268394 Metal Nanoparticle Growth by Molecular Dynamics

Tuesday, October 30, 2012: 3:59 PM
Conference A (Omni )
Beat Buesser1,2 and Sotiris E. Pratsinis2, (1)Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, (2)Department of Mechanical and Process Engineering, ETH Zurich, Zurich, Switzerland

Metal Nanoparticle Growth by Molecular Dynamics

B. Buesser1,2 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 dp = 8 nm were observed. The evolution of smaller primary particles (dp < 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 dp = 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.

a)Beschreibung: picture_3x3nm_800K_0ns.jpg

b)Beschreibung: picture_3x3nm_800K_100ns.jpg

Figure 1 Snapshots of 3 silver nanoparticles with a diameter of dp = 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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2.         Sotiriou GA, Sannomiya T, Teleki A, Krumeich F, Vörös J, Pratsinis SE. Non-Toxic Dry-Coated Nanosilver for Plasmonic Biosensors. Advanced Functional Materials. 2010;20(24):4250-4257.

3.         Buesser B, Pratsinis SE. Design of Nanomaterial Synthesis by Aerosol Processes. Annual Review of Chemical and Biomolecular Engineering. 2012;3:103-127.

4.         Shimada M, Seto T, Okuyama K. Size change of very fine silver agglomerates by sintering in a heated flow. Journal of Chemical Engineering of Japan. 1994;27(6):795-802.

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6.         Zachariah MR, Carrier MJ. Molecular dynamics computation of gas-phase nanoparticle sintering: a comparison with phenomenological models. Journal of Aerosol Science. 1999;30(9):1139-1151.

7.         Buesser B, Gröhn AJ, Pratsinis SE. Sintering Rate and Mechanism of TiO2 Nanoparticles by Molecular Dynamics. J. Phys. Chem. C. 2011;115:11030-11035.

8.         Koch W, Friedlander SK. The effect of particle coalescence on the surface area of a coagulating aerosol. Journal of Colloid and Interface Science. 1990;140(2):419-427.

 

 


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