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

Abstract

The transformation of Ni nanoparticles (NPs) of different sizes (average diameters of 9, 26, 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 h at 200-500 degrees 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 approximately 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. Differences in the void formation and growth processes cause size-dependent nanostructural evolution: For 9 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. Because of the differences in the Ni/NiO volume ratios for the 9 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.

Cobalt Fischer–Tropsch Catalyst Regeneration: The Crucial Role of the Kirkendall Effect for Cobalt Redispersion

“…The formation of hollow oxide particles upon oxidation has been reported previously, on several metals including Co, Ni, Fe and Cu [15,16,[18][19][20], and the mechanism responsible for this is the Kirkendall effect, where diffusivity differences of the reactants in solid state reactions results in void formation [21][22][23]. In the case of oxidation of metallic nanoparticles the outward diffusion of the metal (cations) through the growing oxide layer is much faster than the inward diffusion of oxygen.…”

Engineered inorganic core/shell nanoparticles

“…Even though the chemical route is the standard procedure for hollow structure synthesis [86,87], a physical route based from the conversion of solid to hollow NPs known as the "nanoscale Kirkendall effect" was successfully investigated [88]. The Kirkendall effect is the motion of a boundary layer between two elements that occurs as the consequence of the sizeable diffusion rates between the two elements at high temperature.…”

Characterization, purification, and stability of gold nanoparticles

“…Overall, compared to other chemically unstable NPs (e.g. Fe, Al, Cu, Co and Ni) that could be oxidized at room temperature [28,29] and at elevated temperature (100e500 C) [30,31], or release metal ions and induce toxicity (e.g. CuO, ZnO, and CdSe) [32e34], AuNPs examined in this study can be an appropriate reference material without interference of intrinsic toxicity to evaluate effects or functions of other NPs.…”