Combining theoretical approaches in understanding the ionisation potentials of metal oxides Xingfan Zhang 1 , C. Richard A. Catlow 1,2 , Alexey A. Sokol 1 1 Kathleen Lonsdale Materials Chemistry, Department of Chemistry, University College London, London WC1H 0AJ, United Kingdom. 2 School of Chemistry, Cardiff University, Park Place, Cardiff CF10 1AT, United Kingdom The energies associated with the removal (ionisation potential, IP) and addition (electron affinity, EA) of an electron offer a wealth of information about the electronic, optical, and transport properties of molecules and solids. While molecules have well-defined IPs and EAs, obtaining the absolute band-edge positions in solid-state materials is much more challenging in both experimental and theoretical studies. An exceptionally interesting system is ceria (CeO2), which has a wide range of applications in heterogeneous catalysis and solid oxide fuel cells. Previous experiments observed an extremely large deviation in the measured IP of CeO 2 , ranging from 5.5 eV to 9.1 eV. Such a deviation was assumed to be the consequence of the highly tunable surface chemistry of CeO2, which can easily form oxygen vacancies in reduction conditions. Here, we combined several theoretical approaches in understanding the IPs of CeO 2 and other metal oxides. Apart from the variable surface chemistry, we emphasise the critical role of intrinsic polarisation and structural relaxation in the formation of surfaces, which is not considered in previous research but can be clearly understood using electrostatic analysis supported by a recently developed polarisable shell model (PSM) potential. 1 We determined a theoretical bulk IP of 5.38 eV for CeO 2 using the hybrid QM/MM embedded-cluster model that excludes any surface effects, which is much lower than most experimental measurements. However, surface terminations account for the remaining deviations and can vary the IPs from 4.2 eV to 8.2 eV, as revealed by plane-wave DFT calculations and PSM-based electrostatic analysis. Similar conclusions were also seen in other MO 2 -type oxides with high dielectric constants (TiO 2 , ZrO 2 , and HfO 2 ), which show a distinctive surface polarisation mechanism compared with low-dielectric-constant oxides such as MgO. The consistent predictions by our classical electrostatic analysis and modern electronic-structure techniques provide a universal understanding of the surface effects on the band structures of metal oxides. References 1. X. Zhang et al., Chem. Mater., 2023, 35, 207-227.
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