Titanium Dioxide Nanoparticles and Nanostructures

94

Current Inorganic Chemistry, 2012, 2, 94-114

Titanium Dioxide Nanoparticles and Nanostructures Tarek A. Kandiel1,2, Ralf Dillert1 and Detlef Bahnemann*,1 1 2

Institute of Technical Chemistry, Leibniz Universität Hannover, Callinstrasse 3A, 30167 Hannover, Germany; Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt Abstract: Titanium dioxide (TiO2) is the most intensely investigated photocatalyst and until today the only one that has already been commercialized and that is involved in many applications such as self-cleaning materials, dye-sensitized solar cells, as well as water and air purification. Consequently, an exponential growth of research activities concerning the nanoscience and nanotechnology of TiO2 has been observed during the last decades. These raising research activities have recently lead to the synthesis of nanosized TiO2 single crystals with well-defined shapes and with specific exposed surfaces. Thus, the present review will focus mainly on the synthesis of these nanomaterials. The thermodynamic stability, the transition between different TiO2 polymorphs, and the surface properties of these polymorphs are presented with the aim to utilize these informations for a better understanding of the mechanism of the formation of shape-defined TiO2 nanomaterials and their various applications in photocatalysis.

Keywords: Anatase, Brookite, photocatalysis, rutile, shape-defined titanium dioxide nanomaterials. 1. INTRODUCTION The commercial production of titanium dioxide (TiO2) has been initiated from its traditional use as a pigment, e.g., in paints, sunscreens, and toothpaste [1, 2]. During the last decades, new applications of TiO2 nanomaterials have been discovered. For example, TiO2 nanomaterials are being employed in photocatalyic systems, dye-sensitized solar cells, lithium batteries, gas sensors, as well as in photovoltaic, electrochromic, and photoelectrochromic devices [3]. These new applications lead to an exponential growth of research activities concerning the nanoscience and nanotechnology of TiO2. It was discovered that new physical and chemical properties emerge when the size of the material becomes smaller and smaller, finally approaching the nanometer scale [4, 5]. These properties also vary as the shapes of the shrinking nanomaterials change [6]. The specific surface area and the surface-to-volume ratio increase dramatically as the size of a material decreases. The high surface area achieved by a small particle size is beneficial to many TiO2-based devices, as it facilitates the reaction/interaction between the devices and the interacting media as this mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. Thus, the performance of TiO2-based devices is found to be largely influenced by the size and shape of the TiO2 building units, apparently at the nanometer scale. Since continued breakthroughs have been made in the synthesis of TiO2 nanomaterials within the past few years, the present review will mainly focus on the synthesis of TiO 2 nanoparticles and structures with well-defined shapes. The thermodynamic stability, the transition between different TiO2 polymorphs, and the surface properties of these *Addess correspondence to this author at the Institute of Technical Chemistry, Leibniz Universität Hannover, Callinstrasse 3A, 30167 Hannover, Germany; Tel: +49-(0)511-762-5560; Fax: +49-(0)511-762-3004; E-mail: [email protected]

1877-945X/12 $58.00+.00

polymorphs are presented with the aim to utilize these informations for a better understanding of the mechanism of the formation of shape-defined TiO2 nanomaterials and their various applications in photocatalysis. 2. THERMODYNAMIC STABILITY OF TITANIUM DIOXIDE POLYMORPHS 2.1. Titanium Dioxide Polymorphs In nature, TiO2 exists mainly in three crystal phases: anatase, rutile, and brookite. However, other structures exist as well, for example, cotunnite TiO2 has been synthesized under high pressure and is considered as one of the hardest polycrystalline materials known [7]. TiO2(B) has been synthesized by the hydrolysis of K2Ti4O9 followed by thermal treatment at 500 ºC [8]. However, although TiO2 exists in many polymorphs, only rutile and anatase have been extensively involved for most applications of TiO2 and are being extensively studied due to the ease of their synthesis. Fig. (1) shows the basic unit cell structures of these two common phases [9]. They are both tetragonal, containing six and twelve atoms per unit cell, respectively. In both structures, each Ti atom is coordinated with six O atoms and each O atom is coordinated to three Ti atoms. For both phases, the TiO6 octahedron is slightly distorted, with two Ti–O bonds being a little longer than the other four, and with some of the O–Ti–O bond angles deviating from 90°. The distortion is more pronounced for anatase than for rutile. The structure of rutile and anatase crystals is frequently being described in terms of chains of TiO6 octahedra sharing common edges with two and four edges being shared in rutile and anatase, respectively [10]. The third naturally known polymorph of TiO2, brookite, has a more complicated structure [11] exhibiting eight formula units in an orthorhombic cell. The interatomic distances and the O–Ti–O bond angles are similar to those of rutile and anatase with the essential difference being that there are six different Ti–O bond lengths ranging from 1.87 to 2.04 Å. Accordingly, there are twelve different O– © 2012 Bentham Science Publishers

Titanium Dioxide Nanoparticles and Nanostructures

Current Inorganic Chemistry, 2012, Vol. 2, No. 2

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Rutile

[001] o

1.946 A titanium 90O

oxygen 98.93 O

[100]

[01 0]

o

1.983 A

[010]

[100]

[001]

Anatase

[010]

o

1.966 A 102.308 o 92.604 o o

1.937 A [001]

[001]

[100]

[010]

Fig. (1). Bulk structures of rutile and anatase. The tetragonal bulk unit cell of rutile has the dimensions, a = b = 4.587 Å, c = 2.953 Å, and the one of anatase a = b = 3.782 Å, c = 9.502 Å. The bond lengths and angles of the octahedrally coordinated Ti atoms are indicated and the stacking of the octahedra in both structures is shown on the right side. Reprinted with permission from ref.[9], Copyright (2003) Elsevier.

Ti–O bond angles ranging from 77° to 105°. In contrast, there are only two kinds of Ti–O bonds and O–Ti–O bond angles in rutile and anatase. One can also envision brookite as being formed by joining together distorted TiO6 octahedra sharing three edges [12]. 2.2. Phase Transitions Between the TiO2 Polymorphs As a bulk material, rutile is the thermodynamically stable phase; however, solution-phase preparation methods for TiO2 generally favour the anatase structure. These observations are attributed to two main effects: surface energy and solution chemistry. At very small particle dimensions (nanoscale), the transformation sequence among the three TiO2 phases was reported to be size and pH dependent. This was explanined by the fact that the energies of all three phases are sufficiently close to one another and can thus be reversed by small differences in surface energy [13, 14]. If particle sizes of the three nanocrystalline phases are equal, anatase is found to be thermodynamically most stable at diameters below 11 nm, brookite is most stable for crystal sizes between 11 and 35 nm, and rutile is most stable at sizes exceeding 35 nm. However, rutile is stabilized relative to anatase in very acidic solutions, whereas in very alkaline solutions anatase is stabilized relative to rutile and brookite [15, 16]. Finnegan et al. have investigated the phase transformation in aqueous solution [16] and reported that both, the interfacial tension and the surface charge of TiO2 nanoparticles change significantly with the solution pH, in turn changing the phase stability of nanoparticulate TiO2. At pH values far below the point of zero proton condition (pHZPC) of TiO2, nanoparticles of rutile are more stable than those of anatase; at pH values far above the pH ZPC, nanoparticles of anatase are more stable than those of rutile. The

transformation rate from one nanophase to another increases with increasing temperature and with the deviation of the solution pH from the pHZPC of the TiO2 nanoparticles, and it is found to be higher for smaller nanoparticles than for larger ones. Combining heating conditions with particle growth, the following transformations between the three TiO2 polymorphs have been reported: anatase to brookite to rutile [14], brookite to anatase to rutile [17], anatase to rutile [18, 19], and brookite to rutile [20]. These transformation sequences imply very closely balanced energetics being themselves a function of the particle size. The surface enthalpies of the three polymorphs are sufficiently different that a crossover in thermodynamic stability can occur under conditions that preclude particle growth, with anatase and/or brookite being more stable at small particle size [13, 14]. However, abnormal behaviour and inconsistent results are occasionally observed. The crystal structure of TiO2 nanoparticles is usually found to depend strongly on the preparation method and on the precursor used. For example, the selective synthesis of nanocrystalline brookite, rutile, and anatase nanoparticles, respectively, has been carried out employing the hydrothermal treatment of the hexaammonium hexaglycolatodioxotetraperoxotetratitanate(iv) tetrahydrate precursor at different pH values [21]. Size tailored anatase nanoparticles with mean sizes in the range 5–10 nm have been prepared by the precipitation of TiCl4 in aqueous medium in the range from pH 2 to pH 6 [22]. The resulting particle size, dependent on the acidity, is closely related to the electrostatic surface charge density of the particles. This size variation has been interpreted as resulting from a lowering of the interfacial tension due to the protonation of particle surface groups. An interesting phenomenon involving surface energy that

96 Current Inorganic Chemistry, 2012, Vol. 2, No. 2

Kandiel et al. 2

surface area (m /g)

enthalpy w.r.t. bulk rutile (kJ/mol)

16

0

50

100 ru

12

til

150

e bro

8

oki

te

a n a ta s e

4 0

0

8000

4000

12000

2

surface area (m /mol) Fig. (2). Enthalpy of nanocrystalline TiO2. Reprinted from ref. [26], Copyright (2002), with permission from National Academy of Sciences, U.S.A.

allows the stabilization of anatase nanoparticles with respect to their conversion to rutile upon thermal treatment up to 1000 ºC has been reported. The growth of anatase nanoparticles can be hindered by their dispersion in a silica matrix thus preventing the sintering and keeping the total surface free energy of the system constant. The energetically more favorable scenario for the system is obviously to maintain a metastable phase possessing the lowest surface free energy. This observation is in agreement with reported calculations, indicating that anatase particles with a particle size below 14 nm are thermodynamically more stable than rutile [13, 23]. Banfield et al. have studied the impact of particle size on phase stability and phase transformation during the growth of nanocrystalline TiO2 aggregates employing TiO2 samples consisting of nanocrystalline anatase (46.7 wt %, 5.1 nm) and brookite (53.3 wt %, 8.1 nm). The respective phase transformations were studied isochronally at reaction times of 2 h in the temperature range 598-1023 K and isothermally at 723, 853, and 973 K by X-ray diffraction. The results showed that anatase is transformed to brookite and/or rutile before brookite transforms to rutile. Once rutile is formed, the respective particles tend to grow much faster than anatase nanoparticles. For most TiO2 samples, the transformation sequence between anatase and brookite depends on the respective initial particle sizes determining the thermodynamic phase stability in particular at ultrafine sizes. 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 as the dominant phase above 1123 K [24]. 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., [25]. Ranade et al. investigated the energetics of the TiO2 polymorphs, i.e., rutile, anatase, and brookite, by high-temperature oxide melt drop solution calorimetry, and reported that the energetic stability crossed over between the three phases as shown in Fig. (2) [26].

