Thermogravimetric analysis of cobalt-filled carbon nanotubes
deposited by chemical vapour deposition
Babu P. Ramesh a, W.J. Blau a, P.K. Tyagi b, D.S. Misra b, N. Ali c,
J. Gracio c, G. Cabral c, E. Titus c,*
a
c
Materials Ireland Polymer Research Centre, Trinity College, Dublin-1, Ireland
b
Department of Physics, Indian Institute of Technology, Bombay, India
Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract
In this paper, we report results from an investigation studying the purification of Co-filled carbon nanotubes (CNTs) using Thermogravimetric
analysis (TGA). The as-grown CNTs were prepared using Microwave Plasma Chemical Vapour Deposition (MPCVD). Transmission electron
microscopy (TEM), Fourier Transform Infrared (FTIR) and Raman spectroscopy were used to characterise the CNT samples. The CNTs produced
by MPCVD were filled with cobalt and consisted of thick multi-walls. After TGA purification at 900 -C, 30 wt.% Co-filled CNTs remained in the
TGA pan. However, while investigating the un-filled commercial CNTs (thin multiwalled), the sample completely burnt out at around 650 -C in
the TGA furnace. The high thermal stability and the ability of thick-walled CNTs to act as an effective protective shield which prevents the
oxidation of encapsulated cobalt have been demonstrated.
Keywords: Carbon nanotubes; Chemical vapour deposition; Thermogravimetric analysis
1. Introduction
Carbon nanotubes filled with ferromagnetic materials, such
as Fe, Ni, or Co, hold great potential for use in nano-electronics
and biomedical applications [1]. To date, a very limited number
of researchers have reported results on the synthesis and
properties of ferromagnetic filled CNTs [2 –7]. In our recent
work, we achieved nickel filling of CNTs by using CVD [8 –
10]. Ferromagnetic filled CNTs are attractive since the carbon
shells provide an effective barrier against oxidation, ensuring
long term stability of the ferromagnetic filling. However,
normally deposited CNTs contain other carbonaceous and
transition metal particles. The presence of impurities can
significantly influence the properties of CNTs and the
behaviour of any device built from this new class of materials.
Therefore, knowledge of the purity of ferromagnetic filled
CNTs is highly important when they are used as nano-devices
and in biomedical applications. It is reported that the impurities
associated with CNTs may be eliminated by gas phase
oxidation and/or thermal annealing in air or an oxygen
atmosphere [11]. These purification processes are based on
the phenomenon that the oxidation temperature of impurities is
different from that of CNTs. On the other hand, transition metal
and carbon particles adhered to the walls of CNTs could affect
the performance of many practical applications.
In this paper, we report results on the purification of thick,
multiwalled, Co-filled CNTs using TGA. In addition, we have
characterised the as-grown and purified CNTs using a number
of analytical characterization techniques, such as TEM, FTIR
and Raman spectroscopy. CNT purity is typically explained in
terms of the amount of amorphous carbon and metal present.
In our case, the fully encapsulated cobalt catalyst in the CNT
is considered to be pure and its purity is confirmed by the
absence of any catalyst or other particles adhered/attached to
its surface. CNTs consist of covalently bonded carbon atoms
[12] and they are chemically inert. However, the carbonaceous particles grown along with the CNTs are typically
amorphous in nature and are formed of loosely packed carbon
atoms. This type of loosely bonded carbon can easily get
129
bonded to the hydrogen during the growth process itself and
thus form C –H bonds.
2. Experimental
Depositions of Co-filled, thick and multi-walled carbon
nanotubes (MWCNTs) were performed using a conventional
MPCVD system. The conditions employed together with the
details on the MPCVD system used to deposit the CNTs can be
found elsewhere [10]. Thermogravimetric analysis was performed using Perkin Elmer Pyris 1 TGA thermo balance. In the
TGA experiments, the sample was heated in air from 30 to
900 -C at 10 -C/min. The Raman spectra were recorded at
room-temperature using a Renishaw 1000 micro-Raman
system equipped with a Leica microscope. The 50-magnifying objective of the microscope focused a laser beam to a spot
of ¨ 1 Am diameter. The excitation wavelength used was 514.5
nm from an Ar+ ion laser. A 1800 line/mm grating was used in
all measurements, which allowed one to obtain a resolution of
¨ 1 cm 1. TEM analysis was performed using a JEOL JEM 7
microscope at 200KV using a double tilt holder. FTIR analysis
was carried out using Nicolet NEXUS bench machine with 128
scans. The samples were prepared by mixing with KBr powder
into pellets.
