C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5

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Electrically conductive carbon nanopipe-graphite nanosheet/polyaniline composites

G. Venkata Ramana a,b, Balaji Padya a, Vadali V.S.S. Srikanth b, P.K. Jain a,*,G. Padmanabham a, G. Sundararajan a

a Center for Carbon Materials, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI),

Hyderabad 500005, Indiab School of Engineering Sciences and Technology (SEST), University of Hyderabad, Hyderabad 500046, India

A R T I C L E I N F O

Article history:

Received 25 May 2011

Accepted 21 July 2011

Available online 28 July 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.07.041

* Corresponding author: Fax: +91 40 24442699E-mail addresses: pkjain@arci.res.in, pkja

A B S T R A C T

Carbon nanopipe (CNP)–graphite nanosheet (GNS)/polyaniline (PANI) composites are syn-

thesized by in situ chemical oxidative polymerization. The structural analysis (electron

microscopy, Raman and X-ray diffraction) reveal that PANI is uniformly coated on both

CNP and GNS structures resulting in the formation of a network of uniform composite

structures. Thermogravimetric analysis shows that CNP–GNS/PANI composites are ther-

mally stable up to 300 �C; the polymeric backbone degrades above 300 �C. CNP–GNS/PANI

composites doped with m-cresol, a mixture of camphor sulfonic acid (CSA) and chloroform,

and a mixture of CSA and m-cresol are electrically conductive. The electrical conductivity

strongly depends on the dopants and about six orders of variation in conductivity can be

achieved through the choice of the dopant.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent past, carbon nanotube (CNT)/conducting polymer

(CP) composites [1–6] have gained importance owing to the

combination of unique properties that come with CNT [7–11]

and CP [12–15] materials. Amongst CNT/CP composites, multi

wall carbon nanotube (MWCNT)/polyaniline (PANI) composite

has been extensively studied [16–18]. It has been shown that

MWCNT/PANI composite is electrically more conductive than

the individual components; the increased electrical conduc-

tivity is due to an effective site-selective interaction between

MWCNT and quinoid ring of PANI which leads to an easy

charge transfer between them [16]. However, it is very difficult

to disperse nanotubes in most of the organic solvents and

polymeric matrices due to the rigidity, chemical inertness,

and strong p–p interactions associated with nanotubes [19].

Even though in situ polymerization method offers a better

approach to synthesize homogeneous nanotube/polymer

er Ltd. All rights reserved

.in1966@gmail.com (P.K. J

composites, dispersion of nanotubes is still a major problem.

In this work, synthesis of electrically conductive carbon nano-

pipe (CNP)–graphite nanosheet (GNS)/PANI composites by

in situ chemical oxidative polymerization will be reported.

The idea is to use the uniqueness of CNP and GNS structures

and obtain thermally stable CNP–GNS/PANI composites with

increased and controllable (by doping) electrical conductivity.

CNP structure is made up of a series of short pipes paral-

leling each other between two adjacent compartments. Each

pipe is composed of several graphite layers with one end

closed and the other open resulting in numerous defects on

the outer surface of each CNP all along its length. In order

to differentiate this structure from the regular CNT structure,

an apt nomenclature CNP is being used instead of using the

previously coined name carbon nanobell [20–22]. GNS struc-

tures are an interesting class of extremely thin flat materials

with high surface area, and extraordinary thermal and elec-

trical properties [23–25]. It is thus apparent that CNP and

.ain).

5240 C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5

GNS structures will facilitate increased interaction of poly-

mers (here PANI) with them in turn resulting in a composite

material with increased properties. Additionally, owing to its

structure, CNP is expected to disperse properly in the mono-

mer solution and help in uniform polymerization on its sur-

face. This work is an effort to develop two new carbon

nanostructures/CP based composites namely CNP–GNS/PANI

composites.

