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Synthesis of Cerium Molybdate Hierarchical Architectures and Their Novel Photocatalytic and Adsorption Performances Mingyan Dong, Qiang Lin, Haiming Sun, Dan Chen, Ting Zhang, Qingzhi Wu,* and Shipu Li State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and Biomedical Material and Engineering Center, Wuhan University of Technology, Wuhan 430070, China
bS Supporting Information ABSTRACT: Cerium molybdate (CeMo) hierarchical architectures (such as the flowerlike, microspheric, and bundlelike structure) are successfully synthesized via a facile route with the assistance of amino acid (lysine, Lys). The influences of reaction parameters on the crystal structure and morphology of CeMo hierarchical architectures are investigated. Samples obtained are characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), and thermogravimetric analysis (TGA). Furthermore, the photocatalytic and adsorption performances of samples obtained are investigated using different dyes, such as Cationic red X-GTL, Congo red, Methylene blue, Acid blue 80, and Methyl orange, as the model. The results show that CeMo hierarchical architectures exhibit remarkably high efficiency to photocatalytically decompose Congo red under visible light irradiation, and significant adsorption performance on Cationic red X-GTL and Methylene blue. Contrarily, neither photocatalytic nor adsorption performance was observed on Methyl orange and Acid blue 80. Therefore, the as-synthesized CeMo hierarchical architectures display promising potential for the removal of organic contaminants for environmental protection.
1. INTRODUCTION Thousands of dyes with complicated structures have been fabricated and utilized ubiquitously from textile and cosmetics to food and pharmaceutical, etc. These organic compounds greatly improve our living qualities, unfortunately, at the cost of serious environmental pollution due to their resistance to decompose. Some dye families, such as azo dyes, have been reported to be transformed into genotoxic and carcinogenic species in different mechanisms.13 Therefore, it is one of the most emergent challenges to explore novel photocatalysts for the removal of organic pollutants in high efficiency and at low cost. In the past few decades, various nanomaterials have been explored for applications as novel and high efficient photocatalysts. For instance, titanium dioxide (TiO2) nanostructures have been systematically investigated and well demonstrated as promising photocatalysts under UV irradiation due to their high efficiency, low cost, and low toxicity.4,5 Currently, TiO2 doped with different elements has been further investigated in order to photocatalytically decompose organic compounds under visible light irradiation.610 The photocatalytic performance of various semiconductor nanostructures, such as ZnO, α-Fe2O3 (hematite), and SnO2 nanocrystals, has also been studied extensively.1114 Rare earth elements are well-known for their excellent optical, electronic, and magnetic properties derived from the unique incompletely occupied 4f electronic orbital and the empty 5d r 2011 American Chemical Society
electronic orbital.1,15 The cerium-doped titania photocatalyst has been extensively studied in many reactions, such as the oxidation of CO,16 the conversion of CO2 to hydrocarbons,17 and the decomposition of organic dye pollutants.18 Cerium oxide was also reported as effective catalysts for the catalytic oxidation of different hydrocarbons and the removal of organic compounds derived from wastewater. For example, Aranda and Garcia et al.19 revealed that nanocrystalline ceria catalysts prepared by different methods exhibit remarkable catalytic behavior for the total oxidative decomposition of naphthalene and the selectivity toward CO2. Besides, molybdate compounds with moderate catalytic activity have been reported in many publications.20,21 For instance, Le et al.22 have reported the synthesis and the selective oxidation of propylene by bismuth molybdate nanostructures (including α-Bi2Mo3O12, β-Bi2Mo2O9, γ-Bi2MoO6). Recently, cerium molybdate nanocontainers were synthesized using polystyrene nanospheres as the template, which can load and release 8-hydroxyquinoline and 1-H-benzotriazole-4-sulfonic acid.23 We, herein, reported a facile synthesis of cerium molybdate (CeMo) nanostructures, including the flowerlike, microspheric, and fanlike structure, with the assistance of amino acid (lysine, Lys). Received: July 15, 2011 Revised: September 21, 2011 Published: September 22, 2011 5002
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Figure 1. The typical SEM images of samples synthesized at different conditions. (ac) The Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 1:1, and the EG/H2O volume ratio of 1:1; (df) the Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 3:2 and in the absence of EG; (gi) the Mo/Ce3+ molar ratio of 3:2 and in the absence of Lys and EG.
