Protein-induced ultrathin molybdenum disulfide (MoS₂) flakes for a water-based lubricating system

Abstract

Monolayer or ultrathin transition metal dichalcogenides cover a wide range of atomically thin two-dimensional (2D) materials, whose fascinating properties have made them promising candidates for many applications. In this work, ultrathin MoS₂ flakes were successfully exfoliated, induced by bovine serum albumin (BSA) or hemoglobin (HB), under vigorous ultra-sonication. They were used as additives for water-based lubricating systems and related investigations indicated that they could improve the friction performance and anti-wear abilities at a relatively low concentration: 0.10 wt% of additive contents can make the average friction coefficient stabilize in the range between 0.06 and 0.08, which results from the formation of a dynamic protective film on the friction surfaces. The protein-induced ultrathin MoS₂ flakes are also expected to be useful for other hydrophilic lubrication systems, which possess great scientific value and promising application prospects compared with bulk or microscale lubricating additives.

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1. Introduction

Monolayer or few-layer 2-D transition metal dichalcogenides (TMDS, illustrated as NX₂, N = Mo, W, etc.; X = S, Se, etc.) have attracted more and more attention due to their special properties, such as their semiconducting effect, sizable band gap and large surface area, and their promising applications in sensing catalysis, hydrogen storage, and lubricating additives.^1–5 As a fascinating member of TMDS, MoS₂ has elicited numerous studies because of its unique layered structure and interesting functional properties.^6–8 Natural MoS₂ occurs as a lamellar hexagonal architecture composed of stacked atomic layers (S–Mo–S), which are held together by weak van der Waals interactions.^9–11 Interesting properties which are different from bulk MoS₂ will appear when it is downsized to single or few layer structures.^12,13 For example, exfoliated MoS₂ slices possess excellent catalytic properties for the electrochemical hydrogen evolution reaction,^14–16 and ultrathin MoS₂ exhibits photo-electrochemical and photo-response activity.¹⁷ It is believed that 2-D MoS₂ with ordered structures will have more promising applications in super lubrication through band gap transformation from an indirect way to a direct way for monolayers.^18–21 Obviously, these special properties of 2-D MoS₂ are owing to the ultrathin thickness at atomic scale, thus thickness control is a crucial factor for ultrathin MoS₂.

There are many ways to achieve ultrathin MoS₂ flakes.^22,23 Mechanical exfoliation by scotch-tape is just a simple way, but its yield is too low, and the shape and thickness control is difficult. Chemical vapor deposition (CVD) is another important method which has been successfully used to synthesize high-quality MoS₂ sheets in a controllable manner with needed compositions and controlled number of layers, although it is not easy to scale-up till now.²⁴ Compared with mechanical exfoliation and CVD, liquid phase exfoliation (LPE) is a convenient and cost-effective way to obtain exfoliated MoS₂, which meet the requirements of high yield and large-scale production at the same time.²⁵ Researchers have tried many different ways to develop LPE method. Highly exfoliated MoS₂ sheets can be prepared by lithium ion intercalation and subsequent thermal shock or ultrasonication in a liquid phase.²⁶ Liu et al. gave a resolution of solvo-thermal method for producing excellent nanosheets from bulk MoS₂, hexagonal boron nitride (h-BN) and graphite,²⁷ while Li et al. prepared 2-D MoS₂ from sodium molybdate dehydrate via hydrothermal method and the subsequent water-exfoliation.²⁸ In addition, sonication LPE assisted by proteins is showing a wide, convenient and efficient application in 2-D MoS₂ materials. In this case, proteins adsorbed MoS₂ nanoparticles slide against the layers below, and the freshly exposed surfaces are covered to prevent the reverse sliding subsequently; finally bulk MoS₂ is stripped and ultrathin MoS₂ comes into being.¹ Furthermore, protein adsorbed MoS₂ exhibited excellent dispersion capability in water. The protein molecules possess a variety of functional groups, including benzene rings, carboxyl, hydroxyl and amino group, etc., parts of these groups can stably bind or adsorb on the surface of MoS₂ crystals, resulting hydrophilic MoS₂ ultrathin flakes.

