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Synthesis and Characterization of Multiwall-Carbon NanotubesDecorated with Nickel Ferrite Hybrid

B. Unal • A. Baykal • M. Senel • H. Sozeri

Received: 15 October 2012 / Accepted: 19 November 2012 / Published online: 2 December 2012

� Springer Science+Business Media New York 2012

Abstract Acid functionalized multiwall carbon nanotube,

(MWCNT)-COOH/nickel ferrite (NiFe2O4), magnetic hybrids

were synthesized by a co-precipitation method. X-ray dif-

fraction, Fourier transform infrared spectrometry, thermal

gravimetry, transmission electron microscopy, vibrating sam-

ple magnetometry and Impedance Spectroscopy were used to

characterize the physical and electrical properties of the

MWCNT-COOH/NiFe2O4 hybrid. NiFe2O4 NPs are stably

attached to the surface of via carboxyl groups (COOH). The

magnetic saturation value of the product was found as 8 emu/g.

A tunneling conduction mechanism was believed to occur in

the hybrid. The real modules (M0) of the product illustrate

power law variation with a power exponent of approximately

unity. These magnetic MWCNT-COOH/NiFe2O4 hybrids

exhibit a promising prospective in the application of bio-

nanoscience and technology.

Keywords Magnetic materials � Chemical synthesis �Impedance spectroscopy � Dielectric properties � Electrical

1 Introduction

MWCNT hybrid materials are important materials due to

their promising applications in fields such as catalysis,

biosensors, magnetic data storage, and electronic devices

[1]. For the synthesis of magnetic hybrid materials;

Fe3O4, CoFe2O4 and NiFe2O4 etc. are used. Nickel ferrite

(NiFe2O4) has attracted considerable attention owing to its

broad application in several technological fields involving

Ferro fluids, catalysts, magnetic drug delivery, and mag-

netic high-density information storage [2–4]. NiFe2O4 is a

soft ferrite with low coercivity, high saturation magneti-

zation, chemical stability and electrical resistivity, which

can make it an excellent material for magnetic resonance

imaging (MRI) enhancement, magnetic recording media

and electronic devices [5–7].

The applications of materials reinforced with carbon

nanotubes (CNT) take advantage of the novel properties of

these nanomaterials that in low concentration, if they are well

dispersed, may enhance and modify the mechanical, electric,

dielectric, thermal, and crystallization properties of the host

material due to their high surface area, ultra light, unique

electronic properties, high mechanical strength, and excellent

chemical and thermal stability [8–12]. Decoration of CNTs

with magnetic nanoparticles, such as coating or loading

CNTs with d-Fe2O3, NiFe2O4 and Fe3O4 [13], can improve

or impart new optical, magnetic and electrochemical prop-

erties of CNTs. Theoretical and experimental results show

superior electrical properties of CNTs with an electric-cur-

rent carrying capacity 1,000 times higher than copper wires

[14]. Therefore it can be foreseen that CNTs will find

important applications as additives to improve electrical

properties of hybrid materials. The matrices used in CNTs-

incorporated hybrids for improved electrical properties can

be polymer, metal or metal oxide [15, 16].

B. Unal

Department of Electrical and Electronic Engineering,

Faculty of Engineering, Fatih University, B. Cekmece,

34500 Istanbul, Turkey

A. Baykal (&) � M. Senel

Department of Chemistry, Faculty of Art and Science,

Fatih University, B. Cekmece, 34500 Istanbul, Turkey

e-mail: [email protected]

H. Sozeri

TUBITAK-UME, National Metrology Institute, 41470 Gebze,

Kocaeli, Turkey

123

J Inorg Organomet Polym (2013) 23:489–498

DOI 10.1007/s10904-012-9803-8

Hua et al. [17] stated that the synthesis of nanosized

magnetic materials has been a field of intense study owing

to their biocompatibility and potential applications in

cancer therapy, magnetic fluid separation, electrophoto-

graphic developer, extraction, and so on [18–21]. There-

fore, it can be foreseen that magnetic nanoparticles/

MWCNT hybrid could have potential applications as a

novel magnetic material by combining the superparamag-

netism of NiFe2O4 nanoparticles and the outstanding

mechanical properties of MWCNTs.

