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M. Setvin, U. Aschauer, P. Scheiber, Y. Li, W. Hou, M. Schmid, A. Selloni, U. Diebold:
"Reaction of O₂ with Subsurface Oxygen Vacancies on TiO₂ Anatase (101)";
Science, 341 (2013), 988 - 991.

Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical
sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen
vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed,
transformed, and, in conjunction with theory, identified the nature of O2 molecules on the
(101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the
bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope
tip. O2 adsorbed as superoxo (O2
-) at fivefold-coordinated Ti sites was transformed to peroxo (O2
2-)
and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called
bridging dimer also formed when O2 directly reacted with VOs at or below the surface.