Nanocarbons [6]

TY - JOUR

T1 - Nanocarbons [6]

AU - Inagaki, Michio

AU - Radovic, Ljubisa R.

N1 - Funding Information: A. Activated carbon Carbon fibers Carbon nanotubes Fullerene Intercalation compounds In November 2002 an international symposium on nanocarbons was held in Nagano, Japan. Recent research on a perhaps surprisingly wide range of carbon materials, from carbon nanotubes to activated carbons, was reviewed and there were lively informal discussions among more than 400 participants from all over the world. This symposium was held as a part of the activities of the research project ‘Basic Science on Nanocarbons for Energy Devices’ (project leader: Prof. Morinobu Endo, Shinshu University) within the framework of the ‘Research for the Future’ program of the Japanese Society for the Promotion of Science (JSPS). During a more formal panel discussion on nanocarbons at the end of the symposium, eight panelists from different scientific backgrounds presented their views on nanocarbons. Indeed, throughout the symposium discussions, most of the participants felt the need to have a common understanding or a definition of what nanocarbons exactly are. Here we would like to present our understanding of nanocarbons in terms of their structure and texture. Carbon materials (e.g. soot, charcoal, graphite, diamond) have almost the same history as human beings, since the pre-historic era, as pointed out by several authors [1,2] . Since the 1960s various novel carbon materials have been developed. Carbon fibers derived from polyacrylonitrile (PAN), pyrolytic carbons produced by chemical vapor deposition of hydrocarbons and glass-like carbons derived from non-graphitizable precursors were invented. These had very different structures and textures from the conventional carbon materials (exemplified by graphite electrodes, carbon blacks and activated carbons). Because of profound differences in the production procedures, the raw materials used and the properties of resultant carbon materials, they came to be known as new carbons , in contrast to conventional or classic carbons . Additional novel carbon products were also introduced in the intervening decades: needle coke, mesocarbon microbeads, vapor-grown carbon fibers, high-density isotropic graphite, carbon-fiber-reinforced concrete, molecular sieve carbons, diamond-like carbons, and others [2] . More recently, fullerenes and carbon nanotubes were discovered, and this opened a completely different perspective from that of carbon materials based on flat graphite-like hexagonal layers. They gave us a chance to revisit the hybrid orbitals (sp+2π, sp 2 +π and sp 3 ) for constructing C–C bonds [2] . These newly developed carbon materials promoted still newer developments in carbon science; in Japan the concept of carbon alloys has been proposed [3,4] and a large research project was organized [5] . Carbon remains indeed ‘an old but new material’ [6] . The controlling factors in the production of various carbon materials, both classic and new ones, are summarized in . These can be conveniently classified based on the dominant aggregation state during carbonization, the conditions of processing and the key structural or textural features of the resulting products. Table 1 During carbonization (pyrolysis) in the gas phase, a variety of structures and textures can be produced. When the concentration of the precursor molecules is high, carbon black particles of nanometer size are formed. Under more controlled conditions, pyrolytic carbon is deposited on an inert substrate surface, sometimes resulting in a highly oriented crystallite arrangement, which may be converted to HOPG by annealing at high temperature and pressure, and other times having a more random crystallite orientation. By virtue of the coexistence of active metallic particles with precursor gases, several closely related fibrous carbon materials are obtained: vapor-grown carbon fibers (VGCF) and filaments (or nanofibers), with a crystallite orientation that is often described as tubular, or platelet, or herring-bone. From carbon vapor produced in a high-energy environment (e.g. an electric arc) various sizes of fullerene molecules and carbon nanotubes are formed. The latter are formed also at the onset of VGCF growth, which was already recognized when their structure was first investigated, although it was not designated as such [7] . Under carbon supersaturation conditions at ambient pressure, but in the presence of hydrogen in the excited state (e.g. plasma), sp 3 bonding is kinetically favored over sp 2 bonding and diamond-like carbons are obtained. In the production of fullerenes and carbon nanotubes, the nanometer scale (nano-size) of