2.2.4. Ethylene glycol (EG)

from electrolyte to form oxide, contributing to the formation of long nanotube with thick walls. However, high H2O amount (> 2 wt%) dominates chemical dissolution and thus reduces the nanotube length (Figure 3c). Figure 3d shows that nanotube length increased up to 70 h and then decreased continuously. The shorter nanotube length after 70 h is caused by slow diffu-

Figure 3. Variation of TiO2 nanotube array length as a function of (a) applied voltage for a 70 h anodization using DMSO containing 2% HF with and without a pre-anodization step to template the surface, (b) HF concentration in DMSO for 40 and 60 V 70 h anodization, (c) variation of nanotube length obtained from a 2.0% HF-DMSO containing different deionized water concentrations (40 V, 40 h), and (d) duration for a 40 V anodization in 2.0% HF-DMSO electrolytes with

Formamide (FA) and N-methylformamide (NMF) have been used as electrolyte for fabricating long nanotubes with high growth rate [15]. FA and NMF are both protophilic like DMSO, meaning that their acidity nature is weaker than water [21]. The dielectric constant of FA and NMF are 111 and 182.4, respectively, which are much higher than water; ~78.39. Higher dielectric constant of electrolyte induces more charges on the oxide layer and thus improves

significant loss at atop of nanotube by high chemical dissolution [5, 13].

2.2.2. Amide – Formamide (FA) and N-methylformamide (NMF)

, due to the accumulation of reaction products at the tube bottom, as well as

sion of F–

0, 3, and 5% deionized water content [33].

122 Titanium Dioxide - Material for a Sustainable Environment

The anodization in high viscosity (μv) electrolyte of about 945 cP at 25C resulted in relatively shorter nanotube length as compared to that formed in aqueous electrolyte. Hence, the potential of low viscosity polyol solvent, such as ethylene glycol (EG, μ<sup>v</sup> = 16 cP at 25C) for the formation of TNA was investigated by [34]. TNA with ~1.6 μm in length were obtained by anodization in EG containing 0.5 wt% NH4F at 12 V for 3 h. The significant increase in the nanotube length in EG-based electrolyte could be attributed to lower diffusion resistance in electrolyte with lower viscosity [21]. The potential of EG as efficient electrolyte was later confirmed by [32]. The anodization in EG containing 0.25 wt% NH4F at 60 V allowed the rapid formation of nanotube arrays with the growth rate up to 131.4 nm min<sup>1</sup> . The resulting 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 nanotube arrays.

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

also an important available. The nanotube length increased with increasing anodization time, and achieved a maximum of 360 μm at 96 h. The growth rate was found as 241.7 nm min<sup>1</sup> at 4 h. It decreased exponentially and reached a minimum of 62.5 nm min<sup>1</sup> at 96 h. Paulose and co-workers [15] later achieved the formation of ultra-long TNA with 1000 μm in length by

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

http://dx.doi.org/10.5772/intechopen.74193

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Table 2. Summary of nanotube inner diameter, outer diameter and length obtained by anodization in fresh-EG

The investigation of organic electrolyte was extended to several types of polyol solvents, for example diethylene glycol (DEG), triethylene glycol (TEG) and polyethylene glycol (PEG) in comparison to EG [39]. Large amount of oxy and hydroxyl groups in TEG and PEG allows higher self-association via intra- and intermolecular hydrogen bonds, and consequently forms anatase crystallite by bridging the oxygen atom and hydroxyl groups in TiO2. However, their high viscosity and molecular weight retard the growth of nanotubes, and thus does not satisfy the requirement of practical applications. In 2010, Sreekantan et al. [3] demonstrated extremely fast-formation of TNA with approximately ~15 μm in length by anodization in EG containing excessive-fluoride 0.5 wt% NH4F and 1 wt% H2O. The nanotubes were formed with high

excessive-fluoride limits the anodization time at 1 h. Recently, the addition of alkali species appears as alternative approach to improve the balance between electrochemical oxidation and chemical dissolution. The incorporation of sodium carbonate (NaCO3) into EG allowed the rapid growth of TNA with the growth rate up to 1 μm min<sup>1</sup> [40]. It is noteworthy that the anodic growth of TNA in EG is only method that induces the carbon species into the nanotube through the pyrogenation of ethylene (C2H2) in EG [41], thereby enabling visible-light absorp-

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

. However, high chemical dissolution in EG containing

anodization in EG containing 3.5 wt% H2O and 0.6 wt% NH4F at 60 V for 216 h.

containing 2 and 0.3 wt% NH4F at different applied potential for 17 h [35].

growth rate up to 308.3 nm min<sup>1</sup>

tion [42] without urther processing [43, 44].

3. Conclusion

2.2.5. Polyol – Diethylene glycol (DEG), triethylene glycol (TEG), polyethylene glycol (PEG)

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

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].


Table 2. Summary of nanotube inner diameter, outer diameter and length obtained by anodization in fresh-EG containing 2 and 0.3 wt% NH4F at different applied potential for 17 h [35].

also an important available. The nanotube length increased with increasing anodization time, and achieved a maximum of 360 μm at 96 h. The growth rate was found as 241.7 nm min<sup>1</sup> at 4 h. It decreased exponentially and reached a minimum of 62.5 nm min<sup>1</sup> at 96 h. Paulose and co-workers [15] later achieved the formation of ultra-long TNA with 1000 μm in length by anodization in EG containing 3.5 wt% H2O and 0.6 wt% NH4F at 60 V for 216 h.
