graphene nanosheet hybrid materials with high lithium storage performance

graphene nanosheet hybrid materials with high lithium storage performance

G Model ARTICLE IN PRESS CATTOD-9285; No. of Pages 7 Catalysis Today xxx (2014) xxx–xxx Contents lists available at ScienceDirect Catalysis Today...

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ARTICLE IN PRESS

CATTOD-9285; No. of Pages 7

Catalysis Today xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Synthesis of graphene-like MoS2 nanowall/graphene nanosheet hybrid materials with high lithium storage performance Jin Guo, Xiao Chen, Shaohua Jin, Mingming Zhang, Changhai Liang ∗ Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 5 June 2014 Received in revised form 25 September 2014 Accepted 26 September 2014 Available online xxx Keywords: Graphene-like MoS2 Graphene Nanocomposites Lithium ion battery

a b s t r a c t A facile method to synthesize graphene-like MoS2 nanowall/graphene sheet (GL-MoS2 nanowall/GNS) composites has been developed through a solvothermal method, and followed by annealing at 400 ◦ C under Ar. The characterization results indicate that the thin flaky wall type GL-MoS2 nanowall was supported well on the crumpled graphene flakes with good dispersion. The as-obtained nanowall-nanosheet hybrids are more robust and still retain the reversible capacity of 700 mA h g−1 after 100 cycles at a charge–discharge rate of 500 mA g−1 . The outstanding performance in electrochemistry is attributed to both the robust nanocomposite structure and the synergistic interactions between graphene and GL-MoS2 nanowall, which makes it an efficient stable lithium storage material. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable Li-ion batteries (LIBs) have aroused a wide variety of research interests around the world because they are the key components in portable electronic devices and stationary energy storage systems [1–7]. Graphite is the most popular commercial anode in LIBs owning to their flat potential profile for lithium intercalation and good structural stability during cycling. However, the theoretical maximum capacity of 372 mA h g−1 can hardly achieve the demands associated with the use in electric devices [8]. To meet the requirements in higher power densities and better cycling stability, significant efforts have been made to develop new electrode materials [9]. Molybdenum disulfide (MoS2 ), a typical layered transition metal sulfide with an analogous structure to graphite, has recently received particular attention as a promising electrode material for LIBs because of their high specific capacities, unusual electronic and physical properties [10–14]. Generally, a bulk MoS2 crystal is composed of three atom layers, a Mo layer sandwiched between two S layers, and the triple layers are stacked and held together by weak van der Waals interactions [15]. Precisely similar to the layered structure of graphite indicates that single-layer or fewlayer MoS2 flakes can be prepared. Rao and coworkers named the structure of MoS2 and WS2 with a single layer or a few layers

∗ Corresponding author. Fax: +86 411 84986353. E-mail address: [email protected] (C. Liang).

graphene-like nanostructure [15]. In addition, it has been confirmed that the few-layer (particularly single-layer) MoS2 nanosheets exhibit distinctively different chemical and physical properties in comparison with the bulk materials, owning to the changes in electronic properties with decreasing layer number [16–18]. Especially, the luminescence quantum efficiency of singlelayer MoS2 is about 104 greater than that of bulk MoS2 . GL-MoS2 also has been investigated as a good cathode material for Li-ion batteries. For instance, Du et al. have prepared an exfoliated–restacked MoS2 electrode with the superior cycle stability [19]. Lemmon and coworkers had obtained exfoliated MoS2 –PEO (polyethylene oxide) composite electrode exhibiting the improved cyclic performance and rate capability [20]. However, the single layer MoS2 is easy to aggregate or restack during repetitive cycling and even in the drying process of electrodes due to the van der Waals interactions, which would lose its unique properties and give a negative effect on their performance in LIBs. To overcome these obstacles, many research groups introduced carbon based conductive additives into MoS2 to improve the cyclability and rate capabilities of the material [13,21–27]. Among these carbon based conductive additive, graphene is a desirable matrix for improving the electrochemical performance owning its superior electrical conductivity, large surface area, excellent mechanical flexibility, and high thermal and chemical stability [28]. Thus, graphene composites with MoS2 have been explored as LIB anode materials. The results generally showed some good promises such as high specific capacity and improvement of the cycle ability of MoS2 [13,22]. However, to the best

http://dx.doi.org/10.1016/j.cattod.2014.09.028 0920-5861/© 2014 Elsevier B.V. All rights reserved.

