One-Step Synthesis of Ag2O/Fe3O4 Magnetic Photocatalyst for Efficient Organic Pollutant Removal via Wide-Spectral-Response Photocatalysis–Fenton Coupling

Photocatalysis holds great promise for addressing water pollution caused by organic dyes, and the development of Ag2O/Fe3O4 aims to overcome the challenges of slow degradation efficiency and difficult recovery of photocatalysts. In this study, we present a novel, environmentally friendly Ag2O/Fe3O4 magnetic nanocomposite synthesized via a simple coprecipitation method, which not only constructs a type II heterojunction but also successfully couples photocatalysis and Fenton reaction, enhancing the broad-spectrum response and efficiency. The Ag2O/Fe3O4 (10%) nanocomposite demonstrates exceptional degradation performance toward organic dyes, achieving 99.5% degradation of 10 mg/L methyl orange (MO) within 15 min under visible light irradiation and proving its wide applicability by efficiently degrading various dyes while maintaining high stability over multiple testing cycles. Magnetic testing further highlighted the ease of Ag2O/Fe3O4 (10%) recovery using magnetic force. This innovative approach offers a promising strategy for constructing high-performance photocatalytic systems for addressing water pollution caused by organic dyes.


Introduction
With the acceleration of global industrialization, water pollution caused by organic dyes has become an increasingly urgent issue of public concern. Photocatalytic technology has been widely researched owing to its advantages, such as high efficiency and the absence of secondary pollution [1][2][3][4][5][6][7][8] Among them, silver oxide (Ag 2 O) nanoparticles are extensively employed to degrade water pollutants due to their simple preparation, stable properties, and environmental friendliness [9][10][11][12]. However, the narrow bandgap and low photogenerated carrier-separation efficiency of Ag 2 O limit its application in water treatment [13][14][15][16]. Furthermore, using Ag 2 O as a powder photocatalyst makes it challenging to recycle. Therefore, it is practically significant to develop a magnetic material to couple with Ag 2 O through heterojunctions to enhance the photogenerated carrier separation efficiency and fabricate an efficient and magnetically recoverable Ag 2 Obased photocatalyst.
Fe 3 O 4 nanoparticles possess unique magnetic properties and nanoscale characteristics, which make them highly versatile for various potential applications, including drug delivery, MRI contrast agents, biomolecule separation, biosensing, and catalysis, due to their magnetism and biocompatibility [17][18][19][20][21][22]. Additionally, Fe 3 O 4 nanoparticles demonstrate strong light-absorption ability, absorbing most of the light in the UV-visible range [23]. Furthermore, the Fe 2+ ions in Fe 3 O 4 can react with hydrogen peroxide (H 2 O 2 ) through the Fenton reaction to generate a large number of free radical groups, which can oxidize many known organic compounds, such as carboxylic acids, alcohols, and esters, into inorganic forms, exhibiting a significant oxidation ability to remove refractory organic pollutants [24][25][26][27]. It is known that the Fenton reaction can produce a large number of oxygen-related species through the following reactions: Additionally, Fe 3 O 4 is a magnetic material that can be recycled and reused by an external magnetic field, thereby reducing the cost of recovery treatment [28][29][30]. In contrast, the use of Fe 3 O 4 in the Fenton reaction to oxidize organic pollutants in water has significant limitations, including the need to consume externally provided H 2 O 2 in the reaction and the requirement of an acidic environment to generate free radicals through the Fenton reaction. However, when Fe 3 O 4 is coupled with a photocatalyst, the H 2 O 2 generated at the interface of the photocatalyst can be used for the Fenton reaction with Fe 3 O 4 . Another advantage of selecting Fe 3 O 4 is that it contains both Fe 2+ and Fe 3+ ions, facilitating the continuous progress of the Fenton reaction. Consequently, numerous researchers have employed Fe 3 O 4 as a cocatalyst in the photocatalyst system [31][32][33][34].
In this article, the Ag 2 O/Fe 3 O 4 binary magnetic nanoparticles were synthesized using a simple chemical coprecipitation method with FeCl 2 ·4H 2 O, FeCl 3 ·6H 2 O, and AgNO 3 as raw materials, and they were applied to degrade the organic dyes in water. The results showed that the introduction of Fe 3 O 4 to load Ag 2 O could generate a type II heterojunction at the contact interface, facilitating the fast transfer of photogenerated carriers. At the same time, the photocatalysis-Fenton combined reaction was also constructed to improve the utilization efficiency of photogenerated carriers, further enhancing the degradation efficiency of the photocatalyst. The Ag 2 O/Fe 3 O 4 nanoparticles exhibited very high efficiency in degrading dyes, such as methyl orange (MO), under visible light irradiation.

