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Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

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Science  30 Aug 2013:
Vol. 341, Issue 6149, pp. 988-991
DOI: 10.1126/science.1239879

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Oxide Chemistry Below the Surface

Although metal oxides, such as titanium dioxide (TiO2), are used for catalytic oxidation reactions and photocatalysis, the O2 does not react directly with substrates. Vacancies in the surface region of the TiO2 rutile phase can transfer a negative charge to adsorbed O2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO2 particles—subsurface vacancies form. Setvin et al. (p. 988) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O2 into a peroxo species and a bridging O2 dimer.

Abstract

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 (O22–) 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.

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