Solvothermal Synthesis of TiO2

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Solvothermal Synthesis of TiO2 Nanocrystal Colloids from Peroxotitanate Complex Solution and Their Photocatalytic Activities Jianhua Liao, Liyi Shi,* Shuai Yuan,* Yin Zhao, and Jianhui Fang Research Center of Nanoscience and Nanotechnology, Shanghai UniVersity, 99 Shangda Road, Shanghai 200444, People’s Republic of China ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: August 13, 2009

TiO2 nanocrystal colloids with different shapes and sizes have been prepared from a peroxotitanate complex solution starting from titanium(IV) sulfate and hydrogen peroxide (H2O2) in water/alcohol media by a solvothermal process. The TiO2 nanocrystals were characterized by transmission electron microscopy, highresolution transmission electron microscopy, X-ray diffraction, FT-IR spectroscopy, zeta potential, and nitrogen adsorption. The TiO2 nanocrystal colloids prepared by a solvothermal process at 120 °C show a highly crystallized anatase phase. The particle sizes decreased with the increase of the alcohol/water ratio. The shapes of the particles have been controlled from the “rodlike” in pure water solvent to a “rectangular” shape in pure ethanol solvent. The photocatalytic activities of TiO2 nanocrystals were evaluated by the degradation of phenol. The photocatalytic activity of the anatase nanocrystal was closely related to the particle diameter. In addition, the possible growth mechanism of this anatase was illustrated. 1. Introduction Nanocrystals have been widely studied because of their potential applications in solar energy conversion, batteries, catalysis, ductile ceramics, etc.1-4 The ability to prepare sizeand shape-controlled nanocrystals enables these crystals to be utilized as key components for the fabrication of advanced nanodevices and nanosystems, as the nanocrystals with tailored shapes have attracted extensive research interest in the past decade due to their many intrinsic shape-dependent properties.5 It is important to develop synthetic strategies that control both the size and the shape of nanocrystals with high crystallinity and structural morphologies. TiO2 is an important semiconductor material with numerous applications in photocatalysts, catalyst supports, solar energy cells, gas sensors, and pigments.6 These applications are dependent on the particle size, morphology, phase, and crystallinity. The potential application of TiO2 is expected to be remarkably extended if a fine-tuning of nanocrystal morphology is achieved.7 Consequently, designing TiO2 nanocrystals with different shapes and sizes is of significant importance. To date, various synthetic approaches have been developed to fabricate TiO2 nanocrystals, such as the sol-gel template,8 surfactantdirected method,9 hydrothermal technique,10 solvothermal method,11 chemical vapor deposition,12 and reverse micelle methods.13 Among these preparation methods, the solvothermal method normally has better control of the size and shape distributions and the crystallinity of the TiO2 nanocrystal because a great variety of organic solvents with high boiling points can be chosen.14 For example, Wu et al. obtained anatase TiO2 nanocrystal colloids of two different shapes by hydrolysis of titanium isopropoxide under hydrothermal conditions in an acidic or tetramethylammonium hydroxide (Me4NOH) basic environment at 210-270 °C.15 Yang et al. developed a sol-solvothermal process with the starting precursor of TiCl4 and Ti(OBu)4 mixture based on benzene-water interfacets for * To whom correspondence should be addressed. E-mail: shiliyi@ shu.edu.cn (L.S.), [email protected] (S.Y.). Fax: +86-21-66134852.

