Preparation of BiVO 4-Graphene Nanocomposites and Their Photocatalytic Activity

We prepared BiVO 4 -graphene nanocomposites by using a facile single-step method and characterized the material by xray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, ultraviolet-visible diffuse-reflection spectroscopy, and three-dimensional fluorescence spectroscopy.The results show that graphene oxide in the catalyst was thoroughly reduced. The BiVO 4 is densely dispersed on the graphene sheets, which facilitates the transport of electrons photogenerated in BiVO 4 , thereby leading to an efficient separation of photogenerated carriers in the coupled graphene-nanocomposite system. For degradation of rhodamine B dye under visible-light irradiation, the photocatalytic activity of the synthesized nanocomposites was over ∼20% faster than for pure BiVO 4 catalyst. To study the contribution of electrons and holes in the degradation reaction, silver nitrate and potassium sodium tartrate were added to the BiVO 4 -graphene photocatalytic reaction system as electron-trapping agent and hole-trapping agent, respectively. The results show that holes play the main role in the degradation of rhodamine B.


Introduction
In recent years, nanometer photocatalysis technology has attracted widespread interest because of its potential applications in water splitting and environmental remediation [1,2].To prepare high-activity photocatalysts, researchers have made every effort to prepare new types of photocatalysts such as TiO 2 [3], Bi 2 WO 6 [4], ZnO [5], and BiVO 4 [6].To date, from among the various photocatalysts, TiO 2 has been investigated most extensively because it is nontoxic, chemically inert, and photostable.However, its application is limited by its wide band gap (3.2 eV), which means that ultraviolet (UV) irradiation is required to activate photocatalysis.This is problematic because UV accounts for only about 4% of the entire terrestrial solar spectrum, whereas visible light accounts for about 45% of this spectrum [7,8].Therefore, much research has recently gone into developing photocatalysts that can be activated by visible light.
For many years, BiVO 4 was widely used to produce yellow paint.However, recent studies have found that monoclinic BiVO 4 , which has a band gap of 2.4 eV, is an ideal visiblelight photocatalytic material for recycling polluted water.In addition, it also has the advantages of low cost, environmental friendliness, and high stability against photocorrosion [9,10].However, the conduction band of BiVO 4 is just slightly above 0 eV, which would make it hard for O 2 to capture electrons photoexcited by visible light [11].In addition, in the photocatalytic reaction, the oxidation-reduction reaction competes with electron-hole recombination [12].Eventually, the electron-hole recombination rate increases.Finally, the low photocatalytic activity of pure BiVO 4 restricts its wide application for photocatalytic degradation of organic contaminants [13][14][15][16].
As a new type of two-dimensional carbon material, graphene possesses a number of excellent intrinsic properties.For example, its band gap is almost zero and it has high electrical conductivity, high specific surface area (2630 m 2 /g), and high carrier mobility (200 000 cm 2 /V) [17][18][19][20][21].
In searching for new strategies to promote the photoactivity of semiconductors, graphene-based nanocomposites have recently emerged as the most prominent candidates.Notably, the abundance of delocalized electrons in the -conjugated electronic structure of graphene endows it with outstanding electronic conductivity.In coupled graphene-nanocomposite systems, this high conductivity facilitates the transport of charges photogenerated in the attached semiconductors, thereby leading to efficient separation of photogenerated charge carriers [14,22,23].
Graphene oxide is an important derivative of graphene.Its structure and properties are similar to those of graphene and it is easily produced and commonly used as a precursor for grapheme [13][14][15]24].
Inspired by these concepts, we report herein the design and synthesis of graphene-based nanocomposites, using BiVO 4 -graphene nanocomposites as an example, with the goal of achieving highly efficient photocatalytic properties driven by visible light.

Synthesis of BiVO 4 .
Typically, the synthesis of BiVO 4 proceeds as follows: 4.8 mmol hexadecyl trimethyl ammonium bromide and 1.6 mmol Bi(NO 3 ) 3 ⋅5H 2 O are added in that order to distilled water (40 mL).Next, 1.6 mmol Na 3 VO 4 ⋅12H 2 O in 40 mL distilled water is added to the above solution.After vigorous stirring for 10 min, the mixture is transferred into a 50 mL Teflon-lined autoclave and then sealed and heated at 120 ∘ C for 10 h.The system is allowed to naturally cool down to room temperature.The final product was collected by centrifuging the mixture.It was then washed with distilled water six times and then dried under vacuum overnight at 60 ∘ C for 12 h.

