In this blog, we’ll walk through a study that looks at a very specific but exciting question:

What happens to the gas-sensing performance of Ti₂CTₓ MXene when it is partially oxidized into TiO₂?

If that sounded a bit dense, don’t worry. We’ll unpack everything step by step in simple, logical language, without assuming you are an expert in spectroscopy or solid-state physics. The goal is a clear, useful, and consistent story – no imaginary figures or made-up test results, only what the paper actually did and found.

1. Background: MXenes and Gas Sensors

MXenes are a family of two-dimensional (2D) materials made from transition metal carbides, nitrides, or carbonitrides. They are usually written with a general formula like:

Mₙ₊₁XₙTₓ,
where M = a transition metal (like Ti), X = C or N, and Tₓ = surface groups such as –O, –OH, –F, etc.

In this work, the focus is on Ti₂CTₓ, a titanium carbide MXene.

Why are MXenes interesting?

• They have a very large surface area, which is useful for any surface-based process (like adsorption of gas molecules).

• They can be highly electrically conductive, sometimes metallic in behavior.

• Their surfaces are naturally decorated with functional groups (–OH, –O, –F, etc.), and these groups strongly affect how gases interact with them.

• Their layered 2D structure allows interlayer spaces that can host water, ions, and gas molecules.

These properties make MXenes promising for:

• Batteries and supercapacitors

• Fuel cells

• Photocatalysis

• Gas sensing

In gas sensing, the materials studied here work as chemoresistive sensors. That means the material changes its electrical resistance when gas molecules are adsorbed or desorbed from its surface. The sensor simply measures resistance and correlates the change to gas concentration.

2. MXenes vs Classical Metal Oxide Gas Sensors

Traditionally, chemoresistive gas sensors use metal oxide semiconductors (MOS) like SnO₂, ZnO, TiO₂, etc. In those materials:

• The surface adsorbs oxygen species (O²⁻, O⁻, etc.).

• These oxygen species pull electrons from the semiconductor near the surface.

• That creates an electron depletion layer, which changes resistance when target gases react with these oxygen species.

This model is quite well established for MOS materials.

For MXenes, things are more complex:

• They often show metallic-like conductivity, not typical semiconductor behavior.

• Their surfaces are fully covered with terminal groups.

• Their structure is layered and often contains structural water and hydroxyl groups.

As a result, the classical MOS model doesn’t fully describe MXene sensing. Researchers are still building theoretical and experimental models to explain how MXenes respond to gases.

One key fact: MXenes are very sensitive to moisture. Water molecules and hydroxyls (–OH) can strongly influence their electronic structure and gas interaction.

3. MXenes, Water, and Hydrogen Bonding

Previous studies (summarized in the article) have shown that:

• Water can be adsorbed on the MXene surface and in the interlayer spaces.

• Structural water and –OH groups can create an electrostatic field that hinders charge transfer, which increases resistance as humidity rises.

• The interlayer spacing in Ti₃C₂Tₓ, for example, accommodates water and other molecules without causing capillary condensation (unlike many bulk porous materials).

MXenes tend to show strong responses to gases that can form hydrogen bonds, such as:

• Ammonia (NH₃)

• Ethanol (C₂H₅OH)

• Acetone (C₃H₆O)

• Other polar organics

Hydroxyl groups on the surface often play a key role in these interactions.

Most reported titanium carbide MXene sensors show a positive (p-type) response to such gases – meaning resistance increases when these gases are introduced.

However, there are some exceptions where negative (n-type) responses are seen. This suggests that the detailed surface chemistry and synthesis conditions (for example, what terminal groups dominate: –Cl, –F, –O, –OH) can flip the behavior.

4. Why Combine Ti₂CTₓ with TiO₂?

Another important direction is the creation of MXene/metal oxide nanocomposites, especially MXene/TiO₂.

These can be made by:

• Thermal oxidation of MXene in air

• Oxygen plasma treatment

• Hydrothermal approaches

• Light-assisted oxidation in solution

• Spraying TiO₂ onto MXene surfaces

Earlier work has shown that MXene/TiO₂ composites can:

• Improve sensitivity and selectivity to certain gases

• Change the operating temperature range

• Modify response and recovery times

The idea is that combining a conductive 2D MXene with a semiconducting oxide (TiO₂) creates heterojunctions that change how charges move under gas exposure.

This paper focuses specifically on Ti₂CTₓ MXene and its partial oxidation to form Ti₂CTₓ/TiO₂ nanocomposites, and then examines how this affects gas-sensing performance.

5. How Ti₂CTₓ MXene Was Synthesized and Processed

The starting point is the MAX phase Ti₂AlC.

