Pseudo-Cube Shaped Brookite (TiO2) Nanocrystals Synthesized by

Sep 14, 2011 - Synopsis. Single-phase brookite nanoparticles were synthesized using a water-soluble titanium−glycolate complex as the precursor and ...
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Pseudo-Cube Shaped Brookite (TiO2) Nanocrystals Synthesized by an Oleate-Modified Hydrothermal Growth Method Yukiaki Ohno,† Koji Tomita,† Yukihiro Komatsubara,‡ Takaaki Taniguchi,# Ken-ichi Katsumata,*,‡ Nobuhiro Matsushita,‡ Toshihiro Kogure,^ and Kiyoshi Okada‡ †

Department of Chemistry, School of Science, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan # Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan ^ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡

bS Supporting Information ABSTRACT: Single-phase nanoparticles of brookite, a less common polymorph of TiO2 than rutile and anatase, were successfully synthesized using a water-soluble titanium glycolate complex as the precursor and nanoparticle growth by an oleate-modified hydrothermal growth method. The synthesized particles were approximately 30 nm in size, had high crystallinity, and were highly dispersible in water. The morphology of the synthesized particles was a pseudocube surrounded mainly with four {210} and two {001} faces, accompanying truncation with small {111} facets. It is considered that the preferential absorption of oleate molecules on {210} and {001} faces resulted in the pseudocubic crystal shape.

’ INTRODUCTION Titanium dioxide (TiO2) is a well-known photocatalyst material1 that when exposed to ultraviolet (UV) light irradiation, generates electron hole pairs, which reduce and oxidize adsorbates on the surface, respectively, thereby producing radical species, such as OH radicals and O2 . These radicals have strong oxidation activity and can decompose most organic compounds and bacteria.2 6 The photoinduced hydrophilicity of TiO2 was discovered in 1995;7 10 the TiO2 surface becomes highly hydrophilic with UV light irradiation. This unique property has already been applied to various industrial products, such as self-cleaning exterior tiles, antibeading automobile side mirrors, and antifogging glasses. Three natural polymorphs of TiO2 are known: anatase (space group: I41/amd), rutile (P42/mnm), and brookite (Pbca). Rutile is the most stable phase at any temperature, whereas the other two phases are metastable and transform to rutile when heated at high temperatures by the provision of activation energy. The syntheses and applications of anatase and rutile have been extensively studied because they are relatively easy to synthesize.11 20 In contrast, brookite is a rare phase of the TiO2 polymorphs and is generally not easily synthesized. Recently, the synthesis of brookite nanoparticles with different morphologies such as nanorods, nanotubes, and nanoflowers has been reported.21 30 Kominami et al.21 reported the synthesis of nanosized brookite particles (ca. 50 nm) by a solvothermal method. Buonsanti et al. 29 reported a surfactant-assisted r 2011 American Chemical Society

nonhydrolytic strategy to synthesize anisotropic brookite TiO2 nanocrystals with tunable aspect ratios in the hardly accessible brookite crystal structure. They also reported a seeded-growth approach to fabricate topologically controlled magnetic-semiconductor heterodimer hybrid nanocrystals that individually comprise a single spherical FexOy domain epitaxially grown at either one apex or any location along the longitudinal sidewalls of one rod-like brookite TiO2 section.30 Although a few papers have reported the synthesis of single-phase brookite, its properties are not yet fully understood. Ohtani et al.31 reported higher photocatalytic activity for brookite than rutile and anatase. Kandiel et al.26 also reported that the photocatalytic hydrogen evolution activity of a pure brookite suspension was higher than that of pure anatase, despite the former having a lower surface area. Koelsch et al.32 suggested that brookite is a good candidate for photovoltaic devices. However, these brookite nanoparticles have been synthesized by using TiCl3, TiCl4, and titanium alkoxides as a Ti source. It is difficult to handle due to their strong toxicological and corrosive properties. Tomita et al.33 successfully synthesized single-phase brookite nanoparticles using a water-soluble titanium complex as the precursor. The nanoparticles had high crystallinity, but the morphology was not controlled. We have succeeded in the syntheses of Received: May 18, 2011 Revised: September 14, 2011 Published: September 14, 2011 4831

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highly crystalline functional nanoparticles with controlled morphology, such as Fe3O4, CeO2, and YVO4, using an oleatemodified growth technique.34 36 In this study, brookite nanoparticles with controlled morphology were synthesized using a titanium complex aqueous solution and the oleate-modified hydrothermal growth process. The morphology of the nanoparticles was precisely determined using transmission electron microscopy (TEM).

