Synthesis of Uniform Bi2WO6-Reduced Graphene Oxide

Jan 21, 2015 - Synthesis of Uniform Bi2WO6-Reduced Graphene Oxide ..... and normalized concentration changes of BPA (C/C0) were obtained based on the ...
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Synthesis of Uniform Bi2WO6‑Reduced Graphene Oxide Nanocomposites with Significantly Enhanced Photocatalytic Reduction Activity Juan Yang,* Xiaohan Wang, Xiaolei Zhao, Jun Dai, and Shengran Mo Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454003, P. R. China S Supporting Information *

ABSTRACT: In this work, the uniform B2WO6-reduced graphene oxide (BWO−RGO) nanocomposites are prepared via electrostatic self-assembly of positively charged BWO with negatively charged GO sheets and then the composited GO is reduced via the hydrothermal treatment. The close interfacial contact and strong electronic interaction between BWO and RGO are achieved by this facile and efficient self-assembly route. Photocatalytic degradation of pollutant bisphenol A, selective oxidation of benzyl alcohol, removal of heavy metal ion Cr(VI), and selective reduction of 4-nitrophenol are selected as the probe reactions to investigate the photocatalytic activities of as-obtained BWO−RGO nanocomposites. The experimental results demonstrate the photocatalytic redox activities of BWO−RGO composites are predominantly dependent on the energy levels of photoinduced electrons or holes. In particular, the upshift of the valence band and conduction band edge of catalysts induced by the electronic interaction between BWO and RGO has an inconsistent influence on the photocatalytic reduction and oxidation reactions, respectively. As a result, the photocatalytic activity of reduction reactions is significantly enhanced, owing to the synergetic effect of the upshift of conduction band edge and the improved separation of photogenerated electrons/holes, while the oxidation ability of BWO−RGO nanocomposite is improved to a slight extent compared with bare BWO. The energy levels of photogenerated carriers should be the origins accounting for the different enhancement of photocatalytic activities for the different reactions. According to the discussion, one important conclusion can be drawn, that is, the results should be analyzed on the basis of specific reactions when discussing the effect of graphene or RGO on the photocatalytic properties of semiconductor particles.

1. INTRODUCTION The ever-increasing global environment contamination and energy crisis are becoming topical issues for sustainable development of human society. Semiconductor-based photocatalysis is considered to be an effective strategy in retarding the energy shortage and environment pollution since it is a facile and environmentally friendly way to take advantage of solar energy.1,2 Among the various studied photocatalysts, Bi2WO6 (abbreviated as BWO) is regarded as a promising candidate and has potential in splitting water for O2 evolution and decomposing organic contaminations.3−6 Improvement of the photocatalytic activity of Bi2WO6 has been an interesting research topic. Recently, a few of approaches have been developed to synthesize different morphology of Bi2WO6 particles to obtain high photocatalytic efficiency. For instance, Zhang and colleagues reported the flowerlike Bi2WO6 particles possessed higher performance for the decomposition of rhodamine with visible light illumination than helixlike or platelike Bi2WO6.7 Huang and co-workers prepared porous Bi2WO6 microsphere via the ultrasonic spray pyrolysis method, © XXXX American Chemical Society

which exhibited excellent photocatalytic activity for removal of nitric oxide with either visible light or simulated sunlight.8 Square Bi2WO6 nanoplates and hierarchical nestlike structures constructed by the nanoplates have been found to possess superior photoactivity with the irradiation of visible light.3,9 Although the strategy of shape control can improve the photocatalytic performance of Bi2WO6, the enhancement remains limited owing to its low separation efficiency of photoinduced electron−hole pairs in photocatalytic process. To address this problem, different methods, such as ions doping, nanocomposites construction, noble-metal deposition, and heterostructure assembly, have been studied.10−17 Among the methods, fabricating graphene-based nanocomposite has been regarded as an effective approach to promote the efficiency of photocatalysis. In the obtained nanocomposites, the two-dimensional layered structure of graphene can inhibit Received: October 4, 2014 Revised: January 20, 2015

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approach by using (3-aminopropyl)-triethoxysilane and the significantly improved performance of photocatalytic reduction reactions has been reported for the first time. The positively charged BWO NPs are first adhered on the surface of graphene oxide by electrostatic attraction, and then the composited GO sheets are reduced into RGO via hydrothermal treatment. The photocatalytic activity of as-obtained BWO−RGO nanocomposites has been investigated under artificial solar light irradiation by using several probe reactions, including the oxidative degradation of bisphenol A, selective oxidation of benzyl alcohol to benzaldehyde, photoreduction toxic Cr(VI), and selective reduction of 4-nitrophenol to 4-aminophenol. The results indicate that for photocatalytic degradation of BPA and selective oxidation of BA, the activities of BWO−RGO nanocomposite are enhanced slightly, which are comparable with that of bare BWO and BWO−RGO-U 2.0% sample synthesized by mixing unmodified BWO NPs with GO. Importantly, the significant enhancement in photoreduction of Cr(VI) and selective reduction of 4-NP is observed on BWO−RGO nanocomposites compared with bare BWO. On the basis of experimental results, a possible mechanism on the considerable improvement of photocatalytic reduction activity is finally proposed.

