titanium oxide nanomaterials

69
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: [email protected]. E-mail: [email protected]. 2891 Chem. Rev.  2007,  107,  28912959 10.1021/cr0500535 CCC: $65.00 © 2007 American Chemical Society Published on Web 06/23/2007

Upload: naufan-nurrosyid-p

Post on 10-Oct-2015

75 views

Category:

Documents


1 download

TRANSCRIPT

  • 5/20/2018 Titanium Oxide Nanomaterials

    1/69

    Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, andApplications

    Xiaobo Chen* and Samuel S. Mao

    Lawrence Berkeley National Laboratory, and University of California, Berkeley, California 94720

    Received March 27, 2006

    Contents

    1. Introduction 28912. Synthetic Methods for TiO2 Nanostructures 2892

    2.1. SolGel Method 28922.2. Micelle and Inverse Micelle Methods 28952.3. Sol Method 28962.4. Hydrothermal Method 28982.5. Solvothermal Method 29012.6. Direct Oxidation Method 2902

    2.7. Chemical Vapor Deposition 29032.8. Physical Vapor Deposition 29042.9. Electrodeposition 2904

    2.10. Sonochemical Method 29042.11. Microwave Method 29042.12. TiO2 Mesoporous/Nanoporous Materials 29052.13. TiO2 Aerogels 29062.14. TiO2 Opal and Photonic Materials 29072.15. Preparation of TiO2 Nanosheets 2908

    3. Properties of TiO2 Nanomaterials 29093.1. Structural Properties of TiO2 Nanomaterials 29093.2. Thermodynamic Properties of TiO2

    Nanomaterials2911

    3.3. X-ray Diffraction Properties of TiO2Nanomaterials 2912

    3.4. Raman Vibration Properties of TiO2Nanomaterials

    2912

    3.5. Electronic Properties of TiO2 Nanomaterials 29133.6. Optical Properties of TiO2 Nanomaterials 29153.7. Photon-Induced Electron and Hole Properties

    of TiO2 Nanomaterials2918

    4. Modifications of TiO2 Nanomaterials 29204.1. Bulk Chemical Modif ication: Doping 2921

    4.1.1. Synthesis of Doped TiO2 Nanomaterials 29214.1.2. Properties of Doped TiO2 Nanomaterials 2921

    4.2. Surface Chemical Modifications 29264.2.1. Inorganic Sensitization 2926

    5. Applications of TiO2Nanomaterials 29295.1. Photocatalytic Applications 2929

    5.1.1. Pure TiO2 Nanomaterials: FirstGeneration

    2930

    5.1.2. Metal-Doped TiO2 Nanomaterials:Second Generation

    2930

    5.1.3. Nonmetal-Doped TiO2 Nanomaterials:Third Generation

    2931

    5.2. Photovoltaic Applications 29325.2.1. The TiO2 Nanocrystalline Electrode in

    DSSCs2932

    5.2.2. Metal/Semiconductor Junction SchottkyDiode Solar Cell

    2938

    5.2.3. Doped TiO2Nanomaterials-Based SolarCell

    2938

    5.3. Photocatalytic Water Splitting 29395.3.1. Fundamentals of Photocatalytic Water

    Splitting2939

    5.3.2. Use of Reversible Redox Mediators 29395.3.3. Use of TiO2 Nanotubes 29405.3.4. Water Splitting under Visible Light 29415.3.5. Coupled/Composite Water-Splitting

    System2942

    5.4. Electrochromic Devices 29425.4.1. Fundamentals of Electrochromic Devices 29435.4.2. Electrochromophore for an Electrochromic

    Device2943

    5.4.3. Counterelectrode for an ElectrochromicDevice

    2944

    5.4.4. Photoelectrochromic Devices 29455.5. Hydrogen Storage 29455.6. Sensing Applications 2947

    6. Summary 29487. Acknowledgment 29498. References 2949

    1. Introduction

    Since its commercial production in the early twentiethcentury, titanium dioxide (TiO2) has been widely used as apigment1 and in sunscreens,2,3 paints,4 ointments, toothpaste,5

    etc. In 1972, Fujishima and Honda discovered the phenom-enon of photocatalytic splitting of water on a TiO2electrodeunder ultraviolet (UV) light.6-8 Since then, enormous effortshave been devoted to the research of TiO2material, which

    has led to many promising applications in areas ranging fromphotovoltaics and photocatalysis to photo-/electrochromicsand sensors.9-12 These applications can be roughly dividedinto energy and environmental categories, many of whichdepend not only on the properties of the TiO2material itselfbut also on the modifications of the TiO2material host (e.g.,with inorganic and organic dyes) and on the interactions ofTiO2materials with the environment.

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

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

    * Corresponding author. E-mail: [email protected]. E-mail: [email protected].

    2891Chem. Rev. 2007, 107, 28912959

    10.1021/cr0500535 CCC: $65.00 2007 American Chemical SocietyPublished on Web 06/23/2007

  • 5/20/2018 Titanium Oxide Nanomaterials

    2/69

    the nanometer scale. Properties also vary as the shapes ofthe shrinking nanomaterials change. Many excellent reviewsand reports on the preparation and properties of nanomaterialshave been published recently.6-44 Among the unique proper-ties of nanomaterials, the movement of electrons and holesin semiconductor nanomaterials is primarily governed by the

    well-known quantum confinement, and the transport proper-ties related to phonons and photons are largely affected bythe size and geometry of the materials.13-16 The specificsurface area and surface-to-volume ratio increase dramati-cally as the size of a material decreases.13,21 The high surfacearea brought about by small particle size is beneficial to manyTiO2-based devices, as it facilitates reaction/interactionbetween the devices and the interacting media, which mainlyoccurs on the surface or at the interface and strongly dependson the surface area of the material. Thus, the performanceof TiO2-based devices is largely influenced by the sizes ofthe 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

    many serious environmental and pollution challenges. TiO2also bears tremendous hope in helping ease the energy crisisthrough effective utilization of solar energy based onphotovoltaic and water-splitting devices.9,31,32 As continuedbreakthroughs have been made in the preparation, modifica-tion, and applications of TiO2nanomaterials in recent years,especially after a series of great reviews of the subject inthe 1990s.7,8,10-12,33,45 we believe that a new and compre-hensive review of TiO2nanomaterials would further promote

    TiO2-based research and development efforts to tackle theenvironmental and energy challenges we are currently facing.Here, we focus on recent progress in the synthesis, properties,modifications, and applications of TiO2nanomaterials. Thesyntheses of TiO2 nanomaterials, including nanoparticles,nanorods, nanowires, and nanotubes are primarily categorizedwith the preparation method. The preparations of mesopo-rous/nanoporous TiO2, TiO2 aerogels, opals, and photonicmaterials are summarized separately. In reviewing nanoma-terial synthesis, we present a typical procedure and repre-sentative transmission or scanning electron microscopyimages to give a direct impression of how these nanomate-rials are obtained and how they normally appear. For detailedinstructions on each synthesis, the readers are referred to

    the corresponding literature.The structural, thermal, electronic, and optical propertiesof 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 alsothe transitions between different phases of TiO2 underpressure or heat become size dependent. The dependence ofX-ray diffraction patterns and Raman vibrational spectra onthe size of TiO2nanomaterials is also summarized, as theycould help to determine the size to some extent, althoughcorrelation of the spectra with the size of TiO2nanomaterialsis not straightforward. The review of modifications of TiO2nanomaterials is mainly limited to the research related tothe modifications of the optical properties of TiO2nanoma-

    terials, since many applications of TiO2nanomaterials areclosely related to their optical properties. TiO2nanomaterialsnormally are transparent in the visible light region. By dopingor sensitization, it is possible to improve the optical sensitiv-ity and activity of TiO2 nanomaterials in the visible lightregion. Environmental (photocatalysis and sensing) andenergy (photovoltaics, water splitting, photo-/electrochromics,and hydrogen storage) applications are reviewed with anemphasis on clean and sustainable energy, since the increas-ing energy demand and environmental pollution create apressing need for clean and sustainable energy solutions. Thefundamentals and working principles of the TiO2nanoma-terials-based devices are discussed to facilitate the under-standing and further improvement of current and practical

    TiO2nanotechnology.

    2. Synthetic Methods for TiO2Nanostructures

    2.1. SolGel Method

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

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

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

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    3/69

    substrate by spin-coating or dip-coating. A wet gel will formwhen the sol is cast into a mold, and the wet gel is convertedinto a dense ceramic with further drying and heat treatment.A highly porous and extremely low-density material calledan aerogel is obtained if the solvent in a wet gel is removedunder a supercritical condition. Ceramic fibers can be drawnfrom the sol when the viscosity of a sol is adjusted into aproper viscosity range. Ultrafine and uniform ceramicpowders are formed by precipitation, spray pyrolysis, or

    emulsion techniques. Under proper conditions, nanomaterialscan be obtained.TiO2nanomaterials have been synthesized with the sol-

    gel method from hydrolysis of a titanium precusor.51-78 Thisprocess normally proceeds via an acid-catalyzed hydrolysisstep of titanium(IV) alkoxide followed by condensa-tion.51,63,66,79-91 The development of Ti-O-Ti chains isfavored with low content of water, low hydrolysis rates, andexcess titanium alkoxide in the reaction mixture. Three-dimensional polymeric skeletons with close packing resultfrom the development of Ti-O-Ti chains. The formationof Ti(OH)4 is favored with high hydrolysis rates for amedium amount of water. The presence of a large quantityof Ti-OH and insufficient development of three-dimensional

    polymeric skeletons lead to loosely packed first-orderparticles. Polymeric Ti-O-Ti chains are developed in thepresence of a large excess of water. Closely packed first-order particles are yielded via a three-dimensionally devel-oped gel skeleton.51,63,66,79-91 From the study on the growthkinetics of TiO2 nanoparticles in aqueous solution usingtitanium tetraisopropoxide (TTIP) as precursor, it is foundthat the rate constant for coarsening increases with temper-ature due to the temperature dependence of the viscosity ofthe solution and the equilibrium solubility of TiO2.63 Second-ary particles are formed by epitaxial self-assembly of primaryparticles at longer times and higher temperatures, and thenumber of primary particles per secondary particle increaseswith time. The average TiO2nanoparticle radius increaseslinearly with time, in agreement with the Lifshitz-Slyozov-Wagner model for coarsening.63

