J Supercond Nov MagnDOI 10.1007/s10948-013-2376-0

O R I G I NA L PA P E R

Polyaniline–MnFe2O4-CTAB Nanocomposite in Ionic Liquid:Electrical Properties

S. Shafiu · B. Ünal · A. Baykal

Received: 13 September 2013 / Accepted: 15 September 2013© Springer Science+Business Media New York 2013

Abstract Polyaniline–MnFe2O4-CTAB nanocompositewas successfully synthesized by using 1-butyl-3-methyl-imidazolium trifluoromethane sulfonate (RTILs) as ionicliquid and Cetyl trimethylammonium bromide (CTAB) assurfactant via in-situ polymerization. The calculated aver-age crystallite size, DXRD, of the product was 26 ± 4 nm.Conductivity and permittivity properties of Polyaniline–MnFe2O4 nanocomposite was also exemplified by meansof an impedance spectroscopy, which would be evaluatedat frequency ranges up to 3 MHz for temperature range of20–120 °C. In general, ac conductivity remains almost un-changed until it reaches up to 160 kHz, and then reducesslightly almost for all temperatures except for some slightfluctuation somehow at lower temperatures. The values fluc-tuate between 1.1–1.6 mS/cm at above all temperatures.

Keywords Ionic liquid · Conductivity · Dielectricproperties · MnFe2O4 · Nanocomposite

S. ShafiuKano Univ. Science and Techn., Wudil, Kano State, Nigeria

S. Shafiu · A. BaykalChemistry Department, Fatih University, 34500B. Çekmece-Istanbul, Turkey

B. ÜnalDepartment of Electrical & Electronics Engineering, FatihUniversity, 34500 B. Çekmece-Istanbul, Turkey

B. Ünal · A. Baykal (B)BioNano Technology R&D Center, Fatih University, 34500B. Çekmece-Istanbul, Turkeye-mail: hbaykal@fatih.edu.tr

1 Introduction

Nowadays, inorganic-organic nanocomposite with a welldefined electrical and magnetic characteristics has shownso much concern due to their distinct electrical, magneticas well as optical properties and how they plays an impor-tant role in the present world’s technology such as chemi-cal sensors, photoelectric device [1, 2], and electromagneticinterference (EMI) shielding [3]. The use polymers duringthe synthesis of nanocomposite are not only aim at obtain-ing the compounds, but also it plays an important role forthe size control and preventing the resulting metal-oxygennanocompound from agglomeration, which is not requiredin this research. In addition to that, the use of polymer in-creases the chemical stability of the crystalline nanoparticlesobtained [4, 5].

Being a magnetic nanocrystalline compound, MnFe2O4

may be used as an ultrasensitive MR imaging probe. Thisis due to its excellent saturation magnetization, high initialpermeability, and high resistivity in comparison to other fer-rites [6, 7]. Moreover, magnetic nanocrystalline compoundhas a wide range of application in high density informationstorage, microwave devices, permanent magnets, magneticfluids, and drug delivery [8, 9].

Among conducting polymers, polyaniline (PANI) has astable band gap of 2.8 eV showing strong absorption forvisible light; many people used it in photocatalysis [10, 11],doped polyaniline has an optical band gap 2.21 eV and roomelectrical conductivity (σAC = 3.12 × 10−2 (� cm)−1 [12].Due to its ease of preparation, low cost, light weight, goodelectronic, optical properties its stability in air as well asthe solubility in so many solvent make it useful in differentaspect in modern technology such as biosensors, corrosiondevices, microwave absorption, optoelectronic device, elec-trochromic displays, and chemical sensors [13, 14]. Some

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conjugated polymers like polyaniline have excellent thermalstability and good oxidation resistance; this make it suit-able as electrode materials in electro-catalysis in solar en-ergy conversion [15, 16]. Having an electrical conductivityof 10−10–10−2 (� cm)−1 PANI is used as hole injection lay-ers in flexible light emitting diodes [17, 18].

CTAB as cationic detergent, it is soluble in water andalso used in the synthesis of crystalline nanocompound. Asa surfactant, it may acts as a surface coating to the resultingnanomaterials so as to prevent it from agglomeration beingmagnetic in nature, thereby controlling the size of the nano-materials [19, 20].

