Synthesis of Magnetic Fe3O4/ZnWO4 and Fe3O4/ZnWO4/CeVO4 Nanoparticles: The Photocatalytic Effects on Organic Pollutants upon Irradiation with UV-Vis Light

Magnetic Fe3O4/ZnWO4 and Fe3O4/ZnWO4/CeVO4 nanoparticles with different molar ratios of CeVO4 to other inorganic components were synthesized through co-precipitation with a sonochemical-assisted method. X-ray diffraction, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, vibrating sample magnetometry, and scanning electron microscopy (SEM) methods were used for the physico–chemical characterization of the obtained nanoparticles. As shown in the SEM images, the average sizes of the Fe3O4/ZnWO4 and Fe3O4/ZnWO4/CeVO4 nanoparticles that formed aggregates were approximately 50–70 nm and 80–100 nm, respectively. The photocatalytic performance of these nanoparticles was examined by measuring methylene blue degradation under visible light (assisted by H2O2). The sample with a mass ratio of 1:2:1 (Fe3O4/ZnWO4/CeVO4, S4) exhibited optimal photocatalytic performance, and thus this sample was subsequently used for the photodegradation of different organic pollutants upon irradiation with ultraviolet (UV) and visible light. Approximately 90% and 70% degradation of methyl violet and methylene blue, respectively, was observed after visible light irradiation. Additionally, the mechanism of the photocatalytic reaction was investigated by measuring ·OH release under UV light in a system with terephthalic acid and by measuring the release of ·O2−, ·OH, and hole scavengers. Catalysts 2020, 10, 494; doi:10.3390/catal10050494 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 494 2 of 17


Introduction
The development of human civilization across the earth in the form of population growth and increasing industrialization has resulted in a shortage of healthy wastewater. Wastewater may contain many pathogenic microorganisms and pharmaceutical and chemical residues [1][2][3][4]. A large variety of organic compounds are capable of being completely degraded and mineralized into CO 2 and harmless inorganic anions. The chemical and pharmaceutical pollutants may include, but are not limited to salts, metalloids, metals, organic compounds, residual drugs, endocrine-disrupting compounds [5][6][7], and other compounds. Therefore, different methods have been proposed to treat water resources and restore good and healthy properties to water. Different chemical and biological methods have been used to purify industrial, agricultural, and pharmaceutical wastewater and increase the number of existing resources available [8][9][10].
Photocatalytic processes have become an attractive method for researchers to use to degrade pollutants [11][12][13][14][15]. "Photocatalysis" is a catalytic process driven by light in nature and uses materials called "photocatalysts". Photocatalysis was first reported at the beginning of the nineteenth century, but a boom took place in the field of heterogeneous photocatalysis after an article reported photo-assisted catalysis of water upon irradiation of TiO 2 [16]. Semiconductors are frequently used for this process, due to their optical properties, electronic structure, and intrinsic nature [17][18][19][20][21]. Many physical and chemical parameters affect the efficiency of photocatalytic processes, including (i) their chemical composition, such as the elemental composition and chemical state/structure; (ii) physical properties, such as the physical and crystal structures, optical absorption, charge dynamics, defects, and thermal stability; and (iii) band structure (band gap, band edges and Fermi level) [22]. An appropriate band gap is one of the main principles of a photocatalyst [23]. A suitable band gap is important because of the ability to capture light and its effect on redox reactions that are performed by photocatalytic systems designed by incorporating a matrix into the semiconductor or by adding a co-catalyst [24][25][26][27].
Photocatalysis, in which a photoinduced chemical reaction occurs on the surface of semiconductor materials (photocatalyst) upon exposure to photons, is a heterogeneous process [14,28]. Photocatalysts are typically composed of metal oxides, oxysulfides, metal sulfides, oxynitrides, their composites [23,[29][30][31][32][33][34][35], and other materials. The chemical structure and the well-developed surface (morphology) of the materials exert significant effects on the photocatalytic process. The ability to tune the functional and morphological properties of nanocrystals is one of the most important issue in photocatalysis [15], mainly because the size, shape, and porous characteristics of synthesized nanomaterials affect the photocatalytic properties of these materials [34,[36][37][38][39].
