Surface Terminated MXene: Engineering Functional Groups For Advanced Material Performance

Chemical Composition And Structural Characteristics Of Surface Terminated MXene

Surface terminated MXene materials possess the general formula Mn+1XnTx, where M represents an early transition metal (Ti, Nb, V, Ta, Mo, Cr, Zr, Hf, W, Sc, Y, Lu), X denotes carbon and/or nitrogen, n ranges from 1 to 4, and Tx designates the surface termination groups with x indicating their quantity (typically 0 < x < 2)138. The Tx component is not merely a passive surface feature but an integral structural element that profoundly influences the material's physicochemical behavior1012.

The most commonly encountered termination groups include:

• Hydroxyl groups (-OH): Impart hydrophilicity and facilitate aqueous dispersion; typical content ranges from 15-35 at% depending on synthesis conditions27
• Fluorine (-F): Originates from HF-based etching; occupies face-centered cubic (FCC) adsorption sites and modulates work function values; content typically 5-20 at%112
• Oxygen (=O): Forms during synthesis or environmental exposure; can induce TiO2 formation leading to increased contact resistance; content 10-30 at%212
• Chlorine (-Cl): Introduced via electrochemical or HCl-based etching methods; content 2-10 at%110
• Other terminations: Bromide (-Br), iodide (-I), selenide (-Se), telluride (-Te), and sulfide (-S) can be engineered through specialized synthesis routes610

The distribution and ratio of these terminations are defined during the MAX phase etching process and significantly impact subsequent material performance110. For instance, Ti3C2Tx—the most extensively studied MXene—typically exhibits a mixed termination of approximately 20-30% -OH, 10-15% -F, and 15-25% =O when synthesized via conventional HF etching719. The surface termination density directly correlates with interlayer spacing, which ranges from 0.98 nm to 1.35 nm depending on intercalated species and termination chemistry914.

The layered structure comprises near-close-packed M-element layers interleaved with X atoms in octahedral coordination, with termination groups bonded to the outer M-layer surfaces through covalent, ionic, or mixed bonding character811. This accordion-like stacking morphology, consisting of tightly packed layers, can be expanded through intercalation with various ions and molecules, facilitating exfoliation into single or few-layer nanosheets37. The interlayer interaction forces are relatively weak (van der Waals and electrostatic), enabling mechanical or chemical delamination while the in-plane M-X bonding exhibits strong covalent-ionic-metallic hybrid character conferring mechanical robustness411.

Crystallographically, MXene materials adopt a hexagonal layered structure with P63/mmc symmetry inherited from their MAX phase precursors8. The surface termination groups do not form a perfectly ordered sublattice but rather exhibit statistical distribution across available surface sites, with local ordering influenced by synthesis conditions, post-treatment protocols, and environmental exposure history1012.

Synthesis Routes And Surface Termination Control For MXene Materials

Conventional Etching Methods And Resulting Terminations

The predominant synthesis approach involves selective etching of the A-layer from MAX phase precursors (Mn+1AXn, where A is typically Al, Si, Ga, or Ge) using corrosive solutions138. The choice of etchant critically determines the resulting surface termination profile:

• Hydrofluoric acid (HF) etching: Aqueous HF solutions (concentration 10-50 wt%, etching time 18-72 hours at 25-55°C) yield MXene with mixed -F, -OH, and =O terminations; this method produces high-quality single-layer yields (60-80%) but introduces substantial fluorine content (10-20 at%)1719
• In-situ HF generation: Mixing LiF with HCl (molar ratio 5:1 to 10:1, reaction time 24-48 hours at 35-45°C) generates HF in situ, offering safer handling while producing similar termination profiles with slightly reduced fluorine content (8-15 at%)29
• Electrochemical etching: Applying anodic potential (2-5 V vs. Ag/AgCl) in aqueous electrolytes (NH4Cl, HCl) selectively removes A-layers without HF, yielding -Cl, -OH, and =O terminations with minimal fluorine (<2 at%); etching time 2-6 hours110
• Hydrothermal methods: High-temperature aqueous treatment (150-200°C, 12-24 hours) in alkaline or acidic media produces predominantly -OH and =O terminations with negligible halide content18

Each method presents trade-offs between termination control, material quality, scalability, and safety considerations. The HF-based routes remain most prevalent in research settings due to their high efficiency and well-established protocols, though industrial adoption increasingly favors safer alternatives210.

