Realizing Photocatalytic Overall Water Splitting by Modulating Thickness-Induced Reaction Energy Barrier of Fluorenone-based Covalent Organic Frameworks.

Direct photocatalytic hydrogen and oxygen evolution from water splitting is an attractive approach for producing chemical fuels. For the first time, a novel fluorenone-based covalent organic framework (COF-SCAU-2) was successfully exfoliated into ultrathin three-layer nanosheets (UCOF-SCAU-2) for photocatalytic overall water splitting (OWS) under visible light. The ultrathin structures of UCOF-SCAU-2 greatly enhance carrier separation, utilization efficiency, and the exposure of active surface sites. Surprisingly, UCOF-SCAU-2 exhibited efficient photocatalytic OWS, with hydrogen and oxygen evolution rates reaching 0.046 and 0.021 mmol h -1 g -1 , respectively, under visible light irradiation, whereas bulk COF-SCAU-2 showed no activity for photocatalytic OWS. Charge carrier kinetic analysis and DFT calculation confirmed that reducing the thickness of COFs nanosheets increased the number of accessible active sites, reduced the distance for charge migration, prolong the lifetimes of photogenerated carriers and decrease the rate-limiting step Gibbs free energy than non-exfoliated COFs. This work offers new insights into the effect of the layer thickness of COF on photocatalytic OWS. This article is protected by copyright. All rights reserved.