Preparation of Two Dimensional Atomic Crystals BN, WS2, and MoS2

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Preparation of Two Dimensional Atomic Crystals-BN, WS2 and MoS2 by Supercritical CO2 Assisted with Ultrasonic Yan Wang, Chenghong Zhou, Wucong Wang, and Yaping Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie303633c • Publication Date (Web): 07 Mar 2013 Downloaded from http://pubs.acs.org on March 11, 2013

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Industrial & Engineering Chemistry Research

Preparation of Two Dimensional Atomic Crystals-BN, WS2 and MoS2 by Supercritical CO2 Assisted with Ultrasonic

Yan Wang1, Chenghong Zhou2, Wucong Wang1 and Yaping Zhao1* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China 2

Beijing institute of tracking and telecommunication technology, Beijing, China, 100094, China

Tel: +86-021-54743274; Fax: +86-021-54741297; *E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT A facile and general approach was developed to exfoliate layer materials to produce two dimensional atomic crystals. A single and few layers of BN, MoS2 and WS2 were obtained via directly exfoliating their powder materials using supercritical CO2 assisted with ultrasound. The effects of supercritical CO2 coupled with ultrasound play a key role in exfoliation process. The layer numbers and sizes of the BN, MoS2 and WS2 can be easily controlled by adjusting the power and the time of the ultrasonication. The AFM images suggest single-layered BN, MoS2 and WS2 were produced, respectively. Their lateral sizes are about 0.5~2µm and almost 90% BN sheets are less than 5 layers. The electric diffraction patterns demonstrate the crystallinities of the produced samples were kept as the same as that of the raw material during the exfoliating process. This novel technique is cost-effective and scalable, which can be widely used in production of two dimensional atomic crystals with high quality.

Keywords: boron nitride •layered compounds• molybdenum disulfide •supercritical carbon dioxide •tungsten disulfide • ultrasonic

1. INTRODUCTION With the rapid development of graphene, two-dimensional (2D) materials have caused widely interest in recent years.1-3 Like graphene, 2D atomic crystals will play a crucial role in developing advanced functional materials, such as hexagonal boron nitride (h-BN),

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tungsten disulfide (WS2 ) and molybdenum disulfide (MoS2 )8 2


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called “graphene analogues” which show the similar layered structure with graphene. The “white graphene”-- monolayer h-BN is a promising dielectric because of its remarkable physical properties and chemical stability and it can be used as a complementary two-dimensional dielectric substrate.6 With the improving electron mobility, the few layered MoS2 film is a perfect transistor material and can be used as thin transparent semiconductors like optoelectronics and energy harvesting.9 The monolayer WS2 can also be applied in making field-effect transistors and electronic and optical devices.10 However, up to now, except for graphene, few papers were published for producing high-yield and high-quality few layered 2D materials. Few-layered hexagonal BN was obtained by using a thermal catalytic chemical vapor deposition (CVD) method,9 but the processing temperature need be higher than 1000 °C. Single-layer BN sheets was reported to be made by plasma etching,5 but this method would destroy the material internal structure on account of bringing in some chemical reagent and would not be feasible for fabrication of large amounts. Single-layer MoS2 and WS2 were reported to be obtained by the method of using n-butyl lithium in hexane as the intercalation agent to insert lithium ions into the layered structures following by exfoliation in water with ultrasonication.8,11 The drawbacks of this method are not only the long reaction time and high reaction temperature but also the lack of controllability of lithium insertion.12 Like the methods of making graphene, few-layer nanosheets of graphene analogues BN, MoS2, WS2 were produced by mechanical cleavage.13,14 Recently liquid exfoliation of layered materials for making two dimension atomic materials was reported.15 3



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However, these methods are either time-consuming, high cost, or not scalable. So it is highly demanded to explore efficient approaches to produce them with high quality, cost-effective and large quantities. Here, we demonstrated a general technique of supercritical CO2 assisted with ultrasound for producing 2D layer materials through exfoliation of BN, WoS2, MoS2. This facile method can be scaled up to prepare large amount of exfoliated materials. Supercritical CO2 has been widely applied in many fields because of its various good features, such as low interfacial tension, low viscosity, outstanding wetting of surfaces and high diffusion coefficients.16 Recently, we developed a novel approach for producing nanoparticles by combining supercritical CO2 with ultrasound.17,18 The ultrasound intensifies mass transfer and atomization in supercritical CO2. Being inspired by this approach, we investigated in this article the exfoliation of layer materials to make two dimensional atomic crystals using supercritical CO2 assisted with ultrasonic.