TEM Analysis
Transmission electron microscopy (TEM) was used to characterize the microstructure of the samples. Figure 1 shows the results. Firstly, TEM analysis was performed on Fe 3 O 4 nanoparticles, and the average particle size was found to be 15 ± 5 nm with a typical spherical morphology, as shown in Figure 1a. Due to the large surface area, the Fe 3 O 4 nanoparticles exhibited obvious aggregation in the image. Figure 1b shows the TEM image of the Ag 2 O nanoparticles, which had a particle size distribution ranging from 30 to 80 nm and a polyhedral morphology that differed greatly from Fe 3 O 4 . The nanoparticles of the two materials could be easily distinguished. Figure 1c shows the TEM image of binary Ag 2 O/Fe 3 O 4 (10%) nanoparticles, which demonstrates that Fe 3 O 4 nanoparticles with smaller size and spherical morphology could encapsulate Ag 2 O nanoparticles with larger size and polyhedral morphology, indicating good compatibility between Fe 3 O 4 and Ag 2 O. A high-resolution TEM (HRTEM) analysis was performed on a circular region indicated in Figure 1c to confirm the successful formation of Ag 2 O/Fe 3 O 4 (10%). Figure 1d shows the results. The clear interface between Fe 3 O 4 nanoparticles and Ag 2 O nanoparticles was observed with lattice spacing of 0.25 nm (corresponding to the (311) plane of/Fe 3 O 4 ) and 0.29 nm (corresponding to the (220) plane of Ag 2 O), respectively, further demonstrating the successful formation of Ag 2 O/Fe 3 O 4 (10%).

SEM and EDS Analysis
Ag2O/Fe3O4 (10%) was analyzed through scanning electron microscopy to better verify the successful coupling of Ag2O and Fe3O4 to form Ag2O/Fe3O4 binary nanoparticles, as shown in Figure 2. A relatively large size was selected for characterization to better analyze the overall morphology and surface element distribution of the binary nanoparticles. As shown in Figure 2a, it can be observed that the smaller Fe3O4 nanoparticles with approximately spherical shape were well loaded onto the surface of larger-sized Ag2O nanoparticles with polyhedral shape, forming a compact structure of binary nanoparticles with good structural stability, which is consistent with the conclusion obtained from the TEM image analysis. Additionally, the loading of Fe3O4 increased the number of reaction sites. Note that almost no Fe3O4 spherical nanoparticles were present in unoccupied areas on the surface of Ag2O, indicating that Ag2O has a good ability to capture Fe3O4. EDS analysis was performed in this area to analyze the surface element distribution of Ag2O/Fe3O4 (10%) binary nanoparticles. Figure 2b-e shows the results, where all Fe elements of Fe3O4 are uniformly distributed on the surface of Ag2O, indicating that Fe3O4 was successfully loaded onto the surface of Ag2O to form Ag2O/Fe3O4 binary nanoparticles. EDS data statistics were conducted to further demonstrate the contents of Fe3O4 and Ag2O. Table 1 presents the results. It can be seen that ratio of the number of Ag atoms and Fe atoms is approximately 6:1, indicating that the mass ratio of Ag2O to Fe3O4 is approximately 9:1, and Fe3O4 accounts for 10% of the total mass.

