**1. Introduction**

Titanium dioxide (TiO2 ) is the most widely used one for applications in photocatalysis, dyesensitized solar cells, and biomedical devices owing to some outstanding properties [1–8]. The fascinating properties like strong oxidizing abilities, chemical stability, nontoxicity, ease of preparation, favorable band-edge positions, water insolubility, multifaceted electronic properties, and super hydrophilicity. TiO2 has three metastable phases of rutile, brookite, and anatase, in which rutile (Eg = 3.0 eV) and anatase (Eg = 3.2 eV) are the most commonly synthesized

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and used phases for applications. Rutile is the most stable phase for bulk TiO2 , while it is anatase phase for nanoscale size crystallite TiO2 due to surface energy effect [9]. Anatase can transfer into rutile by annealing temperatures in range of 500–700°C. Compared to rutile phase of TiO2 , anatase generally possesses better photocatalytic activity owing to its wider bandgap (~3.2 eV), lower electron–hole recombination rate, and higher charge carrier mobility.

However, the tube structure was not highly organized, and the tubes showed considerable sidewall inhomogeneity [16]. Moreover, the thickness of TNA layer did not increase with anodizing time, and the limited thickness was ascribed to an oxide growth/chemical dissolu-

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

**Figure 1** illustrates the setup of an anodic anodization process in our experiments that is also a typical process in the literature. Titanium foils (99.9% purity, 0.5 mm thickness) are

ultrasonically cleaned in acetone, methanol, and deionized water (each solvent 10 min) and

is carried out at room temperature using a two-electrode system with the Ti foil as an anode and a stainless steel foil (SS304 or platinum plate for researches) as a cathode. The electrolytes

TNA growth is driven by two simultaneously occurring reactions in the anodization pro-

Cathodic reaction:4H+ + 4e = 2H2 (2)

gas. The TNA film area on Ti foil is 1 × 1.5 cm. The anodization

F in solution of ethylene glycol and water (99:1 in wt.%). The applied

layer [8, 14]. The reactions are given below:

O–4e = TiO2 + 4H+ (1)

<sup>+</sup> = TiF6

. Prior to anodization, Ti foil was

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133

and (2) the electric field-

<sup>2</sup><sup>−</sup> + 2H2

O (3)

tion equilibrium (steady state).

then dried by a purging N2

consisted of 0.5 wt% NH4

*2.1.1. Formation mechanism of TiO2*

assisted chemical dissolution of the TiO2

Anodic reaction:Ti + 2H2

**Figure 1.** A schematic of an electrochemical anodization process.

used as the substrate for growing 1D nanostructures of TiO2

voltage of 30–60 V and anodizing time of 0.5–6 hrs are usually used.

cess: (1) the electric field-driven anodic oxidation of Ti to form TiO<sup>2</sup>

Chemical etching (dissolution) reaction:TiO2 + 6F<sup>−</sup> + 4NH4

 *nanotubes*

For many applications of TiO2 , it is crucial to maximize the specific surface area to enhance catalytic reactions and achieve a maximum overall efficiency. Therefore, nanoparticulated forms of TiO2 are widely used [1]. Though providing high surface area, the charge carrier transport of nanoparticulated forms is limited due to the structural disorders. Instead, 1D TiO2 nanostructures such as TiO2 nanotube arrays (TNAs) and TiO2 nanowires on nanotube arrays (NTWs/TNAs) are of great interests because of the higher control of the chemical or physical behavior. By diminishing dimensions to the nanoscale, TNAs and TNWs/TNAs not only provide large surface-to-volume ratio and unidirectional electrical channel but also may change the electronic properties owing to quantum size effects' strong contribution of surface reconstruction or surface curvature. Complex hybrid nanostructures such as TNWs/ TNAs [10], brush-type nanostructure [11], and bamboo-type TiO2 nanotube [12] have been fabricated successfully by anodic oxidation, which have a great potential to simultaneously provide high surface area, good charge transport, and usually have better performance than simple nanostructures. It is worthy to mention that the form of nanostructured TiO2 film on Ti substrate offers a great advantage that it is able to retrieve after usage in photocatalytic applications. TiO2 nanotubes were readily fabricated by template-assisted processes, hydrothermal method, and electrochemical anodic oxidation [1, 3, 6, 13]. Among these methods, TNAs prepared by electrochemical anodization are simple, cost-effective, recyclable, suitable for rapid production, and notable to yield uniform TNAs with vertical alignment and diverse morphology [1, 6, 10, 14, 15].

