A Green Route to a Low Cost Anisotropic MoS2 ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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A Green Route to a Low Cost Anisotropic MoS2/Poly(Vinylidene Fluoride) Nanocomposite with Ultrahigh Electroactive Phase and Improved Electrical and Mechanical Properties Kai Cai,*,† Xiao Han,‡ Yan Zhao,*,‡ Ruilong Zong,∥ Fei Zeng,⊥ and Dong Guo*,‡,§ †

Beijing Center for Physical and Chemical Analysis, No. 27 North West Third Ring Road, Beijing 100089, China School of Materials Science & Engineering, Beihang University, No. 37 Xueyuan Road, Beijing 100191, China § Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China ∥ Department of Chemistry, Tsinghua University, Beijing 100084, China ⊥ Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: Environment issues due to growing energy consumption have motivated great research efforts on new materials for efficient energy storage and their low cost fabrication. This study reports an energy-efficient solution route for the fabrication of a unique high permittivity nanocomposite film consisting of molybdenum disulfide (MoS2) nanosheets spontaneously aligned in poly(vinylidene fluoride) (PVDF) via super-2D-confinement and gravity sedimentation. A simple thermal lamination was further developed to get anisotropic films with controllable thickness. Interestingly, an ultrahigh fraction (∼86% as confirmed by synchrotron radiation XRD) of β-phase PVDF was directly obtained by only 3.4 vol % of orientated MoS2 nanosheets due to possible crystallization disturbance and synergistically reinforced electrostatic interaction and super-2D-confinement. This reveals a greener route to the desirable electroactive phase PVDF, whose formation usually requires giant electrical field or mechanical stresses. Simulation of the permittivity perpendicular to the nanosheets (up to 146 @ 100 Hz with 19.8 vol % MoS2) also revealed anisotropy due to alignment. The permittivity and conductivity parallel to the nanosheets were much higher, showing anisotropic ratios of 3.96 and 6.14 (9.5 vol % MoS2), respectively. Furthermore, the nanocomposite with a suitable composition showed simultaneously increased tensile strength, elongation, and energy storage density, making it promising for multifunctional field applications. The results may also improve the understanding of polymer polymorph transition and provide hints on new green pathways for novel composites. KEYWORDS: Poly(vinylidene fluoride), Energy storage density, Self-alignment, Dielectric properties, Polymorph

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ing of anisotropic conductive fillers and fluoropolymer have been tried. Anisotropic fillers like nanotubes and nanofibers can form a percolation route required for high permittivity at a much smaller loading.3 This also guarantees much better mechanical performance and processability.4 However, so far, anisotropic fillers are limited to relatively expensive graphene, and the anisotropic characteristics remain largely unexplored. More importantly, to achieve filler alignment, usually a high electrical/magnetic field and very complicated particle preprocessing like surface encapsulation or epitaxy are needed,5−8 On the other hand, the prototype matrix poly(vinylidene fluoride) (PVDF) of such composites usually forms in the nonelectroactive α-phase, while the formation of the β-

INTRODUCTION As the world moves toward alternative environmentally friendly solutions, sustainable energy storage technologies have become a high priority issue. The ever-increasing demand for compact, reliable, and hazard-free transient energy storage devices used in power electronics and power delivery systems has stimulated great research interest in energy efficient, new dielectric materials. Polymer-based new high permittivity composites and their energy-efficient manufacture has recently become a research focus1,2 Compared to their inorganic counterpart, these composites have advantages of a much lower fabrication cost and easy formation in bulk or thin film form. In addition, their flexibility and much lower processing temperature make them easily adaptable to emerging portable devices, wearable healthcare products, green electronics, etc. Nevertheless, greener strategy is still urgently needed to further increase manufacture efficiency. Hence, anisotropic composites consist© XXXX American Chemical Society

Received: December 12, 2017 Revised: January 31, 2018 Published: February 14, 2018 A

DOI: 10.1021/acssuschemeng.7b04697 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the fabrication procedure of the self-aligned MoS2/PVDF composite film.

