Construction of organic heterojunctions as metal-free photocatalysts for enhancing water splitting and phenol degradation by regulating charge flow

Metal-free photocatalysts derived from earth-abundant elements have drawn significant attention owing to their ample supply for potential large-scale applications. However, it is still challenging to achieve highly efficient photocatalytic performance owing to their sluggish charge separation and lack of active catalytic sites. Herein, we designed and constructed a series of covalently bonded organic semiconductors to enhance water splitting and phenol degradation. Experimental and theoretical results revealed that the charge transfer mechanism transformed from type II in the physical mixture to a Z-scheme in the covalently bonded composite, resulting from the interfacial electric field formed at the interface between a β-ketoenamine-linked covalent organic framework (TP-COF) and a urea linked perylene diimide (PDI) semiconductor (UP) linked by amide bonds. The Z-scheme charge transfer route not only improved charge separation but also preserved the high redox ability of both semiconductors. Moreover, more active catalytic sites were created owing to the net charge transfer from the UP to TP-COFs with the amide bonds, contributing to improved photocatalytic performance. As a result, high HER, OER and phenol degradation rates of 613.30 μmol g^-1 h^-1, 1169.36 μmol g^-1 h^-1, and 0.81 h^-1 were achieved, respectively. This work provides a new strategy to develop metal-free photocatalysts with simultaneously improved charge separation efficiency and catalytic site activity.