Self-Assembly and Enhanced Photocatalytic Properties of BiOI Hollow

Dec 29, 2010 - (1, 2) Bismuth oxyiodine (BiOI) is an important V−VI−VII ternary compound with a layer structure characterized by [Bi2O2] slabs int...
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Self-Assembly and Enhanced Photocatalytic Properties of BiOI Hollow Microspheres via a Reactable Ionic Liquid Jiexiang Xia,†,‡ Sheng Yin,† Huaming Li,*,† Hui Xu,‡ Yongsheng Yan,† and Qi Zhang† †

School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, P. R. China, and ‡School of the Environment, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, P. R. China Received October 8, 2010. Revised Manuscript Received December 9, 2010

BiOI uniform flowerlike hollow microspheres with a hole in its surface structures have been successfully synthesized through an EG-assisted solvothermal process in the presence of ionic liquid 1-butyl-3-methylimidazolium iodine ([Bmim]I). The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), nitrogen sorption, and diffuse reflectance spectroscopy (DRS). A possible formation mechanism for the growth of hollow microspheres was discussed. During the reactive process, ionic liquid not only acted as solvents and templates but also as an I source for the fabrication of BiOI hollow microspheres and was vital for the structure of hollow microspheres. Additionally, we evaluated the photocatalytic activities of BiOI on the degradation of methyl orange (MO) under visible light irradiation and found that asprepared BiOI hollow microspheres exhibited higher photocatalytic activity than BiOI nanoplates and TiO2 (Degussa, P25) did. On the basis of such analysis, it can be assumed that the enhanced photocatalytic activities of BiOI hollow microspheres could be ascribed to its energy band structure, high BET surface area, high surface-to-volume ratios, and light absorbance.

1. Introduction Recently, the controlled synthesis of hollow and porous structure photocatalysts has attracted increasing attention for clean hydrogen energy production and environment decontamination. Many efforts have been devoted to a precise control of hollow and porous structures with uniform sizes due to their low effective density, high specific surface area, high-energy conversion efficiencies, and large light-harvesting capacities in photocatalysts.1,2 Bismuth oxyiodine (BiOI) is an important V-VI-VII ternary compound with a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms. In fact, BiOI has initiated more interest and made it an ideal option as a new visible light photocatalyst because of its unique and excellent electrical and optical properties.3-5 Several methods have been reported for the preparation of BiOI micro/nanostructures so far. Zhang’s group6 prepared BiOI hierarchical microspheres by employing KI as an I source with one-pot EGassisted solvothermal process. It was found that the BiOI sample showed higher photocatalytic activities than BiOBr and BiOCl materials for degrading methyl orange (MO) organic pollutants. Lei7 demonstrated the synthesis of flowerlike BiOI hierarchical structures by a solution route at room temperature, and BiOI architectures showed higher photocatalytic activities toward three types of dye under visible light irradiation. At the same time, BiOI microspheres composed of nanoplatelets were synthesized at low *Corresponding author: e-mail [email protected], Tel 86-511-88791108, Fax 86-511-88791108. (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (2) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625–627. (3) Wang, W. D.; Huang, F. Q.; Lin, X. P.; Yang, J. H. Catal. Commun. 2008, 9, 8–12. (4) An, H. Z.; Du, Y.; Wang, T. M.; Wang, C.; Hao, W. C.; Zhang, J. Y. Rare Met. (Beijing, China) 2008, 27, 243–250. (5) Zhao, K.; Zhang, X.; Zhang, L. Z. Electrochem. Commun. 2009, 11, 612–615. (6) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. J. Phys. Chem. C 2008, 112, 747– 753. (7) Lei, Y. Q.; Wang, G. H.; Song, S. Y.; Fan, W. Q.; Pang, M.; Tang, J. K.; Zhang, H. J. Dalton Trans. 2010, 39, 3273–3278.