basic spherical or cylindrical molecules is the origin of their attractive characteristics, from the smallest buckminsterfullerene C 60 to the giant fullerenes such as C 240 and C 540 , and from single-wall to multi-walled nanotubes. Such molecular architecture defines the unique structural and textural properties of this new family of curved carbon materials, including multi-fullerenes, metal-containing fullerenes, armchair, chiral and zig-zag nanotubes, fullerene-containing nanotubes and still others. During carbonization in the liquid phase (e.g. of a petroleum or coal tar pitch), cokes of various degrees of structural order are produced and used in the fabrication of graphitic materials. By subjecting the pitch to shear stresses, preferred orientation of the precipitating crystallites is introduced, and the resulting needle coke improves the performance of graphite electrodes. By spinning the pitch, carbon fibers are produced, isotropic pitches producing random structures (isotropic-pitch-based carbon fibers), but liquid crystalline pitches resulting in highly ordered structures (radial, concentric, etc.) of the highly coveted mesophase-pitch-based carbon fibers [8,9] . During thermal decomposition in the solid phase, the limited mobility of the incipient crystallites is responsible for the formation of amorphous structures consisting of randomly oriented, though largely parallel graphene layers. If the evolution of volatile products is rapid, many pores are created and are maintained open, most of them retain their nanometer size and a very large surface area is achieved upon subsequent controlled gasification, to produce activated carbons. The texture and thus the performance of classic activated carbons has been traditionally controlled by a judicious selection of the precursor and the activation protocol. More recently, for more demanding applications (e.g. molecular sieving gas separation, double layer capacitors, specific adsorption of environmental pollutants), a more refined control of pore size distribution—and even of surface chemistry [10] —is accomplished by activation with KOH or other reactive fluids, carbonization of polymer blends, doping the carbon precursors with heteroatoms, intercalation, or template techniques. In these traditionally called microporous carbons (because the dominant pores are the so-called micropores, <2 nm), which are increasingly being referred to as ‘nanoporous’ carbons (because the majority of the pores are of nanometer size), the size and the orientation of the crystallites are also very important because they define the material’s texture. Thus, for example, in activated carbon fibers both pore diameters and pore lengths can be controlled, making them very attractive materials because of both high adsorption capacity and high adsorption rates. Similar flexibility is offered by graphite intercalation compounds: the size of the pores can be controlled by the size of the intercalating atoms and the chemical affinity of the pore walls by their electron donor or acceptor character [11] . Nanometer-scale structure is also controlled by doping carbons or carbon precursors with nitrogen or boron [11] , the latter already having resulted in improved materials for lithium ion rechargeable batteries [12] . Also, even when a ‘nongraphitizable’ organic polymer possessing high molecular orientation (e.g. polyimide film) is used as the carbon precursor, a highly oriented graphitic film is obtained during its solid-state carbonization; the degree of molecular orientation in the precursor was found to be an important factor that controls the structure of the resultant carbon film [13] . In summary, . Classic carbons relied mostly on nature to produce the various crystallite and/or pore sizes and structures; and only some of them are nanocarbons. In contrast, many among the newly developed carbons are nanocarbons, because we have learned how to control either their nano-structure or their nano-size, or both. Table 1 highlights the fact that the crucial factor in the development of novel carbon materials is the control of not only the size but also the structure at the nanometer scale. Indeed, nanocarbons can be defined as carbon materials produced when either their size or their structure is controlled at the nanometer scale. This definition explains why they include much more than the fullerenes and nanotubes, and this is illustrated in Table 2

PY - 2002

Y1 - 2002

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U2 - 10.1016/S0008-6223(02)00204-X

DO - 10.1016/S0008-6223(02)00204-X

M3 - Letter

AN - SCOPUS:0036052894

SN - 0008-6223

VL - 40

SP - 2279

EP - 2282

JO - Carbon

JF - Carbon

IS - 12

ER -