Fluorination of Carbon Nanotubes

Because of the low reactivity of the surface of CNTs, fluorination was taken into consideration as one of the first sidewall functionalization reactions [27]. Fluorine as the most electronegative element in its elemental form is a powerful oxidizer. Mickelson et al. reported in 1998 extensive controlled and nondestructive sidewall fluorination of SWCNTs (Fig. 1.3) [28]. The functionalized F-SWCNTs dissolved well in alcohol and gave long-living metastable solutions [29]. 

Most of the experimental work on fluorination has been performed using elemental fluorine diluted in an inert gas at elevated temperatures [28, 30, 31], Comparative fluorination of open- and closed-end SWCNTs was performed with ele- 

---

The fluorination of carbon nanotubes was first reported by Margrave and is traditionally performed by using elemental fluorine in the presence of small amounts of HF which serves as catalyst [24]. The loading turns out to be very high, with one fluorine atom every two carbon atoms. 

Halogenation The fluorination of carbon nanotubes is an important primary functionalization because it may be conducted even as heterogeneous process between gaseous and solid phase. The reaction of fluorine with SWNT, for example, can be performed in a tube furnace at about 150°C. It yields perfluori-nated nanotubes with a degree of fluorination of up to 100%. 

Taking into consideration that only the inner wall surface of carbon nanotubes is exposed to atmosphere in the stage of carbon-deposited alumina film, it would be possible to modify only the inner surface if the carbon-deposited alumina film is chemically treated. On the basis of this concept, Hattori et al. tried to fluorinate only the inner surface of carbon nanotubes (42). It is well known that fluorination is quite an effective way to introduce strong hydrophobicity to carbonaceous materials, and it perturbs the carbon it electron system (43,44). Thus, by the selective fluorination of nanotube s inner surface, it would be possible to produce carbon nanotubes whose inner surface is highly hydrophobic and electrically insulating while their outer... 

Fig. 10.1.15 Schematic diagram of the fluorination process of carbon nanotube.

In Figure 10.1.17, the Cls XPS spectrum of the carbon-deposited film fluorinated at 175°C is compared with that of the corresponding fluorinated tubes after the HF washing. The former spectrum and the latter one give information on the external flat surface of the fluorinated film and the outer surface of the fluorinated tube, respectively. In spectrum (a), the most intense component is the peak assigned to covalent CF bond. On the other hand, spectrum (b) exhibits a clear peak at 284.4 eV, which corresponds to the sp2 carbon of the nanotube. These findings indicate selective fluorination of carbon tubes the covalent CF bonds were formed almost exclusively on the inner surfaces of nanotubes, but the outer surfaces retained their sp2 hybridization. 

Thus, for the first time it is shown that carbonic nanomaterials (fullerene, single-and multiwall nanotubes, nanofibers) demonstrate high activity at cryogenic conditions (77K) in reactions of chain halogenation (F2, Cl2) with kinetic chain length up to 104 -105. The ESR spectra of active free- radical intermediates were recorded. The presence of vibration bands of C-Cl bonds in products has been indicated by IR method. For the first time chain fluorination of carbonic nanofibers, mono- and multiwall nanotubes has been performed at low temperatures. 

Fig. 5. TEM image of carbon nanotube fluorinated at room temperature, Stage 1 C5 3F (reproduced with permission from Eur. J. Solid State Inorg. Chem., 33 (1996) 831 [27]).

Fullerene Cgo is easily fluorinated by elemental fluorine at temperatures less than 100°C [29,30] probably because of its nearly spherical surface and exposed it bonding. Direct fluorination of organic compounds by fluorine gas demonstrated that fluorine was preferentially bonded to a carbon atom with a high electron density [31,32], This may be a main reason why fullerene C60 is more easily fluorinated than graphite. On the contrary, the fluorination behavior of carbon nanotube is similar to that of graphite as already shown. 

Figure 3.70 Halogenation of carbon nanotubes. The reaction at least partly takes place as 1,4-addition. Fluorinated nanotubes are good starting materials for further functionalization (bottom).

Fedoseeva, Y. V., Bulusheva, L. G., Okotrub, A. V., Vyalikh, D. V., Fonseca, A. 2010. High reactivity of carbon nanotubes and fluorinated carbon nanotubes irradiated by Ar+ ions. Phys. Status Solidi B 247 (11-12) 2691-2694. 

For a very nice summary of specific stiffness/ specific strength regions for various materials classes see http //www-materials.eng.cam.ac.uk/mpsite/ interactive charts/spec-spec/basic.html Fluorination has recently been used to attach amine-terminated polymers to the sidewalls of carbon nanotubes for instance, see (a) Dillon, E. P. Crouse, C. A. Barron, A. R. AC5 Nano 2008,2,156. 

Kane, C. L., Mele, E. J. (1997). Size, shape, and low energy electronic structure of carbon nanotubes. Physical Review Letters, 78(10), 1932-1935. Kataura, H., Kumazawa, Y., Maniwa, Y, Umezu, I., Suzuki, S., Ohtsuka, Y, Achiba, Y. (1999). Optical properties of single-wall carbon nanotubes. Synthetic Metals, 103, 2555-2558. Khabashesku, V., Billups, W, Margrave, J. (2002). Fluorination of single-wall carbon nanotubes and subsequent derivatization reactions. Accounts of Chemical Research, 35,1087-1095. Kim, Y, Choi, J., Chang, K., Tomanek, D. (2003). Defective fullerenes and nanotubes as molecular magnets An ab initio study. Physical Review B, 68,125420. 

Chen, X., Burger, C Fang, D Sics, I., Wang, X., He, W. et al. (2006) In-situ X-ray deformation study of fluorinated multiwalled carbon nanotube and fluorinated ethylene-propylene nanocomposite fibers. Macromolecules, 39, 5427-5437. 

Fig. 3.7 Fluorination and further substitution of pristine carbon nanotubes.

T. Nakajima, S. Kasamatsu, Y. Matsuo, Synthesis and characterization of fluorinated carbon nanotubes, European Journal of Solid State Inorganic Chemestry, vol. 33, pp. 831-840,1996. 

An XPS scan of flame annealed nanotubes revealed the presence of carbon in all samples and a summary of the carbon content and carbon state information is provided in Table 5.5 [98], Fluorine was... 

A carbon-deposited film was prepared from the alumina film with 30-nm channels by the CVD technique using propylene. Fluorination was carried out by direct reaction of the film with dry fluorine gas (purity 99.7%). The film was placed in a nickel reactor and was allowed to react with 0.1 MPa of fluorine gas for 5 days at a predetennined temperature in the range of 50 to 200°C. Then the fluorinated carbon nanotubes were separated by dissolving the alumina film with HF. A schematic drawing of the fluorination process is given in Fig. 10.1.15. 

Fig. 10.1.17 Cls XPS spectra of (a) the fluorinated carbon deposited film and (b) the carbon nanotubes from the film (fluorination temperature 175°C). (From Ref. 42.)...

---