Figure 2. AFM image of as-exfoliated MoS2 nanosheets (a) and height profiles of two lines as indicated in the topography images (b and c). TEM image of as-exfoliated MoS2 nanosheets (d) and the SAED (e) and HR-TEM (f) patterns.

and thermal properties, and its cost is much lower than that of graphene. Interestingly, single layer MoS2 may exhibit piezoelectricity due to strain-induced lattice distortion and ion charge polarization.22 Considering the critical role of polarization in dielectric behavior, it is reasonable to select 2D MoS2 for polymer-based composite dielectrics. Here, a novel PVDF based high-permittivity flexible nanocomposite film with improved and unique anisotropic performance containing spontaneously aligned 2D MoS2 nanosheets was fabricated by a green route without complicated electro-magnetic or mechanical facilities. Interestingly, the results also reveal a new greener strategy for direct fabrication of electroactive PVDF with only small amount of aligned 2D fillers, which is crucial for the design and manufacture of new device. The comprehensive analysis of the microstructure and electrical and mechanical properties of the MoS2/PVDF nanocomposites may also improve the understanding of polymer polymorph transition and provide hints on new energy-efficient pathways for novel composites.

phase that is crucial for functionalities usually needs very high electrical or mechanical stress,9,10 which in turn limits device design and application. Therefore, its greener fabrication is also an urgent issue. An intriguing fundamental issue for polymerbased composites is the filler induced complicated transition between different polymorphs of the polymer.11 Despite the significance of such a phenomenon in PVDF, the underlying mechanisms are still unclear,12 although the nucleation effect13,14 and electrostatic interaction15 etc. have been proposed. Particularly, in textured composites, the structural evolution is further complicated by the alignment, while the phenomenon is rarely studied. Consequently, the relevant mechanisms and influence on performance are still far from being clear. Transition metal dichalcogenides (TMDs) have superior mechanical, electrical and chemical properties16,17 and have been used as fillers for multifunctional composites for applications like saturable absorber for lasers,18 flame retardance,19 electromechanical actuatotion,20 and electrodes for batteries.21 However, so far, polymer based dielectric composites consisting of aligned 2D TMDs and their energy storage performance are very rarely studied. The challenge exists in achieving good dispersion, selecting greener raw materials, and an energy-efficient route for alignment control. The 2D molybdenum disulfide (MoS2) is a rather cheap dichalcogenide that possesses unique mechanical, electronic,

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EXPERIMENTAL SECTION

Preparation of MoS2 Nanosheets and MoS2/PVDF Nanocomposites. The nanocomposite films were prepared by a highly efficient solution route as schematically shown in Figure 1. The MoS2 nanosheets were prepared by liquid phase exfoliation similarly to the literature using commercial grade MoS2 powders without further processing.23 As an improved approach for higher efficiency, first 1B


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Figure 3. XRD profiles of MoS2/PVDF composites. (a) XRD profiles in the 2θ range of 12.5−35° with logarithm intensity. (b) Magnified XRD profiles in the 2θ range of 17−27° with multiple peak fitting of the diffraction at 2θ of ∼20°. (c) Synchrotron radiation based XRD profiles of pure PVDF and composite sample with a MoS2 fraction of 3.4 vol %. (d) Magnified profiles in the range of 19.00−21.75°. methyl-2-pyrrolidinone (NMP) was used as the medium for exfoliation. Then, the sediment and NMP were partially removed, and DMF was added under sonication. Then, a suitable amount of PVDF powders (Tm ≈ 170 °C, Alfa Aesar Co., Ltd.) was directly added under stirring and heating to give the precursor solution for fabricating the composite films. Here, DMF can act as both an exfoliation liquid and good solvent for PVDF. Finally, the solution was cast on a glass substrate ; then, after heating at 180 °C for 10 min plus 200 °C for 15 min, highly crystalline and uniform composite films containing self-aligned MoS2 nanosheets with a controllable thickness of ∼10 to 70 μm were formed. Films with a thickness of 70-μm-to-mm scale with high anisotropy were also successfully fabricated by thermal lamination of thinner films at ∼200 °C. Characterization. The microstructure of the MoS2 nanosheets was identified by High-Resolution Transmission Electron Microscopy (HR-TEM) and Selected Area Electron Diffraction (SAED) with a FEI Tecnai G2 F30 microscope and by tapping mode Atomic Force Microscopy (AFM, Bruker Dimension Icon). Field-Emission Scanning Electron Microscopy (FE-SEM) and element analysis were conducted with an S4800 microscope (Hitachi, Japan) equipped with an energy dispersive X-ray (EDX) spectrometer. Attenuated total reflectioninfrared Infrared spectra (ATR-IR) were recorded using a Bruker spectrometer. XRD profiles were recorded under a step size of 0.04° and a rather low speed of 6 s per step using a D8 Advance diffractometor (Bruker, Germany) with Cu Kα radiation. To check the phase structure, narrow range XRD profiles of 18−22° with a resolution of 0.005° were also recorded at Beijing Synchrotron

