Preparation of Two-Dimensional Molybdenum Disulfide Nanosheets

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Preparation of Two-Dimensional Molybdenum Disulfide Nanosheets by High-Gravity Technology Xianglu Yin, Yuewei Li, Wei Wu,*,† Guangwen Chu, Yong Luo, and Hong Meng*,‡ †

State Key Laboratory of Organic−Inorganic Composites and ‡College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: In this paper, a high-gravity technology was described to prepare two-dimensional molybdenum disulfide nanosheets from bulk materials. This high-gravity field, producing by a rotating packed bed, provides shear force and collision force to exfoliate layered bulk molybdenum disulfide into nanosheets. The effects of rotational speed, surfactant concentration, exfoliation time, packing volume fraction, and exfoliation time on the concentration were systematically studied. The results showed that the concentration of the asprepared suspension could be easily controlled. The concentration is 0.37 mg/mL under optimum conditions. Moreover, the dispersion of two-dimensional molybdenum disulfide nanosheets could be stabile for a long time. The lateral size of asproduced molybdenum disulfide nanosheets ranged from 50 to 800 nm, and the mean lateral size was 380 nm. Almost 84% of nanosheets were less than five layers. We also demonstrated that high-gravity technology also can be applied to tungsten disulfide. This technology provides a facile, efficient, and universal approach to prepare two-dimensional materials from layered bulk precursors.

1. INTRODUCTION Recently, due to the expanding interest in two-dimensional layered materials, two-dimensional molybdenum disulfide (MoS2),1 the so-called “graphene analogue”, has received intensive attention because of its combination of exciting physical properties and potential applications in nanoelectronics,2 optoelectronics,3 transistors,4 flexible devices,5 lubricant additives,6 etc. To promote the large-scale application of two-dimensional MoS2 nanosheets, it is necessary to develop a scalable and ecofriendly preparation method. Up to now, there are many methods to prepare two-dimensional MoS2 nanosheets, mainly including mechanical exfoliation, liquid phase exfoliation, chemical vapor deposition, and intercalation exfoliation. Among them, liquid-phase exfoliation is the most suitable way to produce two-dimensional MoS2 on a large scale.7 Twodimensional MoS2 nanosheets produced by liquid-phase exfoliation tend to be free of basal plane defects,11 which are suited to produce functional structures such as thin films,12 layered hybrid,13 and nanocomposites.14 In this exfoliation process, the bulk MoS2 sheets are usually ultrasonicated in stabilizing liquids, such as suitable solvents8 or aqueous surfactant solutions9 or polymers,10 and the weak van der Waals forces between sheets are disrupted. Thus, twodimensional MoS2 nanosheets were obtained and stabilized against aggregation via the interaction with the liquid.8 However, ultrasonic exfoliation is a comparatively intense process that can generate high local temperatures, extreme pressure, and a rapid heat transfer rate. These strict conditions could be damaging to flakes. Thus, two-dimensional MoS2 © XXXX American Chemical Society

produced by sonication has been verified to have many more defects and smaller size.15 Moreover, bulk MoS2 sheets cannot be efficiently exfoliated, and many MoS2 flakes precipitate to the bottom of the reaction vessel without exfoliating because the position of the sonication-induced cavitation is almost static, resulting in a low production rate. Thus, an efficient method to produce two-dimensional MoS2 is urgently desired. Fluid dynamics is a novel shear exfoliation method for preparing two-dimensional nanosheets that mainly include use of a vortex fluidic device,16 pressure-driven fluid dynamics,17 and mixer-driven fluid dynamics.18 In this process, the bulk sheet moves with the high-speed liquid and can be exfoliated at different positions. This technology is a promising candidate for the production of less-defective nanosheets in larger quantity. However, preparing two-dimensional MoS2 nanosheets with fewer defects on a large scale is still expected. A rotating packed bed is a novel high-gravity device that can generate a high gravity field of several orders of magnitude as a result of the large centrifugal force created by the rotating packed bed. It can create strong homogeneous and controllable shear force and thus greatly intensify micromixing and mass transfer processes.19 We had used it to exfoliate graphite oxide to prepare graphene oxide.20 In this paper, we use a rotating packing bed to exfoliate bulk MoS2 to prepare stable dispersions of two-dimensional MoS2 sheets. It was demonReceived: Revised: Accepted: Published: A

