Proper Combination of Catalyst Materials and Ethanol for High Yield in CVD Growth of Carbon Nanotubes
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1081-P01-03
Proper Combination of Catalyst Materials and Ethanol for High Yield in CVD Growth of Carbon Nanotubes Fumihiko Maeda, and Yoshihiro Kobayashi NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa, 243-0198, Japan ABSTRACT Carbon nanotubes (CNTs) were grown by using a combination of ethanol and metallic Fe, which was not exposed to air before the growth. CNTs were also grown using intentionally oxidized Fe. The reaction products by these growths were analyzed by in situ x-ray photoelectron spectroscopy. We found that the oxidized Fe partially remained and did not completely reduce, resulting in low CNT yields. At higher growth temperature, oxide formation was found even when metallic Fe was used. These oxides induced a lowering of the decomposition efficiency of ethanol, resulting in difficulty of CNT growth by the combination of pure ethanol and Fe. INTRODUCTION To shed light on the growth mechanism, we have been studying the reaction of catalysts during chemical vapor deposition (CVD) of carbon nanotubes (CNTs) by analyzing reaction products using in situ x-ray photoelectron spectroscopy (XPS). This in situ XPS is one of the most suitable methods for the analysis of reactions during CNT growth and several researchers also have been investigating that by this analysis [1, 2]. We have already investigated CNT growth by the combination of Co and ethanol because the use of ethanol is advantageous for the growth of single-walled CNTs (SWCNTs) when Co is used. This investigation resulted in the high yield synthesis of SWCNTs. We have shown that the metallic state is stable for Co under ethanol-CVD ambient [3, 4]. However, for the combination of ethanol and Fe, it is empirically known that the CNTs can be grown under the determinate condition. When other hydrocarbon gases, such as methane, ethylene, or acetylene, are used as a growth gas, Fe acts efficiently as a catalyst for the synthesis of CNTs. For instance, the low-temperature and low-pressure growth of SWCNT has been performed by using these combinations of growth gases and catalyst metal [5, 6]. We therefore think that understanding the cause of the low CNT yield with the combination of ethanol and Fe would provide a clue to obtain a high yield of SWCNTs, like in the case of CVD growth with the combination of ethanol and Co. In this work, we therefore investigated the reactions of Fe in ethanol ambient. Particularly, we achieved the growth of SWCNTs using the as-deposited metallic catalysts (not oxidized) without exposing the catalysts to air. This allowed us to grow CNTs from intentionally oxidized metal catalysts under uniform conditions and to elucidate the effects of the oxidation of metal catalysts induced by exposing them to air. We also grew SWCNTs at various substrate temperatures and found that the Fe partially oxidized during growth at a high substrate temperature. The growth using the metallic Fe catalyst also allowed to find this oxidation.
EXPERIMENT We used an XPS analysis system [7] equipped wi
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