Chapter 1. Background and Method
1.2 Trends and challenges in synthesis of nano-carbon materials
Fullerene synthesis and challenges remain
The first discovery of C60 fullerene was during the experiments for studying the long carbon chains in interstellar space17 as mentioned in the last section. A vaporization laser beam at 532 nm was used to strike a rotating graphite disk, where the produced carbon vaporization was later carried to a nearby integration cup by the pulsed helium carrier gas17 (Fig. 1.2b). The vapor was then cooled and reacted in the cup to form clusters, and later detected by time-of-flight mass spectrometry (Fig. 1.2c-e) where a dominant peak of C60 was recognized. For this stable C60 molecule, the authors proposed an “unusually beautiful” model (Fig. 1.2f and g) of a truncated icosahedron just like a football17. Because of the trace amount of the products, at that time it was hard to prove the assumption of the molecular structure. It was confirmed the soccer- ball shape of C60 five years later when Taylor et al. characterized the pure C60 sample using 13C NMR spectra19, and Kratschmer et al. provided the evidence by infrared and ultraviolet absorption spectra61.
In 1990, Kratschmer and Huffman61, 62 for the first time get the access to synthesize usable quantities of C60, which opens up the way for further studying the properties of fullerenes including C70, C76 and other larger cages. The method used graphite rods as electrodes to create an electric arc in the reaction chamber of Helium atmosphere, where a hot carbon plasma was generated and carbon vapor shot out to form a jet (Fig. 1.5a-c). Finally a black soot was produced and scraped from the chamber surface inside. The soot can be dissolved in benzene or other non-polar solvents, and then concentrated62. With further purification, mg scale sample in solid form could be produced and be studied by microscopes. The time-of-flight mass spectrum (Fig. 1.5d) shows the dominant C60 peak and a relatively lower C70 peak, as well as peaks for some smaller even-numbered carbon cluster. The X-ray diffraction pattern of the C60
powder (Fig. 1.5e) and infrared absorption spectrum of the solid C60 sample on a silicon substrate (Fig. 1.5f) demonstrate the molecules are indeed fullerenes with very few CH impurities62.
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Figure 1.5 (a) Photograph of the arc heating device. (b) Inside of the chamber of (a) there are graphite anode and cathode which would produce soot at high temperature. (c) Schematic diagram showing the detailed structure of (b) which during the synthesis will produce an arc jet. (d) Time-of-flight mass spectra of the sample after arc heating. (e) X-ray diffraction pattern of the sample with C60. (f) Infrared absorption spectrum of the solid C60 sample on silicon substrate. (d-f) Reproduced from Ref. 62. Copyright@1990, Springer Nature.
Figure 1.6 (a) Synthesis route to fullerene C60. (b-d) Mass spectra with increasing laser fluences. (a-d) Reproduced from Ref. 63. Copyright@2001, The American Association for the Advancement of Science.
Although this method is much better and has been widely adapted, even after condensation the yield is still very low, where many other carbon clusters also exist. Not mention the harsh
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conditions it requires during the synthesis. So to produce C60, C70, or some other fullerene cages, further treatments such as purification is required64. And for many applications, it is required for a high concentration of some specific fullerene isomers which makes it even more impossible as the reaction parameters are quite uncontrollable65.
Later after that, scientists have been devoted to finding new methods that can be controlled and adapted to produce specific pure fullerene isomers. Among all, pyrolysis of polycyclic aromatic hydrocarbons (PAHs) is one of the most successful approaches66. The C60H30 PAHs molecule in the Fig. 1.6a has all the 60 carbon atoms, 13 of the 20 hexagons, and 3 of the 12 pentagons needed for the fullerene C60. The only thing to do is to delete the 30 hydrogen atoms and form carbon-carbon bonds. Boorum et al. used nine steps by conventional methods to synthesize this C60H30 molecule, and got rid of the hydrogen and “rolled up” the molecule to form fullerene under laser irradiation at 337 nm63. Their positive-ion LDI mass spectra (Fig. 1.6b-d) shows the process of the molecule losing hydrogen atoms or hydrocarbon fragments with the increase of the laser fluences63. Finally the formation of C60 molecule was confirmed by the Fig. 1.6c with the C60+ peak.
