High-yield microplasma synthesis of monodisperse sub-3 nm diameter metal nanoparticles explained by a charge-mediated formation mechanism

The size and composition of nanoparticles are instrumental to many catalytic processes; for example, their controlled synthesis is key to achieving high yield and quality in aerosol chemical vapor deposition (CVD) of carbon nanotubes (CNTs). Using a custom-built atmospheric pressure DC microplasma reactor, we synthesize sub-3 nm iron nanoparticle aerosols at high number concentration (> 10(9) #/cm(3)) with narrow size distribution (sigma(g) < 1.3) from a ferrocene vapor precursor. We demonstrate precise diameter control down to 1.1 nm and maximum yield near unity. We invoke a charge-mediated formation mechanism to show that the similar to 10 mu s plasma residence time is sufficient to dissociate the precursor and partially ionize the resulting iron vapor, yet too short for the aggregation of clusters beyond 10 atoms. Thus, particle growth occurs primarily downstream of the plasma domain through aggregation of neutral and ionized vapor and clusters. This model closely reproduces the observed particle size distributions with the selection of an appropriate fractional ionization, yet is insufficient to entirely explain the rapid growth rate observed experimentally. This mechanism is different from that of existing microplasma processes, which have a longer residence time allowing particle formation to occur within the plasma, and may explain the demonstrated capability to co-optimize nanoparticle yield, throughput, and diameter control at a level exceeding the performance of previously published thermal and microplasma methods.

Automated summary

The size and composition of nanoparticles are crucial for catalytic processes. This study focuses on the controlled synthesis of sub-3 nm iron nanoparticle aerosols and the mechanisms behind their growth. By using a custom-built atmospheric pressure DC microplasma reactor, the researchers were able to achieve precise diameter control and high yield of the nanoparticles. A charge-mediated formation mechanism was proposed to explain the growth process, although it couldn't fully explain the observed rapid growth rate. This research is important for optimizing nanoparticle yield, throughput, and diameter control.