One-step synthesis of Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide multicomponent composite with enhanced visible-light photocatalytic activity

Abstract

In this study, the characterization and photocatalytic activity of Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide under visible-light irradiation was investigated in detail. The results suggested that the Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide can be synthesized by a facile one-step solvothermal process. Through the characterization of the composite photocatalyst by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, UV-vis diffuse reflectance spectra and photoluminescence spectra, it was found that Bi₂WO₆/Bi₂O₃@RGO composite was formed, meanwhile GO was completely reduced to graphene and bonded with Bi₂WO₆/Bi₂O₃ with by C–O to form composite. The as-prepared composite owned enhanced absorption in the UV to visible-light region and exhibited decreased radiative recombination of photogenerated charge carriers. Moreover, it was expected that the as-prepared composites exhibited enhanced photocatalytic activity for the degradation of Rhodamine B under visible-light irradiation. Among them, BWO@R3 (GO% = 5%) owned the best photocatalytic activity, which can photodegrade RhB (10⁻² g L⁻¹) reaching 99.6% in 20 min and high concentration RhB (30⁻² g L⁻¹) reaching 99.2% in 180 min. It can be ascribed to their improved light absorption property and the reduced recombination of the photogenerated electron–holes during the photocatalytic reaction.

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1. Introduction

Among the novel photocatalysts, the multicomponent metal oxides containing bismuth are regarded as excellent visible-light-driven photocatalysts.^1,2 Very recently, Bi₂WO₆ with high visible-light responsivity has been widely investigated by many teams in the field of photocatalysis.^3,4 Bi₂WO₆ as one of the simple members of the Aurivillius family is composed of accumulated layers of corner-sharing WO₆²⁺ octahedral sheets and bismuth oxide sheet,⁵ and it has been found that Bi₂WO₆ exhibits the highest photocatalytic activity among the above Bi³⁺-based oxides under visible-light irradiation.^6,7

However, the slow electron transfer and highly efficient electron–hole recombination limits its applications in photocatalysis has greatly limited.⁸ To overcome these drawbacks, it is essential to improve the charge separation efficiency and extend the spectral responsive range for real applications in the environment. Among the various studies for modifying Bi₂WO₆, Bi₂WO₆/Bi₂O₃ (ref. 9–11) as the typical p–n composite photocatalysts with narrow bandgap and improved charge separation efficiency get more attention. However, there are still some drawbacks, such as the limited region of visible-light photoresponse and the highly efficiency of electron–hole recombination prohibits the desirable catalytic activity under visible-light irradiation.^12,13 To solve these problems, multicomponent composite have been developed,^14–16 in which two or more visible-light active components and an electron-transfer system are spatially integrated,^13,17 such as S–Bi₂WO₆/Bi₂O₃.⁴ S–Bi₂WO₆/Bi₂O₃ exhibited enhanced photocatalytic activity for RhB under visible-light irradiation and decreased recombination of photogenerated charge carriers, however the composite did not own well adsorbability and practicability. Hence, in order to further enhance the stability Bi₂WO₆/Bi₂O₃ adsorbability for the practical application, the functional material is chosen to integrate into Bi₂WO₆/Bi₂O₃. Graphene has a perfect sp²-hybridized two dimensional carbon structure with excellent conductivity and large surface area,² so that graphene owns excellent electron conductivity and high adsorption.¹⁸ Hence, graphene-modified semiconductor nanocomposites were regarded as novel photocatalysts for degradation of pollutants.^19–21 For ideal graphene-semiconductor photocatalysts, semiconductors should be loaded firmly onto the surface of the individual two-dimensional sheets of graphene, which will maximize the utilization of electron conductivity of graphene.^22,23

Nevertheless, several works investigating^2,23–26 the preparation and properties of bismuth tungstate–graphene composite materials showed that photocatalytic activity is enhanced in the composite photocatalysts. However, so far, these composite photocatalysts were prepared by a two-step method or the synthesis amount was little, which were inconvenient for the practical application. The purpose in this study is to find a more convenient method to prepare bismuth tungstate–graphene composite photocatalysts.²⁷

Based on the above theories and through modification of preparation conditions, Bi₂WO₆/Bi₂O₃-loaded reduced graphene oxide (RGO) composite photocatalysts with high photocatalytic activity were prepared via a one-step solvothermal process. During the preparation, graphene oxide (GO) was directly reduced without secondary reduction. This route is simple and highly productive, so that the determined experimental parameters are more suitable for the actual mass production.

