Stability of subsurface oxygen at Rh(111)

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 (θₜₒₜ≈1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested octahedral subsurface 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 subsurface islands directly below the first metal layer. Inside these islands, the metastable hcp/octahedral (on-surface/subsurface) 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 subsurface 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.