Alchemists dreamed of turning base metals to gold, but probably wouldn't have been too sad at the modern achievement – turning silver to something with a striking likeness to the element below it on the periodic table. While it's probably not time to sell gold stocks, the discovery may lead to silver's substitution for some current uses of gold.
"When we first encountered the optical spectrum of the silver nanocluster, we thought that we may have inadvertently switched the chemical reagents for silver with gold, and ended up with gold nanoparticles instead,” Dr. Osman Bakr of King Abdullah University of Science and Technology, Saudi Arabia, told Phys.org. Bakr is the senior author of the paper in the Journal of the American Chemical Society.
Particles of a handful of atoms from a single element sometimes reveal properties very different from macroscopic collections of the same element. Solutions of gold nanoparticles vary in color – from deep red to bright pink to purple – depending on particle size.
The structure of silver nanoparticles are much less understood than their gold equivalents, but to the naked eye they do something similar; at certain sizes, silver nanoparticles look more like what you would expect of a solution of gold.
However, Bakr's observation goes beyond mere looks. “This synthesized nanocluster is the only silver nanoparticle that has a virtually identical analogue in gold, in terms of number of metal atoms, ligand count, superatom electronic configuration, and atomic arrangement,” the paper reports.
The nanoparticle in question is [Ag25(SR)18]−, 25 silver atoms with 18 carbon-sulfur ligands (molecules that attach to the central complex of atoms).
Where silver nanoparticles with other numbers of atoms structure differently from gold, the behavior of [Ag25(SR)18]− is so similar in structure to its well-studied gold equivalent that the authors argue it “offers the first model nanoparticle platform to investigate the centuries-old problem of understanding the fundamental differences between silver and gold in terms of nobility, catalytic activity, and optical property.” The question is important because silver is so toxic, it is used for antibacterial coatings; whereas gold is so unreactive, it is biologically safe.
Bakr told Phys.org that the similarities in properties arise from the 25 atoms behaving like a single “superatom” whose structure determines interactions with the outside world. “The size scale of nanoparticles lies in between atoms/molecules and bulk material, where the absolute rule of neither quantum nor classical physics is observed,” he said.
The authors probed the structure of the two nanoparticles with X-ray diffraction, finding that each have a single atom at the center of a 12-pointed icosahedron, nine "atoms occupy the nine triangular face centers of the core, whereas the remaining three can be found facing away from triangular face centers," the paper notes. The last three represent the only difference from the structure of Au25, where 12 gold atoms occupy triangular faces, explaining the strong similarity in properties.
Bakr suggested that just as silver can be made to mimic gold, the reverse may also be achievable, although likely less commercially attractive.
American politics once turned on the demand to substitute silver for the “cross of gold,” but no one guessed that at the smallest scale, the difference disappears.