1. MAX phase synthesis (Ti₂AlC)

   • Powders of titanium, aluminum, graphite, and potassium bromide (KBr) are mixed in a specific ratio.

   • The mixture is pressed into tablets and heat treated at 1000 °C in molten KBr.

   • This forms Ti₂AlC MAX phase.

2. Etching to obtain Ti₂CTₓ MXene

   • Ti₂AlC powder is added to a mixture of NaF (sodium fluoride) and HCl (hydrochloric acid).

   • This selectively removes the aluminum layers from Ti₂AlC.

   • The mixture is stirred at about 40 °C for 24 hours.

   • The product is washed repeatedly with distilled water until the pH is around 5–6.

   • This produces multilayer Ti₂CTₓ MXene.

3. Delamination of MXene

   • A tetramethylammonium hydroxide (TMAOH) solution and ultrasound are used to delaminate the multilayer MXene into thinner, low-layer stacks or flakes.

4. Storage and film preparation

   • The resulting Ti₂CTₓ dispersion is stored in water at 4–6 °C for 7 days.

   • Then, a small volume of the dispersion (about 50 µL) is drop-cast onto a specialized Al₂O₃ (alumina) sensor substrate.

   • This substrate has platinum interdigital electrodes on the front and a platinum microheater on the back.

   • After drop casting, the film is dried stepwise between 25 and 150 °C under reduced pressure.

This produces a Ti₂CTₓ MXene film on an Al₂O₃/Pt sensor platform, suitable for in situ Raman heating experiments and for gas-sensing tests.

6. Studying Oxidation with In Situ Raman Spectroscopy

To understand how Ti₂CTₓ transforms into TiO₂ as temperature increases, the authors used in situ Raman spectroscopy. They heated the MXene film on the sensor substrate and recorded Raman spectra at different temperatures.

They considered two areas:

• On top of the platinum electrodes

• On the ceramic Al₂O₃ surface between the electrodes

This distinction is important because the substrate material affects heat distribution, local environment, and even Raman signal intensity.

Key findings on platinum:

• At room temperature, the Raman spectrum shows:

  • Modes associated with Ti₂CTₓ MXene (and possibly some residual MAX phase).

  • Features related to rutile-like Ti–O bonds, likely from TiO₂ species formed during the aqueous storage step.

  • Carbon D and G bands, consistent with sp² carbon (from MXene or byproducts).

  • Signals from TMAOH residues and nitrate groups (formed during previous NO₂ exposure).

• As the temperature increases:

  • Above about 147 °C, the TMAOH and nitrate-related modes disappear, likely due to desorption or decomposition of those groups.

  • At temperatures of 372 °C and above, distinct Raman modes corresponding to TiO₂ anatase (especially a strong mode around 156 cm⁻¹) appear.

  • At the same time, more pronounced modes from rutile TiO₂ become visible.

• At 447 °C, the Raman spectrum is dominated by anatase and rutile TiO₂ modes, indicating that the MXene has been fully oxidized to TiO₂ in this region.

Key findings on Al₂O₃ ceramic:

• Signals from the Al₂O₃ substrate itself are always present.

• The Ti₂CTₓ MXene signal is weaker and partly masked, but:

  • At 316 °C, anatase TiO₂ modes already appear.

  • This means oxidation begins earlier on the ceramic surface (316 °C) than on the platinum electrodes (372 °C).

• At 410–447 °C, the film on Al₂O₃ is essentially TiO₂ rich, mainly in the anatase form, with only a weak rutile signal.

Important takeaway:

• On Al₂O₃: the onset of anatase (TiO₂) appears at 316 °C.

• On Pt: it appears at 372 °C.

• At 447 °C, the film is fully oxidized to TiO₂.

For gas sensing, the area between the electrodes (on Al₂O₃) is the most critical, because that is where current flows from one electrode to the other through the film.

7. How the Microstructure Changes with Oxidation

The microstructure of the films was studied with:

• Scanning Electron Microscopy (SEM)

• Atomic Force Microscopy (AFM)

• Kelvin Probe Force Microscopy (KPFM) for work function mapping.

As-prepared Ti₂CTₓ MXene film

• Shows a wavy, folded, layered structure, like “sea waves”.

• The surface is highly developed, which is good for gas adsorption.

• Some small spherical particles (~200 nm) appear, probably impurity or oxidation products.

After oxidation at 316 °C (Ti₂CTₓ/TiO₂ composite)

• The wavy structure remains, but the film becomes more uniform.

• Larger spherical particles (1–3 µm) appear, attributed to TiO₂ aggregates formed during oxidation.

• AFM confirms:

  • A combination of wave-like features and spherical aggregates.

