2015 Theses Doctoral

Surface Chemistry Studies of Transition Metal Oxides: Titanium Oxide and Iron Oxide

Li, Zhisheng

Surface chemistry studies of two transition-metal oxides: titanium oxide and iron oxide are presented, which are focused on thermal induced chemistry using proximal probe imaging and spectroscopy. In the first, using single crystal of rutile TiO₂ (110), arrays of nano-scale locally varying surface strain field were generated by introducing highly pressurized nanoscale argon clusters 4-11 layers below the surface. The characteristics of the argon clusters are explored through STM tip-assisted surface excavation, combining with a continuum mechanical model.

This work experimentally demonstrates that surface elastic strain influences the adsorption energy of adsorbates significantly and, thus, can be used for applications of surface nanopatterning. As a comparison with work on nanoscale, two forms of titanium oxide in reduced dimensionalities are experimentally synthesized and investigated for their surface reactivity: 3D nano TiO₂ crystals and monolayer TiO films, both of which are supported on single crystal Au(111) surface. This work demonstrates that both nano crystals and ultrathin films of titanium oxide exhibit distinctive surface structural and catalytic properties compared to the bulk surface terminations. In particular, TiO₂ nano crystals are more catalytically active and provide a new dissociation channel for adsorbed 2-propanol, a probe molecule chosen for this study. In the process of undertaking this research, it was found that monolayer TiO film can be used to employ moire varied chemistry. In particular, a long range pinwheel-shaped surface moiré pattern due to gradual shift of atom registry on Au (111), was found to further influence the adsorption geometry of adsorbates and to cause thereby smoothly varying sites for reactions.

In the case of the second transition metal oxide surface, Fe3O4 (111), a comparison was made with rutile TiO2 (110) surface, Fe3O4 (111) is a polar surface with apparent surface charge, and thus undergoes various surface reconstructions. Therefore, its surface structure is of great complexity. Our work shows that the reaction of methanol on this iron-oxide surface is highly sensitive to atomic-level surface reconstructions.