S-scheme heterojunction of black TiO2 and covalent-organic framework for enhanced photocatalytic hydrogen evolution

Covalent-organic frameworks (COFs), assembled by covalent connection of organic molecules, have emerged as a new type of organic photocatalysts with porous and ordered structure, high surface area and excellent light absorption. In this work, we firstly report a class of COF based S-scheme heterojunction hybrids with oxygen vacancy (OVs) for efficient visible-light-driven hydrogen evolution. The black TiO2 (TiO2-x) was chosen to integrate with TpPa-1-COF to facilitate the efficient separation of photogenerated charges and enhance photocatalytic activity. The existence of oxygen defect was confirmed by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Free radical trapping experiments confirmed the successful integration of COF based S-scheme heterojunction hybrids with oxygen defect. The results of H2 evolution experiments verify that H2 evolution rate of TiO2-x/TpPa-1-COF (6:4) reaches an optimum of 15.33 mmol·h−1·g−1 (with the TOF of 235.74 h−1), which is about 8 and 10.5 times that of pure TpPa-1-COF and TiO2-x, respectively. Moreover, it was demonstrated that TiO2-x/TpPa-1-COF also exhibited apparent higher H2 evolution activity (3.5 times) than that of TiO2/TpPa-1-COF (6:4) without oxygen defect in TiO2. Further investigations about photoelectric properties verify that S-scheme heterojunction with oxygen defect can better promote separation of photogenerated charges from COF in comparison with defect-less TiO2.