Cu2−xSe Modification onto Monoclinic BiVO4 for Enhanced Photocatalytic Activity Under Visible Light
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The rapid recombination of electron-hole pairs in BiVO4 has limited its performance as a photocatalysis. In this paper, BiVO4 is combined with Cu2−xSe semiconductor to slow down the recombination process, and thus improve its photocatalytic activity. This is enabled by careful band structure design. The work function of Cu2−xSe is larger than that of BiVO4. Therefore, electrons flow to Cu2−xSe from BiVO4 after the composition. Accordingly, an inner field could be built, which facilitates the separation of electrons and holes. The experimental result shows that the photocatalytic efficiency of the 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times than that of pure BiVO4.
KeywordsPhotocatalysis Hydrothermal Bismuth-based semiconductor
Scanning electron microscope
With the developing of modern industry, environmental pollution has become more and more severe. Utilizing solar energy, photocatalytic decomposition of organic matter is an environmentally friendly and efficient technology to solve pollution [1, 2, 3, 4, 5, 6]. The Bi-based semiconductor photocatalytic material has a suitable band gap, which enables it to absorb visible light sufficiently and possess superior photocatalytic performance [7, 8, 9, 10]. Among them, monoclinic BiVO4 has a narrow band gap of 2.4 eV and good photocatalytic activity, which has been nominated as an efficient material for decomposing organic pollutions [11, 12, 13, 14, 15]. The rapid electron-hole recombination rate, however, leads to a low photocatalytic activity for pure BiVO4 [16, 17, 18]. An effective approach to slow down the recombination of electrons and holes is to combine two different semiconductor materials, given the band structures of the two combined materials match a specific condition.
As a p-type semiconductor, Cu2−x Se has an indirect bandgap of 1.4 eV, which is beneficial to absorb visible light [19, 20, 21]. When BiVO4 semiconductor is compounded with Cu2−xSe, redistribution of charges is caused. The work function of Cu2−xSe is larger than that of BiVO4, and the Fermi energy is lower than that of BiVO4 [22, 23]. Therefore, electrons flow to Cu2−xSe from BiVO4 while holes flow the other way around. Accordingly, an inner field could be built pointing from BiVO4 to Cu2−xSe, which facilitates the separation of electrons and holes. When under illumination, the photo-generated electrons in BiVO4 and photo-generated holes in Cu2−xSe will recombine preferentially, due to the band bending and inner field, leaving useful holes in BiVO4. The useful holes possess higher energy level, which can benefit the generation of •OH species. These •OH species can break down long chains of organic matter into small molecules. Hence, the Cu2−xSe/BiVO4 composites are expected to have high visible light photocatalytic activity.
In this work, we have fabricated Cu2−x Se/BiVO4 composites and made use of it for the degradation of RhB under visible light irradiation (> 420 nm) for the first time. After compounding with Cu2−x Se, the photocatalytic activity becomes much higher than pure BiVO4. Specifically, the photocatalytic efficiency of 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times that of pure BiVO4. Furthermore, after adding low concentration H2O2 into the organic solution, RhB completely degraded within 50 min. This work provides evidence that Cu2−xSe is an effective co-catalysis for the development of new composite semiconductor photocatalysts.
Preparation of Cu2−xSe/BiVO4 Composites
XRD (X-ray diffraction) measurement of the as-prepared samples was performed by a PANalytical X’pert Pro diffractometer with Cu Kα radiation. The morphology of the sample was obtained by an SEM (scanning electron microscope) Hitachi S-4800. XPS (X-ray photoelectron spectroscopy) of the samples was characterized on a Pekin Elmer PHI-5300 instrument. The photoluminescence emission spectra of the samples were committed using a Cary Eclipse fluorescence spectrophotometer.
The photocatalytic performance was characterized by an XPA photochemical reactor. Additionally, a Xe lamp with a power of 500 W and a cut-off wavelength of 420 nm is utilized to simulate natural light, while a solution of test dye RhB is used to mimic organic solutions. During the degradation process, 60 mg Cu2−xSe composite powder was placed in a 60-mL RhB solution. The suspension was stirred in a dark environment for 2 h before light irradiation to realize an adsorption-desorption balance. Then, light illumination is added with stirring remaining and about 6 mL of the suspension was taken out at intervals of 10 min. Subsequently, the suspension was centrifuged twice. The absorbance spectrum of the solution was characterized on a Shimadzu UV-2450 spectrometer.
The photocurrent is measured by a CHI 660E electrochemical workstation. To make the illumination consistent with that in the degradation process, the light source is still selected as a Xe lamp with a power of 500 W and a cut-off wavelength of 420 nm. The photoelectrochemical measurement is detailed as follows. First, 10 mg of the photocatalyst and 20 μL of Nafion solution were ultrasonically dispersed in 2 mL of ethyl alcohol. Then, 40 μL of the above solution was deposited on an ITO conductive glass with 0.196 cm2, which was sequentially heated at 200 °C for 1 h to obtain the working electrode. Besides, Pt foil is chosen as the counter electrode. A saturated solution of mercury and mercurous chloride in an aqueous solution of potassium chloride as the reference electrode, and 0 .5M Na2SO4 solution is used for the electrolyte.
Results and Discussion
In summary, the Cu2−xSe/BiVO4 composites have been successfully prepared and examined for degrading organic pollutions. Experimental data shows that the photocatalytic activity is largely improved after the combination. The photocatalytic efficiency of 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times that of pure BiVO4. Furthermore, after adding low concentration H2O2, RhB can be completely degraded within 50 min. The SEM and XPS results confirm the presence of Cu2−xSe in the Cu2−xSe/BiVO4 composites. The results of photoluminescence indicate that the Cu2−xSe/BiVO4 composites have higher electron-hole separation efficiency. The results of EIS indicate that Cu2−xSe/BiVO4 composites have smaller charge transfer resistance and faster interface electron transfer. This work shows that Cu2−xSe is an effective co-catalysis for the development of new composite semiconductor photocatalysts.
We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51572183 and 51502188).
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding author on request.
XL and WZ designed this work. XG and RG performed the experiments. RG analyzed the data. ZL and WZ wrote this paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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