Novel magnetically separable of Fe3O4/Ag3PO4@WO3 nanocomposites for enhanced photocatalytic and antibacterial activity against Staphylococcus aureus (S. aureus)

Background Iron oxide nanocomposites have received a great attention for their application in various fields like physics, medicine, biology, and material science etc., due to their unique properties, such as magnetism, electrical properties, small size, biocompatibility and low toxicity. Methods Fe3O4/Ag3PO4@WO3 nanocomposites with different weight percent of Ag3PO4 were successfully prepared through fabricated Ag3PO4/Fe3O4 with WO3 via in situ fabrication method, electrospinning involved precursor solution preparation and spinning to enhance photocatalyst performance under simulated sunlight for the degradation of methylene blue (MB) and antibacterial activity against Staphylococcus aureus (S. aureus). Results The photocatalytic degradation of methylene blue (MB) under simulated light irradiation indicated that the nanocomposite with 0.25 mg of Ag3PO4 has the best activity. An additional advantage of these photocatalysts is magnetic recoverability, using external magnetic field and photocatalytic stability of the nanocomposites was evaluated for three cycles. In addition, using different scavengers, holes (h+) and superoxide radical (O2·−) radicals and hydroxide radical (·OH) were identified the main oxidative species in the degradation reaction of methylene blue. Conclusions The results reveal that Fe3O4/Ag3PO4@WO3-0.25 nanocomposites have photocatalytic and antibacterial activity against S. aureus. The photocatalyst and mechanism based on the enhancement of electron transfer processes between Ag3PO4 and WO3 nanoparticles.


Introduction
In recent years, research attention has been focused on processes that lead to an improved oxidative degradation of organic pollutants. Therefore, semiconductor photocatalysis technology has aroused scientists' interest in environmental remediation. Although several semiconductors have proven to be ideal candidates for the treatment of water pollution, the efficient separation and recycling of this fine powdered photocatalyst is still a scientific problem when applied in practice, including separation process, selectivity, and dispersion [1,2]. A photocatalyst with magnetic properties allow the use of the technique of magnetic separation, which is one of the most effective and simple methods for removing suspended solids from wastewater without need for further separation processes. The magnetic photocatalyst allows its use as a suspended material and providing the advantage to have a high surface area for reaction light photocatalyst. Fe 3 O 4 has been broadly applied as a significant ferromagnetic material for extensive application areas, including catalysis, recording materials, pigment, magnetocaloric refrigeration, and drug delivery carrier, because of its promising properties such as low cost, good hydrophilicity, and biocompatible properties [4]. Although Fe 3 O 4 is not a suitable semiconductor for the photocatalyst process, it wasn't expensive and possess high band gap energies as they willingly go through photocathodic corrosion [9,10]. But the magnetic Ag 3 PO 4 /TiO 2 /Fe 3 O 4 heterostructured nanocomposite was enhanced photocatalytic activity, cycling stability, and long-term durability in the photodegradation of acid orange 7 (AO7) under visible light. Moreover, the antibacterial activity toward Escherichia coli (E. coli) cells under visible-light irradiation [11]. Ag 3 PO 4 has a relatively narrow band gap (2.36-2.43 eV) and is thus active under visible-light irradiation (λ < 530 nm). Therefore, as a highly efficient photocatalyst, Ag 3 PO 4 could behave as a potential antimicrobial material and could have promise in various antimicrobial applications [12,13]. Magnetically separable composite photocatalysts Fe 3 O 4 -Ag 3 PO 4 [14], Ag 3 PO 4 / NiFe 2 O 4 [15] and Fe 3 O 4 @LDH@Ag/Ag 3 PO 4 [16] with high photocatalytic activity. Recently, magnetic and silver phosphate core-shell photocatalysts composed of a magnetic core and photocatalytic shell have attracted great interest Fe 3 O 4 @Ag 3 PO 4 /AgCl under simulated solar light [17] and Fe 3 O 4 @SiO 2 @Ag 3 PO 4 with excellent visible-light-responding photocatalytic activity [18]. WO 3 has a narrow band gap (2.6-2.7 eV), is very hopeful visible light active photocatalyst. It has been used in the photocatalytic degradation of organic contaminants and in the photocatalytic development of O 2 [19][20][21]. Micro and nanoscaled core-shell materials have attracted great interest because their essential properties can be conveniently adjusted by controlling the morphology and chemical composition of both core and shell [22][23][24]. Moreover, the interactions between the various ingredients of the core and shell can significantly improve the overall performance of the core-shell system and even produce beneficial synergistic effects [25,26]. For example, metal-semiconductor as core-shell photocatalysts have been synthesized and shown to display superior photocatalytic efficiency, because a metallic core can accelerate charge separation and the large-surfaced nanostructures enhance light absorption [27]. However, the photocatalytic activity of WO 3 is not reasonable because of its relatively low conduction band level. There are some reports on the synthesis of chestnut, such as Fe 3 O 4 /WO 3 hierarchical core-shell structures that integrate conductive Fe 3 O 4 microspheres and visible light active WO 3 nanoplates as a magnetically recyclable visible light active photocatalyst [27]. In another study the fabrication of a magnetically recyclable Fe 3 O 4 /WO 3 core-shell visible-light photocatalyst has been developed [28]. Nanosilver-decorated WO 3 nanofibers incorporating paramagnetic CoFe 2 O 4 nanoparticles were fabricated for the first time as model solar light-active photocatalyst with potent antibacterial property and recoverability. Formation of semiconductor composites is an effective way to enhance the photo-induced charges separation efficiency and the photocatalytic performance, which has been extensively studied for the last decades [20].
Magnetic photocatalysts are recognized as low cost, efficient, and robust techniques desirable to remove dyes from contaminated water before their discharge and to produce clean water on a large scale. In principle, the rapid separation of photogenerated electrons and holes can be achieved by forming heterojunctions within the semiconductors. Further, multiscale structuring can beneficially increase the light scattering and absorption in such heterojunctions, thus increasing the light absorption range [29]. Hence, such a strategy can be successfully used for the fabrication of high-efficiency semiconductor composites.
In the present work, through in situ ion exchange method, we fabricated Ag 3 PO 4 /Fe 3 O 4 with WO 3 for the first time as model simulated light-active photocatalyst for the degradation of methylene blue (MB). The antibacterial activity of Ag 3 PO 4 /Fe 3 O 4 @WO 3 composite toward Staphylococcus aureus (S. aureus) was studied. The as synthesized Ag 3 PO 4 /Fe 3 O 4 @WO 3 composite exhibited high antibacterial performance than the other prepared composites. The effect of the presence of the reactive species on the photocatalytic activity was studied, and the possible photocatalytic mechanism of Fe 3 O 4 / Ag 3 PO 4 @WO 3 and photodegradation pathway of MB was suggested.

