If you own gold jewelry, you might notice that it doesn’t tarnish as easily as other materials, like silver. For a long time, scientists understood that this was because gold doesn’t interact strongly with oxygen, although the exact physical mechanisms behind this property weren’t as well understood.
But a new discovery, published today in Physical Review Letters, finally identifies how gold retains its golden glow for so long. Essentially, gold’s surface atoms rearrange themselves into distinct patterns that suppress oxygen reactions by a factor of a billion to a trillion. This microscopic barrier helps gold retain its characteristic shininess, according to a press release. What’s more, because gold is a key element for many important chemical reactions, the new understanding could open new avenues for research in chemistry.
“People have generally thought gold doesn’t tarnish simply because it doesn’t interact strongly with oxygen,” Matthew Montemore, the study’s co-author and a chemical engineer at Tulane University, said in the release. “What we show is that for two of the most common gold surface types, the surface atoms actually rearrange themselves in a way that makes the gold much more resistant to oxidation.”
Bouncing electrons
The reason any visible object has a certain color largely depends on its molecular chemistry, namely how light interacts with an object’s electrons. In the case of metals, a delocalized sea of electrons in metallic bonds absorbs and re-emits photons (very simply, particles of light) over a wide range of frequencies, according to Chris Schaller, a retired physicist with the College of Saint Benedict and Saint John’s University, in a blog post.
Gold is special in that relativistic effects cause its electrons to travel at over half the speed of light, which ultimately leads to the absorption of lower-energy blue photons. And if “blue is removed, we see yellow,” explained Mark Lorch, a biochemist at the University of Hull in the U.K., for BBC’s Science Focus.
Peering into the depths
So it made sense to investigate how the minute movements of molecules potentially affected gold’s long-lasting shine. For the new study, Montemore and co-author Santu Biswas, a postdoctoral fellow at Tulane, used computer simulations to predict how atoms and electrons on gold’s surface would behave upon meeting oxygen molecules. They performed analyses on two common types of gold surfaces, Au(110) and Au(100).
According to the study, gold’s “inherent weak interaction with oxygen is by itself not enough to make it resistant to oxidation.” What really keeps the oxygen away is a hexagonal structure generated by the surface atoms. Fascinatingly, similar procedures that resulted in rectangular or squarelike barriers were nowhere near as sturdy, and oxygen molecules broke apart and reacted with gold, the team reported.
A mystery and a plan
The researchers are eyeing more practical implications of the findings. Namely, they’re taking note of gold’s primary role in catalysis, a branch of chemistry focusing on improving the rate and efficiency of various reactions. While gold’s natural resistance to oxidation makes it ideal for jewelry, that “same trait” can “limit its usefulness in chemical manufacturing and energy applications,” the team noted in the statement.
And the stakes are definitely there. For instance, gold-palladium catalysts are important ingredients for vinyl acetate, which are basic building blocks for many plastic materials. Some recent work explored usage of gold catalysts in producing renewable fuels. The latest findings suggest that there’s not even a need to find complex chemical routes for these ventures; physical manipulations of gold’s surface geometry might be enough.
“If you can trick gold into dissociating oxygen, it can actually become a very effective catalyst for certain reactions,” Montemore said. “Our work suggests a new strategy for potentially doing that by preventing or reversing these surface rearrangements.”
