Titanium Dioxide Nanomaterials Synthesis, Properties, Modifications

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Titanium Dioxide Nanomaterials:Synthesis,Properties,Modifications,and

Applications

Xiaobo Chen*and Samuel S.Mao?

Lawrence Berkeley National Laboratory,and University of California,Berkeley,California94720

Received March27,2006

Contents

1.Introduction2891

2.Synthetic Methods for TiO2Nanostructures2892

2.1.Sol?Gel Method2892

2.2.Micelle and Inverse Micelle Methods2895

2.3.Sol Method2896

2.4.Hydrothermal Method2898

2.5.Solvothermal Method2901

2.6.Direct Oxidation Method2902

2.7.Chemical Vapor Deposition2903

2.8.Physical Vapor Deposition2904

2.9.Electrodeposition2904

2.10.Sonochemical Method2904

2.11.Microwave Method2904

2.12.TiO2Mesoporous/Nanoporous Materials2905

2.1

3.TiO2Aerogels2906

2.14.TiO2Opal and Photonic Materials2907

2.15.Preparation of TiO2Nanosheets2908

3.Properties of TiO2Nanomaterials2909

3.1.Structural Properties of TiO2Nanomaterials2909

3.2.Thermodynamic Properties of TiO2

Nanomaterials

2911

3.3.X-ray Diffraction Properties of TiO2

Nanomaterials

2912

3.4.Raman Vibration Properties of TiO2

Nanomaterials

2912

3.5.Electronic Properties of TiO2Nanomaterials2913

3.6.Optical Properties of TiO2Nanomaterials2915

3.7.Photon-Induced Electron and Hole Properties

of TiO2Nanomaterials

2918

4.Modifications of TiO2Nanomaterials2920

4.1.Bulk Chemical Modification:Doping2921

4.1.1.Synthesis of Doped TiO2Nanomaterials2921

4.1.2.Properties of Doped TiO2Nanomaterials2921

4.2.Surface Chemical Modifications2926

4.2.1.Inorganic Sensitization2926

5.Applications of TiO2Nanomaterials2929

5.1.Photocatalytic Applications2929

5.1.1.Pure TiO2Nanomaterials:First

Generation

2930

5.1.2.Metal-Doped TiO2Nanomaterials:

Second Generation

2930

5.1.3.Nonmetal-Doped TiO2Nanomaterials:

Third Generation 2931

5.2.Photovoltaic Applications2932

5.2.1.The TiO2Nanocrystalline Electrode in

DSSCs

2932

5.2.2.Metal/Semiconductor Junction Schottky

Diode Solar Cell

2938

5.2.3.Doped TiO2Nanomaterials-Based Solar

Cell

2938

5.3.Photocatalytic Water Splitting2939

5.3.1.Fundamentals of Photocatalytic Water

Splitting

2939

e286576414791711cd791702e of Reversible Redox Mediators2939

e286576414791711cd791702e of TiO2Nanotubes2940

5.3.4.Water Splitting under Visible Light2941

5.3.5.Coupled/Composite Water-Splitting

System

2942

5.4.Electrochromic Devices2942

5.4.1.Fundamentals of Electrochromic Devices2943

5.4.2.Electrochromophore for an Electrochromic

Device

2943

5.4.3.Counterelectrode for an Electrochromic

Device

2944

5.4.4.Photoelectrochromic Devices2945

5.5.Hydrogen Storage2945

5.6.Sensing Applications2947

6.Summary2948

7.Acknowledgment2949

8.References2949

1.Introduction

Since its commercial production in the early twentieth

century,titanium dioxide(TiO2)has been widely used as a

pigment1and in sunscreens,2,3paints,4ointments,toothpaste,5

etc.In1972,Fujishima and Honda discovered the phenom-

enon of photocatalytic splitting of water on a TiO2electrode

under ultraviolet(UV)light.6-8Since then,enormous efforts

have been devoted to the research of TiO2material,which

has led to many promising applications in areas ranging from

photovoltaics and photocatalysis to photo-/electrochromics

and sensors.9-12These applications can be roughly divided

into“energy”and“environmental”categories,many of which

depend not only on the properties of the TiO2material itself

but also on the modifications of the TiO2material host(e.g.,

with inorganic and organic dyes)and on the interactions of

TiO2materials with the environment.

An exponential growth of research activities has been seen

in nanoscience and nanotechnology in the past decades.13-17

New physical and chemical properties emerge when the size

of the material becomes smaller and smaller,and down to

*Corresponding author.E-mail:XChen3@e286576414791711cd791702.?E-mail:SSMao@e286576414791711cd791702.

2891 Chem.Rev.2007,107,2891?2959

10.1021/cr0500535CCC:$65.00?2007American Chemical Society

Published on Web06/23/2007

the nanometer scale.Properties also vary as the shapes of the shrinking nanomaterials change.Many excellent reviews and reports on the preparation and properties of nanomaterials have been published recently.6-44Among the unique proper-ties of nanomaterials,the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement,and the transport proper-ties related to phonons and photons are largely affected by the size and geometry of the materials.13-16The specific surface area and surface-to-volume ratio increase dramati-cally as the size of a material decreases.13,21The high surface area brought about by small particle size is beneficial to many TiO 2-based devices,as it facilitates reaction/interaction between the devices and the interacting media,which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material.Thus,the performance of TiO 2-based devices is largely influenced by the sizes of the TiO 2building units,apparently at the nanometer scale.As the most promising photocatalyst,7,11,12,33TiO 2mate-rials are expected to play an important role in helping solve

many serious environmental and pollution challenges.TiO 2also bears tremendous hope in helping ease the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices.9,31,32As continued breakthroughs have been made in the preparation,modifica-tion,and applications of TiO 2nanomaterials in recent years,especially after a series of great reviews of the subject in the 1990s.7,8,10-12,33,45we believe that a new and compre-hensive review of TiO 2nanomaterials would further promote TiO 2-based research and development efforts to tackle the environmental and energy challenges we are currently facing.Here,we focus on recent progress in the synthesis,properties,modifications,and applications of TiO 2nanomaterials.The syntheses of TiO 2nanomaterials,including nanoparticles,nanorods,nanowires,and nanotubes are primarily categorized with the preparation method.The preparations of mesopo-rous/nanoporous TiO 2,TiO 2aerogels,opals,and photonic materials are summarized separately.In reviewing nanoma-terial synthesis,we present a typical procedure and repre-sentative transmission or scanning electron microscopy images to give a direct impression of how these nanomate-rials are obtained and how they normally appear.For detailed instructions on each synthesis,the readers are referred to the corresponding literature.

The structural,thermal,electronic,and optical properties of TiO 2nanomaterials are reviewed in the second section.As the size,shape,and crystal structure of TiO 2nanomate-rials vary,not only does surface stability change but also the transitions between different phases of TiO 2under pressure or heat become size dependent.The dependence of X-ray diffraction patterns and Raman vibrational spectra on the size of TiO 2nanomaterials is also summarized,as they could help to determine the size to some extent,although correlation of the spectra with the size of TiO 2nanomaterials is not straightforward.The review of modifications of TiO 2nanomaterials is mainly limited to the research related to the modifications of the optical properties of TiO 2nanoma-terials,since many applications of TiO 2nanomaterials are closely related to their optical properties.TiO 2nanomaterials normally are transparent in the visible light region.By doping or sensitization,it is possible to improve the optical sensitiv-ity and activity of TiO 2nanomaterials in the visible light region.Environmental (photocatalysis and sensing)and energy (photovoltaics,water splitting,photo-/electrochromics,and hydrogen storage)applications are reviewed with an emphasis on clean and sustainable energy,since the increas-ing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions.The fundamentals and working principles of the TiO 2nanoma-terials-based devices are discussed to facilitate the under-standing and further improvement of current and practical TiO 2nanotechnology.

