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Introducing a small amount of water vapor in the catalytic chemical vapor deposition (CVD) growth of single- and double-walled carbon nanotubes (SWNTs and DWNTs) extends the catalyst lifetime and increases the nanotube yield. We study the mechanism of this water-assisted nanotube growth over a Fe−Mo/MgO catalyst, based on analysis of the effluent gas, in terms of chemistry of the water-induced oxidation and its effects on catalytic activity. Water vapor was found to etch away carbon precipitate covering the metal catalyst, based primarily on the chemical reaction C + H2O → CO + H2, thus maintaining the catalytic activity. This oxidative etching was strongly dependent on the CVD temperature, and the balance between the etching and carbon precipitation was important for effective nanotube growth. With an optimized water concentration, the etching rate of the carbon precipitate was estimated to be ca. 1/1000 of the formation rate of carbon precipitate consisting mainly of SWNTs and DWNTs.

Horizontally aligned growth of single-walled carbon nanotubes (SWNTs) on single-crystal surfaces has attracted great interest in terms of nanoelectronic applications, but their growth mechanism is not fully understood. We report on the 13C/12C isotope-labeled growth of SWNTs on a sapphire surface to visualize their growth process. Switching carbon feedstock from 13CH4 to 12CH4 during SWNT growth induces a gradient distribution of the carbon isotopes along the tube axis. From the Raman mapping analysis, we succeeded to observe the gradual change in the isotope distribution of individual SWNTs. The results indicate the base-growth mode for the horizontally aligned SWNTs, which suggests that nanotube−sapphire interaction is essential to alignment. This method offers a unique technique to analyze the nanotube growth mechanism and kinetics.

We report controlled horizontal alignment of single-walled carbon nanotubes (SWNTs) directly grown on trenched SiO2/Si substrate. The nanotubes were found to align along the trenches, which were created via electron beam lithography followed by reactive ion etching. From the experimental observations, the alignment mechanism was proposed. Furthermore, field-effect transistors fabricated from these substrates showed acceptable mobility and on/off ratio as high as 104. The method offers the possibility of large-scale integrated SWNT electronics for mass production.

Carbon nanotubes have been widely synthesized by chemical vapor deposition (CVD) in the presence of transition-metal catalysts, such as Fe and Co, because they have strong catalytic activities for both the decomposition of hydrocarbon feedstock and the formation of graphitic carbons. Recently, gold (Au) was reported to catalyze the growth of multi-wall and single-wall carbon nanotubes (MWNTs and SWNTs, respectively). Because of the low solubility of carbon in Au and the unique catalytic activity observed for Au nanoparticles, the growth mechanism and yield of nanotubes over Au catalysts are intriguing. In this paper, we investigated the catalytic activity of Au supported on various metal oxides, alumina (Al 2 O 3 ), silica (SiO 2 ), titania (TiO 2 ), and magnesia (MgO), with different hydrocarbon feedstocks, methane (CH 4 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ). We found that Au-supported catalysts have limited activity for the decomposition of the carbon source from the gas analyses and that only the catalyst supported on Al 2 O 3 and SiO 2 gave a small number of MWNTs when reacted with C 2 H 4 or C 2 H 2 . The formation mechanism of MWNTs over the Au-supported catalysts is also discussed.

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The selective electrochemical reduction of carbon dioxide (CO 2 ) was demonstrated over single-crystal copper membranes. The crystalline copper membranes were deposited on specific support substrates that were c-plane sapphire for Cu(111), MgO(100) for Cu(100), and MgO (110) for Cu(110). Methane (CH 4 ) and ethylene (C 2 H 4 ) formation in CO 2 electrochemical reduction was more favorable on Cu(111) membranes than that on polycrystalline copper plates. The electrolyte type that CO 2 gas was effectively dissolved in, was important to significantly maintain electrochemical activity. The product selectivity for CO 2 conversion was strongly dependent on the crystal structure of the copper surface.

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