Formation mechanism of noble metal nanoparticles in aqueous solution by solution plasma

Noble metal nanoparticles (NPs) were successfully prepared by the solution plasma in aqueous solution containing Au, Pd, or Pt chrolo-complexes. According to the spectral changes, the formation rate of Au NPs was faster than those of Pd and Pt NPs. The sizes of the prepared NPs were 20-30 nm. The reduction of Au ions, which were easily monitored by the colorimetric determination, was accelerated by the addition of anionic sodium dodecylsulfate (SDS), whereas the rate was slow in the absence of SDS, showing that SDS is the source of reducing species. ESR-spin trapping experiments suggested that radicals originated from SDS decomposition in the solution plasma contributed to the reduction. In contrary to SDS, non-ionic polyethyleneglycol monostearate (PEG-MS) did not facilitate the Au ion reduction. Comparing with SDS system, larger amount of CO₂ and smaller amount of CO were generated in the PEG-MS system. This fact indicates that the Au ion reduction by the solution plasma depends on the kinds and the amounts of the chemical species from the decomposition of the surfactants. It is thought that Pd and Pt ions are also reduced via similar process.