In recent years, use of semiconductor photocatalysts in wastewater treatment has attracted an increasing amount of attention [1-4]. A variety of materials have been used as photocatalysts, including oxides, sulfides and nitrides [5]. Among these materials, Bi-based semiconductor oxides, such as Bi₂O₃ [6], BiFeO₃ [7], and BiOCl [8], have been intensively investigated because of their excellent photocatalytic activity in the field of environmental depollution. In addition, Bi₂O₃ has attracted much attention due to its narrow-band gap, high refractive index, oxygen ion conductivity, and great photoconductivity [9]. However, its high recombination rate of photogenerated electron-hole pairs limits its photocatalytic activity and practical applications. To solve these problems, it is necessary to combine Bi₂O₃ with other semiconductor materials to construct a heterojunction photocatalyst [10], such as Bi₂O₃/g-C₃N₄ [11] or Bi₂O₃/BiVO₄ [12].

ZnCo₂O₄ is a new spinel oxide semiconductor with a band gap of approximately 1.77 eV, and it can absorb visible light to degrade organic [13]. In its band structure, the valence band (VB) of ZnCo₂O₄ is the O 2p level, and the conduction bands (CBs) is composed of the Co 3d-e_g and Co 3d-t_2g levels [14]. This particular band structure is beneficial for internal electron transitions, which are helpful for decreasing the recombination rate of photogenerated electron-hole pairs. It has also been found that ZnCo₂O₄ is an ideal candidate for constructing an effective heterojunction with other photocatalysts [15]. Inspired by this consideration, it is supposed that the band edges of ZnCo₂O₄ and Bi₂O₃ match well with each other and that a constructed heterojunction consisting of two semiconductors should have a higher ability to absorb visible light and faster charge carrier transfer. Thus, a novel ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst will be developed in this study, and the composite's photocatalytic activity in MG degradation under visible light illumination will be investigated for the first time.

The preparation process of the sample is as follows: (1) preparation of the photocatalyst: the ZnCo₂O₄ precursor was prepared in a previous report [16]. Pure Bi₂O₃ was purchased from Aladdin Biochemical Technology Co., Ltd. (2) preparation of the ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst: typically, Bi₂O₃ (2.0 g) and ZnCo₂O₄ (0.434 g) precursors were mixed and ball milled for 30 min. After drying at 80 ℃, the powder mixture was calcined at 400 ℃ for 1 h to prepare the 13% ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst.

X-ray diffraction patterns were recorded using a D8 Advance (XRD, Rigaku-TTRⅢ) diffractometer with Cu Kα radiation (λ = 0.1546 nm). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were recorded on a Tecnai G2 F20 electron microscope. UV–vis diffuse reflection spectra (DRS) were obtained using a Shimadzu UV2550 spectrophotometer. Photoluminescence (PL) spectroscopy was measured at room temperature using a fluorescence spectrophotometer (Jobin Yvon Fluorolog 3–21). Electrochemical impedance spectroscopy (EIS) was collected at a frequency of 1000 Hz.

The photocatalytic activity of the obtained ZnCo₂O₄/Bi₂O₃ was evaluated via MG degradation under visible light irradiation. Before irradiation, 0.12 g of the as-prepared ZnCo₂O₄/Bi₂O₃ was added into 60 mL of 10 mg/L MG solution, which was magnetically stirred in the dark for 30 min to establish equilibrium between MG and the catalyst. The concentration of MG was monitored at given time intervals by measuring the maximum UV absorbance at 619 nm after commencing irradiation.

XRD patterns of as-prepared samples are shown in Fig. 1a. The strong diffraction peaks of Bi₂O₃ at 21.72°, 25.757°, 26.922°, 27.377°, 28°, and 33.241° are in accordance with the (020), (002), (111), (120), (012), and (200) planes of monoclinic Bi₂O₃ corresponding to JCPDS No. 41-1449, respectively. The XRD pattern of pure ZnCo₂O₄ is in good agreement with the reported data (JCPDS No. 23-1390). However, the characteristic diffraction peaks of ZnCo₂O₄ in 13% ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst are very weak because of its low content.

Fig. 1c shows an overview of a typical TEM image of the heterojunction photocatalyst. Different kinds of lattice fringes are clearly seen in Fig. 1d. Specifically, distances of 0.29 and 0.261 nm correspond respectively to the (-241) and (-232) lattice planes of Bi₂O₃, and distances of 0.3 and 0.321 nm correspond respectively to the (422) and (511) planes of ZnCo₂O₄, indicating heterojunction was indeed formed between ZnCo₂O₄ and Bi₂O₃. In addition, the EDX spectrum of the photocatalyst (Fig. 1b) shows 13% ZnCo₂O₄/Bi₂O₃ is only composed of Bi, Zn, Co, and O.

