IIT Madras: designer alloys by chemical reactions

Nothing whatsoever will happen if large chunks of silver and gold kept in proximity even for infinite period. But the same cannot be said when the two metals are at nanoscale.

A path-breaking work by a team led by Prof. T. Pradeep, Department of Chemistry, IIT Madras has shown that two nanoscale pieces of metals react spontaneously at room temperature even in solution to make new alloys of well-defined composition. The study was published recently in the Journal of American Chemical Society (JACS).

“This means that metals with well-defined properties may be created in solution, just by chemical reactions. From a scientific perspective, this study shows that nanoparticles are similar to molecules and reactions between them can be written like chemical reactions:

A + B → C + D

This is a big step forward in the chemistry of nanomaterials,” said Prof. Pradeep. “So far we wrote such reactions only with organic and inorganic molecules, as:

2H2 + O2 → 2H2O

Alloys are made mostly by heating the constituents, often to high temperatures. But in this case, the researchers have been able to make alloys in solution at room temperature with precise composition control, to the extent that each atom is controlled.

The team of researchers led by Prof. Pradeep has shown that nanometre-sized particles or clusters of gold and silver react spontaneously in solution at room temperature to make alloy clusters of precise structure and composition.

“Metal clusters comprise a few atoms of matter and are very different from their bulk powders. Reactions happen between clusters and not between bulk powders of gold and silver. This is because nanomaterials are ‘metastable’ and are more reactive than their bulk counterparts,” he said.

As a result, clusters of gold and silver produce new alloys when in solution. The clusters of gold (Au) and silver (Ag) used in the study were Au25 and Ag44. These are precise molecules composed of 25 atoms of gold and 44 atoms of silver respectively, and are under 2 nanometresin diameter.

A chemical reaction between Au25 and Ag44 in solution will give rise to alloys of AgAu and AuAg composition. The reaction can be represented as:

Au25 + Ag44 → AgAu24 + AuAg43

In this case, the reaction products are alloys of precise composition. “The final composition of the product will depend on the composition of the reactants. So more of Ag and less of Au will give rise to an alloy of Ag type,” Prof. Pradeep explained.

Such reactions may be made to happen to make alloys of many different metals such as platinum, palladium, iridium, nickel, etc. and new materials may be made, all in solution, at room temperature.

“The excitement in nanotechnology is due to the unusual properties of materials when sizes of particles reduce to nanometres. This happens because of ‘electronic confinement’ — an effect when electron motion is limited to extremely short distances, of the order of nanometres,” he said.

This kind of reactions would mean that metallurgical processes may be done differently tomorrow. “Many nanoscale materials have new properties which are useful in catalysis. They could be luminescent or magnetic. Designer alloys with new properties may be made by these reactions,” he added.

Published in The Hindu on January 3, 2016

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