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.