Design of Donor–Acceptor Type Covalent Triazine/Heptazine-Based Frameworks for Enhanced Photocatalytic Water Splitting

Developing photocatalytically competent and stable covalent-organic frameworks with integrated redox centers remains challenging. Herein, we design 12 experimentally feasible donor-acceptor (D-A) type two-dimensional (2D) covalent triazine/heptazine-based framework (CTFs/CHFs) by linking electron-deficient triazine/heptazine moieties with electron-rich benzoheterocyclic units, and systematically explore their photocatalytic properties toward overall water splitting (OWS) via first-principles computations. The present computational estimates show that these frameworks exhibit semiconducting behavior with tunable bandgaps (2.10-2.88 eV). A detailed analysis of the band gaps, band alignments, and the thermodynamic feasibility of the HER and OER reveals that only CTF-5 (incorporating triazine and benzotrioxazole) and CHF-5 (featuring heptazine and benzotrioxazole) are thermodynamically capable of spontaneously driving OWS under their respective light-induced bias potentials. As predicted, the HER and OER active sites exhibit complete spatial separation, where the HER preferentially occurs predominantly on the triazine/heptazine moieties, while the OER takes place at the benzotrioxazole units. This separation helps reduce the electron-hole recombination and enhances the overall photocatalytic efficiency, which has been verified by carrier mobility calculations. As a result, the theoretical energy conversion efficiencies of CTF-5 and CHF-5 reach 10.3% and 15.8%, respectively, with CHF-5 outperforming not only CTF-5 but also several previously reported COFs. These findings pave the way for designing eco-friendly D-A type photocatalysts for OWS and provide valuable insights for future experimental research.