a  – OCSiAl, Group, 1, rue de la Poudrerie, 3364 Leudelange, Grand Duchy of Luxembourg
b – Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88003, United States
c – NM Devices, LLC, 690 Canyon Point, Las Cruces, NM 88003, United States

Abstract

AbstractThis is not a comprehensive review on the topic of single walled carbon nanotubes (SWCNTs). Because it is very broad – a simple search on ‘carbon nanotubes’ provides well over a million citations – we are focusing on a brand of SWCNTs produced by our company, OCSiAl, trademarked as TUBALL™ or just Tuball. It is a well-known product on the market and there is a good citation record on the applications of Tuball in industry and research labs, see Fig. 1. Despite this, there is no published data in the literature properly referencing its quality and the examples of its applications are often left without sufficient characterization. Because of that, we feel that it is well overdue to have a proper summary accessible to many researchers. Besides, the beneficial properties of SWCNT augments in many applications very strongly depend on the quality of SWCNTs and the methods of their introduction into materials. We have a few successful demonstrations here, some of which have not been published yet, and a plenty of additional published results with Tuball nanotubes are included as well.

Introduction. 

There are two sections in this Review: the first one, after introduction, describes the methods of synthesis and discusses in detail the structural and physico-chemical properties of SWCNTs with the emphasis on Tuball. Here we also address the health and safety issues for SWCNT, often confused misconceptions and distinctions between SWCNTs, MWCNTs (multi walled carbon nanotubes), and other carbon materials. The second part [1] describes the methods for dispersing SWCNTs in various media suitable for maximizing the positive effect on the mechanical, electrical, and other functional properties of the composites, including materials for batteries, thermosets, thermoplastics, and elastomers. 

Fig. 1. Annual peer reviewed publications that used Tuball in the study

Brief History of CNTs 

Carbon is reach on stable at ambient conditions allotropes which have dramatically different properties – from very hard dielectric diamond to flaky conductive graphite. Twentieth century was fruitful in discoveries the new allotropes that are not naturally abundant. Synthesis of ‘buckmisterfullerenes’ (C60 and C70) was experimentally confirmed [2,3] in 1984 followed by extending the rich family of buckyballs to a wider range of closed carbon fullerenes. Synthesis of carbyne (–C≡C–)n chains in pure neutral carbon material was also reported [4-7] but is still disputed [8,9]. Attempts to ‘thin graphite down’ to a single layer graphene were reported as early as in 1948 [10]; they were later rediscovered for engineering purposes [11] and for scientific community [12]. Exfoliation of graphene in the form of graphene oxide was first reported in the nineteenth century [13] and eventually improved to a viable technique [14]. In the meantime, synthesis of single layer graphene via chemical vapor deposition was also introduced in 1997 [15] which eventually expanded into a successful method of chemical vapor deposition for preparing high quality monocrystalline single graphene of large dimensions [16]. All of this allowed evolving graphene into a mature industry [17]. 

Discovery of carbon nanotubes (CNTs) is usually attributed to Iijima of NEC who reported the findings in 1991 but earlier reports mentioning CNTs can also be found. In 1952, Radushkevich and Lukyanovich published images of 50 nm diameter tubes made of carbon [18]. Oberlin et al. identified hollow tubes of rolled up graphite sheets synthesized by a chemical vapor-growth technique in 1976 [19]. In 1981 “carbon multi-layer tubular crystals” were suggested to be formed with rolled graphene layers into cylinders during thermocatalytic disproportionation of carbon monoxide [20]. There was even a U.S. patent issued in 1987 describing production of "cylindrical discrete carbon fibrils" with a "constant diameter between about 3.5 and about 70 nanometers..., length 100 times the diameter...." [21].

The real appreciation for the new type of material started in 1991 after Iijima described [22] synthesis of measurable quantities of multi-walled carbon nanotubes (MWCNTs) in arc-discharge evaporation of graphite rods. The interest was further solidified by a prediction of Mintmire et al. that single walled CNTs (SWCNTs) should exhibit remarkable conducting properties [23]. Experimental discoveries of SWCNTs [24-25] assisted the excitement and brought new improved methods for production of SWCNTs by adding transition-metal catalysts to the graphite in an arc discharge.  

The interest was especially amplified by subsequent confirmation that carbon nanotubes are the strongest and stiffest materials in terms of tensile strength and elastic modulus: MWCNTs were shown to have a tensile strength of 63 GPa [26] and SWCNTs - up to 100 GPa [27]. Although the recent experiments have lowered it down to 30-70 GPa, stating that the strength depends on the SWCNT diameter and chirality[28]. The chirality reflects how a graphene sheet is rolled up into a nanotube, as shown in Fig. 2. Depending on that orientation (chirality), SWCNTs can exhibit metallic or semiconducting properties. Statistically, among the defect-free SWCNTs 1/3 are metallic and 2/3 - semiconducting.

Fig. 2. Graphene honeycomb network with lattice vectors, a1 and a2, illustrating different options for the chiral vector, ch, constructed from their linear combination and representing possible wrappings of the two-dimensional graphene sheet into tubular forms, some of which are shown at the bottom (adapted from [29]).

The electrical properties of CNTs are also remarkable and depend on their chirality. Metallic SWCNTs represent one-dimensional conductors with maximum conductance twice that of a single ballistic quantum channel, i.e., 2G0 = 4e2/h [29,30]. Due to the thermal fluctuations and electron collisions with the surface bound molecules, the coherence length capable of sustaining such ballistic conductions at room temperature is only on the order of 200 nm [31], much shorter than typical few micrometer lengths of SWCNTs. Nevertheless, SWCNTs have unsurpassed electrical properties and, because of a strong carbon-carbon bond, metallic SWCNTs can carry an electric current density more than 1,000 times greater than those of metals such as copper [30], where current densities are limited by electromigration. 

Despite the rich history and the outstanding mechanical and electrical properties of CNTs, their commercialization has been quite slow, mainly because of the prohibitively high cost and poor quality of CNT materials with scantily defined standards and their control. 

