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Article

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

1
College of Science & Key Laboratory of Low-Dimensional Structural Physics and Application, Education Department of Guangxi Zhuang Autonomous Region, Guilin University of Technology, Guilin 541004, China
2
School of Electronics Engineering, Guangxi University of Science and Technology, Liuzhou 545006, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(10), 4155; https://doi.org/10.3390/molecules28104155
Submission received: 16 April 2023 / Revised: 5 May 2023 / Accepted: 10 May 2023 / Published: 17 May 2023
(This article belongs to the Special Issue Photocatalytic Materials and Photocatalytic Reactions)

Abstract

:
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.

1. 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 (Ag2O) 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 Ag2O limit its application in water treatment [13,14,15,16]. Furthermore, using Ag2O as a powder photocatalyst makes it challenging to recycle. Therefore, it is practically significant to develop a magnetic material to couple with Ag2O through heterojunctions to enhance the photogenerated carrier separation efficiency and fabricate an efficient and magnetically recoverable Ag2O-based photocatalyst.
Fe3O4 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, Fe3O4 nanoparticles demonstrate strong light-absorption ability, absorbing most of the light in the UV–visible range [23]. Furthermore, the Fe2+ ions in Fe3O4 can react with hydrogen peroxide (H2O2) 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:
2 F e 2 + + H 2 O 2 + O 2 2 F e 3 + + · O 2 + ( O H ) + · O H
H 2 O 2 + 2 F e 3 + 2 F e 2 + + O 2 + 2 H +
Additionally, Fe3O4 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 Fe3O4 in the Fenton reaction to oxidize organic pollutants in water has significant limitations, including the need to consume externally provided H2O2 in the reaction and the requirement of an acidic environment to generate free radicals through the Fenton reaction. However, when Fe3O4 is coupled with a photocatalyst, the H2O2 generated at the interface of the photocatalyst can be used for the Fenton reaction with Fe3O4. Another advantage of selecting Fe3O4 is that it contains both Fe2+ and Fe3+ ions, facilitating the continuous progress of the Fenton reaction. Consequently, numerous researchers have employed Fe3O4 as a cocatalyst in the photocatalyst system [31,32,33,34].
In this article, the Ag2O/Fe3O4 binary magnetic nanoparticles were synthesized using a simple chemical coprecipitation method with FeCl2·4H2O, FeCl3·6H2O, and AgNO3 as raw materials, and they were applied to degrade the organic dyes in water. The results showed that the introduction of Fe3O4 to load Ag2O 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 Ag2O/Fe3O4 nanoparticles exhibited very high efficiency in degrading dyes, such as methyl orange (MO), under visible light irradiation.

2. Results and Discussion

2.1. 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 Fe3O4 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 Fe3O4 nanoparticles exhibited obvious aggregation in the image. Figure 1b shows the TEM image of the Ag2O nanoparticles, which had a particle size distribution ranging from 30 to 80 nm and a polyhedral morphology that differed greatly from Fe3O4. The nanoparticles of the two materials could be easily distinguished. Figure 1c shows the TEM image of binary Ag2O/Fe3O4 (10%) nanoparticles, which demonstrates that Fe3O4 nanoparticles with smaller size and spherical morphology could encapsulate Ag2O nanoparticles with larger size and polyhedral morphology, indicating good compatibility between Fe3O4 and Ag2O. A high-resolution TEM (HRTEM) analysis was performed on a circular region indicated in Figure 1c to confirm the successful formation of Ag2O/Fe3O4 (10%). Figure 1d shows the results. The clear interface between Fe3O4 nanoparticles and Ag2O nanoparticles was observed with lattice spacing of 0.25 nm (corresponding to the (311) plane of/Fe3O4) and 0.29 nm (corresponding to the (220) plane of Ag2O), respectively, further demonstrating the successful formation of Ag2O/Fe3O4 (10%).

2.2. 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.

