A Simple Guide to MXenes and How We Make Them

1. Why Are People So Excited About MXenes?

In the world of nanomaterials, we are constantly trying to make things smaller, lighter, and smarter. One of the big steps in this direction was the discovery of two-dimensional (2D) materials – sheets of atoms that are only a few atoms thick, or even a single atom thick.

The most famous 2D material is graphene, but it is no longer alone. Researchers have discovered many other 2D materials with very different properties. Among these, MXenes have become one of the most promising families.

MXenes are:

Extremely thin, like atomic-scale sheets
Excellent electrical conductors
Mechanically strong and flexible
Chemically tunable — their surfaces can be modified in many ways

Because of this unique combination, MXenes are being studied for:

Energy storage (batteries, supercapacitors)
Catalysis (speeding up chemical reactions)
Sensors (for gases, chemicals, and biosensing)
Electromagnetic shielding
Flexible and wearable electronics

However, to use MXenes in real products, we first need safe, scalable, and controllable ways to make them. The review article you provided focuses exactly on this: how MXenes are synthesized, what the main methods are, and how the field is moving away from dangerous chemicals like hydrofluoric acid (HF) toward safer and greener approaches.

This blog will walk through those ideas in clear, simple language.

2. What Exactly Are MXenes?

MXenes come from another family of layered materials called MAX phases.

M = a transition metal (Ti, V, Mo, Nb, etc.)
A = an “A-group” element, usually from groups 13–16 of the periodic table (e.g., Al, Si, Ga)
X = carbon and/or nitrogen

MAX phases have a formula Mₙ₊₁AXₙ and a layered crystal structure. Within these stacked layers, the bonds between M and X are strong, while the bonds between M and A are weaker.

To obtain MXenes, scientists use selective etching:

They chemically remove the A layers (for example, the Al layers in Ti₃AlC₂).
The remaining layers of M and X become 2D sheets called MXenes, with the formula Mₙ₊₁XₙTₓ.
Tₓ represents surface groups that stick to the MXene during etching, such as –O, –OH, –F, or –Cl.

By changing:

which metal (M) is used,
whether X is carbon, nitrogen, or both,
and which surface groups (Tₓ) are present,

researchers can fine-tune the properties of MXenes. There are already more than 30 experimentally reported MXenes, and theoretically there could be hundreds.

Some types include:

Single-metal MXenes like Ti₃C₂Tₓ, Ti₂CTₓ, V₂CTₓ
Double-metal MXenes where two different metals are ordered in the layers
Solid-solution MXenes where metals are mixed randomly
MXenes with vacancies, where some metal atoms are deliberately missing to create special properties

These subtle differences allow engineers to design MXenes for specific jobs: lighter electrodes, more active catalysts, better gas sensors, and so on.

3. Why Synthesis Matters So Much

The way you make a MXene strongly affects:

Its surface chemistry (which groups are attached)
The size and thickness of its flakes
Its defects, stability, and tendency to oxidize
Its electrical and electrochemical performance

Traditional MXene synthesis has mostly relied on hydrofluoric acid (HF), a very aggressive and dangerous acid. HF works well for etching, but it has serious drawbacks:

It is highly toxic and corrosive.
It creates a lot of fluorine-terminated surfaces, which are not always ideal for applications like energy storage or catalysis.
It complicates handling and scaling up to industrial production.

Because of this, the MXene community has been searching for safer and greener methods, while still keeping the quality and performance high.

The review groups the synthesis strategies into:

Top-down approaches – starting from a bulk MAX phase and cutting/etching it down into 2D layers.
Bottom-up approaches – building MXenes more directly from atoms or small molecules, layer by layer or via chemical growth.

Both have advantages and limitations, and both are now active areas of research.

4. Top-Down Approaches: Starting from MAX and Etching Down

4.1. Classic HF Etching

The most established method is to immerse MAX powders (like Ti₃AlC₂) in an aqueous HF solution.

What happens?

HF attacks and dissolves the A element (e.g., Al).
Hydrogen gas is released, and layers begin to separate.
Surface terminations –O, –OH, and –F attach to the remaining Ti₃C₂ layers, creating Ti₃C₂Tₓ.

This method has been used to produce many different MXenes, including carbides and nitrides of Ti, V, Nb, Mo, Ta, W, and others. It can work at room temperature or slightly elevated temperatures and generally gives high yields and well-defined layered structures.

Advantages of HF etching:

Relatively simple and well-understood
Often high yield
Produces layered structures that can be further exfoliated into single or few-layer sheets

Disadvantages:

HF is extremely dangerous to handle and bad for the environment.
Fluorine-rich surfaces can be less suitable for some electrochemical applications.
Fluorine terminations may hinder interfacial reactions or raise resistance at interfaces.

Because of these issues, researchers have been actively developing HF-related but gentler variants, and also HF-free methods.

