Synthesis, fractionation and optical characterization of Au-Ag composite nanorods

We report on a synthesis procedure and optical properties of composite Au-Ag spherical and rod-like nanoparticles. The synthesis protocol is based on a seed-mediated growth in the presence of soft templates in micellar aqueous solution of ionic surfactant (CTAB). Variation of Au/Ag molar ratio allows one to produce nanorods (NRs) with different aspect ratio. The disadvantage of the method is formation of appreciable amount of spherical nanoparticles. To separate rodlike particles from spheres and surfactant, we used a fractionation procedure that involves centrifugation of samples in the density gradient of glycerol. The separated NRs were suspended in water or 25% glycerol solutions and their extinction and differential light scattering (at 90 degrees) spectra were recorded for 450-850 nm wavelengths. Theoretical spectra were calculated by T-matrix method as applied to randomly oriented gold cylinders with semispherical ends. The simulated spectra for water and glycerol suspensions can be brought in close agreement with experimental observations if the aspect ratio is used as a fitting parameter. We discuss also the absorption and light scattering contribution to the total extinction spectra and deviation of the exact solution from the classical electrostatic approximation by Gans.