Characterization of the state of nanoparticle aggregation in non-equilibrium plasma synthesis systems

Non-equilibrium plasmas offer a unique environment for nanoparticle synthesis. Particles are homogeneously nucleated and grow at near room temperature as a result of non-thermal decomposition of vapor precursors by electrons and other plasma-excited species. Despite their widespread use, several features regarding particle growth in these systems remain poorly understood. In particular, particle aggregation (the formation of non-spherical entities composed of primary particles) is assumed to be negligible because of unipolar particle charging and subsequent Coulombic repulsion, which would hinder collisional growth. Here, we apply ion mobility-mass spectrometry (IM-MS) to a non-thermal, atmospheric pressure DC microplasma to study the state of aggregation of as-synthesized nanoparticles. Under all examined synthesis conditions, we find the presence of highly branched, chain-like aggregates at the reactor outlet, with a primary particle radius below 10 nm that is relatively insensitive to synthesis conditions. The aggregates are polydisperse, with mean masses and mobility diameters increasing with both increasing precursor concentration and increasing flow residence time within the system. TEM structural characterization shows that the aggregates can be described by a quasifractal model, with a fractal dimensions in the 1.6-2.0 range. The mass-mobility relationship inferred from IM-MS and TEM agrees well with Langevin dynamics simulations where coulomb interactions are not considered. We suggest that particle aggregation occurs either in the plasma volume due to the scavenging of smaller neutral or positively charged particles by growing aggregates or outside the reactor where the plasma density is lower and electrons are not available to maintain high levels of unipolar charge. The methods applied here additionally demonstrate the potential of IM-MS and TEM structural characterization in analyzing gas-phase nanoparticle production processes.