Enhanced photocatalytic performance by regulating the Ce³⁺/Ce⁴⁺ ratio in cerium dioxide

Cerium dioxide (CeO2) photocatalysts are used in treating environmental pollution and addressing the energy crisis due to their excellent oxygen storage capacities and abundant oxygen vacancies. In this paper, CeO2 precursors were synthesized with different water-alcohol ratios via a solvothermal method, and CeO2 photocatalysts with different Ce3+/Ce4+ ratios were obtained by changing the precursor calcination atmospheres (air, Ar) as well as the calcination time. The effects of CeO2 with different Ce3+/Ce4+ ratios in photocatalytic degradations of methylene blue under visible light were investigated. X-ray photoelectron spectroscopy results showed that the surfaces of the samples calcined under Ar had higher Ce3+/Ce4+ ratios and oxygen vacancy concentrations, which reduced the band gaps of the catalysts and improved their utilization of visible light. In addition, the many Ce3+/Ce4+ redox centers and oxygen vacancies on the sample surfaces improved the separation and transfer efficiencies of the photogenerated carriers. The sample C2-Ar calcined under Ar showed a high adsorption capacity and excellent photocatalytic activity by removing 96% of the methylene blue within 120 min, which was more than twice the degradation rate of the sample (C2-air) prepared via calcination under air. Trapping experiments showed that photogenerated holes played a key role in the photocatalytic process. In addition, a synergistic photocatalytic mechanism for the Ce3+/Ce4+ redox centers and oxygen vacancies was elucidated in detail, and the sensitization of cerium dioxide by dyes aided the degradation of methylene blue.