SEM and EDS Analysis
Ag 2 O/Fe 3 O 4 (10%) was analyzed through scanning electron microscopy to better verify the successful coupling of Ag 2 O and Fe 3 O 4 to form Ag 2 O/Fe 3 O 4 binary nanoparticles, as shown in Figure 2. A relatively large size was selected for characterization to better analyze the overall morphology and surface element distribution of the binary nanoparticles. As shown in Figure 2a, it can be observed that the smaller Fe 3 O 4 nanoparticles with approximately spherical shape were well loaded onto the surface of larger-sized Ag 2 O nanoparticles with polyhedral shape, forming a compact structure of binary nanoparticles with good structural stability, which is consistent with the conclusion obtained from the TEM image analysis. Additionally, the loading of Fe 3

XRD Analysis
X-ray diffraction (XRD) was used to characterize the pure Ag2O and Fe3O4 nanoparticles as well as Ag2O/Fe3O4 (10%) nanocomposites to investigate their crystal structure. Figure 3 shows the results. The diffraction peaks of Ag2O nanoparticles at X-ray diffraction angles (2θ) of 26

XRD Analysis
X-ray diffraction (XRD) was used to characterize the pure Ag 2 O and Fe 3 O 4 nanoparticles as well as Ag 2 O/Fe 3 O 4 (10%) nanocomposites to investigate their crystal structure. Figure 3 shows the results. The diffraction peaks of Ag 2 O nanoparticles at X-ray diffraction angles (2θ) of 26

XPS Elemental Analysis
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of the Ag2O/Fe3O4 (10%) sample. Figure 4a shows the XPS spectrum of the sample, exhibiting distinct peaks at around 285.2 eV (C 1s), 368.8 eV (Ag 3d), 530.08 eV (O 1s), and 711.08 eV (Fe 2p), which indicate the presence of four elements, namely C, Ag, O, and Fe. The presence of C is attributed to the fixation of CO2 from air during the preparation of the binary composite material. XPS fine-spectrum measurement was performed to investigate the elemental state in detail. Figure 4b shows the Ag 3d fine spectrum, exhibiting binding energies of 368.2 eV and 374.0 eV for Ag 3d5/2 and Ag 3d3/2, respectively. These binding energies correspond to the orbit peaks of Ag + in Ag2O, confirming the existence of Ag2O in the compound. As depicted in Figure 4c, the Fe 2p XPS spectrum reveals two spin-orbit doublets. The first doublet, attributed to Fe 2+ , is observed at 710.58 eV (Fe 2p3/2) and 723.78 eV (Fe 2p1/2), while the second doublet, assigned to Fe 3+ , is observed at 712.18 eV (Fe 2p3/2) and 726.12 eV (Fe 2p1/2). This mixed phase confirms the formation of Fe3O4. Figure 4d shows the O 1s fine spectrum, in which the peak at 532.11 eV is attributed to external −OH groups or adsorbed water molecules on the surface, the peak at 531.11 eV corresponds to the lattice oxygen atoms in Ag2O, and the peak at 529.39 eV is attributed to the Fe-O bond [35,36]. Therefore, XPS analysis confirms the presence of Ag2O and Fe3O4 in the Ag2O/Fe3O4 (10%) binary nanocomposite material and their successful composition.

XPS Elemental Analysis
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of the Ag 2 O/Fe 3 O 4 (10%) sample. Figure 4a shows the XPS spectrum of the sample, exhibiting distinct peaks at around 285.2 eV (C 1s), 368.8 eV (Ag 3d), 530.08 eV (O 1s), and 711.08 eV (Fe 2p), which indicate the presence of four elements, namely C, Ag, O, and Fe. The presence of C is attributed to the fixation of CO 2 from air during the preparation of the binary composite material. XPS fine-spectrum measurement was performed to investigate the elemental state in detail. Figure 4b shows the Ag 3d fine spectrum, exhibiting binding energies of 368.2 eV and 374.0 eV for Ag 3d5/2 and Ag 3d3/2, respectively. These binding energies correspond to the orbit peaks of Ag + in Ag 2 O, confirming the existence of Ag 2 O in the compound. As depicted in Figure 4c, the Fe 2p XPS spectrum reveals two spin-orbit doublets. The first doublet, attributed to Fe 2+ , is observed at 710.58 eV (Fe 2p3/2) and 723.78 eV (Fe 2p1/2), while the second doublet, assigned to Fe 3+ , is observed at 712.18 eV (Fe 2p3/2) and 726.12 eV (Fe 2p1/2). This mixed phase confirms the formation of Fe 3 O 4 . Figure 4d shows the O 1s fine spectrum, in which the peak at 532.11 eV is attributed to external −OH groups or adsorbed water molecules on the surface, the peak at 531.11 eV corresponds to the lattice oxygen atoms in Ag 2 O, and the peak at 529.39 eV is attributed to the Fe-O bond [35,36]. Therefore, XPS analysis confirms the presence of