2. Experimental

2.1. Synthesis of CNP, GNS, and PANI

All the chemicals used in this work were of analytical grade and

were used without further purification. CNP synthesis was car-

ried out by thermal chemical vapor deposition technique. Iron

(Fe) was used as the catalyst whilst acetonitrile and argon gases

were used as the precursor gases for the CNP synthesis.

Reaction gas pressure and temperature were maintained at

1.5 kg/cm2 and 850 �C, respectively. The reaction was carried

out for a period of 30 min. In the case of GNS synthesis, graphite

flakes (natural graphite flakes, 99% pure) were first intercalated

with the nitration mixture and then subjected to a thermal

shock at 900 �C. As a final step, the thermally expanded graph-

ite flakes were subjected to ultra-sonication in acetone for 30 h

to obtain the GNS structures.

PANI was synthesized in a round-bottom flask (1L) equipped

with a mechanical stirrer and a dropping funnel. Two hundred

fifty milliliter of aqueous hydrochloric acid (HCl) was taken in

the flask and 20 ml of vacuum distilled aniline was added to

it. The solution was then cooled to 0 �C using an ice bath.

Ammonium persulfate (APS) solution placed in the dropping

funnel was then added drop by drop to the aniline solution in

the flask for 1 h. During APS addition, the temperature of the

reaction mixture was maintained in the range 0–5 �C. After

the addition was complete, the reaction mixture was left stir-

ring for 1 h. The solid PANI was collected from the reaction

mixture by filtration and subsequently washed with water,

then with methanol, and finally with diethyl ether. The solid

was then dried at 90 �C under vacuum for 48 h.

2.2. Synthesis of CNP–GNS/PANI composite

The composites were synthesized on similar lines as PANI

synthesis. Here, the amounts of HCl and aniline used were

the same as in PANI synthesis. However, 1 wt.% of CNP and

GNS structures (relative to the amount of aniline monomer)

is added to the reaction mixture. The polymerization was ini-

tiated by drop wise addition of APS solution to the reaction

mixture (now, aniline + HCl + CNP–GNS) and reaction was al-

lowed for 2 h. After the polymerization was complete, the

polymer was isolated by filtration. The wet cake so obtained

was subsequently dried at 90 �C under dynamic vacuum for

48 h.

2.3. Preparation of CNP–GNS/PANI composite thin films

Thin films of PANI and CNP–GNS/PANI composites were pre-

pared by spin coating technique. In order to study the effects

of dopants m-cresol and camphor sulfonic acid (CSA) on the

electrical conductivity of CNP–GNS/PANI composites, PANI

and CNP–GNS/PANI were dispersed in three different solvents

namely m-cresol, a mixture of CSA and chloroform, and a

mixture of CSA and m-cresol solvents prior to spin coating.

The solution mixtures were then spin coated onto indium-

tin oxide (ITO) coated glass substrates (1 · 1 cm2). SCU2007A,

Apex Instruments Co. spin coater was used and the coating

was carried out at a speed of 2000–2500 rpm. The spin coated

films were then heat treated at 60 �C under vacuum.

2.4. Characterization

Field emission scanning electron microscope (FESEM) (Model

Zeiss Ultra 55) operated at an accelerating voltage of 5 kV

was used to study the surface morphology of the synthesized

materials. Transmission electron microscope (TEM) (Model

FEI Technai G2 S-Twin) operated at 200 kV was also used to

study the morphology of the samples. Micro Raman spectro-

scopic study was carried out using LabRam HR800 Raman

spectrometer; 514.5 nm green line of Ar + ion laser was used

as the excitation source. X-ray diffraction (XRD) patterns were

recorded from 10 to 60� using Cu Ka as the X-ray source

(k = 1.54 A). Bruker’s AXS Model D8 Advance System was used

to carry out the XRD experiments. Thermal stability of the

samples in Argon gas flow (100 cm3/min) at a heating rate of

10 �C/min was examined by thermogravimetric analysis

(TGA) (Model NETZSCH STA 449 Jupiter).