2. Materials and methods

2.1 Materials

Bi(NO₃)₃·5H₂O was purchased from Guangdong Xilong Chemical Co., Ltd. Na₂WO₄·2H₂O were purchased from Shanghai Chemical Company. Sodium oleate, ethylene glycol (EG) and ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical reagent grade without further purification.

Graphene oxide (GO) was synthesized according to the modification of Hummers' methods and the process was described previously.²⁸

2.2 Synthesis of Bi₂WO₆/Bi₂O₃@RGO

Sodium oleate (4 mmol) and Bi(NO₃)₃·5H₂O (4 mmol) were successively dissolved to 40 mL ethylene glycol, at the same time an appropriate mass ratio of GO (GO% = 1%, 2%, 5%, 10%) was ultrasonically dissolved in 20 mL ethylene glycol, and then the two solution were mixed together and stirred 0.5 h. 20 mL ethylene glycol dissolved with Na₂WO₄·2H₂O (4 mmol) was then injected into the above mixture solution. After vigorous stirring for 2 h, the mixture was transferred to a 100 mL Teflon-lined autoclave, sealed, and heated at 180 °C for 20 h. After that, the system was cooled down to room temperature. Then, the solid precipitate were filtered and washed with the distilled water and ethyl alcohol for three times. The products were dried at 60 °C for 12 h. The solids were ground, and then black powdered Bi₂WO₆/Bi₂O₃ loaded RGO multicomponent composite (abbreviation as BWO@R1-4) were prepared. For comparison, Bi₂WO₆/Bi₂O₃ (abbreviation as BWO) without RGO was also prepared as contrast sample.

The abbreviations of as-prepared composites according to the different GO% were listed in Table 1.

Table 1 Bi₂WO₆/Bi₂O₃@RGO composite with the different GO%

BWO@R	BWO	BWO@R1	BWO@R2	BWO@R3	BWO@R4
GO%	0	1%	2%	5%	10%

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2.3 Characterization

X-ray diffractometry (XRD) was conducted by a Japan Rigaku Rotaflex diffractometer with a monochromatic Cu Kα radiation source, under 40 kV and 100 mA (λ = 0.15418 Å). Scanning electron microscopy (SEM) images were observed by a JEOLJSM-6700F SEM device. The energy-dispersive X-ray spectroscopy (EDS) was also performed during the SEM measurement. Transmission electron microscopy (TEM) was recorded on a FECNAI F20 microscope. X-ray photoelectron spectroscopy (XPS) measurement was carried out by an ESC ALAB-250I-XL device. Fourier transform infrared spectra (FT-IR) were recorded by a Nicolet 500 FT-IR analyzer using KBr pellets in the region 4000–400 cm⁻¹. Ultraviolet-visible diffuse reflectance spectra (UV-DRS) were recorded by a UV-visible spectrophotometer (TU-1901) using BaSO₄ as a reference. Photoluminescence (PL) spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer (λ_Ex = 338 nm) at room temperature. Electron spin resonance (ESR) signals of radicals spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were recorded at room temperature on a JES FA200 spectrometer. Mott–Schottky plots were measured using a three-electrode CHI660E electrochemical workstation at a fixed frequency of 1 kHz in 0.5 M Na₂SO₄ solution.

