One-pot preparation of Bi/Bi₂WO₆/reduced graphene oxide as a plasmonic photocatalyst with improved activity under visible light

Abstract

A novel nanocomposite, Bi nanorods and Bi₂WO₆ nanosheets supported on reduced graphene oxide (Bi/Bi₂WO₆/rGO), was synthesized via a facile one-pot solvothermal method. In the reaction process, Bi₂WO₆ nanosheets and Bi nanorods were grown in situ on the rGO sheets, which were simultaneously achieved by the reduction of GO. Such a synthetic strategy can form effective close interfacial contacts and strong interactions among Bi₂WO₆, Bi and rGO, leading to efficient separation and transfer of photogenerated electron–hole pairs. As a result, the ternary plasmonic photocatalyst exhibits a much higher photocatalytic activity than pure Bi₂WO₆ and the binary composites in the photocatalytic degradation of rhodamine B and p-chlorophenol under visible light irradiation, which could be ascribed to the synergic effects of the improved electron–hole pair separation efficiency, enhanced visible-light harvesting and the good adsorptive capacity toward dye molecules.

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Introduction

Semiconductor-based photocatalysis is a promising technology to solve the current energy shortage and environmental pollution using abundant solar light.^1–3 Undoubtedly, the semiconductor TiO₂ is known as one of the best photocatalysts for redox decomposition of a range of organic pollutants.^4,5 However, TiO₂ has a wide band gap, which has limitations for visible light absorption.^6–8 Therefore, it is important to develop visible-light-driven photocatalysts to efficiently utilize solar light in the visible region.

As a new visible light-responding non-titania-based photocatalyst, Bi₂WO₆ with a unique layered structure has attracted increasing interest recently.^9–11 However, there are two main drawbacks that limit the application of the Bi₂WO₆ photocatalyst. First, pure Bi₂WO₆ can only absorb light with wavelengths shorter than 420 nm.^12,13 Secondly, the high recombination of photogenerated charges after light absorption severely limits the light energy conversion efficiency.^14,15 It was suggested that if a high optical absorption could be maintained while charge recombination could be significantly suppressed, a good photocatalytic activity would be anticipated. To solve these problems, one way is to fabricate heterojunction composite photocatalysts by coupling of another material with the appropriate Fermi energy. The recombination of photogenerated electron–hole pairs can thus be effectively suppressed, resulting in increased probability of the photogenerated charges being available for photocatalytic reactions.^16,17 Reduced graphene oxide (rGO) has recently attracted tremendous attention owing to its fascinating electrical and chemical properties.^18,19 When rGO is hybridized with a photocatalyst, the excellent electronic conductivity of rGO will promote charge transfer through its conjugated structure, inhibiting the recombination of the photoexcited electron–hole pairs^20,21 Furthermore, rGO nanosheets can act as adsorptive centers due to their large π-conjugation system and 2D planar structure, which make dye molecules adsorb easily onto their surface via strong π–π interactions between rGO and the dye molecules.²²

Lately, bismuth (Bi) has been observed to exhibit surface plasmon resonance (SPR) properties.²³ Compared to the general noble metals such as Au and Ag, Bi is much cheaper and easily prepared. It is documented that several Bi-modified photocatalysts, such as Bi/g-C₃N₄ and Bi/Bi₂O₂CO₃ exhibited highly promoted photocatalytic performance, which is ascribed to the SPR effect of Bi.^24,25 Considering the salient characteristics of Bi, Bi₂WO_6, and rGO, it prompts us to consider whether we can use a facile method to prepare a Bi/Bi₂WO₆/rGO triple nanocomposite, in which synergic effects could improve the photocatalytic activity.

In this study, we have developed a one-pot hydrothermal method to prepare a Bi/Bi₂WO₆/rGO nanocomposite, in which the formation of Bi₂WO₆, reduction of graphene oxide (GO) into rGO, and reduction of Bi³⁺ ions to Bi metal are simultaneously accomplished in the same synthetic system. Notably, besides being a solvent, ethylene glycol (EG) also acts as a mild reductant for the reduction of GO and Bi³⁺ ions, and therefore no additional reductants are required. The Bi/Bi₂WO₆/rGO nanocomposite exhibits a high photocatalytic activity to degrade rhodamine B (RhB) solutions under visible light irradiation. And the photocatalytic mechanism has been proposed as well.