    Highly crystalline anatase TiO2nanoparticles with differentsizes and shapes could be obtained with the polycondensationof titanium alkoxide in the presence of tetramethylammoniumhydroxide.52,62 In a typical procedure, titanium alkoxide isadded to the base at 2 C in alcoholic solvents in a three-neck flask and is heated at 50-60C for 13 days or at 90-100C for 6 h. A secondary treatment involving autoclaveheating at 175 and 200 C is performed to improve thecrystallinity of the TiO2nanoparticles. Representative TEMimages are shown in Figure 1 from the study of Chemseddineet al.52

    A series of thorough studies have been conducted bySugimoto et al. using the sol-gel method on the formationof TiO2nanoparticles of different sizes and shapes by tuningthe reaction parameters.67-71 Typically, a stock solution ofa 0.50 M Ti source is prepared by mixing TTIP withtriethanolamine (TEOA) ([TTIP]/[TEOA] ) 1:2), followedby addition of water. The stock solution is diluted with ashape controller solution and then aged at 100 C for 1 dayand at 140 C for 3 days. The pH of the solution can betuned by adding HClO4or NaOH solution. Amines are usedas the shape controllers of the TiO2nanomaterials and actas surfactants. These amines include TEOA, diethylenetri-amine, ethylenediamine, trimethylenediamine, and triethyl-enetetramine. The morphology of the TiO2 nanoparticles

    changes from cuboidal to ellipsoidal at pH above 11 withTEOA. The TiO2nanoparticle shape evolves into ellipsoidalabove pH 9.5 with diethylenetriamine with a higher aspectratio than that with TEOA. Figure 2 shows representativeTEM images of the TiO2nanoparticles under different initialpH conditions with the shape control of TEOA at [TEOA]/[TIPO] ) 2.0. Secondary amines, such as diethylamine, andtertiary amines, such as trimethylamine and triethylamine,act as complexing agents of Ti(IV) ions to promote the

    growth of ellipsoidal particles with lower aspect ratios. Theshape of the TiO2nanoparticle can also be tuned from round-cornered cubes to sharp-edged cubes with sodium oleate andsodium stearate.70 The shape control is attributed to the tuningof the growth rate of the different crystal planes of TiO2nanoparticles by the specific adsorption of shape controllersto these planes under different pH conditions.70

    A prolonged heating time below 100C for the as-preparedgel can be used to avoid the agglomeration of the TiO2nano-particles during the crystallization process.58,72 By heatingamorphous TiO2in air, large quantities of single-phase ana-tase TiO2nanoparticles with average particle sizes between7 and 50 nm can be obtained, as reported by Zhang andBanfield.73-77 Much effort has been exerted to achieve highly

    crystallized and narrowly dispersed TiO2nanoparticles usingthe sol-gel method with other modifications, such as asemicontinuous reaction method by Znaidi et al.78 and a two-stage mixed method and a continuous reaction method byKim et al.53,54

    By a combination of the sol-gel method and an anodicalumina membrane (AAM) template, TiO2 nanorods havebeen successfully synthesized by dipping porous AAMsinto a boiled TiO2 sol followed by drying and heatingprocesses.92,93 In a typical experiment, a TiO2sol solution isprepared by mixing TTIP dissolved in ethanol with a solutioncontaining water, acetyl acetone, and ethanol. An AAM isimmersed into the sol solution for 10 min after being boiled

    in ethanol; then it is dried in air and calcined at 400 C for10 h. The AAM template is removed in a 10 wt % H 3PO4aqueous solution. The calcination temperature can be usedto control the crystal phase of the TiO2nanorods. At lowtemperature, anatase nanorods can be obtained, while athigh temperature rutile nanorods can be obtained. The poresize of the AAM template can be used to control the size ofthese TiO2nanorods, which typically range from 100 to 300nm in diameter and several micrometers in length. Appar-ently, the size distribution of the final TiO2 nanorods islargely controlled by the size distribution of the pores ofthe AAM template. In order to obtain smaller and mono-sized TiO2nanorods, it is necessary to fabricate high-qualityAAM templates. Figure 3 shows a typical TEM for TiO2

    nanorods fabricated with this method. Normally, the TiO2nanorods are composed of small TiO2 nanoparticles ornanograins.

    By electrophoretic deposition of TiO2colloidal suspensionsinto the pores of an AAM, ordered TiO2 nanowire arrayscan be obtained.94 In a typical procedure, TTIP is dissolvedin ethanol at room temperature, and glacial acetic acid mixedwith deionized water and ethanol is added under pH ) 2-3with nitric acid. Platinum is used as the anode, and an AAMwith an Au substrate attached to Cu foil is used as thecathode. A TiO2sol is deposited into the pores of the AMMunder a voltage of 2-5 V and annealed at 500 C for 24 h.After dissolving the AAM template in a 5 wt % NaOHsolution, isolated TiO2nanowires are obtained. In order to

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2893

  • 5/20/2018 Titanium Oxide Nanomaterials

    4/69

    fabricate TiO2nanowires instead of nanorods, an AAM withlong pores is a must.

    TiO2 nanotubes can also be obtained using the sol-gelmethod by templating with an AAM95-98 and other organiccompounds.99,100 For example, when an AAM is used as thetemplate, a thin layer of TiO2sol on the wall of the pores of

    the AAM is first prepared by sucking TiO2sol into the poresof the AAM and removing it under vacuum; TiO2nanowiresare obtained after the sol is fully developed and the AAM isremoved. In the procedure by Lee and co-workers,96 a TTIPsolution was prepared by mixing TTIP with 2-propanol and2,4-pentanedione. After the AAM was dipped into this

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

    Figure 2. TEM images of uniform anatase TiO2nanoparticles. Reprinted from Sugimoto, T.; Zhou, X.; Muramatsu, A.J. Colloid InterfaceSci. 2003, 259, 53, Copyright 2003, with permission from Elsevier.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    5/69

    solution, it was removed from the solution and placed undervacuum until the entire volume of the solution was pulledthrough the AAM. The AAM was hydrolyzed by water 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 cooledto room temperature with a temperature ramp of 2 C/h. PureTiO2nanotubes were obtained after the AAM was dissolvedin a 6 M NaOH solution for several minutes.96 Alternatively,TiO2 nanotubes could be obtained by coating the AAMmembranes at 60C for a certain period of time (12-48 h)with dilute TiF4 under pH ) 2.1 and removing the AAMafter TiO2nanotubes were fully developed.97 Figure 4 showsa typical SEM image of the TiO2nanotube array from theAAM template.97

    In another scheme, a ZnO nanorod array on a glasssubstrate can be used as a template to fabricate TiO2nanotubes with the sol-gel method.101 Briefly, TiO2sol is

    deposited on a ZnO nanorod template by dip-coating with aslow withdrawing speed, then dried at 100 C for 10 min,and heated at 550 C for 1 h in air to obtain ZnO/TiO2nanorod arrays. The ZnO nanorod template is etched-up byimmersing the ZnO/TiO2nanorod arrays in a dilute hydro-chloric acid aqueous solution to obtain TiO2nanotube arrays.Figure 5 shows a typical SEM image of the TiO2nanotubearray with the ZnO nanorod array template. The TiO2nanotubes inherit the uniform hexagonal cross-sectionalshape and the length of 1.5m and inner diameter of 100-120 nm of the ZnO nanorod template. As the concentrationof the TiO2sol is constant, well-aligned TiO2nanotube arrayscan only be obtained from an optimal dip-coating cyclenumber in the range of 2-3 cycles. A dense porous TiO2thick film with holes is obtained instead if the dip-coatingnumber further increases. The heating rate is critical to theformation of TiO2nanotube arrays. When the heating rateis extra rapid, e.g., above 6 C min-1, the TiO2 coat willeasily crack and flake off from the ZnO nanorods due togreat tensile stress between the TiO2 coat and the ZnOtemplate, and a TiO2film with loose, porous nanostructureis obtained.

    2.2. Micelle and Inverse Micelle Methods

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

    Figure 3. TEM image of anatase nanorods and a single nanorodcomposed of small TiO2 nanoparticles or nanograins (inset).

    Reprinted from Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.;Tanemura, M. J. Cryst. Growth 2004, 264, 246, Copyright 2004,with permission from Elsevier.

    Figure 4. SEM image of TiO2nanotubes prepared from the AAOtemplate. Reprinted with permission from Liu, S. M.; Gan, L. M.;Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14,1391. Copyright 2002 American Chemical Society.

    Figure 5. SEM of a TiO2nanotube array; the inset shows the ZnOnanorod array template. Reprinted with permission from Qiu, J. J.;Yu, W. D.; Gao, X. D.; Li, X. M.Nanotechnology2006,17, 4695.Copyright 2006 IOP Publishing Ltd.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2895

  • 5/20/2018 Titanium Oxide Nanomaterials

    6/69

    directed outward toward the nonaqueous media. There is noobvious CMC for reverse micelles, because the number ofaggregates is usually small and they are not sensitive to thesurfactant concentration. Micelles are often globular androughly spherical in shape, but ellipsoids, cylinders, andbilayers are also possible. The shape of a micelle is a functionof the molecular geometry of its surfactant molecules andsolution conditions such as surfactant concentration, tem-perature, pH, and ionic strength.