Radiation Facility (BSRF), Institute of High Energy Physics, with focused X-rays monochromated to a wavelength of 1.5476 Å. The relative dielectric constant (εr) and loss tangent (tan δ) were measured using samples with silver electrodes by an Agilent HP4294A impedance analyzer. Electric breakdown strength was measured by a dielectric withstanding voltage tester (CS2671AX, Nanjing Changsheng Electronic Co. China). Conductivity was measured using a HP4140B analyzer. To measure the anisotropic characteristics, the dielectric properties and conductivity along the film plane were measured by platelet samples cut from thermal laminated thick samples. Tensile properties were evaluated using 30 mm × 15 μm strip films with a Hengyi testing machine at a crosshead speed of 2.0 mm/ min. For this measurement, a 40 mm × 20 mm paper clamp with a middle hollow area of 20 mm × 10 mm was used to control the sample dimension.

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RESULTS AND DISCUSSION Structure of the Exfoliated MoS2 Nanosheets. The AFM topography of the exfoliated MoS2 nanosheets dip coated on a flat mica is shown in Figure 2. The irregularly shaped nanosheets (Figure 2a) show height profiles (Figure 2b and c) corresponding to a thickness of approximately 1.2−1.3 nm, which is in good agreement with the reported thickness of two monolayer MoS2’s.24 The TEM and HR-TEM images and SAED pattern of the exfoliated MoS2 are shown in Figure 2d−f. The edges of the MoS2 nanosheets tend to form an included angle of 60° or 120° well-defined by the hexagonal lattice of C


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Figure 4. (a) ATR-IR spectra of the MoS2/PVDF composites. (b) MoS2 content dependence of β phase fraction F(β) and relative intensity ratio I838/I763 for the bands at 838 and 763 cm−1.

MoS2. Although aggregation is inevitable, the SAED pattern and HR-TEM image in Figure 2e and f clearly demonstrate the presence of single-crystalline and even monolayer MoS2 nanosheets with hexagonal symmetry and periodic atomic arrangement with an interplanar distance of 0.280 nm due to the (100) plane family.25,26 Accurate size distribution data of the exfoliated MoS2 are difficult to derive due to the errors related to sample selection and image analysis. Nevertheless, these results show that high quality well exfoliated MoS2 nanosheets with a micron scale lateral dimension are successfully obtained. Structural Analysis of the MoS2/PVDF Nanocomposites. As clearly seen from the XRD profiles of the MoS2/ PVDF nancomposites in Figure 3, all the samples have a prominent peak at around 14.5° due to the (002) plane of the 2H MoS2 phase. The relative intensities of the characteristic peaks of MoS2 in the composites are largely different from those of single phase 2H MoS2 (PDF #37-1492). The (100) and (101) diffractions of MoS2 were largely suppressed in the composite, while the (004) diffraction was greatly enhanced. Because the (004) and (100) planes are, respectively, parallel and perpendicular relative to the MoS2 nanosheet plane, the change in relative intensity of the out-of-plane XRD suggests that the 2D MoS2 nanosheets are preferentially oriented parallel with the film plane; namely, they on average have a lying down orientation. Magnified XRD profiles in the 2θ range of 17−27° are presented in Figure 3b. The peaks corresponding to different phases of pure PVDF are denoted by dashed lines. The signal intensity of the PVDF matrix decreases with the increase in filler content similar to those in other composites.13 This is largely caused by the reduced relative content of PVDF in the composites. In addition, the change in surface roughness, the appearance of a polymer/filler interface, and interfacial regions with the addition of the highly aligned fillers may also reduce the XRD intensity. Meanwhile, the center of the 19.8° peak tends to shift to a higher angle. This is caused by the appearance of the β-phase (Figure 3b), whose overlapped (200) and (110) diffractions are located at 20.4°. When the MoS2 content increases to 3.4 vol % and 5.0 vol %, the β-phase becomes dominant, while the α-phase signal even disappeared. A further increase of MoS2 to more than 9.5 vol % leads to the shift of the center of this broad peak (which consists of two peaks) back to a lower angle, indicating the transformation again to an α-phase main structure. A clearer image of the MoS2