This book chapter provides some of the spectacular and surprising 1D nanostructures of TiO2 prepared by anodic oxidation method for photocatalytic applications. The first section will focus exclusively on the growths of self-organized TNAs and TNWs/TNAs using conventional one-step anodic oxidation as well as their photocatalytic performances. The second section will review some novel modified TNA structures fabricated by two-step anodic oxidation, including highly ordered TNAs, the modified TNAs, and their photoelectrochemical water splitting performance.

#### **2. One-dimensional nanostructured TiO2 grown by one-step anodic oxidation**

#### **2.1. TiO2 nanotube arrays (TNAs): formation mechanism and photocatalytic activity**

In 1999, Zwilling and coworkers reported the first self-organized anodic oxides on titanium in chromic acid electrolytes containing hydrofluoric acid [16]. In this work, an organized nanotube layer with thickness up to approximately 500 nm was grown on a Ti sheet at low voltage. However, the tube structure was not highly organized, and the tubes showed considerable sidewall inhomogeneity [16]. Moreover, the thickness of TNA layer did not increase with anodizing time, and the limited thickness was ascribed to an oxide growth/chemical dissolution equilibrium (steady state).

**Figure 1** illustrates the setup of an anodic anodization process in our experiments that is also a typical process in the literature. Titanium foils (99.9% purity, 0.5 mm thickness) are used as the substrate for growing 1D nanostructures of TiO2 . Prior to anodization, Ti foil was ultrasonically cleaned in acetone, methanol, and deionized water (each solvent 10 min) and then dried by a purging N2 gas. The TNA film area on Ti foil is 1 × 1.5 cm. The anodization is carried out at room temperature using a two-electrode system with the Ti foil as an anode and a stainless steel foil (SS304 or platinum plate for researches) as a cathode. The electrolytes consisted of 0.5 wt% NH4 F in solution of ethylene glycol and water (99:1 in wt.%). The applied voltage of 30–60 V and anodizing time of 0.5–6 hrs are usually used.

#### *2.1.1. Formation mechanism of TiO2 nanotubes*

and used phases for applications. Rutile is the most stable phase for bulk TiO2

TNAs [10], brush-type nanostructure [11], and bamboo-type TiO2

fer into rutile by annealing temperatures in range of 500–700°C. Compared to rutile phase of

catalytic reactions and achieve a maximum overall efficiency. Therefore, nanoparticulated

transport of nanoparticulated forms is limited due to the structural disorders. Instead, 1D

arrays (NTWs/TNAs) are of great interests because of the higher control of the chemical or physical behavior. By diminishing dimensions to the nanoscale, TNAs and TNWs/TNAs not only provide large surface-to-volume ratio and unidirectional electrical channel but also may change the electronic properties owing to quantum size effects' strong contribution of surface reconstruction or surface curvature. Complex hybrid nanostructures such as TNWs/

fabricated successfully by anodic oxidation, which have a great potential to simultaneously provide high surface area, good charge transport, and usually have better performance than

Ti substrate offers a great advantage that it is able to retrieve after usage in photocatalytic

thermal method, and electrochemical anodic oxidation [1, 3, 6, 13]. Among these methods, TNAs prepared by electrochemical anodization are simple, cost-effective, recyclable, suitable for rapid production, and notable to yield uniform TNAs with vertical alignment and diverse

This book chapter provides some of the spectacular and surprising 1D nanostructures of TiO2 prepared by anodic oxidation method for photocatalytic applications. The first section will focus exclusively on the growths of self-organized TNAs and TNWs/TNAs using conventional one-step anodic oxidation as well as their photocatalytic performances. The second section will review some novel modified TNA structures fabricated by two-step anodic oxidation, including highly ordered TNAs, the modified TNAs, and their photoelectrochemical

 **nanotube arrays (TNAs): formation mechanism and photocatalytic activity**

In 1999, Zwilling and coworkers reported the first self-organized anodic oxides on titanium in chromic acid electrolytes containing hydrofluoric acid [16]. In this work, an organized nanotube layer with thickness up to approximately 500 nm was grown on a Ti sheet at low voltage.

nanotubes were readily fabricated by template-assisted processes, hydro-

simple nanostructures. It is worthy to mention that the form of nanostructured TiO2

(~3.2 eV), lower electron–hole recombination rate, and higher charge carrier mobility.

