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Simple Solvothermal Routes to Synthesize 3D BiOBrxI1-x Microspheres and Their Visible-Light-Induced Photocatalytic Properties Zhifang Jia, Fumin Wang,* Feng Xin, and Baoquan Zhang School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R.China
bS Supporting Information ABSTRACT: The band structure-controlled solid solution of BiOBrxI1-x was successfully synthesized by a simple solvothermal route. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, UVvis diffuse reflectance spectroscopy, and nitrogen sorption/desorption. The resulting BiOBrxI1-x samples were phase-pure and of three-dimensional (3D) microspheres composed of nanoplates. The samples with different x values exhibited composition-dependent absorption properties in the visible light region and the bandgaps were estimated to be between 1.89 and 2.53 eV. Rhodamine B (RhB) photocatalytic degradation experiments showed that these samples possessed excellent and composition-dependent performance. The highest catalytic performance of the 3D BiOBr0.2I0.8 microspheres may derive from a synergetic effect, including higher surface area, porous structure, and enhancement of light absorbance. Moreover, on the basis of the analysis of the valence band and conduction band, a possible mechanism of photocatalytic activity of BiOBrxI1-x samples was also proposed.
1. INTRODUCTION Considerable research during the past few years has shown that photocatalytic decomposition of organic pollutants using semiconductors is a potential way of solving environmental issues. In the past few years, TiO2 has been used extensively as a photocatalyst for degradation of dyes in wastewater due to its high photocatalytic activity, low cost, and nontoxicity.1 However, TiO2 responds only to the ultraviolet light (λ < 400 nm) that accounts for only about 4% of the sunlight due to its wide band gap (3.2 eV). This limits the efficient utilization of solar energy for TiO2 photocatalysis. Therefore, the development of efficient visible-light-induced photocatalysts for dye photodegradation has been an urgent issue from the viewpoint of using solar energy. Recently, a great deal of effort has been devoted to developing photocatalysts containing bismuth with high activities for environmental applications and/or water splitting, such as BiVO4,2 Bi12TiO20,3 Bi2MoO6,4 Bi2WO6,5 and BiYO3.6 Bismuth oxyhalide compounds, BiOX (X = Cl, Br, I), have recently been found to possess remarkable photocatalytic activities under UV or visible-light illumination. BiOX (X = Cl, Br, I) compounds all crystallize in the tetragonal matlockite (PbFCl) structure; in other words, its structural feature comprises a layer of [Bi2O2] slabs interleaved by double slabs of halogen atoms. The internal static electric fields between the [Bi2O2]2þ and halogen anionic layers are believed to induce the efficient separation of photogenerated electronhole pairs, which favors the photocatalytic activity of the catalysts.7 They may serve as a new family of promising photocatalysts. It is well-known that photocatalytic activity is closely related with the size, morphology, and structure of photocatalysts. For example, nanoscale materials are believed to perform better than their bulk counterparts due to the larger surface area and the faster arrival to the reaction sites of the photogenerated electrons and holes.8 Three-dimensional (3D) microscale architectures fabricated from nanosized building blocks hold many advantages, r 2011 American Chemical Society
such as high photocatalytic activity, abundant transport paths for small organic molecules, and easy separation and good recyclability.9 Therefore, catalysts with 3D nanostructures have attracted much research interest. In addition, the bandgap also has an influence on controlling the properties of semiconductors. Several preparation methods have been used to control the bandgaps and band positions, such as metal doping,10 nonmetal doping,1 and solid solution.11 Among them, solid solution has a series of different bandgaps because its components can change in a big proportional band. So it is a feasible and effective method to obtain suitable conduction band (CB) and valence band (VB) for photocatalytic decomposition of organic pollutants using visible light. The band structure-controlled solid solution of BiOBrxI1-x was designed in our present work. In the present study, we have synthesized 3D BiOBrxI1-x microspheres constructed by nanoplates via a simple one-pot solvothermal method. The synthesized BiOBrxI1-x series microspheres exhibit composition-dependent optical properties. The photodegradation of rhodamine B (RhB) was employed to evaluate the photocatalytic activities of the BiOBrxI1-x samples under visible light illumination (λ > 400 nm). It is demonstrated that these samples exhibit excellent and composition-dependent performance. According to the information of the valence band and conduction band, the possible mechanism of photocatalytic activity on BiOBrxI1-x samples is discussed.
