Zhang, Xingfan; (2024) Electronic and Defect Properties of Cerium Oxides. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The complex defect and surface chemistry of ceria (CeO2) are closely linked to its widespread applications in ionic conduction and heterogeneous catalysis. Despite extensive studies using periodic Density Functional Theory (DFT), challenges remain in accurately modelling localised states and highly disordered structures. This work bridges this gap by integrating plane-wave DFT, interatomic potential (IAP) methods, and the hybrid quantum mechanics/molecular mechanics (QM/MM) embedded-cluster approach, to explore fundamental, yet underexplored, aspects of cerium oxides. We first developed a set of robust shell-model IAPs that reproduces accurately the structure and elastic, dielectric, phonon, defect, and surface properties of CeO2. This potential enables accurate modelling of intrinsic charged point defects in CeO2 using the MM Mott-Littleton approach, generating results in good agreement with hybrid QM/MM calculations, and can be potentially extended to model highly disordered, reduced ceria. Determining the absolute band edge positions in solids is challenging, as seen in ceria, where experiments show substantial discrepancies in the ionisation potential (IP) and work function (Φ). We separated the bulk and surface contributions to the IPs of metal oxides, suggesting a general perspective that electron binding energies in metal oxides are surfacedependent variables, which can be clearly understood from IAP-based electrostatic analyses and explicitly calculated through suitable QM methods. Moreover, we examined the environmental effects, demonstrating a trend in the variation of IP and Φ in ceria under different oxygen environments. The distributions of defects significantly affect the IP and Φ of ceria, while surface adsorbates and impurities can further increase this variability. The IP of nano ceria was finally explored, explaining the size and facet effects. These studies provide insights into the origins of variability in experimental measurements. Overall, this work provides a comprehensive understanding of the defect and electronic properties of cerium oxides, along with useful atomistic and QM/MM models for future research.

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