Characteristics of the Pores in Carbon Materials

As already mentioned in the Introduction, the small pores in carbon materials are spaces between stacked aromatic layers or interfacials/interstitials of the crystallites consisting of these stacked layers. Thus, it has been generally accepted that micropores in the carbonaceous materials like activated carbons possess slitlike structure (Fig. 1a).

The carbon nanotube was discovered by Iijima in 1991 [18]. Since a single walled carbon nanotube (SWNT) is composed of a monolayer graphene sheet rolled up into a long seamless cylinder, the substance has a structure of packed bundles of the tubes, in which the pores are located inside of the tubes as well as at intertubular channels. These pores shown in Figure 5 are of round shape, being considerably different from those in the conventional carbons [19]. In 1999 also Iijima et al. have found an analogous type of nano-tube called a nanohorn, which was obtained by CO2 laser

Figure 5. Schematic structure of a single nanotube bundle showing the pore, groove, channel, and surface sites. Reprinted with permission from [19], B. K. Pradhan et al., Mater. Res. Soc. Symp. Proc. 633 (2001). © 2001, Materials Research Society.

ablation of a graphite rod [20]. TEM images of particles prepared suggest that each particle consists of a tubularlike structure with cone caps. Based on the TEM observation they proposed a model for the packing of carbon nanohorns in a dahlia particle [20]. Furthermore, Murata et al., have measured the N2 adsorption isotherms of these nanohorns and reported geometrical models for the horn and its aggregate structure shown in Figure 6 [21]. Accordingly, these modern carbon materials have round-shaped pores, as easily supposed from the structure of Figures 5 and 6.

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