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

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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 4.2.1. Inorganic Sensitization 2926 5. Applications of TiO2Nanomaterials 2929 5.1. Photocatalytic Applications 2929

5.1.1. Pure TiO2 Nanomaterials: First Generation

2930 5.1.2. Metal-Doped TiO2Nanomaterials:

Second Generation

2930 5.1.3. Nonmetal-Doped TiO2 Nanomaterials:

Third Generation

2931

5.2. Photovoltaic Applications 2932 5.2.1. The TiO2Nanocrystalline Electrode in

DSSCs

2932 5.2.2. Metal/Semiconductor Junction Schottky

Diode Solar Cell

2938 5.2.3. Doped TiO2Nanomaterials-Based Solar

Cell

2938 5.3. Photocatalytic Water Splitting 2939

5.3.1. Fundamentals of Photocatalytic Water Splitting

2939 5.3.2. Use of Reversible Redox Mediators 2939 5.3.3. Use of TiO2Nanotubes 2940 5.3.4. Water Splitting under Visible Light 2941 5.3.5. Coupled/Composite Water-Splitting

System

2942

5.4. Electrochromic Devices 2942

5.4.1. Fundamentals of Electrochromic Devices 2943 5.4.2. Electrochromophore for an Electrochromic

Device

2943 5.4.3. Counterelectrode for an Electrochromic

Device

2944 5.4.4. Photoelectrochromic Devices 2945

5.5. Hydrogen Storage 2945

5.6. Sensing Applications 2947

6. Summary 2948

7. Acknowledgment 2949

8. References 2949

1. Introduction

Since its commercial production in the early twentieth century, titanium dioxide (TiO2) has been widely used as a pigment¹and in sunscreens,^2,3paints,⁴ointments, toothpaste,⁵ etc. In 1972, Fujishima and Honda discovered the phenom- enon of photocatalytic splitting of water on a TiO₂electrode under ultraviolet (UV) light.^6-8Since then, enormous efforts have been devoted to the research of TiO₂material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors.^9-12These applications can be roughly divided into “energy” and “environmental” categories, many of which depend not only on the properties of the TiO₂material itself but also on the modifications of the TiO2material host (e.g., with inorganic and organic dyes) and on the interactions of TiO2materials with the environment.

An exponential growth of research activities has been seen in nanoscience and nanotechnology in the past decades.^13-17 New physical and chemical properties emerge when the size of the material becomes smaller and smaller, and down to

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

†E-mail: SSMao@lbl.gov.

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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 properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials.^13-16 The specific surface area and surface-to-volume ratio increase dramatically as the size of a material decreases.^13,21The 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 TiO2mate- rials are expected to play an important role in helping solve

with the preparation method. The preparations of mesoporous/nanoporous TiO₂, TiO₂ aerogels, opals, and photonic materials are summarized separately. In reviewing nanoma- terial synthesis, we present a typical procedure and representative transmission or scanning electron microscopy images to give a direct impression of how these nanomaterials are obtained and how they normally appear. For detailed instructions on each synthesis, the readers are referred to the corresponding literature.

The structural, thermal, electronic, and optical properties of TiO₂nanomaterials are reviewed in the second section.

As the size, shape, and crystal structure of TiO2nanomate- rials vary, not only does surface stability change but also the transitions between different phases of TiO2 under pressure or heat become size dependent. The dependence of X-ray diffraction patterns and Raman vibrational spectra on the size of TiO₂nanomaterials is also summarized, as they could help to determine the size to some extent, although correlation of the spectra with the size of TiO₂nanomaterials is not straightforward. The review of modifications of TiO2

nanomaterials is mainly limited to the research related to the modifications of the optical properties of TiO2nanoma- terials, since many applications of TiO2nanomaterials are closely related to their optical properties. TiO₂nanomaterials normally are transparent in the visible light region. By doping or sensitization, it is possible to improve the optical sensitiv- ity and activity of TiO2 nanomaterials in the visible light region. Environmental (photocatalysis and sensing) and energy (photovoltaics, water splitting, photo-/electrochromics, and hydrogen storage) applications are reviewed with an emphasis on clean and sustainable energy, since the increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. The fundamentals and working principles of the TiO₂nanomaterials-based devices are discussed to facilitate the under- standing and further improvement of current and practical TiO2nanotechnology.

