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Preparation of Few-Layer MoS2 Nanosheets via an Efficient Shearing Exfoliation Method Yuewei Li, Xianglu Yin, and Wei Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04087 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Industrial & Engineering Chemistry Research

Preparation of Few-Layer MoS2 Nanosheets via an Efficient Shearing Exfoliation Method Yuewei Li, Xianglu Yin, Wei Wu*

* Research Center of the Ministry of Education for High Gravity of Engineering and

Technology, Beijing University of Chemical Technology, Beijing 100029, China.

* E-mail: [email protected].

Abstract: In this paper, we selected less studied high-speed dispersive homogenizer as shear-exfoliating device and selected NMP which matches the surface energy of MoS2 as solvent to prepare few-layer MoS2 nanosheets. The effects of operating parameters on the concentration of few-layer MoS2 nanosheets were systematically studied. The results showed that the change of operating conditions has a direct influence on the exfoliation effects. The concentration of MoS2 nanosheets was 0.96 mg/mL in pure NMP under the optimized conditions. The concentration reached 1.44 mg/mL and the highest yield was 4.8% after adding sodium citrate. Particularly, their lateral size are about 50 to 200 nm, in which almost 65% of MoS2 nanosheets are less than 4 layers and 9% are monolayer. It was verified that as-used exfoliation method is simple and high efficient.

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Keywords: MoS2 nanosheets; High-speed dispersive homogenizer; NMP; Liquid phase shear exfoliation.

Table of Contents:

1. Introduction

The rise of graphene has set off new research upsurge of two-dimensional (2D) layered materials. Graphene has excellent mechanical properties, electrical conductivity and thermal conductivity.1 However, its zero band gap limits its application in electronic device.2 Molybdenum disulfide (MoS2) as typical transition metal disulfides (TMDs) possesses similar laminar structure to graphene. Notably, MoS2 has a tunable band gap (1.2 eV~1.8 eV) and is superior to graphene as a future electronic materials.3, 4 To date, 2D MoS2 nanosheets has been successfully applied in electrocatalytic hydrogen evolution (HER),5,

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optoelectronic devices,7

field-effect transistors,8, 9 sensors,10 solar cells,11, 12 Li- and Na-Ion battery,13, 14 etc. In 2


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order to achieve extensive applications, it is very essential to prepare few-layer MoS2 nanosheets with good quality and high yield. At present, the mature preparation methods mainly include hydrothermal methods,15-17 chemical vapor deposition (CVD),18, 19 lithium intercalation20, 21 and liquid phase exfoliation(LPE).22-24 MoS2 nanosheets generally are of high quality by the first three methods. However, their operating conditions are strict and the number of layers is uncontrollable, which is still inadequate for low-cost and large-scale production. Among these methods, LPE is a common, promising and simple method for large-scale production of 2D materials nanosheets.25 LPE is insensitive to air and does not involve chemical reactions, this method can prepare 2D nanosheets with high-crystallinity and intrinsic structure.26 The liquid phase exfoliation commonly involves two conditions: exfoliation equipment and dispersant. The commonly used equipment includes ultrasound device3, 27

and shear device (e.g., variable speed blender,28 high-shear mixer,29 rotating packed

bed30). The dispersant includes organic solvent, cosolvent, surfactant, ionic liquid, etc, 31, 32

which is pivotal to overcome the weak Van der Waals forces between the layers

and maintain the stability of the nanosheets. The research results of mentioned devices are listed in Table S1 of the Supporting Information. For example, Yao et al. combined grind and ultrasonic to exfoliate MoS2 in ethanol/water, the yield of MoS2 nanosheets was 67%.27 It is obvious that this method can prepare 2D nanosheets with the high yield, but the low scalability of ultrasound and the defect rich of nanosheets limit their further industrial production.33 While the rotating packed bed can produce a strong centrifugal force 3



through the high-speed rotation to strengthen the micro-mixing and work efficiency.34, 35

