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Size-Dependent Nanoscale Kirkendall Effect During the Oxidation of Nickel Nanoparticles


Image of a half-oxidized 26 nm nanoparticle. The Ni region is colored red, and the NiO is colored blue and green.

Image of a half-oxidized 26 nm nanoparticle.
The Ni region is colored red, and the NiO is colored blue and green.

Authors: Justin. G. Railsback, Aaron C. Johnston-Peck, Junwei Wang, Joseph B. Tracy, North Carolina State University

Published: April 2, 2010, ACS Nano

Abstract: The transformation of Ni nanoparticles (NPs) of different sizes (average diameters of 9 nm, 26 nm, and 96 nm) during oxidation to hollow (single void) or porous (multiple voids) NiO through the nanoscale Kirkendall effect was observed by transmission electron microscopy. Samples treated for 1-4 hours at 200-500 °C show that the structures of the completely oxidized NPs do not depend on the temperature, but oxidation proceeds more quickly at elevated temperatures. For the Ni/NiO system, after formation of an initial NiO shell (of thickness ~3 nm), single or multiple voids nucleate on the inner surface of the NiO shell, and the voids grow until conversion to NiO is complete.
Hollow or porous NiO nanoparticles nanoparticles of different sizes (6, 26, and 96 nm)

Hollow or porous NiO nanoparticles nanoparticles
of different sizes (6, 26, and 96 nm)

Differences in the void formation and growth processes cause size-dependent nanostructural evolution: For 9 nm and 26 nm NPs, a single void forms beneath the NiO shell, and the void grows by moving across the NP while conversion to NiO occurs opposite the site where the void initially formed.

Images of 26 nm nanoparticles as the oxidation process progressed from 90-210 minutes at 300 °C.

Images of 26 nm nanoparticles as the oxidation process
progressed from 90-210 minutes at 300 °C.

Due to differences in the Ni/NiO volume ratios for the 9 nm and 26 nm NPs when the void first forms, they have distinct nanostructures: The 9 nm NPs form NiO shells that are nearly radially symmetric, while there is a pronounced asymmetry in the NiO shells for 26 nm NPs. By choosing an intermediate oxidation temperature and varying the reaction time, partially oxidized Ni(core)/NiO(shell) NPs can be synthesized with good control. For 96 nm NPs, multiple voids form and grow, which results in porous NiO NPs.