2. Synthetic Methods for TiO

2

Nanostructures 2.1. Sol−Gel Method

The sol-gel method is a versatile process used in making various ceramic materials.^46-50In a typical sol-gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase. Thin films can be produced on a piece of 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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TiO2nanomaterials have been synthesized with the sol- gel method from hydrolysis of a titanium precusor.^51-78This process normally proceeds via an acid-catalyzed hydrolysis step of titanium(IV) alkoxide followed by condensation.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 developed 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 temperature due to the temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO2.⁶³Second- ary particles are formed by epitaxial self-assembly of primary particles at longer times and higher temperatures, and the number of primary particles per secondary particle increases with time. The average 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, ethylenediamine, trimethylenediamine, and triethyl- enetetramine. The morphology of the TiO2 nanoparticles

sodium stearate.⁷⁰The shape control is attributed to the tuning of the growth rate of the different crystal planes of TiO₂ nanoparticles by the specific adsorption of shape controllers to these planes under different pH conditions.⁷⁰

A prolonged heating time below 100°C for the as-prepared gel can be used to avoid the agglomeration of the TiO2nano- particles during the crystallization process.^58,72 By heating amorphous TiO₂in air, large quantities of single-phase anatase TiO2nanoparticles with average particle sizes between 7 and 50 nm can be obtained, as reported by Zhang and Banfield.^73-77Much effort has been exerted to achieve highly crystallized and narrowly dispersed TiO₂nanoparticles using the sol-gel method with other modifications, such as a semicontinuous reaction method by Znaidi et al.⁷⁸and a two- stage mixed method and a continuous reaction method by Kim et al.^53,54

By a combination of the sol-gel method and an anodic alumina membrane (AAM) template, TiO2 nanorods have been successfully synthesized by dipping porous AAMs into a boiled TiO2 sol followed by drying and heating processes.^92,93In a typical experiment, a TiO₂sol solution is prepared by mixing TTIP dissolved in ethanol with a solution containing water, acetyl acetone, and ethanol. An AAM is immersed into the sol solution for 10 min after being boiled in ethanol; then it is dried in air and calcined at 400°C for 10 h. The AAM template is removed in a 10 wt % H₃PO₄ aqueous solution. The calcination temperature can be used to control the crystal phase of the TiO₂ nanorods. At low temperature, anatase nanorods can be obtained, while at high temperature rutile nanorods can be obtained. The pore size of the AAM template can be used to control the size of these TiO₂nanorods, which typically range from 100 to 300 nm in diameter and several micrometers in length. Appar- ently, the size distribution of the final TiO₂ nanorods is largely controlled by the size distribution of the pores of the AAM template. In order to obtain smaller and mono- sized TiO2nanorods, it is necessary to fabricate high-quality AAM templates. Figure 3 shows a typical TEM for TiO2

nanorods fabricated with this method. Normally, the TiO2

nanorods are composed of small TiO2 nanoparticles or nanograins.

By electrophoretic deposition of TiO₂colloidal suspensions into the pores of an AAM, ordered TiO2 nanowire arrays can be obtained.⁹⁴In a typical procedure, TTIP is dissolved in ethanol at room temperature, and glacial acetic acid mixed with deionized water and ethanol is added under pH ) 2-3 with nitric acid. Platinum is used as the anode, and an AAM with an Au substrate attached to Cu foil is used as the cathode. A TiO2sol is deposited into the pores of the AMM under a voltage of 2-5 V and annealed at 500°C for 24 h.

After dissolving the AAM template in a 5 wt % NaOH solution, isolated TiO2nanowires 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 AAM^95-98and other organic compounds.^99,100For example, when an AAM is used as the template, a thin layer of TiO2sol 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; TiO₂nanowires are obtained after the sol is fully developed and the AAM is removed. In the procedure by Lee and co-workers,⁹⁶a TTIP solution was prepared by mixing TTIP with 2-propanol and 2,4-pentanedione. 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 Sci. 2003, 259, 53, Copyright 2003, with permission from Elsevier.