Here we demonstrate that recent developments in the synthesis of high quality SWCNTs offer a broad spectrum of promising applications. We believe that their affordability should ensure market acceptance and propose some convenient solutions in a form of intermediate products. The first part focuses on the characterization of SWCNTs themselves, their electrical, thermal and mechanical properties, while in the second part [1] application of SWCNTs as additives for improving, electrical and mechanical properties will be discussed. We do not intend to provide a detail review of all applications but rather will focus on formulations developed in our company or/and with the use of TUBALLTM SWCNTs in the literature. When appropriate, comparison with other sources of SWCNTs and MWCNTs will be made as well.

Methods of SWCNTs Production and Tuball

Among the various allotropic modifications of carbon, the most stable under normal pressure are sp2-hybridized forms, which include carbon nanotubes. Therefore, the formation of nanotubes should not require significant tricks: they are formed in various common processes along with soot formation, for example, during the operation of internal combustion engines [32-34] or burning wood [35]. However, for the targeted production of nanotubes, especially single-walled ones, it is important to ensure the selectivity of synthesis allowing elimination of the more thermodynamically stable modifications - graphite and soot. Such kinetic selectivity arises from the use of carbon “nuclei” on the catalyst, initiating the growth of carbon nanotubes [36].

Such nascent CNT nuclei are carbon "caps" - fragments of graphene planes, curved because of a few hexagonal rings being replaced by pentagons. In the most common methods for synthesizing CNTs, nuclei are formed from carbon-containing source either under high-temperature conditions of carbon plasma in arc discharge or during laser ablation in non-catalytic synthesis of multi-walled nanotubes, or on the surface of a catalytic nanoparticle (most often transition metals) during catalytic synthesis in arc discharge during laser ablation or in chemical vapor deposition (CVD). Less commonly, artificially made molecular nuclei, for example, C50H10 [37], were used as well.

Synthesis of CNTs is a complex multi-stage process, the details of which are extensively discussed in numerous publications, but many important subtleties are obviously kept secret. An interested reader is referred to recent reviews [38-40] and the references therein. The selection of a catalyst [41] is considered to be of fundamental importance, as it should contribute to a) decomposition of the molecular carbon source, b) formation of a carbon “cap”, c) separation of the cap from the catalytic particle surface, and d) facilitated supply of hydrocarbon to the edge of the growing nanotube.Among the methods of CNT synthesis, the most suitable for scaling up and further development is believed to be the catalytic CVD synthesis involving decomposition of hydrocarbons on a catalytic surface into CNT and hydrogen. However, the problem of an efficient catalyst in the industrial CVD synthesis is far from completion. The method for producing catalytic metal nanoparticles patented by OCSiAl [42] is uniquely suited for synthesis of single-walled carbon nanotubes and allows the production capacity in a flow reactor on the scale of several kilograms per hour. The technology is described in several patents [43-46] and generally can be applied for the synthesis of various carbon nanomaterials [47] including double- and multi-walled CNTs but OCSiAl has tuned it to production of exclusively SWCNTs and distributes it under trade name TUBALLTM. The trade name “TuballTM graphene nanotubes” was chosen to emphasize its distinction from other SWCNTs that were on the market at the time. Compared to other technologies, OCSiAl’s uniqueness has three key advantages:

a) high quality material
b) unlimited scaling of production
c) low cost and, therefore, competitive pricing.

The quality of the 'as-produced' standard Tuball material is illustrated by the physical properties given in Table 1. In November 2013, OCSiAl had launched a pilot industrial plant for the synthesis of carbon nanotubes Graphetron 1.0 with a projected annual production capacity of 10 tonnes of SWCNTs. Improving the technology increased the production by 2018 to 12 tonnes of SWCNTs annually. Such large - scale industrial production of SWCNTs brought the price of nanotubes down from the hundreds of thousands to just $ 2,000 per kilogram currently offered by OCSiAl. In 2019 the next generation Graphetron 50 has been launched with a projected annual production of 50 tonnes SWCNTs [48].

Since the CVD process can lead to the formation of various carbon materials ranging from amorphous carbon to graphite-like particles and the material can contain a significant amount of catalyst in the form of metal and carbide particles, the next important issue beyond the capacity is the purity of the obtained single-walled carbon nanotubes. Tuball SWCNTs manufactured by OCSiAl are characterized by a low concentration of defects and a low content of carbon impurities. However, the product directly obtained from the reactor contains up to 15 wt% of metallic impurities, mainly iron. For many applications, residual iron nanoparticles do not pose a problem, but for some it is very undesirable. In recent development of the expanding range of OCSiAl products, the chemically purified Ultrapure Tuball nanotubes are also offered, where the metal content is reduced to less than 1% by mass. Various methods of CNT purification have been proposed in the literature [49-52], which vary in the quality of purification and the extent of undesired CNT modification. For large volume industrial production one has to balance it with the cost effective scalability restrictions. OCSiAl has developed its own industrial scale processes of Tuball purification based on a combination of oxidative and acidic treatments and for purification of Tuball suspensions from magnetic and weakly magnetic impurities. [53] Table 1 compares the physicochemical properties of the standard Tuball and Ultrapure Tuball and next chapter discusses the details of distinction of these properties. It was also demonstrated that the commercial version of Tuball with <0.1% of impurities is a commercially viable product that will be available soon as well.   

Table 1. Summary of the main structural properties of Tuball SWCNTs.

*- the values may vary with exposure to humid environment but the shipped in a sealed container material has typically less than < 3 wt% of moisture.

**- viscosity and length measured in chlorosulfonic acid (CSA). 

Characterization of Tuball. 

Evaluation of Tuball purity

Electron microscopes (SEM and TEM) are among the direct methods for assessing the quality of single-walled carbon nanotubes. Shown in Fig. 3 SEM images of as-produced Tuball illustrate a uniform mesh of SWCNT bundles with diameters of up to 100 nm and rare impurities. A high degree of SWCNT aggregation is caused by the π - π interaction between graphene-like walls in SWCNT. According to the theoretical estimates [54,55], the binding energy between the neighboring SWCNTs is on the order of 1 eV per nanometer of length. For the bundles of several microns comprising thousands of nanotubes, this binding energy can exceed several MeV per bundle, which is the energy that would be required for separating them.   

Fig. 3.  SEM images of as produced Tuball at different scales.

