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Synthetic Metals 157 (2007) 374–379

Polypyrrole coated carbon nanotubes: Synthesis, characterization,and enhanced electrical properties

Nanda Gopal Sahoo a, Yong Chae Jung b, Hyang Hwa So b, Jae Whan Cho b,∗a Artificial Muscle Research Center, Konkuk University, Seoul 143-701, Republic of Korea

b Department of Textile Engineering, Konkuk University, Seoul 143-701, Republic of Korea

Received 26 December 2006; received in revised form 6 April 2007; accepted 12 April 2007Available online 25 May 2007

bstract

We describe a simple approach to the synthesis of MWNT/polypyrrole nanotubes by the in situ chemical polymerization of pyrrole on the carbonanotubes using ferric chloride as an oxidant. The effects of pyrrole concentration on the coating and properties of the resulting complex nanotubesere studied by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron

icroscopy (TEM), X-ray diffraction, and thermal gravimetric analysis. The coated PPy layers could be controlled easily by adjusting a feed ratio

f pyrrole to MWNTs. FT-IR results suggested an existence of interaction between the –COOH groups of chemically modified MWNTs and NHroups of the PPy. SEM and TEM studies indicated that each individual MWNT could be coated with PPy. The resultant nanotubes enhancedlectrical conductivity compared to PPy and MWNT which was strongly influenced by the feed ratio of pyrrole to MWNTs.

2007 Elsevier B.V. All rights reserved.

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eywords: Carbon nanotube; FT-IR; Polypyrrole; Raman spectroscopy; TEM;

. Introduction

Since the discovery of carbon nanotubes in 1991 by Iijima1], carbon nanotubes have received much attention for theirany potential applications such as ultra-strong wires, nanoelec-

ronic devices, field electron emitters, nanocomposite materials,nd more [2,3]. Carbon nanotubes exhibit excellent mechani-al, electrical, thermal and magnetic properties [3]. The exactagnitude of these properties depends on the diameter and chi-

ality of the nanotubes and whether they are single-walled orulti-walled form. Advances in the synthesis of multi-walled

MWNTs) and single-walled carbon nanotubes (SWNTs) con-inue to rapidly improve both their quality and quantity, andeduce cost. They enable new applications of materials contain-ng carbon nanotubes.

Polypyrrole (PPy) is one of the more important conducting

olymers due to its relatively easy processability, electrical con-uctivity, and environmental stability [4,5]. Polypyrrole offersotential applications in the domain of composite materials [6],

∗ Corresponding author. Tel.: +82 2 450 3513; fax: +82 2 457 8895.E-mail address: jwcho@konkuk.ac.kr (J.W. Cho).

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379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2007.04.006

issue engineering [7], actuator [8,9], supercapacitors [10], elec-ronic and electro optic devices [11]. Recently, a variety of

ethods have been reported for producing composites of PPynd carbon nanotubes such as chemical, electrochemical oxi-ation method [10,12,13]. Xiao and Zhou [10] deposited PPynd poly (3-methylthiophene) uniformly on MWNTs in organicystem by chemical methods. Han et al. [13] synthesized theanocomposite films of PPy–MWNT by electrochemically andtudied the morphology, conductivity and thermal stability ofhis composite. Karim et al. [14] demonstrated complex nan-tubes of SWNTs coated with polyaniline. They showed that theonductivity and thermal stability of complex nanotubes wereigher than polyaniline but lower than CNT. To date, only aew studies concerning PPy-coated carbon nanotubes have beeneported. Long et al. [15] synthesized the CNT/PPy nanocableshrough an in situ chemical oxidative polymerization method.hey showed that the conductivity of nanocables increased with

ncreasing nanotube weight percentage. An et al. [16] fabricatedn SWNT/PPy nanocomposite by in situ chemical polymeriza-

ion of pyrrole monomer with the SWNTs and showed the gasensitivity of nanocomposite was about 10 times higher than thatf PPy. Fan et al. [17] synthesized PPy-coated carbon nanotubesy in situ polymerization methods where the mass ratio of the

N.G. Sahoo et al. / Synthetic Metals 157 (2007) 374–379 375

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ig. 1. (a) FT-IR spectra of PPy, MWNT-COOH and PPy-coated MWNTs; (WNT-COOH.

yrrole and MWNT was 3.35:1. The electrical and thermal prop-rties of the resultant nanotubes were higher compared to purePy but lower than CNTs.

