Hollow Hemisphere Shell Formation by Pure Kirkendall Porosity

Nanoshell formation has been studied experimentally in Ag/Au and Ag/Pd systems in a hemispherical geometry at different temperatures. The void formation in these systems is the result ofpureKirkendall-porosity formation, because it is caused mainly by the inequality of the intrinsic atomic fluxes and other effects (e.g. stresses), inevitably present during nanoshell formations in solid state reactions (oxides, sulphides), can be less important or can be neglected. The kinetics of the process was followed by Transmission Electron Microscopy. Both the growth and shrinkage regimes of the process were observed at the same temperature and even the temperature dependence of the characteristic time (tcr) describing the crossover of the two different regimes was observed. We succeeded to show that tcrshifts to smaller values with increasing temperature. This confirms the theoretical results:the growth and the shrinkage regimes are controlled by the faster as well as the slower diffusion coefficients (DAas well as DB), respectively. It is also illustrated that, confirming recent theoretical predictions, the pore radius linearly depends on the initial particle radius and the slope of this straight line increases with the average composition of the faster component.

Formation of a Hollow Binary Alloy Nanosphere: A Kinetic Monte Carlo Study

Results of kinetic Monte Carlo simulation of the formation of a hollow nanosphere by interdiffusion from a core-shell binary system are presented for the first time. The faster diffusing species is located in the core whilst the slower diffusing species form the shell. With its self-generated vacancy composition all stages of the hollow sphere formation process are observed in our model: interdiffusion, the supersaturation of the core of the nanosphere by vacancies, precipitation of pores and eventual void formation. Results of this simulation confirm the experimental conclusions that interdiffusion accompanied by the Kirkendall effect and Kirkendall porosity is one of the mechanisms responsible for the formation of hollow nano-objects.