## 3 Results and Discussion
### Photocatalytic Performance Analysis
#### 3.3.2 Stability Analysis of Photocatalysts  
A series of techniques were used to explore the changes in functional groups and elemental valence of CdS/g-C\({}_{3}\)N\({}_{4}\)-2 composites before and after use. Figure 9a presents the SEM image of the original composite, and Figure 9b displays the SEM image of the composite after five cycles. SEM scans revealed slight damage to the surface microstructure of the composites after use. Still, the basic morphology remained largely unchanged, indicating that the material's microstructure experienced minimal damage after cycling and the physical properties of the material were stable. The IR spectra shown in Figure 9c also showed that the surface functional groups of the materials remained almost unchanged before and after the reaction, indicating that the composites had relatively stable chemical properties. It has been previously reported in the literature that CdS and its derived composite photocatalysts suffer from photocorrosion, and some Cd\({}^{2+}\) is released in the reaction solution during the photocatalytic process [47].  
Figure 9: Scanning electron micrograph of (**a**) pristine CdS/g-C\({}_{3}\)N\({}_{4}\)-2 composite and (**b**) the material after five cycles; IR spectrum of (**c**) pristine CdS/g-C\({}_{3}\)N\({}_{4}\)-2 composite and the material after five cycles; Results of cyclic degradation (**d**) of TCH by CdS/g-C\({}_{3}\)N\({}_{4}\)-2 photocatalytic materials; Cd 3d fine spectrum of (**e**) pristine CdS/g-C\({}_{3}\)N\({}_{4}\)-2 composite and (**f**) the material after five cycles.  
In this experiment, five photocatalytic degradation experiments of TCH were conducted to analyze the degradation rate of TCH using CdS/g-C3N4-2 composite material. The catalytic material of the fifth cycle was analyzed using the XPS energy spectrum (XPS, Thermo Scientific K-Alpha, USA). Figure 9d illustrates a degradation efficiency of 81.46% for the CdS/g-C3N4-2 material during the first cycle, which declined to 76.26% in the second cycle. The reason is that some of the contaminants occupied the adsorption sites of the composite material after the first reaction, and it was difficult to remove the adsorbed contaminants from the material with simple water and alcohol washing. Degradation rates after the third cycle, fourth cycle, and fifth cycle were 73.66%, 70.86%, and 70.19%, respectively. The adsorption performance reduced slightly but not significantly, and the degradation rate was always retained above 70%. This indicates that such catalysts have high stability and the adsorption mainly affects the first use. One should find ways to elute the adsorbed pollutant molecules to maintain a high adsorption capacity.  
To verify the occurrence of photocorrosion in CdS, the concentration of Cd2+ within the system was measured using ICP-OES. Results revealed an accumulated Cd2+ level of 0.875 mg/L following photocatalytic degradation (Table 1), indicating that photocorrosion was an important factor in the degradation of cycling performance. To verify the change of chemical composition before and after catalysis of CdS/g-C3N4, XPS energy spectroscopy was performed on pristine and fifth-cycle catalytic degraded Cd 3d samples, respectively (Figure 9e,f). After the reaction, the peaks of Cd2+ at 405.6 eV and 412.4 eV were slightly shifted to the right, reaching 406.0 eV and 412.7 eV. This may be caused by electron transfer during the reaction and can be neglected, so the positions of the Cd2+ peaks remained largely unchanged before and after the reaction, indicating that the substance had not changed the nature of the valence state. However, the peak intensities were weakened, which was consistent with the detection of Cd2+ in the solution and with the slight decrease in degradation efficiency after each cycle, indicating that there was some photocorrosion in the system.