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of our knowledge, there are few reports on the synthesis of GLMoS2 nanowall/GNS nanocomposites (GL-MoS2 vertically growth on the graphene surface). Hence, morphology-controlled synthesis of well-aligned graphene-like MoS2 nanowall still remains a challenge by means of a simple, effective, and economical approach. Herein, nanowall architectures constructed of GL-MoS2 nanowall arrays on the graphene were firstly prepared by a facile in situ solvothermal reduction method. Mo(dedtc)4 was used as a single source precursor to prepare GL-MoS2 without the addition of any surfactants. The as-prepared GL-MoS2 nanowall/GNS can not only prevent the agglomeration of MoS2 flakes, but also restrict the growth of Mo nanoparticles during lithiation by the bond formed between MoS2 nanowall and graphene. The limited growth of Mo nanoparticles can further suppress the lithiation product Li2 S to react with the electrolyte by the adsorption effect of Mo nanoparticles [29]. What is more, the vertical MoS2 nanowall provides a shorter pathway for Li-ion and electrons. At the same time, the vertical MoS2 nanowall can more effectively inhibit the aggregation of graphene sheets so that the conductivity of the composites can be further enhanced. As a result, GL-MoS2 nanowall/GNS exhibit outstanding reversible capacity and excellent rate performance (700 mA h g−1 after 100 cycles at a charge–discharge rate of 500 mA g−1 and reversible discharging capacity 380 mA h g−1 at 2000 mA g−1 ), when applied as the anode material for lithium storage. 2. Experimental 2.1. Synthesis of GL-MoS2 nanowall/GNS nanocomposites Graphene oxide (GO) was prepared by a modified Hummers method [30] from natural graphite powder. Then exfoliation was carried out by ultrasonicating the GO dispersion in acetonitrile under ambient conditions. For the preparation of GL-MoS2 nanowall/GNS composites, 40 mL of a 1 mg mL−1 GO suspension was ultrasonicated for 0.5 h in acetonitrile, then 200 mg precursor Mo(dedtc)4 was added into the suspension under vigorous stirring for 0.5 h. The precursor was prepared by our previous work according to a reported method with some modifications [31,32]. 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 mass spectroscopy and molecular structure of precursor are shown in Fig. S1. The molecular weight of as-prepared Mo(dedtc)4 is 689 g mol−1 . The mixture was vigorously stirred for 0.5 h and sealed in a 50 mL Telfon-lined autoclave solvothermally treated at 220 ◦ C for 12 h. After cooling naturally, the black precipitates were collected by centrifugation, and dried in a vacuum oven at 80 ◦ C for 12 h. The GL-MoS2 nanowall/GNS nanocomposites were obtained after annealing in conventional tube furnace at 400 ◦ C for 4 h under Ar. 2.2. Characterizations The materials were recorded with CuK␣ radiation by powder X-ray diffraction (XRD) using a D/MAX 2400 diffractometer. The 2Â angular regions between 5◦ and 80◦ were investigated at a scan rate of 8◦ min−1 with a step of 0.02◦ . The morphology of the samples was investigated by field–emission scanning electron microscope (FEI Nova Nano SEM 450) and transmission electron microscope (TEM, Philips CM200 electron microscope. The sample was

Fig. 1. XRD patterns of graphene, bulk MoS2 and GL-MoS2 nanowall/GNS nanocomposites.