3. Results and discussion
Fig. 1 shows the TGA graph for the as-grown CNTs. The
solid and the dash dotted lines correspond to thermogravimetric
(TG) and differential thermo-gravimetric (DTG) curves,
respectively. From this figure, it is found that the initial
burning temperature was at around 580 -C and this occurred
mainly due to the presence of amorphous carbon in the CNTs
mixture [13]. The burning of CNTs themselves begins at
around 650 -C. It was noted that approx. 30% of the sample
remained behind after performing TGA up to 900 -C. This
residue remains mainly due to the presence of cobalt found in
between and within the inner walls of the CNTs. There was no
weight gain observed during thermal treatment, since no
oxidation of the metal particles is expected to take place
[14]. The weight gain [15] due to oxidation occurs in samples
where the metal particles are exposed to or are adhered on the
surface regions of the CNTs. In our case, the catalyst is
completely encapsulated and is rid of any external or tip
catalytic particles, as evident from the TEM results. As a
comparison, Fig. 1 also shows the TGA graph displaying the %
weight loss of commercial CNTs (Thin MultiWall with purity
>90%—as obtained from ‘‘Nanocyl’’, Belgium) with temperature. It should be noted that the grade of commercial CNT
100
6
90
Asgrown CNT
4
Commercial CNT
Weight loss (%)
70
2
60
0
50
PeakX = 581.16 °C
-2
40
Area = -62.989 %
30
-4
Peak = 589.20 °C
Derivative weight percentage (per minute)
80
20
-6
10
0
100
200
300
400
500
600
700
800
Temperature (°C)
Fig. 1. Graph showing the thermal stability of thick, multi-walled Co-filled CNTs, deposited using MPCVD, and commercial thin-walled, un-filled CNT sample. The
solid and dashed lines represent TG and DTG curves for the CNTs sample.
130
filled CNTs for use in biomedical applications and investigating for magnetic properties of these advanced materials.
Fig. 2 displays TEM micrographs of the as-grown (a), TGAtreated CNTs (b) and single Co-filled CNT (c). From Fig. 2(a),
it is evident that the as-grown CNTs are entangled with each
other and contain amorphous carbon and nanoparticles. The
TEM micrograph (Fig. 2(b)) of the TGA-treated tubes showed
that the CNTs separated and became free from entanglement
and impurities, such as amorphous carbon phases and other
carbon particles. Fig. 2(c) displays the TEM image of isolated
Co filled carbon nanotube. TEM analysis showed that the
CNTs were uniformly filled with the Co-catalyst. The Co-filled
CNTs displayed a dark and uniform contrast with the
surrounding CNT. The filling of the cobalt is uniform and is
up to 10 Am.
Fig. 3 shows the Raman spectra, in the range 1000 – 1800
cm 1, of the as grown (a) and TGA-treated sample (b). The
Raman spectra of the samples indicate that TGA had a
noticeable influence in the structure of the nanotubes, in terms
of the carbon-phases present. It is apparent that the Raman
spectra presents two broad bands centred at around 1350 and
1577 cm 1 wavenumbers, which correspond to the D and G
bands of graphite. The G mode is associated with the ordered
graphite in CNT and the D mode is attributed to disordered
graphitic carbon. The disordered carbon may be present within
CNT or any other carbon-based impurities associated with the
CNTs. The spectrum of the as-grown CNTs sample indicates
that D band is intense and broad, which means that the raw
material contains quite large amount of disordered carbon or
impurities. The broadness of the tube can be correlated to the
Fig. 2. TEM micrographs showing the as-grown CNTs using MPCVD (a),
purified CNTs after TGA (b) and a single CNT (c), as obtained after the TGA
treatment.
sample used in this investigation consisted of thin multi-walled
tubes, whereas, the CNTs prepared using our MPCVD system
were thick multi-walled tubes. Also, it needs to be noted that in
the commercial CNT sample, the tubes were un-filled and no
ferromagnetic material was present, in the form of a filling,
within the inner walls of the tubes. It can be seen that the initial
burning of the commercial CNT sample started at around
580 -C, which is similar to our finding for the CNTs sample
prepared using MPCVD. Also, similar to the as-grown CNTs,
there was no oxidation step for the commercial CNT sample.