January 3, 2017 March 24, 2017 April 5, 2017 April 5, 2017 DOI: 10.1021/acs.iecr.7b00030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

3. RESULTS AND DISCUSSION During the exfoliating process, the suspension quickly goes through the meshy packing as a thin film laminar flow in the surface of packing and numerous small droplets in the space packing due to the intense centrifugal force and shear force under high rotator speed.21 The thickness of the liquid thin film and the size of droplets can be either micrometer or nanometer.22 When MoS2 flakes moved with the liquid, they mainly suffer from two forces on their surface as shown in Figure 1: shear force and collision force. The shear force, producing by a velocity gradient and parallel to the surface of the MoS2 sheets, can conquer the weak van der Waals force attraction between sheets and obtain monolayer or few-layer MoS2 from bulk sheets. The collision effects favor an efficient fragmentation and exfoliation for MoS2 flakes.23 Compared with exfoliation by sonication or high-shear mixer in large quantity, this method possesses an obvious advantage that the rotating packed bed provides a homogeneous force field for exfoliating MoS2 sheets. 3.1. Effects of Process Parameters on the Concentration. In order to understand how to control the amount of two-dimensional MoS2 produced and to optimize the conditions of the process, we prepared a series of dispersions, controllably varying five exfoliation parameters, including rotational speed, TX-100 concentration, exfoliation time, packing volume fraction, and initial MoS2 concentration. All experiments were carried out at room temperature. 3.1.1. Rotational Speed. The most important and straightforward exfoliation parameter is the rotational speed due to the force field depending on it. We investigated its influence on the concentration of two-dimensional MoS2. The others parameters were fixed. With a bulk MoS2 concentration of 40 mg/mL, TX-100 concentration of 30 mg/mL, packing volume fraction of 100%, and exfoliation time of 10 h, the concentration decreases from 0.75 to 0.21 mg/mL with an increase of the rotational speed from 500 to 2500 rpm, as shown in Figure 2a. The higher rotational speed generates a more intense force field, and more sheets of MoS2 were exfoliated into nanosheets. Thus, the concentrations increase with the rotational speed. 3.1.2. Triton TX-100 Concentration. The TX-100 plays an important role as surfactant in exfoliating bulk MoS2 sheets. It can assist in exfoliating sheets and stabilize MoS2 nanosheets. It is necessary to be certain about the effect of surfactant concentration and optimize the amount of surfactant required before scaling up this process. To do this, we prepared a number of two-dimensional MoS2 dispersions in different surfactant concentrations. The others parameters were fixed: bulk MoS2 concentration of 40 mg/mL, rotational speed of 2000 rpm, packing volume fraction of 100%, and exfoliation time of 10 h. Figure 2b shows the concentration of MoS2 nanosheets versus the concentration of TX-100. As the surfactant concentration is increasing, the concentration of nanosheets increased up to 37 mg/mL and then down slightly. When the surfactant concentration was low, the increase of surfactant concentration could improve the coverage of surfactant molecules per nanosheets. Thus, the MoS 2 concentration was increased with surfactant concentration. However, when this surfactant concentration reached critical concentration, the surfactant coverage peak stopped increasing. Then the MoS2 concentration reached its peak value. As the surfactant concentration was increasing further, the micelle

strated that the as-prepared MoS2 nanosheets had only a few layers. This method is a promising candidate for large-scale production. In order to prove the scalability and applicability of this method, we also applied the optimum conditions used for MoS2 to bulk WS2 and obtained two-dimensional WS2.

