What are MXenes?
Content
Key Takeaways About MXenes
- Definition: MXenes are two-dimensional materials made from transition metal carbides and nitrides, first discovered in 2011
- Origin: They are produced by selectively removing aluminum layers from MAX phases through an etching process
- Composition: MXenes could exist in over a million possible stable configurations of transition metals, carbon, and nitrogen
- Properties: Unlike most 2D ceramics, MXenes have excellent electrical conductivity, high surface area, and hydrophilic surfaces
- Manufacturing: The MILD (minimally intensive layer delamination) technique shows promise for manufacturing-scale production
- Applications: MXenes excel in energy storage, water purification, conductive coatings, sensors, and triboelectric nanogenerators
- Advantage: Their unique combination of metallic conductivity with ceramic properties enables performance that surpasses many existing materials
MXenes – pronounced 'max-eens' – first discovered in 2011, are ceramics that comprise one of the largest families of two-dimensional (2D) materials.
MXenes are made from a bulk crystal called MAX. 2D layered materials derived from MAX or non-MAX phases were not predicted to exist before this discovery. Unlike most 2D ceramics, MXenes have inherently good conductivity and excellent volumetric capacitance because they are molecular sheets made from the carbides and nitrides of transition metals like titanium. MXenes have already found applications ranging from energy storage to medicine and optoelectronics.
What makes MXenes so interesting is the fact that this material class could conceivably consist of any of millions of possible arrangements of transition metals (like molybdenum or titanium), carbon and nitrogen. The trick is to find the ones that are stable.
By using a high-throughput computational platform and scanning through the formation energies of millions of alloying configurations, researchers estimate that there are more than a million of stable MXene compounds remaining to be discovered (ACS Nano, "High-Throughput Survey of Ordering Configurations in MXene Alloys Across Compositions and Temperatures").
MAX phases
There is a large family of ternary carbides (ternary is an adjective meaning 'composed of three') with the general formula Mn+1AXn, where n = 1-3, M denotes a transition metal, A is an element such as aluminum or silicon, and X is either carbon or nitrogen. Researchers have termed these ductile and machineable ceramics MAX phases.
As a consequence of their layered structure, these materials kink and delaminate during deformation and also exhibit an unusual, and sometimes unique, combination of properties; they are not sure whether they want to be metals or ceramics. While they conduct heat and electricity like metals, they are elastically stiff, strong, brittle, and heat-tolerant like ceramics. They are resistant to chemical attack, readily machinable, and thermal shock, damage tolerant, and sometimes fatigue, creep, and oxidation resistant.
MXene discovery
Two-dimensional (2D) structures, like graphene and molybdenum disulfide, are known to have unique properties. Therefore, having a new family of 2D structures with a wide range of chemistries can open the door for better understanding of differences between properties of 2D and 3D materials, lead to identification of useful properties of 2D carbides, nitrides, oxycarbides and other related structures, and finally result in new applications.
MAX Phases have been researched for years and dozens of layered carbides, nitrides and carbonitrides with a variety of properties have been synthesized.
However, these ceramics have always been produced as three-dimensional materials, until researchers placed titanium-aluminum carbide (Ti3AlC2) powders in hydrofluoric acid at room temperature to selectively remove the aluminum. The result of this chemical process – referred to as exfoliation – essentially spreads out the layered carbide material and yields two-dimensional Ti3C2 nanosheets, which have since been coined MXene, as a kin to graphene.
In a 2011 paper in Advanced Materials ("Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2"), researchers first demonstrated this ability to transform three-dimensional titanium-aluminum carbide, a typical representative of MAX phases, into a two dimensional structure with greatly different properties.
How to make MXene
MXenes are created by selectively removing aluminum from layered MAX phases. Through this exfoliation process, the carbide layers are separated into two MXene sheets just a few atoms thick. MXenes can accommodate various ions and molecules between their layers by a process known as intercalation, which is sometimes a necessary step in order to exploit the materials?? unique properties.