Advanced Surface Termination Engineering Strategies

Recent innovations enable precise post-synthesis modification of surface terminations, expanding the functional design space beyond synthesis-defined terminations:

Molten salt treatment: Immersing MXene in molten alkali metal halide mixtures (e.g., KBr-NaCl eutectic at 500-700°C for 2-6 hours) facilitates halide exchange reactions, replacing -F and -OH with -Br, -Cl, or -I terminations; this approach enables covalent surface modification and tailored electronic properties110. For example, treating Ti3C2Tx in molten ZnBr2/KBr/NaCl (molar ratio 1:2:2) at 550°C for 4 hours yields predominantly -Br terminated surfaces with <5 at% residual oxygen1.

Thermal treatment in controlled atmospheres: Annealing MXene in calcium vapor (600-800°C, 1-4 hours) selectively removes oxygen-containing terminations, reducing total oxygen content from 25-30 at% to 5-10 at% while preserving structural integrity12. Similarly, annealing in NH3 atmosphere (400-600°C) can introduce -NH2 terminations15.

Chemical functionalization: Reacting MXene with organosilanes, thiols, or phosphonic acids enables grafting of organic functional groups, creating hybrid organic-inorganic surface chemistries; for instance, treatment with 3-aminopropyltriethoxysilane (APTES) in ethanol (reflux 6 hours) introduces amine functionalities that enhance polymer compatibility216.

Intercalation-assisted modification: Inserting small organic molecules (molecular weight ≤300 g/mol) such as dimethyl sulfoxide (DMSO), tetrabutylammonium hydroxide (TBAOH), or hydrazine between MXene layers (soaking 12-48 hours at room temperature) expands interlayer spacing from ~1.0 nm to 1.2-1.5 nm and modifies surface chemistry through coordination interactions914. This approach simultaneously addresses restacking issues and tunes surface properties.

The selection of surface modification strategy should consider target application requirements, processing constraints, and compatibility with subsequent integration steps. For energy storage applications prioritizing high capacitance, maximizing -OH and =O terminations while minimizing -F content is advantageous, as fluorine negatively impacts lithium adsorption12. Conversely, electromagnetic shielding applications benefit from high electrical conductivity, favoring reduced oxygen content and metallic-like terminations1113.

Physical And Chemical Properties Governed By Surface Termination

Electronic And Electrical Characteristics

Surface termination profoundly modulates MXene's electronic structure and transport properties. Pristine MXene with balanced mixed terminations exhibits metallic conductivity ranging from 1,500 to 10,000 S/cm depending on composition and termination profile513. Specifically:

• Fluorine termination effects: -F groups increase work function by 0.3-0.8 eV compared to -OH terminated surfaces, shifting the Fermi level and reducing electron density at the surface; this results in 20-40% lower conductivity (measured via four-point probe on pressed pellets: 3,000-6,000 S/cm for F-rich vs. 6,000-9,000 S/cm for F-poor Ti3C2Tx)125
• Oxygen termination effects: Excessive =O content (>25 at%) promotes TiO2 formation at grain boundaries, introducing insulating phases that increase contact resistance by 50-200% and degrade overall conductivity to 800-2,000 S/cm212
• Hydroxyl termination effects: -OH groups maintain relatively high conductivity (5,000-8,000 S/cm) while enhancing hydrophilicity and electrochemical activity; they serve as active sites for pseudocapacitive charge storage717

The temperature dependence of conductivity exhibits unusual linear behavior between 100 K and 500 K for certain termination profiles, suggesting a charge transport mechanism distinct from conventional organic semiconductors13. This characteristic, combined with tunable work functions (ranging from 4.2 eV for -OH rich to 5.0 eV for -F rich Ti3C2Tx), enables applications in electronic devices requiring specific band alignment512.

Hydrophilicity And Dispersion Behavior

Surface termination chemistry dictates MXene's interaction with solvents and dispersibility:

• Aqueous dispersion: -OH and =O rich MXene (combined content >40 at%) readily disperses in water at concentrations up to 10-20 mg/mL without surfactants, forming stable colloidal suspensions (zeta potential -30 to -45 mV) for weeks under inert atmosphere2717
• Organic solvent compatibility: Reducing -OH content and increasing -F or organic terminations enhances dispersibility in non-polar solvents (toluene, chloroform) and improves compatibility with hydrophobic polymer matrices (polyvinylidene fluoride, polypropylene, epoxy resins)216
• Contact angle measurements: Water contact angles range from <5° for highly hydroxylated surfaces to 40-60° for fluorine-rich or organically modified surfaces, directly correlating with termination composition716

The hydrophilic nature of conventional MXene, while advantageous for aqueous processing, poses challenges for long-term stability as water molecules and dissolved oxygen catalyze oxidative degradation219. Surface modification strategies that reduce -OH content or introduce hydrophobic terminations can extend shelf life from days to months, though often at the cost of reduced electrochemical activity1219.

