<<

commentary The era of allotropes Andreas Hirsch

Twenty-five years on from the discovery of 60C , the outstanding properties and potential applications of the synthetic carbon allotropes — , nanotubes and — overwhelmingly illustrate their unique scientific and technological importance.

arbon is the element in the periodic consist of extended networks of sp3- and 1985, with the advent of fullerenes (Fig. 1), table that provides the basis for sp2 -hybridized carbon , respectively. which were observed for the first time by Con . It is also important for Both forms show unique physical properties Kroto et al.3. This serendipitous discovery many technological applications, ranging such as hardness, , marked the beginning of an era of synthetic from drugs to synthetic materials. This lubrication behaviour or electrical carbon allotropes. Now, as we celebrate role is a consequence of carbon’s ability conductivity. Conceptually, many other ’s 25th birthday, it is to bind to itself and to nearly all elements ways to construct carbon allotropes are also the time to reflect on a growing family in almost limitless variety. The resulting possible by altering the periodic binding of synthetic carbon allotropes, which structural diversity of organic compounds motif in networks consisting of sp3-, sp2- includes the synthesis of carbon nanotubes and is accompanied by a broad and sp-hybridized carbon atoms1,2. As a in 19914 and the rediscovery of graphene range of chemical and physical properties. consequence of the expected remarkable in 20045. Keeping in mind the numerous The tools of modern synthetic chemistry physical properties of these elusive carbon possible carbon modifications and the allow the tailored design of these properties allotropes, it has been appealing, for number of scientists investigating this by the controlled combination of structural some time, to develop concepts for their challenge, these revelations have certainly and functional building blocks in new preparation in macroscopic quantities. not come to an end. target systems. However, and represented Elemental carbon exists in two natural the only known for synthesis of allotropes allotropes, diamond and graphite, which a long time. This situation changed in The fullerenes, of which the smallest stable

and most prominent is C60, are molecular carbon allotropes. C60 consists of a spherical network of sixty structurally equivalent sp2-hybridized carbon atoms in the shape of a (or, a soccer ball or, more

technically, an Ih-symmetrical ) composed of 12 Many undiscovered and 20 . Following the isolation allotropes for example sp-sp2- and structural determination of C60 by 20?? Kroto et al., the first successful preparation of 0-dimensional fullerenes in macroscopic Graphene 2004 quantities was reported five years later by Krätschmer and Huffman6. This involved the evaporation and recondensation of graphite using an arc-discharge method to Carbon nanotubes give C60. Now, fullerenes can be prepared on 1991 the ton scale. Many representatives of higher 7 fullerenes such as D5h-C70, chiral D2-C76, Fullerenes D6h-C84 and more recently D5h-C90 (ref. 8) 1985 have been isolated and structurally characterized. Furthermore, there are several Figure 1 | The world of synthetic carbon allotropes. In this family, fullerenes represent the most intensively possible ways of derivatizing investigated class. is a mature field and many well-defined derivatives with including creating fullerene salts, open-cage outstanding properties have been synthesized. The first fullerene-based products such as organic solar fullerenes and quasi-fullerenes (Fig. 2). cells have already entered the market. The materials properties of the carbon nanotubes and especially The latter are spherical, closed structures those of graphene are considered to be even more promising. However, it is still difficult to control their composed of pentagons and hexagons chemistry and also the bulk production of uniform monodisperse samples, for example, mass production combined with rings of other sizes. Another of tubes with single helicities remains a challenge. Graphene chemistry is in its early infancy. Thinking family of fullerenes, namely endohedral to the future, there are a huge number of elusive carbon allotropes whose predicted properties are fullerenes (Fig. 2f), incorporates guest atoms unprecedented. Synthetic chemists are developing concepts at present for their preparation and have or molecules inside the carbon framework. already synthesized partial structures. Examples of endohedral fullerenes include

