Water Concentration Influence on Catalytic Growth of Carbon Nanotubes in a Suspended Bed Reactor
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Water Concentration Influence on Catalytic Growth of Carbon Nanotubes in a Suspended Bed Reactor V.Z.Mordkovich1,2, A.R.Karaeva1,2, M.A.Khaskov1, I.G. Solomonik1, E.B.Mitberg1,2, B.A.Kulnitskiy1 and I.A.Perezhogin1 1 Technological Institute for Superhard and Novel Carbon Materials, 7A Centralnaya street, Troitsk 142190, Russia 2 INFRA Technologies Ltd., Mokhovaya 11-3B, Moscow 125993, Russia ABSTRACT One of the most important problems in the synthesis of carbon nanotubes is the problem of controlling their morphology, namely: length, aspect ratio, alignment, etc. Catalytic synthesis of carbon nanotubes in a suspended bed reactor allowed to study the possibilities of controlling the growth of nanotubes by introducing a certain amount of water vapor and carbon-containing materials in the reaction zone. The synthesized long carbon nanotubes were studied by Raman spectroscopy, transmission and scanning electron microscopy. We found that water concentration influences both yield and the structure of nanotubes. It is shown that the yield of centimeter-long nanotubes can be maximized at an optimum H2O/C ratio, while deviations dramatically change morphology and thickness of the nanotubes. INTRODUCTION A pathway to longer nanotubes of millimeter or centimeter length leads through optimization of several important growth parameters. The following parameters are usually named in this quality, i.e. catalyst composition, feedstock composition, feedstock flow velocity, contact time, synthesis temperature and synthesis duration [1-3]. Other factors are sometimes cited as important ones such the use of catalyst activation by sulfur-containing chemicals, by laser irradiation or by microwaves [4-6]. The main purpose of optimization is to overcome limitations, which prevent nanotubes from reaching greater length, namely [7, 8]: (a) Slow growth (usually not faster than millimeters per hour); (b) Termination of growth due to catalyst encapsulation or poisoning; (в) Competition between axial and radial growth processes. It is shown in literature that contact time is very much instrumental in controlling CNT elongation during synthesis. There are however indications in early works [9, 10] that longer contact time may result in domination of radial growth, which, in turn, leads to formation of carbon microfibers rather than carbon nanotubes. The formation of microfibers is unwelcome in this respect due to significant difference in properties in comparison with CNT [1, 5-11]. The suppression of radial growth can be realized in many ways, some of them suggested by theoretical considerations [12] and some by experience [13-15]. In particular, introduction of oxygen-containing compounds (water, alcohols) into reaction zone could be one of good solutions. In addition, it is known that water vapor suppresses formation of amorphous carbon and stimulates growth of straight CNT [16, 17]. Since no systematic data on the water influence on CNT growth has been reported up to date, it is of interest to do such research in a wide range of water concent
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