  TEM images of TUBALLTM SWCNTs shown in Fig. 4 present a higher resolution of these bundles, where are indeed single walled nanotubes are clearly visible with diameters in the range 1.3 - 2.0 nm. There are also dark, roughly spherical features (impurities), mostly iron, as confirmed by EDX (energy-dispersive X-ray spectroscopy). The bundles have relatively clean surface corresponding to very low content of amorphous carbon impurity. The statistical analysis of such TEM images at high resolution gives the average diameter of nanotubes ca. 1.7 ± 0.3 nm, as shown in Fig. 5.  Note that there is obvious variation of the diameters from batch to batch and due to the technological developments aimed at improving the yield and purity of the product, but they all fall into the range given in Table 1. For comparison, HiPCO SWCNTs typically have smaller diameters of 0.8 - 1.1 nm and CoMoCATs SWCNTs are even smaller, 0.7 - 0.9 nm in diameter, while arc-discharge SWCNTs have diameters closer to those of Tuball, 1.2 - 1.7 nm.

Fig. 4.  High resolution TEM images of as produced Tuball at different scales. Residual particles of the catalyst are clearly visible as dark nanospheres.  

Fig. 5. TEM micrograph of a “cross-section” of a SWCNT bundle in as-produced Tuball and histogram of distribution of nanotubes’ diameters within the bundle.

Information on the distribution of nanotubes by diameter can be also retrieved from XRD (X-ray diffraction) patterns [56]. Hexagonally packed bundles of nanotubes give the diffraction peaks in the range of 2θ < 10o, the shape of the pattern depends on the bundle thickness besides the actual distribution of the nanotubes’ diameters, d. Comparison of the experimental and computed patterns showed that the diameters of SWCNTs in the bundle can be presented by lognormal distribution of Eq. (1): 

fd=1d2e-ln d -ln <d>+2/2 222 (1)

with the average diameter <d> and the standard deviation of logarithm σ. 

Fig. 6. XRD spectrum (A) and its fit (B) for a bundle of SWCNTs with distribution from Eq. (1) with lognormal distribution of 100 nanotubes with <d>=1.67 nm and σ = 0.22. The fit is shifted up for convenience. 

For example, Fig. 6 illustrates that for a typical unpurified sample with <d> = 1.67 nm and σ = 0.22. The corresponding mode diameter in this case is 1.63 nm.

Purified Ultrapure Tuball, presented in Fig. 7, shows practically no impurities and no other than nanotubes forms of carbon. A trained eye can recognize only one graphite-like spherical particle, likely leftover carbon shell of the removed iron nanoparticle, highlighted in Fig. 7a.

Fig. 7.  High resolution TEM images of Ultrapure Tuball. The arrow indicates a leftover carbon shell of a removed iron nanoparticle.

Overall high purity of the material is also confirmed by the thermal gravimetric analysis (TGA) in oxygen-containing gas flow (5% O2 in Ar), as shown in Fig. 8.  More illustrative is the derivative of the mass change (DTG), which for Tuball (Fig. 8a) shows relatively narrow two consecutive peaks of mass loss with maxima in the ranges 610-670°C and 700-820°C. Both peaks correspond to oxidation of SWCNTs and shift to higher temperatures with the increasing rate of temperature change. The residual mass after oxidation, ca. 20 wt%, corresponds to iron oxide produced from the residual iron catalyst in raw material. Formation of iron oxide is seen as the weight gain at 250-350 oC due to oxidation of the metallic iron particles. The amount of residual Fe2O3 matches that calculated for the 14 wt% of iron from EDX.    

Fig. 8.  Thermal gravimetric analysis (TGA) curves in 5% O2/Ar at different heating rates (2,3,10, and 20 K/min) and their derivatives (DTG) for (a) as-produced Tuball and (b) the same Tuball batch after purification, i.e., Ultrapure Tuball. 

The TGA and DTG curves of the purified Ultrapure Tuball SWCNTs, shown in Fig. 8b, are significantly different -- the effect of increasing weight is absent, in agreement with minute amount of iron. The ash residue for this particular sample is only 0.7 wt% of the original mass of Ultrapure Tuball, corresponding to the iron content ~ 0.4 wt%, consistent with the EDX  estimate of 0.46 wt%. Similarly revealing is that in the purified material the low-temperature mass loss is suppressed to a mere shoulder on the main high-temperature peak, which remains in the same range of 670-810 ° C.

Analysis of the exothermic reactions in a temperature-programmed mode is often analyzed using the Kissinger equation [57]:

ln Tm2 =-EaRTm+ const (2)

where β is the rate of temperature increase, Tm is the temperature of the maximum conversion rate for that β, and Ea is the activation energy of the reaction. Such dependences are shown in Fig. 9, from where the activation energies for the low temperature peak with Tuball is 292 kJ/mol and for the high temperature peak – 171 and 138 kJ/mol for Tuball and Ultrapure Tuball, respectively. The plausible reason for the difference between the last two activation energies is the peaks’ overlap for the unpurified Tuball. 

Fig. 9. Evaluation of the dependence of the maximum reaction rate temperature peak in accordance with the Kissinger equation (2) for as-produced Tuball (circles) and Ultrapure Tuball (triangles). The equations next the lines show parameters for linear approximations.

The disappearance of the low-temperature peak after purification indicates that this process is caused by the presence of iron oxide serving as a catalyst for the SWCNT oxidation, while the high-temperature one is due to the non-catalytic oxidation of SWCNTs. This hypothesis may seem contradicting the significantly lower activation energy of the high-temperature process, but it can be explained by the higher preexponential factor of the rate for the first peak. The direct high-temperature oxidation of SWCNTs proceeds on a very small number of sites, probably the edges or defects in SWCNTs, the concentration of which in Tuball is extremely low.

Apparently, the activation energy for reaction of molecular oxygen with the edge or defect carbons is much lower than the activation energy of oxidation of a regular graphene layer. The value is close to that of natural occurring graphite [58,59] or amorphous carbons [60], while the activation energy of oxidation of very pure well crystallized nuclear graphite is considerably higher [61]. The low-temperature peak in the region of 600–700 °C corresponds to SWCNT oxidation with the catalytic help of iron oxide nanoparticles. An important consequence of this conclusion is that, in the absence of the catalyst, an increasing rate of SWCNT oxidation and, therefore, lowering the characteristic temperature of SWCNT oxidation is an indicator of the high concentration of defects in its hexagonal structure, while lowering the number of such defects reduces the rate and increases the characteristic temperature of SWCNT oxidation.

