# Synthesis of Single-Walled Carbon Nanotubes: Effects of Active Metals, Catalyst Supports, and Metal Loading Percentage

Authors: Wei-Wen Liu; Azizan Aziz; Siang-Piao Chai; Abdul Rahman Mohamed; U. Hashim
Journal: Journal of Nanomaterials
 (2013)

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## Abstract

The effects of active metals, catalyst supports, and metal loading percentage on the formation of single-walled carbon nanotubes (SWNTs) were studied. In particular, iron, cobalt, and nickel were investigated for SWNTs synthesis. Iron was found to grow better-quality SWNTs compared to cobalt and nickel. To study the effect of catalyst supports, magnesium oxide, silicon oxide, and aluminium oxide were chosen for iron. Among the studied supports, MgO was identified to be a suitable support for iron as it produced SWNTs with better graphitisation determined by Raman analysis. Increasing the iron loading decreased the quality of SWNTs due to extensive agglomeration of the iron particles. Thus, lower metal loading percentage is preferred to grow better-quality SWNTs with uniform diameters.


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## Body

## 1. Introduction

Besides laser evaporation and electric arc discharge methods, the catalytic chemical vapor deposition (CVD) is a very efficient method to synthesize single-walled carbon nanotubes (SWNTs) on a large scale but a low cost [1–3]. It is well known that the catalyst is indispensable for the growth of SWNTs in CVD process. A variety of metal catalysts such as Au, Ag, Pt, Pd, Mn, Mo, Cu, Rh, Zn, Cr, Ru, Mg, and Al have been used for the production of SWNTs [4–11]. However, Fe, Co, Ni, and their alloys are the most of the widely used transition metal catalysts for SWNT production [12–14]. So far, various substances have been tested as supports of catalytically active metals, for instance, zeolites [15, 16], SiO2 [17], Al2O3 [18], CaCO3 [19], and MgO [20]. The interactions between the support and active metal are greatly important in the synthesis of SWNTs because it can affect the quality of SWNTs. In addition, the dependency of structure upon property causes the structure control and large-scale production of SWNTs to become one of the crucial topics in carbon related scientific communities [21–25]. Clear understanding of underlying growth mechanisms is therefore of significant importance in promoting the development and innovations of SWNTs.Thus, there is a need to investigate the effects of active metals, catalyst supports, and metal loading percentage on the production of SWNTs. Our previous study shows that Fe3O4/MgO was an effective catalyst for growing SWNTs [26–29]. We had also reported that the quality of SWNTs was affected by the types of carbon precursors used [27]. In this work, the effects of active metals such as Fe, Ni, and Co were chosen because they are believed to yield high quality of SWNTs, but we are unsure which active metals can synthesise the highest quality of SWNT when supported by MgO under our reaction conditions. It is because a minor change on the reaction parameters can influence the active metals’ performances. In the second study, the best performance active metal was supported by MgO, SiO2, and Al2O3 to investigate the interaction between supports and active metal which can affect the yield of SWNTs. At last study, different metal loading percentages were set to examine the effect on the quality of SWNTs.


## 2. Experimental Procedures

### 2.1. Preparation of the Catalyst

The preparation method for Fe3O4 nanoparticles has been previously reported [29]. The molar ratios of Fe3O4 to MgO, NiO to MgO, and CoO to MgO were set at 1 : 9. Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts were prepared by dispersing MgO (99.99% trace metal basis: Aldrich) in distilled water and subsequently adding the required amount of Fe3O4 nanoparticles, Ni (NO3)2·6H2O (99.999% trace metal basis: Aldrich) and Co (NO3)2·6H2O (99.999% trace metal basis: Aldrich), respectively, into the MgO solution to get three different types of mixture solutions. For the preparation of Fe3O4/Al2O3 and Fe3O4/SiO2 catalysts, the molar ratios of Fe3O4 to Al2O3 and Fe3O4 to SiO2 were set at 1 : 9. The required amount of Fe3O4 nanoparticles was dispersed in distilled water and followed by adding Al2O3 (analytical reagents: Ajax) and SiO2 nanopowder (analytical reagents: RdH), respectively, to form two different types of mixture solutions. To prepare different molar ratios of Fe3O4/MgO catalysts at 1.75 : 8.25, 2.5 : 7.5, and 3.25 : 6.75, the same procedure as above was used. All mixture solutions were then sonicated for 10 min, and water was removed by rotary evaporation. Finally, the dried catalyst was ground with a pestle and mortar to break the agglomerates into a very fine powder.


