BiVO4-rGO with a novel structure on steel fabric used as high-performance photocatalysts

A high-performance and novel photocatalyst of BiVO4-reduced Graphene Oxide (BiVO4-rGO) nanocomposite was prepared by a facile hydrothermal method. The photocatalyst was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electronic microscopy, UV-Vis diffusion reflectance spectroscopy, photoluminescence spectroscopy and UV-Vis adsorption spectroscopy, respectively. The visible-light photocatalytic activity was evaluated by oxidation of methyl orange (MO) under simulated sunlight irradiation. The results show that the BiVO4-rGO nanocomposites exhibit enhanced photocatalytic performance for the degradation of MO with a maximum removal rate of 98.95% under visible light irradiation as compared with pure BiVO4 (57.55%) due to the increased light absorption intensity and the degradation of electron-hole pair recombination in BiVO4 with the introduction of the rGO.

graphene-based nano-materials have been utilized as photocatalysts to enhance the photocatalytic efficiencies because the such a supporting matrix with excellent electric conductivity and a super-high surface area make excellent contact with water or target pollutants to provide plenty of reactive sites [32][33][34][35][36][37] .
In this work, we present a simple hydrothermal method to prepare BiVO 4 -rGO composites using graphene oxide (GO) and Bi(NO 3

Results and Discussion
The crystallographic structure and phase purity of the as-obtained samples are first examined by powder X-ray diffraction (XRD) analysis (Fig. 1a). All the diffraction peaks can be indexed as the body-centered monoclinic phase of BiVO 4 with lattice constants of a = 5.195 Å, b = 11.70 Å and c = 5.092 Å (JCPDS card no. 14-0688) [38][39][40] . The XRD patterns are similar for the BiVO 4 -rGO composites and the BiVO 4 . An increase in the content of GO results in no obvious changes in the XRD patterns of the samples, suggesting that the introduction of GO has little influence on the crystalline structure of BiVO 4. The Raman spectra at room temperature under green laser excitation (532 nm) are shown in Fig. 1(b). The main Raman peaks of monoclinic BiVO 4 are observed around 210, 325, 366, 707 and 827 cm −1 , which are consistent with typical vibrational bands of monoclinic BiVO 4 41, 42 . The dominating peak at 827 cm −1 and the inconspicuous peak at 707 cm −1 are assigned to the symmetric and antisymmetric V-O stretching mode, respectively. The Peak centered at 366 and 325 cm −1 is attributed to the typical symmetric and antisymmetric bending modes of the vanadate anion, respectively. The GO exhibits Raman shifts at 1591 and 1355 cm −1 , corresponding to the G-and D-bands, respectively. As for the BiVO 4 -0.057 nanocomposites, aside from the distinctive peaks assigned to BiVO 4 , the G-and D-bands of rGO are located at 1588 and 1350 cm −1 , respectively, indicating shifts toward lower wavenumbers as compared to GO 43,44 . Figure 2 present typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the BiVO 4 and BiVO 4 -0.057 products. As shown in Fig. 2a, it can be seen that the as-synthesized BiVO 4 has a nanowire structure. The TEM image presented in Fig. 2b reveals that the diameter of BiVO 4 is about 66 nm and that the interplanar distance of the (121) plane of the monoclinic BiVO 4 is 0.31 nm (Fig. 2c) 45 . Interestingly, after BiVO 4 was coupled with rGO, the morphology of the nanowires disappeared completely and sheets-like structures appeared, as shown in Fig. 2d. In the TEM images ( Fig. 2e-g), there is a central BiVO 4 nanowire axis, which is covered by BiVO 4 /rGO nanosheets. The thickness of the nanosheets is about 10 nm. The functional groups such as hydroxyl, carboxyl, and carbonyl groups of the GO may provide the reaction sites for the nucleation and growth of the BiVO 4 -0.057 nanosheets 46 . In the TEM-EDS image of the BiVO 4 -0.057 nanosheets is tested and presented (Fig. 2h), from which Bi, V, O and C signals are clearly observed.
For a comparison, the morphological structures of the BiVO 4 -rGO nanocomposites with different synthesis conditions are characterized by the SEM technique (Fig. 3). When the concentration of GO is 0.029 gL −1 , the images show that part of the nanowires are covered by nanosheets ( Fig. 3(a)). When the GO amount is increased to 0.057 gL −1 (Fig. 2c), the nanowires disappear completely and 500-nm wide exhibit nanosheet structures are formed. With a further increase in the GO to 0.086 gL −1 (Fig. 3(b)), the nanosheet structure morphology is retained. Figure 3(c) presents the formation diagram of the BiVO 4 nanowires and the BiVO 4 -rGO nanosheets process. Initially, the nanowire-like BiVO 4 is obtained on a Ti fabric under hydrothermal condition. When the rGO is incorporated, a portion of the nanowires are covered by nanosheets. As the rGO amount is further increased, more nanosheets are formed on the nanowires. In the detailed structure of the nanosheets, the rGO nanosheets are covered by BiVO 4 , i.e., a sandwich structure is formed with rGO in the central part.