Experimental

Synthesis of graphene oxide (GO) and Bi/Bi₂WO₆/rGO composites

GO was synthesized via a modified Hummer's method (see ESI† for Experimental details). For the synthesis of Bi/Bi₂WO₆/rGO, an optimal mass ratio of Bi(NO₃)₃, Na₂WO₄ to GO of 460 : 98 : 1 was adopted. Typically, 3 mg of GO was dispersed in 20 mL of ethylene glycol followed by ultrasonication for 1 h. Then, 1 g of PVP (polyvinylpyrrolidone K-30) and 3.5 mmol of Bi(NO₃)₃·5H₂O were added with stirring. Complete dissolution was required to obtain a uniform mixture. Subsequently, 0.5 mL of Na₂WO₄ solution (2 mol L⁻¹) was added into the mixture and stirred for 30 min, giving a homogeneous suspension. After that, the suspension was transferred to a 25 mL Teflon-lined stainless steel autoclave. The autoclave was heated to 180 °C and kept at this temperature for 24 h. After the autoclave was cooled naturally to room temperature, the product was separated, washed with absolute ethanol and distilled water for several cycles, and dried at 60 °C in vacuum. According to the added dosage of GO in the synthesis process, four Bi/Bi₂WO₆/rGO samples were synthesized and denoted as Bi/Bi₂WO₆/rGO-1, Bi/Bi₂WO₆/rGO-2, Bi/Bi₂WO₆/rGO-3 and Bi/Bi₂WO₆/rGO-4 corresponding to the addition of 1, 2, 3 and 4 mg of GO, respectively. For comparison, pure Bi₂WO_6, Bi₂WO₆/rGO and Bi/Bi₂WO₆ were prepared using similar methods (see ESI† for Experimental details).

Characterization

X-ray powder diffraction (XRD) analysis was conducted using a Bruker D8 Advance X-ray powder diffractometer with Cu-K_α radiation. The morphology was observed with a JEOL JEM-100C II transmission electron microscope (TEM) with an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) were measured using a Kratos AXIS Ultra DLD XPS system with an Al-K_α (1486.6 eV) line at 150 W. UV-Vis diffuse reflectance spectra (DRS) were recorded using a LAMBDA 950 UV/Vis/NIR spectrophotometer. Fluorescence (FL) spectra were determined by a Hitachi F-4500 FL spectrophotometer. Raman spectra were obtained on a Thermo Scientific DXR Raman microscope at room temperature with 532 nm laser excitation. Nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface areas were measured at 77 K using an ASAP 2020 system (Quantachrome Instruments). Photo-electrochemical measurements were performed in a constructed three electrode quartz cell system. Pt sheet was used as a counter electrode, Ag/AgCl was used as reference electrode, and the thin film on fluorine doped tin oxide (FTO) was used as the working electrode for investigation. The photoelectrochemical experimental results were obtained with a CHI 760C electrochemical system. An aqueous solution of 0.2 M Na₂SO₄ was used as the electrolyte.

Photocatalytic experiments

The photocatalytic activities of the as-prepared samples were evaluated by the degradation of rhodamine B (RhB) or colorless p-chlorophenol (4-CP) under visible light irradiation using a 500 W metal halide lamp with a 420 nm cutoff filter as the light source. In a typical process, 20 mg of the photocatalyst was dispersed in 50 mL RhB (10 mg L⁻¹) or 4-CP (10 mg L⁻¹) solution under magnetic stirring. Prior to irradiation, the suspension was agitated in darkness for 30 min to ensure adsorption–desorption equilibrium. Subsequently, the suspension was kept magnetically stirred under visible light irradiation, and 2 mL of the sample solution was taken from the reaction system at a certain time interval. After the catalyst was isolated by centrifugation, the supernatant was analyzed using a UV-Vis spectrometer (UV-8000). The degradation efficiency was calculated in accordance with C/C₀, where C is the concentration of the remaining pollutant solution at each time interval and C₀ is the initial concentration.