The dark solid line in Fig. (2) represents the phases of lowest enthalpy as a function of the surface area. Rutile was found to be the energetically stable polymorph at surface area < 592 m2/mol (7 m2/g or >200 nm). Brookite was considered as the energetically stable polymorph in the range from 592 to 3174 m2/mol (7-40 m2/g or 200-40 nm), while anatase was the energetically stable polymorph for greater surface areas or smaller sizes ( 40 m2/g is initially formed during the synthesis of nanocrystalline TiO2, it is metastable with respect to both anatase and rutile, and the sequence brookite to anatase to rutile during coarsening is energetically downhill. If anatase is formed as the primary product, it could coarsen and is transformed first to brookite (at 40 m2/g) and then to rutile. The energetic driving force for the latter reaction (brookite to rutile) is very small, explaining the natural persistence of coarse brookite. In contrast, the absence of coarse-grained anatase is consistent with the much larger driving force for its transformation to rutile [26]. Li et al. reported only the phase transformation from anatase to rutile in the temperature range between 973 and 1073 K [27]. Both anatase and rutile particle sizes increased with increasing temperature, but the growth rates were found to be different for the two polymorphs. Rutile had a much higher growth rate than anatase. The growth rate


1.4

97

a

1.2

Surface stress (N/m)


1.0 0.8 0.6 0.4 0.2 0.0

D0

0

D1

10

20

D (nm)

30

Fig. (3). Surface free energy of nanocrystalline anatase as a function of the particle size. Reproduced from ref.[28] with permission of the PCCP Owner Societies.

of anatase leveled off at 800 ºC. Rutile particles, after nucleation, grew rapidly, whereas the size of anatase particles remained practically unchanged. With the decrease of initial particle size, the onset transition temperature decreased. An increasing 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 TiO2 nanoparticles, respectively. The decreased thermal stability in smaller nanoparticles was primarily due to the reduced activation energy as the size-related surface enthalpy and stress energy increased. An equation relating the surface free energy and the surface stress of nanoparticles have been derived by Zhang et al., [28]. The surface free energies of nanocrystals have been obtained from an integration of the size-dependent surface stress values determined from XRD. Application of this relationship to nanocrystalline anatase revealed a size dependence of the surface free energy with a maximum value of ca. 1.0 J/m at ca.14 nm as shown in Fig. (3). (001)

(011) (101)

(110)

(110) (100)

Fig. (4). The equilibrium shape of a macroscopic crystal of TiO2 using the Wulff construction and the surface energies. Reprinted from ref.[29], Copyright (1994), with permission from American Physical Society.

2.3. The Surfaces of TiO2 Polymorphs The energetics of several low-index surfaces of TiO2 have been calculated by Ramamoorthy et al. using firstprinciples total-energy calculations methods [29]. These calculations indicate that the (110) surface has a much lower surface energy than the (001) surface while the energies of the (100) and the (011) surface lie in between these extremes. The (100) surface is clearly stable with respect to form facets of the (110) orientation, while the (001) surface appears to be marginally stable with respect to form macroscopic (011) facets. From the calculated energies a threedimensional (3D) Wulff plot was constructed as shown in Fig. (4). The Wulff construction gives the equilibrium crystal shape of a macroscopic crystal. For comparison with experimental crystal shapes it has to take into account that only four planes were considered and that the calculations are strictly valid only at absolute zero. For rutile, the (110), (001) and (100) surfaces have been studied, with (110) being the most stable one. For anatase, typically, (101) and (100)/(010) surface planes are found, together with some (001) [9, 30]. Several theoretical studies have predicted the stability of the different low-index anatase surfaces [30, 31]. The (101) face is the thermodynamically most stable surface as concluded from the calculated surface energies values (See Table 1) [9, 32, 33]. The calculated Wulff shape of an anatase crystal, based on these values, compares well with the shape of naturally grown mineral samples as shown in Fig. (5) [9, 34]. The (101) and (001) facets are widely assumed to make up the majority of exposed facets on anatase nanoparticles surfaces. However, Feldhoff et al. have investigated a series of commercial anatase nanoparticles and they found that the (101) and (001) facets are the minority [35]. Surfaces of powder nanoparticles are rather dominated by facets of the type (100) or (111), and they show also (110), (112), (102), (103), (104), (106), and (108) facets, depending on the particle shape as shown in Fig. (6). These investigations clearly show that the shape of naturally occurring macroscopic anatase is not the key to a better understanding of nanocrystalline materials. The shapes of the synthetic crystallites vary with preparation techniques and procedures. While most of


Kandiel et al.

Comparison of Calculated Surface Formation Energies (J/m2) for Relaxed, Unreconstructed TiO2 Surfaces [9, 32, 33].

Table 1.

Rutile

Anatase

(110)

(101)

(100)

(001)

(103)f

(103)s

(110)

0.31

0.44

0.53

0.90

0.84

0.93

1.09

Two different structures for the (1 0 3) surfaces (a 'faceted' and a 'smooth' one) have been considered. (001)

(100) (103)

(101))

(011)

(101) (011)

(a)

(b)

MM CM

1

2

Fig. (5). (a) The equilibrium shape of a TiO2 crystal in the anatase phase, according to the Wulff construction and surface energies given in Table 1, (b) Photograph of an anatase mineral crystal. Reprinted with permission from ref.[34], Copyright (2003) Elsevier.

the reported syntheses of anatase nanocrystals are reports from different laboratories employing different methods, the experimental results concerning one specific TiO2 sample do not allow a general extrapolation to other TiO2 materials. Theoretical studies often justify investigations of specific crystal faces with their abundance in the bipyramidal shape of naturally occurring macroscopic anatase crystals. However, it seems that these model calculations correspond to well-defined single-crystal surfaces, and not to nanoparticles. a) 0-1-3

00-1

15.5 nm 12.8 nm

16 nm

b) -100

010 0-10

13.5 nm

13.5 nm 0-10

-100 0-13

0-18

001

15 nm

13.7 nm 0-13 5 nm

01-3

12.8 nm 013 12.5 nm 001 0-18 d)

c) 016

002

011 011 16.5 nm 15.5 nm 0-12 01-1 17 nm 17 nm 16 nm0-11 16 nm 01-1 17 nm 15 nm 15.5 nm 0-1-1 01-6 16.5 nm 5 nm 00-2 0-1-4

016

001

0-12 0-11

-100

00-1

0-1-4

The surface alignment of the titanium and oxygen atoms for the lowest energy surface of the anatase and the rutile polymorph, i.e., the (101) and (110) surfaces for anatase and rutile, respectively, is shown in Fig. (7). These two surfaces are similar in several ways. For instance, both surfaces have 5-fold coordinated Ti cations and 2-fold coordinated O anions with a surface concentration of about 5.21014 sites/cm2 for each species on both crystallographic planes (5.161014 sites/cm2 for anatase and 5.201014 sites/cm2 for rutile) [36]. Both 6-fold coordinated Ti cations and 3-fold coordinated O anions make up the remainder of the sites for both surfaces. A significant difference between the two surface terminations is that the anatase surface has a saw tooth profile in which the 2-fold coordinated O anions are bound to the 5fold coordinated Ti cations, whereas the rutile surface is flat with 2-fold-coordinated bridging oxygens bound to 6-foldcoordinated Ti cations and projected out of the surface plane. Removal of one 2-fold coordinated bridging oxygen results in two 4-fold coordinated Ti3+ cations and two 5-fold coordinated Ti3+ cations for the anatase and rutile surfaces, respectively. These highly under-coordinated Ti cations in anatase are likely to be less stable than the respective Ti cations in rutile and may explain why the number of oxygen vacancies usually observed by STM at the surface of anatase (101) is much lower than that observed for rutile (110) [37]. 3. SYNTHESES OF SHAPE-DEFINED TiO2 POLYMORPHS

01-6 0-1-1

Fig. (6). HRTEM micrographs and reconstructed shapes of anatase nanoparticles from PC 50 TiO2 powder: a), b) truncated cube; c), d) prism. Reprinted with permission from ref.[35]. Copyright (2007) Wiley-VCH.

