MXene: Basic Knowledge Analysis of Revolutionary Two-dimensional Materials

The Origin and Importance of MXene

MXene represents a two-dimensional material consisting of transition metal carbides/nitrides and follows the chemical formula M_n+1X_nT_z where M stands for transition metal and X represents C or N while Tz refers to a surface functional group. Selective etching of the A layer from MAX phases like Ti₃AlC₂ creates this material and its surface binds -O, -OH, and -F functional groups which provide special hydrophilicity and modifiable chemical properties. Since scientists synthesized MXene in 2011 research scientists have focused heavily on this material within materials science. The name MXene originates from the MAX phase which consists of a transition metal M combined with a main group element A and either carbon or nitrogen as element X. The "MXene" structure emerges from the extraction of the A layer resulting in a material that combines graphene-like two-dimensional traits with superior transition metal performance.

The unique properties of MXene include:

High conductivity: MXene exhibits metal-like conductivity which qualifies it as an optimal choice for electrode materials.

Adjustable surface chemistry: Synthetic conditions enable the regulation of surface functional groups (-O, -OH, -F) to optimize material properties.

Mechanical strength: Its high elastic modulus and compressive strength make MXene appropriate for fabricating flexible devices.

Hydrophilicity: Surface functional groups enable effortless dispersion in aqueous solutions and improve processing capabilities.

MXene's unique features enable revolutionary advances in energy storage (including batteries and supercapacitors), catalysis, electronic devices and environmental cleanup.

Structure and Composition of MXene

MXene features atomic layers that display hexagonal symmetry by alternating transition metal layers (M) with carbon/nitrogen layers (X) while functional groups (T_z) modify its surface. The MXene Ti₃C₂T_x stands as one of the most researched materials due to its structure which consists of three titanium layers encasing two carbon layers and its surface modified with functional groups such as -O, -OH or -F.

MXene production needs MAX phase precursors which follow the chemical formula M_n+1AX_n, for instance Ti₃AlC₂ serves as such a precursor. Layered MXene emerges from the selective A layer removal (such as Al) by acid etching (such as HF) or molten salt processing. The procedure maintains the M-X strong bonding framework and adds functional groups to the surface.

The properties and structure of MXene materials can be adjusted by manipulating several customizable dimensions.

N value regulation: MXene structures like M₂X, M₃X₂ and M₄X₃ develop based on n values of 1, 2, and 3 which directly influences both their conductivity and mechanical properties.

Surface functional groups: The ratio of surface functional groups like -OH and -F determines the hydrophilicity and chemical stability of MXene as well as its conductivity.

3D structure design: The assembly of two-dimensional nanosheets into porous networks or aerogels prevents interlayer agglomeration while boosting electrochemical active site density.

Synthesis Methods of MXene

The synthesis methods of MXene are mainly divided into two categories: traditional etching technology and emerging synthesis strategies. Product performance and safety levels along with efficiency demonstrate major variations across different methods.

Traditional Etching Technology

HF etching: The traditional method for creating MXene involves using hydrofluoric acid (HF) etching. MXene production involves removing A-layer atoms like Al from the MAX phase structure through selective etching. The MAX phase material titanium aluminum carbide Ti₃AlC₂ reacts with hydrofluoric acid HF to form Ti₃C₂T_x which contains surface functional groups denoted by T_x. The high toxicity and corrosiveness of HF makes waste treatment expensive while also posing substantial environmental contamination risks. The size of MXene sheets reduces and defects increase when subjected to high concentration or long-term etching.

Molten salt etching: The molten salt etching technique eliminates A-layer atoms using high-temperature molten salts (such as CuCl₂ and SnF₂) to prevent HF risk while providing a suitable fluorine-free etching environment. The reaction of Ti₃AlC₂ with CuCl₂ at 680°C produces Ti₃C₂Cl₂ while subsequent oxidation eliminates remaining metal particles. The etching method enables precise control over surface functional groups (-Cl, -Br) yet requires extended processing time which leaves some MAX phase material unreacted.

