Green synthesis of carbon quantum dots from stale soy milk composited zinc oxide (ZnO/CQDs) for photodegradation of malachite green

Visible light-driven photocatalysts are widely investigated to produce high removal efficiency in removing organic pollutants. Carbon quantum dots (CQDs) are a plausible candidate for enhancing photocatalytic activity and play an essential role in malachite green (MG) degradation. Biomass waste, stale soy milk, contains lactic acid, which is utilized as a carbon precursor to prepare CQDs. ZnO photocatalysts were composited with CQDs derived from stale soy milk by green synthesis for the first time. The presence of CQDs and their effect on morphology, surface area, decrease in band gap energy, and reduced electron-hole recombination were confirmed by FESEM, TEM, BET, Uv-Vis DRS, and PL spectroscopy characterization. The photocatalytic activity of ZnO/CQDs in the MG degradation was confirmed, and reached 84% degradation after 90 min irradiation with a reaction rate constant of 0.01137 k/min-1. Furthermore, the reusability study after four reaction cycles revealed that ZnO/CQDs were stable, and scavenger tests were performed to identify the active sites. As a result, we believe that CQDs from stale soy milk composited with ZnO is an excellent photocatalyst candidate for removing organic pollutants.