Investigation of Water Structure at the TiO2/Aqueous Interface

TiO2 thin films coated on SiO2 substrates were prepared for the investigation of water structure at the TiO2/aqueous interface by vibrational sum-frequency spectroscopy (VSFS). The films were first characterized by X-ray photoelectron spectroscopy, ellipsometry, and atomic force microscopy. Film thickness could be readily varied between 0.9 and 5.9 nm with the thinnest film displaying the least degree of surface roughness. The 0.9 nm films were employed in the VSFS investigations and showed characteristic 3200 and 3400 cm-1 OH stretch frequencies of highly aligned interfacial water. Such peaks were reminiscent of those known from SiO2/aqueous interfaces; however, the chemistry at TiO2/aqueous interfaces was significantly richer. In the presence of Cl anions, the surface had an isoelectric point near pH 5.5 and showed the least degree of water organization near this pH. Almost equally strong 3200 cm-1 features could be produced at pH 2.0 and 12.0. On the other hand, the spectra were dramatically altered in the presence of phosphate-buffered saline. The phosphate ions specifically bound to the substrate surface and shifted the isoelectric point of the interface to pH 2.0. In this case, the intensity of the 3400 cm-1 peak was significantly increased in comparison with the Cl ion data at both neutral and acidic pH values. This is presumably because of a lack of sites directly adjacent to the oxide for forming tetrahedrally coordinated water when phosphate ions are present.