Absence of a pressure gap and atomistic mechanism of the oxidation of pure Co nanoparticles

TL;DR

This study provides an atomistic understanding of how pure Co nanoparticles oxidize, revealing a two-step process without a pressure gap and highlighting implications for their magnetic and catalytic properties.

Contribution

It offers the first detailed atomistic mechanism of Co nanoparticle oxidation, showing the absence of a pressure gap and linking structure evolution to magnetism.

Findings

Early oxidation involves formation of CoO crystallites

Complete oxide shell formation occurs via coalescence

Nanoparticles remain reactive at low pressures

Abstract

We present a detailed atomistic picture of the oxidation mechanism of Co nanoparticles and its impact on magnetism by experimentally following the evolution of the structure, chemical composition, and magnetism of individual, gas-phase grown Co nanoparticles during controlled oxidation. The early stage oxidation occurs in a twostep process characterized by (i) the initial formation of small CoO crystallites randomly distributed across the nanoparticle surface, until their coalescence leads to structural completion of the oxide shell and passivation of the metallic core; (ii) progressive conversion of the CoO shell to Co3O4, accompanied by void formation due to the nanoscale Kirkendall effect. The Co nanoparticles remain highly reactive toward oxygen during phase (i), demonstrating the absence of a pressure gap whereby a low reactivity at low pressures is postulated. Our results provide an important benchmark for an improved understanding of the magnetism of oxidized cobalt nanoparticles, with potential implications on their performance in catalytic reactions.