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Ordered 2D layered MoS2/conjugated polymer nanocomposites: influences of sulfonated β-cyclodextrin on the preparation and properties

  • Jin WangEmail author
  • Zongchao Wu
  • Ruihong Xie
  • Yuanyuan Zhu
  • Xueting Liu
Original Research
  • 113 Downloads

Abstract

Nanocomposites of MoS2 and conjugated polymers are excellent candidates for optical limiters, solid electrodes, electrolytes, and other purposes. The ordered layered structures of nanocomposites are essential. A strategy to prepare the regular two-dimensional (2D) layered MoS2/conjugated polymer nanocomposites was developed based on the β-cyclodextrin (β-CD) template. The complete intercalation nanocomposites of molybdenum disulfide (MoS2) with poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3-methylthiophene) (P3MT), and polypyrrole (PPy) were prepared successfully. MoS2 is in the form of a monolayer or no more than trilayer with conjugated polymers inserted into the interlayers. In comparison with that of β-CD, sulfonated β-CD (β-CDSO3) has better water-soluble and act as dopants to improve the electrical and electrochemical performances of these nanocomposites simultaneously. The conductivities of nanocomposites based on β-CDSO3 template increase by one to three orders of magnitude, and their capacitance performances are superior. As an innovative route to prepare the regular 2D layered MoS2/polymer nanocomposites, the methodology is expected to be applicable to a wide range of layered materials and hydrophobic monomers.

Graphical abstract

A strategy to prepare regular 2D layered MoS2/conjugated polymer composites with high conductivity was developed via β-CDSO3 template.

Keywords

MoS2 Conjugated polymer Nanocomposites Sulfonated β-cyclodextrin Ordered layered structure 

1 Introduction

Two-dimensional (2D) composites, based on layered inorganic materials and organic polymers, have been attracting great attention due to their novel characteristic combining the advantages of both. As most commonly used semiconducting transition metal dichalcogenides (TMDs), layered MoS2 has been extensively explored for various applications such as in sensors, catalysis, and energy storage [1, 2, 3]. MoS2 are stacks of triple layers with a molybdenum atoms layer between two sulfur atoms layers, and the triple layers can be mechanically/chemically exfoliated due to weak Van der Waals bonding between the layers. Macromolecules can be intercalated into MoS2 by the exfoliation/restacking method, and a number of both saturated and conjugated polymers have been inserted in the Van der Waals interlaminar spaces of MoS2 [4, 5, 6, 7]. The very small thickness and an extremely large surface of exfoliated 2D nanosheets can induce strong interaction with diverse guest species and create unexpected functionality. For instance, the intercalation of poly(ethylene oxide) (PEO) into MoS2 via exfoliation/restacking method, like that of other electron pair donors, leads to mixed ionic-electronic conductors. The intercalate shows better properties of electrical and lithium-ion conductivities than that of MoS2 and bulk PEO [4]. MoS2/polydimethylsiloxane (PDMS) nanocomposites were prepared by applying mechanical force on the MoS2 in deionized water, and their reversible photomechanical responses to the near infrared light are realized, suggesting an excellent candidate for energy conversion [5]. Composites of MoS2 and conjugated polymers are excellent candidates for the optical limiter, solid electrodes, electrolytes, and other purposes. MoS2/poly(3-hexylthiophene) (P3HT) nanohybrid has been prepared via the P3HT-assisted sonicated exfoliation process. This nanohybrid exhibits unexpected optical limiting properties in contrast to the saturated absorption behavior of both P3HT and MoS2, showing potential in the photoelectric applications [6]. The nanocomposites of MoS2 and polypyrrole (PPy) were prepared by combining in situ oxidative polymerization of pyrrole monomers and chemically exfoliation MoS2, and exhibit a high specific capacitance, remarkable rate capability, and improved cycling stability compared with that of the reported conducting polymer-based electrode materials [7].