Materials
All materials have analytical purity and used as received without further purification. All of the solutions were prepared using deionized water ( For the preparation of Fe 3 O 4 /Ag 3 PO 4 , room temperature in situ anion-exchange method was used. Typically, 0.5 g of the as prepared Fe 3 O 4 was dispersed into 50 ml distilled water and sonicated for 1 h to give black aqueous suspension and then 10 ml of three different amount of AgNO 3 (0.25 g, 0.50 g and 0.75 g) was added into the suspension solution and stirred for 0.5 h. Subsequently, 10 ml of aqueous solution with different content of Na 2 HPO 4 (0.13 g, 0.26 g, 0.39 g) was added drop-by-drop with continues stirring for more 0.

Preparation of tungsten oxide WO 3
Fabrication of WO 3 square like nanoplates by electrospinning involved precursor solution preparation and spinning. Combination of a polymer solution and a tungsten oxide precursor solution resulted in precursor gel for electrospinning. The polymer solution could be prepared by dissolving a selected polymer in a solvent while the tungsten oxide precursor solution by dissolving the salt in a suitable solvent. The following protocol is given as a specific example; A PVP solution was prepared by adding 0.7 g of polyvinylpyrrolidone into 7 ml of ethanol under magnetic stirring. In a separate beaker, 0.2 g of tungstic acid was added to 2 ml of DMF. Then, the two solutions were mixed followed by magnetic stirring at room temperature for 15 min. Then the precipitates were separated by a permanent magnet and washed with distilled water several time and dried under vacuum for 24 h.

