J. Phys. Chem. C 2007, 111, 2709-2714
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Low Temperature Synthesis and Photocatalytic Activity of Rutile TiO2 Nanorod Superstructures Yawen Wang,† Lizhi Zhang,*,† Kejian Deng,‡ Xinyi Chen,§ and Zhigang Zou*,§ Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, Key Laboratory of Catalysis and Materials Science of Hubei ProVince, College of Chemistry and Materials Science, South-Central UniVersity for Nationalities, Wuhan 430074, People’s Republic of China, and Ecomaterials and Renewable Energy Research Center, Department of Physics, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: October 4, 2006; In Final Form: NoVember 22, 2006
Pure rutile nanorods were synthesized by hydrolysis of TiCl4 ethanolic solution in water at 50 °C. The assembly of rutile nanorods could be controlled through simply changing the molar ratios of TiCl4, ethanol, and water, resulting in different superstructures with flower- or urchinlike morphologies. The resulting samples were characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, highresolution transmission electron microscopy, nitrogen sorption, and UV-vis diffuse reflectance spectrum. A possible mechanism for the growth and assembly of rutile nanorod superstructures was proposed on the basis of characterization results. More importantly, we found that those low temperature synthesized superstructures showed significantly higher photocatalytic activities than commercial photocatalyst P25 on degradation of rhodamine B in water under artificial solar light. This study provides a simple and inexpensive way to prepare high active rutile nanorods superstructures photocatalysts on a large scale.
1. Introduction Nanosized titania has received much research attention because of its unique physicochemical properties in the applications of pigments, cosmetics, fine ceramics, catalyst supports, dielectric materials, and photocatalysts for environmental purification1,2 TiO2 is a polymorphous compound, crystallizing as rutile, anatase, or brookite. All of them have the same fundamental structural octahedral units with different arrangements.3 Rutile and anatase are most studied, whereas brookite are less known due to problems in its preparation in pure nanocrystalline form. There is a structural inter-relation between rutile and anatase, which was reported in theoretical and experimental studies.4,5 Anatase is usually more widely used in photocatalysis. Rutile is a thermodynamically stable phase and possesses a smaller band gap than anatase phase. In some cases rutile was found to be more active for photocatalysis than anatase.6 Rutile can often be obtained via high-temperature calcination of anatase nanoparticles. However, calcination unavoidably leads to agglomeration and growth of the nanocrystalline particles.7-9 So fabrication of rutile titania at low temperatures is of great importance. Recently, synthesis of pure rutile at low temperature has been developed by some groups. For example, Cheng and co-workers reported a hydrothermal method to prepare rutile phase titania at 200 °C.3 Chu et al. prepared rutile nanorods from TiCl4 in high acidic aqueous solution in the absence or presence of PEG-1000 at 100 °C.10 Yang et al. synthesized rodlike nanocrystalline rutile titania by ultrasonication of TiCl4 in hydrochloric acid solution at room temperature.11 Watson et * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn;
[email protected]. Tel/Fax: +86-27-6786 7535. † Central China Normal University. ‡ South-Central University for Nationalities. § Nanjing University.