In the present work, we describe a facile and eco-friendlymethod of synthesizing PANI–MnFe2O4 nanocompositeswith the assistance of (1-butyl-3-methyl-imidazolium tri-fluoromethane sulfonate) as ionic liquid. The effect ofimidazolium-based ionic liquid on electrical properties ofPANI–MnFe2O4 nanocomposite was investigated in de-tail. This route can be called as green synthesis due to itsreduced pollution effect during the synthesis. Both waterand -(1-butyl-3-methyl-imidazolium trifluoromethane sul-fonate) as room temperature ionic liquid (RTILs), which areenvironmentally benign solvents were used.

2 Experimental

2.1 Chemicals

Iron (III) chloride hexahydrate (98 %), FeCl3·6H2O (98 %),manganese chloride (98 %), MnCl2·4H2O, aniline monomer(≥99.5 %), Cetyl trimethylammonium bromide (CTAB)(99 %) and Sodium hyroxide (NaOH) were all obtainedfrom Merck and 1-butyl-3-methyl-imidazolium trifluoro-methane sulfonate (RTILs) (99 %) from Alfa–Aesar. Theywere used as-received, without further purification.

2.2 Instrumentations

X-ray powder diffraction (XRD) analysis was conducted ona Rigaku Smart Lab operated at 40 kV and 35 mA usingCu Kα radiation (λ = 1.54059 Å).

Fourier transform infrared (FT-IR) spectra of the sampleswere recorded with a Perkin Elmer BX FT-IR infrared spec-trometer in the range of 4000–400 cm−1.

The electrical conductivities of the PANI–MnFe2O4-CTAB nanocomposite in RTILs was studied in the tempera-ture range of 20–120 ◦C with a heating rate of 10 ◦C/s. Thesample was used in the form of circular pellets of 13 mm di-ameter and 3 mm thickness. The pellets (both nanocompos-ite and pristine) were sandwiched between gold electrodesand the conductivities were measured using Novocontrol

dielectric impedance analyzer in the frequency range 1 Hz–3 MHz, respectively. The temperature (between −100 and250 ◦C) was controlled with a Novocool Cryosystem.

Transmission electron microscopy (TEM) analysis wasperformed using a FEI Tecnai G2 Sphera microscope.A drop of diluted sample in alcohol was dripped on a TEMgrid.

The thermal stability of the nanocomposite was deter-mined by thermo-gravimetric analysis (TGA, Perkin ElmerInstruments model, STA 6000). The TGA thermogramswere recorded for 5 mg of powder sample at a heating rate of10 ◦C/min in the temperature range of 30 ◦C–750 ◦C undernitrogen atmosphere.

2.3 Procedure

2.3.1 Synthesis of MnFe2O4 NP’s

MnFe2O4 NP’s were prepared by the hydrothermal methodusing Cetyl trimethylammonium bromide (CTAB) as thesurfactant. 0.01 mol MnCl2·6H2O and 0.02 mol FeCl3·6H2Ometal precursors were dissolved in 50 ml distilled water and0.7 g CTAB was also dissolved in 20 ml water. Then twosolutions were mixed and its pH was arranged with 2 MNaOH until to pH = 11 under vigorous stirring at 80 ◦C for1 h. The obtained clear solution was transferred into 50 mLstainless steel autoclave; after sealing, the autoclave was putinto an oven heated at 180 ◦C for 12 h, and then cooled nat-urally to room temperature. Finally, the product was washedwith distilled water and ethanol several times to remove theimpurities and dried in an oven at 80 ◦C for 4 h.

2.3.2 Synthesis of PANI–MnFe2O4-CTAB Nanocompositein RTILs

1 g as-prepared MnFe2O4 NP’s and 5.0 mL 1-butyl-3-methyl-imidazolium trifluoromethane sulfonate (RTILs),were dispersed in dilute 30 ml 0.001 M HCl solution in athree-neck round-bottomed flask fitted with ultrasonic vi-bration for 1 h, then 1.8 mL aniline monomer was added tothe above mixture, and an ultrasonic vibration was contin-ued for another 30 min. The reaction system was then cooledin an ice bath. Under the protection with nitrogen gas, theammonium peroxydisulfate (5 g, dissolved in 1.8 M HClsolution), which serves as an oxidant, was added drop-wiseinto the above mixture. The reaction was continued for 18 hat 0 ◦C (Scheme 1).