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Figure 1.7 (a) Flow chart for the production of fullerene compounds in organic solar cell. (b) Energy flow chart for the production of fullerene using pyro-tetralin synthesis method with C60PCBM and C70PCBM (PCBM: [6,6]-phenyl-C61-butyric acid methyl ester). (a-b) Reproduced from Ref. 67. Copyright@2011, American Chemical Society.
However, all these techniques used for producing fullerene for industrial applications are facing the same problem of high cost and low efficiency. To provide isomer-pure sample, after synthesis and condensation, separation of the fullerene mixture and further purification to a high concentration is required (Fig. 1.7a)67. For specific use, functionalization of the pristine fullerenes need even greater effort. A comprehensive analysis (Fig. 1.7b) shows that the energy cost of production for all fullerenes are an order of magnitude higher than most bulk chemicals67. Thus to finally industrialize fullerenes, there is still a long way.
Early carbon nanotube synthetic approaches
The discovery of CNTs was directly related to the synthesis of fullerenes. At the beginning, simple methods using DC arc discharge were already able to synthesize MWCNTs68, which is just the same technique used for fullerene. Ebbesen et al. used the standard arc-discharge equipment with graphite rods (with diameter of 5mm, Fig. 1.8a) for fullerene at Helium atmosphere69. On the surface of one of the rods they got high yield of CNTs, which under High- resolution TEM could be recognized as MWCNTs with number of wall layers ranging from two to many (Fig. 1.8b)69. Usually in the synthesis of MWCNTs, no catalysts are used. While for SWCNTs, it is necessary to add transition metal as catalysts. The metals like Ni, Fe, Co, Ag, Pt, etc. or their mixtures are added as the component of the anode of the arc discharge device70. For example, both Bethune et al.71 and Ajayan et al.72 reported the use of Co as the catalyst in arc discharge synthesis for SWCNT with small diameters of around 1-2 nm. In Fig. 1.8c and d, high-resolution transmission electron microscope (TEM) images both showed clear feature of single wall of the CNTs. In 1995, Smalley et al. also introduced the use of laser ablation in CNT synthesis73, where the principles of this method are very similar to that in the arc discharge method (Fig. 1.8e-f). The only difference is that they used a laser, rather than arc, to vaporize the graphite rods containing transition-metal to produce SWCNT, where the transition metals used are usually Ni or Co, or their mixtures73, 74.
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Figure 1.8 (a) Graphite rod for large-scale synthesis of MWCNT with arc discharge method. (b) High- resolution electron micrographs of MWCNT prepared by arc discharge method. (a-b) Reproduced from Ref. 69. Copyright@1992, Springer Nature. (c-d) TEM image of SWCNT prepared by arc discharge method. (c) Reproduced from Ref. 71. Copyright@1993, Springer Nature. (d) Reproduced from Ref. 72. Copyright@1993, Published by Elsevier B.V. (e) Laser vaporization device for CNT growth. (f-g) TEM images of SWCNT bundles synthesized using Co/Ni catalyst. (e-g) Reproduced from Ref. 73. Copyright@1995, Published by Elsevier B.V.
Figure 1.9 CNT growth models: (a) tip growth and (b) base growth. (a-b) Reproduced from Ref. 75. Copyright@2010, American Scientific Publishers.
Although scientists are already able to produce large quantities of CNTs with these methods, disadvantages of them, the same as those for fullerene synthesis, are still hindering the development of the CNT community, such as the high temperature, harsh condition, high cost, and lost yield. Therefore, nowadays, methods like arc discharge or laser ablation were gradually
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replaced by chemical vapor deposition (CVD) approach in which relatively low temperature is required, and the synthesis seems more under control in terms of the diameter, purity, length, and alignment of the as-grown CNTs70.
CVD growth of carbon nanotube
The basic mechanism of CVD growth is to decompose carbon precursors in the gas phase with the liquid or solid catalyst, transport the dissociated carbon sources to the growth front of CNT on the catalyst surface, and catalyze the incorporation of carbon into the CNT wall for the continuous growth75. There are basically two growth modes that are widely accepted: tip growth and base growth. When the interaction between catalyst and substrate is weak, CNT tends to grow with the catalyst particle on top leaving the substrate surface (Fig. 1.9a)76; when the interaction between catalyst and substrate is strong enough, catalyst will anchor at the substrate and the tube will be pushed away to the other side and grow longer gradually (Fig. 1.9b)77. During the growth, many factors will influence the final structure and properties of the as- grown CNTs, such as carbon precursors, catalyst, temperature, pressure and substrate, etc. But this is also the advantage of CVD method since understanding these factors could help to control the growth of CNT and finally get the specific CNT we need.
