Dual S-Scheme Charge Transfer Pathway in g-C ₃ N ₄ Quantum Dots/HKUST-1/TiO ₂ Ternary Heterojunctions for Photocatalytic Pollutant Degradation

Constructing dual S-scheme heterojunctions is an innovative strategy to boost photocatalysis by promoting photoinduced charge migration and retaining high redox potentials. Herein, we developed a facile green method to prepare g-C₃N₄ quantum dots (QDs)/HKUST-1/TiO₂ dual S-scheme heterojunctions via depositing g-C₃N₄ QDs <i>in situ</i> on spherical HKUST-1/TiO₂. 15% g-C₃N₄ QDs/HKUST-1/TiO₂ exhibited a significantly enhanced tetracycline (TC) degradation performance (97.02% degradation rate, 0.02177 min^-1 reaction rate constant (<i>k</i>_app)) compared to g-C₃N₄ QDs and HKUST-1/TiO₂, attributed to the enlarged specific surface area and accelerated charge separation (verified by experiments). It also showed excellent photostability. ESR confirmed ^•O₂^-, ^•OH, and h⁺ as key active species. LC-MS and online FTIR spectroscopy analyzed TC degradation intermediates, pathways, and toxicity. Finally, experimental combined with theoretical calculation results further elucidated the possible dual S-scheme mechanism. The dual S-scheme mechanism endows this system with a relatively high redox capacity, fully utilizing the reducing ability of electrons on the conduction band of g-C₃N₄ QDs and the oxidizing ability of holes on the valence band of TiO₂. This work is contributory to designing novel dual S-scheme photocatalysts and efficient purification of wastewater in environmental remediation.