2.Synthetic Methods for TiO 2Nanostructures

2.1.Sol ?Gel Method

The sol -gel method is a versatile process used in making various ceramic materials.46-50In a typical sol -gel process,a colloidal suspension,or a sol,is formed from the hydrolysis and polymerization reactions of the precursors,which are usually inorganic metal salts or metal organic compounds such as metal e286576414791711cd791702plete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase.Thin films can be produced on a piece of

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

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

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

Mao

substrate by spin-coating or dip-coating.A wet gel will form when the sol is cast into a mold,and the wet gel is converted into a dense ceramic with further drying and heat treatment.A highly porous and extremely low-density material called an aerogel is obtained if the solvent in a wet gel is removed under a supercritical condition.Ceramic fibers can be drawn from the sol when the viscosity of a sol is adjusted into a proper viscosity range.Ultrafine and uniform ceramic powders are formed by precipitation,spray pyrolysis,or emulsion techniques.Under proper conditions,nanomaterials can be obtained.

TiO 2nanomaterials have been synthesized with the sol -gel method from hydrolysis of a titanium precusor.51-78This process normally proceeds via an acid-catalyzed hydrolysis step of titanium(IV)alkoxide followed by condensa-tion.51,63,66,79-91The development of Ti -O -Ti chains is favored with low content of water,low hydrolysis rates,and excess titanium alkoxide in the reaction mixture.Three-dimensional polymeric skeletons with close packing result from the development of Ti -O -Ti chains.The formation of Ti(OH)4is favored with high hydrolysis rates for a medium amount of water.The presence of a large quantity of Ti -OH and insufficient development of three-dimensional polymeric skeletons lead to loosely packed first-order particles.Polymeric Ti -O -Ti chains are developed in the presence of a large excess of water.Closely packed first-order particles are yielded via a three-dimensionally devel-oped gel skeleton.51,63,66,79-91From the study on the growth kinetics of TiO 2nanoparticles in aqueous solution using titanium tetraisopropoxide (TTIP)as precursor,it is found that the rate constant for coarsening increases with temper-ature due to the temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO 2.63Second-ary particles are formed by epitaxial self-assembly of primary particles at longer times and higher temperatures,and the number of primary particles per secondary particle increases with time.The average TiO 2nanoparticle radius increases linearly with time,in agreement with the Lifshitz -Slyozov -Wagner model for coarsening.63

Highly crystalline anatase TiO 2nanoparticles with different sizes and shapes could be obtained with the polycondensation of titanium alkoxide in the presence of tetramethylammonium hydroxide.52,62In a typical procedure,titanium alkoxide is added to the base at 2°C in alcoholic solvents in a three-neck flask and is heated at 50-60°C for 13days or at 90-100°C for 6h.A secondary treatment involving autoclave heating at 175and 200°C is performed to improve the crystallinity of the TiO 2nanoparticles.Representative TEM images are shown in Figure 1from the study of Chemseddine et al.52

A series of thorough studies have been conducted by Sugimoto et e286576414791711cd791702ing the sol -gel method on the formation of TiO 2nanoparticles of different sizes and shapes by tuning the reaction parameters.67-71Typically,a stock solution of a 0.50M Ti source is prepared by mixing TTIP with triethanolamine

(TEOA)([TTIP]/[TEOA])1:2),followed The diluted with a

shape controller solution °C for 1day and at 140°C for

3days.The pH of the solution

can be tuned by adding HClO 4or NaOH solution.Amines are used as the shape controllers of the TiO 2act as surfactants.amines include TEOA,and triethyl-enetetramine.The morphology of the TiO 2nanoparticles changes from cuboidal to ellipsoidal at pH above 11with TEOA.The TiO 2into ellipsoidal above pH 9.5with diethylenetriamine with a higher aspect ratio than that with TEOA.Figure 2shows representative TEM images of the TiO 2nanoparticles under different initial pH conditions with the shape control of TEOA at [TEOA]/[TIPO])2.0.Secondary amines,such as diethylamine,and tertiary amines,such as trimethylamine and triethylamine,act as complexing agents of Ti(IV)ions promote the ratios.The shape of the TiO 2nanoparticle can also be tuned from round-cornered cubes to sharp-edged cubes with sodium oleate and sodium stearate.70The shape control is attributed to the tuning of the growth rate of the different crystal planes of TiO 2nanoparticles by the to these planes under different pH conditions.70

A prolonged heating time below 100°C for the as-prepared be used to avoid the the TiO 2nano-during the crystallization process.58,72By heating amorphous TiO 2in ana-2with average particle sizes between 7and 50nm can be obtained,as reported by Zhang and Banfield.73-77Much effort has been exerted to achieve highly crystallized and narrowly dispersed TiO 2nanoparticles using the sol -gel method with other modifications,such as a semicontinuous reaction method by Znaidi et al.78and a two-stage mixed method and a continuous reaction method by Kim et al.53,54

By a combination of the sol -gel method and an anodic alumina membrane (AAM)template,TiO 2nanorods have dipping porous into a boiled TiO 2sol followed processes.92,93In a typical experiment,a TiO 2sol solution is prepared by mixing TTIP dissolved in ethanol with a solution containing water,acetyl acetone,and ethanol.AAM is immersed into the boiled it is dried in air and calcined at 400°C for 10h.The AAM template is removed in a 10wt %H 3PO 4aqueous solution.The calcination temperature can be used to control the crystal phase of the TiO 2nanorods.At low temperature,anatase nanorods can be obtained,while at high temperature rutile nanorods can be obtained.The pore size of the AAM template can be used to control the size of these TiO 2nanorods,which typically range from 100to 300nm in diameter and several micrometers in length.Appar-ently,the size distribution of the final TiO 2nanorods is largely controlled by the size distribution of the pores of the AAM template.In order to obtain smaller and mono-sized TiO 2nanorods,it is necessary fabricate high-quality AAM templates.Figure 3shows a for TiO 2nanorods fabricated with this method.Normally,the TiO 2nanorods are composed of small TiO 2nanoparticles or electrophoretic deposition of TiO 2colloidal suspensions into 2nanowire arrays can be obtained.94In a typical procedure,in ethanol at room temperature,with deionized water and ethanol 2-3with nitric acid.is used as the anode,and an AAM with attached to Cu foil is used as the cathode.A TiO 2sol is deposited into the pores of the AMM under a voltage of 2-5V and annealed at 500°C for 24h.After the AAM a 5wt %NaOH solution,isolated TiO 2nanowires are obtained.In order to

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72893

fabricate TiO 2nanowires instead of nanorods,an AAM with

long pores is a must.

TiO 2nanotubes can also be obtained using the sol -gel

method by templating with an AAM 95-98and other organic

compounds.99,100For example,when an AAM is used as the

template,a thin layer of TiO 2sol on the wall of the pores of the AAM is first prepared by sucking TiO 2sol into the pores of the AAM and removing

it under vacuum;TiO 2nanowires are obtained after the sol is fully developed and the AAM is removed.In the procedure by Lee and co-workers,96a TTIP solution was prepared by mixing TTIP with 2-propanol and 2,4-pentanedione.the AAM was dipped into this Figure 1.TEM images of TiO 2nanoparticles prepared by hydrolysis of Ti(OR)4in the presence of tetramethylammonium hydroxide.Reprinted with permission from Chemseddine,A.;Moritz,T.Eur.J.Inorg.Chem.1999,235.Copyright 1999Wiley-VCH.

2894Chemical Reviews,2007,Vol.107,No.7Chen and Mao

solution,it was removed from the solution and placed under vacuum until the entire volume of the solution was pulled through the AAM.The AAM

hydrolyzed by water vapor over a HCl solution for 24h,and then calcined in

a furnace at 673K for 2h and cooled to room temperature

temperature ramp of

2°C/h.Pure TiO 2nanotubes were dissolved in a 6M NaOH solution for several minutes.96Alternatively,TiO 2nanotubes could be obtained by coating the AAM membranes at 60°C for a certain period of time (12-48h)4under pH )2.1and removing the AAM after TiO 2nanotubes were developed.97Figure 4shows a typical SEM image of the 2array from the AAM template.97

In another scheme,a ZnO nanorod array on a glass substrate can be used as a template to fabricate TiO 2nanotubes with the sol -gel method.101Briefly,TiO 2sol is

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

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

Figure 3.TEM image of anatase nanorods and a single nanorod composed of small TiO 2nanoparticles or nanograins (inset).Reprinted from Miao,L.;Tanemura,S.;Toh,S.;Kaneko,K.;Tanemura,M.J.Cryst.Growth 2004,264,246,Copyright 2004,with permission from Elsevier.