Nevertheless, the yield of products is low and needs to be further improved. Thus in

term of yield, there is still a long way to go. In addition to the shear-exfoliating devices, the yield and stability of the exfoliated nanosheets also depend on three main basic factors: solid-liquid interface energy, Hansen solubility parameter (HSP) and solvent molecular size.23 The specific surface energy of the few-layer MoS2 is estimated to be about 46.5 mJ/m2 by contact angle measurement.36 A suitable solvent can be initially selected by surface energy matching and above three basic factors. Among numerous organic solvents, the surface tension of NMP is about 40 mJ/m2, which is similar to the surface energy of MoS2.24 NMP is considered to be the most effective solvent. The yield is higher after addition of the additive (e.g., alkali metal hydroxide,13 PVP37 etc.) compared with the conventional preparation. Therefore, we preferred liquid phase shear exfoliation to prepare few-layer MoS2 nanosheets in NMP. The high-speed dispersive homogenizer is known to be a high shear device that produces localized energy consumption and cutting speed. It has been widely used in process such as homogenization, emulsification, dispersion, cell disruption and grinding.38 Homogenizer has the characteristics of high shearing strength, simple equipment, easy operation and amplification scale. It has obvious advantages for shear-exfoliating MoS2. In this paper, we prepared the MoS2 nanosheets in pure NMP and optimally chose sodium citrate as an assistant to further improve the yield. It was confirmed that this method is simple, high efficient and easy to be scaled up. 4


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2. Experimental Section 2.1. Materials

All chemicals and solvents were used without further purification. MoS2 bulk powder (with flake sizes between 8 and 10 µm) was purchased from Shanghai Shen Yu Co., Ltd., Shanghai, China. N-Methyl pyrrolidone (NMP) (analytical pure regents) was purchased from Tianjin large-mao Chemical Reagent Factory, Tianjin, China. Sodium citrate (Na3C6H5O7·2H2O), Sodium hydroxide (NaOH) and Sodium chloride (NaCl) were purchased from Beijing Chemical Works.

2.2. Methods 2D MoS2 nanosheets were exfoliated by high-speed dispersive homogenizer (Shanghai specimen model factory: FJ300-SH).The executive part of the shear homogenizer is a homogenizing shaft, the tip of which is composed of a stator-rotor. In our opinion, bulk MoS2 is first immersed in NMP, when the rotor starts high-speed rotation, the suspension will enter the high shear region (i.e., the gap between the stator and the rotor) from the internal cavity of the homogenizer. Under the severe centrifugal force, the suspension rapidly diffuses from the working area to the stator channel. During the diffusion process, the rotor and the MoS2 suspension produce strong shear force along the tangential direction of the rotor (including the lateral shear force, the longitudinal shear force and the collision), among them, the lateral

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shear force can overcome the weaker van der Waals forces between the MoS2 layers to get less or even single layer of MoS2, in addition, the longitudinal shear force makes MoS2 sheets fracture and the collision has a synergistic effect. At the same time, the shear force also makes NMP form nano-droplets, which assist in exfoliating and stabilizing MoS2. MoS2 is evenly dispersed and thinned by repeated lateral shear, longitudinal shear and collision. Finally, the bulk MoS2 is shear-exfoliated into nanosheets which stably disperse under the encircling of NMP molecules. Figure 1 showed the detailed mechanism diagram. According to the Navier-Stokes equation, the shear rate γ of any point in the high shear region is obtained, as in equation 139-41

γ=

= 1 − (1)

where γ is the shear rate (s-1),ωis the rotor angular velocity(rad/s), R0 is the rotor outer diameter (12.8×10-3 m), R1 is the stator inner diameter (13.1×10-3 m), r is the high shear region at any point radius (R0≤r≤R1). In this experiment, the shear rates at different speeds were calculated as the maximum shear rate on the outside of the rotor (i.e., r = R0). 350 mL of NMP and the required amount of the bulk MoS2 powders were added into a flat-bottomed beaker and mixed until the suspension was homogeneous. The MoS2 suspension concentration changed from 20 to 60 mg/mL. The suspension was then shear-exfoliated at different sheer rate and kept the ambient temperature during exfoliation. The collected supernatant was centrifuged at 4000 rpm for 20 minutes to remove no-exfoliated material and then further characterized. Furthermore, the vacuum filtration treatment was used to completely remove NMP. And then we selected freeze-drying to collect nanosheets powder, using 6


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low-temperature drying to maintain a good structure of the powder and avoid agglomeration during high-temperature drying. Finally, we optimized the operating conditions, including shear rate, initial suspension concentration and exfoliation time. In order to improve the yield of MoS2 nanosheets, the MoS2 suspension was prepared by liquid phase shear exfoliation with sodium citrate (1~5 mg/mL) as assistant under the optimal operating conditions.

