Physico-chemical studies of amorphous carbon nanotubes synthesized at low temperature

Article abstract of DOI:10.1016/j.materresbull.2012.04.073

This work provides better understanding on the nature of amorphous carbon nanotubes, which are synthesized via a simple chemical route. Amorphous carbon nanotubes (α-CNTs) are successfully synthesized by heating a mixture of ferrocene and ammonium chloride at temperature as low as 200°C and are treated with hydrochloric acid. Transmission and field emission scanning electron microscopy techniques are performed to examine the morphology and dimension of the samples. X-ray diffraction tests confirm the amorphous structure of the nanotubes. The Fourier transform infrared spectroscopy and Raman studies indicate that the treated α-CNTs consist of many defective walls and are more amorphous compared with the untreated α-CNTs. Ultraviolet-visible absorption studies reveal that the untreated and treated α-CNTs exhibit plasmon absorbance with high bandgaps of 4 eV and 4.35 eV, respectively.

Full text of DOI:10.1016/j.materresbull.2012.04.073

Materials Research Bulletin 47 (2012) 1849–1854
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Physico-chemical studies of amorphous carbon nanotubes synthesized at
low temperature
*
Kim Han Tan , Roslina Ahmad, Bey Fen Leo, Ming Chian Yew, Bee Chin Ang, Mohd Rafie Johan
Nanomaterials Engineering Research Group, Advanced Materials Research Laboratory, Department of Mechanical Engineering, University of Malaya, Lembah Pantai,
Kuala Lumpur 50603, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history:
This work provides better understanding on the nature of amorphous carbon nanotubes, which are
Received 13 October 2011
Received in revised form 24 February 2012
Accepted 18 April 2012
synthesized via a simple chemical route. Amorphous carbon nanotubes (a-CNTs) are successfully
synthesized by heating a mixture of ferrocene and ammonium chloride at temperature as low as 200 8C
and are treated with hydrochloric acid. Transmission and field emission scanning electron microscopy
techniques are performed to examine the morphology and dimension of the samples. X-ray diffraction
tests confirm the amorphous structure of the nanotubes. The Fourier transform infrared spectroscopy and
Available online 26 April 2012
Keywords:
Raman studies indicate that the treated
a-CNTs consist of many defective walls and are more amorphous
A. Amorphous materials
B. Chemical synthesis
C. Electron microscopy
C. Raman spectroscopy
D. Optical properties
compared with the untreated -CNTs. Ultraviolet–visible absorption studies reveal that the untreated
a
and treated
a-CNTs exhibit plasmon absorbance with high bandgaps of 4 eV and 4.35 eV, respectively.
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the synthesis temperature. However, longer reaction duration and
poisonous reagents are inevitable in this approach [11,12]. In
Carbon nanotubes (CNTs) have undeniably attracted great
attention since their early discovery [1,2]. CNTs appear to be one of
the most important materials due to their unique structures, which
gives extraordinary strength, excellent electrical properties and
efficient thermal conductivity. Due to these unique properties,
CNTs are suitable for a tremendously diverse range of applications.
In addition, their optical properties are also significant, which leads
to various optical and electronic applications [3]. Various growth
techniques for CNTs such as arc discharge, laser vapourization,
pyrolysis, plasma-enhanced and thermal chemical vapour deposi-
tion (CVD) have been well established [4–6]. Previous works were
mostly focused on either multi-walled carbon nanotubes
(MWCNTs) or single-walled carbon nanotubes (SWCNTs). Howev-
er, conventional techniques such as arc discharge and CVD may
general, requirements of high synthesis temperature and pressure,
complicated processing steps, catalyst supports, longer synthesis
period or expensive costs may still the major drawbacks arising
from the above-mentioned techniques [7–13].
The optical properties of CNTs have been investigated for a
relatively long time, which include absorption, photoluminescence
and Raman characteristics [14,15]. However, the exploitation of
the desirable optical properties of these nanoscopic carbon
cylinders is necessary as the fundamental understanding of how
these functional properties are affected by the morphological
characteristics of the inherent CNTs remain elusive. Literature
survey indicates that relatively few optical absorption studies are
carried out and almost exclusively involve SWCNTs [6]. In
comparison to crystalline CNTs, amorphous CNTs (
gradually receiving attention due to their simple synthesis
conditions. Additionally, -CNTs have gained increasing interest
a-CNTs) are
also be employed to synthesize
a-CNTs [7,8]. In addition, other
methods have been developed which include anodic aluminium
oxide (AAO) template and solvothermal methods. Catalysts are not
required in these approaches. The dimensions of nanotubes such as
diameter, length, graphitization degree and orientation of gra-
phene layer can be manipulated by controlling the shape of the
AAO template [9,10]. The solvothermal method has been used to
a
due to their potential development in field-emission display
devices and nanodevices such as gaseous adsorbents and catalyst
supports. These applications are possible due to the defects within
their amorphous walls [16,17].
The optical properties of CNTs typically possess several key
features, from long to short wavelengths (low to high energy). The
first spectral feature refers to vibrational transitions due to carbon–
carbon bond stretches whereas the other pertains to vibrationally
active modes in the infra-red (IR) [18]. Band-gap transitions
have been observed at the near infra-red–visible–ultraviolet
obtain
a-CNTs from ferrocene in benzene to significantly reduce
* Corresponding author. Tel.: +60 3 79676873; fax: +60 3 79675317.
E-mail address: kimhan8419@gmail.com (K.H. Tan).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.04.073