Note that even the low-temperature oxidation peak for Tuball SWCNTs is observed at higher temperatures than those reported earlier in several publications for other SWCNT sources [30,34,35,62-66], including those produced by arc discharge [64-66], Co-Mo catalyst synthesis (CoMoCAT) [34,35], and high pressure CO disproportionation (HiPCO) [30,34,35]. The typical temperature of the maximum oxidation rate for them was reported between 360 and 550 °C. Nanotubes from SWeNT [67] are the only known to us exception - the maximum oxidation rate of their nanotubes was reported to be close to 630 °C, which is not far from the low-temperature peak in the unpurified Tuball, but still lower than the high-temperature peak for Ultrapure Tuball. This indicates a significantly better quality of Tuball and Ultrapure Tuball compared to other reported SWCNT products. The conclusion of a very high quality of Tuball and, especially, Ultrapure Tubal is supported by various spectroscopic techniques like EDX, Raman scattering, NIR-UV-Vis absorption, which is discussed next.

Vibrational and electron spectroscopy of Tuball. 

Raman spectrum of “as produced” Tuball SWCNTs at laser excitation wavelength 532 nm is presented in Fig. 10. The four major bands, the low frequency radial breathing mode (RBM), in the range of 100-250 cm-1, the strong (allowed) G mode near 1590 cm-1, equivalent to the tangential E2g stretching mode in graphite, the ideally forbidden but often observed at defects the so-called D mode at ~1340 cm-1, and its allowed overtone 2D mode, can be used for detailed characterization of the SWCNT quality and dimensions [68]. The D band is related to the defects and disorder of the continuous honeycomb carbon structure, which causes the lattice symmetry breaking and the relaxation of the selection rules. This band has strong dispersion as a function of laser excitation energy due to a double resonance process and its relative intensity increases with increasing the wavelength of laser excitation. In typical Tuball samples, the ratio of intensities of G and D modes (G/D) at 532 nm excitation in Fig. 10 is above 100, which is a clear indication of a low number of defects and a small amount of amorphous carbon in SWCNTs. The peak positions in the RBM band allow calculation of the representative nanotube diameters in the sample, although more accurate assessment requires analysis of a series of Raman spectra at various excitation wavelengths. The average diameter of nanotubes in Fig. 10 corresponds to dt ≈1.6 nm, as estimated from Eq. (3): [69]

  RBMcm-1248 dtindividual or RBMcm-1234 dt+10 (bundles) (3),

Information about the nanotube diameters theoretically can also be obtained from a more detailed analysis of the G band, which actually has multiple components: the G+ group is associated with atomic vibrations along the nanotube axis, and is independent of the tube diameter, whereas the G− group is associated with vibrations along the nanotube circumference, and its frequency decreases with decreasing diameter [70]: 

   G-cm-1 =1591- C/dt2 (4),

where C = 47.7 nm2/cm-1 for semiconducting and C = 79.5 nm2/cm for metallic nanotubes, respectively. Because of a distribution of SWCNTs by diameter and chirality, this approach is problematic for characterization of industrial size samples.

The Raman spectra of purified Ultrapure Tuball do not show recognizable difference from the original spectra of Tuball before purification. 

Fig. 10. Raman spectrum of Tuball SWCNT powder with excitation at 532 nm.
Characteristic bands are labeled: G, RBM, 2D, and practically invisible D.

Electron absorption in the visible and NIR regions carries information about the electronic structure of SWCNTs, which relates to their diameter and chirality [70,71]. For measuring that, SWCNTs can be dispersed in solution [72] or deposited on a transparent glass slide as a thin film. The example of Tuball SWCNTs’ electron spectrum is shown in Fig. 12a. 

Fig. 11. a) Optical absorption spectra of Ultrapure Tuball suspension in water with PVP (blue line), and as a dry film on a slide before (black line) and after exposure to doping by HClO4 (red). Spectra above 4000 cm-1 were taken on a glass slide and an identical film on Si (below 5000 cm-1). The absorption features labeled S11, S22, and M11 are clearly displayed. b) Concentration dependence of the absorbance at 500 nm for solutions of Ultrapure Tuball with poly(vinylpyrrolidone) (PVP) dispersant (PVP : Tuball = 2). In each case, 2kWh/L of ultrasound energy was used for dispersion. The red dashed line corresponds to 5.9 m2/g cross-section; the black solid line is a guide to eye.

To interpret the spectrum, we refer to Fig. 12 illustrating how the density of states (DOS) in SWCNT changes with its chirality. One can see that both metallic and semiconducting nanotubes have well pronounced maxima of the so-called van Hove singularities and the transitions between them, S11 and S22 for semiconducting and M11 - for metallic, have a well-defined correlation with the nanotubes’ diameters, dt, The correlation cannot be treated as exact but, because of a relatively broad distribution of nanotubes’ diameters, it can be used as a good characterization of that distribution with the most representative diameter roughly corresponding to the energy of S11 and M11 transitions from Eq. (5):  

 E11S20aC-Cdt   and   E11M60aC-Cdt   (5),

for semiconducting and metallic nanotubes, respectively, where γ0 = 3 eV and the bond length, aC-C = 0.142 nm [71].          

Fig. 12. Density of states (DOS) for a) armchair (10, 10) SWCNT, b) chiral (11, 9) SWCNT, and c) zigzag (22, 0) SWCNT. d) The resonant electronic transition energies, Eii, for all the (n,m) SWCNTs with diameters between 0.4 and 3.0. Reproduced with permission from [73].

Note that the characteristic S11 and S22 bands of semiconducting nanotubes in Fig. 11 are quite broad, representing a broad distribution of the nanotube diameters, which also prevents the distinction of their chiralities. Nevertheless, the similarly broad M11 band indicates the presence of metallic nanotubes with close to the statistical abundance of 1/3. Such a statistical representation is typical for Tuball, although some regimes of synthesis allow modest shifting from such a statistical representation.