In order to carry out room temperature direct current (DC)

electrical conductivity measurements, copper wire contacts

were made on the CNP/GNS–PANI composite thin films and

substrate using silver paste to establish a two probe electrical

conductivity measuring geometry. A fixed (1 V) voltage was ap-

plied across the contacts and the corresponding DC current

was recorded in each case. Resistance of the samples was then

calculated using Ohm’s law. The electrical conductivities of the

thin films were calculated using the relation rDC = L/RA where

rDC, L, R, and A are electrical conductivity (S/cm), length of

the sample (1 cm), resistance and cross-sectional area of the

sample normal to direction of current flow, respectively.

3. Results and discussion

3.1. Structural and composition analyses

FESEM and TEM plane view images of the starting materials

namely CNP, GNS, PANI are shown in Fig. 1. The electron

micrograph shown in Fig. 1a reveals that the diameters of

the CNP structures are in the range 20–30 nm whilst the

length is about several lm. Fig. 1b shows the CNP structure

as described in the introduction; the short pipes and the hol-

low space are indicated with the letters G and H, respectively.

Fig. 1c shows the electron micrograph of randomly aggre-

gated, thin, and flat GNS structures. Most of the GNS structures

are more than 1 lm2 in lateral dimensions. It can also be

observed that some of the regions are folded. These structures

as shown in Fig. 1d are transparent to the electron beam as

observed by TEM indicating the presence of only several sheets

(multilayered along the c axis) in a single GNS structure. When

Fig. 1 – (a), (c), and (e) are FESEM plane view images of CNP, GNS, and PANI structures, respectively; (b), (d), and (f) are the

corresponding TEM plane view images.

C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5 5241

carefully observed in high resolution mode the folded regions

of the sheets are about 2–5 nm in thickness. The electron

micrographs shown in Fig. 1e and f of PANI in emeraldine salt

(ES) form reveal the existence of highly agglomerated globular

morphology; the particulate sizes are in sub-micron range.

This is due to the large proportion of solution polymerized PANI

existing in the agglomerated form. According to the classical

theory of nucleation [26], the polymerization of aniline in the

absence of any additional structures like MWCNT gets initiated

with the formation of PANI nuclei which are homogeneously

distributed in the acidic solution. In the subsequent stage, het-

erogeneous growth of PANI takes place on the already formed

nuclei.

FESEM and TEM plane view images of CNP–GNS/PANI com-

posite structures are shown in Fig. 2. As anticipated, it is evi-

dent from the electron micrographs that PANI is uniformly

coated on both CNP and GNS structures in turn resulting in

the formation of uniform networks of composite structures.

Here, it is noteworthy to observe that the original shape of

the starting materials is maintained even after the in situ

polymerization. However, in the case of CNP structures, the

lengths are observed to be decreased while the diameters

have increased (due to PANI coating). The unchanged shape

of GNS structures in the composite indicates that the

polymerization of aniline monomers has taken place pre-

dominantly on the GNS surfaces; if the polymerization had

taken place predominantly in the space between the graphite

multilayers of individual GNS structures, the layers would

have delaminated and a definite shape change would have ta-

ken place. The mechanism leading to such observations will

be elucidated in a separate work. However, it is worthwhile

to briefly discuss the possible reasons based on the available

literature. It has been shown that anilines are easily adsorbed

onto the surface of carbon nanotubes due to electron donor/

acceptor interaction leading to well-dispersed nanotubes in

different solvents [27]. The presence of CNP–GNS structures

or to that matter any carbon nano structures in the acidic

solution promotes the attachment of anilines to the surface

of the nano structures, which initializes the polymerization

with the surfaces as the nucleation sites. Through p–p stack-

ing between CNP structures and aniline, the CNP structures

also serve as templates for the growth of PANI. This behavior

can be the plausible reason responsible for the observed mor-

phological change as seen for PANI alone. As observed by FES-

EM and TEM, the high specific surface area of the CNP–GNS

structures provides large number of adsorption sites to the

aniline monomers which can polymerize to form a coating

over the surfaces of CNP–GNS structures.

