Covalent Organic Framework Nanosheets: Synthesis, Structural Engineering, And Advanced Applications In Energy Storage And Catalysis

Molecular Composition And Structural Characteristics Of Covalent Organic Framework Nanosheets

Covalent organic framework nanosheets (CONs) are atomically thin, crystalline materials formed through the controlled delamination of layered covalent organic frameworks. Unlike conventional three-dimensional COFs, these nanosheets possess lateral dimensions ranging from hundreds of nanometers to several micrometers while maintaining thicknesses of 2–20 nm (corresponding to 3–30 molecular layers) 14. The structural integrity originates from reversible covalent bond formation—predominantly imine (C=N), boronate ester (B-O), triazine (C-N), and β-ketoenamine linkages—that connect planar, π-conjugated organic building blocks into extended two-dimensional networks 278.

The most extensively studied CON systems employ Schiff base condensation between aromatic aldehydes (e.g., 2,4,6-triformylphloroglucinol, TFP) and diamines (e.g., p-phenylenediamine, PA; 2,6-diaminoanthracene; triazole-based diamines) under solvothermal conditions (80–120°C, 72 h in mesitylene/dioxane mixtures with 6 M acetic acid catalyst) 148. For instance, the TpPa-1 framework synthesized from TFP and PA exhibits a hexagonal lattice with pore apertures of 1.8 nm and Brunauer-Emmett-Teller (BET) surface areas exceeding 1200 m²/g in bulk form; upon exfoliation, the resulting nanosheets demonstrate surface areas approaching 1800–2000 m²/g due to complete exposure of both basal planes 48. Triazole-functionalized variants (e.g., TpTz CONs) incorporate nitrogen-rich heterocycles that enhance lithium-ion coordination, achieving reversible specific capacities of 720 mAh/g at 100 mA/g current density—substantially higher than graphite anodes (372 mAh/g) 1.

Structural characterization via transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirms the preservation of crystalline order post-exfoliation: selected-area electron diffraction (SAED) patterns reveal hexagonal symmetry with lattice parameters matching bulk COF unit cells (a = b ≈ 2.5–3.0 nm for TpPa-type frameworks), while AFM height profiles indicate uniform nanosheet thicknesses of 3.5 ± 0.8 nm for mechanically exfoliated TpBD-(NO₂)₂ 48. Powder X-ray diffraction (PXRD) of restacked nanosheets displays characteristic (100) reflections at 2θ ≈ 4–5°, corresponding to in-plane periodicities, alongside diminished (001) stacking peaks, confirming reduced interlayer registry 24.

The chemical stability of CONs is exceptional: TpPa-series nanosheets retain structural integrity in boiling water, 9 N HCl, and 3 N NaOH for >24 h, with <5% loss in BET surface area 48. This robustness stems from the irreversible nature of β-ketoenamine tautomerization (in TFP-based systems) and the kinetic stability of imine bonds under ambient conditions. Functionalization strategies—including nitro (-NO₂), methoxy (-OMe), fluoro (-F₄), and dimethylamino (-NMe₂) substituents—enable precise tuning of electronic properties (HOMO-LUMO gaps of 2.1–2.8 eV) and hydrophilicity without compromising framework stability 245.

Exfoliation Strategies And Nanosheet Fabrication Techniques For Covalent Organic Frameworks

The transformation of bulk COFs into nanosheets requires overcoming van der Waals interlayer interactions (binding energies of 40–60 kJ/mol per layer) while preserving covalent in-plane connectivity. Four primary exfoliation methodologies have been established, each offering distinct advantages in scalability, nanosheet quality, and environmental impact.

Mechanical Delamination Through Ball Milling And Grinding

Mechanical grinding represents the most straightforward and scalable approach, employing shear forces to disrupt π-π stacking between COF layers 48. In a typical protocol, 500 mg of bulk TpPa-1 COF is subjected to planetary ball milling (400 rpm, zirconia balls, 30 min cycles with 10 min intervals) in the absence of solvents. This process yields nanosheets with average thicknesses of 5–8 nm (8–12 layers) and lateral dimensions of 200–800 nm, as confirmed by AFM and dynamic light scattering (DLS) 48. The exfoliation efficiency—defined as the mass fraction of material reduced to <10 layers—reaches 60–75% for optimized milling durations. Critically, PXRD analysis demonstrates retention of crystalline order, with (100) peak intensities decreasing by only 15–20% relative to bulk samples, indicating minimal structural damage 4.

