30th Computational Science Seminar
Reactivity of valence band holes at the rutile TiO2/water interface
Michiel Sprik

Professor,
Department of Chemistry,
University of Cambridge

2011/12/1(THU) 15:30-16:30

Knowledge Science Lecture room, K3, 4

Photo-active wide gap metal oxides, such as TiO2, have remarkably deep lying valence band maxima (VBM). The VBM of aqueous rutile TiO2 is -7.1 eV at PZC (the pH for which the net proton charge of a surface vanishes. PZC=5 for rutile TiO2 (110). An ionization potential of 7.1 eV corresponds to an electrode potential of 2.6 V vs the normal hydrogen electrode (NHE). Free holes should therefore be just about able to oxidize a water molecule to a hydroxyl radical, which requires a minimum potential of 2.4 V vx NHE at PZC. Such photo-generated hydroxyl radicals are considered to act as intermediates in the photo-oxidation of aqueous pollutants by titania. However, self trapping of free holes is assumed to be a fast process, reducing the oxidative power of a photogenerated hole. As a result, photo-excitation may not produce enough energy for dehydrogenation of water molecules in the bulk liquid. This raises the question whether dehydrogenation of a water molecule adsorbed on the TiO2 surface is easier.

We have investigated this question using the latest advances in the CP2K code (cp2k.berlios.de), which allows us to use hybrid density functionals containing a fraction of exact exchange. Indeed, under PZC and flatband conditions, we find that adsorption at the aqueous (110) interface reduces the dehydrogenation potential of a water molecule by 0.5 V. However, the hole trapping energy was found to be almost the same. Therefore, according to our calculations, the oxidative power of the self-trapped hole matches the dehydrogenation energy of an adsorbed water molecule. The implication is that photogenerated holes have little surplus energy to overcome possible overpotetials for abstraction of a hydrogen atom from water. After an outline of the density functional theory based molecular dynamics method (DFTMD) we have applied [1,2], we will analyze these perhaps somewhat unexpected results by an energy decomposition method borrowed from the study of homogeneous proton coupled electron transfer (PCET) reactions [3].

Remarks

[1] J. Cheng and M. Sprik, J. Chem. Theory Comp. 6, 880 (2010.
[2] J. Cheng and M. Sprik, Phys. Rev. B 82, 081406R (2010).
[3] F. Costanzo, M. Sulpizi, R. G. Della Valle and M. Sprik, J. Chem. Phys.
134, 244508 (2011).

Contact Person

T. Ozaki and K. Terakura