2.3. 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 26.9°, 33.0°, 38.3°, 55.1°, 65.7°, and 68.7° were indexed to the (110), (111), (200), (220), (311), and (222) crystal planes of Ag2O, respectively, which were consistent with the JCPDS card (PDF#75-1532) for Ag2O. The diffraction peaks of Fe3O4 nanoparticles at X-ray diffraction angle (2θ) of 28.26°, 34.53°, 44.01° and 61.88° were indexed to the (220), (311), (400), and (440) crystal planes of Fe3O4, respectively, which were consistent with the JCPDS card (PDF#19-0629) for Fe3O4. The XRD pattern of Ag2O/Fe3O4 (10%) nanocomposites showed the same diffraction peaks at 26.9°, 33.0°, 38.3°, 55.1°, 65.7°, and 68.7° for Ag2O and at 28.26°, 34.53°, 44.01°, and 61.88° for Fe3O4, indicating a good coupling of Ag2O and Fe3O4 and showing no change in their crystal structure. Additionally, no other impurity phases were observed, indicating that Ag2O/Fe3O4 (10%) is a two-phase composite.

2.4. 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 Fe2+, is observed at 710.58 eV (Fe 2p3/2) and 723.78 eV (Fe 2p1/2), while the second doublet, assigned to Fe3+, 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.

2.5. UV–Vis and PL Analysis

UVvis and PL tests were conducted to determine the optical properties of the synthesized nanomaterials. UVvis testing was used to measure the absorbance of the synthesized nanomaterials. Figure 5a shows the results. Ag2O exhibits strong absorption in the ultraviolet and near-ultraviolet regions, with a peak at a wavelength of 500 nm [13,14,15]. Fe3O4 exhibits a strong optical response across the whole examined spectral range, indicating that the strong visible light-absorption capability of Ag2O/Fe3O4 binary composite catalysts is undoubtedly due to the optical properties of Fe3O4 [23,31,32,33]. Furthermore, compared with Ag2O, a gradual redshift was observed at the absorption edge of Ag2O/Fe3O4 binary composite catalysts, and a significant increase in absorption was observed in the near-infrared region of 600–800 nm, indicating a strong interaction between Ag2O and Fe3O4 in the binary composite catalyst. It is worth noting that as the loading amount of Fe3O4 increases, the light absorption ability of Ag2O/Fe3O4 binary photocatalysts in the UVvisible spectral range also increases. Ag2O/Fe3O4 (15%) exhibits the best light-absorption ability, followed by Ag2O/Fe3O4 (10%) and Ag2O/Fe3O4 (5%). The Kubelka–Munk equation was used to calculate the bandgap energy of the semiconductor:
a h v 2 n = A h v E g
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. The indirect semiconductors Ag2O and Fe3O4 both have n values of 4. The Kubelka-Munk function was used to derive the absorption spectra of all the synthesized 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 to be excited to the conduction band, generating photogenerated electron–hole pairs. Due to the low optical bandgap of Fe3O4, which is only 1.6 eV, photogenerated electrons and holes are prone to recombine, producing strong light emission.

