MXenes, a family of two-dimensional materials forming layered structures analogous to stacked potato chips, is arousing a vortex of worldwide activities due to their outstanding performance in versatile applications: batteries, supercapacitors, catalysts, electronics, optics and more. An increasing number of scientists are joining the research team led by Professor Yury Gogotsi at Drexel University in Philadelphia, PA, USA. The Drexel team reported the first MXene in 2011. However, the events leading up to the year of 2011, that is how the first MXene was discovered, remained uncharted until I personally meet Professor Gogotsi at the Spring 2017 MRS meeting in Phoenix, AZ, USA.
[Tianyu Liu with Professor Yury Gogotsi (right) at the Spring 2017 MRS meeting in Phoenix, AZ]
Me: First I really want to thank you for your time and willingness to share the story about the discovery of MXene. Let’s begin with your MXene story. Y.G.: As you probably know, I have been working for a long time on selective extraction of metal from metal carbides to make carbide-derived carbons. Specifically, I have been working on making graphene from various low-dimensional structures such as silicon carbide. This process involves removal of silicon to yield the two-dimensional materials on surface. A typical example was published in 2006. We showed in this paper that by immersing 3C polytype of silicon carbide in a mixture of HF and nitric acid, we could selectively etch some of the silicon carbide and leave lamellar silicon carbide with 5-10 nm in thickness behind. It is a process with high selectivity. MXene synthesis is the continuation of selective elimination to produce new materials. When I joined Drexel University in the year of 2000, I started collaboration with Prof. Michel W. Barsoum, who is the father of the entire family of ductile and machinable ceramic materials known as MAX phases, where M is a transition metal, A is mostly a group 13 and 14 element of the periodic table (e.g., Si and Al) and X is C or N. Knowing that MAX phases possess layered structures, I asked Michel if it was possible to engineer them into single layers or nanotubes. He said no. It’s impossible because unlike graphite that depends on weak Van der Waals force binding adjacent layers, MAX layers are held together by strong metallic or covalent bonds. Hence the exfoliation strategies which are applicable for producing graphene from graphite are ineffective for producing layered MAX sheets from MAX bulk phases. I had to leave this idea for a while. At that time, many researchers started to work on silicon (Si) anodes for lithium-ion batteries. We also joined this trend but I had my own an idea associated with the MAX phases, specifically titanium silicon carbide (Ti3SiC2). I envisioned that it would be a promising anode material for lithium-ion batteries because of the three factors. First, it is electrically conductive. Second, it contains Si and Si has been shown to exhibit very high capacity for lithium ion storage. Third, it possesses a layered structure like graphite which is a typical configuration for the anode material. I conceived this idea and I asked one of my students, Murat Kurtoglu, to do density functional theory (DFT) calculations to evaluate the potential performance of Ti3SiC2. His calculation results showed that the energy barrier for lithium ion intercalation into the compound wasn’t very high so lithiation of Ti3SiC2 should be possible. Later we submitted a proposal to the Battery for Advanced Transportation Technologies (BATT) program hosted by US Department of Energy (DOE), suggesting that we would employ MAX compounds as anode materials for batteries. It turned out that DOE approved our proposal so we obtained the funding. The first student who worked on my proposed project is Michael Naguib. He is the one who discovered MXenes. He carried out the experiments as we designed, but the capacity of the electrode was extremely small. He didn’t give up and tried several more times, but again, no impressive results. We were disappointed, but in the meantime, we brainstormed possible ways to address this challenge. We eventually reached the point after rounds of discussions, that we need to provide diffusion channels for lithium ions to intercalate layers of MAX, and based on my previous experience in selective etching, we came up with the etching method. To begin with this idea, we made a list of possible etchants we could think of and started to try one by one. Fluorine gas, hydrofluoride gas, and some molten salts are among the various etchants we tested. All the attempts were not successful in terms of etching. The only product Michael obtained was a new cubic phase in the Ti-C-O-F system and we were not satisfied. Progress was made when Michael selected aqueous HF to etch not Ti3SiC2, but Ti3AlC2. By immersing Ti3AlC2 in aqueous HF solutions, the Al atoms were removed and left Ti3C2, which is the first MXene discovered behind. In parallel, Murat Kurtoglu performed DFT calculations and confirmed that the yielded Ti3C2 was a stable form, and he also predicted that it should be metallic. We then carried out X-ray photoelectron spectroscopy and X-ray diffraction to investigate the structure of Ti3C2. In addition, Michel Barsoum asked his Swedish colleagues to do high-resolution transmission electron microscopy and proved the 2D layered structure of Ti3C2. We therefore published the paper in Advanced Materials, announcing our discovery of the first 2D MXene. Afterwards, Michael managed to synthesize a dozen MXenes and included corresponding works in his PhD thesis. Then, researchers from all over the world started to follow our works. 