Hydrothermal-assisted shearing exfoliation for few-layered MoS₂ nanosheets

Abstract

The exfoliation of bulk MoS₂ into few layers has attracted considerable attention as 2D nanomaterials in the past decade. We developed a facile approach for producing MoS₂ nanosheets by hydrothermal-assisted shearing exfoliation based on organic-free strategy. This original exfoliation was highly efficient for large-scale production and sustainable for the environment. The thickness of the as-exfoliated MoS₂ nanosheets was about 4–6 layers, and the lateral size became smaller from hydrothermal processing to shearing. The hydrothermal processing with the participation of ammonium carbonate played an important role in hydrothermal-assisted shearing exfoliation. As a prospective application, the antifriction performance of the as-exfoliated MoS₂ nanosheets in oil was evaluated using a ball-on-ball mode. Evidently, the average friction coefficient and wear scar diameter of 0.08 wt% MoS₂-based oil dropped to about 20.66% and 47.27% relative to those of the base oil, which exhibited an excellent antifriction and antiwear ability.

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1. Introduction

MoS₂ nanosheets, as inorganic analogues of graphene, have excellent properties that are useful in many practical applications as catalysts,^1,2 in batteries,^3,4 sensors,⁵ nanotransistors,⁶ hydrogen storage devices,⁷ supercapacitors,⁸ and lubricants,^9,10 due to the band gap of 1.2–1.8 eV and the layered structure with S–Mo–S monolayers interacted via van der Waals forces. At present, the bulk MoS₂ has been widely used as an anti-wear additive in solid greases,¹¹ but it cannot be used in liquid lubricants due to its unstable dispersion. The specific surface area of the bulk MoS₂ was increased through nanotechnology to obtain MoS₂ nanosheets and enhanced the stable dispersion in liquid lubricants. Rajendhran et al.¹² exfoliated the bulk MoS₂ to Ni–MoS₂ nanosheets by ultrasonication and reflux techniques. The friction results revealed that the average friction coefficient (COF) and average wear scar diameter (AWSD) of 0.5 wt% Ni–MoS₂-based oil decreased by 40–50% and 15–20% compared to those of the base oil due to its small size and surface modification behavior. To date, many top-down designs for preparing monolayer or multilayer MoS₂ nanosheets have been successfully achieved, such as micromechanical peeling,¹³ electrochemical exfoliation,¹⁴ liquid-phase ultrasound,¹⁵ ion intercalation¹⁶ and shearing method.¹⁷ In contrast to bottom-up methods with high cost and serious pollution,^18–20 the exfoliation method for MoS₂ nanosheets was more conducive to exert its application in oil as additive due to the lateral size being of less influence on the friction of the oil. The liquid-phase exfoliation assisted with physical means such as ultrasonication and shearing is of preferable advantage for high quality and scalable output.^21–23 In particular, a kitchen blender was used to exfoliate bulk MoS₂ into nanosheets with a mean thickness of 4.6 nm (ref. 24) and the number of MoS₂ nanosheet layers is mostly less than four layers.²⁵ However, these liquid-phase exfoliations for nanosheets based on organic solvents were expensive and time consuming because the removal of the toxic and high boiling organic solvents after an exfoliating process accounted for an unprecedented challenge for large scale productions.²⁶ Hence, it is urgent to develop a highly efficient method for exfoliating MoS₂ nanosheets.

In this study, we adopted a facile exfoliation method based on hydrothermal-shearing exfoliation to access MoS₂ nanosheets. The structure and change of the exfoliated MoS₂ nanosheets were intensively determined by a series of characterizations. The tribological properties of the as-exfoliated MoS₂ nanosheets were examined by adding oil as an additive. Finally, we discuss the friction reducing and forming mechanisms of the as-exfoliated MoS₂ nanosheets.

