Ionic Conductivity Measurements - A Powerful Tool for Monitoring Polyol Reduction Reactions

The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex situ ionic conductivity measurements. Using commonly used platinum precursors (K₂PtCl₆, H₂PtCl₆, and K₂PtCl₄) as well as iridium and ruthenium precursors (IrCl₃ and RuCl₃), we demonstrate that their reduction in ethylene glycol at elevated temperatures is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride-containing metal precursors (at ≈120 °C). On the basis of these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H₂PtCl₆ in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.