Molecular Design of Two-Dimensional Covalent Heptazine Frameworks for Photocatalytic Overall Water Splitting under Visible Light

Photocatalytic water splitting sustainably offers clean hydrogen energy, but it is challenging to produce low-cost photocatalysts that split water stoichiometrically into H₂ and O₂ without sacrificial agents under visible light. Here, we designed 17 two-dimensional (2D) covalent heptazine frameworks (CHFs) by topologically assembling heptazine and benzene-containing molecular units that provide active sites for hydrogen and oxygen evolution reactions, respectively. Among them, 12 CHFs have band gap values of <3.0 eV with band margins straddling the chemical reaction potential of H₂/H⁺ and O₂/H₂O. In particular, a 2D H@DBTD CHF based on heptazine and 4,7-diphenyl-2,1,3-benzothiadiazole is a potential photocatalyst with a band gap of 2.47 eV for overall water splitting, which was confirmed with the calculated Gibbs free energy, non-adiabatic molecular dynamics, and preliminary experiment. This study presents an experimentally feasible molecular design of 2D CHFs as metal-free photocatalysts for overall water splitting under visible light.