Hydrothermal Synthesis and High Photocatalytic Activity of 3D Wurtzite

10 Oct 2008 - The as-prepared flowerlike nanostructures efficiently catalyze the photodegradation of methylene blue and ethyl violet present in aqueou...
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J. Phys. Chem. C 2008, 112, 17095–17101

17095

Hydrothermal Synthesis and High Photocatalytic Activity of 3D Wurtzite ZnSe Hierarchical Nanostructures Feng Cao, Weidong Shi, Lijun Zhao, Shuyan Song, Jianhui Yang, Yongqian Lei, and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Graduate School of the Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed: May 29, 2008; ReVised Manuscript ReceiVed: August 28, 2008

Novel 3D wurtzite ZnSe hierarchical nanostructures have been synthesized by a mild hydrothermal treatment. The as-prepared flowerlike nanostructures efficiently catalyze the photodegradation of methylene blue and ethyl violet present in aqueous solutions under UV light irradiation, exhibiting higher photocatalytic activity than the commercially available photocatalysts P25 and ZnSe microspheres. We also demonstrate that flowerlike morphology is important for the excellent photocatalytic activity. The high photoactivity of the hierarchical nanoflowers can be attributed to the combined effects of several factors, namely, large surface areas, the existence of mesopores, and the high band gap energy. In addition, the photocatalysts work effectively under natural sunlight as well. To the best of our knowledge, this is the first time that wurtzite ZnSe 3D flowerlike nanostructures with high photocatalytic activity have been synthesized in solution. 1. Introduction Since Honda and Fujishima first found a titanium dioxide (TiO2)-photoassisted electrochemical splitting of water in 1972, semiconductor photocatalysts have attracted considerable attention for a long time from the fields of catalysis, electrochemistry, and photochemistry, because of their significant effects on solving environmental problems (such as air or water pollution).1 TiO2-based photocatalysts suspended in water have been proven to be one of the most active photocatalysts. However, it has been shown that the photocatalytic activity of TiO2 is limited by fast charge-carrier recombination and low interfacial charge-transfer rates of photogenerated carriers. On the other hand, since its band gap is large (Eg ) 3.2 eV), TiO2 cannot absorb visible light and only makes use of 3-5% of the solar beam that can reach the earth.2 Therefore, the exploration of new UV- and visible-light-responsive photocatalysts with high photoactivity is currently an intensive and hot research topic. One-pot direct synthesis of inorganic materials with specific morphology and orientation via simple route has attracted intensive interest in material science.3 Generally, the choice of initial materials or organic additives in which they organize greatly affected the product’s novel chemical and physical properties. Thus, these results offered new opportunities for investigating a product’s size- and shape-dependent properties, which are extremely important to industrial applications. The traditional hydrothermal technique occupies a unique place in modern science and technology. Recent studies confirm that the mild hydrothermal processes show extraordinary ability in the direct preparation of advanced nanostructures.4 It is believed that this method generates highly crystalline products with high purity, narrow size distribution, and low aggregation. Moreover, the morphology and crystal form of the products can also be controlled by adjusting the hydrothermal reaction conditions. This facile wet chemical route has also been widely * To whom correspondence should be addressed. Tel: +86-43185262127. Fax: +86-431-85698041. E-mail: [email protected].

employed to prepare nanoscale crystals with high purity and novel morphologies to enhance photocatalytic activity.5 For example, 3 nm GdCoO3 particles showed better activity for the degradation of pollutants (such as phenol) than TiO2 (P-25) under UV light irradiation.6 Nanosized Bi2WO6 sheets exhibited relatively high photochemical activity for the decomposition of rhodamine-B under visible light irradiation (λ¨ > 420 nm).7 These reports may suggest that it is possible to synthesize photocatalysts with high activity by this facile method. Of the various II-VI semiconductors, zinc selenide (ZnSe) with a direct band gap of 2.70 eV (460 nm) is especially interesting because it is widely used for various applications like light-emitting diodes (LEDs), biomedical sensors, photovoltaic solar cells, and photocatalysts.8 “Nanostructuring” of the photocatalytic materials can be considered as a primarily promising strategy to gain factorial enhancements in photocatalysts due to both higher surface-to-volume ratios and higher redox potentials with an increase in band gap energy as a result of the so-called “quantum size effect”.9 More recently, Qian et al. synthesized ZnSe nanobelts with high photocatalytic activity in the degradation of fuchsine acid solution under UV light irradiation through a solvothermal approach.10 This is promising for the use of ZnSe as an effective catalyst for photocatalytic degradation of organic pollutants. However, to the best of our knowledge, no further studies have been reported on the photocatalytic activities of ZnSe nanostructures with other morphologies, although they are strongly desired. Here, we present a simple hydrothermal route for the synthesis of wurtzite 3D flowerlike ZnSe nanostructures in high yield. The photocatalytic performance of these ZnSe nanostructures for the photodegradation of various dyes (methylene blue and ethyl violet) has been investigated and found to be distinctly more active than that of Degussa P-25 and ZnSe microspheres reported by Li et al. under UV light irradiation.11 In addition, the photocatalysts work effectively under natural sunlight as well.

