Morphological and crystallinity differences in nitrogen-doped carbon nanotubes grown by chemical vapour deposition decomposition of melamine over coal fly ash

Abstract

Millions of tons of coal fly ash (CFA) are produced each year in thermoelectric coal powered stations as a waste-product. Until recently few researchers have endeavored to use CFA as a catalyst in the formation of carbon nanomaterials (CNMs). In this study a two-stage tube furnace was used to synthesize N-doped carbon nanotubes (NCNTs) by chemical vapor deposition using melamine as the source of nitrogen and carbon, with CFA as a catalyst, at temperatures ranging from 800 to 900 °C. The masses of the NCNTs formed were found to have increased with increased synthesis temperature. The morphology and crystallinity of the NCNTs along with the amount of nitrogen incorporated into these were found to vary with the synthesis temperature. NCNTs synthesized at 800 °C were found to be typical multiwalled carbon nanotubes by transmission electron microscopy, whereas those at 850 and 900 °C were found to be chain-like and bamboo-like compartmentalised nanotubes respectively. The NCNTs synthesized at 800 °C were found to contain the least incorporation of nitrogen by elemental analysis and were the most crystalline (as determined by using the I_G/I_D ratio and the G-band position from laser Raman spectroscopy), whereas those at 850 °C were the least crystalline but had the highest incorporation of nitrogen. NCNTs synthesised at 800 °C were the most thermally stable, whereas those synthesized at 850 °C were the least stable. NCNTs synthesized at 900 °C had a crystallinity, thermal stability and nitrogen content which lay between the other two.