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Carbon Nanomaterials
Andrew R. Barron
This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License †
1 Introduction
Although nanomaterials had been known for many years prior to the report of C 60 the eld of nanoscale science was undoubtedly founded upon this seminal discovery. Part of the reason for this explosion in nanochemistry is that while carbon materials range from well-de ned nano sized molecules (i.e., C 60 ) to tubes with lengths of hundreds of microns, they do not exhibit the instabilities of other nanomaterials as a result of the very high activation barriers to their structural rearrangement. As a consequence they are highly stable even in their unfunctionalized forms. Despite this range of carbon nanomaterials possible they exhibit common reaction chemistry: that of organic chemistry. The previously unknown allotrope of carbon: C 60 , was discovered in 1985, and in 1996, Curl, Kroto, and Smalley were awarded the Nobel Prize in Chemistry for the discovery. The other allotropes of carbon are graphite (sp^2 ) and diamond (sp^3 ). C 60 , commonly known as the buckyball or Buckminsterfullerene, has a spherical shape comprising of highly pyramidalized sp^2 carbon atoms. The C 60 variant is often compared to the typical soccer football, hence buckyball. However, confusingly, this term is commonly used for higher derivatives. Fullerenes are similar in sheet structure to graphite but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar. The unusual structure of C 60 led to the introduction of a new class of molecules known as fullerenes, which now constitute the third allotrope of carbon. Fullerenes are commonly de ned as any of a class of closed hollow aromatic carbon compounds that are made up of twelve pentagonal and diering numbers of hexagonal faces. The number of carbon atoms in a fullerene range from C 60 to C 70 , C 76 , and higher. Higher order fullerenes include carbon nanotubes that can be described as fullerenes that have been stretched along a rotational axis to form a tube. As a consequence of dierences in the chemistry of fullerenes such as C 60 and C 70 as compared to nanotubes, these will be dealt with separately herein. In addition there have also been reports of nanohorns and nano bers, however, these may be considered as variations on the general theme. It should be noted that fullerenes and nanotubes have been shown to be in ames produced by hydrocarbon combustion. Unfortunately, these naturally occurring varieties can be highly irregular in size and quality, as well as being formed in mixtures, making them unsuitable for both research and industrial applications.
2 Fullerenes
Carbon-60 (C 60 ) is probably the most studied individual type of nanomaterial. The spherical shape of C 60 is constructed from twelve pentagons and twenty hexagons and resembles a soccer ball (Figure 1a). The next stable higher fullerene is C 70 (Figure 1b) that is shaped like a rugby or American football. The progression of higher fullerenes continues in the sequence C 74 , C 76 , C 78 , etc. The structural relationship between each
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involves the addition of six membered rings. Mathematically (and chemically) two principles de ne the existence of a stable fullerene, i.e., Euler's theorem and isolated pentagon rule (IPR). Euler's theorem states that for the closure of each spherical network, n (n ≥ 2) hexagons and 12 pentagons are required while the IPR says no two pentagons may be connected directly with each other as destabilization is caused by two adjacent pentagons.
Figure 1: Molecular structures of (a) C 60 and (b) C 70.
Although fullerenes are composed of sp^2 carbons in a similar manner to graphite, fullerenes are soluble in various common organic solvents. Due to their hydrophobic nature, fullerenes are most soluble in CS 2 (C 60 = 7.9 mg/mL) and toluene (C 60 = 2.8 mg/mL). Although fullerenes have a conjugated system, their aromaticity is distinctive from benzene that has all C-C bonds of equal lengths, in fullerenes two distinct classes of bonds exist. The shorter bonds are at the junctions of two hexagons ([6, 6] bonds) and the longer bonds at the junctions of a hexagon and a pentagon ([5,6] bonds). This dierence in bonding is responsible for some of the observed reactivity of fullerenes.
2.1 Synthesis of fullerenes
The rst observation of fullerenes was in molecular beam experiments at Rice University. Subsequent studies demonstrated that C 60 it was relatively easy to produce grams of fullerenes. Although the synthesis is relatively straightforward fullerene puri cation remains a challenge and determines fullerene's commercial price. The rst method of production of measurable quantities of fullerenes used laser vaporization of carbon in an inert atmosphere, but this produced microscopic amounts of fullerenes. Laboratory scales of fullerene are prepared by the vaporization of carbon rods in a helium atmosphere. Commercial production ordinarily employs a simple ac or dc arc. The fullerenes in the black soot collected are extracted in toluene and puri ed by liquid chromatography. The magenta C 60 comes o the column rst, followed by the red C 70 , and other higher fullerenes. Even though the mechanism of a carbon arc diers from that of a resistively heated carbon rod (because it involves a plasma) the He pressure for optimum C 60 formation is very similar. A ratio between the mass of fullerenes and the total mass of carbon soot de nes fullerene yield. The yields determined by UV-Vis absorption are approximately 40%, 10-15%, and 15% in laser, electric arc, and solar processes. Interestingly, the laser ablation technique has both the highest yield and the lowest productivity and, therefore, a scale-up to a higher power is costly. Thus, fullerene commercial production is a challenging task. The world's rst computer controlled fullerene production plant is now operational at the MER Corporation, who pioneered the rst commercial production of fullerene and fullerene products.