Biomolecules, such as various amino acids, have been extensively employed in the synthesis of nanostructures due to their good biocompatibility and abundance of functional groups in the molecular structure.2426 The products were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), and thermogravimetric analysis (TGA). Furthermore, photocatalytic and adsorption performance of samples obtained was measured by monitoring the decomposition of different dyes, such as Congo red, Cationic red X-GTL, Methylene blue, Methyl orange, and Acid blue 80 under visible light irradiation within different time intervals at room temperature.
2. EXPERIMENTAL SECTION Synthesis of Cerium Molybdate Hierarchical Architectures. In a typical synthesis, Lys (0.0146 g, 0.1 mmol) was dissolved in ethylene glycol (20 mL) under magnetic stirring, and then cerium(III) nitrate hexahydrate [Ce(NO3)3 3 6H2O] (0.0434 g, 0.1 mmol) was added to Lys solution. Ammonium molybdate tetrahydrate [(NH4)6Mo7O24 3 4H2O] (0.1236 g, 0.1 mmol) was dissolved in deionized water (20 mL), and the solution containing Lys and cerium nitrate was added dropwise into ammonium molybdate solution. After 30 min of stirring, the mixture was transferred to and sealed in a 50 mL Teflon-lined autoclave, heated to 100 °C for 10 h, and finally cooled to room temperature. In a
series of syntheses, the synthesis parameters, including the reaction time, the reaction temperature, and the ratio of reactants and solvents, were changed. The precipitate was collected by centrifugation (10 000 rpm, 5 min), washed alternately with deionized water and ethanol, and dried in air at 60 °C for 4 h. Characterization. The crystal structure of samples was characterized on a power X-ray diffraction (XRD) (PANalytical B. V., Netherlands) using Cu Kα radiation (λ) 1.5406 Å. The morphology of as-synthesized samples was observed using field-emission scanning electron microscopy (FESEM, Sirion 200, FEI Corp., Netherlands) and transmission electron microscopy (TEM, Tecnai G220, FEI Corp., Netherlands). FT-IR spectra were measured with a Nexus Fourier transform infrared spectrophotometer (Thermo Nicolet, America). Thermogravimetric analysis (TGA) was carried out on a simultaneous thermal analyzer (NETZSCH STA 449C, Germany). Samples were heated from room temperature to 1000 °C with 10 °C min1 in the air. UVvis measurement was recorded on a UVvis spectrophotometer (Shimadzu Corp., UV-2550 PC).
Photocatalytic and Adsorption Performance of CeMo Nanostructures. The photocatalytic and adsorption performance of CeMo nanostructures was measured using different dyes, such as Congo red, Cationic red X-GTL, Methylene blue, Methyl orange, and Acid blue 80, as the model under visible light irradiation. In a typical experiment, the sample (20 mg) was added into 50 mL of dye aqueous solution (Congo red, 32 mg/L; Cationic red X-GTL, 40 mg/L; Methylene blue, 8 mg/L; Acid blue 80, 64 mg/L; and Methyl orange, 20 mg/L). The suspension was continuously stirred for up to 96 h at 5003
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Figure 3. FT-IR spectra of CeMo nanostructures synthesized at different conditions. (a) The Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 1:1, and the EG/H2O volume ratio of 1:1; (b) the Lys/Ce3 + molar ratio of 1:1, the Mo/Ce3+ molar ratio of 3:2 and in the absence of EG; (c) the Mo/Ce3+ molar ratio of 3:2 and in the absence of Lys and EG; (d) pure Lys molecules. Figure 2. XRD patterns of CeMo nanostructures synthesized at different conditions. (a) The Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 1:1, and the EG/H2O volume ratio of 1:1. (b) The Lys/ Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 3:2 in the absence of EG, (c) the Mo/Ce3+ molar ratio of 3:2 in the absence of both Lys and EG. room temperature under visible light irradiation. The supernatant was collected at designed time intervals for UVvis measurement after centrifugation (10 000 rpm, 3 min). Photocatalytic and adsorption performance of different samples was assessed by monitoring the change of UVvis absorption.