content dependence phase composition can be illustrated by the variation of the peak-top position of this broad peak (Figure S1a, SI) and the intensity ratio (Figure S1b, SI) of the deconvoluted peaks at 20.4° and 19.9° (I20.4/I19.9) or 18.3° (I20.4/I18.3). The dominance of β-phase PVDF at a MoS2 loading of 3.4−5.0 vol % and a transitional region at higher or lower content of MoS2 is clear in the figures. The total crystallinity (∼40−50%) estimated from the XRD profiles is similar to those in other PVDF-based composites. In the textured MoS2/PVDF composites, the self-aligned MoS2 filler sheets should restrain the movement of macromolecular chains, because the limited space between MoS2 sheets may exert weak compression or confinement during the crystallization and self-alignment process. On the other hand, the all-trans β-phase PVDF has more regular atomic arrangement with slightly higher density and smaller interplanar distance than those of the TGTG′ α-phase (the density of βand α-phase are 1.973 and 1.925 g/cm3, respectively).13 This is evident from the peak position of the two phases. Therefore, based on the XRD results, it is reasonable to propose that the aligned MoS2 induced stress or geometry confinement would promote the formation of the more compact and regularly packed β-phase as similar to those in nanostructured PVDF.27 In addition, because the two phases have different dielectric and piezoelectric properties, an important implication of the polymorph change is that the properties can be tailored by controlling the filler content. A more accurate estimation of the phase fraction of a typical sample can be made from the synchrotron radiation based XRD (SR-XRD) pattern in Figure 3c. A MoS2 content of 3.4 vol % induced a rather high fraction of β-crystallites in PVDF. On the basis of the deconvoluted profiles in Figure 3d, the (110) intensity of α-phase at 19.99° and the sum of the diffractions in (110) and (200) planes of the β-phase at 20.48° were derived, then, based on equations for the integrated XRD intensity in the Supporting Information (eq S1) and the structure factor FHKL, multiplicity factor P and Lorentz-polarization factor φ(θ) etc. for PVDF.28 The relative content of different phases can be calculated as shown in the inset of Figure 3d. Surprisingly, a MoS2 nanosheet content of 3.4 vol % induced a ferroelectric βphase fraction as high as about 86%. The ATR-IR spectra are shown in Figure 4. The bands at 614 cm−1, 763 cm−1, and 794 cm−1 are related to the mixed CF2 bending and CCC skeletal vibration, the in-plane bending or D


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Figure 5. SEM images of the fracture surface (cross-section) of different samples: (a) Pure PVDF; (b1) 2.1 vol % MoS2/PVDF sample; (b2) enlarged image of the area indicated in b1; (c1) 5.0 vol % MoS2/PVDF sample; (c2) enlarged image of the area indicated in c1; (d) 9.5 vol % MoS2/PVDF; (e1) 19.8 vol % MoS2/PVDF with local EDX mapping; (e2) The white arrows in b1, b2, c1, and c2 indicate the film plane direction.