the aggregation of composited samples as well as improve the separation of photogenerated electron−hole pairs because of the superior electron mobility.18,19 Wang et al. first reported the hydrothermal method followed by ethylene glycol reduction to fabricate graphene−Bi2WO6 nanocomposites, which showed the improved photocatalytic performance in the decomposition of dye pollutant rhodamine B (RhB).17 A refluxing method has been utilized for the preparation of BWO−graphene, and a three times enhancement of photocatalytic RhB degradation was obtained compared to pure Bi2WO6.18 Xu and co-workers synthesized RGO−Bi2WO6 composites through a one-step solvothermal approach.20 However, the particle size of obtained BWO was a few hundreds of nanometers, and the distribution on the RGO sheets was not uniform. Very recently, an in situ sonochemical method has been reported to prepare graphene− Bi2WO6 composites, which showed high photocatalytic performance to water splitting under visible-light irradiation.19 It can be noticed most of the BWO−graphene composites have been fabricated via an in situ synthesis strategy,17,19,20 which can weaken the growth of BWO nanoparticles on the layered graphene nanosheets to some extent and facilitate the transfer of photogenerated electrons. On the other hand, it is equally important to develop new synthetic strategies for the fabrication of graphene-based nanocomposite photocatalysts. Notably, the electrostatic self-assembly approach holds unique advantages on controlling the morphology and distribution of semiconductor particles. Meanwhile, this strategy can also satisfy the demands for effective interfacial contact between graphene sheets and semiconductor particles.21,22 More importantly, the negatively charged surface of graphene oxides (GO) resulted from deprotonation of surface carboxyl groups (−COOH) can be utilized in this approach.23,24 However, to our knowledge, preparation of uniform Bi2WO6 nanoparticles− reduced graphene oxide composites (BWO−RGO) by such a facile self-assembly approach has been not reported until now. In addition, the photocatalytic research of graphene−BWO nanocomposites primarily focused on nonselective decomposition of dye contaminants and splitting of water.17−20 On the contrary, research on employing RGO−BWO composites as photocatalysts of selective redox reactions has been lacking. Besides, compared to the oxidizing reactions, photocatalytic reducing reactions with graphene−BWO photocatalysts are less studied. It is generally accepted that the energy level of conduction band electrons and the transfer of photogenerated electrons from the excited photocatalyst play key roles in the photocatalytic reduction reactions. Thus, decreasing the conduction band potential and improving the utilization efficiency of photogenerated electrons are expected to enhance the photocatalytic performance of reduction reactions. Due to the excellent electronic conductivity of layered graphene sheets, the compounding of graphene with traditional photocatalysts through a proper approach should improve the transfer of photogenerated electrons in principle. Meanwhile, the electronic interaction between the composited particles and graphene might result in the upshift of the Fermi level, which consequently decreases the conduction band potential of graphene-based composites. As a result, the constructed graphene−BWO nanocomposites can be reasonably expected to possess excellent photocatalytic activity for reduction reactions. In this paper, the preparation of uniform BWO−RGO nanocomposites through a facile electrostatic self-assembly

2. EXPERIMENTAL SECTION 2.1. Materials. Bi(NO3)3·5H2O, Na2WO4·2H2O, NaNO3, ethylene glycol, K2Cr2O7, KMnO4, hydrogen peroxide (H2O2, 30%), and bisphenol A (BPA) were obtained from Beijing Chemical Works, P. R. China. (3-Aminopropyl)triethoxysilane (C9H23NO3Si, APTES), ammonium oxalate ((NH4)2C2O4), 4nitrophenol (4-NP), trifluorotoluene (BTF), and benzyl alcohol (BA) were purchased from Aladdin Industrial Inc. (Shanghai, P. R. China). Graphite powder was supplied from Nanjing XFNANO Materials Tech Co., Ltd. (P. R. China). All chemicals were used without further purification. 2.2. Catalyst Preparation. The procedure for synthesizing BWO−RGO consists of four main steps: solvothermal synthesis of BWO NPs, preparation of GO nanosheets, surface charge modification of BWO NPs, and hydrothermal reduction of GO. 2.2.1. Preparation of BWO Nanoparticles. A 1.46 g amount of Bi(NO3)3·5H2O and 0.50 g of Na2WO4·2H2O were dissolved in 40 mL of ethylene alcohol. After mixing the two solutions, a transparent mixture (80 mL) was obtained. The mixture was transferred into a 100 mL autoclave with a PTEE container inside, which was maintained at 160 °C for 24 h. The precipitate was centrifuged, repeatedly washed with ethanol and water, and then dried at 60 °C to obtain BWO NPs. 2.2.2. Synthesis of Graphene Oxide Sheets. Graphene oxide sheets were obtained using a modified Hummer’s method from natural graphite.25,26 The detailed synthetic process is displayed in the Supporting Information. 2.2.3. Fabrication of BWO−RGO Nanocomposites. BWO− RGO nanocomposites were prepared through a surface charge modification method and a simple hydrothermal treatment. Briefly, 0.6 g of BWO powder was first dispersed in 300 mL of ethanol and sonicated for 0.5 h. After 2 mL of APTES was added into BWO suspension, the obtained mixture was heated at 70 °C for 4 h using an oil bath. After that, the powder was collected, washed with ethanol several times, and then dried at 60 °C. Then, 0.2 g of as-prepared APTES-modified BWO was added into 100 mL of deionized water and sonicated for 10 min. An appropriate amount of graphene oxide solution (1 mg/ B

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using a Millipore filter (pore size 0.45 μm) to remove catalyst particles. The filtrates were analyzed by an Agilent 7890 gas chromatograph to measure the concentration change of reactant and product. Anaerobic selective reduction of 4-NP was performed under argon bubbling with a flow rate of 20 mL·min−1. Typically, 20 mg of photocatalyst and 20 mg of (NH4)2C2O4 were added into 40 mL of 4-NP solution (20 mg/L) in a round-bottomed flask. Prior to irradiation, the solution was stirred for 1 h in the dark to ensure adsorption equilibrium between the catalyst and 4-NP. About 2 mL of suspension was collected at a given time interval and filtered to remove the catalyst completely. Reactant and product identification is confirmed by using an Agilent high-performance liquid chromatograph (1200 HPLC) equipped with a diode-array detector (G1315C) and C18 column. As for the photocatalytic reduction of Cr(VI), 50 mg of photocatalyst was dispersed into 50 mL of Cr(VI) solution (20 mg·L−1), based on Cr in a dilute K2Cr2O7 solution. The obtained mixture was stirred in the dark for 1 h to ensure adsorption equilibrium before illumination with a 300 W Xe lamp. During photocatalytic reduction, 3 mL of sample solution was taken out at certain intervals and separated by centrifugation. The supernatant was analyzed on a UV−vis spectrophotometer, and the temporal concentration change of Cr(VI) was calculated on the basis of the change in the absorbance at 371 nm.