10.1021/jp8047345 CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

17096 J. Phys. Chem. C, Vol. 112, No. 44, 2008 2. Experimental Section 2.1. Sample Preparation. All chemicals used in this experiment were of analytical grade. The preparation of ZnSe nanoflowers was performed by a one-pot approach. In a typical synthesis, 14 mmol of NaOH was dissolved in 14 mL of distilled water and then 0.35 mmol of Zn(NO3)2 · 7H2O and 3 mmol of ethylenediaminetetraacetic acid (EDTA) were added successively into the above solution. The resulting mixture was sonicated until a clear solution was obtained. Afterward, 0.34 mmol of Na2SeO3 and 7 mL of hydrazine hydrate N2H4 · H2O (80%) were sequentially added into this reaction. After the mixture was magnetic stirred for 10 min, the final solution was then transferred into a Teflon-lined autoclave of 35 mL capacity. After heating at 180 °C for 2 h, the tank was cooled down to room temperature naturally. Large quantities of yellowish floccules of well-crystallized ZnSe were formed by treatment of the reaction with 1 M HCl solution. The resultant products were washed with distilled water and absolute ethanol and finally dried under vacuum at 60 °C for 2 h. 2.2. Characterization and Measurements. The X-ray diffraction pattern of the products was collected on a Rigaku-D/ max 2500 V X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å), with an operation voltage and current maintained at 40 kV and 40 mA. The samples were prepared by sonicating powdered samples in ethanol and then evaporating one drop of the suspension on a glass slide. X-ray photoelectron spectrometry (XPS) analysis was measured by using a Thermo ESCALAB 250 electron spectrometer; samples for XPS measurement were prepared by adding several drops of condensed solution to a glass substrate and then leaving them to dry in air. Field-emission scanning electron microscopy (FESEM) images were obtained with a XL30 ESEM FEG microscope. The samples were prepared by sonicating powdered samples in ethanol, evaporating one drop of suspension on a Si slide, and then evaporating a thin gold film on the slide. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope. N2 adsorption-desorption isotherms were measured at liquid-nitrogen temperature (77 K) using a Quantachrome Instruments NONA 1000 instrument. Samples were degassed at 120 °C overnight before measurements. SpecificsurfaceareaswerecalculatedusingtheBrunauer-Emmett-Teller (BET) model, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) model. Photocatalytic experiments in aqueous solution were performed in a water-cooled quartz vessel. The UV light was generated from a 300 W high-pressure mercury lamp. A suspension containing a powdered catalyst (160 mg) and fresh dye aqueous solution (400 mL, 1.0 × 10-4 M) was magnetically stirred in the dark for 5 min to establish an adsorption/desorption equilibrium of the dye species. At given irradiation time intervals, a series of aqueous solutions in a certain volume were collected and filtered through a Millipore filter for analysis. The absorption spectrum of the filtrate was measured on a Hitachi U-4100 spectrophotometer. The concentration of dye was determined by monitoring the changes in the main absorbance centered at 663 nm for methylene blue and 593.5 nm for ethyl violet, respectively. ZnSe microspheres and commercial Degussa P25 TiO2 powder were used to compare the photocatalytic activity under the same experimental conditions. In addition, photodecomposition of fuchsine acid under direct sunlight was tested using the same concentration of fuchsine acid solution.

