Synthesis and properties of monometallic and bimetallic silver and gold nanoparticles

Au, Ag monometallic, and Au-Ag bimetallic nanoparticles have been synthesized by two different methods, the first one was the polyol method and stabilized with poly(vinylpyrrolidone) (PVP), modifying the temperature of synthesis. Interesting structure changes were observed in the nanoparticles as the temperature was varied. The second method consisted of successive reduction of metal salts with ascorbic acid on premade seeds in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). In the first method at low temperatures, no bimetallic nanoparticles were detected, but as the temperature increased bimetallic nanoparticles started to appear, commonly obtaining core-shell nanoparticles, always covered by the polymer. This originates the modification of the optical response of the system in the ultraviolet (UV)-visible region. An absorption peak centered at 520 nm at low temperatures was observed (100-110°C); at higher temperatures (130-170°C) there were nondetectable absorption peaks, and finally at the two highest temperatures (180-190°C) the reappearance of an absorption feature centered at 510 nm was noticed. These UV-visible results indirectly imply the composition of the surface of the particle. The structure of the particles has been determined using transmission electronmicroscopy and high-angle annular dark field (HAADF), the latter being a powerful technique to determine the structural composition of the particles and allowing a direct correlation of the optical response with their structural composition. X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) studies were also performed on the samples and their results support the idea of a Aucore-Agshell structure with the elements segregated from each other. The combination of these experimental techniques with calculated UV-visible absorption spectra allowed, in a reliable way, the elucidation of the nanoparticles structure and elemental distribution. In the second method, the coverage of the seeds is extremely uniform, although in some cases deviations from a spherical shape are observed with the formation of nanorods or nanoprisms. The evolution of the optical properties as further metal layers are deposited is very dramatic and can be modeled using Mie theory for multilayer spheres. However, preliminary results using high-resolution STEM-XEDS elemental mapping suggest that the actual distribution of the two metals within the multilayer spheres may involve (partial) alloying of the metals.