Ultrasonic Spray Pyrolysis Fabrication of Solid and Hollow PbWO4

Dec 13, 2010 - College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing, 400067, People's Republic...
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J. Phys. Chem. C 2011, 115, 241–247

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Ultrasonic Spray Pyrolysis Fabrication of Solid and Hollow PbWO4 Spheres with Structure-Directed Photocatalytic Activity Fan Dong,†,‡,| Yu Huang,‡ Shichun Zou,§ Jiang Liu,‡ and S. C. Lee*,‡ College of EnVironmental and Biological Engineering, Chongqing Technology and Business UniVersity, Chongqing, 400067, People’s Republic of China, Department of CiVil and Structural Engineering, Research Center for EnVironmental Technology and Management, The Hong Kong Polytechnic UniVersity, Hong Kong, People’s Republic of China, Department of EnVironmental Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and School of Marine Sciences, Sun Yat-Sen UniVersity, Guangzhou, 510275, People’s Republic of China ReceiVed: August 30, 2010; ReVised Manuscript ReceiVed: NoVember 15, 2010

Solid and hollow PbWO4 spheres were fabricated by a one-step and template-free ultrasonic spray pyrolysis (USP) using lead nitrate and ammonium metatungstate hydrate as precursors. The characteristics of the resulting samples were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2 adsorption/desorption, and UV-vis diffuse reflectance spectroscopy in detail. The addition of citric acid (CA) to the precursor solution played a crucial role in producing hollow structured spheres. The hollow PbWO4 spheres were composed of single-crystal nanoparticles. On the basis of the characterization and experimental observations, a possible mechanism on the formation of solid and hollow PbWO4 spheres was proposed. Gaseous products generated due to the decomposition of CA in the pyrolysis process shaped the final hollow morphology. The results also indicated that the CA addition could decrease the crystal size of hollow PbWO4. The specific surface areas and pore volume of hollow PbWO4 spheres were increased simultaneously due to the CA decomposition compared with solid PbWO4 spheres produced without CA addition. The PbWO4 spheres were tested as photocatalyst for NO removal in air. The hollow structured PbWO4 spheres were found to exhibit superior photocatalytic activity to solid spheres due to the differences in microstructure and morphology. The result acquired may shed light on general fabrication strategy for designing hollow structured materials by USP. 1. Introduction Micro/nanostructured materials, such as solid and hollow spheres, have found diverse applications in nanoscale chemical reactors, adsorbents, sensors, (photo)catalysts, drug delivery, and biomedical imaging because of their unique characteristics.1-8 Consequently, they have been the subject of many recent investigations. Compared to the solid spheres, hollow spheres combining the merits of hollow and porous structures could present improved mass transfer and high surface areas.9,10 The interior void of hollow spheres could encapsulate various substances or reflect light as a space or a reaction chamber.11,12 Conventional methods for fabrication of hollow spheres usually require removable or sacrificial templates (hard or soft).13-18 However, the use of templates usually suffers from disadvantages related to high cost and tedious synthetic procedures, which may prevent them from large-scale applications. In comparison with template methods, which involve multisteps, a facile one-pot template-free method for the controlled preparation of hollow structures is highly attractive and desirable.19 Ultrasonic spray pyrolysis (USP) is a one-step powerful synthetic method for the fabrication of a diverse range of microand nanostructured products, including quantum dots, catalysts, metal oxides, silica, porous carbons, and nanocomposite materials.20-29 In a typical process, ultrasound is applied to a * To whom correspondence should be addressed, [email protected]. † Chongqing Technology and Business University. ‡ The Hong Kong Polytechnic University. | Zhejiang University. § Sun Yat-Sen University.