### 2.2. Synthesis of SWNTs

The synthesis of SWNTs was carried out at atmospheric pressure in a quartz tube placed vertically in a stainless-steel housing [26–29]. For the synthesis of SWNTs, 0.2 g of the catalyst was distributed in the centre of the reactor for each run. The reactor was heated in a tubular furnace to 900°C in a nitrogen atmosphere (99.999% purity) at a flow rate of 40 mL/min. Subsequently, high-purity methane (99.999% purity) was mixed with nitrogen at a ratio of 1 : 1 (v/v) before introducing the mixed gas into the reactor at a total flow rate of 80 mL/min. The reaction was kept under these conditions for 30 min. After the reaction, the furnace was cooled to room temperature in a nitrogen flow.


### 2.3. Characterization of SWNTs

The carbon samples, in black powder form, were collected from the quartz reactor for detail characterisation. The morphology of the SWNTs was investigated using transmission electron microscopy (TEM; Philips CM12) and scanning electron microscopy (SEM; Supra 35 VP). Raman spectrum measurements were conducted on the carbon samples using a Raman spectroscopy (inVia Renishaw) at a wavelength of 633 nm.


## 2.1. Preparation of the Catalyst

The preparation method for Fe3O4 nanoparticles has been previously reported [29]. The molar ratios of Fe3O4 to MgO, NiO to MgO, and CoO to MgO were set at 1 : 9. Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts were prepared by dispersing MgO (99.99% trace metal basis: Aldrich) in distilled water and subsequently adding the required amount of Fe3O4 nanoparticles, Ni (NO3)2·6H2O (99.999% trace metal basis: Aldrich) and Co (NO3)2·6H2O (99.999% trace metal basis: Aldrich), respectively, into the MgO solution to get three different types of mixture solutions. For the preparation of Fe3O4/Al2O3 and Fe3O4/SiO2 catalysts, the molar ratios of Fe3O4 to Al2O3 and Fe3O4 to SiO2 were set at 1 : 9. The required amount of Fe3O4 nanoparticles was dispersed in distilled water and followed by adding Al2O3 (analytical reagents: Ajax) and SiO2 nanopowder (analytical reagents: RdH), respectively, to form two different types of mixture solutions. To prepare different molar ratios of Fe3O4/MgO catalysts at 1.75 : 8.25, 2.5 : 7.5, and 3.25 : 6.75, the same procedure as above was used. All mixture solutions were then sonicated for 10 min, and water was removed by rotary evaporation. Finally, the dried catalyst was ground with a pestle and mortar to break the agglomerates into a very fine powder.


## 2.2. Synthesis of SWNTs

The synthesis of SWNTs was carried out at atmospheric pressure in a quartz tube placed vertically in a stainless-steel housing [26–29]. For the synthesis of SWNTs, 0.2 g of the catalyst was distributed in the centre of the reactor for each run. The reactor was heated in a tubular furnace to 900°C in a nitrogen atmosphere (99.999% purity) at a flow rate of 40 mL/min. Subsequently, high-purity methane (99.999% purity) was mixed with nitrogen at a ratio of 1 : 1 (v/v) before introducing the mixed gas into the reactor at a total flow rate of 80 mL/min. The reaction was kept under these conditions for 30 min. After the reaction, the furnace was cooled to room temperature in a nitrogen flow.


## 2.3. Characterization of SWNTs

The carbon samples, in black powder form, were collected from the quartz reactor for detail characterisation. The morphology of the SWNTs was investigated using transmission electron microscopy (TEM; Philips CM12) and scanning electron microscopy (SEM; Supra 35 VP). Raman spectrum measurements were conducted on the carbon samples using a Raman spectroscopy (inVia Renishaw) at a wavelength of 633 nm.


