titanium dioxide nano materials synthesis, properties, modifications, and

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

Contents1. 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

2912

3.4. Raman Vibration Properties of TiO2Nanomaterials

2912

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 29415.3.5. Coupled/Composite Water-Splitting

System2942

5.4. Electrochromic Devices 29425.4.1. Fundamentals of Electrochromic Devices 29435.4.2. 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. IntroductionSince its commercial production in the early twentieth

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

etc. In 1972, Fujishima and Honda discovered the phenom-enon 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 fromphotovoltaics and photocatalysis to photo-/electrochromicsand sensors.9-12 These applications can be roughly dividedinto “energy” and “environmental” categories, many of whichdepend not only on the properties of the TiO2 material itselfbut also on the modifications of the TiO2 material host (e.g.,with inorganic and organic dyes) and on the interactions ofTiO2 materials with the environment.

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

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

* 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 ofthe 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 thewell-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 ofthe 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. TiO2

also bears tremendous hope in helping ease the energy crisisthrough effective utilization of solar energy based onphotovoltaic and water-splitting devices.9,31,32As continuedbreakthroughs have been made in the preparation, modifica-tion, and applications of TiO2 nanomaterials in recent years,especially after a series of great reviews of the subject inthe 1990s.7,8,10-12,33,45 we believe that a new and compre-hensive review of TiO2 nanomaterials would further promoteTiO2-based research and development efforts to tackle theenvironmental and energy challenges we are currently facing.Here, we focus on recent progress in the synthesis, properties,modifications, and applications of TiO2 nanomaterials. Thesyntheses of TiO2 nanomaterials, including nanoparticles,nanorods, nanowires, and nanotubes are primarily categorizedwith the preparation method. The preparations of mesopo-rous/nanoporous TiO2, TiO2 aerogels, opals, and photonicmaterials are summarized separately. In reviewing nanoma-terial synthesis, we present a typical procedure and repre-sentative transmission or scanning electron microscopyimages to give a direct impression of how these nanomate-rials are obtained and how they normally appear. For detailedinstructions on each synthesis, the readers are referred tothe corresponding literature.

The structural, thermal, electronic, and optical propertiesof TiO2 nanomaterials are reviewed in the second section.As the size, shape, and crystal structure of TiO2 nanomate-rials vary, not only does surface stability change but alsothe transitions between different phases of TiO2 underpressure or heat become size dependent. The dependence ofX-ray diffraction patterns and Raman vibrational spectra onthe size of TiO2 nanomaterials is also summarized, as theycould help to determine the size to some extent, althoughcorrelation of the spectra with the size of TiO2 nanomaterialsis not straightforward. The review of modifications of TiO2

nanomaterials is mainly limited to the research related tothe 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 dopingor sensitization, it is possible to improve the optical sensitiv-ity and activity of TiO2 nanomaterials in the visible lightregion. Environmental (photocatalysis and sensing) andenergy (photovoltaics, water splitting, photo-/electrochromics,and hydrogen storage) applications are reviewed with anemphasis on clean and sustainable energy, since the increas-ing energy demand and environmental pollution create apressing need for clean and sustainable energy solutions. Thefundamentals and working principles of the TiO2 nanoma-terials-based devices are discussed to facilitate the under-standing and further improvement of current and practicalTiO2 nanotechnology.

2. Synthetic Methods for TiO 2 Nanostructures

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

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

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 Mao

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, oremulsion 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 ofthe 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 ofa 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 withTEOA. The TiO2 nanoparticle shape evolves into ellipsoidalabove pH 9.5 with diethylenetriamine with a higher aspectratio than that with TEOA. Figure 2 shows representativeTEM images of the TiO2 nanoparticles under different initialpH conditions with the shape control of TEOA at [TEOA]/[TIPO] ) 2.0. Secondary amines, such as diethylamine, andtertiary amines, such as trimethylamine and triethylamine,act as complexing agents of Ti(IV) ions to promote thegrowth of ellipsoidal particles with lower aspect ratios. Theshape of the TiO2 nanoparticle can also be tuned from round-cornered cubes to sharp-edged cubes with sodium oleate andsodium stearate.70 The shape control is attributed to the tuningof the growth rate of the different crystal planes of TiO2

nanoparticles by the specific adsorption of shape controllersto 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 ana-tase TiO2 nanoparticles with average particle sizes between7 and 50 nm can be obtained, as reported by Zhang andBanfield.73-77 Much effort has been exerted to achieve highlycrystallized and narrowly dispersed TiO2 nanoparticles usingthe sol-gel method with other modifications, such as asemicontinuous reaction method by Znaidi et al.78 and a two-stage mixed method and a continuous reaction method byKim 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 AAMsinto 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 solutioncontaining water, acetyl acetone, and ethanol. An AAM isimmersed into the sol solution for 10 min after being boiledin ethanol; then it is dried in air and calcined at 400°C for10 h. The AAM template is removed in a 10 wt % H3PO4

aqueous solution. The calcination temperature can be usedto control the crystal phase of the TiO2 nanorods. At lowtemperature, anatase nanorods can be obtained, while athigh temperature rutile nanorods can be obtained. The poresize of the AAM template can be used to control the size ofthese TiO2 nanorods, which typically range from 100 to 300nm in diameter and several micrometers in length. Appar-ently, the size distribution of the final TiO2 nanorods islargely controlled by the size distribution of the pores ofthe AAM template. In order to obtain smaller and mono-sized TiO2 nanorods, it is necessary to fabricate high-qualityAAM templates. Figure 3 shows a typical TEM for TiO2

nanorods fabricated with this method. Normally, the TiO2

nanorods 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 dissolvedin ethanol at room temperature, and glacial acetic acid mixedwith deionized water and ethanol is added under pH) 2-3with nitric acid. Platinum is used as the anode, and an AAMwith an Au substrate attached to Cu foil is used as thecathode. 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 % NaOHsolution, 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 AAM isremoved. In the procedure by Lee and co-workers,96 a TTIPsolution was prepared by mixing TTIP with 2-propanol and2,4-pentanedione. After the AAM was dipped into this

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

Figure 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 vaporover 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 MNaOH 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 TiO2

nanotubes with the sol-gel method.101 Briefly, TiO2 sol is

deposited on a ZnO nanorod template by dip-coating with aslow 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-up byimmersing 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 TiO2

nanotubes inherit the uniform hexagonal cross-sectionalshape and the length of 1.5µm and inner diameter of 100-120 nm of the ZnO nanorod template. As the concentrationof the TiO2 sol is constant, well-aligned TiO2 nanotube arrayscan only be obtained from an optimal dip-coating cyclenumber in the range of 2-3 cycles. A dense porous TiO2

thick film with holes is obtained instead if the dip-coatingnumber further increases. The heating rate is critical to theformation of TiO2 nanotube arrays. When the heating rateis extra rapid, e.g., above 6°C min-1, the TiO2 coat willeasily crack and flake off from the ZnO nanorods due togreat tensile stress between the TiO2 coat and the ZnOtemplate, and a TiO2 film with loose, porous nanostructureis obtained.

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

colloid are called micelles when the surfactant concentrationexceeds the critical micelle concentration (CMC). The CMCis the concentration of surfactants in free solution inequilibrium with surfactants in aggregated form. In micelles,the hydrophobic hydrocarbon chains of the surfactants areoriented toward the interior of the micelle, and the hydro-philic groups of the surfactants are oriented toward thesurrounding aqueous medium. The concentration of the lipidpresent in solution determines the self-organization of themolecules of surfactants and lipids. The lipids form a singlelayer on the liquid surface and are dispersed in solution belowthe CMC. The lipids organize in spherical micelles at thefirst CMC (CMC-I), into elongated pipes at the second CMC(CMC-II), and into stacked lamellae of pipes at the lamellarpoint (LM or CMC-III). The CMC depends on the chemicalcomposition, mainly on the ratio of the head area and thetail length. Reverse micelles are formed in nonaqueousmedia, and the hydrophilic headgroups are directed towardthe core of the micelles while the hydrophobic groups are

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. Growth2004, 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, J. J.;Yu, W. D.; Gao, X. D.; Li, X. M.Nanotechnology2006, 17, 4695.Copyright 2006 IOP Publishing Ltd.


directed outward toward the nonaqueous media. There is noobvious CMC for reverse micelles, because the number ofaggregates 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 TiO2

nanoparticles.103 The values of H2O/surfactant, H2O/titaniumprecursor, ammonia concentration, feed rate, and reactiontemperature were significant parameters in controlling TiO2

nanoparticle size and size distribution. Amorphous TiO2

nanoparticles 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.104 The 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 TiO2

nanoparticles 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 TiO2

nanoparticles 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.108 This procedure could producecrystalline TiO2 nanoparticles with unchanged physicaldimensions and minimal agglomeration and allows thepreparation 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 titaniumchloride with a variety of different oxygen donor molecules,e.g., a metal alkoxide or an organic ether.111-119

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

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

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, andtert-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 forTiI 4. 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 monodispersenonaggregated TiO2 nanoparticles in the 1-5 nm range wereobtained through hydrolysis of titanium butoxide in thepresence of acetylacetone andp-toluenesulfonic acid at 60°C.120 The resulting nanoparticle xerosols could be dispersedin water-alcohol or alcohol solutions at concentrationshigher than 1 M without aggregation, which is attributed tothe complexation of the surface by acetylacetonato ligandsand 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 TiO2

nanorods can be synthesized.122-130 For example, the growthof high-aspect-ratio anatase TiO2 nanorods has been reportedby Cozzoli and co-workers by controlling the hydrolysisprocess 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 kept at80-100 °C for 6-12 h with stirring. The bases employedincluded organic amines, such as trimethylamino-N-oxide,trimethylamine, tetramethylammonium hydroxide, tetrabut-ylammonium hydroxyde, triethylamine, and tributylamine.In this reaction, by chemical modification of the titaniumprecursor with the carboxylic acid, the hydrolysis rate oftitanium alkoxide was controlled. Fast (in 4-6 h) crystal-lization in mild conditions was promoted with the use ofsuitable catalysts (tertiary amines or quaternary ammoniumhydroxides). A kinetically overdriven growth mechanism ledto 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 used togenerate 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 ofa TiO2 nanorod. Reprinted with permission from Cozzoli, P. D.;Kornowski, A.; Weller, H.J. Am. Chem. Soc.2003, 125, 14539.Copyright 2003 American Chemical Society.


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 ofnanorods 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 thereaction in aqueous solutions. The temperature can beelevated above the boiling point of water, reaching thepressure of vapor saturation. The temperature and the amountof solution added to the autoclave largely determine theinternal pressure produced. It is a method that is widely usedfor the production of small particles in the ceramics industry.Many groups have used the hydrothermal method to prepareTiO2 nanoparticles.131-140 For example, TiO2 nanoparticlescan be obtained by hydrothermal treatment of peptizedprecipitates of a titanium precursor with water.134 Theprecipitates were prepared by adding a 0.5 M isopropanolsolution 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 (peptizer).After filtration and treatment at 240°C for 2 h, theas-obtained powders were washed with deionized water andabsolute ethanol and then dried at 60°C. Under the sameconcentration of peptizer, the particle size decreased withincreasing alkyl chain length. The peptizers and theirconcentrations influenced the morphology of the particles.Typical 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 acidicethanol-water solution.132 Briefly, TTIP was added dropwiseto a mixed ethanol and water solution at pH 0.7 with nitricacid, 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, X.; Liu,S.; 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 ofTi 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 inorganicsalts.141,143-146 Figure 13 shows a typical TEM image of theTiO2 nanorods prepared with the hydrothermal method.141

The morphology of the resulting nanorods can be tuned withdifferent 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 TiO2

nanorods were prepared at 160°C for 2 h byhydrothermaltreatment of a titanium trichloride aqueous solution super-saturated with NaCl.

TiO2 nanowires have also been successfully obtained withthe hydrothermal method by various groups.147-151 Typically,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 14shows the SEM images of TiO2 nanowires and a TEM imageof a single nanowire prepared by Zhang and co-workers.150

TiO2 nanowires can also be prepared from layered titanateparticles using the hydrothermal method as reported by Wei

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.; Ferreira,J. M. F. Mater. Sci. Eng. C2001, 15, 183, Copyright 2001, withpermission from Elsevier.

Figure 13. TEM image of TiO2 nanorods prepared with thehydrothermal method. Reprinted with permission from Zhang, Q.;Gao, 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 inthe 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 inan autoclave. TiO2 nanotubes are obtained after the productsare washed with a dilute HCl aqueous solution and distilledwater. They proposed the following formation process ofTiO2 nanotubes.154 When 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 theformation of a tube structure. In this mechanism, the TiO2

nanotubes were formed in the stage of the acid treatmentfollowing the alkali treatment. Figure 16 shows typical TEMimages of TiO2 nanotubes made by Kasuga et al.153 However,Du and co-workers found that the nanotubes were formedduring 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.171 It statedthat the raw TiO2 was first transformed into lamellarstructures and then bent and rolled to form the nanotubes.For the formation of the TiO2 nanotubes, the two-dimensionallamellar TiO2 was essential. Yao and co-workers furthersuggested, based on their HRTEM study as shown in Figure

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 inset.Reprinted from Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.;Arakawa, H.Chem. Phys. Lett.2004, 400, 231, Copyright 2004,with permission from Elsevier.

