  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.

Materials Science