- Email: [email protected]
Production of mono- to few-layer MoS2 nanosheets in isopropanol by a salt-assisted direct liquid-phase exfoliation method
Journal of Colloid and Interface Science 515 (2018) 27–31
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Production of mono- to few-layer MoS2 nanosheets in isopropanol by a salt-assisted direct liquid-phase exfoliation method Huaizhi Liu a, Liao Xu a, Weixu Liu a, Bo Zhou a, Yinyan Zhu a, Lihua Zhu b, Xiaoqing Jiang a,⇑ a b
Jiangsu Key Laboratory of New Power Batteries, College of Chemistry and Materials Science, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, PR China Department of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
g r a p h i c a l a b s t r a c t Preparation of MoS2 nanosheets in isopropanol with salt as assistant.
a r t i c l e
i n f o
Article history: Received 15 November 2017 Revised 29 December 2017 Accepted 5 January 2018 Available online 6 January 2018 Keywords: MoS2 nanosheet Salt-assisted exfoliation Liquid-phase exfoliation Potassium ferrocyanide
a b s t r a c t Here, we report a facile salt-assisted direct liquid-phase exfoliation method for mass production of MoS2 nanosheets. We choose organic solvent isopropanol (IPA) as exfoliation media and potassium ferrocyanide, potassium sodium tartrate, or sodium tartrate as salt, the assistant. The selected salts show universal and efficient assistant effect for the exfoliation of MoS2 in IPA. Especially, potassium ferrocyanide (K4Fe(CN)6) can enhance the exfoliation efficiency up to 73 times and a dispersion of MoS2 nanosheets with concentration as high as 0.240 mg mL 1 can be easily obtained in IPA-K4Fe(CN)6 system. Transmission electron microscopy, atomic force microscopy (AFM), and Raman spectroscopy show that bulk MoS2 has been successfully exfoliated into mono- to few-layer MoS2 nanosheets. AFM analysis indicates that nearly 60% flakes are monolayer in MoS2 dispersion. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Over the past few years, the study of layered two-dimensional (2D) nanomaterials has become one of the most key areas of mate⇑ Corresponding author. E-mail address: [email protected] (X. Jiang). https://doi.org/10.1016/j.jcis.2018.01.023 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.
rials science. Among these layered 2D nanomaterials, mono- and few-layered molybdenum disulfide (MoS2), a typical representative of transition metal dichalcogenides, has recently attracted much attention due to the novel and unique properties in electronics, optoelectronics, photonics, catalysis, energy storage, and solar cells [1–7], upon exfoliation from its bulk counterparts. Substantial efforts have been devoted to the mass production of MoS2
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H. Liu et al. / Journal of Colloid and Interface Science 515 (2018) 27–31
nanosheets. At present, however, it is still a big challenge to produce massive quantities of MoS2 nanosheets of high quality in a commercially viable way. Generally, mono- to few-layer MoS2 nanosheets were prepared through either a bottom-up method such as chemical vapor deposition (CVD) [8], or a top-down method such as micromechanical cleavage [9], chemical lithiumintercalation exfoliation [3], and electrochemical lithiumintercalation exfoliation [10]. However, CVD method involves harsh condition (high temperature and high vacuum); lithiumintercalation method fails to keep MoS2 nanosheets structure integrity; micromechanical cleavage method is quantity-limited [6,11]. To overcome these disadvantages, in 2011 Coleman and co-workers introduced a new technique, liquid-phase exfoliation (LPE) method to prepare MoS2 nanosheets [12]. LPE is a potential effective method for the mass production of MoS2 with structure of mono- to few-layer nanosheets. A series of organic solvents, such as N-methyl-pyrrolidinone (NMP), dimethylsulphoxide, and dimethylformamide, were favored in both exfoliation and dispersing of MoS2 [12]. However, most of these solvents are toxic and have a high boiling point. To facilitate the application and reduce pollution, some less toxic solvents with a low boiling point, for example, ethanol and isopropanol (IPA), were selected as exfoliation media. However, the maximum concentrations obtained in these pure solvents are commonly very low. The concentration was as low as 0.0014 mg mL 1 in ethanol [13] and 0.0035 mg mL 1 in IPA [14]. Such a concentration is too low to meet the practical requirement. Long time sonication may increase the concentration. For example, Coleman and co-workers once increased the concentration of MoS2 in NMP to 40 mg mL 1 after 140 h sonication [15]. However, such time- and energy-consuming process is not suitable for application in practice. Here, we demonstrate that the exfoliation efficacy of MoS2 in IPA can be dramatically and easily increased by just adding some common and cheap salts into IPA. This method is in part inspired by our previous work, in which a salt-assisted method was successfully developed to exfoliate graphite into high-quality graphene [16]. In this work, with this salt-assisted exfoliation method, the concentration of MoS2 in IPA can be increased up to 0.240 mg mL 1, as many as 73 times compared with those in pure IPA without salts. This method is simple, environment-friendly, effective, and economical. 2. Experimental 2.1. Materials MoS2. powder was purchased from Aladdin (particle sizes < 2
