Preparation and Properties of Nanostructure Anatase TiO2 Monoliths

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Preparation and Properties of Nanostructure Anatase TiO2 Monoliths Using 1-Butyl-3-methylimidazolium Tetrafluoroborate Room-Temperature Ionic Liquids as Template Solvents

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1643-1649

Yang Liu,†,‡ Jun Li,‡ Meijia Wang,† Zhiying Li,† Hongtao Liu,† Ping He,† Xiurong Yang,† and Jinghong Li*,†,‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Department of Chemistry, Tsinghua University, Beijing 100084, China Received January 17, 2005;

Revised Manuscript Received February 27, 2005

ABSTRACT: Nanostructure anatase titanium oxide (TiO2) monoliths were prepared using 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-) room-temperature ionic liquids (RTILs) as template solvents by a simple sol-gel method with a peptization process at ambient temperature. The as-prepared products exhibited wormlike pore structures and large surface areas, which ranged from ca. 118 to ca. 498 m2 g-1 by adding various amounts of BMIM+BF4-. Mesoporous structures of the product with a surface area of ca. 260 m2 g-1 were retained upon calcining to 450 °C, showing excellent thermal stability. Furthermore, the products revealed higher photocatalytic activities than that of the commercially available TiO2, Degussa P25. Compared with the rutile TiO2 crystal of microporous structure prepared without RTIL, it was thought that BMIM+BF4- acted as a structure-directing agent and enhanced the polycondensation and crystallization rate, resulting in the facile formation of anatase crystal. The nanostructure TiO2 has promising applications in photocatalysis, solar energy conversion, sensor, mesoporous membranes, etc. Introduction Titanium oxide (TiO2) has attracted much attention for its various applications, such as gas sensors, dielectric ceramics, photovoltaic solar cells, catalysis for thermal or photoinduced processes, and pigments.1-6 The physical and chemical properties of TiO2 can be controlled by its particle size, morphology, and crystalline phase. Recently, fabrication of nanostructure TiO2 with ultrafine crystallite sizes and high surface areas has been widely studied due to its unusual optical, electrical, and catalytic properties.7-9 Titania can be synthesized by several methods, such as flame synthesis by TiCl4 oxidation, hydrolysis precipitation of titanium alkoxidesor chlorides, oxidation-hydrothermal synthesis, and so on.10-14 As an alternative way, the sol-gel method is one of the most convenient technologies to prepare various metal oxides, owing to its microstructural tailoring allowing low cost, ease of execution, and low processing temperature, and is widely used to prepare TiO2 for films, particles, or monoliths. The solgel process consists of the hydrolysis of titanium alkoxides and subsequent polycondensation to form the metal oxide gel. On the other hand, using the templating effect of mesophases produced by the self-assembly of amphiphilic molecules controlled porosity can be obtained. Much work has been reported recently on the sol-gel synthesis of mesoporous titanium oxide using surfactant or triblock copolymers or diblock templates.15-19 However, sol-gel synthesis based on both hydrolytic and non-hydrolytic reaction pathways typically provides amorphous titanium oxides. Calcination and heat treat* To whom correspondence should be addressed. Tel and Fax: +8610-62795290; e-mail: [email protected]. † Chinese Academy of Sciences. ‡ Tsinghua University.

ments are therefore required to induce crystallization of the products and are associated with the breakage of the previously existing nanostructures and the growth of TiO2 grains. Room-temperature ionic liquids (RTILs), which are generally exemplified by the combination of a large organic cation with a weakly coordinating anion, have attracted significant attention in many fields of chemistry and industry as environmentally benign solvents.20-22 Since RTILs have advantageous chemical and physical properties, such as negligible vapor pressure, low toxicity, low melt points, and high chemical and thermal stability, they have been widely used in organic synthesis, electrochemistry, molecular selfassembly, and biocatalysis.23-31 Recently, RTILs have been used to prepare nanoporous inorganic materials for their unique properties.32-37 RTILs provide hydrophobic regions and a high directional polarizability, which is oriented parallel or perpendicular to the dissolved species, as well as extended hydrogen-bond systems in the liquid state resulting in a highly structured frame. These advantages would encourage the preparation of well-defined and extended ordering of nanostructures. In this paper, we prepared a nanostructure anatase TiO2 monolith at mild temperatures using 1-butyl-3methylimidazolium tetrafluoroborate (BMIM+BF4-) RTIL as a template solvent by a simple sol-gel method with a peptization process, instead of the rutile TiO2 prepared without RTIL solvent. To study the morphologies, structures, and properties of the products, various technologies were employed, including X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The photo-

