Oxygen vacancy and valence engineering in CeO2 through distinct sized ion doping and their impact on oxygen reduction reaction catalysis

Defect tuning in ceria to enhance its catalytic properties is a subject of great interest for the scientific community owing to the growing demand for catalytic materials in drug, automobile and chemical industries. Doping induced defect engineering was found to be one of the most sought out strategies particularly in oxides for achieving multifunctionality. Here, in this study, we have doped ceria with distinct sized trivalent rare-earth ions, namely, Y³⁺, Eu³⁺ and La³⁺, using combustion techniques. Positron annihilation lifetime spectroscopy (PALS) suggested enhanced defect density with doping in general and higher concentration of oxygen vacancies in La³⁺ doped ceria compared to Y³⁺ and Eu³⁺ counterparts. X-ray photoelectron spectroscopy (XPS) suggested the existence of both Ce³⁺ and Ce⁴⁺, with the former having higher fraction in CeO₂:La³⁺ compared to CeO₂:Y³⁺. The electron transfer resistance (R_ct) reduced in all the doped samples when compared to undoped ceria and they demonstrated improved catalytic activity towards the oxygen reduction reaction (ORR). The highest reduction in R_ct was seen in the 5% La doped sample owing to the very high concentration of oxygen vacancies and Ce³⁺/Ce⁴⁺ ratio and CeO₂:5.0% La³⁺ showed the best performance towards ORR electrocatalysis. The studies are expected to help in further tuning the catalysts in terms of dopant concentrations, and in future work, the strategy will be to control the Ce³⁺/Ce⁴⁺ ratio and see its implication in both catalytic and magnetic applications.