Facile Synthesis of Magnetic Photocatalyst Ag/BiVO4/Mn1−xZnxFe2O4 and Its Highly Visible-Light-Driven Photocatalytic Activity

Ag/BiVO4/Mn1−xZnxFe2O4 was synthesized with a dip-calcination in situ synthesis method. This work was hoped to provide a simple method to synthesis three-phase composite. The phase structure, optical properties and magnetic feature were confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectrophotometer (UV-vis DRS), and vibrating sample magnetometer (VSM). The photocatalytic activity was investigated by Rhodamine B (RhB) photo-degradation under visible light irradiation. The photo-degradation rate of RhB was 94.0~96.0% after only 60 min photocatalytic reaction under visible light irradiation, revealing that it had an excellent visible-light-induced photocatalytic activity. In the fifth recycle, the degradation rate of Ag/BiVO4/Mn1−xZnxFe2O4 still reached to 94.0%. Free radical tunnel experiments confirmed the dominant role of •O2− in the photocatalytic process for Ag/BiVO4/Mn1−xZnxFe2O4. Most importantly, the mechanism that multifunction Ag could enhance photocatalytic activity was explained in detail.


Introduction
Semiconductor photocatalysts have been paid more attention in the application of research of clean energy exploration and environmental pollutants removal. These photocatalysts possess superior physicochemical and magneto-optical properties [1][2][3][4]. Non-toxic bismuth vanadate (BiVO 4 ), with good light absorption and high ionic conductivity [5][6][7][8], has attracted strong interest from scientists. Although the visible-light sensitivity and photocatalytic activity of monoclinic scheelite BiVO 4 (m-BiVO 4 ) are the largest among three crystals including additionally tetragonal zircon and tetragonal scheelite [9][10][11][12], the light absorption and the catalytic property of m-BiVO 4 can be further improved by various strategies. The photocatalytic activity can be greatly improved when the photo-generated electrons and holes are efficiently separated. BiVO 4 -based composites with a high separation efficiency of photo-generated electrons and holes have been developed to enlarge the quantum efficiency of BiVO 4 and the photocatalytic activity of BiVO 4 . Meanwhile, doping noble metal in photocatalysts is an effective way of promoting the efficient separation of photo-generated electrons and holes. Researchers have reported that the electron-hole separation in doping compounds was strengthened by the charge transfer between semiconductor and noble metal [13][14][15][16][17]. Ag is the most promising noble metal because of the low cost and strong electron trapping ability. Ag-doped catalyst could induce surface plasmon resonance, involving in a better absorption of visible light [18].
For the convenience of recovery and separation of photocatalysts after reaction, magnetic photocatalysts have been fabricated in recent years [19][20][21][22]. Magnetic photocatalysts could be recovered with an external magnetic field, and a high recovery ratio would be conducive to promote their industrial application. In our previous research, both the soft-magnetic Mn 1−x Zn x Fe 2 O 4 /Bi 2 O 3 [21] and hard-magnetic SrFe 12 O 19 /BiVO 4 [22] with photocatalytic properties were prepared with dip-calcination method. Further exploration is necessary to synthesize m-BiVO 4 -based composite with high photocatalytic activity as well as large recovery ratio. In this work, Ag was doped in BiVO 4 [19]. The RhB degradation reaction using BiVO 4 /Mn 1−x Zn x Fe 2 O 4 as photocatalyst was very slow (take 3 h). The incorporation of Ag could enhance the photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 .

Synthesis of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4
Mn 1−x Zn x Fe 2 O 4 was prepared according to the literature [19,21]. The precursor of BiVO 4 was produced by the chemical co-precipitation way [22]. 486 mg Mn 1−x Zn x Fe 2 O 4 was dispersed into the precursor of BiVO 4 and dried at 80 • C for 12 h.

