Ultrasound-Assisted Alkaline Solution Reflux for As-Exfoliated MoS2 Nanosheets.

A facile approach was developed to produce MoS2 nanosheets by ultrasound-assisted reflux exfoliation, which was highly efficient for large-scale production and sustainable for environment. The interlayer force of bulk MoS2 was first exhausted in employing LiOH/NaOH solution by reflux and thereafter quickly exfoliated by ultrasound. The lateral size of the as-prepared MoS2 nanosheets with about 2-9 layers became smaller. Definitely, the average friction coefficient and wear scar diameter of 0.08 wt % MoS2-based oil decreased by about 21.87 and 38.09% relative to the base oil, which displayed better antifriction and antiwear performances.

Introduction

MoS2 has attracted the attention of many researchers, owing to the interlayer van der Waals force and intralayer covalent bond,[1,3] which showed excellent properties in the field of catalyst,[4,5] battery,[6,7] sensor,[8] nanotransistor,[9] hydrogen storage,[10] supercapacitor,[11]and so on. More importantly, MoS2 also exhibited excellent lubrication performance due to the interlayer van der Waals force.[12,13] At present, the bulk MoS2 has been widely used as an antiwear additive in solid greases,[14] but it cannot be used in liquid lubricants due to its unstable dispersion. The specific surface area of the bulk MoS2 is increased through nanotechnology to obtain MoS2 nanosheets and enhance the stable dispersion in liquid lubricants. Definitely, various MoS2 nanomaterials have proven to exhibit preferable friction-reducing performance.[15−17] This finding showed that the structure of MoS2 nanomaterials obviously affected the friction-reducing properties. Yi et al.[18] synthesized three morphologies of MoS2 nanomaterials, including flower-like, microspheres, and nanosheets, by hydrothermal and solvothermal methods. The as-synthesized MoS2 in liquid paraffin improved the tribological properties. MoS2 nanosheets exhibited excellent tribological performances compared to flower-like MoS2 and MoS2 microspheres under the same testing conditions. As far as the antifriction and antiwear in oil are concerned, the quality of nanosheets required is much lower than for other purposes. Conversely, too thin or too large, both are improper to be used in oil.

Up to now, many highly efficient exfoliating methods have been exploited to prepare MoS2 nanosheets. Krishnamoorthy et al.[19] successfully prepared MoS2 nanosheets with few layers using 1-methyl-2-pyrrolidinone (NMP) as an exfoliated solvent and via a ball milling method. Varrla et al.[20] demonstrated that the bulk MoS2 in the aqueous surfactant solution was massively sheared and exfoliated to MoS2 nanosheets with a mean thickness of 4.6 nm using a kitchen blender. Bang et al.[21] provided an easy liquid-phase exfoliation method with NMP and NaOH to improve the yield of single-layered MoS2 nanosheets and the thickness of the as-prepared MoS2 nanosheets varied from 1 to 9 nm. Additionally, the Li-intercalated exfoliation has been successfully achieved in many types of nanosheets.[22] Therefore, Liu et al.[22] obtained ultrathin and high-yield MoS2 nanosheets with a uniform thickness of 4.68 nm through the hydrothermal exfoliation route using Li+ and ethylene glycol. However, the expensive Li source and hazardous organic solvent still hindered the scalable production. Meanwhile, the superimposing of MoS2 nanosheets usually took place after cleaning the organic solvent.[2] Recently, we successfully exfoliated BN to few layers via the hydrothermal intercalation and exfoliation method with NaOH/KOH solutions.[23] Due to this idea, the exfoliation of bulk MoS2 in mixed alkaline solution became a feasible method.

In this paper, we provided an efficacious exfoliation method based on the well-established ultrasound-assisted reflux method to prepare MoS2 nanosheets. The as-exfoliated MoS2 nanosheets were detected through a series of characterization methods. Then the friction and wear performances of MoS2 nanosheets in oil were tested through a four-ball friction machine.