precursor solution in order to create an aerosol that is then swept through a furnace by a carrier gas. Upon heating, the precursor solvent evaporates and precursors decompose resulting in a product with generally spherical shape. Each droplet in the aerosol can be considered as an individual microreactor (submicrometer diameter) that directs the resulted morphology.30 As a synthetic tool, USP has several advantages over other traditional methods, such as production of micrometer- or submicrometer-sized spherical particles, high product purity, continuous operation, and ease of controlling composition. PbWO4 is one of the most interesting tungstates, which has found wide applications in electromagnetic calorimetry, photoluminescence, thermoluminescence, and stimulated Raman scattering behavior and photocatalysis.31-33 PbWO4 material can be prepared by a template-assisted solution method,31 solid-state reaction method,32 and hydrothermal method.33 Here, we use USP as a continuous, one-step process for the fabrication of solid and hollow PbWO4 spheres for the first time. The structural properties were investigated in detail by various characterization tools. On the basis of the experimental results, the formation mechanism of solid and hollow spheres was proposed. Citric acid (CA) was found to play a key role in the formation of hollow structure. As expected, the hollow PbWO4 spheres as photocatalyst exhibited enhanced photocatalytic activity on the removal of NO in air compared to the solid one. 1.1. Fabrication of Solid and Hollow PbWO4 Spheres Photocatalysts. Solid and hollow PbWO4 spheres were prepared by the ultrasonic spray pyrolysis (USP) of aerosols. The aerosols were generated by an ultrasonic nebulizer. The schematic

10.1021/jp108221v  2011 American Chemical Society Published on Web 12/13/2010

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illustration of the aerosol reactor setup is shown in Figure S1 (Supporting Information). Lead nitrate Pb(NO3)2 and ammonium metatungstate hydrate (NH4)6H2W12O4 · H2O were obtained from Sigma-Aldrich and used as received. For the fabrication of solid PbWO4 spheres, 0.001 mol of lead nitrate was dissolved in 25 mL of 1 M HNO3 solution and stirred for 15 min (solution A). Ammonium metatungstate hydrate (0.001 mol) was dissolved in 25 mL of deionized water and stirred for 15 min (solution B). Subsequently, solution B was added into solution A and stirred for another 30 min. The mixed solution was nebulized using an ultrasonic nebulizer at 1.7 MHz (10% (YUYUE 402AI, Shanghai). Aerosol droplets generated by the nebulizer were carried through a tube furnace at 700 °C by an air flow with a suction vacuum pump. As the droplets passed through the furnace, the precursor decomposed and produced fine particles, which were collected in a series of percolators containing deionized water. The product was isolated from water by centrifugation, washed with ethanol and deionized water several times, and then dried at 60 °C for 8 h with no further treatment. For fabrication of hollow PbWO4 spheres, 0.004 mol of citric acid (CA) was added into solution B under other identical conditions. The solid and hollow PbWO4 spheres are denoted as PbWO4-S and PbWO4-H, respectively. 1.2. Characterization. The crystal phases of the sample were analyzed by X-ray diffraction with Cu KR radiation (XRD, Philips Xpert, USA). A scanning electron microscope (SEM, JEOL model JSM-6490, Japan) was used to characterize the morphology of the obtained products. The morphology, structure, and grain size of the samples were examined by transmission electron microscopy (TEM, JEM-2010, Japan). The samples for TEM were prepared by dispersing the final powders in ethanol, and the dispersion was then dropped onto carbon copper grids. The UV-vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV-vis spectrophotometer (UV-vis DRS, Varian Cary 100, China) equipped with an integrating sphere assembly, using BaSO4 as reflectance sample. The spectra were recorded at room temperature in air and ranged from 250 to 800 nm. Nitrogen adsorption-desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP 2020, USA). All the samples were degassed at 200 °C prior to measurements. 1.3. Evaluation of the Photocatalytic Activity. The photocatalytic activity of the resulting samples was investigated by oxidation of NO at parts per billion levels in a continuous flow reactor at ambient temperature under UV light irradiation.34,35 The volume of the rectangular reactor, which was made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). Two 6 W UV lamps (Cole-Parmler) emitting a primary wavelength at 365 nm were vertically placed outside and above the reactor. For each photocatalytic activity test experiment, one sample dish (with a diameter of 12 cm) containing the photocatalyst powders was placed in the center of the reactor. The photocatalyst samples were prepared by coating an aqueous suspension of the samples onto the glass dish. The weight of the photocatalysts used for each experiment was kept at 0.2 g. The dishes containing the photocatalyst were pretreated at 70 °C for several hours until complete removal of water in the suspension and then cooled to room temperature before the photocatalytic test. The NO gas was photodegraded at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance, BOC gas) with traceable National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc., model 111).

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Figure 1. XRD patterns of PbWO4-S and PbWO4-H prepared by USP.

