Transparent Conductive Ti₂Cₜₓ (MXene) Films: Spin-Coated from Simple Water-Based Inks

What are MXenes and why Ti₂Cₜₓ?

After graphene was isolated, scientists became very interested in two-dimensional (2D) materials – extremely thin layers of atoms that can conduct electricity, interact with light, and be stacked or combined in clever ways.

One large and growing family of 2D materials is called MXenes. They are produced by chemically etching special layered ceramics known as MAX phases.

MAX phases have the formula:
Mₙ₊₁AXₙ
- M = early transition metal (e.g., Ti, V, Nb)
- A = an element from groups 13–14 (like Al, Si)
- X = carbon and/or nitrogen
When the A layer is selectively removed (usually in a fluoride-containing acid), what remains is a layered structure called a MXene, with formula:
Mₙ₊₁XₙTₓ
Here Tₓ represents surface terminations such as –O, –OH and –F that form during the etching.

So for Ti₂Cₜₓ:

The parent MAX phase is Ti₂AlC.
Etching removes the Al atom layers.
The result is Ti₂CTₓ, a 2D titanium carbide with surface terminations.

Most of the research so far has focused on another titanium carbide MXene: Ti₃C₂Tₓ, which is already known to make highly conductive, solution-processed films. This paper instead examines Ti₂Cₜₓ as a transparent conductor, which is a material that is both electrically conductive and optically transparent, useful in:

Touchscreens
Displays
Solar cells
Transparent heaters
Flexible electronics

The standard industrial material here is indium tin oxide (ITO), which is excellent but uses indium, a relatively rare and expensive element. So alternatives are very attractive.

2. How did they make Ti₂Cₜₓ flakes and the “ink”?

Step 1 – Synthesize the Ti₂AlC MAX phase

The researchers first prepared Ti₂AlC, the parent ceramic:

Mixed elemental powders: titanium (Ti), aluminum (Al), and carbon (C) in a molar ratio of 2 : 1.1 : 1.
Pressed the mixture into a compact at 30 MPa.
Heated it in a tube furnace with flowing argon up to 1400 °C (10 °C/min), held it there for 1 hour, then let it cool in the furnace.
The slightly sintered block was drilled and milled into powder and sieved to get a fine powder.

This powder is Ti₂AlC – the starting point for Ti₂Cₜₓ.

Step 2 – Etch away the Al to form Ti₂Cₜₓ

To turn Ti₂AlC into Ti₂Cₜₓ, they used an in situ HF method, where hydrofluoric acid is generated from fluoride salts and HCl:

They mixed 4 g of LiF (lithium fluoride) with 40 mL of 11.7 M HCl.
Ti₂AlC powder was placed in this solution and kept for 24 hours at 35 °C.

The LiF + HCl mixture creates HF in solution, which selectively removes the Al layers from Ti₂AlC, forming multilayer Ti₂Cₜₓ.

Step 3 – Washing and delamination

After etching:

The suspension of Ti₂Cₜₓ flakes was washed repeatedly with deionized water.
Each time, it was centrifuged at 5000 rpm for 3 minutes, the supernatant decanted, and fresh water added.
Washing continued until the pH of the supernatant was about 6.

Next, to delaminate the layers into mostly single flakes:

The washed powder was dispersed in about 20 mL of distilled water.
The suspension was sonicated for 1 hour at room temperature, while argon gas was bubbled through to limit oxidation.
Then it was centrifuged at 3500 rpm for 30 minutes.
The supernatant (top liquid) contained a colloidal solution of mostly single-layer Ti₂Cₜₓ flakes, which was collected and stored in sealed bottles under argon.

The starting concentration of this colloidal “ink” was about 25 mg/mL.

From this stock, they prepared diluted solutions with concentrations:

12.5, 8.33, 5, 2.5, 1.25, 0.5 and 0.25 mg/mL

by adding deionized water and carefully measuring solid content via vacuum filtration and weighing.

3. Making the transparent films: spin coating

To turn the Ti₂Cₜₓ ink into solid films, the team used spin coating (spincasting), a common thin-film technique:

Glass substrates (2.5 × 2.5 cm²) were cleaned with water and ethanol.
A measured amount of the Ti₂Cₜₓ colloidal solution was dropped onto the glass.
The substrate was spun at different speeds, ω, between 500 and 4000 rpm for 30 seconds, followed by 2000 rpm for 5 seconds (to help remove excess solvent).
After spin coating, the films were stored overnight in a dry nitrogen glove box with less than 1 ppm oxygen or moisture.

They also made some thicker “drop-cast” films by simply dropping the 25 mg/mL solution onto glass and letting it dry in air, mainly to study stability.

So the main variables they controlled were:

Concentration of the Ti₂Cₜₓ ink (c, mg/mL)
Spin speed (ω, rpm)

These two together determine film thickness, which then determines transparency and conductivity.

