**3.1. Highly ordered TNAs**

The SEM images of the highly ordered nanotubes, bamboo-type nanotubes, and lotus rootshaped nanostructure of TiO2 are shown in **Figure 9**. These nanostructures are prepared by two-step anodization method, in which electrolyte contained ethylene glycol (C2 H6 O2 ), 0.25% (in mass) NH4 F and 1% (in volume) H2 O. Noticeably, prior to performing the anodization, the electrolyte was aged under a voltage of 60 V for 60 h. In the first step, a Ti foil was anodized at 60 V for 24 h to grow a layer of TiO2 nanotubes, and then the layer was removed ultrasonically in deionized water. **Figure 9(a)** and **(c)** shows the top view and side view of the TNAs prepared under an anodizing voltage of 60 V in the two-step anodization, which is much more uniform in alignment and length than the TNAs fabricated by the corresponding one-step anodization [15]. **Figure 9b** presents the imprint pattern left on the Ti surface after peeling off from the Ti foil ultrasonically. Clearly, each nanotube is surrounded by six nearest neighbors, and each nanotube has six protrusions with hexagonal 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, and consequently the preparation of long nanotubes is possible.

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 synthesized in EG electrolytes containing 0.3 wt % NH4 F and 5 vol % H2 O under different 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 of bamboo-type TiO2 nanotubes is illustrated by the schematic in **Figure 9e**, where ridge 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 diffusion gradient inside TiO<sup>2</sup> nanotubes and induces formation of bamboo ridges on outer 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 of high-voltage step [12].

**3. Highly ordered TNAs and modified TNAs designed by two-step** 

nanoparticles 4.22 100% 14.38 100%

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

includes (1) the one-step anodization for growth TiO2

Using one-step anodization, the produced TNAs usually present disparity in tube lengths and relatively rough surfaces. It has been recently demonstrated that a two-step anodization is possible for preparation of highly ordered and uniform TNAs [14, 15] or unique hierarchical top layer/TNAs with significantly better performances in dye-sensitized solar cells [21] and in photoelectrochemical (PEC) water splitting [6]. A typical process of two-step anodization

**Change (%) relative to nanoparticles**

3.21 76% 12.54 87%

3.58 85% 13.05 91%

 nm TNAs 1.22 29% 5.49 38% nm TNAs 1.65 39% 6.73 47% nm TNAs 2.33 55% 8.48 59% nm TNAs 2.72 64% 9.80 68%

**Reaction rate constants (k (×10−2 h−1))**

**Change (%) relative to nanoparticles**

the as-grown TNAs by ultrasonication, and (3) the two-step anodization to grow TiO2

The SEM images of the highly ordered nanotubes, bamboo-type nanotubes, and lotus root-

ization, the electrolyte was aged under a voltage of 60 V for 60 h. In the first step, a Ti foil

removed ultrasonically in deionized water. **Figure 9(a)** and **(c)** shows the top view and side view of the TNAs prepared under an anodizing voltage of 60 V in the two-step anodization, which is much more uniform in alignment and length than the TNAs fabricated by the corresponding one-step anodization [15]. **Figure 9b** presents the imprint pattern left on the Ti surface after peeling off from the Ti foil ultrasonically. Clearly, each nanotube is surrounded by six nearest neighbors, and each nanotube has six protrusions with hexagonal

by two-step anodization method, in which electrolyte contained ethylene glycol (C2

F and 1% (in volume) H2

was anodized at 60 V for 24 h to grow a layer of TiO2

nanotubes, (2) subsequent removal of

films.

are shown in **Figure 9**. These nanostructures are prepared

O. Noticeably, prior to performing the anod-

nanotubes, and then the layer was

NTs

H6 O2 ),

**anodic oxidation**

60 nm TNAs/20 nm

40 nm TNAs/20 nm

**TiO2**

TNWs

TNWs

TiO2

 **film Dye adsorption**

142 Titanium Dioxide - Material for a Sustainable Environment

**(×10−8 mol/cm2**

**)**

from the same substrate [14, 15].

shaped nanostructure of TiO2

**3.1. Highly ordered TNAs**

0.25% (in mass) NH4

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

**3.2. Modified TNAs designed by two-step anodic oxidation**

annealed at 450°C for 1 h in the air.

while the TiF6

second step [6].

assisted chemical dissolution of the TiO2

trolled anodizing voltages [6]. The electrolytes consisted of 0.5 wt% NH4

driven anodic oxidation [reaction (1) above, *νelectro*] of Ti to form TiO2

develop into TNA structures [8, 14]. At certain relative rates between TiO2

anodic oxidation rate is very fast and dominated over the NH4

**Figure 10.** (a) Two-step anodization synthesis of the hierarchical TiO2

scanning electron microscopy (SEM) images of the hierarchical TiO2

**Figure 10(a)** presents the two-step anodization process and morphological characteristics under three different voltage regimes of the one-step anodization. Fascinating hierarchical top layer/TNAs has been successfully fabricated by using two-step anodization with con-

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

2 vol% water. The one-step anodization was conducted at anodizing voltages of 60, 80, or 100 V for 60 min to grow TNAs on Ti sheet, and then the as-grown TNAs were ultrasonically removed in DI water. The Ti sheet surface has a regular hexagonally packed round concave morphology (see **Figure 10a**). Next, the second-step anodization was performed using the same Ti sheet at various potentials of 20–100 V and tunable processing time to control the thickness of TNAs. Finally, samples were cleaned with DI water, dried with nitrogen gas, and

The hierarchical top layer/TNAs are an outcome of the competition between the electric field-

oxidation reaction occurs as Ti4+ ejection and deposition on the surface in the form of TiO2

a thin oxide layer in the early stage [14, 15]. In the late stage, the deposition rate of TiO2

the entrance of the nanotubes slows down, while field-induced random dissolution of the surface becomes more significant or dominant to form pore-like structures, which further

60 V and various two-step voltages from 20 to 100 V. (g) the TNAs prepared at 100 V for the first step and 80 V for the

2− etching reaction occurs from the top to bottom of the as-grown TiO<sup>2</sup>

F in EG solution with

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

and the electric field-

F etching rate, resulting in

nanotubes. (b–f) Plan-view and cross-sectional

nanotubes prepared at a fixed one-step voltage of

deposition and

,

145

. The

at

layer [reaction (3) above, *νdis*] [8, 14]. The anodic

**Figure 9.** Nanotube array generated in the two-step anodization under a 60 V voltage: (a) top view and (b) the exposed Ti surface after the removal of the nanotube layer. Side view of the TiO2 nanotubes generated in the two-step anodization: (c) the side view of the nanotubes and (d) zoom-in view of the nanotubes with the ripple features [15]. (c) the bamboo-shaped tubes were synthesized in EG electrolytes containing 0.3 wt % NH4 F and 5 vol % H2 O under different anodization sequences of 60 V for 2 min and 10 V for 2 min. (inset) Anodization sequence for the formation of bamboo-type TiO2 nanotube arrays. Schematic showing growth of bamboo-type TiO2 nanotubes via anodic oxidation: Ridge formation between the second section and third section of nanotubes at the third high-voltage step [12]. (f) the bamboo-shaped tubes generated under another squarewaved anodizing voltage (the 30 V voltage lasted 90 s and the 60 V voltage still lasted 10 s). (g) and (h) Lotus root-shaped nanostructure obtained under a 30 V anodizing voltage in the two-step anodization: (a) top view and (b) side view [15].
