Galvanically replaced hollow Au-Ag nanospheres: study of their surface plasmon resonance.

We synthesized hollow Au-Ag nanospheres (NSs) by employing a galvanic replacement reaction between HAuCl4 and Ag NSs. Uniform Ag NSs with controllable sizes were synthesized as sacrificial templates by a seed-mediated strategy. The atomic ratio of Au to Ag in Au-Ag NSs was tunable by controlling the reagent concentration. UV-vis extinction spectra acquired from well-dispersed colloidal NS solutions were used to investigate the optical properties of the solutions. In addition to a common dipole mode exhibited on most transition metal nanoparticles, we observed a quadrupole plasmon resonance mode when the diameters of the Ag and Au-Ag NSs were larger than 100 nm. The quadrupole and dipole peaks both shifted to longer wavelengths with increased Au content in Au-Ag NSs. The experimental observation of optical properties of hollow Au-Ag NSs was compared with the theoretical simulation using DDA calculation, showing a good agreement.