Constructing benzothiadiazole-based donor‒acceptor covalent organic frameworks for efficient photocatalytic H2 evolution

Abstract

Donor‒acceptor covalent organic frameworks (D‒A COFs) have been regarded as promising materials for photocatalytic water splitting because of their tunable band gaps. However, their efficiency is hindered by fast charge recombination and low photostability. Herein, we proposed a donor structural engineering strategy for improving the photocatalytic activity of D‒A COFs to tackle these problems. Two benzothiadiazole-based D‒A COFs (DHU-COF-BB and DHU-COF-BP) with distinct donors were prepared for photocatalytic H₂ evolution reaction (HER). As a comparison, DHU-COF-TB without benzothiadiazole moieties was also designed and synthesized. Impressively, the photocatalytic H₂ production rate of DHU-COF-BB reaches 12.80 mmol g⁻¹ h⁻¹ under visible light irradiation (≥420 nm), which was nearly 2.0 and 3.1 times higher than that of DHU-COF-BP (6.47 mmol g⁻¹ h⁻¹) and DHU-COF-TB (4.06 mmol g⁻¹ h⁻¹), respectively. In addition, the apparent quantum efficiency (AQE) of DHU-COF-BB was up to 5.04% at 420 nm. Photocatalytic and electrochemical measurements indicate that the enhanced hydrogen evolution activity of DHU-COF-BB can be ascribed to the introduction of appropriate benzene moiety into the donors, which increases the charge separation efficiency and thereby suppresses the electron‒hole recombination. Density functional theory (DFT) calculations revealed that both triphenylamine and benzothiadiazole units are the main active sites for HER over the DHU-COF-BB. This work provides new insight into the photocatalytic hydrogen production activity of D‒A COFs by a donor structural engineering strategy.