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” NPs that can be tailored, e.g., in size. The standard procedure of reducing H2PtCl6 or H2PtBr6 in presence of NaOH has been intensively investigated as well as the influence of the counter cation of the base. However, the base anion influence has not been explored yet. The results of this work show that the size control rather depends on the precursor (metal salt) and surprisingly not the anion type of the base. The latter is surprising as the concentration of the base anion is often an important factor for the size control. The thermal reduction of the precursors H2PtCl6, H2Pt(OH)6, or Pt(acac)2 in presence of the bases NaOH or Na(acac) lead to comparable size control depending on the precursors, i.e., precursor anions. The reduction of H2PtCl6 or H2Pt(OH)6 in presence of NaOH supports the idea of the size being determined by the OH-/Pt molar ratio. Reducing the two precursors in presence of Na(acac) leads to similar results in particle size control for the same acac-/Pt or OH-/Pt ratios. 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 seems originate predominantly in the character of the precursor (precursor anion dependence) and then for certain precursors as H2PtCl6 or H2Pt(OH)6 in the base anion/Pt ratio determining the NP size in the reduction.