Hydroxyl Terminated MXene: Synthesis, Surface Chemistry, And Advanced Applications In Energy Storage And Catalysis

Molecular Composition And Structural Characteristics Of Hydroxyl Terminated MXene

Hydroxyl terminated MXene belongs to the broader family of two-dimensional transition metal carbides, nitrides, or carbonitrides with the general formula M_n+1X_nT_x, where M represents early transition metals (Ti, V, Nb, Ta, Zr, Hf, Cr, Mo, Sc), X denotes carbon and/or nitrogen, n ranges from 1 to 3, and T_x signifies surface termination groups including hydroxyl (-OH), oxygen (=O), fluorine (-F), and chlorine (-Cl) 1,6,13. The hydroxyl terminations are predominantly introduced during the chemical etching process when aluminum (Al) or silicon (Si) layers are selectively removed from the parent MAX phase (M_n+1AX_n) using hydrofluoric acid (HF) or fluoride-containing etchants 1,15,18.

The layered architecture of hydroxyl terminated MXene consists of alternating atomic layers: n layers of X atoms sandwiched between n+1 layers of M atoms, with M atoms forming the outermost surfaces 20. Each unit cell comprises multiple formula units arranged in a hexagonal crystal structure with P6₃/mmc symmetry 6. The surface hydroxyl groups emerge as a consequence of charge balance requirements following the removal of the A-layer element, with water molecules and dissolved oxygen participating in surface reactions during aqueous-phase synthesis 1,5.

Key structural parameters include:

• Layer thickness: Single-layer MXene nanosheets typically measure 1–2 nm in thickness, while multilayer assemblies range from 5 to 50 nm 3,11.
• Lateral dimensions: Chemically exfoliated hydroxyl terminated MXene nanosheets exhibit lateral sizes between 100 nm and several micrometers, depending on synthesis conditions and delamination methods 7,14.
• Interlayer spacing: The d-spacing between adjacent MXene layers varies from 0.95 to 1.35 nm, influenced by the degree of hydration and intercalation of guest species 9,19.

The hydroxyl functional groups are not uniformly distributed but preferentially occupy edge sites and defect regions, where coordinative unsaturation of metal atoms facilitates -OH bonding 1,16. Spectroscopic studies (FTIR, XPS) confirm that -OH groups coexist with -F and =O terminations, with the relative abundance of each species dependent on etching chemistry and post-treatment conditions 7,12. For instance, Ti₃C₂T_x synthesized via LiF-HCl etching exhibits a higher -OH/-F ratio compared to direct HF etching, resulting in enhanced hydrophilicity and electrochemical activity 7,18.

The presence of hydroxyl terminations imparts several critical properties:

• Hydrophilicity: Hydroxyl groups render MXene surfaces highly hydrophilic, enabling spontaneous dispersion in aqueous media without surfactants, with contact angles typically below 20° 1,5,11.
• Electronegativity: Surface -OH and =O groups confer negative surface charge (zeta potential ranging from -30 to -50 mV at neutral pH), facilitating electrostatic interactions with cationic species and proteins 5,12.
• Redox activity: Hydroxyl terminated MXene exhibits intrinsic reducing properties, capable of scavenging reactive oxygen species (ROS) and participating in redox reactions, which is advantageous for antioxidant applications and catalysis 5,14.

However, the abundance of -OH groups also renders hydroxyl terminated MXene susceptible to oxidative degradation in aqueous and humid environments, where water molecules and dissolved oxygen progressively convert MXene into metal oxides (e.g., TiO₂) and amorphous carbon, compromising its metallic conductivity and structural integrity over time 1,11,14.

Precursors And Synthesis Routes For Hydroxyl Terminated MXene

The synthesis of hydroxyl terminated MXene predominantly follows a top-down approach involving selective etching of the A-layer element from MAX phase precursors, followed by delamination to yield single- or few-layer nanosheets 6,15,18. The choice of MAX phase composition, etchant chemistry, and processing conditions critically determines the surface termination profile and hydroxyl group density.

MAX Phase Precursors

Common MAX phase precursors include Ti₃AlC₂, Ti₂AlC, Ta₄AlC₃, V₂AlC, Nb₂AlC, Nb₄AlC₃, Ti₃SiC₂, and their solid-solution variants such as (V_0.5Cr_0.5)₃AlC₂ and (Ti_0.5Nb_0.5)₂AlC 7,18,19. Ti₃AlC₂ remains the most extensively studied precursor due to its commercial availability and well-established etching protocols 15,18. MAX phases are typically synthesized via high-temperature solid-state reactions (1300–1600°C) of elemental powders or carbothermal reduction of metal oxides in inert atmospheres 18.

