“We discovered this effect by accident, when we added sulphur to a solution of cobalt nanocrystals,” Paul Alivisatos told nanotechweb.org. “The goal was to see if sulphur would catalyse polymerization of the organic molecules that coat the outside of the particles. Instead, we found that hollow cobalt-sulphide nanocrystals formed. We followed up on it and realized that it is a very general effect, based on the well known formation of voids by the ‘Kirkendall Effect’.”
The Kirkendall Effect, discovered by Ernest Kirkendall in 1947, provided the first evidence that atomic diffusion occurs by a process of vacancy exchange, rather than by direct swapping of atoms. According to Alivisatos, the oxidation and sulphidation of cobalt under vapour is mainly controlled by the outward diffusion of cobalt cations. For a nanocrystal, that should lead to a hollow structure.
Alivisatos and colleagues prepared hollow nanospheres of cobalt sulphide by injecting a solution of sulphur in an organic solvent into a dispersion of cobalt nanocrystals at 455 K. The reaction was complete within a few seconds, giving a black solution of hollow colbalt-sulphide nanocrystals.
The researchers say that outward flow of cobalt through the sulphide shell creates a supersaturation of vacancies that condense to form a single hole in each nanoparticle. The size distribution of the hollow sulphide particles was similar to that of the initial cobalt nanocrystals. The team also made hollow cobalt-selenide nanocrystals by a similar process.
To form hollow cobalt-oxide nanocrystals, on the other hand, the team flowed an oxygen/argon mixture through a colloidal solution of cobalt at 455 K. They also made hollow structures by oxidizing iron nanospheres and by sulphidation of cadmium nanospheres, and created hollow nanodisks with cylindrical pores by sulphidation of disk-shaped cobalt nanocrystals.
In addition, the researchers formed platinum-cobalt oxide yolk-shell nanostructures by coating a platinum nanocrystal with cobalt and then oxidizing the cobalt to form a hollow cobalt-oxide structure. These structures acted as catalysts for the hydrogenation of ethylene, even though hollow cobalt-oxide nanocrystals without a platinum core did not.
“In the long term there are potential applications in a wide variety of fields, ranging from medical diagnostics and drug delivery to the formation of advanced catalysts,” said Alivisatos.
The researchers reported their work in Science.