524
V. S. Simi et al.
possible to obtain nanotubes over 100 μm by extending the anodization time [64].
However, the length of the nanotubes has no noteworthy influence on the biocompatibility of nanotubes as the cells contact initially on the upper surface of the tubular
structure. On the other side, it was reported that the tube length has a great effect on
drug release rate which can be controlled by tuning the aspect ratio of the nanotubes
[65].
7.3 Effect of Electrolyte Composition
The nanostructures are usually formed by a variety of fluoride-containing aqueous
[51, 66] or organic [67, 68] electrolyte by anodizing titanium at different voltages. It
has been reported that the geometry of the formed nanotubes has a direct influence on
the basic electrolyte parameters such as pH, fluoride ion concentration temperature
and water content [48, 61, 69]. The fluoride ion concentration in the electrolyte has an
important role in the self-ordering of nanotubes. The fluoride concentration should
be minimum and sufficient to ensure the growth of nanotubes. The studies have
shown that the typical F
− concentration leading to the self-ordering of nanotubes is
in the range of 0.3–0.5 wt% [70, 71]. The presence of fluoride ions in the electrolyte
selectively etches the surface of titanium during the growth of oxide layer, which
allows the formation of nanotubes. An increase in the concentration of fluoride ions
obtained from the addition of HF or NH 4 F increases nanotube diameter and length
[72]. The influence of fluoride ion concentration in two different electrolytes 1 M
(NH 4 ) 2 SO 4 and 1 M Na 2 SO 4 on TiO 2 nanotubes was reported. Nanotubes grown in
organic electrolyte tend to be longer, homogenous and have smoother walls as the
chemical dissolution and fluoride ion transmission rate is relatively low in viscous
organic electrolytes [73, 74].
The water content in the electrolyte plays a vital role in the formation of titania
nanotubes. Water content, the source of oxygen in most electrolytes is used to generate
the initial oxide layer and has been reported to have a direct effect on nanotube
wall thickness and length [48]. Raja et al. [75] reported that, to generate nanotube
arrays on titanium, a minimum water content of 0.18 wt% is necessary in an electrolyte containing NH 4 F and ethylene glycol. Mohammed et al. [76] reported that,
an increase in nanotube diameter, wall thickness and length, when the water content
increases up to 30% in an HH 4 F containing electrolyte at a constant potential of
20 V. Various studies [62, 70, 77] reported that the nanotubes formed from low water
content electrolyte results in smooth tube walls, whereas side wall ripples are formed
at higher water content. This could be attributed to the faster etching of the fluoriderich layer between the tubes than the growth speed of the tubes into the underlying
substrate.
To evaluate this aspect, the water content in the electrolyte was varied by holding
the anodization voltage and anodization time to 20 V and 1 h respectively [69]. A
mixed solution of glycerol and ammonium fluoride with different water contents
10, 20 and 40 volume % was used as the electrolyte. The obtained nanotubes were
V. S. Simi et al.
possible to obtain nanotubes over 100 μm by extending the anodization time [64].
However, the length of the nanotubes has no noteworthy influence on the biocompatibility of nanotubes as the cells contact initially on the upper surface of the tubular
structure. On the other side, it was reported that the tube length has a great effect on
drug release rate which can be controlled by tuning the aspect ratio of the nanotubes
[65].
7.3 Effect of Electrolyte Composition
The nanostructures are usually formed by a variety of fluoride-containing aqueous
[51, 66] or organic [67, 68] electrolyte by anodizing titanium at different voltages. It
has been reported that the geometry of the formed nanotubes has a direct influence on
the basic electrolyte parameters such as pH, fluoride ion concentration temperature
and water content [48, 61, 69]. The fluoride ion concentration in the electrolyte has an
important role in the self-ordering of nanotubes. The fluoride concentration should
be minimum and sufficient to ensure the growth of nanotubes. The studies have
shown that the typical F
− concentration leading to the self-ordering of nanotubes is
in the range of 0.3–0.5 wt% [70, 71]. The presence of fluoride ions in the electrolyte
selectively etches the surface of titanium during the growth of oxide layer, which
allows the formation of nanotubes. An increase in the concentration of fluoride ions
obtained from the addition of HF or NH 4 F increases nanotube diameter and length
[72]. The influence of fluoride ion concentration in two different electrolytes 1 M
(NH 4 ) 2 SO 4 and 1 M Na 2 SO 4 on TiO 2 nanotubes was reported. Nanotubes grown in
organic electrolyte tend to be longer, homogenous and have smoother walls as the
chemical dissolution and fluoride ion transmission rate is relatively low in viscous
organic electrolytes [73, 74].
The water content in the electrolyte plays a vital role in the formation of titania
nanotubes. Water content, the source of oxygen in most electrolytes is used to generate
the initial oxide layer and has been reported to have a direct effect on nanotube
wall thickness and length [48]. Raja et al. [75] reported that, to generate nanotube
arrays on titanium, a minimum water content of 0.18 wt% is necessary in an electrolyte containing NH 4 F and ethylene glycol. Mohammed et al. [76] reported that,
an increase in nanotube diameter, wall thickness and length, when the water content
increases up to 30% in an HH 4 F containing electrolyte at a constant potential of
20 V. Various studies [62, 70, 77] reported that the nanotubes formed from low water
content electrolyte results in smooth tube walls, whereas side wall ripples are formed
at higher water content. This could be attributed to the faster etching of the fluoriderich layer between the tubes than the growth speed of the tubes into the underlying
substrate.
To evaluate this aspect, the water content in the electrolyte was varied by holding
the anodization voltage and anodization time to 20 V and 1 h respectively [69]. A
mixed solution of glycerol and ammonium fluoride with different water contents
10, 20 and 40 volume % was used as the electrolyte. The obtained nanotubes were