Oscillations of Cerium Oxidation State Driven by Oxygen Diffusion in Colloidal Nanoceria (CeO_{2 - x} )

The redox performance of CeO_{2 - x} nanocrystals (nanoceria) is always accompanied by the switching of cerium oxidation state between Ce³⁺ and Ce⁴⁺. We monitored Ce³⁺ → Ce⁴⁺ oxidation of nanoceria stimulated by oxidant in aqueous colloidal solutions controlling the luminescence of Ce³⁺ ions located at different distances from nanoceria surface. The observed Ce³⁺ luminescence changes indicate that Ce³⁺ → Ce⁴⁺ 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 Ce³⁺ luminescence intensity arising from Ce³⁺ ↔ Ce⁴⁺ 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.