Surface Reactions of Metal Catalysts in Ethanol-CVD Ambient at Low-Pressure Studied by in-situ Photoelectron Spectroscop
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0963-Q05-04
Surface Reactions of Metal Catalysts in Ethanol-CVD Ambient at Low-Pressure Studied by in-situ Photoelectron Spectroscopy Fumihiko Maeda1, Satoru Suzuki1, Yoshihiro Kobayashi1, Daisuke Takagi2, and Yoshikazu Homma2 1 NTT Basic Research Laboratories, NTT Corporation, and CREST, JST, 3-1 MorinosatoWakamiya, Atsugi-shi, Kanagawa, 243-0198, Japan 2 Department of Physics, Tokyo University of Science, and CREST, JST, 1-3 Kagurazaka, Shinjuku, Tokyo, 162-8601, Japan
ABSTRACT We succeeded in growing carbon nanotubes in a photoelectron spectroscopy analysis system using thermal chemical vapor deposition and analyzed the chemical states of the Co catalysts by in-situ x-ray photoelectron spectroscopy before and after the growth. We found that almost all of the Co particles are metallic after the growth in both cases; Co particles are formed from a Co oxide thin film and a metallic Co thin film. This shows that the metallic state is stable for Co under low-pressure ethanol ambient in our growth condition for carbon nanotubes. INTRODUCTION Chemical vapor deposition (CVD) is one of the most controllable techniques for carbon nanotube (CNT) growth. An understanding of the mechanism of CNT growth by CVD is the key to controlling the CNT structure, such as its chirality. This mechanism has therefore attracted a great deal of interest. In the CVD growth of CNTs, nanoparticles are required as catalysts and CNTs grow from their surfaces. Therefore, there is a consensus in the various proposed growth mechanisms that metal catalysts play an important role. However, a clear understanding of the chemical and physical states of catalysts during CVD growth has not been established. In the several reported transmission electron microscope (TEM) studies [1-3], attempts were made to observe the behavior of growth-material atoms on the surface of catalyst nanoparticles. For the CVD growth, the growth process has been analyzed by x-ray diffraction (XRD) and carbide formation was observed during the CNT growth [4]. However, these studies explained multi-walled (MW) CNTs, and there is uncertainty that the growth mechanism of single-walled (SW) CNTs is the same as that of MWCNTs. Although some attempts at in-situ analyses of SWCNT growth were made [5, 6], due to technical difficulties, the states of catalysts are still unclear in the case of SWCNTs. X-ray photoelectron spectroscopy (XPS) is a powerful technique for chemical analyses at surfaces and therefore one of the most suitable analysi methods for this type of investigation. To understand the catalyst state during CVD growth, a real-time analysis is desirable but technically difficult. Therefore, in this work, we looked at the reaction process by which the nanoparticles of catalysts are formed on a substrate surface by analyzing the reaction products in a stable state in an ultra-high vacuum (UHV). Even for such an analysis, in-situ analysis without exposure to air is essentially important because surface analysis methods are surface sensitive. However, for
XPS measurements, an
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