Membranes are key components in chemical
purification, biological
separation, and water desalination. Traditional polymeric membranes
are subjected to a ubiquitous trade-off between permeance and selectivity,
which significantly hinders the separation performance. Nanoporous
atomically thin membranes (NATMs), such as graphene NATMs, have the
potential to break this trade-off. Owing to their uniqueness of two-dimensional
structure and potential nanopore structure controllability, NATMs
are expected to have outstanding selectivity through molecular sieving
while achieving ultimate permeance at the same time. However, a drastic
selectivity discrepancy exists between the proof-of-concept demonstrations
and scalable separation applications in graphene membranes. In this
paper, we offer a possible solution to narrow this discrepancy by
tuning the pore density and pore size separately with two successive
plasma treatments. We demonstrate that by narrowing the pore size
distribution, the selectivity of graphene membranes can be greatly
increased. Low-energy argon plasma is first applied to nucleate high
density of defects in graphene. Controlled oxygen plasma is then utilized
to selectively enlarge the defects into nanopores with desired sizes.
This method is scalable, and the fabricated 1 cm2 graphene
NATMs with sub-nanometer pores can separate KCl and Allura Red with
a selectivity of 104 and a permeance of 1.1 × 10–6 m s–1. The pores in NATMs can be further tuned
from gas-selective sub-nanometer pores to a few nanometer size. The
fabricated NATMs show a selectivity of 35 between CO2 and
N2. With longer enlargement time, a selectivity of 21.2
between a lysozyme and bovine serum albumin can also be achieved with
roughly four times higher permeance than that of a commercial dialysis
membrane. This research offers a solution to realize NATMs of tunable
pore size with a narrow pore size distribution for different separation
processes from sub-nanometer in gas separation or desalination to
a few nanometers in dialysis.