It may be possible to manage, at least partially, some of the challenges facing us (such as how to use energy in an eco-friendly manner and how to deal with water shortages) with the aid of new materials. Researchers are now attempting to design such materials to ensure that their properties are as useful as possible. A promising group of new materials is known as “MXenes”, whose unique properties have aroused the curiosity of researchers.
“What is so fascinating about MXenes are the possibilities of controlling the material’s properties such that they are suitable for different applications. They have a great potential for use in many fields, such as batteries, water purification and the desalination of seawater, energy-efficient transistors and diodes, and as supercapacitors to store energy”, says Martin Magnuson, associate professor in the Department of Physics, Chemistry and Biology.
Challenging graphene
MXenes first saw the light of day as recently as 2011 at a laboratory in the US. They have several similarities with another, more widely known material, namely graphene. Both are 2-dimensional, and exist as extremely thin sheets consisting of a single layer of atoms. Graphene, however, consists solely of carbon, while the MXenes consist of a transition metal such as titanium, and either carbon or nitrogen. They are strong and brittle, like ceramics, but conduct heat and electricity, like metals. Various surface termination groups can be attached to the core of titanium and carbon, and these influence the properties of MXenes. They may, for example, affect the electrical conductivity or the elasticity.
The material is constructed layer by layer. The layers are held together by very weak forces, and the researchers believe it will be possible to insert other elements between them. One attractive idea is to insert lithium ions, so that the material can be used as electrodes in lithium ion batteries (the pioneers of which, it may be remembered, were awarded this year’s Nobel Prize in Chemistry).
“MXenes can compete with graphene, but they are more versatile, since we can combine the material in many different ways and tailor its properties”, says Martin Magnuson.
Electrons at the speed of light (nearly)
In their quest to find out more about the function of MXenes, Martin Magnuson and his colleagues Lars-Åke Näslund and Joseph Halim travelled to the MAX IV synchrotron facility, which can be thought of as a huge microscope, just outside Lund. It allows researchers in materials science, biology, chemistry, environmental sciences, geology and medicine to investigate details of molecular surfaces and structures. In order to investigate atoms, a microscope that uses radiation with a wavelength shorter than the size of the atoms is needed. Thus, wavelengths around 0.1 nanometre, or one tenth of a billionth of a metre, are needed. The MAX IV facility can generate radiation of this type.MAX IV in Lund. Photo credit Perry Nordeng
The most noticeable part of the synchrotron is the circular building, which is home to the larger of the two storage rings. The circumference of the ring is 528 metres, about the same as the Colosseum in Rome. Electrons travel around the ring at speeds that nearly reach the speed of light. They have been accelerated in a linear accelerator, which leads the electrons underground into the storage ring. Once in the ring, their path is bent with the aid of magnets to create the radiation that the “microscope” uses – extremely intense and energetic synchrotron radiation.
Research stations are located around the ring, nine of them at present, where scientists can carry out their experiments. The radiation is led out from the storage ring through special beamlines to where the experiments are located. The beamlines have been adapted for different wavelengths and energies of synchrotron radiation, and are therefore used for different types of investigation – everything from investigating historical and archaeological items without damaging them to seeing how the atoms are arranged in a material.
Reforming the material
One of the investigations the LiU researchers performed was to heat two forms of MXenes to 700 °C to see what happened to the surface termination groups on the material surface.Scientist Lars-Åke Näslund assembles a sample in a gas cell in a glove bag (an airtight plastic bag filled with nitrogen and equipped with gloves) at the Balder beamline on the 3 GeV ring at MAX IV. Photo credit Martin Magnuson
“If we want to design materials for different applications, we must first know in detail the locations of the different atoms on the surface. The experiment taught us a lot. The surface termination groups initially consist of fluorine and oxygen. Now we know that the fluorine atoms are always in a certain location, while the oxygen atoms are more mobile”, says Martin Magnuson.
As the temperature rises, the surface termination groups start to move, and eventually become completely free at high temperatures. The researchers can reform the material by inserting different gases, such as carbon monoxide (CO) and carbon dioxide (CO2), into the chamber. The molecules in the inserted gas can replace the original surface termination groups.
“We hope to be able to create a surface that functions as a catalyst, facilitating chemical reactions, so that the material can be used for exhaust emission control. We are also hoping to test in situ intercalation of ions for battery applications of MXenes in an experimental electrochemical cell at MAX IV”, says Martin Magnuson.
The researchers are now continuing to analyse the exact positions of the atoms, and comparing the data from the MAX IV experiments with theoretical calculations. They are also helping companies who want to improve their manufacturing methods. They are collaborating in research with Gränges and Seco Tools, to eliminate substances that are harmful for the environment and for people who work in manufacturing. They can in this way make the processes more environmentally friendly.
Linköping University has contributed to the finance of beamlines at MAX IV. The facility itself is a national research facility, financed by the Swedish Research Council, Vinnova, Lund University and Region Skåne.
Update September 30th 2020: A scientific article on research performed at MAX IV has now been published: "Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s x-ray absorption spectroscopy", Martin Magnuson and Lars-Åke Näslund, (2020), PHYSICAL REVIEW RESEARCH 2, 033516 (2020), doi: 10.1103/PhysRevResearch.2.033516
Translated by George Farrants
