Investigating the Growth Process of Vertically Aligned Single-Walled Carbon Nanotubes Synthesized from Alcohol
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Investigating the Growth Process of Vertically Aligned Single-Walled Carbon Nanotubes Synthesized from Alcohol Erik Einarsson, Rong Xiang, Kazuaki Ogura, Jun Okawa, Zhengyi Zhang, and Shigeo Maruyama Dept. of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan ABSTRACT We have performed a systematic investigation of the influence of growth parameters on the synthesis of vertically aligned single-walled carbon nanotubes (VA-SWNTs) by the alcohol catalytic chemical vapor deposition (ACCVD) method. The growth process of the VA-SWNTs was monitored using an in situ optical absorbance technique and the effects of CVD temperature and ethanol pressure on the initial growth rate and the catalyst lifetime were investigated. We found that for a given CVD temperature, there is an optimum pressure at which VA-SWNT film growth is maximized, and this pressure increases with temperature. Below this optimum pressure, the growth reaction is first-order, with the arrival of ethanol to the catalyst being the rate-limiting step. The activation energy of the growth reaction was determined to be approximately 1.5 eV. The root-growth mechanism of VA-SWNTs synthesized by the alcohol CVD method was also confirmed by a two-stage growth process. Following a short growth period using normal ethanol, 13C -labeled ethanol was introduced to continue the growth. The location of the 13C was determined from resonance Raman spectra, confirming the root-growth mechanism. INTRODUCTION Single-walled carbon nanotubes (SWNTs) have attracted much attention in various scientific fields due to their unique physical properties, which arise from their one-dimensional nature. Some of these properties, such as near-ballistic transport [1], nonlinear optical absorption [2,3] and high thermal conductivity [4-6] make SWNTs particularly well-suited for many potential applications. Many of these properties, however, are highly anisotropic, thus it is desirable to control the orientation of SWNTs during synthesis. Significant advancements in this area have been made in the past few years, particularly regarding the synthesis of vertically aligned (VA-) SWNTs [7-12]. These synthesis methods, however, depend on several parameters such as temperature, pressure, flow rate, etc., and the underlying mechanism is not yet well understood. Here we report some recent findings related to the growth mechanism of VA-SWNTs, particularly the effects of growth temperature and pressure in the alcohol catalytic chemical vapor deposition (ACCVD) method [13].
EXPERIMENT The VA-SWNTs used in this study were synthesized on quartz substrates by the catalytic reaction between ethanol vapor and cobalt nanoparticles on the substrate surface. The catalyst nanoparticles were prepared [7,14] by dip-coating optically polished quartz substrates into two solutions. The first solution was molybdenum acetate, (CH3COO)2Mo, dissolved in ethanol (Mo content 0.01 wt%). After dip-coating, the substrate was baked in air at 400 °C for 5 min, and then
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