Oxygen Terminated MXene: Surface Engineering, Synthesis Strategies, And Advanced Applications In Catalysis And Energy Storage

Molecular Composition And Surface Termination Chemistry Of Oxygen Terminated MXene

Oxygen terminated MXene belongs to the broader MXene family with the general formula M_n+1X_nT_x, where M denotes early transition metals (Ti, Nb, V, Ta, Mo, W, Zr, Hf, Cr, Sc, Y, Lu) 1,4,6, X represents carbon and/or nitrogen 2,10, and T_x signifies surface termination groups including ═O, -OH, -F, -Cl, -Br, -I, -Se, -Te, and -S 5,16. The integer n (typically 1, 2, or 3) defines the number of M atomic layers interleaved by X 7,15. In oxygen terminated MXene, the surface is predominantly functionalized with oxygen-based groups (═O and -OH), which arise during selective etching of the A-layer (commonly Al, Si, Ga) from the parent MAX phase (M_n+1AX_n) using aqueous acidic etchants such as HF, LiF/HCl mixtures, or electrochemical methods 1,3,12.

The prevalence of oxygen termination profoundly influences MXene's electronic properties. Oxygen-rich surfaces exhibit higher electronegativity and hydrophilicity compared to fluorine-terminated variants, facilitating aqueous dispersion and protein/cell adhesion resistance in biomedical contexts 8. Density functional theory (DFT) calculations reveal that ═O termination reduces the work function and enhances charge transfer kinetics, critical for electrocatalytic applications 2. However, oxygen termination also introduces structural instability: MXene nanosheets in aqueous or humid environments undergo progressive oxidation, converting to metal oxides (e.g., TiO₂) and losing metallic conductivity within days to weeks 12,14,20. This degradation is accelerated by dissolved oxygen, light exposure, and elevated temperatures 20.

Key structural characteristics of oxygen terminated MXene include:

• Lateral dimensions: 100–500 nm for exfoliated nanosheets 1, with thickness ranging from single-layer (~1 nm) to few-layer (<10 nm) assemblies 6.
• Surface functional group density: Oxygen content can reach 20–46 wt% in heavily oxidized graphene oxide analogs, though typical MXene oxygen termination is lower (5–15 wt%) depending on synthesis conditions 7.
• Interlayer spacing: Oxygen-terminated MXene exhibits d-spacing of 0.98–1.2 nm (compared to 0.85 nm for fluorine-terminated Ti₃C₂T_x), enabling facile ion intercalation 13,18.

Synthesis Routes And Surface Termination Control For Oxygen Terminated MXene

Selective Etching Of MAX Phases: Conventional And Fluoride-Free Methods

The predominant synthesis pathway involves selective removal of the A-layer from MAX precursors (e.g., Ti₃AlC₂, Nb₂AlC, V₂AlC) 1,2,13. Traditional HF etching (40–50% aqueous HF, 18–72 h, room temperature) yields MXene with mixed -F, -OH, and ═O terminations 3. To enrich oxygen termination while minimizing fluorine content, researchers employ:

1. LiF/HCl etching: Mixing LiF (1.6 g) with 9 M HCl (20 mL) and Ti₃AlC₂ powder (1 g) at 35–40°C for 24–48 h generates in-situ HF at controlled concentrations, producing Ti₃C₂T_x with higher -OH/-O ratios and reduced -F content 12,20. Post-etching washing (5–10 cycles, deionized water, centrifugation at 3500 rpm) removes residual salts and excess fluoride 1.
2. Electrochemical etching: Applying anodic potentials (3–5 V vs. Ag/AgCl) in NH₄Cl or (NH₄)₂SO₄ electrolytes selectively oxidizes Al layers, yielding MXene with predominantly -OH and ═O groups and trace -Cl 3,5. This method avoids HF but introduces chloride terminations.
3. Molten salt etching: Immersing MAX phases in eutectic mixtures of alkali halides (e.g., LiF-NaF-KF, 750–850°C, 2–6 h) with transition metal bromide salts (e.g., CuBr₂, ZnBr₂) enables halide-terminated MXene synthesis, which can be post-treated in oxygen-rich atmospheres to convert -Br/-Cl to ═O 5.

