Tuneable Sieving of Ions Using Graphene Oxide Membranes

TL;DR

This paper presents a method to precisely control the interlayer spacing of graphene oxide membranes, enabling tunable ion sieving with high salt rejection and minimal impact on water permeability, advancing desalination technology.

Contribution

It introduces a scalable technique to achieve adjustable interlayer spacing in graphene oxide membranes for effective ion sieving in water.

Findings

Interlayer spacing d can be tuned from ~9.8 to 6.4 Angstroms.

Ion permeation is thermally activated with energy barriers of 10-100 kJ/mol.

Membranes achieve 97% NaCl rejection with minimal water transport reduction.

Abstract

Graphene oxide membranes show exceptional molecular permeation properties, with a promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ~9 Angstrom, which is larger than hydrated ion diameters for common salts. The cutoff is determined by the interlayer spacing d ~13.5 Angstrom, typical for graphene oxide laminates that swell in water. Achieving smaller d for the laminates immersed in water has proved to be a challenge.Here we describe how to control d by physical confinement and achieve accurate and tuneable ion sieving. Membranes with d from ~ 9.8 Angstrom to 6.4 Angstrom are demonstrated, providing the sieve size smaller than typical ions' hydrated diameters.In this regime, ion permeation is found to be thermally activated with energy barriers of ~10-100 kJ/mol depending on d. Importantly, permeation rates decrease exponentially with decreasing the sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for water molecules entry and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.