Mighty MXenes are ready for launch

Michael Naguib remembers with humor and humility the day he accidentally discovered a remarkable new material. It was a hot August morning in 2010. Naguib, a PhD student at Drexel University, had been wrangling a material called a MAX phase, trying to make it work as a lithium-ion battery electrode. But the material refused to cooperate.

MAX phases are layered carbides or nitrides of a transition metal (M) interleaved with an element such as aluminum or silicon (A). X stands for carbon or nitrogen. The titanium aluminum carbide material that Naguib was using would not take up lithium ions, a critical need for lithium-ion battery electrodes. So his research advisers, the materials science professors Yury Gogotsi and Michel W. Barsoum, suggested etching out the aluminum to make space for lithium.

On that August day, Naguib tried to do that using hydrofluoric acid, a powerful etching agent. Suddenly, “everything in the container just bubbled up and fell into the spill tray,” Naguib, now a Tulane University engineering physics professor, recalls. “I shut the fume hood door right away, but it was scary for a few seconds.”

The smoke settled to reveal a black powder. Curious to learn what his debacle had produced, Naguib examined the particles with a transmission electron microscope and was surprised to find that the particles were stacks of sheets just a few atoms thick. Naguib thought he had made graphene. But chemical analysis showed that the 2D flecks contained titanium and carbon. In his hands was a brand-new 2D material.

The Drexel team dubbed the nanomaterial, formula Ti₃C₂, a MXene (pronounced “maxine”) given its similarity to graphene, which was all the rage at that time. There are dozens of MAX phases, Naguib says. “We knew if we could do this for all MAX phases, we would have a huge family of 2D materials. We knew we had something big.”

Labs around the world caught on soon after the Drexel team reported the first MXene in 2011 (Adv. Mater. 2011, DOI: 10.1002/adma.201102306) and the MXene family a year later (ACS Nano 2012, DOI: 10.1021/nn204153h). Since then, scientists have discovered more than 50 MXene compositions. Computers have predicted thousands more. Nowadays, about 70,000 scientists from 100-plus countries study MXenes. Research organizations dedicate scientific symposia and special issues of journals to the materials.

Research papers list a staggering range of potential applications: long-lasting batteries, flexible and wearable electronics, gas sensors, antennas, catalysts, photodetectors, and devices for water purification, green hydrogen production, and kidney dialysis.

Nearly 15 years since their discovery, MXenes are on the verge of commercial debut. Big corporations such as Samsung and Intel have MXene-related patents. And the Japanese electronic component manufacturer Murata Manufacturing plans to launch a MXene-based product within the next 3 years, a spokesperson says.

These first commercial products will decide the fate of the materials.

“The only bottleneck now is to find the killer application that requires tons of the material,” Naguib says.

Hot properties

The discovery of an entirely new material is a rare event. The big materials discovery of the 20th century was fullerenes in 1985. In the early years of the 21st century, graphene shook the materials world when Andre Geim and Konstantin Novoselov of the University of Manchester isolated the atom-thick carbon nanosheets from graphite. Graphene’s unique electronic and mechanical properties and the ease of making it in the lab quickly made it the darling of the materials community.

Twenty years and billions of R&D dollars since graphene’s discovery, however, the material’s use is limited. A few companies make heat sinks for mobile phones and niche sports products with graphene-based composites. But a groundbreaking application remains elusive.

Gogotsi argues that the MXene story will be different. Discovered the year Geim and Novoselov won the 2010 Nobel Prize in Physics, MXenes were initially the overshadowed siblings of the carbon superstar. But they have come into their own. And they outshine graphene and other 2D materials in several ways, Gogotsi says. “MXenes will become a game changer because they allow us to do things no other materials can. Nanotechnology is not about making things small; it’s about controlling things at the nanoscale. And this is where MXenes excel.”

Consider their mind-boggling tunability. The materials are layer cakes containing two to five layers of transition metals connected by one to four layers of nonmetal atoms. The general formula is M_n + 1X_nT_x, where n varies from 1 to 4, and T_x (x is variable) indicates terminations on the surface of the outer transition-metal layers. Those terminations can be oxygen, hydroxyls, amines, halogens, and chalcogens.

By tuning all those knobs—structure, elemental composition, and surface terminations—researchers can create an infinite number of 2D materials and a dizzying range of properties. “The beauty of MXenes is this vast colorful palette where you can pick the right element for the right reactions, and it allows you to target specific applications,” Naguib says.