2. Experimental section

2.1 Materials and methods

MoS₂ nanosheets were achieved by hydrothermal-shearing exfoliation. Initially, 0.98 g of ammonium carbonate ((NH₄)₂CO₃, AC) was dissolved in 20 mL deionized water and then 0.10 g of bulk MoS₂ (Sinopharm Chemical Reagent Co., Ltd) was added into the above solution with magnetic stirring at room temperature for 2 h. Next, the suspension was transferred to a 100 mL Teflon-lined autoclave and heated at 10 rpm and 220 °C for 2 h, thus obtaining the sample denoted as MoS₂-AC. Alternatively, the hydrothermal-treated MoS₂ solution was swiftly added into the jug with double four-leaf rotary blades and sheared by the rotary blades at 18 000 rpm for 5 rounds at 2 min on/15 min off. Under severe centrifugal forces, the suspension rapidly diffused from the working area to the stator channel. During the diffusion process, the MoS₂ suspension produced a strong shear force along the tangential direction of the blades (including the lateral shear force, the longitudinal shear force, and the collision). Finally, the upper suspension was washed by deionized water and anhydrous ethanol three times; it was then dried at 60 °C for 24 h and denoted as MoS₂-AC-S. Scheme 1 shows the forming mechanism of as-exfoliated MoS₂ nanosheets. A variety of anions and cations expanded the layer spacing of MoS₂ in the hydrothermal process. During the shearing process, the lateral shear force overcame the weaker van der Waals forces between the MoS₂ layers to obtain less or even a single layer of MoS₂, the longitudinal shear force made MoS₂ nanosheets fracture, and the collision had a synergistic effect.²⁷

Scheme 1 The forming mechanism of the as-exfoliated MoS₂ nanosheets.

2.2 Testing of tribological properties

The bulk MoS₂ and as-prepared MoS₂ nanosheets were added in 150 SN base oil using an ultrasonic dispersion, which was marked as MoS₂-based oil, with different concentrations of MoS₂ (0.02 wt%, 0.04 wt%, 0.06 wt%, 0.08 wt% and 0.10 wt%). The tribological performance was detected by a four-ball friction tester (Jinan Chenda Ltd. Co., in China). The testing parameters were set at a speed of 1200 rpm, a stable applied load of 100 N and a duration time of 2 h. Every test was repeated at least three times under the same conditions. At the end of testing, the wear scar diameter was measured using an optical microscope.

2.3 Characterization

XRD patterns, Raman, UV-vis and FTIR spectra were inspected by powder X-ray diffractometer (Bruker AXS, German), inVia Raman spectrometer (Renishaw, Britain), Cary 5000 spectrophotometer (Varian, USA) and Cary 610/670 micro infrared spectrometer (Varian, USA), respectively. The SEM, TEM and HRTEM images were recorded by a S-4800II field-emission scanning electron microscope (Hitachi, Japan), a Tecnai 12 transmission electron microscope (Philips, Netherlands) and a Tecnai G2 F30 S-TWIN field-emission transmission electron microscope (FEI, USA), respectively. The AFM analysis was conducted on a Nanoscope (Digital Instruments, USA). The wear scar micrographs were obtained using a LSM 700 3D laser scanning microscope (CARL ZEISS, Germany).

3. Results and discussion

Fig. 1 shows the XRD patterns and Raman spectra of the bulk MoS₂, MoS₂-AC and MoS₂-AC-S. As shown in Fig. 1a, all the MoS₂ samples show nine peaks at 2θ = 14.4°, 32.7°, 33.5°, 35.9°, 39.5°, 44.2°, 49.9°, 58.3° and 60.1°, corresponding to the (002), (100), (101), (102), (103), (006), (105), (110) and (008) planes of 2H MoS₂ (JCPDS no. 37-1492). Every peak is assigned to the lattice of representative MoS₂ and there is no extra peak corresponding to MoO₃. This demonstrates that the as-exfoliated MoS₂ is a single phase.²⁸ However, all lattice planes of MoS₂-AC and MoS₂-AC-S show weaker peak intensities than the bulk MoS₂. The possible reason is that the MoS₂-AC and MoS₂-AC-S have fewer layers than the bulk MoS₂. In Fig. 1b, the bulk MoS₂, MoS₂-AC and MoS₂-AC-S have two stronger characteristic peaks at about 381 cm⁻¹ (E¹_2g mode) and 406 cm⁻¹ (A_1g mode).^29,30 However, the frequency difference between E¹_2g and A_1g peaks for the as-exfoliated MoS₂ sample is less than that of the bulk MoS₂. This indicates that the number of as-exfoliated MoS₂ layers decreases after two exfoliation processes.^31,32 In addition, MoS₂-AC-S shows less frequency difference than MoS₂-AC, indicating that the layer of MoS₂-AC-S decreases after shearing. Furthermore, the characteristic peaks of the 1T phase are not observed after the exfoliation process, indicating that there is no phase transition.

Fig. 1 XRD patterns (a) and Raman spectra (b) of the bulk MoS₂, MoS₂-AC and MoS₂-AC-S.

