Physica B 406 (2011) 1727–1732

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Preparation and characterization of multiwall carbon nanotube/polypyrrolecoaxial fibrils

Ravi Bhatia n, C.S. Suchand Sangeeth, V. Prasad, Reghu Menon

Department of Physics, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Article history:

Received 6 July 2010

Received in revised form

4 February 2011

Accepted 5 February 2011Available online 12 February 2011

Keywords:

Multiwall carbon nanotubes

Composites

Electron microscopy

Electrical characterizations

26/$ - see front matter & 2011 Elsevier B.V. A

016/j.physb.2011.02.016

esponding author. Tel.: +91 80 2293 2313; fa

ail address: bhatia.phy@gmail.com (R. Bhatia)

a b s t r a c t

Multiwall carbon nanotube (MWNT)/polypyrrole (PPy) fibrils were fabricated by template-free in situ

electrochemical deposition of PPy over MWNTs, and characterized by electron microscopy and

electrical measurements. Scanning and transmission electron microscopy studies reveal that PPy

coating on the surface of nanotube is quite uniform throughout the length, with the possibility of

forming unique Y-junctions. Current (I)–voltage (V) characteristics at various temperatures show

nonlinearity due to tunneling and hopping contributions to transport across the barriers. AC

conductivity measurements (300–4.2 K) show that the onset frequency scales with temperature, and

the nanoscale connectivity in MWNT/PPy fibrils decreases with the lowering of temperature.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

In nanocomposite systems the features of different materialscan be easily combined to tailor the properties to achieve thedesired performance. In recent years, composites based on carbonnanotubes (CNTs) and semiconducting polymers have been ofgreat interest for developing new class of advanced multi-func-tional materials. Particularly, these composite materials havebeen used for the development of electromechanical actua-tors [1,2], pH and organic vapor sensors [3–5], organic photo-voltaic cells [6,7] and in field emission devices [8]. It has beenfound that multiwall carbon nanotubes (MWNT)/polypyrrole(PPy) composite films have exceptional charge storage capacities(several times larger than that of either MWNT or PPy) and havepotential applications in supercapacitors and secondary bat-teries [9,10]. CNTs are of interest for composite materials becauseof their excellent electronic, mechanical properties and thermalstability. At the same time conducting polymer microtubeshave attracted much attention because of their applications inelectronic and electro-optical devices [11]. Among conductingpolymers, PPy has unique properties such as high conductivityand ease of preparation. Thus, it is expected that the composite ofCNTs with conducting PPy can be used as new type of materialsfor several potential applications. Recently, there have beencertain attempts to produce CNT/conducting polymer compositesby dissolving polymers in a suspension of CNTs in organic

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solvents, wrapping nanotubes with polymer chains using in situ

polymerization, grafting macromolecules onto CNTs and surfac-tant-directed polymerization [12–14]. Alternatively, template-directed method has been used for the fabrication of complexnanostructures [15].

In this work we report on the synthesis of MWNT/PPy fibrils bytemplate-free method. We have carried out detailed characteriza-tion of these fibrils by using scanning and transmission electronmicroscopy. Current (I)–voltage (V) characteristics were studiedat various temperatures, and AC conductivity measurements weredone in the temperature range of 300–4.2 K. The charge transportmechanism follows a tunneling/hopping model.

2. Experimental details

The synthesis of MWNT/PPy fibrils is done by electrochemicalpolymerization in the present work. MWNTs were synthesizedby thermally assisted chemical vapor deposition of toluene–ferrocene mixture following a simple approach reported in aprevious work [16]. SEM and TEM micrographs of as-obtainedMWNTs are shown in Fig. 1. These MWNTs were used to formMWNT/PPy fibrils without any acid treatment. Propylene carbo-nate (PC) was used as the solvent in electrochemical polymeriza-tion. Pyrrole monomer and PC were distilled under reducedpressure. Tetrabutylammonium hexafluorophosphate (TBA-PF6)was used as a dopant in the electrochemical polymerization. Thepolymerization solution was prepared by adding 0.06 M solutionsof pyrrole and TBA-PF6 in PC. Specific amount of (�1 mg) MWNTswas dispersed in the polymerization solution by sonication. The

Fig. 1. (a) SEM images of MWNTs. (b) TEM images of MWNTs.

Fig. 2. Schematic of MWNT/PPy fibrils formation.

R. Bhatia et al. / Physica B 406 (2011) 1727–17321728

electrodeposition of PPy on MWNTs was carried out at roomtemperature by passing a current of 3 mA for 1.5 h. MWNT/PPyfibrils are deposited at the anode as a black film. The obtainedfilm was gently peeled off and dried under vacuum at roomtemperature for 24 h.

