Science.-Amalgam nanocrystals open up new technological applications

08-05-2021 The production process of an intermetallic nanocrystal (top row: schematic, bottom row: electron microscope images). RESEARCH AND TECHNOLOGY POLICY CMD

MADRID, 5 (EUROPA PRESS)

Researchers at ETH Zurich have succeeded in producing nanocrystals made of two different metals through an amalgam process whereby a liquid metal penetrates a solid one.

This new and surprisingly intuitive technique makes it possible to produce a wide range of intermetallic nanocrystals with customized properties for various applications, according to a statement from the Swiss entity.

Nanocrystals are nano-sized spheres consisting of regularly arranged atoms. Due to their advantageous properties, they are increasing in various technologies. Semiconductor nanocrystals, for example, are used in next-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 to medicine.

In theory, there are tens of thousands of possible combinations of metals that could form such nanocrystals, with a correspondingly large number of different material properties. Until now, however, it has only been possible to make nanocrystals from a few such pairs.

A team of researchers from ETH Zurich led by Maksym Yarema and Vanessa Wood at the Institute of Electronics has developed a new technique that, in principle, allows almost all possible combinations of intermetallic nanocrystals to be made. Their results were published in the scientific journal Science Advances.

“Our method is simple and intuitive – so intuitive, in fact, that we were surprised that no one had come up with this idea before us,” says Yarema. In conventional processes for producing nanocrystals made of a single metal, the metal atoms are introduced in molecular form, for example as salts, into a solution in which the nanocrystals are formed. “Theoretically that can also be done with two different metals, but in practice it is difficult, or even impossible, to combine clearly different metals in the reactor,” explains Yarema. Therefore, the ETH scientists turned to a procedure that has been used for centuries: amalgam, a particular type of fusion or combination of metals.

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Amalgams are especially known in dentistry, where they are used as a filling material, and also in gold extraction. In both cases, liquid mercury is added to dissolve other metals (for dental fillings, a mixture of copper, zinc, and silver). However, amalgam also works with any other liquid metal. In addition to mercury, which is liquid even at room temperature, there are several metals with relatively low melting points, such as gallium (30 degrees Celsius), indium (157 degrees) or tin (232 degrees).

Yarema and his colleagues use the nanoscale fusion approach. The reaction begins with the dispersion of nanocrystals that contain a single metal, for example, 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, with the mixture heated to about 300 degrees.

Initially, the high temperature causes the chemical bonds in the gallium amide to break, allowing the liquid gallium to accumulate in the silver nanocrystals. Now, the actual melting process begins, during which liquid gallium seeps into solid silver. Over time a new crystal lattice forms, in which eventually silver and gallium atoms are regularly arranged. Then everything cools down again, and after ten minutes the nanocrystals are ready. “We are amazed at how efficient fusion is at the nanoscale. Having a liquid metal component is the key to fast and uniform alloying within each nanocrystal,” says Yarema.

Using the same technique, researchers have already produced different intermetallic nanocrystals such as gold-gallium, copper-gallium, and palladium-zinc.

The researchers anticipate great potential for technological applications due to the exact controllability of the composition and size of the nanocrystals along with the possibility of combining the metals almost at will. “Because nanocrystal amalgam synthesis allows for so many new compositions, we can’t wait to see them work in enhanced catalysis, plasmonics, or lithium-ion batteries,” says Yarema. Catalysts made from nanocrystals, for example, can be precisely tailored and optimized for a particular chemical process that they are supposed to accelerate.

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