dissolved in ethanol and the suspension was dropped onto a copper grid.). Raman measurements were obtained by an inVia Reflex Laser Micro-Raman spectroscope (Renishaw) with a 532 nm Ar laser. 2.3. Electrochemical measurements The working electrodes were prepared by mixing 80 wt % active materials (GL-MoS2 nanowall/GNS nanocomposites), 10 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, 1:1:1in volume). The working electrode, the separator, the electrolyte, and the counter electrode, were assembled to a coin-type cell (2016) in an argon-filled glovebox (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) after the catalytic material is stabilized at room temperature. 3. Results and discussion The synthetic route of GL-MoS2 /GNS nanocomposites is schematically illustrated in Scheme 1. Oxygen-containing functional groups on graphene oxide (GO) sheets make it soluble and easily processable. The Mo(dedtc)4 molecules adsorbed firstly on the oxygen-containing groups on the surface of GO through impregnation process in the acetonitrile. Then, GNS with uniformly dispersed MoS2 nanosheets are generated by solvothermal treatment. The MoS2 nanoflakes gradually convert to nanowall with curling up and growing vertically on the surface of the GNS as the reaction progress, and finally the GL-MoS2 nanowall/GNS nanocomposites were obtained after annealing at 400 ◦ C for 4 h under Ar. For the as-prepared GL MoS2 /GNS nanocomposites and bulk MoS2 , all diffraction peaks are consistent with those of typical MoS2 phase (JCPDS Card No.37-1492), demonstrating that the MoS2 phases were formed after annealing at 400 ◦ C. The broad and weak peaks of the (1 0 0) and (1 1 0) planes indicated the low crystallinity of MoS2 in nanocomposites. Compared to bulk MoS2 and pure graphene, as shown in Fig. 1, the (0 0 2) peaks located at 2Â = 14.4◦ and 25.3◦ could not be found for the GL-MoS2 /GNS composites sample, which demonstrated that MoS2 nanosheets and graphene

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Scheme 1. Schematic illustration of synthesis of GL-MoS2 nanowall/GNS nanocomposites.

were not stacked along the c axis. This also indicates that MoS2 in the composites should have the structure of a single layer or a few layers [33], and in accordance with the definition given by Rao and coworkers [15]. For this kind of structure of MoS2 , the as-synthesized MoS2 could be entitled graphene-like MoS2 (GLMoS2 ). In addition to the wide-angle reflections described above, a new diffraction peak centered at 2Â = 9.1◦ in the low-angle range was observed, and the corresponding lattice spacing was calculated to be about 0.97 nm by using the Bragg equation. It could be attributed to the spacing between two adjacent single-layer MoS2 sheets, which was confirmed by HRTEM. Raman spectrum of graphene derived from the reduction of GO exhibits two prominent bands at 1590 cm−1 (G band) and 1350 cm−1 (D band), which corresponds to the vibration of sp2-bonded carbon atoms and the dispersive, defect-induced vibrations, respectively (Fig. 2 a) [34]. The intensity ratio of the welldocumented D band and G band of graphene in the nanocomposite was enhanced after reduction compared with that of graphene, demonstrating the improvement of the disordered graphene sheets [35]. The G band shift in carbon-based composites relates to the

charge transfer between the carbon and other compounds present [36,37]. Therefore, the peak shifts of G band to 1578 cm−1 proves the presence of a charge transfer from graphene to MoS2 . Recently Raman spectroscopy has also been used for identifying single- and few-layer MoS2 sheets [38]. Fig. 2 b shows the Raman spectra of bulk MoS2 and GL-MoS2 /GNS composite in ambient air using an excitation wavelength of 532 nm. The two dominant peaks of bulk MoS2 at 383 and 408 cm−1 correspond to the E1 2g and A1g modes of the hexagonal MoS2 crystal respectively. The GLMoS2 /GNS nanocomposite displays the E1 2g peak at 377 cm−1 and the A1g peak at 401 cm−1 . The E1 2g mode involves the in-layer displacement of Mo and S atoms, whereas the A1g mode involves the out-of-layer symmetric displacements of S atoms along the c axis [12]. Most strikingly, both E1 2g and A1g of bulk MoS2 mode are more stiffened (blue-shift) than GL-MoS2 in Figure 2 b, which is consistent with a classical model for coupled harmonic oscillators that the E1 2g and A1g models are expected to stiffen as additional layers, because the interlayer van der Waal interactions increase the effective restoring forces acting on the atoms [39]. It is further proved that the nanocomposites contain GL-MoS2 which is consistent with our previous XRD result.

