Hydrogen pairing on graphene

Achieving high hydrogen uptake values in graphite has always been a topic of much interest, founded on the question of how hydrogen atoms end up as they get trapped on a carbon surface. The foundations on this subject have long been established – isolated hydrogen atoms impinging on the graphite surface end up at single-coordination top sites, and given appropriate time to relax reaches a very stable chemisorbed state associated with carbon atoms pulled out of the initial planar geometry [1–4]. Stable hydrogen molecule adsorption on graphene [5] in a related matter involves dissociation as a prerequisite, and the pairs on the graphite surface end up with the hydrogen atoms at slightly off-top positions, brought about by the relatively close H– H atomic separation. Breaking up the incoming molecules requires a large amount of energy, but reconstruction of the substrate reduces the barrier to reaching stable adsorbed states. The case with atoms attaching to opposite corners of a graphite hexagon was found to be the most stable configuration for an adsorbed pair, and from the constructed potential energy surface was additionally found to be the most accessible.