Distribution and structure of N atoms in multiwalled carbon nanotubes using variable-energy X-ray photoelectron spectroscopy

We investigated the inhomogeneous distribution of concentration and electronic structure of the nitrogen (N) atoms doped in the multiwalled carbon nanotubes (CNTs) by variable-energy X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure, and electron energy-loss spectroscopy. The vertically aligned N-doped CNTs on the substrates were grown via pyrolysis of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) in the temperature range 750-1000 degrees C. They usually have a bamboo-like structure, and the diameter is in the range of 15-80 nm. As the photon energy of XPS increases from 475 to 1265 eV, the N content increases up to 8 atomic %, indicating a higher N concentration at the inside of nanotubes. We identified three typed N structures: graphite-like, pyridine-like, and molecular N-2. The pyridine-like N structure becomes significant at the inner walls. Molecular N would exist as intercalated forms in the vicinity of hollow inside. The XPS valence band analysis reveals that the pyridine-like N structure induces the metallic behaviors. The CNTs grown using NiPc contain the higher content of pyridine-like structure compared to those grown using FePc and Cope, so they exhibit more metallic properties.