Figure 6. (a) Schematic illustration of the mechanisms of spontaneous alignment of the 2D MoS2 nanosheets. (b) Illustration of the mechanisms for the filler induced formation of polar β-phase in the PVDF matrix.

rocking vibration, and the CH2 rocking vibration, respectively.29 These are characteristic bands of the α-phase. The band at 838 cm−1 has been identified as typical for β-phase PVDF, which is due to the mixed CF2 asymmetric stretching and CH2 rocking vibrations.30 To estimate the phase composition, MoS2 content dependence of the ratio between the intensity of the two typical bands at 763 and 838 cm−1 for the α and β phases, respectively, is derived (see Figure 4b). Consistent with those from XRD profiles, the ratio reaches a maximum at a MoS2 content of 3.4 vol %, where the characteristic bands corresponding to the crystal α phase at 763 and 794 cm−1 almost disappear, and another β-phase band at 1280 cm−1 becomes strong suddenly. According to the Lambert−Beer law and assuming that only the α- and β-phases are present, we may calculate the phase composition through the deconvolution method13,31 using the absorbance of α- and β-phases at 763 and 838 cm−1 and their respective absorption coefficients of 6.1 × 104 cm2 mol−1 and 7.7 × 104 cm2 mol−1. As shown in Figure 4b, at a MoS2 content of 3.4 vol %, the β-phase fraction reaches the maximum of ∼92% (mole ratio). A value very close to that (∼86%) derived

by SR-XRD is shown in the inset of Figure 3d. The value is much higher compared with the data of other PVDF based composite systems.32,33 The maximum β-phase fraction appears when MoS2 is around 3.4−5.0 vol %, consistent with the data from XRD in Figure S1b, SI. These imply a new greener approach for directly fabricating β-phase PVDF with only a small amount of aligned 2D fillers. Furthermore, other solvents for PVDF like DXL and DMPU reported recently may be tried in a further study to develop a greener method for fabricating the materials.34,35 Morphology and Discussion of Alignment and Phase Transition Mechanisms. As can be seen from the SEM images in Figure 5, the neat PVDF sample consists of homogeneous lamellar grains. For the composites, when the filler content is lower than 5.0 vol %, high-aspect-ratio MoS2 nanosheets with a lateral size of approximately 1 μm and a thickness of several nanometers homogeneously distributed in the matrix can be observed. As shown in Figure 5d and e1, when the MoS2 content is up to 9.5 vol % and 19.8 vol %, nonuniform morphology with slight aggregation appears. For E


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Figure 7. Electrical properties of different samples: (a) Frequency dependence of dielectric constants and (b) dielectric loss; (c) MoS2 content dependence of dielectric constant and the linear fitting results based on percolation theory (inset); (d) MoS2 content dependence of conductivity and the logarithm plot (MoS2 volume fraction p)−1/3 (inset).

the filler has affected polymer crystallization. Such filler-induced change in crystallization kinetics is common in polymers. Therefore, the SEM images together with the XRD intensity change may indicate a filler-disturbed crystallization mechanism for phase transition. Polymer nucleation can be usually promoted by additives that act as heterogeneous nuclei.41 For instance, a highly oriented “transcrystalline layer” was found on carbon fiber due to its high nucleating ability in the composite. Furthermore, the filler induced specific phase that was not favorable in neat polymer was observed in systems like claynylon6 composites,42 where the β-crystallites prefer to nucleate near the clay surface. In agreement with these, Figure 5 indicates that MoS2 nanosheets should have acted as initial nuclei and enhanced the nucleation of PVDF β-crystallites. The nucleation effect of the MoS2 nanosheets may be further affected by electrostatic interaction between the negatively charged MoS2 nanosheet surface and the positive CH2 side of the dipoles of the polymer chain. This leads to the alignment of dipoles and facilitating a TTTT conformation of the β-phase.14 This is consistent with previous studies on PVDF-based composites with different oxide fillers.43−45 What deserves noting is that the universal electrostatic interaction is “reinforced” for this highly aligned anisotropic filler particles, which may be another driving force that is synergistic with the geometry confinement in promoting β-phase. When the MoS2 content exceeds a certain value, we propose that these nanosheets may limit PVDF segmental mobility and suppress

