Synergistic Ag/g–C3N4 H2O2 System for Photocatalytic Degradation of Azo Dyes

Graphitic carbon nitride (g-C3N4), known for being nontoxic, highly stable, and environmentally friendly, is extensively used in photocatalytic degradation technologies. Silver nanoparticles effectively capture the photogenerated electrons in g-C3N4, enhancing the photocatalytic efficiency. This study primarily focused on synthesizing graphitic carbon nitride via thermal polymerization and depositing noble metal silver onto g-C3N4 through photoreduction. Methyl orange (MO) and methylene blue (MB) were targeted as the pollutants in the photocatalytic experiments under visible light in conjunction with a H2O2 system. The characteristics peaks, structure, and morphology were analyzed using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). g-C3N4 loaded with 6% Ag exhibited superior photocatalytic performance; the photocatalytic fraction of the degraded materials of the MO and MB solutions reached 100% within 70 and 80 min, respectively, upon adding 1 mL and 2 mL of H2O2. ·OH and ·O2− were the primary active free radicals in the dye degradation process within the synergistic system. Stability tests also demonstrated that the photocatalyst maintained good reusability under the synergistic system.


Introduction
In recent years, rapid industrialization and population growth have led to an everincreasing exploitation of natural resources, causing environmental pollution to become a progressively escalating global challenge [1][2][3], where water pollution is particularly serious [4][5][6].Industrial wastewater contains various toxic substances, among which dyes are particularly hazardous toxic wastes.They are characterized by high acidity, intense coloration, and strong toxicity, potentially causing severe harm to water resources and human health [7].Consequently, it has become a research focus for scholars to render wastewater containing azo dyes harmless, removing the dye components for discharge into natural water bodies or for reuse.
Photocatalysis, as an advanced oxidation technology, utilizes the radicals generated during the photoreaction process to oxidize pollutants into harmless small molecules such as H 2 O and CO 2 [8,9].Researchers like Kun [10] have used photocatalytic technology to degrade typical atmospheric pollutants, with the fraction of removed materials exceeding 80%.Zeng et al. [11] applied photocatalytic techniques to treat low-concentration ammonia nitrogen wastewater, achieving a fraction of removed materials of above 90%.In 1967, Akira Fujishima and his mentor Kenichi Honda discovered that light irradiation pf titanium dioxide (TiO 2 ) could split water into hydrogen and oxygen, but the efficiency was not high, so the discovery was temporarily shelved.It was not until 1972 that Fujishima Akira and Honda discovered that ultraviolet light irradiation of a single-crystal titanium dioxide semiconductor electrode could facilitate the photolytic reaction of water, producing hydrogen and oxygen, thus sparking a surge of research into TiO 2 photocatalytic materials for degrading pollutants.In 2009, graphitic carbon nitride (g-C 3 N 4 ), an organic conjugated semiconductor material, was first used as a novel photocatalyst for the production of hydrogen from water [12].Recently, g-C 3 N 4 , which has a two-dimensional layered structure, narrow bandgap (about 2.7 eV), high chemical stability, nontoxicity, and low production costs, has been widely applied in photocatalytic technology [13,14].Researchers like Saeed [15] utilized photocatalysis as an effective tool for dye degradation, while Zhou et al. [16] produced photocatalytic concrete based on g-C 3 N 4 , degrading 80% of the methylene blue within 30 min; Pandey et al. [17] achieved 96% and 93% removal of P-nitrophenol and rhodamine dyes, respectively.Li et al. [18], using simulated sunlight, prepared a CeTiO 4 /g-C 3 N 4 composite material that degraded 95.7% of rhodamine B within 140 min.
Ahad et al. [19] prepared zinc oxide materials that, in conjunction with a H 2 O 2 system under UV light, decomposed methylene blue solution.After 120 min of irradiation, the photocatalytic efficiency of the multipod and tetrapod ZnO nanostructures increased by approximately 97% and 94%, respectively.Carmen et al. [20] enhanced the photocatalytic performance of SmFe 0.7 Co 0.3 O 3 thin films through Sr doping and synergistic action with H 2 O 2 , achieving a 100% fraction of degraded material within 120 min.Gong et al. [21] improved the photocatalytic degradation of carbamazepine using FeS2/Fe2O3/organic acids with in situ generated H 2 O 2 .Ryma et al. [22] used fluid dynamics cavitation and TiO 2 -coated glass fibers for the photocatalytic degradation of methyl orange in conjunction with H 2 O 2 .Pan et al. [23] prepared MIL-Fe(53)/modified g-C 3 N 4 photocatalysts with H 2 O 2 for the degradation of tetracycline, finding that the 3% Fe-MOF/CM-H 2 O 2 system could degrade 100% of the tetracycline (10 ppm) within 60 min, 3.6 times the fraction degraded using pure g-C 3 N 4 .Wang et al. [24] conducted the photocatalytic degradation of organic dyes and plant hormones with a Cu(II) composite powder catalyst in conjunction with H 2 O 2 , achieving a high fraction of degraded material of 84.9% for MB under UV light after 90 min, while the degradation efficiency of indoleacetic acid (IAA) exceeded 60.4% after 4 h of irradiation.Yang et al. [25,26] have used graphitic carbon nitride in conjunction with H 2 O 2 for the photocatalytic degradation of methyl orange and rhodamine B, enhancing the fraction of degraded materials by 2.5 and 3.5 times, respectively, achieving excellent results.
This study synthesized g-C 3 N 4 through thermal polymerization using urea, followed by the preparation of Ag/g-C 3 N 4 composite material using AgNO 3 and g-C 3 N 4 as precursors through an in situ photoreduction method.Further studies on the morphology, structure, and optical absorption properties of the Ag/g-C 3 N 4 composite photocatalyst were conducted using XRD, FT-IR, SEM, PL, UV-Vis/DRS, XPS, and other characterization methods.The effects of light source, H 2 O 2 dosage, initial concentration of target pollutants, dosage of composite material, and solution pH on the photocatalytic degradation of the MO and MB solutions were examined.The stability and reusability of the composite material were tested, and the mechanism of photocatalysis was discussed.

XRD Analysis
As shown in Figure 1, XRD was used to determine the crystal structures of CN (g-C 3 N 4 ) and Ag-X/CN.The characteristic peaks for sample CN at 13.10 • and 27.55 • , corresponding to the (100) and (002) crystal planes, respectively, can be observed in the diagram.The first diffraction peak is due to the stacking of triazine structure layers within the plane, while the second peak results from the stacking of layers in the conjugated aromatic system, which is a distinctive feature of the graphite structure, indicating the successful preparation of CN.The XRD spectrum of Ag-X/CN is similar to that of CN, suggesting that the loading of Ag did not alter the original growth environment or the existing structure of CN.
suggesting that the loading of Ag did not alter the original growth environmen existing structure of CN.

