Solid State Communications 159 (2013) 93–97

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Solid State Communications

0038-10

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n Corr

E-m1 Pr

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

Charge transport and magnetic properties of coaxial composite fibrilsof polypyrrole/multiwall carbon nanotubes at low temperature

Ravi Bhatia n,1, I. Sameera, 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 12 October 2012

Received in revised form

21 January 2013

Accepted 29 January 2013

by H. Akaithe electrochemical polymerization process. Electron microscopy studies reveal that PPy coating on the

Available online 8 February 2013

Keywords:

A. Composites

C. Electron microscopy

D. Electrical properties

D. Magnetic properties

98/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.ssc.2013.01.029

esponding author. Tel.: þ91 80 2293 2313; fa

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

esently at Dept. of MSE, National University

a b s t r a c t

We report the low temperature electrical and magnetic properties of polypyrrole (PPy)/multiwall

carbon nanotube (MWNT) coaxial composite fibrils synthesized by the electro-polymerization method.

The iron-filled MWNTs were first grown by chemical vapor deposition of a mixture of liquid phase

organic compound and ferrocene by the one step method. Then the PPy/MWNT fibrils were prepared by

surface of nanotube is quite uniform throughout the length. The temperature dependent electrical

resistivity and magnetization measurements were done from room temperature down to 5 and 10 K,

respectively. The room temperature resistivity (r) of PPy/MWNT composite fibril sample is �3.8 Ocm

with resistivity ratio [R5 K/R300 K] of �300, and the analysis of r(T) in terms of reduced activation

energy shows that resistivity lies in the insulating regime below 40 K. The resistivity varies according to

three dimensional variable range hopping mechanism at low temperature. The magnetization versus

applied field (M–H loop) data up to a field of 20 kOe are presented, displaying ferromagnetic behavior at

all temperatures with enhanced coercivities �680 and 1870 Oe at room temperature and 10 K,

respectively. The observation of enhanced coercivity is due to significant dipolar interaction among

encapsulated iron nanoparticles, and their shape anisotropy contribution as well.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the fabrication of composite materials has receivedgreat attention as they offer multi-functionality as well as anopportunity to tailor the physical properties of materials to achievethe desired material performance. For the fabrication of compositematerials the selection of both suitable filler and host materials isvery important. For last two decades, carbon nanotubes (CNTs) arethought to be most promising reinforcing material in compositeindustry due to their extraordinary properties (low density, highaspect ratio and electrical conductivity, superior mechanical proper-ties etc.) [1–3]. For an instance, addition of multiwall carbonnanotubes (MWNTs) in small quantities to the insulating polymermatrix can decrease its resistivity by �5 orders of magnitude andhence results into conducting composite materials with superiormechanical properties [1–3]. In such systems the electrical conduc-tivity solely depends on the MWNT concentrations and the sampleprocessing techniques; therefore by suitably varying the MWNTloading resistivity of composite materials can be accordingly tai-lored [4]. Similarly, incorporation of magnetic particles into a non-

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magnetic material in a small amount can result into a magneticmaterial with novel magnetic properties [4–6].

Chemical vapor deposition (CVD), being a simple and inexpen-sive method, is widely used to grow CNTs using metallic magneticparticles (MMPs) as catalyst that remain trapped within the nano-tubes [4]. Nanotubes encapsulated with MMPs are fantastic fillers tobe used for fabrication of composite materials. Since reinforcementof CNTs filled with MMPs into polymer could lead to compositeswith novel and interesting magnetic properties in addition toelectrical and mechanical properties due to two reasons. First, thetrapped MMPs are prevented from oxidation as the nanotube shellprovides an effective barrier against oxidation while it is quitedifficult to prevent MMPs from oxidation at conventional experi-mental conditions. Second, MMPs trapped within the nanotubes areprevented from aggregation. Both conducting [polyaniline, polypyr-role (PPy), poly(p-phenylenevynilene) etc.] and insulating [polystyr-ene, polymethylmethaacrylate, polyurethane etc.] polymers havebeen mainly used as host materials for composite fabrication [1–6].Since PPy can be easily processed and prepared among variousconducting polymers, it is more favored for composite fabrication.The PPy/CNT composites have many potential applications such asgas sensors, field emission devices, supercapacitors and batteries[7–10] etc. These composites can be prepared by solution processingand casting, melt mixing, in situ polymerization, grafting polymerchains onto CNTs using electrochemistry etc. [1–3].

R. Bhatia et al. / Solid State Communications 159 (2013) 93–9794

In the present work, we report the fabrication of coaxial PPy/MWNT composite fibrils by electro-polymerization of the uniformPPy films over the surface of iron (Fe) nanoparicle-filled MWNTsgrown by CVD. Morphology of as-prepared fibril sample wascharacterized by electron microscopy, and the temperature depen-dent electrical resistivity and magnetization measurements werecarried out from 300 to 5 and 10 K, respectively.

