titanium dioxide nanomaterials synthesis, Properties, Modifications, and Applications

Figure 19 shows a typical TEM image of TiO2 nanorods prepared from the solutions with the weight ratio of precursor/solvent/surfactant 1:5:3.183 Similar to the hydrothermal method, the

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Chemical Reviews is published by the American Chemical Society 1155 Sixteenth Street N.W., Washington, DC 20036

Properties, Modifications, and Applications

Xiaobo Chen, and Samuel S Mao

Chem Rev., 2007, 107 (7), 2891-2959• DOI: 10.1021/cr0500535 • Publication Date (Web): 23 June 2007

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2 Synthetic Methods for TiO2 Nanostructures 2892

2.2 Micelle and Inverse Micelle Methods 2895

2.6 Direct Oxidation Method 2902

2.7 Chemical Vapor Deposition 2903

2.8 Physical Vapor Deposition 2904

2.14 TiO2 Opal and Photonic Materials 2907

2.15 Preparation of TiO2 Nanosheets 2908

3 Properties of TiO2Nanomaterials 2909

3.1 Structural Properties of TiO2 Nanomaterials 2909

3.2 Thermodynamic Properties of TiO2

Nanomaterials

29113.3 X-ray Diffraction Properties of TiO2

Nanomaterials

29123.4 Raman Vibration Properties of TiO2

3.5 Electronic Properties of TiO2 Nanomaterials 2913

3.6 Optical Properties of TiO2Nanomaterials 2915

3.7 Photon-Induced Electron and Hole Properties

of TiO2Nanomaterials

2918

4 Modifications of TiO2 Nanomaterials 2920

4.1 Bulk Chemical Modification: Doping 2921

4.1.1 Synthesis of Doped TiO2Nanomaterials 2921

4.1.2 Properties of Doped TiO2Nanomaterials 2921

4.2 Surface Chemical Modifications 2926

Second Generation

29305.1.3 Nonmetal-Doped TiO2 Nanomaterials:

Third Generation

2931

5.2 Photovoltaic Applications 29325.2.1 The TiO2Nanocrystalline Electrode in

DSSCs

29325.2.2 Metal/Semiconductor Junction Schottky

Diode Solar Cell

29385.2.3 Doped TiO2Nanomaterials-Based SolarCell

29385.3 Photocatalytic Water Splitting 29395.3.1 Fundamentals of Photocatalytic Water

Splitting

29395.3.2 Use of Reversible Redox Mediators 29395.3.3 Use of TiO2Nanotubes 29405.3.4 Water Splitting under Visible Light 29415.3.5 Coupled/Composite Water-Splitting

29435.4.3 Counterelectrode for an Electrochromic

Device

29445.4.4 Photoelectrochromic Devices 2945

An exponential growth of research activities has been seen

in nanoscience and nanotechnology in the past decades.13-17New physical and chemical properties emerge when the size

of the material becomes smaller and smaller, and down to

* Corresponding author E-mail: XChen3@lbl.gov.

† E-mail: SSMao@lbl.gov.

Published on Web 06/23/2007

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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.13-16 The 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

TiO2-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 TiO2-based devices is largely influenced by the sizes of

the TiO2building units, apparently at the nanometer scale

As the most promising photocatalyst,7,11,12,33 TiO2

mate-rials are expected to play an important role in helping solve

many serious environmental and pollution challenges TiO2also 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 TiO2nanomaterials 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 TiO2nanomaterials 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 TiO2nanomaterials 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 properties

of TiO2nanomaterials are reviewed in the second section

As the size, shape, and crystal structure of TiO2rials 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 TiO2nanomaterials is also summarized, as theycould help to determine the size to some extent, althoughcorrelation of the spectra with the size of TiO2nanomaterials

nanomate-is not straightforward The review of modifications of TiO2nanomaterials is mainly limited to the research related tothe modifications of the optical properties of TiO2nanoma-terials, since many applications of TiO2nanomaterials areclosely related to their optical properties TiO2nanomaterialsnormally are transparent in the visible light region By doping

or sensitization, it is possible to improve the optical 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 TiO2nanoma-terials-based devices are discussed to facilitate the under-standing and further improvement of current and practicalTiO2nanotechnology

sensitiv-2 Synthetic Methods for TiO2 Nanostructures 2.1 Sol − Gel Method

The sol-gel method is a versatile process used in makingvarious ceramic materials.46-50In 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 loss

of 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 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

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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.51,63,66,79-91 The development of Ti-O-Ti chains is

favored with low content of water, low hydrolysis rates, and

excess titanium alkoxide in the reaction mixture

Three-dimensional polymeric skeletons with close packing result

from the development of Ti-O-Ti chains The formation

of Ti(OH)4 is favored with high hydrolysis rates for a

medium amount of water The presence of a large quantity

of Ti-OH and insufficient development of three-dimensional

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 TiO2 nanoparticles 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 tempertemper-ature dependence of the viscosity of

the solution and the equilibrium solubility of TiO2.63

Second-ary particles are formed by epitaxial self-assembly of primSecond-ary

particles at longer times and higher temperatures, and the

number of primary particles per secondary particle increases

with time The average TiO2 nanoparticle radius increases

linearly with time, in agreement with the

Lifshitz-Slyozov-Wagner model for coarsening.63

Highly crystalline anatase TiO2nanoparticles with different

sizes and shapes could be obtained with the polycondensation

of titanium alkoxide in the presence of tetramethylammonium

hydroxide.52,62 In 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 13 days or at

90-100°C for 6 h A secondary treatment involving autoclave

heating at 175 and 200 °C is performed to improve the

crystallinity of the TiO2nanoparticles Representative TEM

images are shown in Figure 1 from the study of Chemseddine

et al.52

A series of thorough studies have been conducted by

Sugimoto et al using the sol-gel method on the formation

of TiO2nanoparticles of different sizes and shapes by tuning

the reaction parameters.67-71Typically, a stock solution of

a 0.50 M Ti source is prepared by mixing TTIP with

triethanolamine (TEOA) ([TTIP]/[TEOA] ) 1:2), followed

by addition of water The stock solution is diluted with a

shape controller solution and then aged at 100°C for 1 day

and at 140°C for 3 days The pH of the solution can be

tuned by adding HClO4or NaOH solution Amines are used

as the shape controllers of the TiO2nanomaterials and act

as surfactants These amines include TEOA,

diethylenetri-amine, ethylenedidiethylenetri-amine, trimethylenedidiethylenetri-amine, and

triethyl-enetetramine The morphology of the TiO nanoparticles

sodium stearate The shape control is attributed to the tuning

of the growth rate of the different crystal planes of TiO2nanoparticles by the specific adsorption of shape controllers

to these planes under different pH conditions.70

A prolonged heating time below 100°C for the as-preparedgel can be used to avoid the agglomeration of the TiO2nano-particles during the crystallization process.58,72 By heatingamorphous TiO2in air, large quantities of single-phase ana-tase TiO2nanoparticles with average particle sizes between

7 and 50 nm can be obtained, as reported by Zhang andBanfield.73-77Much effort has been exerted to achieve highlycrystallized and narrowly dispersed TiO2nanoparticles usingthe sol-gel method with other modifications, such as asemicontinuous reaction method by Znaidi et al.78and 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 TiO2sol 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 boiled

in ethanol; then it is dried in air and calcined at 400°C for

10 h The AAM template is removed in a 10 wt % H3PO4aqueous solution The calcination temperature can be used

to 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 TiO2nanorods, which typically range from 100 to 300

nm in diameter and several micrometers in length 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 TiO2nanorods, it is necessary to fabricate high-qualityAAM templates Figure 3 shows a typical TEM for TiO2nanorods fabricated with this method Normally, the TiO2nanorods are composed of small TiO2 nanoparticles ornanograins

Appar-By electrophoretic deposition of TiO2colloidal suspensionsinto the pores of an AAM, ordered TiO2 nanowire arrayscan be obtained.94In a typical procedure, TTIP is dissolved

in 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 TiO2sol 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 TiO nanowires are obtained In order to

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fabricate TiO2nanowires instead of nanorods, an AAM with

long pores is a must

TiO2 nanotubes can also be obtained using the sol-gel

method by templating with an AAM95-98and other organic

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

template, a thin layer of TiO sol on the wall of the pores of

the AAM is first prepared by sucking TiO2sol into the pores

of the AAM and removing it under vacuum; TiO2nanowiresare obtained after the sol is fully developed and the AAM isremoved In the procedure by Lee and co-workers,96a 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

Figure 2 TEM images of uniform anatase TiO2nanoparticles Reprinted from Sugimoto, T.; Zhou, X.; Muramatsu, A J Colloid Interface

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solution, it was removed from the solution and placed under

vacuum until the entire volume of the solution was pulled

through the AAM The AAM was hydrolyzed by water vapor

over a HCl solution for 24 h, air-dried at room temperature,

and then calcined in a furnace at 673 K for 2 h and cooled

to room temperature with a temperature ramp of 2°C/h Pure

TiO2nanotubes were obtained after the AAM was dissolved

in a 6 M NaOH solution for several minutes.96Alternatively,

TiO2 nanotubes could be obtained by coating the AAM

membranes at 60°C for a certain period of time (12-48 h)

with dilute TiF4under pH ) 2.1 and removing the AAM

after TiO2nanotubes were fully developed.97Figure 4 shows

a typical SEM image of the TiO2nanotube array from the

AAM template.97

In another scheme, a ZnO nanorod array on a glass

substrate can be used as a template to fabricate TiO2

nanotubes with the sol-gel method.101Briefly, TiO 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/TiO2nanorod arrays The ZnO nanorod template is etched-up byimmersing the ZnO/TiO2nanorod arrays in a dilute hydro-chloric acid aqueous solution to obtain TiO2nanotube arrays.Figure 5 shows a typical SEM image of the TiO2nanotubearray with the ZnO nanorod array template The TiO2nanotubes 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 concentration

of the TiO2sol is constant, well-aligned TiO2nanotube arrayscan only be obtained from an optimal dip-coating cyclenumber in the range of 2-3 cycles A dense porous TiO2thick 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 rate

is 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 TiO2film with loose, porous nanostructure

is obtained

2.2 Micelle and Inverse Micelle Methods

Aggregates of surfactant molecules dispersed in a liquidcolloid are called micelles when the surfactant concentrationexceeds the critical micelle concentration (CMC) The CMC

is 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 nanorod

composed of small TiO2 nanoparticles or nanograins (inset)

Reprinted from Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.;

with permission from Elsevier

Figure 4 SEM image of TiO2nanotubes 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,

Figure 5 SEM of a TiO2nanotube array; the inset shows the ZnOnanorod array template Reprinted with permission from Qiu, J J.;

Yu, W D.; Gao, X D.; Li, X M Nanotechnology 2006, 17, 4695.