In this study, an in situ hydrothermal method was

employed to synthesize novel carbon MWCNT-NiFe2O4

hybrid. This study would help to develop a new hybrid

material with excellent electrical properties employed in

CNT-based electronic devices. A specific attention was

paid on the effect of surface oxidation on the preparation

and properties of the hybrids. Furthermore, the micro-

structure and electrical conductivity of the as-prepared

hybrids were examined from the viewpoint of the surface

treatment of CNTs.

2 Experimental

2.1 Materials and Instrumentations

MWCNT was provided from Cheap-Tube Inc. from USA.

Ferric chloride hexahydrate and ferrous chloride tetrahy-

drate were purchased from Fluka and used without further

purification. All other chemicals were of analytical grade

and were used without further purification.

2.2 Instrumentations

X-ray powder diffraction (XRD) analysis was conducted

on a Rigaku Smart Lab Diffractometer operated at 40 kV

and 35 mA using Cu Ka radiation.

Transmission electron microscopy (TEM) analysis was

performed using a JEOL JEM microscope. A drop of

diluted sample in alcohol was dripped on a TEM grid.

The thermal stability was determined by thermogravi-

metric analysis (TGA, Perkin Elmer Instruments model,

STA 6000). The TGA thermograms were recorded for

5 mg of powder sample at a heating rate of 10 �C/min in

the temperature range of 30–800 �C under nitrogen

atmosphere.

Fourier transform infrared (FT-IR) spectra were recor-

ded in transmission mode with a Perkin Elmer BX FT-IR

infrared spectrometer. The powder samples were ground

with KBr and compressed into a pellet. FT-IR spectra in

the range 4,000–400 cm-1 were recorded in order to

investigate the nature of the chemical bonds formed.

The real (e0) and imaginary (e00) parts of complex

dielectric permittivity e� ¼ e0 xð Þ þ ie00 xð Þ½ � were measured

with a Novocontrol dielectric-impedance analyzer. The

dielectric data (e0, e00) were collected as a function of

temperature and frequency. The films were sandwiched

between gold blocking electrodes and the conductivities

were measured in the frequency range of 0.1 Hz–3 MHz at

10 �C intervals. The temperature was controlled between

20 and 120 �C with a Novocontrol cryosystem.

VSM measurements were performed by using a

Vibrating sample magnetometer (LDJ Electronics Inc.,

Model 9600). The magnetization measurements were car-

ried out in an external field up to 15 kOe at room

temperature.

2.3 Preparation of MWCNT-COOH/NiFe2O4 Hybrid

The MWCNTs were carboxylic acid functionalized as

follows: 1 g of MWCNTs was added in a mixture of

concentrated nitric acid (10 ml) and sulfuric acid (30 ml)

and ultrasonicated over 12 h, then added 100 ml of dis-

tilled water into the mixture. It was then centrifuged,

washed with distilled water until the dispersion turned

neutral and dried under vacuum at 50 �C over night. The

MWCNT-COOH/NiFe2O4 hybrid was prepared according

to the following procedure. Briefly, 0.5 g of preoxidized

MWCNTs was dispersed in 200 ml of distilled water in an

ultrasonic bath for 20 min. Then 3.5 g of FeCl3•6H2O was

added under stirring. After the mixture was stirred vigor-

ously for 30 min under N2 atmosphere, 0.65 g of NiCl2was added and continued stirring under N2 atmosphere for

