No abstract

Tracking the same region of the reduced TiO2(110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature (RT) confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. The majority of O adatoms (∼81%) are found separated from the original vacancy positions by up to two lattice constants along the [001] direction. Since at RT the thermal diffusivity of O adatoms has been found to be rather small, with an experimentally estimated activation energy of ∼1.1 eV, we conclude that the observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility during the course of O2 dissociation. Unlike for other known cases of the dissociation of diatomic molecules where both “hot” adatoms accommodate at equivalent sites, in the studied system, the oxygen atoms filling the vacancies are locked into the bridging oxygen rows, and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2(110) surface occurs exclusively along the Ti troughs.

O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature-programmed desorption, and electron-stimulated desorption. On the annealed surface, two oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e., 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ∼20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of 2 when the OHb concentration is enhancedwithout changing the concentration of subsurface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.