Fig. 2. Raman spectra of graphene, bulk MoS2 , and graphene-like MoS2 nanowall/GNS nanocomposites.

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Fig. 3. SEM images of GO (a), GL-MoS2 nanowall/GNS nanocomposites (b, c) and bulk MoS2 (d).

Fig. 4. HRTEM images of GL-MoS2 /GNS nanocomposites (a) and bulk MoS2 (b).

The morphologies of the GO, bulk MoS2 and GL-MoS2 /GNS nanocomposites were compared in Fig. 3. From Fig. 3 a, the wrinkled surfaces and folding of two-dimensional GO are clearly observed, meanwhile FEG-SEM images (shown in Fig. 3 b and c) illustrated that thin flaky wall type GL-MoS2 nanowall well supported on the crumpled flakes with good dispersion had a smooth surface. Compared to GL-MoS2 , the bulk-MoS2 possessed a large micrometer-size scaled area, and the sheets were stacked together

as shown in Fig. 3d. The elemental composition of the annealed GL-MoS2 –GNS nanocomposites as determined by EDX as follows in Table 1. Therefore, the content of MoS2 in GL-MoS2 –GNS nanocomposites was calculated to be about 34.57%. The microstructures of samples were further revealed by TEM characterization as shown in Fig. 4. It shows a lot of single-layer MoS2 sheets highly dispersed in the MoS2 /GNS composites. (0 0 2) plane originated from the stacking of MoS2 is hardly found. The

Table 1 Elemental composition of annealed GL-MoS2 /GNS nanocomposite. Sample MoS2

C (wt%) 55.2

O (wt%) 10.3

Mo (wt%) 20.8

S (wt%) 13.7

MoS2 (wt%) 34.6

Mo:S 1:1.97

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Fig. 5. Capacity–voltage profiles for the first, second, and third cycles of GL-MoS2 /GNS nanocomposites (a) and bulk MoS2 (b), cycling performance for GL-MoS2 /GNS nanocomposites and bulk MoS2 at a current density of 500 mA g−1 between 0.01 and 3.0 V (c), rate behavior of the GL-MoS2 /GNS nanocomposites and bulk MoS2 at various current densities (d), and Coulombic efficiency of GL-MoS2 nanowall/GNS nanocomposites at a current density of 500 mA g−1 (e).

spacing between two adjacent single-layer MoS2 sheets is measured to be about 0.97 nm. According to previous report [13], it should be the amorphous carbon intercalated between MoS2 layers enlarging its inter-reticular distance, which is in good agreement with previous XRD analysis. Comparing with the TEM image of GL-MoS2 flake grown on the surface of graphene, the bulk MoS2 is thicker and consists of many stacking layers, which is quite larger than GL-MoS2 flake. It displays a perfect layered crystal with an interlayer distance of the (0 0 2) plane of 0.62 nm, which is consistent with the SEM observation and XRD consequence. For checking possibility of GL-MoS2 -based material as LIB electrodes, the electrochemical properties of the GL-MoS2 /GNS

nanocomposite were also investigated. Fig. 5 a and b shows voltage–capacity curves at a current density of 500 mA g−1 within a cutoff voltage window of 0.01–3.00 V. In the initial discharge process of GL-MoS2 nanowall/GNS, a voltage plateau at about 0.6 V was observed, which is attributed to the irreversible reaction between the electrolyte and MoS2 [40]. The conversion and the formation of a gel-like polymeric layer lead to an irreversible capacity in the initial discharge process [41]. In the latter discharge processes, two potential plateaus at 2.1 and 1.3 V are shown, and the plateau at 0.6 V disappears. In the following charging process, two potential plateaus at 1.7 and 2.2 eV can be seen, consistent with the reported lithiation and delithiation profiles of MoS2 [13,42]. Recently, much attention has been focused on

