ABSTRACT
Figure 5.10 Top-view (left) and cross-section (right) SEM images of ntTiO2after annealing at 450°C in air for 3 h. 5.4.2 TiO2 Polymorph: The Interest of Anatase and
Amorphous TitaniaSeveral TiO2 polymorphs have been reported and named as rutile, anatase, brookite, TiO2-B (bronze), TiO2-R (ramsdellite), TiO2-H (hollandite), TiO2-II (columbite), and TiO2-III (baddeleyite). Since a long time, the practical use of TiO2 had been as white pigment in paint substituting the toxic lead oxides. One of the reasons for this is its whiteness attributed to the relatively high refractive index, which, in combination with small particle sizes, results in strong light scattering in a broad range of wavelengths. Other commercial applications are in products such as sun lotion and toothpaste due to its remarkable optical properties and chemical stability and as a pigment when the crystalline form has a high refractive index (rutile). Concerning energy conversion and storage applications, titanium dioxide (TiO2) is one of the most important wide-gap semiconductors and is widely envisaged for use in photoelectrochemical solar cells, supercapacitors, and Li-ion batteries [41, 42], among others. Photoelectrochemical solar cells: The use of TiO2 in solar energy conversion is directly related with the early development of photoelectrocemical cells to either water cleavage and H2 production or electricity generation. In the early 1970s, Fujishima and Honda
proposed a design of solar cell benefiting from the semiconductor properties of TiO2. Its bandgap energy is suitable for adsorbing ultraviolet-light-promoting electrons to conduction bands. The potential difference created in the electrode is appropriated to oxidize oxide anions while H2 is produced in the Pt counterelectrode
[43]. However, the absorption efficiency is rather low and its development was further abandoned for commercial purposes. In the 1990s, dye-sensitized photoelectrochemical cells proposed combining the better absorption properties of dyes with TiO2 semiconductivity [44, 45], even though efficiency was limited by the deposition of thick dye films deposited on titania compact layers. In this way, photon absorption was enhanced, but recombination was also promoted.A notorious improvement was achieved by using nanostructured TiO2 providing large surface area for the deposition of dye thin films. Thus, an enhanced absorption was allowed without being penalized by recombination. Parellel TiO2 nanotube arrays prepared by anodization of metallic titanium layer not only provide a large surface area but also ordered pore geometry. The nanotubes disposal seems to be more suitable for pore geometry of the nanotubes for solid-state cells manufacturing.
