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Solid, liquid and gaseous – these are the familiar main forms of matter. Scientists at Ulm University and the University of Nottingham have now proven the existence of a completely new state of matter in which both solid and liquid properties are combined. Liquids enclosed by stationary atoms remain liquid even far below their freezing point. The discovery could lead to more efficient and sustainable catalysts.
When metal melts, the atoms within it behave similarly to individuals in a crowd, moving around freely. Now, however, a German-British research team involving Ulm University has made a surprising discovery: in liquid metal, some atoms remain fixed in their position, thereby influencing the solidification process. This results in the characteristics of solids and liquids being combined in the same material. The study was published in ACS Nano.
“With our unique low-voltage microscope SALVE, we were able to observe for the first time how molten metal droplets behave at the atomic level”, explains Dr. Christopher Leist, the lead author from Ulm. “We heated metal nanoparticles such as platinum, gold and palladium that were applied to an atomically thin substrate – graphene.” When the particles melted, their atoms began to move rapidly, as expected. “To our surprise, however, we found that individual atoms remained fixed in certain places.” The reason for this is defects in the crystal structure of the carrier material, where the fixed atoms are strongly bonded to the graphene.
The researchers also discovered that the number of these defects in the carrier material, and thus the number of stationary metal atoms, can be specifically manipulated and increased using the electron microscope beam. “If only a few atoms are fixed, a crystal forms from the liquid and gradually grows,” explains Senior Professor Ute Kaiser, head of the SALVE Centre at Ulm University. “However, if there are many stationary atoms, the solidification process is slowed down and crystal formation is prevented.”
This solidification phase is also particularly important in industrial applications, as it determines the structure and functional properties of a material.
Fixed atoms form a fence around liquid matter
It's particularly exciting when the fixed atoms form a circular corral around the liquid matter, as the research team has succeeded in doing. “Once the liquid is trapped in this ‘atomic enclosure’, it can remain liquid even when the temperature drops well below the point at which the material normally solidifies,” emphasises the head of the research team, Professor Andrei Khlobystov from the University of Nottingham. In the case of platinum, this means that it can still be liquid at 350 degrees Celsius – a completely unexpected behaviour, as this is more than 1000 degrees colder than the point at which platinum normally solidifies. Professor Elena Besley, an expert in theoretical chemistry at the University of Nottingham, adds: “With our molecular dynamics approach, we were able to show that the confined liquid is indeed stable.”
Dr Jesum Alves Fernandes, a catalysis specialist at the University of Nottingham, sees great opportunities as platinum on carbon catalysts are among the most widely used catalysts worldwide. “If we understand how the fixed atoms arrange and move, we could potentially develop catalysts that clean themselves and remain effective for much longer,” says Alves Fernandes.
“Our achievement could herald a new form of matter that combines the properties of solids and liquids in a single material,” the team is convinced. The researchers hope that manipulating the positions of stationary atoms will enable longer and more complex enclosures to be formed in the future. This would allow rare metals to be used more efficiently, for example in energy conversion and storage.
The study was funded by the EPSRC programme ‘Metal Atoms on Surfaces and Interfaces (MASI) for Sustainable Future,’ which addresses the challenges of sustainable use of rare elements in the future.
Prof. Dr Ute Kaiser, Institute of Quantum Optics, Mail: ute.kaiser@uni-ulm.de
Christopher Leist, Sadegh Ghaderzadeh, Emerson C. Kohlrausch, Johannes Biskupek, Luke T. Norman, Ilya Popov, Jesum Alves Fernandes, Ute Kaiser, Elena Besley, and Andrei N. Khlobystov
ACS Nano Article ASAP
DOI: 10.1021/acsnano.5c08201
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