Optical absorption can also be used for measuring SWCNT concentration in solution, i.e., characterization of the quality and stability of SWCNT dispersions. For the often-used wavelength 500 nm, the absorbance is in the pseudo continuous spectral region and thus is less sensitive to the diameter distribution. From the concentration dependence, as in Fig. 11b, we measured the extinction coefficient, ε500 = 5.9 ± 0.5 m2/g, a slightly higher than the previously reported [74, 75]. The value corresponds to SWCNTs in water with either poly(vinylpolypyrrolidone) (PVP) or sodium dodecylbenzenesulfonate (SDBS) achieved upon increasing the dispersing energy of treatment by ultrasound (sonication). 

The low frequency (far infrared) part of the spectrum, often referred to as ‘intraband’ transitions, as opposed to the ‘interband’ transitions associated with S11, S22, M11, etc. The interband peaks’ intensities depend on the concentrations of free carriers and decrease with doping of nanotubes, as shown in Fig. 11a with the perchloric acid dopant. The other features in this region are defined by the lengths of individual nanotubes, their bundles, and their distribution. The broad peak maximum at low frequencies for individual noncontacting nanotubes and/or noncontacting bundles could be related to their lengths; presumably the maximum optical density should be at approximately 3-4 times their length, i.e., at frequencies <400 cm-1, outside the region presented in Fig. 11, but it is difficult to realize. Instead, for overlapping SWCNTs and bundles, when the films are conductive, the optical density at very low frequencies is stronger and broader and shows no maximum [76]. Correspondingly, the low frequency absorption is increased for more conductive film after doping, like with HClO4 in Fig.11a.  If nanotubes are chemically functionalized (not the case here) the characteristic peaks for IR allowed vibrational transitions can be observed as well. Pristine nanotubes do not have such vibrations. 

Note that the intensities of interband van Hove peaks are suppressed upon doping with a degree that depends on the redox capabilities of the dopant and its concentration. Some acids, like perchloric acid in Fig. 11a, are very good p-dopants but even ambient oxygen or some solvents can affect the intensity of S11 transition. For that reason also, calibration of SWCNT concentrations is more convenient at higher frequencies/shorter wavelength, like the suggested above 500 nm.       

 Electrical and thermal conductivity of Tuball.

As was reported previously for experiments with individual single-walled carbon nanotubes, their electrical conductivity can exceed that of copper and silver [77-79] and the thermal conductivity along their axes surpasses that of diamond [80]. However, these properties, determined by the electronic structure, are also affected by the presence of defects and functional groups, as well as by impact of the environment. The electrical and thermal conductivities of macroscopic ensembles of SWCNTs, e.g., in ropes, buckypaper or in more diluted composites, also depends on the distribution of SWCNTs by their chirality, concentration, spatial spreading or topology of the conducting network, as well as by their doping. A mixture of metallic and semiconducting SWCNTs with diameters > 1.2 nm in typical Tuball samples is present in a statistical ratio of 1: 2 [81]. Obviously, SWCNTs having different types of conductivity differ in the magnitude of the resistivity and its change with temperature -- the resistance of metallic nanotubes increases with temperature, while for semiconducting it decreases [82]. As discussed above, the nanotubes are often assembled into bundles because of the strong van der Waals interaction between them. Therefore, in many cases, the characteristics of such bundles, and not just of the nanotubes themselves, are affecting the overall performance. 

The effect from adding carbon nanotubes as a filler in various matrices would be strongly dependent on how uniform is their distribution over the volume of matrix, which makes the means of dispersing nanotubes of great importance. Dispersion to individual nanotubes requires vary intense processing and is rarely achieved in practice. Typically, the dispersions of SWCNTs consist of the nanotube bundles with dimensions dependent on the processing conditions, the nature of the matrix, and the dispersing agents used. These issues are discussed in detail in Part 2 [1]. For different applications, the optimal degree of debundling (the length and diameter of the bundles) may vary as well. Therefore, the characteristics of the obtained composites significantly differ from the ideal model of the individual SWCNTs network.

One of the convenient ways to obtain individual SWCNTs is through dissolving them in chlorosulfonic acid (CSA), a superacid, as described previously [83,84]. Strong doping of SWCNTs with protons makes them charged as a result and practically separates/dissolves into individual nanotubes with the help of very mild additional mechanical energy. At ambient conditions, the superacid decomposes causing nanotubes to reagglomerate but leaves remnant doping on them or it can be also totally removed.  Macroscopic fibers can be drawn with oriented nanotubes from such solutions. They were reported to have a very high conductivity after doping with iodine vapor to exceed 8.5 MS/m (about 15% of that for copper) and the tensile strength exceeding 2.4 GPa [84]. For comparison, the same group reported that fibers drawn from an aqueous dispersion of functionalized SWCNTs had an order of magnitude lower conductivity, 0.58 MS/m and the strength of ‘only’ 1 GPa [85].

Using SWCNT solutions in superacid, one can also deposit thin conductive films on various substrates. For example, conductive films of SWCNTs with surface resistivity, Rs = 850 Ω/sq, and transmission in the visible range T = 97% were reported in [86]. Using these parameters allows obtaining the insightful characteristic, the ratio of the optical conductivity, σop, and dc current conductivity, σdc, from Eq. (6):  

(6), where Z0 is the so-called characteristic impedance of free space, Z0 = (μo/εo)1/2 ≈ 377 Ω. The ratio, σop/σdc = 0.042 ± 0.013, obtained from the above Rs, corresponds to σdc = 0.48 MS/m and σop = 0.02 MS/m [87]. The ratio σop/σdc is the figure of merit for transparent conductive films, which strongly depends on the SWCNT quality. When Tuball is used in a similar approach, i.e., depositing on a quartz substrate from the CSA solution using doctor blade method and further reducing with acetone, the results shown in Fig. 13 appear highly competitive. The obtained from the fit value of σop/σdc = 0.0053 ± 0.001 corresponds to σdc = 3.8 MS/m, which, to our knowledge, is the best ever reported and indicates a high degree of purity, sufficient length, and minute amount of defects for Tuball SWCNTs. The resulting films have a significant anisotropy -- resistances along and across the directions of deposition differ by a factor of 10. The optical absorbance is similarly anisotropic. Similar studies with SWCNT Tuball in literature [88] are less impressive, with Rs = 60 Ω/sq at 90% transmittance compared to our 18 Ω/sq. Despite the high orientation of nanotubes in the film, its maximal conductivity is approximately 25 times lower than the expected based on the conductivity of individual SWCNTs. The film resistance is mostly determined by the contacts between nanotubes [87] and it remains so even in the films with highly oriented nanotubes. In any case, such conductive windows are quite competitive with the current state of the art indium tin oxide (ITO), which is hard to prepare on flexible polymeric substrates.                               

