Researchers in the US and Korea reported the first efficient flexible light-emitting diodes with a two-dimensional titanium carbide MXene as a flexible and transparent electrode. This MXene-based light-emitting diodes (MX-LED) with high efficiency and flexibility have been achieved via precise interface engineering from the synthesis of the material to the application (Advanced Materials,2020, 2000919).
Flexible displays have been developing with a high pace and the global flexible display market has been expanding quickly over the years. Development of flexible transparent conducting electrodes (TCEs) with outstanding flexibility and electrical conductivity is one of the key requirements for the next-generation displays because indium tin oxide (ITO), the conventional TCE, is brittle. Diverse materials such as graphene, conducting polymers and metal nanowires have been suggested but their insufficient electrical conductivity, low work function and complicated electrode fabrication limited their practical use.
MXenes, a new family of two-dimensional materials
MXenes, a new class of two-dimensional materials discovered at Drexel University in 2011, consist of few-atoms-thick layers of transition metal carbides or nitrides. They have shown impressive properties such as metal-like electrical conductivity and tunable surface and electronic properties, offering new possibilities to the various fields of technology. Since their discovery, their use has been explored in a number of areas, such as metal ion batteries, sensors, gas and electrochemical storage, energy devices, catalysts and medicine. MXenes have exhibited potential as flexible electrodes because of their superior flexibility. However, exploration of MXenes in flexible electrodes of optoelectronic devices just started recently because the conventional MXene films do not meet the requirements of work function and conductivity in LEDs and solar cells and can degrade when they are exposed to the acidic water-based hole injection layer (HIL).
MXene for flexible LED application
An international team of scientists from Seoul National University and Drexel University, led by Tae-Woo Lee and Yury Gogotsi focused on the surface and interface modulation of the solution-processed MXene films to make an ideal MXene/HIL system. They tuned the surface of the MXene film to have high work function (WF) by low-temperature vacuum annealing and the HIL is designed to be pH-neutral and be diluted with alcohol, preventing detrimental surface oxidation and degradation of the electrode film. The MXene/HIL system suggested by the team provides advantages to the device efficiency due to efficient injection of holes to the emitting layer by forming a nearly ideal Ohmic contact.
Using the MXene/HIL system, the team fabricated high-efficiency green organic LEDs (OLEDs) exceeding 100 cd/A, which agrees well with the theoretical maximum values and is quite comparable with that of the conventional ITO-based devices. Finally, flexible MXene-LEDs on a plastic substrate show outstanding bending stability while the ITO-LEDs could not stand the bending stress. It is the first report that demonstrates highly efficient OLEDs using a single layer of 2D titanium carbide MXene as a flexible electrode.
This progressive research is published in the prominent journal 'Advanced Materials' (IF: 25.809). The authors explain further: "The results of interface engineered MXene film and the MXene electrode-based flexible organic LEDs show the strong potential of the solution-processed MXene TCE for use in next-generation optoelectronic devices that can be manufactured using a low-cost solution-processing technology."
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 ofsingle-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.
For more than a decade, two-dimensional nanomaterials, such as graphene, have been touted as the key to making better microchips, batteries, antennas and many other devices. But a significant challenge of using these atom-thin building materials for the technology of the future is ensuring that they can be produced in bulk quantities without losing their quality. For one of the most promising new types of 2D nanomaterials, MXenes, that’s no longer a problem. Researchers at Drexel University and the Materials Research Center in Ukraine have designed a system that can be used to make large quantities of the material while preserving its unique properties.
The team recently reported in the journal Advanced Engineering Materials that a lab-scale reactor system developed at the Materials Research Center in Kyiv, can convert a ceramic precursor material into a pile of the powdery black MXene titanium carbide, in quantities as large as 50 grams per batch.
Proving that large batches of material can be refined and produced with consistency is a critical step toward achieving viability for manufacturing. For MXene materials, which have already proven their mettle in prototype devices for storing energy, computing, communication and health care, reaching manufacturing standards is the home stretch on the way to mainstream use.
“Proving a material has certain properties is one thing, but proving that it can overcome the practical challenges of manufacturing is an entirely different hurdle — this study reports on an important step in this direction,” said Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel’s College of Engineering, who has pioneered the research and development of MXene and is a lead author of the paper. “This means that MXene can be considered for widespread use in electronics and energy storage devices.”
Researchers at Drexel have been making MXene in small quantities — typically one gram or less — since they first synthesized the material in 2011. The layered nanomaterial, which looks like a powder in its dry form, starts as a piece of ceramic called a MAX phase. When a mixture of hydrofluoric and hydrochloric acid interacts with the MAX phase it etches away certain parts of the material, creating the nanometer-thin flakes characteristic of MXenes.
