Bi2WO6

Oct 9, 2015 - Varied morphologies and compositions of bismuth tungstate nanocomposites have been investigated as promising materials for photocatalyti...
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Research Article pubs.acs.org/journal/ascecg

Facile Synthesis of Hierarchically Structured Bi2S3/Bi2WO6 Photocatalysts for Highly Efficient Reduction of Cr(VI) Ali Rauf,†,∇ Md. Selim Arif Sher Shah,†,∇ Gwan Hyun Choi,† Usama Bin Humayoun,‡ Dae Ho Yoon,‡ Jong Wook Bae,† Juhyun Park,∥ Woo-Jae Kim,⊥ and Pil J. Yoo*,†,§ †

School of Chemical Engineering, ‡School of Advanced Materials Science and Engineering, and §SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea ∥ School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea ⊥ Department of Chemical and Environmental Engineering, Gachon University, Songnam 461-701, Republic of Korea S Supporting Information *

ABSTRACT: Varied morphologies and compositions of bismuth tungstate nanocomposites have been investigated as promising materials for photocatalytic applications. Among these nanocomposites, hierarchically structured bismuth sulfide (Bi2S3)/bismuth tungstate (Bi2WO6) hybrids have significant photocatalytic efficiency toward heavy metal ions. To simplify the synthetic procedure for this desirable composite, we developed a robust single-step hydrothermal synthesis for the formation of hierarchically structured heterocatalysts of Bi2S3/ Bi 2 WO 6 with a high yield (>95%). The synthesized heterostructures were characterized by various spectroscopic, microscopic, and surface area analysis techniques, which confirmed the successful incorporation of Bi2S3 into the Bi2WO6 matrix and were used to optimize pore size for enhanced catalytic activity. The resulting Bi2S3/Bi2WO6 heterocatalysts were used to remove toxic Cr(VI) ions via reduction to water insoluble Cr(III) utilizing visible-light irradiation. We also investigated the role of citric acid as a hole scavenger in the reduction of Cr(VI) with minimizing the rate of electron−hole recombination during photocatalysis. Likewise, the observed catalytic activity was significantly enhanced under a condition of an appropriate balance between hierarchical structure of catalysts and the amount of hole scavenger. KEYWORDS: Hierarchical heterostructures, Photocatalysts, Bi2S3/Bi2WO6, Hydrothermal synthesis, Cr(VI) reduction

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INTRODUCTION Titanium dioxide (TiO2) in various structures has been investigated for several decades with respect to its potential as a catalyst for photochemical applications.1−5 However, materials containing TiO2 have shortcomings with respect to their ability to only absorb light in the UV and near-UV regions due to their high band gap (∼3.2 eV). Indeed, the UV region covers only 4% of the solar spectrum, whereas the visible region covers about 43%. For these reasons, there is significant interest in exploring visible-light-sensitive photocatalysts. In addition to TiO2, several other photocatalysts comprising binary metal oxides (Fe2O3, Co3O4, PbO, and Bi2O3) also have intrinsic limitations such as high resistivity or indirect band gap formation caused by small polaron dominated low conductivity. On the other hand, functionalized ternary oxide compounds consisting of multiple cations can serve the purpose of spanning the range of absorbing spectra.6 Bismuth tungstate (Bi2WO6) is one such ternary oxide, having a band gap of 2.8 eV. Bi2WO6 has been utilized primarily for dye degradation and water splitting appications.7−10 In addition, by varying the morphologies of Bi2WO6 or forming different heterostructures through hybridization, such as with BiVO4, BiOCl, and Bi2O3, it has © 2015 American Chemical Society

