(190w) Sintering Rate and Mechanism of TiO2 Nanoparticles by Molecular Dynamics

Sintering Rate and Mechanism of TiO₂
Nanoparticles

by Molecular Dynamics

B. Buesser, A.J. Gröhn and S.E. Pratsinis

Particle Technology Laboratory, Institute of Process Engineering,

Department of Mechanical and Process Engineering, ETH
Zürich, 8092 Zürich, Switzerland

Titania (TiO₂)
nanoparticles have many attractive applications in photovoltaic¹ and photocatalytic² processes, to name a few.
The performance of TiO₂ nanoparticles depends considerably on their
size and composition which are determined by the sintering rate during their synthesis.
The sintering rate is crucial for designing of gas-phase processes with
controlled product particle size, structure, composition and eventual
performance in a number of applications³.

Kobata et
al.⁴ and Seto et al.⁵ have proposed sintering
rates for TiO₂ and validated them by accounting for the detailed
fluid mechanics of their hot wall reactors forming rather large TiO₂
nanoparticles (d_p = 10 ? 100 nm). Little is known, however,
for the sintering rate of small TiO₂ nanoparticles (d_p
< 10 nm) as it is difficult to reliably measure it.

On the other
hand molecular dynamics (MD) simulations have been used to study the sintering
to full coalescence of metallic and metalloid nanoparticles^6,7 though much less has been
done for ceramic ones as their force fields and potentials are difficult to
determine. Koparde and Cummings⁸ investigated the
sintering of two TiO₂ nanoparticles up to t = 0.5 ns by
tracking the shrinkage of the center-to-center distance and the growth of the
sintering neck using the force field of Matsui and Akaogi⁹. They
compared MD with phenomenological sintering models and investigated the melting
of TiO₂ nanoparticles¹⁰.

The above MD of
TiO₂ sintering have reached up to 0.5 ns residence time, a duration
that is not sufficient for complete coalescence. So most of the surface area
reduction of such small particles has taken place by adhesion and neck growth
that limited the detailed understanding of sintering mechanisms and, most
importantly, the extraction of quantitative sintering rates that are needed in
process design of nanoparticle manufacturing.

Here,
sintering of rutile TiO₂ nanoparticles is investigated by graphical
processing unit (GPU) accelerated MD¹¹ from adhesion and neck
growth to finally full coalescence up to several hundred nanoseconds. This
allows determining the sintering rate of very small TiO₂
nanoparticles (d_p < 5 nm) by monitoring the evolution of
their surface area (Figure 1). For the smallest particle diameters, the
MD-obtained sintering rates were smaller than that predicted by theory
developed for larger particles4,5. Ions on the particle
surface exhibited higher net displacement than bulk ones revealing that surface
diffusion is the dominant sintering mechanism of TiO₂ nanoparticles.
An expression for the sintering rate of rutile TiO₂ nanoparticles
has been extracted from MD, bridging the gap of knowledge from a few molecules
to several nanometers, the key size range for nanoparticle properties and
performance. This MD-derived sintering rate facilitates the use of phenomenological
models¹² in design of processes
for large scale manufacture and processing of small nanoparticles¹³.

Financial support from the Swiss National Science
Foundation (SNF) grant # 200021-119946/1 and European Research Council is
gratefully acknowledged.

References

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Figure
1 Evolution of
surface area of two TiO₂ nanoparticles (d₀ = 3 nm)
at 1800 K.