Figure 4.SEM image of TiO 2nanotubes prepared from the AAO template.Reprinted with permission from Liu,S.M.;Gan,L.M.;Liu,L.H.;Zhang,W.D.;Zeng,H.C.Chem.Mater.2002,14,1391.Copyright 2002American Chemical Society.

Figure 5.SEM of a TiO 2nanotube array;the inset shows the ZnO nanorod array template.Reprinted with permission from Qiu,J.J.;Yu,W.D.;Gao,X.D.;Li,X.M.Nanotechnology 2006,17,4695.Copyright 2006IOP Publishing Ltd.

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72895

directed outward toward the nonaqueous media.There is no obvious CMC for reverse micelles,because the number of aggregates is usually small and they are not sensitive to the surfactant concentration.Micelles are often

globular roughly spherical in shape,but ellipsoids,cylinders,and bilayers are also possible.The shape of a

micelle is a function

molecular geometry of its surfactant molecules and surfactant concentration,tem-perature,pH,and ionic strength.

Micelles and inverse micelles are commonly employed to synthesize TiO 2nanomaterials.102-110A statistical experi-mental design method conducted by Kim et al.to for the preparation of TiO 2nanoparticles.103The values of H 2O/surfactant,H 2O/titanium precursor,ammonia concentration,feed rate,and reaction temperature were significant TiO 2nanoparticle size and size distribution.Amorphous TiO 2nanoparticles with diameters of 10-20nm were synthesized and converted to the anatase phase at 600°C and to the more thermodynamically stable rutile phase at 900°C.Li et al.developed TiO 2nanoparticles with the chemical reactions between TiCl 4solution and ammonia in a reversed micro-emulsion system consisting of cyclohexane,poly(oxyethyl-5ether,and poly(oxyethylene)9nonyle phenol ether.104The produced amorphous TiO 2nanoparticles transformed into anatase when heated at temperatures from 200to 750°C and into rutile at temperatures higher than 750°Agglomeration and growth also occurred at elevated Shuttle-like crystalline TiO 2nanoparticles were synthesized by Zhang et al.with hydrolysis of titanium tetrabutoxide in the presence of acids (hydrochloric acid,nitric acid,sulfuric acid,and phosphoric acid)in NP-5(Igepal CO-520)-room temperature.110The particle size of the TiO 2nanoparticles were largely controlled by the reaction condi-tions,and the key factors affecting the formation of rutile at room temperature included the acidity,the type of acid used,and the microenvironment of the reverse micelles.Ag-glomeration of the particles occurred with prolonged reaction times and increasing the [H 2O]/[NP-5]and [H 2O]/[Ti-(OC 4H 9)4]ratios.When suitable acid was applied,round TiO 2nanoparticles could also be obtained.Representative TEM images of the shuttle-like and round-shaped TiO 2nanopar-ticles are In the study carried out by Lim et al.,TiO 2nanoparticles were prepared by the controlled hydrolysis of TTIP in reverse micelles formed in CO 2with the surfactants ammonium carboxylate perfluoropolyether (PFPECOO -4+ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl meth-acrylate)(PDMAEMA-b-PFOMA).106It was found that the crystallite size prepared in the presence of reverse micelles increased as either the water to surfactant or the precursor to The TiO 2nanomaterials prepared with the above micelle and reverse micelle methods normally have amorphous structure,and calcination is usually necessary in order to induce high crystallinity.However,this process usually leads to the growth and agglomeration of TiO 2nanoparticles.The crystallinity of TiO 2nanoparticles initially (synthesized by controlled hydrolysis of titanium alkoxide in reverse micelles in a hydrocarbon solvent)could be improved by annealing in the presence of the micelles at temperatures considerably lower than those required for the traditional calcination

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

2.3.Sol Method

The sol method here refers to the nonhydrolytic sol -gel processes and usually involves the reaction of titanium chloride with a variety of different oxygen donor molecules,Figure 6.TEM images of the shuttle-like and round-shaped (inset)TiO 2nanoparticles.From:Zhang,D.,Qi,L.,Ma,J.,Cheng,H.J.Mater.Chem.2002,12,3677(e286576414791711cd791702/10.1039/b206996b).s Reproduced by permission of The Royal Society of Chemistry.

Figure 7.HRTEM images of a TiO 2nanoparticle 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 2002American Chemical Society.

TiX 4+Ti(OR)4f 2TiO 2+4RX (1)TiX 4+2ROR f TiO 2+4RX

(2)

2896Chemical Reviews,2007,Vol.107,No.7Chen and Mao

The condensation between Ti -Cl and Ti-OR leads to the formation of Ti -O -Ti bridges.The

alkoxide groups can be provided by titanium alkoxides or

by reaction of the titanium chloride with alcohols or ethers.In the method by Trentler and

Colvin,119a metal alkoxide was rapidly injected into the hot solution of titanium halide mixed with trioctylphosphine oxide (TOPO)

heptadecane at 300°C dry inert gas protection,completed alkyl substituents including rate of R,while average particle sizes were relatively unaffected.Variation of X yielded a clear trend in average particle size,but without a discernible trend in reaction rate.Increased nucleophilicity resulted in Average sizes ranged from 9.2nm for TiF 4to 3.8nm for TiI 4.The amount of passivating agent influenced the chemistry.was slower and resulted in smaller particles,while reactions without TOPO were much quicker and yielded mixtures of brookite,and anatase with average particle sizes nm.Figure 8shows typical TEM images of TiO 2nanocrystals developed by Trentler et al.119

In the method used by Niederberger and Stucky,111TiCl 4was slowly added to anhydrous benzyl alcohol vigorous stirring at 40-150°C for 1-21days in the reaction vessel.The precipitate was calcinated at 450°C for 5h after thoroughly washing.The reaction between TiCl 4and benzyl alcohol was found suitable for the synthesis of highly crystalline anatase phase TiO 2nanoparticles with nearly uniform size and shape at very low temperatures,such as 40°C.The particle size could be selectively adjusted in the range of 4-8nm with the appropriate thermal conditions and a proper choice of the and titanium tetrachloride.The particle growth depended strongly on temperature,and lowering the titanium tetrachloride led to a considerable 111

Surfactants have been widely used in the preparation of a variety of nanoparticles with good size distribution and Adding different surfactants acetic acid and acetylacetone,can help synthesize monodispersed TiO 2nanoparticles.120,121For example,Scolan and Sanchez found that monodisperse nonaggregated TiO 2nanoparticles in the 1-5nm range were obtained through hydrolysis of titanium butoxide in the presence of acetylacetone and p -toluenesulfonic acid at 60°C.120The dispersed in water -alcohol or alcohol solutions at concentrations higher than 1M without aggregation,which is attributed to the complexation of the surface by acetylacetonato ligands hybrid made with acetylacetone,p -toluenesulfonic acid,and wa-ter.120

With the aid of surfactants,different sized and shaped TiO 2nanorods can be synthesized.122-130For example,the growth of high-aspect-ratio anatase TiO 2nanorods has been reported by Cozzoli and co-workers by controlling the hydrolysis process of TTIP in 122-126,130Typically,TTIP was added into 80-100°C under inert gas protection (nitrogen flow)and stirred for 5min.A 0.1-2M aqueous base solution was then rapidly injected and kept at with stirring.The bases employed included organic amines,such as trimethylamino-N-oxide,trimethylamine,tetramethylammonium hydroxide,tetrabut-ylammonium hydroxyde,triethylamine,and tributylamine.precursor with the carboxylic acid,the hydrolysis rate of titanium alkoxide (in 4-6h)crystal-lization in mild conditions was promoted with the use of suitable catalysts (tertiary amines or quaternary ammonium hydroxides).A kinetically overdriven growth mechanism led 2Typical TEM images of the TiO 2nanorods are shown in Figure 9.123

Recently,Joo et al.127and Zhang et al.129reported similar procedures in obtaining TiO 2nanorods without the use of catalyst.Briefly,a mixture of TTIP and OA was used to generate OA complexes of titanium at 80°C 1-octadecene.