demonstrated its promising potential in highly efficient photocatalytic applications.11−14 Similar to bismuth tungstate, bismuth sulfide (Bi2S3) is a metal sulfide with a lamellar-like structure with a direct band gap of 1.3 eV, and exhibits promising photocatalytic activity toward dye degradation and heavy metal ion reduction reactions.15,16 Bi2S3 has also been shown to form heterostructures with BiOCl, BiVO4, Bi2O2CO3, and Bi2WO6 in photochemical applications.17−20 Photocatalytic activity of catalyst materials can be improved by manipulating the synthetic route to have desired morphologies or by imparting specific structural features, such as porosity or structural hierarchy, that are capable of boosting the light absorption and photochemical reactions even inside the interior region of the catalyst. In photocatalysts, electrons and holes are the main charge carriers, generated as a result of photoexcitation. However, they generally have quite high recombination rates, thereby showing very short lifetimes. A common method to increase the lifetime of charge carriers by Received: July 29, 2015 Revised: September 21, 2015 Published: October 9, 2015 2847

DOI: 10.1021/acssuschemeng.5b00783 ACS Sustainable Chem. Eng. 2015, 3, 2847−2855

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ACS Sustainable Chemistry & Engineering

and then subjected to vigorous mixing for 20 min. The second solution, B, contained 0.5 mmol of sodium tungstate with various quantities of thiourea in 7.5 mL of DI water, and was prepared and ultrasonicated for 20 min. When both solutions A and B became clear, they were combined and mixed for 10 min. The resulting solution was then placed in a Teflon lined stainless steel autoclave and heated in an oven at 180 °C for 3 h. Next, the samples were cooled to room temperature, washed sequentially with ethanol and DI water three times each, and then dried overnight in an oven at 60 °C. Overall yield of the synthesis is greater than 95% on a basis of the amount of Bi atoms. Different samples with various sulfur contents were synthesized using the same procedure by altering the quantity of thiourea, and these samples were named 5 mol % Bi2S3/Bi2WO6, 10 mol % Bi2S3/ Bi2WO6, 20 mol % Bi2S3/Bi2WO6, and 30 mol % Bi2S3/Bi2WO6 based on thiourea content. Nonhybridized bismuth tungstate and bismuth sulfide were also synthesized following the same procedures described above. In addition, for a comparison, cubic structured and nonhierarchical Bi2S3/Bi2WO6 were also synthesized according to a reported procedure.20 Characterization. The crystal structure of the synthesized heterostructures were characterized using a powder X-ray diffractometer (XRD, D8 Focus, Bruker instrument, Germany) with Cu Kα radiation (1.540 Å) in a 2θ range of 2° to 80° at a scan rate of 0.05° s−1. Transmission electron microscopy (TEM, JEM-3010, JEOL, Japan) was performed with an acceleration voltage of 300 kV. Field emission scanning electron microscopy (FESEM, JSM-7600F, JEOL) was used to observe particle sizes and morphologies. The Brunauer− Emmett−Teller (BET) specific surface areas were evaluated on the basis of nitrogen adsorption isotherms and measured at −196 °C using a gas adsorption apparatus (ASAP 2000, Micromeritics, USA). All samples were degassed at 180 °C before nitrogen adsorption measurements. The BET surface area was determined using adsorption data in the relative pressure (P/P0) in the range of 0.06−0.2. The Barrett−Joyner−Halenda (BJH) pore size distribution was determined from desorption data. X-ray photoelectron spectroscopy (XPS) characterization was performed (ESCA 2000 instrument, VG Microtech, UK) with Al Kα X-rays. Binding energy values were corrected and calibrated to a C 1s peak of 284.6 eV. High resolution peaks were deconvoluted using Gaussian−Lorentzian functions with identical full width at half maxima (fwhm) after Shirley background subtraction. UV−visible absorption spectra were collected from the UV−vis-NIR spectrophotometer (UV-3600, Shimadzu, Japan). For photoluminescence measurements, a PL spectrometer (Sinco, FS-2, South Korea) equipped with a 150 W xenon lamp as an excitation source was used. Photocatalytic Experiment. In a typical experiment, 20 mL of a 10 ppm solution of chromium Cr(VI) was prepared by dissolving K2Cr2O7 salt in DI water. Next, 1.56 mM citric acid was added to the Cr(VI) solution, followed by maintaining a constant pH of 2 using a 1 M HCl solution. A violet color was developed in the Cr(VI) solution via the addition of a 19.77 mM diphenylcarbazide solution in acetone.25,26 After color change, 20 mg of catalyst was added to the solution, and the resulting mixture was ultrasonicated for 4 min followed by incubation in the dark for 30 min under stirring to achieve an adsorption−desorption equilibrium. A solar simulator with a Xe lamp (LS-150 Xe Abet Technologies, Inc. Milford, CT) was used as the visible-light source (λ > 400 nm). The experimental mixture was placed at a distance of 100 mm from the light source, and 3.5 mL of the experimental mixture was sequentially removed at various time points and centrifuged. The concentration of Cr(VI) was measured with a spectrophotometer at λ = 540 nm (UV-3600, Shimadzu, Japan) using the diphenylcarbazide (DPZ) method.

