Ultrasound sonication-mediated fabrication of thin molybdenum disulfide nanosheets from their bulk

2.1. Materials

MoS₂ powder was acquired from Aladdin, with particle sizes around 7 μm. (DMF = Di-methyl-formamide) acquired from Sigma Aldrich. with a high grade of purity (99%).

2.2. Characterization

The exfoliated nanosheets of MoS₂ were analyzed using AFM, operating using tapping mode (a Nanoscope 8.10 instrument), Raman spectroscopy (Witec Raman, 532 nm), SEM, using (a FEI Quanta), TEM & high-resolution-TEM (HRTEM). EM was operated on a TECNAI 20 microscope operating at 120 and 200 kV. XPS data was acquired using a UHV instrument with a Phoibos 100 hemispherical analyzer (SPECS) operating under a base pressure of 10⁻⁹ mbar. XRD analysis was conducted using a Bruker Advanced D8 diffractometer with (Co-Kα; λ = 1.7889 Å) radiation. As well as Elemental mapping was performed via an aberration-corrected FEI-Titan-Themis TEM, equipped with an EDX detector, operating at 200 kV.

3.1. Synthesis of exfoliated MoS₂ sheets

This work presents a straightforward, scalable, and cost-effective method for the exfoliation and fabrication of MoS₂ nanosheets from their bulk form, utilizing DMF as the solvent. The approach is not only economical but also practical for implementation in standard laboratory settings. As illustrated schematically in Figure 1(a), MoS₂ nanosheets were fabricated from the bulk material via ultrasonication. The bulk MoS₂ crystals exhibit a large lateral dimension, typically ranging from 5 to 10 µm, with considerable size nonuniformity (Figure 1b). To reduce the crystal size, the material was mechanically ground for 10 min, resulting in sub-micrometer particles approximately 1=2 µm in size (Figure 1c). Subsequently, the MoS₂ powder was dispersed in DMF at a concentration of 3 mg/mL and subjected to low-frequency ultrasonication (20 kHz) for 60 min. This process yielded a high exfoliation efficiency of approximately 90%, producing nanosheets with uniform morphology and consistent dimensions (Figures 1d-e). The isolated yield of exfoliated MoS₂ nanosheets was estimated to be approximately 90% following ultrasonic probe sonication at a low frequency of 20 kHz for 60 min. This calculation was based on the initial volume and concentration of the bulk MoS₂ dispersion (3 mg/mL), relative to the mass of the isolated and dried nanosheets obtained after vacuum-assisted solvent removal.

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Figure 1.

(a-e) Schematic of the exfoliation of 2D MoS₂ nanosheets, which used a single solvent of DMF with MoS₂ concentration 3 mg/mL, processing using a prob sonication for 60 min.

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The fabrication technique of MoS₂ nanosheets employed probing ultrasonication at 20 kHz to prevent damage or alteration of the crystal structure of MoS₂. The procedure for fabricating MoS₂ nanosheets from bulk MoS₂ is as follows: The bulk MoS₂ is initially exfoliated by the force produced from the collapse of cavitation bubbles. The collapse of cavitation-induced bubbles releases substantial energy, producing shock stress waves that rapidly affect the bulk MoS₂ crystal. This process results in the fragmentation of bulk MoS₂ into smaller flakes in the solution and the exfoliation of MoS₂ into nanosheets, happening after 60 min of sonication [17,18].

The morphology, structure, and lateral dimensions of exfoliated MoS₂ nanosheets were characterized using SEM, AFM, TEM, and STEM. Figures 2(a, b) present SEM images at different magnifications, revealing that the exfoliated MoS₂ nanosheets exhibit lateral sizes below 200 nm. In contrast, the bulk MoS₂ material displays particles with irregular shapes and sizes ranging from approximately 1 to 3 µm, Figure 1(c). Following probe sonication at a low frequency of 20 kHz, the bulk MoS₂ was fully exfoliated into nanosheets, with no observable remains of the original bulk material. The exfoliation process yielded a high conversion efficiency, reaching up to 90%. Figures 2(c, d) show high-magnification SEM images of the exfoliated nanosheets, which appear relatively uniform in shape and size, typically ranging from ∼50 to 150 nm. The observed variation in nanosheet dimensions presumably indicates the inherent heterogeneity in the size distribution of the bulk MoS₂.

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Figure 2.

(a-b) Low magnification and (c-d) high magnification of SEM images of exfoliated MoS₂ nanosheets.

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To further investigate the morphology and thickness of the exfoliated MoS₂ nanosheets, tapping mode AFM was employed. A solution containing exfoliated MoS₂ was drop-cast onto a silicon wafer substrate and subsequently dried under ambient conditions. Figures 3(a, b) show the topographical images of the exfoliated MoS₂ nanosheets along with their corresponding height profiles, (Figures 3c, d). Furthermore, statistical analysis of the AFM data confirms that the sheet thickness is around 2.5 nm, (Figure 3e), revealing the presence of monolayer and few-layer MoS₂ sheets [19,20]. These findings demonstrate that the fabricated nanosheets primarily consist of few-layer sheets. The lateral dimensions of the nanosheets were assessed by analysing 360 individual sheets, revealing an average size of approximately 150 nm (Figure 3f); this is consistent with the SEM observations.

