Control of Surface Cation Enrichment in Perovskite-Type Oxides for Energy Conversion Devices

Among the phenomena related to the surface rearrangement of cations in perovskite-based oxide materials (ABO3), cation enrichment near the surface has been frequently observed. Upon annealing in an oxidizing atmosphere, an A-site cation, Sr or La in particular, is often enriched on the surface as compared to the bulk composition of the material, which eventually forms additional phases or rearranges the crystal structure of the surface. This segregation has been suggested to be the key reason behind the chemical instability of perovskite oxide surfaces and the corresponding performance degradation of solid oxide electrochemical cell (SOC) O2-electrodes. In addition to B-site cations, some transition/precious metals can be partially reduced and decomposed into nano-sized metallic particles on the oxide surface upon high-temperature reduction. This in-situ synthesis process of metal nanoparticles is referred to as the redox ex-solution phenomenon and has widely been studied as a way to fabricate metal nanocatalyst-decorated oxide electrodes for SOCs. However, despite much effort by researchers, the underlying mechanisms related to these phenomena are not completely understood. Accordingly, practical solutions that effectively inhibit A-site cation segregation or accelerate B-site cation ex-solution have not yet been proposed.<br/>Here, to confirm the effect of lattice strain on the degree of surface cation enrichment, various types of perovskite oxides were epitaxially fabricated with controlled lattice strain and analyzed their surface compositions. Heterostructured films with a composition of ABO3 (A = Sr and La, B = Ti, Co, Fe, and Zr) were grown onto single crystal substrates with difference lattice parameters by pulsed laser deposition (PLD), and their lattice strains and surface compositions were characterized by high-resolution X-ray diffraction (HR-XRD) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS), respectively. The surface activity of the films for oxygen exchange and CO oxidation was also analyzed by electrical conductivity relaxation (ECR) and quadrupole mass spectroscopy (QMS) respectively. As a result, the lattice strain greatly changes the degree of A-site enrichment and that the tensile strain inhibits Sr enrichment but promotes La-excess. Density functional theory (DFT) calculations revealed that Sr or La atoms are intrinsically unstable despite the fact that the overall perovskite structure is stable. And the extent of deviation from the optimal M-O bond length in the most stable state of the A/B-site cation can be a driving force behind surface enrichment. Based on these findings, an isovalent dopant is added with controlling on bond length between the nearest M-O in the perovskite lattice, Sr or La-excess can be remarkably alleviated, improving the electrochemical/chemical activity of perovskite surface.