3. RESULTS AND DISCUSSION The influences of synthesis parameters, such as the reaction temperature and the reaction time, the molar ratio of reactants, and the volume ratio of solvents, have been investigated in detail. Figure 1 shows typical SEM images of samples synthesized at different reaction conditions. The flowerlike structures ca. 6.7 μm in diameter were obtained when the Lys/Ce3+ molar ratio was at 1:1, the Mo/Ce3+ molar ratio was at 1:1, and the EG/H2O volume ratio was at 1:1, as shown in Figure 1ac. A close-up view shows that the flowerlike structure consisted of numerous irregular nanosheets. It is noticeable that, in the same conditions but in the absence of Lys, the bundles consisting of irregular rods were obtained in larger size (see Figure S1, Supporting Information). When the Mo/Ce3+ molar ratio was adjusted to 3:2 and in the absence of EG (i.e., deionized water as the uniform solvent) without changing the other parameters, the microspheres ca. 670 nm in diameter were obtained (Figure 1df). A close-up view shows that many nanoparticles were attached on the rough surface of microspheres. In order to investigate the role of Lys molecules in the formation of CeMo nanostructures, the synthesis was further carried out in the absence of both Lys and
EG without changing the other parameters, the bundle of nanorods attached with nanoparticles was obtained, as shown in Figure 1gi. These results suggest that Lys molecules play a crucial role in the formation of CeMo nanostructures. Figure 2 shows XRD patterns of samples synthesized at different conditions. All peaks in Figure 2a could be indexed to Ce2(MoO4)3 3 4.5H2O (JCPDS card No. 31-0333), corresponding to the flowerlike structure in Figure 1ac. While the peaks in Figure 2b,c could be indexed to Ce2Mo3O13 (JCPDS card No. 31-0331), corresponding to the microspheric and bundlelike structure in Figure 1df and Figure 1gi, respectively. The sharp peaks in XRD patterns suggest that the well crystallized structures were obtained in different reaction conditions. In a series of experiments, the synthesis was carried out at conditions similar to the flowerlike structure (i.e., the Lys/Ce3+ molar ratio at 1:1, the EG/H2O volume ratio at 1:1) except the Mo/Ce3+ molar ratio was at 3:2. The same product [Ce2(MoO4)3 3 4.5H2O] was obtained (XRD pattern as shown in Figure S2, Supporting Information). Therefore, XRD characterizations suggest that solvent system plays an important role in the formation of phase structure. The structural information of as-synthesized CeMo nanostructures was measured through FT-IR spectra. Figure 3d shows FT-IR spectrum of Lys molecules. The peak at 3350 cm1 can be attributed to NH stretching vibration of Lys molecules, while the peaks at 2937 and 2863 cm1 may be derived from CH stretching vibration in molecular skeleton. The peaks at 1582, 1517, and 1408 cm1 can be attributed to the antisymmetrical and symmetrical stretching of carboxylate ions.27 The peaks near 33633422 cm1 and 16211631 cm1 in Figure 3ac, corresponding to samples in Figure 1, respectively, are the characteristic absorption of the OH stretching and bending mode derived from the physically adsorbed water. A peak near 897 cm1 can be attributed to the vibration mode of molybdate ion.23,28 5004
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Scheme 1. Molecular Structures of Different Dyesa
Figure 4. TGA curves of CeMo nanostructures synthesized at different conditions. (a) The Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 1:1, and the EG/H2O volume ratio of 1:1; (b) the Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 3:2 in the absence of EG; (c) the Mo/Ce3+ molar ratio of 3:2 in the absence of both Lys and EG.
