Defect-free MoS2 nanosheets: Advanced nanofillers for polymer nanocomposites

Composites: Part A 81 (2016) 61–68

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Defect-free MoS2 nanosheets: Advanced nanofillers for polymer nanocomposites Xiaming Feng a,b, Panyue Wen a,b, Yuan Cheng a,b, Lu Liu a,b, Qilong Tai a,b,⇑, Yuan Hu a,b,⇑, Kim Meow Liew b,c a

State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China USTC-CityU Joint Advanced Research Center, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China c Department of Architectural and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b

a r t i c l e

i n f o

Article history: Received 29 July 2015 Received in revised form 19 October 2015 Accepted 1 November 2015 Available online 10 November 2015 Keywords: A. Layered structures A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Thermal analysis

a b s t r a c t The preparation of defect-free MoS2 nanosheets is a key challenge and essential for practical applications. Herein the dodecanethiol was firstly performed as the antioxidant and surface modifier to produce the defect-free MoS2 by direct ultrasonication of bulk MoS2 in N,N-dimethylformamide. Incorporating defect-free MoS2 into polyethylene obviously improved the properties of PE/MoS2 nanocomposites. For crystallization under quiescent condition, the half crystallization time (t0.5) of nanocomposites containing 0.2 wt% MoS2 was reduced by 87.0% compared to that of neat PE. A 54.3 °C increase in the temperature of maximum weight loss (Tmax) was observed by inclusion of as low as 0.7 wt% defect-free MoS2 nanosheets. In addition, the uniformly distributed MoS2 can considerably improve the mechanical properties of composites. These observations suggest that the robust nature, dramatic barrier action of defectfree MoS2 and the strong nanosheets/matrix interfacial adhesion would be the motivation to improve the performance of the polymeric nanocomposites. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recently, the hexagonal MoS2 compound has aroused an enormous amount of interest, due to its wide range of applications, such as its optical properties [1], semiconductor characteristics [2] and catalytic ability [3]. These excellent properties of MoS2 mainly depend on its atomic-layer thickness and twodimensional (2D) morphology. Like graphene, for MoS2 a number of techniques have been used to exfoliate the nanosheets from bulk materials [4,5]. Typically, solution based exfoliation methods are often used to produce layered MoS2 in scalable quantities; these mainly involve the direct liquid exfoliation with sonication and chemical exfoliation using lithium intercalation. In these two methods, it is difficult to avoid employing of ultrasonic equipment. Especially in the former method, to obtain large quantities of single-layer MoS2 sheets, high intensity and long time ultrasonication is necessary. Because of the violent nature of high intensity ultrasound, the crystal structure of MoS2 easily becomes deformed, ⇑ Corresponding authors at: State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China. Tel./fax: +86 551 63601664. E-mail addresses: [email protected] (Q. Tai), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.compositesa.2015.11.002 1359-835X/Ó 2015 Elsevier Ltd. All rights reserved.

and internal edges (tears, pinholes and defects) would become visible [6]. In general, the concentration of MoS2 nanosheets in dispersion increased with the extension of the sonication time. However, longer ultrasonication treatments always result in layered structure with a higher number of defects and reduction of the sheet size [7]. In the catalysis field, it may be a satisfactory choice to obtain the defect containing MoS2 by sonication method. But for some fields, such as electrical and mechanical applications, largescale preparation of high-quality and defect-free MoS2 nanosheet remains an outstanding scientific problem. Specifically, ultrasounds generate cavities whose implosion releases sufficient energy to form high-energy intermediates and free radicals that can drive chemical reactions, thus reduces the size of the 2D sheets and damage the physical properties that are usually sought after [8,9]. In order to fabricate the large defectfree graphene, the author performed the tiopronin as antioxidant to trap the radicals generated by sonication process, thus preventing the graphene from further oxidization and the formation of defects [10]. For now, there are few researches on overcoming the adverse impacts of sonication on MoS2 nanosheets and preparation of defect-free MoS2 nanosheets reinforced polymer composites.

