Facile construction of MoO₃@ZIF-8 core-shell nanorods for efficient photoreduction of aqueous Cr (VI)

Recently, hexavalent chromium (Cr(VI)) in wastewater has become a threat to the ecosystem and human health. The synthesis of high-performance and recyclable photocatalysts still remains a challenge. The photoreduction efficiency of Cr(VI) is inhibited by the high rate of recombination of electron-hole pairs and the low adsorption capacity of Cr(VI). In this study, three-dimensional (3D) MoO₃/ZIF-8 core-shell nanorod composite photocatalysts were prepared via a facile two-step method and applied to the reduction of Cr(VI). The chemical state of the elements, microstructure, surface elements, and optical properties of the MoO₃/ZIF-8 core-shell nanorods were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy. The as-prepared MoO₃@ZIF-8 catalysts displayed superior photocatalytic activity for Cr(VI) reduction under visible light, compared to the pure ZIF-8 and MoO₃ nanowires. Further, MoO₃@ZIF-8 (with 15% of ZIF-8) exhibited the best photocatalytic activity, and promoted 100% reduction of Cr(VI) (15 mg L⁻¹) within 45 min. The 3-D core-shell structure not only provided a large surface area, but also separated the electron-hole pairs effectively. The photocatalytic activity of the composite remained almost unchanged after four cycles. The mechanism of Cr(VI) reduction is also discussed in detail.