titanium dioxide nanomaterials synthesis, Properties, Modifications, and Applications

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Titanium Dioxide Nanomaterials: Synthesis,
Properties, Modifications, and Applications
Xiaobo Chen, and Samuel S. Mao
Chem. Rev., 2007, 107 (7), 2891-2959• DOI: 10.1021/cr0500535 • Publication Date (Web): 23 June 2007
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Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and
Applications
Xiaobo Chen* and Samuel S. Mao
†
Lawrence Berkeley National Laboratory, and University of California, Berkeley, 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:
†
E-mail:
2891
Chem. Rev.
2007,
107,
2891
−
2959
10.1021/cr0500535 CCC: $65.00 © 2007 American Chemical Society
Published on Web 06/23/2007
the nanometer scale. Properties also vary as the shapes of
the shrinking nanomaterials change. Many excellent reviews
and reports on the preparation and properties of nanomaterials
have been published recently.

6-44
Among the unique proper-
ties of nanomaterials, the movement of electrons and holes
in semiconductor nanomaterials is primarily governed by the
well-known quantum confinement, and the transport proper-
ties related to phonons and photons are largely affected by
the size and geometry of the materials.
13-16
The specific
surface area and surface-to-volume ratio increase dramati-
cally as the size of a material decreases.
13,21
The high surface
area brought about by small particle size is beneficial to many
TiO
2
-based devices, as it facilitates reaction/interaction
between the devices and the interacting media, which mainly
occurs on the surface or at the interface and strongly depends
on the surface area of the material. Thus, the performance
of TiO
2
-based devices is largely influenced by the sizes of
the TiO
2
building units, apparently at the nanometer scale.
As the most promising photocatalyst,
7,11,12,33
TiO
2

mate-
rials are expected to play an important role in helping solve
many serious environmental and pollution challenges. TiO
2
also bears tremendous hope in helping ease the energy crisis
through effective utilization of solar energy based on
photovoltaic and water-splitting devices.
9,31,32
As continued
breakthroughs have been made in the preparation, modifica-
tion, and applications of TiO
2
nanomaterials in recent years,
especially after a series of great reviews of the subject in
the 1990s.
7,8,10-12,33,45
we believe that a new and compre-
hensive review of TiO
2
nanomaterials would further promote
TiO
2
-based research and development efforts to tackle the
environmental and energy challenges we are currently facing.
Here, we focus on recent progress in the synthesis, properties,
modifications, and applications of TiO
2
nanomaterials. The
syntheses of TiO
2

nanomaterials, including nanoparticles,
nanorods, nanowires, and nanotubes are primarily categorized
with the preparation method. The preparations of mesopo-
rous/nanoporous TiO
2
, TiO
2
aerogels, opals, and photonic
materials are summarized separately. In reviewing nanoma-
terial synthesis, we present a typical procedure and repre-
sentative transmission or scanning electron microscopy
images to give a direct impression of how these nanomate-
rials are obtained and how they normally appear. For detailed
instructions on each synthesis, the readers are referred to
the corresponding literature.
The structural, thermal, electronic, and optical properties
of TiO
2
nanomaterials are reviewed in the second section.
As the size, shape, and crystal structure of TiO
2
nanomate-
rials vary, not only does surface stability change but also
the transitions between different phases of TiO
2
under
pressure or heat become size dependent. The dependence of
X-ray diffraction patterns and Raman vibrational spectra on
the size of TiO
2

nanomaterials is also summarized, as they
could help to determine the size to some extent, although
correlation of the spectra with the size of TiO
2
nanomaterials
is not straightforward. The review of modifications of TiO
2
nanomaterials is mainly limited to the research related to
the modifications of the optical properties of TiO
2
nanoma-
terials, since many applications of TiO
2
nanomaterials are
closely related to their optical properties. TiO
2
nanomaterials
normally are transparent in the visible light region. By doping
or sensitization, it is possible to improve the optical sensitiv-
ity and activity of TiO
2
nanomaterials in the visible light
region. Environmental (photocatalysis and sensing) and
energy (photovoltaics, water splitting, photo-/electrochromics,
and hydrogen storage) applications are reviewed with an
emphasis on clean and sustainable energy, since the increas-
ing energy demand and environmental pollution create a
pressing need for clean and sustainable energy solutions. The
fundamentals and working principles of the TiO
2

nanoma-
terials-based devices are discussed to facilitate the under-
standing and further improvement of current and practical
TiO
2
nanotechnology.
2. Synthetic Methods for TiO
2
Nanostructures
2.1. Sol
−
Gel Method
The sol-gel method is a versatile process used in making
various ceramic materials.
46-50
In a typical sol-gel process,
a colloidal suspension, or a sol, is formed from the hydrolysis
and polymerization reactions of the precursors, which are
usually inorganic metal salts or metal organic compounds
such as metal alkoxides. Complete polymerization and loss
of solvent leads to the transition from the liquid sol into a
solid gel phase. Thin films can be produced on a piece of
Dr. Xiaobo Chen is a research engineer at The University of California at
Berkeley and a Lawrence Berkeley National Laboratory scientist. He
obtained his Ph.D. Degree in Chemistry from Case Western Reserve
University. His research interests include photocatalysis, photovoltaics,
hydrogen storage, fuel cells, environmental pollution control, and the related
materials and devices development.
Dr. Samuel S. Mao is a career staff scientist at Lawrence Berkeley National
Laboratory and an adjunct faculty at The University of California at

Berkeley. He obtained his Ph.D. degree in Engineering from The University
of California at Berkeley in 2000. His current research involves the
development of nanostructured materials and devices, as well as ultrafast
laser technologies. Dr. Mao is the team leader of a high throughput
materials processing program supported by the U.S. Department of Ener-
gy.
2892 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
substrate by spin-coating or dip-coating. A wet gel will form
when the sol is cast into a mold, and the wet gel is converted
into a dense ceramic with further drying and heat treatment.
A highly porous and extremely low-density material called
an aerogel is obtained if the solvent in a wet gel is removed
under a supercritical condition. Ceramic fibers can be drawn
from the sol when the viscosity of a sol is adjusted into a
proper viscosity range. Ultrafine and uniform ceramic
powders are formed by precipitation, spray pyrolysis, or
emulsion techniques. Under proper conditions, nanomaterials
can be obtained.
TiO
2
nanomaterials have been synthesized with the sol-
gel method from hydrolysis of a titanium precusor.
51-78
This
process normally proceeds via an acid-catalyzed hydrolysis
step of titanium(IV) alkoxide followed by condensa-
tion.
51,63,66,79-91
The development of Ti-O-Ti chains is
favored with low content of water, low hydrolysis rates, and

excess titanium alkoxide in the reaction mixture. Three-
dimensional polymeric skeletons with close packing result
from the development of Ti-O-Ti chains. The formation
of Ti(OH)
4
is favored with high hydrolysis rates for a
medium amount of water. The presence of a large quantity
of Ti-OH and insufficient development of three-dimensional
polymeric skeletons lead to loosely packed first-order
particles. Polymeric Ti-O-Ti chains are developed in the
presence of a large excess of water. Closely packed first-
order particles are yielded via a three-dimensionally devel-
oped gel skeleton.
51,63,66,79-91
From the study on the growth
kinetics of TiO
2
nanoparticles in aqueous solution using
titanium tetraisopropoxide (TTIP) as precursor, it is found
that the rate constant for coarsening increases with temper-
ature due to the temperature dependence of the viscosity of
the solution and the equilibrium solubility of TiO
2
.
63
Second-
ary particles are formed by epitaxial self-assembly of primary
particles at longer times and higher temperatures, and the
number of primary particles per secondary particle increases
with time. The average TiO

2
nanoparticle radius increases
linearly with time, in agreement with the Lifshitz-Slyozov-
Wagner model for coarsening.
63
Highly crystalline anatase TiO
2
nanoparticles with different
sizes and shapes could be obtained with the polycondensation
of titanium alkoxide in the presence of tetramethylammonium
hydroxide.
52,62
In a typical procedure, titanium alkoxide is
added to the base at 2 °C in alcoholic solvents in a three-
neck flask and is heated at 50-60 °C for 13 days or at 90-
100 °C for 6 h. A secondary treatment involving autoclave
heating at 175 and 200 °C is performed to improve the
crystallinity of the TiO
2
nanoparticles. Representative TEM
images are shown in Figure 1 from the study of Chemseddine
et al.
52
A series of thorough studies have been conducted by
Sugimoto et al. using the sol-gel method on the formation
of TiO
2
nanoparticles of different sizes and shapes by tuning
the reaction parameters.
67-71

Typically, a stock solution of
a 0.50 M Ti source is prepared by mixing TTIP with
triethanolamine (TEOA) ([TTIP]/[TEOA] ) 1:2), followed
by addition of water. The stock solution is diluted with a
shape controller solution and then aged at 100 °C for 1 day
and at 140 °C for 3 days. The pH of the solution can be
tuned by adding HClO
4
or NaOH solution. Amines are used
as the shape controllers of the TiO
2
nanomaterials and act
as surfactants. These amines include TEOA, diethylenetri-
amine, ethylenediamine, trimethylenediamine, and triethyl-
enetetramine. The morphology of the TiO
2
nanoparticles
changes from cuboidal to ellipsoidal at pH above 11 with
TEOA. The TiO
2
nanoparticle shape evolves into ellipsoidal
above pH 9.5 with diethylenetriamine with a higher aspect
ratio than that with TEOA. Figure 2 shows representative
TEM images of the TiO
2
nanoparticles under different initial
pH conditions with the shape control of TEOA at [TEOA]/
[TIPO] ) 2.0. Secondary amines, such as diethylamine, and
tertiary amines, such as trimethylamine and triethylamine,
act as complexing agents of Ti(IV) ions to promote the

growth of ellipsoidal particles with lower aspect ratios. The
shape of the TiO
2
nanoparticle can also be tuned from round-
cornered cubes to sharp-edged cubes with sodium oleate and
sodium stearate.
70
The shape control is attributed to the tuning
of the growth rate of the different crystal planes of TiO
2
nanoparticles by the specific adsorption of shape controllers
to these planes under different pH conditions.
70
A prolonged heating time below 100 °C for the as-prepared
gel can be used to avoid the agglomeration of the TiO
2
nano-
particles during the crystallization process.
58,72
By heating
amorphous TiO
2
in air, large quantities of single-phase ana-
tase TiO
2
nanoparticles with average particle sizes between
7 and 50 nm can be obtained, as reported by Zhang and
Banfield.
73-77
Much effort has been exerted to achieve highly

crystallized and narrowly dispersed TiO
2
nanoparticles using
the sol-gel method with other modifications, such as a
semicontinuous reaction method by Znaidi et al.
78
and a two-
stage mixed method and a continuous reaction method by
Kim et al.
53,54
By a combination of the sol-gel method and an anodic
alumina membrane (AAM) template, TiO
2
nanorods have
been successfully synthesized by dipping porous AAMs
into a boiled TiO
2
sol followed by drying and heating
processes.
92,93
In a typical experiment, a TiO
2
sol solution is
prepared by mixing TTIP dissolved in ethanol with a solution
containing water, acetyl acetone, and ethanol. An AAM is
immersed into the sol solution for 10 min after being boiled
in ethanol; then it is dried in air and calcined at 400 °C for
10 h. The AAM template is removed in a 10 wt % H
3
PO

