# Synergistic Effect of Nonthermal Plasma and ZnO Nanoparticles on Organic Dye Degradation
## 4 Catalysis Performance
\begin{split} e^{-}+O_{2}\overset{yields}{\rightarrow}O+O+e^{- }\\ e^{-}+O_{2}\overset{yields}{\rightarrow}O_{2}^{*}+e^{-}\\ O_{2}^{*}+O_{2}\overset{yields}{\rightarrow}O_{3}+e^{-}\\ e^{-}+H_{2}O\overset{yields}{\rightarrow}H+OH^{+}+e^{-}\\ Ar^{*}+H_{2}O\overset{yields}{\rightarrow}Ar+OH^{+}+2e^{-}\\ OH+OH\overset{yields}{\rightarrow}H_{2}O_{2}\end{split}  \tag{2}\]  
The generated species moved with the Ar flow to the plasma-liquid interface, diffused in the MB solution, and played crucial roles in the degradation of the MB dye molecules. The oxidation potential of the \(OH\) (Hydroxyl radical), \(O\), \(O_{3}\) (Ozone), and \(H_{2}O_{2}\) (Hydrogen peroxide) species are 2.8, 2.42, 2.07, and 1.77, respectively. Hence, \(OH\) was found to be the main oxidative species in the plasma \"alone\" water treatment [57]. For instance, the \(OH\) radicals in the MB aqueous solutions absorbed electrons from the double bonds of methylene blue molecules, leading to the degradation of the MB solution [58]. Further, the high-energy electrons interacted with MB molecules, removing the methyl group (\(CH_{3}\)) via demethylation [59]. As shown, the availability of \(OH\) radicals determine the efficiency of the MB degradation process. Hence the addition of \(OH\) source radicles combined with that generated with plasma would improve the degradation of MB dye. Indeed, the MB degradation improved in the presence of the 0.2 gL\({}^{-1}\) ZnO photocatalyst in the MB solution, as shown in Figure 6. As depicted in Figure 1, the plasma emission contains UV photons << 400 nm), which have enough energy to activate ZnO photocatalysts; electrons (\(e^{-}\)) move from the valence band (VB) to the conduction band (CB) and leave holes \(h^{+}\) in the (VB) of ZnO NPs. Figure 7: Schematic diagram of the effects of plasma and ZnO NP photocatalysis reaction pathways for MB degradation. Both \(e^{-}\) and \(h^{+}\) migrate to the ZnO NPs' surface. The holes' \(h^{+}\) oxides \(H_{2}O\) and/or OH' molecules are adsorbed on the ZnO NPs' surface and produce reactive hydroxyl radical anions (\(OH\)) and H\({}^{+}\) ions. Meanwhile, electrons reduced to oxygen molecules \(O_{2}\) exist on the photocatalyst surface and generate super-oxide anions (\(\cdot\)\(O_{2}^{-}\)) [60; 61; 62; 63]. The \(O_{2}^{-}\) might interact with H\({}^{+}\) in the presence of H\({}_{2}\)O and generate H\({}_{2}\)O\({}_{2}\). In addition, oxygen vacancies in the ZnO NPs (Figure 3b) act as electron \(e^{-}\) donors that promote further generation of \(OH\). The reactive \(OH\)- and \(O_{2}^{-}\) oxidize the MB dye molecules into de-colorized unharmful species. The possible reaction pathways for MB degradation in the presence of ZnO NPs under plasma irradiation are summarized in the following equations. \[ \begin{split} ZnO+plasma\ emission\ \ \stackrel{{ yields}}{{\rightarrow}} ZnO\ (e^{-}+h^{+})\\ h^{+}+H_{2}O\ \stackrel{{ yields}}{{\rightarrow}} OH^{+}+\\ h^{+}+OH^{-}\ \stackrel{{ yields}}{{\rightarrow}} OH^{-}\\ e^{-}+O_{2}\ \stackrel{{ yields}}{{\rightarrow}} O_{2}^{-}\\ O_{2}^{-}+H_{2}O+H^{+}\ \stackrel{{ yields}}{{ \rightarrow}} H_{2}O_{2}+O_{2}\\ O_{2}^{-}+MB\ molecule\ \ \stackrel{{ yields}}{{ \rightarrow}} MB\ molecule-OO\\ OH^{+}+MB\ molecule\ \ \stackrel{{ yields}}{{ \rightarrow}} oxidized\ products\end{split}  \tag{3}\]  
Surprisingly, when the amount of the photocatalyst (ZnO NPs) increased from 0.2 to 0.4 gL\({}^{-1}\), the MB degradation efficiency slightly decreased, as shown in Figure 5. The increase in ZnO NPs might induce turbidity in the MB solution, which would decrease the number of UV photons reaching the ZnO NPs' surface, leading to less generation of \(e^{-}\)--\(h^{+}\) pairs and, hence, lower \(OH\) radicals generated and a decrease in the degradation efficiency. Alternatively, the synthesized ZnO NPs contain some ZnO (OH)\({}_{2}\), i.e., an OH group, attached to the ZNO's surface.