Discussing MXenes with Yury Gogotsi

Yury Gogotsi is a pioneer of the burgeoning field of 2D MXenes. Here he offers his insight on the history of MXene development, promising applications and what he is excited about.

Credit: Image courtesy of Yury Gogotsi

Tell us about your scientific background and research interests.

I’m a chemist and materials scientist. I received my MS in metallurgy and PhD in physical chemistry from Kyiv Polytechnic, where I studied high-temperature properties of carbide and nitride ceramics and became the youngest chemistry PhD in Ukraine at that time. I also obtained extensive post-doctoral training as a Humboldt Fellow at the University of Karlsruhe, JSPS Fellow at the Tokyo Institute of Technology, and NATO/NRC Fellow at the University of Oslo, before joining the University of Illinois in Chicago as Assistant Professor in 1996. Since 2000, I’ve been working at Drexel University, where I founded the A.J. Drexel Nanomaterials Institute (DNI).

My research over the past three decades focused on the discovery and development of new inorganic nanomaterials, such as carbide-derived carbons, conical and polyhedral graphite crystals, surface-modified nanodiamonds, and 2D carbides and nitrides (MXenes). Together with my students, we reported the first carbon nanotubes free of amorphous carbon, made the first multi-wall nanotubes by hydrothermal synthesis, and conducted the first electron microscopy studies of water trapped inside carbon nanotubes. Materials with controlled spaces (pores) produced by selective extraction of metals from carbides allowed an improved understanding of electrolytes in confinement and transformed the field of electrochemical capacitors. Materials that we developed impacted many fields, ranging from tribology to electromagnetic interference shielding.

Please can you give us a brief background of your early work on MXenes and what lead to the discovery of the first 2D MXene in 2011?

Serendipity played a major role in MXene discovery. My group worked on developing carbon-based materials for electrochemical energy storage by selective etching (extraction) of metals from carbides, including MAX phases, forming carbide-derived carbons, nanometer-thin SiC flakes, graphene, and carbon nanotubes. After an unsuccessful attempt to fill carbon nanotubes with silicon for developing Li-ion battery anodes with a higher energy density than graphite, I looked for alternative ways to encapsulate Si and considered using a well-known MAX phase, Ti₃SiC₂, developed and studied in the lab of my Drexel colleague Professor Michel W. Barsoum, as an anode material. It comprises layers of Si, known to alloy with Li, interleaved with layers of electronically conductive titanium carbide. Preliminary DFT calculations conducted by one of my PhD students showed promise, and together with Michel we secured funding from the US Department of Energy, which was searching for alternative Li-ion battery anode materials beyond graphite, particularly Si-based ones. However, during the initial screening of MAX phases as battery anodes, we found that their experimentally measured Li-ion capacity was very low. To decrease the strain resulting from the initial insertion of Li ions into a very rigid MAX phase lattice, we explored several selective etching approaches to remove (at least from the surface layer) Si and then other A elements from MAX. After multiple unsuccessful attempts, Michael Naguib, the PhD student who worked with us on this DOE project, tried a concentrated HF solution, which ultimately etched the Al layer from Ti₃AlC₂, leaving layers of Ti₃C₂ with O, OH, and F terminations. You can find a full description of this discovery in the Introduction to our first MXene book (2D Metal Carbides and Nitrides (MXenes): Structure, Properties and Applications, B. Anasori and Y. Gogotsi, eds. (Springer, 2019)).

How do MXenes compare to the other 2D materials?

2D materials possess physical phenomena that differentiate them from their 3D counterparts, and they are at the forefront of nanotechnology. 2D material space greatly expanded with the discovery of MXenes, a new large family. Their composition is M_n+1X_nT_x, where M is an early transition metal, X is C and/or N, n = 1-4, and T_x stands for surface terminations (e.g., O, OH, halogens, chalcogens, etc.). Well over 100 distinct M_n+1X_n chemical compositions with in-plane or out-of-plane ordering possibilities on the M sites have been predicted, and about 50 have already been produced. Adding known surface terminations, one goes over a thousand compositions. Together with possible mixed terminations and solid solutions on M and X sites (dozens already reported, including high-entropy 2D structures), the permutations are practically infinite. Due to their unique structure and surface terminations, MXenes, unlike most other 2D materials, do not have bulk analogs when restacked.

