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Nanocarbons between past, present and future

Nanocarbons between past, present and future

 

Bernadette Bensaude-Vincent, Paris 1 Panthéon-Sorbonne University

 

Nanocarbons feature as stars of Materials Resarch. They are manufactured from an earth-abundant element, very simple in their chemical composition (carbon molecules with a few dopants), and above all they are multi-purpose wonder materials. They have been indirectly distinguished by many awards over the past decades:  the 1996 Nobel prize for chemistry was about the discovery of fullerenes, the 2008 Kavli Prize about the discovery of carbon nanotubes, and the 2010 Nobel prize in physics about graphene. Nanocarbons are one the most studied materials today worth billions of dollars of investments, special journals and international research programs aimed at accelerating industrial applications. Some of them are already manufactured for a variety of applications and commercialized.

Nanocarbons are undoubtedly novel materials, at the cutting edge of research and innovation. Yet their birthdate remains elusive. I am not alluding to the disputes about who should be given the credit for the discovery of nanocarbons, although they are part of the story. I am not concerned with priority disputes or Nobel Prize negotiations. Rather the question I would like to raise here is: when did nanocarbons come into existence as materials? I mean what are the steps to be taken for carbon allotropes to become materials?  How do they articulate the past, and the future to assert their presence in contemporary societies? In raising such questions, the paper provides a case study inviting reflections on the conditions for materials to come into existence as actors in social history.

Over the past decades the wonder molecules of nanocarbons entered the public world as promises of solutions to the major challenges that confront our societies: energy, environment, and health.  They exist as promises, and it is via the future that they act on the present, that they became matters of concern around the world.  Does it mean that they are just hypothetical anticipations of future materials, matters of juicy affairs? In fact their present mode of existence is a strange combination of past and future because they already existed in the past as mere curiosities, or matters of fact.

In order to disentangle their complex relations between past, present and future, I will first focus on the interactions between past and present, by pointing out the difficulties related to the birth dates of fullerenes, nanotubes, and graphene. Then I will consider the interactions between future and present in the economy of promises that prevailed in nanotechnology from their onset. I will conclude that the puzzling mode of existence of nanocarbons is an invitation to revisiting our standard definition of materials and enriching materials research.

Uncertain birth dates of three nanocarbons

In standard history, 1985 is the birth date of fullerenes discovered by Robert Curl, Harry Kroto and Richard Smalley; 1991 is the birth date of carbon nanotubes discovered by Sumio Iijima; and 2004 the birth date of graphene isolated by Andre Geim and Konstantin Novoselov. However all three nanocarbons pre-existed their official birth date.

Fullerenes

Let us first summarize the heroic story of fullerenes.1 In the early 1980s, Harry Kroto, an organic chemist interested in astrochemistry was searching the origin of long linear carbon chain molecules discovered in interstellar space. At a conference in 1984, he met a friend Robert Curl who was working with Richard Smalley at Rice University and talked about their instrument called AP2 to investigate molecular clusters. Kroto convinced them to collaborate. In 1985, they observed among the long carbon chains an unknown molecule of pure carbon. Using a mass spectrometer and the work time of graduate students in the laboratory, they identified a stable molecule made of 60 carbon atoms. They determined that the sixty-carbon molecule had a perfect spherical structure that they compared to the dome designed by the American architect Buckminster Fuller for the 1967 World Fair in Montreal. While standard chemistry papers usually include a structural formula with a graph of the described molecule, Kroto and his collaborators published their result in Nature with just a photograph of a soccer ball on a Texas lawn. The molecule named ‘Fullerene’ imposed itself by its beauty. “It was such a beautiful and perfect structure, how could it be wrong,” said Kroto a few years later.2 It was an aerial beauty however because these fullerenes were no more than traces seen in a helium flow. They existed only as laboratory curiosities. They came to be seen differently five years later, when Wolfang Krästmer and his team managed to produce a macroscopic crystal of solid footballs made of 90% of C60 and 10% of C70, with an electric arc. In fact, these molecules named fullerites had been around for ever in the soot of chimneys and the places of impacts of lightnings. But Krätschmer’s simple and reproducible method of synthesis changed their status: from mere curiosities they became potential materials, opening up a new world, the world of nanocarbons.

