Rutile TiO₂(110) surface is extensively studied and it is considered a model transition-metal oxide surface. The adsorption of molecular O₂ has been long investigated with various techniques, but still even the basic adsorption configurations remain unclear. In this research [Sokolović et al, PNAS 2020, 117, 14827] , we used non-contact atomic force microscopy (nc-AFM) to directly observe the adsorption of molecular O₂dosed at low sample temperatures, and unambiguously determine the adsorption configurations and properties. We demonstrate that an oxygen-terminated AFM tip provides excellent lateral resolution, and that it does not perturb the adsorbates . Using density functional theory (DFT) we simulated the observed adsorption configurations and determined the charge state of the adsorbed molecules, which is 2- (peroxo) in all cases. We subjected the O₂-covered surface to thermal annealing, charge injection/removal with the tip, ultraviolet light irradiation, and we directly observed the reaction products in each case (see the Figure). The various adsorbed O₂ species behave differently, which explains their rich behavior and answers several long-standing questions.

Atomically resolved nc-AFM images of the O<sub>2</sub> covered TiO<sub>2</sub>(110) surface before (left) and after (right) a small UV irradiation dose. The excellent lateral resolution is attributed to the sharp, O-terminated AFM tip apex, while the atomic mechanisms acting during UV irradiation are identified by observing the reaction products.

Atomically resolved nc-AFM images of the O₂ covered TiO₂(110) surface before (left) and after (right) a small UV irradiation dose. The excellent lateral resolution is attributed to the sharp, O-terminated AFM tip apex, while the atomic mechanisms acting during UV irradiation are identified by observing the reaction products.

Martin Setvin

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