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis
Figure 2 presents the FT-IR spectra of CN and Ag-X/CN, showing the functio face groups of the materials prepared with different composite ratios.The FT-IR of the composites containing varying amounts of Ag are identical to that of pure dicating the successful loading of Ag on CN.The absorption peak at 817 cm −1 is att to the deformation vibration of the N-C=N bond in the triazine ring.Additiona absorption peaks between 1235 and 1660 cm −1 are due to the stretching vibrations bonds, while the broad peak ranging from 3000 to 3500 cm −1 is associated with the ing vibrations of -OH groups on the surface of CN [27].

Scanning Electron Microscopy (SEM)
Figure 3 shows the SEM images of CN at different magnifications, revealing it ular porous tubular morphology, which resembles the fluffy porous structure o This morphology increases the specific surface area of CN, providing more active s degrading target pollutants and thus potentially enhancing the photocatalytic fra degraded material [28].

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis
Figure 2 presents the FT-IR spectra of CN and Ag-X/CN, showing the functional surface groups of the materials prepared with different composite ratios.The FT-IR spectra of the composites containing varying amounts of Ag are identical to that of pure CN, indicating the successful loading of Ag on CN.The absorption peak at 817 cm −1 is attributed to the deformation vibration of the N-C=N bond in the triazine ring.Additionally, the absorption peaks between 1235 and 1660 cm −1 are due to the stretching vibrations of C-N bonds, while the broad peak ranging from 3000 to 3500 cm −1 is associated with the stretching vibrations of -OH groups on the surface of CN [27].
suggesting that the loading of Ag did not alter the original growth environmen existing structure of CN.

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis
Figure 2 presents the FT-IR spectra of CN and Ag-X/CN, showing the functio face groups of the materials prepared with different composite ratios.The FT-IR of the composites containing varying amounts of Ag are identical to that of pure dicating the successful loading of Ag on CN.The absorption peak at 817 cm −1 is att to the deformation vibration of the N-C=N bond in the triazine ring.Additiona absorption peaks between 1235 and 1660 cm −1 are due to the stretching vibrations bonds, while the broad peak ranging from 3000 to 3500 cm −1 is associated with the ing vibrations of -OH groups on the surface of CN [27].

Scanning Electron Microscopy (SEM)
Figure 3 shows the SEM images of CN at different magnifications, revealing it ular porous tubular morphology, which resembles the fluffy porous structure o This morphology increases the specific surface area of CN, providing more active s degrading target pollutants and thus potentially enhancing the photocatalytic fra degraded material [28].

Scanning Electron Microscopy (SEM)
Figure 3 shows the SEM images of CN at different magnifications, revealing its irregular porous tubular morphology, which resembles the fluffy porous structure of coral.This morphology increases the specific surface area of CN, providing more active sites for degrading target pollutants and thus potentially enhancing the photocatalytic fraction of degraded material [28].The optical absorption properties are also a factor affecting the photocatalytic activity of photocatalysts.Therefore, the ultraviolet-visible diffuse reflectance spectra of CN and Ag-6/CN were plotted, as shown in Figure 5a.In this figure, the maximum absorption wavelengths for CN and Ag-6/CN are 448 nm and 457 nm, respectively.Compared to CN, Ag-6/CN exhibits a redshift, indicating that CN has weaker absorption capability for visible light.The loading of 6% Ag on CN broadened the visible light absorption range, enhancing its capacity to absorb visible light and, consequently, improving the photocatalytic performance.The bandgap widths obtained through the Tauc plot method are shown in Figure 5b.The bandgap widths for CN and Ag-6/CN are 2.78 eV and 2.73 eV, respectively.Compared to that of CN, the bandgap width of Ag-6/CN is reduced, facilitating the transfer of photogenerated electrons, which is beneficial for enhancing photocatalytic performance.The optical absorption properties are also a factor affecting the photocatalytic activity of photocatalysts.Therefore, the ultraviolet-visible diffuse reflectance spectra of CN and Ag-6/CN were plotted, as shown in Figure 5a.In this figure, the maximum absorption wavelengths for CN and Ag-6/CN are 448 nm and 457 nm, respectively.Compared to CN, Ag-6/CN exhibits a redshift, indicating that CN has weaker absorption capability for visible light.The loading of 6% Ag on CN broadened the visible light absorption range, enhancing its capacity to absorb visible light and, consequently, improving the photocatalytic performance.The bandgap widths obtained through the Tauc plot method are shown in Figure 5b.The bandgap widths for CN and Ag-6/CN are 2.78 eV and 2.73 eV, respectively.Compared to that of CN, the bandgap width of Ag-6/CN is reduced, facilitating the transfer of photogenerated electrons, which is beneficial for enhancing photocatalytic performance.The optical absorption properties are also a factor affecting the photocatalytic activity of photocatalysts.Therefore, the ultraviolet-visible diffuse reflectance spectra of CN and Ag-6/CN were plotted, as shown in Figure 5a.In this figure, the maximum absorption wavelengths for CN and Ag-6/CN are 448 nm and 457 nm, respectively.Compared to CN, Ag-6/CN exhibits a redshift, indicating that CN has weaker absorption capability for visible light.The loading of 6% Ag on CN broadened the visible light absorption range, enhancing its capacity to absorb visible light and, consequently, improving the photocatalytic performance.The bandgap widths obtained through the Tauc plot method are shown in Figure 5b.The bandgap widths for CN and Ag-6/CN are 2.78 eV and 2.73 eV, respectively.Compared to that of CN, the bandgap width of Ag-6/CN is reduced, facilitating the transfer of photogenerated electrons, which is beneficial for enhancing photocatalytic performance.

Photoluminescence (PL) Spectroscopy Analysis
Photoluminescence spectroscopy is used to characterize the recombination fraction of photogenerated electron-hole pairs.Lower fluorescence intensity is indicative of more effective separation of photogenerated electrons and holes.To explore the fraction of recombined electron-hole pairs, PL analysis was performed on CN and Ag-6/CN, and the PL spectra were obtained, as shown in Figure 6.Under excitation at 321 nm, the samples exhibited a strong emission peak at approximately 465 nm.The fluorescence intensity of Ag-6/CN was significantly lower than that of CN, indicating that the loading of Ag on the composite material notably suppressed the recombination of photogenerated electronhole pairs, which was beneficial for reducing the fraction of the recombination carrier.