Many techniques such as the sol-gel, micelle and inverse micelle, hydrothermal, solvothermal, sonochemical and microwave-assisted methods, as well as direct oxidation, chemical vapor deposition, physical vapour deposition, and electrodeposition, have been employed for the synthesis of TiO2 nanoparticles and nanostructures mainly using titanium tetrachloride, titanium trichloride, titanium oxysulphate, tita-


anatase (101)

Current Inorganic Chemistry, 2012, Vol. 2, No. 2 (101)

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99

3.1.1. Anatase Nanoparticles with (101) Facets

thermal method has been developed for the preparation of transparent, oriented, and well-defined (101)-plane dominated nanocrystalline anatase TiO2 films by using a functionalized surface to assist anatase film growth [39]. The success of the film preparation depended strongly on the control of the hydrolysis of the titanium trichloride employed as precursor in this work, on the functional groups (e.g. NH2, -SO3H, -COOH, and –OH) attached to the surface of the substrate, as well as on the use of additives, i.e. poly(vinylpyrrolidone) and sodium sulfate. Oriented and well-defined nanocrystalline TiO2 films were only obtained on amine (-NH2) functionalized surfaces. The films were built from octahedron shaped anatase nanocrystals bounded by eight triangular surfaces, approximately 90-150 nm wide (see Fig. (8)). On -COOH and other functionalized surfaces, agglomerated crystals or noncontinuous films were obtained. The continuous, transparent, and oriented nanocrystalline film was the direct result of a surface-mediated nucleation process. At pH values between 1 and 2, the amine terminated groups on the substrate are positively charged [40]. Thus, negatively charged SO42- ions are preferentially attached to the -NH2 substrate. These SO42- ions likely serve as the nucleation sites for the anatase crystals [41]. Based on the titanyl sulfate structure, the sulfate tetrahedra prefered binding to the vertexes of the Ti-O octahedra. The anatase (001) plane contains edge-sharing octahedra with the highest density of vertexes to bind to the sulfate ions on this plane. This explains the origin of the [001] orientation in the TiO2 films. The orientation has also been suggested to be a result of grain boundary motion during film growth or kinetic anisotropic crystal growth in which the fast growth orientation prevails. Other functionalized groups (-SO3H, -COOH, and OH) are not highly charged at the low pH values, and therefore do not provide the electrostatic attraction mentioned earlier for the crystal growth. The well-defined octahedral pyramidal nanocrystals have only been observed when poly(vinylpyrrolidone) was present in the solution. This suggests that poly(vinylpyrrolidone) might be preferably adsorbed on the (101) anatase surface, resulting in the formation of (101)-plane dominated octahedral bipyramids.

The surface free energy of the (101) lattice plane is the smallest amongst all possible planes of an anatase single crystal. In agreement with the shape of natural minerals, a truncated octahedral bipyramid, exposing eight (101) facets as well as two (001) facets, has been shown to be the thermodynamically most stable shape of an anatase crystallite based on Wulff construction [32, 34]. Recently, a hydro-

Amano et al. reported the synthesis of mesoparticles of anatase octahedral bipyramids by a hydrothermal reaction of titanate nanowires prepared by a preceding hydrothermal treatment of TiO2-P25 particles in a potassium hydroxide solution [42]. The hydrothermal reaction of the titanate nanowires resulted in the formation of mesoparticles of an octahedral morphology, i.e., bipyramidal shape, with the size

Ti(5)

rutile (110)

O(2)

[101] [010]

(110)

Ti(5)

O(2) [001]

[110]

Fig. (7). Schematic representation of the anatase (101) and the rutile (110) surfaces, top and bottom, respectively. The large open circles represent oxygen anions, and the small filled circles represent titanium cations. Reprinted with permission from ref.[36]. Copyright (2003) American Chemical Society.

nium alkoxide and titanium foil as precursors. Utilizing these methods, TiO2 with different morphologies and structures have been synthesized [3, 38]. In particular, new synthetic routes employing the hydrothermal method for the preparation of TiO2 single crystal with well-defined shape that exhibits specific exposed faces have recently published. Since well-defined TiO2 single crystals are very interesting from the scientific and the technological point of view for many applications such as solar cells, photonic and optoelectronic devices, sensors and photocatalysis, we are focusing in the following sections on the preparation of these nanoparticles and structures. 3.1. Synthesis of Anatase Nanoparticles with Defined Structure

(b)

(a)

(c)

PNNL 200 nm

500 nm

TiO2

1 mm

Fig. (8). (a) Top-view SEM image of an octahedral nanocrystalline TiO2 film (200 nm thick) on amine functionalized glasses, (b) Photograph of an octahedral pyramidal nanocrystalline TiO2 film in 200 nm (up) and 100 nm (bottom) thicknesses on amine functionalized glasses. (c) Top-view SEM image of agglomerated TiO2 nanocrystals grown on –COOH functionalized glasses. Reprinted with permission from ref.[39]. Copyright (2006) American Chemical Society.


Kandiel et al.

of the long axis being > 100 nm (see Fig. (9)). The fraction of regular octahedral bipyramids in the obtained mixture of particles was about 70 %. The remaining fraction of particles was similar to octahedral bipyramids but their shape was rounded or irregular, suggesting the presence of high-index lattice planes. The orientation of fringes was consistent to that of the equilibrium shape of anatase crystallites. The octahedral bipyramids exhibited predominantly anatase (101) facets, while the edges of the crystallites became rounded to some extent and other facets such as (100) were also exposed. The specific surface area of the anatase particles containing octahedral crystallites was 40 m2/g. When tightly bound titanate nanowires were employed as the TiO2 precursor, larger octahedral particles with a specific surface area of 25 m2/g were obtained, and the size of the particles was not uniform. The fraction of octahedral bipyramids was only about 60 %, less than that of particles prepared from lessbundled titanate nanowires, indicating that the nanowire structure influences the process of octahedral crystallite production. (b)

(a)

(011) (101) (101)

(011)

200 nm

200 nm (c)

(d)

004

103 101 200

[001

]

[101]

100 nm 20 nm

)

(101)

(002

for the truncated octahedral samples determined by nitrogen adsorption measurement the total area of exposed (001) facets was calculated to be about 4.8 m2 /g, which was much larger than previously reported values [44, 45]. By using 0.1 M CTAB as the capping reagent, the obtained product exhibited a spindle-like morphology with sizes from tens to hundreds of nanometers Fig. (10c). The transformation of ammonium-exchanged titanate nanowires (ATNWs) upon hydrothermal treatment to the nanosized anatase TiO2 with well-defined facets was reported to proceed through the dissolution and nucleation mechanism. At the beginning of the transformation the surface of ATNWs gradually decomposed and produced Ti-(OH)4 fragments, ammonia, and potassium ions under hydrothermal conditions at higher temperature. Then, Ti-(OH)4 fragments were rearranged through a dehydration reaction between Ti-OH and HO-Ti in an edgesharing manner, resulting in the formation of anatase crystal nuclei. Subsequently, the TiO2 nuclei gradually grew up to form TiO2 nanocrystals when Ti(OH)4 fragments diffuse to their surfaces. Due to anisotropy in adsorption stability of the capping reagents, the additives adsorbed onto a certain crystallographic plane more strongly than onto other planes, thus lowering the surface energy of the bound plane and hindering the growth of crystals or some crystal planes.

Fig. (9). SEM image of (a) titanate nanowires and (b, c) particles after hydrothermal reaction of titanate nanowires. (d) TEM image and electron diffraction pattern of an octahedral bipyramid. Reproduced from ref.[42] by permission of The Royal Society of Chemistry.

Li et al. have used modified titanate nanowires, that is, ammonium-exchanged titanate nanowires to prepare nanosized anatase TiO2 with well-defined facets [43]. Octahedral, truncated octahedral, and spindle-like TiO2 particles have been synthesized by kinetically controlling the growth rates of various facets of TiO2 particles with appropriate capping agents, e.g. hexamethylenetetramine (HMTA), ammonium fluoride, and hexadecyltrimethylammonium bromide (CTAB). In the presence of 0.05 M HMTA, uniform octahedral TiO2 nanocrystals with sharp edges (see Fig. (10a)) were obtained in a relative high yield (>80%). When 0.1M ammonium fluoride was used as the capping reagent, most (> 90%) of the particles exhibited truncated octahedra exposing high energy (001) facets (see Fig. (10b)). In order to define the degree of truncation, the ratio of the edge lengths A and B of the truncated octahedral TiO2 crystal (cf. Fig. (10e)) was calculated to be approximately 0.60. Thus, the percentage of (001) facets was estimated to be approximately 17%. On the basis of the BET surface area of 28 m2/g

Anatase TiO2 microspheres with controlled surface morphologies and exposed (101) crystal facets have been recently synthesized by Zhang et al., [46]. The anatase TiO 2 microspheres were directly grown on titanium foil substrates by controlling the hydrothermal reaction time without the need to use templating reagents. Changes of surface morphology of these microspheres were accompanied by changes of the surface crystal facets during the hydrothermal process (see Fig. (11)). Microspheres with exposed squareshaped, plane (001) facets and nanosheet (101) facets were obtained by controlling the reaction times of 1 and 24 h, respectively. These transformations of the surface morphology and crystal facets during the hydrothermal processes were governed by the compositional changes of the reaction media and driven by dynamically shifted dissolution/deposition equilibria. 3.1.2. Anatase Particles with (001) Facets Theoretical studies predicted that the minority (001) facets in the equilibrium state would be especially reactive in heterogeneous reactions [30, 47, 48]. This is based on the fact that the (001) facet exhibits a high density of undercoordinated Ti atoms and shows very large Ti–O–Ti bond angles at the surface [49]. Unfortunately, (001) facets with high surface energy and reactivity usually exist as the minority in most available anatase TiO2 particles. The formation of dominant (001) facets in anatase is therefore scientifically and technologically significant. Many attempts have been made with various adsorbate atoms to change the relative stabilities of different crystal facets [3, 50-55]. Amongst its oxygenated surfaces the (100) facets are the most stable for anatase TiO2, whereas under clean and hydrogenated conditions, the (101) facets are the most stable [9, 52, 56]. However, both H- and O-terminated anatase surfaces possess high surface energies restricting the formation of large anatase single crystals. High values of surface energies are mainly attributed to the high bonding energies of H–H



(b)

(a)

(c)

200 nm

200 nm

100 nm

(e)

101

001

(f) 101

1

(d)

101

00

B A

Fig. (10). SEM images of TiO2 particles prepared by hydrothermal reaction of ammonium-exchanged titanate nanowires precursors in (a) 0.05 M hexamethylenetetramine; (b) 0.1 M ammonium fluoride; (c) 0.1 M hexadecyltrimethylammonium bromide. (d-f) Schematic drawings of shapes of TiO2 particles in panels a, b, and c, respectively. Reprinted with permission from ref.[43]. Copyright (2010) American Chemical Society.

