Identification of the optimal site for oxygen adsorption in chemisorbed and oxidized states on pseudomorphic Fe/Ru(0001) surface: A density functional theory computational investigation

Abstract

The oxidation state and charge distribution of FexOy binaries, including strained monolayers on transition metal surfaces, is a topic of significant interest. The p(2 × 2) and c(4 × 2) superstructures are two stable surface structures of chemisorbed oxygen on the pseudomorphic Fe/Ru(0001) surface that may coexist at 0.25 ML oxygen coverage. Density functional theory calculations were used to investigate the possibility of these two structures occurring on the surface. The calculations considered the effect of magnetic ordering on the choice of adsorption site for oxygen. Paramagnetic ordering of the Fe monolayer favors oxygen adsorption at the hcp site, while antiferromagnetic ordering favors oxygen adsorption at the fcc site at 0.25 ML oxygen coverage. Interestingly, in the case of antiferromagnetic ordering, although the adsorption of 1 ML coverage of oxygen for the oxidation reaction energetically prefers the hcp site, the chemisorbed structures at 0.25 ML coverage are found to prefer the fcc site on the pseudomorphic Fe/Ru(0001) surface. The DFT calculations suggest that both the p(2 × 2) and c(4 × 2) structures of oxygen are exothermic, indicating the possibility of coexistence on the pseudomorphic Fe/Ru(0001) surface and occurrence at room temperature. However, the use of the Hubbard potential parameter, which is used to correctly describe the electronic band structure of FeO and other Mott insulators, also revealed that the oxidation reaction of the pseudomorphic Fe monolayer is endothermic on the Ru(0001) surface.