Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 44

3.1 Background of TiO 2 as a Semiconductor Photocatalyst

TiO 2 is a semiconductor and has three distinct polymorph crystalline structures,
namely rutile, anatase, and brookite. Both rutile and anatase have a tetragonal structure,
containing 6 and 12 atoms per unit cell with a c/a ratio of 0.64 and 2.51, respectively
(Figure 3.2). In contrast, the brookite has an orthorhombic structure. The detailed
crystalline structures of these polymorphs are shown in Table 3.1. Due to easy
preparation of rutile and anatase by a variety of synthesis techniques, these two
semiconductors have been studied insensitively for their photocatalytic and
photoelectrochemical performances in both single and poly-crystalline formations. The
activation energy of the phase transformation of anatase to rutile, dependent on the
particle size, synthesis method, the presence of foreign atoms, and the type of substrates,
is normally in the range of 150 to 600 kJ/mol (Li et al., 2004). Brookite, however, is
relatively less abundant in nature and very difficult to synthesize in pure form.
Therefore, study on the photocatalytic performance of this TiO 2 structure is very limited
(Di Paola et al., 2008).

Figure 3.2 Crystal structures of TiO 2 . (a) Rutile; (b) anatase; and (c) brookite (Replotted
from Mo and Ching, 1995).

Due to its relatively large bandgap (i.e., 3.0 to 3.2 eV), titanium dioxide (TiO 2 ) is
opaque to UV light and transparent to visible light. The optical property of TiO 2 gives it
a high gloss and rich depth of color. Thus, this material has been used to replace
common paint. In addition to its optical property, TiO 2 is also a popular material for
food coloring, UV filtering, UV protection cosmetics and many other products.
Since the discovery of photovoltaic property of titanium dioxide by Fujishima
and Honda (1972), a new era in heterogeneous photocatalysis using this material has
begun. Great efforts have focused on elucidating the electronic structure (Brus, 1984,