(581b) The Crystal Structure and Surface Composition of Coalescing Ag-Au Nano-Alloys By Molecular Dynamics Simulations

Bimetallic nanoparticles have gained significant commercial interest because of their superior or even novel electronic, chemical and plasmonic properties compared to the monometallic counterparts making them excellent candidates for many biomedical, sensory or catalytic applications. For example, gold nanoparticles that are used extensively in catalysis exhibit reduced electron transfer on Au(111) surface, hindering the adsorption of O₂. However, formation of gold-based bimetallic nanoparticles (e.g. with Ag) can enhance the affinity with O₂ compared to pure gold nanocatalysts. Furthermore, Ag nanoparticles exhibit remarkable antibacterial properties but are highly cytotoxic rendering their use as therapeutic agents challenging. Alloyed Ag-Au is an interesting example in biomedicine with potential in theranostic applications since addition of Au in Ag nanoparticles improves biocompatibility without destroying the antibacterial activity of nano-silver (Sotiriou et al., 2014).

Here, the evolution of surface composition of free-standing but coalescing Ag-Au nanoparticles is investigated for different particle sizes and temperatures by atomistic molecular dynamics (MD) simulations. The MD method is validated by the attainment of the melting point of Ag-Au core-shell nanoparticles that increases with increasing particle size and follows closely the trend of the size-dependent melting temperature of pure Au (Goudeli et al., 2016) and Ag nanoparticles (Buesser et al., 2015). Silver atoms exhibit increased mobility upon coalescence and occupy gradually the surface of the segregated particle, consistent with experiments. When Ag nanoparticles are sufficiently smaller than Au ones, a patchy Ag layer forms at the Au particle surface. Sintering of equally-sized Ag and Au nanoparticles results in the formation of segregated nano-alloys with Ag-enriched surface, consistent with the literature. The initial particle morphology affect the sintering rate but also the particle crystallinity. The X-ray diffraction patterns of Ag-Au nanoparticles are calculated during sintering, revealing that even though core-shell configurations and segregated structures exhibit the characteristic peaks of pure Ag and Au, the alloyed nanoparticles exhibit only the (111) and (200) peaks.

Keywords: crystallinity, XRD, surface composition, silver-gold, molecular dynamics.

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.