4.2. In-Situ HF Generation and Fluoride-Salt + Acid Routes

Instead of using HF directly, one can mix a fluoride salt (such as LiF, NaF, KF, NH₄F, or NH₄HF₂) with a weaker acid like HCl. These mixtures generate HF locally in solution, but more controllably and at lower overall risk.

Typical systems include:

LiF + HCl
NH₄HF₂ solutions
Various other fluoride salts combined with acids

These methods can:

Etch the A layers effectively
Introduce –F, –Cl, –O, and –OH terminations
Intercalate metal cations (Li⁺, Na⁺, K⁺, etc.) between layers, which helps delamination (layer separation)
Sometimes avoid the need for strong sonication

Many studies have shown that these milder fluoride-salt systems can produce high-quality MXenes with good performance in batteries, supercapacitors, gas sensors, and catalysis. They are widely used as an alternative to direct HF.

However, because they still generate HF in solution and still introduce fluorine terminations, there is ongoing interest in fluoride-free methods.

4.3. Fluoride-Free Etching

Fluoride-free methods aim to remove the A layers without any fluorine-containing chemicals. This has two big benefits:

Safer and more environmentally friendly
Avoids F-termination, often leading to better electrochemical performance

Some strategies include:

Strong alkali (NaOH) hydrothermal etching
- Uses very concentrated NaOH at high temperature and pressure.
- Can remove Al layers from Ti₃AlC₂ and other MAX phases.
- Often operates like a “Bayer process” used in aluminum production.
HCl hydrothermal etching
- Uses hot, pressurized HCl to etch certain MAX phases like Mo₂Ga₂C.
- Can produce Mo₂CTₓ with mostly oxygen and hydroxyl terminations.
Electrochemical fluoride-free etching (discussed more below).
Supercritical CO₂–based ternary systems and other special solvents
- Used to etch and modify the surface in a more controlled and gentle way.

These approaches are still emerging, and conditions must be carefully optimized (temperature, time, concentration). But they show that MXenes do not have to be tied to fluorine chemistry, which opens doors for better battery electrodes, catalysts, and biocompatible materials.

4.4. Electrochemical Etching

Electrochemical etching uses electric current to drive the removal of the A layer.

A MAX phase (for example, Ti₃AlC₂ or Nb₂AlC) is used as the working electrode in an electrochemical cell.
The electrolyte may contain salts like LiF, or may be fluoride-free (for instance, HCl or ionic liquids).
When a suitable voltage is applied, the A element (often Al) is selectively oxidized and removed.

Benefits of electrochemical etching:

Better control over etching depth and surface terminations.
Can avoid concentrated HF, and in some cases be completely fluoride-free.
Suitable for making MXenes directly as films on conductive substrates.

Researchers have used this method to make Ti₃C₂Tₓ, Nb₂CTₓ, and other MXenes for applications such as supercapacitors, capacitive deionization, gas sensors, and lithium-ion batteries.

The main challenges are:

Lower speed compared to purely chemical etching
More complexity when scaling up to large volumes

But as power supplies and process control improve, electrochemical routes are becoming increasingly attractive.

4.5. Ball Milling–Assisted Approaches

Ball milling is a mechanical process where powders are placed with hard milling balls in a sealed jar and subjected to high-energy impacts.

In MXenes research, ball milling is used in several ways:

To reduce the particle size of MAX or MXene into nanostructures (nanodots, nanoribbons, porous structures).
To assist etching by pre-damaging or refining the MAX structure, making subsequent chemical etching easier and more uniform.
To mix MXenes with other materials (like metal oxides or carbon) to form composites.

Benefits include:

Simplicity and scalability
Control over particle size and morphology by adjusting milling time, ball size, and speed
Ability to produce porous MXenes with large surface areas, which improve performance in batteries and supercapacitors

However, the process must be optimized carefully to avoid damaging the MXene or introducing unwanted contamination from the milling media.

4.6. Ultrasonication and MXene Quantum Dots

Ultrasonication (using high-frequency sound waves) is commonly used after etching to help separate layers and break up larger flakes.

Under the right conditions, ultrasonication can also convert MXenes into quantum dots (MQDs) – tiny particles just a few nanometers in size. These MQDs have:

High surface area
Quantum confinement effects
Strong optical and fluorescent properties

Solvents like N-methyl-2-pyrrolidone (NMP), DMSO, DMF, or TBAOH are often used because their high boiling points and interactions with the MXene help overcome stacking forces.

Ultrasonication is also used to:

Assist etching by improving penetration of etchants
Introduce nitrogen or other dopants when used with specific precursors
Improve energy-storage performance by helping ion intercalation

Because it is relatively simple and can be combined with green solvents, ultrasonication is now a standard part of many MXene production workflows.

5. Bottom-Up Approaches: Building MXenes from the Ground Up

While most MXenes today are made by top-down etching of MAX phases, there is growing interest in bottom-up methods. These strategies build MXenes directly from atoms or molecules, without first making a MAX phase.