30 min. Then, the MWCNT-COOH/NiFe2O4 hybrid was

obtained in a reactor with high-speed mechanical stirring

by adding the 0.5 M NaOH solution, which was preheated

to 70 �C before the co-precipitation reaction. N2 was used

during the reaction to prevent critical oxidation. Black

hybrid product was collected by sedimentation with a help

of an external magnetic field and washed several times with

Milli-Q water until stable ferrofluid was obtained. Finally,

the MWCNT-COOH/NiFe2O4 hybrid was dried under

vacuum at 50 �C over night (Fig. 1). MWCNTs were

adhered by carbonyl and carboxyl groups, which was

demonstrated to be an efficient method for increasing sol-

ubility, chemical reactivity and surface roughness of

MWCNTs [22, 23]. This process also played an important

role on the deposition of metal ions. Ni2?, and Fe3? were

added in the reaction solution, and then adhered on the

surface of MWCNTs with high density of carbonyl and

carboxyl groups due to electrostatic interaction between the

charged species [23–25]. With the extension of there action

time, negatively charged functional groups acted as the

nucleation center and the nuclei could initiate the growth of

490 J Inorg Organomet Polym (2013) 23:489–498

123

NiFe2O4 nanocrystallites which were attached tightly to

MWCNTs [23].

3 Result and Discussion

3.1 FT-IR Analysis

The FT-IR spectra of pristine and surface oxidized MWCNTs

are shown in Fig. 2. The absence of hydroxyl groups and

carbonyl groups in the pristine MWCNTs is obvious from the

FT-IR spectrum (Fig. 2a). On the other hand, characteristic

bands due to generated polar functional groups are observed in

the FT-IR spectrum of the MWCNTs after chemical oxidation

in H2SO4/HNO3 (mOH = 3,433 cm-1, mC=O = 1,706 cm-1,

mHbonded (C=O) = 1,635 cm-1 m–C=C– = 1,570 cm-1, and

mC–C–C) = 1,180 cm-1 [13, 26, 27]. FT-IR was used to char-

acterize the molecular structure of MWCNT-COOH/NiFe2O4

hybrid and preoxidized MWCNT. A characteristic peak of

MWCNTs at 1,570 cm-1 ascribed to C = C bonding is also

present. The absorption band at 592 cm-1 is associated with

the Fe–O stretching vibrations related to the magnetite

nanoparticles [28, 29].

3.2 XRD Analysis

The XRD powder pattern and line profile fitting of

MWCNT and MWCNT-COOH/NiFe2O4 hybrid were

presented in Fig. 3a and b respectively. The diffraction

peaks at 2h = 26.088 and 43.088 are attributed to the

graphite structure (002) and (100) planes of the MWCNTs

[1, 30]. It can be seen that almost all the diffraction peaks

of NiFe2O4 can be assigned to spinel-type NiFe2O4

(JCPDS 54-0964). The peaks at the 2h values of 35.44�,

43.4�, 47.0�, 53.86�, 57.3�, and 62.2o can be indexed to

Fig. 1 Simplified representation of the preparation of MWCNT-COOH/NiFe2O4 hybrid

4000 3500 3000 2500 2000 1500 1000 500

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

(a)

(b)

3433

17061570 1180

Fig. 2 FT-IR spectrum of a MWCNT-COOH/NiFe2O4 hybrid and

b MWCNT-COOH

20 30 40 50 60 70

(b)

(a)

2 Theta (deg.)

CP

S (

a.u

.) 400 44

0

511

422

311

MW

CN

T

D= 17+7 exp. line profile fit

331

–

Fig. 3 XRD powder pattern and line profile fitting of a MWCNT-

COOH b MWCNT-COOH/NiFe2O4 hybrid

J Inorg Organomet Polym (2013) 23:489–498 491

123

(311), (400), (331), (422), (511) and (440) crystal planes of

spinel NiFe2O4, respectively[31]. This indicates that

incorporation of a small amount of MWCNT into NiFe2O4

cannot lead to the change of its phase. The mean size of the

crystallites of MWCNT-COOH/NiFe2O4 hybrid (assuming

spherical morphology) were estimated from the diffraction

pattern given below by line profile fitting using the Eq. (1)

given in [32] and calculated as 17 ± 7 nm.

3.3 TG Analysis

TGA was used to characterize the NiFe2O4 NPs content in

the as-obtained hybrids. For the nickel ferrite sample

(Fig. 4a), the weight increase at a broad temperature is

attributable to the oxidation of the NiFe2O4. The TGA

curve of MWCNTs (Fig. 4c) shows mass loss of about

13 % due to the removal of absorbed water and the func-

tional groups on the surface of MWCNT-COOH. The

35 %wt loss was observed for MWCNT-COOH/NiFe2O4

hybrid in the temperature range of 100–700 �C. Up to

150 �C, the observed weight loss is due to absorbed water.

