Hydrogen interactions with low-index surface orientations of tungsten

We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (111) surface. Stable hydrogen adsorption sites were found for all surfaces. For the reconstructed W(100), there are two primary adsorption sites: namely, the long-bridge and short-bridge sites. The threefold hollow site (3F) was found to be the most stable for W(110), while the bond-centered site between the first and second layer was found to be most stable for the W(1 1 1) surface. No bound adsorption sites for H-2 molecules were found for the W surfaces. Hydrogen (H) migration on both the (100) and (110) surfaces is found to have preferred pathways for 1D motion, whereas the smallest migration barrier for net migration of H on the W(111) surface leads to 2D migration. Although weaker H interactions are predicted for the W(111) surface compared to the (100) or (110) surfaces, we observe higher H surface concentrations of circle minus = 4.0 at zero K, possibly due to the corrugated surface structure. These results provide insight into II adsorption, surface saturation coverage and migration mechanisms necessary to describe the evolution from the dilute limit to concentrated coverages of H.