like mercury to form C 60 -^ and C 60 2-. In addition, salts can also be synthesized with organic molecules, for example [TDAE+][C 60 - ] possesses interesting electronic and magnetic behavior. Geometric and electronic analysis predicted that fullerene behaves live an electro-poor conjugated poly- ole n. Indeed C 60 and C 70 undergo a range of nucleophilic reactions with carbon, nitrogen, phosphorous and oxygen nucleophiles. C60 reacts readily with organolithium and Grignard compounds to form alkyl, phenyl or alkanyl fullerenes. Possibly the most widely used additions to fullerene is the Bingel reaction (Figure 3), where a carbon nucleophile, generated by deprotonation of α-halo malonate esters or ketones, is added to form a cyclopropanation product. The α-halo esters and ketones can also be generated in situ with I 2 or CBr 4 and a weak base as 1,8-diazabicyclo[5.4.0]unde-7ene (DBU). The Bingel reaction is considered one of the most versatile and e cient methods to functionalize C 60.
Figure 3: Bingel reaction of C 60 with 2-bromoethylmalonate.
Cycloaddition is another powerful tool to functionalize fullerenes, in particular because of its selectivity with the 6,6 bonds, limiting the possible isomers (Figure 4). The dienophilic feature of the [6,6] double bonds of C 60 enables the molecule to undergo various cycloaddition reactions in which the monoadducts can be generated in high yields. The best studies cycloadditon reactions of fullerene are [3+2] additions with diazoderivatives and azomethine ylides (Prato reactions). In this reaction, azomethine ylides can be generated in situ from condensation of α-amino acids with aldehydes or ketones, which produce 1,3 dipoles to further react with C 60 in good yields (Figure 5). Hundreds of useful building blocks have been generated by those two methods. The Prato reactions have also been successfully applied to carbon nanotubes.
Figure 4: Geometrical shapes built onto a [6,6] ring junction: a) open, b) four-membered ring, c) ve-membered ring, and d) six-membered ring.
Figure 5: Prato reaction of C 60 with N-methyglycine and paraformaldehyde.
The oxidation of fullerenes, such as C 60 , has been of increasing interest with regard to applications in photoelectric devices, biological systems, and possible remediation of fullerenes. The oxidation of C 60 to C 60 On (n = 1, 2) may be accomplished by photooxidation, ozonolysis, and epoxidation. With each of these methods, there is a limit to the isolable oxygenated product, C 60 On with n < 3. Highly oxygenated fullerenes, C 60 On with 3 ≤ n ≤ 9, have been prepared by the catalytic oxidation of C 60 with ReMeO 3 /H 2 O 2.
3 Carbon nanotubes
A key breakthrough in carbon nanochemistry came in 1993 with the report of needle-like tubes made ex- clusively of carbon. This material became known as carbon nanotubes (CNTs). There are several types of nanotubes. The rst discovery was of multi walled tubes (MWNTs) resembling many pipes nested within each other. Shortly after MWNTs were discovered single walled nanotubes (SWNTs) were observed. Single walled tubes resemble a single pipe that is potentially capped at each end. The properties of single walled and multi walled tubes are generally the same, although single walled tubes are believed to have superior mechanical strength and thermal and electrical conductivity; it is also more di cult to manufacture them. Single walled carbon nanotubes (SWNTs) are by de nition fullerene materials. Their structure consists of a graphene sheet rolled into a tube and capped by half a fullerene (Figure 6). The carbon atoms in a SWNT, like those in a fullerene, are sp2 hybridized. The structure of a nanotube is analogous to taking this graphene sheet and rolling it into a seamless cylinder. The dierent types of SWNTs are de ned by their diameter and chirality. Most of the presently used single-wall carbon nanotubes have been synthesized by the pulsed laser vaporization method, however, increasingly SWNTs are prepared by vapor liquid solid catalyzed growth.
Figure 7: TEM image of an individual multi walled carbon nanotube (MWNTs). Copyright of Nanotech Innovations.
Figure 8: SEM image of vapor grown carbon nano bers.
