Yong Sic Kim

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Master's Thesis

Yong Sic Kim

School of Energy and Chemical Engineering (Chemical Engineering)

Ulsan National Institute of Science and Technology

2022

Carbon Nanotube as a One-Dimensional

Catalyst Support

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Yong Sic Kim

School of Energy and Chemical Engineering (Chemical Engineering)

Ulsan National Institute of Science and Technology

Carbon Nanotube as a One-Dimensional

Catalyst Support

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A thesis/dissertation submitted to

Ulsan National Institute of Science and Technology in partial fulfillment of the

requirements for the degree of Master of Science

Yong Sic Kim

Chang Young Lee

Carbon Nanotube as a One-Dimensional Catalyst Support

01.14.2022 of submission Approved by

Advisor

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Yong Sic Kim

This certifies that the thesis/dissertation of Yong Sic Kim is approved.

01.14.2022 of submission

Signature

___________________________

Advisor: Chang Young Lee

Signature

___________________________

Kwangjin An

Signature

___________________________

Dong Woog Lee

*three signatures total in case of master

Carbon Nanotube as a One-Dimensional

Catalyst Support

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Carbon Nanotube as a One-Dimensional Catalyst Support

Abstract

The catalyst is a substance that does not participate in the reaction and simultaneously controls the reaction rate. Among the many catalyst fields, the supported catalyst anchored to the support increases the possibility of exposure to the active site. Thus, this increases the reaction rate very efficiently. These supported catalysts can be divided into three types depending on the dimension. 3D and 2D supports are the most studied supports. Since the surface area is large, a lot of catalyst particles can be loaded at once. However, the risk of aggregation is high. The 1D material has a small surface area but has a small contact area with the catalyst particles, so it is less likely to form a cluster. Therefore, many studies on 1D catalyst support have been conducted recently. However, materials such as nanotubes, nanowires, and nanorods so far have not acted as actual 1D materials for nanoparticles due to their large diameter and short length. In addition, most of them were synthesized in a liquid phase and transferred to a 2D substrate to proceed with a chemical reaction. In this paper, we proved that suspended single-walled carbon nanotubes can be used as a 1D support for catalysts using iron catalysts and cobalt nanoparticle catalysts. In addition, in the synthesized carbon nanotube supported catalyst, the secondary carbon nanotube was synthesized to a length of several tens of um using chemical vapor deposition (CVD) to confirm the catalyst reaction.

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Figures

Figure 1 Various dimensions of catalyst support ... 5

Figure 2 Previous 1D catalyst support ... 5

Figure 3 Apparatus to make fullerenes and CNTs ... 7

Figure 4 Graphene carbon atom structure scheme and hybrid orbital ... 8

Figure 5 Chirality of CNT ... 8

Figure 6 Basic structures of CNT ... 9

Figure 7 Energy dispersion of graphene ... 10

Figure 8 Chiral vector ... 10

Figure 9 Kataura plot... 12

Figure 10 Raman spectrum ... 12

Figure 11 CNT Raman peak ... 13

Figure 12 laser ablation method scheme ... 16

Figure 13 CVD method scheme ... 16

Figure 14 CNT growth model ... 20

Figure 15 CoNP size control method scheme ... 24

Figure 16 CoNP TEM image ... 25

Figure 17 The size of CoNPs in nanoreactor chip ... 25

Figure 18 photolithography scheme ... 26

Figure 19 Fe catalyst pretreatment condition ... 27

Figure 20 CNT synthesis condition ... 28

Figure 21 CNT synthesis result SEM image ... 29

Figure 22 AFM height comparison depending on catalyst ... 30

Figure 23 AFM image of carbon nanotube by using Fe catalyst ... 31

Figure 24 SWCNT diameter & RBM ... 32

Figure 25 Preparation for drop CoNP solution ... 33

Figure 26 SEM image of half-cat. treated TEM grid ... 34

Figure 27 Fe deposited CNT strands ... 35

Figure 28 Secondary CNTs synthesized from Fe catalysts ... 35

Figure 29 Comparing between before and after dropping CoNP catalyst solution. ... 36

Figure 30 Secondary CNTs synthesized from CoNP catalysts ... 37

Figure 31 Synthesis of another CNT using previous CNT as a catalyst support. ... 37

Figure 32 After dropping CoNP catalyst solution on CNT ... 38

Figure 33 Synthesis of secondary CNT ... 39

Figure 34 Future perspectives ... 40

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4 1. Introduction

1.1. Catalyst support

Catalytic research is a field of study with a very long history. The catalyst has the characteristic of adjusting thereaction rate by changing the active energy without directly participating in the chemical reaction. Therefore, catalysts are almost essential to many chemical reactions. There are two things that are always considered when studying these catalysts(Julkapli & Bagheri, 2015). First, the catalyst used should be easily recovered. This is because the reuse of catalysts is important in terms of economy.

Second, the more active sites of the catalyst are exposed, the better. Chemical reactions take place at the active site of the catalyst. Therefore, as the exposed area of the active site increases, the effect on the reaction rate increases.

The catalyst is largely divided into a homogeneous catalyst and a heterogeneous catalyst (Julkapli &

Bagheri, 2015). Homogeneous catalysts refer to physical phases such as reactants and catalysts.

Conversely, the heterogeneous catalyst has a physical phase in which the reactant and the catalyst are different. Therefore, heterogenous is more advantageous to reuse the catalyst. However, the initial Heterogenic catalyst had a problem in that the catalyst aggregated to form a cluster. The solution designed to solve this problem is supported catalyst. This physically anchored the catalyst particles to the support to reduce the agglomeration of the catalyst particles. The initial catalyst support is very diverse, such as silica and zeolite. However, these early supports are generally characterized by being weak against acids and bases. Catalytic supports that must come into contact with reactants under various conditions must have strong characteristics in this environment. Therefore, the designed material is a carbon material. Carbon materials are less reactive and have relatively strong properties in extreme environments such as high temperatures, acids, and bases.

Catalyst support can be largely divided into three types according to dimension. Three-dimensional and two-dimensional materials are the most studied and used supports so far. In particular, the 3D support made using a 3D printer can design a structure suitable for the desired reaction. In addition, since it has a large area, it is possible to load a large amount of catalyst particles at once. Various materials such as activated carbon, zeolite, silica, and graphene are currently being studied. However, the risk of agglomeration of particles increases due to the large contact area with the catalyst. To compensate for this, various one-dimensional materials are being studied as catalyst supports.

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Figure 1 Various dimensions of catalyst support

The one-dimensional support can significantly reduce the contact area with the particles compared to the multi-dimensional one (Esteves, Oliveira, Xing, & Passos, 2021). Carbon nanotubes, nanorods, and nanowires are the most studied one-dimensional supports. First, because the diameter is very small, it can be synthesized in a size similar to the size of the catalyst particle size. This means that the contact area between the particle and the support is very small. In addition, the risk of forming a cluster is reduced because other particles are not surrounded. However, large amounts of catalysts, such as multidimensional materials, cannot be loaded at once.

