Positive and negative impacts of interfacial hydrogen bonds on photocatalytic hydrogen evolution

Understanding the behavior of water molecules at solid–liquid interfaces is crucial for various applications such as photocatalytic water splitting, a key technology for sustainable fuel production and chemical transformations. Despite extensive studies conducted in the past, the impact of microscopic structure of interfacial water molecules on photo-catalytic reactivity has not been directly examined. In this study, using real-time mass spectrometry and Fourier-transform infrared spectroscopy, we demonstrated the crucial role of hydrogen bond (H-bond) networks on the photo-catalytic hydrogen evolution in thickness-controlled water adsorption layers on various TiO2 photocatalysts. Under controlled water vapor environments with a relative humidity (RH) below 70%, we observed a monotonic increase in the H2 formation rate with increasing RH, indicating that reactive water molecules were present not only in the first adsorbed layer but also in several overlying layers. In contrast, at RH > 70%, when more than three water layers covered the catalyst surface, the H2 formation rate turned to decrease dramatically because of the structural rearrangement and hardening of the interfacial H-bond network induced during further water adsorption. This unique many-body effect of interfacial water was consistently observed for various TiO2 particles with different crystalline structures, including brookite, anatase, and a mixture of anatase and rutile. Our results demonstrated that depositing several water layers in a water vapor environment with RH ~70% is optimal for photocatalytic hydrogen evolution rather than liquid-phase reaction conditions in aqueous solutions. This study provides molecular-level insights for designing interfacial water conditions to enhance photocatalytic performance.