In the lab, this process would take place in a 60 ml container with the ingredients added and mixed by hand. To more carefully control the process at a larger scale, the group uses a one-liter reactor chamber and a screw feeder device to precisely add MAX phase. One inlet feeds the reactants uniformly into the reactor and another allows for gas pressure relief during the reaction. A specifically designed mixing blade ensures thorough and uniform mixing. And a cooling jacket around the reactor lets the team adjust the temperature of the reaction. The entire process is computerized and controlled by a software program created by the Materials Research Center team.
The group reported successfully using the reactor to make just under 50 grams of MXene powder from 50 grams of MAX phase precursor material in about two days (including time required for washing and drying the product). And a battery of tests conducted by students at Drexel’s Materials Science and Engineering Department showed that the reactor-produced MXene retains the morphology, electrochemical and physical properties of the original lab-made substance.
This development puts MXenes in a group with just a handful of 2D materials that have proven they can be produced in industrial-size quantities. But because MXene-making is a subtractive manufacturing process — etching away bits of a raw material, like planing down lumber —
it stands apart from the additive processes used to make many other 2D nanomaterials.
“Most 2D materials are made using a bottom-up approach,” said Christopher Shuck, PhD, a post-doctoral researcher in the A.J. Drexel Nanomaterials Institute. “This is where the atoms are added individually, one by one. These materials can be grown on specific surfaces or by depositing atoms using very expensive equipment. But even with these expensive machines and catalysts used, the production batches are time-consuming, small and still prohibitively expensive for widespread use beyond small electronic devices.”
MXenes also benefit from a set of physical properties that ease their path from processed material to final product — a hurdle that has tripped up even today’s widely used advanced materials.
“It typically takes quite a while to build out the technology and processing to get nanomaterials in an industrially usable form,” Gogotsi said. “It’s not just a matter of producing them in large quantities, it often requires inventing completely new machinery and processes to get them in a form that can be inserted into the manufacturing process — of a microchip or cell phone component, for example.”
But for MXenes, integrating into the manufacturing line is a fairly easy part, according to Gogotsi.
“One huge benefit to MXenes is that they be used as a powder right after synthesis or they can be dispersed in water forming stable colloidal solutions,” he said. “Water is the least expensive and the safest solvent. And with the process that we’ve developed, we can stamp or print tens of thousands of small and thin devices, such as supercapacitors or RFID tags, from material made in one batch.”
This means it can be applied in any of the standard variety of additive manufacturing systems — extrusion, printing, dip coating, spraying — after a single step of processing.
Several companies are looking developing the applications of MXene materials, including Murata Manufacturing Co, Ltd., an electronics component company based in Kyoto, Japan, which is developing MXene technology for use in several high-tech applications.
“The most exciting part about this process is that there is fundamentally no limiting factor to an industrial scale-up,” Gogotsi said. “There are more and more companies producing MAX phases in large batches, and a number of those are made using abundant precursor materials. And MXenes are among very few 2D materials that can be produced by wet chemical synthesis at large scale using conventional reaction engineering equipment and designs.”
Posted By Graphene Council,
Sunday, August 11, 2019
Updated: Sunday, August 4, 2019
For the first time, a team of researchers, from the School of Materials and the National Graphene Institute at The the University of Manchester have formulated inks using the 2D material MXene, to produce 3D printed interdigitated electrodes.
As published in Advanced Materials, these inks have been used to 3D print electrodes that can be used in energy storages devices such as supercapacitors.
MXene, a ‘clay-like’ two-dimensional material composed of early transition metals (such as titanium) and carbon atoms, was first developed by Drexel University. However, unlike most clays, MXene shows high electrical conductivity upon drying and is hydrophilic, allowing them to be easily dispersed in aqueous suspensions and inks.
Graphene was the world’s first two-dimensional material, more conductive than copper, many more times stronger than steel, flexible, transparent and one million times thinner than the diameter of a human hair.
Since its isolation, graphene has opened the doors for the exploration of other two-dimensional materials, each with a range of different properties. However, in order to make use of these unique properties, 2D materials need to be efficiently integrated into devices and structures. The manufacturing approach and materials formulations are essential to realise this.
Dr Suelen Barg who led the team said: “We demonstrate that large MXene flakes spanning a few atoms thick, and water can be independently used to formulate inks with very specific viscoelastic behaviour for printing. These inks can be directly 3D printed into freestanding architectures over 20 layers tall. Due to the excellent electrical conductivity of MXene, we can employ our inks to directly 3D print current collector-free supercapacitors. The unique rheological properties combined with the sustainability of the approach open many opportunities to explore, especially in energy storage and applications requiring the functional properties of 2D MXene in customized 3D architectures.”
