Synthesis of Carbon Nanomaterials Using Catalytic Chemical Vapor Deposition Technique

CHAPTER 1

Synthesis of Carbon Nanomaterials Using Catalytic Chemical Vapor Deposition Technique Ferial Ghaemi*, May Ali†, Robiah Yunus†,‡, Raja Nor Othman§,¶ *

Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Malaysia Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia ‡ Institute of Advanced Technology, Universiti Putra Malaysia, Serdang, Malaysia § Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Kuala Lumpur, Malaysia ¶ Centre for Defence Research and Technology, Universiti Pertahanan Nasional Malaysia, Kuala Lumpur, Malaysia †

1.1 CARBON NANOMATERIALS AND THEIR PROPERTIES Carbon is one of the three basic elements (C, H, and O) that form many kinds of organisms with other elements to constitute life on earth. Among the pure elements in the periodic table, carbon can assume different structures such as one-dimensional carbon nanotube (CNT), two-dimensional graphene, three-dimensional diamond, and even zero-dimensional fullerenes. With nanometer-scale dimensions, the properties of carbon nanomaterials are strongly dependent on their atomic structures and interactions with other materials. The tubular morphology of carbon includes carbon fibers, whiskers, CNT, and carbon nanofiber (CNF). Other forms of carbon are graphene, carbon blacks, carbon dots, and carbon nanoparticles, just to name a few. Fig. 1.1 shows various structures of carbon nanomaterials with their nomenclature [1], with their extraordinary properties, and with their potential applications that are discussed extensively in this book. It was the Kroto-Smalley work [2] that pioneered the field of nanostructured carbon science era and further stimulated the search of finding new carbon materials. In this experiment, graphite was laser vaporized, and a new structure was accidentally found. Fullerenes or C60, a dominant structure of the product, was detected by mass spectrometry. They soon realized that it was a closed cluster containing precisely 60 carbon atoms, which would have a structure of unique stability and symmetry. This discovery led to the Nobel Prize Award for Chemistry in 1996. This success further inspires research of finding other novel carbon nanomaterials. Although research in the area of CNT became intensified in the 1990s, the first evidence of the existence of CNT was actually reported back in 1952 by Radunshkevich and Lukyanovich in the Journal of Physical Chemistry ([3] and references therein). However, due to the cold war and the fact that this journal was published in Russian language, access Synthesis, Technology and Applications of Carbon Nanomaterials https://doi.org/10.1016/B978-0-12-815757-2.00001-2

© 2019 Elsevier Inc. All rights reserved.

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Synthesis, Technology and Applications of Carbon Nanomaterials

Diamond

Graphene

Fullerene

Graphene oxide

Carbon nanotube

Graphite

Carbon dot

Fig. 1.1 Various structures of carbon nanomaterials with their nomenclature. (Reproduced with permission from Q.-L. Yan, et al., Highly energetic compositions based on functionalized carbon nanomaterials, Nanoscale 8 (9) (2016) 4799–4851.)

to this important discovery remained unknown only until the 21st century [3]. Approximately two decades later, Endo et al. [4] also published transmission electron microscope (TEM) images of CNT who described the structure as “…hollow tubes…” with “…turbostratic stacks of carbon layers, parallel to the fibre axis, and arranged in concentric sheets….” However, this finding failed to capture the interest of carbon research community at that time who was more involved in the carbon-fiber-related work [5]. Nevertheless, it was Iijima’s work that caused a worldwide interest in this material who described this carbon structure that consisted of several coaxial tubes (from two to seven concentric graphene cylinders) and a hollow core, with outer diameter ranged from 5.5 nm (two graphene cylinders) to 6.5 nm (seven graphene cylinders) [6]. This tubular structure is capped by a half fullerene at each end. According to Monthioux and Kuznetsov [3], Iijima’s paper [6] created such a tremendous impact as it was published in a high-impact journal accessible by various researchers involved from basic research to fundamental physicists and also the fact that the paper emerged after the discovery of fullerenes. Since then, the progress of CNT has accelerated with the publication on singlewalled carbon nanotube (SWNT) production using transition-metal catalyst by Bethune [7] and Iijima [8]. Fig. 1.2 shows the TEM images published by these three reports. It becomes clear that only the images published from Iijima (Fig. 1.2C) clearly show the graphene fringes and also the number of walls. CNT can be further categorized into two main structures, which are SWNT and MWNT. The main difference between these two is SWNT consists of one rolled-up sheet of graphene (a sheet of carbon atoms arranged in hexagonal rings) while MWNT

Synthesis of Carbon Nanomaterials

Fig. 1.2 TEM images of CNT published by (A) Radunshkevich and Lukyanovich in 1952, (B) Endo et al. in 1976 [4], and (C) Iijima in 1991 [6]. ((A and B) Reproduced with permission from N. Grobert, Carbon nanotubes—becoming clean. Mater. Today 10 (1) (2007) 28–35. (C) Reproduced with permission from S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56.)

has more than one rolled-up graphene sheet. The way the graphene sheet is rolled determines the transport properties, particularly the electronic properties of the SWNT. It is represented by a pair of indexes (n,m), which is known as chiral vector. The integers n and m represent the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. The values of m ¼ 0 and n ¼ m refer to “zigzag” and “armchair” configuration, respectively. Other configuration is known as “chiral.” For a given (n,m) SWNT, if (2n + m) is a multiple of 3, then the SWNT is metallic; otherwise, it possesses semiconductor properties. Since MWNT consists of more than two layers with each layer that can have different chiralities, the prediction of transport properties is more complicated compared with SWNT. Ideally, crystalline CNT should have walls and caps without any defects and missing or added atoms. Unfortunately, this type of CNT is difficult to obtain with the existing synthesis technique. A common CNT synthesized with defects includes “bamboo”-type MWNT, where the different walls cap at different lengths that appear as stacked, which could only be visualized via TEM. Other common defects include the presence of amorphous carbon coating the outer wall of CNT, which would deteriorate the crystallinity and transport properties of the CNT. As catalyst was used to synthesize CNT, they also appear within the enclosed structure of CNT, which might also affect the properties of the CNT. CNFs could be defined as sp2-based linear filaments that are characterized by flexibility and their aspect ratio (above 100). CNFs are cylindrical or conical structures that have diameters varying from a few to hundreds of nanometers and lengths ranging from less than a micron to millimeters [9–11]. In general, a nanofiber consists of stacked curved graphite layers that form cones or “cups” [12]. The stacked-cone structure is regularly referred to as herringbone or fishbone as their cross-sectional transmission electron

