Adsorption, Diffusion, And Vibration Of Oxygen On Ag(110)

We have employed ab initio density functional theory (DFT) to study the adsorption, dissociation, diffusion, and vibration of oxygen on Ag(110). We find that the fourfold-hollow site is the preferred site for O2 adsorption and that the O2 molecular axis marginally prefers to align along the [11¯0] direction (for which the adsorption energy is -0.41eV) rather than along the [001] (for which it is -0.37eV). By weakening the O-O bond, the net charge transfer of ∼0.9e from Ag to the antibonding orbital (2π∗) of O2 facilitates dissociation of O2 on Ag(110). Contrary to a previous theoretical suggestion (subsequently incorporated into interpretation of an experiment), our DFT calculations for adsorption energies and dissociation energy barriers, taken together with findings concerning vibrational frequencies and charge transfer, indicate that, when adsorbing, O2 prefers to align along the [11¯0] direction, and when dissociating, does align along the [001] direction with the dissociated O atoms adsorbing onto threefold-hollow sites rather than short-bridge sites. Importantly, our calculations clearly show that dissociation of O2 on Ag(110) is coupled with the surface Rayleigh mode (∼4meV) and O2 dissociation along the [001] direction is more strongly coupled with the substrate motion than O2 dissociation along the [11¯0] direction indicating that O2 dissociation (particularly, O2[001]) can be activated by surface phonon even at quite low temperature. Once thus dissociated, the O atoms can more easily diffuse in the [11¯0] direction, from one fourfold-hollow site to the next owing to a smaller diffusion barrier in the latter