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temperature using ethanol-water mixed solvent with NH3 3 H2O as pH adjustment. The BiOI microspheres showed much higher photocatalytic activity degradation of phenol under visible light irradiation than the random BiOI platelets.8 In addition, AgI/ BiOI,9 TiO2/BiOI,10 and Pt/BiOI11 composites have been synthesized in order to improve the photocatalytic properties of the materials. It is well-known that porous and hollow solids have excellent adsorptive properties that possess numerous applications in catalysis. Despite these advances, the diversity of desired geometry for BiOI porous and hollow shape materials with high photocatalytic activity remain to be explored to meet the everincreasing demand. Meanwhile, ionic liquids as “designer liquids” have attracted great interest for the synthesis of inorganic materials as its tunable physicochemical properties by changing cations and anions, including inherent high conductivity, wide electrochemical window, and environmental benefits deriving from the negligible vapor pressure and straightforward separation procedures12-14 and so on. A variety of different materials have been synthesized in ILs, such as metal nanoparticles,15,16 metal oxides,17 metal chalcogenides,18 and metal salts.19,20 Moreover, hollow structures (8) Xiao, X.; Zhang, W. D. J. Mater. Chem. 2010, 20, 5866–5870. (9) Cheng, H. F.; Huang, B. B.; Dai, Y.; Qin, X. Y.; Zhang, X. Y. Langmuir 2010, 26, 6618–6624. (10) Zhang, X. L.; Zhang, Z.; Xie, T. F.; Wang, D. J. J. Phys. Chem. C 2009, 113, 7371–7378. (11) Yu, C. L.; Yu, J. C.; Fan, C. F.; Wen, H. R.; Hu, S. J. Mater. Sci. Eng., B 2010, 166, 213–219. (12) Welton, T. Chem. Rev. 1999, 99, 2071–2084. (13) Dahl, J.; Maddux, B. L. S.; Hutchison, J. E. Chem. Rev. 2007, 107, 2228– 2269. (14) Ma, Z.; Yu, J. H.; Dai, S. Adv. Mater. 2010, 22, 261–285. (15) Li, Z. H.; Liu, Z. M.; Zhang, J. L.; Han, B. X.; Du, J. M.; Gao, Y. N.; Jiang, T. J. Phys. Chem. B 2005, 109, 14445–14448. (16) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410–1414. (17) Ding, K. L.; Miao, Z. J.; Liu, Z. M.; Zhang, Z. F.; Han, B. X.; An, G. M.; Miao, S. D.; Xie, Y. J. Am. Chem. Soc. 2007, 129, 6362–6363. (18) Jiang, Y.; Zhu, Y. J. J. Phys. Chem. B 2005, 109, 4361–4364. (19) Wang, W. W.; Zhu, Y. J. Cryst. Growth Des. 2005, 5, 505–507. (20) Xia, J. X.; Li, H. M.; Luo, Z. J.; Xu, H.; Wang, K.; Yin, S.; Yan, Y. S. Mater. Chem. Phys. 2010, 121, 6–9.

Published on Web 12/29/2010

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like TiO2 hollow microspheres,21 CuS nestlike hollow spheres,22 hollow silica spheres,23 and ZnSe hollow nanospheres24 have been obtained from ionic liquid. In these works, ILs sometimes acted not only as solvents but also as templates for the fabrication of inorganic materials. The potential of ILs in the controlled synthesis of hollow structures materials remains to be fully explored. Up to now, no literature has made references to the hollow BiOI materials synthesized with the ionic liquid system. Herein, we demonstrate novel reactable ionic liquids (RILs) synthesis of BiOI hollow microspheres by an EG-assisted solvothermal process. The possible formation mechanism for the growth of hollow microspheres was proposed. Moreover, the asprepared BiOI hollow microspheres exhibited higher photocatalytic activity than BiOI nanoplates and TiO2 (Degussa, P25) on the degradation of MO under visible light irradiation. After systematical characterizations, the relationship between the structure of the photocatalyst and the photocatalytic activities was also discussed in detail. It can be assumed that the enhanced photocatalytic activities of BiOI hollow microspheres could be a synergetic effect.