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directed outward toward the nonaqueous media There is no

obvious CMC for reverse micelles, because the number of

aggregates is usually small and they are not sensitive to the

surfactant concentration Micelles are often globular and

roughly spherical in shape, but ellipsoids, cylinders, and

bilayers are also possible The shape of a micelle is a function

of the molecular geometry of its surfactant molecules and

solution conditions such as surfactant concentration,

tem-perature, pH, and ionic strength

Micelles and inverse micelles are commonly employed to

synthesize TiO2 nanomaterials.102-110 A statistical

experi-mental design method was conducted by Kim et al to

optimize experimental conditions for the preparation of TiO2

nanoparticles.103The values of H2O/surfactant, H2O/titanium

precursor, ammonia concentration, feed rate, and reaction

temperature were significant parameters in controlling TiO2

nanoparticle size and size distribution Amorphous TiO2

nanoparticles with diameters of 10-20 nm were synthesized

and converted to the anatase phase at 600°C and to the more

thermodynamically stable rutile phase at 900 °C Li et al

developed TiO2 nanoparticles with the chemical reactions

between TiCl4 solution and ammonia in a reversed

micro-emulsion system consisting of cyclohexane,

poly(oxyethyl-ene)5 nonyle phenol ether, and poly(oxyethylene)9 nonyle

phenol ether.104The produced amorphous TiO2nanoparticles

transformed into anatase when heated at temperatures from

200 to 750 °C and into rutile at temperatures higher than

750°C Agglomeration and growth also occurred at elevated

temperatures

Shuttle-like crystalline TiO2nanoparticles were synthesized

by Zhang et al with hydrolysis of titanium tetrabutoxide in

the presence of acids (hydrochloric acid, nitric acid, sulfuric

acid, and phosphoric acid) in NP-5 (Igepal

CO-520)-cyclohexane reverse micelles at room temperature.110 The

crystal structure, morphology, and particle size of the TiO2

nanoparticles 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 [H2O]/[NP-5] and [H2

O]/[Ti-(OC4H9)4] ratios When suitable acid was applied, round TiO2

nanoparticles could also be obtained Representative TEM

images of the shuttle-like and round-shaped TiO2

nanopar-ticles are shown in Figure 6 In the study carried out by Lim

et al., TiO2nanoparticles were prepared by the controlled

hydrolysis of TTIP in reverse micelles formed in CO2with

the surfactants ammonium carboxylate perfluoropolyether

(PFPECOO-NH4+) (MW 587) and poly(dimethyl amino

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 molar ratio of water to surfactant or

the precursor to surfactant ratio increased

The TiO2nanomaterials 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 TiO2nanoparticles The

crystallinity of TiO2nanoparticles 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 producecrystalline TiO2 nanoparticles with unchanged physicaldimensions and minimal agglomeration and allows thepreparation of highly crystalline TiO2nanoparticles, 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-gelprocesses 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)

TiO2nanoparticles 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 TiO2nanoparticle 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.

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The condensation between Ti-Cl and Ti-OR leads to the

formation of Ti-O-Ti bridges The alkoxide groups can

be provided by titanium alkoxides or can be formed in situ

by reaction of the titanium chloride with alcohols or ethers

In the method by Trentler and Colvin,119a metal alkoxide

was rapidly injected into the hot solution of titanium halide

mixed with trioctylphosphine oxide (TOPO) in heptadecane

at 300°C under dry inert gas protection, and reactions were

completed within 5 min For a series of alkyl substituents

including methyl, ethyl, isopropyl, and tert-butyl, the reaction

rate dramatically increased with greater branching of R, while

average particle sizes were relatively unaffected Variation

of X yielded a clear trend in average particle size, but without

a discernible trend in reaction rate Increased nucleophilicity

(or size) of the halide resulted in smaller anatase nanocrystals

Average sizes ranged from 9.2 nm for TiF4 to 3.8 nm for

TiI4 The amount of passivating agent (TOPO) influenced

the chemistry Reaction in pure TOPO was slower and

resulted in smaller particles, while reactions without TOPO

were much quicker and yielded mixtures of brookite, rutile,

and anatase with average particle sizes greater than 10 nm

Figure 8 shows typical TEM images of TiO2nanocrystals

developed by Trentler et al.119

In the method used by Niederberger and Stucky,111TiCl4

was slowly added to anhydrous benzyl alcohol under

vigorous stirring at room temperature and was kept at

40-150°C for 1-21 days in the reaction vessel The precipitate

was calcinated at 450°C for 5 h after thoroughly washing

The reaction between TiCl4and benzyl alcohol was found

suitable for the synthesis of highly crystalline anatase phase

TiO2 nanoparticles with nearly uniform size and shape at

very low temperatures, such as 40°C The particle size could

be selectively adjusted in the range of 4-8 nm with the

appropriate thermal conditions and a proper choice of the

relative amounts of benzyl alcohol and titanium tetrachloride

The particle growth depended strongly on temperature, and

lowering the titanium tetrachloride concentration led to a

considerable decrease of particle size.111

Surfactants have been widely used in the preparation of a

variety of nanoparticles with good size distribution and

dispersity.15,16Adding different surfactants as capping agents,

such as acetic acid and acetylacetone, into the reaction matrix

can help synthesize monodispersed TiO2nanoparticles.120,121For example, Scolan and Sanchez found that monodispersenonaggregated TiO2nanoparticles in the 1-5 nm range wereobtained through hydrolysis of titanium butoxide in the

presence of acetylacetone and p-toluenesulfonic acid at 60

°C.120The resulting nanoparticle xerosols could be dispersed

in 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 layer

made with acetylacetone, p-toluenesulfonic acid, and

wa-ter.120With the aid of surfactants, different sized and shaped TiO2nanorods can be synthesized.122-130For example, the growth

of high-aspect-ratio anatase TiO2nanorods has been reported

by 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 led

to the growth of TiO2nanorods instead of nanoparticles.123Typical TEM images of the TiO2 nanorods are shown inFigure 9.123

Recently, Joo et al.127and Zhang et al.129reported similarprocedures in obtaining TiO2nanorods 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 TiO2nanoparticles derived from reaction

of TiCl4and 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.;

American Chemical Society

Figure 9 TEM of TiO2nanorods The inset shows a HRTEM of

a TiO2nanorod Reprinted with permission from Cozzoli, P D.;

Kornowski, A.; Weller, H J Am Chem Soc 2003, 125, 14539.

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The injection of a predetermined amount of oleylamine at

260 °C led to various sized TiO2 nanorods.129 Figure 10

shows TEM images of TiO2nanorods with various lengths,

and 2.3 nm TiO2nanoparticles prepared with this method.129

In the surfactant-mediated shape evolution of TiO2

nano-crystals in nonaqueous media conducted by Jun et al.,128it

was found that the shape of TiO2 nanocrystals 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 Briefly, a dioctyl ether solution containing TOPO

and lauric acid was heated to 300°C followed by addition

of titanium chloride under vigorous stirring The reaction

was initiated by the rapid injection of TTIP and quenched

with cold toluene At low lauric acid concentrations,

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 bullet-and nanorods were

elon-gated along the [001] directions The TiO2 nanorods were

found to simultaneously convert to small nanoparticles as a

function of the growth time, as shown in Figure 11, due to

the minimization of the overall surface energy via dissolution

and regrowth of monomers during an Ostwald ripening

2.4 Hydrothermal Method

Hydrothermal synthesis is normally conducted in steel

pressure vessels called autoclaves with or without Teflon

liners under controlled temperature and/or pressure with thereaction in aqueous solutions The temperature can beelevated above the boiling point of water, reaching thepressure of vapor saturation The temperature and the amount

of 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 prepareTiO2nanoparticles.131-140For example, TiO2nanoparticlescan 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 TiO2nanoparticles made with thehydrothermal method are shown in Figure 12.134

In another example, TiO2nanoparticles were prepared byhydrothermal reaction of titanium alkoxide in an acidicethanol-water solution.132Briefly, TTIP was added dropwise

to a mixed ethanol and water solution at pH 0.7 with nitricacid, and reacted at 240°C for 4 h The TiO nanoparticles

Figure 10 TEM images of TiO2nanorods with lengths of (A) 12 nm, (B) 30 nm, and (C) 16 nm (D) 2.3 nm TiO2nanoparticles Inset

in parts C and D: HR-TEM image of a single TiO2nanorod and nanoparticle Reprinted with permission from Zhang, Z.; Zhong, X.; Liu,

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synthesized under this acidic ethanol-water environment

were mainly primary structure in the anatase phase without

secondary structure The sizes of the particles were controlled

to the range of 7-25 nm by adjusting the concentration of

Ti precursor and the composition of the solvent system

Besides TiO2nanoparticles, TiO2nanorods have also beensynthesized with the hydrothermal method.141-146Zhang et

al obtained TiO2nanorods by treating a dilute TiCl4solution

at 333-423 K for 12 h in the presence of acid or inorganicsalts.141,143-146Figure 13 shows a typical TEM image of theTiO2 nanorods prepared with the hydrothermal method.141The morphology of the resulting nanorods can be tuned withdifferent surfactants146or by changing the solvent composi-tions.145A film of assembled TiO2nanorods deposited on aglass wafer was reported by Feng et al.142 These TiO2nanorods were prepared at 160°C for 2 h by hydrothermaltreatment of a titanium trichloride aqueous solution super-saturated with NaCl

TiO2nanowires have also been successfully obtained withthe hydrothermal method by various groups.147-151Typically,TiO2nanowires are obtained by treating TiO2white powders

in 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 TiO2nanowires and a TEM image

of a single nanowire prepared by Zhang and co-workers.150TiO2nanowires 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 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.

