Paper SS+EN-MoA10
Direct Imaging of the Amphiprotic Nature of Rutile (110) Surfaces in Solution

Monday, November 10, 2014, 5:00 pm, Room 309

High-profile applications of nanocrystalline TiO₂, such as next-generation solar cells and self-cleaning surfaces, have triggered extensive studies on the structure and chemical reactivity of rutile surfaces. But are UHV-prepared clean surfaces a good representation of technologically relevant surfaces? In this study, we show that a simple aqueous procedure produces near-ideal hydrocarbon-free rutile (110) surfaces characterized by well-defined terraces and nearly straight, single-layer-high steps without high-temperature annealing. The structure of rutile surfaces after water exposure is very different from that observed on UHV-prepared clean surfaces. In particular, the O vacancies that dominate the reactivity of surfaces in UHV are not present, as evidenced by both XPS spectra and STM images. This water-induced “healing” of O vacancies is consistent with chemical intuition. The step structures, too, are quite different. In addition, the amphiprotic nature of the surface is directly observed in STM. Depending on the pH of the solution, a low density of protrusions decorate either the Ti rows or the bridging oxygen atoms, consistent with pH dependent protonation/deprotonation of the basic/acidic sites on the surface. No evidence of the corresponding counterions is observed.