Magnetic CoFe1.95Y0.05O4-Decorated Ag3PO4 as Superior and Recyclable Photocatalyst for Dye Degradation

The Ag3PO4/CoFe1.95Y0.05O4 nanocomposite with magnetic properties was simply synthesized by the hydrothermal method. The structure and morphology of the prepared material were characterized, and its photocatalytic activity for degradation of the methylene blue and rhodamine B dyes was also tested. It was revealed that the Ag3PO4 in the nanocomposite exhibited a smaller size and higher efficiency in degrading dyes than the individually synthesized Ag3PO4 when exposed to light. Furthermore, the magnetic properties of CoFe1.95Y0.05O4 enabled the nanocomposite to possess magnetic separation capabilities. The stable crystal structure and effective degradation ability of the nanocomposite were demonstrated through cyclic degradation experiments. It was shown that Ag3PO4/CoFe1.95Y0.05O4–0.2 could deliver the highest activity and stability in degrading the dyes, and 98% of the dyes could be reduced within 30 min. Additionally, the photocatalytic enhancement mechanism and cyclic degradation stability of the magnetic nanocomposites were also proposed.


Introduction
The main hazardous substances in wastewater come from chemical dyes, which can pollute the environment, destroy the ecological balance, and increase the risk of cancer in humans. Therefore, it is extremely urgent to develop efficient wastewater treatment technologies to degrade these chemical dyes. Semiconductor photocatalytic oxidation has been widely considered as one of the most promising photocatalytic technologies to achieve this goal [1][2][3][4][5]. Traditional TiO 2 -or ZnO-based materials have been studied for a long time as photocatalysts due to their exceptional merits of high electron mobility, high stability, non-toxicity, and low cost. However, both substances have relatively high band gap energies that can only be activated by UV irradiation, limiting their potential applications within the wavelength range of sunlight [6][7][8][9].
Therefore, many efforts have been made to explore new photocatalysts with narrow band gaps under certain conditions to achieve a higher light energy conversion efficiency [10][11][12][13]. One of the most promising photocatalysts is silver phosphate (Ag 3 PO 4 ), which has an extremely high photooxidation capacity, a high quantum efficiency of around 90%, and a low band gap of approximately 2.5 eV. As a result, Ag 3 PO 4 has been extensively studied in degrading organic pollutants in water due to its good photocatalytic activity [14][15][16][17]. Nevertheless, the reported Ag 3 PO 4 usually has a larger particle size and smaller specific surface area, and has a lower stability with prolonged exposure to light, leading to an unsatisfactory photocatalytic activity and stability. Furthermore, Ag 3 PO 4 is difficult to recycle, leading to a high cost of the photocatalyst as well as secondary pollution to water resources.
In order to optimize the photocatalytic properties of Ag 3 PO 4 in practical applications, it is essential to design matrix materials with a smaller particle size and long-term stability.
Typically, 2.850 g of cobalt nitrate hexahydrate (15.6 mmol), 7.429 g of iron (III) nitrate nonahydrate (30.60 mmol), 6.245 g of citrate (32.50 mmol), and 0.018 g of yttrium nitrate hexahydrate (0.05 mmol) were mixed in a 150 mL beaker, then 50 mL of deionized water was poured into the beaker and magnetically stirred at room temperature for one hour to form a transparent solution. Next, 12 mol·L −1 of ammonia was dropwise-added to the solution until reaching a pH value of 6-7. The solution was evaporated at a constant temperature of 100 • C for 24 h to obtain a dry gel. Afterward, the gel was calcinated at 700 • C for 3 h with a heating rate of 2.5 • C·min −1 . Finally, the sample was cooled to room temperature and ground for future use.

Measurement of Photocatalytic Activity
To achieve adsorption equilibrium between the photocatalyst and the dye, 0.1 g of each catalyst was added to 50 mL of 1.0 × 10 −5 mol·L −1 solution containing either methylene blue (MB) or rhodamine B (RhB), respectively. The mixture was stirred away from light for one hour. Subsequently, the dye solution was subjected to catalytic degradation evaluation under a 500 W xenon lamp. In the photocatalytic process, approximately 5.0 mL of the solution was extracted every 5 min and its absorbance was measured using a UV-vis spectrometer at the maximum absorption wavelength of MB (665 nm) and RhB (550 nm). Furthermore, the photocatalytic effect of the catalyst on various concentrations of dyes was also examined. The TOC values throughout the degradation process were determined by a Shimadzu TOC-5000A analyzer.