4
aqueous solution. The calcination temperature can be used
to control the crystal phase of the TiO
2
nanorods. At low
temperature, anatase nanorods can be obtained, while at
high temperature rutile nanorods can be obtained. The pore
size of the AAM template can be used to control the size of
these TiO
2
nanorods, which typically range from 100 to 300
nm in diameter and several micrometers in length. Appar-
ently, the size distribution of the final TiO
2
nanorods is
largely controlled by the size distribution of the pores of
the AAM template. In order to obtain smaller and mono-
sized TiO
2
nanorods, it is necessary to fabricate high-quality
AAM templates. Figure 3 shows a typical TEM for TiO
2
nanorods fabricated with this method. Normally, the TiO
2
nanorods are composed of small TiO
2
nanoparticles or
nanograins.
By electrophoretic deposition of TiO
2

colloidal suspensions
into the pores of an AAM, ordered TiO
2
nanowire arrays
can be obtained.
94
In a typical procedure, TTIP is dissolved
in ethanol at room temperature, and glacial acetic acid mixed
with deionized water and ethanol is added under pH ) 2-3
with nitric acid. Platinum is used as the anode, and an AAM
with an Au substrate attached to Cu foil is used as the
cathode. A TiO
2
sol is deposited into the pores of the AMM
under a voltage of 2-5 V and annealed at 500 °C for 24 h.
After dissolving the AAM template ina5wt%NaOH
solution, isolated TiO
2
nanowires are obtained. In order to
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2893
fabricate TiO
2
nanowires instead of nanorods, an AAM with
long pores is a must.
TiO
2
nanotubes can also be obtained using the sol-gel
method by templating with an AAM
95-98
and other organic

compounds.
99,100
For example, when an AAM is used as the
template, a thin layer of TiO
2
sol on the wall of the pores of
the AAM is first prepared by sucking TiO
2
sol into the pores
of the AAM and removing it under vacuum; TiO
2
nanowires
are obtained after the sol is fully developed and the AAM is
removed. In the procedure by Lee and co-workers,
96
a TTIP
solution was prepared by mixing TTIP with 2-propanol and
2,4-pentanedione. After the AAM was dipped into this
Figure 1. TEM images of TiO
2
nanoparticles prepared by hydrolysis of Ti(OR)
4
in the presence of tetramethylammonium hydroxide.
Reprinted with permission from Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. Copyright 1999 Wiley-VCH.
Figure 2. TEM images of uniform anatase TiO
2
nanoparticles. Reprinted from Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface
Sci. 2003, 259, 53, Copyright 2003, with permission from Elsevier.
2894 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
solution, it was removed from the solution and placed under

vacuum until the entire volume of the solution was pulled
through the AAM. The AAM was hydrolyzed by water vapor
over a HCl solution for 24 h, air-dried at room temperature,
and then calcined in a furnace at 673 K for 2 h and cooled
to room temperature with a temperature ramp of 2 °C/h. Pure
TiO
2
nanotubes were obtained after the AAM was dissolved
ina6MNaOH solution for several minutes.
96
Alternatively,
TiO
2
nanotubes could be obtained by coating the AAM
membranes at 60 °C for a certain period of time (12-48 h)
with dilute TiF
4
under pH ) 2.1 and removing the AAM
after TiO
2
nanotubes were fully developed.
97
Figure 4 shows
a typical SEM image of the TiO
2
nanotube array from the
AAM template.
97
In another scheme, a ZnO nanorod array on a glass
substrate can be used as a template to fabricate TiO

2
nanotubes with the sol-gel method.
101
Briefly, TiO
2
sol is
deposited on a ZnO nanorod template by dip-coating with a
slow withdrawing speed, then dried at 100 °C for 10 min,
and heated at 550 °Cfor1hinairtoobtain ZnO/TiO
2
nanorod arrays. The ZnO nanorod template is etched-up by
immersing the ZnO/TiO
2
nanorod arrays in a dilute hydro-
chloric acid aqueous solution to obtain TiO
2
nanotube arrays.
Figure 5 shows a typical SEM image of the TiO
2
nanotube
array with the ZnO nanorod array template. The TiO
2
nanotubes inherit the uniform hexagonal cross-sectional
shape and the length of 1.5 µm and inner diameter of 100-
120 nm of the ZnO nanorod template. As the concentration
of the TiO
2
sol is constant, well-aligned TiO
2
nanotube arrays

can only be obtained from an optimal dip-coating cycle
number in the range of 2-3 cycles. A dense porous TiO
2
thick film with holes is obtained instead if the dip-coating
number further increases. The heating rate is critical to the
formation of TiO
2
nanotube arrays. When the heating rate
is extra rapid, e.g., above 6 °C min
-1
, the TiO
2
coat will
easily crack and flake off from the ZnO nanorods due to
great tensile stress between the TiO
2
coat and the ZnO
template, and a TiO
2
film with loose, porous nanostructure
is obtained.
2.2. Micelle and Inverse Micelle Methods
Aggregates of surfactant molecules dispersed in a liquid
colloid are called micelles when the surfactant concentration
exceeds the critical micelle concentration (CMC). The CMC
is the concentration of surfactants in free solution in
equilibrium with surfactants in aggregated form. In micelles,
the hydrophobic hydrocarbon chains of the surfactants are
oriented toward the interior of the micelle, and the hydro-
philic groups of the surfactants are oriented toward the

surrounding aqueous medium. The concentration of the lipid
present in solution determines the self-organization of the
molecules of surfactants and lipids. The lipids form a single
layer on the liquid surface and are dispersed in solution below
the CMC. The lipids organize in spherical micelles at the
first CMC (CMC-I), into elongated pipes at the second CMC
(CMC-II), and into stacked lamellae of pipes at the lamellar
point (LM or CMC-III). The CMC depends on the chemical
composition, mainly on the ratio of the head area and the
tail length. Reverse micelles are formed in nonaqueous
media, and the hydrophilic headgroups are directed toward
the core of the micelles while the hydrophobic groups are
Figure 3. TEM image of anatase nanorods and a single nanorod
composed of small TiO
2
nanoparticles or nanograins (inset).
Reprinted from Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.;
Tanemura, M. J. Cryst. Growth 2004, 264, 246, Copyright 2004,
with permission from Elsevier.
Figure 4. SEM image of TiO
2
nanotubes prepared from the AAO
template. Reprinted with permission from Liu, S. M.; Gan, L. M.;
Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14,
1391. Copyright 2002 American Chemical Society.
Figure 5. SEM of a TiO
2
nanotube array; the inset shows the ZnO
nanorod array template. Reprinted with permission from Qiu, J. J.;
Yu, W. D.; Gao, X. D.; Li, X. M. Nanotechnology 2006, 17, 4695.

Copyright 2006 IOP Publishing Ltd.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2895
directed outward toward the nonaqueous media. There is no
obvious CMC for reverse micelles, because the number of
aggregates is usually small and they are not sensitive to the
surfactant concentration. Micelles are often globular and
roughly spherical in shape, but ellipsoids, cylinders, and
bilayers are also possible. The shape of a micelle is a function
of the molecular geometry of its surfactant molecules and
solution conditions such as surfactant concentration, tem-
perature, pH, and ionic strength.
Micelles and inverse micelles are commonly employed to
synthesize TiO
2
nanomaterials.
102-110
A statistical experi-
mental design method was conducted by Kim et al. to
optimize experimental conditions for the preparation of TiO
2
nanoparticles.
103
The values of H
2
O/surfactant, H
2
O/titanium
precursor, ammonia concentration, feed rate, and reaction
temperature were significant parameters in controlling TiO
2

nanoparticle size and size distribution. Amorphous TiO
2
nanoparticles with diameters of 10-20 nm were synthesized
and converted to the anatase phase at 600 °C and to the more
thermodynamically stable rutile phase at 900 °C. Li et al.
developed TiO
2
nanoparticles with the chemical reactions
between TiCl
4
solution and ammonia in a reversed micro-
emulsion system consisting of cyclohexane, poly(oxyethyl-
ene)
5
nonyle phenol ether, and poly(oxyethylene)
9
nonyle
phenol ether.
104
The produced amorphous TiO
2
nanoparticles
transformed into anatase when heated at temperatures from
200 to 750 °C and into rutile at temperatures higher than
750 °C. Agglomeration and growth also occurred at elevated
temperatures.
Shuttle-like crystalline TiO
2
nanoparticles were synthesized
by Zhang et al. with hydrolysis of titanium tetrabutoxide in

the presence of acids (hydrochloric acid, nitric acid, sulfuric
acid, and phosphoric acid) in NP-5 (Igepal CO-520)-
cyclohexane reverse micelles at room temperature.
110
The
crystal structure, morphology, and particle size of the TiO
2
nanoparticles were largely controlled by the reaction condi-
tions, and the key factors affecting the formation of rutile at
room temperature included the acidity, the type of acid used,
and the microenvironment of the reverse micelles. Ag-
glomeration of the particles occurred with prolonged reaction
times and increasing the [H
2
O]/[NP-5] and [H
2
O]/[Ti-
(OC
4
H
9
)
4
] ratios. When suitable acid was applied, round TiO
2
nanoparticles could also be obtained. Representative TEM
images of the shuttle-like and round-shaped TiO
2
nanopar-
ticles are shown in Figure 6. In the study carried out by Lim

et al., TiO
2
nanoparticles were prepared by the controlled
hydrolysis of TTIP in reverse micelles formed in CO
2
with
the surfactants ammonium carboxylate perfluoropolyether
(PFPECOO
-
NH
4
+
) (MW 587) and poly(dimethyl amino
ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl meth-
acrylate) (PDMAEMA-b-PFOMA).
106
It was found that the
crystallite size prepared in the presence of reverse micelles
increased as either the molar ratio of water to surfactant or
the precursor to surfactant ratio increased.
The TiO
2
nanomaterials prepared with the above micelle
and reverse micelle methods normally have amorphous
structure, and calcination is usually necessary in order to
induce high crystallinity. However, this process usually leads
to the growth and agglomeration of TiO
2
nanoparticles. The
crystallinity of TiO

2
nanoparticles initially (synthesized by
controlled hydrolysis of titanium alkoxide in reverse micelles
in a hydrocarbon solvent) could be improved by annealing
in the presence of the micelles at temperatures considerably
lower than those required for the traditional calcination
treatment in the solid state.
108
This procedure could produce
crystalline TiO
2
nanoparticles with unchanged physical
dimensions and minimal agglomeration and allows the
preparation of highly crystalline TiO
2
nanoparticles, as shown
in Figure 7, from the study of Lin et al.
108
2.3. Sol Method
The sol method here refers to the nonhydrolytic sol-gel
processes and usually involves the reaction of titanium
chloride with a variety of different oxygen donor molecules,
e.g., a metal alkoxide or an organic ether.
111-119
Figure 6. TEM images of the shuttle-like and round-shaped (inset)
TiO
2
nanoparticles. From: Zhang, D., Qi, L., Ma, J., Cheng, H. J.
Mater. Chem. 2002, 12, 3677 ( />s Reproduced by permission of The Royal Society of Chemistry.
Figure 7. HRTEM images of a TiO

