Introduction

Over the last quarter century, conducting polymers have turned out to be significant materials due to their wide range of technological applications viz., electronic devices [1], rechargeable batteries, sensors [2,3,4], drug delivery [5], conductive paints, super capacitors [6], and light-emitting devices [7]. These polymers possess alternating single and double bonds with π-conjugated polymer chains, which in turn has gifted the special electrical properties [8]. One of the major drawbacks of conducting polymers is their poor mechanical strength which can be overcome by filling the polymer matrix with metal/metal oxides and metalloids through chemical, electrochemical and mechanical mixing routes [9]. Conducting polymers like polypyrrole, polyaniline, polythiophene, and polyacetylene have got overwhelmed considerations in many fields [10]. Among them, polypyrrole and polyaniline polymers have emerged as outstanding materials due to their special transport properties, good environmental stability and facile synthesis. Polypyrrole and polyaniline composites of PbTiO3 [11], SnO2 [12], MgFe2O4 [13], Y2O3 [14], Nb2O5 [15], ZnO [16], ZnWO4 [17], NaVO3 [18], Fe2O3 [19] have been studied for their AC electrical properties so as to explore their potential applications in vivid fields. Apart from these oxides, novel conducting polymer composites of carbon nanotubes [20], graphene [21], and graphene oxide [22] have also been studied in the recent years. However, the synthesis and studies on AC electrical properties of conducting polymer-based composites filled with transition metal dichalcogenides (TMDs) are limited. The TMDs such as MoS2, WS2, TaS2 [23,24,25,26,27], and phosphorene [28] are generally inorganic 2D layered materials which have been reported to play outstanding role in the fields of energy storage, sensors and optoelectronics applications [28, 29]. In this perspective, to enhance the scope of such TMDs based conducting polymer composites, we have embarked to study AC transport studies of PPy/WS2 composites over a wide range of frequency. Here, we have preferred PPy because of its ease of synthesis, good stability, economic viability, and tunable surface charge properties which coup for the best synergy with the filling material [27]. Since WS2 being a well-known TMD and in one of our recent studies, WS2 has exhibited interesting results in AC conductivity, dielectric properties and humidity sensing characteristics with the polyaniline [31, 32]. Encouraged by the interesting results of WS2, in the present paper we report the synthesis, structural–morphological characterization studies, AC conductivity studies over a wide range of frequency in PPy/WS2 novel composites.

Experimental

Materials

Pyrrole [C4H4NH (98% purity)], ammonium persulphate [(NH4)2S2O8] were procured from S.D. Fine Chemicals, Mumbai, India. WS2 (99.8%) metal basis is purchased from Alfa Aesar.

Synthesis of polypyrrole/tungsten disulphide composites

PPy/WS2 composites were synthesized using chemical polymerization method as illustrated in the schematic diagram (Fig. 1). To synthesize the composites; finely ground tungsten disulphide powder was added (10, 20, 30 and 40 wt%) to pyrrole (0.03 M) and amalgamated by using magnetic stirrer. To get PPy/WS2 composites, ammonium persulphate (0.06 M) was added drop-wise while uniform stirring. The precipitated product was purified by acetone and distilled water. To remove the water content, it was kept in oven at 100 °C till the constant weight was achieved. For different wt% of WS2, namely 10, 20, 30 and 40%, the samples have been designated as PPy/WS2-10%, PPy/WS2-20%, PPy/WS2-30% and PPy/WS2-40% respectively. Similar procedure was followed to prepare polypyrrole without adding tungsten disulphide to the above reaction.

Fig. 1
figure 1

Schematic representation of formation of PPy/WS2 composites

Full size image

Characterization

FESEM images were captured by using instrument FEI Nova Nano SEM 600 by placing WS2 on carbon tape while PPy and PPy/WS2-30% on aluminium foil. Energy-dispersive X-ray spectrum (EDX) was obtained using EDAX genesis apparatus, fitted to the FESEM device. Raman spectra were obtained by Horiba Jobin Yvon lab Ram HR 800 spectrometer at a power of 20 mw using argon laser (514.5 nm) in the range 200 – 2000 cm−1. X-ray diffraction (XRD) patterns of WS2, PPy, and PPy/WS2-30% samples were obtained by using Bruker D8 advance X-ray diffractometer, scanned in the 2θ range 10—80° at the rate of 1.1°min−1 using copper Kα as X-ray source of wavelength 1.541 Å. Fourier-transform infrared (FTIR) spectra were obtained in the wavenumber range 450–4000 cm−1 using Frontier Perkin Elmer FTIR spectrometer with a resolution of 4 cm−1 and 16 scans. Transmission electron microscopy (TEM) image for PPy/WS2-40% composite sample was captured using JEOL-3010 instrument.

