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

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• Activation of molecular oxygen (O2) is crucial for energy conversion processes.
• Experimental understanding of oxygen activation at the atomic scale remains limited.
• Anatase titanium dioxide (TiO2) is a relevant material for catalytic and energy applications.

Purpose of the Study:

• To achieve atomic-scale resolution of oxygen species on a TiO2 surface.
• To experimentally control the charge state of adsorbed oxygen molecules.
• To elucidate the mechanisms of electron transfer in oxygen activation.

Main Methods:

• Combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) for high-resolution imaging and force measurements.
• Kelvin probe spectroscopy to study electron transfer dynamics.
• Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) for thermal stability and chemical identification.

Main Results:

• AFM/STM successfully distinguished between neutral triplet O2 molecules and negatively charged superoxide (O2-) radicals.
• The study demonstrated the ability to controllably charge and discharge adsorbed oxygen species.
• Tip-generated (O2-) radicals were found to be identical to those formed via electron transfer from dopants or photo-generated electrons.

Conclusions:

• Atomic-scale control and differentiation of oxygen species on TiO2 are achievable.
• Electron transfer is governed by the interplay between O2 electron affinity and band bending induced by (O2-).
• The findings offer critical insights into the fundamental steps of oxygen activation at surfaces.