Figure 1. Schematic diagram of liquid phase shear-exfoliating MoS2 by high-speed dispersive homogenizer: (a) bulk MoS2 is immersed in NMP, (b) MoS2 is mainly subjected to lateral shear, longitudinal shear, collision and then repeated shear-exfoliating, (c) finally, the successfully shear-exfoliated MoS2 nanosheets are stably dispersed in NMP, (d) introduction to the structure of high-speed dispersive homogenizer. 7



2.3 Characterization The UV-Vis spectrum of the supernatant was measured by a UV-5200 instrument. The morphologies of the MoS2 nanosheets were observed by using a SEM (Hitachi S-4700). The AFM images were measured on a DMFASTSCAN2-SYS instrument. TEM (T20) and HRTEM (JEM3010) were operated with an accelerating voltage of 200 kV. The surface area of the nanosheets was measured by Brunauer-Emmett-Teller (BET, JW-BK200C). Raman spectra were obtained using a Renishaw instrument excited at 633 nm. The XRD measurements used Cu Kα radiation (λ=0.154 nm) at 40 kV and 40 Ma (Shimadzu LabX XRD-6000 diffractometer). FTIR spectra of samples were measured on a Nicolet 8700 spectrometer. The thermal behavior of samples was analyzed using thermogravimetric analysis (TGA) on a Netzsch STA 409PC/PG thermal analyser at a heating rate of 10K min-1 from room temperature to 800℃ in a nitrogen flow. 3. Results and Discussion 3.1 Optimization of operating conditions. During the liquid phase shear exfoliation process, the outer side of the rotor and the inner side of the stator will move relatively while introducing the suspension into the gap. High-speed rotation provides sufficient energy (e.g., shear force and turbulence force) to overcome the weak van der Waals forces between the MoS2 layers, making the sheets mixed evenly and the size decreasing.38, 42 The process parameters have important effects. In this part, the effects of shear rate, initial suspension concentration and exfoliation time were studied in detail. 8


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3.1.1. Shear Rate. In order to study the effect of shear rate on the concentration of MoS2 nanosheets, the MoS2 suspension (40 mg/mL) was exfoliated using the high-speed dispersive homogenizer at different exfoliation speeds for 4 h (2, 4, 6 and 8 krpm/min, the shear rates of these exfoliation speeds were: 209, 419, 628, and 837 s-1, respectively) and sampled at different times for UV-vis characterization. Figure 2a showed the UV-vis spectra of the MoS2 nanosheets supernatant obtained by shear-exfoliating at a shear rate of 419 s-1 (4 krpm/min). The MoS2 nanosheets have two characteristic peaks at 610 nm (B-exciton) and 670 nm (A-exciton) by UV-vis characterization, due to the direct excitation transition at the K-point of the Brillouin zone.32 In order to estimate the concentration of the supernatant, we used Lambert– Beer law to calculate: A/l=αC, where A/l is the absorbance of each cell length, C is the concentration, α is the extinction coefficient. The supernatant concentration C can be measured by filtration and weighing, while eliminating the influence of the background and the extinction coefficient (670 nm) is estimated to be α = 51 mL mg-1 m-1,

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it has been described in detail in a previous article.24, 32, 43, 44 As shown in

Figure 2b, the concentration of MoS2 nanosheets increased from 0.06 to 0.59 mg/mL with the increase of shear rate. We observed that MoS2 nanosheets supernatant was transparent, showing yellow/dark-green in color. The color of the MoS2 nanosheets supernatant darkened gradually with the increase of shear rate, indicating the concentration enhanced continuously (Figure 2b, inset).

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We speculate that the

higher shear rate will form a strong turbulence and vortex, the MoS2 is subjected to a stronger shearing force and have a better exfoliation. At certain shear rate, the 9



concentration of MoS2 nanosheets increases first and then decreases, the reason may be shear-exfoliating MoS2 reaches a certain level and cannot continue to produce more nanosheets. MoS2 nanosheets are more likely to gather and settle under the influence of the repeated shear force and the subsequent centrifugal force. In general, the MoS2 nanosheets concentration increases as the shear rate increases.

