Engineered bismuth oxyhalides/g-C₃N₄ S-scheme heterojunction for tetracycline photodegradation: An experimental and DFT study

Heterojunction construction and band gap modulation through solid-solution fabrication are effective strategies for enhancing photocatalytic performance. Herein, graphitic carbon nitride (g-C₃N₄) nanosheets (NSCNs) with a remarkably high specific surface area (323.2 m² g⁻¹) were synthesized and coupled with compositionally engineered bismuth oxyhalides (BiOCl_xBr_(1-x), x = 0, 0.2, 0.5, 0.8, and 1.0) to construct a novel photocatalyst with a proposed step-scheme (S-scheme) charge-transfer pathway. Among the prepared nanocomposites, NSCN/BiOCl_0.5Br_0.5 exhibited excellent photocatalytic activity, achieving 94% tetracycline degradation within 75 min under simulated solar irradiation, outperforming NSCN/BiOCl (67%) and NSCN/BiOBr (88%). This enhanced performance is attributed to the optimized balance between light-harvesting capability and redox potential achieved through Cl/Br ratio tuning. Radical scavenging experiments identified photogenerated holes (h⁺) as the dominant active species in tetracycline photodegradation. Density functional theory (DFT) calculations predicted charge redistribution at the NSCN/BiOCl_0.5Br_0.5 interface consistent with an internal electric field, supporting the proposed S-scheme charge-transfer mechanism. These findings highlight the importance of interfacial charge redistribution and compositional optimization in developing efficient photocatalysts for environmental remediation.