Ion selectivity of graphene nanopores

Abstract

As population growth continues to outpace development of water infrastructure in many countries, desalination (the removal of salts from seawater) at high energy efficiency will likely become a vital source of fresh water. Due to its atomic thinness combined with its mechanical strength, porous graphene may be particularly well-suited for electrodialysis desalination, in which ions are removed under an electric field via ion-selective pores. Here, we show that single graphene nanopores preferentially permit the passage of K+ cations over Cl− anions with selectivity ratios of over 100 and conduct monovalent cations up to 5 times more rapidly than divalent cations. Surprisingly, the observed K+/Cl− selectivity persists in pores even as large as about 20 nm in diameter, suggesting that high throughput, highly selective graphene electrodialysis membranes can be fabricated without the need for subnanometer control over pore size.

Modulation of Molecular Flux Using a Graphene Nanopore Capacitor

“…Note that a free-standing graphene membrane may carry a net negative charge, which could be attributed to partial oxidation of graphene or adsorption of contaminant material; 54 the charge densities considered in our work are well within the range of experimental estimates. 54 The microscopic model of the negatively charged capacitor system, Fig.…”

“…In our system, the mechanism of the ionic current regulation relies on modulation of the surface currents, similar to an ion selectivity mechanism suggested for atomically thin, single layer graphene membranes. 54 The main difference, however, is that, in our system, charging the surface layer of the graphene-dielectricgraphene membrane alters the effective charge of the pore lumen that in turn modulates surface currents along the full length of the nanopore. This mechanism differs substantially from the electrostatic 21,77,78 or concentration polarization 53,79,80 mechanisms that regulate ionic current through nanopores in thick dielectric membranes.…”

“…53 Recent experimental investigations suggest that a single layer graphene membrane can carry a negative surface charge in solution, causing ion selective transport through nanopores with diameters as large as 5.0 nm at high salt concentrations. 54 In the case of nanopores in semiconductor or metallic membranes, the nanopore surface charge can be externally controlled via electrical bias, 22,55 making it possible to change the ionic conductivity of a nanopore without altering its geometry.…”

Maxwell-Hall access resistance in graphene nanopores

“…Novel fabrication strategies and designs are under development to create large-scale, controllable porous membranes [25,26,28] and graphene laminate devices [23,24]. Moreover, their single atom thickness makes these systems ideal for interrogating ion dehydration [29,30], which both sheds light on recent experiments on ion selectivity in porous graphene [25,26,28] and will help analyze the behavior of biological pores [29,30]. Dehydration has been predicted to give rise to ion selectivity and quantized conductance in long, narrow pores [31][32][33][34][35] but the energy barriers are typically so large that the currents are minuscule, which is rectified by the use of membranes with single-atom thickness [29,30].…”

“…Novel fabrication strategies and designs are under development to create large-scale, controllable porous membranes [25,26,28] and graphene laminate devices [23,24]. Moreover, their single atom thickness makes these systems ideal for interrogating ion dehydration [29,30], which both sheds light on recent experiments on ion selectivity in porous graphene [25,26,28] and will help analyze the behavior of biological pores [29,30].…”