2D layered materials derived from MAX or non-MAX phases were not predicted to exist before our discovery. Without Michael’s persistence in working on the seemingly impossible project, we would not have discovered the MXenes. I cannot imagine when the MXene phases will be identified. At this moment, we are excited to publish a paper a month ago with collaborators from Singapore that there are more than millions of stable MXene compounds remaining to be discovered. These compounds will open up new opportunities in chemistry, material science, batteries, supercapacitors, optics, electronics, catalysts, medical applications and you name it. I am excited to witness such an important family of materials is emerging. Me: You mentioned that MAX phases were not ideal lithium-ion battery electrode materials. Can you please elaborate on this point? Y.G.: MAX phases are electrically conductive, so electrical conductivity shouldn’t be a problem. The problem is the bonding between adjacent layers is too strong to be broken and to host guest ions. We showed in ACS Energy Letters in 2016 that it is possible to intercalate MAX phases if the size of the intercalants is small to fit into the channels of MAX phases. But we are still on our way to fully elucidate the mechanism of the interactions between MAX phases and lithium ions. Does it involve the intercalation process or related to conversion reactions? We don’t know at this time. Me: You said your team tried gaseous hydrofluoride to remove silicon inTi3SiC2 but it did not work efficiently. Any ideas about the reason? Y.G.: Again, it is an issue we don’t fully understand. Several possibilities include: 1) other elements of the MAX phases are etched faster or about the same rate as Si so the selective etching is not applicable; 2) a passive layer formed on surface. For example, transition metals react with HF and yield metal fluorides. But if the formed fluorides are non-volatile, they can easily encapsulate the entire surface and prevent it from being further etched. We hypothesize that this reasoning may be valid not only for Ti3SiC2, but also for the Al-containing MAXs or the Sn-containing MAXs: Intuitively one would believe basic aqueous solutions such as sodium hydroxide solutions should be able to etch the Al or the Sn layers, but the reality is that basic solutions are still ineffective for selective etching. On another hand, we are developing new etching methods together with our external collaborators. One of my students has found that a certain molten salt is capable of selective etching of MAXs under certain optimal conditions including specific composition of the molten salt and etching temperature. But again we are not quite sure about exactly the etching mechanism. Things that are intuitively possible judging by basic chemistry could be limited by thermodynamics, kinetics, and formation of passive layers or other unknown reasons. Questions outnumber answers at this moment. Me: You mentioned that the discovery of MXenes was motivated by battery-related research. As a researcher who works on supercapacitors, I am interested to know how your team employed MXenes as supercapacitor electrodes. Y.G.: It is a story with another student, Maria Lukatskaya. She was working on carbon-metal oxide composite supercapacitor electrodes when we discovered MXenes and started numerous works related to MXenes in my group. Actually, many other students in my group wanted to do something using MXenes. Michel called it the “MXene Vortex” that sucked everyone around. Maria mentioned to me that she would like to test the capacitive performance of MXene, because we had already known that ion intercalation was possible for MXenes. At that time I was not quite optimistic about her idea because I knew MXenes did not have very high surface area for charge storage and the performance should not be promising. But I let her do her project. She tested several MXene samples and calculated their capacitance in aqueous electrolytes. The gravimetric capacitance (capacitance normalized to unit mass of material used) values were from single digits in farad per gram (F/g) to maximum around one hundred F/g. Although we did not expect to get very high capacitance, the very first experiment Maria did presented promising capacitance which exceeded 100 F/g. More significantly, the volumetric capacitance (capacitance normalized to the volume of material used) exceeded 300 F/cm3, much higher compared to commercial activated carbons (60-80 F/cm3 in organic electrolytes). The outstanding performance was even comparable to the best performance achieved by graphene supercapacitor electrodes. This research was published in Science in 2013 to state that MXenes were promising supercapacitor electrodes. Then a year later, we pushed the volumetric capacitance to 900 F/cm3. The ultrahigh volumetric capacitance can be attributed to pseudocapacitance correlated to the surface redox reactions on the surface of MXene. Afterwards, research started to branch out. Our research on