MXenes without surface terminations are conductive, like metals. But alter the type of transition metal or tailor the surface chemistry, and MXenes behave like semiconductors (Nat. Commun. 2019, DOI: 10.1038/s41467-018-08169-8).

From the transition metals that make up MXenes’ core, the materials inherit useful redox properties: the ability to take up, release, and rearrange electrons to enter various oxidation states. These properties underpin the electrochemical energy-storing reactions in batteries and electrocatalysis.

Then there are the physical traits. MXenes are exceptionally durable, elastic, and hydrophilic. Researchers can disperse MXene flakes in water and organic solvents to make inks. They can print and spray these inks to pattern MXene-based devices on curved, flexible surfaces or soak them into fabrics.

What’s more, films made of overlapping MXene flakes retain the properties of the individual flakes—a feature that Purdue University materials and mechanical engineering professor Babak Anasori calls “a killer point of MXenes.” Graphene films, by contrast, have lower conductivity than their constituent sheets.

“We’re not talking about a fancy nanomaterial that’s beautiful only at 1 nm,” says Anasori, who was a PhD student in Barsoum’s group at Drexel at the time of MXenes’ discovery and was later a postdoctoral scholar in Gogotsi’s lab. “That ease of scalable production is key.”

The material that keeps on giving

Every year, researchers report new MXenes and uncover surprising properties. In 2021, a team led by Anasori and Subramanian Sankaranarayanan of the University of Illinois Chicago discovered the first high-entropy MXenes. High-entropy materials combine four or more elements in high and near-equal concentrations. These MXenes boast exceptional hardness, stiffness, and resistance to oxidation and wear, making them excellent for extreme environments such as those experienced by nuclear reactors and spacecraft (ACS Nano 2021, DOI: 10.1021/acsnano.1c02775).

Other researchers have changed the spacing between MXene layers and added various ions and nanomaterials between to make MXenes that grab on to some molecules while allowing others to pass. They are putting this ability to use in membranes for water treatment, desalination, and gas separation (ACS Mater. Lett. 2023, DOI: 10.1021/acsmaterialslett.2c00914).

One of the most exciting findings in recent years stems from MXenes’ strong interaction with electromagnetic waves ranging from microwaves to the ultraviolet. Some MXenes convert electrical power to electromagnetic waves. Others cause the waves to bounce around between the materials’ layers until they fizzle out. Ti₃C₂, for instance, absorbs infrared radiation, which means it could be heated deep in the body and used for therapy. Meanwhile, Nb₄C₃ is an excellent emitter of IR radiation, making it useful for heating its environment.

Researchers have demonstrated MXenes that interact with visible light in all sorts of ways: reflect it like metals do, let it through like transparent glass does, or absorb it. One group led by researchers at King Abdullah University of Science and Technology found that Ti₃C₂ converts visible light to heat with 100% efficiency (ACS Nano 2017, DOI: 10.1021/acsnano.6b08415).

Gogotsi is harnessing this photothermal power in a new project with researchers at Khalifa University. The team intends to use MXenes for solar water distillation. The aim is to use the materials to absorb sunlight and produce heat to evaporate and purify seawater. “The country needs to desalinate seawater, and they are looking for better materials to do that,” he says.

Alexandra Boltasseva, a Purdue University electrical and computer engineer, was immediately intrigued by MXenes when she heard about them in 2015. Her group had been searching for new materials for optic and photonic applications. She had studied noble metals, semiconductors, and ceramics. “Then I met Yury and learned about MXenes,” she says.

Boltasseva recognized that MXenes combined the traits of the various materials she had been studying. At the time, she was investigating the unusual optical properties of bulk titanium nitride and zirconium nitride.

“The MXene family is just a transition metal plus nitrogen or carbon,” she says. “So I was expecting them to have an interesting optical response. With conventional materials, we cannot change their properties much; we’re kind of stuck with the optical response. But MXenes are really striking. They are truly remarkable in terms of changing their properties.”

Her group now tailors MXenes to enhance their light-harvesting properties for solar energy and solar distillation. “At certain frequency windows, they show really funny and exotic optical behavior,” Boltasseva says. For example, they can transmit light along one plane while blocking it in another. The researchers want to harness these unusual traits for next-generation optics applications such as nanolasers, quantum photonics, and metamaterials that could bend light in unnatural ways à la invisibility cloaks.