    Micelles and inverse micelles are commonly employed tosynthesize TiO2 nanomaterials.102-110 A statistical experi-mental design method was conducted by Kim et al. tooptimize experimental conditions for the preparation of TiO2nanoparticles.103 The values of H2O/surfactant, H2O/titaniumprecursor, ammonia concentration, feed rate, and reactiontemperature were significant parameters in controlling TiO2nanoparticle size and size distribution. Amorphous TiO2nanoparticles with diameters of 10-20 nm were synthesizedand converted to the anatase phase at 600 C and to the morethermodynamically stable rutile phase at 900 C. Li et al.developed TiO2 nanoparticles with the chemical reactionsbetween TiCl4solution and ammonia in a reversed micro-emulsion system consisting of cyclohexane, poly(oxyethyl-

    ene)5 nonyle phenol ether, and poly(oxyethylene)9 nonylephenol ether.104 The produced amorphous TiO2nanoparticlestransformed into anatase when heated at temperatures from200 to 750 C and into rutile at temperatures higher than750C. Agglomeration and growth also occurred at elevatedtemperatures.

    Shuttle-like crystalline TiO2nanoparticles were synthesizedby Zhang et al. with hydrolysis of titanium tetrabutoxide inthe presence of acids (hydrochloric acid, nitric acid, sulfuricacid, and phosphoric acid) in NP-5 (Igepal CO-520)-cyclohexane reverse micelles at room temperature.110 Thecrystal structure, morphology, and particle size of the TiO2nanoparticles were largely controlled by the reaction condi-

    tions, and the key factors affecting the formation of rutile atroom temperature included the acidity, the type of acid used,and the microenvironment of the reverse micelles. Ag-glomeration of the particles occurred with prolonged reactiontimes and increasing the [H2O]/[NP-5] and [H2O]/[Ti-(OC4H9)4] ratios. When suitable acid was applied, round TiO2nanoparticles could also be obtained. Representative TEMimages of the shuttle-like and round-shaped TiO2nanopar-ticles are shown in Figure 6. In the study carried out by Limet al., TiO2 nanoparticles were prepared by the controlledhydrolysis of TTIP in reverse micelles formed in CO2withthe surfactants ammonium carboxylate perfluoropolyether(PFPECOO-NH4+) (MW 587) and poly(dimethyl aminoethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl meth-

    acrylate) (PDMAEMA-b-PFOMA).106

    It was found that thecrystallite size prepared in the presence of reverse micellesincreased as either the molar ratio of water to surfactant orthe precursor to surfactant ratio increased.

    The TiO2nanomaterials prepared with the above micelleand reverse micelle methods normally have amorphousstructure, and calcination is usually necessary in order toinduce high crystallinity. However, this process usually leadsto the growth and agglomeration of TiO2nanoparticles. Thecrystallinity of TiO2nanoparticles initially (synthesized bycontrolled hydrolysis of titanium alkoxide in reverse micellesin a hydrocarbon solvent) could be improved by annealingin the presence of the micelles at temperatures considerablylower than those required for the traditional calcination

    treatment in the solid state.108 This procedure could producecrystalline TiO2 nanoparticles with unchanged physicaldimensions and minimal agglomeration and allows thepreparation of highly crystalline TiO2nanoparticles, as shownin Figure 7, from the study of Lin et al. 108

    2.3. Sol Method

    The sol method here refers to the nonhydrolytic sol-gelprocesses and usually involves the reaction of titaniumchloride with a variety of different oxygen donor molecules,e.g., a metal alkoxide or an organic ether.111-119

    Figure 6. TEM images of the shuttle-like and round-shaped (inset)TiO2nanoparticles. From: Zhang, D., Qi, L., Ma, J., Cheng, H.J.

    Mater. Chem.2002,12, 3677 (http://dx.doi.org/10.1039/b206996b).s Reproduced by permission of The Royal Society of Chemistry.

    Figure 7. HRTEM images of a TiO2nanoparticle after annealing.Reprinted with permission from Lin, J.; Lin, Y.; Liu, P.; Meziani,M. J.; Allard, L. F.; Sun, Y. P.J. Am. Chem. Soc.2002,124, 11514.Copyright 2002 American Chemical Society.

    TiX4 + Ti(OR)4 f2TiO2 + 4RX (1)

    TiX4 + 2ROR fTiO2 + 4RX (2)

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    7/69

    The condensation between Ti-Cl and Ti-OR leads to theformation of Ti-O-Ti bridges. The alkoxide groups canbe provided by titanium alkoxides or can be formed in situby reaction of the titanium chloride with alcohols or ethers.In the method by Trentler and Colvin,119 a metal alkoxidewas rapidly injected into the hot solution of titanium halidemixed with trioctylphosphine oxide (TOPO) in heptadecaneat 300C under dry inert gas protection, and reactions werecompleted within 5 min. For a series of alkyl substituentsincluding methyl, ethyl, isopropyl, and tert-butyl, the reactionrate dramatically increased with greater branching of R, whileaverage particle sizes were relatively unaffected. Variationof X yielded a clear trend in average particle size, but withouta discernible trend in reaction rate. Increased nucleophilicity

    (or size) of the halide resulted in smaller anatase nanocrystals.Average sizes ranged from 9.2 nm for TiF4 to 3.8 nm forTiI4. The amount of passivating agent (TOPO) influencedthe chemistry. Reaction in pure TOPO was slower andresulted in smaller particles, while reactions without TOPOwere much quicker and yielded mixtures of brookite, rutile,and anatase with average particle sizes greater than 10 nm.Figure 8 shows typical TEM images of TiO2nanocrystalsdeveloped by Trentler et al.119

    In the method used by Niederberger and Stucky,111 TiCl4was slowly added to anhydrous benzyl alcohol undervigorous stirring at room temperature and was kept at 40-150C for 1-21 days in the reaction vessel. The precipitatewas calcinated at 450C for 5 h after thoroughly washing.

    The reaction between TiCl4and benzyl alcohol was foundsuitable for the synthesis of highly crystalline anatase phaseTiO2 nanoparticles with nearly uniform size and shape atvery low temperatures, such as 40C. The particle size couldbe selectively adjusted in the range of 4-8 nm with theappropriate thermal conditions and a proper choice of therelative amounts of benzyl alcohol and titanium tetrachloride.The particle growth depended strongly on temperature, andlowering the titanium tetrachloride concentration led to aconsiderable decrease of particle size.111

    Surfactants have been widely used in the preparation of avariety of nanoparticles with good size distribution anddispersity.15,16 Adding 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 monodispersenonaggregated TiO2nanoparticles in the 1-5 nm range wereobtained through hydrolysis of titanium butoxide in thepresence of acetylacetone and p-toluenesulfonic acid at 60C.120 The resulting nanoparticle xerosols could be dispersedin water-alcohol or alcohol solutions at concentrationshigher than 1 M without aggregation, which is attributed tothe complexation of the surface by acetylacetonato ligands

    and through an adsorbed hybrid organic-

    inorganic layermade with acetylacetone, p-toluenesulfonic acid, and wa-ter.120

    With the aid of surfactants, different sized and shaped TiO2nanorods can be synthesized.122-130 For example, the growthof high-aspect-ratio anatase TiO2nanorods has been reportedby Cozzoli and co-workers by controlling the hydrolysisprocess of TTIP in oleic acid (OA).122-126,130 Typically, TTIPwas added into dried OA at 80-100 C under inert gasprotection (nitrogen flow) and stirred for 5 min. A 0.1-2 Maqueous base solution was then rapidly injected and kept at80-100 C for 6-12 h with stirring. The bases employedincluded organic amines, such as trimethylamino-N-oxide,trimethylamine, tetramethylammonium hydroxide, tetrabut-

    ylammonium hydroxyde, triethylamine, and tributylamine.In this reaction, by chemical modification of the titaniumprecursor with the carboxylic acid, the hydrolysis rate oftitanium alkoxide was controlled. Fast (in 4-6 h) crystal-lization in mild conditions was promoted with the use ofsuitable catalysts (tertiary amines or quaternary ammoniumhydroxides). A kinetically overdriven growth mechanism ledto the growth of TiO2nanorods instead of nanoparticles.123

    Typical TEM images of the TiO2 nanorods are shown inFigure 9.123

    Recently, Joo et al.127 and Zhang et al.129 reported similarprocedures in obtaining TiO2nanorods without the use ofcatalyst. Briefly, a mixture of TTIP and OA was used togenerate OA complexes of titanium at 80C in 1-octadecene.

    Figure 8. TEM image of TiO2nanoparticles derived from reactionof TiCl4and TTIP in TOPO/heptadecane at 300C. The inset showsa HRTEM image of a single particle. Reprinted with permissionfrom Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.;Colvin, V. L.J. Am. Chem. Soc. 1999,121, 1613. Copyright 1999American Chemical Society.

