MXene Few Layer: Advanced Synthesis, Structural Engineering, And Multifunctional Applications In Energy Storage And Beyond

Structural Characteristics And Formation Mechanisms Of MXene Few Layer Materials

MXene materials are synthesized by selectively etching the "A" layer (typically Al, Si, or Ga from Groups IIIA/IVA) from MAX phase ceramics (Mn+1AXn, where M = transition metal, X = C/N, n = 1–3) using strong oxidizing agents such as hydrofluoric acid (HF), Lewis acid salts (e.g., LiF/HCl), or halogen-based etchants19. The resulting multilayer MXene retains an accordion-like morphology with M-X layers held together by weak van der Waals forces and residual surface terminations (Tx = -OH, -F, -O)210. The general formula Mn+1XnTx accurately describes MXene, where Tx denotes surface functional groups introduced during etching212.

Few-layer MXene (typically ≤10 layers, often ≤20 layers as defined in patent literature) is obtained through subsequent delamination processes involving intercalation agents (e.g., tetraalkylammonium hydroxides, dimethyl sulfoxide, isopropylamine) that expand interlayer spacing from ~1 nm to >1.5 nm, weakening interlayer interactions and facilitating mechanical or ultrasonic exfoliation137. Patent 1 describes a method combining Lewis acid salt etching with potassium chloride, followed by intercalation and freeze-drying to produce single-layer or few-layer Ti₃C₂Tx nanosheets with interlayer distances sufficient for efficient ion sieving in nanofiltration membranes (achieving 99.9% rejection of Alzheimer's blue dye and permeation flux of 65.1 L/m²·h after 120 minutes)1. Patent 3 employs electrostatic self-assembly with cation-assisted precipitation (e.g., NH₄⁺ ions) to rapidly aggregate few-layer MXene from liquid-phase exfoliated suspensions, enabling scalable powder production via freeze-drying and annealing for battery electrode applications3.

The layer number critically influences properties: monolayer MXene exhibits maximum specific surface area (up to several hundred m²/g) and shortest ion diffusion paths, while few-layer configurations (2–10 layers) balance mechanical robustness with electrochemical accessibility316. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) confirm layer thicknesses of ~1–2 nm per MXene sheet, with lateral dimensions ranging from hundreds of nanometers to several micrometers depending on sonication intensity and intercalation efficiency716. Patent 16 reports high-intensity focused ultrasound (HIFU) treatment to produce single-layer small-size Ti₃C₂Tx samples with enhanced purity, dispersion, and specific surface area compared to conventional methods16.

Surface termination engineering is pivotal: as-synthesized MXene surfaces are rich in -F and -OH groups from HF etching, which can be partially removed or replaced via thermal annealing in inert/reducing atmospheres (e.g., Ar, H₂ at 200–600°C) to expose more electrochemically active sites and improve conductivity1417. Patent 14 describes annealing MXene in inert atmospheres to eliminate surface -F/-OH groups, thereby increasing ion exchange capacity for flow battery membranes14. Oxygen-rich surface terminations can be intentionally introduced via controlled oxidation or alternative etchants (e.g., NH₄HF₂, elemental halogens) to tailor hydrophilicity and catalytic activity912.

Synthesis Routes And Process Optimization For Few-Layer MXene Production

HF-Based Etching And Delamination

The conventional synthesis begins with immersing MAX phase powders (e.g., Ti₃AlC₂, Nb₂AlC, Mo₂TiAlC₂) in concentrated HF (20–50 wt%) at room temperature to 60°C for 12–72 hours, selectively dissolving the A-layer and yielding multilayer MXene with accordion morphology19. Patent 1 specifies mixing Ti₃AlC₂ with Lewis acid salts (e.g., FeCl₃, AlCl₃) and KCl, followed by molten-salt etching at elevated temperatures (e.g., 550–650°C for 2–5 hours) to simultaneously etch and pre-intercalate cations, reducing subsequent delamination steps1. The solid product is dissolved in dilute HCl (1–6 M) and washed with deionized water until pH ~6–7, then mixed with intercalators (e.g., tetrabutylammonium hydroxide, TBAOH; tetramethylammonium hydroxide, TMAOH) at mass ratios of 1:5 to 1:20 (MXene:intercalator) and stirred for 6–48 hours at room temperature to 80°C17. Freeze-drying or vacuum filtration isolates few-layer MXene powders or suspensions13.