Results and discussion

The crystalline structure of the products was investigated by the XRD method. The results shown in Fig. 1 indicate that the diffraction peaks of pure Bi₂WO₆ and Bi are in good agreement with the orthorhombic phase of Bi₂WO₆ (JCPDS card no. 39-0256) and the hexagonal phase of Bi (JCPDS card no. 44-1246), respectively. As for the Bi/Bi₂WO₆ composite, all the diffraction peaks can be indexed to the orthorhombic phase of Bi₂WO₆ and the hexagonal phase of Bi, respectively. Furthermore, the diffraction peak intensities of Bi in the Bi/Bi₂WO₆ composites strengthened gradually with increasing Bi(NO₃)₃ in the precursor solutions (Fig. S1†), suggesting that the Bi content in the Bi/Bi₂WO₆ composite can be controlled in the synthesis process. The XRD pattern of the Bi/Bi₂WO₆/rGO-3 composite is comparable to that of Bi/Bi₂WO₆ (Fig. 1f). However, the full width at half maximum (FWHM) of the diffraction peaks of Bi and Bi₂WO₆ is broadened for the Bi/Bi₂WO₆/rGO composites (Fig. S2†), which is attributed to the existence of rGO restricting the degree of crystallization in the synthesis process.²⁶ In addition, no characteristic diffraction peaks for rGO are observed in the pattern because of the low amount and relatively low diffraction intensity.²⁷ However, the existence of rGO in the nanocomposite can be confirmed by the Raman spectra.

Fig. 1 XRD patterns of (a) GO, (b) Bi, (c) Bi₂WO₆, (d) Bi/Bi₂WO₆, (e) Bi₂WO₆/rGO, and (f) Bi/Bi₂WO₆/rGO-3. Triangles and solid dots refer to Bi and Bi₂WO₆, respectively.

As shown in Fig. 2, the Raman peak at 303 cm⁻¹ is assigned to the translation modes involving simultaneous motions of Bi³⁺ and WO₆⁶⁻.²⁸ These two peaks at 763 and 817 cm⁻¹ are attributed to antisymmetric and symmetric A_g modes of the O–W–O group terminus, respectively.²⁹ Compared to Bi₂WO₆, the intensity of Raman peaks of Bi/Bi₂WO₆ composites are slightly enhanced. This phenomenon could be attributed to the surface-enhanced Raman scattering effect of metallic Bi.³⁰ As for rGO, there are two typical Raman bands at ∼1611 cm⁻¹ (G band) and at ∼1359 cm⁻¹ (D band) for the graphitized structure.³¹ In the Raman spectrum of Bi/Bi₂WO₆/rGO-3, these two characteristic bands were also observed. In comparison with GO (I_D/I_G = 0.98), an increased D/G intensity ratio (I_D/I_G = 1.03) was observed, which revealed that GO has been partially reduced after the solvothermal reaction.³² Furthermore, the positions of D band and G band in the ternary composites were slightly shifted (inset of the Fig. 2), due to the reduction of GO and the combination of Bi and Bi₂WO₆ with rGO.^33,34

Fig. 2 Raman spectra of (a) GO, (b) Bi₂WO₆, (c) Bi/Bi₂WO₆ and (d) Bi/Bi₂WO₆/rGO-3. Inset: the enlarged Raman spectra of GO and Bi/Bi₂WO₆/rGO-3.

The chemical states and the surface composition of the Bi/Bi₂WO₆/rGO-3 sample has been analyzed by XPS (Fig. 3). In the full XPS spectrum of Fig. 3A, different binding energies were assigned to W 4f, Bi 4f, W 4d, Bi 4d, C 1s, O 1s, and Bi 4p state emission peaks. These peaks at 34.9 and 35.1 eV in Bi/Bi₂WO₆/rGO-3 correspond to W 4f_5/2 and W 4f_7/2 (Fig. 3B), both of which can be assigned to the W⁶⁺ oxidation state.³⁵ The O 1s peaks for Bi/Bi₂WO₆/rGO-3 (Fig. 3C) can be deconvoluted into two bands at 529.8 and 530.7 eV, which are associated with hydroxyl (OH) groups and oxygen species in the lattice oxygen, respectively.³⁶ Two main peaks with binding energies of 158.8 eV and 164.1 eV can be observed in Fig. 3D, which can be ascribed to the Bi³⁺ 4f_7/2 and Bi³⁺ 4f_5/2 binding energies, respectively.³⁷ Moreover, apart from the two peaks discussed above, there are two peaks at 162.5 eV and 157.2 eV with low intensity, which can be ascribed to metallic Bi.³⁸ As shown in Fig. 3E, the peak at 288.7 eV for the C=O bonding exists, but its peak intensity is very low. To illustrate the reduction degree of the GO sheets in the Bi/Bi₂WO₆/rGO-3 sample, the peak area ratios of C=O bonds to the total area are calculated to be 0.16 by the XPS C 1s peak area analysis, proving the transformation from GO to rGO.^39,40 Therefore, the results confirm the successful reduction of Bi³⁺ and GO into metal Bi and rGO via the facile hydrothermal process.

