Biomedical Engineering Reference
In-Depth Information
(iii) The growth of nanotubes that corresponds to the slow current
decrease.
i
et al
. [11] observed that with a decreasing chemical
dissolubility (increasing pH) of the electrolytes, the current peak
disappears, and more time was therefore required for the nanotube
formation.
The electrochemical oxidation forms a dense oxide ilm (barrier
layer), which is typically conirmed by the oxide evolution. The
increase in the thickness of the barrier layer gave rise to an ohmic
resistance proportional to the thickness of the barrier layer. The
ohmic resistance is also related to the surface morphology. Cai
et
al
. [11] observed that the oxide layer formed in pH 2.8 electrolyte
(Fig. 9.53) is less dense than those formed in stronger acidic
electrolyte (pH < 1) and even in pH 4.2 electrolyte. The less density
of the oxide ilm formed in pH 2.8 electrolyte is related with the
relatively weaker hydrolysis ability compared with pH 4.2 electrolyte,
or the relatively weaker chemical dissolubility compared with the
strong acidic solution [11]. The decrease in anode mass during the
etching indicates that the chemical dissolution is signiicant in the
presence of F
−
(Eq. 9.9). The nanotube formation strongly depends on
the chemical dissolubility of the electrolytes. Increasing pH reduces
the dissolubility of the electrolytes. More time was desired for the
formation of nanotubes in higher pH electrolytes [11]. In pH 2.8
electrolyte, nanotubes were not completely formed after anodization
for 125 min. In absence of chemical dissolution, anodization
would inally be terminated with the barrier layer growing, while
no nanotubes could be formed at low anodic potential [11]. Long
nanotubes can be fabricated with the protection of the oxide
layer because the chemical dissolution proceeded mainly at the
outer surface of the oxide layer while the nanotubes were formed
under the barrier layer. By increasing the electrolyte pH, an oxide
layer can be maintained along the anodization [11].
Bestetti
et al
. [6] investigated the electrochemical formation
of nanotubular titanium oxide ilms in 1M H
2
SO
4
+ 0.05-0.4 wt%
HF electrolytes. In Fig. 9.54, the corresponding pore diameter and
spacing as a function of voltage are presented [6]. Increasing the
anodization voltage from 10 to 15, 20, and 25 V results in pore
diameter increasing to 28, 60, 92, and 103 nm, respectively. For
voltages of 30 V or higher, nanotubular structure disappears [6].
