Molecular Design of Two-Dimensional Covalent Heptazine Frameworks for Photocatalytic Overall Water Splitting under Visible Light
Haiyun Wang, Hao Cheng, Haifeng Lv, Hangxun Xu, Xiaojun Wu, Jinlong Yang
Abstract
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.