Unraveling the Mechanisms of Gold Nanoparticle Synthesis in Plasma-Microdroplet Interactions

Plasma-liquid interactions can be leveraged as a promising alternative green nanoparticle synthesis approach. Especially, plasma microdroplet reactors provide significant advantages for rapid nanoparticle synthesis and enable complete ion reduction by overcoming transport limitation. To gain a better understanding of the reduction and particle growth mechanisms in microdroplets, we developed a one-dimensional reaction-diffusion model simulating gold nanoparticle synthesis in a microdroplet treated by RF plasma. The model includes gold ion precursor reduction processes, nucleation, particle growth and diffusive transport and the results are in good agreement with experiments performed with our on-demand microdroplet reactor. A critical gas phase reducing species current to induce particle formation was identified to be an order of magnitude higher than the measured electron flux. Despite the uncertainties of parameters such as solubility of gold monomers, the model suggests the participation of other short-lived species or VUV photolysis in the ion precursor reduction process. The timescale analysis of the simulation revealed that nucleation is enabled by fast reduction of gold ions, and autocatalytic growth facilitated by hydrogen peroxide contributes to the particle growth. The model also provides a comprehensive view into the contribution of each of the different processes underpinning particle growth, which provides valuable insights for process design and improvement.