M ANY FACES OF MXENE A family of 2D materials discovered by mistake is proving useful for everything from conductive inks to wearable electrical devices and water desalination to the current Covid-19 pandemic, as Jon Evans finds out
A doctored image of MXenes made to resemble a nano-volcano
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n science, failure can often be just as stimulating as success. About 10 years ago, Yury Gogotsi, professor of materials science and engineering at Drexel University in Philadelphia, US, was trying to develop silicon anodes for lithium-ion batteries. Graphite is the conventional material used for battery anodes, with lithium ions becoming incorporated, or intercalated, between the graphite layers during charging. But whereas six of the carbon atoms in graphite are required to take in a single lithium ion, each silicon atom can accept up to four lithium ions. This means silicon anodes offer the promise of batteries able to store much more energy. The problem with silicon anodes, though, is that all those lithium ions moving in and out causes the anodes to expand and contract, by up to 300%, which quickly damages them and stops them working properly. Gogotsi and his team were investigating whether a silicon-containing version of a layered material known as a MAX phase might prove less prone to this expansion and contraction. MAX phases are so-called because they comprise repeating layers of a transition metal such as titanium or chromium (M), a metal from groups 13 or 14 of the periodic table, such as aluminium or silicon (A), and a layer of either carbon or nitrogen atoms (X). ‘I came up with an idea of inserting lithium into a MAX phase like Ti3SiC2 and using this layered conductor to replace graphite in lithium-ion batteries,’ Gogotsi recalls. ‘This material contains atomically thin layers of silicon, which can take more lithium per atom compared with carbon, and layers of titanium carbide, which is known as a good electronic conductor.’ Unfortunately, it didn’t work. ‘Initial lithium intercalation experiments were not successful, and so we decided to create space for SIMGE UZEN, DREXEL UNIVERSITY, USA
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MXenes are 2D materials with the general formula Mn+1XnTx, in which up to three layers of carbon or nitrogen atoms (X) are sandwiched between layers of a transition metal such as titanium or chromium (M), while the surface is covered in oxygen, hydroxide and fluorine groups (Tx).
Removal of surface terminations from various MXenes by heating them in a vacuum increases their conductivity by up to a factor of 10.
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lithium by using selective etching and removing monolayers of silicon from the MAX phase,’ says Gogotsi. This involved exposing the MAX phase to hydrofluoric acid to etch away the silicon layers, and then washing the acid and any debris away with water. But what they ended up producing, after some trial and error, was a hitherto unknown 2D material, made up of three layers of titanium atoms separated by two layers of carbon atoms (Ti3C2). In addition, the surface of this 2D material was covered in oxygen, hydroxide and fluorine groups (Tx), derived from the acid and water, known as surface terminations. Intrigued by this discovery, Gogotsi and his colleagues tried the same etching process on MAX phases with different compositions, which produced various other 2D materials, all with the general formula Mn+1XnTx. By failing to develop a novel silicon anode, they had succeeded in discovering a whole new family of 2D materials, which they termed MXenes (pronounced ‘maxenes’). In the wake of graphene, which had already garnered its discoverers a Nobel Prize in Physics, the discovery of a brand new 2D material was always going to create ripples. But it was when Gogotsi and his colleagues began to test the properties of the first MXene, Ti3C2Tx, that they began to realise quite what they had discovered. For a start, this MXene is highly electrically conductive, even more so than graphene, as well as being mechanically stiffer. Because of the surface terminations, it is also soluble in water, unlike graphene, which tends to clump together. As if that wasn’t enough, it is also able to absorb electromagnetic radiation. ‘This is a unique combination of properties,’ says Gogotsi. A combination that has led him and others to explore the use of Ti3C2Tx for an impressively wide range of different applications, perhaps the widest of any 2D material. Due to its high conductivity and large surface area, Ti3C2Tx quickly proved its ability as a supercapacitor material for storing electrical charge. Furthermore, it allowed those supercapacitors to
