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Research ArticleReceived: 29 August 2013 Revised: 22 October 2013 Accepted article published: 29 October 2013 Published online in Wiley Online Library: 27 November 2013

(wileyonlinelibrary.com) DOI 10.1002/pi.4643

A study on the synthesis, characterization andproperties of polyaniline/magnesium boridenanocompositesShumaila, a,b Masood Alam,b Azher M Siddiquia and Mushahid Husaina,c∗

Abstract

The present paper reports the novel synthesis of polyaniline/magnesium boride (PAni/MgB2) nanocomposites. Nanowires50–100 nm in diameter grown by the sol–gel technique were incorporated in the PAni to prepare PAni/MgB2 nanocomposites,which yielded an enhancement of conductivity by 5 orders of magnitude. PAni was synthesized through the chemical oxidativepolymerization method. The composition of the prepared nanocomposites was tunable, i.e. the amount of dopant was variedand the effects on various parameters were observed by different techniques. The morphology of PAni/MgB2 nanocompositeswas determined using SEM. The temperature dependence of the conductivity of all composites was measured in the temperaturerange 300–450 K and it was observed that samples having a high concentration of MgB2 show the highest conductivity. Themolecular structure of the nanocomposites was further characterized by Fourier Transform IR spectroscopy which showed smallstructural changes in the backbone of PAni. I−V measurements showed that the current increases on increasing MgB2 content.UV−visible spectra exposed the occurrence of an indirect optical transition in the composite.c© 2013 Society of Chemical Industry

Keywords: PAni/MgB2 nanocomposites; DC conductivity; FTIR; UV−visible

INTRODUCTIONIn recent years the development of inorganic/polymer hybridmaterials has received significant attention due to a wide rangeof potential applications in optoelectronic devices and in fieldeffect transistors.1,2 The inorganic fillers at nanoscale exhibit ahigh surface to volume ratio and thus are expected to modifythe electrical, optical and microscopic properties of polymersdrastically. In general, the synthesis of a hybrid polymer/inorganicmaterial aims to obtain a new composite material with synergeticor complementary behaviour between the polymer and inorganicmaterial. Inorganic nanoparticles of different natures and sizescan be combined with the conducting polymers giving rise to ahost of nanocomposites with interesting physical properties andimportant application potential. Depending upon the synthesismethod and the characteristics of the inorganic materials, ultimateproperties of the resulting composites can be achieved. Recentlyconducting polymers have attracted much attention not onlyfrom fundamental viewpoints but also from a practical interestas materials for electrical and optoelectronic devices such aselectroluminescence cell and photocells. They exhibit numerousunique phenomena such as a drastic change in electrical,optical and magnetic properties upon doping.3 The presenceof conjugated π electrons along the backbone of the conductingpolymer gives them the ability to support positive as well asnegative charge carriers with high mobility along the chain.

PAni is regarded as the most salient conjugated polymer sinceit is air and moisture stable and has high electrical conductivity,in both its doped conducting form and its dedoped insulatingform.4 The aim of the present work was to enhance the propertiesof PAni by using a new dopant. A number of nanoparticles of

transition metals such as Ag, Au and Cu etc. have been usedfor incorporation as a dopant in the PAni matrix.5 Plenty ofwork on PAni nanocomposites and their applications has been

reported by various researchers,6–15 but no work has been doneon interactions of MgB2 with PAni. MgB2 materials have attracteda lot of attention because of their remarkable physical propertiesand wide possible technological applications. Special interestin crystalline MgB2 was ignited after the discovery of MgB2

superconductivity at 39 K with a high transition temperature (Tc)and a high critical current density.16 These properties have madeMgB2 a suitable material for fabricating Josephson junctions andsuperconducting nano-devices that can operate above 20 K.17

We believe that combination of MgB2 nanowires and conductingpolymer may result in new materials with exceptional properties.In this paper we report the synthesis and characterization ofPAni/MgB2 nanocomposites, which to the best of our knowledgeis reported for the first time.The method employed here offers anextremely simple and scalable route for the synthesis of PAni/MgB2

