Covalent organic frameworks (COFs), as highly porous crystalline structures, are newly emerging materials designed with tuneable features.

Achieving hierarchical nanomaterials from a bottom-up approach remains challenging. Here, we report closed-cage, onion nanostructure of covalent organic framework (COF) obtained through low-temperature solvothermal synthesis. Atomic resolution transmission electron microscopy revealed the atomic arrangement in this COF onion, which rich nitrogen was uniformly embedded periodic porous graphitic ...