Molecular and Dissociative Adsorbed Water Concentration and Surface Protonic Conduction in Nanocrystalline TiO₂

Surface protonic conduction in porous nanocrystalline oxides is commonly involved in catalytic processes. The configuration of surface adsorbed water on oxides plays a crucial role in surface protonic conduction. However, studies on the impact of complex surface adsorbed water configuration on the surface water concentration and diffusivity remain limited, and hinder an in-depth understanding of surface proton transport mechanisms, and the design of better surface proton conductors. Here, in situ Raman spectroscopy is utilized to quantitatively identify the contribution of dissociative and molecular adsorbed water components on porous nanocrystalline TiO₂ surfaces between 25 and 200 °C. The variations in molecular and dissociative adsorbed water concentration agree with the predominant surface proton conduction mechanisms at three different temperature stages. From 40 to 125 °C, the reduced coverage of molecular adsorbed water layer results in the decreasing proton diffusivity. Water dissociation on the nanocrystalline TiO₂ surface is easier in wet N₂ than in wet O₂, resulting in higher proton conductivity in wet N₂; while the surface proton diffusivities in these two atmospheres are similar. The in situ spectroscopy technique enables the improvement of surface proton conducting oxides through quantitative evaluation and modulation of the surface proton concentration and diffusivity.