A scanning electron microscope [FEI Quanta 200] is used formorphological studies of MWNT/PPy fibrils at high vacuum. Atransmission electron microscope [Technai F30, 300 kV] is used toobtain the TEM images of the samples. A standard two-probemethod was used for I–V and AC conductivity measurements ofMWNT/PPy fibrils. The sample dimensions were 6 mm�3 mm,and thickness�150 mm. The electrical contacts were made byconducting silver paste. A Keithley 2400 sourcemeter was usedfor the I–V measurements. The AC conductivity of MWNT/PPyfibrils, in the frequency range 100 kHz–10 MHz, was measuredusing an Agilent 4285A LCR meter, down to 4.2 K in a Janiscontinuous flow cryostat. The low frequency impedances(100 Hz–100 kHz) were measured using a dual channel lock-inamplifier (SR 830).

3. Results and discussion

The formation of MWNT/PPy fibrils can be understood asfollows. In the polymerization solution the monomer getsadsorbed or attached onto the surface of MWNTs via non-covalent interactions, known as P-P stacking [13]. As an electricfield is applied pyrrole and MWNTs migrate towards anode andformation of PPy starts by anodic oxidation of pyrrole [17]. As aresult a thin layer of PPy gets deposited on the anode over whichMWNTs coated with PPy films deposit. On the basis of theelectron microscopy studies, step-by-step process of MWNT/PPyfibrils formation is schematically shown in Fig. 2.

The fibril morphology of MWNT/PPy is studied using scanningelectron microscopy (SEM). The SEM micrographs of MWNT/PPyfibrils at various magnifications are shown in Fig. 3. The SEMimages show that PPy is firmly coated on the surface of MWNTs,and even after sonication the PPy layer does not get detachedfrom MWNT. This is indicative of good interaction between MWNTand pyrrole monomer, which helps the polymer to grow quiteuniformly on the nanotube surface. The diameter of MWNT/PPyfibrils is significantly larger than that of MWNT and its lengthis nearly identical to that of MWNTs. A large number ofthese randomly oriented nanotubes are coated with thepolymer. Though the MWNT/PPy fibrils are quite entangled, thethickness of the deposited PPy layer looks rather uniform asshown in Fig. 3. This signifies that MWNTs act as secondarynanoelectrodes that provide a large surface area for polymeriza-tion to occur. Fig. 3(c) and (d) shows the magnified SEM micro-graphs of MWNT/PPy fibrils. Coaxial coating of PPy film on thesurface of MWNT can be easily observed. SEM images confirmthat PPy layers are deposited continuously over the length ofMWNT. In several cases the tips of MWNTs are left barely coatedby PPy layer, as in Fig. 3(d), and in mid-portions the coating isobserved to be quite uniform as in Fig. 3(c). A careful examinationof SEM micrographs indicates that Y-junctions of MWNT/PPyfibrils are also present in the sample. It should be noted thatthe pristine MWNTs do not have any Y-junctions (see Fig. 1), sothe branching off to form the Y-junction is not expected. Thisimplies that though the polymerization process mainly occursalong the length of MWNTs, there exists the possibility to formshort random branches as shown in Fig. 4(a)–(d). A possiblereason for the formation of Y-junction fibrils is due to the overlapof crossed MWNTs in the polymerization solution, and theformation of PPy layer could fuse these disconnected Y-junctions.To the best of our knowledge, Y-junction growth of MWNT/PPy

Fig. 3. SEM images of MWNT/PPy coaxial fibrils at different magnifications.

Fig. 4. SEM images of Y-junctions in MWNT/PPy fibrils.

R. Bhatia et al. / Physica B 406 (2011) 1727–1732 1729

Fig. 5. TEM images of MWNT/PPy fibrils.

Fig. 6. (a) I–V characteristics of MWNT/PPy fibrils at various temperatures.

(b) Positive side of the I–V plot. The lines show the fits to Eq. (1).

R. Bhatia et al. / Physica B 406 (2011) 1727–17321730

cables has not been reported yet [9,14,15]. In Fig. 5 the TEMimages of fibrils are shown, which clearly indicate the presence ofthese Y-junctions and the bare portion of the MWNT tips. More-over, in earlier works on the nanowires/nanotubes of conductingpolymers like PPy and polyaniline (PANI), no such Y-junctionshave been observed [18,19]. These findings may need moredetailed investigations in the near future.

Fig. 6(a) shows the I–V characteristics of MWNT/PPy fibrils.The I–V characteristics are non-linear and quite symmetric uponreversal of the voltage direction, at all temperatures. In the case ofMWNTs the I–V characteristics are linear at all temperatures,which point towards its metallic (ohmic) nature. However, whenthe MWNT are coated with PPy films the I–V characteristics resultfrom conduction via outer surface (i.e. PPy coating). In PPy thecharge transport results from the hopping of delocalized electronsthrough small potential barriers/gaps, which give rise to the non-linear I–V profiles at all temperatures. As temperature is increasedfrom 90 to 300 K, it is easy for the electrons to hop to give highconductivity and qualitatively this accounts for the observation ofnon-linear trend in the I–V characteristics as a function oftemperature.