minimizing their recombination is to employ compound semiconductors with a suitable band gap to form a heterostructure. Indeed, heterostructure interfaces act as a separation barrier and decrease the likelihood of electron−hole recombination. In this way, heterostructures can be used to improve charge transfer efficiency. In photocatalytic applications, degradation of organic compounds and dyes has been the focus of significant research efforts, including works by our group.21−24 Likewise, photocatalytic applications targeting on the removal of heavy metal ions (Cr(VI), Ni(II), Fe(III), Hg(II)) represent a promising area of research. Among these heavy metal ions, hexavalent chromium is highly carcinogenic and poses serious environmental concerns. Furthermore, hexavalent chromium is readily soluble in a number of solvents and is thus difficult to remove from effluent streams. One way to resolve this problem is to reduce hexavalent chromium to its trivalent form, which is less toxic and easily precipitates under alkaline or neutral pH conditions.16,25−27 Toward achieving this conversion, heterostructured Bi2WO6 hybridized with other semiconducting species has been extensively investigated.28,29 In this study, we present a facile and robust means to synthesize a Bi2S3/Bi2WO6 photocatalyst with a hierarchical structure to attain highly enhanced photocatalytic activity with respect to Cr(VI) ion reduction. Although a previous study reported the synthesis of a similar yet nonporous composite material with cubic structures, the present study is distinct in that it describes a cosynthesis of Bi2S3/Bi2WO6 with a shape of porous hierarchically structured microspheres, which are highly desirable for facilitating surface reactions of Cr(VI) ion reduction.20 The synthesized materials had a high surface area (31.22 m2/g) with optimally tuned pore sizes, which were comparatively superior to reported characteristics of Bi2WO6based heterostructures.12,30 In addition, the conventionally employed two-step synthetic approach was readily altered to a simple one-step hydrothermal synthesis with high yield (>95%). Further, mixed solvents (deionized water, ethanol, and acetic acid) were employed to form the liquid−liquid interface between two immiscible esterification products, i.e., water and ethyl acetate, which acted as a soft template for the formation of Bi2S3/Bi2WO6 microspheres.31 Different ratios of Bi2S3 and Bi2WO6 were evaluated with respect to Cr(VI) reduction to determine the optimum composition of the photocatalyst. Our results were corroborated by photoluminescence (PL) characterization, which revealed a substantially minimized electron−hole recombination rate at the optimum composition. Finally, the synthesized hierarchically structured Bi2S3/Bi2WO6 heterocatalysts exhibited outstanding photocatalytic activity and appeared to have a strong potential for use in heavy metal ion removal.