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degradation of rhodamine B was also carried out using the products as catalysts. It was thought that the BMIM+BF4- acted more than a template solvent and a catalyst of the polycondensation in the sol-gel process. Experiments +

Chemicals. BMIM BF4- was purchased from Solvent Innovation and dried under vacuum at 60 °C for 24 h. Titanium isopropoxide (98+%) was purchased from Acros Organics. Ethanol was analytical purity and purchased from Shanghai Chemical Regent Corporation. HCl was of chemical purity and purchased from Beijing Chemical Regent Corporation. Water was purified with a Milli-Q (18.3 MΩ) water system. Synthesis of Nanostructure Anatase TiO2 Monoliths. The titanium precursor was titanium isopropoxide, which was significantly less reactive than titanium chloride. The titanium isopropoxide was dissolved in ethanol first and was hydrolyzed under vigorous stirring by adding an aqueous solution of hydrochloric acid. The titanium concentration was 1.0 mol L-1, the mole ratio of HCl/Ti was 1, and the hydrolysis ratio of H2O/ Ti was 20. As the HCl solution was added, the white precipitation occurred immediately, and a clear aqueous sol was obtained in agitate. Various volume ratios (VRs) of BMIM+BF4to titanium isopropoxide were chosen. The BMIM+BF4- was added to the sol after being aged for 30 min. A homogeneous solution was obtained and became semi-transparent gradually. Finally, the white TiO2 gels were obtained followed by standing the sol for 24 h. After that, the samples were treated at 80 °C under vacuum conditions for 6 h, and semi-transparent milk white gels were obtained. The entrapped BMIM+BF4- were extracted by refluxing the products in acetonitrile for 3 days. The subsequent products were dried at 50 °C under vacuum. The final products were white monoliths. On the other hand, heat treatments were carried out by calcining the products at different temperatures for 4 h. Characterization. Both small-angle X-ray powder diffraction (SAXRD) and wide-angle X-ray powder diffraction measurements were performed on a D/max 2500V PC X-ray diffractometer using Cu-KR-radiation (1.5406 Å) of 40 kV and 200 mA. The transmission electron micrographs (TEM) and the selected area electron diffraction (SAED) were taken with a JEOL-JEM-2010 operating at 200 kV (JEOL, Japan). Samples for TEM were prepared by dropping a diluted suspension of the sample powders onto a standard carbon-coated (20-30 nm) Formvar film on a copper grid (230 mesh). The thermal gravimetry analysis (TGA) and differential thermal analysis (DTA) were carried out using a Perkin-Elmer thermal analysis TG/DTA system. Measurements were made while heating from 20 to 700 °C, at a heating rate of 10 °C/ min in air. Raman spectra were measured with a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter (Renishaw Ltd., Gloucestershire, U.K.). The microscope attachment was based on a Leica DMLM system, and a 50× objective was used to focus the laser beam onto a spot approximately 1 µm in diameter. Radiation of 514.5 nm from an air-cooled argon ion laser was used for the SERS excitation with power of not more than a few mW at the sample position. The Brunauer-Emmett-Teller (BET) surface area and pore volume were measured on a Quantachrome NOVA 1000 Ver 6.11 system at 77.4 K. All samples were first degassed in a vacuum at 200 °C for 2 h before analysis. The BET-specific area was calculated from the nitrogen adsorption data in the relative pressure range from 0.01 to 0.3. The total volume was estimated from the amount adsorbed at a relative pressure of about 0.99. XPS was conducted using a VG ESCALAB MK Π spectrometer (VG Scientific, UK) employing a monochromatic MgKR X-ray source (hυ ) 1253.6 eV). Peak position was internally referenced to the C1s peak at 284.6 eV.

Figure 1. The small-angle XRD patterns of the as-prepared TiO2 with VRs of 1.89 (A) and 1 (B). The UV-visible absorption spectroscopic measurements were performed on a Cary-500 UV-Vis spectrophotometer (VARIAN, USA). The photocatalytic experiments of the products were evaluated from an analysis of the photodegradation of rhodamine B. The ultraviolet irradiation source was a 15 W ultraviolet analysis instrument with a main emission at λ of 290 nm and placed on a glass container. A total of 15 mg of the products was dispersed in 6 mL of rhodamine B aqueous solutions with a concentration of 1 × 10-5 mol L-1 under stirring conditions. Solutions were analyzed by UV-visible absorption spectroscopy after centrifugation.

