3 years ago

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

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 (N-CNTs) 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 ID1/IG and the decrease in ID3/IG 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°С resulting in the formation of pyridine and pyrrolic nitrogen species. Above 500 °C these two nitrogen species transform into graphitic nitrogen which decomposes at temperatures above 1000°С. 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.

Publisher URL: www.sciencedirect.com/science

DOI: S0008622317308898

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