Mechanical Properties And Structural Stability

The surface termination influences mechanical behavior through several mechanisms:

• Interlayer shear strength: -OH and -F terminations create stronger interlayer interactions (shear modulus 2-4 GPa) compared to -Cl or -Br terminations (shear modulus 0.8-1.5 GPa), affecting exfoliation ease and film mechanical integrity410
• In-plane elastic modulus: Ranges from 200-400 GPa for Ti3C2Tx depending on termination, with oxygen-rich surfaces exhibiting higher stiffness due to stronger M-O bonding compared to M-F or M-OH411
• Tribological properties: MXene exhibits low friction coefficients (0.05-0.15) and excellent wear resistance attributed to weak interlayer bonding and self-lubricating behavior; -OH terminations enhance these properties compared to -F terminations416

Thermal stability varies significantly with termination: -F terminated MXene remains stable to ~400°C in inert atmosphere, while -OH rich variants begin decomposing at ~250°C through dehydroxylation reactions712. Thermogravimetric analysis (TGA) of Ti3C2Tx in nitrogen atmosphere shows mass loss onset at 280-320°C for hydroxyl-rich samples versus 380-420°C for fluorine-rich samples, with total mass loss by 800°C ranging from 8-15% depending on initial termination composition719.

Applications Leveraging Surface Terminated MXene

Energy Storage Systems: Supercapacitors And Batteries

Surface termination engineering is critical for optimizing MXene's performance in electrochemical energy storage:

Supercapacitor electrodes: -OH and =O terminations provide redox-active sites for pseudocapacitive charge storage, enabling volumetric capacitances of 900-1,500 F/cm³ in aqueous electrolytes (H2SO4, KOH)914. Specific performance metrics include:

• Ti3C2Tx with optimized -OH/-O ratio (1.2:1) achieves gravimetric capacitance of 380-450 F/g at 2 mV/s scan rate in 1 M H2SO4, retaining 85-90% capacitance at 100 mV/s914
• Intercalation with DMSO or TBAOH expands interlayer spacing to 1.3-1.5 nm, improving ion accessibility and increasing capacitance by 20-35% compared to pristine MXene914
• Cycling stability exceeds 10,000 cycles with <10% capacitance fade when oxygen content is controlled below 20 at%, as excessive oxygen promotes irreversible TiO2 formation912

Lithium-ion battery anodes: Reducing -F content is essential, as fluorine terminations hinder lithium adsorption and diffusion12. Calcium-treated Ti3C2Tx with reduced oxygen (8-12 at% O, <3 at% F) delivers reversible capacity of 320-410 mAh/g at 0.1 C rate, compared to 180-250 mAh/g for untreated fluorine-rich MXene12. The improved performance stems from enhanced Li+ intercalation kinetics (diffusion coefficient increased from 10⁻¹² to 10⁻¹⁰ cm²/s) and reduced charge transfer resistance (decreased from 80-120 Ω to 20-40 Ω)12.

Zinc-ion batteries: MXene serves as a surface-mediating material for zinc ion intercalation compounds; -OH rich terminations facilitate Zn²⁺ diffusion and stabilize the electrode-electrolyte interface5. Graphene oxide/MXene hybrid electrodes with controlled termination chemistry achieve specific capacities of 280-350 mAh/g at 0.5 A/g with excellent rate capability (180-220 mAh/g at 5 A/g)5.

For all energy storage applications, controlling surface termination to maximize electrochemically active sites while minimizing resistive oxide formation represents a key optimization parameter. Recommended termination targets: 25-35 at% -OH, 15-25 at% =O, <5 at% -F, with interlayer spacing >1.2 nm achieved through intercalation91214.

Electromagnetic Interference Shielding And Conductive Composites

MXene's metallic conductivity and 2D morphology make it exceptional for EMI shielding, with performance strongly dependent on surface termination:

Shielding effectiveness: Ti3C