Chemical composition and oxidation study of nitrogen doped carbon nanotubes

In this work, carbon nanotubes (CNTs) and nitrogen doped carbon nanotubes (N-CNTs) were synthesized by aerosol chemical vapour deposition and the synthesis parameters such as temperature, catalyst concentration, precursor concentration, gas flow rate and reaction time were studied. N incorporation was extensively investigated on the basis of N bonding configurations detected by X-ray photoelectron spectroscopy (XPS), N concentration distribution measured by electron energy loss spectroscopy (EELS) and N spatial distribution by aberration corrected scanning transmission electron microscopy coupled with EELS (STEM-EELS). Pyridinic, graphitic, gaseous and N-oxide configurations were detected. EELS revealed N concentration differed from nanotube to nanotube within a given sample and also differed from region to region on a given nanotube. N is mostly concentrated regions of high curvature such as dome of repetitive compartments occurring within the inner channel of nanotubes. N was also found towards the innermost graphitic shells. The effects of N incorporation on nanotube outer and inner diameter distributions, wall thickness, morphology types, and morphology distributions are demonstrated. Correlations were found between nanotube morphology and chemical composition. Gaseous N tends to occur in thin-walled N-CNTs with large inner diameters and to be found in hollow and incomplete compartments morphologies. Si occurrence was also detected in CNT and N-CNT samples. Oxidation resistance of CNTs and N-CNTs synthesized from a variety of C/N ratios were studied by thermogravimetric analysis. Undoped CNTs and N-CNTs were classified into three groups of oxidation in terms of the number of oxidation steps they followed. CNTs showed single step oxidation whereas N-CNTs showed two or three steps, indicating the non-uniform chemical composition of N-CNTs. This might result from the N concentration differing from nanotube to nanotube within a given sample and the N concentration differing from region to region in a given nanotube. Partially oxidised CNTs and N-CNTs were characterised by aberration corrected STEM-EELS to map their elemental compositions and understand the chemical changes they undergo during oxidation, which revealed that oxidation starts at specific sites in CNTs and N-CNTs.