([TTIP]/[TEOA]) 1:2), followedby addition of water. The stock solution is diluted with ashape controller solution and then aged at 100 °C for 1 dayand at 140 °C for 3 days. The pH of the solution can betuned by adding HClO4 or NaOH solution. Amines are usedas the shape controllers of the TiO2 nanomaterials and actas surfactants. These amines include TEOA, diethylenetri-amine, ethylenediamine, trimethylenediamine, and triethyl-enetetramine. The morphology of the TiO2 nanoparticles

changes from cuboidal to ellipsoidal at pH above 11 wTEOA. The TiO2 nanoparticle shape evolves into ellipsoidabove pH 9.5 with diethylenetriamine with a higher aspratio than that with TEOA. Figure 2 shows representatTEM images of the TiO2 nanoparticles under different initiapH conditions with the shape control of TEOA at [TEO[TIPO]) 2.0. Secondary amines, such as diethylamine, atertiary amines, such as trimethylamine and triethylamiact as complexing agents of Ti(IV) ions to promote

growth of ellipsoidal particles with lower aspect ratios. Tshape of the TiO2 nanoparticle can also be tuned from roundcornered cubes to sharp-edged cubes with sodium oleate sodium stearate.70The shape control is attributed to the tuninof the growth rate of the different crystal planes of T2nanoparticles by the specific adsorption of shape controlto these planes under different pH conditions.70

A prolonged heating time below 100 °C for the as-preparedgel can be used to avoid the agglomeration of the TiO2 nano-particles during the crystallization process.58,72 By heatingamorphous TiO2 in air, large quantities of single-phase anatase TiO2 nanoparticles with average particle sizes betwe7 and 50 nm can be obtained, as reported by Zhang aBanfield.73- 77 Much effort has been exerted to achieve highcrystallized and narrowly dispersed TiO2 nanoparticles usingthe sol- gel method with other modifications, such assemicontinuous reaction method by Znaidi et al.78 and a two-stage mixed method and a continuous reaction methodKim et al.53,54

By a combination of the sol- gel method and an anodicalumina membrane (AAM) template, TiO2 nanorods havebeen successfully synthesized by dipping porous AAinto a boiled TiO2 sol followed by drying and heatingprocesses.92,93In a typical experiment, a TiO2 sol solution isprepared by mixing TTIP dissolved in ethanol with a solucontaining water, acetyl acetone, and ethanol. An AAMimmersed into the sol solution for 10 min after being boin ethanol; then it is dried in air and calcined at 400 °C for10 h. The AAM template is removed in a 10 wt % H3PO4aqueous solution. The calcination temperature can be uto control the crystal phase of the TiO2 nanorods. At lowtemperature, anatase nanorods can be obtained, whilehigh temperature rutile nanorods can be obtained. The psize of the AAM template can be used to control the sizthese TiO2 nanorods, which typically range from 100 to 3nm in diameter and several micrometers in length. Appently, the size distribution of the final TiO2 nanorods islargely controlled by the size distribution of the poresthe AAM template. In order to obtain smaller and mosized TiO2 nanorods, it is necessary to fabricate high-qualAAM templates. Figure 3 shows a typical TEM for T2

nanorods fabricated with this method. Normally, the T2nanorods are composed of small TiO2 nanoparticles ornanograins.

By electrophoretic deposition of TiO2 colloidal suspensionsinto the pores of an AAM, ordered TiO2 nanowire arrayscan be obtained.94 In a typical procedure, TTIP is dissolvein ethanol at room temperature, and glacial acetic acid miwith deionized water and ethanol is added under pH) 2- 3with nitric acid. Platinum is used as the anode, and an Awith an Au substrate attached to Cu foil is used as cathode. A TiO2 sol is deposited into the pores of the AMMunder a voltage of 2- 5 V and annealed at 500 °C for 24 h.After dissolving the AAM template in a 5 wt % NaOsolution, isolated TiO2 nanowires are obtained. In order to

Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2893



fabricate TiO2 nanowires instead of nanorods, an AAM withlong pores is a must.

TiO2 nanotubes can also be obtained using the sol- gelmethod by templating with an AAM95- 98 and other organiccompounds.99,100For example, when an AAM is used as thetemplate, a thin layer of TiO2 sol on the wall of the pores of

the AAM is first prepared by sucking TiO2 sol into the poresof the AAM and removing it under vacuum; TiO2 nanowiresare obtained after the sol is fully developed and the AAMremoved. In the procedure by Lee and co-workers,96 a TTIPsolution was prepared by mixing TTIP with 2-propanol 2,4-pentanedione. After the AAM was dipped into t

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

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




solution, it was removed from the solution and placed undervacuum until the entire volume of the solution was pulledthrough 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 cooledto room temperature with a temperature ramp of 2 °C/h. PureTiO2 nanotubes were obtained after the AAM was dissolvedin a 6 M NaOH solution for several minutes.96 Alternatively,TiO2 nanotubes could be obtained by coating the AAMmembranes at 60 °C for a certain period of time (12- 48 h)with dilute TiF4 under pH ) 2.1 and removing the AAMafter TiO2 nanotubes were fully developed.97 Figure 4 showsa typical SEM image of the TiO2 nanotube array from theAAM template.97

In another scheme, a ZnO nanorod array on a glasssubstrate can be used as a template to fabricate TiO2nanotubes with the sol- gel method.101 Briefly, TiO2 sol is

deposited on a ZnO nanorod template by dip-coating wislow withdrawing speed, then dried at 100 °C for 10 min,and heated at 550 °C for 1 h in air to obtain ZnO/TiO2

nanorod arrays. The ZnO nanorod template is etched-upimmersing the ZnO/TiO2 nanorod arrays in a dilute hydro-chloric acid aqueous solution to obtain TiO2 nanotube arrays.Figure 5 shows a typical SEM image of the TiO2 nanotubearray with the ZnO nanorod array template. The Ti2nanotubes inherit the uniform hexagonal cross-sectioshape and the length of 1.5 µm and inner diameter of 100-

120 nm of the ZnO nanorod template. As the concentratof the TiO2 sol is constant, well-aligned TiO2 nanotube arrayscan only be obtained from an optimal dip-coating cynumber in the range of 2- 3 cycles. A dense porous TiO2thick film with holes is obtained instead if the dip-coatnumber further increases. The heating rate is critical to formation of TiO2 nanotube arrays. When the heating ratis extra rapid, e.g., above 6 °C min- 1, the TiO2 coat willeasily crack and flake off from the ZnO nanorods duegreat tensile stress between the TiO2 coat and the ZnOtemplate, and a TiO2 film with loose, porous nanostructuris obtained.

2.2. Micelle and Inverse Micelle MethodsAggregates of surfactant molecules dispersed in a liq

colloid are called micelles when the surfactant concentratexceeds the critical micelle concentration (CMC). The Cis the concentration of surfactants in free solution equilibrium with surfactants in aggregated form. In micelthe hydrophobic hydrocarbon chains of the surfactants

oriented toward the interior of the micelle, and the hydphilic groups of the surfactants are oriented toward tsurrounding aqueous medium. The concentration of the lpresent in solution determines the self-organization of molecules of surfactants and lipids. The lipids form a sinlayer on the liquid surface and are dispersed in solution bethe CMC. The lipids organize in spherical micelles at first CMC (CMC-I), into elongated pipes at the second C(CMC-II), and into stacked lamellae of pipes at the lamepoint (LM or CMC-III). The CMC depends on the chemcomposition, mainly on the ratio of the head area and tail length. Reverse micelles are formed in nonaqueomedia, and the hydrophilic headgroups are directed towthe core of the micelles while the hydrophobic groups

Figure 3. TEM image of anatase nanorods and a single nanorodcomposed 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 4. SEM image of TiO2 nanotubes prepared from the AAOtemplate. 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 5. SEM of a TiO2 nanotube array; the inset shows the ZnOnanorod array template. Reprinted with permission from Qiu, Yu, W. D.; Gao, X. D.; Li, X. M. Nanotechnology 2006 , 17 , 4695.Copyright 2006 IOP Publishing Ltd.




directed outward toward the nonaqueous media. There is noobvious CMC for reverse micelles, because the number of aggregates is usually small and they are not sensitive to thesurfactant concentration. Micelles are often globular androughly spherical in shape, but ellipsoids, cylinders, andbilayers are also possible. The shape of a micelle is a functionof the molecular geometry of its surfactant molecules andsolution conditions such as surfactant concentration, tem-perature, pH, and ionic strength.

Micelles and inverse micelles are commonly employed tosynthesize TiO2 nanomaterials.102- 110 A statistical experi-mental design method was conducted by Kim et al. tooptimize experimental conditions for the preparation of TiO2nanoparticles.103The values of H2O/surfactant, H2O/titaniumprecursor, ammonia concentration, feed rate, and reactiontemperature were significant parameters in controlling TiO2nanoparticle size and size distribution. Amorphous TiO2nanoparticles with diameters of 10- 20 nm were synthesizedand converted to the anatase phase at 600 °C and to the morethermodynamically stable rutile phase at 900 °C. Li et al.developed TiO2 nanoparticles with the chemical reactionsbetween TiCl4 solution and ammonia in a reversed micro-emulsion system consisting of cyclohexane, poly(oxyethyl-ene)5 nonyle phenol ether, and poly(oxyethylene)9 nonylephenol ether.104The produced amorphous TiO2 nanoparticlestransformed into anatase when heated at temperatures from200 to 750 °C and into rutile at temperatures higher than750 °C. Agglomeration and growth also occurred at elevatedtemperatures.

Shuttle-like crystalline TiO2 nanoparticles were synthesizedby Zhang et al. with hydrolysis of titanium tetrabutoxide inthe presence of acids (hydrochloric acid, nitric acid, sulfuricacid, and phosphoric acid) in NP-5 (Igepal CO-520)-

cyclohexane reverse micelles at room temperature.110 Thecrystal structure, morphology, and particle size of the TiO2nanoparticles were largely controlled by the reaction condi-tions, and the key factors affecting the formation of rutile atroom temperature included the acidity, the type of acid used,and the microenvironment of the reverse micelles. Ag-glomeration of the particles occurred with prolonged reactiontimes and increasing the [H2O]/[NP-5] and [H2O]/[Ti-(OC4H9)4] ratios. When suitable acid was applied, round TiO2nanoparticles could also be obtained. Representative TEMimages of the shuttle-like and round-shaped TiO2 nanopar-ticles are shown in Figure 6. In the study carried out by Limet al., TiO2 nanoparticles were prepared by the controlledhydrolysis of TTIP in reverse micelles formed in CO2 withthe surfactants ammonium carboxylate perfluoropolyether(PFPECOO- NH4

+ ) (MW 587) and poly(dimethyl aminoethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl meth-

acrylate) (PDMAEMA-b-PFOMA).106

It was found that thecrystallite size prepared in the presence of reverse micellesincreased as either the molar ratio of water to surfactant orthe precursor to surfactant ratio increased.

The TiO2 nanomaterials prepared with the above micelleand reverse micelle methods normally have amorphousstructure, and calcination is usually necessary in order toinduce high crystallinity. However, this process usually leadsto the growth and agglomeration of TiO2 nanoparticles. Thecrystallinity of TiO2 nanoparticles initially (synthesized bycontrolled hydrolysis of titanium alkoxide in reverse micellesin a hydrocarbon solvent) could be improved by annealingin the presence of the micelles at temperatures considerablylower than those required for the traditional calcination

treatment in the solid state.108This procedure could producecrystalline TiO2 nanoparticles with unchanged physicadimensions and minimal agglomeration and allows preparation of highly crystalline TiO2 nanoparticles, as shownin Figure 7, from the study of Lin et al.108

2.3. Sol MethodThe sol method here refers to the nonhydrolytic sol- gel

processes and usually involves the reaction of titanichloride with a variety of different oxygen donor molecue.g., a metal alkoxide or an organic ether.111- 119

Figure 6. TEM images of the shuttle-like and round-shaped (inTiO2 nanoparticles. From: Zhang, D., Qi, L., Ma, J., Cheng, H J. Mater. Chem. 2002, 12, 3677 (http://dx.doi.org/10.1039/b206996bs Reproduced by permission of The Royal Society of Chemi

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

TiX4 + Ti(OR)4 f 2TiO2 + 4RX (1)

TiX4 + 2RORf TiO2 + 4RX (2)




The condensation between Ti-

Cl and Ti-OR leads to theformation of Ti- O- Ti bridges. The alkoxide groups canbe provided by titanium alkoxides or can be formed in situby reaction of the titanium chloride with alcohols or ethers.In the method by Trentler and Colvin,119 a metal alkoxidewas rapidly injected into the hot solution of titanium halidemixed with trioctylphosphine oxide (TOPO) in heptadecaneat 300 °C under dry inert gas protection, and reactions werecompleted within 5 min. For a series of alkyl substituentsincluding methyl, ethyl, isopropyl, and tert -butyl, the reactionrate dramatically increased with greater branching of R, whileaverage particle sizes were relatively unaffected. Variationof X yielded a clear trend in average particle size, but withouta 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 TiF4 to 3.8 nm forTiI4. The amount of passivating agent (TOPO) influencedthe chemistry. Reaction in pure TOPO was slower andresulted in smaller particles, while reactions without TOPOwere 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 TiO2 nanocrystalsdeveloped by Trentler et al.119

In the method used by Niederberger and Stucky,111 TiCl4was slowly added to anhydrous benzyl alcohol undervigorous stirring at room temperature and was kept at 40-

150 °C for 1- 21 days in the reaction vessel. The precipitatewas calcinated at 450 °C for 5 h after thoroughly washing.

The reaction between TiCl4 and benzyl alcohol was foundsuitable for the synthesis of highly crystalline anatase phaseTiO2 nanoparticles with nearly uniform size and shape atvery low temperatures, such as 40 °C. The particle size couldbe selectively adjusted in the range of 4- 8 nm with theappropriate thermal conditions and a proper choice of therelative amounts of benzyl alcohol and titanium tetrachloride.The particle growth depended strongly on temperature, andlowering the titanium tetrachloride concentration led to aconsiderable decrease of particle size.111

Surfactants have been widely used in the preparation of avariety of nanoparticles with good size distribution anddispersity.15,16Adding different surfactants as capping agents,such as acetic acid and acetylacetone, into the reaction matrix

can help synthesize monodispersed TiO2 nanoparticles.120,121

For example, Scolan and Sanchez found that monodispenonaggregated TiO2 nanoparticles in the 1- 5 nm range wereobtained through hydrolysis of titanium butoxide in presence of acetylacetone and p-toluenesulfonic acid at 60°C.120The resulting nanoparticle xerosols could be dispersin water- alcohol or alcohol solutions at concentrationhigher than 1 M without aggregation, which is attributethe complexation of the surface by acetylacetonato ligaand through an adsorbed hybrid organic- inorganic layermade with acetylacetone, p-toluenesulfonic acid, and wa-ter.120

With the aid of surfactants, different sized and shaped T2nanorods can be synthesized.122- 130For example, the growthof high-aspect-ratio anatase TiO2 nanorods has been reportedby Cozzoli and co-workers by controlling the hydrolyprocess of TTIP in oleic acid (OA).122- 126,130Typically, TTIPwas added into dried OA at 80- 100 °C under inert gasprotection (nitrogen flow) and stirred for 5 min. A 0.1- 2 Maqueous base solution was then rapidly injected and kep80- 100 °C for 6- 12 h with stirring. The bases employeincluded organic amines, such as trimethylamino-N-oxitrimethylamine, tetramethylammonium hydroxide, tetrab

ylammonium hydroxyde, triethylamine, and tributylamiIn this reaction, by chemical modification of the titaniprecursor with the carboxylic acid, the hydrolysis ratetitanium alkoxide was controlled. Fast (in 4- 6 h) crystal-lization in mild conditions was promoted with the usesuitable catalysts (tertiary amines or quaternary ammonihydroxides). A kinetically overdriven growth mechanismto the growth of TiO2 nanorods instead of nanoparticles.123

Typical TEM images of the TiO2 nanorods are shown inFigure 9.123

Recently, Joo et al.127 and Zhang et al.129 reported similarprocedures in obtaining TiO2 nanorods without the use ofcatalyst. Briefly, a mixture of TTIP and OA was usedgenerate OA complexes of titanium at 80 °C in 1-octadecene.

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

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




The injection of a predetermined amount of oleylamine at260 °C led to various sized TiO2 nanorods.129 Figure 10shows TEM images of TiO2 nanorods with various lengths,and 2.3 nm TiO2 nanoparticles prepared with this method.129

In the surfactant-mediated shape evolution of TiO2 nano-crystals in nonaqueous media conducted by Jun et al.,128 itwas found that the shape of TiO2 nanocrystals could bemodified by changing the surfactant concentration. Thesynthesis was accomplished by an alkyl halide eliminationreaction between titanium chloride and titanium isopro-poxide. Briefly, a dioctyl ether solution containing TOPOand lauric acid was heated to 300 °C followed by additionof titanium chloride under vigorous stirring. The reactionwas initiated by the rapid injection of TTIP and quenchedwith cold toluene. At low lauric acid concentrations, bullet-and diamond-shaped nanocrystals were obtained; at higherconcentrations, rod-shaped nanocrystals or a mixture of nanorods and branched nanorods was observed. The bullet-and diamond-shaped nanocrystals and nanorods were elon-gated along the [001] directions. The TiO2 nanorods werefound to simultaneously convert to small nanoparticles as afunction of the growth time, as shown in Figure 11, due tothe minimization of the overall surface energy via dissolutionand regrowth of monomers during an Ostwald ripening.

2.4. Hydrothermal MethodHydrothermal synthesis is normally conducted in steel

pressure vessels called autoclaves with or without Teflon

liners under controlled temperature and/or pressure with reaction in aqueous solutions. The temperature can elevated above the boiling point of water, reaching tpressure of vapor saturation. The temperature and the amoof solution added to the autoclave largely determine internal pressure produced. It is a method that is widely ufor the production of small particles in the ceramics indusMany groups have used the hydrothermal method to prepTiO2 nanoparticles.131- 140 For example, TiO2 nanoparticlescan be obtained by hydrothermal treatment of peptizprecipitates of a titanium precursor with water.134 Theprecipitates were prepared by adding a 0.5 M isopropasolution of titanium butoxide into deionized water ([H2O]/ [Ti] ) 150), and then they were peptized at 70 °C for 1 h inthe presence of tetraalkylammonium hydroxides (peptizAfter filtration and treatment at 240 °C for 2 h, theas-obtained powders were washed with deionized water absolute ethanol and then dried at 60 °C. Under the sameconcentration of peptizer, the particle size decreased wincreasing alkyl chain length. The peptizers and thconcentrations influenced the morphology of the particTypical TEM images of TiO2 nanoparticles made with thehydrothermal method are shown in Figure 12.134

In another example, TiO2 nanoparticles were prepared byhydrothermal reaction of titanium alkoxide in an aciethanol- water solution.132Briefly, TTIP was added dropwiseto a mixed ethanol and water solution at pH 0.7 with niacid, and reacted at 240 °C for 4 h. The TiO2 nanoparticles

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




synthesized under this acidic ethanol- water environmentwere mainly primary structure in the anatase phase withoutsecondary structure. The sizes of the particles were controlledto 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 beensynthesized with the hydrothermal method.141- 146 Zhang etal. obtained TiO2 nanorods by treating a dilute TiCl4 solutionat 333- 423 K for 12 h in the presence of acid or inorgansalts.141,143- 146 Figure 13 shows a typical TEM image of thTiO2 nanorods prepared with the hydrothermal method141

The morphology of the resulting nanorods can be tuned wdifferent surfactants146 or by changing the solvent composi

tions.145

A film of assembled TiO2 nanorods deposited on aglass wafer was reported by Feng et al.142 These TiO2nanorods were prepared at 160 °C for 2 h by hydrothermaltreatment of a titanium trichloride aqueous solution supsaturated with NaCl.

TiO2 nanowires have also been successfully obtained wthe hydrothermal method by various groups.147- 151Typically,TiO2 nanowires are obtained by treating TiO2 white powdersin a 10- 15 M NaOH aqueous solution at 150- 200 °C for24- 72 h without stirring within an autoclave. Figure shows the SEM images of TiO2 nanowires and a TEM imageof a single nanowire prepared by Zhang and co-workers150

TiO2 nanowires can also be prepared from layered titanparticles using the hydrothermal method as reported by W

Figure 11. Time dependent shape evolution of TiO2 nanorods:(a) 0.25 h; (b) 24 h; (c) 48 h. Scale bar ) 50 nm. Reprinted withpermission 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 12. TEM images of TiO2 nanoparticles prepared by thehydrothermal method. Reprinted from Yang, J.; Mei, S.; FerrJ. M. F. Mater. Sci. Eng. C 2001 , 15 , 183, Copyright 2001, withpermission from Elsevier.

Figure 13. TEM image of TiO2 nanorods prepared with thehydrothermal method. Reprinted with permission from ZhangGao, L. Langmuir 2003 , 19, 967. Copyright 2003 AmericanChemical Society.




et al.152 In their experiment, layer-structured Na2Ti3O7 wasdispersed into a 0.05- 0.1 M HCl solution and kept at 140-

170 °C for 3- 7 days in an autoclave. TiO2 nanowires wereobtained after the product was washed with H2O and finallydried. In the formation of a TiO2 nanowire from layeredH2Ti3O7, there are three steps: (i) the exfoliation of layeredNa2Ti3O7; (ii) the nanosheets formation; and (iii) the nanow-ires formation.152 In Na2Ti3O7, [TiO6] octahedral layers areheld by the strong static interaction between the Na+ cationsbetween the [TiO6] octahedral layers and the [TiO6] unit.When the larger H3+ O cations replace the Na+ cations in

the interlayer space of [TiO6] sheets, this static interactionis weakened because the interlayer distance is enlarged. Asa result, the layered compounds Na2Ti3O7 are graduallyexfoliated. When Na+ is exchanged by H+ in the dilute HClsolution, numerous H2Ti3O7 sheet-shaped products areformed. Since the nanosheet does not have inversion sym-metry, an intrinsic tension exists. The nanosheets split to formnanowires in order to release the strong stress and lower thetotal energy.152 A representative TEM image of TiO2nanowires from Na2Ti3O7 is shown in Figure 15.152

The hydrothermal method has been widely used to prepareTiO2 nanotubes since it was introduced by Kasuga et al. in1998.153- 175 Briefly, TiO2 powders are put into a 2.5- 20 MNaOH aqueous solution and held at 20- 110 °C for 20 h in

an autoclave. TiO2 nanotubes are obtained after the productsare washed with a dilute HCl aqueous solution and distilledwater. They proposed the following formation process of TiO2 nanotubes.154When the raw TiO2 material was treatedwith NaOH aqueous solution, some of the Ti- O- Ti bondswere broken and Ti- O- Na and Ti- OH bonds were formed.New Ti- O- Ti bonds were formed after the Ti- O- Na andTi- OH bonds reacted with acid and water when the materialwas treated with an aqueous HCl solution and distilled water.The Ti- OH bond could form a sheet. Through the dehydra-tion of Ti- OH bonds by HCl aqueous solution, Ti- O- Tibonds or Ti- O- H- O- Ti hydrogen bonds were generated.The bond distance from one Ti to the next Ti on the surfacedecreased. This resulted in the folding of the sheets and the

connection between the ends of the sheets, resulting in formation of a tube structure. In this mechanism, the T2nanotubes were formed in the stage of the acid treatmfollowing the alkali treatment. Figure 16 shows typical Timages of TiO2 nanotubes made by Kasuga et al.153However,Du and co-workers found that the nanotubes were formduring the treatment of TiO2 in NaOH aqueous solution.161

A 3D f 2D f 1D formation mechanism of the TiO2nanotubes was proposed by Wang and co-workers.171It statedthat the raw TiO2 was first transformed into lamellastructures and then bent and rolled to form the nanotubFor the formation of the TiO2 nanotubes, the two-dimensionalamellar TiO2 was essential. Yao and co-workers furthesuggested, based on their HRTEM study as shown in Fig

Figure 14. SEM images of TiO2 nanowires with the inset showinga TEM image of a single TiO2 nanowire with a [010] selected areaelectron diffraction (SAED) recorded perpendicular to the long axisof 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 15. TEM images of TiO2 nanowires made from the layeredNa2Ti3O7 particles, with the HRTEM image shown in the insReprinted from Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, Arakawa, H. Chem. Phys. Lett. 2004 , 400 , 231, Copyright 2004,with permission from Elsevier.

Figure 16. TEM image of TiO2 nanotubes. Reprinted withpermission from Kasuga, T.; Hiramatsu, M.; Hoson, A.; SekT.; Niihara, K. Langmuir 1998 , 14, 3160. Copyright 1998 AmericanChemical Society.