Figure 2 Previous 1D catalyst support

Previous research cases of one-dimensional supports have several things in common. First, most of them are short and have a large diameter. This may feel like a multidimensional material from the perspective of nano-sized catalyst particles. Second, most of the supported catalyst synthesis is carried out in liquid phase. This not only increases the possibility of cluster formation of catalyst particles, but

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also in hollow structures such as nanotubes, catalyst particles exist regardless of internal and external distinction of the structure. Since the position of the catalyst particles is important depending on the type of specific chemical reaction (Esteves, Oliveira, & Passos, 2018), conditions that cannot be controlled may be disadvantages. For example, in the case of secondary carbon nanotube synthesis, synthesis is possible only in catalysts outside nanotubes. Therefore, the particles present inside are unnecessary. Finally, studies so far have studied support that has not been suspended. There was no suspended support in the air because most catalytic support was synthesized in liquid phase and transferred over the 2D substrate. However, when transferred over the 2D substrate, an interaction between the catalyst and the substrate occurs, which affects the chemical reaction. In addition, the possibility of catalyst clumping on the substrate increases.

In this paper, the suspended single-wall carbon nanotubes were synthesized to show the potential as a one-dimensional support. In addition, the existing limitations were solved by synthesizing carbon nanotube supported catalysts in the atmosphere.

1.2. Discovery of Carbon nanotube

Carbon nanotube was discovered by Iijima in 1991 through arc-discharge evaporation synthesis (Iijima, 1991). However, it was Yoshinori Ando who first synthesized a material containing carbon nanotube (Ando, 2010). This is because mass production of fullerenes using alternating current (AC) was reported in 1990 by Krätschmer et al.(Krätschmer, Lamb, Fostiropoulos, & Huffman, 1990).

Yoshinori Ando wanted to study fullerene production in a different way than Krätschmer. He, therefore, tried a new method using DC arc discharge, where he replaced the silicon block with a graphite electrode (Ando, 2010). As a result, he discovered carbon deposits that had accumulated on the cathode, a phenomenon that had never been reported before. After passing it on to my friend Sumio Iijima, he accidentally confirmed the presence of fullerene and carbon nanotube inside the deposit through HRTEM. The carbon nanotube discovered at this time was multi-walled. Carbon nanotube was not called by that name from the beginning. The first given name was microtubule. When Iijima published his third paper, it was named carbon nanotube (Iijima, Ajayan, & Ichihashi, 1992). After that, Ando was able to obtain MWCNT deposits without fullerene by using methane in addition to an inert gas (He, Ar). Later, they researched how to make SWCNTs. The first SWCNTs could be synthesized by adding Fe and Co metal catalysts to the anode and using the arc plasma jet (APJ) method. The reason why the arc plasma jet (APJ) method got its name is as follows. The temperature factor of arc discharge, which normally proceeds at 180 degrees, was changed to 30 degrees. Then, arc plasma was generated like a jet between the two electrodes. Through this method, the amount of deposit was reduced and more soot containing SWCNT was produced. As a result, a higher yield of SWCNTs could be synthesized (Ando, 2010). Since then, as interest in the properties of SWCNTs has increased, research

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on methods capable of selectively synthesizing them has begun. In addition, as the potential as a next- generation semiconductor material has been proven, the amount of research has exploded.

Figure 3 Apparatus to make fullerenes and CNTs. Copyrighted by Ando et al (2010)

1.3. Carbon nanotube properties

1.3.1. CNT structure

Carbon nanotubes have different properties depending on how twisted they are. The degree of twist is determined by a parameter denoted by (m,n) (Odom, Huang, Kim, & Lieber, 1998). To do this, it is necessary to first look at the atomic structure of CNTs. Usually, CNT is described as graphene rolled into a cylinder. At this time, two vectors that take one of the hexagonal lattice intersections of graphene as the starting point and the other hexagonal lattice intersection that are skipped one by one end-point as the end point and set as a unit vector. Therefore, the vector expressed as 𝐶 = 𝑚𝑎 + 𝑛𝑏 determines the characteristics of one CNT. These are called chiral indices (m,n) (Ivchenko & Spivak, 2002).

According to the vector (curling direction) value, most of the physical properties such as the diameter, electrical properties, and optical properties of CNT are determined (Jouguelet, Mathis, & Petit, 2000).

The CNT structure can be divided into three main types. First, it is a structure of (m,0) when n=0, which

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is called an armchair structure (Natsuki, Tantrakarn, & Endo, 2004). CNT with this structure has metallic properties. Second, when n = m (zig-zag structure) (C.-Y. Su et al., 2009; Yuan & Ding, 2015).

The CNT of this structure has both metallic and semi-metallic properties. Finally, n is non-zero and is different from m. The last case is called chiral. It has different electrical characteristics according to each chirality. According to statistics, it is reported that in the CNT mixture, 1/3 is metallic and the remainder is semi-metallic chirality.

Figure 5 Chirality of CNT. Copyrighted by AZoNetwork

CNTs can be distinguished according to the number of overlapping walls. One or two or more walls are called single-walled CNT (SWCNT), double-walled CNT (DWCNT), and multi-walled CNT (MWCNT), respectively.

Figure 4 Graphene carbon atom structure scheme and hybrid orbital. Copyrighted by Sarkar et al. (2011)

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Figure 6 Basic structures of a single-walled, b double-walled, and c multi-walled CNTs

1.3.2. Electrical properties

The electrical characteristics of CNT are the most important characteristics among the various characteristics of pepper. To understand this, it is necessary to first look at the band structure of graphene.

The graphene band structure has a valence band and a conduction band attached to it. This means that the energy bandgap is zero. This advantage in the bandgap is called the K point, and in the case of graphene, it has a k point at the sp2 bonding point of the hexagonal structure. Therefore, we have 6 k points per grid. In the case of normal metal, these bands are overlapped with each other, and in the case of general semiconductors, there is a gap between the two types of bands. CNT is a structure made by drying such graphene. Therefore, additional boundary conditions are required when calculating the energy bandgap. Additionally, assuming that CNT is an infinite cylindrical structure, two types of wave vectors are needed: a vector perpendicular to the cylinder axis and a vector parallel to the cylinder axis.

In this case, the wave vector perpendicular to the axis can be obtained through boundary conditions.

The bandgap of CNT can be found by obtaining the cross-section of the bandgap of graphene at the endpoint of this vector (Figure 5). Therefore, CNT has two structures, one without bandgap and one with bandgap, and it can be divided into metallic and semi-metallic according to the direction of curling (Figure 6). However, in the case of MWCNTs, the conductivity is complicated because the distribution of current is uneven due to the inter-wall interaction. Therefore, research fields such as semiconductors, transistors, and batteries are paying attention to SWCNTs. In 1998, Frank measured the conductivity in the mercury liquid phase using scanning probing microscopy (SPM) and reported that carbon nanotubes exhibit quantum behavior and have revolutionary ballistic conductance. Sanvito et al. measured the conductivity of MWNTs using the scattering technique, reconfirmed Frank's results, and observed that the quantum conductive channels in the MWNTs were reduced by the interwall reaction. By this reaction, the electron flow of each carbon nanotube was grown was observed. Thess et al. observed that the resistance of rope-type metallic SWNTs was about 10^-4Ω-cm at 300K using the four-point technique, and this value was found to be higher than the currently known high-conductivity carbon nanofibers.

