## 3 Results
Figure 7a shows the time-dependent optical absorption spectra of MO with an initial MO concentration of 10 mg/L and Na\({}_{2}\)SO\({}_{4}\) concentration of 12.5 g/L in the mixture solution of MO-Na\({}_{2}\)SO\({}_{4}\) before and after photocatalytic degradation with Ag/TiO\({}_{2}\)-5 W-50 s. It can be found that the degradation of pure MO in pure solution was completed in 55 min, and the degradation rate of MO can reach 100% while the degradation rate of MO in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution can reach 100% in 65 min, as shown in Figure 7b. By comparison, it can be found that the degradation effect of MO in pure MO solution is better than that in MO-Na\({}_{2}\)SO\({}_{4}\) mixed solution. The results suggest that Na\({}_{2}\)SO\({}_{4}\) has an inhibitory effect on the degradation of MO. The possible reason may be one of the following: (1) When Na\({}_{2}\)SO\({}_{4}\) is added to the MO solution, the pH value of the solution increases. The photocatalytic degradation efficiency of Ag/TiO\({}_{2}\) at low pH is higher than that at high pH due to the fact that MO is an anionic molecule and positively charged photocatalyst surfaces are  
Figure 5: UV-Vis spectra of MO solution after the photocatalytic degradation reaction via **(a)** TiO\({}_{2}\) and Ag/TiO\({}_{2}\); **(b)** time-dependent UV-Vis spectra of MO degraded with Ag/TiO\({}_{2}\)-5 W-50 s in pure MO solution during the photocatalytic degradation process; **(c)** degradation rate of MO degraded with Ag/TiO\({}_{2}\) in pure MO solution. more likely to absorb dye molecules. The dye structure of MO was prone to oxidation over the azo structure as the sulfonic groups (-SO4\({}^{-}\)) can help capture hydrogen protons and further enhance the hydrophobicity of the TiO2 surface [34,35]. (2) When Na2 SO4 is added to the MO solution, the saturation adsorption amount may decrease due to the competition adsorption of anions added (SO4\({}^{2-}\)) and anion dye molecules onto the surface of TiO2, which inactivates part of the catalyst and reduces the degradation efficiency [36, 37, 38, 39]. (3) SO4\({}^{2-}\) is adsorbed on the surface of the photocatalyst, acting as a pore-trapping agent to increase the content of SO4\({}^{-}\) generated, which accelerates the oxidation of Ag and increases the recombination rate of photogenerated electron-hole pairs. This speculation can be supported by the XPS results in the following text. In order to study the elemental composition and chemical status of the Ag/TiO2 before and after photocatalytic degradation, for the high-resolution XPS spectra of C, the peak at 284.80 eV appeared in all the samples, which can be assigned to the adventitious carbon. Figure 8 shows the XPS spectra of Ag/TiO2-5 W-50 s before and after the photocatalytic degradation in different solutions. Figure 8a shows the wide-scan XPS spectra of the Ag/TiO2-5 W-50 s before the photocatalytic degradation. According to the wide-scan XPS  
Figure 6: UV-Vis spectra of MO solution after the different cycles of photocatalytic degradation reaction in pure MO solution using (**a**) Ag/TiO2-5 W-50 s; (**b**) the degradation rate of MO using Ag/TiO2-5 W-50 s with different cycles. Figure 7: (**a**) Time-dependent UV-Vis spectra of MO degraded with Ag/TiO2-5 W-50 s in MO-Na2 SO4 mixed solution during the photocatalytic degradation process. (**b**) The degradation rates of MO in pure MO solution and MO-Na2 SO4 mixed solution, respectively. spectra, Ti, Ag, O, and C are found as the surface elements of the prepared composites. Figure 8b shows the Ag 3d\({}_{5/2}\) peak and Ag 3d\({}_{3/2}\) peak of the Ag/TiO\({}_{2}\)-5 W-50 s before the photocatalytic degradation. For Ag 3d corresponding to Ag\({}^{0}\), the binding energies for 3d\({}_{5/2}\) and 3d\({}_{3/2}\) electrons are observed around 367.7 eV and 373.7 eV, respectively [32]. The spin energy presents a characteristic value of 6.0 eV, which can be applied to confirm the presence of Ag\({}^{0}\) on the surface of Ag/TiO\({}_{2}\)-5 W-50 s in Figure 8b.