’ EXPERIMENTAL SECTION Synthesis of Brookite Nanoparticles. Titanium powder (Wako Pure Chemical Industries, Tokyo; 2.0 mmol) was dissolved in a cooled 28% NH3 aqueous solution (Wako Pure Chemical Industries, Tokyo; 2.0 mL) containing 30% H2O2 (Wako Pure Chemical Industries, Tokyo; 8.0 mL) to obtain peroxo-titanium complex ions ([Ti(OH)3O2] ). After the solution was stirred for 2 h, the titanium powder was completely dissolved, and glycolic acid (HOCH2COOH, Kanto Chemicals Co. Inc., Tokyo; 3.0 mmol) was then added to the solution as a complexant agent to obtain glycolate peroxotitanium complex ions [Ti4(C2H2O3)4(C2H3O3)2(O2)4O2]6 .33 The solution was dried at 65 C to remove excess H2O2 and NH3, forming an orange colored gel. The gel was easily dissolved by the addition of distilled water and formed a yellow transparent aqueous solution. The initial pH of the glycolate titanium complex solution was approximately 6. Sodium oleate (C17H33COONa; Wako Pure Chemical Industries, Tokyo; 2.0 mmol) and sodium acetate (CH3COONa; Wako Pure Industries, Tokyo; 20.0 mmol) dissolved in distilled water (18 mL) were then added to the solution containing glycolate titanium complex ions. The total volume of the solution was adjusted to 20 mL using distilled water, making a pH of 8. The solution was sealed in a Teflon-lined stainless steel autoclave (50 mL) and heated at 130 200 C for 1 6 h. After the hydrothermal reaction, the white precipitate formed and was separated from the solution by centrifuging (6000 rpm for 30 min) and was then washed with distilled water several times. Characterization of the Nanoparticles. The crystalline phase of the samples was identified using a powder X-ray diffractometer (XRD, RINT-TTR3B; Rigaku, Japan) with monochromated Cu Kα radiation. The applied voltage and current to the Cu target was 50 kV-200 mA. The crystalline phase was also identified using Raman spectroscopy (RAMANOR T64000; Jobin-Yvon S.A.S., France) with an Ar laser (514.5 nm) operated at 50 mW. The particle size and morphology of the samples were investigated using TEM (HF-2000; Hitachi, Japan and JEM-2010UHR, JEOL, Japan) operating at 200 kV. The samples for TEM were prepared in two different ways: (1) one drop of the sample dispersed in cyclohexane or water was deposited on an amorphous carbon grid, and (2) the sample powder was embedded in epoxy resin between two glass slides. After hardening, the glass slides were cut using a diamond wheel to laths of ca. 1 mm thickness. After mechanical thinning down to approximately 70 μm, the specimen was further thinned by conventional argon ion-thinning (Dual Ion Mill model 600, Gatan, USA). High-resolution TEM (HRTEM) images were mainly obtained using the JEM-2010UHR and recorded on films or a Gatan MSC 794 bottom-mounted CCD camera. Fourier transform infrared spectroscopy (FT-IR) was performed using a JIR-7000, JEOL, Japan. Hydrodynamic particle size was measured using a dynamic laser scattering apparatus (DLS, Zetasizer Nano ZS, Malvern Instruments Ltd., UK).

’ RESULTS AND DISCUSSION Figure 1 shows XRD patterns of the samples synthesized using sodium acetate and sodium oleate by hydrothermal treatment at 200 C for 6 h. All reflections in the XRD patterns can be assigned to brookite (ICDD#00-029-1360). The XRD pattern of

Figure 1. XRD patterns of samples synthesized by hydrothermal treatment at 200 C for 6 h with (a) sodium acetate and (b) sodium oleate additives. The standard pattern for brookite (ICDD#00-0291360) is shown at the bottom as a reference.

