Fabrication and Characterization of a Novel Composite Magnetic Photocatalyst β-Bi2O3/BiVO4/MnxZn1-xFe2O4 for Rhodamine B Degradation under Visible Light.

β-Bi2O3/BiVO4/MnxZn1−xFe2O4 (BV/MZF) composite magnetic photocatalyst was first synthesized using the hydrothermal and calcination method. BV/MZF was a mesoporous material with most probable pore size and specific surface area of 18 nm and 17.84 m2/g, respectively. Due to its high saturation magnetization (2.67 emu/g), the BV/MZF composite can be easily separated and recovered from solution under an external magnetic field. The results of photo-decomposition experiments show that the decomposition rate of Rhodamine B (RhB) by BV/MZF can reach 92.6% in 3 h under visible light. After three cycles, BV/MZF can still maintain structural stability and excellent pollutant degradation effect. In addition, analysis of the photocatalytic mechanism of BV/MZF for RhB shows that the p-n heterojunction formed in BV/MZF plays a vital role in its photocatalytic performance. This work has potential application in the future for solving environmental pollution.


Introduction
Dye wastewater discharge is increasing with the continuous expansion of the textile and dyeing industries, resulting in higher and higher content of organic pollutants in water [1,2]. As organic pollutants are non-biodegradable, highly toxic, and carcinogenic, they have a serious impact on human health and the ecosystem [3,4]. For example, the basic cationic dye, Rhodamine B (Rhodamine B, RhB), has high chroma, high toxicity, teratogenicity, and hard degradation [5,6]. Therefore, the effective treatment of organic dyes in industrial wastewater is becoming an urgent problem. In recent years, photocatalysts were widely used in wastewater treatment due to their green, environmentally friendly, sustainable, and economic advantages [7,8]. Various semiconductor materials were applied to photocatalysts, such as TiO 2 , ZnO, CdS, and BiVO 4 [9][10][11][12]. Among them, non-toxic BiVO 4 was widely studied due to its good optical absorption performance, high ionic conductivity, and excellent band gap energy (2.4 eV) [13]. However, BiVO 4 has a fast electron-hole pairs recombination rate, a narrow visible light response range, and poor photocatalytic activity. Compounding other semiconductor materials is an effective solution for separating photogenerated electron-hole pairs, thereby greatly improving its catalytic activity [14]. In recent research, the photocatalytic activity of composite photocatalysts such as BiFeO 3 /BiVO 4 , M-BiVO 4 /T-BiVO 4 , and TiO 2 /BiVO 4 was significantly improved [15][16][17]. Nevertheless,

Materials and Methods
Analytical reagents of Bi(NO 3 (RhB), and tartaric acid were utilized as primeval materials. The above were all purchased from Chengdu Kelong Chemical Ltd. (Chengdu, China). All experimental water used was deionized water.

Synthesis of BV/MZF
BiVO 4 , β-Bi 2 O 3 , and MZF, were prepared according to the literatures [22][23][24], respectively. Primarily, 2 mol/L HNO 3 was used to dissolve 0.01 mol Bi(NO 3 ) 3 ·5H 2 O by ultrasonic vibration as solution A. Solution B was gained by dissolving 0.02 mol tartaric acid in hot water at 80 • C. Finally, 0.01 mol NH 4 VO 3 was added to 80 • C hot water and dissolved as solution C. Afterwards, solution B was slowly added dropwise to solution C, and the resulting solution was referred to as solution D. Subsequently, solution A was added dropwise to solution D. After cooling the mixture to room temperature, the pH was adjusted to 7.5 with NH 3 ·H 2 O, and BiVO 4 precursor was formed. Depending on the mass ratio of 10:100 (the theoretical generation value of BiVO 4 ), as-prepared MZF was added to the precursor solution, and the reaction was performed in a water-bath at 80 • C for 30 min. Deionized water was used to wash the resulting precipitates five times and dried at 60 • C for 12 h. Lastly, the precipitates were placed in a muffle furnace at 450 • C for 3 h to obtain BiVO 4 /MZF (10 wt%).
The different mass ratios of BV/MZF were obtained according to the following procedures. Firstly, 4 mmol Bi(NO 3 ) 3 ·5H 2 O was dissolved in 10 mL of 2 mol/L HNO 3 solution, and the solution was stirred with a mechanical stirrer for 30 min at ambient temperature. Then, the stirred solution was slowly added to 40 mL of 0.6 mol/L Na 2 CO 3 solution, and the mixture was mechanically stirred at ambient temperature for 2 h, as a precursor solution. Then, the mass ratios of 5, 10, 15, 20, and 25:100 (β-Bi 2 O 3 generate theoretically), as-synthesized BiVO 4 /MZF, were added to the precursor solution. The resulting mixture was filtered and washed with deionized water. Finally, the filter residues that had been dried at 60 • C for 12 h were roasted in a muffle furnace at 380 • C for 10 min. The synthesis scheme of BV/MZF is displayed in Figure 1.