∗ Correspondenceto:MushahidHusain,DepartmentofPhysics,FacultyofNaturalSciences, Jamia Millia Islamia, New Delhi-110025, India.E-mail: mush_reslab@rediffmail.com

a Department of Physics, Faculty of Natural Sciences, Jamia Millia Islamia, NewDelhi, 110025, India

b Department of Applied Sciences and Humanities, Faculty of Engineering andTechnology, Jamia Millia Islamia, New Delhi, 110025, India

c Centre for Nanoscience & Nanotechnology, Jamia Millia Islamia, New Delhi,110025, India

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nanocomposites. Results indicate an effective interaction of PAniwith MgB2 nanowires with an increase in conductivity by 5 ordersof magnitude.

EXPERIMENTALMaterialsAniline (Merck India, AR grade) was used after distillation andstored in a refrigerator. Ammonium peroxodisulfate (APS) (MerckIndia, AR grade), hydrochloric acid (Qualigens Fine Chemicals,India, LR grade), ammonia solution in water (99%, MerckIndia), MgBr2.6H2O (Sigma-Aldrich, St. Louis, USA), NaBH4 (MerckIndia), cetyltrimethylammonium bromide (CTAB) (Merck India),tetrahydrofuran (Merck India) and ethanol (Loba Chemie, India)were used as received without further purification.

Synthesis and doping of polyanilinePAni was synthesized by the chemical polymerization methodusing APS as an oxidizing agent.18 APS was added to an acidifiedsolution of distilled aniline which was constantly stirred for 12 hat a temperature between 0 and 5 ◦C. At the end of reaction agreen colored powder, emeraldine salt, was obtained which is theconducting form of PAni. This emeraldine salt was then convertedto insulating blue emeraldine base by treating it with ammoniasolution. Emeraldine base was washed several times with distilledwater and finally dried in an oven to a powdered form.

Synthesis of MgB2 nanowiresMgB2 nanowires were prepared by the sol–gel technique in a two-step process. In this procedure, MgBr2.6H2O (2 mmol) and NaBH4

(4 mmol) were dissolved in 10 mL and 15 mL ethanol respectively;6 × 10−2 mmol of CTAB was added to each solution followed bysonication for 30 min. After sonication the bromide solution wasadded drop by drop to the sodium borohydride solution. Thisreaction was left in an open environment for about 14–15 h untila gel was obtained. This gel was loaded in a quartz tube andheated in a furnace at 810 ◦C for 10 min. Pyrolysis was carriedout in an argon atmosphere as the presence of even a minuteamount of oxygen may cause the formation of MgO. After heattreatment, the sample was slowly cooled to room temperatureover a period of 12–14 h. The resultant black powder of MgB2

nanowires was washed with deionized water and ethanol severaltimes in order to remove the impurities formed during the reaction.In this procedure, nanowires of ca 50–100 nm in diameter wereobtained. The chemical reaction taking place during the processmay be represented as

MgBr2 + 2NaBH4 → MgB2 + 2NaBr2 + 4H2 (1)

Synthesis of nanocompositesFinally, dried emeraldine base was chemically doped with differentconcentrations of MgB2 nanowires. Dried MgB2 nanowires weredispersed in tetrahydrofuran for 20 min in an ultrasonic bath atroom temperature and then added to swollen PAni (swollen by theeffect of solvent) in different ratios (90:10, 92:8, 94:6, 96:4 and 98:2by weight). This solution was stirred for 24 h at room temperature.The doped PAni/MgB2 nanocomposites were dried followed bygrinding and sieving. Samples were prepared in the form of pellets.Pellets with a diameter of 20 mm and average thickness of about1.0 mm were formed by using a hydraulic press at a pressure of767 kPa.