Chemical Etching Methods

HF Etching: The pioneering method involves immersing MAX phase powders in concentrated aqueous HF (40–50 wt%) at room temperature for 18–72 hours 15,18. This process selectively dissolves the Al layer, generating MXene nanosheets with mixed -OH, -F, and =O terminations. The reaction for Ti₃AlC₂ can be represented as:

Ti₃AlC₂ + 3HF → Ti₃C₂(OH)ₓFᵧOᵤ + AlF₃ + H₂

While effective, direct HF etching yields MXene with high fluorine content (-F/-OH ratio >2), which may limit certain applications requiring predominantly hydroxyl terminations 1,7.

In Situ HF Generation (LiF-HCl Method): To mitigate safety concerns and enhance -OH content, the in situ HF generation approach employs lithium fluoride (LiF) and hydrochloric acid (HCl) mixtures 7,18. Typical conditions involve stirring MAX phase powders in 6–12 M HCl containing 1–2 equivalents of LiF at 35–60°C for 24–48 hours. This method produces MXene with higher -OH/-F ratios (approaching 1:1) and improved colloidal stability 7. The milder etching kinetics also reduce structural defects and preserve crystallinity 18.

Fluoride-Free Etching: Alternative routes using molten salts (e.g., NaOH, KOH at 500–700°C), electrochemical etching, or Lewis acidic molten salts (e.g., ZnCl₂) have been explored to eliminate fluorine contamination entirely 18. However, these methods often require post-treatment to introduce hydroxyl groups, as the as-synthesized surfaces may be terminated with -O or -Cl instead 6.

Delamination And Intercalation

Following etching, multilayer MXene is delaminated into single- or few-layer nanosheets via intercalation of bulky cations or molecules that expand the interlayer spacing 9,15. Common intercalants include:

• Dimethyl sulfoxide (DMSO): Stirring etched MXene in DMSO for 18–24 hours, followed by sonication in water, yields colloidal dispersions of delaminated nanosheets with lateral sizes of 0.5–2 μm 15.
• Tetraalkylammonium hydroxides: Tetramethylammonium hydroxide (TMAOH) or tetrabutylammonium hydroxide (TBAOH) solutions facilitate rapid delamination (1–6 hours) and enhance -OH content by replacing -F terminations through ion exchange 1,7.
• Organic solvents with hydroxyl groups: Ethanol, isopropanol, and ethylene glycol not only assist delamination but also promote hydroxyl functionalization via solvent-surface interactions, as demonstrated in Patent 2, where organic compounds with 2–8 carbon atoms and hydroxyl/carbonyl groups were used to disperse MXene and form conductive films with enhanced bonding strength 2.

Post-delamination, MXene dispersions are typically centrifuged (3500–5000 rpm, 30–60 min) to remove unexfoliated particles, yielding stable colloidal suspensions with concentrations of 1–10 mg/mL 15,18.

Surface Modification To Enhance Hydroxyl Content

To maximize hydroxyl termination density and mitigate oxidation, several post-synthesis modification strategies have been developed:

• Oxidative treatment: Controlled oxidation using H₂SO₄:H₂O₂ mixtures or ozone exposure converts -F terminations to -OH and =O, increasing hydrophilicity and electrochemical activity 4,17. For example, Patent 17 describes oxidized MXene with enhanced -OH content for lithium-sulfur battery cathodes, where surface hydroxyl groups facilitate sulfur wetting and polysulfide adsorption 17.
• Alkali treatment: Immersion in dilute NaOH or KOH solutions (0.1–1 M) at 60–80°C for 2–6 hours promotes -F/-OH exchange, yielding MXene with -OH/-F ratios exceeding 2:1 1,7.
• Hydrogen annealing: Exposure to H₂ atmospheres (30–100 Pa) at 300–500°C reduces surface oxygen content and introduces -H terminations, which can subsequently be converted to -OH via mild oxidation 4.

These modifications must be carefully controlled to avoid over-oxidation, which degrades MXene into TiO₂ and compromises conductivity 1,11.

