(76b) The Crystal Structure of Coalescing Au and Its Alloy Nanoparticles By Ab Initio Molecular Dynamics

Abstract

Gold nanoparticles are used in a range of applications including electronics, catalysis, sensors, plasmonic biosensing and target-specific drug delivery as therapeutic agents. Physical properties (e.g. reactivity and ligand adsorption of gold nanorods) of such nanostructures depend on the particles crystal structure and surface facet orientation. Even though particle characteristics such as size distribution, primary particle and agglomerate size and morphology have been investigated extensively (Goudeli et al., 2015) little is known about particle crystallinity in atomistic scale. Experimental investigation of particles’ nanocrystallinity with electron microscopy leads to incomplete characterization as it is based on the analysis of 2D projections of nanoparticles. However, Molecular Dynamics (MD) simulations can be used to provide physical insight in crystal structure changes and dynamics with atomistic detail (Buesser and Pratsinis, 2015) and can complement the above experiments, especially for very small nanoparticles.

Here, ab initioMD simulations are used to systematically investigate the sintering mechanism and crystallinity dynamics of gold and its alloy nanoparticles of various size and temperatures. The stage of crystallinity is theoretically investigated by the deviation of each gold atom from a perfect face cubic centered crystal. This deviation is quantified by the so-called bond order parameters (Steinhardt et al, 1983) which are measures of the local and extended orientational symmetries of the particle. The local degree of distortion of coalescing Au particles is described by the disorder variable (Kawasaki and Onuki, 2011). During adhesion, particles reveal increased degree of distortion compared to later stages of sintering, regardless of particle size and sintering temperature, while they form grains of different size and orientation. Large particles (e.g. 4 nm in diameter) form twin boundaries, consistent with experiments of Au nanoparticles coalescing by electron beam irradiation (Yuk et al., 2013). At high temperatures and sufficiently long time these grains transform to a single crystal.

The grain size and crystal direction depend on the synthesis process conditions (e.g. temperature) and lead to the exposure of different facets affecting surface chemistry and product particle morphology and their catalytic properties (e.g. catalytic activity and selectivity) which can be tuned by controlling particle structure and nanocrystallinity. Defects such as grain boundaries at the atomic scale can strongly affect electrical, optical and mechanical properties of 2D materials.

References

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Yuk, J.M., Jeong, M., Kim, S.Y., Seo, H. K., Kim, J., Lee, J.Y. (2013) Chem. Commun. 49, 11479-11481.