Figure 12 TEM images of TiO2 nanoparticles prepared by thehydrothermal method Reprinted from Yang, J.; Mei, S.; Ferreira,

permission from Elsevier

Figure 13 TEM image of TiO2 nanorods prepared with thehydrothermal method Reprinted with permission from Zhang, Q.;

Chemical Society

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et al.152In their experiment, layer-structured Na2Ti3O7was

dispersed into a 0.05-0.1 M HCl solution and kept at

140-170°C for 3-7 days in an autoclave TiO2nanowires were

obtained after the product was washed with H2O and finally

dried In the formation of a TiO2 nanowire from layered

H2Ti3O7, there are three steps: (i) the exfoliation of layered

Na2Ti3O7; (ii) the nanosheets formation; and (iii) the

nanow-ires formation.152In Na2Ti3O7, [TiO6] octahedral layers are

held by the strong static interaction between the Na+cations

between the [TiO6] octahedral layers and the [TiO6] unit

When the larger H3+O cations replace the Na+cations in

the interlayer space of [TiO6] sheets, this static interaction

is weakened because the interlayer distance is enlarged As

a result, the layered compounds Na2Ti3O7 are gradually

exfoliated When Na+is exchanged by H+in the dilute HCl

solution, numerous H2Ti3O7 sheet-shaped products are

formed Since the nanosheet does not have inversion

sym-metry, an intrinsic tension exists The nanosheets split to form

nanowires in order to release the strong stress and lower the

total energy.152 A representative TEM image of TiO2

nanowires from Na2Ti3O7is shown in Figure 15.152

The hydrothermal method has been widely used to prepare

TiO2nanotubes since it was introduced by Kasuga et al in

1998.153-175Briefly, TiO2powders are put into a 2.5-20 M

NaOH aqueous solution and held at 20-110°C for 20 h in

an autoclave TiO2nanotubes are obtained after the products

are washed with a dilute HCl aqueous solution and distilled

water They proposed the following formation process of

TiO2nanotubes.154When the raw TiO2material was treated

with NaOH aqueous solution, some of the Ti-O-Ti bonds

were broken and Ti-O-Na and Ti-OH bonds were formed

New Ti-O-Ti bonds were formed after the Ti-O-Na and

Ti-OH bonds reacted with acid and water when the material

was treated with an aqueous HCl solution and distilled water

The Ti-OH bond could form a sheet Through the

dehydra-tion of Ti-OH bonds by HCl aqueous soludehydra-tion, Ti-O-Ti

bonds or Ti-O-H-O-Ti hydrogen bonds were generated

The bond distance from one Ti to the next Ti on the surface

decreased This resulted in the folding of the sheets and the

connection between the ends of the sheets, resulting in theformation of a tube structure In this mechanism, the TiO2nanotubes were formed in the stage of the acid treatmentfollowing the alkali treatment Figure 16 shows typical TEMimages of TiO2nanotubes made by Kasuga et al.153However,

Du and co-workers found that the nanotubes were formedduring the treatment of TiO2in NaOH aqueous solution.161

A 3D f 2D f 1D formation mechanism of the TiO2nanotubes was proposed by Wang and co-workers.171It statedthat the raw TiO2 was first transformed into lamellarstructures and then bent and rolled to form the nanotubes.For the formation of the TiO2nanotubes, 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 TiO2nanowires with the inset showing

a TEM image of a single TiO2nanowire 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,

Figure 15 TEM images of TiO2nanowires made from the layered

Na2Ti3O7 particles, with the HRTEM image shown in the inset.Reprinted from Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.;

with permission from Elsevier

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

Chemical Society

Trang 12

17, that TiO2nanotubes were formed by rolling up the

single-layer TiO2 sheets 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 scrolling or wrapping of single layer nanosheets

followed by crystallization of successive layers.156 In the

mechanism proposed by Wang et al., the formation of TiO2

nanotubes 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 was formed when the free octahedras

shared edges between the Ti ions with the formation of

hydroxy bridges, leading to the growth along the [100]

direction of the anatase phase Two-dimensional crystalline

sheets formed from the lateral growth of the formation of

oxo bridges between the Ti centers (Ti-O-Ti bonds) in the

[001] direction and rolled up in order to saturate these

dangling bonds from the surface and lower the total energy,

resulting in the formation of TiO2nanotubes.176

2.5 Solvothermal Method

The solvothermal method is almost identical to the

hydrothermal method except that the solvent used here is

nonaqueous However, the temperature can be elevated much

higher than that in hydrothermal method, since a variety of

organic solvents with high boiling points can be chosen The

solvothermal method normally has better control than

hy-drothermal methods of the size and shape distributions and

the crystallinity of the TiO2nanoparticles The solvothermal

method has been found to be a versatile method for the

synthesis of a variety of nanoparticles with narrow sizedistribution and dispersity.177-179The 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,184TTIP was mixed with toluene at the weight ratio of 1-3:10and kept at 250°C for 3 h The average particle size of TiO2powders tended to increase as the composition of TTIP inthe solution increased in the range of weight ratio of 1-3:

10, while the pale crystalline phase of TiO2was not produced

at 1:20 and 2:5 weight ratios.184By controlling the lyzation reaction of Ti(OC4H9)4and linoleic acid, redispers-ible TiO2nanoparticles and nanorods could be synthesized,

hydro-as found by Li et al recently.177The decomposition of NH4HCO3could 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 TiO2.177Figure

-18 shows a representative TEM image of TiO2nanoparticlesfrom their study.177

TiO2nanorods with narrow size distributions can also bedeveloped 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 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, due

to the oriented growth of particles along the [001] axis At

a fixed precursor to surfactant weight ratio of 1:3, theconcentration of rods in the nanoparticle assembly increased

as 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 from

B D Yao, Y F Chan, X Y Zhang, W F Zhang, Z Y Yang, N

American Institute of Physics

Figure 18 TEM micrographs of TiO2nanoparticles 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,

Trang 13

rods were obtained from the solution with a precursor/

surfactant weight ratio of more than 1:3 for a precursor/

solvent weight ratio of 1:10 or from the solution with a

precursor/solvent weight ratio of more than 1:5 for a

precursor/surfactant weight ratio of 1:3 The diameter and

length of these nanorods were in the ranges of 3-5 nm and

18-25 nm, respectively Figure 19 shows a typical TEM

image of TiO2 nanorods prepared from the solutions with

the weight ratio of precursor/solvent/surfactant ) 1:5:3.183

Similar to the hydrothermal method, the solvothermal

method has also been used for the preparation of TiO2

nanowires.180-182Typically, a TiO2powder suspension in an

5 M NaOH water-ethanol solution is kept in an autoclave

at 170-200°C for 24 h and then cooled to room temperature

naturally TiO2 nanowires are obtained after the obtained

sample is washed with a dilute HCl aqueous solution and

dried at 60 °C for 12 h in air.181 The solvent plays an

important role in determining the crystal morphology

Solvents with different physical and chemical properties can

influence the solubility, reactivity, and diffusion behavior

of the reactants; in particular, the polarity and coordinating

ability of the solvent can influence the morphology and the

crystallization behavior of the final products The presence

of ethanol at a high concentration not only can cause the

polarity of the solvent to change but also strongly affects

the ζ potential values of the reactant particles and the

increases solution viscosity For example, in the absence of

ethanol, short and wide flakelike structures of TiO2 were

obtained instead of nanowires When chloroform is used,

TiO2nanorods were obtained.181Figure 20 shows

representa-tive TEM images of the TiO2nanowires prepared from the

solvothermal method.181Alternatively, bamboo-shaped

Ag-doped TiO2nanowires were developed with titanium

butox-ide as precursor and AgNO3 as catalyst.180 Through the

electron diffraction (ED) pattern and HRTEM study, the Ag

phase only existed in heterojunctions between single-crystalTiO2nanowires.180

2.6 Direct Oxidation Method

TiO2 nanomaterials can be obtained by oxidation oftitanium metal using oxidants or under anodization Crystal-line TiO2nanorods have been obtained by direct oxidation

of a titanium metal plate with hydrogen peroxide.186-191Typically, TiO2nanorods on a Ti plate are obtained when acleaned Ti plate is put in 50 mL of a 30 wt % H2O2solution

at 353 K for 72 h The formation of crystalline TiO2occursthrough a dissolution precipitation mechanism By theaddition of inorganic salts of NaX (X ) F-, Cl-, and SO42-),the crystalline phase of TiO2 nanorods can be controlled.The addition of F-and SO42-helps the formation of pureanatase, while the addition of Cl- favors the formation ofrutile.189 Figure 21 shows a typical SEM image of TiO2nanorods prepared with this method.186

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

Figure 19 TEM micrographs and electron diffraction patterns of

products prepared from solutions at the weight ratio of precursor/

solvent/surfactant ) 1:5:3 Reprinted from Kim, C S.; Moon, B

K.; Park, J H.; Choi, B C.; Seo, H J J Cryst Growth 2003, 257,

Figure 20 TEM images of TiO2 nanowires synthesized by the

solvothermal method From: Wen, B.; Liu, C.; Liu, Y New J.

