Segregation Effects in 2D Mixed Oxide Nano-Islands: Edge Structure and Composition in V2-xFexO3 Monolayers

Low-coordinated atoms residing at the edges of oxide nanostructures play an important role for adsorption and reaction processes in heterogeneous catalysis. While elucidation of their local configuration is challenging already for binary oxides, hardly any information is available for ternary materials, for which the variable stoichiometry introduces an additional degree of freedom. In this study, low-temperature scanning tunneling microscopy is employed to analyze the edge configurations of V_2-xFe_xO₃ honeycomb islands grown on Pt(111) as a function of film composition. The islands are delimited by zigzag and armchair edges, the latter with a tendency to reconstruct into sequences of four, five, six and seven-membered rings. Scanning tunneling spectroscopy is used to identify the chemical nature of the edge atoms to be either vanadium or iron. The thermodynamic forces for V or Fe segregation to the edges, both oxygen and cation terminated, are analyzed by density functional theory calculations. In an oxidizing environment, formation of vanadyl-terminated edges is energetically favorable, while Fe atoms segregate toward the edges at O-poor conditions. The observed behavior is explained by the significantly higher oxygen affinity of V versus Fe edge cations, and reflected in the higher stability of terminal vanadyl compared to ferryl groups. Our findings may indicate a pathway to tailor the chemical composition and thus the catalytic reactivity of oxide island edges via cationic mixing.