Evolution of Visible Photocatalytic Properties of Cu-Doped CeO2 Nanoparticles: Role of Cu2+-Mediated Oxygen Vacancies and the Mixed-Valence States of Ce Ions

We report the contribution of oxygen vacancies for enhancing the optical and visible photocatalytic properties of Cu-doped CeO2 nanoparticles (NPs) synthesized through a low-temperature coprecipitation method. Doping Cu ions in the ceria lattice in different mole percentages, 0, 3, 5, 7, 9, and 15 wt %, results in enhancement of visible photocatalytic properties even under natural sunlight. Transmission electron microscopy and X-ray diffraction studies showcase the monodispersive nature of Cu-doped CeO2 NPs in the size range of 3–7 nm with face-centered cubic structure. The Cu-based defect states induce a narrow band function in ceria nanostructures and influence the red shift in absorption with the Cu concentrations. Visible photocatalytic degradation of methylene blue was investigated in the presence of pure CeO2 NPs, CuO NPs, and Cu-doped CeO2 NPs. These studies revealed that the 7 wt % of Cu-doped CeO2 NPs exhibit the degradation rates of 1.41 × 10–2 and 1.12 × 10–2 min–1 under exposure to natural sunlight and visible light (Xe light source), respectively. This is nearly 23.5 and 1.61 times faster than the undoped CeO2 and CuO NPs, respectively. The inclusion of more Cu2+ ions in the CeO2 structure leads to the interaction and spatial distribution of oxygen vacancies with a Ce4+/Ce3+ ratio defect. This promotes the narrowing of the band function to the visible photocatalytic characteristics. Detailed investigations from X-ray absorption spectroscopy support the fact that the oxygen vacancies may strongly affect the valences of Ce ions in CeO2, which improves the carrier mobility and visible response.