Electron transfer between anatase TiO ₂ and an O ₂ molecule 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 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>i</i>) an O₂ molecule accepts an electron from a near-surface dopant or (<i>ii</i>) 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.