Hydrogen absorption in Pd thin-films

Hydrogen absorption isotherms for Pd thin films were modeled at atomistic scale by Monte Carlo (MC) simulation in the TPμN ensamble and by Molecular Dynamics (MD) simulations at 300 K. The interaction among atoms was modeled by embedded atom method (EAM) potentials. Simulated samples consisted of monocrystalline nanofilms with different thickness (2-8 nm) and two crystallographic surface orientations, (001) and (111). The isotherms were compared to bulk Pd and a few available experimental results. Instead of the plateau corresponding to the α-β PdH equilibrium in the bulk, the isotherms at nano-films show a two-plateaux behavior: a small one corresponding to a surface-subsurface hydride formation, and a larger one for the subsequent bulk hydride formation. This is strongly correlated with the atomic stress distribution induced within the thin film. The equilibrium pressures at the isotherms depend on the thin-film thickness, with pressure being larger for thicker films. The isotherms of the (001) films display lower equilibrium pressures than those for (111) films.