2.4 Photocatalytic test

Photocatalytic activities of the as-prepared composites were evaluated via the photodegradation of RhB under the irradiation of a 500 W Xe lamp in a sealed photocatalytic reactor. In each experiment, 1.00 g L⁻¹ of catalyst was added to the RhB solution (10 mg L⁻¹). Before irradiation, the solution was stirred for 75 min in the dark to reach adsorption–desorption equilibrium. After that, the mixture solution containing the photocatalyst and RhB was placed under the Xe lamp. At each time interval, a 3.5 mL mixture was sampled and centrifuged to remove the photocatalyst particles. Then the UV-visible absorbance of the centrifuged solution was recorded by UV-visible spectrophotometer.

3. Results and discussion

3.1 Characterization of the catalysts

The phase and composition of BWO@R were characterized by XRD, as shown in Fig. 1. All diffraction peaks of the as-prepared composites can be well indexed to the standard phase-pure orthorhombic Bi₂WO₆ (a = 5.457 Å, b = 16.435 Å, c = 5.438 Å, JCPDS no. 39-0256) and cubic Bi₂O₃ (a = b = c = 5.450 Å, JCPDS no. 76-2478). It suggested that Bi₂WO₆/Bi₂O₃ composite photocatalyst was synthesized in the solvothermal process.⁹ However, no characteristic diffraction peak for graphene was observed in the patterns. It is attributed to the relatively low amount and diffraction intensity of graphene compared with that of Bi₂WO₆/Bi₂O₃. The conclusion is consistent with some literatures.^15,25,27 These results indicated that new catalyst was not formed and the original crystal form was not changed by loading RGO.²⁸ However, it was shown that the intensity of the diffraction peaks of the composite photocatalysts were reduced due to loading RGO. It indicated that the crystallinity of BWO@R was weakened due to the covering of RGO.²³

Fig. 1 The XRD patterns of BWO@R multiple composite with the different GO%.

The crystallite size of the composite photocatalysts were estimated using the Scherrer equation:^23,29

D = Kλ/B cos θ (1)

where D is crystallite size; B is full width at half-maximum (FWHM); K is Scherrer constant 0.89; λ = 0.154056 nm, which is X-ray wavelength; θ is the angle of diffraction.³⁰ By calculating with the peaks which had the largest peak areas in XRD, as shown in Table 2, the crystallite size of the as-prepared composites were 8.6 of BWO@R1, 5.2 of BWO@R2, 5.4 of BWO@R3, 13.7 of BWO@R4 and 8.8 of BWO. The results show that the lattice dimensions of BWO@R is effectively reduced due to loading RGO. It is attributed to that the crystal growth of Bi₂WO₆/Bi₂O₃ is suppressed by the spatial isolation effect of graphene.²³ It is obvious that the lattice dimensions of BOW@R2 (GO% = 2%) is the smallest among the as-prepared composites. However, the lattice dimension significantly increases due to too excessive addition of RGO (GO% = 10%). It is presumed that the agglomeration of the particles occurred by the excess of RGO.²³

Table 2 The crystallite size of BWO@R

Samples	BWO	BWO@R1	BWO@R2	BWO@R3	BWO@R4
Lattice dimensions (nm)	8.8	8.6	5.2	5.4	13.7

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The appearances of BWO@R were observed by SEM, as shown in Fig. 2. It is obvious that BWO@R is piled up to a sheet-shape, which is consistent with the special sheet-like structure of graphene. It indicates that Bi₂WO₆/Bi₂O₃ is successfully loaded onto the surface of RGO. There is a grain-like structure rather than a evident sheet-like structure at GO% = 1%, which is likely to results from the low dosage (Fig. 2a). With further increase of GO%, the sheet-like structure is increasingly evident (Fig. 2b and c). It is the most evidently that BWO@R2 disperses with the smallest size and the thinnest sheet at GO% = 2%, which indicates the modest dispersing effect of the GO.²³ However, it is shown obviously that the agglomeration of BWO@R4 occurs at GO% = 10% due to the excess of RGO, which is probably attributed to that GO could not be dispersed well in the solution by the excessive dosage (Fig. 2d). Meanwhile, C, Bi, W and O were confirmed as major elements in BWO@R by EDX elemental microanalysis, which indicates that RGO existed in the composite photocatalyst.