Results and Discussion The bulk structure of the as-prepared TiO2 was characterized by small-angle XRD. Figure 1 shows the small-angle XRD patterns of the TiO2 prepared with VRs of 1.89 (A) and 1 (B). As the VR was lower than 1, no obvious peak was observed. A broad and weak peak at approximately 2θ ) 3° was obtained for the sample prepared with a VR of 1. Otherwise, an intense peak with a d-spacing corresponding to ca. 10 nm was obtained for the sample prepared with a VR of 1.89. It indicated that the nanoporous structures were achieved. Patterns of both the as-prepared TiO2 presented one peak, which indicated the mesostructure frameworks without long-range order in the pore arrangement. Figure 2 shows the TEM images of the mesoporous TiO2 prepared with VRs of 1 (A) and 1.89 (B). It was apparent that both of the samples were composed of the wormlike mesoporous structures of titanium oxide like the mesoporous structures of silica gel prepared by using BMIM+BF4- as a template,33 which was consistent with the small-angle XRD results. The pore diameters of the as-prepared TiO2 with VRs of 1 and 1.89 were ca. 2.3 nm and ca. 2.7 nm, respectively. In comparison to the sample prepared with a VR of 1, an intense pore structure was shown for the sample prepared with a VR of 1.89. The electron diffraction pattern in the selected area in Figure 2A (inset) shows the typical tetragonal anatase lattice. On the other hand, a set of weak rings in the inset of Figure 2B corresponding to the anatase phase were obtained,

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Figure 4. The wide-angle XRD patterns of the as-prepared TiO2 with a VR of 1 calcined at 300 °C (a), 450 °C (b), and 600 °C (c).

Figure 2. The TEM morphologies of the as-prepared TiO2 monoliths with VRs of 1 (A) and 1.89 (B). The inset was the electron diffraction patterns corresponding to the selected area.

Figure 3. The XRD patterns of the as-prepared TiO2 without ILs (a) and with BMIM+BF4- of different VRs (b, 0.5; c, 1; d, 1.89).

which indicated the polycrystalline nature of the nanostructure TiO2 with low crystallization. Figure 3 shows the wide-angle X-ray powder diffraction (XRD) patterns of the as-prepared TiO2 with different amounts of BMIM+BF4-. The rutile phase TiO2 prepared without the addition of RTIL (Figure 3a) was compared to the anatase TiO2 enriched with the addition of BMIM+BF4. As the VR was 1 or larger, anatase

TiO2 was obtained (Figure 3c,d). The peaks are indexed as (101), (004), (200), (105), (204) in order of increasing diffraction angles, indicating a body centered tetragonal crystalline structure of TiO2 crystal.38 No other phase such as rutile and brookite was detected. The peaks were apparently broad, showing that the small-sized nanocrystalline TiO2 was obtained. The crystal size of the resulting TiO2 with a VR of 1 was around 10 nm calculated by using the Scherrer equation based on the XRD peak broadening analysis at the (101) peak,39

Dhkl ) 0.89λ/(βhkl cos θ) where Dhkl is the crystallite size, λ is the wavelength of incident ray, βhkl is the full width at half-maximum of the peak, and θ is the position of plane peak. Figure 4 shows the WAXRD of the samples prepared with a VR of 1 at different temperatures. It was obvious that further heat treatment improved the crystalline size of the prepared TiO2. The inset of Figure 4 shows the crystal size of the sample prepared with a VR of 1 at various temperatures. It can be seen that the crystal size changed little upon increasing the temperature to 300 °C and increased slowly until 450 °C. After that, the crystal size increased quickly. The rutile crystalline TiO2 occurred after 800 °C. Figure 5 shows the typical thermogravimetric/differential thermal analysis (TG/DTA) of a TiO2 gel with a VR of 1 without washing. The TG curve can be divided into four stages. The first stage was from room temperature to 140 °C. The weight loss was approximately 6%, which was caused by dehydration and evaporation of alcohol existing in the gel. The second stage was from 140 to 330 °C. The weight loss was about 6%, which was attributed to the removal of the strongly bound water or the surface hydroxyl groups in the gel. The third stage was from 330 to 450 °C. The weight loss was about 65%, which was assigned to the decomposition of BMIM+BF4-. The fourth stage was from 450 to 550 °C. The weight loss was 6%, which was attributed to the oxidation of residue carbon. In the DTA curve, a broad

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Figure 5. DTA/TG curves of the as-prepared TiO2 with a VR of 1.