3.1.2. Initial Suspension Concentration and Exfoliation Time. To test the effect of initial suspension concentration and exfoliation time on the concentration of MoS2 nanosheets, we prepared a range of dispersions, keeping all but one of the operating conditions constant. The shear rate was fixed at 837 s-1. Figure 2c shows the as-produced MoS2 nanosheets concentration versus exfoliation time at four initial suspension concentrations respectively. When the initial suspension concentration was 20，30 ,40 and 60 mg/mL, the highest concentration of MoS2 nanosheets was up to 0.89 (2 h) ,0.96 (2 h) , 0.59 (40 min) and 0.58 mg/mL (40 min) respectively. This phenomenon suggests that the higher the initial concentration is not always conducive to increasing the MoS2 nanosheets concentration. The reasons may be that the bulk MoS2 are prone to collision and shear in the high concentration suspension, which could shorten the exfoliation time, but MoS2 nanosheets are more likely to aggregate, thereby reducing the yield. Obviously, the optimal initial concentration of the suspension is 30 mg/mL and the optimal exfoliation time is 2 h.

3.1.3. Sodium Citrate Concentration. As mentioned above, some additives can intensify the shear-exfoliating effect. Thus we investigated the effect of species and its concentration on shear exfoliation includes sodium chloride, sodium hydroxide and 10


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sodium citrate. We found that shear-exfoliating effect is stronger after adding sodium citrate than others. Then we prepared MoS2 nanosheets dispersion under different concentration of sodium citrate as shown in Figure 2d. Other parameters keep constant: the suspension initial concentration of 30 mg/mL, shear rate of 837 s-1and exfoliation time of 2 h. As shown in Figure 2d, the concentration of MoS2 nanosheets increased from 0.96 to 1.44 mg/mL with an increase of the sodium citrate concentration from 0 to 5 mg/mL. The MoS2 nanosheets concentration increases directly as the sodium citrate concentration increases. The reason could be that the sodium citrate powder in the suspension increases the degree of turbulence and adsorbs on the surface of the MoS2 to form a composite, which facilitates the exfoliation of bulk MoS2 and the dispersion stability of MoS2 nanosheets in the suspension. According to the above results, the high-speed dispersive homogenizer can successfully prepare MoS2 nanosheets in pure NMP. The optimum operating conditions are as follows: the shear rate is 837 s-1, the initial suspension concentration is 30 mg/mL and the exfoliation time is 2 h. The maximum MoS2 nanosheets concentration is about 0.96 mg/mL in pure NMP. The yield (defined as: Y = Ci/C0; C0: the initial MoS2 concentration; Ci: the MoS2 nanosheets concentration) is 3.2%. In addition, taking into account the problem of costs, the amount of sodium citrate should not be too much. Therefore, the addition of 5 mg/mL sodium citrate is more appropriate. At this point, the concentration of MoS2 nanosheets reached 1.44 mg/mL. The highest yield was 4.8% in this experiment. 11



Figure 2. (a) UV-Vis Spectra of MoS2 nanosheets supernatant exfoliated at different exfoliation time under the shear rate of 419 s-1, (b) The concentration of supernatant varies with exfoliating time at different shear rates Inset: photo of the suspensions, (c) The changes of supernatant concentration with the exfoliation time under different initial MoS2 concentrations, (d) The concentration of MoS2 nanosheets versus the concentration of sodium citrate. 3.2 Characterization Herein, under the optimum operating conditions, the as-prepared MoS2 nanosheets were characterized by SEM, AFM, TEM, HRTEM, Raman, XRD, FT-IR and TGA. To determine the lateral size and thickness of the MoS2 nanosheets, we deposited the MoS2 supernatant on the Si/SiO2 wafer until it was dried in an oven at 50 ° C, then 12