2. EXPERIMENT SECTION

2.1. Materials

Boron nitride (BN, Strem, Massachusetts, USA), molybdenum disulfide (MoS2, Alfa Aeser, Massachusetts, USA), and tungsten (IV) disulfide (WS2, Aladdin, Shanghai, China) were purchased from SJTU store. Absolute ethanol was purchased from the SJTU store. CO2 with 99.5% was purchased from the SJTU store.

2.2 Exfoliation device and process 4


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Exfoliation device was designed and fabricated at our lab. It is mainly consisted of two parts. One is the high pressure vessel with a visual window, which can be pressured up to 35MPa. The other is ultrasonic probe whose power can be adjusted. The schematic drawing of the device is shown in Figure 1. All experiments were carried out in the device shown in Figure 1 whose volume is 200 mL into which 100mg powder material was added. Then the device was heated up to 45°C. After that CO2 was added into it up to 10MPa. After the pressure and temperature of the device reached the preset ones the ultrasonic was started to run under the preset power (60 W for BN, 12 W for MoS2 and WS2) for 40min. The pressure and the temperature were kept at 10MPa and 45 °C, respectively, during the exfoliation process. After ultrasonicaion for 40 min, the exfoliated products were sprayed with CO2 either into the absolute ethanol solvent or onto the substrate for further analysis.

Figure 1. Schematic drawing of the exfoliation device

2.3 Characterization

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Optical images of the samples were recorded with DM4000 Optical Microscope. AFM measurements were carried on Environment Control Scanning Probe Microscope instrument, SII Nanonavi E-Sweep. Samples for AFM measurements were prepared by dropping BN, MoS2 and WS2 solutions onto mica substrate, respectively. TEM and high-resolution TEM analyses were performed on JEOL JEM-2100F. A drop of a solution containing the produced 2D nano materials was placed on a holey carbon-coated copper grid and then dried in air prior to its characterization by transmission electron microscopy. Raman spectra of BN, WS2samples were recorded with a 532 nm laser in a Renishaw inVia-reflex spectrometer and those of MoS2 samples were obtained with a 632 nm laser in a Jobin Yvon labram-1b spectrometer.

3. Results and Discussion

The exfoliation process is shown in Figure 2. When the temperature and pressure reach the critical conditions of CO2, supercritical CO2 will diffuse easily among the layer material interspace owing to its high diffusivity, zero interface force, low viscosity and small molecule size (smaller than the space of between the layer materials). The sonication irradiation weakens the van der Waals inter planar interactions. The supercritical CO2 deposited in the layer interspace further expands the adjacent layer distance and finally delaminates the layers into a single or few layers under the cooperation with ultrasound. Then single layer or/and few layer materials can be formed. 6


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Figure 2. The exfoliation schematic of two dimensional atomic crystals with supercritical CO2 assisted with ultrasonic.

Figure 3 shows the digital photos of the ethanol solutions of the exfoliated materials of BN, MoS2 and WS2, respectively. We can see clearly these exfoliated materials can be dispersed in ethanol. They were able to be stable for over several hours and longer time depending on their concentrations ( they are not shown in this paper). The dispersed samples are also used for AFM characterization.

Figure 3. Digital photographs of the ethanol solutions of the exfoliated materials of BN, MoS2 and WS2, respectively Figure 4 is the high-resolution optical microscope images of BN, MoS2 and WS2 sheets. The layer numbers of the samples can be identified by the colour and contrast 7



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variations between the layers and the SiO2 substrate with a 300nm thick SiO2 layer using the optical microscope images19. We can clearly see that the contrasts between the samples and substrates are low and the colour of the samples on the substrates is almost the same (Figure 3 a-c), indicating that few layers were formed with the uniform layer numbers.

Figure 4. Optical microscope images of the exfoliated samples of (a) BN, (b) MoS2, and (c) WS2 sheets. AFM is a powerful non-destructive tool for precisely identifying the layer number via measuring the height of the sample. Figure 5 displays typical tapping-mode AFM images of the exfoliated samples deposited on a mica substrate. Figure 5(a) shows that the step of the exfoliated BN sheets is 0.5nm height close to the theoretical thickness value of single layer BN 0.34nm. Figure 5(b) and (c) reveals the MoS2 and WS2 sheets have an average thickness of 0.75~0.8nm, which are close to the theoretical thickness value of MoS2 (0.61nm) and WS2 (0.65nm), respectively. They are a little thicker than the theoretical thickness of a monolayer because an instrumental offset of ~0.5 nm (caused by different interaction forces) always exists 20. So the AFM images suggest single-layered BN, MoS2 and WS2 have been formed. Their lateral sizes are about 0.5~2µm. We carefully chose 131 pieces over the BN sheets to calculate the layer number statistically. As shown in Figure 5(d), almost 90%

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BN sheets are less than 5 layers, among which about 20% are single layer, 40% are two layers.