2. EXPERIMENTAL DETAILS 2.1. Synthesis of the Samples. All the reagents used in the experiments were analytical grade and used without further purification. In a typical synthesis, 3.0 mmol of Bi(NO3)3 3 5H2O was Received: November 18, 2010 Accepted: April 17, 2011 Revised: March 30, 2011 Published: April 17, 2011 6688
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Industrial & Engineering Chemistry Research added to 40 mL of ethylene glycol (EG) with stirring at room temperature. After Bi(NO3)3 3 5H2O was dissolved fully, appropriate molar ratios of NaBr and KI with a total molar amount of 3 mmol were added to the previous solution. Finally, the mixture was transferred into a Teflon-lined stainless steel autoclave of 60-mL capacity. The autoclave was sealed and maintained at 160 °C for 10 h and then cooled to room temperature naturally. The precipitate was collected by centrifugation, washed several times with deionized water and ethanol, and dried in an oven at 80 °C for 5 h. 2.2. Characterization. The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2500 diffractometer with Cu KR radiation at a scanning speed of 8°/min ranging from 10 to 80°. The scanning electron microscope (SEM) characterizations were performed on a Nanosem 430 field emission scanning electron microscope. The UVvis absorption spectra were measured on a Shimadzu UV-2550 spectrophotometer using BaSO4 as a reference in wavelength of 200700 nm. Nitrogen adsorption desorption isotherms were collected on Quantachrome Quadrasorb SI instrument at 77 K after the sample had been degassed in a flow of N2 at 100 °C overnight 2.3. Photocatalytic Test. Photocatalytic activity was evaluated by the degradation of RhB under visible-light irradiation using a 300-W tungsten halogen lamp with a UV-cutoff filter (λ > 400 nm). An amount of 0.02 g of photocatalyst was suspended in a 100 mL aqueous solution of 10 mg/L RhB. Before illumination, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorptiondesorption equilibrium between the photocatalysts and RhB. At given time intervals, 3 mL of the reaction solution was sampled and centrifuged to remove photocatalyst powders. Then, the UVvis adsorption spectrum of the centrifuged solution was recorded using a UVvis spectrophotometer. The total organic carbon (TOC) concentration was determined with a TOC analyzer (TOC-VCPH, Shimadzu Corporation, Japan).
3. RESULTS AND DISCUSSIONS 3.1. Crystalline Phase and Morphology. Figure 1 shows the XRD patterns of the as-prepared samples. No characteristic peaks of other impurities were observed. As we can see, BiOBr and BiOI are in line with the standard spectra (JCPDS, card nos. 09-0393 and 10-0445), respectively. Despite the different compositions, all the samples display the same tetragonal phase. The diffraction peaks shifted to larger-angle side as the x value increased. The successive shift of the XRD pattern indicated that the crystals obtained were not mixtures of BiOBr and BiOI phases but BiOBrxI1-x solid solutions.12 The radii of Br (1.95 Å) are smaller than those of I(2.16 Å), so it is considered that the Br incorporated in BiOI lattice or entered its interstitial sites. It is observed that the intensity of the strongest diffraction peak in the solid solutions was different, suggesting that the crystallite behavior could be changed. The morphology of as-synthesized BiOBrxI1-x products was characterized by SEM, results of which is shown in Figure 2. During the solvothermal process, addition of I ion directly affects the morphology of BiOBrxI1-x. When none of I was doped, BiOBr exhibited a lot of separate microspheres with diameters of 28 μm, as shown in Figure 2A. Further observation showed that all the microspheres were composed of numerous nanoplates with a thickness of about 25 nm (Figure 2B), these nanoplates aggregated compactly together to form a microsphere structure. When
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Figure 1. XRD patterns of as-prepared BiOBrxI1-x photocatalysts.
