The rearrangement of atoms on the surface of the metal helps prevent reactions with oxygen
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Unlike other metals which oxidize in air, gold (photo) does not react. Scientists now better understand why.
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Steel rusts over time; copper turns green. But gold seems impervious to the elements. Scientists have now discovered a new detail explaining why gold remains so intact.
The atoms on the surface of gold rearrange themselves into a geometry that prevents oxidation, a process that causes many metals to tarnish. Without this rearrangement, gold would start to oxidize within secondsreport the researchers on May 21 in Physical Examination Letters.
So that a metal to oxidizeit must first split the oxygen molecules in the ambient air, each made up of two oxygen atoms. The oxygen atoms can then form compounds that stick to the surface of the metal. The researchers therefore calculated to what extent the gold surfaces could split oxygen.
Once a new gold surface is exposed, for example by cutting it, the atoms lose their original arrangement in the atom lattice, a process called reconstruction. Different arrangements of atoms can occur outside of gold, and researchers have studied two streams of them, in which atoms are initially arranged in squares, but reconstructed into hexagons.
Based on quantum mechanical calculations, the researchers found that the square layout was much better at splitting oxygen than the hexagonal layout. For the hexagonal structure to split oxygen, it would first have to deform back to its original square shape, an obstacle that prevents oxidation. (A third common surface structure of gold, already known for its poor oxidizing power, is intrinsically hexagonal.)
Gold oxide itself is unstable, so even if the square arrangement could be maintained under certain conditions, the material would likely only form a thin oxide layer, says chemical engineer Matthew Montemore, co-author of the study. But these results could help scientists understand how to better design catalysts, materials that promote the progression of chemical reactions.
The reluctance of the reconstructed gold to oxidize was “really a surprise,” says Montemore, of Tulane University in New Orleans. The tiny changes in position make a huge difference: “Oxidation is between a billion and a trillion times slower when rearranged. »