Carbon sources in the CVD growth of CNTs are very important. Their type, state, concentration, etc. will greatly affect the morphology, density, and quality of CNTs. For example, if too high concentration of the precursor was adopted, the as-grown CNTs will be contaminated with large quantity of amorphous carbon resulting in the low yield and the poor quality78. The most commonly used carbon precursors are methane79, ethane, ethylene, acetylene76, etc. In 2002, Maruyama et al. used ethanol as the carbon source to synthesize SWCNTs at low temperature, and found that the presence of OH radical could suppress the impurities in the product80. Therefore, high-quality SWCNTs were grown as shown in Fig. 1.10a and b. Hata et al., later in 2004, synthesized high-density SWCNT forest with ethylene using Ar or He with H2 that contained a small amount of water81. The density, and carbon purity of their vertically aligned SWCNT forests are extremely high as shown in Fig. 1.10c-g. Another widely used carbon source is carbon monoxide. In 2013, He et al. realized chiral-selective growth of SWCNT using Co nanoparticles in an ambient CO atmosphere82. It was reported that they could grow a high concentration of (6,5), almost 30 (6,5) tubes out of the totally 57 individual SWCNTs (Fig.
1.10h-j). Actually, any materials containing carbon could be used as precursors for CNT growth nowadays. It is very crucial to find a cheap and efficient type of carbon precursors that could help control the growth of CNTs.
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Figure 1.10 TEM (a) and SEM (b) image of SWCNTs grown from alcohol. (a-b) Reproduced from Ref.
80. Copyright@2002, published by Elsevier B.V. (c) Photograph of SWCNT forest on silicon wafer grown by ethylene. (d-e) Scanning electron microscopy (SEM) image of the SWCNTs forest. Scale bar is 1 mm and 1 μm, respectively. (f-g) TEM images of the as grown SWCNTs. Scale bar is 100 nm and 5 nm, respectively. (c-g) Reproduced from Ref. 81. Copyright@2004, Springer Nature. (h) Photoluminescence (PL) spectroscopy for the SWCNTs grown with CO. (i) Histogram of diameter distribution of the SWCNTs in (h). (j) Chirality map of the sample in (h). (h-j) Reproduced from Ref. 82. Copyright@2013, Springer Nature.
Catalyst plays a quite important role in the CVD growth of CNTs. It almost takes part in all the processes during synthesis: catalyze the dissociation of carbon sources; manage the diffusion of decomposed carbon precursors, facilitate the incorporation of carbon into the CNT wall, support the CNT for continuous growth and prevent it from closure, etc. Therefore, the selection and preparation of the catalyst would influence the CNT growth significantly. For example, there were reports showing that effective dispersion, as well as the size of the catalyst particles, would greatly affect the yield and the diameter of the grown CNTs (Fig. 1.11a)83, 84. Many transition metal particles, except the most widely used Fe, Ni and Co75, have been used as catalysts to synthesize CNTs, such as Pd, Pt, Au, Cu, Mg, Al, etc85. Except for the monometallic catalysts, later scientists found that alloy catalysts have great advantages for the CNT growth.
In 2003, Bachilo et al. reported the successful synthesis of SWCNTs with a Co-Mo alloy catalyst called CoMoCAT using silica as substrate86. The catalyst showed high selectivity for (6,5) and (7,5) nanotubes as well as a high yield, and therefore led to the commercial production of SWCNTs (Fig. 1.11b-c). Chiang et al. later discovered that chirality distribution can be tuned by varying the composition of NixFe1-x nanoparticles87 (Fig. 1.11f-h). The authors argued that it is iron’s incorporation into Ni that affects the lattice match of the catalyst and leads to the chirality change87 (Fig. 1.11d-e).
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Figure 1.11 (a) TEM image of the dispersion of CoNi alloy catalyst for the growth of SWCNTs.
