(458c) Impact of Fractal-Like Morphology On Surface Oxidation of Nanoparticles Synthesized Via Aerosol Route: A Kinetic Monte Carlo Study

Nanoparticle manufacturing via aerosol routes has become increasingly conducive for the fabrication of nano-structured materials and thin films with controlled porosity and specific surface coatings. This either calls for production of spherical nanoparticles for low surface area compact structures (electronic device fabrications, sensors, etc.) or fractal-like aggregates for high surface area reactive structures (nanocatalysts, energetic materials, H2 storage, etc.). Yet, the complex inter-play between various chemical/physical pathways such as collision, coalescence and surface reactivity leading to the evolution of these structures is not clearly understood. Here, we present a kinetic Monte Carlo (KMC) based model that simulates the growth and evolution of fractal-like nanoparticles by particle?particle collision, subsequent non-isothermal coalescence and surface reaction, i.e., oxidation. The interesting aspect of the present KMC model lies in its ability to capture with high fidelity the interactions between competing mechanisms like the exothermic nature of particle coalescence events coupled with the inter-particle coagulation events and enhanced surface oxidation at nano-scale, all in the framework of complex particle morphology.

Specifically, the objective of the present study being focused on its ability to capture the influence of most energetically active processes namely, surface energy driven coalescence and oxidation on the particle evolution, we use surface fractal dimension, Ds varying between 2 (perfect spheres) to 3 (complete fractals) to define the morphological complexity of our nanoparticle systems. Results from our simulation indicate that at intermediate and later stages of nanoparticle growth, wherein the role of competing events such as collision, exothermic coalescence and surface oxidation becomes significant, the particle Ds value increasingly deviates from the spherical assumption, thereby increasing the surface area. This in turn strongly enhances surface area driven oxidation. In turn, the heat of reaction triggers particle temperature rise that quickly couples non-linearly to the coalescence driven energy release. This mechanism enhances particle coalescence, therefore, drastically driving the resulting oxidation. This self reinforcing process is eventually quenched by the oxide shell formation and heat loss to surroundings. The aforementioned non-linear coupling between coalescence and oxidation on complex particle surface morphologies in the framework of particle collisions based on a kernel modified by the fractal-like nano-aggregate structures, makes the physics of the present study highly intriguing and challenging. To demonstrate the fidelity of our new approach, we present results from the simulation of Al nanoparticles grown in air at different temperatures and pressures. Our results indicate that particle evolution and extent of oxidation differ significantly in the case of fractal-like nanoparticle structures from those with the spherical assumptions. The results are also consistent with experimental studies of Al nanoparticle oxidation.