mL) was added to as-obtained BWO suspension, which was stirred for 1 h. The formed homogeneous suspension was transferred into an autoclave with a PTEE container inside and heated at 160 °C for 12 h. The obtained products were collected, washed with water, and then dried at 60 °C to obtain BWO−RGO samples. According to this approach, different weight ratios of GO to BWO composites at 0.005:1, 0.01:1, 0.02:1, and 0.05:1 were fabricated and labeled as BWO−RGO 0.5%, BWO−RGO 1.0%, BWO−RGO 2.0%, and BWO−RGO 5.0%. For comparison, BWO−RGO-U was prepared by simple mixing GO with BWO NPs obtained from the above step (section 2.2.1) instead of the modified ones in the same way. 2.3. Characterization and Measurements. X-ray diffraction (XRD) patterns were obtained using a D8 Advance diffractometer (Bruker) with Cu Kα radiation (λ = 0.15405 nm) in the range of 10−80° (2θ). Raman spectra were performed using a Renishaw inVia Reflex Raman spectrometer with a He−Ne excitation source of 633 nm. X-ray photoelectron spectroscopy (XPS) images were recorded on a Thermo Scientific ECALAB 250xi system with Mg Kα source. Transmission electron microscopy (TEM) and high-resolution TEM images were observed by using a FEI Tecnai G2 TEM at 200 kV. Fourier transformation infrared spectra (FT-IR) were acquired in the range 400−4000 cm−1 with a NICOLET 750 FTIR spectrometer. Nitrogen adsorption−desorption measurements were performed on a Micromeritics ASAP 2020 system at 77 K. Photoluminescence spectra (PL) of the samples were obtained using a PerkinElmer LS55 Fluorescence Spectrophotometer with a 325 nm excitation wavelength. Ultraviolet− visible diffuse reflectance spectra (UV−vis DRS) were recorded using a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan) using BaSO4 as reflectance standard. Zeta potential measurements were measured using a Zetasizer (NANO ZS) at room temperature. Briefly, a suspension was prepared by dispersing 10 mg of as-synthesized powder into 50 mL of deionized water. The pH value of the measured solution was kept at 6 during the testing process of zeta potential. The electrochemical measurements were performed in a threeelectrode system, including the working electrode prepared according to the previous report,27 the counter electrode (platinum foil), and the reference electrode (saturated calomel electrode). The measurements were carried out using a CHI Electrochemical Workstation (CHI 760D, Shanghai, China), and the electrolyte was 0.5 mol·L−1 Na2SO4. 2.4. Photocatalytic Reactions. The light source for photocatalytic reaction was a 300 W Xe lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) without filter. The initial concentration of BPA was 10 mg·L−1. Before irradiation, the suspension was stirred for 1 h in the dark to establish adsorption equilibrium between BPA molecule and the surface of photocatalysts. About 3 mL of solution was taken out at given time intervals and subsequently separated by centrifugation. The obtained supernatant was analyzed by using a UV−vis spectrophotometer (Cary-50, Varian Co.), and normalized concentration changes of BPA (C/C0) were obtained based on the maximum absorption at 278 nm. The selective oxidation of BA was conducted in a 25 mL round-bottomed quartz flask equipped with a sealed spigot. Typically, the catalyst (50 mg) was added into the mixture of BA (0.35 mmol) and BTF (5 mL) as the solvent. Before the Xe lamp was turned on, this system was stirred for 1 h in the dark to achieve adsorption equilibrium of reagents. About 0.5 mL suspensions were collected at given time intervals and filtered

3. RESULTS AND DISCUSSION Crystal Phase, Composition, Morphology, and Formation of BWO−RGO Nanocomposites. The XRD patterns of bare BWO NPs and BWO−RGO nanocomposites are presented in Figure 1. It can be noted that all as-prepared

Figure 1. XRD patterns of bare BWO and BWO−RGO nanocomposites with different weight ratios of GO: (a) BWO NPs, (b) BWO−RGO 0.5%, (c) BWO−RGO 1.0%, (d) BWO−RGO 2.0%, (e) BWO−RGO 5.0%.

samples show characteristic peaks assigned to (131), (200), (002), (260), (202), (331), (262), (400), (103), and (204) crystallographic planes of orthorhombic Bi2WO6 (JCPD file no. 39-0256), demonstrating that the introduction of RGO has no effect on the structure of Bi2WO6.13 On the basis of the Scherrer equation, the average crystalline size was estimated from the (131) peak in Figure 1, which is about 10.5, 8.3, 8.5, 8.0, and 7.9 nm for the sample of BWO NPs, BWO−RGO 0.5%, BWO−RGO 1.0%, BWO−RGO 2.0%, and BWO−RGO 5.0% nanocomposites, respectively. It demonstrates that the C

