A convenient ultrasonic path for van der Waals heterostructure construction: Study on MoS₂/graphene as an example

Since the discovery of graphene, a growing number of layered 2D materials have been explored and synthesized because of their unique electronic properties and structures. 2D heterostructure enables the investigation of various intriguing physical phenomena in layered 2D materials, and opens up vast potential for practical applications by assembling different 2D layered materials.^[1] In contrast to bulk materials with limited interfaces, vdW heterostructure fully leverages the unique electronic properties of the interface region, which spans from a few to tens of nanometers.^[2,3] The generation, restriction, and transport of excitons, phonons, photons and carriers in the atomic-level interface can be effectively controlled through the vdW heterostructures.^[4–7]

Unlike 3D materials, where the lattice structure is periodically arranged and connected by chemical bonds, in 2D layered materials, one dimension is compressed to the atomic scale, and the layers are held together by weak vdW forces rather than chemical bonds.^[8] Previous research has suggested that ultrasound with sufficient power can disrupt the weak vdW forces between layers of 2D materials, making it a viable method for synthesizing few-layer 2D materials.^[9–11] Furthermore, the assembly of heterostructures via liquid-phase ultrasound has also been proven to be feasible.^[12,13] Currently, there are four other common methods for synthesizing vdW heterostructures: high-temperature solid-state reaction, chemical vapor deposition (CVD), wet-chemical method and mechanical exfoliation.^[3,14–16] In contrast, the liquid-phase ultrasound assembly method offers the advantages of exceptional simplicity and energy efficiency. A simple comparison of the methods is provided in Table 1. It is clear from the table that, both in terms of the equipment required for synthesizing the materials and the atmospheric and temperature conditions needed, the ultrasonic assembly method has lower requirements.

Molybdenum disulfide (MoS₂) is a representative 2D layered material comprising Mo layers sandwiched between two S layers.^[17] Due to its direct narrow band gap, excellent electrical, chemical and mechanical properties, MoS₂ is considered highly promising for applications in power nanodevices, photonics, energy storage and flexible electronics.^[18–20] Graphene, a 2D material renowned for its high electron mobility, comprises carbon atoms arranged in a hexagonal honeycomb lattice formed by sp²-hybridized orbitals, and can be used as an attractive support layer.^[21] Due to the relatively low charge separation rate and conductivity of MoS₂, it is often combined with graphene to form a composite structure for enhanced performance.^[18,22–24] The MoS₂/graphene heterostructure finds wide applications in electronics, photocatalysis, photonics, etc.^[25–27]

In this study, 2D layered MoS₂ and graphene dispersions are first prepared using ultrasonic water bath exfoliation. Next, an ultrasonic assembly method synthesizes MoS₂/graphene vdW heterostructures with varying mass ratios. SEM, XRD and Raman spectroscopy were used to characterize the synthesized materials. The characterization results confirmed the successful synthesis of the heterostructures and validated the feasibility of the ultrasonic assembly method.

Analytical-grade MoS₂ powder and graphene powder were used directly without further purification. Bulk MoS₂ powder was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd (Shanghai, China), with a crystallinity of 99.995% and approximate crystal dimensions of 0.5 cm × 0.5 cm × 0.1–0.15 mm. Layered graphene powder was obtained from Hengqiu Tech. Inc. (Suzhou, China), featuring flake diameters in the range of several tens of micrometers and a specific surface area of 600–1000 m²/g. Ultrasonic exfoliation was performed to obtain few-layer graphene. Analytical grade Rhodamine B (RhB) dye was purchased from Shanghai Aladdin Technology Co. Ltd (Shanghai, China). All absolute ethanol (analytical grade, AEA) used in the experiments was purchased from Tianjin Kermel Chemical Reagent Co. Ltd (Tianjin, China).

Similar to previous studies, both graphene nanosheets and 2D sheet-like MoS₂ were prepared via ultrasonic exfoliation using a water bath-assisted method.^[28] The detailed procedures are as follows.

