Refined protective effect: Gold is extremely durable and does not lose its shine even after centuries. Chemists have now discovered a new explanation for this during experiments with gold surfaces. What is crucial is the ability of gold to flexibly rearrange the atoms on its surface. This turns the normal cubic lattice structure into a hexagonal pattern. This rearrangement then acts as an effective barrier to oxidation by preventing its first, crucial step, the researchers found.
Gold is a very special element in many ways. The precious metal resists almost all chemical reactions, shows a unique shine and is as malleable and yet durable as no other metal. Because of its inertness, gold retains its shimmer even after thousands of years. While copper oxidizes to verdigris, iron rusts and silver turns black, corrosion and other environmental influences seem to have no effect on gold.
Why doesn’t gold tarnish?
But what gives gold its great durability? “Until now, it was thought that gold doesn’t tarnish because it doesn’t interact much with oxygen,” says Matthew Montemore of Tulane University in New Orleans. In addition, the heavy gold core binds its outer electrons particularly tightly, making other chemical reactions more difficult. But is that really the only reason? Montemore and his colleague Santu Biswas suspected that the poor oxidizability of gold had another cause.
To follow up on this lead, the researchers conducted virtual reaction tests on gold surfaces using computer simulations. Typically, the atomic lattice of gold has a face-centered cubic lattice shape, the so-called Au(100). However, this lattice shape changes on the gold surface: the atoms rearrange themselves so that they can assume an energetically more favorable state in this boundary layer. Through this so-called reconstruction, the top atomic layer of gold forms a hexagonal lattice shape, Au(110). In their simulations, the chemists analyzed what happens when these gold surfaces come into contact with oxygen molecules (O2).
Hexagonal atom arrangement as protection against oxidation
The tests revealed that the rearrangement of the gold surface into the hexagonal Au(110) variant has a direct effect on the reaction with oxygen. “The rearrangement of the surface atoms makes the gold significantly more resistant to oxidation,” reports Montemore. In order for such oxidation to take place, the diatomic oxygen molecules must first be broken down. “However, the hexagonal gold structure forms a particularly high barrier to this oxygen dissociation,” explain the researchers.
Specifically, the virtual experiments showed that O2 dissociation is slowed down a billion to a trillion times by changing to a quasi-hexagonal structure, as the researchers report. However, if they prevented the reconstruction of the gold surface in their simulation, this inhibiting effect did not occur and the oxygen was able to react with the gold. The hexagonal lattice shape is not only energetically more favorable for the gold atoms, but its rearrangement also makes the precious metal less chemically vulnerable.
The results thus provide a completely new explanation for one of the most well-known properties of gold. The precious metal is so durable because it can flexibly rearrange the atoms on its surface. “The surface atoms arrange themselves into a structure that makes the gold more resistant to oxidation,” explains Montemore. This means that gold retains its shine even after thousands of years.
Also a starting point for gold catalysts
At the same time, the new findings also offer valuable starting points for another use of gold: catalysis. The actually inert precious metal can accelerate chemical reactions as nanoparticles or in gold complexes. Gold-palladium catalysts are used, for example, in the chemical industry to produce the plastic raw material vinyl acetate. But nanogold can also oxidize carbon monoxide to CO2 and help with recycling.
Until now, however, it was unclear whether and how gold’s catalytic abilities could be further improved. It now appears that the surface geometry of the precious metal could be a starting point. “If we could get gold to dissociate oxygen better, it could become an even more effective catalyst,” says Montemore. “Our results show us that we could achieve this by preventing or reversing the rearrangement of surface atoms.”
Source: Matthew Montemore and Santu Biswas (Tulane University, New Orleans), Physical Review Letters, 2026; doi: 10.1103/g3bc-t1qv