synthesis and characterization of conductive polypyrrole multi-walled carbon

Transmission electron microscopy Multi-walled carbon nanotubes a b s t r a c t High conductivity and solubility of polypyrrole PPy/multi-walled carbon nanotubes MWCNT compos-ites has bee

Synthesis and characterization of conductive polypyrrole/multi-walled carbon nanotubes composites with improved solubility and conductivity

Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan

a r t i c l e i n f o

Article history:

Received 16 September 2008

Received in revised form 14 December 2008

Accepted 17 December 2008

Available online 25 December 2008

Keywords:

A Nano composites

A Polymers

B Electrical properties

D Transmission electron microscopy

Multi-walled carbon nanotubes

a b s t r a c t

High conductivity and solubility of polypyrrole (PPy)/multi-walled carbon nanotubes (MWCNT) compos-ites has been successfully synthesized by in situ chemical oxidation polymerization using various con-centrations of cationic polyelectrolyte poly(styrenesulfonate) (PSS) and ammonium peroxodisulfate (APS) Raman spectroscopy, FTIR, EPR, FESEM and HRTEM were used to characterize their structure and morphology These images of FESEM and HRTEM showed that the fabricated PPy/MWCNT compos-ites are one-dimensional core-shell structures with the average thickness of the PPy/MWCNT composcompos-ites without PSS is about 250 nm and considerably decreases to 100–150 nm by adding the PSS content The results of Raman spectrum, FTIR and UV–Vis indicate the synthesized PPy/MWCNT composites are in the doped state The conductivities of PPy/MWCNT composites synthesized with the weight ratio of PSS/pyr-role monomer at 0.5 are about two times of magnitude higher than that of PPy/MWCNT composites with-out PSS These results are perhaps due to the part of cationic electrolyte served as a dopant can be incorporated to the PPy structure to improve the conductivity of fabricated PPy/MWCNT composites

Ó 2008 Elsevier Ltd All rights reserved

1 Introduction

Carbon nanotubes (CNTs) have recently attracted considerable

interest in consequence of their potential applications in field

emitters, nanoelectronic devices, probe tips for scanning probe

microscopies and nanotube-based composites due to their

excel-lent structural, mechanical and electronic properties[1–4]

Exper-imentally introducing CNTs into a polymer matrix could

significantly improve the mechanical and electrical properties of

the neat polymer matrix[5,6] Many reports have also shown that

the formation of polymer/CNT composites can be considered as a

useful approach for the fabrication of polymer-based devices

[7,8] Among these polymer/CNT composites, a lot of studies have

focused on the combination of CNT and intrinsic conducting

poly-mers (ICPs) for forming hole-conducting layers in organic

light-emitting diodes and highly efficient photovoltaic cells

Among these ICPs, polypyrrole (PPy) has potential uses in

syn-thesizing polymer/CNT composites due to its environmental

stabil-ity and excellent electrical conductivstabil-ity[9,10] PPy can be prepared

by chemical or electrochemical oxidation of pyrrole in various

or-ganic solvent and in aqueous media [10–12] Although

electro-chemical polymerization leads to formation of a conductive PPy

thin film on the working electrode, it is not appropriate for the

mass production In contrast, chemical oxidative polymerization

is simple, fast, cheap, and easily scaled up In a typical chemical

oxidative polymerization of PPy, many oxidants, such as ferric per-chlorate, ferric chloride and ammonium peroxydisulfate, have been used[13] The properties of fabricated conducting polymers are strongly dependent on the preparation conditions and various additives introduced into reaction mixture[14,15] Nevertheless,

it is necessary to point out that chemically and electrochemically synthesized PPy generally contains very poor solubility It is almost insoluble in all common organic solvents and in water that re-stricts its processibility Many investigations have been made to enhance the solubility of PPy by designing colloidal forms using surfactant and the protonation with an organic acid[16–18] In a previous report[18], the chemically synthesized PPy doped with

a bulky anion of dodecylbenzenesulfonic acid (DBSA) was soluble

in m-cresol The conductivity of PPy was about 1 S/cm After disso-lution in a polar solvent, the conductivity reduced into 102S/cm when cast into a film Several reports also reveal that the physical properties of fabricated PPy strongly depend on the types of surfac-tants/organic acids[19,20] Nevertheless, conductive PPy with one-dimensional nanostructure are seldom mentioned among these reports