Figure 16. TEM image of TiO2 nanotubes. Reprinted withpermission from Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino,T.; Niihara, K.Langmuir1998, 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.172 Bavykin 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 TiO2

nanotubes involved several steps.176 During 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 ofhydroxy bridges, leading to the growth along the [100]direction of the anatase phase. Two-dimensional crystallinesheets formed from the lateral growth of the formation ofoxo 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 oforganic 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 sizedistribution 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-workers,184

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 TiO2

powders tended to increase as the composition of TTIP inthe 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 synthesized,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 coordinationsurfactant in the synthesis of nanoparticles. Triethylaminecould act as a catalyst for the polycondensation of the Ti-O-Ti inorganic network to achieve a crystalline product andhad little influence on the products’ morphology. The chainlengths of the carboxylic acids had a great influence on theformation 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 bedeveloped with the solvothermal method.177,183For example,in a typical synthesis from Kim et al., TTIP was dissolvedin anhydrous toluene with OA as a surfactant and kept at250 °C for 20 h in an autoclave without stirring.183 Longdumbbell-shaped nanorods were formed when a sufficientamount of TTIP or surfactant was added to the solution, dueto the oriented growth of particles along the [001] axis. Ata fixed precursor to surfactant weight ratio of 1:3, theconcentration of rods in the nanoparticle assembly increasedas the concentration of the titanium precursor in the solutionincreased. The average particle size was smaller and the sizedistribution was narrower than is the case for particlessynthesized without surfactant. The crystalline phase, diam-eter, and length of these nanorods are largely influenced bythe precursor/surfactant/solvent weight ratio. Anatase nano-

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 Li, X.L.; Peng, Q.; Yi, J. X.; Wang, X.; Li, Y. D.Chem.sEur. 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 TEMimage 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 TiO2

nanowires.180-182 Typically, 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 coordinatingability 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 ú potential values of the reactant particles and theincreases solution viscosity. For example, in the absence ofethanol, short and wide flakelike structures of TiO2 wereobtained instead of nanowires. When chloroform is used,TiO2 nanorods were obtained.181 Figure 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-crystalTiO2 nanowires.180

2.6. Direct Oxidation MethodTiO2 nanomaterials can be obtained by oxidation of

titanium metal using oxidants or under anodization. Crystal-line TiO2 nanorods have been obtained by direct oxidationof a titanium metal plate with hydrogen peroxide.186-191

Typically, TiO2 nanorods on a Ti plate are obtained when acleaned 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 theaddition of inorganic salts of NaX (X) F-, Cl-, and SO4

2-),the crystalline phase of TiO2 nanorods can be controlled.The addition of F- and SO4

2- helps the formation of pureanatase, while the addition of Cl- favors the formation ofrutile.189 Figure 21 shows a typical SEM image of TiO2

nanorods prepared with this method.186

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

Figure 19. TEM micrographs and electron diffraction patterns ofproducts 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. Growth2003, 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 Chemistry(RSC) on behalf of the Centre National de la Recherche Scientifique(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.192

The 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 10µm) 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). InCVD processes, thermal energy heats the gases in the coatingchamber and drives the deposition reaction.

Thick crystalline TiO2 films with grain sizes below 30 nmas well as TiO2 nanoparticles with sizes below 10 nm canbe prepared by pyrolysis of TTIP in a mixed helium/oxygenatmosphere, using liquid precursor delivery.230 When depos-ited on the cold areas of the reactor at temperatures below90 °C with plasma enhanced CVD, amorphous TiO2 nano-particles can be obtained and crystallize with a relativelyhigh surface area after being annealed at high temperatures.231

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 TTIPas the precursor.232

Figure 24 shows the TiO2 nanorods grown on fused silicasubstrates with a template- and catalyst-free MOCVDmethod.233 In a typical procedure, titanium acetylacetonate(Ti(C10H14O5)) vaporizing in the low-temperature zone of afurnace 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 and

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.; Gong,D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A.AdV.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 Society.


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,247 laser-induced pyrolysis,248,249and ultronsic-assisted hydrolysis,250,251among others.

2.8. Physical Vapor DepositionIn PVD, materials are first evaporated and then condensed

to 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 TiO2

nanowires,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 ofreduction 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, andtitanium and/or its compound are deposited into the poresof the AAM. By heating the above deposited template at500°C for 4 h and removing the template, pure anatase TiO2

nanowires can be obtained. Figure 26 shows a representativeSEM image of TiO2 nanowires.256

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

range of nanostructured materials, including high-surface-area transition metals, alloys, carbides, oxides, and colloids.The chemical effects of ultrasound do not come from a directinteraction with molecular species. Instead, sonochemistryarises from acoustic cavitation: the formation, growth, andimplosive collapse of bubbles in a liquid. Cavitationalcollapse produces intense local heating (∼5000 K), high pres-sures (∼1000 atm), and enormous heating and cooling rates(>109 K/s). The sonochemical method has been applied toprepare various TiO2 nanomaterials by different groups.257-269

Yu et al. applied the sonochemical method in preparinghighly photoactive TiO2 nanoparticle photocatalysts withanatase and brookite phases using the hydrolysis of titaniumtetraisoproproxide in pure water or in a 1:1 EtOH-H2Osolution under ultrasonic radiation.109 Huang et al. found thatanatase and rutile TiO2 nanoparticles as well as their mixturescould be selectively synthesized with various precursorsusing ultrasound irradiation, depending on the reactiontemperature and the precursor used.259 Zhu et al. developedtitania whiskers and nanotubes with the assistance ofsonication as shown in Figure 27.269 They found that arraysof TiO2 nanowhiskers with a diameter of 5 nm and nanotubeswith 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 and adilute HNO3 aqueous solution.

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

form of high-frequency electromagnetic waves. The principal

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.; Huang, K.Sol. Energy Mater. Sol. Cells2004, 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 TiO2

nanomaterials.270-276 Corradi 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.270 Ma 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.275 Normally, 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.28,80,264,265,277-312 Barbe 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 undervigorous stirring and at room temperature. A white precipitateformed instantaneously. Immediately after the hydrolysis, thesolution was heated to 80°C and stirred vigorously for 8 hfor peptization. The solution was then filtered on a glass fritto remove agglomerates. Water was added to the filtrate toadjust 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 rotaryevaporator and evaporated to a final TiO2 concentration of11 wt %. The precipitation pH, hydrolysis rate, autoclavingpH, and precursor chemistry were found to influence themorphology of the final TiO2 nanoparticles.

Alternative procedures without the use of hydrothermalprocesses have been reported by Liu et al.292 and Zhang etal.311 In the report by Liu et al., 24.0 g of titanium(IV)n-butoxide ethanol solution (weight ratio of 1:7) wasprehydrolyzed in the presence of 0.32 mL of a 0.28 M HNO3

aqueous solution (TBT/HNO3 ∼ 100:1) at room temperaturefor 3 h. 0.32 mL of deionized water was added to theprehydrolyzed solution under vigorous stirring and stirredfor an additional 2 h. The sol solution in a closed vesselwas kept at room temperature without stirring to gel andage. After aging for 14 days, the gel was dried at roomtemperature, ground into a fine powder, washed thoroughlywith water and ethanol, and dried to produce porous TiO2.Upon calcination at 450°C for 4 h under air, crystallizedmesoporous TiO2 material was obtained.292

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

Briefly, monodispersed TiO2 nanoparticles were formedinitially by ultrasound-assisted hydrolysis of acetic acid-modified titanium isopropoxide. Mesoporous spherical orglobular particles were then produced by controlled conden-

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.; Comte,P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am.Ceram. Soc.1997, 80, 3157. Copyright 1997 Blackwell Publishing.


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 ofmesoporous TiO2 include tetradecyl phosphate (a 14-carbonchain) by Antonelli and Ying277 and 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 AerogelsThe 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 titaniumn-butoxide in methanol with thesubsequent removal of solvent by supercritical CO2.315 Fora typical synthesis process, titaniumn-butoxide was addedto 40 mL of methanol in a dry glovebox. This solution wascombined with another solution containing 10 mL ofmethanol, nitric acid, and deionized water. The concentrationof the titaniumn-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 of24.6 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 TiO2

P25.316,317Figure 30 shows a typical SEM image of a TiO2

aerogel with a surface area of 447 m2/g and an interporestructure constructed by near uniform grains of ellipticalshapes with 30 nm× 50 nm axes.326

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


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 anillustration 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)4

in 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 formeroctahedral 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.354 The 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 ofnanostructured 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 layerof gold. After removing the PS spheres with toluene, ZnOnanorods were grown using a vapor-liquid-solid process.Finally, a TiO2 layer was deposited on the ZnO nanorodsby introducing TiCl4 and water vapors into the atomic layerdeposition 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 TiO2

opals using opal gel templates under uniaxial compressionat ambient temperature during the TiO2 sol/gel process.337

The aspect ratio was controllable by the compression degree,R. Polystyrene inverse opal was template synthesized usingsilica opals as template. The silica was removed with 40 wt% aqueous hydrofluoric acid. Monomer solutions consistingof dimethylacrylamide, acrylic acid, and methylenebisacryl-amide in 1:1:0.02 weight ratios were dissolved in a water/

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 TiO2inversed 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 1998AAAS.


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 pro-duced. Water was completely removed from the opalhydrogel by repeatedly rinsing it with a large amount ofethanol. Afterward, the opal gel was put into a large amountof tetrabutyl titanate (TBT) at ambient temperature for 24h. The TBT-swollen opal gel was then immersed in a water/ethanol (1:1 wt/wt) mixture for 5 h to let the TiO2 sol/gelprocess proceed. Figure 35A shows the opal structure of thegel/titania composite spheres formed. After calcination, TiO2

opal with distinctive spherical contours could be found. Thecompression degree,R, was adjusted by the spacer heightwhen the substrates were compressed. When the substrateswere slightly compressed against each other to the extent ofproducing a 20% reduction in the thickness of the composi-tion opal, the deformation of the template-synthesized titaniaspheres was not substantial (Figure 35B). When the com-pression degree was increased to the point of reaching 35%deformation in the opal gel, noticeably deformed titania opalscould be obtained (Figure 35C and D).

2.15. Preparation of TiO 2 NanosheetsThe preparation of TiO2 nanosheets has also been explored

recently.359-368 Typically, TiO2 nanosheets were synthesizedby delaminating layered protonic titanate into colloidal singlelayers. A stoichiometric mixture of Cs2CO3 and TiO2 wascalcined at 800°C for 20 h to produce a precursor, cesiumtitanate, Cs0.7Ti1.82500.175O4 (0: vacancy), about 70 g ofwhich was treated with 2 L of a 1 M HClsolution at roomtemperature. This acid leaching was repeated three times byrenewing the acid solution every 24 h. The resulting acid-exchanged product was filtered, washed with water, and air-dried. The obtained protonic titanate, H0.7Ti1.82500.175O4‚H2O,was shaken vigorously with a 0.017 M tetrabutylammoniumhydroxide solution at ambient temperature for 10 days. Thesolution-to-solid ratio was adjusted to 250 cm3 g-1. Thisprocedure yielded a stable colloidal suspension with an

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 E2003, 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 ZnOnanorod array on a sapphire substrate. Inset: An optical image ofthe aligned ZnO nanorods over a large area. (B) SEM image ofthe TiO2-coated ZnO nanorod array. Reprinted with permission fromWang, X.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger,L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J.AdV. 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 TiO 2 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 anatase,the octahedron is significantly distorted so that its symmetryis 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 contactwith 10 neighbor octahedrons (two sharing edge oxygen pairsand eight sharing corner oxygen atoms), while, in the anatasestructure, each octahedron is in contact with eight neighbors(four sharing an edge and four sharing a corner). Thesedifferences in lattice structures cause different mass densitiesand electronic band structures between the two forms ofTiO2.

Hamad et al. performed a theoretical calculation on TinO2n

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

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


annealing, Monte Carlo basin hopping simulation, andgenetic algorithms methods.369 They found that the calculatedglobal minima consisted of compact structures, with titaniumatoms reaching high coordination rapidly asn 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 sizes,an anatase-baddeleyite transition regime at intermediatecrystallite sizes, and an anatase-R-PbO2 transition regimecomprising large nanocrystals to macroscopic single crystals.

Barnard et al. performed a series of theoretical studies onthe phase stability of TiO2 nanoparticles in different environ-ments by a thermodynamic model.371-375 They found thatsurface passivation had an important impact on nanocrystalmorphology and phase stability. The results showed thatsurface hydrogenation induced significant changes in theshape of rutile nanocrystals, but not in anatase, and that thesize at which the phase transition might be expected increaseddramatically when the undercoordinated surface titaniumatoms were H-terminated. For spherical particles, the cross-over point was about 2.6 nm. For a clean and faceted surface,at low temperatures (a phase transition pointed at an averagediameter of approximately 9.3-9.4 nm for anatase nano-crystals), the transition size decreased slightly to 8.9 nm whenthe surface bridging oxygens were H-terminated, and the sizeincreased significantly to 23.1 nm when both the bridgingoxygens and the undercoordinated titanium atoms of thesurface trilayer were H-terminated. Below the cross point,the anatase phase was more stable than the rutile phase.371

In their study on TiO2 nanoparticles in vacuum or waterenvironments, they found that the phase transition size inwater (15.1 nm) was larger than that under vacuum (9.6nm).373 In their predictions on the transition enthalpy ofnanocrystalline anatase and rutile, they found that thermo-chemical results could differ for various faceted or spherical

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, Y.; Watanabe,M.; Oikawa, T.J. Phys. Chem. B2001, 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. ReV. 1995, 95, 735. Copyright 1995 American ChemicalSociety.


nanoparticles as a function of shape, size, and degree ofsurface 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 TiO2

nanostructures 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.377 The 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 ofthe preparation methods.378 The 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 TiO 2Nanomaterials

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 coarsen-ing, the following transformations are all seen: anatase tobrookite to rutile, brookite to anatase to rutile, anatase torutile, and brookite to rutile. These transformation sequencesimply very closely balanced energetics as a function ofparticle size. The surface enthalpies of the three polymorphsare sufficiently different that crossover in thermodynamicstability can occur under conditions that preclude coarsening,with anatase and/or brookite stable at small particle size.73,74

However, abnormal behaviors and inconsistent results areoccasionally 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 reachinga certain particle size.73,380Once rutile was formed, it grewmuch faster than anatase. They found that rutile became morestable than anatase for particle size> 14 nm.

Ye et al. observed a slow brookite to anatase phasetransition below 1053 K along with grain growth, rapidbrookite to anatase and anatase to rutile transformationsbetween 1053 K and 1123 K, and rapid grain growth of rutileabove 1123 K as the dominant phase.381 They concluded thatbrookite could not transform directly to rutile but had totransform to anatase first. However, direct transformationof brookite nanocrystals to rutile was observed above 973K by Kominami et al.382

In a later study, Zhang and Banfield found that thetransformation sequence and thermodynamic phase stabilitydepended on the initial particle sizes of anatase and brookitein their study on the phase transformation behavior ofnanocrystalline aggregates during their growth for isothermaland isochronal reactions.74 They concluded that, for equallysized nanoparticles, anatase was thermodynamically stablefor sizes< 11 nm, brookite was stable for sizes between 11and 35 nm, and rutile was stable for sizes> 35 nm.