Measurement of the Photocatalytic Performance
The photocatalytic activity of the as-prepared composites was investigated by the photodegradation of simulated dye wastewater (Rhodamine B, RhB) under visible light irradiation. 100 mg photocatalyst was put into 100 mL RhB solution of 5 mg/L. Then the suspension liquid was placed in dark for 0.5 h with stirring to reach the adsorption-desorption equilibrium. A 500 W Xe lamp, equipping with UV cut-off filter, was used as the visible light source (λ ≥ 420 nm). At the given irradiation time intervals, a series of the reaction solution was sampled and measured the absorption with the UV-vis spectrophotometer (TU-1901). The photocatalytic mechanism of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was explored by holes-radical trapping experiments with p-benzoquinone (BZQ, •O 2 − radical scavenger), EDTA-Na 2 (hole scavenger), and tert-butanol (t-BuOH, •OH radical scavenger). The repeatability of the photocatalyst was detected by cycling tests. After each cycle, the catalyst was separated by an external magnetic field, then washed and dried for the next cycle.

Synthesis Condition and Structure Identification
The appropriate mass ratio of Mn  Figure 1. It was noticed that each diffraction peak of Mn 1−x Zn x Fe 2 O 4 was indexed to the franklinite (cubic spinel) phase [23] which belonged to the Fd-3m (227) space group with a lattice size of 0.8474 nm. Three peaks at 28.9 • , 35.2 • , and 46.0 • were clearly attributed to the iconic twin peaks of monoclinic scheelite BiVO 4 (JCPDS 14-0688) [9]. The lattice parameters of the prepared BiVO 4 was a = 5.1175 nm, b = 11.6697 nm, and c = 5.1084 nm. The peak at 28.9 • (121) was used to calculate the average crystallite size that was 27. with the UV-vis spectrophotometer (TU-1901). The photocatalytic mechanism of Ag/BiVO4/Mn1−xZnxFe2O4 was explored by holes-radical trapping experiments with p-benzoquinone (BZQ, •O2 − radical scavenger), EDTA-Na2 (hole scavenger), and tert-butanol (t-BuOH, •OH radical scavenger). The repeatability of the photocatalyst was detected by cycling tests. After each cycle, the catalyst was separated by an external magnetic field, then washed and dried for the next cycle.

