AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Size Focusing of Metal Nanoparticles in Low-Temperature Plasma
NECIP BERKER UNER, Elijah Thimsen, Washington University in St. Louis
Abstract Number: 221 Working Group: Nanoparticles and Materials Synthesis
Abstract Gas phase synthesis methods constitute a major route to production of nanomaterials with high throughput. These methods usually employ a thermal environment, like a flame, a furnace or an arc to convert vapor or liquid precursors into nuclei. The consequent growth of nanoparticles is well understood by the aerosol community, where nucleation is followed by condensation, surface growth and coagulation. Although the thermal environment is flexible in terms of producing particles of various sizes and compositions, high temperatures favor coagulation and in most cases the broad self-preserving size distributions are obtained, leading to polydisperse particles and/or aggregates. Such products are valuable when high surface area is the desired parameter, as in the case of SiO2 and TiO2. However, for applications that require monodispersity, gas-phase methods fall behind when compared to colloidal methods.
In this work, we show that aerosols undergo unique transformations within a low temperature plasma. Upon sending a pre-generated polydisperse aerosol of metal nanoparticles into a low temperature plasma, we observed that the size distribution of the plasma-treated aerosol becomes significantly narrower, with a geometric standard deviation of less than 1.1. The mass yield was found to be as high as 65%. Although measured gas temperatures were below 120°C, particles were shown to vaporize at relatively low power inputs, due to bombardment by ions and electrons. We hypothesize that a mechanism, in which the resultant supersaturated vapor produces new particles by nucleation and re-condenses on the remaining fragments, is responsible for uniform growth. The uniformity of particle growth is assisted by the absence of coagulation due to unipolar charging in the plasma. This mechanism, which is unique since a polydisperse to monodisperse transformation is involved, is supported with a quantitative model of aerosol dynamics that involves nucleation, condensation, charging, sputtering and thermal evaporation. We will present the effects of particle material and inlet mass concentration. Ways of tuning the final size and increasing productivity will be discussed.