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(A)

(B)

(C)

Carbon nanotubes (CNT)

(D)

(E)

(F)

(G)

(H)

Carbon nanofibers (CNF)

Fig. 1.3 Various structures of carbon-based fibrous nanomaterials. (A) Single-walled CNT, (B) multiwalled CNT, (C) stacked-cup CNF, (D) fishbone solid CNF, (E) fishbone hollow-core CNF, (F) ribbon CNF, (G) platelet CNF, and (H) spiral CNF. (Reproduced with permission from I. Guseva Canu, et al., Human exposure to carbon-based fibrous nanomaterials: a review, Int. J. Hyg. Environ. Health 219 (2) (2016) 166–175.)

micrographs look like a fish skeleton, while the stacked-cup structure is frequently referred to as a bamboo sort, like the compartmentalized structure of a bamboo stem [9]. Parallel layers in single-layer CNF were also observed using HRTEM [13]. The d-spacing of the graphene sheets was reported as 0.34 nm (the same as that in MWNTs and graphite platelets). Other wall structures of CNF include ribbon, platelet, and spiral, as shown in Fig. 1.3 [14]. The ability to form different wall arrangements with respect to the axis is possible, due to the growth mechanism of the CNF that depends on the geometric facets of a metallic catalyst particle and the gaseous carbon feedstock (hydrocarbon or CO gas) that is introduced during CNF processing. CNF can be synthesized via chemical vapor deposition (CVD) method and electrospinning. The production method greatly affects the structure and morphology of the CNF, with CNF produced via CVD that constitutes ultrahigh modulus properties [15]. Compared with CNT, CNF has received less research attention as nanofillers because CNTs have better mechanical properties, smaller diameter, and lower density compared with CNF. Nevertheless, it continues to receive attention due to its relatively lower cost, compared with CNT. In terms of electric applications, CNF is also competitive filler with carbon fibers (CF) and high-structure carbon black (CB), owing to the lower loading of CNF compared with CF and CB required to achieve percolation threshold. Graphene has become the focus of research recently, due to various properties reported. It is an allotrope of carbon that consists of carbon atoms arranged in hexagonal

Synthesis of Carbon Nanomaterials

lattice. The structure consists of sp2-hybridized carbon atoms that possess extended honeycomb network. Its free-standing form was reported in 2004 by Novoselov et al. [16], who successfully isolated this single layer of carbon atom structure mechanical exfoliation from graphite. This work resulted in their winning in Nobel Prize Award in Physics in 2010. Graphene forms basic building block of all graphitic forms of carbon materials encountered before. Its two-dimensional structure can be changed to three-dimensional as a graphite or can be rolled up to produce one-dimensional nanotube or wrapped to form zero-dimensional fullerene [17, 18]. The lateral dimension of graphene can be in nanoscale and macroscale [19]. Table 1.1 shows the properties of various carbon nanomaterials [20]. Due to their extraordinary mechanical, electric, and thermal properties, CNT, CNF, and graphene are being investigated for a wide range of applications. For example, CNT has been investigated for various applications such as microelectronic interconnects [21] and structural composites [22]. The combined excellent mechanical, electric, and thermal properties suggest CNT as potential filler in producing composites. These composites could be used as conductive glue, gas storage devices, sensors, energy storage devices, lightweight aircraft applications, and defense applications, just to name a few [23, 24]. For example, Connolly et al. reported excellent electron emission obtained at 0.7 vol% SWNT in polymer, which has potential use for field-emission device [25]. Similar work was also performed where triode-type field-emitting arrays were fabricated using CNT/polypyrrole nanocomposites Table 1.1 Properties of various carbon nanomaterials DoubleSinglewalled walled carbon carbon nanotube nanotube Property

Tensile strength (GPa) Elastic modulus (TPa) Elongation at break (%) Density (g/cm3) Electric conductivity (S/m) Thermal conductivity at room temperature (W/m K) Thermal stability (in air) Typical diameter (nm) Specific surface area (m2/g)

Multiwalled carbon nanotube

Graphene

130 1 20

Carbon nanofiber

50–100 1 5.8 1.3–1.5 106

23–63 – 28 1.5 106

10–60 0.3–1 – 1.8–2.0 106

6  105

3–7 0.5 0.5–2.5 2.25 103

6000

3000

2000

5000

1900

>700

>700

>700

450–650

1 10–20

5 10–20

20 10–20

2675

50–500 50

Reproduced with permission from S. Liu, et al., A review of extending performance of epoxy resins using carbon nanomaterials, Compos. Part B 136 (2018) 197–214, with data compiled from various sources.

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[26]. Structural properties of this CNT/polymer are also aimed in aerospace field as paints, antiradar protectors, and antistatic. Due to its heat-absorbing capability, these nanocomposites can be used in aerospace industries as electromagnetic wave absorption materials [27]. Similar to CNT, CNF has also been studied for applications in the area of polymerbased nanocomposite [28] and electrochemical energy storage [29]. This is due to the excellent conductivities, extremely large surface areas, and structural stability possessed by CNF. Saleh and Sundararaj provide a comprehensive review on the use of CNF as a filler in composite, aimed to increase conductivity values [30]. The potential application of CNF/polymer composites as sensors for organic vapors has been demonstrated by Zhang et al. [31]. The composite conductivity changed when exposed to an organic vapor because of swelling of the polymer matrix. Organic vapor was then detected based on the composite conductivity after certain time of exposure [31]. In the area of automotive industry, the use of CNF-based composite offers applications such as electrostatic painting of exterior panels, shielding of automotive electronics, and improving tire stiffness after adding VGCNFs to tires to improve stiffness [30]. The significant properties of graphene generated huge interest in the possible implementation of graphene in various applications. These include future generations of highspeed and radio-frequency logic devices and thermally and electrically conductive reinforced composites, sensors, and transparent electrodes for displays and solar cells [17]. Graphene-based nanocomposites have also been prepared for various applications such as electrochemical applications [32–34], lithium-ion batteries [35–37], sensors [38–40], solar cell [41–43], water purification [44–46], supercapacitors [47–49], drug delivery [50–52], and tissue engineering [53–55].

