| AVS 63rd International Symposium & Exhibition | |
| Plasma Science and Technology | Tuesday Sessions |
| Session PS+2D-TuA |
| Session: | Plasma Processing for Nanomaterials and 2D Materials |
| Presenter: | Caroline De Vos, Université Libre de Bruxelles, Belgium |
| Authors: | C. De Vos, Université Libre de Bruxelles, Belgium M.J. Gordon, University of California, Santa Barbara R.M. Sankaran, Case Western Reserve University F. Reniers, Université Libre de Bruxelles, Belgium |
| Correspondent: | Click to Email |
The remarkable stability of microplasmas at atmospheric pressure and their non-thermal operation facilitate the introduction of liquids such as water for water treatment, medical, and material applications. Recently, there has been interest in the reduction of metal salts in aqueous solutions by microplasmas to produce colloidal nanoparticles (NPs). It is generally accepted that some active species from the plasma react with the solution phase and either directly reduce the metal cation or produce a reducing species. However, it remains unclear how exactly the metal cation is reduced to produce NPs.
In this study, we carried out experiments to understand the formation of silver (Ag) and gold (Au) NPs from their respective metal salt precursors with or without stabilizing capping molecule by reactions at the interface of a microplasma and the aqueous solution phase. The NPs were characterized after synthesis by ultraviolet-visible (UV-vis) absorption spectroscopy and transmission electron microscopy (TEM), and the chemical composition of the solution was characterized before and after microplasma treatment by ionic conductivity, electrochemical potential, and UV-vis absorption spectroscopy.
Our results show that both Ag and Au NP formation are directly proportional to the plasma current and process time. The calculated reduction efficiency based on the number of electrons injected and the number of Ag+ reduced is only ~50% while the reduction efficiency for the Au precursor was ~25%. Another difference between the two metals is that plasmon band for Au was found to increase even after the plasma treatment was stopped. This was corroborated by a measured decrease in the concentration of the Au complex, confirming that reduction continues to occur without the plasma, presumably because of a long-lived reducing species generated in solution.
Assuming electrons are the important charge carriers, electrons can reduce metal cations, but can also reduce water to form OH radicals which in turn react to form hydrogen peroxide (H2O2). H2O2 is known to be a weak reducing agent and could also reduce the metal salts but based on our experiments and reduction potentials, we believe that H2O2 could only induce the formation of Au NPs.
To gain more insight into the different mechanisms involved, the gas phase above the plasma-liquid system was analysed by optical emission spectroscopy (OES); a variety of species were observed (OH, O, H, NO, N2) and ultimately linked to the reactions occurring in the liquid phase.
This work was supported by the Belgian Federal Government (IAP research project P7/34 – Physical Chemistry of Plasma Surface Interactions).