A novel approach has been developed for synthesizing nitrogen-doped carbon nanotubes (N-CNTs) from flames using liquid amines as fuels, such as isopropylamine, n-propylamine and n-butylamine, which not only created a high reaction temperature but also provided a source of C and N. The microstructure and morphologies of the N-CNTs were characterized by scanning and transmission electron microscopy, laser Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. These N-CNTs were different from the conventional N-CNTs from a chemical vapor deposition process with primarily a 'pyridine-like' structure at N substitutions (one N atom only bonded to two C atoms). They were dominantly 'graphite-like' (one N atom substituted for C in graphite layers and bonded to three C atoms) with few C≡N bonds due to the special formation conditions of high temperature and oxidative environment. It was found that: (1) amine fuel with a side chain structure was less suitable for preparation of orderly grown N-CNTs; (2) amine fuels with a higher N content generally introduced a larger quantity of N dopant; (3) the proportion of the 'graphite-like' N dopant decreased with increasing flame temperature, which provided a possibility for controllable N doping in N-CNTs from amine flames. The growth mechanism of the N-CNTs was also simulated and discussed.
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