, anatase generally possesses better photocatalytic activity owing to its wider bandgap

are widely used [1]. Though providing high surface area, the charge carrier

nanotube arrays (TNAs) and TiO2

tase phase for nanoscale size crystallite TiO2

132 Titanium Dioxide - Material for a Sustainable Environment

For many applications of TiO2

nanostructures such as TiO2

TiO2

TiO2

forms of TiO2

applications. TiO2

morphology [1, 6, 10, 14, 15].

water splitting performance.

**oxidation**

**2.1. TiO2**

**2. One-dimensional nanostructured TiO2**

, while it is ana-

nanowires on nanotube

nanotube [12] have been

film on

due to surface energy effect [9]. Anatase can trans-

 **grown by one-step anodic** 

, it is crucial to maximize the specific surface area to enhance

TNA growth is driven by two simultaneously occurring reactions in the anodization process: (1) the electric field-driven anodic oxidation of Ti to form TiO<sup>2</sup> and (2) the electric fieldassisted chemical dissolution of the TiO2 layer [8, 14]. The reactions are given below:

$$\text{Anodic reaction:}\\\text{Ti} + 2\text{H}\_2\text{O} + 4\text{e} = \text{TiO}\_2 + 4\text{H}^\* \tag{1}$$

$$\text{Cathodic reaction:}\\4\text{H}^+\\+4\text{e}=\ 2\text{H}\_2\tag{2}$$

 Chemical etching (dissolution) reaction:TiO2 + 6F<sup>−</sup> + 4NH4 <sup>+</sup> = TiF6 <sup>2</sup><sup>−</sup> + 2H2 O (3)

**Figure 1.** A schematic of an electrochemical anodization process.

In the anodic oxidation process, the current density (j) changes with anodizing time (t). **Figure 2(a)** illustrates the current transient curve recorded during the anodization of titanium foil. Initially, the current density rapidly decreases, then slightly increases, and finally remains a constant [1]. According to the variations of current density with reaction time, three stages of the TNA growth process were defined as shown in **Figure 2(a)**. In the early stages (I), the current density rapidly decreases because of the formation of the non-conductive thin oxide layer (see also **Figure 2(b)**). Next, there is a slight increase in the current density owing to the local growth of pits. Finally, the current density remains a constant due to reaching the balance between field-assisted anodic oxidation and chemical−/ field-assisted dissolution current, and the initial pits will grow and develop into nanotube arrays. Notably, the diameter and length of the nanotubes gradually increase when the dissolution rate of the wall of the nanopores is slower than that of the growth rate of nanopores. And, the diameter and length of NTs no longer change when the growth rate is equal to the dissolution rate [17].

#### *2.1.2. An application of TiO2 nanotubes in photoelectrocatalytic degradation of model environmental pharmaceuticals*

A selected optimal TiO2 nanotube was prepared at 30 V for 16 h in ethylene glycol solution containing 0.20 mol L−1 NH4 F and 0.50 mol L−1 HAc. **Figure 3(a)** shows structure-morphology of the TNAs through the FESEM and HRTEM images. Clearly, perfect NTAs are obtained with average inner diameter approximately 70 nm length of about 7.5 μm. TNAs have anatase phase which is confirmed by the selected area electron diffraction pattern recorded from the circled area. In addition, the lattice fringes spacing of 0.352 nm corresponds to {101} planes of anatase TiO2 . To evaluate the photocatalytic activity of the TNA photoanodes, the authors

performed experiments using a model compound of pharmaceutical, acyclovir (20 mg L−1), in a three-electrode thin layer photoelectrochemical reactor with a reaction volume of 100 μL

different processes including electrolysis (EC), photocatalysis (PC), and photoelectrocatalysis (PEC) using the TiO<sup>2</sup>

nanotubes prepared at 30 V for 16 h in ethylene glycol solution containing 0.50 Mol L−1 HAc and