2. Experimental Section 2.1. Material and Sample Preparation. All chemicals were analytical grade and used as received without purification. The ionic liquid [Bmim]I (1-butyl-3-methylimidazolium iodine) (99%) was purchased from Shanghai Chengjie Chemical Co. Ltd. 2.2. Preparation of BiOI Hollow Microspheres. In a typical procedure, 0.001 mol of Bi(NO3)3 3 5H2O was dissolved into an EG solution containing stoichiometric amounts of ionic liquid [Bmim]I. The mixture was stirred for 30 min and then was transferred into a 25 mL Teflon-lined autoclave up to 80% of the total volume. The autoclave was then heated at 140 °C for 24 h and cooled down to room temperature. The final product was separated by centrifugation, washed with distilled water and absolute ethanol four times, and dried under vacuum at 50 °C for 24 h before further characterizations. 2.3. Preparation of BiOI Nanoplates. In a typical procedure, 0.001 mol of Bi(NO3)3 3 5H2O was dissolved into an aqueous solution containing stoichiometric amounts of KI; the mixture was stirred for 30 min and then was transferred into a 25 mL Teflon-lined autoclave up to 80% of the total volume. The autoclave was then heated at 140 °C for 24 h and cooled down to room temperature. The final product was separated by centrifugation, washed with distilled water and absolute ethanol four times, and dried under vacuum at 50 °C for 24 h before further characterizations. 2.4. Characterization. X-ray powder diffraction (XRD) analysis was carried out on a Bruker D8 diffractometer with high-intensity Cu KR (λ = 1.54 A˚). The nitrogen adsorption-desorption isotherms at 77 K were investigated using a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corp.). The field-emission scanning electron microscopy (FE-SEM) measurements were carried out with a fieldemission scanning electron microscope (Hitachi, S-4800) equipped with an energy-dispersive X-ray spectroscope (EDS) which was operated at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) micrographs were taken with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. UV-vis diffuse reflectance spectroscopy was recorded on UV-2450 spectrophotometer (Shimadzu Corp., Kyoto, Japan). (21) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386– 6387. (22) Ge, L.; Jing, X. Y.; Wang, J.; Jamil, S. B.; Liu, Q.; Song, D. L.; Wang, J.; Xie, Y.; Yang, P. P.; Zhang, M. L. Cryst. Growth Des. 2010, 10, 1688–1692. (23) Yuan, J.; Bai, X. T.; Zhao, M. W.; Zheng, L. Q. Langmuir 2010, 26, 11726– 11731. (24) Liu, X. D.; Ma, J. M.; Peng, P.; Zheng, W. J. Langmuir 2010, 26, 9968–9973.

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Figure 1. (a) XRD pattern of the as-prepared BiOI hollow microspheres synthesized with ionic liquid [Bmim]I. (b) EDS of the hollow BiOI microspheres structures.

2.5. Photocatalytic Activity Measurement. Photocatalytic activities of the BiOI powders were evaluated by the photocatalytic degradation of methyl orange (MO) under the visible light irradiation. Experiments were conducted in a Pyrex photocatalytic reactor with two 150 W tungsten halogen lamps as the visiblelight source and an electric fan to prevent thermal catalytic effects. Aeration was performed using an air pump to ensure a constant supply of oxygen and to complete mixing of the solution and the photocatalysts during photoreactions. In a typical experiment, 0.03 g of BiOI powders was dispersed into 100 mL of MO (10 mg/L) solutions. Visible light illumination was provided after the suspension was strongly magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium of MO on the catalyst surfaces. During irradiation, about 3 mL of the suspension continually was taken from the reaction cell at given time intervals. During the photocatalytic process, the intense yellow color of the MO solution gradually faded with increasingly longer exposure times. The photocatalyst powders and the MO solution were separated by a centrifugal machine. The MO concentration was eventually through a UV-vis spectrophotometer (Shimadzu UV-2450) by checking the absorbance at 464 nm.

