Observation of vacancy-related polaron states at the surface of anatase and rutile TiO2 by high-resolution photoelectron spectroscopy

Defects in the surface region of a reducible oxide, as TiO2, have a profound effect on applications, while their nature is very much influenced by the possibility of small polaron formation. Here, we probe rutile (110) and anatase (101) single crystals via high-resolution ultraviolet photoelectron spectroscopy and resolve multiple components of the well-known defect state in the band gap. In rutile, we find two at VBM+2.1 eV and VBM+1.4 eV, which we assign to subsurface polaron traps and vacancy-bound states, respectively, confirming the predicted partial suppression of polaron formation at high vacancy concentration. New defects are created in situ on the anatase surface by the synchrotron beam. We assign a component at VBM+2.3 eV, which can be removed by annealing, to polaron states associated with surface oxygen vacancies. We also identify a second component at VBM+1.6 eV, which can not be removed by annealing, and is too deep to be associated with oxygen vacancies.