Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 271

Ichihashi, 1993). A SWNT is close to an ideal fullerene fiber, as it has single-layer
cylinders with small diameters (typically ∼1 nm). In fact, SWNTs self-assemble into
larger bundles composed of several to tens of nanotubes (Ajayan, 1999).

(a) (b)

Figure 10.2 (a) Single-walled nanotubes, and (b) multiwalled nanotubes.

MWNTs can be synthesized by the carbon arc method, as discovered by Iijima.
MWNTs are formed on cathode surfaces without the use of a metal catalyst. They
typically have outer diameters in the range of 3 - 50 nm, and inner diameters in the 1-4
nm range. However, the amount of material produced is limited and is usually
accompanied with a substantial amount of nanoparticles (Ajayan, 1999). MWNTs can
also be produced by the catalytic pyrolysis of hydrocarbon over metals, which play the
role of catalyst. Other methods in use for the production of MWNTs include template
based approaches and plasma-assisted hot filament chemical vapor deposition (Andrews
et al., 2002; Loutfy et al., 2002). On the other hand, SWNTs can be produced by the arc,
laser ablation, or chemical vapor deposition process in the presence of a catalyst (Dai,
2002; Loutfy et al., 2002). SWNTs produced by these methods possess a diameter in the
range 1 - 5 nm. After synthesis, both MWNTs and SWNTs need to be purified to
remove fullerenes, partially graphitized carbon (glassy carbon), amorphous carbon, and
possibly metal catalyst particles (if a catalyst is present). Usually, a purification
procedure follows these common steps: (1) removing large particles by preliminary
filtration; (2) dissolution to remove fullerenes in organic solvents and metal catalysts in
concentrated acids; and (3) microfiltration again to separate MWNTs or SWNTs from
nanoparticles and amorphous carbon (Ajayan, 1999).

10.1.3 Mesoporous Carbon

First, we should differentiate mesoporous carbon and nanoporous carbon.
Mesoporous carbon refers to porous carbon with mesopores (2 - 50 nm), whereas