Oscillations of Cerium Oxidation State Driven by Oxygen Diffusion in Colloidal Nanoceria (CeO ).

The redox performance of CeO nanocrystals (nanoceria) is always accompanied by the switching of cerium oxidation state between Ce and Ce. We monitored Ce → Ce oxidation of nanoceria stimulated by oxidant in aqueous colloidal solutions controlling the luminescence of Ce ions located at different distances from nanoceria surface. The observed Ce luminescence changes indicate that Ce → Ce reaction develops inside nanoceria being triggered by the diffusing oxygen originated from the water splitting on oxidized nanoceria surface. We present the first observation of the pronounced oscillations of Ce luminescence intensity arising from Ce ↔ Ce reversible switching. This threshold effect is to be driven by uptaking and releasing oxygen by nanoceria, when the concentration of oxygen vacancies in nanoceria lattice, oxidant concentration in colloidal solution, and temperature reach certain critical values. So, the ability of nanoceria to uptake and release oxygen depending on the environmental redox conditions really makes it the self-sufficient eternal antioxidant.