Visible-Light-Assisted Photocatalytic CO2 Reduction and N2-Fixation over TiO2/Covalent Organic Framework Heterojunction Photocatalyst

The potential applications of covalent organic frameworks (COFs) in the field of photocatalysis are constrained by the fast recombination rate of the photoinduced carriers and their limited visible light absorption capacity. Design of Z-scheme heteroframework utilizing COFs is believed to be an innovative and effective approach to assist the charge separation efficacy and improve the photocatalytic activity of the materials. Herein, the imine-based 2D COF (referred to as TP-TAPM COF) and TiO2 were effectively coupled together by covalent bonding using a simple solvothermal approach to construct a novel heterojunction TiO2/TP-TAPM photocatalyst. The resulting TiO2/TP-TAPM heterostructure was well characterized by a sequence of experimentations to investigate the compositional, structural, and morphological characteristics. Interestingly, the as-synthesized hybrid photocatalyst was applied for the first time to efficiently reduce CO2 to CH3OH as well as N2 to NH3 under visible light illumination at ambient reaction conditions without the need of specific organic scavengers and cocatalysts. Using the TiO2/TP-TAPM hybrid photocatalyst (8 mg), a substantially higher yield of methanol was produced with a formation rate of 281.25 mmol gcat–1 h–1 after 4 h of visible light irradiation. Whereas, the generation rate of NH4+ was found to be 747 μmol L–1 h–1 after irradiation for 5 h using 5 mg of as-synthesized heterojunction photocatalyst. The development of a covalent interaction in the 2D–2D heterojunction between COF and TiO2, mostly as a result of their close interfacial contact, can be attributed to the greatly improved photocatalytic efficiency. Hence, the constructed 2D–2D-layered structure offers a high contact area, which substantially facilitates the separation and transportation of photogenerated charge carriers, boosts light absorption, and enhances photocatalytic activity. This work sheds light on the development of a promising approach for the artful integration of organic materials (COFs) with inorganic semiconductors into a single hybrid with a 2D–2D interface as effective photocatalysts for CO2 reduction as well as N2 fixation, holding significant implications for extensions to other material systems.