Optimize the Properties of Carbon Nanotubes Synthesized using a
Microwave Oven
AHMAD M. AL-DIABAT1*, NATHEER A. ALGADRI2, NASER M. AHMAD3,
ADNAN H. ALRAJHI3, ABDULSALAM ABUELSAMEN4,
AMAL MOHAMED AHMED ALI3, SALMA ABDULRHMAN AL-WASLI3
1Department of Physics,
Al-Zaytoonah University of Jordan,
Amman,
JORDAN
2Department of Physics,
Isra University,
Amman,
JORDAN
3School of Physics,
Universiti Sains Malaysia,
Penang,
MALAYSIA
4Aqaba University of Technology,
Aqaba,
JORDAN
*Corresponding Author
Abstract: - In this paper, carbon nanotubes (CNT) are synthesized using the microwave oven method, which
offers several advantages, including a simple, quick, inexpensive, and solvent-free growing method. To
produce CNT, a mixture of graphite and ferrocene catalysts had to be flattened inside a microwave oven for
five seconds at room temperature. CNT was produced using various ratios of graphite and ferrocene, and
analyses indicated that a 70:30 graphite/ferrocene ratio produced better nanocrystalline CNT. To optimize CNT
properties, five processes of purification were used to dispose of impurities like metal particles and support
material from the as-produced carbon nanotubes. Raman spectroscopy, field emission scanning electron
microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier
transform infrared spectroscopy (FTIR) was used to characterize the CNTs both after and before purification.
After acid treatment and centrifugation, the amount of amorphous carbon and iron particles significantly
decreased. Additionally, following the purification process, the ID/IG decreased by 0.14 and the I2D/ID increased
by 0.55 for the purified CNTs. Furthermore, the FTIR spectra of the untreated and functionalized CNTs
confirm the presence of carboxyl groups on pure CNTs and -OH moieties in sorbed water.
Key-Words: -CNT, functionalization, purification, nitric acid, microwave.
Received: February 8, 2023. Revised: May 23, 2023. Accepted: June 22, 2023. Published: July 19, 2023.
1 Introduction
Several methods for carbon nanotube (CNT)
growth have been used to produce CNTs with the
appropriate features. The techniques that are most
frequently utilized are chemical vapor deposition
(CVD), laser ablation, electrolysis, hydrothermal,
and arc discharge, [1], [2].
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DOI: 10.37394/232015.2023.19.68
Ahmad M. Al-Diabat, Natheer A. Algadri,
Naser M. Ahmad, Adnan H. Alrajhi,
Abdulsalam Abuelsamen, Amal Mohamed Ahmed Ali,
Salma Abdulrhman Al-Wasli
E-ISSN: 2224-3496
705
Volume 19, 2023
Furthermore, the microwave oven technique is
often employed since it is a simple and inexpensive
method for the production of CNTs at ambient
pressure and room temperature (RT). In contrast to
the other techniques, the microwave oven technique
is extremely quick and easy to implement, [3], [4].
All the synthesis methods produce CNTs that
contain different impurities like carbon
nanoparticles, amorphous carbon, metal particles,
and other unwanted particles, [5], [6]. Subject to
the type of impurity, a mixture of chemical and
physical techniques is necessary to achieve
complete purification. The main methods for the
purification of CNTs include ultrasonication,
oxidation, annealing, magnetic purification, acid
treatment, centrifugation, filtration, and
functionalization, [5], [7], [8], [9], [10]. Oxidation
can be used to clean the metal surface and remove
carbonaceous impurities (C-impurities), including
amorphous carbon particles from carbon nanotubes.
Wet and dry oxidation are two different types of
oxidations. The wet oxidation treatment involves
refluxing CNTs in strong acids such as H2O2,
HClO4, HNO3, H2SO4+KMnO4, and HNO3+H2SO4.
for 24 hours at RT. Dry oxidation is conducted
under atmospheric air or oxygen gas at high
temperatures (500–750 oC), [5], [11], [12].
In this study, carbon nanotubes were produced in
one step using the conventional microwave oven
for a short time in the absence of inert or feedstock
gases. Then, a five-step purification process was
used to improve the CNT's properties by
eliminating impurities like metal particles and
support material. The CNTs were characterized
using different analytical techniques after and
before purification.
