Efficient Photoreduction of Hexavalent Chromium Using the Reduced Graphene Oxide–Sm2MoO6–TiO2 Catalyst under Visible Light Illumination

In past years, the presence of toxic heavy metal ions in water and soil has caused major health problems. The ternary type semiconductor material, reduced graphene oxide (rGO)-Sm₂MoO₆-TiO₂, has been investigated as a photocatalyst for the reduction of soluble chromium(VI) into (III) for the first time. The as-synthesized rGO-Sm₂MoO₆-TiO₂ catalyst was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy, X-ray photoelectron spectroscopy, FT-Raman, Fourier transform infrared, and optical spectroscopy. The maximum Cr(VI) reduction of 96% was achieved within 70 min under visible light illumination. The powder XRD analysis confirmed the formation of anatase TiO₂. Field-emission SEM images depicted well-dispersed rGO sheets, and TiO₂ and Sm₂MoO₆ particles are randomly distributed onto rGO. The reduction of Cr(VI) by rGO-Sm₂MoO₆-TiO₂ was considerably greater than the reduction by Sm₂MoO₆, TiO₂, Sm₂MoO₆-rGO, TiO₂-rGO, and Sm₂MoO₆-TiO₂. Sm₂MoO₆ acts as an effective cocatalyst to TiO₂ to enhance the separation of photo-generated electron-holes. Even after six consecutive cycles, the photoreduction of Cr(VI) was more than 85%, which reveals that the excellent reusability performance of the catalyst for practical applications. The photogenerated electron plays an important role in the reduction of Cr(VI) into nontoxic Cr(III), and the synergistic effect of rGO greatly improved the separation of h⁺ and e^- pairs.