Cao et al. UV-vis absorption spectra of the samples with and without 3D ZnSe nanoflowers were taken after 6 h of bright sunlight exposure. 3. Results and Discussion 3.1. Morphology and Structure. The morphology and size of the as-synthesized products were characterized by fieldemission scanning electron microscopy (FESEM). The lowmagnification FESEM image (Figure 1a) demonstrates that the typical products consist of a large quantity of 3D flowerlike structures. The flowerlike particles have a diameter of about 2-4 µm with a relatively narrow size distribution. The yield of hierarchical nanostructures is high (>98%). The high-magnification FESEM image (Figure 1b) reveals that the thickness of a leaf is quite thin, ca. 28 nm. The chemical composition of these nanoflowers is further characterized by using energydispersive spectroscopy (EDS, Figure 1c). Peaks of the elements Zn and Se are detected in the EDS pattern and the molar ratio is about 1:1 (the weak signal of O might derive from O2 absorbed onto the surface of 3D flowers from air, and the Si and Au signals come from the substrate). Further evidence for the quality and composition of the products can be provided by using X-ray photoelectron spectroscopy (XPS), and the XPS spectrum of ZnSe was identified. The binding energies obtained in the XPS analysis were corrected for specimen charging, through referencing the C 1s to 284.60 eV. In Figure 2a, there is a strong peak at about 1022 eV, which is attributed to Zn 2p3/2. In Figure 2b, there is a strong peak at about 54.00 eV, which corresponds to Se 3d. The data are consistent with the values reported for ZnSe in the literature.12 Further insight into the morphology and microstructure of flower-shaped ZnSe nanostructures were gained by using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 3a shows the low-magnification TEM image of a single ZnSe nanoflower. A selected area electron diffraction (SAED) pattern (inset of Figure 3a) taken from the edge of nanoflower can be indexed as a polycrystalline structure. Figure 3b shows a typical HRTEM image of a single nanoplate. Lattice fringes showing the nature of highly crystalline ZnSe and mesopores with a pore size of about 4 nm (black arrows are pointing to the regions) on the leaves of nanoflower can be clearly observed. The mesopore nature of ZnSe nanoflowers has been further confirmed by the N2 adsorption and desorption isotherm, and the corresponding BJH pore size distribution curve is shown in Figure 4. We can find that the isotherm of obtained ZnSe nanoflowers can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.5-1.0 P/P0 according to IUPAC classification. The Brunauer-Emmett-Teller (BET) surface area of these nanoflowers is calculated to be 166.9 m2 · g-1. The BJH calculations for the pore size distribution reveals that a distribution for the nanoflowers centers at 4.2 nm (inset in Figure 4), which is in agreement with the HRTEM observation. The phases and purities of the as-prepared samples were investigated by the XRD analysis. Figure 5a shows the XRD pattern of the typical samples. All the peaks in this figure can be identified as a hexagonal wurtzite ZnSe (space group P63mc), with lattice constants a ) 3.996 Å and c ) 6.550 Å (JCPDS Card No. 15-0105). The relatively small size of the crystals of the products makes the diffraction peaks be much broader than that of the corresponding standard pattern. In contrast to the XRD pattern of the product before washing by HCl solution in Figure 5b, we found easily that the post-treatment with HCl solution is crucial to improve crystal quality of ZnSe nanostructures.

3D Wurtzite ZnSe Hierarchical Nanostructures

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Figure 1. (a) Low-magnification SEM image of ZnSe nanoflowers. (b) High-magnification SEM image of ZnSe nanoflowers. Inset: the thickness of leaf is about 27.6 nm. (c) EDS pattern that indicates these nanostructures are composed of ZnSe.

3.2. Effect of Reaction Conditions on the Growth of ZnSe. A series of control experiments were carried out to determine the parameters that may affect the formation of ZnSe hierarchical

nanostructures. We have studied the influence of several parameters (such as additives, concentration of additives, reaction time, and temperature) on the evolution of this unusual

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Figure 5. XRD patterns of the products (a) after and (b) before washing by HCl solution.

Figure 2. XPS analysis of the 3D nanoflowers: (a) zinc region and (b) selenium region.

Figure 6. (a) Low- and (b) high-magnification SEM images of the as-obtained ZnSe products without EDTA. Figure 3. (a) TEM image of a single ZnSe nanoflower. Inset: SAED pattern of the nanoflower. (b) HRTEM image of the edge area of the ZnSe nanoflower (the framed area of image a).

Figure 4. N2 adsorption-desorption isotherms for the ZnSe nanoflowers. The inset shows BJH pore size distributions of the ZnSe nanoflowers.

structure. We found that the EDTA plays a key role in the formation of the 3D flowerlike ZnSe nanostructures. Figure 6 shows the FESEM images of the obtained products without using EDTA. Smooth ZnSe microspheres with diameter of about 1 µm are the dominant products in the batch solution, which is similar to the ZnSe microstructures reported by Li et al.11 When the reactions are carried out in the presence of other additives, such as ammonia, cationic surfactant cetyltrimethylammonium bromide (CTAB), anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), similar results are observed as without surfactants (Supporting Information). In the synthesis of inorganic nanostructures, many organic additives have been employed for the modifications of certain crystallographic surfaces.

