BiVO4(010)/rGO Nanocomposite and Its Photocatalysis Application
By interfacial coupling effect, the reduced graphene oxide (rGO) was constructed on (010) facet of BiVO4, which is a characteristic of the dominant separation of photogenerated electrons. The prepared samples were characterized by X-ray diffractometry and Raman spectra, which indicating that the added rGO plays an important role in regulating of V–O bond. By the X-ray photoelectron spectroscopy, it was found that the valence band value of BGO180 is increased compared with BiVO4, suggesting that there is an effective coupling present between rGO and BiVO4. Under simulated sunlight, the photocatalytic properties of BiVO4/rGO nanocomposite are evaluated by the O2 evolution. The carrier transfer effect between BiVO4 and newly formed coupling layer (BiVO4 and rGO) explains the excellent photoactivity of nanocomposite.
KeywordsBiVO4 rGO Photocatalytic activity
The increasingly energy crisis and environmental pollution are becoming the topic issues in the sustainable development of human society [1, 2, 3, 4, 5, 6, 7]. Hence, it is of great importance to eliminate organic dyes from aquatic circumstances but no easy solution available for this problem and this challenge is still persisted. Recently, one of the most promising technologies, photocatalysis has been widely used in environmental purification and provides an effective method for converting dyes into harmless compounds . Generally, low charge separation efficiency is a major hindered in practical applications of photocatalysts. One of effective strategies for efficiently promoting charge separation in semiconductor (SC) based photocatalysts is to construct reduced graphene oxide (rGO)/SC photocatalysts (e.g. ZnO/rGO [9, 10, 11], ZnxCd1−xS/rGO , (Ru/SrTiO3:Rh)/(photoreduced graphene oxide(PRGO)/BiVO4) , BiVO4/rGO ) due to mobility of rGO in excess of 2 × 105 cm2 V−1 s−1 . The high photocatalytic activity of rGO/SC under illumination irradiation can be explained as follows, first of all electrons are excited from the valence band (VB) to the conduction band (CB), thereby creating holes in the VB, secondly low carrier recombination rate. Normally, the charge carriers recombine rapidly, resulting in low photocatalytic activity. While SC nanoparticles are immobilized on the surface of rGO, those photogenerated electrons in CB of SC nanoparticles could transfer to rGO thereby improving the separation efficiency of hole–electron pair . That is to say, the added rGO could play a role of electron collector and transporter to prolong the lifetime of the charge carriers, thus inducing improved charge separation and photocatalytic activity [7, 17]. Indeed, this view has been widely accepted and demonstrated in many works [18, 19] and is a driving force to obtain novel structures, such as Cu2O/Pd/rGO , α-Fe2O3/Graphene/BiV1−xMoxO4 core/shell heterojunction  and g-C3N4/BiVO4 . As reported, numerous rGO/SC architectures were prepared by hydrothermal or solvothermal processes. Most of attentions were focused on preparations of new systems with superior properties. Sometimes the role of rGO is still controversial in enhancing photocatalytic activity of SC [23, 24]. In previous work, Gan et al. demonstrated that rGO plays an important role in enhanced photocatalytic performance that reveals the photothermal characteristics of graphene-based nanocomposite . Recently, several groups have reported some excellent works about bismuth vanadate (BiVO4) [26, 27, 28, 29], because of its appropriate band gap and favorably positioned band edges [30, 31, 32, 33, 34, 35, 36]. It was also reported that the (010) facet of BiVO4 was undergone reduction reaction . On this framework, we attempted to obtain a coupling between the (010) facet of BiVO4 and rGO to improve the photocatalytic activity of BiVO4.
This paper describes the synthesis of BiVO4/rGO nanocomposite by hydrothermal process and subsequent post-treatment that is featured by thin rGO sheets covering exposed BiVO4 (010) facets and demonstrating well interfacial coupling between rGO and BiVO4, increased charge separation efficiency and resultant positive effects on photocatalytic activities.
2.1 Preparation of rGO/BiVO4 Nanocrystals
Briefly, Bi(NO3)3·5H2O (2.5 mmol) was dissolved in 5 mL nitric acid, and added the certain amount of rGO solution (ultrasonic for 10 min), after stirred for 30 min get the solution (A) Secondly, 2.5 mmol NH4VO3 was dissolved in 5 mL NaOH solution (2 mol L−1) under stirring for 30 min to form solution (B) Thirdly, solution B was added drop wise into solution A and formed a yellow mixture suspension. The pH of the prepared mixture suspension was adjusted to 6 by slowly adding 2 mol L−1 NaOH solution. The final mixture was then sealed in a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was heated and maintained at 180 °C for 12 h, and cooled to room temperature. Powder was collected after centrifugation, washed with distilled water repeatedly, and then dried in a vacuum oven. According to the adding content of rGO (0, 0.0090, 0.0135, 0.0180, 0.0270 g), the samples were named as BGO0, GBO90, BGO135, BGO180, BGO270, respectively.