A most effective method of fabricating one-dimensional nano-structure is using the template-directed synthesis in which reac-tant materials are located within or in the immediate vicinity of the templates [21] Many appropriate nanoscale templates have been reported, including channels in porous inorganic material and existing nanowires served as hard templates and block copoly-mers or self-assembled organic surfactants served as soft tem-plates [22–24] CNTs served as hard templates have recently

0266-3538/$ - see front matter Ó 2008 Elsevier Ltd All rights reserved.

* Corresponding author Tel.: +886 4 2287 2482; fax: +886 4 2285 7017.

E-mail address: tmwu@dragon.nchu.edu.tw (T.-M Wu).

Contents lists available atScienceDirect

Composites Science and Technology

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o m p s c i t e c h

been used to generate one-dimensional nanomaterials with

nano-tubelike morphologies[25,26]

In previous studies [27,28], we have synthesized conducting

polyaniline (PANI)/carbon nanotubes (CNTs) composites without

surfactant In order to improve the solubility of the CNTs in

solu-tion, the as-prepared CNTs were chemically treated to contain

car-boxylic functional groups at the defect sites Structural analysis

using FESEM and HRTEM showed that PANI/CNT composites are

core (CNT)-shell (PANI) one-dimensional tubular structures On

the other hand, PPy/CNT film or nanoparticles are synthesized by

miniemulsion/inverse microemulsion polymerization with anionic

surfactant or by in situ electropolymerization[29,30] Some results

reveal that the physical properties of synthesized PPy/CNT

com-posites significantly depend on the types of surfactants/organic

acids or preparations[31,32] Nevertheless, it is necessary to point

out that the fabricated PPy/CNT composites is approximately

insol-uble in all common organic solvents and in water that limits its

processibility

In this study, we reported a simple procedure for the fabrication

of high-conductivity PPy/multi-walled carbon nanotubes

(MWCNTs) composites with well-dispersion in ethanol by in situ

chemical oxidative polymerization with the presence of various

amount of cationic electrolyte poly(styrenesulfonate) (PSS) and

milder oxidant ammonium peroxodisulfate (APS) The effect of

electrolyte PSS on the conductivity, morphology and structure of

chemically synthesized PPy/MWCNT composites was also

discussed

2 Experimental

2.1 Synthesis of PPy/MWCNT composites

2.1.1 Synthesis of PPy

The MWCNTs were prepared by ethylene chemical vapor

depo-sition using Al2O3 supported Fe2O3 catalysts The diameter of

MWCNT is about 40 nm and the purity of MWCNTs is higher than

90% Pyrrole monomer (98%, Aldrich Chemical Co.) was purified by

distillation under reduced pressure Other reagents, including

poly(styrenesulfonate) (PSS) and ammonium peroxodisulfate

(APS), were used without further purification

The polypyrrole (PPy)/MWCNT composites were synthesized

using in situ chemical oxidative polymerization In a typical

syn-thesis experiment, various weight ratios of PSS was prepared in

distilled water in a reaction vessel containing a magnetic stirring

bar and the 1 wt% MWCNT was then mixed with the surfactant

solution and ultrasonicated (240 W) over 3 h to form PSS/

MWCNT template in solution The freshly distilled 0.5 g of

pyr-role monomer was slowly added dropwise into the stirred

solu-tion and continuously stirred for 30 min The 2.04 g of APS was

first dissolved in 10 ml distilled water and then slowly added

into the solution Therefore, the polymerization was carried out

for 3 h below 5 °C with constant mechanical stirring The

synthe-sized PPy/MWCNT composites was filtered and rinsed several

times with distilled water and methanol The powder thus

ob-tained was vacuum dried at 40 °C for 24 h The electron

