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

**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 controlled anodizing voltages [6]. The electrolytes consisted of 0.5 wt% NH4 F in EG solution with 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 annealed at 450°C for 1 h in the air.

The hierarchical top layer/TNAs are an outcome of the competition between the electric fielddriven anodic oxidation [reaction (1) above, *νelectro*] of Ti to form TiO2 and the electric fieldassisted chemical dissolution of the TiO2 layer [reaction (3) above, *νdis*] [8, 14]. The anodic oxidation reaction occurs as Ti4+ ejection and deposition on the surface in the form of TiO2 , while the TiF6 2− etching reaction occurs from the top to bottom of the as-grown TiO<sup>2</sup> . The anodic oxidation rate is very fast and dominated over the NH4 F etching rate, resulting in a thin oxide layer in the early stage [14, 15]. In the late stage, the deposition rate of TiO2 at 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 develop into TNA structures [8, 14]. At certain relative rates between TiO2 deposition and

**Figure 10.** (a) Two-step anodization synthesis of the hierarchical TiO2 nanotubes. (b–f) Plan-view and cross-sectional scanning electron microscopy (SEM) images of the hierarchical TiO2 nanotubes prepared at a fixed one-step voltage of 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 second step [6].

**Figure 9.** Nanotube array generated in the two-step anodization under a 60 V voltage: (a) top view and (b) the exposed Ti

side view of the nanotubes and (d) zoom-in view of the nanotubes with the ripple features [15]. (c) the bamboo-shaped tubes

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].

for 2 min and 10 V for 2 min. (inset) Anodization sequence for the formation of bamboo-type TiO2

F and 5 vol % H2

nanotubes via anodic oxidation: Ridge formation between the second section and

nanotubes generated in the two-step anodization: (c) the

O under different anodization sequences of 60 V

nanotube arrays. Schematic

surface after the removal of the nanotube layer. Side view of the TiO2

were synthesized in EG electrolytes containing 0.3 wt % NH4

144 Titanium Dioxide - Material for a Sustainable Environment

showing growth of bamboo-type TiO2

dissolution, a layer of interconnected nanopores can be constructed on the top of TiO2 , as shown in **Figure 10** [6]. The highly ordered hexagonal imprints after ultrasonication plays as a template for the subsequent growth of TNAs with a hierarchical porous top layer [15].

**Figure 10(b)**–**(f)** shows SEM images of the modified TNAs prepared at a fixed voltage of 60 V for the one-step anodization and at different voltages from 20 to 100 V for the two-step anodization. **Figure 10(g)** is the TNAs prepared at 100 V for the first step and 80 V for the second step. When the density of applied electric field increases, *νelectro* increases, whereas *νdis* decreases due to Coulomb force on the TiF6 2− anion. Thus, the unique nanoring/nanotube hierarchical nanostructures is formed at the low two-step voltages of 20 and 40 V because of the relatively high *νdis*, relatively little accumulation/deposition of Ti(OH)<sup>4</sup> occurred at the openings of the nanotubes where the nanotubes met the bulk electrolyte (**Figure 10(b)**–**(c)**). As the anodization voltage increases further, the accumulation/deposition of Ti(OH)4 at the entrance of the nanotubes increases to form nanopore and nanohole top structures, with the diameters of the openings decreasing with increasing voltages. Furthermore, at high anodization voltages, an increase in *νelectro* favors the formation of a thicker oxide layer owing to the preferred growth at the bottom of the concavity generated in the one-step anodization where the electrical resistance is the lowest. Consequently, the diameters of the bottom tubes of TNAs increase with increasing voltages (**Figure 10(d)**–**(f)**) [6]. To investigate the effect of the voltage in the one-step anodization step, a comparison between **Figure 10e** (S-60-80) and **10 g** (S-100-80) can be made. Clearly, S-60-80 exhibits a nanopore/TNA structure, whereas S-100- 80 presents a nanohole-nanocave/TNA structure, which consists of holes with a pore size of ~50 nm and concaves with closed bottoms. The formation of a thick oxide barrier layer in the one-step anodization step at the high voltage (i.e., 100 V) should be attributed to the formation of S-100-80 morphology. The insets in **Figure 10** confirm that all samples had a uniform length of TNAs in the range of 1.3–1.5 μm for minimizing the effect of tube length disparity on PEC performance.