17, that TiO2 nanotubes were formed by rolling up the single-layer TiO2 sheets with a rolling-up vector of [001] andattracting other sheets to surround the tubes.172Bavykin andco-workers suggested that the mechanism of nanotubeformation involved the wrapping of multilayered nanosheetsrather than scrolling or wrapping of single layer nanosheetsfollowed by crystallization of successive layers.156 In themechanism proposed by Wang et al., the formation of TiO2nanotubes involved several steps.176During the reaction withNaOH, the Ti- O- Ti bonding between the basic buildingblocks of the anatase phase, the octahedra, was broken anda zigzag structure was formed when the free octahedrasshared 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 crystallinesheets 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 thesedangling bonds from the surface and lower the total energy,resulting in the formation of TiO2 nanotubes.176

2.5. Solvothermal MethodThe solvothermal method is almost identical to the

hydrothermal method except that the solvent used here isnonaqueous. However, the temperature can be elevated muchhigher than that in hydrothermal method, since a variety of organic solvents with high boiling points can be chosen. Thesolvothermal method normally has better control than hy-drothermal methods of the size and shape distributions andthe crystallinity of the TiO2 nanoparticles. The solvothermalmethod has been found to be a versatile method for the

synthesis of a variety of nanoparticles with narrow sdistribution and dispersity.177- 179 The solvothermal methodhas been employed to synthesize TiO2 nanoparticles andnanorods with/without the aid of surfactants.177- 185 Forexample, in a typical procedure by Kim and co-workers184

TTIP was mixed with toluene at the weight ratio of 1- 3:10and kept at 250 °C for 3 h. The average particle size of TiO2powders tended to increase as the composition of TTIPthe solution increased in the range of weight ratio of 1- 3:10, while the pale crystalline phase of TiO2 was not producedat 1:20 and 2:5 weight ratios.184 By controlling the hydro-lyzation reaction of Ti(OC4H9)4 and linoleic acid, redispers-ible TiO2 nanoparticles and nanorods could be synthesize

as found by Li et al. recently.177

The decomposition of NH4-HCO3 could provide H2O for the hydrolyzation reaction, andlinoleic acid could act as the solvent/reagent and coordinasurfactant in the synthesis of nanoparticles. Triethylamcould act as a catalyst for the polycondensation of the T-

O- Ti inorganic network to achieve a crystalline product ahad little influence on the products’ morphology. The chlengths of the carboxylic acids had a great influence on formation of TiO2, and long-chain organic acids wereimportant and necessary in the formation of TiO2.177 Figure18 shows a representative TEM image of TiO2 nanoparticlesfrom their study.177

TiO2 nanorods with narrow size distributions can also developed with the solvothermal method.177,183For example,

in a typical synthesis from Kim et al., TTIP was dissolin anhydrous toluene with OA as a surfactant and kep250 °C for 20 h in an autoclave without stirring.183 Longdumbbell-shaped nanorods were formed when a sufficiamount of TTIP or surfactant was added to the solution, to the oriented growth of particles along the [001] axis.a fixed precursor to surfactant weight ratio of 1:3, tconcentration of rods in the nanoparticle assembly increaas the concentration of the titanium precursor in the soluincreased. The average particle size was smaller and the distribution was narrower than is the case for particsynthesized without surfactant. The crystalline phase, dieter, and length of these nanorods are largely influencedthe precursor/surfactant/solvent weight ratio. Anatase na

Figure 17. (a) HRTEM images of TiO2 nanotubes. (b) Cross-sectional view of TiO2 nanotubes. Reused with permission fromB. 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 18. TEM micrographs of TiO2 nanoparticles prepared withthe solvothermal method. Reprinted with permission from LL.; Peng, Q.; Yi, J. X.; Wang, X.; Li, Y. D. Chem. s Eur. J. 2006 ,12 , 2383. Copyright 2006 Wiley-VCH.




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 aprecursor/solvent weight ratio of more than 1:5 for aprecursor/surfactant weight ratio of 1:3. The diameter andlength of these nanorods were in the ranges of 3- 5 nm and18- 25 nm, respectively. Figure 19 shows a typical TEM

image of TiO2 nanorods prepared from the solutions withthe weight ratio of precursor/solvent/surfactant ) 1:5:3.183

Similar to the hydrothermal method, the solvothermalmethod has also been used for the preparation of TiO2nanowires.180- 182Typically, a TiO2 powder suspension in an5 M NaOH water- ethanol solution is kept in an autoclaveat 170- 200 °C for 24 h and then cooled to room temperaturenaturally. TiO2 nanowires are obtained after the obtainedsample is washed with a dilute HCl aqueous solution anddried at 60 °C for 12 h in air.181 The solvent plays animportant role in determining the crystal morphology.Solvents with different physical and chemical properties caninfluence the solubility, reactivity, and diffusion behaviorof the reactants; in particular, the polarity and coordinating

ability of the solvent can influence the morphology and thecrystallization behavior of the final products. The presenceof ethanol at a high concentration not only can cause thepolarity of the solvent to change but also strongly affectsthe ζ potential values of the reactant particles and theincreases solution viscosity. For example, in the absence of ethanol, short and wide flakelike structures of TiO2 wereobtained instead of nanowires. When chloroform is used,TiO2 nanorods were obtained.181Figure 20 shows representa-tive TEM images of the TiO2 nanowires prepared from thesolvothermal method.181 Alternatively, bamboo-shaped Ag-doped TiO2 nanowires were developed with titanium butox-ide as precursor and AgNO3 as catalyst.180 Through theelectron diffraction (ED) pattern and HRTEM study, the Ag

phase only existed in heterojunctions between single-cryTiO2 nanowires.180

2.6. Direct Oxidation MethodTiO2 nanomaterials can be obtained by oxidation

titanium metal using oxidants or under anodization. Crys

line TiO2 nanorods have been obtained by direct oxidatiof a titanium metal plate with hydrogen peroxide.186- 191

Typically, TiO2 nanorods on a Ti plate are obtained whencleaned Ti plate is put in 50 mL of a 30 wt % H2O2 solutionat 353 K for 72 h. The formation of crystalline TiO2 occursthrough a dissolution precipitation mechanism. By addition of inorganic salts of NaX (X) F- , Cl- , and SO42- ),the crystalline phase of TiO2 nanorods can be controlled.The addition of F- and SO42- helps the formation of pureanatase, while the addition of Cl- favors the formation of rutile.189 Figure 21 shows a typical SEM image of TiO2nanorods prepared with this method.186

At high temperature, acetone can be used as a good oxysource and for the preparation of TiO2 nanorods by oxidizing

Figure 19. 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 20. TEM images of TiO2 nanowires synthesized by thesolvothermal method. From: Wen, B.; Liu, C.; Liu, Y. New J.Chem. 2005 , 29, 969 (http://dx.doi.org/10.1039/b502604k) sReproduced by permission of The Royal Society of Chemi(RSC) on behalf of the Centre National de la Recherche Scienti(CNRS).

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




a Ti plate with acetone as reported by Peng and Chen.192The oxygen source was found to play an important role.Highly dense and well-aligned TiO2 nanorod arrays wereformed when acetone was used as the oxygen source, andonly crystal grain films or grains with random nanofibersgrowing from the edges were obtained with pure oxygen orargon mixed with oxygen. The competition of the oxygenand titanium diffusion involved in the titanium oxidationprocess largely controlled the morphology of the TiO2. Withpure oxygen, the oxidation occurred at the Ti metal and theTiO2 interface, since oxygen diffusion predominated becauseof the high oxygen concentration. When acetone was usedas the oxygen source, Ti cations diffused to the oxide surfaceand reacted with the adsorbed acetone species. Figure 22shows aligned TiO2 nanorod arrays obtained by oxidizing atitanium substrate with acetone at 850 °C for 90 min.192

As extensively studied, TiO2 nanotubes can be obtainedby anodic oxidation of titanium foil.193- 228 In a typicalexperiment, a clean Ti plate is anodized in a 0.5% HFsolution under 10- 20 V for 10- 30 min. Platinum is usedas counterelectrode. Crystallized TiO2 nanotubes are obtainedafter the anodized Ti plate is annealed at 500 °C for 6 h inoxygen.210 The length and diameter of the TiO2 nanotubescould be controlled over a wide range (diameter, 15- 120nm; length, 20 nm to 10 µm) with the applied potentialbetween 1 and 25 V in optimized phosphate/HF electro-lytes.229 Figure 23 shows SEM images of TiO2 nanotubescreated with this method.208

2.7. Chemical Vapor DepositionVapor 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 alterthe mechanical, electrical, thermal, optical, corrosion resis-tance, and wear resistance properties of various substrates.They are also used to form free-standing bodies, films, andfibers and to infiltrate fabric to form composite materials.Recently, they have been widely explored to fabricate variousnanomaterials. Vapor deposition processes usually take placewithin a vacuum chamber. If no chemical reaction occurs,this process is called physical vapor deposition (PVD);

otherwise, it is called chemical vapor deposition (CVD)CVD processes, thermal energy heats the gases in the coachamber and drives the deposition reaction.

Thick crystalline TiO2 films with grain sizes below 30 nmas well as TiO2 nanoparticles with sizes below 10 nm cabe prepared by pyrolysis of TTIP in a mixed helium/oxyatmosphere, using liquid precursor delivery.230When depos-ited on the cold areas of the reactor at temperatures bel

90 °C with plasma enhanced CVD, amorphous TiO2 nano-particles can be obtained and crystallize with a relativhigh surface area after being annealed at high temperature231

TiO2 nanorod arrays with a diameter of about 50- 100 nmand a length of 0.5- 2 µm can be synthesized by metalorganic CVD (MOCVD) on a WC-Co substrate using Tas the precursor.232

Figure 24 shows the TiO2 nanorods grown on fused silicasubstrates with a template- and catalyst-free MOCVmethod.233 In a typical procedure, titanium acetylacetona(Ti(C10H14O5)) vaporizing in the low-temperature zone offurnace at 200- 230 °C is carried by a N2 /O2 flow into thehigh-temperature zone of 500- 700 °C, and TiO2 nanostruc-tures are grown directly on the substrates. The phase a

Figure 22. SEM images of large-scale nanorod arrays preparedby 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 TheRoyal Society of Chemistry.

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

Figure 24. 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 Socie




morphology of the TiO2 nanostructures can be tuned withthe reaction conditions. For example, at 630 and 560 °Cunder a pressure of 5 Torr, single-crystalline rutile andanatase TiO2 nanorods were formed respectively, while, at535 °C under 3.6 Torr, anatase TiO2 nanowalls composedof well-aligned nanorods were formed.233

In addition to the above CVD approaches in preparingTiO2 nanomaterials, other CVD approaches are also used,such as electrostatic spray hydrolysis,234 diffusion flamepyrolysis,235- 239 thermal plasma pyrolysis,240- 246 ultrasonicspray pyrolysis,247laser-induced pyrolysis,248,249and ultronsic-assisted hydrolysis,250,251among others.

2.8. Physical Vapor Deposition

In PVD, materials are first evaporated and then condensedto form a solid material. The primary PVD methods includethermal deposition, ion plating, ion implantation, sputtering,laser vaporization, and laser surface alloying. TiO2 nanowirearrays have been fabricated by a simple PVD method orthermal deposition.252- 254 Typically, pure Ti metal powderis on a quartz boat in a tube furnace about 0.5 mm awayfrom the substrate. Then the furnace chamber is pumpeddown to∼300 Torr and the temperature is increased to 850°C under an argon gas flow with a rate of 100 sccm andheld for 3 h. After the reaction, a layer of TiO2 nanowirescan be obtained.254 A layer of Ti nanopowders can bedeposited on the substrate before the growth of TiO2nanowires,252,253and Au can be employed as catalyst.252 Atypical SEM image of TiO2 nanowires made with the PVDmethod is shown in Figure 25.252

2.9. ElectrodepositionElectrodeposition 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 usedas cathode and immersed into a solution which contains asalt of the metal to be deposited. The metallic ions areattracted to the cathode and reduced to metallic form. Withthe use of the template of an AAM, TiO2 nanowires can beobtained by electrodeposition.255,256In a typical process, theelectrodeposition is carried out in 0.2 M TiCl3 solution with

pH ) 2 with a pulsed electrodeposition approach, atitanium and/or its compound are deposited into the poof the AAM. By heating the above deposited template500 °C for 4 h and removing the template, pure anatase T2nanowires can be obtained. Figure 26 shows a representaSEM image of TiO2 nanowires.256

2.10. Sonochemical MethodUltrasound has been very useful in the synthesis of a w

range of nanostructured materials, including high-surfaarea transition metals, alloys, carbides, oxides, and colloThe chemical effects of ultrasound do not come from a diinteraction with molecular species. Instead, sonochemisarises from acoustic cavitation: the formation, growth, implosive collapse of bubbles in a liquid. Cavitatiocollapse produces intense local heating (∼5000 K), high pres-sures (∼1000 atm), and enormous heating and cooling ra(> 109 K/s). The sonochemical method has been appliedprepare various TiO2 nanomaterials by different groups.257- 269

Yu et al. applied the sonochemical method in preparihighly photoactive TiO2 nanoparticle photocatalysts withanatase and brookite phases using the hydrolysis of titantetraisoproproxide in pure water or in a 1:1 EtOH- H2Osolution under ultrasonic radiation.109Huang et al. found thatanatase and rutile TiO2 nanoparticles as well as their mixturecould be selectively synthesized with various precursusing ultrasound irradiation, depending on the reacttemperature and the precursor used.259 Zhu et al. developedtitania whiskers and nanotubes with the assistance sonication as shown in Figure 27.269 They found that arraysof TiO2 nanowhiskers with a diameter of 5 nm and nanotuwith a diameter of ∼5 nm and a length of 200- 300 nm couldbe obtained by sonicating TiO2 particles in NaOH aqueoussolution followed by washing with deionized water andilute HNO3 aqueous solution.

2.11. Microwave MethodA dielectric material can be processed with energy in

form of high-frequency electromagnetic waves. The princ

Figure 25. SEM images of the TiO2 nanowire arrays prepared bythe PVD method. Reprinted from Wu, J. M.; Shih, H. C.; Wu, W.T. Chem. Phys. Lett. 2005 , 413 , 490, Copyright 2005, withpermission from Elsevier. Figure 26. Cross-sectional SEM image of TiO2 nanowires elec-

trodeposited in AAM pores. Reprinted from Liu, S.; HuangSol. Energy Mater. Sol. Cells 2004 , 85 , 125, Copyright 2004, withpermission from Elsevier.




frequencies of microwave heating are between 900 and 2450MHz. At lower microwave frequencies, conductive currentsflowing within the material due to the movement of ionic con-stituents can transfer energy from the microwave field to thematerial. At higher frequencies, the energy absorption is pri-marily due to molecules with a permanent dipole which tendto reorientate under the influence of a microwave electricfield. This reorientation loss mechanism originates from theinability of the polarization to follow extremely rapid rever-sals of the electric field, so the polarization phasor lags theapplied electric field. This ensures that the resulting currentdensity has a component in phase with the field, and thereforepower is dissipated in the dielectric material. The majoradvantages of using microwaves for industrial processing arerapid heat transfer, and volumetric and selective heating.

Microwave radiation is applied to prepare various TiO2nanomaterials.270- 276Corradi et al. found that colloidal titaniananoparticle suspensions could be prepared within 5 min to1 h with microwave radiation, while 1 to 32 h was neededfor the conventional synthesis method of forced hydrolysisat 195 °C.270Ma et al. developed high-quality rutile TiO2 nano-rods with a microwave hydrothermal method and found thatthey aggregated radially into spherical secondary nanopartic-les.272 Wu et al. synthesized TiO2 nanotubes by microwaveradiation via the reaction of TiO2 crystals of anatase, rutile,or mixed phase and NaOH aqueous solution under a certainmicrowave power.275Normally, the TiO2 nanotubes had thecentral hollow, open-ended, and multiwall structure withdiameters of 8- 12 nm and lengths up to 200- 1000 nm.275

2.12. TiO2 Mesoporous/Nanoporous MaterialsIn the past decade, mesoporous/nanoporous TiO2 materials

have been well studied with or without the use of organic

surfactant templates.28,80,264,265,277- 312Barbe et al. reported thepreparation of a mesoporous TiO2 film by the hydrothermalmethod as shown Figure 28.80 In a typical experiment, TTIPwas added dropwise to a 0.1 M nitric acid solution unvigorous stirring and at room temperature. A white precipiformed instantaneously. Immediately after the hydrolysis,solution was heated to 80 °C and stirred vigorously for 8 hfor peptization. The solution was then filtered on a glassto remove agglomerates. Water was added to the filtrate

adjust the final solids concentration to∼5 wt %. The solutionwas put in a titanium autoclave for 12 h at 200- 250 °C.After sonication, the colloidal suspension was put in a roevaporator and evaporated to a final TiO2 concentration of 11 wt %. The precipitation pH, hydrolysis rate, autoclavpH, and precursor chemistry were found to influence morphology of the final TiO2 nanoparticles.

Alternative procedures without the use of hydrothermprocesses have been reported by Liu et al.292 and Zhang etal.311 In the report by Liu et al., 24.0 g of titanium(IVn-butoxide ethanol solution (weight ratio of 1:7) wprehydrolyzed in the presence of 0.32 mL of a 0.28 M HN3aqueous solution (TBT/HNO3∼ 100:1) at room temperaturefor 3 h. 0.32 mL of deionized water was added to

prehydrolyzed solution under vigorous stirring and stirfor an additional 2 h. The sol solution in a closed veswas kept at room temperature without stirring to gel aage. After aging for 14 days, the gel was dried at rotemperature, ground into a fine powder, washed thorougwith water and ethanol, and dried to produce porous Ti2.Upon calcination at 450 °C for 4 h under air, crystallizedmesoporous TiO2 material was obtained.292

Yu et al. prepared three-dimensional and thermally stamesoporous TiO2 without the use of any surfactants.265

Briefly, monodispersed TiO2 nanoparticles were formedinitially by ultrasound-assisted hydrolysis of acetic acmodified titanium isopropoxide. Mesoporous sphericalglobular particles were then produced by controlled cond

Figure 27. 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 bypermission of The Royal Society of Chemistry.

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




sation and agglomeration of these sol nanoparticles underhigh-intensity ultrasound radiation. The mesoporous TiO2 hada wormhole-like structure consisting of TiO2 nanoparticlesand a lack of long-range order.265

In the template method used by the Stuckygroup278- 280,287,295,302,306- 307,313and other groups,264,293,297,303,309

structure-directing agents were used for organizing network-forming metal oxide species in nonaqueous solutions. Thesestructure-directing agents were also called organic templates.

The most commonly used organic templates were amphi-philic poly(alkylene oxide) block copolymers, such as HO-(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designatedEO20PO70EO20, called Pluronic P-123) and HO(CH2CH2O)106-(CH2CH(CH3)O)70(CH2CH2O)106H (designated EO106PO70-EO106, 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. Theresulting sol solution was gelled in an open Petri dish at 40°C in air for 1- 7 days. Mesoporous TiO2 was obtained afterremoving the surfactant species by calcining the as-madesample at 400 °C for 5 h in air.306 Figure 29 shows typicalTEM images of the mesoporous TiO2. Besides triblock co-polymers as structure-directing agents, diblock polymers werealso used such as [CnH2n- 1(OCH2CH2) yOH, Brij 56 (B56, n / y) 16/10) or Brij 58 (B58, n / y ) 16/20)] by Sanchez et al.285

Other surfactants employed to direct the formation of mesoporous TiO2 include tetradecyl phosphate (a 14-carbonchain) by Antonelli and Ying277and commercially availabledodecyl phosphate by Putnam and co-workers,298 cetyltri-methylammonium bromide (CTAB) (a cationic surfac-tant),281,283,296the recent Gemini surfactant,294 and dodecyl-amine (a neutral surfactant).304 Carbon nanotubes310 andmesoporous SBA-15286 have also been used as the skeletonfor mesoporous TiO2.

2.13. TiO2 Aerogels

The study of TiO2 aerogels is worthy of special men-tion.314- 326 The combination of sol- gel processing withsupercritical drying offers the synthesis of TiO2 aerogels withmorphological and chemical properties that are not easilyachieved by other preparation methods, i.e., with high surfacearea. Campbell et al. prepared TiO2 aerogels by sol- gelsynthesis from titanium n-butoxide in methanol with thesubsequent removal of solvent by supercritical CO2.315 Fora typical synthesis process, titanium n-butoxide was addedto 40 mL of methanol in a dry glovebox. This solution wascombined with another solution containing 10 mL of methanol, nitric acid, and deionized water. The concentrationof the titanium n-butoxide was kept at 0.625 M, and themolar ratio of water/HNO3 /titanium n-butoxide was 4:0.1:

1. The gel was allowed to age for 2 h and then extracted ina standard autoclave with supercritical CO2 at a flow rate of 24.6 L/h, at 343 K under 2.07× 107 Pa for 2- 3 h, resultingin complete removal of solvent. After extraction, the samplewas heated in a vacuum oven at 3.4 kPa and 383 K for 3 hto remove the residual solvent and at 3.4 kPa and 483 K for3 h to remove any residual organics. The pretreated samplehad a brown color and turned white after calcination at 773K or above. The resulting TiO2 aerogel, after calcination at773 K for 2 h, had a BET surface area of > 200 m2 /g,contained mesopores in the range 2- 10 nm, and was of thepure anatase form. Dagan et al. found the TiO2 aerogelsobtanied by using a Ti/ethanol/H2O/nitric acid ratio of 1:20:3:0.08 could have a porosity of 90% and surface areas of

600 m2 /g, as compared to a surface area of 50 m2 /g for TiO2P25.316,317Figure 30 shows a typical SEM image of a TiO2aerogel with a surface area of 447 m2 /g and an interporestructure constructed by near uniform grains of elliptishapes with 30 nm × 50 nm axes.326

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




2.14. TiO2 Opal and Photonic MaterialsThe syntheses of TiO2 opal and photonic materials have

been well studied by various groups.327- 358 Holland et al.

reported the preparation of TiO2 inverse opal from thecorresponding metal alkoxides, using latex spheres astemplates.334,335Millimeter-thick layers of latex spheres weredeposited on filter paper in a Buchner funnel under vacuumand soaked with ethanol. Titanium ethoxide was addeddropwise to cover the latex spheres completely while suctionwas applied. Typical mass ratios of alkoxide to latex werebetween 1.4 and 3. After drying the composite in a vacuumdesiccator for 3 to 24 h, the latex spheres were removed bycalcination in flowing air at 575 °C for 7 to 12 h, leavinghard and brittle powder particles with 320- to 360-nm voids.The carbon content of the calcined samples varied from 0.4to 1.0 wt %, indicating that most of the latex templates hadbeen removed from the 3D host. Figure 31 shows an

illustration of the simple synthesis of TiO2 inverse opal andan SEM image of TiO2 inverse opals. Similar studies havealso been carried out by other researchers.327,356

Dong and Marlow prepared TiO2 inversed opals with askeleton-like structure of TiO2 rods by a template-directedmethod using monodispersed polystyrene particles of size270 nm.328- 330,345Infiltration of a titania precursor (Ti(i-OPr)4in EtOH) was followed by a drying and calcination proce-dure. The precursor concentration was varied from 30% to100%, and the calcination temperature was tuned from 300to 700 °C. A SEM picture of the TiO2 inversed opal is shownin Figure 32.329 The skeleton structure consists of rhombo-hedral windows and TiO2 cylinders forming a highly regularnetwork. The cylinders connect the centers of the former

octahedral and tetrahedral voids of the opal. These voids forma CaF2 lattice which is filled with cylindrical bonds con-necting the Ca and F sites.

Wang et al. reported their study on the large-scalefabrication of ordered TiO2 nanobowl arrays.354The processstarts with a self-assembled monolayer of polystyrene (PS)spheres, which is used as a template for atomic layerdeposition of a TiO2 layer. After ion-milling, toluene-etching,and annealing of the TiO2-coated spheres, ordered arrays of nanostructured TiO2 nanobowls can be fabricated as shownin Figure 33.

Wang et al. fabricated a 2D photonic crystal by coatingpatterned and aligned ZnO nanorod arrays with TiO2.355 PSspheres were self-assembled to make a monolayer mask on

a sapphire substrate, which was then covered with a laof gold. After removing the PS spheres with toluene, Znanorods were grown using a vapor- liquid- solid process.

Finally, a TiO2 layer was deposited on the ZnO nanorodby introducing TiCl4 and water vapors into the atomic layedeposition chamber at 100 °C. Figure 34 shows SEM imagesof a ZnO nanorod array and the TiO2-coated ZnO nanorodarray.

Li et al. reported the preparation of ordered arrays of T2opals using opal gel templates under uniaxial compressat ambient temperature during the TiO2 sol/gel process.337

The aspect ratio was controllable by the compression deg R. Polystyrene inverse opal was template synthesized ussilica opals as template. The silica was removed with 40% aqueous hydrofluoric acid. Monomer solutions consisof dimethylacrylamide, acrylic acid, and methylenebisacamide in 1:1:0.02 weight ratios were dissolved in a wa

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

Figure 31. (A) Schematic illustration of the synthesis of a Ti2inversed opal. (B) SEM image of the TiO2 inversed opal. Reprintedwith permission from Holland, B. T.; Blanford, C.; Stein, A. Science1998 , 281 , 538 (http://www.sciencemag.org). Copyright 19AAAS.




ethanol mixture (4:7 wt/wt) with total monomer content 30wt %. Ethanol was used to facilitate diffusion of themonomer solution into the inverse opal polystyrene. Afterthe inverse opal was infiltrated by the monomer solutioncontaining 1 wt % of the initiator AIBN and a subsequentfree radical polymerization at 60 °C for 3 h, a solid compositeresulted. The initial inverse opal polystyrene template wasthen removed with chloroform in a Soxhlet extractor for 12h, whereupon the opal gel was formed. By using differentcompositions of the monomer solution, hole sizes, andstacking structures of the starting inverse opal templates, opal

gels with correspondingly different properties can be pduced. Water was completely removed from the ophydrogel by repeatedly rinsing it with a large amountethanol. Afterward, the opal gel was put into a large amoof tetrabutyl titanate (TBT) at ambient temperature forh. The TBT-swollen opal gel was then immersed in a waethanol (1:1 wt/wt) mixture for 5 h to let the TiO2 sol/gelprocess proceed. Figure 35A shows the opal structure ofgel/titania composite spheres formed. After calcination, T2

opal with distinctive spherical contours could be found. compression degree, R, was adjusted by the spacer heighwhen the substrates were compressed. When the substrawere slightly compressed against each other to the extenproducing a 20% reduction in the thickness of the comption opal, the deformation of the template-synthesized titaspheres was not substantial (Figure 35B). When the copression degree was increased to the point of reaching 3deformation in the opal gel, noticeably deformed titania ocould be obtained (Figure 35C and D).

