Solar-Driven Cr(VI) Detoxification via a Holmium-Modulated In₂S₃ Strategy: A Rare-Earth Approach to Sustainable Photocatalysis

The photocatalytic reduction of hexavalent chromium Cr(VI) under visible light has gained considerable attention due to its relevance to environmental remediation. In this study, Ho-doped In2S3 photocatalysts were synthesized via a photodeposition method to improve the Cr(VI) reduction performance. The characterizations of the synthesized catalysts were performed using various techniques, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy, which confirmed the successful doping of holmium and provided insights into the material’s structures and optical properties. Subsequent experiments demonstrated that the Ho-doped In2S3 exhibited enhanced photocatalytic activity with the reaction rates 1.74 and 1.98 times higher than that of undoped In2S3 at Cr(VI) concentrations of 10 and 20 mg/L, respectively. It could be attributed to improving charge separation and light absorption efficiency, ultimately leading to higher reduction rates of Cr(VI) in aqueous solutions. Trapping experiments confirmed that electrons were the dominant reactive species in the photoreduction process. Furthermore, DFT calculations showed that Ho incorporation modulated the electronic structure, facilitating photoinduced charge transfer. These findings offer a useful reference for the rational design of rare-earth-doped metal sulfide photocatalysts for environmental applications.