Modeling Defects in TiO2 and Their Importance in Surface Chemistry

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 (O_v) or Ti interstitials (Ti_int), 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 (HO_b) and O_v'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 O_v's, Ti_int's and HO_b's, and show how electrons resulting from these defects affect the adsorption and surface chemistry of TiO₂ through surface-to-adsorbate electron transfer^5,6. 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.