Marigold-like Zn₂In₂S₅ for photocatalytic degradation of rhodamine B: analysis of active species and mechanism investigation

Semiconductor photocatalysis offers an environmentally sustainable approach for the elimination of organic contaminants from wastewater. However, the photocatalytic performance of Zn₂In₂S₅ (ZIS) is often limited by nanosheet aggregation, insufficient exposure of active sites, and the incomplete understanding of reactive oxygen species (ROS) evolution. In this study, marigold-like ZIS microspheres with a 2D-3D hierarchical porous structure were fabricated, and their performance in the photocatalytic degradation of rhodamine B (RhB) was investigated. The structural and physicochemical properties of the obtained ZIS were systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy (DRS), and Mott–Schottky analysis. The results demonstrated that this unique structure integrates the advantages of the short charge migration distance of 2D materials and multiple light scattering/adsorption sites of 3D structures. Under AM 1.5 simulated sunlight irradiation, the as-prepared ZIS photocatalyst exhibited excellent photocatalytic activity, with a RhB degradation efficiency of up to 99.7% within 60 min and an apparent reaction rate constant (k) of 0.09 min⁻¹, which compares favorably with most previously reported ZIS-based photocatalysts. Through trapping experiments, electron spin resonance (ESR) spectroscopy and qualitative detection of hydrogen peroxide (H₂O₂), the photocatalytic reaction mechanism of ZIS was elucidated: superoxide radical (·O₂⁻) is the primary active species, followed by photogenerated hole (h⁺), whereas hydroxyl radical (·OH) make the least contribution. More importantly, a cascade mechanism of “ O₂⁻– H₂O₂–·OH” was proposed in this work, which not only resolves the inherent contradiction of insufficient valence band position of ZIS but also demonstrates the potential of hierarchical zinc indium sulfide photocatalysts for efficient environmental remediation applications.