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Exfoliated MoS2–Polyaniline Nanocomposites: Synthesis and Characterization

  • Erin S. Lyle
  • Cody McAllister
  • Douglas C. Dahn
  • Rabin BissessurEmail author
Article
  • 110 Downloads

Abstract

Exfoliated MoS2PANI nanocomposites with varying percentage by weight of exfoliated 2HMoS2 were synthesized and characterized by several techniques. The characterization techniques used were powder X-ray diffraction, electrical conductivity, Seebeck coefficient measurements, thermogravimetric analysis, and transmission electron microscopy. An intriguing observation was observed in the conductivity data as several of the nanocomposites that contained 5 to 15% by weight of exfoliated MoS2 yielded higher conductivity than a sample of pure PANI synthesized under the same conditions. Exfoliated 2HMoS2 has very low conductivity due to the disorder of the system, so this increase was not expected. This may indicate that the presence of MoS2 may improve the conductivity of PANI by altering its doping, or by enhancing PANI ordering; or, that PANI may be stabilizing MoS2 in its 1T zero band gap metallic form, which is higher in conductivity.

Keywords

Polyaniline Molybdenum disulfide Nanocomposite electronic conductivity 

1 Introduction

There has been a considerable amount of interest in the field of inorganic/organic hybrid nanomaterials [1]. Nanocomposite materials based on layered structures (intercalated or exfoliated) are an important class of hybrid materials since they possess a plethora of applications such as in catalysis [2], separation [3], solid lubrication/wear-resistance [4] and as materials in battery applications [5].

Molybdenum disulfide is one of the layered transition metal dichalcogenides (TMDs) that has been used for the synthesis of intercalated [6] and exfoliated nanocomposites [7]. In general, TMDs have a molecular formula of MX2, where M signifies a transition metal, such as molybdenum, and X signifies a chalcogen element, such as sulfur. TMDs can be found in several crystal structures, including the 2H-MX2 and 1T-MX2 forms. The atoms in the 2H form of TMDs arrange in a honeycomb format and thus possess trigonal prismatic coordination with D3h symmetry, whereas atoms of TMDs in their 1T form arrange in a centered honeycomb format where the atoms can be found in an octahedral coordination with Oh symmetry [8]. The usual, stable form of MoS2 is 2H, however, a metastable 1T form of MoS2 also exists [9]. Factors such as temperature [9] and pressure [10] can affect the metastablity of 1T-MoS2.

Electronically conducting polymers are attractive candidates for use in energy storage devices such as rechargeable batteries and supercapacitors [11]. Among the various conducting polymers, polyaniline (PANI), polythiophene (PTh) and polypyrrole (PPy) have several advantages, and these include high electronic conductivity, high storage capacity, low cost, easy synthesis, and good environmental stability [12]. However, these polymers exhibit relatively low cycle life and poor specific capacitance since the polymer backbones are not sufficiently stable for many repeated redox processes [13]. Hence, PANI, PTh and PPy are often intimately mixed with other inorganic or organic materials to form composites that can deliver better cycling stability, specific capacitance and improved mechanical stability [14, 15, 16, 17, 18].

Nanocomposites based on polyaniline-MoS2 have been reported in the literature and these were shown to be crystalline, where the polyaniline is either intercalated or adsorbed on the surface of the MoS2 [19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. However, we could find only one report on polyaniline-MoS2 nanocomposites that are completely exfoliated, where the MoS2 was prepared in an exfoliated state by a solvothermal method at 180 °C for 24 h [31]. In this paper we report on the synthesis of exfoliated polyaniline–MoS2 nanocomposites, however a solid-state method was used to prepare the MoS2 in a completely exfoliated state. The experimental procedure does not necessitate the use of solvents, and no isolation and/or purification steps are needed, clearly illustrating the simple and green approach of the synthesis. The preparation involves reacting molybdic acid with an excess of thiourea at high temperature for 3 h [32]. The MoS2 disperses readily in water, and when mixed with an acidified solution of aniline followed by ammonium peroxydisulfate, led to the formation of exfoliated MoS2–PANI. The amount of the MoS2 in the reaction mixture was tailored in order to produce a series of nanocomposites with varying compositions. The nanocomposites were characterized by powder X-ray diffraction, electron microscopy, thermogravimetric analysis, conductivity and thermopower measurements.

2 Materials

Molybdic acid, thiourea, and ammonium peroxydisulfate were purchased from Sigma-Aldrich and were used as received without any further purification. Aniline was also purchased from Sigma-Aldrich and was distilled under vacuum prior to use.