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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, TiO2sol is

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

nanorod arrays. The ZnO nanorod template is etched-up by immersing the ZnO/TiO2nanorod arrays in a dilute hydrochloric acid aqueous solution to obtain TiO₂nanotube arrays.

Figure 5 shows a typical SEM image of the TiO2nanotube array with the ZnO nanorod array template. The TiO₂ nanotubes inherit the uniform hexagonal cross-sectional shape 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 arrays can only be obtained from an optimal dip-coating cycle number in the range of 2-3 cycles. A dense porous TiO2

thick film with holes is obtained instead if the dip-coating number further increases. The heating rate is critical to the formation of TiO₂ nanotube arrays. When the heating rate is extra rapid, e.g., above 6 °C min^-1, the TiO2 coat will easily crack and flake off from the ZnO nanorods due to great tensile stress between the TiO2 coat and the ZnO template, and a TiO₂film with loose, porous nanostructure is obtained.

2.2. Micelle and Inverse Micelle Methods

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

Figure 4. SEM image of TiO2nanotubes prepared from the AAO template. Reprinted with permission from Liu, S. M.; Gan, L. M.;

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

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

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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(oxyethylene)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 conditions, and the key factors affecting the formation of rutile at room temperature included the acidity, the type of acid used, and the microenvironment of the reverse micelles. Ag- glomeration of the particles occurred with prolonged reaction times and increasing the [H2O]/[NP-5] and [H2O]/[Ti- (OC₄H₉)₄] ratios. When suitable acid was applied, round TiO₂ nanoparticles could also be obtained. Representative TEM images of the shuttle-like and round-shaped TiO₂nanoparticles 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 methacrylate) (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 produce crystalline TiO2 nanoparticles with unchanged physical dimensions and minimal agglomeration and allows the preparation 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-gel processes and usually involves the reaction of titanium chloride with a variety of different oxygen donor molecules, e.g., a metal alkoxide or an organic ether.^111-119

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.

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

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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.^120,121 For example, Scolan and Sanchez found that monodisperse nonaggregated TiO₂nanoparticles in the 1-5 nm range were obtained through hydrolysis of titanium butoxide in the presence of acetylacetone and p-toluenesulfonic acid at 60

°C.¹²⁰The resulting nanoparticle xerosols could be dispersed in water-alcohol or alcohol solutions at concentrations higher than 1 M without aggregation, which is attributed to the complexation of the surface by acetylacetonato ligands and through an adsorbed hybrid organic-inorganic layer made with acetylacetone, p-toluenesulfonic acid, and wa- ter.¹²⁰

With the aid of surfactants, different sized and shaped TiO2

nanorods 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 hydrolysis process of TTIP in oleic acid (OA).122-126,130Typically, TTIP was added into dried OA at 80-100 °C under inert gas protection (nitrogen flow) and stirred for 5 min. A 0.1-2 M aqueous base solution was then rapidly injected and kept at 80-100°C for 6-12 h with stirring. The bases employed included organic amines, such as trimethylamino-N-oxide, trimethylamine, tetramethylammonium hydroxide, tetrabut- ylammonium hydroxyde, triethylamine, and tributylamine.

In this reaction, by chemical modification of the titanium precursor with the carboxylic acid, the hydrolysis rate of titanium alkoxide was controlled. Fast (in 4-6 h) crystallization in mild conditions was promoted with the use of suitable catalysts (tertiary amines or quaternary ammonium hydroxides). A kinetically overdriven growth mechanism led to the growth of TiO₂nanorods instead of nanoparticles.¹²³ Typical TEM images of the TiO2 nanorods are shown in Figure 9.¹²³

Recently, Joo et al.¹²⁷and Zhang et al.¹²⁹reported similar procedures in obtaining TiO₂nanorods without the use of catalyst. Briefly, a mixture of TTIP and OA was used to generate OA complexes of titanium at 80°C in 1-octadecene.