2.15. Preparation of TiO2 NanosheetsThe preparation of TiO2 nanosheets has also been explored

recently.359- 368

Typically, TiO2 nanosheets were synthesizedby delaminating layered protonic titanate into colloidal sinlayers. A stoichiometric mixture of Cs2CO3 and TiO2 wascalcined at 800 °C for 20 h to produce a precursor, cesiumtitanate, Cs0.7Ti1.8250 0.175O4 (0 : vacancy), about 70 g of which was treated with 2 L of a 1 M HCl solution at rotemperature. This acid leaching was repeated three timesrenewing the acid solution every 24 h. The resulting acexchanged product was filtered, washed with water, and dried. The obtained protonic titanate, H0.7Ti1.8250 0.175O4‚H2O,was shaken vigorously with a 0.017 M tetrabutylammonhydroxide solution at ambient temperature for 10 days. solution-to-solid ratio was adjusted to 250 cm3 g- 1. Thisprocedure yielded a stable colloidal suspension with

Figure 32. SEM picture of a TiO2 skeleton with a cylinder radiusof 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 33. (A) Experimental procedure for fabricating TiO2nanobowl arrays. (B) Low- and high- (inset) magnification SEMimage of TiO2 nanobowl arrays. Reprinted with permission fromWang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers,C. J. Nano Lett. 2004 , 4, 2223. Copyright 2004 American ChemicalSociety.

Figure 34. (A) SEM images of short and densely aligned Znnanorod array on a sapphire substrate. Inset: An optical imag

the aligned ZnO nanorods over a large area. (B) SEM imagthe TiO2-coated ZnO nanorod array. Reprinted with permission frWang, X.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; PranL. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J. Ad V . Mater.2005 , 17 , 2103. Copyright 2005 Wiley-VCH.




opalescent appearance. Figure 36 shows TEM and AFMimages of TiO2 nanosheets with thicknesses of 1.2- 1.3 nm,which is the height of the TiO2 nanosheet with a monolayerof water molecules on both sides (0.70+ 0.25× 2) thick.366

3. Properties of TiO 2 Nanomaterials

3.1. Structural Properties of TiO2 NanomaterialsFigure 37 shows the unit cell structures of the rutile and

anatase TiO2.11 These two structures can be described interms of chains of TiO6 octahedra, where each Ti4+ ion issurrounded by an octahedron of six O2- ions. The two crystalstructures differ in the distortion of each octahedron and bythe assembly pattern of the octahedra chains. In rutile, the

octahedron shows a slight orthorhombic distortion; in anatthe octahedron is significantly distorted so that its symmeis lower than orthorhombic. The Ti- Ti distances in anataseare larger, whereas the Ti- O distances are shorter than thosein rutile. In the rutile structure, each octahedron is in conwith 10 neighbor octahedrons (two sharing edge oxygen pand eight sharing corner oxygen atoms), while, in the anastructure, each octahedron is in contact with eight neighb(four sharing an edge and four sharing a corner). Thdifferences in lattice structures cause different mass densiand electronic band structures between the two formsTiO2.

Hamad et al. performed a theoretical calculation on TinO2n

clusters (n ) 1- 15) with a combination of simulate

Figure 35. SEM of the TiO2 opals. (A) A gel/titania composite opal fabricated without compressing the opal gel template dsol/gel process. (Inset) Image of the sample after calcination at 450 °C for 3 h. (B- D) (Main panel) Oblate titania opal materials afcalcination 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 frasurfaces containing the direction of applied compression. (Inset) Image of the same sample, but with the fracture surface perthe direction of applied compression. From: Ji, L.; Rong, J.; Yang, Z. Chem. Commun. 2003 , 1080 (http://dx.doi.org/10.1039/b300825hs Reproduced by permission of The Royal Society of Chemistry.




annealing, Monte Carlo basin hopping simulation, andgenetic algorithms methods.369They found that the calculatedglobal minima consisted of compact structures, with titaniumatoms reaching high coordination rapidly as n increased. Forn g 11, the particles had at least a central octahedronsurrounded by a shell of surface tetrahedra, trigonal bipyra-mids, and square base pyramids.

Swamy et al. found the metastability of anatase as afunction of pressure was size dependent, with smallercrystallites preserving the structure to higher pressures.370

Three size regimes were recognized for the pressure-inducedphase transition of anatase at room temperature: an anatase-

amorphous transition regime at the smallest crystallite sian anatase- baddeleyite transition regime at intermediatcrystallite sizes, and an anatase-R -PbO2 transition regimecomprising large nanocrystals to macroscopic single cryst

Barnard et al. performed a series of theoretical studiesthe phase stability of TiO2 nanoparticles in different environments by a thermodynamic model.371- 375 They found thatsurface passivation had an important impact on nanocrymorphology and phase stability. The results showed tsurface hydrogenation induced significant changes in shape of rutile nanocrystals, but not in anatase, and thatsize at which the phase transition might be expected increadramatically when the undercoordinated surface titaniatoms were H-terminated. For spherical particles, the croover point was about 2.6 nm. For a clean and faceted surf

at low temperatures (a phase transition pointed at an averdiameter of approximately 9.3- 9.4 nm for anatase nano-crystals), the transition size decreased slightly to 8.9 nm wthe surface bridging oxygens were H-terminated, and the increased significantly to 23.1 nm when both the bridgoxygens and the undercoordinated titanium atoms of surface trilayer were H-terminated. Below the cross pothe anatase phase was more stable than the rutile phase371

In their study on TiO2 nanoparticles in vacuum or waterenvironments, they found that the phase transition sizewater (15.1 nm) was larger than that under vacuum (nm).373 In their predictions on the transition enthalpy nanocrystalline anatase and rutile, they found that thermchemical results could differ for various faceted or spher

Figure 36. (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, M.; Oikawa, T. J. Phys. Chem. B 2001 , 105 , 6116. Copyright 2001 American Chemical Society.

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

Society.




nanoparticles as a function of shape, size, and degree of surface passivation.372 Their study on anatase and rutiletitanium dioxide polymorphs passivated with completemonolayers of adsorbates by varying the hydrogen to oxygenratio with respect to a neutral, water-terminated surfaceshowed that termination with water consistently resulted inthe lowest values of surface free energy when hydrated orwith a higher fraction of H on the surface on both anataseand rutile surfaces, but conversely, the surfaces generallyhad a higher surface free energy when they had an equalratio of H and O in the adsorbates or were O-terminated.375

They demonstrated that, under different pH conditions fromacid to basic, the phase transition size of a TiO2 nanoparticlevaried from 6.9 to 22.7 nm, accompanied with shape changesof the TiO2 nanoparticles as shown in Figure 38.374

Enyashin and Seifert conducted a theoretical study on thestructural stability of TiO2 layer modifications (anatase andlepidocrocite) using the density-functional-based tight bind-ing method (DFTB).376 They found that anatase nanotubeswere the most stable modifications in a comparison of single-walled nanotubes, nanostrips, and nanorolls. Their stabilityincreased as their radii grew. The energies for all TiO2nanostructures relative to the infinite monolayer followed a1/ R2 curve.

Chen et al. found that severe distortions existed in Ti siteenvironments in the structures of 1.9 nm TiO2 nanoparticlescompared to those octahedral Ti sites in bulk anatase Ti usingK-edge XANES.377The distorted Ti sites were likely to adopta pentacoordinate square pyramidal geometry due to thetruncation of the lattice. The distortions in the TiO2 latticewere mainly located on the surface of the nanoparticles andwere responsible for binding with other small molecules.

Qian et al. found that the density of the surface states onTiO2 nanoparticles was likely dependent upon the details of the preparation methods.378The TiO2 nanoparticles preparedfrom basic sol were found to have more surface states thanthose prepared from acidic sol based on a surface photo-voltage spectroscopy study.

3.2. Thermodynamic Properties of TiO2Nanomaterials

Rutile is the stable phase at high temperatures, but anataseand brookite are common in fine grained (nanoscale) natural

and synthetic samples. On heating concomitant with coarsing, the following transformations are all seen: anatasebrookite to rutile, brookite to anatase to rutile, anataserutile, and brookite to rutile. These transformation sequenimply very closely balanced energetics as a function particle size. The surface enthalpies of the three polymorare sufficiently different that crossover in thermodynamstability can occur under conditions that preclude coarsenwith anatase and/or brookite stable at small particle size.73,74

However, abnormal behaviors and inconsistent results occasionally observed.

Hwu et al. found the crystal structure of TiO2 nanoparticlesdepended largely on the preparation method.379 For smallTiO2 nanoparticles (< 50 nm), anatase seemed more stableand transformed to rutile at > 973 K. Banfield et al. foundthat the prepared TiO2 nanoparticles had anatase and/orbrookite structures, which transformed to rutile after reacha certain particle size.73,380Once rutile was formed, it grew

much faster than anatase. They found that rutile became mstable than anatase for particle size > 14 nm.Ye et al. observed a slow brookite to anatase pha

transition below 1053 K along with grain growth, rabrookite to anatase and anatase to rutile transformatiobetween 1053 K and 1123 K, and rapid grain growth of ruabove 1123 K as the dominant phase.381They concluded thatbrookite could not transform directly to rutile but hadtransform to anatase first. However, direct transformatof brookite nanocrystals to rutile was observed above 9K by Kominami et al.382

In a later study, Zhang and Banfield found that ttransformation sequence and thermodynamic phase stabidepended on the initial particle sizes of anatase and broo

in their study on the phase transformation behaviornanocrystalline aggregates during their growth for isotherand isochronal reactions.74 They concluded that, for equallysized nanoparticles, anatase was thermodynamically stafor sizes< 11 nm, brookite was stable for sizes between and 35 nm, and rutile was stable for sizes > 35 nm.

Ranade et al. investigated the energetics of the Ti2polymorphs (rutile, anatase, and brookite) by high-tempature oxide melt drop solution calorimetry, and they fouthe energetic stability crossed over between the three phaas shown in Figure 39.383 The dark solid line represents thephases of lowest enthalpy as a function of surface area. Ruwas energetically stable for surface area < 592 m2 /mol (7m2 /g or > 200 nm), brookite was energetically stable fro

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

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




592 to 3174 m2 /mol (7- 40 m2 /g or 200- 40 nm), and anatasewas energetically stable for greater surface areas or smallersizes (< 40 nm). The anatase and rutile energetics cross at1452 m2 /mol (18 m2 /g or 66 nm). Assuming sphericalparticles, the calculated average diameters of rutile andbrookite for a 7 m2 /g surface area were 201 and 206 nm,and those of brookite and anatase for a 40 m2 /g surface areaare 36 and 39 nm. These differences in particle size at thesame surface area existed because of the differences in

density. If the phase transformation took place without furthercoarsening, the particle size should be smaller after thetransformation. Phase stability in a thermodynamic sense isgoverned by the Gibbs free energy (∆ G ) ∆ H - T ∆ S ) ratherthan 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 TiO2,if the initially formed brookite had surface area> 40 m2 /g,it was metastable with respect to both anatase and rutile,and the sequence brookite to anatase to rutile duringcoarsening was energetically downhill. If anatase formedinitially, it could coarsen and transform first to brookite (at40 m2 /g) and then to rutile. The energetic driving force forthe latter reaction (brookite to rutile) was very small,

explaining the natural persistence of coarse brookite. Incontrast, the absence of coarse-grained anatase was consistentwith the much larger driving force for its transformation torutile.383

Li et al. found that only anatase to rutile phase transforma-tion occurred in the temperature range of 973- 1073 K.384

Both anatase and rutile particle sizes increased with theincrease of temperature, but the growth rate was different,as shown in Figure 40. Rutile had a much higher growthrate than anatase. The growth rate of anatase leveled off at800 °C. Rutile particles, after nucleation, grew rapidly,whereas anatase particle size remained practically unchanged.With the decrease of initial particle size, the onset transitiontemperature was decreased. An increased lattice compressionof anatase with increasing temperature was observed. Largerdistortions existed in samples with smaller particle size. Thevalues for the activation energies obtained were 299, 236,and 180 kJ/mol for 23, 17, and 12 nm TiO2 nanoparticles,respectively. The decreased thermal stability in finer nano-particles was primarily due to the reduced activation energyas the size-related surface enthalpy and stress energyincreased.

3.3. X-ray Diffraction Properties of TiO2Nanomaterials

XRD is essential in the determination of the crystalstructure and the crystallinity, and in the estimate of the

crystal grain size according to the Scherrer equation

where K is a dimensionless constant, 2θ is the diffractionangle, λ is the wavelength of the X-ray radiation, and β isthe full width at half-maximum (fwhm) of the diffractionpeak.385 Crystallite size is determined by measuring thebroadening of a particular peak in a diffraction patternassociated with a particular planar reflection from within thecrystal unit cell. It is inversely related to the fwhm of anindividual peaks the narrower the peak, the larger thecrystallite size. The periodicity of the individual crystallite

domains reinforces the diffraction of the X-ray bearesulting in a tall narrow peak. If the crystals are randomarranged or have low degrees of periodicity, the result broader peak. This is normally the case for nanomateassemblies. Thus, it is apparent that the fwhm of diffraction peak is related to the size of the nanomateri

Figure 41 shows the XRD patterns for TiO2 nanoparticlesof different sizes111 and for TiO2 nanorods of differentlengths.129As the nanoparticle size increased, the diffractipeaks became narrower. In the anatase nanoparticle ananorods developed by Zhang et al., the diameters of TiO2 nanoparticles and nanorods were both around 2.3 nThe nanorods were elongated along the [001] direction wpreferred anisotropic growth along the c-axis of the anataselattice, which was indicated by the strong peak intensity narrow width of the (004) reflection and relatively lowintensity and broader width for the other reflections. Wan increase in length of the nanorods, the (004) diffractpeak became much stronger and sharper, whereas other peremained similar in shape and intensity.129 Similar resultshave been observed by other groups.123,127,177,183

3.4. Raman Vibration Properties of TiO2Nanomaterials

As the size of TiO2 nanomaterials decreases, the featureRaman scattering peaks become broader.255,318,370,386- 395Thesize effect on the Raman scattering in nanocrystallTiO2 is interpreted as originating from phonon confin

D ) K λ

β cos θ (3)

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




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

selection rule for the excitation of Raman active optiphonons with long-range order and crystallite size.318,370Ina perfect “infinite” crystal, conservation of phonon momtum requires that only optic phonons near the Brillouin z(BZ) center (q ≈ 0) are involved in first-order Ramanscattering. In an amorphous material lacking long-ranorder, the q vector selection rule breaks down and the Ramspectrum resembles the phonon density of states. Fnanocrystals, the strict “infinite” crystal selection rule

replaced by a relaxed version. This results in a rangeaccessible q vectors (as large as ∆ q ≈ 1/ L ( L diameter))due to the uncertainty principle.

The anatase TiO2 has six Raman-active fundamentals ithe vibrational spectrum: three Eg modes centered around144, 197, and 639 cm- 1 (designated here Eg(1), Eg(2), and Eg(3),respectively), two B1g modes at 399 and 519 cm- 1 (desig-nated B1g(1) and B1g(2d)), and an A1g mode at 513 cm- 1.370

As the particle size decreases, the Raman peaks shincreased broadening and systematic frequency shifts (Fig42).370The most intense Eg(1) mode shows the maximum blueshift and significant broadening with decreasing crystalsize. A small blue shift is seen for the Eg(2) mode, while theB1g(1) mode and the B1g(2)+ A1g modes show very small blueshifts and red shifts (the latter peak represents a combieffect of two individual modes), respectively. Whereas frequency shifts for the A1g and B1g modes are not pro-nounced, increased broadening with decreasing crystalsize is clearly seen for these modes. The Eg(3) mode showssignificant broadening and a red shift with decreascrystallite size.

Choi et al. found a volume contraction effect in anatTiO2 nanoparticles due to increasing radial pressure particle size decreases, and they suggested that the effeof decreasing particle size on the force constants avibrational amplitudes of the nearest neighbor bonds ctributed to both broadening and shifts of the Raman bawith decreasing particle diameter.388

3.5. Electronic Properties of TiO2 NanomaterialsThe DOS of TiO2 is composed of 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), as shown in Figure 43A.396The uppervalence bands can be decomposed into three main regiothe σ bonding in the lower energy region mainly due topσ bonding; the π bonding in the middle energy region; anO pπ states in the higher energy region due to O π nonbonding states at the top of the valence bands wherehybridization with d states is almost negligible. The conbution of the π bonding is much weaker than that of the σ bonding. The conduction bands are decomposed into Tg

(> 5 eV) and t2g bands (< 5 eV). The d xy states are dominantlylocated at the bottom of the conduction bands (the vertdashed line in Figure 43A). The rest of the t2g bands areantibonding with p states. The main peak of the t2g bands isidentified to be mostly d yz and d zx states.

In the molecular-orbital bonding diagram in Figure 43a noticeable feature can be found in the nonbonding stanear the band gap: the nonbonding O pp orbital at the of the valence bands and the nonbonding d xy states at thebottom of the conduction bands. A similar feature canseen in rutile; however, it is less significant than in anatase397

In rutile, each octahedron shares corners with eight neighband shares edges with two other neighbors, forming a linchain. In anatase, each octahedron shares corners with f

Figure 41. (A) Powder XRD patterns of TiO2 samples of differentdiameters: (a) 5 nm; (b) 7 nm; (c) 13 nm. Reprinted with permissionfrom 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. Copyright2005 Wiley-VCH.




neighbors and shares edges with four other neighbors,forming a zigzag chain with a screw axis. Thus, anatase isless dense than rutile. Also, anatase has a large metal- metal

distance of 5.35 Å. As a consequence, the Ti d xy orbitals atthe bottom of the conduction band are quite isolated, whilethe t2g orbitals at the bottom of the conduction band in rutileprovide the metal- metal interaction with a smaller distanceof 2.96 Å.

The electronic structure of TiO2 has been studied withvarious experimental techniques, i.e., with X-ray photoelec-tron and X-ray absorption and emission spectroscop-ies.379,398- 405 Figure 44 shows a schematic energy leveldiagram of the lowest unoccupied MOs of a [TiO6]8- clusterwith Oh , D2h (rutile), and D2d (anatase) symmetry and the TiK-edge XANES and O K-edge ELNES spectra for rutile andanatase.398 The anatase structure is a tetragonally distortedoctahedral structure in which every titanium cation is

surrounded by six oxygen atoms in an elongated octahedgeometry ( D2d ). The further splitting of the 3d levels of Ti3+

due to the asymmetric crystals is shown for rutile and ana

structures. The fine electronic structure of TiO2 can bedirectly probed by Ti K-edge X-ray-absorption near-edstructure (XANES), and the right panel of Figure 4contains O K-edge experimental electron-energy-loss needge structure (ELNES) spectra.398

Hwu et al. found that the crystal field splitting nanocrystal TiO2 was approximately 2.1 eV, slightly smallethan that of bulk TiO2, as shown in Figure 45A.379 Luca etal. found that 1sf np transitions broadened as particle siz(increased or decreased) in the postedge region in the X-absorption spectroscopy for TiO2 nanoparticles.403 Also, aclear trend in the X-ray absorption spectroscopy for diffesized TiO2 nanoparticles was observed, as shown in Figu45B from the study by Choi et al.401

Figure 42. (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 versuscrystallite size. Reprinted with permission from Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D.B. C. Phys. Re V . B 2005 , 71 , 184302/1 (http://link.aps.org/abstract/PRB/v71/p184302). Copyright 2005 by the American Physica




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

nanoparticle falls below the Bohr radius of the first excitastate or becomes comparable to the de Broglie wavelenof the charge carriers, the charge carriers begin to behaquantum mechanically and the charge confinement leada series of discrete electronic states.408 However, there is adiscrepancy in this critical size below which quantizateffects are observed for TiO2 nanomaterials with indirectband gaps. The estimated critical diameter depends criticon the effective masses of the charge carriers.409 Kormann

et al. estimated the excitation radii for titania particles tobetween 7.5 and 19 Å.84 Quantum confinement size effectswere observed for TiO2 nanoparticles with a small apparenband gap blue shift (< 0.1- 0.2 eV) caused by quantum sizeeffects for spherical particles sizes down to 2 nm.58,60Suchsmall effects are mainly due to the relatively high effectmass of carriers in TiO2 and an exciton radius in theapproximate range 0.75- 1.90 nm.84 On the other hand,Serpone et al. suggested that the blue shifts in the effecband gap of TiO2 with particle sizes of 21, 133, and 267 Åmay in fact not be a quantum confinement effect.410 Mon-ticone et al. did an excellent study on the quatum size effin anatase nanoparticles and found no quantum size effin anatase TiO2 nanoparticles for sizes 2 R g 1.5 nm, but

they did find unusual variation of the oscillator strengththe first allowed direct transition with particle size.411

3.6. Optical Properties of TiO2 NanomaterialsThe main mechanism of light absorption in pure sem

conductors is direct interband electron transitions. Tabsorption is especially small in indirect semiconductors, TiO2, where the direct electron transitions between the bacenters are prohibited by the crystal symmetry. Braginsand Shklover have shown the enhancement of light absotion in small TiO2 crystallites due to indirect electrontransitions with momentum nonconservation at the intface.412 This effect increases at a rough interface when t

share of the interface atoms is larger. The indirect transitiare allowed due to a large dipole matrix element and a ladensity of states for the electron in the valence baConsiderable enhancement of the absorption is expectedsmall TiO2 nanocrystals, as well as in porous and micrcrystalline semiconductors, when the share of the interfatoms is sufficiently large. A rapid increase in the absorptakes place at low (hν < E g + W c, where W c is the width of the conduction band) photon energies. Electron transitito any point in the conduction band become possible whν ) E g + W c. Further enhancement of the absorption occudue to an increase of the electron density of states in othe valence band. The interface absorption becomes the mmechanism of light absorption for the crystallites that

smaller than 20 nm.412

Sato and Sakai et al. showed through calculation ameasurement that the band gap of TiO2 nanosheets was largerthan the band gap of bulk TiO2, due to lower dimensionality,i.e., a 3D to 2D transition, as shown in Figure 46.360,413Fromthe measurement, it was found that the lower edge of conduction band for the TiO2 nanosheet was approximately0.1 V higher, while the upper edge of the valence band w0.5 V lower than that of anatase TiO2.360 The absorption of the TiO2 nanosheet colloid blue shifted (> 1.4 eV) relativeto that of bulk TiO2 crystals (3.0- 3.2 eV), due to a size-quantization effect, accompanied with a strong photolunescence of well-developed fine structures extending ithe visible light regime.362,363The band gap energy shift,∆ E g,

Figure 43. (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 valenceband (the vertical solid line) is taken as the zero of energy. Thevertical dashed line indicates the conduction-band minimum as aguide to the eye. (B) Molecular-orbital bonding structure for anataseTiO2: (a) atomic levels; (b) crystal-field split levels; (c) finalinteraction states. The thin-solid and dashed lines represent largeand small contributions, respectively. Reprinted with permissionfrom 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.




by exciton confinement in anisotropic two-dimensionalcrystallites is formulated as follows:

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

crystallite dimensions in the parallel and perpendicudirections with respect to the sheet, respectively. Since first term can be ignored, the blue shift is predominangoverned by the sheet thickness. The onset of a 270 nm pin the photoluminescence of TiO2 nanosheets was assignedto resonant luminescence. The series of peaks extending a longer wavelength region were attributed to interband legenerated by the intrinsic Ti site vacancies. The contrast

Figure 44. (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 froOuvrared, G.; Gressier, P.; Natoli, C. R. Phys. Re V . B 1997 , 55 , 10382 (http://link.aps.org/abstract/PRB/v55/p10382). Copyright 199the American Physical Society.

∆ E g ) h2

8 µ xz ( 1 L x2

+ 1 L z

2)+ h2

8 µ y L y2 (4)




sharp peaks were also attributed to the subnanometerthickness and its uniformity.362

Bavykin et al. studied the optical absorption and photolu-minescence of colloidal TiO2 nanotubes with internal diam-eter in the range of 2.5- 5 nm, and they found that, in spiteof the different diameters, all the TiO2 nanotubes had similaroptical properties.158 They attributed this to the completesmearing of all 1-dimensional effects due to the largeeffective mass of charge carriers in TiO2, which resulted in

an apparent 2D behavior of TiO2 nanotubes. Figure 47 showsthe absorption, photoluminescence, and luminescence excita-tion spectra of TiO2 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 andvalence bands, respectively, E G is the energy gap, p isPlanck’s constant, and me and mh are the effective massesof the electrons and holes, respectively.