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in the nanocomposites. As shown in Figure 3A and 3B, the peaks at 159.1 and 164.4 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, can be assigned to Bi3+ of bare BWO. The doublet peaks at 37.6 and 35.4 eV belong to W 4f5/2 and W 4f7/2, respectively, which are the features of W6+ in bare BWO.13 It can be clearly observed from Figure 3A and 3B that the characteristic peaks of Bi 4f and W 4f over the BWO−RGO nanocomposites shift toward higher binding energy, compared to those of pure Bi2WO6, suggesting a certain electronic interaction between BWO and RGO in the nanocomposites.19 Furthermore, the shift of these peaks is dependent on the weight ratio of GO. For the sample BWO−RGO 0.5%, there is almost no change for the binding energy of Bi 4f and W 4f. When the weight ratio increases up to 5.0%, the characteristic peaks of Bi 4f and W 4f shift up about 0.2 eV compared with BWO. Besides, two peaks located at 157.3 and 162.6 eV with quite low intensities can be observed from XPS spectra of Bi 4f over BWO−RGO composites (Figure 3A), which might be attributed to metallic Bi0 being formed32 in the hydrothermal process for the reduction of GO. The effect of the presence of Bi0 on the photocatalytic ability of BWO−RGO nanocomposites would be discussed in the following mechanism section. In addition, the C 1s XPS spectra of GO and BWO−RGO 2.0% are depicted in Figure 3C and 3D. For bare GO, besides the symmetric C 1s peak at 284.8 eV for C−C, there are an abundance of oxygenous groups on the surface of GO nanosheets, such as C−O, epoxy or hydroxyl (at 286.9 eV), and CO or carboxyl (at 288.6 eV). For the BWO−RGO 2.0% composite, the peak intensity of the oxygenated functional groups shows a dramatic decrease (Figure 3D), which is very consistent with FT-IR results displayed in Figure S2 (Supporting Information). According to the C 1s XPS spectrum of BWO−RGO 2.0%, the obvious decrease in oxygenous groups suggests that GO sheets have been sufficiently reduced into RGO after coupling with BWO NPs followed by the hydrothermal process.33 The morphologies of as-obtained samples are observed using TEM. From Figure 4a, it is clearly seen that bare BWO is composed of nanoparticles with an average size of 10−15 nm. Typical TEM images of different BWO−RGO composites are depicted in Figure S3, Supporting Information. It can be easily found that the weight ratio of GO significantly affects the distribution of BWO NPs on the RGO nanosheets. The obvious aggregation of BWO NPs is observed for the sample BWO−RGO 0.5%, and the aggregation of BWO NPs decreases with increasing weight ratio of GO up to 2.0%. Excessive GO also results in the uneven distribution of BWO NPs, as displayed in Figure S3(d), Supporting Information. While for the sample BWO−RGO 2.0%, BWO NPs are adhered uniformly on the surface of RGO sheets by using the present electrostatic self-assembly method, as shown in Figure 4b and 4c. The as-prepared composites are entirely different from the inhomogeneous deposition of Bi2WO6 with several hundreds of nanometers on RGO or the irregular square nanoplate morphology of Bi2WO6 on graphene in the literature.18,20 From the high-resolution TEM image of BWO−RGO 2.0% nanocomposite (Figure 4d), the clear lattice fringes with a d spacing of ca. 0.315 nm can be assigned to (131) lattice planes of orthorhombic Bi2WO6, which is in good agreement with XRD results.16 The homogeneous attachment of BWO NPs on RGO sheets is beneficial for the separation of photogenerated charge and the transfer of photoelectron because of superior electron mobility of graphene materials.

weight ratios of GO have little effect on the crystalline size of BWO for the BWO−RGO nanocomposites. Additionally, in comparison with the XRD pattern of GO (shown in Figure S1, Supporting Information), no diffraction peaks corresponding to either GO or RGO is observed, which might be attributed to the low weight content and weak diffraction intensity of GO or RGO.28,29 However, the presence of RGO in as-prepared BWO−RGO can be identified using the corresponding FT-IR spectra. As displayed in Figure S2, Supporting Information, pure Bi2WO6 exhibits main absorption peaks at 400−800 cm−1, which are assigned to the stretching vibration of Bi−O, W−O, and W−O−W. The FT-IR spectrum of GO shows representative absorption bands attributed to oxygenic functional groups, including C−O−C (1054 cm−1), C−OH (1230 cm−1), and CO (1721 cm−1), whereas in the FT-IR spectrum of RGO−BWO nanocomposite the absorption peaks of the oxygenic groups were noticed at similar wavelength, but the corresponding intensity is far lower than that of GO. The results confirm the successful reduction of GO into RGO via facile hydrothermal process. The samples were further characterized by Raman spectroscopy. As indicated in Figure 2, two main peaks are noticed at

Figure 2. Raman spectra of GO, BWO−RGO 2.0%, and BWO−RGO 5.0% nanocomposites.

around 1598 and 1345 cm−1, which should be attributed to the G band and D band of graphene. It clearly indicates the presence of graphene structure in as-synthesized BWO−RGO nanocomposites. In particular, an increased ID/IG ratio of BWO−RGO composites is noted, compared to that of GO sample. It can be obtained from Figure 2 that the ratio of ID/IG in BWO−RGO samples is ca. 1.29, which is obviously higher than ca. 0.96 in blank GO. Generally, the graphitization degree of carbon materials can be represented by using ID/IG obtained from Raman spectra. The higher ratio of ID/IG in BWO−RGO composites, in comparison with that of graphene oxide, implies that GO sheets are successfully reduced to RGO.17,30 Besides, it can be also noticed that the G band of BWO−RGO composites (1598 cm−1) shifts toward higher wavelength, compared to that of GO (1613 cm−1). The shift of the G band suggests there is a chemical doping for carbonaceous materials in general.31 In our study, the inconspicuous band shift might be attributed to weak chemical bonding or electronic interaction between BWO and RGO. XPS analyses were carried out to study the reduction degree of GO and the electronic interaction between BWO and RGO D

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Figure 3. XPS spectra of (A) Bi 4f and (B) W 4f of BWO−RGO nanocomposites; XPS spectra of C 1s in the (C) original GO and (D) BWO−RGO 2.0% nanocomposite.

Scheme 1. Illustration for the Synthesis of BWO−RGO Nanocomposites via Electrostatic Self-Assembly Approach Coupling with a Hydrothermal Reduction Process

BWO NPs holds amine functional groups, which can be corroborated by FT-IR spectra (Figure S4, Supporting Information). Under the effect of electrostatic attraction, BWO NPs that are positively charged are driven and anchored on the surface of negatively charged GO nanosheets. The typical TEM images of BWO−RGO 2.0% and BWO−RGO-U 2.0% are shown in Figure S5, Supporting Information. For the sample BWO−RGO 2.0%, it can be seen that RGO nanosheets are uniformly and densely covered by BWO NPs. It implies that BWO NPs and RGO sheets are compounded and consequently form a close interfacial contact through the surface charge modification method. However, as indicated in Figure S5(a,b),

Figure 4. TEM images of bare Bi2WO6 NPs (a), and TEM images of BWO−RGO 2.0% nanocomposite at different resolutions (b−d).