At around 655 -C, the commercial CNT sample had completely
disappeared due to the sample ‘‘burn out’’ in the TGA furnace.
This suggests that thin-walled and un-filled CNTs are more
vulnerable to ‘‘burning out’’ when exposed to temperatures of
up to 900 -C in a TGA furnace. However, the Co-filled CNTs
did not burn out completely due to their thick protective walls
and the weight percentage remained (30%) was mainly due to
the weight of the metal cobalt, since CNTs are ultra light
weight. This is advantageous while considering using such
Fig. 3. Raman spectra representing the as-grown CNTs (a) and the same sample
after TGA treatment (b).
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the sample to air. By treating/purifying the CNTs using TGA,
the amorphous carbon and other impurities associated with
CNTs are almost completely removed. The peak at 1650 cm 1
is unaffected since it is due to CfC stretching of CNTs.
4. Conclusion
From the TGA results, we observed that the as-grown CNTs
contained carbonaceous and amorphous carbon impurities,
which burnt out when exposed to temperatures up to 900 -C in
a TGA furnace. However, around 30% of the CNTs, deposited
by MPCVD, remained after TGA, unlike the commercial grade
of CNTs, which was used as a comparison and which burnt out
completely at around 650 -C. The thick walled CNTs prevented
cobalt inside the tubes from burning out and the weight
percentage remained after TGA treatment was mainly due to
the weight of the cobalt. The TEM observations revealed that
the TGA purified the CNTs prepared by MPCVD and isolated
Co-filled tubes remained behind in the residue sample material.
We have also confirmed the presence of cobalt from the
electron diffraction patterns (results not shown in this paper).
The removal of impurities from the as-grown CNTs was
confirmed by using results obtained from FTIR and Raman
spectroscopy analyses.
Fig. 4. FTIR spectra for the CNTs sample in the as-grown state (a) and after the
TGA treatment (b).
amount of amorphous carbon contained in it. The G band
displays a highly intense and sharp peak, which is due to the
highly ordered graphite structure. Compared to the Raman
spectrum corresponding to the as-prepared material, the
purified CNTs sample displayed relatively weaker D band
but similar intensity G bands. This observation suggests that
TGA purified the CNTs and removed impurities and/or
suppressed the degree of disordered graphite and other forms
of carbon nanoparticles that may have been present in the
sample.
FTIR spectra of the as-deposited and TGA-treated CNTs are
shown in Fig. 4. The FTIR spectrum of as-grown CNTs
displayed a broad absorption peak centred at 3450 cm 1,
which is related to the OH group. A small peak at around 1650
cm 1 is associated with the CfC stretching of CNT. We have
confirmed that this peak is not associated with OH bending,
since it did not disappear along with OH stretching band after
TGA treatment. The peaks appearing between 2700 and 3000
cm 1 are due to the CH stretching bands [16]. It was observed
that the peaks appearing due to the OH and CH bands
disappeared completely after the TGA treatment (Fig. 4).
The chemical structure of CNTs consists of carbon only and
does not contain any functional group(s). CNTs are chemically
inactive due to their strong covalent bonds. However, the
amorphous carbon associated with the CNTs can easily form
bonds with oxygen and hydrogen. There is an increased
probability that the hydrogen bond formation must have
occurred during the CVD growth process, since hydrogen is
one of the key precursor gases in CNT synthesis by CVD
methods [17]. The OH peak can be expected due to exposure of
Acknowledgement
The authors are grateful to Dr. A. Lopes, Dept. of Ceramics
and Glass Engineering, University of Aveiro (Portugal) for the
TEM analysis. FCT (Portugal) is highly acknowledged for
funding this work.
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