Green‐Synthesized Carbon Dot–Decorated ZnO/SnO ₂ Heterojunctions for Highly Efficient Solar Photocatalytic Removal of Malachite Green

Dyes are among the most persistent organic pollutants in industrial wastewater owing to their complex aromatic structures and high resistance to biodegradation. In this study, carbon dot–modified ZnO–SnO ₂ nanocomposites were synthesized as efficient sunlight‐active photocatalysts for the degradation of malachite green dye. Carbon dots were prepared via a microwave‐assisted green synthesis using Moringa oleifera leaf extract and subsequently integrated with ZnO–SnO ₂ nanoparticles. The structural, optical, textural, and surface properties of the prepared materials were comprehensively characterized using UV–vis spectroscopy, zeta potential analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDX), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques. Using response surface methodology (RSM), the effects of dye concentration, photocatalyst dosage (mg/L), and irradiation time (min) were systematically investigated and optimized. The results revealed that the highest degradation efficiency was achieved under a dye concentration of 10 mg/L, photocatalyst dose of 110 mg, and an irradiation time of 100 min under natural sunlight. At these optimal conditions, the ZnO–SnO ₂ –carbon dot nanocomposite achieved a maximum photocatalytic degradation efficiency of approximately 97%, as predicted by the RSM model, with experimental results reaching up to 99% removal.