para-magnetic resonance (EPR) spectra of the conducting polymer

PPy were performed on a Bruker EMX-10 spectrometer operating

at X-band (m= 9.6 GHz), with 100 KHz field modulations The EPR

g-value of the unknown sample can be determined from g = gs–

(DH/H0)gs, whereDH is the separation of the centers of the two

spectra, H0is the strength of the applied external field, and gsis

the g-value of the reference sample The spin-spin relaxation

process is the energy difference (DE) transferred to neighboring

electrons, and the relaxation time (T2) can be determined from

the peak-to-peak linewidth according to

1

T2

¼gbDH1=2



ffiffiffi 3

p

where b is the Bohr magneton (9.274  1021erg G1),DH1/2the linewidth (in G) at half-height of the absorption peak, ⁄ a constant (1.054  1027ergs) The A/B value is the ratio of the height of posi-tive to the negaposi-tive peak and the EPR spin number (Ns) was calcu-lated as the productDH2

ppph, where h is peak-to-peak height[33] 2.2 Structural and morphological analysis

The molecular structures of the resulting PPy/MWCNT com-posites were measured by FTIR, Raman and UV–Vis spectrosco-pies FTIR spectra were recorded on a Perkin–Elmer Spectrum One spectrometer with the resolution of 4 cm1 The samples were pressed into tablets with potassium bromide (KBr) Raman spectra were analyzed with a TRIAX 550 Jobin-Yvon monochro-mator equipped with a liquid nitrogen cooled CCD detector, using a He–Ne laser operating at 633 nm as the excitation source The Raman signals were collected through a long-work-ing distance 50  objective UV–Vis spectra were obtained with

a Hitachi U-3010 double beam spectrophotometer using a quartz cell and deionized water was used as a blank The peak position of the Raman, FTIR and UV–Vis spectra was determined using the peakfit software package The presented spectrum is

an average of three spectra measured at different regions over the entire sample range Thermal stabilities of the resulting PPy/MWCNT composites were performed from 50 to 800 °C at

a heating rate of 10 °C/min using a Perkin–Elmer thermogravi-metric analysis (TGA) and all experiments were operated under

a nitrogen atmosphere at a purge rate of 100 ml/min All spec-imens weighed about 6 mg Linear h/2h X-ray intensity scans of these specimens were recorded using a Mac MXT III diffractom-eter with Ni-filtered Cu Karadiation in the reflection mode The morphology of all samples was characterized by field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) FESEM measure-ments were conducted at 3 kV using a JEOL JSM-6700 F field-emission instrument HRTEM experiments were performed on

a JEOL JSM-2010 instrument with an accelerating voltage of

200 kV The samples for HRTEM images were prepared by cast-ing a drop of the sample suspended in ethanol on a copper grid covered with carbon

2.3 Electrical properties The samples of MWCNT, PPy and PPy/MWCNT composites were pressed into pellet form under 20 MPa Furthermore, the conduc-tivity at room temperature was measured by a programmable DC voltage/current detector with four probe method The data shown here are the mean values of measurements from at least three samples

3 Results and discussion 3.1 Morphological analysis of PPy/MWCNT composites

Fig 1shows the photos of vials for the synthesized PPy/MWCNT composites with various PSS content in ethanol solution For com-parison, the vial of PPy/MWCNT composites without PSS is also shown in this figure It can be seen that the resulting solution of fabricated PPy/MWCNT composites with the presence of PSS re-mained well-dispersed in ethanol for at least 24 h There is no sed-imentation or aggregation of PPy/MWCNT composites observed in these samples By contrast, we were unable to prepare stable PPy/ MWCNT composites suspensions without PSS Our results

