Unraveling the “Seesaw” Competition between the Electrically Driven Reduction and Reoxidation Processes in Ceria with In Situ Electron Microscopy

Abstract

An improved mechanistic understanding of the redox behavior of ceria (CeO2) is central to controlling its catalytic and ionic properties, which are involved in a variety of technologically important applications. Here, as enabled by the real‐time temporal resolution of in situ electron microscopy, we present strong evidence to help elucidate the complex interplay and dynamics of the electrically driven redox equilibria in ceria. Under an external electric field, a CeO2 film sandwiched between a Nb‐doped SrTiO3 substrate (cathode) and a tungsten probing tip (anode) was observed to dynamically exhibit a seesaw‐like, reversible forward/backward propagation of the modulation wave formed in the cubic ceria lattice as a consequence of ordering and migration of oxygen vacancies. Such an interesting seesaw‐like behavior indicates subtle competition between the electrically driven reduction (forming oxygen vacancies) of ceria and its dynamic reoxidation (repairing oxygen vacancies) by residual oxygen gas in the chamber (≈10−6 Pa). This result provides important new insight that can be used to understand the redox chemistry of ceria, for which, in previous studies, the reduction and oxidation processes were assumed to be two seemingly separate events that could occur only after switching the external thermodynamic conditions.