Fig. 13. Transmittance, T, at 550 nm as a function of the surface resistance, Rs, for films prepared from Tuball dissolved in superacid. Solid line is the best fit using Eq. (6) with σop/σdc = 0.005. 

The advantages of stretchable conductive films can be fully realized on stretchable substrates, as was demonstrated in recent works [89,90] with SWCNT films produced using commercial Tuball ink, an aqueous dispersion of Tuball SWCNTs with surfactant. Another illustration of a conductive SWCNT coating that is potentially useful in designing antiviral/antibacterial masks is given in Fig. 14. Tuball suspension with SDBS sprayed over N95 polypropylene filter after washing off SDBS retains its hydrophobic character but gains a conductance that can be adjusted to a desired level. For example, the coating in Fig. 14 has 400 Ω/sq, which is sufficient for heating above 80 oC by modest 40 V AC. This temperature is sufficient for elimination of practically all bacteria and viruses in a few minutes.                                   

Fig. 14. Nonwoven polypropylene N95 filter with CNT coating obtained by spraying (400 Ω/sq in this case). The electrodes allow applying heating by AC current and the IR imaging on the right demonstrate that the temperature can exceed 85 oC. 

High values of light transmission in conductive windows require thin films of nanotubes, 0.01-0.1 μm thick. Films with 30-100 μm thickness can be also produced aiming at applications for high conductance and strength. Films of that thickness are optically opaque and often nicknamed buckypaper [91]. Different methods can be employed for that, e.g., doctor blade, vacuum filtration, and others. A liquid dispersion of SWCNTs for casting films can be prepared in various solvents with and without surfactants or additional polymer matrix, all of which affect the properties of the resulting buckypaper. Good quality dispersions are difficult to obtain in typical solvents without surfactants, but sufficient quality can be achieved in some organic solvents such as N-methylpyrrolidone (NMP), dimethyl formamide (DMF), dichlorobenzene, isopropanol, and others [92].

For aqueous solutions, surfactants like sodium dodecyl benzene sulfonate (SDBS) improve the SWCNT solubility but, upon film drying, the surfactant needs to be removed as it contributes to the contact resistance between the nanotubes. The density of films casted from organic solvents depends on the capillary forces induced shrinkage and thus correlates with SWCNT solubility in them. Since the contact resistance between nanotubes and a significant fraction of semiconducting nanotubes (~2/3) are strongly contributing factors, appropriate doping improves the film conductivity, which can reach > 1.3 MS/m. Even greater conductivity improvement, >2.5 MS/m, can be achieved by better orienting nanotubes in the buckypaper [91] and should eventually achieve that of SWCNT fibers, up to 4.3-5.6 MS/m, as shown in Fig. 15 [88,93]. High degree of orientation in neat SWCNT fiber or film effectively reduces the number of cross contacts between them but extends the side-by-side contacts that have lower contact resistance. Among the different sources of SWCNTs, Tuball SWCNT, additionally purified in Rice University, showed 4.3 MS/m conductivity and 0.9 GPa strength.  Slightly better values in Fig. 15 are for double walled CNTs can be attributed to their typically greater lengths.                  

Fig. 15. Experimental tensile strength versus electrical conductivity for fibers from CNT arrays (black circles), CVD floating catalyst CNT fibers (blue triangles), carbon fiber (green diamonds), wet spinning CNT fiber (yellow plus signs), CSA solution-spun CNT fiber (red squares), and hand-twisted CNT fibers (red stars). The green arrow points to data with Tuball (spun from chlorsulfonic acid without twisting). Reprinted with permission from [88].

Measuring the length of nanotubes is notoriously difficult – they need to be separated first before the length can be measured because spectroscopically it is not identifiable, unlike the diameter. Separation of nanotubes from their bundles requires energy, application of which affects their length; they become shorter [94]. Even mild exposure to ultrasound cleaning bath was reported to break apart double walled nanotubes [95]. The only known relatively benign option is dissolution in chlorosulfonic acid (CSA), where complete separation of nanotube bundles without ultrasound can take days or more depending on their lengths and density/type of defects [88,93]. The resulting solution can be analyzed by such methods as extensional viscosity [88,93] and dynamic light scattering (DLS). The length of Tuball nanotubes varies from batch to batch but is typically exceeding 5 μm. In [88] it was measured that the aspect ratio for purified Tuball was L/d ~ 2940, which for the diameter d ~1.8 nm, corresponds to L ~ 5.3 μm. A similar batch characterized in [93] gave L/d ~ 3100 before purification and dropped down to 2310 and 1260 upon purification using their methods.  Figure 16 shows our own measurements using dynamic light scattering (DLS) on a batch of Tuball dissolved in CSA. At low concentrations, dispersion of SWCNTs in CSA is a true solution and its scattering pattern in DLS can be used for characterizing the ensemble of individual SWCNTs. The standard analysis of the correlation curve provides a histogram of hydrodynamic diffusion coefficients, Dh, which can be related to particles of different lengths, L, for a given diameter, d, using Eq. (7) [96]: 

