(388l) Growth of Metal Nanoparticles by Molecular Dynamics

Growth of Metal Nanoparticles 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.

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 dominates⁵ 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 models⁶ 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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