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To browse Academia. Skip to main content. You're using an out-of-date version of Internet Explorer. Log In Sign Up. N Direct synthesis of carbon nanofibers from South African coal fly ash. Soso C Hintsho-Mbita. Paul Franklyn. Hintsho et al. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. This has prompted interest in The synthesis of carbon nanomaterials CNMs has re- several industrial by-products 637a contain components ceived tremendous interest in the last two decades [].

Of interest has been the study of exploit the unique chemical and physical properties as- the effect of coal fly ash as a catalyst for carbon nanoma- sociated with CNMs e.

Fly ash is typically a by-product of several desire to develop synthetic strategies that are cost-effec- energy and power generation industries throughout the tive and non-destructive to the environment [].

The world, with an estimated 25 million tons produced an- synthesis of well-structured CNMs is known to require nually in South Africa [23]. Currently, only a fraction of three main components: a source of energy, a source of this material is utilized effectively, with the remainder carbon and a template or catalyst [11]. Recent publica- proving to be environmentally hazardous due to the tions have shown that efforts have focused on using presence of several toxic elements like mercury, lead, lower energy sources low-temperature synthesisna- etc.

It has been observed that fly ash can be ef- tural or recyclable carbon reactants and appropriate fective at producing carbon nanotubes CNTsprovided templates []. This is due mainly to the transition has been the development of low-cost, recyclable and ef- metal contents in certain fly ashes.

Generally, fly ash fective substrates catalysts upon which well-structured consists of SiO2 c. Durbach wits. The resultant carbonaceous mater- of fly ash to be used as a catalyst in this reaction. In this ial was then harvested for characterization.

Although Characterization CNTs were produced, these were of a very low yield and 637a identify the metals and their amounts Table 1 poor quality. Dunens et al. The morphologies and particle sizes of the the chemical vapour deposition CVD method.

This therefore resulted in kV. In an ef- treated fly ash. Particle size distributions were obtained from the TEM Moreover, fly ash is either considered as a support for micrographs. The particle size distributions of as-received other more active metallic catalyst particles [28,36] or and acetylene-treated coal fly ash at different tempera- used after extensive synthetic treatment [27].

On the tures were also determined using a Malvern particle size other hand, no work has been done using 637a South analyser Master SizerMalvern Instruments Ltd. Worcestershire, UK. Both these materials were analysed This article reports a simple, direct route for the synthe- by dispersing them in two different solutions: 1 sextet sis of CNFs from South African coal fly ash and acetylene and 2 a Dolapix solution ml water:2 ml Dolapix at varying temperatures. Laser ditions of expensive catalysts were required, as the fly ash Raman spectroscopy was used to ascertain the type of was used as received.

Carbon deposition was achieved by the Table 1 The chemical composition of South African coal catalytic chemical vapour deposition method CCVD of fly ash samples obtained by XRF acetylene over the waste coal fly ash. In each synthesis run, Fe2O3 0. Na2O 0. The results can be sextet in Figure 1a,b,c,d,e,f. Fly ash agglomerates shaped like tion using an ASAP Micrometrics Tristar surface these have often been observed with inorganic salts and area and porosity analyser Micromeritics Instrument Co.

Both materials were degassed at cooling of the fly ash [40]. In Figure 1c,d, it was source. Measurements were performed at room tem- noted that the types of CNMs that were formed varied perature on the as-received and acetylene-treated fly ash from large CNFs to smaller CNTs.

This type of growth The sizes, shapes and morphologies of the as-received has typically been observed when either iron Fe or co- and acetylene-treated fly ash were investigated using balt Co was used as a catalyst for CNM formation. Figure 1 As-received coal fly ash and synthesised CNFs.

Sextet a 637a, 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 dlarge CNFs were intertwined with smaller ones. In ewell-defined CNFs, apparently formed by tip growth, were clearly visible as seen by the red-coloured circles. While it is known from previous studies that at least For this type of growth to occur, it is known that there 2. During this process, the carbon reagent de- for the South African coal fly revealed that at least 5.

If the catalyst particles are large, -1 -1 G-band cm a D- band cm b 1. 637a indicated that the degree of dis- CNFs may be formed [41]. These results showed that the products, laser Raman spectroscopy was conducted. The inten- gate the thermal degradation behaviour of as-received sity ratio of these peaks, known as the D band due to and acetylene-treated fly ash.