The peaks near 1000, 930, 845, and 656 cm1 were due to the stretching vibration of the MoO band. It is noteworthy that two peaks at ca. 1510 and 1420 cm1 were observed at FT-IR spectra of the flowerlike and microspheric sample (Figure 3a,b), which could be attributed to the stretching of carboxylate ions of Lys. Thus, these results imply that Lys molecules were probably adsorbed on the surface of CeMo nanostructures. This result was further confirmed by TGA measurement. The mass loss of samples obtained up to 1000 °C is illustrated in Figure 4. The main mass loss of samples near 200 °C was, to a great extent, attributed to the removal of physically adsorbed water. It is noticeable that the mass loss of the flowerlike structure in this stage is far less than that of the microspheric and bundlelike structure. We speculate that the crystal water contained in the molecular structure [Ce2(MoO4)3 3 4.5H2O] probably decreases the physical adsorption of water. At higher temperatures from 300 to 600 °C in Figure 4ac, an obvious mass increase was observed at ca. 500 °C for the flowerlike structure, and ca. 420 °C for the microspheric and bundlelike structure, indicating that some new CeMo oxide species were formed under thermal decomposition conditions. Above 800 °C, the mass loss is due to further decomposition of CeMo oxides. A slight increase of loss mass of the microspheric structure compared with that of the bundlelike structure is probably due to the influence of Lys adsorbed on the surface. TGA results also imply that both CeMo nanostructures display a different thermal decomposition mechanism and result in the formation of different products. Amino acid molecules-directed synthesis and assembly of various hierarchical structures have been widely reported due to their unique advantages, such as desirable biocompatibility, water solubility, and active functional groups contained in the molecular structure (such as COOH and NH2). On the other hand, molybdate anions acting as templates in the synthesis of various structures have also been reported in many publications
a
(a) Methyl orange; (b) Congo red; (c) Acid blue 80; (d) Methylene blue; (e) Cationic red X-GTL.
due to their complicated polyacid structure. For example, Bu et al.29 reported the synthesis of NaLa(MoO4)2 tetragonal bipyramid nanocrystals using oleic acid and oleylamine as the mixed surfactant. SrMoO4 nanoparticles, hollow microspheric, and flowerlike CdMoO4 structures have been synthesized using MoO42 as templates.30,31 Moreover, (NH4)6Mo7O24 3 4H2O has been utilized to synthesize bismuth molybdate catalysts with a higher surface area by the combination of a complexation method and spray drying technology.32 In the present synthesis, trivalent cerium ions were first coordinated with Lys molecules to form Ce(Lys)3+ complex [Ce3+ + Lys f Ce(Lys)3+; log K1 = 2.6],33 which released Ce3+ gradually under the subsequent solvothermal conditions. Then the free Ce3+ ions were intercalated into Mo7O246 polyoxometalate structure and resulted in the formation of CeMo compound. The morphology of the CeMo compound was probably templated by the polyoxometalate scaffold. The solvent played a crucial role in the formation of the final morphology and phase structure of products. Yu et al.34 have prepared α-Fe2O3 nanostructures with controllable morphologies, such as the spherical, mulberry-like, nanospherical structure by adjusting the ratio of the mixed solvents. Fan et al.35 have demonstrated that the shape and structure of Mg(OH)2 one-dimensional (1D) were strongly affected by the solvent. Qualitative understanding of the acetone-based mixed solvent effect on the morphology of Ag nanoparticles was also systematically studied.36 In this synthesis, polyoxometalate (Mo7O246) predominated an oriented growth of cerium molybdate nuclei, resulting in the formation of nanorods. Nanorods were further assembled into bundles as shown in Figure 1gi. The oriented growth was suppressed in the presence of Lys molecules, which coordinated with Ce3+ ions and resulted in the isotropic growth of nuclei and the formation of nanoparticles. The nanoparticles were further assembled into microspheres, as shown in Figure 1df. While in a mixture solvent system, the coordination between Lys molecules and Ce3+ was weakened, the partial 5005
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Figure 5. Photocatalytic performances of different CeMo nanostructures using Congo red as the model. (a) The flowerlike sample; (b) the microspheric sample; (c) the bundlelike sample.