AC conductivity and impedance measurements

At first, all the powder samples were grinded for two hours and then pressed under hydraulic pressure of 4 to 6 tons to form the pellets. Each pellet, having 10 mm diameter and 1 mm thickness was silver coated on either side of it, to get ohmic contacts. AC response parameters of each sample were recorded by a computer-controlled LCR metre (Hioki model 3532–50) with a frequency scale ranging from 50 to 1 MHz.

Results and discussion

FESEM and EDX studies

The as-prepared PPy/WS2 composites have been subjected to FESEM to study the morphological properties of them. Figure 2 shows the morphology of PPy, WS2, and PPy/WS2-30% composite. Figure 2a depicts the FESEM image of pure polypyrrole revealing its granular morphology. It can be noted that the granules are consistently distributed throughout the sample [33]. Figure 2b shows 2D layered structure of WS2 in the form of hexagonal sheets. These bundles of hexagonal sheets are having an area of few micrometres. Figure 2c shows the PPy/WS2-30% composite wherein WS2 hexagonal sheets are uniformly distributed in the PPy matrix. The incorporation of WS2 into the PPy matrix has resulted in substantial morphological changes in the composite. As a result, chain orders in PPy have increased with the enhancement of electron delocalization of the composite evidencing increased conductivity. Figure 2d shows the EDX spectrum of PPy/WS2 -30% composite confirming the presence of peaks related to the elements carbon, nitrogen and oxygen, which are the constituents of polypyrrole and the peaks related to tungsten and sulphur elements, which are the constituents of tungsten disulphide.

Fig. 2
figure 2

FESEM images of a Polypyrrole, b WS2, c PPy/WS2-30% composite morphology, and d EDX spectrum of the composite

Full size image

The distribution of grain size of PPy/WS2-30% composite was studied using image J software and the related histogram is shown in Fig. 3. Histogram showing number of grains versus the grain size has revealed the average grain size in the composite, and it is found to be 2.6 µm.

Fig. 3
figure 3

Grain size distribution in PPy/WS2-30% composite

Full size image

Raman spectra

Figure 4 shows the Raman spectra of PPy, WS2, and PPy/WS2-30% composite. Figure 4a depicts the PPy manifested peaks at 1360 cm−1 which represent anti-symmetrical C–N•+ stretching in PPy and 1572 cm−1 represent C = C stretching vibrations of PPy backbone chains [34]. Figure 4b shows the Raman spectrum of WS2 which has exhibited its signature peaks at 356 cm−1 owing to in-plane phonon mode \({E}_{2g}^{1}\) and 421 cm−1 peak corresponding to out-of-plane phonon mode A1g. Figure 4c of PPy/WS2-30% composite shows the peaks at 356 and 421 cm−1 related to WS2 and the broad bands of PPy have been suppressed in the composite due to the crystalline nature of WS2 as revealed in XRD and SAED pattern of TEM image [35].

Fig. 4
figure 4

Raman spectra of a Polypyrrole, b WS2, and c PPy/WS2-30% composite

Full size image

XRD studies

Figure 5 shows the X-ray diffraction studies of PPy, WS2 and PPy/WS2-30% composite. Figure 5a of PPy shows a broad diffraction peak around Bragg angle 25.77˚, exhibiting its amorphous nature. Figure 5b of WS2 with designated (hkl) values evidence it’s important peaks at Bragg angles 14.4˚, 28.9˚, 32.7˚, 33.5˚, 39.5˚, 44˚, 49.7˚, 58.4˚, and 60.5˚ with respect to their interplanar spacing distances; 6.18, 3.08, 2.73, 2.66, 2.27, 2.06, 1.83, 1.57, 1.52 Å respectively. Obtained data have been confirmed using standard JCPDS file No-08–237 [36]. Figure 5c of PPy/WS2-30% composite reveals the pronounced peaks of WS2 and the negligible peak of PPy in the composite, revealing its crystalline nature. The broad peak of PPy is vanished and is negligible due to the predominance of crystalline WS2 in the composite, supporting the PPy wrapped WS2. Similar structure has been observed in polyaniline/WS2 composite in our earlier study [31].

