Based upon this information and observations made from this research, the reaction scheme in Figure 2 has been proposed. Figure 1 As-received coal fly ash and synthesised CNFs. Images of as-received coal fly ash (a) and CNFs synthesized at (b) 400°C, (c) 500°C, (d) 600°C and (e, f) 700°C. In (a), Selleck MAPK Inhibitor Library the as-received coal fly ash was observed to be glassy, smooth and spherical in nature. The glassy, smooth-shaped fly ash became covered with regularly and irregularly shaped CNFs. In (c) and (d), large CNFs were intertwined with smaller ones. In (e), well-defined
CNFs, apparently formed by tip growth, were clearly visible as seen by the red-coloured circles. Figure 2 Proposed reaction scheme for CNF growth, using South African coal fly ash as a catalyst. For this type of growth to occur, it is known that there is normally a weak interaction between Everolimus solubility dmso the catalyst and support [41]. During this process, the carbon reagent decomposes on the metal particle under specific reaction conditions. The carbon deposited on the metal then either dissolves/re-precipitates to form either CNT/CNFs, or the carbon migrates over the metal particle to form a tube/fibre [41]. If the catalyst particles are large, then multi-walled carbon nanotubes (MWCNTs) and CNFs may be formed [41]. To determine the graphitic nature of the carbonaceous products, laser Raman spectroscopy was conducted. Figure 3 shows the laser Raman
spectra that were used to determine the structural information of CNFs produced by the exposure of coal fly ash to acetylene. As expected, the spectrum of the as-received Carnitine dehydrogenase fly ash did not show any peaks, but in the fly ash exposed to acetylene, peaks at 1,350 and 1,590 cm−1 were observed. The intensity ratio of these peaks, known as the D band (due to disordered carbon features) and G band (due to the ordered graphitic carbon features), respectively, represents the degree of graphitization of carbon in the reaction products [36]. A low intensity ratio (I D/I G) indicates a greater degree of wall graphitization, leading to a superior quality of CNFs and/or CNTs. The intensity ratios of the D and G bands (I D/I G) are depicted in Figure 3b. The I D/I G ratio was
found to be low at 400°C, indicating that the products contained more graphitic carbon than non-graphitic (non-crystalline) carbon. However, when the reaction temperature was increased to 500°C, the I D/I G ratio was observed to have increased to 1.1 (to the highest value observed in these studies). The results of the TGA analyses (Figure 4) of the carbonaceous products formed at 500°C revealed the presence of two combustion peaks, i.e. two separate CNM products. While the exact reason for the formation of two types of CNMs at this temperature is not fully known, it is believed that this observation most likely accounts for the anomalous increase in the I D/I G ratio. Thereafter, when the reaction temperature was increased to 600°C and 700°C, the I D/I G ratio decreased.
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