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EXPERIMENTAL SECTION

Materials. Bismuth nitrate pentahydrate, sodium tungstate, thiourea, polyvinylpyrrolidone (PVP, MW 40 000), potassium dichromate (K2Cr2O7), citric acid, and diphenylcarbazide were purchased from Sigma-Aldrich. Absolute ethanol was purchased from Emsure ACS ISO and acetic acid was purchased from Samchun Chemicals, Korea. All reagents were of analytical grade and used without any further purification. Deionized (DI) water with a resistance of 18.2 MΩ was used in all experiments. Synthetic Procedure. In a typical heterostructure synthesis, two different solutions (A and B) were prepared. For the first solution, A, 1 mmol of bismuth nitrate pentahydrate and 1 g of PVP were added to a mixture of 22.5 mL of DI water, ethanol, and acetic acid in a 3:1:1 ratio

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RESULTS AND DISCUSSION Synthesis Mechanism. The formation of Bi2S3/Bi2WO6 microspheres is schematically depicted in Scheme 1. The final growth of the hierarchically structured spheres followed a stepwise sequence as described previously.9 Briefly, the process started with the formation of nanoparticles, followed by 2848

DOI: 10.1021/acssuschemeng.5b00783 ACS Sustainable Chem. Eng. 2015, 3, 2847−2855

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration for the Synthesis of Hierarchically Structured Microspheres of Bi2S3/Bi2WO6 Photocatalysts

reorganization and self-assembly into nanoplatelets via oriented aggregation. Here, the oriented aggregation implies a phenomenon that primary nanoparticles first self-organizes, reassembles themselves to form new secondary particles which are nanoplatelets in this case.32 These nanoplatelets subsequently self-organized in a mutual merging and growing process. Because the outer part of the aggregates was less stable than the inner part, Ostwald ripening takes place, resulting in formation of hierarchically structured microspheres. Polyvinylpyrrolidone (PVP) acts both as stabilizer and capping agent, thus preventing agglomeration of microspheres, which is experimentally confirmed by carrying out comparative experiments without PVP addition (Figure S1a).33 It should be noted that the synthesis of hierarchical microspheres in the present study was carried out in a ternary solvent mixture. This mixture facilitated an esterification reaction between ethanol and acetic acid, thereby producing ethyl acetate and water. Since water was present in excess in the mixed solvent, ethyl acetate acted as a minor organic phase. Under elevated temperature (180 °C) for the hydrothermal synthesis, increased mobility of two liquid phases induced the formation of increased number of liquid−liquid interfaces. In this way, ethyl acetate microdroplets in water acted as soft templates for the formation of porously structured hierarchical microspheres.31 The synthesis was also carried out in binary solvent media of ethanol/water and acetic acid/water, as shown in Supporting Information (Figure S1b,c). In both the cases, there observed no formation of hierarchical microspheres in the final product, supporting above proposed mechanism with a role of soft template formed from the mixed solvent for creating hierarchical microspheres. Material Characterization. The crystallographic structures of the synthesized heterostructured materials of Bi2S3/Bi2WO6 were characterized by XRD (Figure 1). Sharp diffraction peaks indicated the formation of crystalline structures. Likewise, the peaks observed in diffraction pattern of Bi2WO6 were in accordance with Bi2WO6, JCPDS 73-1126.9 As a reference to Bi2S3, Figure 1 also shows diffraction patterns of JCPDS 0170320. The peaks of the 2θ values at 15 and 31° were assigned to the (200) and (221) planes of orthorhombic Bi2S3, respectively. These peaks unambiguously indicated the presence of Bi2S3 in the hierarchical heterostructures.34,35 We performed XPS analysis to elucidate the surface chemical states and composition of heterostructures. Figure 2a shows the low-resolution XPS spectrum of the 20 mol % Bi2S3/Bi2WO6 heterostructures, indicating the presence of Bi, W, and O species. However, S was not present in the spectrum because it overlaps with Bi and was present in relatively small amounts given the Bi2S3 content of the composites.19 Although the presence of S was supported by XRD data, more convincing evidence was obtained from energy dispersive X-ray (EDX)