0.5 % (v/v) HF

Ti substrate

24 h

12 h

6h

3h

2h

1h

reaction at 180 o C

Fig. (11). Schematic illustration of the surface morphological evolution of anatase TiO2 microspheres with hydrothermal reaction time. The scale bar: 200 nm. Reprinted with permission from ref.[46]. Copyright (2011) Wiley-VCH.

(436.0 kJ/mol) and O–O (498.4 kJ/mol) [57]. Therefore, using an element with low bond energy in the molecule but with strong bonding to Ti might provide an effective means for stabilizing the surfaces. Fluor with a F-F bond energy of 158.8 kJ/mol and a F-Ti bond energy of 569.0 kJ/mol is such an element [57, 58]. Zhang et al. have carried out a systematic investigation of 12 non-metallic atoms X (where X can represent H, B, C, N, O, F, Si, P, S, Cl, Br or I) based on first-principles calculations in order to explore the effect of various adsorbate atoms. The calculated surface energies values for different adsorbates indicate that among the 12 Xterminated surfaces and the clean surfaces, termination with F atoms not only yields the lowest value of surface energies for both the (001) and (101) surfaces, but also results in (001) surfaces that are more stable than (101) surfaces. These results have suggested the possibility to achieve anatase TiO2 single crystals with a high percentage of anatase (001) facets if their surfaces are surrounded by F atoms. Based on these calculations a high-quality truncated anatase bipyramids with a high percentage of (001) facets (47%) on a micrometre scale were prepared employing an aqueous solution of titanium fluoride (TiF4) as the precursor and hydrofluoric acid as the crystallographic controlling agent (see Fig. (12) left) [54]. The percentage of the (001) facets was further improved to 64% by employing an alternative solvothermal route using 2-propanol as a synergistic capping agent and reaction medium together with HF for the synthe-

sis of anatase TiO2 single-crystal nanosheets (see Fig. (12) right) [44]. The function of 2-propanol and HF in facilitating the growth of anatase TiO2 single-crystal nanosheets is to enhance the stabilization effect associated with fluorine adsorption at the (001) surface as explored by the firstprinciple theoretical calculations. A synergistic functionality of chemisorbed F to lower the (001) surface energy and 2propanol to enhance this stabilization and to act as a protective capping agent led to the formation of anatase TiO2 single-crystal nanosheets, which is the thermodynamically favored morphology under this condition. Recently, Zhang et al. have developed a green synthetic route for the preparation of a micro-sheet anatase TiO2 single crystal with enhanced level of reactive (001) facets (80%) by using a microwaveassisted method involving an aqueous solution of titanium tetrafluoride and a tetrafluoroborate-based ionic liquid [59]. Microwave heating provides energy to the reactants by means of the molecular interaction with high frequency electromagnetic radiation, which is a different mechanism than convective heating in the conventional thermal treatment [60]. The advantage of this method, beside reducing the time required for the synthesis and enhancing the crystallinity of the product, is to avoid the usage of the extremely corrosive and toxic hydrofluoric acid [61]. Amano et al. have prepared single-crystalline anatase particles through a gas-phase reaction with a relatively high yield of decahedral particles [62]. The TiO2 crystallites were


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Fig. (12). SEM image of anatase single crystals synthesized in presence of TiF4 aqueous solution (left) and anatase single crystals nanosheets synthesized in presence of HF and 2-propanol (right). Reprinted with permission from ref.[54] and ref.[44], respectively. Copyright (2008) Nature Publishing Group and Copyright (2009) American Chemical Society, respectively.

fabricated from titanium(IV) chloride (TiCl4) by a gas-phase reaction process with rapid heating and quenching. The vapour of TiCl4 was liberated by bubbling argon (200 mL/min) into a TiCl4 solution at 358 K, mixing it with an oxygen stream (1200 mL/min), and feeding into a quartz glass tube heated from the outside by an oxygen-hydrogen flame burner. The tube was rotated around the cylindrical axis at a speed of 55 rpm to assure homogeneous heating at 1573 K. The TiO2 particles generated by this thermal oxidation of TiCl4 were collected downstream by a filter cup made from glass fibers. Uniform and rapid heating at the high temperature applied in this study enabled homogeneous nucleation and subsequent growth to well faceted TiO2 crystals with few defects. The low concentration of TiCl4 and the narrow heating zone prevented the formation of large particles and polycrystalline aggregates with grain boundaries. The shape of these particles resembled those of particles prepared by Lu and co-workers by hydrothermal reaction of titanium tetrafluoride (TiF4) under acidic conditions [54], but they are of smaller size and exhibit an enhanced surface area (see Fig. (13)), which is preferred in photocatalytic reactions.

1 nm by employing a facile non-aqueous route [63]. Although terminated by the dominant (001) facets, these ultrathin TiO2 nanosheets are stabilized by large amounts of oleylamine (43 wt %) and form stacked lamellar structures. Surfactant removal by calcination to obtain pure anatase TiO2 from the ultrathin TiO2 nanosheets lamellar structure is assumed to be unattractive because (i) it will likely lead to complete condensation and destruction of the nanosheets, and (ii) it might promote the formation of irregular, large anatase TiO2 crystals, which to prevent is the very purpose of the oleylamine stabilizing layer. One way to obtain thermodynamically stable TiO2 anatase single crystal with exposed (001) facets is to grow them into a three-dimensionally selforganized architecture, as nicely demonstrated for ultrathin zeolite nanosheets very recently [64]. Hierarchical spheres self-organized from ultrathin anatase TiO2 nanosheets with nearly 100% exposed (001) facets have been recently synthesized by Chen et al. (see Fig. (14)) [65]. They have been prepared employing a mixture of diethylenetriamine and isopropyl alcohol as the solvent and titanium(IV) isopropoxide as the titanium source under solovothermal conditions. The resulting hierarchical spheres showed a threedimensional nanoporous structure with a high specific surface area of 170 m2/g. Anatase TiO2 nanosheets-based hierarchical spheres with over 90% (001) facets have also been recently synthesized by using a diethylene glycol-

Fig. (13). SEM image of TiO2 particles prepared by gas-phase reaction of TiCl4 with oxygen. Reprinted with permission from ref.[62]. Copyright (2009) American Chemical Society.

The specific surface area of the TiO2 anatase single crystal with exposed (001) facets is usually low because of the relatively large crystal thickness in the [001] direction. For example, the specific surface area of anatase TiO 2 nanosheets (1.09 m in width, 260 nm in thickness) with 64% exposed (001) facets is only 1.6 m2 g-1 [44]. Reducing the thickness in the [001] direction and increasing the twodimensional lateral size of the (001) planes results in a simultaneous increase in the percentage of exposed (001) facets and the specific surface area of sheet-like anatase TiO2 crystals. Stucky and co-workers used this approach to synthesize anatase TiO2 nanosheets with thicknesses of less than

Fig. (14). (A, B) FESEM images and (C, D) TEM images of asprepared anatase TiO2 nanosheets hierarchical spheres. Reprinted with permission from ref.[65]. Copyright (2010) American Chemical Society.


solvothermal route and TiF4 as titanium source [66]. 3D hierarchical structures of single-crystalline anatase TiO2 nanosheets dominated by well-faceted (001) facets using isobutyl alcohol, water and titanium(IV) butoxide as reactants in presence of HF have also been reported [67]. Anatase TiO2 nanosheets act as the building units of the 3D hierarchical structure and the morphology is controlled by the HF present in the reaction media. Three-dimensional, hollow anatase TiO2 boxes, each one enclosed by six single-crystalline TiO2 plates exposed with highly reactive (001) facets, have been synthesized by calcination of a cubic TiOF2 solid precursor at 500–600 ºC [68]. The formation of such particular nanostructures was attributed to the hard self-template restriction and the adsorption of F- ions from the TiOF2. The percentage of the reactive (001) facets have been estimated from the geometric calculation to be higher than 83 % in this structure. Anatase TiO 2 microspheres with exposed mirror-like plane (001) facets have been prepared by a hydrothermal method using metal titanium foil as Ti source in 0.5% (v/v) HF solution [69]. TiO2 films composed of flower-like TiO2 microspheres with exposed (001) facets have been synthesized by Xiang et al., [70]. Recently, Wang et al. have prepared a TiO2 (001) facet oriented film on a Ti foil substrate by a facile hydrothermal route (see Fig. (15)) [71]. It was found that the TiO2 film grew on the upper face of the Ti foil. The exposed surface of the film consisted of truncated tetrahedrons with well patterned major flat and square top surfaces and minor four isosceles trapezoidal lateral surfaces. Based on the geometric symmetry of anatase TiO2, the top and lateral surfaces were (001) and (101) facets, respectively. The average size of (001) facets was as small as ca. 130 nm, which should be advantageous to enhance the reactivity of the (001) facets.