The additional weight loss after 200 �C (in the temperature

range of 635–730 �C) is caused by the oxidization of the

nanotubes and decomposition of organic layer on the sur-

face of organic layer. Compared with the pristine MW

CNT-COOHs, the MWCNT-COOH/NiFe2O4 hybrid lost

its weight before 700 �C (Fig. 4b), leaving NiFe2O4 resi-

due weight of around *35 % for MWCNT-COOH/

NiFe2O4 hybrid. This is attributed to the catalytic role of

metal oxide nanoparticles in the oxidation of carbon

materials [33].

3.4 TEM Analysis

The morphology and distribution of nickel ferrite nano-

particles on the MWCNTs-COOH can be further explored

by TEM. As shown in Fig. 5, it can be seen that the

MWCNT-COOH have been covered with NiFe2O4 nano-

particles. It should be pointed out that NiFe2O4 nanopar-

ticles are anchored on the surface of MWCNT-COOH even

after ultrasonic treatment. The strong combination of

NiFe2O4 NPs and MWCNT-COOH can be attributed to

electrostatic interaction through the functional groups on

the surface of the modified MWCNT-COOH.

3.5 Magnetization

VSM was used to record hysteresis loops of the NiFe2O4

nanoparticles and the synthesized MWCNT-COOH/NiFe2O4

hybrid (Fig. 6). The saturation magnetization of MWCNT-

COOH/NiFe2O4 hybrid is *8 emu/g, significantly smaller

than those of bulk NiFe2O4 (80 emu/g) [34]. The reduction in

the saturation magnetization can be attributed to a low con-

tent of NiFe2O4 in the hybrid. MWCNT-COOH/NiFe2O4

hybrid shows super paramagnetic behavior at 300 K with

0 100 200 300 400 500 600 700 80050

60

70

80

90

100

110

(a)

(c)

(b)

Wei

ght l

oss

%

Temparature °C

Fig. 4 TGA thermograms of a NiFe2O4 NPs, b MWCNT-COOH and

c MWCNT-COOH/NiFe2O4 hybrid

Fig. 5 TEM micrographs of

MWCNT-COOH/NiFe2O4

hybrid


123

near-zero coercivity and remanence [35]. There is no pro-

nounced hysteresis loop, which indicates that both the

resistivity and the coercivity of the hybrid are zero. This

observation is consistent with super-paramagnetic behavior

[17, 35, 36]. Inset shows the degree of magnetization of

MWCNT-COOH/NiFe2O4 hybrid.

3.6 Electrical and Dielectric Properties

The broad band dielectric spectroscopy is the best tech-

nique to study the electrical properties of hybrids since its

results can be differentiated in dielectric constant, con-

ductivity, or electric modulus domain in the wide-ranging

frequency windows and temperature ranges. The imped-

ance spectroscopy was used to span the conductivity

spectrum at frequencies ranging from 1 Hz to 3 MHz. The

isotherms varied from 293 to 393 K by 20 K steps. The

temperature stability was kept in a reasonable range during

measurement. The sample with gold electrodes was placed

between electrodes. The data was collected in dielectric

constant and electric conductivity domains. The electrical

conductivities and dielectric properties of both MWCNT-

COOH and MWCNT-COOH/NiFe2O4 hybrid as function

of angular frequency were investigated over a broad fre-

quency up to 3 MHz and temperatures ranging from 20 to

120 �C.

3.6.1 Ac and Dc Conductivity

The electrical conductivity of percolating system is eval-

uated as a function of frequency, f, and temperature, T. The

frequency and temperature dependences of the electrical

behavior can be exhibited in various domains. The

response signal to a sinusoidal stimulus is explored by

Fourier Transform by calculating the complex impedance.

