Nanoscale Kirkendall effect and oxidation kinetics in copper nanocrystals characterized by real-time, in-situ optical spectroscopy
Katherine P. Rice, Andrew Paterson, Mark Stoykovich
The low-temperature oxidation of ~10 nm diameter copper nanocrystals was characterized using in-situ UV-vis absorbance spectroscopy and observed to lead to hollow copper oxide shells via the nanoscale Kirkendall effect. The kinetics of the oxidation of solid Cu nanocrystals to hollow Cu2O nanoparticles was monitored in real-time via the localized surface plasmon resonance response of the metallic Cu component. A reaction-diffusion model for the nanoscale Kirkendall effect was fit to the measured time for complete Cu nanocrystal oxidation, and was used to quantify the diffusion coefficient of Cu in Cu2O and the activation energy of the oxidation process. The diffusivity measured here in single-crystalline nanoscale systems was 1~5 orders of magnitude greater than in comparable systems in the bulk, and had an Arrhenius dependence on temperature with an activation energy for diffusion of 37.5 kJ/mol for 85 ≤ T ≤ 205°C. These diffusion parameters have been measured in some of the smallest metal systems and at the lowest oxidation temperatures yet reported, and were enabled by the unique nanoscale single-crystalline material and the in-situ characterization technique. In addition, the oxidation of Cu nanocrystals at higher temperatures (200 ≤ T ≤ 300°C) was observed to produce solid rather than hollow Cu2O nanoparticles due to structural collapse of the thermodynamically unstable Cu2O nanoshell morphology.