As shown in Fig. 2, the bulk MoS₂ had a larger lateral size and thickness (Fig. 2a and b). Evidently, the lateral size and thickness of the MoS₂-AC and MoS₂-AC-S became smaller and thinner (Fig. 2c–f). More importantly, after the hydrothermal process using (NH₄)₂CO₃, the exfoliation of bulk MoS₂ into multi-layer occurred. This suggests that (NH₄)₂CO₃ in the interlayer of MoS₂ under hydrothermal processing served as the exfoliation agent. In comparison of MoS₂-AC and MoS₂-AC-S, the lateral size and thickness of the latter are further lessened and filmy. The result confirms that the shearing after the hydrothermal process accounts for the deep exfoliation into fewer layers and a smaller size. Accordingly, the combination of the two processes is propitious to the curling of the flake edge and prevents the secondary assembly of nanosheets.³³ The surface area was measured to confirm the changes of the bulk MoS₂ and MoS₂-AC-S nanosheets. The results show that the surface area of MoS₂-AC-S is about 62.86% larger than that of bulk MoS₂, indicating that the exfoliation can increase the surface area (Table 1).

Fig. 2 SEM and TEM images of the bulk MoS₂ (a and b), MoS₂-AC (c and d) and MoS₂-AC-S (e and f).

Table 1 The BET surface area of bulk MoS₂ and MoS₂-AC-S

Samples	Surface area (m^2 g^−1)
Bulk MoS2	15.475
MoS2-AC-S	25.203

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In Fig. 3a and b, the edge layers of the bulk MoS₂ are far more than 20, and the layers of MoS₂-AC are more than 10 and less than the layers of the bulk MoS₂. As displayed in the HRTEM image of Fig. 3c, the thickness of MoS₂-AC-S with 0.62 nm of interlayer basal spacing is up to about 4–6 layers, which corresponds to the (002) plane for MoS₂. Additionally, the 0.27 nm of (100) plane, the 0.23 nm of (103) plane and the 0.16 nm of (110) plane for MoS₂-AC-S is in good agreement with the XRD spectra. The result indicates that the as-exfoliated MoS₂ nanosheets still retain a good crystallinity. Fig. 3d shows the corresponding SAED pattern of MoS₂-AC-S, which reveals the presence of a ring-like pattern, thus corroborating the existence of multi-layer polycrystalline nanosheets.³⁴ The size and thickness of the as-exfoliated MoS₂-AC-S was further verified by AFM (Fig. 3e). The as-exfoliated MoS₂-AC-S had a larger irregular-shaped size and about 2.70–3.59 nm thickness (calculated layer number about 4–6), which is consistent with the HRTEM result.

Fig. 3 HRTEM images of bulk MoS₂ (a) and MoS₂-AC (b); HRTEM image (c); SAED pattern; (d) and AFM image and the corresponding height (e) of MoS₂-AC-S.

Fig. 4 shows the average friction coefficients (COFs) and wear scar diameters (AWSDs) of bulk MoS₂ and as-exfoliated MoS₂ under a rotating speed of 1200 rpm and a load of 100 N. As disclosed in Fig. 4a, the average COFs of the bulk MoS₂-based oil and MoS₂-AC-S-based oil with the increase of adding content are descended prior to 0.06 wt% and 0.08%, respectively. With further increase of adding content, the COFs thereupon rise fleetly. The optimal average COFs of these two samples decreased by about 19.01% and 20.66% compared to that of the 150 SN base oil. This is because MoS₂-AC-S with few layers and small size can more easily enter into the contact surface than the bulk MoS₂, probably forming a tribofilm on the contact surface to reduce the friction coefficient and wear scar diameter.³⁵ However, excessive MoS₂ can result in a poor dispersion in oil, and the larger agglomerates consisting of nanosheets conversely increase the friction. Moreover, the AWSDs of the bulk MoS₂ and MoS₂-AC-S decrease with the increase in the content up to 0.08 wt%, but the excessive increase in the content could give rise to abrasive wear, for instance at 0.1 wt%. Furthermore, the AWSDs of the base oil containing 0.08 wt% bulk MoS₂ and MoS₂-AC-S are about 40.00% and 47.27% lower than the base oil, respectively.

Fig. 4 The average COFs (a) and AWSDs (b) of the bulk MoS₂-based oil and MoS₂-AC-S-based oil as a function of adding content.