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uncovering the lithium-storage mechanism for the MoS2 /Li cell system [23,43]. Therefore, the lithium-storage mechanism of the GL-MoS2 nanowall/GNS nanocomposite in our case should follow the following steps: (1) during the initial discharge process, lithium ions are inserted into the MoS2 interlayer to form Lix MoS2 ; (2) when further discharging to 0.01 V, MoS2 is reduced to metallic Mo and Li2 S. During subsequent cycles, the Li2 S↔S + 2Li + 2e− reaction plays a leading role in the electrode. In addition, for GLMoS2 nanowall/GNS at the first cycle, the charge (lithiation) and discharge (delithiation) capacities are 1169.0 and 972.2 mA h g−1 , while those of bulk MoS2 are 919.4 and 403.5 mA h g−1 . As a result, the coulombic efficiency of GL-MoS2 nanowall/GNS nanocomposite and bulk MoS2 are 83.2 and 43.9%, respectively. The cycling performance of GL-MoS2 nanowall/GNS nanocomposite and bulk MoS2 is shown in Fig. 5 c, at a discharge current density of 500 mA g−1 , the specific capacity of bulk MoS2 shows a rapid decrease to about only 130 mA h g−1 during the initial 35th cycle. This may be due to two main reasons. Firstly, the bulk MoS2 layers have a trend to aggregate or restack during repetitive cycling due to the van der Waals interactions, which may result in the loss of the unusual properties and negative effects on their electrochemical properties. Secondly, MoS2 reacts electrochemically with lithium according to the following conversion reaction: MoS2 + 4Li+ + 4e- →Mo + 2Li2 S. Li2 S as the lithiation product is prone to react with the electrolyte to produce a thick gel-like polymeric layer, which result to the particle aggregation. This also leads to the capacity fading and inferior rate capability. In contrast, GLMoS2 nanowall/GNS nanocomposite sample shows significantly improved cycling performance under the same conditions, as characterized by stable capacity retention of 700 mA h g−1 for the 100th cycle. This value is almost twice of the graphite-based materials (the theoretical capacity of graphite is about 372 mA h g−1 ). The cycling performances of pure graphene nanosheets are shown in Fig. S2, which exhibit good stability during cycling, but the capacity is only 320 mA h g−1 which is much lower than the present GL-MoS2 nanowall/GNS nanocomposite. Simultaneously, high coulombic efficiency of around 97–99% can be achieved during cycling performance as show in Fig. 5e. In addition to the cycling stability, benefitted from the unique nanostructures and synergistic effects between GL-MoS2 and graphene, the GL-MoS2 nanowall/GNS nanocomposites materials also show excellent cycling performance with the variation of current density, as shown in Fig. 5d. For example, GL-MoS2 nanowall/GNS nanocomposites shows discharge capacities (delithiation) of approximately 833, 695, 575, 493 mA h g−1 at various current of 200, 500, 1000, 1500 mA g−1 , respectively. Even cycled at high rates of 2000 mA g−1 , discharge capacities of 380 mA h g−1 can still be maintained. After deep cycling at 3000 mA g−1 , a stable capacity of around 590 mA h g−1 can be restored when the current density is reduced back to 500 mA g−1 , which is much better than bulk-MoS2 . The superior performance of GL-MoS2 nanowall/GNS nanocomposites may be primarily attributed to synergistic effects between ultrathin GL-MoS2 and graphene. The bond formed between MoS2 nanowall and graphene restraining the aggregation of MoS2 and the growth of Mo nanoparticles occurred during lithiation, thus suppressing the growth of the gel-like polymeric layer and accommodating their volume expansion during the cycling. The ultrathin MoS2 nanowall in the 2D architectures provides a short pathway for Li-ion and electrons diffusion, and decreases activation energy for the Li-ion intercalation reaction. GL-MoS2 nanowall can more effectively inhibit the aggregation of graphene sheets so that the conductivity of the composites can be further enhanced. The synergistic effects between ultrathin GL-MoS2 nanowall and graphene increased contact area for electron transfer due to the ultrathin sheets. As a result, the GL-MoS2 nanowall/GNS nanocomposites

