SURFACE DIFFUSION INDUCED BULK INTER-MIXING AS A HOLLOWING MECHANISM OF Ag-Au CORE-SHELL METAL NANOPARTICLES ON SAPPHIRE

Hollow metallic nanoparticles attract great deal of attention due to their possible applications in various fields of nanotechnology. In the present work we propose a novel method of fabricating of hollow Au nanoparticles based on surface and grain boundary diffusion, which require significantly lower temperatures than bulk diffusion. We produced an array of single crystal Ag nanoparticles employing the solid state dewetting process of a thin Ag film deposited on sapphire substrate. A thin Au film was then deposited on the dewetted sample. During subsequent heat treatment at 170 °C the Ag atoms from the core of the core-shell Ag-Au nanoparticles diffused along grain boundaries toward the free surface of Au film, leaving behind hollow Au nanoparticles. We proposed a model which explained the hollowing of the nanoparticles by considering the diffusion fluxes of the different species along the surfaces and grain boundaries, resulting in formation of the Au-Ag solid solution layer on the top of the Au film. The thickness of this solution layer was comparable with the thickness of original Au film (15 nm), yet it formed exclusively by the process of surface interdiffusion of Au and Ag. We coined a term “surface diffusion induced bulk intermixing” (SDIBI) to describe this phenomenon. The decrease of the Gibbs free energy of the system caused by surface intermixing of Au and Ag dully overcompensated the increase of energy caused by formation of the new internal surface in the hollow nanoparticle. The kinetics of particles hollowing predicted by our model was in a good agreement with the experimental results.