Ranade et al. investigated the energetics of the TiO2

polymorphs (rutile, anatase, and brookite) by high-temper-ature oxide melt drop solution calorimetry, and they foundthe energetic stability crossed over between the three phasesas shown in Figure 39.383 The dark solid line represents thephases of lowest enthalpy as a function of surface area. Rutilewas energetically stable for surface area< 592 m2/mol (7m2/g or >200 nm), brookite was energetically stable from

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.; Ban-field, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.;Doran, G. S.; Whitfield, H. J.Proc. Natl. Acad. Sci. U.S.A.2002,99, 6476. Copyright 2002 National Academy of Sciences, U.S.A.


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 indensity. 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, theT∆Swill not significantly perturb the sequence ofstability 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 TiO 2Nanomaterials

XRD is essential in the determination of the crystalstructure and the crystallinity, and in the estimate of thecrystal grain size according to the Scherrer equation

whereK is a dimensionless constant,2θ is the diffractionangle,λ is the wavelength of the X-ray radiation, andâ 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 peaksthe narrower the peak, the larger thecrystallite size. The periodicity of the individual crystallite

domains reinforces the diffraction of the X-ray beam,resulting in a tall narrow peak. If the crystals are randomlyarranged or have low degrees of periodicity, the result is abroader peak. This is normally the case for nanomaterialassemblies. Thus, it is apparent that the fwhm of thediffraction peak is related to the size of the nanomaterials.

Figure 41 shows the XRD patterns for TiO2 nanoparticlesof different sizes111 and for TiO2 nanorods of differentlengths.129 As the nanoparticle size increased, the diffractionpeaks became narrower. In the anatase nanoparticle andnanorods developed by Zhang et al., the diameters of theTiO2 nanoparticles and nanorods were both around 2.3 nm.The nanorods were elongated along the [001] direction withpreferred anisotropic growth along thec-axis of the anataselattice, which was indicated by the strong peak intensity andnarrow width of the (004) reflection and relatively lowerintensity and broader width for the other reflections. Withan increase in length of the nanorods, the (004) diffractionpeak became much stronger and sharper, whereas other peaksremained similar in shape and intensity.129 Similar resultshave been observed by other groups.123,127,177,183

3.4. Raman Vibration Properties of TiO 2Nanomaterials

As the size of TiO2 nanomaterials decreases, the featuredRaman scattering peaks become broader.255,318,370,386-395 Thesize effect on the Raman scattering in nanocrystallineTiO2 is interpreted as originating from phonon confine-

D ) Kλâ cosθ

(3)

Figure 40. (A) Changes in particle sizes of anatase and rutilephases as a function of the annealing temperatures. (B) Arrenhiusplot of ln(AR/A0) vs 1/T for activation energy calculations as afunction of the size of the TiO2 nanoparticles.AR andA0 are theintegrated diffraction peak intensity from rutile (110), and the totalintegrated anatase (101) and rutile (110) peak intensity, respectively.Reused with permission from W. Li, C. Ni, H. Lin, C. P. Huang,and S. Ismat Shah,Journal of Applied Physics, 96, 6663 (2004).Copyright 2004, American Institute of Physics.


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 ofphonons in nanocrystals.255,318,370,386,387,394,395The phononconfinement model is also referred to as the spatial correla-tion model orq vector relaxation model. It links theq vector

selection rule for the excitation of Raman active opticalphonons with long-range order and crystallite size.318,370Ina perfect “infinite” crystal, conservation of phonon momen-tum requires that only optic phonons near the Brillouin zone(BZ) center (q ≈ 0) are involved in first-order Ramanscattering. In an amorphous material lacking long-rangeorder, theq vector selection rule breaks down and the Ramanspectrum resembles the phonon density of states. Fornanocrystals, the strict “infinite” crystal selection rule isreplaced by a relaxed version. This results in a range ofaccessibleq vectors (as large as∆q ≈ 1/L (L diameter))due to the uncertainty principle.

The anatase TiO2 has six Raman-active fundamentals inthe 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 showincreased broadening and systematic frequency shifts (Figure42).370 The most intense Eg(1) mode shows the maximum blueshift and significant broadening with decreasing crystallitesize. 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 combinedeffect of two individual modes), respectively. Whereas thefrequency shifts for the A1g and B1g modes are not pro-nounced, increased broadening with decreasing crystallitesize is clearly seen for these modes. The Eg(3) mode showssignificant broadening and a red shift with decreasingcrystallite size.

Choi et al. found a volume contraction effect in anataseTiO2 nanoparticles due to increasing radial pressure asparticle size decreases, and they suggested that the effectsof decreasing particle size on the force constants andvibrational amplitudes of the nearest neighbor bonds con-tributed to both broadening and shifts of the Raman bandswith decreasing particle diameter.388

3.5. Electronic Properties of TiO 2 NanomaterialsThe DOS of TiO2 is composed of Ti eg, Ti t2g (dyz, dzx,

and dxy), O pσ (in the Ti3O cluster plane), and O pπ (out ofthe Ti3O cluster plane), as shown in Figure 43A.396 The uppervalence bands can be decomposed into three main regions:the σ bonding in the lower energy region mainly due to Opσ bonding; theπ bonding in the middle energy region; andO pπ states in the higher energy region due to O pπnonbonding states at the top of the valence bands where thehybridization with d states is almost negligible. The contri-bution of theπ bonding is much weaker than that of theσbonding. The conduction bands are decomposed into Ti eg

(>5 eV) and t2g bands (<5 eV). The dxy states are dominantlylocated at the bottom of the conduction bands (the verticaldashed 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 dyz and dzx states.

In the molecular-orbital bonding diagram in Figure 43B,a noticeable feature can be found in the nonbonding statesnear the band gap: the nonbonding O pp orbital at the topof the valence bands and the nonbonding dxy states at thebottom of the conduction bands. A similar feature can beseen in rutile; however, it is less significant than in anatase.397

In rutile, each octahedron shares corners with eight neighborsand shares edges with two other neighbors, forming a linearchain. In anatase, each octahedron shares corners with four

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-metaldistance of 5.35 Å. As a consequence, the Ti dxy 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), andD2d (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 octahedralgeometry (D2d). The further splitting of the 3d levels of Ti3+

due to the asymmetric crystals is shown for rutile and anatasestructures. The fine electronic structure of TiO2 can bedirectly probed by Ti K-edge X-ray-absorption near-edgestructure (XANES), and the right panel of Figure 44Bcontains O K-edge experimental electron-energy-loss near-edge structure (ELNES) spectra.398

Hwu et al. found that the crystal field splitting ofnanocrystal TiO2 was approximately 2.1 eV, slightly smallerthan that of bulk TiO2, as shown in Figure 45A.379 Luca etal. found that 1sf np transitions broadened as particle size(increased or decreased) in the postedge region in the X-rayabsorption spectroscopy for TiO2 nanoparticles.403 Also, aclear trend in the X-ray absorption spectroscopy for differentsized TiO2 nanoparticles was observed, as shown in Figure45B 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 theEg(1) mode versuscrystallite size. Reprinted with permission from Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle,B. C. Phys. ReV. B 2005, 71, 184302/1 (http://link.aps.org/abstract/PRB/v71/p184302). Copyright 2005 by the American Physical Society.


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 excitationstate or becomes comparable to the de Broglie wavelengthof the charge carriers, the charge carriers begin to behavequantum mechanically and the charge confinement leads toa series of discrete electronic states.408 However, there is adiscrepancy in this critical size below which quantizationeffects are observed for TiO2 nanomaterials with indirectband gaps. The estimated critical diameter depends criticallyon the effective masses of the charge carriers.409 Kormannet al. estimated the excitation radii for titania particles to bebetween 7.5 and 19 Å.84 Quantum confinement size effectswere observed for TiO2 nanoparticles with a small apparentband gap blue shift (<0.1-0.2 eV) caused by quantum sizeeffects for spherical particles sizes down to 2 nm.58,60 Suchsmall effects are mainly due to the relatively high effectivemass 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 effectiveband 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 effectsin anatase nanoparticles and found no quantum size effectin anatase TiO2 nanoparticles for sizes 2R g 1.5 nm, butthey did find unusual variation of the oscillator strength ofthe first allowed direct transition with particle size.411

3.6. Optical Properties of TiO 2 NanomaterialsThe main mechanism of light absorption in pure semi-

conductors is direct interband electron transitions. Thisabsorption is especially small in indirect semiconductors, e.g.,TiO2, where the direct electron transitions between the bandcenters are prohibited by the crystal symmetry. Braginskyand Shklover have shown the enhancement of light absorp-tion in small TiO2 crystallites due to indirect electrontransitions with momentum nonconservation at the inter-face.412 This effect increases at a rough interface when theshare of the interface atoms is larger. The indirect transitionsare allowed due to a large dipole matrix element and a largedensity of states for the electron in the valence band.Considerable enhancement of the absorption is expected insmall TiO2 nanocrystals, as well as in porous and micro-crystalline semiconductors, when the share of the interfaceatoms is sufficiently large. A rapid increase in the absorptiontakes place at low (hν < Eg + Wc, whereWc is the width ofthe conduction band) photon energies. Electron transitionsto any point in the conduction band become possible whenhν ) Eg + Wc. Further enhancement of the absorption occursdue to an increase of the electron density of states in onlythe valence band. The interface absorption becomes the mainmechanism of light absorption for the crystallites that aresmaller than 20 nm.412

Sato and Sakai et al. showed through calculation andmeasurement 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 theconduction band for the TiO2 nanosheet was approximately0.1 V higher, while the upper edge of the valence band was0.5 V lower than that of anatase TiO2.360 The absorption ofthe 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 photolumi-nescence of well-developed fine structures extending intothe visible light regime.362,363The band gap energy shift,∆Eg,

Figure 43. (A) Total and projected densities of states (DOSs) ofthe anatase TiO2 structure. The DOS is decomposed into Ti eg, Tit2g (dyz, dzx, and dxy), 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,µxz and µy are the reducedeffective masses of the excitons, andLx, Ly, andLz are the

crystallite dimensions in the parallel and perpendiculardirections with respect to the sheet, respectively. Since thefirst term can be ignored, the blue shift is predominantlygoverned by the sheet thickness. The onset of a 270 nm peakin the photoluminescence of TiO2 nanosheets was assignedto resonant luminescence. The series of peaks extending intoa longer wavelength region were attributed to interband levelsgenerated by the intrinsic Ti site vacancies. The contrasting

Figure 44. (A) Schematic energy level diagram of the lowest unoccupied MOs of a [TiO6]8- cluster withOh, D2h (rutile), andD2d (anatase)symmetry. (B) Ti K-edge XANES and O K-edge ELNES spectra for rutile (a) and anatase (b). Reprinted with permission from Wu, Z. Y.;Ouvrared, G.; Gressier, P.; Natoli, C. R.Phys. ReV. B 1997, 55, 10382 (http://link.aps.org/abstract/PRB/v55/p10382). Copyright 1997 bythe American Physical Society.

∆Eg ) h2

8µxz( 1

Lx2

+ 1

Lz2) + h2

8µyLy2

(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 inan 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 of2D TiO2 nanosheets can be described by eq 10, where the“plus” and “minus” signs correspond to the conduction andvalence bands, respectively,EG is the energy gap,p isPlanck’s constant, andme andmh 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 indicesn(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/πp2, 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 edgeEn(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 electronsme can varybetween 5m0 and 30m0, and the mass of holesmh 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 statesG1D(E) (Figure 48) is less than24 meV ford ) 2.5 nm and 6 meV ford ) 5 nm, which aretoo small to be resolved in room-temperature experimentsdue to the thermal fluctuations ofkT ) 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.376 The valence band of both bulkTiO2 and their nanostructures was composed of 3d Ti-2pO states, and the lower part of the conduction band wasformed by 3d Ti states. The differences between thesenanostructures were insignificant. All anatase systems weresemiconductors with a wide direct band gap (∼4.2 eV), whilethe lepidocrocite nanotubes were semiconductors with anindirect band gap (∼4.5 eV). Independent from the specifictopology of the titania nanostructures, the band gap ap-proached the band gap of the corresponding nanocrystals withradii of about 25 Å.376

In addition to the above investigation on the bulk electronicstructures for various TiO2 nanomaterials, Mora-Sero´ andBisquert investigated the Fermi level of surface states in TiO2

nanoparticles by the nonequilibrium steady-state statistics ofelectrons.414 They found that the electrons trapped in surfacestates did not generally equilibrate to the free electrons’ Fermilevel, EFn, and a distinct Fermi level for surface states,EFs,could be defined consistent with Fermi-Dirac statistics,determining the surface states’ occupancy far from equilib-rium. The difference between the free electrons’ Fermi level

E2D( ) (

EG

2( p2k2

2me,h(5)

En1D( ) (

EG

2( p2

2me,h[k|

2 + (2nd )2] (6)

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

2π2p2[E - En(0)]1/2

(7)

∆EG ) EG1D - EG

2D ) 2p2

d2 ( 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. Y.;Lin, H. M. Nanostruct. Mater.1997, 9, 355, Copyright 1997, withpermission from Elsevier. (B) TiL2.3 absorption of TiO2 nano-crystals with different sizes. Reprinted with permission from Choi,H. C.; Ahn, H. J.; Jung, Y. M.; Lee, M. K.; Shin, H. J.; Kim, S.B.; Sung, Y. E.Appl. Spectrosc.2004, 58, 598. Copyright 2004Society for Applied Spectroscopy.


and the surface Fermi level (∆EFn - EFs) 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 TiO 2 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 fundamentalprocesses 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. B2003, 107, 9824. Copyright 2003 AmericanChemical Society. (B) Schematic illustration of electronic band structure: (a) TiO2 nanosheets; (b) anatase. Reprinted with permissionfrom Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T.J. Am. Chem. Soc.2004, 126, 5851. Copyright 2004 American Chemical Society. (C)UV-visible spectra of (a) TiO2 sheets and (b) a film of nanosheets on a SiO2 glass substrate. The data for the colloidal suspension isdenoted by a dashed trace. Reprinted with permission from Sasaki, T.; Watanabe, M.J. Phys. Chem. B1997, 101, 10159. Copyright 1997American Chemical Society.

