H₂ Physisorbed on Graphane

Abstract

We study the zero-temperature phase diagrams of H₂ adsorbed on the three structures predicted for graphane (chair, boat and washboard graphane), using a diffusion Monte Carlo technique. Graphane is the hydrogenated version of graphene, in which each carbon atom changes its hybridization to sp ³ and forms a covalent bond with a hydrogen atom. Our results show that the ground state of H₂ adsorbed on all three types of graphane is a $\sqrt{3}\times\sqrt{3}$ solid, similar to the structures found both for H₂ and D₂ on graphene. When the H₂ density increases, the system undergoes a first order phase transition to a triangular incommensurate solid. This change is direct in the case of washboard graphane, but indirect via different commensurate structures in the other cases. The total hydrogen weight percentage on the three graphane types in their ground states is in the range 10 % to 12 %, depending on if one or both graphane surfaces are covered with H₂.