Role of Subsurface Oxygen in Oxide Formation at Transition Metal Surfaces

We present a density-functional theory trend study addressing the incorporation of oxygen into the basal plane of the late $4d$ transition metals (TMs) from Ru to Ag. Occupation of subsurface sites is always connected with a significant distortion of the host lattice, rendering it initially less favorable than on-surface chemisorption. Penetration starts only after a critical coverage ${\theta }_c$, which is lower for the softer metals towards the right of the TM series. The computed ${\theta }_c$ are found to be very similar to those above which the bulk oxide phase becomes thermodynamically more stable, thus suggesting that the initial incorporation of O actuates the formation of a surface oxide on TM surfaces.