Two-dimensional (2D) materials continue to reshape modern materials science, giving researchers the ability to design structures with exceptional properties for advanced technologies. Within this class of materials, MXenes have emerged as one of the most exciting discoveries of the last decade. Their chemical versatility, tunable structure, high conductivity, and broad application potential—ranging from energy storage to catalysis—have positioned them as key candidates for next-generation devices.

In the study summarized here, researchers introduced a new approach for creating MXenes with randomly distributed atomic vacancies. This work centers around a specific MXene known as Nb1.33C, which is synthesized from a quaternary MAX phase, (Nb₂/₃Sc₁/₃)₂AlC. By etching both the aluminum (Al) and scandium (Sc) atoms from the parent material, the authors were able to create a 2D structure full of vacancies—empty positions where atoms used to be—leading to unusual and potentially useful properties.

This blog aims to make the entire study easy to understand, even for readers who do not have a deep background in solid-state chemistry. We’ll explore why vacancies matter, how the researchers produced this new MXene, and what makes this discovery important for future scientific and industrial applications.

Understanding the Foundation: MXenes and MAX Phases

Before diving into Nb1.33C, it’s important to understand how MXenes originate.

MXenes are typically made by chemically removing the A-layer (commonly aluminum or gallium) from a 3D layered material known as a MAX phase. A MAX phase has the general formula:

Mn+1AXn,
where:

• M = transition metal (such as Ti, Nb, Mo, V)

• A = group 13 or 14 element (such as Al)

• X = carbon and/or nitrogen

• n = 1, 2, or 3

When the A-layer is removed, the remaining material rearranges into thin, 2D sheets called MXenes. These sheets contain surface terminations, denoted as Tx, such as –O, –OH, and –F, depending on the etching conditions.

What makes MXenes fascinating is that they can be tuned on three levels:

1. Composition – changing which metals or nonmetals are present

2. Surface Terminations – altering the chemical groups binding to the surface

3. Structure – modifying the morphology, thickness, or introducing defects

This third category—structural tuning—forms the heart of the study.

Why Vacancies Matter in 2D Materials

In 2D materials, every atom plays a crucial role in determining the final properties. When a material contains vacancies (missing atoms), several things can change, including:

• Electrical conductivity

• Surface reactivity

• Catalytic behavior

• Energy storage capacity

• Ion transport

• Mechanical properties

Vacancies can therefore be used deliberately to enhance or redesign functionalities. Previous research has shown that ordered vacancies in MXenes—like those in Mo1.33C—can significantly increase conductivity and improve energy storage performance.

In this new research, scientists applied a similar idea but achieved randomly distributed vacancies instead of ordered patterns. This randomness can offer different opportunities for property manipulation.

Introducing a New Route to Vacancy-Rich MXenes

The authors developed a way to produce a vacancy-rich MXene by starting with a quaternary solid-solution MAX phase that contains two different metals on its M-site: niobium (Nb) and scandium (Sc).

The initial MAX phase they synthesized was:

(Nb₂/₃Sc₁/₃)₂AlC

In this structure:

• Nb and Sc atoms share the same metal layer

• Sc acts as the “minority” metal and is more chemically reactive than Nb

• Al is also present, as in traditional MAX phases

The idea is that if both Sc and Al can be selectively etched away, the resulting MXene will have Nb layers containing vacancies where Sc used to be.

This approach can, in principle, be generalized to any MAX phase containing different M-site metals where one is easier to remove than the other. This makes the method a promising blueprint for creating other vacancy-engineered MXenes.

Synthesis of the Quaternary MAX Phase

The team synthesized the (Nb₂/₃Sc₁/₃)₂AlC phase by reacting elemental Nb, Sc, Al, and carbon at high temperature in an argon atmosphere.

During synthesis, multiple phases formed, including:

• The desired quaternary MAX phase

• NbC

• NbAl₃

• Two previously unreported phases:

  • (Nb₂/₃Sc₁/₃)₃AlC₂

  • (Nb₂/₃Sc₁/₃)₄AlC₃

The identification of these new phases contributes to the broader understanding of quaternary MAX chemistry.

Because Nb and Sc are randomly distributed in the metal layers, the structure maintains the symmetry of conventional MAX phases. The presence of Sc increases some lattice parameters because Sc atoms are larger than Nb atoms.

