An Insight into the Coating Behavior of Bimetallic Silver and Gold Core-Shell Nanoparticles

Bimetallic Ag(core)/Au(shell) and Au(core)/Ag(shell) core-shell nanoparticles are synthesized in aqueous phase using simple citrate reduction method with variations for both sets in the reaction conditions during the fabrication of shell structure, i.e., change in salt concentration, temperature (from 25 to 100 °C), and pH for salt solutions (from 2 to 12). The surface plasmon resonance effect, size and morphology, elemental composition, crystalline structure, and crystallite size were observed for these bimetallic core-shell nanoparticles using techniques such as UV-Vis spectroscopy, SEM, EDX, and XRD respectively. The size observed for Ag(core)/Au(shell) nanoparticles was 50 ± 6 nm, and Au(core)/Ag(shell) nanoparticles was 64 ± 8 nm. Increasing the concentration of salt for Au or Ag shows better coating and increase in shell thickness as observed by surface plasmon resonance (SPR) peaks. At lower temperatures, generally, agglomeration occurs or in case of Au(core)/Ag(shell) nanoparticles, formation of Ag3O4 occurs; however, at higher temperatures, homogenous small-sized nanoparticles with higher crystallinity are formed. For Ag(core)/Au(shell) nanoparticles, at pH 4, best uniformly distributed nanoparticles are formed with higher crystallinity; however for Au(core)/Ag(shell) nanoparticles, pH 7 is the optimal pH, where uniformly sized nanoparticles with higher crystalline structure are formed.