## 3. Results and Discussion

### 3.1. Effect of Fe3O4, NiO, and CoO Supported on MgO

The ability of Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts to produce SWNTs was investigated. The study was based on the ID/IG ratio from Raman spectra of SWNTs produced over these catalysts. The ID/IG ratio is commonly used as a measure of the quality of CNTs produced: a smaller ID/IG ratio corresponds to a higher quality structure or fewer defects [30]. The ID/IG ratios for samples produced by Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts were 0.11, 0.65, and 0.34, respectively, decreasing in the order of NiO/MgO > CoO/MgO > Fe3O4/MgO. A high ID/IG ratio of 0.65 was obtained for the NiO/MgO catalyst, indicating that a low-quality structure was formed. The Fe3O4/MgO catalyst formed better-quality CNTs where ID/IG ratio of 0.11 was obtained. Figure 1 shows the Raman spectra of the three different catalyst systems after reaction. The presence of the radial breathing mode (RBM) and G- and D-bands in the Raman spectra confirms the presence of SWNTs. The frequency of the RBM is inversely proportional to the diameter of the nanotube from which it arises. It has been found empirically that the diameter of the tube (dt/nm) can relate to the frequency of RBM by ϖRBM=A/dt+B, where A=234 cm−1, B=10 cm−1, and dt = diameter of SWNTs in nanometer [31].Figure 1
Raman spectra of the (a) NiO/MgO, (b) CoO/MgO, and (c) Fe3O4/MgO catalysts after reaction.It is important to note the significant peaks at Raman shifts of 203 and 190 cm−1, which correspond to the nanotube diameters of 1.22, 1.30, and 1.31 nm produced by the NiO/MgO, CoO/MgO, and Fe3O4/MgO catalysts, respectively. The nanotube samples were also characterised using transmission electron microscopy (TEM) and their results are shown in Figure 2. It can be observed that bundles of SWNTs with nearly uniform diameters were synthesised by Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts. Bundles that consist of more than 20 SWNTs were obtained from the Fe3O4/MgO catalyst. In addition to the SWNT bundles, MWNTs were also observed under TEM for the NiO/MgO catalyst but not for the Fe3O4/MgO and CoO/MgO catalysts.TEM images showing SWNT bundles synthesised by the (a) NiO/MgO, (b) CoO/MgO, and (c) Fe3O4/MgO catalysts.
(a)
(b)
(c)To arrive at the understanding why the Fe3O4/MgO catalyst formed SWNTs with a lower ID/IG ratio, we have to understand the carbon solubility for Fe, Ni, and Co. Moisala et al. [31] reported that the carbon solubilities for Fe, Co, and Ni are 20.2, 13.9, and 10.7%, respectively, at a given temperature. Fe has the highest carbon solubility as compared with Co and Ni. The higher carbon solubility of Fe influences CNT nucleation and growth in several ways: (i) increasing the carbon availability for CNT growth; (ii) creating a higher concentration driving force to accelerate the CNT formation rate; (iii) affecting nucleation of CNT caps; and (iv) determining the type of CNT grown [32]. By thermodynamic analysis, Kuznetsov et al. [33] reported that the morphology of nanotubes is a function of two parameters: the diameter of the catalyst particle and the carbon supersaturation ratio. When the supersaturation value is high, carbon caps can be formed easily from the multiple carbon nuclei on the same particle surface. Due to the van der Waals forces, the SWNTs may be reoriented to form SWNT bundles. Therefore, Fe tends to have a higher carbon supersaturation ratio compared with Co and Ni because of its higher carbon solubility of Fe, which promotes the nucleation and growth of SWNTs and also explains why broad bundles of SWNTs with a more highly graphitised structure were formed on the Fe3O4/MgO catalyst.