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.;

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 TiO₂nanocrystals 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 isopropoxide. Briefly, a dioctyl ether solution containing TOPO and lauric acid was heated to 300°C followed by addition of titanium chloride under vigorous stirring. The reaction was initiated by the rapid injection of TTIP and quenched with cold toluene. At low lauric acid concentrations, bullet- and diamond-shaped nanocrystals were obtained; at higher concentrations, rod-shaped nanocrystals or a mixture of nanorods and branched nanorods was observed. The bullet- and diamond-shaped nanocrystals and nanorods were elongated 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 the reaction in aqueous solutions. The temperature can be elevated above the boiling point of water, reaching the pressure of vapor saturation. The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced. It is a method that is widely used for the production of small particles in the ceramics industry.

Many groups have used the hydrothermal method to prepare TiO₂nanoparticles.^131-140For example, TiO₂nanoparticles can be obtained by hydrothermal treatment of peptized precipitates of a titanium precursor with water.¹³⁴ The precipitates were prepared by adding a 0.5 M isopropanol solution of titanium butoxide into deionized water ([H2O]/

[Ti] ) 150), and then they were peptized at 70°C for 1 h in the presence of tetraalkylammonium hydroxides (peptizer).

After filtration and treatment at 240 °C for 2 h, the as-obtained powders were washed with deionized water and absolute ethanol and then dried at 60°C. Under the same concentration of peptizer, the particle size decreased with increasing alkyl chain length. The peptizers and their concentrations influenced the morphology of the particles.

Typical TEM images of TiO2nanoparticles made with the hydrothermal method are shown in Figure 12.¹³⁴

In another example, TiO2nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic ethanol-water solution.¹³²Briefly, TTIP was added dropwise to a mixed ethanol and water solution at pH 0.7 with nitric acid, and reacted at 240°C for 4 h. The TiO2nanoparticles 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, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright 2005 Wiley-VCH.

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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 been synthesized 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 inorganic salts.141,143-146Figure 13 shows a typical TEM image of the TiO2 nanorods prepared with the hydrothermal method.¹⁴¹ The morphology of the resulting nanorods can be tuned with different surfactants¹⁴⁶or by changing the solvent composi- tions.¹⁴⁵A film of assembled TiO2nanorods deposited on a glass wafer was reported by Feng et al.¹⁴² These TiO₂ nanorods were prepared at 160°C for 2 h by hydrothermal treatment of a titanium trichloride aqueous solution super- saturated with NaCl.

TiO₂nanowires have also been successfully obtained with the 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 for 24-72 h without stirring within an autoclave. Figure 14 shows the SEM images of TiO2nanowires and a TEM image of a single nanowire prepared by Zhang and co-workers.¹⁵⁰ TiO2nanowires can also be prepared from layered titanate particles 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 the hydrothermal method. Reprinted from Yang, J.; Mei, S.; Ferreira, J. M. F. Mater. Sci. Eng. C 2001, 15, 183, Copyright 2001, with permission from Elsevier.

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

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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 nanowires 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 solution, Ti-O-Ti bonds or Ti-O-H-O-Ti hydrogen bonds were generated.

The bond distance from one Ti to the next Ti on the surface decreased. This resulted in the folding of the sheets and the

connection between the ends of the sheets, resulting in the formation of a tube structure. In this mechanism, the TiO₂ nanotubes were formed in the stage of the acid treatment following the alkali treatment. Figure 16 shows typical TEM images of TiO2nanotubes made by Kasuga et al.¹⁵³However, Du and co-workers found that the nanotubes were formed during the treatment of TiO2in NaOH aqueous solution.¹⁶¹ A 3D f 2D f 1D formation mechanism of the TiO2

nanotubes was proposed by Wang and co-workers.¹⁷¹It stated that the raw TiO₂ was first transformed into lamellar structures and then bent and rolled to form the nanotubes.

For the formation of the TiO₂nanotubes, the two-dimensional lamellar TiO2 was essential. Yao and co-workers further suggested, 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.;

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.;

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

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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 hydrothermal 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 size distribution and dispersity.^177-179The solvothermal method has been employed to synthesize TiO2 nanoparticles and nanorods with/without the aid of surfactants.^177-185 For example, in a typical procedure by Kim and co-workers,¹⁸⁴ TTIP was mixed with toluene at the weight ratio of 1-3:10 and kept at 250°C for 3 h. The average particle size of TiO2

powders tended to increase as the composition of TTIP in the solution increased in the range of weight ratio of 1-3:

10, while the pale crystalline phase of TiO₂was not produced at 1:20 and 2:5 weight ratios.¹⁸⁴By controlling the hydrolyzation reaction of Ti(OC4H9)4and linoleic acid, redispers- ible TiO₂nanoparticles and nanorods could be synthesized, as found by Li et al. recently.¹⁷⁷The decomposition of NH4- HCO₃could provide H₂O for the hydrolyzation reaction, and linoleic acid could act as the solvent/reagent and coordination surfactant in the synthesis of nanoparticles. Triethylamine could act as a catalyst for the polycondensation of the Ti- O-Ti inorganic network to achieve a crystalline product and had little influence on the products’ morphology. The chain lengths of the carboxylic acids had a great influence on the formation of TiO2, and long-chain organic acids were important and necessary in the formation of TiO₂.¹⁷⁷Figure 18 shows a representative TEM image of TiO2nanoparticles from their study.¹⁷⁷

TiO₂nanorods with narrow size distributions can also be developed with the solvothermal method.^177,183For example, in a typical synthesis from Kim et al., TTIP was dissolved in anhydrous toluene with OA as a surfactant and kept at 250 °C for 20 h in an autoclave without stirring.¹⁸³ Long dumbbell-shaped nanorods were formed when a sufficient amount of TTIP or surfactant was added to the solution, due to the oriented growth of particles along the [001] axis. At a fixed precursor to surfactant weight ratio of 1:3, the concentration of rods in the nanoparticle assembly increased as the concentration of the titanium precursor in the solution increased. The average particle size was smaller and the size distribution was narrower than is the case for particles synthesized without surfactant. The crystalline phase, diameter, and length of these nanorods are largely influenced by the precursor/surfactant/solvent weight ratio. Anatase nano- 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.

Figure 18. TEM micrographs of TiO2nanoparticles prepared with the solvothermal method. Reprinted with permission from Li, X.

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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 representative TEM images of the TiO₂nanowires prepared from the solvothermal method.¹⁸¹Alternatively, bamboo-shaped Ag- doped TiO₂nanowires were developed with titanium butoxide 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-crystal TiO₂nanowires.¹⁸⁰

2.6. Direct Oxidation Method

TiO₂ nanomaterials can be obtained by oxidation of titanium metal using oxidants or under anodization. Crystal- line TiO2nanorods have been obtained by direct oxidation of a titanium metal plate with hydrogen peroxide.^186-191 Typically, TiO2nanorods on a Ti plate are obtained when a cleaned Ti plate is put in 50 mL of a 30 wt % H₂O₂solution at 353 K for 72 h. The formation of crystalline TiO2occurs through a dissolution precipitation mechanism. By the addition of inorganic salts of NaX (X ) F^-, Cl^-, and SO42-), the crystalline phase of TiO₂ nanorods can be controlled.

The addition of F^-and SO42-helps the formation of pure anatase, while the addition of Cl^- favors the formation of rutile.¹⁸⁹ Figure 21 shows a typical SEM image of TiO2

nanorods prepared with this method.¹⁸⁶

At high temperature, acetone can be used as a good oxygen source and for the preparation of TiO2nanorods 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.

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 directly oxidizing a Ti plate with a H2O2solution. Reprinted from Wu, J.

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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 resistance, and wear resistance properties of various substrates.

They are also used to form free-standing bodies, films, and fibers and to infiltrate fabric to form composite materials.

Recently, they have been widely explored to fabricate various nanomaterials. Vapor deposition processes usually take place within a vacuum chamber. If no chemical reaction occurs, this process is called physical vapor deposition (PVD);

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

Thick crystalline 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/oxygen atmosphere, using liquid precursor delivery.²³⁰When deposited on the cold areas of the reactor at temperatures below 90°C with plasma enhanced CVD, amorphous TiO₂nanoparticles can be obtained and crystallize with a relatively high surface area after being annealed at high temperatures.²³¹ TiO2nanorod arrays with a diameter of about 50-100 nm and a length of 0.5-2 µm can be synthesized by metal organic CVD (MOCVD) on a WC-Co substrate using TTIP as the precursor.²³²