In this paper, we report the controlled coating of conductingolypyrrole on MWNTs by in situ polymerization with variousyrrole feed ratio. The molecular structure of the resulting com-lex nanotubes (MWNTs–PPy) has been characterized and theirhysical properties, including thermal and electrical propertiesave been discussed. Especially, the effect of the monomer con-entration on the coating and properties of the resulting complexanotubes have been investigated.

. Experimental

The MWNTs used in this study were purchased fromljin Nanotech, Korea. Their diameter and length were about0–20 nm and 20 �m, respectively. MWNTs were treated in aixture of concentrated H2SO4/HNO3 (3:1) at 90 ◦C for 10 min

s described in our previous paper [18]. These modified MWNTs

ere used in all experiments.Polypyrrole was synthesized from its monomer, pyrrole

purity 98%, Aldrich) by chemical oxidative procedure [5], usingeCl3·6H2O as an oxidant. Polypyrrole-coated MWNTs were

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ossible interaction of hydrogen bonding existed between for PPy chain and

ynthesized by in situ polymerization of pyrrole on MWNTs.he following procedure was used to synthesize PPy coated-WNTs. The necessary weight fractions of MWNTs were

rst dispersed in the mixed solvent of methanol and acetoni-rile solution by sonication at room temperature for 2 h usingn ultrasonic homogenizer. Thereafter, a calculated amountf pyrrole was added into this solution and stirred for 0.5 h.eCl3·6H2O solution was added dropwise to the above solutionith constant sonication at ambient temperature. The mixtureas sonicated again for 2 h. The Fe3+/pyrrole molar ratio was.3. The resulted MWNTs–PPy powder was filtered and thenashed by methanol until the solution became colorful. Finally

he powder was dried under vacuum at 60 ◦C for 24 h. Theeeding mass ratios of the pyrrole and MWNTs were 1:1, 2:1,nd 5:1.

Fourier transform infrared (FT-IR) spectroscopic measure-ents were performed using Jasco FT-IR 300E with an

ttenuated total reflectance method. A Raman spectroscopyBRUKER, RFS, 100/s) was used to investigate the structural

hanges of carbon nanotubes by acid treatment. A 632.8 nme–Ne laser was used as the light source. Standard four probeethod was used to measure the electrical conductivity of the

amples at room temperature. X-ray diffraction was studied

376 N.G. Sahoo et al. / Synthetic Metals 157 (2007) 374–379

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Fig. 3. X-ray diffractograms of PPy, MWNT, and PPy-coated MWNTs:(M

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Fig. 2. Raman spectra of PPy, MWNT-COOH and PPy-coated MWNTs.

sing 2100 series X-ray diffractometer with Cu K� targets atscan rate of 4◦/min.

The morphology of some selected samples were observedsing a scanning electron microscope (JSM 6700-F, Jeol Co.).ransmission electron microscopy (TEM) analysis was per-ormed on a JEM-2010F (JEOL Co.) electron microscope at20 kV. A thermogravimetric analysis (TGA) was carried out inTA Q50 system TGA. The samples were scanned from 0 to

00 ◦C at a heating rate of 10 ◦C/min in the presence of nitrogen.

