Electronic Structure of Ce-Containing Oxides by X-Ray Absorption Spectroscopy

The valence change of cations constituting metal oxides plays an important role in the chemical and electronic properties of functional materials. Among them, Ce exhibits high oxygen storage capacity, oxygen mobility, and oxidation catalytic activity due to its ability to oscillate between Ce⁴⁺/Ce³⁺ valence while maintaining a nearly constant ionic radius [1]. The Ce electronic state depends on the complex interaction between localized 4f electrons and the oxygen and vacancy orbitals [2], directly related to the material functionality. In this study, X-ray absorption spectroscopy was performed on CeO₂ nanoparticles (NPs) [3] to discuss the correlation between Ce local structure and electronic state.<br/>The correlation between Ce-O bonding distance and Ce valence was investigated using CeO₂ with different particle sizes. In the Ce L₃-edge, although the Ce³⁺ increased slightly with decreasing grain size, the X-ray absorption fine structure (XAFS) spectral shape was nearly constant, suggesting that the CeO₂ state was maintained. On the other hand, in the Ce M_5,4-edge, the XAFS spectral shape changes significantly with grain size, and the peak intensity of Ce₃₊ remarkedly increases as the grain size decreases. This indicates that the electrons are localized in the Ce4f orbitals. Here, we investigated the O-Ce interaction from O K-edge XAFS spectra measurements. The O2p-Ce5d,eg and O2p-Ce5d,t2g peaks broadened with decreasing grain size. This suggests that the local structure of the CeO₈ hexahedron is disordered. On the other hand, the O2p-Ce4f peak intensity decreased significantly with grain size. This implies a decrease in the interaction between O2p and Ce4f orbitals. These results suggest that lattice expansion and distortion of CeO₂ may localize electrons in the Ce4f orbitals.<br/>This research was partially supported by JSPS Grant-in-Aid for Scientific Research Grant Number 21H05010 and JP22H01764.<br/><br/>[1] F. Zhang, X. Zhang, <i>et al.</i>, <i>Chemical Engineering Journal</i>, <b>348</b> (2018) 831–839.<br/>[2] V. K. Paidi, D. L. Brewe, J. W. Freeland, C. A. Roberts, J. van Lierop, <i>Phys. Rev. B</i>, <b>99</b>, 180403 (2019).<br/>[3] X. Hao <i>et al.</i>, <i>Small</i>, 14 (2018) 1802915.