Magnetizing photocatalytic materials is a procedure to easily split a photocatalyst after use. One of the challenges facing the use of photocatalysts is the leakage of metal ions from the catalyst. The use of magnetic nanoparticles reduces the likelihood of metal leakage because the particles might have accumulated an external magnetic field. The most effective magnetic particles are those containing iron oxide core because of their low cost, easy separation, and excellent reusability [40][41][42][43][44][45]. Fe 3 O 4 has unique electronic and magnetic characteristics that are the result of complicated electron-electron and electron-phonon interactions [42]. The introduction of other metals into these materials containing iron oxides creates multiferroic systems, which can be developed by exploiting the epitaxial strain, oxygen vacancy, charge, or composition gradients [42]. To conclude, multiferroic heterostructures may have enhanced heterogeneous catalytic activity and selectivity compared with monometallic materials [40,41,44] Among the substantial number of multiferroic heterostructures, one of the main groups is metal tungstates, with the formula of MWO 4 (M = Co, Cu, Zn, Pb, and Mn), and transition metal-based orthovanadates (MVO 4 ) [42,[46][47][48]. Metal tungstates possess interesting structural and photoluminescence properties, mainly including ferroelasticity, ionic conductivity, Catalysts 2020, 10, 494 3 of 17 and photoluminescence [47,49]. Due to their very interesting physico-chemical and nanostructural properties, they have attracted the interest of researchers in many areas. In particular, ZnWO 4 has been used in various applications, such as sensors, scintillators, laser hosts, and phase-change optical recording devices [50,51]. In some situations, ZnWO 4 is used to photodegrade organic pollutants [37]. Additionally, MVO 4 materials are a cohesive class of materials with potential applications in various fields, such as optoelectronics, spintronics, catalysis, solar cells, gas sensors, photoluminescence, and photocatalysis [35,[52][53][54]. Although these two groups of inorganic oxides possess very good photocatalytic properties and degrade organic pollutants, a major problem after their use is their removal from wastewater after the redox reaction. The solution to this problem is to use a photocatalyst with magnetic properties. By preparing nanoparticles or composites with simultaneous photocatalytic and magnetic properties, the used photocatalysts can be extracted for recycling with an external magnetic field [55].
Here, the facile synthesis of magnetic Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles at different molar ratios of CeVO 4 to Fe 3 O 4 /ZnWO 4 is reported. The molar ratios of subsequent layers were as follows: 1:2:0.5 (designated as S 2 ), 1:2:0.75 (designated as S 3 ), 1:2:1 (designated as S 4 ), and 1:2:1.5 (designated as S 5 ). The photocatalytic activities of these samples upon irradiation with visible light were compared with Fe 3 O 4 /ZnWO 4 (designated as S 1 ) as a reference. The structural, morphological, optical, and magnetic properties of the photocatalysts were determined for the nanoparticles with the highest photocatalytic activity-S 4 (1:2:1). Three dye pollutants, methylene blue (MB), methyl orange (MO), and methyl violet (MV), were employed to measure the photocatalytic activity of the samples after irradiation with visible light (assisted by H 2 O 2 , MB was used in an optimization step as a probe). The efficiency of the synthesized samples illuminated with ultraviolet light was tested using MB, fenitrothion (FNT), MO, and rhodamine-B (RhB). The conversion of terephthalic acid (TA) to 2-hydroxy-terephthalic acid in the presence of hydroxyl radicals and with the assistance of · O 2 − , · OH and hole scavengers was analysed to investigate the mechanism of the photocatalytic reaction.