al. synthesized rutile nanorods from titanium isopropoxide in HNO3-isopropanol mixed aqueous solutions (pH ) 1.2) at 60 °C.12 Fei et al. fabricated rutile nanorods by hydrolysis and condensation of titanium isopropoxide in aqueous solution at room temprature.13 Wang et al. presented a method to synthesize rutile nanoparticles with a rodlike shape by thermal hydrolysis of TiCl4 in hydrochloric acid-alcohol aqueous solution at a relatively low temperature.14 Li and co-workers prepared flowerlike rutile titania nanocrystals via an aqueous phase stirring for 24 h at the low temperature of 75 °C. In their preparation, HCl was utilized.15 Obviously, most of the reported methods for the synthesis of rutile TiO2 nanorods involved special treatments (hydrothermal or ultrasound) and/or addition of strong acid. In this paper we present that pure rutile nanorods can be synthesized by direct hydrolysis of TiCl4 ethanolic solution in water at 50 °C. We found that the ethanol played an important role in controlling the hydrolysis of TiCl4 in water and tuning the final structure of rutile nanorods, and the organization of rutile nanorods could be controlled to obtain superstructures with different morphologies via varying the molar ratios of TiCl4, ethanol, and water without any seeds and surfactants and special treatment processes. In most cases, rutile showed poor photocatalytic activities.16,17 Interestingly, the rutile TiO2 nanorod superstructures prepared in this study displayed excellent photocatalytic activities on degradation of organic dye pollutant under artificial solar light. Their activities were even 70% higher than that of the famous commercial photocatalyst P25. 2. Experimental Sections 2.1. Sample Preparation. All of the chemicals were analytical grade. In a typical synthesis, titanium tetrachloride (98%) was slowly added dropwise into ethanol under stirring. During the addition process, a large quantity of yellowish gas, presum-
10.1021/jp066519k CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007
2710 J. Phys. Chem. C, Vol. 111, No. 6, 2007 ably EtCl and HCl, was released as a consequence of the predominant alcoholysis of TiCl4 with ethanol and a partial hydrolysis of TiCl4 with the residual water. After stirring at ambient conditions for several minutes, a transparent yellowish sol was formed, which was then slowly added to distilled water under stirring. After the addition was finished, the solution turned colorless. After being stirred about 30 min, the resulting solution was maintained in a closed system at 50 °C in an oven for 24 h, resulting in a white precipitate. The resulting precipitate was harvested by centrifugation, followed by washing with distilled water several times and dried in an oven at 50 °C. To control the morphologies of the samples and to understand the growth process, we investigated the final morphologies of the samples as a function of molar ratios of TiCl4, ethanol and water. Samples synthesized in the molar ratios of 1:20:280, 2:40:280, and 2:20:280 were denoted as T1, T2, and T3, respectively. Moreover, in order to study the effects of H+, Cl-, and ethanol on the morphologies of the resulting TiO2, four control samples were prepared with the similar procedures by replacing TiCl4 with titanium tetraisoproxide (TTIP) and adding some additives such as KCl, HNO3, and H2SO4. The molar ratios of the reagents were controlled as follows: TTIP:KCl:C2H5OH:H2O ) 1:4:20: 280 (denoted as T4); TTIP:HNO3:C2H5OH:H2O ) 1:4:20:280 (denoted as T5); TTIP:H2SO4:C2H5OH:H2O ) 1:2:20:280 (denoted as T6); (d) TiCl4:C2H5OH:H2O ) 1:0:330 (denoted as T7). 2.2. Characterization. X-ray powder diffraction (XRD) patterns were obtained using a Philips MPD 18801 diffractometer using Cu KR radiation. Scanning electron microscopy (SEM) measurements were performed used a JSM-5600 SEM. Transmission electron microscopy (TEM) study was carried out on a Philips CM-120 electron microscopy instrument. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon copper grids. Furthermore, the obtained powders deposited on a copper grid were observed by high-resolution TEM (HRTEM; JEOL JSM-2010 microscope) operating at 200 kV. The nitrogen adsorption and desorption isotherms at 77 K were measured using Micrometrics ASAP2010 system after samples were vacuum dried at 180 °C overnight. A Varian Cary 100 Scan UV-visible system equipped with a labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts over a range of 200-600 nm. Labsphere USRS99-010 was employed as a reflectance standard. 2.3. Photocatalytic Activity Test. The photocatalytic activities of the samples were evaluated by the degradation of RhB in an aqueous solution. A 500-W tungsten halogen lamp was positioned inside a cylindrical vessel and surrounded by a circulating water jacket to cool it. A 0.1 g amount of photocatalyst was suspended in a 100 mL of aqueous solution of 5 mg/L RhB. The solution was continuously stirred for about 1 h to ensure the establishment of an adsorption-desorption equilibrium among the photocatalyst, RhB, and water before irradiation, then the solution was shined with artificial solar light from the tungsten halogen lamp. The distance between light source and the bottom of the solution was about 15 cm, and the temperature of the RhB solution stirred by a dynamoelectric stirrer in an open reactor was about 25 °C. The concentration of RhB was monitored by colorimetry with a U-3310 UV-vis spectrometer (HITACHI). 3. Results and Discussion 3.1. XRD Patterns. Figure 1 shows the XRD patterns of the as-prepared samples obtained at temperature as low as 50 °C.
