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