Abstract
We prepared BiVO4-graphene nanocomposites by using a facile single-step method and characterized the material by x-ray 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 BiVO4 is densely dispersed on the graphene sheets, which facilitates the transport of electrons photogenerated in BiVO4, 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 BiVO4 catalyst. To study the contribution of electrons and holes in the degradation reaction, silver nitrate and potassium sodium tartrate were added to the BiVO4-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.
1. 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 TiO2 [3], Bi2WO6 [4], ZnO [5], and BiVO4 [6]. To date, from among the various photocatalysts, TiO2 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, BiVO4 was widely used to produce yellow paint. However, recent studies have found that monoclinic BiVO4, which has a band gap of 2.4 eV, is an ideal visible-light 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 BiVO4 is just slightly above 0 eV, which would make it hard for O2 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 BiVO4 restricts its wide application for photocatalytic degradation of organic contaminants [13–16].
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 m2/g), and high carrier mobility (200 000 cm2/V) [17–21]. 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 π-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 [13–15, 24].
Inspired by these concepts, we report herein the design and synthesis of graphene-based nanocomposites, using BiVO4-graphene nanocomposites as an example, with the goal of achieving highly efficient photocatalytic properties driven by visible light.
2. Experiment
2.1. Experimental Materials
The following analytically pure chemicals were used: graphite powder (≥99.85%, Shanghai HuaYi Company), sodium nitrate (NaNO3, Chengdu Kelon Chemical Reagent Company), potassium permanganate (KMnO4, Chongqing Chuandong Chemical Company), sulfuric acid (H2SO4, Chongqing Chuandong Chemical Company), hydrogen peroxide (H2O2, Chongqing Chuandong Chemical Company), bismuth nitrate [Bi(NO3)3·5H2O, Chengdu Kelon Chemical Reagent Company], sodium vanadate (Na3VO4·12H2O, Sinopharm Chemical Reagent Company), hexadecyl trimethyl ammonium bromide (C19H42BrN, Chengdu Kelon Chemical Reagent Company), sodium chloride (NaCl, Chongqing Chuandong Chemical Company), hydrochloric acid (HCl, Chongqing Chuandong Chemical Company), cyclohexane (C6H12, Chongqing Chuandong Chemical Company), glucose (C6H12O6, Chongqing Boyi Chemical Company), and 25% of ammonia solution (NH3·H2O, Chongqing Chuandong Chemical Company).
2.2. Synthesis of BiVO4
Typically, the synthesis of BiVO4 proceeds as follows: 4.8 mmol hexadecyl trimethyl ammonium bromide and 1.6 mmol Bi(NO3)3·5H2O are added in that order to distilled water (40 mL). Next, 1.6 mmol Na3VO4·12H2O 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.
2.3. Synthesis of BiVO4-Graphene
Graphene oxide was synthesized according to the modified Hummers method [13, 24]. To synthesize BiVO4-graphene, 200 mg BiVO4, 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.
2.4. Characterization
Powder X-ray diffraction (XRD) spectra were acquired with a Rigaku D/Max-rB diffractometer with Cu Kα radiation. The 2θ 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.
2.5. 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.
3. Results and Discussion
3.1. X-Ray Diffraction
Figure 1 shows XRD patterns of the pure BiVO4 and BiVO4-graphene. Almost all the diffraction peaks of pure BiVO4 and BiVO4-graphene can be assigned to monoclinic BiVO4 (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 BiVO4 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 BiVO4-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].
3.2. SEM
Figure 2 shows SEM images of different samples. The images show clearly that pure BiVO4-particles form a fine structure composed of BiVO4 crystal grains. At the same time, some BiVO4 aggregates and BiVO4-graphene particles appear. BiVO4 and graphene sheets are in close contact, which facilitates the transport of electrons photogenerated in BiVO4, thereby leading to efficient separation of photogenerated carriers in the coupled graphene-nanocomposite system. As a result, we expect this material to have enhanced photocatalytic activity [25].
(a) BiVO4
(b) BiVO4-graphene
3.3. FT-IR
Figure 3 shows the Fourier-transform infrared (FTIR) spectra of graphene, graphene oxide, pure BiVO4, and a BiVO4-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 BiVO4-graphene and pure BiVO4, the broad absorptions at low frequency (such as 729 cm−1) are attributed to [14]. However, no peak typical of graphite or graphene oxide is observed in the FT-IR spectra of BiVO4-graphene. We attribute this result to the low amount of graphene (1%).
3.4. UV-Vis DRS
Figure 4 shows representative spectra of pure BiVO4 and a BiVO4-graphene composite. It is clear that the absorption spectrum of the BiVO4-graphene nanocomposite is almost the same as that of pure BiVO4. The spectrum of the BiVO4-graphene composite lies above that of pure BiVO4 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 BiVO4.
3.5. PL
Photoluminescence (PL) spectra reflect the migration, transfer, and recombination processes of the electron-hole pairs [28–30]. Figure 5 shows 3D fluorescence spectra of pure BiVO4 and BiVO4-graphene. The plots show that both catalysts have a maximum fluorescence peak near = (202 nm, 310 nm) and = (202 nm, 505 nm), which is attributed to the recombination of holes and electrons across the band gap of BiVO4. The BiVO4-graphene composites absorb more weakly than pure BiVO4, which implies that the recombination of photogenerated electrons and holes is much less in the BiVO4-graphene composites.
(a)
(b)
3.6. Photocatalytic Activity of Catalysts
The photodegradation rates of RhB on BiVO4-graphene and on pure BiVO4 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 BiVO4 with 1% graphene, the photodegradation of RhB reaches 80% after 20 h of irradiation. For BiVO4 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 BiVO4 is 68% after irradiation for 20 h under the same conditions. The photodegradation of RhB with BiVO4-graphene is more complete than that of pure BiVO4, which is attributed to the graphene facilitating the transport of electrons photogenerated in the BiVO4, thereby leading to an efficient separation of photogenerated carriers in the coupled BiVO4-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 BiVO4 surface , followed by rapid transfer of photogenerated electrons to graphene sheets via a percolation mechanism [31]. Next, the negatively charged graphene sheets can activate to produce , while the holes can react with to form . Finally, the active species (holes, and ) oxidize the dye molecules adsorbed on the active sites of the BiVO4-graphene nanocomposite photocatalyst [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 (AgNO3, 0.2 mmol) and potassium sodium tartrate (C4H4O6KNa·4H2O, 0.2 mmol) were added to the BiVO4-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.
4. Conclusions
We prepared BiVO4-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 BiVO4 catalyst, the synthesized nanocomposite catalyst is a more active photocatalyst for degrading rhodamine B dye under visible-light 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 BiVO4-graphene system.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The financial support of the National Natural Science Foundation of China (No. 51108483), the Natural Science Foundation Project of CQ CSTC (Grant No. cstcjjA20002), the 111 Project (No. B13041), and the Fundamental Research Funds for the Central Universities (CDJZR12210018, 106112013CDJZR210002) is gratefully acknowledged.