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

1 Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP), 01000 Kangar, Perlis, Malaysia 2 School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Seberang Perai Selatan, 14300 Nibong Tebal, Pulau Pinang, Malaysia 3 School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia 4 School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seberang Perai Selatan, 14300 Nibong Tebal, Pulau Pinang, Malaysia


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][2][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][5][6][7][8][9][10][11].However, Fe, Co, Ni, and their alloys are the most of the widely used transition metal catalysts for SWNT production [12][13][14].So far, various substances have been tested as supports of catalytically active metals, for instance, zeolites [15,16], SiO 2 [17], Al 2 O 3 [18], CaCO 3 [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][22][23][24][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 Fe 3 O 4 /MgO was an effective catalyst for growing SWNTs [26][27][28][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, SiO 2 , and Al 2 O 3 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.O 4 /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.

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][27][28][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.

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.   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 (  /nm) can relate to the frequency of RBM by RBM = /  + , where  = 234 cm −1 ,  = 10 cm −1 , and   = diameter of SWNTs in nanometer [31].

Results and Discussion
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 Fe 3 O 4 /MgO catalysts, respectively.The nanotube samples were also characterised using transmission electron microscopy (TEM) and their results are shown in Figure 2   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 Fe 3 O 4 /MgO catalyst.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.The diameter of SWNTs obtained was in the range of 1.09-2.40nm.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 Figure 4. CNTs with diameters greater than 2.48 nm were produced when SiO 2 was used as a catalyst support (Figure 4 5).The bundle produced by the MgO-supported catalyst consisted of more SWNTs compared with that produced by Al 2 O 3 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.

Effect of Al
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 Fe 3 O 4 /MgO catalyst could be attributed to the strong metal interaction between the Fe 3 O 4 nanoparticles and MgO [37,38].This strong interaction restricted the mobility of Fe 3 O 4 nanoparticles during CVD, thus preventing the extensive agglomeration of Fe 3 O 4 nanoparticles at high temperatures to form larger-sized clusters, leading to good dispersion of Fe 3 O 4 [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 (Fe 3 O 4 ) 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 > Al 2 O 3 > SiO 2 [40].Thus, MgO represents the best electron donor compared with Al 2 O 3 and SiO 2 .As a result, the interaction between Fe 3 O 4 and MgO was stronger than the interactions between Fe 3 O 4 and Al 2 O 3 and between Fe 3 O 4 and SiO 2 [40].With the strong interaction between the Fe 3 O 4 and MgO, Fe 3 O 4 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  D / G ratio was the lowest compared with the samples formed by SiO 2 and Al 2 O 3 .The second highest intensity of RBM peaks was observed for samples formed by SiO 2 used as the catalyst support compared with the samples produced by Al 2 O 3 used as the catalyst support.However, a higher  D / G ratio was observed in the Raman spectra of the sample produced by using an SiO 2 catalyst support than that formed when using an Al 2 O 3 catalyst support.This indicates that more defective structures were formed in the samples due to the fact that SiO 2 is a less active support compared with alumina [41,42].Because Al 2 O 3 is an amphoteric material having the characteristics of both an acid and a base, it may take part in the strong interaction.Therefore, Fe 3 O 4 particles had a better dispersion on the Al 2 O 3 compared with SiO 2 due to this strong interaction and formed a higher degree of CNT graphitisation.