Figure 1. XRD patterns of Bi2WO6, 5 mol % Bi2S3/Bi2WO6, 10 mol % Bi2S3/Bi2WO6, 20 mol % Bi2S3/Bi2WO6, and 30 mol % Bi2S3/Bi2WO6.

analysis, as described in the latter. Figure 2b shows the high resolution XPS spectrum of Bi 4f, which revealed two peaks present at 158.46 and 163.78 eV corresponding to Bi 4f7/2 and Bi 4f5/2 spin states, respectively. The peak positions closely matched the literature values for Bi3+ in Bi2S3.36 For the case of Bi 4f, peaks in the high energy region originated from Bi2WO6 (given in the Supporting Information, Figure S2), and a slight peak shift was observed indicating the presence of interfacial chemical interactions between Bi2S3 and Bi2WO6.13 Moreover, Figure 2b shows a weak peak at 159.65 eV, which was assigned to the binding energy of S 2p3/2.37 Importantly, this observation confirmed the presence of S2− species in the composites. Figure 2c shows deconvoluted XPS spectra of W 4f. Two peaks appeared at binding energies of 34.6 and 36.7 eV, which were assigned to W 4f5/2 and 4f7/2, respectively, and were in accordance with standard values for W6+ in Bi2WO6.38 Finally, Figure 2d depicts the spectra for O 1s, with peak positions captured at 529.6 and 531 eV corresponding to the lattice oxygen in Bi2WO6 and surface adsorbed oxygen from the atmosphere, respectively.39 We next investigated the structural characteristics of the synthesized materials using different microscopic techniques. Field-emission scanning electron microscopy (FESEM) images along with a transmission electron microscopy (TEM) image of the samples are presented in Figure 3. Intriguingly, morphologies of the Bi2S3/Bi2WO6 particles did not appear to be greatly affected by the addition of different amounts of thiourea (Figure 3a−d). Specifically, the size distribution of the heterostructured particles ranged from 1.5 to 3.5 μm. The high magnification image shown in Figure 3e, clearly revealed the presence of a hierarchical structure present on the surface of microspheres consisting of a number of stacked nanoplatelets. This self-organization process was generally accompanied by the formation of porous structures. Consistently, loosely assembled and pore-incorporated internal regions were captured by TEM (Figure 3f). EDX data (Table 1) confirmed the presence of S atom in the heterostructures of all four Bi2S3/Bi2WO6 samples. Table 1 also shows the molar ratio between Bi2S3 and Bi2WO6. As shown in Table 1, the amount of Bi2WO6 in the composites overwhelmed that of Bi2S3, which was consistent with the observation that morphological characteristics were not significantly different despite varied Bi2S3 incorporation in the 2849

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Figure 2. (a) Survey XPS spectrum of 20 mol % Bi2S3/Bi2WO6. (b) Core level Bi 4f spectra. (c) Core level XPS spectra of W 4f. (d) Core level XPS spectra of O 1s.

