In Situ NRA, RBS, XPS and UPS Studies of Hydrogen Diffusion in TiO2 Single Crystals

(1) Environmental Molecular Sciences Laboratory, Richland, WA 99352, USA
(2) National Isotope Centre, GNS Science, New Zealand
(3) Qatar Environment and Energy Research Institute, Qatar Foundation, Doha, Qatar
(4) Pacific Northwest National Laboratory, Richland, WA 99352, USA

The intrinsic point defects associated with oxygen vacancies and Ti3+ ions play a crucial role in the usage of titanium dioxide (TiO2) in various technological applications including catalysis and photochemistry. It is well known that the interactions between H atoms and surface oxygen in TiO2 lead to the formation of Ti3+ ions at elevated temperatures.  However the Ti3+ ion formation and accumulation as a function of elevated temperatures in UHV conditions during hydrogen diffusion in TiO2 is not well understood. In this study, we have used ion implantation method to incorporate hydrogen in single crystal TiO2 (110) samples and investigated the behavior of point defects in both pure and hydrogen implanted TiO2 as a function elevated temperatures using Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), x-ray photoelectron spectroscopy (XPS) and ultra violet photoemission spectroscopy (UPS).  TiO2 single crystals were implanted with 40 keV hydrogen ions at room temperature with ion fluences of 1x1015, 1x1015 and 1x1017 atoms/cm2.  Samples were isochronally annealed in vacuum for 30 minutes at each temperature up to 1100K and hydrogen and Ti3+ defects were quantified. Hydrogen depth profile measurements obtained from 1x1017 atoms/cm2 implanted sample reveal that hydrogen diffused towards the surface at lower temperatures and it slowly diffuses out from the samples at higher temperatures. XPS and UPS measurements from the hydrogen implanted samples show significantly higher Ti3+ defects in comparison to pure TiO2 at these temperatures under UHV conditions. These defects reach a maximum around 880 K in which almost all hydrogen was removed from the sample. When the implanted sample further annealed to high temperatures, the amount of Ti3+ in hydrogen implanted samples started to decrease and reaches the values from the pure TiO2 samples around 1100K.