2
nanoparticle after annealing.
Reprinted with permission from Lin, J.; Lin, Y.; Liu, P.; Meziani,
M. J.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2002, 124, 11514.
Copyright 2002 American Chemical Society.
TiX
4
+ Ti(OR)
4
f 2TiO
2
+ 4RX (1)
TiX
4
+ 2ROR f TiO
2
+ 4RX (2)
2896 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
The condensation between Ti-Cl and Ti-OR leads to the
formation of Ti-O-Ti bridges. The alkoxide groups can
be provided by titanium alkoxides or can be formed in situ
by reaction of the titanium chloride with alcohols or ethers.
In the method by Trentler and Colvin,
119
a metal alkoxide
was rapidly injected into the hot solution of titanium halide
mixed with trioctylphosphine oxide (TOPO) in heptadecane
at 300 °C under dry inert gas protection, and reactions were
completed within 5 min. For a series of alkyl substituents
including methyl, ethyl, isopropyl, and tert-butyl, the reaction

rate dramatically increased with greater branching of R, while
average particle sizes were relatively unaffected. Variation
of X yielded a clear trend in average particle size, but without
a discernible trend in reaction rate. Increased nucleophilicity
(or size) of the halide resulted in smaller anatase nanocrystals.
Average sizes ranged from 9.2 nm for TiF
4
to 3.8 nm for
TiI
4
. The amount of passivating agent (TOPO) influenced
the chemistry. Reaction in pure TOPO was slower and
resulted in smaller particles, while reactions without TOPO
were much quicker and yielded mixtures of brookite, rutile,
and anatase with average particle sizes greater than 10 nm.
Figure 8 shows typical TEM images of TiO
2
nanocrystals
developed by Trentler et al.
119
In the method used by Niederberger and Stucky,
111
TiCl
4
was slowly added to anhydrous benzyl alcohol under
vigorous stirring at room temperature and was kept at 40-
150 °C for 1-21 days in the reaction vessel. The precipitate
was calcinated at 450 °C for 5 h after thoroughly washing.
The reaction between TiCl
4

and benzyl alcohol was found
suitable for the synthesis of highly crystalline anatase phase
TiO
2
nanoparticles with nearly uniform size and shape at
very low temperatures, such as 40 °C. The particle size could
be selectively adjusted in the range of 4-8 nm with the
appropriate thermal conditions and a proper choice of the
relative amounts of benzyl alcohol and titanium tetrachloride.
The particle growth depended strongly on temperature, and
lowering the titanium tetrachloride concentration led to a
considerable decrease of particle size.
111
Surfactants have been widely used in the preparation of a
variety of nanoparticles with good size distribution and
dispersity.
15,16
Adding different surfactants as capping agents,
such as acetic acid and acetylacetone, into the reaction matrix
can help synthesize monodispersed TiO
2
nanoparticles.
120,121
For example, Scolan and Sanchez found that monodisperse
nonaggregated TiO
2
nanoparticles in the 1-5 nm range were
obtained through hydrolysis of titanium butoxide in the
presence of acetylacetone and p-toluenesulfonic acid at 60
°C.

120
The resulting nanoparticle xerosols could be dispersed
in water-alcohol or alcohol solutions at concentrations
higher than 1 M without aggregation, which is attributed to
the complexation of the surface by acetylacetonato ligands
and through an adsorbed hybrid organic-inorganic layer
made with acetylacetone, p-toluenesulfonic acid, and wa-
ter.
120
With the aid of surfactants, different sized and shaped TiO
2
nanorods can be synthesized.
122-130
For example, the growth
of high-aspect-ratio anatase TiO
2
nanorods has been reported
by Cozzoli and co-workers by controlling the hydrolysis
process of TTIP in oleic acid (OA).
122-126,130
Typically, TTIP
was added into dried OA at 80-100 °C under inert gas
protection (nitrogen flow) and stirred for 5 min. A 0.1-2M
aqueous base solution was then rapidly injected and kept at
80-100 °C for 6-12 h with stirring. The bases employed
included organic amines, such as trimethylamino-N-oxide,
trimethylamine, tetramethylammonium hydroxide, tetrabut-
ylammonium hydroxyde, triethylamine, and tributylamine.
In this reaction, by chemical modification of the titanium
precursor with the carboxylic acid, the hydrolysis rate of

titanium alkoxide was controlled. Fast (in 4-6 h) crystal-
lization in mild conditions was promoted with the use of
suitable catalysts (tertiary amines or quaternary ammonium
hydroxides). A kinetically overdriven growth mechanism led
to the growth of TiO
2
nanorods instead of nanoparticles.
123
Typical TEM images of the TiO
2
nanorods are shown in
Figure 9.
123
Recently, Joo et al.
127
and Zhang et al.
129
reported similar
procedures in obtaining TiO
2
nanorods without the use of
catalyst. Briefly, a mixture of TTIP and OA was used to
generate OA complexes of titanium at 80 °C in 1-octadecene.
Figure 8. TEM image of TiO
2
nanoparticles derived from reaction
of TiCl
4
and TTIP in TOPO/heptadecane at 300 °C. The inset shows
a HRTEM image of a single particle. Reprinted with permission

from Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.;
Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. Copyright 1999
American Chemical Society.
Figure 9. TEM of TiO
2
nanorods. The inset shows a HRTEM of
a TiO
2
nanorod. Reprinted with permission from Cozzoli, P. D.;
Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539.
Copyright 2003 American Chemical Society.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2897
The injection of a predetermined amount of oleylamine at
260 °C led to various sized TiO
2
nanorods.
129
Figure 10
shows TEM images of TiO
2
nanorods with various lengths,
and 2.3 nm TiO
2
nanoparticles prepared with this method.
129
In the surfactant-mediated shape evolution of TiO
2
nano-
crystals in nonaqueous media conducted by Jun et al.,
128

it
was found that the shape of TiO
2
nanocrystals could be
modified by changing the surfactant concentration. The
synthesis was accomplished by an alkyl halide elimination
reaction between titanium chloride and titanium isopro-
poxide. Briefly, a dioctyl ether solution containing TOPO
and lauric acid was heated to 300 °C followed by addition
of titanium chloride under vigorous stirring. The reaction
was initiated by the rapid injection of TTIP and quenched
with cold toluene. At low lauric acid concentrations, bullet-
and diamond-shaped nanocrystals were obtained; at higher
concentrations, rod-shaped nanocrystals or a mixture of
nanorods and branched nanorods was observed. The bullet-
and diamond-shaped nanocrystals and nanorods were elon-
gated along the [001] directions. The TiO
2
nanorods were
found to simultaneously convert to small nanoparticles as a
function of the growth time, as shown in Figure 11, due to
the minimization of the overall surface energy via dissolution
and regrowth of monomers during an Ostwald ripening.
2.4. Hydrothermal Method
Hydrothermal synthesis is normally conducted in steel
pressure vessels called autoclaves with or without Teflon
liners under controlled temperature and/or pressure with the
reaction in aqueous solutions. The temperature can be
elevated above the boiling point of water, reaching the
pressure of vapor saturation. The temperature and the amount

of solution added to the autoclave largely determine the
internal pressure produced. It is a method that is widely used
for the production of small particles in the ceramics industry.
Many groups have used the hydrothermal method to prepare
TiO
2
nanoparticles.
131-140
For example, TiO
2
nanoparticles
can be obtained by hydrothermal treatment of peptized
precipitates of a titanium precursor with water.
134
The
precipitates were prepared by adding a 0.5 M isopropanol
solution of titanium butoxide into deionized water ([H
2
O]/
[Ti] ) 150), and then they were peptized at 70 °Cfor1hin
the presence of tetraalkylammonium hydroxides (peptizer).
After filtration and treatment at 240 °Cfor2h,the
as-obtained powders were washed with deionized water and
absolute ethanol and then dried at 60 °C. Under the same
concentration of peptizer, the particle size decreased with
increasing alkyl chain length. The peptizers and their
concentrations influenced the morphology of the particles.
Typical TEM images of TiO
2
nanoparticles made with the

hydrothermal method are shown in Figure 12.
134
In another example, TiO
2
nanoparticles were prepared by
hydrothermal reaction of titanium alkoxide in an acidic
ethanol-water solution.
132
Briefly, TTIP was added dropwise
to a mixed ethanol and water solution at pH 0.7 with nitric
acid, and reacted at 240 °C for 4 h. The TiO
2
nanoparticles
Figure 10. TEM images of TiO
2
nanorods with lengths of (A) 12 nm, (B) 30 nm, and (C) 16 nm. (D) 2.3 nm TiO
2
nanoparticles. Inset
in parts C and D: HR-TEM image of a single TiO
2
nanorod and nanoparticle. Reprinted with permission from Zhang, Z.; Zhong, X.; Liu,
S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright 2005 Wiley-VCH.
2898 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
synthesized under this acidic ethanol-water environment
were mainly primary structure in the anatase phase without
secondary structure. The sizes of the particles were controlled
to the range of 7-25 nm by adjusting the concentration of
Ti precursor and the composition of the solvent system.
Besides TiO
2

nanoparticles, TiO
2
nanorods have also been
synthesized with the hydrothermal method.
141-146
Zhang et
al. obtained TiO
2
nanorods by treating a dilute TiCl
4
solution
at 333-423 K for 12 h in the presence of acid or inorganic
salts.
141,143-146
Figure 13 shows a typical TEM image of the
TiO
2
nanorods prepared with the hydrothermal method.
141
The morphology of the resulting nanorods can be tuned with
different surfactants
146
or by changing the solvent composi-
tions.
145
A film of assembled TiO
2
nanorods deposited on a
glass wafer was reported by Feng et al.
142

These TiO
2
nanorods were prepared at 160 °Cfor2hbyhydrothermal
treatment of a titanium trichloride aqueous solution super-
saturated with NaCl.
TiO
2
nanowires have also been successfully obtained with
the hydrothermal method by various groups.
147-151
Typically,
TiO
2
nanowires are obtained by treating TiO
2
white powders
ina10-15 M NaOH aqueous solution at 150-200 °C for
24-72 h without stirring within an autoclave. Figure 14
shows the SEM images of TiO
2
nanowires and a TEM image
of a single nanowire prepared by Zhang and co-workers.
150
TiO
2
nanowires can also be prepared from layered titanate
particles using the hydrothermal method as reported by Wei
Figure 11. Time dependent shape evolution of TiO
2
nanorods:

(a) 0.25 h; (b) 24 h; (c) 48 h. Scale bar ) 50 nm. Reprinted with
permission from Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S.
Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981.
Copyright 2003 American Chemical Society.
Figure 12. TEM images of TiO
2
nanoparticles prepared by the
hydrothermal method. Reprinted from Yang, J.; Mei, S.; Ferreira,
J. M. F. Mater. Sci. Eng. C 2001, 15, 183, Copyright 2001, with
permission from Elsevier.
Figure 13. TEM image of TiO
2
nanorods prepared with the
hydrothermal method. Reprinted with permission from Zhang, Q.;
Gao, L. Langmuir 2003, 19, 967. Copyright 2003 American
Chemical Society.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2899
et al.
152
In their experiment, layer-structured Na
2
Ti
3
O
7
was
dispersed into a 0.05-0.1 M HCl solution and kept at 140-
170 °C for 3-7 days in an autoclave. TiO
2
nanowires were

obtained after the product was washed with H
2
O and finally
dried. In the formation of a TiO
2
nanowire from layered
H
2
Ti
3
O
7
, there are three steps: (i) the exfoliation of layered
Na
2
Ti
3
O
7
; (ii) the nanosheets formation; and (iii) the nanow-
ires formation.
152
In Na
2
Ti
3
O
7
, [TiO
6

] octahedral layers are
held by the strong static interaction between the Na
+
cations
between the [TiO
6
] octahedral layers and the [TiO
6
] unit.
When the larger H
3
+
O cations replace the Na
+
cations in
the interlayer space of [TiO
6
] sheets, this static interaction
is weakened because the interlayer distance is enlarged. As
a result, the layered compounds Na
2
Ti
3
O
7
are gradually
exfoliated. When Na
+
is exchanged by H
+

in the dilute HCl
solution, numerous H
2
Ti
3
O
7
sheet-shaped products are
formed. Since the nanosheet does not have inversion sym-
metry, an intrinsic tension exists. The nanosheets split to form
nanowires in order to release the strong stress and lower the
total energy.
152
A representative TEM image of TiO
2
nanowires from Na
2
Ti
3
O
7
is shown in Figure 15.
152
The hydrothermal method has been widely used to prepare
TiO
2
nanotubes since it was introduced by Kasuga et al. in
1998.
153-175
Briefly, TiO