On the other hand, friction is a general phenomenon in nature and industry, while lubrication is a ways to improve energy efficiency. On the surface of friction pairs, there are “ridges”, “valleys”, “asperities” and “depressions” at micro- or nano- scale. Effective controls or reductions of the friction in mechanical systems are important for a sustainable consideration. Lubricating oils and water-based lubricating systems are widely used in most kinds of machines for friction reduction, and how to enhance their lubricating properties has been an important challenge. The design of new friction modification and lubrication additives based on 2-D materials which offered solutions to lubrication challenges in a variety of economically and technologically critical sectors of society represents some unique and challenging opportunities. From the viewpoint of additives, more close combination of novel 2-D materials and base oil can offer new approaches to enhancing the friction reduction capabilities of liquid lubricants, in which solubility, dispersibility and particle degradation will be improved when compared with other conventional solid additives. Similar solutions may be on the horizon for improving the tribological properties of water based lubricating system using 2-D MoS₂ materials. In this work, protein-induced ultrathin MoS₂ flakes which possess good hydrophilicity were involved for this attempt.

2. Experimental

2.1 Materials

BSA and HB from bovine blood were provided by Shanghai Shi fang biological technology Co. LTD. Bulk MoS₂ was purchased from Sinopharm Chemical Reagent Company (Shanghai, China). All chemicals were used as received and the aqueous solution was prepared by deionized water.

2.2 Preparations

The protein-induced exfoliation of ultrathin MoS₂ was carried out via an ultrasonic way: certain amount of bulk MoS₂ powders were added into 20 mL deionized water containing 20 mg BSA, followed by ultra-sonication for 48 hour under controlled temperature at 25 °C. After ultra-sonication, the suspension was centrifuged at 7000 rpm for 40 minutes; the collected precipitants were re-dispersed into water under ultra-sonication for 20 minutes. The resulting solution was then centrifuged for 40 minutes to remove unexfoliated MoS₂ residues, and the obtained suspension was denoted as BSA–MoS₂. Similarly, HB–MoS₂ can be obtained when BSA was replaced by HB in the above procedure. For comparison, bulk MoS₂ was also dealt with under similar ultra-sonication and centrifugation procedure without protein addition.

2.3 Techniques

X-ray diffraction (XRD) patterns were obtained by using a Rigaku K/max-γA X-ray diffractometer with a Cu Kα (λ = 1.5415 Å) at the scanning rate of 0.2° s⁻¹; thermogravimetric analysis (TGA) was performed on a Netzsch STA-409c Thermal Analyzer under a 50 × 10³ mm³ min⁻¹ nitrogen flow with the heating rate of 10 °C min⁻¹; UV-Vis absorption spectra was recorded on a UV-3600 spectrometer; scanning electron microscopy (SEM) images were obtained by using the FEI Inspect F50 instrument. Transmission electron microscopy (TEM) images were performed on a JEM-2100 transmission electron microscope. Optical photographs were taken by a digital camera. The tribological properties were tested on an MSR-10A four-ball test machine. The friction and wear tests were conducted at a rotating speed of 1200 rpm under the applied loads for 147 N for duration of 15 min.

3. Results and discussion

3.1 Characterizations of ultrathin MoS₂

Ultrathin MoS₂ can be obtained by protein-induced exfoliation of bulk MoS₂ powders under ultra-sonication, as illustrated in Fig. 1. Firstly, protein molecules are strongly adsorbed on MoS₂ surface via binding energy. During ultra-sonication process, layer-by-layer exfoliation and fracture of MoS₂ flakes take place gradually. The obtained protein-induced MoS₂ were characterized and analyzed by several ways, which can give confirmation of its structure and morphology.

Fig. 1 Ultrathin MoS₂ sheets obtained by protein-induced exfoliation under ultra-sonication.

UV-Vis absorption spectra of protein-induced ultrathin MoS₂ and bulk MoS₂ are shown in Fig. 2. The characteristic absorption peaks for HB (at 413 nm) and BSA (at 291 nm) in the visible radiation region are clearly observed. After ultra-sonication, two absorptive peaks at 618 nm and 678 nm can be observed in the curves for BSA–MoS₂ and HB–MoS₂, which are attributed to the direct-gap absorptions and excitonic transitions in MoS₂ sheets generated from energy split from valence band spin–orbital coupling.^29–31 In comparison to bulk MoS₂, the wide peak between 400 nm and 600 nm is owing to the direct transition from the deep valence band to the conduction band.