2. Experimental details

For the fabrication of PPy/MWNT fibrils, a two step approachwas adopted. In the first step, Fe-filled MWNTs were synthesizedby thermal assisted CVD of a mixture of toluene–ferrocene [4].Here, toluene acts as carbon source material and ferrocene pro-vides the essential catalytic Fe-nanoparticles (NPs). The synthesismechanism of MWNTs is reported in an earlier work [4]. Scanningand transmission electron microscopy (SEM and TEM) images ofas-obtained MWNTs are presented in Fig. 1(a) and (b). The highquality of MWNT sample was confirmed by Raman spectroscopy,reported elsewhere [4]. The inset of Fig. 1(a) displays the energydispersive analysis of X-rays of MWNT sample indicating thepresence of Fe in the MWNT sample, which is consistent withTEM micrograph [see Fig. 1(b)] revealing the encapsulated Fe-NPwithin nanotube shell. The length and diameter of the trapped Fe-NP is �20 and 100 nm, respectively. The second step involved theelectro-polymerization of a mixture of distilled pyrrole monomerand as-prepared MWNTs. The MWNT-monomer mixture was

Fig. 1. (Color online) (a) SEM micrograph of as-grown Fe-filled MWNT sample. Ins

(c) Low magnification SEM micrograph of PPy/MWNT fibrils, and (d) High magnificatio

sonicated for 10 min in order to disperse MWNTs. Propylenecarbonate (PC) was used as the solvent while tetrabutylammoniumhexafluorophosphate (TBA-PF6) was used as dopant in the electro-chemical polymerization. The polymerization solution contained0.06 M solutions of pyrrole and TBA-PF6 in PC, and 1 mg ofMWNTs. The polymerization process was carried out at roomtemperature by passing a current of 3 mA for 1.5 h. As a productPPy/MWNT composite fibrils were deposited at the anode in theform of a black film which was gently scrapped off and dried undervacuum at room temperature for 24 h.

A standard two probe method was used to perform theelectrical resistivity measurements of the fibrils. The sampledimensions were 6 mm�3 mm�0.15 mm. The electrodes weremade of copper wire (thickness 50 mm) and the contacts weremade using silver paste. The low temperature electrical resistivitymeasurements were done in Janis liquid helium cryostat from 300to 5 K connected to Lakeshore temperature controller, Keithley2000 multimeter and Keithley 220 current source. The constantcurrents are typically in the range of 0.1–1 mA, chosen to avoid anysample heating at low temperatures. Magnetization hysteresis loopmeasurements were done in a Quantum Design Squid magnet-ometer (MPMS XL) from 300 to 10 K, in applied fields up to 20 kOe.

3. Results and discussion

The growth process of PPy/MWNT fibrils can be understood asfollows. In the polymerization solution the monomer gets attached

et: EDAX pattern of MWNT sample, (b) TEM micrograph of Fe-filled MWNTs,

n SEM micrograph of PPy/MWNT fibrils.

R. Bhatia et al. / Solid State Communications 159 (2013) 93–97 95

onto the MWNT surface via non-covalent interactions. As anelectric current is passed, pyrrole and MWNTs move towardsanode and formation of PPy starts by anodic oxidation of pyrroleand monomer polymerize uniformly along the length of the tube.As a result a thin layer of PPy gets deposited on the anode overwhich MWNTs coated with PPy films deposit [11]. The thickness ofthe ppy coating significantly depends on the applied current andtime duration. The morphology of as-prepared PPy/MWNT wasstudied by SEM using the FEI Quanta 200 scanning electronmicroscope. The SEM micrographs shown in Fig. 1(c) and (d),clearly reveal that PPy film is coated on the MWNTs surface. FromFig. 1(d), it can be noticed that the coating of PPy film is quiteuniform and continuous. In fact this indicates towards a stronginteraction between MWNT surface and pyrrole monomer, whichhelps the polymer to grow quite uniformly on the nanotubesurface. A comparison of SEM micrographs of MWNT and fibrilsindicate that the diameter of PPy/MWNT fibrils is nearly ten timesthat of MWNT while its length is nearly identical to that ofMWNTs. Detailed electron microscopy studies give a clear idea ofthe growth of PPy/MWNT fibrils. It seems as if MWNTs act as nano-electrodes for PPy polymerization.

In order to get finer details of the fibril morphology, TEM wasemployed using Technai F30 equipment. Fig. 2 presents the TEMmicrographs of PPy/MWNT fibrils. Coaxial nature of PPy/MWNTsample can be easily noticed in Fig. 2(a). Also, it confirms that PPyfilms are deposited continuously over the length of MWNT. Thediameter of the coaxial fibrils is �1 mm, which is consistent withthe SEM studies. The surface of the fibril seen in Fig. 2(b) appearsquite featureless and uniform.

