Subscriber access provided by LIBRARY OF CHINESE ACAD SCI Chemical Reviews is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Titanium Dioxide Nanomaterials: Synthesis, 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 Downloaded from http://pubs.acs.org on March 2, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: • Supporting Information • Links to the 71 articles that cite this article, as of the time of this article download • Access to high resolution figures • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications Xiaobo Chen* and Samuel S. Mao † Lawrence Berkeley National Laboratory, and University of California, Berkeley, California 94720 Received March 27, 2006 Contents 1. Introduction 2891 2. Synthetic Methods for TiO 2 Nanostructures 2892 2.1. Sol − Gel Method 2892 2.2. Micelle and Inverse Micelle Methods 2895 2.3. Sol Method 2896 2.4. Hydrothermal Method 2898 2.5. Solvothermal Method 2901 2.6. Direct Oxidation Method 2902 2.7. Chemical Vapor Deposition 2903 2.8. Physical Vapor Deposition 2904 2.9. Electrodeposition 2904 2.10. Sonochemical Method 2904 2.11. Microwave Method 2904 2.12. TiO 2 Mesoporous/Nanoporous Materials 2905 2.13. TiO 2 Aerogels 2906 2.14. TiO 2 Opal and Photonic Materials 2907 2.15. Preparation of TiO 2 Nanosheets 2908 3. Properties of TiO 2 Nanomaterials 2909 3.1. Structural Properties of TiO 2 Nanomaterials 2909 3.2. Thermodynamic Properties of TiO 2 Nanomaterials 2911 3.3. X-ray Diffraction Properties of TiO 2 Nanomaterials 2912 3.4. Raman Vibration Properties of TiO 2 Nanomaterials 2912 3.5. Electronic Properties of TiO 2 Nanomaterials 2913 3.6. Optical Properties of TiO 2 Nanomaterials 2915 3.7. Photon-Induced Electron and Hole Properties of TiO 2 Nanomaterials 2918 4. Modifications of TiO 2 Nanomaterials 2920 4.1. Bulk Chemical Modification: Doping 2921 4.1.1. Synthesis of Doped TiO 2 Nanomaterials 2921 4.1.2. Properties of Doped TiO 2 Nanomaterials 2921 4.2. Surface Chemical Modifications 2926 4.2.1. Inorganic Sensitization 2926 5. Applications of TiO 2 Nanomaterials 2929 5.1. Photocatalytic Applications 2929 5.1.1. Pure TiO 2 Nanomaterials: First Generation 2930 5.1.2. Metal-Doped TiO 2 Nanomaterials: Second Generation 2930 5.1.3. Nonmetal-Doped TiO 2 Nanomaterials: Third Generation 2931 5.2. Photovoltaic Applications 2932 5.2.1. The TiO 2 Nanocrystalline Electrode in DSSCs 2932 5.2.2. Metal/Semiconductor Junction Schottky Diode Solar Cell 2938 5.2.3. Doped TiO 2 Nanomaterials-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 TiO 2 Nanotubes 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 (TiO 2 ) has been widely used as a pigment 1 and in sunscreens, 2,3 paints, 4 ointments, toothpaste, 5 etc. In 1972, Fujishima and Honda discovered the phenom- enon of photocatalytic splitting of water on a TiO 2 electrode under ultraviolet (UV) light. 6-8 Since then, enormous efforts have been devoted to the research of TiO 2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors. 9-12 These applications can be roughly divided into “energy” and “environmental” categories, many of which depend not only on the properties of the TiO 2 material itself but also on the modifications of the TiO 2 material host (e.g., with inorganic and organic dyes) and on the interactions of TiO 2 materials 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. 2891 Chem. Rev. 2007, 107, 2891 − 2959 10.1021/cr0500535 CCC: $65.00 © 2007 American Chemical Society Published on Web 06/23/2007 the nanometer scale. Properties also vary as the shapes of the shrinking nanomaterials change. Many excellent reviews and reports on the preparation and properties of nanomaterials have been published recently. 6-44 Among 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. 13,21 The high surface area brought about by small particle size is beneficial to many TiO 2 -based devices, as it facilitates reaction/interaction between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. Thus, the performance of TiO 2 -based devices is largely influenced by the sizes of the TiO 2 building units, apparently at the nanometer scale. As the most promising photocatalyst, 7,11,12,33 TiO 2 mate- rials are expected to play an important role in helping solve many serious environmental and pollution challenges. TiO 2 also bears tremendous hope in helping ease the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices. 9,31,32 As continued breakthroughs have