Photocatalytic Tests
Under the irradiation of the 300 W xenon lamp (CEL-HXF 300, Beijing CEAULIGHT Co., Ltd., Beijing, China), with a 420 nm cutoff filter was selected as visible light source, the photocatalytic activity of BV/MZF for RhB solutions as simulated dye wastewater was studied. The temperature and pH of the photocatalytic tests were room temperature and natural pH value, respectively. The prepared 100 mg samples were added to 50 mL of the 10 mg/L RhB aqueous solution, and the dark reaction for 30 min reached the adsorption and desorption equilibrium. During the photocatalytic process, the liquid level of RhB and the light source of the xenon lamp were maintained at 20 cm, and the light experiment was performed under magnetic (normal materials) or mechanical (magnetic materials) stirring. After every 30 min, 5 mL the reaction solution was absorbed and centrifuged for 3 min at 4000 rpm. The photodegradation rate of all samples was the average of three experimental results. Eventually, the decomposition efficiency of the target contaminant was calculated by Equation (1):

Photocatalytic Tests
Under the irradiation of the 300 W xenon lamp (CEL-HXF 300, Beijing CEAULIGHT Co., Ltd., Beijing, China), with a 420 nm cutoff filter was selected as visible light source, the photocatalytic activity of BV/MZF for RhB solutions as simulated dye wastewater was studied. The temperature and pH of the photocatalytic tests were room temperature and natural pH value, respectively. The prepared 100 mg samples were added to 50 mL of the 10 mg/L RhB aqueous solution, and the dark reaction for 30 min reached the adsorption and desorption equilibrium. During the photocatalytic process, the liquid level of RhB and the light source of the xenon lamp were maintained at 20 cm, and the light experiment was performed under magnetic (normal materials) or mechanical (magnetic materials) stirring. After every 30 min, 5 mL the reaction solution was absorbed and centrifuged for 3 min at 4000 rpm. The photodegradation rate of all samples was the average of three experimental results. Eventually, the decomposition efficiency of the target contaminant was calculated by Equation (1): where η, A 0 , and A t are degradation efficiency, and the equilibrium concentration of the target contaminant before and after visible light irradiation, respectively.

Synthesis Optimization
The effect of BV/MZF with different mass ratios on the treatment of RhB and UV-visible spectrum of optimal samples are shown in Figure 2. As seen from Figure 2a, the photocatalytic efficiency for RhB with BV/MZF (5-25 wt%) composite catalysts are 92.6%, 92.8%, 86.3%, 85.1%, and 56.2%, respectively, after irradiating for 3 h. This result was caused by the lessening of the effective photocatalytic component β-Bi 2 O 3 in the photocatalyst as the mass of BiVO 4 /Mn x Zn 1−x Fe 2 O 4 increases. At the same time, the photodegradation rates of BiVO 4 , β-Bi 2 O 3 , and 20 wt% BV/MZF were relatively close, while it improved for 10 wt% BV/MZF samples. In Figure 2b, the absorbance of the maximum peak at 554 nm decreased with time, indicating that RhB continuously degraded, and confirming that the degradation rate of RhB by BV/MZF (10 wt%) was uniform and stable. Ultimately, the BV/MZF (10 wt%) with the best photocatalytic performance was used as the analytical test sample. where η, A0, and At are degradation efficiency, and the equilibrium concentration of the target contaminant before and after visible light irradiation, respectively.