Physico-Chemical Characterization of Fe 3 O 4 /ZnWO 4 and Fe 3 O 4 /ZnWO 4 /CeVO 4 Nanoparticles
X-ray diffraction (XRD) was performed to identify the phase composition and to determine the metal crystallite size and size distribution of the synthesized inorganic nanoparticles (Fe 3 O 4 , Fe 3 O 4 /ZnWO 4 , and Fe 3 O 4 /ZnWO 4 /CeVO 4 (S 4 )). The results of the XRD analyses are shown in Figure 1. Figure 1a shows the diffraction pattern for Fe 3 O 4 , with the main diffraction peaks observed at 2θ of 35.6 • (line (311)) and of 63.1 • (line (220)), which correspond to the cubic phases of Fe 3 O 4 (72-2303 JCPDS) [56]. The average crystallite size (D c ) was calculated using the well-known Scherrer formula: where λ is the X-ray wavelength (nm), β is the breadth of the observed diffraction line at half its intensity maximum [57,58], and K is a constant (0.9) related to the crystallite shape for any 2θ peak in the pattern [59,60]. The average D c value was calculated and was ca. 15 nm.  The Fe3O4/ZnWO4/CeVO4 nanoparticles presented a combination of three pure phases of Fe3O4 (JCPDS 72-2303) [61], ZnWO4 (JCPDS 15-0774) [62], and CeVO4 (JCPDS 12-0757) [63]. The diffraction peaks were observed at 2θ of 24.03° (line (200)), 32.04° (line (112)), and 47.86° (line (312)), consistent with a pure phase tetragonal CeVO4 nanostructure [64]. The Dc value for the nanohybrid system increased to 58 nm ( Figure 1c). All calculated parameters are summarized in Table 1. In summary, the profiles of all inorganic nanoparticles confirmed the presence of Fe3O4, ZnWO4, and CeVO4 components in the obtained materials.   [62], and CeVO 4 (JCPDS 12-0757) [63]. The diffraction peaks were observed at 2θ of 24.03 • (line (200)), 32.04 • (line (112)), and 47.86 • (line (312)), consistent with a pure phase tetragonal CeVO 4 nanostructure [64]. The D c value for the nanohybrid system increased to 58 nm ( Figure 1c). All calculated parameters are summarized in Table 1. In summary, the profiles of all inorganic nanoparticles confirmed the presence of Fe 3 O 4 , ZnWO 4, and CeVO 4 components in the obtained materials.     The light absorption properties of materials play a significant role in their photocatalytic activities [63]. The relationship between the energy and absorption edge was calculated using Tauc's Equation [65]: where h is Planck's constant, υ is the frequency of light, α is the absorption coefficient, Eg is the optical band gap energy (OBGE), A is a constant, and η is 0.5 or 2 for direct or indirect transitions, respectively. The value of η = 2 was considered in this experiment [66]. Based on the data shown in Tauc's plot of the S1 and S4 samples (Figure 4), the OBGE values were ca. 3.1 and 2.6 eV, respectively. Generally, the OBGE values may be affected by the surface and interface effect, change in crystal structure, and lattice strain in the structure [67]. However, the S1 sample had a higher OBGE value; a combination of Fe3O4 conductivity with visible light active semiconductivity is frequently required for photocatalytic purposes. Herein, we report Fe3O4/ZnWO4/CeVO4 hierarchical core-shell structures that enhanced the electron-hole separation (Fe3O4) and light harvesting (ZnWO4/CeVO4) due to their multifunctionality and synergetic effect on every component [68]. The light absorption properties of materials play a significant role in their photocatalytic activities [63]. The relationship between the energy and absorption edge was calculated using Tauc's Equation [65]: where h is Planck's constant, υ is the frequency of light, α is the absorption coefficient, E g is the optical band gap energy (OBGE), A is a constant, and η is 0.5 or 2 for direct or indirect transitions, respectively. The value of η = 2 was considered in this experiment [66]. Based on the data shown in Tauc's plot of the S 1 and S 4 samples (Figure 4) optical band gap energy (OBGE), A is a constant, and η is 0.5 or 2 for direct or indirect transitions, respectively. The value of η = 2 was considered in this experiment [66]. Based on the data shown in Tauc's plot of the S1 and S4 samples (Figure 4), the OBGE values were ca. 3.1 and 2.6 eV, respectively. Generally, the OBGE values may be affected by the surface and interface effect, change in crystal structure, and lattice strain in the structure [67]. However, the S1 sample had a higher OBGE value; a combination of Fe3O4 conductivity with visible light active semiconductivity is frequently required for photocatalytic purposes. Herein, we report Fe3O4/ZnWO4/CeVO4 hierarchical core-shell structures that enhanced the electron-hole separation (Fe3O4) and light harvesting (ZnWO4/CeVO4) due to their multifunctionality and synergetic effect on every component [68]. Fourier transform infrared spectroscopy (FTIR) was performed to assess the chemical structure of Fe3O4, Fe3O4/ZnWO4 (S1), Fe3O4/ZnWO4/CeVO4 (S4) (before and after calcination), and S4 after the   Figure 5e shows the FTIR spectrum of S4 after the photodegradation of MB (25 ppm) upon irradiation with UV light. The lack of change in the peak indicates that the photocatalyst was not altered during the photocatalytic reaction. Based on the FTIR data, light irradiation in the presence of organic pollutants did not affect or destroy the chemical structure of the obtained inorganic nanoparticles. Therefore, the FTIR studies confirmed the incorporation of the three inorganic phases into one system and their high environmental stability under light irradiation, indicating that these nanostructures were successfully used as photocatalytic materials to remove pollutants.   