Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti₃C₂ MXene

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti₃C₂ MXene

MXenes are a family of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides. They have attracted major attention because of their excellent electrical conductivity, tunable surface chemistry, high mechanical strength, and ability to disperse easily in water. These properties make MXenes ideal for applications such as:

  • energy storage (batteries, supercapacitors)

  • EMI shielding

  • sensors

  • composites

  • biomedical technologies

  • catalysis

Among all MXenes, Ti₃C₂Tₓ is the most widely used and studied. It has some of the highest conductivity reported for solution-processed 2D materials, making it important for future electronic, energy, and composite technologies.

However, one of the biggest limitations of Ti₃C₂Tₓ, and MXenes generally, is that they oxidize quickly, especially when stored as water-based suspensions. Even freshly made Ti₃C₂Tₓ typically lasts:

  • 1–2 weeks in water

  • sometimes only a few days for some compositions

  • even as powders, they degrade over time

Oxidation leads to loss of conductivity, structural breakdown, formation of TiO₂, and complete loss of MXene performance.

This study focuses on solving that problem.

2. What the Researchers Discovered

The researchers found a surprisingly simple and effective way to dramatically increase the stability of Ti₃C₂ MXene:

They added extra aluminum during MAX phase synthesis.

This modified synthesis produces a better MAX precursor called Al-Ti₃AlC₂, which leads to:

  • MXene flakes with far fewer defects

  • much more ordered MAX crystal structure

  • smoother MXene edges

  • significantly reduced vacancy defects

  • higher flake quality

  • outstanding oxidation resistance

As a result, Ti₃C₂Tₓ MXene produced from Al-Ti₃AlC₂ (called Al-Ti₃C₂) has:

  • over 10 months of shelf life in water (versus 1–2 weeks normally)

  • much higher conductivity (up to 20,000 S/cm)

  • oxidation starts 100–150 °C later than normal Ti₃C₂

  • stronger thermochemical stability

  • extremely clean, defect-free flake edges

This is a breakthrough that can allow MXenes to finally be used reliably and commercially.

3. Why Adding Extra Aluminum Helps

Traditionally, MAX phases (the precursors of MXenes) are made using precise stoichiometric ratios. Scientists previously assumed that phase-pure MAX would produce the best MXene.

However, this study overturns that assumption.

When you add extra aluminum (Al) during the MAX synthesis:

  • a molten aluminum phase forms during high-temperature sintering

  • molten Al increases diffusion rates of atoms

  • this helps Ti, C, and Al atoms arrange in a more ordered structure

  • the final MAX grains have better crystallinity and fewer defects

Even though the extra Al introduces intermetallic impurities, those impurities can be easily washed away with HCl.

The end result is a higher quality MAX crystal, and therefore, a higher quality MXene after etching.

4. Characterization of the Modified MAX (Al-Ti₃AlC₂)

4.1 XRD Analysis

The X-ray diffraction showed:

  • initial impurities like TiAl₃

  • but after HCl washing, these impurities disappear

  • the MAX phase peaks remain, indicating a purified structure

4.2 Raman Spectroscopy

Raman data revealed:

  • slight shifts in vibrational modes

  • broader Al-related vibrational peaks

  • signals that the Al layer environment has changed

  • evidence for improved structural ordering

4.3 SEM Imaging

SEM images showed:

  • large, clean, well-formed hexagonal grains

  • smooth surfaces

  • better grain geometry than conventional MAX

These observations confirm that the molten Al improved the grain formation.

5. Producing MXene from the Modified MAX: Al-Ti₃C₂

After washing the MAX, the authors etched it using HF/HCl and delaminated it using LiCl, following standard MXene protocols.

The resulting MXene flakes:

  • kept the clean hexagonal shape of the MAX

  • had smoother edges compared to conventional Ti₃C₂

  • showed fewer defects and pinholes

  • had cleaner basal planes

  • were highly dispersible in water

TEM images clearly showed that the Al-Ti₃C₂ flakes have dramatically reduced defects, particularly at the edges.

This is a major reason for the improved stability.

6. Record-Breaking Conductivity

Typically, Ti₃C₂Tₓ MXene films have conductivities between 8,000 and 15,000 S/cm.

But films made from Al-Ti₃C₂ reached:

> 20,000 S/cm — the highest conductivity ever reported for solution-processed 2D materials.

This improvement comes from:

  • fewer defects

  • better flake alignment

  • improved crystallinity

  • smoother edge geometry

This makes Al-Ti₃C₂ one of the most electrically conductive 2D materials available today.