Wenji and Jae, PhD students at the Nano3D Lab at the University, said: “Additive manufacturing offers one possible method of building customised, multi-materials energy devices, demonstrating the capability to capture MXene’s potential for usage in energy applications. We hope this research will open avenues to fully unlock the potential of MXene for use in this field.”
The unique rheological properties combined with the sustainability of the approach open many opportunities to explore, especially in energy storage and applications requiring the functional properties of 2D MXene in customized 3D architectures. Dr Suelen Barg, School of Materials
The performance and application of these devices increasingly rely on the development and scalable manufacturing of innovative materials in order to enhance their performance.
Supercapacitors are devices that are able to produce massive amounts of power while using much less energy than conventional devices. There has been much work carried out on the use of 2D materials in these types of devices due to their excellent conductivity as well as having the potential to reduce the weight of the device.
Potential uses for these devices are for the automotive industry, such as in electric cars as well as for mobile phones and other electronics.
The latest lithium-ion batteries on the market are likely to extend the charge-to-charge life of phones and electric cars by as much as 40 percent. This leap forward, which comes after more than a decade of incremental improvements, is happening because developers replaced the battery’s graphite anode with one made from silicon. Research from Drexel University and Trinity College in Ireland now suggests that an even greater improvement could be in line if the silicon is fortified with a special type of material called MXene.
This adjustment could extend the life of Li-ion batteries as much as five times, the group recently reported in Nature Communications. It’s possible because of the two-dimensional MXene material’s ability to prevent the silicon anode from expanding to its breaking point during charging — a problem that’s prevented its use for some time.
Silicon anodes are projected to replace graphite anodes in Li-ion batteries with a huge impact on the amount of energy stored,” said Yury Gogotsi, PhD, Distinguished University and Bach Professor in Drexel’s College of Engineering and director of the A.J. Drexel Nanomaterials Institute in the Department of Materials Science and Engineering, who was a co-author of the research. “We’ve discovered adding MXene materials to the silicon anodes can stabilize them enough to actually be used in batteries.”
In batteries, charge is held in electrodes — the cathode and anode — and delivered to our devices as ions travel from anode to cathode. The ions return to the anode when the battery is recharged. Battery life has steadily been increased by finding ways to improve the electrodes’ ability to send and receive more ions. Substituting silicon for graphite as the primary material in the Li-ion anode would improve its capacity for taking in ions because each silicon atom can accept up to four lithium ions, while in graphite anodes, six carbon atoms take in just one lithium. But as it charges, silicon also expands — as much as 300 percent — which can cause it to break and the battery to malfunction.
Most solutions to this problem have involved adding carbon materials and polymer binders to create a framework to contain the silicon. The process for doing it, according to Gogotsi, is complex and carbon contributes little to charge storage by the battery.
By contrast, the Drexel and Trinity group’s method mixes silicon powder into a MXene solution to create a hybrid silicon-MXene anode. MXene nanosheets distribute randomly and form a continuous network while wrapping around the silicon particles, thus acting as conductive additive and binder at the same time. It’s the MXene framework that also imposes order on ions as they arrive and prevents the anode from expanding.
“MXenes are the key to helping silicon reach its potential in batteries,” Gogotsi said. “Because MXenes are two-dimensional materials, there is more room for the ions in the anode and they can move more quickly into it — thus improving both capacity and conductivity of the electrode. They also have excellent mechanical strength, so silicon-MXene anodes are also quite durable up to 450 microns thickness.”
MXenes, which were first discovered at Drexel in 2011, are made by chemically etching a layered ceramic material called a MAX phase, to remove a set of chemically-related layers, leaving a stack of two-dimensional flakes. Researchers have produced more than 30 types of MXene to date, each with a slightly different set of properties. The group selected two of them to make the silicon-MXene anodes tested for the paper: titanium carbide and titanium carbonitride. They also tested battery anodes made from graphene-wrapped silicon nanoparticles.
All three anode samples showed higher lithium-ion capacity than current graphite or silicon-carbon anodes used in Li-ion batteries and superior conductivity — on the order of 100 to 1,000 times higher than conventional silicon anodes, when MXene is added.
“The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change but also well resolves the mechanical instability of Si,” they write. “Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.”
Chuanfang Zhang, PhD, a post-doctoral researcher at Trinity and lead author of the study, also notes that the production of the MXene anodes, by slurry-casting, is easily scalable for mass production of anodes of any size, which means they could make their way into batteries that power just about any of our devices.
“Considering that more than 30 MXenes are already reported, with more predicted to exist, there is certainly much room for further improving the electrochemical performance of battery electrodes by utilizing other materials from the large MXene family,” he said.