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We have studied the oxidation of a reduced rutile TiO2(110) surface in detail by means of ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunnelling microscopy (STM). Sputtering a perfect TiO2(110) surface at room temperature followed by annealing at 873 K gives rise to a seriously reduced rough surface. Oxidation of the seriously reduced surface at 873 K following by annealing the surface at 873 K fully oxidizes the surface and greatly enhances the surface order. No Ti3+ feature is present in the UPS spectrum, but A weak but clear band at ca. 2.4 eV below the Fermi level appears. XPS and the HREELS results exclude the existence of contaminants and surface hydroxyls likely introduced during the oxidation process. Therefore, the feature at 2.4 eV below Fermi level demonstrates the existence of an intrintic occupied defect state besides the commonly observed oxygen vacancy/Ti3+ on the TiO2(110) surface. We attribute it to the F centers, oxygen vacancy occupied by one or two electrons. Annealing the surface at higher temperatures leads to the disapperance of F centers, simultaneously oxygen vacancy/Ti3+ accumulates on the surface. HREELS results also indicates the existence of F centers on the TiO2(110) surface treated by oxidation and annealing at 873 K, which exhibits the fundamental surface phonon at 98 meV. Annealing the surface at higher temperatures gives rise to the fundamental fundamental phonons at 95, 55, and 46 meV. The higher energy in surface phonon might due to the interactions between electrons in F centers with surface phonons of TiO2.
In summary, our spectroscopic results demonstrate the likely existence of F centers on a ruitle Tio2(110) surface under appropriate conditions. |