Lawrence Berkeley National Laboratory

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.