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K.H. Tan et al. / Materials Research Bulletin 47 (2012) 1849–1854
(NIR–vis–UV) regions within a wavelength range of 2500–350 nm,
and are affected by both specific diameter and chiralities of CNTs.
Strongabsorptionsarealsopresentatsignificantlyhigherenergy,i.e.
in the UV and far-UV regions, due to plasmon resonance [19,20].
electron detector. An EDX spectrometer equipped in the FESEM
was used for elemental analysis of the -CNTs at room
temperature. Structural analysis was carried out using XRD
a
(SIEMENS D5000, German) with Cu-K
a X-ray of wavelength
˚
In this paper,
a
-CNTs are synthesized via a simple chemical
1.54056 A at 60 kV and 60 mA. The diffraction was conducted with
route [16,17]. The low temperature synthesis is capable of
eliminating the potential disadvantages associated with the
aforementioned growth techniques. The goal of the present work
Bragg angles between 58 and 1008.
2.2.2. Optical studies
is to examine the optical properties of
a
-CNTs using ultraviolet–
Ultrasonication was performed on a sample for 1 h using
methanol as the solvent in order to provide better dispersion for
the sample. The ultraviolet ray absorption was then recorded using
UV–vis spectrophotometer (Cary Win UV 50, Australia). By using a
1 cm quartz cuvette, the optical absorption and transmittance
measurements were then scanned at a slow rate over the range of
190–800 nm (ultraviolet, infrared, visible and adjacent regions).
visible (UV–vis), Fourier transform infrared (FTIR) and Raman
spectrometers. Morphological, structural and elemental studies
are also carried out by using transmission electron microscope
(TEM), field emission scanning electron microscope (FESEM),
energy-dispersive X-ray spectrometer (EDX) and X-ray diffrac-
tometer (XRD). The results are subsequently correlated with those
obtained from optical studies.
Raman characteristics of the a-CNTs were examined using Raman
microscope (RENISHAW, United Kingdom). The He–Ne laser was
excited on the nanotubes with a wavelength of 633 nm. FTIR
spectroscopy (PerkinElmer, Spectrum 400, USA) was carried out
within the range of 400–4000 cmꢀ1 to study the attachment of
bonding groups in the nanotubes.
2. Experimental
2.1. Materials and sample preparation
In this work, all as-prepared
a-CNT samples were synthesized
via a modified reduction process. Briefly, 8 g of ammonium
chloride (NH4Cl) and 16 g of ferrocene (Fe(C5H5)2) were mixed
homogeneously. The mixture was transferred into a Parr reactor,
with a capacity of 125 mL. The Parr reactor was sealed and heated
to 200 8C inside a furnace. The heating process was maintained at
200 8C for 30 min. Following this, the Parr reactor was cooled down
to room temperature and the mixture was taken out and labelled
3. Results and discussion
3.1. FESEM, TEM and EDX results
Fig. 1 presents both low and high magnification FESEM images
of nanotubes prior to treatment with hydrochloric acid, i.e.
untreated
a-CNTs [21]. The magnified structure can hardly be
‘‘untreated a-CNTs’’. In addition, a portion of the untreated sample
recognized as nanotubes as the structure has irregular shapes with
a disordered arrangement. Some of the nanotubes agglomerated
heterogeneously. Additionally, the surface of the nanotubes is
rough, indicating the presence of many defects within the
structure. However, it can be seen that the nanotubes have
uniform diameters and open ends after being treated with diluted
acid, as shown in Fig. 2. The nanotubes in this sample exist in
bundles. The tubular shape of the nanotubes is more apparent due
to the removal of residual reactants (Fe and NH4Cl) by purification
treatment with diluted acid.
was soaked and washed with concentrated hydrochloric acid (HCl)
(5 M), followed by methanol (CH3OH) (99.8%) and purified water
(>15.0 M
V
cm) for several times. The sample was subsequently
m) with the
filtered and collected on a nylon filter membrane (0.2
m
aid of a pump. Dehydration was carried out using a vacuum oven at
80 8C for 10 h to obtain the final sample, which was labelled
‘‘treated
a-CNTs’’. Both untreated and treated samples were then
ready for further characterizations.
2.2. Characterizations
Fig. 3 shows that the dominant element in both untreated and
treated
a-CNTs is carbon. Impurities such as Fe, O, N, and Cl
2.2.1. Morphological, elemental and structural studies
elements are present in both samples and their respective weight
percentages are presented in Table 1. It is expected that both N and
Cl originate from the unreacted NH4Cl compound (one of the
starting materials) during reaction. The traces of Fe may have
originated either from the ferrocene compound or from the inner
body of the stainless steel Parr reactor. The presence of oxygen
indicates that the samples are oxidized to some extent. The soaking
Both untreated and treated samples (a-CNTs) were observed
under TEM and FESEM for qualitative analysis on their surface
morphologies and microstructures. The TEM (LIBRA1 120,
Germany) was operated at an accelerating voltage of 120 kV.
The FESEM (AURIGA, ZEISS, Germany) was operated at an
accelerating voltage of 10 kV, with a working distance of 3.5–
4.5 mm. The FESEM images were generated by a backscattered
Fig. 1. FESEM images of untreated
a-CNTs under different magnifications: (a) 5000ꢁ and (b) 20,000ꢁ.