demon-strate the fabricated PPy/MWCNT composites with the presence of

PSS show better solubility compared to that without PSS.Fig 2

shows the FESEM images of PPy/MWCNT composites synthesized

by in situ chemical oxidative polymerization using various

concen-trations of cationic polyelectrolyte PSS and MWCNT served as

tem-plate for the formation of one-dimensional PPy/MWCNT

nanostructure The average thickness of fabricated

one-dimen-sional PPy/MWCNT composites without PSS is about 250 nm and

significantly decreases to hundreds of nanometer with increasing

PSS contents It is necessary to point out that the synthesis of

PPy/MWCNT composites with the presence of PSS is a

size-control-lable process, which is strongly dependent on the content of PSS

Closer inspection of HRTEM images of PPy/MWCNT composites

shown inFig 3reveals that the resulting PPy/MWCNT composites

have one-dimensional core-shell structures Moreover, the average

thickness of the PPy/MWCNT composites without PSS is about

250 nm and considerably decreases to 100–150 nm by adding

the PSS content The above data exhibits the formation of

amor-phous PPy layer on the surface of MWCNT can be influenced by

various contents of PSS

3.2 Physical properties of PPy/MWCNT composites The molecular structure of the resulting PPy/MWCNT compos-ites synthesized by cationic polyelectrolyte PSS was characterized using Raman and IR spectra.Fig 4exhibits the Raman spectra of PPy/MWCNT composites with various concentrations of PSS All data demonstrate that the synthesized PPy/MWCNT composites with the presence of PSS have approximately identical peak posi-tions associated with the structure of the PPy The peaks at 935 and 1080 cm1have been attributed to the quinonoid bipolaronic structure and those at 970 and 1055 cm1 with the quinonoid polaronic structure, exhibiting the presence of the doped PPy structures[34] The peak at 1240 cm1is considered to the anti-symmetrical C–H in-plane bending and the C@C stretching peak

at 1600 cm1is related to be an overlap of the two oxidized struc-ture.Fig 5shows the FTIR spectrum of PPy/MWCNT composites with various concentrations of PSS Normally, this spectrum shows

a rich-band fingerprint region, revealing seven strong intensity bands[35] All results demonstrate almost the same peak positions

of the main IR bands which are associated with the structure of the

Fig 1 Vials (6 mL) containing aqueous dispersion of 1 wt% PPy/MWCNT composites synthesized with weight ratio of PSS/pyrrole monomer of (b) 0.1, (c) 0.3 and (d) 0.5 For comparison, the vial of (a) primary PPy/MWCNT composites is also shown in this figure.

Fig 2 FESEM images of 1 wt% PPy/MWCNT composites synthesized with weight ratio of PSS/pyrrole monomer of (c) 0.1, (d) 0.3 and (e) 0.5 For comparison, the FESEM image

of (a) pure MWCNT and (b) primary PPy/MWCNT composites is also shown in this figure.

Fig 3 HRTEM image of 1 wt% PPy/MWCNT composites synthesized with weight ratio of PSS/pyrrole monomer of (c) 0.1, (d) 0.3 and (e) 0.5 For comparison, the HRTEM

PPy The peaks at 1545 and 1455 cm1could be attributed to C–N

and C–C asymmetric and symmetric ring-stretching, respectively

Additionally, the strong peaks near 1170 and 900 cm1 present

the doping state of polypyrrole, the peak at 1040 cm1is attributed

to C–H deformation and N–H stretching vibrations, and the broad

band at 1300 cm1demonstrates the C–H and C–N in-plane

defor-mation vibration, respectively

UV–Vis spectroscopy was performed to ascertain the interfacial

interaction of PPy/MWCNT composites and PSS.Fig 6exhibits the

spectra of PPy/MWCNT composites with various contents of PSS

There is no absorption peak in the 300–900 nm range for MWCNT

sample The typical absorption peak of the PPy/MWCNT

compos-ites without PSS at approximately 494 nm is attributed to the

tran-sition from the valence band to the anti-bonding polaron state

[36], showing that the synthesizing PPy/MWCNT composites are

in the doped state The typical absorption peaks of PPy/MWCNT

composites slightly shift to 492 nm as the loading of the weight

ra-tio of cara-tionic electrolyte PSS/pyrrole monomer at 0.1 While the

structure of PPy was continuously doped with high content of

PSS, the absorption peak associated with the polaron-ptransition

was significantly shifted to a smaller wavelength with increasing the PSS content These results exhibit the possible interaction be-tween the quinoid rings of PPy and SO2

