# Synergistic Effect of Nonthermal Plasma and ZnO Nanoparticles on Organic Dye Degradation
## 3 Results and Discussion
### Characterization of the ZnO Nanoparticles  
Figure 2a shows SEM images of the synthesized ZnO nanostructures at a growth temperature of 160 \({}^{\circ}\)C, a pH of 10, and a synthesis time of 5 h. The SEM images show that the ZnO nanostructures have a homogeneous shape (NPs) with a diameter in the range of 30-50 nm. Furthermore, the ZnO NPs agglomerate and form nano crystallites, as shown in Figure 2a, which is like that reported in the literature [34]. Figure 2b depicts the XRD pattern of the synthesized ZnO (NPs). As observed, the pattern exhibits sharp peaks with high intensities, confirming the high crystallinity of the studied sample. The observed peaks are located at positions 20 = 31.72\({}^{\circ}\), 33.66\({}^{\circ}\), 36.21\({}^{\circ}\), 47.48\({}^{\circ}\), 56.51\({}^{\circ}\), 62.75\({}^{\circ}\), and 67.84\({}^{\circ}\). The observed peaks agree well with the typical hexagonal wurtzite crystalline ZnO with space group _P63 mc_ (Card No. 96-900-4180). Therefore, the diffracted peaks were labeled 100, 002, 101, 102, 110, 103, and 112. The XRD pattern of the synthesized ZnO NPs, shown in Figure 2b, displays other additional peaks at 20-28.6\({}^{\circ}\), 30.9\({}^{\circ}\), 32.6\({}^{\circ}\), 42.7\({}^{\circ}\), and 50.6\({}^{\circ}\), which were assigned to Zinc oxyhydrooxides Zn (OH)\({}_{2}\) (Card No. 96-451-7838). In other words, the synthesized ZnO NPs are composed of Zincite 99% and Zinc hydroxide (1%). The grain size \(D\) was estimated using Scherrer formula \(D=\frac{0.9\cdot\lambda}{\beta\cdot\cos\theta_{B}}\), where \(\lambda\), \(\beta\), and \(\theta_{B}\) are the X-ray wavelength (\(\lambda\) = 1.543 A) and the full width at half-maximum (FWHM) of the diffraction peak and the diffraction angle, respectively [32]. The strongest reflecting peak, 002, was selected to estimate the average size of ZnO. The crystallite size was about 35.05 nm, which slightly matches that observed from the SEM image. The lattice parameters \(a\) and \(c\) of the hexagonal (wurtzite) were calculated using \(a=\frac{\lambda}{1.73\cdot\sin\theta_{100}}\) and \(c=\frac{\lambda}{\sin\theta_{002}}\), respectively [35]. The lattice parameters \(a\) and \(c\) were found to be 3.2533 A and 5.2073 A, respectively, which are comparable to the parameters of pure ZnO (\(a\) = 3.24 A, \(c\) = 5.206 A). Figure 1: Typical emission spectra of the atmospheric-pressure plasma jet at Ar flow rate of 3 L/min. Figure 2c displays the Raman modes of a ZnO nanoparticle using a green laser \(\lambda\) = 532 nm at room temperature. One can observe six major peaks, located at 98.5, 210, 333, 393, 438.5, 538, and 577.5 cm\({}^{-1}\), that were assigned to modes E\({}_{2}\)(low), 2E\({}_{2}\)(low), E\({}_{2}\)(high)-E\({}_{2}\) (low), A\({}_{1}\)(TO), E\({}_{2}\)(high), A\({}_{1}\)(LO), and E\({}_{1}\)(LO), respectively [36]. The high-intensity E\({}_{2}\)(low) and E\({}_{2}\)(high) modes reflect the hexagonal wurtzite structure of synthesized ZnO nanoparticles [37]. The 2E\({}_{2}\)(low) is the second order of E\({}_{2}\)(low mode). The mode E\({}_{2}\)(high), which was assigned to the oxygen vibration, is a fingerprint of the degree of crystallinity of ZnO nanoparticles, i.e., a sharp high-intensity peak indicates the high-crystalline nature of the synthesized ZnO NPs, which is consistent with the XRD result. The peak E\({}_{1}\)(LO) indicates the presence of oxygen-vacancy defects [38], implying proper improvements in the ZnO NPs' photocatalysis performance. The A\({}_{1}\) (TO) and A\({}_{1}\)(LO) modes are first-order optical modes of wurtzite ZnO. The mode E\({}_{2}\)(high)-E\({}_{2}\)(low) is a second feature of the ZnO structure. The Raman mode at \(\sim\)160 cm\({}^{-1}\) is probably attributed to Zn (OH)\({}_{2}\)[39]. Figure 3a shows the XPS survey spectrum of the synthesized ZnO NPs. The apparent high-intensity Zn2p3\({}_{2}\), Zn2p1\({}_{1/2}\), and O1s peaks confirm the chemical composition of the synthesized ZnO NPs, i.e., 43.8% Zn, 44.65% oxygen, and 11% carbon.