Figure 6. The Raman spectra of the as-prepared TiO2 with (A, VR ) 1) and without (B) BMIM+BF4-.

weak endothermic peak at ca. 100 °C was attributed to the desorption of water and alcohol. Another broad weak endothermic peak at ca. 300 °C came from the removal of the strongly bound water or the surface hydroxyl groups in the gel. At about 401 °C, a small endothermic peak was observed due to the carbonization of organic materials.40 A broad weak exothermic peak at 448 °C was due to the decomposition of BMIM+BF4-. The broad exothermic peak at around 500 °C was assigned to the oxidation of residue carbon or the further crystallization of anatase TiO2. No exothermal peak existed in the range of 150 to 400 °C, which indicated that there was no phase transformation in this temperature range, and the as-prepared TiO2 was pure anatase, and it was consistent with the XRD results. Figure 6 shows the Raman spectra of the as-prepared TiO2 with (A, VR ) 1) and without (B) RTIL. The XRD usually shows the information related to the long-range symmetry of the structure, while the Raman spectrum provides the local symmetry of the chemical bond. The amorphous TiO2 component did not seem to exhibit any

Liu et al.

Figure 7. The isotherms for as-prepared TiO2 with VRs of 0 (b), 0.25 (3), 0.5 ([), 1 (O), 1.89 (0).

characteristic Raman signal.2 The Raman peaks of the as-prepared TiO2 with a VR of 1 (Figure 6A) at around 156 cm-1 (Eg), 400 cm-1 (B1g), 506 cm-1 (A1g and B1g), and 630 cm-1(Eg) clearly showed an anatase phase of the as-prepared sample,41 which matched the XRD data. Otherwise, the sample prepared without RTIL (Figure 6B) represented the peaks at 156, 254, 440, and 604 cm-1, indicating a rutile phase TiO2. Upon progressive calcinations, shifts of the bands centered on 156 cm-1 to the frequency value of well-crystallized anatase, 144 cm-1, in the Raman spectra were observed (the inset of Figure 6A). The shifts could be attributed to the oxygen deficiency of the titanium oxide crystal.42 Figure 7 shows the isotherms for the samples prepared with various amount of BMIM+BF4-. The asprepared TiO2 with BMIM+BF4- template exhibited typical type VI isotherms with type H2 hysteresis loops, which are characteristics of mesoporous material.43 These characteristics were similar to those inorganic oxides prepared with a BMIM+BF4- template,33,34,36,37 which can be attributed to the capillary condensation within the pores of the gels. The sharp hysteresis loop was related to a narrow pore size distribution of the products. As a comparison, the sample prepared without a RTIL template shows a type I isotherm, indicating a microporous or nonporous structure.43 Figure 8 presents the isotherm for the sample prepared with a VR of 1 calcined at different temperatures. As the temperature increased, the hysteresis loop was transformed to near H1, and the steep region of the desorption branch was close to P/P0 ) 1. The transformation of the isotherm was attributed to the agglomerates of the TiO2 crystal and the shrinkage of the mesoporous structure during the calcinations process.43 The BET surface area of the sample prepared with a VR of 1 and without RTIL were decreased to 263 and 46 m2 g-1, respectively, upon heat treatment to 450 °C, exhibiting good thermal stability of the mesoporous TiO2 with a BMIM+BF4- template. The main porous characteristics of the samples obtained at various conditions are listed in Table 1. The surface areas of the mesoporous TiO2 prepared with BMIM+BF4increased from 118 to 497 m2 g-1 depending on the increased amounts of BMIM+BF4-. Furthermore, the total pore volumes of the samples prepared with the BMIM+BF4- template increased with the addition of BMIM+BF4- except for the products with a VR of

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Figure 8. The isotherms of the TiO2 (VR ) 1) as-prepared (b) and calcined at 300 °C (0) and 450 °C (3). Table 1. The Main Porous Characteristics of the Samples Obtained at Various Conditions volume ratio (IL/TIPP)a 0 0 0.25 0.5 1 1 1 1.89

heat treatment no 350 °C no no no 300 °C 450 °C no

specific surface area (m2 g-1)

total pore volume (cm3 g-1)

BJH average pore diameter (nm)

154 46 118 171 368 342 263 497

0.08 0.129 0.19 0.173 0.337 0.664 0.468 0.636

2.4 10.2 4.0 4.0 3.6 7.7 7.1 5.1

a IL: 1-butyl-3-methylimidazolium tetrafluoroborate; TIPP: titanium isopropoxide.