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Scheme 1. Schematic illustration for exfoliation of defect-free MoS2 and corresponding production of polymer based nanocomposites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Many works have indicated that reinforcing polymer materials with 2D nano-materials is attractive, which could efficiently enhance the physical properties of polymer materials for a variety of engineering applications [11,12]. For example, robust graphene has been considered to be the ideal candidate to reinforce the mechanical and thermal properties of polymer composites [13]. However, for some applications it is required to effectively reinforce mechanical properties and thermal stability of polymers while also maintaining their electrical insulation property and the high dielectric constant. The high electric conductivity of graphene seems to be an impenetrable barrier to their utilization in these applications such as electronic packaging and transmission lines. Due to its semiconductor characteristic, the MoS2 nanosheet may be an excellent alternative to graphene used in the aforementioned applications [14]. In our previous report [15], the MoS2 nanosheets have been used to reinforce the polymer matrix with the help of good dispersibility, inherent excellent mechanical strength and thermal stability. Some papers also have proved that the surface defects could cause significant damage to the mechanical strength of 2D nanosheets [16,17]. Therefore, preparation of dispersible and defect-free nanosheets is a committed step to reinforce polymer composites with superior performances. The presence of reducing agent or antioxidant can obviously impact the sonication process, such as preparing the MWNTs by ultrasonication of graphite in ferrocene aldehyde/DMF solution [10,18]. Moreover, previous work has suggested these sulfur vacancies of MoS2 to possess higher molecular affinities, with theoretical suggestions of thiol edge absorption [6,19]. Here, we speculate that dodecanethiol as antioxidant and surface modifier could simultaneously trap the radicals and modify the MoS2 once the sulfur vacancies generated during the sonication of bulk MoS2 in DMF. It is anticipated that defect-free MoS2 sheets can substantially improve the physical performances of nonpolar polymer materials,

due to their inherent superior properties and good dispersibility, and thus promoting the development of polymer/MoS2 nanocomposites. 2. Experiment section 2.1. Materials Molybdenum disulfide (MoS2, AP), dodecanethiol, N,Ndimethylformamide (DMF), xylene were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). PE (melt flow rate = 2.0 g/10 min) was supplied by Yangzi Petrochemical Co., Ltd. (China). 2.2. Exfoliation of MoS2 1 g of MoS2 powder and 100 ml of dodecanethiol solution (5% w/v solution of the dodecanethiol and DMF) were put into a 250 ml glass vial and treated with bath sonication to exfoliate MoS2. The mixture was carried out in the sonication bath with mechanical stirring for 8 h. Subsequently, the mixture was centrifuged at 1500 rpm for 45 min to isolate stable MoS2 dispersion from the MoS2 sediments. The exfoliated MoS2 was obtained by vacuum filtration of the MoS2 dispersion, washing with DMF repeatedly and drying in vacuum oven at 90 °C for 24 h. 2.3. Preparation of PE/MoS2 nanocomposites PE/MoS2 composites with different MoS2 content (0.2, 0.4, and 0.7 wt%) were prepared by solution mixing method. In general, the previously obtained MoS2 was ultrasonic dispersed in a certain volume of xylene to yield a homogeneous dispersion. The calculated PE was dissolved in the MoS2 dispersion and treated with

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Fig. 1. (a) AFM image and (b) TEM image of exfoliated MoS2 nanosheets. (c) Height profiles along the lines in image (a). (d) Size distributions of exfoliated MoS2 sheets. Photographs of (e) exfoliated MoS2 re-dispersed in various solvents after settling for one week and (f) the dumbbell-shaped PE/MoS2 nanocomposite samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

sonication for 60 min, then poured into a glass petri-dish and dried at 80 °C for 24 h and 100 °C in vacuum oven for 12 h to completely remove the solvent. The samples were cut into pellets and finally hot-pressed into sheets. 2.4. Characterization Fourier transform infrared (FT-IR) spectra were obtained from a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, US). X-ray diffraction (XRD) was performed using a Japan Rigaku