The absorption of the organic additives on those surfaces will eliminate part of the dangling bonds and result in the reduction of their surface energy, therefore suppressing the growth along these surfaces and making them appear in the final morphology.13 EDTA is a strong chelator for metal ions. It reacts with Zn2+ to form stable Zn-EDTA complexes, decreasing the generation rate of ZnSe nanoparticles in solution. The relatively slow generation rate of ZnSe nanoparticles is favorable for the subsequent growth of 3D hierarchical nanostructures. Compared with stable Zn-EDTA complexes, the interactions between Zn2+ and other additives are quite weak. These relatively weaker interactions and therefore faster releases of Zn2+ will induce the formation of spheres, analogous to the case without additives. In addition, it is worthwhile to mention that the quantity of EDTA has strong effects on the formation of well-defined hierarchical nanostructures. Figure 7 shows FESEM images of ZnSe samples obtained when different amounts of EDTA (from 0.3 to 5.1 mmol) are introduced into the reaction system. When 0.1 g (0.3 mmol) of EDTA is added, some 3D flowerlike structures come out in the products, although the dominant product remains irregular microspheres (Figure 7a). Further increasing the amount of EDTA (1.4 and 2.4 mmol), more and more 3D nanoflowers appear in the products together with a decreased amount of spherelike structures (Figure 7b,c). When the addition of EDTA is up to 0.875 g (3 mmol), 3D flowerlike architectures are the only morphology. When the quantity of EDTA is further increased (5.1 mmol), the morphology remains flowerlike without an obvious change (Figure 7d). The 3D flowerlike ZnSe nanostructures can be formed over a range of

3D Wurtzite ZnSe Hierarchical Nanostructures

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Figure 7. SEM images of the obtained ZnSe products adding different amounts of EDTA: (a) 0.1 g (0.3 mmol), (b) 0.4 g (1.4 mmol), (c) 0.7 g (2.4 mmol), and (d) 1.5 g (5.1 mmol).

temperatures (100-180 °C) and times (40 min to 12 h) without a significant change in product morphology (Supporting Information). The reactions to form ZnSe nanostructures can be formulated as follows:

3SeO32- + 3N2H4 f 3Se + 3N2 + 3H2O + 6OH-

(1)

3Se + 6OH- f 2Se2- + SeO32- + 3H2O

(2)

Se + Zn (EDTA) f ZnSe

(3)

2-

II

According to the literature,8,10,11 Se and Na2SeO3 are the frequently used selenium sources in the solution preparation of ZnSe. In our reaction, the reason that Na2SeO3 is used instead of Se powder is that it can dissolve easily in water and be reduced to highly reactive Se quickly by hydrazine hydrate upon heating. These highly reactive Ultrafine Se particles produced in situ form Se2- much easier than commercial Se powders do. Furthermore, Se powders cannot dissolve into the solution completely, the synthetic reactions are incomplete and the obtained sample contains an impurity of Se, which is not easy to eliminate from the products. In the present case, although further investigation is necessary to elucidate the mechanism of the growth of the 3D nanostructures, we believe that the interaction between inorganic materials and EDTA is also the main driving force for the formation of a flowerlike structure, which can be confirmed by the FT-IR spectrum of the flowerlike product (Supporting Information).14 3.3. Photocatalytic Activity of ZnSe Nanostructures. Organic dyes are used extensively to color products in the textile industry, such as nylon, wool, cotton, and silk, as well as for coloring oil, fats, waxes, varnish, and plastics. Many dyes are nontoxic themselves at the concentration discharged into the receiving waters. However, they can easily form highly toxic complexes with some heavy metal ions (i.e., Cr, Al, and Cu) in wastewater to pollute water resource.15 Methylene blue, ethyl violet, and malachite green are the representations of aryl methane basic dyes; meanwhile, fuchsine acid is a typical acid dye. Because their structures contain one or several benzene rings, these dyes are too stable to be decomposed easily by traditional chemical and biological treatment. Their molecular structures and chemical properties of the four dyes photocatalytically destroyed are illustrated in the Supporting Information (Table S1). In our case, we found the obtained 3D wurtzite ZnSe flowerlike nanomaterials had extraordinary photocatalytic activ-