Fig. 2 SEM images of BWO@R with the different GO% (a) BWO@R1; (b) BWO@R2; (c) BWO@R3; (d) BWO@R4; (e) EDS spectra of BWO@R3.

The microstructure of BWO@R2 was further investigated by TEM and high resolution TEM (HRTEM), as shown in Fig. 3. The 2D graphene sheets and Bi₂WO₆/Bi₂O₃ nanoparticles are clearly observed in Fig. 3a. The well-crystallized Bi₂WO₆/Bi₂O₃ nanoparticles present on the surface of graphene sheets, which is illustrated by characteristic wrinkles on the edge. The selected area electron-diffraction (SEAD) pattern (Fig. 3a) reveals that Bi₂WO₆/Bi₂O₃ is polycrystalline nature ingredient in BWO@R2. From the HRTEM image of BWO@R (Fig. 3b), it is illustrated that a set of clear lattice fringes with the interspacing of 0.315 nm corresponding to the (131) crystal plane of orthorhombic Bi₂WO₆ and 0.272 nm corresponding to the (200) crystal plane of cubic phase Bi₂O₃ are found, which are consistent with the lattices that were calculated by Bragg equation in XRD. Furthermore, an interconnected well nanoparticulate morphology is observed, indicating that heterojunctions were formed in the composite photocatalyst.⁸ The well-defined fringes and the high crystallinity of BWO@R2 can facilitate the separation of the photogenerated carriers, improving the corresponding photocatalytic activities.^29,31

Fig. 3 (a) TEM images and inserted SAED pattern of BWO@R2 (b) high-resolution image with the magnification of the Bi₂WO₆ and Bi₂O₃ lattice.

The surface element composition and the chemical states of all elements were analyzed with XPS, as shown in Fig. 4. The chemical states of Bi, W, O and C in BWO@R were investigated, as well as the reduction state of organic carbon in RGO. The full spectrum spectra of BWO@R2 shows that C 1s, O 1s, Bi 4f and W 4f are all presented in the as-prepared composites (Fig. 4a). The XPS spectra of C 1s (Fig. 4b) shows the reductive degree of organic carbon in RGO of BWO@R2 during the preparation. The C 1s of GO shows four XPS peaks, which correspond to the C–C in non-oxidative carbon (284.6 eV), C–O in epoxy and hydroxy (286.99 eV), C=O in carbonyl (288.95 eV), and O–C=O in carboxyl (290.46 eV),³² indicating that GO contains abundant C and O organic groups. The XPS spectra of C 1s in BWO@R2 shows that the characteristic peaks of C–O, C=O and O–C=O basically disappeared, but the characteristic peak of C–C almost did not change. These results indicate that the organic groups of C and O were reduced during the preparation, and the GO in BWO@R was reduced to graphene. Compared with C–O from GO, the peak of C–O in BWO@R migrated to lower binding energy, indicating that GO interacted with Bi₂WO₆/Bi₂O₃ by C–O to form BWO@R composite.² The XPS spectra of O 1s from the BWO@R2. The XPS spectra of O 1s from the BWO@R2 (Fig. 4c) can be deconvoluted the two peaks representing O 1s in Bi₂WO₆ (530.29) and Bi₂O₃ (532.2 eV), but the two peaks are neither consistent with the peaks of O 1s in GO. These results further indicates that the O-containing groups in GO were reduced. In conclusion, during the synthesis of BWO@R, ethylene glycol with sodium oleate can enhanced the reduction reaction so that GO can be completely reduced to graphene. Moreover, graphene bonded with Bi₂WO₆/Bi₂O₃ with by C–O to form BWO@R composite.

Fig. 4 XPS spectra of BWO@R3 composite and GO (a) survey spectra; (b) C 1s; (c) O 1s.