Effect of Ag-X/CN Composite Ratio on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the laboratory's visible light source was a 300 W xenon lamp, the dark adsorption time was 30 min, the light irradiation time was 2 h, the dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-X/CN, the concentration of the target degradant was 20 mg/L (100 mL), and the pH was 7.Among them, the loading ratio of Ag in the catalyst was an experimental variable, which was CN, Ag-2/CN, Ag-4/CN, Ag-6/CN, and Ag-8/CN, respectively.Notably, the dark reaction involved the adsorption of a part of the dye, and each independent experiment was repeated three times.

Photoluminescence (PL) Spectroscopy Analysis
Photoluminescence spectroscopy is used to characterize the recombination fraction of photogenerated electron-hole pairs.Lower fluorescence intensity is indicative of more effective separation of photogenerated electrons and holes.To explore the fraction of recombined electron-hole pairs, PL analysis was performed on CN and Ag-6/CN, and the PL spectra were obtained, as shown in Figure 6.Under excitation at 321 nm, the samples exhibited a strong emission peak at approximately 465 nm.The fluorescence intensity of Ag-6/CN was significantly lower than that of CN, indicating that the loading of Ag on the composite material notably suppressed the recombination of photogenerated electron-hole pairs, which was beneficial for reducing the fraction of the recombination carrier.

Photoluminescence (PL) Spectroscopy Analysis
Photoluminescence spectroscopy is used to characterize the recombination fraction of photogenerated electron-hole pairs.Lower fluorescence intensity is indicative of more effective separation of photogenerated electrons and holes.To explore the fraction of recombined electron-hole pairs, PL analysis was performed on CN and Ag-6/CN, and the PL spectra were obtained, as shown in Figure 6.Under excitation at 321 nm, the samples exhibited a strong emission peak at approximately 465 nm.The fluorescence intensity of Ag-6/CN was significantly lower than that of CN, indicating that the loading of Ag on the composite material notably suppressed the recombination of photogenerated electronhole pairs, which was beneficial for reducing the fraction of the recombination carrier.

Effect of Ag-X/CN Composite Ratio on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the laboratory's visible light source was a 300 W xenon lamp, the dark adsorption time was 30 min, the light irradiation time was 2 h, the dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-X/CN, the concentration of the target degradant was 20 mg/L (100 mL), and the pH was 7.Among them, the loading ratio of Ag in the catalyst was an experimental variable, which was CN, Ag-2/CN, Ag-4/CN, Ag-6/CN, and Ag-8/CN, respectively.Notably, the dark reaction involved the adsorption of a part of the dye, and each independent experiment was repeated three times.

Study on Photocatalytic Effects of Composite Materials 2.2.1. Effect of Ag-X/CN Composite Ratio on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the laboratory's visible light source was a 300 W xenon lamp, the dark adsorption time was 30 min, the light irradiation time was 2 h, the dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-X/CN, the concentration of the target degradant was 20 mg/L (100 mL), and the pH was 7.Among them, the loading ratio of Ag in the catalyst was an experimental variable, which was CN, Ag-2/CN, Ag-4/CN, Ag-6/CN, and Ag-8/CN, respectively.Notably, the dark reaction involved the adsorption of a part of the dye, and each independent experiment was repeated three times.
As shown in Figure 7a, the MO degradation effect by the Ag-X/CN composites at different ratios increased in the following order: CN, Ag-2/CN, Ag-4/CN, Ag-6/CN, and Ag-8/CN, with the fraction of degraded materials increasing correspondingly from 31.6%, 49.8%, 52.3%, 57.6%, to 58.0%.However, the fraction of the degraded materials of Ag-6/CN, and Ag-8/CN differed by only 0.4%, and the content of loaded Ag differed by 2%.Since Ag belongs to the precious metals, Ag-6/CN was thus selected as the optimal composite material for degrading MO.
different ratios increased in the following order: CN, Ag-2/CN, Ag-4/CN, Ag-6/CN, and Ag-8/CN, with the fraction of degraded materials increasing correspondingly from 31.6%, 49.8%, 52.3%, 57.6%, to 58.0%.However, the fraction of the degraded materials of Ag-6/CN, and Ag-8/CN differed by only 0.4%, and the content of loaded Ag differed by 2%.Since Ag belongs to the precious metals, Ag-6/CN was thus selected as the optimal composite material for degrading MO.
As shown in Figure 7c, the MB degradation effect by the Ag-X/CN composites at different ratios increased in the following order: CN, Ag-2/CN, Ag-4/CN, Ag-8/CN, and Ag-6/CN, with fraction of degraded materials increasing from 65.03%, 71.7%, 80.3%, 81.7%, to 81.8%.Therefore, Ag-6/CN was chosen as the best composite material for degrading MB.It was evident that both excessively low and high amounts of Ag loading reduced the photocatalyst's degradation effect.Too low Ag loading may not support much CN, while too high Ag loading could cover some active sites on the CN surface.An appropriate amount of Ag loading can improve the separation efficiency of electrons and holes [29-31].As shown in Figure 7b,d and Table 1, pseudo-first-order reaction kinetics were used to fit the MO and MB degradation processes by different Ag-X/CN composites.The fitting effect and fraction of MB degraded material were better compared to those MO, possibly due to the influence of the adsorption during the reaction process.According to the k values and R 2 listed in Table 1, Ag-6/CN had the highest reaction rate constants during the photocatalytic degradation of MO and MB, which were 0.44 × 10 −2 cm −1 and 1.14 × 10 −2 As shown in Figure 7c, the MB degradation effect by the Ag-X/CN composites at different ratios increased in the following order: CN, Ag-2/CN, Ag-4/CN, Ag-8/CN, and Ag-6/CN, with fraction of degraded materials increasing from 65.03%, 71.7%, 80.3%, 81.7%, to 81.8%.Therefore, Ag-6/CN was chosen as the best composite material for degrading MB.It was evident that both excessively low and high amounts of Ag loading reduced the photocatalyst's degradation effect.Too low Ag loading may not support much CN, while too high Ag loading could cover some active sites on the CN surface.An appropriate amount of Ag loading can improve the separation efficiency of electrons and holes [29][30][31].
As shown in Figure 7b,d and Table 1, pseudo-first-order reaction kinetics were used to fit the MO and MB degradation processes by different Ag-X/CN composites.The fitting effect and fraction of MB degraded material were better compared to those MO, possibly due to the influence of the adsorption during the reaction process.According to the k values and R 2 listed in Table 1, Ag-6/CN had the highest reaction rate constants during the photocatalytic degradation of MO and MB, which were 0.44 × 10 −2 cm −1 and 1.14 × 10 −2 cm −1 , respectively.From Figure 7a,b, it can be observed that Ag-6/CN was more effective in degrading MB than MO.The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the dosage of photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the synergistic system was H 2 O 2 , the target degradation product concentration was 20 mg/L (100 mL), and the pH was 7.
As shown in Figure 8a, under illumination conditions, the fraction of the Ag-6/CN degraded material of the MO solution was 57.6%.With the addition of H 2 O 2 , the fraction of the degraded material rose to 100%.The introduction of H 2 O 2 played a significant role in the photocatalytic degradation of the MO solution, facilitating redox reactions between the MO solution and active free radicals, resulting in the degradation of dye wastewater.In the dark reactions, both the Ag-6/CN and H 2 O 2 exhibited good adsorption properties for the MO solution, but almost no degradation occurred after reaching adsorption-desorption equilibrium.In the other systems, the MO solution was almost undegraded.
Molecules 2024, 29, x FOR PEER REVIEW 7 of 18 cm −1 , respectively.From Figure 7a,b, it can be observed that Ag-6/CN was more effective in degrading MB than MO.The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the dosage of photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the synergistic system was H2O2, the target degradation product concentration was 20 mg/L (100 mL), and the pH was 7.
As shown in Figure 8a, under illumination conditions, the fraction of the Ag-6/CN degraded material of the MO solution was 57.6%.With the addition of H2O2, the fraction of the degraded material rose to 100%.The introduction of H2O2 played a significant role in the photocatalytic degradation of the MO solution, facilitating redox reactions between the MO solution and active free radicals, resulting in the degradation of dye wastewater.In the dark reactions, both the Ag-6/CN and H2O2 exhibited good adsorption properties for the MO solution, but almost no degradation occurred after reaching adsorption-desorption equilibrium.In the other systems, the MO solution was almost undegraded.
As depicted in Figure 8b, with Ag-6/CN and visible light, the fraction of the material of the MB solution degraded by Ag-6/CN after 70 min of illumination was 81.80%.After the dark reaction and the addition of H2O2, the fraction of material degraded in the MB solution increased to 99.7%.In the absence of a catalyst, with only H2O2 and light, the MB solution degraded to some extent, with a degraded material fraction of 56.9%.This may have been due to the production of •OH under visible light irradiation by H2O2, which further promoted the degradation of the MB solution.As depicted in Figure 8b, with Ag-6/CN and visible light, the fraction of the material of the MB solution degraded by Ag-6/CN after 70 min of illumination was 81.80%.After the dark reaction and the addition of H 2 O 2 , the fraction of material degraded in the MB solution increased to 99.7%.In the absence of a catalyst, with only H 2 O 2 and light, the MB solution degraded to some extent, with a degraded material fraction of 56.9%.This may have been due to the production of •OH under visible light irradiation by H 2 O 2 , which further promoted the degradation of the MB solution.

