Potential dependent ionic sieving through functionalized laminar MoS₂ membranes

Laminar MoS₂ membranes show outstanding potential for practical applications in energy conversion/storage, sensing, and as nanofluidic devices. The re-stacking of exfoliated MoS₂ creates nanocapillaries between the layers of MoS₂ nanosheets. These MoS₂ membranes have been shown to possess a unique combination of ionic rejection properties, high water permeation rates, and long-term solvent stability, with no significant swelling when exposed to aqueous or organic solvents. Chemical modification of MoS₂ membranes has been shown to improve their ionic rejection properties, however the mechanism behind this improvement is not well understood. In this work, we elucidate the ion-sieving mechanism by the study of potential-dependent ion transport through functionalized MoS₂ membranes. The ionic permeability of the MoS₂ membrane is transformed by chemical functionalization with a simple naphthalene sulfonate dye (sunset yellow) and with a resultant attenuation of permeability by at least an order of magnitude, compared to the pristine MoS₂ membranes and permeability reported for graphene oxide and Ti₃C₂T_x (MXene) membranes. The effects of pH, solute concentration, and ionic size/charge on the ionic selectivity of the functionalized MoS₂ membranes are also reported. Understanding the mechanism of ionic sieving within functionalized MoS₂ membranes will enable future applications in electro-dialysis and ion exchange for water treatment technologies. (paper)