Molecular engineering in tunable crystallinity of 2D covalent organic frameworks for efficient photocatalytic hydrogen evolution

Abstract

The strategy of molecular engineering has been employed to build the structure–property–activity relationship for covalent organic frameworks (COFs) with efficient photocatalytic water splitting for hydrogen production. However, previous reports focus on the impact of molecular design on COFs’ physicochemical properties, the crystallinity differences are usually ignored and related studies remain rare. Herein, we describe the preparation of a series of structurally related COFs with similar chemical compositions but have different numbers of nitrogen (N) atoms in the central aromatic ring via an efficient crystallization strategy using of 2,4,6-triformyl resorcinol (TFR). On one hand, the introduction of N atoms reduces π-π stacking interaction during the formation of framework, leading to a decrease in crystallinity. On the other hand, it contributes to enhancing the photocatalytic performance of the materials by varying the electronic structures. Consequently, our results revealed that the photocatalytic hydrogen evolution (PHE) rate is 61.22 mmol g−1 h−1 for COF-905-1N, significantly higher than COF-905-3N (40.60 mmol g−1 h−1) and COF-905 (26.23 mmol g−1 h−1). This study not only provide fundamental insights into formation mechanism of COFs, but also provide a practical approach to enhance COF-based photocatalytic performance.