Template Methods

Template methods essentially consist of the following three steps: charging of organic materials into nanospaces of the template inorganic materials, carbonization of the organic-inorganic nanocomposites, and liberation of carbons by removing the template inorganic materials [68]. Knox et al. prepared mesoporous carbons by carbonization of phenolic resin impregnated silica gel or porous glasses followed by removing the inorganic templates [69-71]. This carbon is commercially available for column packing materials for liquid chromatography. Kamegawa and Yoshida also carbonized organic-silica composite and obtained carbon materials that can be swollen by contact with organic vapors [72, 73]. In this case, they made very thin organic layers that were fixed by estrification to the surface of template silica.

In the previous examples, the pores in the porous materials were utilized as templates, in other words, carbon precursors penetrated into the template's pores. There are examples including embedded templates. Hyeon et al. used a colloidal silica (ca. 12 nm in diameter) as a template to be embedded in the resorcinol resin [74]. They obtained carbons of which surface area, total pore volume, and mesopore volume were 950 m2/g, 5.5 cm3/g, and 4.8 cm3/g, respectively, when the resorcinol/silica ratio was 7.5. Li and Jaroniec developed an imprinting method in which template silica particles were imprinted into mesophase pitch spheres [75]. In this case, carbonization and template removal were also included. This method gave a possibility of controlling the spatial distribution of the pores such as uniform distribution or biased distribution to surfaces depending on the extent of the imprinting depth into the mesophase pitch, as depicted in Figure 20. Kawashima et al. prepared mesoporous carbons from polymerized furfuryl alcohol in the presence of silica sol made by hydrolysis of tetraethoxysilane [76].

There are many attempts to use ordered inorganic materials as templates. Clays and zeolites form a class of porous materials with ordered pore structures defined by crystalline ordering. Kyotani et al. synthesized very thin carbon film by carbonization of the in-situ polymerized polyacrylonitrile or poly(furfuryl alcohol) in the interlayer space of a mont-morillonite [77-79]. The obtained carbon was easily graphi-tized even though the carbon precursors were polymers that usually gave nongraphitizing carbons. Actually, this is the starting point of this type of study. Kyotani et al. tried to prepare carbon material with controlled pore distribution by using these microporous crystalline templates [80]. They

Volume imprinted carbon

Bi-porous imprinted carbon

Bi-porous imprinted carbon

Catalyst particle

Colloid imprinted catalyst

Colloid imprinted catalyst

Figure 20. Concept of colloid imprinting method for preparing spatially regulated porous carbons. Reprinted with permission from [75], Z. Li and M. Jaroniec, J. Am. Chem. Soc. 123, 9208 (2001). © 2001, American Chemical Society.

could obtain high surface area carbon (2200 m2/g) by using Y-zeolite as a template [81]. They also made another effort to fill the zeolite's pores with carbon as much as possible by utilizing both in-situ polymerization of furfuryl alcohol and chemical vapor deposition of propylene into pore spaces. The obtained microporous carbon had a periodical ordering of 1.4 nm, which is the same as the spacing of the 111 plane of Y-zeolite as shown in Figure 21.

Figure 21. (a) High resolution transmission electron micrographs of a carbon prepared by using Y-zeolite as a template and (b) its electron diffraction pattern. Reprinted with permission from [81], Z. Ma et al., Chem. Commun. 2365 (2000). © 2000, Royal Society of Chemistry.

Apart from the utilization of microporous inorganic material as template, two Korean research groups reported that they successfully prepared regularly ordered mesoporous carbons by using mesoporous silica as templates. Lee et al. used an aluminum substituted mesoporous silica, MCM-48, as a template [82]. Phenol and formaldehyde were polymerized in the template and subjected to carbonization and removal of template. The obtained carbon showed an X-ray diffraction pattern resembling the template. Electron microscopic analysis revealed that there were small pores of 2 nm in diameter separated by a carbon wall with a thickness of 2 nm. Ryoo et al. also reported the formation of highly ordered porous carbon by using the same mesoporous silica, MCM-48 [83]. However, they employed different carbon precursor materials, sucrose and sulfuric acid. Figure 22 shows the X-ray diffraction of the template (a), template with carbon (b), and carbon itself (c). Highly ordered pores can be observed in an electron micrograph presented in Figure 23. Recently Joo et al. compared the carbon precursor for the mesoporous silica templated carbon [84]. When furfuryl alcohol was used as a precursor, rodlike carbon was obtained, while in the case of sucrose with sulfuric acid, like Ryoo et al., pipelike carbon was obtained. They loaded platinum on the pipelike carbon and found that the activity for oxygen reduction in fuel cells was extremely high (100 A/g-Pt).

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