## 3 Results and Discussion  
### ZnO/g-C\({}_{3}\)N\({}_{4}\) Composites Characterization  
The ZnO/g-C\({}_{3}\)N\({}_{4}\) composites synthesized using DES as a precursor were initially characterized to analyze their structural properties and confirm a successful synthesis. Therefore, the morphological and chemical characterization was performed using XRD, FTIR, XPS, SEM, and EDS mapping techniques. The crystalline structure of the catalyst was analyzed by XRD. Figure 4a shows the XRD pattern obtained for the synthesized composites. The sharp diffraction peaks indicate a high degree of crystallinity in the prepared material. The existence of diffraction peaks at 20 = 13.1\({}^{\circ}\) and 27.5\({}^{\circ}\), which can be assigned to the (100) and (002) planes, respectively, are characteristic of g-C\({}_{3}\)N\({}_{4}\)[36; 37]. This confirmed the formation of g-C\({}_{3}\)N\({}_{4}\) during the pyrolysis process. However, when compared with the standard pattern, the intensity of the peak corresponding to the (002) plane was significantly weakened. This may indicate that the introduction of ZnO could restrict the growth of pure g-C\({}_{3}\)N\({}_{4}\) crystals [38]. Additionally, the spectrum also showed characteristic patterns that can be attributed to the theoretical crystal structure of ZnO at 20 = 31.9\({}^{\circ}\), 36.4\({}^{\circ}\), 47.8\({}^{\circ}\), 56.8\({}^{\circ}\), 63.2\({}^{\circ}\), and 66.7\({}^{\circ}\), corresponding to the (010), (011), (012), (110), (013), and (020) planes. These results are consistent with others reported in the literature, where catalytic materials containing ZnO were synthesized [39; 40; 41]. The functional groups of the composites were explored by FTIR (Figure 4b). The spectra corresponding to g-C\({}_{3}\)N\({}_{4}\) and the ZnO/g-C\({}_{3}\)N\({}_{4}\) composites presented several adsorption peaks in common. Thus, this allowed for ensuring the presence of g-C\({}_{3}\)N\({}_{4}\) in the composites. The intensity of the adsorption peak at 785 cm\({}^{-1}\) was attributed to the out-of-plane bending of the tris-s-triazine ring [42; 43]. The peak at 1632 cm\({}^{-1}\) corresponded to stress in the C=O group and in the amide group (CONH) and the peak at 1567 cm\({}^{-1}\) to stress in the C=N bonds [42; 44; 45]. The peaks appearing in the range 3100-3300 cm\({}^{-1}\) indicated N-H bond stresses of non-condensed amine groups [42; 46], while those present between 3300-3600 cm\({}^{-1}\) would correspond to the O-H bond stresses of the hydroxyl group of adsorbed water molecules. On the other hand, the characteristic bands of ZnO appeared in the range of 400-800 cm\({}^{-1}\), corresponding to the vibration of the Zn-O bond [19]. Thus,the spectrum of the composites revealed vibrational bands characteristic of pure g-C\({}_{3}\)N\({}_{4}\) and ZnO, confirming the generation and stability of the ZnO/g-C\({}_{3}\)N\({}_{4}\) composites. To confirm the results obtained by FTIR and to better understand the structural details of the ZnO/g-C\({}_{3}\)N\({}_{4}\) composites, XPS analyses of the C, N, O, and Zn elements were performed. As shown in Figure 4c, the C 1s peaks could be fitted to three peaks located at 284.8, 287.8, and 289.5 eV, which were attributed to C-C or C-H bonds, sp\({}^{2}\) carbon-nitrogen (N-C=N) bonds of aromatic ring systems and ester (O-C=O) and carbamate (O-C(N)=O)-type bonds, respectively [47; 48]. The N 1s spectrum (Figure 4d) was fitted into two peaks. A major one was located around 398.59 eV, which can be assigned to the sp\({}^{2}\) bonded nitrogen of the triazine rings (C=NH), and another one at 399.97 eV is associated with the pyridine-type nitrogen atoms [47; 48]. Meanwhile, the O 1s spectrum in Figure 4e could be divided into two peaks at 531.4 and 532.74 eV, corresponding to N-C-O bonding, OH groups, and even to O\({}_{2}\) in the lattice and carbon-oxygen functionalities [35].