Structural evolution of nitrogen-doped carbon nanotubes: From synthesis and oxidation to thermal defunctionalization

Mechanism of the oxidation and thermal defunctionalization of nitrogen-doped carbon nanotubes (NCNTs) was studied. TGA-MS, low temperature nitrogen adsorption, Raman spectroscopy, TEM and XPS were used to trace the changes in the morphology, surface properties and defectiveness of N-CNTs during HNO3 treatment. The first hour of treatment resulted in the intensive oxidation of N-CNTs due to the transformation of surface nitrogen groups into oxidized pyridone ones. Complete destruction of segmented structure of N-CNTs and a decrease in both oxygen and nitrogen content was observed after prolonged oxidation. The changes in the defectiveness were confirmed by the increase in I-D1/I-G and the decrease in I-D3/I-G ratio in Raman spectra. The thermal stability of nitrogen species in N-CNTs was found to increase in the following sequence: pyridone, pyridine and graphitic nitrogen. Decomposition of pyridone nitrogen starts at 200-400 degrees C resulting in the formation of pyridine and pyrrolic nitrogen species. Above 500 degrees C these two nitrogen species transform into graphitic nitrogen which decomposes at temperatures above 1000 degrees C. NO release in different temperature ranges was associated with both the destruction of nitro-, nitrito-, nitroso-groups and oxidation of HCN. The mechanism of functionalization and thermal destruction of oxygen-and nitrogen-containing groups was proposed. (C) 2017 Elsevier Ltd. All rights reserved.