Infrared spectroscopy of the TiO2/aqueous solution interface

An innovative approach has been used to probe the molecular nature of the m etal oxide/aqueous solution interface. Internal reflection spectroscopy of thin colloidal TiO2 films, under aqueous solutions of pH 11.7-2.3, has been used to obtain differential in situ infrared spectra related to interfacia l species. An alkaline solution wash of the TiO2 films was shown to remove contaminants arising from the sol evaporation-film deposition process. Spec ific infrared absorptions have been assigned to terminal Ti-OH and Ti-OH2+, adsorbed water Ti-OH2, and bridging Ti-OH+-Ti species from the pH dependen ce of spectra and from deuteration experiments. These surface species deter mine the pH-dependent surface charge and the enhanced interfacial ionic con centrations observed in our previously published STIRS results. The enhance d interfacial ionic concentrations were also observed to have spectral effe cts related to interfacial water structure which are similar to those obser ved from concentrated aqueous solutions containing these ions. The proposed interfacial species have long been components of models of hydrous oxide s urfaces, but this is the first in situ vibrational spectroscopic analysis a imed at their identification.