the fabrication of nanostructured TiO2 with various morphologies.16 Chae and his partners prepared the TiO2 particles with the range of 7-25 nm by a solvothermal reaction of titanium alkoxide in acidic ethanol/water solution at 240 °C.17 Chemseddine et al. controlled the crystal structure, size, shape, and organization of TiO2 nanocrystals by means of hydrolysis and polycondensation of Ti(OR)4 in the presence of tetramethylammonium hydroxide.18 Ryu et al. synthesized TiO2 particles from peroxotitanate solution using different amine group-containing organics that demonstrated well crystallinity after being calcined at 400 °C.19 Additionally, many reports have described the shapecontrolled synthesis of TiO2 nanocrystals in the presence of surfactants that selectively bind to specific crystalline facets. Jun et al. used a surface-selective surfactant (lauric acid) to synthesize TiO2 nanocrystals with the shape of a bullet, diamond, rod, and branched rod.20 Li et al. synthesized highly crystalline, near monodispersed TiO2 nanoparticles and nanorods in the presence of linoleic acid surfactant by solvothermal reactions.21 Cozzoli et al. fabricated shaped-controlled organic-capped TiO2 nanocrystals by hydrolysis of titanium tetraisopropoxide (TTIP) in oleic acid (OLEA) as surfactant at 80 °C and found that the presence of tertiary amines or quaternary ammonium hydroxides as catalysts promoted fast crystallization.22 So far, considerable progress has been achieved in the synthesis of TiO2 nanocrystal colloids, but further studies still have to be exerted for the control of particle size, shape, and crystallization. Besides, the convenience of its wide applications is very important because it is difficult to remove the organics that are introduced by the surfactant and the solvothermal media. Up to now, few reports demonstrated the ability to control the high purity, highly crystalline, and morphology of stable anatase phase TiO2 in the neutral range. Herein, we reported a solvothermal route using the peroxotitanate solution that is a benefit for high crystallinity to synthesize organic-free TiO2 nanocrystals with a controllable morphology in the mixed water/ alcohol solvent. The prepared anatase TiO2 nanocrystal colloids are structurally uniform, stable, of high purity, and show high

10.1021/jp905720g CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

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crystallinity in the neutral range. Additionally, we discussed the relationship between the TiO2 structure and the photocatalytic activity by the degradation of phenol. 2. Experimental Section 2.1. Materials. All chemicals were used as received without further purification. Titanium(IV) sulfate (Ti(SO4)2, CR) was provided from Shanghai Chemical Reagent Co., China. Aqueous ammonia solution (NH4OH, 25%, AR), aqueous hydrogen peroxide solution (H2O2, 30%, AR), absolute ethanol (C2H5OH, AR), n-propanol (n-C3H7OH, AR), n-butanol (n-C4H9OH, AR), and phenol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used for solution preparation. 2.2. Preparation of TiO2 Nanocrystal Colloids. In a typical synthesis, 6 g of titanium(IV) sulfate was diluted with deionized water to be a 0.2 M Ti(SO4)2 solution. Under vigorous stirring, 3 M ammonia solution was slowly added, adjusting the pH value to 7.5, which produced a white precipitate in the solution immediately. After aging for 20 h, the resulting suspension was filtered and washed until the electric conductivity of the filtrate was below 100 µs/cm. A purified white precipitate was then obtained. The obtained cake was redispersed in 125 mL of mixed water/alcohol solvent and was ultrasonically dispersed for 0.5 h. Under continuous magnetic stirring, 2.5 mL of H2O2 (30 wt %) was added dropwise to the suspension in an ice-water bath. The mixture was refluxed at 78 °C for 4 h to gain the solvothermal precursor. Finally, the resulting solvothermal precursor was transferred to a Teflon autoclave lined with Teflon, 70% filled and tightly closed, and then held at 120 °C for 15 h. After the solvothermal crystallization, the prepared TiO2 nanocrystal was filtered again and was redispersed in 250 mL of deionized water. The TiO2 nanocrystal colloid was thus obtained. 2.3. Characterization. The TiO2 powders were prepared through rotatory evaporation of the TiO2 colloids at 45 °C. FTIR spectra were measured on an AVATAR 370-IR spectrometer (Thermo Nicolet, America) with a wavenumber range of 4000 to 400 cm-1. The morphologies of the products were viewed by transmission electron microscopy (TEM). The XRD analysis was performed using a Rigaku D/MAX-2000 X-ray diffractometer at room temperature, operating at 30 kV and 30 mA, using Cu kR radiation (λ ) 0.15418 nm). The TEM images were recorded on a JEOL JEM-200 CX microscope at an acceleration voltage of 200 kV. The particle size was estimated by the measurement of at least 200 particles in the TEM images. The HRTEM images were obtained using a JSM-2010F microscope (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. The zeta potential of the TiO2 nanoparticles was measured on a Zetasizer 3000HS (Malvern Instruments Ltd., U.K.). The surface area of the TiO2 nanoparticles was determined using a nitrogen adsorption apparatus (model 3H-2000III, China). 2.4. Photocatalytic Degradation of Phenol. Phenol is a representative phenolic compound in aqueous solutions and a major industrial pollutant, which causes severe environment problems. Hence, extensive studies on the photocatalytic degradation of phenol have been studied.23 The photocatalytic activities of the different TiO2 nanocrystals were evaluated by photocatalytic degradation of phenol under UV light illumination. As the requirement of high crystallinity for the photocatalytic process, the TiO2 nanocrystals prepared in both pure n-propanol and butanol solvents were not considerable for photocatalytic experiments because of their poor crystallinities. The photocatalysts were obtained through rotatory evaporation of the TiO2 colloids at 45 °C and then dried at 110 °C for 1 h