The surface chemical composition and the chemical states of BiVO 4 and BiVO 4 -0.057 were analyzed by X-ray photoelectron spectroscopy(XPS) (Fig. 4a). Seven obvious peaks corresponding to Bi5d, Bi4f, C1s, Bi4d 5/2 , Bi4d 3/2 , O1s and V2p 3/2 are detected in both samples.  and 159.2 eV (Bi 4f 5/2 ) are detected (Fig. 4b), which confirm that the Bi species exist as Bi 3+47-51 . The signal of V 2p 1/2 and V 2p 3/2 is located at 524.8 and 516.8 eV, respectively (Fig. 4c), indicating that the V species are in the state of V 5+ 52, 53 . Thus, the electron couples of Bi 3+ and V 5+ coexist in the orthorhombic BiVO 4 structures, where the total atomic ratio of the Bi and V elements is about 1:1, corresponding to the molecular formula of BiVO 4 . In the high resolution spectrum of C 1 s (Fig. 4d), carbons in the form of sp 2 bonds (284.6 eV) are dominated and oxygen-containing functional group is also observed at 288.6 eV (C = O), which may represent the absorption of atmospheric CO 2 54, 55 . The normalized temporal concentration changes (C/C 0 ) of MO during the photocatalytic process are proportional to the normalized maximum absorbance (A/A 0 ), which can be derived from the change of the MO absorption profile at a given time interval. Figure 5a shows that the adsorption-desorption equilibrium attained in 210 minutes and the adsorption capacity were 7.30% and 5.10% of the MO for BiVO 4 -rGO nanocomposite arrays (black) and the BiVO 4 nanowire arrays (red), respectively. As can be seen in Fig. 5b, the degradation of the MO solution exhibited a small decrease without the photocatalyst under visible-light irradiation, this decrease was 9.4% for 150 min of irradiation, indicating that the MO was stable. The photocatalytic performance of the BiVO 4 -rGO composites is dependent on the proportion of rGO in the composite. Under simulated sunlight irradiation for 150 min, pure rGO and BiVO 4 exhibit 59.50% and 57.55% degradation efficiency for MO, respectively. When rGO is introduced into BiVO 4 , the removal rate is increased to 85.03% for BiVO 4 -0.029, and reaches a maximum value of 98.95% for BiVO 4 -0.057, while the removal rate is 89.37% and 88% for BiVO 4 -0.086 and BiVO 4 -0.114, respectively. The BiVO 4 -rGO composites exhibit a slightly lower activity, which is still significantly higher than that of the pure BiVO 4 sample. It is known that during photocatalysis, the light absorption and the charge transportation and separation are crucial factors; these energy levels are beneficial for the transfer of photo-induced electrons from the BiVO 4 conduction band to the rGO, which can efficiently separate the photo-induced electrons efficiently and hinder the charge recombination in the electron-transfer processes 56 , thus enhancing the photocatalytic performance. However, when the rGO content is further increased above its optimum value, the photocatalytic performance deteriorates. This is ascribed to the following reasons: (i) rGO may absorb some visible light and thus cause a light harvesting competition between BiVO 4 and rGO with the increase of the rGO content, which leads to the decrease in the photocatalytic performance 57,58 ; (ii) the excessive rGO can act as a kind of recombination center instead of providing an electron pathway and promoting the recombination of electron-hole pairs in the rGO 59 .
UV-vis spectra are used to characterize the optical properties of the samples (Fig. 6a). According to the spectra, all the samples express absorbance in the visible regions. The pure BiVO 4 exhibits an absorption edge at   In addition, the photoluminescence (PL) spectrum is regarded as a significant emission signal of carrier recombination. The transfer property of the photogenerated carriers (electron-hole pairs) can be evaluated by this method. Usually, a weaker PL intensity indicates a stronger ability for the separation of photo-generated carriers 62 . Figure 6c shows the PL emission spectra of pure BiVO 4 and the BiVO 4 -0.057 photocatalysts monitored at an excitation wavelength of 320 nm. The peak at ~535 nm corresponds to the recombination of the hole formed in the O 2p band and the electron in the V 3d band 63 , corresponding to the near band edge emission (NBE) of BiVO 4 64 . A decrease in PL intensity is clearly evident for the BiVO 4 -0.057 due to the effective separation of electron-hole pairs. The photo-generated electrons in the excited BiVO 4 are transferred to the rGO nanosheets immediately after the photo-production, separating the photo-generated electrons and holes and inhibiting their recombination (Fig. 6d). This may be the reason that the BiVO 4 -rGO sample exhibits an enhanced photocatalytic efficiency under visible light irradiation. Figure 6e illustrates the photocurrent responses of the BiVO 4 -rGO as photoelectrode under intermittent illumination by simulated sunlight and compared with that of the bare BiVO 4 nanowire arrays. The photocurrent density is much higher for the BiVO 4 -rGO nanocomposite arrays than for the BiVO 4 nanowire arrays, suggesting that the charge carriers that are photogenerated for the BiVO 4 -rGO persist than those for the BiVO 4 nanowire arrays. This is not surprising because the photo-responsive rGO contributes to the photocurrent. Further, the rGO possesses an enhanced charge mobility compared with the BiVO 4 nanowire arrays. Figure 6f shows the results of five successive runs for the photo-degradation of MO for the BiVO 4 -rGO composite photocatalyst under the same experimental conditions. There is no apparent loss of photoactivity after six consecutive photo-degradation cycles. Therefore, the BiVO 4 -rGO composites possess excellent stability and are not prone to be suffer from photo-corrosion during the degradation process.

