In-plane polarity boosts photocatalytic overall water splitting in two-dimensional covalent organic frameworks with polarized linkages

Abstract

Benefiting from their modular nature and tunable optoelectronic properties, two-dimensional (2D) covalent organic frameworks (COFs) have emerged as promising candidates for photocatalytic overall water splitting (OWS). However, the rapid recombination of photogenerated carriers usually results in poor photocatalytic performance in many of these COFs. Herein, we propose that introducing in-plane polarity in COFs with homogeneous cores can effectively address this challenge. Using first-principles calculations, we designed two types of 2D imine-linked COF monolayers (Im-TPB and Im-TPP) for OWS. Interestingly, both COFs exhibit type-II-like band alignments induced by polarized imine linkages, which significantly inhibit electron–hole recombination. This advantageous characteristic was further confirmed in ten additional COFs. The predicted lifetimes of photogenerated carriers for Im-TPB and Im-TPP are 13.5 and 8.7 times longer than those of non-polar azo-linked TPB and TPP, respectively. Furthermore, the band edges of Im-TPB and Im-TPP not only straddle the water redox potentials at pH = 7 and pH = 14, but also afford a sufficiently high driving force and enhanced light absorption ability. Of particular note is Im-TPB's capability to spontaneously split pure water into hydrogen and oxygen under visible light without the need for cocatalysts and sacrificial reagents. Our results highlight the role of in-plane polarity in the polarized-bond-linked COFs and open a promising avenue to design efficient COF-based photocatalysts for OWS.