Transition (Mn, Fe) and rare earth (La, Pr) metal doped ceria solid solutions for high performance photocatalysis: Effect of metal doping on catalytic activity

In this work, various transition and rare earth metal ions (M3+; M = Mn3+, Fe3+, La3+, and Pr3+) doped CeO2 solid solutions were prepared by a simple sol-gel method for the degradation of an organic dye, Rhodamine B (RhB) as a model pollutant. The as-prepared samples were thoroughly studied by various characterization techniques to understand the surface and optical properties. The XRD results suggested formation of solid solutions, and TEM studies confirmed the nanosized nature of the ceria particles. The Raman and XPS results revealed that the doping of metal ions enhanced the formation of Ce3+ ions associated with the oxygen vacancies. The doping of M3+ ions in the CeO2 lattice strongly influenced the band gap tuning of undoped CeO2 (3.01 eV) from the UV to Visible region (i.e., 2.45-2.90 eV). Photoluminescence studies suggested that doping of M3+ cations suppresses the recombination rate of photogenerated electron-hole pairs. The photocatalytic activity results indicated that the doped CeO2 samples exhibit substantially enhanced photocatalytic performance for the degradation of RhB compared to undoped CeO2. The better catalytic activity of doped CeO2 samples could be attributed to the presence of defects (Ce3+ ions and oxygen vacancies), which play a prominent role as trapping centres for excited electrons and inhibit the recombination process. The scavengers tests confirmed that the generation of highly reactive hydroxyl ((OH)-O-center dot) and super oxide (O-center dot(2) (-)) radicals are actively involved in the photodegradation process. This work rendered a new concept for rational design and development of doped CeO2-based materials as better photocatalysts.