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(101) surfaces [54]. As a result, hydrofluoric acid was suggested to play a triple role: to dissolve Ti powder, to retard the hydrolysis of the titanium precursor, and to reduce the surface energy. Beside the importance of fluoride ions, the addition of H2O2 was found to be essential for the formation of single crystals with exposed (001) and (110) facets. In absence of H2O2 an irregular and extensively aggregated TiO2 was formed. The aggregation of the TiO2 was suggested to be due to the tendency to reduce the surface energy. It was reported that Ti4+ reacts with H2O2 to form yellow peroxotitanium acid [74]. The formation of this complex has been suggested to further retard the hydrolysis of the titanium precursor. The relatively slow hydrolysis rate could provide sufficient time for a better packing of the Ti–O–Ti chains resulting in a growth of well-formed anatase single crystals, and a better adsorption of fluoride ions on the surface of the TiO2 crystals thus reducing the surface energy.

Fig. (15). Top-view SEM images of TiO2 (001) facet oriented film (a) and partially scrapped film (b). Reproduced from ref.[71] by permission of The Royal Society of Chemistry.

3.1.3. Anatase Particles with (110) or (100) Facets Following a report of Lu and co-workers on the preparation of anatase TiO2 single crystals with exposed (001) [54], several research groups have prepared anatase TiO2 single crystals with exposed (001) facets from different raw materials as reviewed in the previous section, while little experimental work on the synthesis of the highest surface-energy (110) facets on anatase TiO2 single crystals has been reported. Recently, Liu et al. have reported on the synthesis of anatase TiO2 single crystals with exposed (001) and (110) facets from Ti powder employing a mixture of water, hydrogen peroxide, and hydrofluoric acid as the reactive medium (see Fig. (16)) [72]. Metallic Ti reacted with HF forming TiO2 under hydrothermal conditions [73] and according to theoretical calculations fluoride ions reduced the surface energy of the (001) surface to a level lower than that of the

Fig. (16). (a) XRD pattern of the anatase single crystals. (b) FESEM image of the anatase single crystals. (c) Interfacial angle between (001) and (101) facets. The white dashed lines indicate the (001) and (101) crystal planes of anatase TiO2, respectively. (d) Small rhombus (110) facets (indicated by red circle) of the anatase single crystals. (e) Schematic diagram of the anatase single crystal. Scale bars in (b), (c) and (d): 1 mm. Reproduced from ref.[72] by permission of The Royal Society of Chemistry.

However, although theoretical studies demonstrated that anatase (100) facets are more active and accordingly exhibit higher catalytic activity than (001) and (101) facets [75, 76], they have been rarely synthesized. Based on a theoretic consideration Barnard et al. have reported that hydroxyl groups


are able to lower the surface free energy of (100) facets [52, 77]. However, this process has never been verified experimentally, because most available titanium precursors are very reactive under basic conditions. Recently, Li and Xu have synthesized single-crystalline anatase nanorods with a large percentage of higher-energy {100} facets by hydrothermal treatment of Na-titanate nanotubes (see Fig. (17)) [78]. Upon hydrothermal treatment of the Na-titanate nanotubes the transformation to anatase TiO2 single crystals proceeded by dissolution and nucleation. At the beginning of the transformation the surface of the Na-titanate nanotubes was gradually decomposed and Ti(OH)4 fragments, hydroxyl and sodium ions were produced under the hydrothermal condition employed in this study. Subsequently, the Ti(OH) 4 fragments were rearranged through dehydration reaction between Ti–OH and HO–Ti so as to share edges, resulting in the formation of anatase crystal nuclei. Finally, these TiO2 nuclei gradually grew while Ti(OH)4 fragments diffuse to their surface forming nanocrystalline TiO2. Due to the anisotropy in adsorption stability of the capping reagents, the adsorbates were bound onto certain crystallographic planes more strongly than on others, lowering the surface energy of the bound plane and, therefore, hindering the growth of crystals or some crystal planes. According to theoretical calculations [52, 77], under alkaline conditions O-terminated (100) facets exhibit lower surface free energy than O-terminated (101) facets and (001) facets. Therefore, the hydroxyl ions gradually released from Na-titanate nanotubes were preferentially adsorbed onto anatase (100) facets lowering their surface energy and thus limiting the crystal growth along the a- and b-axis.

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rutile unit cell. The synthesis of rutile TiO2 rods exposing (110) and (111) facets have also been reported by Bae et al., [82].

Fig. (17). (a) Lower and (b) higher magnifications SEM images of tetragonal faceted nanorods. The inset of (b) is the top view of a single tetragonal faceted nanorod. Reproduced from ref.[78] by permission of The Royal Society of Chemistry.

3.2. Synthesis of Rutile Nanoparticles with Defined Structure 3.2.1. Rutile Particles with (110) and (111) Facets Rutile (110) facets are the most stable ones and a high percentage of them is usually encountered during the synthesis of rutile TiO2 nanomaterials [9, 79]. During crystal growth, a fast growing plane generally tends to disappear leaving behind slower growing planes with lower surface energies. The (110) plane of rutile therefore survives during the growth. According to the atomistic simulation reported by Oliver et al. [80], rutile TiO2 shows four surfaces of (011), (110), (100), and (221) with surface energies of 1.85, 1.78, 2.08, and 2.02 J m-2, respectively, in the relaxed equilibrium morphology. In contrast to the calculations performed by Oliver et al., Kakiuchi et al. have reported the synthesis of rutile-type TiO2 rods through heterogeneous nucleation in TiCl3 aqueous solutions in presence of high concentration of NaCl at low temperatures of 80 or 200 ºC under hydrothermal conditions [81]. These rods are characterized by fast growth of the (110)-faceted surface within 1.5 h followed by the formation of the (111)-faceted top surface. After the deposition for 24 h, the tops of all the rods are pyramidally capped, as shown in Fig. (18). A TEM analysis of these rods indicates that the plane of the pyramid has an angle of approximately 130º with the lateral (110) plane. The most appropriate plane is considered to be (111) under this condition because the angles between (110) and (101), (110) and (111), and (110) and (221) planes are calculated to be 112.5º, 132.3º, and 151.2º, respectively, using the tetragonal

Fig. (18). (a) An FE-SEM and (b) a TEM image of the deposits obtained by the reaction at 200 ºC for 24 h. Reprinted with permission from ref.[81]. Copyright (2006) Elsevier.

In a similar synthesis process, rutile TiO2 nanorods with specific exposed crystal faces(110) and/or (001) facets have been prepared by hydrothermal treatment of aqueous TiCl3 and NaCl mixture in presence of poly(vinyl pyrrolidone, (PVP) as a shape-controlling reagent. Preferred adsorption of PVP occured on (111) and (001) rutile crystal faces, resulting in the formation of the (001) face of TiO2 nanorods.[83] Instead of using just Cl- anions as additive, Murakami et al. recently have used a two-step synthesis process, i.e., hydrolysis followed by hydrothermal treatment, using TiCl3 as


titanium precursor and Cl- and ClO4- anions as additives.[84] They have found that the hydrolysis of TiCl3 in presence of Cl- or ClO4- anions or both of them for one week resulted in the formation of rutile needles, however, the shape, the size, and the aggregation state of the products strongly depended on the kind and concentrations of the additive employed during crystal growth. The observed changes in the needle-like structure have been attributed to the effect of both Cl- and ClO4- anions during nucleation. Cl- has a larger influence than ClO4- during the hydrolysis because titanium complexes coordinated with ClO4- ions are not stable [85]. In the second step, the hydrothermal treatment did not induce the formation of different crystal phases; instead, it increased the crystallization of the rutile phase and induced the formation of rod-like particles with exposed crystal faces attributed as (110) side faces and (111) and/or (001) edge surfaces. The rod length primarily depended on the additive used in the preceding hydrolysis step. The samples initially hydrolyzed in the presence of ClO4- ions consisted of relatively long rods, while the samples hydrolyzed in the presence of Clions consisted of rods whose length was strongly depending on the Cl- concentration employed in the first step. The addition of ClO4- ions during the hydrothermal steps retarded sintering and further growth of rutile TiO2 rods. Direct hydrothermal synthesis of TiO2 particles employing ClO4- as the additive led to the formation of the anatase phase whereas in the presence of Cl- anions only rutile nanorods were formed. ClO4- anions both induced anatase formation and changed the shape of the rutile TiO2 particles as a result of retardation of crystal growth along the [001] direction, presumably due to the coordination of ClO4- ions with a specific site in the nucleation process. Thus, the first synthesis step has been proposed to be important in that the anatase formation and the crystal growth along the [001] direction are being retarded in the presence of ClO4- ions under the hydrothermal conditions employed in the second step of the synthetic sequence. Several groups have prepared rutile needles with urchinlike structures by hydrolysis, hydrothermal, and microwave assisted hydrothermal processes from TiCl3, titanium(IV) chloride (TiCl4), and titanium(IV) oxychloride (TiOCl2) precursors [52, 86-88]. Although the concentration of chloride ions (Cl-) in the precursor solution is considered as the crucial parameter to achieve morphological control over the rutile nanocrystal [86, 89], rutile TiO2 nanorods have been recently prepared employing aqueous solutions of titanium bis(ammoniumlactate) dihydroxide (TALH) at natural pH (8.0) without any additives (see Fig. (19)) [90]. Condensation-dehydration reaction between the -OH ligands of the TALH complex leading to the formation of TiO-Ti oxo species followed by competing formation of anatase and rutile seeds were proposed as the first step followed by anisotropic growth of the rutile TiO2 seeds as the thermodynamically more stable phase, accompanied by the consumption of the small anatase TiO2 nanoparticles. The anisotropic crystal growth along the c-axis (001) direction of the rutile TiO2 seeds led to the formation of rutile nanorods with primarily (110)-type facets as confirmed by TEM analysis. This was explained by the well-established fact that during crystal growth a fast growing plane with a high surface energy tends to disappear, leaving behind slower grow-


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ing planes with lower surface energies. It is known that the (110) plane of rutile TiO2 has the lowest surface energy whereas the (001) plane has the highest surface energy. Additionally, the anisotropic crystal growth observed during the thermal hydrolysis of the TALH complex might be facilitated by the presence of lactate anions acting as chelating ligands attached to the surfaces of the nanoparticles and thus possibly inhibiting the growth along other crystallographic directions [91].