the I was doped, BiOBrxI1-x (x < 1) products still appeared in the shape of microspheres composed of numerous nanoplates but become incompact (Figure 2C2L). The gaps between neighbor nanoplates were observed in the range of several tens to hundreds of nanometers and the thickness of nanoplates were estimated to be about 10 nm.When x changed from 0.8 to 0.4, the sizes of these spheres were not uniform. Their diameters were in the range of several micrometers. For example, the diameters were about14 μm for x = 0.8, 17 μm for x = 0.6, and 18 μm for x = 0.4. As x decreased further, the sizes of these spheres became more uniform; the average diameters were about 3.5 μm for x = 0.2, and 1.5 μm for x = 0, respectively. It was found that the surfaces of the microspheres are rough as they are composed of numerous radically grown nanoplates. The nanoplates interweave together to form a flower-like 3D spheres. More importantly, this (3D) architecture with many pores may provide abundant transport paths and active sites for small organic molecules, which is considered to be favorable to the photocatalytic performance. To reveal the growth process of 3D BiOBrxI1-x microspheres, time-dependent experiments were carefully conducted when x = 0.2. The products were collected at different stages from the reaction mixture, and then their morphologies were investigated by SEM. Figure S1 shows SEM images of BiOBr0.2I0.8 powders collected at different reaction stages. In the initial stage (30 min), the primary nanoparticles tended to aggregate together to form submicroscaled solid spheres, as presented in Figure S1(A). As the reaction proceeded, the primary nanoparticles grew into nanoplates, which may be due to the directing ability of EG (from Figure S1(B) to Figure S1(D)).13 With the reaction time increasing, the nanoplates continued to grow and assembled together to form microspheres (Figure S1(E)) through a dissolution recrystallization process of the preformed nanoparticles. When the reaction time increased to 2 h, perfect hierarchical microspheres composed of nanoplates were produced (Figure S1(E)). After 2 h reaction, the morphology of the sample did not change obviously (Figure S1(F)). 3.2. Optical Absorption Property and Pore Distribution. Diffuse reflectance spectroscopy (DRS) is a useful tool for characterizing the electronic states in semiconductor materials. Figure 3 displays the diffuse reflectance spectra of BiOBrxI1-x samples prepared by solvothermal method at 160 °C. The absoption 6689
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Figure 2. SEM images of as-prepared BiOBrxI1-x photocatalysts with different values of x: (A,B) 1.0, (C,D) 0.8, (E,F) 0.6, (G,H) 0.4, (I,J) 0.2, (K,L) 0.
Figure 3. UVvis diffuse reflectance spectra of as-prepared BiOBrxI1-x photocatalysts.
edges of BiOBrxI1-x solid solutions were gradually red shiftted as the amount of I increased. The maximal absorbance wavelengths were 578.5, 567.8, 554.8, 506.3, 469.7, and 436.6 nm, corresponding to x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0, respectively. Interestingly, the products obtained show a gradient evolution in color from offwhite to brick red (inset of Figure 3), which is in agreement
with the red shift with the increasing amount of I. For a crystalline semiconductor, it was shown that the optical absorption near the band edge follows the equation14 ahv = A(hv Eg)n/2, where a, v, Eg, and A are the absorption coefficient, the light frequency, the bandgap, and a constant, respectively. Among them, n decides the characteristics of the transition in a semiconductor. The value of n is 1 and 4 for a direct and indirect bandgap material, respectively. To distinguish the transition character (direct or indirect) of the sharp absorption edge, we analyzed its shape. In semiconductors, the square of absorption coefficient is linear with energy for direct optical transitions in the absorption edge region; whereas the square root of absorption coefficient is linear with energy for indirect transitions.7 Data plots of absorption2 versus energy and absorption1/2 versus energy in the absorption edge region are shown in Figure S2 for BiOBrxI1-x samples prepared at x = 0.2, 0.4, 0.6, and 0.8. The absorption1/2 versus energy plot is nearly linear, whereas the absorption2 versus energy plot is not linear. This feature suggests that the absorption edge of BiOBrxI1-x samples (0 < x < 1.0) is caused by indirect transitions, namely, BiOBrxI1-x samples (0 < x < 1.0) are indirect semiconductors.7 Because former studies showed that BiOX (X = Br, I) is an indirect bandgap material,15 the bandgap can be estimated by extrapolation of the plots of (ahv)1/2 versus hv. Plots of (ahv)1/2 versus photon energy (hv) of BiOBrxI1-x powders are shown in Figure 4. The obtained bandgap values are 1.89, 1.97, 2.00, 2.05, 2.21, and 2.53 eV, corresponding to 6690
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Figure 4. Plots of (ahv)1/2 versus photon energy (hv) for BiOBrxI1-x photocatalysts.