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Figure 7 A conic energy dispersions of graphene with at six K points. The k⊥ states in CNT are showed by dashed lines. The band structure of CNT is obtained by cross-sections; b) a metallic CNT and c) a semiconducting CNT. Copyrighted by Javey and Kong (2009)

Figure 8 Chiral vector, C = n1a1 + n2a2

CNT has a structure in which each atom is strongly attached through sp² chemical bonds. For this reason, CNT has a very high modulus of elasticity and is expected to be a high-strength fiber. The highest young's modulus reported is 1.8 TPa (Hsieh et al. 2006). Depending on the measurement method and the chirality of SWCNTs, 1.22 TPa-1.26 TPa have been reported (Dresselhaus et al. 2001). This is very high considering that the young's modulus of ordinary steel is 0.2 TPa. Many studies have been conducted on this property because of its high utility as high-resolution scanning probe tips like AFM.

(Find papers) However, in the case of MWCNT, it has a value of 0.2-0.95 TPa, which is reported to increase as the number of walls decreases (Forro et al. 2002).

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11 1.3.3. Raman spectroscopy of carbon nanotube

Rao et al. reported the Raman spectrum of SWCNTs for a various energy terrain. Optical absorption spectra of SWCNTs have different values depending on the CNT diameter due to pi-plasmon was offset.

(Kataura et al., 1999). Thus, diameter could be estimated from the peak of Raman spectrum. A plenty of Raman spectroscopy research has been done for the characterization of carbon nanotubes and their electronic and vibrational properties (Anglaret et al., 1998; Kasuya, Sasaki, Saito, Tohji, & Nishina, 1997; Pimenta et al., 1998; Rao et al., 1997; Richter & Subbaswamy, 1997). Raman scattering of SWCNT has illuminated a resonant process related to allowed optical transition (AOT) between peaks of the one-dimensional electric density. The energy value of AOT has been reported by several groups.

the result indicated that the energy has correlated to the diameter and electrical properties of carbon nanotubes. The most representative research is Kataura plot. the plot showed relation between transition energies, diameter and wavelength which means the helicity of carbon nanotube (chiral indices) could be estimated from the Kataura plot. But the problem is resonance occurs probabilistically. Scatterd phonon is the main factor of the resonance. However, it doesn’t follow the rule. That’s why the RBM(radical breathing modes) occurring is the fortune. RBM is the specialty of carbon nanotube which make a difference from graphene. Raman spectrum of RBM has a range from 100 to 200 𝑐𝑚⁻¹ which is not presented from other graphene materials. The cylindrical structure of carbon nanotube generates the unique vibration wavelength resulted in bond stretching. That’s why the RBM is directly connected to the diameter of carbon nanotube (Dresselhaus, Dresselhaus, Saito, & Jorio, 2005). In other words, the diameter of carbon nanotube can be calculated via RBM though the value is affected by amount of nested carbon nanotubes (Bandow et al., 1998). The carbon nanotube diameter calculation method presented in Bandow et al. is as follows.

𝑣(𝑐𝑚⁻¹) = 224/𝑑(𝑛𝑚)

ν is RBM wavelength and d is diameter of CNT.

However, nanotube has specific property of self-assembled on the bundle which influenced the RBM value. Fundamentally the packing of carbon nanotube affected the RBM. L. Alvarez et al. considered wan ders waals interactions and proposed another equation.

𝑣(𝑐𝑚⁻¹) = 6.5 + 232/𝑑(𝑛𝑚)

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Figure 9 Kataura plot copyrighted by Kataura et al.

Figure 10 Raman spectrum. RBM peak is located between 200 and 300 ccd

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Figure 11 CNT Raman peak copyrighted by Nano photon

Raman spectroscopy projected a lot of other information of carbon nanotubes like phonon structure, defection of CNT and electrical structure. The elasticity and mechanical properties referred above are also determined by phonons (Dresselhaus et al., 2005). In Raman spectrum there are some unique peaks for carbon nanotube. The first peak is G-peak which is first-order Raman scattering in graphene (1582 𝑐𝑚⁻¹). This is the tangential shear mode of carbon atoms, which is consistent with the stretching mode seen in the graphite plane. The G-peak is separated in other peaks around 1582 𝑐𝑚⁻¹ at CNT because of cylindrical structure. G⁺ has no correlation with the diameter of the tube, but G^-, which appears in the lower region, tends to soften as the diameter decreases. If G^- appears as a broad and symmetrical peak, it can be expected that the sample is a metallic tube. The stronger the metallicity of the tube, the lower the G^- frequency, and when it appears in the lower region of 100 cm^-1 or more, it means that the diameter of the carbon nanotube is small. On the other hand, G⁺ appears at a constant position regardless of the diameter of the metallic or semiconducting CNT. RBM peak is also first-order Raman scattering peak which is the strongest evidence of existence of SWCNT.

In second-order Raman scattering, D-peak is a longitudinal optical (LO) acoustic quantum, known as imperfection or defect because preserving momentum process needs stretch scatter. It usually appears at 1330~1360 cm^-1 and has linearity according to the laser excitation state. D-peak appears in all carbon allotropes including amorphous carbon of sp² and sp³ hybridized bond. In carbon nanotubes, D-peak is activated from first order scattering of sp²carbons by in-plane substitution of non-carbon atoms, vacancies, grains, defects, and limited sizes. These properties lower the crystal symmetry of the quasi- infinite lattice. When observing MWNTs, D-peak is also considered as evidence of disorder in the carbon nanotube. The quality of a sample can be evaluated by D/G intensity. A high-purity sample without defects or amorphous carbon has a low D/G ratio (less than 2%). The D width of CNTs is usually 10-20 cm^-1.

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In Raman spectroscopy, for minimizing sample damage, it is necessary to consider the phenomenon of CNT heating by the laser. Which is called Laser power heating is associated with easy and accidental overheating of CNT samples. Changing the wavelength reduces overheating of the sample, and through the experiment, it was concluded that 5 mW is appropriate for the CNT sample. For High-throughput spectroscopy analyzes, low laser intensity is used to accumulate the spectrum signal. Mixed CNTs or liquid CNTs do not overheat even at high laser intensities.

Carbon nanotubes mixed with impurities are more likely to be heated by the laser. Whether or not the sample is heated can be determined by moving to a region where G is higher or when D disappears.

Granular nanotubes containing a lot of amorphous carbon also tend to heat up easily, which can be confirmed by a large baseline change when analyzed in continuous mode.

1.3.4. CNT synthesis methods

Carbon nanotubes have influence on purity and chirality depending on how they are synthesized. When CNTs are synthesized, carbon compounds (such as CH4, C2H2 etc.) are usually used as carbon sources, but the rate of decomposition differs depending on the temperature at which the synthesis proceeds, and the degree of accumulation of amorphous carbon is also different. However, it seems that the role of the catalyst is greater than the synthesis method on carbon nanotube synthesis. There are three main methods for synthesizing carbon nanotube.