Fig. 2 UV-Vis spectra of the protein-induced ultrathin MoS₂ nanosheets and bulk MoS₂ dispersed in deionized water. Inserted picture: digital camera images of as-prepared MoS₂ nanosheets dispersed in deionized water after sonication (A), 1 day later (B), 7 days later (C), in each image from left to right is bottle of BSA–MoS₂, HB–MoS₂ and bulk MoS₂ suspensions, respectively.

The dispersion stability and durability of BSA–MoS₂ and HB–MoS₂ in water can be improved after protein modification. These dispersion samples have been examined in a simple way: the bulk MoS₂, BSA–MoS₂ and HB–MoS₂ suspensions after 48 h bath ultra-sonication are allowed to settle for several days. It is found that there is no obvious precipitation in the colloid suspensions of as-prepared MoS₂ after 7 days (as shown in Fig. 2C_ins). According to previous studies,¹ protein molecules are strongly adsorbed on MoS₂ surface through its functional group, including carboxyl, hydroxyl and amino group, etc. When protein modified MoS₂ is dispersed in water, these hydrophilic functional groups play a very important role in improving its dispersion stability and durability. Due to the existing of hydrophilic functional group adsorbed on the MoS₂ flakes, the modified MoS₂ becomes more hydrophilic, thus better dispersion stability. However, bulk MoS₂ suspension exhibited only short-term stability and precipitated completely after one day (as shown in Fig. 2B_ins). This indicates that protein-induced ultrathin MoS₂ sheets have good dispersion stability in water.

Fig. 3 gives XRD results of bulk MoS₂, BSA–MoS₂, and HB–MoS₂. The diffractive peaks at 32.7°, 39.0°, 56.8° are in good agreement with (100), (103), (110) planes of the MoS₂ hexagonal structure, although these peaks are quite weaker and broader in the patterns of BSA–MoS₂ and HB–MoS₂. Furthermore, the diffractive peak corresponding to (002) plane of MoS₂ is significantly broadened and its intensity is decreased much, which suggests less stacking and more ultrathin MoS₂ sheets in the direction of the Z-axis perpendicular to atomic layers produced. These results are consistent with the microscopic morphology of bulk MoS₂, BSA–MoS₂ and HB–MoS₂ characterized by SEM, as shown in the inserted picture of Fig. 3. Obviously, bulk MoS₂ presents a clear multilayer with large lateral flake dimension (Fig. 3A_ins). However, protein-induced MoS₂ samples (as shown in Fig. 3B_ins and C_ins) exhibit a reassembled sheet-like structure with the lateral sizes of hundreds of nanometers, and ultrathin layer with a smooth surface can be observed from the exfoliated MoS₂. In comparison, the efficiency of HB on MoS₂ exfoliation is higher than that of BSA, it may be due to the HB molecule has stronger binding energy than BSA on MoS₂ edge.

Fig. 3 X-ray diffraction spectra of the bulk MoS₂, BSA–MoS₂ and HB–MoS₂. Inserted picture: scanning electron microscope (SEM) images of (A) bulk MoS₂, (B) BSA–MoS₂ and (C) BSA–MoS₂.

TGA results of bulk MoS₂, BSA–MoS₂ and HB–MoS₂ shown in Fig. 4 give supplementary and supportive evidences for protein-induced exfoliation. TGA curves for BSA–MoS₂ and HB–MoS₂ can be divided into three stages: little weight loss below 100 °C can be assigned to the absorbed water; the weight loss from 200 °C to 360 °C is mainly due to the thermal decomposition of protein. The weight loss from 400 °C and 800 °C is correspond to the decomposition of MoS₂, which is similar to the curve for bulk MoS₂.

Fig. 4 TGA curves of bulk MoS₂, BSA–MoS₂ and HB–MoS₂.

The ultrathin structure is confirmed by TEM. From Fig. 5A and B, transparent BSA–MoS₂ and HB–MoS₂ flakes with an average size of about 100 nm can be observed, implying that the sheets are quite thin when comparing with bulk MoS₂ shown in Fig. 5C. In addition, HR-TEM images of protein-induced MoS₂ indicate perfect hexagonal structure with a lattice parameter of 0.27 nm, corresponding to its lattice spacing of (100) plane. Selected area electron diffraction (SAED) patterns (the inset of Fig. 5A and B) reveal that ultrathin MoS₂ possesses a hexagonal symmetry structure, which indicate that protein-induced treatment does not damage its original nanostructure.