The crystal structures of various photocatalysts were determined by X-ray diffraction (XRD; Model D/MAX-3B, Tokyo, Japan) excited by Cu Kα radiation, a sampling interval of 0.02°, and a scan speed of 4° min−1. Their morphology and crystalline structure were also performed by transmission electron microscopy (TEM, JEM2010) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) performed by means of a G3 F20 (FEI Tecnai) with operating voltage of 300 kV. Using BaSO4 as a reference, the UV–Vis diffuse reflectance spectra (DRS) were recorded on a Shimadzu UV-2401PC UV/Vis scanning spectrophotometer equipped with an integrating sphere. The fluorescence spectrophotometer (Shimadzu, model RF-5301 PC) was used to analyze the effective separation of photogenerated electron–hole pairs. Total organic carbon (TOC) was measured using a TOC analyzer (TOC-VCPN). X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Escalab 250 system with a monochromatic Al Kα source and a charge neutralizer. Raman spectra were collected by using LabRam HR 800 using 532 nm excitations of laser at room temperature.
2.3 Photocatalytic Reactions
The photocatalytic activities of the samples were evaluated for the photocatalytic oxygen evolution reaction from water in an AgNO3 aqueous solution (0.02 M). 100 mg as-prepared powder was dispersed in 300 mL aqueous solution. The reaction temperature was maintained around 20 °C to prevent any significant evaporation of the solvent. Then, the lamp (300 W Xe lamp) was switched on, and the effluent gases were analyzed to quantify O2 production by gas chromatography (Agilent Technologies, 6890N). Photocatalytic experiments were also performed with MB under ambient conditions. In a typical procedure, 0.1 g of as-prepared photocatalyst was added into 100 mL of MB (15 mg L−1) aqueous solution. The mixture was maintained under dark conditions for 60 min to achieve the adsorption–desorption equilibrium between the samples and the reactant prior to the illumination. Afterwards, the suspension was irradiated with a 500 W Xe arc lamp. The degree of photodegradation was monitored by checking the absorption spectrum of each sample through its specific wavelength (664 nm) using an UV–Vis spectrophotometer (UV757CRT).
3 Results and Discussion
XRD patterns were recorded for the BiVO4/rGO powders to confirm the crystallographic phase of msBiVO4 in the composite and investigate the influence of rGO on the position of BiVO4 diffraction peak. Figure 1a shows that the XRD patterns of BiVO4/rGO composites synthesized with rGO compared to BiVO4. The main diffraction peaks (110), (121), and (040) correspond to the planes of msBiVO4 (JCPDS 14-0688). The splitting diffraction peaks of msBiVO4 appear at 2θ = 18.58, 35.8 and 46.8 , which are related to distorted BiO8 dodecahedron in msBiVO4 . After introducing rGO, it was found that the XRD peaks of BiVO4/rGO samples shift higher diffraction angle. On the basis of Eq. 2, it is probably that the C element enters into lattice of BiVO4 under high temperature and pressure.
Compare to one of Bi3+ (103 pm), V5+ (54 pm) and O2− (140 pm), the C (+ 4, 16 pm) has a smaller radius. It is possible to result into lattice constant of BiVO4 becoming smaller, consequently those diffraction peaks shifting to higher diffraction angle. Thus, with increasing the amount of rGO, their diffraction peaks (121, 040) gradually shift higher diffraction angle (Fig. 1b, c). In the Fig. 1d, it can be seen that the msBiVO4 includes VO4 tetrahedron. As we known, the sp3 hybridization of C element makes it center of tetrahedron, which coincides the role of V5+ in VO4 tetrahedron. Of course, the doping induces a changing of BiVO4 crystal symmetry and introduces some polarons , benefiting for higher charge carrier concentration.