lm), potassium ferrocyanide (K4Fe(CN)63H2O) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Potassium sodium tartrate (C4H4O6KNa4H2O) and sodium tartrate (C4H4O6Na22H2O) were purchased from Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd. IPA was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade. 2.2. Methods Different amount of MoS2 powder was mixed with different amount of one of K4Fe(CN)63H2O, C4H4O6KNa4H2O, or C4H4O6Na22H2O. Then the mixture was added into a centrifugation tube containing 3.0 mL IPA, and sonicated in an ultrasonic cleaner (DTC-15, 40 kHz, 200 W) for 2 h at ambient temperature. After sonication, the exfoliated solution was centrifuged at 3000 rpm, relative centrifugal force (RCF) = 850g, for 20 min using a centrifuge (Hettich Universal 320) to remove the unexfoliated or thick flakes
of MoS2. Afterwards, the top 4/5 of the dispersion was collected carefully, then centrifuged at 6500 rpm (RCF = 4010g) for 5 min, and the sediment was collected. The sediment was washed with 10 mL deionized water by sonicating for 2 min, then centrifuged at 6500 rpm for 5 min, and re-collected. The wash process was repeated for five times to remove salt and then the sediment was re-dispersed into 3.0 mL fresh IPA by sonication for 5 min. The final obtained MoS2 dispersions were deep brown green, stable at least for 30 days without aggregation, and were used for different characterizations. 2.3. Characterization Concentration of the final suspension of MoS2 nanosheets was measured by using UV–Vis spectrometer (TU-1900) with quartz cuvettes (path length 0.5 or 0.1 cm) at 672 nm [17]. It should be mentioned that the UV–Vis spectra of all the suspensions of MoS2 nanosheets obtained from different IPA-salt system were measured with the corresponding IPA-salt solution as blank (prepared with the same exfoliation process to that of the MoS2 sample but without adding MoS2 powder). Transmission electron microscopy (TEM, Hitachi-7650) was used to examine the morphology of the exfoliated MoS2 nanosheets. A drop of the MoS2 dispersion was casted on a holey carbon grid (400 meshes) for the TEM measurement. Atomic force microscopy (AFM, Agilent) and Raman spectroscopy (JY Labram HR 800, 514 nm) were also used to characterize the exfoliated MoS2 nanosheets. A drop of MoS2 dispersion was deposited on a freshly cleaved mica sheet, dried at room temperature, and then used for Raman and AFM analysis. 3. Result and discussion Fig. 1a shows the photo of these final MoS2 dispersions described above obtained from pure IPA and IPA assisted with different salts. The dispersion from pure IPA is transparent and almost colorless, whereas all those from the three different IPA-salt systems show very deep brown green color. The concentration of these final MoS2 dispersions can be estimated using the optical absorption spectrum and the Lambert-Beer law, which relates the measured absorbance (A) at a given wavelength to the concentration (C) of the dispersion by the formula A/l = aC, where l is the optical path length, a is the absorption coefficient [12]. However, the reported values for the absorption coefficient of MoS2 nanosheet are quite different, which has been shown in Table 1 [5,12,15,17–19]. Table 1 includes two kinds of absorption coefficient. One kind of absorption coefficient, a1, is calculated directly with the measured absorbance, and the other one, a2, is estimated from the absorbance after correction of scattering background. However, both a1 and a2 change greatly, a1 ranges from 490 to 6820 mL mg 1 m 1 and a2 from 95.70 to 1020 mL mg 1 m 1. The huge variation of absorption coefficient may be attributed to the influence of scattering [5,12,15], which depends sensitively on flake size [12,15]. During the process of LPE, flake size was closely related to ultrasonic time, ultrasonic power, centrifugal time, centrifugal speed, etc. Therefore, under the different LPE conditions, the absorption coefficient may be different. So, it is necessary to measure the absorbance coefficient. In Fig. 1b, A/l scaled linearly with C for all the three IPA-salt systems, allowing for the calculation of absorption coefficient values. It should be mentioned that the concentration of MoS2 dispersions in Fig. 1b was accurately measured by gravimetric method. The absorption coefficient of MoS2 nanosheet obtained here in IPA-K4Fe(CN)6, IPA-C4H4O6Na2, and IPA-C4H4O6KNa (without correction of scattering background) are 3769, 3735, and 3475 mL mg 1 m 1, respectively. These values are close with each other and comparable to those reported a1 in
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H. Liu et al. / Journal of Colloid and Interface Science 515 (2018) 27–31
Fig. 1. (a) Photograph of the final MoS2 dispersions obtained in pure IPA and IPA assisted with different salts. (b) Absorbance (k = 672 nm) per unit length (A/l) as a function of MoS2 dispersion concentration. (c) The concentrations of the final MoS2 dispersions in pure IPA and IPA assisted with three kinds of salts. (d) UV–Vis spectra of the MoS2 dispersions obtained in pure IPA and IPA assisted with different salts.