Reproduced from Ref. 83. Copyright@1999, AIP Publishing. (b) Chirality map for the SWCNTs grown by CoMoCAT method. (c) PL contour of the SWCNTs grown by CoMoCAT method (top) and HiPco method (bottom). (b-c) Reproduced from Ref. 86. Copyright@2003, American Chemical Society. (d-e) TEM images of Ni0.27Fe0.73 nanoparticle used to grown SWCNTs. (f-h) Micro-Raman spectra of the SWCNTs grown using the catalyst in (d-e). (d-h) Reproduced from Ref. 87. Copyright@2009, Springer Nature. (i) Picture of the aligned ultra-long CNTs grown by air-assisted CVD method. (j-l) SEM images of the CNT film in (i) from (j) bottom, (k) middle, and (l) top, respectively. Scale bar is 1 μm. (i-l) Reproduced from Ref. 88. Copyright@2008, IOP Publishing, LTD. (m) Optical images of CNT forest grown with Fe/Al2O3 catalyst on Si substrate. (n) Thickness of the CNT forest grown with the same method in (m) was a function of time at different positions. (o) SEM images of the CNT forests at different positions using the same growth method as in (m). (m-o) Reproduced from Ref. 89. Copyright@2008, American Chemical Society.
In addition to the dispersion, size, and type of the particle, the effect of the atmosphere on catalyst particle was also studied. It was already mentioned in the above discussion that the addition of very little water vapor would greatly increase the efficiency of the catalysis of the Fe nanoparticles, resulting in the dense SWCNT forest81 (Fig. 1.10c-h). Research work by Yamada et al. showed that the trace amount of water could help remove the excessive carbon coating on the catalyst surface and prevent the particle from being deactivated90. Later in 2008, Li et al. also found that mixing a small amount of air in the CVD furnace could lead to the
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super-long growth of CNTs (Fig. 1.11i-l)88. They believed that it was the oxygen in the air that maintains the catalyst activity and supports the CNTs for longer lifetime.
During the growth, a suitable substrate also affects the yield and quality of the as-grown CNTs.
Various substrates have been tested applicable for CNT growth, such as silicon, magnesium oxide, silicon carbide, zeolite, graphite, alumina, quartz, silica, and so on85. For instance, Mattevi et al. found alumina a good support for CNT growth as it has a stronger catalyst- substrate interaction than that on SiO2 (Fig. 1.11m), and also narrows the catalyst particle size distribution which leads to a higher tube density as shown in their research result (Fig. 1.11n- o)89.
Controlled growth of carbon nanotube and the remaining challenges
Compared with arc discharge and laser ablation methods, CVD is obviously more economically friendly and practical for further large scale industrial production, and therefore, it becomes the most popular preparation technique for CNT synthesis with low cost, high quality, high yield, and easy controllability. With this method, nowadays, people start moving forward, trying to control the structure of the grown CNTs, in terms of the diameter, chirality, and the corresponding properties.
Scientists began with controlling the diameter of the grown CNTs. In 2001, Li et al. prepared uniform Fe-Mo catalyst particles with the size ranging from 3 to 14 nm by using a mixture of protective agents with different ratio of acid/amine91. TEM images in Fig. 1.12a-c show the narrow distributions of the nanoparticles. With these catalyst particles, SWCNTs with narrower diameter distribution were synthesized. Except for synthesize uniform nanoparticles, preventing aggregation of the catalyst particles on the substrate is also very important. An et al.
used 3-aminopropyltriethoxysilane to treat the silicon dioxide substrate surface to prevent the Fe-Mo catalyst particles from growing larger92. The as-grown SWCNTs were found to have a narrow diameter distribution from 0.7 to 1.5 nm with a small standard deviation.
As the semiconducting SWCNTs (s-SWCNTs) have the great potential in electronics, techniques to selectively grow this kind of SWCNTs were also reported. Kang et al. synthesized horizontally aligned s-SWCNTs with high density and high purity using ethanol and methane by CVD method93. They argued the active H radicals would inhibit metallic SWCNTs and finally got over 91% s-SWCNTs with this method. Later Wang et al. introduced a very interesting method--twisting the chirality of the CNTs by switching the direction of an applied electric field, and increased the purity of the grown s-SWCNTs to be 99.9% (Fig. 1.12d)94.
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Figure 1.12 TEM images and corresponding histograms of Fe-Mo nanoparticles produced using typical condition (a) and 2.50 mmol (b) or 5.00 mmol (c) protective agent. Scale bars of all the inserts are 4 nm.
