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immobilized nanoparticles), lowering of photodegradation performance in
the immobilized case was due to reduction in active surface area for adsorp-
tion and subsequent catalyst action. However, the recovery of nanoparticles
is easy when the particles are immobilized.
Degradation of nitrobenzene by using nano-TiO
2
and ozone was studied
by Yang
et al.
(2007). They compared the effect of nano-TiO
2
catalysed plus
ozone and ozone only and found that the catalysed ozonation was more
effi cient than ozone alone. Sobana
et al.
(2006) prepared silver nanoparti-
cles doped with TiO
2
and used them for the photodegradation of direct azo
dyes.
16.4.4 Carbon nanotubes
A carbon nanotube (CNT) is a one-atom thick sheet of graphite (called
graphene) rolled up ino a seamless cylinder with diameter of the order of
a nanometer and capped at both ends by hemispheres of fullerene. This
results in a nanostructure where the length-to-diameter ratio exceeds
10,000. CNTs can be categorized by their structures as single-walled nano-
tubes (SWNT) and multi-walled nanotubes (MWNT). In the most general
way, the CNT can be shown as composed of a concentric arrangement of
several cylinders (Fig. 16.4). The high curvature of the graphene sheets
increases the total energy of the tubules per carbon atom, but this is more
16.4
Multi-walled carbon nanotube shown as composed of a
concentric arrangement of several graphene cylinders.
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