Here the complex dielectric constant e � ðx; TÞ ¼e0ðx; TÞ � ie00ðx; TÞ and the complex conductivity r �ðx; TÞ ¼ r0ðx; TÞ � ir00ðx; TÞ are calculated where

x = 2pf. The real part of the ac conductivity as a function

of the angular frequency, x, is calculated from the imagi-

nary part of the dielectric constant e00(x) through the

relation r0(x) = e0xe00(x), where e0 is the vacuum per-

mittivity. The alternating current (ac) conductivities,

rac(x), of both MWCNT-COOH and MWCNT-COOH/

NiFe2O4 hybrid as a function of frequency for various

temperatures ranging from 20 �C up to 120 �C were

measured using impedance spectroscopy and characterized

in this section. Frequency dependent ac conductivities,

rac(x), could be acquired from the following equation:

r0ðx; TÞ ¼ racðx; TÞ ¼ e00ðx; TÞxe0

where r’(x) is the real part of conductivity, x(=2pf) is the

angular frequency of the applied potentials across the

electrodes, e00 is the imaginary part of complex dielectric

permittivity (e*) and e0 (=8.852 9 10-14 F/cm) is the

vacuum permittivity. The frequency dependence of the

alternative current (ac) conductivity follows a power law

behavior. r0(x, T) or the total ac conductivity can be then

represented by the following equation [37, 38]

r0ðx; TÞ ¼ rdcð0; TÞ þ racðx; TÞ ¼ rdc þ rðTÞxn

where x the angular frequency, rdc is the independent

frequency conductivity or dc conductivity (at x ? 0),

r(T) is the constant dependent on temperature T, and n is

an exponent dependent on both frequency and temperature

with values in the range around unity. The conductivity

measured at the lowest frequency used here (1 Hz) was

taken as the DC value, rDC(T) r0(1 Hz, T), and its variation

with x is described as above. This type of behavior was

noted by Jonscher, who called it as ‘‘UDR’’ (Universal

dynamic response) [39, 40] because of a wide variety of

materials that displayed such behavior.

The ac conductivity of MWCNT-COOH as function of

angular frequency of applied potentials across two elec-

trodes is measured for temperatures ranging from 20 to

120 �C respectively, as shown in Fig. 7a. It is clear that ac

conductivity at lower temperatures remain unchanged up to

a frequency value of 130 kHz. After that, it drops sharply

with a peak at a frequency of 1.1 MHz for all temperatures

concerned. It tends to be similar behavior but remain out of

range of our measurements in our system’s limit. There can

also be seen some electrical changes at about 1,926 Hz for

all the temperatures. When some NiFe2O4 NPs was added

to the MWCNT-COOH to make ferromagnetic hybrid,

frequency dependent electrical characteristics of MWCNT-

COOH/NiFe2O4 hybrid change for temperatures ranging

from 20 to 120 �C as shown Fig. 7b. Below a frequency of

Fig. 6 The room temperature hysteresis loops of MWCNT-COOH/

NiFe2O4 hybrid


123

88 kHz, ac conductivities remain unchanged for all tem-

peratures studied here while over this frequency, there

seem to represent a lot of chemical and physical properties

within the measurement results. The ac conductivity drops

sharply until 382 kHz. After this value of frequency they

still become temperature-dependency of some (between 20

and 50 �C) and the rest owns temperature-independency

and continues like that again until a frequency of

1.48 MHz. In our whole frequency window range, another

intersection of the conductivity variation for both fre-

quency- and temperature-dependencies (T = 60 and

70 �C) of the hybrid occurs a minimum conductivity at

2.35 MHz (between 80 and 120 �C) as well as at 840 kHz

(between 20 and 50 �C) as before.

rdcðTÞ ¼ rð0Þ exp½�Ea

kT�

According to the Arrhenius plots, the activation energy

for MWCNT-COOH seems to be temperature-range

(about below and above 85 �C) dependent of and is best

regarded as an experimentally resolved parameter that

reveals the sensitivity of the reaction rate to temperature.