Fig. 3 XPS spectra of the Bi/Bi₂WO₆/rGO-3 sample: (A) full spectrum, (B) W 4f, (C) O 1s, (D) Bi 4f, and (E) C 1s.

TEM images showing the morphology and microstructure of the as-prepared Bi/Bi₂WO₆/rGO-3 are presented in Fig. 4. It can be seen that the rGO sheets exhibit a typical rippled and crumpled structure and the Bi/Bi₂WO₆ composites spread over the rGO surface (Fig. 4a). The apparent contrast between the darker nanorods and brighter nanosheets, as illustrated in Fig. 4b, suggests the formation of Bi nanorods and Bi₂WO₆ nanosheets on the surface of rGO. The Bi nanorods appear distinctly darker than the Bi₂WO₆ due to the higher electron density, which could be further confirmed by HRTEM (Fig. 4c).^41–43 The Bi nanorods with an average length of 50 nm have been observed in the TEM images (Fig. 4b and S6a†). As can be observed in Fig. 4c, the fringe interval of 0.315 nm is in accordance with the interplanar spacing of the (131) plane of Bi₂WO₆, whereas the periodic fringe spacing of 0.325 nm can be indexed as the (102) plane of the hexagonal Bi. To further obtain information about the Bi structure, the special region of Bi nanorods was analyzed (Fig. 4d). The spacing of 0.227 nm between neighboring lattice fringes corresponds to the distance between two (110) planes, indicating [100] as the growth direction for the Bi nanorods.⁴⁴ These results agree well with the XRD results. Moreover, the stacking width is calculated to be 0.37 nm for rGO layers corresponding to the spacing of the (002) lattice planes.^45,46 This spacing is very close to pristine graphite, indicating that GO has been well reduced to graphene (rGO).

Fig. 4 (a and b) TEM and (c and d) HRTEM images of Bi/Bi₂WO₆/rGO-3.

Fig. 5 shows the nitrogen adsorption–desorption isotherms for samples of Bi₂WO₆, Bi/Bi₂WO₆, Bi₂WO₆/rGO and Bi/Bi₂WO₆/rGO-3. The typical type IV isotherms and hysteresis loops of the sample are characteristic of mesoporous materials.^37,38 The BET surface areas of the as-prepared Bi₂WO₆, Bi/Bi₂WO₆, Bi₂WO₆/rGO, and Bi/Bi₂WO₆/rGO-3 are 27.87, 39.12, 67.42, and 67.67 m² g⁻¹, respectively. Notably, the higher surface area of Bi/Bi₂WO₆/rGO-3 is attributed to the presence of rGO nanosheets in the nanocomposite. Moreover, the pore volume determined by the Barrett–Joyner–Halenda (BJH) model indicated broad distributions with pore volumes of 24.65, 35.14, 68.43, and 69.26 cm³ g⁻¹ for Bi₂WO₆, Bi/Bi₂WO₆, Bi₂WO₆/rGO, and Bi/Bi₂WO₆/rGO-3, respectively. The pore size distribution of Bi/Bi₂WO₆/rGO-3 is shown in the inset in Fig. 5, which exhibits an average pore diameter of 20 nm. Owing to its high surface area and large pore volume, the mesoporous photocatalyst will provide efficient adsorption sites and enhance transportation of charge carriers, which would lead to an improvement of the photocatalytic activity.

Fig. 5 Nitrogen adsorption–desorption isotherms of various samples. Inset: the pore size distribution of Bi/Bi₂WO₆/rGO-3.