    Figure 9. TEM of TiO2nanorods. The inset shows a HRTEM ofa TiO2 nanorod. Reprinted with permission from Cozzoli, P. D.;Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539.Copyright 2003 American Chemical Society.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2897

  • 5/20/2018 Titanium Oxide Nanomaterials

    8/69

    The injection of a predetermined amount of oleylamine at260 C led to various sized TiO2 nanorods.129 Figure 10shows TEM images of TiO2nanorods with various lengths,and 2.3 nm TiO2nanoparticles prepared with this method.129

    In the surfactant-mediated shape evolution of TiO2nano-crystals in nonaqueous media conducted by Jun et al.,128 itwas found that the shape of TiO2 nanocrystals could bemodified by changing the surfactant concentration. Thesynthesis was accomplished by an alkyl halide eliminationreaction between titanium chloride and titanium isopro-poxide. Briefly, a dioctyl ether solution containing TOPOand lauric acid was heated to 300 C followed by additionof titanium chloride under vigorous stirring. The reactionwas initiated by the rapid injection of TTIP and quenched

    with cold toluene. At low lauric acid concentrations, bullet-and diamond-shaped nanocrystals were obtained; at higherconcentrations, rod-shaped nanocrystals or a mixture ofnanorods and branched nanorods was observed. The bullet-and diamond-shaped nanocrystals and nanorods were elon-gated along the [001] directions. The TiO2nanorods werefound to simultaneously convert to small nanoparticles as afunction of the growth time, as shown in Figure 11, due tothe minimization of the overall surface energy via dissolutionand regrowth of monomers during an Ostwald ripening.

    2.4. Hydrothermal Method

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

    liners under controlled temperature and/or pressure with thereaction in aqueous solutions. The temperature can beelevated above the boiling point of water, reaching thepressure of vapor saturation. The temperature and the amountof solution added to the autoclave largely determine theinternal pressure produced. It is a method that is widely usedfor the production of small particles in the ceramics industry.Many groups have used the hydrothermal method to prepareTiO2nanoparticles.131-140 For example, TiO2nanoparticlescan be obtained by hydrothermal treatment of peptizedprecipitates of a titanium precursor with water.134 Theprecipitates were prepared by adding a 0.5 M isopropanolsolution of titanium butoxide into deionized water ([H2O]/[Ti] ) 150), and then they were peptized at 70 C for 1 h in

    the presence of tetraalkylammonium hydroxides (peptizer).After filtration and treatment at 240 C for 2 h, theas-obtained powders were washed with deionized water andabsolute ethanol and then dried at 60 C. Under the sameconcentration of peptizer, the particle size decreased withincreasing alkyl chain length. The peptizers and theirconcentrations influenced the morphology of the particles.Typical TEM images of TiO2nanoparticles made with thehydrothermal method are shown in Figure 12.134

    In another example, TiO2nanoparticles were prepared byhydrothermal reaction of titanium alkoxide in an acidicethanol-water solution.132 Briefly, TTIP was added dropwiseto a mixed ethanol and water solution at pH 0.7 with nitricacid, and reacted at 240C 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 TiO 2nanoparticles. Insetin 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.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    9/69

    synthesized under this acidic ethanol-water environmentwere mainly primary structure in the anatase phase withoutsecondary structure. The sizes of the particles were controlledto the range of 7-25 nm by adjusting the concentration ofTi precursor and the composition of the solvent system.

    Besides TiO2nanoparticles, TiO2nanorods have also beensynthesized with the hydrothermal method.141-146 Zhang etal. obtained TiO2nanorods by treating a dilute TiCl4solutionat 333-423 K for 12 h in the presence of acid or inorganicsalts.141,143-146 Figure 13 shows a typical TEM image of theTiO2 nanorods prepared with the hydrothermal method.141

    The morphology of the resulting nanorods can be tuned withdifferent surfactants146 or by changing the solvent composi-

    tions.145 A film of assembled TiO2nanorods deposited on aglass wafer was reported by Feng et al.142 These TiO2nanorods were prepared at 160 C for 2 h by hydrothermaltreatment of a titanium trichloride aqueous solution super-saturated with NaCl.

    TiO2nanowires have also been successfully obtained withthe hydrothermal method by various groups.147-151 Typically,TiO2nanowires are obtained by treating TiO2white powdersin a 10-15 M NaOH aqueous solution at 150-200 C for24-72 h without stirring within an autoclave. Figure 14shows the SEM images of TiO2nanowires and a TEM imageof a single nanowire prepared by Zhang and co-workers.150

    TiO2nanowires can also be prepared from layered titanateparticles using the hydrothermal method as reported by Wei

    Figure 11. Time dependent shape evolution of TiO2 nanorods:(a) 0.25 h; (b) 24 h; (c) 48 h. Scale bar ) 50 nm. Reprinted withpermission from Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S.Y.; Cheon, J.; Alivisatos, A. P.J. Am. Chem. Soc.2003,125, 15981.Copyright 2003 American Chemical Society.

    Figure 12. TEM images of TiO2 nanoparticles prepared by thehydrothermal method. Reprinted from Yang, J.; Mei, S.; Ferreira,J. M. F. Mater. Sci. Eng. C 2001, 15, 183, Copyright 2001, withpermission from Elsevier.

    Figure 13. TEM image of TiO2 nanorods prepared with thehydrothermal method. Reprinted with permission from Zhang, Q.;Gao, L. Langmuir 2003, 19, 967. Copyright 2003 AmericanChemical Society.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2899

  • 5/20/2018 Titanium Oxide Nanomaterials

    10/69

    et al.152 In their experiment, layer-structured Na2Ti3O7wasdispersed into a 0.05-0.1 M HCl solution and kept at 140-170C for 3-7 days in an autoclave. TiO2nanowires wereobtained after the product was washed with H2O and finallydried. In the formation of a TiO2 nanowire from layeredH2Ti3O7, there are three steps: (i) the exfoliation of layeredNa2Ti3O7; (ii) the nanosheets formation; and (iii) the nanow-ires formation.152 In Na2Ti3O7, [TiO6] octahedral layers areheld by the strong static interaction between the Na+ cationsbetween the [TiO6] octahedral layers and the [TiO6] unit.When the larger H3+O cations replace the Na+ cations in

    the interlayer space of [TiO6] sheets, this static interactionis weakened because the interlayer distance is enlarged. Asa result, the layered compounds Na2Ti3O7 are graduallyexfoliated. When Na+ is exchanged by H+ in the dilute HClsolution, numerous H2Ti3O7 sheet-shaped products areformed. Since the nanosheet does not have inversion sym-metry, an intrinsic tension exists. The nanosheets split to formnanowires in order to release the strong stress and lower thetotal energy.152 A representative TEM image of TiO2nanowires from Na2Ti3O7is shown in Figure 15.152

    The hydrothermal method has been widely used to prepareTiO2nanotubes since it was introduced by Kasuga et al. in1998.153-175 Briefly, TiO2powders are put into a 2.5-20 MNaOH aqueous solution and held at 20-110C for 20 h in

    an autoclave. TiO2nanotubes are obtained after the productsare washed with a dilute HCl aqueous solution and distilledwater. They proposed the following formation process ofTiO2nanotubes.154 When the raw TiO2material was treatedwith NaOH aqueous solution, some of the Ti-O-Ti bondswere broken and Ti-O-Na and Ti-OH bonds were formed.New Ti-O-Ti bonds were formed after the Ti-O-Na andTi-OH bonds reacted with acid and water when the materialwas treated with an aqueous HCl solution and distilled water.The Ti-OH bond could form a sheet. Through the dehydra-tion of Ti-OH bonds by HCl aqueous solution, Ti-O-Tibonds or Ti-O-H-O-Ti hydrogen bonds were generated.The bond distance from one Ti to the next Ti on the surfacedecreased. This resulted in the folding of the sheets and the

    connection between the ends of the sheets, resulting in theformation of a tube structure. In this mechanism, the TiO2nanotubes were formed in the stage of the acid treatmentfollowing the alkali treatment. Figure 16 shows typical TEMimages of TiO2nanotubes made by Kasuga et al.153 However,Du and co-workers found that the nanotubes were formedduring the treatment of TiO2in NaOH aqueous solution.161

    A 3D f 2D f1D formation mechanism of the TiO2nanotubes was proposed by Wang and co-workers.171 It statedthat the raw TiO2 was first transformed into lamellarstructures and then bent and rolled to form the nanotubes.For the formation of the TiO2nanotubes, the two-dimensionallamellar TiO2 was essential. Yao and co-workers furthersuggested, based on their HRTEM study as shown in Figure

    Figure 14. SEM images of TiO2nanowires with the inset showinga TEM image of a single TiO2nanowire with a [010] selected areaelectron diffraction (SAED) recorded perpendicular to the long axisof the wire. Reprinted from Zhang, Y. X.; Li, G. H.; Jin, Y. X.;

    Zhang, Y.; Zhang, J.; Zhang, L. D. Chem. Phys. Lett. 2002, 365,300, Copyright 2002, with permission from Elsevier.

    Figure 15. TEM images of TiO2nanowires made from the layeredNa2Ti3O7 particles, with the HRTEM image shown in the inset.Reprinted from Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.;Arakawa, H. Chem. Phys. Lett. 2004, 400, 231, Copyright 2004,with permission from Elsevier.

