Accepted Manuscript A controlled anion exchange strategy to synthesize core/shell β-Bi2O3/Bi2S3 hollow heterostructures with enhanced visible-light photocatalytic activity Yunhui Yan, Zhaoxian Zhou, Xiaohua Zhao, Jianguo Zhou PII: DOI: Reference:

S0021-9797(14)00584-0 http://dx.doi.org/10.1016/j.jcis.2014.08.027 YJCIS 19762

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

20 May 2014 14 August 2014

Please cite this article as: Y. Yan, Z. Zhou, X. Zhao, J. Zhou, A controlled anion exchange strategy to synthesize core/shell β-Bi2O3/Bi2S3 hollow heterostructures with enhanced visible-light photocatalytic activity, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.08.027

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A controlled anion exchange strategy to synthesize core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures with enhanced

visible-light photocatalytic activity Yunhui Yana,b, Zhaoxian Zhouc, Xiaohua Zhaoa, Jianguo Zhoua* a School of Environment, Henan Normal University, Xinxiang, Henan 453007, PR China b Department of Chemistry, Xinxiang Medical University, Xinxiang, Henan 453003, PR China c School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China

Abstract: Heterojunction construction is an exciting direction to pursue for highly active photocatalysts. In this study, novel core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures were successfully synthesized through a simple and economical ion exchange method between ȕ-Bi2O3 hollow microspheres and thioacetamide (CH3CSNH2, TAA), and characterized by multiform techniques, such as XRD, XPS, SEM, TEM, BET, DRS and PL. The results indicated that the core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures exhibited strong absorption in visible light region and excellent photocatalytic performance for decomposing rhodamine B (RhB) compared with pure ȕ-Bi2O3 under visible light irradiation. Among the ȕ-Bi2O3/Bi2S3 photocatalysts with different molar percentage of Bi2S3 to initial ȕ-Bi2O3, the ȕ-Bi2O3/Bi2S3 (10%) heterostructures exhibited the highest photocatalytic activity, which was about 3.3 times higher than that of pure ȕ-Bi2O3 sample. Moreover, the study on the mechanism suggested that the enhanced photocatalytic activity mainly resulted from the role of ȕ-Bi2O3-Bi2S3 heterojunction formed in the ȕ-Bi2O3/Bi2S3, which could lead to efficient separation of photoinduced carriers. Additionally, the photosensitization of Bi2S3 and the hollow nature of ȕ-Bi2O3 were also responsible for the high photocatalytic activity. Keywords: In situ, ion-exchange, ȕ-Bi2O3, Bi2S3, heterostructure, photocatalysis. 1

1. Introduction During the past decades, immense effort has been devoted to the design and controlled fabrication of core-shell structured nanomaterials due to their special structures and unique properties. By fabricating a core-shell semiconductor, their electronic properties can be engineered [1-3]. Recently, bismuth (III) compounds have attracted intensive attention due to their low toxicity, low cost, good stability, and high catalytic efficiency [4-6]. Both bismuth sulfide (Bi2S3) and bismuth oxide (Bi2O3) are two important semiconductor materials and have been extensively researched. Bi2S3 is a direct band gap semiconductor with a narrow bandgap (Eg) of 1.25-1.7 eV (depending on the shape, size and uniformity), a large absorption coefficient and reasonable incident photon to photocurrent conversion efficiency (~15%) [7, 8]. As a typical n-type semiconductor, Bi2S3 has wide potential applications, such as electrochemical hydrogen storage, hydrogen sensor, biomolecule detection and photoresponsive materials [9]. In recent years, the photosensitive properties of Bi2S3 with different morphologies and structures have been studied in details. For example, A. A. Tahir et al. [7] had investigated photosensitive and photoresponsive properties of Bi2S3 nanotubes and nanoparticles. L. S. Li et al. [10] studied the photoresponsive properties of the hierarchical Bi2S3 core-shell microspheres. H. F. Cheng, et al. [9] synthesized Bi2S3 nanocrystals/BiOCl hybrid architectures and they displayed highly efficient visible light photoactivities. All these indicate that Bi2S3 is a good candidate for photosensitizer and optoelectronic material. Bi2O3 is a p-type semiconductor oxide with suitable band gap of 2.0-3.6 eV