Oxygen Enrichment Via Post-Synthesis Treatments

To maximize oxygen termination and introduce oxygen vacancies (OVs), the following strategies are employed:

• Thermal annealing in controlled atmospheres: Heating as-synthesized MXene (e.g., Ti₃C₂T_x) at 300–600°C under Ar/H₂ (95:5 vol%) or pure Ar for 1–4 h promotes desorption of -F and -OH, leaving ═O as the dominant termination 9,12. Higher temperatures (>600°C) risk complete oxidation to TiO₂ 20.
• Hydrothermal oxidation: Treating MXene dispersions in deionized water at 120–180°C for 6–24 h in autoclaves gradually replaces -F with -OH and ═O, though prolonged exposure degrades the MXene lattice 3.
• Chemical reduction followed by re-oxidation: Reducing MXene with NaBH₄ or hydrazine removes surface oxygen, then controlled air exposure at 80–150°C re-introduces ═O selectively 9.

Exfoliation And Colloidal Stabilization

Delamination of multilayer MXene into single/few-layer nanosheets is achieved via:

1. Sonication-assisted exfoliation: Dispersing etched MXene (0.5–2 mg/mL) in water or polar solvents (DMSO, NMP) and sonicating (bath or probe, 100–400 W, 30–120 min) under inert atmosphere (Ar or N₂) 1,11. Centrifugation (3500–5000 rpm, 1 h) separates unexfoliated particles.
2. Intercalation-assisted exfoliation: Intercalating TMAOH (tetramethylammonium hydroxide), DMSO, or urea between MXene layers (12–48 h, room temperature) expands interlayer spacing, facilitating mechanical shaking or mild sonication 11,20.
3. Freeze-drying: Rapidly freezing MXene dispersions in liquid N₂ followed by lyophilization (−50°C, <10 Pa, 24 h) prevents restacking and yields fluffy powders suitable for composite fabrication 12.

Critical process parameters for oxygen-rich MXene synthesis:

• Etching temperature: 35–40°C (LiF/HCl) vs. 750–850°C (molten salt) 5,12.
• Etching duration: 24–72 h for aqueous methods; 2–6 h for molten salt 1,5.
• Washing cycles: ≥5 cycles to achieve pH 6–7 and remove residual etchants 1,20.
• Annealing atmosphere: Ar/H₂ (95:5) or pure Ar to prevent over-oxidation 9,12.
• Sonication power and duration: 100–400 W, 30–120 min; excessive sonication induces defects 11.

Oxygen Vacancies (OVs) As Active Sites In Oxygen Terminated MXene

Oxygen vacancies—defects where lattice oxygen atoms are missing—emerge as pivotal active centers in oxygen terminated MXene, particularly for catalytic and electrochemical applications 9. OVs are generated during:

• High-temperature annealing: Heating Ti₃C₂T_x at 400–600°C under reducing atmospheres (Ar/H₂) desorbs surface -OH and ═O, creating coordinatively unsaturated Ti sites 9.
• Chemical doping: Incorporating heteroatoms (e.g., Au, Mo, Fe, Co) via hydrothermal or solvothermal routes introduces lattice strain and facilitates OV formation 9,12.
• Electrochemical cycling: Repeated charge-discharge in aqueous electrolytes induces surface reconstruction and OV generation 13.

Functional roles of OVs in oxygen terminated MXene:

1. Enhanced O₂ adsorption and activation: OVs serve as Lewis acid sites, binding O₂ molecules and lowering the energy barrier for O-O bond cleavage in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) 2,9.
2. Improved electron transfer: OVs increase the density of states near the Fermi level, enhancing electrical conductivity (up to 10⁴ S/cm for Ti₃C₂T_x with optimized OVs) 2.
3. ROS scavenging: OVs exhibit superoxide dismutase (SOD)-, catalase (CAT)-, and glutathione peroxidase (GPX)-like activities, enabling antioxidant applications in biomedical devices 12.

Quantitative characterization of OVs employs:

• X-ray photoelectron spectroscopy (XPS): Deconvolution of O 1s spectra reveals peaks at 530.5 eV (lattice O), 531.8 eV (OVs), and 533.2 eV (-OH) 9.
• Electron paramagnetic resonance (EPR): g-factor ~2.003 signals confirm unpaired electrons at OV sites 9.
• Positron annihilation spectroscopy (PAS): Quantifies vacancy concentration (typically 10¹⁸–10²⁰ cm⁻³ in annealed MXene) 9.