The Impact of H 2 O 2 Dosage on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, and the dark adsorption time was 30 min.After the dark reaction, different volumes of H 2 O 2 were added.The open light irradiation times of the MO and MB solutions were 70 min and 80 min, respectively.The dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the target pollutant concentration was 20 mg/L (100 mL), and the pH was 7.
As shown in Figure 9a, with the addition of 0 mL, 0.05 mL, 0.1 mL, 0.5 mL, and 1 mL of H 2 O 2 , the fractions of degraded materials for the MO solution were 49.5%, 79.4%, 85.3%, 99.4%, and 100%, respectively.It was evident that the fraction of the degraded MO material increased progressively with the increase in H 2 O 2 volume.Particularly, 1 mL of H 2 O 2 enabled the complete degradation of the MO solution within 70 min of illumination, representing an increase of 50.5% in the fraction of degraded material compared to the scenario without H 2 O 2 .
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, and the dark adsorption time was 30 min.After the dark reaction, different volumes of H2O2 were added.The open light irradiation times of the MO and MB solutions were 70 min and 80 min, respectively.The dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the target pollutant concentration was 20 mg/L (100 mL), and the pH was 7.
As shown in Figure 9a, with the addition of 0 mL, 0.05 mL, 0.1 mL, 0.5 mL, and 1 mL of H2O2, the fractions of degraded materials for the MO solution were 49.5%, 79.4%, 85.3%, 99.4%, and 100%, respectively.It was evident that the fraction of the degraded MO material increased progressively with the increase in H2O2 volume.Particularly, 1 mL of H2O2 enabled the complete degradation of the MO solution within 70 min of illumination, representing an increase of 50.5% in the fraction of degraded material compared to the scenario without H2O2.
As depicted in Figure 9b, with the addition of 0 mL, 0.05 mL, 0.1 mL, 0.5 mL, 1 mL, and 2 mL of H2O2, the fraction of degraded materials in the MB solution were 68.9%, 74.6%, 81.1%, 84.9%, 95.4%, and 99.7%, respectively.Clearly, the fraction of degraded MB material also gradually increased with the increase in H2O2 volume.Notably, compared to the condition without H2O2, 2 mL of H2O2 under 80 min of illumination enhanced the fraction of degraded MB material by 30.7%.Compared to the conditions for the MO solution, the degradation process for the MB solution involved an additional 1 mL of H2O2 and 10 more minutes of light exposure, yet the fraction of the degraded material was 0.3% lower, indicating that the synergistic H2O2 system had a more effective degradation impact on the MO solution.

Impact of Initial Solution Concentration on the Photocatalysis by Composite Material
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, 1 mL and 2 mL H2O2 were added to the MO and MB solutions after the dark reaction, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the volume of the target degradation products was 100 mL, and the pH was 7.Among them, different initial concentrations of the MO and MB solutions were used as experimental variables, which were 20 mg/L, 25 mg/L, 30 mg/L, 35 mg/L, and 40 mg/L.
As indicated in Figure 10, the fraction of degraded materials for the MO and MB solutions ranged from 86.2% to 100% and 62.7% to 99.7%, respectively, from low to high As depicted in Figure 9b, with the addition of 0 mL, 0.05 mL, 0.1 mL, 0.5 mL, 1 mL, and 2 mL of H 2 O 2 , the fraction of degraded materials in the MB solution were 68.9%, 74.6%, 81.1%, 84.9%, 95.4%, and 99.7%, respectively.Clearly, the fraction of degraded MB material also gradually increased with the increase in H 2 O 2 volume.Notably, compared to the condition without H 2 O 2 , 2 mL of H 2 O 2 under 80 min of illumination enhanced the fraction of degraded MB material by 30.7%.Compared to the conditions for the MO solution, the degradation process for the MB solution involved an additional 1 mL of H 2 O 2 and 10 more minutes of light exposure, yet the fraction of the degraded material was 0.3% lower, indicating that the synergistic H 2 O 2 system had a more effective degradation impact on the MO solution.