Table 1 Absorption coefficient of MoS2 nanosheet. Exfoliation media
NMP NMP IPA NMP IPA/H2O H2O/surfactant IPA
Absorption coefficient (mL mg
1
m
1
a1 (without
a2 (with
correction)
correction) 95.70 1020
490 3400 3302 6820 3475 3769
)
Reference
[5] [15] [18] [12] [17] [19] This work
NMP [12] and IPA/H2O mixture [17]. It is interesting to find that in both reference 12 and 17, the MoS2 flakes have similar lateral size of several hundred nm and most fakes are mono- to few-layer, which is quite comparable to those obtained in this work (see below). Fig. 1c shows the concentrations of the final MoS2 dispersions obtained from pure IPA and IPA assisted with different salts. The incorporation of any of the three salts can effectively increase the concentration of MoS2 in IPA. The concentration reaches 0.240, 0.202, and 0.152 mg mL 1 in IPA incorporated with K4Fe(CN)6, C4H4O6Na2, and C4H4O6KNa, respectively. These concentrations are much higher than that in pure IPA, 0.00326 mg mL 1, a value quite close to the reported value of 0.0035 mg mL 1 in literature [14]. Up to 73 times enhanced exfoliation efficiency is achieved in IPA-K4Fe (CN)6. The influences of the amount of each salt and the dosage of initial MoS2 powder on the concentration of final MoS2 dispersion are also investigated in detail in this work. The corresponding results are shown in Fig. S1. The highest concentration of MoS2 dispersion could be obtained from MoS2-IPA-K4Fe(CN)6 when the amount of K4Fe(CN)6 and the initial dosage of MoS2 powder were both 40 mg mL 1. For MoS2-IPA-K4Fe(CN)6 and MoS2-IPAC4H4O6KNa exfoliation systems, the highest concentration of
MoS2 dispersions could be obtained when the amount of K4Fe (CN)6 and the initial dosage of MoS2 powder were both 30 mg mL 1. The concentration values of final MoS2 dispersions in this work and previously reported values are listed in Table 2 [5,12–14,19– 21]. It can be seen clearly that the concentration in our experiment is relatively higher than many previously reported values, although it is still lower than in NMP [5], a toxic and high-boiling point solvent, and in a H2O/surfactant system [19], in which the hard-toremove surfactant was used. Fig. 1d shows the UV–Vis spectra of the MoS2 dispersions obtained from pure IPA and IPA assisted with the three different salts. It should be mentioned that in Fig. 1d, the spectra for those MoS2 nanosheets obtained from the three IPA-salt systems were measured with dispersions diluted 3 times of the final ones, since
Table 2 MoS2 dispersions prepared in this work and in previous reports. Media
Concentration (mg mL 1)
Parameters
Reference
NMP
0.6
[5]
NMP
0.16
H2O/surfactant
0.4
H2O
0.13
Alcohol/H2O
0.01
C2H5OH/H2O
0.018
IPA/H2O
0.017
IPA/salt
0.240
Sonication 2 h (200 W), 2000 rpm 30 min Sonication 1 h (285 W), 500 rpm 90 min Shear exfoliation (8335 rpm), 1500 rpm 90 min Sonication 60 h, RCF 600 g 30 min Sonication 3 h, 14,500 rpm 20 min Sonication 8 h, 3000 rpm 20 min Sonication 1 h (280 W), 3000 rpm 30 min Sonication 2 h (200 W), 3000 rpm 20 min
[12] [19] [20] [21] [13] [14] This work
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the concentration of those final dispersions are too high to be measured correctly. Although the spectra of MoS2 obtained from pure IPA was measured directly (the magenta line), it is still much lower than the other spectra. In Fig. 1d, the two distinct peaks of 2HMoS2 approximately locating at 670 (A-exciton) and 615 nm (Bexciton) [7,18] are clearly observed for those MoS2 nanosheets obtained from the three IPA-salt systems. A- and B-exciton direct transitions belong to 2H-MoS2 at the Κ point of the Brillouin zone [18]. It should be mentioned that both IPA-C4H4O6KNa and IPAC4H4O6Na2 show almost no absorption at 672 nm, however, IPAK4Fe(CN)6 shows weak absorption at 672 nm. Fig. S2 shows the spectra of the three IPA-salt dispersions with IPA as blank. The three IPA-salt dispersions were prepared with the same exfoliation process to that for preparing the corresponding MoS2 sample but only without adding MoS2 powder. It should be noted that in this work all the UV–Vis spectra and the absorption at 672 nm for those MoS2 nanosheets obtained from the three IPA-salt systems were measured with the corresponding IPA-salt solution as blank. It should be also mentioned that the MoS2 dispersions obtained from the three IPA-salt systems were very stable. All of them could keep stable at least for 30 days without significant aggregation. Fig. S3 shows the concentration retention versus time of the MoS2 dispersions obtained from the three IPA-salt systems. After 30 days the concentration retained 89%, 88%, and 89% for the dispersions obtained from IPA-K4Fe(CN)6, Na2C4H4O6, and KNaC4H4O6, respectively. The morphology of the MoS2 nanosheets was investigated by TEM. Fig. 2a shows the TEM image of MoS2 sheets obtained from
MoS2-IPA-K4Fe(CN)6. Due to partial fold, MoS2 nanosheets generally show wrinkled sheets [5]. Similar images were observed for MoS2 nanosheets obtained from other exfoliation systems. Fig. S4 shows the TEM images of MoS2 nanosheets obtained from MoS2IPA-K4Fe(CN)6 (S4a), MoS2-IPA-C4H4O6Na2 (S4b), MoS2-IPAC4H4O6KNa (S4c), and pure IPA (S4d). Raman spectroscopy was also performed to estimate the thickness of MoS2 nanosheets. Fig. 2b shows the Raman spectra of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6, in which two distinct Raman active modes, E12g (in-plane mode, 381 cm 1 for MoS2 nanosheets) and A1g (out-of-plane mode, 405.5 cm 1 for MoS2 nanosheets) are clearly observed [22]. For bulk MoS2, these two Raman active modes (E12g and A1g) were located around at 378 and 404.5 cm 1, respectively. The layer number of MoS2 nanosheets can be estimated according to the position of the two Raman peaks and the separation between them [6,22]. It has been reported that the location of E12g and A1g and the wavenumber difference between them become sensitive to the sheet thickness only when the layer number is less than four [22]. In Fig. 2b, for MoS2 obtained from MoS2IPA-K4Fe(CN)6, both Raman peaks shift to higher wavelength number compared with those of bulk MoS2 and the difference between the two peaks decrease from 26.5 cm 1 (bulk) to 24.5 cm 1 (nanosheets). The clear peak shift and obvious variation of the peak difference therefore evidence that the as-prepared MoS2 nanosheets are less than four layers. As shown by the AFM images in Fig. 2c, the MoS2 sheet obtained from MoS2-IPA-K4Fe(CN)6 exhibits a height of 0.8 nm, consistent with the theoretical thickness
Fig. 2. (a) TEM image of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6. (b) Raman spectra of MoS2 bulk and exfoliated MoS2 nanosheets obtained from MoS2-IPA-K4Fe (CN)6. (c) AFM image of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6 and the corresponding line profile taken along the green line. (d) Statistical analysis obtained by AFM measurements for the thickness of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6.