    Figure 16. TEM image of TiO2 nanotubes. Reprinted withpermission from Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino,T.; Niihara, K.Langmuir1998,14, 3160. Copyright 1998 AmericanChemical Society.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    11/69

    17, that TiO2nanotubes were formed by rolling up the single-layer TiO2 sheets with a rolling-up vector of [001] andattracting other sheets to surround the tubes.172 Bavykin andco-workers suggested that the mechanism of nanotubeformation involved the wrapping of multilayered nanosheetsrather than scrolling or wrapping of single layer nanosheetsfollowed by crystallization of successive layers.156 In themechanism proposed by Wang et al., the formation of TiO2nanotubes involved several steps.176 During the reaction withNaOH, the Ti-O-Ti bonding between the basic buildingblocks of the anatase phase, the octahedra, was broken anda zigzag structure was formed when the free octahedrasshared edges between the Ti ions with the formation ofhydroxy bridges, leading to the growth along the [100]direction of the anatase phase. Two-dimensional crystallinesheets formed from the lateral growth of the formation ofoxo bridges between the Ti centers (Ti-O-Ti bonds) in the[001] direction and rolled up in order to saturate thesedangling bonds from the surface and lower the total energy,resulting in the formation of TiO2nanotubes.176

    2.5. Solvothermal Method

    The solvothermal method is almost identical to thehydrothermal method except that the solvent used here isnonaqueous. However, the temperature can be elevated muchhigher than that in hydrothermal method, since a variety oforganic solvents with high boiling points can be chosen. Thesolvothermal method normally has better control than hy-drothermal methods of the size and shape distributions andthe crystallinity of the TiO2nanoparticles. The solvothermalmethod has been found to be a versatile method for the

    synthesis of a variety of nanoparticles with narrow sizedistribution and dispersity.177-179 The solvothermal methodhas been employed to synthesize TiO2 nanoparticles andnanorods with/without the aid of surfactants.177-185 Forexample, in a typical procedure by Kim and co-workers,184

    TTIP was mixed with toluene at the weight ratio of 1-3:10and kept at 250C for 3 h. The average particle size of TiO2powders tended to increase as the composition of TTIP inthe solution increased in the range of weight ratio of 1-3:10, while the pale crystalline phase of TiO2was not producedat 1:20 and 2:5 weight ratios.184 By controlling the hydro-lyzation reaction of Ti(OC4H9)4and linoleic acid, redispers-ible TiO2nanoparticles and nanorods could be synthesized,

    as found by Li et al. recently.177

    The decomposition of NH4-HCO3could provide H2O for the hydrolyzation reaction, andlinoleic acid could act as the solvent/reagent and coordinationsurfactant in the synthesis of nanoparticles. Triethylaminecould act as a catalyst for the polycondensation of the Ti-O-Ti inorganic network to achieve a crystalline product andhad little influence on the products morphology. The chainlengths of the carboxylic acids had a great influence on theformation of TiO2, and long-chain organic acids wereimportant and necessary in the formation of TiO2.177 Figure18 shows a representative TEM image of TiO2nanoparticlesfrom their study.177

    TiO2nanorods with narrow size distributions can also bedeveloped with the solvothermal method.177,183 For example,

    in a typical synthesis from Kim et al., TTIP was dissolvedin anhydrous toluene with OA as a surfactant and kept at250 C for 20 h in an autoclave without stirring.183 Longdumbbell-shaped nanorods were formed when a sufficientamount of TTIP or surfactant was added to the solution, dueto the oriented growth of particles along the [001] axis. Ata fixed precursor to surfactant weight ratio of 1:3, theconcentration of rods in the nanoparticle assembly increasedas the concentration of the titanium precursor in the solutionincreased. The average particle size was smaller and the sizedistribution was narrower than is the case for particlessynthesized without surfactant. The crystalline phase, diam-eter, and length of these nanorods are largely influenced bythe precursor/surfactant/solvent weight ratio. Anatase nano-

    Figure 17. (a) HRTEM images of TiO2 nanotubes. (b) Cross-sectional view of TiO2 nanotubes. Reused with permission fromB. D. Yao, Y. F. Chan, X. Y. Zhang, W. F. Zhang, Z. Y. Yang, N.Wang, Applied Physics Letters 82, 281 (2003). Copyright 2003,American Institute of Physics.

    Figure 18. TEM micrographs of TiO2nanoparticles prepared withthe solvothermal method. Reprinted with permission from Li, X.L.; Peng, Q.; Yi, J. X.; Wang, X.; Li, Y. D. Chem.sEur. J. 2006,12, 2383. Copyright 2006 Wiley-VCH.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2901

  • 5/20/2018 Titanium Oxide Nanomaterials

    12/69

    rods were obtained from the solution with a precursor/surfactant weight ratio of more than 1:3 for a precursor/solvent weight ratio of 1:10 or from the solution with aprecursor/solvent weight ratio of more than 1:5 for aprecursor/surfactant weight ratio of 1:3. The diameter andlength of these nanorods were in the ranges of 3-5 nm and18-25 nm, respectively. Figure 19 shows a typical TEM

    image of TiO2 nanorods prepared from the solutions withthe weight ratio of precursor/solvent/surfactant ) 1:5:3.183

    Similar to the hydrothermal method, the solvothermalmethod has also been used for the preparation of TiO2nanowires.180-182 Typically, a TiO2powder suspension in an5 M NaOH water-ethanol solution is kept in an autoclaveat 170-200C for 24 h and then cooled to room temperaturenaturally. TiO2 nanowires are obtained after the obtainedsample is washed with a dilute HCl aqueous solution anddried at 60 C for 12 h in air.181 The solvent plays animportant role in determining the crystal morphology.Solvents with different physical and chemical properties caninfluence the solubility, reactivity, and diffusion behaviorof the reactants; in particular, the polarity and coordinating

    ability of the solvent can influence the morphology and thecrystallization behavior of the final products. The presenceof ethanol at a high concentration not only can cause thepolarity of the solvent to change but also strongly affectsthe potential values of the reactant particles and theincreases solution viscosity. For example, in the absence ofethanol, short and wide flakelike structures of TiO2 wereobtained instead of nanowires. When chloroform is used,TiO2nanorods were obtained.181 Figure 20 shows representa-tive TEM images of the TiO2nanowires prepared from thesolvothermal method.181 Alternatively, bamboo-shaped Ag-doped TiO2nanowires were developed with titanium butox-ide as precursor and AgNO3 as catalyst.180 Through theelectron diffraction (ED) pattern and HRTEM study, the Ag

    phase only existed in heterojunctions between single-crystalTiO2nanowires.180

    2.6. Direct Oxidation Method

    TiO2 nanomaterials can be obtained by oxidation oftitanium metal using oxidants or under anodization. Crystal-

    line TiO2nanorods have been obtained by direct oxidationof a titanium metal plate with hydrogen peroxide.186-191

    Typically, TiO2nanorods on a Ti plate are obtained when acleaned Ti plate is put in 50 mL of a 30 wt % H 2O2solutionat 353 K for 72 h. The formation of crystalline TiO2occursthrough a dissolution precipitation mechanism. By theaddition of inorganic salts of NaX (X ) F-, Cl-, and SO42-),the crystalline phase of TiO2 nanorods can be controlled.The addition of F- and SO42- helps the formation of pureanatase, while the addition of Cl- favors the formation ofrutile.189 Figure 21 shows a typical SEM image of TiO2nanorods prepared with this method.186

    At high temperature, acetone can be used as a good oxygensource and for the preparation of TiO2nanorods by oxidizing

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

    Figure 20. TEM images of TiO2 nanowires synthesized by thesolvothermal method. From: Wen, B.; Liu, C.; Liu, Y. New J.Chem. 2005, 29, 969 (http://dx.doi.org/10.1039/b502604k) sReproduced by permission of The Royal Society of Chemistry(RSC) on behalf of the Centre National de la Recherche Scientifique(CNRS).

    Figure 21. SEM morphology of TiO2 nanorods by directlyoxidizing a Ti plate with a H2O2solution. Reprinted from Wu, J.M. J. Cryst. Growth 2004, 269, 347, Copyright 2004, withpermission from Elesevier.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    13/69

    a Ti plate with acetone as reported by Peng and Chen.192The oxygen source was found to play an important role.Highly dense and well-aligned TiO2 nanorod arrays wereformed when acetone was used as the oxygen source, andonly crystal grain films or grains with random nanofibersgrowing from the edges were obtained with pure oxygen orargon mixed with oxygen. The competition of the oxygenand titanium diffusion involved in the titanium oxidationprocess largely controlled the morphology of the TiO2. Withpure oxygen, the oxidation occurred at the Ti metal and theTiO2interface, since oxygen diffusion predominated becauseof the high oxygen concentration. When acetone was usedas the oxygen source, Ti cations diffused to the oxide surfaceand reacted with the adsorbed acetone species. Figure 22

    shows aligned TiO2nanorod arrays obtained by oxidizing atitanium substrate with acetone at 850 C for 90 min.192

    As extensively studied, TiO2nanotubes can be obtainedby anodic oxidation of titanium foil.193-228 In a typicalexperiment, a clean Ti plate is anodized in a 0.5% HFsolution under 10-20 V for 10-30 min. Platinum is usedas counterelectrode. Crystallized TiO2nanotubes are obtainedafter the anodized Ti plate is annealed at 500 C for 6 h inoxygen.210 The length and diameter of the TiO2nanotubescould be controlled over a wide range (diameter, 15-120nm; length, 20 nm to 10 m) with the applied potentialbetween 1 and 25 V in optimized phosphate/HF electro-lytes.229 Figure 23 shows SEM images of TiO2 nanotubescreated with this method.208

    2.7. Chemical Vapor Deposition

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

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

    Thick crystalline TiO2films with grain sizes below 30 nmas well as TiO2nanoparticles with sizes below 10 nm canbe prepared by pyrolysis of TTIP in a mixed helium/oxygenatmosphere, using liquid precursor delivery.230 When depos-ited on the cold areas of the reactor at temperatures below

    90C with plasma enhanced CVD, amorphous TiO2nano-particles can be obtained and crystallize with a relativelyhigh surface area after being annealed at high temperatures.231

    TiO2nanorod arrays with a diameter of about 50-100 nmand a length of 0.5-2 m can be synthesized by metalorganic CVD (MOCVD) on a WC-Co substrate using TTIPas the precursor.232

    Figure 24 shows the TiO2nanorods grown on fused silicasubstrates with a template- and catalyst-free MOCVDmethod.233 In a typical procedure, titanium acetylacetonate(Ti(C10H14O5)) vaporizing in the low-temperature zone of afurnace at 200-230C is carried by a N2/O2flow into thehigh-temperature zone of 500-700C, and TiO2nanostruc-tures are grown directly on the substrates. The phase and

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

    Figure 23. SEM images of TiO2nanotubes prepared with anodicoxidation. Reprinted with permission from Varghese, O. K.; Gong,D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV.