A total of 100 mg of MoS₂ powder was weighed using an electronic balance (±0.1 mg accuracy) and transferred into a 100 mL glass beaker containing 50 mL of AEA. The dispersion was stirred using a magnetic stirrer at 600 rpm for 10 min to ensure preliminary uniform dispersion. The beaker was subsequently placed in an ultrasonic water bath (KQ-300DE, Kunshan Ultrasonic Instruments Co. Ltd, China) and sonicated continuously for 16 hours at a frequency of 40 kHz and power of 300 W. After sonication, the mixture was left to settle for 30 min to allow the precipitation of unexfoliated particles. The supernatant was carefully decanted and transferred to a clean glass dish. It was then dried in an electric oven at 60 °C for 12 hours to obtain MoS₂ nanosheets.

Multilayer graphene powder was treated by following a similar protocol. Specifically, 100 mg of graphene was added to 50 mL of absolute ethanol in a 100 mL beaker. After magnetic stirring at 600 rpm for 10 min, the suspension was sonicated in the same water bath ultrasonic cleaner at 40 kHz and 300 W for 30 min. After treatment, the dispersion was left undisturbed for sedimentation, and the upper suspension was collected and dried at 60 °C for 12 hours for further use.

The specific steps of the ultrasonic assembly process are illustrated in Fig. 1.

MoS₂ and graphene nanosheets were first dispersed separately. Specifically, 50 mg of MoS₂ nanosheets and 50 mg of few-layer graphene were each weighed using a precision electronic balance (±0.1 mg), and then added into separate 100 mL glass beakers containing 50 mL of AEA. The dispersions were stirred magnetically at 600 rpm for 10 min to ensure uniform mixing. Both beakers were then placed into an ultrasonic water bath cleaner and subjected to sonication at 40 kHz frequency and 180 W power for 20 min. After this initial treatment, the beaker containing the MoS₂ dispersion was removed from the ultrasonic bath, while the beaker containing the graphene dispersion remained under continuous sonication.

The MoS₂ dispersion was then slowly added dropwise into the graphene dispersion over a period of approximately 20 min using a glass dropper with a constant flow rate (about one drop every 2–3 s). Throughout the process, the graphene-containing beaker remained under ultrasonic treatment at the same frequency and power. This continuous sonication helps facilitate the uniform and stable integration of MoS₂ nanosheets onto the surface of graphene via non-covalent interactions such as vdW forces and π–π stacking.

Once the MoS₂ dispersion had been completely added, the mixed solution continued to be ultrasonicated under the same conditions for an additional 20 min to allow the heterostructure system to stabilize and undergo self-adjustment. This step enhances the uniformity and interfacial compatibility of the resulting heterostructures.

After the completion of the self-adjustment stage, the resulting MoS₂/graphene dispersion was transferred to a clean glass dish and dried in an electric oven at 60 °C for 12 hours. The final product was collected as a dry powder, representing the successfully synthesized MoS₂/graphene heterostructure. By changing the mass ratio of graphene, MoS₂/graphene heterostructures with different graphene content were prepared and named MG1, MG2 and MG3. Specifically, the graphene content in the vdW heterostructures was set to 50 wt% for MG1, 65 wt% for MG2 and 80 wt% for MG3.

Scanning electron microscopy (SEM) was carried out on a Zeiss ΣIGMA/VP instrument operated at an accelerating voltage of 5 kV and a working distance of 6–8 mm to observe the surface morphology and microstructure of the samples. The images were captured in secondary electron mode with a resolution of approximately 3.0 nm. X-ray diffraction (XRD) analysis was performed using a SHIMADZU 6100 diffractometer with Cu Kα radiation (λ = 1.5406 Å) to investigate the crystal structure of the samples at room temperature. The measurements were conducted over a 2θ range of 10°–80°, with a scanning rate of 4 °/min, an accelerating voltage of 40 kV and an applied current of 30 mA. Raman spectroscopy was performed using a Renishaw via confocal Raman microscope with a 514 nm laser excitation source, under ambient conditions. The laser power was set to 1%, and the spectral resolution was better than 1 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific ESCALAB Xi+ (Japan) equipped with a micro-focused monochromatic Al Kα x-ray source (hν = 1486.6 eV). The pass energy for the survey and high-resolution scans were set to 100 eV and 20 eV, respectively. All binding energies were calibrated using the C 1s peak at 284.6 eV as a reference.

