Biomedical Engineering Reference
In-Depth Information
formed nanotubes. The growth mechanism of graded TiO
2
nanotube
arrays is schematically presented in Fig. 9.37. After the step-1
anodization (in H
3
PO
4
+ HF), the highly ordered TiO
2
nanotubes
grow upon a barrier layer of the titanium oxide and hydroxide
(Fig. 9.37a). This layer of TiO
2
nanotubes has rough side-wall and
wide diameter. The formation of rough side-wall may be due to
the voltage oscillations during the anodization process in aqueous
electrolytes. After immersing the already formed TiO
2
nanotubes in
glycerin + NH
4
F electrolyte, the electrochemical environment alters
due to the changes of the electrolyte composition. Dissolution and
breakdown of the barrier layer at the bottom of the already formed
TiO
2
nanotubes occur in the initial stage of the step-2 anodization
(Fig. 9.37b). The formation of the breakdown sites is due to the
high electric ield intensity at the bottom of the already formed
TiO
2
nanotubes. The breakdown sites act as seeds to the growth of
a new layer of TiO
2
nanotubes (Fig. 9.37c) and the new nanotubes
grow directly from the breakdown sites, which are the bottom of
the already existing TiO
2
nanotubes. By extending the time of the
step-2 anodization, graded TiO
2
nanotube arrays can be formed
(Fig. 9.37d). The anodization in step-1 can produce higher electric
ield intensity and hence faster chemical dissolution rate. In the
step-2 anodization, at lower electric ield intensity, a slower
chemical dissolution rate dominates.
Figure 9.37
Growth mechanism of graded TiO
2
nanotube arrays: (a)
already formed TiO
2
nanotubes by the step-1 anodization,
(b) breakdown of the barrier layer in the bottom of the
already formed TiO
2
nanotubes, (c) growth of a new layer of
TiO
2
nanotubes, (d) graded nanotubular structure [113].