In order to understand the non-linear I–V characteristics inquantitative manner, we have used the model proposed by Kaiseret al. [19]. This model is observed to fit well in nanoscaleconductors hindered by small potential barriers and is expressedby the following equation:

G¼ I=V ¼G0 expðV=V0Þ

½1þhðexpðV=V0Þ�1Þ�ð1Þ

where G0, h and V0 are parameters. G0 is the temperaturedependent low-field conductance. The parameter h¼G0/Gh

(ho1) and Gh is the saturated conductance at high-fields. V0 isa voltage scale factor; there is an exponential increase in con-ductance as voltage increases, and this strongly depends on thebarriers at the intersections of nanowires/nanotubes. It has beenshown that this model gives a very good description of the nonlinearities observed in the I–V characteristics of polyacetylenefibers, CNT networks, vanadium pentoxide fibers, individual PPy,PANI and cadmium sulphide nanotubes [19,20]. Fig. 6(b) showsthe fits according to this model. It can be seen that the experi-mental data fit quite well to this model at all temperatures, andthe fit parameters are listed in Table 1. It can be observed that thelow-field conductance G0 increases from 0.05 to 0.49 mS astemperature increases from 90 to 300 K. Also an increase in the

R. Bhatia et al. / Physica B 406 (2011) 1727–1732 1731

value of h from 0.38 to 0.81 is noticed as temperature is increasedfrom 90 to 300 K, which corresponds to the increase in conduc-tance at low fields. The voltage scale factor V0 also increases from7.45 (T¼90 K) to 39.78 V (T¼300 K), which indicates the decreasein nonlinearity in I–V characteristics as temperature increases.Since V0 depends on the potential barriers present at the inter-fibrilar junctions, its variation with temperature is as expectedwithin this model (Eq. (1)).

The AC transport is a useful tool to probe the extent ofnanoscale connectivity in disordered systems. The frequencydependence of normalized AC conductivity of MWNT/PPy fibrilsand MWNT sample at various temperatures is presented inFig. 7. All the curves show two regions (R(1) and R(2)) in bothMWNT/PPy fibril and MWNT sample. At low frequencies theconductivity maintains a constant value up to an onset frequency

Table 1Fitting parameters to Eq. (1) for MWNT/PPy fibrils at different temperatures.

T (K) 90 115 150 175 205 225 265 280 300

G0 (mS) 0.05 0.08 0.13 0.15 0.17 0.20 0.30 0.38 0.49

V0 (V) 7.45 9.68 13.02 15.91 16.28 20.13 28.53 33.01 39.78

h 0.38 0.42 0.48 0.57 0.65 0.72 0.73 0.78 0.81

Fig. 7. Normalized AC conductivity vs. frequency plots of (a) MWNT/PPy fibrils

and (b) pristine MWNT. Inset: onset frequency vs. temperature.

(o0) and then it increases monotonically at higher frequencies(o4o0) for both samples. But the AC conductivity has pro-nounced temperature dependence at higher frequencies in thecase of fibrils, which is normally observed in semiconducting andinsulating systems. But in the case of MWNT there is hardly anychange in conductivity at any temperature for higher frequency(o4o0), which is observed in metallic systems. The onsetfrequency is almost independent of temperature for MWNTsample [see inset of Fig. 7(b)], but in the case of fibrils the onsetfrequency vs. temperature (log–log plot) data suggest a directrelationship of the form o0pTp and the shift in onset frequencyto lower values at lower temperatures implies that the connec-tivity in the system is reduced at lower temperatures [21–25].Thus, AC conductivity data support the observed non-linear I–V

characteristics for MWNT/PPy fibril sample.

4. Conclusions

We have successfully synthesized MWNT/PPy fibrils by asimple template-free electropolymerization technique. SEM andTEM investigations confirmed that the fibrils are randomlyoriented with consistent and uniform coating of PPy films.Interestingly Y-junctions of MWNT/PPy fibrils are also observed,which may find some potential applications in organic electro-nics. I–V characteristics were non-linear due to the presenceof barriers, which cause a combined tunneling and hoppingtransport at various temperatures. AC conductivity studiesshowed that connectivity is reduced at lower temperatures andthe temperature dependence of onset frequency follows apower law.

Acknowledgements

The authors are thankful to Institute Nanoscience Initiative,Indian Institute of Science, Bangalore, for the TEM and SEMcharacterization. R. Bhatia would like to thank I. Sameera,Jugeshwar and Vaibhav for fruitful discussions. R. Bhatia grate-fully acknowledges UGC, New Delhi, for contingency support.CSSS wishes to acknowledge CSIR, New Delhi, for financialsupport.

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