K.H. Tan et al. / Materials Research Bulletin 47 (2012) 1849–1854
1851
Fig. 2. FESEM images of treated
a-CNTs under different magnifications: (a) 10,000ꢁ and (b) 20,000ꢁ.
Fig. 3. EDX spectra of: (a) untreated and (b) treated
a-CNTs.
and washing treatments substantially reduced the foreign
elements such as N, Cl and Fe.
soaking and washing with acid. Nanotubes are clearly observed
(Figs. 2 and 4(b)) as the NH4Cl phase is absent within the treated
CNTs (Fig. 5). The XRD pattern of treated -CNTs is very broad with
low intensity, although some residual reactant of Fe2O3 is still
present within the sample.
a
-
Fig. 4 shows the untreated
different magnifications. The nanotubes have straight dimensions,
whereby the outer and inner diameters of -CNTs are estimated to
be about 90 and 65 nm, respectively. The length of the nanotubes is
about 8–10 m. The average length and diameter of the -CNTs
a
-CNTs at room temperature under
a
a
m
a
3.2. FTIR study
are in good agreement with the FESEM images (Figs. 1 and 2). The
nanotubes are completely amorphous, which agrees well with the
XRD patterns presented in Fig. 5. On the other hand, Fig. 4(a)
FTIR study was performed on the
from their powders. Based on Fig. 6, both the treated and untreated
-CNTs exhibit peaks at 1360 and 1259 cmꢀ1, which correspond to
a-CNTs by fabricating pellets
indicates that the untreated
a
-CNTs are surrounded by the
a
unreacted NH4Cl salt, which corresponds to its XRD pattern
shown in Fig. 5, where all diffraction peaks are attributable to the
NH4Cl phase (JCPDS 73-0365). This compound is removed after
C55O vibrational bands. Additionally, the peaks at 1588 and
1582 cmꢀ1 also indicate the presence of C55C vibrational bands due
to internal defects [16]. An additional band in the 2800–3500 cmꢀ1
region is observed only in the treated
a-CNTs, which corresponds
Table 1
to the presence of hydroxyl group (–OH). This band is associated
with amorphous carbon, which forms bonds easily with oxygen
and hydrogen. The OH peak may be due to exposure of the sample
to air. The amorphous walls of the nanotubes have many defects,
which absorb moisture conveniently. On the other hand, the OH
peak for untreated CNTs is hardly observed due to the relatively
lower intensity. This indicates that a lesser extent of amorphous
Weight and atomic percentage of elements within untreated and treated samples.
Sample
Elements
C
O
Cl
Fe
N
Untreated
a
-CNTs
wt%
at%
43.78
63.27
9.92
12.20
5.97
23.98
7.45
10.12
12.54
10.77
Treated
a
-CNTs
wt%
at%
82.30
90.85
6.02
4.99
10.18
3.81
1.50
0.36
0
0
walls was formed during growth for the untreated
this sample is not completely amorphous (Fig. 5).
a-CNTs, since