CharacterizationThe electrical conductivity of the pellets was measured in thetemperature range 300–450 K. The pellets were placed betweentwo steel electrodes situated inside a metallic sample holderfor conductivity measurements. A voltage of 1.5 V (through aregulated DC supply) was applied to the pellet and the resultingcurrent was measured with a Keithley electrometer model 6157A.The temperature measurements were done through a calibratedchromel alumel thermocouple mounted near the samples. Anneal-ing of the samples was done before conductivity measurements ina vacuum of 10−3 Torr, to avoid the effect of moisture absorption,up to 373 K for 1 h. I − V characteristics of all the composites weredetermined with a Keithley Semiconductor Characterization Sys-tem Model 4200-SCS at room temperature. SEM was used to studythe morphology of the sample using a JEOL JSM 6380 scanningelectron microscope at 10 kV. Fourier transform IR (FTIR) spectra ofundoped and doped PAni MgB2-KBr pellets were obtained using aPerkin Elmer FTIR spectrometer in the range from 400 to 4000 cm−1

at a resolution of 4 cm−1. The composites were dissolved in chlo-roform to obtain UV − visible absorption spectra using a CamspecM-550 double beam scanning UV − visible spectrophotometer.

RESULTS AND DISCUSSIONThe temperature dependence of the electrical conductivity of thenanocomposites follows the Arrhenius law and is given by19,20

σ = σ0 exp

(−�E

kT

)(2)

where σ 0 is the pre-exponential factor, �E is the activation energyof the electrical conduction, k is the Boltzmann constant and T is theabsolute temperature. The Meyer − Neldel (MN) rule states that ina thermally activated process the DC conductivity is exponentiallyrelated to the activation energy as given by Eqn (2). The pre-factorσ 0 correlates with the activation energy �E in the form

σ0 = σ00 exp

(�E

kT0

)(3)

where σ 00 is a constant for a set class of materials and T0 is theMN characteristic temperature.21 In organic semiconductors, theMN rule is directly related to the charge transport mechanismand possibly even to the polaronic carriers.22,23 The electricalconduction in organic semiconducting polymers having acomplex molecular structure is not completely understood.The experimental results obtained were interpreted in termsof various conduction mechanisms such as bond conduction,variable hopping conduction etc. Introduction of dopants intopolymers modifies the structure of the polymer and so a changein polaronic states occurs which brings a change in electricalconductivity. The temperature dependence of the conductivityfor PAni/MgB2 composites was measured in the temperaturerange 300–450 K and it was found that the results are morepronounced for nanocomposites than for pure PAni, whichsuggests that the delocalization of charge carriers is greater in thePAni/MgB2 nanocomposites. Furthermore, as the concentrationof MgB2 nanowires increases, the temperature dependence of theconductivity becomes stronger. The Arrhenius plot ofσ is a straightline (Fig. 1), which indicates that conduction in samples is takingplace via an activated process, having a single activation energy inthe temperature range 300–450 K. The activation energy (�E) was

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Figure 1. Temperature dependence of DC conductivity in the temperaturerange 300–450 K for undoped PAni and PAni/MgB2 nanocomposites withdifferent concentrations of MgB2 nanowires (wt%).

Figure 2. Variation of DC conductivity ln σ and activation energy �E versusundoped and doped PAni composites with different concentrations ofMgB2 nanowires (wt%) at 350 K.

determined by plotting ln σ versus 1000/T and a decrease in thevalues was observed after doping. Fig. 2 shows the DC conductivityand activation energy versus concentration of dopant. It is clearfrom the figure that the activation energy decreases whereas theconductivity increases with increase in MgB2 concentration up to10% (w/w). The lowering of the activation energy with increasingMgB2 concentration indicates a strong charge transfer caused bythe delocalization of charge carriers accumulated at the interfacealong a long range conjugated length. The values of the pre-exponential factor, �E and the DC conductivity of the undopedand doped PAni samples were calculated and are given in Table 1.

