Paper SS-FrM9
Probing Shallow Trapped Electrons in TiO₂ with UHV-IRRAS

Friday, November 1, 2013, 11:00 am, Room 201 A

Understanding photochemistry on oxide surfaces, a topic of pronounce importance in the context of solar fuel synthesis, includes tracking the path of the charge carriers photoexcited in the bulk to the surface where both, the electron and the hole, may induce chemical transformations in adsorbed molecular species. Shallow trap states which in the case of TiO₂ are known to store charge over extended periods of time (several hours) have a pronounced influence on this bulk-to-surface processes.[1-2] In this contribution we demonstrate that grazing-incidence IR-spectroscopy can be used to probe electrons localized in such states. Loading was carried out by either irradiation with UV-light or exposure to atomic hydrogen. For both, powders as well as a (110) oriented single crystal substrate of rutile titanium dioxide (r-TiO₂), distinct IR absorption features were observed at around 940 and 970 cm^-1. For the single crystal substrates, the IR absorption bands arising from an excitation of the trapped electrons into higher lying final states, are substantially sharper and, in addition, two new bands are resolved at 1205 cm^-1 and 1375 cm^-1. On the basis of this new high-resolution data we propose that the trap state has to be described as a polaron state and that the final states correspond to hydrogenic states within the polaron potential.

[1] M. Xu, Y. Gao, E. M. Moreno, M. Kunst, M. Muhler, Y. Wang, H. Idriss, C. Wöll, Phys. Rev. Lett.106, 138302 (2011).

[2] D. A. Panayotov and J. T. Yates Jr, Chem. Phys. Lett.436, 204 (2007).