Synergic Effects of the Nanopore Size and Surface Charge on the Ion Selectivity of Graphene Membranes

Ion selectivity of nanoporous graphene membranes is of great importance in separation technologies including ion sieving and seawater desalination. In this study, we put forward a method to produce high-density nanopores in graphene membranes with tunable sizes in the sub-nanometer range by two-step plasma-etching processes, where the graphene membrane is treated by argon plasma first, followed by oxygen plasma or hydrogen plasma. Besides the pore size, the surface charges of nanopores can also be regulated by the different functional groups at the pore rims created by oxygen plasma or hydrogen plasma. The results show that the fabricated nanoporous graphene membranes can not only select cations with different hydration radii but also distinguish cations from anions with a high selectivity value, which are attributed to the synergic effects of nanopore sizes and surface charges. Additionally, the nanoporous graphene membranes generated by oxygen plasma permit the passage of Na+ over Cl– with selectivity ratios of over 8, while the graphene membranes generated by hydrogen plasma have almost no Na+/Cl– selectivity. This work provides an efficient method to fabricate nanoporous graphene membranes with tunable ion selectivity and looks into the effects of nanopore sizes and surface charges on ion selectivity.