Figure 8.TEM image of TiO 2nanoparticles derived from reaction of TiCl 4and TTIP in TOPO/heptadecane at 300°C.The inset shows a HRTEM image of a single particle.Reprinted with permission from Trentler,T.J.;Denler,T.E.;Bertone,J.F.;Agrawal,A.;Colvin,V.L.J.Am.Chem.Soc.1999,121,1613.Copyright 1999American Chemical Society.

Figure 9.TEM of TiO 2nanorods.The inset shows a HRTEM of a TiO 2nanorod.Reprinted with permission from Cozzoli,P.D.;Kornowski,A.;Weller,H.J.Am.Chem.Soc.2003,125,14539.Copyright 2003American Chemical Society.

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72897

The injection of a

predetermined amount of oleylamine at 260°C led

to various sized TiO 2nanorods.129Figure 10shows TEM

images of TiO 2nanorods with various lengths,and 2.3nm

TiO 2nanoparticles prepared with this method.129In the surfactant-mediated shape evolution of TiO 2nano-crystals in nonaqueous media conducted by Jun et al.,128it was found that the shape of TiO 2nanocrystals could be modified by changing the surfactant concentration.The synthesis was accomplished by an alkyl halide elimination reaction between titanium chloride and titanium isopro-poxide.a dioctyl ether solution containing TOPO by addition of under vigorous stirring.The reaction was initiated by the rapid injection of TTIP and quenched with cold toluene.At low lauric acid concentrations,bullet-and diamond-shaped nanocrystals were obtained;at higher concentrations,rod-shaped nanocrystals or a mixture of nanorods and branched nanorods was observed.The bullet-and diamond-shaped nanocrystals and nanorods were elon-gated along the [001]directions.The TiO 2nanorods were found to simultaneously convert to small nanoparticles as a function of the growth time,as shown in Figure 11,due to the minimization of the overall surface energy via dissolution and regrowth of monomers during an Ostwald ripening.

2.4.Hydrothermal Method

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

liners under controlled temperature and/or pressure with the reaction in aqueous solutions.The temperature can be elevated above the boiling point of water,reaching the pressure of vapor saturation.The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced.It is a method that is widely used for the production of small particles in the ceramics industry.Many groups have used the hydrothermal method to TiO 2nanoparticles.131-140For example,TiO 2nanoparticles can be obtained by hydrothermal treatment of peptized a titanium precursor with water.134The prepared by adding a 0.5M isopropanol solution of titanium butoxide into deionized water ([H 2O]/1h in the presence of tetraalkylammonium hydroxides (peptizer).After 2h,the as-obtained powders were washed with deionized water and absolute ethanol and then dried at 60°C.Under the same concentration of peptizer,the particle size decreased with increasing The peptizers and their of the particles.Typical TEM images of TiO 2nanoparticles made with the hydrothermal method are shown in Figure 12.134

In another example,TiO 2nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic ethanol -water solution.132Briefly,TTIP was added dropwise to a mixed ethanol and water solution at pH 0.7with nitric acid,and reacted at 240°C for 4h.The TiO 2nanoparticles

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

2898Chemical Reviews,2007,Vol.107,No.7Chen and Mao

synthesized under this acidic ethanol -water environment were mainly primary structure in the anatase phase without secondary structure.The sizes of the particles were controlled to the range of 7-25nm by adjusting the concentration of Ti precursor and the composition of the solvent system.Besides TiO 2nanoparticles,TiO 2nanorods have also been synthesized with the hydrothermal method.141-146Zhang et al.obtained TiO 2nanorods by treating a dilute TiCl 4solution at 333-423K for 12h in the presence of acid or inorganic salts.141,143-146Figure 13shows a typical TEM image of the TiO 2nanorods prepared with the hydrothermal method.141The morphology of the resulting nanorods can be tuned with different surfactants 146or by changing the solvent composi-tions.145A film of assembled TiO 2nanorods deposited on a glass wafer was reported by Feng et al.142These TiO 2nanorods were prepared at 160°C for 2h by hydrothermal treatment of a titanium trichloride aqueous solution super-

saturated with NaCl.2also been successfully obtained with the hydrothermal method by various groups.147-151Typically,TiO 2nanowires are obtained by treating TiO 2white powders in a 10-15M NaOH aqueous solution at 150-200°C for 24-72h without stirring within an autoclave.Figure 14shows the SEM images of TiO 2nanowires and a TEM image of a single nanowire prepared by Zhang and co-workers.150TiO 2nanowires can also be prepared from layered titanate

particles using the hydrothermal method as reported by Wei Figure 11.Time dependent shape evolution of TiO 2nanorods:(a)0.25h;(b)24h;(c)48h.Scale bar )50nm.Reprinted with permission from Jun,Y.W.;Casula,M.F.;Sim,J.H.;Kim,S.Y.;Cheon,J.;Alivisatos,A.P.J.Am.Chem.Soc.2003,125,15981.Copyright 2003American Chemical Society.

Figure 12.TEM images of TiO 2nanoparticles prepared by the hydrothermal method.Reprinted from Yang,J.;Mei,S.;Ferreira,J.M.F.Mater.Sci.Eng.C 2001,15,183,Copyright 2001,with permission from Elsevier.Figure 13.TEM image of TiO 2nanorods prepared with the hydrothermal method.Reprinted with permission from Zhang,Q.;Gao,e286576414791711cd791702ngmuir 2003,19,967.Copyright 2003American Chemical Society.Titanium Dioxide Nanomaterials

Chemical Reviews,2007,Vol.107,No.72899

et al.152In their experiment,layer-structured Na 2Ti 3O 7was dispersed into a 0.05-0.1M HCl solution and kept at 140-170°C for 3-7days in an autoclave.TiO 2nanowires were obtained after the product was washed with H 2O and finally dried.In the formation of a TiO 2nanowire from layered H 2Ti 3O 7,there are three steps:(i)the exfoliation of layered

the nanosheets formation;and (iii)the nanow-152In Na 2Ti 3O 7,[TiO 6held by strong static interaction between between 6and the [TiO 6]unit.When the larger H 3+O cations replace the Na +cations in the interlayer space of [TiO 6]sheets,this static interaction is weakened because the interlayer distance is enlarged.As a result,the layered compounds Na 2Ti 3O 7are gradually exfoliated.When Na +is exchanged by H +in the dilute HCl solution,numerous H 2Ti 3O 7sheet-shaped products are formed.Since the nanosheet does not have inversion sym-metry,intrinsic tension exists.The nanosheets split to form the strong stress and lower the total energy.152A representative TEM image of TiO 2nanowires from Na 2Ti 3O 7is shown in Figure 15.152

The hydrothermal method has been widely used to prepare TiO 2nanotubes since it was introduced by Kasuga et al.in 1998.153-175Briefly,TiO 2powders are put into a 2.5-20M NaOH aqueous solution and held at 20-110°C for 20h in an autoclave.TiO 2nanotubes are obtained after the products are washed with a dilute HCl aqueous solution and distilled water.They proposed the following formation process of TiO 2nanotubes.154When the raw TiO 2material was treated with NaOH aqueous of the Ti -O -Ti bonds were broken and Ti -O -Na and Ti -OH bonds were formed.New Ti -O -Ti bonds were formed after the Ti -O -Na and Ti -OH bonds reacted with acid and water when the material was treated with an aqueous HCl solution and distilled water.The Ti -OH bond could form a sheet.Through the dehydra-tion of Ti -OH bonds by HCl aqueous solution,Ti -O -Ti or Ti -O -H -O -Ti hydrogen bonds were generated.The bond distance from one Ti to the next Ti on the surface decreased.This resulted in the folding of the sheets and the

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

Figure 14.SEM images of TiO 2nanowires with the inset showing a TEM image of a single TiO 2nanowire with a [010]selected area electron diffraction (SAED)recorded perpendicular to the long axis of the wire.Reprinted from Zhang,Y.X.;Li,G.H.;Jin,Y.X.;Zhang,Y.;Zhang,J.;Zhang,L.D.Chem.Phys.Lett.2002,365,300,Copyright 2002,with permission from Elsevier.

Figure 15.TEM images of TiO 2nanowires made from the layered Na 2Ti 3O 7particles,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 TiO 2nanotubes.Reprinted with permission from Kasuga,T.;Hiramatsu,M.;Hoson,A.;Sekino,T.;Niihara,e286576414791711cd791702ngmuir 1998,14,3160.Copyright 1998American Chemical Society.

