  Laminar MoS2 membranes show outstanding potential for practical applications
in energy conversion/storage, sensing, and as nanofluidic devices. For water
purification technologies, MoS2 membranes can form abundant nanocapillaries
from layered stacks of exfoliated MoS2 nanosheets. These MoS2 membranes have
previously demonstrated excellent ionic rejection with high water permeation
rates, as well as long-term stability with no significant swelling when exposed
to aqueous or organic solvents. Chemical modification of these MoS2 membranes
has been shown to improve their ionic rejection properties, however the
mechanism behind this improvement is not well understood. To elucidate this
mechanism we report the potential dependant ion transport through
functionalized MoS2 membranes. The ionic permeability of the MoS2 membrane was
transformed by chemical functionalization with a simple naphthalene sulfonate
dye (sunset yellow) and found to decrease by over a factor of ~10 compared to
the pristine MoS2 membranes and those reported for graphene oxide and Ti3C2Tx
(MXene) membranes. The effect of pH, solute concentration, and ionic
size/charge on the ionic selectivity of the functionalized MoS2 membranes is
also reported. The potential dependant study of these dye functionalized MoS2
membranes for ionic sieving with charge selectivity should enable future
applications in electro-dialysis and ion exchange for water treatment
technologies.