The desired relative humidity (RH) level of the NO flow was controlled at 70% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 3.3 L/min by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases, and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments, Inc., model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of the photocatalyst. 2. Results and Discussion 2.1. Phase Structure and Morphology. XRD patterns of both samples (Figure 1) correspond to PbWO4 with tetragonal stolzite structure (JCPDS 21-1272). The unit cell parameters are a ) b ) 0.545 nm, c ) 1.205 nm, and R ) β ) γ ) 90°. The crystallographic planes are also shown in Figure 1. No other phases can be seen in the XRD pattern, and the narrow line widths indicate a high degree of crystallinity. The (112) peak intensity of PbWO4-S is much higher than that of PbWO4-H, indicating that the crystallinity of solid PbWO4 spheres is higher. The crystal sizes of PbWO4-S and PbWO4-H are calculated to be 31.6 and 26.6 nm, respectively, by using Scherrer’s equation, which indicates that the CA in the USP process inhibits the crystal growth. Materials prepared by USP usually contain particles with a spherical shape. Figure 2a shows a typical SEM image of the solid PbWO4, clearly revealing that the sample consists of large amounts of polydispersed spheres ranging from several hundred nanometers to 1 µm in diameter. The surface of the spheres is not smooth, as shown in Figure 2b, suggesting that the resulting microspheres are formed by the aggregation of nanoparticles. Huang et al. observed similar phenomenon in the USP prepared Bi2WO6 spheres.22 The addition of CA to the precursor solution induced an obvious change of the morphology, as shown in panels c and d of Figure 2. First, the agglomeration of the primary nanosized particles is prevented by adding CA. The spheres are porous owing to the presence of large cavities. The crystal size of the PbWO4 nanoparticles decreased compared to samples prepared without the addition of CA, which is confirmed by the XRD results. Second, a substantial difference between the spherical particles obtained with and without the addition of CA is that the former ones are hollow spheres

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Figure 2. SEM images of PbWO4-S (a, b) and PbWO4-H (c, d) illustrating typical morphologies.

(Figure 2d). Some broken hollow spheres and some fragments can also be observed. The resulting hollow PbWO4 spheres are further investigated by TEM. As shown in Figure 3a, the PbWO4 spheres are lying side by side with different diameters. The spheres present a cagelike hollow structure. Panels b and c of Figure 3 indicate that the hollow sphere is formed by many nanoparticles through aggregation, which are in intimate contact with many other crystals in a continuous structure. Consistent with SEM, the sphere is porous due to large cavities and small pores. The HRTEM image of a single particle on the edge of a wellcrystallized PbWO4 sphere in Figure 3b is shown in Figure 3d. The fringe spacing is ca. 0.272 nm, which corresponds to the (200) plane of PbWO4. The inset of Figure 3d yields an SAED pattern, which can be indexed to the (200) and (116) diffraction planes, respectively. The EDS spectrum (Figure 3e) clearly indicates that the tungstate spheres are composed solely of Pb, W, and O. The C and Cu peaks originate from the TEM grid. 2.2. BET Surface Areas and Pore Structure. Figure 4 shows the nitrogen adsorption-desorption isotherms and Barret-Joyner-Halenda (BJH) pore-size distribution curves (inset) of the solid and hollow PbWO4 spheres. For both samples, the distinct hysteresis loop observed in the range of 0.4 < P/P0 < 1.0 indicates the existence of a mesoporous structure.36-38 The BET specific surface areas of the solid and hollow PbWO4 spheres are about 2.0 and 3.5 m2/g, respectively. The BJH pore-size distribution for the desorption isotherm shows that the solid and hollow PbWO4 spheres possess mesopores (3.0-8.0 nm), which are produced by the agglomeration of primary particles in the spheres. The pore volumes of the solid and hollow PbWO4 spheres are calculated to be 0.0038 and 0.0085 cm3/g, respectively. The above results indicate that the addition of CA in the USP process could increase the specific surface area and the pore volume due to