2
powders are put into a 2.5-20 M
NaOH aqueous solution and held at 20-110 °C for 20 h in
an autoclave. TiO
2
nanotubes are obtained after the products
are washed with a dilute HCl aqueous solution and distilled
water. They proposed the following formation process of
TiO
2
nanotubes.
154
When the raw TiO
2
material was treated
with NaOH aqueous solution, some of the Ti-O-Ti bonds
were broken and Ti-O-Na and Ti-OH bonds were formed.
New Ti-O-Ti bonds were formed after the Ti-O-Na and
Ti-OH bonds reacted with acid and water when the material
was treated with an aqueous HCl solution and distilled water.
The Ti-OH bond could form a sheet. Through the dehydra-
tion of Ti-OH bonds by HCl aqueous solution, Ti-O-Ti
bonds or Ti-O-H-O-Ti hydrogen bonds were generated.
The bond distance from one Ti to the next Ti on the surface
decreased. This resulted in the folding of the sheets and the
connection between the ends of the sheets, resulting in the
formation of a tube structure. In this mechanism, the TiO
2
nanotubes were formed in the stage of the acid treatment
following the alkali treatment. Figure 16 shows typical TEM

images of TiO
2
nanotubes made by Kasuga et al.
153
However,
Du and co-workers found that the nanotubes were formed
during the treatment of TiO
2
in NaOH aqueous solution.
161
A3Df 2D f 1D formation mechanism of the TiO
2
nanotubes was proposed by Wang and co-workers.
171
It stated
that the raw TiO
2
was first transformed into lamellar
structures and then bent and rolled to form the nanotubes.
For the formation of the TiO
2
nanotubes, the two-dimensional
lamellar TiO
2
was essential. Yao and co-workers further
suggested, based on their HRTEM study as shown in Figure
Figure 14. SEM images of TiO
2
nanowires with the inset showing
a TEM image of a single TiO

2
nanowire with a [010] selected area
electron diffraction (SAED) recorded perpendicular to the long axis
of the wire. Reprinted from Zhang, Y. X.; Li, G. H.; Jin, Y. X.;
Zhang, Y.; Zhang, J.; Zhang, L. D. Chem. Phys. Lett. 2002, 365,
300, Copyright 2002, with permission from Elsevier.
Figure 15. TEM images of TiO
2
nanowires made from the layered
Na
2
Ti
3
O
7
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 TiO
2
nanotubes. Reprinted with
permission from Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino,
T.; Niihara, K. Langmuir 1998, 14, 3160. Copyright 1998 American
Chemical Society.
2900 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
17, that TiO
2
nanotubes were formed by rolling up the single-
layer TiO

2
sheets with a rolling-up vector of [001] and
attracting other sheets to surround the tubes.
172
Bavykin and
co-workers suggested that the mechanism of nanotube
formation involved the wrapping of multilayered nanosheets
rather than scrolling or wrapping of single layer nanosheets
followed by crystallization of successive layers.
156
In the
mechanism proposed by Wang et al., the formation of TiO
2
nanotubes involved several steps.
176
During the reaction with
NaOH, the Ti-O-Ti bonding between the basic building
blocks of the anatase phase, the octahedra, was broken and
a zigzag structure was formed when the free octahedras
shared edges between the Ti ions with the formation of
hydroxy bridges, leading to the growth along the [100]
direction of the anatase phase. Two-dimensional crystalline
sheets formed from the lateral growth of the formation of
oxo bridges between the Ti centers (Ti-O-Ti bonds) in the
[001] direction and rolled up in order to saturate these
dangling bonds from the surface and lower the total energy,
resulting in the formation of TiO
2
nanotubes.
176

2.5. Solvothermal Method
The solvothermal method is almost identical to the
hydrothermal method except that the solvent used here is
nonaqueous. However, the temperature can be elevated much
higher than that in hydrothermal method, since a variety of
organic solvents with high boiling points can be chosen. The
solvothermal method normally has better control than hy-
drothermal methods of the size and shape distributions and
the crystallinity of the TiO
2
nanoparticles. The solvothermal
method has been found to be a versatile method for the
synthesis of a variety of nanoparticles with narrow size
distribution and dispersity.
177-179
The solvothermal method
has been employed to synthesize TiO
2
nanoparticles and
nanorods with/without the aid of surfactants.
177-185
For
example, in a typical procedure by Kim and co-workers,
184
TTIP was mixed with toluene at the weight ratio of 1-3:10
and kept at 250 °C for 3 h. The average particle size of TiO
2
powders tended to increase as the composition of TTIP in
the solution increased in the range of weight ratio of 1-3:
10, while the pale crystalline phase of TiO

2
was not produced
at 1:20 and 2:5 weight ratios.
184
By controlling the hydro-
lyzation reaction of Ti(OC
4
H
9
)
4
and linoleic acid, redispers-
ible TiO
2
nanoparticles and nanorods could be synthesized,
as found by Li et al. recently.
177
The decomposition of NH
4
-
HCO
3
could provide H
2
O for the hydrolyzation reaction, and
linoleic acid could act as the solvent/reagent and coordination
surfactant in the synthesis of nanoparticles. Triethylamine
could act as a catalyst for the polycondensation of the Ti-
O-Ti inorganic network to achieve a crystalline product and
had little influence on the products’ morphology. The chain

lengths of the carboxylic acids had a great influence on the
formation of TiO
2
, and long-chain organic acids were
important and necessary in the formation of TiO
2
.
177
Figure
18 shows a representative TEM image of TiO
2
nanoparticles
from their study.
177
TiO
2
nanorods with narrow size distributions can also be
developed with the solvothermal method.
177,183
For example,
in a typical synthesis from Kim et al., TTIP was dissolved
in anhydrous toluene with OA as a surfactant and kept at
250 °C for 20 h in an autoclave without stirring.
183
Long
dumbbell-shaped nanorods were formed when a sufficient
amount of TTIP or surfactant was added to the solution, due
to the oriented growth of particles along the [001] axis. At
a fixed precursor to surfactant weight ratio of 1:3, the
concentration of rods in the nanoparticle assembly increased

as the concentration of the titanium precursor in the solution
increased. The average particle size was smaller and the size
distribution was narrower than is the case for particles
synthesized without surfactant. The crystalline phase, diam-
eter, and length of these nanorods are largely influenced by
the precursor/surfactant/solvent weight ratio. Anatase nano-
Figure 17. (a) HRTEM images of TiO
2
nanotubes. (b) Cross-
sectional view of TiO
2
nanotubes. Reused with permission from
B. D. Yao, Y. F. Chan, X. Y. Zhang, W. F. Zhang, Z. Y. Yang, N.
Wang, Applied Physics Letters 82, 281 (2003). Copyright 2003,
American Institute of Physics.
Figure 18. TEM micrographs of TiO
2
nanoparticles prepared with
the 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
rods were obtained from the solution with a precursor/
surfactant weight ratio of more than 1:3 for a precursor/
solvent weight ratio of 1:10 or from the solution with a
precursor/solvent weight ratio of more than 1:5 for a
precursor/surfactant weight ratio of 1:3. The diameter and
length of these nanorods were in the ranges of 3-5nmand
18-25 nm, respectively. Figure 19 shows a typical TEM
image of TiO

2
nanorods prepared from the solutions with
the weight ratio of precursor/solvent/surfactant ) 1:5:3.
183
Similar to the hydrothermal method, the solvothermal
method has also been used for the preparation of TiO
2
nanowires.
180-182
Typically, a TiO
2
powder suspension in an
5 M NaOH water-ethanol solution is kept in an autoclave
at 170-200 °C for 24 h and then cooled to room temperature
naturally. TiO
2
nanowires are obtained after the obtained
sample is washed with a dilute HCl aqueous solution and
dried at 60 °C for 12 h in air.
181
The solvent plays an
important role in determining the crystal morphology.
Solvents with different physical and chemical properties can
influence the solubility, reactivity, and diffusion behavior
of the reactants; in particular, the polarity and coordinating
ability of the solvent can influence the morphology and the
crystallization behavior of the final products. The presence
of ethanol at a high concentration not only can cause the
polarity of the solvent to change but also strongly affects
the ζ potential values of the reactant particles and the

increases solution viscosity. For example, in the absence of
ethanol, short and wide flakelike structures of TiO
2
were
obtained instead of nanowires. When chloroform is used,
TiO
2
nanorods were obtained.
181
Figure 20 shows representa-
tive TEM images of the TiO
2
nanowires prepared from the
solvothermal method.
181
Alternatively, bamboo-shaped Ag-
doped TiO
2
nanowires were developed with titanium butox-
ide as precursor and AgNO
3
as catalyst.
180
Through the
electron diffraction (ED) pattern and HRTEM study, the Ag
phase only existed in heterojunctions between single-crystal
TiO
2
nanowires.
180

2.6. Direct Oxidation Method
TiO
2
nanomaterials can be obtained by oxidation of
titanium metal using oxidants or under anodization. Crystal-
line TiO
2
nanorods have been obtained by direct oxidation
of a titanium metal plate with hydrogen peroxide.
186-191
Typically, TiO
2
nanorods on a Ti plate are obtained when a
cleaned Ti plate is put in 50 mL of a 30 wt % H
2
O
2
solution
at 353 K for 72 h. The formation of crystalline TiO
2
occurs
through a dissolution precipitation mechanism. By the
addition of inorganic salts of NaX (X ) F
-
,Cl
-
, and SO
4
2-
),

the crystalline phase of TiO
2
nanorods can be controlled.
The addition of F
-
and SO
4
2-
helps the formation of pure
anatase, while the addition of Cl
-
favors the formation of
rutile.
189
Figure 21 shows a typical SEM image of TiO
2
nanorods prepared with this method.
186
At high temperature, acetone can be used as a good oxygen
source and for the preparation of TiO
2
nanorods by oxidizing
Figure 19. TEM micrographs and electron diffraction patterns of
products prepared from solutions at the weight ratio of precursor/
solvent/surfactant ) 1:5:3. Reprinted from Kim, C. S.; Moon, B.
K.; Park, J. H.; Choi, B. C.; Seo, H. J. J. Cryst. Growth 2003, 257,
309, Copyright 2003, with permission from Elsevier.
Figure 20. TEM images of TiO
2
nanowires synthesized by the

solvothermal method. From: Wen, B.; Liu, C.; Liu, Y. New J.
Chem. 2005, 29, 969 ( s
Reproduced by permission of The Royal Society of Chemistry
(RSC) on behalf of the Centre National de la Recherche Scientifique
(CNRS).
Figure 21. SEM morphology of TiO
2
nanorods by directly
oxidizing a Ti plate with a H
2
O
2
solution. Reprinted from Wu, J.
M. J. Cryst. Growth 2004, 269, 347, Copyright 2004, with
permission from Elesevier.
2902 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
a Ti plate with acetone as reported by Peng and Chen.
192
The oxygen source was found to play an important role.
Highly dense and well-aligned TiO
2
nanorod arrays were
formed when acetone was used as the oxygen source, and
only crystal grain films or grains with random nanofibers
growing from the edges were obtained with pure oxygen or
argon mixed with oxygen. The competition of the oxygen
and titanium diffusion involved in the titanium oxidation
process largely controlled the morphology of the TiO
2
. With

