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

Abstract

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.

Quantitative Study of Electrochemical Reduction of Ag+to Ag Nanoparticles in Aqueous Solutions by a Plasma Cathode

“…[14] A recent publication by Kondeti et al reported Ag nanoparticle formation using an RF-driven Ar plasma operating in air even when separated from the solution by a VUV transparent window, suggesting that VUV photolysis creates H · , which then reduces Ag + . 43 The publication also reports large densities of H · in the plasma phase measured via laser-induced fluorescence, which might solvate and reduce Ag + . However, the free energy of hydration for H · , G hyd = 18 kJ/mol, is positive, meaning that H · is hydrophobic and solvation from the plasma is energetically unfavorable.…”

“…21 We also note that the plasma source used in Ref. 43 was not configured as an electrolytic cell, as there was no net flow of charge from the plasma into solution. This makes it difficult to compare the two systems and determine how significant Reaction 12 is in our plasma-based electrolytic cell.…”

Production of antimicrobial silver nanoparticles modified by alkanethiol self‐assembled monolayers by direct current atmospheric pressure glow discharge generated in contact with a flowing liquid anode

“…A convenient solution to the abovementioned problems can be to apply a cold atmospheric pressure plasma (CAPP)‐based bottom‐up approach, based on generation of selected reactive oxygen and nitrogen species along with hydrated electrons and H radicals, which are involved in reducing AgNPs precursor ions . CAPP‐based methods have been utilized for environment‐friendly AgNP production by several research groups . In the above‐cited studies, different CAPPs are operated either in liquids or in contact with liquids, but in all cases stationary (nonflow‐through) reaction‐discharge systems are used.…”

“…CAPP‐based methods have been utilized for environment‐friendly AgNP production by several research groups . In the above‐cited studies, different CAPPs are operated either in liquids or in contact with liquids, but in all cases stationary (nonflow‐through) reaction‐discharge systems are used.…”

“…Ag nanostructures’ production via the CAPP‐based bottom‐up approach has been typically carried out in the presence of capping agents, preventing their sedimentation and aggregation . Natural stabilizers such as gelatin, dextran, pectins, sucrose, fructose, or surfactants like sodium dodecyl sulfate, have been used to stabilize NPs synthesized in CAPP‐based systems. Alkanethiols are example of ligands that have been used to cap versatile noble metal nanostructures produced with the aid of non‐CAPP based synthesis procedures .…”