First-principles study of surface and subsurface O structures at Al(111)

The structural and electronic properties of oxygen atoms adsorbed in on-surface and subsurface sites at the Al(111) surface are investigated from first principles using the density functional theory within the generalized gradient approximation and a supercell approach for a range of oxygen coverages $0<~\Theta <~1,$ in some cases in two layers. For on-surface adsorption the binding energy increases with Θ, predicting formation of islands in agreement with earlier calculations and experiments. The most stable subsurface adsorption site is found for the $(1\mathrm{}\times 1)$ structure, i.e., $\Theta =1,$ in tetrahedral sites 1.92 Å below the topmost, 25% relaxed, Al atomic plane and 0.4 eV/atom higher in energy than the most preferred energy state in the on-surface fcc hollow site. The adsorption of O has a significant effect on surface buckling relaxation. Oxygen atoms adsorbed in subsurface octahedral sites induce very large (60%) outward relaxation of the topmost layer spacing, which points to the weakening of metal-metal bonds between the two outer Al layers. For the simultaneous subsurface and on-surface adsorption at $\Theta =1,$ the binding energy in the subsurface site is 0.2 eV/atom lower than the binding energy in over-surface fcc hollow sites. The sizable work-function changes for different structures are presented and discussed.