Towards a fine-tuning of surface chemistry in aligned carbon nanotubes induced by nitrogen plasma discharge post-treatment: a combined microscopic and spectroscopic study

The modification of the surface chemistry of individual nanotubes inside vertically aligned carbon nanotube (VACNT) arrays is a key challenge in developing successful active materials for many electronic, photonic and catalytic applications. To this end, an efficient, clean, non-destructive and adjustable process for nitrogen functionalization and doping of the VACNTs in a controlled manner is highly required. In this study, the results of a systematic study using plasma discharge for this purpose by varying the N-2/H-2 gas volume ratio, discharge exposure time, and the radio-frequency (r.f.) power are presented. This process resulted in the generation of a few defects induced deliberately by the nitrogen radicals into the graphitic framework, mainly as in; amine, amide, pyridinic, pyrrolic, graphitic-type nitrogen. The parameters of plasma discharge were adjusted in a way that the densities and the relative ratios of nitrogen-containing functional groups can be selectively controlled. Evidence for the induced structural and chemical bonding changes, and the formation of different functional groups on the surface of the VACNTs are examined as a function of nitrogen content, using proper combination of analytical methods of high-resolution transmission electron microscopy (HRTEM), Raman, X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopy. At the relatively low level of r.f. power and discharge nitrogen flow rate in the feed, a preservation of the graphitic framework of the VACNTs has been demonstrated. The current study, therefore, sets the stage for selective control of the densities and the relative ratios of different nitrogen-containing acidic and basic surface functional groups on or within the nanotube framework under defined process parameters without causing a dramatic loss of graphitic structures.