In 2010, Researchers at Drexel University developed a 2D material that in comparison to other 2D materials, like graphene, received little fanfare.
Michel W. Barsoum and Yury Gogotsi at Drexel University’s A.J. Drexel Nanomaterials Institute, dubbed the material a “MXene” because of its origin from the process of etching and exfoliating atomically thin layers of aluminum from layered carbides called “MAX phases.” The M is for transition metal, the A for "A group" metal, and the X for carbon and/or nitrogen.
In the decade since that discovery the properties and range of applications for the material have multiplied, making it a key feature in the 2D material landscape.
The A.J. Drexel Materials Institute recently became an institutional member of The Graphene Council so we took that opportunity to ask Yury Gogotsi, Director of the Drexel Nanomaterials Institute some questions about MXene’s and how they are changing the landscape for 2D materials going forward.
Q: First off, can you tell us a little bit about MXenes? I understand the term “MXene” itself is based on its origin from the process of etching and exfoliating atomically thin layers of aluminum from layered carbides called “MAX phases.” [The M is for transition metal, the A for "A group" metal, and the X for carbon and/or nitrogen.] Is that correct? What else should we know about these materials?
A: MXenes (pronounced “maxenes”) are carbides and nitrides of transition metals, a fast-growing and already very large family of 2D materials. In a 2D flake of MXene, n + 1 (n = 1−4) layers of early transition metals (M) are interleaved with n layers of carbon or nitrogen (X, elements in gray in Figure 1), with a general formula of Mn+1XnTx. The Tx in the formula represents the surface terminations, such as O, OH, F, and/or Cl, which are bonded to the outer M layers. MXenes are currently produced by selective chemical etching of aluminum silicon, gallium or aluminum carbide layers form layered ceramics such as MAX phases and related structures. The key features of the archetypical MXenes, such as Ti3C2Tx, include their high metallic conductivity, hydrophilicity and high negative surface charge that allows dispersion in water, forming stable colloidal solutions of single-layer flakes or liquid crystal slurries with rheological behavior of clay with no surfactants or additives. So, they combine the best properties of graphene oxide (GO) and reduced graphene oxide (rGO) and take those to extreme (like 5-10 times higher conductivity compared to rGO films). And since there are so many MXene structures and compositions, their optical, catalytic, electrochemical and other properties can be tuned in a very wide range.
Q: It appears that you first isolated these MXenes in 2010. Within the first few years of your research with these materials, you had already isolated nine different forms of them. How many forms of MXenes have you created at this point? How does each of these forms differ, i.e. range of different properties, range of different potential applications, different synthesis methods, etc.?
A: The first MXene, Ti3C2, was synthesized by Michel Naguib, a PhD student advised by Prof. Michel Barsoum and myself, in 2010 and published on 2011. There are at least 30 stoichiometric MXenes reported so far (from more than 100 predicted) and a dozen of solid solutions. Most of them were first synthesized at Drexel University, but discoveries are being made around the world, in particular Chinese and Swedish researchers contributed significantly to making new MXene structures. Since solid solutions are possible on both, X site (carbonitrides) and M site, an infinite number of compositions can be made. This is very important as one can tune finely properties by “alloying” a particular MXene, just as it’s done with metal alloys. Also, different synthesis methods lead to different surface terminations, which allow further control over the properties. I also expect many other related 2D structures that are different from MXene stoichiometries to be discovered (2D borides, dicarbides, layered carbide/nitride structures, oxycarbides, oxynitrides, etc.).
There are large and very distinct differences in their properties – several orders of magnitude differences in conductivity of MXene films, plasmon resonance across the entire visible and far into infrared range, very different chemical properties determined by the chemistry of specific transition metal in the surface layer. MXenes have a large variety of colors covering the entire visible spectrum offering a potential for many optoelectronic, plasmonic and photonic applications. Very efficient light-to-heat conversion has already attracted attention in photodynamic cancer therapy. Chemically tunable in a very wide range work functions is very valuable for solar cells, light emitting diodes and other optoelectronic devices. Some MXenes have a wide range of electrochemical stability (good for use in supercapacitor electrodes) and some other split water under very low overpotential (good for electrocatalytic water splitting). This is the beauty of having a compositional and structural variety.
Q: An early application for MXenes was thought to be in energy storage. How has that application developed over time? Are there commercial uses of the material for these applications? What other applications are demonstrating potential and has there been interest in developing them commercially?
A: The use of MXenes in batteries was the first application explored because our initial work and the discovery of MXenes at Drexel was funded by the US Department of Energy. A major company has acquired an exclusive license for the use of MXenes in supercapacitors. Bothe applications are very promising and MXenes offer advantages of conductivity exceeding all other electrochemical energy storage materials (high rate/high power advantage) and redox reactions of transition metals. However, those are challenging applications requiring very large volume of material and large-scale commercial production of energy storage devices will probably become economically justified a few years down the road. I expect the initial growth to be driven by smaller-volume applications in conductive films, inks, optoelectronics and medicine, which will increase the availability of the material and push the price down. This will allow applications in energy storage and composites to follow.