Structure and Morphology
The XRD patterns of each sample are displayed in Figure 1a      2a-c display the SEM images of the pure Ag3PO4, CoFe1.95Y0.05O4, and Ag3PO4/CoFe1.95Y0.05O4-0.2 nanocomposites, and it is evident that the individually synthesized Ag3PO4 has an irregular granular morphology with a large size of approximately 5-10 μm, while the Ag3PO4 in the composite exhibits a significantly smaller size around 1 μm, demonstrating that the CoFe1.95Y0.05O4 nanoparticles plays an important role in reducing the size of Ag3PO4. The nanoparticles decorated on the microparticles are CoFe1.95Y0.05O4, because the lattice spacing in the TEM image is indexed to the (311) plane of CoFe1.95Y0.05O4 (Figure 2d). It is difficult to observe the lattice planes of the microsized Ag3PO4 particles. The EDS mapping ( Figure 2e) shows that the Ag, Y, P, Fe, Co, and O elements are all distributed in Ag3PO4/CoFe1.95Y0.05O4-0.2. These findings suggest that the CoFe1.95Y0.05O4 nanoparticles can effectively regulate the formation and growth of the Ag3PO4 particles, resulting in the formation of uniformly coupled Ag3PO4/CoFe1.95Y0.05O4 composites. The smaller particle size of Ag3PO4 will result in a larger surface area, thus facilitating the utilization of the incident irradiation energy and photocatalytic reactions through effectively separating the electrons and holes [15].

Optical Properties of the Samples
It is worth noting that Ag 3 PO 4 has the main ability of absorbing light in the UV region, but with minimal absorption of incident light when the wavelength exceeds 500 nm [14]. To explore the light absorption capacity of the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites, Diffuse Reflectance Spectrum (DRS) tests are conducted and are presented in Figure 3a, and it can be seen that pure Ag 3 PO 4 has the ability to absorb light in both the UV and visible regions, with a majority of the absorption occurring in the UV region. When the wavelength of the incident light exceeds 500 nm, there is almost no absorption anymore. Comparatively, the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites exhibit a high absorption efficiency in both the UV and visible regions, particularly in the visible region, where the absorption intensity is significantly amplified.
strates that coupling with CoFe1.95Y0.05O4 significantly improves the light absorption ability of Ag3PO4, and simultaneously reduces its band gap. Furthermore, the conduction band (CB) and valence band (VB) energy levels can be given by equations of ECB = X -Ee − 0.5Eg and EVB = X -Ee + 0.5Eg, respectively, where the value of Ee is 4.5 eV and X is absolute electronegativity, which is 4.97 for Ag3PO4 and 5.83 for CoFe1.95Y0.05O4. Therefore, the CB is calculated as 0.21 and 0.73 eV, while the VB is calculated as 2.71 and 0.93 eV for Ag3PO4 and CoFe1.95Y0.05O4, respectively. To investigate the photo-separation efficiency of electron-hole pairs in the prepared samples, we conduct Photoluminescence Spectra (PL) tests on Ag3PO4/CoFe1.95Y0.05O4 nanocomposites. As shown in Figure 3c, the pure Ag3PO4 displays a prominent emission peak at 535 nm, which results from the complexation of excited state electrons in the conduction band and holes in the valence band. Comparatively, the emission intensity of the Ag3PO4/CoFe1.95Y0.05O4 nanocomposites is found to be significantly reduced. This is due to the heterojunction structure of the Ag3PO4/CoFe1.95Y0.05O4 nanocomposites, which suppresses the complexation of the electrons and holes, thereby enhancing the photocatalytic efficiency [16].

