**4. Conclusions**

dissolution, a layer of interconnected nanopores can be constructed on the top of TiO2

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*

hierarchical nanostructures is formed at the low two-step voltages of 20 and 40 V because

openings of the nanotubes where the nanotubes met the bulk electrolyte (**Figure 10(b)**–**(c)**).

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

The photocurrent densities of different samples were determined by linear sweep photovoltammetry measurements using a three-electrode electrochemical system in a 1 M KOH

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

As the anodization voltage increases further, the accumulation/deposition of Ti(OH)4

of the relatively high *νdis*, relatively little accumulation/deposition of Ti(OH)<sup>4</sup>

2− anion. Thus, the unique nanoring/nanotube

) illumination. Three major conclusions can be

decreases due to Coulomb force on the TiF6

146 Titanium Dioxide - Material for a Sustainable Environment

on PEC performance.

electrolyte under AM 1.5 G (100 mW/cm2

, as

occurred at the

at the

It is demonstrated that various spectacular and interesting 1D nanostructures of TiO2 have been grown and designed by anodic oxidation. TiO2 nanotube arrays and TiO2 nanowires/ nanotubes are generally obtained by conventional one-step anodic oxidation with suitable conditions. The effects of anodizing voltage, processing time, and electrolytes on the morphologies of anodic oxidation TiO2 nanomaterials are reported. Meanwhile, two-step anodic oxidation allows growing some spectacular TiO2 nanostructures such as highly ordered TNAs, bamboo-type TNAs, and lotus root-shaped TNAs. In addition, the formation mechanisms and photocatalytic activities of some TNA-based nanomaterials prepared by anodic oxidation are presented and discussed.

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