A wavy AMXene-assisted leaf-like asymmetric polyamide membrane with fast permeability and high selectivity

Rapid water permeability and precise solute–solute separation are both important for high-end nanofiltration (NF) membranes. Yet, current NF membranes suffer from high energy consumption or low efficiency because of a trade-off between membrane permeability and selectivity. An intricately designed polyamide (PA) separation layer can be used to construct high permeability-selectivity NF membranes. Inspired by the asymmetric structure between the surface and bottom layers of leaves, an innovative asymmetric leaf-like PA composite membrane with a highly ordered nanochannel bottom layer and loose PA surface layer structure was produced using alkali-induced MXene (AMXene) nanosheets. In this endeavor, small amorphous AMXene nanosheets were embedded in the bottom layer of the PA network via electrostatic attraction and hydrogen bonding to create short low-resistance 2D water transport nanochannels. These shorter nanochannels overcame the disadvantages of conventionally constructed nanofiltration membranes with long tortuous nanochannels comprising 2D nanosheets by reducing the transverse transmission resistance during NF. Furthermore, a loose ultrathin (18.48 nm) hydrophilic PA surface with low intrinsic resistance to water transport was obtained by limiting the diffusivity of interfacial-polymerized piperazine via the highly hydrophilic and electronegative AMXene nanosheets. Consequently, the water permeability of a thin-film nanocomposite membrane fabricated with 0.002 w/v% AMXene significantly increased by 1.63 times (17.48 L m^-2h⁻¹ bar⁻¹) over that of one prepared without the nanosheets owing to the reduction of transverse and normal energy barriers. Moreover, the asymmetric leaf-like PA composite membrane conferred a more homogeneous pore size distribution and more precise selectivity for targeted ion species of various sizes compared to conventional nanofiltration membranes. These findings provide new insights for manufacturing fast water-permeable NF membranes.