Page 108 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

94 Principles and Methods

O O O O O O
Na + Na +
O O O O O O
- -
Na/Hg, toluene
SWNT SWNT
O O O -
O O O
O O O
Na +
O O O
Figure 3.48 Representation of the reaction between Na/Hg amalgam, dibenzo-
18-crown-6, and purified SWNTs in toluene and the formation of the [Na(dibenzo-
18-crown-6)] n [SWNT] complex.

polymers [194], or surfactants [195, 196]. These methods allow the sidewall
of the nanotube to remain untouched, and conserve the tube’s electronic
structure. However, upon drying of the solution, bundles re-form. SWNTs
may be made soluble in a range of organic solvents without sidewall
functionalization via their reduction by Na/Hg amalgam in the presence
of dibenzo-18-crown-6 (Figure 3.48) [197]. The [Na(dibenzo-18-crown-6)]
n n
[SWNT] complex shows solubility in CH Cl and DMF being comparable
2
2
to surfactant-dispersed SWNTs; however, measurable solubilities are also
observed in hexane, toluene, and alcohols.
A noncovalent functionalization is mainly based on supramolecu-
lar interaction using various adsorption forces, such as van der
Waals and -stacking interactions. Covalent functionalization relies
on the chemical reaction at either the sidewall or end of the SWNT.
The high aspect ratio of nanotubes, sidewall functionalization is
much more important than the functionalization of the cap. Direct
covalent sidewall functionalization is associated with a change of
2
3
hybridization from sp to sp and a simultaneous loss of conjugation
[198]. Defect functionalization takes advantage of chemical trans-
formations of defect sites already present. Defect sites can be the
open ends and holes in the sidewalls, and pentagon and heptagon
irregularities in the hexagon graphene framework. All these func-
tionalizations are exohedral derivatizations. Taking the hollow struc-
ture of nanotubes into consideration, endohedral functionalization
of SWNTs is possible—that is, filling the tubes with atoms or small
molecules [198–200].
Different application of nanotubes requires varied, specified modifi-
cation to achieve processibility and accessibility of nanotubes. Thus,
the covalent functionalization can provide a higher degree of fine-tuning
the chemistry and physics of SWNTs than noncovalent functionalization.
Until now, a variety of methods have been used to achieve the func-
tionalization of nanotubes (Figure 3.49).