In a breakthrough, a team of researchers from the Indian Institute of Technology (IIT) Madras has successfully transformed nanoscale pieces of silver to gold and gold to silver by replacing their atoms one at a time. The shape and structure of these materials before and after transformation are identical, although they are completely different chemically. The result were published today (November 10) in the journal Nature Communications.

“This is like transforming a silver Nataraja sitting on your table to a gold equivalent, by atom-by-atom changes. Although this is possible only in the nanoscale, that too with limited systems today, there is a hope that such changes can occur in the macroscopic world in future” says Prof. T. Pradeep, from the Department of Chemistry, IIT Madras and the corresponding author of the paper.

When nanoparticles of gold and silver, which have different mass but identical atomic arrangements, are mixed in solution at room temperature an atom by atom replacement takes place. Within a few minutes the silver nanoparticle becomes a gold nanoparticle and the gold nanoparticle becomes a silver nanoparticle. Generally, nanoscale materials are more reactive as they have higher energy compared with bulk matter.

A structure of gold just becomes another identical structure of silver or vice versa. No principle of science is violated.“If changing objects atom by atom is easily possible, tomorrow we can produce novel alloys that might have very different, unknown properties,” says Prof. Pradeep.

“This is not the medieval magic of converting everything to gold. Here, gold does not become silver. Instead, a structure of gold just becomes another identical structure of silver or vice versa. Number of atoms of gold and silver are the same. No principle of science is violated,” he says. “We are only creating conditions such that one structure transforms to another.”

During such transformations, alloys of different compositions of gold (Au) and silver (Ag) are formed. For instance, when silver nanoparticle composed of 25 atoms react with a gold nanoparticle composed of 25 atoms, one atom from the silver nanoparticle is replaced with one atom of gold particle, to form an AuAg24 alloy. The silver atom removed from the silver nanoparticle in turn takes the place of the gold atom in the gold nanoparticle to form an AgAu24 alloy.

As the reaction proceeds, the number of atoms of one metal in an alloy keeps increasing while the other metal keeps decreasing. In other words, the gold-rich alloy gradually gets richer in silver, and by successive single atom changes it becomes a pure silver nanoparticle. Similarly, the silver-rich alloy gradually becomes richer in gold and becomes a pure gold nanoparticle.

This chemistry occurs between a 25-atom piece of gold protected by molecular groups called ligands and a corresponding silver piece composed of 25 atoms of silver and the same number of ligands. These two nanoparticles, also called clusters, are made separately in solution.

Various alloys can be made and their composition can be controlled by controlling the ratio of the two clusters used. “The properties of alloys with different composition could be very new. We do not know what such capabilities give us,” he says. “The most fascinating aspect of this science is that it demonstrates the molecular nature of nanoscale matter.”

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Published in The Hindu on November 10, 2016