Rapid Photochemical Synthesis of Sea-Urchin-Shaped Hierarchical Porous COF-5 and Its Lithography-Free Patterned Growth

Abstract

Despite potential advantages of covalent organic frameworks (COFs) in wide area applications, several limitations in conventional solvothermal synthesis, such as long reaction time and high reaction temperature, reduce reaction efficiency and prohibit technical processes for practical applications. Therefore, the development of a novel synthesis method that provides better reaction efficiency and spatial controllability has become a critical challenge. Herein, a photochemical synthesis of C9H4BO2 (COF-5) is demonstrated for the first time, by which "sea urchin-shaped" COF-5 (UV-COF-5) with uniform size is synthesized with a highly enhanced growth rate, approximate to 48 times faster than that of the solvothermal method for 75% yield. In addition, an enlarged surface area is measured from UV-COF-5, which originates from its hierarchical morphology. The selectively increased growth rate of UV-COF-5 in the [001] direction observed by microscopic analysis results in the local 1D morphology of the hierarchical structure. Density functional theory calculations determine that the enhanced growth rate along the [001] direction can be understood by the characteristic of the interlayer orbital coupling at the frontier energy region. In addition, this study successfully demonstrates the preparation of COF-5 patterns without any complicated postsynthesis lithography process, but simply by utilizing optical masks during the photochemical method.