The mechanism for the formation of uniform BWO−RGO nanocomposites is illustrated in Scheme 1. First, small-sized BWO NPs (10−15 nm) have been prepared via a solvothermal approach. After being modified with APTES, the surface of E

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Figure 5. (A) Photocatalytic degradation of BPA over bare RGO, BWO, and BWO−RGO nanocomposites with different weight ratios of GO under artificial solar light irradiation. (B) Comparison of rate constants over bare BWO and BWO−RGO composites.

prepared BWO−RGO-U 2.0% suspension. The zeta potential analysis can also explain TEM images displayed in Figure S5, Supporting Information. The weak repulsive interaction between bare BWO NPs and RGO gives rise to the insufficient interfacial contact, including the obvious aggregation and partially anchoring of BWO NPs on the RGO surface. Photocatalytic Oxidation/Reduction Reactions. The photocatalytic oxidation activity of as-synthesized uniform BWO−RGO nanocomposite has been evaluated using BPA as the target pollutant. Figure 5A shows the photocatalytic degradation of BPA by bare RGO, BWO, and various BWO− RGO composites under artificial solar light irradiation. No obvious photodegradation of BPA is observed after illumination for 80 min in the absence of photocatalysts (direct photolysis). When RGO alone is used as a photocatalyst, only 15% of BPA is degraded. Bare BWO NPs exhibit moderate photocatalytic activity toward BPA degradation, and 70% of BPA is removed within 80 min of irradiation. All BWO−RGO nanocomposites show the higher photocatalytic activity, compared to bare BWO. Additionally, it is found that pseudo-first-order kinetics is obeyed for photocatalytic degradation of BPA. Therefore, the first-order kinetic equation ln(Ct/C0) = kt is used to fit the experimental data in Figure 5A, where k, Ct, and C0 are the apparent rate constant, initial concentration, and concentration of BPA at specific time intervals, respectively. On the basis of the kinetic plots indicated in Figure S8, Supporting Information, the rate constants over bare BWO and BWO− RGO nanocomposites are obtained and indicated in Figure 5B. Clearly, the rates of photodegradation follow the order BWO− RGO 2.0% > BWO−RGO 1.0% > BWO−RGO 5.0% > BWO− RGO 0.5% > BWO−RGO-U 2.0% > bare BWO. For the best photocatalyst of BWO−RGO 2.0%, the rate constant is 2.48 × 10−2 min−1, which is 1.5 times higher than that of BWO NPs (1.59 × 10−2 min−1). Photocatalytic oxidation performance of BWO−RGO nanocomposites is also investigated by selective oxidation of benzyl alcohol (BA), often used as a substrate for aerobic oxidation.34,35 Figure 6 summarizes the results of the selective oxidation of BA after 4 h irradiation over bare BWO, RGO, and different BWO−RGO nanocomposites. It should be noted that almost no benzaldehyde is detected in the presence of bare RGO, suggesting RGO is unlikely active for selective oxidation of BA under our experimental conditions. Selective oxidation of BA can be achieved on all BWO-based photocatalysts with selectivity > 95% to benzaldehyde. The conversion rate of BA

Supporting Information, the case is quite different for BWO− RGO-U 2.0% sample obtained by simple mixing bare BWO NPs with GO and hydrothermal reduction. Obvious aggregation of BWO NPs can be observed, and partial BWO NPs are not anchored on the RGO surface, which clearly suggests that the interfacial contact between BWO NPs and RGO is ineffective for BWO−RGO-U 2.0% nanocomposite. The different interfacial contact of BWO NPs with RGO sheets can be also evidenced by photographs (Figure S6, Supporting Information) taken after the hydrothermal treatment. For the BWO−RGO-U 2.0% sample, a stratification phenomenon between BWO NPs and RGO is observed, while the suspension of BWO−RGO 2.0% obtained by surface charge modification is homogeneous. For as-fabricated BWO−RGO nanocomposites, BWO NPs with small particle size benefit to generate more reaction active sites. Meanwhile, the intimate interfacial contact between RGO sheets and BWO facilitates the separation of photoinduced electron−hole pairs. The above two structural features contribute to improving the photocatalytic performance of BWO−RGO composites. Similar effects of surface charge functionalization on the interfacial contact could also be found in other reports.22,29 In order to explain the effective interfacial interaction obtained from the above-mentioned method, the zeta potential of bare BWO, GO, RGO, and APTES-modified BWO are measured and the results are depicted in Figure S7, Supporting Information. As indicated in Figure S7, Supporting Information, the suspension of GO, bare BWO, and RGO in deionized water at pH = 6 exhibits a negatively charged surface with a zeta potential of −43.3, −24.2, and −19.7 mV, respectively. The zeta potential of RGO increases compared with GO, which derives from the decrease of negatively charged functional groups after hydrothermal treatment. However, the APTES-functionalized BWO NPs show an opposite zeta potential value of +10.5 mV, that is, the negatively charged surface of BWO NPs is changed by modifying with APTES successfully. When mixing APTESmodified BWO NPs with GO suspension, electrostatic interaction makes them integrate into adequately contacted BWO-GO nanocomposites. Although the zeta potential of RGO increases by 23.6 mV compared to that of GO, there is still a repulsive interaction between bare BWO NPs and RGO. For the nanocomposite BWO−RGO-U 2.0%, because of weak electrostatic repulsion between negatively charged BWO NPs and RGO, the interfacial contact of BWO NPs with RGO is inadequate, which leads to inhomogeneous stratification of asF