D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu-Ka tube and a Ni filter (l = 0.1542 nm). X-ray photoelectron spectroscopy (XPS) was carried out with a VGESCALB MK-II electron spectrometer (Al Ka excitation source at 1486.6 eV). Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed and the accelerating voltage was 200 kV. Atomic force microscopy (AFM) observation was performed on the DI Multimode V in tapping-mode. Morphology of the sample was studied by a PHILIPS XL30E scanning electron microscope (SEM). Thermogravimetric analysis (TGA) was carried

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Fig. 2. (a and b) S2p XPS spectra, (c) FT-IR spectra and (d) XRD pattern of the samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., USA) under air flow of 25 ml min
1 and heated from 25 °C to 700 °C at a linear heating rate of 20 °C min
1. Tensile testing was measured by a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., China) at a crosshead speed of 50 mm min
1. Thermal behavior was investigated by a Q2000 differential scanning calorimetry (DSC) (TA Instruments Inc., USA). Samples were heated from 20 to 180 °C, kept at 180 °C for 10 min and then decreased to 20 °C at a linear heating rate of 10 °C min
1. For isothermal crystallization, samples were also first held at 180 °C for 10 min and then cooled to 113 °C at a rate of 100 °C/min.

3. Results and discussion 3.1. Exfoliation of MoS2 Scheme 1 outlines the overall procedures for preparation of the defect-free MoS2 nanosheets and production of composites in this study. The exfoliation of MoS2 was processed by direct ultrasonication of bulk MoS2 powder in DMF. Previous works indicated that sonication in DMF produces CH3 and CH2N(CH3)CHO radicals [20,21]. In air-saturated sonicated solutions, the radicals convert to the corresponding peroxyl radicals, which are likely to cause the damage of MoS2 layers. It would result in the MoS2 sheets with small size and sulfur vacancies in the peripheral edges and internal edges, as shown in Scheme 1. The dodecanethiol was used as antioxidant and surface modifier simultaneously in this study,

which could protect the MoS2 sheets from attacking by the radicals. Moreover, thiol groups of dodecanethiol could conjugate on the MoS2 sheets by chemical binding once the sulfur vacancies generated. This method can easily produce defect-free MoS2 nanosheets, the surface organic modification and robust nature of which ensure the good dispersion and dramatic reinforced effect on polymer materials. The representative AFM and TEM images of exfoliated MoS2 nanosheets are depicted in Fig. 1. Most of the nano-materials are present as two-dimensional thin sheets in Fig. 1a. TEM image (Fig. 1b) also reveals the presence of lamellar morphology with equal optical contrast, confirming the high-quality of obtained MoS2 nanosheets. Fig. 1c gives three height profiles varied from 1.5 to 2.0 nm corresponding to the MoS2 nanosheets shown in Fig. 1a. It is fairly consistent with the thickness of bi-layer or trilayer MoS2 nanosheets. By comparing the optical contrast of MoS2 nanosheets in Fig. 1a, it concludes that the majority of exfoliated MoS2 nanosheets are 2–3 layers thick. The lateral dimension of a large number of flakes was measured and the statistical analysis is plotted in Fig. 1d. The observation reveals that size distribution of MoS2 nanosheets predominantly ranges from 100 to 300 nm. Fig. 1e shows a digital picture of obtained MoS2 uniformly re-dispersed in different solvents, which is attributed to the presence of linear alkyl chain on MoS2 nanosheets. As the MoS2 dispersion has been conveniently prepared, polymer based composites would be easily fabricated by a solution mixing. In Fig. 1f, the color of PE/MoS2 samples turned to black green with increase of MoS2 content. All samples exhibited homogeneity and no sign of phase separation was found.

