Magnetic Photocatalyst BiVO4/Mn-Zn ferrite/Reduced Graphene Oxide: Synthesis Strategy and Its Highly Photocatalytic Activity

Magnetic photocatalyst BiVO4/Mn-Zn ferrite (Mn1−xZnxFe2O4)/reduced graphene oxide (RGO) was synthesized by a simple calcination and reduction method. The magnetic photocatalyst held high visible light-absorption ability with low band gap energy and wide absorption wavelength range. Electrochemical impedance spectroscopies illustrated good electrical conductivity which indicated low charge-transfer resistance due to incorporation of Mn1−xZnxFe2O4 and RGO. The test of photocatalytic activity showed that the degradation ratio of rhodamine B (RhB) reached 96.0% under visible light irradiation after only 1.5 h reaction. The photocatalytic mechanism for the prepared photocatalyst was explained in detail. Here, the incorporation of RGO enhanced the specific surface area compared with BiVO4/Mn1−xZnxFe2O4.The larger specific surface area provided more active surface sites, more free space to improve the mobility of photo-induced electrons, and further facilitated the effective migration of charge carriers, leading to the remarkable improvement of photocatalytic performance. Meanwhile, RGO was the effective acceptor as well as transporter of photo-generated electron hole pairs. •O2− was the most active species in the photocatalytic reaction. BiVO4/Mn1−xZnxFe2O4/RGO had quite a wide application in organic contaminants removal or environmental pollution control.


Introduction
In the recent decade, composite semiconductor materials are considered extraordinarily attractive in the field of solar energy and pollution control engineering. Many kinds of photocatalytic composite materials with superior optical properties and high photo-induced activity have been synthesized and studied [1,2]. However, the utilization efficiency of visible light for some photocatalysts is very low, owing to their large intrinsic band gap energy, which impels scientists to explore new photocatalytic compounds with high visible light-driven photocatalytic activity. Bismuth-based composites with n-type junctions exhibited excellent photocatalytic activity and high stability [3]. Among them, reference electrode was the saturated calomel electrode (SCE), setting AC voltage amplitude of 5 mV and a frequency range of 1 × 10 5 -1 × 10 −2 Hz.

Photocatalytic Activity, Stability, and Corresponding Mechanism
The photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was investigated by the rhodamine B (RhB) degradation under visible light irradiation [22]. Ninety milligrams of composite photocatalyst (named fresh photocatalyst) was put into 5.0 mg/L RhB solution (100.0 mL). The solution was placed for 0.5 h with stirring in the dark to reach the adsorption-desorption equilibrium. A 500 W Xe lamp was used as the visible light source, equipped with ultraviolet (UV) light cut-off filter (λ ≥ 400 nm). At given irradiation time intervals, a series of the reaction solution was sampled and the absorption spectrum was measured.
The stability for the photocatalyst were assessed by cycling tests. After each cycle, the photocatalyst was separated and recovered by means of an external magnet. The recovered catalyst was respectively washed with ethanol and deionized water, then dried at the end of each cycle.

Optimal Synthesis Condition
The Mn 1−x Zn x Fe 2 O 4 , prepared in advance, had a strong magnetization. In order to completely form BiVO 4 precursor and reduce the impurity, Mn 1−x Zn x Fe 2 O 4 was put into the precursor instead of Bi(NO 3 ) 3 solution, in other words, BiVO 4 precursor was already formed before magnetic substance was added. Mn 1−x Zn x Fe 2 O 4 /BiVO 4 was assembled via calcination at only 450 • C. This temperature was lower than that of Mn 1−x Zn x Fe 2 O 4 formation (1200 • C), as well as BiVO 4 formation (500 • C). Therefore, the calcination approach was indeed low-cost and economical.
GO was dispersed in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 with deionized water under room temperature, RGO was produced by NH 3 H 2 O + N 2 H 4 H 2 O reduction of GO without heating. This in situ synthesis method was simple and with low-energy consumption.