The formation mechanism of Bi/Bi₂WO₆/rGO nanocomposites

Based on the above experimental results and analysis, a possible synthesis process of Bi/Bi₂WO₆/rGO is proposed. The in situ one-step synthetic route of Bi/Bi₂WO₆/rGO nanocomposites is illustrated in Scheme 1. The reactions involved in the formation of Bi/Bi₂WO₆/rGO structures can be summarized as follows:

Bi(NO₃)₃ → Bi³⁺ + 3NO₃⁻ (1)

Na₂WO₆ → 2Na⁺ + WO₄²⁻ (2)

2Bi³⁺ + WO₄²⁻ + 2H₂O → 4H⁺ + Bi₂WO₆ (3)

2HOCH₂CH₂OH → 2CH₃CHO + 2H₂O (6)

6CH₃CHO + 2Bi³⁺ → 3CH₃COCOCH₃ + Bi + 6H⁺ (7)

Scheme 1 Schematic of the synthesis procedure of Bi/Bi₂WO₆/rGO composites.

The overall synthesis can be divided into four consecutive stages. At the early stage of the reaction process, when Bi(NO)₃ is dissolved in the GO solution (eqn (1)), the positively charged Bi³⁺ could be uniformly adsorbed onto the surfaces of the GO sheets, which are negatively charged due to the existence of OH and C=O functional groups (Scheme 1A and B). After Bi³⁺ ions have been adsorbed, the reaction between Bi³⁺ and EG takes place on the surface of GO, forming EG–Bi³⁺ complexes via the coordination between Bi³⁺ ions and OH groups (eqn (4) and Scheme 1C).⁴⁷ Then, in the following solvothermal treatment at high operating temperature, the complex is gradually decomposed to release the Bi³⁺ ions, which then react with WO₄²⁻ ions to form Bi₂WO₆ nanosheets on the surface of the graphene components (eqn (5)).⁴⁸ Moreover, metal Bi and rGO can be formed simultaneously (eqn (6) and (7), Scheme 1D) during the hydrothermal treatment. When Bi(NO₃)₃ is in excess in the reaction, there are still many residual Bi³⁺ ions absorbed on the surface of GO after the formation of Bi₂WO₆. These residual Bi³⁺ ions can be reduced into Bi nanorods by the reductant EG in the presence of PVP. Therefore, the content of Bi nanorods can be easily tuned just by varying the precursor ratios. It has been reported that only urchin-like bismuth structures were obtained by self-assembly of Bi nanorods in pure EG solution, in which a typical coordinative reduction course was suggested.⁴⁹ It is believed that the anisotropic effects of crystal and the face inhibitor function of PVP play crucially important roles in the formation of 1D nanostructures.⁵⁰ In the solvothermal reaction process, PVP molecules could preferentially adsorb onto the primary surfaces of Bi crystals through O–Bi bonding, as reported in the previous work.⁴⁴ The polymer molecules adsorbed on some surfaces of the Bi crystals could significantly decrease their growth rates and lead to highly anisotropic growth.⁵¹ Therefore, the isolated Bi nanorods were formed in the presence of PVP under appropriate reaction conditions.

UV-Vis diffuse reflectance spectra (DRS) of the as-prepared samples are shown in Fig. 6. Compared with that of pure Bi₂WO₆, the light-harvesting ability of Bi/Bi₂WO₆ is apparently enhanced, ranging from the UV to visible light region. This enhancement is attributed to the SPR effect of plasmonic Bi.⁵² In addition, the absorption of Bi/Bi₂WO₆ is gradually intensified with increasing content of Bi (Fig. S3†). For Bi/Bi₂WO₆/rGO, the continuous background absorption from 400–800 nm is enhanced due to the black body effect of graphene.⁵³ Moreover, this observation also clearly indicates that more visible light can be absorbed in the prepared Bi/Bi₂WO₆/rGO nanocomposite to generate more electron–hole pairs, and thus its photocatalytic activity will be improved under visible light irradiation. This hypothesis was confirmed by measuring the degradation of RhB over the obtained samples under the same conditions. With the increasing amount of rGO, the Bi/Bi₂WO₆/rGO composites show a continuously enhanced visible-light absorption in the range of 400–800 nm, which is in agreement with the previous report.⁵⁴ Therefore, the incorporation of Bi nanorods and rGO sheets increased the light absorption of the ternary composites in the visible region.

Fig. 6 UV-Vis diffuse reflectance spectra of (a) Bi₂WO₆, (b) Bi/Bi₂WO₆, (c) Bi₂WO₆/rGO, (d) Bi/Bi₂WO₆/rGO-1, (e) Bi/Bi₂WO₆/rGO-2, (f) Bi/Bi₂WO₆/rGO-3, and (g) Bi/Bi₂WO₆/rGO-4.

