Direct synthesis of carbon nanofibers from South African coal fly ash
1 DST-NRF Centre of Excellence in Strong Materials, University of the Witwatersrand (Wits), Private Bag 3, Johannesburg 2050, South Africa
2 Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand (Wits), Private Bag 3, Johannesburg 2050, South Africa
3 School of Physics, University of the Witwatersrand (Wits), Private Bag 3, Johannesburg 2050, South Africa
Nanoscale Research Letters 2014, 9:387 doi:10.1186/1556-276X-9-387Published: 10 August 2014
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.