(192av) The Crystal Structure and Surface Composition of Coalescing Ag-Au Nano-Alloys By Molecular Dynamics Simulations

Bimetallic nanoparticles exhibit catalytic, optical, electronic and magnetic synergy between their constituents. Typically, that synergy is traced to the structure and surface characteristics of such particles. Core-monolayer shell bimetallics exhibit often unique properties that are not an interpolation between those of their parent metals. This behavior is attributed to the lattice mismatch (strain effect) and charge transfer between layers (ligand effect) resulting in different electronic properties. Furthermore, the addition of an ad-metal may alter also the reactant adsorption geometries (ensemble effect) and the degree of ad-metal coverage and its diffusion into the bulk alter the adsorbate binding energies. Yet, there is only limited understanding of the structure and mixing of the two parent metals. Furthermore, understanding metal-adsorbate interactions is a key issue to controlling and improving the functionality of bimetallic nanoparticles in energy applications. Adsorption, however, depends on nanoparticle morphology.

Here, the effect of temperature and initial particle morphology on the sintering rate and crystalline structure is elucidated during sintering of such silver-gold nanoparticles by Molecular Dynamics (MD) simulations. The MD method is validated by the attainment of the melting temperature of pure Ag (Buesser and Pratsinis, 2015), Au (Goudeli and Pratsinis, 2016) and Ag-Au bimetallic nanoparticles as function of their size. The sintering rate of particle pairs with different initial morphology (Ag-Au alloys, segregated or Ag@Au and Au@Ag core-shell nanoparticles) is determined by tracking the evolution of the total surface area and the changes in surface composition of the formed nanoparticle are elucidated during coalescence at T = 500 – 1000 K. The XRD pattern of Ag@Au and Au@Ag core-shell nanoparticles is calculated and compared with experiments (Sotiriou et al., 2014) as well as to those of pure Ag & Au and segregated or alloyed composites while their crystallite size is obtained by fitting these MD-obtained XRD patterns.

Pure Ag and segregated Ag-Au nanoparticles have larger XRD size than pure Au or alloyed Ag-Au ones. This can provide indication of the detailed structure of flame-made nanoparticles at the atomistic level as experimental measurements of Ag-Au particle size revealed smaller XRD diameter than pure Au or Ag particles (Sotiriou et al., 2014). Furthermore, sintering of Ag@Au core-shell nanoparticles leads to slight reduction (less than 10%) in Ag surface composition, in contrast to Au@Ag ones that increase their Ag surface composition more than 20%.

References

Buesser, B., and Pratsinis, S.E. (2015) J. Phys. Chem. C, 119, 10116-10122.

Goudeli, E., and Pratsinis, S.E. (2016) AIChE J., 62, 589-598.

Sotiriou, G. A, Etterlin, G. D, Spyrogianni, A., Krumeich, F., Leroux, J-C., and Pratsinis, S. E. (2014) Chem. Commun. 50, 13559-13562.