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Figure 6. Pore size distribution curves of the BiOBrxI1-x photocatalysts.
Table 1. Physicochemical Properties of As-Prepared BiOBrxI1-x Samples x value
adsorption (%) Eg (eV)
Figure 5. Nitrogen adsorption/desorption isotherms of the BiOBrxI1-x photocatalysts.
x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0, respectively.These results indicate that BiOBrxI1-x has a bandgap suitable for photocatalytic applications in the visible range. The nitrogen adsorptiondesorption isotherms and porosity of the 3D microspheres were further investigated. Figure 5 shows the nitrogen adsorption and desorption isotherms of the as-prepared samples with varying x. The isotherm can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.61.0 P/P0, which is characteristic of mesoporous materials. According to previous reports,16 a bimodal mesopore size distribution results from two different aggregates in the powders. The hysteresis loop in the lower relative pressure range (0.4 < P/P0 < 0.8) is related to finer intra-aggregated pores formed between intra-agglomerated primary particles, and that in the higher relative pressure range (0.8 < P/P0 < 1) is associated with larger interaggregated pores produced by interaggregated secondary particles. This bimodal mesopore size distribution was further confirmed by the corresponding pore size distributions shown in Figure 6. The samples BiOBrxI1-x (0.2 < x e 1.0) contained two small mesopores (ca. 2.5 and 3.8 nm), with decreasing the x value to 0.2, BiOBr0.2I0.8
0
0.2
0.4
0.6
0.8
1.0
58.5 1.89
38.6 1.97
21.5 2.00
15.4 2.05
10.8 2.21
5.15 2.53
EVB (eV)
2.39
2.42
2.53
2.61
2.74
2.94
ECB (eV)
0.50
0.45
0.53
0.56
0.53
0.41
ABET (m2 g1)
56.29
55.37
24.70
19.21
21.28
12.03
VBJH (cm3 g1)
0.254
0.251
0.113
0.073
0.074
0.031
contained small mesopores of ca. 3.8 nm and larger mesopores with a maximum pore diameter of ca. 9.7 nm. When none of Br was doped, BiOI contained small mesopores of ca. 2.5 nm and larger mesopores with a maximum pore diameter of ca. 12.7 nm. The presence of iodine in the BiOBr exerted significant influence on the pore structure and BET surface areas of the obtained products. With varying x value, the shapes of nitrogen adsorption and desorption isotherms underwent several obvious changes, implying a significant variation of the pore structure. First, with decreasing x value, the isotherms showed higher absorption at high relative pressures (P/P0 approaching 1), indicating the formation of more macropores and/or an increasing in the pore volume, as confirmed by a gradual increase in the corresponding pore volume (as shown in Table 1). Second, when the x value was decreased from 1 to 0, the relative areas of the hysteresis loops in the higher relative pressure (P/P0) region increased gradually and eventually dominated the total areas of all the hysteresis loops; these results suggest that the pore volume of the interaggregated pores increased and became the main portion of the total pore volume. As further confirmed in Figure 6, when the x values were 0.2 or 0, the pore volumes of intra-aggregated mesopores were negligible compared with those of the interaggregated ones, based on their corresponding integral areas. The BET surface areas of BiOBrxI1-x products were also found to be highly dependent on x value. Each sample showed an increase in the BET surface area with decreasing x value (as shown in Table 1). 3.3. Photocatalytic Activities. To determine the photocatalytic activity of the as-prepared BiOBrxI1-x photocatalysts, it was evaluated by using RhB as the model pollutant with an initial 6691
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Industrial & Engineering Chemistry Research concentration of 10 mg/L. The photocatalytic degradation process is monitored by examining the variations in maximal absorption in UVvis spectra at 553 nm. C was the absorption of RhB at the wavelength of 553 nm and C0 was the absorption of RhB after the adsorption equilibrium on BiOBrxI1-x photocatalysts before irradiation. Figure 7 displays the concentration changes of Rhodamine B at 553 nm as a function of irradiation time during the degradation process in aqueous BiOBrxI1-x. BiOBr0.8I0.2, BiOBr0.6I0.4, BiOBr0.4I0.6, and BiOBr0.2I0.8 show a far higher
Figure 7. Photodegradation efficiencies of RhB as a function of irradation time by BiOBrxI1-x photocatalysts.
