The great appeal of this nanoscale solid-state chemistry lies in its promise of generality across the periodic table. Iron and cadmium nanocrystals also react through the same reaction pathway, as evidenced by formation of hollow iron oxide and cadmium sulfide nanospheres upon exposure to oxygen and sulfur, respectively. The formation of hollow spheres can be further favored by tailoring the experimental conditions such that the starting nanocrystals are placed in a dilute solution of the second reactant, which provides a greater driving force for diffusion of metal atoms from the nanoparticle core to the reactive interface. Thus, hollow nanostructures of a variety of shapes and compositions—including insulators, semiconductors, and metals—should be accessible via this wide-ranging chemistry. Efficient and dependable approaches toward synthesizing inorganic hollow nanoparticles not only offer important insights into solid-state reactivity, but also are expected to be useful for the production of tailored nanoreactors or nanocapsules (8). Porous materials have many present and potential future applications for catalysis, separation technologies, and hydrogen storage, among others (9). As a demonstration of the utility of the synthetic method described here, Yin et al. encapsulated catalytically active platinum metal nanoparticles about 5 nm in diameter within their own cobalt oxide shells. To form this structure, they deposited cobalt metal on the outside of small platinum nanoparticles (diameter 3 nm) to form a core-shell structure. Upon reaction with oxygen, the cobalt reacted to form the cobalt oxide shell, pulling away from the platinum nanoparticle center as a result of migration outward, leaving an egg-like structure with the metallic platinum yolk in the center. These encapsulated platinum nanoparticles were catalytically active for ethylene hydrogenation and were not isolated from the surrounding medium. Reactants and products could diffuse in and out of the polycrystalline cobalt oxide shell through the grain boundaries. Thus, such systems may serve as unique nanoreactors, effectively isolating the individual reactive metal nanoparticles from each other but still allowing free diffusion of small molecules.

The dividing line between fundamental science and applications is a fine one in this work. The authors show that a well-established solid-state reaction can be analyzed in detail in these nanocrystalline systems that are as close to perfection as can be achieved in any laboratory. This accomplishment raises the question of which other solid-state principles can be examined and what they will reveal. At the same time, this very general synthetic strategy to produce hollow particles with high surface area has tremendous promise for a range of applications. Investigations into the range of materials that can be produced and of synergies between core-shell structures have only just begun.