*2.2.4. Formation mechanism of TNWs/TNAs*

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

∂*c* \_\_\_*i*

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

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.

<sup>∂</sup>*<sup>x</sup>* <sup>−</sup> *ui ci <sup>E</sup>* (4)

is the mobility of species *i*, and *E* is the electric

is the diffusion coeffi-

in direction *x*, *Di*

tube wall when the applied voltage is sufficiently low and

*<sup>i</sup>* = −*Di*

expressed as [10]

where *Ji*

cient, ∂*ci*

*J*

138 Titanium Dioxide - Material for a Sustainable Environment

would be too low to break up TiO2

is the flux of species *i* of concentration *ci*

/∂*x* is the concentration gradient, *ui*

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 fieldenhanced chemical drilling by a high H+ concentration at the pore bottom of nanotubes, in 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> ions toward the electric field to the bottom of anode is inhibited by the highly viscous solution, which results in the much higher F<sup>−</sup> concentration at the tube than at the tube bottom. 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].

(< 10 nm), it can be etched through under high dissolution reaction with high concentration of F<sup>−</sup> ions (**Figure 7(b)**). By increasing processing time, strings of through holes are formed on the tube wall, and they would initiate and propagate downward from the top to bottom of TNAs (or along the F<sup>−</sup> migration direction), while the holes near the top expand and connect to split into nanowires (**Figure 7(c)**) as the anodizing time increases. With further increasing anodizing time, the nanowires exhibit smooth at the edges and narrower wire width due to the sufficient chemical etching. When the wire length is too long (i.e., over ∼5 μm), the nanowires would collapse onto TNAs, as illustrated in **Figure 7d**. Briefly, the proposed formation mechanism of TNWs/TNAs by one-step anodic oxidation follows four stages: (a) thinning the tube wall thickness with high roughness near the TNA's mouths, (b) forming strings through holes in the top section of the TNAs, (c) splitting into nanowires, and (d) collapsing and further thinning of nanowires [10].

and film thickness (~11 μm) are kept almost the same between TNAs and TNWs/TNAs by adjusting processing conditions. **Figure 8** shows the photocatalytic activity of TNAs and

C0 = 2.5 × 10−5 M. We used the pseudo first-order kinetics as described by the Langmuir-Hinshelwood model [20]. Thus, the photodegradation rate of MB could be expressed by the

*Ct* = *Co e* <sup>−</sup>*kt* (5)

concentration and the reaction concentration of MB. The reaction rate constants (k) were calculated from the experimental data using a linear regression. For TNAs with various tube diameters at a fixed tube thickness, the *k* values were 9.80 × 10−2, 8.48 × 10−2, 6.73 × 10−2, and 5.49 × 10−2 h−1 for tube diameter of 40, 60, 80, and 100 nm, respectively. Clearly, a smaller tube diameter results in a higher reaction rate [10]. In comparison, at the same tube diameter of 60 nm, the k value of 20 nmTNWs/60 nmTNAs was 48% higher than that of pure TNAs (12.54 × 10−2 vs. 8.40 × 10−2 h−1). Similarly, at a fixed tube diameter of 40 nm, the rate constant k for 20 nm TNWs/40 nm TNAs enhanced approximately 33% over the pure TNA one (13.05 × 10−2 vs. × 9.8 × 10−2 h−1) [10]. It is found that the photocatalytic activity of TNWs/TNAs enhanced remarkably (up to 33–48%) over the corresponding pure TNAs.

**Table 1** summarizes the calculated dye adsorption and the *k* values of various TiO2

TNAs, a smaller tube diameter yields a higher dye adsorption because of its larger surface area, which results in enhanced photocatalytic efficiency. The difference in surface area of TNAs comes from different densities of nanotube. As the anodizing voltage increases, tube density decreases with increasing tube diameter to result in a decrease of dye adsorption and photocatalytic efficiency. The dye adsorption of TNWs/TNAs is significantly higher than that of the corresponding TNA film because the former that offers larger surface area than the latter. Indeed, the congregation of the bundled nanowires connected to the nanotube mouths induces surface area enhancement over the TNAs. It is also found from **Table 1** that the change percentages of the reaction rate constants for both TNAs and TNWs/TNAs are higher than those of dye adsorption. This indicates that besides the predominant factor of surface area, the 1D nanostructures of TNAs and TNWs/TNAs offer the promotion of charge transport and reduced recombination rate. Though having smaller surface area than

 nanoparticles, both the faster transport and slower recombination of 1D structures and the providing additional surface area from TNWs, the photocatalytic performance of TNWs/ TNAs has achieved a very high efficiency (*k* = 13.05 × 10−2 h−1) that is very close to the *k* value

nanoparticles (13.05 × 10−2 h−1) [10]. It is worthy to mention that the immobilized film

The photocatalytic performance of TNWs/TNAs may be further improved by optimizing the

forms (i.e., TNAs and TNWs/TNAs on Ti plates) are more practical than TiO2

width and density of TNWs covering on TNAs.

as a function of UV irradiation time, in a MB solution with

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

/*Co* = −*kt* (6)

and Ct

http://dx.doi.org/10.5772/intechopen.77063

are the initial

141

films. For

powdery form.

