  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.
