Identification and quantification of oxygen vacancies in CeO2 nanocrystals and their role in formation of F-centers

Abstract In this work we present synthesis and extensive characterization of Cerium oxide (CeO 2 ) nanocrystals. Comparison between the properties of as-prepared and air annealed nanoparticles has been carried out, with a goal to clearly identify the effect of oxygen vacancies on crystal, electronic and band structure. Detail crystal and electronic structural analysis was employed to quantify oxygen vacancies. Structural analysis confirmed that the formation of single phase cubic Fluorite structure for both as-prepared and annealed samples. Crystal and electronic structural studies confirmed that Ce ions exists in two oxidation states, Ce +3 and Ce +4 . Concentration of oxygen vacancies was larger in as-synthesis nanocrystal. A drastic decrease in oxygen vacancy concentration was observed for the sample annealed in air at 550 °C. For the as-prepared sample, the Raman allowed F 2g mode shifted towards lower wavenumber, exhibiting mode softening with broader and asymmetric peak. Observation of absorption edge revealed presence of 4f band within the band gap. Absorption with different band edge, confirmed different energy position of 4f level for the sample possessing oxygen vacancies. Blue shift of the band edge for as-prepared sample has been discussed in terms of increase in lattice parameter, formation of Ce +3 ions, quantum confinement effect etc. Photoluminescence emission studies revealed presence of F-centers with corresponding energy level located below 4f band as a result of oxygen vacancies. It was found that within the measured experimental energy window, transitions associated by the F-center are mainly associated with 4f 0 to 4f 1 , F ++ to 4f 1 and 4f 0 to F + .