Single-phase Gd0.2La0.2Ce0.2Hf0.2Zr0.2O2 and Gd0.2La0.2Y0.2Hf0.2Zr0.2O2 nanoparticles as efficient photocatalysts for the reduction of Cr(VI) and degradation of methylene blue dye

Highlights: • Single-phase noble metal-free multi-component equiatomic oxide nanoparticles with a crystallite size of 3 nm is obtained. • Better Cr(VI) remediation and methylene blue degradation under natural sunlight. • Formic acid acts as hole scavenger for Cr(VI) reduction. • Good structural stability of catalyst after multiple recycling. -- Abstract: We report for the first time the use of noble metal-free multi-component equiatomic oxide as photocatalysts with excellent performance for natural sunlight driven degradation of Cr(VI) and methylene blue dye. The multi-component oxide nanoparticles with a composition Gd_0.2La_0.2Ce_0.2Hf_0.2Zr_0.2O₂ and Gd_0.2La_0.2Y_0.2Hf_0.2Zr_0.2O₂ were synthesized by simple co-precipitation followed by peptization in acid to obtain nanoparticle sol and calcined at 500 °C. The nanopowders were characterized by x-ray diffraction (XRD), UV–Visible spectroscopy (UV–Vis), and high-resolution transmission electron microscopy (HRTEM). The complete (∼100%) reduction of Cr(VI) to Cr(III) was observed after 90 and 100 min for the calcined Gd_0.2La_0.2Ce_0.2Hf_0.2Zr_0.2O₂ and Gd_0.2La_0.2Y_0.2Hf_0.2Zr_0.2O₂ respectively, under exposure to natural sunlight. In addition, 70% degradation of methylene blue is observed in 180 min. The effective photocatalytic activity of multi-component oxides can be attributed to their unique composition containing five components in equimolar amounts. The role of oxygen vacancies in photocatalytic reduction of Cr(VI) and the degradation of methylene blue is also discussed.