Stability Challenges And Mitigation Strategies For Oxygen Terminated MXene

Oxidation Mechanisms And Degradation Kinetics

Oxygen terminated MXene is inherently susceptible to oxidation in aqueous and humid environments, driven by:

• Hydrolysis of surface Ti-C bonds: Water molecules attack Ti-C bonds at sheet edges, forming Ti-OH and releasing CH₄ or CO₂ 14,20.
• Dissolved oxygen-mediated oxidation: O₂ dissolved in water oxidizes Ti³⁺ to Ti⁴⁺, nucleating TiO₂ nanoparticles on MXene surfaces 20.
• Photo-induced oxidation: UV/visible light generates electron-hole pairs, accelerating surface oxidation 20.

Degradation kinetics follow pseudo-first-order models: for Ti₃C₂T_x dispersions (1 mg/mL, deionized water, ambient conditions), conductivity drops by 50% within 3–7 days and >90% within 14–21 days 14,20. Transmission electron microscopy (TEM) reveals amorphous TiO₂ layers (2–5 nm thick) after 7 days 20.

Stabilization Approaches

1. Surface passivation with organic molecules: Functionalizing MXene with hydroxyl-rich compounds (polyvinyl alcohol, chitosan, glucose) or ionic liquids (1-ethyl-3-methylimidazolium acetate) forms protective layers, reducing water/O₂ access 14. For example, glucose-modified Ti₃C₂T_x retains 85% conductivity after 30 days in water 14.
2. Heterostructure formation: Coupling MXene with chemically stable 2D materials (MoS₂, graphene, h-BN) via van der Waals assembly or covalent bonding shields reactive surfaces 12. MXene/MoS₂ heterostructures prepared by thermal annealing (450°C, Ar/H₂, 2 h) exhibit <10% conductivity loss over 60 days 12.
3. Inert atmosphere storage: Storing MXene powders or dispersions under Ar or N₂ at −20°C to 4°C extends shelf life to >6 months 20.
4. Antioxidant additives: Adding ascorbic acid (0.1–1 mM) or hydroquinone (0.5 mM) to MXene dispersions scavenges dissolved O₂ and free radicals, delaying oxidation 20.
5. Optimized MAX precursor synthesis: Reducing oxygen impurities in MAX phases (e.g., Ti₃AlC₂ with <0.5 wt% O) via high-purity raw materials and controlled sintering (1400–1600°C, vacuum or Ar) yields MXene with lower intrinsic oxidation susceptibility 19,20.

Recommended storage and handling protocols:

• Store MXene dispersions at 4°C in amber glass vials under Ar headspace; use within 7 days for critical applications 20.
• For long-term storage, freeze-dry MXene and seal under vacuum or Ar; reconstitute in degassed solvents immediately before use 12,20.
• Minimize light exposure during synthesis and processing; conduct operations in dark or amber-lit environments 20.

Catalytic Applications Of Oxygen Terminated MXene: Oxygen Evolution Reaction (OER) And Beyond

OER Catalysis: Mechanism And Performance Metrics

Oxygen terminated MXene, particularly when integrated with transition metal oxides or hydroxides, exhibits exceptional OER activity in alkaline electrolytes (0.1–1 M KOH) 1,2. The OER mechanism on MXene-based catalysts follows the four-electron pathway:

1. M-OH + OH⁻ → M-O + H₂O + e⁻
2. M-O + OH⁻ → M-OOH + e⁻
3. M-OOH + OH⁻ → M-OO + H₂O + e⁻
4. M-OO → M + O₂ + e⁻

where M represents exposed Ti, Nb, or V sites at OVs 2,9. Oxygen termination (═O) stabilizes M-O intermediates, lowering the overpotential for the rate-determining step (typically step 2 or 3) 2.

Case Study: MXene/MOF Composite OER Catalyst 1

A Ti₃C₂T_x/Ni-MOF (metal-organic framework) composite synthesized via in-situ growth of Ni-based MOF nanoparticles (10–100 nm) on MXene nanosheets (100–500 nm lateral size) demonstrates:

• Overpotential (η₁₀): 280 mV at 10 mA/cm² in 1 M KOH, compared to 350 mV for bare Ni-MOF and 420 mV for Ti₃C₂T_x alone