The interaction of atmospheric pressure plasma discharges with liquid or vapour phases is a hot-topic in plasma science and technology. Synthesis and processing of nanomaterials using atmospheric pressure plasma jets attracts more attention because of its simplicity and cost effectiveness. The plasma containing reactive electrons and active chemical species activates physical processes and reduction reaction of metal salts which leads to the formation of metal nanoparticles. Among the unique features of atmospheric pressure plasmas, 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 could be achieved by the reactions with reactive species of the precursors. Plasma compliments the production of nanomaterials in vapour phase or from aerosols and in solutions of the precursors. The interface between plasmas 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 employed an atmospheric pressure plasma jet setup operating with a mixture of He and Ar gases to produce gold nanocrystals onto a silicon wafer substrate. In this study, it reveals an easy and reliable synthesis mechanism of gold nanoparticles from chloroauric acid using atmospheric pressure plasma jet working with a high voltage source. A solution of the precursor in water acts as the feed solution for the process which is introduced to plasma in the form of microdroplets and metal nanoparticles were formed inside the plasma from the vapours containing the precursor molecules. Since the plasma-liquid/vapour 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 collected on to a Si wafer substrate downstream of the plasma jet and characterized via scanning electron microscopy, Raman spectroscopy, transition electron microscopy etc. 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 microdroplets inside the plasma the nanoparticles tend to deposit in a pattern like galaxies on the silicon wafer substrate. These results would lead to the development of low cost and mobile, user-friendly atmospheric pressure plasma devices for the synthesis of nanoparticles without any toxic chemicals for the reduction of metal salt.

The challenge in this process is in creating a vapour-plasma interface directly. This problem has been solved by incorporating a vaporizer directly to plasma. The vaporizer would introduce a new phase which would be a combination of gaseous plasma and plasma-liquid interface. To ease the transportation and diffusion of precursor molecules at the interface an inert gas has been introduced to the vaporizer as a carrier. At the interface, high energy electrons are introduced to the vapors containing the precursor metal ions and these wet electrons play the key role as initiators for the reduction process inside the plasma. Nanoparticles are formed inside the plasma by simple reduction mechanism carried out by short-lived reducing species formed at the interface. From the qualitative analysis carried our during the process implies the formation of 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 residence time of the particles inside the plasma and the population density of the metal ions. Whereas the purity in turn the reduction rate of metal ions are dependent upon the feed gas flow as well as the applied voltage, i.e. plasma power.

Concisely, atmospheric pressure plasma jets are outstanding choice for the synthesis of gold nanoparticles from chloroauric acid without any external reducing agents. The synthesized gold nanoparticles are promising for different applications because of its geometry and shape. Because of its polygonal geometries the particles are plasmonic active surfaces, the functional for highly specific sensing applications and nanoelectronics devices. However, the low-temperature plasma-vapour 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.