the decomposition of CA, which could improve the mass transfer of reactant and reaction products. 2.3. Formation Mechanism of Solid and Hollow PbWO4 Spheres. It is known that the aerosol assisted UPS synthesis process generally involves several stages: nebulization, solvent evaporation, solute pyrolysis, crystallization, and particle shaping. In many cases, the resulting products are solid spheres. The formation process for solid spheres is illustrated in Figure 5. When the nebulized aerosols carrying solutes of lead nitrate and ammonium metatungstate hydrate undergo high temperature treatment, solvent evaporation and solute pyrolysis occur. The decomposed products react with each other quickly to produce crystallized PbWO4 particles, which are then assembled into solid spheres through free aggregation (Figure 2b). With CA-improved UPS, the process is different and hollow spheres can be produced. Zhang et al. reported the H3BO3-induced formation of metal oxide hollow spheres in flowing aerosols, which can be attributed to the in situ emission of gas phase HBO2 from the inner part of the metal oxide microspheres.39 Lee et al. prepared hollow silica particle by a polystyrene latex (PSL) modified UPS process, where the removal of PSL shaped the final morphology.40 We found that the addition of CA to the precursor solution induced formation of cagelike PbWO4 hollow spheres. The mechanism behind the hollow PbWO4 sphere particle formation is of particular interest considering that the hollow morphology is obtained without the use of template. CA is well-known to dynamically modify the crystal growth along specific crystallographic facets in solution preparation method as chelating ligand.41 Thus, it can be used to as “shape modifier” to adjust and control the size and morphology of the metal oxide products.41,42 CA is also reported as a fuel supporting decomposition of precursors during ultrasonic spray combustion to metal oxide, and the mechanism of the effect of CA decomposition on the metal

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Figure 3. TEM (a-c) and HRTEM images (d) of PbWO4-H. The inset in (d) shows the corresponding SAED pattern. (e) EDS data for PbWO4-H.

oxide morphology is not known.43 On the basis of the characterization and experimental observations, we proposed the possible formation processes of the hollow structured spheres highlighting the role of CA (Figure 5). During the formation of hollow PbWO4 spheres, the decomposition of CA would take place simultaneously in the droplet at high

temperature, subsequently producing gaseous product, e.g., CO2 and CO, which are very volatile. The resulting gas would escape from the PbWO4 spheres to create inner hollow structure or broken fragments, leaving large cavities and small pores in the hollow PbWO4 spheres (panels b and c of Figure 3), as revealed in Figure 5. In this sense, the role of CA in

Fabrication of Solid and Hollow PbWO4 Spheres

Figure 4. Pore-size distribution and N2 adsorption-desorption isotherms (inset) of PbWO4-S and PbWO4-H samples.

morphology control is different from than of CA as chelating ligand; a new mechanism revealing the role of CA in forming hollow spheres is therefore discovered. This concept can also be applied to controlled fabrication of other oxide hollow spheres. 2.4. UV-vis Diffuse Reflectance Spectroscopy (DRS). Figure 6 shows UV-vis DRS for PbWO4-S and PbWO4-H samples. It can be seen that both samples show a huge optical absorption in the UV region. It can be further observed that the absorption edge for PbWO4-H is blue-shifted slightly. For the solid spheres, light can only reach on the outer surface. However, light can enter into the interior space of hollow spheres and can be reflected in the hollow space several times. Such multiple reflections could allow more efficient use of the light source, which may improve the photocatalytic activity. The observed UV-vis DRS differences may be due to the changes in how light is reflected from the hollow spheres. The direct band gap energy can be estimated from the intercept of the tangent to the plot of (Rhν)1/2 vs photon energy, as shown in the inset in Figure 6.44 The estimated band gap is 3.71 and 3.81 eV for solid and hollow PbWO4, respectively, which is consistent with the DFT calculation result.32 According to the DFT calculation result, the top of the valence band consists of O 2p, Pb 6s, and W 5d and the bottom of the conduction band is composed of the W 5d band hybridized with O 2p and Pb 6p orbitals.32 2.5. Photocatalytic Property. To evaluate the performance of the solid and hollow PbWO4 spheres as photocatalyst, the photocatalytic degradation of NO in air under UV irradiation

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Figure 6. UV-vis DRS and plots of (Rhν)1/2 vs photon energy (inset) for PbWO4-S and PbWO4-H samples.

Figure 7. Photocatalytic degradation of NO in air under UV irradiation over solid and hollow PbWO4 spheres.

is used as a probe. Prior to the light irradiation, the adsorption/desorption equilibrium between the indoor air with NO and photocatalysts had been reached. When the lamp was switched on, the photodegradation reaction of NO was initiated. Figure 7 shows the relative of NO removal rate against irradiation time in the presence of PbWO4 photocatalysts under UV irradiation. As shown in Figure 7, direct photolysis of NO does not occur since NO removal rate is nearly zero in the absence of photocatalyst with UV. The NO removal rate reached as high as 98 ppb/min in the

Figure 5. Schematic illustration of the formation processes of the solid and hollow PbWO4 spheres fabricated by USP.

