# Synthesis and Optimization of MWCNTs on Co-Ni/MgO by Thermal CVD

Authors: H. Ryu; B. K. Singh; K. S. Bartwal
Journal: Advances in Condensed Matter Physics
 (2008)

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## Abstract

Multiwalled carbon nanotubes (MWCNTs) were prepared by the thermal chemical vapor deposition (CVD) technique. Monometallic and bimetallic Co and Ni combinations were used as a catalyst on MgO support. The mixer ofH2/C2H2 was used as a carbon source. The prepared CNTs were found to possess different shapes, morphologies, and sizes. Maximum yield was found for 50% Co (MgO: 50% and Ni: 0%) catalyst at 600C∘. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) techniques were used for structural analysis. Raman spectra were taken to investigate the quality and crystalline perfection of the prepared CNTs. The ratio of D- and G-bands (ID/IG) was measured from these spectra.


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## Body

## 1. Introduction

Carbon nanotubes (CNTs), discovered in 1992 by Iijima [1], are
tiny, hollow tubes made of pure carbon just a few nanometers in diameter and up
to few hundreds microns in length. Since the discovery of carbon nanotubes,
researchers were attracted to work on these fascinating materials. Carbon
nanotubes exhibit unique electrical properties, efficient heat conductivity,
and excellent mechanical strength. Due to their outstanding physical properties,
CNTs are one of nanotech’s most promising molecular building blocks. CNTs have
recently emerged as an attractive cold cathode material due to their excellent
field emission properties such as low turn-on field, high current density, and
long-term stability [2]. The other forms of carbon nanostructures, such as the carbon
nanofibers (CNFs) [3] and carbon nanoparticles (CNPs) [4], also have excellent
electron emission characteristics. Carbon nanotubes imagined to be made out of
a slice of a graphite monolayer, which is rolled up into a seamless cylinder.
Three electrons per carbon atom are used for covalent bonding in sp2 hybridization, while the forth electron is smeared out over the wire-like
structure and supports current transport along the carbon scaffold. Due to the
one-dimensional nature of the CNT, scattering is absent at moderate current
densities giving rise to ballistic transport. As a consequence, the current
density in CNTs can be a factor of 102–103 higher than in metals. Nanostructured
graphitic materials such as single-wall carbon nanotubes (SWNT)
and multiwalled carbon nanotubes (MWNT) [5], fullerenes, onion-like carbon, and
related structures [6] are mainly obtained by evaporating solid carbon targets
by means of arc-discharge and laser ablation.Several methods have been employed for
the synthesis of CNTs, which include arc discharge, laser ablation, plasma-enhanced chemical
vapor deposition (PECVD) and thermal chemical vapor deposition (CVD). Thermal chemical vapor
deposition (CVD) is a most promising synthesis approach in view of the lower
temperatures requirement, good control of deposit morphology, capability of
scaling up the process [7, 8]. Various monometallic and bimetallic catalytic
systems with different supports have been explored for the production of CNTs. The
Fe/MgO, Co/MgO, Fe/Al2O3, Fe-Mo/MgO, Co-Mo/MgO, and
Co-Ni/SiO2 catalysts have produced different types of CNTs ranging
from SWNT, DWNT, and MWNT [9–12]. It is
to be mentioned here that the morphology of the metallic particles deposited on
the substrate films are principally depends on the substrate effective
resistance [13]. The forms of CNTs produced depend upon the carbon source and various
experimental conditions. The bimetallic catalysts combinations have proved to
be better candidates for CNT synthesis. The different process routes can favor
the selective production of other curled carbon nanostructures [14–16].We report results on optimization of Co- and Ni-based monometallic and
bimetallic catalytic systems for the synthesis of MWNTs by decomposition of acetylene
(C2H2). We also correlate the morphology and structural
features with the employed catalytic combination.