Fig. 5 TEM (A-1, B-1, C-1, C-2), HRTEM (A-2, B-2) images of the ultrathin MoS₂ prepared by different methods. BSA–MoS₂ (A-1, A-2); HB–MoS₂ (B-1, B-2); bulk MoS₂ sonication for 48 h (C-1, C-2). Inserted picture: selected area electron diffraction (SAED) pattern of each sample.

3.2 Tribological properties

According to our experimental results, the boundary lubrication would occur under a load of 147 N. The lubrication will be failed in 3 minutes for pure water (the calculated average value of friction coefficient is 0.383); while water-based system containing BSA–MoS₂ or HB–MoS₂ are quite effective in reducing the friction coefficient. The real-time changing tendency of friction coefficient for water-based system containing BSA–MoS₂ is shown in Fig. 6. With BSA–MoS₂ concentration increasing, the friction coefficient is dropped remarkably. When concentration is more than 0.10 wt%, the average values of friction coefficient are stabilized in the range between 0.06 and 0.08 (Fig. 7), which gives a reduction of 85% when compared with pure water system. As we known, when the worn surface is lubricated by pure water, friction pairs contact directly due to poor separation ability of water, the rough bumps can form rapidly and further scratched the surface, and then dry friction will occur in the friction experiment. Ultrathin MoS₂ has great specific surface area, which helps to form protective film and prevent direct metal friction and abrasion wear. In addition, ultrathin MoS₂ sheets can easily enter and stay in the contact area of the worn surfaces, ensure the integrity of the lubricating film, thus the stability of the friction coefficient. At the same time, the curve of the wear scar diameter (WSD) gives similar tendency with the friction coefficient, which indicates that BSA–MoS₂ can improve the anti-wear performance of water-based lubricating system. As shown in the inserted picture of Fig. 7, the surfaces of friction pairs lubricated by water-based system containing 0.10 wt%, 0.13 wt%, and 0.16 wt% of BSA–MoS₂ are smoother than that lubricated with water-based system containing 0.01 wt%, 0.04 wt% of BSA–MoS₂, and the related furrows on wear scar surfaces are shallower and narrower. The reason can be attributed to “separate effect” caused by ultrathin MoS₂. At a proper concentration, a homogenous and dense protective film will form on the frictional surface, which can partly avoid the direct contact of rough surfaces; as a result, shallower and narrower furrows can be achieved. On the other hand, the dense lubrication film is difficult to form at a lower concentration, some ‘peaks’ on the rubbing surfaces will be in contact directly, resulting in thick and deep furrows.

Fig. 6 Real-time changing tendency of friction coefficient with time obtained from water-based lubricating systems with different BSA–MoS₂ concentration. (a) 0.01 wt%; (b) 0.04 wt%; (c) 0.07 wt%; (d) 0.10 wt%; (e) 0.13 wt%; (f) 0.16 wt%; (g) 0.19 wt%; (h) 0.22 wt%; (i) 0.25 wt%.

Fig. 7 Average friction coefficient and wear scar diameter obtained from water-based lubricating systems with different BSA–MoS₂ concentration (the unit of WSD is 0.01 mm). Inserted picture: the wear scar photos of the steel balls after friction test for water-based lubricating systems containing (A) 0.01 wt%; (B) 0.04 wt%; (C) 0.07 wt%; (D) 0.10 wt%; (E) 0.13 wt%; (F) 0.16 wt% of BSA–MoS₂.

Meanwhile, similar behaviors in improving the friction performance can be observed in the water-based system containing ultrathin BSA–MoS₂ flakes (in our experiments, 0.10 wt% is the optimal addition ratio, corresponding to the smallest friction coefficient of 0.07 similarly) when compared with BSA–MoS₂, as shown in Fig. 8. On the other hand, 0.10 wt% of bulk MoS₂ cannot decrease the friction coefficient of water-based lubricating system. These obvious contrast data reconfirms that ultrathin BSA–MoS₂ and HB–MoS₂ flakes are useful additives for improving the friction-reduction and anti-wear ability of water-based lubricating system.