The electronic band structure of a TiO2 nanotube can beobtained from this relation by zone-folding and is given bya series of quasi-1D sub-bands with different indices n(Figure 48b):

This transition from the 2D to the quasi-1D energy spectrumhas a dramatic effect on the energy density of states. In the2D case, the density of states, G2D ) mc.h / π p 2, has a constantvalue for energies outside the energy gap (see Figure 48c).In the quasi-1D case, however, the density of states of eachsub-band

diverges at the band edge E n(0), leading to van Hovesingularities. The resulting density of state is formed by aseries of sharp peaks with long overlapping tails (Figure 48c).The energy gap between the valence and conductance bandsin the quasi-1D case is larger than that in the parental 2Dmaterial, and the difference increases with decreasingdiameter of the nanotube. The change in the energy gapsbetween a nanosheet and a nanotube is

In TiO2, the effective masses of electrons me can varybetween 5m0 and 30m0, and the mass of holes mh is morethan 3m0. With me ) 9m0 and mh ) 3m0, the differencebetween energy gaps of nanotubes with diameters 2.5 and 5nm is 8 meV. The energy difference between the two firstpeaks in the density of states G1D( E ) (Figure 48) is less than24 meV for d ) 2.5 nm and 6 meV for d ) 5 nm, which aretoo small to be resolved in room-temperature experimentsdue to the thermal fluctuations of kT ) 26 meV.158

In the theoretical study conducted by Enyashin and Seifertrecently, the band structures for anatase nanotubes, nano-strips, and nanorolls were similar to the DOS of the

corresponding bulk phase.376The valence band of both bulkTiO2 and their nanostructures was composed of 3d Ti- 2pO states, and the lower part of the conduction band wformed by 3d Ti states. The differences between thenanostructures were insignificant. All anatase systems wsemiconductors with a wide direct band gap (∼4.2 eV), whilethe lepidocrocite nanotubes were semiconductors with

indirect band gap (∼4.5 eV). Independent from the specifitopology of the titania nanostructures, the band gap proached the band gap of the corresponding nanocrystals wradii of about 25 Å.376

In addition to the above investigation on the bulk electrostructures for various TiO2 nanomaterials, Mora-Sero andBisquert investigated the Fermi level of surface states in T2nanoparticles by the nonequilibrium steady-state statisticelectrons.414They found that the electrons trapped in surfastates did not generally equilibrate to the free electrons’ Felevel, 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 equirium. The difference between the free electrons’ Fermi le

E 2D( ) (

E G2 (

p 2k 2

2me ,h(5)

E n1D( ) (

E G2 (

p 2

2me ,h [k |

2 + (2nd )

2] (6)

Gn,1D( E ) ) ( me ,h

2π 2p 2[ E - E n(0)]1/2 (7)

∆ E G ) E G1D - E G

2D )2p 2

d 2 ( 1me

+ 1mh)

(8)

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




and the surface Fermi level (∆ E Fn - E Fs) was found todepend on the rate constants for charge transfer and detrap-ping and could reach several hundred millielectron-volts.414

3.7. Photon-Induced Electron and Hole Propertiesof TiO2 Nanomaterials

After TiO2 nanoparticles absorb, impinging photons withenergies equal to or higher than its band gap (> 3.0 eV),electrons are excited from the valence band into the unoc-cupied conduction band, leading to excited electrons in theconduction band and positive holes in the valence band.These charge carriers can recombine, nonradiatively orradiatively (dissipating the input energy as heat), or gettrapped and react with electron donors or acceptors adsorbedon the surface of the photocatalyst. The competition betweenthese processes determines the overall efficiency for various

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

Figure 46. (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 AmericanChemical Society. (B) Schematic illustration of electronic band structure: (a) TiO2 nanosheets; (b) anatase. Reprinted with permissifrom Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004 , 126 , 5851. Copyright 2004 American Chemical Society. UV- visible spectra of (a) TiO2 sheets and (b) a film of nanosheets on a SiO2 glass substrate. The data for the colloidal suspensiondenoted by a dashed trace. Reprinted with permission from Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997 , 101 , 10159. Copyright 1997American Chemical Society.

TiO2 + hυ 98 e-

+ 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)

h+ +12 O2-

latticeT 14 O2(g)+ vacancy (12)

e-| + O2 ,s f O2 ,s

- (13)




Reaction 8 is the photon absorption process. Reactions 10-

14 are photocatalytic redox pathways, whereas reactions 15-

17 represent the recombination channels. Reactions 11 and12 are the competition pathways for holes, leading to boundOH radicals and O vacancies, respectively. The reverse of reaction 12 generates O adatom intermediates upon exposingdefective surfaces to O2- (g).415Electrons and holes generatedin TiO2 nanoparticles are localized at different defect siteson the surface and in the bulk. Electron paramagneticresonance (EPR) results showed that electrons were trappedas two Ti(III) centers, while the holes were trapped asoxygen-centered radicals covalently linked to surface tita-nium atoms.416- 419 Howe and Gratzel found that irradiationat 4.2 K in vacuo produced electrons trapped at Ti4+ sites

within the bulk and holes trapped at lattice oxide ionsimmediately below the surface, which decayed rapidly inthe dark at 4.2 K. In the presence of O2, trapped electronswere removed and the trapped holes were stable to 77 K.Warming to room-temperature caused loss of trapped holesand formation of O2- at the surface.416,417Hurum et al. foundthat, upon band gap illumination, holes appeared at thesurface and preferentially recombined with electrons insurface trapping sites for mixed-phase TiO2, such as DegussaP25, and recombination reactions were dominated by surfacereactions that followed charge migration.419

Colombo and Bowman studied the charge carrier dynamicsof TiO2 nanoparticles with femtosecond time-resolved diffusereflectance spectroscopy and found a dramatic increase in

the population of trapped charge carriers within the first fewpicoseconds.420,421Skinner et al. found that the trapping timefor photogenerated electrons on 2 nm TiO2 nanoparticles inacetonitrile by ultrafast transient absorption was about 180fs.422 Serpone et al. found that localization (trapping) of theelectron as a Ti3+ species occurred with a time scale of about30 ps and about 90% or more of the photogenerated electron/ hole pairs recombined within 10 ns.409 They suggested thatphotoredox chemistry occurring at the particle surfaceemanated from trapped electrons and trapped holes ratherthan from free valence band holes and conduction bandelectrons. Bahnemann et al. found that, in 2.4 nm TiO2nanoparticles, electrons were instantaneously trapped withinthe duration of the laser flash (20 ns). Deeply trapped holes

were rather long-lived and unreactive, and shallowly trappedholes were in a thermally activated equilibrium with freeholes which exhibited a very high oxidation potential.423

Szczepankiewicz and Hoffmann et al. found that O2 wasan efficient scavenger of conduction band electrons at thegas/solid interface and the buildup of trapped carrierseventually resulted in extended surface reconstruction in-volving Ti- OH functionalities.415 They found that photo-generated free conduction band electrons were coupled withacoustic phonons in the lattice and their lifetimes werelengthened when dehydrated.424 The photoexcited chargecarriers in TiO2 nanoparticles produced Stark effect intensityand wavelength shifts for surface TiO- H stretching vibra-tions. Although deep electron-trapping states affected certain

types of TiO- H stretch, shallow electron-trapping stateproduced a homogeneous electric field and were suggesnot to be associated with localized structures, but ratdelocalized across the TiO2 surface.424

Berger et al. studied UV light-induced electron- hole pairexcitations in anatase TiO2 nanoparticles by electron para-magnetic resonance (EPR) and IR spectroscopy.425 Thelocalized states such as holes trapped at oxygen anions (- )

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)

Figure 47. (A) (a) Absorption spectrum and (b) luminescenexcitation spectrum (wavelength of emission light is 400 nmcolloidal TiO2 nanotubes of different mean diameters: (1) 2.5 n(2) 3.1 nm; (3) 3.5 nm; (4) 5 nm. The curves are shifted vertifor clarity. (B) Photoluminescence spectra of colloidal T2nanotubes of different mean diameters: (1) 2.5 nm; (2) 3.1 (3) 3.5 nm; (4) 5 nm. Room temperature, excitation wavele237 nm, slits width 5 nm. The range of wavelengths, 455- 490nm, in the spectra is omitted due to the high signal of the seharmonic from scattered excitation light. The curves are shivertically for clarity. Vertical lines (5) show the positions ofpeaks in the PL spectrum of the nanosheets. Reprinted wpermission from Bavykin, D. V.; Gordeev, S. N.; MoskalenkoV.; Lapkin, A. A.; Walsh, F. C. J. Phys. Chem. B 2005 , 109 , 8565.Copyright 2005 American Chemical Society.




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

trapped at localized sites, giving paramagnetic Ti3+ centers,or remained in the conduction band as EPR silent specwhich may be observed by their IR absorption and that EPR-detected holes produced by photoexcitation were -

species, produced from lattice O2- ions. It was also foundthat, under high-vacuum conditions, the majority of phoexcited electrons remained in the conduction band. At K, all stable hole and electron states were lost.

4. Modifications of TiO 2 Nanomaterials

Many applications of TiO2 nanomaterials are closelyrelated to its optical properties. However, the highly efficuse of TiO2 nanomaterials is sometimes prevented by its wiband gap. The band gap of bulk TiO2 lies in the UV regime(3.0 eV for the rutile phase and 3.2 eV for the anatase phawhich 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 performaof TiO2 nanomaterials is to increase their optical activity

shifting the onset of the response from the UV to the visregion.21,426- 428 There are several ways to achieve this goaFirst, doping TiO2 nanomaterials with other elements canarrow the electronic properties and, thus, alter the optproperties of TiO2 nanomaterials. Second, sensitizing TiO2with other colorful inorganic or organic compounds cimprove its optical activity in the visible light region. Thcoupling collective oscillations of the electrons in tconduction band of metal nanoparticle surfaces to thosethe conduction band of TiO2 nanomaterials in metal- TiO2nanocomposites can improve the performance. In additithe modification of the TiO2 nanomaterials surface with othersemiconductors can alter the charge-transfer propertbetween TiO2 and the surrounding environment, thus im

Figure 48. Schematic presentation of the transformation of the electron band structure of the nanosheet semiconductor accomformation of nanotubes: (a) band diagram of a 2-dimensional nanosheet; (b) band diagram of quasi-1-D nanotubes; (c) enerstates 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 arethe wave vectors. Reprinted with permission from Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A. V.; Lapkin, A. A.; Walsh, F J. Phys.Chem. B 2005 , 109 , 8565. Copyright 2005 American Chemical Society.

Figure 49. Scheme of UV-induced charge separation in TiO2.Electrons from the valence band can either be trapped (a) by defectstates, which are located close to the conduction band (shallow

traps), or (b) in the conduction band, where they produce absorptionin the IR region. Electron paramagnetic resonance spectroscopydetects 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 AmericanChemical Society.




proving the performance of TiO2 nanomaterials-based de-vices.

4.1. Bulk Chemical Modification: DopingThe optical response of any material is largely determined

by its underlying electronic structure. The electronic proper-ties of a material are closely related to its chemicalcomposition (chemical nature of the bonds between the atomsor ions), its atomic arrangement, and its physical dimension(confinement of carriers) for nanometer-sized materials. Thechemical composition of TiO2 can be altered by doping.Specifically, the metal (titanium) or the nonmetal (oxygen)component can be replaced in order to alter the material’soptical properties. It is desirable to maintain the integrity of the crystal structure of the photocatalytic host material andto produce favorable changes in electronic structure. Itappears easier to substitute the Ti4+ cation in TiO2 with othertransition metals, and it is more difficult to replace the O2-

anion with other anions due to differences in charge statesand ionic radii. The small size of the nanoparticle is beneficialfor the modification of the chemical composition of TiO2due to the higher tolerance of the structural distortion thanthat of bulk materials induced by the inherent lattice strainin nanomaterials.426,429

4.1.1. Synthesis of Doped TiO 2 Nanomaterials 4.1.1.1. Metal-Doped TiO 2 Nanomaterials. Different

metals have been doped into TiO2 nanomaterials.313,430- 465

The preparation methods of non-metal-doped TiO2 nanoma-terials can be divided into three types: wet chemistry, high-temperature treatment, and ion implantation on TiO2 nano-materials. Wet chemistry methods usually involve hydrolysisof a titanium precursor in a mixture of water and other

reagents, followed by heating. Choi et al. performed asystematic study of TiO2 nanoparticles doped with 21 metalions by the sol- gel method and found the presence of metalion dopants significantly influenced the photoreactivity,charge carrier recombination rates, and interfacial electron-transfer rates.434Li et al. developed La3+ -doped TiO2 by thesol- gel process and found that the lanthanum doping couldinhibit the phase transformation of TiO2, enhance the thermalstability of the TiO2, reduce the crystallite size, and increasethe Ti3+ content on the surface.442 Nagaveni et al. preparedW, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO2 nanopar-ticles by a solution combustion method and found that thesolid solution formation was limited to a narrow range of concentrations of the dopant ions.448 Wang et al. prepared

Nd3+ -doped and Fe(III)-doped TiO2 nanoparticles with ahydrothermal method and found that anatase, brookite, a trace of hematite coexisted at lower pH (1.8 and 3.6) wthe Fe(III) content was as low as 0.5% and the distributof iron ions was nonuniform between particles, but at higpH (6.0), the uniform solid solution of iron- titanium oxideformed.460,463

Anpo et al. prepared TiO2 nanoparticles doped with Crand V ions with an ion-implantation method.466- 471Bessekh-

ouad et al. investigated alkaline (Li, Na, K)-doped Ti2nanoparticles prepared by sol- gel and impregnation technol-ogy and found that the crystallinity level of the products largely dependent on both the nature and the concentratof the alkaline, with the best crystallinity obtained for doped TiO2 and the lowest for K-doped TiO2.430 Cao et al.prepared Sn4+ -doped TiO2 nanoparticle films by the plasma-enhanced CVD method and found that, after doping by more surface defects were present on the surface.433 Graciaet al. synthesized M (Cr, V, Fe, Co)-doped TiO2 by ion beaminduced CVD and found that TiO2 crystallized into theanatase or rutile structures depending on the type and amoof cations present with partial segregation of the cationthe form of M2On after annealing.438Wang et al. synthesized

Fe(III)-doped TiO2 nanoparticles using oxidative pyrolysiof liquid-feed organometallic precursors in a radiatifrequency (RF) thermal plasma and found that the formaof rutile was strongly promoted with iron doping compato the anatase phase being prevalent in the undoped TiO2.246

4.1.1.2. Nonmetal-Doped TiO 2 Nanomaterials. Variousnonmetal elements, such as B, C, N, F, S, Cl, and Br, hbeen successfully doped into TiO2 nanomaterials. C-dopedTiO2 nanomateirals have been obtained by heating titanicarbide472- 474 or by annealing TiO2 under CO gas flow athigh 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 bhydrolysis of TTIP in a water/amine mixture and the ptreatment of the TiO2 sol with amines426,428,477

-482or directlyfrom a Ti- bipyridine complex483or by ball milling of TiO2

in a NH3 water solution.484N-doped TiO2 nanomaterials werealso obtained by heating TiO2 under NH3 flux at 500- 600°C485,486 or by calcination of the hydrolysis product oTi(SO4)2 with ammonia as precipitator487or by decompositionof gas-phase TiCl4 with an atmosphere microwave plasmtorch488 or by sputtering/ion-implanting techniques winitrogen489,490or N2

+ gas flux.491

S-doped TiO2 nanomaterials were synthesized by mixinTTIP with ethanol containing thiourea492- 494 or by heatingsulfide powder495,496or by using sputtering or ion-implantintechniques with S+ ion flux.497- 499Different doping methodscan induce the different valence states of the dopants. example, the incorporated S from thiourea had S4+ or S6+

state,492- 494 while direct heating of TiS2 or sputtering withS+ induced the S2- anion.496- 499

F-doped TiO2 nanomaterials were synthesized by mixinTTIP with ethanol containing H2O- NH4F,500- 502 or byheating TiO2 under hydrogen fluoride503,504 or by spraypyrolysis from an aqueous solution of H2TiF6505,506or usingion-implanting techniques with F+ ion flux.507 Cl- and Br-co-doped nanomaterials were synthesized by adding Ti4to ethanol containing HBr.508

4.1.2. Properties of Doped TiO 2 Nanomaterials 4.1.2.1. Electronic Properties of Doped TiO 2 Nanoma-

terials. 4.1.2.1.1. Metal-Doped TiO 2 Nanomaterials. Ac-

Figure 50. 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. Re V . 1995 , 95, 735. Copyright 1995 AmericanChemical Society.




cording to Soratin and Schwarz’s study, the electronic statesof TiO2 can be decomposed into three parts: the σ bondingof the O pσ and Ti eg states in the lower energy region; theπ bonding of the O pπ and Ti eg states in the middle energyregion; and the O pπ states in the higher energy region(Figure 51A).397,509The 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 t2g- Ti t2gstates. At the top of the lower CB, the rest of the Ti2g states

are antibonding with the O pπ states. The upper CB consistsof the σ antibonding orbitals between the O pσ and Ti egstates.

The electronic structures, i.e., the densities of states(DOSs), of V-, Cr-, Mn-, Fe-, Co-, and Ni-doped TiO2 wereanalyzed by ab initio band calculations based on the densityfunctional theory with the full-potential linearized augmentedplane wave (FLAPW) method by Umebayashi et al. (Figure51B).509 They found that when TiO2 was doped with V, Cr,Mn, Fe, or Co, an electron occupied level formed and theelectrons were localized around each dopant. As the atomicnumber of the dopant increased, the localized level shiftedto lower energy. The energy of the localized level due toCo doping was low enough that it lay at the top of the valence

band while the other metals produced midgap states. Theelectrons from the Ni dopant were somewhat delocalized,thus significantly contributing to the formation of the valenceband with the O p and Ti 3d electrons. The states due to the3d dopants shifted to a lower energy as the atomic numberof the dopant increased. For Ti1- xV xO2: two localized levelsoccurred at 1.5 eV above the VB (a) and between the lowerand upper CBs (b). Level a was occupied by one electronconsisting of the V t2g and O pπ states and was localizedaround V. Level b consisted of the V eg and O pσ statesforming the σ antibonding orbital. For Cr- and Mn-dopedTiO2, state c was localized at 1.0 eV (0.5 eV for Mn) abovethe VB due to Cr (Mn) t2g and O pπ , the former of whichwas occupied by 2 (3) electrons. The σ antibonding orbital

formed by the Cr (Mn) eg and O pσ states occurred withinthe lower CB. For Fe- and Co-doped TiO2, the localized level(e) was situated 0.2 eV above the VB (or at the top of VB for Co) due to the π antibonding of the Fe eg and O pπ states. This level was occupied by four (or five for Celectrons. The Fe (Co) eg state was split into d z2 (f) and d x2- y2

(g) orbitals in the band gap. For Ni-doped TiO2, the π antibonding of the Ni t2g and O pπ states was somewhatdelocalized and appeared within the VB (h) due to the Ng

states from the d z2 and d x2- y2 orbitials situated in the bandgap. The electron densities around the dopant were largthe VB and small in the CB compared to the case of pTiO2. The metal- O interaction strengthened, and the metal-

metal interaction became weak as a result of the 3d medoping.

Li et al. found that 1.5 at % Nd3+ -doped TiO2 nanoparticlesreduced the band gap by as much as 0.55 eV and that band gap narrowing was primarily attributed to the subtutional Nd3+ ions, which introduced electron states into thband gap of TiO2 to form the new lowest unoccupiedmolecular orbital (LUMO).444 Wang and Doren found thatNd 4f electrons changed the electronic structure of Nd-doTiO2 into the half-metallic or the insulating ground state510

and that V 3d states were located at the bottom of tconduction band of the TiO2 host in V-doped TiO2, whichwas shown to be a half-metal or an insulator from ththeoretical studies.511

4.1.2.1.2. Nonmetal-Doped TiO 2 Nanomaterials. Recenttheoretical and experimental studies have shown that desired band gap narrowing of TiO2 can also be achievedby using nonmetal dopants (refs 385, 428, 444, 489, 4482, 484, 503, 504, and 512- 547). Asahi and co-workerscalculated the electronic band structures of anatase TiO2 withdifferent substitutional dopants, including C, N, F, P, orusing the FLAPW method in the framework of the lodensity approximation (LDA) as shown in Figure 52.489 Inthis study, C dopant introduced deep states in the gap489

Figure 51. (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 linetotal DOS. Black solid lines: dopant’s DOS. The states are labeled (a) to (j). Reprinted from Umebayashi, T.; Yamaki, T.; ItK. J. Phys. Chem. Solids 2002 , 63 , 1909, Copyright 2002, with permission from Elsevier.




Nakano et al. found three deep levels located at ap-proximately 0.86, 1.30, and 2.34 eV below the conductionband in C-doped TiO2, which were attributed to the intrinsicnature of TiO2 for the first one and the two levels newlyintroduced by the C doping.522In particular, the pronounced2.34 eV band contributed to band gap narrowing by mixingwith the O 2p valence band.522 Lee et al., in their first-principles density-functional LDA pseudopotential calcula-

tions of electronic properties of C-doped TiO2, found thatthe bands originating from C 2p states appeared in the bgap of TiO2; however, the mixing of C with O 2p statewas too weak to produce a significant band gap narrowing517

In Asahi’s study, the substitutional doping of N was most effective in the band gap narrowing because its p stmixed with O 2p states, while the molecularly existispecies, e.g., NO and N2 dopants, gave rise to the bondingstates below the O 2p valence bands and antibonding stdeep in the band gap (Ni and Ni+ s), and were well screenedand hardly interacted with the band states of TiO2.489 DiValentin et al. found that, for nitrogen doping in both anaand rutile polymorphs, N 2p localized states were just abovethe top of the O 2p valence band.512,513In anatase, thesedopant states caused a red shift of the absorption band etoward the visible region, while, in rutile, an overall bshift was found by the N-induced contraction of the Oband.512Experimental evidence supported the statement thnitrogen-doped TiO2 formed nitrogen-induced midgap levelslightly above the oxygen 2p valence band.486 Lee et al., intheir first-principles density-functional LDA pseudopotencalculations of electronic properties of N-doped TiO2, foundthat the bands originating from N 2p states appeared in band gap of TiO2; however, the mixing of N with O 2p statwas too weak to produce a significant band gap narrowing517

Wang and Doren found that N doping introduced some stat the valence band edge and thus made the original bagap of TiO2 smaller, and that a vacancy could induce somstates in the band gap region, which acted as shalldonors.510 Nakano et al. found that, in N-doped TiO2, deeplevels located at approximately 1.18 and 2.48 eV belowconduction band were attributed to the O vacancy statean efficient generation- recombination center and to the Ndoping which contributed to band gap narrowing by mixwith the O 2p valence band, respectively.523 Okato et al.found that, at high doping levels, N was difficult to substifor O to contribute to the band gap narrowing, instead givrise to the undesirable deep-level defects.524

S dopant induced a similar band gap narrowing nitrogen,489 and the mixing of the sulfur 3p states with thvalence band was found to contribute to the increased wof the valence band, leading to the narrowing of the bagap.495,497When S existed as S4+ , replacing Ti4+ , sulfur 3sstates induced states just above the O 2p valence states, S 3p states contributed to the conduction band of TiO2 asshown in Figure 53A.494

When F replaced the O in the TiO2 lattice, F 2p stateswere localized below the O 2p valence states without mixing with the valence or conduction band as shownFigure 53B, and additional states appeared just below

conduction edge, due to the electron occupied level compoof the t2g state of the Ti 3d orbital.507 The electronic changeinduced by F dopant was considered to be similar to thvacancy, thus reducing the effective band gap and improvvisible light photoresponse.507 Li et al. found that F dopingproduced several beneficial effects including the creationsurface oxygen vacancies, the enhancement of surfacidity, and the increase of Ti3+ ions, and doped N atomsformed a localized energy state above the valence bandTiO2, whereas doped F atoms themselves had no influenon the band structure in N- F-co-doped TiO2.519

4.1.2.2. Optical Properties of Doped TiO 2 Nanomate-rials. 4.1.2.2.1. Optical Properties of Metal-Doped TiO 2

Nanomaterials. A red shift in the band gap transition or

Figure 52. (A) Total DOSs of doped TiO2 and (B) the projectedDOSs into the doped anion sites, calculated by FLAPW, for thedopants F, N, C, S, and P located at a substitutional site for an Oatom in the anatase TiO2 crystal (eight TiO2 units per cell). Ni-doped stands for N doping at an interstitial site, and Ni+ s-dopedstands 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.




visible light absorption was observed in metal-doped TiO2(refs 433- 435, 438, 444, 445, 448, 449, 460- 463, 465, 466,470, 509, 548, and 549). For V-, Mn-, or Fe-doped TiO2,the absorption spectra shifted to a lower energy region with

an increase in the dopant concentration.434,445,460

This red shiftwas attributed to the charge-transfer transition between thed electrons of the dopant and the CB (or VB) of TiO2. Metal-ion doped TiO2 prepared by ion implantation with varioustransition-metal ions such as V, Cr, Mn, Fe, and Ni wasfound to have a large shift in the absorption band towardthe visible light region, with the order of the effectivenessin the red shift being V> Cr> Mn> Fe > Ni.466- 471Anpoet al. found that the absorption band of Cr-ion-implantedTiO2 shifted smoothly toward the visible light region, withthe extent of the red shift depending on the amount of metalions implanted as shown in Figure 54A.470 Impregnated orchemically Cr-ion-doped TiO2 showed no shift in theabsorption edge of TiO2; however, a new absorption band

appeared at around 420 nm as a shoulder peak due to theformation 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 absorptionof V-doped TiO2 was due to the transition between the VBand the V t2g level.509 The holes in the VB produced ananodic photocurrent. The photoexcitation processes undervisible light of V-, Cr-, and Mn-doped TiO2 are illustratedin Figure 55. Photoexcitation for V-, Cr-, Mn-, and Fe-dopedTiO2 occurred via the t2g level of the dopant. The visiblelight absorption for Mn- and Fe-doped TiO2 was due to theoptical transitions from the impurity band tail into the CB.The Mn (Fe) t2g level was close to the VB and easily

overlapped in highly impure media. The visible liabsorption for the Cr-doped TiO2 can be attributed to a donortransition from the Cr t2g level into the CB and the acceptotransition from the VB to the Cr t2g level.

