Two-dimensional (2D) materials have become a huge topic in materials science over the last 15–20 years. It started with graphene, a single layer of carbon atoms, and quickly expanded to many other 2D families like boron nitride, transition metal dichalcogenides (MoS₂, WS₂, etc.), oxides, and more.

One of the youngest and most exciting members of this family is MXenes – 2D transition metal carbides, carbonitrides and nitrides. They were first discovered in 2011, and since then research on MXenes has exploded, especially because they combine:

• High electrical conductivity

• Hydrophilic (water-loving) surfaces

• Easy, solution-based processing

• The ability to be made in relatively large quantities in water

This combination makes MXenes strong candidates for applications such as:

• Batteries and supercapacitors

• Electromagnetic interference (EMI) shielding

• Transparent conductive electrodes

• Water purification and filtration

• Sensors and electrocatalysis

In this blog, we’ll walk through how MXenes and related ultra-thin 2D carbides/nitrides are made, focusing on two big families of methods:

1. Top-down synthesis – starting from a layered 3D solid (like a MAX phase), then selectively etching and exfoliating it into 2D sheets.

2. Bottom-up synthesis – building 2D carbides/nitrides directly as thin films or nanosheets using methods like chemical vapor deposition (CVD), template methods or plasma-based techniques.

The goal is to give you a clear, simple overview without diving into unnecessary mathematical detail, and without inventing any test results or images that are not actually described in the text.

What Are MXenes Exactly?

MXenes generally have the formula:

• Mn+1XnTz or M1.33XTz

where:

• M = early transition metal (e.g., Ti, V, Cr, Nb, Mo, Ta, etc.)

• X = carbon (C) and/or nitrogen (N)

• Tz = surface terminations (often –OH, –O, –F, and sometimes others)

MXenes are usually produced from a parent family of layered solids called MAX phases. These have the formula:

• Mn+1AXn

where:

• M = transition metal

• A = element from groups 13–14 (like Al, Si, etc.)

• X = C and/or N

The MAX phases are layered, hexagonal carbides and nitrides. The idea behind MXene synthesis is:

1. Start from a MAX phase

2. Selectively remove the A-element layers (for example Al)

3. What remains is a stacked 2D carbide/nitride, now terminated with –O, –OH, –F, etc.

So you can think of MXenes as chemically “peeled” versions of MAX phases.

Two Big Strategies: Top-Down vs Bottom-Up

1. Top-Down: Etch + Exfoliate

Top-down methods start from a layered 3D crystal and use chemistry to “open it up” into 2D sheets. For MXenes, almost all top-down work starts from MAX phases.

The general steps are:

1. Synthesize or obtain a suitable MAX phase

2. Etch the A-layers (often Al) using a chemical that can dissolve or remove them

3. Get multilayer “MXene” powder (flakes stacked like a deck of cards)

4. Exfoliate these multilayers into single or few-layer MXene flakes, usually via intercalation (inserting ions/molecules between layers) and some mechanical energy (shaking, stirring, sonication)

We’ll come back to the details in a moment.

2. Bottom-Up: Grow the 2D Carbides/Nitrides Directly

Bottom-up methods do not start from MAX phases. Instead, they build thin carbide or nitride layers atom-by-atom or layer-by-layer. These methods include:

• Chemical vapor deposition (CVD)

• Template-assisted synthesis

• Plasma-enhanced pulsed laser deposition (PEPLD)

The important point: bottom-up 2D carbides and nitrides are often not strictly “MXenes”, because they don’t come from selective etching of MAX phases. But structurally and functionally, they can behave like ultra-thin 2D carbides/nitrides with interesting properties (e.g., superconductivity).

Precursors: What Do We Start From?

For top-down MXene synthesis, the main precursors are:

• Al-containing MAX phases, e.g.

  • Ti₃AlC₂ → Ti₃C₂Tz

  • Ti₂AlC → Ti₂CTz

  • Nb₂AlC → Nb₂CTz, etc.

There are also special layered precursors like:

• Mo₂Ga₂C → Mo₂CTz (via Ga etching)

• Layered compounds like Zr₃Al₃C₅ → Zr₃C₂Tz after etching Al-rich “blocks”

In recent years, more complex quaternary MAX phases have been discovered, where two different metals share the M site and can be:

• Out-of-plane ordered (o-MAX) – different metals in different layers

• In-plane ordered (i-MAX) – different metals ordered in the same layer

When these ordered MAX phases are etched, the resulting MXenes can have:

• Ordered metal layers

• Ordered vacancies (missing atoms)

• Mixed or “solid solution” metallic compositions

This dramatically expands MXene chemistry beyond simple Ti–C systems.