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(depending on the phase, shape and size). And it has been widely used in superconductor ceramic glass manufacturing, photocatalyst, electrolytes, and sensor optical coatings due to its special properties including high refractive index, dielectric permittivity, marked photoconductivity and photoluminescence [11, 12]. The photoactivity of Bi2O3 strongly depends on its crystalline structure and morphology. In the last decades, Bi2O3 with different morphologies such as nanoparticles [13], nanoflakes [14], nanorods [15], nanotubes [16], nanowires [17] and nanofibers [18], microspheres [19] and microflowers [20] have been synthesized by means of wet chemical, sono-chemical, microwave-assisted, hydro- and/or solvothermal methods. Very recently, our group has prepared uniform Į and ȕ-Bi2O3 hollow microspheres by template-free solvothermal process and found that ȕ-Bi2O3 had superior photoactivity to Į-Bi2O3 for decomposing organic dye [21]. Although previous research indicates that Bi2O3 is a good candidate for water splitting and pollutant decomposing under visible light irradiation, the photocatalytic activity of Bi2O3 is low due to the fast recombination of photogenerated charges. Therefore, the photodegradation efficiency of Bi2O3 catalyst needs to be further improved [22]. Combining two or more semiconductors with appropriate band positions can reduce the recombination of photogenerated charges and enhance the interfacial charges transfer efficiency [23]. Especially, fabrication of a p-n junction is believed to be the most effective method due to the existence of internal electric field [24]. In order to explore the property of heterostructure between Bi2S3 and Bi2O3, some researches integrate these two materials together to form new composites. P. M.

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Sirimanne et al. [25] managed to synthesize the Bi2O3/Bi2S3 electrodes by sulfurizing Bi2O3 pellets and showed that the composites possess enhanced photocurrent and photoresponse. F. A. Liu [8] and F. Y. Lu [26] obtained the Bi2O3/Bi2S3 composites by surface sulfurization and hydrothermal method, respectively, and investigated their photosensitive activity. However, to the best of our knowledge, the investigation about photocatalytic performance of the ȕ-Bi2O3/Bi2S3 heterostructure has never been reported previously. In this paper, we applied a simple and economical ion exchange method to prepare the core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures using monodispersed ȕ-Bi2O3 hollow microspheres as a starting reactant and in situ template. The photocatalytic property of the as prepared ȕ-Bi2O3/Bi2S3 hollow heterostructures was studied and they displayed enhanced photocatalytic activities compared with the pure Bi2O3. The possible mechanism of enhanced photoactivity was also discussed.

2. Experimental 2.1. Synthesis of ȕ-Bi2O3 hollow sphere All chemical reagents in our experiments were analytical and used as received without further purification. The synthesis of ȕ-Bi2O3 hollow spheres adopted our previous method [21]. In a typical experiment, Bi(NO3)3•5H2O (2.91 g, 6 mmol) was dissolved in a mixed solution of glycerol (30 mL) and ethanol (30 mL) under vigorous stirring. After 40 min of stirring, the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, heated to 160 Ԩ and maintained for 3 h. After the autoclave was cooled to room temperature, the products were separated centrifugally and

