| AVS 60th International Symposium and Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS+EN-ThM |
| Session: | Photocatalysis and Photochemistry at Surfaces |
| Presenter: | L. Österlund, Uppsala University, Sweden |
| Authors: | A. Mattsson, Uppsala University, Sweden S. Hu, Uppsala University, Sweden K. Hermansson, Uppsala University, Sweden L. Österlund, Uppsala University, Sweden |
| Correspondent: | Click to Email |
Adsorption of formic acid on rutile TiO2 (110) were studied with reflection absorption infrared spectroscopy (RAIRS) in UHV with both s- and p-polarized IR light, incident along either the <001> or <1-10> direction. Experiments were conducted on surfaces prepared with different pre-treatments to obtain stoichiometric (s-TiO2), oxidized (o-TiO2) and reduced (r-TiO2) surfaces. Experiments were compared with density functional theory (DFT) calculations as implemented in the Vienna ab initio simulation package (VASP).
With p-polarized light, transmission and absorption peaks are observed due to the symmetric and asymmetric O-C-O stretch and C-H wagging modes in formate bonded to the Ti-atoms between the bridging oxygen rows in the <001> direction, in agreement with the measurements made by Hayden and co-workers.[1] This orientation of the formate molecule is dominant on for all surface preparations studied here. Employing s-polarized light reveals that the C-H wagging occurs in the plane of the molecule, since it is only seen with s-polarized light incident in the <1-10> direction. In the earlier work by Hayden, weak absorption peaks were observed with p-polarized light incident in the <1-10> direction, and attributed to a minority specie, oriented perpendicular to the majority specie, and bonded to bridging oxygen vacancies. This interpretation is at variance with our results for s-polarized light, and furthermore an equally weak band is seen regardless of the surface preparation. We attribute this to rapid hydroxylation of the bridging oxygen vacancies in good agreement with recent STM studies, which show that bridging oxygen vacancies become hydroxylated within a few minutes at pressures of 3x10-10 mbar.[2] DFT calculations support the above assignments, and in particular show that the minority species inferred from RAIRS is due to formate bonded to OH groups and not to bridging oxygen vacancies. We discuss the implications of our results for the photo-induced decomposition of formic acid on TiO2(110).
[1] B.E. Hayden, A. King, M.A. Newton, Journal of Physical Chemistry B 103 (1999) 203-208.
[2] S. Wendt, et. Al. Surf. Science 598 (2005) 226-245.