## 3 Results and Discussion
Additionally, the 174 cm\({}^{-1}\) and 668 cm\({}^{-1}\) modes are also related to ZnS, respectively, indicating a group of second-order modes in the region of 130 cm\({}^{-1}\) to 180 cm\({}^{-1}\) and the mode 2(LO) associated with the cubic structure of ZnS [35]. Analyzing the samples with rGO, the active vibrational modes discussed before are attenuated, agreeing with SEM-FEG images. For these samples, the presence of two vibrational modes is notable in 1344 cm\({}^{-1}\) and 1591 cm\({}^{-1}\), which are related to the D (disorder in the structure) and G (sp\({}^{2}\) carbon structure) bands characteristics of carbon-based materials, indicating success in the preparation of ZnO/ZnSr:GO heterostructure. The I\({}_{D}\)/I\({}_{G}\) ratio increased for samples with more rGO in the structure, indicating an increase in the structural defects of these materials. The samples with a more effective conversion of ZnO to ZnS (2% and 4% rGO) exhibit more defects in the rGO [36]. The samples with the highest I\({}_{D}\)/I\({}_{G}\) ratio value were those with the highest amount of rGO in the system. The increase in this rate may be directly related to the degree of coating of the rGO structure with ZnS nanoparticles, which have the potential to change the vibration characteristics of rGO because they are deposited on its surface. In addition, a shoulder is observed on the left side of the G band in the ZnO/ZnS:1% rGO sample. This mode is associated with the D\({}^{\prime\prime}\) band, which, according to the literature, has its intensity amplified due to the decrease in the crystallinity of the carbon structures since it is characteristic of amorphous materials [37]. The FTIR spectra of the ZnO/ZnS, GO, and ZnO/ZnSr:rGO samples are shown in Figure 5. In the FTIR spectrum, vibrations typical of functional groups in the structure are observed. The broad band between 3450 cm\({}^{-1}\) and 3300 cm\({}^{-1}\) indicates the stretching  
Figure 4: Raman spectroscopy of samples: (**a**) ZnO/ZnS; (**b**) ZnO/ZnS:0.5% rGO; (**c**) ZnO/ZnS:1% rGO; (**d**) ZnO/ZnS:2% rGO; and (**e**) ZnO/ZnS:4% rGO. modes of the -OH bond. Furthermore, the bands at 1624 cm\({}^{-1}\) and 1421 cm\({}^{-1}\) are related to C=C vibrations, and the band at 1031 cm\({}^{-1}\) indicates the presence of the vibrational mode of the C-O functional group. Additionally, the band at 2388 cm\({}^{-1}\) suggests the presence of CO\({}_{2}\) molecules on the sample's surface [38]. Analyzing the spectra in Figure 5, we noticed the extraction and decrease in the intensity of some oxygenated groups for ZnO/ZnSr:GO samples concerning GO. This result suggests that the microwave-assisted thermal process reduced GO and formed rGO. However, the reduction was incomplete, which was likely due to the presence of the remaining oxygenated groups. Furthermore, other factors demonstrate the GO reduction process, such as the compound's non-solubility and as well as the change in the color from brown to black, which are characteristics related to rGO. Vibrational modes related to characteristic metal bonds are identified at 718 cm\({}^{-1}\) and in the 420-530 cm\({}^{-1}\) range. The first is associated with the Zn-S bond characteristic of the cubic phase, and the second indicates metal-oxygen vibrational modes, specifically the Zn-O bonds. Moreover, a decrease in the intensity of Zn-S and Zn-O bonds was observed for samples with a higher amount of rGO in the structure [39]. The photoluminescence spectra of the ZnO/ZnS and ZnO/ZnS:rGO samples are presented in Figure 6. A broad peak centered at 478 nm indicates a blue emission for ZnO/ZnS. The energy associated with this emission is below the compound's optical bandgap energy; this emission is attributed to synergistic interactions between the ZnO core and ZnS shell, confirming the type II indirect band alignment [40; 41; 42]. Additionally, emissions in the spectrum's orange region are observed and associated with interstitial oxygen, highlighting the excess oxygen in the ZnO/ZnS sample.