Structure and Phase Identification
The XRD spectra of the obtained samples were shown in Figure 1 [20]. The diffraction peaks of Mn 1−x Zn x Fe 2 O 4 patterns were hardly observed in Figure 1c,d. Not only was the amount (15.0%) of the magnetic matrix low, but also, the diffraction patterns location of Mn 1−x Zn x Fe 2 O 4 overlapped with the domain diffraction patterns of BiVO 4 . The diffraction peak of GO (Figure 1e) was observed at 10.8 • (crystal plane (001)) [23]. However, the peak (Figure 1d) disappeared after GO was mostly reduced to RGO under the reduction of NH 3 H 2 O and N 2 H 4 H 2 O [24]. Moreover, the amount (3%, w/w) of RGO was not enough to be detected in X-ray diffraction. In short, it was deduced that the prepared samples totally exhibited good crystallinity.
The peak-intensity ratio (I D /I G ) of D band (~1364.0 cm −1 , originating from disorder-activated Raman mode) and G band (~1598.0 cm −1 , corresponding to sp 2 hybridized carbon) in RGO was usually used to assess the reduction extent. Figure 2 showed the Raman spectra of the above-obtained samples. It was seen that G-band of RGO was shifted from 1598.0 cm −1 to 1589.0 cm −1 , while the D-band shorted from 1364.0 cm −1 to 1352 cm −1 after the thermal reduction finished. The I D /I G ratio of GO was 1.10, and that of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO decreased to 0.84. The relative low I D /I G ratio of RGO implied high reduction efficiency in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO [17]. Typical Raman bands of BiVO 4 were located at 120.0, 210.0, 324.0, 366.0, and 826.0 cm -1 in Figure 2. The two bands at 324.0 cm −1 and 366.0 cm −1 changed into one wide band in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 , as well as in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO. The result was also consistent with the results of the previous report [25].  To investigate the valence state and the surface property of BiVO4/Mn1-xZnxFe2O4/RGO, XPS spectrum characterization was employed. As displayed in Figure 3a, the spectrum intensity of C 1s in BiVO4/Mn1-xZnxFe2O4/RGO was larger than that in BiVO4/Mn1-xZnxFe2O4, namely, the introduction of RGO brought the intensity increase of C 1s. The spectrum intensity of oxygen-containing functional groups in Figure 3b was larger than that in Figure 3c, meaning the decrease of GO and the increase of RGO in BiVO4/Mn1-xZnxFe2O4/RGO sample. This feature confirmed the efficient reduction of GO and the valence states for various elements in BiVO4/Mn1-xZnxFe2O4/RGO. Figure 4 was the transmission electron microscopy (TEM) images of the as-synthesized BiVO4/Mn1-xZnxFe2O4/RGO. In detail, there were the black core of Mn1-xZnxFe2O4 and the gray shell of BiVO4, and RGO sheets had good interfacial contact with BiVO4/Mn1-xZnxFe2O4 spherical particle. In   To investigate the valence state and the surface property of BiVO4/Mn1-xZnxFe2O4/RGO, XPS spectrum characterization was employed. As displayed in Figure 3a, the spectrum intensity of C 1s in BiVO4/Mn1-xZnxFe2O4/RGO was larger than that in BiVO4/Mn1-xZnxFe2O4, namely, the introduction of RGO brought the intensity increase of C 1s. The