Molar-Fraction-Tunable Synthesis of Ag–Au Alloy Nanoparticles via a Dual Evaporation–Condensation Method as Supported Catalysts for CO Oxidation

Abstract

The properties of nanoparticles composed of two metallic elements are affected by their synergy as well as the composition and structure of nanocrystals. Therefore, precise adjustment of the molar fractions of the constituent elements and crystal structure is required for their successful synthesis. An evaporation–condensation method, which represents an aerosol nanoparticle synthesis method, is based on the reagglomeration of metallic elements in the gas phase. In this study, we utilized a dual evaporation–condensation method with two furnaces to adjust the molar fractions of Ag–Au nanoparticles with an alloy-type nanocrystalline structure and obtain spherical particles with sizes smaller than 10 nm. The molar fraction of Ag atoms in the synthesized particles varied between 0 and 80% depending on the heating temperature. Energy-dispersive X-ray spectroscopy data revealed that Ag and Au elements were present in all particles and formed an alloy structure, suggesting that alloyed composite particles with different stoichiometries can be easily fabricated via the dual evaporation–condensation method. In addition, the nanoparticles generated in the gas phase were successfully recovered by immobilization on a substrate and served as effective bimetallic catalysts for CO oxidation. It is noteworthy that the preparation of nanoparticles and their fixation to a substrate in the proposed method were performed in one step.