Fig. (19). Shape analysis of as-synthesized nanocrystalline TiO 2 powder obtained by thermal hydrolysis of aqueous solutions of the TALH precursor for 48 h: (a) TEM bright-field micrograph, (b, c) high resolution TEM micrographs, (d) Fourier transform of (c), and (e) sketch of the shape. Reprinted with permission from ref.[90]. Copyright (2010) American Chemical Society.

3.2.2. Rutile Particles with (110), (011), and/or (001) Facets Rutile TiO2 particles with 1 m size and developed crystal faces have been synthesized by Ohno et al., [92]. The microscopic analysis of these particles indicated that the rutile particles exhibit a tetragonal prism structure with four planes assigned to the (110) faces. Each end of the prism was capped by four planes assigned to the (011) faces as shown in Fig. (20). Films of crystalline (nearly single-crystalline) rectangular parallelepiped rutile TiO2 in a submicrometer scale have been prepared by hydrothermal treatment of TiCl3 in the presence of high concentration of NaCl (10 M) [79]. These parallelepipedoid rutile exhibits remarkably enhanced (101) and (002) diffraction peaks indicating that the film was oriented against the substrate surface as confirmed by XRD analysis. The TEM analysis of these parallelepiped crystals indicated that the crystals were grown along the [001]-axis that is perpendicular to the [110]-axis. This explained the


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through adsorption of Cl- onto the (110) plane of rutile TiO 2 [96]. The growth of TiO2 can then be suppressed and accelerated in the [110] and the [001] directions, respectively. Oriented, single-crystalline rutile TiO2 nanorods on FTO substrates were also prepared by hydrothermal treatment of titanium butoxide in presence of HCl (see Fig. (21)) [97]. XRD analysis of these films indicated that the (002) diffraction peak was significantly enhanced. Some diffraction peaks including (110), (111), and (211) were absent indicating that the as-deposited film was highly oriented with respect to the substrate surface and that the TiO2 nanorods grow in the [001] direction with the growth axis parallel to the substrate surface normal. The absence of diffraction peaks that are normally present in polycrystalline or powder samples is a strong indication that the nanorods are not only aligned but are also single crystalline throughout their length.

Fig. (20). SEM and TEM images, and electron diffraction patterns (from top to bottom) of a rutile particle. Reproduced from ref.[92] by permission of The Royal Society of Chemistry for the Centre National de la Recherche Scientifique.

enhanced (002) peak which resulted from the crystals that were orientated perpendicular to the substrate surface, while the (101) peak was assigned to 33º-inclined crystals. The presence of molecular oxygen during the synthesis process was found to be necessary for the film formation. The film deposition was promoted in the presence of oxygen-saturated solution, while the nitrogen-saturated solution did not lead to the formation of the rutile film. Ti(IV) oxo species are usually assumed to be an intermediate between TiO2+ and TiO2, consisting of partly dehydrated polymeric Ti(IV) hydroxide [93]. TiCl3 is hydrolyzed producing TiOH2+ which needs to be further oxidized to Ti(IV) oxo species. This oxidation process can be a reaction between TiOH2+ and dissolved molecular oxygen. The film deposition from the solutions followed heterogeneous nucleation driven by lower degrees of supersaturation [94]. A low degree of supersaturation by the Ti(IV) oxo species was achieved by a slow oxidation process consuming dissolved oxygen. Films were not deposited when aqueous TiCl4 solutions were used. Furthermore, the aqueous TiCl3 solution without added NaCl led apparently to higher degrees of supersaturation, resulting in homogeneous nucleation of TiO2 as precipitated powders. The role of NaCl is a twofold: First retarding formation of TiO 2 by changing the composition or coordination structure of the growing unit [95], and second, influencing the morphology

Fig. (21). FESEM images of oriented rutile TiO2 nanorod film grown on FTO substrate: (a) top view, (b) cross sectional view, (c) and (d) tilted cross-sectional views. Reprinted with permission from ref.[97]. Copyright (2009) American Chemical Society.

3.3. Synthesis of Brookite Nanoparticles with Defined Structure Because of the relatively narrow energy range of stability for brookite, reaction conditions that can guarantee both selective nanocrystal nucleation and growth in this crystal phase are inherently more restrictive than those required for obtaining either anatase or rutile [98]. The conventional aqueous precipitation or hydrothermal approaches frequently yield brookite as a minor contamination byproduct, while a number of experimental variables (e.g., water to precursor ratio, pH, catalysts, ionic strength, nature and concentration of chelating agents, temperature) need to be delicately adjusted in order to increase phase selectivity, however, at the cost of reproducibility [3]. Buonsanti et al. have developed a surfactant-assisted nonaqueous strategy by which highquality anisotropically shaped TiO2 nanocrystals are synthesized in the exclusive brookite crystal structure with geometric features tunable over an exceptionally wide dimensional range (30-200nm) (see Fig. (22)). This method relies on suitable temporal manipulation of the aminolysis reaction of titanium oleate complexes, which takes place during the high-temperature treatment of mix-



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regime. The time variation of the chemical potential for the monomers in the solution, the size dependence of thermodynamic structural stability of the involved TiO2 polymorphs, and the reduced activation barrier for brookite nucleation onto initially formed anatase seeds are considered to play key roles in the phase-controlled growth of the nanostructures by this approach. Recently, Kandiel et al. have synthesized high quality brookite nanorods by thermal hydrolysis of aqueous solutions of titanium bis(ammonium lactate) dihydroxide (TALH) in the presence of high concentrations of urea ( 6.0 M) as the in situ OH- source [99]. Biphasic anatase/brookite powders were obtained at lower urea concentrations. The ratio between anatase and brookite was readily tailored by controlling the urea concentration as confirmed by XRD and SEM measurements (see Fig. (23)).

Fig. (22). Low- and high-magnification (left and right panels, respectively) TEM overview of TiO2 nanocrystals synthesized at higher oleyl amine/oleic acid ratios. Reprinted with permission from ref.[98]. Copyright (2008) American Chemical Society.

tures of titanium tetrachloride, oleic acid, and oleyl amine under water- and oxygen-free conditions. Simultaneous tailoring of the size, themorphology, and the crystal-phase of TiO2 nanostructures was demonstrated by means of a precursor supply technique combining a slow-heating step with subsequent delivery of additional reactants at a controllable low rate. The formation of brookite nanorods has been explained within the frame of a self-regulated phase-changing seed-catalyzed mechanism, in which homogeneous nucleation, on one side, and heterogeneous nucleation and growth processes, on the other side, are properly balanced while switching from the anatase to the brookite structures, respectively, in a continuous unidirectional crystal development

On the basis of spacing and angles of the lattice fringes, rod-like particles with a diameter of about 25 nm were identified to be of the brookite structure [12, 100]. The nanorod is imaged along the [-1-2-1]B zone axis, and it is faceted by (210)B, 0.35 nm; (111)B, 0.35 nm; and (-110)B, 0.45 nm. Also, a small facet of (012)B occurs just above the tip (see Fig. (24)). On the basis of the analysis of fast Fourier transforms (FFTs) of the image area, all these facets are drawn as contour lines into the experimental micrograph according to the indexing given in the inset. The longitudinal growth direction appears to be along [001]B, which is the c-axis, and which is tilted by ca. 25º out of the image plane. Adherent to the tip of the brookite nanorod, there is a smaller particle, which is identified as anatase imaged along [1-11]A. The anatase particle exhibits (0-11)A and (10-1)A planes, which share an angle of 98º and show spacings of 0.35 nm. Brookite TiO2 nanoflowers consisting of single crystalline nanorods have been synthesized by hydrothermal treatment of tetrabutyl titanate (TBOT) hydrolyzed in a solution of sodium chloride and aqueous ammonia [101]. The crystalline phase of the TiO2 products was changed in a step-wise fashion by adjusting the concentration of NaCl in the hydrothermal system. When TBOT alone was directly hydrolyzed and hydrothermally treated in aqueous ammonia, the product synthesized was almost pure anatase TiO2. Adding a small amount of sodium chloride led to the formation of brookite/anatase mixtures. By increasing the concentration of NaCl, pure brookite TiO2 was obtained when the concentration of sodium chloride was about 0.25 M. A formation mechanism for the brookite TiO2 nanoflowers that follows Ostwald’s step rule was proposed [102].

