Surface Science Study of Water and Hydrogen Adsorbed on Rutile TiO2(110)-(1x1)

The dissociative adsorption of water at oxygen-vacancy defect sites on the TiO2(110) surface spatially redistributes the defect electron density originally present at subsurface sites near the defect sites. This redistribution of defect-electrons makes them more accessible to Ti4+ ions surrounding the defects. The redistribution of electron density decreases the O+ desorption yield from surface lattice O2- ions in TiO2, as excited by electron stimulated desorption (ESD). A model in which OH formation on defect sites redistributes defect electrons to neighboring Ti4+ sites is proposed. This switches off the Knotek-Feibelman mechanism for ESD of O+ ions from lattice sites. Conversely, enhanced O+ reneutralization could also be induced by redistribution of defect electrons. The redistribution of surface electrons by adsorption is further verified by the use of donor and acceptor molecules which add or remove electron density.
We have found a new form of chemically-bound hydrogen on the TiO2(110) surface (H/TiO2), which is produced by exposure to atomic H at 87 K. This chemisorbed hydrogen differs significantly in its physical properties from OH/TiO2 produced by H2O adsorption. The H/TiO2 species produces a normal beam of H+ upon electron stimulated desorption whereas OH/TiO2 species produce inclined H+ ESD beams. An inclined O-H bond in OH/TiO2 results in an elliptic H+ ion angular distribution pattern. H/TiO2 is thermally less stable than OH/TiO2. By ~ 350 K, H/TiO2 has disappeared, probably by thermal desorption or diffusion into the bulk, whereas OH/TiO2 begins to desorb as H2O only above ~350 K.

This is a master thesis.