In modern materials science, there is a big trend toward multi-functional materials – single materials that can do more than one job in electronic, energy, or sensing devices. One of the most exciting families in this area is MXenes, especially titanium carbide MXene (Ti₃C₂Tₓ).

This review article focuses on how Ti₃C₂Tₓ MXene can be used in two major areas:

1. As an electron or hole transport material in perovskite solar cells (PSCs)

2. As an electrode material in electrochemical sensors and biosensors

The goal of the review is to collect recent results, explain how Ti₃C₂Tₓ is made and how it works, and highlight why it is so powerful in both solar and sensing technologies.

In this blog, we’ll walk through the main ideas in a simple, logical way.

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1. Why Ti₃C₂Tₓ MXene Is So Interesting

1.1 What Are MXenes?

MXenes are a large family of two-dimensional (2D) materials made from transition metal carbides, nitrides, or carbonitrides. Their general formula is:

Mₙ₊₁XₙTₓ,
where:

• M is a transition metal (like Ti, Mo, Cr, Nb, etc.)

• X is carbon (C) and/or nitrogen (N)

• Tₓ represents surface groups such as −O, −OH, −F, −Cl

MXenes are produced by etching out the “A” layer from layered ceramics called MAX phases. During this process, surface functional groups (−O, −OH, −F, etc.) are introduced, giving MXenes special surface chemistry.

1.2 Why Ti₃C₂Tₓ Specifically?

Among all MXenes, Ti₃C₂Tₓ is the most widely studied. It has several important features:

• High electrical conductivity (metallic-like behavior)

• High carrier mobility (electrons move quickly through it)

• Good mechanical properties

• Optical transparency in thin films

• Tunable work function (it can be adjusted between about 1.6 and 6.25 eV using different synthesis and post-treatment methods)

• Good chemical and thermal stability

Because of this unique combination of properties, Ti₃C₂Tₓ is very attractive for:

• Perovskite solar cells (as electron transport layer (ETL), hole transport layer (HTL), or additive)

• Electrochemical sensors and biosensors (as electrode material or support)

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2. Perovskite Solar Cells and the Need for Better Transport Layers

2.1 Basics of Perovskite Solar Cells (PSCs)

Perovskite solar cells are a type of thin-film photovoltaic technology that has rapidly reached very high power conversion efficiencies (PCEs). A typical PSC has:

• A perovskite light absorber layer (e.g., MAPbI₃ – methylammonium lead iodide)

• An electron transport layer (ETL) to extract and move electrons

• A hole transport layer (HTL) to extract and move holes

PSCs can achieve high efficiency, but they face some major challenges:

• Stability issues (sensitivity to moisture, oxygen, heat)

• Charge recombination due to poor transport layers or defective interfaces

To improve both performance and stability, researchers look for better ETL/HTL materials and better interfacial engineering.

2.2 Role of 2D Materials in PSCs

Before MXenes, many 2D materials were already explored for PSCs:

• Graphene

• MoS₂, WS₂, TiS₂, SnS₂

• Other layered materials

These materials can help:

• Provide uniform, smooth interfaces

• Improve charge transport

• Reduce recombination losses

Examples mentioned in the article:

• MoS₂ used as ETL gave a PCE of about 20.55%

• WS₂ used as ETL gave a PCE of about 12.44%

• TiS₂ and SnS₂ have also been used as ETLs with promising results

This background paved the way for testing Ti₃C₂Tₓ MXene in PSCs.

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3. How Ti₃C₂Tₓ MXene Is Made and What It Looks Like

3.1 Etching: From MAX Phase to MXene

Ti₃C₂Tₓ originates from the MAX phase Ti₃AlC₂. The transformation can be summarized in two main steps:

1. Etching (removal of Al layer)

2. Exfoliation (separating layers)

In a typical HF-based etching process, Ti₃AlC₂ is treated with hydrofluoric acid (HF). The reactions involve:

• Removing Al and forming AlF₃

• Producing hydrogen gas

• Introducing −OH and −F surface terminations on Ti₃C₂

However, using concentrated HF is dangerous. So, alternative and safer methods were developed:

• Using NH₄HF₂ instead of direct HF

• Using LiF + HCl mixtures to generate HF in situ in a more controlled way

• Adjusting etching temperature, time, and acid concentration

These methods allow:

• Better control over etching

• Less damage to the structure

• Larger, better-quality Ti₃C₂Tₓ flakes

3.2 Exfoliation: Getting Few-Layer or Single-Layer Ti₃C₂Tₓ

After etching, the material looks like an accordion – stacked sheets separated but still attached. To fully separate them, exfoliation is needed.