TiO2 + hυ 98e- + h+ (9)

e- + Ti(IV)O-H f Ti(III)O -H-(X) (10)

h+ + Ti(IV)O-H f Ti(IV)O •-H+(Y) (11)

h+ + 12

O2-lattice T

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 ofreaction 12 generates O adatom intermediates upon exposingdefective surfaces to O2-(g).415 Electrons 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 Gra¨tzel found that irradiationat 4.2 K in vacuo produced electrons trapped at Ti4+ siteswithin 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 inthe 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 TiO2

nanoparticles, electrons were instantaneously trapped withinthe duration of the laser flash (20 ns). Deeply trapped holeswere 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 statesproduced a homogeneous electric field and were suggestednot to be associated with localized structures, but ratherdelocalized 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 (O-)

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) luminescenceexcitation spectrum (wavelength of emission light is 400 nm) ofcolloidal TiO2 nanotubes of different mean diameters: (1) 2.5 nm;(2) 3.1 nm; (3) 3.5 nm; (4) 5 nm. The curves are shifted verticallyfor clarity. (B) Photoluminescence spectra of colloidal TiO2nanotubes of different mean diameters: (1) 2.5 nm; (2) 3.1 nm;(3) 3.5 nm; (4) 5 nm. Room temperature, excitation wavelength237 nm, slits width 5 nm. The range of wavelengths, 455-490nm, in the spectra is omitted due to the high signal of the secondharmonic from scattered excitation light. The curves are shiftedvertically for clarity. Vertical lines (5) show the positions of thepeaks in the PL spectrum of the nanosheets. Reprinted withpermission from Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A.V.; Lapkin, A. A.; Walsh, F. C.J. Phys. Chem. B2005, 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 specieswhich may be observed by their IR absorption and that theEPR-detected holes produced by photoexcitation were O-

species, produced from lattice O2- ions. It was also foundthat, under high-vacuum conditions, the majority of photo-excited electrons remained in the conduction band. At 298K, 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 efficientuse of TiO2 nanomaterials is sometimes prevented by its wideband 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 phase),which is only a small fraction of the sun’s energy (<10%),as shown in Figure 50.11

Thus, one of the goals for improvement of the performanceof TiO2 nanomaterials is to increase their optical activity byshifting the onset of the response from the UV to the visibleregion.21,426-428 There are several ways to achieve this goal.First, doping TiO2 nanomaterials with other elements cannarrow the electronic properties and, thus, alter the opticalproperties of TiO2 nanomaterials. Second, sensitizing TiO2

with other colorful inorganic or organic compounds canimprove its optical activity in the visible light region. Third,coupling collective oscillations of the electrons in theconduction band of metal nanoparticle surfaces to those inthe conduction band of TiO2 nanomaterials in metal-TiO2

nanocomposites can improve the performance. In addition,the modification of the TiO2 nanomaterials surface with othersemiconductors can alter the charge-transfer propertiesbetween TiO2 and the surrounding environment, thus im-

Figure 48. Schematic presentation of the transformation of the electron band structure of the nanosheet semiconductor accompanying theformation of nanotubes: (a) band diagram of a 2-dimensional nanosheet; (b) band diagram of quasi-1-D nanotubes; (c) energy density ofstates for nanosheets (G2D) and nanotubes (G1D). EG1D andEG2D are the band gaps of the 1D and 2D structures, respectively.kx andky arethe wave vectors. Reprinted with permission from Bavykin, D. V.; Gordeev, S. N.; Moskalenko, A. V.; Lapkin, A. A.; Walsh, F. C.J. Phys.Chem. B2005, 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 (shallowtraps), 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. B2005, 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 ofthe 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 TiO2

due 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 TiO2 Nanomaterials

4.1.1.1. Metal-Doped TiO2 Nanomaterials. Differentmetals 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 otherreagents, 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.434 Li 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 ofconcentrations 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, anda trace of hematite coexisted at lower pH (1.8 and 3.6) whenthe Fe(III) content was as low as 0.5% and the distributionof iron ions was nonuniform between particles, but at higherpH (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.466-471 Bessekh-ouad et al. investigated alkaline (Li, Na, K)-doped TiO2

nanoparticles prepared by sol-gel and impregnation technol-ogy and found that the crystallinity level of the products waslargely dependent on both the nature and the concentrationof the alkaline, with the best crystallinity obtained for Li-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 Sn,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 amountof cations present with partial segregation of the cations inthe form of M2On after annealing.438 Wang et al. synthesizedFe(III)-doped TiO2 nanoparticles using oxidative pyrolysisof liquid-feed organometallic precursors in a radiation-frequency (RF) thermal plasma and found that the formationof rutile was strongly promoted with iron doping comparedto the anatase phase being prevalent in the undoped TiO2.246

4.1.1.2. Nonmetal-Doped TiO2 Nanomaterials.Variousnonmetal elements, such as B, C, N, F, S, Cl, and Br, havebeen successfully doped into TiO2 nanomaterials. C-dopedTiO2 nanomateirals have been obtained by heating titaniumcarbide472-474 or by annealing TiO2 under CO gas flow athigh temperatures (500-800 °C)475 or by direct burning ofa titanium metal sheet in a natural gas flame.476

N-doped TiO2 nanomaterials have been synthesized byhydrolysis of TTIP in a water/amine mixture and the post-treatment of the TiO2 sol with amines426,428,477-482 or directlyfrom a Ti-bipyridine complex483 or by ball milling of TiO2

in a NH3 water solution.484 N-doped TiO2 nanomaterials werealso obtained by heating TiO2 under NH3 flux at 500-600°C485,486 or by calcination of the hydrolysis product ofTi(SO4)2 with ammonia as precipitator487or by decompositionof gas-phase TiCl4 with an atmosphere microwave plasmatorch488 or by sputtering/ion-implanting techniques withnitrogen489,490or N2

+ gas flux.491

S-doped TiO2 nanomaterials were synthesized by mixingTTIP with ethanol containing thiourea492-494 or by heatingsulfide powder495,496or by using sputtering or ion-implantingtechniques with S+ ion flux.497-499 Different doping methodscan induce the different valence states of the dopants. Forexample, 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 mixingTTIP with ethanol containing H2O-NH4F,500-502 or byheating TiO2 under hydrogen fluoride503,504 or by spraypyrolysis from an aqueous solution of H2TiF6

505,506or usingion-implanting techniques with F+ ion flux.507 Cl- and Br-

co-doped nanomaterials were synthesized by adding TiCl4

to ethanol containing HBr.508

4.1.2. Properties of Doped TiO2 Nanomaterials4.1.2.1. Electronic Properties of Doped TiO2 Nanoma-

terials. 4.1.2.1.1. Metal-Doped TiO2 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. ReV. 1995, 95, 735. Copyright 1995 AmericanChemical Society.


cording to Soratin and Schwarz’s study, the electronic statesof TiO2 can be decomposed into three parts: theσ 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 dxy orbital contributes to the metal-metal interactions due to theσ bonding of the Ti t2g-Ti t2g

states. At the top of the lower CB, the rest of the Ti2g statesare 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 valenceband 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-xVxO2: 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 theσ 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. Theσ 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 theVB for Co) due to theπ antibonding of the Fe eg and O pπstates. This level was occupied by four (or five for Co)electrons. The Fe (Co) eg state was split intodz2 (f) anddx2-y2

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

states from thedz2 and dx2-y2 orbitials situated in the bandgap. The electron densities around the dopant were large inthe VB and small in the CB compared to the case of pureTiO2. The metal-O interaction strengthened, and the metal-metal interaction became weak as a result of the 3d metaldoping.

Li et al. found that 1.5 at % Nd3+-doped TiO2 nanoparticlesreduced the band gap by as much as 0.55 eV and that theband gap narrowing was primarily attributed to the substi-tutional Nd3+ ions, which introduced electron states into theband 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-dopedTiO2 into the half-metallic or the insulating ground state510

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

4.1.2.1.2. Nonmetal-Doped TiO2 Nanomaterials.Recenttheoretical and experimental studies have shown that thedesired band gap narrowing of TiO2 can also be achievedby using nonmetal dopants (refs 385, 428, 444, 489, 481,482, 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, or S,using the FLAPW method in the framework of the localdensity approximation (LDA) as shown in Figure 52.489 Inthis study, C dopant introduced deep states in the gap.489

Figure 51. (A) Bonding diagram of TiO2. (B) DOS of the metal-doped TiO2 (Ti1-xAxO2: A ) V, Cr, Mn, Fe, Co, or Ni). Gray solid lines:total DOS. Black solid lines: dopant’s DOS. The states are labeled (a) to (j). Reprinted from Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai,K. J. Phys. Chem. Solids2002, 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.522 In 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 bandgap of TiO2; however, the mixing of C with O 2p stateswas too weak to produce a significant band gap narrowing.517

In Asahi’s study, the substitutional doping of N was themost effective in the band gap narrowing because its p statesmixed with O 2p states, while the molecularly existingspecies, e.g., NO and N2 dopants, gave rise to the bondingstates below the O 2p valence bands and antibonding statesdeep 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 anataseand rutile polymorphs, N2p localized states were just abovethe top of the O2p valence band.512,513 In anatase, thesedopant states caused a red shift of the absorption band edgetoward the visible region, while, in rutile, an overall blueshift was found by the N-induced contraction of the O 2pband.512 Experimental evidence supported the statement thatnitrogen-doped TiO2 formed nitrogen-induced midgap levelsslightly above the oxygen 2p valence band.486 Lee et al., intheir first-principles density-functional LDA pseudopotentialcalculations of electronic properties of N-doped TiO2, foundthat the bands originating from N 2p states appeared in theband gap of TiO2; however, the mixing of N with O 2p stateswas too weak to produce a significant band gap narrowing.517

Wang and Doren found that N doping introduced some statesat the valence band edge and thus made the original bandgap of TiO2 smaller, and that a vacancy could induce somestates in the band gap region, which acted as shallowdonors.510 Nakano et al. found that, in N-doped TiO2, deeplevels located at approximately 1.18 and 2.48 eV below theconduction band were attributed to the O vacancy state asan efficient generation-recombination center and to the Ndoping which contributed to band gap narrowing by mixingwith the O 2p valence band, respectively.523 Okato et al.found that, at high doping levels, N was difficult to substitutefor O to contribute to the band gap narrowing, instead givingrise to the undesirable deep-level defects.524

S dopant induced a similar band gap narrowing asnitrogen,489 and the mixing of the sulfur 3p states with thevalence band was found to contribute to the increased widthof the valence band, leading to the narrowing of the bandgap.495,497When S existed as S4+, replacing Ti4+, sulfur 3sstates induced states just above the O 2p valence states, andS 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 anymixing with the valence or conduction band as shown inFigure 53B, and additional states appeared just below theconduction edge, due to the electron occupied level composedof the t2g state of the Ti 3d orbital.507 The electronic changeinduced by F dopant was considered to be similar to the Ovacancy, thus reducing the effective band gap and improvingvisible light photoresponse.507 Li et al. found that F dopingproduced several beneficial effects including the creation ofsurface oxygen vacancies, the enhancement of surfaceacidity, and the increase of Ti3+ ions, and doped N atomsformed a localized energy state above the valence band ofTiO2, whereas doped F atoms themselves had no influenceon the band structure in N-F-co-doped TiO2.519

4.1.2.2. Optical Properties of Doped TiO2 Nanomate-rials. 4.1.2.2.1. Optical Properties of Metal-Doped TiO2

Nanomaterials.A red shift in the band gap transition or a

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. Science2001, 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 withan increase in the dopant concentration.434,445,460This 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.466-471 Anpoet 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 bandappeared 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 lightabsorption for the Cr-doped TiO2 can be attributed to a donortransition from the Cr t2g level into the CB and the acceptortransition 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 theTiO2 electrons within their band gap led to nonradiativeenergy 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 TiO2

Nanomaterials.Nonmetal doped TiO2 normally has a colorfrom white to yellow or even light gray, and the onset ofthe absorption spectra red shifted to longer wavelengths (refs385, 426, 478, 483, 489, 494, 495, 497, 498, 505, 506, 512,516, 518, 519, 521, and 529). In N-doped TiO2 nanomate-rials, the band gap absorption onset shifted 600 nm from380 nm for the undoped TiO2, extending the absorption upto 600 nm, as shown in Figure 56.426 The optical absorptionof N-doped TiO2 in the visible light region was primarilylocated between 400 and 500 nm, while that of oxygen-deficient TiO2 was mainly above 500 nm from their density-functional theory study.520 N-F-co-doped TiO2 prepared byspray pyrolysis absorbs light up to 550 nm in the visiblelight spectrum.518 The S-doped TiO2 also displayed strongabsorption in the region from 400 to 600 nm.494 The red shiftsin the absorption spectra of doped TiO2 are generallyattributed to the narrowing of the band gap in the electronicstructure 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 bandgap of the oxide which were responsible for visible light

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. A2004, 265, 115,Copyright 2004, with permission from Elsevier. (B) Total DOSsof F-doped TiO2 calculated by FLAPW.Eg 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. B2003, 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 ofimplanted 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 method.The amount of doped Cr ions (wt%) was (a) 0, (b′) 0.01, (c′) 0.1,(d′) 0.5, or (e′) 1. Reprinted from Anpo, M.; Takeuchi, M.J. Catal.2003, 216, 505, Copyright 2003, with permission from Elsevier.