## 2. Experimental Details

Thermal
chemical vapor deposition (CVD) technique is relatively simple and most common
for the commercial production of CNTs. Thermal CVD was employed to synthesize carbon nanotubes using
Co-Ni/MgO as a combination of catalyst and support. The high-purity acetylene gas (C2H2)
was used as a carbon source. Various combinations of metal ions were supported on MgO in order
to see their effect on yield and quality of the CNTs. MgO (50%) was kept
constant and varying combinations of Ni and Co were tried and investigated. The
catalyst ratios were taken as 50% Co, (10% Ni  +  40% Co), (20% Ni  +  30% Co), (25% Ni  +  25% Co), (30% Ni  +  20% Co), (40% Ni  +  10% Co), and 50% Ni.
The nitrates were used for the starting materials as Co(NO3)2⋅6H2O,
Ni(NO3)2⋅6H2O with Mg(OH)2. Desired
stoichiometric amount of nitrates were ground in ethanol in an agate mortar and
the resulting slurry was dried at 100°C for 10 hours in an oven. The as-prepared catalyst
powder (0.1 g) was uniformly dispersed in an alumina boat that was placed in
the central region of a horizontal 100 cm long quartz tube furnace with inner
diameter of 4.5 cm. The furnace was flushed with argon while it was heated at
10°C/min rate till the desired temperature (~600–700°C)
was attained. After the attainment of desired temperature, H2 was
introduced in the furnace at 100 mL/min rate for 60 minutes so as to generate
active Co/Ni nanoparticles on MgO support. With maintaining the hydrogen stream
for the desired period, C2H2 at 10–50 mL/min rate was
introduced for 1 hour while keeping the H2 flow rate constant.The prepared CNTs were
purified by refluxing them in 3M HNO3 for 2 hours at 60°C
and subsequently stirred in 5M HCl for 6 hours at 120°C. The
product was filtered and washed successively with distilled water and isopropyl
alcohol. The purified samples were used for structural characterization. Powder
XRD, SEM, and TEM studies were done to investigate morphology, shape, size, and
crystallinity of the prepared CNTs. Sample for TEM observation
was prepared by suspending the CNTs in ethanol by ultrasonification and drying
a drop of the suspension on a carbon-coated copper grid. Philips Tecnai G2-20 (FEI)
electron microscope operating at 200 kV was used for TEM experiments. Raman
spectrometer (Bruker, FRA106, excitation beam wavelength- 1064 nm) was used to
confirm the formation of the CNTs.


