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

Abstract

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, CO 2 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. Covalent organic frameworks (COFs) have proven to be an appealing photocatalyst due to their high crystallinity and tunable optical and electronic properties. Although a variety of excellent papers have been published on the exploitation of COF-based photocatalysts, there has been no one comprehensive review focusing on both deep photocatalytic mechanisms and rational design of COF-based photocatalysts. Herein, the considerate design principles, the diversified synthesis and engineering-modification strategies, 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 surface reaction mechanisms, as well as the kinetics of charge-carrier separation and transport), and significant and urgent points in different emerging applications are thoroughly summarized and addressed, which should be crucial for rationally and precisely designing advanced COF-based photocatalysts. This review highlights four important aspects for rationally and precisely designing highly efficient and selective COF-based photocatalysts, in the sequence of considerate design principles based on the understanding of photocatalytic fundamentals, deep insights into the photocatalytic mechanisms, diversified synthesis and engineering-modification strategies, and significant and urgent points in different emerging applications.