Fig. 5a and b show the effect of the load on the average COFs and AWSDs of the steel balls lubricated by 0.08 wt% MoS₂-AC-S-based oil with 2 h of friction time. As can be seen from Fig. 5a, the average COFs of MoS₂-AC-S-based oil are decreasing with the increase in the applied load. Furthermore, the COF of MoS₂-AC-S-based oil at 300 N is about 27.18% lower than that of MoS₂-AC-S-based oil at 50 N. This demonstrates that the MoS₂-AC-S-based oil under a higher applied load is more effective to improve the friction ability. On the contrary, the AWSDs of MoS₂-AC-S-based oil increased in the range of testing. The AWSD of MoS₂-AC-S-based oil at 50 N is about 58.33% smaller than that of MoS₂-AC-S-based oil at 300 N. Overall, the MoS₂-AC-S-based oil shows better anti-friction ability at higher applied load and better anti-wear ability at a lower applied load.

Fig. 5 The average COFs (a) and AWSDs (b) of base oil with 0.08 wt% MoS₂-AC-S as a function of load.

Fig. 6 illustrates the COF curves of the base oil, 0.08 wt% bulk MoS₂-based oil and 0.08 wt% MoS₂-AC-S-based oil tested for 2 h. Fig. 6a shows that the COFs of the bulk MoS₂-based oil and MoS₂-AC-S-based oil are lower than that of the 150 SN base oil throughout the testing time. More importantly, the COF and AWSD of the MoS₂-AC-S-based oil are distinctly lower than those of the bulk MoS₂-based oil. Consequently, the average COF and AWSD of MoS₂-AC-S-based oil decreased to about 15.79% and 12.12% compared to those of the bulk MoS₂. The result indicates that the bulk MoS₂ slides difficultly onto the contact surface of the steel ball with the flow of the base oil to reduce the COF and WSD due to the bulk MoS₂ with a large size, greater thickness and poor dispersibility in base oil. Owing to the MoS₂-AC-S with fewer layers and smaller size, it is easy to permeate into the friction surfaces of the counterpart to form a tribofilm.³⁶ Accidentally, the COF of MoS₂ nanosheets is rising with the friction time. This suggests that the stability of MoS₂ nanosheets in oil is poor in running-in period.

Fig. 6 The COFs (a) and AWSDs (b) of base oil, base oil with 0.08 wt% MoS₂ and base oil with 0.08 wt% MoS₂-AC-S as a function of time.

The 3D laser scanning micrograph was used to measure the morphologies of the worn surface for clearly understanding the tribological behavior. Fig. 7 displays the 3D profiles of the worn surfaces of steel balls tested by the base oil, 0.08 wt% bulk MoS₂-based oil and 0.08 wt% MoS₂-AC-S-based oil. The contact areas of the testing balls are all severely damaged to a different degree after 2 h. For the base oil, the worn surface has a very rough wear with a deep and wide hollow along the rubbing direction. However, the worn surfaces of steel balls tested by the bulk MoS₂ and MoS₂-AC-S as additives are much smaller than that of the base oil because of forming the tribofilm on the contact surface, which is derived from the interaction between MoS₂ and friction pairs.³⁷ For further demonstration of the tribofilm, the worn surfaces of these steel balls were examined by Raman spectroscopy. The spectra corroborate the E¹_2g and A_1g feature peaks of MoS₂ observed on the worn surfaces tested by the bulk MoS₂-based oil and MoS₂-AC-S-based oil. The friction reducing mechanism suggests that the MoS₂ nanosheets in oil can smoothly slide onto the touching surface to avoid the wear of the tested steel ball.^38,39

Fig. 7 3D laser scanning micrographs (left) and the corresponding Raman spectra (right) of worn steel ball tested by (a-1 and a-2) base oil, (b-1 and b-2) base oil with 0.08 wt% MoS₂ and (c-1 and c-2) base oil with 0.08 wt% MoS₂-AC-S.

4. Conclusions

In summary, 2D MoS₂ nanosheets with 4–6 layers were successfully prepared by the hydrothermal-assisted shearing exfoliation method. During exfoliation processing, the strategy of organic-free was thoroughly achieved. The MoS₂ nanosheets as additives were applied in oil for the sake of anti-friction and anti-wear. The results revealed that the average COF and AWSD of the 150 SN base oil with 0.08 wt% MoS₂ nanosheets decreased to about 20.66% and 47.27% compared to those of the base oil and exhibited better anti-friction and anti-wear performances.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by Jiangsu Industrial-Academic-Research Prospective Joint Project (BY2016069-02). The authors also acknowledge the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The data of this paper originated from the Test Center of Yangzhou University.

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