This is usually done by:

• Intercalating large ions or molecules (like TBAOH, DMSO, urea, NH₄⁺, etc.) between layers

• Then using ultrasonication or mechanical shaking to peel off individual layers

The result is:

• Stable colloidal suspensions of Ti₃C₂Tₓ flakes in water

• With few-layer or single-layer sheets

3.3 Structure and Charge Transport Properties

Ti₃C₂Tₓ MXene consists of:

• Intra-layer skeleton of Ti and C atoms (forming strong Ti–C bonds)

• Interlayer regions connected via hydrogen bonding and van der Waals forces

• Surface terminating groups like −O, −OH, −F, which strongly influence the electronic and chemical properties

Key electrical features:

• Metallic conductivity

• High carrier mobility

• Tunable work function

This makes Ti₃C₂Tₓ ideal as:

• Electron transport layer (ETL): extracting and transporting electrons

• Hole transport layer (HTL): in some device architectures

Because the energy levels of Ti₃C₂Tₓ can be aligned with those of perovskite and other layers, it can help reduce barriers, improve charge extraction, and suppress recombination.

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4. Ti₃C₂Tₓ MXene in Perovskite Solar Cells

The article reviews several ways Ti₃C₂Tₓ MXene is used in PSCs:

1. As an additive

2. As part of or modifying the electron transport layer (ETL)

3. As part of or modifying the hole transport layer (HTL) or electrode

4.1 Ti₃C₂Tₓ as an Additive in PSCs

One strategy is to add small amounts of Ti₃C₂Tₓ into other layers to improve their properties.

For example, Ma and co-workers:

• Used Ti₃C₂Tₓ as an additive in the SnO₂ electron transport layer

• Adjusted the weight percentage of MXene in SnO₂

• Found that Ti₃C₂Tₓ addition enhanced the optical and electronic properties of SnO₂

• Achieved a PCE of 16.8% in MAPbI₃-based PSCs

The improvement was attributed to:

• Better electron transfer

• Lower charge transfer resistance

• More favorable interface between SnO₂ and perovskite

Other researchers have used Ti₃C₂Tₓ MXene to:

• Tune the work function of perovskite or transport layers

• Improve interfacial contact without damaging the perovskite

In all these cases, Ti₃C₂Tₓ is not the main ETL or HTL, but it modifies and improves existing layers.

4.2 Ti₃C₂Tₓ as an Electron Transport Layer (ETL)

Because Ti₃C₂Tₓ is highly conductive and has suitable energy levels, it can be used as part of the ETL or as a composite ETL.

Some key examples:

• Ti₃C₂Tₓ quantum dots at the ETL/perovskite interface

  • These improve interfacial contact and charge extraction

  • PCEs over 21% (e.g., ~21.64%) have been reported in such engineered structures

• Ti₃C₂Tₓ/SnO₂ composite ETL (Yang and co-workers):

  • MXene was combined with SnO₂ to create a new ETL

  • Pure Ti₃C₂Tₓ devices had low PCE (~5.28%)

  • Pure SnO₂ devices had ~17.23%

  • Ti₃C₂Tₓ/SnO₂ composite ETL reached 18.34% PCE

  • This shows MXene can improve electron extraction and transport when properly combined

• Multi-dimensional conductive network (MDCN) ETLs using Ti₃C₂Tₓ:

  • These networks help form efficient electron paths

  • A PCE of about 18.44% and good stability over 45+ days in air were reported

• Ti₃C₂Tₓ-modulated electrode/SnO₂ interface (Wang and co-workers):

