Tuning Pore Size in Graphene in the Angstrom Regime for Highly Selective Ion-Ion Separation

Zero-dimensional pores spanning only a few angstroms in size in two-dimensional materials such as graphene are some of the most promising systems for designing ion-ion selective membranes. However, the key challenge in the field is that so far a crack-free macroscopic graphene membrane for ion-ion separation has not been realized. Further, methods to tune the pores in the & Aring;-regime to achieve a large ion-ion selectivity from the graphene pore have not been realized. Herein, we report an & Aring;-scale pore size tuning tool for single layer graphene, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 & Aring; while minimizing the nonselective pores above 10 & Aring;. These pores impose a strong confinement for ions, which results in extremely high selectivity from centimeter-scale porous graphene between monovalent and bivalent ions and near complete blockage of ions with the hydration diameter, D-H, greater than 9.0 & Aring;. The ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with the barrier increasing with D-H. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of the dehydrated ions. These results underscore the promise of porous two-dimensional materials for solute-solute separation when & Aring;-scale pores can be incorporated in a precise manner.