Sanjay Agarwal, B. Yamini Sarada and Kamal K. Kar
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1833-122X) Volume 6 December 2010
Results and Discussion
EDAX and SEM Analysis
The innovation of carbon nanotubes and coils shows highly useful electrical properties. Their nanostructured geometry
equips them in efficient nanoelectronic devices leading to many industrial applications. To fabricate such electronic
devices, it is important to grow nanotubes/nanocoils/microcoils with controlled density, length, and alignment. In this work
we have grown carbon nanotubes (CNTs), carbon nanocoils and carbon microcoils by catalytic CVD using
C2H2, H2, and N2 as precursors. The carbon structures were synthesized at 700°C
under different conditions i.e. in the presence or absence of the sulphur. The metallic wire (Fe-Cr-Al alloy) was coated with
Ni catalyst using electroless dip coating technique in basic medium followed by pre-treatment process. We have grown 3D
spring-like carbon structures with the diameter 100-900 nm, coil pitch 20-600 nm and the length of several mm. This simple
and novel approach involves no toxic gases and gives controlled growth of perfect coiled CNTs which can be used in variety of
vacuum electronic applications.
Carbon Nanotube, Carbon Nanocoil, Carbon Microcoil, Chemical Vapor Deposition, Electron Microscopy, Electroless Plating
The unique properties make carbon nanotubes (CNTs) attractive nanostructures for a variety of potential applications. The
recent achievements towards practical applications in nanoelectronics such as logic gates assembled from nanowires , CNT
based logic circuits , and in field emission devices. CNTs have been proposed as new materials for luminescent bulbs [3-
6], electron-field emitters in panel displays [7-9], single-molecular transistors , scanning probe microscope tips ,
high-power capacitors , and next generation molecular electronic devices . The coiled carbon structures were first
predicted by Ihara and Dunlap [14-17]. The experimental observations were reported for the production of coiled nanotubes
[18-20]. These types of structures are obtained due the presence of pentagon and heptagon rings at the curved portion in the
structure of CNTs. The pentagon carbon ring is responsible for the formation of positive curvature whereas the heptagon
carbon ring forms a negative curvature . Computer simulations with molecular dynamics calculations have shown that coiled
nanotubes are energetically and thermodynamically stable . Many researchers have been practiced important methods for the
growth of carbon nanocoils. Pan et al. reported synthesis of CNCs in the presence of iron-coated indium tin oxide as a
catalyst . Xie et al. synthesized regular coiled nanotubes over three types of catalyst, Fe-magnesium carbonate, Fe-
silica, and Ni-zeolite . Koos et al. reported coiled nanotubes growth by laser evaporation of a fullerene /Ni particle
mixture in vacuum using 532 nm laser pulses of 12–28 m J from Nd YAG laser on freshly cleaved graphite (HOPG) surface .
Many researchers have worked to produce carbon microcoils and proposed that sulphur and phosphorous elements will promote the
microcoiled structure and they used thiophene for the production of the CMCs. Mukhopadhyay et al. produced CMCs using
thiophene [26-27] as the promoter of sulphur in order to attain the CMCs. In this work we have practiced a simple coating
method to attain the growth of the coiled CNTs in the absence of these promoters. We have grown coiled carbon structures by
novel, simple and economical method. Nanotubes are synthesized by catalytic chemical vapor deposition using
C2H2, H2, and N2 as precursors. We have coated metallic kanthal wire with Ni
catalyst using electroless dip coating technique in basic medium followed by pre-treatment process. This novel approach
involves no toxic gases and gives controlled growth of perfect coiled CNTs which can be used in variety of vacuum electronic
Firstly samples were polished with a solution containing HNO3 (1 mL), H2O2 (1 mL) and
oxalic acid (1 g) in a 50 mL de-ionized water for 5 min. This solution removes contaminants and oily or fatty impurities from
the surface of the wire. In our experiment we have used Ni catalyst on the surface of the wire by dipping it into a bath
comprised of 30 g/L nickel sulphate (NiSO4.6H2O), 25 g/L sodium hypophosphite
(NaH2PO2.H2O), 40 g/L ammonium chloride (NH4Cl) and 20 g/L tri sodium citrate
(Na3C6H5O7.2H2O). Ammonium hydroxide (NH4OH) was used to
adjust the pH of the bath. The Ni catalyst deposition was tried for different bath temperatures and pH parameters. The Ni-
film deposition was optimized for 80°C and 8 pH of the bath.