Synthesis Optimization
The effect of BV/MZF with different mass ratios on the treatment of RhB and UV-visible spectrum of optimal samples are shown in Figure 2. As seen from Figure 2a, the photocatalytic efficiency for RhB with BV/MZF (5-25 wt%) composite catalysts are 92.6%, 92.8%, 86.3%, 85.1%, and 56.2%, respectively, after irradiating for 3 h. This result was caused by the lessening of the effective photocatalytic component β-Bi2O3 in the photocatalyst as the mass of BiVO4/MnxZn1−xFe2O4 increases. At the same time, the photodegradation rates of BiVO4, β-Bi2O3, and 20 wt% BV/MZF were relatively close, while it improved for 10 wt% BV/MZF samples. In Figure 2b, the absorbance of the maximum peak at 554 nm decreased with time, indicating that RhB continuously degraded, and confirming that the degradation rate of RhB by BV/MZF (10 wt%) was uniform and stable. Ultimately, the BV/MZF (10 wt%) with the best photocatalytic performance was used as the analytical test sample.  Furthermore, there is high intensity reflection at 24.5° in BV/MZF, which is caused by the conversion of monoclinic BiVO4 to tetragonal BiVO4 during the roasting process [25]. However, only the (440) characteristic diffraction peaks of MZF could be found in BV/MZF. This may be owing to the fact that other diffraction peaks in MZF are so weak that they were covered by the diffraction peaks of β-Bi2O3 and BiVO4 [26]. The characteristic peaks of MZF, BiVO4, and β-Bi2O3 were observed in the composite sample, which indicates that BV/MZF was successfully synthesized.  there is high intensity reflection at 24.5 • in BV/MZF, which is caused by the conversion of monoclinic BiVO 4 to tetragonal BiVO 4 during the roasting process [25]. However, only the (440) characteristic diffraction peaks of MZF could be found in BV/MZF. This may be owing to the fact that other diffraction peaks in MZF are so weak that they were covered by the diffraction peaks of β-Bi 2 O 3 and BiVO 4 [26]. The characteristic peaks of MZF, BiVO 4 , and β-Bi 2 O 3 were observed in the composite sample, which indicates that BV/MZF was successfully synthesized.  Figure 4 shows the FTIR spectra of MZF, BiVO4, β-Bi2O3, and BV/MZF. The stretching and bending vibration peaks provided by the hydroxyl groups (-OH) from the surface water molecules can be attributed to 3434.0 cm −1 and 2359.3 cm −1 of MZF, respectively [27]. The bands at 568.0 cm −1 and 1399.4 cm −1 are the stretching vibration peaks of the zinc-oxygen bond and typical Raman bands of the ferrite bond, respectively [28]. The characteristic absorption peaks at 1387.6 cm −1 , 851.7 cm −1 , and 582.8 cm −1 are caused by the bending vibration of the bismuth oxygen bond [29,30]. The absorption peak at 468.3 cm −1 is closely related to the stretching vibration of the bismuth oxygen bond [24]. The absorption peak at 472.0 cm −1 in Figure 4b and at 641.3 cm −1 in Figure 4a,b arise from the stretching vibration peak of the bismuth oxygen bond and the stretching vibration peak of the vanadium oxygen bond, respectively. Due to the low MZF content, the characteristic absorption peak of MZF does not appear in BV/MZF (Figure 4a). The elemental composition and state of the BV/MZF are analyzed by XPS. Figure 5 shows the XPS full spectrum