The typical Raman spectra of BiVO4 and BiVO4/rGO (BGO180) are presented in the Fig. 2. The dominated peak at 820 cm−1 is assigned to the symmetric V–O stretching mode. However the inconspicuous peak at 708 cm−1 is assigned to the antisymmetric V–O stretching mode. It is obvious that the main peak located at 820 cm−1 of BGO180 show a slight shift towards to low wavenumber compared with BiVO4. Generally, the atom with small radius and low quality substituting for atom with large radius and high quality would lead to an increasing of Raman wavenumber. However, the abnormal shift was observed as rGO being adding into BiVO4 system. A possible reason is that the effective C doping is only comes from the edge or part of the added rGO. The doped C atom belongs to the added rGO nanosheet, which resulting into the abnormal Raman shift. Peaks centered at 362 and 323 cm−1 are attributed to the typical symmetric and antisymmetric bending modes of the vanadate anion, respectively. The peaks at 130 and 206 cm−1 are external modes. The peaks at 820, 708, 362, 323, 206 and 130 cm−1 for BiVO4 are in agreement with previous report .As for the BGO180, besides the distinctive peaks assigned to BiVO4, the G and D bands of rGO are located at 1540 and 1354 cm−1, respectively, indicating existing of the added rGO.
To further highlight this effect of adding rGO, we used UV–Vis DRS to characterize the electronic states of BiVO4 and BiVO4/rGO samples. Here, a comparison of the UV–Vis DRS of samples is displayed in Fig. 3a. It was observed that there was a marked increase in the absorbance in the visible region (530 nm) for BiVO4/rGO. With the increasing of rGO content, the absorbance ability of BiVO4/rGO increases. It is obvious that the BiVO4 exhibits stronger absorbance ability when a certain amount of rGO was introduced. As shown in Fig. 3, the energy gap (Eg) values for the BiVO4 and BiVO4/rGO samples were calculated from the UV–Vis spectra by the Tauc formula .
Here, MB was firstly adopted as a probe dye to evaluate the photocatalytic activity of as-prepared BiVO4 and BiVO4/rGO composites under simulated sunlight irradiation. Figure 4a shows the discoloration of MB dye as a function of irradiation time. Without a photocatalyst, it can be seen clearly that photocatalysis of MB itself is negligible after an exposure time of 360 min. When using pure BiVO4 as the photocatalyst, only 25% of MB is discolored in 360 min. However, the discoloration rates of MB solution runs up to 80% for BGO180, indicating much higher photocatalytic discoloration than pure BiVO4 sample. In addition, further increase in the rGO content led to deterioration of the catalytic performance. It can be attributed to the excess rGO leading to shielding of the active sites on the catalyst surface and also rapidly decreased the intensity of light through the depth of the reaction solution . The results are well in accordance with the PL analysis (see following Fig. 5). As a consequence, suitable rGO content is crucial for optimizing the photocatalytic activity of BiVO4/rGO. The further comparing reaction kinetics of the photo-degradation process of MB was carried out by the Langmuir–Hinshelwood (L–H) kinetics model, as shown in the following equation.
The PL spectra of BiVO4/rGO and BiVO4 samples are illustrated in Fig. 5. The origin of the maximum intensity peak about 530 nm is the recombination from Bi3+ (6p) to O2− (2p) and represents the band gap of BiVO4. By the comparison, a significant decrease of the PL intensity was observed for the BiVO4/rGO samples compared with BiVO4, which was considered from the suppressing the recombination process of electron and hole . It was also noticed that the emission from BiVO4 is located at 530 nm. However, the emission from BiVO4/rGO samples appears in the form of a broad band between 370 and 520 nm. In the graphene/BiOCl  composites, an abnormal blue shift of PL peak of coupled samples does not be discussed compared to pure sample. In the case of Zhuo et al. prepared GQDs with upconverted emission is reported . Therefore, this blue shift of PL peaks may originate from the upconverted emission of added rGO in the BiVO4/rGO.
As shown in Fig. 6a, the C 1s peak was located at 284.6 eV in the BiVO4 sample. It can be seen that there is a symmetrical characteristic, indicating this peak is arising from adventitious carbon. In Fig. 5b, it was found that the wider C 1s peak was fitted with four peaks at binding energies of 282.2, 284.6, 286.3 and 288.4 eV, respectively, where the major peak at 284.6 eV was assigned to surface adventitious carbon, the peak centered at 288.2 eV was identified as sp2-bonded carbon (C=O) coordination, while peak at 286.3 eV was attributed to the C–O bond . Interestingly, an additional shoulder-peak located at 282.2 eV was observed, which was usually assigned to a carburet such as TiC. This binding energy region refers to the presence of a M–C bond in the as-prepared GR/BiOCl .
The VB maximum (VBM) energy positions in the BiVO4/rGO and BiVO4 were determined to be 1.64 and 1.38 eV, respectively (Fig. 6d). It indicates that the valence band of BiVO4 is regulated by the addition of rGO, which has an important effect on carrier transport.