0.5 and were obviously larger than those without a BMIM+BF4- template. The average pore diameters of the products with a VR below 1 were nearly sizable and increased to ca. 5.1 nm depending on the increase of the VR to 1.89. It was thought that the increased amount of RTIL would induce the stacking arrangement of its cation or anion and result in the increase of pore diameter of the as-prepared TiO2 for the supramolecular structure of RTIL. The as-prepared mesoporous anatase sample was also characterized by XPS for the evaluation of the surface composition. Figure 9A exhibits the high-resolution XPS spectrum for the Ti 2p levels. There are two peaks detected in the Ti 2p region and the one located at 464.3 eV was corresponding to the Ti 2p1/2 and another one located at 458.7 eV was assigned to Ti 2p3/2. The position of Ti 2p3/2 was 0.5 eV less than the literature value for Ti4+ of rutile/anatase at 459.2 eV.44 This shift could be due to a change in microenvironments of titanium. This phenomenon was also observed in the mesoporous TiO2 samples prepared by using surfactant as template45,46 and was thought to be the result of the interaction between the remaining BMIM+BF4- and TiO2. The results show that the as-prepared mesoporous anatase TiO2 was in a normal state of Ti4+. Figure 9B gives the O1s photoelectron peaks of the as-prepared sample. This peak can be resolved into two components (inset of Figure 9B). The peak located at 530.1 eV corresponded to the lattice oxygen, and another peak at 532 eV

Figure 9. X-ray photoelectron spectroscopic scan survey in the region of Ti2p (A) and O1s (B) of the as-prepared TiO2 with a VR of 1.

corresponded to the adsorbed oxygen on the as-prepared TiO2 surface. The adsorbed oxygen on the TiO2 surface allowed the H+ hydroxylation to form -Ti(OH)-O-Ti(OH)-, which made the photogenerated holes H+ change into OH• free radicals.4 The proportion of the lattice oxygen to adsorbed oxygen was calculated to be 0.68 by the proportion of the area in the inset of Figure 9B. The sol-gel process of a titanium alkoxide to form a titanium oxide polymer consists of hydrolysis and polycondesation. The structures and properties of TiO2 were influenced intensively by the preparation conditions. It was reported that the crystalline transformation was affected by crystallization rate. Fast crystallization at room temperature would encourage the formation of anatase TiO2.15,47 The RTILs are salts entirely consisting of large cations and anions, which might allow it to capture protons or hydroxides by hydrogen bonds or electrostatic function, acting as an acid-base pair, and accelerate the polycondensation in the sol-gel process. It has also been reported that the salt consisted of a weak acidic cation, and a weak basic anion would accelerate the polycondensation in the sol-gel process.48 On the other hand, although RTILs are polar, they can have low interface tensions resulting in high nucleation rates and the generation of small particles, which facilitate the formation of anatase TiO2.49 This fact was

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ized before. Upon calcination as shown in Figure 10 ([), the samples exhibited excellent photoactivity, which was obviously higher than that of P25 due to its higher surface area retained and its good anatase crystal structure. Conclusion

Figure 10. Evolution of the photodegradation efficiency as a function of UV irradiation time in the presence of the TiO2 prepared without BMIM+BF4- (O) and with BMIM+BF4- [VR ) 1, without calcinations (b) and calcinations at 450 °C ([)], and Degussa P25 TiO2 powder (∇).

