A S-scheme heterojunction of Co9S8 decorated TiO2 for enhanced photocatalytic H2 evolution

Construction of heterojunction photocatalyst is an effective strategy for optimizing the light absorption, charge carrier separation and surface reaction efficiency during the photocatalytic processes. Herein, an S-scheme Co9S8/TiO2 heterojunction was fabricated by an in-situ deposition hydrothermal method. The experimental results indicate that Co9S8 can broaden the light absorption range of TiO2 from 400 to 800 nm, accelerate charge carrier separation using Co-O-Ti chemical bonds as the transfer bridge. The enhanced H2 evolution reaction (HER) performance over 20 wt% Co9S8/TiO2 is 12.6 and 283.0 times than that of pure TiO2 and Co9S8 after electrochemical active surface area (ECSA) normalization, the HER over 20 wt% Co9S8/TiO2 can reach up to 3982 μmol·g–1·h–1 in absence of cocatalysts, which is 13.2 and 73.7 times than that of pristine TiO2 and Co9S8, respectively. Further investigation shows that the water contact angle (WAC) of TiO2 is obviously reduced from 23.3° to 5.4° after introducing Co9S8, this super-hydrophilic surface benefits the adsorption of water molecules on the catalyst surface. This composite also exhibits an excellent stability during three cycles of H2 production tests. Based on band structure analysis and ·OH/·O2- radical trapping experiments, the charge carrier separation process is in accord with the S-scheme route. The useless e– on the CB of Co9S8 are driven to combine with h+ on the VB of TiO2. Simultaneously, the e– with strong reduction ability are preserved in the CB of TiO2 to participate in HER, while the h+ on the VB of Co9S8 are consumed for the oxidation of the sacrificial agent. This work demonstrates Co9S8 is a potential material for fabricating heterojunction with desired activity, although the activity of Co9S8 is unsatisfactory alone.