1.2 CHEMICAL VAPOR DEPOSITION It is to be kept in mind that for the abovementioned applications, highly reliable synthesis techniques are required that should be capable of obtaining large quantities of high-purity materials. Therefore, large-scale production with high-purity product is imperative. CNT could be produced by using arc discharge and laser ablation methods [56]. Graphene could be synthesized via several techniques such as mechanical exfoliation, chemical reduction of graphite oxide, epitaxial growth on silica carbide, liquid-phase exfoliation, and CNT unzipping [57]. Among these techniques, CVD has been considered as the most effective method and has been applied to grow CNT, CNF, and graphene by many researchers, due to its high scalability and high quality of the products produced. In the case of CNT, CVD is the only technique where CNT could be specifically grown on a substrate tailored for specific application. The advantages of CVD technique compared with other techniques to grow CNT, CNF, and graphene have been reviewed elsewhere [56–58], which includes its simplicity and economical and

Synthesis of Carbon Nanomaterials

potential scalability for mass production. Besides, various types of hydrocarbon in any form that is solid, liquid, and gas can be grown on a wide range of substrates. CVD is perhaps the most versatile technique to produce CNT, CNF, and graphene as different morphologies of carbon nanomaterials can be tailored by changing the operating parameters such as types of catalysts, types of substrates, types of hydrocarbon, reaction time, and reaction temperature. CVD method has been used to grow carbon fibers since 1890 by passing cyanogen over red-hot porcelain ([5] and references therein). The growth of carbon fibers and carbon nanofibers via this method is also reported in the 1960s and 1970s [58]. In 1993, Endo et al. reported the growth and structure of pyrolytic CNT [59]. Although the CNT appeared to exhibit defects and covered with thick amorphous carbon, this paper marked as the preliminary study of CNT via CVD. The growth process of CNT, CNF, and graphene via CVD method involves decomposition of carbon source in the presence of catalysts. The carbon sources could be in solid, liquid, or gaseous forms. The CVD process can be described as follows. First, the hydrocarbon source decomposed to form carbon atoms due to the presence of heat or plasma. This is followed by their adsorption on the surface of catalyst. Finally, these carbon atoms formed nanomaterials depending on the shape and size of catalysts. The CVD setup that utilizes plasma source is known as plasma-enhanced CVD [60]. The CVD process is known as thermal CVD in the case where hydrocarbon is decomposed utilizing only heat, which is described further in this chapter. For easy reference, the term CVD is used in this chapter to dictate thermal CVD. A typical setup for CVD process to produce CNT, CNF, and graphene is as shown in Fig. 1.4. It consists of hydrocarbon source gas, carrier gas, furnace, and also outlet tube. Catalyst is usually predeposited on a substrate and placed within the reaction zone of the

Furnace Inlet gas

Carbon Carrier source gas gas

Fig. 1.4 Schematic of the typical CVD reactor.

Reactor

Outlet gas

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reactor before the reaction starts. At the end of the reaction, the carbon nanomaterials will be collected when the system reaches room temperature. There are other CVD variation setups depending on the types of hydrocarbon and catalysts used. In the case of solid carbon source, a dual-reactor configuration is used where the carbon source is placed in the first reactor of lower temperature setting, where the second reactor is set at the actual reaction temperature, and where the growth occurs. The temperature of the first reactor is set to the sublimation temperature of the solid hydrocarbon source, while the temperature of the second reactor is set to the growth temperature of the nanomaterials. Besides placing catalyst within the furnace before the reaction starts, it can also be introduced into the reactor during reaction. This is applied for organometallic catalyst such as metallocenes. There are several choices of furnace configurations that have been used to grow CNT and CNF. Horizontal furnace configuration type is shown in Fig. 1.4, where the furnace is positioned horizontally. As the name implied, vertical furnace refers to the setup where the furnace is positioned vertically [61]. The fluidized bed reactor is a variation of the vertical furnace, where supported catalysts are usually placed in the center of the furnace and an upward flow of carbon feedstock gases is used. The fluidized process involves the supported catalysts to remain much longer in the vertical floating technique [62]. Up to the writing of this chapter, fluidized bed CVD is employed mostly for the production of CNT and CNF. Graphene is produced via CVD of horizontal configuration. The next subsections discuss the prominent CVD parameters that greatly influence the morphology of CNT, CNF, and graphene.

1.2.1 Carbon Source The carbon sources have a main role in the quality, quantity, and properties of the produced carbon nanomaterials due to their own binding energy, reactive group, and thermodynamic properties. The molecular structure of the carbon source strongly affects the morphology of the carbon nanomaterials produced, and by proper selection of carbon precursor and its vapor pressure, both nanomaterial growth rate and catalyst lifetime can be increased. Depending on the types of carbon nanomaterials used, different carbon sources exerted different outcomes. In the case of CNT, linear hydrocarbons like methane, ethylene, and acetylene produce straight, hollow, or plane nanomaterials as they thermally decompose into atomic carbons or linear dimers/trimmers of carbon, whereas cyclic hydrocarbons like benzene, xylene, fullerene, and cyclohexene produce relatively curved or entangled nanomaterials having bridged tube walls often inside [58]. Generally, the production of multiwalled CNT occurred at a relatively low temperature, while SWNT is produced at a higher temperature as they require high-energy formation due to small diameters. Several researchers reported the following gaseous hydrocarbon to synthesize CNT, which

