Growth and characteristics of carbon nanotubes obtained under different C2H2/H2/NH3 concentrations

Aligned carbon nanotubes were synthesized under a combination of 20 different C₂H₂/H₂/NH₃ compositions at 700 °C using a thermal chemical vapor deposition method. Thin film Fe was used as the catalyst, which was pretreated with H₂ or NH₃ prior to the growth of carbon nanotubes. The use of different pretreatment gases results in little difference in the growth and characteristics of the carbon nanotubes except that the carbon nanotubes grown on H₂ treated catalysts have smaller diameters. The growth rate of the CNTs does not depend on the NH₃ concentration but on the ratio of NH₃/C₂H₂. There is a critical NH₃/C₂H₂ ratio that is independent of the C₂H₂ concentration and at which the peak growth rate occurs. The critical value was found to be 4.7 ± 1.2. Microstructural analysis indicates that the carbon nanotubes obtained at higher NH₃ concentrations contain defects and disorder. Field emission tests show that the carbon nanotubes exhibit a turn-on field of 2.36 V/μm and a maximum current density of 1.91 mA/cm². The field emission properties were found to be stable after 15 test cycles.