Dual Non-Noble-Metals-Immobilized Covalent Organic Frameworks for Visible-Light-Driven Photocatalytic Hydrogen Evolution

Abstract

Covalent organic frameworks (COFs) have emerged as promising heterogeneous platform to coordinately immobilize metals for the photocatalytic hydrogen evolution reaction (HER). Herein, we integrate both Cu(I)diimine complex-based photosensitizer unit and Co(II)bipyridine-based catalyst unit into a classic two-dimensional COF, TpBpy, which functions as a macro-ligand. The resulting hybrid material, TpBpy-Cu/Co, achieved an optimized photocatalytic H2 evolution rate of 12.16 mmol h-1 g-1, which is 25 times higher than that of the pristine TpBpy. We performed ultrafast transient optical spectroscopy, electrochemical, and photoluminescence studies to understand their properties’ changes and photocatalytic reaction mechanism. The introduction of Cu(I)diimine unit in TpBpy leads to the improvement in the photosensitizing ability. Meanwhile, the excited-state lifetime of Cu(I)diimine photosensitizer is enlonged due to the confinement effect. Furthermore, compare to TpBpy-Cu/Co, where the Co(II) catalyst unit is also immobilized in a single COF, the significant reduced photocatalytic activity for TpBpy-Cu and Co(bpy)Cl2 system, underscore the role of metal coordination in facilitating the intra-framework charge transfer from photosensitizers to catalysts. This study offers strategic insights for optimizing COF-metal coordination systems and highlights the significance of multi-functional units in modulating photocatalytic processes.