From a single value of activation energy it is clear that theconduction is through the carrier concentration at the Fermilevel. But the activation energy (�E) alone does not provide anyinformation on whether the conduction takes place in extendedstates or by hopping in localized states. This can be explainedon the basis of the values of the pre-exponential factor (σ 0).According to Mott and Davis,24,25 a value of σ 0 in the range103 − 104 S cm−1 indicates that the conduction takes place mostlyin extended states. A smaller value of σ 0 indicates a wide range oflocalized states and the conduction is taking place by a hoppingprocess. In our case, the values of σ 0 are found to be of the orderof 10−1. Therefore the conduction takes place by the hoppingprocess due to the extensive range of localized states present in

Table 1. Electrical and optical parameters for PAni/MgB2nanocomposites doped with different concentrations of MgB2 at350 K

No. Sample

DC

conductivity

σ (S cm−1)

Activation

energy

�E (eV)

Pre-exponential

factor σ 0

(S cm−1)

1 Undoped PAni 4.36 × 10−9 0.49 3.53 × 10−1

2 2% doped PAni 1.05 × 10−5 0.37 5.08 × 10−1

3 4% doped PAni 1.89 × 10−5 0.31 6.88 × 10−1

4 6% doped PAni 4.04 × 10−5 0.29 8.81 × 10−1

5 8% doped PAni 5.46 × 10−5 0.28 7.20 × 10−1

6 10% doped PAni 1.01 × 10−4 0.33 3.70 × 10−1

7 H2SO4 doped PAni 0.75 × 10−6 0.10 2.35 × 10−5

Figure 3. I − V characteristics of pristine PAni and PAni/MgB2nanocomposites.

the sample. From these results we can conclude that the hoppingmechanism is responsible for the increase in the conductivity ofall the composites. A smaller value of σ 0 indicates that the densityof defect states increases in the sample and further supportsour argument that conduction mostly takes place by a hoppingprocess in the MgB2-doped PAni.

The I–V characteristic curves for pure PAni and PAni/MgB2

nanocomposites at room temperature and in the voltage rangefrom −2 V to +2 V are presented in Fig. 3. This shows forwardand reverse bias characteristics of all the samples. The linearnature of these I–V curves indicates that all the composites followohmic behaviour and the current increases linearly with increasingapplied voltage. The I–V characteristics of the nanocompositesshow that the current level increases with increasing MgB2

concentration, which is due to the increasing contacts betweenPAni and MgB2 nanowires.

Morphology is a key factor in controlling the physicochemicalproperties of materials. The morphology of the obtainedPAni/MgB2 nanocomposites was studied through SEM and theimages are shown in Figs 4 and 5. Figures 4(a) and 4(b) showthat as-synthesized PAni consists of an irregular mixture of shortfibres and granular particles. Figures 4(c) and 4(d) indicate thatMgB2 powder is composed of nanowires with different diametersin the range from 50–100 nm to several micrometres in length.Figures 5(a) − 5(d) show low and high magnification images ofPAni/MgB2 nanocomposites with different dopant percentages. It

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(a) (b)

(c) (d)

Figure 4. (a), (b) SEM micrographs of pristine PAni and (c), (d) SEM micrographs of MgB2 nanowires at different magnifications.

(a) (b)

(c) (d)

Figure 5. SEM micrographs of PAni/MgB2 nanocomposites with different contents of MgB2: (a) 2, (b) 2, (c) 4 and (d) 8 wt%.

can be seen that there is no distinctive and dominant interparticlecontact for the polymer and dopant. It is evident from these figuresthat the resultant composites are likely to be porous in nature andare able to increase the liquid–solid interfacial area, providing apath for the movement of ions.

The molecular structure of the obtained PAni/MgB2

nanocomposites was studied in the FTIR spectra. Figure 6 shows the

FTIR spectra of all the nanocomposites. All spectra have vibrationpeaks at 823, 1143, 1300, 1398, 1504 and 1589 cm−1, whichcorrespond to most of the characteristic peaks for PAni26 revealingthat PAni exists in the composite. The strong absorption peakscorresponding to the stretching vibration of quinonoid rings areassigned at 1589 cm−1, the stretching vibration of benzenoid ringsat 1504 cm−1, C − N stretching mode at 1300 and 1389 cm−1, the

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Figure 6. FTIR spectra of doped PAni at different MgB2 nanowireconcentrations (w/w).

