28 June 2021

[Materials] – MXenes – game changers in the materials field

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MXenes move nanotechnology from using a few “wonder materials” to manipulation of hundreds and even thousands of 2D building blocks to assemble designer materials and devices. Those materials can contribute to solving the key problems in the fields of energy (generation, harvesting, storage), water (desalination, purification), food (longer storage, sensing of degradation/bacteria), environment (clean air and water) and medicine (treatment, diagnostics, artificial organs).
From the Stone Age to the Silicon Age, humans crafted tools by carving them out of large pieces of materials, such as bone or single-crystalline silicon. In the nanotechnology era, scientists and engineers began creating–atomic layer by atomic layer–materials, building blocks, and even entire devices that offer new and unique combinations of properties required by new technologies, from renewable energy harvesting to quantum computing.
Two-dimensional (2D) nanomaterials, such as graphene, have just one to a dozen of atoms in cross-section. With the layer thickness of about a billionth of a meter (hundred thousand times thinner than a human hair), these materials show unusual physical and electronic properties and are becoming building blocks for advanced technologies of the future, including flexible and transparent electronics for Internet of Things (IoT) and wearable gadgets, conformal batteries, printable antennas, sensors and electrodes for epidermal electronics, water desalination and gas separation membranes, etc.
Among 2D materials discovered to date, carbides and nitrides of transition metals, known as MXenes, occupy a special place. The majority of 2D materials are dielectrics, semiconductors, or semi-metals. The famous graphene, which is often praised for its high electronic conductivity, is in fact a zero-band-gap semiconductor, not a metal.MXenes added a large number of truly metallic 2D structures to the family, closing the huge gap (lack of metallic 2D materials with high conductivity and high strength) which was limiting the development of devices and systems made entirely of 2D materials. Moreover, they are water-soluble metals – like metallic clay, which allows their environmentally-friendly processing from aqueous solutions/slurries with no surfactants, binders or additives.
MXenes are made by selective etching of bulk ceramics. Titanium carbide (Ti3C2) was the first MXene discovered at Drexel University and published in 2011. Ten years later, there are more than 50 different experimentally made MXene compositions with an infinite number of compositions theoretically possible. This enormous variety of MXene structures and compositions opens a new era in the design and discovery of materials.
Many MXenes are made of abundant elements such as titanium, carbon and/or nitrogen, which should eventually allow large-volume and low-cost manufacturing. Of hundreds of 2D materials reported, only a handful (graphene, boron nitride, molybdenum disulfide and a few others) can be produced in amounts allowing their use beyond microelectronics. Since MXenes are made by etching common ceramics in acidic solutions or molten salts, large-volume production is possible.To mark the tenth anniversary of the discovery of these materials, researchers from Drexel University, the birthplace of MXenes, in the USA and Linköping University in Sweden, have published a forward-looking review article in Science magazine (“The World of Two-Dimensional Carbides and Nitrides (MXenes)”), discussing fundamentals, properties, and applications of MXenes, the progress in the field, ways to overcome the hype accompanying new discoveries, as well as to address challenges related to synthesis, scale-up, and commercialization of these materials. Publication of a review of MXenes in Science is a clear statement of the impact and promise of this material family.
MXenes can also be combined with graphene and other 2D materials. Assembly of hybrid materials with combinations of properties that no conventional (single) material can provide will allow: (i) Building hybrid and composite materials by self-assembly or additive manufacturing techniques; (ii) Development of extremely anisotropic materials being, e.g., thermal or electrical insulator out of plane and conductor in plane; (iii) Device assembly without waste; (iv) Creation of complex shapes and integration of 2D, 0D, and 1D materials into 3D materials and structures with unique properties.
Because of their diverse structures and compositions, as well as unique electrochemical, physical, and optoelectronic properties that they offer, they are expected to find numerous applications.
MXenes are mainly metallic conductors and have been widely used in devices ranging from supercapacitors and batteries to antennas, water purification, dialysis, and sensors. Many MXenes are biocompatible, non-toxic and environmentally friendly and therefore are being explored in biomedical applications.A few of the most promising applications include the use of MXenes to build wearable kidney and free millions of people from stationary dialysis machines, improving their quality of life and saving lives of people where there is no access to dialysis facilities.
Another potential medical application includes replacement of epidermal electrodes made of platinum, gold or silver. In electronics and communication, electromagnetic interference shielding (titanium carbide and carbonitride MXenes outperform all other materials, including bulk metals) and thin-film and printed antennas for 5G communication and beyond.
The world of 2D metal carbides and nitrides has grown at an unprecedented rate, but many exciting properties and applications of these materials are yet to be discovered and demonstrated. Therefore, we can expect to see the pinnacle of MXene research in the next few years with wide commercialization and use of these materials.

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