Figure 5e shows the FTIR spectrum of S 4 after the photodegradation of MB (25 ppm) upon irradiation with UV light. The lack of change in the peak indicates that the photocatalyst was not altered during the photocatalytic reaction. Based on the FTIR data, light irradiation in the presence of organic pollutants did not affect or destroy the chemical structure of the obtained inorganic nanoparticles. Therefore, the FTIR studies confirmed the incorporation of the three inorganic phases into one system and their high environmental stability under light irradiation, indicating that these nanostructures were successfully used as photocatalytic materials to remove pollutants.  Figure 6b shows a plot of the kinetics of −ln(C/C 0 ) versus the irradiation time. Figure 6c shows temporal changes in the absorption spectra of MB during the photodegradation mediated by the S 4 sample. The slope of the linear regression curve was calculated as the first-order reaction rate constant. As shown in Figure 6a, S 4 exhibited the best performance among all synthesized samples and was capable of removing the organic pollutant via photocatalysis. Similar experiments for S4 were conducted to measure the photodegradation of MV and MO, and the results are shown in Figure 6d. The photodegradation of MV was much higher than that of MO. After of exposure to light irradiation for 100 min, 90% of MV was degraded. Similarly, under the same experimental conditions, less MO was degraded, approximately 50% of the initial concentration, by the S4 sample.

Photocatalytic Reactions
A photocatalytic degradation test was performed using different contaminants, MB, MO, fenitrothion (FNT, O,O-dimethyl-O-4-nitro-m-tolyl phosphorothioate), and rhodamine-B (RhB), to examine the photocatalytic efficiency of S4 upon irradiation with UV light. The results are shown in Figure 7. After approximately 70 min of exposure to light, the lowest photocatalytic activity of S4 was observed for FNT. This is an organophosphorus pesticide that exhibits high chemical stability and resistance to biodegradation [72,73]. Pesticides degrade very slowly under light irradiation compared with the reaction rates obtained using photocatalysts [74].  Figure 7. After approximately 70 min of exposure to light, the lowest photocatalytic activity of S 4 was observed for FNT. This is an organophosphorus pesticide that exhibits high chemical stability and resistance to biodegradation [72,73]. Pesticides degrade very slowly under light irradiation compared with the reaction rates obtained using photocatalysts [74].
Similar experiments for S4 were conducted to measure the photodegradation of MV and MO, and the results are shown in Figure 6d. The photodegradation of MV was much higher than that of MO. After of exposure to light irradiation for 100 min, 90% of MV was degraded. Similarly, under the same experimental conditions, less MO was degraded, approximately 50% of the initial concentration, by the S4 sample.
A photocatalytic degradation test was performed using different contaminants, MB, MO, fenitrothion (FNT, O,O-dimethyl-O-4-nitro-m-tolyl phosphorothioate), and rhodamine-B (RhB), to examine the photocatalytic efficiency of S4 upon irradiation with UV light. The results are shown in Figure 7. After approximately 70 min of exposure to light, the lowest photocatalytic activity of S4 was observed for FNT. This is an organophosphorus pesticide that exhibits high chemical stability and resistance to biodegradation [72,73]. Pesticides degrade very slowly under light irradiation compared with the reaction rates obtained using photocatalysts [74].  The addition of some inorganic and organic matter is necessary to increase the photodegradation efficiency of photocatalysts because these substances may affect the photodegradation rate due to their adsorption on the catalyst surface [75]. On the other hand, efficient photodegradation of MB, MO, and RhB by S 4 was observed.
The mechanisms of photocatalytic reactions were examined by capturing hydroxyl radicals ( · OH). Hydroxyl radicals are usually sensed with the terephthalic acid (TA) photoluminescence probing method [76][77][78]. Following the adsorption of · OH by TA, 2-hydroxyl-terephthalic acid is produced and displays fluorescence emission; therefore, · OH is able to be sensed by monitoring the changes in the fluorescence intensity of the TA solution. Figure 8 shows the variations in the fluorescence intensity of 2-hydroxyl-terephthalic acid at 443 nm. As shown in Figure 8, with increasing UV irradiation time, the fluorescence intensity increased. However, in the first 10 min without irradiation, the intensity of the · OH signal was minimized, indicating that · OH was produced over the photocatalyst during UV irradiation.