For this, at lower temperature the activation energy is

found to be around 16 meV while it has a value of

67 meV at higher temperatures. On the other hand, in the

case of MWCNT-COOH/NiFe2O4 hybrid at lower tem-

perature below 50 �C, there has been a fluctuation in

conductivity, however, above it dc conductivity remains

unchanged and activation energy is calculated to be

almost zero from the Arrhenius plots (Fig. 8). This can be

attributed to the existence of a conventional temperature

independent tunneling conduction mechanism, which can

be also explained that the metallic conduction is a dom-

inant mechanism around room temperature. Especially, a

tunneling conductive mechanism could be expected to

occur in this type of hybrid. The ac conductivity of

MWCNT-COOH/NiFe2O4 hybrid followed the predic-

tions of the UDR and the n exponents could be determined

with lower concentration in the hybrid.

100 101 102 103 104 105 106 107 108

5,0

5,2

5,4

5,6

5,8

6,0

f=1926 Hzω=2πf

f=1.1MHz

f=130kHz 20°C 30°C

40°C 50°C

60°C 70°C

80°C 90°C

100°C 110°C

120°C

(a)

Co

nd

uct

ivit

y (m

S/c

m)x

10-3

Angular frequency, ω (Hz)

100 101 102 103 104 105 106 107 108


-4,5

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

f=2.

35 M

Hz

f=1.48 MHz

f=38

2 kH

z

f=84

0 kH

z

f=88 kHz

20°C 30°C

40°C 50°C

60°C 70°C

80°C 90°C

100°C 110°C

120°C

ac c

on

du

ctiv

ity

(μS

/cm

)

(b)

Fig. 7 Ac Conductivity of a MWCNT-COOH and b MWCNT-

COOH/NiFe2O4 hybrid as function of angular frequency of applied

potentials across two electrodes, for temperature ranging from 20 to

120 �C, respectively

3,4 3,2 3,0 2,8 2,6 2,4-5,115

-5,120

-5,125

-5,130

-5,135

-5,140

-5,145

-5,150

85°C

Ea=16 meV

Ea=67 meV

(a)

(b)

Temperature, 1000/T(K-1)

3,4 3,2 3,0 2,8 2,6 2,4

Temperature, 1000/T(K-1)

-32

-31

-30

-29

-28

-27

-26

Ea=1.5 μeV

50°C

ln(σ

dc)

ln( σ

dc)

Fig. 8 Dc conductivity of a MWCNT-COOH and b MWCNT-

COOH/NiFe2O4 hybrid as function of reciprocal temperature ranging

from 20 to 120 �C, respectively


123

3.6.2 Permittivity Modulus and Tangent Loss

In general, relative permittivity of all compound nanopar-

ticles of MWCNT-COOH/NiFe2O4 hybrid is found to be

dependent to both temperature and frequency and can be

expressed as follows

e0ðx; TÞ ¼ e0ð0; TÞx�nðx;TÞ

together with power exponents, n(x, T), of both frequency

and temperature. Each doping level and type of Fe3O4

nanoparticles with the dopants of cobalt and zinc represent

very complicated characteristics as function of applied

frequency for elevated temperatures. Dielectric study of

hybrid was carried out at different frequencies (1 Hz–

3 MHz) by using LCR meter.

Figure 9 shows the tangent loss of both MWCNT-

COOH and MWCNT-COOH/NiFe2O4 hybrid as a function

of angular frequency of applied potentials across two

electrodes for temperatures ranging from 20 to 120 �C,

respectively. The variation of the tangent loss of MWCNT-

COOH for all the temperatures studied here almost stayed

unchanged at lower and higher frequency ranges except for

medium range of the frequency windows between 1 Hz and

3 MHz. The fluctuation of tangent loss of MWCNT-COOH

is observed at a frequency range between 200 Hz and

2.5 kHz. The inset of Fig. 9a also shows the temperature

dependent fluctuation and shifts. It seems to be resonant

type variation at certain frequency values with temperature

dependencies. When tangent loss value is examined for the

MWCNT-COOH/NiFe2O4 hybrid as seen in Fig. 9b, this

attitude is completely found to be different from the host

material MWCNT-COOHs. Their similarity is that varia-

tion of tangent loss is mainly invariant with frequency

except for the value taken at lower frequency for all the

temperatures between 20 and 120 �C (inset Fig. 9b).