Fig. (23). FE-SEM micrographs of as-synthesized nanocrystalline TiO2 powders obtained by thermal hydrolysis of aqueous solutions of the TALH precursor at 160 ºC for 24 h in the presence of different concentrations of urea: (a) 0.1 M, (b) 1.0M, (c) 2.0 M, and (d) 6.0 M. Reprinted with permission from ref.[99]. Copyright (2010) American Chemical Society.


Fig. (24). High-resolution transmission electron micrograph of assynthesized nanocrystalline TiO2 powder obtained by thermal hydrolysis of aqueous solutions of the TALH precursor at 160 ºC for 24 h in the presence of 1.0 M urea. The rodlike particle of ca. 25 nm in diameter is brookite imaged along the [-12-1]B zone axis (see inset). The smaller particle of ca. 15 nm in size is anatase imaged along [1-11]A. The labels A and B refer to anatase and brookite, respectively. Reprinted with permission from ref.[99]. Copyright (2010) American Chemical Society.

Hu et al. have also synthesized brookite flowers by hydrothermal treatment of titanate at adjusted concentrations of Na+ and OH- species [103]. The synthetic conditions resulting in the formation of flower-like brookite were monitored by a series of time-resolved experiments. On the basis of these time-resolved experiments, three steps were proposed for the formation of the flower-like brookite TiO2: (1) the transformation of layer titanate into brookite nanoparticles, (2) the growth of brookite particles up to a spindle-like shape, and (3) the assembly of these spindle-like particles into flower morphology. In the first step, the layered titanate showed TiO6 octahedral layers held by the strong static interaction between Na+ cations and TiO6 units, and were therefore assumed to act as the key precursor to brookite TiO2 nanophases such as nanotubes [104] and brookite TiO 2 nanoparticles [105]. The Na+ or H+ ions located in the interlayer spaces of the layered titanate could be gradually released disturbing the static interaction and introducing the internal structural tension. With an increase of this tension, the layered structure would become unstable and finally transform into anatase under acidic condition or brookite under alkaline condition. In the subsequent process, spindleand flower-like brookite TiO2 were formed, which underwent a preferential coarsening of the nanoparticles governed by the surface charge and surface energy. The equilibrium morphology of nanophases could be spherical when considering the liquid model. Namely, if the surface energy is isotropic, the obtained crystal should be nearly spherical, while if the surface energy is anisotropic, the energy-minimizing shape could be formed by the limiting planes of the possible lowest surface energy. The observed spindle-like shape should be related to the surface energy difference of the lattice planes, since as indicated by a theoretical study, among

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(100), (010), (001), (110), (011), and (111) planes of brookite TiO2, the (001) plane holds the smallest surface formation energy of 0.62 eV, while the (100) plane exhibits a larger value of 0.88 eV [106]. Based on this calculation it was suggested that brookite crystal prefers to grow along the [001] direction to give a nonspherical morphology. On the other hand, coarsening of brookite TiO2 nanoparticles strongly depends on the surface charges. Once the brookite TiO2 nanocrystals are formed, Na+ and OH- ions in the reaction solution adhere at the brookite surfaces destroying the local equilibrium concentrations to give rise to gradient distributions of charges [107]. Thus, the bigger particles grow up to show a spindle-like shape at the expense of the shrinkage of the smaller particles, typical for the Ostwald ripening process [107]. With a prolonged aging time, these spindlelike brookite TiO2 particles weld together reducing the surface energy and finally producing brookite flowers. Hu et al. have also found that pH values and Na+ are two key factors for the formation of brookite flowers. When the pH value was reduced slightly to 12 and the reaction time kept the same at 48 h, the product was nearly pure brookite TiO2 but with obviously weakened assembly of particles. As discussed above, the assembling of small particles affects the surface charge of the crystallites. Decreasing the pH value reduced the concentration of Na+ and OH- ions further weakening the interactions of small spindle-like particles. Therefore, the building rate of brookite flowers was lower at pH 12 compared to that at pH 12.5. When NaOH was replaced by LiOH or KOH, the products did not contain any brookite phase but a mixture of anatase and rutile with a small amount of unknown phases even though the solution was kept at pH 12.5. Brookite TiO2 is thus transformed from layered titanate. When LiOH or KOH was introduced into the reaction system, an ionic exchange process between Na+ ions in the TiO2 precursor and Li+ (or K+) ions in the solution occur. The newly formed layered titanates containing Li+ (or K+) were transformed into anatase or rutile under hydrothermal conditions. From these formation reactions, it was assumed to be likely that alkali ions play an important role in nucleation or nucleus growth of given phases of TiO2. It is well documented that sodium and potassium ions could lead to the formation of brookite and anatase, respectively. Therefore, the authors concluded that sodium ions promote the brookite nucleation, while other alkali ions prefer the nucleation of anatase and rutile, thus explaining why pure brookite TiO2 is difficult to obtain without having sodium involved. Employing titanate Na2H2-xTi3O7 nanotubes, brookite nanotubes have been synthesized (see Fig. (25)) [104]. It is well known that titanates are composed of edge-sharing [TiO6 ] octahedral layers [108, 109], and the interspaces between the layers are occupied by Na+ or H+. Nagase et al., [105] reported the hydrothermal synthesis of brookite TiO2 as an almost single phase at 200 ºC with a TiO2/NaOH mole ratio = 1. They considered that the brookite phase can be formed from sodium titanate by releasing Na+ and H+ from the surface accompanied by oxidation of Ti in the structure. Thus, Na+ and H+ located in the interlayer spaces of titanate can be released and consequently the brookite TiO2 phase is formed. A similar explanation for the formation of brookite nanotubes has been given by Deng et al., [104].



TiO 2 hv eCB + h+VB

(1)

eCB + h+VB heat

(2)

h+VB + D D•+

(3)

h+VB + TiIVOH/H 2 O TiIVOH•+ /(OH• + H+ ) TiIVOH•+ + D D•+ + TiIVOH •

e

Fig. (25). SEM (a, b) and TEM (c, d) images of the brookite nanotubes synthesized at 200 ºC for 20 h at pH 10.9. Reproduced from ref.[104] by permission of The Royal Society of Chemistry.

4. APPLICATIONS OF SHAPE-DEFINED TiO2 NANOPARTICLES AND STRUCTURES IN PHOTOCATALYSIS

Titanium dioxide as a semiconductor can act as a sensitizer for light-induced redox processes due to its electronic structure, which is characterized by a filled valence band and an almost empty conduction band being an n-type semiconductor. Absorption of a photon with an energy exceeding the bandgap energy leads to the formation of an electron/hole pair (Eq. 1). In the absence of suitable scavengers, the stored energy is dissipated within a few nanoseconds by recombination (Eq. 2). If a suitable scavenger or surface defect state is available to trap the electron or hole, recombination can be prevented and subsequent redox reactions may occur. The valence band holes are powerful oxidants (~ +3.0V vs. NHE) [120], while the conduction band electrons are rather good reductants (0.0 to -0.5 V vs. NHE depending on the TiO2 phases). Most organic photodegradation reactions (electron donor, D) utilize the oxidizing power of the holes either directly through the reaction with the photogenerated holes (Eq. 3) or indirectly through the reaction with the so called trapped holes (TiIVOH•+) or with •OH radicals formed as highly reactive intermediates (Eqs. 4, 5, and 6); however, to prevent a buildup of charge, a reducible species (electron acceptor, A) must also be present to react with the electrons (Eq. 7).

(6)

•

+ A A

(7)

At the surface of bulk semiconductor electrodes only one species, either the hole or the electron, is available for reactions due to band bending. However, in very small semiconductor particles both species are present on the surface. Therefore, careful consideration of both, the oxidative and the reductive paths is required. Fig. (26) shows a schematic presentation of the photocatalytic processes [121, 122]. It consists of a superposition of the energy bands of TiO2 semiconductor (valence band VB, conduction band CB) and the geometrical image of a spherical particle. Absorption of a photon with an energy h greater or equal to the bandgap energy Eg generally leads to the formation of an electron/hole pair in the semiconductor particle. These charge carriers subsequently either recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and acceptors adsorbed on the surface or present within the electrical double layer. E eV

4.1. Photocatalysis Employing TiO2 A number of review papers have summarized the current knowledge about the mechanism of photocatalytic reactions both, from a theoretical and an experimental perspective [110-119]. It is commonly accepted that the reaction progresses via a multi-step route starting with photon absorption, exciton formation, charge separation and charge transfer to the electrolyte species at the anodic and the cathodic sites. Other processes that may occur include charge recombination or trapping, reactant adsorption and desorption.

(4) (5)

OH + D D•+ + OH CB

109

A E CB

0 Eg

+1.5 +3

CB A

Ef hn > Eg

E VB VB

E

VS.

NHE / V

D

D

Fig. (26). Principle mechanism of photocatalysis and superposition of the energy band diagram with the geometrical image of a spherical TiO2 particle. Note: for simplicity, the indirect oxidation of electron donor (D) by trapped holes (TiIVOH•+) or by •OH radicals is represented by the hole oxidation step.

