Synthesis, characterization and electrochemical behavior of polypyrrole/carbonnanotube composites using organometallic-functionalized carbon nanotubes

Hongyu Mi a,c, Xiaogang Zhang b,**, Youlong Xu a,*, Fang Xiao a

a Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an 710049, PR Chinab College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR Chinac School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China

Applied Surface Science 256 (2010) 2284–2288

A R T I C L E I N F O

Article history:

Received 27 June 2009

Received in revised form 16 October 2009

Accepted 17 October 2009

Available online 31 October 2009

Keywords:

Organometallic complex

Carbon nanotube

Polypyrrole

Electrochemical property

A B S T R A C T

Thorn-like, organometallic-functionalized carbon nanotubes were successfully developed via a novel

microwave hydrothermal route. The organometallic complex with methyl orange and iron (III) chloride

served as reactive seed template, resulting in the oriented polymerization of pyrrole on the modified

carbon nanotubes without the assistance of other oxidants. Morphological and structural characteriza-

tions of the carbon nanotube/methyl orange–iron (III) chloride and polypyrrole/carbon nanotube

composites were examined using transmission electron microscopy (TEM), energy dispersive

spectroscopy (EDS), infrared spectroscopy and X-ray diffraction (XRD). The electrochemical property

of the polypyrrole/carbon nanotube composite was elucidated by cyclic voltammetry and galvanostatic

charge–discharge. A specific capacitance of 304 F g�1 was obtained within the potential range of �0.5–

0.5 V in 1 M KCl solution.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Supercapacitors occupy important position among variouspower systems due to their advantages over the second batteriesand conventional capacitors, and potential applications in a seriesof electronic devices and electrical vehicles [1]. It is well knownthat carbon nanotubes (CNTs) possess high surface area, tunablesurface function, high electrical conductivity and environmentalstability, thereby are widely used as the electrode materials forelectric double-layer capacitors. On the other hand, the con-trollable structures and outstanding properties (e.g., high dopinglevel and quick electrochemical switching) of electronicallyconducting polymers (ECPs) [2–5] endow them the possibilityfor the application as prospective materials in pseudocapacitors.Recently, much effort was devoted to the combination of ECP withCNT to improve the electrochemical performance of the materials[6–11]. However, some drawbacks of CNTs, including insolubility,poor dispersibility and bad compatibility, hinder their uniformincorporation with ECP matrixes [12–15]. Accordingly, extensiveattention has been focused on the enhancement of the interactionbetween CNT and ECP [16–21]. The most common method is to

* Corresponding author. Tel.: +86 29 82665161; fax: +86 29 8266 0010..

** Corresponding author. Tel.: +86 25 52112902.

E-mail addresses: azhangxg@163.com (X. Zhang), ylxuxjtu@mail.xjtu.edu.cn

(Y. Xu).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.10.053

treat CNTs with strong inorganic acids, such as HNO3, H2SO4 or themixture of HNO3/H2SO4 to generate carboxylic group capped CNTsbefore their direct association with ECPs [22]. The control ofmorphology of ECPs on CNTs is another challenging issue. Theutilization of organometallic-functionzied CNTs as morphology-guiding agent is a possible strategy to tune the growth of ECPs onCNTs. Unfortunately, scarce investigation has been conductedalong this line.

In the present study, a microwave hydrothermal strategy hasbeen employed to functionalize acid-treated CNTs with a thorn-like organometallic, the methyl orange–iron (III) chloride (MO–FeCl3) complex. This complex can serve as both morphology-guiding agent and oxidant, thereby polypyrrole nanoparticles canbe attached directionally on CNTs by the polymerization of pyrrolein the absence of extra oxidants. Fig. 1 illustrates the chemicalstructure of MO [23,24] and the possible mechanism for thesynthesis of CNT/MO–FeCl3 and the resulting polypyrrole/carbonnanotube (PPy/CNT) composite. To the best of our knowledge, thepreparation of such organometallic-functionized CNTs (CNT/MO–FeCl3) and the corresponding PPy/CNT composite has not beenreported by far. This investigation offers a new prospect for thepreparation of 1D nanostructured polymer perpendicular-mod-ified CNTs. The resulted PPy/CNT composite has been characterizedby transmission electron microscopy (TEM), energy dispersivespectroscopy, infrared spectroscopy and X-ray diffraction, and itsapplication as electrode for supercapacitors has also beenestimated by electrochemical investigation.

