Hydrogen induced trap states in TiO2probed by resonant X-ray photoemission

The standard approach to surface analysis is X-ray photoelectron spectroscopy (XPS), which is used to follow electronic structure changes of the catalyst material TiO2 upon hydrogenation, however, without conclusion whether the effect can be traced back to the hydrogen treatment. Resonant photoemission experiments using a tunable synchrotron X-ray source yields further insights. The integration of the electron yield over all kinetic energies results in X-ray absorption spectra (XAS). Furthermore, in resonant conditions, electrons are excited from a core level to the conduction band and can subsequently be trapped by specific defect states. From this, the observed shallow trap states can be identified as Ti3+ states. We quantify the Ti3+/Ti4+ ratio both from XPS and XAS and the oxygen to titanium elemental ratio. The correlation of the results from resonant and non-resonant photoemission reveals that hydrogen defects serve as trap centers, while defects associated with oxygen vacancies serve as recombination centers suppressing trap state emission. The main effect of hydrogen in TiO2 is the increased disorder in the material.