Transient Mobility of Oxygen Adatoms upon O₂ Dissociation on Reduced TiO₂(110)

Tracking the same region of the reduced TiO 2 (110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature (RT) confirms that O 2 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 O 2 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 TiO 2 (110) surface occurs exclusively along the Ti troughs.