(a-c) Reproduced from Ref. 91. Copyright@2001, American Chemical Society. (d) Schematic diagram showing how electric field could twist the chirality of CNT. Reproduced from Ref. 94. Copyright@2018, Springer Nature. (e) Templated growth of CNTs by ethanol CVD using the special W-Co nanocrystal catalyst. (f) Chirality map showing the predominant abundance of (12,6) tube using the catalyst prepared as shown in (e). (e-f) Reproduced from Ref. 95. Copyright@2014, Springer Nature. (g) Raman spectra in the RBM region with an excitation of 633 nm. (h) Ultraviolet-visible-near-infrared absorption spectrum of the same sample in (g). (g-h) Reproduced from Ref. 96. Copyright@2017, Springer Nature. (i) Raman spectra in the RBM region with an excitation of 514 nm. (j) Chirality distribution map of the CNTs in (i). (i-j) Reproduced from Ref. 97. Copyright@2019, Elsevier Inc. (k) Schematic diagram showing how PAH precursor C96H54 forms specific (6,6) SWCNT. (l) Raman spectra shift when the PAH precursor C96H54 was elongated to be (6,6) SWCNT. (m) Raman spectra of the as-grown tube in (l) with the insets show the RBM peak at 295 cm-1, 1518 cm-1 and 1591 cm-1. (k-m) Reproduced from Ref. 98. Copyright@2014, Springer Nature.
Taking one more step, scientists devoted to aiming at SWCNTs with specific chirality. As mentioned, bimetal catalyst particles have been reported to be able to tune the chirality distribution87. With the alloy catalysts, researchers further carefully chose catalysts with unique structures. Yang et al. was able to template the growth of CNTs on a special Co7W6 solid catalyst which exhibits a unique rhombohedral crystal structure, resulting in a good structural match between a specific (12,6) tube and the catalyst (Fig. 1.12e)95. Therefore, the synthesis
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reached a high (12,6)-tube abundance of 92% (Fig. 1.12f). Other than catalyst design, good kinetic control also leads to chirality control. For example, Zhang’s group found that by increasing the concentration of carbon source, ethanol, the produced SWCNTs contained 90%
(12,6) tubes (Fig. 1.12g-h)96; while by decreasing the carbon feeding rate of CO and using Co catalyst, a preferential growth of (n,n-1) tubes could be realized (Fig. 1.12i-j)97. Also there were reports about growing single chirality tubes. In 2014, Sanchez-Valencia et al. synthesized the targeted (6,6) tube with a few hundred nanometers by converting a special nanocarbon molecule—C96H54 into the singly capped (6,6) tube seed (Fig. 1.12k-m)98.
The entire society is moving fast forward and has achieved a lot during the last few decades as mentioned above. However, challenges still exist as for synthesizing high-quality high-purity SWCNTs with high efficiency and low cost99. Many questions remain to be answered: is the chirality determined in nucleation or during growth? How does the nature and geometry of the catalyst support alter the mechanism of nanotube nucleation of nanotube nucleation, growth, chirality and chirality selection? What determines the catalytic efficiency of free and supported catalyst nanoparticles for nanotube nucleation and growth? What are the rate-limiting steps in SWCNT growth? 100
Fabrication of graphene and the CVD method
Various synthesis strategies have been proposed and tested since the first fabrication of free standing graphene in 2004 by Novoselov and Geim47. Mechanical exfoliation, as the simplest and most straightforward method, has provides the society the first chance to study the intrinsic properties of this extraordinary material with highest-quality samples. The idea is to break the weak van der Waals interaction between graphene layers by repeatedly applying the micro- mechanical cleavage. As shown in Fig. 1.13a-c, with increasing times of tape peeling, the graphene films’ thickness decreased from ~3 nm to 0.8 nm, and finally it was able to use the produced single layer graphene for novel electronic devices (Fig. 1.13d-e)47. Though with high quality products, this method is still quite limited as for its extremely low yield. Therefore, another top-down exfoliation technique emerged, naming as liquid-phase exfoliation. The bulk layered materials (such as graphite) were dispersed into liquid solvents like N-methyl- pyrrolidone, and intercalated into graphene layers (Fig. 1.13f-g)101. The yield of monolayer graphene was reported as 1 wt%, which at that time was described as “high yield” but still far from large production. There are also other chemical synthetic methods like graphite oxide (GO) reduction, where graphite is first oxide to GO, and then GO was exfoliated into GO solution in water and reduced to graphene at higher temperature102. Though this method is more efficient and easy to process, most of the chemical reactions contain uncertainties, and thus, the quality of the as-grown graphene films is expected to be degraded.