Azobenzene-assisted exfoliation of 2D covalent organic frameworks into large-area, few-layer nanosheets for high flux and selective molecular separation membrane

Abstract

The porous 2D covalent organic nanosheets (CONs) membrane has become a new generation of membrane material due to its ultrahigh permeation flux. The main challenge in fabricating CONs membrane is how to exfoliate covalent organic frameworks (COFs) into large-scale, few-layer nanosheets. Herein, we presented a simple and economical azobenzene template approach for exfoliating 2D COFs into large nanosheets and achieved fabrication of ultrathin CONs membrane successfully. Three COFs with different skeleton structures were chosen to explore the influencing factors for effective exfoliating. We found that the compatibility between azobenzene molecules and COFs skeleton structure was the key point for exfoliation. When the COF skeleton matching with the azobenzene molecular structure, COFs can be easily exfoliated into larger-scale CONs than nanosheets prepared by conventional exfoliation methods. The as-exfoliated nanosheets membrane showed a higher water flux (596 L m2 h-1 bar-1) than a graphene oxide (GO) membrane (4.3 L m2 h-1 bar-1) and high rejection of dye molecules, due to the nano-channels perpendicular to the surface of the membrane.