The first synthesis method is arc discharge method. Generating arc discharge between two electrodes made by graphite was the first method for producing carbon nanotubes. This method used a direct current between electrodes which has a circumstance of inert gas (He, Ar) at around 500 torr. For MWCNT synthesis, there is no need to add any other metal catalysts. But metal catalysts are essential for formation of SWCNTs. Normally, carbon nanotubes synthesized by arc discharge represent a high- quality structure. Nonetheless, a variety of factors (temperature, the density and composition of the catalyst, the existence of hydrogen, etc.) influence their size and shape (Imasaka, Kanatake, Ohshiro, Suehiro, & Hara, 2006). Recently, other chemicals and electrodes have been applied for the producing of carbon nanotubes via arc discharge (A, M, D, & Haridoss, 2015; Ben Belgacem, Hinkov, Yahia, Brinza, & Farhat, 2016; Sagara, Kurumi, & Suzuki, 2014; Y. Su & Zhang, 2015). MWCNTs doped with B and N2 through the arc discharge method was reported by Belgacem et al. (Ben Belgacem et al., 2016).

SWCNT-SWCNT hybridization by arc discharge is reported by other research groups in air condition at more less cost (A et al., 2015).

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The second synthesis method is laser ablation method. Guo et al. presented the laser ablation method for CNT synthesis at first (Guo, Nikolaev, Thess, Colbert, & Smalley, 1995). In this synthesis a quartz tube is coated by solid graphite substance at high temperature (800-1500 °C ) where the circumstance is filled with inert gas which is vaporized by a laser beam .After the target substance is vaporized, the carbon-included residue is collected (Kingston & Simard, 2003). The CNTs arranged by this method present a high quality of structure, according to the high temperatures arranged in this method. SWCNTs can be synthesized alone without the byproduct of MWCNTs. The main foibles of this method are the high energy required and smaller amounts of CNTs produced than the arc discharge method (Kumar, Rani, Dilbaghi, Tankeshwar, & Kim, 2017). Several factors of laser ablation influence the amount and quality of CNTs produced, such as wavelength and laser power, chemical proportion of the target material, the distance between the substances and the substrates etc (Arepalli, 2004; Dai, Hafner, Rinzler, Colbert, & Smalley, 1996).

The third synthesis method is chemical vapor deposition method (CVD). Nowadays, CVD is the actively researched synthesis method for the mass production of various chirality of CNTs. In this last synthesis method, a gas phase hydrocarbon source (propylene, ethanol, ethylene, methane, acetylene, carbon monoxide, etc.) is heated to about 1000°C with transition metal catalysts (Fe, Ni, Co, etc.) to accelerate the synthesis of carbon nanotubes. CVD allows mass production, fine alignment, synthesis and controlled growth of carbon nanotubes (José‐Yacamán, Miki‐Yoshida, Rendón, & Santiesteban, 1993). Furthermore, controlling diameter of CNTs and accumulated layer number of MWCNT is also reported (Hou, Liu, & Cheng, 2008). The foible of this synthesis method is higher defection possibility of the fabricated carbon nanotubes (amorphous carbon) when compared to above former methods (Collins, Arnold, & Avouris, 2001). The electrical and thermal properties is influenced by these disorders along with the structural characteristics of the carbon nanotubes (Avouris, Chen, & Perebeinos, 2007; Ferreira et al., 2016). Currently, the CVD method has been applied for controlling of the fabrication of long carbon nanotubes with no defected structure. Marvelous thermal, electrical and mechanical properties of carbon nanotubes could be attained by these perfect structure of CNTs.

Thermal plasma reactor is also reported for using CVD method by other authors (Szymanski, Kolacinski, Wiak, Raniszewski, & Pietrzak, 2017) at pressure of atmosphere. However, The synthesis of carbon nanotubes fall into a deadlock in commercial application field. For fabrication of enormous high quality of carbon nanotubes, a decisive leap of progress is essential. And low production cost is also necessary to application for their commercial advantages. These advances will certainly be achieved by the large market opportunities (Kumar et al., 2017).

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Figure 12 laser ablation method scheme copyrighted by Zan Lu et al.

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Figure 13 CVD method scheme copyrighted by T.Guo et al.

1.4. CNT catalysts

Usually when synthesize CNTs, transition metal nano particles are essential to enable decomposition of hydrocarbon at a low temperature compared to the autonomous decomposition temperature. The most employed metal nanoparticles are Fe, Ni, Co. These metal catalysts are mainly used as CNT synthesis catalysts because the high saturation of carbon to these metals at high temperature and the rate of carbon diffusion are very fast in these metal catalysts. Recent research presented that Fe, Ni and Co have more adhesive interaction with the growing carbon nanotubes (than other transition metals catalysts). Thus these metal catalysts are more efficient in formation of smaller diameter carbon nanotubes which is equivalent with high curvature like SWCNTs (Ding et al., 2008).

There is another sort of catalysts. Solid organometallocenes such as ferrocene, cobaltocene, nickelocene are broadly used as a carbon nanotube catalyst. Because that kind of catalysts have free metal-nanoparticles which accelerate decomposition of the hydrocarbon more efficiently. From the experiments, the diameter of CNTs is directly correlated to catalyst particle size. Thus, controlling the size of metal nanoparticles by reliable synthesis methods can be applied to control the CNT diameter.

Various substrates coated by thin films of catalyst are also reported as authentic method to obtain

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uniform carbon nanotubes (Fan et al., 1999). There are two key point to attain high purity of CNTs. One is processing hydrocarbon decomposition on the catalyst surface exclusively. Second is repression of the aerial pyrolysis. Without using any additional catalyst a high-yield CNT synthesis has been reported from acetylene hydrocarbon source at 730 °C on a stainless steel substrate (Camilli et al., 2011). This research appointed that the catalyst could be another instead of nanoparticle morphology. Even a bulk size metal could be a catalyst. Nanometer scale roughness of substrate surface can provide the appropriate environment for synthesis of carbon nanotubes. Furthermore, other research showed alloys are more efficient than pure metals due to higher catalytic activity.

In 2008, ballistic growth of carbon nanotube was reported by Kumar et al. Fe-Co catalyst on zeolite support was used for CNT synthesis. The result showed a volume gain of 10,000 % and weight gain of 1000% relative to the zeolite substrate. Thus, by fabricating alloy metals in different composition and controlling the catalyst synthesis conditions, it is prospect to develop new crystallographic phases which could express even higher catalytic activity for CNTs growth. In 2011, Nunez et al. showed Ni-Mo and Co-Mo catalyst which have high active crystallographic phases have been performed on MgO support, yielding ~3000 % of weight CNT growth. In 2003, Moisala et al. showed, besides the usual transition metals (Fe, Co, Ni), other metal catalyst such as Pd, Cu, Ag, Pt, Au also catalyze CNT growth from diverse hydrocarbons compounds. As a result, transition metals are authenticated as efficient catalysts not only in arc-discharge but also in CVD and laser ablation methods.