Q: MXenes belong to a fairly rich and expanding landscape of 2D materials. What role do you see MXenes playing in this 2D landscape, i.e. a complimentary material with other 2D materials or the basis for new devices on its own?
A: MXenes can perform extremely well in many applications. The key advantage explored so far is their high metallic electronic conductivity, also in transparent films. They are the best available materials for electromagnetic interference shielding or printable 5G and other antennas. However, their metallic conductivity can be combined with semiconducting properties of transition metal dichalcogenides, dielectric properties of boron nitride or oxidation resistance of rGO. MXenes can act as active materials (electrodes in batteries and supercapacitors or gas sensors) but also as current collectors, interconnect or catalyst supports.
Q: Along the lines of the last question, how do you see the world of graphene and 2D materials working out? Currently, graphene has some real commercial markets, primarily in composites. However, other 2D materials seem to have more limited commercial use. Are these 2D materials still looking to take a foothold in electronics applications, or can they compete with graphene in non-electronics applications?
A: Graphene has found large-volume applications in composites largely because strong and conductive multilayer sheets can be produced in quantities by mechanical shearing of natural graphite. Additives to paint for corrosion protection, conductive additives, heat spreaders for cell phones, etc., are among applications where graphene derivatives outperform other materials. In my opinion, industrial applications of graphene will continue expanding. The hype will be over after a few years and applications in composites, sorbents, protective coating, and conductive additives will keep growing in volume. In many of those applications, graphene-based products will replace carbon black, nanotubes or clay in polymer-matrix composites, but unique applications in flexible and wearable devices, as well as printable electronics are expect to emerge. GO and rGO membranes look promising for many separation applications. It will be interesting to see if applications of single-layer CVD graphene will make a difference in technology one day. It’s still not obvious to me, but hope this is going to happen.
TMDs are being widely researched, but except electronic applications, which may still be very far away, there are always other materials that can outperform them in practice (graphene is stronger and cheaper, oxides are more stable, Ti3C2 is more conductive, Mo2C is a better HER catalyst, etc.). No other 2D material is expected to have the same low price as multilayer graphene simply because there are no equally abundant and inexpensive natural precursors for other 2D materials and more expensive synthesis processes are often involved. However, in application in computer electronics, cell phones, sensors or wearable electronics, internet of things devices, the weight of the material used is negligible, so the performance and manufacturability become the key factors. This is where TMDs and other 2D materials may find a foothold. We need to find out which materials can perform better in a particular application, making the devices smaller and adding new functions, and can be manufactured into the desired components. Processing of MXenes from aqueous colloidal solution without any additives or surfactants is a huge plus – you can print, spray- or spin-coat safely, and no burning of the binder/surfactant if needed. Making ink-jet printed patterns with conductivity ten times that of printed graphene and not requiring heat treatment opens many opportunities.
I also look at 2D materials as convenient building blocks. They are like bricks that can be laid in the required order and this can be done by simple solution processing, e.g., spray coating. For example, printable batteries and supercapacitors when layers or 2D materials forming (1) current collector, (2)anode, (3) separator, (4) cathode, (5) current collector, (6) sealant are sprayed sequentially. This is one of the reasons industry will use a variety of 2D materials when building devices in the future.
Q: As an academic, what remains an issue of miscommunication between the research and business communities as it relates to 2D materials? How can this issue (or issues) be overcome?
A: Academics should not oversell new materials that they discovered just because they are so excited about their babies (I feel the same way about MXenes, nanodiamond or carbide-derived carbons that I’ve been exploring), they need to understand where the use of their materials is practical and justified. Yes, it’s hard to expect a researcher like myself to say that while MXene can do a better job than graphene in a certain adsorption application, the company should still go with multilayer graphene or even clay because of a much lower cost. This is the decision for the business to make. On the other hand, the business community and especially investors, should not go after hype (yesterday- nanotubes, today - graphene, or tomorrow - MXene), but after useful properties that enable applications. We also need more dialogue between business and research communities, where inventors of new materials can talk to potential users of those materials and figure out what properties are really needed.
There are clear technological advantages that 2D materials offer. If a micron-thin titanium carbide MXene film processed from water solution can replace a 15-30 micron copper or aluminum foil as a current collector, interconnect, antenna or electromagnetic interference shielding, there is a very obvious technological advantage that can be used in any devices where size and weight reduction is of importance. When the same MXene film replaces a gold or platinum metal electrode in medical technology, there is not only performance, but also a significant price advantage as well. Those are the applications industry should go after.