Ag⁺ reduction and silver nanoparticle synthesis at the plasma-liquid interface by an RF driven atmospheric pressure plasma jet: Mechanisms and the effect of surfactant

The involvement of plasma produced species in the reduction of silver ions at the plasma-liquid interface is investigated using a well-characterized radio-frequency driven atmospheric pressure plasma jet. The absolute gas phase H density was measured using two photon absorption laser induced fluorescence in the free jet. Broadband absorption and transmission electron microscopy were used to study the synthesis of silver nanoparticles (AgNPs). It is shown that fructose, an often used surfactant/stabilizer for AgNP synthesis, also acts as a reducing agent after plasma exposure. Nonetheless, surfactant free AgNP synthesis is observed. Several experimental findings indicate that H plays an important role in the reduction of silver ions for the plasma conditions in this study. Vacuum ultraviolet photons generated by the plasma are able to reduce silver ions in the presence of fructose. Adding H₂ to the argon feed gas leads to the production of a large amount of AgNPs having a particle size distribution with a maximum at a diameter of 2-3 nm, which is not observed for argon plasmas. This finding is consistent with a smaller concentration of reducing species at the plasma-liquid interface for Ar with the H₂ admixture plasma. The smaller flux of reactive species to the liquid is in this case due to a less strong interaction of the plasma with the liquid. The formation of the nanoparticles was observed even at a distance of 6-7 mm below the tip of the plasma plume, conditions not favoring the injection of electrons.