Exfoliated MXene: Synthesis, Surface Modification, And Advanced Applications In Energy Storage And Functional Coatings

Structural Characteristics And Chemical Composition Of Exfoliated MXene

Exfoliated MXene materials possess a general formula of Mn+1XnTx (n = 1, 2, or 3), where M denotes early transition metals such as Ti, V, Nb, Ta, Cr, Mo, or W; X represents carbon and/or nitrogen; and Tx signifies surface termination groups including —OH, —F, —O, and occasionally —Cl introduced during the etching process 127. The parent MAX phase (Mn+1AXn) is a ternary layered compound in which the A-layer (typically Al, Si, or Ga) is selectively removed via chemical etching, yielding accordion-like multilayer MXene that can be further delaminated into single- or few-layer nanosheets 346. The most extensively studied member, Ti3C2Tx, exhibits a single-layer thickness of approximately 1 nm and lateral dimensions ranging from 500 nm to 5 μm depending on precursor particle size and exfoliation conditions 417.

The interlayer spacing of as-etched MXene is typically 0.9–1.2 nm, which can be expanded to 1.5–2.0 nm through intercalation with organic molecules (e.g., dimethyl sulfoxide, DMSO) or inorganic cations (e.g., Li+, Na+) to facilitate mechanical or sonication-assisted exfoliation 410. Surface terminations play a critical role in determining hydrophilicity, electrochemical activity, and compatibility with polymer matrices: —OH and —O groups enhance hydrophilicity and enable hydrogen bonding with aqueous or polar solvents, whereas —F terminations, introduced by HF-based etchants, can reduce volumetric capacitance and biocompatibility 315. The metallic conductivity of exfoliated Ti3C2Tx MXene (approximately 6,500–24,000 S/cm) arises from the delocalized d-electrons of transition metal atoms, making it superior to conventional carbon-based electrode materials 3514.

Key structural features include:

• Layer Thickness: Single-layer MXene nanosheets are 1.0 ± 0.2 nm thick; multilayer stacks range from 3 to 10 nm 417.
• Lateral Size: Controlled by MAX precursor particle size (typically 1–10 μm after sieving through 400-mesh screens) and exfoliation intensity 19.
• Surface Area: Exfoliated Ti3C2Tx exhibits BET surface areas of 20–100 m²/g, significantly lower than activated graphene but sufficient for high volumetric capacitance (up to 1,500 F/cm³ in hydrogel form) 20.
• Interlayer Spacing: Expandable from 0.98 nm (as-etched) to 1.8 nm (DMSO-intercalated) or 2.5 nm (oleylamine-modified) 117.

The chemical composition and surface termination distribution can be tailored by adjusting etching conditions (etchant type, concentration, temperature, and duration), post-treatment (washing pH, intercalation agents), and storage environment (inert atmosphere, low temperature) to optimize performance for specific applications 21014.

Synthesis Routes And Exfoliation Strategies For Exfoliated MXene

Selective Etching Of MAX Phase Precursors

The predominant synthesis route for exfoliated MXene involves selective removal of the A-layer from MAX phase precursors using fluoride-containing etchants. Traditional methods employ concentrated hydrofluoric acid (HF, 40–50 wt%) at room temperature for 18–72 hours, achieving near-complete Al removal from Ti3AlC2 to yield Ti3C2Tx 1018. However, HF is highly corrosive and toxic, posing severe safety hazards and environmental concerns 315. To mitigate these risks, in-situ HF generation via LiF/HCl mixtures has become the preferred approach: typical molar ratios of Ti3AlC2:LiF:HCl are 1:20:1.6, reacted at 35–60°C for 24–48 hours, followed by centrifugal washing until pH reaches 5.5–6.5 1810. This method reduces direct HF exposure while maintaining high etching efficiency and product purity 18.

Alternative fluoride-free etching strategies have emerged to address toxicity and fluorine contamination. Sodium fluoroborate (NaBF4) in polyol-water solutions enables safe, scalable production without HF, yielding high-purity MXene with reduced fluorine surface terminations 20. Elemental halogen etching (e.g., Br2 or I2 in anhydrous organic solvents under inert atmosphere) selectively removes Al while introducing —Br or —I terminations, offering tunable surface chemistry and improved biocompatibility for supercapacitor electrodes 15. However, halogen-based methods require strict anhydrous conditions and longer reaction times (up to 90°C for 24 hours) 15.

