Adsorption and diffusion energetics of hydrogen atoms on Fe(1 1 0) from first principles

Spin-polarized density functional theory (DFT) has been used to characterize hydrogen atom adsorption and diffusion energetics on the Fe(1 1 0) surface. The Kohn-Sham equations are solved with periodic boundary conditions and within the all-electron projector-augmented-wave (PAW) formalism, using a generalized gradient approximation (GGA) to account for electron exchange and correlation. We investigate the site preference of H on Fe(1 1 0) for 0.25, 0.50, and 1.0 ML coverages and find that the quasi three-fold site is the only stable minimum (in agreement with experiment). We find the long and short bridge sites to be transition states for H diffusion on Fe(1 1 0), while the on top site is a rank-2 saddle point. The preference of the three-fold site is rationalized via an analysis of the site- and orbital-resolved density of states. An analysis of charge density differences suggests that the H–Fe interaction is quite covalent, with only ∼0.1 electron transferred from Fe atoms to H in the three-fold site of Fe(1 1 0). We also compare two experimentally observed 0.50 ML phases for H/Fe(1 1 0): a graphitic (2 × 2)-2H and a (2 × 1) phase. We confirm the LEED data that the Fe(1 1 0)-(2 × 2)-2H superstructure is more stable at low temperature. The predicted adsorption structure and weak substrate reconstruction for the Fe(1 1 0)-(2 × 2)-2H phase roughly agree with experiment, though discrepancies do exist regarding the H-surface height and the H–H distance. Moreover, trends in work function with coverage are predicted to be qualitatively different than older measurements, with even the sign of the work function changes in question. Lastly, a zig–zag diffusion path for H atoms on Fe(1 1 0) is proposed, involving a very low (<0.2 eV) barrier.