Direct synthesis of carbon nanofibers from South African coal fly ash

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 effect of CO2 on the CVD synthesis of carbon nanomaterials using fly ash as a catalyst

“…20 In these reactions, coal y ash was used as a catalyst, CO 2 and C 2 H 2 were used as the carbon sources and hydrogen as the carrier gas, in order to create an optimal reaction environment. Carbon depositions were achieved by CVD reactions of: (1) only CO 2 , (2) CO 2 together with C 2 H 2 (i.e.…”

“…C 2 H 2 ) improves the yield of CNMs. 20 Disposal of this material has become a major issue due to the expense. On the other hand the addition of water, which acts as an etching agent that prevents the encapsulation of catalyst particles by amorphous carbon, increased the CNT yield, extended lifetime of the catalyst and an enhanced the initial growth rate as compared to those of classical C 2 H 2 decomposition reactions.…”

“…19 For example, the addition of oxygen which has been shown to act as a scavenger of hydrogen radicals, has provided conditions that were suitable for CNM synthesis. 12,13,20,21 In recent years, owing to the transition metal content in y ash, this waste material has been at the forefront of CNM (in particular CNT/CNF) synthesis. 28 Fly ash is an inorganic waste material generated from the burning of coal in the production of electricity.…”

“…On the other hand the addition of water, which acts as an etching agent that prevents the encapsulation of catalyst particles by amorphous carbon, increased the CNT yield, extended lifetime of the catalyst and an enhanced the initial growth rate as compared to those of classical C 2 H 2 decomposition reactions. 12,13,20,21 In particular, Dunens et al have shown that CNTs and CNFs could be synthesised by reaction of acetylene and Australian coal y ash. It is an heterogeneous material with a variable composition depending on its source and the processes used.…”

Direct growth of vertically aligned carbon nanotubes on silicon substrate by spray pyrolysis of Glycine max oil

“…G 1 band at 2696.56 cm -1 is a second order of two phonon can be used as an indicator of the extent of defect or disorder within the value of as grown CNTs is 0.858. Generally, lower I D /I G value indicates a higher degree of graphitization [33].…”