Emerging Synthesis Strategies

Electrochemical etching: Green synthesis of the MAX phase is achieved through voltage application in a fluorine-free electrolyte like diluted HCl followed by adjusting both voltage and electrolyte composition. Boron-doped diamond electrodes enable electrochemical etching to produce MXene effectively in low-toxic electrolytes while maintaining high yield and minimal liquid waste hazards. Over-etching occurs in this method when voltage control is not properly managed which results in material integrity problems.

CVD method: MXene films with high purity and large dimensions created through Chemical Vapor Deposition (CVD) facilitate large-scale manufacturing applications. Through chemical vapor deposition (CVD) scientists can produce Mo₂C MXene on substrates while controlling thickness and minimizing defects. The equipment required for this process comes with high costs and complex procedures which limits its use to laboratory research settings.

Unique Properties of MXene

The outstanding performance of MXene arises from its two-dimensional layered structure combined with its rich surface chemistry and adjustable physical properties which enable potential applications in diverse fields.

The metallic-level conductivity of MXene which includes Ti₃C₂T_x reaching up to 15,000 S/cm combined with its electronic state density near the Fermi level makes it an excellent electrode material for supercapacitors and lithium-ion batteries. The V₂CT_x MXene developed through molten salt etching demonstrates exceptional performance in energy storage devices because of its elevated conductivity and reduced fluorine content.

The combination of MXene's bendability to 180° with its robust strength makes it ideal for flexible electronic applications. Ti₃C₂T_x nanosheets serve as the basis for creating flexible sensors that keep their conductivity stable under stretching or folding. MXene surface groups include functional groups such as -O, -OH, -F and -Cl among others. The surface functional groups of MXenes (-O, -OH, -F, -Cl, etc.) undergo regulation through etching methods to modify its hydrophilicity and catalytic activity. The electrochemical etching process allows -Cl groups to attach to MXene through electrolyte composition adjustments which increase MXene's efficiency in catalytic reactions.

Mo₂CT_x MXenes display semiconductor characteristics with tunable band gaps ranging from 0.2 to 1.8 eV which makes them suitable for photocatalysis and photodetector applications. Transition metal-containing MXenes like those with Cr and Fe elements display ferromagnetic properties and show significant potential for use in spintronic technologies.

Application Fields of MXene

Energy Storage

Supercapacitors: Through its 3D porous structure MXene achieves 1,500 F/g of specific capacitance with 90% cycle stability across 10,000 cycles when used in 3D printed electrodes. The rapid ion transport channel and pseudocapacitive mechanism serve as major advantages.

Lithium/sodium ion batteries: MXene possesses an interlayer spacing of 1.3 nm which speeds up Li+ embedding, and Ti₃C₂T_x demonstrates a lithium ion diffusion coefficient that exceeds graphite's by 2 orders of magnitude as the negative electrode. The MXene sodium battery with SnS₂ composite achieved 85% capacity retention after undergoing 500 cycles at 1 A/g.

Electronics and Sensors

Flexible electronic devices: MXene/PDMS composite materials demonstrate a fracture strain of 400% making them ideal for wearable strain sensors while achieving a sensitivity value of 180 GF.

Electromagnetic shielding: MXene multilayer films with a thickness of 10 μm display a shielding effectiveness of 60 dB making them appropriate for 5G communication equipment. The operational process involves both electromagnetic wave absorption and multiple reflections.

Environment and Catalysis

Photocatalytic hydrogen production: The MXene/TiO₂ heterojunction achieves a photocurrent density five times greater than pure TiO₂ since MXene accelerates carrier separation.

Water purification: MXene's -O groups show an adsorption capacity of 450 mg/g for Pb²⁺ ions while their organic dye removal performance achieves over 95% effectiveness for substances like methylene blue. Surface oxidation treatment provides a pathway to enhancing the adsorption selectivity of this material.