For example, placing lithium ions between MXene sheets has been shown to render them promising materials for both lithium-ion batteries and electrochemical capacitors.
To synthesize free-standing MXene flakes, a research team from Drexel University improved their initial technique from 2011 using acid, which they termed minimally intensive layer delamination (MILD). They treated bulk MAX with an etchant of fluoride salt and hydrochloric acid to selectively remove unwanted layers of aluminum from between titanium carbide layers.
Then they manually shook the etched material to separate and collect the titanium carbide layers. Each layer is five atoms thick and is made of carbon atoms binding three titanium sheets. Etching and exfoliating MAX produces many of these free-standing MXene layers. This relatively simple technique may enable manufacturing-scale production.
Since then, the continued exploration has revealed their exceptional ability to store energy, block electromagnetic interference, purify water and even ward off bacteria. And, as recent research suggests, MXenes are also very durable – the strongest material of its kind.
Although there are many possible MXene alloy compositions, most will not be stable. The challenge faced by material scientists has been how to efficiently sweep through the huge number of alloy configurations to identify those with the lowest formation energy and hence highest stability. Conventional ??first principles?? calculation approaches are too computationally intensive for such a scan to be feasible.
A high-throughput scan of possible compositions for MXenes gives researchers invaluable direction for picking the best candidate from the millions of possible material recipes.
MXene uses and applications
MXene could be used in energy storage devices such as electrodes of Li-ion batteries, pseudo capacitors, etc. The researchers also envision its use as reinforcement in composites, similar to clays or graphene, which increase mechanical properties and decrease gas permeability of polymers. A variety of surface chemistries, presence of transition metal oxides and high surface area make MXene potentially attractive for catalytic applications.
Desalination and waste water treatment
The materials' remarkable properties open new possibilities for MXenes in water desalination and wastewater treatment. This excitement comes from findings that Ti3C2 can trap the energy of sunlight to purify water by evaporation with an energy efficiency that is state-of-the-art.
To investigate MXene??s possibilities in water purification, researchers fabricated a thin and flexible Ti3C2 membrane incorporating a polystyrene heat barrier to prevent the heat energy from escaping. This created a system that could float on water and evaporate some of the water with 84% efficiency at the illumination levels of natural sunlight.
Battery technology and energy storage
Computational studies have suggested that fully exfoliating, or delaminating, certain MXenes would yield layers with exceptional charge capacities for use in battery anodes. In one report, scientists demonstrated successful intercalation of MXenes with several organic molecules, including dimethyl sulfoxide (DMSO), which allowed them to fully exfoliate stacked layers into MXene sheets and ultimately create MXene 'paper' by filtering flakes from solution.
This flexible and electrically conductive paper showed a lithium ion capacity of four times that of typical MXene material, with extremely high charging rates and a cyclability superior to graphite, which is used in commercial lithium-ion batteries. Critically, this work demonstrates that such material can be synthesized on a large scale.
Researchers also have developed new electrode designs with MXene material, that will allow batteries to charge much faster. Their design could make energy storage devices like batteries, viewed as the plodding tanker truck of energy storage technology, just as fast as the speedy supercapacitors that are used to provide energy in a pinch – often as a battery back-up or to provide quick bursts of energy for things like camera flashes.
Triboelectric nanogenerators
Researchers have demonstrated that MXenes could be used to harvest wasted frictional energy, for example, from muscle contractions during typing or walking. MXenes possess high electrical conductivity and the ability to uptake electrons when in contact with polymers and other materials.
This unusual combination of properties makes them useful as components for triboelectric nanogenerators (TENG), which turn muscle movements into electric power. The research suggests these advanced materials could be incorporated into mobile phones, handheld electronics, wearable devices and laptops, ultimately making them self-powering.
Conductive coatings
MXenes have been used to developed a mechanically robust conductive coating that can maintain performance under heavy stretching and bending. This research exploited the fact that MXene multilayer coatings that can undergo large-scale mechanical deformation while maintaining a high level of conductivity. In this work, the researchers also successfully deposited the MXene multilayer coatings onto flexible polymer sheet, stretchable silicones, nylon fiber, glass and silicon.