  • The film is electrically inhomogeneous, with local work function values between ~4.57 and 5.00 eV.

  • The average work function (~4.71 eV) is close to previously reported values for Ti₂CTₓ MXene, indicating that at 316 °C, a true Ti₂CTₓ/TiO₂ nanocomposite is formed (not fully oxidized).

After oxidation at 447 °C (fully oxidized TiO₂ film)

• The wavy MXene-like structure disappears.

• The surface becomes smoother, formed mostly by spherical aggregates ~2–4 µm in size, plus smaller particles (~45–95 nm) on top.

• The roughness is relatively low (around 20 nm RMS over 100 µm²).

• KPFM shows a more uniform work function, with an average around 5.00 eV, consistent with anatase TiO₂.

This confirms the structural transition from a MXene-dominated film to a TiO₂-dominated oxide film as oxidation temperature rises.

8. Strong Sensitivity to Humidity

Before testing gases, the authors measured how the films respond to humidity.

Conditions:

• Room temperature

• Relative humidity (RH) varied from 30% up to 93%

Both Ti₂CTₓ and Ti₂CTₓ/TiO₂ films showed:

• Very large changes in resistance with humidity.

• As RH increased, the response to humidity increased significantly (up to around 90–96% at 93% RH depending on the sample).

The original Ti₂CTₓ film was slightly more sensitive at very high humidity than the oxidized composite.

This is consistent with what is already known about MXenes: their surfaces and interlayers strongly interact with water, forming hydroxyl groups and structural water that directly affect electrical transport.

Because humidity has such a strong effect, the authors chose to perform all gas tests at a fixed 50% RH, to mimic realistic conditions and keep humidity as a controlled background.

9. Gas Sensing of the Original Ti₂CTₓ Film

The authors then tested chemoresistive responses at room temperature and 50% RH for the initial Ti₂CTₓ film.

They exposed the film to:

• 100 ppm: CO, NH₃, benzene (C₆H₆), acetone (C₃H₆O), ethanol (C₂H₅OH)

• 1000 ppm: H₂, CH₄

• Various oxygen concentrations (0.4–10% O₂)

Key observations:

• The strongest response was to NH₃ (~24% change).

• Significant responses were also seen for:

  • CO (~15%)

  • Benzene (~13%)

  • Ethanol (~11.5%)

  • Acetone (~8.3%)

• Responses to H₂ and CH₄ were smaller (about 4%).

• The film also responded to oxygen: as O₂ concentration increased from 1% to 10%, the response grew from about 4% to 33%.

Most importantly:

For all gases (including reducing gases and oxygen), the resistance decreased when the gas was introduced.
This is an n-type response.

This is unusual for titanium carbide MXenes, which often show p-type behavior (resistance increases when exposed to many gases).

The authors point out that:

• The Ti₂CTₓ surface already contains oxygen-containing groups.

• There is also an impurity of rutile TiO₂ even in the “initial” film (from storing the dispersion in water).

• These factors may cause the MXene film to behave differently, more like an n-type oxide-influenced system.

The exact mechanism behind this n-type response is not fully clear yet and is left as an open question in the paper.

10. Gas Sensing of the Partially Oxidized Ti₂CTₓ/TiO₂ Composite (316 °C)

Next, the same type of measurements were performed on the film oxidized at 316 °C, which is a Ti₂CTₓ/TiO₂ nanocomposite.

Same general conditions:

• Room temperature

• 50% RH

• Same sets of gases and concentrations

Key findings:

• The composite shows higher responses to almost all gases (except O₂) compared to the original MXene film.

• Again, the highest response is to NH₃, reaching about 61% at 100 ppm.

• The response to NO₂ is strongly enhanced (about ten times higher than for the unoxidized MXene).

• The response to ethanol is also significantly improved.

• Interestingly, for this composite:

  • For gases like CO, H₂, CH₄, benzene, acetone, and ethanol, the film shows a p-type response (resistance increases).

  • For NH₃ and NO₂, the film shows an n-type response (resistance decreases).

So, oxidation not only boosts sensitivity, especially to NH₃ and NO₂, but also changes the sign of the response for most gases.

Ammonia sensing in more detail

The authors specifically looked at NH₃ concentrations from 4 ppm to 100 ppm for the composite film.

• As NH₃ concentration increased, the response increased from about 16% to 61%.

• The response was reproducible, although there was some baseline drift, which they attribute to the strong interaction between NH₃ and the receptor material.

This demonstrates that the composite can detect ammonia at comparatively low concentrations and with a large signal at room temperature and realistic humidity.