Figure 2. Raman spectra of samples synthesized by hydrothermal treatment at 200 C for 6 h with (a) sodium acetate and (b) sodium oleate additives. Anatase powder was obtained from Wako Pure Chemical Industries, and brookite powder was prepared by the hydrothermal method.28

the sample using sodium acetate shows a narrower peak width than that using sodium oleate, which indicates slightly higher crystallinity or larger crystal size. No reflections assignable to other TiO2 phases such as anatase and rutile were observed in the XRD patterns. When the titanium complex/sodium oleate molar ratio was changed, brookite could not be obtained as a main phase (see Supporting Information section Figure S1). Therefore, the optimum molar ratio to obtain the brookite phase is considered to be 1.0. However, it is difficult to clearly confirm that the obtained samples are single-phase brookite only from XRD data because the (101) reflection of anatase at 2θ = 25.28 (ICDD#00-021-1272), which is the strongest reflection, overlaps with the (120) reflection of brookite at 2θ = 25.34. Figure 2 shows Raman spectra of the samples, with one strong Raman peak at 156 cm 1 and 13 weak peaks at ca. 126, 197, 215, 245, 287, 320, 365, 414, 460, 502, 548, 589, and 637 cm 1. The 37 space group of brookite is Pbca or D15 2h symmetry. From group theory, brookite has the following optical modes: 9A1g + 9B1g + 9B2g + 9B3g + 9A1u + 8B1u + 8B2u + 8B3u, where A1g, B1g, B2g, and B3g modes are Raman active, Bu, B2u, and B3u modes are infrared active, and the A1u mode is inactive in both Raman and infrared.38 Brookite shows 15 vibration bands in the range of 100 to 700 cm 1, which are A1g (155, 194, 247, 412, and 636 cm 1), 4832

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B1g (213, 322, and 501 cm 1), B2g (366, 395, 460, and 583 cm 1), and B3g (172, 287, and 545 cm 1).39 On the other hand, anatase has six Raman active modes; A1g + 2B1g + 3Eg and Ohsaka et al.40 reported the following bands at 144 cm 1 (Eg(1)), 197 cm 1 (Eg(2)), 399 cm 1 (B1g(1)), 513 cm 1 (A1g), 519 cm 1 (B1g(2)), and 639 cm 1 (Eg(3)). Rutile has four Raman active modes: A1g + B1g + B2g + Eg. The allowed modes reported were at 143 cm 1 (B1g), 447 cm 1 (Eg), 612 cm 1 (A1g), and 826 cm 1 (B2g).41 The Raman peaks observed in the present samples were in good agreement with the Raman active modes of brookite, and no peaks inherent in anatase (399 cm 1 (B1g) and 519 cm 1 (A1g)) and rutile (447 cm 1 (Eg) and 612 cm 1 (A1g)) were apparent. Therefore, the synthesized samples were concluded to be single-phase brookite, also considering the Raman spectra. Figure 3 shows IR spectra of the samples. A broad band at 3200 3600 cm 1 is attributed to the O H stretching of physisorbed water on the sample surfaces, and a band at 1630 cm 1 corresponds to the O H bending modes of water molecules.42 The bands at 400 800 cm 1 are attributed to the Ti O stretching vibrations of crystalline TiO2.43 For the sample synthesized using sodium acetate, the band at 1700 cm 1 can be attributed to the CdO bond.44 On the other hand, a series of bands for the sample prepared using sodium oleate, at 2840 2940 cm 1, are attributed to the symmetric and asymmetric CH2 stretching modes and the asymmetric CH3- stretching mode,45 which indicates that organics assigned to oleic acid were contained in the obtained sample. The bands at 1430 and 1556 cm 1 are attributed to the symmetric and asymmetric COO stretching, the band at 1400 cm 1 is attributed to CH2 bonds, and the band at 1715 cm 1 is attributed to the CdO bond;44 therefore, oleate molecules are thought to bond with the sample surface.

