*2.2.3. Effect of anodization time*

In order to explore the formation mechanism of TNWs/TNAs, we observe the morphology evolution of TiO2 film as a function of anodizing time from 30 to 120 min at a fixed anodizing voltage of 40 V (**Figure 5**). At 30 min, a highly ordered TNA structure is obtained, and it has a tube length of 12.2 μm, diameter of 60 nm, and wall thickness of 18 nm. The steady-state growth rate of the TNA length at up to 30 min is ~0.4 μm/min. As further increasing the anodizing time to 35, 38, and 40 min, the surface morphology near the top of the TNA exhibits a gradual change. Indeed, the wall thickness decreases from 18 nm at 30 min to 12 nm at 35 min near the top of the TNAs due to the increased electrochemical etching with anodizing time. Noticeably, the wall thickness of the 35-min TNAs almost remains the same value of ∼18 nm in the middle section of nanotubes (see the inset), implying that enhanced electrochemical etching occurs near the top of the nanotube mouths, with respect to the middle section. It is a transitional stage at 38 and 40 min, where the wall thicknesses are even thinner and especially the tubes start disintegrating. After 45 min, TNAs near the top surface are broken up, along with thinning of wall thickness down to ~10 nm. At 60 min, TNWs with a width of ∼50 nm is covered on surface. The nanowires fell down on the top of the TNAs as its length is over ∼2 μm. At longer anodizing time of 90 and 120 min, TNWs are completely covered on the top of TNAs. Due to chemical etching, the width of TNWs decreased from 50 nm (at 60 min) down to ∼30 nm (at 90 min) and then to ∼20 nm (at 120 min). Moreover, as the time increase from 90 to 120 min, the nanowire structure emerges upon further etching, while the length of the TNAs is slightly increased from 11 μm (90 min) to 12 μm (120 min) [10].

The formation of TNWs/TNAs is governed by two key factors of (1) the strength of the electric field and (2) the processing time. In fact, **Figure 6(a)** summarizes the required conditions for forming TNWs/TNAs as functions of anodizing voltage and processing time in a fixed

Recent Advances in TiO2 Nanotube-Based Materials for Photocatalytic Applications Designed…

films prepared by anodic oxidation at a

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137

films prepared by anodic oxidation under at a

**Figure 4.** Anodizing voltage dependence of surface morphology for the TiO2

**Figure 5.** Anodizing time dependence of surface morphology of the TiO2

constant anodizing time of 1 h [10].

constant anodizing voltage of 40 V [10].

Recent Advances in TiO2 Nanotube-Based Materials for Photocatalytic Applications Designed… http://dx.doi.org/10.5772/intechopen.77063 137

**2.2. TiO2**

TiO2

of 2°C/min.

of 80 V [10].

evolution of TiO2

*2.2.1. Fabrication of TiO2*

*2.2.2. Effect of anodization voltage*

*2.2.3. Effect of anodization time*

**Figure 4** shows the surface morphology of the TiO2

voltages from 20 to 80 V, in a 0.5 wt% NH4

136 Titanium Dioxide - Material for a Sustainable Environment

 **nanowires on nanotube arrays (TNWs/TNAs)**

 *films and other experimental details*

The electrolyte consists of ethylene glycol and water (99:1 in wt%) and 0.5 wt% NH4

films with morphology of TNWs/TNAs were grown on Ti foils by anodic oxidation.

the anodizing voltage was varied from 20 to 80 V at a fixed processing time of 1 h. Then, the anodizing time varied from 30 to 120 min at a fixed voltage of 40 V. After fabrication of the TNWs/TNAs, thermal annealing was performed in the air at 450°C for 2 h, at a heating rate

At 20 V, it exhibits highly ordered TNAs with a tube inner wall diameter of 40 nm and a wall thickness of 20 nm. Meanwhile, as increasing the voltage to 40 V, TNWs with a wire width ∼50 nm appears on the top of TNAs which has tube diameters of 60 nm in diameter and ∼10 nm in wall thickness. When the voltage increases to 60 V, TNWs with a width of ∼20 nm fully covered on the TNAs with a tube diameter of 80 nm and a wall thickness of ∼10 nm. Strikingly, TNW-free TNAs with a tube diameter of ∼110 nm are obtained at a high voltage

In order to explore the formation mechanism of TNWs/TNAs, we observe the morphology

voltage of 40 V (**Figure 5**). At 30 min, a highly ordered TNA structure is obtained, and it has a tube length of 12.2 μm, diameter of 60 nm, and wall thickness of 18 nm. The steady-state growth rate of the TNA length at up to 30 min is ~0.4 μm/min. As further increasing the anodizing time to 35, 38, and 40 min, the surface morphology near the top of the TNA exhibits a gradual change. Indeed, the wall thickness decreases from 18 nm at 30 min to 12 nm at 35 min near the top of the TNAs due to the increased electrochemical etching with anodizing time. Noticeably, the wall thickness of the 35-min TNAs almost remains the same value of ∼18 nm in the middle section of nanotubes (see the inset), implying that enhanced electrochemical etching occurs near the top of the nanotube mouths, with respect to the middle section. It is a transitional stage at 38 and 40 min, where the wall thicknesses are even thinner and especially the tubes start disintegrating. After 45 min, TNAs near the top surface are broken up, along with thinning of wall thickness down to ~10 nm. At 60 min, TNWs with a width of ∼50 nm is covered on surface. The nanowires fell down on the top of the TNAs as its length is over ∼2 μm. At longer anodizing time of 90 and 120 min, TNWs are completely covered on the top of TNAs. Due to chemical etching, the width of TNWs decreased from 50 nm (at 60 min) down to ∼30 nm (at 90 min) and then to ∼20 nm (at 120 min). Moreover, as the time increase from 90 to 120 min, the nanowire structure emerges upon further etching, while the length of the

TNAs is slightly increased from 11 μm (90 min) to 12 μm (120 min) [10].

film as a function of anodizing time from 30 to 120 min at a fixed anodizing

F. First,

films prepared at different anodizing

F solution, and at a constant anodizing time of 1 h.