Synthesis Condition and Structure Identification
The appropriate mass ratio of Mn1−xZnxFe2O4 and BiVO4 was essential for BiVO4/Mn1−xZnxFe2O4. Thus, the composite possessed not only a good magnetization but also a high photocatalytic activity. By the comparison experiments, it is found that the composite held the largest magnetic property without the reduction of photocatalytic activity when 15.0 wt % Mn1−xZnxFe2O4 was loaded in BiVO4 by the dip-calcination approach. The doping quantity of Ag was not only closely related to the photocatalytic activity but also affected the cost of Ag/BiVO4/Mn1−xZnxFe2O4. PVP was confirmed as an efficient stabilizer and reductant in the synthesis process of Ag/BiVO4/Mn1−xZnxFe2O4. Its suitable dosage was 1.0 g in 50 mL ethanol solution. With a series of tests, the optimized doping dosage of Ag in the magnetic composite was determined to be 12.0 wt %.
XRD patterns of Mn1−xZnxFe2O4, BiVO4, BiVO4/Mn1−xZnxFe2O4, and Ag/BiVO4/Mn1−xZnxFe2O4 were illustrated in Figure 1. It was noticed that each diffraction peak of Mn1−xZnxFe2O4 was indexed to the franklinite (cubic spinel) phase [23] which belonged to the Fd-3m (227) space group with a lattice size of 0.8474 nm. Three peaks at 28.9°, 35.2°, and 46.0° were clearly attributed to the iconic twin peaks of monoclinic scheelite BiVO4 (JCPDS 14-0688) [9]. The lattice parameters of the prepared BiVO4 was a = 5.1175 nm, b = 11.6697 nm, and c = 5.1084 nm. The peak at 28.9° (121) was used to calculate the average crystallite size that was 27.2 nm, while the average size of BiVO4/Mn1−xZnxFe2O4 and Ag/BiVO4/Mn1−xZnxFe2O4 was 29.0 nm and 32.8 nm, respectively. To gain further insight into the structure of Ag/BiVO4/Mn1−xZnxFe2O4, we carried out the measurement of bonds vibration absorption with Fourier transform infrared spectroscopy. Figure 2 showed the FTIR spectra of the composites.    To discern the element contents in Ag/BiVO4/Mn1−xZnxFe2O4 and determine their valence states, XPS study was carried out. The binding energy peaks of Ag, O, V, Bi, Fe, Zn, and Mn were recorded in Figure 3. The peaks of O, V, Bi, Fe, Mn, and Zn elements were clearly observed in Figure 3a. Further comparing the fully scanning XPS spectra, it can be seen that the characteristic profile of Ag3d was obvious in Ag/BiVO4/Mn1−xZnxFe2O4 while Ag peak in BiVO4/Mn1−xZnxFe2O4 was not observed. Thus, it was deduced that the doping Ag in BiVO4/Mn1−xZnxFe2O4 was successful [25]. The peaks at the binding energy of 373.9 eV and 367.9 eV in Figure 3b was severally ascribed to Ag 3d3/2 and 3d5/2 [19], revealing the existence of Ag + . In Figure 3c, the peaks of O1s, V2p3/2, and V2p1/2 were located at 530.5 eV, 516.5 eV, and 523.4 eV, which were assigned to O2 − and V-O bands. There were peaks of Bi4f5/2 and 4f7/2 at 164.1 eV and 158.3 eV in Figure 3d, indicating the presence of bismuth species of Bi 3+ in BiVO4. Figure 3f displayed peaks at the binding energy of 641.5 eV (Mn2p3/2) and 653.1 eV (Mn2p1/2). The high resolution spectra of Fe2p as well as Zn2p were shown in Figure 3e,g. These peaks verified the presence of Mn1−xZnxFe2O4, which was consistent with the results of XRD and FTIR detection [26]. So, Ag/BiVO4/Mn1−xZnxFe2O4 was successfully assembled by in situ wet-chemistry synthesis method. This synthesis approach was simple, low cost, and environmentally friendly. To discern the element contents in Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 and determine their valence states, XPS study was carried out. The binding energy peaks of Ag, O, V, Bi, Fe, Zn, and Mn were recorded in Figure 3. The peaks of O, V, Bi, Fe, Mn, and Zn elements were clearly observed in Figure 3a. Further comparing the fully scanning XPS spectra, it can be seen that the characteristic profile of Ag3d was obvious in Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 while Ag peak in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was not observed. Thus, it was deduced that the doping Ag in BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was successful [25]. The peaks at the binding energy of 373.9 eV and 367.9 eV in Figure 3b was severally ascribed to Ag 3d 3/2 and 3d 5/2 [19], revealing the existence of Ag + . In Figure 3c, the peaks of O1s, V2p 3/2 , and V2p 1/2 were located at 530.  [26]. So, Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was successfully assembled by in situ wet-chemistry synthesis method. This synthesis approach was simple, low cost, and environmentally friendly. To discern the element contents in Ag/BiVO4/Mn1−xZnxFe2O4 and determine their valence states, XPS study was carried out. The binding energy peaks of Ag, O, V, Bi, Fe, Zn, and Mn were recorded in Figure 3. The peaks of O, V, Bi, Fe, Mn, and Zn elements were clearly observed in Figure 3a. Further comparing the fully scanning XPS spectra, it can be seen that the characteristic profile of Ag3d was obvious in Ag/BiVO4/Mn1−xZnxFe2O4 while Ag peak in BiVO4/Mn1−xZnxFe2O4 was not observed. Thus, it was deduced that the doping Ag in BiVO4/Mn1−xZnxFe2O4 was successful [25]. The peaks at the binding energy of 373.9 eV and 367.9 eV in Figure 3b was severally ascribed to Ag 3d3/2 and 3d5/2 [19], revealing the existence of Ag + . In Figure 3c, the peaks of O1s, V2p3/2, and V2p1/2 were located at 530.5 eV, 516.5 eV, and 523.4 eV, which were assigned to O2 − and V-O bands. There were peaks of Bi4f5/2 and 4f7/2 at 164.1 eV and 158.3 eV in Figure 3d, indicating the presence of bismuth species of Bi 3+ in BiVO4. Figure 3f displayed peaks at the binding energy of 641.5 eV (Mn2p3/2) and 653.1 eV (Mn2p1/2). The high resolution spectra of Fe2p as well as Zn2p were shown in Figure 3e,g. These peaks verified the presence of Mn1−xZnxFe2O4, which was consistent with the results of XRD and FTIR detection [26]. So, Ag/BiVO4/Mn1−xZnxFe2O4 was successfully assembled by in situ wet-chemistry synthesis method. This synthesis approach was simple, low cost, and environmentally friendly.  The morphological analysis of Ag/BiVO4/Mn1−xZnxFe2O4 was studied with transmission electron microscopy (TEM), and the results were displayed in Figure 4. By comparative experiments, it was demonstrated that the surface property of Ag/BiVO4/Mn1−xZnxFe2O4 was significantly improved when the appropriate dosage of polyvinylpyrrolidone (PVP) was used in the fabrication process of the composite. The improvement in properties resulted from the surface activity of PVP and the full uniform dispersion of Ag ions in the reaction solution. In addition, ethanol (solvent) further promoted the complete interface reaction of the ions with BiVO4/Mn1−xZnxFe2O4 particles. Namely, PVP could prompt the formation of nano-structural particles through in situ wet-chemistry method.  It was noted in Figure 4a,b that the bright surface sphere of BiVO4 attached the dark particles of Mn1−xZnxFe2O4. A small amount of Ag granular particles uniformly dispersed in the spherical surface in Figure 4c,d. As estimated from the images of BiVO4/Mn1−xZnxFe2O4, the average size of Ag granular particles was about 30 nm. The granular nanostructure particle of Ag favored production of rich active sites in the photocatalyst.