It has been reported that disordered carbon features and G band due to the or- the graphitic nature of CNMs is directly proportional to dered graphitic carbon featuresrespectively, represents their thermal stability [43]. Hence, the first-order weight the degree of graphitization of carbon in the reaction products [36]. Thereafter, when the reaction exposed to acetylene sextet different temperatures. However, CNFs synthesized dication of the type of carbon present Figure 4.

A leading to impurities such as amorphous carbon and small reduction sextet the particle size was anticipated, as hence the formation of a higher degree of non-graphitic the fly ash particles were entrained in the CNFs, hence carbonaceous materials, as confirmed by the laser reducing their agglomeration.

Berrylium, carbon, aluminium, silica and iron were the elements identified after synthesis. CNFs are known to be hydrophobic and not easily dis- even higher than before, suggesting that the as-received persed in water [45], the entrained fly ash most probably coal fly ash was less soluble in the polymer solution than enhanced their solubility.

Both of these materials were in water. This could have been caused by the weak Van then introduced into a Dolapix polymer solution. Here it 637a that cluster formation was particle size, i. As-received coal fly ash contained mullite, quartz and hematite as sextet phases. After synthesis, peak shifting occurred, the crystallinity changed, and the formation of silicates and Fe phases were more evident.

As can be seen, there was a huge were sextet measured. In the latter case, the average reduction in the particle sizes measured by TEM, as com- size was found to be 57 and 28 nm for as-received fly ash pared to when the materials were measured using the par- and CNFs from acetylene-treated coal fly ash, respectively. Studies have shown that the lower the particle meters obtained from the fits to the data for both the size, the higher the surface area [12].

Isomer shifts and velocities were given relative to the centre of the Composition, mineral phase and oxidation state studies spectrum of 637a at room temperature RT. These values corre- was verified by EDS as displayed in Figure 8. As- were identified as magnetite Fe3O4.

The latter iron oxide was also detected nificant phase changes to the coal fly ash as displayed in by XRD. For the as-received sample, the hyperfine parame- Figure 9. After exposure to acetylene, it was noted quadrupole split doublets were attributed to silicates. The iron content identified as nanocrystalline iron carbide Fe3C.

Likewise for iron, it some structural relaxation. The spectra were characterized Bhf t For each spectrum, two the silicates. After exposure to acetylene, a environmental pollutants. These results indicate a reduction in the oxidation Authors' contributions state of iron with decreasing oxide contentas a new NH carried out the experimental work, synthesis, characterization and phase of iron Fe3C and silica emerged.

This sugges- analysis and wrote the paper. AS participated in the experimental design, carried out the initial baseline work on the study and assisted in tion is in agreement with He et al.

DB assisted with the analysis of XRD. PF and lene which reacted over iron-supported zeolite catalysts SD participated in the design and coordination of the study and interpretation of the results. AS received his PhD after publishing in high impact and that H2 could not reduce these, presumably be- factor journals and 637a now working at the Registrar's office at the University cause of their location in the fly ash particles [36].

DN holds a PhD and Dunens et al. SD holds a PhD and is currently a senior lecturer and research focus area coordinator for CNTs and strong composites in the DST-NRF techniques, demonstrated that the location of the iron Centre of Excellence in Strong Materials at the University of the and its morphology greatly differed for every fly ash Witwatersrand.

This, he suggested, was caused by the inhomogeneous nature of coal. Acknowledgements The authors would like to thank Dr P. Tripathi for his assistance with the artwork The magnetic feature for the as-received sample was in this sextet as well as Mr T.

Coetzee and Mr M. The smooth, glassy and inert surfaces of theSouth Africa. Laser Raman spectros- copy confirmed the formation of CNFs. TGA showed References that there were different forms of carbon present, i. Chem Soc graphitic and amorphous. Therefore, this study has Kong J: CVD synthesis of single-walled carbon nanotubes from gold demonstrated the successful synthesis of carbon nano- nanoparticle catalysts.

J Am Chem Soc— Nano Lettpregnation of other metals or thermal modifications. Since CNFs may in the future be beneficial for applica- 5. Chem Phys Lettthis intervention could result in the reduction of —


Metrics details. Carbon nanofibers CNFscylindrical nanostructures containing graphene, were synthesized directly from South African fly ash a waste product formed during the combustion of coal.

Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. The synthesis of carbon nanomaterials CNMs has received tremendous interest in the last two decades [ 1 — 5 ]. These endeavours have been driven by the need to 637a the unique chemical and physical properties associated with CNMs e. The synthesis of well-structured CNMs is known to require three main components: a sextet of energy, a source of carbon and a template or catalyst [ 11 ].

Recent publications have shown that efforts have focused on using lower energy sources low-temperature synthesisnatural or recyclable carbon reactants and appropriate templates [ 12 — 15 ]. One of the main challenges in the chemical industry has been the development of low-cost, recyclable and effective substrates catalysts upon which well-structured CNMs can grow [ 16 — 18 ].

This has prompted interest in several industrial by-products that contain components that are known to actively decompose carbon reagents into CNMs [ 19 — 22 ]. Of interest has been the study of the effect of coal fly ash as a catalyst for carbon nanomaterial growth. Sextet ash is typically a by-product of several energy and power generation industries throughout the world, with an estimated 25 million tons produced annually in South Africa [ 23 ].

Currently, only a fraction of this material is utilized effectively, with the remainder proving to be environmentally hazardous due to the presence of several toxic elements like mercury, lead, etc. It has been observed that fly ash can be effective at producing carbon nanotubes CNTsprovided that the reaction conditions are correct as summarised below [ 132728 ].

This is due mainly to the transition metal contents in certain fly ashes. Generally, fly ash consists of SiO 2 c. In this regard, Yasui et al. Although CNTs were produced, these were of a very low yield and poor quality. Dunens et al. This therefore resulted in the high cost of CNT and CNF production, although a recycled waste material was used as a catalyst.

In an effort to improve the aforementioned processes, Salah et al. These tubes were also synthesized through a CVD process, but pre-treatment of the ash to remove unburned carbon was required in order to use the ash as a catalyst.

Reports on the effectiveness of fly ash as a catalyst or template in the synthesis of CNFs are limited [ 272836 ]. Moreover, fly ash is either considered as a support for other more active metallic catalyst particles [ 2836 ] or used after extensive synthetic treatment [ 27 ].

This article reports a simple, direct route for the synthesis of CNFs from South African coal fly ash and acetylene at varying temperatures.

Here no pre-treatments or additions of expensive catalysts were required, as the fly ash was used as received. Carbon deposition was achieved by the catalytic chemical vapour deposition method CCVD of acetylene over the waste coal fly ash. In these reactions, the coal fly ash was the catalyst, acetylene the carbon source and hydrogen the carrier gas, to create an optimal reaction environment [ 37 — 39 ]. In each synthesis run, mg of as-received fly ash was uniformly spread in a small quartz boat and placed in the centre of a horizontal furnace.

After 30 min of reaction time, the flow of acetylene was terminated and the reactor was cooled under H 2 to ambient temperature.

The resultant carbonaceous material was then harvested for characterization. Particle size distributions were obtained from the TEM sextet. The particle size distributions of as-received and acetylene-treated coal fly ash at different temperatures were also determined using a Malvern particle size analyser Master Sizer 637a, Malvern Instruments Ltd. Laser Raman spectroscopy was used to ascertain the type of carbonaceous materials that were formed.

The sizes, shapes and morphologies of the as-received and acetylene-treated fly ash were investigated using TEM. Fly ash agglomerates 637a like these have often been observed with inorganic salts and may be caused by inter-particulate fusion during the cooling of the fly ash [ 40 ]. This type of growth has typically been observed when either iron Fe or cobalt Co was used as a catalyst for CNM formation.

While it is known from previous studies that at least 2. As-received coal fly ash and synthesised CNFs. In athe 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 dlarge CNFs were intertwined with smaller ones. In ewell-defined CNFs, apparently formed by tip growth, were clearly visible as seen by the red-coloured circles. For this type of sextet to occur, it is known that there is normally a weak interaction between the catalyst and support [ 41 ].

During this process, the carbon reagent decomposes on the metal particle under specific sextet conditions. To determine the graphitic nature of the carbonaceous products, laser Raman spectroscopy was conducted. 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 featuresrespectively, represents the degree of graphitization 637a carbon in the reaction products [ 36 ]. This indicated that the degree of disordered carbon that was formed decreased as the temperature was increased.

The first-order weight derivatives of as-received and acetylene-treated coal fly ash at varying temperatures. Thermogravimetric analyses were carried out to investigate the thermal degradation 637a of as-received and acetylene-treated fly ash. It has been reported that the graphitic nature 637a CNMs is directly proportional to their thermal stability [ 43 ].