1.4.1. CNT catalyst supports

The substrate materials are another main point of catalyst activity. Silica, silicon carbide, alumina, silicon, quartz, zeolite and magnesium oxide are the most considered candidates. This, the research of interaction between support materials and catalyst is essential to high quantity of carbon nanotubes. The activity of catalyst behavior is inhibited by the chemical reaction between transition metal catalyst and substrate materials (chemical bond formation). Especially, the texture and surface morphology of support materials influence the quality and quantity of carbon nanotubes. For example, In 2005, Kumare et al. reported that nanoparticle metal catalyst on the zeolite support nanopores presented phenomenal high amount of carbon nanotubes which have significantly narrow distribution of diameter.). In case of comparing alumina support to silica, alumina substrate has a better support for catalysts due to stronger support- metal interaction which make easier metal dispersion and high density of active catalyst area.

The metal-substrate interaction is needed to be considered. Because this prevent metal catalysts from being conglomerate and unintended cluster residue which could lead to amorphous carbon (defects in carbon nanotubes). This hypothesis was authenticated by in-situ XPS CNT growth analysis with a variety of carbon source on iron catalyst and silica/alumina substrate. In 2008, Mattevi et al. showed

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0.04-0.4 µm alumina flake and Fe nanoparticles presented high quantities of aligned and CNTs which have high aspect ratio.

Oxide substrate which commonly used as metal catalyst support showed special chemical effect in CNT growth (Noda et al., 2007) Thus, the crystal structure of substrate, especially the crystallographic orientation on the support surface, and chemical state are also main factors of controlling CNT synthesis.

n. For instance, on Si (100) surface, CNT growth showed the perfect direction to 90°. However, the growth of carbon nanotubes in the 60° direction from the Si (111) surface is weak (Su et al., 2000).

Although plane sapphire of c and m showed low quantity of carbon nanotube growth, sapphire of a- plane showed normal growth of carbon nanotubes to (001) direction (Han et al., 2005). Similarly, on single crystal MgO (110) direction presented strong preference growth of carbon nanotubes (Maret et al., 2007).

On alumina micro particle, CNT showed orthogonal growth to crystal direction at 600°, CVD synthesis method of ferrocene-xylene atmosphere (He et al., 2011). These research stipulate crystal morphology and grain boundary of substrate material have an important role in carbon nanotube synthesis.

1.4.2. New CNT catalysts

Recent research of nanomaterials characterization and synthesis get new kinds of catalysts for carbon nanotube synthesis field. Besides commonly used transition metal catalysts (Co, Fe, Ni), a variety of other metals (Cu, Mg, Al, Pt, Mo, Cr, Sn, Pd, Mn, Au,) have also been reported for horizontal synthesis of SWCNT growth on quartz support materials (Yuan et al., www.intechopen.com152 Carbon Nanotubes - Synthesis, Characterization, Applications 2008). Noble metals with very low carbon solubility (Au, Pt, Pd, Ag) are not as highly soluble in carbon as transition metals. But when the noble metal particle size is under 5 nm, carbon can be dissolved efficiently for CNT growth. In 2008, Takagi et al. reported controlled CNTs synthesis method through Au nanoparticles which is dropped on the atomic layer of Silicon (Takagi et al., 2008). Among the total deposited Au nanoparticle catalysts, the active catalyst was observed to be about 80% Au-Si alloy. Copper is included in the transition metal.

But copper shows meager catalytic activity in CNT growth (Vanderwal et al., 2001). In fact, it has been considered a harmful pollutant. Although Vanderwal et al. presented scanty catalytic activity of Cu, according to another research Cu has been illuminated to promote the CNT growth effectively. On condition Cu nanoparticles exist, ethanol and methane are decomposed at 825–925° on silicon wafers substrate fabricating high density of crystallized-SWCNT up to 1 cm long (Zhou et al., 2006). The reduction of CuCl2 was the method of Cu nanoparticles synthesis. The circumstance with Cu2O nanoparticles is produced through pyrolysis of cupric formate in solvents. As such, the novelty of the

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above method lies in a new catalyst synthesis method. The use of rhenium (Re) catalysts in CNT synthesis is very rare. In 2007, Rachel et al. proposed the possibility of synthesizing antimagnetic SWCNTs and MWCNTs through CNT synthesis experiments using Re catalysts and methane carbon sources (Ritchel et al., 2007).

These results suggest that just as carbon sources are important variables for CNTs growth, the type of metal catalyst can also be an essential variable that can promote CNT growth through optimization of experimental conditions. For this reason, extensive research has been conducted to find a new metal candidate for CNT growth catalysts.

1.5. CNT growth mechanism

Since the moment carbon nanotubes were discovered, there has been constant debate over the synthesis mechanism. Depending on the reaction conditions and post-deposition product analysis, several conflicting mechanism possibilities have often been proposed by several research groups. Therefore, a single-walled CNT growth mechanism has not been well established to date. Nevertheless, the most widely accepted mechanism can be summarized as follows. When hydrocarbon vapor encounters metal nanoparticles heated to a very high temperature, hydrocarbons are decomposed into carbon and hydrogen. At this time, hydrogen flies away and carbon accumulates in the metal catalyst, which is usually referred to as dissolution. When the amount of carbon accumulation increases and the carbon solubility of the metal exceeds the carbon solubility at the corresponding temperature, the decomposed carbon gradually crystallizes to form a cylindrical network structure with stable energy. At this stage, chemical bonds between carbons are not properly formed. In the carbon crystallization (temperature resistance process), heat in the carbon accumulation region of the metal is absorbed, while thermal decomposition of hydrocarbons is an exothermic reaction, so heat is emitted in the exposed region of the metal. Energy circulation inside metal particles through this process continues to promote the process (CNT growth). There are two CNT synthesis mechanisms presented based on this heat circulation theory. The first is when the catalyst-substrate interaction is weak. In this case, the contact angle formed between the metal catalyst particles and the substrate is relatively large. When hydrocarbons are thermally decomposed from the exposed surface of metal nanoparticles, hydrogen diffuses into the surrounding atmosphere and disappears, and carbon diffuses into the substrate contact surface with the nanoparticles through the metal nanoparticles. If the dissolved carbon has a concentration equivalent to or higher than the solubility of the metal, the cylindrical crystallization proceeds, and the carbon network pushes the metal particles out of the substrate (Figure 12 a). If the upper end of the metal nanoparticle is exposed to the atmosphere and mint hydrocarbon could be decomposed continuously, the diffusion of carbon, which is caused by the energy concentration gradient

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of the metal, persists and CNTs prolong to be extended (Figure 12 a (ii)). However, if the top is covered by carbon exceeding the metal nanoparticles solubility, the activity of the catalyst gradually decreases and consequently suspension of CNT growth (Figure 12 a (iii)). This is called a ‘tip growth model.’