The oxygen-containing groups and chemical interaction of BWO@R was illustrated by FT-IR spectra, as shown in Fig. 5. The absorption peaks at 400–1100 cm⁻¹ are attributed to bridging stretching modes of metal–O,^33,34 in this range the peak at 489 cm⁻¹ is related to the stretching vibration of Bi–O,³⁰ and the peaks at 569 and 727 cm⁻¹ are attributed to the stretching vibration of WO₆ in Bi₂WO₆.³⁵ It is found that the absorption peaks positions were not significantly different among all catalysts, nevertheless there were some differences at 1520 cm⁻¹, 1615 cm⁻¹ and 1079 cm⁻¹. The IR absorption of the peaks at 1615 and 1079 cm⁻¹ decreased with the increasing amount of GO, which is associated with the decreasing of the stretching vibration mode of C=O on the surface of the as-prepared composites.^14,32,36 However, the IR absorption of the peaks at 1520 cm⁻¹ rose with the increasing amount of GO, which indicates that the simple substance carbon species increased on the as-prepared heterojunctions.³⁴ These suggested that GO be reduced to graphene in the reaction system, of which C=O be mainly reduced to the simple substance carbon species. This further confirmed the XPS analysis.

Fig. 5 FT-IR spectra of (a) BWO@R; (b) BWO@R2 after adsorption and photocatalysis of RhB.

The UV-vis diffuse-reflectance spectrum revealed that the absorption edge of Bi₂WO₆/Bi₂O₃ extended to the visible-light region, as shown in Fig. 6, which implied the possibility of enhanced photocatalytic activity of these materials under visible-light irradiation. According to the spectra (Fig. 6a), all BWO@R exhibit high UV-vis absorbance, which indicates that the as-prepared composites possess the high UV-vis responses.²⁷ Within the visible range (400–800 nm), however, BWO@R exhibits higher absorbance than BWO, indicating that the loading RGO can enhanced the visible-light response of the as-prepared composites. The reasons are that black body properties typical of graphite-like materials own enhance absorption intensity, and meanwhile the reintroduction of graphite can modify the original formation of photoelectrons–holes.²⁸ Moreover, graphene as a visible-light sensitizing agent can broaden the absorption range under visible-light.²⁴ The steep lines in the graphs indicate that the enhancement of visible-light response is attributed to the energy level transition rather than the impurity transition.⁵ These results also indicate that the charge carrier inside BWO@R occurred the energy level transition, which can enhanced the range of visible-light adsorbability.

Fig. 6 (a) UV-vis DRS spectra of BWO@R multiple composite with the different GO% (b) estimated band gap by Kubelka–Munk function.

For a crystalline semiconductor, the band gap energy of a semiconductor can be calculated by the Kubelka–Munk equation:^8,37,38

αhν = A(hν − E_g)^n/2 (2)

where, α is the absorption coefficient, h is the Planck constant, ν is the light frequency, E_g is the band gap, and A is a constant. Among them, n is determined from the type of optical transition of a semiconductor, and the value of n for the direct semiconductor (Bi₂WO₆) is 1.³⁹ The band gaps were estimated from the (αhν)² versus photon energy (hν) plots were 2.60 eV (BWO@R1), 2.28 eV (BWO@R2), 1.87 eV (BWO@R3), 2.61 eV (BWO@R4), respectively, as shown in the Fig. 6b. It is obviously shown that the band gap of BWO@R3 (GO% = 5%) is the smallest, suggesting that loading RGO can effectively shorten the gap band of the as-prepared composites. Meanwhile, it illustrated that the separation and migration of photogenerated carriers are improved by the high conductibility of RGO. Hence, loading RGO can enhance the visible-light response and photogenerated carriers transfer of the composites.

