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 °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 μmol/g/h at λ = 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.