Sol–Gel Synthesis of SnO ₂ /Zn ₂ –SnO ₄ Heterostructure for the Photocatalytic Degradation of Malachite Green

The discharge of contaminated effluents containing organic dyes is a persistent environmental issue due to their toxicity, stability, and resistance to biodegradation. Among these dyes, malachite green (MG) is particularly concerning because of its widespread industrial use and severe ecological and health impacts. To address this problem, SnO₂/Zn₂-SnO₄ heterostructures were synthesized via sol-gel and evaluated for MG degradation under ultraviolet irradiation. The SnO₂/Zn₂-SnO₄ catalyst has been characterized by X-ray diffraction (XRD), photoluminescence (PL), field-emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FE-SEM-EDS), ultraviolet-visible (UV-vis) spectroscopy, Brunauer-Emmett-Teller method, and electrochemical impedance spectroscopy (EIS). Structural properties revealed that the catalyst presents a polycrystalline structure and crystallite sizes in the range of 11-25 nm. The band gap energy of the Sn:Zn (1:2) catalyst was 2.88 eV, which was lower than that of pure SnO₂ (3.33 eV), indicating enhanced light absorption. MG photocatalysis degradation tests were conducted under ultraviolet irradiation. The ZnSn 1:2 sample achieved a degradation efficiency of approximately 96% after 100 min, while the pure SnO₂, ZnSn 1:4, and ZnSn 1:6 samples reached only 81%, 88%, and 92%, respectively. This could be due to the presence of h⁺ and ^•OH species, which were identified as the most active radicals during the photocatalytic process. Furthermore, the 1:2 ZnSn photocatalyst demonstrated good stability and maintained its photocatalytic performance after five successive degradation cycles. These results indicate that the Zn/Sn 1:2 ratio results in the highest photocatalytic efficiency, confirming the superior effect of Zn₂-SnO₄ structure in enhancing charge separation and accelerating the degradation of malachite green compared with the other compositions.