Light Absorption Property and Magnetic Property
UV-vis diffuse reflectance spectrophotometry was a suitable and important technique to determine the light absorption for semiconductor photocatalysts [27]. Figure 5 showed UV-vis diffuse reflectance spectra (UV-vis DRS) of BiVO4, BiVO4/Mn1−xZnxFe2O4, and Ag/BiVO4/Mn1−xZnxFe2O4. It can be discovered from Figure 5a that the maximum absorption edge of Ag/BiVO4/Mn1−xZnxFe2O4 shifted to red light region, leading to the main absorption edge around 400 nm. First, the red shift was directly related to the electrons and Ag + transformation between the conduction band and the valence band of BiVO4 [17]. Second, Ag particles had darkened color to enhance absorption of the visible-light for BiVO4/Mn1−xZnxFe2O4. Third, Ag particles could produce a strong surface plasmon resonance absorption. The band-gap energy (Eg) for Ag/BiVO4/Mn1−xZnxFe2O4 was about 2.25 eV. Eg values of BiVO4 and BiVO4/Mn1−xZnxFe2O4 in Figure 4b were about 2.36 eV. The incorporation of Mn1−xZnxFe2O4 did not change the optical properties of BiVO4 [18]. The relatively low Eg of Ag/BiVO4/Mn1−xZnxFe2O4 appeared to strengthen the absorption and sensitivity response for visible light. The significant enhancement of optics properties would be conducive to bringing high photocatalytic activity.

Light Absorption Property and Magnetic Property
UV-vis diffuse reflectance spectrophotometry was a suitable and important technique to determine the light absorption for semiconductor photocatalysts [27].  Hysteresis loops are the key way to characterize magnetization for magnetic materials. Figure 6 recorded the hysteresis loops of Ag/BiVO4/Mn1−xZnxFe2O4 and pure Mn1−xZnxFe2O4. The saturation magnetization (Ms) of Ag/BiVO4/Mn1−xZnxFe2O4 in Figure 6 was 10.04 emu/g. It was noted that large Ms was conducive towards the separation and recovery with an external magnet. Compared with Mn1−xZnxFe2O4, the Ms of Ag/BiVO4/Mn1−xZnxFe2O4 was declined by 87.5% due to the decrease of magnetic content per unit mass. More importantly, Ms of Ag/BiVO4/Mn1−xZnxFe2O4 was larger than that (7.01 emu/g) of Mn1−xZnxFe2O4/Bi2O3 [21]. The magnetic property was conducive to the stable activity of Ag/BiVO4/Mn1−xZnxFe2O4. The result revealed that the as-prepared magnetic photocatalyst was easily recovered by an external magnet. Therefore, it was concluded that Ag/BiVO4/Mn1−xZnxFe2O4 with good magnetic property possessed a high recovery rate.