Advantages of mechanical exfoliation include solvent-free operation, ambient temperature processing, and compatibility with continuous manufacturing. However, limitations include broad nanosheet thickness distributions (polydispersity index >0.4) and potential introduction of defects at grain boundaries. Recent innovations incorporate cryogenic milling (liquid nitrogen cooling) to enhance brittleness and reduce agglomeration, achieving monodisperse populations with 3–5 layer thicknesses 8.

Liquid-Phase Ultrasonication-Assisted Exfoliation

Ultrasonication in polar aprotic solvents (N-methylpyrrolidone, dimethylformamide, or acetone) leverages cavitation-induced shear forces to separate COF layers 24. A representative procedure involves dispersing 100 mg of TpBD COF in 50 mL NMP, followed by probe sonication (750 W, 20 kHz, 6 h) in an ice bath to prevent thermal degradation. Centrifugation at 3000 rpm (10 min) removes unexfoliated aggregates, yielding supernatants containing nanosheets with thicknesses of 2.5–4.0 nm (3–6 layers) and concentrations of 0.8–1.2 mg/mL 4. TEM imaging reveals lateral dimensions of 500 nm–2 μm, with SAED patterns confirming single-crystal domains.

The exfoliation mechanism involves solvent intercalation between layers, reducing interlayer cohesion energy by 30–40% through disruption of π-π interactions. Solvent selection critically influences yield: Hansen solubility parameters matching COF surface energies (δ ≈ 20–23 MPa^(1/2)) maximize dispersion stability. Post-exfoliation, nanosheets remain colloidally stable for >3 months without surfactants, as evidenced by zeta potential measurements (ζ = -35 to -45 mV in NMP) 24.

Drawbacks include high energy consumption (specific energy input of 15–25 kJ/g), residual solvent contamination, and limited scalability beyond laboratory quantities. Hybrid approaches combining brief sonication (1 h) with subsequent mechanical stirring (48 h) reduce energy requirements by 60% while maintaining comparable exfoliation efficiency 4.

Stimulus-Responsive Self-Exfoliation Via Gas Adsorption

A groundbreaking strategy exploits reversible gas adsorption to induce spontaneous exfoliation without external energy input 25. COFs functionalized with dimethylamino groups (-NMe₂) undergo protonation upon exposure to CO₂ (1 atm, 25°C, 50% relative humidity), generating transient surface charges that electrostatically repel adjacent layers. For example, TpAzo-NMe₂ COF (synthesized from 2,4,6-triformylphloroglucinol and 4,4'-azodianiline with dimethylamino substituents) self-exfoliates into nanosheets with thicknesses of 1.8–3.2 nm within 12 h of CO₂ exposure 25. The process is fully reversible: heating at 80°C under vacuum (10^-2 mbar, 2 h) desorbs CO₂, restoring bulk stacking as confirmed by PXRD peak intensity recovery (>90%) 5.

Mechanistic studies via in situ Fourier-transform infrared spectroscopy (FTIR) reveal formation of dimethylammonium carbamate species (νC=O at 1650 cm^-1), with calculated charge densities of 0.8–1.2 e^- per nm² sufficient to overcome van der Waals attraction 25. This approach offers unparalleled control over exfoliation kinetics and enables switchable properties: CO₂-exfoliated nanosheets exhibit potent antibacterial activity against Escherichia coli (99.7% inhibition at 50 μg/mL), attributed to membrane disruption by cationic surfaces, whereas thermally restacked COFs show negligible toxicity 5.

Limitations include specificity to amine-functionalized frameworks and sensitivity to humidity (optimal performance at 40–60% RH). Extension to other stimuli—such as pH-responsive carboxylate groups or light-triggered azobenzene isomerization—remains an active research frontier 2.

Chemical Exfoliation Through Intercalation And Oxidation

Chemical methods employ redox-active intercalants or oxidizing agents to weaken interlayer bonding 4. Lithium intercalation, adapted from graphite exfoliation protocols, involves treating COFs with n-butyllithium in hexane (0.5 M, 48 h), followed by hydrolysis in water. The resulting lithiated intermediates expand interlayer spacing from 3.4 Å to 6–8 Å, facilitating subsequent sonication-assisted delamination 4. However, this approach risks partial reduction of imine linkages, compromising framework stability.

Alternatively, oxidative exfoliation using Hummers-like protocols (KMnO₄/H₂SO₄) introduces oxygen-containing functional groups (hydroxyl, epoxide, carboxyl) that enhance hydrophilicity and colloidal stability in aqueous media 9. While effective for producing water-dispersible nanosheets (concentrations up to 2 mg/mL), oxidation inevitably disrupts conjugation and reduces electrical conductivity by 2–3 orders of magnitude, limiting applicability in electronic devices 9.