Chem 2005, 29, 969 (http://dx.doi.org/10.1039/b502604k) s

Reproduced by permission of The Royal Society of Chemistry(RSC) on behalf of the Centre National de la Recherche Scientifique(CNRS)

Figure 21. SEM morphology of TiO2 nanorods by directlyoxidizing a Ti plate with a H2O2solution Reprinted from Wu, J

permission from Elesevier

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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 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 TiO2 With

pure oxygen, the oxidation occurred at the Ti metal and the

TiO2interface, since oxygen diffusion predominated because

of the high oxygen concentration When acetone was used

as the oxygen source, Ti cations diffused to the oxide surface

and reacted with the adsorbed acetone species Figure 22

shows aligned TiO2nanorod arrays obtained by oxidizing a

titanium substrate with acetone at 850°C for 90 min.192

As extensively studied, TiO2nanotubes can be obtained

by anodic oxidation of titanium foil.193-228 In a typical

experiment, a clean Ti plate is anodized in a 0.5% HF

solution under 10-20 V for 10-30 min Platinum is used

as counterelectrode Crystallized TiO2nanotubes are obtained

after the anodized Ti plate is annealed at 500°C for 6 h in

oxygen.210The length and diameter of the TiO2nanotubes

could be controlled over a wide range (diameter, 15-120

nm; length, 20 nm to 10 µm) with the applied potential

between 1 and 25 V in optimized phosphate/HF

electro-lytes.229Figure 23 shows SEM images of TiO2nanotubes

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) InCVD processes, thermal energy heats the gases in the coatingchamber and drives the deposition reaction

Thick crystalline TiO2films with grain sizes below 30 nm

as well as TiO2nanoparticles with sizes below 10 nm can

be prepared by pyrolysis of TTIP in a mixed helium/oxygenatmosphere, using liquid precursor delivery.230When depos-ited on the cold areas of the reactor at temperatures below

90°C with plasma enhanced CVD, amorphous TiO2particles can be obtained and crystallize with a relativelyhigh surface area after being annealed at high temperatures.231TiO2nanorod arrays with a diameter of about 50-100 nmand a length of 0.5-2 µm can be synthesized by metal

nano-organic CVD (MOCVD) on a WC-Co substrate using TTIP

as the precursor.232Figure 24 shows the TiO2nanorods grown on fused silicasubstrates with a template- and catalyst-free MOCVDmethod.233In a typical procedure, titanium acetylacetonate(Ti(C10H14O5)) vaporizing in the low-temperature zone of afurnace at 200-230°C is carried by a N2/O2flow into thehigh-temperature zone of 500-700°C, and TiO2nanostruc-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 90 min From:

Peng, X.; Chen, A J Mater Chem 2004, 14, 2542 (http://

dx.doi.org/10.1039/b404750h) s Reproduced by permission of The

Royal Society of Chemistry

Figure 23 SEM images of TiO2nanotubes prepared with anodicoxidation Reprinted with permission from Varghese, O K.; Gong,

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

Figure 24 SEM images of TiO2 nanorods grown at 560 °C

Reprinted with permission from Wu, J J.; Yu, C C J Phys Chem.

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morphology of the TiO2 nanostructures can be tuned with

the reaction conditions For example, at 630 and 560 °C

under a pressure of 5 Torr, single-crystalline rutile and

anatase TiO2nanorods were formed respectively, while, at

535 °C under 3.6 Torr, anatase TiO2nanowalls composed

of well-aligned nanorods were formed.233

In addition to the above CVD approaches in preparing

TiO2nanomaterials, other CVD approaches are also used,

such as electrostatic spray hydrolysis,234 diffusion flame

pyrolysis,235-239 thermal 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 TiO2nanowire

arrays have been fabricated by a simple PVD method or

thermal deposition.252-254Typically, pure Ti metal powder

is on a quartz boat in a tube furnace about 0.5 mm away

from the substrate Then the furnace chamber is pumped

down to∼300 Torr and the temperature is increased to 850

°C under an argon gas flow with a rate of 100 sccm and

held for 3 h After the reaction, a layer of TiO2nanowires

can be obtained.254 A layer of Ti nanopowders can be

deposited on the substrate before the growth of TiO2

nanowires,252,253and Au can be employed as catalyst.252A

typical SEM image of TiO2nanowires 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, TiO2nanowires can be

obtained by electrodeposition.255,256In a typical process, the

electrodeposition is carried out in 0.2 M TiCl solution with

pH ) 2 with a pulsed electrodeposition approach, andtitanium and/or its compound are deposited into the pores

of the AAM By heating the above deposited template at

500°C for 4 h and removing the template, pure anatase TiO2nanowires can be obtained Figure 26 shows a representativeSEM image of TiO2nanowires.256

2.10 Sonochemical Method

Ultrasound has been very useful in the synthesis of a widerange 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

(>109K/s) The sonochemical method has been applied toprepare various TiO2nanomaterials 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.109Huang et al found thatanatase and rutile TiO2nanoparticles as well as their mixturescould be selectively synthesized with various precursorsusing ultrasound irradiation, depending on the reactiontemperature and the precursor used.259Zhu et al developedtitania whiskers and nanotubes with the assistance ofsonication as shown in Figure 27.269They found that arrays

of TiO2nanowhiskers with a diameter of 5 nm and nanotubeswith a diameter of∼5 nm and a length of 200-300 nm could

be obtained by sonicating TiO2particles in NaOH aqueoussolution followed by washing with deionized water and adilute HNO3aqueous solution

2.11 Microwave Method

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

Figure 25 SEM images of the TiO2nanowire arrays prepared by

the PVD method Reprinted from Wu, J M.; Shih, H C.; Wu, W

permission from Elsevier Figure 26 Cross-sectional SEM image of TiO2nanowires

elec-trodeposited in AAM pores Reprinted from Liu, S.; Huang, K

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frequencies of microwave heating are between 900 and 2450

MHz At lower microwave frequencies, conductive currents

flowing within the material due to the movement of ionic

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 TiO2

nanomaterials.270-276Corradi et al found that colloidal titania

nanoparticle suspensions could be prepared within 5 min to

1 h with microwave radiation, while 1 to 32 h was needed

for the conventional synthesis method of forced hydrolysis

at 195°C.270Ma et al developed high-quality rutile TiO2

nano-rods with a microwave hydrothermal method and found that

they aggregated radially into spherical secondary

nanopartic-les.272Wu et al synthesized TiO2nanotubes by microwave

radiation via the reaction of TiO2crystals of anatase, rutile,

or mixed phase and NaOH aqueous solution under a certain

microwave power.275Normally, the TiO2nanotubes had the

central hollow, open-ended, and multiwall structure with

diameters of 8-12 nm and lengths up to 200-1000 nm.275

2.12 TiO2 Mesoporous/Nanoporous Materials

In the past decade, mesoporous/nanoporous TiO2materials

have been well studied with or without the use of organic

surfactant templates.28,80,264,265,277-312Barbe et al reported thepreparation of a mesoporous TiO2film by the hydrothermalmethod as shown Figure 28.80In 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 frit

to remove agglomerates Water was added to the filtrate toadjust the final solids concentration to∼5 wt % The solution

was put in a titanium autoclave for 12 h at 200-250°C.After sonication, the colloidal suspension was put in a rotaryevaporator and evaporated to a final TiO2concentration of

11 wt % The precipitation pH, hydrolysis rate, autoclaving

pH, and precursor chemistry were found to influence themorphology of the final TiO2nanoparticles

Alternative procedures without the use of hydrothermalprocesses have been reported by Liu et al.292and Zhang et

al.311 In the report by Liu et al., 24.0 g of titanium(IV)

n-butoxide ethanol solution (weight ratio of 1:7) was

prehydrolyzed in the presence of 0.32 mL of a 0.28 M HNO3aqueous solution (TBT/HNO3∼ 100:1) at room temperature

for 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 TiO2material was obtained.292

Yu et al prepared three-dimensional and thermally stablemesoporous TiO2 without the use of any surfactants.265Briefly, 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 TiO2nanotubes (A) and nanowhiskers

(B) prepared with the sonochemical method From: Zhu, Y.; Li,

H.; Koltypin, Y.; Hacohen, Y R.; Gedanken, A Chem Commun.

2001, 2616 (http://dx.doi.org/10.1039/b108968b) s Reproduced by

permission of The Royal Society of Chemistry

Figure 28 SEM image of the mesoporous TiO2film 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.

Trang 17

sation and agglomeration of these sol nanoparticles under

high-intensity ultrasound radiation The mesoporous TiO2had

a wormhole-like structure consisting of TiO2nanoparticles

and a lack of long-range order.265

In the template method used by the Stucky

group278-280,287,295,302,306-307,313and 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-(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designated

EO20PO70EO20, 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 TiCl4precursor was added with vigorous stirring The

resulting sol solution was gelled in an open Petri dish at 40

°C in air for 1-7 days Mesoporous TiO2was obtained after

removing the surfactant species by calcining the as-made

sample at 400°C for 5 h in air.306Figure 29 shows typical

TEM images of the mesoporous TiO2 Besides triblock

co-polymers as structure-directing agents, diblock co-polymers were

also used such as [CnH2n-1(OCH2CH2)y OH, Brij 56 (B56, n/y

) 16/10) or Brij 58 (B58, n/y ) 16/20)] by Sanchez et al.285

Other surfactants employed to direct the formation of

mesoporous TiO2include tetradecyl phosphate (a 14-carbon

chain) by Antonelli and Ying277and commercially available

dodecyl phosphate by Putnam and co-workers,298

cetyltri-methylammonium bromide (CTAB) (a cationic

surfac-tant),281,283,296the recent Gemini surfactant,294and

dodecyl-amine (a neutral surfactant).304 Carbon nanotubes310 and

mesoporous SBA-15286have also been used as the skeleton

for mesoporous TiO2

2.13 TiO2 Aerogels

The study of TiO2 aerogels is worthy of special

men-tion.314-326 The combination of sol-gel processing with

supercritical drying offers the synthesis of TiO2aerogels with

morphological and chemical properties that are not easily

achieved by other preparation methods, i.e., with high surface

area Campbell et al prepared TiO2 aerogels by sol-gel

synthesis from titanium n-butoxide in methanol with the

subsequent removal of solvent by supercritical CO2.315For

a typical synthesis process, titanium n-butoxide was added

to 40 mL of methanol in a dry glovebox This solution was

combined with another solution containing 10 mL of

methanol, nitric acid, and deionized water The concentration

of the titanium n-butoxide was kept at 0.625 M, and the

molar ratio of water/HNO3/titanium n-butoxide was 4:0.1:

1 The gel was allowed to age for 2 h and then extracted in

a standard autoclave with supercritical CO2at a flow rate of

24.6 L/h, at 343 K under 2.07× 107Pa for 2-3 h, resulting

in complete removal of solvent After extraction, the sample

was heated in a vacuum oven at 3.4 kPa and 383 K for 3 h

to remove the residual solvent and at 3.4 kPa and 483 K for

3 h to remove any residual organics The pretreated sample

had a brown color and turned white after calcination at 773

K or above The resulting TiO2aerogel, after calcination at

773 K for 2 h, had a BET surface area of >200 m2/g,

contained mesopores in the range 2-10 nm, and was of the

pure anatase form Dagan et al found the TiO2 aerogels

obtanied by using a Ti/ethanol/H2O/nitric acid ratio of 1:20:

3:0.08 could have a porosity of 90% and surface areas of

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

Figure 29. TEM micrographs of two-dimensional hexagonalmesoporous TiO2recorded 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 TiO2accompanied 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.