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photocatalytic activity than that of BiOBr and BiOI, which affirms that the photocatalytic activity is really induced by the BiOBrxI1-x. Figure 8 shows the temporal evolution of the spectral changes of the RhB mediated by BiOBr0.8I0.2, BiOBr0.6I0.4, BiOBr0.4I0.6, and BiOBr0.2I0.8, respectively. With the photocatalytic degradation of RhB, the absorption peak at 553 nm blueshifts and turns broadened at the same time. This agrees well with the report of Zhao and co-workers in the TiO2/RhB process.17 According to their report, the blue-shift of absorption band is caused by de-ethylation of RhB because of the attack by one of the active oxygen species on the N-ethyl group. When the deethylated process is fully completed, the absorption band shifts to 498 nm at wavelength and RhB turns to rhodamine. Then rhodamine is gradually decomposed due to the further destruction of the conjugated structure. Moreover, the different decomposed processes of RhB are observed in Figure 8. The sample BiOBr0.2I0.8 presents a slower change in de-ethylation of RhB with concomitance of a rapid destruction of the conjugated structure. Correspondingly, the color of dye solution changes from initial red to colorless. However, the samples BiOBr0.8I0.2, BiOBr0.6I0.4, and BiOBr0.4I0.6 show a rapid process in de-ethylation of RhB and then the slower destruction of the conjugated structure. Correspondingly, the color of dye solution changes gradually from initial red to a light greenyellow, which can be observed by the naked eye. The extent of mineralization of the RhB can be reflected by the reduction of TOC. To minimize experiment errors, the initial concentration of RhB was promoted from 10 to 20 mg/L. The TOC variations of RhB solution during the photocatalytic degradation by the BiOBrxI1-x photocatalysts under visible light
Figure 8. Temporal evolution of the spectra during the photodegradation of RhB mediated by the BiOBrxI1-x (x = 0.2, 0.4, 0.6, 0.8) photocatalysts. 6692
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Industrial & Engineering Chemistry Research irradiation are shown in Figure S3. After degradation for 4 h, the TOC concentration of RhB solution decreased from 12.48 mg/L to 7.343, 9.714, 9.397, and 7.726 mg/L, corresponding to x = 0.2, 0.4, 0.6, 0.8, respectively, which indicates that the RhB was mineralized to CO2 partly. The photocatalytic activity is affected by many factors which could cooperate with each other and enhance the photocatalytic activity. Among them, the valence band (VB) of a photocatalyst is an important factor for the effective photocatalytic decomposition of organic contaminants.18 The valence band-edge of a semiconductor at the point of zero charge can be predicted by the following equation18 EVB ¼ X Ee þ 0:5Eg where X is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy, Ee is the energy of free electrons on the hydrogen scale (∼4.5 eV), and Eg is the bandgap of he semiconductor. According to this empirical expression, the EVB of the BiOBrxI1-x samples were calculated and are listed in Table1. As can be seen, with decreasing x value, the EVB decreases from 2.94 to 2.39 eV, which means that the oxidation ability becomes weaker. Consequently, the photocatalytic activity is expected to decline from this aspect. However, the absorption spectra of the as-prepared sample showed a strong absorption in the visible light region and a red shift with decreasing x value. Therefore, with decreasing x value, the BiOBrxI1-x samples increased the number of photogenerated electrons and holes to participate in the photocatalytic reaction, which could partly explain the enhanced photoactivity. That is to say, a balance between the level of valence band and the light absorption ability of BiOBrxI1-x solid solutions is an important factor for the effective