Diffusion Mechanisms for Ions over Hydroxylated Surfaces: Cu on Î-Al₂O₃

The energies involved in the diffusion of Cu²⁺ and Cu⁺ over hydroxylated Î-alumina were modeled with density functional theory using explorative molecular dynamics. This is the first time that the mechanism for diffusion of ions over hydroxylated surfaces is studied. It is found that the crucial requirement for feasible activation energies for ion diffusion is the prevention of charge separation. This can be realized either by counterion codiffusion or proton contradiffusion. Furthermore, the effects of the cation valency, hydroxylation level, and nature of the counterions were studied and general trends for diffusing cations on hydroxylated surfaces were postulated. At full hydroxylation, all charge compensation is performed by proton contradiffusion, while at intermediate hydroxylation levels, a combination of proton contradiffusion and counterion codiffusion occurs. Finally, energy barriers for codiffusion are related to the bonding strength of the counterions to the surface, which depends on the counterion and the hydroxylation level.