3 Instrumentation

Powder X-ray diffraction (XRD) was run on a Bruker AXS D8 Advance diffractometer which was equipped with a graphite monochromator, variable divergence slit, variable anti-scatter slit, and a scintillation detector. Cu (Kα) radiation (λ = 1.542 Å) was used. The powdered samples were adhered on glass slides using double sided tape. The samples were run in air from 2 to 60° (2θ), with an increment of 0.05°.

Pellets of the various nanocomposites were pressed by applying 1500–2000 psi of pressure, through the use of a hydraulic press. The pellets were 12.7 mm in diameter, and 0.3–0.7 mm thick. Pellets were then subjected to the van der Pauw electrical conductivity measurements. Wires were attached to the pellets with silver paste. Room-temperature measurements were carried out in air. For some samples, variable-temperature electrical conductivity measurements were also made, with the samples in vacuum. Details of the method used have been published elsewhere [33, 34].

Thermopower (Seebeck coefficient) was also measured on the pressed pellets, using a home-built apparatus similar to the one described by Hitchcock et al. [35]. A pellet was sandwiched between two copper block electrodes, one of which was electrically heated so that the temperature difference between the blocks rose to about 5 K after a few minutes. The temperature difference between the blocks was measured with a type K differential thermocouple. The thermoelectric voltage developed across the sample was measured and plotted as a function of the temperature difference. The slope of this plot is the thermopower of the sample, relative to copper. The thermopower of copper (+ 1.8 μV/K) was then added, to give the absolute thermopower of the sample.

Thermogravimetric analysis (TGA) was used to investigate the thermal properties of numerous samples through the use of a Q500 model TGA from TA Instruments. All samples were subjected to two separate analyses: under nitrogen (up to 1000 °C) and in air (up to 800 °C), with a heating rate of 10 °C/minute.

High resolution transmission electron microscopy was performed on a Hitachi 7500 Bio-TEM, using an accelerating voltage of 80 kV. The powdered samples were dispersed in deionized water with the help of ultrasonication, and the dispersed samples were cast on carbon-coated copper grids.

4 Experimental

4.1 Synthesis of Exfoliated MoS2

The synthetic procedure was adapted from reference 32. Molybdic acid was combined with an excess of thiourea in a ceramic reaction vessel. The reaction vessel was then inserted in a ceramic tube that was placed in a Lindberg Hevi-Duty split furnace. The reaction vessel was then steadily heated to 500 °C and was allowed to remain at this temperature for 3 h, while a nitrogen flow was maintained throughout the reaction. The furnace was turned off after 3 h and the reaction vessel remained under a nitrogen rich atmosphere until cool enough for collection of the grey powder.

4.2 Synthesis of Exfoliated MoS2–PANI Nanocomposites

Exfoliated MoS2 was ultrasonicated in deionized water for 20 min at 30% amplitude with a Cole-Parmer 750 W ultrasonic processor. The MoS2 suspension was then added to a magnetically stirred solution of aniline in 1 M HCl, kept at 0 °C. Thereafter, a cooled solution of ammonium peroxydisulfate in 1 M HCl was added dropwise to the reaction vessel. The reaction mixture was allowed to stir for 90 min at 0 °C before isolation of the product by vacuum filtration. The obtained product was a dark (almost black) blue-green powder. This procedure was duplicated in order to create at least two trials of nanocomposites which contained 1, 5, 10, 15, 20, 30, 35, 40, 45, and 50% exfoliated MoS2 by weight. Pure PANI was also synthesized by using the same procedure without the addition of MoS2.

5 Results and Discussion

5.1 Powder X-ray Diffraction

Powder X-ray diffraction was used to characterize the synthesized MoS2. As shown in Fig. 1d, the powder pattern of our synthesized MoS2 clearly indicates a lack of order/stacking between the MoS2 layers, thus confirming that the material is highly disordered, and is, in fact, in an exfoliated state. The broad feature around 20° seen prominently in Fig. 1d is due to the glass slide and tape used to support the powdered sample, as can be seen in a scan of an empty sample holder (Fig. 1g). In contrast, the powder pattern of pristine MoS2 (Fig. 1e), reveals the highly crystalline, and layered character of the material with an interlayer spacing value of 0.615 nm. Powder X-ray diffraction also confirms that the synthesized PANI-MoS2 samples are amorphous and, thus exfoliated. Chemically synthesized bulk polyaniline is amorphous as shown in Fig. 1f. However, it is interesting to note that at low MoS2 content (1–15%), the powder patterns are very similar to pure polyaniline as expected, since the samples are mostly polyaniline. At higher MoS2 content (e.g., 45%), the pattern has features similar to that of the exfoliated MoS2, with peaks at 33.4° and 59.0°.
Fig. 1

