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Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy

Significance Molecular O2 on semiconducting metal oxides, its adsorption, response to thermal treatment, UV irradiation, and electron/hole injection is at the heart of a wide range of technologies. These processes were studied molecule by molecule on the prototypical model oxide TiO2(110), using noncontact AFM. We show that the nonintrusive nature of the nc-AFM and its exceptional spatial resolution due to the functionalization of the tip with a terminal O atom allow identifying oxygen adsorption geometries which were not previously determined by other techniques. In combination with density functional theory calculations, we explain the charge states of the various adsorbed oxygen species, their dynamics upon annealing and UV irradiation, and the role of electron exchange with the TiO2 substrate. Interaction of molecular oxygen with semiconducting oxide surfaces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to multiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combination of noncontact atomic force microscopy (AFM) and density functional theory (DFT) to resolve O2 adsorption on the rutile TiO2(110) surface, which presents a longstanding challenge in the surface chemistry of metal oxides. We show that chemically inert AFM tips terminated by an oxygen adatom provide excellent resolution of both the adsorbed species and the oxygen sublattice of the substrate. Adsorbed O2 molecules can accept either one or two electron polarons from the surface, forming superoxo or peroxo species. The peroxo state is energetically preferred under any conditions relevant for applications. The possibility of nonintrusive imaging allows us to explain behavior related to electron/hole injection from the tip, interaction with UV light, and the effect of thermal annealing.