been made in the preparation, modifica- tion, and applications of TiO 2 nanomaterials in recent years, especially after a series of great reviews of the subject in the 1990s. 7,8,10-12,33,45 we believe that a new and compre- hensive review of TiO 2 nanomaterials would further promote TiO 2 -based research and development efforts to tackle the environmental and energy challenges we are currently facing. Here, we focus on recent progress in the synthesis, properties, modifications, and applications of TiO 2 nanomaterials. The syntheses of TiO 2 nanomaterials, including nanoparticles, nanorods, nanowires, and nanotubes are primarily categorized with the preparation method. The preparations of mesopo- rous/nanoporous TiO 2 , TiO 2 aerogels, opals, and photonic materials are summarized separately. In reviewing nanoma- terial synthesis, we present a typical procedure and repre- sentative transmission or scanning electron microscopy images to give a direct impression of how these nanomate- rials are obtained and how they normally appear. For detailed instructions on each synthesis, the readers are referred to the corresponding literature. The structural, thermal, electronic, and optical properties of TiO 2 nanomaterials are reviewed in the second section. As the size, shape, and crystal structure of TiO 2 nanomate- rials vary, not only does surface stability change but also the transitions between different phases of TiO 2 under pressure or heat become size dependent. The dependence of X-ray diffraction patterns and Raman vibrational spectra on the size of TiO 2 nanomaterials is also summarized, as they could help to determine the size to some extent, although correlation of the spectra with the size of TiO 2 nanomaterials is not straightforward. The review of modifications of TiO 2 nanomaterials is mainly limited to the research related to the modifications of the optical properties of TiO 2 nanoma- terials, since many applications of TiO 2 nanomaterials are closely related to their optical properties. TiO 2 nanomaterials normally are transparent in the visible light region. By doping or sensitization, it is possible to improve the optical sensitiv- ity and activity of TiO 2 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 increas- ing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. The fundamentals and working principles of the TiO 2 nanoma- terials-based devices are discussed to facilitate the under- standing and further improvement of current and practical TiO 2 nanotechnology. 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. 46-50 In 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 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. 2892 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao substrate by spin-coating or dip-coating. A wet gel will form when the sol is cast into a mold, and the wet gel is converted into a dense ceramic with further drying and heat treatment. A highly porous and extremely low-density material called an aerogel is obtained if the solvent in a wet gel is removed under a supercritical condition. Ceramic fibers can be drawn from the sol when the viscosity of a sol is adjusted into a proper viscosity range. Ultrafine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques. Under proper conditions, nanomaterials can be obtained. TiO 2 nanomaterials have been synthesized with the sol- gel method from hydrolysis of a titanium precusor. 51-78 This 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. 51,63,66,79-91 From the study on the growth kinetics of TiO 2 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 temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO 2 . 63 Second- ary particles are formed by epitaxial self-assembly of primary particles at longer times and higher temperatures, and the number of primary particles per secondary particle increases with time. The average TiO 2 nanoparticle radius increases linearly with time, in agreement with the Lifshitz-Slyozov- Wagner model for coarsening. 63 Highly crystalline anatase TiO 2 nanoparticles 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 TiO 2 nanoparticles. 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 TiO 2 nanoparticles of different sizes and shapes by tuning the reaction parameters. 