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Nanomaterials 2020, 10, 555 5 of 12 3.1. Synthesis Pathways In order to gain more detailed insights into the formation of silver nanoparticles, some process-determining kinetic parameters and the synthesis routes are explained below. The coloration of the stock solution by the plasma is an indication of the formation of silver nanoparticles [25]. The generated electrons, ions, radicals, and electromagnetic fields provided at the plasma/solution interface lead to the reduction of metal ions dissolved in water and ultimately to the nucleation and formation of nanoparticles [6,18]. In the literature, the formation of silver nanoparticles via plasma out of a salt solution is mainly attributed directly to emitted electrons from the plasma discharge [18] and solvated electrons [36,37]: Ag+ + e− → Ag0 (1) By contrast, nanoscale gold particles are primarily formed by plasma ion radiation [38] or plasma-induced liquid chemistry [39]. From the investigations presented in this publication, the influence of long-living species and liquid chemistry on the reduction of silver ions is low or not process-determining. This assumption is supported by the fact that there is no significant change in absorbance in the near future after switching off the plasma. During the whole treatment time, the pH-value stays constant (6.2 ± 0.1). Kondeti et al. [40] showed that a constant pH-value is a strong indicator for a direct reduction without reacting mainly via intermediates like H−, H•, or H2O2. A citrate-based stabilization system was used, which can also create buffer systems, that keep the pH-value in a constant range. These two circumstances made it almost impossible to estimate whether intermediate species formed or not. There is still less knowledge about the concrete reduction pathways. Some research groups claim that the reduction is realized by solvated electrons. In fact, this means that the electrons show strong interactions with the lowest unoccupied molecular orbital (LUMO) of the solvent molecules in the water [36]. Therefore, this reaction pathway is probably responsive for more or less direct reactions between the plasma discharge and the silver ions, whereby the solvent molecules serve as an electron carrier. Other influences on particle synthesis by (i) UV radiation, (ii) temperature, and (iii) the stabilizer sodium citrate as a reducing agent are discussed beloi) A 3 mm-thick fused silica (SiO2) and also a 5 mm-thick window of calcium fluoride (CaF2) were positioned above the stock solution. The two glasses differ in their transmission characteristics, which allows the effect of UV radiation on the solution to be observed in defined wavelength ranges. In this modification of the experimental setup, the electrons, ions, and radicals generated in the plasma were excluded and only the UV radiation could interact with the stock solution. While using the SiO2-window, the stock solution remained unchanged. The use of the CaF2-window showed that the UV radiation in the wavelength range of 130–190 nm emitted from the plasma source is capable of reducing silver ions. When the stock solution was brought into direct contact with the CaF2-window, a silver mirror formed after a plasma treatment time of t = 120 s. If there is a volume of air between the window and the solution, a thin layer of a deep yellow veil formed after t = 120 s, which was not comparable to the result generated by the direct plasma treatment. This circumstance can be explained partly due to the absence of convection mechanisms [40]. It cannot be completely excluded that electrons were formed by the plasma discharge in the air gap between the window and the solution, which could lead to a reduction process of the silver ions. The low formation rate of silver nanoparticles is potentially an indicator for generating a small number of free electrons in the air gap. (ii) The used plasma source generates a cold, atmospheric non-equilibrium plasma. After a treatment time of t = 120 s, the treated liquid surface heated up by about 5 K. This temperature increase does not explain the fast and controlled silver nanoparticle synthesis. (iii) The stabilizer sodium citrate is not capable of acting as a reducing agent under the experimental conditions. Therefore, the plasma activation of the pure stabilizer solution and subsequent addition to the silver nitrate solution also did not lead to the formation of silver nanoparticles.PDF Image | Formation Kinematics of Plasma-Generated Silver Nanoparticles
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