Synthesis of Carbon Nanomaterials

includes ethylene [63, 64], acetylene [65, 66], methane [67, 68], and ethane [69]. Propane was also used to synthesize CNT and SWNT, where the temperature greatly affects the amount of these CNT and SWNT. Liquid hydrocarbon has also been used to grow CNT such as xylene [70], toluene [71], and benzene [72]. Besides hydrocarbon, CNT can also be synthesized from CO [73] and alcohol [74]. In addition to this, several works have reported the growth of CNT from “green” resources such as camphor, turpentine oil, eucalyptus oil, castor oil, coconut oil, palm oil, and green grass [75–79]. Further, fuels such as kerosene, liquefied petroleum gas, coal gas, and natural gas have also been reported by others as a source to grow CNT [80–82]. In some cases, solid-state polymers have been used as carbon source to produce CNT. Han et al. [83] reported the growth of CNT via carbonization of polymer carbon sources placed on alumina templates of well-defined pore size. The process was conducted for 3 h under temperature range of 400–600°C in the absence of catalyst. Their TEM images show the presence of CNT, with 20 nm diameter, prepared with slow heating to 600°C. Carbonization of polypyrrole within alumina and zeolite membrane produced CNT, with the nitrogen-doped CNT type that shows a better hydrogen storage capacity [84]. In a separate study, Tang et al. reported the growth of SWNT, with 0.4 nm diameter via pyrolysis of tripropylamine within nanochannels of aluminophosphate crystals [85]. Often, various additives have also been added to the carbon source to enhance the growth of CNT. For example, nitrogen has been added to acetylene by Li et al. to obtain vertically aligned CNT at 900°C [86]. The pretreatment of nitrogen was found to be crucial to control the surface morphology of the Ni catalyst, which could assist the growth of vertically aligned CNT. Besides nitrogen, thiophene was also added to acetone to obtain aligned double-walled CNT [87]. The addition of air to xylene carbon source yielded the growth of ultralong CNT bundles (1.5 cm) [88]. It was proposed that the oxygen in air oxidized the catalyst, hence maintaining the catalyst activity. It was found that water also exerted similar effects with air, where long and aligned CNT bundles up to 1.5 cm were successfully synthesized [89]. Other researchers also reported similar effect in employing water to their CVD system [90, 91], with catalyst activity reported to prolong up to 12 h, yielding 7 mm aligned CNT with high density [90]. Similar to CNT, various carbon sources can also be used to grow CNF via CVD, such as natural gas, propane, acetylene, benzene, and ethylene [92, 93]. The study on the growth of CNF via acetylene was performed by Nasibulin et al. [94], on the surface of cement particle. They obtained CNF with diameter of about 30 nm and average length of 3 μm on the surface of the cement in the temperature range from 550 to 750°C. The group also performed the growth of CNF on silica fumes and obtained diameters varied from 30 to 50 nm at 550°C [94]. By using fluidized bed reactor, the group also managed to grow CNF with an average length of 10 μm by varying growth time from 6 to 30 min. Ludvig et al. also utilized acetylene to grow CNF with diameter of 30–60 nm on

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cement [95]. Besides these hydrocarbon sources, “green” source such as linseed oil is also capable of growing CNF via CVD. In the process as described by [96], hydrogen was used to transport linseed oil into the reactor that was maintained at 850°C. The CNF obtained had diameter of about 500 nm, with length measured to be 30–40 μm. There are a wide range of carbon precursors that can be used to produce graphene via CVD. Methane and ethane are among the common carbon precursors used to synthesize graphene. Ethylene and acetylene could also be used to grow graphene at a temperature of 650°C [97]. It was found that acetylene and ethylene is able to attach on the metals firmly [98]. Various types of liquid oxygenated precursors with low pyrolysis temperature such as methanol, ethanol, and propanol have been utilized to grow graphene at low temperature. The amorphous carbon formed during graphene growth could be etched by OH radicals [99]. Benzene was also employed to grow graphene on Cu catalyst at the growth temperature of 300°C [100]. Aromatic hydrocarbons such as pyridine, hexachlorobenzene, bianthryl, hexabenzocoronene, and corroenene have also been utilized to grow graphene via CVD at lower temperature [101–103]. “Green” sources such as food, solid waste, and insect parts have also been demonstrated as a carbon source to grow graphene [104, 105]. The CdH bond energies for polystyrene (PS) and poly(methyl methacrylate) (PMMA) are calculated to be 292–305 and 283–288 kJ/mol, respectively, which are lower than typical gaseous hydrocarbon [106, 107]. This implied that these polymer precursors have a lower decomposition temperature, which eventually enables the growth of graphene to take place between the temperatures of 400 and 1000°C, as shown in Fig. 1.5 [100]. Single- and multilayer graphenes were reported to obtain in between the growth temperatures of 300 and 500°C [108] by employing carbon source consisting of PMMA dissolved in chlorobenzene. Zhu et al. reported the growth of graphene at 400°C by using polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) as starting materials [109]. The group reported the formation of ordered structure of graphene with small amount of defects on various substrates including Fe, Cu, Ni, and CuNi, due to lowtemperature dehydrogenation possessed by PEG and PVP [109]. This finding is particularly useful to synthesize large-area, high-quality graphene at low temperature to allow direct integration of graphene into the manufacturing technologies of complementary metal-oxide semiconductor or flexible devices.

1.2.2 Catalyst and Substrate Catalyst plays a major role in affecting the morphology and quality of CNT, CNF, and graphene produced via CVD. In general, filamentous carbon growth occurs as postulated by Baker [110]; hydrocarbon gas is decomposed on the front surface of the catalyst to release carbon and hydrogen. Carbon atoms then dissolve and diffuse through the catalyst to precipitate at its other end, forming tubular carbon [110]. Therefore, a catalyst particle

Synthesis of Carbon Nanomaterials

Fig. 1.5 (A) Image showing graphene produced from PMMA on a SiO2/Si substrate, with (B) the corresponding Raman spectra and (C) the corresponding optical transmittance spectra. Scanning electron microscopy (SEM) images of graphene grown at (D) 1000, (E) 800, (F) 700, and (G) 400°C. The sharp edges of graphene on the SiO2/Si substrate are shown in the inset of (F). The scale bars in panels (D–G) and in the inset of (F) are 2 μm. Reproduced with permission from Z. Li, et al., Lowtemperature growth of graphene by chemical vapor deposition using solid and liquid carbon sources, ACS Nano 5 (4) (2011) 3385–3390.

