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Titanium dioxide (TiO 2 ) has a number of uses in catalysis, photochemistry, and sensing that are linked to the reducibility of the oxide. Usually, bridging oxygen (O br ) vacancies are assumed to cause the Ti 3d defect state in the band gap of rutile TiO 2 (110). From high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements, we propose that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. We argue that these donor-specific sites play a key role in and may dictate the ensuing surface chemistry, such as providing the electronic charge required for O 2 adsorption and dissociation. Specifically, we identified a second O 2 dissociation channel that occurs within the Ti troughs in addition to the O 2 dissociation channel in O br vacancies. Comprehensive density functional theory calculations support these experimental observations.

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We studied the nucleation of gold clusters on TiO2(110) surfaces in three different oxidation states by high-resolution scanning tunneling microscopy. The three TiO2(110) supports chosen were (i) reduced (having bridging oxygen vacancies), (ii) hydrated (having bridging hydroxyl groups), and (iii) oxidized (having oxygen adatoms). At room temperature, gold nanoclusters nucleate homogeneously on the terraces of the reduced and oxidized supports, whereas on the hydrated TiO2(110) surface, clusters form preferentially at the step edges. From interplay with density functional theory calculations, we identified two different gold-TiO2(110) adhesion mechanisms for the reduced and oxidized supports. The adhesion of gold clusters is strongest on the oxidized support, and the implications of this finding for catalytic applications are discussed.