UV-Vis and PL Analysis
UV-vis and PL tests were conducted to determine the optical properties o synthesized nanomaterials. UV-vis testing was used to measure the absorbance o synthesized nanomaterials. Figure 5a shows the results. Ag2O exhibits strong absor in the ultraviolet and near-ultraviolet regions, with a peak at a wavelength of 500 nm 15]. Fe3O4 exhibits a strong optical response across the whole examined spectral r indicating that the strong visible light-absorption capability of Ag2O/Fe3O4 b composite catalysts is undoubtedly due to the optical properties of Fe3O4 [23,31 Furthermore, compared with Ag2O, a gradual redshift was observed at the absor edge of Ag2O/Fe3O4 binary composite catalysts, and a significant increase in absor was observed in the near-infrared region of 600-800 nm, indicating a strong intera between Ag2O and Fe3O4 in the binary composite catalyst. It is worth noting that a loading amount of Fe3O4 increases, the light absorption ability of Ag2O/Fe3O4 b photocatalysts in the UV-visible spectral range also increases. Ag2O/Fe3O4 (15%) ex the best light-absorption ability, followed by Ag2O/Fe3O4 (10%) and Ag2O/Fe3O4 (5% Kubelka-Munk equation was used to calculate the bandgap energy of the semicondu ( ℎ ) = ℎ where α represents the absorption coefficient of the semiconductor, h is a constan stands for the Planck constant, v represents the frequency of light, A is a constan represents a constant term, and n is closely related to the semiconductor tran process. The indirect semiconductors Ag2O and Fe3O4 both have n values of 4 Kubelka-Munk function was used to derive the absorption spectra of all the synthe

UV-Vis and PL Analysis
UV-vis and PL tests were conducted to determine the optical properties of the synthesized nanomaterials. UV-vis testing was used to measure the absorbance of the synthesized nanomaterials. Figure 5a shows the results. Ag 2 O exhibits strong absorption in the ultraviolet and near-ultraviolet regions, with a peak at a wavelength of 500 nm [13][14][15].
where α represents the absorption coefficient of the semiconductor, h is a constant and stands for the Planck constant, v represents the frequency of light, A is a constant and represents a constant term, and n is closely related to the semiconductor transition process. catalysts, which were then used to generate Tauc plots. As shown in Figure 5b, the results of Tauc plots show that the optical bandgaps of Ag2O and Fe3O4 are 2.0 eV and 1.2 eV, respectively, while the bandgaps of Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) are 1.6 eV, 1.5 eV, and 1.4 eV, respectively. These results are in good agreement with the increasing trend in the redshift observed at the absorption edge with the increase in the Fe3O4 loading amount shown in Figure 5a. When Ag2O and Fe3O4 are exposed to light, valence band electrons absorb photon energy and transition to the conduction band, forming photogenerated electron-hole pairs. PL emission occurs when conduction band electrons recombine with valence band holes. Therefore, PL intensity is proportional to the separation of photogenerated charge carriers; lower PL intensity reflects a reduction in recombination probability. As shown in Figure 5c, when the samples of Ag2O, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) were subjected to PL testing under 260nm excitation light, their emission peak positions were all at 400 nm. Although the emission peak intensity of Ag2O was higher, it decreased with the loading of Fe3O4. Compared with Ag2O, the emission peak intensity of Ag2O/Fe3O4 (5%) decreased to 90%, while that of Ag2O/Fe3O4 (10%) decreased significantly to 60%. However, as the loading of Fe3O4 continued to increase, the emission peak intensity of Ag2O/Fe3O4 (15%) was higher than that of the original Ag2O. Therefore, it can be concluded that photogenerated electron-hole pairs are generated when light is irradiated onto the surface of Ag2O. When Fe3O4 with a low loading is coupled to the surface of Ag2O, they enhance the separation efficiency of photogenerated electron-hole pairs generated by Ag2O. However, when the loading of Fe3O4 exceeds 15% of the total mass, an excess Fe3O4 forms a thick covering layer on the surface of Ag2O. Under light irradiation, Fe3O4 absorbs photons, causing the electrons on the valence band When Ag 2 O and Fe 3 O 4 are exposed to light, valence band electrons absorb photon energy and transition to the conduction band, forming photogenerated electron-hole pairs. PL emission occurs when conduction band electrons recombine with valence band holes. Therefore, PL intensity is proportional to the separation of photogenerated charge carriers; lower PL intensity reflects a reduction in recombination probability. As shown in Figure 5c

