## 3 Results and Discussion
### XPS Results  
Figure 7A-D show the C 1s XPS spectrum of the GO/Zn (OH)\({}_{\rm x}\) samples with different Zn (OH)\({}_{\rm x}\) concentrations of 0, 25, 50, and 75 wt%, respectively. The C1S XPS results showed a peak centered around 285.7 eV, which was deconvolved into three separate peaks located at 284.2, 285.7, and 287.7. These peaks represent the C-C/C=C, the C-O, and the C=O coordination environments [63; 64]. No large changes were observed in the binding environment of the graphene oxide, indicating that the Zn is not directly attached to the carbon in the graphene oxide sheets.  
Figure 8A-D show the O1S XPS spectrum of the GO/Zn (OH)\({}_{\rm x}\) samples with Zn (OH)\({}_{\rm x}\) concentrations of 0, 25, 50, and 75 wt.%, respectively. The O1 spectra showed a peak centered around 523 eV, which was deconvolved into three peaks located at 531.3, 532.4, and 533.2 eV for the GO sample. These peaks were consistent with the binding environments for the C-O, OH, and C=O, respectively. The XPS results in Figure 8A are consistent with those reported in the literature on graphene oxide. Interestingly, when the main peak of the O1 spectra was deconvolved for each of the GO/Zn (OH)\({}_{\rm x}\) samples, a slight shift in the peak was observed with changing Zn (OH)\({}_{\rm x}\) concentration. For the GO/Zn (OH)\({}_{\rm x}\) sample with 25 wt.% concentration, the O1S spectrum was deconvolved into three peaks located at 531.5, 532.5, and 533.2 eV, which were consistent with O-Zn/O-H, C-OH, and C=O binding environments [63; 64]. The O1S peak of the GO/Zn (OH)\({}_{\rm x}\) samples with 50 and 75 wt.% of Zn(OH)\({}_{\rm x}\) (Figure 8C) was deconvolved into two individual peaks located at 531.9 and 534.1, which were consistent with Zn-OH and C-O/C=O binding environments [65; 66].  
Figure 7: C 1s XPS spectrum of the GO/Zn (OH)\({}_{\rm x}\) samples with different Zn (OH)\({}_{\rm x}\) concentrations of: (**A**) (0 wt. %), (**B**) (25 wt.%), (**C**) (50 wt.%), and (**D**) (75 wt.%).  
Figure 9A-C show the Zn 2P\({}_{3/2}\) and Zn 2P\({}_{1/2}\) XPS spectra for the GO/Zn (OH)x samples with different Zn (OH)\({}_{\rm X}\) concentrations of 0, 25, 50, and 75 wt%, respectively. The Zn 2P showed two peaks centered around 1022.4 and 1045.7 eV, representing the Zn 2P\({}_{3/2}\) and Zn2 P\({}_{1/2}\) binding energies, respectively. The Zn2 P\({}_{3/2}\) peak was deconvolved into two peaks, which were centered around 1022.2 and 1024.5 eV. The two Zn2P\({}_{3/2}\) peaks indicated that the Zn is present as the Zn\({}^{2+}\) and is bound to an oxygen ligand on graphene oxide [67; 68]. It is well-known that the shifts in XPS peaks are common due to many factors, such as a metal binding to a specific site or complex formation or the formation of nanoparticles and size, and the latter is a well-known contributor to binding energy. Interestingly, the shifts away from the peak observed for the GO indicate that the oxygen functional groups are involved in the binding of the Zn2+ ions to the GO [69; 70]. In fact, shifts in the O1s spectrum, up to 534 eV, have been observed in the literature for transition metals bound to graphene oxide as well as in the formation of Zn(hydroxy)oxide-GO composites [71].  
The results reported by [PERSON] et al. showed that the binding of Zn onto biochar had a binding energy of 1024 eV in aged biochar, indicating that the binding occurs through an oxygen moiety [72]. The attachment of the Zn to the graphene oxide is through the oxygen on the surface, which was indicated in the appearance of the O-Zn bond observed in the O1S spectrum. The Zn 2P\({}_{1/2}\) spectrum represents the same information as in the binding of the Zn-O on the graphene oxide surface (1047.5 eV) and the presence of the Zn-OH located at (1045.2 eV).  
Figure 8: O1S XPS spectrum of the GO/Zn (OH)x samples with different Zn (OH)x concentrations: (A) (0 wt. %), (B) (25 wt.%), (C) (50 wt.%), and (D) (75 wt.%).