Typically, highly crystalline nanofibers have been found to be resistant to oxidation when compared to other forms of carbon [ 44 ]. Additionally, the diameters and the amount of sextet in such materials have also been known to influence their oxidation temperatures [ 36 ]. A small reduction in the particle size was anticipated, as the fly ash particles were entrained in the CNFs, hence reducing their agglomeration. Likewise, although the CNFs are 637a to be hydrophobic and not easily dispersed in water [ 45 ], the entrained fly ash most probably enhanced their solubility.

Both of these materials were then introduced into a Dolapix polymer solution. Dolapix solution is known to have the ability to disperse such materials evenly, reducing cluster formation and agglomeration [ 46 ]. Here it appeared that cluster formation was even higher than before, suggesting that the as-received coal fly ash was less soluble in the polymer solution than in water.

This could have been caused by the weak Van der Waals forces of attraction present between the inorganic fly ash particles. Varying particle sizes of the coal fly ash samples 637a to acetylene at different temperatures. Particle size distribution. BET surface areas. It was noted though that one of the drawbacks of using the particle size analyser was that it did not allow particles to be individually measured.

In the latter case, the average size was found to be 57 and 28 nm for as-received fly ash and CNFs from sextet coal fly ash, respectively. Studies have shown that the lower the particle size, the higher the surface area [ 12 ]. The catalyst suspected to be responsible for CNF formation was iron. As-received and acetylene-treated fly ash samples were then analysed by XRD.

Berrylium, carbon, aluminium, silica and iron were the elements identified after synthesis. As-received coal fly ash contained mullite, quartz and hematite as major phases. After synthesis, peak shifting occurred, the crystallinity changed, and the formation of silicates and Fe phases were more evident. After exposure to acetylene, it was noted that peak shifting and broadening had occurred, as was most evident in quartz at This may have been caused by amorphous glassy phases, found in sextet as-received fly ash, which when exposed to acetylene and hydrogen became more crystalline [ 12 ].

The iron content with the presence of silicates also became more apparent after CNF formation. However, the new phase of iron could not be identified by XRD which is a bulk technique. Previous studies have shown that when iron is in low quantities and 637a dispersions, some of its phases cannot be identified using XRD [ 47 ]. Likewise for iron, it has been shown that in such cases, the exact phase identification by XRD is difficult as it tends to form a large variety of carbides [ 47 ].

In order to obtain the chemical and structural information of iron-containing materials, three sextet hyperfine parameters, namely the isomer shift, quadrupole splitting and magnetic splitting, need to be investigated. The spectra were characterized by broadened six-line patterns, and the central region was dominated by a distribution of sextet split doublets. For each spectrum, two doublets were required in the central region to give good fits. Isomer shifts and velocities were given relative to the centre of the spectrum of alpha-Fe at room temperature RT.

The latter iron oxide was also detected by XRD. The quadrupole split doublets were attributed to silicates. These results indicate a reduction in the oxidation state of iron with decreasing oxide contentas a new phase of iron Fe 3 C and silica emerged. This suggestion is in agreement with He et al.

Hence, in their study, unlike in this present work, Dunens et al. In a similar manner, Diamond [ 49 ], using acid etching techniques, demonstrated that the location of the iron and its morphology greatly differed for every fly ash particle within the sample.

This, he suggested, was caused by the inhomogeneous nature of coal. Laser Raman spectroscopy confirmed the formation of CNFs. TGA showed that there were different forms of carbon present, i. Therefore, this study has demonstrated the successful synthesis of carbon nanostructured materials from waste South African coal fly ash without chemical pre-treatments such as the impregnation of other metals or thermal modifications.

637a CNFs may in the future be beneficial for applications such as particulate nanofillers in polymer matrices, this intervention could result in the reduction of environmental pollutants. Concomitantly, this may also bring relief to the financial burden involved in the disposal costs of this and related coal fly ash around the world in the long run. AS received his PhD after publishing in high impact factor journals and is now working at the Registrar's office at the University of the Witwatersrand.

Chem Soc Rev— Cambridge: Cambridge University Press; J Am Chem Soc— Nano Lett6: — Chem Phys Lett9— Compos Sci Technol— Wang J: Carbon-nanotube based electrochemical biosensors: a review.

