Constructing S-scheme heterojunctions by integrating covalent organic frameworks with transition metal sulfides for efficient noble-metal-free photocatalytic hydrogen evolution

Abstract

Two-dimensional covalent organic frameworks (COFs) are considered among the most potential crystalline porous materials for solar-driven hydrogen production. However, it is usually necessary to introduce noble metal cocatalysts to boost the hydrogen evolution capacity of COFs. In this work, a unique S-scheme heterojunction structured TtTfp-COF/NiS composite material was effectively developed by growing metal sulfide on the typical two-dimensional covalent organic framework TtTfp-COF through a simple solvothermal synthesis method. In this structure, linear structure of rod-like NiS is more stable and convenient for further surface modification. It also provides key active sites and promotes efficient electron transfer, significantly enhancing the hydrogen evolution efficiency. The covalent organic framework enhances charge carrier transport efficiency by controlling the spatial organization of precursors and ligands. It is indicated by the experimental findings that a hydrogen evolution rate of 5978 μmol g⁻¹ h⁻¹ can be achieved for the NT-20 sample, which about 11.5 times higher than that of the initial TtTfp-COF (520 μmol g⁻¹ h⁻¹). In addition, the material exhibits a notable quantum efficiency of 1.96 % when exposed to 420 nm illumination. Both experimental results and theoretical analyses have been confirmed to improve the hydrogen evolution rate via photocatalysis and the charge transfer mechanism within the S-scheme heterojunction has been thoroughly elucidated. The design and development of non-precious metal COF-based photocatalysts are provided with new insights in this article, and new ideas for the construction of S-scheme heterojunctions are offered by the synergistic combination of inorganic and organic materials in photocatalysis.