Anion Dependent Particle Size Control of Platinum Nanoparticles Synthesized in Ethylene Glycol

The polyol synthesis is a well-established method to form so-called "surfactant-free" nanoparticles (NPs). In the present study, the NP size resulting from the thermal reduction of the precursors H₂PtCl₆, H₂Pt(OH)₆, or Pt(acac)₂ in presence of the bases NaOH or Na(acac) at different concentrations is studied. It is shown that the size control depends more strongly on the nature of the precursor (metal salt) than on the anion present in the base. The latter is surprising as the concentration of the base anion is often an important factor to achieve a size control. The reduction of H₂PtCl₆ or H₂Pt(OH)₆ in presence of NaOH and Na(acac) confirm the observation that the NP size is determined by the OH^-/Pt molar ratio and expands it to the base anion/Pt molar ratio. In contrast, the reduction of Pt(acac)₂ in presence of the bases NaOH (previous reports) or Na(acac) (shown in the present work) leads to larger NPs of ca. 3 nm, independent of the concentration of the base anions. Hence, the anion effect observed here seems to originate predominantly from the nature of the precursor (precursor anion dependence) and only for certain precursors as H₂PtCl₆ or H₂Pt(OH)₆ the size control depends on the base anion/Pt molar ratio.