Layer-controlled synthesis of graphene-like MoS₂ from single source organometallic precursor for Li-ion batteries

Abstract

Herein, we report an new approach to synthesis of graphene-like MoS₂ flakes and tunable layers (from mono- to multi-layer) easily controlled by the thermal decomposition temperature of a single source (Mo(Et₂NCS₂)₄). The approach opens a new way to controlled large-scalable synthesis of graphene-like transition metal sulfides for energy storage, nanoelectronics and optoelectronics.

---

Transition metal dichalcogenides have attracted great attention owing to their two-dimensional layer structure analogous to graphene. MoS₂ is a typical inorganic layered material, in which each layer consists of a covalently bonded S–Mo–S hexagonal quasi-two-dimensional network,^1,2 with weak van der Waals attraction between the layers. To date, a large number of researches have confirmed that the internal relationship between the shape and number of layers of MoS₂ and its properties. Among them, few-layer MoS₂ (especially mono-layer MoS₂) has been successfully demonstrated as a promising material, which could complement graphene for a variety of applications such as lubricants, transistors,^3,4 anode materials for lithium ion batteries,^5,6 and photo/electro-catalysts^7–10 due to its low cost, high chemical stability, good mechanical, distinctive electrical and optical properties.

However, the controlled synthesis of single and few layers MoS₂ (named graphene-like MoS₂) still remains highly challenging. Recently, significant efforts have been devoted to prepare mono- and few-layer MoS₂. Novoselov et al.¹¹ reported that few-layer MoS₂ was synthesized by Scotch-tape assisted micromechanical exfoliation, but it was small scale and has poor repeatability. In addition, solution-phase production of MoS₂ by exfoliation or hydrothermal method is often time-consuming.^12–16 The synthetic routes also include epitaxial growth,^17,18 intercalation assisted exfoliation,^19–21 chemical vapor deposition (CVD),^3,22 and sulfurization of molybdenum, phosphomolybdic acid, and molybdenum oxides.^4,23 However, these current methods always involve high temperatures dangerous gases, including H₂ and H₂S, and complicated procedures. Therefore, developing simple and large scalable methods which graphene-like MoS₂ can be produced in a controlled manner is highly needed.

The single source approach using stable organometallic precursors may be considered as an elegant and pertinent alternative to easily control the shape and layers. Indeed, the two-dimensional layer structure of MoS₂ is anticipated to form through adjusting the metal–ligand interactions and decomposition temperatures. Thus, above-mentioned work shows the limits of the currently developed methods in terms of layer control, and further suggests that the rational design of precursors for layer number control is necessary to overcome these challenges. Here, we provide an effective approach to synthesis of the graphene-like MoS₂ with controlled number of layers by thermal decomposition of a single source precursor tetrakis(diethylaminodithiocarbomato)molybdate(IV) (Mo(Et₂NCS₂)₄). Compared with previous method, the controllable synthesis offers three advantages (i) the layer of the graphene-like MoS₂ flakes can be easily controlled, (ii) the whole preparation process is very facial and has a good repeatability, and (iii) the few layer MoS₂ flakes are highly dispersed which could offer high surface areas and has a better electrochemical performance than bulk MoS₂. The resulting 2D graphene-like MoS₂ flakes with uniform dispersion, which could offer high surface areas. As a consequence, the favorable graphene-like structure exhibit a storage capacity of 970 mA h g⁻¹ in the initial cycles at a current density of 500 mA h g⁻¹, and the cycling performance is much better than multi-layer MoS₂ based anode.

