Coverage-dependent absorption of atomic hydrogen into the sub-surface of Cu(1 1 1) studied by density-functional-theory calculations

With density-functional-theory calculations, we have studied coverage-dependent absorption of H atoms into the sub-surface below a face-centered-cubic (fcc) hollow site of Cu(1 1 1). Both frozen and relaxed surface lattices were considered when the atomic H migrated from the surface to the sub-surface. The potential energy curve for the absorbing H shows that the surface site is in general favored over the sub-surface site, and this trend varies little with the H coverage (0.11-0.67 ML). If the hexagonal-close-packed (hcp) hollow sites immediately vicinal to the absorbing H are pre-adsorbed with other H atoms, the surface adsorption potential is greatly increased, because of the repulsive H-H interaction, to a value near, or even greater than, the sub-surface absorption potential; when two or three H atoms (on the hcp sites) are beside the absorbing H, the energy barrier for the sub-surface absorption is decreased, whereas that for diffusion from the sub-surface to the surface is enhanced. These results indicate that, on an H-saturated Cu(1 1 1) surface (0.67 ML), the sub-surface sites below the fcc sites with two or three neighboring H atoms can trap the sub-surface H.