take on a variety of different guises. For example, because of its surface terminations, Ti3C2Tx readily forms a conductive ink when dispersed in water or organic solvents such as ethanol. In contrast, conductive inks made with 2D materials like graphene and molybdenum disulfide require additives such as solvents or surfactants to stop the materials clumping together and to ensure the ink flows and prints properly. After printing, these solvents and surfactants need to be removed by a special heating step. Together with scientists from Trinity College Dublin in Ireland, Gogotsi and his colleagues showed that these additives aren’t required for conductive inks made from Ti3C2Tx. With a conventional inkjet printer, Gogotsi and his team used their inks to print working versions of various electronic components, including resistors, conductive tracks and supercapacitors, on both paper and plastic (Nature Commun., 2019, 10, 1795). In addition to printed supercapacitors, Gogotsi and his colleagues have also produced knitted supercapacitors from yarn containing Ti3C2Tx. Together with scientists at Deakin University in Australia, they coated natural yarns made from bamboo, cotton and linen with Ti3C2Tx, by simply dipping them in a colloidal solution of the MXene. This solution contained both small and large MXene flakes, allowing the small flakes to bind to the fibres making up the yarns and the large flakes to coat the surface of the yarns, maximising the loading of MXene. The resulting coated yarns were highly conductive and could be turned into textiles with an industrial knitting machine. The coating was also highly robust, allowing the textiles to be washed 45 times at temperatures up to 80°C without losing their conductivity. These MXene yarns could thus be used to produce wearable electrical devices, which Gogotsi and his team demonstrated by producing a supercapacitor from two coated cotton yarns and knitting a pressure sensor from coated bamboo yarns (Adv. Funct. Mat., 2019, 29, 1905015). As this shows, Ti3C2Tx is perfectly amenable to being combined with other materials, which can make its properties even more impressive than they already are. The etching process
that produces Ti3C2Tx from its parent MAX phase usually does so as lots of flakes, each comprising multiple sheets. These flakes can be forced apart into their individual sheets by intercalating various molecules between them, but often they are used just as they are, as was the case with the conductive inks and yarns. The flakes can also be used to produce conductive films, but these films tend to be quite weak, with a mechanical strength of around 30MPa. To increase their strength and flexibility, a team of scientists from Sweden and the US, led by Mahiar Hamedi at the KTH Royal Institute of Technology in Stockholm, decided to try combining the flakes with nanofibrils of cellulose, the main structural component in plants. Hamedi and his colleagues had already tried combining cellulose nanofibrils with graphene and carbon nanotubes to produce conductive nanocomposites that were tough and flexible, but thought that Ti3C2Tx might offer certain advantages. ‘We chose MXenes to overcome some of the shortcomings that we had experienced with these other nanomaterials, mainly to achieve higher conductivity and pseudo-capacitance, but also having a material with more well defined, homogeneous geometries at the nanoscale,’ Hamedi explains. They were able to combine the two materials by simply mixing solutions of Ti3C2Tx and cellulose nanofibrils, finding that the two formed strong interactions that caused them to bind together, with the cellulose nanofibrils essentially entangling the Ti3C2Tx flakes. This produced a nanocomposite with a strength more than 10 times greater than the Ti3C2Tx film, at 341MPa. Using dispersions of this nanocomposite, Hamedi and his colleagues were able to print a robust, flexible supercapacitor (Adv. Mat., 2019, 31, 1902977). As a 2D material, Ti3C2Tx is also being combined with other 2D materials to produce novel heterostructures with useful properties. ‘We can use 2D materials like bricks in the wall, combining nanometer and subnanometer-thin sheets with metallic, dielectric and semiconducting properties to generate artificial materials and assemble entire devices,’ says Gogotsi. For example, a team of chemists from the US and Taiwan, led by
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They also represent the applications for just a single MXene. Ti3C2Tx may have received most of the attention so far, being the subject of around 70% of studies into MXenes, but scientists have predicted there could be hundreds of different MXenes, many of which have already been synthesised. For not only can different MXenes be produced with different transition metals, but two different transition metals can exist in a single MXene, either forming separate layers or mixed layers. And these different MXenes are likely to have a range of different properties, which could prove of use for a whole new set of applications, especially as many transition metals are catalytic. On top of this, there are also several ways to alter the properties of existing MXenes. One option is to modify the surface terminations, by either binding other molecules to them or getting rid of them entirely. When Gogotsi and his colleagues removed the surface terminations from various MXenes, including Ti3C2Tx, by heating them in a vacuum, they found that this increased their conductivity by up to a factor of 10 (Nature Commun., 2019, 