In Situ ETEM Investigation of Size-Dependent Metallic Nanoparticle Oxidation—A Unified Oxidation Mechanism

Oxidation of metallic nanoparticles is a fundamental gas-solid reaction, with significant scientific and technological impact in a broad range of applications including catalysis, energy storage, and nanomaterial fabrication. Although establishing reaction models based on kinetic measurements is commonly applied to study the oxidation mechanisms of bulk metals, its feasibility to nanoscale metal particles remains to be validated especially for nanoparticles with different sizes. As a result, a consensus has not yet been reached on the oxidation mechanism of Ni nanoparticle oxidation.<br/><br/>In this talk, we present the direct <i>in situ</i> gas-cell ETEM observation of the dynamic oxidation process of Ni nanoparticles ranging from 4 to 50 nm, under technically relevant atmospheric pressure of 2%O₂/N₂ at 600 °C. Our observations revealed distinct structural evolution in the course of the oxidation and different morphology of the product NiO, both depending strongly on the size of the starting Ni nanoparticles. To gain a mechanistic understanding of the Ni nanoparticle oxidation and its size dependence, we first performed kinetic model fitting to experimental kinetic curves obtained at an individual nanoparticle level. To validate the best-suitable solid-state models, we further examined the microstructural evolution characteristics, which are uniquely obtained using the time-resolved <i>in situ </i>ETEM real-space imaging. Our kinetic model fitting and complementary microstructural evolution examination at an individual nanoparticle level provide a unified oxidation theory that reconciles this size-dependent Ni nanoparticle oxidation. Specifically, we identified a two-stage process for the oxidation of Ni nanoparticles: stage 1 is the early NiO nucleation (AE model) and initial thickening, and stage 2 is the Wagner diffusion-balanced NiO shell thickening (Wanger model).<br/><br/>This time-resolved <i>in situ</i> ETEM real-space imaging coupled with phase segmentation allowed the extraction of individual nanoparticle-level oxidation kinetics together with the information of microstructural evolution that are impossible to obtain from common collective (diffraction/mass-based) kinetic analysis. In particular, this method offers a unique insight into the transformation kinetics and mechanisms for a variety of size-dependent reactions, providing guiding principles for nanoparticles reaction and morphology control.