2900Chemical Reviews,2007,Vol.107,No.7Chen and Mao

17,that TiO 2nanotubes were formed by rolling up the single-layer TiO 2sheets with a rolling-up vector of [001]and attracting other sheets to surround the tubes.172Bavykin and co-workers suggested that the mechanism of nanotube formation involved the

wrapping of multilayered nanosheets rather than

followed crystallization of successive layers.156mechanism

of TiO 2nanotubes involved several steps.176During the reaction with NaOH,the Ti -O -Ti bonding between the basic building blocks of the anatase phase,the octahedra,was broken and a zigzag structure formed when the free octahedras the Ti ions with the formation of hydroxy bridges,leading to the growth along the [100]direction of the anatase phase.Two-dimensional crystalline sheets formed from the lateral growth of the formation of the Ti centers (Ti -O -Ti bonds)in the and rolled up in order to saturate these dangling bonds from the surface and lower the total energy,of TiO 2nanotubes.176

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

synthesis of a variety of nanoparticles with narrow size distribution and dispersity.177-179The solvothermal method has been employed to synthesize TiO 2nanoparticles and nanorods with/without the aid of surfactants.177-185For example,in a typical procedure by Kim and co-workers,184TTIP was mixed with toluene at the weight ratio of 1-3:10and kept at 250°C particle size of TiO 2powders tended to increase as the composition of TTIP in the solution increased in the range of weight ratio of 1-3:10,while the pale crystalline phase of TiO 2was not produced at 1:20and 2:5weight ratios.184By controlling the hydro-lyzation reaction of Ti(OC 4H 9)4and linoleic acid,redispers-2nanoparticles and by Li et al.recently.177The decomposition of NH 4-HCO 3could provide H 2O for the hydrolyzation reaction,and linoleic acid could act as the solvent/reagent and coordination surfactant in the synthesis of nanoparticles.Triethylamine a catalyst for the polycondensation of O -Ti inorganic network to and had little influence on the products’morphology.The chain lengths of the carboxylic acids had a great influence on the formation of TiO 2,and long-chain organic acids were important and necessary in the formation of TiO 2.177Figure 18shows a representative TEM image of TiO 2nanoparticles from their study.177

TiO 2nanorods with narrow size distributions can also be developed with the solvothermal method.177,183For example,in a typical synthesis from Kim et al.,TTIP was dissolved in anhydrous toluene with OA as a surfactant and kept at 250°C for 20h in an autoclave without stirring.183Long dumbbell-shaped nanorods were formed when a sufficient amount of TTIP or surfactant was added to the solution,due to the oriented growth of particles along the [001]axis.At a fixed precursor to surfactant weight ratio of 1:3,the concentration of rods in the nanoparticle assembly increased as the concentration of the titanium precursor in the solution increased.The average particle size was smaller and the size distribution was narrower than is the case for particles synthesized without surfactant.The crystalline phase,diam-eter,and length of these nanorods are largely influenced by the precursor/surfactant/solvent weight ratio.Anatase nano-

Figure 17.(a)HRTEM images of TiO 2nanotubes.(b)Cross-sectional view of TiO 2nanotubes.Reused with permission from B.D.Yao,Y.F.Chan,X.Y.Zhang,W.F.Zhang,Z.Y.Yang,N.Wang,Applied Physics Letters 82,281(2003).Copyright 2003,American Institute of Physics.

Figure 18.TEM micrographs of TiO 2nanoparticles prepared with the solvothermal method.Reprinted with permission from Li,X.L.;Peng,Q.;Yi,J.X.;Wang,X.;Li,Y.D.Chem.s Eur.J.2006,12,2383.Copyright 2006Wiley-VCH.

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

phase only existed in heterojunctions between single-crystal TiO 2nanowires.180

2.6.Direct Oxidation Method

TiO 2nanomaterials can be obtained by oxidation of titanium metal using oxidants or under anodization.Crystal-line TiO 2nanorods have been obtained by direct oxidation of a titanium metal plate with hydrogen peroxide.186-191Typically,TiO 2nanorods on a Ti plate are obtained when a cleaned Ti plate is put in 50mL of a 30wt %H 2O 2solution at 353K for 72h.The formation of crystalline TiO 2occurs through a dissolution precipitation mechanism.By the addition of inorganic salts of NaX (X )F -,Cl -,and SO 42-),the crystalline phase of TiO 2nanorods can be controlled.The addition of F -and SO 42-helps the formation of pure anatase,while the addition of Cl -favors the formation of rutile.189Figure 21shows a typical SEM image of TiO 2nanorods prepared with this method.186

At high temperature,acetone can be used as a good oxygen source and for the preparation of TiO 2nanorods by

oxidizing

Figure

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

Figure 20.TEM images of TiO 2nanowires synthesized by the solvothermal method.From:Wen,B.;Liu,C.;Liu,Y.New J.Chem.2005,29,969(e286576414791711cd791702/10.1039/b502604k)s Reproduced by permission of The Royal Society of Chemistry (RSC)on behalf of the Centre National de la Recherche Scientifique (CNRS).

2902Chemical Reviews,2007,Vol.107,No.7Chen and Mao

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

2.7.Chemical Vapor Deposition

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

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

Thick crystalline TiO 2films with grain sizes below 30nm as well as TiO 2nanoparticles with sizes below 10nm can be prepared by pyrolysis of TTIP in a mixed helium/oxygen atmosphere,using liquid precursor delivery.230When depos-ited on the cold areas of the reactor at temperatures below 90°C with plasma enhanced CVD,amorphous TiO 2nano-particles can be obtained and crystallize with a relatively high surface area after being annealed at high temperatures.231TiO 2nanorod arrays with a diameter of about 50-100nm and a length of 0.5-2μm can be synthesized by metal organic CVD (MOCVD)on a WC-Co substrate using TTIP as the precursor.232

Figure 24shows the TiO 2nanorods grown on fused silica substrates with a template-and catalyst-free MOCVD method.233In a typical procedure,

titanium acetylacetonate (Ti(C 10H 14O 5))vaporizing in the furnace at 200-230°C is carried by a N 2/O 2flow into the high-temperature zone of 500-700°C,and TiO 2nanostruc-tures are grown directly on the substrates.The phase and

Figure 22.SEM images of large-scale nanorod arrays prepared by oxidizing a titanium with acetone at 850°C for 90min.From:Peng,X.;Chen, A.J.Mater.Chem.2004,14,2542(e286576414791711cd791702/10.1039/b404750h)s Reproduced by permission of The Royal Society of Chemistry.

Figure 23.SEM images of TiO 2nanotubes prepared with anodic oxidation.Reprinted with permission from Varghese,O.K.;Gong,D.;Paulose,M.;Ong,K.G.;Dickey,E.C.;Grimes,C.A.Ad V .Mater.2003,15,624.Copyright 2003Wiley-VCH.

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morphology of the TiO 2nanostructures can be tuned with the reaction conditions.For example,at 630and 560°C under a pressure of 5Torr,single-crystalline rutile and anatase TiO 2nanorods were formed respectively,while,at 535°C under 3.6Torr,anatase TiO 2nanowalls composed of well-aligned nanorods were formed.233

In addition to the above CVD approaches in preparing TiO 2nanomaterials,other CVD approaches are also used,such as

electrostatic spray hydrolysis,234diffusion flame pyrolysis,235-239thermal plasma pyrolysis,240-246ultrasonic spray pyrolysis,247laser-induced pyrolysis,248,249and ultronsic-assisted hydrolysis,250,251among others.