Wang et al.
Figure 1. XRD patterns of (a) T1, (b) T2, and (c) T3.
These patterns can be well indexed to tetragonal rutile (JCPDS No. 21-1276, space group: P42/mnm(136)). No peaks of anatase or brookite phase are detected, indicating the high purity of the products. By applying the Debye-Scherrer formula on the rutile (110) diffraction peaks, the average crystallite sizes of the samples were found to be 9.5, 6.9, and 6.8 nm for the samples T1, T2, and T3, respectively. This result indicated that the crystal sizes of the resulting rutile nanorods decreased with increasing the amounts of TiCl4 and/or ethanol. 3.2. SEM Images. Figure 2 shows SEM images of the samples T1, T2, and T3. A lot of flowerlike structures were observed in sample T1 (Figure 2a). These flowerlike structures connected each other so that it was difficult to estimate the size of an individual flower. After magnification, we found that these flowerlike structures were composed of many nanorods with sharp tips. These nanorods were of 10-15 nm in diameter and 50-70 nm in length (Figure 2b). When the amounts of TiCl4 and ethanol became double, the resulting sample T2 also consisted of nanoflowers of about 500 nm in sizes (Figure 2c). These nanoflowers possessed more or less spherical shape and were more complex than those of T1. Figure 2d reveals that each petal of nanoflowers in T2 is assembled by several nanorods of lengths up to 80-100 nm and diameters of 4-7 nm, which are obviously smaller than the nanorods of T1. This observation was consistent with the XRD results. That is, the crystal sizes of the resulting rutile nanorods decreased with increasing the amounts of TiCl4 and/or ethanol. When only the amount of TiCl4 doubled, the resulting sample T3 consisted of numerous well-defined spheres of 200-600 nm in sizes (Figure 2e). Figure 2f displays that these spheres of T3 possess rough surface and are radically covered with acicular particles, which look like urchins. The image of a broken sphere in Figure 3g reveals that these urchinlike particles are also composed of nanorods with acicular tips. Therefore, we conclude that different superstructures of rutile TiO2 nanorods can be obtained by controlling the molar ratios of TiCl4, ethanol, and water. 3.3. TEM and HRTEM Images. The size and morphology of the products were further analyzed by TEM measurements. The TEM image (Figure 3a) reveals that each flower of sample T1 consists of tens of short nanorods with sharp tips. The widths and lengths of the nanorods are in the range of 10-15 nm and about 50 nm, respectively. With the increases of TiCl4 and/or ethanol amounts, the organization of nanorods into complex superstructures became more obvious in samples T2 and T3 (parts b and c of Figure 3). The nanorods could be easily
Rutile TiO2 Nanorod Superstructures
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2711
Figure 2. SEM images of (a, b) T1, (c, d) T2, and (e, f, g) T3.
