Investigation of the influence of the location of oxygenated functional groups in graphene nanostructures on water permeation via molecular dynamics simulations

The oxygenated functional groups are typically distributed on the basal plane in graphene oxide (GO) but only exist at the plane edges in graphene nanoplatelets (GNPs). Experimentally, the water permeability of GNPs outperforms that of GO despite the larger interlayer spacing and higher oxidation level of the latter. Here, we employ molecular dynamics (MD) simulations to investigate this contradiction. The MD results align with the experimental results and they can be explained by scrutinizing the trajectories of water molecules and the characteristics of hydrogen bonds. It shows that water molecules encounter a much lower energy barrier in the GNPs during the permeation process where the translation paths and wandering time of water molecules through the GNPs nanostructures are shorter. There exists a threshold amount of the hydroxyls in both GO and GNPs, retarding the water transport. For GO, exceeding the threshold amount induces the pinning effect where the water molecules are pinned by the hydroxyls while for GNPs, the isolation effect which deteriorates the hydrogen bonding strength between the hydroxyls and the water molecules is induced. Therefore, two distinctive water permeation mechanisms prevail due to the location of the oxygenated functional groups in the graphene nanostructures.