From MAX to MXene: Producing Nb1.33C

To convert the MAX phase into the MXene Nb1.33C, the researchers etched the material in 48% hydrofluoric acid (HF) at room temperature.

Both Sc and Al were successfully removed, leaving behind:

• A 2D MXene composed primarily of Nb and C

• A significant number of vacancies where Sc atoms previously resided

These vacancies appeared as randomly distributed empty spaces within the MXene lattice, giving the final material its formula:

Nb1.33C

The vacancy concentration varied locally but was found to be within the range of 20–35%.

Improved Etching Efficiency Due to Scandium

One interesting outcome of the quaternary design was that the presence of Sc greatly improved the MXene synthesis process.

For reference:

• A traditional Nb2CTx MXene (made from Nb2AlC) required 100 hours of etching.

• The quaternary MAX phase needed only 30 hours under similar conditions.

Additionally, the yield of delaminated MXene flakes was higher.

This efficiency comes from the fact that Sc–C and Sc–Al bonds are weaker and easier to break compared to Nb–C and Nb–Al bonds.

Forming Delaminated and Free-Standing MXene Films

After etching, the multilayer Nb1.33CTx material was intercalated with TBAOH, allowing the layers to separate into near-single-layer flakes. When the researchers filtered the suspension, they obtained:

• Flexible, free-standing films of Nb1.33CTx

• In contrast, Nb2CTx formed brittle films

This difference highlights how vacancies and composition can influence mechanical behavior.

Chemical Composition and Surface Groups

Using established analytical techniques, the researchers identified the approximate surface chemistry of the MXene. The key findings include:

• Oxygen-based terminations dominate

• –OH and –F groups are also present in smaller amounts

• Some terminations may occupy vacancy sites rather than surface sites

These chemical details help determine the MXene’s electronic, electrochemical, and surface reactivity properties.

Electrical Behavior and Transport Mechanism

The researchers analyzed how the MXene’s electrical resistivity changed with temperature. As the temperature decreased from room temperature to very low temperatures, the resistivity increased significantly.

This behavior suggests:

• Electrical conduction is not metallic

• Instead, it follows a variable-range hopping (VRH) mechanism

VRH is common in disordered materials where electrons move between localized states rather than flowing freely.

Additionally, at low temperatures under a magnetic field, the MXene exhibited negative magnetoresistance. This is unusual compared to many MXenes but has been observed in other disordered materials.

Key Outcomes of the Study

This work provides several important contributions to the field of MXene research:

1. A New Vacancy-Rich MXene Was Successfully Synthesized

The Nb1.33C MXene with randomly distributed vacancies opens the door to tuning properties in new ways.

2. The Method Can Be Generalized

Any quaternary MAX phase with a more reactive minority metal could potentially be used to produce a vacancy-engineered MXene.

3. Etching Time and Yield Improved Dramatically

The addition of Sc significantly reduced etching duration and increased overall yield, helping overcome a major barrier in MXene synthesis.

4. Vacancies Influence Properties and Film Formation

The presence of vacancies not only affects electronic behavior but also gives rise to more flexible films—a desirable feature for practical applications.

5. The Work Revealed New MAX Phases

The discovery of (Nb₂/₃Sc₁/₃)₃AlC₂ and (Nb₂/₃Sc₁/₃)₄AlC₃ expands the structural diversity of known MAX phases.

Why This Matters for Future Technologies

Vacancy engineering is a powerful tool for improving or tailoring material performance. With this new method, researchers can:

• Modify electronic properties

• Enhance catalytic activity

• Improve adsorption behavior

• Increase ion storage capacity

• Tune mechanical flexibility

This makes vacancy-rich MXenes attractive for applications such as:

• Batteries and supercapacitors

• Hydrogen evolution and other catalytic processes

• Electronic and sensing devices

• Flexible and wearable technologies

Moreover, the idea of “dialing in” vacancy concentrations could lead to MXenes engineered with extremely high precision for specialized needs.

Conclusion: A Step Forward in MXene Design

This study demonstrates a simple, effective, and scalable way to introduce random vacancies into MXenes by using quaternary MAX phases with reactive minority metals. The result—Nb1.33C MXene—exhibits unique structural and electronic characteristics that distinguish it from vacancy-free MXenes.

As the field continues to grow, these insights will likely inspire new strategies for designing MXenes with controlled defects, enabling the next generation of functional materials for advanced energy, environmental, and electronic technologies.