Electrochemical Characterization Analysis
Mott-Schottky analysis, photocurrent response analysis, and EIS were performed to determine the electrochemical properties of the prepared samples. The Mott-Schottky plot is the most commonly used method to distinguish between n-type and p-type semiconductors [37]. A positive slope and a negative slope indicate an n-type and a p-type semiconductor, respectively. Additionally, the Mott-Schottky plot can be extrapolated to estimate the flat-band potential (Efb) of the semiconductor, which can be used to estimate the position of the Fermi level [38]. Assuming that the Fermi level is very close to the band edge, the extrapolated flat-band potential (Efb) can be utilized as the position of the edge of either the n-type semiconductor (E CB ) or the p-type semiconductor (E VB ). Figure 6a,

Photocatalytic Performance Analysis
Fe3O4, Ag2O, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) were pl under a xenon lamp light source (λ > 420nm) to simulate visible light in sunlight an photocatalyze a 10 mol/L MO solution to better demonstrate the visible photocatalytic performance of different samples. Figure 7a shows the results. When    The photocatalytic rate of Ag2O changed significantly after Fe3O4 was loaded onto surface. When the Fe3O4 loading amount was 5wt% of the overall weight, the Ag2O/Fe (5%) binary catalyst was formed, degrading 99.1% of the MO solution after 15 min visible light irradiation. When the Fe3O4 loading amount was 10wt% of the overall weig the Ag2O/Fe3O4 (10%) binary catalyst was formed, degrading 99.5% of the MO solut after 15 min of visible light irradiation. It is worth noting that when the Fe3O4 load amount continued to increase to 15wt% of the overall weight, the Ag2O/Fe3O4 (15%) bin catalyst did not increase but decreases the degradation rate of the MO solution. Moreov it only degraded 75.1% and 85.2% of the MO solution after 15 min and 30 min of visi light irradiation, respectively. This indicates that during the Fe3O4 loading, a covera layer forms on the surface of Ag2O, and Fe3O4 absorb photons and produce electron-h pairs under light irradiation, which are then be transferred from type-II heterojunction the electrode of Ag2O to participate in the reaction. However, when there is too mu  O, blocks the entry of photons, and shields the surface of Ag 2 O from light, thereby reducing the generation of photoinduced carriers. In addition, in photocatalytic reactions, electrons and holes are transmitted through the surface conductor, thereby participating in redox reactions. The reduction in the available surface area of oxidized silver increases the resistance encountered by electrons and holes during transmission, resulting in a slower charge transfer rate and reduced reaction efficiency under visible light irradiation. Figure 7b shows the UV-vis absorption spectra during the photocatalytic degradation of the MO solution using Ag 2 O/Fe 3 O 4 (10%). It can be observed that the absorption peak at 464 nm of MO decreases significantly with the irradiation time, and the peak intensity almost reaches zero after 15 min of irradiation. No new absorption peaks were generated, indicating that MO was completely degraded into inorganic substances without the formation of other organic compounds. Figure 7c shows the degradation of MO by different photocatalysts using a pseudo-first-order kinetics model. It can be seen that the degradation rate of Ag 2 O/Fe 3 O 4 (10%) is the fastest, reaching 0.183 min −1 , which is 2.3 times higher than that of pure Ag 2 O (0.078 min −1 ), 3 times higher than that of Ag 2 O/Fe 3 O 4 (15%) (0.061 min −1 ), and 1.17 times higher than that of Ag 2 O/Fe 3 O 4 (5%) (0.156 min −1 ). Four photocatalytic cycling tests were conducted to verify the structural stability of the Ag 2 O/Fe 3 O 4 (10%) sample. Figure 7d shows the results. After four cycles, the catalytic rate of Ag 2 O/Fe 3 O 4 (10%) slightly decreased but still exhibited a fast catalytic rate, indicating good structural stability.