X-ray diffractograms of (a) 1% MoS2-PANI, (b) 15% MoS2–PANI, (c) 45% MoS2–PANI, (d) exfoliated 2H–MoS2, (e) pristine MoS2 (Sigma Aldrich), (f) chemically synthesized bulk PANI, (g) empty sample holder (double-sided tape on glass slide)

5.2 Electron Microscopy

Transmission electron microscopy (TEM) provides further evidence that we have produced genuine exfoliated systems. First and foremost, pure PANI as viewed under TEM (Fig. 2a) shows that it is amorphous, and this is consistent with the XRD data. TEM also reveals that the incorporation of exfoliated MoS2 within the polymer matrix results in the formation of completely exfoliated systems, and no stacking of the layers is observed in the micrographs (Fig. 2b–d).
Fig. 2

TEM of a pure PANI—scale bar 100 nm, b 1%MoS2–PANI—scale bar 100 nm, c 15%MoS2–PANI—scale bar 100 nm, d 20%MoS2–PANI—scale bar 500 nm

5.3 Thermogravimetric Analysis

By comparing the thermogram of PANI to those of the synthesized nanocomposites, an understanding of the effect of combining PANI and exfoliated MoS2 can be developed. As observed in Fig. 3, PANI begins to decompose in air at approximately 360 °C and the decomposition of PANI in several of the higher content MoS2 nanocomposites, also shown in Fig. 3, begins at 360 °C as well, thus showing that the presence of MoS2 within the polymer matrix does not affect its thermal stability in air.
Fig. 3

Thermogram of PANI and several MoS2–PANI nanocomposites in air. The derivatives of the thermograms are also shown

Under nitrogen, PANI begins to decompose at approximately 500 °C, as seen in Fig. 4, which is where the decomposition of this polymeric material also begins in the various nanocomposites. Hence, the incorporation of exfoliated MoS2 into the polymer network does not affect its thermal stability in a nitrogen atmosphere.
Fig. 4

Thermogram of PANI and several MoS2–PANI nanocomposites under nitrogen. The derivatives of the thermograms are also shown

5.4 Electrical Properties

5.4.1 Room-Temperature Conductivity Measurements

Two groups of samples were used for conductivity measurements. The first group was synthesized in 2014 and had compositions of 1, 5, 10, 15, and 20% exfoliated MoS2–PANI. Pressed pellets of these samples were created in 2016 shortly before conductivity measurements were made. The second group of samples (from 1 to 50%) was made in 2017/18, with pellets being pressed and the conductivity measurements made within a few weeks after the nanocomposites were synthesized.

Some of the samples with MoS2 content in the 1% to 15% range attracted water out of the laboratory air, to the point where droplets of liquid could be seen on the surface of the pellet. When this occurred, van de Pauw conductivity measurements were not possible, as offset voltages were observed even when no current was flowing through the sample. This suggests that the presence of the liquid may have enabled electrochemical reactions between the sample and the wires. When sample wetting was observed in open laboratory air, the room-temperature conductivity measurements were made with the sample in a desiccator.

Another indication that water has an influence on the conductivity in these materials is a tendency for conductivity to drop when the samples were exposed to vacuum. For the variable-temperature conductivity measurements that were made on some samples, these samples were in vacuum, typically for 16 to 24 h before the measurements were started. As a result of exposure to vacuum, the room-temperature conductivity of PANI and the nanocomposite materials was observed to decrease, typically by 40 to 60%, but when the samples were returned to air the conductivity was gradually restored. A possible reason for this observation could be that as the pellet is subjected to a low pressure environment, lower conductivity is observed as HCl, the dopant eliciting the conductive properties, is being removed by vacuum, although when the pellet is re-exposed to air it is most likely wicking moisture from the atmosphere which causes the sample to be re-doped which results in the observed increase in conductivity.

