considerable attention mainly for its applications in electronics, electrochemical
systems, including photoelectrochemical solar cells, electrocatalysis, optoelectronic
sensors devices, and high-performance photocatalytic films (Fox and Dulay 1993;
Hoffmann et al. 1995; Linsebigler et al. 1995; Chen and Mao 2007).
2.3.1 Structure and Properties of TiO 2
TiO 2 exhibits three main polymorphs: anatase, rutile, and brookite (Fig. 2.3)
(Landmann et al. 2012; Pelaez et al. 2012). The TiO 6 octahedron is the common
primary structural unit of these phases. The structural difference of these phases is
due to the different stacking arrangements of their octahedra. Rutile crystallizes in
the tetragonal system (P4 2 /mnm space group). In rutile, each octahedron has ten
neighbors: two share edges and eight share corners. The neighbors share corners
along the [110]-type direction. Anatase belongs also to the tetragonal system but to
the I4 1 /amd space group. It is composed of TiO 6 octahedra sharing faces. The
corner-sharing octahedron forms [001] planes. Bulk brookite is a metastable
phase, which crystallizes in the orthorhombic system (Pbca space group). At highFig. 2.2 Dye classification on the basis of dye chemical constitution and its application. (Reprinted
with permission from Ajmal et al. 2014. Copyright 2014 RSC, RSC Advances)
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