Stucky et al. found that up to 8 mol % Eu3+ ions could bedoped into mesoporous anatase TiO2, and excitation of the

TiO2 electrons within their band gap led to nonradiatienergy transfer to the Eu3+ ions with a bright red lumines-cence.287 The mesoporous TiO2 acted as a sensitizer.

4.1.2.2.2. Optical Properties of Nonmetal-Doped TiO 2

Nanomaterials. Nonmetal doped TiO2 normally has a colorfrom white to yellow or even light gray, and the onsetthe absorption spectra red shifted to longer wavelengths (385, 426, 478, 483, 489, 494, 495, 497, 498, 505, 506, 5516, 518, 519, 521, and 529). In N-doped TiO2 nanomate-rials, the band gap absorption onset shifted 600 nm fr380 nm for the undoped TiO2, extending the absorption upto 600 nm, as shown in Figure 56.426The optical absorptionof N-doped TiO2 in the visible light region was primarillocated between 400 and 500 nm, while that of oxyg

deficient TiO2 was mainly above 500 nm from their densitfunctional theory study.520N- F-co-doped TiO2 prepared byspray pyrolysis absorbs light up to 550 nm in the visilight spectrum.518 The S-doped TiO2 also displayed strongabsorption in the region from 400 to 600 nm.494The red shiftsin the absorption spectra of doped TiO2 are generallyattributed to the narrowing of the band gap in the electrostructure after doping.489 C-doped TiO2 showed long-tailabsorption spectra in the visible light region.472,543Cl-, Br-,and Cl- Br-doped TiO2 had increased optical responsecompared to the case of pure TiO2 in the visible region.508

Livraghi et al. recently found that N-doped TiO2 containedsingle atom nitrogen impurity centers localized in the bgap of the oxide which were responsible for visible li

Figure 53. (A) Total DOS of S-doped TiO2. Reprinted withpermission 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 DOSsof 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.; Yama-moto, S.; Maekawa, M.; Kawasuso, A.; Itoh, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2003 , 206 , 254, Copyright 2003, withpermission from Elsevier.

Figure 54. (A) The UV- vis absorption spectra of TiO2 (a) andCr 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-dopedTiO2 (b′- d′) photocatalysts prepared by an impregnation methThe 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 Elsevi




the TiO2- xN x has a typical photoelectrochemical behaviorof a material with states in the band gap which act asrecombination centers for light-induced charge carriers.521

In another study, the action spectrum of N-doped TiO2 alsodisplayed a higher response in the visible region than thatof pure TiO2.486 The photocurrent spectra for the pure andS-doped crystals showed that the photocurrent spectrum edgeshifted to the low-energy region below 2.9 eV for theS-doped crystal, compared to 3.0 eV for pure TiO2, due to

the transition of electrons across the narrowed band gapbetween the VB and the CB.497

4.2. Surface Chemical ModificationsWhen a photocurrent is generated with light energy less

than that of the semiconductor band gap, the process isknown as sensitization and the light-absorbing dyes arereferred to as sensitizers.9,10TiO2 is a semiconductor with awide band gap, with optical absorption in the UV region(< 400 nm). Any materials with a narrower band gap orabsorption in the visible or infrared regime can be used as asensitizer for TiO2 materials. These materials include inor-ganic semiconductors with narrow band gaps, metals, andorganic dyes. How efficiently the sensitized TiO2 can interactwith the light depends largely on how efficiently thesensitizer interacts with the light. A common and key stepin the photosensitization of TiO2 is the efficient chargetransfer from the excited sensitizer to TiO2, and the resultingcharge separation. The match between the electronic struc-tures of the sensitizer and TiO2 plays a large role in thisprocess, as does the structure of the interface, including thegrain boundaries and bonding between the sensitizer andTiO2. Careful design is needed to avoid the charge trappingand recombination which eventually harm the performanceof sensitized TiO2.9,10,550

4.2.1. Inorganic Sensitization

4.2.1.1. Sensitization by Narrow Band Gap Semicon-ductors. Narrow band gap semiconductors have been usedas sensitizers to improve the optical absorption propertiesof TiO2 nanomaterials in the visible light region by variousgroups.551- 559 The preparation method for these inorganicsemiconductor sensitized TiO2 nanomaterials systems isusually the sol- gel method.551- 558 Hoyer et al. reported thesensitization of a nanocrystalline TiO2 matrix by small PbSnanoparticles (< 2.5 nm), and they found that the photogen-erated excess electrons could be directly injected from thePbS to the TiO2, resulting in strong photoconductance in thevisible region.553 Fitzmaurice et al. found that excitation of the sensitizer AgI on TiO2 nanoparticles resulted in astabilization of electron- hole pairs with a lifetime well

beyond 100 µs and in electron migration from AgI to TiO2.551

Vogel et al. studied the sensitization of nanoporous TiO2 byCdS, PbS, Ag2S, Sb2S3, and Bi2S3 and found that the relativepositions of the energetic levels at the interface between thequantum size particles and TiO2 could be optimized forefficient charge separation by using the size quantizationeffect and that the photostability of the electrodes could besignificantly enhanced by surface modification of the TiO2nanoparticles with CdS nanoparticles.558

Qian et al. found from surface photovoltage spectra (SPS)measurements that the large surface state density present onthe TiO2 nanoparticles could be efficiently decreased bysensitization using CdS nanoparticles and that the slowphotocurrent response disappeared and the steady-state

photocurrent increased drastically after the TiO2 nanopar-ticulate thin film was sensitized using CdS nanopacles.378,555Sant and Kamat found that quantum size effecplayed an important role in interparticle electron transfethe CdS- TiO2 semiconductor systems in that electrotransfer from photoexcited CdS to TiO2 was found to dependon the size of TiO2 nanoparticles.560Charge transfer occurredonly when TiO2 nanoparticles were sufficiently large (> 1.2nm) that the conduction band of the nanoparticles was loc

below that of CdS nanoparticles.560

Shen et al. studiednanostructured TiO2 electrodes with different nanocrystalsizes sensitized with CdSe nanoparticles and found tphotoelectrochemical currents in the visible region in CdSe-sensitized TiO2 nanostructured electrodes were largeldependent on both the structure and electron diffuscoefficient of the TiO2 electrodes.556 Zaban et al. studiednanocrystalline TiO2 electrodes sensitized with InP quantumdots, and found they exhibited strong photoconductionthe visible region and had a photocurrent action spectrconsistent with the absorption spectrum of the InP Qindicating electron transfers from InP QDs into TiO2 nano-particles under visible light illumination.559

Kamat et al. recently reported the sensitization of mescopic TiO2 films using bifunctional surface modifiers (SHR-COOH) linked with CdSe nanoparticles. Upon visible lexcitation, CdSe nanoparticles injected electrons into T2nanocrystallites.561 The TiO2- CdSe composite exhibited aphoton-to-charge carrier generation efficiency of 12% wemployed as a photoanode in a photoelectrochemical c

4.2.1.2. Sensitization by Metal Nanoparticles. Ohko etal. found that when the TiO2 nanoparticle films weresensitized with Ag nanoparticles, the color of the film cobe reversely switched back and forth between brownish-gunder UV light and the color of illuminating visible lidue to the oxidation of Ag by O2 under visible light andreduction of Ag+ under UV light.562 The color of the filmunder visible light could be tuned from green to red and wby changing the size of the Ag nanoparticles due to plasmon-based absorption of Ag and the dielectric confiment of the TiO2 nanoparticle film matrix. Figure 58 showabsorption spectra and photographs of Ag- TiO2 films.

Naoi et al. found that the chromogenic properties of Ag- TiO2 films could be improved by simultaneous irradtion during Ag deposition with UV and blue lightssuppress the formation of anisotropic Ag particles and tnonvolatilization of a color image could be achieved removing Ag+ that was generated during the irradiation wia colored light.563The color of the film was further found tbe affected by the resonance wavelengths of the Ag particthe TiO2 film, and the nanopores in the TiO2 film. They

found that the photochromism and rewritability of Ag-

TiO2films could be deactivated by modification of Ag nanopticles with thiols to make it possible to retain color imadisplayed on the films, and that the deactivated propercould be fully reactivated by UV irradiation (Figure 59A564

Kawahara et al. proposed the mechanism of charseparation at the interface between Ag and TiO2 nanoparticlesshown in Figure 59B.565 They found that, in the multicolorphotochromism of TiO2 nanoporous films loaded withphotocatalytically deposited or electrodeposited and comercially available Ag nanoparticles, visible light-induelectron transfer from Ag to oxygen molecules playedessential role. Some of the photoexcited electrons on Ag wtransferred to oxygen molecules via TiO2 and nonexcited




Ag, and replacement of the nonexcited Ag with Pt acceleratedthe electron transport from the photoexcited Ag to oxygenmolecules and the photochromic behavior.

Tian and Tatsuma found that nanoporous TiO2 filmsloaded with Ag and Au nanoparticles exhibit negativepotential changes and anodic currents in response to visiblelight irradiation, with potential applications for photovoltaiccells, photocatalysts, and plasmon sensors.566They found thatfor the Au- TiO2 system photoaction spectra for open-circuit

potential and short-circuit current agreed with the absorptionspectrum of Au nanoparticles in the TiO2 film. After the Aunanoparticles were photoexcited due to plasmon resonance,charge separation occurred by the transfer of photoexcitedelectrons from the Au particle to the TiO2 conduction bandwith the simultaneous transfer of compensating electronsfrom a donor in solution to the Au particles.567 Cozzoli etal. found that, following UV illumination, TiO2 nanorodssensitized with Ag or Au nanoparticles could sustain a higherdegree of conduction band electron accumulation than pureTiO2.126

4.2.1.3. Organic Dye Sensitization. Organic dyes havebeen widely employed as sensitizers for TiO2 nanomaterialto improve its optical properties, i.e., in dye-sensitized

nanocrystalline solar cells (DSSCs).18,246,312,568- 673Organicdyes are usually transition metal complexes with low lyexcited states, such as polypyridine complexes, phthaloc

nine, and metalloporphyrins.568- 673

The metal centers for thedyes include Ru(II), Zn(II), Mg(II), Fe(II), and Al(III), wthe ligands include nitrogen heterocyclics with delocaliπ or aromatic ring systems.

These organic dyes are normally linked to TiO2 nanopar-ticle surfaces via functional groups by various interactibetween the dyes and the TiO2 nanoparticle substrate: (a)covalent attachment by directly linking groups of interesvia linking agents, (b) electrostatic interactions via exchange, ion-pairing, or donor- acceptor interactions, (c)hydrogen bonding, (d) van der Waals forces, etc. Mostthe dyes of interest link in the first way. Groups suchsilanyl (- O- Si- ), amide (- NH- (Cd O)- ), carboxyl (- O-

(Cd O)- ), and phosphonato (- O- (HPO2)- ) have been

shown to from stable linkages with the surface hydrogroups on TiO2 substrates.610Carboxylic and phosphonic acidderivatives react with the hydroxyl groups to form estewhile amide linkages are obtained via the reaction of amderivatives and dicyclohexyl carbodiimide on TiO2. The mostcommon and successful functional groups are based carboxylic acids. Qu and Meyer found spectroscopic edence for ester linkages after carboxylic acids react withsurface titanol groups dehydratively.674 Metal cyano com-pounds in acidic solutions were found to link to TiO2 surfacesby a single cyanide ligand with a C 4V symmetry, i.e., TiIV-

NC- FeII(CN)5.668,669,675

The interfacial charge separation between the adsorbdyes and TiO2 nanomaterials involves one of three mecha

Figure 58. (A) Absorption spectra of a Ag- TiO2 film byultraviolet 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 substrateirradiated successively with monochromatic lights. A xenon lampand an ultraviolet-cut filter (blocking light below 400 nm) was usedwith 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 permissionfrom Ohko, Y.; Tatsuma, T.; Fujii, T.; Naoi, K.; Niwa, C.; Kubota,Y.; Fujishima, A. Nature Mater. 2003 , 2, 29. Copyright 2003 NaturePublishing Group.

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




nisms, which differ by the nature of the donor that transfersthe electron to the semiconductor: (1) excited state; (2)reduced state; or (3) molecule-to-particle charge-transfercomplex.550 For complete knowledge of the charge transferbetween the dye sensitizers and the TiO2 nanomaterials,please refer to other excellent reviews.10,18,19,550,676,677Ultrafastelectron transfer from metal-to-ligand charge transfer (MLCT)excited states to anatase TiO2 is the most common categoryin dye-sensitized TiO2.572,595,606,618,619,677- 683 The mechanism

of the dye sensitization of TiO2 nanoparticles normallyinvolves the excitation of the dye and the charge transferfrom the dye to TiO2 nanoparticles. The low-lying MLCTand ligand-centered (π - π *) excited states of these complexesare fairly long-lived, allowing them to participate in electron-transfer processes. As an efficient photosensitizer, the dyehas to meet several requirements. First, the dye should havehigh absorption efficiency and a wide spectral range of coverage of light absorption in the visible, near-IR, and IRregions. Second, the excited states of the dye should have along lifetime and a high quantum yield. Third, the dye shouldhave matched electronic structures for the ground and excitedstates with TiO2 nanoparticles to ensure the efficient chargetransfer between them; that is, the energy level of the excited-state should be well matched to the lower bound of theconduction band of TiO2 to minimize energetic losses duringthe electron-transfer reaction.550

The electron transfer from the dye to TiO2 usually is veryfast, in the range of tens of femtoseconds. Hannappel et al.found that electron transfer from the excited electronic singletstate of chemisorbed ruthenium(II) cis-di(isothiocyanato)-bis(2,2′-bipyridyl-4,4′-dicarboxylate) into empty electronicstates in a colloidal anatase TiO2 film was on the time scaleof < 25 fs.595 Rehm et al. found the charge injection from asurface-bound coumarin 343 to the conduction band of TiO2occurred on a time scale of ∼200 fs due to strong electroniccoupling between the dye and TiO2 energy levels.684 The

electron transport and recombination in dye-sensitized TiO2solar cells with different electrolytes had been investigated,including iodine-doped ionic liquids (diethylmethylsulfo-nium, dibutylmethylsulfonium, or 3-hexyl-1-methylimida-zolium iodide) and an organic solvent (3-methoxypropion-itrile with LiI, I2, and 1-methylbenzimidazole).685 The mostviscous electrolytes showed a clear limitation in photocurrentattributed to a low diffusion coefficient for the triiodide thattransports positive charge to the counterelectrode. Theelectron transport of the solar cells appeared to be dominatedby the properties of the nanostructured TiO2 film, and theelectron lifetime depended on the type of cation used in theionic liquid. Bulky, less absorptive cations seem to givelonger lifetimes. Schwarzburg et al. found a time constant

of 13 fs for electron transfer from the excited singlet stateof the chromophore perylene bonded to the surface via acarboxyl group into anatase TiO2.686 The electron-transfertime of perylene became much longer (3.8 ps) at a distanceof about 1.3 nm. Wenger et al. found that carefully controlleddeposition of Ru(II) complex dye molecules onto nanocrys-talline TiO2 consistently yielded monophasic injection dy-namics with a time constant shorter than 20 fs.687 Theanchoring 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 TiO2 filmsoccurs by a bidentate binuclear coordination mode.577 Theelectron injection process from the dye excited state into theTiO2 conduction band was biexponential with a fast com-

ponent (200 ( 50 fs) and a slow component (20 ps)attributed to the electron injection from the initially formand the relaxed dye excited states, respectively.577

In the reduced sensitizer injection mechanism, the sentizer excited state(s) is first quenched by an external donand subsequently the reduced state of the dye, S- , transfersan electron across the semiconductor interface.550,688 Apotential advantage of this mechanism is that the reducsensitizer is a stronger reductant than the MLCT excited sttypically by 0.3- 0.5 eV. Thus, sensitizers that are weakphotoreductants may sensitize TiO2 efficiently after reductivequenching. This mechanism may be exploited to prodularge open-circuit photovoltages or enhanced light harvesin the near-IR regions. The observation of ultrafast electinjection coupled with the weak oxidizing power of excited sensitizers currently in use strongly suggests thaexcited-state injection mechanism is operative in regenerasolar cells based on these materials. The reduced sensitiinjection mechanism was reported by Thompson,688Haque,598

and Wang.659

The metal-to-particle charge-transfer mechanism involvinterfacial chemistry between the compounds and the T2surface which produces color changes, observed by Graand identified as molecule-to-particle charge-transfer trantions.689 Metal cyanides, [M(CN) x]4- (M ) FeII, RuII, OsII,ReIII, MoIV, or WIV, x ) 6, 7, or 8), such as ferrocyanideFeII(CN)64- , bind to TiO2 through ambidentate cyano ligandsFor example, FeII(CN)64- does not absorb light above 380nm, but a deep orange color with an absorption maximcentered at 420 nm was observed for FeII(CN)64- /TiO2, dueto a MPCT complex formed between FeII(CN)64- and surfaceTi4+ ions, Fe(II)f Ti(IV).550 The metal-to-particle charge-transfer mechanism was consistent with the subpicosecoinfrared spectroscopy study on the FeII(CN)64- /TiO2 nano-particle by Weng et al., where a mid-infrared absorption wassigned to TiO2 electrons in the semiconductor.690 Theinjection rate constant could not be time-resolved with a fs instrument response function. The MPCT was also foby Yang et al. in their study on Fe(bpy)(CN)42- -sensitizedTiO2, where the absorption spectra were well modeled bsum of MLCT (Fe f bpy) and metal-to-particle (Fe(II) f Ti(IV)) bands. The MLCT bands were solvatochromic, wthe MPCT bands were not.668,669Benkoe et al. found thatthe larger the TiO2 particle and the better its overallcrystallinity, the faster the process of electron injection frthe dye fluorescein 27 to the anatase TiO2 film.578 Haque etal. found that a supramolecule dye with a remarkably lolived (4 s) charge-separation state could be obtained controlling the spatial separation between the cation cenof the dye and the electrode surface.597 The dyes were Ru-

(II) complexes containing carboxylated polypyridyl chmophores and a bipyridyl ligand with aromatic amine-baelectron donor substituents.597

The kinetics and mechanisms of the injections, transport,combination, and photovoltaic properties of electrons in nastructured TiO2 solar cells have been thoroughly discussein recent reviews676,691- 694and will only be briefly mentionedbelow. Considerable effort has been devoted to the kineand energetics of transport and recombination in dye-sentized solar cells with various techniques, such as intensmodulated photocurrent spectroscopy (IMPS),640,695- 702inten-sity modulated photovoltage spectroscopy (IMVS),639,700,703,704

electrical impedance spectroscopy (EIS),700,705- 710 transientphotocurrent,695,706,711- 718and transient photovoltage.715,719For




example, Searson and Cao studied the photocurrent responseof dye-sensitized, porous nanocrystalline TiO2 cells withphotocurrent transient measurements and intensity-modulatedphotocurrent spectroscopy and found that the electrontransport in the TiO2 film can be fitted with a diffusion modelwhere the diffusion coefficient for electrons in the particlenetwork was a function of the light intensity.695 Usingintensity modulated photocurrent spectroscopy, Vanmaekele-bergh et al. found that the electronic transport was controlled

by trapping and detrapping of photogenerated electrons ininterfacial band gap states, distributed in energy, and thatthe localization time of a trapped electron was controlledby the steady-state light intensity and interfacial kinetics.696,697

Peter et al. recently found that the electron transport in dye-sensitized nanocrystalline solar cells appeared to be a slowdiffusion-controlled process, attributed to multiple trappingat energy levels distributed exponentially in the band gap of the nanocrystalline TiO2.720

Frank et al. summarized that the electron motion isessentially ambipolarly diffusional and the morphology anddefect structure of the TiO2 film had a strong influence onelectron transport.676The recombination predominates at theinterface and depends on the spatial region of photoinjected

charge buildup in the cell, the redox electrolyte, and thesurface properties of both the TiO2 nanoparticle film andthe TCO substrate. For the recombination, two mechanismsassume either a dismutation reaction or an interfacialelectron-transfer reaction as rate-limiting, while the thirdmechanism states electron transport limits recombination.676

The spatial location of the traps limiting electron transportin nanocrystalline TiO2 has been a long standing issue. Thesetraps have been speculated to locate either at the particlesurface,640,721,722in the bulk of the particles,723 or at inter-particle grain boundaries.568 Kopidakis et al. recently inves-tigated the dependence of the electron diffusion coefficientand the photoinduced electron density on the internal surfacearea of TiO2 nanoparticle films in dye-sensitized solar cellsby photocurrent transient measurements.724They found thatthe density of electron traps in the films changed in directproportion with the internal surface area, which was variedby altering the average particle size of the films, and thescaling of the electron diffusion coefficient with the internalsurface area. They suggested that the traps were locatedpredominately at the surface of TiO2 particles instead of inthe bulk of the particles or at interparticle grain boundaries,and that surface traps limited transport in TiO2 nanoparticlefilms. Kopidakis et al. found that the traps were locatedpredominately at the surface of TiO2 particles instead of inthe bulk of the particles or at interparticle grain boundariesand that surface traps limited transport in TiO2 nanoparticlefilms in the dye-sensitized TiO2 solar cell.724

5. Applications of TiO 2 Nanomaterials The existing and promising applications of TiO2 nanoma-

terials include paint, toothpaste, UV protection, photoca-talysis, photovoltaics, sensing, and electrochromics as wellas photochromics. TiO2 nanomaterials normally have elec-tronic band gaps larger than 3.0 eV and high absorption inthe UV region. TiO2 nanomaterials are very stable, nontoxic,and cheap. Their optical and biologically benign propertiesallow them to be suitable for UV protection applications.725- 730

A surface is defined as superhydrophilic or superhydro-phobic 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 pructs, i.e., mirrors and eyeglasses, having superhydrophor superhydrophobic surfaces.332,732- 734 For example, Fenget al. found that reversible superhydrophilicity and suphydrophobicity could be switched back and forth for T2nanorod films.142 When the TiO2 nanorod films wereirradiated with UV light, the photogenerated hole reacwith lattice oxygen to form surface oxygen vacancies. Wmolecules kinetically coordinated to these oxygen vacanc

and the spherical water droplet filled the grooves along nanorods and spread out on the film with a contact anglabout 0°, resulting in superhydrophilic TiO2 films. After thehydroxy group adsorption, the surface transformed intoenergetically metastable state. When the films were plain the dark, the adsorbed hydroxy groups were graduareplaced by atmospheric oxygen, and the surface evolvback to its original state. The surface wettability converfrom superhydrophilic to superhydrophobic.142 Stain-proof-ing, self-cleaning properties can also be bestowed on mdifferent types of surfaces due to the superhydrophilicsuperhydrophobic surfaces.735- 744 TiO2 nanomaterials havealso been used as sensors for various gases and humiddue to the electrical or optical properties which change u

adsorption.745- 751

One of the most important research areas for future cleenergy applications is to look for efficient materials for production of electricity and/or hydrogen. When sensitiwith organic dyes or inorganic narrow band gap semicductors, TiO2 can absorb light into the visible light regioand convert solar energy into electrical energy for solar applications.28,30,752For example, an overall solar to currenconversion efficiency of 10.6% has been reached by group led by Gratzel with DSSC technology.31 TiO2 nano-materials have been widely studied for water splitting ahydrogen production due to their suitable electronic bastructure given the redox potential of water.198,475,476,753- 770

Another application of TiO2 nanomaterials when sensitizedwith dyes or metal nanoparticles is to build photochrodevices.562,565,771- 777Of course, one of the many applicationof TiO2 nanomaterials is the photocatalytic decompositiof various pollutants.

