Application of BiVO 4 Nanocomposite for Photodegradation of Methyl Orange †

In this work, BiVO4–graphene photocatalyst was prepared by a facile one-step hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The photocatalytic activity of this compound was investigated by destruction of methyl orange (MO) under visible light irradiation. The photodegradation results show that the prepared BVG compound has higher photocatalytic activity than the pure BiVO4 compound. This compound can degrade 98% of MO under visible light irradiation. This work indicates that BiVO4 compounds are excellent compounds for pollutant degradation under visible light irradiation.


Introduction
The development of semiconductor photocatalysts for the degradation of organic pollutants under visible light irradiation is a challenging and necessary topic.In recent years, semiconductor photocatalysts have attracted great attention due to their potential applications in water splitting and organic pollutant photodegradation [1].To date, TiO2 as a useful material for many photocatalytic reactions has been intensively investigated.However, its application is limited by its wide band gap (3.2 eV) which requires ultraviolet irradiation for photocatalytic activation [2,3].Therefore, developing alternative materials that are capable of using solar energy has become one of the most challenging topics today.Great efforts have been exerted to develop visible-light-responsive photocatalysts for the degradation of dyes.Various semiconductor photocatalysts, such as WO3, ZnO, BiVO4, and CdS, have been investigated for the photocatalytic degradation of water pollutants under visible light irradiation [4].
Among the photocatalysts investigated in these works, BiVO4 is an attractive semiconductor with three crystalline phases: tetragonal zircon-type (z-t), tetragonal sheelite-type (s-t), and monoclinic sheelite-type (s-m).Monoclinic BiVO4 is generally considered to be more active than the other phases [5].BiVO4 is nontoxic and chemically stable in aqueous solution, and the narrow band gap (2.4 eV) of BiVO4 can provide effective utilization of the visible light region [6].
Graphene materials have shown great potential as catalysts in the photocatalysis process in recent years.Its unique properties such as 2D structure, high mobility of charge carriers, theoretically large surface area (~2600 m 2 /g), etc., have made graphene as a prospective candidate for the design of photocatalytically active composites [7,8].
In this work, we synthesized monoclinically structured BiVO4 and BiVO4-graphene compound.The photocatalytic activities of the prepared samples were evaluated by methyl orange decomposition under visible irradiation.

Synthesis of Bismuth Vanadate
Bi(NO3)3•5H2O (1 mmol, 0.485 g) was dissolved in 20 mL of 1 M HNO3 by stirring for 30 min at room temperature.An alkaline solution of NH4VO3 was prepared by dissolving 0.117 g (1 mmol) of NH4VO3 in 20 mL of 0.5 M NaOH under stirring for 30 min at room temperature.The NH4VO4 solution was added dropwise to the Bi(NO3)3 solution with stirring to get a clear solution.The resultant solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, sealed, and heated at 120 °C for 24 h under autogenous pressure.The obtained samples were filtered and washed with deionized distilled water followed by absolute ethanol.Finally, the obtained solid samples were dried in a hot air oven [9].

Preparation of the BiVO4-Graphene Nanocomposite
The prepared solution was named A. A facile one-step solvothermal method was used to synthesize the BiVO4-graphene nanocomposite by dissolving 0.5 wt % of graphene.Then, 0.485 g of the prepared BiVO4 powder was added into solution A. The suspensions were then transferred into a 100 mL Teflon-sealed autoclave and kept at 120 °C for 24 h.The resulting products were recovered by centrifugation, rinsed with DI water and ethanol, and dried at 60 °C for 6 h in a hot air oven.

X-ray Powder Diffraction
The XRD pattern of the prepared BiVO4-graphene nanocomposite is shown in Figure 1.The characteristic peak of graphene is at 2θ = 25°.In the XRD pattern of the BiVO4-graphene photocatalyst, the broad peak of graphene is observed at 2θ = 25°.Due to the high crystallinity of BiVO4 and, subsequently, its sharp peaks, the intensity of the graphene peak is low.Due to the high purity of the synthesized samples, no impurity peaks are observed in the X-ray diffraction pattern of this compound.

DRS Analysis of the Prepared Sample
The DRS analysis of the prepared sample is shown in Figure 2. Based on the DRS analysis, the band gap of BiVO4 was calculated as 2.47 eV.Graphene with its high charge mobility can decrease the recombination parameter and decrease the band gap, consequently increasing the photocatalytic activity.

Morphological Characterization
A SEM image of the BiVO4-graphene sample is shown in Figure 3.The sheet-like morphology of the BiVO4-graphene sample is obvious in the SEM image of this compound.Due to the sheet-like morphology of the graphene plates, the BiVO4 plates decorated on the graphene sheets arrange in the sheet-like morphology.

IR Spectrum of the Prepared Sample
The IR spectrum of BiVO4-graphene is shown in Figure 4.In this IR spectrum, the strong and broad peak at 1261 cm −1 is in response to the BiVO4 vibrational system.

Photocatalytic Activity of the Prepared Samples
The photocatalytic activity of BiVO4 and BiVO4-graphene samples was evaluated by the degradation of methyl orange in aqueous solution (10 ppm) under visible light irradiation.As shown in Figure 5, under visible light irradiation, 69% of the methyl orange dye was destroyed in the presence of the BiVO4 sample after 240 min, and 98% of the methyl orange dye was destroyed in the presence of the BiVO4-graphene sample after 180 min.The results showed that the BiVO4-graphene nanocomposite has higher photocatalytic activity than pure BiVO4.
In the BiVO4-graphene nanocomposite, electron-hole pairs are generated after excitation of BiVO4 with visible light irradiation.Due to the low band gap of BiVO4, this low-power visible light is sufficient to excite the BiVO4 photocatalyst.One of the important parameters of photocatalysis reactions is the recombination process.This especially happens in the low-band-gap semiconductors.The recombination process decreases the efficiency of the photocatalytic reactions.In a BiVO4graphene nanocomposite, due to the presence of graphene, the prepared electron-hole pairs move to graphene and reach the photocatalyst surface.Therefore, the recombination process is reduced in the photocatalytic reaction of this nanocomposite.
The high charge conductivity of graphene increases the migration of charges to the photocatalyst surface.The synergistic effect between graphene and BiVO4, proper adjustment between the valence and conduction bands of graphene and BiVO4, and proper composition between these two components yield the high photocatalytic activity of the prepared BiVO4-graphene nanocomposite.

Conclusions
In this work we prepared BiVO4 and BiVO4-graphene via a simple hydrothermal method.The photocatalytic activity of the prepared BiVO4 is low due to the low band gap of this compound.The occurrence of the recombination process is high in BiVO4.The photodegradation results show that the composition of graphene and BiVO4 effectively increases the photocatalytic activity of the prepared photocatalyst.Thus, the recombination process of BiVO4 decreases after composition with graphene.

Figure 5 .
Figure 5.The photodegradation results of methyl orange (MO) over (a) BiVO4 and (b) BiVO4graphene nanocomposite under visible light irradiation.