and narrow scan area map of Zn 2p, Mn 2p, Fe 2p, O 1s, Bi 4f, and V 2p. In Figure  5a, the characteristic peaks of Mn, Zn, Fe, Bi, V, O, and C were detected on the surface of BV/MZF,   [27]. The bands at 568.0 cm −1 and 1399.4 cm −1 are the stretching vibration peaks of the zinc-oxygen bond and typical Raman bands of the ferrite bond, respectively [28]. The characteristic absorption peaks at 1387.6 cm −1 , 851.7 cm −1 , and 582.8 cm −1 are caused by the bending vibration of the bismuth oxygen bond [29,30]. The absorption peak at 468.3 cm −1 is closely related to the stretching vibration of the bismuth oxygen bond [24]. The absorption peak at 472.0 cm −1 in Figure 4b and at 641.3 cm −1 in Figure 4a,b arise from the stretching vibration peak of the bismuth oxygen bond and the stretching vibration peak of the vanadium oxygen bond, respectively. Due to the low MZF content, the characteristic absorption peak of MZF does not appear in BV/MZF (Figure 4a).  Figure 4 shows the FTIR spectra of MZF, BiVO4, β-Bi2O3, and BV/MZF. The stretching and bending vibration peaks provided by the hydroxyl groups (-OH) from the surface water molecules can be attributed to 3434.0 cm −1 and 2359.3 cm −1 of MZF, respectively [27]. The bands at 568.0 cm −1 and 1399.4 cm −1 are the stretching vibration peaks of the zinc-oxygen bond and typical Raman bands of the ferrite bond, respectively [28]. The characteristic absorption peaks at 1387.6 cm −1 , 851.7 cm −1 , and 582.8 cm −1 are caused by the bending vibration of the bismuth oxygen bond [29,30]. The absorption peak at 468.3 cm −1 is closely related to the stretching vibration of the bismuth oxygen bond [24]. The absorption peak at 472.0 cm −1 in Figure 4b and at 641.3 cm −1 in Figure 4a,b arise from the stretching vibration peak of the bismuth oxygen bond and the stretching vibration peak of the vanadium oxygen bond, respectively. Due to the low MZF content, the characteristic absorption peak of MZF does not appear in BV/MZF (Figure 4a).  Figure 5 shows the XPS full spectrum and narrow scan area map of Zn 2p, Mn 2p, Fe 2p, O 1s, Bi 4f, and V 2p. In Figure  5a, the characteristic peaks of Mn, Zn, Fe, Bi, V, O, and C were detected on the surface of BV/MZF,  Figure 5 shows the XPS full spectrum and narrow scan area map of Zn 2p, Mn 2p, Fe 2p, O 1s, Bi 4f, and V 2p. In Figure 5a, the characteristic peaks of Mn, Zn, Fe, Bi, V, O, and C were detected on the surface of BV/MZF, indicating the presence of these elements. This is consistent with the bonds that contained related elements in the FTIR results. The signal of carbon from the apparatus was responsible for the peak observed at C 1s around 284.06 eV. The two major peaks centered at 158.41 eV and 163.71 eV belong to bismuth 4f 5/2 and bismuth 4f 7/2 , respectively, which means that the bismuth state of BV/MZF is Bi 3+ cation (Figure 5b). The peaks at 515.86 eV and 523.47 eV appearing in Figure 5c are generally considered to be vanadium 2p 3/2 and vanadium 2p 1/2 , which means that vanadium is V 5+ in BV/MZF. In the end, the bond energy at 529.02 eV, 640.20 eV, 1051.30 eV, and 710.77 eV were generated by the signals of O 1s, Mn 2p, Zn 2p, and Fe 2p, respectively (Figure 5d-g). In summary, the production of BV/MZF was confirmed.