3. Results and Discussion 3.1. XRD Analysis. The purity and crystallinity of as-prepared samples were examined using powder XRD measurements. Figure 1a is the XRD patterns of the as-prepared BiOI hollow DOI: 10.1021/la104054r

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Figure 2. SEM images of the hollow BiOI microsphere structure: (a) the low-magnification SEM image; (b) the high-magnification SEM images; (c) top view SEM image; (d) TEM images of the hollow BiOI microsphere structures.

microspheres synthesized with [Bmim]I ionic liquid. All the peaks for the BiOI sample were readily indexed to the tetragonal phase of BiOI (JCPDS Card No. 10-0445), with lattice constants of a = 3.994 A˚ and c = 9.149 A˚. No characteristic peaks of the other impurities were observed. Interestingly, the intensity ratio of the (110) peak to the (012) peak in our result of BiOI hollow microspheres structures was 1.20, obviously larger than that of BiOI nanoplates and the data of the BiOI JCPDS card which was only 0.62 and 0.53, respectively (Figure S1). This important result indicated that the hollow structure BiOI crystal had special anisotropic growth along the (110) plane. The EDS analysis on the BiOI hollow microspheres crystal suggested that the sample contained only the elements of Bi, O, and I (Figure 1b). The atomic ratio of Bi:O:I was nearly about 1:1:1, which further proved that the hollow structure sample was pure BiOI. 3.2. SEM and TEM Analysis. Figure 2 is the typical SEM and TEM images of the hollow BiOI microspheres architectures synthesized by ionic liquid [Bmim]I solvothermal treatment at 140 °C for 24 h. It clearly showed that a spherelike BiOI structure with an average diameter of 1-2 μm. After careful examination for numerous SEM images of the sample, it could be found that all the BiOI products were spherelike in shape. As can be shown from the high-magnification SEM image (Figure 2b,c), the entire spherelike BiOI structures were composed of numerous BiOI nanosheets, and there was typically just one hole in the shell of most BiOI microspheres. These observations would lead to a conclusion that the BiOI microspheres were hollow in structure. Their hollowness was further investigated by TEM, as shown in Figure 2d. The obvious contrast between the dark and relatively bright parts further confirmed their hollow nature, and the hole in the shell of BiOI also could found in TEM (inset in Figure 2d), while the structure of BiOI hollow microshperes was different from the structures of BiOI materials synthesized with KI as I source in the early literatures.6,7 Therefore, apart from being the I source, ionic liquid [Bmim]I was also used as a soft template, directing the growth of BiOI hollow microspheres structures, and it played an important role in the formation of flowerlike BiOI 1202 DOI: 10.1021/la104054r

Figure 3. Nitrogen absorption-desorption isotherms of BiOI hollow microspheres.

hollow microspheres. As is well-known that the hollow structure materials could enhance the surface areas and then enhance the photocatalytic activities, it agreed well with our results of photocatalytic degradation of MO under the visible light irradiation as described later. 3.3. Nitrogen Adsorption Analysis. The Brunauer-EmmettTeller (BET) specific surface areas and porosity of the BiOI sample were investigated by using nitrogen adsorption and desorption isotherms. Figure 3 displays the nitrogen absorption-desorption isotherms of BiOI hollow microspheres. The isotherm to the sample was of type IV (BDDT classification) with a hysteresis loop observed in the range of (0.6-1.0)p/p0, which was characteristic of mesoporous materials (Figure 3). The BET specific surface area of the sample was calculated to be 61.63 m2/g, which was much larger than the other structures of BiOI (5.2-22.7 m2/g) which were reported in the literatures.6,7 The Langmuir 2011, 27(3), 1200–1206

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Figure 4. SEM images of the BiOI hollow structures synthesized at 140 °C in the presence of ionic liquid [Bmim]I: (a, b) 30 min; (c, d) 1 h; (e) 6 h. (f) XRD pattern of products synthesized with different reaction time.

pore volumes of BiOI hollow microspheres were determined by using the Barrett-Joyner-Halenda (BJH) method (inset in Figure 3); the result showed that BiOI hollow structures contained small mesopores of ca. 9.3 nm and large mesopores with maximum pore diameters of ca. 51 nm. The small pores presumably related to finer intra-aggregated pores formed by nanosheets, whereas the large pores may be attributed to the hollow structure and the hole in the shell of BiOI crystal. This confirms the result of SEM analysis. The BET specific surface areas of BiOI sample indicated that the hollow microspheres had a relatively high surface-to-volume ratio. 3.4. Possible Formation Process of BiOI Hollow Microspheres. To investigate the intermediates and the mechanism of the hollow BiOI microspheres architectures, a systematic timedependent experiment was conducted to track the formation of Langmuir 2011, 27(3), 1200–1206