Preparation of Fe
The fabrication of magnetic with silver phosphate involved WO 3 precursor solution preparation and electrospinning was chosen as a facile and an effective approach to fabricate plates with diameters at nanometer length scale. The precursor solutions were derived from mixtures Fe 3 O 4 /Ag 3 PO 4 nanoparticles with different concentration and tungsten oxide precursor. A typical method is as follows. A PVP solution was prepared by adding 0.7 g of polyvinylpyrrolidone into 7 ml of ethanol under magnetic stirring. In a separate beaker, 0.2 g of tungsten acid was added to 2 ml of DMF. Then, the two solutions were mixed followed by magnetic stirring at room temperature for 15 min. To the mixture above, determined about 0.1 mg of different amounts of Fe 3 O 4 @ Ag 3 PO 4 nanoparticles were added. The final mixture was then stirred by a magnetic stirrer at room temperature for 15 min, followed by sonication for 10 min. Then

Characterization
The morphology and the composition were characterized; Transmission Electron Microscopy (TEM, FEI TECNAI-G2 operating at 300 kV) and Field-emission Scanning Electron Microscope (FESEM, JEOL, JSM-7001F) were used to determine the morphology and size of the magnetite particles, The structures of composites were characterized by X-ray diffractometer (XRD) using a Brucker 8 Advanced, Germany with a Cu Kα (λ = 1.5406˚ A) source in the 2θ range of 10 ̊ to 80 ̊ at room temperature, A vibration sample magnetometer VSM (Model EV9System) was employed for magnetic properties of the samples at room temperature, The Fourier Transform Infrared (FTIR) spectra of the samples were recorded on a Bruker Vertex 70 FT-IR spectrophotometer using the KBr method, The UV-visible diffuse reflectance spectroscopy (DRS) spectra of photocatalyst powder was obtained for the dry-pressed disk samples using scan Shimazu, Japan UV-2450 spectrometer equipped with the integrated sphere accessory for diffuse reflectance spectra, using BaSO 4 as the reflectance sample, UV-vis absorption spectra of the as-prepared samples were obtained using a Hitachi U-4500 spectrophotometer (Hitachi High-Technology Corporation). The electron transfer properties of the synthesized composites were studied using electrochemical impedance spectrometer (EIS) VMP2 multi-potentiostat with ZsimpWin program (Princeton Applied Research, USA) and it's frequencies swept from 10 kHz to 100 mHz.

Evaluation of the antibacterial activity Preparation of photocatalyst film
In general, suspensions of the as-prepared Fe 3 O 4 / Ag 3 PO 4 @WO 3 composites (0.001 mg/1 ml) were dripped onto filter paper (Φ 6 mm), and then placed onto the bottom of a Petri dish (nutrient agar plate) for antibacterial experiments. For comparison, the same film was prepared using Fe 3 O 4 , Ag 3 PO 4 or WO 3 .

Antibacterial tests
The antibacterial activity was evaluated using S. aureus and E. coli as representative microorganisms. Before the antimicrobial experiments, all glass wares were sterilized by autoclaving at 120 °C for 30 min. To measure antibacterial activity, 100 μl of the bacterial suspension was serially diluted with sterile water to make sure the final colony count is not more than 10 −7 colony-forming units per milliliter (CFU ml −1 ). Then 100 μl aliquots were spread onto nutrient agar plates that were prepared already and incubated at 37 °C for 24 h. The experiment was further repeated. For each antibacterial experiment, the prepared photocatalyst film was placed onto the bottom of a Petri dish (onto an agar plate) seeded with 100 μl of S. aureus or E. coli. After 24 h of incubation at 37 °C, the diameters of the inhibition zones were measured.

Photocatalytic activity
The photocatalytic evaluation study was carried out at room temperature. Typically, MB solution (50 ml, 5 mg l −1 ) and 15 mg of the catalyst were placed in a sealed glass. The suspension was ultrasonicated in the dark for 10 min before irradiation to reach an adsorption/desorption equilibrium between MB and the surface of the catalyst under room temperature. Then the mixture was illuminated with 350 Xe lamp as a light source system equipped with a UV cutoff filter (λ > 420 nm). The distance between the light source and the reaction containers was fixed at 10 cm. After simulated light irritation, 3 ml of the mixture was taken out at a regular time interval of 1 min. Then centrifuged to remove the photocatalyst and the concentration of MB was measured with UV-Vis spectrophotometer at 664 nm.

Analysis of hydroxyl radical ( · OH)
Terephthalic acid photoluminescence probe technique was used in the detection of · OH. Terephthalic acid readily reacted with OH to produce highly fluorescent product, 2-hydroxy-terephthalic acid. The method relied on the PL signal at 426 nm of 2-hydroxyterephthalic acid. The PL intensity of 2-hydroxyterephthalic acid was proportional to the amount of · OH formed. Experimental procedures were reported in early reports [30,31], a basic terephthalic acid solution was added to the reactor and the concentration of terephthalic acid was set at 5 × 10 −4 M in 2 × 10 −3 M NaOH solution. The solution was irradiated for intervals time 10 min using ultraviolet light 365 for 50 min under magnetic stirring. The PL spectra of generated 2-hydroxyterephthalic acid were measured on a Hitachi F-4500 fluorescence spectrophotometer. The reaction solution was used to measure the increase of the PL intensity at 426 nm excited by 365 nm ultraviolet light.