The catalyst coated samples were placed inside the middle zone of the CVD furnace chamber. Acetylene gas was used as the
carbon source, N2 gas was used to provide the inert atmosphere and H2 gas was used as a carrier gas and
to reduce the catalyst film in to particles. The conditions under which the CNCs were grown were at 700°C temperature, gas
flow rates of 200 sccm for N2, 25 sccm for H2 and 60 sccm for C2H2. The CVD was
operated with two different processing conditions i.e. precursors were passed through absence or presence of
H2SO4. CNT coated kanthal wire is shown is Fig. 1. The characterization of Ni film deposited and
tubular carbon structures grown samples was carried using different characterization tools i.e. X-ray diffraction (XRD)
technique, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), transmission electron microscopy
(TEM), and current-voltage (I-V) measurement. XRD analysis of different samples were performed with CuKα (λ =
1.54184 Ao) radiation. The entire samples were characterized at room temperature with scanning range 30° to 70°, applied
voltage 40 KV and scanning speed 3° per min. Electron microscopy analysis was carried out under vacuum 8.5X10-5
Torr and applied voltage of 20 KV. I-V measurements were done on the surface of CNT coated kanthal wire. For I-V measurements
the voltage was applied from 0 to 7 V.
Figure 1. Fabricated CNT cathode
Results and Discussion
Fig. 2 (a) shows two peaks at 2θ = 44.54° which corresponds to a Ni (111) and also a small peak of Ni (200) at
2θ=51.2° which explores the polycrystalline structure of the Ni coating. At higher coating time the increase in the
broadness of the peak was noticed which caused the decrease in grain size. Fig. 2 (b) shows XRD pattern for different bath
temperature. The optimized bath temperature was observed 800C. The grain size increased initially and reduced further due to
the higher amount of Ni ions concentration in the solution. Electroless nickel deposition process advances by hydrogen ions
generation resulting a drop in bath pH . The electroless nickel reaction kinetics is controlled by hydrogen ions
concentration in the catalyst bath. The H2 evolution rate increases due to the increase of H2 ion
concentration, as a consequence the elemental deposition rate is expected to decrease drastically with coating time and
affects in the decrease of grain size.
Figure 2. XRD pattern of Ni coated kanthal substrate for (a) different bath temperature (b) different
EDAX and SEM Analysis
Fig. 3 shows EDAX of Ni catalyst coated kanthal substrate. The surface morphology of Ni coated kanthal wire which was
analyzed through SEM for different coating time, as shown in Fig. 4. The thickness of the film was observed from 500 nm to 20
μm for the catalyst coating time ranges 5 to 20 min. The cracks in the coating were observed due to the presence of the
phosphorous. These cracks were probably induced by the internal stresses, generated by the adsorption–desorption processes
involved in the redox reactions and the co-deposition of P. Fig. 5 (a), (b) and Fig. 6 (a), (b) show the SEM micrographs of
coiled tubes over Ni catalyst coated kanthal wire for 5 and 10 min coating time. In the absence of sulphur compound the
length of CMCs and CNCs varies from 30 μm to 50 μm. The diameter of CMCs varies from 200 nm to 500 nm and CNCs
diameter ranges from 700 nm to 900 nm. Now the Ni catalyst coating time was further increased from 10 to 15 min. All other
processing conditions were same but no significant improvement with respect to the yield of CNTs/CNCs/CMCs was observed.