The second case (Figure 12 b) is when the catalyst-substrate interaction is strong. When the interaction between the catalyst particles and the substrate is strong, the metal has a relatively dull contact angle with the substrate. In the early stages of this mechanism, hydrocarbon decomposition and carbon diffusion occur similarly to the tip-growth case. However, unlike the case of tip-growth, the contact angle is wide, so carbon precipitation does not occur under the metal nanoparticles. Therefore, precipitation inevitably occurs on the exposed surface of the metal nanoparticles. The CNT thus formed has crystallization from the vertices of the metal nanoparticles (separated from the substrate and has minimal interaction with the substrate). Carbon produced by pyrolysis in the initial stage of this mechanism forms a hemispherical dome in the form of a seamless grafitic cylinder. This spherical form is the most advantageous closed carbon network in nanoparticles (i.e., means the most stable energy state). Thereafter, contrary to the tip-growth model mechanism, hydrocarbon decomposes on the lower peripheral surface of the metal nanoparticles, and the decomposed carbon diffuses into the upper part of the metal particles. CNTs of this mechanism grow with foundation of base catalyst particles, so this is called a ‘base growth model.’ However, there are several uncertainties in the most likely CNT growth mechanism mentioned above. Previous studies have not found whether the carbon diffusion of the metal is volume diffusion or surface diffusion during CNT growth, whether the metal is solid or liquid, or whether the actual catalyst for CNT growth is carbide or pure metal particles.

Figure 14 CNT growth model. (a) Tip-growth model, (b) Base-growth model.

Copyrighted by Mukul et al. (2011)

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21 1.6. CNT application

CNTs are known to have a wide range of applications because they have remarkable properties. For example, circuits with electric field effect transistors, electric field effect transistors (Bachtold, Hadley, Nakanishi, & Dekker, 2001), electrocatalytic electrodes (Gong, Du, Xia, Durstock, & Dai, 2009), transparent conductive films (Yu, Shearer, & Shapter, 2016), smart materials (I. Kang, Schulz, Kim, Shanov, & Shi, 2006), electrical dissipation materials (Kaul, Wong, Epp, & Hunt, 2006), reinforcement enhanced polymer nanocomposites (Thostenson, Ren, & Chou, 2001), biocompatible devices (X. Cao et al., 2016), electrochemical biosensors (J. Wang, 2005), molecular channels for water and protons(Brady et al., 2016), battery storage devices and others (Krissanasaeranee, Wongkasemjit, Cheetham, & Eder, 2010) etc. The preparation of an even CNT dispersion is known as a limitation that needs to be addressed in order to expand its applicability.

1.6.1. Reinforcement of polymer nanocomposites

Polymers, which are readily available, are one of the main components of manufacturing materials (Ferreira, Cividanes, Gouveia, & Lona, 2019). Currently, polymers are already being used as a substitute for various materials due to their low production cost and easy post-processing properties (Kreiger & Pearce, 2013). However, these replacement polymers must have specific properties such as electrical, mechanical, optical, and thermal to be meaningful as an alternative (C. Zhang, Garrison, Madbouly, & Kessler, 2017). Currently, polymer materials are being applied to various applications, but there are limitations in performance. Nanofillers such as CNTs are in the spotlight as the most suitable means to overcome these limitations (Ajayan, Stephan, Colliex, & Trauth, 1994; Loos, 2015).

In particular, the high modulus, high aspect ratio and strength of CNTs are the most useful properties.

Typically, CNT-based nanocomposites made of plastic matrices are prepared by three methods: melt mixing, solvent casting, and in situ polymerization, whereas a curing agent is used as a thermosetting agent (Hussain, Hojjati, Okamoto, & Gorga, 2006).

1.6.1.1. Synthesis of CNT-Based Nanocomposites

1.6.1.1.1. Melt Mixing

Melt mixing is one of the most applied methods characterized by environmental affinity, simplicity, and low cost. Another reason why this method is attractive is that it is easier to use in industrial plants such as injection molding and extrusion (Chafidz, Latief, Samad, Ajbar, & Al-Masry, 2016). The key to this method is to mix CNTs with molten polymers. In order to prevent excessive degradation of the

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polymer during the process, the rotation and melting temperature of the mixer should be optimized.

After that, the melted ingredients are extruded and injected (Lagarón, López-Rubio, & José Fabra, 2016).

The weak interaction of the polymer matrix and the imperfections of dispersion of CNTs are limitations to be overcome in the future. To solve this problem, multiple melt mixing methods and process conditions (higher temperature, processing shear, and mixing time) have been studied (Lin et al., 2016).

1.6.1.1.2. Solvent Casting

Another nanocomposite synthesis method, solvent casting, is widely used due to composite processing facilities. The key point of this method is to dissolve the polymer in the solvent and disperse the CNTs in the mixture of the solvent and the polymer. The solution is stirred to homogenize, poured into a mold, and the solvent is evaporated. At this time, most of the solvents are evaporated using vacuum drying.

Variation of solvents has a great influence on the properties of nanocomposites(Venugopal et al., 2016).

Solvent evaporation may be slow during the solution casting procedure and may cause CNT aggregation.

Since the spin casting technique can reduce the solvent evaporation time, CNT dispersion can be maintained in the polymer matrix (Filipe V Ferreira et al., 2019).

1.6.2. Electronic Devices

The electrical properties of CNTs have attracted much attention in the field of electronic equipment.

In particular, the high electrical conductivity, high intrinsic carrier mobility, and mechanical flexibility of CNTs further inspired the application plan for electronic devices. (Park, Vosguerichian, & Bao, 2013) Many studies related to these properties have been conducted and these properties are mostly related to the carbon nanotube structure (Bachilo et al., 2002; Rutherglen, Jain, & Burke, 2009).In general, electrical conduction of metal SWCNTs and MWCNTs occurs by electron transfer, unlike most materials, there is little scattering of energy acid electrons throughout the length of carbon nano tubes due to low resistance. Therefore, a high current may be transferred without heating by electrical conduction (White & Todorov, 2001). Important parameters such as defects in the structure of CNTs, tube diameter, wrapping angle, and metal content affect the ON/OFF ratio and charge carrier mobility, so proper structural control of CNTs is required depending on the device application (Snow, Novak, Campbell, & Park, 2003). Many studies have already been conducted on the electronic structure and physical properties of CNTs. The technique of wrapping and applying CNTs to a substrate may be applied to nitrocellulose paper (Yoon et al., 2017), thin film (F. Wang, Gu, & Swager, 2008), and fiber (Yang, Deng, Chen, Ren, & Peng, 2013). In the case of a polymer thin film-based device containing CNT, the polymer is deposited in the pre-manufactured CNT device pattern (Star, Han, Joshi, Gabriel,

& Grüner, 2004). In general, polymer coatings increase selectivity, while the manufacturing process

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has become more complex (Capek, 2009). In addition, there have been reports that the production of aligned MWCNT-based polymer nanocomposite films has been successful in the dissolution process.

Electrical conductivity, transparency, and flexibility are characteristics of this composite film. CNT thin-film transistors synthesis method with usage of floating catalyst at CVD was reported by Sun et al.