was attributed to the intrinsic luminescence of Bi2WO6.42 As expected, the electron−hole recombination rate decreased with increasing concentrations of Bi2S3 in the composite due to the formation of heterostructures. However, when the complexation ratio of Bi2S3 was significantly increased (30 mol % Bi2S3/ Bi2WO6), the recombination rate increased due to the formation of additional junction sites, which in turn acted as sites of recombination. Moreover, the smaller band gap of Bi2S3 (∼1.3 eV) facilitated recombination, which was particularly apparent for 30 mol % Bi2S3/Bi2WO6 but not for the other composites.18,43 Photocatalytic Activity. The synthesized Bi2WO6 and Bi2S3/Bi2WO6 composites with hierarchical heterostructures were used as photocatalysts for a reduction of Cr (VI) ions. Deng et al. previously reported high reduction efficiency for Cr(VI) using Bi 2WO 6 with different morphologies by irradiating LED white light rather than using simulated solar light.11 Figure 7 shows the photoreduction efficiencies of Cr(VI) in the presence of different catalysts after exposure to visible light for 60 min at room temperature and ambient pressure. For comparison, photocatalytic experiments were also performed with no catalyst, nonporous and cubic structured Bi2S3/Bi2WO6 and P25 of TiO2 as reference photocatalysts.26 C0 was taken as the concentration of Cr(VI) after achieving equilibrium in the dark. Notably, reaction rates increased with the addition of Bi2S3 to Bi2WO6 to form heterostructures. For a bare Bi2WO6 and 5 mol % Bi2S3/Bi2WO6, the reaction rate were almost the same with 0.0176 min−1, whereas the reaction rate increased to 0.0260 min−1 for the sample 10 mol % Bi2S3/ Bi2WO6. Likewise, the reaction rate for 20 mol % Bi2S3/ Bi2WO6 was marginally increased to 0.0266 min−1. The increased reaction rates observed above were presumably due to the increased pore diameter of hierarchical structures as shown in Table 2. Specifically, the increase in pore size to some extent provided faster mass transport as well

cosynthesis. To determine the specific BET surface area of the porous structured Bi2S3/Bi2WO6 composites, a nitrogen adsorption−desorption isotherm was measured, as depicted in Figure 4. The pore size distribution was also estimated using the BJH method from the desorption branch of the isotherm. In general, desorption isotherm is thermodynamically more stable than the adsorption case. A representative pore size distribution curve is shown in the inset of Figure 4. The specific surface area and pore size distribution of different samples are summarized in Table 2. Notably, increasing the amount of Bi2S3 inclusion led to decreased BET surface area, whereas pore size increased. In addition, the pore size distribution of heterostructures was much broader than that of parental Bi2WO6, indicating the increased generation of both macropores and mesopores (shown in the Supporting Information, Figure S3). Similarly, opposite behavior between the surface area and the average pore size was observed for single phase Bi2S3, which has an intrinsically low surface area and large pore size. The band gaps of the synthesized heterostructured materials were determined by applying the Kubelka−Munk equation to diffuse reflectance spectra (DRS) data.40 The estimated band gap energies along with UV−vis DRS spectra are shown in Figure 5, and the values were close to the reported values of related Bi2WO6-based nanocomposites.9,41 A slight decrease in the band gap of the composites with increasing the Bi2S3 content clearly indicates the positive role of Bi2S3 for shifting the overall bandgap to a smaller value of the composite materials. PL spectra were investigated to measure the electron−hole recombination rate of the photocatalytic systems. In this method, a smaller PL intensity reflects a more effective suppression of the electron−hole recombination. The synthesized materials were excited with a wavelength of 300 nm and emission spectra were collected in the range of 340−560 nm, as shown in Figure 6. The emission band centered at ∼440 nm 2850

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Figure 3. FESEM images of (a) Bi2WO6, (b) 10 mol % Bi2S3/Bi2WO6, (c) 20 mol % Bi2S3/Bi2WO6, and (d) 30 mol % Bi2S3/Bi2WO6. (e) High magnification SEM image of 20 mol % Bi2S3/Bi2WO6. (f) TEM image of 20 mol % Bi2S3/Bi2WO6.

Table 1. EDX Analysis for Synthesized Samples (at. %) and Corresponding Molar Ratio between Bi2S3 and Bi2WO6 samples

Bi

W

S

O

Bi2WO6:Bi2S3

Bi2WO6 5 mol % Bi2S3/Bi2WO6 10 mol % Bi2S3/Bi2WO6 20 mol % Bi2S3/Bi2WO6 30 mol % Bi2S3/Bi2WO6

16.71 14.41 13.98 15.25 21.23

17.82 26.10 24.39 14.17 9.82

N/A 1.39 1.72 1.91 2.16

65.46 57.79 59.91 68.67 66.78

N/A 58.8 43.2 22.2 13.8

Table 2. Parameters Obtained from N2 Desorption Isotherm Measurements samples

BET surface area (m2/g)

average pore diameter (nm)