The separation efficiency of photogenerated charge carriers in Bi₂WO₆/Bi₂O₃ loaded RGO composites were surveyed by photoluminescence spectra (excited at 300 nm), as shown in Fig. 7. The PL emissions are originated from the radiative recombination of photogenerated electrons–holes,^10,40 thus a higher PL emission peak indicates that it is easier to combination of the charge carriers.⁴¹ It is obvious that the intensity of the PL emission peaks of the as-prepared composites were dramatically weakened with the increase of GO%, and the PL peaks of BWO@R decrease as follows: BWO > BWO@R1 > BWO@R2 > BWO@R3 ≈ BWO@R4. Therefore, these results indicate that the as-prepared composites could more effectively inhibit the recombination of photogenerated charge carriers by loading RGO than that Bi₂WO₆/Bi₂O₃, and that the inhibiting abilities of BWO@R3 and BWOR4 were most strong. It indicates that loading RGO is the major reason for inhibiting combination of photogenerated electrons–holes. This is because graphene possesses superior properties, such as a unique two-dimensional layer structure and high electrical conductivity, as well as large specific surface area, which is beneficial to enhance separation efficiency of photogenerated charge carriers and inhibit recombination of photogenerated electrons–holes.¹⁸ Thereby, the as-prepared composites can effectually enhanced the photocatalytic activity,²⁸ further confirming the existence of covalent bonding interactions between RGO and BWO and smaller, which is consistent with the results of FT-IR and XPS.²⁴ Meanwhile, it was determined that BWO@R3 has the best ability to inhibit the recombination of photogenerated charge carriers, which was matched by the result of UV-DRS, indicating that BWO@R3 owns the best photocatalytic activity.

Fig. 7 Room-temperature PL spectra of BWO@R composite with the different GO%.

3.2 Photocatalytic activities

The adsorbed ability of BWO@R was tested by the adsorption of RhB under dark conditions, as showed in Fig. 8. It is shown that the adsorption of RhB was enhanced in the presence of BWO@R2 compared with that of BWO, reaching 88.2% in 120 min. It illustrates obviously that loading of graphene significantly enhanced the adsorption of BWO@R3. This is because the two-dimensional planar structure and large conjugated π bond of graphene can make RhB molecules easily adsorb on the surface of BWO@R3 via strong π–π conjugation interactions between graphene and RhB.²⁶

Fig. 8 The adsorption curve of RhB by BWO@R3 (BWO@R: 1.00 g L⁻¹; RhB: 10⁻² g L⁻¹).

The photocatalytic activities of BWO@R were evaluated by the degradation of RhB (10⁻² g L⁻¹) under visible-light irradiation, as showed in Fig. 9a. It is shown that BWO@R exhibited enhanced photocatalytic activity in degradation of RhB. Compared with the adsorption, the photocatalysis of BWO@R is mainly responsible for the degradation of RhB. It is obvious that BWO@R2 and BWO@R3 exhibited the highest photocatalytic activity with degradation ratio of RhB reaching 99.6% in 20 min, followed by BWO@R4, BWO@R4 and BWO. The photocatalytic activity of the as-prepared composites in the study is significantly improved compared with that of Bi₂WO₆/Bi₂O₃ (degradation ratio 60% in 95 min). These results indicate that the loading RGO can evidently enhance the visible-light catalytic activity. The reasons are that with a typical large conjugated π bond and high conductibility, the combination of RGO with Bi₂WO₆/Bi₂O₃ can efficiently transfer the photoelectrons, inhibit its combination with holes, and largely enhance the photocatalytic activity of the composites.²⁵ The photocatalytic activity of the as-prepared composites in the study was significantly improved compared with that of Bi₂WO₆/Bi₂O₃ (degradation ratio 60% in 95 min). However, the photocatalytic activity of the corresponding BWO@R4 (GO% = 10%) was less weakened than that of BWO@R2 (GO% = 2%), probably because the excessive RGO covered the active centers of BWO@R4. Therefore, the optimal GO% in synthesis of BWO@R composites is determined to be 5%. In addition, in the same experimental condition, the photocatalytic activity of BWO@R3 is better than the results of the Bi₂WO₆/Bi₂O₃ composites that have been reported,⁴ which further indicated that the photocatalytic activity of the as-prepared composites in the study was better improved.

Fig. 9 (a) Photodegradation for RhB with BWO@R composite under visible-light irradiation (BWO@R: 1.00 g L⁻¹, RhB: 10⁻² g L⁻¹); (b) UV-vis spectra of the reaction solution with BWO@R3; (c) photodegradation for RhB with BWO@R3 under visible-light irradiation (BWO@R: 1.00 g L⁻¹, RhB: 30⁻² g L⁻¹).