The regioregular structure and deliberate coupling between the hybridized species are critically important for the functionality and scalable physicochemical properties of hybridized components [8]. Many efforts have been made for preparing the ordered layered structure composites of MoS2 and polymers, and these approaches are mainly in the following aspects: (a) Mix the guest species solution with the exfoliated MoS2 suspension in the co-solvent system directly. This is suitable for these systems in which both organic and inorganic phases are molecularly compatible. Water-soluble polymers, such as PEO, poly(ethylene glycol) (PEG), and poly(vinyl alcohol) (PVA) [9, 10, 11, 12], can be introduced into the single-layered dispersion of MoS2 directly for their miscible with water. Few oil-soluble polymers can dissolve in the high boiling point solvent such as N-methylformamide (NMF) which is miscible with water [13]. Nanocomposites were prepared by merely stirring the mixture of aqueous polymer solutions and exfoliated MoS2 suspension. This method is convenient for the hydrophilic polymers, but not for hydrophobic polymers. The reactions depend on the intercalation procedures, and the two phases easy to separate and form the incomplete intercalation. (b) Another synthetic approach involves insertion of monomer first, followed by polymerization in the present of single-layered MoS2 suspension. This monomer guest species may be hydrophobic such as thiophene but needs to be soluble in the co-solvent system of the exfoliated MoS2 [14]. These co-solvents include N-methylpyrrolidinone (NMP)/H2O, methanol (MeOH)/H2O, NMF/H2O, etc. This method is limited for hydrophobic monomer by reason of their poor miscibility with water. The molecularly incompatible between organic and inorganic phases always results in the lower molecular weight of the intercalated polymer. The co-solvent is not eco-friendly, and some of them have a high boiling point and hard to remove. (c) Form the interreaction between the polymer and single-layered MoS2 nanosheets. The exfoliation process of MoS2 is a chemical process in which the lithiated matrix is partially oxidized leading to a polyanion [(MoS2)−x]n and the negative charge on the single layers to form a colloidal alkaline suspension [15]. Obviously, this approach is appropriate only for those positively charged polymers such as structure contains amine and amino groups [13, 16]. (d) Treat the inorganic MoS2 nanosheets organic functionalization and modified the surface chemical state of MoS2 to match the organic polymers [17, 18]; for example, MoS2 nanosheets were chemically modified with cationic or polycationic surfactant such as cetyltrimethyl ammonium bromide (CTAB), and oleylamine and chitosan can be acted by electrostatic interaction and result in good compatibility with polymer. This approach is simple and convenient but is not eco-friendly. Meanwhile, the organic modified compound may be an insulating layer and impede the effective charge or energy transfer between the inorganic and organic component when in their photoelectric applications.

In this paper, sulfonated β-cyclodextrin (β-CDSO3) as the template was introduced to prepare the ordered 2D layered MoS2/conjugated polymer nanocomposites. β-CD is cyclic oligosaccharides built up from 1,4-glucopyranose units that exhibit a torus-shaped structure with a hydrophobic cavity and a hydrophilic exterior. Thus, β-CD is an attractive design element, as it can form the complexes with hydrophobic guest molecules primarily in aqueous solution without any chemical modification of the guest molecule [19]. This ability has been utilized for the modification of polymer functionality, polymer composition, and polymer topology. The species includes homopolymer, block copolymer, star polymer, brush/comb polymer, and their functionalized polymers [20, 21, 22, 23]. Few reports about β-CD were applied in inorganic/organic composites; they involve in the composites of polymer with silica, graphene, Fe3O4, etc. [24, 25, 26], and yet, a lot of works in this area are still required to do. Conjugated polymers are insoluble in polar solvents and the addition of a polymer solution into an aqueous suspension of single-layer MoS2 results in undesirable macroscopic phase separation. Those four types of measures which mentioned above all have their limitations. β-CD using for the template can resolve the incompatibility between nonpolar monomer/polymer materials and the polar inorganic materials effectively. Hydrophobic monomers, such as 3,4-ethylenedioxythiophene (EDOT), 3-methylthiophene (3MT), and pyrrole (Py), can form inclusion with β-CD or its derivative first, and then mixed with water-soluble single-layered MoS2 suspension uniformly. The residual of β-CD without electronic conductivity in the nanocomposites would impede the electronic transportation. Improving the water solubility of β-CD can help its de-clathration and can be removed from the solution thoroughly. β-CD can be modified with hydroxypropyl, amino alcohol, and sulfoacid [27, 28, 29]. Here, the sulfonated β-CD has better water solubility than that of β-CD; meanwhile, it also acts as a dopant for conjugated polymers to improve the electrical performances.