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Figure 3.

(a-b) AFM images of exfoliated MoS₂ nanosheets with (c-d) their height profiles and (e-f) the histogram of their thickness and lateral size.

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Low-magnification TEM and STEM images of the exfoliated MoS₂ nanosheets, (Figures 4a, b), reveal that the nanosheets are flat, ultrathin, and consist of only a few layers. Figure 4(c) presents a high-magnification high angle annular dark field (HAADF)- STEM image that confirms the presence of thin exfoliated MoS₂, with elemental mapping indicating the uniform distribution of molybdenum (Mo) and sulfur (S) across the nanosheet surface. Additionally, Figure 4(d) displays HRTEM analysis, demonstrating that the exfoliated MoS₂ nanosheets possess an interplanar spacing of approximately 0.27 nm. This value corresponds to the d-spacing of the (100) planes in hexagonal MoS₂, as directly observed in the HRTEM image [21,22].

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Figure 4.

(a) TEM images and (b-c) STEM image with elemental mapping and (d) HRTEM images of exfoliated MoS₂ nanosheets.

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Raman spectroscopy was employed to investigate the crystalline structural characteristics of exfoliated MoS₂ nanosheets produced via probe sonication, in comparison to the bulk material. The conventional Raman spectrum of MoS₂ exhibits two distinct peaks, corresponding to the E_2g and A_1g vibrational modes. These modes represent the in-plane vibrations of molybdenum and sulfur atoms (E_2g) and the out-of-plane vibrations of sulfur atoms (A_1g), respectively [23,24]. The Raman spectrum of the exfoliated MoS₂ nanosheets displays a blue shift, with the E_2g mode appearing at approximately 382 cm^⁻1 and the A_1g mode at around 407 cm⁻¹, as shown in Figure 5(a). This spectral shift is indicative of the exfoliated nature of the nanosheets. Moreover, the observed band displacement suggests a reduction in the overall number of MoS₂ layers [25,26].

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Figure 5.

(a) Raman spectra and (b) XRD patterns for bulk MoS₂ (black line) and exfoliated MoS₂ nanosheets (red line).

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XRD analysis was performed to assess the crystalline quality and phase purity of MoS₂ fabricated via the ultrasound sonication method, as illustrated in Figure 5(b). The bulk MoS₂ exhibited distinct XRD peaks at 2θ values of 17°, 33.6°, 39.7°, 58.5°, 72.2°, and 73.8°, corresponding to the (002), (101), (103), (110), (108), and (203) crystallographic planes of MoS₂, respectively. These reflections are characteristic of the 2H-MoS₂ crystal structure [27,28]. Following dispersion in DMF and subsequent probe sonication for 60 min (Figure 4b), only two prominent diffraction peaks remained at 2θ = 39.7° and 73.8°, associated with the (103) and (203) planes. After dispersing MoS₂ in DMF and processing using probe sonication for 60 min (Figure 4b), only two prominent diffraction peaks remained at 2θ = 39.7° and 73.8°, associated with the (103) and (203) planes. Notably, the disappearance of most peaks compared to the bulk material, along with the reduced intensity of the remaining (103) and (203) peaks, confirms the successful exfoliation of MoS₂ nanosheets via prob sonication. This suggests the formation of monolayer and few-layer structures [29,30]. The presence of sharp and well-defined peaks further indicates that the exfoliated MoS₂ retains high crystallinity and phase purity [31].

The elemental composition of both materials was investigated using XPS. The survey spectra of the bulk and exfoliated MoS₂ nanosheets, shown in Figures 6(a,b), indicate that both samples contain molybdenum (Mo) and sulfur (S), as highlighted in blue. Figures 6(c, d) illustrate the XPS high-resolution scans for Mo 3d and S 2p in both the bulk MoS₂ and exfoliated nanosheets. Figure 6(c) presents the high-resolution Mo 3d spectrum of the bulk MoS₂, revealing two main peaks corresponding to the Mo 3d_5/2 and Mo 3d_3/2 states, located at binding energies of approximately 229 eV and 233 eV. These peaks are characteristic of the 2H phase of MoS₂ [32]. Similarly, the S 2p XPS spectrum of the bulk MoS₂, shown in Figure 6(d), displays two peaks at 162.75 eV and 163.93 eV, corresponding to the S 2p_3/2 and S 2p_1/2 states, respectively. After exfoliation and the formation of MoS₂ nanosheets, both Mo 3d_5/2 and Mo 3d_3/2 peaks are observed (Figure 6e), exhibiting shifts to higher binding energies at approximately 233 eV and 237 eV, respectively. Similarly, the S 2p XPS spectra of the MoS₂ nanosheets display the same two main peaks, S 2p_3/2 and S 2p_1/2, shifted to higher binding energies at 164 eV and 169 eV, respectively (Figure 6f). These shifts in both Mo and S peaks following exfoliation are attributed to the formation of molybdenum oxides and oxidized sulfur species during the exfoliation process [33-35].

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Figure 6.

XPS survey spectrum of (a) Bulk MoS₂ and (b) Exfoliated MoS₂ nanosheets. (c-d) High resolution XPS spectra of Mo 3d and (e-f) S 2p for the bulk MoS₂ and exfoliated nanosheets.

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