Photocatalytic Properties
The photodegradation performance of the Ag3PO4/CoFe1.95Y0.05O4 nanocomposites is analyzed and presented in Figure 4. The results show that all the samples exhibit excellent degradation behaviors for both MB and RhB (Figure 4a,b), The RhB and MB can be degraded within 40 and 30 min, respectively. Specifically, the Ag3PO4/CoFe1.95Y0.05O4 nanocomposites demonstrate a higher degradation efficiency compared to pure Ag3PO4.  To investigate the photo-separation efficiency of electron-hole pairs in the prepared samples, we conduct Photoluminescence Spectra (PL) tests on Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites. As shown in Figure 3c, the pure Ag 3 PO 4 displays a prominent emission peak at 535 nm, which results from the complexation of excited state electrons in the conduction band and holes in the valence band. Comparatively, the emission intensity of the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites is found to be significantly reduced. This is due to the heterojunction structure of the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites, which suppresses the complexation of the electrons and holes, thereby enhancing the photocatalytic efficiency [16].

Photocatalytic Properties
The photodegradation performance of the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposites is analyzed and presented in Figure 4. The results show that all the samples exhibit excellent degradation behaviors for both MB and RhB (Figure 4a,b)     are 1.81, 3.14, and 3.42 times higher than those of Ag3PO4/CoFe1.95Y0.05O4-0.3, Ag3PO4/CoFe1.95Y0.05O4-0.1, and Ag3PO4, respectively. Figure 5a,b display the TOC changes during the degradation of MB and RhB. Although the catalyst successfully decolorizes all dye solutions, the desired level of TOC removal is not achieved. This suggests that not all dye molecules can be catalyzed to inorganic molecules, and the dye concentration has an impact on the degradation efficiency. It is indicated that the Ag3PO4/CoFe1.95Y0.05O4-0.2 nanocomposite exhibits the most effective photocatalytic performance. The TOC removal rates of Ag3PO4/CoFe1.95Y0.05O4-0.2 are approximately 0.35 and 0.20 for MB and RhB, respectively, after 60 min of degradation. Figure 5c-d display the degradation efficiencies of Ag3PO4/CoFe1.95Y0.05O4-0.2 for various concentrations of MB and RhB dyes. The initial degradation during the first 5 min is slightly slower under higher dye concentrations. The main reason is that the dark color of the dye shields the light. When the dye concentrations reduce to 4 × 10 −5 mol·L − 1 , the photocatalyst is able to degrade all of the RhB dye within 30 min and completely degrade the MB dye within 50 min.

Enhancement Mechanism and Stability
As shown in Figure 6a, the Ag3PO4/CoFe1.95Y0.05O4-0.2 catalyst exhibits a saturation magnetization intensity of 6.48 A‧m 2 ‧kg −1 . As a result, the catalyst can be conveniently separated from the dye using a magnet once the degradation process is finished, which is a benefit for the recycling and reuse of the photocatalyst, effectively preventing any secondary pollution that may result from the addition of the photocatalyst [20,23]. In this study, MB is selected as the target dye for five cycles of catalytic degradation. The results of the degradation process are illustrated in Figure 6b. It can be seen that there is