observed obviously in our case. As BMIM+BF4- was added, the sol became viscid quickly. And a homogeneous white gel formed a few minutes later. In contrast, the sol lasted a long time. These facts indicate that the addition of BMIM+BF4- encourages the formation of anatase TiO2. Furthermore, for the supermolecular structure of RTIL,49 a mesoporous TiO2 monolith might be formed with the self-assembled BMIM+BF4- in the TiO2 sol. Moreover, the pore diameters of TiO2 prepared with VRs of 1 and 1.89 obtained from the TEM patterns were about 2.3 and 2.7 nm, respectively, which were about two times longer than the molecular length of BMIM+BF4- (1.2 nm).33 It was thought that BMIM+ should interact with the bulk by hydrogen bonds or electrostatic force, which would induce the oriented arrangement of the BF4- anion along the wall, and pile up and stack, possibly by π-π interactions or other noncovalent interactions between the imidazole rings and the mesoporous structure formed. Rhodamine B, one of the most important xanthene dyes, was used in a variety of applications such as paper and dye-lasers and has become a common organic pollution resulting in a series of environmental problems. Photocatalysis degradation of organic dyes using semiconductors as catalysts is attractive for its rapid and convenient procedure. Titanium oxide has been the most popular photocatalyst because of its chemical inertness, nontoxicity, low cost, and photostability.4 Generally speaking, the photocatalytic activity of amorphous TiO2 is negligible, and the rutile form of titania is less photoactive than anatase.4 In this case, the TiO2 prepared without IL calcined at 300 °C was selected for comparison because of its better crystal structure and higher photoactivity than those without calcinations. On the other hand, the commercially available Degussa P25 TiO2 powder (specific surface area of 50 m2 g-1) was also selected. Figure 10 shows the photodegradation efficiency as a function of the irradiation time for the samples. It indicated that the activity of TiO2 prepared with BMIM+BF4- was higher than that of the samples prepared without RTIL calcined at 350 °C but was lower than that of P25. The lower activity of TiO2 (VR ) 1) should be on account of the low amount of crystal, even though it has a much higher surface area as character-

In conclusion, anatase mesoporous TiO2 monoliths with a high surface area have been synthesized using BMIM+BF4- as the template agent by a sol-gel method with a peptization process. The as-prepared products showed wormhole-like mesoporous structures. On the other hand, the products exhibited good thermal stabilities and photodegradation ability towards rhodamine B. The existence of BMIM+BF4- enhanced the polycondensation and crystallization rate, which encouraged the formation of anatase crystal. This mesoporous TiO2 has promising applications in photocatalysis, solar energy conversion, sensor, mesoporous membranes, etc. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20125513, No. 20435010), Li Foundation Prize, USA, and A Foundation for the Author of National Excellent Doctoral Dissertation of PR China. References (1) Baraton, M. I.; Merhari, L. Nanostruct. Mater. 1998, 10, 699. (2) Kavan, L.; Rathousky, J.; Gratzel, M.; Shklover, V.; Zukal, A. J. Phys. Chem. B 2000, 104, 12012. (3) Vettraino, M.; Trudeau, M.; Antonelli, D. M. Inorg. Chem. 2001, 40, 2088. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (5) Tuller, L. J. Electroceram. 1997, 1, 211. (6) Forx, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (7) Yun, H. S.; Miyazawa, K.; Zhou, H.; Honma, I.; Kuwabara, M. Adv. Mater. 2001, 13, 1377. (8) Haseloh, S.; Choi, S. Y.; Mamak, M.; Coombs, N.; Petrov, S.; Chopra, N.; Ozin, G. A. Chem. Commun. 2004, 1460. (9) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943. (10) Morrison, P. W., Jr.; Raghavan, R.; Timpone, A. J. Chem. Mater. 1997, 9, 2702. (11) Ragai, J.; Lotfi, W. Colloid Surf. 1991, 61, 97. (12) Chen, Q.; Qian, Y.; Chen, Z.; Zhou, G.; Zhang, Y. Mater. Lett. 1995, 22, 77. (13) Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. Nano Lett. 2002, 2, 717. (14) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (15) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15, 2463. (16) Lee, S.; Jeon, C.; Park, Y. Chem. Mater. 2004, 16, 4292. (17) Hwang, Y. K.; Lee, K. C.; Kwon, Y. U.; Chem. Commun. 2001, 1738. (18) Yun, B. S.; Miyazawa, K.; Zhou, H.; Honma, I.; Kuwabara, M. Adv. Mater. 2001, 13, 1377. (19) Yang, P.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (20) Welton, T. Chem. Rev. 1999, 99, 2071. (21) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351. (22) Schofer, S. H.; Kaftzik, N.; Wasserscheidt, P.; Kragl, U. Chem. Commun. 2001, 425. (23) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106. (24) Buijsman, R. C.; Van Vuuren, E.; Sterrenburg, J. G. Org. Lett. 2001, 3, 3785. (25) Bosmann, A.; Datsevish, L.; Jess, A.; Lauter, A. E.; Wasserscheid, P. Chem. Commun. 2001, 2494. (26) Yanes, E. G.; Gratz, S. R.; Bardwin, M. J.; Robison, S. E.; Stalcup, A. M. Anal. Chem. 2001, 73, 3838.

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