| AVS 59th Annual International Symposium and Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS+OX-WeM |
| Session: | Synthesis and Characterization of Oxides |
| Presenter: | J. Jupille, Institut des Nanosciences de Paris, France |
| Authors: | P. Krueger, Institut Carnot de Bourgogne, France J. Jupille, Institut des Nanosciences de Paris, France S. Bourgeois, Institut Carnot de Bourgogne, France B. Domenichini, Institut Carnot de Bourgogne, France A. Verdini, Laboratorio TASC, Italy L. Floreano, Laboratorio TASC, Italy A. Morgante, Laboratorio TASC, Italy |
| Correspondent: | Click to Email |
Titanium dioxide, an inert insulator in stoichiometric form, can be easily reduced into an n-type semiconductor TiO2-x with the many electronic, photocatalytic and chemical properties that make the material of huge technological relevance. Formally associated with Ti4+ + e- → Ti3+, the reduction of titania results in excess electrons (EE) that populate localized Ti 3d band gap states. Puzzling issues are the surface or subsurface distribution of EE and the lattice or interstitial nature (Tiint) of the Ti3+ ions. Despite a number of experimental and theoretical studies, the reduced archetypal TiO2(110) has not been unambiguously pictured yet. Regarding the location of Ti3+ ions, density functional theory (DFT), DFT + U scheme and DFT-Hartree-Fock hybrid functionals (the two latter including a better account of self interaction corrections) are far from consensus. EE that are suggested to be trapped either on sixfold (Ti1) and fivefold (Ti2) coordinated surface Ti, or on subsurface beneath Ti1 or beneath Ti2. In such context, the unique capability of resonant photoelectron diffraction (RPED) to map out the spatial distribution of Ti 3d gap states was previously demonstrated in the study of a TiO2(110) surface involving vacancies in bridging oxygen rows (Ob-vac) [1]. However, our conclusion that EE mostly occupy subsurface Ti sites was later challenged by the suggestion that Tiint atoms play a key role in the formation of EE [2,3]. This has prompted us to analyse the Na-covered TiO2(110) surface on which Na adatoms are predicted to produce similar EE as by direct injection of electrons [4], while the formation of Tiint is not expected.
The pivotal observation was that the Na/TiO2 RPED pattern [4] was almost perfectly similar to the Ob-vac RPED pattern [1]. Data were fitted on the basis of the location of the Ti3+ ions on the Ti lattice sites. Indeed, attempts to model EE on Tiint failed. A unified model of the reduced TiO2(110) surface emerges, with EE located on subsurface beneath Ti2 > second subsurface beneath Ti1 > Ti1 [4] (See supplemental document). As shown by the qualitative agreement of the present findings with DFT approaches [5,6], the charge distribution of the Ti 3d states is dictated by electrostatics. It is essentially an intrinsic property of the titania surface that is independent on the way EE are created.
[1] Krueger et al. Phys. Rev. Lett. 100 (2008) 055501.
[2] Wendt et al., Science 320 (2008) 1755.
[3] Papageorgiou et al., Proc. Natl. Acad. Sci. U.S.A. 107 (2010) 2391.
[4] Krueger et al. Phys. Rev. Lett. 108 (2012) 126803.
[5] Albaret et al., Phys. Rev. B 65 (2001) 035402.
[6] Deskins et al., J. Phys. Chem. C 113 (2009) 14583.