Conclusions
In summary, a novel BiVO 4 -rGO photocatalyst was successfully synthesized via a simple one-step hydrothermal method. Based on the narrow band gap (2.34 eV) and the relatively low PL intensity, the added rGO can effectively suppress the complex of light-generated electron-hole and increase the separation efficiency of photon-generated carrier, thereby enhancing the catalytic activity of the composite photocatalyst. The synthesized composite photocatalysts showed much higher photocatalytic activity than that of pure BiVO 4 with regarding to MO degradation under visible light. The present recoverable BiVO 4 -rGO composite photocatalysts can be regards as one of the ideal photocatalysts for the various potential applications.

Experimental Section
Synthesis of a uniform BiVO 4 nanowires. All reagents were of analytical grade and used as received without further purification. In a typical procedure, NH 4 VO 3 , oxalic acid, hexamethylenetetramine and Bi (NO 3 ) 3 5H 2 O (the molar ratio 30:60:6:1) were dissolved into deionized water under ultrasonication for 1 h at room temperature. The dark blue mixture solution and a piece of pretreated Ti fabric, which has been rinsed with pure ethanol and deionized water for 1 h, were transferred into an autoclave, and then kept at 150 °C for 1 h in an oven. Finally, after cooling to room temperature, the Ti fabric with the as-prepared samples was rinsed with deionized water and dried at 80 °C for 12 h.

Synthesis of a BiVO 4 -rGO nanocomposite photocatalysts.
First of all, graphene oxide (GO) was prepared by a modified Hummer's method 30 , graphene oxide (0.057 g) was sonicated in 100 mL water for 10 min. The nanocomposites were prepared by mixing the prepared graphene oxide suspension into the solution with NH 4 VO 3 , oxalic acid, hexamethylenetetramine and Bi (NO 3 ) 3 5H 2 O (the molar ratio 30:60:6:1), which was followed by vigorous magnetic stirring at room temperature for 1 h. Finally, the resulting mixture and a piece of pretreated Ti foil were then transferred to an autoclave and kept at 150 °C for 1 h. The final products were collected by centrifugation, and washed with deionized water and ethanol for three times, before drying at 80 °C for 12 h. Photocatalytic Activity Measurements. Photocatalytic activities of the samples were evaluated by the degradation of methyl orange (MO) solution under simulated sunlight (λ ≥ 420 nm) in a homemade reactor with a cooling water circulator assembled to keep the reactor at a constant temperature. Experiments were performed at ambient temperature as follows: BiVO 4 or BiVO 4 -rGO nanocomposites (2 cm × 2 cm) grown on Ti fabric catalyst was added into 50 mL of 10 mg/L methyl orange (MO) solution. Before illumination, the solution was stirred for 30 min in the dark in order to reach the adsorption-desorption equilibrium for MO and dissolved oxygen. A 300 W xenon lamp with a 420 nm cutoff filter to remove any irradiation below 420 nm was used as the visible light source to trigger the photocatalytic reaction. The concentrations of the MO were monitored using a UV-2003 UV-vis spectrophotometer by checking the absorbance at 464 nm during the photodegradation process. A sample in approximately 2 mL was taken at the designed time interval during irradiation for chromatographic analysis.
The photocurrent measurements had been taken on a electrochemical working station (CHI-660C, China). The active area of the specimen was 2 × 2 cm 2 and the supporting electrolyte was 0.25 M Na 2 SO 4 aqueous solution. A 300 W Xe-lamp was used to provide the simulated sunlight.
Characterization. The samples were characterized with X-ray diffraction (XRD; Bruker D8 X-ray diffractometer). The morphology of the sample was investigated by a field-emission scanning electron microscope (FE-SEM; Hitachi S-4800) and a transmission electron microscope (TEM; JEOL-2100F at 200 kV). Energy Dispersive Spectroscopy (EDS) were used to determine morphology and elemental composition of the sample in the TEM. Raman spectra of GO, BiVO 4 , and BiVO 4 -rGO were recorded using a Raman spectroscope (JY-HR800, the excitation wavelength of 633 nm). UV-vis diffuse reflectance spectra (DRS) of the as-prepared samples were obtained using a Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere using BaSO 4 as the reflectance standard. The chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS) using KAlpha 1063 (Thermo Fisher Scientific, UK). The photoluminescence (PL) spectral measurements were carried out on a Hitachi F-2500 fluorescence spectrophotometer with a Xe lamp as the light source.