3 Results and Discussion

3.1 XRD Analysis

The XRD pattern of PANI-MnFe2O4-CTAB nanocompositein RTILs was presented in Fig. 1. All of the observed diffrac-tion peaks are indexed by the cubic structure of MnFe2O4

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Scheme 1 The synthesis of PANI–MnFe2O4-CTAB nanocomposite in RTILs

Fig. 1 XRD powder pattern of PANI–MnFe2O4-CTAB nanocompos-ite in RTILs

spinel (JCPDS card no. 10-0319) phase. The broadeningof the diffraction peaks distinctly indicates the nanocrys-talline nature of the materials. To determine the crystallitesize of the sample, the XRD profile was fitted according tothe Eq. (1) in Wejrzanowski et al. [21] and Pielaszek [22],which allows the estimation of average crystallite size andits standard deviation from XRD. The experimental line pro-file, shown in Fig. 1 was fitted for seven peaks (220), (311),(400), (105), (312), (511), and (440). The calculated averagecrystallite size, DXRD, of the product was 26 ± 4 nm.

Fig. 2 FT-IR spectra of (a) CTAB, (b) PANI and (c) PANI–MnFe2O4-CTAB nanocomposite in RTILs

3.2 FT-IR Analysis

The FT-IR spectra of PANI–MnFe2O4-CTAB nanocompos-ite in RTILs, CTAB, and PANI were presented in Fig. 2a,b, and c, respectively. The symmetric and asymmetric C–H scissoring vibrations of a CH3–N+ moiety between pureCTAB molecules (1482, 1430 cm−1) (Fig. 2a) was alsoobserved in the FT-IR spectra of nanocomposite (Fig. 2c)[23]. The peaks at 1564 and 1488 cm−1 are attributed tothe characteristic C=C stretching of the quinoid and ben-zenoid rings, the peaks at 1303 and 1246 cm−1 are assignedto C–N stretching of the benzenoid ring, the broad peak at1143 cm−1 [24]. The peak at around 566 cm−1 due to the

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Fig. 3 TEM images ofPANI–MnFe2O4-CTABnanocomposite in RTILs withdifferent magnifications

strong M–O absorption band [24], along with the FT-IR fea-tures of PANI molecules for the NPs further supports the sta-bilizing role played by PANI. Conjugation scheme of PANIonto the surface of MnFe2O4 is presented in Scheme 1.

3.3 TEM analysis

Morphology of PANI–MnFe2O4-CTAB nanocomposite inRTILs has been investigated by TEM and few micrographstaken at various magnifications are presented in Fig. 3.MnFe2O4 nanoparticles were observed to have a mixtureof near spherical and polygonic morphology with particleshaving sizes in the range of 10 and 150 nm. Nanoparticlesare visible in the micrographs, which in turn will affect themagnetic interaction between the nanoparticles.

3.4 Electrical and permittivity properties

The conductivity and permittivity of PANI–MnFe2O4-CTAB nanocomposite in RTILs were quantified after two-electrode connection was used for measuring at different

Fig. 4 Plot of ac conductivity of PANI–MnFe2O4-CTAB nanocom-posite in RTILs at different temperature ranging from 20 to 120 ◦C infrequency range of up to 3 MHz

temperatures ranging from 20 to 120 ◦C in a frequencyrange of up to 3 MHz. The ac conductivities of samples

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Fig. 5 Plot of dc conductivity of PANI–MnFe2O4-CTAB nanocom-posite in RTILs against reciprocal temperature together with activationenergies given

used in this study are depicted in Fig. 4. It is seen clearlythat conductivity in general remains almost unchanged untilit reaches up to 160 kHz, and then reduces slightly almostfor temperatures over 50 ◦C, except for tremendous fluctu-ations somehow at lower temperatures. Conductivity variesbetween values of 1.15–1.55 mS/cm at all temperatures forall frequency ranges interested.

