Biomedical Engineering Reference
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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].
 
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