In situ XANES Analysis of Co and Ni Catalysts during Single-Walled Carbon Nanotube Growth
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In situ XANES Analysis of Co and Ni Catalysts during Single-Walled Carbon Nanotube Growth
Makoto Kumakura1, Hoshimitsu Kiribayashi1, Takahiro Saida2, Shigeya Naritsuka1 and Takahiro Maruyama2
1 Department of Materials Science and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya 468-8502, Japan
2
Department of Applied Chemistry, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya 468-8502, Japan
ABSTRACT
In situ X-ray absorption near edge structure (XANES) measurements were conducted to elucidate the chemical states of Co and Ni catalysts during single-walled carbon nanotube (SWCNT) growth via chemical vapor deposition (CVD). XANES spectra indicated that both Co and Ni catalysts partially oxidized before heating. It was found that Co catalysts formed carbides during the SWCNT growth. In contrast, Ni catalysts remained metallic state even after the SWCNT growth had begun. These results indicate that during SWCNT growth, carbon atoms dissolve into Co particles, whereas for Ni particles, they diffuse on the surface region. It was concluded that the growth mechanisms of SWCNTs formed by CVD differed for either Co or Ni catalyst.
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INTRODUCTION
Single-walled carbon nanotubes (SWCNTs) [1] are an interesting class of nanomaterials that exhibit some remarkable properties, such ballistic transport [2], high mobility [3], and high current carrying capacity [4]. These properties allow for their potential use in various future nanoelectronics applications [5-8]. At present, the chemical vapor deposition (CVD) method using metal catalyst nanoparticles is used for SWCNT growth [9, 10]. In particular, Fe, Co, and Ni are widely used as catalysts. For electronics devices, SWCNT structure control is essential since the electronic state of an SWCNT is uniquely determined by its structure (diameter and chirality). An understanding of the growth process of SWCNTs is therefore indispensable. This understanding is also necessary to investigate the catalyst properties such as particle size, crystallinity, and chemical states during SWCNT growth. Recently, a density functional theory (DFT) calculation pointed out that the dissolution of carbon atoms into catalyst particles affects the diameters of SWCNTs [11]. However, the chemical states of catalyst particles cannot be accurately investigated during SWCNT growth because the particle size of catalysts is only 1–3 nm. So far, the chemical states of catalyst particles during SWCNT growth were analyzed via in situ observations using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy. Using in situ TEM, Picher et al. reported that carbon atoms dissolved into Co catalysts during SWCNT growth and that the nanotubes were grown by precipitation [12]. In contrast, Lin et al. used in situ TEM to
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