Covalent organic frameworks: Fundamentals, mechanisms, modification, and applications in photocatalysis

Covalent organic frameworks (COFs) as efficient photocatalysts have attracted extensive attention due to their high crystallinity and tunable optical and electronic properties. This review first summarizes the fundamental aspects of COF photocatalysts—including the basic physical, chemical, optical, and electronic properties—and key considerations in the synthesis of crystalline COFs. Subsequently, deep insights into the photocatalytic mechanisms of COF-based photocatalysts (i.e., adsorption and activation mechanisms of reagents, the identifications of exact active sites, and the surface reaction mechanisms, as well as the kinetics of charge-carrier separation and transport) and engineering-modification strategies of COF-based photocatalysts are thoroughly addressed. Finally, various applications of COF-based photocatalysts are discussed, including photocatalytic hydrogen production, CO2 reduction, pollutant degradation, and organic transformation. A perspective on the challenges and future exploration of COF-based photocatalysts is also proposed. It is expected that this review will inspire new thoughts and will trigger exciting progress in COF-based photocatalysis.