Fig. 5
figure 5

XRD spectra of a Polypyrrole, b WS2, and c PPy/WS2-30% composite

Full size image

FTIR studies

Figure 6 shows the FTIR spectra of PPy, WS2, and PPy/WS2-30% composite. Figure 6a depicts FTIR spectrum of PPy showing its significant bands at 603, 867, and 919 cm−1 related to C-H wagging. The bands at 1050 cm−1 related to = C-H deformation, band at 1217 cm−1 correspond to C-N stretching vibrations, bands at 1365 and 1566 cm−1 corresponds to C-H in-plane stretching and ring stretching modes, respectively, and also the band at 1737 cm−1 is attributed to the C-N stretching all confirm the signature peaks of PPy. [37]. Figure 6b depicts the FTIR spectrum of WS2 with the absorption bands at 588 and 871 cm−1 representing W-S stretching and S–S stretching vibrations, respectively [31]. FTIR spectrum of PPy/WS2-30% composite has manifested the absorption bands of both WS2 and PPy with small shifts validating the interaction between WS2 and PPy due to the synergistic effects. The significant bands at 1717, 1328, 1203, 912, and 585 cm−1 in the composite corresponding to 1737, 1365, 1217, 919 and 603 cm−1 of PPy, respectively, all have been downshifted due to the chemical interaction between the active sites of PPy and WS2 leading to the change in polymer conformation. Interestingly, the charge delocalization band of PPy at 1217 cm−1 has appeared in the composite at 1203 cm−1 with increased intensity indicating more conductive mature of the composite [38].

Fig. 6
figure 6

FTIR spectra of a Polypyrrole, b WS2, and c PPy/WS2-30% sample

Full size image

TEM analysis

Figure 7 shows the TEM image and SAED pattern of PPy/WS2-30% composite. In Fig. 7a, the darker portion of the image represents WS2, and the lighter region represents PPy, which is an indicative of formation of PPy epitomized WS2 sheets. This has also been observed in terms of FESEM results as discussed in Sect. 3.1. Similar kind of epitomized conducting polymers of vivid filling materials are reported in the literature [39]. Figure 7b shows selected area electron diffraction pattern of the PPy/WS2-30% composite. The bright spots in the pattern reveal the crystalline nature of the composite as evidenced from the XRD studies.

Fig. 7
figure 7

a TEM image of PPy/WS2-30% composite, b selected area electron diffraction captured on the darken regions of the composite

Full size image

Impedance spectroscopy

The complex plane impedance plots of PPy and PPy/WS2 composites are depicted in Fig. 8. Cole–Cole plots of all the samples manifests a deprived semicircle indicating the type of conduction mechanism involved in the composite system [40]. Here, the area under each curve was found to be decreasing with increasing wt% of WS2 in PPy matrix, as seen in Fig. 8a. The area under Cole–Cole plot of PPy was observed to be much larger than the composites indicating its least conductivity which can be seen in Fig. 8b. PPy has exhibited a semicircle with lower conductivity which is the reminiscent of simple Debye-type relaxation mechanism. However, the composites show a moderate deviation from the perfect semicircle. These composites are considered to exhibit nearly Debye-type relaxation. The relaxation time for PPy and the composites were calculated using Eq. 1. Relaxation time for PPy was found to be 3.9 μs; however, for all the composites, it is found to be less than PPy, with a relaxation time range 0.15—0.39 μs as depicted in Table 1.

Fig. 8
figure 8

Cole–Cole impedance plots for a PPy/WS2 composites, b PPy

Full size image
Table 1 AC response behaviour of PPy/WS2 composites
Full size table

Relaxation time (τ),

$$\tau = \frac{1}{{2\pi {f_p}}},$$
(1)

here fp is the frequency corresponding to the peak point of Cole–Cole curve and it is also termed as relaxation frequency which is given in Table 1 for all the samples.

Nyquist plots of PPy and the composites were plotted as shown in Fig. 9. Variation of imaginary part of impedance with the applied AC frequency reveals compelling relaxation of the systems [41]. These traditional Nyquist plots show a single peak, specifying unique relaxation in the materials. Here, the peak position shifts from lower frequency to higher frequencies as the wt% of WS2 is increased in polypyrrole. The highest frequency can be observed for 40 wt% composite with fastest relaxation time of 0.15 µs. Lower value of relaxation time indicates decrease in hopping length of charge carriers and hence an increase in conductivity [42].