the hollow structures of BiOI crystal (Figure 4). The synthesis was done in EG solution just containing [Bmim]I and Bi(NO3)3. During the synthesis process, a clear solution could be obtained when Bi(NO3)3 and [Bmim]I were dissolved into the EG. As shown in Figure 4a, after 30 min reaction, BiOI nanosheets were attained at first, the thickness of which was about 5-10 nm. Since BiOI has a layered structure characterized by [Bi2O2] slabs interleaved by double slabs of I atoms, it resulted in the formation of platelet morphology.25 In addition, further observation showed that many BiOI nanosheets began to oriented aggregate to form flowerlike structures (Figure 4b). When the reaction time lasted for 1 h, the anomalous BiOI nanosheets continued to (25) Zhang, J.; Shi, F. J.; Lin, J.; Chen, D. F.; Gao, J. M.; Huang, Z. X.; Ding, X. X.; Tang, C. C. Chem. Mater. 2008, 20, 2937–2941.

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Figure 5. Schematic illustration of proposed formation mechanism of BiOI hollow structures.

aggregate and transformed from flowerlike structures into BiOI hollow microspheres (Figure 4c,d). After treatment for 6 h, large numbers of uniform BiOI microspheres were obtained, and no other morphology was observed (Figure 4e). After 24 h reaction, hierarchical hollow BiOI microspheres with a hole in the shell were produced. The corresponding XRD pattern confirmed that all the products were tetragonal phase of BiOI (JCPDS Card No. 10-0445), with lattice constants of a = 3.994 A˚ and c = 9.149 A˚. (Figure 4f). The diffraction peaks demonstrated stronger intensity and the crystallinity of the products improved, with increased reaction time. In addition, the intensity ratio of the (110) peak in the result of BiOI hollow structures was also increased. This important result indicated that the crystal had special anisotropic growth along the (110) plane, which could would in turn lead to the transition of morphology of BiOI crystal from nanosheets into hollow microspheres. On the basis of the above discussion, the possible mechanistic pathway in the formation of hollow BiOI microspheres architectures could be best summarized in the schematic illustration as shown in Figure 5. It is well-known that ionic liquid based on 1-alkyl-3-methylimidazolium cations could show aggregation behavior and form micelles in solutions.26 At the beginning, the reagents were added into the reaction system, with large amounts of nuclei of BiOI formed on the surface of ionic liquid micelles. Then these BiOI nuclei grew and transformed into nanosheets with uniform size on the surface of ionic liquid micelles. When the BiOI nanosheets formed, these small nanosheets were oriented aggregated on the surface of ionic liquid micelles by an aggregation-based mechanism and then self-assembled into flowerlike BiOI structures. With increased reaction time, the aggregates of flowerlike BiOI structures continuously grew to form BiOI hollow microspheres through a self-assembly process. When the reaction finished, the external temperature of the hollow microspheres was reduced more quickly than that of the internal. Hence, the pressure difference between the inner and outer of hollow structures was increased gradually. Finally, ionic liquid pushed out from the unsubstantial point of BiOI crystal, and a hole developed on the shell of the hollow microspheres. When the hole formed on the shell, the pressure difference between the inner and outer of hollow structures will disappear, and the hollow microspheres with a hole on the shell were produced. During the process, ionic liquid [Bmim]I played an important role as solvent, reactant, and template and was vital for the structure of BiOI hollow microspheres. The growing process was similar to the previous report of hollow CuS structures.22 Further work is still needed to investigate the exact growth mechanism.