2 Experimental method

2.1 Preparation of MoS2/conjugated polymer nanocomposites

LixMoS2 was prepared with reference to the literature method [30]. One gram MoS2 (purchased from Hefei Kehua Fine Chemical Industry Research Institute) and three equivalents N-butyllithium (n-BuLi) (2.5 M solution in hexane, was obtained from Energy Chemical Inc.) were kept in a nitrogen atmosphere for 3 days. Then, the product was exfoliated in 150 mL water through ultrasonication. A certain amount of monomer was injected into the β-CDSO3 solution, and the feed ratio of β-CDSO3 to monomer is 1:5 and 1.5:1 (in molar, written as cond.1 and cond.2), respectively. The preparation of β-CDSO3 is according to the literature [31]; typically, 9 g β-CD (Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 30 mL 50% NaOH solution, and then 21.8 g 1,3-propanesultone (≥ 99.9%, was from Suzhou Yanke Chemical Reagent Co., Ltd.) was added in it. After stirring for 18 h under 50 °C, the product was dissolved in water and the mixture was adjusted to pH = 5~6 by sulfuric acid. Then, 400 mL ethanol was added to the mixture, and the precipitated product was separated by filtration and washed by methanol and alcohol, respectively. The white product was dried in a vacuum oven overnight at 40 °C. FTIR characterization of β-CD and β-CDSO3 is shown in Fig. S1, respectively. Complete the capture of the monomer under the continuous agitation for 2 h. Then, add a certain amount of MoS2 suspension (the same feed ratio of MoS2/monomer in mole) into the above solution and continue stirring for 1 h. Ammonium persulfate (APS) which from Sinopharm Chemical Reagent Co., Ltd. (monomer: APS = 1:2 in molar ratio) was dissolved into deionized water and added dropwise to the flask followed by mechanical stirring for 72 h at 25 °C (EDOT, Aladdin Industrial, Inc.), for 24 h at 25 °C (3MT, Alfa Aesar) and for 24 h in an ice bath (Py, Aladdin Industrial, Inc.), respectively. The residues were collected by centrifugation and washed successively with hot water, methanol/acetone mixture (methanol: acetone = 2:1, in volume ratio), and distilled water, and then vacuum dried at 60 °C for 24 h. The reaction products were named as MoS2/PEDOT-CDSO3, MoS2/P3MT-CDSO3, and MoS2/PPy-CDSO3, respectively. By contrast, pure PEDOT, P3MT, and PPy were prepared, and the composites of MoS2 and conjugated polymer based on the β-CD template were also prepared in similar ways and named as MoS2/PEDOT-CD, MoS2/P3MT-CD, and MoS2/PPy-CD, respectively. The related data are shown in the Supporting Information.

2.2 Characterization

The infrared spectra were determined by FT-IR200 (Thermo Nicolet Co.). Raman spectra were recorded with a confocal Raman micro-spectrometer (HR Evolution, HORIBA JOBIN YVON) in the range of 200–600 cm−1 under a 532-nm diode laser excitation. X-ray diffraction (XRD) data were recorded using a Rigaku D/Max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). The morphology of MoS2 and MoS2/conjugated polymer nanocomposites were examined using a field emission scanning electron microscope (FE-SEM, SU8020, Hitachi, Japan), operated at an acceleration voltage of 5 kV. Electrical conductivity at room temperature was determined using a conventional four-probe method on pressed pellets, formed under a pressure of 20 MPa with a diameter of 10 mm. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were conducted using a CHI660B electrochemical workstation with a three-electrode system. Blank platinum was used as the counter electrode, and Ag/AgCl was used as the reference electrode. Experiments were carried out at ambient temperature using 1 M H2SO4 aqueous solution as an electrolyte.

