Solid, liquid and gaseous – these are the known main forms of matter. Scientists at Ulm University and the University of Nottingham have now demonstrated a completely new state of matter in which matter has both solid and liquid properties. Liquids trapped by stationary atoms thus remain liquid even far below their freezing point. The discovery could lead to more efficient and sustainable catalysts.
When metal melts, atoms in it actually move around freely like individuals in a crowd. However, a German-British research team involving the University of Ulm has now made a surprising discovery: In liquid metal, some atoms remain firmly in position and thus influence the solidification process. As a result, characteristics of solids and liquids can be found combined in the same material. The study was published in the specialist publication ACS Nano.
“Using 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 first author carried out the experiments in Ulm using the SALVE electron microscope. “We heated metal nanoparticles such as platinum, gold and palladium, which were deposited on an atomically thin carrier – graphene.” As the particles melted, their atoms began to move rapidly, as expected. “To our surprise, however, we found that individual atoms remained stuck in certain places.” The reason for this is defects in the crystal structure of the substrate 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 manipulated and increased using the electron microscope beam. “If only a few atoms are fixed, the liquid forms a crystal that gradually grows,” explains Senior Professor Ute Kaiser, Head of the SALVE Center at the University of Ulm. “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 for industrial applications, as it determines the structure and functional properties of a material.
It becomes particularly exciting when the fixed atoms form a circular fence 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 far below the point at which the material normally solidifies,” emphasizes 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 behavior, as this is more than 1000 degrees colder than the point at which platinum usually solidifies. Professor Elena Besley, an expert in theoretical chemistry at the University of Nottingham, adds: “Using our molecular dynamics approach, we were able to show that the fenced liquid is indeed stable.”
Dr. Jesum Alves Fernandes, catalysis specialist at the University of Nottingham, sees great opportunities in this. After all, platinum on carbon catalysts are among the most widely used catalysts in the world. “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 one material,” the team is convinced. The researchers hope that by manipulating the positions of the stationary atoms, longer and more complex enclosures can be formed in the future. This could allow rare metals to be used more efficiently, for example in energy conversion and storage.
The study was funded by the EPSRC program “Metal Atoms on Surfaces and Interfaces (MASI) for Sustainable Future”, which deals with the challenges of sustainable use of rare elements in the future.