Nanotubes

The case of nanotubes is even more complex since their molecular structure has been observed and described by a number of chemists.3 Carbon filaments were first discovered in the soot of the furnaces of steel-plant in the early 19th century, then rediscovered in the deposits of ovens in a manufacture of metallurgical coke as well as in the process of decomposition of cyanogen. “Metallized” carbon filaments obtained by “flash carbonization” of bamboo had a brand and brief industrial career in Swann and Edison light bulbs. They disappeared during the Interwar period, replaced by coiled tungsten filament lamps. They resurfaced in the 1950s under the electron microscope together with X-ray diffraction and electron diffraction. A crystallography paper published in 1958 by Hillert and Lange established that carbon filaments are hollow, built up of lamellar units bent into single or multi-wall cylinders, and that they display a variety of shapes while keeping the crystalline structure of graphite.4

However these hollow structures did not catch the attention of chemists and materials scientists who were focused on the production of carbon fibres for reinforcing polymers. Roger Bacon from Union Carbide grew graphite whiskers to use these filaments as possible precursors for carbon fibres to reinforce plastics more effectively than glass fibres.  He later confessed that he had made nanotubes without discovering them. In 1976, Morinobu Endo, a young Japanese chemical engineer was trying to enter the business of carbon fibres, particularly flourishing in Japan. His project was to find a cheapest alternative to the mainstream PAN process.. He developed a new process of vapor-grown carbon fibres by catalytic decomposition of benzene. While using a powerful electron microscope to characterize his fibre, he clearly established that these filaments were “hollow tubes”. Twelve years later, he realized that this hollow tube was not a by-product of fibre manufacture, but an essential initial step in fibre formation. He established that the process occurs in two stages: first, the catalytic growth of a hollow tube, then thickening by deposition of pyrolitic carbon. The hollow structure no longer was a defect to be got rid of, but the core of carbon fibres, and perhaps the cause of their remarkable mechanical properties. Has he discovered carbon nanotubes? It took more than that for them to exist as nanomaterials.

The missing eureka came from Sumio Iijima, a researcher in electron microscopy in the Japanese microelectronics company NEC. His short paper published in 1991 totalized more 30.000 quotations.5 Its success was not due to the magics of the prefix “nano” since Iijima talked about “microtubules” or “needle-like tubes”. The article did not promise a bright future of industrial applications, it simply described tubes obtained by the electric arc. The true “discovery” of nanotubes as materials resulted from a dual move;  of distanciation from the industrial world of composite materials on the one hand, and of rapprochement with the hollow structure of fullerenes and the process used by Krätschmer, on the other hand. Iijima included the pre-existing hollow tubes in a family of hollow objects produced by electric arc. When he demonstrated that nanotubes could be “capped” at one end by a half-fullerene, the family of carbon how structures captured the attention of materials scientists because it could indefinitely be extended: from single- or multi-wall nanotubes, to onion-shaped, sea urchin-shaped, necklace-shaped, serpentine, helix-shaped, spiral-shaped, tripod-shaped nanocarbons.

Graphene

No revolutionary claims signalled the entrance of graphene into the scientific world. It came into existence as a creature of the laboratory, potentially useful for measurements and catalytic applications. It is made of a single monoatomic layer of graphite, a familiar carbon allotrope that has been known and used for centuries to make pencil leads – hence its name graphite from the Greek verb ‘graphein’, which means to write. In the 20th century graphite became a strategic material in nuclear technology and was also used for making intercalation compounds. The term ‘graphene’ was coined by Hans-Peter Boehm, a German intercalation chemist who had reported a method of production of flakes of “a thin carbon film” as early as 1962. He coined the term ‘graphene’ (from graphite+benzene) , referring to a single layer of polyaromatic carbon.6 The term was officially endorsed by IUPAC in 1994.

If graphene already existed as a common material used by chemists and solid state physicists in graphite  intercalation compounds research, was it really discovered  in 2004 by  Andre Geim and Konstantin Novoselov, from Manchester University ? So what is the achievement celebrated by the 2010 Nobel Prize of Physics?