Transmission electron microscopy
This synthesis procedure of Fe 3 O 4 /Ag 3 PO 4 @WO 3 nanocomposites is presented Schematic diagram in Fig. 1. A Fe 3 O 4 nanoparticle was first synthesized by co-precipitation method and fabrication of magnetic with silver phosphate involved WO 3 precursor solution preparation and electrospinning method.  Fig. 2c, the as prepared Fe 3 O 4 /Ag 3 PO 4 well cabled with the WO 3 nanoplates, indicating the Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite has been successfully fabricated spherical shape with wide size distribution, and the aggregation of the particles is dense and the size range of bare Fe 3 O 4 nanoparticles is 29 nm Fig. 2d-f shows the SEM images of the Ag 3 PO 4 , Fe 3 O 4 / Ag 3 PO 4 , and Fe 3 O 4 /Ag 3 PO 4 @WO 3 composites, respectively. As been seen in (Fig. 2e)

X-Ray energy-dispersive spectroscopy analysis
The purity of the samples was investigated using EDS analysis and the results are shown in Fig. 3a. As can be seen the peaks in the spectra of the Fe 3 O 4 /Ag 3 PO 4 @ WO 3 -0.25 composite are described to O, W, P, Fe, and Ag elements. Moreover, the EDS elemental mapping further elucidated the composition of the Fe 3 O 4 /Ag 3 PO 4 @ WO 3 -0.25 nanocomposite. As can be seen in Fig. 3b-

X-ray diffraction
The XRD analysis was used to investigate the different crystalline structures of the synthesized composites (Fig. 4)    respectively, which can be indexed to the standard XRD data of the cubic-phase Ag 3 PO 4 crystal (JCPDS File, no: 06-0505, 74-1876) [33,34]. The square-like WO 3 nanoplates indicate that the as-synthesized WO 3 crystal structure with obvious diffraction peaks at 2θ value of 16 3 , and no other peaks were detected. This further confirms the fabrication of the synthesized composite.
The particle size play important role on the photocatalytic performance of the semiconductors, the less particle sizes the higher catalytic activity. To calculate the particle size of the synthesized composites, Scherrer's Eq. 1 [36] was used.
where β is full width at half-maximum, D is crystallite size, λ is X-ray wavelength, θ is Bragg's diffraction angle.
The average sizes of the WO 3 were estimated to be 450, 411 and 383 corresponding to the patterns of (1) D = K /βcosθ

Fig. 3 a EDS spectra for the F 3 O 4 /Ag 3 PO 4 @WO 3 , b-g EDS mapping for the F 3 O 4 /Ag 3 PO 4 @WO 3 nanocomposite
From the calculations, the crystalline size of Fe 3 O 4 / Ag 3 PO 4 markedly decreased after the synthesis of Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite. Which it's beneficial enhance the photocatalytic activity.

Magnetic properties
The magnetic properties have been quantified by using a superconducting quantum interference device a vibrating sample magnetometer (VSM Model EV9System) at room temperature. Figure 5, displayed the hysteresis loops at 300 K of the as prepared Fe 3 O 4 microspheres and Fe 3 O 4 /Ag 3 PO 4 @WO 3 -0.25 composites catalysts. The saturation magnetization (Ms), remanence (Mr), coercivity (Hc) and loop Squarenses ratio (Mr/Ms) of samples is shown in (Tables 1 and 2