11. Why Does Partial Oxidation Improve Gas Sensing?

The paper doesn’t propose a very detailed microscopic model, but it gives a clear physical picture:

1. Ti₂CTₓ alone already has surface functional groups, structural water, and some TiO₂ impurities. It is highly sensitive to humidity and gases, but its behavior can be complex and sometimes non-typical (such as n-type response to many gases).

2. Partial oxidation at 316 °C creates a Ti₂CTₓ/TiO₂ composite:

   • You have MXene regions that are more conductive, with layered structures.

   • You have TiO₂ particles, mostly anatase, which behave as n-type semiconductors.

   • At the interfaces between Ti₂CTₓ and TiO₂, heterojunctions (Schottky-like barriers) form.

3. These interfaces are very sensitive to changes in surface charge and adsorbed species:

   • When a gas is adsorbed and transfers charge (or modifies surface states), the barrier height and width change, which strongly affects the overall resistance.

   • This can explain why the partially oxidized composite has higher responses and why the sign of the response changes for many gases.

At room temperature, the MXene part likely provides the main conduction channel, while TiO₂ regions and their interfaces with MXene help modulate the current in a more pronounced way when gases are adsorbed.

In short:

A little bit of oxidation makes Ti₂CTₓ more useful as a gas sensor than no oxidation or complete oxidation.

Complete oxidation (447 °C) turns the film into basically TiO₂, which at room temperature has very high resistance and becomes difficult to use in this configuration.

12. How This Work Compares to Other MXene/TiO₂ Sensors

The article also briefly compares its results with other reported Ti₃C₂Tₓ/TiO₂ and Ti₂CTₓ/TiO₂ composite sensors.

• Most previous work focuses on Ti₃C₂Tₓ, not Ti₂CTₓ.

• Many prior sensors operate at dry or low-humidity conditions, while this study uses 50% RH, which is closer to real environments.

• Ammonia often emerges as the gas with the highest response in MXene/TiO₂ systems, consistent with theoretical predictions about strong adsorption and charge transfer.

The authors emphasize that their Ti₂CTₓ/TiO₂ composite:

• Shows strong ammonia sensitivity across a relatively wide concentration range.

• Performs well at room temperature and moderate humidity.

• Achieves responses that are comparable to or better than many previously reported systems.

13. Practical Takeaways and Future Directions

From a practical standpoint, this study suggests several useful ideas:

• MXene films should not necessarily be kept “perfectly pristine” if you want good gas sensing. Controlled partial oxidation can actually improve performance.

• Substrate choice matters:

  • Oxidation starts earlier on Al₂O₃ than on Pt.

  • For sensor design, the combination of MXene, oxide, and substrate must be considered as one integrated system.

• Humidity cannot be ignored:

  • MXenes are strongly humidity-sensitive.

  • Real-world gas sensors based on MXenes must work with or compensate for varying RH.

The work also shows that:

• There is room to tune gas selectivity by controlling oxidation conditions and the MXene/TiO₂ ratio.

• Particularly, highly sensitive ammonia sensors operating at room temperature and realistic humidity can be designed using Ti₂CTₓ/TiO₂ nanocomposites.

14. Conclusion

To summarize the story in simple terms:

• The authors synthesized Ti₂CTₓ MXene from a Ti₂AlC MAX phase and formed films on Al₂O₃/Pt sensor substrates.

• They used in situ Raman spectroscopy to carefully monitor how these films oxidize when heated in air.

  • On Al₂O₃, anatase TiO₂ started to form at 316 °C.

  • On Pt, anatase appeared at 372 °C.

  • At 447 °C, the film was fully oxidized to TiO₂ (anatase + rutile).

• Microstructural analysis (SEM, AFM, KPFM) confirmed the transformation:

  • From layered, wavy Ti₂CTₓ structures

  • To a Ti₂CTₓ/TiO₂ composite with mixed morphology

  • Finally to a mostly TiO₂ film with smoother spherical aggregates

• Both Ti₂CTₓ and Ti₂CTₓ/TiO₂ films were strongly sensitive to humidity.

• At room temperature and 50% RH:

  • The initial Ti₂CTₓ film showed n-type responses to all gases tested, with maximum sensitivity to NH₃ and noticeable sensitivity to CO, benzene, ethanol, acetone and even oxygen.

  • The partially oxidized Ti₂CTₓ/TiO₂ composite showed much stronger responses, especially to NH₃ and NO₂, and the response type switched to p-type for most gases (except NH₃ and NO₂).

• The best performance was obtained with partial oxidation at 316 °C, not with fully oxidized films.

The central message is clear:

Partial, controlled oxidation of Ti₂CTₓ MXene into a Ti₂CTₓ/TiO₂ composite can greatly enhance gas sensing performance, especially for ammonia, at room temperature and realistic humidity levels.