N, S co-doped graphene quantum dot–decorated Fe₃O₄ nanostructures: Preparation, characterization and catalytic activity

The sonocatalytic decolorization of the dye methylene blue was considered in the present study. In this regard, as a novel approach, Fe₃O₄ nanoparticles synthesized through a coprecipitation method were decorated with nitrogen and sulfur co-doped graphene quantum dots (NS-GQDs) and used as a sonocatalyst. The use of NS-GQD-decorated/Fe₃O₄ (NS-GQD/Fe₃O₄) nanoparticles significantly increased the decolorization efficiency (99.0%) compared with the use of pure Fe₃O₄ nanoparticles (43.7%). The band gap of the nanostructured NS-GQD/Fe₃O₄ (1.923 eV) was lower than that of pure Fe₃O₄ nanoparticles (2.147 eV). An optimal dopant amount of 3 wt% was used in the experiments. The highest sonocatalytic decolorization of 99.0% was attained at an initial pH of 8. As a result, the adverse effect of scavenging compounds on the sonocatalytic decolorization was in the following order: sulfate < carbonate < chloride. In the case of enhancers, the presence of potassium persulfate and hydrogen peroxide resulted in complete decolorization in a reaction time of 60 min. A drop in the decolorization efficiency of only 4% was observed after the fifth consecutive cycle, thereby implying the appropriate reusability potential of the NS-GQD/Fe₃O₄ sonocatalyst for use in full-scale operation. Gas chromatography–mass spectrometry analysis was used for the identification of intermediate by-products generated during the sonocatalytic decolorization of methylene blue.