Alloy formation of gold-silver nanoparticles and the dependence of the plasmon absorption on their composition

Gold-silver alloy nanoparticles with varying mole fractions are prepared in aqueous solution by the co-reduction of chlorauric acid HAuCl4 and silver nitrate AgNO3 with sodium citrate. As the optical absorption spectra of the ir solutions show only one plasmon absorption it is concluded that mixing o f gold and silver leads to a homogeneous formation of alloy nanoparticles. The maximum of the plasmon band blue-shifts linearly with increasing silver content. This fact cannot be explained by a simple linear combination of t he dielectric constants of gold and silver within the Mie theory. On the ot her hand, the extinction coefficient is found to decrease exponentially rat her than linearly with increasing gold mole fraction x(Au). Furthermore, th e size distribution of the alloy nanoparticles is examined using transmissi on electron microscopy (TEM). High-resolution TEM (HRTEM) also confirms the formation of homogeneous gold-silver alloy nanocrystals.