2.6. 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 (ECB) or the p-type semiconductor (EVB). Figure 6a,b shows the Mott–Schottky plots of Ag2O and Fe3O4 with Ag/AgCl as the reference electrode. It can be seen that both Ag2O and Fe3O4 have positive slopes, indicating that they are p-type semiconductors. By extrapolation, the Mott–Schottky plots of Ag2O and Fe3O4 intersect the x-axis at 1.84 eV and 1.95 eV, respectively. Considering the difference between the reference electrode (Ag/AgCl) and the standard value of 0.19 eV (relative to the normal hydrogen electrode), the Evb values of Ag2O and Fe3O4 are estimated to be 2.03 eV and 2.14 eV, respectively. Furthermore, based on the previously obtained data, the optical bandgaps of Ag2O and Fe3O4 are 2.0 eV and 1.2 eV, respectively. Therefore, the EVB of Ag2O and Fe3O4 can be calculated using the following equation:
E V B = E g + E C B
where Eg represents the optical bandgap energy. By substituting the values of Eg as 2.0 eV and EVB as 2.03 eV for Ag2O in the formula, the value of ECB is calculated as −0.03 eV. Similarly, by substituting the values of Eg as 1.2 eV and EVB as 2.14 eV for Fe3O4 in the formula, the value of ECB is calculated as 0.94 eV. The photogenerated current response analysis can be used to verify the efficiency of the photogenerated carriers in the samples. The research results show that when a small amount of Fe3O4 is loaded on the surface of Ag2O, the photocurrent intensity of the sample is significantly improved, and the photocurrent intensity of Ag2O/Fe3O4 (10%) is the highest, as shown in Figure 6c. However, when the loading amount of Fe3O4 reaches 15%, the photocurrent intensity of the formed Ag2O/Fe3O4 (15%) is lower than that of Ag2O. This indicates that too much Fe3O4 loading will reduce the utilization efficiency of photogenerated carriers. In addition, EIS measurements were also conducted to study the charge-transfer resistance and transfer efficiency of photogenerated carriers. As shown in Figure 6d, it can be observed that the Nyquist semicircle diameters of the Ag2O/Fe3O4 (5%) and Ag2O/Fe3O4 (10%) nanocomposites are smaller than those of Ag2O and Ag2O/Fe3O4 (15%). The Nyquist semicircle diameter of Ag2O/Fe3O4 (10%) is the lowest, indicating that its resistance is lower than that of Ag2O and the other samples. Therefore, loading a small amount of Fe3O4 can improve the transfer efficiency of photogenerated carriers in Ag2O, which is a favorable condition for enhancing the photocatalytic activity. However, when the loading amount of Fe3O4 reaches 15%, the Nyquist semicircle diameter of Ag2O/Fe3O4 (15%) is larger than that of Ag2O, indicating that too much Fe3O4 loading will reduce the available surface area of oxidized silver, leading to an increase in the resistance encountered by electrons and holes during transmission and a decrease in the transfer efficiency of photogenerated carriers.

2.7. Photocatalytic Performance Analysis

Fe3O4, Ag2O, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) were placed under a xenon lamp light source (λ > 420nm) to simulate visible light in sunlight and to photocatalyze a 10 mol/L MO solution to better demonstrate the visible light photocatalytic performance of different samples. Figure 7a shows the results. When pure Fe3O4 was placed in the MO solution and irradiated with visible light, no MO degradation was observed, indicating that Fe3O4 alone does not have the ability to degrade the MO solution under visible light. When pure Ag2O was placed in the MO solution and irradiated with visible light, MO was significantly degraded. Approximately 80% of the MO solution was degraded in 15 min of light irradiation, and 99.1% of the MO solution was degraded in 30 min of light irradiation, indicating that Ag2O can absorb photon energy and produce photocatalytic reactions under visible light, which is a considerable catalytic rate for the MO solution.
The photocatalytic rate of Ag2O changed significantly after Fe3O4 was loaded onto its surface. When the Fe3O4 loading amount was 5wt% of the overall weight, the Ag2O/Fe3O4 (5%) binary catalyst was formed, degrading 99.1% of the MO solution after 15 min of visible light irradiation. When the Fe3O4 loading amount was 10wt% of the overall weight, the Ag2O/Fe3O4 (10%) binary catalyst was formed, degrading 99.5% of the MO solution after 15 min of visible light irradiation. It is worth noting that when the Fe3O4 loading amount continued to increase to 15wt% of the overall weight, the Ag2O/Fe3O4 (15%) binary catalyst did not increase but decreases the degradation rate of the MO solution. Moreover, it only degraded 75.1% and 85.2% of the MO solution after 15 min and 30 min of visible light irradiation, respectively. This indicates that during the Fe3O4 loading, a coverage layer forms on the surface of Ag2O, and Fe3O4 absorb photons and produce electron–hole pairs under light irradiation, which are then be transferred from type-II heterojunction to the electrode of Ag2O to participate in the reaction. However, when there is too much Fe3O4 loading, the thicker coverage layer it forms reduces the available surface area of Ag2O, blocks the entry of photons, and shields the surface of Ag2O 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 Ag2O/Fe3O4 (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 Ag2O/Fe3O4 (10%) is the fastest, reaching 0.183 min−1, which is 2.3 times higher than that of pure Ag2O (0.078 min−1), 3 times higher than that of Ag2O/Fe3O4 (15%) (0.061 min−1), and 1.17 times higher than that of Ag2O/Fe3O4 (5%) (0.156 min−1). Four photocatalytic cycling tests were conducted to verify the structural stability of the Ag2O/Fe3O4 (10%) sample. Figure 7d shows the results. After four cycles, the catalytic rate of Ag2O/Fe3O4 (10%) slightly decreased but still exhibited a fast catalytic rate, indicating good structural stability.
In general, the reactive species in photocatalytic processes are often considered to be holes (h+), hydroxyl radicals (·OH), and superoxide ion radicals (·O2). 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 Ag2O/Fe3O4 (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 ·O2 and ·OH have a significant impact on the degradation of MO in the Ag2O/Fe3O4 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 Ag2O/Fe3O4 (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 Ag2O/Fe3O4 (10%) photocatalyst has a good applicability and a good catalytic effect on various types of organic pollutants in water.