ity for the degradation of methylene blue and ethyl violet. Before the photocatalytic reaction, the dye solutions were first photolyzed in the absence of the photocatalysts to examine their stability. The results show that the dyes are not decomposed, even after long illumination with UV light. In addition, the concentrations of dyes almost do not change under dark conditions after the ZnSe and dye solutions reach the adsorption-desorption equilibrium. Therefore, the presence of both catalysts and illumination is necessary for efficient degradation. Figure 8a,b shows absorption spectra of an aqueous solution of various dyes (initial concentration: 1.0 × 10-4 M, 400 mL) in the presence of as-prepared ZnSe nanoflowers under exposure to the 300 W high-pressure mercury (UV light) lamp for various durations, respectively. The characteristic absorption of the methylene blue at λ ) 663 nm and ethyl violet at λ ) 593.5 nm is chosen to monitor the photocatalytic degradation process, respectively. The activity of ZnSe nanoflowers for degrading methylene blue and ethyl violet in water irradiated by UV light is about twice that of using the well-known Degussa P25 TiO2 photocatalysts. The color-change sequence during the photocatalytic degradation of both dyes by using ZnSe nanoflowers is shown in the inset of Figure 8, parts a and b, respectively. It is clear that the intense color of the starting solution gradually decreases with increasing exposure time. Additionally, the acid dye fuchsine acid and basic dye malachite green were used to probe the photoactivity of ZnSe nanoflowers. As a result, the ZnSe photocatalysts show good activity for these two dye photodegradations. The photodegradation results of an aqueous solution of these two dyes in the presence of photocatalysts under the same conditions for various durations are listed in the Supporting Information. It can be seen that the degradations of basic dyes are faster as compared to that of acid dyes. Further experiments were carried out to compare the catalytic activity of the ZnSe nanoflowers, commercial Degussa P25 TiO2 powder, and ZnSe microspheres (obtained without EDTA) under the same conditions. The data of curves in Figure 8c,d clearly indicate that the ZnSe nanoflower photocatalysts show much higher photocatalytic activity than that of P25 TiO2 powder and ZnSe microspheres in the degradation of those dyes. This high photocatalytic activity of ZnSe nanoflowers might be attributed to the following reasons: (1) Flowerlike superstructure and mesopores in the leaves make them possess higher BET surface area than P25 TiO2 powder and ZnSe microspheres, which results in an increase of adsorption percentages and benefits the enhancement of the photocatalytic activity of ZnSe.16 (2) The energy of the band gap of ZnSe nanoflowers estimated from the main absorption edges of the UV-vis diffuse reflectance spectrum is 2.8 eV (Supporting Information), which is slightly higher than the well-known band gap of 2.70 eV for bulk ZnSe due to the so-called “quantum size effect”. As known, the optimum band gap plays a major role in the photocatalytic activities of semiconductors. Electron hole pair generation is dependent on the band gap of ZnSe. The increase in band gap is useful in obtaining higher electron hole pair generation. The results show that the hydroxyl (OH•) groups present in nanophotocatalysts are higher as compared with bulk photocatalysts which can capture the photoproduced h+, preventing the recombination of the h+ and e-, and consequently improve the photocatalytic activity.17 On the other hand, the results of the increase in band gap will lead to an uplift of redox potential, thereby enhancing the charge-transfer rates in the system and reducing the volume recombination, that is, radiationless recombination of the electron-hole pair within the semiconduc-

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Figure 8. (a) Absorption spectrum of (a) methylene blue and (b) ethyl violet in the presence of ZnSe nanoflowers under UV light. Curves of the concentration of residual dyes (c) methylene blue and (d) ethyl violet solution with different UV light irradiation time over the as-prepared ZnSe nanoflowers (9), commercial Degussa P25 TiO2 powder (b), and ZnSe microspheres (2).

4. Conclusions

Figure 9. UV-vis absorption spectra of fuchsine acid after placing the samples under natural bright sunlight for 6 h (a) with and (b) without the presence of ZnSe nanoflower photocatalysts.

tor particles.18 Therefore, the photocatalytic properties have been greatly improved. To further extend the photocatalytic applicability of these nanoflowers in a more practical situation, the photodegradation of fuchsine acid using these nanoflowers under natural sunlight was carried out. As illustrated in Figure 9, about 93% of fuchsine acid is decomposed after exposure to 6 h under bright sunlight in the presence of ZnSe nanoflowers. In contrast, no obvious change can be seen in UV-vis absorption spectrum of the system without addition of ZnSe nanoflowers. The results demonstrate the excellent photocatalytic ability of these welldefined nanoflowers without artificial light source.

