Ni Ingress and Egress in SrTiO3 Single Crystals of Different Facets

Abstract

Metal-ion surface interactions and/or doping in host perovskite-oxides are techniques that are widely employed for electronic structure tuning purposes and in developing novel heterogeneous catalysts; however, an in-depth understanding of the different elementary steps and factors involved in these processes is lacking. Herein, we use Atomic Force Microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), and ab initio thermodynamics through density functional theory (DFT) to specifically investigate Ni surface adsorption, ingress, migration, segregation, and egress processes across different SrTiO3 (STO) single-crystal facets and terminations, specifically the (001), (110), and the (111). Under oxidizing and reducing conditions at different temperatures, Ni egress is observed on (110) STO samples, but not the (001). DFT results demonstrate Ni to have a higher thermodynamic egress propensity, specifically through an oxygen-terminated (110) facet in comparison to other (001) terminations, whereas for the (111)-Ti terminated facet, Ni is likely to remain in the bulk post ingress. We suggest that the observed uniqueness of the (110) surface facet toward Ni egress is possibly a consequence of a surface phase transition. These results can help guide design interests with regard to Ni surface stabilization, ingress/egress suppression, or facilitation in STO by elucidating the nuances involved across different facets.