This nomenclature has been widely used since an international symposium on nanocarbons was held in Nagano, Japan in November 2002. Inagaki and Radovic interpret it in terms of the structural and textural features [126]. They summarize the controlling factors for the production of various classic and new carbon materials containing the nanocarbons, as indicated in Figure 32. In regard to the pore structure, they explain that what are traditionally called microporous carbons in which the dominant pores are the so-called micropores, <2 nm, are increasingly being referred

State during carbonization

Carbon materials

Carbon blacks Pyrolytic carbons Vapor-grown carbon fibers (carbon filaments/nanofibers)

Fullerenes Carbon nanotubes

Diamond-like carbons

Cokes (e.g., needle coke) Graphite

Carbon fibers (e.g., pitch) Activated carbons Molecular sieve carbons Carbon fibers (e.g., PAN) Glass-like carbons Highly oriented graphite

Controlling factor for production

High concentration of precursor Deposition on a substrate Presence of a catalyst

Condensation of carbon vapor Condensation of carbon vapor (with or without catalyst) Condensation of organic vapors

Shear stress

High-temperature treatment Spinning

Rapid carbonization/activation Selective pore development Slow carbonization Slow carbonization High molecular orientation

Key structural and/or textural features

Nano-size particles Preferred orientation Catalyst particle size/shape

Nano-size molecules Armchair, zig-zag, chiral; Single-wall or multi-wall

Mesophase formation and growth Mesophase formation and growth Mesophase formation and growth Nano-size pores

Nano-size pores and constrictions Nano-size random crystallites, nonporous Nano-size random crystallites,impervious Highly oriented crystallites

Figure 32. Controlling factors for the production of various carbon materials and the key points for their structural and/or textural features. Reprinted with permission from [126], M. Inagaki and L. R. Radovic, Carbon 40, 2263 (2002). © 2002, Elsevier Science.

sp3 and sp2 bonding to as "nanoporous" carbons, because the majority of the pores are of nanometer size. Also the category of nano-carbons is proposed on the basis of nanometer-scale structure and size of newly developed carbons, as illustrated in Figure 33. In this section we focus only on the carbon nano-tubes in the nanocarbons.

Carbon nanotubes and their related materials are forming a new category of nanoporous materials. In 1997, Dillon et al. first clamed that SWNTs have a high reversible hydrogen storage capacity [127]. It was the start of the following a large number of studies on hydrogen storage. Here we will not review these studies, but we explain the overview of these materials from the viewpoint of pore structure. SWNT is a carbonaceous tube with a diameter of 1 nm and usually makes hexagonally packed bundles (Fig. 5). Figure 34 shows a configuration of H2 molecules on carbon nanotube arrays by a Monte Carlo calculation [128]. It illustrates that H2 molecules adsorb in the interstitial cavity and in inner-tube cavities. The attraction potential inside the tube is calculated to be larger because the curvature of the tube increases the number of nearest neighbor carbon atoms.

Eswaramoorthy et al. were interested in the adsorptive nature of carbon nanotubes without worrying about the surrounding graphitic layers [129]. So they conducted a study by selecting a SWNT rather than using a multiwalled carbon nanotube that was known to possess mesopores [130]. They prepared HNO3-treated SWNTs as well and compared the adsorption nature of N2 and benzene. The fact that the HNO3-treated SWNT showed a larger amount of benzene and the calculation of the cross-sectional area of benzene lead them to conclude that the inner-tube space was also used for adsorption.

Barisci et al. studied the electrochemical nature of SWNTs from the standpoint of electrochemical capacitance [131].

Figure 34. Configuration of hydrogen molecules in a bundle of singlewalled carbon nanotubes. Reprinted with permission from [150], H. M. Cheng et al., Carbon 39, 1447 (2001). © 2001, Elsevier.

The capacitance of the nanotube paper (NTP) made from SWNTs was independent of the sizes of the cation or anion and of the voltage sweep rate for the measurements. These results indicated that the NTPs investigated are characterized by an interbundle open structure with relatively large pores that can be readily accessed by ions with a wide range of sizes and charges. Possibly, this is due to the pores being interconnected spaces in the entangled nanotube network rather than cavities and micropores as in other carbon materials.

The adsorption nature of SWNTs is not so simple as is judged from its appearance. It is important to modify the

Conventional carbons

Newly-developed carbons (including nanocarbons)

Graphite electrodes Carbon blacks Activated carbons

Classic ■ carbons

New carbons

Natural diamonds

Newly-developed carbons (including nanocarbons)

Classic ■ carbons

New carbons

Figure 33. Proposed scheme for classification of carbon materials, including nanocarbons. Reprinted with permission from [126], M. Inagaki and L. R. Radovic, Carbon 40, 2263 (2002). © 2002, Elsevier Science.

higher order arrangement as well as the bundle structure and the inner-tube spaces depending on the applications of the SWNT. The situation of multiwalled carbon nanotubes seems more complex, because this substance is known to include mesopores.

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