4 ion of PSS

The thermal stability of the PPy/MWCNT composites prepared

in the presence of PSS was studied by TGA analysis.Fig 7presents the curves of weight loss versus temperature of PPy/MWCNT com-posites with various concentrations of PSS For comparison, the TGA analysis of PSS and MWCNT are also shown in this figure The first significant weight loss PPy/MWCNT composites which corresponds to polymer degradation starts at about 200 °C Although these curves of PPy/MWCNT composites symthesized with various concentrations of PSS have the same shape, PPy/ MWCNT composites with high PSS content seems to be slightly more stable if we compare its TGA curve with the curve of PPy/ MWCNT composites with low PSS content in the whole tempera-ture range This data demonstrates that the addition of high ther-mal stability of PSS is more stable for the all temperature range

of measurement Clearly, the 10% loss temperature (T10%) of the PPy/MWCNT composites fabricated with various contents of PSS

is higher than that of PPy fabricated without any addition of PSS

Fig 4 Raman spectroscopy of 1 wt% PPy/MWCNT composites synthesized with

weight ratio of PSS/pyrrole monomer of (b) 0.1, (c) 0.3 and (d) 0.5 For comparison,

the Raman spectroscopy of (a) primary PPy/MWCNT composites and (e) MWCNT is

also shown in this figure.

Fig 5 FTIR spectrum of 1 wt% PPy/MWCNT composites synthesized with weight

ratio of PSS/pyrrole monomer of (b) 0.1, (c) 0.3 and (d) 0.5 For comparison, the FTIR

spectra of (a) primary PPy/MWCNT composites and (e) cationic polyelectrolyte PSS

are also shown in this figure.

Fig 6 UV–Vis spectrum of 1 wt% PPy/MWCNT composites synthesized with weight ratio of PSS/pyrrole monomer of (c) 0.1, (d) 0.3 and (e) 0.5 For comparison, the UV– Vis spectrum of (a) MWCNT and (b) primary PPy/MWCNT composites is also shown

in this figure.

Fig 7 TGA data of 1 wt% PPy/MWCNT composites synthesized with weight ratio of PSS/pyrrole monomer of (b) 0.1, (c) 0.3 and (d) 0.5 For comparison, the Raman spectroscopy of (a) primary PPy/MWCNT composites, (e) cationic polyelectrolyte PSS and (f) MWCNT is also shown in this figure (Inserted the temperature scale in the range of 200–350 °C of each curve).

The T10%of PPy/MWCNT composites without PSS is 249.5 °C and

extensively increases to 261.3 °C as the loading of the weight ratio

of cationic electrolyte PSS/pyrrole monomer at 0.1 As the addition

of the weight ratio of PSS/pyrrole monomer at 0.3 and 0.5, the T10%

continuously increases to 279.1 and 298.1 °C, respectively This

re-sult demonstrates that introduction of PSS into PPy/MWCNT

com-posites can improve the thermal stability of PPy due to the

presence of high thermal stability PSS and MWCNT

The electrical conductivities of PPy/MWCNT composites were

measured using the standard four-probe method The

room-tem-perature conductivities of MWCNT and PPy/MWCNT composites

without PSS were 28.4 and 40 S/cm In the meantime, the

conduc-tivity of PPy/MWCNT composites with various contents of PSS at

room temperature clearly depends on the contents of PSS By

add-ing the weight ratio of cationic electrolyte PSS/pyrrole monomer at

0.1, the conductivity at room temperature slightly increases from

40 S/cm to 49 S/cm With the continuous increase in the loading

of PSS, the conductivities at room temperature continuously

in-crease from 49 Scm for the PPy/MWCNT composites with the

weight ratio of cationic electrolyte PSS/pyrrole monomer at 0.1

to 73 and 91 S/cm for these synthesizd PPy/MWCNT composites

with the weight ratio at 0.3 and 0.5, respectively It is necessary

to point out that all conductivities of PPy/MWCNT composites

pre-pared with the presence of PSS are in the range between 50 S/cm

and 90 S/cm, which is at least one order in magnitude higher than

those synthesized PPy/MWCNT composites reported in the

litera-tures[37,38] The conductivities of PPy/MWCNT composites

fabri-cated with the weight ratio of cationic electrolyte PSS/pyrrole

monomer at 0.5 at room temperature are about two times of

mag-nitude higher than that of PPy/MWCNT composites prepared

with-out PSS, perhaps because the part of cationic electrolyte can be

incorporated to the PPy structure served as a dopant to enhance

the conductivity of synthesized PPy/MWCNT composites

Increas-ing PSS content in these conductIncreas-ing polymers also improves their

conductivities and these results may be due to the decrease in the thickness of PPy with the presence of high content of PSS