Photocatalytic Activity
It was well-known that the photocatalytic ability was vital to photocatalytic materials, which was the base property for their industrial application. Generally, the photocatalytic activity was assessed with the degradation reaction of dyes.

Visible-Light-Driven Photocatalytic Activity
The photocatalytic performance of the samples under visible light irradiation was evaluated with the RhB photodegradation, and the result was shown in Figure 7. There was only a little degradation rate in the blank test (without any photocatalyst), indicating the poor self-degradation of RhB. The degradation rate for BiVO4 and BiVO4/Mn1−xZnxFe2O4 was approximately 45.0% after 60 Hysteresis loops are the key way to characterize magnetization for magnetic materials. Hysteresis loops are the key way to characterize magnetization for magnetic materials. Figure 6 recorded the hysteresis loops of Ag/BiVO4/Mn1−xZnxFe2O4 and pure Mn1−xZnxFe2O4. The saturation magnetization (Ms) of Ag/BiVO4/Mn1−xZnxFe2O4 in Figure 6 was 10.04 emu/g. It was noted that large Ms was conducive towards the separation and recovery with an external magnet. Compared with Mn1−xZnxFe2O4, the Ms of Ag/BiVO4/Mn1−xZnxFe2O4 was declined by 87.5% due to the decrease of magnetic content per unit mass. More importantly, Ms of Ag/BiVO4/Mn1−xZnxFe2O4 was larger than that (7.01 emu/g) of Mn1−xZnxFe2O4/Bi2O3 [21]. The magnetic property was conducive to the stable activity of Ag/BiVO4/Mn1−xZnxFe2O4. The result revealed that the as-prepared magnetic photocatalyst was easily recovered by an external magnet. Therefore, it was concluded that Ag/BiVO4/Mn1−xZnxFe2O4 with good magnetic property possessed a high recovery rate.

Photocatalytic Activity
It was well-known that the photocatalytic ability was vital to photocatalytic materials, which was the base property for their industrial application. Generally, the photocatalytic activity was assessed with the degradation reaction of dyes.

Visible-Light-Driven Photocatalytic Activity
The photocatalytic performance of the samples under visible light irradiation was evaluated with the RhB photodegradation, and the result was shown in Figure 7. There was only a little degradation rate in the blank test (without any photocatalyst), indicating the poor self-degradation of RhB. The degradation rate for BiVO4 and BiVO4/Mn1−xZnxFe2O4 was approximately 45.0% after 60

Photocatalytic Activity
It was well-known that the photocatalytic ability was vital to photocatalytic materials, which was the base property for their industrial application. Generally, the photocatalytic activity was assessed with the degradation reaction of dyes.

