All that glitters is not gold, but gold sure does glitter, holding a shine far longer than most metals. And now two researchers have explained why.
In a paper published today in Physical Review Letters, Santu Biswas and Matthew Montemore of Tulane University reveal the reason gold is harder to oxidize than similar metals. They key, they say, is the same chemical trickery that gives it a beautiful zigzag structure when viewed under a scanning tunneling microscope.
“Everyone knows that gold is difficult to oxidize,” Biswas says. “The thing is, why? What is the proper reason for that?”
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Oxidation is the process in which oxygen (or another element, such as sulfur) reacts with a metal and attaches to its surface. For iron, we call this buildup of bonded oxygen “rust,” while oxidation of other metals is referred to as “tarnish.” How easily oxygen sticks to a metal depends on how well the metal’s atomic structure holds onto electrons. In particular, gold treasures its electrons and spurns donations from outside, preserving its shine and making it a cherished material for jewelry and various industrial applications.
But gold’s close clutch of electrons isn’t enough to explain just how reticent it is to oxidize. Biswas and Montemore suspected that the additional reason for gold’s near invulnerability involved the strange way it behaves when you break it. Cleave a chunk of gold, and the newly exposed face will microscopically reshape itself in seconds. The atoms rearrange themselves to produce a zigzagging “herringbone” pattern—a phenomenon called “surface reconstruction.”
The authors calculated the energy required to oxidize gold before and after this unusual reshuffling. They found that oxygen molecules from the air—which are each composed of two oxygen atoms bonded together—more easily break apart and adhere to the gold atoms on the surface before the pattern is reestablished. The reaction requires much less energy in the brief instant that a new surface is exposed.
Physically, the reconstruction pulls more gold atoms out from the bulk of the metal, jamming them into the surface and turning a simple, square-shaped atomic lattice into a denser hexagonal shape—which gives rise to the pattern of bumps and ridges. The reconstruction brings gold’s surface closer to thermodynamic equilibrium, making it easier for the metal’s atoms to exchange heat with one another. But the flip side is that it’s harder for oxygen to wedge itself into the mix.
The team thinks its finding may be a boon to chemists. “You can prevent reconstruction by putting some absorbent on top of the surface,” Biswas says. “And then the gold can easily oxidize.” This could make the use of gold surfaces a new way to capture oxygen from gases, which is important for keeping gases pure.