2.8. Magnetic Properties Analysis

Vibrating sample magnetometer (VSM) measurements were performed on Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) to determine the magnetic properties of the samples. Figure 8a–c show the results. As shown in Figure 8a, the magnetic properties of the Ag2O/Fe3O4 binary nanocomposites gradually increase with the increase in Fe3O4 surface loading content. Ag2O/Fe3O4 (15%) exhibited the strongest magnetism, with a maximum saturation magnetization of 1.01 emu/g and a hysteresis loop showing a clear bent shape without a saturation region. In contrast, Ag2O/Fe3O4 (10%) and Ag2O/Fe3O4 (5%) exhibited the maximum saturation magnetization of 0.31 emu/g and 0.15 emu/g, respectively. Figure 8b,c show the hysteresis loop characteristics of Ag2O/Fe3O4 (5%) and Ag2O/Fe3O4 (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 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%).

2.9. 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 e- on 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 e- on 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+.
Second, the Fenton reaction occurs on Fe3O4. Fe2+ in Fe3O4, H2O2 generated in the photocatalytic reaction, and dissolved O2 in water react to generate Fe3+, ·O2, and ·OH, respectively. Next, H2O2 can reduce Fe3+ to replenish the consumed Fe2+ 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:
A g 2 O / F e 3 O 4 + h v e + h +
h + + H 2 O · O H + H +
2 e + O 2 + 2 H 2 O H 2 O 2 + 2 O H
· O 2 + M O D e g r a d e d   p r o d u c t s
H 2 O 2 + 2 F e 3 + 2 F e 2 + + O 2 + 2 H +
· O 2 + M O D e g r a d e d   p r o d u c t s
· O H + M O D e g r a d e d   p r o d u c t s

3. Materials and Methods

3.1. Material

Silver nitrate (AgNO3, 99%) was purchased from National Pharmaceutical Group Co., Ltd. (Shanghai, China). Iron(II) chloride tetrahydrate (FeCl2·4H2O, ≥92.0%), Iron(III) chloride hexahydrate (FeCl3·6H2O, ≥98.1%), sodium hydroxide (NaOH, ≥96.0%), and methyl orange (MO) were purchased from Xilong Science Co., Ltd. (Guangdong, China). All the raw materials were of analytical grade and used without any additional purification. Deionized water was used for all the experiments.

3.2. Preparation of Fe3O4

Approximately 0.198 g of FeCl3·6H2O and 0.072 g of FeCl2·4H2O 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 Fe3O4.

3.3. Preparation of Ag2O

Approximately 0.5 g of AgNO3 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 Ag2O.

