Synthesis and Characterization of Multiwall-Carbon Nanotubes Decorated with Nickel Ferrite Hybrid B. Unal • A. Baykal • M. Senel • H. So ¨zeri Received: 15 October 2012 / Accepted: 19 November 2012 / Published online: 2 December 2012 Ó Springer Science+Business Media New York 2012 Abstract Acid functionalized multiwall carbon nanotube, (MWCNT)-COOH/nickel ferrite (NiFe 2 O 4 ), 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/NiFe 2 O 4 hybrid. NiFe 2 O 4 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 (M 0 ) of the product illustrate power law variation with a power exponent of approximately unity. These magnetic MWCNT-COOH/NiFe 2 O 4 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; Fe 3 O 4 , CoFe 2 O 4 and NiFe 2 O 4 etc. are used. Nickel ferrite (NiFe 2 O 4 ) 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]. NiFe 2 O 4 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-Fe 2 O 3 , NiFe 2 O 4 and Fe 3 O 4 [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. So ¨zeri 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
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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
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
492 J Inorg Organomet Polym (2013) 23:489–498
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
J Inorg Organomet Polym (2013) 23:489–498 493
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
Angular frequency, ω (Hz)
-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
494 J Inorg Organomet Polym (2013) 23:489–498
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