Visible-Light-Assisted\nPhotocatalytic CO₂ Reduction and N₂‑Fixation\nover TiO₂/Covalent Organic Framework Heterojunction Photocatalyst

Abstract

The potential applications of covalent\norganic frameworks\n(COFs)\nin the field of photocatalysis are constrained by the fast recombination\nrate of the photoinduced carriers and their limited visible light\nabsorption capacity. Design of Z-scheme heteroframework utilizing\nCOFs is believed to be an innovative and effective approach to assist\nthe charge separation efficacy and improve the photocatalytic activity\nof the materials. Herein, the imine-based 2D COF (referred to as TP-TAPM\nCOF) and TiO₂ were effectively coupled together by covalent\nbonding using a simple solvothermal approach to construct a novel\nheterojunction TiO₂/TP-TAPM photocatalyst. The resulting\nTiO₂/TP-TAPM heterostructure was well characterized by\na sequence of experimentations to investigate the compositional, structural,\nand morphological characteristics. Interestingly, the as-synthesized\nhybrid photocatalyst was applied for the first time to efficiently\nreduce CO₂ to CH₃OH as well as N₂ to NH₃ under visible light illumination at ambient reaction\nconditions without the need of specific organic scavengers and cocatalysts.\nUsing the TiO₂/TP-TAPM hybrid photocatalyst (8 mg), a substantially\nhigher yield of methanol was produced with a formation rate of 281.25\nmmol g_cat^–1 h^–1 after\n4 h of visible light irradiation. Whereas, the generation rate of\nNH₄⁺ was found to be 747 μmol L^–1 h^–1 after irradiation for 5 h using 5 mg of as-synthesized\nheterojunction photocatalyst. The development of a covalent interaction\nin the 2D–2D heterojunction between COF and TiO₂, mostly as a result of their close interfacial contact, can be attributed\nto the greatly improved photocatalytic efficiency. Hence, the constructed\n2D–2D-layered structure offers a high contact area, which substantially\nfacilitates the separation and transportation of photogenerated charge\ncarriers, boosts light absorption, and enhances photocatalytic activity.\nThis work sheds light on the development of a promising approach for\nthe artful integration of organic materials (COFs) with inorganic\nsemiconductors into a single hybrid with a 2D–2D interface\nas effective photocatalysts for CO₂ reduction as well as\nN₂ fixation, holding significant implications for extensions\nto other material systems.