Impact of the Crystallinity of Covalent Organic Frameworks on Photocatalytic Hydrogen Evolution

The design and synthesis of crystalline porous materials is a frontier research area, especially in photo(electro)catalysis, due to their fascinating semiconducting properties. In recent years, crystalline metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have received much research attention in heterogeneous catalysis, ranging from chemical conversion to solar energy production. In addition, COFs with well-ordered framework structures have been extensively studied in the field of heterogeneous photocatalysis for water splitting to produce hydrogen (H2) and oxygen (O2). Due to the synergistic effect of crystallinity, porosity, and conjugation in their frameworks, COFs have been widely explored. In this Review, we aim to discuss the effect of crystallinity of COFs on hydrogen generation via photocatalytic water splitting. We then briefly summarize the basic mechanisms of photocatalytic hydrogen generation, the advantages of crystalline semiconductors over amorphous materials, and the strategic designs of crystalline COFs. In addition, the state-of-the-art developments of crystalline COFs as organic semiconductors for photocatalytic hydrogen evolution have been systematically reviewed. We believe that understanding the structure–property relationship and photocatalytic performance for hydrogen evolution with respect to the long-range structural order of COFs is essential for the further development of innovative crystalline COF-based semiconductors in real-time hydrogen generation applications.