Atomic-Scale Degradation Mechanisms during Nanoparticle Exsolution in Thin Films

Exsolution, defined as the emergence of nanoparticles from a host matrix under reducing conditions, has become a key strategy in catalysis and energy applications, owing to its exceptional particle dispersion and stability. Although extensive research has focused on the early stage of exsolution, the atomic-scale degradation processes during prolonged nanoparticle exsolution remain poorly understood. Here, using in situ transmission electron microscopy, we directly visualize the sequential transformation and degradation processes during exsolution in nonstoichiometric La_0.2Sr_0.7Ni_0.1Ti_0.9O_3-δ thin films. At the early stage, Ni nanoparticles were exsolved through a two-step crystallization process mediated by surface segregation. Remarkably, the nonstoichiometric perovskite lattice undergoes the annihilation of antiphase boundaries, known as fast diffusion channels for exsolution, through a transition toward a stoichiometric state induced by exsolution of Ni from the lattice. Upon continued high-temperature reduction, the film undergoes surface pit formation and progressive sublimation of Ni and Sr, accompanied by the emergence of a secondary La₂TiO₅ phase via surface ledge migration. These sequential transformations and degradation processes, intimately coupled to nanoparticle stability, provide mechanistic insight into the dynamic nature of exsolution and establish guiding principles for the design of catalytic and energy materials.