Synthesis of BiVO 4 -Graphene.
Graphene oxide was synthesized according to the modified Hummers method [13,24].To synthesize BiVO 4 -graphene, 200 mg BiVO 4 , 2 mg of graphene sheets, and 60 mL cyclohexane were added to a 100 mL beaker, and then the mixture was sonicated in an ultrasonic bath for about 30 min.The precipitate was filtered and dried in a vacuum oven at 40 ∘ C for 12 h [13].The resulting dark-green powder was collected for further characterization.

Characterization.
Powder X-ray diffraction (XRD) spectra were acquired with a Rigaku D/Max-rB diffractometer with Cu K radiation.The 2 scanning angle ranged from 10 ∘ to 70 ∘ .Scanning electron microscopy (SEM) images were acquired with a Zeiss AURIGA FIB (EDT = 3 kV; WD = 5.2 nm).Fourier-transform infrared (FT-IR) spectra were recorded on a Shimadzu IR Prestige-1 spectrometer by using the KBr-pellet technique.UV-visible diffuse-reflectance spectroscopy (UV-Vis DRS) was done with a Hitachi U-3010 UV-Vis spectrometer.Three-dimensional (3D) fluorescence spectra were obtained with a Hitachi F-7000 fluorescence spectrophotometer with a 150 W Xe lamp as excitation source.The EX and EM slits were both set at 5 nm, and the photomultiplier-tube voltage was 400 V.

Photocatalytic-Activity Measurement.
The photocatalytic activity of the samples was determined by the degradation of rhodamine B (RhB) under visible-light irradiation.A 450 W high-pressure mercury lamp was used as the visible-light irradiation source.For the experiments, 0.15 g of catalyst was first added to 400 mL of 5 mg/L RhB aqueous solution.Before irradiation, the reaction mixture was stirred for 30 min in the dark to reach adsorption-desorption equilibrium between dye and catalyst.After 2 h sessions of irradiation, 8 mL aliquots were withdrawn and centrifuged to remove essentially all catalysts.The concentration of the remnant dye was spectrophotometrically monitored by measuring the absorbance of solutions at 552 nm during the photodegradation process.

Results and Discussion
3.1.X-Ray Diffraction.Figure 1 shows XRD patterns of the pure BiVO 4 and BiVO 4 -graphene.Almost all the diffraction peaks of pure BiVO 4 and BiVO 4 -graphene can be assigned to monoclinic BiVO 4 (JCPDS 14-0688), which is the most active photocatalyst under visible-light irradiation [14].This explains why the photocatalysts remain in a monoclinic structure and why the phase of BiVO 4 does not change after adding the graphene-oxide solution.However, no diffraction peak typical of graphite or graphene oxide appears in the XRD pattern of BiVO 4 -graphene.We attribute this result to the fact that graphene oxide can be thoroughly reduced to graphene, which has no XRD peaks in this range [23].system.As a result, we expect this material to have enhanced photocatalytic activity [25].

FT-IR.
Figure 3 shows the Fourier-transform infrared (FTIR) spectra of graphene, graphene oxide, pure BiVO 4 , and a BiVO 4 -graphene composite.Graphene oxide exhibits a peak at 1731 cm −1 for C=O stretching vibrations.The peaks at 1127 and 1196 cm −1 are assigned to phenolic hydroxyl groups.Tertiary C-OH at the edges appear at 1335 cm −1 and C-O stretching vibrations of the epoxy groups appear at 1031 and 1051 cm −1 [26,27].For graphene, the adsorption around 1567 cm −1 may be assigned to stretching vibrations of the unoxidized carbon backbone, which suggests that the graphene oxide has been reduced [14].It is clear that, for graphene, almost all the peaks characteristic of graphene oxide disappear except for C-OH.For BiVO 4 -graphene and pure BiVO 4 , the broad absorptions at low frequency (such as 729 cm −1 ) are attributed to VO 4 3− [14].However, no peak typical of graphite or graphene oxide is observed in the FT-IR spectra of BiVO 4 -graphene.We attribute this result to the low amount of graphene (1%).

