A facile liquid foam based synthesis of nickel nanoparticles and their subsequent conversion to Ni_coreAg_shell particles: structural characterization and investigation of magnetic properties

Abstract

A facile route for the synthesis of nickel nanoparticles in stable aqueous foams is reported. The Ni nanoparticles were roughly 12-15 nm in size and were stable as aqueous suspensions or powders when oleic acid was used as a capping agent. These Ni nanoparticles were subsequently coated with a silver shell in view of the extra stability and the enhanced manipulative ability afforded by the silver nanocoating. This was accomplished by a simple transmetallation reaction wherein the nanoparticle surface nickel atoms act as localized reducing agents for the silver ions in solution. As the silver shell is formed through the surface reaction a reduction in the average size of the Ni_core occurs. After the core-shell structure formation, the Ni_core has an average diameter of 10-20 nm while the Ag_shell has a thickness of 2-4 nm. The pristine oleic acid coated Ni and Ni_coreAg_shell nanoparticles were probed for their magnetic characteristics by a vibrating sample magnetometer. The nascent, oleic acid coated Ni nanoparticles display a low superparamagnetic blocking temperature, T_B, of 20 K. The field dependent magnetic behaviour above and below T_B displays the standard features corresponding to superparamagnetism, as expected for very small Ni crystallites suggesting again that each 12 nm particle is polycrystalline. The magnetic contribution in the Ni_coreAg_shell system comes from only the Ni core and predictably, the blocking temperature of this system is below 12 K due to the smaller size of the Ni core.