Preparation and optical/electrical/electrochemical properties of expanded graphite-filled polypyrrole nanocomposite

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Abstract

Expanded graphite filled polypyrrole (PPy/EG) conducting composites were prepared by in situ polymerization of pyrrole by the addition of expanded graphite in various proportions (0.25%, 0.50% and 1.0%). The synthesized samples were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), ultraviolet–visible absorption (UV–vis), X-ray diffraction (XRD) and electrical conductivity measurements. FTIR spectroscopy revealed the interaction between expanded graphite and polypyrrole (PPy). PPy/EG composites showed crystalline nature with rhombohedral structure. The band gaps of pure PPy and PPy/EG composites were determined by UV–vis absorption spectrophotometry and cyclic voltammetry. The electrical conductivities of the composites were enhanced dramatically to 110.04 S cm−1 compared to pure PPy. The composites also showed excellent electrochemical reversibility at the scan rate of 0.1 V s−1 and maximum reversible electrochemical response intaking charge capacity almost unchanged even up to 125th cycles.

Highlights

► We synthesized polypyrrole/expanded graphite composite via in situ polymerization. ► Expanded graphite dramatically increased the conductivity of the polymer composite. ► Increased conductivity is due the excess surface provided by the EG. ► The composites possess good thermal stability up to 750 K. ► The composite shows gratifying reversible electrochemical response.

Introduction

Conducting polymer composites with various conductive fillers have recently attracted much attention. They can exhibit significant levels of electrical conductivity suitable for use in electronic devices, rechargeable batteries, functional electrodes, electrochromic devices, sensors, conductive inks and so on [1], [2], [3], [4], [5], [6]. Virgin conducting polymers like polyaniline, polypyrrole (PPy), polythiophene, etc. have poor processability and the lack of essential mechanical properties. To overcome these drawbacks, some fillers are incorporated into these polymer matrices to form composite materials [7], [8], [9], [10].

PPy is one of the most studied conducting polymers because of its rather straightforward preparation methods. PPy materials are reasonably stable in air, and possess good electrochemical properties and thermal stability. PPy exhibits a wide range of volume conductivities (10−3 S cm−1 < σ < 100 S cm−1) depending on the functionality and substitution pattern of the monomer and the nature of the counter ion or dopant [11], [12]. PPy shows its capability to store electrical charges. The stored electrical charges can be recovered upon demand, for that reason PPy can be considered as a good candidate of super-capacitors [13], [14], [15].

Graphite, which is naturally abundant and low cost, has widely been used as electronically conducting filler in preparing conducting polymer composites [16], [17]. In most of the cases relatively large quantities of graphite are required to reach a critical percolation value. Large amount of graphite concentration is always leading to the poor mechanical property of the composite materials [18]. To overcome this problem the concept of expanded graphite (EG) has been extensively employed [19], [20], [21], [22], [23], [24], [25]. EG is produced from graphite flakes intercalated with concentrated H2SO4 followed by rapid thermal treatment which could lead expansion up to several hundred times of the original volume. This expansion splits up the graphite sheet into nanoplates with a very high aspect ratio [26]. The graphite intercalation compounds may provide a possible source for nanocomposite formation with polymers [27], [28]. It is also well known that expanded graphite (EG) is much lighter, and has a higher specific surface area than carbon powder and carbon nanotubes [29].

In this paper, we have showed the effort of expanded graphite (EG) on the electrical and electrochemical behaviour of the polypyrrole/expanded graphite (PPy/EG) nanocomposite. The EG has been prepared by the standard method and this modified EG has been used for preparation of highly conducting PPy/EG nanocomposite. To the best of our knowledge, the synthesis of PPy/EG functional composites and study of their electrical and electrochemical behaviour have not been reported so far. The concept of optical and electrochemical band gap of the PPy/EG nanocomposite and comparison of both the techniques was studied thoroughly. Compared to the previous work of PPy/G composite [30], we have got the dramatic enhancement of the electrical conductivity, better electrochemical and optical behaviour of this modified nanocomposite.

Section snippets

Materials

Pyrrole was obtained from Aldrich Co. and used without further purification. The natural graphite flake of size −50 +100 BS mesh, hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3) and anhydrous ferric chloride (FeCl3) were obtained from Merck and used as received. The solvent methanol (CH3OH) was distilled before used. Acetonitrile (CH3CN) was obtained from Merck and purified by standard methods. Lithium perchlorate (LiClO4) was obtained from Fluka and used as received. For

Characterization

Fourier transform infrared spectroscopy (FTIR) was used to record FTIR spectra by Impact 410, Nicolet, USA, using KBr pellets. The ultraviolet–visible (UV–vis) absorption spectroscopy of the samples in 1-methyl-2-pyrrolidon solvent was recorded using Shimadzu UV-2550 UV–vis spectrophotometer in the range of 300–800 nm. The surface morphology of the composites was observed by scanning electron microscope (SEM) of model JSM-6390LV, JEOL, Japan. The surface of the sample was coated with platinum

Results and discussion

The EG filled PPy composites were prepared by in situ oxidative polymerization technique. The monomer pyrrole may get adsorbed on the surface of the dispersed expanded graphite sheets. This adsorbed pyrrole on expanded graphite as well as remaining free pyrrole gets polymerized in the presence of oxidizing agent FeCl3 to yield PPy/EG composites.

Conclusion

In conclusion, the conducting composites of expanded graphite filled polypyrrole have been fabricated via oxidative polymerization successfully. XRD reveals the incorporation of EG into the PPy matrix. The PPy/EG composites possess good thermal stability up to 750 K. The optical band gap of PPy/EG composites has been decreased from 1.93 to 1.85 eV and the electrochemical band gap has been decreased from 2.18 to 1.98 eV on addition of EG (0.25, 0.5 and 1.0%). The DC electrical conductivity of the

Acknowledgments

The authors would like to thank the Department of Atomic Energy (DAE) and the Board of Research in Nuclear Sciences (BRNS), India, for their financial support in the research under contract no. 2008/37/37/BRNS/2470.

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