Imaging surface reactions at molecular level on TiO₂ surfaces.
Understanding the structure and properties of TiO₂ surfaces is critical to achieve a better understanding of heterogeneous catalytic reactions on TiO₂. In this thesis, rutile TiO₂ (110) and anatase TiO₂ (001) surfaces have been investigated using scanning tunneling microscopy (STM). The approach of comparison of the same nanoscale area during reaction allows the site-specific molecular-level understanding of surface catalytic reactions. The rutile TiO₂(110) (1×1) surface prepared by cycles of Ar-ion sputtering and e-beam annealing was studied using acetone as a probe molecule. It was found that acetone molecules preferably adsorb on the oxygen vacancy sites at room temperature. The sequential isothermal STM images show acetone either diffuse along the bridge-bonded oxygen row or from the oxygen row to the titanium row and then moving along the titanium row. By exposing the reduced TiO₂(110) (1×1) surface to O₂, the surface became partially oxidized. The consecutive reaction steps of acetone molecules with the partially oxidized surface was studied. It was found that acetone molecules react with the oxygen adatoms and the bridge-bound oxygen vacancies. As a result of reactions, two different types of species were formed. With more cleaning cycles at higher temperatures, TiO₂(110) transformed from the reduced (1×1) phase to the cross-linked (1×2) phase. The structure of cross-linked (1×2) TiO₂(110) was studied through the interaction of trimethyl acetic acid (TMAA) with various sites on the surfaces. At low coverage, TMAA molecules mostly adsorb in troughs and on cross-links. With increasing coverage, they form a chain in the trough. At higher coverage, TMAA molecules adsorb on strands in both centered and off-centered configurations. Comparing three different models, the adsorption of TMAA strongly supports the Ti₂O model. The atomic structure of anatase TiO₂(001) expitaxial thin films has been studied. Three types of features were found on the bright rows of the (1×4) reconstructed surface from the high bias STM images. High resolution STM images taken at the same area at different bias voltages show that these features are originated from two basic atomic building blocks. Based on that, a modified added molecule model was proposed.