Figure 7 shows the TEM images of BGO0 and BGO180. It can be seen that the TEM and HAADF-STEM images of pure BiVO4 show characteristic of plate-like morphology (Fig. 7a, b) as reported case . HRTEM image in the inset of Fig. 7a was also presented. It can be seen that interlayer distance of 2.6 Å, corresponding to the (200) facet of BiVO4. The BGO180 also shows similar plate-like morphology as BiVO4 (Fig. 7c). Here, the included angle between dot line and solid line is 66°, this is corresponding to the included angle between (010) and (110) facet. It was observed that the rGO was loaded on the (010) facet of plate-like BiVO4. Thus, these results demonstrated that BGO180 composite was successfully built between BiVO4(010) and rGO. Coincidentally, it was considered that the reduction reaction with photogenerated electrons could take place on the (010) facet of BiVO4 [54, 55]. So, the coupling of rGO on the (010) facet could enormously enhance the separation of photogenerated electrons in BiVO4. Additionally, before the TEM observing, the samples undergo ultrasonic dispersion for ten minutes. This further suggests there is a strong coupling between (010) facet of BiVO4 and added rGO.
Figure 8a shows the evolution of O2 over the as-synthesized samples with different samples. It can be found that the added rGO exhibited a significant influence on the photocatalytic activity of BiVO4. BGO0 just presented weak O2 evolution, indicating the poor oxidizing power of the photogenerated holes in BiVO4. Under the same conditions, there was higher O2 evolution being detected for the BGO180 (292.3 µmol after 8 h), indicating the photocatalytic performance of BGO180 was enhanced compared to BGO0. This rapid increase in the O2 evolution activity should partly originate from selective separation of the electron by coupled rGO (it can be seen in Fig. 9). It demonstrated that the presence of rGO on (010) facet of BiVO4 can be advantageous in increasing electron–hole separation by allowing rapid electron extraction from the (010) facet of BiVO4. Here, the O2 evolution reaction for sample is as follow .
For the BGO0 and BGO180 samples, the EIS spectra were presented in the form of a Nyquist diagram as shown Nyquist plots in Fig. 8b. As we known, the water splitting processes occur at the interface between photoanodes and electrolytes. And these processes can be interpreted by simple Randles–Ershler (R–E) circuit model [57, 58] where the RS is solution resistance, Rct is charge transfer resistance and CPE is double layer capacitance . These data revealed that the charge transfer resistance on the photoanode surface was 24.1, 12.4 for the BGO0 and BGO180, respectively. It is obvious that the smaller arc of BGO180 represents more favorable carrier transport than that of BGO0, revealing that the BGO180 sample has a lower charge transfer resistance compared with BGO0, thus effectively improving O2 evolution reaction rate.
As a result, the energy band diagram of the charge-separation process in photocatalysis was proposed (Fig. 9b). By the interface electric field effect , it was inferred that the photogenerated electrons in matrix have a flowing direction from matrix to CL. Moreover, rGO can serves as an electron acceptor for BiVO4, and effectively decrease the recombination probability of the photogenerated electron–hole pairs. In the case of Bolotin and co-workers, the carrier mean free path of graphene approaches to ~ 1.2 µm . The long diffusing distance can facilitate the efficient separation of photogenerated electrons. In the work of He et al. the carrier diffusion length of BiVO4 was determined to be ~ 70 nm . In Fig. 7b, it was observed that the typical thickness of BiVO4 is about 400 nm. Due to the electrons could diffuse along opposite direction from the b axis, the overall diffusing distance approximates to ~ 140 nm for BiVO4. Overall, it is obvious that the added rGO could suppress the charge recombination and facilitate the separation efficiency of e−–h+ pairs. Of course, by adjusting the thickness, an improved photocatalytic activity of BiVO4 can be achieved in the future work.
In summary, we have successfully constructed BiVO4/rGO composite photocatalysts with excellent photocatalytic activities under irradiation of simulated sunlight. Morphology characterizations reveal that the rGO covers on the (010) facet of BiVO4. As a consequence, the advantages of BiVO4/rGO system are promoting the transfer of photogenerated electrons generated in the (010) facet of BiVO4. Besides, the interface electric field between BiVO4 and CL plays a key role on the transport of photogenerated carrier. The VB difference among them could facilitate the efficient separation of photogenerated electrons and holes. This work may provide an effective approach for design of high performance photocatalysts and some new understandings for the photocatalytic mechanism of rGO based photocatalysts.
The authors gratefully acknowledge financial supports from the Education Department Program of Heilongjiang Province (12541111) and Postdoctoral Scientific Developmental Fund of Heilongjiang Province (LBH-Q16122).
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no competing financial interest.
- 8.M. Yan, Y.L. Wu, Y. Yan, X. Yan, F.F. Zhu, Y.Q. Hua, W.D. Shi, ACS Sustain. Chem. Eng. 4, 757–766 (2015)Google Scholar
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