2.1. TNA synthesis using aqueous electrolytes

Gong and co-workers [20] first obtained self-organized, highly-uniform TNA by anodizing Ti in aqueous electrolyte containing 0.5 wt% HF under applied potential of 10–20 V. The pore size increased with increasing applied potential while the nanotube length was dependent on the anodization time. Titanium undergoes electrochemical oxidation process as follow: Ti þ 2H2O ! TiO2 þ 4H<sup>þ</sup> (Reaction 1) and chemical dissolution process: TiO2 þ 6HF ! ½ � TiF6 <sup>2</sup>� <sup>þ</sup>2H2O <sup>þ</sup> 2H<sup>þ</sup> (Reaction 2) to form nantubular structure. However, the length of nanotubes was limited to a maximum of ~500 nm, due to the restriction imposed by high chemical dissolution of formed oxide by strong acidity of HF aqueous electrolyte, c.a., pH < 2 [14, 21]. The chemical dissolution of the formed oxide is determined by the F� concentration and the pH value of the solution (Reaction 2). The model proposed by Macak and co-workers suggest that F� and H+ are attracted toward the pore bottom during anodization (Figure 1a), followed by establishment of pH profile as in (Figure 1b). The variation in pH profile triggers different degree of dissolution rate along the tube inner surface (Figure 1c).

(H2SO4) addition could reduce the chemical dissolution, and thus allows the formation of porous TiO2 with improved structural morphologies. This basis was later used to obtain TNA with well-defined tubular structure over applied potential range of 10–25 V [14]. The nanotube length and pore diameter increased linearly with increasing applied potential. In summary, the structural morphologies of anodic growth oxides can be controlled by the acidity of

Factor Affecting Geometry of TiO2 Nanotube Arrays (TNAs) in Aqueous and Organic Electrolyte

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<sup>+</sup> and F that creates a local acidification, (b) pH profile

Macak et al. [12] investigated the influence of the electrolyte acidity on the growth of nanotube arrays using HF, potassium fluoride (KF) and sodium fluoride (NaF) as F sources. TNA with ~280 nm long and ~40 nm pore diameter were formed in strong acidity (pH < 1) HF aqueous electrolyte after 2 h of anodization at 10 V. The increase in the pH value of electrolyte to 1.3 and 2.8 by using KF and NaF instead of HF resulted in the formation of TNA with the nanotube length of ~320 and 590 nm, respectively. TNA with a maximum length of 1.5 μm were achieved at 25 V. Moreover, the nanotube length is proportional to the anodization time, and thus resulted in the formation of nanotube arrays with length of ~800 nm, ~1.8 μm and ~2.3 μm after exposure for 20, 60 and 90 h in electrolyte with pH of 3.8. It is interesting to note that the acidity of electrolyte plays an essential role for the growth of long nanotube arrays instead of

electrolyte [26].

Figure 1. (a) Schematic diagram to illustrate the presence of NH4

within the nanotube, (c) rate of dissolution of the tube.

This work directly leads to the use of buffer solution as electrolyte to control pH and increase the length of nanotube produced. Reaction 2 permits the dissolution reaction and become the core foundation of field-assisted dissolution model. Since then, many authors have adapted the dissolution process into their research framework such that field-assisted dissolution model is gradually established for TNA anodization [22–24].

Large number of F� and H+ ions in HF aqueous electrolyte dominates high chemical dissolution and thus hinders the equilibrium growth of the nanotube arrays [12]. Beranek et al. [25] found that the incorporation of low concentration of HF aqueous electrolyte with acetic acid

defined arrangement and a vertical and homogeneous alignment over the entire substrate surface, based on the self-organized anodic oxidation is pursued. Electrochemical anodization is widely used because of its controllable, reproducible results and simplicity of the process. The feasibility to tune the size and shape of nanotubular arrays to the desired dimensions and meeting the demands of specific applications by means of controlled anodic oxidation of the metal substrate have widen the application of TNA. Furthermore, it is a cost-effective method

The anodic growth of self-organized tubular structure is well known as the equilibrium reaction between electrochemical oxidation at the metal/electrolyte interface and chemical dissolution at the oxide/electrolyte interface [12, 13]. The structural characteristics of the nanotube arrays can be controlled through various synthesis parameters, including applied potential [14–16], anodization time [14, 17, 18], electrolyte type and chemical composition [12, 13, 19]. Therefore, this section provides a comprehensive review on the influence of aforemen-

Gong and co-workers [20] first obtained self-organized, highly-uniform TNA by anodizing Ti in aqueous electrolyte containing 0.5 wt% HF under applied potential of 10–20 V. The pore size increased with increasing applied potential while the nanotube length was dependent on the anodization time. Titanium undergoes electrochemical oxidation process as follow: Ti þ 2H2O !