  • MXene was used at the electrode/ETL interface

  • The device reached a PCE of 20.6% and remained stable for up to 3 months

Other studies show that:

• The energy levels of Ti₃C₂Tₓ can match well with ITO, TiO₂, and perovskite

• The surface of Ti₃C₂Tₓ can be optimized (e.g., via UV–ozone) to improve work function and interfacial contact without harming mobility

• Planar PSCs using Ti₃C₂Tₓ as ETL can reach PCEs around 17–18%

Overall, as an ETL or ETL modifier, Ti₃C₂Tₓ:

• Improves charge extraction

• Reduces recombination

• Enhances efficiency and stability

4.3 Ti₃C₂Tₓ as Hole Transport Layer (HTL) or in Electrode Engineering

Although most work focuses on Ti₃C₂Tₓ as an ETL or additive, it can also play a role in hole-selective structures or electrode engineering:

• MXene-based electrodes in HTL-free PSCs have shown PCEs around 13.8%

• Inverted p–i–n PSCs with Ti₃C₂Tₓ decorated with NiO reported a PCE of 19.2%

These results indicate that, with the right architecture, Ti₃C₂Tₓ can also assist in:

• Hole extraction and transport

• Building HTL-free or simplified device structures

The review concludes that Ti₃C₂Tₓ is a powerful toolbox material for PSC engineering – as additive, ETL, HTL, and interfacial modifier.

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5. Ti₃C₂Tₓ MXene in Electrochemical Sensing

Beyond solar cells, Ti₃C₂Tₓ is also very successful in the field of electrochemical biosensors and non-biosensors.

5.1 Why Ti₃C₂Tₓ Is Good for Electrochemical Sensors

The review lists several reasons:

• High electrical conductivity → faster electron transfer at the electrode

• Easy to disperse in solution → simple fabrication by drop-casting/printing

• Good mechanical flexibility → suitable for flexible or wearable devices

• Rich surface chemistry → easy to combine with enzymes, nanoparticles, polymers, etc.

• Good biocompatibility → can host proteins and enzymes without destroying their activity

• Photothermal effects → can enable dual-mode sensing in some designs

Because of these features, Ti₃C₂Tₓ is used as:

• A direct electrode material

• A support for enzymes, nanoparticles, and functional polymers

• A component in printed, flexible, or screen-printed sensors

The review divides the sensing applications into:

1. Enzymatic biosensors (enzyme-based)

2. Non-biosensors (no enzymes; direct electrochemical detection)

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6. Ti₃C₂-Based Enzymatic Biosensors

In enzymatic biosensors, enzymes are immobilized on an electrode to detect a specific analyte. For these to work well, you need:

• Good direct electron transfer (DET) between enzyme and electrode

• A stable and biocompatible surface

• High surface area and conductivity

Ti₃C₂Tₓ MXene provides all of these.

Some key examples from the review:

• First Ti₃C₂-based biosensor (2014):

  • A hemoglobin (Hb)-based sensor for H₂O₂ detection

  • Ti₃C₂ served as a support for Hb, enabling direct electron transfer

  • Demonstrated that Ti₃C₂ can host enzymes and preserve their activity

• Ti₃C₂/TiO₂-based biosensor:

  • TiO₂ nanoparticles attached to Ti₃C₂ increased surface area and maintained enzyme stability

  • Used for Hb-based H₂O₂ detection

  • Achieved a low detection limit of 14 nM

• Au/Ti₃C₂-based glucose biosensor:

  • Gold nanoparticles (Au NPs) + Ti₃C₂ MXene used to immobilize glucose oxidase (GOx)

  • Showed high sensitivity and a low micromolar LOD for glucose

• Ag@Ti₃C₂ composite for acetylcholinesterase (AChE):

  • Used for detection of pesticides like malathion

• 3D MnO₂@Mn₃O₄/MXene/AuNPs composites:

  • Used in an AChE-based sensor for organophosphorus pesticides

  • Achieved extremely low detection limits (sub-picomolar) for methamidophos

• Ti₃C₂–PLL–GOx nanoreactor:

  • Combined Ti₃C₂, poly-L-lysine (PLL), and GOx

  • Ti₃C₂ catalyzed H₂O₂ decomposition, and GOx oxidized glucose

  • Formed a cascade system for glucose detection with an LOD of 2.6 µM

• Dual-enzyme inosine monophosphate (IMP) biosensor:

  • Ti₃C₂ + Au@Pt nanoflowers

  • Two enzymes immobilized for cascade reactions

  • Achieved low LOD in real food samples (meat)

Ti₃C₂Tₓ is also used as:

• Part of inkjet-printed electrodes

• Components in screen-printed electrodes (SPCE/SPE)

In all of these, Ti₃C₂ provides:

• High surface area for enzyme loading

• Good electron pathways

• Flexibility and printability

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7. Ti₃C₂-Based Non-Biosensors (Enzyme-Free)

Non-biosensors rely on the direct electrochemical interaction between the analyte and the electrode surface, without enzymes.

Some applications reviewed:

• First non-biosensor (2018):

  • Ti₃C₂ MXene used for detecting bromate (BrO₃⁻) in drinking water

  • Ti₃C₂ acted as a signal amplifier and reducing agent

  • Showed strong electrocatalytic activity for BrO₃⁻ reduction

• Carbendazim and dopamine detection:

  • Ti₃C₂-modified glassy carbon electrodes (GCE)

  • Achieved lower overpotentials and better sensitivity than graphene-based sensors

  • Dopamine detection with LOD around 3 nM

• Adrenaline detection using Ti₂C MXene/GC paste electrodes:

  • First use of Ti₂C MXene in sensing

  • LOD ~ 9.5 nM

  • Good recovery in pharmaceutical samples

• Simultaneous detection of acetaminophen (ACOP) and isoniazid (INZ):

  • Ti₃C₂-modified screen-printed electrodes

  • Showed clear separate peaks and low detection limits

Many composite materials were also developed:

• NiO/Ti₃C₂ for enzyme-free H₂O₂ detection

• Ti₃C₂/NiCo-LDH for enzyme-free glucose sensing

• Au/Ti₃C₂ for nitrite detection

• Pd/Ti₃C₂ for l-cysteine detection

• Mn₃(PO₄)₂/Ti₃C₂ for superoxide anions from cells

• Ti₃C₂/MWCNT for hydroquinone and catechol

• MB/Cu/Ti₃C₂ for piroxicam

A common challenge with MXenes is restacking of layers, which reduces surface area. To prevent this, researchers:

• Insert spacers like carbon nanohorns (CNHs)

• Use nitrogen-doped porous carbon to separate Ti₃C₂ layers

• Perform alkaline treatments to introduce more −OH groups and improve interactions

These strategies improve:

• Stability

• Active surface area

• Electron and ion transport

As a result, very low detection limits (down to the nanomolar level) can be achieved for many analytes, including drugs, pollutants, and biomolecules.

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8. Conclusions and Future Outlook

The review concludes that Ti₃C₂Tₓ MXene is one of the most important 2D materials for:

• Perovskite solar cells, thanks to:

  • Tunable work function

  • High conductivity

  • Good stability

  • Ability to act as ETL, HTL, additive, or interfacial layer

  • Improvement in both PCE and long-term device stability

• Electrochemical sensors and biosensors, thanks to:

  • Excellent electrical properties

  • Processability in solution

  • Rich surface chemistry and good biocompatibility

  • Easy integration with enzymes, nanoparticles, and polymers

  • Suitability for flexible and printed devices

For the future, the authors highlight some directions:

• Better surface functionalization of Ti₃C₂Tₓ (e.g., amination, carboxylation) to create stronger, more specific chemical links with biomolecules

• Exploring more MXene compositions (with different metals and terminations) for tailored properties in solar and sensing applications

• Designing new PSC architectures using Ti₃C₂Tₓ-assisted metal oxides to further improve efficiency and stability

• Developing more advanced electrochemical sensors using MXenes as key functional materials

In short, Ti₃C₂Tₓ MXene is not just another 2D material. It is a multi-functional platform that can boost the performance of both energy devices like perovskite solar cells and analytical devices like biosensors and chemical sensors.