Abstract
Strain has been widely recognized as important for tuning the behavior of defects in metal oxides since properties such as defect configuration, electronic structure, excess charge localization, and local atomic distortions may be affected by surface strain. In CeO2, the most widely used promoter in three-way catalysts and solid state electrolyte in fuel cells, the behaviors of oxygen vacancies, and associated Ce3+ polarons are crucial in applications. Recent STM and AFM investigations as well as DFT-based calculations have indicated that in the near-surface of CeO2 (111), at low temperatures and vacancy concentrations, subsurface oxygen vacancies are more stable than surface ones, and the Ce3+ ions are next-nearest neighbors to both types of vacancies, which can be explained by the better ability of the system to relax the lattice strain induced by vacancy formation as well as by the excess charge localization. The results also revealed that the interaction between first-neighbor vacancies is repulsive. In this work, the relative stability of surface and subsurface oxygen vacancies at the CeO2 (111) surface under in-plane strain is investigated by means of DFT+U calculations. The tensile strain favors isolated surface vacancies with next nearest neighbor polarons, whereas isolated subsurface vacancies with nearest neighbor polarons are energetically favored under compressive strain. In addition, the formation of both surface and subsurface dimers is favored over having corresponding isolated species under compressive strain, which implies the possibility of controlling the formation of vacancy clusters using strain. In many applications, ceria is employed as a supported thin film or within a heterostructure in which ceria can be strained, and this study shows that strain can be a useful handle to tune properties of such materials.
Highlights
Ceria is widely used as solid oxide fuel cell electrolyte (Inaba and Tagawa, 1996) and in catalytic applications (Trovarelli, 1996; Fu et al, 2003; Vayssilov et al, 2011; Montini et al, 2016) largely due to its facile oxygen vacancy formation and diffusion, either within the bulk or at its surfaces
For oxygen vacancies in the near-surface of CeO2 (111), it has been predicted from Density Functional Theory (DFT) calculations that the two Ce3+ polarons are not necessarily NN (NearestNeighbor) to the oxygen vacancies, but rather prefer to locate at next-nearest neighbor (NNN) (Next-Nearest-Neighbor) cationic sites (GandugliaPirovano et al, 2007, 2009; Li et al, 2009; Paier et al, 2013), which has been later validated in Scanning Tunneling Microscope (STM) experiments (Jerratsch et al, 2011)
The preference for the NNN positions upon localization of the excess charge has been explained by the better ability of the system to relax the lattice strain induced by the presence of the vacancies as well as by the excess charge localization; a Ce3+ ion is more spacious than its Ce4+ counterpart and at NN sites Ce3+ – O bonds would be compressed (Ganduglia-Pirovano et al, 2009)
Summary
Ceria is widely used as solid oxide fuel cell electrolyte (Inaba and Tagawa, 1996) and in catalytic applications (Trovarelli, 1996; Fu et al, 2003; Vayssilov et al, 2011; Montini et al, 2016) largely due to its facile oxygen vacancy formation and diffusion, either within the bulk or at its surfaces It has been previously reported (Skorodumova et al, 2002), and later supported by subsequent research (Ganduglia-Pirovano et al, 2007, 2009; Li et al, 2009; Shoko et al, 2010; Jerratsch et al, 2011; Paier et al, 2013), that the formation of an intrinsic neutral oxygen vacancy. Under zero applied stress, isolated subsurface vacancies are more stable than surface ones, and the interaction between nearest neighbor vacancies at the surface or in the subsurface is repulsive
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