Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 45

1986; Mo and Ching, 1995; Asahi et al., 2000), catalytic reactivity (Anpo et al., 1987;
Oregan and Gratzel, 1991; Hagfeldt et al., 1995) and surface property (Bourikas et al.,
2001) of TiO 2 . Being inexpensive and thermal-dynamically stable at room temperature,
this semiconductor material has been used widely in heterogeneous photocatalysis and
proven to be capable of decomposing a host of organic pollutants such as phenolic
compounds (Ilisz et al., 2002; Nevim et al., 2002), metal ethylene diamine tetra acetate
(

EDTA) complexes (Muhammad and Allen, 2000; Yang and Davis, 2001), airborne
microbes (Sato et al., 2003) and odorous chemicals (Canela et al., 1998). Most of these
studies involved ultraviolet (UV) photons as the major exciting light sources.
Considering that there is only 5% of solar irradiation within the UV range, intuitively it
is desirable to enhance the photocatalytic performance of TiO 2 by enabling the use of
photons from the near-visible UV to visible region (Figure 3.3). It has been suggested
that this can be achieved by manipulating the particle size of photocatalyst (Brus, 1984;
Anpo et al., 1987; Kormann et al., 1988), or doping the TiO 2 with foreign ions (Choi et
al., 1994; Asahi et al., 2001; Yamaki et al., 2002).

Based on the first principle of the orthogonalized linear-combinations-of-atomic-
orbital (OLCAO) method, Mo and Ching (1995) computed the density of states (DOSs)
for the rutile, anatase, and brookite phases of TiO 2 as to evaluate the electronic
properties of TiO 2 of different crystalline structures. Their results suggested that, among
the three polymorphs of TiO 2 , the upper valance band and the lowest conduction band
consist mainly of O 2 p and Ti 3 d states, respectively. Although most of the theoretical
calculations performed by local-density-approximation (LDA) under-estimate the
bandgap values comparing to the experimental results, the theoretical numbers provide
the relative differences in bandgap energies between different phases. In addition, DOS
calculation offers the capability in the prediction of the electronic band structure in the
presence of dopant impurities.

Table 3.1 Crystal structure data of TiO 2 (Mo and Ching, 1995).

Rutile Anatase Brookite

Crystal Structure tetragonal tetragonal orthorhombic

a = 9.184
b = 5.447
c = 5.145

a = 4.5936
c = 2.9587

a = 3.784
c = 9.515

Lattice Constants (Å)

Molecules/Cell 2 4 8

Volume/Molecule (Å 3 ) 31.216 34.061 32.172

Density (g/cm 3 )

4.13 3.79 3.99

1.949*
1.980**

1.937*
1.965**

Ti-O Bond Length (Å) 1.872.04

* Brus, 1986
** Asahi, et al., 2000