According to the official report of the Nobel Committee, the award was “for groundbreaking experiments regarding the two-dimensional material graphene”. In his Nobel lecture, Andre K. Geim rather reported what looks like a kitchen experiment: peeling a piece of graphite with Scotch adhesive tape. He insisted: “After all, we now know that isolated monolayers can be found in every pencil trace, if one searches carefully enough in an optical microscope. Graphene has been literally before our eyes and our noses for many centuries but was never recognized fro what it really is”.7  So what is it really? Let us go back to the official definition provided in the IUPAC Compendium of Chemical Terminology, which mentioned that: “It is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed”.8

Geim rightly claimed to have understood the true nature of graphene as an individual layer of graphite, a 2D material. The Manchester team obtained ‘freestanding’ graphene, a surface area of a few square centimetres at room temperature. Its prowess was to make a material that is a pure surface. Thermodynamically speaking, it had been established in the 1930s that two-dimensional crystals cannot exist in the free state. A crystal minimises its surface energy and small three-dimensional islands are formed. In short, the growth of a 2D structure was a physical impossibility. It did not stop researchers from growing graphene crystals in the laboratory, either by epitaxy on a metal substrate, or in the form of rolls, cones or folded like origamis, which minimises their surface energy. They also obtained graphene in the form of flakes in a liquid solvent, but never was it free-standing. No miracle: graphene could not float upright, like Christ walking on water! The Manchester team managed to obtain a metastable state of graphene, an isolated plane that undulates a little but can be transferred without alteration from one substrate to another, maintaining its stability under ambient conditions. Just as carbon nanotubes derive their properties from their curvature, the planar morphology of graphene affords a particular band structure that is usually observed only in high-energy particles. The electrons behave as if they had no mass.

In addition, the Manchester team observed the great ‘versatility’ of its electrical properties, i.e. its ability to switch on demand from almost zero conductivity to extreme conductivity (and vice versa), and demonstrated that the electronic properties of graphene remained unaltered in the open air. Prior to 2004 graphene existed as an academic material, a laboratory tool. It provided a theoretical model for understanding the stacking of sheets and the electronic exchanges in the intercalation and a textbook case for explaining the theory of energy bands that every solid-state physicist needs to master. From then on, the academic material became an interesting technical device for microelectronics industry.  

To sum up this section, none of the three star nanocarbons were born as materials. They were first described as remarkable exotic structures. Fullerenes first appeared as beautiful laboratory curiosities. Graphene first existed as a research tool in the academic world. While carbon nanotubes already existed in the industrial world as filaments in the 19th century, then as fibres in the 20th century, they started their career as nanomaterials only through connecting them with the hollow structure of fullerenes and with a process of synthesis (electric arc). They thus played a central role in the constitution of nanocarbons as a family of novel materials bringing about a cornucopia of technological innovations.9 

Nanocarbons as promise of a renewable future

Carbon acted as a major actor in the launching of nanotechnology and contributed to the hype surrounding its debut. The question driving the US National Initiative Brochure subtitled “Shaping the World atom by atom” was: what if we (humans) could assemble the basic ingredients of the material world the way nature does – atom by atom, molecule by molecule? Since self-assembly is the Graal of the announced revolution, carbon atoms are good players because they spontaneously form bonds with other carbon atoms. As Richard Smalley noted in his Nobel Lecture:

The discovery that garnered the Nobel Prize was the realization that carbon makes the truncated icosahedral molecule, and larger geodesic cages, all by itself. Carbon has wired within it, as part of its birth right ever since the beginning of this universe, the genius for spontaneously assembling into fullerenes. (Smalley 1996, 90)

Given the “genius” of carbon atoms, it comes to no surprise that carbon nanotubes (CNTs) were immediately considered as promising nanomaterials “for applications ranging from new structural materials that are stronger and lighter to electronic compounds for new supercomputers to drug delivery systems”.10 Single-wall CNTs are praised for their extraordinary thermal, mechanical, and electrical properties. Multi-wall CNTs are especially interesting because nano-engineers can play with the degree of entanglement of the walls, for instance to get electrical conductivity only on the outer side of the tube and not inside. In addition, they have a high thermal stability. As they travel through all biological barriers, they can be used for a variety of medical applications. Graphene is branded as the world’s thinnest material (one million times thinner than a human hair) with a unique combination of properties: stronger than steel and diamond, light and flexible like polymers, transparent like glass, conducting heat and electricity,  and above all it is a pure surface. Ideal for composites, for wearable devices and foldable electronics, batteries, conductive inks, etc..

