# Synergistic Effect of Nonthermal Plasma and ZnO Nanoparticles on Organic Dye Degradation
## 3 Results and Discussion
### Characterization of the ZnO Nanoparticles
The carbon content is probably contaminated during the synthesis processes. The XPS Zinc spectral lines were assigned to the Zn 3D, Zn 3p, Zn 3 s, Zn LMM (Auger), and Zn 2p peaks. The oxygen line  
Figure 2: (**a**) SEM images, (**b**) X-ray diffraction pattern, and (**c**) Raman spectra of ZnO nanoparticles. O 1s at 530 eV was attributed to O\({}^{2}\)- ions in the wurtzite ZnO structure, while Zinc spectral lines were assigned to the Zn 3D, Zn 3p, Zn 3 s, Zn LMM (Auger), and Zn 2p peaks [40]. To reveal the oxidation state of the Zn and O components of ZnO NPs, Figure 3b,c provide a high-resolution spectra of the O 1s and Zn2p spectral lines. The O 1s spectrum is asymmetric with a shoulder at higher binding energy, and it is decomposed into two peaks; at the lower B.E. of 530.1 eV, it is ascribed to oxygen anions O\({}^{2}\)- in the wurtzite ZnO crystal lattice (Zn-O), while at a higher B.E. of 531.5 eV, it represents oxygen vacancies (V\({}_{\rm o}\)) [41; 42; 43] and/or OH species adsorbed on the surface of ZnO [44; 45]. In fact, the XRD (Figure 2b) and Raman (Figure 2c) results support the presence of the oxygen vacancies and the OH species on the synthesized ZnO NPs. Figure 3c shows the Zn2p spectra splitting into peaks at 1021.86 eV and 1044.98 eV, which can be indexed to Zn2p\({}_{3/2}\) and Zn2p\({}_{1/2}\), respectively. The Zn2p spectra corresponds to the Zn atom at the regular lattice site in ZnO. Here, the binding energy separation between the Zn2p\({}_{3/2}\) and Zn2p\({}_{1/2}\) peaks was found to be 23.06 eV, which is consistent with the emissions from Zn\({}^{2}\) ions in the ZnO crystal. Figure 4a displays the UV-Vis absorption spectrum of ZnO NPs. The spectrum exhibits strong absorption in the UV range \(<\) 400 nm, with the absorption peak at 366 nm assigned to the intrinsic band-gap absorption of ZnO [46]. The absorption peak at 366 nm is a characteristic of the hexagonal wurtzite ZnO structure. Further, ZnO NPs' UV-Vis spectrum reveals a blue shift compared with defect-free ZnO (368 nm) [47; 48], probably due to the quantum size effect. Figure 3: (**a**) Survey spectra ZnO nanoparticles, (**b**) high-resolution spectra for oxygen O1s, and (**c**) high-resolution spectra for Zn 2p. The optical direct band gap energy, \(E_{\rm g}\), for the synthesized ZnO NPs was estimated using Tauc law [49],  
\[h\
u\cdot\alpha=A\cdot\left(h\
u-E_{g}\right)^{n}\]  
where \(\alpha\) is the absorption coefficient, \(h\
u\) is the photon energy, \(A\) is constant, and \(n=1/2\) for the direct band gap. The \(E_{\rm g}\) value was obtained via an extrapolation of the linear part of the \((h\
u\alpha)^{2}\) versus \(h\
u\) to \(h\
u\alpha=0\). The value of \(\alpha\) was computed from the absorption spectrum. As seen in Figure 4b, the \(E_{\rm g}\) value was 3.17 eV, which is smaller than the defect-free bulk ZnO (\(E_{\rm g}=3.3\) eV) [50]. The decrease in the \(E_{\rm g}\) value is probably attributed to the formation of a shallow level within the band gap due to the presence of some defects in ZnO NPs, such as the oxygen vacancies as observed in the O 1s spectrum of the XPS results, as shown in Figure 3b, and the attached hydroxyl as observed in the XRD and Raman results.