    Mater.2003, 15, 624. Copyright 2003 Wiley-VCH.

    Figure 24. SEM images of TiO2 nanorods grown at 560 C.Reprinted with permission from Wu, J. J.; Yu, C. C.J. Phys. Chem.

    B 2004, 108, 3377. Copyright 2004 American Chemical Society.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2903

  • 5/20/2018 Titanium Oxide Nanomaterials

    14/69

    morphology of the TiO2nanostructures can be tuned withthe reaction conditions. For example, at 630 and 560 Cunder a pressure of 5 Torr, single-crystalline rutile andanatase TiO2nanorods were formed respectively, while, at535 C under 3.6 Torr, anatase TiO2nanowalls composedof well-aligned nanorods were formed.233

    In addition to the above CVD approaches in preparingTiO2nanomaterials, other CVD approaches are also used,such as electrostatic spray hydrolysis,234 diffusion flamepyrolysis,235-239 thermal plasma pyrolysis,240-246 ultrasonicspray pyrolysis,247 laser-induced pyrolysis,248,249 and ultronsic-assisted hydrolysis,250,251 among others.

    2.8. Physical Vapor Deposition

    In PVD, materials are first evaporated and then condensedto form a solid material. The primary PVD methods includethermal deposition, ion plating, ion implantation, sputtering,laser vaporization, and laser surface alloying. TiO2nanowirearrays have been fabricated by a simple PVD method orthermal deposition.252-254 Typically, pure Ti metal powderis on a quartz boat in a tube furnace about 0.5 mm awayfrom the substrate. Then the furnace chamber is pumpeddown to 300 Torr and the temperature is increased to 850C under an argon gas flow with a rate of 100 sccm andheld for 3 h. After the reaction, a layer of TiO2nanowirescan be obtained.254 A layer of Ti nanopowders can bedeposited on the substrate before the growth of TiO2nanowires,252,253 and Au can be employed as catalyst.252 Atypical SEM image of TiO2nanowires made with the PVDmethod is shown in Figure 25.252

    2.9. Electrodeposition

    Electrodeposition is commonly employed to produce acoating, usually metallic, on a surface by the action ofreduction at the cathode. The substrate to be coated is usedas cathode and immersed into a solution which contains asalt of the metal to be deposited. The metallic ions areattracted to the cathode and reduced to metallic form. Withthe use of the template of an AAM, TiO2nanowires can beobtained by electrodeposition.255,256 In a typical process, theelectrodeposition is carried out in 0.2 M TiCl3solution with

    pH ) 2 with a pulsed electrodeposition approach, andtitanium and/or its compound are deposited into the poresof the AAM. By heating the above deposited template at500C for 4 h and removing the template, pure anatase TiO2nanowires can be obtained. Figure 26 shows a representativeSEM image of TiO2nanowires.256

    2.10. Sonochemical Method

    Ultrasound has been very useful in the synthesis of a widerange of nanostructured materials, including high-surface-area transition metals, alloys, carbides, oxides, and colloids.The chemical effects of ultrasound do not come from a directinteraction with molecular species. Instead, sonochemistryarises from acoustic cavitation: the formation, growth, andimplosive collapse of bubbles in a liquid. Cavitationalcollapse produces intense local heating (5000 K), high pres-sures (1000 atm), and enormous heating and cooling rates(>109 K/s). The sonochemical method has been applied toprepare various TiO2nanomaterials by different groups.257-269

    Yu et al. applied the sonochemical method in preparinghighly photoactive TiO2 nanoparticle photocatalysts withanatase and brookite phases using the hydrolysis of titaniumtetraisoproproxide in pure water or in a 1:1 EtOH-H2Osolution under ultrasonic radiation.109 Huang et al. found thatanatase and rutile TiO2nanoparticles as well as their mixturescould be selectively synthesized with various precursorsusing ultrasound irradiation, depending on the reactiontemperature and the precursor used.259 Zhu et al. developedtitania whiskers and nanotubes with the assistance ofsonication as shown in Figure 27.269 They found that arraysof TiO2nanowhiskers with a diameter of 5 nm and nanotubeswith a diameter of5 nm and a length of 200-300 nm couldbe obtained by sonicating TiO2particles in NaOH aqueoussolution followed by washing with deionized water and adilute HNO3aqueous solution.

    2.11. Microwave Method

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

    Figure 25. SEM images of the TiO2nanowire arrays prepared bythe PVD method. Reprinted from Wu, J. M.; Shih, H. C.; Wu, W.T. Chem. Phys. Lett. 2005, 413, 490, Copyright 2005, withpermission from Elsevier. Figure 26. Cross-sectional SEM image of TiO2nanowires elec-

    trodeposited in AAM pores. Reprinted from Liu, S.; Huang, K.Sol. Energy Mater. Sol. Cells 2004,85, 125, Copyright 2004, with

    permission from Elsevier.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    15/69

    frequencies of microwave heating are between 900 and 2450MHz. At lower microwave frequencies, conductive currentsflowing within the material due to the movement of ionic con-stituents can transfer energy from the microwave field to thematerial. At higher frequencies, the energy absorption is pri-marily due to molecules with a permanent dipole which tendto reorientate under the influence of a microwave electricfield. This reorientation loss mechanism originates from theinability of the polarization to follow extremely rapid rever-sals of the electric field, so the polarization phasor lags theapplied electric field. This ensures that the resulting currentdensity has a component in phase with the field, and thereforepower is dissipated in the dielectric material. The majoradvantages of using microwaves for industrial processing arerapid heat transfer, and volumetric and selective heating.

    Microwave radiation is applied to prepare various TiO2nanomaterials.270-276 Corradi et al. found that colloidal titaniananoparticle suspensions could be prepared within 5 min to1 h with microwave radiation, while 1 to 32 h was needed

    for the conventional synthesis method of forced hydrolysisat 195C.270 Ma et al. developed high-quality rutile TiO2nano-rods with a microwave hydrothermal method and found thatthey aggregated radially into spherical secondary nanopartic-les.272 Wu et al. synthesized TiO2nanotubes by microwaveradiation via the reaction of TiO2crystals of anatase, rutile,or mixed phase and NaOH aqueous solution under a certainmicrowave power.275 Normally, the TiO2nanotubes had thecentral hollow, open-ended, and multiwall structure withdiameters of 8-12 nm and lengths up to 200-1000 nm.275

    2.12. TiO2 Mesoporous/Nanoporous Materials

    In the past decade, mesoporous/nanoporous TiO2materialshave been well studied with or without the use of organic

    surfactant templates.28,80,264,265,277-312 Barbe et al. reported thepreparation of a mesoporous TiO2film by the hydrothermalmethod as shown Figure 28.80 In a typical experiment, TTIPwas added dropwise to a 0.1 M nitric acid solution undervigorous stirring and at room temperature. A white precipitateformed instantaneously. Immediately after the hydrolysis, thesolution was heated to 80 C and stirred vigorously for 8 hfor peptization. The solution was then filtered on a glass fritto remove agglomerates. Water was added to the filtrate to

    adjust the final solids concentration to 5 wt %. The solutionwas put in a titanium autoclave for 12 h at 200-250 C.After sonication, the colloidal suspension was put in a rotaryevaporator and evaporated to a final TiO2concentration of11 wt %. The precipitation pH, hydrolysis rate, autoclavingpH, and precursor chemistry were found to influence themorphology of the final TiO2nanoparticles.

    Alternative procedures without the use of hydrothermalprocesses have been reported by Liu et al.292 and Zhang etal.311 In the report by Liu et al., 24.0 g of titanium(IV)n-butoxide ethanol solution (weight ratio of 1:7) wasprehydrolyzed in the presence of 0.32 mL of a 0.28 M HNO3aqueous solution (TBT/HNO3 100:1) at room temperaturefor 3 h. 0.32 mL of deionized water was added to the

    prehydrolyzed solution under vigorous stirring and stirredfor an additional 2 h. The sol solution in a closed vesselwas kept at room temperature without stirring to gel andage. After aging for 14 days, the gel was dried at roomtemperature, ground into a fine powder, washed thoroughlywith water and ethanol, and dried to produce porous TiO2.Upon calcination at 450 C for 4 h under air, crystallizedmesoporous TiO2material was obtained.292

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

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

    Figure 27. TEM images of 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 bypermission of The Royal Society of Chemistry.

    Figure 28. SEM image of the mesoporous TiO2film synthesizedfrom the acetic acid-modified precursor and autoclaved at 230 C.Reprinted with permission from Barbe, C. J.; Arendse, F.; Comte,P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gratzel, M. J. Am.Ceram. Soc.1997,80, 3157. Copyright 1997 Blackwell Publishing.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2905

  • 5/20/2018 Titanium Oxide Nanomaterials

    16/69

    sation and agglomeration of these sol nanoparticles underhigh-intensity ultrasound radiation. The mesoporous TiO2hada wormhole-like structure consisting of TiO2nanoparticlesand a lack of long-range order.265

    In the template method used by the Stuckygroup278-280,287,295,302,306-307,313 and other groups,264,293,297,303,309

    structure-directing agents were used for organizing network-forming metal oxide species in nonaqueous solutions. Thesestructure-directing agents were also called organic templates.