The organometallic precursor Mo(Et₂NCS₂)₄ was synthesized from Mo(CO)₆ and bis(diethylthiocarbamoyl)disulfide following a modified procedure from the literature,²⁴ as confirmed by the mass spectroscopy and X-ray single-crystal diffraction. As shown in Fig. S1 and Table S1,† the molecular weight of as-prepared Mo(Et₂NCS₂)₄ is 689 g mol⁻¹. The molecular structure of Mo(Et₂NCS₂)₄ presumed from X-ray single-crystal diffraction is shown in the inserted of (Fig. 1) one Mo atom was coordinated with four diethylaminodithio-carbonato (S₂N-(C₂H₅)₂) ligands. The result is similar to the reference reported.²⁴ Thermogravimetric analysis was applied to verify volatility characteristics (Fig. 1). The as-prepared precursor can be totally decomposed when the calcined temperature is above 374 °C. After the four decomposition steps, the residual black solid weight accounts for 25.5% of the initial weight. This is in good agreement with the calculated weight percent loss to produce pure MoS₂ (23.5% calculated), the additional 2% material possibly attributed to decomposed ligand. The redundant sulfur, nitrogen and carbon in the precursor escaped as gaseous sulfur, hydrogen cyanide and ethylene.

Fig. 1 Molecular structure of Mo(Et₂NCS₂)₄ precursor (a) and thermogravimetric analysis of the precursor (b).

XRD patterns of the as-obtained product graphene-like MoS₂ (GL-MoS₂) and bulk MoS₂ (B-MoS₂) are shown in Fig. 2a. All the diffraction peaks in the pattern can be assigned as hexagonal MoS₂ (JCPDS card no. 37-1492). The reflection peak at 2θ = 14.2° (002), corresponding to a d-spacing of 0.62 nm, indicates a well-staked layered structure. However, the MoS₂ (002) reflection peak is not shown in the XRD patterns of the GL-MoS₂ (320 °C, 400 °C, 600 °C), only two peaks at 2θ = 33.0°, 58.8° attributed to MoS₂ (100) and (110) can be observed (as shown in Fig. 2a), which suggests the predominant formation of single layer or few layers MoS₂.¹³ On the contrary, the sample of GL-MoS₂ (800 °C) and B-MoS₂ show a typical (002) reflection, which indicate a well-stacked layer structure.

Fig. 2 XRD patterns (a) and Raman spectra (b) of GL-MoS₂ from the decomposition of Mo(Et₂NCS₂)₄ as a function of temperature and B-MoS₂ from the decomposition of (NH₄)₂MoS₄. The left and right dashed lines indicate the position of the E¹_2g and A_1g.

The Raman spectra of samples excited by 532 nm line is shown in (Fig. 2b), which also confirmed the formation of MoS₂ by thermal decomposition of a single source precursor Mo(Et₂NCS₂)₄. Both B-MoS₂ and GL-MoS₂ exhibited sharp peaks at about 384 cm⁻¹ (E¹_2g) and 408 cm⁻¹ (A_1g) that were due to the first order Raman vibration modes within the S–Mo–S layer.²⁵ Most strikingly, the E¹_2g vibration and the A_1g vibration of GL-MoS₂ were stiffened (blue shift) with increasing annealing temperature. The result is consistent with a classical model for coupled harmonic oscillators that the E¹_2g and A_1g modes are expected to stiffen as additional layers, because the interlayer van der Waal interactions increase the effective restoring forces acting on the atoms.²⁶ This result indicated the layers of GL-MoS₂ increased with thermal-decomposition temperature increasing from 320 °C to 800 °C, which was consistent with the XRD result.

The morphology and microstructure of as-prepared GL-MoS₂ and B-MoS₂ were investigated via field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM). As shown in Fig. 3a, the GL-MoS₂ (400 °C) flakes are uniformly dispersed, which was main due to the spillover of redundant sulfur, nitrogen and carbon gas from the precursor Mo(Et₂NCS₂)₄ during the calcination process, thus suppressing the sheet stacking together. However, the B-MoS₂ possessed a large micrometer-sized scaled area, and the sheets were stacked together (as shown in Fig. 3b).

Fig. 3 Field-emission SEM images of GL-MoS₂ (400 °C) (a) and B-MoS₂ (b).