Impact of Initial Solution Concentration on the Photocatalysis by Composite Material
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, 1 mL and 2 mL H 2 O 2 were added to the MO and MB solutions after the dark reaction, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the dosage of the photocatalyst was 50 mg, the photocatalyst was Ag-6/CN, the volume of the target degradation products was 100 mL, and the pH was 7.Among them, different initial concentrations of the MO and MB solutions were used as experimental variables, which were 20 mg/L, 25 mg/L, 30 mg/L, 35 mg/L, and 40 mg/L.
As indicated in Figure 10, the fraction of degraded materials for the MO and MB solutions ranged from 86.2% to 100% and 62.7% to 99.7%, respectively, from low to high concentration.As the initial concentration of the MO and MB solutions increased, the fraction of the degraded materials gradually decreased.This was likely due to the higher initial concentrations of the target pollutants, which meant more pollutant molecules, and to the limited capacity of the Ag-6/CN photocatalyst to process these pollutants, thereby affecting the photocatalytic performance and resulting in a smaller fraction of degraded materials.
Molecules 2024, 29, x FOR PEER REVIEW 9 of 18 concentration.As the initial concentration of the MO and MB solutions increased, the fraction of the degraded materials gradually decreased.This was likely due to the higher initial concentrations of the target pollutants, which meant more pollutant molecules, and to the limited capacity of the Ag-6/CN photocatalyst to process these pollutants, thereby affecting the photocatalytic performance and resulting in a smaller fraction of degraded materials.

Impact of Composite Material Dosage on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, 1 mL and 2 mL of H2O2 were added to the MO and MB solutions after the dark reaction, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the photocatalyst was Ag-6/CN, the target pollutant concentration was 20 mg/L (100 mL), and the pH was 7.Among them, the dosage of the catalyst was the experimental variable, which was 0 mg, 10 mg, 20 mg, 30 mg, mg, 50 mg, or 60 mg.
As shown in Figure 11a, without the addition of the Ag-6/CN photocatalyst, the MO solution was almost undegraded, with a fraction of degraded material of only 3.6%.When the dosages of the Ag-6/CN photocatalyst were 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg, the fraction of degraded materials of the MO solution were 79.1%, 98.3%, 99.6%, 100%, and 100%, respectively.Both 40 mg and 50 mg of the Ag-6/CN photocatalyst achieved the complete degradation of the MO solution within 70 min of light exposure, but from an economic standpoint, the addition of 40 mg of Ag-6/CN photocatalyst is preferred.
As depicted in Figure 11b, with catalyst dosages of 0 mg, 10 mg, 20 mg, 30 mg, 40 mg, 50 mg, and 60 mg, the fraction of degraded materials of the MB solution were 56.9%, 86.1%, 90.8%, 94.5%, 99.7%, and 98.9%, respectively.As the dosage of the catalyst increased, the fraction of the degraded material of the MB solution also increased, but when the catalyst dosage reached 60 mg, the fraction of degraded material slightly decreased.This decrease may have been due to the excessive amount of catalyst affecting the active sites on the catalyst surface or worsening the light transmittance of the MB solution, thereby impacting the photocatalytic effect.Therefore, the optimal catalyst dosage for the MB solution was 50 mg.

Impact of Composite Material Dosage on Photocatalysis
The reaction conditions regulated in this study were as follows: at room temperature, the visible light source in the laboratory was a 300 W xenon lamp, the dark adsorption time was 30 min, 1 mL and 2 mL of H 2 O 2 were added to the MO and MB solutions after the dark reaction, the open light irradiation times of the MO and MB solutions were 70 min and 80 min, the photocatalyst was Ag-6/CN, the target pollutant concentration was 20 mg/L (100 mL), and the pH was 7.Among them, the dosage of the catalyst was the experimental variable, which was 0 mg, 10 mg, 20 mg, 30 mg, 40 mg, 50 mg, or 60 mg.
As shown in Figure 11a, without the addition of the Ag-6/CN photocatalyst, the MO solution was almost undegraded, with a fraction of degraded material of only 3.6%.When the dosages of the Ag-6/CN photocatalyst were 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg, the fraction of degraded materials of the MO solution were 79.1%, 98.3%, 99.6%, 100%, and 100%, respectively.Both 40 mg and 50 mg of the Ag-6/CN photocatalyst achieved the complete degradation of the MO solution within 70 min of light exposure, but from an economic standpoint, the addition of 40 mg of Ag-6/CN photocatalyst is preferred.

Impact of Solution pH on the Photocatalysis of Composite Material
As shown in Figure 12a, when the initial pH values of the MO solution were 3, 5, 7, 9, and 11, the fractions of degraded materials were 100%, 99.9%, 100%, 100%, and 99.2%, respectively.During the first 40 min of light exposure, the degradation of the MO solution at pH 11 was notably effective but slowed down subsequently.This could have been due to a small amount of alkali promoting the reaction, while an excess of alkali inhibited the redox reaction.Although the MO solutions at pH values of 3, 7, and 9 were completely As depicted in Figure 11b, with catalyst dosages of 0 mg, 10 mg, 20 mg, 30 mg, 40 mg, 50 mg, and 60 mg, the fraction of degraded materials of the MB solution were 56.9%, 86.1%, 90.8%, 94.5%, 99.7%, and 98.9%, respectively.As the dosage of the catalyst increased, the fraction of the degraded material of the MB solution also increased, but when the catalyst dosage reached 60 mg, the fraction of degraded material slightly decreased.This decrease may have been due to the excessive amount of catalyst affecting the active sites on the catalyst surface or worsening the light transmittance of the MB solution, thereby impacting the photocatalytic effect.Therefore, the optimal catalyst dosage for the MB solution was 50 mg.

Impact of Solution pH on the Photocatalysis of Composite Material
As shown in Figure 12a, when the initial pH values of the MO solution were 3, 5, 7, 9, and 11, the fractions of degraded materials were 100%, 99.9%, 100%, 100%, and 99.2%, respectively.During the first 40 min of light exposure, the degradation of the MO solution at pH 11 was notably effective but slowed down subsequently.This could have been due to a small amount of alkali promoting the reaction, while an excess of alkali inhibited the redox reaction.Although the MO solutions at pH values of 3, 7, and 9 were completely degraded, the degradation of the material at pH 3 was somewhat faster.This indicated that under weakly acidic conditions, the photocatalytic performance was slightly worse within 70 min of light exposure, possibly because a small amount of acid inhibited the reaction.

