Hydrogen intercalation in MoS2

We investigate the structure and energetics of interstitial hydrogen and hydrogen molecules in layered 2H-MoS2, an issue of interest both for hydrogen storage applications and for the use of MoS2 as an (opto)electronic material. Using first-principles density functional theory we find that hydrogen interstitials are deep donors. H-2 molecules are electrically inactive and energetically more stable than hydrogen interstitials. Their equilibrium position is the hollow site of the MoS2 layers. The migration barrier of a hydrogen molecule is calculated to be smaller than 0.6 eV. We have also explored the insertion energies of hydrogen molecules as a function of hydrogen concentration in MoS2. For low concentrations, additional inserted H-2 molecules prefer to be located in hollow sites (on top of the center of a hexagon) in the vicinity of an occupied site. Once two molecules have been inserted, the energy cost for inserting additional H-2 molecules becomes much lower. Once all hollow sites are filled, the energy cost increases, but only by a modest amount. We find that up to 13 H-2 molecules can be accommodated within the same interlayer spacing of an areal 3 x 3 supercell.