Promising breakthrough: Nanocrystals made of amalgam

Researchers at ETH have managed to produce nanocrystals made of two different metals using an amalgamation process whereby a liquid metal penetrates a solid one. This new and surprisingly intuitive technique makes it possible to produce a vast array of intermetallic nanocrystals with tailored properties for diverse applications.

Intermetallic nanocrystals (electron microscope images) made from different combinations of metals. The white bar indicates 10 nanometres. (Images: Chemistry and Materials Design group)
Intermetallic nanocrystals (electron microscope images) made from different combinations of metals. The white bar indicates 10 nanometres. (Images: Chemistry and Materials Design group)

Nanocrystals are nanometre-sized spheres consisting of regularly arranged atoms. Owing to their advantageous properties, they are on the rise in several technologies. Semiconductor nanocrystals, for instance, are used in new generation television screens. More recently, so-called intermetallic nanocrystals, in which two different metals combine to form a crystal lattice, have made a name for themselves as they promise improved and unique applications. Those applications range from catalysis to data storage and medicine.

In theory, there are tens of thousands of possible combinations of metals that could make up such nanocrystals, with a correspondingly large number of different material properties. So far, however, it has only been possible to make nanocrystals out of a few such pairings. A team of researchers at ETH Zurich led by Maksym Yarema and Vanessa Wood at the Institute for Electronics have now developed a new technique that, in principle, allows one to realize nearly all possible combinations of intermetallic nanocrystals. Their results were recently published in the scientific journal Science Advances.

Surprisingly intuitive method

“Our method is simple and intuitive – so intuitive, in fact, that we were surprised that no one had had this idea before us”, says Yarema. In conventional procedures for producing nanocrystals made of a single metal, the metal atoms are introduced in molecular form, for instance as salts, into a solution in which the nanocrystals then form. “Theoretically that can also be done with two different metals, but in practice it is difficult, or even impossible, to combine distinctly dissimilar metals in the reactor”, Yarema explains. Therefore, the ETH scientists resorted to a procedure that has been used for centuries: amalgamation, a particular kind of fusing or blending metals.

Liquid metals

Amalgams are particularly well-known from dentistry, where they are used as a filling material, and also from gold mining. In both cases, liquid mercury is added to dissolve other metals (for tooth fillings, a mixture of copper, zinc and silver). However, amalgamation also works with any other liquid metal. Besides mercury, which is liquid even at room temperature, there are a number of metals with relatively low melting points, such as gallium (30 degrees centigrade), indium (157 degrees) or tin (232 degrees).

Amalgamation approach for nanocrystals

Yarema and his colleagues use the amalgamation approach at the nanoscale. The reaction starts with the dispersion of nanocrystals containing a single metal, for instance silver. Then, the atoms of the second metal – say, gallium – are added in molecular form (in this case as amides, a compound of carbon, hydrogen, and nitrogen), while the mixture is heated to around 300 degrees.

We are amazed how efficient the amalgamation is at the nanoscale. Having one liquid metal component is the key to fast and uniform alloying within each nanocrystal.

Maksym Yarema

Maksym Yarema

Durch die hohe Temperatur brechen zunächst die chemischen Verbindungen des Gallium-Amids auf, und flüssiges Gallium lagert sich auf den Silber-Nanokristallen ab. Nun beginnt der eigentliche Amalgamierungsprozess, bei dem flüssiges Gallium in das feste Silber hineinkriecht. Dadurch bildet sich nach und nach ein neues Kristallgitter, in dem am Ende Silber- und Galliumatome regelmässig angeordnet sind. Dann wird das Ganze wieder abgekühlt, und nach zehn Minuten sind die Nanokristalle fertig. “Wir sind erstaunt, wie effizient die Amalgamierung auf der Nanoskala funktioniert. Die flüssige Metallkomponente ist dabei der Schlüssel zur schnellen und gleichmässigen Bildung einer Legierung in den einzelnen Nanokristallen”, sagt Yarema.

Kontrollierbarer Prozess

Mit derselben Technik haben die Forschenden bereits verschiedene intermetallische Nanokristalle hergestellt, unter anderem Gold-Gallium, Kupfer-Gallium und Palladium-Zink. Der Amalgamierungsprozess ist dabei genau steuerbar. Durch die Menge an Zweitatomen, die als Amide in die Lösung gegeben werden, kann man das Mengenverhältnis der Metalle in den Nanokristallen exakt kontrollieren. Am Beispiel von Gold-Gallium (chemische Symbole Au und Ga) haben die Wissenschaftler gezeigt, dass sich so Nanokristalle mit verschiedensten Mengenverhältnissen herstellen lassen, etwa 1:2 (AuGa2), 1:1 (AuGa) oder 7:2 (Au7Ga2). Auch die Grösse der fertigen intermetallischen Nanokristalle lässt sich aus der Grösse der ursprünglichen Nanokristalle aus dem ersten Metall und dem Grössenzuwachs durch das Zweitmetall genau vorhersagen.

Massgeschneiderte Nanokristalle für Anwendungen

In der genauen Kontrollierbarkeit der Zusammensetzung und Grösse der Nanokristalle, gepaart mit der nahezu beliebigen Kombinierbarkeit der Metalle, sehen die Forschenden ein grosses Potenzial für technologische Anwendungen. “Da die Synthese von Nanokristallen mittels Amalgamierung so viele neue Zusammensetzungen ermöglicht, können wir es kaum erwarten, bis diese in verbesserten Katalysatoren, in der Plasmonik oder in Lithium-Ionenbatterien zum Einsatz kommen”, sagt Yarema. So können etwa Katalysatoren aus Nanokristallen auf einen bestimmten chemischen Prozess, der durch sie beschleunigt werden soll, genau massgeschneidert und optimiert werden.

(Images: CMD)” data-size=”1564×782″>
The production process of an intermetallic nanocrystal (upper row: schematic, lower row: electron microscope images). To the solution containing nanocrystals of the first material (left), the second metal (
The production process of an intermetallic nanocrystal (upper row: schematic, lower row: electron microscope images). To the solution containing nanocrystals of the first material (left), the second metal (“M”) is added as an amide and subsequently accumulates as a liquid on the nanocrystals (centre). Amalgamation finally results in intermetallic nanocrystals (right). (Images: CMD)

Reference

Clarysse, J, Moser, A, Yarema, O, Wood V, Yarema M. Size- and composition-controlled intermetallic nanocrystals via amalgamation seeded growth. Science Advances, 28 Jul 2021, Vol. 7, no. 31, eabg1934. DOI: 10.1126/sciadv.abg1934 .

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