Donor–acceptor covalent organic framework hollow submicrospheres with a hierarchical pore structure for visible-light-driven H2 evolution

Abstract

Two-dimensional (2D) covalent organic frameworks (COFs) have received much attention due to their tunable electronic structures and superior surface area in visible-light-driven water splitting. Compared to traditional photocatalysts, 2D COFs for photocatalytic water splitting show the advantages of controllable light absorption and easy carrier separation. Here, we fabricate a highly crystalline 2D COF with hierarchical pore structures and donor–acceptor (D–A) moieties consisting of electron donor tetraphenylethylene (4PE) and electron acceptor thiazolo[5,4-d]thiazole (TZ), the PETZ–COF. Owing to the specific molecular configuration and microstructures, the PETZ–COF demonstrates remarkable properties in visible-light-driven H₂ evolution and a suitable energy band structure for O₂ generation. The PETZ–COF shows an excellent hydrogen evolution rate of 7324.3 μmol g⁻¹ h⁻¹ in the presence of a Pt co-catalyst and using ascorbic acid as the hole sacrificial reagent. Density functional theory (DFT) calculations show that the site with the lowest hydrogen-binding free energy (ΔG_H*) is the N atom in the –NC– (−0.0418 eV) group, which suggests that the electron withdrawing–pushing interaction in D–A parts may facilitate hydrogen adsorption and thus contribute to H₂ formation. Besides, the hierarchical pore structure involving micropores, mesopores and macropores was also thought to exert an important influence on the photocatalytic performance. This work provides an effective design and synthetic strategy to prepare highly active COF-based photocatalysts for solar energy harvesting and conversion.