Understanding the Aqueous Stability and Filtration Capability of MoS₂ Membranes

Membranes made of layer-stacked two-dimensional molybdenum disulfide (MoS₂) nanosheets have recently shown great promise for water filtration. At present, the reported water fluxes vary significantly, while the accountable structure and properties of MoS₂ nanochannels are largely unknown. This paper aims to mechanistically relate the performance of MoS₂ membranes to the size of their nanochannels in different hydration states. We discovered that fully hydrated MoS₂ 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 MoS₂ 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 MoS₂ membranes in aqueous solution is maintained by comparable van der Waals and hydration forces, thereby ensuring the aqueous stability of MoS₂ membranes without the need of cross-linking. In addition, we attributed the high water flux (30-250 L m^-2 h^-1 bar^-1) of MoS₂ membranes to the low hydraulic resistance of smooth, rigid MoS₂ nanochannels. We also concluded that compaction of MoS₂ 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 MoS₂ membranes with the widely studied graphene oxide membranes to highlight the uniqueness and advantages of MoS₂ membranes for water-filtration applications.