868 materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials

© 2010 Macmillan Publishers Limited. All rights reserved

nmat_Commentary_NOV10.indd 868 12/10/10 14:14:35 commentary

La@C and Sc N@C and, most recently, Over the past couple of decades, 82 3 80 a + the first containing –– thousands of well-characterized adducts an I -symmetrical C cage, Li@C , was of C and other fullerenes have been h 60 60 a+ + 60 structurally characterized9. Exohederal synthesized and show remarkable + + fullerenes — those that have been modified – – properties. For example, the exohedral externally — are discussed in further R R covalent binding of organic donor f + b detail below. molecules such as porphyrins has attracted Soon after the discovery of the fullerenes, a lot of attention7, to simulate natural the formation of quasi-one-dimensional photosynthesis or to transform light carbon nanotubes out of evaporated graphite into chemical energy. Donor–acceptor 4 was reported . Later, a series of alternative C60 R R dyads or oligoads such as compound 1 production methods for carbon nanotubes, e c (Fig. 3) undergo photoinduced such as chemical vapour deposition N N R R transfer from the porphyrin donor methods, were developed10. Depending on to the fullerene acceptor and serve as the preparation conditions, either single- d ideal model compounds for organic walled or multiwalled carbon nanotubes solar cells. Indeed, no other compound are obtained. Carbon nanotubes can be class surpasses fullerenes as electron considered as seamless, rolled-up graphite acceptors in photovoltaic devices11.

sheets. These tubular networks of bent Adduct 2, namely phenyl-C61-butyric 2 sp -hybridized atoms are characterized by a Figure 2 | Possible derivatizations of C60 acid methyl ester (PCBM), serves as an pronounced one-dimensionality because of (ref. 6): a, fullerene salts; b, exohedral adducts; n-type conductor in very inexpensive a very high aspect ratio: typical diameters of c, open-cage fullerenes; d, quasi-fullerenes; and comparatively efficient solar 11 single-walled carbon nanotubes are around e, heterofullerenes; f, endohedral fullerenes. cells (bulk heterojunctions) that have one to two nanometres, but their lengths can already entered the market. Water-soluble easily reach millimetres and above. Present fullerenes, such as adduct 3 and 4, exhibit production methods of carbon nanotubes sheer manifold of aesthetically pleasing remarkable biological properties with to the formation of mixtures consisting structures but their outstanding, and often regard to anti-HIV activity12 and metal-free of tubes with many different helicities, unprecedented, properties. Common superoxide dismutation, respectively. The characterized by so-called m,n-values of the to fullerenes, carbon nanotubes and latter is a principal mechanism for efficient roll-up vectors10. graphene is the presence of fully conjugated neuroprotection13. Further applications π- confined in either zero-, quasi- of fullerene adducts in Synthetic carbon allotropes one- or two-. This characteristic include the fields of non-linear optical conjugation to pronounced properties14 and liquid crystals15. represent a growing activity and outstanding electronic Other types of fullerene derivative show family of fascinating and properties. However, what about the unprecedented properties: for example, chemical properties of the synthetic carbon salts, such as K3C60 or (TDAE)C60 are aesthetically pleasing allotropes and their (and derivatives’) superconductors16 and ferromagnets17, architectures with outstanding application as high-performance respectively. -opened fullerenes materials? Let us start by taking a look at have been prepared that can be filled with