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 18 The addition of some inorganic and organic matter is necessary to increase the photodegradation efficiency of photocatalysts because these substances may affect the photodegradation rate due to their adsorption on the catalyst surface [75]. On the other hand, efficient photodegradation of MB, MO, and RhB by S4 was observed.
The mechanisms of photocatalytic reactions were examined by capturing hydroxyl radicals (˙OH). Hydroxyl radicals are usually sensed with the terephthalic acid (TA) photoluminescence probing method [76][77][78]. Following the adsorption of ˙OH by TA, 2-hydroxyl-terephthalic acid is produced and displays fluorescence emission; therefore, ˙OH is able to be sensed by monitoring the changes in the fluorescence intensity of the TA solution. Figure 8 shows the variations in the fluorescence intensity of 2-hydroxyl-terephthalic acid at 443 nm. As shown in Figure 8, with increasing UV irradiation time, the fluorescence intensity increased. However, in the first 10 min without irradiation, the intensity of the ˙OH signal was minimized, indicating that ˙OH was produced over the photocatalyst during UV irradiation.
The principal oxidative species in the photocatalytic process was identified by performing trapping tests of ˙OH, superoxide radical (˙O2 − ), and holes using tert-butanol, benzoquinone, and citric acid, respectively [64,71]. The results of the experiments conducted using these scavengers to measure the photodegradation of MB (25 ppm) are shown in Figure 9a. After the addition of the superoxide scavenger, the photocatalyst performance was substantially reduced (by approximately 75%). Furthermore, the addition of the hydroxyl scavenger decreased the photocatalytic performance by approximately 50%. In contrast, the addition of the hole scavenger exerted a slight effect on photocatalytic performance. Based on these results, the ˙OH and ˙O2 − radicals were the main oxidative species photogenerated by the synthesized nanoparticles. Figure 9b presents the proposed graphical mechanism of the photocatalytic reaction in the presence of H2O2. This mechanism was determined and described in detail in our previous study [65].  The principal oxidative species in the photocatalytic process was identified by performing trapping tests of · OH, superoxide radical ( · O 2 − ), and holes using tert-butanol, benzoquinone, and citric acid, respectively [64,71]. The results of the experiments conducted using these scavengers to measure the photodegradation of MB (25 ppm) are shown in Figure 9a. After the addition of the superoxide scavenger, the photocatalyst performance was substantially reduced (by approximately 75%). Furthermore, the addition of the hydroxyl scavenger decreased the photocatalytic performance by approximately 50%. In contrast, the addition of the hole scavenger exerted a slight effect on photocatalytic performance. Based on these results, the · OH and · O 2 − radicals were the main oxidative species photogenerated by the synthesized nanoparticles. Figure 9b presents the proposed graphical mechanism of the photocatalytic reaction in the presence of H 2 O 2 . This mechanism was determined and described in detail in our previous study [65].   The photocatalytic activity of semiconductors is very complicated and can be affected by factors such as the chemical structure, composition, material size, surface area, and a suitable band gap. Briefly, considering a magnetic nanoparticle as a photocatalyst, the magnetic core is useful for the separation of the photocatalyst from treated water whereas the outer inorganic layer is beneficial for the photocatalytic reaction [44]. Some examples of Fe 3 O 4 -based photocatalysts are presented in Table 2. Application of pristine Fe 3 O 4 in photocatalysis is limited owing to its easy aggregation and its easy electron-hole recombination [44]. To eliminate these shortcomings, the modification of Fe 3 O 4 is required by designing core-shell nanostructures ( Table 2). The Fe 3 O 4 -based nanocomposites have higher chemical and photochemical stability, higher photocatalytic activity, and are reusable and magnetically recoverable. For example, introducing inert substances, such as ceramics (SiO 2 ), can protect Fe 3 O 4 from chemical dissolution and can affect photocatalytic processes [79]. By controlling the thickness of the outer inorganic shell, increased degradation of pollutants might be achieved [80]. The smaller photocatalytic nanoparticles are more active than larger ones. With decreasing size of the semiconductor crystal size, the recombination rate of e − /h + pairs will decrease [67]. Additionally, smaller nanoparticles with larger specific areas have a higher photon absorption rate on the surface of photons. These findings were observed for our Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles with the different molar ratios of the three components ( Figure 2). Briefly, comparison of the pristine inorganic nanoparticles with the nanocomposites clearly showed that the improved photocatalytic activity of the multilayered nanoparticles was ascribed to efficient electron-hole separation.
