# Synergistic Effect of Nonthermal Plasma and ZnO Nanoparticles on Organic Dye Degradation
## 4 Catalysis Performance  
The catalytic performance of atmospheric argon plasma W/O ZnO NPs has been evaluated from the MB absorption spectra. The absorbance spectra of the MB dye solution treated with Ar plasma W/O ZnO NPs at different treatment-time values are presented in Figure 5a-c. As evident, the absorption spectra of pristine MB dye solutions (Figure 5a) have a characteristic absorption peak at 663 nm associated with an MB monomer, and a shoulder at a lower wavenumber of 616 nm assigned to the MB dimer. The intensity of the peak at 663 nm is proportional to the concentration of the undissociated MB dye molecules in the solution sample. After exposure to the Ar plasma W/O ZnO NPs, the overall MB absorption intensity decreases, as shown in Figure 5a-c. In particular, the peak intensity at 663 nm dramatically decreases, implying the degradation of the chromophore group in MB molecules [50]. The wavelength at which the MB absorption peak is located (663 nm) is related to the length of the conjugated systems in the MB molecule [51]. Figure 5d shows the position of the MB absorption peak as a function of treatment time for the Ar plasma W/O ZnO NPs' treatment processes as it clears the peak shifts to a lower value (blue shift), particularly during the Ar plasma \"alone\" treatment. This behavior is attributed to the destroying of the conjugated system of MB and the formation of by-products such as Azure and Thionine [52] through the demethylation/degradation of MB during the treatment process [53]. The pH value of the pristine MB was slightly acidic at 5.4 and turned to be neutral with a pH of 6, 6.33, and 6.5 for the samples treated with Ar plasma and Ar plasma combined with 0.2 and 0.4 gL\({}^{-1}\) ZnO, respectively. Knowing that, plasma increased the acidity of the exposure DI water [54]; the increases in the pH values of plasma-treated MB solutions imply physiochemical changes in the treated solution, which is consistent with the shift observed in the MB absorption peak. Figure 4: (a) UV-visible optical absorption, (b) plots of \((h\
u\alpha)^{2}\) versus h\(\
u\) of ZnO nanoparticles. To quantify the photocatalytic performance of Ar plasma W/O ZnO NPs, the degradation efficiency was calculated from Equation (1), and the results are depicted in Figure 6. The rate of MB degradation was found to depend on the presence of ZnO NPs as well as the concentration of ZnO NPs. For example, the degradation efficiency after 60 min of treatment was found to be 86% (plasma plasma), 95% (plasma combined with 0.2 gL\({}^{-1}\) of ZnO), and 74% (plasma combined with 0.4 gL\({}^{-1}\) of ZnO). Figure 5: Absorption spectra of MB dye (**a**) sole plasma exposure, (**b**) plasma + 0.2 gL\({}^{-1}\) ZnO, (**c**) plasma + 0.4 gL\({}^{-1}\) ZnO solution mixture, and (**d**) the position of the MB absorption peaks as a function of exposure time. Figure 6: The degradation efficiency of MB dye as a function of exposure time for (a) plasma “alone” exposure, (b) plasma + 0.2 gL\({}^{-1}\) ZnO, and (c) plasma + 0.4 gL\({}^{-1}\) ZnO solution mixture. The mechanism of MB degradation is illustrated in Figure 7 and can be explained as follows. Considering the plasma emission and the subsequent chemical reactions taking place at the Ar plasma\(-\)MB solution interface, the OES measurements (Figure 1) confirm the existences of OH-, N\({}_{2}\)-, and OI-reactive radicals. In addition, the emitted Ar I lines imply the presence of high-energy electrons >> 11 eV and the Ar metastable A\({}^{*}\) species. The dissociation energy of \(H_{2}O\) molecules into \(OH\), \(H_{2}\), and \(O\): are 5.0-5.1 eV [32; 33]. Hence, one would expect various interaction pathways between plasma active species and the surrounding air (gas phase), as well as at the plasma-water interface. Some examples of possible reaction pathways [55; 56] are listed in Equation (2)  
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