Mechanisms of controlled stabilizer-free synthesis of gold nanoparticles in liquid aerosol containing plasma

Abstract

The interaction between low-temperature plasma and liquid enables highly reactive solution phase chemistry and fast reaction kinetics. In this work, we demonstrate the rapid synthesis of stabilizer-free, spherical and crystalline gold nanoparticles (AuNP). More than 70 % of gold ion complex (AuCl_4^-) conversion is achieved within a droplet residence time in the plasma of ~10 ms. The average size of the AuNPs increases with an increase in the droplet residence time and the particle synthesis showed a power threshold effect suggesting the applicability of the classical nucleation theory. Leveraging UV-vis absorption and emission spectroscopy, and nanoparticle size distributions obtained from TEM measurements, we showed that the AuCl_4^- conversion exceeded by 250 times the maximum Faradaic efficiency. We identified important roles of both short-lived reducing species including solvated electrons and possibly vacuum ultraviolet (VUV) photons, and long-lived spcies, H2O2, in the reduction of AuCl_4^-. A quantitative investigation was performed by a 1-D reaction-diffusion model which includes transport, plasma-enabled interfacial reduction of AuCl_4^-, classical nucleation, monomer absorption and autocatalytic surface growth enabled by H2O2. The model shows good agreement with the experimental results. The timescale analysis of the simulation revealed that nucleation is enabled by fast reduction of gold ions, and autocatalytic growth mainly determines the particle size and is responsible for the majority of the ion precursor conversion while also explaining the excessively large Faradaic efficiency found experimentally.