Dh=kBTln(L/d)+0.313L    (7),

where η is the viscosity. Such a model works relatively well for L/d >>1, and L > 1 μm, when (nL/λ) sin(θ/2)  > 1, where θ is the scattering angle of the laser light at the wavelength λ and n is the solution refractive index at this wavelength. Eq.(7) presumes a rigid rotor approximation, which actually underestimates the length as a result. The rigidity of nanotubes grows as ~ d3 and for the typical diameters of Tuball, d ~1.5 nm, approximation of the rigid rotors is thought to be suitable up to L < 10 μm [97]. The matter can be verified by employing DLS at different scattering angles [97,98] and we intend to present detailed study on the subject in the near future. Briefly, the correlation curve and the corresponding effective diffusion coefficients are least affected by the flexibility at small scattering angles θ. The discrepancy can be quite large for measurements at higher angles.  Figure 16 shows autocorrelation curves for the dynamic light scattering of 0.0002 wt% Tuball dispersion in chlorosulfonic acid at two scattering angles, θ =13° and θ =173°. The fit for θ =13° gives the diffusion coefficient Dh = 2.5 ×10-9 cm2/s and the corresponding from Eq.(7) length is L = 12 μm. The fit for θ =173° requires two exponentials and the analysis is more complicated. Note that θ =173° is the default scattering angle on a Zetasizer Nano from Malvern used in the study and its use makes the analysis not obvious. The longer exponential is concentration dependent and is due to entanglements of long rods,[98] its magnitude declines with dilution. The shorter exponential for rigid rods should be ‘faster’ by the factor of sin2(θ/2) difference, but it is actually even faster. The discrepancy is due to noticeable flexibility of such long SWCNTs.   

Fig. 16. Autocorrelation curves for the dynamic light scattering (DLS) of 0.0002 wt% Tuball dispersion in chlorosulfonic acid at two scattering angles, θ =13°(red points) and θ =173° (black points). The solid curves represent the best fits with exponentials for each. The fit for θ =13° corresponds to L = 12 μm. See text for details.   

The lengths can vary from batch to batch and due to the technological developments aimed at improving the yield and purity of the product, but they always fall into the range >5 μm given in Table 1 and the majority is greater than 10 μm.   

The greatest advantage of employing high SWCNT conductance is using them as additive to give composite materials antistatic and conductive properties. The advantage of carbon SWCNTs over other types of nano-fillers is their high aspect ratio for nanotubes, which allows much lower concentrations necessary to reach the electrical percolation threshold not affecting negatively the original color. The use of Tuball in conductive composites is discussed in Part 2 [1].

High thermal conductivity, which is not due to conducting electrons, is another unique property of CNT. Pop et al. [80] reported that suspended metallic SWCNTs with diameters near 1.7 nm have intrinsic thermal conductivity near κ = 3500 Wm-1K-1 at room temperature, where it presumably has a maximum. They stated that it varied within ca. 20% for other chiralities and the length becomes of importance for L < 2.5 μm. This thermal conductivity is higher than that of a carbon fiber, κ = 620 Wm-1 K-1 [99], and even of graphene. Figure 17 compares κ for SWCNT, MWCNT, and graphene of different forms, single layer (SLG), bilayer (BLG), and triple layer (3LG), measured by different groups. 

Thermal conductivity in SWCNT bundles is greatly impeded and, according to Feng et al. [100], it declines from 4500 Wm-1 K-1 for isolated SWCNTs at room temperature down to 1800 Wm-1 K-1 for 4-SWCNTs bundle and further to 280 and 180 Wm-1 K-1 for 8-SWCNTs and 13-SWCNTs bundles, respectively.                      

Fig. 17. Experimental thermal conductivity, κ, as a function of temperature T for carbonaceous materials. Reprinted with permission from [101]. 

Employing Tuball in production of buckypaper with unaligned nanotubes gave κ = 30 Wm-1K-1 but for aligned by magnetic field nanotubes κ was significantly greater, 200 Wm-1K-1 [102]. All of this indicates that the thermal conductivity also is very much affected by the contacts between CNTs. Indeed, pressure induced density increase in aligned SWCNT buckypaper raised from κ = 472 Wm-1K-1 for density ρ = 0.8 g/cm3 to κ = 766 W·m-1K-1 at ρ = 1.39 g/cm3 [103]. Note that the graph in Fig. 16 for SWCNTs extends up to ~500 oC, illustrating gradual decline of κ, but SWCNTs can actually withstand up to 1000 oC beyond which they start converting into MWCNTs [104].

As with electrical conductivity, addition of small amounts of SWCNTs can have positive impact on the thermal conductance in composite materials. In [105] it was shown that the addition of Tuball linearly improves the thermal conductance of dispersions up to 17% at 0.2 wt% of Tuball and addition to ionic liquids increases their thermal conductance by up to 68% at 1 wt% of Tuball [106]. 

Mechanical properties of Tuball.

The mechanical properties of composites can be enhanced by addition of small amounts of SWCNT. It will be discussed in detail in Part 2 [1] but here we focus on the mechanical properties of ‘neat’ SWCNT samples. According to Fig.15, the strength of SWCNT fibers strongly correlates with their electrical conductivity. It emphasizes the importance of good contacts between CNTs for both properties. The contacts are even more important for mechanical strength and the authors of [101] found it rising as a power 1.2 in correlation with the conductivity. Notably, the maximum tensile strength values do not exceed 7 GPa, which is far less than the values for individual CNT and comparable to those of carbon fibers. Nevertheless, bundles of extremely long nanotubes can be quite strong; in [105] it was measured to increase from 50 GPa to ~80 GPa for bundles of a few nanotubes, see Fig.18. By a stretching treatment, which eliminated weak nanotubes in a bundle, the authors were able to narrow the distribution down and improve the strength of even large bundles to 80 GPa, i.e., corresponding to the strength of individual nanotubes. It exceeds the claimed 30-50 GPa strength of single nanotubes of different chiralities reported in [28] and likely illustrates the sensitivity of the measured values to the residual defects in nanotubes.               

Fig. 18. The experimental (EV) and theoretical (TV) values for the mean tensile strength of CNT bundles vs the number of nanotubes in it before and after stretching treatment (STR) for very long CNT. Statistic symbols at the bottom are the values for CFs (carbon fibers) and bindles of shorter nanotubes prepared by different techniques: spinning from aerogels (ACNTFs), spinning from vertically aligned CNT arrays (VACNTFs), and spinning from CNT solutions (SCNTFs). Reproduced with permission from [107].

Health and Safety aspects of Tuball.  

