We investigate the evolution of structures that result when spherical Cd nanoparticles of a few hundred nanometers in diameter react with dissolved molecular sulfur species in solution to form hollow CdS. Over a wide range of temperatures and concentrations, we find that rapid Cd diffusion through the growing CdS shell localizes the reaction front at the outermost CdS/S interface, leading to hollow particles when all the Cd is consumed. When we examine partially reacted particles, we find that this system differs significantly from others in which the nanoscale Kirkendall effect has been used to create hollow particles. In previously reported systems, partial reaction creates a hollow particle with a spherically symmetric metal core connected to the outer shell by filaments. In contrast, here we obtain a lower symmetry structure, in which the unreacted metal core and the coalesced vacancies separate into two distinct spherical caps, minimizing the metal/void interface. This pattern of void coalescence is likely to occur in situations where the metal/vacancy self-diffusivities in the core are greater than the diffusivity of the cations through the shell.

In the present work, we investigate the magnetic properties of ferrimagnetic and non-interacting maghemite hollow nanoparticles obtained by the Kirkendall effect. From the experimental characterization of their magnetic behavior, we find that polycrystalline hollow maghemite nanoparticles exhibit low blocked-to-superparamagnetic transition temperatures, small magnetic moments, significant coercivities and irreversibility fields, and no magnetic saturation on external magnetic fields up to 5 T. These results are interpreted in terms of the microstructural parameters characterizing the maghemite shells by means of atomistic Monte Carlo simulations of an individual spherical shell. The model comprises strongly interacting crystallographic domains arranged in a spherical shell with random orientations and anisotropy axis. The Monte Carlo simulation allows discernment between the influence of the polycrystalline structure and its hollow geometry, while revealing the magnetic domain arranggement in the different temperataure regimes.

Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It was found that performing the reaction at >120oC leads to fast formation of a single void ins ide each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature-dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, significant inward transport of sulfur anions can be inferred to occur as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells implying significant porosity and/or a defective structure. Indirect evidence for shells to fracture during growth at lower temperatures was observed in shell size statistics and TEM of as-grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.