3 Results and discussion

3.1 Preparation mechanism of MoS2/conjugated polymer nanocomposites

The preparation process of MoS2/conjugated polymer nanocomposites is illustrated in Fig. 1. In an aqueous solution, β-CDSO3 owns the capacity for capturing appropriate “guest molecules” which are less polar than water [32]. The inclusion complexes of β-CDSO3 and guest molecules such as EDOT, 3MT, and Py monomers possess much higher solubility in water than those of guest molecules monomer themselves. Mixing these complexes with the single-layered MoS2 suspension and the more homogeneous solution can be obtained due to the enhanced solubility of hydrophobic guest molecules in MoS2 water suspension. The uniformity is quite necessary for the construction of the ordered 2D MoS2/conjugated nanocomposites. As the reaction progresses, EDOT, 3MT, and Py monomers were first initiated into the cationic radicals in the presence of APS, respectively, and then these monomers are rapidly expelled from the host β-CDSO3 once the formation of cationic radicals [33]. The single layer MoS2 exfoliated by n-BuLi charged negative charges, so the cationic radicals can be anchored onto the MoS2-layered surface by means of electrostatic interaction; thus, the polymerization would proceed between the MoS2 layers. β-CDSO3 which departed from the inclusion complexes can capture other monomers continually and finally be removed by washing. Even though there is some residual β-CDSO3 which can act as a dopant in conductive polymers. The acidic APS also plays a role of pH regulator which causes the restacked of MoS2 single layers [34]. Combining the electrostatic interactions between MoS2 single layer and conjugated macromolecules with the driving force of restacked MoS2 which is from the acidic condition, the ordered sandwich structures of MoS2/conjugated polymer composites be obtained.
Fig. 1

The preparation schematic diagram of MoS2/conjugated polymer composites

3.2 Structure of MoS2/conjugated polymer nanocomposites

The structure of MoS2/conjugated polymer composites was characterized by FTIR, and the spectra of MoS2/PEDOT-CDSO3, MoS2/P3MT-CDSO3, and MoS2/PPy-CDSO3 are shown in Fig. S2, respectively. The intercalation of conjugated polymers into the MoS2 host layers is demonstrated by powder X-ray diffraction. As shown in Fig. 2, the (002) diffraction peak of restacked MoS2 at 2θ ≈ 14.3° corresponds to d ≈ 6.18 Å according to the Bragg equation, which indicates that MoS2 layers are regularly restacked along the c-axis. However, the (002) diffraction peak at 2θ ≈ 14.3° was disappeared in curves b, c, and d of Fig. 2, and the (001) diffraction peaks of MoS2/conjugated polymer nanocomposites are all observed instead, which shifts to lower angles that indicate that the MoS2 layers were intercalated completely and an increase in interlayer spacing of MoS2 caused by the intercalation of conjugated polymer between MoS2 layers. The narrow diffraction peaks suggesting the regularity of the crystallite, i.e., the composites, still have ordered 2D layered structure after intercalation polymerization. The XRD patterns of MoS2/conjugated polymer nanocomposites based on β-CD template are shown in Fig. S3 for comparison. The data (2θ, d, and Δd) of MoS2/conjugated polymer nanocomposites are listed in Table 1. It can be shown that β-CDSO3 template could cause bigger interlayer space than that of β-CD template. The more β-CDSO3 template could cause bigger interlayer space of MoS2 except for MoS2/P3MT-CDSO3 nanocomposites, which it may be ascribed to the different inclusion effect according to their different water solubilities. Besides, the composites based on β-CDSO3 template possess narrower and more intense (001) diffraction peaks than that composites based on the β-CD template, and the (001) diffraction peaks are sharper and more intense with increasing the feed ratio of β-CDSO3 (comparison curve is shown in Fig. S4), i.e., the composites based on β-CDSO3 template have more regularity in a 2D ordered structure. It could be attributed to the increase of the water solubility of β-CDSO3. The water solubilities of β-CD and β-CDSO3 have been determined by dissolving them into a certain volume water solution until they cannot dissolve, and the value is ca. 18.5 g/L and 740 g/L (25 °C), respectively. The more β-CDSO3 can also provide the hydrophobic monomers in inclusion complexes with better water-soluble properties and is in favor of forming a homogeneous system.
Fig. 2

XRD patterns in of restacked MoS2 (a), MoS2/PEDOT-CDSO3 (b), MoS2/P3MT-CDSO3 (c), and MoS2/PPy-CDSO3 (d)

Table 1

XRD data of MoS2 and MoS2/conjugated polymer composites

 

