Initial Subsurface Incorporation of Oxygen into Ru(0001): A Density Functional Theory Study

The adsorption and diffusion of oxygen on Ru(0001) surfaces as a function of coverage are systematically investigated by using density functional theory. A high incorporation barrier of low-coverage adsorbed oxygen into the subsurface is discovered. Calculations show that the adsorption of additional on-surface oxygen can lower the penetration barrier dramatically. The minimum penetration barrier obtained is 1.81 eV for a path starting with oxygen in mixed on-surface hcp and fcc sites at an oxygen coverage of 0.75 ML, which should be regarded as close to 1 ML. Energy diagrams show that oxygen-diffusion barriers on the surface and in the subsurface are much lower than the penetration barrier. Oxygen diffusion on the surface is an indispensable step for its initial incorporation into the subsurface.