pure oxygen, the oxidation occurred at the Ti metal and the
TiO
2
interface, since oxygen diffusion predominated because
of the high oxygen concentration. When acetone was used
as the oxygen source, Ti cations diffused to the oxide surface
and reacted with the adsorbed acetone species. Figure 22
shows aligned TiO
2
nanorod arrays obtained by oxidizing a
titanium substrate with acetone at 850 °C for 90 min.
192
As extensively studied, TiO
2
nanotubes can be obtained
by anodic oxidation of titanium foil.
193-228
In a typical
experiment, a clean Ti plate is anodized in a 0.5% HF
solution under 10-20 V for 10-30 min. Platinum is used
as counterelectrode. Crystallized TiO
2
nanotubes are obtained
after the anodized Ti plate is annealed at 500 °Cfor6hin
oxygen.
210
The length and diameter of the TiO
2
nanotubes
could be controlled over a wide range (diameter, 15-120

nm; length, 20 nm to 10 µm) with the applied potential
between 1 and 25 V in optimized phosphate/HF electro-
lytes.
229
Figure 23 shows SEM images of TiO
2
nanotubes
created with this method.
208
2.7. Chemical Vapor Deposition
Vapor deposition refers to any process in which materials
in a vapor state are condensed to form a solid-phase material.
These processes are normally used to form coatings to alter
the mechanical, electrical, thermal, optical, corrosion resis-
tance, and wear resistance properties of various substrates.
They are also used to form free-standing bodies, films, and
fibers and to infiltrate fabric to form composite materials.
Recently, they have been widely explored to fabricate various
nanomaterials. Vapor deposition processes usually take place
within a vacuum chamber. If no chemical reaction occurs,
this process is called physical vapor deposition (PVD);
otherwise, it is called chemical vapor deposition (CVD). In
CVD processes, thermal energy heats the gases in the coating
chamber and drives the deposition reaction.
Thick crystalline TiO
2
films with grain sizes below 30 nm
as well as TiO
2
nanoparticles with sizes below 10 nm can

be prepared by pyrolysis of TTIP in a mixed helium/oxygen
atmosphere, using liquid precursor delivery.
230
When depos-
ited on the cold areas of the reactor at temperatures below
90 °C with plasma enhanced CVD, amorphous TiO
2
nano-
particles can be obtained and crystallize with a relatively
high surface area after being annealed at high temperatures.
231
TiO
2
nanorod arrays with a diameter of about 50-100 nm
and a length of 0.5-2 µm can be synthesized by metal
organic CVD (MOCVD) on a WC-Co substrate using TTIP
as the precursor.
232
Figure 24 shows the TiO
2
nanorods grown on fused silica
substrates with a template- and catalyst-free MOCVD
method.
233
In a typical procedure, titanium acetylacetonate
(Ti(C
10
H
14
O

5
)) vaporizing in the low-temperature zone of a
furnace at 200-230 °C is carried by a N
2
/O
2
flow into the
high-temperature zone of 500-700 °C, and TiO
2
nanostruc-
tures are grown directly on the substrates. The phase and
Figure 22. SEM images of large-scale nanorod arrays prepared
by oxidizing a titanium with acetone at 850 °C for 90 min. From:
Peng, X.; Chen, A. J. Mater. Chem. 2004, 14, 2542 (http://
dx.doi.org/10.1039/b404750h) s Reproduced by permission of The
Royal Society of Chemistry.
Figure 23. SEM images of TiO
2
nanotubes prepared with anodic
oxidation. Reprinted with permission from Varghese, O. K.; Gong,
D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV.
Mater. 2003, 15, 624. Copyright 2003 Wiley-VCH.
Figure 24. SEM images of TiO
2
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
morphology of the TiO
2

nanostructures can be tuned with
the reaction conditions. For example, at 630 and 560 °C
under a pressure of 5 Torr, single-crystalline rutile and
anatase TiO
2
nanorods were formed respectively, while, at
535 °C under 3.6 Torr, anatase TiO
2
nanowalls composed
of well-aligned nanorods were formed.
233
In addition to the above CVD approaches in preparing
TiO
2
nanomaterials, other CVD approaches are also used,
such as electrostatic spray hydrolysis,
234
diffusion flame
pyrolysis,
235-239
thermal plasma pyrolysis,
240-246
ultrasonic
spray 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 condensed
to form a solid material. The primary PVD methods include
thermal deposition, ion plating, ion implantation, sputtering,
laser vaporization, and laser surface alloying. TiO
2
nanowire
arrays have been fabricated by a simple PVD method or
thermal deposition.
252-254
Typically, pure Ti metal powder
is on a quartz boat in a tube furnace about 0.5 mm away
from the substrate. Then the furnace chamber is pumped
down to ∼300 Torr and the temperature is increased to 850
°C under an argon gas flow with a rate of 100 sccm and
held for 3 h. After the reaction, a layer of TiO
2
nanowires
can be obtained.
254
A layer of Ti nanopowders can be
deposited on the substrate before the growth of TiO
2
nanowires,
252,253
and Au can be employed as catalyst.
252
A
typical SEM image of TiO

2
nanowires made with the PVD
method is shown in Figure 25.
252
2.9. Electrodeposition
Electrodeposition is commonly employed to produce a
coating, usually metallic, on a surface by the action of
reduction at the cathode. The substrate to be coated is used
as cathode and immersed into a solution which contains a
salt of the metal to be deposited. The metallic ions are
attracted to the cathode and reduced to metallic form. With
the use of the template of an AAM, TiO
2
nanowires can be
obtained by electrodeposition.
255,256
In a typical process, the
electrodeposition is carried out in 0.2 M TiCl
3
solution with
pH ) 2 with a pulsed electrodeposition approach, and
titanium and/or its compound are deposited into the pores
of the AAM. By heating the above deposited template at
500 °C for 4 h and removing the template, pure anatase TiO
2
nanowires can be obtained. Figure 26 shows a representative
SEM image of TiO
2
nanowires.
256

2.10. Sonochemical Method
Ultrasound has been very useful in the synthesis of a wide
range of nanostructured materials, including high-surface-
area transition metals, alloys, carbides, oxides, and colloids.
The chemical effects of ultrasound do not come from a direct
interaction with molecular species. Instead, sonochemistry
arises from acoustic cavitation: the formation, growth, and
implosive collapse of bubbles in a liquid. Cavitational
collapse produces intense local heating (∼5000 K), high pres-
sures (∼1000 atm), and enormous heating and cooling rates
(>10
9
K/s). The sonochemical method has been applied to
prepare various TiO
2
nanomaterials by different groups.
257-269
Yu et al. applied the sonochemical method in preparing
highly photoactive TiO
2
nanoparticle photocatalysts with
anatase and brookite phases using the hydrolysis of titanium
tetraisoproproxide in pure water or in a 1:1 EtOH-H
2
O
solution under ultrasonic radiation.
109
Huang et al. found that
anatase and rutile TiO
2

nanoparticles as well as their mixtures
could be selectively synthesized with various precursors
using ultrasound irradiation, depending on the reaction
temperature and the precursor used.
259
Zhu et al. developed
titania whiskers and nanotubes with the assistance of
sonication as shown in Figure 27.
269
They found that arrays
of TiO
2
nanowhiskers with a diameter of 5 nm and nanotubes
with a diameter of ∼5 nm and a length of 200-300 nm could
be obtained by sonicating TiO
2
particles in NaOH aqueous
solution followed by washing with deionized water and a
dilute HNO
3
aqueous solution.
2.11. Microwave Method
A dielectric material can be processed with energy in the
form of high-frequency electromagnetic waves. The principal
Figure 25. SEM images of the TiO
2
nanowire arrays prepared by
the PVD method. Reprinted from Wu, J. M.; Shih, H. C.; Wu, W.
T. Chem. Phys. Lett. 2005, 413, 490, Copyright 2005, with
permission from Elsevier.

Figure 26. Cross-sectional SEM image of TiO
2
nanowires 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
frequencies of microwave heating are between 900 and 2450
MHz. At lower microwave frequencies, conductive currents
flowing within the material due to the movement of ionic con-
stituents can transfer energy from the microwave field to the
material. At higher frequencies, the energy absorption is pri-
marily due to molecules with a permanent dipole which tend
to reorientate under the influence of a microwave electric
field. This reorientation loss mechanism originates from the
inability of the polarization to follow extremely rapid rever-
sals of the electric field, so the polarization phasor lags the
applied electric field. This ensures that the resulting current
density has a component in phase with the field, and therefore
power is dissipated in the dielectric material. The major
advantages of using microwaves for industrial processing are
rapid heat transfer, and volumetric and selective heating.
Microwave radiation is applied to prepare various TiO
2
nanomaterials.
270-276
Corradi et al. found that colloidal titania
nanoparticle suspensions could be prepared within 5 min to
1 h with microwave radiation, while 1 to 32 h was needed
for the conventional synthesis method of forced hydrolysis

at 195 °C.
270
Ma et al. developed high-quality rutile TiO
2
nano-
rods with a microwave hydrothermal method and found that
they aggregated radially into spherical secondary nanopartic-
les.
272
Wu et al. synthesized TiO
2
nanotubes by microwave
radiation via the reaction of TiO
2
crystals of anatase, rutile,
or mixed phase and NaOH aqueous solution under a certain
microwave power.
275
Normally, the TiO
2
nanotubes had the
central hollow, open-ended, and multiwall structure with
diameters of 8-12 nm and lengths up to 200-1000 nm.
275
2.12. TiO
2
Mesoporous/Nanoporous Materials
In the past decade, mesoporous/nanoporous TiO
2
materials

have been well studied with or without the use of organic
surfactant templates.
28,80,264,265,277-312
Barbe et al. reported the
preparation of a mesoporous TiO
2
film by the hydrothermal
method as shown Figure 28.
80
In a typical experiment, TTIP
was added dropwise to a 0.1 M nitric acid solution under
vigorous stirring and at room temperature. A white precipitate
formed instantaneously. Immediately after the hydrolysis, the
solution was heated to 80 °C and stirred vigorously for 8 h
for peptization. The solution was then filtered on a glass frit
to remove agglomerates. Water was added to the filtrate to
adjust the final solids concentration to ∼5 wt %. The solution
was put in a titanium autoclave for 12 h at 200-250 °C.
After sonication, the colloidal suspension was put in a rotary
evaporator and evaporated to a final TiO
2
concentration of
11 wt %. The precipitation pH, hydrolysis rate, autoclaving
pH, and precursor chemistry were found to influence the
morphology of the final TiO
2
nanoparticles.
Alternative procedures without the use of hydrothermal
processes have been reported by Liu et al.
292

and Zhang et
al.
311
In the report by Liu et al., 24.0 g of titanium(IV)
n-butoxide ethanol solution (weight ratio of 1:7) was
prehydrolyzed in the presence of 0.32 mL of a 0.28 M HNO
3
aqueous solution (TBT/HNO
3
∼ 100:1) at room temperature
for 3 h. 0.32 mL of deionized water was added to the
prehydrolyzed solution under vigorous stirring and stirred
for an additional 2 h. The sol solution in a closed vessel
was kept at room temperature without stirring to gel and
age. After aging for 14 days, the gel was dried at room
temperature, ground into a fine powder, washed thoroughly
with water and ethanol, and dried to produce porous TiO
2
.
Upon calcination at 450 °C for 4 h under air, crystallized
mesoporous TiO
2
material was obtained.
292
Yu et al. prepared three-dimensional and thermally stable
mesoporous TiO
2
without the use of any surfactants.
265
Briefly, monodispersed TiO

