Rational design of a cascade CdS/C₃N₄/COF heterostructure for high-performance Cr(VI) photoreduction

Photocatalytic reduction of highly toxic Cr(VI) is fundamentally limited by inefficient charge separation and nonselective carrier recombination in conventional heterojunctions. Here we introduce a cascade CdS/C₃N₄/COF architecture in which charge recombination is deliberately engineered rather than suppressed. As inferred from bond structure analysis and PL spectroscopy, a triazine-based nanoporous COF is employed as an electronic mediator that facilitates preferential recombination of low-energy charge carriers, while preserving high-energy electrons in g-C₃N₄ for reductive reactions. Structural and electrochemical analyses reveal strong interfacial coupling and Fermi-level equilibration across the ternary interfaces, giving rise to a COF-mediated S-scheme charge-transfer pathway. Under visible-light irradiation, the optimized heterostructure achieves rapid and efficient Cr(VI) removal (≈92% within 90 min at pH 3), markedly exceeding the performance of binary analogues. Kinetic and scavenger studies confirm that the reaction is governed by direct electron transfer rather than adsorption or radical-driven pathways. This work establishes charge-preferential recombination as a powerful design principle for constructing high-performance photocatalysts and highlights the unique role of covalent organic frameworks as programmable electronic mediators in cascade heterostructures.