The degree of charge recombination can be evaluated by photoluminescence (PL) spectroscopy (Fig. 7). The lower PL intensity reflects the lower recombination rate of photogenerated electrons and holes.⁵⁵ Evidently, Bi/Bi₂WO₆ shows a significantly diminished PL intensity in comparison with pure Bi₂WO₆, which suggests that the deposition of Bi results in a remarkable decline in the recombination rate of photogenerated electron–hole pairs.²⁵ In Bi₂WO₆/rGO, photogenerated electrons could be transferred efficiently from Bi₂WO₆ to rGO due to the high charge carrier mobility of the rGO sheets, leading to a low charge recombination rate. The PL results demonstrated that the rGO layers with a two-dimensional π-conjugated structure could serve as an effective electron-accepting material.⁵⁶ A more efficient separation of electron–hole pairs was observed in Bi/Bi₂WO₆/rGO-3, which may be due to the charge transfer between Bi/Bi₂WO₆ and rGO. These results clearly indicate that the recombination of photogenerated charge carriers of Bi₂WO₆ is greatly restrained by the coupling of Bi and rGO.

Fig. 7 Photoluminescence spectra of the various photocatalysts.

Fig. 8 shows the transient photocurrent responses of the obtained samples. The photocurrents were stable and reversible at light-on and light-off for all samples. In comparison with Bi₂WO₆, Bi/Bi₂WO₆, and Bi₂WO₆/rGO, Bi/Bi₂WO₆/rGO-3 composite exhibited an increased current density, about 1.6, 1.2, and 1.15 times that of Bi₂WO₆, Bi/Bi₂WO₆, and Bi₂WO₆/rGO, respectively. The improvement of photocurrent indicated an enhanced photogenerated electron–hole separation efficiency, which is in good agreement with the PL measurements.

Fig. 8 Photocurrent responses of (a) Bi₂WO₆, (b) Bi/Bi₂WO₆, (c) Bi₂WO₆/rGO, and (d) Bi/Bi₂WO₆/rGO-3.

The photocatalytic activities of the samples were evaluated by the degradation of RhB under visible light irradiation (λ > 420 nm), as shown in Fig. 9. A blank control test (without photocatalyst) confirmed that the self-degradation of RhB can be regarded as negligible. The Bi/Bi₂WO₆ composite has a higher photocatalytic activity than that of Bi₂WO₆, which is due to the SPR effect of Bi. In addition, the photocatalytic activity was further enhanced by increasing the Bi amount in the composites (Fig. S4†), which is consistent with the results of DRS. Compared with pure Bi₂WO₆ and Bi/Bi₂WO₆, the Bi/Bi₂WO₆/rGO photocatalysts exhibited much higher photocatalytic activity (Fig. 9A). About 99.9% of RhB molecules were decomposed in 120 min when Bi/Bi₂WO₆/rGO-3 was used as the photocatalyst, whereas only 49% and 83% of the RhB molecules were degraded when pure Bi₂WO₆ and Bi/Bi₂WO₆ were employed, respectively. Moreover, the photocatalytic activity of the Bi/Bi₂WO₆/rGO composites increases with increasing the rGO content, and Bi/Bi₂WO₆/rGO-3 shows the highest degradation efficiency. However, when more GO was added, the photocatalytic activity of the Bi/Bi₂WO₆/rGO photocatalyst was found to decrease. It is probably because the dense black layers of excessive rGO shield the light irradiation on Bi and Bi₂WO₆, which can weaken the photocatalytic activity of the composite. Furthermore, excessive rGO also can act as a recombination center, which then decreases the efficiency of charge separation.⁵⁷

Fig. 9 (A) Photocatalytic degradation curves of RhB as a function of irradiation time over various photocatalysts under visible light irradiation (the time before 0 min means in the dark). (B) Kinetic curves of photocatalytic degradation of RhB fitted from the data of (A).