. Results and discussion

The FT-IR spectra of chemically modified MWNTs, PPynd PPy-coated MWNTs are shown in Fig. 1(a). The char-cteristic bands due to newly formed polar functional groups

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Fig. 4. SEM photographs of PPy, MWNT, and PPy-coated MW

a) MWNT-COOH; (b) MWNT:Py = 1:1; (c) MWNT:Py = 1:2; (d)WNT:Py = 1:5; (e) PPy.

ere observed in the FT-IR spectra of the MWNTs aftercid treatment in H2SO4/HNO3. The IR bands at 3440, 1637,nd 1182 cm−1 were consistent with the OH stretching, C Otretching and C C O stretching in modified MWNTs, respec-ively [18]. In case of PPy, the characteristic band for the ringundamental vibration appeared at 1546 cm−1, and the C Hn-plane vibration and C N stretching vibration appeared at310 and 1045 cm−1, and at 1184 cm−1, respectively [19]. TheWNTs–PPy nanotubes showed the characteristics peaks for

oth MWNTs and PPy. But, it can be clearly seen that the

eak for C O groups was shifted from 1637 cm−1 in modi-ed MWNTs to 1605 cm−1 in case of MWNTs–PPy nanotubes.he presence of the carboxylic groups on the nanotube surface

NTs: (a) PPy; (b) MWNT-COOH; (c) MWNT:Py = 1:2.

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N.G. Sahoo et al. / Synthe

s likely to give the interfacial interaction between the polymernd the nanotubes in MWNTs–PPy nanotubes. It is due to theydrogen bonds which were formed between –COOH groupsf chemically modified MWNTs and NH groups of the PPys shown in Fig. 1(b). Also the Raman spectra measurementsFig. 2) showed the two bands at around 1579 and 1325 cm−1

hich were assigned to tangential mode (G-band) and disorderode (D-band), respectively [20]. For PPy, the characteristicaman bands appeared at about 1584 and 1345 cm−1 due to the

C backbone stretching of PPy [13] and the ring-stretchingode of PPy [21], respectively. In the case of MWNT–PPy

anotubes, the combined bands related to MWNTs and PPyere shown together. In addition, the intensity of the bandsecreased with the increase of the feeding mass ratio of pyr-ole to MWNTs, suggesting a increase of PPy content in the

WNTs–PPy nanotubes.X-ray diffraction patterns of MWNTs, PPy, and MWNT–PPy

anotubes are presented in Fig. 3. The X-ray pattern of theWNT displayed the presence of two peaks at 2θ = 25.80◦

3.47 A) and 42.75◦ (2.12 A) corresponding to the (0 0 2) and

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ig. 5. TEM photographs of PPy, MWNT, and PPy-coated MWNTs: (a) PPy; (b) MW

tals 157 (2007) 374–379 377

1 0 0) reflections of the carbon atoms, respectively, in goodgreement with the literatures [22,23]. The pure PPy showedroad diffraction peaks at 2θ = 25.4◦ due to the pyrrole inter-olecular spacing [17]. For the MWNTs–PPy nanotubes, the-ray diffractograms showed both the characteristic PPy broadeaks and the strong MWNTs peaks. From Fig. 3(b)–(d), it islear that the peaks intensity decreased with the increase of theeeding mass ratio of pyrrole to MWNTs.

The SEM images of resulting PPy, MWNTs andWNTs–PPy are shown in Fig. 4. Fig. 4(a) shows a typicalorphology of PPy. The particle size of PPy was 200–400 nmith spherical morphology. In case of MWNTs, many nan-tubes are loosely entangled together without any particle-likempurities as shown Fig. 4(b). It is clear from Fig. 4(c) thatPy was found to coat each individual MWNTs. Comparedith the MWNTs, the diameter of MWNT–PPy (1:2) nanotubes

ncreased from 10–20 nm to 35–60 nm, and their external surface

as less smooth.The tubular morphology of the MWNTs–PPy was imaged by

transmission electron microscopy as shown in Fig. 5. It was

NT-COOH; (c) MWNT:Py = 1:1; (d) MWNT:Py = 1:2; (e) MWNT:Py = 1:5.