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investigated by SEM and AFM. Figure 3 shows typical SEM images of MoS2, in which above 100 flakes were measured. The lateral size of bulk MoS2 was significantly reduced from 9 µm to 5 µm after liquid phase shear exfoliation. It indicates that bulk sheets were efficiently exfoliated by this method. Moreover, Figure 3c shows that the MoS2 nanosheets in the supernatant (i.e., MoS2 nanosheets-p) had an average lateral size of about 160 nm among which about 70% are 50 to 200 nm. Figure 4a-b shows that the AFM images of MoS2 nanosheets-p. According to the monolayer thickness of MoS2 nanosheets is 0.7 to 1.2 nm, the thickness of MoS2 nanosheets relatively distribute wide by statistical data, while the average layer number of MoS2 nanosheets is about 6 layers.46, 47 In particular, the lateral size of MoS2 nanosheets are smaller and about 118 nm among which about 93% are 50 to 200nm after the addition of sodium citrate (Figure 3d) (i.e., MoS2 nanosheets-sc). Figure 4c-d shows that the AFM images of MoS2 nanosheets-sc, the thickness of MoS2 nanosheets is also relatively thin with narrow distribution and the average layer number of MoS2 nanosheets layers is about 4 layers in which about 9% are monolayer. From the analysis of SEM and AFM, MoS2 must be subject to lateral shear, longitudinal shear and continuous collision at the high-speed region, so that its size and thickness are simultaneously reduced. The results are consistent with our assumed shear-exfoliation mechanism (Figure 1a-c). We also tested the specific surface area and bulk density to confirm the changes of the MoS2 nanosheets after shear-exfoliating. The results show that the specific surface area of MoS2 nanosheets-sc is about 5 times larger than the specific surface area of bulk MoS2, 13



whereas the bulk density of MoS2 nanosheets-sc is smaller than bulk MoS2 (Supporting Information Table S2). We speculate that MoS2 nanosheets-sc become more lose after freeze-drying, resulting in a decrease of bulk density.

Figure 3. Typical SEM images: (a) The bulk MoS2, (b) the MoS2 precipitate and (c) nanosheets supernatant were prepared with the optimum operating conditions in pure NMP (recorded as: MoS2 nanosheets-p), (d) the MoS2 supernatant was prepared by the addition of 5 mg/mL sodium citrate (recorded as: MoS2 nanosheets-sc). (Inset: the histogram of statistical size; : the average lateral size of MoS2).

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Figure 4. Typical AFM images (left) and the histogram of layer number of MoS2 nanosheets calculated from height measurements of AFM images of above 100 flakes (right): (a) (b) MoS2 nanosheets-p, (c) (d) MoS2 nanosheets-sc. (: the average layer number of MoS2 nanosheets). In order to more intuitively characterize the thickness of MoS2 nanosheets, we deposited the MoS2 supernatant on a copper mesh until dried in an oven at 50°C and then observed by TEM and HRTEM. It can be seen from Figure 5a-b that the typical MoS2 nanosheets exhibit a nearly transparent nanomorphology. It indicates that the 15



exfoliated MoS2 nanosheets are relatively thin. Figure 5c-d shows typical HRTEM images of MoS2 nanosheets-sc to further confirm the ultrathin nanosheets morphology and the nanosheets still have a hexagonal symmetrical single crystal structure.

Figure 5. Typical TEM images: (a) MoS2 nanosheets-p, (b) MoS2 nanosheets-sc. Typical HRTEM image of MoS2 nanosheets-sc (c) show that MoS2 nanosheets-sc are single crystalline and top right insets is SAED images (d). In order to further verify the existence of MoS2 nanosheets in supernatant, we prepared the above two kinds of MoS2 nanosheets for Raman analysis. As shown in Figure 6, the bulk MoS2 and MoS2 nanosheets-p appear two distinct peaks at 379 and 405 cm−1, which correspond to the in-plane vibration of the Mo-S bond (E12g) and out-of-plane vibrations (A1g) 13, 32, 37, the peak separation between E12g and A1g is the same (~26 cm-1). However, the characteristic peaks of the MoS2 nanosheets-p are sharper than the bulk MoS2。 Due to the average thickness of MoS2 nanosheets-p is 16


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more than 4 layers, it is not significantly different from the Raman spectra of the bulk MoS2.48 This result further confirms the correctness of the AFM statistics. On the other hand, poor crystallinity, defects and particle size can lead to broadening of Raman peak.36 It is suggested that the MoS2 nanosheets prepared in pure NMP have few defects and good crystallinity. In addition, the peak separation of the MoS2 nanosheets-sc between E12g and A1g (∼24 cm−1) also proves the thickness of MoS2 nanosheets-sc is range from 3 to 6 layers, keeping with the AFM observation.49, 50