reasonably show wonderful cycling performance and rate capability for reversible Li ion storage. 4. Conclusion In summary, GL-MoS2 nanowall/GNS nanocomposites are successfully fabricated through a solvothermal process followed by annealing treatment. The MoS2 nanowalls are grown vertically on both sides of graphene sheets, and furthermore the synergistic effects of the GNS and MoS2 nanowall allowing the 2D architectures to be more stable and display excellent performance rates and charge/discharge cycle ability. These results demonstrate that GL-MoS2 nanowall/GNS nanocomposites are promising anode electrodes for lithium ions batteries. Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21373038 and 21403026), the China Postdoctoral Science Foundation (2014M551068), and the Fundamental Research Funds for the Central Universities (DUT12YQ03 and DUT14RC(3)007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cattod.2014.09.028. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors, Chem. Rev. 104 (2004) 4245–4270. [3] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190–193. [4] H. Wu, G. Chan, J.W. Choi, I. Ryu, Y. Yao, M.T. McDowell, S.W. Lee, A. Jackson, Y. Yang, L. Hu, Stable cycling of double-walled silicon nanotube battery anodes through solidelectrolyte interphase control, Nat. Nanotechnol. 7 (2012) 310–315. [5] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2009) 587–603. [6] D.L. Ma, Z.Y. Cao, H.G. Wang, X.L. Huang, L.M. Wang, X.B. Zhang, Threedimensionally ordered macroporous FeF3 and its in situ homogenous polymerization coating for high energy and power density lithium ion batteries, Energ. Environ. Sci. 5 (2012) 8538. [7] Y. Yang, G. Zheng, S. Misra, J. Nelson, M.F. Toney, Y. Cui, High-capacity micrometer-sized Li2 S particles as cathode materials for advanced rechargeable lithium-ion batteries, J. Am. Chem. Soc. 134 (2012) 15387–15394. [8] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [9] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (2012) 19–29. [10] R. Tenne, Advances in the synthesis of inorganic nanotubes and fullerene-like nanoparticles, Angew. Chem. Int. Ed. 42 (2003) 5124–5132. [11] X. Zhou, L.J. Wan, Y.G. Guo, Facile synthesis of MoS2 @CMK-3 nanocomposite as an improved anode material for lithium-ion batteries, Nanoscale 4 (2012) 5868–5871. [12] Z. Wang, T. Chen, W. Chen, K. Chang, L. Ma, G. Huang, D. Chen, J.Y. Lee, CTAB-assisted synthesis of single-layer MoS2 -graphene composites as anode materials of Li-ion batteries, J. Mater. Chem. A 1 (2013) 2202–2210. [13] K. Chang, W. Chen, Single-layer MoS2 /graphene dispersed in amorphous carbon: towards high electrochemical performances in rechargeable lithium ion batteries, J. Mater. Chem. 21 (2011) 17175–17184. [14] H. Hwang, H. Kim, J. Cho, MoS2 nanoplates consisting of disordered graphenelike Layers for high rate lithium battery anode materials, Nano Lett. 11 (2011) 4826–4830. [15] H.R. Matte, A. Gomathi, A.K. Manna, D.J. Late, R. Datta, S.K. Pati, C. Rao, MoS2 and WS2 analogues of graphene, Angew. Chem. Int. Ed. 122 (2010) 4153–4156. [16] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [17] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Emerging photoluminescence in monolayer MoS2 , Nano Lett. 10 (2010) 1271–1275. [18] K.F. Mak, C. Lee, J. Hone, J. Shan, T.F. Heinz, Atomically thin MoS2 : a new directgap semiconductor, Phys. Rev. Lett. 105 (2010) 136805.

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Please cite this article in press as: J. Guo, et al., Synthesis of graphene-like MoS2 nanowall/graphene nanosheet hybrid materials with high lithium storage performance, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.028