The photocatalytic activity of the material is evaluated by irradiating the RhB solution with visible light. 30 mg of catalyst is added to 50 mL of 15 mg/L RhB solution. Before illumination, the reaction mixture is continuously stirred in the dark for 30 min to reach the adsorption–desorption equilibrium. Then, the mixture is illuminated with a 10 W white LED lamp (400 nm ≤ λ ≤ 800 nm) in a photocatalytic reactor. Every 10 min, 5 mL of the reaction solution is collected and centrifuged to remove the photocatalyst for further measurement. The residual RhB concentration is measured using a UV–vis spectrophotometer.

SEM images of MoS₂ nanosheets, graphene films and assembled MoS₂/graphene heterostructures are shown in Fig. 2. From the SEM image of MoS₂ nanosheets, it can be seen that MoS₂ nanosheets like snowflakes were successfully synthesized. Carefully observing the image of the MoS₂ nanosheets given in Fig. 2(a), it is found that the size of the exfoliated MoS₂ nanosheets is mostly between 1 μm and 2 μm. The morphology of the graphene films is presented in Fig. 2(b). The graphene films in the image do not show a flat film morphology but have many wrinkles. This is because the graphene films we measured are dried graphene films rather than stretched graphene films in solution. At the same time, the overlap of different graphene films may also promote the generation of this rugged morphology. The SEM images of the assembled MoS₂/graphene vdW heterostructures with different graphene content are shown in Figs. 2(c) and 2(d). Snow-like MoS₂ nanosheets are dispersed on the curly and rugged graphene film. Due to the curling and overlapping of the graphene film after drying, it can be seen that some composited MoS₂ nanosheets are wrapped in the graphene film. At the same time, the heterostructures after drying remain stable. This is because although the dryness and the overlap of different graphene films make the surface of the graphene rugged and full of wrinkles, it still has a huge surface area compared to the snowflake-shaped MoS₂ nanosheet graphene film. This feature of graphene provides enough attachment area for the small flake-shaped MoS₂ nanosheets so that they can maintain a heterogeneous structure through vdW force bonding. Therefore, even if the composite material is dried, the MoS₂/graphene heterostructure still exists.

In addition, it can be seen from Figs. 2(c)–2(e) that as the proportion of graphene gradually increases, the probability of heterostructure synthesis gradually decreases. When the graphene content is 50%, the ultrasonically assembled MoS₂/graphene heterostructure has the best quality and the highest material utilization rate. Since there are not enough graphene films to provide bonding points for the small MoS₂ nanosheets, as shown in Fig. 2(e), more MoS₂ nanosheets are combined with each other instead of forming a heterostructure with the graphene film. However, the MG3 sample in Fig. 2(e) has the highest graphene content. Perhaps it is precisely because the graphene content is too high that the highly extended and overlapping graphene films during assembly hinder the dispersion of MoS₂ in the solution, thereby further hindering the combination of MoS₂ and graphene. This results in the undispersed MoS₂ being able to only combine with itself, rather than with more graphene to form heterogeneous structures.

