Electron transfer between anatase TiO ₂ and an O ₂ molecule directly observed by atomic force microscopy

Significance Molecular oxygen is an inert species, unable to enter chemical reactions. Activation occurs through the acceptance of an extra electron; this catalytic step plays a major role in applications such as heterogeneous catalysis and fuel cells. It is also used by all living organisms. We show that the two different charge states of O 2 can be easily distinguished by atomic force microscopy (AFM). We directly injected or removed electrons into/from the O 2 molecule by the AFM tip, switching the O 2 reactivity. These results open new possibilities for studying catalytic and photocatalytic processes.