UV-Vis DRS.
Figure 4 shows representative spectra of pure BiVO 4 and a BiVO 4 -graphene composite.It is clear that the absorption spectrum of the BiVO 4 -graphene nanocomposite is almost the same as that of pure BiVO 4 .The spectrum of the BiVO 4 -graphene composite lies above that of pure BiVO 4 because of the graphene in the composite.These results illustrate that incorporating graphene could significantly increase the absorption of visible light, meaning that visible light could be better utilized simply by combining graphene with BiVO 4 .
3.5.PL.Photoluminescence (PL) spectra reflect the migration, transfer, and recombination processes of the electronhole pairs [28][29][30].Figure 5 shows 3D fluorescence spectra of pure BiVO 4 and BiVO 4 -graphene.The plots show that both catalysts have a maximum fluorescence peak near ( ex ,  em ) = (202 nm, 310 nm) and ( ex ,  em ) = (202 nm, 505 nm), which is attributed to the recombination of holes and electrons across the band gap of BiVO 4 .The BiVO 4graphene composites absorb more weakly than pure BiVO 4 , which implies that the recombination of photogenerated electrons and holes is much less in the BiVO 4 -graphene composites.

Photocatalytic Activity of Catalysts.
The photodegradation rates of RhB on BiVO 4 -graphene and on pure BiVO 4 under visible-light irradiation are shown in Figure 6.For reference, the result for no catalyst is also shown.From Figure 6, it is clear that the concentrations of RhB gradually decrease as a result of visible-light irradiation, whereas the concentration of RhB with no catalyst decreases negligibly.This phenomenon indicates that the degradation of the RhB solution is due to a photocatalytic reaction that happens upon irradiation by visible light.For BiVO 4 with 1% graphene, the photodegradation of RhB reaches 80% after 20 h of irradiation.For BiVO 4 with 0.25% graphene, the photodegradation of RhB reaches 87% after 20 h of irradiation, which demonstrates that the RhB molecules in solution have decomposed.In contrast, the photodegradation of RhB with pure BiVO 4 is 68% after irradiation for 20 h under the same conditions.The photodegradation of RhB with BiVO 4 -graphene is more complete than that of pure BiVO 4 , which is attributed to the graphene facilitating the transport of electrons photogenerated in the BiVO 4 , thereby leading to an efficient separation of photogenerated carriers in the coupled BiVO 4 -graphene system.The end result is an increase in photoconversion efficiency [14].
A possible reaction process is proposed in Figure 7. Upon visible-light excitation, electron-hole pairs are generated on the BiVO 4 surface (R1), followed by rapid transfer of photogenerated electrons to graphene sheets via a percolation mechanism (R2) [31].Next, the negatively charged graphene sheets can activate O 2 to produce O 2 •− (R3), while the holes can react with H 2 O to form • OH (R4).Finally, the active species (holes, • OH and O 2 •− ) oxidize the dye molecules adsorbed on the active sites of the BiVO 4graphene nanocomposite photocatalyst (R5) [13,14].The entire sequence is summarized here: However, increasing the graphene content did not lead to an increase in the photocatalytic activity of composites, because too much graphene leads to the formation of recombination centers of electrons and holes.This parallel recombination pathway reduces the probability that photoexcited charges participate in the photocatalytic reaction.As a result, a high graphene content reduces the photocatalytic activity [32].
To study the contribution of electrons and holes to the degradation reaction, silver nitrate (AgNO 3 , 0.2 mmol) and potassium sodium tartrate (C 4 H 4 O 6 KNa⋅4H 2 O, 0.2 mmol) were added to the BiVO 4 -graphene photocatalytic reaction system as electron-trapping agent and hole-trapping agent, respectively.This approach allows us to observe the degradation of RhB in the presence of either only electrons or only holes.As shown in Figure 8, adding silver nitrate clearly increases the degradation rate of RhB.In contrast, adding potassium sodium tartrate reduces the photocatalytic effect.These results suggest that the holes play the main role in the degradation of RhB in this system.

Conclusions
We prepared BiVO 4 -graphene nanocomposites by using a facile single-step method and characterized the resulting samples by a variety of analytical methods.The results show that the graphene oxide in the catalyst is thoroughly reduced to graphene.In comparison with pure BiVO 4 catalyst, the synthesized nanocomposite catalyst is a more active photocatalyst for degrading rhodamine B dye under visiblelight irradiation.We attribute the significant enhancement in photoactivity to the efficient separation of photogenerated carriers because of the high carrier mobility provided by graphene in the coupled BiVO 4 -graphene system.