First-principles study of atomic hydrogen adsorption and initial hydrogenation of Zr(0001) surface

Abstract

The atomic hydrogen adsorption on Zr(0001) surface is systematically investigated by using density functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics is studied in detail for a wide range from 0.11 to 2.0 monolayer. At low coverage of 0<Θ≤1.0, the most stable adsorption site is identified as the on-surface hcp site followed by the fcc site, and the adsorption energy gradually increases with the coverage, thus, indicating the higher stability of on-surface adsorption and the tendency to form H clusters. The origin of this stability is carefully analyzed by the projected density of states and the charge distribution showing the Zr-H chemical bonding with a mixed ionic/covalent feature during the surface hydrogenation. In addition, the minimum energy paths as well as the activation barriers of the on-surface diffusion and the penetration from on-surface sites to subsurface sites are also calculated. At high coverage of 1.0<Θ≤2.0, it is found that the co-adsorption configuration with 1.0 monolayer H residing on the surface hcp sites and the remaining (Θ-1) monolayer H occupying the sub-surface octahedral sites is most energetically favorable. The electronic structure properties of the resultant H-Zr-H sandwich structures at the coverage range of 1.0<Θ≤2.0 reveal the similar characteristics to the bulk hydride ZrH2, providing a detailed microscopic understanding for the Zr surface hydrogenation phenomenon.