Engineering charge transfer by tethering halogens onto covalent organic framework for photocatalytic sacrificial hydrogen evolution

Abstract

Covalent organic frameworks (COFs) are promising versatile organic semiconductors for the photocatalytic hydrogen evolution reaction (HER), but usually rely on the utilization of expensive Pt species as the co-catalyst and exhibit inferior activity in seawater relative to pure water. Herein, different halogen atoms (F, Cl, and Br) were integrated into the COFs framework to optimize the photochemical properties, carrier thermodynamics, and kinetics. The Cl-containing COF TpPaCl achieved the effective co-catalyst-free HER performance under visible light irradiation, resulting in a high HER rate of 16.3 mmol g-1 h-1 in artificial seawater (11.5 mmol g-1 h-1 in real seawater), outperforming that in pure water. The remarkable performance comes from the fine and efficient adjusting of the local environment of the COF framework by the electronegative halogen atoms. Theoretical calculation and soft X-ray absorption spectroscopy (XAS) indicated that the carbon atoms adjacent to the halogen group exhibited strong interaction towards the key intermediate (H*) in the HER process and served as the photoactive H2 evolution sites. The strong polarization derived from the halogen also greatly improves the formation of the long-live electron and accelerates the successive charge transfer. The kinetics were enhanced after salt adsorption in seawater, contributing to the superior HER rate in seawater.