Fig. 8 Real-time changing tendency of friction coefficient with time obtained from water-based lubricating systems with different MoS₂ concentration: (a) 0.01 wt% of HB–MoS₂; (b) 0.10 wt% of HB–MoS₂; (c) 0.19 wt% of HB–MoS₂; (d) 0.10 wt% of bulk MoS₂. Inserted picture on the top left corner: average friction coefficient for pure water; water-based lubricating systems with 0.10 wt% of BSA–MoS₂; 0.10 wt% HB–MoS₂; 0.10 wt% of bulk MoS₂ Inserted picture on the top right corner: the wear scar photos of the steel balls after friction test water-based lubricating systems with (A) 0.01 wt% HB–MoS₂; (B) 0.10 wt% of bulk MoS₂; (C) 0.10 wt% HB–MoS₂; (D) 0.19 wt% HB–MoS₂ respectively.

3.3 Related mechanism

Fig. 9 suggests one possible mechanism for ultrathin MoS₂ flakes improving lubricating performance of water-based system. When the worn surface lubricated by pure water, many ‘peaks’ on the rubbing surfaces will be in contact directly due to poor separation ability of pure water, thus a rough surface corresponds to rather higher friction coefficient in the friction experiment.^32,33 When the worn surface lubricated with pure water containing certain concentration (below 0.10 wt%) of ultrathin MoS₂ (Fig. 9b), lower friction coefficient and smooth worn surface can be achieved, because MoS₂ flakes can separate ‘peaks’ on the rubbing surfaces. When the friction pairs are lubricated by water-based system with higher concentration (above 0.10 wt%) of ultrathin MoS₂ (Fig. 9c), ultrathin MoS₂ would separate all ‘peaks’ and fill the micro-scratches on the rubbing surface.

Fig. 9 Illustration of the lubrication mechanism. (a) Pure water with no additives; (b) water-based lubricating system with small concentration of ultrathin MoS₂; (c) water-based lubricating system with higher concentration of ultrathin MoS₂; (d) water-based lubricating system with bulk MoS₂.

Consequently, the thick, deep furrows on the worn surface cannot be found as shown in Fig. 7 (inserted picture D, E, F) and Fig. 8 (inserted picture on the top right corner, C, D). Together with the good dispersion in pure water, ultrathin MoS₂ sheets can easily enter and stay in the contact area of the worn surfaces and serve as ‘nanofillers’ to form a surface protective and lubricious layer, which can result in effective friction reduction. Furthermore, ultrathin MoS₂ sheets could provide quite easier shear and slider between friction pairs. As for 0.10 wt% of bulk MoS₂ (Fig. 9d), their being difficult to be exfoliated makes it hard to enter the contact areas and work as an effective lubricant, which leads to an unexpected lubricating effects.

4. Conclusions

Protein-induced ultrathin MoS₂ (BSA–MoS₂ and HB–MoS₂) were successfully synthesized via ultra-sonication based exfoliation. Both BSA–MoS₂ and HB–MoS₂ possess good dispersion and stability in pure water, which was advantageous to be used as water-based lubricating additives. In comparison to pure water and water-based lubricating system containing bulk MoS₂, 0.10 wt% of BSA–MoS₂ and HB–MoS₂ can decrease the friction coefficient and wear scar diameter effectively. These results indicated that protein-induced ultrathin MoS₂ had a good potential for the application as water-based lubricating additives.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (U1332134, 51635004), the Natural Science Foundation of Jiangsu Province (BK20150505), the Natural Science Foundation of Suzhou (SYG201329), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project and the International Foundation for Science, Stockholm, Sweden, the Organization for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu (F/4736-2) and the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF15A11).