Fig. 9
figure 9

Nyquist plots for PPy and PPy/WS2 composites

Full size image

AC conductivity

AC conductivity for PPy and the composites were calculated using the equation as reported in our earlier papers [31, 43]. Figure 10 shows the frequency dependence of AC conductivity of PPy and the PPy/WS2 composites. Initially, the conductivity was found to be consistent up to critical frequency. Above the critical frequency (fc), conductivity was found varying linearly with increasing frequency which is in accordance with power law [44],

$${\sigma^\prime }(f) \propto \;{f^s},\quad {\text{when}}\;f\; > \;{f_c}$$
(2)
Fig. 10
figure 10

Real part of AC conductivity versus applied frequency for PPy and its composites with 10, 20, 30 and 40 wt% of WS2

Full size image

From the above figure, it is clear that conductivity of the composites increase with increase in wt% of WS2 in the matrix of PPy [45]. Incorporation of WS2 in the matrix of PPy might have favoured the charge carriers to polarize at the suitable localized sites in the embedded structure of the composite as revealed in TEM, leading to the enhanced electrical conductivity. The conductivity pertaining to 10, 20, 30, and 40 wt% of the composite was observed to increase and for 40wt% composite, conductivity is maximum which is of nearly one order compared to the pristine PPy. However, for 50 wt% composite which is not shown in the figure, conductivity was found to decrease which may be ascribed to the increase in disorderliness in PPy chains and reduction in favourable localized sites due to the conformational changes in the PPy matrix [31].

AC conductivity dependence on wt% of WS2 in PPy composites is depicted in Fig. 11. Conductivity values at different frequencies (10, 100, and 1000 kHz) were plotted with increase in wt% of WS2. Conductivity was observed to increase with the wt% of WS2 in the composites. For 10, 20 and 30 wt% composites, the conductivity values were found to increase linearly till 30 wt%. However, for 40 wt% sample, the conductivity was found to increase enormously as depicted in Fig. 11. AC conductivities of all the samples at 10 kHz are shown in Table 1. Also, by considering least square fitting method for the conductivity values above the critical frequency, power-law index was calculated. The value obtained is an exponent mentioned in the power law as in Eq. 2. The index values (s) for the present samples lie in between 0.19 and 0.26 given in Table 1. From Fig. 12, we can see the behaviour of power-law index s, critical frequency fc and relaxation time τ with respect to the wt% of WS2 in the composite. The value of critical frequency has been found to decrease with increase in wt% of WS2 in PPy. Among different composites, 40 wt% composite exhibited high conductivity and a less relaxation time. In general, as the conductivity of the composites increases, the relaxation time decreases and the same has been given in Table 1.

Fig. 11
figure 11

Real part of AC conductivity versus wt% of WS2 in PPy

Full size image
Fig. 12
figure 12

AC response changes in critical frequency, power-law index, and relaxation time with wt% of WS2 in PPy composites

Full size image

Dielectric loss

Figure 13 shows the frequency dependence of dielectric loss tangent of PPy and the composites. All the samples exhibited dielectric relaxation peak in the applied frequency range and these peaks are well described by using Rezlescu model, wherein the origin of relaxation peaks can be attributed to the total charge contribution both due to the p-type and n-type charge carriers [43]. Formation of dielectric loss peaks could also be realized based on polarons in the solid materials containing a narrow conduction band. Development of these polarons in the composite lead to the charge polarization. The relaxation peak is due to the relaxation loss at the resonant frequency between the applied AC field and the frequency of the hopping charge carriers. At 100 Hz, PPy exhibited dielectric loss tangent value of 21, whereas 10, 20, 30, and 40 wt% composites have shown dielectric loss tangent values of 874, 897, 942, and 1025 respectively. It is observed that with the increase in wt% of WS2 in the composite the relaxation peak position has also been found to increase as given in Table 1. The highest dielectric loss at low frequencies can be explored for their potential applications in the fabrication of medium frequency devices.

Fig. 13
figure 13

Loss tangent peaks for PPy and PPy/WS2 composites

Full size image

Conclusion

A set of PPy/WS2 based novel composites have been synthesized using in situ chemical polymerization technique. The morphological properties of pristine PPy and the PPy/WS2 composites have been studied by FESEM technique. The room temperature AC conductivity and dielectric properties of the composites over a wide range of frequency has been investigated. Interestingly, PPy/WS2-40% sample has exhibited highest conductivity among all the samples. The decrease in conductivity beyond 40 wt% of WS2 may be attributed to the disorderliness of PPy matrix. AC conductivity in the composites is ascribed to the hopping of charge carriers and the frequency response behaviour has been explained using power law applicable to the disordered materials. Impedance studies through Cole–Cole and Nyquist plots suggest that the conductivity is electronic in nature. The dielectric behaviour has been understood by using Rezlescu model. These results suggest that the PPy/WS2 composites have the potential to find applications in the design of medium frequency devices and the related fields. In conclusion, conducting polymer composites with TMDs form a novel class of hybrid materials providing huge opportunities to explore their potential applications in the fields of low-to-medium frequency devices, humidity, and gas sensors.