Synthesis of Polythiophene/Manganese Dioxide Nanocomposites by In-situ Core-shell Polymerization Method and Study of their Physical Properties

INTRODUCTION

Conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh) with extensive π-electron delocalization along their backbones are considered to be the most important semiconducting materials because of their fascinating chemical and physical properties useful for various applications [1,2]. These materials may combine the process ability and outstanding mechanical characteristics of polymers with the readily-tailored electrical, optical, and magnetic properties of functional organic molecules. In particular, the potential use of these materials in light-emitting diodes [3], field-effect transistors [4], photovoltaic cells [5], and other opto-electronic devices has motivated the development of synthesis and processing methods of conjugated polymer materials with unique properties [6].

PTh and its derivatives have attracted much consideration because of their easy preparation, environmental stability, higher conductivity and photoconduction [7,8]. PTh is produced from the polymerization of thiophene, as a sulphur heterocycle, by connecting thiophene through its 2,5 positions. Three approaches to polymerization of thiophene have been reported in the literature: (1) electropolymerization, (2) metal-catalyzed coupling reactions, and (3) chemical oxidative polymerization [9]. When PTh was combined with different inorganic materials such as metal or metal oxide nanoparticles, it produced nano-composite materials. Nanocomposites are the special class of materials which exhibit unique physical, chemical and biological properties [10]. The investigation of polymer-inorganic nanocomposites is motivated by many reasons, including the need for novel electronic anisotropic materials [11], better performing battery [12], cathode materials [13], functional structural materials with superior mechanical and thermal properties [14]. It is expected that the uses of new functional inorganic nano-fillers will lead to new polymer-inorganic nanocomposites with unique combinations of material properties. In the recent years, synthesis and application of conducting polymers nanocomposites with various inorganic nanoparticles such as CdS [15], NiO [16], SnO₂ [17], SiO₂ [18], V₂O₅ [19], CuO [20], Cu₂O [21], Al₂O₃ [22], ZnO [23], Fe₃O₄ [24], and TiO₂ [25] have been reported.

In recent years, manganese dioxide (MnO₂) has attracted enormous attentions due to its physical and chemical properties such as low cost, abundant availability, high surface area, adsorption ability, good stability under acidic conditions and environmental compatibility [26]. The MnO₂ has been widely used in wastewater treatment [27], molecular/ion sieves [28] and electrode materials in batteries or capacitors [29] because of its different and unique structures. MnO₂ exists in several crystallographic forms, such as α-, β-, γ-, δ-, λ- and ε-type, when the basic unit [MnO₆] octahedron links in different ways [30]. Among these crystallographic forms, 1D α-MnO₂ nanorods have received special attention as cathodic materials for lithium batteries since the large tunnels existing in the crystalline lattice of α-MnO₂ are believed to facilitate the accommodation and transportation of inserting lithium ions [31]. So far, numerous efforts have been devoted to synthesize MnO₂ nanostructures and a variety of strategies have been developed, including thermal decomposition, coprecipitation [32], simple reduction [33], hydrothermal method [34], sol-gel [35] and etc. Among these methods, hydrothermal synthesis has attracted more attention for preparation of nanostructured materials such as metal oxides, chalcogenides, and metals because it is easily controlled on the shape of materials, which are simple processed and in large scale [36-38].

This paper reports the controlled synthesis of α-MnO₂ nanostructures via a facile hydrothermal route without using any physical template and addition of any surfactant. α-MnO₂ nanorods was synthesized based on the hydrothermal reaction of MnSO₄ and KMnO₄ in aqueous medium. Then, a novel serious of PTh/MnO₂ nanocomposites was synthesized by one step in-situ polymerization of thiophene in the presence of different amounts of α-MnO₂ nanorods. The characteristics of the molecular structure, crystallinity, thermal stability, and morphology of the PTh/MnO₂nanocomposites are also discussed. Therefore, synthesis of a new serious of PTh/MnO₂ nanocomposites for the first time and studying their physical properties are the main novelty of this work.

MATERIALS AND METHODS

Materials

Thiophene was purchased from Mreck and distilled to obtain a colorless liquid. Then was kept below 4°C. Chloroform was purchased from Merck and dried by standard procedure before use. Anhydrous FeCl₃, MnSO₄.H₂O, KMnO₄ and other reagents were provided from Merck and used as received. All chemical used were of analytical grade and double distilled water was used for solution preparation.

