Elsevier

Materials Letters

Volume 67, Issue 1, 15 January 2012, Pages 124-127
Materials Letters

Synthesis of multi-walled C nanotubes by Fe–Ni (70 wt.%) catalyzed chemical vapor deposition from pre-heated CH4

https://doi.org/10.1016/j.matlet.2011.09.011Get rights and content

Abstract

This study aims to investigate effect of pre-heating of pure CH4 on the growth of C nanotubes by chemical vapor deposition in the presence of Fe–Ni (70 wt.%) alloy catalyst. It was observed that un-preheated CH4 yielded no C formation at 1050–1150 K owing to its poor decomposition rate, contrary to the results of thermodynamic analysis. Whereas, pre-heating treatment of CH4 at 1200 K prior to the growth increased pyrolysis of the hydrocarbon which promoted synthesis of C nanotubes at 1050–1150 K, possibly due to enhanced formations of intermediate gaseous species. TEM study revealed that the morphologies of the products consist of multi-walled C nanotubes.

Introduction

Carbon nanotubes (CNTs) have received considerable attention owing to their unique electronic and mechanical properties that are expected to lead to breakthrough industrial applications. CNTs have been synthesized generally by chemical vapor deposition (CVD), laser ablation and arc discharge techniques [1]. Among these techniques, CVD has inherent advantages such as better control of process parameters and higher scalability. Various transition metal catalysts and C sources such as CO, hydrocarbons usually diluted with H2, He or Ar have been used for the synthesis of CNTs by CVD [2], [3], [4], [5], [6], [7]. Catalyst composition plays an important role in the growth of this material [4], [7], [8]. For example, carbon morphology changes from fish bone structure to tubular one with increasing Ni content in Fe–Ni catalysts using CO–H2 gaseous mixtures at 923 K [4].

CH4 has been used as a carbon supply for the production of CNTs by CVD. The growth from CH4, however, takes place at elevated temperatures (e.g. 1273 K) owing to its high thermal stability compared to other gaseous C sources. The use of pre-heated CH4 may be advantageous to reduce growth temperatures. To our knowledge, little is known about the effect of pre-heating of CH4 on the synthesis of CNTs. The work reported here was undertaken to investigate this effect at the growth temperatures in the range 1050–1150 K using pure CH4 pre-heated at 1200 K or 1300 K. The purpose of using pre-heated methane was to promote synthesis of C nanotubes at lower temperatures (1050–1150 K). We used undiluted CH4 as a carbon source because it is relatively cheap, abundant (a main component of natural gas) and environmentally friendly.

Section snippets

Materials and methods

The experimental set-up used for the present study essentially consists of a hot-wall furnace with SiC heating elements, a quartz tube (20 mm in diameter, 500 mm in length) and gas flow meters. The details of the system including temperature profile of the furnace were given elsewhere [9]. Fe–Ni (70 wt.%) alloy catalyst with particle sizes < 80 nm used in this study was produced by spray pyrolysis [10]. The particles were mixed with ethanol by ultrasonic agitation. Substrates cut from a silicon

Results and discussion

Fig. 1 shows TEM images of the product grown from un-preheated CH4 at 1200 K for 30 min. As seen from Fig. 1a, hollow multi-walled tubes with an outer diameter of 75–85 nm and inner diameter of ~ 10 nm were synthesized. There is also a fragment of catalyst particle inside the tube (marked by an arrow) with a size about the same as the inner diameter. Clear breaks of the tubes shown in Fig. 1b indicate that the fractures were not caused by the growth process, but by the sample preparation for the TEM

Conclusions

TEM study showed that multi-wall C nanotubes were synthesized by chemical vapor deposition from pure CH4 in the presence of Fe–Ni (70 wt.%). The experimental results indicate that CH4 decomposition is not sufficient to form C at temperatures < 1200 K (e.g. 1050 K) contrary to thermodynamic predictions. Pre-heating of CH4 at 1200 K increases, however, its decomposition rate possibly owing to enhanced formations of intermediate gaseous species. It was demonstrated that C nanotubes were grown at the

Acknowledgements

This work was supported by the Scientific Research Projects Coordination Unit of Istanbul University (Project Number 1459). It was partially based on a Ph.D. thesis pursued by M.C. Altay.

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