Fig. 2 – (a) and (c) are FESEM plane view images of PANI coated CNP and GNS composite structures, respectively; (b) and (d) are

the corresponding TEM plane view images.

Fig. 3 – Raman spectra obtained from different samples

considered in this study.

Table 1 – Raman peak positions and intensity ratiosobtained for different samples.

Sample name D-band (cm�1) G-band (cm�1) ID/IG

CNP 1355.39 1585.51 0.86GNS 1366.28 1597.17 0.22GNS/PANI 1357.72 1597.17 0.90CNP/PANI 1357.72 1597.17 0.91

5242 C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5

The Raman spectra obtained from PANI, CNP/PANI, and

GNS/PANI materials are shown in Fig. 3. Micro Raman spec-

troscopic analysis revealed the following: The Raman spectra

obtained from PANI, CNP/PANI, and GNS/PANI materials show

two characteristic bands at �1358 and �1597 cm�1. The

strong �1597 cm�1 band corresponds to mC = N of the quinoid

units present in PANI (ES form) and tangential vibrations of

the carbon atoms present in graphite layers in CNP/PANI

and GNS/PANI materials. The �1358 cm�1 band corresponds

to mC–N of the polarons in charged phenazine-like and/or oxa-

zine-like rings in PANI (ES form) and defective graphitic struc-

tures in CNP/PANI and GNS/PANI materials. As for the carbon

materials (here CNP & GNS), �1358 cm�1 band is the charac-

teristic D band whilst �1597 cm�1 band is the characteristic

G band. The comparison of the ratios of these two peaks

intensities (ID/IG) gives a measure of the quality of the sam-

ples. Thus, as the ratio approaches zero (as minimum as pos-

sible) the carbon will have a more ordered structure. Table 1

shows the position of the G- and D-bands, and their intensi-

ties ratios. Based on these values it can be concluded that

as synthesized CNP structures have many defects. This com-

plements well with the TEM observations.

ID/IG is �0.86 and 0.22 for CNP and GNS structures, respec-

tively. These ratios show that GNS structures are more gra-

phitic with fewer defects. In the composite, due to the PANI

coating, the disorderness increased for both the CNP and

GNS structures. The ID/IG ratio in the case of CNP–PANI com-

posite is 0.91 whilst in the case of GNS/PANI composite it is

0.90. These observations go hand-in-hand with the morphol-

ogy of the samples explained above.

The powder XRD patterns obtained from CNP, GNS, PANI,

and CNP–GNS/PANI structures are shown in Fig. 4. In the case

of CNP and GNS structures the peaks at 2h = 26.22 and 26.73,

respectively correspond to the diffracting (0 0 2) graphite

plane. In the case of PANI, three broad peaks were observed

at around 2h = 15.48, 20.31, and 25.56 corresponding to

(0 1 1), (0 2 0), and (2 0 0) crystal planes of PANI in its emeral-

dine salt form, respectively. The peak at 20.31 is ascribed to

periodicity parallel to the polymer chain, and the peaks at

Fig. 4 – XRD patterns obtained from different samples

considered in this study.

Fig. 5 – Thermographs obtained from different samples

considered in this study.

C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5 5243

15.8 and 25.56 are due to the periodicity perpendicular to the

polymer chain [18,19]. The XRD patterns obtained from CNP–

GNS/PANI composites show broad peaks at 2h = 15.48, 20.31,

and 26.56 which are similar to pure PANI matrix, revealing

that no additional crystalline order has been introduced into

the composites. The increase in the intensity of the peat at

26.56 (also the presence of a shoulder) can be attributed to

higher X-ray scattering factors due to the presence of CNP–

GNS in the composites. This can also be due to the ordering

of polyaniline macromolecule along CNP–GNS axis. From

the XRD measurements, it can be concluded that CNP–GNS/

PANI composites are semi crystalline in nature. The diffrac-

tion peak at �2h = 43� was not observed, indicating that GNS

and CNP have fully interacted with PANI molecules and are

completely coated with PANI.