7. Dramatically Improved Oxidation Stability

7.1 TGA Data

Thermogravimetric analysis showed:

  • conventional Ti₃C₂ oxidizes early and rapidly

  • Al-Ti₃C₂ resists oxidation 150 °C longer

  • Al-Ti₃C₂ powder oxidizes much more slowly

  • delaminated Al-Ti₃C₂ films resist oxidation up to 450 °C, much higher than usual

Why this matters

MXene was previously unusable in environments above 200 °C.
Now, applications like:

  • high-temperature electronics

  • sensors near engines

  • hybrid composites

  • thermal devices

can become feasible.

8. Extraordinary Long-Term Shelf Stability

The most remarkable result is that Al-Ti₃C₂ solutions remain stable for more than 10 months at room temperature with minimal protection.

Normal Ti₃C₂ lasts:

  • 1–2 weeks, even under careful storage

  • sometimes only a few days for M₂C phases

How the samples were stored

  • Degassed with argon

  • Put into sealed vials

  • Stored in a drawer at room temperature

  • No freezer, no antioxidant additives, no special handling

UV-Vis tracking

Over 10 months:

  • concentration barely changed

  • spectral red-shift was minimal

  • optical features remained stable

  • no significant degradation appeared until after ~6 months

  • even after 10 months, almost no TiO₂ particles were detected

Conductivity after aging

Films made from:

  • 4-month-old solution → >10,000 S/cm

  • 6-month-old solution → >6,000 S/cm

  • 10-month-old solution → still showed low oxidation and minimal pinholes

This performance is unprecedented.

9. Why Al-Ti₃C₂ Is So Much More Stable

The researchers propose several reasons:

9.1 Fewer defects

Traditional MAX synthesis introduces vacancies:

  • Al monovacancies (VAl)

  • Al divacancies

  • divacancies involving Al and C

These turn into defects in MXene after etching.

But adding extra Al reduces these vacancies, leading to:

  • better Ti:C stoichiometry

  • fewer weak spots

  • stronger MXene lattice

  • improved resistance to hydrolysis and oxidation

9.2 Cleaner edges

Oxidation begins at edges and defects.
The improved MXene has:

  • smoother edges

  • no rough protrusions

  • fewer pinholes

  • fewer reactive sites

9.3 Lower oxygen content in MAX

XPS showed reduced oxygen in the MAX, which means:

  • fewer oxycarbides

  • less initial oxidation

  • cleaner Ti–C bonding

  • higher quality flakes

Together, these features make Al-Ti₃C₂ inherently more stable.

10. Implications for MXene Production

This study fundamentally changes how scientists view MAX synthesis:

Before:

  • MAX phase purity was thought to be the most important factor.

Now:

  • Crystal ordering

  • Stoichiometry

  • Defect density

  • Bond uniformity
    are much more important.

Adding excess Al might initially seem counterproductive because it introduces impurities.
However:

  • those impurities are removable

  • the improved structural ordering remains

  • the resulting MXene is dramatically better

This will likely reshape future MXene synthesis protocols.

11. Industrial and Commercial Impact

The improvements in MXene stability and conductivity make large-scale applications far more realistic:

1. Energy storage devices

MXene electrodes that stay stable for months instead of days.

2. Conductive inks

Long-lasting suspensions that can be shipped globally without degrading.

3. EMI shielding coatings

More durable films for aerospace and telecom applications.

4. High-temperature electronics

MXenes that withstand >400 °C.

5. Sensors

More reliable environmental and chemical sensors.

6. Composites

Better reinforcement for metals, polymers, and ceramics.

This study could accelerate MXene commercialization by several years.

12. Conclusion 

This groundbreaking research shows that:

  • Modifying the MAX phase synthesis by adding excess aluminum dramatically enhances the structural quality of Ti₃AlC₂.

  • The resulting MXene (Al-Ti₃C₂) has far better stability, much higher conductivity, and greater resistance to oxidation.

  • Al-Ti₃C₂ MXene remains stable for over 10 months, a record for aqueous MXene suspensions.

  • Conductivity can reach 20,000 S/cm, the highest for any solution-processed 2D material.

  • Oxidation resistance increases by more than 150 °C.

  • This work will enable MXenes to transition from laboratory research materials into reliable industrial materials.

Overall, the study provides a major step forward toward making MXenes practical for real-world technologies.

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