Nanoscale-faceting of metal surfaces induced by adsorbates

Using density functional theory and thermodynamic considerations, adsorbate-induced faceting of high-index metal surfaces such as Ir(210) and Re(112 1) has been studied. Focusing on these two systems we first discuss the adsorption behaviour of oxygen and nitrogen on the various surfaces relevant for the faceting, and afterwards use these energies to evaluate the stability of substrates and facets in the presence of oxygen and nitrogen. The faceting phase diagrams of Ir(210) and Re(112 1) show that both adsorbates enhance the anisotropy in surface free energy, finally causing nanofacets to become the thermodynamically favourable surface structure. We also generated analogous electrochemical phase diagrams for both surfaces in contact with an oxygen- or nitrogen-containing electrolyte and found that the same nanofacets should also become stable at positive electrode potentials. Thus, our calculations not only reproduce the experimentally observed surface faceting under UHV conditions, but also predict facet formation under electrochemical conditions.