WFU Physics Colloquium

TITLE: Defect and Adsorbate Dyanamics on TiO₂ (110)

SPEAKER: Dr. Zhenrong Zhang,

TIME: Monday Feb. 2, 2009 at 4:00 PM

PLACE: Room 101 in Olin Physical Laboratory

The catalytic activity of reducible oxides is often dominated by surface oxygen vacancies (O_V's). Our studies have focused on how such reactive sites affect the chemical activity of the oxide, specifically TiO₂, using atomically resolved scanning tunneling microscopy (STM). Alcohols and water on the rutile TiO₂(110) surface are important model systems used to study oxidation of organic contaminants and photocatalytic hydrogen production via water splitting. New methodology has enabled us to achieve unprecedented understanding of the key elemental steps - adsorption, dissociation, and diffusion - taking place in heterogeneous catalytic reactions.

Specifically, in-situ room temperature imaging provided direct information about sitespecific adsorption and dissociation of alcohols and water on O_V sites of the TiO₂(110) surface. At elevated temperatures, isothermal STM measurements enabled us to carry out the first diffusion studies of defects (O_V's) and surface species (hydroxyls and alkoxys) on any oxide. The sequence of images reveals that O_V diffuse exclusively along oxygen rows. In the case of hydroxyl species, a new hydrogen diffusion channel with a two-step mechanism was discovered. For alkoxy species, we found that diffusion occurs only with the assistance of mobile O_V species. This finding has broad implications about the role of defect sites in mass transport on catalytically important surfaces. Using detailed data analysis as a function of temperature, we extracted the kinetic parameters controlling the diffusion and compared the results with the theoretical DFT calculations. Our studies of the rotational dynamics of alkoxy groups show that at room temperature alkyl chains rapidly rotate among their equivalent orientations which are dependent on the position of the OH group within the alcohol molecule.

The research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the BER Division of DOE and located at Pacific Northwest National Laboratory.