Single-Molecule Vibrational Spectroscopy of H₂O on Anatase TiO₂(101)

Atomic level understanding of water on titanium dioxide (TiO₂), in particular the mineral form anatase, is exceptionally important, considering its practical applications including photocatalytic water-splitting, photoinduced hydrophilicity, and water purification.1-3 Although much effort has been devoted to the study of water adsorption and dissociation on anatase, investigating the fundamental steps of single water splitting remains challenging. These challenges arise from the difficulty of chemical identification of single molecular water and its dissociation products. Here, we have unequivocally identified single water molecules and hydroxyls on the TiO₂ anatase (101) surface using scanning tunneling microscopy (STM) in combination with inelastic tunneling spectroscopy (IETS). Labeling of single molecules on the semiconductor surface was confirmed by demonstrating the isotope shift of the vibrational signatures and by first-principles density functional theory (DFT). The chemical identification of individual water molecules on anatase opens a direct path to follow the water splitting process on the single molecule level.