4.2. Photocatalytic Activities of Shape-defined TiO2 with Specific Exposed Faces Photocatalytic reactions on TiO2 nanoparticles and structure have been utilized in various applications, including environmental remediation [110, 113] and hydrogen production [123-126]. The extensive studies on TiO2 photocatalysts over the past decades indicate that the photocatalytic processes are complex and the activities depend on many parameters such as crystal structure, specific surface area, particle size, defect density, light absorption, crystallinty and exposed faces [6, 122, 127-133]. The latter effect has attracted attention in the last years due to the significant progress in the synthesis of shape-defined TiO2 nanoparticles and nanostructure with specific exposed faces. In the follow-


ing section some results are presented showing the important of the kind of exposed face on the photocatalytic activities of different TiO2 polymorphs. 4.2.1. Rutile Particles As a result of the progress in the preparation of rutile nanorods with defined exposed crystal faces, a correlation between the photocatalytic activities and the surface structure has recently been reported. For example, Ohno et al. have investigated the photocatalytic activity of shapecontrolled rutile rods predominantly exhibiting (110) and (111) exposed crystal faces, which were considered, respectively, as reduction and oxidation sites as confirmed by the photodeposition of Pt and PbO2 (see Fig. (27)) [82, 83]. These rods showed higher photocatalytic activity for degradation of acetaldehyde and toluene than did commercial TiO2 particles. However, the photocatalytic activities of the synthesized TiO2 particles for the degradation of acetaldehyde in the gas phase were different from those for the degradation of toluene. These higher activities of rutile nanorods have been explained by enhanced separation of electrons and holes due to the separation of the oxidation and reduction sites. To further investigate this hypothesis, the photocatalytic activities of rutile rods with different aspect ratios due to preferential growth along the [001] direction have been investigated. The photocatalytic activities for decomposition of acetaldehyde were found to be increased with decreasing aspect ratio (i.e., increasing number of oxidation sites) due not to an increase in the number of adsorption sites but rather to a more optimal ratio of the surface areas of oxidation to reduction sites as proposed by Murakami et al.[84]

Fig. (27). TEM image of rutile TiO2 nanorod on which Pt and PbO2 particles were deposited. Reprinted with permission from ref.[82]. Copyright (2009) Elsevier.

4.2.2. Anatase Particles Recently, it has been reported that largely truncated octahedral shape with two square (001) facets such as decahedral-shaped anatase TiO2 particles exhibit high photocatalytic activities because the (001) surface is more reactive for dissociative adsorption of reactant molecules compared with (101) facets. Amano et al. have investigated the photocatalytic activities of such materials employing the photocatalytic hydrogen production from aqueous methanol solutions

Kandiel et al.

and the photocatalytic oxidative decomposition of organic compounds, i.e. acetic acid and methanol, as test reactions under different conditions [62]. They have found that the faceted decahedral single-crystalline anatase particles with sizes of 50-250 nm exhibit photocatalytic activities higher than or at least comparable to that of Degussa P25 which is a commercial standard photocatalyst, although these particles have a relatively small specific surface area (9.4 m2 /g) compared with that of P25 (48 m2/g). The low density of crystalline defects has been proposed to be the reason for the extremely high photocatalytic activity observed during the H2 evolution from an aqueous methanol solution and during the oxidative decomposition of an organic compound in an aqueous solution. As the photocatalytic activities are expected to be strongly depending on the exposed face as well as on the test reaction employed, anatase crystallites with abundant (101) facets have been also investigated. These anatase particles have shown a high activity for the photocatalytic oxidative decomposition of organic compounds in the presence of molecular oxygen, while, in the absence of molecular oxygen, these anatase particles with exposed (101) surface are not as effective as those with other surfaces for H2 evolution [42]. Li et al. have studied the photocatalytic activities of anatase TiO2 nanocrystals with different morphologies and exposed facets, i. e., octahedral particles, truncated octahedral particles and spindle-like particles, using the photocatalytic oxidative decomposition of methylene blue (MB) under UV irradiation [43]. For comparison, the photocatalytic activity of Degussa P25 has also been measured. They found that octahedral particles exhibit the highest photocatalytic activity, and the photodecomposition rate could be sequenced as octahedral particles > truncated octahedral particles = P25 > spindle-like particles. By using photocatalytic rates, normalized to the surface area, they found that the reaction rates of the samples with morphologies of octahedral, truncated octahedral, and spindle are 5.2, 2.0, and 1.3 times that of the Degussa P25, respectively. The truncated octahedral sample did not exhibit high photocatalytic activity, probably because the high energy (001) facets are terminated by fluorine atoms and the real surface energy could be sequenced as the F-001 < F-101 facet [54]. The high photocatalytic activity of the anatase particles with a lower-energy (101) facet exposed was attributed to their high crystallinity and lower number of defects on the surfaces of the perfect crystals. Pan et al. have synthesized anatase TiO 2 single crystals dominated by the (010), (001) and (101) exposed surfaces, respectively, and their respective photocatalytic activities have been investigated by evaluating the amount of OH radicals generated and the rate of hydrogen evolution [134]. The three samples with fluorine-terminated surfaces show very similar abilities for generating OH radicals and their hydrogen-evolution rates are also nearly identical. After removing the surface fluorine atoms, improved photocatalytic activities have been observed and the order of photoreactivity was found to be (010) > (101) > (001) in the photooxidation and reduction reactions. This difference in the photoreactivity has been explained to depend on the difference in the surface structure and electronic structure of the different surfaces. A lower activity of nanosheets with the (001) exposed surface as compared with P25 has been observed by Han et al. in the first cycle of a photocatalytic test reaction (degradation of methyl orange), which was repeated



111

several times [45]. A gradually accelerating degradation rate and also that the degradation efficiency was higher than that of P25 TiO2 after the first degradation cycle. The abnormal performance of the TiO2 nanosheets in the first cycle was explained by the presence of superficial inorganic species and organic adsorbates on the TiO2 nanosheets. After the first cycle, the inorganic species were desorbed from the surface and the organic adsorbates degraded, resulting in increasing degradation rates in the following degradation cycles. This hypothesis was also verified by directly cleaning the TiO2 nanosheets with 0.1 M NaOH, during which the adsorbed fluorine ions on the surface of TiO2 were replaced by hydroxyl groups. The degradation efficiency of the TiO2 nanosheets after cleaning with alkaline solution was found to be remarkably improved. Conversely, the degradation rates of TiO2 P25 were gradually decreased after three degradation cycles, possibly because of the adsorption of incompletely photodecomposed products at the surface of the P25. Singlecrystalline anatase TiO2 nanobelts with two dominant surfaces of (101) facets have shown enhanced photocatalytic activity as compared with their nanosphere counterparts exhibiting identical crystal phase and similar specific surface area [135]. An ab initio density functional theory (DFT) calculation has shown that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O2, facilitating the generation of superoxide radical which is considered as a pathway for oxidation of the organic compounds beside the •OH radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals as concluded from the DFT calculation; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O2 on the (101) facet. The lower electron-hole recombination rate of the nanobelts as confirmed by measuring the photoluminescence spectra directly arising from the radiative recombination processes originates from the following factors: (i) the charge carriers have higher mobility in the nanobelts than in the nanospheres; (ii) the single-crystal nature along the longitudinal dimension of the nanobelts provides a large space as the pathway for charge separation, while the nanospheres are confined by the space; (iii) the anatase nanobelts with two dominant surfaces terminated by the (101) facet have higher reactivity with molecular O2 than the nanospheres. More superoxide anions are formed by trapping the photogenerated electrons on the (101) faceted nanobelts. Thus more charge carriers are spatially separated on the nanobelt surface.

In the recent years, the tremendous effort put into the synthesis of TiO2 nanomaterials has resulted in the preparation of nanosized TiO2 with well-defined shape and with specific exposed surfaces. Most frequently hydrothermal processes employing mostly fluoride and chloride ions as surface adsorbates for the stabilization of specific surfaces of anatase and rutile single crystals, respectively, were employed. Sodium or potassium titanate structures have also been used as starting materials due to their controllable conversion either to shape defined anatase single crystal or to brookite structure as a result of the release of sodium or potassium ions. Photocatalytic investigation of these nanomaterials showed that they have, in most cases, higher photocatalytic activity than TiO2 P25 despite having lower surface area. The (001) surface has been proposed as the most active surface of TiO2 nanomaterials during the photocatalytic process, however, some reports indicate that the (001) surface exhibits less activity than the (101) and the (010) surface. Synthesis of shape defined TiO2 nanomaterials with different ratios of the exposed specific surfaces and measurement of their photocatalytic activities under the same conditions employing different test reactions are still needed in order to clarify the photocatalytic activities of the different exposed surfaces.

4.2.3. Brookite Particles

REFERENCES

Due to the difficulty encountered in the preparation of shape-defined brookite nanomaterials with specific exposed faces, there is still scarce information about the photocatalytic activity of their different surfaces. Recently, high quality brookite nanorods have been prepared and their photocatalytic activities have been assessed [99]. They show higher activity toward hydrogen production from aqueous methanol solution than anatase nanoparticles prepared under the same conditions despite the latter having higher surface

area. The further characterization of these brookite nanorods indicates that their flatband potential is shifted by 140 mV more cathodically than the flatband potential of the anatase nanoparticles. Thus, in the case of the brookite nanorods, the driving force for the proton reduction is higher than that in the case of the anatase nanoparticles. This explains why brookite nanorods exhibit higher photocatalytic activity than anatase nanoparticles despite their lower surface area. Recently, many reports showed that brookite exhibits comparable photocatalytic activities to that of anatase and even more in some test reactions [101, 131]. 5. CONCLUSIONS

CONFLICT OF INTEREST Author’s declare having no potential competing financial interests. ACKNOWLEDGEMENT Financial support from the Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged (Grant No. 60420819). T. A. Kandiel thanks the Chemistry Department, Faculty of Science, Sohag University for granting him a leave of absence.

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Revised: December 03, 2011

Accepted: December 03, 2011