Introduction

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.

Methods

Preparation of Cu2−xSe/BiVO4 Composites

BiVO4 was synthesized through a chemical precipitation method [24, 25]. The preparation method of Cu2−xSe can be found in our previously reported paper [26]. Then Cu2−xSe/BiVO4 composites were fabricated by a co-precipitation approach. The schematic illustration of the preparation progress is shown in Fig. 1. Firstly, the pre-prepared Cu2−xSe and BiVO4 powders were dispersed in ethanol with constant stirring for 4 h under 60 °C. Secondly, the suspension of the mixture was continuously stirred at 80 °C to remove the ethanol solvent. Finally, the obtained powdery sample was heated at 160 °C for 6 h under a flowing nitrogen atmosphere to form the Cu2−xSe/BiVO4 composite.

Fig. 1
figure 1

The schematic diagram of formation for Cu2−xSe/BiVO4 composite

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Characterization

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.

Photocatalytic Reaction

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.

Photoelectrochemical Measurements

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

We used photodegradation of RhB to examine the photocatalytic properties of the samples. Figure 2a shows the photocatalytic degradation of RhB over Cu2−xSe/BiVO4. When BiVO4 is combined with Cu2−xSe, its photocatalytic performance is significantly improved. The optimum composite ratio is 3%, and the photocatalytic efficiency at this ratio reaches the maximum. Figure 2b shows the degradation rate of the Cu2−xSe/BiVO4 composites, corresponding to the concentration of Cu2−xSe with 0, 2, 3, and 4 wt%, respectively. In Fig. 2b, the slope value of degradation lines is 0.0011, 0.0118, 0.0174, and 0.0045 min−1, respectively. Therefore, the photocatalytic efficiency of the 3 wt% Cu2−xSe/BiVO4 composite is 15.8 times than that of pure BiVO4. Figure 2c shows the recycle runs of photocatalytic degradation of RhB over 3 wt% Cu2−xSe/BiVO4 composite with added H2O2 under visible light irradiation. When a small amount of H2O2 is added (103 μL/100 mL), the 3 wt% Cu2−xSe/BiVO4 composites can degrade RhB completely in 50 min under visible light excitation. It can also be seen from Fig. 2c that the degradation efficiency is not attenuated after 3 cycles.

Fig. 2
figure 2

a Photocatalytic degradation of RhB over Cu2−xSe/BiVO4. b Photocatalytic degradation rate constant of RhB for Cu2−xSe/BiVO4. c Recycle runs of photocatalytic degradation of RhB over 3 wt% Cu2−xSe/BiVO4 composite with H2O2 under visible light irradiation

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In order to analyze the microscopic morphology and grain size of the samples, the samples were characterized by SEM. As shown in Fig. 3a, BiVO4 is a hexagonal bulk with a particle size of 0.2–1 μm. In Fig. 3b, the area circled by the red solid line exhibits a Cu2−xSe sheet with a thickness of 300 nm and a length of 4 μm. After compounding, the Cu2−xSe sheets are randomly distributed on the surface of BiVO4 bulk. The XPS results also reveal the presence of Cu2−xSe (shown below).

Fig. 3
figure 3

The SEM photograph of BiVO4 (a) and Cu2−xSe/BiVO4 (b)

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Figure 4a shows the XRD data for BiVO4 and 3 wt% Cu2−xSe/BiVO4 composite, which exhibits that the BiVO4 has a monoclinic crystal structure. It can be seen that the crystal structure of BiVO4 does not change when BiVO4 is combined with Cu2−xSe. This may be due to the fact that the content of Cu is relatively too small to be detected by XRD. Photoluminescence measurement is a general way to explore the separation and combination of electrons and holes. The relatively low luminescence intensity means a high electron-hole separation efficiency [27, 28]. Figure 4b shows the PL spectra for BiVO4 and Cu2−xSe/BiVO4 composites. After BiVO4 is combined with Cu2−xSe, the relative luminescence intensity of the Cu2−xSe/BiVO4 composite is lower than that of BiVO4, which indicates that the Cu2−xSe/BiVO4 composite has higher electron-hole separation efficiency after the combination of BiVO4 and Cu2−xSe.

Fig. 4
figure 4

The XRD data for BiVO4 and 3% Cu2−xSe/BiVO4 (a), the PL spectra for BiVO4 and Cu2−xSe/BiVO4 composites (b)

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The surface chemical state plays an important role in determining photocatalytic performance. So XPS is used to analyze the surface element valence of the Cu2−xSe/BiVO4 composite. Figure 5a is the XPS survey spectrum of the Cu2−xSe/BiVO4 composite and pure BiVO4, from which characteristic energy of Bi, V, O, Cu, and Se can be observed for Cu2−xSe/BiVO4, and characteristic energy of Bi, V, and O can be observed for BiVO4. The peaks of 159.1 and 164.1 eV can be attributed to the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively (Fig. 5b), which are derived from Bi3+ in BiVO4 [29]. The peaks of 517.0 eV and 525.0 eV correspond to V 2p3/2 and V 2p1/2 band respectively (Fig. 5c), which are derived from the V5+ of BiVO4. The peak of 530.2 eV can be attributed to O 1 s in BiVO4 (Fig. 5d) [30, 31]. The two peaks of 58.6 eV and 53.8 eV correspond to Se 3d3/2 and Se 3d5/2, respectively (Fig. 5e) [32]. The Cu 2p3/2 peak located at 931.9 eV corresponds to Cu0 or CuI (Fig. 5f) [33].

Fig. 5
figure 5

The XPS spectra of Cu2−xSe/BiVO4 composite. a Survey, b Bi, c V, d O, e Cu, and f Se

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To further illustrate the separation efficiency of photo-generated electrons and holes, the sample was subjected to EIS analysis. As shown in Fig. 6, the EIS Nyquist diagram of Cu2−xSe/BiVO4 has a smaller arc radius than Cu2−xSe, indicating that Cu2−xSe/BiVO4 composites have smaller charge transfer resistance and faster interface electron transfer. [34, 35]

Fig. 6
figure 6

The EIS for BiVO4 and Cu2−xSe/BiVO4 under visible light irradiation in 0.5 M Na2SO4 solution

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The reason why Cu2−xSe/BiVO4 composite exhibits high efficiency is explained as follows. As illustrated in Fig. 7, the Fermi level of Cu2−xSe and BiVO4 disagrees with each other. As a result, after the BiVO4 semiconductor surface is compounded with CuSe, the charges will be redistributed. Cu2−xSe has larger work function and lower Fermi energy, so electrons flow to Cu2−xSe from BiVO4 while holes flow the other way around. As a result, the Cu2−xSe is negatively charged and BiVO4 is positively charged until the Fermi level is equal. Meanwhile, the band structure of both materials will bend corresponding to the movement of Fermi levels. Another effect of the redistribution of carriers is the building of an inner field pointing from BiVO4 to Cu2−xSe. Both the Fermi level movement and inner field form the so-called S-scheme heterojunction between Cu2−xSe and BiVO4 [36]. Under illumination, electrons and holes are excited in both materials. In this type of heterojunction, however, 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. The above results indicate that loading Cu2−xSe on the surface of BiVO4 can enhance the visible light photocatalytic activity.

Fig. 7
figure 7

The schematic diagram of photocatalytic mechanism

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Conclusion

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.