10, 522). Another option is to intercalate molecules between individual MXene sheets. In the same Nature Commun. study, Gogotsi and his colleagues tried intercalating molecules of water and tetrabutylammonium between individual MXene sheets. They found that this reduced their conductivity, causing them to behave more like semiconductors. Intercalation can also influence other MXene properties, as demonstrated by a team of Chinese and German researchers led by Haihui Wang at South China University of Technology. They were investigating using Ti3C2Tx to produce a novel membrane for water desalination, by depositing it onto a polymer film. Previous research had shown that MXenes can make effective membranes for various applications, including gas separation and water purification, but they tend to swell in aqueous solutions, hampering their performance. Wang and his team were able to greatly reduce this swelling by simply intercalating aluminium cations between the MXene sheets (Nature Sustainability, 2020, 3, 296). Strong interactions between the cations and
We can use 2D materials like bricks in the wall, combining nanometre and subnanometre-thin sheets with metallic, dielectric and semiconducting properties to generate artificial materials and assemble entire devices. Yury Gogotsi professor of materials science and engineering, Drexel University, Philadelphia, US oxygen groups on the MXene surface essentially held the sheets together, preventing water molecules from forcing them apart. The potential applications of MXene could even extend to the current Covid-19 pandemic, thanks to a Ti3C2Tx membrane developed by Gogotsi and his colleagues for filtering urea from blood during dialysis (ACS Nano, 2018, 12, 10518). ‘It is estimated that 30-40% of people who are hospitalised with Covid-19 have evidence of kidney injury that could require dialysis,’ says Gogotsi. ‘Based on our preliminary data, we believe that MXenes have the potential to address a key limitation of current ambulatory dialysis systems, offering efficient urea adsorption, small size and light weight.’ As a final irony, MXenes might end up providing a solution to the problem Gogotsi was trying to solve in the first place. Together with scientists from Trinity College Dublin, Gogotsi and his colleagues have recently combined silicon with Ti3C2Tx to produce a battery anode with high energy storage that can withstand repeated expansion and contraction without degrading (Nature Commun., 2019, 10, 849). This is because the Ti3C2Tx provides sufficient free space to accommodate the volume change and also helps to stabilise the silicon. A good example of failure breeding success if ever there was one.
Natural yarns coated with MXene Ti3C2Tx are highly conductive and have been turned into textiles that could be washed 45 times at 80°C without losing their conductivity.
A film comprising 24 layers of Ti3C2Tx, with a thickness of just 55nm, could produce 99% shielding from electromagnetic interference.
SIMGE UZEN, DREXEL UNIVERSITY, USA
Lia Stanciu at Purdue University, combined Ti3C2Tx with tungsten diselenide (WSe2) to produce a sensor for detecting volatile organic chemicals (VOCs). WSe2 is a type of 2D material known as a transition metal dichalcogenide (TMD), in which a layer of tungsten atoms is sandwiched between two layers of selenium atoms. Stanciu and her colleagues had tried using both Ti3C2Tx and a related TMD, molybdenum diselenide (MoSe2), as VOC sensors on their own. When VOCs such as ethanol bind to the surface of these 2D materials, they produce a detectable change in electrical conductivity. But neither 2D material was ideal – MoSe2 wasn’t sufficiently conductive while Ti3C2Tx didn’t bind VOCs well enough – so Stanciu and her colleagues decided to combine them. ‘We came up with the idea of trying to take advantage of the abundant active sites for gas adsorption of the 2D dichalcogenides and the higher conductivity of MXenes,’ explains Stanciu. Their idea worked, with the combination of Ti3C2Tx and WSe2 proving 12 times more sensitive at detecting VOCs than either 2D material on its own (Nature Commun., 2020, 11, 1302). They were also able to produce their combined 2D sensor by again simply mixing solutions of the two 2D materials together and then depositing them with an inkjet printer. Due to its ability to absorb electromagnetic radiation, Ti3C2Tx is also being investigated for electromagnetic interference shielding. As the transistors and electrical components on computer chips shrink down to the nanoscale, there is increasing need to be able to shield them from damaging electromagnetic radiation with nanoscale materials. Together with a team of Korean researchers, Gogotsi and his colleagues recently showed that a film comprising 24 layers of Ti3C2Tx, with a thickness of just 55nm, could produce 99% shielding (Adv. Mat., 2020, 32, 1906769). These electronic applications are already beginning to attract commercial interest. ‘Murata, a leading Japanese manufacturer of electronics, has already scaled up MXene production to kilogram quantities,’ says Gogotsi.
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