Biomedical Engineering Reference
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(a)
(b)
200 nm
20 nm
(c)
Semicrystalline
barrier layer
Amorphous
tube wall
Ti substrate
5 nm
FIGURE 5.9
TEM views at various sites of the nanotube (a) and (b) are cross-section views; (c) HRTEM image of Ti sub-
strate barrier layer and nanotube oxide. (From Nguyen et al.,
Electrochim. Acta
, 54, 4340-4344, 2009. With
permission.)
approximately 5 to 10 nm thick can be observed between the tube and metal (Figure 5.9b).
This barrier layer is sculpted not only along the tube barrier interface, but also along the
barrier/metal substrate interface. The high-resolution TEM images disclose the amor-
phous structure of the nanotube wall and that the barrier layer is actually semicrystalline
in nature. Most of the oxides are indeed amorphous, but there are small crystalline regions
embedded in the amorphous matrix.
The elemental distribution and chemical states along the nanotube length of titania nano-
tube are studied by x-ray photoelectron spectroscopy (XPS) and the results are presented in
Figure 5.10. This nanotube is prepared by anodization in an electrolyte containing 1 mol/L
Na
2
SO
4
, 0.1 mol/L NaF, 0.2 mol/L citric acid at a voltage 20 V for 20 h. The surface contains
a high content of O. The Ti and O signals are stable during the initial 6 min of sputtering.
Progressively increased Ti concentrations and quick drop in the O content are observed
after about 58 min corresponding to the thickness of the barrier layer formed by anod-
ization. The Ti 2p signal in the surface region only consists of one chemical state, Ti
4+
/
TiO
2
.
After sputtering for about 6 min, peaks corresponding to Ti
2+
/TiO and Ti
3+
/Ti
2
O
3
also appear. When the sputtering time reaches about 70 min, a high Ti content is observed
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