References

1. G. J. Guan, S. G. Zhang, S. H. Liu, Y. Q. Cai, M. Low, C. P. Teng, I. Y. Phang, Y. Cheng, K. L. Duei, B. M. Srinivasan, Y. G. Zheng, Y. W. Zhang and M. Y. Han, Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides, J. Am. Chem. Soc., 2015, 137, 6152–6155.
2. R. R. Chianelli, E. B. Prestridge, T. A. Pecoraro and J. P. Deneufville, Molybdenum disulfide in the poorly crystalline“Rag” structure, Science, 1979, 203(4385), 1105–1107.
3. Z. Chen, X. W. Liu, Y. H. Liu, S. Gunsel and J. B. Luo, Ultrathin MoS₂ Nanosheets with Superior Extreme Pressure Property as Boundary Lubricants, Sci. Rep., 2015, 5, 12869.
4. X. P. Dai, K. L. Du, Z. Z. Li, H. Sun, Y. Yang, W. Zhang and X. Zhang, Enhanced hydrogen evolution reaction on few-layer MoS₂ nanosheets-coated functionalized carbon nanotubes, Int. J. Hydrogen Energy, 2015, 40, 8877–8888.
5. L. Liu, Y. W. Chen and P. Huang, Preparation and tribological properties of organically modified graphite oxide in liquid paraffin at ultra-low concentrations, RSC Adv., 2015, 5, 90525–90530.
6. X. H. Zhang, X. H. Huang, M. Q. Xue, X. Ye, W. N. Lei, H. Tang and C. S. Li, Hydrothermal synthesis and characterization of 3D flower-like MoS₂ microspheres, Mater. Lett., 2015, 148, 67–70.
7. W. Y. Wang, L. Li, K. Wu, G. H. Zhu, S. Tan, W. S. Li and Y. Q. Yang, Hydrothermal synthesis of bimodal mesoporous MoS₂ nanosheets and their hydrodeoxygenation properties, RSC Adv., 2015, 5, 61799–61807.
8. M. A. Bissett, I. A. Kinloch and R. A. W. Dryfe, Characterization of MoS₂–Graphene Composites for High-Performance Coin Cell Supercapacitors, ACS Appl. Mater. Interfaces, 2015, 7, 17388–17398.
9. X. P. Ren, L. Q. Pang, Y. X. Zhang, X. D. Ren, H. B. Fan and S. Z. Liu, One-step hydrothermal synthesis of monolayer MoS₂ quantum dots for highly efficient electrocatalytic hydrogen evolution, J. Mater. Chem. A, 2015, 3, 10693–10697.
10. S. D. Jiang, G. Tang, Z. M. Bai, Y. Y. Wang, Y. Hu and L. Song, Surface functionalization of MoS₂ with POSS for enhancing thermal, flame-retardant and mechanical properties in PVA composites, RSC Adv., 2014, 4, 3253–3262.
11. G. B. Feng, A. X. Wei, Y. Zhao and J. Liu, Synthesis of flower-like MoS₂ nanosheets microspheres by hydrothermal method, J. Mater. Sci.: Mater. Electron., 2015, 26, 8160–8166.
12. X. P. Dai, Z. Z. Li, K. L. Du, H. Sun, Y. Yang, X. Zhang, X. Y. Ma and J. Wang, Facile Synthesis of In situ Nitrogenated Graphene Decorated by Few-Layer MoS₂ for Hydrogen Evolution Reaction, Electrochim. Acta, 2015, 171, 72–80.
13. A. Ambrosi, Z. Sofer and M. Pumera, Lithium intercalation compound dramatically influences the electrochemical properties of exfoliated MoS₂, Small, 2015, 11, 605–612.
14. A. Y. S. Eng, A. Ambrosi, Z. Sofer, P. Simek and M. Pumera, Electrochemistry of transition metal dichalcogenides: strong dependence on the metal-to-chalcogen composition and exfoliation method, ACS Nano, 2014, 8, 12185–12198.
15. J. Kim, S. Byun, A. J. Smith, J. Yu and J. X. Huang, Enhanced electrocatalytic properties of transition-metal dichalcogenides sheets by spontaneous gold nanoparticle decoration, J. Phys. Chem. Lett., 2013, 4, 1227–1232.
16. Y. D. Liu, L. Ren, X. Qi, L. W. Yang, G. L. Hao, J. Li, X. L. Wei and J. X. Zhong, Preparation, characterization and photoelectrochemical property of ultrathin MoS₂ nanosheets via hydrothermal intercalation and exfoliation route, J. Alloys Compd., 2013, 571, 37–42.