2 Experimental Part
2.1 Synthesized CNT
The CNTs were synthesized using a method that
makes use of a traditional microwave oven. To
achieve the homogeneous distribution of
microwave heat at every point inside the quartz
boat, a mixture of graphite and ferrocene was put in
quartz boats at various ratios (80:20, 70:30, 50:50,
and 40:60), and when the pop light spark appeared,
the microwave heating was turned off.
2.2 Carbon Nanotube Treatment
1 g of as-produced carbon nanotubes (R-CNT)
were treated in 1 L of nitric acid (2.6 M HNO3)
under stirring and blending at room temperature for
17 h (Fig. 1, step 1). The hydrophilicity of CNTs is
enhanced as a result of the acid treatment, which
enables chemical functional groups "(-OH)" to
attach to the sidewalls of CNTs, [13].
Furthermore, it is employed in the purification
process to remove metallic particles and unwanted
material after exposing the metal surface to
oxidation and sonication treatments. The acid
dissolves the metal particle in the catalyst, while
the nanotube remains suspended in the acid, [10],
[14].
N nitric acid, CNTs, and other carbon particles are
unaffected, whereas metal particles are removed.
The benefits of using nitric acid include its low cost
and effectiveness in removing metal particles.
Contrarily, hydrochloric acid treatment slightly
affects the structures and properties of CNTs, [5],
[15], [16].
The ultrasonic process involved the use of
ultrasound to suspend the highly agglomerated
nanotubes. The ultrasonic process typically takes
between 40 and 60 minutes (Fig. 1, step 2).
After the acid treatment, a supernatant of CNT was
obtained by centrifuging at 10,000 rpm for 1 hour
(Fig. 1, step 3), [8], and deionized water was used
to rinse the solution many times until the pH
reached 7. (Fig. 1, step 4). Finally, the as-produced
CNTs were desiccated in a furnace at 70 ˚C for 72
h, as a result, functionalized and purified carbon
nanotube (F-CNT) powder could be obtained (Fig.
1, step 5).
3 Result and Dissociation
3.1 As-produced CNT
3.1.1 Raman Spectroscopy Analysis
Fig. 2 illustrates the Raman spectrum of four CNT
samples grown using different graphite/ferrocene
ratios (80:20, 70:30, 50:50, and 40:60).
The well-ordered carbon atoms in the graphene
layer are related to two separate bands at 1580 cm-1
(G-band), and all sp2 carbon materials have defects
(D) at 1330 cm-1. Additionally, a 2D band of active
CNTs was found to satisfy particular chirality
requirements by double resonance. The intensity
ratio for G to D bands (ID/IG), [17], which measures
the degree of crystallinity and phase purity in the
acquired samples, was used to describe the degree
of ordered structure with respect to defects that
were present in CNTs. A low ID/IG ratio suggested
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Ahmad M. Al-Diabat, Natheer A. Algadri,
Naser M. Ahmad, Adnan H. Alrajhi,
Abdulsalam Abuelsamen, Amal Mohamed Ahmed Ali,
Salma Abdulrhman Al-Wasli
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that the graphitic structure of CNTs was well-
ordered. Since the development of the 2D band in
CNTs was very sensitive to an increase in defect
density, the proposed CNTs' crystallinity and phase
purity were determined using the value of I2D/ID,
[18].
Table 1 provides the full-width half maximum
(FWHM) of the G-band as well as the relative
intensity ratios of the D-to-G and 2D-to-D bands
derived from the Raman spectra shown in Fig. 2.
In comparison to the 80:20, 50:50, and 40:60
ratios, the CNTs made from a graphite/ferrocene
combination with a 70:30 ratio had the highest
values of I2D/ID and FWHM (G), as well as the
lowest values of ID/IG.
The Raman spectra of the CNT powder are
shown in Fig. 3 (a). It displayed two distinct,
identical high-intensity peaks at 1333 and 1582
cm-1. The G-asymmetries bands suggest that the
spectrum has many peak components. Figure 3 (b)
displays the de-convoluted Raman spectra of the
carbon nanotube that were shown using the
Lorentzian curve fit for the G band.
Asymmetric G and G+ components are visible
in the Raman spectra of CNT samples with a 70:30
sample ratio at 1580 and 1615 cm-1, respectively.
The semiconducting component at 1580 cm-1 and
the metallic component band at 1557 cm-1 were the
sources of the G component, which includes the
G(S) band, [19], [20].
3.1.2 FESEM and TEM Observations
The FESEM pictures of CNTs produced using
various amounts of graphite and ferrocene
(80:20, 70:30, 50:50, and 40:60) are shown in Fig.