Visible-Light-Driven Photocatalytic Activity
The photocatalytic performance of the samples under visible light irradiation was evaluated with the RhB photodegradation, and the result was shown in Figure 7. There was only a little degradation rate in the blank test (without any photocatalyst), indicating the poor self-degradation of RhB. The degradation rate for BiVO 4   In fact, most degradation tests are very slow (may take several hours) despite the improvements in visible light absorption of the photocatalyst. Here, the as-prepared Ag/BiVO4/Mn1−xZnxFe2O4 has a highly photocatalytic efficiency. This can be explained with the following three aspects: (1) Ag produced the surface plasmon resonance (plasma energy), which was transferred to BiVO4, leading to more formation of photo-excited electrons and holes. It was helpful to the enhancement of photocatalytic activity; (2) Ag particles acted as holes and accepted photo-produced electrons from the conduction band of BiVO4, extending the wavelength range and preventing the recombination of electrons and holes. The transformation or conversion of the charged particles in the interface was strengthened. In other words, the presence of Ag particles boosted the quantum efficiency for BiVO4/Mn1−xZnxFe2O4; (3) Owing to the nanostructure of Ag particles, Ag/BiVO4/Mn1−xZnxFe2O4 possessed a relatively large specific surface area, which increased the efficient sites and further yielded a high photocatalytic activity [19,25]. Thus, nanostructure Ag particles ensured high photocatalytic property of Ag/BiVO4/Mn1−xZnxFe2O4.
Ag/BiVO4/Mn1−xZnxFe2O4 was recovered with an external magnet in the end of the photocatalytic degradation test. 88~91 mg (after washing and drying) Ag/BiVO4/Mn1−xZnxFe2O4 could recover from initial dosage of 100 mg in each cycle. The average recovery rate of magnetic Ag/BiVO4/Mn1−xZnxFe2O4 was 89%, which was larger than the literature report value (85.0%) [28]. It is worth mentioning that the recovery method was quick with low energy consumption. The high recovery rate effectively avoided the leftover of catalysts in the water solution. Namely, Ag/BiVO4/Mn1−xZnxFe2O4 demonstrated itself as an environmentally friendly photocatalytic material and showed perspective industrial application in removal water-soluble contaminants.
The repeatability and stability were necessary in the practical photocatalytic application [27]. Cycling tests were employed to evaluate the photocatalytic stability of Ag/BiVO4/Mn1−xZnxFe2O4, and the degradation rate of RhB was described in Figure 8. It was clear that the degradation rate during the five cycles was severally 96.0%, 96.0%, 95.0%, 94.0% and 94.0%, which revealed photocatalyst efficiency of Ag/BiVO4/Mn1−xZnxFe2O4 barely decreased in the test process. Experiment results exhibited excellent photocatalytic stability. What is more, 94.0% of the degradation rate during the In fact, most degradation tests are very slow (may take several hours) despite the improvements in visible light absorption of the photocatalyst. Here, the as-prepared Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 has a highly photocatalytic efficiency. This can be explained with the following three aspects: (1) Ag produced the surface plasmon resonance (plasma energy), which was transferred to BiVO 4 , leading to more formation of photo-excited electrons and holes. It was helpful to the enhancement of photocatalytic activity; (2) Ag particles acted as holes and accepted photo-produced electrons from the conduction band of BiVO 4 , extending the wavelength range and preventing the recombination of electrons and holes. The transformation or conversion of the charged particles in the interface was strengthened. In other words, the presence of Ag particles boosted the quantum efficiency for BiVO 4 /Mn 1−x Zn x Fe 2 O 4 ; (3) Owing to the nanostructure of Ag particles, Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 possessed a relatively large specific surface area, which increased the efficient sites and further yielded a high photocatalytic activity [19,25]. Thus, nanostructure Ag particles ensured high photocatalytic property of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 .
Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was recovered with an external magnet in the end of the photocatalytic degradation test. 88~91 mg (after washing and drying) Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 could recover from initial dosage of 100 mg in each cycle. The average recovery rate of magnetic Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was 89%, which was larger than the literature report value (85.0%) [28]. It is worth mentioning that the recovery method was quick with low energy consumption. The high recovery rate effectively avoided the leftover of catalysts in the water solution. Namely, Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 demonstrated itself as an environmentally friendly photocatalytic material and showed perspective industrial application in removal water-soluble contaminants.
The repeatability and stability were necessary in the practical photocatalytic application [27]. Cycling tests were employed to evaluate the photocatalytic stability of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 , and the degradation rate of RhB was described in Figure 8. It was clear that the degradation rate during the five cycles was severally 96.0%, 96.0%, 95.0%, 94.0% and 94.0%, which revealed photocatalyst efficiency of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 barely decreased in the test process. Experiment results exhibited excellent photocatalytic stability. What is more, 94.0% of the degradation rate during the five cycles was very larger than that of the reference report [13]. So, magnetic photocatalyst Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 possessed promising prospect in the photo-decomposition organic dyes (industrial wastewater) field.
Materials 2018, 11, x FOR PEER REVIEW 9 of 12 five cycles was very larger than that of the reference report [13]. So, magnetic photocatalyst Ag/BiVO4/Mn1−xZnxFe2O4 possessed promising prospect in the photo-decomposition organic dyes (industrial wastewater) field.