2. EXPERIMENTAL SECTION 2.1. Materials. Molybdenum disulfide with an average lateral size 2 μm was purchased from Shanghai Shen Yu Co., Ltd., Shanghai, China. Tungsten disulfide with an average lateral size 2 μm was purchased from Shanghai Zai Bang Co., Ltd., Shanghai, China. These materials have not been modified. Triton TX-100 [chemical pure reagent (CR)] and absolute ethanol [analytical pure regents (AR)] were purchased from Beijing Chemical Works. 2.2. Methods. The exfoliation process for preparing the two-dimensional MoS2 nanosheets mainly consisted of two parts, as showed in Figure 1. One is the rotator with packing,

Figure 1. Exfoliation process and mechanism of preparing twodimensional nanosheets.

which can produce a high-gravity environment by rotating and serve as the flow channel. The other is the motor, which could provide precise power for rotation. In the experiment, the total suspension volume was 350 mL, comprising 43 vol % absolute ethanol and 57 vol % water while the surfactant was Triton TX100. The rotation speed of the rotating packed bed could be controlled from 500 to 2500 rpm. This suspension flowed through the porous packing of the rotating packed bed, entering and exiting through an inlet and outlet point, respectively. The outlet suspension, recirculated into the rotating packed bed for a certain time by a peristaltic pump, was collected. The suspension was centrifuged at 4000 rpm for 20 min to remove bulk MoS2 sheets in all experiments. Finally, the stabilized dispersion with two-dimensional MoS2 nanosheets was obtained. 2.3. Characterization. UV−vis spectra of the dispersion were measured by a UV-5200 instrument. Raman spectra were collected by a Renishaw instrument excited at 514 nm. TEM (T20) and HRTEM (JEM3010) were operated with an accelerating voltage of 200 kV. Two-dimensional MoS 2 nanosheets were deposited on a copper microgrid covered with a carbon film. AFM studies were carried out in tapping mode on a DMFASTSCAN2-SYS instrument after depositing a drop of the diluted dispersions on a freshly prepared Si/SiO2 wafer. B

DOI: 10.1021/acs.iecr.7b00030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Effects of exfoliation parameters on the concentration of two-dimensional MoS2 concentration: (a) rotational speed, (b) surfactant TX-100 concentration, (c) exfoliation time, (d) packing volume fraction, and (e) initial bulk MoS2 concentration.

may be that the collision force is greater than the shear stress. The as-produced MoS2 nanosheets are smaller than the low packing volume fraction. These small nanosheets adsorbed on the surface of bulk sheets and were removed. 3.1.5. Initial MoS2 Concentration. The effect of initial MoS2 concentration on the two-dimensional MoS2 concentration was studied at 20, 40, and 60 mg/mL. The other conditions were kept constant: rotational speed of 2000 rpm, packing volume fraction of 50%, TX-100 concentration of 37 mg/mL, and exfoliation time of 4−14h. Figure 2e showed the changes of nanosheet concentration with the exfoliation time under different initial MoS2 concentrations. It showed that an optimization of the initial concentration was 40 mg/mL. This could be because the surfactant molecules interacted with the bulk MoS2 sheets up to a coverage peak and showed a high concentration. The results showed that the concentration of MoS2 nanosheets can be controlled by adjusting the process parameters, and the optimum conditions are an initial MoS2 concentration of 40 mg/mL, a surfactant concentration of 37 mg/mL, a rotation speed of 2000 rpm, a packing volume fraction of 100%, and an exfoliation time of 14 h, where the concentration of nanosheets reached its peak. In our next work we will scientifically research the change of lateral size and layer number with these parameters. 3.2. Characterization. On the basis of section 3.1, we prepared two-dimensional sheets under the optimum con-