Gogotsi admits that MXenes’ blessing of endless tunability can be a curse. Like choosing the right hue in a paint store, picking the right composition for a desired application is hard. The challenge, then, is to find the right application for the right MXene.

And among those many combinations, Anasori and Naguib are confident they know what MXenes’ killer app will be.

Getting to work

“Without a doubt, the key application for MXenes is electromagnetic shielding,” Anasori says. Naguib agrees.

Remember those aluminum foil hats that people in sci-fi movies wore in the hopes of blocking signals to their brain? Electronic and medical devices today use something like that to prevent the escape of harmful radiation and interference from unwanted external radiation. These electromagnetic interference shields are screens made of copper or aluminum that absorb radio waves in the range of a few hundred kilohertz to 20 GHz. MXenes, it turns out, can give those materials a run for their money.

In 2016, Gogotsi, Chong Min Koo of the Korea Institute of Science and Technology, and colleagues showed that a 2 μm thick Ti₃C₂T_x film absorbs nearly all radiation between 8 and 12 GHz (Science 2016, DOI: 10.1126/science.aag2421). They have since shown that Ti₃C₂T_x and Ti₃CNT_x outperform metals and all other synthetic materials of the same thickness, thanks to their high conductivity and layered structure (Science 2020, DOI: 10.1126/science.aba7977).

Metal electromagnetic interference shields require rolling, cutting, and shaping. But MXene shields can be sprayed on. Gogotsi’s group has even soaked cotton and linen in Ti₃C₂ solutions to make fabrics that can protect wearable devices and people from harmful microwave radiation.

Any application that needs a thin, tough, conductive film is the sweet spot for MXenes, he says. Sprayable and printable MXene films could, for instance, be an easy-to-make replacement for the patterned copper antennas sputtered painstakingly today onto the communication circuits in phones and computers. Spray-on MXene antennas work just as well at one-seventh the thickness, a Drexel team found in 2020, and they work better than other new materials being considered for the purpose, including silver ink, carbon nanotubes, and graphene.

Gogotsi has other ideas for MXenes’ key applications. One is energy storage and catalysis. Most research studies, in fact, focus on these applications, which harness the materials’ combination of high conductivity and redox ability.

Because of those properties, Gogotsi doggedly pursued the use of MXenes as battery electrodes. But now he believes that more promising uses are as an additive to electrodes for improving conductivity and as a replacement for the copper or aluminum foils that collect current. “Imagine shrinking the battery by making the current collector 10 times thinner and lighter,” he says. “I think the game-changing applications are printable devices and wearable, flexible devices.”

Many researchers also believe that MXenes could be green hydrogen heroes. In 2016, Gogotsi, Anasori, and their colleagues first reported the use of molybdenum carbide MXenes as electrocatalysts for splitting water to make green hydrogen (ACS Energy Lett. 2016, DOI: 10.1021/acsenergylett.6b00247). Several teams have since tinkered with the mix of transition metals and functional groups in MXenes or changed the structures to increase surface area to make promising hydrogen-evolution catalysts.

Their goal is to replace expensive platinum, the best-known catalyst for splitting water, in electrolyzers. Naguib’s group found that replacing half the carbon in Ti₃C₂ with nitrogen produces a titanium carbonitride MXene with three times the hydrogen-evolution performance of the original carbide. “It’s not as catalytically active as platinum, but then price-wise, it’s not platinum,” Naguib says.

Anasori’s group has made strides with new tungsten-titanium MXene formulations and many others. “We have about 10 new promising compositions we haven’t reported yet,” he says.

Roadblocks

Asia has become the hub of MXene activity in recent years because of government and industry support in China, Japan, and South Korea. Researchers in China have published far more MXene papers in peer-reviewed journals than those in other countries, according to a search on the Web of Science (Graphene and 2D Mater. 2023, DOI: 10.1007/s41127-023-00067-1). Two-thirds of the roughly 1,600 MXene-related patents granted at the end of 2022 were also held by Chinese universities and research institutions, according to a Patsnap analysis from October 2022. Big corporations, many in Asia, hold almost 20% of the patents.

Murata Manufacturing, which licensed MXene technology from Drexel 5 years ago, leads the pack with 30 patents. The company’s list of products includes antennas, batteries, capacitors, sensors, and power converters. Murata is targeting two electronics-related applications with Ti₃C₂, says Takeshi Torita, senior manager in the company’s materials technology center. He declines to disclose specifics, though.