More importantly all nanocarbons can be functionalized. By attaching additional molecules or introducing them inside the hollow space, nanocarbons are proned to be used in microelectronics and optoelectronics, in energy technology for solar cells, in water purification, in medical applications for targeted drug delivery or brain implants or neuroelectronics … In other words, these wonder materials are enabling “the ongoing technological revolution” prompted by the convergence of nano, bio, info and cognitive technology – known by the acronym NBIC. Mihaïl Roco, the promoter of the NBIC program launched in 2004 outlined a roadmap in 3 steps: i) nanomaterials as passive structures; ii) active nanodevices; and iii) nanorobots. The future of nanocarbons was thus anticipated from the onset: once functionalized they were bound to become smart materials with sensors and actuators, then more or less autonomous nanorobots responding to their environment, like living beings. They would thus assist human societies in their effort to monitor the ecological transition and build a more sustainable world. Not only would they re-open the future threatened by climate crisis but they also inspired the perspective of an endlessly renewable future.

Still, to make this bright future happen required heavy public and private investments, as well as human resources (scientists, engineers, business developers…). Billions of dollars, R&D programs, start-ups or spin-offs, capital ventures, have been devoted to nanocarbons all over the world in order to develop these materials, and bridge the gap between the laboratory and the market place. This world mobilization to turn techno-promises into reality, resulted in a flurry of war metaphors in nanocarbons research, such as “target drug delivery”; “graphene flagship” (name of a ten-year European consortium) or “spearhead projects”. The discourses about nanocarbons emphasize control and mastery based on the assumption that can be designed and functionalized, tailored and tuned, on demand. 

However, that there is a gap between discourses and actual practices. Without claiming that the design and synthesis of nanocarbons would result from a will for uncontrol as Jean-Pierre Dupuy argued,11 one has to admit that after twenty years of intensive R&D the present and the future of nanocarbons are still uncertain. Indeed a number of CNTs and graphene layers are already manufactured and used in electronic circuits, solar cells, coatings, inks. However their development as the materials of the 21st-century is limited by two obstacles. On the one hand, the properties of nanocarbons are process-dependent because their atomic structure is determined by the conditions of synthesis. Consequently it is difficult to standardize CNTs for industrial production and commercial applications. The regulations about chemicals or novel materials require that they comply to a number of standards to be certified before entering the market. But the size and properties of CNTs often vary from one batch to the next one, and even in the same batch you never get CNTs with uniform properties. In a batch of CNTs you always find a mixture of structures because it is almost impossible to separate the metallic CNTs (conductors) from the semiconductor CNTs to make only conductors or semi-conductors. With graphene, it is easier since there is just one layer of atoms, but the purification process is also demanding, especially for biomedical applications.

On the other hand, safety is another obstacle for nanocarbons to become materials. Are they toxic to humans and to the environment? The question emerged in the early days of nanotechnology and remains controversial despite significant research efforts to identify the health and environmental impacts of nanoparticles in general. The question did not stem out of the exploration of nanocarbons. It has been channeled into nanomaterials research by societal and economic concerns and by the notion of Responsible Research and Innovation. Nanotoxicology emerged as a new research field bringing together chemists, physicist, toxicologists, epidemiologists, environmental scientists, insurance companies, and regulatory bodies. When the National Science Foundation Center for Biological and Environmental Nanotechnology was created at Rice University (the birthplace of fullerenes) in 2004, Vicky Colvin, head of the program, was confident that nanocarbons were just probes for further exploring what happens to them in different contexts. She expected to learn something about their structure and functions. She invented the concept of “safety by design” to secure the future of nanocarbons, thus avoiding the threat of a societal backlash analogous to that of GM crops. However nanotoxicological research showed that her dear buckyballs and CNTs were toxic. Moreover it is difficult to make realistic environmental scenarios of contamination of soils and lakes because their toxicity depends not only on their own properties but on the action of light, water… on their interaction with the milieu, as well. As Chris Kelty convincingly argued, it was therefore impossible to treat safety as a property of materials on par with other properties such as stress, strain, or conductivity.12 This major obstacle could threaten the future of nanocarbon as materials. Up to now, the emphasis in the field on nanocarbons R&D has been on translational research, i.e. on the translation from the laboratory to the industrial world with the integration of industrial partners and business developers into research programs. This research strategy is in line with the old linear model But the future of materials such as graphene oxide for biomedical coating for instance is dependent on clinical trials, physicians, on regulatory agencies, on nurses and patients expectations. Hospitals and regulatory agencies are not equipped to contribute to the process of creation of new materials. Nanocarbons cannot exist as materials until they comply with their users’ expectations, regulations, and values. 

Enriching Materials Thinking

Materials do not come into existence as atomic structures determining a set of properties, which in turn qualify them for specific performances. Structure and properties characterize ‘academic materials’ that are not fit to enter into technological adventures, as illustrated by the case of graphene before 2004. The case of carbon nanotubes clearly demonstrates two major requirements for molecular structures to become materials: they have to be produced by a standard process of synthesis and they have to overcome the obstacle of the context-dependency of their performances.

