3. Conclusion

confirmed by [32]. The anodization in EG containing 0.25 wt% NH4F at 60 V allowed the rapid

nanotube arrays exhibited length of 134 μm and pore size of 25 nm after 17 h of anodization. The nanotube length was further increased to 220 μm by anodization in EG containing 0.3 wt% NH4F and 2 wt% H2O [13], implying the significant role of H2O addition on the growth of long

The influences of anodization parameters, including H2O content, NH4F concentration, applied potential and anodization time on the growth of TNA in EG-based electrolytes were systematically investigated by Prakasam et al. [35]. The increase in the H2O content and NH4F concentration stimulate electrochemical oxidation at the metal/oxide interface and chemical dissolution at the oxide/electrolyte interface, resulting in longer nanotube length up to 220 μm by anodization in EG containing 2 wt% H2O and 0.3 wt% NH4F. Further increase of water content and NH4F concentration leads to disequilibrium reaction between electrochemical oxidation and chemical

In the presence of optimum H2O content, O<sup>2</sup>– and/or OH– are injected into the body of the oxide layer, and affect the structure efficiently to impede the ionic transport at barrier layer [38]. This greatly accelerates the inward movement of the metal/oxide interface into the metal surface and thus resulting in long nanotubes with thick walls. Higher content of H2O largely increases the number of extracted oxygen and/or hydroxyl ions per unit area of the oxide layer, leading to higher chemical dissolution. The increase in the F concentration provides higher chemical dissolution at the oxide/electrolyte interface, thereby allowing higher number of ion migrate across thin barrier layer. This resulted in the formation of long nanotube with thin walls. However, high chemical dissolution in electrolyte containing high F concentration

The anodic growth of TNA in used electrolyte significantly increased the nanotube length. Lower electrical conductivity of used electrolyte hinders ionic migration across barrier layer, thereby reducing loss at the pore tip by high chemical dissolution. Inner and outer diameters and nanotube length were found to increase with increasing applied potential from 20 V to 60 V (Table 2). Further increase of applied potential to 65 V resulted in further increase in inner and outer diameters but decreased the nanotube length. This implies a significant loss at the pore tip by high chemical dissolution under applied potential higher than 60 V. The anodization time is

Table 1. Summary of nanotube length (in μm) obtained by varying the concentration of H2O of 1–3 wt%, and NH4F of

0.1–0.5 wt% in EG with respect to fresh and once-used solution [35].

dissolution and thus resulted in shorter nanotube in the range of 54–136 μm (Table 1).

induces a significant loss from the pore mouth and thus formed short nanotubes.

. The resulting

formation of nanotube arrays with the growth rate up to 131.4 nm min<sup>1</sup>

nanotube arrays.

124 Titanium Dioxide - Material for a Sustainable Environment

TNA is by far among the most investigated functional metal oxide. The possibility of growing porous anodic nanostructure, particularly self-organized and highly-ordered TNA through anodization has further enlarged the research interest on this TNA due to specific advantages, such as large surface area, relatively long electron diffusion length and biocompatibility. In this chapter, the formation of TNA and various parameters, which affects the tube formation and properties of TNA were discussed. The tube length, thickness, and diameter were varied according to the pH, electrolytes, anodization potential, and time. Today, TNA are used as solar cell, self-cleaning coating, orthopedic and dental implants, and also an effective drug delivery carrier for the cancer therapy.

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