High‑Yield Exfoliation of MoS₂ via Ultrasound Sonication in a Supercritical CO₂–NMP Complex Solvent

Introduction

Two‑dimensional transition metal dichalcogenides (TMDs) have attracted intense interest because their atomically thin sheets exhibit a wide spectrum of electronic behaviours—from semiconducting to superconducting—depending on composition and structure. MoS₂, a prototypical TMD, consists of hexagonal Mo layers sandwiched between S layers. Its strong in‑plane covalent bonds and weak out‑of‑plane van der Waals interactions make it amenable to exfoliation into individual nanosheets, which in turn unlocks new physicochemical properties such as increased surface area, a direct band gap, and enhanced surface reactivity.

Realising these benefits, however, requires scalable, high‑quality production of few‑layer MoS₂. Existing techniques—chemical vapor deposition, micromechanical cleavage, ion intercalation, hydrothermal synthesis—suffer from drawbacks including high cost, low yield, or defect formation. Liquid‑phase exfoliation remains the most promising, yet conventional approaches rely on surfactants that are difficult to remove and long sonication times that promote defects.

We therefore develop a liquid‑phase method that leverages the unique properties of supercritical CO₂ combined with the solvent‑matching capabilities of NMP. This dual‑solvent system reduces the exfoliation energy barrier, facilitates interlayer intercalation, and dramatically accelerates the process, delivering a scalable route to few‑layer MoS₂.

Methods

Materials

Commercial MoS₂ (99.5 %) and NMP (99.9 %) were used without further purification. Absolute ethanol (99.5 %), de‑ionised water, and CO₂ (99.5 %) were sourced from Chengdu and Shanghai suppliers.

Exfoliation process

The exfoliation apparatus consists of a stainless‑steel reactor (max 250 mL) capable of 20 MPa pressure and an ultrasonic probe. In a typical run, 100 mg MoS₂ was dispersed in 150 mL solvent, heated to the target temperature, and pressurised to 14 MPa with CO₂. Sonication (600 W) was then applied for 1 h. After depressurisation, the nanosheets were collected by filtration, washed, and dried.

Characterization

X‑ray diffraction (Rigaku, CuKα) assessed crystallinity; Raman spectroscopy (Thermo Fisher, 532 nm) evaluated layer number; AFM (ANSYS) measured topography; BET surface area (Micromeritics) quantified specific area; XPS (ESCALAB 250Xi) probed chemical states; HRTEM (Quanta) confirmed lattice fringes.

Results and discussion

The exfoliation scheme is illustrated in Fig. 1. Bulk MoS₂ is suspended in a supercritical CO₂–NMP mixture and sonicated. Supercritical CO₂, with its gas‑like diffusivity and liquid‑like density, intercalates between layers, while NMP’s surface tension closely matches that of MoS₂, lowering the enthalpy barrier for exfoliation. Together, they produce a synergistic effect that dramatically increases yield.

A schematic of the exfoliation process and the concerted intercalation of supercritical CO₂ and NMP.

Control experiments (Fig. 2a) demonstrate that CO₂ alone provides minimal exfoliation, whereas miscible co‑solvents (ethanol, NMP) enhance the effect. NMP alone yields the strongest reduction in XRD peak intensity, confirming its role in lowering the exfoliation barrier. When combined with supercritical CO₂, the figure of merit (F.O.M) drops from 0.526 (NMP or CO₂ alone) to 0.152, illustrating a strong synergistic contribution (Table 1).

a XRD patterns for various co‑solvents; b synergistic effect of NMP + CO₂; c Raman spectra.

Raman spectra (Fig. 2c) show broadened E_2g¹ and A_1g peaks, indicative of reduced layer number and phonon confinement. XPS confirms Mo⁴⁺ and S²⁻ oxidation states, matching bulk MoS₂.

XPS survey of Mo 3d and S 2p for exfoliated MoS₂.

AFM (Fig. 4a) reveals nanosheet lateral sizes of 100–450 nm with thicknesses ranging from 3 to 9 nm, corresponding to 5–15 layers. HRTEM (Fig. 4b) confirms ~18–19 layers (≈11 nm). BET analysis (Fig. 5) shows a specific surface area of 36.86 m² g⁻¹, translating to an average of ~17 layers and confirming a high‑yield process (>90 % without centrifugation).

BET surface area of MoS₂ from various solvents.

Re‑dispersion tests (Fig. 6) demonstrate that MoS₂ obtained with the NMP/CO₂ mixture remains stable in NMP for at least 5 h, whereas the CO₂‑only sample settles rapidly, underscoring the superior dispersion quality afforded by the synergistic method.

Dispersion stability of MoS₂ from complex solvent (a,b) and post‑settling images (c,d).

Conclusions

We have demonstrated a liquid‑phase exfoliation strategy that exploits the combined action of supercritical CO₂ and NMP to achieve rapid, high‑yield production of few‑layer MoS₂. The process delivers >90 % yield in just 1 h, preserves layer integrity, and produces a stable colloidal dispersion. This scalable approach opens the door to large‑scale applications of MoS₂ in energy storage, catalysis, sensing and photonics.

Availability of data and materials

Data supporting the findings are available from the corresponding author upon reasonable request.

Abbreviations

AFM:

Atomic force microscopy

BET:

Brunauer–Emmett–Teller

F.O.M:

Figure of merit

FWHM:

Full width at half maximum

HRTEM:

High‑resolution transmission electron microscopy

MoS₂:

Molybdenum disulfide

NMP:

N‑Methyl‑2‑pyrrolidone

TMD:

Transition metal dichalcogenides

XPS:

X‑ray photoelectron spectroscopy

XRD:

X‑ray diffraction