The real parts of the permittivity of both MWCNT-

COOH and MWCNT-COOH/NiFe2O4 hybrid are illus-

trated in Fig. 10a and b as functions of angular frequency

100 101 102 103 104 105 106 107 108-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

103 104

-0,2

0,0

120oC

110oC

100oC

90oC

80oC70oC

60oC

50oC30oC

40oC

20oC

Tan

Los

s x1

06

Angular frequency, ω(Hz)

20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°C

(a)

(b)

Tan

Lo

ss x

106


101 102 103 104 105 106 107 108

-120

-100

-80

-60

-40

-20

0

100 101 102 103 104 105 106 107 108

-0,002

0,000

Tan

Lo

ss


MWCNTNiFe2O

4

20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°C

Tan

Lo

ss


Fig. 9 Tangent loss of a MWCNT-COOH and b MWCNT-COOH/

NiFe2O4 hybrid as function of angular frequency of applied potentials

across two electrodes, for temperature ranging from 20 to 120 �C,

respectively

100 101 102 103 104 105 106 107 108

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

101

-3

-2

-1

0

Rea

l Per

mitt

ivity

x 1

06


20°C 30°C

40°C 50°C

60°C 70°C

80°C 90°C

100°C 110°C

120°C

(a)

(b)

Rea

l Per

mit

tivi

ty x

10

6


100 101 102 103 104 105 106 107 108

-140

-120

-100

-80

-60

-40

-20

0

100 101 102 103 104 105 106 107 108

-0,0030

-0,0025

-0,0020

-0,0015

-0,0010

-0,0005

0,0000

0,0005

0,0010

0,0015

Rel

ativ

e P

erm

ittiv

ity x

1012


MWCNT-NiFe2O

4

20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°CR

elat

ive

Per

mitt

ivity

x10

12


Fig. 10 Real permittivity of both a MWCNT-COOH and

b MWCNT-COOH/NiFe2O4 hybrid as function of angular frequency

of applied potentials across two electrodes, for temperature ranging



123

for temperature ranging from 20 to 120 �C, respectively.

At lower frequency below 15 Hz, the real permittivity

changes tremendously with temperatures ranging from 20

to 120 �C while it remain almost constant over a frequency

of 15 Hz. Similar attitudes can be observed in the case of

compound of MWCNT-COOH/NiFe2O4 hybrid. Magnified

windows can be seen in both Fig. 10a and b as the insets.

For temperature ranging from 20 to 120 �C, the imagi-

nary parts of the permittivity of both MWCNT-COOH and

MWCNT-COOH/NiFe2O4 hybrid as function of angular

frequency of applied potentials across two electrodes are

depicted in Fig. 11a and b, respectively. In general, it

obeys the power law in frequency as given with a power

exponent n which is also temperature independent within

both lower and higher frequency ranges. It is also very

interesting that the power exponent is about unity for all

temperatures and frequency windows studied here.

e00ðx; TÞ ¼ e00ð0; TÞx�n

where x(= 2pf) is the angular frequency, and e00(0,T) is a

both frequency and temperature-independent constants

which is unusual from our expectation from our normal

hybrid attitudes. The semi-log plots of imaginary permit-

tivity of MWCNT-COOH with frequency were depicted in

inset of Fig. 11a. For the case of MWCNT-COOH/

NiFe2O4 hybrid the imaginary part is slightly temperature-

dependents at lower frequency range below 60 Hz, how-

ever, at higher frequency range over 1,6 kHz it becomes

temperature independent except for the temperature value

of 110 �C which is a highly temperature-dependent below

1,6 kHz as can be seen clearly the inset of the Fig. 11b. It

is clear that frequency-dependency of imaginary part of the

permittivity of the MWCNT-COOH with a power exponent

becomes frequency-independent parameters while the

preparation of MWCNT-COOH/NiFe2O4 hybrid with the

exceptional temperature variation of 110 �C.