materials properties. C60: immediately after its availability in small molecules, such as hydrogen, and macroscopic quantities, it was realized subsequently reclosed to form H2@C60 The youngest synthetic carbon allotrope that chemical functionalization was the (ref. 18). This opens the door to a whole is two-dimensional graphene, representing key to full exploitation of its vast potential. 5 a single graphite sheet . Graphene, the For example, chemical functionalization ab ultimate example of expanded aromatic enables a dramatic increase in the N Zn N carbon, was considered for a very long time of fullerenes, in any , which is N N O O O to be an exclusively theoretical material. required for their development as new O O Recently, however, single graphene layers materials or as bioactive redox drugs. O O O were prepared successfully by means of a Most importantly, however, chemical O O simple mechanical exfoliation of graphite functionalization provides the possibility O O using Scotch tape5. Andre Geim and of combining the unique properties of Konstantin Novoselov, who headed the fullerenes with those of other compounds. team behind the discovery of the exfoliation Exohedral addition reactions to the cdHO O method and an associated investigation of conjugated π-system represent the most RROH HO O OO O the electronic properties of graphene, were important functionalization methods O O awarded the 2010 Nobel Prize in Physics. realized so far7 (Fig. 2b). The driving force O O O O Of the three families of carbon allotropes, for exohedral additions is the reduction O O O graphene is the most structurally uniform of strain energy stored in the spherical O material where only the sheet extensions and carbon framework. Additions usually nature of edges can differ. occur regioselectively at fusions of two six-membered rings, because in contrast R = –(CH2)2NHC(O)(CH2)2COOH Properties of fullerenes to the neighbouring [5,6]-bonds, these What makes synthetic carbon allotropes [6,6]-bonds are shorter and have more Figure 3 | Some examples of fullerene derivatives so attractive for scientists is not only the double-bond character. with outstanding materials or biological properties.

nature materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials 869

© 2010 Macmillan Publishers Limited. All rights reserved

nmat_Commentary_NOV10.indd 869 12/10/10 14:14:38 commentary

a The extension of the construction principle by incorporating both sp- and sp3-hybridized atoms into carbon networks bcoffers almost limitless structural possibilities. The simplest example for such synthetic carbon allotropes is one-dimensional carbyne consisting of infinite chains ofsp -hybridized carbon atoms (Fig. 4a). Whereas the parent allotrope itself is still elusive, a number of model compounds, namely end-capped polyynes, have been synthesized and characterized. The combination ofsp -, sp2- and sp3-hybridized carbon atoms leads to many possible two- and three-dimensional carbon allotropes, such as graphyne23 (Fig. 4b). Significantly large molecular segments of graphyne (pictured in red Figure 4 | Some examples of elusive synthetic carbon allotropes: a, one-dimensional sp-carbyne; in Fig. 4b) have been synthesized and b, two-dimensional sp-sp2-graphyne; c, three-dimensional sp-sp3-yne-diamond. their study suggests the material has attractive optoelectronic and self-assembly properties. Allotropes with a skeleton world of new endohedral derivatives. considerably less mature and graphene of sp- and sp3-hybridized carbon are the The substitution of carbon atoms with chemistry is still in its earliest infancy. least-studied class of hybrid allotropes, and heteroatoms such as , realized in Nevertheless, graphene-based technology the only notable example reported thus far 23 the heterofullerene (C59N)2 (Fig. 2e) allows is at present considered to have the is expanded . The study of these for further variation of the electronic greatest potential for applications. In this materials is motivated by the prediction of properties of the fullerene core19. regard, Nobel Laureate Frank Wilczek impressive mechanical properties, such as from the Nobel Symposium on Graphene hardness for allotropes like yne-diamond, nanotubes and graphene held in May 2010 said: “Graphene is which is composed of the hypothetical The electronic and materials properties probably the only system where ideas building block adamantyne (pictured of carbon nanotubes and graphene are at from quantum field theory can lead in red in Fig. 4c). Although a variety least as remarkable as those of fullerenes. to potential applications.” The idea of of low-molecular partial structures of Depending on their helicity, carbon using graphene for digital these networks have been synthesized, nanotubes are either semiconducting or was born in 2004 with the realization of preparative access to these designer metallic. Both properties are appealing a graphene-based field effect transistor5 allotropes is still elusive. for applications in the field of molecular driven mainly by three extraordinary electronics or for the refinement of properties: extremely high charge-carrier Conclusions materials, such as antistatic paints and mobilities at room temperature; an Synthetic carbon allotropes represent shieldings. Indeed, metallic carbon enormous current-carrying capability; and a growing family of fascinating and nanotubes and graphene are the first the fact that graphene is the ultimate thin aesthetically pleasing architectures with representatives of stable organic metals material. Already it has been shown that outstanding materials properties. The where no further activation, doping graphene meets the electrical and optical existing representatives, namely fullerenes, or charge transfer is required for the requirements for transparent conductive carbon nanotubes and graphene, will establishment of extremely high charge- used in touch screens and be complemented in the future by carrier mobilities. A fascinating recent displays22. However, in contrast to the new carbon forms whose construction example for the performance of carbon material used at the moment, indium principles are based on the numerous nanotubes is their ability to act as single- tin , the graphene electrodes are possibilities to assemble carbon atoms electron pumps20. This could be a key for flexible and mechanically more robust. To in one, two and three dimensions. At the development single-electron ; proceed with these developments and to a time when -based technology devices that can confine charges down gain access to mass production, chemical is approaching its fundamental limits, to single-electron level and hence, are functionalization of graphite/graphene the possible incorporation of synthetic applicable for quantized current generation. could be a powerful tool to solve a number carbon allotropes in the production of Carbon nanotubes hold the record for of problems, such as increased solubility electronic devices is welcome. As a result the mechanically strongest material, and processability, stabilization in solution, of their spectacular electronic, thermal making them useful for the reinforcement separation and purification, as well as and mechanical properties they are of polymers in composite materials. For device fabrication. expected to offer an exceptional choice. example, coating cotton thread with However, fully realizing their application in conductive carbon nanotubes leads to the elusive allotropes , sensors, , formation of a lightweight thread that can During the investigation of sp2-hybridized batteries and still requires be easily woven into fabrics21. allotropes, attention has also been much fundamental research, keeping Compared with fullerenes, chemical paid towards the development of chemists, physicists, materials scientists functionalization of carbon nanotubes is further synthetic carbon allotropes23. and engineers busy for years to come. ❐