F: (a) FESEM image and a HRTEM image of nanotube wall (inset), (b) degradation of acyclovir under

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

acyclovir was injected into the reactor under different constant speeds to adjust the reaction time. An applied potential of +1 V constant and illumination of 10 mWcm−2 from UV-LED array were used for the PEC experiment. Photocatalysis (PC) and electrolysis (EC) degradation experiments were conducted under identical experimental conditions as photoelectrocatalysis (PEC) experiments, except the electrochemical system was disconnected and the light

**Figure 3(b)** presents the degradation of acyclovir under EC, PC, and PEC processes using the TNA photoanode. There was no measurable change in acyclovir concentration within residence time up to 370 s for the EC process. The degradation efficiencies of acyclovir in the PC process were 62.0 and 69.0% at residence times of 180 and 370 s, respectively. In comparison, the PEC degradation efficiencies increased remarkably to 83.0% at 180 s and 96.2% at 370 s. The significantly faster degradation of acyclovir for PEC process than those of PC and EC is attributed to two reasons: First, under an appropriate applied potential bias to the photoanode, it is enable to remove the photogenerated electrons and prolong the lifetime of photoholes for direct photohole reactions, and the oxidation power of the photoholes (3.1 V) is greater than that of photogenerated radicals such •OH (2.8 V) [17]. Second, the light adsorption capability and charge transfer is enhanced owing to the tubular structure of the TNA photoanode. Thus, the separation of photogenerated electrons and holes assisted by the application of a potential bias is accelerated, which in turn enhances the concentration of photoholes and promotes effectively the degradation

NaNO3

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

135

and 20 mgL−1

under UVA irradiation [17]. An aqueous solution containing 0.2 mol −<sup>1</sup>

switched off, respectively [17].

**Figure 3.** TiO2

0.20 Mol L−1 NH4

nanotube photoanode [17].

of acyclovir.

**Figure 2.** (a) Typical current–time *(j*–*t*) characteristics after applied voltage in the absence (**−--**) and presence (**―**) of fluoride ions in the electrolyte. Either compact oxide (fluoride free) or porous/tubular metal oxide formation (containing fluoride) formed by different morphological stages (I–III). The growth process of TiO<sup>2</sup> nanotube arrays (TNAs): (b) nonconductive thin oxide layer forming, (c) local growth of the pits, and (d) growth of the semicircle pores and developed nanotube arrays. (e) a typical TNA prepared by using an electrolyte consisting of ethylene glycol and water (99:1 in wt%) with 0.5 wt% NH4 F, at 30 V for 30 mins.

Recent Advances in TiO2 Nanotube-Based Materials for Photocatalytic Applications Designed… http://dx.doi.org/10.5772/intechopen.77063 135

In the anodic oxidation process, the current density (j) changes with anodizing time (t). **Figure 2(a)** illustrates the current transient curve recorded during the anodization of titanium foil. Initially, the current density rapidly decreases, then slightly increases, and finally remains a constant [1]. According to the variations of current density with reaction time, three stages of the TNA growth process were defined as shown in **Figure 2(a)**. In the early stages (I), the current density rapidly decreases because of the formation of the non-conductive thin oxide layer (see also **Figure 2(b)**). Next, there is a slight increase in the current density owing to the local growth of pits. Finally, the current density remains a constant due to reaching the balance between field-assisted anodic oxidation and chemical−/ field-assisted dissolution current, and the initial pits will grow and develop into nanotube arrays. Notably, the diameter and length of the nanotubes gradually increase when the dissolution rate of the wall of the nanopores is slower than that of the growth rate of nanopores. And, the diameter and length of NTs no longer change when the growth rate is equal

 *nanotubes in photoelectrocatalytic degradation of model* 

. To evaluate the photocatalytic activity of the TNA photoanodes, the authors

of the TNAs through the FESEM and HRTEM images. Clearly, perfect NTAs are obtained with average inner diameter approximately 70 nm length of about 7.5 μm. TNAs have anatase phase which is confirmed by the selected area electron diffraction pattern recorded from the circled area. In addition, the lattice fringes spacing of 0.352 nm corresponds to {101} planes

