Chemical vapour deposition synthesis of carbons from halogen and silicon sources

Kao, Mahalieo
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In this study carbonaceous materials were produced using the floating catalyst horizontal CVD method by injection of a hydrocarbon solution into the high temperature zone of the reaction chamber. Various hydrocarbon reagents were used. The reagents included toluene, chlorobenzene (ClBz), bromobenzene (BrBz), chloroform, tertraethylorthosilicate (TEOS), and triethylchlorosilane (TESCl). Catalytic and noncatalytic reactions were investigated. Ferrocene was used as a source of catalyst during catalytic reactions. The reactions were carried out under a flowing Ar/H2 (5 %) (v/v) gas mixture at atmospheric pressure. Carbon materials synthesized in this study were characterized by Raman spectroscopy, TEM analysis, SEM analysis, Infra red spectroscopy, TGA, EDS and XRD analyses. Catalytic pyrolysis of toluene, ClBz, and BrBz resulted in production of multi-walled carbon nanotubes (MWNTs). It was found that the percentage content of the haloarene in the feed impacts on the morphology of carbonaceous materials produced during catalytic pyrolysis. A change in morphology from spheres to a mixture of spheres and tubes and finally to tubes was observed. The influence of the gas flow rate on the type of carbons prepared from catalytic pyrolysis of ClBz has been studied. At high gas flow rates MWNTs were the sole product but at low flow rates carbon spheres resulted. Carbon spheres were also prepared from non-catalytic reactions. It was established that non-catalytic pyrolysis of ClBz or chlorinated species and toluene enhance product yields. It was established that the size distribution of spheres can be controlled by using hydrogen as a carrier gas. The impact of additives such as thiophene and ammonia on the pyrolysis of toluene and ClBz was investigated. They were found to inhibit formation of the graphitic carbon from toluene but not from ClBz. TEOS and TESCl were used as sources of silicon. CNTs were prepared from a TEOS/toluene feed. It was found that the preparation of CNTs from a feed containing low concentrations of TEOS enables control of the diameter distribution and also results in a remarkable improvement in CNT length as compared to CNTs prepared from toluene. However, it was found that high concentrations of TEOS in toluene are destructive. A drastic deterioration of lengths and yield was observed at high concentrations of TEOS in toluene. Non-catalytic decomposition of TEOS produced core/shell structures formed from codeposition of carbon and silica. Subsequent thermal treatment of the carbon/silica core/shell structures in air resulted in formation of hollow core silica structures. Decomposition of TESCl in the absence of a catalyst produced high yields of carbon spheres with a wide diameter distribution. EDS analysis revealed that the spheres prepared from TESCl are composed of carbon, silicon and chlorine. This suggests production of chlorinated silicon/carbon composites. Thermal treatment in air, at 1000 °C produced core/shell structures.