Proton and hydrogen transport through two-dimensional monolayers

© 2016 IOP Publishing Ltd. Diffusion of protons and hydrogen atoms in representative two-dimensional materials is investigated. Specifically, density functional calculations were performed on graphene, hexagonal boron nitride (h-BN),phosphorene, silicene, and molybdenum disulfide (MoS2) monolayersto study the surface interaction and penetration barriers for protons and hydrogen atoms employing finite cluster models. The calculated barrier heights correlate approximately with the size of the opening formed by the three-fold open sitesinthe monolayers considered. They range from 1.56 eV(proton) and 4.61 eV(H) for graphene to 0.12 eV (proton) and 0.20 eV (H) for silicene. The results indicate that only graphene and h-BN monolayers have the potential for membranes with high selective permeability. The MoS2 monolayer behaves differently: protons and Hatoms become trapped between the outer Slayersinthe Mo planeinawell withadepth of1.56 eV (proton) and 1.5 eV(Hatom), possibly explaining why no proton transport was detected, suggesting MoS2 asa hydrogen storage material instead. For graphene and h-BN, off-center proton penetration reduces the barrier to1.38 eV for graphene and 0.11 eVfor h-BN. Furthermore, Pt acting as a substrate was found to have a negligible effect on the barrier height. In defective graphene, the smallest barrier for proton diffusion (1.05 eV) is found for an oxygenterminated defect. Therefore, it seems more likely that thermal protons can penetrate a monolayer of h-BN but not graphene and defects are necessary to facilitate the proton transport in graphene.