S-60-40) because the formers have better degree of crystallinity and higher optical absorption (to be discussed in the later section). (3) Among all of the nanopore/TNA samples, S-60-80 achieved the highest photocurrent density, which attributed to its better optical absorption and its uniform morphology for reducing structural defects that serve as photoelectron/hole

collected with a scan rate of 5 mVs—1 In the dark and under illumination and (b) amperometric *I*–*t* curves at an applied

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

A photocatalyst material with a better optical absorption and higher crystallinity will subsequently result in a better PEC performance. *P. Wang* et al. found that all two-step hierarchical TNA samples exhibited better UV–Vis absorption than one-step TNA sample because of their unique hierarchical nanostructures and improved morphological uniformity [6]. A better surface morphological uniformity should result in enhanced optical absorption because the material has better periodicity of the photonic crystals formed by the top porous layers. It is found that the crystallite size of TNAs increases with increasing anodizing voltages [22]. Therefore, a high anodizing voltage favors the formation of bigger crystal nucleus and thus results in a better crystallinity and less defects to obtain the better PEC performance, which agreed well with the aforementioned results [6]. In addition, the photoelectrode stability of the hierarchical TNAs was assessed via amperometric (I − t) measurements, performing at a fixed electrode potential of 1.23 V vs. RHE under alternating light on and light off with a cycle of 60 s. As can be seen in **Figure 11(b)**, fast photoresponses were recorded for all samples, and

recombination centers (**Figure 10e**) and thus favor high PEC performance.

**Figure 11.** Photoelectrochemical properties of hierarchical TiO2

potential of 1.23 V vs. RHE under illumination with 60 s light on/off cycles [6].

this photocurrent pattern was highly reproducible for many on/off cycles [6].

been grown and designed by anodic oxidation. TiO2

phologies of anodic oxidation TiO2

It is demonstrated that various spectacular and interesting 1D nanostructures of TiO2

nanotubes are generally obtained by conventional one-step anodic oxidation with suitable conditions. The effects of anodizing voltage, processing time, and electrolytes on the mor-

have

nanowires/

nanotube arrays and TiO2

nanotube electrodes: (a) linear-sweep voltammograms

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

147

nanomaterials are reported. Meanwhile, two-step anodic

**4. Conclusions**

The photocurrent densities of different samples were determined by linear sweep photovoltammetry measurements using a three-electrode electrochemical system in a 1 M KOH electrolyte under AM 1.5 G (100 mW/cm2 ) illumination. Three major conclusions can be drawn from the results in **Figure 11(a)**. (1) The photocurrent densities of all samples prepared by two-step anodization are consistently higher than the ones prepared by the conventional one-step method under the same conditions. This implies that the hierarchical TiO2 nanostructures favor better PEC performances. Indeed, the one-step prepared TNAs only obtained a photocurrent density of 0.345 mAcm−2 at 0.23 V vs. Ag/AgCl or 1.23 V vs. RHE, which is the potential often chosen as a metric to evaluate the performance of photoanodes as it corresponds to the water oxidation potential. Among all samples, S-60-80 achieved the highest photocurrent density of 1.59 mAcm−2 at the same potential [6]. (2) The photocurrent densities (I) of different nanostructured TNAs presented a general order of Inanopore/ TNA > Inanoring/TNA > Inanohole-nanocave/TNA. The photocurrent density of the nanohole-nanocave/TNA sample (i.e., S-100-80) possesses a low value of 0.480 mAcm−2 that should be due to the closed nanocave structures on its surface, and thus it effectively blocks the light absorption and solution infiltration. Moreover, the nanopore/TNA samples (i.e., S-60-60, S-60-80, and S-60-100) exhibited higher photocurrent densities than the nanoring/TNA samples (i.e., S-60-20 and Recent Advances in TiO2 Nanotube-Based Materials for Photocatalytic Applications Designed… http://dx.doi.org/10.5772/intechopen.77063 147

**Figure 11.** Photoelectrochemical properties of hierarchical TiO2 nanotube electrodes: (a) linear-sweep voltammograms collected with a scan rate of 5 mVs—1 In the dark and under illumination and (b) amperometric *I*–*t* curves at an applied potential of 1.23 V vs. RHE under illumination with 60 s light on/off cycles [6].

S-60-40) because the formers have better degree of crystallinity and higher optical absorption (to be discussed in the later section). (3) Among all of the nanopore/TNA samples, S-60-80 achieved the highest photocurrent density, which attributed to its better optical absorption and its uniform morphology for reducing structural defects that serve as photoelectron/hole recombination centers (**Figure 10e**) and thus favor high PEC performance.

A photocatalyst material with a better optical absorption and higher crystallinity will subsequently result in a better PEC performance. *P. Wang* et al. found that all two-step hierarchical TNA samples exhibited better UV–Vis absorption than one-step TNA sample because of their unique hierarchical nanostructures and improved morphological uniformity [6]. A better surface morphological uniformity should result in enhanced optical absorption because the material has better periodicity of the photonic crystals formed by the top porous layers. It is found that the crystallite size of TNAs increases with increasing anodizing voltages [22]. Therefore, a high anodizing voltage favors the formation of bigger crystal nucleus and thus results in a better crystallinity and less defects to obtain the better PEC performance, which agreed well with the aforementioned results [6]. In addition, the photoelectrode stability of the hierarchical TNAs was assessed via amperometric (I − t) measurements, performing at a fixed electrode potential of 1.23 V vs. RHE under alternating light on and light off with a cycle of 60 s. As can be seen in **Figure 11(b)**, fast photoresponses were recorded for all samples, and this photocurrent pattern was highly reproducible for many on/off cycles [6].