Photocatalytic Performance Analysis
In general, the reactive species in photocatalytic processes are often considered to be holes (h + ), hydroxyl radicals (·OH), and superoxide ion radicals (·O 2 − ). Therefore, EDTA-2Na, isopropyl alcohol (IPA), and benzene quinone (BQ) were selected as the capture agents to study the capture of these reactive species, as shown in Figure 7e. Through the photodegradation experiment of MO using the Ag 2 O/Fe 3 O 4 (10%) photocatalyst under visible light, in which the original photocatalytic degradation was 18.3 × 10 −2 min −1 , it was observed that the degree of inhibition of the photocatalytic degradation rate decreased in the following order: BQ (1.85 × 10 −2 min −1 ), IPA (2.76 × 10 −2 min −1 ), and EDTA-2Na (11.2 × 10 −2 min −1 ). This reveals that ·O 2 − and ·OH have a significant impact on the degradation of MO in the Ag 2 O/Fe 3 O 4 photocatalytic reaction, while h + has a relatively small degree of participation.
The catalytic rates of phenol, rhodamine B, methyl blue, and basic fuchsin were tested under visible light irradiation to verify the applicability of the Ag 2 O/Fe 3 O 4 (10%) photocatalyst for the degradation of organic pollutants in water. As shown in Figure 7f, the degradation rates of basic fuchsin, rhodamine B, and methyl blue were 10.72 × 10 −2 min −1 , 9.37 × 10 −2 min −1 , and 6.1 × 10 −2 min −1 , respectively. This demonstrates that the Ag 2 O/ Fe 3 O 4 (10%) photocatalyst has a good applicability and a good catalytic effect on various types of organic pollutants in water.  (10%), respectively. It can be observed that although the size of the magnetic moment increases with the external magnetic field, its maximum value is much smaller than the saturation magnetization of ferromagnetic materials. Therefore, the hysteresis loop shows a curve similar to paramagnetism. Since the magnetic moment is very small, the hysteresis loop of Ag 2 O/Fe 3 O 4 nanocomposites is smoother and more symmetrical than that of paramagnetic materials. Therefore, due to the introduction of Fe 3  moment is very small, the hysteresis loop of Ag2O/Fe3O4 nanocomposites is smoother and more symmetrical than that of paramagnetic materials. Therefore, due to the introduction of Fe3O4, it was confirmed that Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) all have superparamagnetic properties [39]. A neodymium magnet adsorption experiment was performed to verify whether Ag2O/Fe3O4 (10%) can be magnetically recovered. Figure 8d shows the results. Specifically, a neodymium magnet was placed next to the Ag2O/Fe3O4 (10%) suspension and was allowed to stand still for 20 min. It was observed that the neodymium magnet clearly adsorbed the gray-brown catalyst powder. Therefore, it was confirmed that magnetic adsorption can recover Ag2O/Fe3O4 (10%).

Photocatalytic Reaction Mechanism Analysis
First, under visible light irradiation, Ag2O and Fe3O4 on the surface of Ag2O/Fe3O4 are excited from the valence band to the conduction band, generating photogenerated electrons (e − ) and leaving behind holes (h + ). As the ECB of Ag2O is −0.67 eV, which is more negative than that of Fe3O4, i.e., 0.54 eV, and the EVB of Ag2O is 2.03 eV, which is more negative than that of Fe3O4, i.e., 2.14 eV, a type-II heterojunction is formed due to the band offset when the two are coupled. The h + on the Fe3O4 valence band transfers to the Ag2O valence band, and the eon the Ag2O conduction band transfers to the Fe3O4 conduction band, thus improving the separation efficiency of the photogenerated electrons and holes. These electrons and holes then participate in other reactions. The eon the Fe3O4 conduction band reacts with the dissolved oxygen and water in the liquid to form H2O2 and OH − . H2O2 can then further participate in the Fenton reaction, while the h + on the Ag2O valence band reacts with H2O to generate ·OH free radicals and H + . A neodymium magnet adsorption experiment was performed to verify whether Ag 2 O/Fe 3 O 4 (10%) can be magnetically recovered. Figure 8d shows the results. Specifically, a neodymium magnet was placed next to the Ag 2 O/Fe 3 O 4 (10%) suspension and was allowed to stand still for 20 min. It was observed that the neodymium magnet clearly adsorbed the gray-brown catalyst powder. Therefore, it was confirmed that magnetic adsorption can recover Ag 2 O/Fe 3 O 4 (10%).