Fig. 1. Illustration of MO structure and the possible mechanism for the synthesis of

CNT/MO–FeCl3 and PPy/CNT composites.

H. Mi et al. / Applied Surface Science 256 (2010) 2284–2288 2285

2. Experimental

2.1. Synthesis of CNT/MO–FeCl3 composites

Pyrrole from Aldrich (99%) was distilled prior to use and storedat �5 8C in a nitrogen atmosphere. CNTs from Shenzhen NanotechPort Co. Ltd. (China) were purified by refluxing in concentratednitric acid. All other reagents including methyl orange (MO,(CH3)2NC6H4-N55NC6H4SO3Na) were of analytical grade and usedas received. Typically, purified CNTs (0.07 g) and methyl orange(MO, 0.4312 g) were dispersed in 240 mL deionized water andsonicated for 30 min so that the MO was uniformly adsorbed on theCNTs. On stirring, FeCl3 (1.944 g) was added into the abovesuspension. After that, the mixture was poured into a self-madeautoclave reactor, which was then heated in a commercialmicrowave oven for 30 min. Finally, a part of the resulting mixturewas washed and dried for further characterization.

2.2. Synthesis of PPy/CNT composites

20 mL of the above mixture with the CNT/MO–FeCl3 compositewas sonicated for 30 min and pyrrole monomer was then droppedinto this solution. The reaction was allowed to occur for 3 min in amicrowave oven to generate a dark precipitate. The precipitate wasfiltered and thoroughly rinsed with deionized water and ethanol,and dried at 50 8C overnight under vacuum to get the powder ofPPy/CNT composite.

2.3. Characterization

Transmission electron microscopy images of the as-preparedsample were obtained using a Hitachi 600 transmission electronmicroscope. Fourier transform infrared (FT-IR) spectra wererecorded on a BRUKER-EQUINOX-55 spectrometer in the fre-quency range of 500–3000 cm�1. X-ray diffraction (XRD) patternswere taken in the 2u range of 5–608 using an M18Xce X-raydiffractometer with CuKa radiation. Energy dispersive spectro-scopy (EDS) spectrum analysis was performed on a LEO 1430VPscanning electron microscope as an energy dispersive X-rayanalyzer.

The working electrode was prepared by mixing the activematerial with 15 wt.% acetylene black and 5 wt.% polytetrafluor-oethylene (based on the total electrode mass) to form a slurry. Theslurry was then cast on a graphite electrode (1.0 cm2). Cyclicvoltammetry and chronopotentiometry were performed in thevoltage of �0.5–0.5 V using a CHI660A electrochemical work-

station (CHI Co., USA) with a three-electrode system. Theelectrolyte was 1 M KCl solution. A Pt foil and a saturated calomelelectrode (SCE) were used as the counter and reference electrodes,respectively.

3. Results and discussion

3.1. Morphology and structure analysis

Typical TEM images of functionalized CNTs prepared by boththe microwave hydrothermal route and the conventional solutionmethod, and the resulted PPy/CNT composite are displayed inFig. 2. Obviously, the CNT surface treated by the microwavehydrothermal technique were very rough (Fig. 2a), as distinctfrom the smooth surface of the raw CNTs. As can be seen from theTEM picture of a single CNT (Fig. 2b), lots of thorns were covereddensely on the surface of the CNT. However, for CNTs functio-nalized by conventional solution route, only few thorns wereattached on the CNTs (Fig. 2c) and the majority of them weredispersed outside CNTs. According to the finding of previousreports [24], it was inferred the MO–FeCl3 was bound on thesurface of acid-treated carbon nanotubes mainly by the electro-static association between negatively charged CNT and positivelycharged MO (as shown in Fig. 1). Such association was not verystrong and the dissociation might be happened at a relatively mildsurrounding. The finding in Fig. 2a suggests that the microwavehydrothermal process presumably enhanced the associationbetween CNT and MO–FeCl3, resulted in the much moreabsorption of MO–FeCl3 on CNTs.