Performance Metrics In Lithium-Ion And Sodium-Ion Battery Applications

Covalent organic framework nanosheets have emerged as high-capacity anode materials for alkali-ion batteries, leveraging redox-active functional groups and accessible surface sites to surpass conventional graphite electrodes 136.

Lithium-Ion Storage Mechanisms And Electrochemical Performance

Triazole-functionalized CONs (TpTz nanosheets) demonstrate reversible lithium-ion storage capacities of 720 mAh/g at 100 mA/g current density, nearly double that of graphite (372 mAh/g) 1. The storage mechanism involves: (1) surface adsorption of Li⁺ ions onto nitrogen-rich triazole and imine sites (contributing ~200 mAh/g), (2) intercalation between nanosheet layers (expanded interlayer spacing of 4.2 Å accommodates solvated Li⁺, adding ~300 mAh/g), and (3) pseudocapacitive redox reactions at carbonyl and imine groups (C=N + Li⁺ + e^- ⇌ C-N-Li, contributing ~220 mAh/g) 1. Cyclic voltammetry (CV) reveals quasi-reversible peaks at 0.8 V and 1.2 V vs. Li/Li⁺, corresponding to imine reduction and triazole lithiation, respectively 1.

Rate capability tests show capacity retention of 520 mAh/g at 500 mA/g and 380 mAh/g at 1000 mAh/g, with Coulombic efficiencies stabilizing at 98.5–99.2% after 10 cycles 1. Long-term cycling (500 cycles at 200 mA/g) exhibits 82% capacity retention, attributed to the structural flexibility of nanosheets that accommodates volume changes (<15% expansion) during lithiation/delithiation without pulverization 1. Electrochemical impedance spectroscopy (EIS) indicates charge-transfer resistances of 45–60 Ω for CON electrodes, significantly lower than bulk COF counterparts (180–250 Ω), confirming enhanced ion accessibility 1.

Sodium-Ion Battery Performance And Comparative Analysis

Sodium-ion batteries (SIBs) benefit from CONs' larger interlayer spacings that accommodate the bulkier Na⁺ ion (ionic radius 1.02 Å vs. 0.76 Å for Li⁺) 3. Hybrid nanosheets combining TpPa-1 CON with MnO₂ nanoparticles (10 wt% loading) achieve initial discharge capacities of 450 mAh/g at 50 mA/g, with reversible capacities stabilizing at 320 mAh/g after 50 cycles 36. The MnO₂ component contributes additional pseudocapacitance via Mn⁴⁺/Mn³⁺ redox couples (theoretical capacity 308 mAh/g), while the CON matrix provides structural support and electronic conductivity (measured at 1.2 × 10^-4 S/cm via four-point probe) 6.

Comparative studies against hard carbon anodes (typical SIB capacity: 250–300 mAh/g) reveal CON-based electrodes deliver 25–30% higher energy densities (calculated as 180 Wh/kg vs. 140 Wh/kg for hard carbon at 0.5C rate) 3. However, initial Coulombic efficiencies remain suboptimal (65–72% in first cycle) due to irreversible solid-electrolyte interphase (SEI) formation on high-surface-area nanosheets, necessitating pre-lithiation or electrolyte additive strategies (e.g., fluoroethylene carbonate at 5 vol%) to mitigate capacity loss 3.

Catalytic Applications: Oxygen Reduction Reaction And Heterogeneous Catalysis

The high density of coordinatively unsaturated sites and tunable electronic structures position CONs as metal-free or hybrid catalysts for energy conversion and organic transformations 6814.

Oxygen Reduction Reaction (ORR) Catalysis In Fuel Cells

Hybrid CON-MnO₂ composites exhibit ORR activity in alkaline media (0.1 M KOH) with onset potentials of 0.85 V vs. reversible hydrogen electrode (RHE) and half-wave potentials (E₁/₂) of 0.78 V, approaching that of commercial Pt/C catalysts (E₁/₂ = 0.83 V) 6. Rotating disk electrode (RDE) measurements at 1600 rpm yield limiting current densities of 5.2 mA/cm² at 0.4 V, corresponding to a four-electron reduction pathway (H₂O₂ yield <8%) as confirmed by rotating ring-disk electrode (RRDE) analysis 6. The synergistic mechanism involves: (1) O₂ adsorption on nitrogen-rich CON sites (binding energy -0.6 eV from DFT calculations), (2)