H. Liu et al. / Journal of Colloid and Interface Science 515 (2018) 27–31
of 0.615 nm for MoS2 monolayer [22]. Another AFM image of three MoS2 sheets from MoS2-IPA-K4Fe(CN)6 and AFM images of MoS2 nanosheets obtained from the other exfoliation systems are shown in Fig. S5. The thicknesses of these MoS2 nanosheets are all approximately 1 nm, indicating the monolayer structure of them. The statistical counts (Fig. 2d) about the thickness of 50 fakes (obtained from MoS2-IPA-K4Fe(CN)6) show that approximately 60% of the flakes are monolayer, indicating that exfoliation of MoS2 with salts as an assistant is a good method to produce MoS2 nanosheets in IPA. In this work, we have demonstrated that compared with the pure IPA, the exfoliation efficiency can be enhanced significantly only after adding some common salts. A possible mechanism of this salt-assisted exfoliation may be attributed to the phenomenon of cavitation and electrostatic repulsion. According to previous report [23], cavitation occurs during the process of sonication owing to abrupt pressure fluctuation. Cavitation is a phenomenon in which bubbles are formed in exfoliation media and collapse within milliseconds. When bubbles collapse, it induces very high temperatures and pressure that could provide sufficient energy to break MoS2 powder, that’s why we could prepare twodimensional nanosheets by liquid-phase exfoliation method. Part of the reason for salt-assistance may be attributed to salt particles (all the three salts have very low solubility in IPA, so most of them keep as solid particles in IPA) continuously striking the edges of MoS2 powder under the enormous energy released by bubble collapse, resulting in enlarging the interlayer space of MoS2, weakening van der Waals forces, making exfoliation successful. What’s more, another reason may be related to electrostatic repulsion. In order to demonstrate that electrostatic repulsion has an effect on the exfoliation process, we test the zeta potential of the MoS2 dispersions without water washing. The results are shown in Fig. S6 and indicate that MoS2 nanosheets in all these dispersions are negatively charged. It is also found that after adding the salts, the zeta potential of those MoS2 dispersions obtained in IPA-salt systems is more negative than that of the MoS2 dispersion obtained from pure IPA. That is to say, during the sonication process, the dispersions assisted with salts have greater electrostatic repulsion than in pure IPA. This makes MoS2 easier to be separated, in other words, to be exfoliated. 4. Conclusions In summary, we have constructed a facile, low-cost, and environment-friendly method to produce MoS2 nanosheets in IPA with salts as an assistant. The incorporation of some common and cheap salts can effectively improve the MoS2 exfoliation efficiency in IPA. The as-made concentration of MoS2 dispersion can be increased up to 0.240 mg mL 1, which is 73 times compared with that in pure IPA. Approximately 60% of the flakes obtained are monolayer. The method is very simple and economical since all the reagents used are cheap and easily available, and the exfoliation is carried out in air without any special treatment, which is beneficial for mass production of MoS2 nanosheets.
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Acknowledgements This work is supported by the opening fund of Hubei Key Laboratory of Bioinorganic Chemistry & Materia Medica (No. BCMM201701), Priority Academic Program Development of Jiangsu Higher Education Institutions, and Jiangsu Collaborative Innovation Center of Biomedical Functional Materials. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.01.023. References [1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat. Nanotechnol. 6 (2011) 147. [2] K.F. Mak, J. Shan, Nat. Photon. 10 (2016) 216. [3] D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V.B. Shenoy, G. Eda, M. Chhowalla, Nano Lett. 13 (2013) 6222. [4] K. Gopalakrishnan, S. Sultan, A. Govindaraj, C.N.R. Rao, Nano Energy 12 (2015) 52. [5] G.S. Bang, K.W. Nam, J.Y. Kim, J. Shin, J.W. Choi, S.Y. Choi, ACS Appl. Mater. Interfaces 6 (2014) 7084. [6] L. Niu, K. Li, H. Zhen, Y.S. Chui, W. Zhang, F. Yan, Z. Zheng, Small 10 (2014) 4651. [7] X. Geng, W. Sun, W. Wu, B. Chen, A. Al-Hilo, M. Benamara, H. Zhu, F. Watanabe, J. Cui, T.P. Chen, Nat. Commun. 