Figure 7. UV−visible absorption spectra of undoped PAni and PAni dopedwith different concentrations of MgB2 nanowires.

characteristic vibrational mode of quinoid rings at 1143 cm−1 andthe C − H out of plane bending of para-disubstituted benzene ringsis assigned at 823 cm−1. However, it is observed that the spectrumof the PAni/MgB2 nanocomposites shows no difference comparedwith that of pure PAni except for the reduced peak intensity. Thereduced peak intensity may result from the interaction betweenPAni and MgB2 even when there are only small amounts of MgB2

in the composites.Typical UV–visible spectra of PAni and PAni/MgB2 nanocom-

posites are shown in Fig. 7. The absorption bands at around 300 nmand 360 nm can be attributed to inter-band n−π* transition ofexcitons in the aromatic ring and charge transfer π−π* transitionof aromatic rings and charge transfer groups respectively.27 Thereis no shift in the π−π* transition of the PAni/MgB2 nanocom-posites with respect to pure polymer. By taking into account thatthe excitation of π electrons requires smaller energy, the bandgap was calculated for the π−π* transition by means of the Taucequation:28

α =(

hν − Eg)n

hν(4)

where α is the absorption coefficient, h is Planck’s constant, ν

is the photon frequency, Eg is the band gap and n takes into

Figure 8. The photon energy hν versus (αhν)1/2.

account different possible electronic transitions responsible forthe light absorption. The index n has discrete values such as 1/2,3/2, 2 or more depending on whether the transition is direct orindirect and allowed or forbidden. In the direct and allowed cases,the index n is 1/2, whereas for the direct but forbidden cases itis 3/2, but for the indirect and allowed cases n = 2 and for theforbidden cases it is 3 or more. In the present case, the photonenergy is plotted against αhν and for n = 2; it gives a straightline fit, which implies that the samples obey indirect transition.From the absorption spectra the band gaps of all the samples areestimated by plotting energy hν versus (αhν)1/2 (Fig. 8). The bandgaps exhibit decreasing orders for all the composites. The decreasein band gap is due to the insertion of doping agents inside thepolymer material. The extinction coefficient is determined by therelation

k = αλ/4π (5)

where λ is the wavelength of the incident photons and α isthe absorption coefficient. The measured values of the opticalband gap and the extinction coefficient at different dopant ratiosare given in Table 2. Since the optical absorption dependson the short range order in the amorphous state and thedefect state associated with it, the decrease in the optical bandgap in the present system may be due to reduction in thedisorderliness of the system and increase in the density of defectstates.

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Table 2. Optical parameters for PAni/MgB2 nanocomposites dopedwith different concentrations of MgB2 at wavelength 550 nm

No. Samples

Optical band

gap Eg (eV)

Extinction

coefficient k (at 550 nm)

1 Undoped PAni 1.39 9.38

2 2% doped PAni 0.96 16.83

3 4% doped PAni 0.83 8.60

4 6% doped PAni 0.78 6.58

5 8% doped PAni 0.48 12.73

6 10% doped PAni 0.27 19.43

CONCLUSIONSPAni/MgB2 nanocomposites with different amounts of MgB2 weresuccessfully synthesized in a simple and template-free method.The diameter of the MgB2 nanowires is in the range 50–100 nm.The presence of MgB2 leads to structural and electrical changesin the polymer, which induce changes in electrical and opticalproperties of the composites. The FTIR spectra exhibit structuralchanges in the backbone of the PAni as the doping processproceeds and UV − visible spectra reveal that an indirect opticaltransition exists in the composites. Their resultant conductivityis found to increase by 5 orders of magnitude confirming theirsemiconducting nature.

