The synthesis of vertically aligned multi-walled carbon nanotube forests by thermal chemical vapor deposition

Despite the many potential applications and attractive characteristics of carbon nanotubes (CNTs), a major challenge on the way toward their commercialization remains their controlled synthesis. In this chapter, we present a parametric study of the synthesis of vertically-aligned CNT forests using an ethylene-based chemical vapor deposition system without any oxidizing agent. We also provide an overview of the complex effects of hydrogen and an interpretation of the non-monotonic effect of the hydrogen flow rate on the resulting carbon nanotube forests. Even though hydrogen plays a partial role as a carrier gas (other than acting as a reducing agent), variations in the hydrogen and argon flow rates lead to distinct trends in the synthesis outcome, pointing to the diverse roles for the reducing and carrier gases: adjusting the proportions of hydrogen and argon for a given total flow rate had a drastic impact. The gas flow profile was optimal at an ethylene-to-total flow ratio of 8% and ethylene-to-hydrogen ratio of 25%, respectively. A strong temperature dependence was also observed: while the growth was similarly successful at 675 C and 725 C, a temperature of 650 C led to severely suppressed yield. According to transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy analysis, the tip-growth mechanism dominates in this synthesis method.