Kinetic description of metal nanocrystal oxidation: a combined theoretical and experimental approach for determining morphology and diffusion parameters in hollow nanoparticles by the nanoscale Kirkendall effect

Abstract

The oxidation of colloidal metal nanocrystals to form hollow shells via the nanoscale Kirkendall effect has been investigated using a combined theoretical and experimental approach. A generalized kinetic model for the formation of hollow nanoparticles describes the phenomenon and, unlike prior models, is applicable to any material system and accounts for the effect of surface energies. Phase diagrams of the ultimate oxidized nanoparticle morphology and the time to achieve complete oxidation are calculated, and are found to depend significantly upon consideration of surface energy effects that destabilize the initial formation of small voids. For the oxidation of Cu nanocrystals to Cu2O nanoparticles, we find that the diffusion coefficients dictate the morphological outcomes: the ratio of to controls the void size, determines the time of oxidation and is largely irrelevant in the kinetics of oxidation. The kinetic model was used to fit experimental measurements of 11 nm diameter Cu nanocrystals oxidized in air from which temperature-dependent diffusivities of and for 100 ≤ T ≤ 200 °C were determined. In contrast to previous interpretations of the nanoscale Kirkendall effect in the Cu/Cu2O system, these results are obtained without any a priori assumptions about the relative magnitudes of and . The theoretical and experimental approaches presented here are broadly applicable to any nanoparticle system undergoing oxidation, and can be used to precisely control the final nanoparticle morphology for applications in catalysis or optical materials.