870 nature materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials

© 2010 Macmillan Publishers Limited. All rights reserved

nmat_Commentary_NOV10.indd 870 12/10/10 14:14:38 commentary

Andreas Hirsch is in the Department of Chemistry 3. Kroto, H. W. et al. Nature 318, 162–163 (1985). 15. Campidelli, S. et al. Chem. Commun. 4282–4284 (2006). and Pharmacy & Interdisciplinary Center of 4. Iijima, S. Nature 354, 56–58 (1991). 16. Holczer, K. et al. Science 252, 1154–1157 (1991). 5. Novoselov, K. & Geim, A. et al. Science 306, 666–669 (2004). 17. Allemand, P. M. et al. Science 253, 301–302 (1991). Molecular Materials (ICMM), Friedrich-Alexander- 6. Krätschmer, W. et al. Nature 347, 354–358 (1990). 18. Komatsu, K. et al. Science 307, 238–240 (2005). University Erlangen-Nuremberg, Henkestrasse 42, 7. Hirsch, A. & Brettreich, M. Fullerenes – Chemistry and Reactions 19. Vostrowsky, O. & Hirsch, A. Chem. Rev. 106, 5191–5207 (2006). 91054 Erlangen, Germany. (Wiley, 2004). 20. Siegle, V. et al. Nano Lett. doi:10.1021/nl101023u (2010). e-mail: [email protected] 8. Yang, H. et al. Angew. Chem. Int. Ed. 49, 886 (2010). 21. Shim, B. S. et al. Nano Lett. 8, 4151–4157 (2008). 9. Aoyagi, S. et al. Nature Chem. 2, 678–683 (2010). 22. Bae, S. et al. Nature Nanotech. 5, 574–578 (2010). 10. Hirsch, A. & Vostrowsky, O. Top. Curr. Chem. 23. Diederich, F. & Kivala, M. Adv. Mat. 22, 803–812 (2010). References 245, 193–237 (2005). 1. Karfunkel, H. R. & Dressler, T. J. Am. Chem. Soc. 11. Dennler, G. et al. Adv. Mater. 21, 1323–1338 (2009). 114, 2285–2288 (1992). 12. Friedman, S. H. et al. J. Am. Chem. Soc. 115 6506–6509 (1993). Acknowledgements 2. Diederich, F. & Rubin, Y. Angew. Chem. Int. Ed. 13. Liu, G-F. et al. Angew. Chem. Int. Ed. 47, 3991–3994 (2008). A.H. thanks GRAPENOCHEM and the Cluster of 31, 1101–1123 (1992). 14. Koudoumas, E. et al. Chem. Eur. J. 9, 1529 (2003). Excellence EAM for financial support.