Meanwhile, the temporal evolution of the spectra during the photodegradation with BWO@R2 for RhB under visible-light irradiation (λ > 420 nm) was displayed in Fig. 9b. A rapid decrease of RhB absorption at the wavelength of 553 nm was observed, and the spectral maximum shifted from 553 to 496 nm. The color of the suspension changed gradually from pink to light green, which is in agreement with the shift of the major absorption. The hypsochromic shifts were caused by the removal of the N-ethyl groups and the destruction of the conjugated structure from RhB during the photodegradation.^42,43 It also proves that photochemical changes of RhB occurred by the as-prepared composite during the degradation process, rather than adsorption. After 20 min reaction, the hypsochromic shifts of the absorption band were considerably insignificant. The major absorption peaks decreased sharply and the color was clear in the following photodegradation. It was inferred that the cleavage of the whole chromophore structure (cycloreversion) of RhB occurred over the photocatalysts.^44,45 These indicate that the as-prepared composite exhibits excellent photocatalytic activity in the degradation of RhB.

In addition, the photodegradation of higher concentration RhB (30⁻² g L⁻¹) by BWO@R3 was shown in Fig. 9c. It is shown that BWO@3 also exhibited well photodegradation to high concentration RhB. The degradation ratio of RhB reached 99.2% in 180 min, which more obviously indicated that BWO@R3 possesses enhanced photocatalytic activity for RhB, even though in higher concentration.

To learn the involvement of active radical species in the photocatalytic oxidation of RhB over the as-prepared composite, the electron spin resonance (ESR) analysis were conducted, as shown in Fig. 10. It is obvious that no signals were observed in the dark. With irradiation for 10 min, there were obvious signals for superoxide radical (Fig. 10a), indicating that DMPO-˙O₂⁻ species were the main oxidizing species during the period of photodegradation reaction.

Fig. 10 DMPO spin-trapping ESR spectra of BWO@R3 (a) in methanol dispersion for DMPO-˙O₂⁻ and (b) aqueous dispersion for DMPO-˙OH.

However, no typical signals with the characteristic intensity of 1 : 2 : 2 : 1 for hydroxyl radical were observed, no matter whether the reaction was conducted in darkness or under light irradiation (Fig. 10b), suggesting that ˙OH would not be the main oxidizing species of the as-prepared composite in the photodegradation reaction. Previously, similar results have also been reported by Zhou et al. and Wang et al.^46,47

3.3 Mechanism of photocatalytic activity enhancement

Based on these experimental results, it is obviously found that the BWO@R exhibited the enhanced visible-light response and photocatalytic activity on the photodegradation of RhB. These results indicated that the evidently improved photocatalytic property of BWO@R could attributes to its multicomponent composite structure. The loaded RGO not only act as a good substrate and stabilizer of Bi₂WO₆/Bi₂O₃, but also could efficiently improve the electron transport and reduce charge recombination. Firstly, the potentials of conductance band (CB) and valence band (VB) edges of Bi₂WO₆, Bi₂O₃ and RGO were investigated. The CB bottoms (E_CB) and the VB top edge (E_B) were calculated empirically according to the formula:^48,49

E_CB = X − E_e − 0.5E_g (3)

E_VB = E_g − E_CB (4)

The results are E_CB = 0.46 eV for Bi₂WO₆, 0.22 eV for Bi₂O₃, and −0.36 eV for RGO,²⁸ respectively. Based on the confirmed forbidden bandwidth of BWO@R (1.72 eV), the corresponding E_VB of Bi₂WO₆, Bi₂O₃ and RGO in the as-prepared composites are determined 2.18, 1.94 and 1.36 eV, respectively. The schematic illustration of the photogenerated carrier transfer and enhanced photocatalytic activity by BWO@R is summarized in Scheme 1.

Scheme 1 Schematic diagram of charge separation in visible-light irradiation and the energy band mechanism of enhanced photocatalytic activity of BWO@R.

