Preparation and Characterization of Palladium Shells with Gold and Silica Cores

This paper describes the structural and optical properties of core−shell particles in which the outer shell is composed of palladium and the inner core is composed of either gold (conducting) or silica (dielectric). Monodispersed gold-core particles having ∼75 nm diameters were prepared by conventional citrate reduction; silica-core particles having diameters ranging from 100 to 500 nm were prepared by the Stöber method. The silica-core particles were functionalized with amine groups and seeded with small gold nanoparticles (∼2−3 nm). Both types of core particles were then coated with palladium to afford controllable sizes of core−shell particles ranging from ∼100 to 600 nm in overall diameter (i.e., the palladium shell thickness could be varied from 10 to 60 nm). The optical properties, morphology, and elemental composition of the composite nanoparticles were characterized by UV−vis, FE-SEM, EDX, DLS, and TEM. The results demonstrate that palladium shell nanoparticles can be reliably prepared in a controlled fashion and that their optical absorptions are broadened and shifted to longer wavelength compared to simple palladium nanoparticles.