The conductivity of PANI and nanocomposite samples is shown in Fig. 5, as a function of the percentage of MoS2. In the figure, each data point represents a different pellet, and the error bars represent the estimated uncertainty in the conductivity of that pellet. Thus, the variability due to differences in water content, pressure used in pellet formation, etc., is reflected in the scatter of the points, not in the error bars. As seen in Fig. 5, the materials made in 2014 and measured in 2016 generally had lower conductivities than the nanocomposites made more recently. This could be due to sample aging, or perhaps to slight differences in starting materials or synthesis conditions.
Fig. 5

Conductivity versus the percentage of MoS2 contained in PANI and the various MoS2–PANI nanocomposites. The nanocomposites prepared in 2014 were measured in 2016 and may have lost conductivity due to aging

The most notable feature seen in Fig. 5 is a trend where many nanocomposite samples in the 5 to 15% MoS2 range had higher conductivity than samples of PANI made at about the same time under the same conditions. As the amount of exfoliated MoS2 was increased beyond 20%, the conductivity of the materials decreased. It was originally hypothesized that as a poorly-conducting material, such as exfoliated MoS2 [33], was introduced into the nanocomposite, that the overall conductivities of the materials would be always be lower than that of pure PANI. The reason for the conductivity increase for small percentages of MoS2 is not fully understood, but there are two possible explanations being considered.

One possibility is that the presence of MoS2 during the polymerization process alters the properties of PANI and enhances its conductivity. Different batches of PANI prepared in our lab have had conductivities at room temperature in air ranging from 2.6 to 19.6 S/cm [36], and even higher values have been reported in the literature [37]. Although in Fig. 5 we have compared the nanocomposites in this study to batches of PANI that were prepared under conditions closest to those used for those nanocomposites, the fact that other PANI samples had a higher conductivity than the nanocomposites suggests that changes to the PANI could perhaps be sufficient to explain the high conductivity of the nanocomposites. Possible changes to PANI that could occur include alterations in the doping level, which are well known to affect conductivity [37], and/or improved ordering of PANI when it is in contact with the MoS2 layers, as was observed in polypyrrole/MoS2 nanocomposites [38] and possibly in other PANI/MoS2 nanococomposites [26].

Another possible explanation for this increase in conductivity relative to that of PANI is the stabilization of the typically-unstable 1T metallic form of MoS2. High conductivity has been seen previously in some layered PANI/MoS2 nanocomposites in which the MoS2 was chemically exfoliated, using a procedure where the first step is lithium interaction to form LiMoS2 [19, 24, 28]. During lithiation, the MoS2 layers undergo a transition from the 2H to 1T polytype, with the transition being driven by electron transfer into the MoS2 layers [39, 40]. The LiMoS2 is then exfoliated, the lithium is removed, and nanocomposites are formed by different methods. MoS2 in these nanocomposites can remain in the metallic, conducting 1T form, and it has been proposed that this contributes to their high conductivity [19, 28].

In this work, the MoS2 was synthesized in an exfoliated 2H form, and layering (re-stacking) did not occur in the nanocomposites. However, it may be that interactions with the polymeric material are reducing and thus stabilizing MoS2 in its higher conducting 1T form. Reduction of 2H–MoS2 also might occur during formation of the nanocomposite, as this involves in situ oxidative polymerization of aniline. Further work is needed to clarify the mechanism responsible for the high conductivity of nanocomposites in the 5% to 15% MoS2 range.

5.4.2 Variable Temperature Conductivity Measurements

Variable temperature conductivity data was obtained for several samples, as seen in Fig. 6. In all cases the conductivity dropped rapidly as the temperature was lowered. This is qualitatively similar to a semiconductor; however, an equation for the Arrhenius-like thermally activated conductivity typical of semiconductors did not fit the data well.
Fig. 6

Conductivity versus temperature for PANI, and exfoliated MoS2–PANI nanocomposites. The percentage of MoS2 in the nanocomposites is indicated in the legend. The lines are fits to a heterogeneous conduction model

Much better fits (shown in Fig. 6) were obtained using a heterogeneous conduction model [37], in which the resistivity ρ and conductivity σ are given by:

$$\rho = \sigma^{ - 1} = f_{c} \rho_{m} \exp \left( { - \frac{{T_{m} }}{T}} \right) + f_{n} \rho_{0} \exp \left[ {\left( {\frac{{T_{0} }}{T}} \right)^{\gamma } } \right],$$
(1)
where T is the absolute temperature and the other parameters are constants. This model gives a good description of the temperature dependence of conductivity of many lower-conductivity conducting polymers. The model assumes there is good conduction, similar to a quasi-one-dimensional metal, in some regions of the material, and that these highly-conducting regions are separated by poorly-conducting disordered regions through which mobile charges travel by a hopping process. The first term in equation [1] represents the resistance within the conducting regions and is often small. In fact, the first term was not required to fit our data. The second term, representing hopping conduction, dominates the resistivity in our materials, as it does in many conducting polymers. It is equivalent to the variable-range hopping (VRH or Mott law) model [41] in three dimensions when γ = 0.25. VRH-like conductivity with γ ≈ 0.25 has been reported in, for example, polypyrrole/MoS2 nanocomposites [38] and polyaniline/FeOCl nanocomposites [42]. However, our fits gave values of γ between 0.4 and 0.6. When γ = 0.5, the second term of equation [1] is mathematically equivalent to the VRH model in one dimension, however, γ  = 0.5 can also occur in three dimensions if there are significant electron–electron interactions or if charging effects are important. We conclude that the resistivity is consistent with a heterogeneous conduction model in which hopping between conducting regions is the dominant factor limiting the rate of charge transport, but we cannot make any definite conclusions about the dimensionality of the conduction process.