2
nanoparticles were formed
initially by ultrasound-assisted hydrolysis of acetic acid-
modified titanium isopropoxide. Mesoporous spherical or
globular particles were then produced by controlled conden-
Figure 27. TEM images of TiO
2
nanotubes (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 ( s Reproduced by
permission of The Royal Society of Chemistry.
Figure 28. SEM image of the mesoporous TiO
2
film synthesized
from the acetic acid-modified precursor and autoclaved at 230 °C.
Reprinted with permission from Barbe, C. J.; Arendse, F.; Comte,
P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am.
Ceram. Soc. 1997, 80, 3157. Copyright 1997 Blackwell Publishing.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2905
sation and agglomeration of these sol nanoparticles under
high-intensity ultrasound radiation. The mesoporous TiO
2
had
a wormhole-like structure consisting of TiO
2
nanoparticles
and a lack of long-range order.
265
In the template method used by the Stucky

group
278-280,287,295,302,306-307,313
and other groups,
264,293,297,303,309
structure-directing agents were used for organizing network-
forming metal oxide species in nonaqueous solutions. These
structure-directing agents were also called organic templates.
The most commonly used organic templates were amphi-
philic poly(alkylene oxide) block copolymers, such as HO-
(CH
2
CH
2
O)
20
(CH
2
CH(CH
3
)O)
70
(CH
2
CH
2
O)
20
H (designated
EO
20

PO
70
EO
20
, called Pluronic P-123) and HO(CH
2
CH
2
O)
106
-
(CH
2
CH(CH
3
)O)
70
(CH
2
CH
2
O)
106
H (designated EO
106
PO
70
-
EO
106

, called Pluronic F-127). In a typical synthesis, poly-
(alkylene oxide) block copolymer was dissolved in ethanol.
Then TiCl
4
precursor was added with vigorous stirring. The
resulting sol solution was gelled in an open Petri dish at 40
°C in air for 1-7 days. Mesoporous TiO
2
was obtained after
removing the surfactant species by calcining the as-made
sample at 400 °Cfor5hinair.
306
Figure 29 shows typical
TEM images of the mesoporous TiO
2
. Besides triblock co-
polymers as structure-directing agents, diblock polymers were
also used such as [C
n
H
2n-1
(OCH
2
CH
2
)
y
OH, Brij 56 (B56, n/y
) 16/10) or Brij 58 (B58, n/y ) 16/20)] by Sanchez et al.
285

Other surfactants employed to direct the formation of
mesoporous TiO
2
include tetradecyl phosphate (a 14-carbon
chain) by Antonelli and Ying
277
and commercially available
dodecyl 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 nanotubes
310
and
mesoporous SBA-15
286
have also been used as the skeleton
for mesoporous TiO
2
.
2.13. TiO
2
Aerogels

The study of TiO
2
aerogels is worthy of special men-
tion.
314-326
The combination of sol-gel processing with
supercritical drying offers the synthesis of TiO
2
aerogels with
morphological and chemical properties that are not easily
achieved by other preparation methods, i.e., with high surface
area. Campbell et al. prepared TiO
2
aerogels by sol-gel
synthesis from titanium n-butoxide in methanol with the
subsequent removal of solvent by supercritical CO
2
.
315
For
a typical synthesis process, titanium n-butoxide was added
to 40 mL of methanol in a dry glovebox. This solution was
combined with another solution containing 10 mL of
methanol, nitric acid, and deionized water. The concentration
of the titanium n-butoxide was kept at 0.625 M, and the
molar ratio of water/HNO
3
/titanium n-butoxide was 4:0.1:
1. The gel was allowed to age for 2 h and then extracted in
a standard autoclave with supercritical CO

2
at a flow rate of
24.6 L/h, at 343 K under 2.07 × 10
7
Pa for 2-3 h, resulting
in complete removal of solvent. After extraction, the sample
was heated in a vacuum oven at 3.4 kPa and 383 K for 3 h
to remove the residual solvent and at 3.4 kPa and 483 K for
3 h to remove any residual organics. The pretreated sample
had a brown color and turned white after calcination at 773
K or above. The resulting TiO
2
aerogel, after calcination at
773 K for 2 h, had a BET surface area of >200 m
2
/g,
contained mesopores in the range 2-10 nm, and was of the
pure anatase form. Dagan et al. found the TiO
2
aerogels
obtanied by using a Ti/ethanol/H
2
O/nitric acid ratio of 1:20:
3:0.08 could have a porosity of 90% and surface areas of
600 m
2
/g, as compared to a surface area of 50 m
2
/g for TiO
2

P25.
316,317
Figure 30 shows a typical SEM image of a TiO
2
aerogel with a surface area of 447 m
2
/g and an interpore
structure constructed by near uniform grains of elliptical
shapes with 30 nm × 50 nm axes.
326
Figure 29. TEM micrographs of two-dimensional hexagonal
mesoporous TiO
2
recorded along the (a) [110] and (b) [001] zone
axes, respectively. The inset in part a is selected-area electron
diffraction patterns obtained on the image area. (c) TEM image of
cubic mesoporous TiO
2
accompanied 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
2.14. TiO
2
Opal and Photonic Materials
The syntheses of TiO
2
opal and photonic materials have
been well studied by various groups.

327-358
Holland et al.
reported the preparation of TiO
2
inverse opal from the
corresponding metal alkoxides, using latex spheres as
templates.
334,335
Millimeter-thick layers of latex spheres were
deposited on filter paper in a Buchner funnel under vacuum
and soaked with ethanol. Titanium ethoxide was added
dropwise to cover the latex spheres completely while suction
was applied. Typical mass ratios of alkoxide to latex were
between 1.4 and 3. After drying the composite in a vacuum
desiccator for 3 to 24 h, the latex spheres were removed by
calcination in flowing air at 575 °C for 7 to 12 h, leaving
hard and brittle powder particles with 320- to 360-nm voids.
The carbon content of the calcined samples varied from 0.4
to 1.0 wt %, indicating that most of the latex templates had
been removed from the 3D host. Figure 31 shows an
illustration of the simple synthesis of TiO
2
inverse opal and
an SEM image of TiO
2
inverse opals. Similar studies have
also been carried out by other researchers.
327,356
Dong and Marlow prepared TiO
2

inversed opals with a
skeleton-like structure of TiO
2
rods by a template-directed
method using monodispersed polystyrene particles of size
270 nm.
328-330,345
Infiltration of a titania precursor (Ti(i-OPr)
4
in EtOH) was followed by a drying and calcination proce-
dure. The precursor concentration was varied from 30% to
100%, and the calcination temperature was tuned from 300
to 700 °C. A SEM picture of the TiO
2
inversed opal is shown
in Figure 32.
329
The skeleton structure consists of rhombo-
hedral windows and TiO
2
cylinders forming a highly regular
network. The cylinders connect the centers of the former
octahedral and tetrahedral voids of the opal. These voids form
a CaF
2
lattice which is filled with cylindrical bonds con-
necting the Ca and F sites.
Wang et al. reported their study on the large-scale
fabrication of ordered TiO
2

nanobowl arrays.
354
The process
starts with a self-assembled monolayer of polystyrene (PS)
spheres, which is used as a template for atomic layer
deposition of a TiO
2
layer. After ion-milling, toluene-etching,
and annealing of the TiO
2
-coated spheres, ordered arrays of
nanostructured TiO
2
nanobowls can be fabricated as shown
in Figure 33.
Wang et al. fabricated a 2D photonic crystal by coating
patterned and aligned ZnO nanorod arrays with TiO
2
.
355
PS
spheres were self-assembled to make a monolayer mask on
a sapphire substrate, which was then covered with a layer
of gold. After removing the PS spheres with toluene, ZnO
nanorods were grown using a vapor-liquid-solid process.
Finally, a TiO
2
layer was deposited on the ZnO nanorods
by introducing TiCl
4

and water vapors into the atomic layer
deposition chamber at 100 °C. Figure 34 shows SEM images
of a ZnO nanorod array and the TiO
2
-coated ZnO nanorod
array.
Li et al. reported the preparation of ordered arrays of TiO
2
opals using opal gel templates under uniaxial compression
at ambient temperature during the TiO
2
sol/gel process.
337
The aspect ratio was controllable by the compression degree,
R. Polystyrene inverse opal was template synthesized using
silica opals as template. The silica was removed with 40 wt
% aqueous hydrofluoric acid. Monomer solutions consisting
of dimethylacrylamide, acrylic acid, and methylenebisacryl-
amide in 1:1:0.02 weight ratios were dissolved in a water/
Figure 30. SEM image of a TiO
2
aerogel. Reprinted with
permission from Zhu, Z.; Tsung, L. Y.; Tomkiewicz, M. J. Phys.
Chem. 1995, 99, 15945. Copyright 1995 American Chemical
Society.
Figure 31. (A) Schematic illustration of the synthesis of a TiO
2
inversed opal. (B) SEM image of the TiO
2
inversed opal. Reprinted

with permission from Holland, B. T.; Blanford, C.; Stein, A. Science
1998, 281, 538 (). Copyright 1998
AAAS.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2907
ethanol mixture (4:7 wt/wt) with total monomer content 30
wt %. Ethanol was used to facilitate diffusion of the
monomer solution into the inverse opal polystyrene. After
the inverse opal was infiltrated by the monomer solution
containing 1 wt % of the initiator AIBN and a subsequent
free radical polymerization at 60 °C for 3 h, a solid composite
resulted. The initial inverse opal polystyrene template was
then removed with chloroform in a Soxhlet extractor for 12
h, whereupon the opal gel was formed. By using different
compositions of the monomer solution, hole sizes, and
stacking structures of the starting inverse opal templates, opal
gels with correspondingly different properties can be pro-
duced. Water was completely removed from the opal
hydrogel by repeatedly rinsing it with a large amount of
ethanol. Afterward, the opal gel was put into a large amount
of tetrabutyl titanate (TBT) at ambient temperature for 24
h. The TBT-swollen opal gel was then immersed in a water/
ethanol (1:1 wt/wt) mixture for5htolettheTiO
2
sol/gel
process proceed. Figure 35A shows the opal structure of the
gel/titania composite spheres formed. After calcination, TiO
2
opal with distinctive spherical contours could be found. The
compression degree, R, was adjusted by the spacer height
when the substrates were compressed. When the substrates

were slightly compressed against each other to the extent of
producing a 20% reduction in the thickness of the composi-
tion opal, the deformation of the template-synthesized titania
spheres was not substantial (Figure 35B). When the com-
pression degree was increased to the point of reaching 35%
deformation in the opal gel, noticeably deformed titania opals
could be obtained (Figure 35C and D).
2.15. Preparation of TiO
2
Nanosheets
The preparation of TiO
2
nanosheets has also been explored
recently.
359-368
Typically, TiO
2
nanosheets were synthesized
by delaminating layered protonic titanate into colloidal single
layers. A stoichiometric mixture of Cs
2
CO
3
and TiO
2
was
calcined at 800 °C for 20 h to produce a precursor, cesium
titanate, Cs
0.7
Ti

1.825
0
0.175
O
4
(0: vacancy), about 70 g of
which was treated with2Lofa1MHClsolution at room
temperature. This acid leaching was repeated three times by
renewing the acid solution every 24 h. The resulting acid-
exchanged product was filtered, washed with water, and air-
dried. The obtained protonic titanate, H
0.7
Ti
1.825
0
0.175
O
4
‚H
2
O,
was shaken vigorously with a 0.017 M tetrabutylammonium
hydroxide solution at ambient temperature for 10 days. The
solution-to-solid ratio was adjusted to 250 cm
3
g
-1
. This
procedure yielded a stable colloidal suspension with an
Figure 32. SEM picture of a TiO