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Fig. 3. (a and b) TEM ultrathin section images of PE/MoS2-0.7 wt% at different magnification. (c and d) SEM images of the fractured surfaces of neat PE at different magnification; and (e and f) PE/MoS2-0.7 wt% at different magnification, respectively.

To verify dodecanethiol conjugation on MoS2 surface through adopted method, XPS analysis was performed. Two representative peaks of primitive MoS2 at 162.8 and 163.9 eV were assigned in Fig. 2a. In contrast, there is a new C–S binding peak appeared at 164.9 eV location in the S2p spectra of exfoliated MoS2 (Fig. 2b). This is indicative of the conjugation of dodecanethiol molecule on the surface of MoS2 sheets during the solvent exfoliation process. FT-IR was performed to confirm conjugation of dodecanethiol on MoS2 nanosheets. As shown in Fig. 2c, both the isolated dodecanethiol molecules and exfoliated MoS2 presented two main absorption peaks in the range of 2800–2950 cm
1, which could be assigned to the C–H aliphatic bands, respectively, indicating the presence of dodecanethiol on the surface of MoS2 nanosheets. Furthermore, as marked by black box the spectra from 2500 to 2600 cm
1 show that the thiol absorption (–SH) at 2580 cm
1 disappeared in the exfoliated MoS2 sample, which confirms the chemical binding between the thiol group of dodecanethiol and the surface defect of MoS2. 3.2. Characterization of PE/MoS2 nanocomposites XRD was performed to characterize the layered structure materials, which also could partially evaluate the dispersion state of layered nanofillers in polymer composite. XRD patterns of the bulk MoS2, exfoliated MoS2 and PE/MoS2 nanocomposites were compared in Fig. 2d. Different from bulk MoS2, the significantly

reduced 002 peak of exfoliated MoS2 caused by destruction of ordered crystalline in the direction of z-axis suggests the efficient exfoliation by sonication method and partial re-stack during the drying process. Moreover, the characteristic peaks of MoS2 were not observed on the XRD patterns of PE/MoS2 nanocomposites, which partially confirm the homogeneous dispersion of MoS2 nanosheets in PE matrix. In order to describe the dispersion state of MoS2 in PE matrix, TEM observations of ultrathin sections obtained from PE/MoS20.7 wt% are depicted in Fig. 3. It is clearly shown that most of the MoS2 with thin morphology are randomly dispersed in the polyethylene matrix without obviously reaggregation in Fig. 3a, providing the direct evidence of exfoliated or intercalated structure. The high magnification TEM image (Fig. 3b) verifies that the majority of MoS2 are basically fully exfoliated into single sheets with bits of few-layers (marked by white box). This observation is in good agreement with the XRD patterns of PE nanocomposites. SEM images were employed to observe the micro-morphology of fracture surface of composite samples. Fig. 3c–f present the fractured surface morphology of neat PE and PE/MoS2-0.7 wt% nanocomposite under the different magnifications. The morphological difference between neat PE and PE/MoS2 composite is clearly visible. The fractured surface of neat PE (Fig. 3c and d) was comparatively smooth. In contrast, the fractured surface of PE/MoS2 nanocomposite (Fig. 3f) shows a fairly rough morphology with many white dots. This pull-out morphology confirms the strong interfacial adhesion between defect-free MoS2 nanosheets

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Fig. 4. DSC (a) melting and (b) cooling curves of PE nanocomposites. (c) DSC heat flow and (d) relative crystallinity as a function of time; (e) Avrami plots of the samples isothermally crystallized at 113 °C, respectively. (f) Half crystallization time (t0.5) from Xc(t) = 50%, the crystallization rate parameter K and crystallization temperature (Tc) against the loading of MoS2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and PE matrix and predicts the performance enhancements of these nanocomposites. 3.3. Properties of PE/MoS2 nanocomposites It has been well reported that nanofillers could act as effective nucleating agents for various semicrystalline polymers and affect the crystallization process [22]. Therefore, it is necessary to characterize the influence of MoS2 on the crystallization behavior of PE/MoS2 composites. Both nonisothermal and isothermal crystallization behaviors were studied by DSC. The second melting and cooling curves of samples are shown in