### 3.2. Effect of Al2O3, SiO2, and MgO Supports

Figure3 shows the Raman spectra for samples produced by Fe3O4 supported on SiO2, Al2O3, and MgO. The ID/IG ratio for samples produced by Fe3O4 supported on SiO2, Al2O3, and MgO were 0.34, 0.29, and 0.11, respectively. RBM in the range of 100–350 cm−1 are attributed to the presence of SWNTs [34]. In Figure 3, RBM peaks were observed for all three catalysts investigated, revealing the formation of SWNTs. The intensity of RBM peaks decreased in the following order of Fe3O4/MgO > Fe3O4/SiO2 > Fe3O4/Al2O3, whereas the ID/IG ratio decreased in the order of Fe3O4/SiO2 > Fe3O4/Al2O3 > Fe3O4/MgO. In other words, compared with SiO2 and Al2O3, MgO-supported Fe3O4 represents the most suitable combination to produce SWNTs, considering that the highest intensity of RBM peaks and the lowest ID/IG ratio were observed. This is in good agreement with the finding reported by Ago et al. [35] and Jin et al. [36] that high quality of SWNTs were synthesised by iron supported on MgO catalyst.Figure 3
Raman spectra of the (a) Fe3O4/SiO2, (b) Fe3O4/Al2O3, and (c) Fe3O4/MgO catalysts after reaction.The diameter of SWNTs obtained was in the range of 1.09–2.40 nm. In this work, CNTs with diameters greater than 2.48 nm were considered large diameter CNTs, whereas CNTs with diameters of 2.48 nm or less were defined as small-diameter CNTs. Scanning electron microscopy (SEM) images for all samples are shown in Figure4. CNTs with diameters greater than 2.48 nm were produced when SiO2 was used as a catalyst support (Figure 4(a)), whereas CNTs with small diameters were formed when Al2O3 and MgO were used (Figures 4(b) and 4(c)). The diameters of CNTs synthesised by Fe3O4 supported on Al2O3 and MgO were measured using TEM, and the results are shown in Figure 5. The diameter ranges were 1.22–1.46 nm and 1.09–2.40 nm for CNTs produced by Al2O3 and MgO when used as catalyst supports, respectively. The diameters were almost uniform, and the CNTs attached to one another in a bundle form (Figure 5). The bundle produced by the MgO-supported catalyst consisted of more SWNTs compared with that produced by Al2O3 as the catalyst support (Figure 5). The TEM observations show that SWNTs synthesised by MgO used as the support were of high quality, which is consistent with the Raman analysis.SEM images showing nanotube structure synthesised using the (a) Fe3O4/SiO2, (b) Fe3O4/Al2O3, and (c) Fe3O4/MgO catalysts.
(a)
(b)
(c)TEM images showing SWNT bundles synthesised using the (a) Fe3O4/Al2O3 and (b) Fe3O4/MgO catalysts.
(a)
(b)To understand why MgO was a superior support, we considered the interaction between the metal oxide nanoparticle and its support. The formation of the highest-quality SWNTs by the Fe3O4/MgO catalyst could be attributed to the strong metal interaction between the Fe3O4 nanoparticles and MgO [37, 38]. This strong interaction restricted the mobility of Fe3O4 nanoparticles during CVD, thus preventing the extensive agglomeration of Fe3O4 nanoparticles at high temperatures to form larger-sized clusters, leading to good dispersion of Fe3O4 [39]. This strong interaction can be explained in terms of Lewis acid/base interactions between metal oxide supports and metal catalyst nanoparticles. A Lewis base is a species that acts as an electron pair donor, whereas a Lewis acid is a species that acts as an electron pair acceptor [39]. In this case, the metal oxide support (MgO) acts as a Lewis base and donates electrons through metal catalyst nanoparticles (Fe3O4) to methane [39]. The metal catalyst nanoparticles play the role of a conduit for transferring negative charge from the metal oxide support to methane. Generally, simultaneous back-donation from methane also takes place that is, methane acts as a Lewis base, whereas MgO acts as a Lewis acid. The electronic structure of methane is then changed in such a way that decomposition of the methane molecule takes place.The surface basicity was reported to decrease in the following order: MgO > Al2O3 > SiO2 [40]. Thus, MgO represents the best electron donor compared with Al2O3 and SiO2. As a result, the interaction between Fe3O4 and MgO was stronger than the interactions between Fe3O4 and Al2O3 and between Fe3O4 and SiO2 [40]. With the strong interaction between the Fe3O4 and MgO, Fe3O4 particles are thought to be well dispersed and promote the formation of SWNTs. This explains the reason of intensity of the RBM peaks for the samples produced by MgO used as a catalyst support was the highest and the ID/IG ratio was the lowest compared with the samples formed by SiO2 and Al2O3. The second highest intensity of RBM peaks was observed for samples formed by SiO2 used as the catalyst support compared with the samples produced by Al2O3 used as the catalyst support. However, a higher ID/IG ratio was observed in the Raman spectra of the sample produced by using an SiO2 catalyst support than that formed when using an Al2O3 catalyst support. This indicates that more defective structures were formed in the samples due to the fact that SiO2 is a less active support compared with alumina [41, 42]. Because Al2O3 is an amphoteric material having the characteristics of both an acid and a base, it may take part in the strong interaction. Therefore, Fe3O4 particles had a better dispersion on the Al2O3 compared with SiO2 due to this strong interaction and formed a higher degree of CNT graphitisation.