Figure 5. AFM images and associated with height profiles of (a) BN layers.( b) MoS2 layers. (c) WS2 layers. (d) Histogram of the number of visual observations of flakes as a function of the number of BN monolayers per flake. The TEM images of all three exfoliated materials are displayed in Figure 6(a) -(c), including the typical elective electron area diffraction patterns (ED). The results inferred from the TEM images are consistent with that inferred from the AFM. Figure 6 reveals that the exfoliated BN, MoS2 and WS2 samples contain few layers. The fold morphology of the films can be seen clearly. ED patterns (inset in Figure 6(a) to (c), respectively) demonstrate the hexagonal symmetry lattices of these samples with excellent crystallinity, which confirmed no distortions in the products through exfoliating process.

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Figure 6. Low-resolution and High-resolution TEM images of flakes of BN, MoS2 and WS2, respectively,(a) to (c). (Insets): Typical electron diffraction pattern.

Raman spectroscopy is a good candidate for quick and nondestructive inspection of the thickness of 2D crystal compounds.6, 21- 23 The Raman spectra of BN, MoS2 and WS2 samples are compared with that of their bulks, respectively, as shown in Figure 7. The analysis samples were prepared by directly spraying the exfoliated products with CO2 of the device onto the SiO2 wafer. Through the Raman mapping of BN，MoS2 and WS2 shown in Fig 7(a)-(c), respectively, we can also confirm that the few-layered sheets were successfully produced. A dominant peak near 1370 cm-1 as shown in figure 7(a) often appears intensely for both the bulk and the few-layer samples because of the B-N vibrational mode (E2g) within the BN layers 24. The Raman peak frequency of the E2g mode shifts to higher frequency since an increase of stress in the few-layer BN, which may be generated by the intrinsic surface wrinkles and the substrate interaction 6. Compared the bulk BN with the few-layer BN shown in Figure 7(a), the E2g mode shifts to higher frequency from 1364 to 1366.77 cm-1. The peak shift range is about 2.77cm-1. Compared the Raman spectra of the exfoliated MoS2 and WS2 samples with that of the bulk MoS2 and WS2 shown in Figure 7(b) and (c), we can see that there are no additional bands observed and no bands dramatically shifted. The bulk MoS2 samples shows bands at 406.51(A1g)and 377.25(E2g)cm-1 while the few-layered MoS2 exhibits corresponding bands at 403.07 and 375.53 cm-1 .Compared to the narrow bands at 351.09(E2g) and 419.73(A1g) cm-1 of the bulk 10


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WS2, the spectrum of few-layer WS2 shows bands at 349.49 and 418.14 cm-1, respectively. However, the intensities of the bands become significantly weak and the A1g and E2g modes of MoS2, WS2 are broadened. It can be clearly seen from the Raman map that the full-widths at half maximum (FWHM) values are obviously wider in the obtained products (>10cm-1) than the bulk samples which are considered to be due to phonon confinement by facet boundaries.24 All of the data above accord with that in the published paper 8 . Raman spectra of these samples show progressive softening of the E2g and A1g bands with decreasing number of layers, which suggest that few-layered MoS2 and WS2 were successfully prepared.

Figure 7. Raman spectra of bulk BN and few layers BN (a), bulk MoS2 and few layers MoS2 (b) and bulk WS2 and few layers WS2 (c).

4. Conclusions In conclusion, we have developed a facile and efficient method to exfoliate layer materials to produce single and few layers of BN, MoS2 and WS2, respectively, using supercritical CO2 assisted with ultrasound. The ultrasound intensifies mass transfer and weakens van der Waals force between the adjacent layers. The coupled effect of supercritical CO2 cooperated with ultrasound plays a key role in exfoliation process. The pressure, temperature, ultrasound power and processing time impact the 11



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delamination. With this technique, graphene inorganic analogues films of BN, MoS2 and WS2 can also be formed, respectively, by spraying them dispersed in supercritical CO2 directly on the substrate without any solvent remains. We believe that this approach can be applied not only to h-BN, MoS2 and WS2 but also to TMDs, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, and Bi2Te3, etc. layer materials. Then, this technology will widely be applied in preparation of the 2D atomic crystals and boost their application. ACKNOWLEDGMENTS There is no potential conflict of interest for this article. We are thankful to the Instrumental Analysis Center of SJTU for assistance on AFM, TEM and Raman analysis. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morosov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.

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Preparation of Two Dimensional Atomic Crystals BN, WS2, and MoS2

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