Patent 7 introduces a polar solvent (e.g., N-methyl-2-pyrrolidone, NMP) and organic solvent (e.g., ethanol) mixture with tetramethylammonium salts to exfoliate multilayer MXene into monolayers, achieving conductive thin films with sheet resistance <10 Ω/sq and optical transmittance >80% at 550 nm7. The method minimizes residual A-element contamination (<1 at% Al by X-ray photoelectron spectroscopy, XPS) and enhances film conductivity by >30% compared to HF-only routes7.

HF-Free And Green Synthesis Approaches

To address environmental and safety concerns associated with HF, alternative etchants have been developed. Patent 9 describes using elemental halogens (Cl₂, Br₂, I₂) in anhydrous organic solvents (e.g., chloroform, carbon tetrachloride) at 30–90°C for ~24 hours to etch MAX phases, producing layered MXene with comparable or superior crystallinity and fewer surface defects than HF-etched counterparts9. The halogen method avoids HF waste and allows tuning of surface terminations by varying halogen type and reaction temperature9. Post-etching, the MXene slurry is washed with ethanol and water, then intercalated with TBAOH or DMSO for delamination9.

Molten-salt etching combined with in-situ cation exchange (e.g., using ZnCl₂, CuCl₂ at 500–700°C) has been explored to directly obtain few-layer MXene with intercalated metal ions, bypassing separate intercalation steps and reducing processing time to <6 hours total118. Patent 18 reports magnetic MXene composites prepared by intercalating Fe²⁺/Fe³⁺ ions during or after etching, achieving oriented layer stacking and enhanced electromagnetic properties (magnetic permeability μ' >1.2 at 1–10 GHz)18.

Scalable Production And Quality Control

Patent 3 emphasizes rapid, scalable synthesis via electrostatic self-assembly: few-layer MXene suspensions (obtained by liquid-phase exfoliation in water or ethanol with sonication power 200–800 W for 1–4 hours) are mixed with electrolyte solutions (e.g., 0.1–1 M NH₄Cl, NaCl) at volume ratios 1:0.5 to 1:5, inducing controlled aggregation within minutes due to charge screening3. The resulting sol or precipitate is collected by low-speed centrifugation (1000–3000 rpm, 5–15 min) or vacuum filtration, then freeze-dried at -40 to -80°C and annealed at 150–400°C in Ar for 2–6 hours to remove residual solvents and stabilize structure3. This method yields few-layer MXene powders with >90% delamination efficiency (confirmed by XRD peak broadening and TEM) and specific capacitance >300 F/g at 1 A/g in lithium-ion battery anodes3.

Quality metrics include: (1) interlayer spacing measured by XRD (002 peak shift from ~9.5° for multilayer Ti₃C₂Tx to ~6–7° for few-layer, corresponding to d-spacing increase from ~0.98 nm to ~1.3–1.5 nm)13; (2) lateral size distribution by dynamic light scattering (DLS) or TEM (target: 100–500 nm for high surface area, >1 μm for mechanical reinforcement)416; (3) surface termination composition by XPS (F/O/C atomic ratios, e.g., F:O ~1:2 for HF-etched, <1:3 after annealing)714; (4) electrical conductivity of pressed pellets or films (>10⁴ S/m for high-quality few-layer Ti₃C₂Tx)711.

Physicochemical Properties And Performance Metrics Of MXene Few Layer Materials

Electrical Conductivity And Charge Transport

Few-layer MXene exhibits metallic conductivity due to partially filled d-orbitals of transition metals and delocalized electrons across M-X bonds211. Ti₃C₂Tx films prepared by vacuum filtration of few-layer suspensions achieve electrical conductivity of 6,500–15,000 S/cm (measured by four-point probe at room temperature), approaching that of graphite (~25,000 S/cm) and surpassing reduced graphene oxide (~1,000–5,000 S/cm)711. Patent 11 reports that washing MXene with dilute acids (e.g., 0.1–1 M HCl, H₂SO₄) to remove residual Li⁺ or Na⁺ ions from intercalation, followed by coating with π-conjugated polymers (e.g., polyaniline, polypyrrole at 1–10 wt%), maintains conductivity >10,000 S/cm even after 30 days of ambient exposure (vs. ~8,000 S/cm for untreated MXene)11. The polymer layer acts as a moisture barrier, preventing oxidation of surface Ti atoms and preserving metallic character11.