3.1 Synthesis of carbon nanotubes
A range of methodologies have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPco), and vapor liquid solid (VLS) growth. All
these processes take place in vacuum or at low pressure with a process gases, although VLS growth can take place at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making SWNTs more commercially viable. The rst observation of nanotubes was in the carbon soot formed during the arc discharge production of fullerenes. The high temperatures caused by the discharge caused the carbon contained in the negative electrode to sublime and the CNTs are deposited on the opposing electrode. Tubes produced by this method were initially multi walled tubes (MWNTs). However, with the addition of cobalt to the vaporized carbon, it is possible to grow single walled nanotubes. This method it produces a mixture of components, and requires further puri cation to separate the CNTs from the soot and the residual catalytic metals. Producing CNTs in high yield depends on the uniformity of the plasma arc, and the temperature of the deposit forming on the carbon electrode. Higher yield and purity of SWNTs may be prepared by the use of a dual-pulsed laser. SWNTs can be grown in a 50% yield through direct vaporization of a Co/Ni doped graphite rod with a high-powered laser in a tube furnace operating at 1200 ◦C. The material produced by this method appears as a mat of ropes, 10 - 20 nm in diameter and up to 100 μm or more in length. Each rope consists of a bundle of SWNTs, aligned along a common axis. By varying the process parameters such as catalyst composition and the growth temperature, the average nanotube diameter and size distribution can be varied. Although arc-discharge and laser vaporization are currently the principal methods for obtaining small quantities of high quality SWNTs, both methods suer from drawbacks. The rst is that they involve evaporating the carbon source, making scale-up on an industrial level di cult and energetically expensive. The second issue relates to the fact that vaporization methods grow SWNTs in highly tangled forms, mixed with unwanted forms of carbon and/or metal species. The SWNTs thus produced are di cult to purify, manipulate, and assemble for building nanotube-device architectures for practical applications. In order to overcome some of the di culties of these high-energy processes, the chemical catalysis method was developed in which a hydrocarbon feedstock is used in combination with a metal catalyst. The catalyst is typically, but not limited to iron, colbalt, or iron/molybdenum, it is heated under reducing conditions in the presence of a suitable carbon feedstock, e.g., ethylene. This method can be used for both SWNTs and MWNTs; the formation of each is controlled by the identity of the catalyst and the reaction conditions. A convenient laboratory scale apparatus is available from Nanotech Innovations, Inc., for the synthesis of highly uniform, consistent, research sample that uses pre-weighed catalyst/carbon source ampoules. This system, allows for 200 mg samples of MWNTs to be prepared for research and testing. The use of CO as a feedstock, in place of a hydrocarbon, led to the development of the high-pressure carbon monoxide (HiPco) procedure for SWNT synthesis. By this method, it is possible to produce gram quantities of SWNTs, unfortunately, eorts to scale beyond that have not met with complete success. Initially developed for small-scale investigations of catalyst activity, vapor liquid solid (VLS) growth of nanotubes has been highly studied, and now shows promise for large-scale production of nanotubes. Recent approaches have involved the use of well-de ned nanoparticle or molecular precursors and many dierent transition metals have been employed, but iron, nickel, and cobalt remain to be the focus of most research. The nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotube. The length of the tube grown in surface supported catalyst VLS systems appears to be dependent on the orientation of the growing tube with the surface. By properly adjusting the surface concentration and aggregation of the catalyst particles it is possible to synthesize vertically aligned carbon nanotubes, i.e., as a carpet perpendicular to the substrate. Of the various means for nanotube synthesis, the chemical processes show the greatest promise for industrial scale deposition in terms of its price/unit ratio. There are additional advantages to the VLS growth, which unlike the other methods is capable of growing nanotubes directly on a desired substrate. The growth sites are controllable by careful deposition of the catalyst. Additionally, no other growth methods have been developed to produce vertically aligned SWNTs.
functionalization method was also used for the puri cation of SWNTs. Under electrochemical conditions, aryl diazonium salts react with SWNTs to achieve functionalized SWNTs, alternatively the diazonium ions may be generated in-situ from the corresponding aniline, while a solvent free reaction provides the best chance for large-scale functionalization this way. In each of these methods it is possible to control the amount of functionalization on the tube by varying reaction times and the reagents used; functionalization as high as 1 group per every 10 - 25 carbon atoms is possible. Organic functionalization through the use of alkyl halides, a radical pathway, on tubes treated with lithium in liquid ammonia oers a simple and exible route to a range of functional groups. In this reaction, functionalization occurs on every 17 carbons. Most success has been found when the tubes are dodecylated. These tubes are soluble in chloroform, DMF, and THF. The addition of oxygen moieties to SWNT sidewalls can be achieved by treatment with acid or wet air oxidation, and ozonolysis. The direct epoxidation of SWNTs may be accomplished by the direct reaction with a peroxide reagent, or catalytically. Catalytic de-epoxidation (Figure 10) allows for the quantitative analysis of sidewall epoxide and led to the surprising result that previously assumed pure SWNTs actually contain ca. 1 oxygen per 250 carbon atoms.