In order to directly show the photocatalytic activity enhancement of Bi/Bi₂WO₆/rGO, the Langmuir–Hinshelwood kinetics model was applied to calculate the apparent pseudo-first order rate constant k.⁵⁸ As can be seen in Fig. 9B, the rate constants corresponding to Bi/Bi₂WO₆, Bi₂WO₆/rGO, Bi/Bi₂WO₆/rGO-1, Bi/Bi₂WO₆/rGO-2, Bi/Bi₂WO₆/rGO-3 and Bi/Bi₂WO₆/rGO-4 were estimated to be 0.0111, 0.0099, 0.0115, 0.0203, 0.0355 and 0.0147 min⁻¹, which are about 2.27, 2.02, 2.35, 4.14, 7.24 and 3 times as much, respectively, as that over Bi₂WO₆ (0.0049 min⁻¹). These results clearly indicated that introducing the appropriate amount of Bi and rGO into the Bi₂WO₆ photocatalyst system can greatly enhance its photocatalytic activity.

It is usually accepted that the degradation of RhB over the photocatalyst would occur partially via a photosensitization pathway. In order to eliminate the photosensitization effect, colorless p-chlorophenol (4-CP) was used as the model pollutant. The photocatalytic degradation results are shown in Fig. 10. In contrast to Bi₂WO₆, Bi/Bi₂WO₆, and Bi₂WO₆/rGO, an enhanced photocatalytic activity was observed for the Bi/Bi₂WO₆/rGO-3 composite. As shown in Fig. 10A, a similar trend to the photocatalytic degradation of RhB was observed under visible light irradiation. The Bi/Bi₂WO₆/rGO-3 composite exhibits the highest photocatalytic performance among all the tested samples for RhB and 4-CP removal. Fig. S5† shows the evolution of UV-Vis spectra for RhB and 4-CP solution in the presence of Bi/Bi₂WO₆/rGO-3 under visible light irradiation. The UV-Vis peak intensity of RhB at 553 nm decreased gradually with irradiation time, whereas its position remained unchanged, suggesting that the RhB molecules were directly destroyed through the cleavage of the chromophore ring structure rather than a de-ethylation process.⁵⁹ The UV-Vis intensity of 4-CP also decreased with irradiation time, but slower as compared with that of RhB. Only 52% 4-CP was removed, whereas almost 99% RhB was degraded after 120 min. Thus, the degradation efficiency for 4-CP is lower than that for RhB (Fig. 10B), which implies that photosensitization plays a role in the RhB degradation.

Fig. 10 (A) Photocatalytic degradation of 4-CP as a function of irradiation time over the different four samples. (B) The comparison of removal efficiencies for RhB and 4-CP over Bi₂WO₆, Bi/Bi₂WO₆, Bi₂WO₆/rGO and Bi/Bi₂WO₆/rGO-3 after 120 min visible light irradiation.

As is known, various active species (˙OH, ˙O₂⁻, and h⁺) generated in the photocatalytic processes play key roles. Therefore, trapping experiments were carried out to explore the main active species. Isopropyl alcohol (IPA), p-benzoquinone (BQ), and triethanolamine (TEOA) were adopted as the traps for ˙OH, ˙O₂⁻, and h⁺, respectively.⁶⁰ Fig. 11 shows the effect of different scavengers on the photocatalytic degradation of RhB and 4-CP over Bi/Bi₂WO₆/rGO-3. It can be easily seen that the addition of IPA does not cause deactivation of the Bi/Bi₂WO₆/rGO-3 photocatalyst in either case, indicating that ˙OH was not the major active species involved in the photocatalytic processes. However, the photocatalytic performance of Bi/Bi₂WO₆/rGO-3 significantly decreases on the addition of TEOA and BQ. Thus, ˙O₂⁻ radicals and h⁺ were proven to be the dominant active species in the photocatalytic degradation of RhB and 4-CP over the Bi/Bi₂WO₆/rGO-3 composite photocatalyst under visible light irradiation. According to the previous report, the CB electrons of Bi₂WO₆ possess a poor reducing power owing to their more positive potential (+0.3 V, vs. SHE) than the one-electron oxygen reduction (O₂ + H⁺ ± e⁻ = HO₂, −0.046 V vs. SHE).⁶¹ It is plausible that the conduction band potential of Bi₂WO₆ is incapable of reducing the dissolved O₂ to ˙O₂⁻ radicals. However, with the addition of rGO, the electronic interaction and charge equilibrium between Bi₂WO₆ and rGO lead to the negative shift of the Fermi level and a change in the energy levels of the conduction band and valence band, indicating that the photocatalytic performance of reduction reactions is enhanced.^62,63

Fig. 11 Effects of different scavengers on (A) RhB and (B) 4-CP degradation over Bi/Bi₂WO₆/rGO-3 under visible light irradiation.