**Figure 4.** Anodizing voltage dependence of surface morphology for the TiO2 films prepared by anodic oxidation at a constant anodizing time of 1 h [10].

**Figure 5.** Anodizing time dependence of surface morphology of the TiO2 films prepared by anodic oxidation under at a constant anodizing voltage of 40 V [10].

The formation of TNWs/TNAs is governed by two key factors of (1) the strength of the electric field and (2) the processing time. In fact, **Figure 6(a)** summarizes the required conditions for forming TNWs/TNAs as functions of anodizing voltage and processing time in a fixed electrolyte solution. For example, TNWs/TNAs are obtained for 120 min at 30 V; meanwhile, it takes only 30 min for anodizing voltage of 50–60 V. The influence of the electric field strength on the formation of TNWs/TNAs is elucidated by considering the ion migration under electric field in the electrolyte. In principle, the flux of ions in the presence of electric field can be expressed as [10]

$$f\_i = -D\_i \frac{dc\_i}{d\mathbf{x}} - \mu\_i c\_i E \tag{4}$$

leads to high dissolution rate to completely remove the top section of TiO2

*2.2.4. Formation mechanism of TNWs/TNAs*

enhanced chemical drilling by a high H+

which results in the much higher F<sup>−</sup>

consequently TNW-free TNAs with a thin wall thickness of ∼8 nm are observed.

At some proper conditions of anodic oxidation, TNW's cover on TNAs can be fabricated. J. H. Lim et al. proposed the "bamboo-splitting" model for the TNW formation mechanism [19]. In addition, the formation TNWs can also be clearly explained "strings of through holes" model by M.Y. Hsu et al., as illustrated in **Figure 7** [10]. It follows four stages. First, the ordered TNAs are formed as processing the anodization, accompanying with field-

Recent Advances in TiO2 Nanotube-Based Materials for Photocatalytic Applications Designed…

conjunction with a protective environment maintained along the pore walls by the highly viscous EG solution. At this stage, field-enhanced further dissolution in the tube bottom is still occurring during anodization process (**Figure 7(a)**). Second, the migration of F<sup>−</sup>

toward the electric field to the bottom of anode is inhibited by the highly viscous solution,

Notably, in the electrolyte with water content, the chemical dissolution reaction of TNAs is further enhanced by hydrogen ions (reaction (3) above). Consequently, the tube mouth part is thinner than the lower sections, as shown in **Figure 7(a)**. The inner tube is found to be nonuniform and rough (**Figure 7(b)** inset), and thus when the tube mouth is thin enough

**Figure 7.** Schematic diagrams and corresponding SEM images of four stages in the TNW/TNA formation mechanism: (a) thinning the tube wall thickness with high roughness near the TNA's mouths, (b) forming strings through holes in the

top section of TNAs, (c) splitting into nanowires, and (d) collapsing and further thinning of nanowires [10].

nanotubes, and

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ions

concentration at the pore bottom of nanotubes, in

concentration at the tube than at the tube bottom.

where *Ji* is the flux of species *i* of concentration *ci* in direction *x*, *Di* is the diffusion coefficient, ∂*ci* /∂*x* is the concentration gradient, *ui* is the mobility of species *i*, and *E* is the electric field strength. Evidently, since ion migration in the electric field is considerable to the ion diffusion process under a concentration gradient, the field strength significantly drives the ion transport in the electrolyte. According to Eq. 4, higher field strength leads to higher ion flux in the electrolyte. Consequently, under a high field strength (or high voltage), the TNW/ TNA structure is formed in a shorter time. Notably, the growth of TNWs with characteristic of longer processing times for lower voltages is only found in the anodizing voltage range of 30–60 V. To get a deeper insight into the role of electric field strength in the formation of TNWs onto TNAs, the voltage dependence of the pore diameter and wall thickness of TNA's top section before the emergence of nanowires is investigated, and the results are shown in **Figure 6(b)**. For the cases with no TNW formation, for example, at ≤20 or ≥ 80 V, the tube diameter and wall thickness are obtained from TNAs prepared at anodizing time of 30 min. As can be seen in **Figure 6(b)**, the pore diameter of the TNA's mouth increases from 30 to 110 nm with increasing the voltage from 10 to 80 V, agreeing with the reported results [18]. Meanwhile, the wall thickness of the TNA's mouths decreases from 20 to 8 nm when the applied voltage is increased from 10 to 80 V. The TNW/TNA film is only found when the tube wall thickness is in range of 10–15 nm. As shown in **Figure 6(a)** and **(b)**, the dissolution rate would be too low to break up TiO2 tube wall when the applied voltage is sufficiently low and thus only TNA structure is found. By contrary, if the anodizing voltage is too high, which

**Figure 6.** (a) Conditions of required anodizing voltage and processing time (shaded zone) for forming TNWs/TNAs. (b) The pore diameter and wall thickness of TNA's top section prior to the emergence of nanowires, as a function of voltage.

leads to high dissolution rate to completely remove the top section of TiO2 nanotubes, and consequently TNW-free TNAs with a thin wall thickness of ∼8 nm are observed.
