Tailoring the Surface Functionalities of Titania …
527
fluid flow of culture media, ions, proteins, nutrients essential for healthy cell growth,
increase spaces for the exchange of gas and cell signaling molecules for an overall
improved cell environment. In human body, due to circulation and repetitive loading
and unloading of bones during various activities, the fluid flow naturally occurs in
the interstitial spaces around the bone cells [78–80].
7.4 Effect of Electrolyte Temperature
Another important parameter that influences the growth of nanotubes is anodization
temperature. In most of the anodization process, the electrolytes are kept at room
temperature to produce the nanotubes. It has been reported that an increase in electrolyte temperature leads to the formation of nanotubes with a larger diameter, shorter
length and thinner walls [48]. The dissolution rates of the nanotubes are affected by
the electrolyte temperature [53]. Prida et al. [81] reported that the growth of nanotubes
is inhibited at lower temperatures (2 °C) in aqueous electrolyte, whereas the temperature range between 0 and 40 °C are reported to be most favorable for the growth of
nanotubes in organic electrolyte [62]. Wang et al. [82] reported that the diameter of
the nanotubes does not depend on the anodizing temperature in aqueous electrolyte,
whereas the nanotube diameter was found to increase at higher temperatures while
using in organic electrolyte. This could be due to the fact that at lower temperatures,
the viscosity increases, which reduces the ion migration especially fluoride ion and
as a result of which etching of TiO 2 gets slowed down, leading to smaller diameter
tubes.
The influence of electrolyte temperature during the anodization of Ti-6Al-7Nb
alloy was studied and reported [61]. Figure 12(a–f) illustrated the FESEM images
of anodic TiO 2 nanotube arrays obtained after anodization of Ti-6Al-7Nb for 1 h at
30 V in an electrolyte consisting of 1 M H 2 SO 4 and 0.08 M HF at 5, 10, 25, 30,
50 and 70 °C. The sample anodized at 5 °C (Fig. 12a) revealed a premature porous
structure with an un-dissolved oxide layer on the surface. At 10 °C, a porous oxide
layer with pore diameter ~35 nm covers the entire surface in an irregular pattern as
shown in Fig. 12b. No clear wall separation and inter-tubular space noticed at this
stage.
In Fig. 12c, TiO 2 nanotube arrays formation with an average inner pore diameter
of 125 nm, length of ~250 nm, the wall thickness of 30 nm and an inter-tube space
35 nm were observed at 25 °C. Also, the morphology of the nanotubes was smooth
and circular without any defect. FESEM images of samples anodized at 30, 50 and
70 °C (Fig. 12d–f) exhibited regular and vertically aligned tube structure.
The effect of decreased electrolyte temperature on anodization was also studied
by Wang et al. [82]. It was reported that there is a decrease in nanotube diameter and
increase in wall thickness on anodization done in glycerol-based aqueous electrolyte
kept in an ice bath.
527
fluid flow of culture media, ions, proteins, nutrients essential for healthy cell growth,
increase spaces for the exchange of gas and cell signaling molecules for an overall
improved cell environment. In human body, due to circulation and repetitive loading
and unloading of bones during various activities, the fluid flow naturally occurs in
the interstitial spaces around the bone cells [78–80].
7.4 Effect of Electrolyte Temperature
Another important parameter that influences the growth of nanotubes is anodization
temperature. In most of the anodization process, the electrolytes are kept at room
temperature to produce the nanotubes. It has been reported that an increase in electrolyte temperature leads to the formation of nanotubes with a larger diameter, shorter
length and thinner walls [48]. The dissolution rates of the nanotubes are affected by
the electrolyte temperature [53]. Prida et al. [81] reported that the growth of nanotubes
is inhibited at lower temperatures (2 °C) in aqueous electrolyte, whereas the temperature range between 0 and 40 °C are reported to be most favorable for the growth of
nanotubes in organic electrolyte [62]. Wang et al. [82] reported that the diameter of
the nanotubes does not depend on the anodizing temperature in aqueous electrolyte,
whereas the nanotube diameter was found to increase at higher temperatures while
using in organic electrolyte. This could be due to the fact that at lower temperatures,
the viscosity increases, which reduces the ion migration especially fluoride ion and
as a result of which etching of TiO 2 gets slowed down, leading to smaller diameter
tubes.
The influence of electrolyte temperature during the anodization of Ti-6Al-7Nb
alloy was studied and reported [61]. Figure 12(a–f) illustrated the FESEM images
of anodic TiO 2 nanotube arrays obtained after anodization of Ti-6Al-7Nb for 1 h at
30 V in an electrolyte consisting of 1 M H 2 SO 4 and 0.08 M HF at 5, 10, 25, 30,
50 and 70 °C. The sample anodized at 5 °C (Fig. 12a) revealed a premature porous
structure with an un-dissolved oxide layer on the surface. At 10 °C, a porous oxide
layer with pore diameter ~35 nm covers the entire surface in an irregular pattern as
shown in Fig. 12b. No clear wall separation and inter-tubular space noticed at this
stage.
In Fig. 12c, TiO 2 nanotube arrays formation with an average inner pore diameter
of 125 nm, length of ~250 nm, the wall thickness of 30 nm and an inter-tube space
35 nm were observed at 25 °C. Also, the morphology of the nanotubes was smooth
and circular without any defect. FESEM images of samples anodized at 30, 50 and
70 °C (Fig. 12d–f) exhibited regular and vertically aligned tube structure.
The effect of decreased electrolyte temperature on anodization was also studied
by Wang et al. [82]. It was reported that there is a decrease in nanotube diameter and
increase in wall thickness on anodization done in glycerol-based aqueous electrolyte
kept in an ice bath.