Etching: How Do We Remove the A-Layers?

Classic Route: HF Etching

The first MXene, Ti₃C₂Tz, was made by placing Ti₃AlC₂ powder in concentrated hydrofluoric acid (HF). HF:

• Selectively attacks and dissolves Al layers

• Leaves behind the M–X skeleton (e.g., Ti–C layers)

• Adds surface terminations like –F, –O, –OH

Over time, HF etching has been used for many different MAX phases to produce a variety of MXenes.

However, HF is:

• Highly corrosive and toxic

• Dangerous to handle and dispose of

So a lot of research is now focused on alternatives that avoid concentrated HF or avoid F entirely.

In-Situ HF: HCl + Fluoride Salt

A widely used safer-feeling alternative is in-situ HF formation using:

• HCl + LiF (most common)

• Or similar mixtures like HCl + NaF, KF, NH₄F

Here:

• HCl provides protons (H⁺)

• LiF (or other fluoride salts) reacts with HCl to form HF inside the solution, not as a separate bulk HF bottle

This method is particularly famous for:

• Ti₃C₂Tz made using LiF/HCl, often called a “clay-like” MXene, because the resulting multilayers swell in water and behave a bit like clays.

• At the same time, Li⁺ ions and water intercalate between the layers, which helps exfoliation later.

Even though HF still exists in the solution, you don’t handle concentrated HF directly. This makes lab work more manageable, though the waste still needs careful treatment.

Other F-Containing Etchants

• NH₄HF₂ – an ammonium bifluoride salt that acts as a HF source

• HF + oxidizers (like H₂O₂) – used, for example, to etch Si out of Ti₃SiC₂ and create Ti₃C₂Tz

F-Free or Low-Fluoride Paths

Because fluorine terminations (–F) can be a problem in some applications, especially biomedical ones, there is growing interest in F-free or F-poor methods.

Some reported routes include:

• Hydrothermal NaOH: High-temperature, concentrated NaOH can etch Ti₃AlC₂ to yield Ti₃C₂-type MXene without F, but the conditions (e.g., ~270 °C in autoclave) are harsh and not yet very scalable.

• Electrochemical etching in dilute HCl: MAX phases are used as electrodes in an electrochemical cell, and the A-layer is removed by anodic corrosion. Early versions had low yield and side products, but newer works have improved yields and performance.

• Electrochemical etching in F-free electrolytes (e.g. NH₄Cl + organic bases): More recent systems achieved over 40% yield for Ti₃C₂Tz without using F in the electrolyte and showed supercapacitor performance comparable to HF-etched MXene.

These approaches are still under development, but promising as safer, more tunable MXene production routes.

Exfoliation: From Multilayers to Single-Layer MXenes

After etching, you don’t immediately get free-floating single layers. You typically get multilayer MXene particles – stacks of sheets still stuck together.

To exfoliate them, researchers often:

1. Wash the etched powder (multiple times) to remove acids and byproducts (like AlF₃, LiF, etc.) and adjust the pH to near neutral.

2. Intercalate molecules or ions between the MXene layers to swell and weaken their interlayer bonding.

3. Apply mechanical energy – shaking, stirring, or sonication – to separate them into single or few-layer flakes.

Common Intercalants

• DMSO (dimethyl sulfoxide) – used for some MXenes like Ti₃C₂Tz and certain Mo-Ti MXenes. It increases the interlayer spacing and helps exfoliation, especially with co-intercalated water.

• Organic bases like TBAOH and TMAOH:

  • TBAOH (tetrabutylammonium hydroxide) works well for many MXenes like V₂CTz, Mo₂CTz, Ti₃CNTz, etc.

  • TMAOH (tetramethylammonium hydroxide) can delaminate Ti₃C₂Tz where TBAOH sometimes fails.

  • These large organic cations insert between layers and push them apart.

• Li⁺ from LiF/HCl etching – in that method, the Li⁺ is already intercalated during etching. With enough washing and sometimes gentle shaking (instead of harsh sonication), the material can spontaneously delaminate into colloidal MXene dispersions.