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

4.1.2.3. Photoelectrical Properties of Doped TiO2 Nano-materials. The photoelectrical properties of a material canbe measured with an “action spectrum” curve using a photo-to-current conversion setup.385,486,497,521In this setup, lightfrom a xenon lamp passing through a monochromator isradiated onto the electrode, and the photocurrents from theelectrodes are measured as a function of wavelength.385,486,497,521

The incident photo-to-current efficiency as a function ofwavelength, IPCEλ, is called an “action spectrum”. IPCEλcan be calculated by

whereIph,λ is the photocurrent, Pλ is the power intensity ofthe light at wavelengthλ, and h, c, and e are Planck’sconstant, the speed of light, and the elementary charge,respectively.385 The IPCEλ curve normally has a similar shapeand trend as the absorption spectrum. When the IPCEλ isdivided by the absorption, the absorbed photon-to-currentefficiency (APCEλ; also called the quantum yield) isobtained.521 Figure 57 shows IPCEλ and APCEλ curves forN-doped TiO2 and TiO2.521 The photoelectrochemical onsetfor TiO2-xNx is shifted to around 550 nm into the visibleregion of the spectrum, and some ultraviolet (UV) efficiencyfor TiO2-xNx is lost compared to that of TiO2, suggesting

Figure 55. Schematic diagram to illustrate the photoexcitationprocess under visible light of metal-doped TiO2: (a) Ti1-xVxO2;(b) Ti1-xFexO2; (c) Ti1-xCrxO2. Reprinted from Umebayashi, T.;Yamaki, T.; Itoh, H.; Asai, K.J. Phys. Chem. Solids2002, 63,1909, Copyright 2002, with permission from Elsevier.

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

Figure 57. IPCEλ and APCEλ curves for N-doped TiO2 and TiO2.SE stands for the substrate/electrode (SE) interface. The actionspectra are recorded with light incident onto the SE interface.Reprinted from Lindgren, T.; Lu, J.; Hoel, A.; Granqvist, C. G.;Torres, G. R.; Lindquist, S. E.Sol. Energy Mater. Sol. Cells2004,84, 145, Copyright 2004, with permission from Elsevier.

IPCEλ ) hce

Iph,λ

Pλλ(18)


the TiO2-xNx 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 tothe 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.9,10 TiO2 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 Sensitization4.2.1.1. 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 ofthe sensitizer AgI on TiO2 nanoparticles resulted in astabilization of electron-hole pairs with a lifetime wellbeyond 100µs 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 TiO2

nanoparticles 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 nanoparti-cles.378,555Sant and Kamat found that quantum size effectsplayed an important role in interparticle electron transfer inthe CdS-TiO2 semiconductor systems in that electrontransfer from photoexcited CdS to TiO2 was found to dependon the size of TiO2 nanoparticles.560 Charge transfer occurredonly when TiO2 nanoparticles were sufficiently large (>1.2nm) that the conduction band of the nanoparticles was locatedbelow that of CdS nanoparticles.560 Shen et al. studiednanostructured TiO2 electrodes with different nanocrystalssizes sensitized with CdSe nanoparticles and found thatphotoelectrochemical currents in the visible region in theCdSe-sensitized TiO2 nanostructured electrodes were largelydependent on both the structure and electron diffusioncoefficient of the TiO2 electrodes.556 Zaban et al. studiednanocrystalline TiO2 electrodes sensitized with InP quantumdots, and found they exhibited strong photoconduction inthe visible region and had a photocurrent action spectrumconsistent with the absorption spectrum of the InP QDs,indicating electron transfers from InP QDs into TiO2 nano-particles under visible light illumination.559

Kamat et al. recently reported the sensitization of meso-scopic TiO2 films using bifunctional surface modifiers (SH-R-COOH) linked with CdSe nanoparticles. Upon visible lightexcitation, CdSe nanoparticles injected electrons into TiO2

nanocrystallites.561 The TiO2-CdSe composite exhibited aphoton-to-charge carrier generation efficiency of 12% whenemployed as a photoanode in a photoelectrochemical cell.

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 couldbe reversely switched back and forth between brownish-grayunder UV light and the color of illuminating visible lightdue 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 whiteby changing the size of the Ag nanoparticles due to theplasmon-based absorption of Ag and the dielectric confine-ment of the TiO2 nanoparticle film matrix. Figure 58 showsabsorption spectra and photographs of Ag-TiO2 films.

Naoi et al. found that the chromogenic properties of theAg-TiO2 films could be improved by simultaneous irradia-tion during Ag deposition with UV and blue lights tosuppress the formation of anisotropic Ag particles and thatnonvolatilization of a color image could be achieved byremoving Ag+ that was generated during the irradiation witha colored light.563 The color of the film was further found tobe affected by the resonance wavelengths of the Ag particles,the TiO2 film, and the nanopores in the TiO2 film. Theyfound that the photochromism and rewritability of Ag-TiO2

films could be deactivated by modification of Ag nanopar-ticles with thiols to make it possible to retain color imagesdisplayed on the films, and that the deactivated propertiescould be fully reactivated by UV irradiation (Figure 59A).564

Kawahara et al. proposed the mechanism of chargeseparation 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 com-mercially available Ag nanoparticles, visible light-inducedelectron transfer from Ag to oxygen molecules played anessential role. Some of the photoexcited electrons on Ag weretransferred 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.566 They found thatfor the Au-TiO2 system photoaction spectra for open-circuitpotential 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 lyingexcited states, such as polypyridine complexes, phthalocya-nine, and metalloporphyrins.568-673 The metal centers for thedyes include Ru(II), Zn(II), Mg(II), Fe(II), and Al(III), whilethe ligands include nitrogen heterocyclics with delocalizedπ or aromatic ring systems.

These organic dyes are normally linked to TiO2 nanopar-ticle surfaces via functional groups by various interactionsbetween the dyes and the TiO2 nanoparticle substrate: (a)covalent attachment by directly linking groups of interest orvia linking agents, (b) electrostatic interactions via ionexchange, ion-pairing, or donor-acceptor interactions, (c)hydrogen bonding, (d) van der Waals forces, etc. Most ofthe dyes of interest link in the first way. Groups such assilanyl (-O-Si-), amide (-NH-(CdO)-), carboxyl (-O-(CdO)-), and phosphonato (-O-(HPO2)-) have beenshown to from stable linkages with the surface hydroxylgroups on TiO2 substrates.610 Carboxylic and phosphonic acidderivatives react with the hydroxyl groups to form esters,while amide linkages are obtained via the reaction of aminederivatives and dicyclohexyl carbodiimide on TiO2. The mostcommon and successful functional groups are based oncarboxylic acids. Qu and Meyer found spectroscopic evi-dence for ester linkages after carboxylic acids react with thesurface titanol groups dehydratively.674 Metal cyano com-pounds in acidic solutions were found to link to TiO2 surfacesby a single cyanide ligand with aC4V symmetry, i.e., TiIV-NC-FeII(CN)5.668,669,675

The interfacial charge separation between the adsorbeddyes 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 ofmulticolored 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 theAg-TiO2 film (a, b) and deactivation (c) and reactivation (d) ofthe 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 Chemistry. (B)Proposed mechanism of the charge separation at the interfacebetween Ag and TiO2 nanoparticles. From: Kawahara, K.; Suzuki,K.; Ohko, Y.; Tatsuma, T.Phys. Chem. Chem. Phys.2005, 7, 3851(http://dx.doi.org/10.1039/b511489f)s Reproduced by 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 mechanismof 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 ofcoverage 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 TiO2

occurred on a time scale of∼200 fs due to strong electroniccoupling between the dye and TiO2 energy levels.684 Theelectron transport and recombination in dye-sensitized TiO2

solar 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, I 2, 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 constantof 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 formedand the relaxed dye excited states, respectively.577

In the reduced sensitizer injection mechanism, the sensi-tizer excited state(s) is first quenched by an external donor,and subsequently the reduced state of the dye, S-, transfersan electron across the semiconductor interface.550,688 Apotential advantage of this mechanism is that the reducedsensitizer is a stronger reductant than the MLCT excited state,typically by 0.3-0.5 eV. Thus, sensitizers that are weakphotoreductants may sensitize TiO2 efficiently after reductivequenching. This mechanism may be exploited to producelarge open-circuit photovoltages or enhanced light harvestingin the near-IR regions. The observation of ultrafast electroninjection coupled with the weak oxidizing power of theexcited sensitizers currently in use strongly suggests that anexcited-state injection mechanism is operative in regenerativesolar cells based on these materials. The reduced sensitizerinjection mechanism was reported by Thompson,688Haque,598

and Wang.659

The metal-to-particle charge-transfer mechanism involvesinterfacial chemistry between the compounds and the TiO2

surface which produces color changes, observed by Gra¨tzeland identified as molecule-to-particle charge-transfer transi-tions.689 Metal cyanides, [M(CN)x]4- (M ) FeII, RuII, OsII,ReIII , MoIV, or WIV, x ) 6, 7, or 8), such as ferrocyanide,FeII(CN)64-, bind to TiO2 through ambidentate cyano ligands.For example, FeII(CN)64- does not absorb light above 380nm, but a deep orange color with an absorption maximumcentered at 420 nm was observed for FeII(CN)64-/TiO2, dueto a MPCT complex formed between FeII(CN)64- and surfaceTi4+ ions, Fe(II)fTi(IV). 550 The metal-to-particle charge-transfer mechanism was consistent with the subpicosecondinfrared spectroscopy study on the FeII(CN)64-/TiO2 nano-particle by Weng et al., where a mid-infrared absorption wasassigned to TiO2 electrons in the semiconductor.690 Theinjection rate constant could not be time-resolved with a 50-fs instrument response function. The MPCT was also foundby Yang et al. in their study on Fe(bpy)(CN)4

2--sensitizedTiO2, where the absorption spectra were well modeled by asum of MLCT (Fef bpy) and metal-to-particle (Fe(II)fTi(IV)) bands. The MLCT bands were solvatochromic, whilethe MPCT bands were not.668,669 Benkoe et al. found thatthe larger the TiO2 particle and the better its overallcrystallinity, the faster the process of electron injection fromthe dye fluorescein 27 to the anatase TiO2 film.578 Haque etal. found that a supramolecule dye with a remarkably long-lived (4 s) charge-separation state could be obtained bycontrolling the spatial separation between the cation centerof the dye and the electrode surface.597 The dyes were Ru-(II) complexes containing carboxylated polypyridyl chro-mophores and a bipyridyl ligand with aromatic amine-basedelectron donor substituents.597

The kinetics and mechanisms of the injections, transport, re-combination, and photovoltaic properties of electrons in nano-structured TiO2 solar cells have been thoroughly discussedin recent reviews676,691-694 and will only be briefly mentionedbelow. Considerable effort has been devoted to the kineticsand energetics of transport and recombination in dye-sensi-tized solar cells with various techniques, such as intensitymodulated photocurrent spectroscopy (IMPS),640,695-702 inten-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 controlledby 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 ofthe 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.676 The recombination predominates at theinterface and depends on the spatial region of photoinjectedcharge 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.724 They 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 NanomaterialsThe 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 prod-ucts, i.e., mirrors and eyeglasses, having superhydrophilicor superhydrophobic surfaces.332,732-734 For example, Fenget al. found that reversible superhydrophilicity and super-hydrophobicity could be switched back and forth for TiO2

nanorod films.142 When the TiO2 nanorod films wereirradiated with UV light, the photogenerated hole reactedwith lattice oxygen to form surface oxygen vacancies. Watermolecules kinetically coordinated to these oxygen vacancies,and the spherical water droplet filled the grooves along thenanorods and spread out on the film with a contact angle ofabout 0°, resulting in superhydrophilic TiO2 films. After thehydroxy group adsorption, the surface transformed into anenergetically metastable state. When the films were placedin the dark, the adsorbed hydroxy groups were graduallyreplaced by atmospheric oxygen, and the surface evolvedback to its original state. The surface wettability convertedfrom superhydrophilic to superhydrophobic.142 Stain-proof-ing, self-cleaning properties can also be bestowed on manydifferent types of surfaces due to the superhydrophilic orsuperhydrophobic surfaces.735-744 TiO2 nanomaterials havealso been used as sensors for various gases and humiditydue to the electrical or optical properties which change uponadsorption.745-751

One of the most important research areas for future cleanenergy applications is to look for efficient materials for theproduction of electricity and/or hydrogen. When sensitizedwith organic dyes or inorganic narrow band gap semicon-ductors, TiO2 can absorb light into the visible light regionand convert solar energy into electrical energy for solar cellapplications.28,30,752For example, an overall solar to currentconversion efficiency of 10.6% has been reached by thegroup led by Gra¨tzel with DSSC technology.31 TiO2 nano-materials have been widely studied for water splitting andhydrogen production due to their suitable electronic bandstructure 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 photochromicdevices.562,565,771-777 Of course, one of the many applicationsof TiO2 nanomaterials is the photocatalytic decompositionof various pollutants.

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

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

TiO2 photocatalysts can also be used to kill bacteria, as hasbeen carried out withE. 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 stud-ied.7,12,20,33,406The principle of the semiconductor photocata-lytic reaction is straightforward. Upon absorption of photonswith energy larger than the band gap of TiO2, electrons areexcited from the valence band to the conduction band,creating electron-hole pairs. These charge carriers migrateto the surface and react with the chemicals adsorbed on thesurface to decompose these chemicals. This photodecom-position process usually involves one or more radicals orintermediate species such as•OH, O2-, H2O2, or O2, whichplay important roles in the photocatalytic reaction mecha-nisms. The photocatalytic activity of a semiconductor islargely controlled by (i) the light absorption properties, e.g.,light absorption spectrum and coefficient, (ii) reduction and


oxidation rates on the surface by the electron and hole, (iii)and the electron-hole recombination rate. A large 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 ofTiO2 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 TiO2 Nanomaterials: First Generation

As the size of the TiO2 particles decreases, the fraction ofatoms 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 TiO2

nanoparticles 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 TiO2

nanoparticles for maximum photocatalytic efficiency in thedecomposition of chloroform.815 They 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 surfacerecombination 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 ofRhodamine B due to the large surface area, small crystalsize, and well-crystallized anatase mesostructure.296 Figure

60 showsthe photocatalytic properties of mesoporous TiO2

samples as prepared and calcined at different temperaturescompared to those of TiO2 P25 nanoparticles. All mesopo-rous TiO2 showed better activity than Deguessa P25 TiO2.The optimum reactivity was obtained with the samplecalcined at 400°C, and the photoactivity gradually decreasedwith further increases in calcination temperature.