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2.14 TiO2 Opal and Photonic Materials

The syntheses of TiO2opal and photonic materials have

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

reported the preparation of TiO2 inverse 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 1.4 and 3 After drying the composite in a vacuum

desiccator for 3 to 24 h, the latex spheres were removed by

calcination in flowing air at 575 °C for 7 to 12 h, leaving

hard and brittle powder particles with 320- to 360-nm voids

The carbon content of the calcined samples varied from 0.4

to 1.0 wt %, indicating that most of the latex templates had

been removed from the 3D host Figure 31 shows an

illustration of the simple synthesis of TiO2inverse opal and

an SEM image of TiO2inverse opals Similar studies have

also been carried out by other researchers.327,356

Dong and Marlow prepared TiO2inversed opals with a

skeleton-like structure of TiO2rods by a template-directed

method using monodispersed polystyrene particles of size

270 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% to

100%, and the calcination temperature was tuned from 300

to 700°C A SEM picture of the TiO2inversed opal is shown

in Figure 32.329The skeleton structure consists of

rhombo-hedral windows and TiO2cylinders forming a highly regular

network The cylinders connect the centers of the former

octahedral and tetrahedral voids of the opal These voids form

a CaF2 lattice which is filled with cylindrical bonds

con-necting the Ca and F sites

Wang et al reported their study on the large-scale

fabrication of ordered TiO2nanobowl 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 TiO2layer After ion-milling, toluene-etching,

and annealing of the TiO2-coated spheres, ordered arrays of

nanostructured TiO2nanobowls 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 TiO2.355PS

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, ZnOnanorods were grown using a vapor-liquid-solid process.Finally, a TiO2 layer was deposited on the ZnO nanorods

by introducing TiCl4and water vapors into the atomic layerdeposition chamber at 100°C Figure 34 shows SEM images

of a ZnO nanorod array and the TiO2-coated ZnO nanorodarray

Li et al reported the preparation of ordered arrays of TiO2opals using opal gel templates under uniaxial compression

at ambient temperature during the TiO2sol/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 40 wt

% aqueous hydrofluoric acid Monomer solutions consisting

of dimethylacrylamide, acrylic acid, and amide in 1:1:0.02 weight ratios were dissolved in a water/

methylenebisacryl-Figure 30. SEM image of a TiO2 aerogel Reprinted with

permission from Zhu, Z.; Tsung, L Y.; Tomkiewicz, M J Phys.

Society

Figure 31 (A) Schematic illustration of the synthesis of a TiO2

inversed opal (B) SEM image of the TiO2inversed opal Reprinted

with permission from Holland, B T.; Blanford, C.; Stein, A Science

AAAS

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ethanol mixture (4:7 wt/wt) with total monomer content 30

wt % Ethanol was used to facilitate diffusion of the

monomer solution into the inverse opal polystyrene After

the inverse opal was infiltrated by the monomer solution

containing 1 wt % of the initiator AIBN and a subsequent

free radical polymerization at 60°C for 3 h, a solid composite

resulted The initial inverse opal polystyrene template was

then removed with chloroform in a Soxhlet extractor for 12

h, whereupon the opal gel was formed By using different

compositions of the monomer solution, hole sizes, and

stacking structures of the starting inverse opal templates, opal

gels with correspondingly different properties can be 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 amount

pro-of tetrabutyl titanate (TBT) at ambient temperature for 24

h The TBT-swollen opal gel was then immersed in a water/ethanol (1:1 wt/wt) mixture for 5 h to let the TiO2sol/gelprocess proceed Figure 35A shows the opal structure of thegel/titania composite spheres formed After calcination, TiO2opal with distinctive spherical contours could be found The

compression degree, R, was adjusted by the spacer height

when 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 TiO2 Nanosheets

The preparation of TiO2nanosheets has also been exploredrecently.359-368Typically, TiO2nanosheets were synthesized

by delaminating layered protonic titanate into colloidal singlelayers A stoichiometric mixture of Cs2CO3and TiO2wascalcined 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 HCl solution 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 TiO2skeleton 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,

Figure 33. (A) Experimental procedure for fabricating TiO2

nanobowl arrays (B) Low- and high- (inset) magnification SEM

image of TiO2nanobowl arrays Reprinted with permission from

Wang, X D.; Graugnard, E.; King, J S.; Wang, Z L.; Summers,

Society

Figure 34 (A) SEM images of short and densely aligned ZnO

nanorod 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.

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opalescent appearance Figure 36 shows TEM and AFM

images of TiO2nanosheets with thicknesses of 1.2-1.3 nm,

which is the height of the TiO2nanosheet with a monolayer

of water molecules on both sides (0.70 + 0.25× 2) thick.366

3 Properties of TiO2 Nanomaterials

3.1 Structural Properties of TiO2 Nanomaterials

Figure 37 shows the unit cell structures of the rutile and

anatase TiO2.11 These two structures can be described in

terms of chains of TiO6octahedra, where each Ti4+ ion is

surrounded by an octahedron of six O2-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 anataseare larger, whereas the Ti-O distances are shorter than those

in 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 degree R of (B) 20%, (C) 35%, and (D) 50% The images were taken for the fractured

surfaces containing the direction of applied compression (Inset) Image of the same sample, but with the fracture surface perpendicular to

the direction of applied compression From: Ji, L.; Rong, J.; Yang, Z Chem Commun 2003, 1080 (http://dx.doi.org/10.1039/b300825h)

s Reproduced by permission of The Royal Society of Chemistry

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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.370

Three size regimes were recognized for the pressure-induced

phase transition of anatase at room temperature: an

anatase-amorphous transition regime at the smallest crystallite sizes,

an anatase-baddeleyite transition regime at intermediatecrystallite sizes, and an anatase-R-PbO2transition regimecomprising large nanocrystals to macroscopic single crystals.Barnard et al performed a series of theoretical studies onthe phase stability of TiO2nanoparticles 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-δO2δ-nanosheets (B and C) AFM image and height scan of the TiO2nanosheets 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,

Figure 37 Lattice structure of rutile and anatase TiO2 Reprinted

with permission from Linsebigler, A L.; Lu, G.; Yates, J T., Jr

Society

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

surface passivation.372 Their study on anatase and rutile

titanium dioxide polymorphs passivated with complete

monolayers of adsorbates by varying the hydrogen to oxygen

ratio with respect to a neutral, water-terminated surface

showed that termination with water consistently resulted in

the lowest values of surface free energy when hydrated or

with a higher fraction of H on the surface on both anatase

and rutile surfaces, but conversely, the surfaces generally

had a higher surface free energy when they had an equal

ratio of H and O in the adsorbates or were O-terminated.375

They demonstrated that, under different pH conditions from

acid to basic, the phase transition size of a TiO2nanoparticle

varied from 6.9 to 22.7 nm, accompanied with shape changes

of the TiO2nanoparticles as shown in Figure 38.374

Enyashin and Seifert conducted a theoretical study on the

structural stability of TiO2layer 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 TiO2

nanostructures relative to the infinite monolayer followed a

1/R2curve

Chen et al found that severe distortions existed in Ti site

environments in the structures of 1.9 nm TiO2nanoparticles

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 TiO2lattice

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

TiO2nanoparticles was likely dependent upon the details of

the preparation methods.378The TiO2nanoparticles 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 TiO2

Nanomaterials

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 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.73,74However, abnormal behaviors and inconsistent results areoccasionally observed

coarsen-Hwu et al found the crystal structure of TiO2nanoparticlesdepended largely on the preparation method.379 For smallTiO2nanoparticles (<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 reaching

a 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.381They concluded thatbrookite could not transform directly to rutile but had totransform to anatase first However, direct transformation

of brookite nanocrystals to rutile was observed above 973

K by Kominami et al.382

In a later study, Zhang and Banfield found that thetransformation sequence and thermodynamic phase stabilitydepended on the initial particle sizes of anatase and brookite

in their study on the phase transformation behavior ofnanocrystalline aggregates during their growth for isothermaland isochronal reactions.74They 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 TiO2polymorphs (rutile, anatase, and brookite) by high-temper-ature oxide melt drop solution calorimetry, and they foundthe energetic stability crossed over between the three phases

as shown in Figure 39.383The dark solid line represents thephases of lowest enthalpy as a function of surface area Rutilewas energetically stable for surface area < 592 m2/mol (7

m2/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) 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.

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,

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592 to 3174 m2/mol (7-40 m2/g or 200-40 nm), and anatase

was energetically stable for greater surface areas or smaller

sizes (<40 nm) The anatase and rutile energetics cross at

1452 m2/mol (18 m2/g or 66 nm) Assuming spherical

particles, the calculated average diameters of rutile and

brookite for a 7 m2/g surface area were 201 and 206 nm,

and those of brookite and anatase for a 40 m2/g surface area

are 36 and 39 nm These differences in particle size at the

same surface area existed because of the differences in

density If the phase transformation took place without further

coarsening, the particle size should be smaller after the

transformation Phase stability in a thermodynamic sense is

governed by the Gibbs free energy (∆G ) ∆H - T∆S) rather

than the enthalpy Rutile and anatase have the same entropy

Thus, the T ∆S will not significantly perturb the sequence of

stability seen from the enthalpies For nanocrystalline 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 during

