Understanding the Aqueous Stability and Filtration Capability of MoS₂ Membranes

Abstract

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.

Highly Stable and Antibacterial Two‐Dimensional Tungsten Disulfide Lamellar Membrane for Water Filtration

“…Even after filtration for two days, the WS 2 laminar membrane was still watertight. Moreover, there was only a slight change in the interlayer spacing of the WS 2 laminar membrane to 0.62 nm (or 0.3 nm free spacing) at 13.76° (Figure d) after soaking in water for 1 h. A similar behavior has also been reported for MoS 2 laminar membranes . However, if the WS 2 laminar membrane was kept wet after the vacuum filtration, the separation performance was excellent.…”

“…A diffraction peak located at 14.4° corresponding to an interlayer spacing of 0.61 nm, was evident in the XRD pattern of the dry WS 2 laminar membrane (Figure d), which was nearly the same as the interlayer spacing of bulk hexagonal 2H‐WS 2 (0.62 or 0.3 nm free spacing) . This behavior coincided well with that of dry MoS 2 laminar membranes with an interlayer spacing of 0.62 nm . The XPS spectra (Figure ) of the WS 2 membrane revealed S and W signals.…”

“…Although the stability of the GO membranes can be enhanced by partial reduction or cross‐linking using a polymer and multivalent metal ions, the fabrication processes are too complex to scale up at a low cost. In addition, the variation of the interlayer spacing resulting from the hydration of the oxygenated functional groups compromised the separation properties of the membranes …”

“…Furthermore, no oxygen‐containing hydrophilic functional groups extruded from their basal plane, resulting in a high surface smoothness of the TMD nanosheets. Therefore, a high water permeance was achieved owing to their low hydraulic resistance . The TMD nanosheets were more rigid than GO nanosheets because of the existence of three atomic layers.…”

Graphene oxide membrane for molecular separation: challenges and opportunities

“…The transport pathways and mechanism through GO membranes still needs further investigations, in which atomic-level characterization techniques and theoretical models would be helpful. The insights gained in GO membranes might pave the avenue to a new family of high-performance separation membranes derived from other two-dimensional materials such as graphene-like MXene [19,20], MoS 2 [21], nanoporous metal-organic framework (MOF) [22] and covalent organic framework (COF) [23] nanosheets. Note that even though the dye rejection increases and approaches ∼90% with increasing CGO membrane thickness their permeance is significantly lower than 8 nm HLGO membranes.…”