Results and Discussion

Figure shows the X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of MoS2 and four kinds of as-exfoliated MoS2. In Figure a, all of the MoS2 samples show four feature peaks at 14.4, 32.7, 39.5, and 58.3°, ascribing to the (002), (100), (103), and (110) planes of MoS2 (JCPDS No. 37-1492), respectively. Every peak is attributed to the lattice of representative MoS2 and there is no additional peak from MoO3. It proves that this MoS2 is a single phase and polycrystalline structure.[24] In particular, all lattice planes of four kinds of as-exfoliated MoS2 show weaker peak intensities than bulk MoS2. The possible reason is that the as-exfoliated MoS2 has fewer layers than bulk MoS2. In Figure b, the bulk MoS2 has larger lateral size and thickness. Obviously, four kinds of as-exfoliated MoS2 exhibit a smaller size and more transparent nanosheets (Figure c–f). More importantly, after the processes using a single alkali solution (Figure c,d) and a single reflux method using alkaline solution (Figure e), the bulk MoS2 is exfoliated into multilayer. In comparison to these as-exfoliated MoS2 samples, the thickness of MoS2–Li+/Na+ is filmiest. This result proves that the ultrasound after the reflux process using mixed alkali solutions accounts for the deep exfoliation of the bulk MoS2 into fewer layers. These results indicate that bulk MoS2 exfoliated by both Li+ and Na+ under the ultrasound-assisted reflux method obtains the most ideal nanosheets compared to the single ion and single method.[23]

Figure 1

(a) XRD patterns, SEM, and TEM (inset) images of (b) MoS2, (c) MoS2–Na+, (d) MoS2–Li+, (e) MoS2–R, and (f) MoS2–Li+/Na+.

Figure displays the Raman spectra and UV–vis spectra of MoS2 and MoS2–Li+/Na+. In Figure b, MoS2 and MoS2–Li+/Na+ obtain two characteristic peaks at about 381 and 406 cm–1.[25] The peak intensities and peak positions of MoS2–Li+/Na+ have changed. Importantly, the frequency difference between E2g1 and A1g peaks of MoS2–Li+/Na+ is 22 cm–1, which is lower than 25 cm–1 of the bulk MoS2. This indicates that the number of layers for MoS2–Li+/Na+ decreases after exfoliation.[26,27] In Figure b, two peaks at about 630 nm (1.97 eV) (B) and 680 nm (1.82 eV) (A) are shown in the UV–vis spectra of MoS2–Li+/Na+ dispersions. The two peaks are assigned to the exciton transitions of MoS2 at the first Brillouin zone.[28] After exfoliation, the peak of MoS2–Li+/Na+ is consistent with the bulk MoS2. However, MoS2–Li+/Na+ shows more pronounced peaks than MoS2, indicating that the ultrasound-assisted alkaline solution reflux route plays an important role in the exfoliation process. In addition, the percentage of the size for MoS2–Li+/Na+ is calculated (Figure d) according to the TEM image of MoS2–Li+/Na+ in 10 μm (Figure c). The percentage of the nanoscale size (<1 μm) is as high as 78.29% and the vast majority size is below 2.0 μm.

Figure 2

(a) Raman and (b) UV–vis spectra of MoS2 and MoS2–Li+/Na+, (c) TEM image and (d) the percentage of size for MoS2–Li+/Na+.

The thickness and size of MoS2–Na+, MoS2–Li+, and MoS2–Li+/Na+ are verified by atomic force microscopy (AFM) (Figure a–c). In Figure a,b, MoS2–Na+ and MoS2–Li+ have about 2.70–5.33 and 2.54–6.15 nm of the height and the number of the corresponding layers is about 4–9 layers and 4–10 layers. MoS2–Li+/Na+ (Figure c) has a larger irregular-shaped size, the height is about 1.00–5.48 nm, and the number of corresponding layers is about 2–9 layers. The as-exfoliated MoS2–Li+/Na+ is further examined by high-resolution transmission electron microscopy (HRTEM) (Figure d). The thickness of MoS2–Li+/Na+ with 0.62 nm of interlayer basal spacing is up to about 2–4 layers due to more easily exfoliated edges of nanosheets, which corresponds to the (002) plane of MoS2. The corresponding selected area electron diffraction pattern (Figure e) of MoS2–Li+/Na+ reveals polylattice diffraction rings, showing the retention of polycrystalline MoS2–Li+/Na+ during exfoliation.[29]

Figure 3

AFM image and the height profiles of (a) MoS2–Na+, (b) MoS2–Li+, (c) MoS2–Li+/Na+, (d) HRTEM image, and (e) the selected area electron diffraction pattern of MoS2–Li+/Na+.

