Environmental Engineering Reference
In-Depth Information
Fullerenes are thermodynamically less stable than graphite and diamonds due to
the strain energy formed by their deviation from planarity, as can be seen in the
spherical shape of an unsaturated carbon network of fullerenes (Hirsch, 1994, 1999).
However, fullerenes are extremely strong molecules, able to resist high pressures of over
3000 atm. Fullerenes also exhibit a very large non-linear optical response (their optical
properties change with exposure to light). Interestingly, the thermal conductivity of
fullerene C
60
was found to be less than that for graphite or diamonds by more than three
orders of magnitude (Dresselhaus et al., 1996). This low value has been attributed to
both the high density of defect states associated with the merohedral disorder and a low
Debye temperature in C
60
as compared to graphite. The high electrical resistivity of
undoped C
60
and C
70
fullerenes implies that no carriers are available for transport in
fullerenes unless they are thermally or optically excited, or doped with a donor species.
For example, doping with alkali metals decreases the electrical resistivity of C
60
by
many orders of magnitude (Dresselhaus et al., 1996). The diamagnetic behavior of C
60
is
unique and distinctly different from that of graphite, because of an unusual cancellation
of ring currents in the molecules. Thus, the introduction of a magnetic dopant iron may
lead to Curie paramagnetism (Dresselhaus et al., 1996). Furthermore, it has been shown
that C
60
behaves like an electron deficient alkene because of the poor electron
delocalization as a result of the lack of double bonds in pentagonal rings, thereby
allowing C
60
to readily react with electron rich species (Hirsch, 1999). Lastly, fullerenes
are soluble in common solvents such as benzene, toluene, or chloroform.
Owing to their exceptional properties, fullerenes can be applied as optical
limiters and have potential for use in important photonic device applications
(Dresselhaus et al., 1996). Moreover, fullerenes can be a starting material for superhard
materials and diamonds, precursors for chemical vapor deposition diamond films and
SiC, lithographic films, solar cells, lubricants, catalysts, fullerene-containing polymers,
and medicines (Dresselhaus et al., 1996; Vul', 2002); a huge number of chemical
transformations of C
60
fullerene and outstanding properties of fullerene derivatives have
been discovered. Fullerenes are now an established compound class in organic chemistry
(Hirsch, 1994, 1999). Recent investigations have focused on the application of fullerenes
in the adsorption and preconcentration of contaminants from the environment, as well as
the toxicity and transport of fullerenes. This will be discussed in further detail in the
following sections.
10.2.2 Carbon Nanotubes
The novel structure of carbon nanotubes, with perfect lattice alignment and
closed topology, endows them outstanding properties such as high electrical
conductivity, excellent strength and stiffness as well as unusual electronic properties,
chemical properties, and thermal conductivity. The electronic properties of nanotubes
are dependent on its helicity structure and diameter; armchair tubes are metallic, whereas
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