3.5. Optical Absorption Properties. The optical properties of BiOI hollow microspheres were measured by UV-vis diffuse reflectance spectroscopy. Figure 6 showed the UV-vis diffuse reflectance spectroscopy of BiOI spherelike structures. The BiOI samples demonstrated a great increase in absorption with wavelengths lower than about 470 nm due to the band gap transition. In comparison with that TiO2 (Degussa, P25), a notable red shift in the absorption edge was observed for BiOI materials, indicating a smaller band gap (Figure 6A). A classical Tauc approach was further employed to estimate the Eg value of BiOI crystals according to the following equation: REphoton = K(Ephoton Eg)n/2, where K is a constant, R is the absorption coefficient, Ephoton is the discrete photon energy, and Eg is the band gap energy. Among them, n depends on the characteristics of the transition in a semiconductor (direct transition n = 1 and indirect transition n = 4). For BiOI, the value of n is 4 for the indirect transition.27 The energy intercept of a plot of (REphoton)1/2 vs Ephoton yields was Eg for a direct transition. As shown in Figure 6B, the band gaps of BiOI hollow microspheres and nanoplates (Figure S2) were calculated to be 1.66 and 1.71 eV, respectively, which were smaller than the band gap of Degussa TiO2 (about 3.2 eV). The DRS of the BiOI samples synthesized with different reaction time showed that a notable red shift in the

(26) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 2191–2198.

(27) Zhang, K. L.; Liu, C. M.; Huang, F. Q.; Zheng, C.; Wang, W. D. Appl. Catal., B 2006, 68, 125–129.

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Figure 6. (A) UV-vis diffuse reflectance spectra and (B) (REphoton)1/2 vs Ephoton curves of BiOI samples: (a) BiOI hollow microspheres; (b) BiOI nanoplates; (c) TiO2 (Degussa, P25).

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Figure 7. (a) Temporal UV-vis absorption spectral changes during the photocatalytic degradation of MO in aqueous solution. (b) Photocatalytic degradation of MO in the presence of BiOI and P25 and photolysis of MO under visible light irradiation.

absorption edge was observed with the increasing of reactive time, and the band gap of the BiOI decreased from 1.88 to 1.74 eV at the same time (Figure S3a). In addition, it was also found that an enhanced optical absorbance in the UV-vis region was achieved for BiOI hollow microspheres (Figure 6A). The hollow structures of BiOI microspheres with a hole in the shell could allow multiple scattering of UV-vis light, suggesting that the optical path length for light transporting through those BiOI hollow structures might be longer than that for the BiOI nanoplates. Thus, longer optical path length could increase the quantity of photogenerated electrons and holes available to participate in the photocatalytic decomposition of the contaminants.28 As a result, it is expected that such hollow structures with a hole in its shell could provide an effective approach for increasing the photoreactivity of semiconductors. 3.6. Photocatalytic Activity. The photocatalytic activities of BiOI samples were measured on the degradation of MO in water under the visible light irradiation. Figure 7 showed the timedependent absorption spectra of MO solution in the presence of BiOI hollow microspheres. As can be seen in Figure 7a, in the presence of BiOI hollow microspheres catalyst, the maximum absorption of MO suspension shifted from 464 to 390 nm, and the color changed gradually from yellow to colorless after irradiation (28) Yu, J. G.; Su, Y. R.; Cheng, B. Adv. Funct. Mater. 2007, 17, 1984–1990.