Figure 1. FT-IR patterns of TiO2 nanocrystals prepared in (a) H2O and (b) ethanol before solvothermal treatment and in (c) n-propanol, (d) n-butanol, (e) ethanol, (f) H2O/ethanol ) 1:4, (g) H2O/ethanol ) 1:1, (h) H2O/ethanol ) 4:1, and (i) H2O after solvothermal crystallization.

in an oven to remove the solvents absolutely. The photocatalytic degradation was performed in an 80 mL test tube with 75 mL of reaction solution. The initial concentration of phenol was 10 mg/L, equivalent to 1.0 g/L of TiO2 in aqueous solution. The adsorption-desorption experiment was operated. The suspension was stirred in the dark for 30 min to obtain the saturated adsorption of phenol before UV light irradiation. The photocatalytic activities of different samples were evaluated in a tube, equipped with a magnetic stirring bar. During the photoreaction process, the mixed solution was irradiated by a UV lamp (16 W, 365 nm) with constant magnetic stirring. The phenol concentration of the solution was measured every 1 h after removing powders by centrifugation at 10 000 rpm for 30 min. The remaining phenol in the solution system was detected by a UV-2501 PC spectrometer at a wavelength of 200-400 nm. For comparison, the direct photolysis of phenol was experimented. The efficiency of degradation was calculated from formula 1 as follows

C/C0 ) A/A0

(1)

where A0 and A are the initial and final absorbance at 269 nm for phenol. 3. Results and Discussion 3.1. Structure Analysis. The FT-IR curves for as-dried powders of TiO2 particles are shown in Figure 1. The broad peaks at 3200-3400 cm-1 are due to water. The peak at 1630 cm-1 is attributed to the bending vibration of water molecules. The peak at 1400 cm-1 is attributed to the stretching vibration of the N-H bonds in NH4+ that remained.24 It is known that a stretching mode frequency of the first-order O-O bond (a peroxo group) is 890 cm-1.25 An absorption peak at 900 cm-1, which is attributed to the peroxo groups, is detected for the powders in pure water solvent (Figure 1, pattern a), whereas no peroxo groups signal peak appears in pure ethanol solvent

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Figure 2. XRD patterns of TiO2 nanocrystals prepared in (a) H2O and (b) ethanol before solvothermal treatment and in (c) n-propanol, (d) n-butanol, (e) ethanol, (f) H2O/ethanol ) 1:4, (g) H2O/ethanol ) 1:1, (h) H2O/ethanol ) 4:1, and (i) H2O after solvothermal crystallization.

(Figure 1, pattern b) before solvothermal treatment and all the samples after the solvothermal process. In pure water solvent, a transparent yellow colloid is obtained after adding H2O2 (30 wt %), whereas a straw yellow precipitation is formed in pure ethanol solvent and the buff suspension is obtained in the mixed solvent before the solvothermal process. The yellow peroxotitanate complexes are easy to form in pure water environment but not in ethanol solution. After solvothermal treatment, the peroxotitanate complexes decompose. Therefore, few peroxo groups are examined by FT-IR. A peroxotitanate complex solution, synthesized by neutral precipitation in different mixed solvent, was used as precursor for the solvothermal process to prepare TiO2 nanocrystals. Figure 2 shows the XRD patterns of the products. The peroxotitanate complex solution in the pure water and in the pure ethanol solvent before crystallinity treatment are mainly composed of an amorphous phase, as indicated in the XRD patterns of Figure 2, patterns a and b. The sizes of the nanoparticles are greatly dependent on the ratio between alcohol and water in the solvent system. The broad baseline observed in the powder XRD of Figure 2, patterns c and d, indicated the presence of a small quantity of amorphous phase. The TiO2 particles were relative poorly crystallized and aggregated with insufficient hydrolysis as the carbon chain length of the alcohol molecule increased. It is thought that n-C3H7OH or n-C4H9OH, with the longer carbon chain, has a lower concentration of OH- to form anatase nuclei,26 and thus, the growth rate of the anatase crystal was slower in comparison with in other solvents. However, in the mixed water/ethanol solution, as shown in the XRD images of Figure 2, patterns e-i, the diffraction lines are relatively broad, indicating the nanosize of high crystallinity. The sizes of TiO2 nanoparticles are gradually decreased with the increase of the alcohol/water ratio. The particles calculated using the Scherrer formula27 are 13.4, 11.0, 10.2, 8.9, and 11.8 nm, respectively. 3.2. Morphology Examination. Further structural characterization of the TiO2 samples was carried out using TEM image analysis. Figure 3 shows the TEM images of products in different solvent systems. As shown in the TEM images of