Figure 4 shows particle size distributions of the samples synthesized using sodium acetate and sodium oleate by hydrothermal treatment at 200 C for 6 h. The average hydrodynamic particle size of the sample synthesized using sodium oleate dispersed in water was 32 nm and smaller than that using sodium acetate (72 nm). This result is consistent with the XRD measurements, in that the crystal size of the samples synthesized using sodium acetate was larger than that using sodium oleate. Figure 5 shows TEM images of the samples synthesized using sodium acetate and sodium oleate. The particle sizes of the samples synthesized using sodium acetate (Figure 5a) were 50 100, nm and the morphology resembled a grain of rice, which inspired the name “nanorice structure”.46 On the other hand, the particle sizes of the samples synthesized using sodium oleate (Figure 5b) were 30 40 nm, and the morphology was a squarish shape without angles. In addition, the particle size of the samples synthesized using sodium acetate was larger than that of the samples synthesized using sodium oleate (see Figure S2, Supporting Information). The observed particle size is in good agreement with the average hydrodynamic particle size measured by DLS (Figure 4), which suggests that the sodium oleate used for the synthesis plays an important role to control the morphology of the grown brookite particles from 1D-rods to pseudocubic habits. Figure 6 shows TEM images of the samples synthesized at various hydrothermal temperatures from 130 to 200 C. XRD and Raman spectra revealed that these reaction conditions strongly favor the yield of brookite phase products rather than other crystalline titania phases (see Figures S3 and S4, Supporting Information). When the hydrothermal temperature was 130 C, the products were mixtures of nanoparticles with larger (40 nm) and smaller (5 nm) sizes. It is considered that the larger and smaller particles were brookite and amorphous titania

Figure 3. FT-IR spectra of samples synthesized by hydrothermal treatment at 200 C for 6 h with (a) sodium acetate and (b) sodium oleate additives.

Figure 5. TEM images of brookite samples synthesized using (a) sodium acetate and (b) sodium oleate additives.

Figure 4. Particle size distributions of samples synthesized using (a) sodium acetate and (b) sodium oleate additives, as measured by DLS using a colloidal dispersion of water/oleate-stabilized brookite nanoparticles. 4833

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Crystal Growth & Design particles, respectively. The XRD pattern (Figure S3, Supporting Information) had hollow peaks assigned to the amorphous phase, and the Raman spectrum (Figure S4, Supporting Information) can be assigned to brookite. An increase of the hydrothermal temperatures to 200 C resulted in a decrease of the smaller particles but an increase in the larger pseudocube shaped particles. Figure 7 shows TEM images of the samples synthesized by changing the hydrothermal time to 3, 6, and 48 h at 200 C. The XRD and Raman spectra revealed that these reaction conditions strongly favor the yield of the brookite phase rather than other crystalline titania phases (see Figures S5 and S6, Supporting Information). For the synthesis time of 3 h, pseudocube shape brookite particles were formed, but the presence ratio was low. TEM images of the samples synthesized at 200 C for 3 h (Figure 7a) were similar to the sample synthesized at 130 C for 6 h (Figure 6a), which contained larger nanoparticles with pseudocube shape and smaller particles. The intensity ratio between the ((210)(111)) and (211) peaks of the 200 C-6 h sample was close to 1, as observed in Figure 1, but was less than 0.5 for the 200 C-3 h sample shown in Figure S5, Supporting Information. These results indicate that the (101) anatase peak overlaps with the brookite peak. Moreover, the (004) anatase peak around 38 was also observed in the XRD pattern (Figure S5, Supporting Information) and anatase peaks at 397 and 516 cm 1 were evident in the Raman spectrum (Figure S6, Supporting Information). Therefore, the smaller particles were anatase particles. The smaller particles disappeared and pseudocube particles appeared (Figure 7b,c) with an increase in the hydrothermal time to 6 and 48 h. On the basis of these results, it is supposed that crystal growth may be dependent on the transformation of the classical Ostwald ripening principle involving dissolution of smaller nanoparticles and reprecipitation into larger particles by increasing the hydrothermal temperature and time. Sugimoto et al.47 reported the

Figure 6. TEM images of brookite nanoparticles synthesized at hydrothermal temperatures of (a) 130 and (b) 200 C.