Sensors and chemical noses
It appears that MXene is one of the most sensitive gas sensors ever reported. This research is significant because it expands the range for detection of common gases allowing us to detect very low concentrations that we were not able to detect before.
Research findings suggest that MXene can pick up chemicals, such as ammonia and acetone, which are indicators of ulcers and diabetes, in much lower traces than sensors currently being used in medical diagnostics.
MXene's advantage over conventional sensor materials lies in its porous structure and chemical composition. The material is good at both allowing gas molecules to move across its surface and snagging, or adsorbing, certain ones that are chemically attracted to it, showing good selectivity.
Major Open Challenges in MXene Research
Despite the rapid progress in MXene research and development, several significant challenges remain before these materials can reach their full commercial potential. These challenges span from fundamental synthesis issues to practical application hurdles.
Large-Scale Production
While laboratory-scale synthesis of MXenes has advanced significantly since their discovery, scaling up production to industrial levels remains challenging. The current synthesis methods involving hydrofluoric acid (HF) present safety concerns and environmental issues that need addressing before widespread manufacturing can occur. Additionally, ensuring consistent quality and properties across large batches of MXene materials is essential for commercial applications.
Stability and Oxidation
MXenes are susceptible to oxidation when exposed to water and oxygen in ambient conditions, which can significantly degrade their performance over time. This oxidation stability issue is particularly problematic for applications requiring long-term reliability. Researchers are exploring various approaches to improve stability, including surface termination engineering and protective coatings, but a definitive solution remains elusive.
Theoretical Understanding
Despite extensive experimental work, a comprehensive theoretical understanding of MXene properties is still developing. The wide variety of possible compositions, surface terminations, and structural variations creates a complex landscape that requires more advanced modeling and simulation efforts. Better theoretical frameworks would accelerate the discovery of new MXene compositions optimized for specific applications.
Sustainable Synthesis
Current MXene synthesis methods often use hazardous chemicals like hydrofluoric acid. Developing greener, more environmentally friendly synthesis routes is crucial for sustainable large-scale production. Research into alternative etching agents and processes that maintain MXene quality while reducing environmental impact is an active area of investigation.
Device Integration
Incorporating MXenes into practical devices presents challenges related to processing compatibility with existing manufacturing technologies. Issues such as uniform deposition, adhesion to various substrates, and integration with other materials need to be addressed. Developing standardized procedures for MXene integration into devices is essential for commercial adoption.
Toxicity and Biocompatibility
For biomedical and environmental applications, a thorough understanding of MXene toxicity and biocompatibility is necessary. While initial studies show promise for certain biomedical uses, comprehensive long-term safety studies are still needed before MXenes can be widely used in biological contexts. The potential environmental impact of MXene production and disposal also requires further investigation.
Cost-Effectiveness
The economic viability of MXenes compared to existing materials remains a significant consideration. Production costs need to decrease substantially through improved synthesis methods and economies of scale. For commercial success, MXenes must not only outperform current materials in technical specifications but also offer competitive pricing.
Despite these challenges, the remarkable progress in MXene research over just a decade since their discovery suggests that solutions will continue to emerge. Collaborative efforts between materials scientists, chemists, engineers, and industry partners will be crucial in overcoming these hurdles and realizing the full potential of MXenes in next-generation technologies.
Environmental Stability and Degradation
Environmental stability remains one of the most significant challenges limiting MXene applications. When exposed to ambient conditions, MXenes can undergo rapid oxidation and degradation. Studies have shown that Ti₃C₂ MXene flakes begin oxidizing within hours of exposure to air and moisture, with complete conversion to TiO₂ possible within weeks. This degradation dramatically alters their electrical, mechanical, and chemical properties, severely limiting shelf-life and long-term performance in applications.