TNW/TNA films and TiO<sup>2</sup>

following Eqs. (5) and (6):

TiO2

of TiO2

, Ct /C0

where *k* is the reaction rate constant, *t* is the irradiation time, and C0

ln *Ct*

#### *2.2.5. Photocatalytic performance*

The photocatalytic performance of TNW/TNA nanostructure in comparison with other morphologies of TNAs and TiO2 nanoparticles is presented in this part. The TiO2 P25 nanoparticle films were fabricated by the doctor-blade method. For making a reliable comparison, the film thickness of TiO<sup>2</sup> nanoparticles was controlled to be comparable to that of TNW/ TNA films (~11 μm) by controlling the height of the blade above the Ti substrate and/or the concentration of TiO2 paste. The effect of various TiO<sup>2</sup> morphologies on the photocatalytic degradation of methylene blue (MB) was examined under the 360 nm wavelength irradiation of an 8 W HeCd lamp. To make a reliable comparison, diameters of nanotubes (40–100 nm)

**Figure 8.** Photocatalytic degradation of methylene blue of various TNAs, TNWs/TNAs, and TiO2 nanoparticle films, C/ C0 vs. reaction time under UV light irradiation (8 W, 360 nm wavelength) [10].

and film thickness (~11 μm) are kept almost the same between TNAs and TNWs/TNAs by adjusting processing conditions. **Figure 8** shows the photocatalytic activity of TNAs and TNW/TNA films and TiO<sup>2</sup> , Ct /C0 as a function of UV irradiation time, in a MB solution with C0 = 2.5 × 10−5 M. We used the pseudo first-order kinetics as described by the Langmuir-Hinshelwood model [20]. Thus, the photodegradation rate of MB could be expressed by the following Eqs. (5) and (6):

(< 10 nm), it can be etched through under high dissolution reaction with high concentration

connect to split into nanowires (**Figure 7(c)**) as the anodizing time increases. With further increasing anodizing time, the nanowires exhibit smooth at the edges and narrower wire width due to the sufficient chemical etching. When the wire length is too long (i.e., over ∼5 μm), the nanowires would collapse onto TNAs, as illustrated in **Figure 7d**. Briefly, the proposed formation mechanism of TNWs/TNAs by one-step anodic oxidation follows four stages: (a) thinning the tube wall thickness with high roughness near the TNA's mouths, (b) forming strings through holes in the top section of the TNAs, (c) splitting into nanowires,

The photocatalytic performance of TNW/TNA nanostructure in comparison with other mor-

ticle films were fabricated by the doctor-blade method. For making a reliable comparison,

TNA films (~11 μm) by controlling the height of the blade above the Ti substrate and/or the

degradation of methylene blue (MB) was examined under the 360 nm wavelength irradiation of an 8 W HeCd lamp. To make a reliable comparison, diameters of nanotubes (40–100 nm)

paste. The effect of various TiO<sup>2</sup>

**Figure 8.** Photocatalytic degradation of methylene blue of various TNAs, TNWs/TNAs, and TiO2

vs. reaction time under UV light irradiation (8 W, 360 nm wavelength) [10].

nanoparticles is presented in this part. The TiO2

nanoparticles was controlled to be comparable to that of TNW/

P25 nanopar-

nanoparticle films, C/

morphologies on the photocatalytic

and (d) collapsing and further thinning of nanowires [10].

 ions (**Figure 7(b)**). By increasing processing time, strings of through holes are formed on the tube wall, and they would initiate and propagate downward from the top to bottom

migration direction), while the holes near the top expand and

of F<sup>−</sup>

of TNAs (or along the F<sup>−</sup>

140 Titanium Dioxide - Material for a Sustainable Environment

*2.2.5. Photocatalytic performance*

phologies of TNAs and TiO2

the film thickness of TiO<sup>2</sup>

concentration of TiO2

C0

$$\mathcal{C}\_t = \mathcal{C}\_o e^{-\lambda t} \tag{5}$$

$$
\ln \mathbf{C}\_t / \mathbf{C}\_o = -kt \tag{6}
$$

where *k* is the reaction rate constant, *t* is the irradiation time, and C0 and Ct are the initial concentration and the reaction concentration of MB. The reaction rate constants (k) were calculated from the experimental data using a linear regression. For TNAs with various tube diameters at a fixed tube thickness, the *k* values were 9.80 × 10−2, 8.48 × 10−2, 6.73 × 10−2, and 5.49 × 10−2 h−1 for tube diameter of 40, 60, 80, and 100 nm, respectively. Clearly, a smaller tube diameter results in a higher reaction rate [10]. In comparison, at the same tube diameter of 60 nm, the k value of 20 nmTNWs/60 nmTNAs was 48% higher than that of pure TNAs (12.54 × 10−2 vs. 8.40 × 10−2 h−1). Similarly, at a fixed tube diameter of 40 nm, the rate constant k for 20 nm TNWs/40 nm TNAs enhanced approximately 33% over the pure TNA one (13.05 × 10−2 vs. × 9.8 × 10−2 h−1) [10]. It is found that the photocatalytic activity of TNWs/TNAs enhanced remarkably (up to 33–48%) over the corresponding pure TNAs.