Figure 6. Photocatalytic performance of different CeMo nanostructures using Cationic red X-GTL as the model. (a) The flowerlike sample; (b) the microspheric sample; (c) the bundlelike sample.
oriented growth of nuclei resulted in the formation of nanosheets. These nanosheets were subsequently assembled into a flowerlike structure as shown in Figure 1ac. More investigations on the precise mechanisms of the formation of CeMo nanostructures are in progress. The mass fabrication and the subsequent applications of dyes have been considered as one of the main sources causing serious environmental pollution and endangering public health due to their resistance to decompose and their easy transformation into genotoxic and carcinogenic amines in different mechanisms.13 In this study, the photocatalytic performance of typical CeMo nanostructures obtained was measured using different dyes, such as Cationic red X-GTL, Congo red, Methylene blue, Acid blue 80, and Methyl orange, as the model under visible light irradiation. The molecular structures of different dyes employed are given in Scheme 1. The change of absorption spectra of dye aqueous solutions was recorded at different time intervals. The photodecomposition and/or adsorption efficiency (D) was evaluated as follows: D ¼ ½ðA0 AÞ=A0 100% where A and A0 was the absorbance at different time intervals and the initial time, respectively, at the maximum absorption wavelength of dye aqueous solutions. As shown in Figure 5a, two major absorption peaks of Congo red aqueous solution appear at ca. 340 and 498 nm. The absorption band at ca. 498 nm can be attributed to the strong chromophoric azo (NdN) group and the large conjugated π system of
the whole dye molecule, while the peak at ca. 340 nm can be attributed to the interaction between aromatic rings and polycyclic aromatic hydrocarbon in the molecular structure.37 A steep absorbance decrease at both 340 and 498 nm occurred immediately (within 5 min) when the Congo red aqueous solution was mixed with the flowerlike sample. The significant blue shift of both absorption peaks, from 498 to 460 nm, and 340 to 320 nm, strongly implies the destruction of the chromophoric azo (-NdN-) bond, as well as the interaction of between aromatic rings and polycyclic aromatic hydrocarbon in the molecular structure. Only a slight further decrease of absorbance was observed at the subsequent 48 h, suggesting that the decomposition photocatalyzed by the flowerlike sample was highly efficient. The total decomposition efficiency of the flowerlike sample was ca. 82% at the end of 48 h (according to A498). It is noteworthy that the remarkable decrease of the maximum absorption (A498 and A340) occurred without a blue shift of the maximum absorption in the first 10 min when Congo red aqueous solution was mixed with the microspheric and bundlelike sample, as shown in Figure 5b,c. This result suggests that the dye was adsorbed on samples. After 20 min, the absorption was further decreased and a significant blue shift was observed, from 498 to 470 nm, and 340 to 320 nm. The total decomposition efficiency of the microspheric and the bundlelike sample was ca. 84%, and 85%, respectively, at the end of 48 h (according to A498). These results show that the photocatalytic performance of the flowerlike structure on Congo red was more rapid and more direct than that of the microspheric and bundlelike structures. The latter two 5006
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Figure 7. Photocatalytic performance of different CeMo nanostructures using Methylene blue as the model. (a) The flowerlike sample; (b) the microspheric sample; (c) the bundlelike sample.