coarsening was energetically downhill If anatase formed

initially, it could coarsen and transform first to brookite (at

40 m2/g) and then to rutile The energetic driving force for

the latter reaction (brookite to rutile) was very small,

explaining the natural persistence of coarse brookite In

contrast, the absence of coarse-grained anatase was consistent

with the much larger driving force for its transformation to

rutile.383

Li et al found that only anatase to rutile phase

transforma-tion occurred in the temperature range of 973-1073 K.384

Both anatase and rutile particle sizes increased with the

increase of temperature, but the growth rate was different,

as shown in Figure 40 Rutile had a much higher growth

rate than anatase The growth rate of anatase leveled off at

800 °C Rutile particles, after nucleation, grew rapidly,

whereas anatase particle size remained practically unchanged

With the decrease of initial particle size, the onset transition

temperature was decreased An increased lattice compression

of anatase with increasing temperature was observed Larger

distortions existed in samples with smaller particle size The

values for the activation energies obtained were 299, 236,

and 180 kJ/mol for 23, 17, and 12 nm TiO2nanoparticles,

respectively The decreased thermal stability in finer

nano-particles was primarily due to the reduced activation energy

as the size-related surface enthalpy and stress energy

increased

3.3 X-ray Diffraction Properties of TiO2

Nanomaterials

XRD is essential in the determination of the crystal

structure and the crystallinity, and in the estimate of the

crystal grain size according to the Scherrer equation

where K is a dimensionless constant, 2 θ is the diffraction

angle,λ is the wavelength of the X-ray radiation, and β is

the full width at half-maximum (fwhm) of the diffraction

peak.385 Crystallite size is determined by measuring the

broadening of a particular peak in a diffraction pattern

associated with a particular planar reflection from within the

crystal unit cell It is inversely related to the fwhm of an

individual peaksthe narrower the peak, the larger the

crystallite size The periodicity of the individual crystallite

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 TiO2nanoparticles

of different sizes111 and for TiO2 nanorods of differentlengths.129As the nanoparticle size increased, the diffractionpeaks became narrower In the anatase nanoparticle andnanorods developed by Zhang et al., the diameters of theTiO2nanoparticles and nanorods were both around 2.3 nm.The nanorods were elongated along the [001] direction with

preferred anisotropic growth along the c-axis of the anatase

lattice, which was indicated by the strong peak intensity andnarrow width of the (004) reflection and relatively lowerintensity and broader width for the other reflections With

an increase in length of the nanorods, the (004) diffractionpeak became much stronger and sharper, whereas other peaksremained similar in shape and intensity.129Similar resultshave been observed by other groups.123,127,177,183

3.4 Raman Vibration Properties of TiO2Nanomaterials

As the size of TiO2nanomaterials decreases, the featuredRaman scattering peaks become broader.255,318,370,386-395Thesize effect on the Raman scattering in nanocrystallineTiO is interpreted as originating from phonon confine-

Figure 40 (A) Changes in particle sizes of anatase and rutile

phases as a function of the annealing temperatures (B) Arrenhius

plot of ln(AR/A0) vs 1/T for activation energy calculations as a

function of the size of the TiO2nanoparticles ARand A0 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).

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ment,255,318,370,386,387,395 nonstoichiometry,391,392 or internal

stress/surface tension effects.390Among these theories, the

most convincing is the three-dimensional confinement of

phonons in nanocrystals.255,318,370,386,387,394,395 The phonon

confinement model is also referred to as the spatial

correla-tion model or q vector relaxacorrela-tion model It links the q vector

The anatase TiO2has six Raman-active fundamentals inthe vibrational spectrum: three Egmodes centered around

144, 197, and 639 cm-1(designated here Eg(1), Eg(2), and Eg(3),respectively), two B1gmodes at 399 and 519 cm-1 (desig-nated B1g(1) and B1g(2d)), and an A1gmode at 513 cm-1.370

As the particle size decreases, the Raman peaks showincreased broadening and systematic frequency shifts (Figure42).370The 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 the

B1g(1)mode and the B1g(2)+A1gmodes 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 effects

of 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 TiO2 Nanomaterials

The DOS of TiO2is 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.396The uppervalence bands can be decomposed into three main regions:theσ bonding in the lower energy region mainly due to O

pσbonding; theπ bonding in the middle energy region; and

O 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 the σ

bonding The conduction bands are decomposed into Ti eg(>5 eV) and t2gbands (<5 eV) The dxystates 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 t2gbands isidentified to be mostly dyzand 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 top

of the valence bands and the nonbonding dxystates 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 TiO2samples of different

diameters: (a) 5 nm; (b) 7 nm; (c) 13 nm Reprinted with permission

from Niederberger, M.; Bartl, M H.; Stucky, G D Chem Mater.

Powder XRD patterns of TiO2samples 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,

2005 Wiley-VCH

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neighbors and shares edges with four other neighbors,

forming a zigzag chain with a screw axis Thus, anatase is

less dense than rutile Also, anatase has a large metal-metal

distance of 5.35 Å As a consequence, the Ti dxyorbitals at

the bottom of the conduction band are quite isolated, while

the t2gorbitals at the bottom of the conduction band in rutile

provide the metal-metal interaction with a smaller distance

of 2.96 Å

The electronic structure of TiO2 has been studied with

various 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 level

diagram of the lowest unoccupied MOs of a [TiO6]8-cluster

with O h , D 2h (rutile), and D 2d(anatase) symmetry and the Ti

K-edge XANES and O K-edge ELNES spectra for rutile and

anatase.398The anatase structure is a tetragonally distorted

octahedral structure in which every titanium cation is

surrounded by six oxygen atoms in an elongated octahedral

geometry (D 2d) 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 TiO2was approximately 2.1 eV, slightly smallerthan that of bulk TiO2, as shown in Figure 45A.379Luca et

al found that 1s f 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 TiO2nanoparticles 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 the Eg(1)mode versuscrystallite size Reprinted with permission from Swamy, V.; Kuznetsov, A.; Dubrovinsky, L S.; Caruso, R A.; Shchukin, D G.; Muddle,

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It is well-known that for nanoparticles the band gap energy

increases and the energy band becomes more discrete with

decreasing size.84,406,407 As the size of a semiconductor

band gap blue shift (<0.1-0.2 eV) caused by quantum sizeeffects for spherical particles sizes down to 2 nm.58,60Suchsmall effects are mainly due to the relatively high 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 TiO2with particle sizes of 21, 133, and 267 Åmay in fact not be a quantum confinement effect.410Mon-ticone et al did an excellent study on the quatum size effects

in anatase nanoparticles and found no quantum size effect

in anatase TiO2nanoparticles for sizes 2R g 1.5 nm, but

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

3.6 Optical Properties of TiO2 Nanomaterials

The main mechanism of light absorption in pure 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.412This 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 absorption

semi-takes place at low (h ν < Eg+ Wc, where Wcis the width ofthe conduction band) photon energies Electron transitions

to any point in the conduction band become possible when

h ν ) 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 TiO2nanosheets 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 TiO2nanosheet was approximately0.1 V higher, while the upper edge of the valence band was0.5 V lower than that of anatase TiO2.360The absorption ofthe TiO2nanosheet colloid blue shifted (>1.4 eV) relative

to that of bulk TiO2 crystals (3.0-3.2 eV), due to a quantization effect, accompanied with a strong photolumi-nescence of well-developed fine structures extending intothe visible light regime.362,363The band gap energy shift,∆E,

size-Figure 43 (A) Total and projected densities of states (DOSs) of

the anatase TiO2structure The DOS is decomposed into Ti eg, Ti

t2g(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 valence

band (the vertical solid line) is taken as the zero of energy The

vertical dashed line indicates the conduction-band minimum as a

guide to the eye (B) Molecular-orbital bonding structure for anatase

TiO2: (a) atomic levels; (b) crystal-field split levels; (c) final

interaction states The thin-solid and dashed lines represent large

and small contributions, respectively Reprinted with permission

from Asahi, R.; Taga, Y.; Mannstadt, W.; Freeman, A J Phys.

ReV B 2000, 61, 7459 (http://link.aps.org/abstract/PRB/v61/p7459).

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by exciton confinement in anisotropic two-dimensional

crystallites is formulated as follows:

where h is Plank’s constant, µ xz and µ y are the reduced

effective masses of the excitons, and L , L , and L 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 peak

in the photoluminescence of TiO2nanosheets was assigned

to resonant luminescence The series of peaks extending into

a 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 with O h , D 2h (rutile), and D 2d(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.;

the American Physical Society

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Within the effective mass model, the energy spectrum of

2D TiO2nanosheets can be described by eq 10, where the

“plus” and “minus” signs correspond to the conduction and

valence bands, respectively, EG is the energy gap, p is

Planck’s constant, and meand mh are the effective masses

of the electrons and holes, respectively

The electronic band structure of a TiO2 nanotube can be

obtained from this relation by zone-folding and is given by

a series of quasi-1D sub-bands with different indices n

(Figure 48b):

This transition from the 2D to the quasi-1D energy spectrum

has a dramatic effect on the energy density of states In the

2D case, the density of states, G2D) mc.h/πp2, has a constant

value for energies outside the energy gap (see Figure 48c)

In the quasi-1D case, however, the density of states of each

sub-band

diverges at the band edge E n(0), leading to van Hove

singularities The resulting density of state is formed by a

series of sharp peaks with long overlapping tails (Figure 48c)

The energy gap between the valence and conductance bands

in the quasi-1D case is larger than that in the parental 2D

material, and the difference increases with decreasing

diameter of the nanotube The change in the energy gaps

between a nanosheet and a nanotube is

In TiO2, the effective masses of electrons me can vary

between 5m0 and 30m0, and the mass of holes mhis more

than 3m0 With me ) 9m0 and mh ) 3m0, the difference

between energy gaps of nanotubes with diameters 2.5 and 5

nm is 8 meV The energy difference between the two first

peaks in the density of states G1D(E) (Figure 48) is less than

24 meV for d ) 2.5 nm and 6 meV for d ) 5 nm, which are

too small to be resolved in room-temperature experiments

due to the thermal fluctuations of kT ) 26 meV.158

In the theoretical study conducted by Enyashin and Seifert

recently, the band structures for anatase nanotubes,

nano-strips, and nanorolls were similar to the DOS of the

corresponding bulk phase.376The valence band of both bulkTiO2and their nanostructures was composed of 3d Ti-2p