5.1. Photocatalytic ApplicationsTiO2 is regarded as the most efficient and environmenta

benign photocatalyst, and it has been most widely usedphotodegradation of various pollutants.121,127,132,430,442,778- 822

TiO2 photocatalysts can also be used to kill bacteria, as hbeen carried out with E. coli suspensions.793,799The strongoxidizing power of illuminated TiO2 can be used to kill tumorcells in cancer treatment.782,785,820,823- 825

The photocatalytic reaction mechanisms are widely stied.7,12,20,33,406The principle of the semiconductor photocatlytic reaction is straightforward. Upon absorption of photwith energy larger than the band gap of TiO2, electrons areexcited from the valence band to the conduction bancreating electron- hole pairs. These charge carriers migratto the surface and react with the chemicals adsorbed on surface to decompose these chemicals. This photodecoposition process usually involves one or more radicalsintermediate species such as •OH, O2- , H2O2, or O2, whichplay important roles in the photocatalytic reaction mecnisms. The photocatalytic activity of a semiconductorlargely controlled by (i) the light absorption properties, elight absorption spectrum and coefficient, (ii) reduction




oxidation rates on the surface by the electron and hole, (iii)and the electron- hole recombination rate. A large surfacearea with a constant surface density of adsorbents leads tofaster surface photocatalytic reaction rates. In this sense, thelarger the specific surface area, the higher the photocatalyticactivity is. On the other hand, the surface is a defective site;therefore, the larger the surface area, the faster the recom-bination. The higher the crystallinity, the fewer the bulkdefects, and the higher the photocatalytic activity is. High-

temperature treatment usually improves the crystallinity of TiO2 nanomaterials, which in turn can induce the aggregationof small nanoparticles and decrease the surface area. Judgingfrom the above general conclusions, the relation between thephysical properties and the photocatalytic activities iscomplicated. Optimal conditions are sought by taking theseconsiderations into account and may vary from case to case.20

5.1.1. Pure TiO 2 Nanomaterials: First Generation As the size of the TiO2 particles decreases, the fraction of

atoms located at the surface increases with higher surfacearea to volume ratios, which can further enhance the catalyticactivity. The increase in the band gap energy with decreasingnanoparticle size can potentially enhance the redox potentialof the valence band holes and the conduction band electrons,allowing photoredox reactions, which might not otherwiseproceed in bulk materials, to occur readily. One disadvantageof TiO2 nanoparticles is that they can only use a smallpercentage of sunlight for photocatalysis. Practically, thereexists an optimal size for a specific photocatalytic reaction.

Anpo et al. investigated the photocatalytic activity of TiO2nanoparticles on hydrogenation reactions of CH3CCH withH2O, and they found the activity increased as the diameterof the TiO2 particles decreased, especially below 10 nm.406

They suggested that the dependence of the yields on theparticle size arose from the differences in the chemicalreactivity and not from the physical properties of thesecatalysts.

Wang et al. found that there was an optimal size for TiO2nanoparticles for maximum photocatalytic efficiency in thedecomposition of chloroform.815They observed an improve-ment in activity when the particle size was decreased from21 to 11 nm, but the activity decreased when the size wasreduced further to 6 nm. They concluded that for thisparticular reaction the optimum particle size was about 10nm. In large TiO2 nanoparticles, bulk recombination of thecharge carriers was the dominant process, which could bereduced by a decrease in particle size; as the particle sizewas lowered below a certain limit, surface recombinationprocesses became dominant, since most of the electrons andholes were generated close to the surface and surface

recombination was faster than interfacial charge carriertransfer processes.826

Chae et al. studied the photocatalytic activity of four sizesof TiO2 nanoparticles on the decomposition of 2-propanol,and they found that 7-nm particles showed 1.6 times betterphotocatalytic activity than TiO2 P25 and that 15- and 30-nm particles showed lower photocatalytic efficiencies.132

Mesoporous TiO2, TiO2 nanorods, and nanotubes havebeen demonstrated to have high photocatalytic performanceunder suitable conditions.127,187,265,281,296,818Peng et al. pre-pared mesoporous TiO2 with a high specific surface area,which showed significant activity on the oxidation of Rhodamine B due to the large surface area, small crystalsize, and well-crystallized anatase mesostructure.296 Figure

60 shows the photocatalytic properties of mesoporous TiO2samples as prepared and calcined at different temperatucompared to those of TiO2 P25 nanoparticles. All mesopo-rous TiO2 showed better activity than Deguessa P25 TiO2.The optimum reactivity was obtained with the samcalcined at 400 °C, and the photoactivity gradually decreasewith further increases in calcination temperature.

Yang et al. found that TiO2 nanotubes treated with H2-SO4 solutions showed photocatalytic activity on degradatof acid orange II in the following order: TiO2 nanotubestreated with 1.0 mol/L H2SO4 solution > TiO2 nanotubestreated with 0.2 mol/L H2SO4 solution > untreated TiO2nanotubes> TiO2 nanoparticles, since TiO2 nanotubes treatedwith H2SO4 were composed of smaller particles and hahigher specific surface areas.818

TiO2 aerogels were also suggested as promising candida

for photocatalysts.316,317,319

Degan et al. prepared TiO2aerogels with a porosity of 90% and surface areas of 6m2 /g, and they found that the photodegradation of salicyacid on TiO2 aerogels, after 1 h of near-UV illuminationwas about 10 times faster than that on the Degussa TiO2.316,317

Figure 61 shows photodegradation profiles for the aerobefore and after annealing, as compared to the commerDegussa P25 powder.

5.1.2. Metal-Doped TiO 2 Nanomaterials: Second Generation

Over the past decades, metal-doped TiO2 nanomaterialshave been widely studied for improved photocatalyperformance on the degradation of various organic polluta

i.e., under visible light irradiation (refs 21, 430, 433- 435,444, 446, 450- 452, 455- 457, 490, 515, 548, 810, 827-

836). Choi et al. conducted a systematic study on tphotocatalytic activity of TiO2 nanoparticles doped with 21transition metal elements on the oxidation of CHCl3 and thereduction of CCl4 and found that the photocatalytic activitwas related to the electron configuration of the dopant in that dopant ions with closed electron shells had littleno effect on the activity.434,435Doping with Fe3+ , Mo5+ , Ru3+ ,Os3+ , Re5+ , V4+ , and Rh3+ at 0.1- 0.5 at % significantlyincreased the photoreactivity, while Co3+ and Al3+ dopingdecreased the photoreactivity. The presence of metal idopants in the TiO2 matrix significantly influenced the chargcarrier recombination rates and interfacial electron-trans

Figure 60. Photocatalytic properties of mesoporous TiO2 samplesas prepared and calcined at different temperature as well as T2P25 nanoparticles (RB, c0 ) 1.0× 10- 5 M, pH) 6.0) under UV-light radiation. Reprinted with permission from Peng, T.; ZD.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005 , 109 , 4947.Copyright 2005 American Chemical Society.




rates. The photoreactivity of doped TiO2 appeared to be acomplex function of the dopant concentration, the energylevel of dopants within the TiO2 lattice, their d electronicconfigurations, the distribution of dopants, the electron donorconcentrations, and the light intensity.

Sn4+ ion-doped TiO2 nanoparticle films prepared by theplasma-enhanced CVD method displayed a higher photo-catalytic activity for photodegradation of phenol than pureTiO2 under both UV and visible light, and the Sn4+ dopantwas found profitable to the separation of photogeneratedcarriers under both UV and visible light excitation.433Figure62 shows the photocatalytic decomposition of phenol withreaction time under UV and visible light using Sn4+ -doped

TiO2 nanoparticles as photocatalyst.433

Fe-doped nanocrystalline TiO2 was shown to displayhigher photocatalytic activity with lower Fe content (optimal0.05% mass fraction) than TiO2 in the treatment of paper-making wastewater,837and it was shown to be more efficientin the photoelectrocatalytic disinfection of E. coli than pureTiO2.827 V-doped TiO2 photocatalyst photooxidized ethanolunder visible radiation and had comparable activity underUV radiation to that of pure TiO2.548 Pt4+ ion-doped TiO2nanoparticles exhibited higher visible light photocatalyticactivities on the degradations of dichloroacetate and 4-chlo-rophenol,830and Ag- TiO2 nanocatalysts displayed enhancedphotocatalytic activity in the degradation of 2,4,6-trichlo-rophenol due to a better separation of photogenerated charge

carriers and improved oxygen reduction inducing a higherextent of degradation of atoms.809

Wei et al. synthesized La- and N-co-doped TiO2 nano-particles with superior catalytic activity under visible light,where N doping was responsible for the band gap narrowingof TiO2 and La3+ doping prevented the aggregation of nanoparticles.833 Chang et al. reported Cr- and N-co-dopedTiO2 nanomaterials with visible light absorbance generallyled to a reduction in photocatalytic efficacy in the decolori-zation of methylene blue, except at the low nitrogen dopingconcentration.490 Bessekhouad et al. found that low concen-tration alkaline (Li, Na, K)-doped TiO2 nanoparticles werepromising materials for organic pollutants degradation.430

Peng et al. found that in Be2+ -doped TiO2 nanomaterials,

when the doping ions were in the shallow surface, the dopwas beneficial, while, in the deep bulk, the doping wdetrimental.451

However, not all the metal-doped TiO2 nanomaterialsshowed higher photocatalytic activities than pure Ti2nanomaterials. Martin found V-doped TiO2 nanoparticles hadreduced photocatalytic activity on the photooxidation4-chlorophenol compared to pure TiO2 nanoparticles. Va-nadium appeared to reduce the photoreactivity of TiO2 bypromoting charge-carrier recombination with electron trping at VO2+ centers or with hole trapping at V4+ impuritycenters, which shunted charge carriers away from the sosolution interface.446Hermann et al. found that although Crdoped (0.85 atomic %) TiO2 absorbed in the visible region,its activity for oxidation of oxalic acid, propene, a2-propanol and for O isotope exchange was null under visillumination and was smaller under UV light than thatpure TiO2, due to an increase in electron- hole recombinationat the Cr3+ ion sites.440 Luo et al. reported that thephotoactivity of TiO2 doped with 1.5 mol % Mo, 1 mol %V, 0.1 mol % V plus 1 mol % Al, or 0.1 mol % V plumol % Pb decreased, since the d electrons of Mo(4d) aV(3d), as majority carriers in TiO2, could effectively quenchthe high-energy photogenerated holes at the impurity levintroduced by doping within the band gap of TiO2.445

5.1.3. Nonmetal-Doped TiO 2 Nanomaterials: Third Generation

Nonmetal-doped TiO2 nanomaterials have been regardedas the third generation photocatalyst. Various nonmet

Figure 61. Photodegradation profiles of salicylic acid on annealed(Ela) and nonannealed (El) TiO2 aerogels as compared to acommercial Degussa P25. Reprinted with permission from Dagan,G.; Tomkiewicz, M. J. Phys. Chem. 1993 , 97 , 12651. Copyright1993 American Chemical Society.

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




doped TiO2 nanomaterials have been widely studied for theirvisible 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-dopedTiO2 nanomaterials have been demonstrated to have im-proved photocatalytic activities compared to those for pureTiO2 nanomaterials, especially in the visible light re-gion.426,428,485,489,833,848

Figure 63 shows the decomposition of methylene blueusing N-doped TiO2 as measured by Asahi and co-wokers.489

It was found that N-doped TiO2 had much higher photo-

catalytic activity than pure TiO2 in the visible light region,while displaying lower activity in the UV-light region. Anitrogen concentration dependent performance of the pho-tocatalytic activity of the nitrogen-doped TiO2 was found inthe visible region, and the active sites of N for photocatalysisunder visible light were identified with the atomic β-N statespeaking at 396 eV in the XPS spectra.489In the study of Irieand co-workers, the concentration dependent photocatalyticactivity of the N-doped TiO2 was attributed to the fact thatthe band structure of the N-doped TiO2 with lower nitrogenconcentration (< 2%) was different from that with higherconcentration.483 It was found that the significant increasein photocatalytic activity in N-doped TiO2 nanoparticles wasdue to the O- Ti- N bond formation as oxynitride during

the substitutional doping process.428,477

The photocatalyticoxidation of organic compounds by N-doped TiO2 undervisible illumination was mainly via reactions with surfaceintermediates of water oxidation or oxygen reduction, notby direct reactions with holes trapped at the N-inducedmidgap level.486N-doped TiO2 nanotubes also exhibited highphotocatalytic oxidation activity for decomposition of gas-eous isopropanol into acetone and carbon dioxide whenilluminated with visible light.528

The photocatalytic activity of sulfur-doped TiO2 has alsobeen studied.492- 494,496 The S-doped TiO2 was found todisplay a higher photocatalytic activity in the visible regionbut a lower photocatalytic activity in the UV region.492- 494

S-doped TiO2 prepared with different methods showed

different photocatalytic activity under visible light due different carrier behavior in these samples.849

A noticeable photocatalytic activity on decompositionsmethylene blue and isopropanal in the visible region wdemonstrated for C-doped TiO2 made from a TiC precur-sor.472,473C-doped TiO2 made by pyrolyzing Ti metal in anatural gas flame displayed a much higher photoactivitywater splitting than pure TiO2.476C-doped TiO2 nanoparticlesalso displayed high photoactivity in degradation of trichroacetic acid under visible light.474

Yu et al. found that F-doped TiO2 showed higher photo-catalytic activity on the oxidation of acetone into CO2 thandid Degeussa P25 in the photodecomposition study acetone under proper preparation conditions.502 N/F-dopedTiO2 nanomaterials had high visible light photocatalyactivities for decompositions of both acetaldehyde atrichloroethylene due to the creation of surface oxygvacancies rather than the improvement of optical absorpproperties.505,506,518,519Luo et al. found that chlorine- andbromine-co-doped TiO2 displayed a much higher photocatalytic activity than chlorine- or bromine-doped TiO2.508

5.2. Photovoltaic Applications

5.2.1. The TiO 2 Nanocrystalline Electrode in DSSCs

Photovoltaics based on TiO2 nanocrystalline electrodeshave been widely studied.9,28- 32 A schematic presentationof the structure and operating principles of the DSSC is gin Figure 64. At the heart of the system is a nanocrystalmesoporous TiO2 film with a monolayer of the chargetransfer dye attached to its surface. The film is placedcontact with a redox electrolyte or an organic hole conducPhotoexcitation of the dye injects an electron into conduction band of TiO2. The electron can be conducted tothe outer circuit to drive the load and make electric powThe original state of the dye is subsequently restored

electron donation from the electrolyte, usually an orgasolvent containing a redox system, such as the ioditriiodide couple. The regeneration of the sensitizer by iodprevents the recapture of the conduction band electronthe oxidized dye. The iodide is regenerated in turn by reduction of triiodide at the counterelectrode, with the cirbeing completed via electron migration through the exteload. The voltage generated under illumination correspoto the difference between the Fermi level of TiO2 and theredox potential of the electrolyte. Overall, the devgenerates electric power from light without suffering apermanent chemical transformation.9,28- 32

Cahen et al. explained the cause for the photocurrent aphotovoltage in nanocrystalline mesoporous dye-sensiti

solar cells in terms of the separation, recombination, atransport of electronic charge as well as in terms of electenergetics.721 The basic cause for the photovoltage is thchange in the electron concentration in the nanocrystallelectron conductor that results from photoinduced chainjection from the dye. Pichot and Gregg found that photovoltage was determined by photoinduced chemipotential gradients, not by equilibrium electric fields.635Themaximum photovoltage is given by the difference in electenergies between the redox level and the bottom of tconduction band of the electron conductor, rather thanany difference in electrical potential in the cell, in the daCharge separation occurs because of the enthalpic aentropic driving forces that exist at the dye/electron condu

Figure 63. Photocatalytic properties of TiO2- xN x and TiO2 basedon decomposition rates [measuring the change in absorption of thereference 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 theaqueous solution under visible light as a function of the ratio of the decomposed area in the XPS spectra with the peak at 396 eVto 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 1.0atom % (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.




interface, with charge transport aided by such driving forcesat the electron conductor/contact interface. The mesoporosityand nanocrystallinity of the semiconductor are important notonly because of the large amount of dye that can be adsorbed

on the very large surface but also for two additionalreasons: (a) they allow the semiconductor small particlesto become almost totally depleted upon immersion in theelectrolyte (allowing for large photovoltages), and (b) theproximity of the electrolyte to all particles makes screeningof injected electrons, and thus their transport, possible.721

Many ruthenium complexes containing anchoring groupssuch as carboxylic acid, dihydroxy, and phosphonic acid onpyridyl ligands have been used as dyes in the DSSCs. Gratzelet al. have been leading the research in this field since theirbreakthrough in the early 1990s. Tris(2,2′-bipyridyl-4,4′-carboxylate) ruthenium(II) was used in DSSCs until theannouncement in 1991 of a sensitized electrochemicalphotovoltaic device with a conversion efficiency of 7.1%

under solar illumination with polypyridyl ruthenium aosmium sensitizers with the general structure ML2(X)2, whereL stands for 2,2′-bipyridyl-4,4′-dicarboxylic acid, M is Ruor Os, and X presents a halide, cyanide, thiocyanate, acacetonate, thiacarbamate, or water substituent.88 The dye-sensitized solar cells with cis-dithiocyanatobis(4,4′-dicar-boxylic acid-2,2′-bipyridine)ruthenium(II) (N3) displayeabsorption maxima at 518 and 380 nm and emission at nm with a lifetime of 60 ns.625,850In 2001 the “black dye”

tri(cyanato)-2,2′,2′′-terpyridyl-4,4′,4′′-tricarboxylate) ruthe-nium(II) was found to achieve 10.4% conversion efficiein full sunlight.631 Amphiphilic heteroleptic N3 equivalendyes were recently applied to solar cells.673These amphiphilicheteroleptic sensitizers had several advantages comparedthe N3 complex: (a) The ground-state pK a of the 4,4′-dicarboxy-2,2′-bpy was higher to enhance the binding of thcomplex onto the TiO2 surface. (b) The decreased chargeon the sensitizer attenuated the electrostatic repulsion aincreased the dye loading. (c) The presence of the hydphobic moiety on the ligand increased the stability of socells toward water-induced desorption. (d) The oxidatpotential of these complexes was cathodically shifted copared to that of the N3 sensitizer, which increased t

reversibility of the ruthenium III/II couple, leading enhanced stability. Combining the N3 dye with guanidinithiocyanate brought a further increase in the open-circvoltage of the solar cell.30,31

Unlike the large amount of effort put forth to optimthe organic dyes in DSSCs in the past decades, attentiononly recently been paid to the TiO2 nanocrystalline electrode,and some important results have been obtained. In tfollowing, various research efforts on the use of the Ti2nanocrystalline electrode for DSSCs are briefly summariz

5.2.1.1. Mesoporous TiO 2 Nanocrystalline Electrodes.Zukalova et al. found that ordered mesoporous TiO2 nano-crystalline films showed enhanced solar conversion efficieby about 50% compared to traditional films of the sathickness made from randomly oriented anatase nanocrtals.312The TiO2 nanocrystalline film was prepared via layeby-layer deposition with Pluronic P123 as template. Tsensitizer used was cis-dithiocyanato(4,4′-dicarboxy-2,2′-bipyridine)(4,4′-di-(2-(3,6-dimethoxyphenyl)ethenyl)-2,2′-bi-pyridine) ruthenium(II), N945. Figure 65 shows the phocurrent-voltage characteristics for solar cells based on ordeand nonordered TiO2 films. When sensitized by N945, th0.95- µm-thick nonorganized anatase film gave a conversiefficiency of only 2.21%, which increased to 2.74% wsurface treatment by TiCl4 prior to dye deposition. Understandard global AM 1.5 solar conditions, the cell withordered mesoporous TiO2 nanocrystallinne film gave a

photocurrent density of I p )

7 mA/cm2

, an open circuitpotential of U OC ) 0.799 V, and a fill factor of ff ) 0.72,yielding 4.04% conversion efficiency. This improvemresulted from a remarkable enhancement of the short cirphotocurrent, due to the huge surface area accessible to bthe dye and the electrolyte.312

5.2.1.2. TiO 2 Nanotube Electrode. Adachi et al. foundthat dye-sensitized solar cells with electrodes made disordered single-crystalline TiO2 nanotubes (10-nm diam-eter, 30- 300-nm length) displayed an efficiency of 4.88%showing more than double the short-circuit current dencompared to those made of TiO2 nanoparticles of DeguessaP-25 in a similar thin-film thickness region.569 Macak et al.found that, for Ru-dye (N3) sensitization of self-organi

Figure 64. (A) Structure and (B) principle of operation and energylevel scheme of the dye-sensitized nanocrystalline solar cell.Photoexcitation of the sensitizer (S) is followed by electron injectioninto the conduction band of an oxide semiconductor film. The dyemolecule is regenerated by the redox system, which itself is

regenerated at the counterelectrode by electrons passed through theload. Potentials are referred to the normal hydrogen electrode(NHE). The open circuit voltage of the solar cell corresponds tothe difference between the redox potential of the mediator and theFermi level of the nanocrystalline film indicated with a dashed line.The energy levels drawn for the sensitizer and the redox mediatormatch the redox potentials of the doubly deprotonated N3 sensitizerground state and the iodide/triiodide couple. Reprinted from Gratzel,M. J. Photochem. Photobiol. A: Chem . 2004 , 164 , 3, Copyright2004, with permission from Elsevier.




TiO2 nanotubes grown by Ti anodization, IPCEmax values(at 540 nm) of 3.3% and 1.6% (at 530 nm) were obtainedfor 2.5- µm- and 500-nm-long nanotubes, respectively.219

Ohsaki et al. found that the higher efficiency of solar cellswith TiO2 nanotube-based electrodes resulted from anincrease in electron density in nanotube electrodes comparedto 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 SnO2-coated glasssubstrate by anodic oxidization.201 After crystallization byoxygen annealing and treatment with TiCl4, the nanotubearrays were integrated into a DSC structure using a com-mercially available ruthenium-based dye N719. The cellgenerated a photocurrent of 7.87 mA/cm2 with a photocurrentefficiency of 2.9%, using a 360-nm-thick electrode underAM 1.5 illumination. They found that the highly orderedTiO2 nanotube arrays had superior electron lifetimes andprovided excellent pathways for electron percolation incomparison to nanoparticulate systems. Figure 66 shows thephotocurrent- photovoltage characteristics of the TiO2 nano-tube DSSC.201 They also found that backside illuminatedsolar cells based on 6- µm-long highly ordered nanotube-

array films sensitized by bis(tetrabutylammonium)-cis -(dithiocyanato)- N , N -bis(4-carboxylato-4-carboxylic acid-2,2-bipyridine)ruthenium(II) (commonly called “N719”) showeda power conversion efficiency of 4.24% under AM 1.5illumination.203

5.2.1.3. Inversed TiO 2 Opal. The relatively low efficiencyobtained in solid-state DSSCs is attributed to the poorpenetration of the material into pores of the thick TiO2 filmsand the consequent noncontact of the hole transport layerwith the titania electrode. A novel approach to increase theefficiency of solid-state Gratzel solar cells was presented bySomani et al., using large-surface titania inverse opal filmsas electrodes in fabricating solid-state dye-sensitized organic-

inorganic hybrid Gratzel solar cells.352 Direct comparison

indicated that light conversion efficiency increased by at l1 order of magnitude by the usage of the inversed opal T2films rather than nanocrystalline TiO2 films (Figure 67). Thebetter performance of inversed opal cells was due to the wand well-connected pores in mesoporous TiO2 films that

Figure 65. 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-layerfilm, 1.0- µm-thick; (2) nonorganized anatase treated by TiCl4, 0.95- µm-thick; (3) nonorganized anatase nontreated by TiCl4, 0.95- µm-thick. The inset shows the SEM image of Pluronic-templated three-

layer TiO2 films. Reprinted with permission from Zukalova, M.;Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gratzel, M. Nano Lett. 2005 , 5, 1789. Copyright 2005 American ChemicalSociety.

Figure 66. Photocurrent- photovoltage characteristics of a TiO2nanotube array DSSC under 100% AM-1.5 illumination. The shows an SEM image of TiO2 nanotubes. Reprinted with permissiofrom Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. Grimes, C. A. Nano Lett. 2006 , 6 , 215. Copyright 2006 AmericanChemical Society.

Figure 67. (A) Current- voltage ( I - V ) curves for an inversed opalcell obtained in the dark and under white light illumination uan 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 T2film. (B) Current- voltage ( I - V ) characteristic of a nanocrystallineTiO2 cell in the dark and under white light illumination usingAM 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, Palles, D.; Nozar, P.; Ruani, G. Sol. Energy Mater. Sol. Cells 2005 ,87 , 513, Copyright 2005, with permission from Elsevier.




allowed easy penetration of the hole transporting material,allowing good contact with the dye and hence the bestefficiency of the cell.