Liao et al. Figure 3a,b, these particles are not uniform. Their edges and morphology are fuzzy. There are large numbers of amorphous phase TiO2 particles, which is in agreement with the XRD results. The TEM images of crystalline TiO2 solvothermally treated at 120 °C for 15 h are shown in Figure 3c-i. In pure n-propanol and pure n-butanol solution, amorphous floccus-like structures were obtained (Figure 3c,d). These are in accordance with the results measured from the XRD. The TiO2 size, morphology, and crystallinity changed by varying the composition of the solvent. In pure water solution, TiO2 nanocrystals consist of “rodlike” anatase particles, which have lengths of about 80.0 nm and widths of about 7.0 nm (Figure 3i). However, in the mixed water/ethanol condition, the particle size decreases from 47.0 to 9.0 nm with the increase of the ethanol to water ratio (Table 1). Pure ethanol solution leads to a shape transformation from “rodlike” to “rectangular” particles, which have a length of about 26.0 nm and a width of about 14.0 nm (Figure 3e). Figure 4 shows the HRTEM images of the TiO2 samples prepared in pure water and pure ethanol solution. The HRTEM image of rodlike anatase in pure water solvent is shown in Figure 4a. This nanocrystal is viewed along the (101) plane and elongated in the (001) direction. The lattice fringes shown in the HRTEM image, corresponding to a distance of 3.4 and 4.8 Å, agree well with the distance of the (101) and (002) lattice planes of anatase TiO2, respectively.28 The clear lattice fringes further confirm the nanorod is single-crystalline and the structure is perfect. The HRTEM image of the rectangular-shaped anatase of TiO2 in pure ethanol is shown in Figure 4b. This nanocrystal is grown in the (101) direction. The lattice fringes shown in the HRTEM image, corresponding to a distance of 3.4 Å, agree well with the distance of the (101) lattice plane of anatase.29 The corresponding fast Fourier transform (FFT) analysis of the two shapes (see inset of Figure 4) confirms that the primary particles are similarly oriented. 3.3. Photocatalytic Degradation of Phenol. Many factors can affect the photocatalytic activity. Liu et al. reported that the bicrystalline framework, high crystallinity, large surface area, mesoporous structure, high visible-light absorption, and the nanocrystals with abundant surface states can make the TiO2 nanocrystals possess enhanced photocatalytic activities.30 Ohno and co-workers found that the photocatalytic activity of TiO2 shows dependence on the surface structure of TiO2 particles with specific exposed crystal faces.31 As shown in Figure 5, after 6 h of UV light illumination, the absorbance of phenol increases slightly in a direct photolysis process without any photocatalyst. It may contribute to the absorbance of catechol,32 the intermediate degradation products, which has the maximum absorbance at 253.4 nm. In the mixed solvent, the photocatalytic degradation efficiency increases with the decrease of the water/ ethanol ratio. The TiO2 particles in the 1:4 water/ethanol ratio solvent exhibit the highest photocatalytic degradation efficiency of phenol, which is up to 87.0%; 33.8% of phenol is photocatalytically degraded when using the rodlike photocatalyst prepared in the pure water solvent, whereas only 29.0% of phenol is degraded in the pure ethanol solvent. It is expected that large surface area contributes to the high photocatalytic activity of TiO2 nanocrystals by creating more possible reactive sites on the surface of the photocatalyst.30a Kominami et al. also reported that the higher activity of the TiO2 photocatalysts is attributed to both high crystallinity and large surface.33 As shown in Table 1, the photocatalyst prepared with the ratio of water/ ethanol equal to 1:4 possesses a relatively high surface area (185.4 m2/g). The surface area is in agreement with the calculated surface area of 175.5 m2/g valued by the formula of

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Figure 3. TEM images of TiO2 nanocrystals prepared in (a) H2O and (b) ethanol before solvothermal treatment and in (c) n-propanol, (d) n-butanol, (e) ethanol, (f) H2O/ethanol ) 1:4, (g) H2O/ethanol ) 1:1, (h) H2O/ethanol ) 4:1, and (i) H2O after solvothermal crystallization.