7 (2016) 10672. [8] Y.H. Lee, X.Q. Zhang, W. Zhang, M.T. Chang, C.T. Lin, K.D. Chang, Y.C. Yu, J.T. Wang, C.S. Chang, L.J. Li, T.W. Lin, Adv. Mater. 24 (2012) 2320. [9] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A. K. Geim, PNAS 102 (2005) 10451. [10] Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, H. Zhang, Angew. Chem. 50 (2011) 11093. [11] L. Niu, J.N. Coleman, H. Zhang, H. Shin, M. Chhowalla, Z. Zheng, Small 12 (2016) 272. [12] J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.Y. Kim, K. Lee, G. T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J.C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Science 331 (2011) 568. [13] K.G. Zhou, N.N. Mao, H.X. Wang, Y. Peng, H.L. Zhang, Angew. Chem. 50 (2011) 10839. [14] M.M. Bernal, L. Álvarez, E. Giovanelli, A. Arnáiz, L. Ruiz-González, S. Casado, D. Granados, A.M. Pizarro, A. Castellanos-Gomez, E.M. Pérez, 2D Materials 3 (2016) 035014. [15] A. O’Neill, U. Khan, J.N. Coleman, Chem. Mater. 24 (2012) 2414. [16] W. Du, J. Lu, P. Sun, Y. Zhu, X. Jiang, Chem. Phys. Lett. 568–569 (2013) 198. [17] J. Shen, Y. He, J. Wu, C. Gao, K. Keyshar, X. Zhang, Y. Yang, M. Ye, R. Vajtai, J. Lou, P.M. Ajayan, Nano Lett. 15 (2015) 5449. [18] X. Hai, K. Chang, H. Pang, M. Li, P. Li, H. Liu, L. Shi, J. Ye, J. Am. Chem. Soc. 138 (2016) 14962. [19] E. Varrla, C. Backes, K.R. Paton, A. Harvey, Z. Gholamvand, J. McCauley, J.N. Coleman, Chem. Mater. 27 (2015) 1129. [20] J. Kim, S. Kwon, D.H. Cho, B. Kang, H. Kwon, Y. Kim, S.O. Park, G.Y. Jung, E. Shin, W.G. Kim, H. Lee, G.H. Ryu, M. Choi, T.H. Kim, J. Oh, S. Park, S.K. Kwak, S.W. Yoon, D. Byun, Z. Lee, C. Lee, Nat. Commun. 6 (2015) 8294. [21] U. Halim, C.R. Zheng, Y. Chen, Z. Lin, S. Jiang, R. Cheng, Y. Huang, X. Duan, Nat. Commun. 4 (2013) 2213. [22] H. Li, Q. Zhang, C.C.R. Yap, B.K. Tay, T.H.T. Edwin, A. Olivier, D. Baillargeat, Adv. Funct. Mater. 22 (2012) 1385. [23] K.S. Suslick, Science (New York, N.Y.) 247 (1990) 1439.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Production of mono- to few-layer MoS2 nanosheets in isopropanol by a salt-assisted direct liquid-phase exfoliation method Huaizhi Liu a, Liao Xu a, Weixu Liu a, Bo Zhou a, Yinyan Zhu a, Lihua Zhu b, Xiaoqing Jiang a,⇑ a b
Jiangsu Key Laboratory of New Power Batteries, College of Chemistry and Materials Science, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, PR China Department of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
g r a p h i c a l a b s t r a c t Preparation of MoS2 nanosheets in isopropanol with salt as assistant.
a r t i c l e
i n f o
Article history: Received 15 November 2017 Revised 29 December 2017 Accepted 5 January 2018 Available online 6 January 2018 Keywords: MoS2 nanosheet Salt-assisted exfoliation Liquid-phase exfoliation Potassium ferrocyanide
a b s t r a c t Here, we report a facile salt-assisted direct liquid-phase exfoliation method for mass production of MoS2 nanosheets. We choose organic solvent isopropanol (IPA) as exfoliation media and potassium ferrocyanide, potassium sodium tartrate, or sodium tartrate as salt, the assistant. The selected salts show universal and efficient assistant effect for the exfoliation of MoS2 in IPA. Especially, potassium ferrocyanide (K4Fe(CN)6) can enhance the exfoliation efficiency up to 73 times and a dispersion of MoS2 nanosheets with concentration as high as 0.240 mg mL 1 can be easily obtained in IPA-K4Fe(CN)6 system. Transmission electron microscopy, atomic force microscopy (AFM), and Raman spectroscopy show that bulk MoS2 has been successfully exfoliated into mono- to few-layer MoS2 nanosheets. AFM analysis indicates that nearly 60% flakes are monolayer in MoS2 dispersion. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Over the past few years, the study of layered two-dimensional (2D) nanomaterials has become one of the most key areas of mate⇑ Corresponding author. E-mail address: [email protected] (X. Jiang). https://doi.org/10.1016/j.jcis.2018.01.023 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.