spectrum intensity of oxygen-containing functional groups in Figure 3b was larger than that in Figure 3c, meaning the decrease of GO and the increase of RGO in BiVO4/Mn1-xZnxFe2O4/RGO sample. This feature confirmed the efficient reduction of GO and the valence states for various elements in BiVO4/Mn1-xZnxFe2O4/RGO. Figure 4 was the transmission electron microscopy (TEM) images of the as-synthesized BiVO4/Mn1-xZnxFe2O4/RGO. In detail, there were the black core of Mn1-xZnxFe2O4 and the gray shell of BiVO4, and RGO sheets had good interfacial contact with BiVO4/Mn1-xZnxFe2O4 spherical particle. In other words, there was an overlap between BiVO4/Mn1-xZnxFe2O4 and RGO. At the same time, energy  Specific surface area of the as-obtained compounds was determined with the adsorption instrument, and the result was shown in Figure 5. The adsorption-desorption isotherms in Figure 5 were the typical isotherm III, agreeing with the reference report [26]. The discrete curve of BiVO4/Mn1-xZnxFe2O4/RGO was in p/p0 range of 0.45-0.55, and the pore diameter distribution was mainly 2-10 nm, and the most probable distribution was located in 4 nm. It was deduced that the introduction of RGO caused the mesopore increase and the macropore decrease. Thus, there was the uniform surface  Specific surface area of the as-obtained compounds was determined with the adsorption instrument, and the result was shown in Figure 5. The adsorption-desorption isotherms in Figure 5 were the typical isotherm III, agreeing with the reference report [26]. The discrete curve of BiVO4/Mn1-xZnxFe2O4/RGO was in p/p0 range of 0.45-0.55, and the pore diameter distribution was mainly 2-10 nm, and the most probable distribution was located in 4 nm. It was deduced that the introduction of Specific surface area of the as-obtained compounds was determined with the adsorption instrument, and the result was shown in Figure 5. The adsorption-desorption isotherms in Figure 5 were the typical isotherm III, agreeing with the reference report [26]. The discrete curve of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was in p/p 0 range of 0.45-0.55, and the pore diameter distribution was mainly 2-10 nm, and the most probable distribution was located in 4 nm. It was deduced that the introduction of RGO caused the mesopore increase and the macropore decrease. Thus, there was the uniform surface structure in the ternary composite. Calculating with the data in Figure 6, the specific surface area of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was 8.84 m 2 /g, and that of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was only 2.22 m 2 /g. The incorporation of RGO enhanced the specific surface area compared with BiVO4/Mn 1−x Zn x Fe 2 O 4 . The larger specific surface area provided more active surface sites, more free space to improve the mobility of photo-induced electrons, and further facilitated the effective migration of charge carriers, leading to the remarkable improvement of photocatalytic performance [27]. The surface structure characterization could demonstrate, in advance, the photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO to some extent.