Characterization on the Structure and Specific Surface Property
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 13 indicating the presence of these elements. This is consistent with the bonds that contained related elements in the FTIR results. The signal of carbon from the apparatus was responsible for the peak observed at C 1s around 284.06 eV. The two major peaks centered at 158.41 eV and 163.71 eV belong to bismuth 4f5/2 and bismuth 4f7/2, respectively, which means that the bismuth state of BV/MZF is Bi 3+ cation (Figure 5b). The peaks at 515.86 eV and 523.47 eV appearing in Figure 5c are generally considered to be vanadium 2p3/2 and vanadium 2p1/2, which means that vanadium is V 5+ in BV/MZF. In the end, the bond energy at 529.02 eV, 640.20 eV, 1051.30 eV, and 710.77 eV were generated by the signals of O 1s, Mn 2p, Zn 2p, and Fe 2p, respectively (Figure 5d-g). In summary, the production of BV/MZF was confirmed.   Figure 6a,b, it is obvious that β-Bi2O3 is composed of flakes, irregular blocks, and particles of different sizes, while BiVO4 shows an irregular dumbbell shape [31]. MZF is irregular and aggregated together, as shown in Figure 6c. The massive flaky β-Bi2O3 can be clearly found through the blue circle in BV/MZF, and the obvious irregular dumbbell-shaped BiVO4 can be found through the red circle in Figure 6d. However, the granular MZF could not be observed because of the relatively low content in BV/MZF.   Figure 6a,b, it is obvious that β-Bi 2 O 3 is composed of flakes, irregular blocks, and particles of different sizes, while BiVO 4 shows an irregular dumbbell shape [31]. MZF is irregular and aggregated together, as shown in Figure 6c. The massive flaky β-Bi 2 O 3 can be clearly found through the blue circle in BV/MZF, and the obvious irregular dumbbell-shaped BiVO 4 can be found through the red circle in Figure 6d. However, the granular MZF could not be observed because of the relatively low content in BV/MZF.   Figure 7c, the energy-dispersive X-ray spectroscopy (EDX) spectrum result proves the existence of Mn, Zn, O, Bi, and V elements in BV/MZF. It is worth noting that there is a strong Cu signal at 8 KeV, which may be from the TEM grid [32]. Combining the analyses of XRD and XPS, it was confirmed that BV/MZF was successfully prepared.    Figure 7c, the energy-dispersive X-ray spectroscopy (EDX) spectrum result proves the existence of Mn, Zn, O, Bi, and V elements in BV/MZF. It is worth noting that there is a strong Cu signal at 8 KeV, which may be from the TEM grid [32]. Combining the analyses of XRD and XPS, it was confirmed that BV/MZF was successfully prepared.   Figure 7c, the energy-dispersive X-ray spectroscopy (EDX) spectrum result proves the existence of Mn, Zn, O, Bi, and V elements in BV/MZF. It is worth noting that there is a strong Cu signal at 8 KeV, which may be from the TEM grid [32]. Combining the analyses of XRD and XPS, it was confirmed that BV/MZF was successfully prepared.   Figure 8 is the N 2 adsorption-desorption isotherm of BV/MZF with its corresponding pore size distribution curve (inset). In Figure 8, the N 2 adsorption-desorption isotherm and hysteresis loop of BV/MZF obtained by the Brunauer isotherm classification method belong to type "IV" and type H3 respectively. Simultaneously, the sample has a mesoporous structure. From the inset in Figure 8, the most likely pore size for BV/MZF is 18 nm. Moreover, the specific surface area of the BV/MZF is 17.84 m 2 /g. Table 1 exhibits the average pore diameter and specific surface area of BiVO 4 , β-Bi 2 O 3 , MZF, and BV/MZF. From Table 1, the pore size (9.49 nm) and specific surface area (17.84 m 2 /g) of BV/MZF are larger than β-Bi 2 O 3 (6.09 nm and 11.81 m 2 /g) and BiVO 4 (6.29 nm and 6.00 m 2 /g). The larger surface area of the BV/MZF nanocomposite provides more active sites during the photocatalytic reaction and enhances the photocatalytic activity. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 13 Figure 8 is the N2 adsorption-desorption isotherm of BV/MZF with its corresponding pore size distribution curve (inset). In Figure 8, the N2 adsorption-desorption isotherm and hysteresis loop of BV/MZF obtained by the Brunauer isotherm classification method belong to type "IV" and type H3 respectively. Simultaneously, the sample has a mesoporous structure. From the inset in Figure 8, the most likely pore size for BV/MZF is 18 nm. Moreover, the specific surface area of the BV/MZF is 17.84 m 2 /g. Table 1 exhibits the average pore diameter and specific surface area of BiVO4, β-Bi2O3, MZF, and BV/MZF. From Table 1, the pore size (9.49 nm) and specific surface area (17.84 m 2 /g) of BV/MZF are larger than β-Bi2O3 (6.09 nm and 11.81 m 2 /g) and BiVO4 (6.29 nm and 6.00 m 2 /g). The larger surface area of the BV/MZF nanocomposite provides more active sites during the photocatalytic reaction and enhances the photocatalytic activity.   Figure 9 presents the magnetization curves of MZF and BV/MZF nanocomposites tested using VSM technology. The saturation magnetization (Ms) of MZF and BV/MZF are 70.61 emu/g and 2.67 emu/g, respectively. The saturation magnetization of BV/MZF only accounts for 3.78% of MZF, owing to the lower content of magnetic material MZF in BV/MZF. However, the magnetic properties of BV/MZF are sufficient for quickly recovering the photocatalyst from the reaction solution under an external magnetic field. Besides, BV/MZF exhibits the same magnetic properties as MZF, which means that the composite material contains MZF [33].   Figure 9 presents the magnetization curves of MZF and BV/MZF nanocomposites tested using VSM technology. The saturation magnetization (Ms) of MZF and BV/MZF are 70.61 emu/g and 2.67 emu/g, respectively. The saturation magnetization of BV/MZF only accounts for 3.78% of MZF, owing to the lower content of magnetic material MZF in BV/MZF. However, the magnetic properties of BV/MZF are sufficient for quickly recovering the photocatalyst from the reaction solution under an external magnetic field. Besides, BV/MZF exhibits the same magnetic properties as MZF, which means that the composite material contains MZF [33].

