Photocatalytic degradation of malachite green over a Fe3O4/ZnO S-scheme heterojunction: experimental and DFT insights

Abstract

Using a combination of computational and experimental methods, a magnetic Fe3O4/ZnO S-scheme heterostructure was established for the effective photocatalytic degradation of malachite green (MG). A mesoporous heterostructure with improved light absorption, effective charge separation, and outstanding magnetic recoverability was confirmed by the thorough characterization of the catalyst's structural, optical, magnetic, and electrochemical properties. By adjusting the irradiation source, catalyst dosage, pH, and coexisting ions, the photocatalytic performance was maximized. The catalyst reached 96.58 ± 0.34% degradation with a pseudo-first-order rate constant of 0.0322 ± 0.0056 min−1 under ideal conditions of 30 mg L−1 catalyst and 200 mg L−1 MG at pH 3 under UV light. While VSM analysis showed an adequate magnetic response for easy catalyst separation, electrochemical investigations also indicated enhanced interfacial charge transfer. Density functional theory (DFT) simulations at the B3LYP/6-311G(d) level indicated an energy gap of 1.5259 eV for MG and identified aromatic rings and electron-rich nitrogen atoms as the preferred locations for oxidative attack, offering molecular-level insights into the degradation mechanism. A structure–reactivity link between MG and Fe3O4/ZnO was revealed by the combined experimental and theoretical data. This work offers a potential approach to sustainable wastewater cleanup by integrating DFT analysis with a recyclable Fe3O4/ZnO S-scheme heterostructure.