Figure 4. Plots of the (RhV)1/2 vs the energy of absorbed light and UV-vis diffuse reflectance spectra (inset) of the sample T1, T2, T3, and P25. Figure 3. TEM images of (a) T1, (b) T2, (c) T3, and (d) HRTEM of an individual nanorod.
observed on the edges of flower and urchin-like superstructures in parts b and c of Figure 3, respectively. It seems that more amounts of TiCl4 and/or ethanol could produce more complicate superstructures, comparing with the superstructures in T1, T2, and T3. The single-crystal nature of the nanorods in superstructures was revealed by HRTEM (Figure 3d). However, the nanarods are randomly oriented with respect to each other according to HRTEM observation. The lattice spacing is about 0.325 Å between adjacent lattice planes of the TiO2 nanorods,
corresponding to the distance between (110) crystal planes of the rutile phase. Therefore, the growth directions of nanorods are concluded to be perpendicular to (110) crystal planes. It is of great importance to obtain rutile TiO2 nanorods with small crystal sizes but high crystallinity at low temperature in the field of photocatalysis. This point will be discussed later. 3.4. UV-Vis Diffuse Reflectance Spectra. Figure 4 displays UV-vis diffuse reflectance spectra of the samples T1, T2, T3, and Degussa P25 (inset). As TiO2 is an indirect transition semiconductor, plots of the (RhV)1/2 vs the energy of absorbed light afford the band gaps of the TiO2 nanorods superstructures. The band gaps optically obtained in such a way were ap-
2712 J. Phys. Chem. C, Vol. 111, No. 6, 2007
Wang et al. TABLE 1: Textural Properties of T1, T2, and T3 sample T1 T2 T3
Figure 5. Nitrogen adsorption-desorption isotherm and pore size distribution curve (inset) of (a) T1, (b) T2, and (c) T3.
proximately 3.05, 3.10, 3.10, and 3.19 eV for the samples T1, T2, T3, and Degussa P25, respectively. It reveals that the band gaps of the samples T1, T2, and T3 are slightly narrower than that of the Degussa P25. These results are easily understood because P25 is a mixture of 75% anatase and 25% rutile and the band gap of anatase is larger than that of rutile. It was reasonable to find that the difference in band gap of the T1
ABET
(m2/g)
94.0 106.1 166.8
VBJH
(cm3/g)
0.29 0.15 0.21
mean pore diameter (nm)
3.2
sample and T2 and T3 samples was about 0.06 eV because the crystal size of T1 was larger than those of T2 and T3. 3.5. Nitrogen Sorption. The porous structures of the resulting samples were studied by nitrogen sorption. Figure 5 presents the nitrogen adsorption-desorption isotherms and BarretJoyner-Halenda (BJH) pore size distribution curves (inset) of the samples. Parts a and b of Figure 5 display type II adsorption-desorption isotherms, which are typical characteristics of macroporous materials. These macropores were produced by interaggregated superstructures. In Figure 5c there are two distinct capillary condensation steps in the nitrogen adsorption-desorption isotherms of T3, which reveal the hierarchical porous system of sample T3. The first hysteresis loop of T3 is at 0.4 < P/P0 < 0.55, corresponding to the filling of the framework confined mesopores formed between intra-agglomerated primary nanorods. The second hysteresis loop is at 0.78 < P/P0 < 1.0, corresponding to the filling of textural macropores produced by interaggregated secondary urchinlike superstructures. The Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes, and mean pore diameters of samples T1, T2, and T3 are summarized in Table 1. 3.6. Photocatalytic Activity. RhB were used for detecting the photocatalytic activity of the three rutile TiO2 nanorods superstructures samples. Under the illumination of artificial solar light from the high-pressure mercury lamp and in the presence of the titania, the photodecomposition of the RhB will proceed in both photocatalytic pathway and photosensitization pathway. Both of these processes are dependent on the photocatalytic activity of the titania.18,19 Therefore, the photodegradation of RhB in water under artificial solar light can reflect the phtocatalytic activity of the titania powders. RhB shows a maximum absorption at about 555 nm. In the photodegradation process, the major absorption band had significant hypsochromic shifts by as much as 29 nm from 555 to 526 nm (not shown).20 The concentration changes of RhB with irradiation time for the superstructures samples, P25, and self-degradation of RhB are shown in Figure 6. It was found that the self-degradation of RhB was slightly under our artificial solar light irradiation. This slight self-degradation should be attributed to the presence of UV light among the artificial solar light. However, the degradation of RhB became obvious in the presence of photocatalysts. It was clearly observed that all three superstructures samples possessed high photocatalytic activities. Their activities were even higher than that of Degussa P25. 79, 60, 65, and 46% degradation of RhB were observed after 5 h’ irradiation for sample T1, T2, T3, and P25, respectively. The order of photocatalytic activities was T1 > T3 > T2 > P25. Among all the photocatalysts in this study, T1 was the most active sample. Its photocatalytic activity was about 70% higher than that of P25. Several reasons may account for the high activities of the rutile TiO2 nanorods superstructures prepared in this study. First, samples T1, T2, and T3 with narrower band gaps could absorb more light than Degussa P25. Moreover, the large specific surface areas and small crystal sizes as well as high crystallinity of the samples T1, T2, and T3 might also play important roles in the enhancement of their photocatalytic activities. Small particle size could shorten the route for an electron migrates from the conduction band of the rutile TiO2 to its surface.