3.2. Thermal stability

TGA thermograms obtained from CNP, GNS, PANI, CNP–PANI,

and GNS–PANI structures are shown in Fig. 5. The thermo-

grams (except for CNP and GNS structures) show several

systematic steps in weight loss corresponding to the loss of

particular species. However, there is always some overlap in

weight loss ranges with no sharp transition between the steps.

The CNP structures have excellent thermal stability up to

�1000 �C which indicates the extent of degree of crystallinity.

Since, the GNS structures were prepared by intercalation of

natural graphite flakes with oxidizing agents followed by

thermal exfoliation and delaminating of graphite sheets by

ultra-sonication process, the oxygen related moieties will be

attached as functional groups on their surfaces. These surfaces

groups will be decomposed at particular oxidation tempera-

ture which results in weight loss as shown in Fig.5. In the com-

posite structures, the first loss step (25–100 �C) is attributed to

the loss of moisture in the structures. The second loss step

(200–300 �C) involves the loss of low molecular weight frag-

ments, cross linking of chains and onset of degradation of

the polymeric backbone. The final loss step (570–700 �C) corre-

sponds to the complete breakdown of the polymeric backbone

as well as heavier fragments into even smaller fractions and

gaseous byproducts. The char residues (remaining above

700 �C) are mainly thermally stable inert materials like CNP,

GNS, and the carbonized polymeric fragments. By adding

CNP and GNS structures, the corresponding char residue also

increases indicating increased incorporation of CNP/GNS

structures inside the PANI matrix.

3.3. DC electrical conductivity

The electrical conductivity values of different cases are

shown in Table 2. It can be clearly observed that the conduc-

tivity of PANI varies with dopants. PANI cast from m-cresol

solution shows the least value of 4.5 · 10�6 S/cm while PANI

doped by CSA and cast from chloroform shows a slightly in-

creased value of 2.65 · 10�4 S/cm. PANI doped with CSA and

cast from m-cresol solution showed the highest value of

4.25 S/cm which is 6 orders greater than the value obtained

from PANI cast from m-cresol solution alone and 4 orders

greater than the value obtained from PANI doped by CSA

and cast from chloroform. It has been recently reported that

CSA and m-cresol are the most effective pairs of dopants

and solvents for PANI doping [28]. Although the concept of

secondary dopants that modifies the conformation of PANI

has been a major description for the role of m-cresol, the

acidity of m-cresol also has been suggested as one of the cru-

cial factors enhancing electron mobility of PANI. Single m-

cresol molecule is small enough to diffuse into PANI cluster,

and it has the ability to exchange a proton with a camphor-

sulfonate ion and PANI, while CSA is relatively big to stay in

PANI crystalline regions. Based on these arguments, in this

work the same dopants were used but different cases were

considered. In the 1st case, PANI, CNP/PANI, GNS/PANI were

dispersed in m-cresol solvent; in the 2nd case, PANI, CNP/

Table 2 – Electrical conductivity of composite thin films with different dopants.

Cases Dopants Sample DC conductivity (S/cm)

1st Case m-cresol PANI 4.5 · 10�6

GNS/PANI 16.25 · 10�6

CNP/PANI 24.85 · 10�6

2nd Case CSA and chloroform PANI 2.65 · 10�4

GNS/PANI 28.25 · 10�4

CNP/PANI 42.54 · 10�4

3rd Case CSA and m-cresol PANI 4.25GNS/PANI 18.52CNP/PANI 26.85

5244 C A R B O N 4 9 ( 2 0 1 1 ) 5 2 3 9 – 5 2 4 5

PANI, GNS/PANI were mixed with CSA and dispersed in chlo-

roform solution and in the 3rd case these were mixed with

CSA and dispersed in m-cresol solution.

