Morphology Engineering for Covalent Organic Frameworks (COFs) by Surfactant Mediation and Acid Adjustment

Two-dimensional covalent organic frameworks (COFs) with specific morphologies including nanofibers and nanoplates are highly desired in both nanoscience research and practical applications. Thus far, however, morphology engineering for COFs remains challenging because the mechanism underlying the morphology formation and evolution of COFs is not well understood. Herein, we propose a strategy of surfactant mediation coupled with acid adjustment to engineer the morphology of a β-ketoenamine-linked COF, TpPa, during solvothermal synthesis. The surfactants function as stabilizers that can encapsulate monomers and prepolymers to create micelles, enabling the formation of fiber-like and plate-like morphologies of TpPa rather than irregularly shaped aggregates. It is also found that acetic acid is important in regulating such morphologies, as the amino groups inside the prepolymers can be precisely protonated by acid adjustment, leading to an inhibited ripening process for the creation of specific morphologies. Benefitting from the synergistic enhancement of surfactant mediation and acid adjustment, TpPa nanofibers with a diameter down to ~20 nm along with a length of up to a few microns and TpPa nanoplates with a thickness of ~18 nm are created. Our work sheds light on the mechanism underlying the morphology formation and evolution of TpPa, providing some guidance for exquisite control over the growth of COFs, which is of great significance for their practical applications.