5.2.1.4. Hybrid TiO 2 Nanocrystalline Electrode. 5.2.1.4.1. Anatase - Rutile TiO 2 Nanocrystalline Electrode. Han et al.found that a hybrid TiO2 electrode composed of a mixtureof anatase and rutile phases showed a higher solar-to-electricenergy conversion efficiency than one made of pure ana-tase.852,853Figure 68 shows the performance of the photo-

electrochemical cells built with pure anatase (TiO2 II) andanatase- rutile (TiO2 I) nanocrystalline TiO2 electrodessensitized by N3. These electrodes had the same crystallinesizes and surface areas (26 nm, BET 57 m2 /g).852TiO2 I had71% anatase phase with 29% rutile phase, while TiO2 II hadpure anatase phase. The anataserutile-based DSSC showedhigher performance (efficiency η ) 6.8%, short-circuitphotocurrents density J sc ) 19.4 mA/cm2, open-circuitphotovoltage V OC ) 652 mV, fill factor ff ) 0.53) than thepure 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 geom-

etry: (a) the high-area cross section for recombination of photoinjected electrons with holes that are transferred to theelectrochemical mediator and (b) the image field opposingthe separation process that is distributed inside the TiO2nanoporous electrode. The conversion efficiency of a DSSCdecreases due to recombination losses of photoinjectedelectrons with oxidized dye molecules or a redox couple atthe surface of nanocrystalline TiO2. Various methods havebeen adopted to prevent this loss. Kang et al. added a bufferlayer of a TiO2- WO3 composite material between a TCOsubstrate (Figure 69) and a TiO2 layer and found that thebuffer layer effectively isolated dye molecules and electro-lytes from directly contacting the conducting substrate.854Inthe presence of the buffer layer having 15- 75 mol % WO3,

both open-circuit photovoltage and short-circuit photocurwere enhanced. In the case of the electrode having a bulayer of less than about 10 mol % WO3, due to the largenegative V FB, a potential barrier to the conduction banelectrons from TiO2 emerged at the TiO2- WO3 /TiO2 junc-tion. This resulted in a drop in photoinjection efficiency

subsequently in the photocurrent. For electrodes having mthan about 75 mol % WO3, the conduction band edge of thebuffer layer lay close to or lower than that of TCO, and relative conduction band energy of the buffer layer was particularly beneficial for the electron injection from conduction band of TiO2.854

5.2.1.4.3. Core - Shell Structured Nanocrystalline Elec-trode. Under the operating conditions of a DSSC, telectrons need to diffuse several micrometers into the T2layer surrounded by electron acceptors at a distance of oseveral nanometers. The nanoporous structure of the T2layer provides a large surface area, allowing absorptionenough dye molecules to achieve significant optical dsity.10,855However, the structure also enhances the recom

bination processes and decreases the total conversion eciency of DSSC.856- 858 The recombination processes arecompletely prohibited due to the lack of a significant elecfield that could assist the separation of electrons from hoin the TiO2 layer, since small TiO2 nanoparticles allow onlylimited band bending at the electrode surface.721,856,857,859

Core- shell TiO2 electrodes consisting of a nanoporous TiO2covered with a shell of another metal oxide have been shoto slow the recombination processes by the formation ofenergy barrier at the TiO2 surface.560,648,649,860- 865 The con-duction band potential of the shell should be more negatthan that of TiO2 in order to generate an energy barrier fothe reaction of the electrons present in TiO2 with the oxidizeddye or the redox mediator in solution. Two approaches

employed to fabricate the nanoporous core- shell electrodes.The first approach involves synthesis of core- shell nano-particles that are applied onto the conducting sustrate.560,648,649,863,866An energy barrier forms not only at thelectrode/electrolyte interface but also between the individTiO2 nanoparticles. The second approach involves a nanporous TiO2 electrode coated with the thin shell layer.670,860- 862,864,865,867The TiO2 nanoparticles are connecteddirectly to each other allowing electron transport throuTiO2.

The approach involving nanoporous electrodes in a wdefined core- shell configuration is usually a TiO2 corecoated with Al2O3,596,865,868- 870 MgO,871 SiO2,865 ZrO2,865 orNb2O5.860,670For example, Zaban et al. found that TiO2 /Nb2O5

Figure 68. Photocurrent density versus voltage for the photoelec-trochemical cells based on the pure anatase (TiO2 II) and anatase-rutile (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 nmand 6 µm, respectively. Conditions: electrolyte, 0.5 M LiI+ 0.04M I2 in propylene carbonate (PC); room temperature; light intensity,98 mW/cm2; AM1.5 spectral radiation. Inset: Performance param-eters 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 69. Schematic band diagrams of working electrodconsisting of a TiO2- WO3 buffer layer between TCO and a P25layer. From: Kang, T. S.; Moon, S. H.; Kim, K. J. J. Electrochem.Soc. 2002 , 149 , E155. Copyright 2002. Reproduced by permissiof The Electrochemical Society, Inc.




nanoporous electrodes could improve the performance of dye-sensitized solar cells by > 35%.670,860Figure 70 showsa bilayer nanoporous electrode which consists of a nanopo-rous TiO2 matrix covered with a thin layer of Nb2O5 andthe performance of three TiO2 electrodes coated with Nb2O5.For the best coating condition, the photocurrent increasedfrom 10.2 to 11.4 mA/cm2, the photovoltage from 661 to730 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 theshell material shifted the conduction band potential of thecore rather than forming an energy barrier. For example,

coating of TiO2 with a SrTiO3 shell resulted in a shift of theTiO2 conduction band in the negative direction.861,862Con-sequently, introduction of a SrTiO3-coated TiO2 electrodeto a DSSC increased the open circuit photovoltage whilereducing the short circuit photocurrent compared to that of the noncoated TiO2 electrode.861,862Diamant et al. found thatthe mechanism by which the shell affected the electrodeproperties depended on the coating material. Coating materi-als included Nb2O5, ZnO, SrTiO3, ZrO2, Al2O3, and SnO2.862

The coating Nb2O5 formed a surface energy barrier, whichslowed the recombination reactions, while the other shellmaterials each formed a surface dipole layer that shifted theconduction band potential of the core TiO2. The shiftdirection and magnitude depended on the dipole parameters

which were induced by the properties of the two materat the core/shell interface.862

Palomares et al. found that the conformal growth ofoverlayer of Al2O3 on a nanocrystalline TiO2 film resultedin a 4-fold retardation of interfacial charge recombinat

and a 30% improvement in photovoltaic device efficiency870

Fabregat-Santiago et al. found that the alumina barrreduced the recombination of photoinjected electrons to bthe dye cations and the oxidized redox couple, due to teffects: (a) almost complete passivation of surface trap stain TiO2 that were able to inject electrons to acceptor specand (b) slowing down by a factor of 3- 4 of the rate of interfacial charge transfer from conduction band states868

O’Regan found that the Al2O3 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 dippingmesoporous nanocrystalline TiO2 films in organic solutionsof their respective alkoxides, followed by sintering at 4°C.865The metal oxide overlayers acted as barrier layers

interfacial electron-transfer processes. The most basic ovlayer coating, Al2O3 (pzc) 9.2), was optimal for retardinginterfacial recombination losses under negative applied bwith an increase in open-circuit voltage of up to 50 mV a 35% improvement in overall device efficiency. Diamet al. found that SrTiO3-coated nanoporous TiO2 electrodesincreased the open circuit photovoltage while reducing short circuit photocurrent and resulting in a 15% improment of the overall conversion efficiency of the solar cel861

The SrTiO3 layer shifted the conduction band of the TiO2 inthe negative direction due to a surface dipole rather thforming an energy barrier at the TiO2 /electrolyte inter-face.861,862The shell having a more negative conduction bapotential acted as an energy barrier that slowed recombina

reactions. Photoexcitation of dye molecules anchoredultrathin (e 1 nm) outer shells of insulators or semiconductoon n-type semiconductor crystallites resulted in electrtransfer to the inner core material.

However, there is still considerable recombination thincreases with the distance between the electron injectpoint and the current collector. In other words, the limilifetime of the injected electron and the slow diffusion rinside the porous structure limit the effective thicknessthe nanoporous electrode. Chappel et al. proposed a electrdesign, shown in Figure 71, with a core shell configuratbased on a conductive ITO or Sb-doped SnO2 matrix coatedwith TiO2.872 In principle, the conducting core extended thcurrent collector into the nanoporous network and w

Figure 70. (A) Schematic view of the new bilayer nanoporouselectrode which consists of a nanoporous TiO2 matrix covered witha thin layer of Nb2O5. The Nb2O5 coating forms an inherent energybarrier at the electrode/electrolyte interface, which reduces therecombination 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 per-mission of The Royal Society of Chemistry. (B) I - V curves of four DSSCs differing by the nanoporous electrodes used to fabricatethem: the TiO2 reference electrode (a), and three bilayer electrodes(b- d). The Nb2O5 coating was made by a 30 s dipping of a 6 µmTiO2 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 2001American Chemical Society.

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




denoted the nanoporous “collector shell electrode”. Conse-quently, the distance between the injection spot and thecurrent collector should decrease to several nanometersthroughout the nanoporous electrode, in contrast to severalmicrometers with the standard electrode. All electronsinjected into the electrode, including those generated severalmicrometers away from the substrate, had to travel a veryshort 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-shellelectrode contained inherent screening capability due to thehigh doping level of the conducting matrix. Theoretically,the new design should enable efficient charge separation andcollection for thick nanoporous layers and solid electro-chemical mediators. They found that, unless the TiO2 coatingwas thicker than 6 nm, the electrode performance was verylow due to fast recombination.872

5.2.1.4.4. Electrode Coupled with Photonic Crystals.Development of photosensitizers with improved spectralresponse at the low-energy end of the solar spectrum hasnot proven so successful because dye molecules with highred 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 inorder to increase the absorbance in the red results in anincrease in the electron transport length and the recombina-tion rate, and a decrease in the photocurrent. An alternativeapproach to improving efficiency was to increase the pathlength of light by enhancing light scattering in the TiO2films.873- 878While the small size of TiO2 nanoparticles (10-

30 nm) employed to ensure a high surface area makesconventional nanocrystalline TiO2 films poor light scatterers,mixing the nanoparticles with larger particles or applying ascattering layer to the nanocrystalline film has been shownto increase light harvesting by enhancing the scattering of light.873- 878

Nishimura347 and Halaoui333 reported an enhancement inthe light conversion efficiency of dye-sensitized TiO2 solarcells by coupling a conventional nanocrystalline TiO2 filmto a TiO2 inverse opal, with a 26% increase in the IPCrelative to that of a nanocrystalline film of the same ovethickness in the 550- 800 nm spectral range. They foundthat the bilayer architecture, rather than enhanced liharvesting within the inverse opal structures, was respons

for the bulk of the gain in the IPCE.333

Figure 72 shows anSEM image of a cross section of the bilayer photoncrystal- nano-TiO2 photoelectrode.347

Figure 73 shows the sketch for the mechanism of tphotonic crystal in enhancing absorption in certain regime347

The fact that light waves were localized in different partthe structure, depending on their energy, implied that absorber in the high dielectric medium should interact mstrongly with light at wavelengths to the red of the stop baand less strongly to the blue. Effectively, the red part of spectrum of this absorber would “borrow” intensity fromblue part.

Figure 74A shows the effect of the TiO2 photonic crystalas compared to a film of nanocrystalline TiO2 on the

absorption spectra when dye is adsorbed to the surface347

In a comparison of the spectrum of dye molecules adsorto the TiO2 photonic crystal film with that of a conventionnanocrystalline TiO2 film, there was a substantial enhancement absorbance on the red side of the stop band, as wela slight attenuation of absorbance on the blue side of stop band. The enhanced absorbance was most pronounbetween 500 and 550 nm, but it persisted to a lesser degat longer wavelengths. Figure 74B shows the enhancemof the performance of a bilayer electrode compared tconventional nanocrystalline TiO2 photoelectrode.347Between400 and 530 nm, there was little difference between the kinds of electrodes. The close similarity in the maximphotocurrent from the two electrodes was consistent w

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




the fact that both contain the same amount of dye. Between540 and 750 nm, the short circuit photocurrent was substan-tially increased in the bilayer electrode. The overall gain,integrated over the visible spectrum (400- 750 nm), wasabout 30%. Localization of heavy photons at the edges of the photonic stop band347,879,880from Bragg diffraction in theperiodic lattice and multiple scattering events at disorderedregions in the photonic crystal or at disordered films ledultimately to enhanced backscattering.333 This largely ac-

counted for the enhanced light conversion efficiency in thered spectral range (600- 750 nm), where the sensitizer wasa poor absorber.333

5.2.2. Metal/Semiconductor Junction Schottky Diode Solar Cell

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

steps. First, light absorption occurred in the surface-absor

photoreceptors, giving rise to energetic electrons. Secoelectrons from the photoreceptor excited state were injecinto the conduction levels of the adjacent conductor, whthey travelled ballistically through the metal at an ener1e, above the Fermi energy, E f . Third, provided that 1e wasgreater than the Schottky barrier height, f , and the carriermean-free path was long compared to the metal thicknthe electrons traversed the metal and entered the conductlevels of the semiconductor (internal electron emission). Tabsorbed photon energy was preserved in the remainexcess electron free energy when it was collected at the bohmic contact, giving rise to the photovoltage, V . Thephotooxidized dye was reduced by transfer of thermalielectrons from states near E f in the adjacent metal. Devices

fabricated by using a fluorescein photoreceptor on an ATiO2 /Ti multilayer structure had typical open-circuit phovoltages of 600- 800 mV and short-circuit photocurrents o10- 18 mA cm- 2 under 100 mW cm- 2 visible light illumina-tion: the internal quantum efficiency (electrons measuper photon absorbed) was 10%. This alternative approto photovoltaic energy conversion might provide the bafor durable low-cost solar cells using a variety of materi

5.2.3. Doped TiO 2 Nanomaterials-Based Solar Cell Lindgren et al. found that N-doped TiO2 nanocrystalline

porous thin films showed visible light absorption in wavelength range from 400 to 535 nm and generatedincident photon-to-current efficiency response in good ag

Figure 73. (A) Simplified optical band structure of a photoniccrystal. Near the Brillouin zone center, light travels with velocityc0 / n, where c0 is the speed of light in a vacuum and n is the averagerefractive index. At photon energies approaching a full band gapor a stop band from the red side, the group velocity of lightdecreases and light can be increasingly described as a sinusoidalstanding wave that has its highest amplitude in the high-refractive-index part of the structure. At energies above the band gap or stopband, the standing wave is predominantly localized in the low indexpart of the photonic crystal, i.e., in the air voids. (B) Illustration of the effect of standing wave localization on dye absorbance. In anisotropic medium, the dye absorbs strongly in the blue but weaklyin the red (heavy line). If the stop band is tuned to the positionshown by the arrow, the blue absorbance is diminished and the redabsorbance is increased when the dye is confined to the high-

refractive-index part of the photonic crystal (dotted line). Reprintedwith 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 2003American Chemical Society.

Figure 74. (A) Absorption spectra of the TiO2 photonic crystal(a), the N719 dye adsorbed on the photonic crystal (b), anddye adsorbed on a film of nanocrystalline TiO2 (c). The positionof the stop band at 486 nm is indicated by the arrow. Wavelength dependence of the short-circuit photocurrent inbilayer electrode (a) and the conventional nanocrystalline T2photoelectrode (b). The position of the stop band maximum inbilayer electrode was 610 nm. Reprinted with permission fNishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; MallT. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am.Chem. Soc. 2003 , 125 , 6306. Copyright 2003 American ChemicSociety.




ment with the optical spectra.385For the best nitrogen-dopedTiO2 electrodes, the photoinduced current due to visible lightand at moderate bias increased around 200 times comparedto the behavior of pure TiO2 electrodes.

5.3. Photocatalytic Water Splitting

5.3.1. Fundamentals of Photocatalytic Water Splitting

An enormous research effort has been dedicated to thestudy of the properties and applications of TiO2 under lightillumination since the discovery of photocatalytic splittingof water on a TiO2 electrode in 1972 (Fujishima andHonda).6- 8 Photocatalytic splitting of water into H2 and O2using TiO2 nanomaterials continues to be a dream for cleanand sustainable energy sources.882

Figure 76 shows the principle of water splitting using aTiO2 photocatalyst.761When TiO2 absorbs light with energylarger than the band gap, electrons and holes are generatedin the conduction and valence bands, respectively. Thephotogenerated electrons and holes cause redox reactions.Water molecules are reduced by the electrons to form H2

and oxidized by the holes to form O2, leading to overall watersplitting.883- 885The width of the band gap and the potentiaof the conduction and valence bands are important. Tbottom level of the conduction band has to be more negathan the reduction potential of H+ /H2 (0 V vs NHE), whilethe top level of the valence band has to be more positthan the oxidation potential of O2 /H2O (1.23 V). The potentialof the band structure of TiO2 is just the thermodynamicalrequirement. Other factors such as charge separation, mo

ity, and lifetime of photogenerated electrons and holes aaffect the photocatalytic properties of TiO2. These factorsare strongly affected by the bulk properties of the matesuch as crystallinity. Surface properties such as surface stasurface chemical groups, surface area, and active reactsites are also important.768 The water-splitting process inreturn affects the local pH environment and surface structuof the TiO2 electrode.769

Salvador conducted a thermodynamic and kinetic conseration of water-splitting and competitive reactions in photoelectrochemical cell, and they found that the overvage for evolution of O must be minimized, which wasthe order of 0.6 eV for n-TiO2 electrodes loaded withRuO2.767 Cocatalysts such as Pt and NiO are often loadon the surface in order to introduce active sites for 2evolution. Thus, suitable bulk and surface properties aenergy structure are demanded for photocatalysts.

Laser-induced photocatalytic oxidation/splitting of waover TiO2 catalysts was studied.883,886,887Sayama and Ara-kawa found that addition of carbonate salts to Pt-loaded T2suspensions led to highly efficient water splitting.888 Thecarbonate ions affected both the Pt particles and the Ti2surface. The Pt was covered with some titanium hydroxcompounds and the rate of the back reaction on the Pt wsuppressed effectively in the presence of carbonate ions. carbonate species aided desorption of O2 from the TiO2surface.888 Khan and Akikusa found that bare n-TiO2nanocrystalline film electrodes were unstable during wasplitting reactions under illumination of light and thstability could be significant improved when covered wMn2O3.759

5.3.2. Use of Reversible Redox Mediators It has been reported that pure TiO2 could not easily split

water into H2 and O2 in the simple aqueous suspensionsystem.413,754,889The main problem is the fast, undesireelectron- hole recombination reaction.762 Therefore, it isimportant to prevent the electron- hole recombination pro-cess. The Pt- TiO2 system could be illustrated as a “shortcircuited” photoelectrochemical cell, where a TiO2 semi-conductor electrode and a platinum- metal counterelectrode

are brought into contact. Well-dispersed metal particles as miniphotocathodes, trapping electrons, which reduwater to hydrogen.

The role of sacrificial reagents is shown in Figure 77761

When the photocatalytic reaction is carried out in aquesolutions including easily oxidizable reducing reagenphotogenerated holes irreversibly oxidize the reducireagents instead of water. This makes the photocatalelectron-rich, and a H2 evolution reaction is enhanced asshown in Figure 77a. On the other hand, in the presenceelectron acceptors such as Ag+ and Fe3+ , the photogeneratedelectrons in the conduction band are consumed by them an O2 evolution reaction is enhanced as shown in Figu77b. These reactions using sacrificial reagents are regar

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

Figure 76. 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 andBusiness Media.




as half reactions and are often employed for test reactionsof photocatalytic H2 or O2 evolution. However, one shouldrealize that the results do not guarantee a photocatalyst tobe active for overall water splitting into H2 and O2 in theabsence of sacrificial reagents.

A sacrificial reagent helps to control the electron- holerecombination process. The photoefficiency of the processcan be improved by the addition of sacrificial reagents.754,889,890

The sacrificial reagents help separation of the photoexcitedelectrons 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 watersplitting under sunlight.753- 755 They designed a new photo-catalytic reaction that split water into H2 and O2 by a two-step photoexcitation system composed of an IO3

- /I- shuttleredox mediator and two different TiO2 photocatalysts: Pt-loaded TiO2-anatase for H2 evolution and TiO2-rutile for O2evolution (Figure 78).753 Simultaneous gas evolution of H2(180 mmol/h) and O2 (90 mmol/h) was observed from a basic(pH ) 11) NaI aqueous suspension of two different TiO2photocatalysts under UV radiation. The overall water splittingproceeded by the redox cycle between IO3

- and I- underbasic conditions as follows: (a) water reduction to H2 and

I- oxidation to IO3- over Pt- TiO2-anatase, and (b) IO3-

reduction to I- and water oxidation to O2 over TiO2-rutile.IO3

- reduction to I- over Pt- TiO2-anatase is an undesirablereaction. If this reaction is suppressed, the total water-splitreaction will take place more efficiently. The advantagethis system is that H2 gas is evolved over the Pt- TiO2-anatase photocatalyst only and that O2 gas is evolved overthe TiO2-rutile photocatalyst only, even from a mixture IO3

- and I- in a basic aqueous solution. Therefore, anoth

undesirable backward reaction, H2O formation from H2 andO2 on Pt particles, was suppressed.753 They found thataddition of a small amount of iodide anion, I- , into theaqueous suspension of Pt- TiO2-anatase photocatalyst sig-nificantly improved the splitting into H2 and O2 with astoichiometric ratio. The iodide anion was adsorbed preentially onto the Pt cocatalyst as iodine atom. This iodlayer effectively suppressed the backward reaction of waformation from H2 and O2 to H2O over the Pt surface.754

Fujihara et al. studied the photochemical splitting of waby combining the reduction of water to hydrogen usibromide ions and the oxidation of water to oxygen usFeIII ions.892The bromide ions were oxidized to bromine oPt-loaded TiO2 nanoparticles, and the FeIII ions were reducedto FeII ions on TiO2 nanoparticles. These two reactions wercarried out in separated compartments and combined platinum electrodes and cation-exchange membranes shown in Figure 79. At the electrodes, FeII ions were oxidizedby bromine, and protons were transported through membranes to maintain the electrical neutrality and pHthe solutions in the two compartments. As a result, wawas continuously split into hydrogen and oxygen unradiation. The reversible reactions on photocatalysts whoften suffered from the effects of back reactions were largprevented due to the low concentration of the productssolution.

Lee et al. found that a considerable amount of photocalytic H2 was produced from water over NiO/TiO2 inproportion to the hole scavenger CN- .890 Galinska andWalendziewski studied water splitting over a Pt- TiO2catalyst with various sacrificial reagents, such as methanNa2S, EDTA, and I- and IO3

- ions, and they found that thesacrificial reagents had a key role in hydrogen productvia the photocatalyzed water-splitting reaction.757Photocata-lytic water splitting was obtained when EDTA and Na2S wereused. They acted as effective hole scavengers, preventoxygen formation and the recombination reaction of oxywith hydrogen.

5.3.3. Use of TiO 2 Nanotubes

Mor et al. found that highly ordered TiO2 nanotube arrays

efficiently decomposed water under UV radiation.198

Theauthors found that the nanotube wall thickness was a kparameter influencing the magnitude of the photoanoresponse and the overall efficiency of the water-splittireaction. For TiO2 nanotubes with 22-nm pore diameter an34-nm wall thickness (Figure 80A), upon 320- 400 nmillumination at an intensity of 100 mW/cm2, hydrogen gaswas generated at the power- time normalized rate of 960mmol/h W (24 mL/h W) at an overall conversion efficieof 6.8% as shown in Figure 80B.198,199They also claimedthat, for illumination at 320- 400 nm (98 mW/cm2), the TiO2nanotube-array photoanodes could generate H2 by H2Ophotoelectrolysis with a photoconversion efficiency 12.25%.212 Park et al. further found that, when doped wi

Figure 77. Photocatalytic H2 (a) or O2 (b) evolution in the presenceof 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 78. Proposed reaction mechanism for overall photocatalyticwater splitting using a IO3

- /I- redox mediator and a mixture of Pt- TiO2-antase and TiO2-rutile photocatalysts. Reprinted withpermission from Abe, R.; Sayama, K.; Domen, K.; Arakawa, H.Chem. Phys. Lett. 2001 , 344 , 339. Copyright 2001 Elsevier.




carbon, TiO2- xC x nanotube arrays showed more efficientwater splitting under UV and visible light illumination (> 420nm) than pure TiO2 nanotube arrays.475

5.3.4. Water Splitting under Visible Light 5.3.4.1. Water Splitting over Doped TiO 2 Nanomate-

rials. In general, the conduction bands of stable oxidesemiconductor photocatalysts consisting of metal cations witha d0 and d10 configuration consist of empty orbitals (LUMO)of the metal cations. On the other hand, the valence bandsconsist of O2p orbitals. The potential of this valence band(about + 3 eV) is considerably more positive than theoxidation potential of H2O to O2 ( E 0 ) 1.23 V). Therefore,the band gaps of oxide semiconductor photocatalysts withthe potential for H2 evolution inevitably become wide.Accordingly, a valence band or an electron donor levelconsisting of orbitals of some element, except for O2p, hasto be formed to make the band gaps or the energy gaps

narrow. New photocatalysts having the band structure shoin Figure 81 are necessary in order to develop materialssplitting water into H2 and O2 under visible light.761 Thecreated levels have to possess not only the thermodynampotential for oxidation of H2O but also the catalytic propertiesfor the four-electron oxidation reaction. The followstrategies can be considered for the development of visilight-driven photocatalysts: (i) forming a donor level aba valence band by doping some element into conventiophotocatalysts with wide band gaps such as TiO2; (ii) creating

Figure 79. (A) Schematic of the photocatalytic reaction cell forsplitting water. (B) Energy diagram of splitting of water bycombined 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 RoyalSociety of Chemistry.

Figure 80. (A) SEM images, top view, of 20 V TiO2 nanotubearrays anodized at 5 °C. (B) Photoconversion efficiency as function of measured potential [vs Ag/AgCl] for 10 V samanodized at four temperatures [i.e., 5, 25, 35, and 50 °C]. Reprintedwith permission from Mor, G. K.; Shankar, K.; Paulose, Varghese, O. K.; Grimes, C. A. Nano Lett. 2005 , 5, 191. Copyright2005 American Chemical Society.

