Alloy Formation of Gold−Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition

TLDR

The observed plasmon behavior cannot be explained by a simple linear combination of gold and silver dielectric constants within Mie theory. Gold–silver alloy nanoparticles of varying composition were synthesized by co‑reduction of HAuCl₄ and AgNO₃ with sodium citrate, and their size distribution was characterized by TEM. The nanoparticles exhibit a single plasmon band that blue‑shifts linearly with increasing silver content, while the extinction coefficient decreases exponentially with gold mole fraction, confirming homogeneous alloy formation by HRTEM.

Abstract

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 their solutions show only one plasmon absorption it is concluded that mixing of 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 the dielectric constants of gold and silver within the Mie theory. On the other hand, the extinction coefficient is found to decrease exponentially rather than linearly with increasing gold mole fraction xAu. Furthermore, the size distribution of the alloy nanoparticles is examined using transmission electron microscopy (TEM). High-resolution TEM (HRTEM) also confirms the formation of homogeneous gold−silver alloy nanocrystals.