Understanding the Aqueous Stability and Filtration Capability of MoS Membranes.

Membranes made of layer-stacked two-dimensional molybdenum disulfide (MoS) nanosheets have recently shown great promise for water filtration. At present, the reported water fluxes vary significantly, while the accountable structure and properties of MoS nanochannels are largely unknown. This paper aims to mechanistically relate the performance of MoS membranes to the size of their nanochannels in different hydration states. We discovered that fully hydrated MoS membranes retained a 1.2 nm interlayer spacing (or 0.9 nm free spacing), leading to high water permeability and moderate-to-high ionic and molecular rejection. In comparison, completely dry MoS membranes had a 0.62 nm interlayer spacing (or 0.3 nm free spacing) due to irreversible nanosheet restacking and were almost impermeable to water. Furthermore, we revealed that the interlayer spacing of MoS membranes in aqueous solution is maintained by comparable van der Waals and hydration forces, thereby ensuring the aqueous stability of MoS membranes without the need of cross-linking. In addition, we attributed the high water flux (30-250 L m h bar) of MoS membranes to the low hydraulic resistance of smooth, rigid MoS nanochannels. We also concluded that compaction of MoS membranes with a high pressure helps create a more neatly stacked nanostructure with minimum voids or looseness, leading to stable water flux and separation performance. Besides, this paper systematically compares MoS membranes with the widely studied graphene oxide membranes to highlight the uniqueness and advantages of MoS membranes for water-filtration applications.