Auto-dissociation of atmospheric water on TiO2: insights from sum-frequency spectroscopy of Ti-O vibrations.

The dissociation of water on TiO2 surfaces, marked by the presence of TiOH groups, is pivotal for environmental and energy applications involving TiO2. Yet characterizing these surface groups has remained a challenge. Here, we employ in situ sum-frequency vibrational spectroscopy (SFVS) to unveil the vibrational signatures of surface TiOH and undercoordinated Ti-O groups in the Ti-O vibrational frequency range, offering a clear structural indicator of TiO2 hydroxylation. Our findings confirm the spontaneous dissociation of water molecules on TiO2 surfaces, a process significantly enhanced by structural defects such as oxygen vacancies. Through methanol titration experiments, we gain molecular-level insights into the adsorption/desorption dynamics, estimating a ∼70% TiOH coverage on amorphous TiO2 under ambient conditions. This work not only deepens our understanding of TiO2/water interactions but also lays the groundwork for future SFVS investigations into these interfaces.