On the nucleation and crystallization of nanoparticles in continuous-flow nonthermal plasma reactors

Abstract

The use of nonthermal plasmas for the processing of nanomaterials remains the focus of ongoing investigations because of the many advantageous properties of this class of processes. Silicon nanoparticles in particular are easily produced using continuous flow nonthermal plasma reactors. This has become a technologically relevant technique especially because silicon is an earth-abundant material with important applications in many energy-related fields. Still, there are significant gaps of knowledge in the dynamics of nucleation, growth, and structural evolution of nanoparticles in continuous flow nonthermal plasma reactors. In this contribution, the authors address these issues by performing in-situ infrared absorption measurements to monitor the rate with which the silicon precursor, silane, is consumed by the plasma. At the same time, the authors extract nanoparticles from the plasma volume at different stages of their growth and monitor their structural evolution. The authors find that particle nucleation occurs rapidly and that as soon as the silane is fully consumed the particles have grown to almost their final size. At the end of this phase, the particles have an amorphous structure. The crystallization then occurs in the plasma on a time scale of 120 ms for the conditions used for this study. By using an experimentally determined crystallization rate for free-standing silicon nanopowders, the authors estimate the particle temperature in the plasma. The authors find that the plasma heats the particle to a temperature of over 1100 K, exceeding estimates based on the reaction between the nanoparticle surface and plasma-generated ions and radicals.

Hollow silicon carbide nanoparticles from a non-thermal plasma process

“…39,40 In Ref. 30, we have performed an indirect measurement of the nanoparticle temperature based on the kinetics of crystallization of amorphous silicon nanoparticles 42 suspended in an argonhydrogen discharge and found the particle temperature to be $1100 K. The value obtained in this contribution is slightly larger than this value. The discrepancy can be explained in terms of additional heating due to the surface reaction with carbon-containing radicals.…”

“…Our previous work on the characterization of silane-containing nonthermal plasmas suggests that the silane precursor is fully consumed in the first stage of the process. 30 The silicon nanocrystals are then exposed to the second plasma to which methane is added. Exposure to the methane-containing plasma leads to first (a) the growth of an amorphous carbon film around the silicon particles (as confirmed by higher magnification images in Figure 4 and by Raman in Figure 5), followed by (b) the formation of beta silicon carbide nanoshells, and (c) the destabilization of the hollow shell structure, which ultimately results in the formation of solid (as opposed to hollow) silicon carbide nanoparticles with diameters smaller than the original silicon particles.…”

“…The details of the interaction between the partially ionized gas and the nanoparticles suspended within it are still actively investigated. 30,39,40 In this work, we do not focus on such details and we treat the second non-thermal plasma as the heating source that provides the heating term G 1 . G 2 is used to account for the energy released during the nucleation and propagation phases, which is equal to the enthalpy of formation of beta silicon carbide (73.223 kJ/mol).…”

On the non‐thermal plasma synthesis of nickel nanoparticles

“…While we do not have experimental data that can provide insight on this issue at this time, we can hypothesize that the particle temperature history may play a role in this system. Theoretical [23] and experimental [10,24] studies for the case of silicon confirm that nanoparticles are likely to achieve high temperatures in a low pressure plasma. For the case of nickel nanoparticles, and in presence of carbon-containing radicals, this may lead to dissolution of carbon into nickel and growth of a graphitic shell around the particles.…”

“…Among several possible candidates, non-thermal plasma-based fabrication techniques have emerged as promising all-gas phase protocols for the synthesis of a broad variety of NPs, including semiconductors, [6] metal oxides, [7] ceramics, [8] and more complex core-shell nanostructures. [9] Such systems can produce particles with tight and controlled size distribution (PSDs) [10] and high-quality crystalline structure. [11] Metal nanoparticles have been successfully produced using atmospheric pressure plasmas.…”