rials science. Among these layered 2D nanomaterials, mono- and few-layered molybdenum disulfide (MoS2), a typical representative of transition metal dichalcogenides, has recently attracted much attention due to the novel and unique properties in electronics, optoelectronics, photonics, catalysis, energy storage, and solar cells [1–7], upon exfoliation from its bulk counterparts. Substantial efforts have been devoted to the mass production of MoS2
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nanosheets. At present, however, it is still a big challenge to produce massive quantities of MoS2 nanosheets of high quality in a commercially viable way. Generally, mono- to few-layer MoS2 nanosheets were prepared through either a bottom-up method such as chemical vapor deposition (CVD) [8], or a top-down method such as micromechanical cleavage [9], chemical lithiumintercalation exfoliation [3], and electrochemical lithiumintercalation exfoliation [10]. However, CVD method involves harsh condition (high temperature and high vacuum); lithiumintercalation method fails to keep MoS2 nanosheets structure integrity; micromechanical cleavage method is quantity-limited [6,11]. To overcome these disadvantages, in 2011 Coleman and co-workers introduced a new technique, liquid-phase exfoliation (LPE) method to prepare MoS2 nanosheets [12]. LPE is a potential effective method for the mass production of MoS2 with structure of mono- to few-layer nanosheets. A series of organic solvents, such as N-methyl-pyrrolidinone (NMP), dimethylsulphoxide, and dimethylformamide, were favored in both exfoliation and dispersing of MoS2 [12]. However, most of these solvents are toxic and have a high boiling point. To facilitate the application and reduce pollution, some less toxic solvents with a low boiling point, for example, ethanol and isopropanol (IPA), were selected as exfoliation media. However, the maximum concentrations obtained in these pure solvents are commonly very low. The concentration was as low as 0.0014 mg mL 1 in ethanol [13] and 0.0035 mg mL 1 in IPA [14]. Such a concentration is too low to meet the practical requirement. Long time sonication may increase the concentration. For example, Coleman and co-workers once increased the concentration of MoS2 in NMP to 40 mg mL 1 after 140 h sonication [15]. However, such time- and energy-consuming process is not suitable for application in practice. Here, we demonstrate that the exfoliation efficacy of MoS2 in IPA can be dramatically and easily increased by just adding some common and cheap salts into IPA. This method is in part inspired by our previous work, in which a salt-assisted method was successfully developed to exfoliate graphite into high-quality graphene [16]. In this work, with this salt-assisted exfoliation method, the concentration of MoS2 in IPA can be increased up to 0.240 mg mL 1, as many as 73 times compared with those in pure IPA without salts. This method is simple, environment-friendly, effective, and economical. 2. Experimental 2.1. Materials MoS2. powder was purchased from Aladdin (particle sizes < 2
lm), potassium ferrocyanide (K4Fe(CN)63H2O) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Potassium sodium tartrate (C4H4O6KNa4H2O) and sodium tartrate (C4H4O6Na22H2O) were purchased from Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd. IPA was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade. 2.2. Methods Different amount of MoS2 powder was mixed with different amount of one of K4Fe(CN)63H2O, C4H4O6KNa4H2O, or C4H4O6Na22H2O. Then the mixture was added into a centrifugation tube containing 3.0 mL IPA, and sonicated in an ultrasonic cleaner (DTC-15, 40 kHz, 200 W) for 2 h at ambient temperature. After sonication, the exfoliated solution was centrifuged at 3000 rpm, relative centrifugal force (RCF) = 850g, for 20 min using a centrifuge (Hettich Universal 320) to remove the unexfoliated or thick flakes
of MoS2. Afterwards, the top 4/5 of the dispersion was collected carefully, then centrifuged at 6500 rpm (RCF = 4010g) for 5 min, and the sediment was collected. The sediment was washed with 10 mL deionized water by sonicating for 2 min, then centrifuged at 6500 rpm for 5 min, and re-collected. The wash process was repeated for five times to remove salt and then the sediment was re-dispersed into 3.0 mL fresh IPA by sonication for 5 min. The final obtained MoS2 dispersions were deep brown green, stable at least for 30 days without aggregation, and were used for different characterizations. 2.3. Characterization Concentration of the final suspension of MoS2 nanosheets was measured by using UV–Vis spectrometer (TU-1900) with quartz cuvettes (path length 0.5 or 0.1 cm) at 672 nm [17]. It should be mentioned that the UV–Vis spectra of all the suspensions of MoS2 nanosheets obtained from different IPA-salt system were measured with the corresponding IPA-salt solution as blank (prepared with the same exfoliation process to that of the MoS2 sample but without adding MoS2 powder). Transmission electron microscopy (TEM, Hitachi-7650) was used to examine the morphology of the exfoliated MoS2 nanosheets. A drop of the MoS2 dispersion was casted on a holey carbon grid (400 meshes) for the TEM measurement. Atomic force microscopy (AFM, Agilent) and Raman spectroscopy (JY Labram HR 800, 514 nm) were also used to characterize the exfoliated MoS2 nanosheets. A drop of MoS2 dispersion was deposited on a freshly cleaved mica sheet, dried at room temperature, and then used for Raman and AFM analysis. 3. Result and discussion Fig. 1a shows the photo of these final MoS2 dispersions described above obtained from pure IPA and IPA assisted with different salts. The dispersion from pure IPA is transparent and almost colorless, whereas all those from the three different IPA-salt systems show very deep brown green color. The concentration of these final MoS2 dispersions can be estimated using the optical absorption spectrum and the Lambert-Beer law, which relates the measured absorbance (A) at a given wavelength to the concentration (C) of the dispersion by the formula A/l = aC, where l is the optical path length, a is the absorption coefficient [12]. However, the reported values for the absorption coefficient of MoS2 nanosheet are quite different, which has been shown in Table 1 [5,12,15,17–19]. Table 1 includes two kinds of absorption coefficient. One kind of absorption coefficient, a1, is calculated directly with the measured absorbance, and the other one, a2, is estimated from the absorbance after correction of scattering background. However, both a1 and a2 change greatly, a1 ranges from 490 to 6820 mL mg 1 m 1 and a2 from 95.70 to 1020 mL mg 1 m 1. The huge variation of absorption coefficient may be attributed to the influence of scattering [5,12,15], which depends sensitively on flake size [12,15]. During the process of LPE, flake size was closely related to ultrasonic time, ultrasonic power, centrifugal time, centrifugal speed, etc. Therefore, under the different LPE conditions, the absorption coefficient may be different. So, it is necessary to measure the absorbance coefficient. In Fig. 1b, A/l scaled linearly with C for all the three IPA-salt systems, allowing for the calculation of absorption coefficient values. It should be mentioned that the concentration of MoS2 dispersions in Fig. 1b was accurately measured by gravimetric method. The absorption coefficient of MoS2 nanosheet obtained here in IPA-K4Fe(CN)6, IPA-C4H4O6Na2, and IPA-C4H4O6KNa (without correction of scattering background) are 3769, 3735, and 3475 mL mg 1 m 1, respectively. These values are close with each other and comparable to those reported a1 in
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Fig. 1. (a) Photograph of the final MoS2 dispersions obtained in pure IPA and IPA assisted with different salts. (b) Absorbance (k = 672 nm) per unit length (A/l) as a function of MoS2 dispersion concentration. (c) The concentrations of the final MoS2 dispersions in pure IPA and IPA assisted with three kinds of salts. (d) UV–Vis spectra of the MoS2 dispersions obtained in pure IPA and IPA assisted with different salts.