67-71 Typically, 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 HClO 4 or NaOH solution. Amines are used as the shape controllers of the TiO 2 nanomaterials and act as surfactants. These amines include TEOA, diethylenetri- amine, ethylenediamine, trimethylenediamine, and triethyl- enetetramine. The morphology of the TiO 2 nanoparticles changes from cuboidal to ellipsoidal at pH above 11 with TEOA. The TiO 2 nanoparticle shape evolves into ellipsoidal above pH 9.5 with diethylenetriamine with a higher aspect ratio than that with TEOA. Figure 2 shows representative TEM images of the TiO 2 nanoparticles under different initial pH conditions with the shape control of TEOA at [TEOA]/ [TIPO] ) 2.0. Secondary amines, such as diethylamine, and tertiary amines, such as trimethylamine and triethylamine, act as complexing agents of Ti(IV) ions to promote the growth of ellipsoidal particles with lower aspect ratios. The shape of the TiO 2 nanoparticle can also be tuned from round- cornered cubes to sharp-edged cubes with sodium oleate and sodium stearate. 70 The shape control is attributed to the tuning of the growth rate of the different crystal planes of TiO 2 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 TiO 2 nano- particles during the crystallization process. 58,72 By heating amorphous TiO 2 in air, large quantities of single-phase ana- tase TiO 2 nanoparticles with average particle sizes between 7 and 50 nm can be obtained, as reported by Zhang and Banfield. 73-77 Much effort has been exerted to achieve highly crystallized and narrowly dispersed TiO 2 nanoparticles using the sol-gel method with other modifications, such as a semicontinuous reaction method by Znaidi et al. 78 and a two- stage mixed method and a continuous reaction method by Kim et al. 53,54 By a combination of the sol-gel method and an anodic alumina membrane (AAM) template, TiO 2 nanorods have been successfully synthesized by dipping porous AAMs into a boiled TiO 2 sol followed by drying and heating processes. 92,93 In a typical experiment, a TiO 2 sol 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 % H 3 PO 4 aqueous solution. The calcination temperature can be used to control the crystal phase of the TiO 2 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 TiO 2 nanorods, which typically range from 100 to 300 nm in diameter and several micrometers in length. Appar- ently, the size distribution of the final TiO 2 nanorods is largely controlled by the size distribution of the pores of the AAM template. In order to obtain smaller and mono- sized TiO 2 nanorods, it is necessary to fabricate high-quality AAM templates. Figure 3 shows a typical TEM for TiO 2 nanorods fabricated with this method. Normally, the TiO 2 nanorods are composed of small TiO 2 nanoparticles or nanograins. By electrophoretic deposition of TiO 2 colloidal suspensions into the pores of an AAM, ordered TiO 2 nanowire arrays can be obtained. 94 In 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 TiO 2 sol 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 ina5wt%NaOH solution, isolated TiO 2 nanowires are obtained. In order to Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2893 fabricate TiO 2 nanowires instead of nanorods, an AAM with long pores is a must. TiO 2 nanotubes can also be obtained using the sol-gel method by templating with an AAM 95-98 and other organic compounds. 99,100 For example, when an AAM is used as the template, a thin layer of TiO 2 sol on the wall of the pores of the AAM is first prepared by sucking TiO 2 sol into the pores of the AAM and removing it under vacuum; TiO 2 nanowires are obtained after the sol is fully developed and the AAM is removed. In the procedure by Lee and co-workers, 96 a 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 TiO 2 nanoparticles prepared by hydrolysis of Ti(OR) 4 in the presence of tetramethylammonium hydroxide. Reprinted with permission from Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. Copyright 1999 Wiley-VCH. Figure 2. TEM images of uniform anatase TiO 2 nanoparticles. Reprinted from Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 53, Copyright 2003, with permission from Elsevier. 2894 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao solution, it was removed from the solution and placed under vacuum until the entire volume of the solution was pulled through the AAM. The AAM 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 TiO 2 nanotubes were obtained after the AAM was dissolved ina6MNaOH solution for several minutes. 