Multiple Stages of Metal Exsolution from Nonstoichiometric Perovskites

Abstract

Exsolution enables controlled solid-state precipitation of metal nanoparticles from metal oxides, resulting in materials that exhibit excellent catalytic properties for electrochemical energy conversion. The diverse characteristics of particles exsolved at different temperatures indicate complex mechanisms that remain unresolved. Through a series of in situ electron microscopy, synchrotron X-ray diffraction, and thermogravimetry experiments on La0.2Ca0.7Ti0.9Sc0.05Cu0.05O3−δ across nanoscale wedges to μm-sized powders, multiple stages of exsolution are deconvoluted with increasing thermal activation: (1) minor exsolution of anchored surface nanoparticles, (2) exsolution of endoparticles within the bulk, and (3) diffusion and coalescing in the bulk and at the surfaces. Reconstruction of the perovskite surface by formation of nanoscopic steps and terraces proceeds concurrently with endoparticle growth. Moreover, distinct exsolution characteristics are observed in the near-surface region and deeper within the bulk, resulting in spherically and orthorhombically shaped endoparticles, respectively.