(337c) Metal Nanoparticle Growth by Molecular Dynamics

Metal Nanoparticle Growth by Molecular Dynamics

B. Buesser^1,2 and S.E. Pratsinis¹

¹Particle Technology Laboratory, Institute of Process
Engineering, Department of Mechanical and Process Engineering, ETH Zürich, 8092
Zürich, Switzerland

²Department of Chemical Engineering, Massachusetts
Institute of Technology, Cambridge, MA 02139, USA

Metal nanoparticles are attractive in catalysis, magnetic
separations¹ or sensors², to name only a few applications. The
performance of these particles, however, depends considerably on their primary
particle size and structure. Gas-phase processes allow making economically such
particles in large quantities with close control of their size and extent of aggregation³. In gas-phase reactors these two
characteristics are determined mostly by particle sintering. The detailed understanding
of sintering is crucial for the development and scale-up of such reactors to
target product particle size and morphology at maximal yield especially when
precious metals are involved.

Silver nanoparticles are one of the most
studied noble nanomaterials. For example Shimada et al.⁴ proposed a sintering rate for silver
nanoparticles in gas-phase. They found their particle dynamics model in good
agreement with the measured particle size evolution in their hot wall reactor,
where primary particle sizes bigger than d_p = 8 nm were
observed. The evolution of smaller primary particles (d_p <
8 nm) is increasingly difficult to determine experimentally although this would
be the key size range where nanoparticle exhibit their extraordinary
performance and exciting new properties.

Molecular dynamics (MD) simulations are
reaching this range of particle sizes and sintering time scales with the
proliferation of high-performance computer hardware. Sintering of metallic⁵, metalloid⁶ and ceramic⁷ nanoparticles has been investigated, but
often the surface area evolution, a key quantity in reactor design for particle
synthesis, has been neglected.

Here, sintering of silver nanoparticles is investigated
using MD simulations accelerated by graphical processing units (GPU) in the
range of d_p = 2 ? 5 nm. The sintering rate is determined by calculating
the surface area evolution comparable to BET surface area measurements. First observations
of the atom trajectories reveal that surface atoms exhibit a much higher mobility
than bulk ones indicating that sintering by surface diffusion dominates⁷ at these particle sizes and temperatures (Figure
1). The dependence of the sintering rate on particle morphology has been investigated
during sintering of straight chains, triangles and stars of three and four
particles.

An expression for the sintering rate as
function of primary particle size and temperature has been extracted from MD, filling
the gap of knowledge between clusters of a few atoms up to particles of several
nanometers. This sintering rate will facilitate the design of large scale
manufacture and processing of these small nanoparticles based on
phenomenological models⁸ or allow engineering estimations of the particle
morphology by comparing it to the coagulation rate³.

Figure 1 Snapshots of 3 silver nanoparticles with a diameter of d_p
= 3 nm sintering at T = 800 K and time t = a) 0 ns and b) 100 ns.
The atoms are colored green at the surface and red in the bulk at t = 0
ns. It is fascinating to see that these surface atoms largely move to the
concave areas between these particles at t = 100 ns, revealing the
dominance of surface diffusion during their sintering or coalescence.

Financial support from the European Research Council
is gratefully acknowledged.

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