Figure 2: Schematic top view of the sample and the orientation of the vanadium crystal.

The epitaxial vanadium layer was grown on a MgO (001) substrate. The vanadium crystal is depicted by yellow spheres, and the in-plane lattice directions are indicated by arrows. The light grey area represents the oxide layer hindering the hydrogen uptake. The catalytically active stripe (dark grey) is parallel to one of the edges of the substrate, which results in a net diffusion of hydrogen in the [110] direction of the vanadium layer.