To further observe the microstructure, the as-prepared GL-MoS₂ samples were characterized by TEM. Their TEM images demonstrated the coexistence of mono- and few-layer nanosheets, and the different layers of GL-MoS₂ in Fig. 4a–d had been statistically analyzed. When the annealing temperature at 320 °C and 400 °C, the majority sheets were mono-layer, and the percentage of mono-layer can be reached about 76% and 70%, respectively. However, the percentage of mono-layer MoS₂ was decreased but that of bilayer MoS₂ was significant increased with increasing calcination temperature to 600 °C. Further increasing the temperature to 800 °C (as shown in Fig. 5d), the GL-MoS₂ with some folded edges exhibited parallel lines corresponding to the different layers of MoS₂ sheets (mainly layers = 6), which could be attributed to the crystallinity of MoS₂ during higher temperature treatment. The structure of GL-MoS₂ (800 °C) is similar to the reference B-MoS₂ (Fig. 4e). This is consistent with the results of Raman and XRD analysis. Fig. 4f shows the varied layers with different temperature, which indicates that the two-dimensional layer structure of MoS₂ can be well controlled by tuning the decomposition temperature ranges of the single source precursor Mo(Et₂NCS₂)₄.

Fig. 4 TEM images of GL-MoS₂ (320 °C) (a), GL-MoS₂ (400 °C) (b), GL-MoS₂ (600 °C) (c), GL-MoS₂ (800 °C) (d), and B-MoS₂ (e), and effect of decomposition temperatures on MoS₂ stacking degree (f).

Fig. 5 Cycling performance of GL-MoS₂ (400 °C) and bulk MoS₂ electrodes at a constant current density of 500 mA g⁻¹ (a), and rate performances of GL-MoS₂ (400 °C) and B-MoS₂ electrodes at different current densities (b).

Recently, we have also synthesized W(Et₂NCS₂)₄ through a similar reaction of W(CO)₆ and bis(diethylthiocarbamoyl)disulfide (Fig. S2†) and subsequently decomposed the single organometallic precursor to successfully prepare graphene-like WS₂. Its 2-D structure has been evidenced by XRD and Raman characterizations, as shown in Fig. S3 and S4.† The results suggest that this method may open a new avenue to the synthesis of a wide range of graphene-like transition metal sulfides and make it convenient to the research of layers-dependent properties of transition metal sulfides.

The charge/discharge characteristics were tested by galvanostatically cycling the cells based on the current density calculated by 500 mA g⁻¹ in the potential range of 0–3 V vs. Li/Li⁺ (as shown in Fig. S5†). For GL-MoS₂ (400 °C) at the first cycle, the charge (lithiation) and discharge (delithiation) capacities are 1093 and 905 mA h g⁻¹, while that of B-MoS₂ are 889 and 859 mA h g⁻¹. As shown in (Fig. 5a), the capacity of the GL-MoS₂ could be kept a stable storage capacity of about 330 mA h g⁻¹ after 100 cycles at a current density of 500 mA g⁻¹ between 0.01 and 3.0 V, which is much higher than the bulk-layer MoS₂. The rate performance of the MoS₂ is shown in Fig. 1. The GL-MoS₂ exhibits a better rate performance than the solid ones. At the current densities of 200, 500, 1000, 1500, 2000, and 3000 mA g⁻¹, the specific capacities of the graphene-like MoS₂ are 890, 730, 540, 360, 235, and 130 mA h g⁻¹, respectively. When the current density goes back to 500 mA g⁻¹, the specific capacity of the graphene-like MoS₂ returns to 450 mA h g⁻¹, which is much better than bulk-MoS₂. This mainly attributes to its optimal 2D graphene-like structure, which will enhance the contact area between MoS₂ and electrolyte, shorten the lithium ion diffusion length, and facilitate the lithium ion ultrafast diffusion. Based on the excellent lithium-storage performance of single graphene-like MoS₂ in our current study results, its long-time performance can be enhanced by deposition onto carbon materials to improve the structure stability and inhibit the MoS₂ aggregation during the lithium ion insertion and extraction processes. As a result, it would be a promising anode material for energy storage applications in high-performance lithium ion battery.