In addition to photocatalytic efficiency, the stability of photocatalysts is another important issue for their practical application. To study the photocatalytic stability and reusability of Bi/Bi₂WO₆/rGO-3, the photocatalysis process was repeated five times under the same conditions. As shown in Fig. 12A, the Bi/Bi₂WO₆/rGO-3 composite had a high and stable activity for photocatalytic degradation of RhB under visible light irradiation. After five cycling runs, the photocatalyst incurs no detectable loss of its photocatalytic activity. XRD patterns of Bi/Bi₂WO₆/rGO-3 before and after the photocatalytic reaction were comparable, and are shown in Fig. 12B. There was no apparent change observed in the XRD patterns. TEM and XPS analysis also proved that there was no apparent change in composition and morphology for Bi/Bi₂WO₆/rGO-3 during the photocatalytic process (Fig. S6 and S7†), which also confirmed the stability of the structure. It can be concluded that Bi/Bi₂WO₆/rGO possesses an efficient photocatalytic activity and can be easily separated for reuse.

Fig. 12 (A) Cycling runs in the photocatalytic degradation of RhB in the presence of Bi/Bi₂WO₆/rGO-3. (B) XRD diffraction patterns of Bi/Bi₂WO₆/rGO-3 measured before and after a photocatalytic reaction under visible light irradiation.

The enhanced photocatalytic activities of the Bi/Bi₂WO₆/rGO composites could be ascribed to the synergic effects of the following factors. Firstly, the introduction of Bi nanorods in the composite enhanced the light absorption in the visible region, which could be ascribed to the SPR effect of metallic Bi. Secondly, due to their matching energy levels, the interfacial charge transfer could effectively enhance the separation efficiency of photogenerated electrons and holes. Finally, the presence of rGO provides more active adsorption sites on the surface of rGO in the photocatalytic reaction. A proposed mechanism for the separation and transportation of photogenerated electron–hole pairs at the Bi/Bi₂WO₆/rGO interface is shown in Scheme 2. Electrons and holes could be generated from both photo-excited Bi₂WO₆ and SPR-excited Bi under visible light irradiation. Since the Fermi energy level of Bi (E_f = −0.17 eV vs. SHE) is more negative than the conduction band (CB) of Bi₂WO₆ (E_CB = 0.24 eV vs. SHE),^64,65 it is energetically favorable for the SPR-excited electrons to transfer from Bi to the CB of Bi₂WO₆. Subsequently, these electrons can transfer from the CB of Bi₂WO₆ to the surface of rGO nanosheets due to their two-dimensional conjugated π-structure and ultrahigh charge carrier mobility. Concomitantly, the holes transfer to the Bi₂WO₆ and Bi surfaces, which corresponds to oxidation of dye molecules into CO₂. Thus, in the case of Bi/Bi₂WO₆/rGO, the rGO sheets served as an acceptor of the photogenerated electrons of Bi₂WO₆ and Bi, which suppressed the recombination of photogenerated electron–hole pairs. The electrons in rGO can be captured by the dissolved O₂ molecules to generate superoxide anion radicals (˙O₂⁻) and then react with RhB (or 4-CP). In the meantime, the holes in the composite can directly oxidize the pollutants into CO₂ and H₂O, and other intermediates.

Scheme 2 Proposed mechanism of degradation of RhB over Bi/Bi₂WO₆/rGO composite.

Conclusion

In summary, the excellent heterojunction structure of rGO-supported Bi nanorods and Bi₂WO₆ nanosheets was obtained via a one-pot solvothermal process. The obtained composite possesses an enhanced visible light harvesting, good adsorptive capacity and efficient charge separation of the photogenerated electron–hole pairs. The ternary composite photocatalyst had a better performance for RhB photocatalytic degradation compared with pristine Bi₂WO₆ and the binary composite Bi/Bi₂WO₆. Such excellent performance should be attributed to the SPR effect of Bi, visible light response of Bi₂WO₆ and vectorial charge carrier transfer in the Bi/Bi₂WO₆/rGO heterojunctions and rGO sheets. In addition, the photocatalyst exhibits a high stability and can be reused five times without any degeneration in its photocatalytic activity. This work provides a new possibility in the investigation of Bi/Bi₂WO₆/rGO composites and promotes their practical application in waste water treatment.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant 20971044) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20316g

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