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tion of BWO NPs can be noticed for BWO−RGO-U 2.0%, while the uniform distribution of APTES-modified BWO NPs can be observed for BWO−RGO 2.0%. It demonstrates the interfacial contact between BWO NPs and RGO is more sufficient in the nanocomposite of BWO−RGO 2.0%. The enhanced photocatalytic activity of BWO−RGO 2.0% might be attributed to the above two aspects. Besides, the compounding appropriate amount of RGO with BWO NPs can improve the photocatalytic oxidation rate of bare BWO for the degradation of BPA and selective conversion of BA. However, based on the experimental results of Figures 5 and 6, the enhanced effect via the formation of BWO−RGO nanocomposites on the photocatalytic oxidation activity is limited. To further test the reduction activity of BWO−RGO composites with different ratios of GO, photoreduction of Cr(VI) is employed as a probe reaction. Figure 7A presents the experimental results of photocatalytic reduction Cr(VI) in the presence of different photocatalysts. In the absence of catalysts, there is no obvious change in Cr(VI) concentration. Photocatalytic reduction of Cr(VI) by BWO−RGO nanocomposites is more effective compared to the reduction by bare RGO or BWO. Similar to the case of photocatalytic degradation of BPA discussed above, the kinetics of photoreduction of Cr(VI) is fitted to a pseudo-first-order reaction ln(Ct/C0) = kt. On the basis of the pseudo-first-order kinetic plots in Figure S9, Supporting Information, the rate constants of photoreduction Cr(VI) over BWO−RGO composites and blank BWO are calculated and depicted in Figure 7B. As indicated in Figure 7B, the rates of photoreduction Cr(VI) follow the sequence BWO−RGO 2.0% > BWO−RGO 5.0% > BWO−RGO 1.0% > BWO−RGO 0.5% > BWO−RGO-U 2.0% > blank BWO. The rate constant calculated for the optimal nanocomposite BWO−RGO 2.0% is 3.22 × 10−2 min−1, which is ∼5 times greater than that of blank BWO NPs (0.59 × 10−2 min−1). This result indicates that by compounding an appropriate amount of RGO the reduction performance of BWO−RGO composites can be promoted significantly compared to bare BWO. The significantly enhanced reduction performance of BWO− RGO nanocomposites is also verified by the selective photocatalytic reduction of aromatic nitro-organics to aminoorganics. Figure 8 shows the photocatalytic activity of blank BWO, RGO, and uniform BWO−RGO composites toward selective reduction of 4-nitrophenol (4-NP) in water, employing ammonium oxalate as scavenger of photoinduced holes and

Figure 6. Photocatalytic performances of bare BWO, RGO, and BWO−RGO nanocomposites for selective oxidation of benzyl alcohol to benzaldehyde after 4 h irradiation.

in Figure 6 clearly indicates that BWO−RGO nanocomposites exhibit higher activity than pure BWO. For the BWO−RGO composites, the weight ratios of graphene have an effect on the selective photocatalytic activity in a way. The conversion rate of BA increases with the increase of GO weight ratio up to 2.0%. The optimal catalyst BWO−RGO 2.0% holds a reaction rate of 63.6 μmol/h, which is slightly higher than bare BWO (47.3 μmol/h). In contrast, BWO−RGO 5.0% exhibits lower catalytic efficiency than pure BWO for selective oxidation of BA. Specifically, for selective oxidation of BA, the conversion rate (63.6 μmol/h) over BWO−RGO 2.0% prepared via an electrostatic self-assembly approach is greater than that of BWO−RGO-U 2.0% (49.8 μmol/h) obtained by the simple mixing GO nanosheets with solid BWO NPs (Figure 6). Similarly, the rate constant of BPA photodegradation with BWO−RGO 2.0% (2.48 × 10−2 min−1) is also higher than that of BWO−RGO-U 2.0% (1.67 × 10−2 min−1). The results demonstrate definitely the preparation methods have an important effect on the catalytic performance of photocatalysts. As depicted in Figure S5, Supporting Information, for the simple mixture of GO suspension and BWO NPs, partial BWO NPs are anchored on the surface of RGO sheets. The amount of attached BWO NPs in BWO−RGO 2.0% sample obtained by surface charge modification is more than that in sample BWO−RGO-U 2.0%. More importantly, the evident aggrega-

Figure 7. (A) Photocatalytic reduction of Cr(VI) in bare BWO, RGO, and BWO−RGO nanocomposites aqueous dispersions under artificial solar light irradiation. (B) Comparison of rate constants over bare BWO and BWO−RGO nanocomposites. G

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mainly undergoes the oxidation process of photogenerated holes or hydroxyl radicals.5,36 The initial step of selective oxidation BA also primarily involves the attack of photogenerated holes.37,38 The oxidizing ability of photoinduced holes is intrinsically governed by the separation efficiency of electron−hole pairs and the valence band potential of semiconductor. It has been reported that BPA can be decomposed by mesoporous Bi2WO6 under simulated sunlight irradiation,5 and benzylic alcohols can be selectively converted to the corresponding aldehydes by flowerlike Bi2WO6 under visible light illumination.39 It demonstrates the edge potential of the valence band in Bi2WO6 is more positive than the standard redox potentials of BPA and BA molecule, and thus, photogenerated holes of blank Bi2WO6 or as-prepared nanocomposites could drive BPA degradation and BA oxidation. On the other hand, incorporation of RGO might accelerate the transfer of photogenerated electrons and suppress the recombination of electron−hole pairs due to its high electron mobility. To study this effect, EIS Nyquist plots of bare Bi2WO6 and BWO−RGO 2.0% sample are measured under artificial solar light irradiation. As shown in Figure 9, the

Figure 8. Photocatalytic selective reduction of 4-NP to 4-AP over blank BWO, RGO, and BWO−RGO nanocomposites after irradiation for 30 min.