Magnetic Performance and Optical Properties
The magnetic hysteresis loops of the samples were displayed in Figure 6. The saturation magnetization (Ms) of Mn1-xZnxFe2O4 and BiVO4/Mn1-xZnxFe2O4/RGO were 84.03 and 8.21 emu g −1 , respectively. Ms of the compounds was lower than that of the pure Mn1-xZnxFe2O4, owing to the amount decrease of the magnetic substance quantity in per unit composite. It was obvious that the prepared composite BiVO4/Mn1-xZnxFe2O4/RGO had a soft-magnetic feature like pure Mn1-xZnxFe2O4, which further confirmed than the synthesized composite must be comprised of Mn1-xZnxFe2O4 component [20,26].
It was worth noting that Ms was no attenuation after BiVO4/Mn1-xZnxFe2O4/RGO was employed after five rounds of recycling, indicating the stable magnetism of the as-prepared composite photocatalyst. More importantly, the compound exhibited outstanding paramagnetism because both coercivity (Hc) and remnant magnetization (Mr) were near to zero. Obviously, the excellent magnetic

Magnetic Performance and Optical Properties
The magnetic hysteresis loops of the samples were displayed in Figure 6. The saturation magnetization (Ms) of Mn1-xZnxFe2O4 and BiVO4/Mn1-xZnxFe2O4/RGO were 84.03 and 8.21 emu g −1 , respectively. Ms of the compounds was lower than that of the pure Mn1-xZnxFe2O4, owing to the amount decrease of the magnetic substance quantity in per unit composite. It was obvious that the prepared composite BiVO4/Mn1-xZnxFe2O4/RGO had a soft-magnetic feature like pure Mn1-xZnxFe2O4, which further confirmed than the synthesized composite must be comprised of Mn1-xZnxFe2O4 component [20,26].
It was worth noting that Ms was no attenuation after BiVO4/Mn1-xZnxFe2O4/RGO was employed after five rounds of recycling, indicating the stable magnetism of the as-prepared composite photocatalyst. More importantly, the compound exhibited outstanding paramagnetism because both

Magnetic Performance and Optical Properties
The magnetic hysteresis loops of the samples were displayed in Figure 6. The saturation magnetization (Ms) of Mn 1 [20,26].
It was worth noting that Ms was no attenuation after BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was employed after five rounds of recycling, indicating the stable magnetism of the as-prepared composite photocatalyst. More importantly, the compound exhibited outstanding paramagnetism because both coercivity (Hc) and remnant magnetization (Mr) were near to zero. Obviously, the excellent magnetic property ensured the high recovery ratio of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO using an external magnet after reaction.
The light absorption ability of the as-prepared samples was investigated with UV-vis DRS, and the diffuse reflectance spectra were recorded in Figure 7. It was seen from Figure 8a that the maximum absorption wavelength (λ max ) of pure BiVO 4 was about 500 nm. The further insights revealed the absorbance (at λ max = 500 nm) of the compounds was higher than that of BiVO 4 . The band gap energy (E g ) was estimated from (Ahv) 1/2~h v plots [5] (Figure 7b) [20]. However, the introduction of RGO could be conducive to lessen E g , leading to the enhancement of visible light absorbance for BiVO 4 /Mn 1−x Zn x Fe 2 O 4 . It is true that the great light absorption was closely related to good photocatalytic activity of catalysts [26].

Electrochemical Performance
Electrochemical impedance spectroscopy (EIS) was an effective approach to evaluate electron

Electrochemical Performance
Electrochemical impedance spectroscopy (EIS) was an effective approach to evaluate electron transfer ability in the interface between solid phase electrodes and electrolyte solution [28]. The typical impedance spectra of the samples were displayed with Nyquist plots. The semicircle diameter in Figure 8 became small when RGO inserted in the work electrode contained the compound. This change implied the resistance decrease and the conductivity increase in the test interface. The chargetransfer resistance (Rct) of the samples was gained by fitting the data from Figure 8. Rct of BiVO4,

Electrochemical Performance
Electrochemical impedance spectroscopy (EIS) was an effective approach to evaluate electron transfer ability in the interface between solid phase electrodes and electrolyte solution [28]. The typical impedance spectra of the samples were displayed with Nyquist plots. The semicircle diameter in Figure 8 became small when RGO inserted in the work electrode contained the compound. This change implied the resistance decrease and the conductivity increase in the test interface. The charge-transfer resistance (R ct ) of the samples was gained by fitting the data from Figure 8. R ct of BiVO 4 , BiVO 4 /Mn 1−x Zn x Fe 2 O 4 , and BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO were 351.0 Ω cm 2 , 206.0 Ω cm 2 , and 103.0 Ω cm 2 , respectively. It was clear that R ct of the ternary composite was the lowest.
The good electron accepting and transporting properties of RGO could contribute to the prevention of charge recombination. It was reasonable that the introduction of RGO was beneficial to the efficient charge separation and transportation in the compound interface. The electrochemical behavior brought high conductivity of the comprising electrode. As a result, the enhancement of conductivity promoted the improvement of photocatalytic activity for BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO.

