Hollow nanostructures are used for various applications including catalysis, sensing, and drug delivery. Methods based on the Kirkendall effect have been the most successful for obtaining hollow nanostructures of various multicomponent systems. The classical Kirkendall effect relies on the presence of a faster diffusing species in the core; the resultant imbalance in flux results in the formation of hollow structures. Here, an alternate non-Kirkendall mechanism that is operative for the formation of hollow single crystalline particles of intermetallic PtBi is demonstrated. The synthesis method involves sequential reduction of Pt and Bi salts in ethylene glycol under microwave irradiation. Detailed analysis of the reaction at various stages indicates that the formation of the intermetallic PtBi hollow nanoparticles occurs in steps. The mechanistic details are elucidated using control experiments. The use of microwave results in a very rapid synthesis of intermetallics PtBi that exhibits excellent electrocatalytic activity for formic acid oxidation reaction. The method presented can be extended to various multicomponent systems and is independent of the intrinsic diffusivities of the species involved.
The authors show a non-classical method for developing hollow nanoparticles. Beginning with a Pt nanoporous core and depositing a Bi layer on the shell which has a high diffusion constant, they are able to create hollow nanoparticles through heating. Using the Fusion system in an FEI T20, they visualized the process in situ. They explain that it is not a Kirkendall process, but a non-classical diffusion process resulting from the physical structure of the particles. Bi has a larger diffusion constant than Pt, so if it were a Kirkendall diffusion process, Bi would diffuse into the Pt core. However, just the opposite occurs. They hypothesize that the Pt prefers to aggregate towards the concave region in the Bi shell, leaving behind a void in the core. This material system is applicable in catalyst systems due to its large surface area, and it’s been shown that when Pt is alloyed with another metal it is less susceptible to CO poisoning.