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washed with deionized water and absolute ethanol for several times, and then dried under vacuum at 80 Ԩ overnight. Finally, the products were further heated at 2Ԩ/min heating rate from room temperature to 270 Ԩ and maintained at 270 Ԩ for 2 h to obtain ȕ-Bi2O3 hollow spheres. 2.2. Synthesis of core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructure The core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructure was synthesized by in situ anion-exchange method. In a typical process, the as-prepared ȕ-Bi2O3 hollow microsphere (0.4660 g, 1 mmol) was dispersed in 50 mL deionized water under vigorous stirring. Then a certain amount CH3CSNH2 (0.0225 g, 0.3 mmol) was added into the solution. After 30 min of stirring at 50 Ԩ, the precipitates were collected, washed and dried under vacuum at 80 Ԩ for 6 h. By controlling the theoretical molar percentage of Bi2S3 to initial Bi2O3, different ȕ-Bi2O3/Bi2S3 heterostructures were obtained. The final samples were labeled as the ȕ-Bi2O3/Bi2S3 (5%, 10%, 25%) heterostructure. 2.3. Characterization The crystal structures of the obtained samples were performed on a Bruker D8 Advance X-ray diffractometer using Cu KĮ radiation (Ȝ=0.15418 nm). The XPS data were taken on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Ka radiation (1486.71 eV). The morphology of the as-prepared samples was examined by transmission electron microscopy (TEM, JEM-2100) and scanning electron micrography (SEM, JSM-6390LV). Nitrogen adsorption-desorption measurements were conducted by using a Micromeritics ASAP 2020 apparatus at 77.35 K, the samples were degassed at 150 Ԩ for 6 h prior to the measurement.

5

UV-vis diffuse reflectance spectra (DRS) were recorded on a Lambda 950 Spectrophotometer (Perkin Elmer) using BaSO4 as reference. The photoluminescence (PL) spectra of photocatalysts were recorded using a Fluorescence Spectrophotometer (FP-6500, Janpan) at room temperature with an excitation wavelength of 325 nm. 2.4. Photocatalytic evaluation The photocatalytic activities of as-prepared pure ȕ-Bi2O3 and core/shell ȕ-Bi2O3/Bi2S3 heterostructures were evaluated by the degradation of RhB under visible light irradiation from 500 W Xe light equipped with a 420 nm cutoff filter. In a typical process, 50 mg photocatalyst was added into 50 mL RhB solution (5 mg/L). Before illumination, the suspensions were magnetically stirred for 60 min in the dark to ensure the adsorption-desorption equilibrium between photocatalyst and dye. The suspension was under constant vigorous stirring with the photoreactor during the process and the temperature of the suspension was maintained at 25 Ԩ by circulation of water. At given time intervals of illumination, about 5 mL of the mixture solution were taken out and centrifuged. After centrifugation, the UV-Vis spectrum of the supernatant was recorded on a UV-vis spectrophotometer (UV-2450) to monitor the absorption behavior. The characteristic absorption peak of RhB at 554 nm was used to determine the extent of its degradation. The degradation efficiency of RhB was calculated by the following equation: Degradation rate = (C0 í C)/ C0×100% = (A0 í A)/ A0×100%

(1)

Here, C is the solution concentration of RhB, C0 is the initial concentration, A is the absorbance of RhB solution after degradation, and A0 is the initial absorbance of RhB solution before degradation.

6

3. Results and discussion 3.1 Phase structure and chemical composition The phase purity and crystal structure of the as-prepared samples were examined by X-ray diffraction (XRD). Fig. 1 presents the XRD patterns of pure ȕ-Bi2O3, Bi2S3 and core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures with different Bi2S3 contents. The sharp diffraction peaks of the XRD patterns indicate good crystallinity of the as-prepared samples. All the diffraction peaks of pure Bi2O3 (Fig.1a) were well-indexed to tetragonal ȕ-Bi2O3 (JCPDS, No. 27-0050) and no traces of other phases were examined. The pattern of Bi2S3 (Fig. 1e) could be readily indexed to the pure orthorhombic phase structure of Bi2S3 (JCPDS, No.17-0320). As for the ȕ-Bi2O3/Bi2S3 heterostructures (Fig.1b to d), it can be found that there was a main tetragonal phase of ȕ-Bi2O3 with a small amount of orthorhombic Bi2S3 in the ȕ-Bi2O3/Bi2S3. Moreover, the intensities of the corresponding diffraction peaks of Bi2S3 strengthened gradually along with the increase of its content in the ȕBi2O3/Bi2S3 core/shell structure, while those of ȕ-Bi2O3 weakened simultaneously as its content decreased. No impurity peaks were detected, indicating that the ȕ-Bi2O3/Bi2S3 heterostructures were only composed of ȕ-Bi2O3 and Bi2S3 phases. According to previous report [27], TAA could decompose into S2- ions at 50 Ԩ in aqueous solution. Bi2S3 is a product with a rather lower solubility (Ksp=1×10-97) and thus Bi2O3 can easily convert into Bi2S3 in the presence of S2- ions. Consequently, the ȕ-Bi2O3/Bi2S3 heterostructure can be formed when the anion exchange is controlled at a limited level, which has been confirmed by the XRD results above.

