## 4 Discussion
At a lower sintering temperature, there were four broad SiO\({}_{4}\) and ZnO\({}_{4}\) bands, whose presence was attributed to the bending modes of the O-Si-O and Si-O-Si bonds, Zn-O stretching vibrational bond, vibrations of Zn-O in the ZnO\({}_{4}\) tetrahedral, the decreasing of the Si-O stretching bonds, and the stretching vibrations of the SiO\({}_{4}\) and ZnO\({}_{4}\) tetrahedron [17]. This four broad SiO\({}_{4}\) and ZnO\({}_{4}\) bands indicated that there was no formation of zinc silicate glass-ceramics, which agrees with the XRD pattern, where all samples were in an amorphous form at 27 \({}^{\circ}\)C and 700 \({}^{\circ}\)C. While, for the IR spectrum of the zinc silicate glass-ceramics, there were eight types of absorption bands located at the 459 cm\({}^{-1}\) asymmetric deformation of the SiO\({}_{4}\), symmetric stretching of ZnO\({}_{4}\) at 576 cm\({}^{-1}\), asymmetric stretching of ZnO\({}_{4}\) at 615 cm\({}^{-1}\), bond vibration of SiO\({}_{4}\) at 697 cm\({}^{-1}\), symmetric stretching of SiO\({}_{4}\) at 865 cm\({}^{-1}\), and asymmetric stretching of SiO\({}_{4}\) at 905 cm\({}^{-1}\), 932 cm\({}^{-1}\), and 978 cm\({}^{-1}\), respectively [18]. From Figure 4, after the sintering temperature reached 750 \({}^{\circ}\)C up to 950 \({}^{\circ}\)C, a sharp band at 456 cm\({}^{-1}\) was spotted at the IR region, showing theasymmetric deformation of the Si-O bond in the SiO\({}_{4}\) units. Then, a band at 574 cm\({}^{-1}\) associated with the Zn-O symmetric stretching vibration in the ZnO\({}_{4}\)\({}^{-}\) was also spotted. While, for the asymmetric stretching of the Zn-O during vibration of the ZnO\({}_{4}\) group was located at 615 cm\({}^{-1}\). The bands positioned at 779 cm\({}^{-1}\) are accredited to the Si-O bond vibration and the bands at 876 cm\({}^{-1}\) are subjected to Si-O symmetric stretching vibration [6; 19]. Lastly, the band that appeared at 980 cm\({}^{-1}\) was transmitted due to the Si-O asymmetric stretching vibration. The band assignation of the IR spectra was tabulated in Table 2. Considering all the IR spectra, it was found that, at first, there were only four broad bands present, proving the formation of the zinc silicate glass. Later, as the sintering temperature increased, there was an additional, sharp band at the IR region, indicating the formation of zinc silicate glass-ceramics, which supports the XRD result. The photoluminescence spectrometer was cast-off to analyse and prove the luminescence properties of the zinc silicate glass and glass-ceramics by looking at the emission spectra of the sample as in Figure 5, where three broad emission peaks at 529, 570, and 682 nm appeared. The glass-ceramics reveals green, yellow, and red emission. It was found that the origin of these emission peaks might originate from the intrinsic or native defects in ZnO [20]. According to [PERSON] and co-researchers, these emissions can be attributed to the following transitions: Zn\({}_{\text{i}}\)\({}^{+}\)\(\rightarrow\) VZn\({}^{-}\) at 529 nm, VZn\({}^{-}\)\(\rightarrow\) VZn\({}^{-}\) at 570 nm, and CB \(\rightarrow\) V\({}_{\text{o}}\)\({}^{+}\)\(/\) V\({}_{\text{o}}\)\({}^{+}\)\(\rightarrow\) VB at 682 nm [21]. The green emission happened due to the electron-hole recombination, in which the electron was trapped at the singly ionized oxygen vacancy (V\({}_{\text{o}}\)) centre; then, it recombined with the hole in the valance band (VB) [22]. The recombination process between the electron and hole releases the green emission. Moreover, based on study by [PERSON] and co-researchers, it has been concluded that the centre/origin of the green luminescence in ZnO can be either the zinc interstitial (Zn\({}_{\text{i}}\)) and/or V\({}_{\text{o}}\)[20]. Apart from that, it has been discussed that the Zn\({}^{2+}\) ions are the ones that caused the green emission to occur and appear in zinc silicate glass and glass-ceramics [22; 23; 24].