Design of covalent organic framework based heterojunctions to inhibit photogenerated carrier dynamics for ultralow recombination and improved photocatalytic hydrogen production

Abstract

Covalent organic frameworks (COFs) possess broad light absorption and a high surface area but are limited by rapid photogenerated carrier recombination in single-component systems, leading to significant energy losses. Here, we strategically engineer p-n heterojunctions by in situ growth of p-type Co₂CuS₄ on a donor–acceptor (DA) COF, establishing a synergistic built-in electric field through interfacial electron transfer from an amine-enriched COF to Co₂CuS₄. The DA-COF (TT-TPCOF) was synthesized via a condensation reaction using 1,3,5-tris (4-aminophenyl) (TAP) benzene as an electron donor and 2,4,6-triformylphloroglucinol (TFP) as an electron acceptor. The presence of interfacial contact barriers at the heterojunction interface induces band bending, creating a staggered Type-II band alignment, effectively suppressing electron backflow via the built-in electric field. Under light irradiation, photogenerated electrons in the Co₂CuS₄ semiconductor rapidly transfer to the COF through the conduction band, initiating an efficient hydrogen evolution reaction (HER). Concurrently, photoexcited holes migrate from the COF back to the valence band of Co₂CuS₄. Notably, the incorporation of the Pt cocatalyst not only dramatically increases the density of surface active sites but also enhances HER efficiency by orders of magnitude through optimizing reaction kinetic pathways. The hydrogen evolution amount of TT-TPCOF/Co₂CuS₄-3 reached 10.76 mmol g⁻¹ h⁻¹ which was 4.89 times that of TT-TPCOF. Bader charge analysis and charge difference calculations unambiguously confirm that the direction of electron transfer is consistent with the formation of a p-n heterojunction in this system. These findings not only validate the charge carrier dynamics mechanism but also open up new avenues for the rational design and functionalization of COF-based heterojunctions.