Inline coating of silicon nanoparticles in a plasma reactor: Reactor design, simulation and experiment

Gas-phase synthesis of nanoparticles (NPs) in hot plasmas is a promising approach to produce pure, highly specific, and complex nanomaterials at large production rates. Post-processing of the material by particle coating, embedding, or surface functionalization is often required to adjust the materials’ properties with respect to their utilization in functional structures. Due to the high surface-to-volume ratio, the nanoparticles’ surface properties strongly influence the processing and thus their applicability. We report on a scalable and continuous gas-phase synthesis process of silicon nanoparticles by a high-temperature single-step plasma process with subsequent inline coating. Our process requires a two-stage supply of process gases: First, silicon nanoparticles (Si-NPs) are formed from the gaseous precursor monosilane (SiH4) after its decomposition in the plasma zone. Secondly, the coating agent ethylene (C2H4) is mixed with the hot, particle-laden gas flow downstream of the plasma zone via a specifically-designed coating nozzle. To facilitate a homogeneous intermixing of C2H4 and the nanoparticle-laden gas stream, fluid dynamics simulations were performed to design and optimize the geometry of the coating nozzle. The process conditions can be varied to tune the decomposition process of gaseous C2H4 in respect to coating the Si-NP surface. As a result, we are able to tune the composition of the nanoparticles. Product characterization by X-ray diffraction, Raman, FTIR and X-ray photoelectron spectroscopy revealed that either SiC, or silicon with a carbon-like or a polyethylene-like shell is produced respectively, with increasing distance of the coating nozzle from the plasma. For all process conditions, spherical, coated particles with a highly-crystalline silicon core were observed as indicated by TEM measurements.