In situ generation of (sub) nanometer pores in MoS2 membranes for ion-selective transport

Ion selective membranes are fundamental components of biological, energy, and computing systems. The fabrication of solid-state ultrathin membranes that can separate ions of similar size and the same charge with both high selectivity and permeance remains a challenge, however. Here, we present a method, utilizing the application of a remote electric field, to fabricate a high-density of (sub)nm pores in situ. This method takes advantage of the grain boundaries in few-layer polycrystalline MoS2 to enable the synthesis of nanoporous membranes with average pore size tunable from <1 to ~4 nm in diameter (with in situ pore expansion resolution of ~0.2 nm2 s−1). These membranes demonstrate selective transport of monovalent ions (K+, Na+ and Li+) as well as divalent ions (Mg2+ and Ca2+), outperforming existing two-dimensional material nanoporous membranes that display similar total permeance. We investigate the mechanism of selectivity using molecular dynamics simulations and unveil that the interactions between cations and the sluggish water confined to the pore, as well as cation-anion interactions, result in the different transport behaviors observed between ions. Atomically thin materials can form ionic sieves, in which certain species pass through the films, and others are prevented. Here, the authors synthesize nanoporous membranes using chemical vapor deposition and electrochemistry and reveal the mechanism behind the large ion selectivity they observe.