The element composition and phases of MoS₂, graphene, and MoS₂/graphene heterostructures were characterized by XRD. The comparison between the XRD pattern of the measured MoS₂ and the standard card is given in Fig. S1. All the diffraction peaks of MoS₂ observed in the figure are in good agreement with the hexagonal MoS₂ of 2H phase (standard JCPDS No. 37-1429).^[29,30] The strongest diffraction peak (002) of MoS₂ appears at θ = 14.378°, and the two small diffraction peaks (103) and (105) appear at θ = 39.538° and θ = 49.787° . The XRD pattern of graphene is basically consistent with the diffraction peaks of 3R phase graphene (standard JCPDS No. 26-1079), and is shown in Fig. S1. The strongest diffraction peak (003) of graphene appears at θ = 26.603°. The XRD results of the heterostructures with different graphene mass ratios are all shown in Fig. 3. The characteristic peaks of MoS₂ and graphene can be found in all composite structures, indicating that the synthesis of MoS₂/graphene heterostructure does not change the crystal structure of MoS₂ and graphene. This also shows that the entire synthesis process of heterostructures is just a physical combination of MoS₂ and graphene using vdW force. At the same time, a new lower-intensity graphene diffraction peak appeared in the spectrum of the MoS₂/graphene composite material, which directly proved the successful synthesis of the heterostructures. Furthermore, comparing the XRD patterns of MG1, MG2 and MG3, the strongest diffraction peak of MoS₂ (002) and the characteristic peak of graphene (003) in the MG1 XRD spectrum are the strongest among the three samples. To further reveal the structure and phase of MoS₂/graphene heterostructures, Raman spectroscopy was measured as present, and is shown in Fig. 4. Compared to the spectrum of MoS₂, the E2g1 and Ag1 modes of heterostructures are red-shifted, with the strongest red-shift of MG3. The phenomenon of red-shift also proves the successful combination of MoS₂ and graphene in another way.^[31–33] G and 2D modes appearing at 1580 cm⁻¹ and 2720 cm⁻¹ are important observational features in the Raman spectrum of graphene. The appearance of the D band is due to the size reduction of the sp² domain in the plane during the oxidation process. Thus, the absence of the D peak in the characterized spectrum indicates that the graphene is pure without chemical bonds and edges.^[34] Similar to MoS₂, the Raman spectral peaks of graphene have a red-shift in the composite structure with the highest being MG3. The Raman characteristic peaks of graphene and MoS₂ both appear in the MoS₂/graphene heterostructures and both have red-shifts. XPS results (Fig. S2) and related analysis (supporting information) reveal binding energy shifts in Mo 3d and S 2p spectra of the MoS₂/graphene heterostructures compared to pure MoS₂, indicating strong electronic interactions and confirming the formation of the heterostructure rather than simple physical mixing.^[33] The presence of characteristic C 1s, Mo and S signals further supports the successful synthesis of the MoS₂/graphene heterostructures.

Based on ultrasound theory, experimental procedures and the results of various characterizations, a potential mechanism is proposed. Similar to the preparation process, the assembly mechanism is divided into three stages: dispersion, assembly and adjustment. Figure 5 presents a schematic diagram of the dispersion stage. In general liquids, ultrasound with a frequency from 20 kHz–50 MHz has wavelengths that range roughly from 10 cm to 100 μm, which are much larger than the molecular scale. Therefore, the chemical and physical effects of ultrasound in the material synthesis process do not arise from the direct interaction between the material and ultrasound, but from the physical phenomenon of acoustic cavitation.^[35,36] This physical phenomenon involves three processes: formation, growth and implosive collapse of bubbles.^[37] When sound waves of sufficient amplitude are transmitted into the liquid, micro air-core cavitation bubbles are generated. Bubbles of critical size (typically around 10 μm) strongly couple with the acoustic field, undergo rapid inertial overgrowth during expansion, and ultimately collapse, releasing a significant amount of energy. This release of internal energy facilitates the uniform dispersion of MoS₂ nanosheets and graphene films in the solution. Moreover, SEM observations revealed that a higher mass ratio of graphene in the sample hindered the dispersion of MoS₂ due to the extensive size and surface area of graphene in the solution, leading to the aggregation of MoS₂ nanosheets, which indicates the presence of a dispersion stage. These findings suggest that the dispersion stage is necessary to overcome the agglomeration of nanosheets.