Figure 10: Catalytic oxidation and de-epoxidation of SWNTs.
One of the easiest functionalization routes, and a useful synthon for subsequent conversions, is the uorination of SWNTs, using elemental uorine. Importantly, a C:F ratios of up to 2:1 can be achieved without disruption of the tubular structure. The uorinated SWNTs (F-SWNTs) proved to be much more soluble than pristine SWNTs in alcohols (1 mg/mL in iso-propanol), DMF and other selected organic solvents. Scanning tunneling microscopy (STM) revealed that the uorine formed bands of approximately 20 nm, while calculations using DFT revealed 1,2 addition is more energetically preferable than 1,4 addition, which has been con rmed by solid state 13 C NMR. F-SWNTs make highly exible synthons and subsequent elaboration has been performed with organo lithium, Grignard reagents, and amines. Functionalized nanotubes can be characterized by a variety of techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), UV-vis spectroscopy, and Raman spectroscopy. Changes in the Raman spectrum of a nanotube sample can indicate if functionalization has occurred. Pristine tubes exhibit two distinct bands. They are the radial breathing mode (230 cm-1) and the tangential mode ( cm-1). When functionalized, a new band, called the disorder band, appears at ca.1350 cm-1. This band is attributed to sp^3 -hybridized carbons in the tube. Unfortunately, while the presence of a signi cant D mode is consistent with sidewall functionalization and the relative intensity of D (disorder) mode versus the tangential G mode (1550 1600 cm-1) is often used as a measure of the level of substitution. However, it has been shown that Raman is an unreliable method for determination of the extent of functionalization since
the relative intensity of the D band is also a function of the substituents distribution as well as concentration. Recent studies suggest that solid state 13 C NMR are possibly the only de nitive method of demonstrating covalent attachment of particular functional groups.
3.3 Coating carbon nanotubes: creating inorganic nanostructures
Fullerenes, nanotubes and nano bers represent suitable substrates for the seeding other materials such as oxides and other minerals, as well as semiconductors. In this regard, the carbon nanomaterial acts as a seed point for the growth as well as a method of de ning unusual aspect ratios. For example, silica bers can be prepared by a number of methods, but it is only through coating SWNTs that silica nano- bers with of micron lengths with tens of nanometers in diameter may be prepared. While C 60 itself does not readily seed the growth of inorganic materials, liquid phase deposition of oxides, such as silica, in the presence of fullerenol, C 60 (OH)n, results in the formation of uniform oxide spheres. It appears the fullerenol acts as both a reagent and a physical point for subsequent oxide growth, and it is C 60 , or an aggregate of C 60 , that is present within the spherical particle. The addition of fullerenol alters the morphology and crystal phase of CaCO 3 precipitates from aqueous solution, resulting in the formation of spherical features, 5-pointed ower shaped clusters, and triangular crystals as opposed to the usual rhombic crystals. In addition, the meta-stable vaterite phase is observed with the addition of C 60 (OH)n. As noted above individual SWNTs may be obtained in solution when encased in a cylindrical micelle of a suitable surfactant. These individualized nanotubes can be coated with a range of inorganic materials. Liquid phase deposition (LPD) appears to have signi cant advantages over other methods such as incorporating surfacted SWNTs into a preceramic matrix, in situ growth of the SWNT in an oxide matrix, and sol-gel methods. The primary advantage of LPD growth is that individual SWNTs may be coated rather than bundles or ropes. For example, SWNTs have been coated with silica by liquid phase deposition (LPD) using a silica/H 2 SiF 6 solution and a surfactant-stabilized solution of SWNTs. The thickness of the coating is dependent on the reaction mixture concentration and the reaction time. The SWNT core can be removed by thermolysis under oxidizing conditions to leave a silica nano ber. It is interesting to note that the use of a surfactant is counter productive when using MWNTs and VGFs, in this case surface activation of the nanotube oers the suitable growth initiation. Pre-oxidation of the MWNT or VGF allows for uniform coatings to be deposited. The coated SWNTs, MWNTs, and VGFs can be subsequently reacted with suitable surface reagents to impart miscibility in aqueous solutions, guar gels, and organic matrixes. In addition to simple oxides, coated nanotubes have been prepared with minerals such as carbonates and semiconductors.
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