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effect of sodium oleate on the shape of anatase particles formed in a solution of Ti(IV) TEOA (triethanolamine) compound at an initial pH of 10.5. The particles obtained were cube-shaped with sharp edges. Sodium oleate may specifically adsorb onto the {001} and {100} faces to reduce their specific surface energies and induce the formation of the cubic shape. We assume that pseudocubic morphology of the present particles was attained by the selective absorption of oleic acid molecules onto the specific crystal faces of brookite to lower the surface energy. This is because the structure of anatase TiO2(100) is similar to the brookite TiO2(001), and the structure of anatase TiO2(001) has a certain similarity to brookite TiO2(210) (see Figure S7, Supporting Information). In contrast, the brookite particles in the samples synthesized without using sodium oleate became larger, because their crystal faces may have higher surface energy than those of the oleate-absorbed faces. Kobayashi et al.48 reported the synthesis of single-phase brookite by hydrothermal treatment using water-soluble titanium complexes. The obtained particles had a one-dimensional (1-D) rod shape. Zhao et al. 25 reported the formation of flower-like textures consisting of agglomerated 1-D rod-shaped brookite nanoparticles by hydrothermal treatment. The reported morphologies of brookite crystals have always been rodshaped,21,25,28 30,33,48,49 and there have been no reports on the formation of the pseudocube morphology discussed here. This study is the first report on the synthesis of brookite nanoparticles with pseudocubic morphology. The three-dimensional morphology of the brookite particles grown with sodium oleate (Figure 5b) was analyzed using highresolution transmission electron microscopy (HRTEM). Most particles dispersed on the carbon gird could be oriented with the incident beam parallel to Æ110æ or Æ120æ by tilting the sample holder several degrees. Figure 8a shows an HRTEM image taken along the Æ110æ direction. Now let us suppose that the direction is [110], one of the four equivalent Æ110æ directions for brookite with mmm point symmetry. Considering the spacing of the lattice fringes and the diffraction pattern (inset of the figure), it is understood that the particle is surrounded with planes parallel to (001), (001), (110), (110), (111), (111), (111), and (111). If the termination feature of the lattice fringes at the crystal edges are examined, it is noted that the lattice fringes terminate sharply at the two {001} and four {111} edges, whereas they are fading toward the (110) and (110) edges, and the edges are not sharp. This implies that the former six edges form facets parallel to the incident beam but not for the latter two. When the same crystal was rotated about the c-axis by ca. 20 to set the [120] direction parallel to the electron beam, the side edges became parallel to (210) and (210) (Figure 8b). In this case, the lattice fringes terminate sharply at these edges, which suggests that these edges form facets on the crystal surface. The idea that {210} facets are

Figure 7. TEM images of brookite nanoparticles synthesized by hydrothermal treatment at 200 C and reaction times of (a) 3, (b) 6, and (c) 48 h. 4834

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Figure 8. (a) HRTEM image of a brookite particle dispersed on a carbon grid, viewed along [110], and the corresponding diffraction pattern. The Miller indices of the lattice planes surrounding the particle are indicated. (b) HRTEM image and diffraction pattern of the same particle as in (a) but viewed along [120] by tilting the particle about the c-axis. (c) HRTEM image of brookite particles in the ion-milled specimen, showing 0.35 nm lattice fringes that correspond to {210} lattice planes. Note the fringes terminate sharply at the crystal edges, as indicated with the white arrows. (d) HRTEM image of a particle in the ion-milled specimen and its Fourier transform, showing the crossed lattice fringes with 0.35 nm spacing and crossing angle of ca. 80.

as indicated by the white arrows. Furthermore, the particle shown in Figure 8d has crossed lattice fringes, both of which have 0.35 nm spacing and the crossed angle is ca. 80. This geometry suggests that the lattice fringes correspond, for example, to (210) and (210) (the calculated angle between the two planes is 80.18) and the crystal orientation is [001]. From this image, it is evident that four equivalent {210} planes form side facets of the crystal. The shape of the crystal is considered approximately a pseudocube because the distance between the two {001} facets and that between the opposite two {210} facets are comparable (Figure 8b), and the angles between adjacent {210} facets are 80.2 and 99.8. From the image in Figure 8a, {111} (the number of the equivalent facets is eight) also forms small facets, to truncate the four corners of the pseudocube with a pair of {111} facets at each corner (Figure 9).