Electrochemical impedance spectroscopy (EIS) on few-layer MXene electrodes reveals low charge-transfer resistance (Rct <1 Ω·cm² at 1 kHz in 1 M H₂SO₄ electrolyte), attributed to abundant surface redox-active sites (-OH, -O terminations) and short ion diffusion distances (<5 nm through few layers)13. Patent 13 demonstrates MXene-based neural electrodes with interfacial impedance <50 Ω at 1 kHz (vs. >500 Ω for Au electrodes of same geometry), enabling high-resolution electrophysiological recording13.

Mechanical Strength And Flexibility

Few-layer MXene nanosheets possess high in-plane Young's modulus (330 ± 20 GPa for Ti₃C₂Tx monolayer, measured by nanoindentation AFM) and tensile strength (~570 MPa for vacuum-filtered films with ~10 layers)410. Patent 4 incorporates 0.5–5 wt% few-layer Ti₃C₂Tx into zinc matrix via electrostatic self-assembly and laser powder bed fusion (LPBF), achieving Zn-MXene composites with ultimate tensile strength 180–250 MPa (vs. 120 MPa for pure Zn) and elongation 8–15% (vs. 3–5% for pure Zn)4. MXene acts as grain refiner during rapid solidification (cooling rate ~10⁶ K/s in LPBF), reducing Zn grain size from ~50 μm to ~10 μm and enhancing dislocation pinning at grain boundaries4. The composite exhibits relative density >99.5% and biocompatible degradation rate (~0.1 mm/year in simulated body fluid), suitable for biodegradable orthopedic implants4.

Flexibility is demonstrated by bending MXene films (thickness 5–50 μm) to radii <1 mm without cracking, with <5% conductivity loss after 10,000 bending cycles (bending angle ±90°)711. This flexibility arises from weak interlayer van der Waals forces allowing layer sliding and the ductile nature of metallic M-X bonds10.

Thermal And Chemical Stability

MXene few-layer materials exhibit thermal stability up to 200–400°C in inert atmospheres (Ar, N₂), beyond which surface terminations (-OH, -F) desorb and partial oxidation to metal oxides (e.g., TiO₂, Nb₂O₅) occurs214. Thermogravimetric analysis (TGA) of Ti₃C₂Tx shows ~5–10 wt% mass loss at 200–300°C (dehydration and defluorination) and ~15–25 wt% loss at 400–600°C (oxidation to TiO₂)1417. Patent 14 reports that annealing Nb₂CTx at 300°C in Ar for 2 hours removes ~70% of surface -F groups (confirmed by XPS F 1s peak reduction) while maintaining layered structure (XRD (002) peak retained), improving ion exchange capacity from 0.8 to 1.5 meq/g for vanadium redox flow battery membranes14.

Chemical stability in aqueous environments is pH-dependent: MXene is stable in neutral to mildly acidic solutions (pH 4–7) for weeks, but degrades in strong acids (pH <2) or bases (pH >10) due to dissolution of M-X bonds or oxidation of surface terminations111. Patent 1 demonstrates that Ti₃C₂Tx nanofiltration membranes maintain >95% flux and rejection performance after continuous operation in pH 5–8 water for 120 hours, but show ~20% flux decline in pH 3 solutions due to partial protonation and swelling1. Coating MXene with hydrophobic polymers (e.g., polydimethylsiloxane, PDMS at 2–5 wt%) or inorganic shells (e.g., Al₂O₃ via atomic layer deposition, 2–10 nm thickness) extends stability to pH 2–12 and reduces oxidation rate by >80% in ambient air1112.

Hydrophilicity And Surface Chemistry

The abundant -OH and -O terminations render MXene highly hydrophilic, with water contact angles <10° for freshly prepared few-layer films112. This facilitates aqueous processing and enables strong interfacial adhesion with polar polymers (e.g., polyvinyl alcohol, PVA; sulfonated poly(ether ether ketone), SPEEK) for composite membranes114. Patent 14 reports SPEEK/MXene (mass ratio 3:1 to 10:1) ion-exchange membranes with proton conductivity 80–150 mS/cm at 80°C (vs. 60 mS/cm for pure SPEEK) and