concentration grows, increasing the importance of the depletion interaction. This destabilizes the surfactant-coated nanosheets, leaded to a decreased dispersed concentration.24 3.1.3. Exfoliation Time. In order to characterize the effects of exfoliation time on the concentration of MoS2 nanosheets, we prepared a number of samples and recorded the concentration changes for a bulk MoS2 concentration of 40 mg/mL, rotational speed of 2000 rpm, packing volume fraction of 100%, and TX100 concentration of 37 mg/mL. Figure 2c showed a line chart of concentration versus exfoliation time. The concentration increased from 0.1 to 0.27 mg/mL with an increase of the exfoliation time from 4 to 14 h. However, the concentration was reduced when the exfoliation was continued. In the course of the exfoliation, the size of the nanosheets was gradually decreased, and the level of agglomeration was increased. Thus, a part of nanosheets was removed during centrifugation and the concentration is decreased. 3.1.4. Packing Volume Fraction. Packing is a hardcore in a rotating packed bed and provides a channel and force field for exfoliating bulk sheets. Thus, it is important to study the effect of the packing volume fraction on the MoS2 nanosheets concentration. The others parameters were fixed, such as the bulk MoS2 concentration of 40 mg/mL, rotation speed of 2000 rpm, TX-100 concentration of 37 mg/mL, and exfoliation time of 14 h. The concentration increased from 0.22 to 0.37 mg/mL with an increase of the packing volume fraction from 17% to 50%, as shown in Figure 2d. When the packing volume fraction exceeded 50%, the concentrations were reduced. The reasons C

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Figure 3. Exfoliated MoS2 nanosheets: (a) digital photograph, (b) extinction spectra at different exfoliation times, (c) the concentration after centrifugation as a function of exfoliation time, and (d) the concentration plotted versus standing times.

ditions. The production was characterized by UV−vis, TEM and HRTEM, and AFM. Figure 3a shows a digital photo of the as-produced dispersion of two-dimensional MoS2 nanosheets. These nanosheets can be well dispersed to produce a dark-green solution. The UV−vis extinction spectra of the dispersions after exfoliation 4, 6, 8, 10, 12, 14, 16, and 18 h were measured as shown in Figure 3b. Two major peaks are evident approximately at 671 and 610 nm in each spectrum, indicating the existence of two-dimensional MoS2 nanosheets in these dispersions.25 This result suggests that a rotating packed bed can provide force enough to transform bulk MoS2 into two-dimensional MoS2 nanosheets. In addition, the optical extinction also can be used to estimate the concentration of well-dispersed nanosheets at a given wavelength. The extinction consisted of the absorbance and scattering of flakes [Ext(λ) = Abs(λ) + Sca(λ)]. The scattering strongly depends on the dimensions of the nanosheets due to edge and confinement effects, so the information cannot be directly extracted from the optical spectra. Here we subtracted the scattering background from the high-wavelength region (dashed line shown in Figure 3b).26 According to the Lambert−Beer law, A/l = αC, where A/l is the absorbance per cell length, C is the dispersed concentration, α is the extinction coefficient, and the value is 51 L mg−1 m−1 at 671 nm after subtraction of scattering background in our experiment. The concentration of dispersions is shown in Figure 3c as a function of exfoliation time. The concentration of dispersion steeply increased in the first 14 h and then leveled off. The maximum concentration is about 0.37 mg/mL. The yield (defined as Y = C/Ci, where Ci is the initial MoS2 concentration and C is the MoS2 nanosheet concentration) is 0.9%. In other words, the reactor produces 9 mg/h. Figure 3d confirmed the stability of the dispersion, since its concentration was maintained at 84 wt % of the initial concentration after a period of 900 h.