MXenes is now guided by modern simulations. When we first synthesized the first MXene, we almost concurrently performed the DFT calculations to confirm the structural stability of the prepared MXene. In the next year, there was almost no experimental report on the synthesis of MXene, yet papers on theoretical studies of MXenes appeared rapidly. At this time, modelling has served as the guide for experimental studies on MXenes to explore functions beyond lithium ion batteries, such as magnesium-ion batteries, magnetics, optics and superconductivity. Theoretical studies present wonderful predictions that avoid physically exhausted examinations of every individual MXene phase and thus save us a tremendous amount of time and effort. As you can see from my publications, we published papers with only my students before, but now, we often publish papers with other groups of experts in theoretical simulations to predict material properties and to interpret experimental observations. Combination of theoretical simulations with experimental works leads to the best results and helps to achieve fast progress. Me: If people call you “the father of MXene”, will you agree? Y.G.: I would not accept the title because the discovery of MXenes is a result of team effort. Michael Naguib, Michel Barsoum and I all contributed to this discovery. The discovery would probably not happen if any of us was missing in this game. Other students and post-doctoral researchers from my group – Murat Kurtoglu, Min Heon, Junjie Niu and Volker Presser, helped us to understand the structure and chemistry of the reaction products. We are a Drexel MXene team, and we are excited to see that this team is expanding. Collaborators from places all over the world: France, China, Korea, Sweden, and other countries are joining our efforts. Collaboration is the trend of modern science research topics, especially for those that are not explored before, which are more complex than ever and need teamwork to propel. Me: So it is a victory of teamwork. Let’s talk about something beyond your research. What pieces of advice you would give to graduate students who are at their early stage of research? Y.G.: First, try to identify things that attract you. If you are doing something you really enjoy, it’s likely that you will do well and eventually succeed. Second, you need to be self-motivated. I often told my students: “I’m not going to look over your shoulder and babysit. I can provide you the research directions; I can help you troubleshoot and solve problems, so you will get a lot of freedom. But giving you freedom means you need to think independently, to read and analyze literature, and to make decisions on your own.” I strongly believe that my students must grow as independent researchers, but not technicians. Third, I am lucky enough to recruit a number of talented and dedicated students. My lab has a culture that I believe plays an important role in my student’s excellence. Students must understand that they need to do world-class research as well as something that they are excited about, something they are willing to do, and something that needs their time and efforts to explore – even those ideas which may not lead to great results in the end. References [Literature citations were added after the conversation to guide readers to relevant publications]  Cambaz, G. Z.; Yushin, G. N.; Gogotsi, Y.; Lutsenko, V. G., Anisotropic Etching of SiC Whiskers. Nano Letters 2006, 6, 548-551.  Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y., 2D metal carbides and nitrides (MXenes) for energy storage. Nature Reviews 2017, 2, 16098.  Naguib, M.; Presser, V.; Tallman, D.; Lu, J.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W.; Zhou, Y., On the Topotactic Transformation of Ti2AlC into a Ti-C-O-F Cubic Phase by Heating in Molten Lithium Fluoride in Air. Journal of the American Ceramic Society 2011, 94 (12), 4556-4561.  Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W., Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Advanced Materials 2011, 23 (37), 4248-4253.  Tan, T. L.; Jin, H. M.; Sullivan, M. B.; Anasori, B.; Gogotsi, Y., High-Throughput Survey of Ordering Configurations in MXene Alloys Across Compositions and Temperatures. ACS Nano 2017, 11 (5) 4407-4418.  Xu, J.; Zhao, M.-Q.; Wang, Y.; Yao, W.; Chen, C.; Anasori, B.; Sarycheva, A.; Ren, C. E.; Mathis, T.; Gomes, L.; Zhenghua, L.; Gogotsi, Y., Demonstration of Li-Ion Capacity of MAX Phases. ACS Energy Letters 2016, 1 (6), 1094-1099.  Urbankowski, P.; Anasori, B.; Makaryan, T.; Er, D.; Kota, S.; Walsh, P. L.; Zhao, M.; Shenoy, V. B.; Barsoum, M. W.; Gogotsi, Y., Synthesis of two-dimensional titanium nitride Ti4N3 (MXene). Nanoscale 2016,8, 11385-11391.  Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall'Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y., Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341(6153), 1502-1505.  Ghidiu, M.; Lukatskaya, M. R.; Zhao, M.-Q.; Gogotsi, Y.; Barsoum, M. W., Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance. Nature 2014, 516, 78-81.  Lukatskaya, M. R.; Bak, S.-M.; Yu, X.; Yang, X.-Q.; Barsoum, M. W.; Gogotsi, Y., Probing the Mechanism of High Capacitance in 2D Titanium Carbide Using In Situ X-Ray Absorption Spectroscopy. Advanced Energy Materials 2015, 5, 1500589.
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