Photocatalytic Reaction Mechanism Analysis
First, under visible light irradiation, Ag 2 O and Fe 3 O 4 on the surface of Ag 2 O/Fe 3 O 4 are excited from the valence band to the conduction band, generating photogenerated electrons (e − ) and leaving behind holes (h + ). As the E CB of Ag 2 O is −0.67 eV, which is more negative than that of Fe 3 O 4 , i.e., 0.54 eV, and the E VB of Ag 2 O is 2.03 eV, which is more negative than that of Fe 3 O 4 , i.e., 2.14 eV, a type-II heterojunction is formed due to the band offset when the two are coupled. generated by the combined photocatalytic and Fenton reactions can further participate in the oxidation and degradation of organic compounds, decomposing them into smaller harmless compounds, as shown in Figure 9. The specific reaction process is as follows: ·O 2 + MO → Degraded products (8) ·O 2 + MO → Degraded products (10) ·OH + MO → Degraded products (11) es 2023, 28, x FOR PEER REVIEW 13 of 17 Second, the Fenton reaction occurs on Fe3O4. Fe 2+ in Fe3O4, H2O2 generated in the photocatalytic reaction, and dissolved O2 in water react to generate Fe 3+ , ·O2 − , and ·OH, respectively. Next, H2O2 can reduce Fe 3+ to replenish the consumed Fe 2+ and O2 and generate H + , so the reaction can be cycled. The large amounts of ·OH, and ·O2 − generated by the combined photocatalytic and Fenton reactions can further participate in the oxidation and degradation of organic compounds, decomposing them into smaller harmless compounds, as shown in Figure 9. The specific reaction process is as follows: Ag O/Fe O + hv → e + h (5) h + H O →· OH + H (6) 2e + O + 2H O → H O + 2OH (7) · O + MO → Degraded products (8) H O + 2Fe → 2Fe + O + 2H (9) · O + MO → Degraded products (10) · OH + MO → Degraded products (11)

Preparation of Fe 3 O 4
Approximately 0.198 g of FeCl 3 ·6H 2 O and 0.072 g of FeCl 2 ·4H 2 O were dissolved in 200 mL of deionized water by ultrasonication for 30 min. Approximately 20 mL of NaOH solution (1 M) was then added. The mixture was sonicated for 1 h, centrifuged, washed three times with deionized water, and then freeze-dried to obtain Fe 3 O 4 .

Preparation of Ag 2 O
Approximately 0.5 g of AgNO 3 was dissolved in 200 mL of deionized water, and 20 mL of NaOH solution (1 M) was then added. The mixture was sonicated for 1 h, centrifuged, washed three times with deionized water, and then freeze-dried to obtain Ag 2 O.

Preparation of Ag 2 O/Fe 3 O 4
The Ag 2 O/Fe 3 O 4 catalyst was prepared using a one-step coprecipitation method. First, 0.5 g of AgNO 3 was dissolved ultrasonically in 200 mL of deionized water, and 0.041, 0.088, and 0.119 g of FeCl 3 ·6H 2 O were dissolved with 0.015, 0.032, and 0.044 g of FeCl 2 ·4H 2 O, respectively, in 200 mL of deionized water as different precursors of Fe 3 O 4 . The precursor of the Ag 2 O solution was then added into the Fe 3 O 4 precursor solutions and treated ultrasonically for 1 h. Next, a 1 M NaOH solution was continuously dripped into the quickly stirred precursor solution until no further color change was observed. Finally, the product was washed three times by centrifugation, freeze-dried, and obtained as Ag 2 O/Fe 3 O 4 (5%, 10%, 15%) binary photocatalysts.

