High-Frequency Mechanical Stirring Initiates Anisotropic Growth of Seeds Requisite for Synthesis of Asymmetric Metallic Nanoparticles like Silver Nanorods

Abstract

High-speed stirring at elevated temperatures is shown to be effective in the symmetry-breaking process needed for the growth of the hard-to-synthesize silver nanorods from the polyol reduction of silver ions. This process competes with the facile formation of more symmetrical, spherical and cubic, nanoparticles. Once the seed is formed, further growth proceeds predominantly along the long axis, with a consequent increase of the particles' aspect ratio (that of the nanorod). When stirring is stopped shortly after seed formation, nanorods with a broad distribution of aspect ratios are obtained, while when the high-frequency stirring continues the distribution narrows significantly. The width of the nanorods can only be increased if the initial concentration of Ag(+) ions increases. Reducing the stirring speeds during seed formation lowers the yield of nanorods. Molecular dynamics simulations reveal that the formation of a nanometer-scale thin boundary region between a solid facet of the nanoparticle and the liquid around it, and the accommodation processes of metal (Ag) atoms transported through this boundary region from the liquid to the solid growth interface, are frustrated by sufficiently fast shear flow caused by high-frequency stirring. This arrests growth on seed facets parallel to the flow, leading, together with the preferential binding of the capping polymer to the (100) facet, to the observed growth in the (110) direction, resulting in silver nanorods capped at the ends by (111) facets and exposing (100) facets on the side walls.