### 3.3. Effect of Fe3O4 Loading

In this study, the molar ratios of Fe to MgO were varied at 1 : 9, 1.75 : 8.25, 2.5 : 7.5, and 3.25 : 6.75, which were denoted as (Fe3O4)1(MgO)9, (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75, respectively. The CVD conducted using a molar ratio of 1 : 9 produced the lowest ID/IG ratio, that is, 0.11. The ID/IG ratios of samples were found to increase from 0.11 to 0.46 as the Fe3O4 loading increased from 1 to 3.25.The Raman spectra shown in Figure6 show the presence of RBM peaks, indicating the presence of SWNTs. The intensity of RBM peaks decreased in the following order for Fe3O4 loading: 1 > 1.75 > 2.5 > 3.25. The ID/IG ratio was found to decrease in the following order: 3.25 > 2.5 > 1.75 > 1. In contrast, the highest intensity of RBM peak and the lowest ID/IG ratio were observed for SWNTs produced by a catalyst with a molar ratio of 1 : 9.Figure 6
Raman spectra of the (a) (Fe3O4)1(MgO)9, (b) (Fe3O4)1.75(MgO)8.25, (c) (Fe3O4)2.5(MgO)7.5, and (d) (Fe3O4)3.25(MgO)6.75 catalysts after reaction.The SEM images, shown in Figures7(a) and 7(b), revealed the presence of narrow-diameter CNTs from (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts. The narrow CNTs grew over the entire catalyst surface with almost uniform diameters. However, large-diameter CNTs were observed in the samples formed by the (Fe3O4)2.5(MgO)7.5 and (Fe3O4)3.25(MgO)6.75 catalysts. The structures of large CNTs were poor and had inconsistent diameters, which can be seen in Figures 7(c) and 7(d).SEM images showing the CVD products synthesised using the (a) (Fe3O4)1(MgO)9, (b) (Fe3O4)1.75(MgO)8.25, (c) (Fe3O4)2.5(MgO)7.5, and (d) (Fe3O4)3.25(MgO)6.75 catalysts.
(a)
(b)
(c)
(d)To measure the diameters of narrow CNTs, samples produced by the (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts were characterised by TEM. The measured diameter was in the range of 1.16–2.10 nm and 1.10–1.85 nm for SWNTs synthesised by (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalyst, respectively. Figure 8 clearly shows SWNTs in a bundle form produced by the (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts. Under TEM observation, the diameter of these SWNTs was almost uniform and in agreement with the SEM observations (Figures 7(a) and 7(b)). Moreover, the SWNTs bundles possessed a high degree of graphitization, which is consistent with the Raman results (Figure 6).TEM images showing SWNT bundles synthesised using the (a) (Fe3O4)1(MgO)9 and (b) (Fe3O4)1.75(MgO)8.25 catalysts.
(a)
(b)From these results, it is clear that there is an optimum Fe3O4 loading that leads to the formation of SWNTs of the desired quality. When increasing the Fe3O4 loading by more than 1.75, the ID/IG ratio also increased significantly, and large CNTs were clearly observed in the samples (Figures 7(c) and 7(d)). Increasing the Fe3O4 content is beneficial only if it enhances the available active sites for CNT growth, rather than increasing the nanoparticle sizes [43–47]. It is well known that the diameters of CNTs are correlated with active metal nanoparticles sizes [32, 40, 48–50]. Therefore, the narrow CNTs in this work were likely produced by small Fe3O4 nanoparticles, and the large CNTs by large Fe3O4 nanoparticles. Therefore, the quality of CNTs decreased with an increase in Fe3O4 loading.The average size of Fe3O4 nanoparticles for (Fe3O4)1(MgO)9 catalyst was 10.33 nm as reported in our previous work [42]. In this work, the size of catalyst nanoparticles for the (Fe3O4)1(MgO)9 catalyst was much smaller than the size of catalyst nanoparticles for (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75, as observed in Figure 7. The large nanoparticle catalysts were formed for the high Fe3O4-loaded: (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75 catalysts due to the sintering of the Fe3O4 nanoparticles during reaction at high temperatures (i.e., 900°C) [33]. Sintering is a thermally activated process that involves the diffusion of atoms [51]. Together with the higher concentration of iron, there would be a significantly higher degree of sintering of iron nanoparticles, causing large nanoparticles to form. However, it is believed that the degree of sintering of iron nanoparticles for (Fe3O4)1(MgO)9 was low compared with those of the (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75 catalysts. As a result, the sizes of iron nanoparticles are small for a molar ratio of 1 : 9.


