The onset of aerosol Au nanoparticle crystallization: accretion & explosive nucleation.

The crystallization of gold nanoparticles is investigated in the gas-phase by molecular dynamics (MD) that is most relevant to their synthesis by aerosol processes (flame, plasma, or cluster beam deposition). A particle size-dependent metastable region, 200-300 °C wide, is revealed between the melting and freezing points of Au. This region decreases as the MD heating or cooling rates decrease. Two separate stages, subcritical and supercritical cluster formation, are distinguished during isothermal crystallization of 2.5-11 nm Au nanoparticles at 500-1000 K. The degree of Au crystallization (face-centered cubic or hexagonal close-packing) is quantified based on the Au atom local crystalline disorder. The onset of crystallization is identified by the steep rise of the fraction of atoms that retain their crystallinity in the largest subcritical cluster, accompanied by a sharp drop of the amorphous fraction of the Au nanoparticle. Crystallization starts from, at least, one atom layer below the surface of the nanoparticle and then quickly expands to its surface and bulk. Two crystallization nucleation pathways are identified: (a) explosive nucleation well below the Au freezing point resulting in many small and broadly distributed crystals; and (b) accretion nucleation near the freezing point where narrowly distributed and larger crystals are formed that grow by accretion and coalescence. X-ray diffraction (XRD) patterns are generated by MD, from which the dynamics of crystal growth are elucidated, consistent with the literature and in excellent agreement with direct tracing of crystal sizes.