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K.H. Tan et al. / Materials Research Bulletin 47 (2012) 1849–1854
Fig. 4. TEM images under different magnifications: (a) untreated
a-CNTs 10,000ꢁ, (b) treated
a-CNTs 12,500ꢁ and (c) 31,500ꢁ.
3.3. Raman analysis
The D band appears at 1372 cmꢀ1 for the untreated sample, and at
1350 cmꢀ1 for the treated sample. The D-band is attributed to the
Raman-inactive A1g in-plane breathing vibration mode. The D-
band can be assigned to the vibrations of carbon atoms with
dangling bonds in plane terminations of disordered graphite, and
thus being associated with the presence of defects within the
hexagonal graphitic layers [22]. The G band appears at 1576 cmꢀ1
in the untreated sample, and at 1570 cmꢀ1 in the treated sample.
The G band is attributed to the Raman-active E2g in-plane vibration
mode, which is related to the vibration of sp2 bonding within the
carbon atoms in a two dimensional hexagonal lattice, such as in a
graphite layer. Nanotubes with crystalline structures consisting of
hexagonal carbon lattice display the same vibration. [21,23]. For
Fig. 7 shows the Raman spectra for both untreated and treated
a
-CNTs. It is apparent that two broad bands are present in the
Raman spectra, which correspond to the D and G bands of graphite.
treated
of the G-band. The G-band (1570 cmꢀ1) is also broader than that of
the untreated -CNTs. The intensity ratio for D-band to G-band (Id/
Ig) is also larger. Therefore, the treated -CNTs are more
a-CNTs, the relative intensity of D-band is larger than that
a
a
amorphous since they have defective tube walls made of
disordered carbon. In fact, the G-band is hardly observed, revealing
that the treated
a-CNTs have a higher degree of disorder after the
soaking and washing treatment. The Raman spectra correlate well
with the XRD spectra (Fig. 5).
Fig. 5. XRD spectra of
a-CNTs at room temperature.
Fig. 6. FTIR spectra of
a-CNTs at room temperature.