    The most commonly used organic templates were amphi-philic poly(alkylene oxide) block copolymers, such as HO-(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designatedEO20PO70EO20, called Pluronic P-123) and HO(CH2CH2O)106-(CH2CH(CH3)O)70(CH2CH2O)106H (designated EO106PO70-EO106, called Pluronic F-127). In a typical synthesis, poly-(alkylene oxide) block copolymer was dissolved in ethanol.Then TiCl4precursor was added with vigorous stirring. Theresulting sol solution was gelled in an open Petri dish at 40C in air for 1-7 days. Mesoporous TiO2was obtained afterremoving the surfactant species by calcining the as-madesample at 400 C for 5 h in air.306 Figure 29 shows typicalTEM images of the mesoporous TiO2. Besides triblock co-polymers as structure-directing agents, diblock polymers were

    also used such as [CnH2n-1(OCH2CH2)yOH, Brij 56 (B56, n/y) 16/10) or Brij 58 (B58,n/y ) 16/20)] by Sanchez et al.285

    Other surfactants employed to direct the formation ofmesoporous TiO2include tetradecyl phosphate (a 14-carbonchain) by Antonelli and Ying277 and commercially availabledodecyl phosphate by Putnam and co-workers,298 cetyltri-methylammonium bromide (CTAB) (a cationic surfac-tant),281,283,296 the recent Gemini surfactant,294 and dodecyl-amine (a neutral surfactant).304 Carbon nanotubes310 andmesoporous SBA-15286 have also been used as the skeletonfor mesoporous TiO2.

    2.13. TiO2 Aerogels

    The study of TiO2 aerogels is worthy of special men-tion.314-326 The combination of sol-gel processing withsupercritical drying offers the synthesis of TiO2aerogels withmorphological and chemical properties that are not easilyachieved by other preparation methods, i.e., with high surfacearea. Campbell et al. prepared TiO2 aerogels by sol-gelsynthesis from titanium n-butoxide in methanol with thesubsequent removal of solvent by supercritical CO2.315 Fora typical synthesis process, titanium n-butoxide was addedto 40 mL of methanol in a dry glovebox. This solution wascombined with another solution containing 10 mL ofmethanol, nitric acid, and deionized water. The concentrationof the titanium n-butoxide was kept at 0.625 M, and themolar ratio of water/HNO3/titanium n-butoxide was 4:0.1:

    1. The gel was allowed to age for 2 h and then extracted ina standard autoclave with supercritical CO2at a flow rate of24.6 L/h, at 343 K under 2.07 107 Pa for 2-3 h, resultingin complete removal of solvent. After extraction, the samplewas heated in a vacuum oven at 3.4 kPa and 383 K for 3 hto remove the residual solvent and at 3.4 kPa and 483 K for3 h to remove any residual organics. The pretreated samplehad a brown color and turned white after calcination at 773K or above. The resulting TiO2aerogel, after calcination at773 K for 2 h, had a BET surface area of >200 m2/g,contained mesopores in the range 2-10 nm, and was of thepure anatase form. Dagan et al. found the TiO2 aerogelsobtanied by using a Ti/ethanol/H2O/nitric acid ratio of 1:20:3:0.08 could have a porosity of 90% and surface areas of

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

    Figure 29. TEM micrographs of two-dimensional hexagonalmesoporous TiO2recorded along the (a) [110] and (b) [001] zoneaxes, respectively. The inset in part a is selected-area electrondiffraction patterns obtained on the image area. (c) TEM image ofcubic mesoporous TiO2accompanied by the corresponding (inset)EDX spectrum. Reprinted with permission from Yang, P.; Zhao,D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D.Chem. Mater.1999, 11, 2813. Copyright 1999 American Chemical Society.

    2906 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao

  • 5/20/2018 Titanium Oxide Nanomaterials

    17/69

    2.14. TiO2 Opal and Photonic Materials

    The syntheses of TiO2opal and photonic materials havebeen well studied by various groups.327-358 Holland et al.

    reported the preparation of TiO2 inverse opal from thecorresponding metal alkoxides, using latex spheres astemplates.334,335 Millimeter-thick layers of latex spheres weredeposited on filter paper in a Buchner funnel under vacuumand soaked with ethanol. Titanium ethoxide was addeddropwise to cover the latex spheres completely while suctionwas applied. Typical mass ratios of alkoxide to latex werebetween 1.4 and 3. After drying the composite in a vacuumdesiccator for 3 to 24 h, the latex spheres were removed bycalcination in flowing air at 575 C for 7 to 12 h, leavinghard and brittle powder particles with 320- to 360-nm voids.The carbon content of the calcined samples varied from 0.4to 1.0 wt %, indicating that most of the latex templates hadbeen removed from the 3D host. Figure 31 shows an

    illustration of the simple synthesis of TiO2inverse opal andan SEM image of TiO2inverse opals. Similar studies havealso been carried out by other researchers.327,356

    Dong and Marlow prepared TiO2 inversed opals with askeleton-like structure of TiO2rods by a template-directedmethod using monodispersed polystyrene particles of size270 nm.328-330,345 Infiltration of a titania precursor (Ti(i-OPr)4in EtOH) was followed by a drying and calcination proce-dure. The precursor concentration was varied from 30% to100%, and the calcination temperature was tuned from 300to 700C. A SEM picture of the TiO2inversed opal is shownin Figure 32.329 The skeleton structure consists of rhombo-hedral windows and TiO2cylinders forming a highly regularnetwork. The cylinders connect the centers of the former

    octahedral and tetrahedral voids of the opal. These voids forma CaF2 lattice which is filled with cylindrical bonds con-necting the Ca and F sites.

    Wang et al. reported their study on the large-scalefabrication of ordered TiO2nanobowl arrays.354 The processstarts with a self-assembled monolayer of polystyrene (PS)spheres, which is used as a template for atomic layerdeposition of a TiO2layer. After ion-milling, toluene-etching,and annealing of the TiO2-coated spheres, ordered arrays ofnanostructured TiO2nanobowls can be fabricated as shownin Figure 33.

    Wang et al. fabricated a 2D photonic crystal by coatingpatterned and aligned ZnO nanorod arrays with TiO2.355 PSspheres were self-assembled to make a monolayer mask on

    a sapphire substrate, which was then covered with a layerof gold. After removing the PS spheres with toluene, ZnOnanorods were grown using a vapor-liquid-solid process.

    Finally, a TiO2layer was deposited on the ZnO nanorodsby introducing TiCl4and water vapors into the atomic layerdeposition chamber at 100C. Figure 34 shows SEM imagesof a ZnO nanorod array and the TiO2-coated ZnO nanorodarray.

    Li et al. reported the preparation of ordered arrays of TiO2opals using opal gel templates under uniaxial compressionat ambient temperature during the TiO2sol/gel process.337

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

    Figure 30. SEM image of a TiO2 aerogel. Reprinted withpermission from Zhu, Z.; Tsung, L. Y.; Tomkiewicz, M. J. Phys.Chem. 1995, 99, 15945. Copyright 1995 American ChemicalSociety.

    Figure 31. (A) Schematic illustration of the synthesis of a TiO2inversed opal. (B) SEM image of the TiO2inversed opal. Reprintedwith permission from Holland, B. T.; Blanford, C.; Stein, A.Science1998, 281, 538 (http://www.sciencemag.org). Copyright 1998AAAS.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2907

  • 5/20/2018 Titanium Oxide Nanomaterials

    18/69

    ethanol mixture (4:7 wt/wt) with total monomer content 30wt %. Ethanol was used to facilitate diffusion of themonomer solution into the inverse opal polystyrene. Afterthe inverse opal was infiltrated by the monomer solutioncontaining 1 wt % of the initiator AIBN and a subsequentfree radical polymerization at 60C for 3 h, a solid compositeresulted. The initial inverse opal polystyrene template wasthen removed with chloroform in a Soxhlet extractor for 12h, whereupon the opal gel was formed. By using differentcompositions of the monomer solution, hole sizes, andstacking structures of the starting inverse opal templates, opal

    gels with correspondingly different properties can be pro-duced. Water was completely removed from the opalhydrogel by repeatedly rinsing it with a large amount ofethanol. Afterward, the opal gel was put into a large amountof tetrabutyl titanate (TBT) at ambient temperature for 24h. The TBT-swollen opal gel was then immersed in a water/ethanol (1:1 wt/wt) mixture for 5 h to let the TiO2sol/gelprocess proceed. Figure 35A shows the opal structure of thegel/titania composite spheres formed. After calcination, TiO2opal with distinctive spherical contours could be found. Thecompression degree, R, was adjusted by the spacer heightwhen the substrates were compressed. When the substrateswere slightly compressed against each other to the extent ofproducing a 20% reduction in the thickness of the composi-tion opal, the deformation of the template-synthesized titaniaspheres was not substantial (Figure 35B). When the com-pression degree was increased to the point of reaching 35%deformation in the opal gel, noticeably deformed titania opalscould be obtained (Figure 35C and D).

    2.15. Preparation of TiO2 Nanosheets

    The preparation of TiO2nanosheets has also been explored

    recently.359-368 Typically, TiO2nanosheets were synthesizedby delaminating layered protonic titanate into colloidal singlelayers. A stoichiometric mixture of Cs2CO3and TiO2wascalcined at 800 C for 20 h to produce a precursor, cesiumtitanate, Cs0.7Ti1.82500.175O4 (0: vacancy), about 70 g ofwhich was treated with 2 L of a 1 M HCl solution at roomtemperature. This acid leaching was repeated three times byrenewing the acid solution every 24 h. The resulting acid-exchanged product was filtered, washed with water, and air-dried. The obtained protonic titanate, H0.7Ti1.82500.175O4H2O,was shaken vigorously with a 0.017 M tetrabutylammoniumhydroxide solution at ambient temperature for 10 days. Thesolution-to-solid ratio was adjusted to 250 cm3 g-1. Thisprocedure yielded a stable colloidal suspension with an

    Figure 32. SEM picture of a TiO2skeleton with a cylinder radiusof about 0.06a. a is the lattice constant of the cubic unit cell.Reprinted from Dong, W.; Marlow, F. Physica E 2003, 17, 431,Copyright 2003, with permission from Elsevier.

