The Onset of Crystallization of Aerosol Au Nanoparticles

Gold nanoparticles have versatile applications in catalysis, chemical and biological sensing, medicine (drug carrier, therapeutic agents and gene delivery), nanolithography and microelectronics [1]. The performance of these nanoparticles is heavily influenced by their crystalline characteristics. Even though Au nanoparticles are routinely made in the liquid phase, their gas-phase (aerosol) synthesis is most attractive for its proven scalability (25 t/h) and high-purity products (i.e. optical fibers) [2]. During aerosol synthesis, the formation of particles occurs at high temperatures, steep cooling rates and short residence times that affect considerably nanoparticle characteristics, including their crystallinity, size, and morphology [3].<br/><br/>Here, the gas-phase crystallization of Au nanoparticles is investigated by molecular dynamics (MD) using the embedded atom method (EAM) that is most relevant to their synthesis by aerosol processes (flame, plasma, laser, etc.). We elucidate the onset of crystallization and growth dynamics of 2.5 – 11 nm Au nanoparticles under isothermal conditions.<br/><br/>The relationship between Au melting point and particle size is used to validate the method.A metastable region of 200 - 300 K wide between the melting point and super-cooled solidifying temperature of Au is revealed as a function of particle size. The crystalline disorder parameters and potential energy are used to elucidate the isothermal crystallization dynamics of Au nanoparticles in three distinct stages at 500 – 800 K (subcritical cluster dynamics, onset of crystallization and crystal growth). The degree of crystallization (predominantly face-centered cubic (FCC) with some hexagonal close-packing (HCP)), is quantified from the local crystalline disorder. In the first stage, before the onset of crystallization, subcritical Au cluster form and deform continuously similar to the subcritical nucleation dynamics of liquid (i.e. water) droplets in the atmosphere. The onset of crystallization is detected by the steep rise of the retained atoms fraction (RAF) of the largest subcritical cluster. This increase is accompanied by a sharp drop of the amorphous fraction of the Au nanoparticle.<br/><br/>In the second stage, crystallization usually starts from the nanoparticle surface as a result of lower kinetic energy barriers there. 3D-snapshots of the Au particle reveal two crystallization nucleation pathways: (A) Catastrophic nucleation when crystallization takes place well below the Au supercooled solidifying temperature resulting in many small crystal domains within the nanoparticle; and (B) Accretion nucleation when crystallization takes place at temperatures near the super-cooled solidifying temperature where fewer nanocrystals are formed that gradually coalesce into larger crystal domains.<br/><br/>In the third stage, after the onset of crystallization, crystals grow via both accretion and cluster merging, with crystal domain reorganization after the crystalline transformation is completed. X-ray diffraction (XRD) patterns are generated, from which the dynamics of crystal growth are elucidated and compared to direct tracing of crystal sizes. The product crystal size is influenced largely by crystallization temperature, with the largest crystal size obtained when crystallization takes place near the supercooled solidifying temperature consistent with the literature [4].<br/><br/>The results of this study can facilitate the process design for aerosol synthesis of Au as well as other metal nanoparticles for optical, electronic, catalytic and biomedical applications.<br/><br/>[1] Goudeli, E. and Pratsinis, S. E. (2016). AIChE J. 62: 589-9.<br/>[2] Kelesidis G.A. and Pratsinis S.E. (2021). Chem Eng J. 421:129884.<br/>[3] Pratsinis, S. E., Zhu, W. and Vemury, S. (1996). Powder Technol. 86: 87-93.<br/>[4] Belahmar, A. and Chouiyakh, A. (2016). J. Nanosci. Nanotechnol. 2: 100-103.