Raman spectroscopy of dispersions was applied to further confirm the nature of these nanosheets. Figure 4 shows

Figure 4. Raman spectra of the dispersion and bulk MoS2.

representative Raman spectra of a dispersion and bulk MoS2 excited at 514 nm. There are two characteristic peaks of ∼382 and ∼408 cm−1, which correspond to E12g and A1g vibration modes of MoS2, respectively. This data indicates that asproduced MoS2 nanosheets maintain the trigonal prismatic coordination of bulk MoS2.25 This also signifies that no chemical reaction (such as oxidation) occurred during the shear-exfoliation process. TEM and HRTEM characterizations were performed on the dispersion to further analyze the quality and dimensions of asprepared MoS2 nanosheets. Figure 5a shows TEM images of typical flakes. From Figure 5a, it can be seen that there are abundant thin nanosheets, verifying that the bulk MoS2 was D

DOI: 10.1021/acs.iecr.7b00030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. (a) Low-magnification TEM images of the prepared MoS2 nanosheets. (b) Histograms showing lateral size statistics for the length of exfoliated MoS2 nanosheets measured by TEM images. (c and d) The high-resolution TEM images; (inset) a typical electron diffraction pattern.

Figure 6. (a) AFM image of MoS2 nanosheets deposited on a Si/SiO2 wafer. (b) Height profile along the red line. (c) Histogram of the layer number of MoS2 nanosheets deduced from height measurements of AFM images of ∼100 flakes.

In order to directly confirm that the MoS2 nanosheets was produced and to precisely identify the layer number of exfoliated MoS2 nanosheets, the exfoliated dispersion was deposited on a Si/SiO2 wafer and analyzed by atomic force microscopy (AFM). Figure 6a shows the outline of typical asexfoliated MoS2 nanosheets in an AFM image. Figure 6b shows the height profile of the MoS2 nanosheets along the red line in Figure 6a, and the thickness of these flakes is about 5 nm. Because the theoretical thickness of a monolayer is about 0.65− 1 nm,27 it can be considered that the MoS2 nanosheets are less than 10 layers. The height profile of MoS2 nanosheets also

efficiently exfoliated in the rotating packed bed. About 100 cross-sectional flakes in the TEM images were measured, and the results are presented in Figure 5b as a histogram. The nanosheets measured are between 50 and 800 nm with a mean size of 380 nm in length. Figure 5c,d shows high-resolution TEM images of the nanosheets. It is evident that these twodimensional MoS2 nanosheets are three to five layers. The typical electron diffraction pattern clearly indicates that nanosheets have a hexagonally symmetric structure with excellent crystallinity. E

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Figure 7. Exfoliated WS2 nanosheets: (a) Digital photograph, (b) extinction spectra at different exfoliation times, and (c and d) TEM images of the prepared WS2 nanosheets.

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indicates that these flakes have a relatively flat and steep terrace, suggesting that bulk MoS2 has been exfoliated into twodimensional MoS2 nanosheets. We randomly chose 114 MoS2 nanosheets from the AFM image and measured, as shown in Figure 6c, that almost 84% of the MoS2 nanosheets are less than five layers, among which about 22% are two layers and 44% are trilayered. 3.3. Other Two-Dimensional Materials. In this experiment, we have described a novel method to produce a dispersion of surfactant-stabilized MoS2 nanosheets by highgravity technology. To prove the scalability and applicability, we applied the optimum conditions used for MoS2 to bulk WS2. Figure 7a shows a digital photo of a WS2 dispersion prepared in this way. To ascertain the nature of the dispersed material, Figure 7b shows the UV−vis extinction spectra of the dispersions after exfoliation for 6, 10, and 14 h. The spectra show features expected for WS2. The excitonic transitions can be easily observed, as for MoS2.28 Figure 7c,d shows typical TEM images of the dispersion. It shows that the dispersion contains WS2 nanosheets. Meanwhile, graphite oxide and graphite were successfully exfoliated in our experiment.20 We think this method can exfoliate layered materials that are relatively soft and have a specific gravity that is relatively small.

AUTHOR INFORMATION

Corresponding Authors

*W.W. e-mail: [email protected]. *H.M. e-mail: [email protected]. ORCID

Wei Wu: 0000-0003-0138-6075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21376025 and 21676023). REFERENCES

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DOI: 10.1021/acs.iecr.7b00030 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX