Evolution of nanoscale morphology in single and binary metal oxide microparticles during reduction and oxidation processes

Abstract

Metal oxide composites are enabling materials for many energy conversion systems such as chemical looping and photocatalysis. Their synthesis into electronic materials and operation in chemical looping technologies are based on reduction and oxidation reactions that involve exchanges of ions and electrons. These processes result in the creation and diffusion of defects that determine the nanoscale crystal phases and morphologies within these materials and their subsequent bulk chemical and electrical behavior. In this study, samples of metal oxide composites undergoing cycles of reduction and oxidation are examined at the nano- and micro-scale; the interfacial characteristics of dissimilar metals and metal oxides within the composites are examined. Specifically, structural transformations during redox processes involving pure Fe, FeNi alloy, and CuNi alloy microparticles are investigated. In Fe and FeNi systems, nanowires and nanopores are observed to simultaneously form on the microparticle surface during oxidation, while no such structuring is observed in the CuNi system. Additionally, uniform FeNi microparticles are transformed into particles with a NiO-rich core and a Fe₂O₃-rich shell during oxidation, due to differences in the oxidation and ion diffusion rates of Ni and Fe. In all material systems, the oxidized form of the microparticles exhibited porous cores due to ion transport described by the Kirkendall effect. A fundamental understanding of these phenomena will help direct the fabrication of electronic oxide materials and the development of metal oxide-based oxygen carriers for chemical looping applications.