As shown in the inset image zoomed in from Fig. 5, while the dispersion process is occurring, assembly is simultaneously taking place. According to the Raman spectrum and XPS characterization results, there is a force interaction between MoS₂ and graphene, rather than a pure physical mixing, indicating the presence of an assembly stage that assembles the two 2D materials into vdW heterostructures. In this stage, numerous dispersed MoS₂ nanosheets and the extended graphene in the solution are attracted to each other through vdW forces, forming a heterostructure. Meanwhile, the energy generated by the acoustic cavitation process is sufficient to break the weak vdW bond between MoS₂ and graphene.^[36,38–47] As a result, the dispersed MoS₂ nanosheets are continuously combined with the graphene surface during the assembly process. However, the energy released by bubble collapse, the shock waves generated by the rebound of the collapsed bubble, the jets produced by the interaction between the bubble and the material, and the intensified thermal motion after the liquid molecules absorb the energy all cause constant interruptions in this combination, leading to a more uniform dispersion of the MoS₂ nanosheets.^{[36,44,48–50]}

As depicted in the adjustment phase of Fig. 6, the assembly stage persists with a critical distinction from prior stages; no additional MoS₂ nanosheets are introduced into the system at this juncture. By this point, both the dispersion equilibrium of MoS₂ nanosheets and their heterogeneous integration with graphene films have been substantially achieved, marking the transition to refinement rather than further material incorporation. Sustained ultrasonic treatment primarily serves to refine the morphology and enhance dispersion of MoS₂ sheets, while simultaneously facilitating their optimal integration with graphene films to engineer a robust vdW heterogeneous nanocomposite interface.

To demonstrate the practical application potential of the synthesized MoS₂/graphene vdW heterostructure, a photocatalytic experiment was conducted to evaluate its photocatalytic performance. Based on the SEM, XRD and Raman spectroscopy results, the MG1 sample was selected as the photocatalyst and tested under illumination from a 10 W white LED lamp. As shown in Fig. 7, the MG1 sample achieved a RhB degradation rate of up to 91.3%. A performance comparison with MoS₂/graphene heterostructures prepared by other methods is presented in Table 2, highlighting the superior photocatalytic efficiency of the material synthesized via the ultrasonic assembly approach.^[51–54]

In summary, MoS₂/graphene vdW heterostructures were successfully fabricated using a convenient ultrasound assembly method, demonstrating the potential of the ultrasonic path for synthesizing 2D heterogeneous structures. The ultrasonic assembly process, based on the ultrasonic path, involves three main stages: assembly preparation, assembly and self-adjustment. The morphology of MoS₂ nanosheets, graphene films and composite materials was observed through SEM. The successful fabrication of heterostructures was confirmed through XRD, Raman and XPS analysis. A red-shift in the Raman spectrum of the composite structures and a decrease in binding energy in the Mo 3d and S 2p XPS spectra indicate successful heterostructure formation. These results provide sufficient evidence that the ultrasonic path is a feasible and effective approach for manufacturing heterostructures. The proposed mechanism for fabrication involves three stages: dispersion, assembly and adjustment. Acoustic cavitation, a key element of ultrasonic sonochemistry, plays a major role in these stages.

However, while the ultrasonic assembly method shows significant promise, some limitations remain. The scalability of the method for large-scale production, the stability of the heterostructures over extended periods, and the control over uniformity at larger scales need further investigation. Moreover, the influence of ultrasonic frequency and power on the final properties of the heterostructures warrants deeper exploration.

Looking ahead, future research will focus on optimizing the ultrasonic assembly process for industrial applications, improving the reproducibility and scalability of the method, and exploring other 2D material combinations for heterostructure fabrication. In addition, further studies on the long-term stability and potential environmental impact of these materials are crucial for their practical use in various applications such as sensors, energy storage and electronic devices.