Boosting visible-light hydrogen evolution of covalent-organic frameworks by introducing Ni-based noble metal-free co-catalyst

Covalent-organic frameworks (COFs) have been recognized as an emerging type of photocatalysts for H2 evolution and some of them have shown really outstanding photocatalytic activity with the help of Pt co-catalyst. To avoid the utilization of noble metal in COF-based photocatalysts, for the first time, we designed and constructed a series of nickel hydroxide-modified COF (TpPa-2) composite materials Ni(OH)2-X%/TpPa-2 (X: molar fraction of Ni(OH)2), which show apparently improved photocatalytic H2 evolution activity than that of the parent COF and the activity is comparable to that with Pt (0.3 wt%) co-catalyst. A series of Ni(OH)2-X%/TpPa-2 were prepared by in-situ adding TpPa-2 into the reaction system of Ni(OH)2, and the resulting Ni(OH)2-X%/TpPa-2 exhibit a novel sandwich-like morphology. The results of photocatalytic hydrogen evolution under visible light irradiation show that the optimized photocatalytic H2-evolution rate for Ni(OH)2–2.5%/TpPa-2 is up to 1895.99 μmol·h−1·g−1, which is about 26.3 times higher than that of the parent TpPa-2 and is one of the best performing 2D COF-based photocatalyst for H2 evolution. Further investigation confirm the improved activity should be attributed to the enhanced visible-light absorption of the composite materials contributed from Ni(OH)2 and the synergetic effect of Ni(OH)2 and metallic Ni derived from the in-situ reduction of Ni(OH)2, which promoted the separation of photogenerated electron–holes of the resulting materials. This work not only presents a series of new photocatalysts for efficient H2 evolution but also open an avenue for future design and synthesis of COF-based composite materials acting as a substitute of noble-metal-containing photocatalytic systems.