K.H. Tan et al. / Materials Research Bulletin 47 (2012) 1849–1854
1853
Fig. 7. Raman spectra of
a
-CNTs at room temperature.
3.4. UV–vis analysis
for an amorphous material is expressed by Eq. (1):
n
According to the UV–vis transmittance spectra shown in Fig. 8,
it is clear that overall, the transmittance intensity of the treated
CNTs in solution is lower than that of the untreated -CNTs. This
shows that the treated -CNTs are well-dispersed in the solution
and have better dispersion characteristics compared to the
untreated -CNTs.
Both treated and untreated
behaviour as shown in Fig. 9. Both samples have intense absorption
peaks at 256 nm (4.85 eV) and 253 nm (4.90 eV), respectively. The
absorption energy for both samples is consistent with those in
previous reports [14,24]. In general, both absorption bands are due
to transitions between spikes in density of states within the
electronic structure of the tubes. It is believed that the results are
ð
ahn
Þ
¼ Bðh
n
ꢀ EgÞ
(1)
a
-
a
where B is a constant, h
and n is the characterization index for the type of optical transition.
The absorption coefficient (
given by Eq. (2):
n
is the photon energy of the incident light
a
a) is defined by the Lambert–Beer law
a
a
-CNTs exhibit similar UV–vis
ꢀ ꢁ
A
a
¼ ꢀln
(2)
t
where A is the absorbance and t is the sample thickness. The optical
band gaps can be obtained from the extrapolation of the best linear
portion of the curves for (
band edge region.
The presence of a metallic element (remaining Fe) in both
samples is believed to modify the electronic states and optical
transitions of the nanotubes, which results in allowed transitions
rather than forbidden transitions [26]. Hence, an index value of
n = 3 is selected to obtain a suitable Tauc and Davis–Mott plot for
both samples, as presented in Figs. 10 and 11. The Eg for untreated
ahn n, at a equals zero near the
)n versus h
attributed to the
associated with collective excitations of
around 310–155 nm (4.0–8.0 eV) in the CNTs’
transition. In other words, this absorption peak is due to plasmon
p
plasmon absorbance. This phenomenon is
electrons which occur at
* electron
p
p–p
resonance in the free electron cloud of the nanotube
[14,19,25].
p electrons
The band gaps (Eg) for the
a-CNTs were estimated using Tauc/
a
a
-CNTs is estimated to be 4 eV, which is lower than that of treated
-CNTs, having a value of 4.35 eV. Both samples are found to have
Davis–Mott model based on optical absorption measurements as
this is an important parameter for opto-electronic applications
[26]. In this model, the relation between Eg and optical absorption
higher bandgaps than that of crystalline nanotubes [27,28].
Fig. 8. UV–vis transmittance spectra for the
a-CNTs at room temperature.
Fig. 9. UV–vis absorbance spectra of a-CNTs at room temperature.

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K.H. Tan et al. / Materials Research Bulletin 47 (2012) 1849–1854
amorphous, with the introduction of numerous defects. A lesser
extent of amorphous walls is formed for untreated -CNTs. There is
a
a higher degree of disorder in the structures (more amorphous) of
the treated nanotubes after being soaked and washed with diluted
acid, as indicated by the Raman spectra. This is attributed to the
higher intensity of the D-band compared with the G-band. The
CNTs demonstrate the plasmon absorbance (E ) phenomenon in
the UV region, which is characteristic for most crystalline CNTs.
The -CNTs also have a higher estimated bandgap compared to
a-
p
p
a
crystalline CNTs.
Acknowledgements
3
This work was financially supported by the Postgraduate
Research Grant (PPP) (PS112-2010A) and UM/MOHE HIR Research
Grant (UM.C/HIR/MOHE/ENG/12) provided by Ministry of Higher
Education of Malaysia. The authors gratefully acknowledge the
financial support.
Fig. 10. Tauc and Davis–Mott plots for (ahn) as a function of hn for untreated a-
CNTs.
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simple technique. TEM, FESEM and XRD observations confirm that
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mm. The average inner and outer diameters
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