4. What do the Ti₂Cₜₓ flakes and films look like?

TEM: individual flakes

Transmission electron microscopy (TEM) was used to image the flakes in the colloidal solution:

At low magnification, the flakes appear well delaminated, with lateral sizes around 1 µm.
The selected area diffraction patterns (SAD) show hexagonal symmetry, consistent with the underlying crystal structure inherited from Ti₂AlC.
From the diffraction pattern, the in-plane lattice parameter (a) was about 0.3 nm, matching the known value (~0.306 nm) for Ti₂AlC.
Some flakes appear flat; others show folding, meaning they are flexible and can bend without obvious damage.
The TEM images reported do not show strong signs of oxidation or large defects like pores, indicating good structural quality.

XRD: layer spacing and phase purity

X-ray diffraction (XRD) was used to check:

Whether any un-etched Ti₂AlC remained.
How the layer spacing (c-lattice parameter) changed.

For the spincast Ti₂Cₜₓ films:

The diffraction patterns show the typical MXene (00ℓ) peaks shifted to lower angles and broadened compared to Ti₂AlC.
No Ti₂AlC peaks were visible, meaning the films were essentially free of un-etched MAX phase.
The c-lattice parameter, c, was found to be around 2.43–2.54 nm, much larger than the ~1.5 nm reported when Ti₂Cₜₓ is etched with HF only.

The larger c suggests that multiple layers of water are intercalated between the Ti₂Cₜₓ layers – and interestingly, this water remains even after at least 100 days in a dry nitrogen glove box.

For drop-cast films stored in air:

The XRD patterns measured immediately and after 100 days in air were essentially identical.
The c-lattice parameter was about 0.5 nm larger than the spincast films, again consistent with more water between the layers (likely because the film stayed wet longer while drying in air).

In both cases, no clear crystalline oxide peaks were detected. If any oxidation occurs, it is likely amorphous or minimal under these conditions.

SEM: film morphology and thickness

Scanning electron microscopy (SEM) was used to look at the surface and cross-sections:

From the top, the films appear smooth overall, with only occasional thicker or multilayer flakes on the surface.
Cross-section images show films with thicknesses around 200 nm in some cases.
Thicknesses measured by SEM agreed reasonably well with those measured by profilometry (within about 10%).

Overall, the films are continuous, relatively uniform, and compact, which is important for good conductivity and optical performance.

5. Controlling thickness and transparency

Two key tunable parameters are:

Spin speed (ω)
Solution concentration (c)

The researchers systematically studied how these control film thickness and optical transmittance.

Thickness vs spin speed

For a fixed concentration of 25 mg/mL, the film thickness h decreases as spin speed increases, following a standard spin-coating relationship:

h (nm) ≈ 1280 · ω⁻⁰·⁵

This means:

Doubling the spin speed reduces thickness by roughly a factor of √2.
Higher ω → thinner, more transparent films.

Thickness vs concentration

At fixed spin speeds, thickness increases with concentration:

At 3500 rpm:
h (nm) ≈ 2.95 · c (mg/mL)
At 800 rpm:
h (nm) ≈ 6.1 · c (mg/mL)

So the same ink makes a thicker film if you spin slower, and a more dilute ink makes a thinner film at the same speed.

Transparency (transmittance τ)

They measured the optical transmittance at 550 nm (green light), a standard wavelength for evaluating transparent conductors.

At fixed concentration 25 mg/mL, varying spin speed from 500 to 4000 rpm produced transmittance values between roughly 43% and 65%.
At fixed spin speed 3500 rpm, reducing concentration from 25 to 0.25 mg/mL increased transmittance from about 62% up to ~99.9%.

In other words:

Thicker films (low spin speed, high concentration) → more conductive but less transparent.
Thinner films (high spin speed, low concentration) → more transparent but less conductive.

This trade-off is typical of all transparent conductors and is the reason we use a figure of merit (FOM) to compare materials fairly.

6. Electrical conductivity and sheet resistance

The films’ sheet resistance (Rₛ) was measured with a four-point probe inside a nitrogen glove box to avoid degradation.

Sheet resistance vs thickness

As expected, thicker films had lower Rₛ (higher conductance).
The reciprocal of sheet resistance, 1/Rₛ, scaled roughly linearly with thickness:

1/Rₛ (Ω/□)⁻¹ ≈ −0.01 + (6 × 10⁻⁴) · h (nm)

There was some scatter, partly due to the difficulty of measuring very thin film thickness precisely or small non-uniformities.

To refine this, they also used the optical data (transmittance τ) and an absorption model to estimate thickness. When they replotted 1/Rₛ vs thickness derived from τ, the correlation improved (R² > 0.97), confirming that the main physics is consistent.