TABLE 1: Summary of the Characterization for Different TiO2 Nanoparticles Vwater/ DTEMc DXRDd SBETe SXRDf samplea Vethanolb (nm) (nm) (m2/g) (m2/g) TW TWE4 TWE1 TWE0.25 TE

1:0 4:1 1:1 1:4 0:1

80.6 47.0 24.0 9.7 26.1

13.4 11.0 10.2 8.9 11.8

117.7 148.2 159.8 185.4 241.9

117.8 142.0 153.2 175.5 132.4

ζg (mV)

Dphenolh (%)

-47.4 -45.5 -43.6 -42.8 -42.0

33.8 50.4 77.4 87.0 29.0

a TW, TWE4, TWE1, TWE0.25, and TE correspond to the samples prepared in the different solvents of pure water, 4:1 water/ethanol ratio, 1:1 water/ethanol ratio, 1:4 water/ethanol ratio, and pure ethanol, respectively. b V is the volume ratio. c DTEM is the diameter size of TiO2 observed from TEM. d DXRD is the diameter size of TiO2 observed from the Scherrer formula. e SBET is the surface area examined using a nitrogen adsorption apparatus. f SXRD is the calculated surface area valued by the formula of S ) 6/d*F, where d is the particle size calculated by the Scherrer formula from XRD results and F is the density of anatase TiO2. g ζ is the zeta potential of the TiO2 colloid. h Dphenol is the photocatalytic degradation efficiency of phenol.

S ) 6/d*F, where d is the particle size examined by XRD and F is the density of anatase TiO2. The surface area of the sample prepared in the pure ethanol solvent is 241.9 m2/g, which is remarkably higher than that calculated from the Scherrer formula, indicating the presence of amorphous TiO2. Therefore, TiO2 nanoparticles with relatively poor crystallinity results in the lowest photocatalytic efficiency. 3.4. Reaction Mechanism. The overall reaction in the system can be expressed by the following equation:32

[Ti(OH)5(OH2)]- + H2O2 f (x > 2) f [Ti(OH)4(OH2)2]0 f Ti2O5(OH)(x-2)x Solvothermal Treatment

(2)

[Ti16O16(OH)32(OH2)32] 98 TiO2

The growth of anatase nanocrystals in the solvothermal solution occurred via four growth stages in series. In the first growth

Figure 4. HRTEM images of the anatase TiO2’s nanorod-like (a) and rectangular shape (b). The inset shows an FFT analysis of the anisotropic crystals.

stage, peroxotitanium complex formed. In the second growth stage, [Ti(OH)4(OH2)2]0 growing units formed. In the third growth stage, small equidimensional anatase nanocrystals formed and grew. The final growth stage involved the nanocrystals’ growth along the specified direction. The dinuclear peroxotitanium complexes are formed and slowly condensed to polynuclear anions at the pH of 3-9 in an aqueous peroxide solution rich environment.34 After the peroxotitanium acid solution is heated at 120 °C for 15 h, the peroxo groups are decomposed and the [Ti(OH)4(OH2)2]0 growing units are formed, which results in small equidimensional anatase nanocrystals by polycondensation.32 In this reaction, the crystal growth occurs during the solvothermal process, and the anatase crystalline generated is well-dispersed without any disperser. The absolute zeta potentials of all the TiO2 colloids with negative charges are higher than 40.0 mV, indicating higher stability of the TiO2 nanocrystal colloids. Many hydroxyl groups are absorbed on the surface of TiO2 anatase particles with mutual repulsion to maintain the high stability. The solvent plays a key role in