Preparation of α-MnO₂ nanorods

α-MnO₂ nanorods were produced by the hydrothermal reaction [39, 40]. In a typical synthesis, MnSO₄.H₂O (1.69 g, 0.1 mol) and KMnO₄ (3.16 g, 0.2 mol) were mixed in the distilled water at room temperature and magnetically stirred to form a homogeneous mixture. When the mixed solution changed to a dark brown gel-like solution, it was immediately transferred into a Teflon-lined stainless steel autoclave and heated at 140°C for 12 h. After the reaction was complete, the reactor was taken out and naturally cooled to room temperature. The resulting brownish-black solid product was filtered off, washed with distilled water to remove ions possibly remaining in the final products, and finally dried at 120°C in air overnight. The production yield of α-MnO₂ nanorods was about 92% based on Manganese quantity in the starting reagents. The chemical reaction is shown in Eq. (1).

3 MnSO₄ + 2 KMnO₄ + 2 H₂O →

5 MnO₂ + K₂SO₄ + 2 H₂SO₄ (1)

Preparation of PTh/MnO₂ nanocomposites

The PTh/MnO₂ nanocomposites were chemically synthesized by in-situ oxidative polymerization of thiophene using FeCl₃ as oxidant under controlled conditions. As-synthesized α-MnO₂ in different weight percentage (5%, 10%, 15% and 20%) was dispersed in flask containing 50 ml of dry CHCl₃ and thiophene (1 g, 48 mmol) was then added into the flask. The mixture was sonicated in an ultrasound bath for 20 minutes to get well dispersed. Then, anhydrous FeCl₃ (7.8 g, 48 mmol) was dissolved in 50 ml of dry CHCl₃ and added dropwise into the mixture at room temperature. The in-situ polymerization was started immediately and continued for 24 h at room temperature. The dark gray precipitate was recovered by filtration and extracted with methanol for 24 h to remove of the residual FeCl₃ and unreacted monomer. During this procedure, the color of nanocomposites changed from black (PTh in oxide state) to red brown (PTh in reduced state), which indicated the successful reduction of PTh shell. A series of PTh/MnO₂ nanocomposites designated as PTh/MnO₂ (5%), PTh/MnO₂ (10%), PTh/MnO₂ (15%), and PTh/MnO₂ (20%) was obtained in high yield (75-90%) after washing with methanol for several times and drying at 80°C for 3 h in vacuum oven.

Pure PTh was prepared applying above procedure in the absence of α-MnO₂ nanoparticles.

Instrumental measurements

FT-IR spectra were recorded on a Bruker Spectrometer Tensor 27 FTIR with KBr pellets. X-ray diffraction (XRD) patterns of α-MnO₂, PTh, and PTh/MnO₂ nanocomposites were measured in the range of 2Ɵ=10-70° by step scanning on the Siemens X-ray diffraction D5000 with Cu Kα radiation (λ=0.154 nm). The elements of samples were measured on energy-dispersive X-ray (EDX) spectroscopy, which was taken on a Leo1430VP microscope with operating voltage 5 kV. The process of EDX measures were carried out with a pellet which was pressed at 200 MPa and then adhered to copper platens. The morphology of α-MnO₂, PTh, and PTh/MnO₂ nanoparticles was investigated by scanning electron microscopy (SEM, LEO-440i). The size of particles was investigated with a Hitachi 600 transmission electron microscope (TEM). Thermal stability of α-MnO₂, pure PTh and nanocompistes was investigated by thermal gravimetric analyzer (Perkin Elmer, TGA-7) under a nitrogen flow (35 ml/min) and heating rate of 10°C/min.

