Nyctanthes arbor-tristis L. mediated sustainable synthesis of α-Fe2O3/g-C3N4 S-scheme heterojunctions for enhanced photocatalytic degradation of tetracycline hydrochloride: a mechanistic insight and DFT study

Abstract

The development of efficient and sustainable photocatalytic systems is critical for addressing emerging pollutants in wastewater. In this study, we report the green synthesis of a novel S-scheme α-Fe₂O₃/g-C₃N₄ heterojunction, engineered to enhance the visible-light-driven photodegradation of tetracycline hydrochloride (TCH). The composite was fabricated using an eco-friendly route involving Nyctanthes arbor-tristis L. leaf extract, ensuring a minimal environmental footprint. Structural and chemical characterization studies via XRD and FTIR confirmed the presence of distinct crystalline phases and key functional groups, including Fe–O and CN bonds. Morphological analysis using FESEM and HRTEM revealed a well-defined interfacial architecture, while XPS confirmed the presence of Fe³⁺, C, and N elements, validating successful heterojunction formation. The heterostructure exhibited a BET surface area of 21 m² g⁻¹ and a narrowed optical band gap of 2.2 eV, using UV-Vis DRS. Photoluminescence (PL) spectroscopy demonstrated significantly reduced recombination of photogenerated electron–hole pairs, indicating efficient charge separation. Under visible-light irradiation, the α-Fe₂O₃/g-C₃N₄ photocatalyst achieved an impressive 93% degradation of TCH within 60 minutes, significantly outperforming pristine g-C₃N₄(32%) and α-Fe₂O₃(43%). The enhanced photocatalytic activity is attributed to a synergistic S-scheme charge transfer pathway that promotes redox potential and suppresses charge recombination. A detailed parametric study was conducted to evaluate the effects of catalyst dosage, initial TCH concentration, heterojunction ratio, and exposure duration. DFT calculations explained the geometrically favorable crystal and band structures. Superoxide radicals were identified as major reactive species from scavenging studies. A plausible mechanism for TCH degradation was established based on LCMS analysis.