(727e) Modeling Defects in TiO2 and Their Importance in Surface Chemistry

Authors: 
Deskins, N. A., Worcester Polytechnic Institute
Dupuis, M., Pacific Northwest National Laboratory
Rousseau, R., Pacific Northwest National Laboratory


TiO2 is often taken as a
model oxide material and is an important photoactive catalyst used for a
variety of applications, such as organic molecule decomposition and
water-splitting. It is also used as a catalyst support, such as for Au or V2O5.
Point defects within the TiO2 structure, such as O vacancies (Ov)
or Ti interstitials (Tiint), lead to unpaired electrons formally associated
with reduced Ti3+ sites. The exact nature and role of these defects
in surface reactions is not however without controversy, and various
descriptions exist for the reduced TiO2 surface1.
We present in this talk density functional theory (DFT) calculations of rutile
TiO2, and give an account of the Ti3+ centers which demonstrates
that the excess electrons at Ti3+ sites freely move about in the TiO2
lattice and can strongly influence surface reactions through electron donation
to adsorbed species.

Relying on DFT+U, a method to ensure
charge localization, we calculated diffusion activation barriers for unpaired electrons
at Ti3+ sites and found them to be small (~0.3 eV)2,3,
indicating that the electrons are quite mobile in the lattice. We also modeled
surfaces with bridging-oxygen hydroxyls (HOb) and Ov's,
and found several electronic states (each with electrons localized at specific
Ti sites) to be nearly degenerate, suggesting that the reduced surface is best
described as having a thermodynamic distribution of Ti3+ sites4.
Interestingly, subsurface Ti3+ sites were found to be more stable
than surface Ti3+ sites. Furthermore we also modeled surfaces with Ov's,
Tiint's and HOb's, and show how electrons resulting from
these defects affect the adsorption and surface chemistry of TiO2 through
surface-to-adsorbate electron transfer5,6.
Finally, we present a model which indicates that adsorbate electronegativity
determines when electron transfer will occur, and our model also shows how
modifying the surface work function can be used to control surface chemistry.

References

(1)        Ganduglia-Pirovano,
M. V.; Hofmann, A.; Sauer, J. Surf Sci Rep 2007, 62, 219.

(2)        Deskins,
N. A.; Dupuis, M. Physical Review B 2007, 75, 195212.

(3)        Deskins,
N. A.; Dupuis, M. J Phys Chem C 2009, 113, 346.

(4)        Deskins,
N. A.; Rousseau, R.; Dupuis, M. The Journal of Physical Chemistry C 2009,
113, 14583.

(5)        Deskins,
N. A.; Rousseau, R.; Dupuis, M. The Journal of Physical Chemistry C 2010,
114, 5891.

(6)        Du,
Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Physical
Chemistry Chemical Physics
2010, In Publication.

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