The dc conductivity of PANI–MnFe2O4-CTAB nanocom-posite in RTILs in the form of Arrhenius plots is shown inFig. 5 together with two activation energies. The activationenergies extracted from a linear fitting at two temperatureranges are given with values of 10 µeV and 1.23 meV obey-ing a standard exponential decay form as expected. This canbe followed by a standard correlation of activation energy asshown in the following Arrhenius formula:

σdc(T ) = σ(0) exp

[−Ea

kT

]

In accordance with Arrhenius plots, the activation energy isbest regarded as an experimentally resolved parameter thatreveals the sensitivity of the reaction rate to temperature.It shows that ac conductivity almost unchanged with fre-quency up to 3,0 MHz as mentioned earlier, yet, initiallyfluctuates at lower temperatures, and then slightly increaseswith temperatures up to 120 ◦C. As explained in our previ-ous study [24], this type of characteristic performance con-firms that conductivity is irrelevant to hopping-style con-duction, but is governed by temperature-assisted conductionmechanisms being more predominance in this manner.

Real component of permittivity of PANI-MnFe2O4-CTAB nanocomposite in RTILs is illustrated in Fig. 6 asa function of angular frequency up to f = 3 MHz for tem-peratures ranging from 20 to 120 ◦C with an increment of

Fig. 6 Plot of real component of the permittivity of PANI–MnFe2O4-CTAB nanocomposite in RTILs against angular frequency for tempera-tures up to 120 ◦C with an interval of 10 ◦C. The inset is the magnifiedpart of lower frequency range to make fluctuations clear

10 ◦C. The inset of the figure represents the magnified fluc-tuations of real permittivity at a lower frequency range. Itcan be seen clear that the real part of the permittivity atlower frequencies fluctuates at all temperatures while it has atemperature independency in medium and higher frequency.In these frequencies, it shows how ac electric field affectsand, is less affected by our dielectric medium. So, less ef-fective real permittivity exists because of less polarizationeffects at higher frequency. The real part of permittivity, rel-evant to the stored energy with the nanocomposites, can beseen that nanocomposite structure at lower temperatures isquite fragile because of noncompleted settling down processduring temperature increments. Long range order processesbecomes dominated structurally, which makes fluctuationshigher.

Plot of an imaginary component of the permittivity ofPANI–MnFe2O4-CTAB nanocomposite in RTILs againstangular frequency up to 3 MHz for temperatures from 20 to120 ◦C with an interval of 10 ◦C is illustrated in a full-scaledlogarithmic form in Fig. 7. It can be simply expressed thateach of the relevant curves obeys the power law of a unityfor all temperatures up to 120 ◦C with a slight shifting-upwith an increment of temperature. From tangent of the plot,it can be expressed as follows [25, 26]:

ε(ω,T ) = ε(0, T )ω−s

with a unity of slope s, which means that imaginary com-ponent of permittivity is said to be inversely proportional tothe frequency. Here, ω (= 2πf ) is the angular frequency,and ε(0, T ) is a constant frequency-independent coefficient,which is within our expectation from our normal nanocom-posite attitudes [27]. This shows that how the dissipation ofenergy of PANI–MnFe2O4-CTAB nanocomposite in RTILs

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Fig. 7 Plot of imaginary component of the permittivity ofPANI–MnFe2O4-CTAB nanocomposite in RTILs against angular fre-quency up to 3 MHz for temperatures from 20 to 120 ◦C with an inter-val of 10 ◦C

varies with frequencies of electric field applied, and almostno influence is recorded for all temperatures.

4 Conclusion

Polyaniline–MnFe2O4-CTAB nanocomposite was success-fully synthesized in ionic liquid. Structural, spectroscopic,and morphological investigations were done by XRD, FT-IR, and TEM respectively. The complex dielectric behav-iors and its electrical properties can be elucidated with areality of temperature-dependent conduction mechanisms.It is revealed that ac conductivity versus frequency up to3.0 MHz remains almost unchanged, and yet, increasesslightly with temperatures up to 120 ◦C, except for fluctu-ations at lower temperatures. This kind of characteristic per-formance proves that conductivity is irrelevant to hopping-type conduction because of independency to frequency and,however, is governed by temperature-assisted conductionmechanisms being more prevalent in this manner.

Acknowledgements This work is supported by Fatih University un-der BAP Grant no. P50021203_Y (2282).

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