UV-vis DRS
The optical properties of a semiconductor photocatalytic material have an important relationship with its band structure, which is one of the important factors determining its degradation performance. The UV-vis DRS spectra of BV/MZF is depicted in Figure 10. The largest absorption wavelength and forbidden band energy of BV/MZF are about 593 nm and 2.29 eV, respectively. The literature [22] shows that the largest absorption wavelength and forbidden band energy of β-Bi2O3 was 530 nm and 2.48 eV, respectively. Similarly, according to reference [23], for BiVO4 it was 508 nm and 2.51 eV, respectively. It can be seen that the composite material expands the range of absorbed light. Compared with β-Bi2O3 and BiVO4, BV/MZF has a significantly lower forbidden band energy, resulting in the larger the visible light response range of BV/MZF, which is more conducive to photocatalytic reactions.

UV-vis DRS
The optical properties of a semiconductor photocatalytic material have an important relationship with its band structure, which is one of the important factors determining its degradation performance. The UV-vis DRS spectra of BV/MZF is depicted in Figure 10. The largest absorption wavelength and forbidden band energy of BV/MZF are about 593 nm and 2.29 eV, respectively. The literature [22] shows that the largest absorption wavelength and forbidden band energy of β-Bi 2 O 3 was 530 nm and 2.48 eV, respectively. Similarly, according to reference [23], for BiVO 4 it was 508 nm and 2.51 eV, respectively. It can be seen that the composite material expands the range of absorbed light. Compared with β-Bi 2 O 3 and BiVO 4 , BV/MZF has a significantly lower forbidden band energy, resulting in the larger the visible light response range of BV/MZF, which is more conducive to photocatalytic reactions.

UV-vis DRS
The optical properties of a semiconductor photocatalytic material have an important relationship with its band structure, which is one of the important factors determining its degradation performance. The UV-vis DRS spectra of BV/MZF is depicted in Figure 10. The largest absorption wavelength and forbidden band energy of BV/MZF are about 593 nm and 2.29 eV, respectively. The literature [22] shows that the largest absorption wavelength and forbidden band energy of β-Bi2O3 was 530 nm and 2.48 eV, respectively. Similarly, according to reference [23], for BiVO4 it was 508 nm and 2.51 eV, respectively. It can be seen that the composite material expands the range of absorbed light. Compared with β-Bi2O3 and BiVO4, BV/MZF has a significantly lower forbidden band energy, resulting in the larger the visible light response range of BV/MZF, which is more conducive to photocatalytic reactions.

Stability and Reusability
To further evaluate the stability and reusability of the BV/MZF, three regeneration experiments were performed. At the end of each experiment, the photocatalyst recovered by the external magnet was washed with anhydrous ethanol and deionized water. After drying at 60 • C for 12 h, the next test was performed until the end of the third experiment. The results of three recovery experiments are shown in Figure 11. After photodegradation of RhB for 3 h, the original degradation rate was 92.8%, which only decreased by 7.9% after three cycles of experiments. This outcome shows that the prepared BM/MZF has favorable stability and reusability.

Stability and Reusability
To further evaluate the stability and reusability of the BV/MZF, three regeneration experiments were performed. At the end of each experiment, the photocatalyst recovered by the external magnet was washed with anhydrous ethanol and deionized water. After drying at 60 °C for 12 h, the next test was performed until the end of the third experiment. The results of three recovery experiments are shown in Figure 11. After photodegradation of RhB for 3 h, the original degradation rate was 92.8%, which only decreased by 7.9% after three cycles of experiments. This outcome shows that the prepared BM/MZF has favorable stability and reusability.