ACKNOWLEDGEMENTSOne of the authors, Shumaila, is grateful to the University GrantsCommission (UGC) for providing financial support in the form of aSenior Research Fellowship (MANF-SRF).

REFERENCES1 Olson DC, Piris J, Colins RT, Shaheen SE and Ginley DS, Thin Solid Films

496:26–29 (2006).2 Zu ZX, Roy VAL, Stallinga P, Muccini M, Toffanin S, Xiang HF et al., Appl

Phys Lett 90: 223509(1–3) (2007).

3 Yoshino K, Kajii H and Araki H, Fullerene Sci Technol 7:695–711 (1999).4 Huang J, Pure Appl Chem 78:15–27 (2006).5 Khanna PK and Kulkarni DK, Synthesis and Reactivity in Inorganic, Metal

Organic and Nano Metal Chemistry 38:459–463 (2008).6 Gu H, Guo J, Sadu R, Huang Y and Haldolaarachchige N, Appl Phys Lett

102:212403–212404 (2013).7 Gu H, Guo J, He Q, Tadakamalla S, Zhang X, Yan X et al., Ind Eng Chem

Res 52:7718–7728 (2013).8 Gu H, Guo J, Zhang X, He Q, Huang Y, Colorado HA et al., J Phys Chem

C 117:6426–6436 (2013).9 Zhu AJ, Chen M, Qu H, Z Xi, Wei H, Luo Z et al., Polymer 53:5953–5964

(2012).10 Zhang X, Wei S, Haldolaarachchige N, Colorado HA, Luo Z, Young DP

et al., J Phys Chem C 116:15731–15740 (2012).11 Zhang X, Zhu J, Haldolaarachchige N, Ryu J, Young DP, Wei S et al.,

Polymer 53:2109–2120 (2012).12 Zhang X, He Q, Gu H, Wei S and Guo Z, J Mater Chem C 1:2886–2899

(2013).13 Z Xi, He Q, Gu H, Colorado HA, Wei S and Guo Z, ACS Appl Mater

Interfaces 5: 898–910 (2013).14 Gu H, Tadakamalla S, Zhang Xi, Huang Y, Jiang Y, Colorado HA et al., J

Mater Chem 1:729–743 (2013).15 Gu H, Guo J, Wei S and Guo Z, J Appl Polym Sci 130:2238–2244 (2013).16 Sorokin PB, Chernozatonskii IA, Avramov PA and Yakobson BI, J Phys

Chem C 114:4852–4856 (2010).17 Seong WK, Huh JY, Jung SG and Kang WN, JKoreanPhysSoc 51:174–177

(2007).18 Wei Y, Jang GW, Hsuesh KF, Scherr EM, MacDiarmid AG and Epstein

AJ, Polymer 33:314–322 (1992).19 Khan ZH, Malik M, Zulfequar M and Husain M, J Phys Condens Mater

7:8979–8991 (1995).20 Shumaila, Lakshmi GBVS, Alam M, Siddiqui AM, Zulfequar M and

Husain M, Sci Adv Mater 4:227–231 (2012).21 Abdel-Wahab F, Turk J Phys 28:133–138 (2004).22 Rusu M, Caplanus I, Mardare D and Rusu GI, J Optoel Adv Mater

7:3149–3154 (2005).23 Meijer EJ, Matters M, Herwig PT, Leeuw DM and Klapwijk TM, Appl Phys

Lett 76:3433–3435 (2000).24 Mott NF and Davis EA, Electronic Processes in Non Crystalline Materials,

2nd edn. Clarendon, Oxford, Ch. 9 (1979).25 Mott NF and Davis EA, Philos Mag 22:903–922 (1970).26 Asturias GE, MacDiarmid AG, MacCall RP and Epstein AJ, Synth Met

29:E157−E162 (1989).27 Ameen S, Lakshmi GBVS and Husain M, J Phys D: Appl Phys 42:

105104(1–5) (2009).28 Pal M, Hirota K and Sakata H, Phys Status Solidi A 196:396–404 (2003).

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