2θ (°)

d (Å)

Δd (Å)

MoS2

14.3

6.18

0

MoS2/PEDOT-CD

8.16

10.83

4.65

MoS2/PEDOT-CDSO3

cond.1

6.8

12.99

6.81

cond.2

6.7

13.18

7.00

MoS2/P3MT-CD

8.82

10.02

3.84

  MoS2/P3MT-CDSO3

cond.1

7.2

12.27

6.09

cond.2

7.28

12.13

5.95

MoS2/PPy-CD

7.34

12.03

5.85

  MoS2/PPy-CDSO3

cond.1

7.6

11.62

5.44

cond.2

7.02

12.58

6.40

In comparison with our previous work [35], MoS2/PEDOT composites were also prepared without β-CD and β-CDSO3 template, and the existence of the (002) diffraction peak at 2θ ≈ 14.3° together with the (001) diffraction peak at 2θ ≈ 6.54° was still suggested the partial intercalation of PEDOT into the MoS2 layers. The methodology with the β-CDSO3 template is obviously superior to the others and facilitates the preparation of the regular 2D layered MoS2/polymer nanocomposites.

The vibrational spectrum is sensitive to the number of atomic layers. Raman spectroscopy has been widely used to determine the number of layers, as well as to examine the changes in material properties with thickness [36]. The Raman spectra of pristine MoS2, MoS2/PEDOT-CDSO3, MoS2/P3MT-CDSO3, and MoS2/PPy-CDSO3 composites with different feed ratio of β-CDSO3 are shown in Fig. 3, respectively. The Raman spectra of composites based on β-CD are shown in Fig. S5 for comparison. For all the samples, there are two characteristic peaks at around 380 cm−1 and 410 cm−1 which are corresponding to the in-plane (E1 2 g) and out-of-plane (A1g) vibration modes, respectively. The separation of E1 2 g and A1g is about 26.2 cm−1 for MoS2 which the E1 2 g is at 379.5 cm−1 and the A1g is at 405.7 cm−1, respectively. MoS2/conjugated polymer nanocomposites exist blue shift of A1g and red shift of E1 2 g. The integrated data of separation of E1 2 g and A1g are listed in Table 2. It can be shown that these composites whether based on β-CD or β-CDSO3 template possess smaller separation of E1 2 g and A1g than that of 2H-MoS2, which indicate that MoS2 was exfoliated to a much greater extent in these composites. Especially, composites based on β-CDSO3 template have the smaller separation of E1 2 g and A1g in comparison with that of β-CD template based, and more β-CDSO3 have the smaller separation of E1 2 g and A1g. Thus, it indicated that β-CDSO3 template has better intercalation and exfoliation effect than that of β-CD, and the thinner MoS2 layer would be obtained with increasing the dosage of β-CDSO3. The numbers of MoS2 layer were confirmed no more than trilayer according to the literature [37]. In combination with XRD patterns, it can be deduced that a large amount of MoS2 is in the form of monolayer and the form of MoS2 monolayers can maintain during polymerization.
Fig. 3

Raman shift of restacked MoS2 (a), MoS2/PEDOT-CDSO3 (b), MoS2/P3MT-CDSO3 (c), and MoS2/PPy-CDSO3 (d)

Table 2

The Raman data of MoS2 and MoS2/conjugated polymer composites

 

A1g − \( {E}_{2g}^1 \) (cm−1)

