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246 Multifunctional Photocatalytic Materials for Energy

Fig. 11.6 SEM images of the anatase TiO 2 NSs synthesized with a reaction time of 11 h (A, B)
[111]. TEM images of TiO 2 NSs through a one-step hydrothermal reaction (C) [112] and TiO 2
NSs-MWCNT composite (D) [113].
have focused on synthetic strategies for controlling TiO 2 nanostructures in three di-
mensions [118–120]. As is typical, these materials can be characterized by their poros-
ity, including their pore-size distribution, their specific nature, and their pore volume
[121–125]. In this section, we will describe recent methods that have been developed
for the synthesis and characterization of 3D TiO 2 materials, such as porous films,
porous spheres, hierarchical hollow spheres, and spherical bundles of NRs or NWs
(Table 11.2).
11.2.4.1 Porous TiO 2 films

Porous TiO 2 films are widely used in various fields, for example, for dye-sensitized
solar cells, electrochromics, and self-cleaning coatings. Different preparations for
various types of porous TiO 2 materials as films on different substrates have been in-
vestigated. Careful control of essential film properties, such as layer thickness, adhe-
sion force, homogeneity, roughness, and mechanical stability, during the formation of
porous films requires a strategic approach. For example, Agrios's group successfully
obtained multihierarchical porous TiO 2 films with a mesoporous layer based on TiO 2
NPs (Fig. 11.7A). After calcinations, the templates could be completely removed, and
films with pore sizes corresponding to the templates were obtained [126]. One of
the interesting techniques enabling fabrication of highly porous TiO 2 films under low