The broad industrial acceptance of the Tuball materials demands evaluation of the health concern regarding its use and the highly reproducible quality of Tuball allows assessment of the SWCNTs effect without compromising it by other factors. In November 2014, the International Agency for Research on Cancer Working Group (IARC WG) evaluated the carcinogenicity of CNTs and classified a type of MWCNTs as a group 2B agent (possibly carcinogenic to humans), with limited evidence for other similar MWCNT types and no evidence of carcinogenicity for SWCNTs [108]. It was highlighted that conclusions cannot be extrapolated to SWCNTs. It should be noted that the number of studies on health effects of SWCNTs is still limited and, unfortunately quite often, it is presumed that the effects for SWCNTs and MWCNTs are similar, which is incorrect. For example, MWCNTs induced an innate immune response at subtoxic doses on mouse macrophage cells suggesting their exposure may result in chronic inflammation and compromised immunity but no such effect was observed for SWCNTs [109]. Another illustrative comparison can be found in two articles authored by a group from multiple Japanese Universities [110,111] who evaluated pulmonary toxicity of well-dispersed single-walled and multi-walled carbon nanotubes after inhalation and intratracheal instillation. They found that SWCNTs did not induce neutrophil inflammation in the lung [110] while MWCNTs induced a pulmonary inflammation, as evidenced by a transient neutrophil response in the low-dose groups, and presence of small granulomatous lesion and persistent neutrophil infiltration in the high-dose groups [111]. They also found that well-dispersed MWCNT can produce pulmonary lesions, including inflammation, thus concerns for their safe use should not be discarded.

The data on SWCNTs are often contradictory, which is mostly related to the difference in quality and purity of studied materials. Early SWCNT-containing materials were often compromised by major impurities of highly active catalyst nanoparticles (e.g., cobalt, manganese, nickel, and other), other carbon allotropes, and polynuclear aromatic hydrocarbons. A recent review [112] tried to analyze data on toxicity of SWCNTs and found them ambiguous and discording: some studies showed cytotoxicity of SWCNTs, while others didn’t show any, even at very high exposures. Unfortunately, in a broader most recent review on toxicity of carbon nanomaterials there is not much information added on SWCNTs and no studies of Tuball can be found discussed either [113]. Nevertheless, there is sufficient amount of literature data confirming that the use of SWCNTs does not bring significant health risks, unlike the case of MWCNTs. For example, it was shown [114-115] using Comet Assay study that SWCNTs and Tuball in particular did not induce DNA damage in the lung cells of rats intratracheally instilled, even at doses that elicited both acute and subacute inflammatory responses concluding that Tuball has no potential for genotoxicity in vivo. 

Similarly, in a study performed in accordance with OECD (The Organization for Economic Co-operation and Development) Guideline 431 on evaluating the corrosivity potential using the EpiDerm™ Human Skin Model after treatment for 3 and 60 minutes, Tuball was found to be neither corrosive nor irritant to skin [116]. Furthermore, Buehler tests performed on guinea pigs in accordance with OECD Guideline 406 found no indication of skin sensitization by Tuball [117]. 

According to [118] SWCNTs of a similar to Tuball dimensions had no mutagenicity in S. typhimurium TA98, TA100, TA1535 or TA1537, or in E. coli WP2uvrA, in the absence or presence of metabolic activation. SWCNTs did not increase the chromosomal aberrations after short-term or continuous exposure. In the micronucleus test using CD-1 mice, SWCNTs did not affect the proportion of immature erythrocytes, the total proportion of erythrocytes or the number of micronuclei in immature erythrocytes. In other words, it was found that SWCNTs did not pose a genotoxic risk.

Inhalation exposure concerns are driven by the industry’s interest in adopting new nanomaterials but recent tests on Tuball inhalation for rats performed at BASF failed to give any negative data, since all standard aerosol generation methods failed to produce respirable aerosol from it [119]. This distinguishes Tuball SWCNTs from other carbon allotropes, such as carbon black, MWCNTs, and graphene platelets, aerosols for which were easily generated. There is even more limited published information on potential toxicity of materials with SWCNTs, which should be addressed more aggressively. But in one study [120] involving SWCNT-polymer nanocomposite it was found that the detected volatile particles in the range 10–1000 nm, released by the friction heat of the grinding process, had no free standing SWCNTs. 

Nevertheless, there are illustrations of beneficial medical use of Tuball. Medical sorbents based on porous aluminum oxide with small amounts of Tuball significantly increase the sorption of undesired chemicals but have no toxic effect on the proliferation of bone marrow stem cells and endothelial cells, yet they promote stable production of metabolites by cells. Such sorbents were suggested for use as carriers for active substances (e.g., vitamins) and simultaneous performing the functions of detoxicants [121].

The exact mechanism of the biological impact of CNTs on living organisms has not been fully elucidated yet but it obviously differs for SWCNTs and MWCNTs, and other carboneous nanomaterials. A recent review [122] summarized studies illustrating that CNTs can be biodegraded by macrophages through enzymatic oxidation and SWCNTs biodegrade with a higher rate than MWCNTs. Many additional factors such as length, surface functionalization, and impurities can also influence their biodegradation, for example, carboxylation significantly enhances this rate. 

In September 2016, Tuball SWCNTs became the first in this category brand registered in accordance with the European Union REACH regulation (ECHA registration number 01-2120130006-75-0000, EC number: 943-098-9), which attests a safe use of SWCNTs and permits their broad production and utilization at industrial volumes in Europe [123].

Conclusions 

Chemical vapor deposition has become a very competitive method for production of single walled carbon nanotubes (SWCNT) in large quantities and exceptional quality. OCSiAl is not the only company capitalizing on the industrial applications of SWCNTs. So far, it is leading the pack - it has been estimated to hold over 90 % of the world production capacity of SWCNTs but newcomers and established players do not stay idle.  One thing is certain, the abundance of high quality SWCNTs on the market illustrates that the time for their industrial applications has arrived and this commodity is here to thrive. It is indeed a new wonder material with a list of exciting properties that open new opportunities in material science, engineering, electronics, medicine, and many others.  There will be more players and new applications yet developed but, as it is discussed in Part 2 [1], even already realized applications open new opportunities in multiple fields.

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