Controlled Growth of Gold Nanoparticles

Title: Synthesis and Isolation of {110}-Faceted Gold Bipyramids and Rhombic Dodecahedra

Authors: Michelle L. Personick, Mark R. Langille, Jian Zhang, Nadine Harris, George C. Schatz, and Chad A. Mirkin

Journal: Journal of the American Chemical Society

Affiliation: Department of Chemistry and International Institute for Nanotechnology, Northwestern University

Au nanostructures: Trigonal Bipyramids (left) and Rhombic Dodecahedra (right)

Nanoparticles have been an interest of many researchers of late, due to their unique chemical and physical properties.  In order to control the size and shape of nanoparticles, many nanoparticle syntheses let the reaction run for a pre-set time before quenching it (here is an epic video of quantum dot synthesis and the difference in properties as a function of reaction time).  Recent research has focused on controlling the size and shape of nano particles and usually involves adding a small amount of a chemical that binds to certain faces of the nanoparticles as they grow.

The Mirkin group is studying the growth and morphology of gold nanoparticles.  It is known that, because of the way that gold atoms pack (face-centered cubic) different faces of the crystal (identified by their miller index) have different surface energies and catalytic properties, so it would be advantageous to have the ability to control which crystal faces are exposed during nanoparticle synthesis.  In this case, the Mirkin group was able to grow gold nanoparticles of two different shapes (trigonal bipyramidal and rhombic dodecahedra) with exposed, high surface energy {110} faces.


Because the {110} faces have a higher surface energy, crystal growth (deposition of more Au atoms) occurs on them at a faster rate than on the other surfaces.  This usually leads to crystals that have no {110} faces.  The Mirkin group found that by adding a small percentage of Ag to the reaction mixture, they could prevent the growth on these surfaces and end up with crystals bound by {110} faces.  Other researchers have studied the effect of adding Ag+ to Au crystal growth solutions, and found that if higher concentrations of Ag were used, crystals containing exotic {730} and {711} faces were formed.

The synthesis was carried out  by adding HAuCl4, AgNO3, HCl, and ascorbic acid to a 10 mL aqueous solution of cetyltrimethylammonium chloride and NaCl, and then adding Au seed crystals.  The Au seed crystals were prepared by quick reduction of HAuCl4 by NaBH4.  Using this method, they were able to obtain the two different kinds of crystals, which could be separated by filtration.  Interestingly, the trigonal bipyramidal crystals are the result of a twinned crystal.

Further work in this group will study how effective these particles are at catalyzing reactions and what other metals may exhibit the same growth and exposure of high energy faces in the presence of Ag or other additives.

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