(398c) Morphology and Crystallinity of Coalescing Nanosilver By Molecular Dynamics

Morphology
and Crystallinity of Coalescing Nanosilver by Molecular Dynamics

B.
Buesser¹ and S. E.
Pratsinis²

¹
Smarter Cities Technology Center, IBM Research -- Ireland, Dublin,
Ireland

²
Department of Mechanical and Process Engineering, ETH Zurich, Zurich,
Switzerland

Metallic
nanoparticles are attractive candidates for catalytic, biomedical and
sensor applications. However their optimal product performance
depends heavily on an application-specific particle size,
crystallinity and morphology. Gas-phase synthesis processes are an
economic and scaleable technology to produce such nanoparticles in
large quantities (ton/hour). There, nanoparticles mainly grow by
coagulation and sintering, whereas the latter is one of the main
influence on the final product particle size and crystallinity and is
not very well understood at the atomic level.

There
fore, to obtain a basic understanding of the detailed, atomic
processes that drive nanoparticle growth and lead to sintering, the
occurrence of atomic defects and twin boundaries in small metallic
nanoparticles during gas-phase synthesis we have investigated the
sintering of silver nanoparticles in vacuo
at different temperatures and initial spatial particle configurations
by molecular dynamics using the Embedded Atom Method (EAM) (Buesser
and Pratsinis, 2015). We have found that early on, sintering of solid
silver nanoparticles is dominated by surface diffusion of highly
mobile surface atoms whereas a transition towards plastic flow
sintering takes place especially near the size-dependent nanosilver
melting temperature. The sintering rate of straight particle chains
seems to be much longer than that of more compact nanoparticle
morphologies The formation of new crystal domains during silver
particle sintering has been observed and conditions leading to the
formation of crystal twins and polycrystalline nanoparticles will be
elucidated (Figure 1). The melting temperature has been investigated
with the present simulations and found to decrease with decreasing
particle size and approach the bulk melting point of Ag for bigger
particles (d_p
> 10 nm) in agreement with literature and a temperature range of
metastable particles, caused by supercooling, has been identified.
The sintering rate and mechanism of the metallic silver nanoparticles
of our present study (Buesser and Pratsinis, 2015) will be compared
with our previous results on the sintering of ceramic, metal-oxide,
TiO₂ nanoparticles
(Buesser et al.,
2011).

Figure 1 Cross-section of a
silver nano-aggregate particle showing an advanced stage of sintering
between two silver nanoparticles with initial diameter d_p
= 3 nm. The spheres represent single silver atoms and their coloring
indicates the crystalline order around each atom. The fcc-crystalline
domains are colored blue, disordered grain-boundaries are highlighted
in green colors and displaced surface atoms are colored red.

References

Buesser, B., and Pratsinis, S. E. Morphology
and Crystallinity of Coalescing Nanosilver by Molecular Dynamics. J.
Phys. Chem. C. 2015,
119
(18), 10116 - 10122

Buesser, B., Grohn, A. J. and
Pratsinis, S. E. Sintering Rate and Mechanism of TiO₂
Nanoparticles by Molecular Dynamics. J.
Phys. Chem. C. 2011,
115
(22), 11030 - 11035