17. J. X. Guo, F. F. Li, Y. F. Sun, X. Zhang and L. Tang, Oxygen-incorporated MoS₂ ultrathin nanosheets grown on graphene for efficient electrochemical hydrogen evolution, J. Power Sources, 2015, 291, 195–200.
18. N. V. Petrova, I. N. Yakovkin and D. A. Zeze, Metallization and stiffness of the Li-intercalated MoS₂ bilayer, Appl. Surf. Sci., 2015, 353, 333–337.
19. E. Mieda, R. Azumi, S. Shimada, M. Tanaka, T. Shimizu and A. Ando, Nanoprobe characterization of MoS₂ nanosheets fabricated by Li-intercalation, Jpn. J. Appl. Phys., 2015, 54, 08LB07.
20. S. H. Song, B. H. Kim, D. H. Choe, J. Kim, D. C. Kim, D. J. Lee, J. M. Kim, K. J. Chang and S. Jeon, Bandgap widening of phase quilted, 2D MoS₂ by oxidative intercalation, Adv. Mater., 2015, 27, 3152–3158.
21. X. X. Wang, F. X. Nan, J. L. Zhao, T. Yang, T. Ge and K. Jiao, A label-free ultrasensitive electrochemical DNA sensor based on thin-layer MoS₂ nanosheets with high electrochemical activity, Biosens. Bioelectron., 2015, 64, 386–391.
22. J. H. Kim, J. Kim, S. D. Oh, S. Kim and S. H. Choi, Sequential structural and optical evolution of MoS₂ by chemical synthesis and exfoliation, J. Korean Phys. Soc., 2015, 66, 1852–1855.
23. X. M. Li, W. Z. Wang, L. Zhang, D. Jiang and Y. l. Zheng, Water-exfoliated MoS₂ catalyst with enhanced photoelectrochemical activities, Catal. Commun., 2015, 70, 53–57.
24. Y. W. Lihui, H. Yu, X. R. Yang, J. J. Zhou, Q. Zhang, Y. Q. Zhang, Z. M. Luo, S. Su and L. H. Wang, Rapid preparation of single-layer transition metal dichalcogenide nanosheets via ultrasonication enhanced lithium intercalation, Chem. Commun., 2016, 52, 529–532.
25. V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N. Coleman, Liquid exfoliation of layered materials, Science, 2013, 340, 1226419.
26. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. W. Chen and M. Chhowalla, et al., Photoluminescence from chemically exfoliated MoS₂, Nano Lett., 2011, 11, 5111–5116.
27. Z. Liu, Y. B. Wang, Z. Y. Wang, Y. G. Yao, J. Q. Dai, S. Das and L. B. Hu, Solvo-thermal microwave-powered two-dimensional material exfoliation, Chem. Commun., 2016, 52, 5757–5760.
28. X. M. Li, W. Z. Wang, L. Zhang, D. Jiang and Y. L. Zheng, Water-exfoliated MoS₂ catalyst with enhanced photoelectrochemical activities, Catal. Commun., 2015, 70, 53–57.
29. R. K. Joshi, S. Shukla, S. Saxena, G.-H. Lee, V. Sahajwalla and S. Alwarappan, Hydrogen generation via photoelectrochemical water splitting using chemically exfoliated MoS₂ layers, AIP Adv., 2016, 6, 015315.
30. F. X. Jiang, J. H. Xiong, W. Q. Zhou, C. C. Liu, L. Y. Wang, F. Zhao, H. X. Liu and J. K. Xu, Use of organic solvent-assisted exfoliated MoS₂ for optimizing the thermoelectric performance of flexible PEDOT: PSS thin films, J. Mater. Chem. A, 2016, 4, 5265–5273.
31. V. Forsberg, R. Y. Zhang, J. Backstrom, C. Dahlstrom, B. Andres, M. Norgren, M. Andersson, M. Hummelgard and H. Olin, Exfoliated MoS₂ in Water without Additives, PLoS One, 2016, 11, 0154522 . (Page 7, Line 7 in the revised paper).
32. Y. Gu, X. Zhao, Y. Liu and Y. X. Lv, Preparation and tribological properties of dual-coated TiO₂ nanoparticles as water-based lubricant additives, J. Nanomater., 2014, 2014, 2.
33. Y. F. Mo, K. T. Turner and I. Szlufarska, Friction laws at the nanoscale, Nature, 2009, 457, 1116–1119.

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