2.8.Physical Vapor Deposition

In PVD,materials are first evaporated and then condensed to form a solid material.The primary PVD methods include thermal deposition,ion plating,ion implantation,sputtering,laser vaporization,and laser surface alloying.2nanowire method or thermal deposition.252-254Typically,pure Ti metal powder is on a quartz boat a tube furnace 0.5mm away from furnace chamber is pumped down to ～300Torr and the to 850°C under an argon gas flow with a rate of 100sccm and held for 3h.After the reaction,a layer of TiO 2nanowires can be obtained.254A layer of Ti nanopowders can be deposited on the substrate before the growth of TiO 2nanowires,252,253and Au can be employed as catalyst.252A typical SEM image of TiO 2nanowires made with the PVD method is shown in Figure 25.252

2.9.Electrodeposition

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

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

2.10.Sonochemical Method

Ultrasound 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 direct interaction with molecular species.Instead,sonochemistry arises from acoustic cavitation:the formation,growth,and implosive collapse of bubbles in a liquid.Cavitational collapse produces intense local heating (～5000K),high pres-sures (～1000atm),and enormous heating and cooling rates (>109K/s).The sonochemical method has been applied to prepare various TiO 2nanomaterials by different groups.257-269Yu et al.applied the sonochemical method in preparing highly photoactive TiO 2nanoparticle photocatalysts with anatase and brookite phases using the hydrolysis of titanium tetraisoproproxide in pure water or in a 1:1EtOH -H 2O solution under ultrasonic radiation.109Huang et al.found that anatase and rutile TiO 2nanoparticles as well as their mixtures could be selectively synthesized with various precursors using ultrasound irradiation,depending on the reaction temperature and the precursor used.259Zhu et al.developed titania whiskers and nanotubes with the assistance of sonication as shown in Figure 27.269They found that arrays of TiO 2nanowhiskers with a diameter of 5nm and nanotubes with a diameter of ～5nm and a length of 200-300nm could be obtained by sonicating TiO 2particles in NaOH aqueous solution followed by washing with deionized water and a dilute HNO 3aqueous solution.

2.11.Microwave Method

A dielectric material can be processed with energy in the form of high-frequency electromagnetic waves.The principal

2904Chemical Reviews,2007,Vol.107,No.7Chen and Mao

frequencies of microwave heating are between 900and 2450MHz.At lower microwave frequencies,conductive currents flowing within the material due to the movement of ionic con-stituents can transfer energy from the microwave field to the material.At higher frequencies,the energy absorption is pri-marily due to molecules with a permanent dipole which tend to reorientate under the influence of a microwave electric field.This reorientation loss mechanism originates from the inability of the polarization to follow extremely rapid rever-sals of the electric field,so the polarization phasor lags the applied electric field.This ensures that the resulting current density has a component in phase with the field,and therefore power is dissipated in the dielectric material.The major advantages of using microwaves for industrial processing are rapid heat transfer,and volumetric and selective heating.Microwave radiation is applied to prepare various TiO 2nanomaterials.270-276Corradi et al.found that colloidal titania nanoparticle suspensions could be prepared within 5min to 1h with microwave radiation,while 1to 32h was needed for the conventional synthesis method of forced hydrolysis at 195°C.270Ma et al.developed high-quality rutile TiO 2nano-rods with a microwave hydrothermal method and found that

they aggregated radially into spherical secondary nanopartic-les.272Wu et al.synthesized TiO 2nanotubes by microwave radiation via the reaction of TiO 2crystals of anatase,rutile,or mixed phase and NaOH aqueous solution under a certain microwave power.275Normally,the TiO 2nanotubes had the central hollow,open-ended,and multiwall structure with diameters of 8-12nm and lengths up to 200-1000nm.275

2.12.TiO 2Mesoporous/Nanoporous Materials

In the past decade,mesoporous/nanoporous TiO 2materials have been well studied with or without the use of organic

surfactant templates.28,80,264,265,277-312Barbe et al.reported the preparation of a mesoporous TiO 2film by the hydrothermal method as shown Figure 28.80In a typical experiment,TTIP was added dropwise to a 0.1M nitric acid solution under vigorous stirring and at room temperature.A white precipitate formed instantaneously.Immediately after the hydrolysis,the solution was heated to 80°C and stirred vigorously for 8h for peptization.The solution was then filtered on a glass frit to remove agglomerates.Water was added to the filtrate to adjust the final solids concentration to ～5wt %.The solution was put in a titanium autoclave for 12h at 200-250°C.After sonication,the colloidal suspension was put in a rotary evaporator and evaporated to a final TiO 2concentration of 11wt %.The precipitation pH,hydrolysis rate,autoclaving pH,and precursor chemistry were found to influence the morphology of the final TiO 2nanoparticles.

Alternative procedures without the use of hydrothermal processes have been reported by Liu et al.292and Zhang et al.311In the report by Liu et al.,24.0g of titanium(IV)n -butoxide ethanol solution (weight ratio of 1:7)was prehydrolyzed in the presence of 0.32mL of a 0.28M HNO 3aqueous solution (TBT/HNO 3～100:1)at room temperature for 3h.0.32mL of deionized water was added to the prehydrolyzed solution under vigorous stirring and stirred for an additional 2h.The sol solution in a closed vessel was kept at room temperature without stirring to gel and age.After aging for 14days,the gel was dried at room temperature,ground into a fine powder,washed thoroughly with water and ethanol,and dried to produce porous TiO 2.Upon calcination at 450°C for 4h under air,crystallized mesoporous TiO 2material was obtained.292

Yu et al.prepared three-dimensional and thermally stable mesoporous TiO 2without the use of any surfactants.265Briefly,monodispersed TiO 2nanoparticles were formed initially by ultrasound-assisted hydrolysis of acetic acid-modified titanium isopropoxide.Mesoporous spherical or globular particles were then produced by controlled conden-

Figure 27.TEM images of TiO 2nanotubes (A)and nanowhiskers (B)prepared with the sonochemical method.From:Zhu,Y.;Li,H.;Koltypin,Y.;Hacohen,Y.R.;Gedanken,e286576414791711cd791702mun.2001,2616(e286576414791711cd791702/10.1039/b108968b)s Reproduced by permission of The Royal Society of Chemistry.

Figure 28.SEM image of the mesoporous TiO 2film synthesized from the acetic acid-modified precursor and autoclaved at 230°C.Reprinted with permission from Barbe,C.J.;Arendse,F.;Comte,P.;Jirousek,M.;Lenzmann,F.;Shklover,V.;Gra ¨tzel,M.J.Am.Ceram.Soc.1997,80,3157.Copyright 1997Blackwell Publishing.

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sation and agglomeration of these sol nanoparticles under high-intensity ultrasound radiation.The mesoporous TiO 2had a wormhole-like structure consisting of TiO 2nanoparticles and a lack of long-range order.265

In the template method used by the Stucky group 278-280,287,295,302,306-307,313and other groups,264,293,297,303,309

structure-directing agents were used for organizing network-forming metal oxide species in nonaqueous solutions.These structure-directing agents were also called organic templates.The most commonly used organic templates were amphi-philic poly(alkylene oxide)block copolymers,such as HO-(CH 2CH 2O)20(CH 2CH(CH 3)O)70(CH 2CH 2O)20H (designated EO 20PO 70EO 20,called Pluronic P-123)and HO(CH 2CH 2O)106-(CH 2CH(CH 3)O)70(CH 2CH 2O)106H (designated EO 106PO 70-EO 106,called Pluronic F-127).In a typical synthesis,poly-(alkylene oxide)block copolymer was dissolved in ethanol.Then TiCl 4precursor was added with vigorous stirring.The resulting sol solution was gelled in an open Petri dish at 40°C in air for 1-7days.Mesoporous TiO 2was obtained after removing the surfactant species by calcining the as-made sample at 400°C for 5h in air.306Figure 29shows typical TEM images of the mesoporous TiO 2.Besides triblock co-polymers as structure-directing agents,diblock polymers were also used such as [C n H 2n -1(OCH 2CH 2)y OH,Brij 56(B56,n /y )16/10)or Brij 58(B58,n /y )16/20)]by Sanchez et al.285Other surfactants employed to direct the formation of mesoporous TiO 2include tetradecyl phosphate (a 14-carbon chain)by Antonelli and Ying 277and commercially available dodecyl phosphate by Putnam and co-workers,298cetyltri-methylammonium bromide (CTAB)(a cationic surfac-tant),281,283,296the recent Gemini surfactant,294and dodecyl-amine (a neutral surfactant).304Carbon nanotubes 310and mesoporous SBA-15286have also been used as the skeleton for mesoporous TiO 2.

