Defect Chemistry of CeO2 Surfaces from First Principles and Space Charge Theory

CeO₂ (ceria) is a diverse oxide with numerous technologically important functional properties. It is one of the most studied oxygen incorporation materials, and exhibits remarkable catalytic properties towards hydrocarbon and CO oxidation, and water splitting reactions.^1-3 Further, acceptor doped ceria is of great interest as electrolyte for solid oxide fuel cells due to its considerable oxide ion conductivity at intermediate temperatures.⁴ Recently, nanocrystalline CeO₂ ceramics were shown to display significant proton conduction even at room temperature,⁵ suggested to be due to surface, or sub-surface proton migration. The majority of these functional properties are affected by, or even originate from the rich surface, or interface defect chemistry of ceria, and corresponding space charge layers.^{3, 6}

In this contribution, we explore the defect chemistry of various surface terminations of CeO₂ at finite temperatures, by combining first principles calculations and space charge theory. Plane-wave based electronic/atomic structure calculations at the GGA+U and hybrid HF-DFT levels are used to determine bulk defect formation energetics and the defects' surface segregation energies in CeO₂. The first principles results are further combined with space charge theory through numerical evaluation of the space charge potential profiles and corresponding defect concentration profiles.

The charged oxygen vacancies and protonic defects exhibit highly favorable surface segregation energies, and therefore tend to accumulate on CeO₂ surfaces. Although such accumulation is partly charge compensated by oxide and hydroxide ad-ions and Ce³⁺ species at the surface, the surface turns out to be overall positively charged, and charge-compensated by a negative sub-surface space charge region, at least for low defect concentrations. Under humid conditions, especially at lower temperatures, the surfaces are saturated by protonic species, forming water layers. Finally, the effect of such surface saturation by protonic species, and also H₂O monolayer and film formation on the sub-surface space charge potential and defect concentration profiles are discussed.

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