O 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), while

the lepidocrocite nanotubes were semiconductors with anindirect band gap (∼4.5 eV) Independent from the specific

topology of the titania nanostructures, the band gap proached the band gap of the corresponding nanocrystals withradii of about 25 Å.376

ap-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 TiO2nanoparticles by the nonequilibrium steady-state statistics ofelectrons.414They found that the electrons trapped in surfacestates did not generally equilibrate to the free electrons’ Fermi

level, 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

permission from Elsevier (B) Ti L 2.3 absorption of TiO2 crystals with different sizes Reprinted with permission from Choi,

nano-H C.; Ahn, nano-H J.; Jung, Y M.; Lee, M K.; Shin, nano-H J.; Kim, S

Society for Applied Spectroscopy

Trang 29

and the surface Fermi level (∆EFn - EFs) was found to

depend 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 Properties

of TiO2 Nanomaterials

After TiO2nanoparticles absorb, impinging photons with

energies equal to or higher than its band gap (>3.0 eV),

electrons are excited from the valence band into the

unoc-cupied conduction band, leading to excited electrons in the

conduction band and positive holes in the valence band

These charge carriers can recombine, nonradiatively or

radiatively (dissipating the input energy as heat), or get

trapped and react with electron donors or acceptors adsorbed

on the surface of the photocatalyst The competition between

these processes determines the overall efficiency for various

applications of TiO2 nanoparticles These 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

Chemical Society (B) Schematic illustration of electronic band structure: (a) TiO2 nanosheets; (b) anatase Reprinted with permission

UV-visible spectra of (a) TiO2 sheets and (b) a film of nanosheets on a SiO2 glass substrate The data for the colloidal suspension is

American Chemical Society

2-4O2(g) + vacancy (12)

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OH radicals and O vacancies, respectively The reverse of

reaction 12 generates O adatom intermediates upon exposing

defective surfaces to O2-(g).415Electrons and holes generated

in TiO2nanoparticles are localized at different defect sites

on the surface and in the bulk Electron paramagnetic

resonance (EPR) results showed that electrons were trapped

as two Ti(III) centers, while the holes were trapped as

oxygen-centered radicals covalently linked to surface

tita-nium atoms.416-419Howe and Gra¨tzel found that irradiation

at 4.2 K in vacuo produced electrons trapped at Ti4+sites

within the bulk and holes trapped at lattice oxide ions

immediately below the surface, which decayed rapidly in

the dark at 4.2 K In the presence of O2, trapped electrons

were removed and the trapped holes were stable to 77 K

Warming to room-temperature caused loss of trapped holes

and formation of O2-at the surface.416,417Hurum et al found

that, upon band gap illumination, holes appeared at the

surface and preferentially recombined with electrons in

surface trapping sites for mixed-phase TiO2, such as Degussa

P25, and recombination reactions were dominated by surface

reactions that followed charge migration.419

Colombo and Bowman studied the charge carrier dynamics

of TiO2nanoparticles with femtosecond time-resolved diffuse

reflectance spectroscopy and found a dramatic increase in

the population of trapped charge carriers within the first few

picoseconds.420,421Skinner et al found that the trapping time

for photogenerated electrons on 2 nm TiO2nanoparticles in

acetonitrile by ultrafast transient absorption was about 180

fs.422Serpone et al found that localization (trapping) of the

electron as a Ti3+species occurred with a time scale of about

30 ps and about 90% or more of the photogenerated electron/

hole pairs recombined within 10 ns.409They suggested that

photoredox chemistry occurring at the particle surface

emanated from trapped electrons and trapped holes rather

than from free valence band holes and conduction band

electrons Bahnemann et al found that, in 2.4 nm TiO2

nanoparticles, electrons were instantaneously trapped within

the duration of the laser flash (20 ns) Deeply trapped holes

were rather long-lived and unreactive, and shallowly trapped

holes were in a thermally activated equilibrium with free

holes which exhibited a very high oxidation potential.423

Szczepankiewicz and Hoffmann et al found that O2was

an efficient scavenger of conduction band electrons at the

gas/solid interface and the buildup of trapped carriers

eventually resulted in extended surface reconstruction

in-volving Ti-OH functionalities.415 They found that

photo-generated free conduction band electrons were coupled with

acoustic phonons in the lattice and their lifetimes were

lengthened when dehydrated.424 The photoexcited charge

carriers in TiO2nanoparticles produced Stark effect intensity

and 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 TiO2surface.424

Berger et al studied UV light-induced electron-hole pairexcitations in anatase TiO2nanoparticles by electron para-magnetic resonance (EPR) and IR spectroscopy.425 Thelocalized states such as holes trapped at oxygen anions (O-)

Figure 47 (A) (a) Absorption spectrum and (b) luminescence

excitation spectrum (wavelength of emission light is 400 nm) ofcolloidal TiO2nanotubes 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 TiO2

nanotubes of different mean diameters: (1) 2.5 nm; (2) 3.1 nm;(3) 3.5 nm; (4) 5 nm Room temperature, excitation wavelength

237 nm, slits width 5 nm The range of wavelengths, 455-490

nm, in the spectra is omitted due to the high signal of the 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 B 2005, 109, 8565.

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and electrons trapped at coordinatively unsaturated cations

(Ti3+ formation) were accessible to EPR spectroscopy

Delocalized and EPR silent electrons in the conduction band

may be traced by their IR absorption, which results from

their electronic excitation within the conduction band in the

infrared region (Figure 49) They found that, during

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 298

K, all stable hole and electron states were lost

4 Modifications of TiO2 Nanomaterials

Many applications of TiO2 nanomaterials are closelyrelated to its optical properties However, the highly efficientuse of TiO2nanomaterials is sometimes prevented by its wideband gap The band gap of bulk TiO2lies 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 50.11Thus, one of the goals for improvement of the performance

of TiO2nanomaterials is to increase their optical activity byshifting the onset of the response from the UV to the visibleregion.21,426-428There are several ways to achieve this goal.First, doping TiO2 nanomaterials with other elements cannarrow the electronic properties and, thus, alter the opticalproperties of TiO2nanomaterials Second, sensitizing TiO2with 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 TiO2nanomaterials in metal-TiO2nanocomposites can improve the performance In addition,the modification of the TiO2nanomaterials surface with othersemiconductors can alter the charge-transfer propertiesbetween TiO and the surrounding environment, thus im-

Figure 48 Schematic presentation of the transformation of the electron band structure of the nanosheet semiconductor accompanying the

formation of nanotubes: (a) band diagram of a 2-dimensional nanosheet; (b) band diagram of quasi-1-D nanotubes; (c) energy density ofstates for nanosheets (G2D) and nanotubes (G1D) EG 1Dand EG 2Dare the band gaps of the 1D and 2D structures, respectively k x and k yare

the wave vectors Reprinted with permission from Bavykin, D V.; Gordeev, S N.; Moskalenko, A V.; Lapkin, A A.; Walsh, F C J Phys.

Figure 49 Scheme of UV-induced charge separation in TiO2

Electrons from the valence band can either be trapped (a) by defect

states, which are located close to the conduction band (shallow

traps), or (b) in the conduction band, where they produce absorption

in the IR region Electron paramagnetic resonance spectroscopy

detects both electrons in shallow traps, Ti3+, and hole centers, O-

Reprinted with permission from Berger, T.; Sterrer, M.; Diwald,

O.; Knoezinger, E.; Panayotov, D.; Thompson, T L.; Yates, J T.,

Chemical Society

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proving the performance of TiO2 nanomaterials-based

de-vices

4.1 Bulk Chemical Modification: Doping

The 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 chemical

composition (chemical nature of the bonds between the atoms

or ions), its atomic arrangement, and its physical dimension

(confinement of carriers) for nanometer-sized materials The

chemical composition of TiO2 can be altered by doping

Specifically, the metal (titanium) or the nonmetal (oxygen)

component can be replaced in order to alter the material’s

optical properties It is desirable to maintain the integrity of

the crystal structure of the photocatalytic host material and

to produce favorable changes in electronic structure It

appears easier to substitute the Ti4+cation in TiO2with other

transition metals, and it is more difficult to replace the O

2-anion with other 2-anions due to differences in charge states

and ionic radii The small size of the nanoparticle is beneficial

for the modification of the chemical composition of TiO2

due to the higher tolerance of the structural distortion than

that of bulk materials induced by the inherent lattice strain

in nanomaterials.426,429

4.1.1.1 Metal-Doped TiO2 Nanomaterials Different

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

The preparation methods of non-metal-doped TiO2

nanoma-terials can be divided into three types: wet chemistry,

high-temperature treatment, and ion implantation on TiO2

nano-materials Wet chemistry methods usually involve hydrolysis

of a titanium precursor in a mixture of water and other

reagents, followed by heating Choi et al performed a

systematic study of TiO2nanoparticles doped with 21 metal

ions by the sol-gel method and found the presence of metal

ion dopants significantly influenced the photoreactivity,

charge carrier recombination rates, and interfacial

electron-transfer rates.434Li et al developed La3+-doped TiO2by the

sol-gel process and found that the lanthanum doping could

inhibit the phase transformation of TiO2, enhance the thermal

stability of the TiO2, reduce the crystallite size, and increase

the Ti3+content on the surface.442Nagaveni et al prepared

W, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO2

nanopar-ticles by a solution combustion method and found that the

solid solution formation was limited to a narrow range of

concentrations of the dopant ions.448Wang et al prepared

largely dependent on both the nature and the concentration

of the alkaline, with the best crystallinity obtained for doped TiO2and the lowest for K-doped TiO2.430Cao et al.prepared Sn4+-doped TiO2nanoparticle films by the plasma-enhanced CVD method and found that, after doping by Sn,more surface defects were present on the surface.433Gracia

Li-et al synthesized M (Cr, V, Fe, Co)-doped TiO2by ion beaminduced CVD and found that TiO2 crystallized into theanatase or rutile structures depending on the type and amount

of cations present with partial segregation of the cations inthe form of M2Onafter annealing.438Wang et al synthesizedFe(III)-doped TiO2nanoparticles using oxidative pyrolysis

of liquid-feed organometallic precursors in a frequency (RF) thermal plasma and found that the formation

radiation-of rutile was strongly promoted with iron doping compared

to the anatase phase being prevalent in the undoped TiO2.246

4.1.1.2 Nonmetal-Doped TiO2 Nanomaterials Various

nonmetal elements, such as B, C, N, F, S, Cl, and Br, havebeen successfully doped into TiO2nanomaterials C-dopedTiO2nanomateirals have been obtained by heating titaniumcarbide472-474or by annealing TiO2 under CO gas flow athigh temperatures (500-800°C)475or by direct burning of

a titanium metal sheet in a natural gas flame.476N-doped TiO2 nanomaterials have been synthesized byhydrolysis of TTIP in a water/amine mixture and the post-treatment of the TiO2sol with amines426,428,477-482or directlyfrom a Ti-bipyridine complex483or by ball milling of TiO2

in a NH3water solution.484N-doped TiO2nanomaterials werealso obtained by heating TiO2under NH3flux at 500-600