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presence of the hollow PbWO4 spheres, whereas NO removal rate on the solid PbWO4 spheres was much lower (35 ppb/ min), revealing the superior photocatalytic activity of the hollow structured PbWO4. If the removal rate is normalized by specific surface areas, the NO removal rate is 28 (ppb/min)/(m2/g) in the presence of the hollow PbWO4 spheres, whereas NO removal rate on the solid PbWO4 spheres was still lower 17.5 (ppb/min)/(m2/ g). In other words, there must be other factors resulting in the different activities between hollow and solid PbWO4 spheres. As known, the hollow architecture with large surface area and pore system plays an important role in catalyst design for its being able to improve the molecular transport of reactants and products.10,46,47 The cagelike hollow structure is also favorable for the multiple reflections of UV light within the sphere interior voids, allowing more efficient light harvesting and therefore offering an enhanced photocatalytic activity (inset in Figure 7).11 The photocatalytic reaction could take place in the interior void, and the resulting products could move out of the inner space through diffusion. Therefore, the enhanced photocatalytic activity of PbWO4 hollow spheres can be ascribed to the synergistic consequence of small crystal size, high surface area, large pore volume, and the special hollow structure.10,11,45-48 Further observation indicates that the NO removal rate decreased slightly after about 15 min, probably due to the accumulation of HNO3 on the catalyst surface that deactivated the photocatalysts.22 As reported by Ai et al., the photocatalytic oxidation of NO to NO3- was the major process with a minor amount of NO2- generated simultaneously.34 Therefore, final oxidation products (HNO2 and HNO3) absorbed on the surface of the photocatalysts were attributable to the decrease of photocatalytic activity of the PbWO4 spheres. Further observation in Figure 7 indicates that decrease of NO removal rate over hollow PbWO4 spheres is much slower than that of the solid spheres in 30 min. The reason for this fact is that the accumulation of final oxidation products on the catalyst surface is slower in the hollow PbWO4 spheres, as the special structure could facilitate the diffusion of the final reaction products. 3. Conclusion USP provides a one-step, continuous, and scalable process for the template-free fabrication of solid and hollow PbWO4 spheres using lead nitrate and ammonium metatungstate hydrate as precursors. The formation of a hollow structure can be ascribed to the decomposition of citric acid and emission of gaseous products during the sphere generation in the pyrolysis process. The solid PbWO4 spheres can be produced without citric acid addition. Photocatalytic testing reveals that hollow PbWO4 spheres are significantly more active for NO removal than that of the solid spheres. The enhanced photocatalytic activity of hollow PbWO4 spheres can be ascribed to the synergistic consequence of small crystal size, high surface areas, large pore volume, and the enhanced use of the light source due to the special hollow structure. The result may provide a general strategy for fabrication of hollow structured materials by the USP method. Acknowledgment. This work is supported by Research Grants from Chongqing Technology and Business University (2010-56-13). The work is also funded by the Research Grants Council of Hong Kong (PolyU 5204/07E and PolyU 5175/09E), The Hong Kong Polytechnic University (GYF08, GYX75, and

