Tunable Ion Sieving of Graphene Membranes through the Control of Nitrogen-Bonding Configuration

Graphene-oxide (GO) membranes with notable ionic-sieving properties have attracted significant attention for many applications. However, the swelling and unstable nanostructure of GO laminates in water results in enlarged interlayer spacing and a low permeation cut-off, limiting their applicability for water purification and desalination. Herein, we propose novel nitrogen-doped graphene (NG) membranes for use in tunable ion sieving that are made via facile fabrication by a time-dependent N-doping technique. Doping reaction time associated variation in atomic content and bonding configurations strongly contributed to the nanostructure of NG laminates by yielding narrower interlayer spacing and a more-polarized surface than GO. These nanostructural features subsequently allowed ion transport through the combined mechanisms of size exclusion and electrostatic interaction. The stacked NG membranes provided size-dependent permeability for hydrated ions and improved ion selectivity by 1-3 orders of magnitude in comparison to that of a GO membrane. For ions small enough to move through the interlayer spacing, the ion permeation is determined by electrostatic properties of NG membranes with the type of N configuration, especially polarized pyridinic N. Due to these properties, the NG membrane functioned as an unconventionally selective graphene-based membrane with better ion sieving for water purification.