Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO2(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO2, 6 to 9% surface vacancies), hydroxylated (h-TiO2, vacancies filled with OH), oxygen covered (ox-TiO2, oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO2, a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO2, 5.2 eV for r-TiO2, 5.35 eV for ox-TiO2 to 5.5 eV for qs-TiO2. We establish the method for an unambiguous characterization of TiO2(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti3+(3d) states.