Sheet resistance vs concentration

At a fixed spin speed of 3500 rpm, the sheet resistance related to concentration roughly as:

log Rₛ (Ω/□) ≈ 3.9 − 1.1 · log c (mg/mL)

So increasing concentration by a factor of 10 reduces Rₛ by about an order of magnitude.

DC conductivity

From thickness and sheet resistance, they extracted the DC conductivity (σ_DC). For example:

A 100 nm-thick Ti₂Cₜₓ film had a conductivity around 5250 S/cm, slightly below Ti₃C₂Tₓ films of similar thickness (~6450 S/cm), but still very high compared to many solution-processed materials.

7. Optical absorption and figure of merit (FOM)

To compare different transparent conductors, a dimensionless figure of merit is used, based on the ratio of DC conductivity (σ_DC) to optical conductivity (σ_OP).

First, they determined the absorption coefficient at 550 nm (α₅₅₀) by looking at how −ln(τ₅₅₀) scales with thickness:

For Ti₂Cₜₓ:
α₅₅₀ ≈ 2.7 × 10⁵ cm⁻¹ (depending slightly on intercept assumptions)
Previously, for Ti₃C₂Tₓ, they had found:
α₅₅₀ ≈ 2.5 × 10⁵ cm⁻¹

So Ti₂Cₜₓ and Ti₃C₂Tₓ absorb light at very similar rates per unit thickness.

Then they used a standard relation connecting transparency and sheet resistance for thin conducting films:

τ = (1 + (Z₀ σ_DC / 2) · (1/Rₛ))⁻²

(They write it in terms of σ_DC/σ_OP, but the idea is that τ and Rₛ are connected through σ_DC and σ_OP, and Z₀ is the impedance of free space, 377 Ω.)

By plotting the experimental data appropriately, the slope gives the ratio σ_DC/σ_OP, which they take as the FOM.

For these Ti₂Cₜₓ films, the FOM ≈ 5.

How does that compare?

They compare this FOM to other transparent conductors:

Ti₂Cₜₓ (this work): FOM ≈ 5
Ti₃C₂Tₓ (spincast from solution): similar FOM
Un-doped CVD graphene: comparable FOM
Doped graphene, ITO, Ag nanowires: higher FOM, so they still perform better overall as transparent conductors.

However, considering that:

This is the first generation of spincast Ti₂Cₜₓ transparent films,
The processing is simple and water-based,
The elements used (Ti, C) are relatively abundant and non-toxic,
The FOM is already comparable to undoped graphene,

the results are very promising.

They also note a subtle advantage: per formula unit, Ti₂Cₜₓ has one fewer titanium atom than Ti₃C₂Tₓ, which may mean lower material cost per area for similar performance, since Ti is the more expensive component here relative to carbon.

8. Stability and practical aspects

A few important practical points from their observations:

Film stability
- Spincast Ti₂Cₜₓ films stored in a dry N₂ glove box for 100 days show no new crystalline oxide phases and maintain their MXene structure (with water intercalation).
- Drop-cast films stored in air for 100 days also showed no change in XRD patterns, suggesting that any oxidation is either slow, amorphous, or does not strongly affect the main layered structure over this timescale.
Hydrophilicity and solution processing
- The films were made from aqueous colloidal solutions, and the MXene surface is hydrophilic.
- This enables simple, scalable wet processing (spin coating, and in principle also spray coating, printing, etc.).
Flexibility (from previous similar work)
- While this paper focuses on Ti₂Cₜₓ, similar Ti₃C₂Tₓ MXene films have been shown previously to keep their properties when flexed.
- It is reasonable to expect that Ti₂Cₜₓ films will behave similarly, though this specific paper focuses more on optical and electrical metrics than mechanical testing.

9. Take-home message

This work shows that:

Ti₂Cₜₓ MXene can be etched from Ti₂AlC using a LiF/HCl system, delaminated into single-layer flakes, and processed into films using spin coating from water.
By tuning solution concentration and spin speed, the researchers can control film thickness, and thus transparency and conductivity, in a predictable way.
The films show:
- High electrical conductivity (thousands of S/cm)
- High optical absorption per thickness, similar to Ti₃C₂Tₓ
- A figure of merit around 5, comparable to un-doped graphene and previous Ti₃C₂Tₓ films.
The films are stable over months (under nitrogen, and to some extent even in air), and the material uses abundant, non-toxic elements.

In simple terms:
The study demonstrates that Ti₂Cₜₓ MXene is a serious candidate for transparent, conductive coatings, offering performance on par with early graphene films, but with simple water-based processing and potentially lower material cost.

As synthesis and processing are further improved, it’s likely that Ti₂Cₜₓ and related MXenes will become even more competitive against established transparent conductors like ITO in future flexible and large-area electronics.