Photocatalytic Mechanism
Radical scavengers were used to study active species in photocatalytic reaction, the result was displayed in Figure 9. In details, the degradation rate of RhB in Ag/BiVO4/Mn1−xZnxFe2O4-EDTA-Na2 (h + scavenger) lowed and reached to 28.0%, which was significantly lower than that of Ag/BiVO4/Mn1−xZnxFe2O4. Under the same condition, the degradation rate steeply went down when BZQ (•O2 − scavenger) in place of EDTA-Na2 was added into the reaction solution, the rate was only 11.0%. However, the introduction of t-BuOH (•OH scavenger) caused a large decrease in the degradation of about 60.0%. Namely, the change of the degradation rate in Ag/BiVO4/Mn1−xZnxFe2O4t-BuOH was the smallest among three radical scavenger tests. The results illustrated that free radicals were major active species, and that •O2 − played the domination role though •OH and h + took part in the photocatalytic reaction.

Photocatalytic Mechanism
Radical scavengers were used to study active species in photocatalytic reaction, the result was displayed in Figure 9.
In details, the degradation rate of RhB in Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 -EDTA-Na 2 (h + scavenger) lowed and reached to 28.0%, which was significantly lower than that of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 .
Under the same condition, the degradation rate steeply went down when BZQ (•O 2 − scavenger) in place of EDTA-Na 2 was added into the reaction solution, the rate was only 11.0%. However, the introduction of t-BuOH (•OH scavenger) caused a large decrease in the degradation of about 60.0%. Namely, the change of the degradation rate in Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 -t-BuOH was the smallest among three radical scavenger tests. The results illustrated that free radicals were major active species, and that •O 2 − played the domination role though •OH and h + took part in the photocatalytic reaction.
Materials 2018, 11, x FOR PEER REVIEW 9 of 12 five cycles was very larger than that of the reference report [13]. So, magnetic photocatalyst Ag/BiVO4/Mn1−xZnxFe2O4 possessed promising prospect in the photo-decomposition organic dyes (industrial wastewater) field.

Photocatalytic Mechanism
Radical scavengers were used to study active species in photocatalytic reaction, the result was displayed in Figure 9. In details, the degradation rate of RhB in Ag/BiVO4/Mn1−xZnxFe2O4-EDTA-Na2 (h + scavenger) lowed and reached to 28.0%, which was significantly lower than that of Ag/BiVO4/Mn1−xZnxFe2O4. Under the same condition, the degradation rate steeply went down when BZQ (•O2 − scavenger) in place of EDTA-Na2 was added into the reaction solution, the rate was only 11.0%. However, the introduction of t-BuOH (•OH scavenger) caused a large decrease in the degradation of about 60.0%. Namely, the change of the degradation rate in Ag/BiVO4/Mn1−xZnxFe2O4t-BuOH was the smallest among three radical scavenger tests. The results illustrated that free radicals were major active species, and that •O2 − played the domination role though •OH and h + took part in the photocatalytic reaction.  The electron transition occurred between valence band and conduction band, generating the photoelectrons and holes when the photon energy was higher than E g of the semiconductor. The possible transition of photo-induced electron and hole was used to express the photocatalytic process under light irradiation. The photocatalytic mechanism of Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was described in Figure 10. In detail, etransferred to the surface of Ag particle, the dissolved oxygen (O 2 ) could capture the electron and form the super oxygen radical ( The electron transition occurred between valence band and conduction band, generating the photoelectrons and holes when the photon energy was higher than Eg of the semiconductor. The possible transition of photo-induced electron and hole was used to express the photocatalytic process under light irradiation. The photocatalytic mechanism of Ag/BiVO4/Mn1−xZnxFe2O4 was described in Figure 10. In detail, etransferred to the surface of Ag particle, the dissolved oxygen (O2) could capture the electron and form the super oxygen radical (•O2 − ) through the Fermi level surface resonance. The adsorbed H2O in the surface of Ag/BiVO4/Mn1−xZnxFe2O4 could be oxidized by holes (h + ), yielding hydroxyl free radical (•OH). Both •O2 − and •OH had a large oxidation ability and decomposed RhB into CO2 and H2O. At the same time, holes themselves prompted the degradation-oxidized of RhB [26]. Thus, the doping Ag was favorable to drive more •O2 − and •OH radicals, strengthening the degradation of RhB in visible light irradiation. In fact, we used Mn1−xZnxFe2O4 as magnetic substrate in order to simplify separation after photocatalytic reaction for BiVO4. The UV-vis DRS shown the incorporation of Mn1−xZnxFe2O4 did not enhance the optical properties of BiVO4. Noble metal-doping and graphene-loading were good ways to improve optical properties and enhance photocatalytic activity for single phase semiconductor. Here, we chose Ag-doping to boost the photocatalytic activity of BiVO4/Mn1−xZnxFe2O4. In addition, we will use graphene to modify BiVO4/Mn1−xZnxFe2O4. These studies will help to choose a better way (the above mentioned) for enhancing photocatalytic activity via comparing their photocatalytic activity and reaction kinetics, and then apply these findings to other signal semiconductors.

