In-situ observation of inkjet droplets in atmospheric-pressure radio frequency argon plasma and demonstration for synthesis of monodisperse submicron gold particles*

  In recent decades, plasmas in contact with liquids have been extensively studied in a wide range of fields such as material synthesis, surface treatment, agriculture, and medicine [1]. Some short-lived reactive species such as O₃ and OH radicals are abundant near the plasma-liquid interface [2], and by using micro-sized droplets with a large specific surface area, magnified interfacial reactions different from that of bulk liquid phase can be promoted. Here, we generated microdroplets by means of an inkjet device with high reproducibility and exposed to atmospheric-pressure radio frequency argon plasma. By controlling the ejection timing of the inkjet droplets and illuminating the droplets by a flash lamp, in-situ observation of each droplet in plasma at various positions became possible by charge-coupled device camera. The droplet size in the plasma decreased rapidly compared to in the atmosphere, and the temporal behavior of the solvent evaporation rate was measured. Furthermore, synthesis of monodisperse gold particles via atmospheric-pressure plasma process with inkjet droplets of chloroauric acid (HAuCl₄) aqueous solution was demonstrated [3]. The variation of inkjet droplets size was within 1–2%, thus synthesized particles had a very narrow size distribution (3–9 % standard deviation). Their diameters can be controlled by adjusting the concentration of the solution. Further details will be presented at the conference.

[1] P. J. Bruggeman et al., Plasma Sources Sci. Technol. 25 (2016) 053002.

[2] C. Chen et al., Plasma Chem. Plasma Process 34 (2014) 403.

[3] K. Nitta et al., J. Phys. D: Appl. Phys. 54 (2021) 33LT01.