Effect of NH3 gas ratio on the formation of nitrogen-doped carbon nanotubes using thermal chemical vapor deposition

Nitrogen-doped carbon nanotubes (N-doped CNTs) were grown on Fe-coated Si substrates through thermal chemical vapor deposition using C2H2, H2, Ar, and NH3 gases. The effects of the flow rate of NH3 gas during the growth of N-doped CNTs were investigated. As the proportion of NH3 gas was increased from 0 to 30%, the growth rate exponentially decreased from 50 nm/s to 5 nm/s, while the diameter (10-17 nm) and wall numbers (∼7 layers) of the N-doped CNTs seldom changed. However, with the increased proportion of NH3, the nitrogen concentration in N-doped CNTs gradually increased. From XPS and Raman results, we observed that pyrrolic and pyridine bonding was dominant at low proportions of NH3, causing an increase in the number of defects and disorders in the CNTs. However, the quaternary bonding appeared at the NH3 proportions of 30%, while the concentration of pyrrolic and pyridine bonding saturated. Consequently, N-corporation inside the carbon honeycomb structure occurred above a specific proportion of NH3. This indicates that a high proportion of NH3 during the CNT growth is necessary to acquire the quaternary bonding between CNTs and nitrogen.