This richness of chemical and structural diversity, in turn, allows for unprecedented property tunability. Moreover, not just discrete but continuous tuning when using solid solutions, as already demonstrated for plasmon resonances and optical properties. The electrical conductivity of a given MXene composition can be tuned from metallic to semiconducting to superconducting by varying its surface terminations. Similarly, the work function changes from ~2 eV to ~6 eV by changing the surface terminations from hydroxyl to oxygen.

Because their chemistry is distinct from other 2D materials, MXenes offer unusual combinations of properties. Many of them can be considered as hydrophilic 2D metals that can be dispersed in water and processed from aqueous colloidal solutions or liquid-crystalline suspensions with no additives. This robust metallic conductivity, coupled with scalable synthesis and, for the most part, earth-abundant elements such as titanium, has been the key to MXenes’ usefulness in a wide range of applications. The latter span from electrode materials and passive components for high-power electrochemical energy storage devices, electromagnetic interference shielding and antennas, to kidney dialysis, water purification and desalination, chemical catalysis and electrocatalysis, gas and pressure sensing, photothermal cancer therapies, conductive and reinforcing additives for ceramic, metal and polymer composites, lasers and LEDs, solid lubricants, implantable electrodes, interconnects for circuits and photodetectors, among many others. Moreover, they allow modulation of metallic properties, which has not been possible with conventional metals. In addition to attractive and often unique optoelectronic and electrochemical properties, they also offer extreme mechanical properties. The strength of multilayer MXene “paper” manufactured from the dispersion of large (5-10 microns) delaminated flakes in water at room temperature is about the same as that of aluminum foil. However, when hybridized with cellulose nanofibers, it can reach the strength of steel, and twice that value with graphene. This opens the opportunity to create multifunctional materials that will shape future technologies, from robotics to space travel.

What are the major gaps in our understanding of MXenes or in their capabilities?

Gaps are almost everywhere, and this is exciting because there is still so much to discover in this field! I could never understand researchers who stay in a very crowded field, working on minor problems left after decades of research and competing with thousands of others to make a small evolutionary step. We are just scratching the surface of this very rich 2D materials field. Control of surface chemistry, defect engineering, and demonstration of theoretically predicted ferromagnetic, thermoelectric, or fascinating quantum properties of MXenes offer great opportunities for exciting fundamental research with major practical implications. With such a huge number of materials in this family, there is plenty of room for discovery!

What are the most promising applications for MXenes and what are the limiting factors to their wider implementation?

I’m not really in the business of forecasting or fortune-telling. However, many believe that electromagnetic interference shielding is currently the most promising application for MXenes due to their extraordinary ability to reflect or absorb (depending on the MXene type and film architecture) microwaves. As a result, thinner and lighter (compared to metals) films and coatings can shield electronics, equipment, and people. This looks like a low-hanging fruit, and the market is ready. Moreover, MXenes can provide shielding across a very broad range of frequencies, from radio waves to terahertz and infrared radiation. I also expect that conductive MXene inks will be very widely used in printed electronics – they offer a variety of colors and attractive optoelectronic properties. In epidermal electronics, which is the future of medical diagnostics and health monitoring, MXenes outperform gold because of lower impedance with skin and tissue. Heat shielding properties of MXenes cover a very wide range, but a combination of low infrared emissivity with very low thermal conductivity in Ti₃C₂ may become the biggest game changer in energy conservation. Submicrometer-thin sprayable thermal insulation that can be applied from an aqueous dispersion to building walls, cars, and clothes may save trillions of dollars in heating and air conditioning costs, and greatly reduce heat emission into the environment. Medical applications in photothermal therapy, tissue engineering, kidney dialysis, and others may take longer to develop, but this does not make them less important. The tunability of properties of these materials via atomistic design makes MXenes versatile and opens an extremely wide range of potential applications.