acts as a nucleation site for CNT growth, and its size generally determines the diameter of the CNT. Transition metals such as Fe, Ni, and Co are often popular choices as catalysts. These catalysts could be placed on a substrate or introduced together with carbon source during reaction, a process known as floating-catalyst CVD. These catalysts are found to be most effective for synthesizing CNT and CNF as carbon has high solubility and high diffusion rate in these metals. In addition to Fe, Ni, and Co, the SWNT could also be synthesized from In, Cu, Ag, Pd, Mn, Mo, Cr, Al, and Au [111, 112]. Furthermore, CNTs have also been produced by using diamond and semiconductor (Si and Ge) nanoparticles as catalysts [113, 114] and on sapphire and scratched thin SiO2 films [115, 116]. The growth of high-density SWNT (130 μm1) was also reported by using Trojan catalyst, which is named by analogy with the soldiers emerging from the Trojan horse in the Greek story [117]. The catalyst may be either predeposited or coated on a substrate (before being placed in a furnace) or supplied via a floating-catalyst method. In the floating-catalyst chemical vapor deposition (FCCVD) method, catalyst enters the furnace together with a hydrocarbon carbon source in vapor form and generates CNTs on a substrate located in the furnace. The type of substrate is also critical, as catalyst-substrate interactions affect the quality and quantity of the CNTs produced. Substrates such as silica oxide (SiO2),

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alumina oxide (Al2O3), and magnesium oxide (MgO), to name just a few, are often used to grow CNTs in CVD techniques [118]. There are many ways that the catalyst could be deposited or coated on a substrate prior to CNT growth. One way is to impregnate the substrate with catalyst. One particular example [67] used is a substrate of fumed-alumina nanoparticles impregnated with a methanol solution that contained ferric nitrate nonahydrate (Fe(NO3)39H2O). The resulting mixture was heated at 150°C overnight before being used to grow bundles of SWNTs by methane decomposition at 1000°C [67]. Catalyst can also be deposited on a substrate via physical vapor deposition by sputtering. Once heated during reaction, the film broke up to form hemispheric islands that seeded the growth of CNTs [119]. Floating-catalyst techniques, where the catalyst is formed in situ during the reaction, offer an alternative method to synthesize CNTs. Metallocene powders (ferrocene, nickellocene, cobaltocene, and their combinations) or iron pentacarbonyl (Fe(CO)5) is usually used as the catalyst. A double-stage furnace is typically needed as the catalyst powder is reduced in situ in a first-stage thermal decomposition before traveling to the second stage of the furnace where CNT growth occurs on a substrate. The powdered catalyst can either be placed in the first stage prior to starting flow of the hydrocarbon carbon source or dissolved in a hydrocarbon liquid and then injected into the furnace together. Rao’s group was one of the earliest to experiment with different types of metallocenes and hydrocarbons to synthesize CNTs [63, 120]. SWNTs were successfully synthesized at a 1100°C growth temperature by employing acetylene as the hydrocarbon source and Fe(CO)5, cobaltocene, nickellocene, cobaltocene-ferrocene, nickellocene-cobaltocene, and ferrocene-nickellocene mixtures as catalysts [120]. In general, the metallocene mixtures favored the formation of “cleaner” SWNTs, whereas Fe(CO)5 favored the formation of SWNT bundles. Aligned CNTs were also obtained using methane, acetylene, or butane as the hydrocarbon and ferrocene catalyst, with the ferrocene-acetylene combination giving more compact bundles. Overall, this group has successfully produced both SWNTs and MWNTs, although detailed analysis of the tubes such as dimensions and thermal stability was not presented. Alternatively, metallocene powder can be dissolved in liquid hydrocarbon before being introduced into a two-stage furnace. These reactants can be introduced into the furnace as a vapor in gas flow [121–123] or as a liquid injected using a syringe pump. Thus, the catalyst/hydrocarbon ratio and feed rate could be set precisely. As such, in a vapor flow setup reported by Mayne et al. [122], aerosol was generated from ferrocene-benzene solutions atomized with compressed argon. The vapor was evenly distributed within the furnace volume, yielding the formation of a large area of aligned CNT on the surface of quartz tube surface. In the case of liquid injection, a ferrocenexylene solution was injected continuously into a two-stage furnace using a syringe pump, as first demonstrated by Andrew et al. [124]. Various parameters, such as furnace

Synthesis of Carbon Nanomaterials

temperature, ferrocene-xylene ratio, feed rate, total reaction time, and sweep-gas flow rate, were adjusted to determine the growth conditions for aligned CNTs. Singh et al. [125] confirmed the practicality of using a syringe pump in a study in which high-purity aligned CNTs were grown on quartz (silica) from a ferrocene-toluene mixture. The dimensions of the CNTs were easily controlled by varying growth parameters such as reaction temperature, time, and catalyst concentration. Due to its simplicity, liquid injection method has also been used by other groups [126–128]. Silica has been used widely as a substrate to provide sites for catalyst deposition that then allows CNT growth in CVD regardless of whether the catalyst was precoated before reaction [65, 129] or reduced in situ during the reaction [125]. Silica substrates have been used to form CNTs from almost all types of catalyst, such as Fe-based [65, 126, 129], Co-based [119], Ni-based [130], and Fe-Co-based [131, 132]. Typically, CNTs grow aligned and perpendicular to a nonporous silica surface regardless of the substrate geometry, which can be either flat [125, 129] or in the form of spheres [71] or fibers [126]. Mesoporous silica can also be used to grow CNTs [65, 131–134]. An early attempt to grow CNTs was reported by Li et al. in 1996 [65] in which the group successfully synthesized aligned CNT arrays from mesoporous iron-silica substrates. Iron was preimpregnated within the 30 nm pores of the mesoporous silica before growing CNTs of 30 nm diameter. SEM images show that the spacing between CNTs was 100 nm, the same spacing as between the pores before growth took place, which indicated that the CNTs grew exclusively from inside the pores. The group further proposed that the pore axis angle determined the direction of CNT growth, that is, if a CNT was seeded from a vertical pore, then growth would be perpendicular to the substrate, while inclined pores would yield nonaligned CNTs [65]. However, not all CNTs grown from mesoporous silica formed aligned arrays. For example, another study [131] obtained double-walled CNTs (DWNT) arranged in bundles, using an Fe-Co catalyst mixture precoated on the substrate. Moreover, SEM images did not show whether the growth of the DWNTs originated from the pores and the diameters of the CNTs were not measured [131]. DWNT bundles have also been produced on mesoporous silica catalyzed by Fe-Co nanoparticles [132], where the DWNT diameter was found to depend on both the reaction temperature and the pore size of the substrate. A further study by the same group [135] showed that the as-grown product consists of only 5 wt% DWNTs with the silica substrate and catalyst making up the remaining 95 wt%. Thus, the amount of product obtained was very small, especially given that the catalyst-substrate preparation procedure was rather lengthy and tedious. Barreca et al. [134] used iron phthalocyanine as a catalyst to grow CNT from acetylene flow at 800°C, which also yielded products that appeared to be nonaligned and with “spaghetti”-like entanglement. One striking observation is reported; the diameter of the CNT did not change when the iron loading was increased. The authors reasoned that this was due to a fixed size of the iron nanoparticles embedded within the pores