this high MoS2 loading sample, its EDX element mapping of an area in Figure 5e1 was shown in Figure 5e2. The homogeneous distribution of Mo and F indicates a well-dispersed microstructure. In addition, Figure 5b−e indicate that in all samples the MoS2 nanosheet plane tends to highly align along the film plane or perpendicular to gravity. This spontaneous alignment of MoS2 nanosheets should be mainly caused by two factors. First, the gravitational force may compel the plate-like nanosheets to adopt a flat-lying orientation, similar to other anisotropic fillers dispersed in a low-viscosity medium.36 Second, the macromolecular chains of solvent cast polymer films are well-known to prefer to orient parallel to the film plane due to geometry confinement.37 Note that the difference in electronegativity between S and Mo atoms gives rise to the negatively charged MoS2 surface, which may interact with the positive CH2 side of the H−C−F polymer dipoles,38,39 Then, it is reasonable to propose that the anisotropic while enhanced van der Waals (electrostatic) interaction between the 2D fillers and the flat-lying PVDF chains further facilitates the lying-down movement of the nanosheets under the shear flow during casting, and we may call it a “super-2D-confinement” effect.40 Such gravitational sedimentation and 2D filler induced unique super-2D-confinement mechanisms for alignment are schematically illustrated in Figure 6a. Interestingly, it is also clear from Figure 5b2 and c2 that the polymer around the MoS2 nanosheets shows a different morphology from those away in the matrix, indicating that F


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ACS Sustainable Chemistry & Engineering Table 1. Dielectric and Conductive Anisotropy of the MoS2/PVDF Nanocompositesa MoS2 vol % 9.5 12.3 a

εr∥ 83.12 61.95

εr⊥ 20.99 27.89

δ∥ 0.24 0.29

δ⊥

σ∥ (S cm−1)

σ⊥ (S cm−1)

εr∥/εr⊥

δ∥/δ⊥

σ∥/σ⊥

−10

7.0 × 10−11 4.6 × 10−9

3.96 2.83

1.19 1.46

6.14 2.39

4.3 × 10 1.1 × 10−8

0.20 0.20

∥ and ⊥ represent that the orientations of the MoS2 nanosheets are parallel and perpendicular to the electric field, respectively.

β-phase change may also slightly increase the dielectric constant due to the higher εr of the latter phase. The insulating behavior of the nanocomposites as evaluated by DC conductivity (σ) is shown in Figure 7d. It shows an increasing trend with increasing MoS2 loading because of its high conductivity. It deserves noting that even at a relatively high filler content (19.8 vol %), σ remains as low as 10−8 S cm−1, which is much smaller compared to the data for graphene/PVDF52 and other similar type composites,53,54 indicating good insulation perpendicular to the alignment direction. This also partly explains the lower loss of the MoS2/ PVDF composites. The relatively low conductivity of the samples makes it possible for poling without large leakage current and hence to get piezoelectric performance. Issues about piezoelectricity and poling efficiency deserve further investigation. Also, we found that ln σ was roughly proportional to p−1/3 when the MoS2 content was in the range of 1.0−9.5 vol % (inset of Figure 7d). This may imply that the charge transport is mainly based on electron tunneling without direct contact of semiconductive fillers.53,55 σ increases largely at higher MoS2 loadings, indicating a gradual transition from aligned while isolated 2D fillers to a contacted network. The breakdown strength first increases slightly from 237.6 kV mm−1 to 268.4 kV mm−1 when the MoS2 content is ≤2.1 vol %, as shown in Figure S2, SI, indicating that a suitable amount of filler may even enhance high voltage endurance. The reduced value with a further increase of MoS2 should be due to the shorter conductive path and the enhanced local electric field due to space charges accumulated on the filler/matrix interface. On average, the nanocomposites show a higher breakdown strength than those with inorganic fillers.56,57 Consequently, a suitable amount of MoS2 filler (see Figure S2, SI) of 2.1 vol % leads to a higher energy density (1.90 J/cm3) compared to that of pure PVDF (1.20 J/cm3) as estimated from eq S10, SI, and similar results were obtained by loop measurements. To estimate the anisotropic dielectric properties and conductivity, thermal laminated thick film samples with silver electrodes on different sides were used. The alignment of such samples is the same as that of cast composite thin film because lamination has no effect on the prealigned nanosheets. The results of two typical samples are shown in Table 1. Evidently, both εr and σ are largely increased along the alignment direction, while the loss shows less change. Anisotropic ratios of 6.14 (9.5 vol %) and 2.39 (12.3 vol %) for σ and 3.96 (9.5 vol %) and 2.83 (12.3 vol %) for εr confirm the effect of the alignment. The anisotropic ratio of the samples with a MoS2 content around 3−5 vol % is around 3. These indicate that εr data several times those in Figure 7a can be obtained. Such unique anisotropic properties are not only useful for energy storage but also promising for special multifunctional fields like electromagnetic interference (EMI) shielding, which is critical for strategic systems like aircraft, nuclear reactors, telecommunication devices, etc. The highly aligned nanofillers may more effectively shield electromagnetic waves that emanate through the thickness direction, as interfacial polarization is higher in the plane direction. Such flexible nanocomposites also have