Stable MXene Colloids

Once exfoliated, MXene flakes can form stable colloidal suspensions in water (near neutral pH) or in polar organic solvents. These dispersions:

• Show a visible “Tyndall effect” (light scattering) when a laser beam passes through them

• Can be processed like an ink: filtered, spin-coated, spray-coated, printed, etc.

The stability of these suspensions depends strongly on:

• pH

• Oxygen content (dissolved O₂ can slowly oxidize MXenes)

• Solvent type

What Controls MXene Properties? Terminations, Order and Defects

Surface Terminations (Tz)

When you remove the A-layers, you don’t get a bare metal carbide surface; you get surfaces covered with –O, –OH, –F (and sometimes –Cl, etc.). These terminations affect:

• Electronic properties (conductivity, band structure)

• Hydrophilicity

• Stability in water

• Electrochemical behavior

Key points:

• HF-etched MXenes typically have more –F terminations.

• LiF/HCl-etched MXenes usually have less –F and more –O/–OH on average.

• Post-treatments (like heating or reaction with bases) can reduce –F content and increase –O/–OH. For example, controlled annealing can remove F at higher temperatures.

Different MXenes and different n values (e.g., Ti₂C vs Ti₃C₂ vs Nb₄C₃) show different balances between –O and –OH. This gives a lot of tuning possibilities, but also makes the system complex.

Chemical Order from o-MAX and i-MAX Precursors

When the parent MAX phase has ordered metal layers or in-plane order, the resulting MXene often preserves that order.

Examples include:

• Out-of-plane ordered MXenes, such as (Cr₂/₃Ti₁/₃)₃C₂Tz or (Mo₂/₃Ti₁/₃)₃C₂Tz – different metals in different layers.

• In-plane ordered MXenes, such as Mo₁.₃₃CTz, derived from (Mo₂/₃Sc₁/₃)₂AlC, with ordered “divacancies” (missing metal sites).

These ordered structures often show unique electrochemical behavior, e.g.:

• Mo₁.₃₃CTz can show higher volumetric capacitance than Mo₂CTz, partly because of the structured vacancies and metal arrangement.

Defects

No MXene is perfectly defect-free. Defects can originate from:

• The parent MAX phase

• Harsh etching conditions

• Aggressive sonication during exfoliation

Defects observed include:

• Atomic-scale vacancies and adatoms (e.g., missing Ti atoms, extra Ti atoms sitting on the surface)

• Pores and holes in the sheets, especially after long HF etching or strong sonication

• Broken or fragmented flakes when mechanical energy is too high

In general:

• Milder etching and gentler exfoliation (shaking instead of long sonication) tend to give larger, less defective flakes.

• Shorter etching times can reduce defects, but they may also leave more unetched MAX material.

Bottom-Up Synthesis: Building 2D Carbides and Nitrides from Scratch

Top-down MXenes are amazing, but they’re not the only way to get 2D carbides and nitrides. Several bottom-up methods can produce ultra-thin TMC (transition metal carbide) and TMN (transition metal nitride) films with high crystallinity.

Important note: These bottom-up products are often very thin films (a few nm thick), not single-layer sheets, but they do show 2D-like properties (e.g., 2D superconductivity).

Chemical Vapor Deposition (CVD)

CVD has been used to grow ultra-thin crystals such as:

• Mo₂C (often α-Mo₂C, sometimes β-Mo₂C)

• WC (tungsten carbide)

• TaC (tantalum carbide)

• TaN (tantalum nitride)

A classic CVD strategy is:

• Use a bilayer metal foil (e.g., Cu/Mo) as substrate

• Heat above or near the melting point of Cu

• Introduce a carbon source (e.g., methane, CH₄)

• Carbon atoms diffuse and react with Mo at the surface to form Mo₂C crystals on top

These CVD-grown crystals:

• Can have regular shapes (triangles, hexagons, rectangles, etc.)

• Are a few nm thick (but laterally microns in size)

• Show high crystalline quality (few defects)

Similar approaches have been extended to grow WC and TaC by swapping Mo for W or Ta.

