Constructing biomass-derived carbon quantum dots modulated S-scheme TiO2 heterojunctions: synergistic effect for enhanced photocatalytic degradation of basic fuchsin dye

Basic Fuchsin (BF), a recalcitrant organic dye, poses severe risks to aquatic ecosystems and human health, necessitating the development of efficient and stable environmental remediation technologies. Herein, a novel direct S-scheme heterojunction photocatalyst (MG-CQDs/TiO 2 ) was prepared by impregnating and calcining TiO 2 with mango peel-derived carbon quantum dots (MG-CQDs). The successful construction of the S-scheme charge-transfer heterostructure was systematically elucidated through band structure analysis and electron paramagnetic resonance (EPR) spectroscopy. Under ultraviolet–visible (UV–Vis) light irradiation, the MG-CQDs/TiO 2 composite exhibited exceptional photocatalytic performance, achieving a remarkable BF degradation efficiency of 96.5% within 180 min. Notably, its reaction rate constant was 2.1 times higher than that of pristine TiO 2 . Mechanistically, the synergistic interaction between MG-CQDs and TiO 2 significantly promotes the spatial separation of photogenerated carriers. Radical trapping experiments further confirmed that superoxide (·O 2 − ) and hydroxyl (·OH) radicals serve as the primary reactive oxygen species driving the oxidation process. Crucially, density functional theory (DFT) calculations combined with Fukui index analyses provided molecular-level insights into the degradation pathway, identifying the nitrogen atoms and the central carbon of the BF molecule as the preferential sites for radical attack. Furthermore, the composite demonstrated outstanding photostability, maintaining highly efficient degradation over multiple consecutive cycles. Supported by both macroscopic experimental evidence and microscopic theoretical calculations, this study presents a highly efficient, cost-effective, and green paradigm for the advanced treatment of industrial dye wastewater. Graphical abstract