96 Alternatively, TiO 2 nanotubes could be obtained by coating the AAM membranes at 60 °C for a certain period of time (12-48 h) with dilute TiF 4 under pH ) 2.1 and removing the AAM after TiO 2 nanotubes were fully developed. 97 Figure 4 shows a typical SEM image of the TiO 2 nanotube 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 TiO 2 nanotubes with the sol-gel method. 101 Briefly, TiO 2 sol 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 °Cfor1hinairtoobtain ZnO/TiO 2 nanorod arrays. The ZnO nanorod template is etched-up by immersing the ZnO/TiO 2 nanorod arrays in a dilute hydro- chloric acid aqueous solution to obtain TiO 2 nanotube arrays. Figure 5 shows a typical SEM image of the TiO 2 nanotube array with the ZnO nanorod array template. The TiO 2 nanotubes inherit the uniform hexagonal cross-sectional shape and the length of 1.5 µm and inner diameter of 100- 120 nm of the ZnO nanorod template. As the concentration of the TiO 2 sol is constant, well-aligned TiO 2 nanotube arrays can only be obtained from an optimal dip-coating cycle number in the range of 2-3 cycles. A dense porous TiO 2 thick film with holes is obtained instead if the dip-coating number further increases. The heating rate is critical to the formation of TiO 2 nanotube arrays. When the heating rate is extra rapid, e.g., above 6 °C min -1 , the TiO 2 coat will easily crack and flake off from the ZnO nanorods due to great tensile stress between the TiO 2 coat and the ZnO template, and a TiO 2 film 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 hydro- philic 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 TiO 2 nanoparticles or nanograins (inset). Reprinted from Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. J. Cryst. Growth 2004, 264, 246, Copyright 2004, with permission from Elsevier. Figure 4. SEM image of TiO 2 nanotubes prepared from the AAO template. Reprinted with permission from Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14, 1391. Copyright 2002 American Chemical Society. Figure 5. SEM of a TiO 2 nanotube array; the inset shows the ZnO nanorod array template. Reprinted with permission from Qiu, J. J.; Yu, W. D.; Gao, X. D.; Li, X. M. Nanotechnology 2006, 17, 4695. Copyright 2006 IOP Publishing Ltd. Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2895 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 TiO 2 nanomaterials. 102-110 A statistical experi- mental design method was conducted by Kim et al. to optimize experimental conditions for the preparation of TiO 2 nanoparticles. 103 The values of H 2 O/surfactant, H 2 O/titanium precursor, ammonia concentration, feed rate, and reaction temperature were significant parameters in controlling TiO 2 nanoparticle size and size distribution. Amorphous TiO 2 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 TiO 2 nanoparticles with the chemical reactions between TiCl 4 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. 104 The produced amorphous TiO 2 nanoparticles 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 TiO 2 nanoparticles 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 TiO 2 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 [H 2 O]/[NP-5] and [H 2 O]/[Ti- (OC 4 H 9 ) 4 ] ratios. When suitable acid was applied, round TiO 2 nanoparticles could also be obtained. Representative TEM images of the shuttle-like and round-shaped TiO 2 nanopar- ticles are shown in Figure 6. In the study carried out by Lim et al., TiO 2 nanoparticles were prepared by the controlled hydrolysis of TTIP in reverse micelles formed in CO 2 with the surfactants ammonium carboxylate perfluoropolyether (PFPECOO - NH 4 + ) (MW 587) and poly(dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl meth- acrylate) (PDMAEMA-b-PFOMA). 106 It 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 TiO 2 nanomaterials prepared with the above micelle and reverse micelle methods normally have amorphous structure, and calcination is usually necessary in order to induce high crystallinity. However, this process usually leads to the growth and agglomeration of TiO 2 nanoparticles. The crystallinity of TiO 2 nanoparticles 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. 