No abstract

Carbon nanofibers (CNFs), cylindrical nanostructures containing graphene, were synthesized directly from South African fly ash (a waste product formed during the combustion of coal). The CNFs (as well as other carbonaceous materials like carbon nanotubes (CNTs)) were produced by the catalytic chemical vapour deposition method (CCVD) in the presence of acetylene gas at temperatures ranging from 400°C to 700°C. The fly ash and its carbonaceous products were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), laser Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurements. It was observed that as-received fly ash was capable of producing CNFs in high yield by CCVD, starting at a relatively low temperature of 400°C. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. Small bundles of CNTs and CNFs observed by TEM and energy-dispersive spectroscopy (EDS) showed that the catalyst most likely responsible for CNF formation was iron in the form of cementite; X-ray diffraction (XRD) and Mössbauer spectroscopy confirmed these findings.

The efficient use of fly ash is a worldwide issue due to its high production and harmful effects on the environment. In this work the synthesis of carbon nanomaterials (CNMs) via the chemical vapour deposition (CVD) method, using fly ash as a catalyst and CO 2 as an alternate carbon source, was investigated. Here CO 2 was employed in three different ways: (1) as a sole carbon source, (2) as an additive to C 2 H 2 and (3) as a carbon source prior to the reaction of C 2 H 2 with fly ash. SEM, TEM and laser Raman spectroscopy confirmed that CNMs were formed in all three cases. In the first case, when CO 2 was used as a sole carbon source, CNMs were formed in low yields with a considerable amount of amorphous carbon. However, in the second case when CO 2 was used as an additive to C 2 H 2 , a drastic increase in CNM formation was observed. In both cases optimal yields were observed at 600 C.However in the third case, when CO 2 was used as a carbon source prior to the reaction with C 2 H 2 , uniform-sized nanofibers of the highest yields of all three cases were formed. Likewise these CNMs were found to be the most thermally stable. Hence this study has shown that the use of waste materials such as fly ash as a catalyst and CO 2 as a carbon source prior to the reaction with C 2 H 2 , results in a very simple and cost efficient process to make uniformally shaped, thermally stable CNMs.