7

Insert Fig.1 near here In order to confirm the products containing both Bi2O3 and Bi2S3, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Fig. 2a shows a typical XPS survey spectrum of the ȕ-Bi2O3/Bi2S3 (10%) heterostructures, in which all peaks can be assigned to Bi, O, S, and C elements. The C 1s signal at 284.8 eV was used as a reference to calibrate the binding energies of other elements. Fig. 2b shows high-resolution XPS spectra of Bi 4f and S 2p of the products. The peaks with binding energies of 164.8 and159.5 eV correspond to Bi 4f5/2 and Bi 4f7/2 in Bi2O3, respectively [28], while the peaks located at 163.8eV and 158.5eV can be attributed to the binding energies of Bi 4f5/2 and Bi 4f7/2 in Bi2S3, respectively [29]. Both are characteristic of Bi3+, indicating that the bismuth ions in the heterostructure were trivalency. The peak at 160.4 eV can be assigned to the binding energy of the S 2p transition. The weaker signal located at 225.8 eV (Fig.2c) can be assigned to S 2s from a trace amount of Bi2S3. Fig.2d presents the high-resolution O 1s spectrum with binding energy of 530.8 eV. Therefore, the XPS results further confirm the coexistence of ȕ-Bi2O3 and Bi2S3 in the ȕ-Bi2O3/Bi2S3 heterostructures. Insert Fig.2 near here 3.2. Morphology and BET surface area The scanning electron microscopy (SEM) image (Fig. 3a) reveals that the as-prepared ȕ-Bi2O3 microspheres have quite smooth surfaces and the microspheres are nearly monodispersed in size with ca. 1.5 ȝm diameter. The broken spheres in Fig. 3a demonstrate the formation of hollow spheres. Fig.3b-d show the SEM images of the ȕ-Bi2O3/Bi2S3 heterostructures with different Bi2S3 contents, from which one can 8

see that the samples remain spherical during the synthesis process. With the addition of TAA at low concentration, it can be seen that the ȕ-Bi2O3/Bi2S3 microsphere surfaces were not as smooth as that of pure Bi2O3, due to the formation of nanosized Bi2S3 particles on the surface of Bi2O3 (Fig. 3b). For the ȕ-Bi2O3/Bi2S3(10%) heterostructure, the surface became rough and Bi2S3 particles can be clearly observed (Fig. 2c). When higher concentration of TAA was added, more Bi2S3 nanorods were deposited, resulting in a relatively rough surface, as shown in Fig. 3d. Insert Fig.3 near here The detailed structural information about the ȕ-Bi2O3/Bi2S3(10%) heterostructure was further investigated by transmission electron microscopy (TEM). In agreement with the SEM image, the TEM image of the ȕ-Bi2O3 hollow sphere shows that the microsphere has a dimension of ca. 1.5 ȝm, and the relatively smooth surfaces (Fig.4a). For the ȕ-Bi2O3/Bi2S3 heterostructure, as presented in Fig. 4b, the TEM image shows that Bi2O3 microspheres have a coarse outer layer, further confirming that Bi2S3 nanoparticles were anchored on the surface of the Bi2O3 through in situ anion exchange. It is believed that the in-situ growth coupling method, which is based on the simple ion-exchange, is extendable for the fabrication of other photocatalysts with specific structures and chemical compositions. Insert Fig.4 near here Itǯs well known that the specific surface area is an important factor influencing the photoactivity of catalysts and the larger BET surface areas will provide more active sites in photocatalysis. Fig. 5 shows the nitrogen adsorption-desorption isotherms and corresponding pore size distributions curves (inset) calculated from the 9

