Dual-layered covalent organic framework/MXene membranes with short paths for fast water treatment

MXenes are receiving growing attention in wastewater treatment, but the preparation of ultrathin MXene membranes with short water pathways on macroporous substrates for improving flux is challenging due to the deformation of the flexible MXene nanosheets. Herein, we demonstrate the fabrication of ultrathin MXene membranes by constructing an intermediate porous covalent organic framework (COF) layer. COF nanosheets are first vacuum filtrated on macroporous substrates to form a porous COF layer, followed by the vacuum deposition of MXene nanosheets to produce the dual-layered COF/MXene composite membranes. The COF layer provides a smooth surface with rich pores and good hydrophilicity for depositing MXene nanosheets, leading to the well-assembled MXene layer as thin as 8 nm. The ultrathin MXene layer and the porous COF layer enable the short water transport pathways, and the well-assembled MXene nanosheets guarantee the narrow interlayer nanochannels. Membranes thereby are endowed with high permeance of 563 L/(m 2 ·h·bar) and high rejection to congo red of 99.6%, outperforming other reported COF- or MXene-based membranes. This work suggests a facile construction strategy of ultrathin MXene-based composite membranes with high separation performances, which are highly desired in water treatment. • Dual-layered COF/MXene composite membranes are fabricated by vacuum filtration. • Intermediate COF layer modifies the surface properties of macroporous substrate. • Integrate MXene layer with 8 nm thickness is obtained on the intermediate COF layer. • Porous COF layer and ultrathin MXene layer shorten water pathways of the membrane. • Improved water permeances and high rejection rates are achieved.