Environmental Engineering Reference
In-Depth Information
conformations: single-walled CNTs (SWCNTs) or multiwalled CNTs (mWCNTs). SWCNTs comprise a single-layer cylindrical
graphite sheet, whereas mWCNTs comprise two or more concentric cylindrical shells of graphite coaxially arranged around a
central hollow area with a space between layers [7, 8]. Both SWCNTs and mWCNTs can generally be synthesized by subject-
ing a precursor, such as graphite, to an arc discharge [9] or laser irradiation [10] in the presence of a catalyst metal particle such
as iron (Fe), cobalt (Co), or nickel (Ni). An alternative synthesis route is chemical vapor deposition (CVD) [11], in which
gaseous hydrocarbon sources are catalytically decomposed with a metal nanoparticle supported on a substrate.
The distinctive properties of the sp
2
-hybridized bonding in CNTs provide unusual mechanical stability, high electrical and
thermal conductivity, and strong chemical reactivity [12]. However, CNTs tend to agglomerate because of strong van der Waals
interactions, causing these CNTs to be insoluble and hard to disperse in almost all solvents. This phenomenon critically affects
the performance of CNTs in water treatments and the quality of their hybrid materials. modification of CNTs by functionaliza-
tion is indispensable to solving the problem. Functionalization is an effective strategy for improving the surface characteristics
of CNTs by introducing suitable functional moieties into CNT walls [13]. Figure 8.1 shows that the various extensively devel-
oped functionalization approaches can be categorized into covalent and noncovalent sidewall functionalization.
The local strain arising from the pyramidalization and misalignment of the π orbitals of the sp
2
-hybridized carbon atom
causes CNTs to become more reactive and more likely to be covalently attached onto other chemical species [14]. The most
popular process in covalent sidewall functionalization can be achieved by defect functionalization. In this process, acidic solu-
tions (such as hydrochloric acid (HCl), nitric acid (HNO
3
), or sulfuric acid (H
2
SO
4
)) or oxidizing agents (such as potassium
permanganate (KmnO
4
) or hydrogen peroxide (H
2
O
2
)) are used to create a defect site in CNTs by opening their end caps and
attaching the caps to hydroxyl, carbonyl, and carboxyl groups [15]. The presence of these oxygen-containing functional groups
exfoliates CNTs from aggregation and facilitates the dispersion of CNTs. These functional groups also serve as anchor sites for
amidation, esterification, thiolation, and silanization reactions. Various functional molecules including biomolecules [16, 17]
and polymers [18, 19] are then enabled to be covalently bonded with CNTs.
CNTs are also known for their reactivity with increased curvature [20]. Thus, the treatment of CNTs with highly reactive
species such as radical aryls, nitrenes, carbines, or halogens [21, 22] can induce a change in hybridization from sp
2
to sp
3
in the
carbon atom and a simultaneous loss of the π conjugation system on the graphene layer [23]. These changes lead to the occur-
rence of direct sidewall functionalization. Direct coupling of functional groups such as fluorine onto the π-conjugated carbon
framework can be further modified by nucleophilic substitution reactions and replaced by hydroxyl, amino, or alkyl groups [6,
24]. This direct coupling is similar to defect functionalization. This approach reduces the formation of defect sites that may
inflict damage to the CNT structures and affect their intrinsic properties. An example of direct sidewall functionalization is
through the N
2
plasma technique. In this approach, active species −C* or −C-N* are produced from the reaction between CNTs
and active nitrogen species (N*) and serve as active sites for grafting functional molecules onto CNT frameworks [25]. This
method has a great advantage of eliminating the use of large amounts of chemicals.
Noncovalent functionalization is particularly attractive because it involves physical interactions between CNTs and
functional molecules without disturbing the bonding structure of CNTs. Van der Waals interaction and π-π stacking are
the physical interactions responsible for noncovalent bonding. Surfactant adsorption is one of the typical methods of
noncovalent functionalization. Surfactants are well known for their amphiphilic properties because of the hydrophilic
region composed of a polar head group and a hydrophobic region composed of an alkyl chain tail in the structure [26] of
surfactants. Thus, dispersing CNTs in surfactants such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfo-
nate, triton x, and siloxane polyether copolymer [27-29] facilitates the interaction of CNTs with the hydrophobic region
Chemical functionalization
Covalent sidewall
functionalization
Noncovalent sidewall
functionalization
Defect
functionalization
Direct sidewall
functionalization
Surfactant
adsorption
Polymer
wrapping
fiGure 8.1
Functionalization of CNTs.