(Reaction 2) to form nantubular structure. However, the length of nanotubes was limited to a maximum of ~500 nm, due to the restriction imposed by high chemical dissolution of formed oxide by strong acidity of HF aqueous electrolyte, c.a., pH < 2 [14, 21]. The chemical dissolution of the formed oxide is determined by the F� concentration and the pH value of the solution (Reaction 2). The model proposed by Macak and co-workers suggest that F� and H+ are attracted toward the pore bottom during anodization (Figure 1a), followed by establishment of pH profile as in (Figure 1b). The variation in pH profile triggers different degree of dissolution rate along the tube

This work directly leads to the use of buffer solution as electrolyte to control pH and increase the length of nanotube produced. Reaction 2 permits the dissolution reaction and become the core foundation of field-assisted dissolution model. Since then, many authors have adapted the dissolution process into their research framework such that field-assisted dissolution

Large number of F� and H+ ions in HF aqueous electrolyte dominates high chemical dissolution and thus hinders the equilibrium growth of the nanotube arrays [12]. Beranek et al. [25] found that the incorporation of low concentration of HF aqueous electrolyte with acetic acid

<sup>2</sup>� <sup>þ</sup>2H2O <sup>þ</sup> 2H<sup>þ</sup>

TiO2 þ 4H<sup>þ</sup> (Reaction 1) and chemical dissolution process: TiO2 þ 6HF ! ½ � TiF6

model is gradually established for TNA anodization [22–24].

and the tubes prepared via this method have good adherent strength.

2. Factor affecting geometry and composition of TNA

tioned parameters on the formation of TNA.

118 Titanium Dioxide - Material for a Sustainable Environment

inner surface (Figure 1c).

2.1. TNA synthesis using aqueous electrolytes

Figure 1. (a) Schematic diagram to illustrate the presence of NH4 <sup>+</sup> and F that creates a local acidification, (b) pH profile within the nanotube, (c) rate of dissolution of the tube.

(H2SO4) addition could reduce the chemical dissolution, and thus allows the formation of porous TiO2 with improved structural morphologies. This basis was later used to obtain TNA with well-defined tubular structure over applied potential range of 10–25 V [14]. The nanotube length and pore diameter increased linearly with increasing applied potential. In summary, the structural morphologies of anodic growth oxides can be controlled by the acidity of electrolyte [26].

Macak et al. [12] investigated the influence of the electrolyte acidity on the growth of nanotube arrays using HF, potassium fluoride (KF) and sodium fluoride (NaF) as F sources. TNA with ~280 nm long and ~40 nm pore diameter were formed in strong acidity (pH < 1) HF aqueous electrolyte after 2 h of anodization at 10 V. The increase in the pH value of electrolyte to 1.3 and 2.8 by using KF and NaF instead of HF resulted in the formation of TNA with the nanotube length of ~320 and 590 nm, respectively. TNA with a maximum length of 1.5 μm were achieved at 25 V. Moreover, the nanotube length is proportional to the anodization time, and thus resulted in the formation of nanotube arrays with length of ~800 nm, ~1.8 μm and ~2.3 μm after exposure for 20, 60 and 90 h in electrolyte with pH of 3.8. It is interesting to note that the acidity of electrolyte plays an essential role for the growth of long nanotube arrays instead of the anodization time. The increase of pH value from 3.8 to 4.5 resulted in significant increase in the nanotube length from ~800 nm to ~4.4 μm after anodization at 10 V for 20 h.

The significance of the pH value of an electrolyte on the growth of TNA was proven by Bauer et al. [27]. Self-organized TNA was successfully formed in 1 M H3PO4 containing 0.3 wt% HF over applied potential range from 1 V to 25 V. Weak acidity of H3PO4 governs the local acidification during pore growth, and thus allowed the control of nanotube geometry over wide range of applied potential [12]. In 2007, Allam and Grimes [28] demonstrated an alternative method to fabricate TNA using hydrochloride (HCl) instead of fluoride-containing electrolyte. The anodization in 3 M HCl aqueous electrolyte at 20 V for 20 min allowed the formation of TNA with the nanotube length of ~600 nm. However, high chemical dissolution of aqueous electrolytes still remains as significant restriction in achieving long nanotubes with smooth walls. This limitation of forming long TNA in aqueous electrolyte has extended the use organic electrolyte to reduce the chemical dissolution process.
