Chromium/Vanadium Mixed Oxide Films on Pt(111): Revealing Oxide Alloying Mechanisms in Two Dimensions.

The mixing characteristics of oxide materials largely depend on the dimensionality of the system, and many oxide-alloy structures in three dimensions (3D) do not have a 2D analog. To unravel fundamental alloying mechanisms in 2D, V/Cr mixing into oxide thin films is investigated on Pt(111) by scanning tunneling microscopy and density functional theory. The experiments reveal flat, double-stack islands made of a compact bottom and a honeycomb top layer with a 4.5 Å total height. The energetically most favorable structure-match comprises an O-Cr-O trilayer at the interface to the Pt(111) capped by a mixed V/Cr honeycomb top layer. The structure is stabilized by strong interlayer adhesion, reinforced by a charge transfer toward the central trilayer from the metal support and the honeycomb plane. A negative V/Cr mixing enthalpy arises from the presence of two distinct surface sites that enable formation of tetrahedrally coordinated V and octahedrally coordinated Cr cations. The identified thin-film structure bears resemblance to a (111) cut of a hypothetical V/Cr spinel, a unique 2D configuration without bulk equivalent that is stabilized solely by its nanoscale thickness and a strong coupling to the Pt support.