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Sotiris Pratsinis1

1, Swiss Federal Inst of Tech, Zurich, , Switzerland

Recent advances in understanding of aerosol formation and growth through discrete element modeling and molecular dynamics allow now optimal reactor design, away from the Edisonian approaches of the past. This leads to scalable synthesis of sophisticated nanoparticles with controlled composition, size and morphology resulting in a few high value products (nanosilver and carbon-coated Co nanoparticles) in the market already, while several promising ones are emerging such as single atom catalysts and breath sensors.
This lecture will highlight the interplay of surface growth and coagulation revealing the evolution of nascent to mature soot formation by mesoscale simulations, in excellent agreement with experimental data in premixed and diffusion flames. Such an understanding facilitates to elucidate the effects of primary particle polydispersity and chemical bonding (aggregation) on soot morphology and optical properties using the Discrete Dipole Approximation. This is of critical importance in monitoring soot emissions, carbon black manufacture, climate modelling and operation of fire detectors. Agglomerates of polydisperse aggregates and spheres produced by agglomeration and surface growth have more compact structure and larger fractal dimension, Df, than agglomerates of monodisperse spheres. Primary particle polydispersity and aggregation enhance soot scattering (of critical importance to fire detectors) up to 50 and 30 %, respectively!
Quantitative understanding of the formation of bimetallic nanoparticles is important as they exhibit catalytic, optical, electronic and magnetic synergy between their constituent metals. Typically, that synergy is traced to the domain structure and surface characteristics of such particles. Here these characteristics of coalescing Ag-Au nanoparticles of various initial sizes, morphologies (segregated or alloys) are investigated by atomistic molecular dynamics (MD). Silver atoms exhibit increased mobility over Au and occupy gradually the surface of the coalesced (or sintered) bimetallic particle, consistent with scanning electron microscopy and selective O2 chemisorption experiments for heterogeneous catalysis of ethylene oxidation. The characteristic sintering time of equally-sized Ag-Au nanoparticles is similar to that of pure Au but shorter than that of Ag nanoparticles. When the latter coalesce with substantially bigger Au ones, a patchy Ag layer is formed at the Au particle surface. However, when Ag are bigger, then Au is rather embedded into Ag consistent with microscopy data. Most notably, X-ray diffraction patterns of Ag-Au nanoparticles are obtained, for the first time to our knowledge by MD distinguishing segregated from alloyed ones. The latter form smaller crystal size (highly polycrystalline) than coalescing pure and segregated Ag & Au nanoparticles, quantitatively explaining the structure of flame-made Ag-Au nanoparticles for biomaterial applications.

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