Stability, structure, and electronic properties of chemisorbed oxygen and thin surface oxides on Ir(111)

Abstract

We present ab initio calculations for atomic oxygen adsorption on Ir(111) for a wide range of oxygen coverages, $\ensuremath{\Theta}$, namely from 0.11 to 2.0 monolayers (ML), including subsurface adsorption and thin surface-oxide-like structures. For on-surface adsorption, oxygen prefers the fcc-hollow site for all coverages considered. Similarly to oxygen adsorption on other transition metal surfaces, as $\ensuremath{\Theta}$ increases from 0.25 ML to 1.0 ML, the binding energy decreases, indicating a repulsive interaction between the adsorbates. For the coverage range of 0.11 to 0.25 ML, there is an attractive interaction, suggesting the possible formation of a local $(2\ifmmode\times\else\texttimes\fi{}2)$ periodicity with a local coverage of $\ensuremath{\Theta}=0.25$ ML. Pure subsurface oxygen adsorption is found to be metastable and endothermic with respect to the free ${\text{O}}_{2}$ molecule. For structures with coverage beyond one full ML, we find the incorporation of oxygen under the first Ir layer to be exothermic. As the subsurface O coverage increases in these structures from 0.5 to 1.0 ML, the energy becomes slightly more favorable, indicating an attractive interaction between the O atoms. The structure with the strongest average O binding energy is however a reconstructed trilayer-like structure that can be described as a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ oxide-like layer in $p(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit cell, with coverage 1.5 ML. Through calculation of the surface Gibbs free energy of adsorption, taking into account the pressure and temperature dependence through the oxygen atom chemical potential, the calculations predict only three thermodynamically stable regions, namely, the clean surface, the $p(2\ifmmode\times\else\texttimes\fi{}2)\text{-O}$ phase, and bulk ${\text{IrO}}_{2}$. Thin trilayer surface oxide structures are predicted only to form when kinetic hindering occurs, in agreement with recent experimental work.