Exfoliated MoS2–Polyaniline Nanocomposites: Synthesis and Characterization
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Exfoliated MoS2–PANI nanocomposites with varying percentage by weight of exfoliated 2H–MoS2 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 2H–MoS2 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.
KeywordsPolyaniline Molybdenum disulfide Nanocomposite electronic conductivity
There has been a considerable amount of interest in the field of inorganic/organic hybrid nanomaterials . 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 , separation , solid lubrication/wear-resistance  and as materials in battery applications .
Molybdenum disulfide is one of the layered transition metal dichalcogenides (TMDs) that has been used for the synthesis of intercalated  and exfoliated nanocomposites . 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 . The usual, stable form of MoS2 is 2H, however, a metastable 1T form of MoS2 also exists . Factors such as temperature  and pressure  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 . 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 . 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 . 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 . 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 . 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.
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.
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. . 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.
4.1 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
5.2 Electron Microscopy
5.3 Thermogravimetric Analysis
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 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 , 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 , and even higher values have been reported in the literature . 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 , and/or improved ordering of PANI when it is in contact with the MoS2 layers, as was observed in polypyrrole/MoS2 nanocomposites  and possibly in other PANI/MoS2 nanococomposites .
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
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.  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 . 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.
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.
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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