adsorption branch of nitrogen isotherm by Barrett-Joyner-Halenda (BJH) method for the pure ȕ-Bi2O3 and ȕ-Bi2O3/Bi2S3 (10%) sample. both samples exhibit a type IV isotherm with a type H3 hysteresis loop, which is the typical characteristic of mesoporous materials. Furthermore, the enhanced adsorption behavior at high relative pressure (P/P0) was observed for both samples, suggesting the existence of large mesopores and macropores within the as-prepared samples. The pore size distribution data (inset) indicate that the main pore sizes are ca. 20 nm. Such mesoporous structure is extremely useful in photocatalysis as they will provide communicable channels for reactant molecules and products [30]. The BET surface area (SBET) and the pore volume of the ȕ-Bi2O3/Bi2S3 (10%) heterostructure are about 4.59 m2/g and 0.0318 cm3/g, and higher than that of pure ȕ-Bi2O3 microspheres with SBET of 3.16 m2/g and pore volume of 0.0190 cm3/g. These results are in accordance with the observation results from SEM and TEM. The high specific surface area and large total pore volume suggest the more active reaction or adsorption sites accessible for the organic dyes. Insert Fig.5 near here 3.3. Optical properties The UV-vis diffuse reflectance spectra (DRS) were measured to investigate the optical absorption property of the samples. Pure ȕ-Bi2O3 hollow spheres and Bi2S3 were also studied for comparison. The absorption onsets are determined by linear extrapolation from the inflection point of the curve to the baseline. As shown in Fig. 6a, the onset of pure ȕ-Bi2O3 hollow spheres at around 530 nm corresponds to the

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band gap of 2.36 eV, while visible light absorption ability of ȕ-Bi2O3 is greatly enhanced after Bi2S3 was introduced. For the ȕ-Bi2O3/Bi2S3(5%), ȕ-Bi2O3/Bi2S3(10%) and ȕ-Bi2O3/Bi2S3(25%) samples, the absorption onsets are about 570nm, 595 nm and 750nm, respectively. Compared to pure ȕ-Bi2O3 hollow spheres, the onsets of the ȕ-Bi2O3/Bi2S3 heterostructures had a significant red shift within the visible light range. Optical absorption property of material is often closely associated with its energy gap. For a semiconductor, the absorbance near the band edge follows the formula [3]: ĮhȞ=A(ԣȞíEg)n/2

(2)

where Į, hȞ, A, and Eg are optical absorption coefficient, the photonic energy, proportionality constant, and the band gap, respectively. The value of n depends on whether the transition is direct (n= 1) or indirect (n= 4) in a semiconductor. Bi2O3 and Bi2S3 are direct transition, thus, n is equal to 1. Fig. 6b shows the curve of (ĮhȞ)2 versus photon energy (hȞ), from which we can estimate the band gap of the products. The band gap values of all the obtained samples were listed in Table 1. The band gap of the ȕ-Bi2O3/Bi2S3 heterostructures ranged from 2.28 to 1.65 eV, and decreased with the increased Bi2S3 content from 5 to 25%. The decrease in band gap energy indicates that ȕ-Bi2O3/Bi2S3 heterostructures has much greater optical absorption region than pure ȕ-Bi2O3, which can be excited to produce more electron-hole pairs under the same visible light illumination and then resulting in higher photocatalytic activity. 

Insert Fig.6 near here

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For semiconductors, the photoluminescence (PL) spectra are related to the migration, transfer, and recombination behavior of the photoinduced electrons and holes, so that it can reflect the separation and recombination of photoinduced charge carriers[31]. A weaker PL intensity represents a lower recombination probability of the electron-hole under light irradiation. Fig. 7 shows the comparison of PL spectra with 325 nm excitation wavelength for ȕ-Bi2O3, Bi2S3 and the ȕ-Bi2O3/Bi2S3 heterostructures at room temperature. For pure ȕ-Bi2O3 and the ȕ-Bi2O3/Bi2S3 heterostructures, the PL peaks are observed at around 535 nm, while pure Bi2S3 displays almost no luminescence peak in the scope of monitoring. The PL intensity decreases

as:

ȕ-Bi2O3

>

ȕ-Bi2O3/Bi2S3(5%)

>

ȕ-Bi2O3/Bi2S3(25%)

>

ȕ-Bi2O3/Bi2S3(10%) > Bi2S3. It can be clearly observed that ȕ-Bi2O3/Bi2S3 shows diminished PL intensity in comparison to pure ȕ-Bi2O3, indicating a reduced charge carrier recombination.[32] This might attribute to the fact that the heterostructures could efficiently prevent the direct recombination of photogenerated carriers and enhance the interfacial charge transfer efficiency, which is of great benefit for enhancing activity in the photocatalytic reaction. X.Q. Lu [33] and F.Y. Lu [26] reported that the combination of Bi2O3 and Bi2S3 could significantly enhance photocurrent response and prevent the recombination of electron and hole for the formation of heterostructure. Our results are consistent with the research results above, indicating that the prepared ȕ-Bi2O3/Bi2S3 heterostructures might possess great photocatalytic capacities. Insert Fig.7 near here

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3.4. Photocatalytic activity The photocatalytic activities of the ȕ-Bi2O3/Bi2S3 heterostructures were evaluated by the degradation of RhB under visible light irradiation. Fig.8 shows UV-vis absorption spectra changes of the RhB degraded by the ȕ-Bi2O3/Bi2S3 (10%) heterostructures for 3 h. As can be seen from the spectra, the major absorption peaks gradually decreased during the photodegradation process, and blue-shifted step by step, indicating the removal of ethyl groups one by one, which was in agreement with the literature [34]. Insert Fig.8 near here Fig. 9(a) presents the curves of RhB concentration changes with irradiation time over different photocatalysts. The blank test confirms that RhB was hardly decomposed by photolysis in the absence of catalysts. The pure ȕ-Bi2O3 samples display a low photocatalytic activity under visible light irradiation and degrade less than 65% of RhB in 3 h, while the RhB removal efficiency was greatly enhanced in the presence of the ȕ-Bi2O3/Bi2S3 heterostructures. Furthermore, loading amounts of Bi2S3 had a crucial influence on photocatalytic activity and a small amount of Bi2S3 over ȕ-Bi2O3 surface could lead to an obvious increase of decomposition efficiency. When the theoretical molar percentage of Bi2S3 to initial ȕ-Bi2O3 was 10%, the photocatalytic degradation efficiency was the highest and reached 90.9% after 3 h irradiation. The obvious enhancement of photocatalytic activity might be attributed to the photosensitization of Bi2S3 and the synergistic effect between ȕ-Bi2O3 core and Bi2S3 shell, which can assist the extension of the visible light absorption region and

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the separation of photoinduced electron-hole pairs. However, the larger Bi2S3 loading is unfavorable to the improvement of photocatalytic activity, which can be ascribed to the fact that the surface of ȕ-Bi2O3 were excessively coated with Bi2S3 and the active sites for the degradation of organic dyes via ȕ-Bi2O3 were reduced, meanwhile, excess Bi2S3 can become a recombination center for photogenerated electrons and holes, leading to photoactivity fading. To further evaluate photocatalytic activity of the ȕ-Bi2O3/Bi2S3 heterostructures, the Langmuir-Hinshelwood model was applied to fit the experimental data. Since the reactant concentration was low, the photocatalytic degradation reaction followed the pseudo first-order kinetics equation [35]. ln(C0/C)= kappt

(3)

where C0 and C are the concentrations of RhB in aqueous solution at time 0 and t, respectively, and kapp is the apparent reaction rate constant. The relation between ln(C0/C) and irradiation time (t) is plotted in Fig.9(b). The kapp values and corresponding correlation coefficients (R2) of different samples were calculated and listed in Table 1. The excellent fitness indicates that the photoreaction follows the way of first-order reaction kinetics. The kapp of ȕ-Bi2O3/Bi2S3 heterostructures was much higher than that of pure Bi2O3 samples. Especially, the ȕ-Bi2O3/Bi2S3 (10%) heterostructures exhibited the highest k (1.23×10-2 min-1) and the kapp values was about 3.3 times higher than that of pure ȕ-Bi2O3 samples, indicating that it had the best photocatalytic activity for decomposing RhB. Insert Fig.9 near here