In order to understand the role of PSS during the formation of PPy/MWCNT composites, EPR analysis has been performed for all specimens.Fig 8shows the EPR spectra of PPy/MWCNT compos-ites with various contents of PSS The EPR parameters (DHpp, g fac-tor, A/B ratio, T2, and Ns) obtained fromFig 8 are illustrated in

Table 1 The EPR spectra exhibit a single narrow and symmetric EPR signal revealing that the free electron existed The g value of the EPR signal is a function of the molecular motion, the paramag-netic properties and the symmetry of ion[39] The g factors of PPy/ MWCNT composites were almost constant (2.0025), suggesting that the spins are delocalized over a few carbon atoms of the rings, since the g value of an electron near a carbon–hydrogen bond is 2.0031 The A/B asymmetric ratio obtained from EPR spectra of PPy/MWCNT composites remains constant (1.07) for all PPy/ MWCNT composites samples, revealing that the spins are free elec-tron type[40] The width of the EPR signal was directly attributed

to the interaction of the spins with their environment and to their motion It is known that the line width is related to the extent of delocalization of unpaired electrons along the polymer chain; i.e the higher the extent of delocalization, the smaller the line width

In this study, the values ofDHppof the PPy with various contents of PSS were larger than that of the PPy, while the T2was reverse[41] These results reveal that PPy/MWCNT composites with various contents of PSS has lower polaron mobility, which coincides with more hydrogen bond between PPy and PSS, while higher polaron mobility of PPy/MWCNT composites without PSS indicates less hydrogen bond between PPy and PSS But the NS of the PPy/ MWCNT composite with various contents of PSS were larger than that of the PPy/MWCNT composite without PSS, indicating an in-crease in spin concentration for PPy with various contents of PSS These results can be assigned to more polaron formation for PPy with various contents of PSS[42] Therefore, we can conclude the conductivity of PPy is dominant by the number of polaron forma-tion during the in situ polymerizaforma-tion by adding H+to the b-posi-tion of pyrrole ring during the in situ doping polymerizab-posi-tion of pyrrole[37]

4 Conclusions High-conductivity polypyrrole (PPy)/multi-walled carbon nano-tubes (MWCNTs) composites with well-dispersion in ethanol has been successfully synthesized by in situ chemical oxidation poly-merization using various concentrations of cationic polyelectrolyte poly(styrenesulfonate) (PSS) and ammonium peroxodisulfate (APS) These images of FESEM and HRTEM showed that the fabri-cated PPy/MWCNT composites are one-dimensional core-shell structures with the average thickness of the PPy/MWCNT compos-ites without PSS is about 250 nm and considerably decreases to 100150 nanometers by adding the PSS content The results of UV–Vis indicate the synthesized PPy/MWCNT composites are in the doped state All conductivities of alcohol-soluble PPy/MWCNT composites are in the range between 50 S/cm and 90 S/cm, which

is at least one order in magnitude higher than those reported in the literatures The conductivities of PPy/MWCNT composites

syn-Fig 8 EPR spectrum of 1 wt% PPy/MWCNT composites synthesized with weight

ratio of PSS/pyrrole monomer of (b) 0.1, (c) 0.3 and (d) 0.5 For comparison, the EPR

spectrum of (a) primary PPy/MWCNT composites is also shown in this figure.

Table 1

EPR data of the conductive 1 wt% PPy/MWCNT composites synthesized with the presence of various amounts of PSS.

Sample Weight ratio of PSS/

pyrrole monomer

DH pp (G) g Factor A/B ratio T 2 (10 9

40

49

thesized with the weight ratio of PSS/pyrrole monomer at 0.5 are

about two times of magnitude higher than that of PPy/MWCNT

composites without PSS These results are perhaps due to the part

of cationic electrolyte served as a dopant can be incorporated to

the PPy structure to improve the conductivity of fabricated PPy/

MWCNT composites

Acknowledgements

The financial support provided by National Science Council

through the Project NSC96-2212-E-005-049 is greatly appreciated

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