Then, a gas filtration and transfer procedure is following. Other research groups have reported that a thin-film semiconductor with aligned-SWCNTs is a great assistant for fabrication of electric devices especially a transistor (S. J. Kang et al., 2007). From small to medium scale digital circuits based on SWCNTs random network (Q. Cao et al., 2008). In this transistor research the result showed excellent properties: high mobility, on/off ratios of 105, low sub-threshold, low operating voltages (<5 V) and high mechanical flexibility. Purified-SWCNT and density-ultracentrifugation were used to concoct thin-film transistor which has high flexibility (C. Wang et al., 2012; J. Zhang et al., 2011). And high mobility was also showed. Hussein et al. produced potentiometric electrochemical sensors by using and MWCNT as transducer between electron and ion. Comparing with poly-aniline nanoparticle and microparticle based sensor, the results presented that the inclusion of MWCNT makes longer life time and enhanced stability. Cases of CNT-based devices being applied and studied in a very wide range have been reported. It has been reported that SWCNTs conformally laminated to linear human skin can be used for the fabrication of flexible electronic devices such as transistors, capacitors, logic gates, and non-volatile memory devices (Son et al., 2015). A thread-based electrical analysis device using fabric wool as a solution channel and detecting electrochemical signals with CNTs was also reported by Ochiai et al. (Ochiai et al., 2017). Such a device can be used for ampere-metric measurement of estriol hormone, which can be used in the pharmaceutical field. A device capable of selectively detecting serum proteins including antibodies and markers, including autoantibodies, was also studied using the high sensitivity of CNTs Chen et al. (Chen et al., 2003; Marchesan, Ballerini, & Prato, 2017).

1.6.3. Biological Applications

In biomedical applications field, CNT is especially attractive material due to their exclusive physical and chemical properties (Marchesan et al., 2017). Surface functionalization has provided a research direction for the development of novel nano-biomaterials that can improve the diagnosis and treatment of various diseases (Ahmadi et al., 2017). In particular, the application of CNTs focused on neurogenesis and regeneration suggests a new research direction different from that of the past. For example, the physicochemical properties of CNTs allow them to interface with neurons (Cellot et al., 2017), and these properties facilitate the design of neural prosthetic devices.

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Although the remarkable properties of CNTs have been studied extensively and enable their application in applications, there are still many limitations. The most frequently mentioned limitation is the dispersion of CNTs (Hirsch & Vostrowsky, 2005). Due to van der Waals forces, CNTs are not well dispersed in the solvent and tend to agglomerate, and it is extremely difficult to separate CNTs from clusters (Ham, Choi, & Chung, 2005). Also, in general, the physical and chemical properties of these clusters are inferior to those of stranded nanotubes (Coleman, Khan, Blau, & Gun’ko, 2006).

Currently, it has been studied that this surface modification improves CNT dispersion, but also affects properties and applications (Hashem Nia et al., 2017). Several papers have reported that the functionalization of the surface induces defects in the CNT structure and consequently reduces the mechanical, electrical and thermal properties (Laudenbach et al., 2017). Moreover, since most of these surface modifications are carried out in toxic solvent environments, cytotoxicity issues must be considered, especially in physiological applications (Zhou, Forman, Ge, & Lunec, 2017).

2. SWCNT Diameter Comparison: Fe nanoparticle catalyst & Co nanoparticle catalyst

2.1. Experiment

2.1.1. Cobalt nanoparticle catalyst preparation

The synthesis of cobalt nanoparticles used a catalyst synthesized in Professor Ahn Kwang-jin's lab.

The synthesis method of this catalyst was the hot injection method (Iablokov et al., 2012). The particle size of the cobalt nanoparticles used in the experiment was about 3 nm. The adjustment of nanoparticle size in the hot injection method is determined by terminating temperature. In the synthesis method, after heating the oleic acid to 180°C, a solution in which Co2(CO)8 (cobalt carbonyl) is dissolved in 1,2-Dichlorobenzene is injected with a syringe. In the process of maintaining and cooling for the next 3 hours, CoNPs having a size of 10 nm are generated when terminated at 168°C, and CoNPs having a size of 3 nm are generated when stopped at 182°C.

Figure 15 CoNP size control method scheme. Copyrighted by Iablokov et al

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Figure 16 CoNP TEM image

Figure 17 The size of CoNPs in nanoreactor chip. The size of CoNPs is increased with adhesion to nanoreactor chip (SMSI effect). Produced by Euiseob Yang

2.1.2. Fe catalyst preparation

In the case of iron catalyst agents, a different approach of depositing iron particles on the substrate was chosen than that for cobalt particles. The synthesis conditions were selected by referring to Professor Lee Chang-young's thesis (Lee, Choi, Han, & Strano, 2010). Silicon dioxide was selected as the carbon nanotube support substrate. This is to better observe the synthesized CNTs with SEM. The operating principle of SEM is to project an electron beam onto an object and project the amount of current reflected into an image. In the case of carbon nanotubes, as mentioned above, they are high conductive materials. Therefore, it is very easy to observe CNTs through SEM. However, if the substrate has similar conductivity, the CNT projected into the SEM image is covered by the substrate, making it difficult to observe properly. In the case of the SiO2 substrate, since the silicon dioxide film acts as an insulator, it has a conductivity different from that of CNT. In this experiment, Hitachi S-4800 SEM equipment was used to observe CNTs, and 10.0 A 1.0 kV was used for observation conditions. An E- beam evaporator was used to place the catalyst on the substrate, and the deposition condition was 5 Å

RT 600 °C 800 °C 950 °C

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thick. At first, a photolithography process was used to expose only the part where the catalyst was raised.

A film mask was used to cover the area where the catalyst was to be deposited with a photoresist (PR).

The type of PR used at this time was AZ5214 used. AZ5214 photo resist is a negative photo resist which has a property crosslinking proceeds to form a polymer network when exposed to UV. Conversely, when the positive PR is exposed to UV, the bond is weakened. Although negative PR lacks resolution, the sensitivity is higher. In addition, AZ5214 which formed a network through baking and UV treatment can be easily dissolved and removed using acetone, making it a very useful material for temporarily covering certain areas. After depositing the Fe catalyst using this, the remaining PR was removed, and then a dicing process was added to cut the substrate to the desired size. Before the dicing process proceeds, a guideline indicating the cutting line of the substrate should be added, and a photolithography process was also used at this time. The precaution before the dicing process is that since impurities can splash and contaminate the substrate, the entire substrate must be covered with PR before proceeding.

The final substrate piece is made in a size that can fit into the CVD insert tube. The substrate piece undergoes a catalyst activation process before being put into the CVD. After heating at room temperature for 53 min in step 1 to 550°C, the catalyst is activated at 550°C for 40 minutes in step 2. In this process, the catalyst conglomerates with the surrounding catalyst particles or forms an alloy with the substrate to adhere more tightly. Without going through this process, the catalyst can move during CVD synthesis for CNT growth to produce unwanted results.

