Sea urchin-like covalent organic frameworks/TiO2 heterostructure for enhanced photocatalytic CO2 conversion

Photocatalytic reduction of CO2 to valuable chemicals is an effective strategy to address the environmental problems and energy crisis. Covalent organic frameworks (COFs) are emerging materials known for their excellent diverse properties, albeit limited by special synthetic methods, including high temperature (120 degrees C) and the necessity of inert gas atmosphere. Herein, a novel synthesis method under room temperature and air was optimized to form TpPa-COF (TP-COF) by p-phenylenediamine (Pa) and 2,4,6-triformyl phloroglucinol (Tp) through electrostatic self-assembly. To further expand the application scope of TP-COF, a heterojunction structure was constructed by in-situ growth of TP-COF onto TiO2 to form TiO2@TP-COF. In the photocatalytic CO2 reaction of TiO2@TP-COF composites, TiO2 acts as a reduction site to reduce CO2 to CO, and triethanolamine (TEOA) acts as a hole-sacrificing reagent. It was demonstrated by in situ X-ray photoelectron spectroscopy (XPS) that the direction of electron transfer in the TiO2@TP-COF composites flowed from TP-COF to TiO2. Meanwhile, TEOA on TP-COF was oxidized to consume holes and produce protons for the reduction of CO2. Combining the advantages of organic and inorganic semiconductors, the heterojunction structure effectively improves the photocatalytic properties of TiO2@TP-COF under visible light irradiation. TiO2@TP-COF demonstrates a remarkable photocatalytic CO2 reduction rate of 133.37 mu mol/g/h at lambda = 420 nm, which is 3.19 and 2.88 times higher than that of TP-COF and TiO2, respectively, while exhibiting a selectivity of 73 % for CO. This convenient method of synthesizing TiO2@TP-COF catalysts will open up new perspectives for future COF-based materials.