Table 1 Absorption coefficient of MoS2 nanosheet. Exfoliation media
NMP NMP IPA NMP IPA/H2O H2O/surfactant IPA
Absorption coefficient (mL mg
1
m
1
a1 (without
a2 (with
correction)
correction) 95.70 1020
490 3400 3302 6820 3475 3769
)
Reference
[5] [15] [18] [12] [17] [19] This work
NMP [12] and IPA/H2O mixture [17]. It is interesting to find that in both reference 12 and 17, the MoS2 flakes have similar lateral size of several hundred nm and most fakes are mono- to few-layer, which is quite comparable to those obtained in this work (see below). Fig. 1c shows the concentrations of the final MoS2 dispersions obtained from pure IPA and IPA assisted with different salts. The incorporation of any of the three salts can effectively increase the concentration of MoS2 in IPA. The concentration reaches 0.240, 0.202, and 0.152 mg mL 1 in IPA incorporated with K4Fe(CN)6, C4H4O6Na2, and C4H4O6KNa, respectively. These concentrations are much higher than that in pure IPA, 0.00326 mg mL 1, a value quite close to the reported value of 0.0035 mg mL 1 in literature [14]. Up to 73 times enhanced exfoliation efficiency is achieved in IPA-K4Fe (CN)6. The influences of the amount of each salt and the dosage of initial MoS2 powder on the concentration of final MoS2 dispersion are also investigated in detail in this work. The corresponding results are shown in Fig. S1. The highest concentration of MoS2 dispersion could be obtained from MoS2-IPA-K4Fe(CN)6 when the amount of K4Fe(CN)6 and the initial dosage of MoS2 powder were both 40 mg mL 1. For MoS2-IPA-K4Fe(CN)6 and MoS2-IPAC4H4O6KNa exfoliation systems, the highest concentration of
MoS2 dispersions could be obtained when the amount of K4Fe (CN)6 and the initial dosage of MoS2 powder were both 30 mg mL 1. The concentration values of final MoS2 dispersions in this work and previously reported values are listed in Table 2 [5,12–14,19– 21]. It can be seen clearly that the concentration in our experiment is relatively higher than many previously reported values, although it is still lower than in NMP [5], a toxic and high-boiling point solvent, and in a H2O/surfactant system [19], in which the hard-toremove surfactant was used. Fig. 1d shows the UV–Vis spectra of the MoS2 dispersions obtained from pure IPA and IPA assisted with the three different salts. It should be mentioned that in Fig. 1d, the spectra for those MoS2 nanosheets obtained from the three IPA-salt systems were measured with dispersions diluted 3 times of the final ones, since
Table 2 MoS2 dispersions prepared in this work and in previous reports. Media
Concentration (mg mL 1)
Parameters
Reference
NMP
0.6
[5]
NMP
0.16
H2O/surfactant
0.4
H2O
0.13
Alcohol/H2O
0.01
C2H5OH/H2O
0.018
IPA/H2O
0.017
IPA/salt
0.240
Sonication 2 h (200 W), 2000 rpm 30 min Sonication 1 h (285 W), 500 rpm 90 min Shear exfoliation (8335 rpm), 1500 rpm 90 min Sonication 60 h, RCF 600 g 30 min Sonication 3 h, 14,500 rpm 20 min Sonication 8 h, 3000 rpm 20 min Sonication 1 h (280 W), 3000 rpm 30 min Sonication 2 h (200 W), 3000 rpm 20 min
[12] [19] [20] [21] [13] [14] This work
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the concentration of those final dispersions are too high to be measured correctly. Although the spectra of MoS2 obtained from pure IPA was measured directly (the magenta line), it is still much lower than the other spectra. In Fig. 1d, the two distinct peaks of 2HMoS2 approximately locating at 670 (A-exciton) and 615 nm (Bexciton) [7,18] are clearly observed for those MoS2 nanosheets obtained from the three IPA-salt systems. A- and B-exciton direct transitions belong to 2H-MoS2 at the Κ point of the Brillouin zone [18]. It should be mentioned that both IPA-C4H4O6KNa and IPAC4H4O6Na2 show almost no absorption at 672 nm, however, IPAK4Fe(CN)6 shows weak absorption at 672 nm. Fig. S2 shows the spectra of the three IPA-salt dispersions with IPA as blank. The three IPA-salt dispersions were prepared with the same exfoliation process to that for preparing the corresponding MoS2 sample but only without adding MoS2 powder. It should be noted that in this work all the UV–Vis spectra and the absorption at 672 nm for those MoS2 nanosheets obtained from the three IPA-salt systems were measured with the corresponding IPA-salt solution as blank. It should be also mentioned that the MoS2 dispersions obtained from the three IPA-salt systems were very stable. All of them could keep stable at least for 30 days without significant aggregation. Fig. S3 shows the concentration retention versus time of the MoS2 dispersions obtained from the three IPA-salt systems. After 30 days the concentration retained 89%, 88%, and 89% for the dispersions obtained from IPA-K4Fe(CN)6, Na2C4H4O6, and KNaC4H4O6, respectively. The morphology of the MoS2 nanosheets was investigated by TEM. Fig. 2a shows the TEM image of MoS2 sheets obtained from
