Selective ion sieving through arrays of sub-nanometer nanopores in chemically tunable 2D carbon membranes

Abstract

Two-dimensional (2D) membranes featuring arrays of sub-nanometer pores have applications in purification, solvent separation and water desalination. Compared to channels in bulk membranes, 2D nanopores have lower resistance to transmembrane transport, leading to faster passage of ions. However, the formation of nanopores in 2D membranes requires expensive post-treatment using plasma or ion bombardment. Here, we study bottom-up synthesized porous carbon nanomembranes (CNMs) of biphenyl thiol (BPT) precursors. Sub-nanometer pores arise intrinsically during the BPT-CNM synthesis with a density of 2 ± 1 pore per 100 nm 2 . We employ BPT-CNM based pore arrays as efficient ion sieving channels, and demonstrate selectivity of the membrane towards ion transport when exposed to a range of concentration gradients of KCl, CsCl and MgCl 2 . The selectivity of the membrane towards K + over Cl − ions is found be 16.6 mV at a 10 : 1 concentration ratio, which amounts to ∼30% efficiency relative to the Nernst potential for complete ion rejection. The pore arrays in the BPT-CNM show similar transport and selectivity properties to graphene and carbon nanotubes, whilst the fabrication method via self-assembly offers a facile means to control the chemical and physical properties of the membrane, such as surface charge, chemical nature and pore density. CNMs synthesized from self-assembled monolayers open the way towards the rational design of 2D membranes for selective ion sieving.

Graphene Nanopores

“…Interestingly, the membrane could differentiate between monovalent and divalent cations by conducting monovalent cations 5 times faster than divalent cations. Ion selective transport through graphene with pores larger than a nanometer was experimentally shown by Rollings and van Deursen et al [74,75].…”

“…The selectivity calculated by the Goldman-Hodgkin-Katz (GHK) model was around 100, a value much higher than the selectivity observed by Jain et al for pore sizes of sub-nm level (0.4 nm) [63]. The membrane potential, specially for biological membranes is typically calculated using the GHK voltage Equation [70][71][72][73][74][75][76]. This equation is applicable for multiple permeating monovalent species and it takes into account the permeability of each specie.…”

Angstrom-scale ion channels towards single-ion selectivity

“…Thus, negatively charged ion channels are cation-selective, and positively charged ion channels are anionselective. Although the electrostatic interaction can be realized at a large scale (4100 nm), high cation/anion selectivities have been only achieved by nanoscale channels 146 and pores 73 with pore sizes comparable to the electric double layer (EDL). 147,148 The thickness of EDL (l D ) is inversely propositional to the ion concentration, [149][150][151]…”

Carbon Material With Ordered Sub-Nanometer Hole Defects

“…Graphene with sub-nanopores has a wide range of applications in ion transport ( Suk and Aluru, 2014 ), selective ion sieving ( van Deursen et al, 2019 ), seawater desalination ( Xu et al, 2019 ), supercapacitors ( Lee et al, 2016 ), etc., Graphene with nanopores has a strong water permeability and can be used as a reverse osmosis desalination membrane ( Cohen-Tanugi and Grossman, 2014 ). The incorporate of three-dimensional nanopore crystals with sub-nano sized aperture size into the two-dimensional graphene laminate greatly improves the separation performance of water ( Guan et al, 2017 ).…”