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Table 1.2 List of types of tubular nanomaterials produced, together with the catalyst and the corresponding substrate Formation of Catalyst Underlayer and substrate carbon structure

Fe Fe Fe Fe Fe Fe Fe Fe, Fe-Ni alloy, Fe-Tb alloy, Ni Fe, Mo/Fe-layered film Fe-Ni alloy Fe-Ni alloy Fe-Mo, Mo powder Al-Fe, Cu-Fe, Fe-Ni-Al Alumina-Ni, alumina-Ni-Cu

Mesoporous Si Porous Si Porous Si Si SiC on Si SiO2 SiO2 on Si Silica, alumina

Vertically aligned CNT Vertically aligned CNT CNF CNF CNT CNF CNT CNT and CNF

Ir, Al, Nb, and Ti on Si Si SiO2 on Si Alumina Quartz tube Quartz tube

Co Co, Co-Cu Mo/Fe-layered film Co, Fe, Fe-Ni alloy, Ni

MgO Ceramic boat Al on Si ITO, Ir, Al, Ti, Ta, and W on Si Quartz tube

SWNT CNF SWNT SWNT CNT, CNF Multidirectional CNT, CNF Y-junction CNF CNF CNT CNT

Cu-Ni alloy

CNF

Reproduced with permission from K.A. Shah, B.A. Tali, Synthesis of carbon nanotubes by catalytic chemical vapour deposition: a review on carbon sources, catalysts and substrates, Mater. Sci. Semicond. Process. 41 (2016) 67–82, with data compiled from various sources.

of the substrate, which in turn determined the diameter of the CNT synthesized [134] (based on Baker’s postulation [136]). Besides silica, there are many other types of substrate used to grow CNT and CNF. This information is summarized and shown in Table 1.2 [58]. Similarly, graphene growth needs the similar transition metals as CNT and CNF fabrication. Various metals each have their own special mechanism for graphene growth. The quality of the synthesized graphene is dependent on the ability of the transition metal to improve dehydrogenation of hydrocarbon and carbon isolation. The produced graphene would have different interaction strengths and growth mechanisms based on the substrate. In 2012, Batzill summarized information on 11 different transition metals that possess different interaction properties with graphene [137]. Ni as one of the most efficient catalysts for growing carbon nanomaterials is widely studied for the synthesis of graphene [99, 138] because of high solubility of carbon atoms

Synthesis of Carbon Nanomaterials

in Ni related to the strong interaction of NidC in comparison with CdC. Co has a structure similar to Ni, as both have in-plane lattice constants that match that of graphene; the lattice mismatch is <2% [139]. Besides, Co has high carbon solubility but a low carbon diffusion coefficient [140]. Fast cooling is required after CVD to enhance the graphene formation; otherwise, the carbon atoms will diffuse deeper into the Co film, which causes no graphene to be formed. Cu as a catalyst has different manner for synthesizing graphene in nucleation and mechanism. A chemisorption/deposition or surface production approach has been considered. The solvency of C atoms into the Cu mass is very low, and the mobility of carbon can be closed to be a purely surface-based process [141]. Furthermore, the diffusion barrier of carbon atoms on Cu is also low [142], which causes Cu to have a different catalytic role compared with other catalysts. Because of the low solubility of carbon in Cu, the chemisorption/deposition or surface growth mechanism describes the graphene formation mechanism, effectively. Hence, in order to produce carbon nanoparticles with high quality, CVD using Cu catalyst is considered as one of the most favorable techniques because of its growth in a large-scale and high-quality graphene, CNF, and CNT [143]. Readers may also refer to a review article published by Seah et al. that discussed extensively on the types of catalyst used to synthesize graphene [138]. Substrates play an important role in the thermocatalytic synthesis of graphene. Metals such as Cu, Co, Ir, and Ni have been widely used as substrates for the growth of graphene [144, 145]. The nature of the substrate used for the synthesis of graphene has a significant effect on the physicochemical properties, crystallography, and morphology of the resultant graphene. The different types of substrates that have been employed for the synthesis of graphene are discussed in the following paragraphs. Graphene has been synthesized through industrial heterogeneous catalysis on transition-metal surfaces [146]. The use of transition metals and their compounds as substrates for graphene synthesis is prompted by their partially filled d orbitals or from the formation of intermediate compounds that enhance the reactivity of the precursor gas [147]. Hence, metallic substrates have the advantage of providing low-activation-energy pathways for reactions through a change in the oxidation state of the metal or the formation of intermediate compounds. Graphene synthesis by CVD on transition-metal substrates, such as Ni, Pd, Ru, Ir, and Cu foils or evaporated films, has been widely investigated [137, 144]. The findings from these studies show that the physicochemical properties of the as-grown graphene films are dependent on the type of substrate used. The difference in activity of the substrates and their tendency to solubilize the formed carbon a function of the different growth mechanisms. The activity of the metallic substrate can be directly related to the decomposition of the hydrocarbons on metals, whereby the active carbon species is dissolved on the metal surface by lowering the activation energy. This was demonstrated by [148], who investigated the growth of graphene on Pt and Cu substrates at a relatively low temperature (750°C) using CH4 as the carbon source.