Impact of Solution pH on the Photocatalysis of Composite Material
As shown in Figure 12a, when the initial pH values of the MO solution were 3, 5, 7, 9, and 11, the fractions of degraded materials were 100%, 99.9%, 100%, 100%, and 99.2%, respectively.During the first 40 min of light exposure, the degradation of the MO solution at pH 11 was notably effective but slowed down subsequently.This could have been due to a small amount of alkali promoting the reaction, while an excess of alkali inhibited the redox reaction.Although the MO solutions at pH values of 3, 7, and 9 were completely degraded, the degradation of the material at pH 3 was somewhat faster.This indicated that under weakly acidic conditions, the photocatalytic performance was slightly worse within 70 min of light exposure, possibly because a small amount of acid inhibited the reaction.
As indicated in Figure 12b, when the initial pH values of the MB solution were 3, 5, 7, 9, and 11, the fractions of degraded materials were 76.2%, 89.2%, 99.7%, 99.9%, and 100%, respectively.As the pH value increased, the fraction of the degraded material of the MB solution also increased gradually.This suggests that alkaline conditions are favorable for the migration of photogenerated carriers at the Ag-6/CN interface, while acidic conditions reduce the photocatalytic activity of Ag-6/CN, impacting the photocatalytic effect.As indicated in Figure 12b, when the initial pH values of the MB solution were 3, 5, 7, 9, and 11, the fractions of degraded materials were 76.2%, 89.2%, 99.7%, 99.9%, and 100%, respectively.As the pH value increased, the fraction of the degraded material of the MB solution also increased gradually.This suggests that alkaline conditions are favorable for the migration of photogenerated carriers at the Ag-6/CN interface, while acidic conditions reduce the photocatalytic activity of Ag-6/CN, impacting the photocatalytic effect.

Mechanism of Photocatalytic Degradation of MO and MB by Ag-6/CN with H 2 O 2 System
To explore the possible photocatalyst mechanisms in under the H 2 O 2 system in degrading MO and MB, diphenylamine (DPA, 0.5 mM), ethylenediaminetetraacetic acid (EDTA, 0.5 mM), and p-benzoquinone (BQ, 0.5 mM) were used as scavengers for •OH, h + , and other free radicals, respectively.These scavengers were added to the target pollutants containing the photocatalyst in the radical-capturing experiments to analyze the active free radicals playing a major role in the photocatalytic degradation of MO and MB, as shown in Figure 13.
of degraded material by 27.4% compared to that of the blank group; EDTA produced slightly more inhibition, only 4.1% more than BQ; DPA produced the most significant inhibition, reducing the fraction of degraded material by 44.8%, about half that compared to the blank group.This suggests that •OH, h + , and •O2 − contribute to the removal of pollutants in the photocatalytic degradation process, with •OH being the most critical active species in the Ag-6/CN with H2O2 system for degrading MB solutions.Photocatalysis refers to the chemical reactions induced by photocatalytic materials under the influence of light, representing an intersection of photochemistry and catalytic sciences.Semiconductor photocatalytic materials possess a conduction band structure (CB) and a valence band structure (VB).When semiconductor photocatalytic materials are irradiated by a light source, the electrons from the VB are excited to the CB only if the photon energy is sufficient to be absorbed by the bandgap.Simultaneously, holes (h + ) are generated on the VB, forming electron-hole pairs (e − -h + ), as shown in Equation ( 1).The h + and e − can independently undergo oxidation and reduction reactions with the substances As shown in Figure 13a, after the addition of scavengers, the fraction of degraded MO material in the blank group remained at 100%.After adding EDTA, DPA, and BQ scavengers, the fractions of degraded MO materials were 100%, 59.6%, and 42.0%, respectively, with fractions of inhibition materials of 0%, 40.4%, and 58.0%, respectively.These data indicated that EDTA produced almost no inhibition in the photocatalytic degradation of MO under H 2 O 2 .Both DPA and BQ exhibited certain inhibitory effects on the photocatalytic degradation of MO, with BQ showing a more significant inhibition, 17.6% higher than that of DPA.This suggests that •OH and •O 2 − play a role in the photocatalytic degradation process, with •O 2 − being the most critical active species in the Ag-6/CN in the H 2 O 2 system for degrading MO solutions.
As shown in Figure 13b, after the addition of scavengers, the fraction of degraded MB material in the blank group was 100%.After adding EDTA, DPA, and BQ scavengers, the fractions of degraded MB materials were 68.5%, 55.2%, and 72.6%, respectively, with fractions of inhibition materials of 31.5%, 44.8%, and 27.4%, respectively.These data indicated that EDTA, DPA, and BQ all exhibited certain inhibitory effects on the photocatalytic degradation of MB.Among them, BQ produced the least inhibition, reducing the fraction of degraded material by 27.4% compared to that of the blank group; EDTA produced slightly more inhibition, only 4.1% more than BQ; DPA produced the most significant inhibition, reducing the fraction of degraded material by 44.8%, about half that compared to the blank group.This suggests that •OH, h + , and •O 2 − contribute to the removal of pollutants in the photocatalytic degradation process, with •OH being the most critical active species in the Ag-6/CN with H 2 O 2 system for degrading MB solutions.
Photocatalysis refers to the chemical reactions induced by photocatalytic materials under the influence of light, representing an intersection of photochemistry and catalytic sciences.Semiconductor photocatalytic materials possess a conduction band structure (CB) and a valence band structure (VB).When semiconductor photocatalytic materials are irradiated by a light source, the electrons from the VB are excited to the CB only if the photon energy is sufficient to be absorbed by the bandgap.Simultaneously, holes (h + ) are generated on the VB, forming electron-hole pairs (e − -h + ), as shown in Equation (1).The h + and e − can independently undergo oxidation and reduction reactions with the substances adsorbed on the surface of the photocatalytic material.Concurrently, the e − -h + pairs that do not participate in the redox reactions may recombine directly on the surface of or inside the catalyst.During the photocatalytic process, H 2 O reacts with holes to produce •OH and O 2 , as depicted in Equations ( 2) and (3).O 2 reacts with electrons to reduce •O 2 − , as shown in Equation ( 4).The addition of H 2 O 2 triggers the following reaction, where H 2 O 2 reacts with e − to produce •OH, as per Equation (5).The generated active free radicals (•OH, •O 2 − ) and holes degrade pollutants, as indicated in Equation ( 6) [32][33][34].in equation ( 4).The addition of H2O2 triggers the following reaction, where H2O2 reacts with e − to produce •OH, as per equation (5).The generated active free radicals (•OH, •O2 − ) and holes degrade pollutants, as indicated in (6) [32][33][34].
Semiconductor + hv → Semiconductor (e -+h + ) (1) As shown in Figure 14, based on previous experimental tests and theoretical analysis, we hypothesize the mechanism through which the target pollutants are photocatalytically degraded by the Ag-6/CN in conjunction with the H2O2 and Na2S2O8 systems under visible light irradiation.The electrons on the VB of Ag-6/CN are excited to the CB, generating photogenerated electrons and holes, e-and h+.The presence of the oxidants Na2S2O8 and H2O2 reduces the recombination fractions of e − and h + , allowing more photogenerated carriers to participate in the degradation of MO and MB.h + , •O2 − , •OH, and •SO4− can all degrade solutions of methyl orange and methylene blue.However, in the synergistic system, the primary active free radicals degrading the pollutants are •OH and •O2 − .O2 and H2O2 undergo reduction reactions to produce •OH and •O2 − ; H2O undergoes oxidation reactions to produce •OH, which then degrades azo dyes (MO and MB) along with "•O2 −" into small molecular substances.