photocatalytic decomposition of RhB. Another contribution to the high photocatalytic activity could be the 3D BiOBrxI1-x spherical architecture. Table 1 shows the BET surface areas, BJH pore volumes, and the adsorption percentages of RhB on the photocatalysts in the dark for 60 min. The BET surface areas of the samples increase with decreasing x value, resulting in an increase of adsorption percentages of RhB molecules. Moreover, a larger surface area not only supplies more active sites for the degradation reaction but also effectively promotes the separation efficiency of the electronhole pairs,19 therefore, the increasing of BET surface areas was beneficial to the enhancement of the photocatalytic activity. There are plenty of pores in this structure that can be considered as transport paths for the RhB molecules. It benefits the reactant molecules to get to the reactive sites on the framework walls of the photocatalysts, which results in good photocatalytic performance.20 Furthermore, we believe that the porous structure may enhance visible-light utilization due to the transmission and multiple reflections of visible light within the interior cavity. According to this general information, we assume that the highest photoactivity of BiOBr0.2I0.8 could be a synergetic effect, including higher surface area, porous structure, and enhancement of light absorbance. Also, the larger diameters of the 3D microspheres will facilitate the separation and recycling of photocatalysts. BiOBr0.2I0.8 could settle naturally in 15 min, as shown in Figure 9, which is very important for environmental application. 3.4. Possible Photocatalytic Mechanism. As is known, in a typical photodegradation procedure, once the semiconductor is
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Figure 9. Experiments of natural settlement. (A) BiOBr0.2I0.8 powder suspended in water, (B) settled naturally after 15 min.
irradiated, the electrons (eCB) can be excited to the conduction band (CB) from the valence band (VB) and the holes (hVBþ) generated in the valence band simultaneously. Then the photoinduced holes (hVBþ) and/or the formed hydroxyl radicals ( 3 OH) directly oxidize the pollutants in the aqueous solution, which are regarded as the main oxidative species responsible for the degradation. At the same time, the photogenerated electrons at the photocatalyst surface can be scavenged by the dissolved O2 in solution to yield the superoxide radical anion(O2 3 ) and other reactive oxygen species, the active oxygen species attack the pollutants and the pollutants will be degraded gradually, therefore, the photogenerated electrons (eCB) can also participate in the oxidation process. To understand the photocatalytic mechanism, conduction band (CB) and valence band (VB) potentials were calculated by the following empirical equation, EVB = X Ee þ 0.5 Eg and ECB = EVB Eg, respectively. The results are listed in Table1. The valence band edge potentials of all the BiOBrxI1-x samples are more positive than E0 ( 3 OH/OH) (2.38 eV vs NHE), thus, it is theoretically speculated that the hVBþ on the surfaces of BiOBrxI1-x samples can oxidize OH to yield 3 OH. Nevertheless, the production of 3 OH is almost impossible due to the standard redox potential of BiV/BiIII (þ1.59 eV)21 being more negative than that of 3 OH/OH (þ2.38 eV). And as shown in Table 1, the conduction band edge potentials of all the BiOBrxI1-x samples are less negative than E0 (O2/ O2 3 ) (0.13 eV vs NHE), which does not allow the yield of O2 3 radicals via the reduction of dissolved O2 by photogenerated electrons (eCB). As a result, it is difficult for O2 to get the electrons (eCB) to take part in the oxidation reactions. Thus, we believe that the photoinduced holes (hVBþ) directly oxidize the organic pollutants and are the dominant active species in the photodegradation process.