Enhancement Mechanism and Stability
As shown in Figure 6a, the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 -0.2 catalyst exhibits a saturation magnetization intensity of 6.48 A·m 2 ·kg −1 . As a result, the catalyst can be conveniently separated from the dye using a magnet once the degradation process is finished, which is a benefit for the recycling and reuse of the photocatalyst, effectively preventing any secondary pollution that may result from the addition of the photocatalyst [20,23]. In this study, MB is selected as the target dye for five cycles of catalytic degradation. The results of the degradation process are illustrated in Figure 6b. It can be seen that there is almost no degradation decay of the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 -0.2 nanocomposite compared to Ag 3 PO 4 . Furthermore, the XRD patterns ( Figure 6c) and SEM images (Figure 6d) show that the phase and morphology of the photocatalyst remain unchanged after five degradation cycles. This suggests that the crystal structure of the composite is stable, and the photocatalyst exhibits both efficient photocatalytic performance and good cycling stability during the photodegradation process. rials 2023, 16, x FOR PEER REVIEW 8 of almost no degradation decay of the Ag3PO4/CoFe1.95Y0.05O4-0.2 nanocomposite co pared to Ag3PO4. Furthermore, the XRD patterns ( Figure 6c) and SEM images (Figu 6d) show that the phase and morphology of the photocatalyst remain unchanged af five degradation cycles. This suggests that the crystal structure of the composite is s ble, and the photocatalyst exhibits both efficient photocatalytic performance and go cycling stability during the photodegradation process. The Nyquist diagram (Figure 7a) demonstrates that Ag3PO4/CoFe1.95Y0.05O4-0.2 h a smaller arc radius compared to Ag3PO4 and CoFe1.95Y0.05O4, suggesting that the co pling structure of Ag3PO4/CoFe1.95Y0.05O4-0.2 is able to quickly transport and effective separate photogenerated carriers. Based on the above discussion, a photocatalyst deg dation mechanism is proposed as shown in Figure 7b; when subjected to continuo light irradiation, the electrons in Ag3PO4 and CoFe1.95Y0.05O4 become excited from t valence band to the conduction band, leading to the creation of valence band hol These holes then react with H2O, generating ‧OH radicals that degrade the dye. And t electrons migrate to the Ag3PO4 layer, where they participate in the photocatalytic re tion. This mechanism enhances the separation of photogenerated electron-hole pa and improves the photocatalytic activity of the Ag3PO4/CoFe1.95Y0.05O4 nanocomposi The conduction band potential of CoFe1.95Y0.05O4 is more negative than that of Ag3PO resulting in a continuous transfer of excited electrons to the conduction band of Ag3PO Additionally, the valence band of Ag3PO4 has a higher energy level than that CoFe1.95Y0.05O4, leading to a continuous transfer of holes from the valence band Ag3PO4 to that of CoFe1.95Y0.05O4. The transferred vacancies react with H2O to produ 2 is able to quickly transport and effectively separate photogenerated carriers. Based on the above discussion, a photocatalyst degradation mechanism is proposed as shown in Figure 7b; when subjected to continuous light irradiation, the electrons in Ag 3 PO 4

Conclusions
In this study, Ag3PO4/CoFe1.95Y0.05O4 magnetic nanocomposites are prepared a used as photocatalysts against the degradation of MB and RhB dyes. It is observed th the particle size of the Ag3PO4 in nanocomposites can be effectively reduced compared the pure Ag3PO4. Furthermore, the addition of CoFe1.95Y0.05O4 can enhance the light a sorption efficiency of Ag3PO4, resulting in a narrower forbidden band width, allowi the effective degradation of dyes within 30 min at concentrations of up to 4 × 10 −5 mol·L The degradation cycling tests show that the Ag3PO4/CoFe1.95Y0.05O4 nanocompos demonstrates superior photocatalytic stability. On the other hand, t Ag3PO4/CoFe1.95Y0.05O4 nanocomposite exhibits exceptional magnetic properties, whi enables the recycling and reuse of the photocatalyst. This study involves the develo ment of a new photocatalyst to achieve efficient, stable degradation and magnetic sep ration, providing a new way for photocatalytic degradation of organic pollutants wastewater.

Conclusions
In this study, Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 magnetic nanocomposites are prepared and used as photocatalysts against the degradation of MB and RhB dyes. It is observed that the particle size of the Ag 3 PO 4 in nanocomposites can be effectively reduced compared to the pure Ag 3 PO 4 . Furthermore, the addition of CoFe 1.95 Y 0.05 O 4 can enhance the light absorption efficiency of Ag 3 PO 4 , resulting in a narrower forbidden band width, allowing the effective degradation of dyes within 30 min at concentrations of up to 4 × 10 −5 mol·L −1 . The degradation cycling tests show that the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposite demonstrates superior photocatalytic stability. On the other hand, the Ag 3 PO 4 /CoFe 1.95 Y 0.05 O 4 nanocomposite exhibits exceptional magnetic properties, which enables the recycling and reuse of the photocatalyst. This study involves the development of a new photocatalyst to achieve efficient, stable degradation and magnetic separation, providing a new way for photocatalytic degradation of organic pollutants in wastewater.

Conflicts of Interest:
The authors declare no conflict of interest.