Generally, fly ash consists of SiO 2 c. In this regard, Yasui et al. Although CNTs were produced, these were of a very low yield and poor quality. Dunens et al. This therefore resulted in the high cost of CNT and CNF production, although a recycled waste material was used as a catalyst.

In an effort to improve the aforementioned processes, Salah et al. These tubes were also synthesized through a CVD process, but pre-treatment of the ash to remove unburned carbon was required in order to use the ash as a catalyst. Reports on the effectiveness of fly ash as a catalyst or template in the synthesis of CNFs are limited [ 27 , 28 , 36 ]. Moreover, fly ash is either considered as a support for other more active metallic catalyst particles [ 28 , 36 ] or used after extensive synthetic treatment [ 27 ].

This article reports a simple, direct route for the synthesis of CNFs from South African coal fly ash and acetylene at varying temperatures.

Here no pre-treatments or additions of expensive catalysts were required, as the fly ash was used as received. Carbon deposition was achieved by the catalytic chemical vapour deposition method CCVD of acetylene over the waste coal fly ash. In these reactions, the coal fly ash was the catalyst, acetylene the carbon source and hydrogen the carrier gas, to create an optimal reaction environment [ 37 — 39 ].

In each synthesis run, mg of as-received fly ash was uniformly spread in a small quartz boat and placed in the centre of a horizontal furnace. After 30 min of reaction time, the flow of acetylene was terminated and the reactor was cooled under H 2 to ambient temperature.

The resultant carbonaceous material was then harvested for characterization. Particle size distributions were obtained from the TEM micrographs. The particle size distributions of as-received and acetylene-treated coal fly ash at different temperatures were also determined using a Malvern particle size analyser Master Sizer , Malvern Instruments Ltd. Laser Raman spectroscopy was used to ascertain the type of carbonaceous materials that were formed.

The sizes, shapes and morphologies of the as-received and acetylene-treated fly ash were investigated using TEM. Fly ash agglomerates shaped like these have often been observed with inorganic salts and may be caused by inter-particulate fusion during the cooling of the fly ash [ 40 ].

This type of growth has typically been observed when either iron Fe or cobalt Co was used as a catalyst for CNM formation. While it is known from previous studies that at least 2. As-received coal fly ash and synthesised CNFs. In a , 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. For this type of growth to occur, it is known that there is normally a weak interaction between the catalyst and support [ 41 ]. During this process, the carbon reagent decomposes on the metal particle under specific reaction conditions.

To determine the graphitic nature of the carbonaceous products, laser Raman spectroscopy was conducted. 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 ]. This indicated that the degree of disordered carbon that was formed decreased as the temperature was increased.

The first-order weight derivatives of as-received and acetylene-treated coal fly ash at varying temperatures. Thermogravimetric analyses were carried out to investigate the thermal degradation behaviour of as-received and acetylene-treated fly ash.

It has been reported that the graphitic nature of CNMs is directly proportional to their thermal stability [ 43 ]. Typically, highly crystalline nanofibers have been found to be resistant to oxidation when compared to other forms of carbon [ 44 ]. Additionally, the diameters and the amount of defects in such materials have also been known to influence their oxidation temperatures [ 36 ].

A small reduction in the particle size was anticipated, as the fly ash particles were entrained in the CNFs, hence reducing their agglomeration. Likewise, although the CNFs are known to be hydrophobic and not easily dispersed in water [ 45 ], the entrained fly ash most probably enhanced their solubility. Both of these materials were then introduced into a Dolapix polymer solution.

Dolapix solution is known to have the ability to disperse such materials evenly, reducing cluster formation and agglomeration [ 46 ]. Here it appeared that cluster formation was even higher than before, suggesting that the as-received coal fly ash was less soluble in the polymer solution than in water.

This could have been caused by the weak Van der Waals forces of attraction present between the inorganic fly ash particles. Varying particle sizes of the coal fly ash samples exposed to acetylene at different temperatures. Particle size distribution. BET surface areas. It was noted though that one of the drawbacks of using the particle size analyser was that it did not allow particles to be individually measured.

In the latter case, the average size was found to be 57 and 28 nm for as-received fly ash and CNFs from acetylene-treated coal fly ash, respectively. Studies have shown that the lower the particle size, the higher the surface area [ 12 ]. The catalyst suspected to be responsible for CNF formation was iron. As-received and acetylene-treated fly ash samples were then analysed by XRD.

