A comparative study of the reduction of silver and gold salts in water by a cathodic microplasma electrode

A comparative study of the reduction of aqueous silver (Ag) and gold (Au) salts to colloidal Ag and Au nanoparticles, respectively, by a gaseous, cathodic, atmospheric-pressure microplasma electrode is presented. The resulting nanoparticles (NPs) were characterized by ultraviolet-visible (UV-vis) absorption spectroscopy and transmission electron microscopy (TEM), and the aqueous solution composition before and after experiments was determined by ionic conductivity, electrochemical potential, and/or UV-vis absorption measurements. TEM showed that Ag and Au NPs were spherical and non-agglomerated when synthesized in the presence of a stabilizer, polyvinyl alcohol. The charge injected by the plasma was correlated to the maximum intensity in the absorbance spectra which in turn depends on the nanoparticle concentration. Separately, the charge injected was correlated to the metal cation concentration. Ag and Au reduction rates were found to be directly proportional to the charge injected, independent of plasma current and process time. Differences in the mechanism for Ag and Au reduction were also observed, and solution species generated by the plasma and their role in the reduction process (e.g. H₂O₂, electrons) is discussed.