5.3 Nanostructures Related to Compounds with Layered Structures 109
and consumed by the growing nanotube. In a simplified way, this process is
depicted in Figure 5.34.
Electron micrographs of such zinc oxide nanotubes are displayed in Figure
5.35. The two ZnO nanotubes have entirely different dimensions. The one
displayed in Figure 5.35a [24], which is rather a micro- than a nanotube, has
an edge length of more than 1 μm and a wall thickness of approximately
200 nm. Entirely different is the nanotube displayed in Figure 5.35b [25]. In
that case, the edge length is ca. 200 nm, and the wall thickness less than 50 nm.
Looking in detail at the micrograph in Figure 5.35b, it is most interesting to
see that, possibly, this nanotube grew as a spiral.
Figure 5.34 Growth of a ZnO nanotube
from a metallic zinc particle having formed
a shape with minimum surface energy.
(According to Xing et al. [23]) (a) The tube
is formed by the oxidation of the lateral
surfaces of the zinc embryo. (b). As long as
there is material supplied from the metallic
embryo, the nanotube grows. The growth
comes to an end when the zinc reservoir is
depleted.
Zn embryo
Growing ZnO nanotube
(a)
(b)
(c)
Figure 5.35 Hexagonal ZnO nanotubes
grown from metallic zinc embryos. In case
(a), the outer surface is very flat, whereas
the inner surface is corrugated [24]
(Reproduced with opermission by John
Wiley & Sons.). In the case of the smaller
zinc oxide nanotube, (b) it is interesting to
see that the tube has grown, possibly, as a
spiral [25]. (Reproduced with permission by
Elsevier.)
1 µm
120 nm
(a)
(b)