5.4.3 Thermopower

The results of thermopower (Seebeck coefficient) measurements are shown in Fig. 7, for pellets of the materials made in 2017–2018. (Seebeck coefficient measurements made in 2018 on samples prepared in 2014 gave similar results but were less consistent and reproducible, perhaps due to the age of the samples). Each data point represents one pellet, thus the scatter in the data represents the variability due to differences between pellets, due to variations in sample preparation conditions, pellet pressing pressure, humidity, etc.
Fig. 7

Seebeck coefficient at room temperature for PANI, and exfoliated MoS2–PANI nanocomposites made in 2017–2018, as a function of the percentage of MoS2 in the nanocomposites

We see the thermopower of PANI and the nanocomposites is consistently small and negative, indicating n-type metallic conductivity. A negative thermopower for PANI was not expected, since most values reported in the literature are positive, especially when the PANI is highly conducting [37, 43]. However, negative thermopowers have also been reported for PANI [37, 44]. For example, Wang, et al. [44] reported data for PANI in the fully doped emeraldine salt form, which was unoriented, and had a thermopower near room temperature of about − 3 μV/K, close to our results. When the PANI was oriented by stretching, its conductivity was improved, and the thermopower became positive [44]. The negative room-temperature thermopower we observe for PANI presumably indicates a disordered, unoriented material.

The thermopower values for the nanocomposites are similar to PANI. This is consistent with the possibility that conduction in the nanocomposites takes place largely through the PANI component. However, the alternative explanation involving the presence of 1T-MoS2 cannot be ruled out, since 1T-MoS2 is metallic and would also be expected to have a small n-type thermopower.

6 Conclusions

XRD and TEM confirmed that that we have produced genuine exfoliated nanocomposites, with the components intimately mixed, and with no significant stacking of the MoS2 layers. TGA showed that the presence of MoS2 within the PANI matrix does not affect its thermal stability in either air or nitrogen. Although the TGA results did not show any enhancements in thermal stability, there was a similarity over all the various trials and compositions of the nanocomposites, which confirmed the similarity of these systems in terms of their chemical structure.

Variable-temperature conductivity and thermopower measurements were consistent with a heterogeneous conduction model that has previously been applied to PANI and other conducting polymers and nanocomposites. When the conductivities of nanocomposites with varying amounts of exfoliated MoS2 were compared, a clear trend was seen when large amounts (more than 20%) were added, where the conductivity dropped. An interesting increase in conductivity in comparison to pure PANI was seen in several nanomaterials with MoS2 content in the 5% to 15% range. Possible reasons for this increase include an improvement in the conductivity of PANI due to interaction with the MoS2 layers, through either changes in doping or increased ordering; or, alternatively, stabilization of MoS2 in its metallic 1T zero band gap form. Although further work is needed to clarify the mechanism responsible for the high conductivity of nanocomposites in the 5% to 15% MoS2 range, the possibility that two-dimensional conducting sheets of 1T-MoS2 are stabilized is intriguing, since this would open up the doors for electronic applications. Potentially, other conductive polymers may also behave in a similar way in nanocomposites with MoS2. But no matter which possible explanation for the enhanced conductivity is correct, it is another example of the synergistic enhancement of material properties that can occur in nanocomposites due to molecular-scale interactions between the components.

In the future we plan to study the mechanical properties of these materials by dynamic mechanical analysis and investigate their electrochemical properties via cyclic voltammetry and battery testing.

Notes

Acknowledgements

The authors acknowledge funding from the University of Prince Edward Island, and thank Bowen Gao for assistance with some of the thermopower measurements.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of Prince Edward IslandCharlottetownCanada
  2. 2.Department of PhysicsUniversity of Prince Edward IslandCharlottetownCanada

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