The stability issue varies across different MXene compositions, with some showing greater resilience than others. For example, Mo- and Nb-based MXenes generally demonstrate better oxidation resistance than Ti-based variants. Surface termination also plays a crucial role, with -F terminated MXenes typically exhibiting better stability than -OH or -O terminated ones.
Researchers are actively pursuing several strategies to improve environmental stability:
- Encapsulation in polymers or protective coatings
- Storage in oxygen-free environments or vacuum-sealed packaging
- Chemical modification of surface terminations
- Development of more inherently stable MXene compositions
- Freeze-drying and other specialized processing techniques
For MXenes to transition from laboratory demonstrations to commercial products, particularly in electronics, energy storage, and sensing applications, solving this stability challenge is imperative. Recent work using antioxidants and specialized storage solutions has shown promise, but a definitive solution that preserves MXene properties while ensuring long-term stability under real-world conditions remains an active research frontier.
Frequently Asked Questions
What are MXenes and how are they pronounced?
MXenes (pronounced 'max-eens') are a family of two-dimensional (2D) materials made from transition metal carbides and nitrides. They were first discovered in 2011 and are derived from a bulk crystal called MAX.
What makes MXenes different from other 2D materials?
Unlike most 2D ceramic materials, MXenes have inherently good electrical conductivity because they are molecular sheets made from the carbides and nitrides of transition metals like titanium. They combine metallic conductivity with hydrophilic surfaces, making them unique among 2D materials.
How are MXenes made?
MXenes are made by selectively removing aluminum layers from MAX phases using an etching process, typically with hydrofluoric acid. This process is called exfoliation. An improved technique called minimally intensive layer delamination (MILD) uses a combination of fluoride salt and hydrochloric acid to selectively remove aluminum layers.
What are MAX phases?
MAX phases are a family of ternary carbides with the general formula Mn+1AXn, where n = 1-3, M denotes a transition metal, A is an element such as aluminum or silicon, and X is either carbon or nitrogen. They are ductile and machinable ceramics that exhibit unusual properties, behaving like both metals and ceramics.
How many possible MXene compositions exist?
Researchers estimate there are more than a million possible stable MXene compounds. MXenes could conceivably consist of any of millions of possible arrangements of transition metals (like molybdenum or titanium), carbon and nitrogen.
What are the main applications of MXenes?
MXenes have applications in energy storage (batteries and supercapacitors), water purification and desalination, electromagnetic interference shielding, triboelectric nanogenerators, conductive coatings, sensors, and medical applications. Their diverse properties make them suitable for a wide range of technological applications.
Why are MXenes promising for energy storage?
MXenes show excellent performance as electrode materials for batteries and supercapacitors due to their high electrical conductivity, large surface area, and ability to intercalate ions between their layers. They have demonstrated lithium ion capacity four times that of typical materials, with extremely high charging rates and superior cyclability compared to graphite used in commercial lithium-ion batteries.
How do MXenes compare to graphene?
While both are 2D materials, MXenes offer more versatility in composition and surface termination than graphene. MXenes have naturally hydrophilic surfaces (unlike graphene), making them easier to process in water. MXenes also exhibit different electronic properties and can be engineered with various transition metals to optimize for specific applications.
Can MXenes be produced at industrial scale?
Research suggests that MXenes can be manufactured at scale. The relatively simple technique called minimally intensive layer delamination (MILD) may enable manufacturing-scale production. Researchers have successfully produced MXene "paper" by filtering flakes from solution, demonstrating potential for larger-scale applications.
Are MXenes safe to use?
The safety profile of MXenes is still being studied. Since they're a relatively new class of materials, long-term toxicity and environmental impact assessments are ongoing. Some research suggests potential applications in antimicrobial coatings and even medical uses, but more research is needed before definitive safety claims can be made.
By
Michael
Berger
– Michael is author of four books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology (2009),
Nanotechnology: The Future is Tiny (2016),
Nanoengineering: The Skills and Tools Making Technology Invisible (2019), and
Waste not! How Nanotechnologies Can Increase Efficiencies Throughout Society (2025)
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