Growth of high-quality covalent organic framework nanosheets at the interface of two miscible organic solvents

Stronger covalent bonds between monomers, relatively more complex growth processes (polymerization, crystallization, assembly, etc.) and π–π stacking interactions between adjacent layers make it extremely difficult to obtain highly ordered crystalline 2D covalent organic framework (COF) nanosheets. So more effective solutions have to be developed to push the methods reported so far beyond their inherent limitations. Herein, we report the first example of growing high-quality 2D COF nanosheets (NS-COF) at the interface of two miscible organic solvents. The novel approach, which is named as a buffering interlayer interface (BII) method, can be achieved by simply adding a low-density solvent interlayer, as a buffer layer, between the two miscible main solvents based on the self-propelled directed motion of the interface driven by the density differences among the solvents involved. The as-synthesized NS-COF exhibits a super-large size and a relatively regular shape with a smooth surface, which have not been observed before. The proposed strategy offers a facile and effective approach for growing well-structured 2D COF nanosheets and also other kinds of nanosheets.