Figure 12a and b illustrate the real part of the complex

dielectric modulus of both MWCNT-COOH and MWCNT-

COOH/NiFe2O4 hybrid as function of angular frequency of

applied potentials across two electrodes for temperature

ranging from 20 to 120 �C, respectively. It is clearly seen that

real part of the modules for MWCNT-COOH keep less

changes at lower frequencies for all temperatures while there is

a big drop at higher frequency ranges. For more detail exam-

ination shows the variation at higher frequency with a slightly

temperature dependency. In the case of linear plots rather than

semi-log plots, real modules show less change but at higher

frequency over approximately 50 kHz it shows an exponential

decay with frequency. When we examine the same attitudes

for MWCNT-COOH/NiFe2O4 hybrid as depicted in Fig. 12b

the real modules illustrate power law variation with a power

exponent of approximately unity (a closure examination was

given in inset). Power law obeys the following equations;

M0ðx; TÞ ¼ M0ð0; TÞxn

where n varies unity with a 5 % deviations depending on

the temperatures between 20 and 120 �C except for a big

fluctuation of the curve of 110 �C.

The real and imaginary components of the modulus are

calculated from the e0 and e00 data;

M� ¼ 1

e�¼ M0 þ iM00 ! M� ¼ e0 þ ie00

e02 þ ie002

Its imaginary part for MWCNT-COOH varies linearly with

the increase of frequency for all the temperatures studied in

this work while that of MWCNT-COOH/NiFe2O4 hybrid

represents completely different tendency. Imaginary modules

below a critical temperatures of 70 �C shows big fluctuation

with temperatures at lower frequency ranges as those above

that critical temperatures imaginary modules tend to be

almost independent of both frequency and temperatures as

0

2

4

6

8

10

12

100 101 102 103 104 105 106 107 108

1E-7

1E-6

1E-5

1E-4

1E-3

0,01

0,1

1

10

100

s=tanθ=-1.007

Imag

inar

y P

erm

ittiv

ity x

10

9


20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°C

(a)

(b)

100 101 102 103 104 105 106 107 108-80

-60

-40

-20

0

20

40

60

100 101 102 103 104 105

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

Imag

inar

y P

erm

ittiv

ity x

1012


MWCNTNiFe2O

4 20°C 30°C 40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°C

Imag

inar

y P

erm

itti

vity

x10

12Im

agin

ary

Per

mit

tivi

ty x

109


100 101 102 103 104 105 106 107 108

Angular frequency, w(Hz)

Fig. 11 Imaginary permittivities of a MWCNT-COOH and





123

illustrated in Fig. 13b. For the MWCNT-COOH, imaginary

modules depend linearly on the angular frequency for all

temperatures obeying the following formulas

M00ðx; TÞ ¼ M00ð0; TÞ þ mx

m is the slope of the curve in Fig. 13a inset, and M00(0, T) is

the initial value of the imaginary part of the complex dc

dielectric modulus at room temperature of 293 K. The

dielectric constants of these MWCNT-COOH and MWCNT-

COOH/NiFe2O4 hybrid hybrid fluctuates with increasing

temperature. The ‘wave’ phenomenon of the temperature

dependence of these dielectric constants can be attributed to

the interfacial polarization under different temperatures for

various frequency windows studied here [41].

4 Conclusion

A simple and effective approach for the decoration of

MWCNT-COOHs by NiFe2O4 NPs was presented. The

conductivity and magnetization measurements proved that

the samples are conductive and have a super paramagnetic

behavior. Electrical properties of the hybrid can be explained

with the existence of a conventional temperature independent

tunneling conduction mechanism. Especially, a tunneling

conductive mechanism could be expected to occur in this

type of hybrid. The AC conductivity of MWCNT-COOH/

NiFe2O4 hybrid followed the predictions of the UDR and the

n exponents could be determined with lower concentration in

the hybrid. MWCNT-COOH/NiFe2O4 hybrids may have

potential to be used as effective MRI contrast agents and

drug carriers for biomedical applications.

Acknowledgments This work is supported by Fatih University

under BAP Grant No. P50021104-B.

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100 101 102 103 104 105 106 107 108

-20

-15

-10

-5

0

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80°C 90°C

100°C 110°C

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