samples exhibited an obvious adsorption performance on Congo red prior to photocatalytically decomposing the dye. In order to further investigate the photocatalytic and adsorption performance of CeMo nanostructures, several different dyes, such as Cationic red X-GTL, Methylene blue, Acid blue 80, and Methyl orange, were used as the model. Remarkable adsorption performance of samples was observed when using Cationic red X-GTL and Methylene blue as the model, as shown in Figures 6 and 7. The total absorption decrease at the end of 96 h was ca. 42%, 66%, 87% and 23%, 70%, 89%, corresponding to the flowerlike, microspheric, and bundlelike sample, respectively, in the case of Cationic red X-GTL and Methylene blue. It seems that the bundlelike and flowerlike structure exhibited the strongest and weakest adsorption performance among samples tested. It is noteworthy that no blue shift of maximum absorption occurred when using Cationic red X-GTL and Methylene blue as the model, implying that CeMo nanostructures exhibited rare photocatalytic performance on these two cationic dyes. On the other hand, neither adsorption nor photocatalytic performance was observed when using Acid blue 80 and Methyl orange as the model (see Figures S3 and S4, Supporting Information). It is generally accepted that photocatalytic reaction is initiated by the photogeneration of electronhole pairs.38 The photogenerated 3 OH radicals are primary oxidative species in a photocatalytic decoloration reaction, which attack -NdN- bonds and cause the decomposition of chromophores.39 It was reported that Ce3+ doping at the optimum amount promoted the photocatalytic decomposition by preventing the recombination of electron hole pairs, while high surface areas provided more dye molecules around TiO2 and a high concentration of hydroxyl.40 On the other hand, adsorption ability was associated with the electrostatic attraction and hydrogen bonding between the dye species and hydroxyl groups on the surface of sorbents.41,42 Molybdate anions as polyoxometalate acid have been reported to exhibit relatively high absorption activities of cationic dye.43 In this study, different dyes, including Cationic red X-GTL, Congo red, Methylene blue, Acid blue 80, and Methyl orange, were used as the model, among which Congo red and Methyl orange are anionic azo compounds containing two or one -NdN- group in the molecular structure, respectively; Acid blue 80 is a kind of anionic dye containing no -NdN- group in the molecular structure, while Methylene blue and Cationic red X-GTL are cationic dyes without a -NdN- group or with one -NdN- group in the molecular structure, respectively. Therefore, the remarkable adsorption performance of samples on Methylene blue and Cationic
red X-GTL could be attributed to the polyoxometalate scaffold through electrostatic interaction.42 The bundlelike and microspheric structure displaying higher adsorption performance than that of the flowerlike structure probably derived from the larger specific surface area and negative charge distribution on the surface of nanorods and nanoparticles. The electrostatic repulsion prevents the adsorption of anionic dyes (Acid blue 80 and Methyl orange) except for Congo red. It seems the large π-conjugated system in the molecular backbone resulted in the partial adsorption of Congo red on microspheric and bundlelike structures. On the other hand, oxidative species generated by Ce3+ played a crucial role in the photocatalytic decomposition of the -NdNgroup contained in Congo red, resulting in the destruction of the chromophoric -NdN- bond, as well as the interaction of between aromatic rings and polycyclic aromatic hydrocarbon in the molecular structure. Although the -NdN- group is also contained in the molecular structure of Methyl orange and Cationic red X-GTL, no photocatalytic activity was observed. It seems that the π-conjugated system of Congo red facilitates the attack of oxidative species to some extent. The precise mechanisms of photocatalytic performance by CeMo nanostructures on dyes need more investigation.
4. CONCLUSIONS In summary, we developed a facile route to synthesize novel cerium molybdate nanostructures with different morphologies, such as the flowerlike, microspheric, and bundlelike structure, by adjusting the reaction parameters. Lys molecules, as well as the mixture solvents, played crucial roles in the formation and crystal growth of different structures. More attractively, CeMo nanostructures obtained exhibited excellent photocatalytic or adsorption performance on different dyes. The adsorption performance is mainly due to the electrostatic interaction between negatively charged polyoxometalate backbone and positively charged dye molecules, while the photocatalytic activity is probably attributed to oxidative species photogenerated by Ce3+, resulting in the destruction of the chromophoric azo (-NdN-) bond, as well as the interaction of between aromatic rings and polycyclic aromatic hydrocarbon in the molecular structure. The synthesis of cerium molybdate nanostructres is low-cost and facile to scale-up, providing promising potential in the effective and economical removal of organic industrial contaminants for environmental protection. 5007
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’ ASSOCIATED CONTENT
bS
Supporting Information. SEM images of the sample synthesized at the Mo/Ce3+ molar ratio of 1:1 and the EG/H2O volume ratio of 1:1 in the absence of Lys, XRD pattern of the sample synthesized at the Lys/Ce3+ molar ratio of 1:1, the Mo/Ce3+ molar ratio of 3:2 and the EG/H2O volume ratio of 1:1, and photocatalytic performance of samples on Acid blue 80 and Methyl orange. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 86-27-87651852; fax: 86-27-87880734.