Effect of Fe 3 O 4
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 (Fe 3 O 4 ) 1 (MgO) 9 , (Fe 3 O 4 ) 1.75 (MgO) 8.25 , (Fe 3 O 4 ) 2.5 (MgO) 7.5 , and (Fe 3 O 4 ) 3.25 (MgO) 6.75 , respectively.The CVD conducted using a molar ratio of 1 : 9 produced the lowest  D / G ratio, that is, 0.11.The  D / G ratios of samples were found to The Raman spectra shown in Figure 6 show the presence of RBM peaks, indicating the presence of SWNTs.The intensity of RBM peaks decreased in the following order for Fe 3 O 4 loading: 1 > 1.75 > 2.5 > 3.25.The  D / G 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  D / G ratio were observed for SWNTs produced by a catalyst with a molar ratio of 1 : 9.
The SEM images, shown in Figures 7(a Moreover, the SWNTs bundles possessed a high degree of graphitization, which is consistent with the Raman results (Figure 6).
From these results, it is clear that there is an optimum Fe 3 O 4 loading that leads to the formation of SWNTs of the desired quality.When increasing the Fe 3 O 4 loading by more than 1.75, the  D / G ratio also increased significantly, and large CNTs were clearly observed in the samples (Figures 7(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 (Fe 3 O 4 ) 1 (MgO) 9 was low compared with those of the (Fe 3 O 4 ) 1.75 (MgO) 8.25 , (Fe 3 O 4 ) 2.5 (MgO) 7.5 , and (Fe 3 O 4 ) 3.25 (MgO) 6.75 catalysts.As a result, the sizes of iron nanoparticles are small for a molar ratio of 1 : 9.

Conclusion
Our experimental findings show that among the types of active metals and supports investigated, Fe 3 O 4 supported on MgO is the most suitable catalyst.High-quality SWNTs were synthesised using an Fe 3 O 4 /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 Fe 3 O 4 and MgO is strong, which prevents agglomeration of Fe 3 O 4 and produces CNTs with a high degree of graphitisation.Using a low molar ratio of Fe 3 O 4 to MgO could reduce the agglomeration of Fe 3 O 4 nanoparticles.Therefore, the most suitable Fe 3 O 4 loading found was at an Fe 3 O 4 to MgO molar ratio of 1 : 9.
(a)), whereas CNTs with small diameters were formed when Al 2 O 3 and MgO were used (Figures 4(b) and 4(c)).The diameters of CNTs synthesised by Fe 3 O 4 supported on Al 2 O 3 and MgO were measured using TEM, and the results are shown in Figure 5.The diameter ranges were 1.22-1.46nm and 1.09-2.40nm for CNTs produced by Al 2 O 3 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
[29]Preparation of the Catalyst.The preparation method for Fe 3 O 4 nanoparticles has been previously reported[29].The molar ratios of Fe 3 O 4 to MgO, NiO to MgO, and CoO to MgO were set at 1 : 9. Fe 3 O 4 /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 Fe 3 O 4 nanoparticles, Ni (NO 3 ) 2 ⋅6H 2 O (99.999% trace metal basis: Aldrich) and Co (NO 3 ) 2 ⋅6H 2 O (99.999% trace metal basis: Aldrich), respectively, into the MgO solution to get three different types of mixture solutions.For the preparation of Fe 3 O 4 /Al 2 O 3 and Fe 3 O 4 /SiO 2 catalysts, the molar ratios of Fe 3 O 4 to Al 2 O 3 and Fe 3 O 4 to SiO 2 were set at 1 : 9.The required amount of Fe 3 O 4 nanoparticles was dispersed in distilled water and followed by adding Al 2 O 3 (analytical reagents: Ajax) and SiO 2 nanopowder (analytical reagents: RdH), respectively, to form two different types of mixture solutions.To prepare different molar ratios of Fe 3 . It can be observed that bundles of SWNTs with nearly uniform diameters were synthesised by Fe 3 O 4 /MgO, NiO/MgO, and CoO/MgO catalysts.Bundles that consist of more than 20 SWNTs were obtained from the Fe 3 O 4 /MgO catalyst.In addition to the SWNT bundles, MWNTs were also observed under TEM for the NiO/MgO catalyst but not for the Fe 3 O 4 /MgO and CoO/MgO catalysts.To arrive at the understanding why the Fe 3 O 4 /MgO catalyst formed SWNTs with a lower  D / G ratio, we have