In summary, we suggest that the synergetic effect of triple-layer inorganic nanoparticles (Fe 3 O 4 and ZnWO 4 and CeVO 4 ) leads to enhancement of photocatalytic activity under visible light irradiation ( Table 2). In this regard, the Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles (with molar ratios of 1:2:1) are found to be effective as a simple recyclable photocatalytic material for removing organic pollutants from wastewater.

Synthesis of Fe 3 O 4 Nanoparticles
The co-precipitation method was used to synthesize the Fe 3 O 4 nanoparticles. First, 16.25 g of FeCl 3 and 6.32 g of FeCl 2 were dissolved in 200 mL of distilled water and stirred with a mechanical stirrer for 60 min. Then, precipitation was induced by adding 2 M NaOH at 30 • C in an N 2 atmosphere. The reaction system was maintained at a temperature of 70 • C for 5 h, while maintaining the pH of the solution at ± 12. Next, the system was cooled to room temperature, and the precipitate was separated with a permanent magnet and washed with distilled water to obtain a neutral pH. Finally, Fe 3 O 4 nanoparticles were washed with acetone and dried at 70 • C [58].

Synthesis of Fe 3 O 4 /ZnWO 4 Nanoparticles
The Fe 3 O 4 /ZnWO 4 nanoparticles were synthesized using the in situ co-precipitation method. First, 0.232 g of the previously obtained Fe 3 O 4 nanoparticles were dispersed in 50 mL of distilled water using ultrasonication for 20 min. Then, 0.274 g of ZnCl 2 dissolved in 50 mL of distilled water was added to the reactor containing Fe 3 O 4 . The reaction mixture was stirred for 10 min at room temperature using a mechanical stirrer. Then, a solution prepared by dissolving 0.66 g of Na 2 WO 4 ·2H 2 O in 50 mL of distilled water was added to this mixture dropwise over 10 min, and the whole solution was mixed using ultrasonication. The resulting precipitate was filtered/separated with a magnet, washed twice with distilled water and once with ethanol, and dried in an oven at 70 • C. Finally, the synthesized Fe 3 O 4 /ZnWO 4 nanoparticles were placed in an oven for calcination (3 h at 550 • C).

Synthesis of Fe 3 O 4 /ZnWO 4 /CeVO 4 Nanoparticles
For this reaction, 0.544 g of Fe 3 O 4 /ZnWO 4 prepared in the previous step was dispersed in 50 mL of distilled water and ultrasonicated for 20 min. Then, 0.058 g of NH 4 VO 3 and 0.217 g of Ce(NO 3 ) 3 ·6H 2 O, were separately dissolved in 50 mL of distilled water to prepare a precipitate with a molar ratio of 1:2:0.5. The NH 4 VO 3 solution was added to the previously dispersed nanoparticles. The mixture was subjected to ultrasonication with a probe with a power of 400 W and frequency of 20 KHz. Afterwards, the Ce(NO 3 ) 3 ·6H 2 O solution was added dropwise to the mixture, and the reaction continued for the next 15 min. Quantities of 0.087 g of NH 4 VO 3 and 0.325 g of Ce(NO 3 ) 3 ·6H 2 O, 0.117 g of NH 4 VO 3 , and 0.44 g of Ce(NO 3 ) 3 ·6H 2 O, as well as 0.175 g of NH 4 VO 3 and 0.651 g of Ce(NO 3 ) 3 ·6H 2 O, were used to prepare precipitates with molar ratios of 1:2:0.75, 1:2:1, and 1:2:1.5, respectively. The obtained precipitates were washed twice with distilled water and once with ethanol and then dried in an oven at 70 • C. Finally, the synthesized Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles were placed in an oven for calcination (3 h at 550 • C).