’ ACKNOWLEDGMENT This work is financially supported by the Natural Science Foundation of China (Grant No. 30800256), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 200804971065), the Natural Science Foundation of Hubei Province of China (Grant No. 2008CDB035), as well as the SelfDetermined and Innovative Research Funds of WUT (Grant No. 2010la012). ’ REFERENCES (1) Schneider, K.; Hafner, C.; J€ager, I. J. Appl. Toxicol. 2004, 24, 83–91. (2) Kantiani, L.; Llorca, M.; Sanchís, J.; Farre, M.; Barcelo, D. Anal. Bioanal. Chem. 2010, 398, 2413–2427. (3) (a) Pal, A.; Gin, K. Y.; Lin A. Y. Reinhard, M. Sci. Total Environ. 2010, 408, 6062–6069. (b) Clarke, B. O.; Smith, S. R. Environ. Int. 2011, 37, 226–247. (c) Zhang, Z. H.; Shan, Y. B.; Wang, J.; Ling, H. J.; Zang, S. L.; Gao, W.; Zhao, Z.; Zhang, H. C. J. Hazard. Mater. 2007, 147, 325–333. (d) Sakkas, V. A.; Islam, M. A.; Stalikas, C.; Albanis, T. A. J. Hazard. Mater. 2010, 175, 33–44. (4) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (5) (a) Konstantinou, I. K.; Albanis, T. A. Appl. Catal. B: Environ. 2004, 49, 1–14. (b) Konstantinou, I. K.; Albanis, T. A. Appl. Catal. B: Environ. 2003, 42, 319–335. (c) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol. C. 2008, 9, 1–12. (d) Arconada, N.; Duran, A.; Suarez, S.; Portela, R.; Coronado, J. M.; Sanchez, B.; Castro, Y. Appl. Catal. B: Environ. 2009, 86, 1–7. (e) Reijnders, L. J. Hazard. Mater. 2008, 152, 440–445. (6) (a) Jung, J. M.; Wang, M. S.; Kim, E. J.; Park, C.; Hahn, S. H. Appl. Catal. B: Environ. 2008, 84, 389–392. (b) Akpan, U. G.; Hameed, B. H. J. Hazard. Mater. 2009, 170, 520–529. (c) Kocí, K.; Mateju, K.; apek, L.; Hospodkova, Obalova, L.; Krejcíkova, S.; Lacny , Z.; Placha, D.; C A.; Solcova, O. Appl. Catal. B: Environ. 2010, 96, 239–244. (7) Reddy, E. P.; Davydov, L.; Smirniotis, P. Appl. Catal. B: Environ. 2003, 42, 1–11. (8) (a) Chatterjee, D.; Dasgupta, S. J. Photochem. Photobiol. C. 2005, 6, 186–205. (b) Shan, A. Y.; Ghazi, T. I. M; Rashid, S. A. Appl. Catal. A-Gen. 2010, 389, 1–8. (9) (a) Fu, C.; Li, T. Z.; Qi, J. S.; Pan, J.; Chen, S. H.; Cheng, C. Chem. Phys. Lett. 2010, 494, 117–122. (b) Wang, J.; Lv, Y. H.; Zhang, L. Q.; Liu, B.; Jiang, R. Z.; Han, G. X.; Xu, R.; Zhang, X. D. Ultrason. Sonochem. 2010, 17, 642–648. (10) (a) Wang, D. P.; Zeng, H. C. Chem. Mater. 2009, 21, 4811–4823. (b) Bamwenda, G. R.; Uesigi, T.; Abe, Y.; Sayama, K.; Arakawa, H. Appl. Catal. A-Gen. 2001, 205, 117–128. (11) (a) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol. C 2008, 9, 171–192. (b) Karunakaran, C.; Dhanalakshmi, R. Sol. Energy Mater. Sol. C 2008, 92, 1315–1321.
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