Steel rusts, copper turns green, silver dulls — yet gold pulled from a shipwreck can still gleam after centuries underwater. Chemists have long put this down to gold simply being unreactive. New research suggests the real story is more active than that: gold's surface atoms quietly rearrange themselves into a geometry that makes it extraordinarily hard for oxygen to attack.
The work, by Matthew Montemore and Santu Biswas of Tulane University in New Orleans, was published on 21 May in Physical Review Letters. Using quantum-mechanical simulations, the pair modelled how oxygen molecules meet two of the most common types of gold surface. Before a metal can tarnish, it first has to break apart the paired oxygen atoms in the air; only then can oxygen bond to the surface. So the researchers asked a simple question: how easily can gold split oxygen?
From squares to hexagons
The answer turned on shape. When a fresh gold surface is exposed — by cutting it, say — its atoms shift from their original square layout into a hexagonal one, a process known as reconstruction. The simulations showed that the square arrangement would readily split oxygen, but the hexagonal one is far more reluctant. To react, the hexagonal surface would first have to distort back into squares — an energy barrier that stops oxidation almost before it starts.
The effect is dramatic. According to the team, the reorganised surfaces slow oxygen reactions by a factor of somewhere between a billion and a trillion. Without that rearrangement, Montemore notes, gold would begin to oxidise within seconds. Even the surprise at how large the difference turned out to be was, he said, considerable.
The finding does more than explain why wedding rings and ancient coins keep their lustre. Gold is also a valuable catalyst, speeding up industrial reactions — but the very inertness that protects jewellery limits its usefulness there. If chemists could coax gold into splitting oxygen on demand, by controlling how its surface atoms sit, they might build better catalysts for tasks such as stripping carbon monoxide from exhaust or making chemicals used in plastics and cleaner energy. Understanding gold's centuries-old shine, in other words, could help put the metal to new work.