MoS2

26.2

MoS2/PEDOT-CD

22.1

  MoS2/PEDOT-CDSO3

cond.1

22.5

cond.2

22.0

MoS2/P3MT-CD

22.8

  MoS2/P3MT-CDSO3

cond.1

22.5

cond.2

22.2

MoS2/PPy-CD

22.4

  MoS2/PPy-CDSO3

cond.1

21.8

cond.2

21.8

The morphology of restacked MoS2 and MoS2/conjugated polymer composites was characterized by SEM. As shown in Fig. 4a, the majority of restacked MoS2 flakes are in micron-grade in lateral dimensions with clear edges. Figure 4b image shows that the restacked MoS2 layers are in a two-dimensional ordered state. The thicknesses of recognizable layers in SEM images are about 100 nm of ten of layers and can estimate that these nanosheets of MoS2 are stacked together compactly according to the average thickness of the restacked MoS2 sheets that is about 1.4 nm which are reported in the literature with Li intercalation technique [38]. Figure 4c~h shows the FE-SEM images of MoS2/PEDOT-CDSO3, MoS2/PPy-CDSO3, and MoS2/P3MT-CDSO3, respectively. All the images show that the structure of MoS2/conjugated polymer as orderly and regularly, and almost no unintercalated polymer was observed. Figure 4d, f, h shows the partial enlarged images of the circle partial area of Fig. 4c, e, g, respectively. The layered restacked MoS2/conjugated polymer nanocomposites have a looser structure in comparison with that of restacked MoS2. The slightly corrugated layers with an expansion in the interlayer spacing indicate that the conjugated polymers include PEDOT, PPy, and P3MT insert into MoS2 layers, respectively. This is in consistent with the results of XRD patterns we have mentioned above. The composites based on β-CD template present the same morphology as above (Fig. S6). Combined with the XRD analysis, the method based on the β-CDSO3 or β-CD template could be generally applicable for the preparation of 2D layered MoS2/conjugated polymer nanocomposites which came from the hydrophobic monomers.
Fig. 4

SEM images of restacked MoS2 (a, b), MoS2/PEDOT-CDSO3 (c, d), MoS2/P3MT-CDSO3 (e, f), and MoS2/PPy-CDSO3 (g, h)

3.3 Electrical and electrochemical properties

Electrical conductivities at room temperature were measured by a conventional four-probe method. The electrical conductivities of MoS2/conjugated polymer nanocomposites are listed in Table 3. The conductivities of MoS2/conjugated polymer nanocomposites based on the β-CDSO3 template are higher than those of the β-CD template by one to three orders of magnitude, which should be related to the residual β-CD template that hind the carrier mobility although it can be produced the regularity composites. In contrast, β-CDSO3 with higher water solubility is easy to remove, even if it has the small residual which can act as the doped agent to enhance the conductivities of the nanocomposites [29]. It is also obvious that the electrical conductivities increase with increasing the content of β-CDSO3. This could be attributed to the doping function of β-CDSO3.
Table 3

Conductivities of MoS2/conjugated polymer composites

 

Conductivity (S/cm)

MoS2/PEDOT

MoS2/P3MT

MoS2/PPy

β-CD

2.05 × 10−5

5.32 × 10−4

1.73 × 10−3

β-CDSO3 (cond.1)

5.26 × 10−5

7.56 × 10−4

9.45 × 10−2

β-CDSO3 (cond.2)

1.85 × 10−2

2.04 × 10−3

1.55 × 10−1

The electrochemical behavior of the samples used as active electrode materials was investigated using CV and EIS in 1.0 M H2SO4 aqueous solution. Figure 5 illustrates the contrast of CV performance of MoS2/conjugated polymer nanocomposites with different contents of β-CDSO3. The CV curves show a deviation from the rectangular shape, whereby the CV profile resembles an “S”-shape, which suggests the coexistence of charge storage mechanisms with double layer charge storage and pseudocapacitive faradic charging. The electrochemical redox reaction is as follows:
$$ {\displaystyle \begin{array}{l} PEDO T+{A}^{-}\overset{\mathrm{ox}}{\to } PEDO{T}^{+}\left({A}^{-}\right)+e\\ {} PEDO{T}^{+}\left({A}^{-}\right)+e\overset{\mathrm{re}}{\to } PEDO T+{A}^{-}\end{array}} $$
$$ {\displaystyle \begin{array}{l}P3 MT+{A}^{-}\overset{\mathrm{ox}}{\to }P3M{T}^{+}\left({A}^{-}\right)+e\\ {}P3M{T}^{+}\left({A}^{-}\right)+e\overset{\mathrm{re}}{\to }P3 MT+{A}^{-}\end{array}} $$
$$ {\displaystyle \begin{array}{l} PP y+{A}^{-}\overset{\mathrm{ox}}{\to } PP{y}^{+}\left({A}^{-}\right)+e\\ {} PP{y}^{+}\left({A}^{-}\right)+e\overset{\mathrm{re}}{\to } PP y+{A}^{-}\end{array}} $$
where A is SO42− of the electrolyte.
Fig. 5