Stability Test of Composite Material
Figure 15 shows the degradation cycle graph for the MO and MB solutions under H2O2 conditions in conjunction with Ag-6/CN.Repeated experiments were conducted under the same reaction conditions.As indicated by Figure 15, after five cycles, the fraction

Stability Test of Composite Material
Figure 15 shows the degradation cycle graph for the MO and MB solutions under H 2 O 2 conditions in conjunction with Ag-6/CN.Repeated experiments were conducted under the same reaction conditions.As indicated by Figure 15, after five cycles, the fraction of material of the MO solution degraded by Ag-6/CN decreased from 100% to 89.1%, and for the MB solution, it decreased from 100% to 82.9%.The fraction of degraded materials for both MO and MB solutions remained above 80%, demonstrating that Ag-6/CN exhibited good photocatalytic stability in the degradation process of the MO and MB solutions.The photocatalytic effects were evaluated by recording the concentration changes in the MO and MB solutions before and after the reaction, with the fraction of degraded material calculated according to Equation (7).
where η is the fraction of the degraded material, in %; C 0 is the initial mass concentration of the solution, in mg/L; C is the mass concentration of the solution at time t, in mg/L.calculated according to Equation (7).
where η is the fraction of the degraded material, in %; C 0 is the initial mass concentration of the solution, in mg/L; C is the mass concentration of the solution at time t, in mg/L.

Experimental Reagents
The reagents used in this experiment are shown in Table 2.All water used in the experiment was deionized water, and the reagents used were all analytical grade.

Experimental Reagents
The reagents used in this experiment are shown in Table 2.All water used in the experiment was deionized water, and the reagents used were all analytical grade.

Experimental Apparatus
The experimental apparatuses used in this study are shown in Table 3.
prepared materials were mounted on black conductive adhesive and then gold-sputtered under a vacuum.The acceleration voltage used was 1.0 kV.In this experiment, a UV-visible diffuse reflectance spectrophotometry (UV-Vis/DRS) was used to test the light absorption range of the solid powder photocatalyst and to plot the UV-visible diffuse reflectance spectra.The scanning range was 200-800 nm.Based on the light absorption range and the Tauc plot method, as shown in Equation ( 7), the bandgap widths of the g-C 3 N 4 photocatalyst and its composites could be deduced.The g-C 3 N 4 used in this paper was a direct bandgap semiconductor; therefore, n was taken as 1/2.
(αhv) 1/n = A hv − E g (8) where α is the absorbance value; h is the Planck constant, equal to 4.1356676969 × 10 −15 ev; ν is the frequency; A is a constant; E g is the semiconductor bandgap width.Photoluminescence (PL) spectroscopy was used in this experiment to obtain the emission spectra of g-C 3 N 4 and Ag/g-C 3 N 4 under 365 nm excitation light to study their electronic structures and optical properties, thus revealing the mechanisms of e − -h + migration, trapping, and recombination.

Preparation of Methyl Orange and Methylene Blue Wastewater
To prepare stock solutions (1 g/L) and working solutions of methyl orange (MO) and methylene blue (MB), firstly, we weighed 1 g each of MO and MB, which we placed into 100 mL beakers.We added a certain amount of deionized water under stirring until completely dissolved.We transferred the solutions into 1000 mL volumetric flasks, which we made up to the mark with deionized water, resulting in 1 g/L solutions of MO and MB (referred to as stock solutions).We stored these solutions in a refrigerator.Before the experiment, we diluted the MO and MB solutions to the required working concentrations.

Activity Test of Photocatalyst
Using a xenon lamp to simulate sunlight conditions, photocatalytic degradation experiments were conducted with MO and MB as the target pollutants in a photocatalytic performance testing device.The maximum absorption wavelengths for MO and MB were 465 nm and 664 nm, respectively.For each experiment, 50 mg of photocatalyst and 100 mL of target degradant (20 mg/L) were weighed, thoroughly mixed, and placed in a reaction container for a dark reaction for 30 min, with stirring provided by a magnetic stirrer inside a light-proof box.Once adsorption-desorption equilibrium was reached, the xenon light source was turned on to initiate the light reaction.During the light reaction, samples of the target pollutants were taken every 10 min and filtered through a 0.45 µm filter head, and the absorbance was measured at specific wavelengths using a UV spectrophotometer.The photocatalytic effects were evaluated by recording the concentration changes of the MO and MB solutions before and after the reaction, with the fraction of degraded material calculated according to Equation (7).
We used quasi-first-order reaction kinetics to simulate the photocatalytic degradation process of MO and MB, and we plotted the relationship between MO and MB solutions and light exposure time t.We studied the photocatalytic degradation kinetic constant k and correlation coefficient R. The first-order reaction kinetics equation is shown in Equation (9).−Ln(C/C 0 ) = kt (9) where C 0 is the initial concentration of the target pollutant, mg/L; C is the concentration of the target pollutant at time t, mg/L.