While structure, properties, performances, and process are necessary parameters for materials, they  are not sufficient. Structures become materials when they have a social life, that is a market. Materials exist as hybrids of nature and society. Most textbooks of Materials Science & Engineering being devoted to purely scientific and technical aspects of materials provide a narrow and partial view of what is needed for a material to become part of the world as partner of technological adventures. Indeed since the 1980s a system approach moving beyond the linear model – from basic science to industrial applications- has prevailed for training students in “materials thinking”. This new thought style has been imagined with the help of a tetrahedron indicating that four variables have to be simultaneously taken into consideration in the design or selection of materials: structure, properties, performances and process (Fig.1)  However, the materials tetrahedron is too narrowly focused on one moment of the life of materials. It overlooks that materials have a life-cycle: where they come from and how they will be disposed or recycled are crucial variables that have to be also taken into account. Due to the increasing importance of environmental and safety issues, many puzzles arise at the end of the value chain. Therefore a fifth parameter should be added that has been named sustainability and criticality.13 Sustainability is incorporated to ensure the life-cycle is taken into account and criticality to address issues of supply and demand, of market and geopolitics. The tetrahedron is consequently reimagined into a pentahedron, i.e. a square pyramid  (Fig. 2). 

               Fig. 1 Materials tetrahedron        Fig.2 Materials square pyramid

In a more practical perspective, new venues are needed for novel materials to come into existence because they have to pass tests, trials, in order to get certifications and approvals by a number of regulatory bodies and to be accepted by users. Politics comes in their destiny and determines their future.

  1. American Chemical Society. The Discovery of Fullerenes. acs.org/…/whatischemistry/landmarks/lesson-plans/discovery-of-fullerenes []
  2. Jeffrey I. Seeman and Stuart Cantrill, Wrong but seminal, Nature Chemistry (8 March 2016): 193-200. []
  3. Sacha Loeve, Point and line to plane. The ontography of carbon, Cahiers François Viète, Series 3, N°2 (2017): 183-216.  Monthioux, M. & Kuznetsov, V. Who should be given the credit for the discovery of carbon nanotubes?, Carbon 44, (2006): 1621-1625. []
  4. Hillert Mats, Lange Nils The structure of graphite filaments. Zeitschrift für KristallographieCrystalline Materials, 111 n° 1-6, (1958). 24-34. []
  5. Iijima Sumio, Helical microtubules of graphitic carbon Nature, 354 (1991) p. 56-58. []
  6. Boehm, H. P., Setton, R., & Stumpp, E. (1986). Nomenclature and terminology of graphite intercalation compounds. Carbon24(2), 241-245. []
  7. Andre K. Geim, Random walk to graphene, Nobel Lecture, December 8, 2020, cit. on p. 86-87. nobelprize.org/uploads/2018/06/geim_lecture.pdf []
  8. McNaught, A. D., & Wilkinson, A. Eds. (1997), Compendium of chemical terminology, Oxford: IUPAC. []
  9. CNTs may epitomize the entire family. For instance in this non-official definition:“Carbon nanotubes (aka CNTs) are made from graphene sheets consisting of a single atomic layer of carbon atoms in a honeycomb framework that can be rolled into a tube measuring about a nanometer, or one billionth of a meter, in diameter. Carbon nanotubes are also known as “Buckytubes” because they resemble R. Buckminster Fuller’s geodesic domes”. http://futuretechreport.com/carbon-nanotubes-applications-uses-future/ []
  10. Mihaïl Roco (ed.) Nanotechnology : Shaping the World Atom by Atom, NSF 2001, p. 7. []
  11. Jean-Pierre Dupuy argued that nantoechnology, like cybernetics, is driven by a vill for uncontrol and that high risks are its raison d’être rather than side effects.(Dupuy, Technology and metaphysics, in In Jan Kyrre Berg Olsen Friis, Stig Andur Pedersen & Vincent F. Hendricks (eds.),A Companion to the Philosophy of Technology Wiley-Blackwell, 2012, Chapter 38 []
  12. Christopher Kelty, Beyond implications and applications : The story of ‘safety by design’, Nanoethics, 3 (2009) 79-96. []
  13. Donahue C.J.. Reimagining the materials tetrahedron. Journal of Chemical Education, 96, (2019) 2682-2689. []