Atomically dispersed metal sites on covalent organic frameworks for photocatalysis

Atomically dispersed metal sites (ADMSs) have attracted substantial attention in photocatalysis owing to their numerous active sites, excellent charge transfer kinetics, and unique properties. Because of the broad light absorption and strong covalent bonds of covalent organic frameworks (COFs), their combination with atomically dispersed metal sites (ADMSs) is ideal. Adjusting the type, proportion, and binding mode of ADMSs can enhance photocatalytic performance. However, the optimal loading amount of different types of single atoms in ADMS-loaded COF materials (A-COFs) for achieving the best photocatalytic performance is still not well-defined, which limits the practical application of A-COFs. This paper summarizes the effects of different loading amounts on photocatalytic performance from the perspectives of noble metal single atoms and transition metal single atoms, elucidating the structure-activity relationship between loading amount and catalytic performance. The aim is to overcome the blind addition of single atoms in A-COFs and to determine the optimal loading amounts for different types of single atoms. We analyzed methods for synthesizing optimally loaded ADMSs. The impact of different loading amounts of ADMSs on the catalytic mechanism was systematically investigated. The challenges faced by high-performance A-COFs with optimal single-atom metal dispersion at active sites in practical applications are discussed, with an outlook on future developments.