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




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 beinduced with visible light in colloidal solutions of Cr-dopedTiO2 nanoparticles deposited with ultrafine Pt or RuO2.431

A pronounced synergistic effect in catalytic activity wasnoted when both RuO2 and Pt were co-deposited onto theparticle. Jin and Lu found that Pt/B-doped TiO2 was a goodsystem for water splitting under a B4O72- environment

without sacrificial electron donor reagents.893

Luo et al. foundthat Br- - and Cl- -co-doped nanocrystalline TiO2 with theabsorption edge shifted to a lower energy region displayedhigher efficiency for water splitting than pure TiO2.508 Jinget al. found that a Ni-doped mesoporous TiO2 photocatalystwith 0.2 wt % Pt accomplished hydrogen evolution at nearly125.6 lmol/h compared to 81.2 lmol/h for TiO2 P25.894 N-,B-doped TiO2 nanomaterials have displayed higher activitythan pure TiO2 in water splitting, i.e., under visible lightillumination.529,889Khan et al. found that a C-doped TiO2nanocrystalline film with visible light response obtained bycontrolled combustion of Ti metal in a natural gas flamehad a high water-splitting performance with a total conver-sion efficiency of 11% and a maximum photoconversionefficiency of 8.35% when illuminated at 40 mW/cm2,476although there were questions about its solar-to-hydrogenconversion efficiency by other researchers.895- 897

Matsuoka et al. developed visible light responsive TiO2nanocrystalline thin films by the radio frequency magnetronsputtering method, which decomposed water when Pt-loadedand in the presence of a sacrificial reagent such as methanolor silver nitrate under visible light.763,764

5.3.4.2. Water Splitting over Dye-Sensitized TiO 2.Duonghong et al. found that TiO2 loaded simultaneously withultrafine Pt and RuO2 displayed extremely high activity asan H2O decomposition catalyst under band gap excitationof the TiO2 and that, when Ru(bipy)32+ or rhodamine B wasused as a sensitizer, H2O was decomposed under visiblelight.898

Abe et al. investigated H2 production over merocyanineor coumarin dye C343 or Ru complex dye N3 dye-sensitizedPt/TiO2 photocatalysts under visible light in a water-

acetonitrile solution containing iodide as an electron donor.756

They found that the rates of H2 evolution decreased withincreasing proportion of water in the solutions because of adecrease in the energy gap between the redox potential of I3

- /I- and the HOMO levels of the dyes, which decreasesthe efficiency of electron transfer from I- to dye. The energydiagram and the mechanism for the H2 production from waterover the dye-sensitized Pt/TiO2 photocatalyst system areshown in Figure 82. The two key electron-transfer steps,electron injection from an excited state of the dye to the TiO2conduction band and oxidation of I- to I3- (steps 2 and 5),occurred efficiently in acetonitrile solvent. The increased ratioof water hindered electron transfer from I- to the HOMOlevel of the oxidized dye (step 5).

In addition, Park and Bard designed two different kindsof cells with bipolar dye-sensitized TiO2 /Pt panels connectedso that their photovoltages added to provide vectorial electrontransfer for unassisted water splitting to yield the separatedproducts 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

combination of single-crystal p-SiC and nanocrystalln-TiO2 photoelectrodes.899Both photoelectrodes (p-SiC andn-TiO2) were placed side by side facing the light source ain contact with an electrolyte of 0.5 M H2SO4. The open

circuit potential was found to be 1.24 V between the n-T2and p-SiC photoelectrodes, with a maximum photocurrdensity of 0.05 mA cm- 2 under a closed circuit potential of0.23 V, corresponding to an efficiency of 0.06%. The lcell photocurrent density and the photoconversion efficiefor the p-SiC/n-TiO2 self-driven system for the water-splittinreaction were due to the high band gap energies of bsemiconductors and high recombination of photogeneracarriers mainly in the covalently bonded p-SiC.

Takabayashi et al. proposed a solar water-splitting systbased on a composite polycrystalline-Si/doped TiO2 thin-film electrode for high-efficiency and low-cost by combinthe advantages of Si and doped TiO2: (1) an n-Si electrodewith surface alkylation and a metal nanodot coating gan efficient and stable photovoltaic characteristic, and TiO2 doped with other elements, such as nitrogen and sulcould cause water photooxidation (oxygen photoevolutiunder visible light illumination.770The structure and workingmechanism of solar water splitting with this system is shoin Figure 83. Although a high solar-to-chemical conversefficiency of more than 10% was calculated for this systseveral major problems needed to be solved before the rdevice could show promising performance.770

5.4. Electrochromic DevicesTiO2 nanomaterials have been widely explored as ele

trochromic devices, such as electrochromic windows a

displays.611,634,772,900- 916

Electrochromism can be defined asthe ability of a material to undergo color change upoxidation or reduction. Electrochromic devices are ablevary their throughput of visible light and solar radiation uelectrical charging and discharging using a low voltagesmall voltage applied to the windows will cause themdarken; reversing the voltage causes them to lighten. Thone can regulate the amount of energy entering throug“smart window” so that the need for air conditioning icooled building decreases. The energy efficiency inherin this technology can be large, provided that the contstrategy is adequate. Additionally, the transmittance regution can impart glare control as well as user control of indoor environment. The absorbance, rather than the ref

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




tance, is modulated so that the electrochromic devices tendto heat up in their low-transparent state.752

Two types of electrochromism of nanocrystalline thin filmTiO2 electrodes have been reported. The first type is theelectrochromism of nanocrystalline TiO2 electrodes in Li-containing electrolytes related to the reversible insertion of Li+ into the anatase lattice of the nanoparticles.912 Hagfeldtet al. found that forward biasing of transparent nanocrystal-

line TiO2 films in lithium ion-containing organic electrolytesled to rapid and reversible coloration due to electronaccumulation and Li+ intercalation in the anatase lattice.912

Absorption of > 90% light throughout the visible and nearIR could be switched on and off within a few seconds. Thenanocrystalline morphology of the film played a role inenhancing the electrochromic process. Ottaviani et al. foundthat the rate of the electrochromic process was controlledby the diffusion of the Li+ ions throughout the TiO2 lattice.914

It was convenient to drive the electrochromic process withpotentiostatic pulses, and under these conditions, many cycleswith initially good color contrast and efficiencies whichapproached 100% were obtained with TiO2 thin filmelectrodes.

The second type is the electrochromism of nanocrytalline TiO2 electrodes modified with viologens and/oanthrachinons equipped with a surface anchorigroup.902- 904,906- 908,910,917- 919 This category usually has fastswitching times and considerable optical dynamics, duethe combination of good conductivity between the Ti2nanoparticles and the fast electron exchange between T2and the monolayer of the electrochromic compound coveeach particle.904Bach et al. demonstrated high-quality pape

like electrochromic displays based on nanostructured T2films modified with electrochromophores with excellent ion-paper optical qualities, fast response times, and low poconsumption.901 Moeller et al. demonstrated electrochrompictures with unprecedented resolution (360 dpi) in transpent and reflective electrochromic displays (ECD) basedink-jet printing technology and cascade-type crosslinkreactions of viologens in the mesopores of a TiO2 electrode,with a completely transparent counterelectrode based mesoporous antimony tin oxide coated with CeO2.913

5.4.1. Fundamentals of Electrochromic Devices Figure 84A shows the principle of the electrochromi

of a molecular monolayer adsorbed on TiO2.902A molecule,which functions as the electrochromophore and exhibdifferent colors in different oxidation states, must be chosuch that its redox potential lies above the conduction bedge of the TiO2 nanocrystalline electrode at the liquid/solinterface. In this way, electrons can be transferred reversfrom the conduction band to the molecule. The Ti2electrode in fact behaves like a conductor for the adsorbelectroactive species. If the redox potential is situated bethe conduction band edge, the reduction process is irrevible. Figure 84B shows the TiO2 nanocrystalline electro-chromic devices based on viologen (solvent: glutarodinitrwith a counterelectrode made of Prussian blue.902The devicecould be switched back and forth between the colorless the colored states within 1 s.

The nanocrystalline structure of the TiO2 film makespossible 100- to 1000-fold amplification compared to a surface as shown in Figure 85.902 The combination of highconductivity of the nanocrystalline TiO2 particles, fastelectron exchange with the molecular monolayer, optiamplification by the porous structure, and fast charcompensation by ions in the contacting liquid makes nanocrystalline electrodes highly attractive electrochromelements. The principle of efficiency relies on fast interfaelectron transfer between the nanocrystalline TiO2 and theadsorbed modifier as well as on the high surface area ofTiO2 support that amplifies optical phenomena by 2 oorders of magnitude.902The investigated TiO2 nanocrystallineelectrodes include ordered905 and disordered902- 904 mesopo-rous films. Ordered mesoporous nanocrystalline TiO2 elec-trodes were found to display enhanced color contrast yet hsimilar conduction band edge energy levels and electrpercolation ability as electrodes made from nanocrystallTiO2, attributed to the uniform and ordered mesopoarchitecture and the large accessible surface area for tetheviologen 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, its remarkable stability in both the oxidized and the redu(radical cation) states (Figure 86).902,904,906,907Oxidized vi-

Figure 83. (A) Schematic illustration and (B) the operationprinciple of solar water splitting with a composite polycrystalline-Si/doped TiO2 semiconductor electrode. Reprinted from Takaba-yashi, S.; Nakamura, R.; Nakato, Y. J. Photochem. Photobiol., A:Chem. 2004 , 166 , 107, Copyright 2004, with permission fromElsevier.




ologen is colorless, while the radical cation can be blue,violet, purple, or green, depending on the substituents. Theassociated first reduction potential is between 0.2 and- 0.6V (vs NHE). The typical absorption spectrum of reduced N , N ′-dialkylviologen in an organic solvent has a maximumaround 600 nm. With N , N ′-diarylviologens, the absorptionband is shifted by about 50 nm to the red. In concentratedsolution or in the solid state, viologen radical cations formdimers, with their blue-shifted absorption maximum in the550 nm region. A second reduced state can be reached atpotentials which are more negative by 0.2- 0.4 V. This stateis neutral and almost colorless (yellowish). This secondreduction is reversible in organic solvents like acetonitrilebut not in water. The anchoring groups with strong affinitytoward TiIV include carboxylates, salicylates, or phospho-nates.902

Bonhote et al. examined phosphonated triarylamine as anelectrochromophore due to its oxidation by the stable

triarylamminum radical cation, which is accompanied bblue coloration with the absorption band at 730 nm903

Vayssieres et al. studied bis(phthalocyaninato)lutetium(Icomplexes (Pc2Lu) as electrochromophores, and they foun

that the typical neutral green state of Pc2Lu was reduced toa brown state at potentials< - 0.3 V vs Ag/AgCl at neutralpH when Pc2Lu was adsorbed onto a nanostructured TiO2electrode.916

Ag- TiO2 films, prepared by loading nanoporous filmwith Ag nanoparticles by photocatalytic means, exhibimulticolor photochromism, which was related to the oxition and reduction of Ag nanoparticles under UV and visradiation.773 Please also see section 4.2.1.2 on Sensitizatioby Metal Nanoparticles.

5.4.3. Counterelectrode for an Electrochromic Device Closed cells are built by combining a transparent nan

crystalline electrode with a counterelectrode able to prov

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




enough electrons to allow complete reduction of viologen.The simplest counterelectrode is conducting glass.

Prussian blue (PB) is an inorganic polymeric material (ironphexacyanoferrate) that is commonly used on a conductingglass substrate as a counterelectrode. Being blue in theoxidized state and colorless in the reduced state, it is asuitable complementary electrochromic material to thenanocrystalline viologen electrode. When the latter electrodeturns blue by reduction, the PB counterelectrode turns blueby oxidation.902,904 Bonhote et al. studied nanocrystalline

WO3 films as counterelectrodes for electrochromic applica-tions since they turn from colorless to blue by reduction andlithium ion insertion.903

Fitzmaurice et al. constructed an electrochromic windowbased on a modified transparent nanostructured TiO2 filmsupported on conducting glass and modified with theelectrochromophore bis(2-phosphonoethyl)-4,4′-bipyridiniumdichloride, the electrolyte LiClO4, and ferrocene in γ-butyro-lactone.906They used a counterelectrode of conducting glass,which had excellent electrochromic performance with acoloration efficiency of 170 cm2 C- 1 at 608 nm, a switchingtime of 1 s, and stability over 10,000 steady test cycles. Theyupgraded this system with a counterelectrode based on atransparent nanostructured SnO2 film supported on conduct-

ing glass and modified by the electrochromophore [ β-(10-phenothiazyl)propoxy]phosphonic acid, which displaycycles-switching times of < 250 ms, a coloration efficiencyof 270 cm2 C- 1, and steady-state currents of < 6 mA cm- 2.908

Zinc can be used as a counterelectrode instead of PB displays applications.902 When the two electrodes are shortcircuited, the electrons flow from the zinc, which oxidito Zn2+ ions, to the viologens of the nanocrystalline electroThe process can be reversed under a potential of 1- 2 V.

5.4.4. Photoelectrochromic Devices Pichot et al. demonstrated a photoelectrochromic sm

window with flexible substrates and solid-state electrolybased on a dye-sensitized TiO2 electrode spin-coated ontoIn- Sn oxide-coated polyester substrates coupled with a W3electrochronic counterelectrode, separated by a cross-linpolymer electrolyte containing LiI (Figure 87).634The devicestypically transmitted 75% of visible light in the bleachstate. After a few minutes of exposure to white light, windows turned dark blue, transmitting only 30% of vislight. They spontaneously bleached back to their initnoncolored state upon removal of the light source. Tphotoelectrochromic device ideally behaved like a capator: There was initially no mobile oxidized species (i.e.2)present in the electrolyte. A schematic representation of components and the electron and ion transfers in the sostate photoelectrochromic device is shown in Figure 87. ultimate electron acceptor (WO3) is localized as an insolublematerial on the back electrode. Only the electron donor (- ),which serves as a regenerator to the oxidized dye, is initipresent in the electrolyte introduced as LiI. Upon coloraof the device at short-circuiting under illumination, I2 isgenerated at the TiO2 electrode and Li+ intercalates in WO3.

5.5. Hydrogen StorageLim et al. found that TiO2 nanotubes could reproducibly

store up to approximately 2 wt % H2 at room temperature

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

Figure 86. Electrochromism mechanism of a viologen chro-mophore. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.;Walder, L.; Gratzel, M. Displays 1999 , 20 , 137, Copyright 1999,with permission from Elsevier.




and 6 MPa.165 About 75% of this stored hydrogen could bereleased when the hydrogen pressure was lowered to ambientconditions due to physisorption. Approximately 13% wasweakly chemisorbed and could be released at 70 °C as H2,and approximately 12% was strongly bonded to oxide ionsand released only at temperatures above 120 °C as H2O. TheP- C isotherms of TiO2 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 lowerhydrogen concentration of ∼0.8 wt % for bulk TiO2. Whenthe pressure was reduced, only∼75% of the stored hydrogencould be released, whereas 25% of adsorbed hydrogenmolecules were retained due to chemical adsorption.

Bavykin et al. studied the sorption of hydrogen betweenthe layers of the multilayered wall of nanotubular TiO2 inthe temperature range of - 195 to 200 °C and at pressuresof 0 to 6 bar.157 Hydrogen could intercalate between layersin the walls of TiO2 nanotubes forming host- guest com-pounds TiO2‚ xH2, where x e 1.5 and decreases at highertemperature. The rate of hydrogen uptake increased withtemperature, and the characteristic time for hydrogen sorptionin TiO2 nanotubes was several hours at 100 °C. The hydrogen

adsorption isotherm for TiO2 nanotubes at- 195 °C is shownin Figure 89. Almost 1.5 hydrogen molecules per one Tiatom could be adsorbed at a hydrogen partial pressure of 2bar. During the desorption of hydrogen, a large hysteresiswas observed; even at 0 bar of pressure, the uptake of hydrogen achieved a 1.25 molar ratio (point B). Theadsorption of hydrogen was a reversible process. Heatingthe sample in a vacuum to 200 °C led to a completedesorption of hydrogen, returning the weight of the sampleto its initial value (point A). The author found that thediffusion of hydrogen molecules in the axial directionbetween the layers in multilayered walls of TiO2 nanotubeswas the rate-limiting step of the process of intercalation andthe rate of hydrogen intercalation depended on the inverse

of the square of nanotube length from their proposdiffusion model.

Recently, Xu et al. studied the hydrogen storage properof a series of five pristine micro- and mesoporous Ti oxmaterials, synthesized from C6, C8, C10, C12, and Camine templates possessing BET surface areas ranging fr643 to 1063 m2 /g, and they found that at 77 K the isothermfor all materials gently rose sharply at low pressure acontinued to rise in a linear fashion from 10 atm onward65 atm and then return on desorption without significhysteresis. Extrapolation to 100 atm could yield total stor

Figure 87. Schematic representation of the components and theelectron and ion transfers in a solid-state photoelectrochromicdevice. Upon illumination, electrons are injected into the TiO2conduction band (CB), travel through the external circuit, and reducethe WO3 counterelectrode. Upon lithium ion intercalation into thereduced WO3 film, a “tungsten bronze” is formed that absorbsvisible and near-IR radiation, presumably via an intervalencecharge-transfer absorption. TCO stands for transparent conductingoxide, 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 permissionof The Electrochemical Society, Inc.

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

Figure 89. 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.; PlucinskK.; Friedrich, J. M.; Walsh, F. C. J. Phys. Chem. B 2005 , 109 ,19422. Copyright 2005 American Chemical Society.




values as high as 5.36 wt % and 29.37 kg/m3, and surface

Ti reduction by the appropriate organometallic reagentprovided an increase in performance, possibly because of aKubas-type interaction.920

5.6. Sensing ApplicationsTiO2 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 TiO2nanotubes.194,196,206- 209,211They found that TiO2 nanotubeswere excellent room-temperature hydrogen sensors not onlywith a high sensitivity of 104 but also with an ability to self-clean photoactively after environmental contamination.196At

24 °C, in response to 1000 ppm of hydrogen, the sensorsshowed a fully reversible change in electrical resistance of approximately 175,000%. The hydrogen-sensing capabilitiesof the sensors were largely recovered by ultraviolet (UV)light exposure after being completely extinguished by a ratherextreme means of sensor contamination: immersion of thesensor in motor oil. Figure 90 shows a plot of real-timevariation of resistance before, during, and after cleaning thecontaminant, motor oil 10W-30, with UV exposure.

Many types of TiO2 nanomaterial-based room-temperaturehydrogen sensors are based on Schottky barrier modulationof devices like Pd/TiO2 or Pt/TiO2.922,947,954Elevated tem-perature hydrogen sensors examine the electrical resistancechange with hydrogen concentration. Birkefeld et al. found

that the resistance of anatase TiO2 varied in the presence of

CO and H2 at temperatures above 500 °C, but on dopingwith 10% alumina it became selective for hydrogen922

Shimizu et al. reported that anodized nanoporous titania fiwith a Pd Schottky barrier were sensitive to hydrogen at °C.951,952 Kobayashi et al. investigated the mechanism hydrogen sensing by Pd/TiO2 Schottky diodes, and theyfound that the formation of adsorbed water from adsorboxygen at the Pd/TiO2 interface was the dominant reactionfor the Pd/TiO2(001) diodes throughout the hydrogen concentration range of 0- 3000 ppm; for the Pd/TiO2(100)diodes, this reaction was dominant only for hydrogconcentrations below 100 ppm and the hydrogen adsorpon bare Pd atoms became dominant for higher hydrogconcentrations.939,940Carney et al. found that sensors baseon SnO2- TiO2 with higher surface areas were more sensitivto H2 in the presence of O2 by measuring the change in theelectrical resistance of the sensor upon exposure to diffehydrogen concentrations under a constant hydrogen gas frate.923 Devi et al. found that ordered mesoporous TiO2exhibited higher H2 and CO sensitivities than sensors madfrom common TiO2 powders due to increased surface areaand the sensitivity could be further improved by loadingsensor with 0.5 mol % Nb2O5.929 Gao et al. found thatnanoscale TiO2 displayed higher performance in H2 sensingthan microscale TiO2 due to larger surface area.932

Oxygen sensors based on TiO2 nanomaterials includeTiO2- x,961 TiO2- Nb2O5,928 CeO2- TiO2,957 and Ta-,935

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




Nb-,949 Cr-,949 and Pt-doped TiO2.950 Pt-doped TiO2 sensorsshowed improved gas sensitivity, low operation temperature(350- 800 °C), and short response time (< 0.1 s).941,950,958Theoxygen-sensing mechanism was the combination of Pt/TiO2interfaces in a Schottky-barrier mechanism and an oxygen-vacancy bulk effect mechanism.959 At high temperatures,

TiO2 devices can be used as thermodynamically controlledbulk defect sensors to detect oxygen over a large range of partial pressures; at low temperatures, Pt/TiO2 Schottkydiodes make extremely sensitive oxygen detection possible.938

In Ta-doped TiO2 sensors, oxygen vacancies formed byphotoirradiation acted as oxygen-sensing sites.935 Sotter etal. found Nb-doped TiO2 nanomaterials to be good sensormaterials for O2.953 Nb- and Cr-doped TiO2 nanocrystallinefilms displayed higher O2 sensitivity than pure TiO2 films949

in that Nb5+ -doped TiO2 showed 65 times enhancement inthe sensitivity compared to undoped material at a loweroperating temperature.474

TiO2 nanomaterials are promising candidates for COsensing927,945and for methanol and ethanol sensing.933,946,955,956

Ruiz et al. found that La-doped TiO2 nanoparticles were goodsensing materials for ethanol based on electrical resistance746

while Cu- or Co-doped TiO2 nanoparticles were goodcandidates for CO sensing.747Garzella et al. found W-dopedTiO2 displayed better performance for ethanol sensing thpure TiO2.934 The addition of Ta and Nb to TiO2 wasbeneficial for stabilization of the nanophase, resultingselectivity enhancement toward CO930,931 and NO2.930,960

Comini et al. found that the sensitivity enhancement tow

ethanol and methanol of TiO2 films could be improved whendoped with Pt and Nb.926

Benkstein and Semancik found that mesoporous Ti2nanoparticle thin films prepared on MEMS micro-hot-pplatforms could be used as high-sensitivity conductomegas sensor materials.921The nanoparticle films were depositeonto selected micro-hot-plates in a multielement array microcapillary pipet and were sintered using the micro-hplate. Figure 91A shows the conductometric response of TiO2 nanoparticle films. The relative thickness of the filwas varied by using one, two, three, or four drops of mass fraction TiO2 to cast the film. Sensitivity was defineas the ratio of the film conductance in the presence ofanalyte to the baseline conductance measured in dry airS

) G / G0). The thicker films showed a higher baselinconductance and a higher overall sensitivity to methanol G / G0(1 drop) ) 4.1, G / G0(4 drops) ) 7.5). Shown in Figure91B are sensitivity responses to µmol/mol levels of methanolof a mesoporous TiO2 nanoparticle film and a CVD TiO2film. The nanoparticle films were found to demonstrhigher sensitivity to target analytes, attributed to the hinternal surface area of the porous nanoparticle films.

Montesperelli et al. found K-doped TiO2 nanocrystallinefilms showed high sensitivity of magnitude of 107 with greatstability over time.943 Yadav et al. fabricated TiO2 nano-crystalline films as optical humidity sensors based on variations in the intensity of light with in humidity change750

The sensor element consisted of a thin U-shaped borosil grod with a film of TiO2 deposited on it. Both the ends of thglass rod were coupled to optical fibers. Light from a H-

Ne laser was launched into the sensing element through of them. Light received from the other fiber was fed intooptical power meter.

6. Summary Over the past decades, the tremendous effort put into T2

nanomaterials has resulted in a rich database for thsynthesis, properties, modifications, and applications. Tcontinuing breakthroughs in the synthesis and modificatiof TiO2 nanomaterials have brought new properties and napplications with improved performance. Accompanied

the progress in the synthesis of TiO2 nanoparticles are newfindings in the synthesis of TiO2 nanorods, nanotubes,nanowires, as well as mesoporous and photonic structuBesides the well-know quantum-confinement effect, thnew nanomaterials demonstrate size-dependent as wellshape- and structure-dependent optical, electronic, thermand structural properties. TiO2 nanomaterials have continuedto be highly active in photocatalytic and photovoltapplications, and they also demonstrate new applicatioincluding electrochromics, sensing, and hydrogen storaThis steady progress has demonstrated that TiO2 nanoma-terials are playing and will continue to play an importrole in the protections of the environment and in the seafor renewable and clean energy technologies.

Figure 91. (A) Conductance response of TiO2 nanoparticle filmsto methanol in concentrations going from zero (baseline) to 50 µmol/mol in 10 µmol/mol steps, sequentially, at a sensor temper-ature of 450 °C. Between the steps, the analyte concentration wasreturned to 0 µmol/mol. The thickness of the films was controlledby the number of drops of the nanoparticle dispersion used todeposit the film. (B) Comparison of the sensitivity of a TiO2 thinfilm deposited by CVD (solid line) and that of a TiO2 thin filmcomposed of 15-nm-diameter anatase nanoparticles (dotted line).Reprinted from Benkstein, K. D.; Semancik, S. Sens. Actuators, B2006 , 113 , 445, Copyright 2006, with permission from Else-vier.




7. Acknowledgment The authors acknowledge the financial support from

Lawrence Berkeley National Laboratory and the U.S.Department of Energy. We thank M. C. Schriver and K. R.Carrington for critical reading of the manuscript.

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CR0500535


2007 titanium dioxide nanomaterials synthesis

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