## 3.1. Effect of Fe3O4, NiO, and CoO Supported on MgO

The ability of Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts to produce SWNTs was investigated. The study was based on the ID/IG ratio from Raman spectra of SWNTs produced over these catalysts. The ID/IG ratio is commonly used as a measure of the quality of CNTs produced: a smaller ID/IG ratio corresponds to a higher quality structure or fewer defects [30]. The ID/IG ratios for samples produced by Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts were 0.11, 0.65, and 0.34, respectively, decreasing in the order of NiO/MgO > CoO/MgO > Fe3O4/MgO. A high ID/IG ratio of 0.65 was obtained for the NiO/MgO catalyst, indicating that a low-quality structure was formed. The Fe3O4/MgO catalyst formed better-quality CNTs where ID/IG ratio of 0.11 was obtained. Figure 1 shows the Raman spectra of the three different catalyst systems after reaction. The presence of the radial breathing mode (RBM) and G- and D-bands in the Raman spectra confirms the presence of SWNTs. The frequency of the RBM is inversely proportional to the diameter of the nanotube from which it arises. It has been found empirically that the diameter of the tube (dt/nm) can relate to the frequency of RBM by ϖRBM=A/dt+B, where A=234 cm−1, B=10 cm−1, and dt = diameter of SWNTs in nanometer [31].Figure 1
Raman spectra of the (a) NiO/MgO, (b) CoO/MgO, and (c) Fe3O4/MgO catalysts after reaction.It is important to note the significant peaks at Raman shifts of 203 and 190 cm−1, which correspond to the nanotube diameters of 1.22, 1.30, and 1.31 nm produced by the NiO/MgO, CoO/MgO, and Fe3O4/MgO catalysts, respectively. The nanotube samples were also characterised using transmission electron microscopy (TEM) and their results are shown in Figure 2. It can be observed that bundles of SWNTs with nearly uniform diameters were synthesised by Fe3O4/MgO, NiO/MgO, and CoO/MgO catalysts. Bundles that consist of more than 20 SWNTs were obtained from the Fe3O4/MgO catalyst. In addition to the SWNT bundles, MWNTs were also observed under TEM for the NiO/MgO catalyst but not for the Fe3O4/MgO and CoO/MgO catalysts.TEM images showing SWNT bundles synthesised by the (a) NiO/MgO, (b) CoO/MgO, and (c) Fe3O4/MgO catalysts.
(a)
(b)
(c)To arrive at the understanding why the Fe3O4/MgO catalyst formed SWNTs with a lower ID/IG ratio, we have to understand the carbon solubility for Fe, Ni, and Co. Moisala et al. [31] reported that the carbon solubilities for Fe, Co, and Ni are 20.2, 13.9, and 10.7%, respectively, at a given temperature. Fe has the highest carbon solubility as compared with Co and Ni. The higher carbon solubility of Fe influences CNT nucleation and growth in several ways: (i) increasing the carbon availability for CNT growth; (ii) creating a higher concentration driving force to accelerate the CNT formation rate; (iii) affecting nucleation of CNT caps; and (iv) determining the type of CNT grown [32]. By thermodynamic analysis, Kuznetsov et al. [33] reported that the morphology of nanotubes is a function of two parameters: the diameter of the catalyst particle and the carbon supersaturation ratio. When the supersaturation value is high, carbon caps can be formed easily from the multiple carbon nuclei on the same particle surface. Due to the van der Waals forces, the SWNTs may be reoriented to form SWNT bundles. Therefore, Fe tends to have a higher carbon supersaturation ratio compared with Co and Ni because of its higher carbon solubility of Fe, which promotes the nucleation and growth of SWNTs and also explains why broad bundles of SWNTs with a more highly graphitised structure were formed on the Fe3O4/MgO catalyst.


