Gold doesn’t tarnish like other metals
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Silver goes dull, copper turns green and iron rusts, but gold always stays shiny. Why this is the case has remained a mystery, but researchers may have finally figured out what makes the valuable metal so resistant to change and how to tarnish it.
Gold is chemically inert, meaning that it doesn’t react with molecules from its surroundings, such as oxygen in the air. This is great news for jewelry, but it limits gold’s usefulness in chemistry, where researchers think it could be a useful catalyst – if only it could be nudged out of its inertness.
Matthew Montemore and Santu Biswas at Tulane University in Louisiana investigated a phenomenon called reconstruction, which happens when a piece of gold is cut, creating a new surface.
“The atoms just hate being on a surface so much that they completely rearrange,” says Montemore. Often, they rearrange into a pattern resembling repeating hexagons, then don’t shuffle further because their energy is low in this arrangement. Reconstruction isn’t common among metals, so the researchers wondered whether it contributes to gold’s inertness.
They used a supercomputer to simulate the quantum states of atoms for several different rearrangements that can happen during reconstruction and to analyse their interactions with oxygen. For a reconstructed gold surface to lose some of its luster, a molecule of oxygen would have to first split in two upon hitting it. The researchers’ simulations revealed that such splitting requires a lot of energy for atoms in a hexagonal pattern, which makes tarnishing very unlikely, but a lot less energy when their arrangement is rectangular.
Because the hexagonal pattern is more common, gold tends to stay shiny. Biswas says that this connection between atoms’ geometry, reconstruction and oxidation has never been considered before.
Understanding it now could help researchers make gold more useful as a catalyst, says Hongliang Xin at Virginia Tech. “The exciting takeaway is that gold’s catalytic behaviour may be tuned by controlling surface reconstruction,” he says. Montemore says that one way to control reconstruction, like nudging atoms into rectangular patterns that are less inert to oxygen, could be by placing a piece of gold in an electrical circuit and applying a voltage.
“[This work] is telling us something that perhaps hadn’t been considered before. Definitely there’s something here for experimentalists to go and look at,” says Andrew Beale at University College London. He says that the idea of using gold as a catalyst has already been proven for certain reactions by using nano-sized particles of the precious metal. The project of making gold useful in this new way is then rather realistic, but questions remain about how the team’s analysis can be connected to objects like such nanoparticles, which tend to have curved surfaces, says Beale.
Going forward, the researchers want to extend their analysis to reactions with molecules other than oxygen, as well as gold alloys in addition to pure gold.
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