Migration of point defects at charged Cu, Ag, and Au (100) surfaces

We report ab initio values for the electrical dipole moment and for the formation and activation energies of adatoms and vacancies migrating on Cu, Ag, and Au (100) surfaces, and we discuss the effect of the electrical dipole energy of these point defects on the rate of mass transport along charged metal-electrolyte interfaces. We find that adatoms and vacancies exhibit positive surface dipole moments, which for positive electrode potentials tend to reduce the formation and activation energies and increase the mobility of the point defects. We also consider atom migration by an exchange process involving the intermediate formation of dimers, and find that the surface dipole moment of the Cu and Ag dimers is negative, which means that they tend to become more mobile for negative potentials. However, because of their large activation energies, we conclude that the exchange process is not likely to provide an energetically favorable mechanism for migration in these systems. The Au dimer has a small positive dipole moment, which implies that the exchange process may contribute to surface transport but only in neutral surfaces.