    Figure 33. (A) Experimental procedure for fabricating TiO2nanobowl arrays. (B) Low- and high- (inset) magnification SEMimage of TiO2nanobowl arrays. Reprinted with permission from

    Wang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers,C. J.Nano Lett.2004,4, 2223. Copyright 2004 American ChemicalSociety.

    Figure 34. (A) SEM images of short and densely aligned ZnOnanorod array on a sapphire substrate. Inset: An optical image of

    the aligned ZnO nanorods over a large area. (B) SEM image ofthe TiO2-coated ZnO nanorod array. Reprinted with permission fromWang, X.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger,L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J.AdV. Mater.2005, 17, 2103. Copyright 2005 Wiley-VCH.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    19/69

    opalescent appearance. Figure 36 shows TEM and AFMimages of TiO2nanosheets with thicknesses of 1.2-1.3 nm,which is the height of the TiO2nanosheet with a monolayerof water molecules on both sides (0.70 + 0.25 2) thick.366

    3. Properties of TiO2Nanomaterials

    3.1. Structural Properties of TiO2 Nanomaterials

    Figure 37 shows the unit cell structures of the rutile andanatase TiO2.11 These two structures can be described interms of chains of TiO6octahedra, where each Ti4+ ion issurrounded by an octahedron of six O2- ions. The two crystalstructures differ in the distortion of each octahedron and bythe assembly pattern of the octahedra chains. In rutile, the

    octahedron shows a slight orthorhombic distortion; in anatase,the octahedron is significantly distorted so that its symmetryis lower than orthorhombic. The Ti-Ti distances in anataseare larger, whereas the Ti-O distances are shorter than thosein rutile. In the rutile structure, each octahedron is in contactwith 10 neighbor octahedrons (two sharing edge oxygen pairsand eight sharing corner oxygen atoms), while, in the anatasestructure, each octahedron is in contact with eight neighbors(four sharing an edge and four sharing a corner). Thesedifferences in lattice structures cause different mass densitiesand electronic band structures between the two forms ofTiO2.

    Hamad et al. performed a theoretical calculation on TinO2nclusters (n ) 1-15) with a combination of simulated

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

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2909

  • 5/20/2018 Titanium Oxide Nanomaterials

    20/69

    annealing, Monte Carlo basin hopping simulation, andgenetic algorithms methods.369 They found that the calculatedglobal minima consisted of compact structures, with titaniumatoms reaching high coordination rapidly as nincreased. Forn g 11, the particles had at least a central octahedronsurrounded by a shell of surface tetrahedra, trigonal bipyra-mids, and square base pyramids.

    Swamy et al. found the metastability of anatase as afunction of pressure was size dependent, with smallercrystallites preserving the structure to higher pressures.370

    Three size regimes were recognized for the pressure-inducedphase transition of anatase at room temperature: an anatase-

    amorphous transition regime at the smallest crystallite sizes,

    an anatase-

    baddeleyite transition regime at intermediatecrystallite sizes, and an anatase-R-PbO2transition regimecomprising large nanocrystals to macroscopic single crystals.

    Barnard et al. performed a series of theoretical studies onthe phase stability of TiO2nanoparticles in different environ-ments by a thermodynamic model.371-375 They found thatsurface passivation had an important impact on nanocrystalmorphology and phase stability. The results showed thatsurface hydrogenation induced significant changes in theshape of rutile nanocrystals, but not in anatase, and that thesize at which the phase transition might be expected increaseddramatically when the undercoordinated surface titaniumatoms were H-terminated. For spherical particles, the cross-over point was about 2.6 nm. For a clean and faceted surface,

    at low temperatures (a phase transition pointed at an averagediameter of approximately 9.3-9.4 nm for anatase nano-crystals), the transition size decreased slightly to 8.9 nm whenthe surface bridging oxygens were H-terminated, and the sizeincreased significantly to 23.1 nm when both the bridgingoxygens and the undercoordinated titanium atoms of thesurface trilayer were H-terminated. Below the cross point,the anatase phase was more stable than the rutile phase.371

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

    Figure 36. (A) TEM of Ti1-O24- nanosheets. (B and C) AFM image and height scan of the TiO2nanosheets deposited on a Si wafer.(D) Structural model for a hydrated TiO2 nanosheet. Closed, open, and shaded circles represent Ti atom, O atom, and H 2O molecules,respectively. All the water sites are assumed to be half occupied. Reprinted with permission from Sasaki, T.; Ebina, Y.; Kitami, Y.; Watanabe,M.; Oikawa, T. J. Phys. Chem. B 2001, 105, 6116. Copyright 2001 American Chemical Society.

    Figure 37. Lattice structure of rutile and anatase TiO2. Reprintedwith permission from Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr.Chem. ReV. 1995, 95, 735. Copyright 1995 American Chemical

    Society.

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

  • 5/20/2018 Titanium Oxide Nanomaterials

    21/69

    nanoparticles as a function of shape, size, and degree ofsurface passivation.372 Their study on anatase and rutile

    titanium dioxide polymorphs passivated with completemonolayers of adsorbates by varying the hydrogen to oxygenratio with respect to a neutral, water-terminated surfaceshowed that termination with water consistently resulted inthe lowest values of surface free energy when hydrated orwith a higher fraction of H on the surface on both anataseand rutile surfaces, but conversely, the surfaces generallyhad a higher surface free energy when they had an equalratio of H and O in the adsorbates or were O-terminated.375

    They demonstrated that, under different pH conditions fromacid to basic, the phase transition size of a TiO2nanoparticlevaried from 6.9 to 22.7 nm, accompanied with shape changesof the TiO2nanoparticles as shown in Figure 38.374

    Enyashin and Seifert conducted a theoretical study on thestructural stability of TiO2layer modifications (anatase andlepidocrocite) using the density-functional-based tight bind-ing method (DFTB).376 They found that anatase nanotubeswere the most stable modifications in a comparison of single-walled nanotubes, nanostrips, and nanorolls. Their stabilityincreased as their radii grew. The energies for all TiO2nanostructures relative to the infinite monolayer followed a1/R2 curve.

    Chen et al. found that severe distortions existed in Ti siteenvironments in the structures of 1.9 nm TiO2nanoparticlescompared to those octahedral Ti sites in bulk anatase Ti usingK-edge XANES.377 The distorted Ti sites were likely to adopta pentacoordinate square pyramidal geometry due to thetruncation of the lattice. The distortions in the TiO2latticewere mainly located on the surface of the nanoparticles andwere responsible for binding with other small molecules.

    Qian et al. found that the density of the surface states onTiO2nanoparticles was likely dependent upon the details ofthe preparation methods.378 The TiO2nanoparticles preparedfrom basic sol were found to have more surface states thanthose prepared from acidic sol based on a surface photo-voltage spectroscopy study.

    3.2. Thermodynamic Properties of TiO2Nanomaterials

    Rutile is the stable phase at high temperatures, but anataseand brookite are common in fine grained (nanoscale) natural

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

    However, abnormal behaviors and inconsistent results areoccasionally observed.

    Hwu et al. found the crystal structure of TiO2nanoparticlesdepended largely on the preparation method.379 For smallTiO2nanoparticles (973 K. Banfield et al. foundthat the prepared TiO2 nanoparticles had anatase and/orbrookite structures, which transformed to rutile after reachinga certain particle size.73,380 Once rutile was formed, it grew

    much faster than anatase. They found that rutile became morestable than anatase for particle size >14 nm.Ye et al. observed a slow brookite to anatase phase

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

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

    in their study on the phase transformation behavior ofnanocrystalline aggregates during their growth for isothermaland isochronal reactions.74 They concluded that, for equallysized nanoparticles, anatase was thermodynamically stablefor sizes < 11 nm, brookite was stable for sizes between 11and 35 nm, and rutile was stable for sizes >35 nm.

    Ranade et al. investigated the energetics of the TiO2polymorphs (rutile, anatase, and brookite) by high-temper-ature oxide melt drop solution calorimetry, and they foundthe energetic stability crossed over between the three phasesas shown in Figure 39.383 The dark solid line represents thephases of lowest enthalpy as a function of surface area. Rutilewas energetically stable for surface area 200 nm), brookite was energetically stable from

    Figure 38. Morphology predicted for anatase (top), with (a)hydrogenated surfaces, (b) hydrogen-rich surface adsorbates, (c)hydrated surfaces, (d) hydrogen-poor adsorbates, and (e) oxygenatedsurfaces, and for rutile (bottom), with (f) hydrogenated surfaces,(g) hydrogen-rich surface adsorbates, (h) hydrated surfaces, (i)hydrogen-poor adsorbates, and (j) oxygenated surfaces. Reprintedwith permission from Barnard, A. S.; Curtiss, L. A. Nano Lett.2005, 5, 1261. Copyright 2005 American Chemical Society.

    Figure 39. Enthalpy of nanocrystalline TiO2. Reprinted withpermission from Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Ban-field, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.;Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002,99, 6476. Copyright 2002 National Academy of Sciences, U.S.A.

    Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2911

  • 5/20/2018 Titanium Oxide Nanomaterials

    22/69

    592 to 3174 m2/mol (7-40 m2/g or 200-40 nm), and anatasewas energetically stable for greater surface areas or smallersizes ( 40 m2/g,it was metastable wit