the molecular packing and crystallization (as reflected by the decreased XRD intensity), leading to a lower fraction of the βphase. Thus, the filler induced complicated phase transition may be well explained by the interplay of crystallization disturbance and reinforced electrostatic effects as shown in Figure 6b. Electrical Characterization and Anisotropic Performance Measurement. The normal dielectric constants εr (perpendicular to the nanosheets) of the composites in the frequency range of 102−107 Hz are shown in Figure 7a. The εr shows a simple decreasing trend with frequency, which should result from the reduction of the dipolar contribution. The dielectric loss first decreases slightly with frequency due to the signature relaxation generated interfacial polarization.46 Then, it increases as a consequence of increased leakage current related to the defects formed at high filler loading, as reflected by the morphology in Figure 5d. At the highest frequency side, the loss keeps stable again, which may be due to the slowdown of the molecular dipole motions at too high a reversal rate of field. With increasing MoS2 nanosheet content, the εr (100 Hz) of the composite gradually increases and reaches up to 146 at a MoS2 loading of 19.8 vol %, about 13 times that of pure PVDF. The slightly higher εr at 3.4 vol % MoS2 versus that at 5.0 vol % may be related to the ultrahigh β-phase content of the former sample, because the permittivity of the β phase is slightly higher than that of the α phase. Several theoretical models were selected to simulate the effective εr of the nanocomposites, including the percolation theory,46 the effective medium theory model (EMT),47 the Maxwell−Garnett (M-G) model,48 and the Jayasundere−Smith model,49 whose mathematical expressions are sequentially listed in eqs S4−S8, SI. The simulation results are shown in Figure 7c. As shown in Figure 7c, the deviation from the Jayasundere− Smith model (interactions between particles is considered) indicates weak interaction perpendicular to the film plane, implying alignment of fillers. The M-G approximation involves the calculation of field induced by spherical or ellipsoidal fillers and approximation of distortion of the field caused by electrostatic interaction between different phases. Thus, it cannot give good simulation due to the 2D shape of the MoS2 filler. The EMT model has been built up by taking into account of the irregular morphology of filler particles. The morphology fitting factor n takes the values of 0 (prolate), 1/3 (sphere), or 1 (oblate) or intermediate values for intermediate shapes.46 Importantly, it gives a rather good fit with n = 0.51, revealing again the influence of a 2D shape of the MoS2 nanosheets. Due to the semiconductive nature of MoS2, the percolation theory gives a rather good fitting as shown in the inset of Figure 7c. The derived percolation threshold of 33.48 vol % is higher than that of the composites containing graphene or carbon nanotube.50,51 This is because the percolation path perpendicular to the MoS2 nanosheets is more difficult to form. Note that the higher this “perpendicular” percolation, the lower the percolation along the MoS2 alignment direction that can be expected due to the anisotropic geometry. The promoted α- to G


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Figure 8. Tensile properties of typical composite samples: (a) typical load-deflection curve and (b) calculated tensile strength and elongation and (c) a photograph of the flexible composite films with 9.5 vol % MoS2.