In summary, a facile and low-cost hydrothermal synthetic method was developed for fabricating novel 3D wurtzite ZnSe nanoflowers. The addition of chelator EDTA played a critical role in the formation of the 3D flowerlike ZnSe nanostructures. The as-prepared ZnSe nanomaterials have also been demonstrated to work as effective photocatalysts and exhibit excellent photocatalytic activity for the degradation of methylene blue and ethyl violet, which were found in different industries’ effluents. Our results may herein confirm that the photocatalytic activity of ZnSe nanoflowers in the photodegradation of methylene blue and ethyl violet was higher than that of commercial P25 TiO2 powder. We also demonstrated that flowerlike morphology was important for the excellent photocatalytic activity. A combination of their unique features of high surface to volume ratios, monodispersion, high photocatalytic activity, and easy recycling may render these 3D ZnSe nanoflowers more practical applications, such as in semiconductor photocatalysis and environmental remediation. Acknowledgment. The authors are grateful to the financial aid from the Natiosnal Natural Science Foundation of China (Grant Nos. 20490210, 20631040, 20602035, and 20610102007) andtheMOSTofChina(“973”Program,GrantNo.2006CB601103).

3D Wurtzite ZnSe Hierarchical Nanostructures Supporting Information Available: SEM images of the obtained ZnSe products for different additives, reaction temperatures, and times; IR spectrum of the ZnSe nanostructures; The photocatalytic degradation of malachite green and fuchsine acid in the presence of ZnSe; UV-vis diffuse reflectance spectra of ZnSe nanoflowers; molecular structures and chemical properties of the four dyes photocatalytically destroyed. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Fujishima, A.; Honda, K. Nature 1972, 37, 238. (b) Sato, T.; Masaki, K.; Sato, K. C.; Fujishiro, Y.; Okuwaki, A. J. Chem. Technol. Biotechnol. 1996, 67, 339. (c) Yu, J. C.; Ho, W. K.; Leung, M. K. P.; Cheng, B.; Zhang, G. K.; Zhao, X. J. Appl. Catal. A 2003, 255, 309. (2) (a) Yu, J. G.; Su, Y. R.; Cheng, B. AdV. Funct. Mater. 2007, 17, 1984. (b) Kemell, M.; Pore, V.; Ritala, M.; Leskela, M.; Linden, M. J. Am. Chem. Soc. 2005, 127, 14178. (c) Wu, C. Z.; Lei, L. Y.; Zhu, X.; Yang, J. L.; Xie, Y. Small 2007, 3, 1518. (3) (a) Rao, C. N. R.; Vivekchand, S. R. C.; Biswasa, K.; Govindaraja, A. Dalton Trans. 2007, 34, 3728. (b) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P. R.; Micic, O. I.; Ellingson, R. J.; Nozik, A. J. J. Am. Chem. Soc. 2006, 128, 3241. (c) Wang, L. Y.; Bao, J.; Wang, L.; Zhang, F.; Li, Y. D. Chem. Eur. J. 2006, 5, 6341. (d) Tura, C.; Coombs, N.; Dag, O. Chem. Mater. 2005, 17, 573. (4) (a) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (b) Shi, W. D.; Yu, J. B.; Wang, H. S.; Zhang, H. J. J. Am. Chem. Soc. 2006, 128, 16490. (5) (a) Niu, X.; Li, H.; Liu, G. J. Mol. Catal. A 2005, 232, 89. (b) Xie, H. D.; Shen, D. Z.; Wang, X. Q.; Shen, G. Q. Mater. Chem. Phys. 2007, 103, 334. (c) Tokunaga, S.; Kato, H.; Kudo, A. Chem. Mater. 2001, 13, 4624. (6) Mahata, P.; Aarthi, T. G.; Madras, G.; Natarajan, S. J. Phys. Chem. C 2007, 111, 1665. (7) Wu, J.; Duan, F.; Zheng, Y.; Xie, Y. J. Phys. Chem. C 2007, 111, 12866. (8) (a) Kim, C. C.; Sivananthan, S. Phys. ReV. B 1996, 53, 1475. (b) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, P. C.; Wang, L. W.; Buhro, W. E.

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