Degradation Mechanism
So as to obtain the possible photocatalytic mechanism, the band position of BV/MZF was first calculated by Equations (2)  The photocatalytic mechanism belonging to BV/MZF is shown in Figure 12. Under visible light, the photon energy absorbed by BV/MZF satisfies the equation: hν ≥ Eg. Then, the valence band electrons of BV/MZF transit to the conduction band, and eventually produces holes (h + ) and photogenerated electrons (e − ). h + and e − moved with the appearance of BV/MZF and reacted accordingly. Dissolved oxygen attached to the outside of BV/MZF traps e − to form superoxide anions (·O2 − ), and h + converts the OH − and H2O adsorbed on the surface of BV/MZF into hydroxyl radicals (·OH). Organic pollutants decomposed into other products due to the strong oxidative properties of ·O2 − and ·OH.

Degradation Mechanism
So as to obtain the possible photocatalytic mechanism, the band position of BV/MZF was first calculated by Equations (2) and (3) [34], where the implications of χ, E e , E g , E VB , and E CB were the negative values of the ground potential chemical potential, energy of the free electrons of hydrogen (4.5 eV), band gap energy of BV/MZF, potential of VB, and potential of CB. The χ values of BiVO 4 and β-Bi 2 O 3 were calculated to be 6.78 eV and 5.90 eV, respectively. The E VB and E CB values of β-Bi 2 O 3 were calculated to be 2.67 eV and 0.16 eV, respectively. Similarly, the corresponding E VB (3.51 eV) and E CB (1.03 eV) values of BiVO 4 were obtained. The photocatalytic mechanism belonging to BV/MZF is shown in Figure 12. Under visible light, the photon energy absorbed by BV/MZF satisfies the equation: hν ≥ E g . Then, the valence band electrons of BV/MZF transit to the conduction band, and eventually produces holes (h + ) and photo-generated electrons (e − ). h + and e − moved with the appearance of BV/MZF and reacted accordingly. Dissolved oxygen attached to the outside of BV/MZF traps e − to form superoxide anions (·O 2 − ), and h + converts the OH − and H 2 O adsorbed on the surface of BV/MZF into hydroxyl radicals (·OH). Organic pollutants decomposed into other products due to the strong oxidative properties of ·O 2 − and ·OH. The improvement of BV/MZF photocatalytic activity can be due to the following reasons. (1) Composition of p-n heterojunction. The ECB and EVB of n-type BiVO4 are more positive and more negative than p-type β-Bi2O3, respectively [35]. e − of p-type β-Bi2O3 will migrate to n-type BiVO4, and h + of n-type BiVO4 will transfer to p-type β-Bi2O3, thereby constructing a p-n-type heterojunction. Meanwhile, there is an internal electric field inside the BV/MZF system, which will further boost the migration and detachment of photo-generated carriers. (2) As a result of the reduced Eg of BV/MZF, the visible light response is enhanced, and the scope of absorbed light is increased. BV/MZF will absorb more incident hν and generate more e − and h + , producing more ·O2 − and ·OH. (3) The stable magnetic field generated by MZF makes e − directional flow, suppresses the recombination of e − and h + , extends the e − life, and raises the photocatalytic activity of BV/MZF. (4) Photosensitive degradation of RhB. Due to the low quantum efficiency and low conversion rate of RhB, there could be weak photodegradation during the degradation process [36].

Conclusions
The composite magnetic photocatalyst BV/MZF was produced by a sample hydrothermal and calcination method and applied for photodegrading RhB. According to the analysis of chemical composition, valence state, and structure, the three-component photocatalyst BV/MZF was successfully compounded. Under visible light irradiation, BV/MZF showed excellent photocatalytic activity. A reasonable photocatalytic mechanism indicates that the p-n type heterojunction formed at the β-Bi2O3 and BiVO4 interface and the stable electric field provided by MZF are helpful for the transfer and separation of photoelectrons. This paper provides a simple and effective method for the synthesis of composite magnetic photocatalysts, and has potential for application in the fields of green chemistry and environmental protection.