## 3.2. Effect of Al2O3, SiO2, and MgO Supports

Figure3 shows the Raman spectra for samples produced by Fe3O4 supported on SiO2, Al2O3, and MgO. The ID/IG ratio for samples produced by Fe3O4 supported on SiO2, Al2O3, and MgO were 0.34, 0.29, and 0.11, respectively. RBM in the range of 100–350 cm−1 are attributed to the presence of SWNTs [34]. In Figure 3, RBM peaks were observed for all three catalysts investigated, revealing the formation of SWNTs. The intensity of RBM peaks decreased in the following order of Fe3O4/MgO > Fe3O4/SiO2 > Fe3O4/Al2O3, whereas the ID/IG ratio decreased in the order of Fe3O4/SiO2 > Fe3O4/Al2O3 > Fe3O4/MgO. In other words, compared with SiO2 and Al2O3, MgO-supported Fe3O4 represents the most suitable combination to produce SWNTs, considering that the highest intensity of RBM peaks and the lowest ID/IG ratio were observed. This is in good agreement with the finding reported by Ago et al. [35] and Jin et al. [36] that high quality of SWNTs were synthesised by iron supported on MgO catalyst.Figure 3
Raman spectra of the (a) Fe3O4/SiO2, (b) Fe3O4/Al2O3, and (c) Fe3O4/MgO catalysts after reaction.The diameter of SWNTs obtained was in the range of 1.09–2.40 nm. In this work, CNTs with diameters greater than 2.48 nm were considered large diameter CNTs, whereas CNTs with diameters of 2.48 nm or less were defined as small-diameter CNTs. Scanning electron microscopy (SEM) images for all samples are shown in Figure4. CNTs with diameters greater than 2.48 nm were produced when SiO2 was used as a catalyst support (Figure 4(a)), whereas CNTs with small diameters were formed when Al2O3 and MgO were used (Figures 4(b) and 4(c)). The diameters of CNTs synthesised by Fe3O4 supported on Al2O3 and MgO were measured using TEM, and the results are shown in Figure 5. The diameter ranges were 1.22–1.46 nm and 1.09–2.40 nm for CNTs produced by Al2O3 and MgO when used as catalyst supports, respectively. The diameters were almost uniform, and the CNTs attached to one another in a bundle form (Figure 5). The bundle produced by the MgO-supported catalyst consisted of more SWNTs compared with that produced by Al2O3 as the catalyst support (Figure 5). The TEM observations show that SWNTs synthesised by MgO used as the support were of high quality, which is consistent with the Raman analysis.SEM images showing nanotube structure synthesised using the (a) Fe3O4/SiO2, (b) Fe3O4/Al2O3, and (c) Fe3O4/MgO catalysts.
(a)
(b)
(c)TEM images showing SWNT bundles synthesised using the (a) Fe3O4/Al2O3 and (b) Fe3O4/MgO catalysts.
(a)
(b)To understand why MgO was a superior support, we considered the interaction between the metal oxide nanoparticle and its support. The formation of the highest-quality SWNTs by the Fe3O4/MgO catalyst could be attributed to the strong metal interaction between the Fe3O4 nanoparticles and MgO [37, 38]. This strong interaction restricted the mobility of Fe3O4 nanoparticles during CVD, thus preventing the extensive agglomeration of Fe3O4 nanoparticles at high temperatures to form larger-sized clusters, leading to good dispersion of Fe3O4 [39]. This strong interaction can be explained in terms of Lewis acid/base interactions between metal oxide supports and metal catalyst nanoparticles. A Lewis base is a species that acts as an electron pair donor, whereas a Lewis acid is a species that acts as an electron pair acceptor [39]. In this case, the metal oxide support (MgO) acts as a Lewis base and donates electrons through metal catalyst nanoparticles (Fe3O4) to methane [39]. The metal catalyst nanoparticles play the role of a conduit for transferring negative charge from the metal oxide support to methane. Generally, simultaneous back-donation from methane also takes place that is, methane acts as a Lewis base, whereas MgO acts as a Lewis acid. The electronic structure of methane is then changed in such a way that decomposition of the methane molecule takes place.The surface basicity was reported to decrease in the following order: MgO > Al2O3 > SiO2 [40]. Thus, MgO represents the best electron donor compared with Al2O3 and SiO2. As a result, the interaction between Fe3O4 and MgO was stronger than the interactions between Fe3O4 and Al2O3 and between Fe3O4 and SiO2 [40]. With the strong interaction between the Fe3O4 and MgO, Fe3O4 particles are thought to be well dispersed and promote the formation of SWNTs. This explains the reason of intensity of the RBM peaks for the samples produced by MgO used as a catalyst support was the highest and the ID/IG ratio was the lowest compared with the samples formed by SiO2 and Al2O3. The second highest intensity of RBM peaks was observed for samples formed by SiO2 used as the catalyst support compared with the samples produced by Al2O3 used as the catalyst support. However, a higher ID/IG ratio was observed in the Raman spectra of the sample produced by using an SiO2 catalyst support than that formed when using an Al2O3 catalyst support. This indicates that more defective structures were formed in the samples due to the fact that SiO2 is a less active support compared with alumina [41, 42]. Because Al2O3 is an amphoteric material having the characteristics of both an acid and a base, it may take part in the strong interaction. Therefore, Fe3O4 particles had a better dispersion on the Al2O3 compared with SiO2 due to this strong interaction and formed a higher degree of CNT graphitisation.


