Bi-functional ionic liquids facilitate liquid-phase exfoliation of porphyrin-based covalent organic frameworks in water for highly efficient CO2 photoreduction

Abstract

Covalent organic nanosheets (CONs) are receiving tremendous interest for their potential in versatile applications. However, the synthesis of CONs usually suffers from the use of toxic solvents, low yields and uncontrollable thickness. Here, we report a green, high yield, and scalable strategy to strip two-dimensional imine-linked covalent organic framework (COF) powders in water using bi-functional ionic liquids as both the intercalator and stabilizer under ambient conditions. It is shown that the exfoliation yield reaches up to 66.6%, which is a record value for the delamination of CONs from bulk COF powder. The thickness of CONs is mainly in the range from 0.4 to 2.0 nm and the lateral size is large up to a few hundred nanometers. Importantly, the aqueous IL solutions can be reused and recycled for the exfoliation of CONs. The acid–base and hydrogen bond interactions between CONs and cations of the ILs are found to play critical roles in the delamination and stability of CONs. Furthermore, the CONs without any additives such as metals, photosensitizers, and sacrificial agents have been used as catalysts for photocatalytic CO₂ reduction with gaseous water to yield CO in the gas phase under visible-light irradiation. The production rate of CO is 132.2 μmol g⁻¹ h⁻¹, which is twice that by the bulk COF powder, and outperforms most visible-light driven metal-free COFs reported so far. Therefore, the liquid phase stripping approach reported here possesses tremendous potentiality in mass production of CONs for high efficiency photocatalysis of CO₂ reduction in the gas phase.