MoS2-IPA-K4Fe(CN)6. Due to partial fold, MoS2 nanosheets generally show wrinkled sheets [5]. Similar images were observed for MoS2 nanosheets obtained from other exfoliation systems. Fig. S4 shows the TEM images of MoS2 nanosheets obtained from MoS2IPA-K4Fe(CN)6 (S4a), MoS2-IPA-C4H4O6Na2 (S4b), MoS2-IPAC4H4O6KNa (S4c), and pure IPA (S4d). Raman spectroscopy was also performed to estimate the thickness of MoS2 nanosheets. Fig. 2b shows the Raman spectra of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6, in which two distinct Raman active modes, E12g (in-plane mode, 381 cm 1 for MoS2 nanosheets) and A1g (out-of-plane mode, 405.5 cm 1 for MoS2 nanosheets) are clearly observed [22]. For bulk MoS2, these two Raman active modes (E12g and A1g) were located around at 378 and 404.5 cm 1, respectively. The layer number of MoS2 nanosheets can be estimated according to the position of the two Raman peaks and the separation between them [6,22]. It has been reported that the location of E12g and A1g and the wavenumber difference between them become sensitive to the sheet thickness only when the layer number is less than four [22]. In Fig. 2b, for MoS2 obtained from MoS2IPA-K4Fe(CN)6, both Raman peaks shift to higher wavelength number compared with those of bulk MoS2 and the difference between the two peaks decrease from 26.5 cm 1 (bulk) to 24.5 cm 1 (nanosheets). The clear peak shift and obvious variation of the peak difference therefore evidence that the as-prepared MoS2 nanosheets are less than four layers. As shown by the AFM images in Fig. 2c, the MoS2 sheet obtained from MoS2-IPA-K4Fe(CN)6 exhibits a height of 0.8 nm, consistent with the theoretical thickness
Fig. 2. (a) TEM image of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6. (b) Raman spectra of MoS2 bulk and exfoliated MoS2 nanosheets obtained from MoS2-IPA-K4Fe (CN)6. (c) AFM image of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6 and the corresponding line profile taken along the green line. (d) Statistical analysis obtained by AFM measurements for the thickness of MoS2 nanosheets obtained from MoS2-IPA-K4Fe(CN)6.
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of 0.615 nm for MoS2 monolayer [22]. Another AFM image of three MoS2 sheets from MoS2-IPA-K4Fe(CN)6 and AFM images of MoS2 nanosheets obtained from the other exfoliation systems are shown in Fig. S5. The thicknesses of these MoS2 nanosheets are all approximately 1 nm, indicating the monolayer structure of them. The statistical counts (Fig. 2d) about the thickness of 50 fakes (obtained from MoS2-IPA-K4Fe(CN)6) show that approximately 60% of the flakes are monolayer, indicating that exfoliation of MoS2 with salts as an assistant is a good method to produce MoS2 nanosheets in IPA. In this work, we have demonstrated that compared with the pure IPA, the exfoliation efficiency can be enhanced significantly only after adding some common salts. A possible mechanism of this salt-assisted exfoliation may be attributed to the phenomenon of cavitation and electrostatic repulsion. According to previous report [23], cavitation occurs during the process of sonication owing to abrupt pressure fluctuation. Cavitation is a phenomenon in which bubbles are formed in exfoliation media and collapse within milliseconds. When bubbles collapse, it induces very high temperatures and pressure that could provide sufficient energy to break MoS2 powder, that’s why we could prepare twodimensional nanosheets by liquid-phase exfoliation method. Part of the reason for salt-assistance may be attributed to salt particles (all the three salts have very low solubility in IPA, so most of them keep as solid particles in IPA) continuously striking the edges of MoS2 powder under the enormous energy released by bubble collapse, resulting in enlarging the interlayer space of MoS2, weakening van der Waals forces, making exfoliation successful. What’s more, another reason may be related to electrostatic repulsion. In order to demonstrate that electrostatic repulsion has an effect on the exfoliation process, we test the zeta potential of the MoS2 dispersions without water washing. The results are shown in Fig. S6 and indicate that MoS2 nanosheets in all these dispersions are negatively charged. It is also found that after adding the salts, the zeta potential of those MoS2 dispersions obtained in IPA-salt systems is more negative than that of the MoS2 dispersion obtained from pure IPA. That is to say, during the sonication process, the dispersions assisted with salts have greater electrostatic repulsion than in pure IPA. This makes MoS2 easier to be separated, in other words, to be exfoliated. 4. Conclusions In summary, we have constructed a facile, low-cost, and environment-friendly method to produce MoS2 nanosheets in IPA with salts as an assistant. The incorporation of some common and cheap salts can effectively improve the MoS2 exfoliation efficiency in IPA. The as-made concentration of MoS2 dispersion can be increased up to 0.240 mg mL 1, which is 73 times compared with that in pure IPA. Approximately 60% of the flakes obtained are monolayer. The method is very simple and economical since all the reagents used are cheap and easily available, and the exfoliation is carried out in air without any special treatment, which is beneficial for mass production of MoS2 nanosheets.
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