Figure 9. Trigonometric drawing indicating the morphology of the brookite crystal synthesized at 200 C with sodium oleate.

developed on brookite particles was confirmed by observing the sample embedded in epoxy resin and thinned by ion milling (Figure 8c,d). In the sample, the brookite particles are oriented in various directions, whereas the particles dispersed on the carbon film are preferentially oriented as the developed facets are parallel to the film. In Figure 8c, the lattice fringes of 0.35 nm, which corresponds to (210), terminate sharply at the crystal boundaries

’ CONCLUSION The oleate-modified hydrothermal growth process has been successfully extended to the synthesis of highly dispersible brookite nanoparticles. The obtained nanoparticles have a pseudocube shape, and the particle size is approximately 40 nm. The particles were surrounded by four {210} and two {001} facets, accompanying small {111} facets. The addition of oleic acid in the synthesis is thought to cause preferential absorption on the {210} and {001} facets, which induces the particles to exhibit pseudocube morphology. The present method is thought to be very effective for the synthesis of nanoparticles with controlled morphology and grain size, and could be applicable to the synthesis of other metal oxides. 4835

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’ ASSOCIATED CONTENT

bS

Supporting Information. XRD patterns, Raman spectra, and TEM images of the samples synthesized by hydrothermal treatment. Structures of anatase TiO2(001), anatase TiO2(100), brookite TiO2(001), and brookite TiO2(210). These materials are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-45-924-5369. Fax: +81-45-924-5358. E-mail: katsumata. [email protected].

’ ACKNOWLEDGMENT We thank Mr. K. Yamamoto (Tokai University, Japan), Mr. S. Omata, and Mr. K. Fuse (Tokyo Institute of Technology, Japan) for fruitful discussions and Prof. Y. Kitamoto (Tokyo Institute of Technology, Japan) for allowing use of the TEM (HF-2000). We are also grateful to E. Fujii (The University of Tokyo) for preparation of the ion-milled sample for TEM observation. ’ REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Kawai, T.; Sakata, T. Nature 1980, 286, 474–476. (3) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633–5638. (4) Sunada, K.; Watanabe, T.; Hashimoto, K. J. Photochem. Photobiol. A: Chem. 2003, 156, 227–233. (5) Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Environ. Sci. Technol. 1998, 32, 726–728. (6) Trapalis, C. C.; Keivanidis, P.; Kordas, G.; Zaharescu, M.; Crisan, M.; Szatvanyi, A.; Gartner, M. Thin Solid Films 2003, 433, 186–190. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, A.; Watanabe, T. Nature 1997, 388, 431–432. (8) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135–138. (9) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188–2194. (10) Watanabe, T.; Nakajima, A.; Wang, R.; Minabe, M.; Koizumi, S.; Fujishima, A.; Hashimoto, K. Thin Solid Films 1999, 351, 260–263. (11) Cozzoli, P. D.; Curri, M. L.; Giannini, C.; Agostiano, A. Small 2006, 2, 413–421. (12) Fisher, A.; Kuemmel, M.; Jarn, M.; Linden, M.; Boissiere, C.; Nicole, L.; Sanchez, C.; Grosso, D. Small 2006, 2, 569–574. (13) Wang, K.; Wei, M.; Morris, M. A.; Zhou, H.; Holmes, J. D. Adv. Mater. 2007, 19, 3016–3020. (14) Hu, Y. S.; Kienle, L.; Guo, Y. G.; Maier, J. Adv. Mater. 2006, 18, 1421–1426. (15) Wang, Q.; Wen, Z. H.; Li, J. H. Adv. Funct. Mater. 2006, 16, 2141–2146. (16) Chen, K. S.; Liu, W. H.; Wang, Y. H.; Lai, C. H.; Chou, P. T.; Lee, G. H.; Chen, K.; Chen, H. Y.; Chi, Y.; Tung, F. C. Adv. Funct.Mater. 2007, 17, 2964–2974. (17) Zhang, D.; Yoshida, T.; Oekermann, T.; Furuta, K.; Minoura, H. Adv. Funct. Mater. 2006, 16, 1228–1234. (18) Zhou, Z.; Shinar, R.; Allison, A, J.; Shinar, J. Adv. Funct. Mater. 2007, 17, 3530–3537. (19) Liu, Z. Y.; Sun, D. D.; Guo, P.; Leckie, J. O. Chem.—Eur. J. 2007, 13, 1851–1855. (20) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. Adv. Mater. 2006, 18, 2807–2824.

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