Fig. 2a shows the PL spectra of pure Bi₂O₃, ZnCo₂O₄, and 13% ZnCo₂O₄/Bi₂O₃ at an excitation wavelength of 320 nm. As shown in Fig. 2a, the emission peak for the 13% ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst has comparatively weaker intensity than that of pure Bi₂O₃, indicating a lower recombination rate of photogenerated electron-hole pairs [17, 18]. As shown in Fig. 2b, the arc radius of the EIS Nyquist plot of the heterojunction photocatalyst is much smaller than that of the EIS Nyquist plot of Bi₂O₃, showing that the heterojunction structure can accelerate the photogenerated electron-hole pair separation. Therefore, it is expected that the as-prepared heterojunction photocatalyst will possess fast photogenerated electron-hole pair separation and transfer capability, which are helpful for significantly enhancing photocatalytic activity.

The UV–vis diffuse reflectance spectra (Fig. 2c) shows the absorption edge of pure Bi₂O₃ is about 454 nm and the absorption edge of the 13% ZnCo₂O₄/Bi₂O₃ heterojunction shifts to longer wavelength (about 788 nm), which indicates that there is an increase of the absorption range of visible-light. Moreover, the absorption intensity of the 13% ZnCo₂O₄/Bi₂O₃ heterojunction is higher than that of Bi₂O₃. Fig. 2d shows the band gap energies of asprepared samples. It can be seen from Fig. 2d that the values of the band gap energy for pure Bi2O3, ZnCo₂O₄, and the 13% ZnCo₂O₄/Bi₂O₃ composite are nearly 2.8 eV, 1.26 eV, and 1.5 eV, respectively. Compared with the band gap of the pure Bi₂O₃ sample, the band gap of the 13% ZnCo₂O₄/Bi₂O₃ is lower, which is beneficial for generating more photogenerated charge carriers under visible light irradiation.

Fig. 3a shows photocatalytic rate for MG of the as-prepared samples. As shown in Fig. 3a, the photocatalytic activity of the ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst with different ZnCo₂O₄ content are higher than that of pure Bi₂O₃ (degradation rate 30% for MG), especially, the degradation rate for MG of the 13% ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst is up to 94% in 120 min. It is worth noting that, the degradation rate for MG of the 13% ZnCo₂O₄/Bi₂O₃ (physically mixed) and pure ZnCo₂O₄ is up to 42% and 98% in 120 min, respectively. The superior photocatalytic activity of pure ZnCo₂O₄ can be ascribed to its special structural feature. The particular structure of the mesoporous fibers provides the mesopores/channels with larger surface area can enhance accessibility of photo-generated electrons and holes [16]. In addition, the ZnCo₂O₄ fibers were built by plenty of nanoparticles and the nanoparticles were interconnected to each other to form stable fibers, which provided more active sites for the photocatalytic reactions [16]. The improving photocatalytic activity of ZnCo₂O₄/Bi₂O₃ results from the heterojunction formation of between ZnCo₂O₄ and Bi₂O₃ which can facilitate charge carrier transfer, suppress the recombination rate of photogenerated electron-hole pairs, and enhance the absorption and utilization of visible light. Recycling reactions for the photodegradation of MG over 13% ZnCo₂O₄/Bi₂O₃ under visible light irradiation were carried out to investigate the stability, and the results are shown in Fig. 3b. The photocatalytic activity of the 13% ZnCo₂O₄/Bi₂O₃ sample had no apparent deactivation even after four successive cycles for MG degradation (only about 8% loss). The high stability of 13% ZnCo₂O₄/Bi₂O₃ can probably be attributed to the formation of the heterojunction.

Fig. 3c shows the active species trapping experiments of the 13% ZnCo₂O₄/Bi₂O₃ heterojunction photocatalyst using isopropanol (IPA), ammonium (AO), and benzoquinone (BQ) as scavengers for ^·OH, h⁺ and ^·O₂^-. As seen in Fig. 3c, ^·O₂^- plays a major role in MG degradation. Fig. 3d shows a possible photocatalytic reaction mechanism. To explain the photocatalytic degradation mechanism, the VB and CB edge positions of Bi₂O₃ and ZnCo₂O₄ were obtained (Fig. S1 in Supporting information). When 13% ZnCo₂O₄/Bi₂O₃ is excited under visible light irradiation, the photogenerated electrons on the CB of Bi₂O₃ transfer to the CB of ZnCo₂O₄, whereas the photogenerated holes on the VB of ZnCo₂O₄ migrate to the VB of Bi₂O₃, and these transfers make electron-hole pair separation more efficient. As a result, the recombination of formed electrons and holes can be inhibited, and this enhances the photocatalytic activity of the 13% ZnCo₂O₄/Bi₂O₃ photocatalyst.

In summary, the novel heterojunction photocatalyst ZnCo₂O₄/Bi₂O₃ was constructed. Degradation results show that the asprepared 13% ZnCo₂O₄/Bi₂O₃ has the highest rate of MG degradation (94%), which is almost 3.1 times higher than that of pure Bi₂O₃. The heterojunction formation between Bi₂O₃ and ZnCo₂O₄ is beneficial for inhibiting the recombination rate of photogenerated electron-hole pairs between Bi₂O₃ and ZnCo₂O₄, and improving the absorption and utilization of visible light. Therefore, 13% ZnCo₂O₄/Bi₂O₃ may be a promising photocatalyst for removing organic pollutants in wastewater.

Acknowledgments

This work was financially supported by Jiana Foundation of Central South University (No. JNJJ201613) and the National Natural Science Foundation of China (No. 51404306).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.08.020.1