Single Step Synthesis and Deposition of Gold Nanoparticles Using Atmospheric Pressure Plasma Jet

Abstract

The interaction of atmospheric pressure plasma discharges with liquid or vapor phase is a hot topic in plasma nanoscience and technology. Synthesis and processing of nanomaterials using atmospheric pressure plasma jets attract wide attention because of its simplicity and cost-effectiveness. The plasma containing reactive electrons and active chemical species activates physical processes and reduction of metal salts, leading to the formation of metal nanoparticles. Among the unique features of atmospheric pressure plasma, the chemical non-equilibrium environment produces highly reactive radicals and ions from the precursors, as well as inside the plasma. Thus the energy transfer could be initiated with high energy electrons and excited species from plasma and by the reactions with reactive species of the precursors. Plasma compliments the production of nanomaterials in the vapor phase or from aerosols and in solutions of the precursors. The interface between plasma and vapors can be employed for rapid, simultaneous reduction and deposition of metal nanoparticles from metal salts, for instance gold nanoparticles from chloroauric acid (HAuCl4).We have demonstrated an atmospheric pressure plasma jet (APPJ) setup operating with a mixture of He and Ar gases to deposit gold nanocrystals on a silicon substrate. In this study, we revealed an easy and reliable synthesis mechanism of gold nanoparticles from chloroauric acid using an APPJ working with a high voltage source and He gas. Chloroauric acid in water is used as the precursor for the process which is introduced to plasma in the form of micro droplets produced by the insertion of Ar gas to the precursor solution. The metal nanoparticles are formed inside the plasma by the plasma-assisted reduction of precursor molecules. Since the plasma-liquid/vapor chemistry allows the lead of accelerated reactions and instantly reduce the metal ions to metal atoms skipping the intermediate steps in the reduction process. The nanoparticles were deposited on a Si wafer substrate downstream of the plasma jet, and characterized via scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The polygonal nanoparticles sized 40-100 nm were obtained and they do not tend to agglomerate or fuse onto the substrate. Due to the low volatility of the precursor solution and the presence of comparatively larger micro droplets inside the plasma the nanoparticles tend to deposit in a pattern like galaxies of gold nanocrystals on the silicon. These results would lead to the development of low cost and mobile, eco-friendly atmospheric pressure plasma devices to synthesis the nanoparticles without any hazardous chemical reducing agents.To create a plasma-vapor interaction, a vaporizer has been employed and the new interaction phase would be a combination of gaseous plasma and plasma-liquid interface. To ease the transportation and diffusion of precursor molecules at the interface Ar gas has been used to as a carrier. At the interface, high energy electrons play the key role as initiators for the reduction of metal ions inside the plasma. Nanoparticles were formed by a simple reduction mechanism carried out by short-lived reducing species formed at the interface. From the optical emission spectra taken during the process implies the formation OH radicals which may form short-lived reducing agents like hydrogen peroxide (H2O2) which is known as a reducing agent for chloroaurate ions in water medium. The size and shape of the nanoparticles are controlled by the plasma parameters by controlling the population density of the metal ions. Whereas the purity, in turn, the reduction rate of metal ions, is dependent upon the feed gas flow as well as the applied voltage, i.e. plasma power.The application of gold nanoparticles are promising for different applications depends on its geometry and shape. Therefore, the synthesized gold nanoparticles in this work with polygonal geometries can be potential candidates for plasmonic active surfaces for highly specific sensing applications and nanoelectronics devices. However, the low-temperature plasma-vapor interphases containing droplets, the evaporation may add new complexities such as quantitative analysis of non-equilibrium chemistry to plasma chemistry which is yet to be understood and developed, which will open a new era of plasma-surface interaction.