  Because they may provide ultrathin, high-flux, and energy-efficient membranes
for precise ionic and molecular sieving in aqueous solution, GO membranes
(partially oxidized, stacked sheets of graphene) have shown great potential in
water desalination and purification, gas and ion separation, biosensors, proton
conductors, lithium-based batteries and super-capacitors. Unlike carbon
nanotube (CNT) membranes, in which the nanotube pores have fixed sizes, the
pores of GO membranes - the interlayer spacing between GO sheets - are of
variable size. This presents a challenge for using GO membranes for filtration.
Despite the great efforts to tune and fix the interlayer spacing, it remains
difficult both to reduce the interlayer spacing sufficiently to exclude small
ions while keeping this separation constant against the tendency of GO
membranes to swell when immersed in aqueous solution, which greatly affects the
applications of GO membranes. Here, we demonstrate experimentally that highly
efficient and selective ion rejection by GO membranes can be readily achieved
by controlling the interlayer spacing of GO membranes using cations (K+, Na+,
Ca2+, Li+ and Mg2+) themselves. The interspacing can be controlled with
precision as small as 1 A, and GO membranes controlled by one kind of cation
can exclude other cations with a larger hydrated volume, which can only be
accommodated with a larger interlayer spacing. First-principles calculations
reveal that the strong noncovalent cation-pi interactions between hydrated
cations in solution and aromatic ring structures in GO are the cause of this
unexpected behavior. These findings open up new avenues for using GO membranes
for water desalination and purification, lithium-based batteries and
super-capacitors, molecular sieves for separating ions or molecules, and many
other applications.
