Segregation Effects\nin 2D Mixed Oxide Nano-Islands:\nEdge Structure and Composition in V<sub>2‑x</sub>Fe<sub><i>x</i></sub>O<sub>3</sub> Monolayers

Low-coordinated atoms residing at the edges of oxide\nnanostructures\nplay an important role for adsorption and reaction processes in heterogeneous\ncatalysis. While elucidation of their local configuration is challenging\nalready for binary oxides, hardly any information is available for\nternary materials, for which the variable stoichiometry introduces\nan additional degree of freedom. In this study, low-temperature scanning\ntunneling microscopy is employed to analyze the edge configurations\nof V<sub>2‑x</sub>Fe<sub><i>x</i></sub>O<sub>3</sub> honeycomb islands grown on Pt(111) as a function of film composition.\nThe islands are delimited by zigzag and armchair edges, the latter\nwith a tendency to reconstruct into sequences of four, five, six and\nseven-membered rings. Scanning tunneling spectroscopy is used to identify\nthe chemical nature of the edge atoms to be either vanadium or iron.\nThe thermodynamic forces for V or Fe segregation to the edges, both\noxygen and cation terminated, are analyzed by density functional theory\ncalculations. In an oxidizing environment, formation of vanadyl-terminated\n edges is energetically favorable, while Fe atoms segregate toward\nthe edges at O-poor conditions. The observed behavior is explained\nby the significantly higher oxygen affinity of V versus Fe edge cations,\nand reflected in the higher stability of terminal vanadyl compared\nto ferryl groups. Our findings may indicate a pathway to tailor the\nchemical composition and thus the catalytic reactivity of oxide island\nedges via cationic mixing.