Conclusions
The results of characterization indicated that CN and Ag-X/CN photocatalytic materials were successfully prepared.It was found that the g-C 3 N 4 synthesized from urea possesses a fluffy porous structure resembling coral, and Ag-6/CN features a cloud-like thin and dense morphology.Compared to CN, Ag-6/CN photocatalytic material has a larger surface area and lower bandgap energy, providing more active sites, enhancing visible light absorption, promoting the separation of the photogenerated electron-hole pairs, and effectively improving the photocatalytic degradation of the target pollutants.Additionally, due to the high Schottky barrier of Ag, e − is transferred to the Ag and finally transferred to the surface of the photocatalyst to participate in the reduction reaction, which increases the specific area as Ag is added.The results showed that g-C 3 N 4 loaded with 6% Ag demonstrated superior photocatalytic performance.
In a synergistic H 2 O 2 system, with initial MO and MB concentrations of 20 mg/L (100 mL), initial pH values of 3 and 11, photocatalyst dosages of 40 mg and 50 mg, and 1 mL and 2 mL of H 2 O 2 added, respectively, Ag-6/CN achieved a 100% photocatalytic fraction of degraded material for both MO and MB solutions within 70 and 80 min, respectively.After five cycles, the fraction of degraded materials of the MO and MB solutions remained above 80%.The radical trapping experiments indicated that the main active species in the photocatalytic degradation of MO and MB by Ag-6/CN under the synergistic H 2 O 2 system are •O 2 − and •OH.

Figure 1 .
Figure 1.XRD patterns of CN and Ag-X/CN series catalysts.

Figure 1 .
Figure 1.XRD patterns of CN and Ag-X/CN series catalysts.

Figure 1 .
Figure 1.XRD patterns of CN and Ag-X/CN series catalysts.

Figure 3 .
Figure 3. SEM images of CN at different magnifications.(a) SEM image of CN at 5 μm; (b) SEM image of CN at 2 μm.

Figure 4
Figure 4 presents the SEM images of Ag-6/CN at different magnifications, showing a thin and dense morphology similar to sheet-like clouds.Compared to CN, Ag-6/CN has a larger specific surface area, allowing more extensive contact with pollutants and providing more active sites, demonstrating a photocatalytic advantage.

Figure 3 .
Figure 3. SEM images of CN at different magnifications.(a) SEM image of CN at 5 µm; (b) SEM image of CN at 2 µm.

Figure 4
Figure 4 presents the SEM images of Ag-6/CN at different magnifications, showing a thin and dense morphology similar to sheet-like clouds.Compared to CN, Ag-6/CN has a larger specific surface area, allowing more extensive contact with pollutants and providing more active sites, demonstrating a photocatalytic advantage.

Figure 4
Figure 4 presents the SEM images of Ag-6/CN at different magnifications, showing a thin and dense morphology similar to sheet-like clouds.Compared to CN, Ag-6/CN has a larger specific surface area, allowing more extensive contact with pollutants and providing more active sites, demonstrating a photocatalytic advantage.

Figure 7 .
Figure 7. Curve plots of Ag-X/CN degradation of MO and MB with different composite ratios.(a) Degradation curve of MO solution; (b) quasi-first-order reaction kinetics diagram for degradation of MO solution; (c) degradation curve of MB solution; (d) quasi-first-order reaction kinetics diagram for degradation of MB solution.

Figure 7 .
Figure 7. Curve plots of Ag-X/CN degradation of MO and MB with different composite ratios.(a) Degradation curve of MO solution; (b) quasi-first-order reaction kinetics diagram for degradation of MO solution; (c) degradation curve of MB solution; (d) quasi-first-order reaction kinetics diagram for degradation of MB solution.

Figure 8 .
Figure 8. Degradation curves of MO (a) and MB (b) under different systems.

Figure 8 .
Figure 8. Degradation curves of MO (a) and MB (b) under different systems.

Figure 9 .
Figure 9. Degradation curves of MO (a) and MB (b) with different amounts of H2O2.

Figure 9 .
Figure 9. Degradation curves of MO (a) and MB (b) with different amounts of H 2 O 2 .

Figure 10 .
Figure 10.Degradation curves of MO (a) and MB (b) solutions for different initial concentrations.

Figure 10 .
Figure 10.Degradation curves of MO (a) and MB (b) solutions for different initial concentrations.

s 18 Figure 11 .
Figure 11.Degradation curves for MO (a) and MB (b) solutions with varying dosages of catalyst.

Figure 11 .
Figure 11.Degradation curves for MO (a) and MB (b) solutions with varying dosages of catalyst.

Figure 11 .
Figure 11.Degradation curves for MO (a) and MB (b) solutions with varying dosages of catalyst.

Figure 12 .
Figure 12.Degradation curves of MO (a) and MB (b) solutions under different pH.Figure 12. Degradation curves of MO (a) and MB (b) solutions under different pH.

Figure 12 .
Figure 12.Degradation curves of MO (a) and MB (b) solutions under different pH.Figure 12. Degradation curves of MO (a) and MB (b) solutions under different pH.

Figure 13 .
Figure 13.Degradation diagrams of MO and MB using different capture agents.(a) H2O2 degradation of MO; (b) H2O2 degradation of MB.

Figure 13 .
Figure 13.Degradation diagrams of MO and MB using different capture agents.(a) H 2 O 2 degradation of MO; (b) H 2 O 2 degradation of MB.

h + /•O − 2 / 6 )
•OH + MO/MB → degradation products (As shown in Figure 14, based on previous experimental tests and theoretical analysis, we hypothesize the mechanism through which the target pollutants are photocatalytically degraded by the Ag-6/CN in conjunction with the H 2 O 2 and Na 2 S 2 O 8 systems under visible light irradiation.The electrons on the VB of Ag-6/CN are excited to the CB, generating photogenerated electrons and holes, e-and h+.The presence of the oxidants Na 2 S 2 O 8 and H 2 O 2 reduces the recombination fractions of e − and h + , allowing more photogenerated carriers to participate in the degradation of MO and MB.h + , •O 2 − , •OH, and •SO 4 − can all degrade solutions of methyl orange and methylene blue.However, in the synergistic system, the primary active free radicals degrading the pollutants are •OH and •O 2 − .O 2 and H 2 O 2 undergo reduction reactions to produce •OH and •O 2 − ; H 2 O undergoes oxidation reactions to produce •OH, which then degrades azo dyes (MO and MB) along with "•O 2 −" into small molecular substances.

Figure 14 .
Figure 14.Diagram of the mechanism of the photocatalytic degradation of MO and MB in synergistic Ag−6/CN oxidation system.

Figure 14 .
Figure 14.Diagram of the mechanism of the photocatalytic degradation of MO and MB in synergistic Ag−6/CN oxidation system.

Figure 15 .
Figure 15.The stability of degradation rate with number of cycles.

Figure 15 .
Figure 15.The stability of degradation rate with number of cycles.

Table 1 .
Quasi-first-order reaction kinetics parameters for the degradation of MO and MB by Ag-X/CN with different composite ratios.

Table 1 .
Quasi-first-order reaction kinetics parameters for the degradation of MO and MB by Ag-X/CN with different composite ratios.

Table 2 .
The list of experimental reagents.

Table 2 .
The list of experimental reagents.