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Table 1 Estimated band gap energy (Eg), degradation rate, reaction rate constant (kapp) and correlation coefficients (R2) of different photocatalysts. Sample

Eg/eV

Degradation rate (%)

kapp/min-1

R2

ȕ-Bi2O3

2.36

63.4%

3.80×10-3

0.9826

ȕ-Bi2O3/Bi2S3 (5%)

2.28

76.2%

7.55×10-3

0.9861

ȕ-Bi2O3/Bi2S3 (10%)

2.12

90.9%

1.2×10-2

0.9874

ȕ-Bi2O3/Bi2S3 (25%)

1.65

79.3%

7.8×10-3

0.9847

blank

/

5%

3.75×10-4

0.9427

To cast light on the effect of the heterostructure on photocatalytic activity, the band structure was studied. The valence band edge of a semiconductor at the point of zero charge can be calculated by the empirical equation [36]: EVB = X í Ee + 0.5Eg

(4)

where EVB is the valence band-edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band-gap energy, and the conduction band edge (ECB) can be determined by ECB = EVB í Eg. The X values for Bi2O3 and Bi2S3 are ca. 5.95 and 5.27 eV, and the band gap energies of ȕ-Bi2O3 and Bi2S3 are 2.36 and 1.30 eV, respectively. Given the equation above, the conduction band bottom of ȕ-Bi2O3 and Bi2S3 is calculated to be 0.27 and 0.12, respectively. Correspondingly, the valence band top of them is 2.63 eV and 1.42 eV, respectively. The band edge positions of ȕ-Bi2O3 and Bi2S3 are shown in Fig. 10(a).

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Insert Fig.10 near here Bi2O3 is recognized as a typical p-type semiconductor and its Fermi energy level is close to its valance band; Bi2S3 is an intrinsic n-type semiconductor and its Fermi energy level is close to the conduction band. When two semiconductors are in contact with each other, there is oriented diffusion of photogenerated electrons and holes between ȕ-Bi2O3 and Bi2S3. The energy bands of the ȕ-Bi2O3 and Bi2S3 shifts upward and downward, respectively, along with the diffusion of carriers until the Fermi levels of Bi2O3 and Bi2S3 reach equilibrium (Fig. 10(b)) [37, 38]. After the p-n junction is formed, an inner electric field is built in the interface between ȕ-Bi2O3 and Bi2S3. Under visible light irradiation, both ȕ-Bi2O3 and Bi2S3 can be excited to generate electron-hole pairs. The photogenerated electrons on the CB of p-type Bi2O3 transfer to that of n-type Bi2S3. Meanwhile, the holes can migrate from the VB of n-type Bi2S3 to that of p-type Bi2O3. Such migrations of the photogenerated carriers can be promoted by the internally formed electric field [39]. Thus, the photogenerated electron-hole pairs can be separated effectively for the formation of the p-n junction between p-type Bi2O3 and n-type Bi2S3 interfaces. The separated electrons and holes are then free to initiate reactions with the reactants adsorbed on the photocatalyst surface, leading to an enhanced photocatalytic activity. In addition, there are another two reasons to explain its superior photocatalytic performance. Firstly, after introducing Bi2S3, the light absorption ability of the core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures was significantly enhanced for the narrow band gap and large absorption coefficient of Bi2S3. More importantly, such a

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hollow structure can also allow multiple reflections of UV-vis light within the hollow center, which can make more efficient use of the light source [40]. The improvement of the catalytic activity due to the existence of the hollow morphology has been previously reported [41]. Secondly, the ȕ-Bi2O3/Bi2S3 heterostructures possessed high surface area (SBET) and mesoporous structure. The larger BET surface areas will provide more active sites for the degradation of organic dyes, while the mesoporous nature is extremely advantageous for photocatalysis as they will provide communicable channels for reactant molecules and products. Effective use of both interior and exterior surfaces of the hollow spheres could lead to superior photocatalytic activities [42].

