A Novel Approach to Synthesize Carbon Nanotubes, Carbon Nanocoils, Carbon Microcoils on the Surface of Metallic Wire: Application in Vacuum Electronic Devices

Sanjay Agarwal, B. Yamini Sarada and Kamal K. Kar

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AZojomo (ISSN 1833-122X) Volume 6 December 2010

Topics Covered

Abstract

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.

Keywords

Carbon Nanotube, Carbon Nanocoil, Carbon Microcoil, Chemical Vapor Deposition, Electron Microscopy, Electroless Plating

Introduction

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 [1], CNT based logic circuits [2], 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 [10], scanning probe microscope tips [11], high-power capacitors [12], and next generation molecular electronic devices [13]. 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 [21]. Computer simulations with molecular dynamics calculations have shown that coiled nanotubes are energetically and thermodynamically stable [22]. 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 [23]. Xie et al. synthesized regular coiled nanotubes over three types of catalyst, Fe-magnesium carbonate, Fe- silica, and Ni-zeolite [24]. 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 [25]. 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 applications.

Experimental

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

XRD Analysis

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 [28]. 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 coating time

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) [29]. 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 Analysis

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

I-V Characteristics

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

Conclusions

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.

References

Contact Details

Sanjay Agarwal, B. Yamini Sarada and Kamal K. Kar
Department of Materials Science, Advanced Nano-Engineering Materials Laboratory,
Indian Institute of Technology Kanpur, India

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 11[2] (2009) 49-56.