From the theoretical view of point, the Fe3+ moieties on theCNT/MO–FeCl3 might serve as the initiator for oxidation poly-merization. To confirm this, the monomer, pyrrole, was added intothe mixture containing the CNT/MO–FeCl3 and without adding anyextra oxidants, and then this mixture was exposed under theirradiation of microwave. The TEM image of the resulted PPy/CNTcomposite is depicted in Fig. 2d and e. One could find that someparticles rather than the above-mentioned thorns, were attachedon CNTs, which was proved to be PPy polymer by IR spectroscopy.The formation of PPy particles on CNTs confirmed that thepolymerization of pyrrole was initiated by the Fe3+ in the CNT–MO–FeCl3 complex and the polymer chain propagated along theCNT–MO–FeCl3 orientation.

One can see from the chemical structure of MO (Fig. 1) that itcontains simultaneously amine and sulfonate groups at two sides,and the amine sides can be converted into positive chargedammoniums under the acidic environment. Consequently, theelectrostatic withdrawing between ammonium and –COO– mightbe helpful for MO to attach on CNTs. The CNT–MO–FeCl3complexes could be obtained with the addition of FeCl3 into theCNT–MO mixture, which was based on the association between thenegative –SO3

� and positive Fe3+ groups [25,26]. EDS investigationwas also conducted to trace the evidence for the formation of CNT/MO–FeCl3 composite. In the Fig. 3, the EDS spectrum confirmed thepresence of Fe, O and S in the CNT/MO–FeCl3. Based on thesefindings, one can induce that the CNT/MO–FeCl3 composite hasbeen successfully obtained.

IR spectra of the CNT/MO–FeCl3 and PPy/CNT composites areshown in Fig. 4. In Fig. 4a, the strong peaks at 1605, 1548, 1414,1280, 1186, 1119, 1131, 1005, 829, 700 and 617 cm�1 were typicalsignals that could be attributed to MO [27]. The PPy/CNT composite(Fig. 4b) exhibited some absorption peaks corresponding to thecharacteristic peaks of the pyrrole ring (1564 cm�1), and to the C–H in-plane vibration (1300 and 1036 cm�1), the C–N vibration(1190 cm�1), and the C–H out-plane bend (928 cm�1) [28,29]. Thepossible reason for the disappearance of MO in Fig. 4b was due tothe automatic degradation of the MO–FeCl3 complex and the

Fig. 2. TEM images of the CNT/MO–FeCl3 composites obtained by: (a, b) the microwave hydrothermal method; and (c) the conventional solution method. TEM images of the

PPy/CNT composite at magnification of (d) �50,000 and (e) �100,000.

H. Mi et al. / Applied Surface Science 256 (2010) 2284–22882286

subsequent liberation of the MO which occurs when pyrrole ispolymerized in the presence of the MO–FeCl3 complex.

Fig. 5 shows XRD patterns for the CNT/MO–FeCl3 and PPy/CNTcomposites. The Bragg diffraction peaks of 2u = 11.88, 17.68, 208,26.48, 28.68, 35.28, 39.38, 46.58 and 55.98 that could be observed inFig. 5a, suggesting a well-crystallized CNT/MO–FeCl3 composite.However, in Fig. 5b, the PPy/CNT composite exhibited a low intensebroad peak at about 12.3–25.28, which indicated that thiscomposite had an amorphous nature. The XRD result illustratedthat, after pyrrole was polymerized on the CNTs, the MO–FeCl3

complex as a self-degraded seed template was finally displaced byPPy. So the obtained product was PPy/CNT composite without MO–FeCl3 complex.