2
skeleton with a cylinder radius
of about 0.06a. a is the lattice constant of the cubic unit cell.
Reprinted from Dong, W.; Marlow, F. Physica E 2003, 17, 431,
Copyright 2003, with permission from Elsevier.
Figure 33. (A) Experimental procedure for fabricating TiO
2
nanobowl arrays. (B) Low- and high- (inset) magnification SEM
image of TiO
2
nanobowl 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 Chemical
Society.
Figure 34. (A) SEM images of short and densely aligned ZnO
nanorod array on a sapphire substrate. Inset: An optical image of
the aligned ZnO nanorods over a large area. (B) SEM image of
the TiO
2
-coated ZnO nanorod array. Reprinted with permission from
Wang, X.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger,
L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J. AdV. Mater.
2005, 17, 2103. Copyright 2005 Wiley-VCH.
2908 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
opalescent appearance. Figure 36 shows TEM and AFM
images of TiO
2
nanosheets with thicknesses of 1.2-1.3 nm,
which is the height of the TiO
2

nanosheet with a monolayer
of water molecules on both sides (0.70 + 0.25 × 2) thick.
366
3. Properties of TiO
2
Nanomaterials
3.1. Structural Properties of TiO
2
Nanomaterials
Figure 37 shows the unit cell structures of the rutile and
anatase TiO
2
.
11
These two structures can be described in
terms of chains of TiO
6
octahedra, where each Ti
4+
ion is
surrounded by an octahedron of six O
2-
ions. The two crystal
structures differ in the distortion of each octahedron and by
the assembly pattern of the octahedra chains. In rutile, the
octahedron shows a slight orthorhombic distortion; in anatase,
the octahedron is significantly distorted so that its symmetry
is lower than orthorhombic. The Ti-Ti distances in anatase
are larger, whereas the Ti-O distances are shorter than those
in rutile. In the rutile structure, each octahedron is in contact

with 10 neighbor octahedrons (two sharing edge oxygen pairs
and eight sharing corner oxygen atoms), while, in the anatase
structure, each octahedron is in contact with eight neighbors
(four sharing an edge and four sharing a corner). These
differences in lattice structures cause different mass densities
and electronic band structures between the two forms of
TiO
2
.
Hamad et al. performed a theoretical calculation on Ti
n
O
2n
clusters (n ) 1-15) with a combination of simulated
Figure 35. SEM of the TiO
2
opals. (A) A gel/titania composite opal fabricated without compressing the opal gel template during the
sol/gel process. (Inset) Image of the sample after calcination at 450 °Cfor3h.(B-D) (Main panel) Oblate titania opal materials after
calcination at 450 °C for 3 h, subject to compression degree R of (B) 20%, (C) 35%, and (D) 50%. The images were taken for the fractured
surfaces containing the direction of applied compression. (Inset) Image of the same sample, but with the fracture surface perpendicular to
the direction of applied compression. From: Ji, L.; Rong, J.; Yang, Z. Chem. Commun. 2003, 1080 ( />s Reproduced by permission of The Royal Society of Chemistry.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2909
annealing, Monte Carlo basin hopping simulation, and
genetic algorithms methods.
369
They found that the calculated
global minima consisted of compact structures, with titanium
atoms reaching high coordination rapidly as n increased. For
n g 11, the particles had at least a central octahedron
surrounded by a shell of surface tetrahedra, trigonal bipyra-

mids, and square base pyramids.
Swamy et al. found the metastability of anatase as a
function of pressure was size dependent, with smaller
crystallites preserving the structure to higher pressures.
370
Three size regimes were recognized for the pressure-induced
phase transition of anatase at room temperature: an anatase-
amorphous transition regime at the smallest crystallite sizes,
an anatase-baddeleyite transition regime at intermediate
crystallite sizes, and an anatase-R-PbO
2
transition regime
comprising large nanocrystals to macroscopic single crystals.
Barnard et al. performed a series of theoretical studies on
the phase stability of TiO
2
nanoparticles in different environ-
ments by a thermodynamic model.
371-375
They found that
surface passivation had an important impact on nanocrystal
morphology and phase stability. The results showed that
surface hydrogenation induced significant changes in the
shape of rutile nanocrystals, but not in anatase, and that the
size at which the phase transition might be expected increased
dramatically when the undercoordinated surface titanium
atoms were H-terminated. For spherical particles, the cross-
over point was about 2.6 nm. For a clean and faceted surface,
at low temperatures (a phase transition pointed at an average
diameter of approximately 9.3-9.4 nm for anatase nano-

crystals), the transition size decreased slightly to 8.9 nm when
the surface bridging oxygens were H-terminated, and the size
increased significantly to 23.1 nm when both the bridging
oxygens and the undercoordinated titanium atoms of the
surface trilayer were H-terminated. Below the cross point,
the anatase phase was more stable than the rutile phase.
371
In their study on TiO
2
nanoparticles in vacuum or water
environments, they found that the phase transition size in
water (15.1 nm) was larger than that under vacuum (9.6
nm).
373
In their predictions on the transition enthalpy of
nanocrystalline anatase and rutile, they found that thermo-
chemical results could differ for various faceted or spherical
Figure 36. (A) TEM of Ti
1-δ
O
2
4δ-
nanosheets. (B and C) AFM image and height scan of the TiO
2
nanosheets deposited on a Si wafer.
(D) Structural model for a hydrated TiO
2
nanosheet. Closed, open, and shaded circles represent Ti atom, O atom, and H
2
O 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 TiO
2
. Reprinted
with 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
nanoparticles as a function of shape, size, and degree of
surface passivation.
372
Their study on anatase and rutile
titanium dioxide polymorphs passivated with complete
monolayers of adsorbates by varying the hydrogen to oxygen
ratio with respect to a neutral, water-terminated surface
showed that termination with water consistently resulted in
the lowest values of surface free energy when hydrated or
with a higher fraction of H on the surface on both anatase
and rutile surfaces, but conversely, the surfaces generally
had a higher surface free energy when they had an equal
ratio of H and O in the adsorbates or were O-terminated.
375
They demonstrated that, under different pH conditions from
acid to basic, the phase transition size of a TiO
2
nanoparticle
varied from 6.9 to 22.7 nm, accompanied with shape changes
of the TiO
2

nanoparticles as shown in Figure 38.
374
Enyashin and Seifert conducted a theoretical study on the
structural stability of TiO
2
layer modifications (anatase and
lepidocrocite) using the density-functional-based tight bind-
ing method (DFTB).
376
They found that anatase nanotubes
were the most stable modifications in a comparison of single-
walled nanotubes, nanostrips, and nanorolls. Their stability
increased as their radii grew. The energies for all TiO
2
nanostructures relative to the infinite monolayer followed a
1/R
2
curve.
Chen et al. found that severe distortions existed in Ti site
environments in the structures of 1.9 nm TiO
2
nanoparticles
compared to those octahedral Ti sites in bulk anatase Ti using
K-edge XANES.
377
The distorted Ti sites were likely to adopt
a pentacoordinate square pyramidal geometry due to the
truncation of the lattice. The distortions in the TiO
2
lattice

were mainly located on the surface of the nanoparticles and
were responsible for binding with other small molecules.
Qian et al. found that the density of the surface states on
TiO
2
nanoparticles was likely dependent upon the details of
the preparation methods.
378
The TiO
2
nanoparticles prepared
from basic sol were found to have more surface states than
those prepared from acidic sol based on a surface photo-
voltage spectroscopy study.
3.2. Thermodynamic Properties of TiO
2
Nanomaterials
Rutile is the stable phase at high temperatures, but anatase
and brookite are common in fine grained (nanoscale) natural
and synthetic samples. On heating concomitant with coarsen-
ing, the following transformations are all seen: anatase to
brookite to rutile, brookite to anatase to rutile, anatase to
rutile, and brookite to rutile. These transformation sequences
imply very closely balanced energetics as a function of
particle size. The surface enthalpies of the three polymorphs
are sufficiently different that crossover in thermodynamic
stability can occur under conditions that preclude coarsening,
with anatase and/or brookite stable at small particle size.
73,74
However, abnormal behaviors and inconsistent results are

occasionally observed.
Hwu et al. found the crystal structure of TiO
2
nanoparticles
depended largely on the preparation method.
379
For small
TiO
2
nanoparticles (<50 nm), anatase seemed more stable
and transformed to rutile at >973 K. Banfield et al. found
that the prepared TiO
2
nanoparticles had anatase and/or
brookite structures, which transformed to rutile after reaching
a certain particle size.
73,380
Once rutile was formed, it grew
much faster than anatase. They found that rutile became more
stable 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, rapid
brookite to anatase and anatase to rutile transformations
between 1053 K and 1123 K, and rapid grain growth of rutile
above 1123 K as the dominant phase.
381
They concluded that
brookite could not transform directly to rutile but had to
transform to anatase first. However, direct transformation
of brookite nanocrystals to rutile was observed above 973

K by Kominami et al.
382
In a later study, Zhang and Banfield found that the
transformation sequence and thermodynamic phase stability
depended on the initial particle sizes of anatase and brookite
in their study on the phase transformation behavior of
nanocrystalline aggregates during their growth for isothermal
and isochronal reactions.
74
They concluded that, for equally
sized nanoparticles, anatase was thermodynamically stable
for sizes < 11 nm, brookite was stable for sizes between 11
and 35 nm, and rutile was stable for sizes > 35 nm.
Ranade et al. investigated the energetics of the TiO
2
polymorphs (rutile, anatase, and brookite) by high-temper-
ature oxide melt drop solution calorimetry, and they found
the energetic stability crossed over between the three phases
as shown in Figure 39.
383
The dark solid line represents the
phases of lowest enthalpy as a function of surface area. Rutile
was energetically stable for surface area < 592 m
2
/mol (7
m
2
/g or >200 nm), brookite was energetically stable from
Figure 38. Morphology predicted for anatase (top), with (a)
hydrogenated surfaces, (b) hydrogen-rich surface adsorbates, (c)

hydrated surfaces, (d) hydrogen-poor adsorbates, and (e) oxygenated
surfaces, and for rutile (bottom), with (f) hydrogenated surfaces,
(g) hydrogen-rich surface adsorbates, (h) hydrated surfaces, (i)
hydrogen-poor adsorbates, and (j) oxygenated surfaces. Reprinted
with permission from Barnard, A. S.; Curtiss, L. A. Nano Lett.
2005, 5, 1261. Copyright 2005 American Chemical Society.
Figure 39. Enthalpy of nanocrystalline TiO
2
. Reprinted with
permission from Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Ban-
field, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.;
Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002,
99, 6476. Copyright 2002 National Academy of Sciences, U.S.A.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2911
592 to 3174 m
2
/mol (7-40 m
2
/g or 200-40 nm), and anatase
was energetically stable for greater surface areas or smaller
sizes (<40 nm). The anatase and rutile energetics cross at
1452 m
2
/mol (18 m
2
/g or 66 nm). Assuming spherical
particles, the calculated average diameters of rutile and
brookite fora7m
2
/g surface area were 201 and 206 nm,

and those of brookite and anatase for a 40 m
2
/g surface area
are 36 and 39 nm. These differences in particle size at the
same surface area existed because of the differences in
density. If the phase transformation took place without further
coarsening, the particle size should be smaller after the
transformation. Phase stability in a thermodynamic sense is
governed by the Gibbs free energy (∆G ) ∆H - T∆S) rather
than the enthalpy. Rutile and anatase have the same entropy.
Thus, the T∆S will not significantly perturb the sequence of
stability seen from the enthalpies. For nanocrystalline TiO
2
,
if the initially formed brookite had surface area > 40 m
2
/g,
it was metastable with respect to both anatase and rutile,
and the sequence brookite to anatase to rutile during
coarsening was energetically downhill. If anatase formed
initially, it could coarsen and transform first to brookite (at
40 m
2
/g) and then to rutile. The energetic driving force for
the latter reaction (brookite to rutile) was very small,
explaining the natural persistence of coarse brookite. In
contrast, the absence of coarse-grained anatase was consistent
with the much larger driving force for its transformation to
rutile.
383