3.1. Temperature dependent electrical resistivity

Fig. 3(a) and (b) show the variation of electrical resistance (R)and resistivity (r) of fibril sample as a function of temperature.It can be seen that R increases appreciably [from 165 O to 50 kO] astemperature decreases from 300 to 5 K, and an increase in R by anorder of magnitude is observed as temperature reduces from 40to 5 K, as in the inset of Fig. 3(a). This kind of electrical response atlow temperatures is a characteristic feature of disordered materi-als [12]. The resistivity ratio [r5 K/r300 K] is an important para-meter that gives an idea about the conduction regime in the caseof disordered materials. In the present case, the [r5 K/r300 K] is�300 [with r300 K �3.8 Ocm, see Fig. 3(b)] which suggests theresistivity of fibril sample lies in the insulating regime at lowtemperatures. The characteristic behavior of resistivity can beexplicitly described by analyzing the low temperature resistivity

Fig. 2. (a) TEM micrograph of coaxial PPy/MWNT fibril, and (b) TEM micrograph

data in terms of reduced activation energy (W). The reducedactivation energy is defined as [12].

W ¼�Td½ln rðTÞ�

dT

� �¼�

d½ln rðTÞ�d½lnðTÞ�

ð1Þ

The temperature dependences of W in various regimes at lowtemperatures are as follows:

(a)

of PP

In the metallic regime, W has a positive temperaturecoefficient.

(b)

In the critical regime, W is temperature independent. (c) In the insulating regime, W has a negative temperature

coefficient.

The W vs. T plot in the present case is shown as inset ofFig. 3(b) which shows that W has a negative temperaturecoefficient. This confirms that the resistivity of PPy/MWNT fibrilsample lies in the insulating regime below 40 K. The physicalmeaning of the negative temperature coefficient is that theamount of energy required for charge carrier to hop from onesite to other increases as temperature goes down, due to whichcharge transport becomes difficult. Furthermore, the enhance-ment in the resistivity with lowering temperature can addition-ally be attributed to the reduction in inter-connectivity of thefibril network making the hopping of the charge carrier moredifficult at low temperatures [11]. It is worth noting that bulkMWNT sample does not display such charge transport behavior atlow temperatures [13]. On the contrary, in the case of MWNT the[r5 K/r300 K] is less than 2, and W has a positive temperaturecoefficient at low temperature which indicates towards metallicnature of MWNTs [13]. In the insulating regime, the variablerange hopping (VRH) model is generally used to describe the lowtemperature charge transport in disordered systems [12]. In thestrongly disordered systems the electrical conductivity approacheszero at sufficiently low temperatures (�0 K). The low temperaturecharge transport follows the VRH model in such materials, which isexpressed as [12]

rðTÞpexpT0

T

� �n

ð2Þ

where T0 is the characteristic temperature that depends on thedensity of states at Fermi level, and n is the exponent whichdepends on the extent of disorder, dimensionality of the system,morphology etc. The parameter n is equal to ¼ in the case of threedimensional (3D) VRH transport. Fig. 3(c) presents ln rðTÞ vs.

y/MWNT fibril showing the uniform coating of PPy over MWNT surface.

Fig. 3. (Color online) (a) Electrical resistance (R) of PPy/MWNT fibril sample as a

function of temperature (T) from 300 to 5 K. Inset. R vs. T plotted in low

temperature regime from 40 to 5 K, (b) Electrical resistivity [r(T)] as a function

of temperature from 300 to 5 K, indicating the resistivity ratio [r5 K/r300 K] �300.

Inset. Reduced activation energy (W) vs. T at the low temperature indicating the

resistivity lies in the insulating regime below 40 K, and (c) ln r vs. T1/4 plot,

suggesting the resistivity follows the 3D VRH model. The red line is fitted curve

according to Eq. (2).

R. Bhatia et al. / Solid State Communications 159 (2013) 93–9796

T1/4 plot suggesting that r(T) of fibril sample follows 3D VRHconduction mechanism as ln r(T) varies linearly with T1/4, herethe red line is the linear fit curve according to Eq. (2).

