ETH researchers have succeeded in producing nanocrystals from two different metals using an amalgamation process in which a liquid metal penetrates a solid one. This new and surprisingly intuitive technology makes it possible to produce a wide range of intermetallic nanocrystals with tailor-made properties for a wide variety of applications.
Nanocrystals are spheres a few nanometers in size made up of regularly arranged atoms. Because of their beneficial properties, they are on the rise in several technologies. Nanocrystals made of semiconductors, for example, are used in the new generation of television screens. Recently, so-called intermetallic nanocrystals, in which two different metals form a common crystal lattice, have become increasingly popular because they promise powerful and unique applications. These range from catalysis and data storage to medicine.
Theoretically, there are tens of thousands of possible combinations of metals that could form such nanocrystals, with correspondingly many different material properties. So far, however, it has only been possible to actually produce nanocrystals from a few pairings. Researchers at ETH Zurich under the leadership of Maksym Yarema and Vanessa Wood at the Institute for Electronics have now developed a new technology with which, in principle, almost all possible combinations of intermetallic nanocrystals can be realized. Their results were recently published in the journal Science Advances.
Surprisingly intuitive method
“Our method is simple and intuitive – so intuitive that we were surprised that no one had come up with this idea before us,” says Yarema. In conventional processes for the production of nanocrystals from a single metal, the metal atoms are added in molecular form, for example as salts, to a solution in which the nanocrystals then grow. “In theory, you can do this with two different metals, but in practice it is difficult or even impossible to combine very different metals in a test tube,” explains Yarema. So the ETH scientists resorted to a process that has been used for centuries: amalgamation, i.e. a certain type of fusing or mixing of metals.
Amalgams are mainly known from dentistry, where they are used as a material for seals, and also from gold mining. In both cases, liquid mercury is added to dissolve other metals in it (a mixture of copper, tin and silver for dental fillings). Amalgamation also works with any other liquid metal. In addition to mercury, which is already liquid at room temperature, there are a number of metals with relatively low melting points such as gallium (30 degrees Celsius), indium (157 degrees) or tin (232 degrees).
Amalgam principle for nanocrystals
Yarema and his colleagues make use of the amalgam principle on the nanoscale. The reaction begins with the dissolution of nanocrystals, which are made up of a single metal, such as silver. Then the atoms of the second metal – gallium, for example – are added in molecular form (in this case as amides, a combination of carbon, hydrogen, nitrogen and oxygen) while the mixture is heated to just under 300 degrees.
We are surprised at how efficient the merging is on the nanoscale. A single liquid metal component is the key to rapid and uniform alloying in every nanocrystal.Maksym Yarema
Due to the high temperature, the chemical compounds of the gallium amide break first, and liquid gallium is deposited on the silver nanocrystals. Now the actual amalgamation process begins, in which liquid gallium creeps into the solid silver. This gradually creates a new crystal lattice in which silver and gallium atoms are regularly arranged at the end. Then the whole thing is cooled down again, and after ten minutes the nanocrystals are ready. “We are amazed at how efficiently amalgamation works on the nanoscale. The liquid metal component is the key to the rapid and uniform formation of an alloy in the individual nanocrystals, ”says Yarema.
Using the same technique, the researchers have already produced various intermetallic nanocrystals, including gold-gallium, copper-gallium and palladium-zinc. The amalgamation process can be precisely controlled. The quantity ratio of the metals in the nanocrystals can be precisely controlled by the amount of second atoms that are added to the solution as amides. Using gold-gallium (chemical symbols Au and Ga) as an example, the scientists have shown that nanocrystals can be produced in a wide variety of proportions, such as 1: 2 (AuGa 2 ), 1: 1 (AuGa) or 7: 2 (Au 7 Ga 2). The size of the finished intermetallic nanocrystals can also be precisely predicted from the size of the original nanocrystals from the first metal and the increase in size due to the second metal.
Customized nanocrystals for applications
The researchers see great potential for technological applications in the precise controllability of the composition and size of the nanocrystals, coupled with the ability to combine the metals in almost any way. “Since the synthesis of nanocrystals by means of amalgamation enables so many new compositions, we can hardly wait until these are used in improved catalysts, in plasmonics or in lithium-ion batteries,” says Yarema. For example, catalysts made from nanocrystals can be precisely tailored and optimized for a specific chemical process that they are supposed to accelerate.
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
Provided by ETH Zurich