Raman Spectra of Polycrystalline CeO2: A Density Functional Theory Study

Abstract

Cerium oxide is an important material for catalytic and fuel cell applications. We present an ab initio density functional theory (DFT) study of the vibrational properties of ceria focusing on the interpretation of Raman spectra of polycrystalline powder samples, with vibrational bands in the frequency region between 250 and 1200 cm –1 . The model systems include the oxidized CeO 2 as well as the reduced CeO 2– x and Ce 2 O 3 bulk materials together with the CeO 2 (111) and oxygen defective CeO 2– x (111) surfaces. The experimentally observed band at 250 cm –1 is assigned to a surface mode of the clean CeO 2 (111) surface, in agreement with our Raman spectra of ceria (CeO 2 ) powders with varying crystal size (Filtschew, A.; Hofmann, K.; Hess, C., J. Phys. Chem. C 2016, 120, 6694). The reduced model systems display signature vibrational bands in the 480–600 cm –1 region associated with the presence of oxygen defects and reduced Ce 3+ ions. In the high-frequency region between 800 and 900 cm –1, characteristic peroxide (O 2 2– ) stretching vibrations at the oxidized and defective ceria surfaces are obtained, and a systematic study with respect to the peroxide coverage provides the basis for a correlation between the position of the peroxide stretching mode and its adsorption geometry and concentration. The present theoretical analysis allows for a consistent description of the experimental Raman spectra of polycrystalline ceria. The outlined approach serves as a reference for the description of vibrational properties of other metal oxides.