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

Abstract

Interaction of molecular oxygen with semiconducting oxide sur- faces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to mul- tiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combina- tion 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 excel- lent 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 pos- sibility 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.