(Left) an transmission electron microscopy (TEM) image of intermediate Fe@void@FexOy nanoparticles formed in the course of oxidation of iron nanoparticles; (Right) image of the corresponding intermediate nanoparticle constructed from the in-situ synchrotron measurement with an ab initio program, highlighting the structural details of the voids.Scientists from the Argonne National Laboratory and Temple University in the US have shown how hollowed nanoparticles are formed during metal oxidation. Combining X-ray imaging and computer modeling and simulation, the study improves our understanding of the behavior of metal nanoparticles when undergoing oxidation, as well as the general processes of oxidation and corrosion, a breakthrough that could influence the development of technologies such as sensors, solar cells and batteries, thermal and electrical insulators, optics and electronics.
In the study, published in Science [Sun et al. Science (2017) DOI: 10.1126/science.aaf6792], the behavior of metal nanoparticles was examined by observing them in real time while they oxidized, and the changes in nanoparticle geometry were then modeled as they occurred. Small-angle X-ray scattering was used to characterize the void structures at a relatively high spatial resolution, while wide-angle X-ray scattering explored the crystalline structure of the nanoparticles, a combination that allowed the team to investigate both the metal lattice and the pore structure, enabling an accurate reconstruction of 3D morphologies of the nanoparticles.
During metal oxidation, the directional flow of material across a solid/gas or solid/liquid interface can lead to holes forming in the atomic lattice, a process known as the Kirkendall effect, a motion that can help in the design of exotic materials at the nanoscale. Here, the Kirkendall effect in solid iron nanoparticles during oxidation at the nanoscale level was demonstrated. At the same time, large-scale reactive molecular dynamics simulations were consistent with the experiments, showing the underlying atomistic mechanism and dynamical evolution of voids during the oxidation of iron nanoparticles. The real-time quantitative results provide unprecedented knowledge of the Kirkendall process at the atomic level, highlighting that the in-situ techniques and simulations were useful for studying many nanomaterials under real working conditions.
“Understanding the details of nanoparticle hollowing process at the atomic level will be beneficial for precise control over properties of the resulting hollow nanostructures, which are promising for applications including catalysis, energy conversion and storage”Yugang Sun
The oxidation of iron is expected to be substantially different when the size of iron objects is reduced down to the nanoscale, and the real-time capturing of the evolution of iron nanoparticles during oxidation under real reaction conditions is important for an understanding of the reaction kinetics involving mass diffusion, crystalline phase transition, and variations of size and morphology.
As researcher Yugang Sun told Materials Today, “Understanding the details of nanoparticle hollowing process at the atomic level will be beneficial for precise control over properties of the resulting hollow nanostructures, which are promising for applications including catalysis, energy conversion and storage”. The team now hope to apply the technique to assess functional nanomaterials under operando conditions to correlate device performance and structures.