Fig. 4a and b. There is 1–2 °C increasement of melting temperature (Tm) for different samples, but in general the addition of MoS2 does not influence the Tm of PE/MoS2 composites. The crystallization temperature (Tc) increased from 104.7 °C of neat PE to 109.2 °C of 0.2 wt% MoS2 (Fig. 4f). This dramatic increase of Tc was attributed to the heterogeneous nucleation induced by the MoS2 nanosheets. The impact of MoS2 on the crystallization kinetics of composites was also investigated. Heat flow and relative crystallinity (Xc) as a function of time for the samples isothermally crystallized at 113 °C are plotted in Fig. 4c and d, respectively. Xc is a relative parameter defined as

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Fig. 5. (a) TG and (b) DTG curves of PE nanocomposites under air atmosphere. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) Stress–strain curves, (b) enlarged stress–strain curves, (c) storage modulus, (d) loss modulus and (e) loss factor of PE/MoS2 nanocomposites with various MoS2 weight fractions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Z X c ¼ X c ðtÞ=X c ðt1 Þ ¼

t 0

dHðtÞ dt= dt

Z 0

1

dHðtÞ dt ¼ DHt =DH1 dt

ð1Þ

where dH/dt is the rate of heat evolution, DHt is the heat generated at time t, and DH1 is the total heat., The peak location in Fig. 4c meaning the time to maximum crystallization rate shifts to a much shorter time region when only 0.2 wt% MoS2 was added. Moreover, the half crystallization time (t0.5) obtained from the Xc(t) curves (Fig. 4d) for samples is plotted in Fig. 4f, which is defined as the time taken to complete 50% of crystallization process and usually chosen as a key parameter to evaluate the overall crystallization kinetics. Similar tendency of an initially rapid decrease (87.0%) of

t0.5 and subsequently the almost constant t0.5 as MoS2 loading further increasing were observed. These results confirmed the remarkable promoting effect of MoS2 on the overall crystallization rate of PE and the peculiar MoS2 loading dependence. The most frequently used analysis of experimental data concerning isothermal crystallization is based on the classic Evans– Avrami equation [23]. The general form is

1
XðtÞ ¼ expð
Kt n Þ

ð2Þ

where X(t) is the relative crystallinity; n is the Avrami exponent and K is the crystallization rate parameter. Fig. 4e shows the Avrami

