Aqueous solutions of Ag−Pt ions and poly(vinyl alcohol) were irradiated with gamma rays at dose rates below 0.5 kGy/h to generate nanoparticles. The nanoparticles were characterized with several experimental techniques. Transmission electron microscopy showed that, surprisingly, the nanoparticles were not spherical but had a high aspect ratio. Wirelike structures were generated with lengths up to 3.5 μm and diameters between 3 and 20 nm. Selected-area diffraction showed that the wires were polycrystalline and that individual grains making up the wires had a face-centered cubic (fcc) structure. The optical absorption of samples, with a Ag/Pt mole ratio higher than 80%, exhibited a Ag surface plasmon absorption band centered around 400 nm. The plasmon band broadened with increasing Pt molar ratio and was replaced by a monotonically decaying background for a Pt molar ratio higher than about 30%. Alloying in the Ag−Pt nanoparticles was investigated with X-ray absorption spectroscopy. The Pt L3 edge (11.564 keV) was excited to determine the local structure around the Pt atoms. A contraction in the first shell of 0.05 Å was observed, which ruled out the formation of a Ag−Pt homogeneous alloy and suggested the formation of core−shell particles. To understand the mechanism of formation of the nanoparticles, several experimental parameters such as the total radiation dose, type of polymer, metal and polymer concentrations, and type of counterions in solution were varied. The most relevant parameters inducing filament growth were the counterions added to the solution, the mole ratio between the two metals, and the capping polymer. For example, spherical particles resulted if AgNO3 was used instead of Ag2SO4, if the Ag/Pt mole ratio was higher than 80% or lower than 20%, and if the degree of hydrolysis of the poly(vinyl alcohol) was higher than 98%.

We report studies of bimetallic nanoparticles with 15%–16% atomic crystal parameters size mismatch. The degree of alloying was probed in a 2-nm Pt core (smallest attainable core size) of Pt–Ag nanoparticles (completely immiscible in bulk) and 20-nm-diameter Pd–Ag nanowires (completely miscible in bulk). Particles were synthesized radiolytically, and depending on the initial parameters, they assume spherical or cylindrical (nanowire) morphologies. In all cases, the metals are seen to follow their bulk alloying characteristics. Pt and Ag segregate in both spherical and wire forms, which indicates that strain due to crystallographic mismatch overcomes the excess surface free energy in the small particles. The Pd–Ag nanowires alloy similar to previously reported spherical Pd–Ag particles of similar diameter and composition.

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