86 5 One- and Two-Dimensional Nanoparticles
following the chemical formula Al 2 O 3 ⋅(SiO 2 ) x ⋅(H 2 O) y , with 1.3 < x < 2 and
2.5 < y < 3. Generally, allophanes crystallize in hollow spherules with diameters
in the range between 3 and 5 nm. Under special conditions of synthesis and
depending on the composition, one obtains tubes with a diameter in the range
from 2 to 5 nm. The diameter of the tubes can be adjusted by the relative aluminum content. Additionally, some of the aluminum may be exchanged by iron ions
with the same valency. This substitution influences the diameter and color of the
tubes.
Comparing the three possibilities of producing one- and two-dimensional
nanostructures, those related to layered compounds are, with respect to technical
applications, the most important ones.
Figure 5.7 Three tungsten disulfide WS 2 nanoparticles, each one consisting only of a few
lattice planes, bonded together to saturate dangling bonds at the circumference (Szabo, D.V.,
and Vollath, D., KIT, Germany, unpublished results (1998).)
5 nm
Box 5.4 Imogolite, a One-Dimensional Silicate
An important compound in the group of one-dimensional silicates is imogolite,
(Al 2 O 3 )⋅(SiO 2 ) x ⋅2.5(H 2 O). Imogolite tubes with this composition have an inside
diameter around 1 nm and outside diameters of ca. 2 nm; both can be adjusted
by the silicon / aluminum ratio. Figure 5.8 displays the atomic arrangement
in a cross section of such an imogolite tube.
This structure of imogolite is characterized by aluminum, silicon, oxygen,
and (OH)
− ions arranged in rings. The (OH)
− ions at the surface allow addition
of (“functionalize”) organic molecules.
As shown in Figure 5.9, imogolite can be synthesized in long fibers.
Figure 5.9 displays wet-chemically synthesized imogolite fiber, which are in
a diameter range from 5 to 30 nm [5]. The length of the tubes is in the microm-
following the chemical formula Al 2 O 3 ⋅(SiO 2 ) x ⋅(H 2 O) y , with 1.3 < x < 2 and
2.5 < y < 3. Generally, allophanes crystallize in hollow spherules with diameters
in the range between 3 and 5 nm. Under special conditions of synthesis and
depending on the composition, one obtains tubes with a diameter in the range
from 2 to 5 nm. The diameter of the tubes can be adjusted by the relative aluminum content. Additionally, some of the aluminum may be exchanged by iron ions
with the same valency. This substitution influences the diameter and color of the
tubes.
Comparing the three possibilities of producing one- and two-dimensional
nanostructures, those related to layered compounds are, with respect to technical
applications, the most important ones.
Figure 5.7 Three tungsten disulfide WS 2 nanoparticles, each one consisting only of a few
lattice planes, bonded together to saturate dangling bonds at the circumference (Szabo, D.V.,
and Vollath, D., KIT, Germany, unpublished results (1998).)
5 nm
Box 5.4 Imogolite, a One-Dimensional Silicate
An important compound in the group of one-dimensional silicates is imogolite,
(Al 2 O 3 )⋅(SiO 2 ) x ⋅2.5(H 2 O). Imogolite tubes with this composition have an inside
diameter around 1 nm and outside diameters of ca. 2 nm; both can be adjusted
by the silicon / aluminum ratio. Figure 5.8 displays the atomic arrangement
in a cross section of such an imogolite tube.
This structure of imogolite is characterized by aluminum, silicon, oxygen,
and (OH)
− ions arranged in rings. The (OH)
− ions at the surface allow addition
of (“functionalize”) organic molecules.
As shown in Figure 5.9, imogolite can be synthesized in long fibers.
Figure 5.9 displays wet-chemically synthesized imogolite fiber, which are in
a diameter range from 5 to 30 nm [5]. The length of the tubes is in the microm-