The main mechanism of the photocatalysis is that the separation of photogenerated carriers is promoted, and then the photogenerated electrons and holes are migrated to the surface of photocatalyst for photocatalytic reaction. The loaded RGO is well blackbody materials which can modify the original formation of photoelectron–hole pairs. In addition, it also broaden the visible-light response range of the prepared heterojunction, which reduce the forbidden bandwidth. With these improvements, the electrons (e⁻) in the VB of BWO@R will more easily transit into the CB by the visible-light excitation, which enhances the photogenerated electron–holes separation efficiently. Moreover, RGO serve as an excellent electron acceptor and mediator in this composite material, photogenerated electrons from Bi₂WO₆ and Bi₂O₃ could transfer to the large RGO sheets quickly through the presence of the delocalized π electrons,² which improve the efficiency in separation and migration of photogenerated carrier charge. The effective carrier charge transfer process can inhibit the photogenerated electrons–holes recombination. Thereby, the process can significantly enhance the photocatalytic activity of BWO@R.

To further analyze the reason why the as-prepared composite exhibited enhanced photocatalytic activity under visible-light, Mott–Schottky (MS) measurements were conducted, as shown in Fig. 11. It can be found that both BWO and BWO@R3 show positive slopes, confirming that the composite is mainly typical for n-type semiconductors. Obviously, the E_CB of BWO@R3 is more negative than that of BWO, which is in good agreement with the data of the above-mentioned calculation results.⁵⁰ Meanwhile, it is obvious that the Mott–Schottky plots slope of BOW@R3 is smaller than BWO. It was reported that the smaller slope suggests a faster charge transfer.⁵¹ The Mott–Schottky plots illustrate that charge transfer ability of BWO@R composite is higher than BWO, which contributes to the higher photocatalytic activity of the as-prepared composites.⁴⁹ It can further indicate that loading RGO which owns excellent electron conductivity can reduce band gap of the as-prepared composites and enhance migration of photogenerated carrier charge, so that the as-prepared composite own better visible-light response ability and enhanced visible-light photocatalytic activity.

Fig. 11 Mott–Schottky plots of BWO and BWO@R2 composites in 0.5 M Na₂SO₄.

4. Conclusions

In this paper, Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide composite photocatalyst was successfully synthesized via a one-step solvothermal process. It was shown that Bi₂WO₆/Bi₂O₃@RGO were synthesized directly from the results of XRD, SEM and EDS. In addition, TEM results determined that multicomponent composites were formed in the composite photocatalyst. In the synthesis process, it was found that GO was completely reduced to graphene and bonded with Bi₂WO₆/Bi₂O₃ with by C–O to form BWO@R composite from XPS and FT-IR. BWO@R owned great light absorption from UV to visible-light region and decreased recombination of photogenerated charge carriers, which suggested that the as-prepared composite had the features that can improved photocatalytic performance. The RhB degradation results showed that BWO@R exhibited enhanced photodegradation for RhB under visible-light irradiation. Among them, BWO@R3 exhibited the best photocatalytic activity for degradation ratio of RhB reaching 99.6% in 20 min. At the same time, it illustrated obviously that loading of graphene significantly enhanced the adsorption of BWO@R3. What is more, these results also illustrated that the best added GO% for synthesis BWO@R was 2%. Finally, it concludes that RGO serve as an excellent electron acceptor and mediator in the composite which can enhance the migration of photogenerated electrons and inhibit the photogenerated electron–holes recombination. These are the reasons that photocatalytic activity can be enhanced. This work provides not only a principle method to synthesize Bi₂WO₆/Bi₂O₃ loaded RGO with the excellent photocatalytic performance for actual produce, but also new possibilities to give some insights into the design of new multicomponent composite of Bi₂WO₆ with high photocatalyst activity, which shows great potential application in energy conversion.

Acknowledgements

The present work was financially supported by Graduate Innovation Fund of Jilin University (No. 2015027), also funded by National Natural Science Foundation of China (Grant No. 41472214) and Jilin Provincial Science & Technology Department (Grant No. 20150204050SF).

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