advantages of light weight, low temperature processing, large area, excellent mechanical properties, etc. Mechanical Properties. The photograph of a typical MoS2/PVDF nanocomposite sample with a special paper clamp used for tensile performance measurement is shown in the inset of Figure 8a. As shown in Figure 8b, tensile strength of the samples under such a condition with a suitable content of MoS2 was even enhanced. Actually, MoS2 itself has superior mechanical properties, with tensile strength and Young’s modulus as high as ∼23 GPa and ∼270 GPa, respectively.58,59 As a consequence, the propagation of cracks in the composite system was blocked by an inorganic filler during stretching,60 leading to a higher tensile strength. In this special sample, the aligned MoS2 nanosheets should have an additional reinforcement effect along the tensile direction because they can sustain more loading along the nanosheet plane. In addition, the stress concentration effect around interfacial defects (if any) may also be reduced along the alignment direction. The deterioration of mechanical performance at a MoS2 content of 12.3 vol % might be partly due to the filler aggregation and formation of more defects around the fillers. Usually, rigid fillers would lead to improved mechanical strength, but this is associated with a lower elongation, while the photograph in Figure 8c indicates that the nanocomposite film still shows excellent flexibility even with a MoS2 content of 9.5 vol %. In addition, the elongation of the MoS2/PVDF nanocomposite film remains rather high, especially when the MoS2 nanosheet content is less than 5.0 vol %. As an analogue to previous studies on the nanoclay/PVDF composite, we assume that the filler induced α- to β-phase transformation of PVDF may have played a critical role in toughening the MoS2/ PVDF nanocomposite.61 As previously mentioned, long chain PVDF in the β-form inclines to nucleate on the MoS2 nanosheet surface, which may give rise to a structure much more conductive to plastic flow under stress, leading to more efficient energy dissipation during deformation.

This reveals a greener route for direct fabrication of electroactive phase PVDF. Both SEM and XRD results indicated that the MoS2 nanosheets were highly oriented parallel to the film plane due to gravity sedimentation and super-2D-confinement specific to the charged 2D fillers. Simulation of the permittivity perpendicular to the nanosheets (up to 146 @ 100 Hz with 19.8 vol % MoS2) also revealed anisotropy. The permittivity and conductivity parallel to the nanosheets were much higher, showing anisotropic ratios of 3.96 and 6.14 (9.5 vol % MoS2), respectively. The films with suitable MoS2 loading also showed increased tensile strength, elongation, and energy density, indicating that the aligned reinforcing networks via the green route are promising for multifunctional composites. The results may provide hints on new energy-efficient pathways for novel composites.

CONCLUSIONS A greener route for the fabrication of unique nanocomposite films consisting of MoS2 nanosheets spontaneously aligned in PVDF was developed. A simple thermal lamination was further developed to get anisotropic films with controllable thickness. XRD and ATR-IR observations indicated that the MoS2 nanosheets induced α-phase to polar β-phase transition in PVDF, and an ultrahigh β-phase fraction of about 86% by only 3.4 vol % of MoS2 was confirmed by synchrotron radiation based XRD. This was attributed to a crystallization disturbance and synergistically reinforced electrostatic and confining effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04697. Comparison of XRD peak position, equations for XRD intensity, equations for different dielectric models, equations for evaluating dielectric breakdown strength and energy density (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-68417645. E-mail: [email protected]. *Tel.: +86-10-82338872. E-mail: [email protected]. *Tel.: +86-10-82338872. E-mail: [email protected]. ORCID

Xiao Han: 0000-0002-1670-9140 Fei Zeng: 0000-0001-8735-8766 Dong Guo: 0000-0001-9938-2624

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51302015, Grant No. 11574346), the National Basic Research Program of China (973 Program, Grant No. 2013CB632900), and the Overseas Talent Foundation of Beijing Academy of Science and Technology (Grant No.OTP-2013-001), and the Fundamental Research Funds for the Central Universities. The authors also H


Research Article


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thank Dr. Chen at Beijing Synchrotron Radiation Facility (BSRF) for support on the relevant experiments.

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