Fourier transforms infrared spectroscopy
The typical vibration peaks of the Fe 3 O 4 , Ag 3 PO 4 , WO 3 , and Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite are shown in the FT-IR spectrum in (Fig. 6). In the spectra of the functionalized Fe 3 O 4 , the presence of OA layer can be confirmed. The bands at 2840 cm −1 and 2917 cm −1 can be ascribed to the stretching modes of -CH 2 and -CH 3 of the OA, respectively. The vibrations at 1699 cm −1 and 1454 cm −1 can be assigned to the stretching modes of -C=O and -C=C-of the OA, respectively. The characteristic band of the pure Fe 3 O 4 usually appears at 570 cm −1 , in the case of the present sample, this peak was shifted to 579 cm −1 , due to the functionalization process. In the spectra of Ag 3 PO 4 , the intense absorption peak at 1016 cm −1 is ascribed to the stretching vibration of the phosphate (PO 4 3− ) group, and the absorption peaks at 574 and 538 cm −1 is ascribed to the bending vibration of the phosphate (PO 4 3− ) group. The absorption peaks at 3450 and 1660 cm −1 are corresponding to the − OH stretching and bending vibrations of physically absorbed H 2 O molecules, respectively. The precursor of WO 3 has strong

UV-vis diffuse reflectance spectroscopy and band structure
The study of the light absorption behaviors clearly shows the efficient photocatalytic properties of the semiconductor [40,41]. The light absorption behaviors of the synthesized photocatalysts were studied by DRS analysis, and the results are shown in (Fig. 7a). The results show that, the light absorptivity of the products shown a trend to increase in the range of 200-520 nm and reached a peak at 510 nm, which belonged to the visible region. The band gap of the synthesized composites was determined by a plot of (αhv) 2 versus energy (hv), (Fig. 7b)

Evaluation of antibacterial activity
Concerning the prospective application of Fe 3 O 4 / Ag 3 PO 4 /WO 3 , the antibacterial properties of the as-synthesized catalyst were investigated in the present work. S. aureus even was chosen as representative microorganism. Following the findings above, it was presumed that successful deposition of Ag 3 PO 4 imparted antibacterial functions on the nanocomposites [42][43][44]. To further assess this function, we employed the test method of antimicrobial in term of the inhibitory zone. In a control sample, bacterial colony growth was induced on an Agar dish as illustrated in (Fig. 8a-f ), following interfacial contact between the test samples and the Agar plate, which should lead to inhibition of the bacterial growth. The radius of the inhibition zones was measured. As seen in Fig. 8a  and Fe 3 O 4 /Ag 3 PO 4 @WO 3 -non composite shows inhibition zone of 12 mm and 17 mm, respectively.

Study of photocatalytic activity
For the evaluation of the photocatalytic activity of the synthesized composites, MB was used as a representative sample. Figure 9a shows the photocatalytic activities of the Fe 3 O 4 /Ag 3 PO 4 @WO 3 photocatalysts with different ratios of Ag 3 PO 4 under simulated light irradiation and the MB solution shows light absorption edge at λ max of 664 nm. In order to investigate the optimum amount of Ag 3 PO 4 on the synthesized composite, Fe 3 O 4 /Ag 3 PO 4 @ WO 3 with different Ag 3 PO 4 was synthesized, and their photocatalytic activity against MB degradation was studied, and the results are shown in (Fig. 9b)

Electrochemical impedance spectroscopy (EIS)
The electrochemical impedance spectroscopy of the WO 3 , Ag 3 PO 4 @WO 3 , Fe 3 O 4 /Ag 3 PO 4 @WO 3 -non and Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite was carried out to further investigated the charge transfer and recombination processes in the Fe 3 O 4 /Ag 3 PO 4 @WO 3 composites under simulated light irradiation, and the result shown in (Fig. 10), a smaller arc radius can be observed on the EIS Nyquist plot of Fe 3 O 4 /Ag 3 PO 4 @WO 3 -non and Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite compared with WO 3 and Ag 3 PO 4 @WO 3 indicating that a more effective separation of the photogenerated electron/hole pairs and a faster interfacial charge transfer occurred on the surface of the Fe 3 O 4 /Ag 3 PO 4 @WO 3 composite.

Kinetic study
In order to investigate the kinetic behavior of the synthesized photocatalysts on the degradation of MB under simulated sunlight irradiation, ln (C/Co) for MB is plotted versus irradiation time according to the following first-order kinetic model 2: where k is the degradation rate constant, Co and C the initial concentration and the concentration at different irradiation time t of the organic dye respectively. From the result presented in (Fig. 11), the disappearance of MB over the Fe 3 O 4 /Ag 3 PO 4 @WO 3 synthesized photocatalysts under simulated light irradiation is shown to fit a pseudo first order kinetics pattern, with degradation (2) ln(C/Co) = −kt

Stability study
The stability of a photocatalyst is important for its practical application. The stability and recyclability of Fe 3 O 4 / Ag 3 PO 4 @WO 3 are evaluated by additional experiments to degrade MB under simulated light irradiation cycled for three times. Figure 12a, shows the repeated visible light photocatalytic activity of Fe 3 O 4 /Ag 3 PO 4 @WO 3 , it can be observed that composite have good stability for the degradation of MB under simulated light irradiation during three cycles.