**Figure 2.** (a) Typical current–time *(j*–*t*) characteristics after applied voltage in the absence (**−--**) and presence (**―**) of fluoride ions in the electrolyte. Either compact oxide (fluoride free) or porous/tubular metal oxide formation (containing

conductive thin oxide layer forming, (c) local growth of the pits, and (d) growth of the semicircle pores and developed nanotube arrays. (e) a typical TNA prepared by using an electrolyte consisting of ethylene glycol and water (99:1 in wt%)

fluoride) formed by different morphological stages (I–III). The growth process of TiO<sup>2</sup>

F, at 30 V for 30 mins.

nanotube was prepared at 30 V for 16 h in ethylene glycol solution

F and 0.50 mol L−1 HAc. **Figure 3(a)** shows structure-morphology

nanotube arrays (TNAs): (b) non-

to the dissolution rate [17].

134 Titanium Dioxide - Material for a Sustainable Environment

*2.1.2. An application of TiO2*

A selected optimal TiO2

of anatase TiO2

with 0.5 wt% NH4

*environmental pharmaceuticals*

containing 0.20 mol L−1 NH4

**Figure 3.** TiO2 nanotubes prepared at 30 V for 16 h in ethylene glycol solution containing 0.50 Mol L−1 HAc and 0.20 Mol L−1 NH4 F: (a) FESEM image and a HRTEM image of nanotube wall (inset), (b) degradation of acyclovir under different processes including electrolysis (EC), photocatalysis (PC), and photoelectrocatalysis (PEC) using the TiO<sup>2</sup> nanotube photoanode [17].

performed experiments using a model compound of pharmaceutical, acyclovir (20 mg L−1), in a three-electrode thin layer photoelectrochemical reactor with a reaction volume of 100 μL under UVA irradiation [17]. An aqueous solution containing 0.2 mol −<sup>1</sup> NaNO3 and 20 mgL−1 acyclovir was injected into the reactor under different constant speeds to adjust the reaction time. An applied potential of +1 V constant and illumination of 10 mWcm−2 from UV-LED array were used for the PEC experiment. Photocatalysis (PC) and electrolysis (EC) degradation experiments were conducted under identical experimental conditions as photoelectrocatalysis (PEC) experiments, except the electrochemical system was disconnected and the light switched off, respectively [17].

**Figure 3(b)** presents the degradation of acyclovir under EC, PC, and PEC processes using the TNA photoanode. There was no measurable change in acyclovir concentration within residence time up to 370 s for the EC process. The degradation efficiencies of acyclovir in the PC process were 62.0 and 69.0% at residence times of 180 and 370 s, respectively. In comparison, the PEC degradation efficiencies increased remarkably to 83.0% at 180 s and 96.2% at 370 s. The significantly faster degradation of acyclovir for PEC process than those of PC and EC is attributed to two reasons: First, under an appropriate applied potential bias to the photoanode, it is enable to remove the photogenerated electrons and prolong the lifetime of photoholes for direct photohole reactions, and the oxidation power of the photoholes (3.1 V) is greater than that of photogenerated radicals such •OH (2.8 V) [17]. Second, the light adsorption capability and charge transfer is enhanced owing to the tubular structure of the TNA photoanode. Thus, the separation of photogenerated electrons and holes assisted by the application of a potential bias is accelerated, which in turn enhances the concentration of photoholes and promotes effectively the degradation of acyclovir.

#### **2.2. TiO2 nanowires on nanotube arrays (TNWs/TNAs)**

#### *2.2.1. Fabrication of TiO2 films and other experimental details*

TiO2 films with morphology of TNWs/TNAs were grown on Ti foils by anodic oxidation. The electrolyte consists of ethylene glycol and water (99:1 in wt%) and 0.5 wt% NH4 F. First, 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 of 2°C/min.

#### *2.2.2. Effect of anodization voltage*

**Figure 4** shows the surface morphology of the TiO2 films prepared at different anodizing voltages from 20 to 80 V, in a 0.5 wt% NH4 F solution, and at a constant anodizing time of 1 h. 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 of 80 V [10].