4. The evaluation of the synthesized CNTs is based
on their propensity for agglomeration and the
formation of stacked bundles. For a 70:30 blend of
graphite and ferrocene, the typical tube diameter is
45 nm.
The CNTs' cavity, as well as the top or bottom,
is injected with encapsulated Fe catalyst
nanoparticles (Fig. 4). According to a previous
study, Fe nanoparticles were found to remain in a
liquid state and be present inside the tube as
dispersed heterogeneous metallic particles, [21].
Furthermore, microwave radiation that moved over
the substrate surface and attached to the base of the
CNTs decomposed the ferrocene into free Fe
atoms[22]. The Fe sticks to the walls of the CNT
due to the tube's strong capillary forces, [23]. Fig. 5
depicts an isolated CNT with graphitic caps and
enclosed Fe nanoparticles aligned transversely to
the CNT length. Additionally, it is shown that due
to the buildup of Fe nanoparticles during CNT
growth, the tube width of CNTs changes slightly
over their length, [24].
3.1.3 X-ray Diffraction Analysis
The XRD analysis of CNT powder synthesized
using graphite/ferrocene combination ratios of
80:20, 70:30, 50:50, and 40:60 is shown in Fig. 6.
The prominent diffraction beak at 26.43o for all
samples reveals the interlayer spacing of 0.34 A° of
CNTs, according to JCPDS card number 00-008-
0415. A decrease in CNT alignment is the cause of
the (002) peak's higher intensity as compared to the
other peaks. The other low-intensity minor peaks
were associated with the hexagonal graphitic
structure's reflections at 35.7°, 42.3°, 50.1°, 54.6°,
and 77.4°, respectively.
Iron (Fe) and Fe carbide (Fe3C) are both
represented by the two peaks at (43.7° and 57°) and
(44.6° and 62°), respectively, [25], [26], [27]. Fe3C
is employed as a progress indicator in the formation
of nanotubes because it is thought to be an
intermediary phase in the configuration of the
graphite layers that make up CNTs, [28].
XRD line broadening can be used to determine
the structural information of CNTs, [29], such as
strain broadening, which is expressed by:
γπ
(1)
where l is the index of reflection, is the interlayer
spacing, and is the FWHM of the distribution of
interlayer spacing. It is clear that the strain
broadening brought on by the CNTs' curvature is
greatly responsible for the reduction in the FWHM
of (002) reflection. The Bragg law can be used to
calculate the d-spacing of the (002) reflection, [30],
[31]:
λ
θ (2)
where is the order of diffraction, is the X-ray
wavelength,2 stands for the scattering angle and d
is the inter-planar spacing between the crystal
planes.
The XRD data analysis of CNT synthesized
using the microwave oven is summarized in Table
2. The strain () of the grown CNT was calculated
using Equation 1 and is listed in Table 2. CNT
prepared using a 70:30 ratio of graphite/ferrocene
showed the lowest strain value, [20], [29],
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Ahmad M. Al-Diabat, Natheer A. Algadri,
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Abdulsalam Abuelsamen, Amal Mohamed Ahmed Ali,
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indicating a better quality crystal structure, the
obtained results coincided with the Raman analysis.
3.2 CNT after Purification
3.2.1 FESEM and EDX Observation
The morphologies of "as-produced" CNTs (R-
CNT) and functionalized CNTs (F-CNT) were
comparatively analyzed using FESEM, as shown in
Fig. 7. The surfaces and inside of the "as-produced"
CNTs are shown to include amorphous carbon and
iron particles (Fig. 7(a) and Figure 7(c)). The
amorphous carbon and iron particles decreased
significantly after the nitric acid treatment, as
shown in Fig. 7(b) and Figure 7(d). The CNT
shown in Fig. 8(b) and Figure 8(d) were analyzed
using energy dispersive X-ray spectroscopy (EDX),
and the results are displayed in Fig. 8(a) and Figure
8(c).
The EDX elemental analysis of the R-CNTs
indicates the presence of C, O, and Fe, as outlined
in Table 3. After oxidation and purification via
HNO3 acid treatment, the weight (%) of Fe in F-
CNT decreased. In contrast, the quantity of O after
treatment (F-CNTs) increased significantly
compared to the “as-prepared” nanotubes. The
CNTs tend to appear as bundles, as shown in the
FESEM images, which is consistent with some
previous studies, [32], [33]. Additionally, the
introduction of a small number of functional
groups, the partition of bundles of CNTs into some
individual tubes, and generally smaller diameter
bundles, as is evident in the FESEM images (Fig.