CV curves of MoS2/conductive polymer composites. (13) Curves a and b are cond.1 and cond.2. (4) Curves a, b, and c are MoS2/PEDOT-CDSO3, MoS2/P3MT-CDSO3, and MoS2/PPy-CDSO

At the high scan rate, the CV curves maintain the same shape at the low scan rate, which indicates that MoS2/conjugated polymer nanocomposites possess the property of rapid charging and discharging (Fig. S7). The CV curves become larger with the increment of β-CDSO3, which indicate that the increase of β-CDSO3 could improve the capacitance of MoS2/conjugated polymer nanocomposites (Fig. 5(1–3)). Figure 5(4) shows the CV curves of MoS2/PEDOT-CDSO3 (a), MoS2/P3MT-CDSO3 (b), and MoS2/PPy-CDSO3 (c) at the scan rate of 50 mV/s. It is obvious that MoS2/PEDOT-CDSO3 possessed the greater capacitance; it should ascribe to the polarity of monomer. The higher polarity the monomer has (Py > EDOT > 3MT), the smaller increment the CV curve has (MoS2/P3MT > MoS2/PEDOT > MoS2/PPy). The monomer which has high polarity has a strong interaction with β-CDSO3, and the effective template function of β-CDSO3 makes the conjugate polymer chain much longer.

Figure 6 illustrates the Nyquist impedance spectra of MoS2/conjugated polymer nanocomposites with different contents of β-CDSO3. Impedance was tested in the frequency range from 0.05 to 100 kHz at open-circuit potential. At high frequency, the plots of MoS2/conjugated polymer nanocomposites all show semicircles which is indicative of the charge transfer phenomena of a faradic redox process. The diameter of the semicircle corresponded to interface charge transfer resistance (Rct). With increasing the content of β-CDSO3, the Rct values of MoS2/PEDOT-CDSO3, MoS2/P3MT-CDSO3, and MoS2/PPy-CDSO3 all decrease which means the better ion/electron mobility through the electrode; meanwhile, the straight portion of the Nyquist plots is more perpendicular which verifying that the composites have a better capacitance characteristic. This conclusion agrees with the capacitance properties above.
Fig. 6

Nyquist impedance spectra of MoS2/conductive polymer composites under cond.1 (a) and cond.2 (b), (1) MoS2/PEDOT-CDSO3, (2) MoS2/P3MT-CDSO3, and (3) MoS2/PPy-CDSO3

4 Conclusions

A strategy to prepare the regular 2D layered MoS2/conjugated polymer nanocomposites with high conductivity was developed based on the β-CDSO3 template. The complete intercalation nanocomposites of MoS2 with PEDOT, P3MT, and PPy were prepared successfully. The electrostatic interaction between negative MoS2 lamella and cationic monomers radicals in the presence of initiator proceeds the polymerization into the MoS2 interlayer. MoS2 is in the form of a monolayer or no more than trilayer with conjugated polymers inserted into the interlayers. The regular 2D layered MoS2/conjugated polymer nanocomposites can be obtained successfully whether based on β-CD template or β-CDSO3 template; however, the latter can effectively solve the problems of the impede current for insulating β-CD. The water solubility of β-CD and β-CDSO3 is ca. 18.5 g/L and 740 g/L (25 °C), respectively, and this different property influences the properties of intercalation, conductivity, and electrochemical performance of MoS2/conjugated polymer nanocomposites. In comparison with β-CD template, nanocomposites based on β-CDSO3 template have better intercalation and exfoliation effect and the thinner MoS2 layer would be obtained with increasing the dosage of β-CDSO3; furthermore, the conductivities are higher by one or three orders of magnitude and also present a better capacitance performance. So, the methodology with β-CDSO3 template is superior and facilitates to prepare the regular 2D layered MoS2/polymer nanocomposites, and is expected to be applicable to a wide range of layered materials/polymer nanocomposites.

Notes

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

42114_2019_90_MOESM1_ESM.docx (2.2 mb)
ESM 1 (DOCX 2275 kb)

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical engineeringHefei University of TechnologyHefeiPeople’s Republic of China

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