In conclusion, we present an simple approach for synthesizing GL-MoS₂ with controlled shape and number of layers by thermal decomposition of a single source precursor Mo(Et₂NCS₂)₄ at different temperature. The obtained GL-MoS₂ flakes with a relatively uniform dispersion, which exhibited high storage capacity of 970 mA h g⁻¹ in the initial cycle at a current density of 500 mA g⁻¹, and the cycling performance is much better than that of B-MoS₂. This simple, large-scalable and reliable approach opens up a new avenue to preparation of graphene-like TMS in a controlled manner. The regular shape and controlled number of layers combined with the uniform dispersion make them a promising material for applications in energy storage devices as well as high-performance nanoelectronics and optoelectronics.

Experimental section

Synthesis of the GL-MoS₂

GL-MoS₂ was prepared by thermal decomposition of a single source precursor Mo(Et₂NCS₂)₄. And single source precursor Mo(Et₂NCS₂)₄ was prepared according to a reported method with some modifications.²⁴ Typically, 1 g (3.8 mmol) Mo(CO)₆ and 2.25 g (7.6 mmol) bis(diethylthiocarbamoyl)disulfide were dissolved in 30 mL acetone under an oxygen-free argon atmosphere. The solution was stirred and refluxed at 58 °C for 2.5 h, after cooling to room temperature naturally for 5 h. The violet precipitate were collected and washed with pentane. Finally, the product was dried under vacuum at 120 °C to evaporate off residual impurities.

The single source precursor was heated to the various temperature (320, 400, 600, and 800 °C) at a rate of 10 °C min⁻¹ under Ar and kept that temperature for 4 h. Then the graphene-like MoS₂ can be obtained. The calcined temperature could be varied to obtain the desired results. To evaluate the electrochemical performances of the GL-MoS₂, the B-MoS₂ was prepared by thermal decomposition of the ammonium thiomolybdate ((NH₄)₂MoS₄) under Ar.

Characterization

Thermogravimetric analysis (TGA) was analyzed by a SDTA851e apparatus at a heating rate of 10 °C min⁻¹ in N₂ atmosphere. XRD patterns were recorded with CuKα radiation on a D/MAX 2400 diffractometer, and the scan rate is 5° min⁻¹ with a step of 0.02°. Raman measurements were obtained by an Invia/Reflex Lasser Micro-Raman spectroscope (Renishaw England) with a 532 nm Ar laser. FESEM images were obtained with an FEI Nova NanoSEM 450. TEM and HRTEM images were obtained with Philips CM200 electron microscope. The sample was dissolved in ethanol and the suspension was dropped onto a copper grid.

Electrochemical measurements

The working electrodes were prepared by mixing 70 wt% active materials (MoS₂), 20 wt% acetylene black (super-p) and 10 wt% poly(vinylidene fluoride) (PVDF) dissolved in N-methyl-2-pyrrolidinone. After coating the above slurry onto a copper foil current collector, it was dried under vacuum at 120 °C for 12 h and cut into pieces with a diameter of 14 mm before use. A Celgard 2300 membrane was used as a separator between the working electrode and the counter electrode (Li foil). The electrolyte was 1 M LiPF₆ in the mixture of ethylene carbonate–dimethyl carbonate–diethyl carbonate (EC–DMC–DEC, 1 : 1 : 1 v/v/v). The working electrode, the separator, the electrolyte, and the counter electrode, were assembled to a coin-type cell (2016) in an argon-filled glove box Unilab(1200/780). Galvanostatic charge/discharge cycles of the cells were conducted between 0.01 and 3.00 V on a LAND CT-2001A battery cycler (Wuhan, China) at room temperature.

Acknowledgements

We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21373038) and the Fundamental Research Funds for the Central Universities (DUT12YQ03).