Etching parameters critically influence MXene morphology and yield:

• Etchant Concentration: Higher LiF concentrations (e.g., 20:1 LiF:Ti3AlC2 molar ratio) accelerate Al removal but may induce over-etching and layer collapse; optimal ratios balance etching completeness and structural integrity 18.
• Temperature: Elevated temperatures (50–90°C) shorten reaction time but increase oxidation risk; room-temperature etching (20–35°C) preserves surface terminations and minimizes defects 1015.
• Duration: Insufficient etching (<18 hours) leaves MAX residues, reducing purity; excessive etching (>72 hours) causes layer restacking and decreased exfoliation efficiency 410.
• Washing Protocol: Centrifugal washing with deionized water (5,000–10,000 rpm, 5–10 cycles) until pH 6.0 ± 0.5 removes residual etchant and soluble byproducts, preventing post-synthesis oxidation 1810.

Intercalation And Mechanical Exfoliation Techniques

Post-etching intercalation with organic molecules or inorganic cations expands interlayer spacing and weakens van der Waals forces, facilitating mechanical exfoliation into single- or few-layer nanosheets. DMSO intercalation is the most widely adopted method: etched MXene is dispersed in DMSO (10–50 mg/mL) and gently shaken by hand or stirred at low speed (200–400 rpm) for 6–18 hours, followed by water bath sonication (100–200 W, 30–60 minutes) or probe sonication (500–1,000 W, 10–30 minutes) to achieve delamination 410. However, hand-shaking yields predominantly multilayer sheets with low single-layer content (<30 wt%), and prolonged sonication induces oxidation and structural damage 414.

High-shear mechanical exfoliation using kitchen blenders or homogenizers offers a scalable, rapid alternative: DMSO-intercalated MXene suspension is blended at 10,000–20,000 rpm for 5–15 minutes, producing few-layer nanosheets (2–5 layers) with lateral sizes of 1–3 μm and yields exceeding 60 wt% 4. This method avoids organic solvent waste and reduces processing time from days to hours, making it suitable for industrial-scale production 4. Alternatively, freeze-thaw cycling (−60°C for 12 hours, then thawing at room temperature, repeated 3–5 cycles) induces ice crystal formation between layers, mechanically separating them without sonication-induced oxidation 18.

Exfoliation efficiency is quantified by:

• Single-Layer Yield: Determined by atomic force microscopy (AFM) height profiling (1.0 ± 0.2 nm) or transmission electron microscopy (TEM) contrast analysis; high-shear blending achieves 40–60 wt% single-layer content 4.
• Lateral Size Distribution: Measured by dynamic light scattering (DLS) or scanning electron microscopy (SEM); optimal exfoliation preserves 70–90% of precursor lateral dimensions 417.
• Colloidal Stability: Assessed by zeta potential (ζ > −30 mV indicates stable dispersion) and sedimentation rate (stable suspensions remain homogeneous for >7 days) 214.

For applications requiring monodisperse single-layer MXene, density gradient ultracentrifugation (e.g., sucrose gradient, 50,000–100,000 rpm, 2–4 hours) separates nanosheets by thickness, yielding fractions with >90% single-layer purity 4.

Surface Modification Strategies For Enhanced Stability And Functionality Of Exfoliated MXene

Oxidation Resistance And Long-Term Stability Enhancement

Exfoliated MXene, particularly Ti3C2Tx, is highly susceptible to oxidation in aqueous environments due to the presence of dissolved oxygen and water molecules, which react with surface Ti atoms to form TiO2, degrading electrical conductivity and electrochemical performance within days to weeks 214. Oxidation kinetics are accelerated by elevated temperatures, acidic or alkaline pH, and exposure to air, necessitating robust stabilization strategies for practical applications 14.

Surface modification with antioxidant biomolecules has proven effective: sericin protein, extracted from silk cocoons, contains abundant polar amino acids (serine, threonine) and acts as a natural radical scavenger. Coating Ti3C2Tx nanosheets with sericin (mass ratio 1:0.5 to 1:2 MXene:sericin) via aqueous mixing and freeze-drying extends aqueous dispersion stability from 2 weeks (unmodified) to >8 weeks at room temperature, as evidenced by unchanged UV-Vis absorption spectra and maintained conductivity (>5,000 S/cm) 14. Sericin's hydroxyl and carboxyl groups form hydrogen bonds with MXene surface terminations, creating a protective hydration shell that inhibits oxygen diffusion 14.