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plots of samples isothermally crystallized at 113 °C. The values of kinetics parameter K for samples are determined by the initial linear part of the Avrami plots and shown in Fig. 4f. It can be obviously seen that K increases significantly by the addition of MoS2. Inclusion of as low as 0.2 wt% MoS2 nanosheets led to two orders of magnitude increase (from 0.57  10
3 to 69.66  10
3 min
n) of K value compared with PE. It is a confirmation of the nucleating activity of MoS2 nanosheets on the crystallization of PE. The thermal stability of neat PE composites was investigated by TGA in Fig. 5. It reveals that the defect-free MoS2 nanosheets are effective in terms of enhancing the thermal stability of the PE composites. The temperature of 5 wt% weight loss (T
5%) is usually used to evaluate the decomposition of polymer materials on the onset stage. T
5% and Tmax (temperature of maximum weight loss) of neat PE were recorded as 347.1 and 410.1 °C, respectively. Inclusion of as low as 0.2 wt% MoS2 led to a 10.8 °C increase of T
5% and a 35.5 °C increase of Tmax. Further increasing MoS2 loading gradually shifted the degradation temperature of composites to higher temperature. A 19.8 °C increase of T
5% and a 54.3 °C increase of Tmax were observed for 0.7 wt% defect-free MoS2 nanosheets. Moreover, the value of DTG peak of samples decreased by as much as 29.2% in Fig. 5b, compared to that of neat PE. This substantial enhancement of thermal stability can be ascribed to homogeneous dispersion of high aspect ratio nanosheets, which could act as barrier to disrupt the oxygen supply from the atmosphere to the bulk and prevent the emission of small gaseous molecules during degradation/ burning. The mechanical behavior of the PE composites was investigated by tensile testing. The stress–strain curves of samples are shown in Fig. 6. Incorporation of as low as 0.7 wt% MoS2 nanosheets with good exfoliation can obviously improve the tensile modulus, yield stress and breaking strength by 37.5%, 16.8% and 9.6%, respectively. Moreover, the elongation-at-break is also increased by 17.0%. The dramatic reinforcement in mechanical property is surely due to the presence of the well-exfoliated, high aspect ratio and defectfree MoS2 nanosheets. The strong interfacial interaction between nanofillers and matrix could efficiently transfer the load from the weak polymer chains to defect-free and robust MoS2 nanosheets. Storage modulus, loss modulus and loss angle tangent of PE/MoS2 composites are presented in Fig. 6c–e, respectively. The storage modulus is a measure of the stiffness and characterized in the temperature range of
100 to 100 °C. The storage modulus and loss modulus of PE nanocomposites are always higher than those of neat PE, and the largest increase (14.1%) in storage modulus and largest increase (46.3%) in loss modulus have been observed in the PE/MoS2-0.7 wt% sample at
100 °C. In Fig. 6e, there only the neat PE exhibited one obvious relaxation at 86.4 °C. The temperature at maximum of tan d is usually taken as the glass transition temperature (Tg). However, it is difficult to determine Tg of PE/MoS2 composites from the tan d peak in Fig. 6e. It may be because of the significant decrease in the peak value of tan d. The peak value of tan d is a measure of the capability of damping at the relaxation temperature. Therefore, this is indicative of the efficient reinforcement of defect-free MoS2 in the PE matrix. 4. Conclusions We used the dodecanethiol as antioxidant and surface modifier to produce defect-free and functionalized MoS2 sheets by ultrasonication of bulk MoS2 in DMF. Because of the conjugation of linear alkyl chain on the MoS2, the obtained MoS2 could be well redispersed in organic solvent and used to fabricate the PE based composites. Incorporating defect-free MoS2 nanosheets obviously promoted the crystallization process of PE, suggesting the high nucleation ability of MoS2 sheets. The considerable barrier action of defect-free MoS2 nanosheets could significantly enhance the

