*2.6.1. Examples*

*2.5.2. Merits*

etching [72].

time and voltage itself [74].

• Ease of fabrication.

nanotubes.

*2.5.3. Demerits*

*2.5.4. Summary of anodization*

• High aspect ratio tubes are formed.

28 Titanium Dioxide - Material for a Sustainable Environment

formed of long lengths and diameters.

• The thickness and lengths of the nanotubes produced can be varied by changing voltage

**Figure 25.** SEM image of foil's surface: (a) After 30 s etching (inset shows anodization after 1 h at 60 V) and (b) after 90 s

**Figure 24.** (a) H-TMS calcinated at different temperatures: (b) 300°C, (c) 400°C, (d) 500°C, (e) 600°C, and (f) 700°C [40].

• The change in anodization conditions does not affect the chemical composition of TiO<sup>2</sup>

• Increasing the electrolyte concentration can cause etching to increase and nanotubes can be

So, hierarchical Titania nanostructures were prepared from anodization technique and nanotube-like structures were prepared. The particle diameter was in nanometer ranges. The diameter and length of nanotubes increase by increasing voltage and voltage time up to optimum

• Structures are highly recommended for DSSCs as high aspect ratio tubes are formed.

• Average growth rate can be decreased with increase in anodization time [74].

• Extended warranties of products are not offered in this process.

Kim et al. have reported HNSs of Titania by photolithography. A negative PR is prepared on 5 mm Ti foil by baking at 120°C. Upon exposure to UV light, PR gets developed. The foil is etched via reactive ion etching. Titanium foil's area that is not covered with the PR is etched out in this process. TiO<sup>2</sup> nanoflowers composed of nanotubes (**Figure 26**) are prepared in this procedure [48].

So, flower-like HNSs are produced via dot patterning technique. The particles are in the micron ranges (diameter). The prepared product is employed in dye-sensitized solar cells and exhibits maximum surface area for dye adsorption. Hence, these show better result for photocatalytic processes.