Figure 3. EDAX of Ni coated kanthal substrate
Figure 4. SEM micrographs of Ni coated kanthal wire for different coating time (a) 5 min (b) 10 min
(c) 15 min
Figure 5. (a), (b) SEM micrographs of carbon nanostructure coated kanthal wire synthesized in the
absence of sulphur at 700°C (catalyst coating for 5 min at 80°C coating temperature)
Figure 6. (a), (b) SEM micrographs of carbon nanostructure coated kanthal wire synthesized in the
absence of sulphur at 700°C (catalyst coating time 10 min at 80°C coating temperature)
Improvement in the yield of coiled tubes was observed when precursors were passed through H2SO4.
Fig. 7 (a) and (b) show the SEM micrographs of coiled CNTs at different magnifications. At 700°C growth temperature there was
a yield of CNTs with 50 nm to 150 nm in diameter and 30 μm to 200 μm in length. The CNCs and CMCs diameter ranges
from 100 nm to 250 nm and 600 nm to 800 nm. The length of CNCs varies from 80 μm to 150 μm. From these observations
it can be explored that the coiled tubes with small diameter and with large length can be made in presence of sulphur.
Fig. 8 (a) and (b) are the SEM micrographs of coiled tubes grown on 10 min catalyst coated substrate at 80°C coating
temperature. No significant difference was observed in this growth condition. Sulphur atoms avoid the formation of amorphous
carbon on the catalyst particles. This helps to get a good yield of long CNTs and CNCs.
For a polycrystalline Ni plating the coil yield is in (111) and (110) directions i.e., there is a large anisotropic nature
among the respective crystal faces for the deposition of the carbon coils. From the XRD results of Ni catalyst coating the
thin film formed was a polycrystalline in form and highly oriented in the (111) direction. Many researchers have suggested
that this anisotropic nature is most effective for coiling the CNTs. Yang et al. have reported for the carbon filament growth
using the Ni catalyst pyrolysis of methane at 700°C that the most favoured face for graphite precipitation is Ni(111) .
From these observations it is proposed that yield based on crystal faces is an important factor for the coiled growth.
Figure 7. (a), (b) SEM micrograph of carbon structure coated kanthal wire synthesized in the presence
of sulphur at 700°C (catalyst coating time 5 min at 80°C coating temperature)
Figure 8. (a), (b) SEM micrograph of carbon structure coated kanthal wire synthesized in the presence
of sulphur at 700°C (catalyst coating time 10 min at 80°C coating temperature)
TEM micrograph of CNTs grown at 700°C temperature is shown in Fig. 9. TEM image confirmed the presence of multi walled
carbon nanotubes (MWNTs).
Figure 9. TEM micrograph of coiled tubes
A two probe I-V measurement was performed on the coiled tubes coated kanthal wire as shown in Fig. 10. The curves in the
plot belong to the coiled CNTs coated samples at 700°C growth temperature in the absence or presence of sulphur. I-V curve
shows a low current flow at the low voltages but it is gradually increased at higher voltages. Also it was observed that at
the low voltages the curves show nonlinearity and becomes linear at higher voltages.
Figure 10. I-V characteristics of CNT coated and uncoated kanthal substrate
We have grown coiled carbon structures by novel, simple and economical method. Various CNTs are synthesised by catalytic
CVD using C2H2, H2, and N2 as precursors. We have coated metallic kanthal wire
with Ni catalyst using electroless dip coating technique in basic medium followed by pre-treatment process. In the presence
of sulphur the yield of coiled carbon material is increased tremendously. This novel approach involves no toxic gases and
gives controlled growth of perfect coiled CNTs. I-V curve shows that fabricated coiled carbon structures coated cathode may
have potential applications in vacuum electronic devices.
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Sanjay Agarwal, B. Yamini Sarada and Kamal K. Kar
Department of Materials Science, Advanced Nano-Engineering Materials
Indian Institute of Technology Kanpur, India
This paper was also published in print form in "Advances in Technology
of Materials and Materials Processing", 11 (2009) 49-56.