Rutile TiO2 Nanorod Superstructures
Figure 6. Comparison of photocatalytic degradation of RhB in the presence of T1, T2, T3, and P25 and the self-degradation of RhB (RB) under artificial solar light.
Figure 7. XRD patterns of the powders prepared at 50 °C for 24 h by the molar ratios of: (a) TTIP:KCl:C2H5OH:H2O ) 1:4:20:280 (denoted as T4); (b) TTIP:HNO3:C2H5OH:H2O ) 1:4:20:280 (denoted as T5); (c) TTIP:H2SO4:C2H5OH:H2O ) 1:2:20:280 (denoted as T6); (d) TiCl4: C2H5OH:H2O ) 1:0:330 (denoted as T7).
Moreover, large surface could provide more active sites and absorb more reactive species. Meanwhile, the high crystallinity means few defects in the photocatalysts. It is well known that these defects may serve as the recombination centers for photoexcited electron-hole pairs during photocatalysis, which would decrease the photocatalytic activity. In most cases the poor photocatalytic activities of rutile TiO2 synthesized by high temperature thermal treatment may be attributed to their low surface areas and large crystal sizes, although these rutile photocatalysts also possessed narrow band gaps (3.0 eV) and good crystallinity.21 In contrast, poor crystallinity should be the main reason for the low photocatalytic activity of rutile TiO2 synthesized at low temperatures. 3.7. Possible Formation Processes of Rutile TiO2 Nanorods Superstructures. In order to understand the formation mechanisms of rutile TiO2 nanorods superstructures, we investigated the influences of H+, Cl-, and ethanol in the growth processes. Figure 7a showed that the sample prepared in the absence of H+ (denoted as T4) was pure anatase TiO2 (JCPDS No. 211272). It was found that the sample T4 was composed of nanoparticles with average size of 20 nm (Figure 8a). Therefore,
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2713
Figure 8. SEM images of the powders prepared at 50 °C for 24 h with different molar ratios. (a) TTIP:KCl:C2H5OH:H2O ) 1:4:20:280 (denoted as T4); (b) TTIP:HNO3:C2H5OH:H2O ) 1:4:20:280 (denoted as T5); (c) TTIP:H2SO4:C2H5OH:H2O ) 1:2:20:280 (denoted as T6); (d) TiCl4:C2H5OH:H2O ) 1:0:330 (denoted as T7).