378 N.G. Sahoo et al. / Synthetic Me

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ig. 6. TGA thermograms of PPy, MWNT, and PPy-coated MWNTs: (a)WNT-COOH; (b) MWNT:Py = 1:1; (c) MWNT:Py = 1:5; (d) PPy.

lear that the surface of carbon nanotubes was coated with PPy.or an oxidative polymerization, the PPy networks are formedn the surface of the samples. During the polymerization, theyrrole is oxidized by FeCl3, as soon as the amounts of PPyre generated on the surface of the carbon nanotubes. Fromig. 5(c)–(e), the thickness of the PPy coating on each nan-tube was observed to increase, which was dependent on theonomer concentration. The diameter of the MWNT–PPy nan-

tubes could be controlled by changing the MWNT/pyrrole massatio.

In order to investigate the thermal stability of the MWNTs,Py and MWNT–PPy nanotubes, thermogravimetric analyzereasurements were carried out, and the results are shown

n Fig. 6. MWNTs were comparatively more stable and didot show dramatic decomposition in the temperature range of0–750 ◦C, and about a 14% mass loss was observed due to theresent functional groups. Pure PPy did not exhibit significantass loss in the temperature range of 50–175 ◦C, and showed

nly 8% mass loss. However, a rapid mass loss occurred in theange of 170–750 ◦C, and only 30% mass remained for purePy at 750 ◦C. In the case of MWNT–PPy nanotubes showedore delay decomposition compared to pure PPy and more mass

emained at 750 ◦C. The thermal stability of MWNT–PPy nan-tubes increased with the decrease of the feeding mass ratio ofyrrole to MWNTs. This indicates that the carbon nanotubesan improve the thermal stability of MWNT–PPy nanotubes.t higher mass ratio of polypyrrole, the trend of the decom-osition curve of MWNT–PPy nanotubes was similar to thatf PPy.

The room temperature electrical conductivity of the modifiedWNTs pellet and PPy synthesized by the chemical method

sing FeCl3 was measured to be 1.94 and 0.40 S/cm, respec-ively. The room temperature conductivity of the MWNT–PPyanotubes (2.40 S/cm for MWNT:Py = 1:1, 2.10 S/cm for

WNT:Py = 1:2) was increased by one order of magnitude com-

ared to that of PPy, and significantly higher than that of neatWNT. The structure of the MWNT–PPy nanotubes is likely

o contribute to its improved electrical conductivity in the fol-

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tals 157 (2007) 374–379

owing ways. First, due to the large surface area of MWNTs,hey may serve as conducting bridge connecting PPy conductingomains and increasing the effective percolation [24]. Second,n accordance with the FT-IR observations, this suggested thathe interaction between PPy and MWNTs could be responsibleor higher conducting than the starting components. However,he electrical conductivity of MWNT–PPy nanotubes (1.50 S/cmor MWNT:Py = 1:5) was slightly decreased with the increasef the feeding mass ratio of pyrrole to MWNTs.

. Conclusions

The PPy-coated MWNTs were prepared from in situ chem-cal polymerization of pyrrole on the carbon nanotubes. Theffect of the monomer concentration on the coating and prop-rties of the resulting complex nanotubes was investigated. Byhanging the pyrrole/MWNT ratio, we could easily control theayer thickness of PPy in MWNT–PPy complex nanotubes.T-IR results suggested an existence of interaction between

he –COOH groups of MWNTs with NH groups of the PPy.he electrical conductivity and thermal stability of PPy-coatedWNTs were dependent on the feed ratio of PPy to MWNTs.

hese results implied that the feed ratio of pyrrole to MWNTslayed more active roles in determining the coated layer as wells the resulting properties. The resulted nanotubes may be usedor supercapacitors and actuators.

cknowledgements

This work was supported by the Korea Research Foundationrant (KRF-2004-005-B00046) and the SRC/ERC program ofOST/KOSEF (R11-2005-065).

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