3.4. Preparation of Ag2O/Fe3O4

The Ag2O/Fe3O4 catalyst was prepared using a one-step coprecipitation method. First, 0.5 g of AgNO3 was dissolved ultrasonically in 200 mL of deionized water, and 0.041, 0.088, and 0.119 g of FeCl3·6H2O were dissolved with 0.015, 0.032, and 0.044 g of FeCl2·4H2O, respectively, in 200 mL of deionized water as different precursors of Fe3O4. The precursor of the Ag2O solution was then added into the Fe3O4 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 Ag2O/Fe3O4 (5%, 10%, 15%) binary photocatalysts.

3.5. 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.

3.6. 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 Ag2O/Fe3O4. In the experiment, 100 mg of Ag2O/Fe3O4 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.

3.7. Photoelectrochemical Measurement

The experiment was conducted using an electrochemical analyzer (CHI660E, Shanghai) equipped with a standard three-electrode system. A 100 mL Na2SO4 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.

4. Conclusions

In summary, a novel type of environmentally friendly magnetic nanocomposite, i.e., Ag2O/Fe3O4, has been synthesized and characterized as a high-performance visible-light-responsive photocatalyst. According to the XRD, SEM, TEM, XPS, UV–vis, PL, and electrochemical characterization, it has been confirmed that Ag2O and Fe3O4 are well compounded and exhibit a good synergistic effect. Loading Fe3O4 onto the surface of Ag2O 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 Ag2O/Fe3O4 (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 Ag2O. 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, Ag2O/Fe3O4 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.

Author Contributions

Conceptualization, J.W., G.H. and T.T.; Methodology, Z.S. and Z.L.; Software, R.X.; Formal analysis, Z.F.; Investigation, L.J.; Writing—original draft, C.S.; Supervision, M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (12164013), the Natural Science Foundation of Guangxi Province (2020GXNSFBA297125), the Science and Technology Base and Talent Special Project of Guangxi Province (AD21220029), Research Foundation of Guilin University of Technology (GUTQDJJ2019011), and innovation Project of Guangxi Graduate Education (YCSW2022331).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data can be made available upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

