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% (k₁ = 0.018 min⁻¹) and 96% (k₁ = 0.048 min⁻¹), 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⁻¹ 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 O₂•- 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.