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for 180 min. It could be assumed that the MO was photodegraded and changed to the intermediates by BiOI hollow microspheres under the visible light irradiation, and then the intermediates were photodegraded completely after irradiation for 180 min. Figure 7b showed the variation in absorption of MO at 464 nm with the irradiation time, and a blank experiment without BiOI photocatalyst indicated that direct photolysis of MO under the same conditions could almost be neglected. When the catalyst was used in the system, after irradiation for 180 min, 21% and 11% of MO were photodegraded by BiOI nanoplate and Degussa P25, respectively. 92% of MO was photodegraded by BiOI hollow microspheres during the same irradiation time. These results suggested that the as-prepared BiOI hollow microspheres showed higher photoactivities for the MO degradation than those of BiOI nanoplates and TiO2 (Degussa P25). Moreover, the photocatalytic activities of BiOI crystal synthesized with different reaction time were improved at the same reactive condition (Figure S3b). The photocatalytic activity of BiOI hollow microspheres was also much higher than that of the flowerlike and solid microspheres BiOI structures which were reported in the literatures.6,7 In addition, in the early report, the photocatalytic activities of BiOI crystal were also evaluated by degradation of phenol in water under visible light, and the results showed that BiOI crystals were able to effectively degraded and mineralized colorless substrate phenol under visible light irradiation.8 To understand the variation of photocatalytic activity of BiOI crystal, several factors may account for the high activity of BiOI hollow microspheres as compared to the randomly nanoplatelets and the literatures. First, it is well-known that the photocatalytic activity is closely related to the adsorption ability and surface area and of the catalyst. The BET of the BiOI hollow microspheres was calculated to be 61.63 m2/g, which was much larger than the other structures of BiOI (5.2-22.7 m2/g) reported in the literatures. These results indicate that the BiOI hollow microspheres held larger specific surface areas and can absorb more active species and reactants on their surface. Moreover, the hollow structures of BiOI microspheres with a hole in the shell could allow multiple scattering of UV-vis light. The longer optical path length could increase the quantity of photogenerated electrons and holes available to participate in the photocatalytic decomposition of the contaminants. At the same time, the pores on its shell could be considered as transport way for reactant and product molecules moving in or out of the material,28,29 so the chemical reaction could be occur more easily, hence enhancing the photocatalytic efficiency. Second, the high surface-to-volume ratios of hollow structures are good at the transfer of electrons and holes to the surface and facilitate the degradation of MO dye. It is helpful for maintaining high active surface area and improving the photocatalytic activity of the material. The third factor could be the narrow band-gap energy. In a typical photodegradation process, when the semiconductor BiOI is irradiated by light, the photogenerated electrons will transfer from the valence band (VB) to the conduction band (CB), leaving the corresponding holes in the valence band. Then the photoinduced holes (hVBþ) will directly oxidize the MO pollutants in the aqueous solution.9 As is known that BiOX (Cl, Br, I) had a layered structure characterized by [Bi2O2] slabs interleaved by double slabs of X atoms, this layered structure could provide the large space to separate and conduct the photogenerated hole-electron pair efficiently.30 On the basis of the DRS results, it can be found that the band gap of BiOI (29) Yu, J. G.; Zhang, L. J.; Cheng, B.; Su, Y. R. J. Phys. Chem. C 2007, 111, 10582–10589. (30) Shang, M.; Wang, W. Z.; Zhang, L. J. Hazard. Mater. 2009, 167, 803–809.

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hollow microspheres is 1.66 eV, which is smaller than the band gap of BiOI nanoplates. The small band gap could excited easier production of active species;6 thus, it can make the photocatalytic process more efficient. On the basis of above analysis, it could be concluded that the enhanced photocatalytic activities of BiOI hollow microspheres could be a synergetic effect, including energy band structure, high BET surface area, high surface-to-volume ratios, and light absorbance.

4. Conclusions In summary, BiOI uniform flowerlike hollow microspheres with a hole in its shell have been successfully synthesized in the presence of ionic liquid [Bmim]I. The average diameter of BiOI flowerlike hollow microspheres was 1-2 μm, and the asprepared hollow structures exhibited larger BET specific surface area (61.63 m2/g) and narrow band gap (1.66 eV). The XRD and SEM analysis of the products showed that the formation of BiOI hollow microspheres mainly went through an oriented aggregation-based process and self-assembly growth mechanism. Ionic liquid acted not only as solvents and templates but also as an I source for the fabrication of BiOI hollow microspheres.

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Moreover, the photocatalytic activities of BiOI samples on the degradation of MO under visible light irradiation showed that the BiOI hollow microspheres exhibited higher photocatalytic activity than that of BiOI nanoplates and P25. On the basis of such analysis, it could be assumed that the enhanced photocatalytic activities of BiOI hollow microspheres could be ascribed to their energy band structure, high BET surface area, high surface-tovolume ratios, and light absorbance. The resulting BiOI hollow structures obtained were very promising photocatalysts for degrading organic pollutants and other applications. Acknowledgment. The present work is supported by the National Natural Science Foundation of China (No. 20876071, No. 21007021, and No. 21076099) and the Doctoral Innovation Fund of Jiangsu (CX09B_210Z). Supporting Information Available: XRD and SEM of BiOI nanoplates, UV-vis diffuse reflectance spectra, (REphoton)1/2 vs Ephoton curves, and photocatalytic activities of BiOI synthesized with different reaction times. This material is available free of charge via the Internet at http://pubs.acs.org.

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