RESULTS AND DISCUSSION

Thiophene and its derivatives can be polymerized by both chemical and electrochemical methods. Chemical method is preferred over electrochemical methods because of its simplicity and scalability. For many years, oxidative polymerization method using FeCl₃ had been the synthetic inexpensive method of choice for the preparation of PTh. Thiophene is not stable in protic acid media, therefore, FeCl₃ as a Lewis acid in dry CHCl₃ is used as oxidant [41]. Fig. 1 shows the proposed mechanism for thiophene oxidative polymerization. The active sites in the polymerization are the crystal Fe³⁺ surface ions. They have one unshared chloride and one empty orbital, which is the source of their Lewis acidity. The soluble part of FeCl₃ is inert because it exists in a dimeric form without empty orbitals. The reaction starts by complexation between the thiophene sulfur and the FeCl₃ to form a cation radical, which upon deprotonation yields the initiating radical. The mechanism also proposes that the combination of thiophene radicals gives mainly the 2,5-disubstituted thiophene moieties in the PTh chains. The effective reaction needs a 4:1 mole ratio of FeCl₃: thiophene. This requirement is because the polymerization process requires solid FeCl₃, and 50% of the FeCl₃ dissolves in the CHCl₃ [42]. In addition, the HCl byproduct consumes FeCl₃ to form the FeCl₄^- complex ion. Homopolymerization also takes place in this reaction mixture, yielding a reddish PTh powder after dedoping with methanol.

Study of FT-IR spectroscopy results

The FT-IR spectra of the PTh, and PTh/MnO₂ nanocomposites are shown in Fig. 2. The band at around 3500 cm⁻¹ corresponds to the O-H vibrating mode of traces of absorbed water. In the FT-IR spectrum of pure PTh, the several low-intensity absorption peaks at 2800-3100 cm^-1 can be attributed to the aromatic C-H stretching vibration bands. The bans at 1565 and 1400 cm^-1 were ascribed to the C=C asymmetric and symmetric stretching vibrations of thiophene ring, respectively. The absorption peak at 784 cm^-1 was ascribed to the =C-H out of plane vibration of α,αʹ-coupling of poly-٢,5-thiophene which confirmed the polymerization of thiophene monomer. The peaks at 1200 and 1040 cm^-1 were due to C-H bending and C-H in-plane deformation. The absorption peak at 684 cm^-1 was assigned to the C-S bending mode, which indicated the presence of thiophene monomer [43]. The PTh/MnO₂ nanocomposites spectrum shows nearly identical numbers and positions of the pure PTh bands. The C-H stretching vibrations and C=C characteristic peaks can be identified almost in the same range at 2800-3100 cm^-1 and 1590 cm^-1, respectively. The band located at 500 cm^-1 can be ascribed to the Mn-O stretching vibration of MnO₂ nanopowder [44]. The peak at 830 cm^-1 should be ascribed to the C-H out of plane stretching vibration mode of PTP.

Study of XRD patterns

The X-ray diffraction (XRD) patterns of the pure PTh and PTh/MnO₂ nanocomposites are shown in Fig. 3. The broad peak in the region of 2Ɵ=25° in XRD pattern of pure PTh (Fig. 3a) shows that the synthesized PTh in the absence of α-MnO₂ nanorods is amorphous. The XRD pattern of the PTh/MnO₂ nanocomposites (Fig. 3b) shows strong sharp diffraction peaks at 12.7°(110), 18.1°(200), 28.8°(310), 37.4°(211), 49.8°(411), 60.2°(521) can be indexed to tetragonal phase of α-MnO₂ (JCPDS Card, No.44-0141) [45]. In addition, no diffraction peaks for impurities are observed, which suggests the high purity of the product. As shown in Fig. 3b, the broad diffraction peak around 2θ=25° which is caused by the periodicity perpendicular to the polymer chains of PTh still exists, but its intensity has been decreased. This means that PTh deposited on the surface of α-MnO₂ nanorods has no effect on the crystallization of α-MnO₂, and each phase maintains his initial structure.

The average crystallite size of as-synthesized α-MnO₂ nanorods and PTh/MnO₂ nanocomposites is calculated by Scherrer formula (Eq. 2) [46-48]:

D = Kλ/β cosƟ (2)

where D is crystallite size of particle, λ is X-ray wavelength (0.154 nm for Cu-Kα), K is the shape factor, which can be assigned a value of 0.89 if the shape is unknown, Ɵ is Bragg diffraction angle and β is the full width at half-height of angle of diffraction in radians. The above equation was introduced for the characteristic (211 plane) peak at 2Ɵ=37.4°. The obtained results showed that the average particle size of α-MnO₂ nanorods and PTh/MnO₂ nanocomposites was 37 nm and 50-70, respectively. These results were in good agreement with the TEM and SEM images.