Figure displays the tribology data of 150 SN base oil and the base oil with 0.08 wt % MoS2–Li+/Na+ inspecting for 6 h. In Figure a, the friction coefficient (COF) of MoS2–Li+/Na+ is less than that of base oil throughout the test time. Compared to the base oil, the average COF and average wear scar diameter (AWSD) of MoS2–Li+/Na+ are decreased by about 21.87 and 38.09%, respectively. The result indicates that the bulk MoS2 slides with difficulty into the contact surface of the steel ball with the flow of the base oil to reduce the COF and WSD due to MoS2 with a large size, a thick thickness and poor dispersibility in base oil. Owing to the MoS2-AC-S with fewer layers and smaller size, it is easy to infiltrate into the contact surfaces of counterpart to become a tribofilm.[30] Surprisingly, the COF of MoS2–Li+/Na+ increases with the friction time. This indicates that the stability of MoS2–Li+/Na+ in base oil is unsatisfied in testing time. Figure c,d show three-dimensional (3D) profiles of wear surfaces of balls tested by base oil and the base oil with 0.08 wt % MoS2–Li+/Na+. The contact areas of testing balls are severely damaged to a different degree after 6 h. Compared with the two kinds of wear mark, the damaged surface of the steel ball after MoS2–Li+/Na+ as additives is decreased owing to preferably becoming the tribofilm on the wear surface, when the MoS2–Li+/Na+ nanosheets in base oil contacted the steel balls.[31] For further demonstration of the tribofilm, the wear surfaces of balls are determined by Raman spectroscopy. The appearance of two characteristic peaks of MoS2 on the contact surfaces examined by MoS2–Li+/Na+. The friction value decreasing mechanism is deduced that MoS2 nanosheets in base oil can smoothly slip into the contact surface to prevent the wear of steel ball.[32,33]

Figure 4

(a) COFs and (b) the AWSDs of base oil and the base oil with 0.08 wt % MoS2–Li+/Na+. The Raman spectra and 3D laser scanning micrographs (inset) of the wear steel ball examined by (c) base oil and (d) the base oil with 0.08 wt % MoS2–Li+/Na+.

Conclusions

In summary, the MoS2 nanosheets with about 2–9 layers were successfully prepared by an ultrasound-assisted reflux exfoliation method. The ultrasound processing played an important role in the union exfoliation. MoS2–Li+/Na+ as lubrication additives were added into 150 SN base oil to evaluate their lubrication properties. COF and WSD of 0.08 wt % MoS2–Li+/Na+-based oil decreased by 21.87 and 38.09% compared to the base oil, revealing better tribological properties.

Experimental Section

Materials and Methods

Two-dimensional MoS2 nanosheets were obtained by the exfoliation method combining the reflux and ultrasound process. In particular, 0.73 g of LiOH and 1.22 g of NaOH were dispersed in 180 mL of deionized water, then 1.02 g of bulk MoS2 (Sinopharm Chemical Reagent Co., Ltd) was added into the above solution and stirred at room temperature for 2 h. Next, the suspension was transferred to a 250 mL three-necked flask with a stirrer and refluxed at 100 °C for 3 h, denoted as MoS2–R. Then, the three-necked flask with MoS2 nanosheets was placed in an ultrasonic bath for 2 h, and the cold water was continuously added into the ultrasonic bath to control the water temperature during the ultrasonic process. Next, the solution was allowed to stand for 30 min and the precipitate was removed. Finally, the upper liquid was cleaned by deionized water and anhydrous ethanol at least three times, dried at 60 °C for 24 h, denoted MoS2–Li+/Na+. In addition, the MoS2–Li+ sample was obtained in the absence of NaOH and the MoS2–Na+ sample was obtained in the absence of LiOH in the same reflux and ultrasound exfoliation method. The exfoliation illustration of MoS2 nanosheets is proposed in Figure .

Figure 5

Exfoliation illustration of MoS2 nanosheets.

Tribological Properties Testing

MoS2–Li+/Na+ nanosheets were added into 150 SN base oil through the ultrasound method to obtain MoS2–Li+/Na+-based oils. The testing of tribological properties was executed in an MMW-1 four-ball machine (Jinan Chenda Ltd. Co., in China). The rotating speed, stably applied load, and testing time of the test parameters were set to 1200 rpm, 100 N, and 6 h, respectively. The wear scar of the steel ball was cleaned with ethanol to remove the base oil and then detected through the Raman spectrometer and 3D laser scanning microscope.

Characterization

XRD analysis was performed by Powder X-ray diffraction (Bruker AXS, German). The SEM images were obtained by an S-4800 II Field emission scanning electron microscope (Hitachi, Japan). The TEM images were obtained by a Tecnai 12 transmission electron microscope (Philips, Netherlands). The Raman spectra were recorded by an InVia Raman spectrometer (Renishaw, Britain). The UV–vis spectra were recorded on a Cary 5000 spectrophotometer (Varian). HRTEM images were obtained by a Tecnai G2 F30 S-TWIN field emission transmission electron microscope (FEI). The AFM images were obtained on a nanoscope (Digital Instruments). The wear scar micrographs were recorded by an LSM 700 3D Laser Scanning Microscope (CARL ZEISS, German).