Photocatalytic Activity, Stability, and Corresponding Mechanism
The photocatalytic activity was probed with photodegradation of RhB dye, and the result was shown in Figure 9. It was found from Figure 9 [19,22]. The high photocatalytic property of the as-produced compound BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was explained as follows: the graphene owned two-dimensional π-π conjugate structure was not only a good electron acceptor, but also a good electronic vector. RGO excited electrons in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 and prompted the transferring of the conduction band in itself [29]. It was more interesting that the photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was obviously better than that of Mn 1−x Zn x Fe 2 O 4 /β-Bi 2 O 3 in previous our group's report [20]. The good electron accepting and transporting properties of RGO could contribute to the prevention of charge recombination. It was reasonable that the introduction of RGO was beneficial to the efficient charge separation and transportation in the compound interface. The electrochemical behavior brought high conductivity of the comprising electrode. As a result, the enhancement of conductivity promoted the improvement of photocatalytic activity for BiVO4/Mn1-xZnxFe2O4/RGO.

Photocatalytic Activity, Stability, and Corresponding Mechanism
The photocatalytic activity was probed with photodegradation of RhB dye, and the result was shown in Figure 9. It was found from Figure 9 that the degradation ratio of RhB with BiVO4/Mn1-xZnxFe2O4/RGO under visible light irradiation reached to 96.0% after only 1.5 h reaction. It is worth noting that the self-degradation of RhB was very weak in the comparative test. It took about 3 h to get the same degradation ratio (96.0%) with pure BiVO4 as well as BiVO4/Mn1-xZnxFe2O4 under identical conditions. Significantly, BiVO4/Mn1-xZnxFe2O4/RGO exhibited more excellent photocatalytic activity than that of BiVO4/Mn1-xZnxFe2O4. Moreover, the activity of BiVO4/Mn1-xZnxFe2O4/RGO was greatly superior to that of SrFe12O19/BiVO4, as well as BiVO4/RGO, in the literature [19,22]. The high photocatalytic property of the as-produced compound BiVO4/Mn1-xZnxFe2O4/RGO was explained as follows: the graphene owned two-dimensional π-π conjugate structure was not only a good electron acceptor, but also a good electronic vector. RGO excited electrons in BiVO4/Mn1-xZnxFe2O4 and prompted the transferring of the conduction band in itself [29]. It was more interesting that the photocatalytic activity of BiVO4/Mn1-xZnxFe2O4/RGO was obviously better than that of Mn1-xZnxFe2O4/β-Bi2O3 in previous our group's report [20]. The stability was a key property in the industrial application of catalytic materials. Each recycling experiment was operated in triplicate, and average values and standard deviations were also shown in Figure 10. The degradation ratio of RhB in the fifth recycling was still 85.0% after 1.5 h of reaction under the same test parameters. The photocatalytic activity was only reduced a little within five recycles. The result revealed good stability of BiVO4/Mn1-xZnxFe2O4/RGO.   Figure 11 belonged to C=C stretch of aromatic group in RGO. The peak at 1629.5cm −1 in Figure 11b was weaker than that in Figure 11a, due to a little loss of RGO quantity after the fifth cycle. The absorption peaks located at around 1400.0 cm −1 and 1065.0 cm −1 illustrated the functional group of RGO [30]. By comparing pattern (a) and (b) in Figure 11, the typical peaks were detected in the spectra of the fresh, as well as the recovered compound. Thereby, it was concluded that the structure of BiVO4/Mn1-xZnxFe2O4/RGO was stable in the process of RhB photocatalytic degradation. The photocatalytic mechanism of BiVO4/Mn1-xZnxFe2O4/RGO was probed with radical scavengers [31]. t-BuOH (•OH scavenger), EDTA-Na2 (h + scavenger), and BZQ (•O2 − scavenger) were employed to ascertain the dominant radical species in the photocatalytic degradation of RhB with the as-synthesized compound. Degradation ratios of RhB under these scavengers were given in Figure 12. The photodegradation ratio of RhB was 72.0% and 65.0% only when 5.0 mM t-BuOH and 1.0 mM EDTA-Na2 were added into the reaction system. Namely, h + or •OH scavenger brought about the decrease of degradation ratios. Thus, the photocatalytic activity of the compound greatly decreased. The inactivation of the photocatalytic test was evidently proven when 1 mM BZQ was added in the same reaction system. The above results demonstrated that the most active species was •O2 − , though •OH and h + also contributed to the photocatalytic activity of BiVO4/Mn1-xZnxFe2O4/RGO.   