## 3.3. Effect of Fe3O4 Loading

In this study, the molar ratios of Fe to MgO were varied at 1 : 9, 1.75 : 8.25, 2.5 : 7.5, and 3.25 : 6.75, which were denoted as (Fe3O4)1(MgO)9, (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75, respectively. The CVD conducted using a molar ratio of 1 : 9 produced the lowest ID/IG ratio, that is, 0.11. The ID/IG ratios of samples were found to increase from 0.11 to 0.46 as the Fe3O4 loading increased from 1 to 3.25.The Raman spectra shown in Figure6 show the presence of RBM peaks, indicating the presence of SWNTs. The intensity of RBM peaks decreased in the following order for Fe3O4 loading: 1 > 1.75 > 2.5 > 3.25. The ID/IG ratio was found to decrease in the following order: 3.25 > 2.5 > 1.75 > 1. In contrast, the highest intensity of RBM peak and the lowest ID/IG ratio were observed for SWNTs produced by a catalyst with a molar ratio of 1 : 9.Figure 6
Raman spectra of the (a) (Fe3O4)1(MgO)9, (b) (Fe3O4)1.75(MgO)8.25, (c) (Fe3O4)2.5(MgO)7.5, and (d) (Fe3O4)3.25(MgO)6.75 catalysts after reaction.The SEM images, shown in Figures7(a) and 7(b), revealed the presence of narrow-diameter CNTs from (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts. The narrow CNTs grew over the entire catalyst surface with almost uniform diameters. However, large-diameter CNTs were observed in the samples formed by the (Fe3O4)2.5(MgO)7.5 and (Fe3O4)3.25(MgO)6.75 catalysts. The structures of large CNTs were poor and had inconsistent diameters, which can be seen in Figures 7(c) and 7(d).SEM images showing the CVD products synthesised using the (a) (Fe3O4)1(MgO)9, (b) (Fe3O4)1.75(MgO)8.25, (c) (Fe3O4)2.5(MgO)7.5, and (d) (Fe3O4)3.25(MgO)6.75 catalysts.
(a)
(b)
(c)
(d)To measure the diameters of narrow CNTs, samples produced by the (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts were characterised by TEM. The measured diameter was in the range of 1.16–2.10 nm and 1.10–1.85 nm for SWNTs synthesised by (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalyst, respectively. Figure 8 clearly shows SWNTs in a bundle form produced by the (Fe3O4)1(MgO)9 and (Fe3O4)1.75(MgO)8.25 catalysts. Under TEM observation, the diameter of these SWNTs was almost uniform and in agreement with the SEM observations (Figures 7(a) and 7(b)). Moreover, the SWNTs bundles possessed a high degree of graphitization, which is consistent with the Raman results (Figure 6).TEM images showing SWNT bundles synthesised using the (a) (Fe3O4)1(MgO)9 and (b) (Fe3O4)1.75(MgO)8.25 catalysts.
(a)
(b)From these results, it is clear that there is an optimum Fe3O4 loading that leads to the formation of SWNTs of the desired quality. When increasing the Fe3O4 loading by more than 1.75, the ID/IG ratio also increased significantly, and large CNTs were clearly observed in the samples (Figures 7(c) and 7(d)). Increasing the Fe3O4 content is beneficial only if it enhances the available active sites for CNT growth, rather than increasing the nanoparticle sizes [43–47]. It is well known that the diameters of CNTs are correlated with active metal nanoparticles sizes [32, 40, 48–50]. Therefore, the narrow CNTs in this work were likely produced by small Fe3O4 nanoparticles, and the large CNTs by large Fe3O4 nanoparticles. Therefore, the quality of CNTs decreased with an increase in Fe3O4 loading.The average size of Fe3O4 nanoparticles for (Fe3O4)1(MgO)9 catalyst was 10.33 nm as reported in our previous work [42]. In this work, the size of catalyst nanoparticles for the (Fe3O4)1(MgO)9 catalyst was much smaller than the size of catalyst nanoparticles for (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75, as observed in Figure 7. The large nanoparticle catalysts were formed for the high Fe3O4-loaded: (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75 catalysts due to the sintering of the Fe3O4 nanoparticles during reaction at high temperatures (i.e., 900°C) [33]. Sintering is a thermally activated process that involves the diffusion of atoms [51]. Together with the higher concentration of iron, there would be a significantly higher degree of sintering of iron nanoparticles, causing large nanoparticles to form. However, it is believed that the degree of sintering of iron nanoparticles for (Fe3O4)1(MgO)9 was low compared with those of the (Fe3O4)1.75(MgO)8.25, (Fe3O4)2.5(MgO)7.5, and (Fe3O4)3.25(MgO)6.75 catalysts. As a result, the sizes of iron nanoparticles are small for a molar ratio of 1 : 9.


## 4. Conclusion

Our experimental findings show that among the types of active metals and supports investigated, Fe3O4 supported on MgO is the most suitable catalyst. High-quality SWNTs were synthesised using an Fe3O4/MgO catalyst with a molar ratio of 1 : 9. It is believed that iron has a higher carbon solubility than either cobalt or nickel, which helps to promote the formation of SWNTs. In addition, the interaction between Fe3O4 and MgO is strong, which prevents agglomeration of Fe3O4 and produces CNTs with a high degree of graphitisation. Using a low molar ratio of Fe3O4 to MgO could reduce the agglomeration of Fe3O4 nanoparticles. Therefore, the most suitable Fe3O4 loading found was at an Fe3O4 to MgO molar ratio of 1 : 9.



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*Source: 592464-2013-06-27.xml*