Stability of sub-surface oxygen at Rh(111)

TL;DR

This study uses density-functional theory to explore how oxygen incorporates below Rh(111), revealing the sequence of surface and sub-surface oxygen structures, their stability, and implications for oxide formation on transition metals.

Contribution

It provides a detailed theoretical analysis of oxygen incorporation mechanisms and site preferences below Rh(111), highlighting elementary steps leading to surface oxide formation.

Findings

Oxygen incorporation begins after a full monolayer of on-surface oxygen.

Octahedral sub-surface site occupancy is energetically favorable.

Oxygen aggregation in sub-surface islands causes stacking faults and site transitions.

Abstract

Using density-functional theory (DFT) we investigate the incorporation of oxygen directly below the Rh(111) surface. We show that oxygen incorporation will only commence after nearly completion of a dense O adlayer (\theta_tot = 1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested octahedral sub-surface site occupancy, inducing a site-switch of the on-surface species from fcc to hcp sites, is indeed found to be a rather low energy structure. Our results indicate that at even higher coverages oxygen incorporation is followed by oxygen agglomeration in two-dimensional sub-surface islands directly below the first metal layer. Inside these islands, the metastable hcp/octahedral (on-surface/sub-surface) site combination will undergo a barrierless displacement, introducing a stacking fault of the first metal layer with respect to the underlying substrate and leading to a stable fcc/tetrahedral site occupation. We suggest that these elementary steps, namely, oxygen incorporation, aggregation into sub-surface islands and destabilization of the metal surface may be more general and precede the formation of a surface oxide at close-packed late transition metal surfaces.