Surfactant-free synthesis of size controlled platinum nanoparticles: Insights from in situ studies

The controlled synthesis of surfactant-free nanoparticles (NPs) remains a challenge. To develop improved and reproducible syntheses, a deeper understanding of the formation mechanism of surfactant-free NP is key. Pt NPs in particular are important catalysts for a wide range of applications. In situ X-ray absorption near edge structure (XANES) spectroscopy, in situ small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) are used to study the formation of surfactant-free Pt NPs in alkaline ethylene glycol. For the first time, different concentrations of H2PtCl6 (4 mM or 10 mM), different NaOH/Pt molar ratios between 0 and 20 and a wide temperature range from 30 to 140 °C are investigated. It is shown that the NaOH/Pt molar ratio influences the kinetics of the NP formation and is a key parameter to tune in order to achieve size control. At higher NaOH/Pt molar ratio the formation of Pt NPs is slower and smaller NPs are formed in a reproducible manner across various experimental conditions. This is related to the surface properties of the NPs. NPs with a diameter ca. 1 nm are easily obtained with a NaOH/Pt molar ratio of 10–20 and their formation takes place faster at higher temperatures.