Photocatalysis over CdS quantum dots/oxygen doped g-C3N4 Z-scheme heterojunction: Revealing the synergistic effects of quantum confinement effect, defect engineering, and Z-scheme mechanism

Polymeric carbon nitride (g-C₃N₄), as an emerging visible-light-responsive semiconductor photocatalyst, has attracted considerable research interest in solar energy conversion technologies. Nevertheless, its practical implementation faces critical challenges including restricted light-harvesting capacity and inefficient charge carrier dynamics, which substantially compromise photocatalytic efficiency. To address these limitations, we propose a novel Z-scheme heterojunction (denoted as OCN/CdS) by strategically integrating cadmium sulfide quantum dots (CdS QDs) with defect-engineered oxygen-doped g-C₃N₄ nanosheets (OCN). The designed architecture integrates quantum confinement effects from CdS QDs, tailored defect states in OCN, and interfacial electric field enhancement through Z-scheme charge transfer pathways. The optimized OCN/CdS-2 heterojunction exhibits enhanced photocatalytic activity, achieving a tetracycline degradation rate (k_app) of 0.031 min⁻¹ and a hydrogen evolution rate of 5270 μmol·g⁻¹·h⁻¹. These values correspond to 3.4-fold and 21-fold improvements compared to pristine g-C₃N₄, respectively. Mechanistic investigations reveal that the OCN/CdS-2 Z-scheme heterojunction simultaneously maximizes active site exposure and establishes robust built-in electric fields at interfaces, which collectively suppress charge recombination while accelerating redox reaction kinetics. A strategic framework is proposed in this work for constructing enhanced-performance g-C₃N₄-based Z-scheme heterojunctions through multidimensional structural engineering strategies.