2.2.1. Dimethyl sulfoxide (DMSO)

Ruan et al. [19] demonstrated the anodic growth of TNA in fluorinated dimethyl sulfoxide (DMSO) and ethanol (EtOH) mixture. Well-aligned TNA with 2.3 μm in length were formed by anodization in 1:1 DMSO and EtOH mixture containing 4 wt% HF at 20 V for 70 h. DMSO is a dipolar aprotic solvent. Unlike protic solvents, aprotic solvents do not have a hydrogen atom directly bonded to an electronegative atom, such as oxygen or nitrogen. DMSO has a very weak ability to donate protons (hydrogen bond donation) and it is more basic than water, implying that DMSO has a protophilic nature [21]. DMSO accepts the hydrogen proton from HF and thus allows the formation of long nanotubes by the reduction of the chemical dissolution. However, the donation of oxygen in organic electrolyte is more difficult than aqueous electrolyte, and thus limited the nanotube length even after long anodization time. Discrete nanotube arrays obtained in 1:1 DMSO and EtOH mixture exhibited fourfold higher photocurrent density as compared to that formed in aqueous electrolyte [20]. This reveals the potential of organic electrolyte for the formation of long TNA for superior photoelectrochemical properties. The nanotube length was later increased up to 93 μm by anodization in DMSO containing 2 wt% HF at 60 V for 70 h refer to Figure 2a-d [13, 15].

decreased the viscosity of electrolyte [3, 34], leading to high chemical dissolution by the higher diffusion rate of reactant ions [35]. The minimum amount of water less than 5 wt% limits the chemical dissolution and thus allows the nanotube to grow deeply into the metal without significant loss at the pore mouth. Besides, thinner barrier layer in such condition allows greater ionic conduction and faster movement of the metal/oxide interface into the metal, thus

Figure 2. (a) Cross-sectional, (b) magnified-cross sectional, (c) surface and (d) pore bottom morphologies of TNA grown

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

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The significance of anodization parameters, including applied potential, F concentration, water content and anodization time on the structural characteristics of TNA were extensively investigated by Yoriya et al. [33] and the finding are presented in Figure 3a-e. Higher applied potentials provide a greater driving force for both electronic and ionic conduction and thus allow the growth of long nanotube arrays. A maximum length of TNA of 101 μm was achieved

Figure 3b reveals that 2 wt% HF concentration allowed the formation of longest nanotube at both 40 and 60 V. HF concentration higher than 2% greatly increases the chemical dissolution, and thus reduces nanotube length. Furthermore, chemical dissolution is relatively lower than electrochemical oxidation in DMSO electrolyte containing less than 2 wt% HF, resulting in the formation of short nanotubes. The addition of H2O facilitates the extraction of O2– and OH–

by anodic growth in DMSO containing 2 wt% HF at 60 V for 70 h.

producing long TNA [13, 32].

in DMSO containing 2 wt% HF at 60 V for 70 h [19].

To enhance the oxidation rate in organic electrolyte, water usually serves as a source of O2 and hydroxyl ion; OH [13, 15]. Several authors reported that small amount of water addition could improve the adhesion between the nanotube and underlying oxide barrier layer. This facilitates the growth of long nanotubes [32, 33]. Large amount of water addition however Factor Affecting Geometry of TiO2 Nanotube Arrays (TNAs) in Aqueous and Organic Electrolyte http://dx.doi.org/10.5772/intechopen.74193 121

the anodization time. The increase of pH value from 3.8 to 4.5 resulted in significant increase in

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

An organic electrolyte is known as efficient solvent in anodic fabrication of highly-ordered selforganized porous alumina (Al2O3) at high applied potential, c.a., 100 V [29, 30]. The incorporation of organic component from the electrolyte into anodic oxide film is known to lower the relative permittivity of oxide and increase its dielectric breakdown potential [18], thereby allowing the anodic growth of oxide film under wide range of applied potential [13, 31].

Ruan et al. [19] demonstrated the anodic growth of TNA in fluorinated dimethyl sulfoxide (DMSO) and ethanol (EtOH) mixture. Well-aligned TNA with 2.3 μm in length were formed by anodization in 1:1 DMSO and EtOH mixture containing 4 wt% HF at 20 V for 70 h. DMSO is a dipolar aprotic solvent. Unlike protic solvents, aprotic solvents do not have a hydrogen atom directly bonded to an electronegative atom, such as oxygen or nitrogen. DMSO has a very weak ability to donate protons (hydrogen bond donation) and it is more basic than water, implying that DMSO has a protophilic nature [21]. DMSO accepts the hydrogen proton from HF and thus allows the formation of long nanotubes by the reduction of the chemical dissolution. However, the donation of oxygen in organic electrolyte is more difficult than aqueous electrolyte, and thus limited the nanotube length even after long anodization time. Discrete nanotube arrays obtained in 1:1 DMSO and EtOH mixture exhibited fourfold higher photocurrent density as compared to that formed in aqueous electrolyte [20]. This reveals the potential of organic electrolyte for the formation of long TNA for superior photoelectrochemical properties. The nanotube length was later increased up to 93 μm by anodization in DMSO

To enhance the oxidation rate in organic electrolyte, water usually serves as a source of O2 and hydroxyl ion; OH [13, 15]. Several authors reported that small amount of water addition could improve the adhesion between the nanotube and underlying oxide barrier layer. This facilitates the growth of long nanotubes [32, 33]. Large amount of water addition however

the nanotube length from ~800 nm to ~4.4 μm after anodization at 10 V for 20 h.

organic electrolyte to reduce the chemical dissolution process.

containing 2 wt% HF at 60 V for 70 h refer to Figure 2a-d [13, 15].

