Nature-inspired Z-scheme SnO2@ZnO n–n heterostructure photocatalyst for reactive oxygen species-mediated degradation of emerging contaminants in wastewater

Abstract

Industrial activities have intensified the discharge of persistent organic pollutants into aquatic systems, compromising water quality and the environment. In this study, a green hydrothermal synthetic approach is employed to fabricate a Z-scheme SnO₂@ZnO n–n type heterojunction nanocomposite using D. cristata leaf extract as a biogenic redox and stabilizing agent. Comprehensive characterization by XRD, FT-IR, UV-vis. DRS, PL, EIS, SEM-EDX, TEM-SAED, and BET confirmed the successful formation of the heterostructure. Under visible light and H₂O₂ assistance, the photocatalyst achieved 97.63 ± 1.82% degradation of ibuprofen (IBU) and 96.35 ± 1.59% of 2,4,6-trinitrophenol (TNP) within 30 min, following pseudo-first-order kinetics with rate constants (k) of 0.09391 and 0.09272 min⁻¹, and quantum yields of 5.63 × 10⁻² and 6.18 × 10⁻² molecules photon⁻¹. ESR and radical quenching studies confirmed a direct Z-scheme charge transfer mechanism, promoting efficient separation of photogenerated carriers and reactive oxygen species (ROS)-mediated degradation. Subsequent liquid chromatography-mass spectrometry (LC-MS), chemical oxygen demand (COD), and total organic carbon (TOC) analyses confirmed extensive mineralization (>92%) of the target pollutants into environmentally benign products such as CO₂, H₂O, and simpler mineral acids, signifying near-complete detoxification. The catalyst retained high activity over five reuse cycles with minimal leaching and no observed ecotoxicity, demonstrating that SnO₂@ZnO is an efficient, durable, and environmentally benign photocatalyst for practical wastewater treatment applications.