title: Electron transfer between anatase TiO2and an O2molecule directly observed by atomic force microscopy.
abstract: 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 O2molecules 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 TiO2surface, we show that the tip-generated (O2)-radicals are identical to those created when (i) an O2molecule 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 TiO2and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O2and 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.
authors.affiliation: Institute of Applied Physics, TU Wien, 1040 Vienna, Austria setvin@iap.tuwien.ac.at.
authors.forename: Martin
authors.initials: M
authors.lastname: Setvin
authors.affiliation: Institute of Applied Physics, TU Wien, 1040 Vienna, Austria.
authors.forename: Jan
authors.initials: J
authors.lastname: Hulva
authors.affiliation: Institute of Applied Physics, TU Wien, 1040 Vienna, Austria.
authors.forename: Gareth S
authors.initials: GS
authors.lastname: Parkinson
authors.affiliation: Institute of Applied Physics, TU Wien, 1040 Vienna, Austria.
authors.forename: Michael
authors.initials: M
authors.lastname: Schmid
authors.affiliation: Institute of Applied Physics, TU Wien, 1040 Vienna, Austria.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Diebold
keywords: AFM; O2; TiO2; adsorption; anatase