thermal stability of PE/MoS2 nanocomposites. Importantly, the strong nanosheets/matrix interfacial adhesion could achieve the efficient stress transfer from the weak polymer chains to the robust nanosheets, thus endowing the PE/MoS2 composites with excellent mechanical properties. It is anticipated that this work will enable MoS2 nanosheets to achieve their whole potential in polymer nanocomposites. Acknowledgements The work was financially supported by the National Natural Science Foundation of China (Nos. 21374111, 51403196), the China Postdoctoral Science Foundation (2013T60621), the Fundamental Research Funds for the Central Universities (WK2320000032), and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914). References [1] Wu SF, Huang CM, Aivazian G, Ross JS, Cobden DH, Xu XD. Vapor-solid growth of high optical quality MoS2 monolayers with near-unity valley polarization. ACS Nano 2013;7(3):2768–72. [2] Ganatra R, Zhang Q. Few-layer MoS2: a promising layered semiconductor. ACS Nano 2014;8(5):4074–99. [3] Li YG, Wang HL, Xie LM, Liang YY, Hong GS, Dai HJ. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J Am Chem Soc 2011;133(19):7296–9. [4] Yao YG, Lin ZY, Li Z, Song XJ, Moon KS, Wong CP. Large-scale production of twodimensional nanosheets. J Mater Chem 2012;22(27):13494–9. [5] Chang K, Chen WX. L-cysteine-assisted synthesis of layered MoS2/graphene composites with excellent electrochemical performances for lithium ion batteries. ACS Nano 2011;5(6):4720–8. [6] Chou SS, De M, Kim J, Byun S, Dykstra C, Yu J, et al. Ligand conjugation of chemically exfoliated MoS2. J Am Chem Soc 2013;135(12):4584–7. [7] O’Neill A, Khan U, Coleman JN. Preparation of high concentration dispersions of exfoliated MoS2 with increased flake size. Chem Mater 2012;24(12):2414–21. [8] Khan U, O’Neill A, Lotya M, De S, Coleman JN. High-concentration solvent exfoliation of graphene. Small 2010;6(7):864–71. [9] Suslick KS, Choe SB, Cichowlas AA, Grinstaff MW. Sonochemical synthesis of amorphous iron. Nature 1991;353(6343):414–6. [10] Quintana M, Grzelczak M, Spyrou K, Kooi B, Bals S, Van Tendeloo G, et al. Production of large graphene sheets by exfoliation of graphite under high power ultrasound in the presence of tiopronin. Chem Commun 2012;48 (100):12159–61. [11] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28(11):1539–641. [12] Feng XM, Xing WY, Song L, Hu Y, Liew KM. TiO2 loaded on graphene nanosheet as reinforcer and its effect on the thermal behaviors of poly(vinyl chloride) composites. Chem Eng J 2015;260:524–31. [13] Liang JJ, Huang Y, Zhang L, Wang Y, Ma YF, Guo TY, et al. Molecular-level dispersion of graphene into poly(vinyl alcohol) and effective reinforcement of their nanocomposites. Adv Funct Mater 2009;19(14):2297–302. [14] Feng XM, Xing WY, Song L, Hu Y. In situ synthesis of a MoS2/CoOOH hybrid by a facile wet chemical method and the catalytic oxidation of CO in epoxy resin during decomposition. J Mater Chem A 2014;2(33):13299–308. [15] Feng XM, Wang X, Xing WY, Zhou KQ, Song L, Hu Y. Liquid-exfoliated MoS2 by chitosan and enhanced mechanical and thermal properties of chitosan/MoS2 composites. Compos Sci Technol 2014;93:76–82. [16] Wang MC, Yan C, Ma L, Hu N, Chen MW. Effect of defects on fracture strength of graphene sheets. Comp Mater Sci 2012;54:236–9. [17] Ataca C, Sahin H, Akturk E, Ciraci S. Mechanical and electronic properties of MoS2 nanoribbons and their defects. J Phys Chem C 2011;115(10):3934–41. [18] Quintana M, Grzelczak M, Spyrou K, Calvaresi M, Bals S, Kooi B, et al. A simple road for the transformation of few-layer graphene into MWNTs. J Am Chem Soc 2012;134(32):13310–5. [19] Makarova M, Okawa Y, Aono M. Selective adsorption of thiol molecules at sulfur vacancies on MoS2(0001), followed by vacancy repair via S–C dissociation. J Phys Chem C 2012;116(42):22411–6. [20] Misik V, Riesz P. Peroxyl radical formation in aqueous solutions of N,Ndimethylformamide, N-methylformamide, and dimethylsulfoxide by ultrasound: implications for sonosensitized cell killing. Free Radical Bio Med 1996;20(1):129–38. [21] Wang J, Huang LY, Chen GA, Huang JL. The sonochemiluminescence of Lucigenin in N,N-dimethylformamide solution under the influence of natural flavonoids. Chem Lett 2005;34(11):1514–5. [22] Chrissopoulou K, Andrikopoulos KS, Fotiadou S, Bollas S, Karageorgaki C, Christofilos D, et al. Crystallinity and chain conformation in PEO/layered silicate nanocomposites. Macromolecules 2011;44(24):9710–22. [23] Avrami M. Kinetics of phase change. I general theory. J Chem Phys 1939;7 (12):1103–12.