Combined effects of direct plasma exposure and pre-plasma functionalized metal-doped graphene oxide nanoparticles on wastewater dye degradation

The current study investigates the combinatorial effect of the photocatalytic performance of plasma pretreated Ti-Cu-Zn doped graphene oxide (TCZ-GO) nanoparticles (NPs) and advanced oxidation processes of a non-thermal atmospheric pressure plasma on the degradation of reactive orange-122 (RO-122) dye compounds. Firstly, in order to enhance the photocatalytic performance of the synthesized composite NPs, they were subjected to glow discharge plasma treatments operating in different gases (Ar, air, O-2 and N-2). Their surface morphology, chemical composition and band gap were examined by means of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis spectrophotometry respectively. XPS results revealed that plasma-treated NPs exhibited a higher content of oxygen vacancies and a variation in their oxidation states (Ti4+ -> Ti3+, Cu+ -> Cu2+). These plasma-induced surface chemical changes hindered the recombination of photo-generated electron-hole pairs which led to a drop in the bandgap of the NPs with N-2 plasma-treated NPs acquiring the lowest bandgap. Lastly, the article examined the actual decomposition of RO-122 dye in wastewater by an Ar plasma treatment alone or combined with the plasma-treated TCZ-GO NPs via spectrophotometric analyses, electrical conductivity, pH and total organic carbon (TOC) removal measurements. Moreover, the reactive species produced during the combined plasma/photocatalysis induced degradation were detected in situ by optical emission spectroscopy. Results revealed that the processes carried out by combining N-2 plasma-treated TCZ-GO NPs and Ar plasma exhibited the highest degradation efficiency (85 %) due to the generation of more OH center dot and H2O2. Overall, it can be concluded that plasma-aided treatment processes used synergistically as indirect surface functionalization of TCZ-GO NPs and direct plasma treatment of wastewater are extremely efficient in the degradation of toxic compounds and can be extrapolated to various environmental applications. (c) 2023 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.