The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile TiO_2(110) Single Crystals

摘要

We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of TiO_2(110) single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile TiO_2(110) specimens were cut from the same crystal and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of bulk defects with electron paramagnetic resonance (EPR), the specimens were studied with scanning tunneling microscopy (STM), low-energy He~+ ion scattering (LEIS), and work function measurements. EPR reveals that darker rutile crystals exhibit higher concentrations of extended Ti~(3+) related bulk defects such as crystallographic shear planes (CSP), with a decrease in substitutional and interstitial defects as compared to lighter crystals. Surface structures with (1 * 2) features are preferably formed upon UHV annealing on these darker crystals. LEIS measurements show that all of the crystals' (110) surfaces are reoxidized upon annealing in ~(18)O_2 (573 K, 1 * 10~(-6) mbar, 10 min) and that the ~(18)O surface content is proportional to the bulk reduction state. UV-visible adsorption spectra and resistivity measurements also scale with the reduction states of crystals. Only the (1 * 1) structure is observed on the surface of slightly reduced crystals. Annealing in oxygen induces additional metastable structures, i.e., TiO_2 clusters on blue crystals and rosette networks on dark blue crystals.