Membrane separation provides an energy-efficient technology for molecular separation. Conventional filtration systems are constrained by a trade-off between permeance and selectivity that results from broad pore size distribution. Recent developments on nanotechnology have demonstrated the potential to overcome this limitation by utilizing well-defined nanoconduits that allow a coordinated passage of water molecules. Fabrication of these materials is still very challenging, but their performance inspires research toward nanofabricated membranes.
Carbon nanomembranes (CNMs) are a special class of 2D materials made by crosslinking of self-assembled monolayers. This work will present the rapid and selective water permeation through a ~1.2 nm thin CNM fabricated from terphenylthiol (TPT) precursors. Molecular transport through TPT CNMs is investigated by mass-loss measurements and gas permeation in vacuum system. TPT CNMs block the passage of most gases and liquids, while permitting water and helium to pass through. In particular, water transits with a remarkably high permeance of ~1.1×10−4 mol·m−2·s−1·Pa−1, 2,500 times faster than helium. Scanning probe microscopy reveals that the membrane consists of sub-nanometer channels with a high areal density of 1018 m−2. Assuming all channels in a TPT CNM are active in mass transport, we find a single-channel permeation of ∼66 water molecules·s−1·Pa−1. This suggests that water molecules translocate fast and cooperatively through the sub-nanometer channels, similar to carbon nanotubes and membrane proteins (aquaporins).
Furthermore, ion transport across these membranes are investigated by conductance measurements using both DC and AC methods. The results show that freestanding TPT CNMs act as ionic insulators, preventing the penetration of ionic species including protons. The specific membrane resistance reaches ~104 Ω·cm2, comparable to the typical high resistance of planar lipid bilayers. The single-channel conductance yields 2×10−18 S in 1 M KCl solution, ~107 lower than that of biological porins. This again confirms the existing of sub-nm channels within TPT CNMs.
Unlike other nanostructured membranes, CNMs are built in a versatile and scalable fabrication process, thus these 2D sieves will inspire the development of various advanced filtration systems that require highly efficient and precise separations.
