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M. Setvin, J. Hulva, G. Parkinson, M. Schmid, U. Diebold:
"Electron transfer between anatase TiO₂ and an O₂ molecule directly observed by atomic force microscopy";
PNAS - Proceedings of the National Academy of Sciences of the United States of America, 114 (2017), 1 - 7.

Activation of molecular oxygen is a key step in converting fuels
into energy, but there is precious little experimental insight into
how the process proceeds at the atomic scale. Here, we show
that a combined atomic force microscopy/scanning tunneling
microscopy (AFM/STM) experiment can both distinguish neutral
O2 molecules in the triplet state from negatively charged (O2)􀀀
radicals and charge and discharge the molecules at will. By measuring
the chemical forces above the different species adsorbed
on an anatase TiO2 surface, we show that the tip-generated (O2)􀀀
radicals are identical to those created when (i) an O2 molecule
accepts an electron from a near-surface dopant or (ii) when a
photo-generated electron is transferred following irradiation of
the anatase sample with UV light. Kelvin probe spectroscopy measurements
indicate that electron transfer between the TiO2 and
the adsorbed molecules is governed by competition between electron
affinity of the physisorbed (triplet) O2 and band bending
induced by the (O2)􀀀 radicals. Temperature-programmed desorption
and X-ray photoelectron spectroscopy data provide information
about thermal stability of the species, and confirm the
chemical identification inferred from AFM/STM