2.1

3.TiO 2Aerogels

The study of TiO 2aerogels is worthy of special men-tion.314-326The combination of sol -gel processing with supercritical drying offers the synthesis of TiO 2aerogels with morphological and chemical properties that are not easily achieved by other preparation methods,i.e.,with high surface area.Campbell et al.prepared TiO 2aerogels by sol -gel synthesis from titanium n -butoxide in methanol with the subsequent removal of solvent by supercritical CO 2.315For a typical synthesis process,titanium n -butoxide was added to 40mL of methanol in a dry glovebox.This solution was combined with another solution containing 10mL of methanol,nitric acid,and deionized water.The concentration of the titanium n -butoxide was kept at 0.625M,and the molar ratio of water/HNO 3/titanium n -butoxide was 4:0.1:

1.The gel was allowed to age for 2h and then extracted in a standard autoclave with supercritical CO 2at a flow rate of 24.6L/h,at 343K under

2.07×107Pa for 2-3h,resulting in complete removal of solvent.After extraction,the sample was heated in a vacuum oven at

3.4kPa and 383K for 3h to remove the residual solvent and at 3.4kPa and 483K for 3h to remove any residual organics.The pretreated sample had a brown color and turned white after calcination at 773K or above.The resulting TiO 2aerogel,after calcination at 773K for 2h,had a BET surface area of >200m 2/g,contained mesopores in the range 2-10nm,and was of the pure anatase form.Dagan et al.found the TiO 2aerogels obtanied by using a Ti/ethanol/H 2O/nitric acid ratio of 1:20:3:0.08could have a porosity of 90%and surface areas of 600m 2/g,as compared to a surface area of 50m 2/g for TiO 2P25.316,317Figure 30shows a typical SEM image of a TiO 2aerogel with a surface area of 447m 2/g and an interpore structure constructed by near uniform grains of elliptical shapes with 30nm ×50nm axes.326Figure 29.TEM micrographs of two-dimensional hexagonal mesoporous TiO 2recorded along the (a)[110]and (b)[001]zone axes,respectively.The inset in part a is selected-area electron diffraction patterns obtained on the image area.(c)TEM image of cubic mesoporous TiO 2accompanied 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 1999American Chemical Society.2906Chemical Reviews,2007,Vol.107,No.7

Chen and Mao

2.14.TiO 2Opal and Photonic Materials

The syntheses of TiO 2opal and photonic materials have been well studied by various groups.327-358Holland et al.reported the preparation of TiO 2inverse opal from the corresponding metal alkoxides,using latex spheres as templates.334,335Millimeter-thick layers of latex spheres were deposited on filter paper in

a Buchner funnel under vacuum and soaked with ethanol.Titanium ethoxide was added dropwise to cover the latex spheres completely while suction was applied.Typical mass ratios of alkoxide to latex were between 1.4and 3.After drying the composite in a vacuum desiccator for 3to 24h,the latex spheres were removed by calcination in flowing air at 575°C for 7to 12h,leaving hard and brittle powder particles with 320-to 360-nm voids.The carbon content of the calcined samples varied from 0.4to 1.0wt %,indicating that most of the latex templates had been removed from the 3D host.Figure 31shows an illustration of the simple synthesis of TiO 2inverse opal and an SEM image of TiO 2inverse opals.Similar studies have also been carried out by other researchers.327,356

Dong and Marlow prepared TiO 2inversed opals with a skeleton-like structure of TiO 2rods by a template-directed method using monodispersed polystyrene particles of size 270nm.328-330,345Infiltration of a titania precursor (Ti(i-OPr)4in EtOH)was followed by a drying and calcination proce-dure.The precursor concentration was varied from 30%to 100%,and the calcination temperature was tuned from 300to 700°C.A SEM picture of the TiO 2inversed opal is shown in Figure 32.329The skeleton structure consists of rhombo-hedral windows and TiO 2cylinders forming a highly regular network.The cylinders connect the centers of the former octahedral and tetrahedral voids of the opal.These voids form a CaF 2lattice which is filled with cylindrical bonds con-necting the Ca and F sites.

Wang et al.reported their study on the large-scale fabrication of ordered TiO 2nanobowl arrays.354The process starts with a self-assembled monolayer of polystyrene (PS)spheres,which is used as a template for atomic layer deposition of a TiO 2layer.After ion-milling,toluene-etching,and annealing of the TiO 2-coated spheres,ordered arrays of nanostructured TiO 2nanobowls can be fabricated as shown in Figure 33.

Wang et al.fabricated a 2D photonic crystal by coating patterned and aligned ZnO nanorod arrays with TiO 2.355PS spheres were self-assembled to make a monolayer mask on

a sapphire substrate,which was then covered with a layer of gold.After removing the PS spheres with toluene,ZnO nanorods were grown using a vapor -liquid -solid process.Finally,a TiO 2layer was deposited on the ZnO nanorods by introducing TiCl 4and water vapors into the atomic layer deposition chamber at 100°C.Figure 34shows SEM images of a ZnO nanorod array and the TiO 2-coated ZnO nanorod array.

Li et al.reported the preparation of ordered arrays of TiO 2opals using opal gel templates under uniaxial compression at ambient temperature during the TiO 2sol/gel process.337The aspect ratio was controllable by the compression degree,R .Polystyrene inverse opal was template synthesized using silica opals as template.The silica was removed with 40wt %aqueous hydrofluoric acid.Monomer solutions consisting of dimethylacrylamide,acrylic acid,and methylenebisacryl-amide in 1:1:0.02weight ratios were dissolved in a water/

Figure 31.(A)Schematic illustration of the synthesis of a TiO 2inversed opal.(B)SEM image of the TiO 2inversed opal.Reprinted with permission from Holland,B.T.;Blanford,C.;Stein,A.Science 1998,281,538(e286576414791711cd791702).Copyright 1998AAAS.

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72907

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

gels with correspondingly different properties can be pro-duced.Water was completely removed from the opal hydrogel by repeatedly rinsing it with a large amount of ethanol.Afterward,the opal gel was put into a large amount of tetrabutyl titanate (TBT)at ambient temperature for 24h.The TBT-swollen opal gel was then immersed in a water/ethanol (1:1wt/wt)mixture for 5h to let the TiO 2sol/gel process proceed.Figure 35A shows the opal structure of the gel/titania composite spheres formed.After calcination,TiO 2opal with distinctive spherical contours could be found.The compression degree,R ,was adjusted by the spacer height when the substrates were compressed.When the substrates were slightly compressed against each other to the extent of producing a 20%reduction in the thickness of the composi-tion opal,the deformation of the template-synthesized titania spheres 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 opals could be obtained (Figure 35C and D).

2.15.Preparation of TiO 2Nanosheets

The preparation of TiO 2nanosheets has also been explored recently.359-368Typically,TiO 2nanosheets were synthesized

by delaminating layered protonic titanate into colloidal single layers.A stoichiometric mixture of Cs 2CO 3and TiO 2was calcined at 800°C for 20h to produce a precursor,cesium titanate,Cs 0.7Ti 1.82500.175O 4(0:vacancy),about 70g of which was treated with 2L of a 1M HCl solution at room temperature.This acid leaching was repeated three times by renewing the acid solution every 24h.The resulting acid-exchanged product was filtered,washed with water,and air-dried.The obtained protonic titanate,H 0.7Ti 1.82500.175O 4?H 2O,was shaken vigorously with a 0.017M tetrabutylammonium hydroxide solution at ambient temperature for 10days.The solution-to-solid ratio was adjusted to 250cm 3g -1.This procedure yielded a stable colloidal suspension with an

Figure 32.SEM picture of a TiO 2skeleton with a cylinder radius of about 0.06a .a is the lattice constant of the cubic unit cell.Reprinted from Dong,W.;Marlow,F.Physica E 2003,17,431,Copyright 2003,with permission from Elsevier.

Figure 33.(A)Experimental procedure for fabricating TiO 2nanobowl arrays.(B)Low-and high-(inset)magnification SEM image of TiO 2nanobowl arrays.Reprinted with permission from Wang,X.D.;Graugnard,E.;King,J.S.;Wang,Z.L.;Summers,C.J.Nano Lett.2004,4,2223.Copyright 2004American Chemical Society.