Figure 11 belonged to C=C stretch of aromatic group in RGO. The peak at 1629.5 cm −1 in Figure 11b was weaker than that in Figure 11a, due to a little loss of RGO quantity after the fifth cycle. The absorption peaks located at around 1400.0 cm −1 and 1065.0 cm −1 illustrated the functional group of RGO [30]. By comparing pattern (a) and (b) in Figure 11, the typical peaks were detected in the spectra of the fresh, as well as the recovered compound. Thereby, it was concluded that the structure of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was stable in the process of RhB photocatalytic degradation.   Figure 11 belonged to C=C stretch of aromatic group in RGO. The peak at 1629.5cm −1 in Figure 11b was weaker than that in Figure 11a, due to a little loss of RGO quantity after the fifth cycle. The absorption peaks located at around 1400.0 cm −1 and 1065.0 cm −1 illustrated the functional group of RGO [30]. By comparing pattern (a) and (b) in Figure 11, the typical peaks were detected in the spectra of the fresh, as well as the recovered compound. Thereby, it was concluded that the structure of BiVO4/Mn1-xZnxFe2O4/RGO was stable in the process of RhB photocatalytic degradation. The photocatalytic mechanism of BiVO4/Mn1-xZnxFe2O4/RGO was probed with radical scavengers [31]. t-BuOH (•OH scavenger), EDTA-Na2 (h + scavenger), and BZQ (•O2 − scavenger) were employed to ascertain the dominant radical species in the photocatalytic degradation of RhB with the as-synthesized compound. Degradation ratios of RhB under these scavengers were given in Figure 12. The photodegradation ratio of RhB was 72.0% and 65.0% only when 5.0 mM t-BuOH and 1.0 mM EDTA-Na2 were added into the reaction system. Namely, h + or •OH scavenger brought about the decrease of degradation ratios. Thus, the photocatalytic activity of the compound greatly decreased. The inactivation of the photocatalytic test was evidently proven when 1 mM BZQ was added in the same reaction system. The above results demonstrated that the most active species was •O2 − , though •OH and h + also contributed to the photocatalytic activity of BiVO4/Mn1-xZnxFe2O4/RGO. The photocatalytic mechanism of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was probed with radical scavengers [31]. t-BuOH (•OH scavenger), EDTA-Na 2 (h + scavenger), and BZQ (•O 2 − scavenger) were employed to ascertain the dominant radical species in the photocatalytic degradation of RhB with the as-synthesized compound. Degradation ratios of RhB under these scavengers were given in Figure 12. The photodegradation ratio of RhB was 72.0% and 65.0% only when 5.0 mM t-BuOH and 1.0 mM EDTA-Na 2 were added into the reaction system. Namely, h + or •OH scavenger brought about the decrease of degradation ratios. Thus, the photocatalytic activity of the compound greatly decreased. The inactivation of the photocatalytic test was evidently proven when 1 mM BZQ was added in the same reaction system. The above results demonstrated that the most active species was •O 2 − , though •OH and h + also contributed to the photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO. Figure 13 described the electron-hole pairs forming process under the light irradiation. The potentials of conduction band (CB) and valence band (VB) of BiVO 4 were 0.46 eV and 2.86 eV, respectively (referring to hydrogen electrode, NHE). The electrons in VB were excited to CB under visible light irradiation, forming driven-electrons (e − ) and holes (h +  Figure 13 described the electron-hole pairs forming process under the light irradiation. The potentials of conduction band (CB) and valence band (VB) of BiVO4 were 0.46 eV and 2.86 eV, respectively (referring to hydrogen electrode, NHE). The electrons in VB were excited to CB under visible light irradiation, forming driven-electrons (e − ) and holes (h + ). The VB potential of BiVO4 was close to E θ of •OH/H2O, and the CB potential was larger than E θ of O2/•O2 − . This meant that electrons were able to directly reduce O2 molecules into superoxide O2 − . Besides, RhB molecules were directly oxidized by the holes on VB of BiVO4. In addition, there were much more active adsorption centers and photocatalytic reaction sites in RGO with a large surface area. These active centers and sites were beneficial to the improvement of the photocatalytic activity. As a good electron accepter and electronic vector, RGO facilitates the transmission of photo-produced electrons, which was conducive to the separation of photo-produced electrons and holes, and further promoted the formation of •O2 − . It was ensured that •O2 − played the main role for the RhB photodegradation, though •OH and h + had a collaborative oxidation role in the photocatalytic reaction of BiVO4/Mn1-xZnxFe2O4/RGO.    Figure 13 described the electron-hole pairs forming process under the light irradiation. The potentials of conduction band (CB) and valence band (VB) of BiVO4 were 0.46 eV and 2.86 eV, respectively (referring to hydrogen electrode, NHE). The electrons in VB were excited to CB under visible light irradiation, forming driven-electrons (e − ) and holes (h + ). The VB potential of BiVO4 was close to E θ of •OH/H2O, and the CB potential was larger than E θ of O2/•O2 − . This meant that electrons were able to directly reduce O2 molecules into superoxide O2 − . Besides, RhB molecules were directly oxidized by the holes on VB of BiVO4. In addition, there were much more active adsorption centers and photocatalytic reaction sites in RGO with a large surface area. These active centers and sites were beneficial to the improvement of the photocatalytic activity. As a good electron accepter and electronic vector, RGO facilitates the transmission of photo-produced electrons, which was conducive to the separation of photo-produced electrons and holes, and further promoted the formation of •O2 − . It was ensured that •O2 − played the main role for the RhB photodegradation, though •OH and h + had a collaborative oxidation role in the photocatalytic reaction of BiVO4/Mn1-xZnxFe2O4/RGO.   In fact, since our group found that the magnetic composite ZnFe 2 O 4 /SrFe 12 O 19 had a highly photocatalytic activity in 2013 [27], we firmly thought that the stable magnetic field from SrFe 12 O 19 itself could promote the separation of photo-generated electrons and holes, and furthermore, that the photocatalyst could produce more photo-generated electrons and holes under identical light irradiation. Thus, the photoelectric transformation efficiency would be boosted. Our group the studied BiVO 4 /SrFe 12 [22,32,33] magnetic heterojunction to confirm previous speculation. However, these studies about its photoelectron transfer mechanism were not enough. In future work, our group will continue to attempt to confirm our speculation via experimental and theoretical calculations. Of course, this work was carried out in order to compare with SrFe 12 O 19 functions.

Conclusions
Magnetic photocatalyst BiVO 4 /Mn 1−x Zn x Fe 2 O 4 /RGO was synthesized with the simple and economical roasting-reduction approach. The photocatalyst exhibited excellent photocatalytic activity and stability. The degradation ratio of RhB reached 96.0% under visible light irradiation after only 1.5 h reaction with the photocatalyst. The degradation ratio of RhB was still maintained at 85.0% after five cycles of photocatalytic reaction. Here, the incorporation of RGO enhanced the specific surface area compared with BiVO4/Mn 1−x Zn x Fe 2 O 4 .The larger specific surface area provided more active surface sites, more free space to improve the mobility of photo-induced electrons, and further facilitated the effective migration of charge carriers, leading to the remarkable improvement of photocatalytic performance. RGO was the effective acceptor as well as transporter of photo-generated electron-hole pairs. •O 2 − was the most active species in this photocatalytic reaction. We hope this photocatalyst has a wide application in organic contaminants removal or environmental pollution control in practical.