Of course, many of those applications require large production volumes and a much lower cost of the material compared to what we have in the current low-volume batch synthesis. On the bright side, selective etching processes used nowadays to produce MXenes for research are fundamentally scalable – the volume of the reactor is the only limiting factor. Their solution processing is similar to graphene oxide or layered double hydroxides – both have already been scaled to industrial volumes. The recently published fluidized bed synthesis of Ti₂CCl₂ MXene from titanium chloride and natural gas can potentially be scaled to thousands and even millions of tons per year, as it is similar to the so-called “chloride process” for the synthesis of titania, or manufacturing of multiwalled carbon nanotubes. Add in the abundance of elements that most MXenes are made of (Ti, C, N, O, H, Cl, etc.), and the economics will work.

What are you most excited about?

About the future. I am convinced that we are moving into an age of assembled and programmable multifunctional nanomaterials. This change will be similar to moving from the Bronze Age to the Iron Age or even bigger because we will erase the border between structural and functional materials. We won’t depend on the properties of single materials any longer. Guided by artificial intelligence, we’ll combine nanoparticles to produce materials that have combinations of properties needed for specific applications that couldn’t be achieved before – such as the high strength and electronic conductivity of metal combined with the ionic conductivity of gel and ultralow (<1 W/mK) thermal conductivity of a porous ceramic. Those properties are no longer mutually exclusive, and we can assemble those materials from solution (in a beaker). Take the already demonstrated MXene films assembled at room temperature which have the strength of metal. They can be combined with nanocellulose or graphene to improve strength and ionic conductivity even further. A material with the strength of steel and extraordinary thermal shielding/insulation ability can be used to build light and strong houses (or planes and spaceships) with thin walls, which can be used for living in extremely hot or cold environments, from Arizona to Mars, as they can maintain temperature gradients of hundreds of Kelvin. Moreover, they can provide radiation protection. Since these assembled materials conduct ions in addition to electrons, they can be used for electrochemical energy storage or as strong but flexible electrochemical actuators, changing their size and shape when ions move under applied potential. Imagine lightweight electric airplanes with the fuselage working as a structural battery and reconfigurable wings enabled by electrochemical actuation. Another example is unmanned aerial vehicles with flapping and shape-morphing wings that fly like birds, not like the current drones. One more example would be electrochemically switchable soft-to-hard robots or exoskeletons for disabled people that use less than 1 V for actuation and don’t require heavy electric motors. MXenes provide important building blocks for those and many other futuristic technologies.

On a final note, many researchers from your lab have taken up academic positions and continue to study MXenes and many other materials – you must be very proud.

Yes, I am proud of them all. Our main product at DNI is people who we teach and train to do research. Three students and postdocs who co-authored the first MXene paper, are now tenured faculty members at US (Michael Naguib and Junjie Niu) and German (Volker Presser) universities. Many others have taken up academic positions in the US and abroad, including some of the world’s best universities such as Princeton, Georgia Tech, Purdue, Rutgers, ETH Zurich, TU Delft, Indian Institute of Technology, Fudan etc. The latest addition to academia is Ruocun (John) Wang who joined the University of North Texas a month ago. Many of our alumni continue researching MXenes. Some of them already send their former students to my lab for post-doc training. Others invite me to speak in symposia that they organize. They frequently win national and international awards for their work. Just a few days ago, I was notified that my former post-doc, Babak Anasori (tenured Assoc. Prof. at Purdue) was awarded a Kavli Foundation Early Career Lectureship in Materials Science. However, I’m equally proud of our alumni who selected careers in industry or national labs. I was glad to see more than 20 of our lab alumni in the latest (2023) Stanford List of top 2% of the most cited researchers in the world. They make a real impact, and this generation of researchers will shape the future of materials science and engineering.

The interview was conducted by John Plummer, Chief Editor of Communications Materials.