Evaluation of Photocatalytic Activities
The photodegradation of MB under visible light was measured for all synthesized samples to choose the nanoparticles with the highest photocatalytic activity. In each experiment, 60 mg of photocatalyst was added to 300 mL of a 25 mg L −1 MB solution. Three milliliters of 25% H 2 O 2 were added to the photoreactor to increase the efficacy. Prior to exposure to visible light (250 W xenon lamp), the solution was stirred in the dark for 20 min to obtain an adsorption/desorption equilibrium between the catalyst and MB. Four milliliters of the solution were collected with a pipette every 10 min for the reaction conducted in the dark and every 20 min for the reaction conducted in the light. Then, the studied solution was centrifuged at 5000 rpm for 5 min to separate the catalyst. The concentration of the MB solution was calculated by recording the absorption with a UV-Vis spectrophotometer to identify the result of the photodegradation of pollutants. Afterwards, the sample with the optimized photocatalytic properties (the highest value of degradation) was used for the photodegradation of 10 mg L −1 MO and 10 mg L −1 MV.
The photocatalytic activity of S 4 was evaluated by analyzing the photodegradation of MB under UV light to determine the efficiency of the synthesized photocatalyst. In each experiment, 30 mg of photocatalyst was added to 300 mL of the 20 mg/L MB solution. Prior to exposure to the UV light, (50 W Hg lamp), the solution was stirred in the dark for 20 min to obtain an adsorption/desorption equilibrium between the catalyst and MB solution. Next, 4 mL of the solution were pipetted every 10 min and centrifuged at 5000 rpm for 5 min to separate the catalyst. As described above, the concentration of the MB solution was calculated by measuring the absorbance with a UV-Vis spectrophotometer to estimate the yield of photodegradation. Afterwards, the material with the optimal photocatalytic properties was used to photodegrade 15 mg L −1 MO, 15 mg L −1 FNT, and 20 mg L −1 RhB.

Photodegradation Mechanism
The release of · OH was measured using a photoluminescence method. 2-Hydroxyterephthalic acid is the product of the reaction of TA with · OH, which has fluorescence properties. Hence, the fluorescence intensity depends on the amount of · OH released. The method is similar to photocatalytic tests conducted under UV light. The solution was prepared by adding 0.03 g of the synthesized catalyst (0.1 g L −1 ) to 300 mL of the aqueous TA solutions prepared at concentrations of 0.0005 M and 0.002 M in NaOH. The principal oxidative factor in the photocatalytic reactions was identified by measuring the amounts of · O 2 − , · OH, and holes trapped by tert-butanol, benzoquinone, and citric acid, respectively. The procedure is similar to the UV irradiation test. Three hundred milliliters of a 25 mg L −1 MB solution were prepared, and then 3 mmol of one of the scavengers were added to that solution. Afterwards, 0.03 g of dispersed photocatalyst was added to the system. Finally, after exposing the solution to UV light, 4 mL of the solution was removed from the reactor every 10 min and the catalyst was separated from the pollutant by centrifugation. The progress of each reaction was monitored using a UV-Vis spectrophotometer.

Conclusions
The Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles were prepared at different molar ratios using the co-precipitation method with the help of the sonochemical method. Identification tests, such as XRD, SEM, FTIR, and EDX, confirmed the presence of the subsequent inorganic phases: Fe 3 O 4 , ZnWO 4, and CeVO 4 . The VSM test allowed us to calculate the MS values for Fe 3 O 4 and the Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles: 50.9 and 1.5 emu g −1 , respectively. Additionally, based on the results of the DRS test, the OBGEs for Fe 3 O 4 and Fe 3 O 4 /ZnWO 4 /CeVO 4 were 3.1 and 2.6 eV, respectively. The Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles with a 1:2:1 molar ratio mediated the highly efficient photocatalytic degradation of different organic pollutants, such as MV with~100% degradation (with visible light) and MO with~100% degradation (under UV light). When Fe 3 O 4 /ZnWO 4 /CeVO 4 (S 4 ) was reacted in the presence of various scavengers, the photogenerated · OH and · O 2 − radicals were the main oxidative forms, and the fluorescence intensities increased over time upon the irradiation of TA with UV-Vis light, which produced · OH as a by-product. These results confirm that the Fe 3 O 4 /ZnWO 4 /CeVO 4 nanoparticles represent a potentially useful photocatalyst for the effective removal of organic pollutants from wastewater.