MoS2 Membranes for Organic Solvent Nanofiltration: Stability and Structural Control.

Layered molybdenum disulfide membranes based on stacked nanosheets have shown potentials for molecular separations in aqueous and gaseous environment. However, the long-term stability of MoS2 membranes and their applications for organic solvent nanofiltration (OSN) have not yet been investigated. This paper aims to reveal the chemical, structural and separation stability of MoS2 membranes, and establish a low-cost and facile approach to develop stable, selective membranes for efficient molecular separation in organic solvent. MoS2 nanoflakes which were dominant by monolayer MoS2 sheets were firstly prepared by direct chemical exfoliation (referred as chem-MoS2 thereafter) and the resultant membranes were found chemically and structurally instable in both the air and solvents. The solute rejection in OSN decreased sharply within a few days. Few-layered MoS2 nanoflakes were then fabricated based on a hydrothermal method (referred to hydro-MoS2 thereafter). A 'supportive' drying process involving glycerol pre-treatment and drying in the oven was established to allow re-alignment of nanoflakes and adjustment of inter-flake spacing. We have shown that the hydro-MoS2 membranes provide a mean inter-flake free spacing of ~ 1 nm, which is ideal for the separation of model solute (Rose Bengal, size: ~ 1.45 nm) from the solvent isopropanol (size: 0.58 nm) with good long-term stability over a 7-day OSN operation.