argon bubbling. When bare RGO is used as a photocatalyst, only a trace amount of 4-NP is reduced in the present experimental conditions. Blank BWO NPs show a 0.26 mg/h conversion rate (equal to 16% conversion) after reaction for 30 min. Compounding of RGO can effectively enhance the reduction activity compared to BWO, but excessive addition of RGO would lower the photoactivity of BWO−RGO nanocomposites. The selectivity of 4-NP to 4-AP is also displayed in Figure 8, and all photocatalysts show excellent selectivity. It can be seen from Figure 8 that the reaction rates of selective reduction 4-NP follow this order: BWO−RGO 2.0% > BWO−RGO 5.0% > BWO−RGO 1.0% > BWO−RGO 0.5% > BWO−RGO-U 2.0% > blank BWO. Under the present conditions, the reduction rate of 1.52 mg/h (about 95% conversion) is achieved for BWO−RGO 2.0% within 30 min, whereas only 16% of 4-NP is reduced over bare BWO NPs, which indicates the significant improvement in photoreduction activity of BWO because of coupling with RGO. Furthermore, similar to the results of photocatalytic degradation BPA and selective oxidation of BA, the reaction rates of photocatalytic reduction of Cr(VI) and 4-NP over BWO−RGO 2.0% are significantly higher than BWO−RGO-U 2.0%. This also suggests that preparation methods and the resulting interfacial interaction between BWO and RGO are essential factors for the improved photocatalytic activity. Notably, based on the experimental results of Figure 7, the rate constant of BWO−RGO 2.0% (3.22 × 10−2 min−1) is ∼5 times greater than that of blank BWO NPs (0.59 × 10−2 min−1) for photoreduction of Cr(VI). As for selective reduction of 4NP, the conversion rate of BWO−RGO 2.0% (1.52 mg/h) is about 6 times of blank BWO NPs (0.26 mg/h), whereas the optimal composite BWO−RGO 2.0% and bare BWO show comparable activity for the photocatalytic degradation of BPA and selective oxidation of BA. The enhanced effect of compounding RGO with BWO on the activity of reduction reactions is more remarkable than that of oxidation reactions, which also suggests introduction of RGO may have a greater influence on the photocatalytic reduction reactions. These inferences can be authenticated by the following characterizations of photocatalyst BWO−RGO 2.0% and bare BWO NPs, which include photoelectrochemical analysis, photoluminescence spectra, and UV−vis diffuse reflectance spectra. Mechanism for Significantly Enhanced Photoreduction Performance. The photocatalytic degradation of BPA

Figure 9. EIS Nyquist plots of bare BWO and BWO−RGO 2.0% nanocomposite photoelectrodes under artificial solar light irradiation.

arc radius of the EIS Nyquist plot of BWO−RGO 2.0% is markedly smaller than that of bare BWO. In general, the arc radius of EIS spectra denotes the interface layer resistance occurring at the surface of the electrode. The smaller arc radius indicates a higher transfer rate of interfacial charge and improved separation of photoinduced charge carriers over BWO−RGO 2.0% photocatalyst. To further investigate the transfer process of photoexcited charge carriers, PL spectra of blank BWO and BWO−RGO 2.0% are measured. As indicated in Figure S10, Supporting Information, the emission spectra of bare BWO and BWO−RGO 2.0% are centered at 465 nm, corresponding to the intrinsic luminescence of Bi2WO6. The PL intensity obtained over BWO−RGO 2.0% is about one-half of bare BWO. The fluorescence quenching via coupling RGO mainly results from improved interfacial charge transfer between BWO NPs and RGO nanosheets. The results are consistent with the above EIS analysis, and improved interfacial charge transfer leads to the enhanced catalytic activity of BPA degradation and BA oxidation. Comparing with the slightly improved photocatalytic activity of oxidizing reactions over BWO−RGO nanocomposites, the catalytic performance of reduction reactions is significantly H

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addition of GO during the hydrothermal reduction and a blank sample of BWO-NG is obtained. To reveal whether the formed Bi0 influences catalytic activity or not, the photocatalytic results over bare BWO, BWO-NG, and optimal BWO−RGO 2.0% are compared, which are displayed in Figure S12, Supporting Information. It can be clearly observed that there are no obvious improvements of photocatalytic activity over BWO− NG for either oxidation or reduction reactions compared with bare BWO. It is probably ascribed to the unmatched energy level between bare BWO and Bi0 species. The work function of Bi is −0.28 V vs NHE,41 which is more negative than the conduction band edge of BWO NPs. The electron transfer from photoexcited BWO to Bi0 would not happen, and thus, formation of Bi0 has no effect on the photocatalytic activity, which is different from the photocatalytic system of Bi/ Bi2MoO6.41 Therefore, in our present system, a more negative conduction band potential and stronger reducing ability achieved by compounding a proper amount of RGO should be the primary reason that BWO−RGO nanocomposites possess significantly enhanced photocatalytic reduction activity. To further investigate the influence of electronic structures on photocatalytic activity, the rough band structures of bare BWO and composite BWO−RGO 2.0% are obtained on the basis of UV−vis absorption spectra and the above MS plots displayed in Figure 10. As indicated in Figure 11A, the band gap absorption edges of BWO NPs and BWO−RGO 2.0% nanocomposite are 432 and 470 nm, respectively. It can be seen introduction of RGO leads to an obvious enhancement of background absorption in the range of 450−800 nm. The band gaps of bare BWO and BWO−RGO 2.0% are estimated to be 2.89 and 2.78 eV, as indicated in the inset of Figure 11A. The band gap narrowing might be ascribed to weak interaction between BWO and RGO, which is in line with the results of the Raman spectra. Besides, it is known that Vfb equals the Fermi level (EF) of n-type semiconductor, which is an intrinsic character of a semiconductor material. Combining with the shift of Vfb shown in Figure 10, a rough band structure diagram of bare BWO and BWO−RGO 2.0% can be obtained and is depicted in Figure 11B. It can be seen that the reduction abilities of BWO−RGO 2.0% should be stronger than bare BWO. As expected, the significantly enhanced photocatalytic activity of reduction Cr(VI) and selective conversion of 4-NP should be ascribed to the synergetic effect of an upshift of the conduction band potential and the improved separation of

enhanced (Figures 7 and 8). Except for the sufficient separation and fast interfacial transfer of charge carriers, the electronic structures of BWO−RGO composites might have a greater influence on the photocatalytic reduction performance. Since photogenerated electrons are the primary reducing agent for the reduction Cr(VI) or 4-nitrophenol, the energy of electrons is considered to be another vital factor affecting the reduction performance. To find the reason why the nanocomposites exhibit a remarkably improved reduction ability compared to blank BWO, Mott−Schottky measurements (MS) of BWO NPs and BWO−RGO nanocomposites are conducted and shown in Figure S11, Supporting Information. MS plots observed of all samples correspond to that of typical n-type semiconductor with an overall shape. The flat band potential (Vfb) of bare BWO and BWO−RGO 2.0% are found to be −0.37 and −0.59 V vs SCE (equivalent to −0.13 and −0.35 V vs NHE), which are obtained from x intercepts of the linear region in Figure 10. A reasonable conclusion can be drawn that