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Their findings show that the Pt substrate has a stronger catalytic ability for CH4 dissociation than Cu. Additionally, studies have shown that Cu substrates display lower activity during graphene synthesis compared with Ni and other transition-metal substrates when using CH4 as the carbon source [149]. The higher activity of Cu compared with Ni can be explained in terms of the electron transfer from the CH bonds to the 3d orbitals of the Cu substrate, as Ni has two 3d unpaired electrons compared with Cu, which has only one unpaired electron available for the interaction [147]. Copper has not been observed to form any carbide phases. The low reactivity of Cu could be due to its inability to form any carbide phases. Moreover, the low reactivity of Cu could also be justified by the fact that it possesses a filled 3d electron shell, which is the most stable configuration [150]. The solubility of carbon on the metallic substrate surface has been reported to be an important parameter for controlling graphene growth on metal substrates [144]. The growth kinetics of graphene are dependent on the solubility of the metal substrate. The conditions of graphene growth on a metal substrate also significantly influence the mechanisms of deposition of the carbon source and invariably affect the synthesized graphene morphology in terms of the domain size and thickness [144]. Confirming this point, comparative studies by [151, 152] revealed that Cu has a very low carbon solubility (0.001–0.008wt% at 1084°C) compared with Co (0.9 wt% at 1320°C) or Ni (0.6 wt% at 1326°C). Metal alloys or bimetallic substrates also have wide applications in the growth of graphene [153]. The use of metal alloys was demonstrated by [154], who synthesized an Ni-Au bimetallic substrate and employed it in the low-temperature synthesis of graphene. The results showed that few-layer graphene (FLG) with moderate crystallinity and domain size was formed via CVD at a low temperature of 450°C. Additionally, the study showed that the Ni-Au bimetallic substrate significantly lowered the graphene nucleation density, resulting in uniform and controlled graphene growth. The findings also showed that the Au-Ni bimetallic substrate enhanced the growth of graphene with a uniform grain size. One shortcoming of using the Au-Ni bimetallic substrate is that high temperature is required to mix the bimetallic alloy. Graphene can also be directly synthesized on dielectric substrates, such as BN, Si, SiO2, Al2O3, GaN, MgO, and Si3N4. Several studies have investigated the growth of graphene films directly on these dielectric substrates [155–157]. However, none of these studies have shown the continuous growth of highly conductive graphene films. Bi et al. investigated the direct growth of graphene films on BN, Si, SiO2, and Al-N dielectric substrates via CVD at 1100–1200°C using a gaseous mixture of CH4, H2, and Ar [158]. The study showed that FLG films were synthesized over the dielectric substrates at 1200°C. The results showed that the FLG synthesized by the CVD approach displayed characteristics comparable with graphene lacking long-range order in the perpendicular direction. The study also showed that single-layer to FLG films were obtained. However, some defects were observed in the graphene film consisting of thicker, graphitic-like material.

Synthesis of Carbon Nanomaterials

Studies have shown that the CVD temperature significantly influences the quality of graphene. Hu et al. investigated the CVD temperature (900 and 1000°C) during graphene growth on a Cu substrate using a CH4 precursor [159]. The authors observed that graphene grown at 900°C over the Cu substrate was composed of few-layer graphene. Similarly, graphene grown at 1000°C also contained few-layer graphene, but it was more uniform without a clear contrast. Raman spectroscopy of the graphene obtained at 900°C showed significant D bands compared with that grown at 1000°C, which is an indication that graphene grown at 900°C possesses more defects compared with that grown at 1000°C. Table 1.3 shows the different CVD methods employed in the synthesis of graphene on different metallic substrates along with their reaction conditions. The different CVD temperatures that have been employed in the synthesis of graphene are depicted in the sixth column of Table 1.3. It can be seen that the temperature ranges from a lower temperature of 300°C to the highest temperature of 1035°C. Interestingly, the quality of graphene synthesized in all of these studies differs and significantly depends on the respective temperature. The various studies reported in Table 1.3 show that graphene synthesis depends on the solubility of carbon in Ni and the kinetics of carbon segregation. Reports have shown that Ni possesses excellent solubility for carbon atoms [170]. The diffusion of carbon atoms is facilitated at slow cooling rates, in which carbon atoms have ample time to diffuse into the bulk Ni, resulting in the absence of segregation on the surface. Table 1.3 The parameters used for synthesizing graphene via CVD method Time Temperature Flow rate (CH4/H2/Ar) (sccm) Substrate (min) (°C) Gas source

Reference

CH4/H2 CH4/H2 CH4/H2 CH4/H2 CH4/H2 CH4/H2/Ar CH4 CH4/H2/Ar CH4/Ar/H2 CH4/H2 CH4/H2/Ar

35:2 7:2 5–25:1500 24:8 15:7 0–450:50–200:0–450 70 250:4000:1000 30:10:0 1:80 100:0–50: Ar

CH4/Ar/H2 CH4/Ar/H2

0.025–142:10:600 20% CH4 in Ar 10–50:600 H2 5–500 ppm 1.3% H2 Ftotal 1500 sccm

CH4/Ar/H2

Cu Cu Ni Cu Cu Cu Cu Ni Cu, Al Ni Ni, Cu on SiO2/Si Cu Cu

1–60 1–60 5–10 30 30 20–30 15 0.5–7 0.5–3 1 1–60

1000 1035 900–1000 1000 1000 1000 1000 960–970 300–400 450–750 900

[151] [160] [161] [162] [163] [163] [164] [165] [166] [166] [147]

– –

1000 1000

[167] [168]

Cu

5–60

1000

[169]

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Synthesis, Technology and Applications of Carbon Nanomaterials