Yang et al. found that TiO2 nanotubes treated with H2-SO4 solutions showed photocatalytic activity on degradationof 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 hadhigher specific surface areas.818

TiO2 aerogels were also suggested as promising candidatesfor photocatalysts.316,317,319 Degan et al. prepared TiO2aerogels with a porosity of 90% and surface areas of 600m2/g, and they found that the photodegradation of salicylicacid on TiO2 aerogels, after 1 h of near-UV illumination,was about 10 times faster than that on the Degussa TiO2.316,317

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

5.1.2. Metal-Doped TiO2 Nanomaterials: SecondGeneration

Over the past decades, metal-doped TiO2 nanomaterialshave been widely studied for improved photocatalyticperformance on the degradation of various organic pollutants,i.e., under visible light irradiation (refs 21, 430, 433-435,444, 446, 450-452, 455-457, 490, 515, 548, 810, 827-836). Choi et al. conducted a systematic study on thephotocatalytic activity of TiO2 nanoparticles doped with 21transition metal elements on the oxidation of CHCl3 and thereduction of CCl4 and found that the photocatalytic activitywas related to the electron configuration of the dopant ionin that dopant ions with closed electron shells had little orno 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 iondopants in the TiO2 matrix significantly influenced the chargecarrier recombination rates and interfacial electron-transfer

Figure 60. Photocatalytic properties of mesoporous TiO2 samplesas prepared and calcined at different temperature as well as TiO2P25 nanoparticles (RB,c0 ) 1.0× 10-5 M, pH ) 6.0) under UV-light radiation. Reprinted with permission from Peng, T.; Zhao,D.; Dai, K.; Shi, W.; Hirao, K.J. Phys. Chem. B2005, 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.433 Figure62 shows the photocatalytic decomposition of phenol withreaction time under UV and visible light using Sn4+-dopedTiO2 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,837 and it was shown to be more efficientin the photoelectrocatalytic disinfection ofE. 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 TiO2

nanoparticles exhibited higher visible light photocatalyticactivities on the degradations of dichloroacetate and 4-chlo-rophenol,830 and Ag-TiO2 nanocatalysts displayed enhancedphotocatalytic activity in the degradation of 2,4,6-trichlo-rophenol due to a better separation of photogenerated chargecarriers 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 ofnanoparticles.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 dopingwas beneficial, while, in the deep bulk, the doping wasdetrimental.451

However, not all the metal-doped TiO2 nanomaterialsshowed higher photocatalytic activities than pure TiO2

nanomaterials. Martin found V-doped TiO2 nanoparticles hadreduced photocatalytic activity on the photooxidation of4-chlorophenol compared to pure TiO2 nanoparticles. Va-nadium appeared to reduce the photoreactivity of TiO2 bypromoting charge-carrier recombination with electron trap-ping at VO2+ centers or with hole trapping at V4+ impuritycenters, which shunted charge carriers away from the solid/solution interface.446 Hermann et al. found that although Cr-doped (0.85 atomic %) TiO2 absorbed in the visible region,its activity for oxidation of oxalic acid, propene, and2-propanol and for O isotope exchange was null under visibleillumination and was smaller under UV light than that ofpure 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 plus 1mol % Pb decreased, since the d electrons of Mo(4d) andV(3d), as majority carriers in TiO2, could effectively quenchthe high-energy photogenerated holes at the impurity levelsintroduced by doping within the band gap of TiO2.445

5.1.3. Nonmetal-Doped TiO2 Nanomaterials: ThirdGeneration

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

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 timeunder (A) UV and (B) visible light: (a) pure TiO2 catalyst; (b)Sn4+-doped TiO2. From: Cao, Y.; Yang, W.; Zhang, W.; Liu, G.;Yue, P.New J. Chem.2004, 28, 218 (http://dx.doi.org/10.1039/b306845e)s Reproduced by permission of The Royal Society ofChemistry (RSC) on behalf of the Centre National de la RechercheScientifique (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 atomicâ-N statespeaking at 396 eV in the XPS spectra.489 In 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 duringthe 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.486 N-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 thedifferent carrier behavior in these samples.849

A noticeable photocatalytic activity on decompositions ofmethylene blue and isopropanal in the visible region wasdemonstrated for C-doped TiO2 made from a TiC precur-sor.472,473 C-doped TiO2 made by pyrolyzing Ti metal in anatural gas flame displayed a much higher photoactivity inwater splitting than pure TiO2.476 C-doped TiO2 nanoparticlesalso displayed high photoactivity in degradation of trichlo-roacetic 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 ofacetone under proper preparation conditions.502 N/F-dopedTiO2 nanomaterials had high visible light photocatalyticactivities for decompositions of both acetaldehyde andtrichloroethylene due to the creation of surface oxygenvacancies rather than the improvement of optical absorptionproperties.505,506,518,519Luo et al. found that chlorine- andbromine-co-doped TiO2 displayed a much higher photocata-lytic activity than chlorine- or bromine-doped TiO2.508

5.2. Photovoltaic Applications

5.2.1. The TiO2 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 givenin Figure 64. At the heart of the system is a nanocrystallinemesoporous TiO2 film with a monolayer of the charge-transfer dye attached to its surface. The film is placed incontact with a redox electrolyte or an organic hole conductor.Photoexcitation of the dye injects an electron into theconduction band of TiO2. The electron can be conducted tothe outer circuit to drive the load and make electric power.The original state of the dye is subsequently restored byelectron donation from the electrolyte, usually an organicsolvent containing a redox system, such as the iodide/triiodide couple. The regeneration of the sensitizer by iodideprevents the recapture of the conduction band electron bythe oxidized dye. The iodide is regenerated in turn by thereduction of triiodide at the counterelectrode, with the circuitbeing completed via electron migration through the externalload. The voltage generated under illumination correspondsto the difference between the Fermi level of TiO2 and theredox potential of the electrolyte. Overall, the devicegenerates electric power from light without suffering anypermanent chemical transformation.9,28-32

Cahen et al. explained the cause for the photocurrent andphotovoltage in nanocrystalline mesoporous dye-sensitizedsolar cells in terms of the separation, recombination, andtransport of electronic charge as well as in terms of electronenergetics.721 The basic cause for the photovoltage is thechange in the electron concentration in the nanocrystallineelectron conductor that results from photoinduced chargeinjection from the dye. Pichot and Gregg found that thephotovoltage was determined by photoinduced chemicalpotential gradients, not by equilibrium electric fields.635 Themaximum photovoltage is given by the difference in electronenergies between the redox level and the bottom of theconduction band of the electron conductor, rather than byany difference in electrical potential in the cell, in the dark.Charge separation occurs because of the enthalpic andentropic driving forces that exist at the dye/electron conductor

Figure 63. Photocatalytic properties of TiO2-xNx and TiO2 basedon decomposition rates [measuring the change in absorption of thereference light (∆abs)] of methylene blue as a function of the cutoffwavelength 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 ofthe 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.Science2001, 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 adsorbedon 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. Gra¨tzelet 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 andosmium 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, acetylacetonate, thiacarbamate, or water substituent.88 The dye-sensitized solar cells withcis-dithiocyanatobis(4,4′-dicar-boxylic acid-2,2′-bipyridine)ruthenium(II) (N3) displayedabsorption maxima at 518 and 380 nm and emission at 750nm with a lifetime of 60 ns.625,850 In 2001 the “black dye”tri(cyanato)-2,2′,2′′-terpyridyl-4,4′,4′′-tricarboxylate) ruthe-nium(II) was found to achieve 10.4% conversion efficiencyin full sunlight.631 Amphiphilic heteroleptic N3 equivalentdyes were recently applied to solar cells.673These amphiphilicheteroleptic sensitizers had several advantages compared tothe N3 complex: (a) The ground-state pKa of the 4,4′-dicarboxy-2,2′-bpy was higher to enhance the binding of thecomplex onto the TiO2 surface. (b) The decreased chargeon the sensitizer attenuated the electrostatic repulsion andincreased the dye loading. (c) The presence of the hydro-phobic moiety on the ligand increased the stability of solarcells toward water-induced desorption. (d) The oxidationpotential of these complexes was cathodically shifted com-pared to that of the N3 sensitizer, which increased thereversibility of the ruthenium III/II couple, leading toenhanced stability. Combining the N3 dye with guanidiniumthiocyanate brought a further increase in the open-circuitvoltage of the solar cell.30,31

Unlike the large amount of effort put forth to optimizethe organic dyes in DSSCs in the past decades, attention hasonly recently been paid to the TiO2 nanocrystalline electrode,and some important results have been obtained. In thefollowing, various research efforts on the use of the TiO2

nanocrystalline electrode for DSSCs are briefly summarized.

5.2.1.1. Mesoporous TiO2 Nanocrystalline Electrodes.Zukalova et al. found that ordered mesoporous TiO2 nano-crystalline films showed enhanced solar conversion efficiencyby about 50% compared to traditional films of the samethickness made from randomly oriented anatase nanocrys-tals.312 The TiO2 nanocrystalline film was prepared via layer-by-layer deposition with Pluronic P123 as template. Thesensitizer used wascis-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 photo-current-voltage characteristics for solar cells based on orderedand nonordered TiO2 films. When sensitized by N945, the0.95-µm-thick nonorganized anatase film gave a conversionefficiency of only 2.21%, which increased to 2.74% withsurface treatment by TiCl4 prior to dye deposition. Understandard global AM 1.5 solar conditions, the cell with anordered mesoporous TiO2 nanocrystallinne film gave aphotocurrent density ofIp ) 7 mA/cm2, an open circuitpotential ofUOC ) 0.799 V, and a fill factor of ff) 0.72,yielding 4.04% conversion efficiency. This improvementresulted from a remarkable enhancement of the short circuitphotocurrent, due to the huge surface area accessible to boththe dye and the electrolyte.312

5.2.1.2. TiO2 Nanotube Electrode.Adachi et al. foundthat dye-sensitized solar cells with electrodes made ofdisordered 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 densitycompared 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-organized

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 isregenerated 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 Gra¨tzel,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-nmlength) 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 TiO2 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 Gra¨tzel solar cells was presented bySomani et al., using large-surface titania inverse opal filmsas electrodes in fabricating solid-state dye-sensitized organic-inorganic hybrid Gra¨tzel solar cells.352 Direct comparison

indicated that light conversion efficiency increased by at least1 order of magnitude by the usage of the inversed opal TiO2

films rather than nanocrystalline TiO2 films (Figure 67). Thebetter performance of inversed opal cells was due to the wideand 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.; Gra¨tzel, 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 insetshows an SEM image of TiO2 nanotubes. Reprinted with permissionfrom Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.;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 usingan AM 1.5 simulator (Isc ) 1.8 × 10-7 A/cm2, Voc ) 0.78 V, FF) 0.33). The inset shows an SEM image of an inverse opal TiO2film. (B) Current-voltage (I-V) characteristic of a nanocrystallineTiO2 cell in the dark and under white light illumination using anAM 1.5 simulator (Isc ) 8.5 × 10-9 A/cm2, Voc ) 0.87 V, FF)0.40). Reprinted from Somani, P. R.; Dionigi, C.; Murgia, M.;Palles, D.; Nozar, P.; Ruani, G.Sol. Energy Mater. Sol. Cells2005,87, 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 TiO2 Nanocrystalline Electrode.5.2.1.4.1.Anatase-Rutile TiO2 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.852,853 Figure 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).852 TiO2 I had71% anatase phase with 29% rutile phase, while TiO2 II hadpure anatase phase. The anatase-rutile-based DSSC showedhigher performance (efficiencyη ) 6.8%, short-circuitphotocurrents densityJsc ) 19.4 mA/cm2, open-circuitphotovoltageVOC ) 652 mV, fill factor ff ) 0.53) than thepure TiO2 (η ) 5.3%,Jsc ) 18.4 mA/cm2, VOC ) 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 ofphotoinjected electrons with holes that are transferred to theelectrochemical mediator and (b) the image field opposingthe separation process that is distributed inside the TiO2

nanoporous 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.854 Inthe presence of the buffer layer having 15-75 mol % WO3,

both open-circuit photovoltage and short-circuit photocurrentwere enhanced. In the case of the electrode having a bufferlayer of less than about 10 mol % WO3, due to the largenegativeVFB, a potential barrier to the conduction bandelectrons from TiO2 emerged at the TiO2-WO3/TiO2 junc-tion. This resulted in a drop in photoinjection efficiency andsubsequently in the photocurrent. For electrodes having morethan about 75 mol % WO3, the conduction band edge of thebuffer layer lay close to or lower than that of TCO, and therelative conduction band energy of the buffer layer was notparticularly beneficial for the electron injection from theconduction band of TiO2.854

5.2.1.4.3. Core-Shell Structured Nanocrystalline Elec-trode. Under the operating conditions of a DSSC, theelectrons need to diffuse several micrometers into the TiO2

layer surrounded by electron acceptors at a distance of onlyseveral nanometers. The nanoporous structure of the TiO2

layer provides a large surface area, allowing absorption ofenough dye molecules to achieve significant optical den-sity.10,855 However, the structure also enhances the recom-bination processes and decreases the total conversion effi-ciency of DSSC.856-858 The recombination processes arecompletely prohibited due to the lack of a significant electricfield that could assist the separation of electrons from holesin 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 TiO2

covered with a shell of another metal oxide have been shownto slow the recombination processes by the formation of anenergy barrier at the TiO2 surface.560,648,649,860-865 The con-duction band potential of the shell should be more negativethan that of TiO2 in order to generate an energy barrier forthe reaction of the electrons present in TiO2 with the oxidizeddye or the redox mediator in solution. Two approaches areemployed to fabricate the nanoporous core-shell electrodes.The first approach involves synthesis of core-shell nano-particles that are applied onto the conducting sub-strate.560,648,649,863,866An energy barrier forms not only at theelectrode/electrolyte interface but also between the individualTiO2 nanoparticles. The second approach involves a nano-porous TiO2 electrode coated with the thin shell lay-er.670,860-862,864,865,867The TiO2 nanoparticles are connecteddirectly to each other allowing electron transport throughTiO2.