## 3. Results and Discussion

Carbon nanotubes were prepared
by thermal chemical vapor deposition method using Co/Ni as catalyst and C2H2 as a carbon source. MgO was used as a support for these catalyst nanoparticles. Ni and Co metal ions in
various combinations were supported on MgO in order to see their effect on the
yield and quality of the CNTs produced. MgO was kept constant at 50% for all
the catalyst combinations. We have investigated and optimized the influence of the various
experimental parameters such as the reaction temperature, time of H2 flow for catalyst activation, flow rate of acetylene, and content of the Co/Ni
catalyst. Figure 1(a) shows the CNT yield (%) as a
function of Co-Ni catalysts loading (Wt%) during 60 minutes growth time with
flow of H2  +  C2H2 (100  +  15 mL/min) at 600°C.
Figure 1(b) shows the representative CNT
yield as a function of the reaction temperature of 25% Co  +  25% Ni catalyst on
MgO support. The yield was calculated
from the as-produced
CNTs. The CNTs yield (%) was calculated by weighing the sample after the reaction
using the following equation: (1)Carbon yield⁢(%)=[MF−MIMI]×100, where MF is
the total weight obtained after the synthesis and MI is the weight
of the catalyst taken for the synthesis. It was observed that the yield of CNTs
increases with increase in Co content in the catalyst. The highest yield was
observed for 50% Co (i.e., MgO: 50% and Ni: 0%). However, the optimized
temperature for the highest yield was 600°C. It suggests that Co is
more effective as catalyst than Ni. The precursor catalyst combinations were
investigated for the phase and crystallinity before being used for the CNTs
synthesis.
Figure 2
shows the representative XRD pattern for catalyst precursor combination 25% Co  +  25% Ni prepared at 600°C. XRD pattern exhibits characteristic peaks
of MgO support and the oxides of Co and Ni. Peaks for Co and Ni oxides are
clearly seen apart from the mean peak for MgO (200) at 42°. This
suggests that the temperature, time, and H2 gas flow rate optimized
for the preparation of the precursor is correct. The average sizes of the crystallites of
Co/Ni metal were roughly estimated from XRD peak broadening to be 15–20 nm. It is important
that complete conversion of oxidic phase into active Co/Ni particles
necessarily occur under the influence of acetylene and hydrogen combination [17].
The representative XRD patterns for purified CNTs prepared with different
catalyst Co/Ni combinations are shown in Figure 3. The XRD patterns show graphite-like peaks
having (002) and (100) reflections at 26° and 44°, respectively. The presence of broad hump at 44° along with C (100) in Figure 3 corresponds
to respective metal nanoparticles. In Co-Mo
and Co-Ti systems, it was reported that Co center promotes the growth while
other metal components are responsible for alignment and thickness of CNTs [18].
In the present investigations, Co alone on MgO shows higher yield than the bimetallic combination
of Co/Ni.CNT
yield as a function of (a) Co-Ni catalysts loading (Wt%) during 60 minutes growth
time with flow of H2  +  C2H2 (100  +  15 mL/min) at
600°C, (b) reaction temperature of 25% Co  +  25% Ni catalyst on MgO
support.
(a)(b)Figure 2
Representative powder XRD pattern for the catalyst 25% Co  +  25% Ni on MgO support.Figure 3
Representative powder XRD patterns for CNTs synthesized
using (a) 50% Co, (b) 25% Co  +  25% Ni, and (c) 50% Ni on MgO support.Scanning electron
microscopy (SEM) was done to investigate morphology and sizes of the
synthesized CNTs. Figures4(a), 4(b), and 4(c) show SEM images of the CNTs grown
with 50% Co, 25% Co  +  25% Ni, and 50% Ni, respectively. From these micrographs it
is observed that the CNTs exhibit different sizes and morphology for different
catalyst combinations without any noticeable carbon nanoparticles. CNTs are oriented
randomly in the bending and twisting forms of tubes with few visible defects. The samples grown with
different composition of catalysts, reveals that the sizes and diameters do not
vary much. The average diameter of about ~25–30 nm was
estimated from these images.Representative SEM images of purified MWNTs for (a) 50% Co,
(b) 25% Co  +  25% Ni, and (c) 50% Ni on MgO support.
(a)(b)(c)Transmission electron microscopy (TEM) in high-resolution
mode is the best tool to investigate local structure, particle size, and
morphology of the material. Crystallinity and morphology of synthesized CNTs
was investigated using high-resolution transmission electron microscopic
(HRTEM) images. Figures5(a), 5(b), and 5(c) show the HRTEM images for the three
different catalyst compositions of 50% Co, 25% Co  +  25% Ni, and 50% Ni,
respectively. Corresponding SAD patterns are inserted in the micrographs. High-resolution images
show that prepared CNTs contain large number of isotropic domains of nanocarbon graphite with
random orientations of crystal direction having curved graphene sheets. The
intershell spacing is found to be 0.34 nm.
The selected area electron diffraction pattern (SAED), inserted in Figure 5(a), shows that the
bright ring diffraction peak is originating from the (002) graphite plane.
Multiwalled character with some structural defects was also revealed from these
micrographs for all the catalyst combinations. MWNTs are highly crystalline in
nature with long fringes that are oriented parallel to each other as evidenced
from the images in Figure 5. A pair of small but strong bright arcs of (002) diffraction
indicates the presence of graphene texture along the tube axis. The outer faint
ring probably reveals the defects in the graphite layers in the outer walls of
CNTs.Representative TEM micrographs for CNTs grown with (a) 50% Co,
(b) 25% Co  +  25% Ni, and (c) 50% Ni
on MgO support.
(a)(b)(c)The quality and crystalline perfection of the prepared CNTs can
also be estimated by Raman spectroscopy [19]. A perfect nanotube would be a
cylindrical graphene sheet composed only of hexagons having minimum defects at
the tips to form a closed seamless structure. We have investigated the prepared
CNT samples by Raman spectroscopy to determine their quality and perfection. Raman spectra for the CNTs for 50% Co, 25% Co  +  25% Ni, 20% Co  +  30% Ni, and 50% Ni on MgO support are shown in Figure 6. All the
spectra have two strong peaks at 1287 cm-1 and at 1595 cm-1.
These two peaks are designated as the tangential modes of carbon [20]. The first
peak corresponds to the defect-induced A1g or D-band. It is
associated with vibrations of carbon atoms with dangling bonds in plane
terminations of disordered graphite or glossy carbon. This
D-band also called as breathing mode.
The second peak known as G-band represents the Raman allowed E2g mode of graphite and is related to the vibration of sp2 bonded
carbon atoms in a two-dimensional
hexagonal lattice [21]. The ratio of the intensities of D and G bands (i.e., ID/IG)
is the indirect measure of the crystallite size and crystalline defects in the
CNTs. The ID/IG ratio in Figure 6, for CNTs with different Co and Ni concentrations was
calculated. The ratio calculated was ~1.24, which estimates the crystallite
size and relative extent of crystalline defects. The intensity of D-band is higher than
G-band indicating that some amount of disordered graphite as defects or
amorphous carbon present in the prepared CNTs.Figure 6
Raman spectra of purified multiwalled CNTs for
(a) 50% Co, (b) 25% Co  +  25% Ni, (c) 20% Co  +  30% Ni, and (d) 50% Ni on MgO
support.


## 4. Conclusion

Reasonably good crystalline quality CNTs
were prepared with different combinations of Co/Ni catalyst on MgO support by
thermal CVD. The
CNTs synthesized over different catalysts combinations have different
sizes/shapes and morphologies with minimal deposition of carbon particles.
HRTEM studies have shown that morphological characteristics are catalyst
sensitive and the purity and crystallinity depends on combination of the
catalyst. The reaction temperature, gas flow rate, catalyst combination, and
support are important parameters to be looked into for quality and efficient
growth of CNTs. We expect that these results can also be extended to the
preparation of metallic nanoparticles on suitable support/substrate apart from
the CNTs



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*Source: 971457-2008-08-18.xml*