108 This procedure could produce crystalline TiO 2 nanoparticles with unchanged physical dimensions and minimal agglomeration and allows the preparation of highly crystalline TiO 2 nanoparticles, 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) TiO 2 nanoparticles. From: Zhang, D., Qi, L., Ma, J., Cheng, H. J. Mater. Chem. 2002, 12, 3677 (http://dx.doi.org/10.1039/b206996b). s Reproduced by permission of The Royal Society of Chemistry. Figure 7. HRTEM images of a TiO 2 nanoparticle after annealing. Reprinted with permission from Lin, J.; Lin, Y.; Liu, P.; Meziani, M. J.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2002, 124, 11514. Copyright 2002 American Chemical Society. TiX 4 + Ti(OR) 4 f 2TiO 2 + 4RX (1) TiX 4 + 2ROR f TiO 2 + 4RX (2) 2896 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao The condensation between Ti-Cl and Ti-OR leads to the formation of Ti-O-Ti bridges. The alkoxide groups can be provided by titanium alkoxides or can be formed in situ by reaction of the titanium chloride with alcohols or ethers. In the method by Trentler and Colvin, 119 a 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 TiF 4 to 3.8 nm for TiI 4 . 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 TiO 2 nanocrystals developed by Trentler et al. 119 In the method used by Niederberger and Stucky, 111 TiCl 4 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 TiCl 4 and benzyl alcohol was found suitable for the synthesis of highly crystalline anatase phase TiO 2 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. 15,16 Adding different surfactants as capping agents, such as acetic acid and acetylacetone, into the reaction matrix can help synthesize monodispersed TiO 2 nanoparticles. 120,121 For example, Scolan and Sanchez found that monodisperse nonaggregated TiO 2 nanoparticles 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. 120 The 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 TiO 2 nanorods can be synthesized. 122-130 For example, the growth of high-aspect-ratio anatase TiO 2 nanorods has been reported by Cozzoli and co-workers by controlling the hydrolysis process of TTIP in oleic acid (OA). 122-126,130 Typically, TTIP was added into dried OA at 80-100 °C under inert gas protection (nitrogen flow) and stirred for 5 min. A 0.1-2M 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) crystal- lization in mild conditions was promoted with the use of suitable catalysts (tertiary amines or quaternary ammonium hydroxides). A kinetically overdriven growth mechanism led to the growth of TiO 2 nanorods instead of nanoparticles. 123 Typical TEM images of the TiO 2 nanorods are shown in Figure 9. 123 Recently, Joo et al. 127 and Zhang et al. 129 reported similar procedures in obtaining TiO 2 nanorods 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 TiO 2 nanoparticles derived from reaction of TiCl 4 and TTIP in TOPO/heptadecane at 300 °C. The inset shows a HRTEM image of a single particle. Reprinted with permission from Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. Copyright 1999 American Chemical Society. Figure 9. TEM of TiO 2 nanorods. The inset shows a HRTEM of a TiO 2 nanorod. Reprinted with permission from Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. Copyright 2003 American Chemical Society. Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2897 The injection of a predetermined amount of oleylamine at 260 °C led to various sized TiO 2 nanorods. 129 Figure 10 shows TEM images of TiO 2 nanorods with various lengths, and 2.3 nm TiO 2 nanoparticles prepared with this method. 129 In the surfactant-mediated shape evolution of TiO 2 nano- crystals in nonaqueous media conducted by Jun et al., 128 it was found that the shape of TiO 2 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 and nanorods were elon- gated along the [001] directions. The TiO 2 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 TiO 2 nanoparticles. 131-140 For example, TiO 2 nanoparticles 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 ([H 2 O]/ [Ti] ) 150), and then they were peptized at 70 °Cfor1hin the presence of tetraalkylammonium hydroxides (peptizer). After filtration and treatment at 240 °Cfor2h,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 TiO 2 nanoparticles made with the hydrothermal method are shown in Figure 12. 134 In another example, TiO 2 nanoparticles were prepared by hydrothermal reaction of titanium alkoxide in an acidic ethanol-water solution. 