(419j) Formation of Fractal Aggregates Among Nanoparticles in Gas-Phase Produced from Non-Equilibrium Plasmas

Non-equilibrium plasmas are well-suited for monodisperse nanoparticle synthesis, particularly for sub-10 nm particles. Being synthesized in the gas phase, these nanoparticles do not contain any organic stabilizers and are therefore extremely attractive in the field of catalysis, drug delivery and pharmaceutical application. The nanoparticle growth in such plasmas (for a given plasma condition) are primarily controlled by the precursor residence time and number concentration. Estimates of particle sizes synthesized from such plasmas are typically quoted from that of the primary particles and does not take into account factors such as coagulation (particle-particle fused joints and necking) and aggregation (non-spherical polylithic particle-particle joints). The particle shape and structure flowing out of the plasmas however, are not isolated individual primary particles and instead are aggregates. The reason for such formation beyond collision remain poorly understood.

In this report we study the aggregation of as-synthesized nanoparticles produced from a DC argon microplasma through a tandem ion mobility and mass spectrometry (IM-MS) system. Particle aggregation and collisional growth among the as-synthesized nanoparticles from these plasmas are typically considered to be negligible due to unipolar charging and coulombic repulsions. In our experiments, and more importantly, for all experimental conditions, we find all as-synthesized structures as highly branched chain-like aggregates. The mass, mobility diameter and number concentration of the aggregates are affected by the synthesis conditions such as residence time and flow rate for the same plasma conditions. For a purely spherical particle, the mass is related to the cube of the mobility diameter. However, for non-spherical particle, the relationship is skewed since the particle shape will affect the radius of gyration and the center of mass. We confirm our findings through three separate analysis techniques.

First, the as-acquired 2D number based IM-MS spectra was analyzed to determine the correlation between the particle mass and mobility diameter using a Twoney-Markowski data-inversion technique. We found that the effective density of the aggregates of a given size and mass are lower than that of a perfect sphere of Ni or C. Second, the structure of the aggregates was confirmed through transmission electron microscopy (TEM). Through careful particle collection and detailed image analysis, the 2D TEM images were correlated with a database of 3D aggregate structures assuming that the aggregates are formed through a collection of hard spheres. Energy dispersive spectroscopy analysis confirmed that the particles are composed of a Ni core with a soft carbon shell. Finally, the aggregate structures, mass and mobility correlation was correlated with Langevin dynamics simulation of aggregate formation. We found that all the measured aggregates follow the predictions from Langevin simulations. From our analysis, we show that the non-equilibrium plasma synthesis leads to the formation of highly branched aggregates with a polydisperse size distribution. Further, these results confirm that unipolar charging of the particles at the plasma exit insufficient to prevent particle aggregation.