Notes and references

1. S. Helveg, J. V. Lauritsen, E. Lægsgaard, I. Stensgaard, J. K. Nørskov, B. Clausen, H. Topsøe and F. Besenbacher, Phys. Rev. Lett., 2000, 84, 951.
2. J. V. Lauritsen, J. Kibsgaard, S. Helveg, H. Topsoe, B. S. Clausen, E. Laegsgaard and F. Besenbacher, Nat. Nanotechnol., 2007, 2, 53–58.
3. X. Wang, H. Feng, Y. Wu and L. Jiao, J. Am. Chem. Soc., 2013, 135, 5304–5307.
4. K. K. Liu, W. Zhang, Y. H. Lee, Y. C. Lin, M.-T. Chang, C. Y. Su, C. S. Chang, H. Li, Y. Shi and H. Zhang, Nano Lett., 2012, 12, 1538–1544.
5. Z. Wang, T. Chen, W. Chen, K. Chang, L. Ma, G. Huang, D. Chen and J. Y. Lee, J. Mater. Chem. A, 2013, 1, 2202–2210.
6. K. Chang, W. Chen, L. Ma, H. Li, H. Li, F. Huang, Z. Xu, Q. Zhang and J.-Y. Lee, J. Mater. Chem., 2011, 21, 6251–6257.
7. Y. Li, Y.-L. Li, C. M. Araujo, W. Luo and R. Ahuja, Catal. Sci. Technol., 2013, 3, 2214–2220.
8. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen and M. Chhowalla, Nano Lett., 2011, 11, 5111–5116.
9. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296–7299.
10. Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578.
11. K. S. Novoselov, A. K. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva and A. Firsov, Science, 2004, 306, 666–669.
12. Y. Peng, Z. Meng, C. Zhong, J. Lu, W. Yu, Y. Jia and Y. Qian, Chem. Lett., 2001, 30, 772–773.
13. H. S. Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati and C. N. Rao, Angew. Chem., Int. Ed., 2010, 49, 4059–4062.
14. 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 and V. Nicolosi, Science, 2011, 331, 568–571.
15. Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun'Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and J. N. Coleman, Nat. Nanotechnol., 2008, 3, 563–568.
16. R. J. Smith, P. J. King, M. Lotya, C. Wirtz, U. Khan, S. De, A. O'Neill, G. S. Duesberg, J. C. Grunlan, G. Moriarty, J. Chen, J. Wang, A. I. Minett, V. Nicolosi and J. N. Coleman, Adv. Mater., 2011, 23(34), 3944–3948.
17. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass and A. N. Marchenkov, Science, 2006, 312, 1191–1196.
18. P. W. Sutter, J.-I. Flege and E. A. Sutter, Nat. Mater., 2008, 7, 406–411.
19. A. Schumacher, L. Scandella, N. Kruse and R. Prins, Surf. Sci., 1993, 289, 595–598.
20. R. Gordon, D. Yang, E. Crozier, D. Jiang and R. Frindt, Phys. Rev. B: Condens. Matter Mater. Phys., 2002, 65.
21. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274–10277.
22. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science, 2009, 324, 1312–1314.
23. H. Liu, D. Su, R. Zhou, B. Sun, G. Wang and S. Z. Qiao, Adv. Energy Mater., 2012, 2(8), 970–975.
24. M. Decoster, F. Conan, J. Guerchais, Y. Le Mest, J. Sala Pala, J. Jeffery, E. Faulques, A. Leblanc and P. Molinié, Polyhedron, 1995, 14, 1741–1750.
25. C. Chang and S. Chan, J. Catal., 1981, 72, 139–148.
26. T. Li and G. Galli, J. Phys. Chem. C, 2007, 111, 16192–16196.

---

Footnote

† Electronic supplementary information (ESI) available: Mass spectrum, molecular structure and the brief crystallographic data and data collection of Mo(Et₂NCS₂)₄. Molecular structure of W(Et₂NCS₂)₄, XRD pattern and Raman spectrum of graphene-like WS₂. CCDC 992256 contains the supplementary crystallographic data for this paper. See DOI: 10.1039/c4ra01318b

---