3.2. Temperature dependent hysteresis loops

Fig. 4(a) and (b) show the M–H loops at various temperatures,which indicate towards the ferromagnetic nature of the PPy/MWNT fibrils. Unambiguously, the presence of Fe-NPs onlycauses this ferromagnetic response since pristine MWNT andpolymer are diamagnetic in nature [4]. At room temperature,the saturation magnetization (Ms) and coercivity (Hc) are0.85 emu/g and 680 Oe, respectively for the fibril sample whilefor as-prepared Fe-MWNTs, Ms and Hc are 3.82 emu/g and 488 Oe,respectively [4]. The value of Hc is quite higher in comparison toreported values for similar systems [4–6,14,15]. However, thevalues of Ms and Hc for bulk Fe are 220 emu/g and 1 Oe,respectively [15,16]. In fact the magnetic response of dispersedmagnetic NPs is reported to be significantly different in compar-ison to their bulk counterpart mainly due to enhanced surface tovolume ratio and dominances of particular magnetic interactionsas a function of shape and size of NPs, anisotropy, inter-particleseparations etc. [4–6]. For instance, the Hc of spherical Fe-NPswith diameter below 9 nm (superparamagnetic limit for Fe) iszero and it has finite value when the size of NPs is above 9 nm[17]. The Hc of elongated rod-shaped Fe-NPs is enhanced due toadditional contribution of shape anisotropy to the total aniso-tropy of the particle [18], and with the increase in the length ofrod-shape particle further enhancement of Hc may be observed atlower temperatures. Also, the composition of the magneticsystem encapsulated within the MWNT significantly affects itsmagnetic response. For example, the presence of iron carbide(Fe3C) at the interface of innermost layer of MWNT and Fe corecontributes to the total anisotropy, and hence to Hc. The value ofHc of MWNTs with magnetic system composed of Fe3C (inter-facial) and Fe (core) is reported to increase with the amountof interfacial Fe3C [19]. In the present case, Ms value for thefibril sample is much lesser which is probably due to presence ofvery small content of Fe in the composite fibril whereasan enhancement in the Hc value can be noticed. This incrementin Hc of fibrils is attributed to dominance of strong dipolarinteractions among Fe-NPs and the shape anisotropy contributionof rod-shaped Fe-NPs.

Further, the magnetization of composite fibril sample isreduced at higher fields as in Fig. 4(c), due to the diamagneticcontribution of MWNT and polymer [4,6]. The blue dotted lineseparates ferromagnetic and diamagnetic regions, indicated by I &II. The reduction in the magnetization at higher fields at alltemperatures can be understood from following arguments. Thecomposite fibril sample is fabricated by electro-polymerization ofPPy over Fe-MWNT surface. As mentioned earlier, Fe-NPs alonecontribute towards the observed ferromagnetic behavior of com-posite fibrils but the Fe content is only a very small fractionwithin fibril sample. It should be noted that PPy is diamagnetic innature. The diamagnetic susceptibility is always small negativevalue which is almost independent of temperature, but thenegative magnetization increases with increase in the strengthof applied field. While for ferromagnetic material the suscept-ibility is large positive value and increases with decrease intemperature, since the magnetic ordering is favored at lowtemperatures. Thus, due to enhanced diamagnetic contributionthe magnetization never saturates for the composite fibril sample,rather reaches a maximum at fields below 20 kOe and thendecreases due to significant diamagnetic contribution. Here, forsimplicity the maximum magnetization is termed as saturationmagnetization (Ms). Furthermore, the field strength required toattain Ms increases as temperature reduces. This is also due tocompetition between the ferromagnetic and diamagnetic contri-butions from Fe-NPs and the polymer. The variation of Ms and Hc

with temperature is presented in Fig. 4(d). Both saturation

Fig. 4. (Color online) (a) The M–H loops in the applied field of �20 to 20 kOe at various temperatures, indicating the maximum value of Ms�1.1 emu/g at 10 K, (b) M–H

loops from �2 to 2 kOe indicating the highest value of Hc�1870 Oe at 10 K, (c) M–H loops in the positive quadrant indicating ferromagnetic and diamagnetic dominances

in the regions I and II, respectively, where the blue dotted line is to guide eyes, and (d) variation of Ms and Hc values with temperature.

R. Bhatia et al. / Solid State Communications 159 (2013) 93–97 97

magnetization and coercivity increase with decrease in tempera-ture, as temperature reduces to 10 K the value of Ms increases to1.1 emu/g and Hc approaches to 1870 Oe. This is because of thefact that at low temperatures the thermal energy is negligiblysmall to affect magnetic ordering.

4. Conclusions

Coaxial composite polypyrrole (PPy)/Fe-filled multiwall car-bon nanotubes (MWNT) fibrils were fabricated by a simple twostep approach, which display interesting electrical and ferromag-netic properties. The analysis of electrical resistivity data as afunction of temperature showed that the resistivity of as-fabricated fibril sample lies in insulating regime at temperaturesbelow 40 K, and follows 3D variable range hopping model.Magnetization studies showed that room temperature Hc valueis 680 Oe, which is significantly higher than the reported valuesfor the similar systems. However, Ms value is lesser due to the factthat very small Fe content is present in the fibril sample.

Acknowledgment

We gratefully acknowledge the Institute Nanoscience Initiative,IISc for the SEM and TEM characterization. The SQUID magnetometryfacility at Department of Inorganic and physical chemistry, IISc isacknowledged, for magnetization measurements.

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