Bi2WO6 10 mol % Bi2S3/Bi2WO6 20 mol % Bi2S3/Bi2WO6 30 mol % Bi2S3/Bi2WO6 Bi2S3

45.76 34.35 31.22 33.16 10.44

12.05 19.83 22.11 20.48 77.03

as more accessible active sites available for reduction reactions, resulting in an increased reaction rate. However, for the 30 mol % Bi2S3/Bi2WO6 composite, the reaction rate decreased to 0.0151 min−1, which may have been due to an increase in the electron−hole recombination rate on account of a decreased separation barrier, which in turn decreased catalyst efficiency. As a result, the improved catalytic performance for hetero-

Figure 4. Nitrogen adsorption−desorption isotherms of 20 mol % Bi2S3/Bi2WO6. Inset shows pore size measurement using the BJH method.

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Figure 5. UV-DRS spectrum of all samples.

(Cr2O72−) to facilitate their reduction to Cr(III). At the same time, holes are consumed by citric acid, a hole scavenger, which minimizes the rate of electron−hole recombination. This reaction mechanism can be expressed as follows: 4Cr2O7 2 − + C6H5O7 3 − + 41H+ + 6e− → 8Cr 3 + + 6CO2 + 23H 2O

(1)

Effect of Citric Acid (Hole Scavenger) Concentration. We next studied the effect of CA as a hole scavenger on Cr(VI) ion reduction, and the results are summarized in Figure 8. Generally, CA is added in an equimolar ratio with the metal ion.46 In our study, the reduction reaction efficiency increased to a certain optimum value upon increasing CA concentration. Conversely, when Cr(VI) ion reduction was carried out in the absence of CA, we observed a minimum level (∼40%) of reduction of Cr(VI) to Cr(III), which was presumably due to a smaller amount of photoexcited electrons being generated. On the other hand, increasing the concentration of CA resulted in a more acidic reaction mixture, which may have provided a more favorable environment for Cr(VI) ion adsorption on the catalyst surface, resulting in a gradual increase in reduction efficiency of the photocatalysis. A 95% photocatalytic reduction efficiency was achieved with a CA/Cr(VI) ratio of 2.10, whereas further increases in CA decreased reduction efficiency of the catalyst. Indeed, when the ratio of CA/Cr(VI) increased to 2.63, 88% of Cr (VI) ions were reduced to Cr(III). A plausible explanation for the observed optimum ratio was that upon exceeding a CA/Cr(VI) ratio of 2.10, the catalyst surface was well covered with carboxylate groups of citric acid, which in turn hindered the surface adsorption of ions due to electrostatic repulsion. This competition may have eventually decreased the space available for Cr(VI) ion reduction on the catalyst surface.45,47 Another advantage of employing CA as a hole scavenger is that CA does not contain any aromatic groups in its structure. On the other hand, hole scavengers that contain aromatic groups such as salicylic acid undergo complexation with Cr(VI) ions, resulting in undesirable deactivation of the catalyst surface.48 A schematic of the energy band diagram of the Bi2WO6/ Bi2 S 3 heterostructure photocatalyst is depicted in the Supporting Information (Figure S6), where the interface between the two materials represents the charge separation barrier.20 The conduction band for Bi2S3 was above than that of Bi2WO6, enabling facilitated transfer of electrons from Bi2S3 to

Figure 6. PL spectrum of different samples excited at 300 nm.