Figure 18 photolithography scheme. Copyrighted by sung-hoon kim et al. (2009)

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Figure 19 Fe catalyst pretreatment condition

2.1.3. Synthesis of horizontal aligned carbon nanotube via CVD method

The synthesis of CNTs was all performed through CVD. The synthetic atmosphere used methane as a carbon source and hydrogen as a gas carrier. The flow rates of hydrogen gas and methane gas were 4.2 sccm : 2.1 sccm, respectively. The synthesis temperature is composed of step 1 heating at room temperature to 950°C for 46 min, step 2 holding at 950°C for 30 min, step 3 heating at 950°C to 1030°C for 20 minutes, and step 4 consists of CNT synthesis at 1030°C for 2.5 hr. The composition of the gas varies slightly by step. Step 1, step 2, and step 3 are heating times for synthesis, so only hydrogen is supplied to the chamber. Then, in step 4, methane gas, a carbon source, is additionally injected to cause pyrolysis of methane at 1030°C (Lee et al., 2010). The most important thing in the synthesis of CNTs using CVD is the setting and environment of synthesis conditions. This is because even if the same conditions are used, synthesis is often not possible when the environment changes. For example, when synthesizing the horizontal aligned carbon nanotube through CVD, in addition to the glass chamber into which gas is injected, there is an insert tube for making the internal atmospheric flow into a laminar flow. If it is a new glass tube that has never been used, it is very unlikely that the desired carbon nanotubes will be synthesized, even with the same synthesis temperature and gas composition used in this experiment. We do not know the exact cause, but based on the fact that synthesis succeeds when the experiment is repeated to some extent, adsorption of carbon sources can be expected to be the cause. When synthesizing carbon nanotubes in the same way, it is observed that stains are continuously formed inside the chamber and in the insert glass tube. After this point, it can be observed that CNTs grow in the desired direction on the substrate. In addition, it is important to note that CNTs grow in the direction of gas flow when using the CVD method. Therefore, it is possible to determine where to place

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the catalyst part of the substrate. After synthesis is completed, cool slowly under a nitrogen atmosphere and drop to room temperature. At this time, if cooled too quickly, amorphous carbons may be entangled and debris may be generated. As mentioned above, as the amorphous carbon increases, the purity decreases and the D peak increases noticeably when Raman analysis is performed.

Figure 20 CNT synthesis condition

2.2. Result

2.2.1. SEM image

Usually, the CNT morphology and structure are investigated by SEM (scanning electron microscopy).

A field emission gun is also used to with SEM (called FEG-SEM) observe structural change and carbon nanotube bundles after functionalization treatment, graphitization and purification. Also, information about dispersion, homogeneity and alignment are investigated through FEG-SEM (Herrera-Basurto, López-Lorente, & Valcárcel, 2015). When improved resolution microscopy is used to analysis, specific characteristics including curvature and entanglement of CNTs, diameter and length of the CNTs, presence of amorphous carbon and other carbon structure forms can also be shown. For instance, studied the He¹²irradiation effect on the intrinsic structure of MWCNT was studied by Liu et al. through the SEM. Prior to irradiation, the MWCNT arrays took parallel directions to each other. After He12 irradiation, the shape of the MWCNT array changed very severely and more damage was observed as the irradiation flow rate increased (Liu et al., 2017). Basurto et al. investigated the effects of ionic liquids on sample preparation of CNTs using FEG-SEM. The authors compared images of nanotubes dispersed in an organic solvent (methanol), a surfactant (tritone X-100), and an ionic liquid to find the

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optimal distribution of MWCNT. The results showed that the dispersion of MWCNTs to methanol was low and aggregates were observed. The use of Triton X-100 improved the stability and homogeneity of the solution. However, some nanotube clusters were also observed. Suspensions made of ionic liquids generated a number of individual MWCNTs and were uniformly distributed (Herrera-Basurto et al., 2015).

In this paper, SEM was used to observe simple external morphology. The most focused point in SEM analysis was to observe the density of CNT and the amount of amorphous carbon. First, in order to increase the density of CNTs, it was helpful to increase the synthesis temperature. It was found that CNTs were synthesized more densely in the order of 1020°C < 1025°C < 1030°C. In addition, there is a way to increase the carbon flow rate, but it shows a pattern of increasing the amount of amorphous carbon, so we chose to keep the temperature high. The choice of synthesis temperature according to the concentration was very useful. For example, in order to synthesize a single strand of CNTs for CNT nanopores, a small density of SWCNTs is required. Therefore, SWCNTs of the desired density could be synthesized by lowering the synthesis temperature and increasing the nitric atmosphere annealing time.

Figure 21 CNT synthesis result SEM image

In the case of AFM analysis, it was not conducted in depth. In this experiment, the use of AFM was simply to obtain the average diameter of the synthesized CNTs. Raman peak analysis uses only RBM peak for diameter measurement, so only data on SWCNT can be accessed. In contrast, AFM physically measures the height (diameter) of all CNTs, so that the diameter of the overall CNTs present in the

PDMS block Punching for dropcasting of CoNP

SiO₂wafer

1 mm

500 um

200 um

Above CoNP cat region

Under CoNP cat region

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sample substrate can be obtained on average. Through this, the diameter distribution according to the catalyst used could be obtained. The Fe catalyst used had an overall wide diameter distribution compared to the CoNP (cobalt nanoparticle) catalyst. This is expected to be due to the difference in size of the catalyst. In the case of the Fe catalyst, the catalyst particle size is not relatively constant by using evaporator deposition. However, in the case of CoNP, it was confirmed by TEM that most of them have a constant size of 3 nm. Since both 'Tip-growth' and 'Base-growth' occur in the synthesis mechanism of CNT, the size of the catalyst is critical to the diameter of the CNT being synthesized. In addition, in the case of CoNP, it seems that the size of the catalyst particles could be anchored as it did not travel on the substrate even at high temperature of 1000°C and combine themself to grow in bigger size. One thing to note is that even with the same CNT strand, the diameter can vary slightly from part to part. In fact, when AFM height of several points was measured, there were no CNTs that all had the same value. This may vary depending on the causes, but it is predicted to occur especially when the lamina flow of the gas is broken. In reality, it is difficult to maintain a constant flow rate of gas. Therefore, the flow rate continues to increase or decrease in the middle, and it seems that the CNT growth direction is partially modified due to the turbulence generated in a small amount.

Figure 22 AFM height comparison depending on catalyst

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Figure 23 AFM image of carbon nanotube by using Fe catalyst

2.2.2. Raman peak

Raman Peak's analysis is the most important part of this experiment. More accurate diameter than AFM can be predicted using RBM peak, which cannot be observed in MWCNT and can only be observed in SWCNT. In addition, using the Kataura plot, it is possible to measure the direction in which the graphene sheet is rolled, that is, chirality. As mentioned above, if the chirality is known, the characteristics of conductors/semiconductors can also be determined, which is directly related to the use of transistor devices. Therefore, many studies have been conducted on alloy catalysts that selectively control this chirality (Liao et al., 2003).A method of determining chirality using the Kataura plot and RBM peak is as follows. First, check the laser wavelength of the Raman equipment used. Each wavelength of the laser has a different energy, so you have to find that energy. In this experiment, a 532 nm wavelength laser was used, which approximately has an energy value of 2.33 eV. In the Katura plot, there are axes for this energy separation (eV), Raman shift value (cm^-1), and CNT diameter (nm) parameters, and chirality dices (m, n) are written at the intersection of each parameter. However, chirality for all values is not specified. Therefore, the approximate value is obtained by incorporating it into the closest value and optimizing it. Usually, chirality, located in a similar neighborhood in a coordinate plane, has similar characteristics.