The incredible growth in global MXene research risks creating hype. But the nanotechnology industry has learned its lessons from graphene, Torita says, “and we can avoid the same situation.”

The community needs to clear a few key hurdles before MXene products can enter the real world. One is improved stability. Hydrophilic MXenes absorb water from the environment, which makes them swell and break apart. Torita, Gogotsi, and others reported last year that treating MXenes with N-methylformamide could be the answer (MRS Commun. 2023, DOI: 10.1557/s43579-023-00350-5). The solvent slips between MXene layers, keeping water molecules out. The resulting MXenes are stable and conductive in high heat and humidity.

The standard requirement for electronic components is to tolerate such conditions for a few thousand hours, Torita says. “We don’t have perfect results, but for some applications, stability is already good enough.”

Perhaps the biggest challenge is identifying low-cost, environmentally friendly ways to make MXenes at large scale.

The standard route to MXenes is to make MAX phases and etch them with hydrofluoric acid. Producing the MAX phases requires expensive transition metals and temperatures above 1,500 °C. Naguib and others recently used cheap precursors: titanium dioxide—the stuff of paints and sunscreen—carbon derived from discarded tires, and scrap aluminum. “We get the same-quality MXenes as those made from high-purity MAX phases,” Naguib says.

Moving away from poisonous, corrosive hydrofluoric acid will be crucial, Torita says. Murata is looking for an industrially feasible way to stop using the acid. One alternative is to chemically neutralize the acid before disposal. Some researchers are exploring other etchants, such as molten metal halide salts, hydroxide salts, and iodine vapors.

Others are exploring ways to circumvent MAX phases altogether. A year ago, chemistry professor Dmitri Talapin at the University of Chicago and colleagues became the first to directly grow carbide and nitride MXenes using chemical vapor deposition (Science 2023, DOI: 10.1126/science.add9204). The fast, easy method does not produce toxic by-products, and it yields MXenes in flower-shaped clusters instead of separate flakes.

In September, the US National Science Foundation (NSF) awarded Talapin almost $2 million to establish the MXenes Synthesis, Tunability, and Reactivity Center for Chemical Innovation, which brings together scientists from the University of Chicago, Drexel, the University of Pennsylvania, Purdue, and Vanderbilt University. The center aims to investigate direct synthesis routes, new properties, and applications for MXenes and, according to the center’s website, “ultimately push the frontier in this enormous chemical space.”

The MXene community also needs to establish standards. Right now, MAX phase and MXene quality vary from laboratory to laboratory, Naguib says. In 2023, Purdue researchers published a step-by-step guide to synthesize Ti₃C₂X MXenes. And in 2021, Drexel helped launch an international trade association called the MXene Association to establish standards and promote MXene R&D.

Several companies, such as Sigma-Aldrich, Carbon-Ukraine, 2D Semiconductors, and ACS Material, sell MXenes for research purposes. And at least 20 companies in China sell MXenes online in batches of over 10 kg. Some are developing reactors to produce 500 g of MXenes per batch. Anasori and colleagues, with a recent NSF award, are exploring what it will take to build even larger reactors. “As the field matures, we need to focus on the engineering side of things,” he says. “The idea is to investigate if and how we can run a 1 kg batch synthesis continuously with the same quality as smaller batches.”

Scaling up will automatically lower costs. That’s true especially for titanium carbide MXenes because carbon and titanium are among the most abundant materials in the earth’s crust, Gogotsi says. “It’s simply economy of scale. In the 19th century, Napoleon served aluminum tableware to his guests because the exotic light metal was more expensive than gold. Now it’s one of the cheapest materials. You wrap your sandwich in it.”

The road from discovery to commercial product is usually circuitous and slow. Quantum dots, the light-emitting nanocrystals used in today’s light-emitting diode lamps and TVs, for example, were discovered in the early 1980s, Gogotsi says. But it took decades for them to enter the market. And just last year, their discovery and development were recognized with the Nobel Prize in Chemistry.

Pointing to the products that are just around the corner, Naguib says progress on MXenes since their discovery has been “amazing—very impressive. When you see that an entire field of nanomaterials has been established, and you look back and remember that you were one of the first people to make it happen, that gives you a great feeling of pride and humility, but more than that, a feeling of excitement.”