Study of EDX spectroscopies

The EDX spectra of as-synthesized α-MnO₂ and PTh/MnO₂ (20%) nanocomposite are shown in Fig. 4. It was found that the major elements of as-synthesized α-MnO₂ are Mn and O. (Fig. 4a). A small amount of Fe was possibly contaminated by instrumentation (autoclave). In EDX spectrum of PTh/MnO₂ nanocomposites (Fig. 4b), the major elements are Mn, O, S and C. This further proves that the PTh/MnO₂ nanocomposites are synthesized successfully. Furthermore, the element of Cl and Fe are observed in nanocomposites, which should be resulted from FeCl₃ which used as an oxidant in polymerization.

Study of SEM and TEM pictures

Fig. 5 shows representative TEM image of the as-synthesized α-MnO₂. The as-synthesized α-MnO₂ contains 100% rod-like morphology with smooth surface and no other morphology has been detected. TEM image shows that diameter of the α-MnO₂ nanorods is in the range of 30-40 nm and the length is about 0.5 μm.

SEM technique was performed to investigate the dimensions and the morphology of PTh/MnO₂ nanocomposites. A typical SEM image of PTh/MnO₂ (20%) nanocomposite is shown in Fig. 6. The nanocomposite has rod-like morphology with average diameter of 50-70 nm which are linked together. In nanocomposites, the thiophene monomers are adsorbed onto the surface of α-MnO₂nanorods due to the electrostatic interaction. Thus, the polymerization has a preferring trend on the surfaces of MnO₂.

Study of thermal stability

The thermal stability behavior of α-MnO₂ nanoparticles, pure PTh and PTh/MnO₂ nano-composites were investigated by the thermal gravimetric analyzer in the temperature range of 20-800°C (Fig. 7). The neat α-MnO₂ is stable and shows a trace weight change in the whole investigated temperature range (Fig. 7a). Around 5% weight loss in α-MnO₂ nanorods is observed up to the temperature 250°C, which can be attributed to the removal of physically adsorbed water. There is a 6% weight loss between 550 and 650°C, which can be ascribed to the reduction of manganese from tetravalent to trivalent form accompanied by the evolution of oxygen [49].

TGA diagram of pure PTh (Fig. 7b) shows three major stages of weight loss. The first loss weight in the range of 20-140°C can be attributed to the evolution of water molecules. The second stage, in the temperature range of 140-320°C, is related to removal of dopant anions from the polymer structure. The last weight loss observed between 320-650°C corresponds to the degradation of the PTh polymer chains. At the ending temperature of 800°C, the weight loss of the pure PTh is 96%.

Study of TGA diagram of PTh/MnO₂ (20%) nanocomposite (Fig. 7c) shows that PTh/MnO₂ nanocomposite is stable up to 400°C, and decomposes completely at 650°C. At the ending temperature of 800°C, the weight loss of PTh/MnO₂ (20%) nanocomposite is 63%. The obtained results indicate that introduction of α-MnO₂ nanoparticles into PTh polymer matrix increases the thermal stability of PTh around 33%. As a result, these data confirm that the presence of α-MnO₂ nanoparticles in the PTh/MnO₂ nanocomposites is responsible for the high thermal stability of the synthesized nanocomposites in comparison with pure PTh.

CONCLUSION

In summary, α-MnO₂ nanorods were prepared with hydrothermal precipitation method at 140°C for 12 h. Then, a novel serious of PTh/MnO₂ nanocomposites was synthesized by one-step in-situ chemical oxidative polymerization of thiophene containing different weight percentage of as-prepared α-MnO₂ nanorods. The FT-IR and EDX spectra showed presence and encapsulation of α-MnO₂ in the PTh matrix. The XRD study revealed that PTh deposited on the surface of α-MnO₂ nanoparticles has no effect on the crystallization of α-MnO₂, and each phase maintains his initial structure. SEM and TEM images of PTh/MnO₂ nanocomposites showed strong effect of α-MnO₂ nanoparticles on the morphology of nanocomposites. PTh/MnO₂ nanocomposites had rod-like morphology similar to α-MnO₂ morphology. TGA plots showed that the synthesized PTh/MnO₂ nanocomposites had higher thermal stability in comparison with pure PTh. Finally, the employed method is very simple and inexpensive in comparison with other applied methods, and it can be easily applied industrially.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.