Figure 10. Mott−Schottky (MS) plots of bare BWO and BWO−RGO 2.0% electrodes.

the conduction band potential of BWO−RGO 2.0% is −0.22 V more negative than that of bare BWO based on the MS plots in Figure 10. Additionally, the conduction band position of bare BWO is estimated to be −0.23 V vs NHE since the conduction band edge is generally more negative by ca. −0.1 V than Vfb for n-type semiconductor.40 Taking into account the possible formation of Bi0 in the hydrothermal reduction of GO (shown in Figure 3A), a reference experiment is performed without

Figure 11. (A) UV−vis DRS of bare BWO and BWO−RGO 2.0% (inset presents the corresponding plots of transformed Kubelka−Munk vs energy of light). (B) Schematic illustration of band structures of bare BWO and BWO−RGO 2.0% sample. I

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RGO nanocomposites with intimate interfacial contact and strong electronic interaction can be obtained. The close interfacial contact promotes the separation of photoexcited e−/h+ pairs and lengthens the lifetime of photoinduced charge carriers. The electronic interaction and charge equilibration between BWO and RGO lead to the shift of Vfb and the change of energy levels of the conduction band e− and valence band h+. Four model reactions are employed to study photocatalytic redox properties of as-synthesized BWO−RGO composites. Integrating the catalytic results of the photocatalytic reduction with oxidation reactions, it can be found that the separation of photogenerated electrons/holes and the energy levels of separated e−/h+ are important factors which influence the photoactivity of BWO−RGO nanocomposites. More importantly, the energy level shift of separated e−/h+ has an inconsistent influence on reduction and oxidation reactions. Hence, the photocatalytic reductive ability is significantly enhanced owing to the synergetic effect of the above two aspects, while the oxidation ability of BWO−RGO is improved slightly. To some extent, when trying to disclose the effect of the addition graphene on the photoactivity of composites, the influence should be discussed based on specific reactions. By aiming at given reaction, finding out the vital factors of catalytic process will contribute to fabrication of more efficient photocatalysts.

charge carriers over BWO−RGO nanocomposites. On the other hand, it can be also noticed that the valence band level of BWO−RGO 2.0% is −0.33 V more negative than that of bare Bi2WO6, and the upshift of the valence band potential could weaken the oxidizing capability of BWO−RGO nanocomposites. The improvement of charge carriers separation and the shift of the valence band position resulting from introduction of RGO have conflicting influences on the photocatalytic activity of oxidation reactions. Hence, photocatalytic degradation of BPA and selective oxidation of BA are slightly accelerated over BWO−RGO composites. On the basis of the above discussion, a possible catalytic mechanism is proposed and displayed in Scheme 2. Under the Scheme 2. Schematic Description of the Enhanced Mechanism for the Photocatalytic Reduction Activity over BWO−RGO Nanocomposites

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of GO synthesis, XRD spectra of GO and RGO, FT-IR spectra of GO and BWO−RGO 2.0%, TEM images of BWO−RGO composites with different GO ratios, FT-IR spectra of BWO and APTES−BWO, TEM images of BWO−RGO-U 2.0% and BWO−RGO 2.0%, photographs of BWO−RGO-U 2.0% and BWO−RGO 2.0% suspension after hydrothermal process, zeta potential of GO, RGO, BWO, and APTES−BWO sample, the pseudo-first-order kinetics of BPA degradation and Cr(VI) reduction, PL spectra of BWO and BWO−RGO 2.0%, Mott−Schottky (MS) plots of bare BWO and BWO−RGO electrodes, and the compared photocatalytic results of bare BWO and BWO-NG sample. This material is available free of charge via the Internet at http://pubs.acs.org.

illumination of artificial solar light irradiation, BWO NPs are photoexcited to generate electron−hole pairs. The photogenerated holes (h+) and electrons (e−) transfer to the surface of BWO NPs and then drive photocatalytic oxidation reactions (degradation of BPA and selective oxidation of BA) or reduction reactions (reduction of Cr(VI) and 4-NP), respectively. The separation of photogenerated charge carriers and the energy levels of separated e−/h+ are two crucial factors affecting photocatalytic redox performance. Due to the introduction of RGO and the close interfacial contact, the recombination of photoinduced electrons and holes can be sufficiently inhibited, resulting in the improved photoactivity. However, the addition of RGO shifts the valence band edge to a more negative position, which undoubtedly decreases the oxidizing ability of holes. The above two factors simultaneously and oppositely influence the oxidation ability of BWO−RGO composites, which can explain the slightly improved photocatalytic performance. In contrast, the upshift of the conduction band potential (Figures 10 and 11B) causes the stronger reductive power of BWO−RGO nanocomposites. As depicted in Scheme 2, the synergetic effect of these two aspects significantly enhance the photocatalytic efficiency of reduction reactions. According to these results, one important conclusion can be drawn, that is, when discussing the effect of introduction graphene or RGO on the photocatalytic activities of semiconductor particles, the results should be analyzed on the basis of specific reactions.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (86)391-3987818. Fax: (86)391-3987811. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This works was supported by the National Nature Science Foundation of China (21307027, 51074067) and the Foundation of Henan Educational Committee (2010B150009).

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REFERENCES

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4. CONCLUSIONS In summary, a facile and efficient electrostatic self-assembly approach is successfully developed to construct BWO−RGO nanocomposites. Through the method, the uniform BWO− J

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