Active species responsible for MB degradation
The recombination of photogenerated electron-hole pairs is an important limiting factor for the performance of photocatalysts [45,46]. To study the photocatalytic mechanism of the Fe 3 O 4 /Ag 3 PO 4 /WO 30 -0.25 composite in detail, the effects of different scavengers on the decomposition of dye molecules were investigated. In order to determined the predominant reactive oxygen species in the photocatalytic process. In this study, p-benzoquinone (BZQ), disodium ethylenediamine tetraacetate (Na 2 -EDTA) and tert-butanol (t.B) were used as scavengers for O 2 ·− , h + , and · OH, respectively [47]. As seen in (Fig. 12b), the addition of B.Q and t.B has slight effect on the inhibition of the dye degradation. Introducing EDTA to the reaction system the photocatalytic activity significantly reduced. Which indicate that the photocatalytic reaction mainly depends on the h + . While O 2 ·− and · OH have negligible effect. Herein, the highly enhanced photodegradation and antimicrobial activities achieved on Fe 3 O 4 /Ag 3 PO 4 @WO 3 can be assigned to its composite structure.
The formation of · OH on the surface photo-illuminated of composite Fe 3 O 4 /Ag 3 PO 4 @WO 3 (0.25, 0.50 and 0.75) was further confirmed by the PL technique using terephthalic acid as a probe molecule. Figure 13a shows that an obvious difference in PL intensity at about ∼ 425-430 nm was observed using different catalysts. It was clear that the formation rate of · OH on the Fe 3 O 4 /Ag 3 PO 4 @ WO 3 -0.25 was higher than that of other composite. This implied that the former has higher photocatalytic activity than the latter. Moreover, the inset of ( Fig. 13a) the pure Ag 3 PO 4 exhibited higher PL intensity than pure WO 3 and pure Fe 3 O 4 , suggesting that doping of Ag 3 PO 4 with WO 3 and Fe 3 O 4 was a good route to accelerate the interfacial charge transfer and inhibit the recombination of electron-hole pairs, which resulted in the increase in · OH formation. Moreover, the Fig. 13b depicts the change of PL spectra with irradiation time for the case of Fe 3 O 4 / Ag 3 PO 4 @WO 3 -0.25. A gradual increase in PL intensity was observed with increasing irradiation time, which suggested that the fluorescence was caused by chemical reactions of terephthalic acid with · OH formed during photo-illuminated reactions. Thus, these results confirmed the evidence of · OH formation and indeed participated in degradation process.

Probable mechanism
As illustrated in Fig. 14 the semiconductors of WO 3 (with an optical band gap of 2.44 eV and Ag 3 PO 4 (with an optical band gap of 2.35 eV, especially, the redox potentials (CB and VB) of WO 3 are more positive than can migrate to that of WO 3 and the photogenerated holes could migrate from the VB of WO 3 to that of Ag 3 PO 4 . The holes could directly oxidize the organic dyes adsorbed on Ag 3 PO 4 surface [48][49][50] and the electrons could be consumed through a multi-electron reaction with oxygen (O 2 + 2H + + 2e − → H 2 O 2 , E 0 = + 0.682 V vs. NHE) [51]. The produced H 2 O 2 reacts with an additional electron to produce · OH (H 2 O 2 + e − → OH − + · OH) [52], which could accelerate the antimicrobial activity.
At the same time, the enhanced photocatalytic activity could be expected the composites Fe 3 O 4 /Ag 3 PO 4 / WO 3 -0.25 catalyst due to the effective separation of photogenerated electron-hole pairs. According to the plot of (Ahν) 2 vs. hν, the band gaps (Eg) of Ag 3 PO 4 and WO 3 are estimated to be 2.35 and 2.44 eV, respectively. The band structure of Fe 3 O 4 /Ag 3 PO 4 /WO 3 composites can be estimated according to the empirical equations as shown below: where EVB and ECB are the valence and conduction band edge potentials, respectively; χ is the electronegativity of the semiconductor; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV vs. NHE). The χ values for bare Fe 3 O 4 , Ag 3 PO 4 and WO 3 are 5.78, 5.96 and 6.49 eV and, respectively [53][54][55]