structures despite the slight decrease in surface area highlights the importance of optimizing pore size, which allows for reactant molecules to be adsorbed on the internal surface of hierarchical heterostructures. A successful reduction reaction of Cr(VI) was visibly identified as shown in the captured image of Figure S4 (Supporting Information). The photocatalytic activity of nonporous and cubic structured Bi2S3/Bi2WO6 and P25 were evaluated under the same condition as a control for photocatalysis. For the case of cubic Bi2S3/Bi2WO6, only 40% reduction of Cr(VI) was obtained, underpinning the role of porous surface and hierarchical structure in the composites for enhancing the photocatalytic activity. Similarly, for the case of the other reference photocatalyst of P25, it only exhibited 37% reduction of the initial concentration. Likewise, a sample prepared without any catalyst had a photoreduction efficiency of 27%. In particular, this observation suggested a partial contribution of citric acid for the Cr(VI) reduction reaction even in the absence of catalysts. Indeed, the reaction may be ascribed in part to a spontaneous photoexcitation of citric acid upon irradiation, which generates electrons and facilitates their transport to Cr(VI) ions for reducing into Cr(III) (Supporting Information, Figure S5). On the other hand, the general reduction reaction supported by photocatalysis is well understood and its reaction mechanism has been investigated extensively.44,45 Specifically, visible light excites the electrons from heterostructures, which in turn attack dichromate groups 2852

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Figure 7. Photoreduction of Cr(VI) under visible light irradiation. (a) Ct/C0 concentration changes of Cr(VI) as a function of time using different catalysts. (b) Logarithmic concentration ratio of the samples as a function of time with different amount of Bi2S3 complexation. (c) Bar plot showing the differences in rate constants.

Figure 8. (a) Ct/C0 photoreduction efficiency of Cr(VI) ions as a function of time for different ratios of CA/Cr(VI). (b) Logarithmic concentration ratio of the samples as a function of time for different ratios of CA/Cr(VI). (c) Bar plot showing the differences in rate constant.

Bi2WO6. Similarly, this mechanism readily enables the transfer of holes from Bi2WO6 to Bi2S3. Specifically, when photoexcited

electrons react with Cr(VI) ions, the holes in Bi2S3 and Bi2WO6 are simultaneously consumed by citric acid through an 2853

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ACS Sustainable Chemistry & Engineering

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oxidative reaction. In turn, the absence of citric acid leaves holes inside the heterostructures, which are responsible for fast electron−hole recombination and decreased photocatalytic activity. Therefore, the addition of a hole scavenger is critical for increasing the lifetime of charge carriers.

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CONCLUSIONS In the present study, hierarchically structured Bi2S3/Bi2WO6 heterocatalysts were synthesized using a single-step hydrothermal method. Optimizing the ratio of solvent mixtures was important for obtaining the desired hierarchical structures with large surface areas. In addition, mesopores present in the heterostructures provided efficient paths for reactant molecules to reach the inner structure of the catalyst. The synthesized samples were successfully used as photocatalysts for reducing highly toxic and carcinogenic Cr(VI) to Cr(III). In addition, we optimized photocatalytic reduction efficiency by varying the amount of Bi2S3. We also systematically investigated the role of an organic species as a hole scavenger in the photocatalytic reaction. Robust efficiencies were achieved with the heterostructured photocatalysts, highlighting the importance of this material design, which may be explored further as a platform for designing high performance heterocatalysts for various applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00783. Additional information about FESEM images of Bi2WO6 synthesized under different conditions, comparative XPS of Bi 4f and W 4f, BET surface area and BJH pore size distribution, digital photograph of Cr(VI) reduction to Cr(III) along with effect of CA on Cr(VI) reduction in dark, and energy level diagram of composite (PDF).

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AUTHOR INFORMATION

Corresponding Author

*P. J. Yoo. E-mail: [email protected]. Author Contributions ∇

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by research grants of the NRF (2012M1A2A2671795, 2014M3A7B4052200, 2014M3C1A3053035) and Basic Science Research Program (2010-0027955) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea.

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DOI: 10.1021/acssuschemeng.5b00783 ACS Sustainable Chem. Eng. 2015, 3, 2847−2855