we concluded that the strong acidic condition (H+ comes from the ethanolysis and hydrolysis of TiCl4 in our synthesis) was crucial for the formation of rutile TiO2 at low temperatures. This is consistent with the previous result.12 When Cl- was replaced by NO3- with keeping the same acidity as that for synthesizing T1, the as-prepared sample T5 was found to be pure rutile TiO2 (Figure 7b). However, the sample T6 synthesized in the presence of SO42- was anatase TiO2. Parts b and c of Figure 8 display that both samples T5 and T6 are composed of nanoparticles, but the particles in T6 were agglomerated more seriously. Therefore, it can further be concluded that under acidic condition the presence of Cl- or NO3- is favorable for the formation of rutile, but SO42- is favorable for the formation of anatase. So the inorganic negative ions also influence the formation of rutile titania besides the acidity of solution. We found that ethanol was an important factor for the formation of rutile TiO2 nanorod superstructures in the absence of no seed nanocrystals or organic surfactant additives. We prepared another control sample (T7) under the same condition as that for sample T1 but without using ethanol. When TCl4 is directly added to water, the hydrolysis reaction proceed fast and white amorphous Ti(OH)4 precipitate was formed at once. After aged at 50 °C for 24 h, the precipitation was crystallinized into rutile TiO2 (Figure 7d). Figure 8d shows that sample T7 possesses only fewer rodlike nanoparticles. Therefore, ethanol plays a crucial role on tuning the morphology of rutile TiO2. From the above analysis, the possible growth process of rutile TiO2 superstructures can be proposed as follows. TiCl4 first dissolved in ethanol to form 6-fold coordinated [TiClm(OC2H5)6-m]2- complex species and HCl. Then the [TiClm(OC2H5)6-m]2- species reacted with water to form [Ti(OH)nClm(OC2H5)6-n-m]2- complex species when the ethanolic solution of TiCl4 was added into distilled water, where n and m are associated with the acidity and [Cl-] in the reaction system, respectively. That is, the higher [Cl-], the bigger the value of m and the higher the acidity, the less the number of OH ligands in [Ti(OH)nClm(OC2H5)6-n-m]. In the reaction process, the [OC2H5-] served as a barrier against the hydrolysis of TiCl4 in H2O. And this is the main reason for no precipitate yielded among the whole ethanolysis of TiCl4 before aging at 50 °C.
2714 J. Phys. Chem. C, Vol. 111, No. 6, 2007 It is accepted that both anatase and rutile titania grow from TiO6 octahedra, and the phase formation proceeds by the rearrangement of these octahedra that the edge-shared bonding lead to anatase and vertex-shared bonding to rutile.22 It was reported that higher acidity in solution could result in the formation of the rutile phase.3,23 In this study, the linking between [TiO6] units was carried out by the dehydration reaction between OH ligands in [Ti(OH)nClm(OC2H5)6-n-m]2- complex species. High acidity and a large amount of Cl- coming from hydrolysis of TiCl4 reduced the number of OH ligands in [Ti(OH)nClm(OC2H5)6-n-m]2-. This caused the edge-shared bonding to be suppressed and the vertex-shared bonding to be enhanced. Finally, rutile titania was obtained. In [Ti(OH)nClm(OC2H5)6-n-m]2- species, the values of n, m, and 6 - n - m all have a role on influencing the dehydration reaction. Therefore, the ethanol not only serve as a barrier against the hydrolysis of TiCl4 in H2O but also have the ability to tune the final structure of rutile titania. This point was further confirmed by the different morphologies of sample T2 and T3 prepared with different concentrations of ethanol. In general, the equilibrium concentration and exact nature of individual complex ions of titanium(IV) are depending directly on the [H+], [Cl-], and [OC2H5-]. These ions can selectively be adsorbed on the side faces of crystals to affect their growths. Like