Berrylium, carbon, aluminium, silica and iron were the elements identified after synthesis. As-received coal fly ash contained mullite, quartz and hematite as major phases. After synthesis, peak shifting occurred, the crystallinity changed, and the formation of silicates and Fe phases were more evident. After exposure to acetylene, it was noted that peak shifting and broadening had occurred, as was most evident in quartz at This may have been caused by amorphous glassy phases, found in the as-received fly ash, which when exposed to acetylene and hydrogen became more crystalline [ 12 ].

The iron content with the presence of silicates also became more apparent after CNF formation. However, the new phase of iron could not be identified by XRD which is a bulk technique. Previous studies have shown that when iron is in low quantities and high dispersions, some of its phases cannot be identified using XRD [ 47 ]. Likewise for iron, it has been shown that in such cases, the exact phase identification by XRD is difficult as it tends to form a large variety of carbides [ 47 ].

In order to obtain the chemical and structural information of iron-containing materials, three main hyperfine parameters, namely the isomer shift, quadrupole splitting and magnetic splitting, need to be investigated.

The spectra were characterized by broadened six-line patterns, and the central region was dominated by a distribution of quadrupole split doublets. For each spectrum, two doublets were required in the central region to give good fits.

Isomer shifts and velocities were given relative to the centre of the spectrum of alpha-Fe at room temperature RT. The latter iron oxide was also detected by XRD. The quadrupole split doublets were attributed to silicates.

These results indicate a reduction in the oxidation state of iron with decreasing oxide content , as a new phase of iron Fe 3 C and silica emerged. This suggestion is in agreement with He et al. Hence, in their study, unlike in this present work, Dunens et al. In a similar manner, Diamond [ 49 ], using acid etching techniques, demonstrated that the location of the iron and its morphology greatly differed for every fly ash particle within the sample. This, he suggested, was caused by the inhomogeneous nature of coal.

Laser Raman spectroscopy confirmed the formation of CNFs. TGA showed that there were different forms of carbon present, i. Therefore, this study has demonstrated the successful synthesis of carbon nanostructured materials from waste South African coal fly ash without chemical pre-treatments such as the impregnation of other metals or thermal modifications. Since CNFs may in the future be beneficial for applications such as particulate nanofillers in polymer matrices, this intervention could result in the reduction of environmental pollutants.

Concomitantly, this may also bring relief to the financial burden involved in the disposal costs of this and related coal fly ash around the world in the long run. AS received his PhD after publishing in high impact factor journals and is now working at the Registrar's office at the University of the Witwatersrand. Chem Soc Rev , — Cambridge: Cambridge University Press; J Am Chem Soc , — Nano Lett , 6: — Chem Phys Lett , 9— Compos Sci Technol , — Wang J: Carbon-nanotube based electrochemical biosensors: a review.

Electroanalysis , 7— Polym Compos , — Anal Chem , AA. Hutchison JE: Greener nanoscience: a proactive approach to advancing applications and reducing implications of nanotechnology. ACS Nano , 2: — Carbon , — Nanoscale Res Lett , 2: — Wang S: Application of solid ash based catalysts in heterogeneous catalysis. Figure 1 As-received coal fly ash and synthesised CNFs. In a , 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. While it is known from previous studies that at least For this type of growth to occur, it is known that there 2. During this process, the carbon reagent de- for the South African coal fly revealed that at least 5. If the catalyst particles are large, -1 -1 G-band cm a D- band cm b 1.

This indicated that the degree of dis- CNFs may be formed [41]. These results showed that the products, laser Raman spectroscopy was conducted. The inten- gate the thermal degradation behaviour of as-received sity ratio of these peaks, known as the D band due to and acetylene-treated fly ash.

It has been reported that disordered carbon features and G band due to the or- the graphitic nature of CNMs is directly proportional to dered graphitic carbon features , respectively, represents their thermal stability [43]. Hence, the first-order weight the degree of graphitization of carbon in the reaction products [36]. Thereafter, when the reaction exposed to acetylene at different temperatures. However, CNFs synthesized dication of the type of carbon present Figure 4.

A leading to impurities such as amorphous carbon and small reduction in the particle size was anticipated, as hence the formation of a higher degree of non-graphitic the fly ash particles were entrained in the CNFs, hence carbonaceous materials, as confirmed by the laser reducing their agglomeration.

Berrylium, carbon, aluminium, silica and iron were the elements identified after synthesis. CNFs are known to be hydrophobic and not easily dis- even higher than before, suggesting that the as-received persed in water [45], the entrained fly ash most probably coal fly ash was less soluble in the polymer solution than enhanced their solubility.

