Biomedical Engineering Reference
In-Depth Information
[10, 18, 84, 120]. It is known that an increase in nanotube length
enhances the effective surface area and the resistance of the oxide
ilm, which is promising in implant applications.
Yang
et al
. [113] have used a two-step anodization procedure at
potential 20 V for the nanotube formation (Fig. 9.36). The two-step
anodization consists of the following:
Figure 9.36
SEM cross-sectional images of TiO
2
nanotube arrays fabricated
by two-step anodization for different step-2 anodization
duration: (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 4 h [113].
(i) Anodization of titanium foil in an aqueous electrolyte (1M
H
3
PO
4
+ 0.5 wt% HF) for 2 h, followed by rinsing with DI water
and drying in ambient air
(ii) Anodization of the specimen in a nonaqueous electrolyte
(glycerin containing 0.5 wt% NH
4
F)
They found that TiO
2
nanotubes formed in aqueous electrolyte
(H
3
PO
4
+ HF) have a much wider diameter compared to nanotubes
formed in nonaqueous electrolyte (glycerin + NH
4
F). The
morphologies of the TiO
2
nanotubes formed by anodization are
strongly dependent upon the electrolyte composition. Figure 9.36
shows the SEM cross-section images of the TiO
2
nanotubes arrays
fabricated by two-step anodization. After anodization for 0.5 h,
another layer of TiO
2
nanotubes with 150 nm in length grows
underneath the already formed nanotubes. The extension of the
anodization time to 4 h results in the 2 μm TiO
2
nanotubes growth.
The two different layers of nanotubes connect closely, exhibiting
any exfoliation. Yang
et al
. [113] suggest that the new layer of the
TiO
2
nanotubes grows directly from the bottom of the already