Green carbon as a bridge to renewable energy James M. Tour, Carter Kittrell and Vicki L. Colvin A green use of carbon-based resources that minimizes the environmental impact of carbon fuels could allow a smooth transition from fossil fuels to a sustainable energy economy.

arbon-based resources (, natural and even in the next 5 to 10 years is still The concept of a bridge to a renewable gas and oil) give us most of the world’s extremely limited when put into the energy future is not new, and many strategies energy today, but the energy economy context of total world use of fossil fuels. have been offered as near-term alternatives C 3 of the future must necessarily be far more For example, the world used the equivalent to conventional carbon resources . Although diverse. Energy generation through solar, of 113,900 terawatts hours [TWh] of fossil some of these concepts appear at first glance wind and geothermal means is developing energy to fuel economic activity, human to be ‘green’ solutions, further scrutiny now, but not fast enough to meet our mobility, and global telecommunications, that takes into account the energy lifecycle expanding global energy needs. We advocate among other modern day activities in and environmental consequences suggests that ‘green carbon’, which enables us to use 2007. Replacing those terawatts hours with a more murky set of trade-offs. Biodiesel, carbon-based sources with high efficiency non-fossil energy would be the equivalent of for example, has led to destruction of vast and in an environmentally friendly manner, constructing an extra 6,020 nuclear plants areas of tropical rainforest to make palm will provide our society time to develop across the globe or 14 times the number of oil4. A shift of transportation to lithium alternative energy technologies and markets nuclear power plants in the world today. battery-powered cars requires enormous without sacrificing environmental or In renewable energy terms, it is 133 times amounts of fossil fuels to mine vast stretches economic quality. Green carbon will help the amount of solar, wind and geothermal of the Earth in search of lithium, and to reduce the loss of our precious carbon energy currently in use on the planet.”1 the vehicle would need to be driven tens resources, which are better reserved for Barring a huge reduction in our global of thousands of kilometres entirely on high-value chemicals, and it will ensure standard of living, we will need to rely renewable energy just to accrue back the that those hydrocarbons used for fuels on carbon-based energy for some time. ~1.7 GJ energy per kilowatt hour capacity will minimize carbon emissions. Through Whether this will last for several decades or used for the lithium and battery intensive research and development in into the next century is unclear, but what manufacture5. The electric grid promised green carbon, our society can guarantee an is apparent is that renewable approaches to serve the purportedly clean electric-car energy future that uses carbon strategically, to energy generation are increasing at an technology may just become a long tailpipe without smokestacks, greenhouse gases and annual rate of 7.2% compared with 1.6% for connecting the plug-in batteries to coal- extensive environmental damage. non-renewable growth2, and the continued fired power plants6. And even if renewable growth of renewables will demand sustained energies could supply the , the Building a bridge government support. During this transition current electrical grids could not sustain There is a chasm between the diminutive we propose a green carbon bridge that the vast increase in demand. We need to proportions of renewable energy currently minimizes the environmental impact of quantify the full environmental or lifecycle available and our overwhelming dependence carbon fuels and lowers our reliance on these consequences of various options for our on fossil fuels that currently propel society. resources for primary energy generation. energy bridge if we are to build it from the The energy policy review of the Obama Ultimately, green carbon will use hydrogen proper technologies. administration makes this soberingly from renewable sources, while at the same We must also consider energy clear: “The use of renewable energy today time producing basic chemical feedstocks. technology options in a broader context. In

nature materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials 871

© 2010 Macmillan Publishers Limited. All rights reserved

nmat_Commentary_NOV10.indd 871 12/10/10 14:14:38