Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy

M. Setvin, J. Hulva, G.S. Parkinson, M. Schmid, U. Diebold

Institut für Angewandte Physik, Technische Universität Wien, 1040 Wien, Austria

Proc. Natl. Acad. Sci. USA 114 (2017) E2556-E2562

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.

Corresponding author: Martin Setvin (setvin at iap_tuwien_ac_at).

You can download a PDF file of this article from Proceedings of the National Academy of Sciences.
The PDF is also available via the homepage of Michael Schmid.