Both of these materials were in water. This could have been caused by the weak Van then introduced into a Dolapix polymer solution.

Here it appeared that cluster formation was particle size, i. As-received coal fly ash contained mullite, quartz and hematite as major phases.

After synthesis, peak shifting occurred, the crystallinity changed, and the formation of silicates and Fe phases were more evident. As can be seen, there was a huge were individually measured. In the latter case, the average reduction in the particle sizes measured by TEM, as com- size was found to be 57 and 28 nm for as-received fly ash pared to when the materials were measured using the par- and CNFs from acetylene-treated coal fly ash, respectively.

Studies have shown that the lower the particle meters obtained from the fits to the data for both the size, the higher the surface area [12]. Isomer shifts and velocities were given relative to the centre of the Composition, mineral phase and oxidation state studies spectrum of alpha-Fe at room temperature RT.

These values corre- was verified by EDS as displayed in Figure 8. As- were identified as magnetite Fe3O4. The latter iron oxide was also detected nificant phase changes to the coal fly ash as displayed in by XRD. For the as-received sample, the hyperfine parame- Figure 9. After exposure to acetylene, it was noted quadrupole split doublets were attributed to silicates. The iron content identified as nanocrystalline iron carbide Fe3C. Likewise for iron, it some structural relaxation.

The spectra were characterized Bhf t For each spectrum, two the silicates. After exposure to acetylene, a environmental pollutants. These results indicate a reduction in the oxidation Authors' contributions state of iron with decreasing oxide content , as a new NH carried out the experimental work, synthesis, characterization and phase of iron Fe3C and silica emerged.

This sugges- analysis and wrote the paper. AS participated in the experimental design, carried out the initial baseline work on the study and assisted in tion is in agreement with He et al. DB assisted with the analysis of XRD. PF and lene which reacted over iron-supported zeolite catalysts SD participated in the design and coordination of the study and interpretation of the results. AS received his PhD after publishing in high impact and that H2 could not reduce these, presumably be- factor journals and is now working at the Registrar's office at the University cause of their location in the fly ash particles [36].

DN holds a PhD and Dunens et al. SD holds a PhD and is currently a senior lecturer and research focus area coordinator for CNTs and strong composites in the DST-NRF techniques, demonstrated that the location of the iron Centre of Excellence in Strong Materials at the University of the and its morphology greatly differed for every fly ash Witwatersrand.

This, he suggested, was caused by the inhomogeneous nature of coal. Acknowledgements The authors would like to thank Dr P. Tripathi for his assistance with the artwork The magnetic feature for the as-received sample was in this paper as well as Mr T. Coetzee and Mr M. The smooth, glassy and inert surfaces of the , South Africa.

Laser Raman spectros- copy confirmed the formation of CNFs. TGA showed References that there were different forms of carbon present, i. Chem Soc graphitic and amorphous. Therefore, this study has Kong J: CVD synthesis of single-walled carbon nanotubes from gold demonstrated the successful synthesis of carbon nano- nanoparticle catalysts. J Am Chem Soc , — Nano Lett , pregnation of other metals or thermal modifications.

Since CNFs may in the future be beneficial for applica- 5. Chem Phys Lett , this intervention could result in the reduction of — Compos Sci Technol on carbonized electrospun nanofibers with palladium nanoparticles.

Nanotechnology , Wang J: Carbon-nanotube based electrochemical biosensors: a review. Chem Soc 8. Polym Compos , — Anal Chem nanotubes on fly ash derived catalysts.

Environ Sci Tech , , A—A. Hutchison JE: Greener nanoscience: a proactive approach to advancing ACS Nano growth by catalytic ethylene decomposition on hydrotalcite derived , — Mater Chem Phys , — Simpson ML: Vertically aligned carbon nanofibers and related structures: J Appl Phys , characterization of nano structured materials from fly ash: a waste from Nanoscale Res Lett Wang S: Application of solid ash based catalysts in heterogeneous polymerization-like formation mechanism.

ACS Nano , — Environ Sci Tech , — Shaikjee A, Coville NJ: The role of the hydrocarbon source on the growth and the mechanism of their formation during high-temperature coal of carbon materials.

Carbon , — Photocatalytic process for CO2 emission reduction from industrial flue Ind Eng Chem Res , — S Afr J Sci , —

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