Carboxylation via chloroacetic acid treatment introduces additional —COOH groups, enhancing compatibility with silicone rubber matrices and enabling esterification reactions that anchor MXene to polymer chains, preventing aggregation and oxidation 1. The carboxylation process involves dispersing MXene (30–50 mass parts) in chloroacetic acid solution (60–80 mass parts) at 60–80°C for 4–8 hours, followed by freeze-drying at −60°C 1. Carboxylated MXene exhibits improved dispersibility in non-polar solvents (e.g., chloroform) and reduced oxidation rates (50% conductivity retention after 4 weeks in air vs. 10% for pristine MXene) 1.

Inorganic salt additives (NaCl, LiCl, Na2CO3) at concentrations of 0.1–1.0 M stabilize MXene aqueous dispersions by screening surface charges and reducing oxygen solubility, extending shelf life to 4–6 weeks 14. However, high ionic strength may interfere with electrochemical applications, requiring thorough washing before device fabrication 14.

Storage under inert atmospheres (Ar, N2, or He) in sealed containers at 4°C or below effectively prevents oxidation, maintaining MXene properties for >6 months 214. For long-term storage, vacuum-sealed packaging with desiccants (silica gel) and oxygen scavengers is recommended 14.

Functionalization For Polymer Composites And Coatings

Hydrophobic surface modification enables MXene integration into non-polar polymer matrices and organic solvents, expanding application scope to waterproof coatings, flexible electronics, and composite materials. Oleylamine (OA) functionalization via liquid-phase ligand exchange transforms hydrophilic Ti3C2Tx into organophilic nanosheets dispersible in chloroform, toluene, and hexane 17. The procedure involves mixing aqueous MXene dispersion (5–20 mg/mL) with oleylamine in chloroform (10–20 mg/mL, 1:1 volume ratio) under inert atmosphere (N2 or Ar) with 20–50 wt% oleic acid as co-stabilizer, stirring for 12–24 hours, then separating the organic phase and washing with ethanol 17. OA-modified MXene forms stable colloids in chloroform (>3 months without sedimentation) and retains 2D morphology during controlled oxidation to amorphous transition metal oxide nanosheets (thickness 2–5 nm, lateral size 500 nm–5 μm) 17.

Metal alkoxide surface modification via covalent bonding of titanium isopropoxide or zirconium butoxide enhances thermal stability and catalytic activity 16. The reaction is conducted in anhydrous toluene at 80–120°C for 6–12 hours, forming M—O—Ti or M—O—Zr bonds that anchor alkoxide ligands to MXene surfaces 16. Alkoxide-modified MXene exhibits improved resistance to hydrolysis and oxidation, maintaining structural integrity at temperatures up to 300°C in air 16.

Hydroxyl-rich compound modification (e.g., polyethylene glycol, PEG; polyvinyl alcohol, PVA) via hydrogen bonding or esterification improves biocompatibility and dispersibility in aqueous media 2. PEG-modified MXene (mass ratio 1:1 to 1:3 MXene:PEG) demonstrates reduced cytotoxicity (cell viability >85% at 100 μg/mL) and enhanced photothermal conversion efficiency (>95% under 808 nm NIR irradiation) for cancer therapy applications 2.

For anti-corrosion coatings on magnesium alloys, MXene is blended with hydrophilic polymers (PVA, sodium alginate, or carboxymethyl cellulose at 5–25 wt%) and coupling agents (silanes, 1–5 wt%) to form composite films via spin-coating, blade-coating, or vacuum filtration 12. The resulting coatings (thickness 5–50 μm) exhibit corrosion current densities of 10⁻⁷–10⁻⁶ A/cm² in 3.5 wt% NaCl solution, 2–3 orders of magnitude lower than uncoated substrates, and wear rates of 1–5 × 10⁻⁵ mm³/N·m under dry sliding conditions 12.

Electrochemical Performance And Energy Storage Applications Of Exfoliated MXene

Supercapacitor Electrodes And Volumetric Capacitance Optimization

Exfoliated MXene, particularly Ti3C2Tx, has emerged as a leading candidate for high-performance supercapacitor electrodes due to its exceptional volumetric capacitance, which surpasses conventional carbon-based materials. In aqueous KOH electrolyte (1–6 M), Ti3C2Tx films prepared by vacuum filtration deliver volumetric capacitances of 340–400 F/cm³ at scan rates of 2–20 mV/s, attributed to pseudocapacitive redox reactions involving surface —OH and —O terminations 20. MXene hydrogels, formed by freeze-thaw gelation or chemical cross-linking, achieve record volumetric capacitances of 1,500 F/cm³, comparable to RuO2 but at significantly lower cost 20.

Fluorine-free MXene electrodes exhibit superior electrochemical activity