4. Conclusions In summary, we have demonstrated a facile and economical ion exchange method to synthesize the core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures using monodispersed ȕ-Bi2O3 hollow microspheres as a starting reactant and in situ template. Compared with pure ȕ-Bi2O3, the core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures exhibits enhanced photocatalytic activities for the degradation of RhB under visible light irradiation. The highest degradation efficiency was observed over the ȕ-Bi2O3/Bi2S3 heterostructure with loading 10% Bi2S3, and the corresponding reaction rate constant was 3.3 times higher than that of pure ȕ-Bi2O3 sample. The enhanced photocatalytic activities were attributed to the effective separation of photogenerated carriers through the p-n heterojunction formed between ȕ-Bi2O3 and Bi2S3, the photosensitization of Bi2S3, and the hollow nature of ȕ-Bi2O3. Consequently,

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it is hoped that the information provided here may also be adapted for preparing other Bi-based semiconductor heterostructures, which may have promising for environment and energy applications. Acknowledgements The authors are grateful for the financial support from the National Science Foundation of China (Grant No. 20871042), the Key Science and Technology Program of Henan Province, PR China (Grant No.122102210232, 122102210233, and 132102210439), the Basic Scientific and Technological Frontier Project of Henan Province, PR China (Grant No.132300410286), Natural Science Foundation of the Education Department of Henan Province (No 2011A150019 and 2008B430012), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20104104110004).

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Figure Captions Fig. 1. XRD patterns of as-prepared samples. (a) pure ȕ-Bi2O3, (b) ȕ-Bi2O3/Bi2S3 (5%), (c) ȕ-Bi2O3/Bi2S3 (10%), (d) ȕ-Bi2O3/Bi2S3 (25%), (e) pure Bi2S3. Fig.2. XPS spectra of the ȕ-Bi2O3/Bi2S3 (10%) sample: (a) survey spectrum, (b) Bi 4f and S 2p, (c) S 2s and (d) O1s. Fig. 3. SEM images of (a) pure ȕ-Bi2O3, (b) ȕ-Bi2O3/Bi2S3 (5%), (c) ȕ-Bi2O3/Bi2S3 (10%), (d) ȕ-Bi2O3/Bi2S3 (25%) heterostructures. Fig. 4. TEM images of pure ȕ-Bi2O3 and the ȕ-Bi2O3/Bi2S3 (10%) heterostructure. Fig. 5. Nitrogen adsorption-desorption isotherms and the corresponding pore size distributions (inset) : (a) pure ȕ-Bi2O3, (b) ȕ-Bi2O3/Bi2S3 (10%) heterostructures. Fig. 6. UV-vis diffuse reflectance spectra (a) and plots of (ĮhȞ)2 versus the photon energy (hȞ) (b) of pure ȕ-Bi2O3, Bi2S3 and the ȕ-Bi2O3/Bi2S3 heterostructures. Fig. 7. Photoluminescence spectra of the as-prepared photocatalysts at excitation wavelength of 325 nm. Fig. 8. Adsorption changes of RhB aqueous solution in the presence of ȕ-Bi2O3/Bi2S3 (10%) heterostructures under visible light irradiation. Fig. 9. (a) Photodegradation efficiencies (C/C0) of RhB as a function of irradiation time by different photocatalysts under visible light irradiation; (b) Kinetics curves for the photodegradation of RhB. Fig. 10. Schematic diagram for photogenerated electron-hole separation and transfer over the ȕ-Bi2O3/Bi2S3 heterostructures under visible light irradiation.

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Highlight ŹNovel core/shell ȕ-Bi2O3/Bi2S3 hollow heterostructures were synthesized by two-steps method. ŹThe ȕ-Bi2O3/Bi2S3 heterostructure exhibited a higher photoactivity than the pure ȕ-Bi2O3. ŹThe p-n heterojunction formed between ȕ-Bi2O3 and Bi2S3 facilitate photogenerated carrier separation ŹThe study provides an effective strategy to develop more efficient heterostructured photocatalyst.

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