It is evident from the DC conductivity values that in the

above mentioned cases, the presence of CNP and GNS struc-

tures along with PANI in the composite enhances the electrical

conductivity. The conductivity of the composites varies over a

wide range. The 3rd case wherein CNP and GNS structures are

mixed with PANI (mixed with CSA and dispersed in m-cresol

solution) showed the highest conductivity values of 18.52 S/

cm for GNS/PANI and 26.85 S/cm for CNP/PANI. These values

are 4 and 6 orders greater than the values obtained in 1st

and 2nd cases for the same composition, respectively. Due to

their highly conducting nature as well as high aspect ratio,

the pipes and nanosheets can act as interconnecting bridges

between the various conducting grains of the polyaniline. As

discussed in Section 3.1, close interaction of CNP and GNS

structures with PANI is evident. This increases the coherence

or coupling between the networks in the composite and leads

to the enhancement of interchain electrical transport. Most of

the literature available argues and shows that the introduction

of carbon nanostructures to PANI enhances the electrical

properties of the composite by facilitating the charge transfer

processes between the two components [17,29]. On the outset,

this is also applicable to the CNP–GNS/PANI system; study to

elucidate the transport mechanism is being pursued.

4. Conclusion

In situ chemical oxidative polymerization was found to be an

effective method to synthesize CNP–GNS/PANI composites.

PANI was uniformly coated on the surfaces of individual

CNP and GNS structures resulting in the network of uniform

composite structures. Due to the low concentration (1 wt.%)

of CNP structures used, PANI coating on them exists as glob-

ular agglomerates. In the case of GNS structures, polymeriza-

tion took place predominantly on the surfaces of the

structures. Raman analysis showed significant interactions

between the CNP–GNS structures and PANI. The CNP–GNS/

PANI composite showed good thermal stability. The compos-

ites constituted with PANI that is doped with CSA and cast

from m-cresol solution showed conductivity 6 orders higher

than the composites constituted with PANI that is doped with

m-cresol solution. This work shows that: (i) the serious obsta-

cles like poor dispersion and lack of interfacial adhesion

while synthesizing similar composites with carbon nano-

tubes (for example: MWCNT) as reinforcement material have

been overcome to a great extent by sysnthesizing CNP/PANI

and GNS/PANI composites, (ii) by properly choosing the start-

ing materials (for example: CNP with intrinsic surface defects

all along its length), additional surface functionalization steps

which are otherwise needed can be avoided to achieve effec-

tive PANI nucleation and thereby uniform coating to finally

obtain networks of uniform composite structures, and (iii)

the proper design and execution of the synthesis steps has

led to the synthesis of CNP/PANI and GNS/PANI composites

with enhanced thermal stability and electrical properties

when compored to the existing composite materials (for

example: MWCNT/PANI composite). Use of these composites

as gas sensors and electromagnetic interference materials

in electronic devices is currently under investigation.

Acknowledgements

The authors are grateful to Prof. K. Bhanu Sankara Rao, Dean,

School of Engineering Sciences and Technology (SEST) Univer-

sity of Hyderabad, for his encouragement and support for this

collaborative work. We also thank Dr. B.V. Sharadha for help-

ing us with Raman measurements. We thank Dr. S. Srinath,

School of Physics (University of Hyderabad) for generously

allowing us to use the FESEM and Mr. M. Durga Prasad, Center

for Nanotechnology (University of Hyderabad), for helping us

with TEM. We would also like to thank the lab technicians

(ARCI) Mr. G. Venkatreddy and Mr. K. Subba Rao for their tech-

nical support. Lastly, we thank the editor in chief (Dr. Thrower)

for generating the discussion on the nomenclature of carbon

nanostructures and suggesting several important corrections.

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