7(b) and Figure 7(d), can significantly increase the
solubility of CNTs in ethanol (without adding
surfactant or polymer conditions).
Fig. 9 shows a TEM image of CNTs after
purification by HNO3. It indicated that the majority
of the iron particles embedded in the CNTs' tips
had been eliminated. It appears that the metal
catalyst was initially removed by nitric acid when
the sealed tip was opened. The effectiveness of
HNO3 in removing metal particles is widely
established, and it can eliminate amorphous carbon
since it is a strong oxidant, [34].
3.2.2 Raman Spectroscopy Analysis
Fig. 10 displays the Raman spectra of the purified
and "as-synthesized" CNTs. The full width at half
maximum (FWHM) of the R-CNT and F-CNT, as
well as the ratio of Raman band intensities, are
shown in Table 4. It shows that the ID/IG ratio and
FWHM of the G band decreased by 0.14 and 4.9,
respectively, while the I2D/ID ratio increased by
0.55 for F-CNTs compared to as-synthesis CNTs.
The ratios of the Raman peaks (ID/IG) generally
decrease after purification because the removal of
amorphous carbon improves the concentration of
nanotubes. Furthermore, the ID/IG ratio is linearly
correlated with the quantity of "disordered" carbon
in the sample of graphitic materials. The decreased
ID/IG and FWHM G as well as increased I2D/ID for
F-CNTs indicate that oxidized amorphous carbon
was transferred to the centrifuged suspension,
where polyhedral carbon particles, Fe particles,
highly crystallized graphite fragments, and a few
large bundles of CNTs were deposited.
According to Cancado et al., the crystallite size
for nano graphitic by Raman spectroscopy was
calculated from the following equation, [35]:
(3)
The excitation laser energy utilized in Raman
spectroscopy is denoted as and has a value of
1.96 eV.
The crystallite size has increased from 5.5 nm for
R-CNT to 7.8 nm for F-CNT, as shown in Table 4.
3.2.3 FTIR Observation Analysis
Fig. 11 displays the FTIR spectra of the
unfunctionalized and functionalized CNTs. The
distinctive absorbance peak at 3553 cm-1 indicates
the presence of -OH moieties or -OH in sorbed
water and carboxyl groups.
The spectral bands observed in the 1550–1750
cm-1 range are indicative of C–C bonds present in
aromatic rings and C–O groups present in different
compounds such as ketone/quinone and carboxylic
acid. On the other hand, the 950–1300 cm-1 range
signifies C–O bonds present in various chemical
environments, [36], [37]. The absorbance band at
1406 cm-1 comprises the OH in sorbed water and
the C-O moiety, which are present as overlapping
bands. The presence of OH sorbed during the
production of carbon nanotubes is indicated by the
weak bands observed at 2929 cm-1 and 1386 cm-1,
[38]. The functionalization of CNTs resulted in a
decrease in the intensity of the -OH band at 3553
cm-1. The absorbance band at 1406 cm-1 is
comprised of the OH in sorbed water and the C-O
moiety, which are present as overlapping bands.
The presence of weak bands at 2929 cm-1 and 1386
cm-1 suggests the sorption of OH during the carbon
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Ahmad M. Al-Diabat, Natheer A. Algadri,
Naser M. Ahmad, Adnan H. Alrajhi,
Abdulsalam Abuelsamen, Amal Mohamed Ahmed Ali,
Salma Abdulrhman Al-Wasli
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nanotube production process. The Fourier-
transform infrared (FTIR) spectra of fluorinated
carbon nanotubes (F-CNTs), as depicted in Fig. 11,
suggest the presence of diverse hydroxyl groups
that are bound to the sidewalls of the CNTs, [39],
[40].
3.2.4 Electrical Properties
Fig. 12 illustrates the electrical performance of R-
CNTs, and F-CNTs-based sensors observed
between -5 V and +5 V. The current of F-CNTs"
reached up to 35 mA as compared to 11 µA in R-
CNTs, as the voltage was swept up to 5 V.
Accordingly, the electrical transport properties of
the F-CNTs are a thousand times greater than the
R-CNTs. As a result of the functional groups
attached to carbon nanotubes, there are more bands
close to the Fermi level, which enhances electron
transport between carbon atoms, [40].