More recent work has also:

• Tuned the shape and thickness of Mo₂C by adjusting CH₄ flow and Cu thickness

• Observed domain structures and structural transformations under electron beams

• Demonstrated that these thin films can show 2D superconductivity behavior

TMC/Graphene Heterostructures

CVD also enables heterostructures where 2D carbides and graphene are stacked or joined:

• Vertical heterostructures: Mo₂C + graphene stacked on top of one another

• In-plane heterostructures: WC domains embedded laterally in a graphene sheet

Two general strategies exist:

1. One-step CVD: grow Mo₂C and graphene simultaneously under controlled CH₄ and temperature conditions.

2. Two-step CVD: first grow continuous graphene on Cu; then grow Mo₂C underneath or above by changing temperature and gas composition.

These heterostructures can show:

• Orientation alignment between the carbide and graphene lattices

• Non-uniform strain in graphene (observable in Raman) due to strong coupling

• Unique electronic and superconducting behaviors not seen in purely stacked artificial layers.

Template-Based Methods

Template methods use 2D oxide nanosheets as precursors and convert them into nitrides or carbides:

• 2D MoO₃ → 2D MoN: by annealing in NH₃

• 2D MoO₂ → N-doped Mo₂C: by heating with a nitrogen-containing carbon source (like dicyandiamide)

In these methods:

• The oxide defines the 2D shape and thickness

• Reaction with NH₃ or carbon/nitrogen sources converts the oxide to nitride or carbide

• The resulting nanosheets can be extremely thin (down to sub-nanometer) but can contain pores or structural disorder depending on conditions.

Plasma-Enhanced Pulsed Laser Deposition (PEPLD)

PEPLD has been used, for example, to grow:

• Large-area, ultrathin fcc-Mo₂C films on sapphire substrates

This technique combines:

• A pulsed laser to vaporize Mo from a target

• A plasma (e.g., CH₄ plasma) as a carbon source

• A heated substrate (e.g., 700 °C)

The films:

• Are uniform and thickness-tunable (2–25 nm)

• Show a specific preferred structure (fcc Mo₂C) due to matching with the sapphire lattice

• Have more stacking faults than the best CVD-grown films, but still are interesting for fundamental studies.

Where Is the Field Going?

The MXene and 2D TMC/TMN fields are still evolving very rapidly. A few key trends and challenges stand out:

1. Beyond Ti₃C₂Tz

   • Ti₃C₂Tz is still the “star” MXene, but there is no reason to think it’s best for all applications.

   • Many newer MXenes (Nb-, Mo-, Cr-, V-, Ta-based; ordered o-MXenes; i-MXenes with vacancies) are just beginning to be explored.

2. Safer, F-Free Synthesis

   • HF is effective but dangerous.

   • In-situ HF (LiF/HCl) was a major step forward, but still uses fluoride.

   • F-free electrochemical and hydrothermal routes are promising but need better scalability, yields, and control over surface terminations.

3. Control Over Terminations and Defects

   • Terminations strongly impact performance in energy storage, catalysis, and biomedicine.

   • Being able to choose or modify terminations (e.g., more –O/–OH, no –F, possibly Cl, etc.) is a big research direction.

   • At the same time, controlling defects and flake size is critical for mechanical, electrical, and electrochemical properties.

4. Exploiting Ordered MAX Phases

   • o-MAX and i-MAX phases open the door to tailored MXenes with designed metal order and vacancies.

   • There are many predicted ordered MXenes that haven’t been made yet.

5. Bottom-Up 2D Carbides/Nitrides

   • CVD, template methods and PEPLD enable high-quality thin films and compositions not reachable by etching (e.g., pure WC, TaC, TaN).

   • The challenge is to reach truly monolayer or near-monolayer, wafer-scale, defect-controlled films and to integrate them into devices.

Closing Thoughts

MXenes started as a clever chemical trick: etch out a layer from a robust, strongly bonded 3D solid to get 2D sheets. That alone was a paradigm shift, because it showed you don’t need weak van der Waals solids to make 2D materials.

Today, MXenes form a rich, expanding family of about 30 experimentally synthesized compositions (and many more predicted). On top of that, bottom-up routes to 2D carbides and nitrides are growing in parallel, offering high-quality thin films and new chemistries.

From a practical point of view, the big questions now are:

• Which MXene or 2D carbide/nitride is best for which application?

• How can we scale production safely, reproducibly, and cheaply?

• How can we precisely tune structure, terminations, and defects to get the properties we want?

As synthesis methods become more refined—both top-down and bottom-up—MXenes and their cousins are very likely to move from “exciting lab materials” to key components in real devices, especially in energy, electronics, shielding, and sensing.