Role of oxygen in the growth of carbon nanotubes on metal alloy fibers by plasma-enhanced chemical vapor deposition
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Seok Joo Park Clean Energy System Research Center, Korea Institute of Energy Research, Yuseong-gu, Daejeon 305-343, Republic of Korea
Tae Gyu Kim and Soo H. Kima) Department of Nanosystem and Nanoprocess Engineering, Pusan National University, Miryang-si, Gyeongnam 627-706, Republic of Korea (Received 25 April 2008; accepted 6 January 2008)
A method allowing for the stable growth of carbon nanotubes (CNTs) on the surface of a fibrous metal mesh substrate (SUS304) was developed with the assistance of the microwave plasma-enhanced chemical vapor deposition process. The controlled addition of up to 13% of O2 to the CH4 plasma reacting gas flow was found to promote the growth of the CNTs by oxidizing the amorphous carbon and removing the active H2 radicals. However, excessive amounts of O2 (i.e., fraction of O2 > 13%) and H2 were found to play a negative role in the growth of the CNTs. The control of the density and length of the CNTs was also achieved by varying the H2 plasma reduction time and CH4 plasma reacting time, respectively. Longer H2 reduction pretreatment of the catalytic metal islands resulted in the formation of a less dense CNT forest with craters. When the growth time of the CNTs was increased to 20 min, their length was increased to 10 mm. However, when the growth time of the CNTs exceeded 20 min, their length was significantly decreased, indicating that the continuous presence of O2 in the CH4 plasma destroys the preformed CNTs due to the oxidation reaction.
I. INTRODUCTION
Thermal chemical vapor deposition (CVD) processes have been used by numerous researchers to grow carbon nanotubes (CNTs) on a catalytic metal medium because of their simplicity.1–3 However, the repeatability of the stable growth of CNTs in thermal CVD processes is not easy to achieve. This is because the relatively hightemperature environment in thermal CVD processes (e.g., tube furnace) causes the hydrocarbon sources to be directly decomposed into amorphous carbon, which results in the coking of the catalytic particles. Unlike thermal CVD processes, plasma-enhanced CVD (PECVD) processes have the advantage of maintaining a precisely controlled local temperature condition on the heating stage and vacuum operating conditions, which minimize the coking of the catalytic particles due to amorphous carbon contamination in the plasma reactor. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0174
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J. Mater. Res., Vol. 24, No. 4, Apr 2009
Given the advantages of PECVD processes, we used a microwave plasma-enhanced chemical vapor deposition (MPECVD) process to grow CNTs directly on a commercially available metal (SUS304) substrate without preformed catalytic metal particles, which can be easily adapted to industrial applications. On the basis of guidelines suggested by numerous researchers,4–7 we performed a series of MPECVD experiments to grow CNTs. However, the growth of CNTs on the surface of a catalytic metal substrate was rarely achieved in our MPECVDbased approach. Instead,
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