The approach involving nanoporous electrodes in a well-defined 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 ofthe sputter deposited layer and the nanocrystalline layer are 20 nmand 6µm, 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 electrodesconsisting 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 permissionof The Electrochemical Society, Inc.


nanoporous electrodes could improve the performance ofdye-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 ofthe 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 materialsat the core/shell interface.862

Palomares et al. found that the conformal growth of anoverlayer of Al2O3 on a nanocrystalline TiO2 film resultedin a 4-fold retardation of interfacial charge recombinationand a 30% improvement in photovoltaic device efficiency.870

Fabregat-Santiago et al. found that the alumina barrierreduced the recombination of photoinjected electrons to boththe dye cations and the oxidized redox couple, due to twoeffects: (a) almost complete passivation of surface trap statesin TiO2 that were able to inject electrons to acceptor speciesand (b) slowing down by a factor of 3-4 of the rate ofinterfacial charge transfer from conduction band states.868

O’Regan found that the Al2O3 layer acted as a tunnel barrier,thus increasingVoc 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 435°C.865 The metal oxide overlayers acted as barrier layers forinterfacial electron-transfer processes. The most basic over-layer coating, Al2O3 (pzc ) 9.2), was optimal for retardinginterfacial recombination losses under negative applied bias,with an increase in open-circuit voltage of up to 50 mV anda 35% improvement in overall device efficiency. Diamantet al. found that SrTiO3-coated nanoporous TiO2 electrodesincreased the open circuit photovoltage while reducing theshort circuit photocurrent and resulting in a 15% improve-ment of the overall conversion efficiency of the solar cell.861

The SrTiO3 layer shifted the conduction band of the TiO2 inthe negative direction due to a surface dipole rather thanforming an energy barrier at the TiO2/electrolyte inter-face.861,862The shell having a more negative conduction bandpotential acted as an energy barrier that slowed recombinationreactions. Photoexcitation of dye molecules anchored toultrathin (e1 nm) outer shells of insulators or semiconductorson n-type semiconductor crystallites resulted in electrontransfer to the inner core material.

However, there is still considerable recombination thatincreases with the distance between the electron injectionpoint and the current collector. In other words, the limitedlifetime of the injected electron and the slow diffusion rateinside the porous structure limit the effective thickness ofthe nanoporous electrode. Chappel et al. proposed a electrodedesign, shown in Figure 71, with a core shell configurationbased on a conductive ITO or Sb-doped SnO2 matrix coatedwith TiO2.872 In principle, the conducting core extended thecurrent collector into the nanoporous network and was

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 offour 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 6µmTiO2 matrix in a 5 mMsolution 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. Thiscore shell electrode consists of a conductive nanoporous matrixthat is coated with TiO2. Reprinted with permission from Chappel,S.; Grinis, L.; Ofir, A.; Zaban, A.J. Phys. Chem. B2005, 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 shownby several studies, transport shorter than 1µm 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 TiO2

films.873-878 While 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 oflight.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 IPCErelative to that of a nanocrystalline film of the same overallthickness in the 550-800 nm spectral range. They foundthat the bilayer architecture, rather than enhanced lightharvesting within the inverse opal structures, was responsiblefor the bulk of the gain in the IPCE.333 Figure 72 shows anSEM image of a cross section of the bilayer photoniccrystal-nano-TiO2 photoelectrode.347

Figure 73 shows the sketch for the mechanism of thephotonic crystal in enhancing absorption in certain regimes.347

The fact that light waves were localized in different parts ofthe structure, depending on their energy, implied that anabsorber in the high dielectric medium should interact morestrongly with light at wavelengths to the red of the stop band,and less strongly to the blue. Effectively, the red part of thespectrum of this absorber would “borrow” intensity from theblue part.

Figure 74A shows the effect of the TiO2 photonic crystalas compared to a film of nanocrystalline TiO2 on theabsorption spectra when dye is adsorbed to the surface.347

In a comparison of the spectrum of dye molecules adsorbedto the TiO2 photonic crystal film with that of a conventionalnanocrystalline TiO2 film, there was a substantial enhance-ment absorbance on the red side of the stop band, as well asa slight attenuation of absorbance on the blue side of thestop band. The enhanced absorbance was most pronouncedbetween 500 and 550 nm, but it persisted to a lesser degreeat longer wavelengths. Figure 74B shows the enhancementof the performance of a bilayer electrode compared to aconventional nanocrystalline TiO2 photoelectrode.347Between400 and 530 nm, there was little difference between the twokinds of electrodes. The close similarity in the maximumphotocurrent from the two electrodes was consistent with

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


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 ofthe 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 SolarCell

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.881 The 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-absorbedphotoreceptors, giving rise to energetic electrons. Second,electrons from the photoreceptor excited state were injectedinto the conduction levels of the adjacent conductor, wherethey travelled ballistically through the metal at an energy,1e, above the Fermi energy,Ef. Third, provided that 1e wasgreater than the Schottky barrier height,f, and the carriermean-free path was long compared to the metal thickness,the electrons traversed the metal and entered the conductionlevels of the semiconductor (internal electron emission). Theabsorbed photon energy was preserved in the remainingexcess electron free energy when it was collected at the backohmic contact, giving rise to the photovoltage,V. Thephotooxidized dye was reduced by transfer of thermalizedelectrons from states nearEf in the adjacent metal. Devicesfabricated by using a fluorescein photoreceptor on an Au/TiO2/Ti multilayer structure had typical open-circuit photo-voltages of 600-800 mV and short-circuit photocurrents of10-18 mA cm-2 under 100 mW cm-2 visible light illumina-tion: the internal quantum efficiency (electrons measuredper photon absorbed) was 10%. This alternative approachto photovoltaic energy conversion might provide the basisfor durable low-cost solar cells using a variety of materials.

5.2.3. Doped TiO2 Nanomaterials-Based Solar CellLindgren et al. found that N-doped TiO2 nanocrystalline

porous thin films showed visible light absorption in thewavelength range from 400 to 535 nm and generated anincident photon-to-current efficiency response in good agree-

Figure 73. (A) Simplified optical band structure of a photoniccrystal. Near the Brillouin zone center, light travels with velocityc0/n, wherec0 is the speed of light in a vacuum andn 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 ofthe 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), and thedye adsorbed on a film of nanocrystalline TiO2 (c). The positionof the stop band at 486 nm is indicated by the arrow. (B)Wavelength dependence of the short-circuit photocurrent in thebilayer electrode (a) and the conventional nanocrystalline TiO2photoelectrode (b). The position of the stop band maximum in thebilayer electrode was 610 nm. Reprinted with permission fromNishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk,T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J.J. Am.Chem. Soc.2003, 125, 6306. Copyright 2003 American ChemicalSociety.


ment with the optical spectra.385 For 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 O2

using TiO2 nanomaterials continues to be a dream for cleanand sustainable energy sources.882

Figure 76 shows the principle of water splitting using aTiO2 photocatalyst.761 When 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.883-885 The width of the band gap and the potentialsof the conduction and valence bands are important. Thebottom level of the conduction band has to be more negativethan the reduction potential of H+/H2 (0 V vs NHE), whilethe top level of the valence band has to be more positivethan 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, mobil-ity, and lifetime of photogenerated electrons and holes alsoaffect the photocatalytic properties of TiO2. These factorsare strongly affected by the bulk properties of the materialsuch as crystallinity. Surface properties such as surface states,surface chemical groups, surface area, and active reactionsites are also important.768 The water-splitting process inreturn affects the local pH environment and surface structuresof the TiO2 electrode.769

Salvador conducted a thermodynamic and kinetic consid-eration of water-splitting and competitive reactions in thephotoelectrochemical cell, and they found that the overvolt-age for evolution of O must be minimized, which was onthe order of 0.6 eV for n-TiO2 electrodes loaded withRuO2.767 Cocatalysts such as Pt and NiO are often loadedon the surface in order to introduce active sites for H2

evolution. Thus, suitable bulk and surface properties andenergy structure are demanded for photocatalysts.

Laser-induced photocatalytic oxidation/splitting of waterover TiO2 catalysts was studied.883,886,887Sayama and Ara-kawa found that addition of carbonate salts to Pt-loaded TiO2

suspensions led to highly efficient water splitting.888 Thecarbonate ions affected both the Pt particles and the TiO2

surface. The Pt was covered with some titanium hydroxidecompounds and the rate of the back reaction on the Pt wassuppressed effectively in the presence of carbonate ions. Thecarbonate species aided desorption of O2 from the TiO2

surface.888 Khan and Akikusa found that bare n-TiO2

nanocrystalline film electrodes were unstable during water-splitting reactions under illumination of light and theirstability could be significant improved when covered withMn2O3.759

5.3.2. Use of Reversible Redox Mediators

It has been reported that pure TiO2 could not easily splitwater into H2 and O2 in the simple aqueous suspensionsystem.413,754,889 The main problem is the fast, undesiredelectron-hole recombination reaction.762 Therefore, it isimportant to prevent the electron-hole recombination pro-cess. The Pt-TiO2 system could be illustrated as a “short-circuited” photoelectrochemical cell, where a TiO2 semi-conductor electrode and a platinum-metal counterelectrodeare brought into contact. Well-dispersed metal particles actas miniphotocathodes, trapping electrons, which reduceswater to hydrogen.

The role of sacrificial reagents is shown in Figure 77.761

When the photocatalytic reaction is carried out in aqueoussolutions including easily oxidizable reducing reagents,photogenerated holes irreversibly oxidize the reducingreagents instead of water. This makes the photocatalystelectron-rich, and a H2 evolution reaction is enhanced asshown in Figure 77a. On the other hand, in the presence ofelectron acceptors such as Ag+ and Fe3+, the photogeneratedelectrons in the conduction band are consumed by them andan O2 evolution reaction is enhanced as shown in Figure77b. These reactions using sacrificial reagents are regarded

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,Ef, and the semiconductor band gap,Eg. 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. SurV. Asia2003, 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 O2

evolution (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 TiO2

photocatalysts 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-splittingreaction will take place more efficiently. The advantage ofthis 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 ofIO3

- and I- in a basic aqueous solution. Therefore, anotherundesirable 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 prefer-entially onto the Pt cocatalyst as iodine atom. This iodinelayer effectively suppressed the backward reaction of waterformation from H2 and O2 to H2O over the Pt surface.754

Fujihara et al. studied the photochemical splitting of waterby combining the reduction of water to hydrogen usingbromide ions and the oxidation of water to oxygen usingFeIII ions.892 The bromide ions were oxidized to bromine onPt-loaded TiO2 nanoparticles, and the FeIII ions were reducedto FeII ions on TiO2 nanoparticles. These two reactions werecarried out in separated compartments and combined viaplatinum electrodes and cation-exchange membranes asshown in Figure 79. At the electrodes, FeII ions were oxidizedby bromine, and protons were transported through themembranes to maintain the electrical neutrality and pH ofthe solutions in the two compartments. As a result, waterwas continuously split into hydrogen and oxygen underradiation. The reversible reactions on photocatalysts whichoften suffered from the effects of back reactions were largelyprevented due to the low concentration of the products insolution.

Lee et al. found that a considerable amount of photocata-lytic H2 was produced from water over NiO/TiO2 inproportion to the hole scavenger CN-.890 Galinska andWalendziewski studied water splitting over a Pt-TiO2

catalyst with various sacrificial reagents, such as methanol,Na2S, EDTA, and I- and IO3

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

5.3.3. Use of TiO2 Nanotubes

Mor et al. found that highly ordered TiO2 nanotube arraysefficiently decomposed water under UV radiation.198 Theauthors found that the nanotube wall thickness was a keyparameter influencing the magnitude of the photoanodicresponse and the overall efficiency of the water-splittingreaction. For TiO2 nanotubes with 22-nm pore diameter and34-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 efficiencyof 6.8% as shown in Figure 80B.198,199 They also claimedthat, for illumination at 320-400 nm (98 mW/cm2), the TiO2

nanotube-array photoanodes could generate H2 by H2Ophotoelectrolysis with a photoconversion efficiency of12.25%.212 Park et al. further found that, when doped with

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

Figure 78. Proposed reaction mechanism for overall photocatalyticwater splitting using a IO3-/I- redox mediator and a mixture ofPt-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-xCx 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 Light5.3.4.1. Water Splitting over Doped TiO2 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 (E0 ) 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 shownin Figure 81 are necessary in order to develop materials forsplitting water into H2 and O2 under visible light.761 Thecreated levels have to possess not only the thermodynamicalpotential for oxidation of H2O but also the catalytic propertiesfor the four-electron oxidation reaction. The followingstrategies can be considered for the development of visiblelight-driven photocatalysts: (i) forming a donor level abovea valence band by doping some element into conventionalphotocatalysts 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 afunction of measured potential [vs Ag/AgCl] for 10 V samplesanodized at four temperatures [i.e., 5, 25, 35, and 50°C]. Reprintedwith permission from Mor, G. K.; Shankar, K.; Paulose, M.;Varghese, O. K.; Grimes, C. A.Nano Lett.2005, 5, 191. Copyright2005 American Chemical Society.

Figure 81. Strategy of the development of photocatalysts with avisible light response. Reprinted Figure 6 from Kudo, A.Catal.SurV. Asia 2003, 7, 31, Copyright 2003, with kind permission ofSpringer 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 B4O7

2- environmentwithout 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.529,889 Khan et al. found that a C-doped TiO2

nanocrystalline 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/cm2,476

although there were questions about its solar-to-hydrogenconversion efficiency by other researchers.895-897

Matsuoka et al. developed visible light responsive TiO2

nanocrystalline 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 TiO2.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)3

2+ 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 ofI3

-/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 TiO2

conduction 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 watersplitting under illumination could be achieved with the

combination of single-crystal p-SiC and nanocrystallinen-TiO2 photoelectrodes.899 Both photoelectrodes (p-SiC andn-TiO2) were placed side by side facing the light source andin contact with an electrolyte of 0.5 M H2SO4. The opencircuit potential was found to be 1.24 V between the n-TiO2

and p-SiC photoelectrodes, with a maximum photocurrentdensity of 0.05 mA cm-2 under a closed circuit potential of0.23 V, corresponding to an efficiency of 0.06%. The lowcell photocurrent density and the photoconversion efficiencyfor the p-SiC/n-TiO2 self-driven system for the water-splittingreaction were due to the high band gap energies of bothsemiconductors and high recombination of photogeneratedcarriers mainly in the covalently bonded p-SiC.