3.2. Electrochemical profiles

Fig. 6a provides cyclic voltammograms (CVs) for the PPy/CNTcomposite tested with scanning rates of 3, 5 and 20 mV s�1 in thepotential window of �0.5–0.5 V vs. SCE. All I–E curves of thecomposite were almost rectangular in shape in the given potentialrange, and the I–E responses were almost symmetric with respectto the zero-current line, showing the characteristic feature ofcapacitive behavior [30,31]. Moreover, with increasing scan ratefrom 3 to 20 mV s�1, the shape of the curves only had a slightchange as the current increased linearly, showing good rate abilityattributed to the effective electrochemical accessibility of electro-lyte through the PPy [32].

Fig. 3. EDS image of the CNT/MO–FeCl3 composites.

Fig. 4. IR spectra of (a) CNT/MO–FeCl3 and (b) PPy/CNT composites.

Fig. 5. XRD patterns of (a) CNTs/MO–FeCl3 and (b) PPy/CNTs composites.

Fig. 6. The electrochemical performance of the PPy/CNT composites: (a) cyclic voltamm

current density of 1 mA cm�2.

H. Mi et al. / Applied Surface Science 256 (2010) 2284–2288 2287

Galvanostatic charge/discharge for the PPy/CNT composite wasperformed with an applied current density of 1 mA cm�2 in thepotential range between �0.5 and 0.5 V, shown in Fig. 6b. Theresulting curve was not an ideal straight line, indicatingpseudocapacitance behavior [33]. The process of the electroche-mical redox reaction at the interface between the electrode andelectrolyte is shown as follows:

½PPy0� þ Cl� ! ½PPyþ�Cl� þ e�

½PPyþ�Cl� þ e� ! ½PPy0� þ Cl�

In addition, there was a small IR drop during discharge causedby small internal resistance. The discharge capacitance (SC) may becalculated from the equation: SC = (I �Dt)/(DV �m), where Cm isthe specific capacitance, I the charge/discharge current, Dt thedischarge time, DV 1 V, and m the mass of active material. A SCvalue as high as 304 F g�1 was gained for the PPy/CNT composite. Apossible explanation for a high SC may be that good dispersion ofthe as-prepared PPy nanoparticles based on the oriented thorns onthe CNTs could help to provide a largely electrolyte-accessiblesurface area to improve the utilization of PPy for redox reaction[34]. Currently, the reported SC value of polypyrrole/carbonnanotube composite prepared by various methods [6,35–38] hasranged from 130 to 506 F g�1, respectively. The difference in thevalue was perhaps related to synthesis method, reaction system,

etry at scan rates of 3, 5 and 20 mV s�1; (b) Galvanostatic charge–discharge at a

H. Mi et al. / Applied Surface Science 256 (2010) 2284–22882288

the structure of the composite, test condition and the mass ofpolymers in the composite. Hence, how to harvest a higherperformance PPy/CNT composite available for supercapacitorapplication still remains a big challenge.

4. Conclusions

A thorn-like MO–FeCl3 complex was successfully grown onCNTs by the microwave hydrothermal technique. Further, PPy/CNTcomposite with controllable morphology was obtained with theaid of the MO–FeCl3 complex. The electrochemical performanceindicated that the prepared composites had a specific capacitanceof 304 F g�1. Through a series of chemical characterizations, it wasclear that the growth of granular-like PPy was closely related to theshape and size of the oxidative compounds on CNTs. If the thornscould be developed into long fibers, the highly oriented PPynanofibers or nanotubes covered on the CNTs could be expected,which will result in a significant improvement of electrochemicalperformance.

Acknowledgments

This work was supported by the Natural Basic ResearchProgram of China (973 Program; No. 2007CB209703) and theNatural Sciences Foundation of Xinjiang Uygur AutonomousRegion (No. 200821123).

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