Goldene in the form of gold monolayer sheets has been prepared by etching away titanium carbide (Ti3C2) slabs from titanium gold carbide (Ti3AuC2).
“If you make a material extremely thin, something extraordinary happens — as with graphene. The same thing happens with gold,” said Dr. Shun Kashiwaya, a researcher at Linköping University.
“As you know, gold is usually a metal, but if single-atom-layer thick, the gold can become a semiconductor instead.”
To create goldene, Dr. Kashiwaya and colleagues used a three-dimensional base material where gold is embedded between layers of titanium and carbon. But coming up with goldene proved to be a challenge.
“We had created the base material with completely different applications in mind,” said Linköping University’s Professor Lars Hultman.
“We started with an electrically conductive ceramics called titanium silicon carbide, where silicon is in thin layers.”
“Then the idea was to coat the material with gold to make a contact. But when we exposed the component to high temperature, the silicon layer was replaced by gold inside the base material.”
This phenomenon is called intercalation and what the researchers had discovered was titanium gold carbide.
For several years, the authors have had titanium gold carbide without knowing how the gold can be exfoliated or panned out.
By chance, they found a method that has been used in Japanese forging art for over a hundred years.
It is called Murakami’s reagent, which etches away carbon residue and changes the color of steel in knife making, for example. But it was not possible to use the exact same recipe as the smiths did.
“I tried different concentrations of Murakami’s reagent and different time spans for etching. One day, one week, one month, several months. What we noticed was that the lower the concentration and the longer the etching process, the better. But it still wasn’t enough,” Dr. Kashiwaya said.
The etching must also be carried out in the dark as cyanide develops in the reaction when it is struck by light, and it dissolves gold. This step was to get the gold sheets stable.
To prevent the exposed two-dimensional sheets from curling up, a surfactant was added. In this case, a long molecule that separates and stabilizes the sheets, i.e. a tenside.
“The goldene sheets are in a solution, a bit like cornflakes in milk. Using a type of ‘sieve,’ we can collect the gold and examine it using an electron microscope to confirm that we have succeeded. Which we have,” Dr. Kashiwaya said.
“The new properties of goldene are due to the fact that the gold has two free bonds when two-dimensional.”
“Thanks to this, future applications could include carbon dioxide conversion, hydrogen-generating catalysis, selective production of value-added chemicals, hydrogen production, water purification, communication, and much more.”
“Moreover, the amount of gold used in applications today can be much reduced.”
The team’s work was published in the journal Nature Synthesis.
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S. Kashiwaya et al. Synthesis of goldene comprising single-atom layer gold. Nat. Synth, published online March 18, 2024; doi: 10.1038/s44160-024-00518-4