132 Briefly, 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 TiO 2 nanoparticles Figure 10. TEM images of TiO 2 nanorods with lengths of (A) 12 nm, (B) 30 nm, and (C) 16 nm. (D) 2.3 nm TiO 2 nanoparticles. Inset in parts C and D: HR-TEM image of a single TiO 2 nanorod and nanoparticle. Reprinted with permission from Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright 2005 Wiley-VCH. 2898 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao synthesized under this acidic ethanol-water environment were mainly primary structure in the anatase phase without secondary structure. The sizes of the particles were controlled to the range of 7-25 nm by adjusting the concentration of Ti precursor and the composition of the solvent system. Besides TiO 2 nanoparticles, TiO 2 nanorods have also been synthesized with the hydrothermal method. 141-146 Zhang et al. obtained TiO 2 nanorods by treating a dilute TiCl 4 solution at 333-423 K for 12 h in the presence of acid or inorganic salts. 141,143-146 Figure 13 shows a typical TEM image of the TiO 2 nanorods prepared with the hydrothermal method. 141 The morphology of the resulting nanorods can be tuned with different surfactants 146 or by changing the solvent composi- tions. 145 A film of assembled TiO 2 nanorods deposited on a glass wafer was reported by Feng et al. 142 These TiO 2 nanorods were prepared at 160 °Cfor2hbyhydrothermal treatment of a titanium trichloride aqueous solution super- saturated with NaCl. TiO 2 nanowires have also been successfully obtained with the hydrothermal method by various groups. 147-151 Typically, TiO 2 nanowires are obtained by treating TiO 2 white powders ina10-15 M NaOH aqueous solution at 150-200 °C for 24-72 h without stirring within an autoclave. Figure 14 shows the SEM images of TiO 2 nanowires and a TEM image of a single nanowire prepared by Zhang and co-workers. 150 TiO 2 nanowires can also be prepared from layered titanate particles using the hydrothermal method as reported by Wei Figure 11. Time dependent shape evolution of TiO 2 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. Copyright 2003 American Chemical Society. Figure 12. TEM images of TiO 2 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 TiO 2 nanorods prepared with the hydrothermal method. Reprinted with permission from Zhang, Q.; Gao, L. Langmuir 2003, 19, 967. Copyright 2003 American Chemical Society. Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2899 [...]... 2p valence bands and antibonding states deep in the band gap (Ni and Ni+s), and were well screened and hardly interacted with the band states of TiO2.489 Di Valentin et al found that, for nitrogen doping in both anatase and rutile polymorphs, N 2p localized states were just above the top of the O 2p valence band.512,513 In anatase, these dopant states caused a red shift of the absorption band edge toward... gap.489 Titanium Dioxide Nanomaterials Figure 52 (A) Total DOSs of doped TiO2 and (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) Nidoped stands for N doping at an interstitial site, and Ni+s-doped stands for doping at both substitutional and interstitial... significant band gap narrowing.517 Wang and Doren found that N doping introduced some states at the valence band edge and thus made the original band gap of TiO2 smaller, and that a vacancy could induce some states in the band gap region, which acted as shallow donors.510 Nakano et al found that, in N-doped TiO2, deep levels located at approximately 1.18 and 2.48 eV below the conduction band were attributed... millielectronvolts.414 3.7 Photon-Induced Electron and Hole Properties of TiO2 Nanomaterials After TiO2 nanoparticles 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 unoccupied conduction band, leading to excited electrons in the conduction band and positive holes in the valence band These charge carriers can recombine,... 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 of states for nanosheets (G2D) and nanotubes (G1D) EG1D and EG2D are the band gaps of the 1D and 2D structures, respectively kx and ky are the wave vectors Reprinted with... electrons in the conduction band of metal nanoparticle surfaces to those in the conduction band of TiO2 nanomaterials in metal-TiO2 nanocomposites can improve the performance In addition, the modification of the TiO2 nanomaterials surface with other semiconductors can alter the charge-transfer properties between TiO2 and the surrounding environment, thus im- Titanium Dioxide Nanomaterials Figure 50 Solar... much weaker than that of the σ bonding The conduction bands are decomposed into Ti eg (>5 eV) and t2g bands (