(287e) Evaluation of Sulfur Role As a Promoter for the Growth of Carbon Nanotube in Chemical Vapor Deposition

Because carbon nanotubes (CNTs) possess a lot of outstanding physical properties, many applications using CNT are studied around the world. Accordingly, a variety of CNT production processes have been proposed until now such as laser ablation, arc discharge, flame synthesis, and chemical vapor deposition (CVD). Among these processes, CVD is recognized as a most promising process for CNT mass production and actually commercial production has been achieved by this method. It has been reported in CVD that the addition of compound including sulfur in its chemical structure facilitated the growth of CNT or carbon fiber, in particular floating catalyst process. Several reasons of such promoted effects have been proposed by many groups, like the reduction of melting temperature of catalyst, decreasing surface free energy of catalyst particles, blocking catalyst active sites to moderately supply carbon atoms for preventing catalyst deactivation, and so on. However, studies to explore the role of sulfur for CNT formation have been insufficient to date. In this work, we aimed to evaluate the effect of sulfur as a promoter for the growth of CNT in CVD using substrate growth method.

In this study, CNTs were synthesized on quartz substrate on which cobalt-molybdenum co-catalyst including sulfur and carbon monoxide as a carbon source were employed. Cobalt acetate and molybdenum acetate dimer used as Co and Mo precursor respectively were dissolved in dehydrated ethanol under an ultrasonic environment for 2 hours so that their concentration to be 0.05 wt%. Before dip coating of quartz substrate into tailored solution, substrate was calcined in air at 900 ^oC for more than one hour in order to remove impurities on it. After immersing substrate in tailored solution for 10 minutes, it was drawn from solution at a constant velocity of 0.96 cm/min. Then, substrate was calcined in air at 400 ^oC for 5 minutes to decompose precursors to form Co-Mo-O particles. After calcination, sulfur loading on substrate was carried out. Substrate was again dip coated into tailored solution that sulfur powder was dissolved in toluene solvent. Immersion time in solution and drawing speed from the solution were the same with that in Co-Mo solution. Substrate was then reduced at 800 ^oC for 30 minutes to form active catalyst particles under the condition that the flow rates of Ar and H₂ were 60 sccm and 40 sccm, respectively. CVD to deposit carbon on substrate was performed at CO of 45 sccm and Ar of 155 sccm. Synthesized CNTs were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Catalysts layer was characterized by several methods such as atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP) analysis, and others.

Growth of CNTs was not confirmed up to CVD time of 60 min under the condition without sulfur, while formation of CNTs started to appear in case of Co-Mo-S system from 20 min as CVD time. When compared at the same CVD time (30 min), the difference was significantly obvious, that is to say, a large number of CNTs were synthesized with sulfur while they were hardly formed without sulfur. These results were supported by G peak of Raman spectroscopy which signifies the degree of graphitization stemmed from CNT formation. When sulfur concentration in dip-coating solution was changed as a parameter, maximum CNT production was found at 2.0 wt%. According to this result, the effect of blocking catalyst active sites for preventing catalyst deactivation caused by sulfur may be ruled out. Arrhenius plot was prepared based on changing reactor temperatures and observing by SEM images, indicating that the activation energy for CNT growth under a sulfur condition was significantly lower than the conventional one which has been reported in many researchers handling CNT growth. It is reasonable to consider that this result may be induced by sulfur, thus catalysts characterization was performed. From these analyses, we discussed the role of sulfur for growing CNT in CVD process using Co-Mo co-catalyst on quartz substrate and CO as a carbon source.