Figure 6.Raman spectra of bulk MoS2, MoS2 nanosheets-p and MoS2 nanosheets-sc. Figure 7a-b shows that the XRD spectra of the bulk MoS2, MoS2 nanosheets-p and MoS2 nanosheets-sc samples. There are three diffraction peaks (2θ) appearing at 14.32° (002), 69.08° (201), 69.26° (108) for crystalline 2H-MoS2 according to PDF#65-0160. Comparing with the bulk MoS2, the (002) peak of MoS2 nanosheets-p and MoS2 nanosheets-sc decreases and broadens due to defects and nanoscale of the MoS2 nanosheets. Furthermore, the (201) peak and (108) peak of MoS2 nanosheets are 17



slightly offset, indicating that MoS2 is effectively shear-exfoliated.13, 51 Especially, The (201) and (108) peak of MoS2 nanosheets-p and MoS2 nanosheets-sc are sharper than pristine MoS2. This phenomenon suggests that the as-produced MoS2 nanosheets exhibit a crystal structure diversity.52 The (201) and (108) peaks of MoS2 nanosheets-sc are relatively weakened, which shows that the presence of sodium citrate has a certain influence on the crystal change of MoS2. We used FT-IR spectra to explore the role of sodium citrate. As shown in Figure 7c, MoS2 nanosheets-sc and bulk MoS2 exhibit similar peak around 470 cm-1 which is on behalf of Mo-S vibration.53 In the range of 1200 to 800 cm-1, MoS2 nanosheets-sc exhibit new and sodium citrate-like peaks which are different from the bulk MoS2. Moreover, the peak around 1600 cm-1 for MoS2 nanosheets-sc becomes stronger and wider than bulk MoS2 due to the effect of sodium citrate. The results show that MoS2 and sodium citrate were efficiently combined during the shear-exfoliation process. Figure 7d shows the results of thermogravimetric analysis (TGA), further demonstrating there are sodium citrate absorbed on the surface of MoS2. The TGA spectrum of bulk MoS2 displays negligible weight loss. In contrast, the MoS2 nanosheets-sc displays a small weigh loss (1.83%) with an onset temperature at 170℃ which was identical to the starting degradation temperature of pure sodium citrate, indicative of the loss of sodium citrate. The EDS analysis is also used to determine the elemental composition of the MoS2 nanosheets-sc which contains the elements of Mo, S, Na, C and O, confirming above analysis (Supporting Information Figure S1). The adsorption of

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sodium citrate reduces the surface energy and alleviating the aggregation of MoS2 nanosheets, resulting in the increasement of concentration and yield.

Figure 7. (a) XRD patterns of bulk MoS2, MoS2 nanosheets-p and MoS2 nanosheets-sc, (b) a partial enlargement of (a), (c) FT-IR spectra for bulk MoS2, MoS2 nanosheets-sc and sodium citrate in the range of 400 to 4000 cm-1, (d) TGA curves of bulk MoS2, MoS2 nanosheets-sc and sodium citrate. 4. Conclusion In this work, we used the homogenizer to successfully prepare MoS2 nanosheets and optimized the operating conditions. The highest yield of MoS2 nanosheets is up to 4.3% , the lateral size of nanosheets is centered in 50 ~ 200 nm and the number of MoS2 nanosheets layers is mostly less than 4 layers. By comparing the data 19



summarized in Table S1(Supporting Information), it can be found that the yield of this method is higher than that of other similar shear preparation methods and achieve better results than the rotating packed bed. Although the yield is not as good as ultrasound preparation, this method is easier to be scaled up and also very simple and efficient. ASSOCIATED CONTENT

Supporting Information

Compare the effects of exfoliating MoS2 under different methods, Energy dispersive X-ray spectrum confirming the elemental composition of MoS2 nanosheets-sc and the specific surface area and bulk density of MoS2.

AUTHOR INFORMATION

Corresponding Author

* Wei Wu. E-mail: [email protected]. ORCID

* Wei Wu: 0000-0003-0138-6075

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Nos. 21676023).

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