4. CONCLUSIONS In summary, self-assembled 3D BiOBrxI1-x microspheres composed of nanoplates have been successfully synthesized by a facile solvothermal method. The variation of composition of BiOBrxI1-x solid solutions leads to changes in their BET surface areas, porous structure, level of valence band, and light absorption ability associated with the changes in band gap. It is shown that changes in optical and structural characteristics of BiOBrxI1-x solid solutions influence photocatalytic activity. Among all the prepared samples, the as-synthesized BiOBr0.2I0.8 microspheres possessed the best photocatalytic activity under visible light irradiation, which is ascribed to their energy band structure, high BET surface area, and porous structure. In addition, the photoinduced holes (hVBþ) are regarded as the main active species during the photodegradation process based on analysis of the 6693
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’ ASSOCIATED CONTENT
bS
Supporting Information. Figures: (S1) SEM images of BiOBr0.2I0.8 powders collected at different reaction stages, (S2) plots of absorption2 vs energy and absorption1/2 vs energy in the absorption edge region for BiOBrxI1-x samples (0 < x < 1.0), and (S3) TOC variations of RhB solution (20 mg/L) under visible light irradiation in the presence of BiOBrxI1-x photocatalysts. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) 20776106 and 20876109 as well as Program for New Century Excellent Talents in University of China MOE and the Program for Changjiang Scholars and Innovative Research Terms in Universities (IRT0936) ’ REFERENCES (1) Xu, J.; Ao, Y.; Chen, M.; Fu, D. Photoelectrochemical property and photocatalytic activity of N-doped TiO2 nanotube arrays. Appl. Surf. Sci. 2010, 256, 4397–4401. (2) Yang, T.; Xia, D.; Chen, G.; Chen, Y. Influence of the surfactant and temperature on the morphology and physico-chemical properties of hydrothermally synthesized composite oxide BiVO4. Mater. Chem. Phys. 2009, 114, 69–72. (3) Zhou, J. K.; Zou, Z. G.; Ray, A. K.; Zhao, X. S. Preparation and characterization of polycrystalline bismuth titanate Bi12TiO20 and its photocatalytic properties under visible light irradiation. Ind. Eng. Chem. Res. 2007, 46, 745–749. (4) Yu, J.; Kudo, A. Hydrothermal Synthesis and Photocatalytic Property of 2-Dimensional Bismuth Molybdate Nanoplates. Chem. Lett. 2005, 34, 1528–1529. (5) Zhang, G.-Q.; Chang, N.; Han, D.-Q.; Zhou, A.-Q.; Xu, X.-H. The enhanced visible light photocatalytic activity of nanosheet-like Bi2WO6 obtained by acid treatment for the degradation of rhodamine B. Mater. Lett. 2010, 64, 2135–2137. (6) Qin, Z. Z.; Liu, Z. L.; Liu, Y. B.; Yang, K. D. Synthesis of BiYO3 for degradation of organic compounds under visible-light irradiation. Catal. Commun. 2009, 10, 1604–1608. (7) Zhang, K.; Liu, C.; Huang, F.; Zheng, C.; Wang, W. Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst. Appl. Catal., B 2006, 68, 125–129. (8) Zhang, J.; Shi, F. J.; Lin, J.; Chen, D. F.; Gao, J. M.; Huang, Z. X.; Ding, X. X.; Tang, C. C. Self-assembled 3-D architectures of BiOBr as a visible light-driven photocatalyst. Chem. Mater. 2008, 20, 2937–2941. (9) Ma, D. K.; Huang, S. M.; Chen, W. X.; Hu, S. W.; Shi, F. F.; Fan, K. L. Self-Assembled Three-Dimensional Hierarchical Umbilicate Bi2WO6 Microspheres from Nanoplates: Controlled Synthesis, Photocatalytic Activities, and Wettability. J. Phys. Chem. C 2009, 113, 4369–4374. (10) Cai, L.; Liao, X. P.; Shi, B. Using Collagen Fiber as a Template to Synthesize TiO2 and Fex/TiO2 Nanofibers and Their Catalytic Behaviors on the Visible Light-Assisted Degradation of Orange II. Ind. Eng. Chem. Res. 2010, 49, 3194–3199.
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