Investigating the nano-scale structure and composition dynamics during the phase transition towards complete separation of CeO2–ZrO2 solid solutions

Abstract

Over the past few decades, CeO₂–ZrO₂ (CZO) solid solutions have rapidly replaced conventional CeO₂ as a support material in three-way catalysts (TWCs) because of their superior oxygen releasing/storing capability. However, the phase stability of CZO has long been controversial as can be inferred from many conflicting reports on its structural or compositional stability. In a recent experimental observation, it was noted for the first time that even the solid solution can be completely separated into ceria and zirconia. This finding adds a new layer to the existing confusing controversy, thereby constraining the appropriate utilization of this material. Therefore, in order to elucidate the precise cause and mechanism behind the controversial phase evolution phenomena associated with phase instability, we performed a well-defined model experiment with epitaxial Ce_0.75Zr_0.25O₂ thin films and systematically observed the detailed structural and compositional changes at each stage of the evolution process. Furthermore, in order to ascertain the precise driving force and the underlying reaction mechanism that can be used to manipulate the breakdown or onset of such phase evolution, we calculated the grand potentials for not only the initial phase but also all intermediate and final phases that appear during the phase evolution reaction by using ab initio thermodynamics based on DFT.