The main advantages:

Precise control over structure, thickness, and composition
Possibility of creating MXenes that are hard or impossible to obtain by etching
Potential for fewer defects and more uniform materials

However, these methods are currently more complex and harder to scale.

5.1. Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) is a technique where materials are grown one atomic layer at a time:

A first precursor is pulsed over the substrate and reacts with the surface.
Excess precursor and byproducts are purged.
A second precursor is pulsed, reacting with the first layer.
Repeating these cycles gradually builds up a film with atomic precision.

In the MXene context, ALD is used in two main ways:

To deposit thin oxides or other materials on top of MXenes, passivating or functionalizing them (for example, ZnO, SnO₂, or battery anode materials).
To carefully grow layered structures that resemble MXenes or serve as precursors for them.

ALD offers excellent conformality (uniform coating even on 3D structures) and precise thickness control. This helps:

Improve MXene stability against oxidation
Create better contacts for sensors and batteries
Tailor interfaces in thermoelectric or electronic devices

5.2. Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) uses gaseous precursors at high temperatures to deposit thin films on substrates. It is widely used in the semiconductor industry.

In MXene research, CVD has been used to:

Grow MXene-like carbides such as Ti₂CCl₂ directly on metal surfaces
Deposit protective or functional coatings on MXenes
Form hybrid structures like MXene + carbon nanotubes or graphene

CVD enables:

Large-area films
Good crystallinity (if conditions are carefully controlled)
Direct integration with industrial processes

However, CVD typically requires high temperatures and sophisticated equipment, so it is still mainly at the research stage for MXenes.

5.3. Pyrolysis

Pyrolysis is a high-temperature decomposition of organic or inorganic precursors in an oxygen-free environment (often under argon or nitrogen).

For MXenes, pyrolysis can be used to:

Convert precursors containing metals and carbon/nitrogen into carbide or nitride nanoparticles with MXene-like surfaces.
Transform MXenes into other functional materials, such as doped TiO₂ or carbon structures, while keeping some of their beneficial morphology.
Form composite structures where MXenes serve as templates or catalysts for carbon quantum dots or metal oxides.

Pyrolysis is:

Simple and effective
Compatible with recycled or low-cost starting materials
Useful for creating zero-dimensional or porous MXene-based structures

However, controlling exact layer structures and terminations is more difficult than in etching-based methods.

6. The Role of AI and Machine Learning in MXene Synthesis

An interesting aspect of the review is the mention of machine learning (ML) and artificial intelligence (AI). These tools can help by:

Analyzing large experimental databases
Predicting which etchants, temperatures, and times are likely to work best
Suggesting new, safer, and more efficient synthesis routes
Optimizing material properties such as conductivity or stability

For example, ML models have been used to:

Predict which MAX phases can form MXenes
Propose fluoride-free etchants
Shorten the trial-and-error phase in the lab

This integration of computation and experiments is expected to accelerate the discovery of new MXenes and make their production more industrially viable.

7. Putting It All Together: Where Is MXene Synthesis Headed?

From all the methods described, we can draw several key conclusions:

Top-down HF etching started the field and is still widely used because it is effective and understood. But its safety and environmental issues push researchers to move beyond it.
Fluoride-salt + acid routes and in-situ HF generation offer gentler versions of HF chemistry and are already common in labs.
Fluoride-free methods (alkali, HCl hydrothermal, supercritical CO₂, etc.) are becoming increasingly important, especially for electrochemical and biological applications where F-terminations are unwanted.
Electrochemical etching provides controllable, often safer synthesis, with interesting possibilities for integrated devices.
Ball milling and ultrasonication are valuable tools to tailor size, porosity, and structure, and to produce MXene quantum dots and advanced composites.
Bottom-up approaches such as ALD, CVD, and pyrolysis expand what is possible beyond MAX-phase etching and will likely play a bigger role as the field matures.
AI and ML will help navigate the huge design space of compositions, conditions, and applications, making synthesis faster and more predictable.

8. Final Thoughts

MXenes are no longer just a laboratory curiosity. They have evolved into a large and diverse family of 2D materials with impressive potential in:

Batteries and supercapacitors
Catalysis and photocatalysis
Gas and biosensing
Electromagnetic interference shielding
Flexible and wearable electronics

However, their future depends heavily on how we make them. Safer, scalable, and tunable synthesis strategies are essential if MXenes are to move from papers and prototypes into real products.

The review you provided offers a comprehensive map of where we are today:

It highlights the strengths and weaknesses of each synthesis route.
It emphasizes the need for fluoride-free, environmentally friendly processes.
It shows how top-down and bottom-up strategies can complement one another.

As research continues, we can expect more novel MXene compositions, better control of surface terminations, improved stability, and more integrated uses in complex devices.

In short, MXenes are a powerful toolbox for next-generation materials science. Understanding their synthesis strategies is the key to unlocking that toolbox — safely, efficiently, and creatively.

A Simple Guide to MXenes and How We Make Them

A Simple Guide to MXenes and How We Make Them