Figure 34.(A)SEM images of short and densely aligned ZnO nanorod array on a sapphire substrate.Inset:An optical image of the aligned ZnO nanorods over a large area.(B)SEM image of the TiO 2-coated ZnO nanorod array.Reprinted with permission from Wang,X.;Neff,C.;Graugnard,E.;Ding,Y.;King,J.S.;Pranger,L.A.;Tannenbaum,R.;Wang,Z.L.;Summers,C.J.Ad V .Mater.2005,17,2103.Copyright 2005Wiley-VCH.

2908Chemical Reviews,2007,Vol.107,No.7Chen and Mao

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

3.Properties of TiO 2Nanomaterials

3.1.Structural Properties of TiO 2Nanomaterials

Figure 37shows the unit cell structures of the rutile and anatase TiO 2.11These two structures can be described in terms of chains of TiO 6octahedra,where each Ti 4+ion is surrounded by an octahedron of six O 2-ions.The two crystal structures differ in the distortion of each octahedron and by the assembly pattern of the octahedra chains.In rutile,the

octahedron shows a slight orthorhombic distortion;in anatase,the octahedron is significantly distorted so that its symmetry is lower than orthorhombic.The Ti -Ti distances in anatase are larger,whereas the Ti -O distances are shorter than those in rutile.In the rutile structure,each octahedron is in contact with 10neighbor octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms),while,in the anatase structure,each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner).These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO 2.

Hamad et al.performed a theoretical calculation on Ti n O 2n clusters (n )1-15)with a combination of simulated

Figure 35.SEM of the TiO 2opals.(A)A gel/titania composite opal fabricated without compressing the opal gel template during the sol/gel process.(Inset)Image of the sample after calcination at 450°C for 3h.(B -D)(Main panel)Oblate titania opal materials after calcination at 450°C for 3h,subject to compression degree R of (B)20%,(C)35%,and (D)50%.The images were taken for the fractured surfaces containing the direction of applied compression.(Inset)Image of the same sample,but with the fracture surface perpendicular to the direction of applied compression.From:Ji,L.;Rong,J.;Yang,e286576414791711cd791702mun.2003,1080(e286576414791711cd791702/10.1039/b300825h)s Reproduced by permission of The Royal Society of Chemistry.

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72909

annealing,Monte Carlo basin hopping simulation,and genetic algorithms methods.369They found that the calculated global minima consisted of compact structures,with titanium atoms reaching high coordination rapidly as n increased.For n g 11,the particles had at least a central octahedron surrounded by a shell of surface

tetrahedra,trigonal bipyra-mids,and square base pyramids.

Swamy et al.found the metastability of anatase as a function of pressure was size dependent,with smaller crystallites preserving the structure to higher pressures.370Three size regimes were recognized for the pressure-induced phase transition of anatase at room temperature:an anatase -

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

Figure 36.(A)TEM of Ti 1-δO 24δ-nanosheets.(B and C)AFM image and height scan of the TiO 2nanosheets deposited on a Si wafer.(D)Structural model for a hydrated TiO 2nanosheet.Closed,open,and shaded circles represent Ti atom,O atom,and H 2O molecules,respectively.All the water sites are assumed to be half occupied.Reprinted with permission from Sasaki,T.;Ebina,Y.;Kitami,Y.;Watanabe,M.;Oikawa,T.J.Phys.Chem.B 2001,105,6116.Copyright 2001American Chemical Society.

Figure e286576414791711cd791702ttice structure of rutile and anatase TiO 2.Reprinted with permission from Linsebigler,A.L.;Lu,G.;Yates,J.T.,Jr.Chem.Re V .1995,95,735.Copyright 1995American Chemical Society.

2910Chemical Reviews,2007,Vol.107,No.7Chen and Mao

nanoparticles as a function of shape,size,and

degree of surface passivation.372Their study on anatase and rutile titanium dioxide polymorphs passivated with complete monolayers of adsorbates by varying the hydrogen to oxygen ratio with respect to a neutral,water-terminated surface showed that termination with water consistently resulted in the lowest values of surface free energy when hydrated or with a higher fraction of H on the surface on both anatase and rutile surfaces,but conversely,the surfaces generally had a higher surface free energy when they had an equal ratio of H and O in the adsorbates or were O-terminated.375They demonstrated that,under different pH conditions from acid to basic,the phase transition size of a TiO 2nanoparticle varied from 6.9to 22.7nm,accompanied with shape changes of the TiO 2nanoparticles as shown in Figure 38.374

Enyashin and Seifert conducted a theoretical study on the structural stability of TiO 2layer modifications (anatase and lepidocrocite)using the density-functional-based tight bind-ing method (DFTB).376They found that anatase nanotubes were the most stable modifications in a comparison of single-walled nanotubes,nanostrips,and nanorolls.Their stability increased as their radii grew.The energies for all TiO 2nanostructures relative to the infinite monolayer followed a 1/R 2curve.

Chen et al.found that severe distortions existed in Ti site environments in the structures of 1.9nm TiO 2nanoparticles compared to those octahedral Ti sites in bulk anatase Ti using K-edge XANES.377The distorted Ti sites were likely to adopt a pentacoordinate square pyramidal geometry due to the truncation of the lattice.The distortions in the TiO 2lattice were mainly located on the surface of the nanoparticles and were responsible for binding with other small molecules.Qian et al.found that the density of the surface states on TiO 2nanoparticles was likely dependent upon the details of the preparation methods.378The TiO 2nanoparticles prepared from basic sol were found to have more surface states than those 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 anatase and brookite are common in fine grained (nanoscale)natural

and synthetic samples.On heating concomitant with coarsen-ing,the following transformations are all seen:anatase to brookite to rutile,brookite to anatase to rutile,anatase to rutile,and brookite to rutile.These transformation sequences imply very closely balanced energetics as a function of particle size.The surface enthalpies of the three polymorphs are sufficiently different that crossover in thermodynamic stability can occur under conditions that preclude coarsening,with anatase and/or brookite stable at small particle size.73,74However,abnormal behaviors and inconsistent results are occasionally observed.

Hwu et al.found the crystal structure of TiO 2nanoparticles depended largely on the preparation method.379For small TiO 2nanoparticles (<50nm),anatase seemed more stable and transformed to rutile at >973K.Banfield et al.found that the prepared TiO 2nanoparticles had anatase and/or brookite structures,which transformed to rutile after reaching a certain particle size.73,380Once rutile was formed,it grew much faster than anatase.They found that rutile became more stable than anatase for particle size >14nm.

Ye et al.observed a slow brookite to anatase phase transition below 1053K along with grain growth,rapid brookite to anatase and anatase to rutile transformations between 1053K and 1123K,and rapid grain growth of rutile above 1123K as the dominant phase.381They concluded that brookite could not transform directly to rutile but had to transform to anatase first.However,direct transformation of brookite nanocrystals to rutile was observed above 973K by Kominami et al.382

In a later study,Zhang and Banfield found that the transformation sequence and thermodynamic phase stability depended on the initial particle sizes of anatase and brookite in their study on the phase transformation behavior of nanocrystalline aggregates during their growth for isothermal and isochronal reactions.74They concluded that,for equally sized nanoparticles,anatase was thermodynamically stable for sizes <11nm,brookite was stable for sizes between 11and 35nm,and rutile was stable for sizes >35nm.

Ranade et al.investigated the energetics of the TiO 2polymorphs (rutile,anatase,and brookite)by high-temper-ature oxide melt drop solution calorimetry,and they found the energetic stability crossed over between the three phases as shown in Figure 39.383The dark solid line represents the phases of lowest enthalpy as a function of surface area.Rutile was energetically stable for surface area <592m 2/mol (7m 2/g or >200nm),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)oxygenated surfaces,and for rutile (bottom),with (f)hydrogenated surfaces,(g)hydrogen-rich surface adsorbates,(h)hydrated surfaces,(i)hydrogen-poor adsorbates,and (j)oxygenated surfaces.Reprinted with permission from Barnard,A.S.;Curtiss,L.A.Nano Lett.2005,5,1261.Copyright 2005American Chemical Society.

Figure 39.Enthalpy of nanocrystalline TiO 2.Reprinted with permission 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 2002National Academy of Sciences,U.S.A.

Titanium Dioxide Nanomaterials Chemical Reviews,2007,Vol.107,No.72911

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