Li et al. found that only anatase to rutile phase transforma-
tion occurred in the temperature range of 973-1073 K.
384
Both anatase and rutile particle sizes increased with the
increase of temperature, but the growth rate was different,
as shown in Figure 40. Rutile had a much higher growth
rate than anatase. The growth rate of anatase leveled off at
800 °C. Rutile particles, after nucleation, grew rapidly,
whereas anatase particle size remained practically unchanged.
With the decrease of initial particle size, the onset transition
temperature was decreased. An increased lattice compression
of anatase with increasing temperature was observed. Larger
distortions existed in samples with smaller particle size. The
values for the activation energies obtained were 299, 236,
and 180 kJ/mol for 23, 17, and 12 nm TiO
2
nanoparticles,
respectively. The decreased thermal stability in finer nano-
particles was primarily due to the reduced activation energy
as the size-related surface enthalpy and stress energy
increased.
3.3. X-ray Diffraction Properties of TiO
2
Nanomaterials
XRD is essential in the determination of the crystal
structure and the crystallinity, and in the estimate of the
crystal grain size according to the Scherrer equation
where K is a dimensionless constant, 2θ is the diffraction
angle, λ is the wavelength of the X-ray radiation, and β is
the full width at half-maximum (fwhm) of the diffraction

peak.
385
Crystallite size is determined by measuring the
broadening of a particular peak in a diffraction pattern
associated with a particular planar reflection from within the
crystal unit cell. It is inversely related to the fwhm of an
individual peaksthe narrower the peak, the larger the
crystallite size. The periodicity of the individual crystallite
domains reinforces the diffraction of the X-ray beam,
resulting in a tall narrow peak. If the crystals are randomly
arranged or have low degrees of periodicity, the result is a
broader peak. This is normally the case for nanomaterial
assemblies. Thus, it is apparent that the fwhm of the
diffraction peak is related to the size of the nanomaterials.
Figure 41 shows the XRD patterns for TiO
2
nanoparticles
of different sizes
111
and for TiO
2
nanorods of different
lengths.
129
As the nanoparticle size increased, the diffraction
peaks became narrower. In the anatase nanoparticle and
nanorods developed by Zhang et al., the diameters of the
TiO
2
nanoparticles and nanorods were both around 2.3 nm.

The nanorods were elongated along the [001] direction with
preferred anisotropic growth along the c-axis of the anatase
lattice, which was indicated by the strong peak intensity and
narrow width of the (004) reflection and relatively lower
intensity and broader width for the other reflections. With
an increase in length of the nanorods, the (004) diffraction
peak became much stronger and sharper, whereas other peaks
remained similar in shape and intensity.
129
Similar results
have been observed by other groups.
123,127,177,183
3.4. Raman Vibration Properties of TiO
2
Nanomaterials
As the size of TiO
2
nanomaterials decreases, the featured
Raman scattering peaks become broader.
255,318,370,386-395
The
size effect on the Raman scattering in nanocrystalline
TiO
2
is interpreted as originating from phonon confine-
D )
Kλ
β cos θ
(3)
Figure 40. (A) Changes in particle sizes of anatase and rutile

phases as a function of the annealing temperatures. (B) Arrenhius
plot of ln(A
R
/A
0
)vs1/T for activation energy calculations as a
function of the size of the TiO
2
nanoparticles. A
R
and A
0
are the
integrated diffraction peak intensity from rutile (110), and the total
integrated anatase (101) and rutile (110) peak intensity, respectively.
Reused with permission from W. Li, C. Ni, H. Lin, C. P. Huang,
and S. Ismat Shah, Journal of Applied Physics, 96, 6663 (2004).
Copyright 2004, American Institute of Physics.
2912 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao
ment,
255,318,370,386,387,395
nonstoichiometry,
391,392
or internal
stress/surface tension effects.
390
Among these theories, the
most convincing is the three-dimensional confinement of
phonons in nanocrystals.
255,318,370,386,387,394,395

The phonon
confinement model is also referred to as the spatial correla-
tion model or q vector relaxation model. It links the q vector
selection rule for the excitation of Raman active optical
phonons with long-range order and crystallite size.
318,370
In
a perfect “infinite” crystal, conservation of phonon momen-
tum requires that only optic phonons near the Brillouin zone
(BZ) center (q ≈ 0) are involved in first-order Raman
scattering. In an amorphous material lacking long-range
order, the q vector selection rule breaks down and the Raman
spectrum resembles the phonon density of states. For
nanocrystals, the strict “infinite” crystal selection rule is
replaced by a relaxed version. This results in a range of
accessible q vectors (as large as ∆q ≈ 1/L (L diameter))
due to the uncertainty principle.
The anatase TiO
2
has six Raman-active fundamentals in
the vibrational spectrum: three E
g
modes centered around
144, 197, and 639 cm
-1
(designated here E
g(1)
,E
g(2)
, and E

g(3)
,
respectively), two B
1g
modes at 399 and 519 cm
-1
(desig-
nated B
1g(1)
and B
1g(2d)
), and an A
1g
mode at 513 cm
-1
.
370
As the particle size decreases, the Raman peaks show
increased broadening and systematic frequency shifts (Figure
42).
370
The most intense E
g(1)
mode shows the maximum blue
shift and significant broadening with decreasing crystallite
size. A small blue shift is seen for the E
g(2)
mode, while the
B
1g(1)

mode and the B
1g(2)
+A
1g
modes show very small blue
shifts and red shifts (the latter peak represents a combined
effect of two individual modes), respectively. Whereas the
frequency shifts for the A
1g
and B
1g
modes are not pro-
nounced, increased broadening with decreasing crystallite
size is clearly seen for these modes. The E
g(3)
mode shows
significant broadening and a red shift with decreasing
crystallite size.
Choi et al. found a volume contraction effect in anatase
TiO
2
nanoparticles due to increasing radial pressure as
particle size decreases, and they suggested that the effects
of decreasing particle size on the force constants and
vibrational amplitudes of the nearest neighbor bonds con-
tributed to both broadening and shifts of the Raman bands
with decreasing particle diameter.
388
3.5. Electronic Properties of TiO
2

Nanomaterials
The DOS of TiO
2
is composed of Ti e
g
,Tit
2g
(d
yz
,d
zx
,
and d
xy
),Op
σ
(in the Ti
3
O cluster plane), and O p
π
(out of
the Ti
3
O cluster plane), as shown in Figure 43A.
396
The upper
valence bands can be decomposed into three main regions:
the σ bonding in the lower energy region mainly due to O
p
σ

bonding; the π bonding in the middle energy region; and
Op
π
states in the higher energy region due to O p
π
nonbonding states at the top of the valence bands where the
hybridization with d states is almost negligible. The contri-
bution of the π bonding is much weaker than that of the σ
bonding. The conduction bands are decomposed into Ti e
g
(>5 eV) and t
2g
bands (<5 eV). The d
xy
states are dominantly
located at the bottom of the conduction bands (the vertical
dashed line in Figure 43A). The rest of the t
2g
bands are
antibonding with p states. The main peak of the t
2g
bands is
identified to be mostly d
yz
and d
zx
states.
In the molecular-orbital bonding diagram in Figure 43B,
a noticeable feature can be found in the nonbonding states
near the band gap: the nonbonding O pp orbital at the top

of the valence bands and the nonbonding d
xy
states at the
bottom of the conduction bands. A similar feature can be
seen in rutile; however, it is less significant than in anatase.
397
In rutile, each octahedron shares corners with eight neighbors
and shares edges with two other neighbors, forming a linear
chain. In anatase, each octahedron shares corners with four
Figure 41. (A) Powder XRD patterns of TiO
2
samples of different
diameters: (a) 5 nm; (b) 7 nm; (c) 13 nm. Reprinted with permission
from Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater.
2002, 14, 4364. Copyright 2002 American Chemical Society. (B)
Powder XRD patterns of TiO
2
samples of diameter 2.3 nm: (a)
spherical particles; (b) 16-nm nanorods; (c) 30-nm nanorods.
Reprinted with permission from Zhang, Z.; Zhong, X.; Liu, S.; Li,
D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. Copyright
2005 Wiley-VCH.
Titanium Dioxide Nanomaterials Chemical Reviews, 2007, Vol. 107, No. 7 2913
neighbors and shares edges with four other neighbors,
forming a zigzag chain with a screw axis. Thus, anatase is
less dense than rutile. Also, anatase has a large metal-metal
distance of 5.35 Å. As a consequence, the Ti d
xy
orbitals at
the bottom of the conduction band are quite isolated, while

the t
2g
orbitals at the bottom of the conduction band in rutile
provide the metal-metal interaction with a smaller distance
of 2.96 Å.
The electronic structure of TiO
2
has been studied with
various experimental techniques, i.e., with X-ray photoelec-
tron and X-ray absorption and emission spectroscop-
ies.
379,398-405
Figure 44 shows a schematic energy level
diagram of the lowest unoccupied MOs of a [TiO
6
]
8-
cluster
with O
h
,D
2h
(rutile), and D
2d
(anatase) symmetry and the Ti
K-edge XANES and O K-edge ELNES spectra for rutile and
anatase.
398
The anatase structure is a tetragonally distorted
octahedral structure in which every titanium cation is

surrounded by six oxygen atoms in an elongated octahedral
geometry (D
2d
). The further splitting of the 3d levels of Ti
3+
due to the asymmetric crystals is shown for rutile and anatase
structures. The fine electronic structure of TiO
2
can be
directly probed by Ti K-edge X-ray-absorption near-edge
structure (XANES), and the right panel of Figure 44B
contains O K-edge experimental electron-energy-loss near-
edge structure (ELNES) spectra.
398
Hwu et al. found that the crystal field splitting of
nanocrystal TiO
2
was approximately 2.1 eV, slightly smaller
than that of bulk TiO
2
, as shown in Figure 45A.
379
Luca et
al. found that 1s f np transitions broadened as particle size
(increased or decreased) in the postedge region in the X-ray
absorption spectroscopy for TiO
2
nanoparticles.
403
Also, a

clear trend in the X-ray absorption spectroscopy for different
sized TiO
2
nanoparticles was observed, as shown in Figure
45B from the study by Choi et al.
401
Figure 42. (A) Ambient pressure Raman spectra of anatase with an average crystallite size of 4 ( 1 nm (A), 8 ( 2 nm (B), 20 ( 8nm
(C), and 34 ( 5 nm (D). The spectrum marked “E” is from a bulk anatase. (B) The Raman line width (fwhm) of the E
g(1)
mode versus
crystallite size. Reprinted with permission from Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle,
B. C. Phys. ReV.B2005, 71, 184302/1 ( Copyright 2005 by the American Physical Society.
2914 Chemical Reviews, 2007, Vol. 107, No. 7 Chen and Mao