Figure 24 shows the TiO₂nanorods grown on fused silica substrates with a template- and catalyst-free MOCVD method.²³³In a typical procedure, titanium acetylacetonate (Ti(C10H14O5)) vaporizing in the low-temperature zone of a furnace at 200-230°C is carried by a N₂/O₂flow into the high-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 anodic oxidation. 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 TiCl3solution with

pH ) 2 with a pulsed electrodeposition approach, and titanium and/or its compound are deposited into the pores of the AAM. By heating the above deposited template at 500°C for 4 h and removing the template, pure anatase TiO2

nanowires can be obtained. Figure 26 shows a representative SEM image of TiO2nanowires.²⁵⁶

2.10. Sonochemical Method

Ultrasound has been very useful in the synthesis of a wide range of nanostructured materials, including high-surface- area transition metals, alloys, carbides, oxides, and colloids.

The chemical effects of ultrasound do not come from a direct interaction with molecular species. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating (∼5000 K), high pres- sures (∼1000 atm), and enormous heating and cooling rates (>10⁹K/s). The sonochemical method has been applied to prepare various TiO₂nanomaterials by different groups.^257-269 Yu et al. applied the sonochemical method in preparing highly photoactive TiO₂ nanoparticle photocatalysts with anatase and brookite phases using the hydrolysis of titanium tetraisoproproxide in pure water or in a 1:1 EtOH-H₂O solution under ultrasonic radiation.¹⁰⁹Huang et al. found that anatase and rutile TiO₂nanoparticles as well as their mixtures could be selectively synthesized with various precursors using ultrasound irradiation, depending on the reaction temperature and the precursor used.²⁵⁹Zhu et al. developed titania whiskers and nanotubes with the assistance of sonication as shown in Figure 27.²⁶⁹They found that arrays of TiO2nanowhiskers with a diameter of 5 nm and nanotubes with a diameter of∼5 nm and a length of 200-300 nm could be obtained by sonicating TiO2particles in NaOH aqueous solution followed by washing with deionized water and a dilute HNO3aqueous solution.

2.11. Microwave Method

A dielectric material can be processed with energy in the form 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 primarily due to molecules with a permanent dipole which tend to reorientate under the influence of a microwave electric field. This reorientation loss mechanism originates from the inability of the polarization to follow extremely rapid 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 TiO₂nanorods with a microwave hydrothermal method and found that they aggregated radially into spherical secondary nanoparticles.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. TiO

2

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 the preparation of a mesoporous TiO2film by the hydrothermal method as shown Figure 28.⁸⁰In a typical experiment, TTIP was added dropwise to a 0.1 M nitric acid solution under vigorous stirring and at room temperature. A white precipitate formed instantaneously. Immediately after the hydrolysis, the solution was heated to 80°C and stirred vigorously for 8 h for peptization. The solution was then filtered on a glass frit to remove agglomerates. Water was added to the filtrate to adjust the final solids concentration to∼5 wt %. The solution was put in a titanium autoclave for 12 h at 200-250°C.

After sonication, the colloidal suspension was put in a rotary evaporator and evaporated to a final TiO2concentration of 11 wt %. The precipitation pH, hydrolysis rate, autoclaving pH, and precursor chemistry were found to influence the morphology of the final TiO₂nanoparticles.

Alternative procedures without the use of hydrothermal processes 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 HNO3

aqueous solution (TBT/HNO₃∼ 100:1) at room temperature for 3 h. 0.32 mL of deionized water was added to the prehydrolyzed solution under vigorous stirring and stirred for an additional 2 h. The sol solution in a closed vessel was kept at room temperature without stirring to gel and age. After aging for 14 days, the gel was dried at room temperature, ground into a fine powder, washed thoroughly with water and ethanol, and dried to produce porous TiO₂. Upon calcination at 450 °C for 4 h under air, crystallized mesoporous TiO₂material was obtained.²⁹²

Yu et al. prepared three-dimensional and thermally stable mesoporous TiO₂ without the use of any surfactants.²⁶⁵ Briefly, monodispersed TiO2 nanoparticles were formed initially by ultrasound-assisted hydrolysis of acetic acid- modified titanium isopropoxide. Mesoporous spherical or globular 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 synthesized from the acetic acid-modified precursor and autoclaved at 230°C.

Reprinted with permission from Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am.