Controlling the Structure of Restacked Two-Dimensional Materials for Ion-Selective Separations

Two dimensional materials such as molybdenum disulfide (MoS₂) are promising candidates for selective liquid-phase separations. Membranes made from restacked MoS₂ nanoflakes are highly scalable and can efficiently separate ions and other molecular species based on size, chemistry and valence. Controlling the interlayer spacing of restacked MoS₂ sheets and mitigating the presence of defects remains a challenge, however. Here, we control the interlayer spacing of MoS₂ membranes via covalent functionalization (Nano Letters, 2020, 20, 11, 7844–7851). The functional groups act as permanent molecular spacers; they prevent local impermeability caused by irreversible restacking and promote the uniform rehydration of the membrane. We use the functionalized membrane as a platform to study the transport of a suite of ions simultaneously and find that specific chemical effects play a strong role in relative uptake and permeance. We employ molecular dynamics simulations to elucidate the interplay of functionalization, equilibrium interlayer spacing, and the structure of water confined in the MoS₂ channels. We also present more recent results in which we control and study the transport of ions through pores in few-layer van der Waals materials. Our work widens the parameter space for studying and utilizing the unique properties of ion and water transport under nanometer-scale confinement.