4 Conclusion
CNT was successfully produced in a microwave
oven using a combination of graphite and ferrocene
as catalysts at room temperature in an ambient
environment and the absence of feedstock or inert
gases. The experimental results indicate that the
structural quality is influenced by the
graphite/ferrocene ratio. According to the ID/IG
ratio values derived from Raman spectra, the CNTs
with the best-improved crystallinity were generated
using a graphite/ferrocene combination ratio of
70:30.
CNTs with an average diameter of 45 nm were
produced, according to FESEM investigations.
XRD analysis confirms that CNTs are stable in the
presence of Fe and Fe3C phases.
The electrical and morphological properties of
the CNT were improved by removing the metal
particles using nitric acid (HNO3, 2.6 M) and the
remaining amorphous carbon and graphite particles
using the centrifugation technique. The amount of
residual catalyst in purified CNT was removed, as
displayed by TEM. Fe concentration in F-CNT was
reduced, and the quantity of O content sharply
increased after oxidation and purification. The
FTIR and measurements exhibit the presence of a
hydroxyl group linked to the surfaces of the CNT.
The functionalization of carbon nanotubes resulted
in an increase in electrical current from 11 µA for
F-CNT to 35 mA for R-CNT.
Acknowledgment:
This work has been supported by Isra University
and Al-Zaytoonah University of Jordan, Amman,
Jordan.
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Appendix
Fig. 1: CNT treatment steps
Fig. 2: Raman spectra for graphite/ferrocene ratios of (a) 80:20 (b) 70:30 (c) 50:50 (d) 40:60 for CNTs
produced by microwave oven in the range of 1000-3000 cm-1
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Fig. 3: (a) Raman spectra for CNTs produced by microwave oven at a ratio of 70:30, obtained in the range
between 1100 and 1900 cm-1,(b) CNTs' Raman spectra show the optical phonon mode G-band at a wavelength
of about 1580 cm-1. Three components are displayed through lorentzian fitting (color lines)
Fig. 4: Shows FESEM images of CNTs produced by microwave irradiating mixtures of graphite and ferrocene
at ratios of (a) 80:20, (b) 70:30, (c) 50:50, and (d) 40:60 for 5 seconds
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Fig. 5: TEM picture of a CNT produced by a microwave oven using a 70:30 powder mixture of graphite and
ferrocene
Fig. 6: X-ray diffraction of CNTs powder produced from a mixture of graphite and ferrocene with ratios of
80:20, 70:30, 50:50, and 40:60
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Fig. 7: FESEM images of untreated CNT (a) and (c), and HNO3-treated CNT (b) and (d).
Fig. 8: Acid treated CNT and as-prepared CNT were analyzed using EDX in (a) and (c) and FESEM in (b) and
(d), respectively
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Fig. 9: TEM image of CNTs after purification by HNO3
Fig. 10: The Raman spectra of “as-prepared” (R-CNT) and functionalized carbon nanotube (F-CNT)
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Fig. 11: The FTIR spectra of “as-produced” (R-CNT) and acid treated carbon nanotube (F-CNT)
Fig. 12: Comparison of I-V characteristics of R-CNTs and F-CNTs at 5V
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Table 1. Comparison of Raman spectrum intensities for CNTs produced using deferent ratio of
graphite/ferrocene
Gr: Fr
80:20
70:30
50:50
40:60
ID/IG
0.64
0.47
0.77
0.70
I2D/ID
1.28
1.45
1.25
1.29
FWHM G
40.65
34.4
41.90
35.82
Table 2. X-ray diffraction analysis for the produced CNT using graphite/ferrocene with different ratio
Sample Ratio
Gr:Fr
2θ
Degree
FWHM
(002) (º)
d(002) (Å)
80:20
26.47
0.64
3.367
1.194
70:30
26.41
0.51
3.375
0.949
50:50
26.53
0.59
3.360
1.103
40:60
26.46
0.56
3.368
1.045
Table 3. EDX analysis of as-produced CNT and acid treated CNT
Element
Weight%
R-CNT
F-CNT
C
71.2
63.71
O
5.18
32.03
Fe
23.61
4.25
Table 4. Comparison of Raman spectrum intensities and FWHM of G-band for R-CNTs and F-CNT
properties
R-CNT
F-CNT
ID/IG
0.47
0.33
I2D/ID
1.45
2
FWHM G
34.4
29.5
La (nm)
5.5
7.8
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Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
The authors equally contributed to the present
research, at all stages from the formulation of the
problem to the final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The authors have no conflicts of interest to declare.
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