Characterization
The samples were subjected to various analytical techniques to investigate their morphologies, chemical environments, structures, microstructures, surface composition, optical features, bandgap, and magnetic performance. Specifically, scanning electron microscopy (SEM, TESCAN, MIRA) equipped with an electron-dispersive spectroscopy (EDS) detector was used to observe the morphologies and chemical environments, while X-ray diffraction (XRD, MiniFlex-600, Rigaku, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL) were used to analyze the structures and microstructures, respectively. X-ray photoelectronic spectroscopy (XPS, ESCALAB-250XI, Thermo Fisher, Waltham, MA, USA) was used to study the surface composition. A photoluminescence spectroscopy (PL, Cary Eclipse, Varian, Cheadle, UK) and UV-vis spectrometer (UV, PerkinElmer (Houston, TX, USA), Lambda 950) were used to analyze the optical features and bandgap, respectively. A vibrating sample magnetometer (VSM, Lake Shore (Westerville, OH, USA), 7404) was used to evaluate the magnetic performance.

Photocatalytic Measurement
The photocatalytic performance test was conducted under a xenon lamp source (PLS-SXE300) with a power of 300 W for the degradation of MO (10 mg/L) by Ag 2 O/Fe 3 O 4 . In the experiment, 100 mg of Ag 2 O/Fe 3 O 4 was dispersed in 100 mL MO solution. After mixing, the mixture was stirred in the dark for 30 min to allow the catalyst to reach adsorption-desorption equilibrium with MO. The mixture containing the photocatalyst was then placed 10 cm away from the xenon lamp source and stirred at a speed of 200 r/min. During the light irradiation, 3 mL of the solution was taken out every 5 min and transferred to a centrifuge tube, and the catalyst powder was removed using a needle filter with a 0.22 µm pore size. A UV-visible spectrophotometer was used to measure the filtered MO concentration. The degradation rate of MO can be expressed as (C0-C)/C0, where C represents the MO concentration after xenon lamp irradiation, C0 represents the original concentration before irradiation, and the concentration of undegraded MO can be expressed as C/C0.

Photoelectrochemical Measurement
The experiment was conducted using an electrochemical analyzer (CHI660E, Shanghai) equipped with a standard three-electrode system. A 100 mL Na 2 SO 4 solution (0.1 M) was used as the electrolyte, with a platinum (Pt) foil as the counter electrode, Ag/AgCl as the reference electrode, and the loading samples on FTO glass as working electrodes.
Electrochemical impedance spectroscopy (EIS), a Mott-Schottky curve, and photocurrent response tests were performed.

Conclusions
In summary, a novel type of environmentally friendly magnetic nanocomposite, i.e., Ag 2 O/Fe 3 O 4 , has been synthesized and characterized as a high-performance visible-lightresponsive photocatalyst. According to the XRD, SEM, TEM, XPS, UV-vis, PL, and electrochemical characterization, it has been confirmed that Ag 2 O and Fe 3 O 4 are well compounded and exhibit a good synergistic effect. Loading Fe 3 O 4 onto the surface of Ag 2 O not only constructs the type II heterojunction but also successfully couples the photocatalysis and Fenton reaction, enhancing its broad-spectrum response and efficiency. Under simulated sunlight irradiation, the Ag 2 O/Fe 3 O 4 (10%) exhibited the fastest MO degradation rate, rapidly degrading 99.5% of 10 mg/L MO within 15 min, which was 2.4 times higher than that of pure Ag 2 O. Furthermore, after four cycles of testing, the sample still exhibited a fast degradation rate, indicating high stability. Magnetic testing emphasized the ease of material recovery using magnetic force, making the nanocomposite suitable for practical applications in water treatment and environmental remediation. Therefore, Ag 2 O/Fe 3 O 4 exhibits magnetic properties, wide spectral response, and high oxidative degradation performance, and its preparation method provides a new approach for the development of future photocatalysts.