Physical And Chemical Properties Of Hydroxyl Terminated MXene

Hydroxyl terminated MXene exhibits a unique combination of metallic conductivity, hydrophilicity, mechanical robustness, and chemical reactivity, making it suitable for diverse applications.

Electrical Conductivity

MXene nanosheets possess metallic or semi-metallic electronic structures, with electrical conductivities ranging from 2,000 to 15,000 S/cm for pristine Ti₃C₂T_x films, depending on synthesis method, termination composition, and film density 3,11. Hydroxyl terminations contribute to conductivity by facilitating charge transfer between adjacent layers and reducing contact resistance in composite electrodes 2,7. However, excessive -OH content can introduce localized states near the Fermi level, slightly reducing conductivity compared to oxygen-terminated MXene 16.

Hydrophilicity And Colloidal Stability

The high density of surface -OH groups (estimated at 2–5 OH/nm² based on XPS quantification) renders hydroxyl terminated MXene superhydrophilic, with water contact angles below 10° for freshly prepared films 1,5. This hydrophilicity enables spontaneous dispersion in water, forming stable colloidal suspensions with zeta potentials of -35 to -50 mV, which prevent aggregation via electrostatic repulsion 5,12. The colloidal stability is critical for solution-processing techniques such as spin-coating, inkjet printing, and vacuum filtration used to fabricate MXene-based devices 2,11.

Mechanical Properties

MXene nanosheets exhibit exceptional mechanical strength, with Young's moduli ranging from 200 to 400 GPa and tensile strengths of 10–30 GPa for single-layer Ti₃C₂T_x, as determined by nanoindentation and molecular dynamics simulations 3,6. Hydroxyl terminations enhance interlayer bonding through hydrogen bonding networks, improving the mechanical integrity of multilayer films and composites 2,14. For instance, Patent 2 reports that MXene films bonded to metal substrates via hydroxyl-mediated interactions exhibit high resilience to bending (>10,000 cycles at 5 mm radius) and bonding strengths exceeding 10 MPa 2.

Thermal Stability And Oxidation Resistance

Hydroxyl terminated MXene is thermally stable up to 200–300°C in inert atmospheres, beyond which -OH groups dehydrate to form =O terminations and interlayer water is expelled 11,14. In air, oxidation initiates at 100–150°C, with edge sites and defects serving as nucleation points for TiO₂ formation 1,11. The oxidation kinetics are accelerated in aqueous environments, where dissolved oxygen and water molecules synergistically attack the MXene lattice, converting it to metal oxides within days to weeks depending on pH and temperature 1,14. Strategies to enhance oxidation resistance include:

• Surface passivation: Coating MXene with polymers (e.g., polyvinyl alcohol, polyvinylidene fluoride) or inorganic layers (e.g., Al₂O₃, SiO₂) via atomic layer deposition or sol-gel methods 1,14.
• Composite formation: Embedding MXene in conductive matrices such as carbon nanotubes, graphene, or metal sulfides (e.g., MoS₂) to shield reactive sites and provide structural reinforcement 14,20.
• Controlled atmosphere storage: Storing MXene dispersions under inert gases (Ar, N₂) or in deoxygenated solvents to minimize oxidation 1,15.

Electrochemical Properties

Hydroxyl terminated MXene demonstrates pseudocapacitive behavior in aqueous electrolytes, with specific capacitances of 200–400 F/g at scan rates of 2–20 mV/s in H₂SO₄, KOH, or neutral electrolytes 7,9. The -OH groups participate in surface redox reactions, contributing to charge storage via proton-coupled electron transfer mechanisms 7,12. In lithium-ion batteries, hydroxyl terminated MXene serves as an anode material with theoretical capacities of 320–450 mAh/g for Li⁺ intercalation, though practical capacities are typically 150–250 mAh/g due to incomplete utilization of active sites and irreversible side reactions with -OH groups 8,20. Strategies to enhance electrochemical performance include:

• Heteroatom doping: Introducing boron, nitrogen, or sulfur into the MXene lattice to increase defect density and electronic conductivity 8,16.
• Nanostructuring: Synthesizing MXene nanoribbons, nanowires, or porous architectures to maximize surface area and ion-accessible sites 16,20.
• Composite electrodes: Combining MXene with metal oxides (e.g., MnO₂, RuO₂), metal-organic frameworks (MOFs), or conductive polymers to synergistically enhance capacity and cycling stability 10,13.

Chemical Reactivity And Functionalization