Solid Phase Microwave Assisted Synthesis N,S-GQDs as Nano-photocatalyst for Efficient Degradation of Nitrofurantoin and Methylene Blue under Visible Light: Comparative Studies, Mechanism Insight, and Extended Scope.

Metal-free nanophotocatalysts generating radical species through indirect association of visible light have gained importance in remediating a variety of pollutants by obeying the principles of "green and sustainable". Herein, four different doped and undoped graphene quantum dots were synthesized using a novel solid phase microwave assisted (SPMW) strategy, namely, graphene quantum dots (GQDs), nitrogen-doped GQDs (N-GQDs), sulfur-doped GQDs (S-GQDs), and nitrogen and sulfur co-doped GQDs (N,S-GQDs). The as-prepared N,S-GQDs with uniform size distribution (∼4.5 nm) and demonstrating excellent visible light entrapment ability resulting from separated electron-hole pairs (e-h) were used for degrading environmentally benign pollutants nitrofurnatoin (NFR) and methylene blue (MB). Comparative assessment revealed N,S-GQDs to have the highest degradation efficiency for both NFR and MB, ∼95% = 0.018 min and 96% = 0.048 min, respectively, within 120 min exposure and without aid of external oxidants. Furthermore, the extended scope of N,S-GQDs prompted a similar trend in degradation of tetracycline (TC) and malachite green (MG) under established optimal conditions. In addition, the designed N,S-GQDs showed excellent recyclability up to five consecutive cycles with catalyst loading only 0.01 mg mL and without any compositional loss over time. The mechanistic study modeled using radical trapping experimentation, lifetime analysis, and fragment mass analysis revealed the indirect association of OH and O created from photogenerated charge carriers have triggered the degradation. Our results potentiate the cost-effective and cheaper preparation of N,S-GQDs using SPMW and open newer avenues for its photocatalysts based applications that are intended for remediating organic pollutants.