the preparation of sample T2 and T3, although the concentration of Ti ions was higher than that for sample T1 in the process of synthesis, their average crystal sizes were smaller than T1. This is because of the increased concentration of [H+] and [Cl-] ions caused by more TiCl4. Meanwhile, the higher the concentration of TiCl4, the more rutile TiO2 nanorods produced, which resulted in more complex nanorods superstructures. Thus various rutile nanorods superstructures would form through changing the moral ratios of TiCl4, ethanol, and H2O. 4. Conclusions We have demonstrated that various rutile TiO2 nanorod superstructures can be selectively synthesized at low temperatures by adjusting the molar ratios of TiCl4, ethanol, and water. The effect of H+, Cl-, and ethanol on the morphology of rutile nanorods were first studied in detail. All of these rutile nanorods superstructures exhibited higher photocatalytic activities than Degussa P25. Their high photocatalytic activities could be attributed to their narrow bang gap, large surface area, small crystal size, and high crystallinity. We proposed possible formation processes for the rutile TiO2 nanorods superstructures through systematically studying the influences of ions and
Wang et al. ethanol in the system. This study provide a simple and inexpensive way to prepare high active rutile nanorods photocatalysts on a large scale. Acknowledgment. The work described in this paper was partially supported by National Science Foundation of China (Grants 20503009, 20373025, and 20528302), Open Funds of Key Laboratory of Catalysis and Materials Science of Hubei Province (Grants CHCL0508 and CHCL06012), Jiangsu Provincial Natural Science Foundation of China (Grants BK2006718 and BK2006127), and Jiangsu Provincial High Technology Research Project (Grant BG2006030). One of the authors (Professor Zou) would like to thank the Talent Project of Jiangsu province. References and Notes (1) Karch, J.; Birriger, R.; Gleiter, H. Nature 1987, 10, 556. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Cheng, H.; Ma, J.; Zhao, Z.; Qi, L. Chem. Mater. 1995, 7, 663. (4) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. ReV. B 2001, 63, 155409. (5) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra¨tzel, M. Phys. ReV. Lett.1998, 81, 2954. (6) Ohno, T.; Tokieda, K.; Higashida, S. Appl. Catal. A Gen. 2003, 244, 383. (7) Yanagisawa, K.; Ovenstone, J. J. Phys. Chem. B 1999, 103, 7781. (8) Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2000, 104, 3481. (9) Wu, M.; Lin, G.; Chen, D.; Wang, G.; He, D.; Feng, S.; Xu, R. Chem. Mater. 2002, 14, 1974. (10) Chu, R. H.; Yan, J. C.; Lian, S. Y.; Wang, Y. H.; Yan, F. C.; Chen, D. W. Solid State Commun. 2004, 130, 789. (11) Yang, K.; Zhu, J. M.; Zhu, J. J.; Huang, S. S.; Zhu, X. H.; Ma, G. B. Mater. Lett. 2003, 57, 4639. (12) Watson, S.; Beydoun, D.; Scott, J.; Amal, R. J. Nanopart. Res. 2004, 6, 193. (13) Fei, B.; Deng, Z. X.; Xin, J. H.; Zhang, Y. H.; Pang, G. Nanotechnology 2006, 17, 1927. (14) Wang, W.; Gu, B. H.; Liang, L. Y.; Hamilton, W. A.; Wesolowski, D. J. J. Phys. Chem. B 2004, 108, 14789. (15) Li, Y. Y.; Liu, J. L.; Jia, Z. J. Mater. Lett. 2006, 60, 1753. (16) Andersson, M.; O ¨ sterlund, L.; Ljungstro¨m, S.; Palmqvist, A. J. Phys. Chem. B 2002, 106, 10674. (17) Sclafani, A.; Herrmann, J. M. J. Phys. Chem. 1996, 100, 13655. (18) Wu, T. X.; Liu, G. M.; Zhao, J. C.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845. (19) Ma, Y.; Yao, J. N. Chemosphere 1999, 38, 2407. (20) Zhao, W.; Chen, C. C.; Li, X. Z.; Zhao, J. C.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 2002, 106, 5022. (21) Wang, X. C.; Yu, J. C.; Ho, C.; Hou, Y. D.; Fu, X. Z. Langmuir 2005, 21, 2552. (22) Jolivet, J. P. Metal Oxide Chemistry and Synthesis: from Solution to Solid State; Wiley: Chichester, 2000. (23) Wu, J. M.; Zhang, T. W.; Zeng, Y. W.; Hayakawa, S.; Tsuru, K. Osaka, A. Langmuir 2005, 21, 6995.