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Figure 1. (a) TEM image of Fe3O4 nanoparticles; (b) TEM image of Ag2O nanoparticles; (c) TEM image of Ag2O/Fe3O4 (10%); (d) HRTEM image of Ag2O/Fe3O4 (10%).
Figure 1. (a) TEM image of Fe3O4 nanoparticles; (b) TEM image of Ag2O nanoparticles; (c) TEM image of Ag2O/Fe3O4 (10%); (d) HRTEM image of Ag2O/Fe3O4 (10%).
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Figure 2. SEM images and element distributions of Ag2O/Fe3O4 (10%) nanoparticles, including (a) SEM image, (b) element distribution, and (ce) O, Fe, and Ag element distributions.
Figure 2. SEM images and element distributions of Ag2O/Fe3O4 (10%) nanoparticles, including (a) SEM image, (b) element distribution, and (ce) O, Fe, and Ag element distributions.
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Figure 3. XRD characterization patterns of Ag2O/Fe3O4 (10%) nanoparticles, Ag2O nanoparticles, and Fe3O4 nanoparticles.
Figure 3. XRD characterization patterns of Ag2O/Fe3O4 (10%) nanoparticles, Ag2O nanoparticles, and Fe3O4 nanoparticles.
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Figure 4. XPS spectra of Ag2O/Fe3O4 (10%), including (a) full-spectrum, (b) Ag3d high-resolution spectrum, (c) Fe2p high-resolution spectrum, and (d) O1s high-resolution spectrum.
Figure 4. XPS spectra of Ag2O/Fe3O4 (10%), including (a) full-spectrum, (b) Ag3d high-resolution spectrum, (c) Fe2p high-resolution spectrum, and (d) O1s high-resolution spectrum.
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Figure 5. Ag2O, Fe3O4, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) of (a) UV–vis spectra and (b) Tauc plots. (c) PL spectra of Ag2O, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%).
Figure 5. Ag2O, Fe3O4, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%) of (a) UV–vis spectra and (b) Tauc plots. (c) PL spectra of Ag2O, Ag2O/Fe3O4 (5%), Ag2O/Fe3O4 (10%), and Ag2O/Fe3O4 (15%).
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Figure 6. Mott–Schottky curves of (a) Ag2O and (b) Fe3O4; (c) sample photocurrent response profiles; (d) sample EIS test graph.
Figure 6. Mott–Schottky curves of (a) Ag2O and (b) Fe3O4; (c) sample photocurrent response profiles; (d) sample EIS test graph.
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Figure 7. (a) Degradation rate of the MO solution by different photocatalyst samples; (b) UV–vis absorption spectra of the MO solution degraded by Ag2O/Fe3O4 (10%); (c) pseudo-first-order kinetic model diagram of the MO solution degradation; (d) cycle test of the Ag2O/Fe3O4 (10%) sample degradation; (e) kinetics rate of the MO solution degradation using Ag2O/Fe3O4 (10%) with different scavengers; (f) kinetics of different water pollutants degradation using Ag2O/Fe3O4 (10%).
Figure 7. (a) Degradation rate of the MO solution by different photocatalyst samples; (b) UV–vis absorption spectra of the MO solution degraded by Ag2O/Fe3O4 (10%); (c) pseudo-first-order kinetic model diagram of the MO solution degradation; (d) cycle test of the Ag2O/Fe3O4 (10%) sample degradation; (e) kinetics rate of the MO solution degradation using Ag2O/Fe3O4 (10%) with different scavengers; (f) kinetics of different water pollutants degradation using Ag2O/Fe3O4 (10%).
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Figure 8. VSM curves of the (a) overall magnetic samples, (b) Ag2O/Fe3O4 (5%) composite, (c) Ag2O/Fe3O4 (10%) composite, and (d) test of Ag2O/Fe3O4 (10%) composite under Nd magnet adsorption.
Figure 8. VSM curves of the (a) overall magnetic samples, (b) Ag2O/Fe3O4 (5%) composite, (c) Ag2O/Fe3O4 (10%) composite, and (d) test of Ag2O/Fe3O4 (10%) composite under Nd magnet adsorption.
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Figure 9. Schematic of the possible photocatalytic reaction mechanism of Ag2O/Fe3O4 under visible light irradiation.
Figure 9. Schematic of the possible photocatalytic reaction mechanism of Ag2O/Fe3O4 under visible light irradiation.
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Table 1. EDS data statistics of Ag2O/Fe3O4 (10%) nanoparticles.
Table 1. EDS data statistics of Ag2O/Fe3O4 (10%) nanoparticles.
ElementLine Typewt%wt% Sigmaat%
OK series21.330.4663.01
FeK series6.170.235.22
AgL series72.490.4731.76
Total 100.00 100.00
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Shan, C.; Su, Z.; Liu, Z.; Xu, R.; Wen, J.; Hu, G.; Tang, T.; Fang, Z.; Jiang, L.; Li, M. One-Step Synthesis of Ag2O/Fe3O4 Magnetic Photocatalyst for Efficient Organic Pollutant Removal via Wide-Spectral-Response Photocatalysis–Fenton Coupling. Molecules 2023, 28, 4155. https://doi.org/10.3390/molecules28104155

AMA Style

Shan C, Su Z, Liu Z, Xu R, Wen J, Hu G, Tang T, Fang Z, Jiang L, Li M. One-Step Synthesis of Ag2O/Fe3O4 Magnetic Photocatalyst for Efficient Organic Pollutant Removal via Wide-Spectral-Response Photocatalysis–Fenton Coupling. Molecules. 2023; 28(10):4155. https://doi.org/10.3390/molecules28104155

Chicago/Turabian Style

Shan, Chuanfu, Ziqian Su, Ziyi Liu, Ruizheng Xu, Jianfeng Wen, Guanghui Hu, Tao Tang, Zhijie Fang, Li Jiang, and Ming Li. 2023. "One-Step Synthesis of Ag2O/Fe3O4 Magnetic Photocatalyst for Efficient Organic Pollutant Removal via Wide-Spectral-Response Photocatalysis–Fenton Coupling" Molecules 28, no. 10: 4155. https://doi.org/10.3390/molecules28104155

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