XPS and TDS studies of trapping states of helium implanted in TiO2

Chemical reduction of TiO2 surface to Ti2+ and Ti3+ was found upon He+ (1.5-11 keV) bombardment. In the thermal desorption spectrum of helium, three peaks were observed at 4600C, 5900C and 7000C, regardless of the bombarding energy. The dependences of the bombarding energies and the ion fluences on the peak intensities, as well as the results of the XPS analyses, revealed that the first peak originates from the helium trapped at the interstitial site. It was also found that the second one originates from the recovery of the surface chemical state, i.e. Ti2+, Ti3+ → Ti4+. The third peak is correlated to the thermal decomposition of TiO2, which was confirmed by the XPS spectral changes. Activation energies for the desorption processes providing the first and second peaks were estimated to be 0.56 eV and 1.68 eV, respectively. (orig.)