Using a hundred-year-old method employed by Japanese smiths, the Linköping group turned the precious metal into a semiconductor.
In detail, they used a three-dimensional base material where gold is embedded between layers of titanium and carbon. Then, serendipity played its part.
“We had created the base material with completely different applications in mind. We started with an electrically conductive ceramic called titanium silicon carbide, where silicon is in thin layers,” Lars Hultman, professor of thin film physics and co-author of the paper, said. “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, they had had titanium gold carbide without knowing how gold could be exfoliated or panned out from it, so to speak.
Murakami’s reagent
By chance, Hultman found the Japanese method called Murakami’s reagent, which etches away carbon residue and changes the colour of steel in knife making, for example. However, it was not possible to use the exact same recipe as the smiths did. Thus, lead author Shun Kashiwaya looked at modifications.
“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,” 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 dissolves gold. The last 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, that is, 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,” Kashiwaya said.
The new properties of goldene are the result of gold having 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, and telecommunications, among others.
Moreover, the researchers believe that the amount of gold used in applications today can be reduced.
The LiU team is also investigating whether it is possible to do the same with other noble metals and identify additional future applications.