Conclusions
Ag/BiVO4/Mn1−xZnxFe2O4 was fabricated with the dip-calcination and in situ wet-chemistry synthesis method that was simple and environmentally-friendly. Element contents and their valence states in Ag/BiVO4/Mn1−xZnxFe2O4 were detected, indicating Ag granular particles dispersed in the spherical surface of BiVO4. The presence of Ag particles boosted the quantum efficiency, and further enhanced the photocatalytic activity. Under visible light irradiation (λ ≥ 400nm), the degradation rate of RhB using Ag/BiVO4/Mn1−xZnxFe2O4 after only 60 min reaction reached to 96.0%, which was greater than that of Mn1−xZnxFe2O4/BiVO4 and pure BiVO4. Most importantly, the degradation rate was close to 94.0% during the fifth recycle. We hope this research can promote the industrial application of BiVO4.  In fact, we used Mn 1−x Zn x Fe 2 O 4 as magnetic substrate in order to simplify separation after photocatalytic reaction for BiVO 4 . The UV-vis DRS shown the incorporation of Mn 1−x Zn x Fe 2 O 4 did not enhance the optical properties of BiVO 4 . Noble metal-doping and graphene-loading were good ways to improve optical properties and enhance photocatalytic activity for single phase semiconductor. Here, we chose Ag-doping to boost the photocatalytic activity of BiVO 4 /Mn 1−x Zn x Fe 2 O 4 . In addition, we will use graphene to modify BiVO 4 /Mn 1−x Zn x Fe 2 O 4 . These studies will help to choose a better way (the above mentioned) for enhancing photocatalytic activity via comparing their photocatalytic activity and reaction kinetics, and then apply these findings to other signal semiconductors.

Conclusions
Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 was fabricated with the dip-calcination and in situ wet-chemistry synthesis method that was simple and environmentally-friendly. Element contents and their valence states in Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 were detected, indicating Ag granular particles dispersed in the spherical surface of BiVO 4 . The presence of Ag particles boosted the quantum efficiency, and further enhanced the photocatalytic activity. Under visible light irradiation (λ ≥ 400nm), the degradation rate of RhB using Ag/BiVO 4 /Mn 1−x Zn x Fe 2 O 4 after only 60 min reaction reached to 96.0%, which was greater than that of Mn 1−x Zn x Fe 2 O 4 /BiVO 4 and pure BiVO 4 . Most importantly, the degradation rate was close to 94.0% during the fifth recycle. We hope this research can promote the industrial application of BiVO 4 .

Conflicts of Interest:
The authors declare that they have no conflict of interest.