Dong et al. GYX0L). The work is also funded by The Program for Chongqing Innovative Research Team Development in University (KJTD201020) and The Chongqing Key Natural Science Foundation (CSTC, 2008BA4012). The authors also thank Mr. Tam for his technical support. Supporting Information Available: The schematic illustration of the aerosol reactor setup. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhu, L. P.; Xiao, H. M.; Zhang, W. D.; Yang, G.; Fu, S. Y. Cryst. Growth Des. 2008, 8, 957. (2) Zheng, Y. H.; Cheng, Y.; Wang, Y. S.; Zhou, L. H.; Bao, F.; Jia, C. J. Phys. Chem. B 2006, 110, 8284. (3) Yu, J. G.; Yu, H. G.; Guo, H. T.; Li, M.; Mann, S. Small 2008, 4, 87. (4) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (5) Caruso, F.; Caruso, R. A.; Moh¨wald, H. Science 1998, 282, 1111. (6) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (7) Martinez, C. J.; Hockey, B.; Montgomery, C. B.; Semancik, S. Langmuir 2005, 21, 7937. (8) Yin, W. Z.; Wang, W. Z.; Shang, M.; Zhou, L.; Sun, S. M.; Wang, L. Eur. J. Inorg. Chem. 2009, 29-30, 4379. (9) Zhao, Y.; Jiang, L. AdV. Mater. 2009, 21, 3621. (10) Lou, X. W.; Archer, L. A.; Yang, Z. C. AdV. Mater. 2008, 20, 3987. (11) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406. (12) Huang, Y.; Ho, W. K.; Ai, Z. H.; Song, X.; Zhang, L. Z.; Lee, S. C. Appl. Catal., B 2009, 89, 398. (13) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem. 2005, 117, 6885. (14) Yu, J. G.; Yu, X. X. EnViron. Sci. Technol. 2008, 42, 4902. (15) Gao, J. H.; Zhang, B.; Zhang, X. X.; Xu, B. Angew. Chem. 2006, 118, 1242. (16) Wang, H. Q.; Wu, Z. B.; Liu, Y. J. Phys. Chem. C 2009, 113, 13317. (17) Buchold, D. H. M.; Feldmann, C. Nano Lett. 2007, 7, 3489. (18) Zhou, J.; Wu, W.; Caruntu, D.; Yu, M. H.; Martin, A.; Chen, J. F.; O’Connor, C. J.; Zhou, W. L. J. Phys. Chem. C 2007, 111, 17473–17477. (19) Yu, X. X.; Yu, J. G.; Cheng, B.; Huang, B. B. Chem.sEur. J. 2009, 15, 6731. (20) Huang, Y.; Zheng, Z.; Ai, Z. H.; Zhang, L. Z.; Fan, X. X.; Zou, Z. G. J. Phys. Chem. B 2006, 110, 19323. (21) Dunkle, S. S.; Helmich, R. J.; Suslick, K. S. J. Phys. Chem. C 2009, 113, 11980. (22) Huang, Y.; Ai, Z. H.; Ho, W. K.; Chen, M. J.; Lee, S. C. J. Phys. Chem. C 2010, 114, 6342. (23) Bian, Z. F.; Huo, Y. N.; Zhang, Y.; Zhu, J.; Lu, Y. F.; Li, H. X. Appl. Catal., B 2009, 91, 247. (24) Bang, J. H.; Helmich, R. J.; Suslick, K. S. AdV. Mater. 2008, 20, 2599. (25) Skrabalak, S. E.; Suslick, K. S. J. Am. Chem. Soc. 2006, 128, 12642. (26) Lu, Y. F.; Fan, H. Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (27) Skrabalak, S. E.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 9990. (28) Zhu, J.; Zhang, D. Q.; Bian, Z. F.; Li, G. S.; Huo, Y. N.; Lu, Y. F.; Li, H. X. Chem. Commun. 2009, 5394. (29) Wei, X.; Hug, P.; Figi, R.; Trottmann, M.; Weidenkaff, A.; Ferri, D. Appl. Catal., B 2010, 94, 27. (30) Bang, J. H.; Suslick, K. S. AdV. Mater. 2010, 22, 1039. (31) Zhang, Q.; Yao, W. T.; Chen, X. Y.; Zhu, L. W.; Fu, Y. B.; Zhang, G. B.; Sheng, L. S.; Yu, S. H. Cryst. Growth Des. 2007, 7, 1423. (32) Kadowaki, H.; Saito, N.; Nishiyama, H.; Kobayashi, H.; Shimodaira, Y.; Inoue, Y. J. Phys. Chem. C 2007, 111, 439. (33) Lei, F.; Yan, B.; Chen, H. H.; Zhang, Q.; Zhao, J. T. Cryst. Growth Des. 2009, 9, 3730. (34) Ai, Z. H.; Ho, W. K.; Lee, S. C.; Zhang, L. Z. EnViron. Sci. Technol. 2009, 43, 4143. (35) Chen, J.; Poon, C. S. EnViron. Sci. Technol. 2009, 43, 8948. (36) Ho, W. K.; Yu, J. C.; Lee, S. C. Chem. Commun. 2006, 1115. (37) Huang, Y.; Ho, W. K.; Lee, S. C.; Zhang, L. Z.; Li, G. S.; Yu, J. C. Langmuir 2008, 24, 3510. (38) Dong, F.; Wang, H. Q.; Wu, Z. B. J. Phys. Chem. C 2009, 113, 16717. (39) Song, X.; Ding, X.; Li, P. N.; Ai, Z. H.; Zhang, L. Z. J. Phys. Chem. C 2009, 113, 5455.

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