°C485,486 or by calcination of the hydrolysis product ofTi(SO4)2with ammonia as precipitator487or by decomposition

of gas-phase TiCl4 with an atmosphere microwave plasmatorch488 or by sputtering/ion-implanting techniques withnitrogen489,490or N2+gas flux.491

S-doped TiO2nanomaterials were synthesized by mixingTTIP with ethanol containing thiourea492-494or by heatingsulfide powder495,496or by using sputtering or ion-implantingtechniques with S+ion flux.497-499Different doping methodscan induce the different valence states of the dopants Forexample, the incorporated S from thiourea had S4+ or S6+state,492-494while direct heating of TiS2or sputtering with

S+induced the S2-anion.496-499F-doped TiO2nanomaterials 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 H2TiF6505,506or usingion-implanting techniques with F+ion flux.507Cl-and Br-co-doped nanomaterials were synthesized by adding TiCl4

to ethanol containing HBr.508

4.1.2.1 Electronic Properties of Doped TiO2

Nanoma-terials 4.1.2.1.1 Metal-Doped TiO Nanomaterials

Ac-Reprinted with permission from Linsebigler, A L.; Lu, G.; Yates,

Chemical Society

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cording to Soratin and Schwarz’s study, the electronic states

of TiO2can be decomposed into three parts: theσ bonding

of the O pσand Ti egstates in the lower energy region; the

π bonding of the O p πand Ti egstates in the middle energy

region; 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 dxyorbital 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 Ti2gstates

are antibonding with the O pπstates The upper CB consists

of theσ antibonding orbitals between the O p σ and Ti eg

states

The electronic structures, i.e., the densities of states

(DOSs), of V-, Cr-, Mn-, Fe-, Co-, and Ni-doped TiO2were

analyzed by ab initio band calculations based on the density

functional theory with the full-potential linearized augmented

plane wave (FLAPW) method by Umebayashi et al (Figure

51B).509They found that when TiO2was doped with V, Cr,

Mn, Fe, or Co, an electron occupied level formed and the

electrons were localized around each dopant As the atomic

number of the dopant increased, the localized level shifted

to lower energy The energy of the localized level due to

Co doping was low enough that it lay at the top of the valence

band while the other metals produced midgap states The

electrons from the Ni dopant were somewhat delocalized,

thus significantly contributing to the formation of the valence

band with the O p and Ti 3d electrons The states due to the

3d dopants shifted to a lower energy as the atomic number

of the dopant increased For Ti1-xVxO2: two localized levels

occurred at 1.5 eV above the VB (a) and between the lower

and upper CBs (b) Level a was occupied by one electron

consisting of the V t2g and O pπ states and was localized

around V Level b consisted of the V eg and O pσ states

forming the σ antibonding orbital For Cr- and Mn-doped

TiO2, state c was localized at 1.0 eV (0.5 eV for Mn) above

the VB due to Cr (Mn) t2gand O pπ, the former of which

was occupied by 2 (3) electrons Theσ antibonding orbital

formed by the Cr (Mn) egand O pσstates 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 the

VB for Co) due to theπ antibonding of the Fe egand O pπ

states This level was occupied by four (or five for Co)electrons The Fe (Co) egstate was split into d z2(f) and d x2-y2

(g) orbitals in the band gap For Ni-doped TiO2, the π

antibonding of the Ni t2g and O pπ states was somewhatdelocalized and appeared within the VB (h) due to the Ni eg

states from the d z2and d x2-y2orbitials 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 TiO2nanoparticlesreduced 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).444Wang and Doren found that

Nd 4f electrons changed the electronic structure of Nd-dopedTiO2into the half-metallic or the insulating ground state510and that V 3d states were located at the bottom of theconduction band of the TiO2host 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 TiO 2 Nanomaterials Recent

theoretical and experimental studies have shown that thedesired band gap narrowing of TiO2can also be achieved

by 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 TiO2withdifferent 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 52.489Inthis 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,

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Nakano et al found three deep levels located at

ap-proximately 0.86, 1.30, and 2.34 eV below the conduction

band in C-doped TiO2, which were attributed to the intrinsic

nature of TiO2for the first one and the two levels newly

introduced by the C doping.522In particular, the pronounced

2.34 eV band contributed to band gap narrowing by mixing

with the O 2p valence band.522 Lee et al., in their

first-principles density-functional LDA pseudopotential

calcula-and rutile polymorphs, N 2p localized states were just above the top of the O 2p 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.512Experimental evidence supported the statement thatnitrogen-doped TiO2formed nitrogen-induced midgap levelsslightly above the oxygen 2p valence band.486Lee 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.517Wang and Doren found that N doping introduced some states

at the valence band edge and thus made the original bandgap of TiO2smaller, and that a vacancy could induce somestates in the band gap region, which acted as shallowdonors.510Nakano 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 as

an efficient generation-recombination center and to the Ndoping which contributed to band gap narrowing by mixingwith the O 2p valence band, respectively.523Okato 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,489and the mixing of the sulfur 3p states with thevalence band was found to contribute to the increased width

of 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, and

S 3p states contributed to the conduction band of TiO2 asshown in Figure 53A.494

When F replaced the O in the TiO2 lattice, F 2p stateswere localized below the O 2p valence states without 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 composed

of the t2gstate of the Ti 3d orbital.507The electronic changeinduced by F dopant was considered to be similar to the Ovacancy, thus reducing the effective band gap and improvingvisible light photoresponse.507Li 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 influence

on the band structure in N-F-co-doped TiO2.519

4.1.2.2 Optical Properties of Doped TiO 2

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

Nanomaterials A red shift in the band gap transition or a

Figure 52 (A) Total DOSs of doped TiO2and (B) the projected

DOSs into the doped anion sites, calculated by FLAPW, for the

dopants F, N, C, S, and P located at a substitutional site for an O

atom in the anatase TiO2 crystal (eight TiO2 units per cell) Ni

-doped stands for N doping at an interstitial site, and Ni+s-doped

stands for doping at both substitutional and interstitial sites

Reprinted with permission from Asahi, R.; Morikawa, T.; Ohwaki,

T.; Aoki, K.; Taga, Y Science 2001, 293, 269

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visible light absorption was observed in metal-doped TiO2

(refs 433-435, 438, 444, 445, 448, 449, 460-463, 465, 466,

470, 509, 548, and 549) For V-, Mn-, or Fe-doped TiO2,

the absorption spectra shifted to a lower energy region with

an increase in the dopant concentration.434,445,460This red shift

was attributed to the charge-transfer transition between the

d electrons of the dopant and the CB (or VB) of TiO2

Metal-ion doped TiO2prepared by ion implantation with various

transition-metal ions such as V, Cr, Mn, Fe, and Ni was

found to have a large shift in the absorption band toward

the visible light region, with the order of the effectiveness

in the red shift being V > Cr > Mn > Fe > Ni.466-471Anpo

et al found that the absorption band of Cr-ion-implanted

TiO2shifted smoothly toward the visible light region, with

the extent of the red shift depending on the amount of metal

ions implanted as shown in Figure 54A.470Impregnated or

chemically Cr-ion-doped TiO2 showed no shift in the

absorption edge of TiO2; however, a new absorption band

appeared at around 420 nm as a shoulder peak due to the

formation of an impurity energy level within the band gap,

with its intensity increasing with the number of Cr ions

(Figure 54B).470

In the study by Umebayashi et al., visible light absorption

of V-doped TiO2was due to the transition between the VB

and the V t2g level.509 The holes in the VB produced an

anodic photocurrent The photoexcitation processes under

visible light of V-, Cr-, and Mn-doped TiO2are illustrated

in Figure 55 Photoexcitation for V-, Cr-, Mn-, and Fe-doped

TiO2 occurred via the t2g level of the dopant The visible

light absorption for Mn- and Fe-doped TiO2was due to the

optical transitions from the impurity band tail into the CB

The Mn (Fe) t level was close to the VB and easily

overlapped in highly impure media The visible lightabsorption for the Cr-doped TiO2can be attributed to a donortransition from the Cr t2glevel into the CB and the acceptortransition from the VB to the Cr t2glevel

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.287The mesoporous TiO2acted as a sensitizer

4.1.2.2.2 Optical Properties of Nonmetal-Doped TiO 2

Nanomaterials Nonmetal doped TiO2normally has a colorfrom white to yellow or even light gray, and the onset ofthe absorption spectra red shifted to longer wavelengths (refs

385, 426, 478, 483, 489, 494, 495, 497, 498, 505, 506, 512,

516, 518, 519, 521, and 529) In N-doped TiO2rials, the band gap absorption onset shifted 600 nm from

nanomate-380 nm for the undoped TiO2, extending the absorption up

to 600 nm, as shown in Figure 56.426The optical absorption

of N-doped TiO2 in the visible light region was primarilylocated between 400 and 500 nm, while that of oxygen-deficient TiO2was mainly above 500 nm from their density-functional theory study.520N-F-co-doped TiO2prepared byspray pyrolysis absorbs light up to 550 nm in the visiblelight spectrum.518The S-doped TiO2 also displayed strongabsorption in the region from 400 to 600 nm.494The red shifts

in 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 TiO2in the visible region.508Livraghi et al recently found that N-doped TiO2containedsingle 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 with

permission from Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai,

K.; Mitsui, T.; Matsumura, M Appl Catal A 2004, 265, 115,

of F-doped TiO2calculated by FLAPW Egindicates 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.

Figure 54 (A) The UV-vis absorption spectra of TiO2(a) and

Cr 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.

Tiêu đề	Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications
Tác giả	Xiaobo Chen, Samuel S. Mao
Trường học	Lawrence Berkeley National Laboratory, and University of California, Berkeley
Chuyên ngành	Materials Science and Nanotechnology
Thể loại	journal article
Năm xuất bản	2007
Thành phố	Berkeley

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Số trang	70
Dung lượng	7,28 MB