Graphene formation can also be enhanced at moderate cooling rates at which the carbon atoms segregate, whereas the formation of graphene at higher rates leads to segregation of the carbon atoms out of Ni with the formation of a less crystalline and defective graphitic structure. A recent study by Losurdo et al. reported the growth of single- to multilayer graphene on polycrystalline Ni by thermal CVD [147]. The Ni film was prepared by evaporation on a SiO2/Si substrate, followed by annealing at 900–1000°C in Ar/H2 atmosphere for 20 min. The result showed that Ni grains of 5–20 μm in size were created during the annealing stage. Graphene formation was observed on the Ni film after CVD at 900–1000°C for 5–10 min employing 5–25 sccm CH4 and 1500 sccm H2. However, the size of the graphene formed was limited by the Ni grain size. The graphene formed was subsequently transferred onto a glass substrate and employed for electronic applications. A similar study by Kim et al. investigated graphene growth on a Ni/SiO2/Si substrate [166]. The authors reported that the Ni film thickness was optimized to obtain good-quality graphene. The numerous breakthroughs reported for the growth of graphene on various substrates by CVD have established the reproducibility of obtaining good-quality graphene. Hence, advances in graphene synthesis via CVD have opened new process routes for graphene applications in gas sensing, photovoltaics, and flexible electronics.

1.3 GROWTH MECHANISM OF MULTILAYER GRAPHENE The growth mechanism of CNT, CNF, and graphene has been discussed in the literature in an attempt to predict the morphology of the resultant product. For this chapter, the scope is limited to the growth mechanism of multilayer graphene. The synthesis of graphene involves a heterogeneous catalytic chemical reaction on a metallic surface [171]. The metal usually plays the role of the substrate/catalyst. Hence, in a typical thermocatalytic CVD procedure, a graphene film is grown on a metal substrate, and the catalytic activity can be reduced as a result of catalyst poisoning or deactivation [172]. Catalyst poisoning/deactivation often leads to the end of the chemical reaction for the formation of the graphene film [144, 147]. If the overall graphene synthetic process occurs on the catalyst surface, steps such as the adsorption, decomposition, and diffusion of molecules from the carbon source are prevalent, leading to the preferential growth of monolayer graphene [138]. This process is known to have a “self-limiting” effect, which has only been reported in Cu to date, depending on the process conditions. On the other hand, it has been demonstrated that the growth of graphene by CVD on metals such as Ni, Co, Ru, and Ir occurs by carbon bulk diffusion resulting from the high carbon solubility and segregation. As the process is quenched, a solid solution of the mixture of elements is formed near the surface of the substrate, which often results in the formation of graphene. The subsequent formation of graphene on or near the metal surface is a function of the kinetic

Synthesis of Carbon Nanomaterials

Table 1.4 Values of activation energies obtained from different studies on graphene growth kinetics Type of Activation graphene Precursor Substrate Growth conditions energy Reference

Single layer Bilayer

H2/CH4

Cu

CH4

Single layer

C2H2

Cu and Ni Ni foil

Few layer

C2H2

Ni foil

Bilayer

CH4

Cu

Temperature, 1035°C; pressure, 160 m torr Temperature, 900°C; pressure, 533 Pa Temperature, 700–950°C; pressure, 933 Pa Temperature, 700–950°C; pressure, 933 Pa Temperature, 1000°C; pressure, 400 Pa

144.7 kJ/mol

[174]

201 kJ/mol

[147]

144.7 kJ/mol

[175]

86.83 kJ/mol

[175]

NA

[176]

parameters chosen for the synthesis. A study showed that a fast cooling rate is one of the most important thermodynamic parameters for preventing the formation of multilayer graphene [144]. Additional gas-phase activation during the CVD process usually leads to a more complex deposition process. A mixture of heterogeneous catalysts and decomposition in the vapor phase in this case governs the chemical reaction. The mechanisms controlling the growth of graphene on Ni and Cu can be explained by the C-metal binary phase diagram, in which the solubility of C in Cu is much lower than that in Ni [147]. The carbon source, which is mainly CH4, catalytically decomposes on the Cu surface. This decomposition path expedites the surface migration and growth of monolayer graphene on the Cu surface. In contrast, many more carbon atoms can be solubilized in a Ni surface [144]. The growth of graphene on a metallic surface usually emerges mainly from precipitation during quenching of the process [173]. One important benefit of using Ni as a substrate is that the carbon solubility and precipitation stage can be controlled to some extent with the annealing, growing, and cooling rates. The activation energies obtained from kinetic studies of graphene growth on substrates such as Cu and Ni are summarized in Table 1.4. It can be seen that the activation energy is a function of the growth conditions and the type of precursor. The obtained activation energies range from 86.83 to 144.7 kJ/mol.

1.4 CONCLUSIONS Since the last few decades, CVD technology has been employed as a large-scale and lowcost technique to produce CNT, CNF, and graphene. The overall process seemed rather simple as it only involves decomposition of carbon precursors over catalyst at high temperature. In spite of this, the knowledge of the synthesis of these materials is still

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insufficient. Table 1.3 listed down various “recipes” to produce graphene. However, different results will be obtained by using CVD of different setups and configurations. The presence of unknown impurities in the carbon source such as NO2, SO2, and NH3 also complicates the process as it may result in the formation of different products. The knowledge on substrate-catalyst relation is also not fully established, causing a variation of products synthesized by merely changing the CVD temperature, reaction time, and pressure. CNT is seen as a promising material for various applications such as for electronic device fabrication. As such, complete control on the chirality of the CNT produced is important. As such, further study on this including technique and report on chirality of the CNT is necessary. Graphene has been demonstrated as potential candidate aimed for various applications. However, knowledge on graphene growth via CVD is still incomplete. For example, the decomposition reaction of the carbon sources and the role of the catalysts are still not fully understood. The knowledge on catalyst interaction with other CVD parameters is also extremely important to achieve controlled graphene production, including the ability to tailor and fabricate the size and number of layers. Hence, thorough understanding on the growth mechanism and the kinetics of the graphene formation is essential. If all of these issues can be addressed, a wide range of applications of these carbon nanomaterials can be realized.

ACKNOWLEDGMENTS This research is partially supported by the Ministry of Higher Education Malaysia (MOHE) under LRGS/ TD/2011/UPM/PG/01, UPM-LL-07-FGRS0211-2010, Modal Insan, and Putra IPB grants.

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