Cu2−xSe Modification onto Monoclinic BiVO4 for Enhanced Photocatalytic Activity Under Visible Light

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.


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 BiVO 4 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 BiVO 4 [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, Cu 2−x Se has an indirect bandgap of 1.4 eV, which is beneficial to absorb visible light [19][20][21]. When BiVO 4 semiconductor is compounded with Cu 2−x Se, redistribution of charges is caused. The work function of Cu 2−x Se is larger than that of BiVO 4 , and the Fermi energy is lower than that of BiVO 4 [22,23]. Therefore, electrons flow to Cu 2−x Se from BiVO 4 while holes flow the other way around. Accordingly, an inner field could be built pointing from BiVO 4 to Cu 2−x Se, which facilitates the separation of electrons and holes. When under illumination, the photo-generated electrons in BiVO 4 and photo-generated holes in Cu 2−x Se will recombine preferentially, due to the band bending and inner field, leaving useful holes in BiVO 4 . 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 Cu 2−x Se/ BiVO 4 composites are expected to have high visible light photocatalytic activity.
In this work, we have fabricated Cu 2−x Se/BiVO 4 composites and made use of it for the degradation of RhB under visible light irradiation (> 420 nm) for the first time. After compounding with Cu 2−x Se, the photocatalytic activity becomes much higher than pure BiVO 4 . Specifically, the photocatalytic efficiency of 3 wt% Cu 2−x Se/BiVO 4 composite is 15.8 times that of pure BiVO 4 . Furthermore, after adding low concentration H 2 O 2 into the organic solution, RhB completely degraded within 50 min. This work provides evidence that Cu 2−x Se is an effective co-catalysis for the development of new composite semiconductor photocatalysts.

Methods
Preparation of Cu 2−x Se/BiVO 4 Composites BiVO 4 was synthesized through a chemical precipitation method [24,25]. The preparation method of Cu 2−x Se can be found in our previously reported paper [26]. Then Cu 2−x Se/BiVO 4 composites were fabricated by a co-precipitation approach. The schematic illustration of the preparation progress is shown in Fig. 1. Firstly, the pre-prepared Cu 2−x Se and BiVO 4 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 Cu 2−x Se/BiVO 4 composite.

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 Cu 2−x Se 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 cm 2 , 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 Na 2 SO 4 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 Cu 2−x Se/BiVO 4 . When BiVO 4 is combined with Cu 2−x Se, 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 Cu 2−x Se/BiVO 4 composites, corresponding to the concentration of Cu 2−x Se 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% Cu 2 −x Se/BiVO 4 composite is 15.8 times than that of pure BiVO 4 . Figure 2c shows the recycle runs of photocatalytic degradation of RhB over 3 wt% Cu 2−x Se/BiVO 4 composite with added H 2 O 2 under visible light irradiation. When a small amount of H 2 O 2 is added (103 μL/100 mL), the 3 wt% Cu 2−x Se/BiVO 4 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.
In order to analyze the microscopic morphology and grain size of the samples, the samples were characterized by SEM. As shown in Fig. 3a, BiVO 4 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 Cu 2−x Se sheet with a thickness of 300 nm and a length of 4 μm. After compounding, the Cu 2−x Se sheets are randomly distributed on the surface of BiVO 4 bulk. The XPS results also reveal the presence of Cu 2−x Se (shown below). Figure 4a shows the XRD data for BiVO 4 and 3 wt% Cu 2−x Se/BiVO 4 composite, which exhibits that the BiVO 4 has a monoclinic crystal structure. It can be seen that the crystal structure of BiVO 4 does not change when BiVO 4 is combined with Cu 2−x Se. 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 BiVO 4 and Cu 2−x Se/BiVO 4 composites. After BiVO 4 is combined with Cu 2−x Se, the relative luminescence intensity of the Cu 2−x Se/BiVO 4 composite is lower than that of BiVO 4 , which indicates that the Cu 2−x Se/ BiVO 4 composite has higher electron-hole separation efficiency after the combination of BiVO 4 and Cu 2−x Se.
The surface chemical state plays an important role in determining photocatalytic performance. So XPS is used to analyze the surface element valence of the Cu 2−x Se/ BiVO 4 composite. Figure 5a is the XPS survey spectrum of the Cu 2−x Se/BiVO 4 composite and pure BiVO 4 , from   (Fig. 5b), which are derived from Bi 3+ in BiVO 4 [29]. The peaks of 517.0 eV and 525.0 eV correspond to V 2p 3/2 and V 2p 1/2 band respectively (Fig. 5c), which are derived from the V 5+ of BiVO 4 . The peak of 530.2 eV can be attributed to O 1 s in BiVO 4 (Fig. 5d) [30,31]. The two peaks of 58.6 eV and 53.8 eV correspond to Se 3d 3/2 and Se 3d 5/2 , respectively (Fig. 5e) [32]. The Cu 2p 3/2 peak located at 931.9 eV corresponds to Cu 0 or Cu I (Fig. 5f ) [33].
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 Cu 2−x Se/BiVO 4 has a smaller arc radius than Cu 2−x Se, indicating that Cu 2−x Se/BiVO 4 composites have smaller charge transfer resistance and faster interface electron transfer. [34,35] The reason why Cu 2−x Se/BiVO 4 composite exhibits high efficiency is explained as follows. As illustrated in Fig. 7, the Fermi level of Cu 2−x Se and BiVO 4 disagrees with each other. As a result, after the BiVO 4 semiconductor surface is compounded with CuSe, the charges will be redistributed. Cu 2−x Se has larger work function and lower Fermi energy, so electrons flow to Cu 2−x Se from BiVO 4 while holes flow the other way around. As a result, the Cu 2−x Se is negatively charged and BiVO 4 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 BiVO 4 to Cu 2−x Se. Both the Fermi level movement and inner field form the so-called S-scheme heterojunction between Cu 2−x Se and BiVO 4 [36]. Under illumination, electrons and holes are excited in both materials. In this type of heterojunction, however, the photo-generated electrons in BiVO 4 and photo-generated holes in Cu 2−x Se will recombine preferentially, due to the band bending and inner field, leaving useful holes in BiVO 4 . 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 Cu 2−x Se on the surface of BiVO 4 can enhance the visible light photocatalytic activity.