2.2. TNA synthesis using organic electrolytes

120 Titanium Dioxide - Material for a Sustainable Environment

2.2.1. Dimethyl sulfoxide (DMSO)

Figure 2. (a) Cross-sectional, (b) magnified-cross sectional, (c) surface and (d) pore bottom morphologies of TNA grown in DMSO containing 2 wt% HF at 60 V for 70 h [19].

decreased the viscosity of electrolyte [3, 34], leading to high chemical dissolution by the higher diffusion rate of reactant ions [35]. The minimum amount of water less than 5 wt% limits the chemical dissolution and thus allows the nanotube to grow deeply into the metal without significant loss at the pore mouth. Besides, thinner barrier layer in such condition allows greater ionic conduction and faster movement of the metal/oxide interface into the metal, thus producing long TNA [13, 32].

The significance of anodization parameters, including applied potential, F concentration, water content and anodization time on the structural characteristics of TNA were extensively investigated by Yoriya et al. [33] and the finding are presented in Figure 3a-e. Higher applied potentials provide a greater driving force for both electronic and ionic conduction and thus allow the growth of long nanotube arrays. A maximum length of TNA of 101 μm was achieved by anodic growth in DMSO containing 2 wt% HF at 60 V for 70 h.

Figure 3b reveals that 2 wt% HF concentration allowed the formation of longest nanotube at both 40 and 60 V. HF concentration higher than 2% greatly increases the chemical dissolution, and thus reduces nanotube length. Furthermore, chemical dissolution is relatively lower than electrochemical oxidation in DMSO electrolyte containing less than 2 wt% HF, resulting in the formation of short nanotubes. The addition of H2O facilitates the extraction of O2– and OH–

the extraction of Ti4+ ions, and facilitates the dissolution of HF for it to be chemically available at the oxide/electrolyte interface [13]. Paulose et al. [15] fabricated TNA of ~69 μm long in FA electrolyte containing 0.27 M NH4F at 35 V for 48 h. It is noticeable that the growth rate

aqueous electrolyte [36]. Shankar et al. [13] further investigated the influence of applied potential at 20–30 V, and NH4F concentration in the range of 0.11– 0.37 M. The nanotube length and pore size increase with increasing applied potential and concentration of NH4F, achieving a maximum length of ~78 μm in FA containing 0.37 M NH4F. Besides, the incorporation of high polarity electrolyte, NMF into FA also accelerates the chemical dissolution and

The diffusion of ionic species in electrolyte greatly affects the growth of TNA by local acidification at the pore bottom [12]. Macak and colleagues [34] investigated the influence of electrolyte viscosity on the formation of TNA by anodization in various types of electrolyte; 1 M ammonium sulfate [(NH4)2SO4] containing 0.5 wt% NH4F, 1:1 glycerol and H2O mixture containing 0.5 wt% NH4F, and glycerol containing 0.5 wt% NH4F. The viscosities of these electrolytes were ~0.001, ~0.004 and ~1.5 Pa.s, respectively. The anodization in (NH4)2SO4 electrolyte allowed the formation of TNA with length of ~2 μm while the anodization in glycerol electrolyte resulted in nanotube arrays with ~1.3 μm in length. However, the incorporation of large amount of water leads to the presence of ripples at the nanotube walls, resulting from high chemical dissolution in electrolyte. The nanotube length formed in glycerol-based electrolyte increases almost linearly with increasing anodization time, and achieved ~6.1 μm

The viscosity of electrolyte can be also affected by anodization temperature [34, 37]. The viscosity of glycerol containing 0.5 wt% NH4F decreases from 12 Pa.s to 1.5 and 0.3 Pa.s by increasing temperature from 0C to 20 and 40C, respectively. Low viscosity facilitates the diffusion of reactants at the pore tip, resulting in long nanotube with large pore. As mentioned earlier, the incorporation of organic species from organic electrolyte into the oxide film during anodization allows the growth of nanotube arrays under a wide range of applied potential. Alivov et al. [31] investigated the formation behavior of TNA in a broad range of applied potential of 5–350 V and F concentration of 0.1–0.7 wt%. TNA were formed in glycerol under applied potential of 10–240 V, and the applied potential is disproportional to F concentration.

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

, which is relatively higher than that formed in DMSO [19] or

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

increased to ~24.3 nm min<sup>1</sup>

after anodization at 20 V for 18 h.

2.2.4. Ethylene glycol (EG)

2.2.3. Glycerol

thus results in nanotubes with larger pore size.

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

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 diffusion of F– , due to the accumulation of reaction products at the tube bottom, as well as significant loss at atop of nanotube by high chemical dissolution [5, 13].

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

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 the extraction of Ti4+ ions, and facilitates the dissolution of HF for it to be chemically available at the oxide/electrolyte interface [13]. Paulose et al. [15] fabricated TNA of ~69 μm long in FA electrolyte containing 0.27 M NH4F at 35 V for 48 h. It is noticeable that the growth rate increased to ~24.3 nm min<sup>1</sup> , which is relatively higher than that formed in DMSO [19] or aqueous electrolyte [36]. Shankar et al. [13] further investigated the influence of applied potential at 20–30 V, and NH4F concentration in the range of 0.11– 0.37 M. The nanotube length and pore size increase with increasing applied potential and concentration of NH4F, achieving a maximum length of ~78 μm in FA containing 0.37 M NH4F. Besides, the incorporation of high polarity electrolyte, NMF into FA also accelerates the chemical dissolution and thus results in nanotubes with larger pore size.
