Linkages Conversion of Imine to Amide Promotes Photocatalytic Overall Water Splitting in Two-Dimensional Covalent Organic Frameworks

Abstract

Imine-based covalent organic frameworks (Im-COFs) have attracted extensive attention in photocatalytic overall water splitting (OWS) due to their facile synthesis and structural stability. However, most of the Im-COFs photocatalysts face the challenge of rapid recombination of photogenerated carriers, leading to low solar-to-hydrogen conversion efficiency. Here, we propose that the oxidation of imine to amide linkages in 2D Im-COFs would be an effective strategy to enhance the in-plane polarity and thus facilitate the separation of photogenerated carriers. Based on first principles calculations, two amide-linked 2,4,6-triphenyl-1,3,5-triazine (TST) and triphenylamine (TA) monolayers (Am-TST, Am-TA) were used as model to compare the photocatalytic performance with the imine-linked ones (Im-TST, Im-TA). Interestingly, both the amide-linked COFs exhibit type-II band alignments, which is absent in the imine-linked ones. Moreover, the NAMD simulations reveal the photogenerated carriers’ lifetime of Am-TST (13.41 ns) is 3.45 times longer than that of Im-TST, indicating the superior ability of amide linkages in suppressing the recombination of photoexcited carriers. In addition, the Am-TST monolayer exhibits visible-light-driven photocatalytic OWS activity without requiring sacrificial agents and co-catalysts. These findings reveal the role of imine-to-amide linkages conversion in promoting the separation of photogenerated carriers, providing a promising strategy for designing amide-based COFs photocatalysts towards OWS.

Keywords: Covalent organic frameworks, Photocatalytic overall water splitting, Linkages conversion of imine to amide, Oxidation of imine linkages, Density functional theory, Nonadiabatic molecular dynamics.