High-pressure pyrolysis of melamine route to nitrogen-doped conical hollow and bamboo-like carbon nanotubes

Nitrogen-doped conical hollow and bamboo-like carbon nanotubes (CNTs) have been prepared by pyrolysis of melamine with NaN3–Fe–Ni and Ni catalysts at high temperature and high pressure, respectively. The conical hollow CNTs with an average diameter of about 70 nm and a length up to 5 μm account for ∼50% of the product, whose N/C atomic ratios are about 0.27. The conical bamboo-like CNTs with the diameter of ∼65 nm and length of 1–4 μm and wall thickness of 10–20 nm account for ∼95% of the product, whose N/C atomic ratios are up to 0.18. The control experiments show that NaN3 plays a key role in keeping high nitrogen content and high conversion ratio in the CNTs. The possible growth mechanisms have been discussed on the base of the experimental observation. The strategy provides an alternative route to nitrogen-doped CNTs and other carbon nitride materials.