Biomedical Engineering Reference
In-Depth Information
During the Ti anodization process in aqueous acidic solution,
Ti could be oxidized into various types of oxides, such as TiO
2
and
Ti
2
O
3
with different valence states, as long as the positive potential
applied on the titanium anode was higher than its theoretical
oxidation potentials (1.630 V for Ti/Ti
2+
, 1.998 V for Ti/Ti
3+
,
2.188 V for Ti/Ti
4+
vs SCE) [108]. The higher applied potential can
results in the complete transformation of Ti into TiO
2
. During anodic
oxidation, a competition between an oxidation reaction (9.4) and
dissolution reaction (9.5, 9.6) occurs:
Ti + 2H
2
O
→
TiO
2
+ 4H
+
(9.4)
TiO
2
+ 6HF
→
[TiF
6
]
2−
+ 2H
2
O + 2H
+
(9.5)
TiO
2
+ H
2
O + H
+
→
[Ti(OH)
3
]
+
(9.6)
Furthermore, reaction generating oxygen from water occurs
(9.7):
2
O + 4e
−
→
O
2
+ 4H
+
(9.7)
The voltage applied to titanium anode, the anodizing time, and
the composition of the electrolyte are the most important factors
to control the surface morphology and crystal structure of the
product TiO
2
/Ti ilms [108].
Jakubowicz [28]
investigated etching of the Ti-electrode at low
voltages in H
3
PO
4
electrolyte containing HF addition. Figure 9.10
presents morphology of the commercially pure titanium after
anodic oxidation in 1M H
3
PO
4
electrolyte containing following
amount of HF: 0.5% (a), 2% (b) and 10% (c). The process carried
out at relatively low voltage of 10 V vs. ocp for 30 min results in
pore formation. When the HF concentration increases, the pore
diameter signiicantly increases, too. For the low HF concentration
0.5% (a), the average pore diameter is in the nanometer range and
is approximately 45 nm. For the 2% and 10% HF concentration, the
pore diameter signiicantly increases up to 1.5 μm (b) and 5.2 μm
(c), respectively. Depending on the electrochemical conditions,
two types of layers can be formed: nanoporous and microporous.
Tsuchiya
et al
. [89] found a two-layers structure, with outer
nanoporous and underneath with larger diameter tubes layer.