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SCHEME 1: Growth Schematic Diagram of TiO2 Nanostructures in Different Experimental Conditions. (The Line Length Is Not the Real Size of the Crystal; It Is Just for Explaining the Relative Ratio of Each Plane)

determining the different growth behavior of TiO2 nanostructures. The interface-solvent interactions depend on the surface energies of different crystal facets and solvent properties. According to the growth model of an anion coordination polyhedron,35 for the growth rates of different facets, the fastest is the face along the vertex of the coordinated polyhedron, the slowest is parallel to the face of the coordinated polyhedron, and the medium is the orientation normal to the edge of the coordinated polyhedron. Along the [001] direction of the anatase structure, each [TiO6] octahedron shares four edges with its two neighboring [TiO6] octahedrons to form the relatively stable octahedron chain along the c-axis direction. Along the c-axis direction, [TiO6] octahedral exposing up the most edges has the fastest growth rate.36 Therefore, under the hydrothermal condition in the pure water solution, a rapid growth occurs along to the [001] direction, which resulted in the morphology of the rodlike particles (Figure 4a). According to the reports, the two TiO2 (001) and (101) surfaces exhibit very different activities as the valence band of TiO2 (001) extends to somewhat higher energies with respect to TiO2 (101).37 Yang et al.38 reported that the higher density of 5-fold Ti on (001) surfaces may lead to more obvious selective adhesion of 2-propanol because 2-propanol tends to heterolytically dissociate to form an alkoxy group bound to coordinatively unsaturated Ti4+ cations on the (001) surfaces, which retards the growth of anatase TiO2 single crystals along the [001] direction.5a Additionally, Hengerer et al. have studied the different adsorptions on the two surfaces by methanol, with the results of easily occurring on TiO2 (001), whereas unfavored on the majority TiO2 (101).39 Methanol is much more strongly adsorbed to the TiO2 (001) surface than water, using density functional theory calculations and firstprinciples molecular dynamics simulations.40 Wang et al. obtained the same experimental result by sum frequency generation spectroscopy.41 Compared with methanol and 2-pro-

Figure 5. Process of photocatalytic degradation of phenol under UV light illumination of different TiO2 photocatalysts.

panol, ethanol has the similar negative charges on the oxygen, which leads to the similar activities of the alkoxy group bound to coordinatively unsaturated Ti4+ cations on the (001) surfaces. Therefore, ethanol adsorbed on the (001) plane depresses the growth rate along the [001] direction, while the growth along the [101] direction is rapid. The nature growth rate along the [001] direction is close to 2.7 times that along the [101] direction (G100 ≈ 2.7G101).38 Thus, rodlike anatase nanoparticles were formed in the pure water condition, whereas the smaller rectangular-shaped particles were obtained in pure ethanol solvent. In the mixed solvents, such as in the 4:1 water/ethanol ratio, growing units continuously grow further on both (001) facets and (101) facets of the truncated octahedral bipyramid seeds, but the water-rich environment enables the crystal to grow faster along the [001] direction than along the [101] direction. The TiO2 particle size in the 4:1 water/ethanol ratio solvent is smaller than that in pure water solution. However, when the (101) surface area reaches a critical value under the kinetically driven regime, the faceted rods are formed.38 The schematic illustration of growth proceeding nanostructure anatase is shown in Scheme 1. 4. Conclusions In summary, TiO2 nanocrystal colloids with a diversity of well-defined morphologies and size-controlled nanoparticles have been successfully fabricated by adjusting the solvent system from the peroxotitanate complex solution. The TiO2 nanocrystals exhibited high stability and high crystallinity by a solvothermal process. Ethanol shows more strongly adsorption to the (001) plane, which depressed the growth rate along the [001] direction. The particle shapes transformed from “rectangular” to “rodlike” with the changes of the water/ethanol ratio. In the mixed water/ethanol condition, the particle size decreases from 47.0 to 9.0 nm with the increase of the ethanol to water ratio. We also examined the activities of TiO2 particles as photocatalysts for the decomposition of phenol. The TiO2 particles prepared in the 1:4 water/ethanol solvent showed the highest activity on the photocatalytic decomposition of phenol. Acknowledgment. The authors acknowledge the support of the Academic Leader Program of Shanghai Science and Technology Committee (07XD14014), Shanghai Leading Academic Discipline Project (S30107), Key Project of Chinese Ministry of Education (208182), International Cooperation Fund of Shanghai Science and Technology Committee (07SU07001), National High Technology Research and Development Program (863 Program) of China (2007AA03Z335), and The Special Project for Nano-Science of Shanghai Science and Technology Committee (0752 nm001, 08520704800, and 0852 nm01800).

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