Oil–Water–Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets

Ionic covalent organic framework nanosheets (iCOFNs) with long‐range ordered and mono‐dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil–water biphase method, herein, we explore an oil–water–oil triphase method based on phase engineering strategy for the bottom‐up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few‐layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra‐high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect‐free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs. An oil–water–oil triphase method based on a phase engineering strategy is developed for the bottom‐up synthesis of ionic covalent organic framework nanosheets (iCOFNs). A favourable reaction region with moderate monomer concentration is achieved in the middle water phase. The resulting three cationic and anionic iCOFNs show potential for ultrathin biogas separation membranes.