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 (dp > 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 dp = 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.
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
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