Controlling the Structure of MoS₂ Membranes via Covalent Functionalization with Molecular Spacers

Restacked two-dimensional (2D) materials represent a new class of membranes for water-ion separations. Understanding the interplay between the 2D membrane's structure and the constituent material's surface chemistry to its ion sieving properties is crucial for further membrane development. Here, we reveal, and tune via covalent functionalization, the structure of MoS₂-based membranes. We find features on both the ∼1 nm (interlayer spacing) and ∼100 nm (mesoporous voids between layers) length scales that evolve with the hydration level. The functional groups act as permanent molecular spacers, preventing local impermeability caused by irreversible restacking and promoting the uniform rehydration of the membrane. Molecular dynamics simulations show that the choice of functional group tunes the structure of water within the MoS₂ channel and consequently determines the hydrated interlayer spacing. We demonstrate that MoS₂ membranes functionalized with acetic acid have consistently ∼92% rejection of Na₂SO₄ with a flux of ∼1.5 lm^-2 hr^-1 bar^-1.