Takabayashi et al. proposed a solar water-splitting systembased on a composite polycrystalline-Si/doped TiO2 thin-film electrode for high-efficiency and low-cost by combiningthe advantages of Si and doped TiO2: (1) an n-Si electrodewith surface alkylation and a metal nanodot coating gavean efficient and stable photovoltaic characteristic, and (2)TiO2 doped with other elements, such as nitrogen and sulfur,could cause water photooxidation (oxygen photoevolution)under visible light illumination.770 The structure and workingmechanism of solar water splitting with this system is shownin Figure 83. Although a high solar-to-chemical conversionefficiency of more than 10% was calculated for this system,several major problems needed to be solved before the realdevice could show promising performance.770

5.4. Electrochromic Devices

TiO2 nanomaterials have been widely explored as elec-trochromic devices, such as electrochromic windows anddisplays.611,634,772,900-916 Electrochromism can be defined asthe ability of a material to undergo color change uponoxidation or reduction. Electrochromic devices are able tovary their throughput of visible light and solar radiation uponelectrical charging and discharging using a low voltage. Asmall voltage applied to the windows will cause them todarken; reversing the voltage causes them to lighten. Thus,one can regulate the amount of energy entering through a“smart window” so that the need for air conditioning in acooled building decreases. The energy efficiency inherentin this technology can be large, provided that the controlstrategy is adequate. Additionally, the transmittance regula-tion can impart glare control as well as user control of theindoor environment. The absorbance, rather than the reflec-

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, R.;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 ofLi + 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 nanocrys-talline TiO2 electrodes modified with viologens and/oranthrachinons equipped with a surface anchoringgroup.902-904,906-908,910,917-919 This category usually has fastswitching times and considerable optical dynamics, due tothe combination of good conductivity between the TiO2

nanoparticles and the fast electron exchange between TiO2

and the monolayer of the electrochromic compound coveringeach particle.904 Bach et al. demonstrated high-quality paper-like electrochromic displays based on nanostructured TiO2

films modified with electrochromophores with excellent ink-on-paper optical qualities, fast response times, and low powerconsumption.901 Moeller et al. demonstrated electrochromicpictures with unprecedented resolution (360 dpi) in transpar-ent and reflective electrochromic displays (ECD) based onink-jet printing technology and cascade-type crosslinkingreactions of viologens in the mesopores of a TiO2 electrode,with a completely transparent counterelectrode based onmesoporous antimony tin oxide coated with CeO2.913

5.4.1. Fundamentals of Electrochromic Devices

Figure 84A shows the principle of the electrochromismof a molecular monolayer adsorbed on TiO2.902 A molecule,which functions as the electrochromophore and exhibitsdifferent colors in different oxidation states, must be chosensuch that its redox potential lies above the conduction bandedge of the TiO2 nanocrystalline electrode at the liquid/solidinterface. In this way, electrons can be transferred reversiblyfrom the conduction band to the molecule. The TiO2

electrode in fact behaves like a conductor for the adsorbedelectroactive species. If the redox potential is situated belowthe conduction band edge, the reduction process is irrevers-ible. Figure 84B shows the TiO2 nanocrystalline electro-chromic devices based on viologen (solvent: glutarodinitrile)with a counterelectrode made of Prussian blue.902 The devicecould be switched back and forth between the colorless andthe colored states within 1 s.

The nanocrystalline structure of the TiO2 film makespossible 100- to 1000-fold amplification compared to a flatsurface as shown in Figure 85.902 The combination of highconductivity of the nanocrystalline TiO2 particles, fastelectron exchange with the molecular monolayer, opticalamplification by the porous structure, and fast chargecompensation by ions in the contacting liquid makes thenanocrystalline electrodes highly attractive electrochromicelements. The principle of efficiency relies on fast interfacialelectron transfer between the nanocrystalline TiO2 and theadsorbed modifier as well as on the high surface area of theTiO2 support that amplifies optical phenomena by 2 or 3orders of magnitude.902 The 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 havesimilar conduction band edge energy levels and electronpercolation ability as electrodes made from nanocrystallineTiO2, attributed to the uniform and ordered mesoporearchitecture and the large accessible surface area for tetheringviologen 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, forits remarkable stability in both the oxidized and the reduced(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 reducedN,N′-dialkylviologen in an organic solvent has a maximumaround 600 nm. WithN,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 by ablue coloration with the absorption band at 730 nm.903

Vayssieres et al. studied bis(phthalocyaninato)lutetium(III)complexes (Pc2Lu) as electrochromophores, and they foundthat 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 TiO2

electrode.916

Ag-TiO2 films, prepared by loading nanoporous filmswith Ag nanoparticles by photocatalytic means, exhibitedmulticolor photochromism, which was related to the oxida-tion and reduction of Ag nanoparticles under UV and visibleradiation.773 Please also see section 4.2.1.2 on Sensitizationby Metal Nanoparticles.

5.4.3. Counterelectrode for an Electrochromic DeviceClosed cells are built by combining a transparent nano-

crystalline electrode with a counterelectrode able to provide

Figure 84. (A) Principle of the electrochromism of a molecular monolayer adsorbed on a semiconductor surface. Electrons are injectedfrom the conducting substrate into the conduction band of the semiconductor and from there reduce the adsorbed electroactive molecule.Provided the redox potential of that molecule lies above the conduction band edge, the process is reversible by application of a positivepotential to the conductive substrate. (B) Nanocrystalline electrochromic devices based on viologen (solvent: glutarodinitrile) with acounterelectrode made of Prussian blue, in the colorless and in the colored state. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.;Walder, L.; Gratzel, M. Displays1999, 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 nanocrystallineWO3 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 inγ-butyro-lactone.906 They 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 [â-(10-phenothiazyl)propoxy]phosphonic acid, which displayedcycles-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 fordisplays applications.902 When the two electrodes are short-circuited, the electrons flow from the zinc, which oxidizesto Zn2+ ions, to the viologens of the nanocrystalline electrode.The process can be reversed under a potential of 1-2 V.

5.4.4. Photoelectrochromic Devices

Pichot et al. demonstrated a photoelectrochromic smartwindow with flexible substrates and solid-state electrolytesbased on a dye-sensitized TiO2 electrode spin-coated ontoIn-Sn oxide-coated polyester substrates coupled with a WO3

electrochronic counterelectrode, separated by a cross-linkedpolymer electrolyte containing LiI (Figure 87).634The devicestypically transmitted 75% of visible light in the bleachedstate. After a few minutes of exposure to white light, thewindows turned dark blue, transmitting only 30% of visiblelight. They spontaneously bleached back to their initialnoncolored state upon removal of the light source. Thephotoelectrochromic device ideally behaved like a capaci-tor: There was initially no mobile oxidized species (i.e., I2)present in the electrolyte. A schematic representation of thecomponents and the electron and ion transfers in the solid-state photoelectrochromic device is shown in Figure 87. Theultimate electron acceptor (WO3) is localized as an insolublematerial on the back electrode. Only the electron donor (I-),which serves as a regenerator to the oxidized dye, is initiallypresent in the electrolyte introduced as LiI. Upon colorationof 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 the equivalent ofhundreds of superposed monolayers. Reprinted from Bonhote, P.; Gogniat, E.; Campus, F.; Walder, L.; Gra¨tzel, M. Displays1999, 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. Displays1999, 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∼2.0wt % 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, wherex 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 hydrogenadsorption 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 ofhydrogen 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 proposeddiffusion model.

Recently, Xu et al. studied the hydrogen storage propertiesof a series of five pristine micro- and mesoporous Ti oxidematerials, synthesized from C6, C8, C10, C12, and C14amine templates possessing BET surface areas ranging from643 to 1063 m2/g, and they found that at 77 K the isothermsfor all materials gently rose sharply at low pressure andcontinued to rise in a linear fashion from 10 atm onward to65 atm and then return on desorption without significanthysteresis. Extrapolation to 100 atm could yield total storage

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, J.Inorg. Chem.2005, 44, 4124. Copyright 2005 American ChemicalSociety.

Figure 89. Isotherm for (9) hydrogen sorption into and (O)desorption out of the pores of TiO2 nanotubes at-196°C. Reprintedwith permission from Bavykin, D. V.; Lapkin, A. A.; Plucinski, P.K.; Friedrich, J. M.; Walsh, F. C.J. Phys. Chem. B2005, 109,19422. Copyright 2005 American Chemical Society.


values as high as 5.36 wt % and 29.37 kg/m3, and surfaceTi 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 TiO2

nanotubes.194,196,206-209,211 They 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.196 At24 °C, in response to 1000 ppm of hydrogen, the sensorsshowed a fully reversible change in electrical resistance ofapproximately 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 ofCO and H2 at temperatures above 500°C, but on dopingwith 10% alumina it became selective for hydrogen.922

Shimizu et al. reported that anodized nanoporous titania filmswith a Pd Schottky barrier were sensitive to hydrogen at 250°C.951,952 Kobayashi et al. investigated the mechanism ofhydrogen sensing by Pd/TiO2 Schottky diodes, and theyfound that the formation of adsorbed water from adsorbedoxygen at the Pd/TiO2 interface was the dominant reactionfor the Pd/TiO2(001) diodes throughout the hydrogen con-centration range of 0-3000 ppm; for the Pd/TiO2(100)diodes, this reaction was dominant only for hydrogenconcentrations below 100 ppm and the hydrogen adsorptionon bare Pd atoms became dominant for higher hydrogenconcentrations.939,940Carney et al. found that sensors basedon SnO2-TiO2 with higher surface areas were more sensitiveto H2 in the presence of O2 by measuring the change in theelectrical resistance of the sensor upon exposure to differenthydrogen concentrations under a constant hydrogen gas flowrate.923 Devi et al. found that ordered mesoporous TiO2

exhibited higher H2 and CO sensitivities than sensors madefrom common TiO2 powders due to increased surface area,and the sensitivity could be further improved by loading thesensor 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 UV exposure.The plot, broken into four parts for clarity, shows (a) the original sensor behavior from time 10 to 1000 s, (b) the behavior of the sensorover time 100-6000 s, during which the sensor is contaminated with oil, losing its hydrogen-sensing capabilities, and is initially exposedto UV light, and (c) the behavior of the sensor from time 5000 s to 45,000 s. At time 7000 s, the UV is turned off, with the sensor regainingits nominal starting resistance of approximately 100,000Ω, at which point it is exposed to 1000 ppm hydrogen and its resistance changesby a factor of approximately 50. The sensor is then again exposed to UV, from roughly time 15,000 s to 29,000 s. After this second UVexposure, the sensor is again exposed to 1000 ppm hydrogen, showing an approximate factor of 500 change in electrical resistance. Thesensor is once again exposed to UV, from time 36,000 s. (d) Sensor behavior from time 45,000 to 70,000 s continues with UV exposureof the sensor until time 52,000 s, after which the sensor is repeatedly cycled between air and 1000 ppm hydrogen, showing a relativechange in impedance of approximately 1000×. Compared to the hydrogen sensitivity of a noncontaminated sensor, the relative response ofthe “recovered” sensor is within a factor of 2. Reprinted with permission from Mor, G. K.; Carvalho, M. A.; Varghese, O. K.; Pishko, M.V.; Grimes, C. A.J. Mater. Res.2004, 19, 628. Copyright 2004 Materials Research Society.


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/TiO2

interfaces 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 ofpartial 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 resistance,746

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

Comini et al. found that the sensitivity enhancement towardethanol and methanol of TiO2 films could be improved whendoped with Pt and Nb.926

Benkstein and Semancik found that mesoporous TiO2

nanoparticle thin films prepared on MEMS micro-hot-plateplatforms could be used as high-sensitivity conductometricgas sensor materials.921The nanoparticle films were depositedonto selected micro-hot-plates in a multielement array viamicrocapillary pipet and were sintered using the micro-hot-plate. Figure 91A shows the conductometric response of fourTiO2 nanoparticle films. The relative thickness of the filmswas varied by using one, two, three, or four drops of 6%mass fraction TiO2 to cast the film. Sensitivity was definedas the ratio of the film conductance in the presence of ananalyte to the baseline conductance measured in dry air (S) G/G0). The thicker films showed a higher baselineconductance 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 toµmol/mol levels of methanolof a mesoporous TiO2 nanoparticle film and a CVD TiO2film. The nanoparticle films were found to demonstratehigher sensitivity to target analytes, attributed to the highinternal 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 thevariations in the intensity of light with in humidity changes.750

The sensor element consisted of a thin U-shaped borosil glassrod with a film of TiO2 deposited on it. Both the ends of theglass rod were coupled to optical fibers. Light from a He-Ne laser was launched into the sensing element through oneof them. Light received from the other fiber was fed into anoptical power meter.

6. SummaryOver the past decades, the tremendous effort put into TiO2

nanomaterials has resulted in a rich database for theirsynthesis, properties, modifications, and applications. Thecontinuing breakthroughs in the synthesis and modificationsof TiO2 nanomaterials have brought new properties and newapplications with improved performance. Accompanied bythe progress in the synthesis of TiO2 nanoparticles are newfindings in the synthesis of TiO2 nanorods, nanotubes,nanowires, as well as mesoporous and photonic structures.Besides the well-know quantum-confinement effect, thesenew nanomaterials demonstrate size-dependent as well asshape- and structure-dependent optical, electronic, thermal,and structural properties. TiO2 nanomaterials have continuedto be highly active in photocatalytic and photovoltaicapplications, and they also demonstrate new applicationsincluding electrochromics, sensing, and hydrogen storage.This steady progress has demonstrated that TiO2 nanoma-terials are playing and will continue to play an importantrole in the protections of the environment and in the searchfor renewable and clean energy technologies.

Figure 91. (A) Conductance response of TiO2 nanoparticle filmsto methanol in concentrations going from zero (baseline) to 50µmol/mol in 10µmol/mol steps, sequentially, at a sensor temper-ature of 450°C. Between the steps, the analyte concentration wasreturned to 0µmol/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. AcknowledgmentThe 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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