**Table 1** summarizes the calculated dye adsorption and the *k* values of various TiO2 films. For TNAs, a smaller tube diameter yields a higher dye adsorption because of its larger surface area, which results in enhanced photocatalytic efficiency. The difference in surface area of TNAs comes from different densities of nanotube. As the anodizing voltage increases, tube density decreases with increasing tube diameter to result in a decrease of dye adsorption and photocatalytic efficiency. The dye adsorption of TNWs/TNAs is significantly higher than that of the corresponding TNA film because the former that offers larger surface area than the latter. Indeed, the congregation of the bundled nanowires connected to the nanotube mouths induces surface area enhancement over the TNAs. It is also found from **Table 1** that the change percentages of the reaction rate constants for both TNAs and TNWs/TNAs are higher than those of dye adsorption. This indicates that besides the predominant factor of surface area, the 1D nanostructures of TNAs and TNWs/TNAs offer the promotion of charge transport and reduced recombination rate. Though having smaller surface area than TiO2 nanoparticles, both the faster transport and slower recombination of 1D structures and the providing additional surface area from TNWs, the photocatalytic performance of TNWs/ TNAs has achieved a very high efficiency (*k* = 13.05 × 10−2 h−1) that is very close to the *k* value of TiO2 nanoparticles (13.05 × 10−2 h−1) [10]. It is worthy to mention that the immobilized film forms (i.e., TNAs and TNWs/TNAs on Ti plates) are more practical than TiO2 powdery form. The photocatalytic performance of TNWs/TNAs may be further improved by optimizing the width and density of TNWs covering on TNAs.


pattern at the fringe of its top end, suggesting that the nanotubes in the two-step anodization directly developed from the imprint pattern left on the Ti surface. It is found that the aging the electrolyte is necessary for improving the quality of the imprint patterns or avoiding the initial random patterns of TNAs. The ordered imprints play the role of template for the nanotube growth. Owing to the regular distribution across the surface of the pretreated Ti foil at the very beginning, the uniformity and orderliness of the nanotube arrays were developed during the two-step anodization [15]. Intriguingly, a thin porous film is covered on the top of the nanotube layer as taking a closer look at **Figure 9a**. It is well known that the length of a nanotube does not increase when the rate of oxidation at the Ti/oxide interface at the bottom equals the rate of dissolution at the oxide/electrolyte interface at the top. Hence, the porous film on the top of the TNA plays a role as the protecting layer from dissolution,

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143

The side walls of the TNAs have obvious thickness variation, which often refers to ripples, as shown in **Figure 9d**. So far, the ripples are formed due to the periodic oscillations of the current in anodization [12]. Indeed, the bamboo-shaped nanotube, a nanostructure with more drastic ripples along the side walls, has been successfully fabricated by using anodic oxidation with ac voltage [12, 15]. **Figure 9e** shows bamboo-shaped tubes which were syn-

anodization sequences of 60 V for 2 min and 10 V for 2 min. The inset illustrates the anodization sequence for the formation of bamboo-type TNAs. The anodic oxidation growth

formation between the second section and third section of nanotubes is at the third highvoltage step [12]. The formation mechanism is that the low-voltage step reduces pH and ion

tube walls when a second high-voltage step is conducted [12]. Length and ridge spacing of bamboo-type nanotubes can be easily tuned by adjusting electrolyte composition and time

**Figure 9(g)** and **(h)** presents the morphology of a lotus root-shaped nanostructure, which was obtained using anodizing voltage of 30 V in the second step and 60 V in the first step. The nanostructure exhibits two levels, and it resembles as a lotus root in shape. The first level consists of cells with size of approximately 0.2 μm, as highlighted by a hexagon in the inset of **Figure 9g** [15]. The second-level structure is constituted by the pores with smaller diameters inside the cells (one of them highlighted by a circle in the inset of **Figure 9g**). Noticeably, the nanopores all evolved in the interior of the cells or such nanopores did not extend across any neighboring cells. By comparing the size and shape, it is found that the first-level structure corresponds to the imprints of Ti surface after the removal of the nanotube layer in the onestep anodization. In addition, this lotus root-shaped nanostructure was only obtained when the second-step voltage was low enough [15]. When the second-step voltage is lower than the first-step voltage, for example, 30 vs. 60 V, the generated nanotubes in the second-step anodization will be thinner than those generated in the first-step anodization. Therefore, several nanotubes simultaneously developed inside one imprint to result in the lotus root-shaped

F and 5 vol % H2

nanotubes is illustrated by the schematic in **Figure 9e**, where ridge

nanotubes and induces formation of bamboo ridges on outer

O under different

and consequently the preparation of long nanotubes is possible.

thesized in EG electrolytes containing 0.3 wt % NH4

nanostructure, as shown in **Figure 9(g)** and **(h)** [15].

of bamboo-type TiO2

diffusion gradient inside TiO<sup>2</sup>

of high-voltage step [12].

**Table 1.** Dye adsorption, reaction rate constants (k), and change percentage of various TiO2 films.
