SrTiO 3 [15]. In general, an ideal photocatalyst should have some basic properties,
such as they must be active under UV, visible light, or solar light, they should have
property of chemical and biological robustness, as well as they should be stable
toward photocorrosion. The other most important characteristic they must have is
that they should be nontoxic and must have low cost and easy availability
[16]. These all are key factors for a good photocatalyst.
1.2 TiO 2 Photocatalysis
TiO 2 is the most common catalyst used in the field of photocatalysis. From the
ancient times, TiO 2 has been used as a white pigment. It can be extracted from the
ilmenite (FeTiO 3 ) [17], perovskite (CaTiO 3 ) [18], and titanite (CaTiSiO 5 ) [19] ores.
TiO 2 is mechanically, thermally, and chemically very stable. It has a very high
melting point of 1855
C and is insoluble in water, HNO 3 , HCl, and dilute H 2 SO 4 . It
can be dissolved in hot concentrated H 2 SO 4 and HF [20]. The reports on TiO 2 as
photocatalysis were found back in 1921 where Renz [21] from the University of
Lugano reported the partial reduction of titania during illumination under sunlight in
the presence of glycerol and in 1929 where Keidel [22] found that the dyes which are
quite stable to light and are not easily degradable are found to be decomposed when
adsorbed on lithopone or white lead in contact with TiO 2 [23]. There they were
called TiO 2 as photosensitizer. Titania is supposed to be an excellent photocatalyst
due to its enormous properties such as high activity, low cost, photostability,
nontoxicity, etc.
1.3 Mechanism of Photocatalytic Oxidation Reactions
In semiconductors the chemical reaction usually occurs due to the transfer of
electrons from the valance band to the conduction band as shown in the Fig. 1.1
As the number of orbitals (N) in the HOMO (highest occupied molecular orbital
(valance band)) and LUMO (lowest unoccupied molecular orbital (conduction
band)) increases, the energy to shift the electrons from the valance band to the
conduction band will decrease.
The detailed mechanism of the photocatalysis is quite a complex subject but the
basic principle is the same. Upon irradiation of the light with the energy equal or
more than the bandgap of the semiconductor photocatalyst, the electrons are excited
from the valance band of the catalyst to the conduction band, leaving the holes
behind in the valance band.
2
1 Mechanism of Photocatalysis
such as they must be active under UV, visible light, or solar light, they should have
property of chemical and biological robustness, as well as they should be stable
toward photocorrosion. The other most important characteristic they must have is
that they should be nontoxic and must have low cost and easy availability
[16]. These all are key factors for a good photocatalyst.
1.2 TiO 2 Photocatalysis
TiO 2 is the most common catalyst used in the field of photocatalysis. From the
ancient times, TiO 2 has been used as a white pigment. It can be extracted from the
ilmenite (FeTiO 3 ) [17], perovskite (CaTiO 3 ) [18], and titanite (CaTiSiO 5 ) [19] ores.
TiO 2 is mechanically, thermally, and chemically very stable. It has a very high
melting point of 1855
C and is insoluble in water, HNO 3 , HCl, and dilute H 2 SO 4 . It
can be dissolved in hot concentrated H 2 SO 4 and HF [20]. The reports on TiO 2 as
photocatalysis were found back in 1921 where Renz [21] from the University of
Lugano reported the partial reduction of titania during illumination under sunlight in
the presence of glycerol and in 1929 where Keidel [22] found that the dyes which are
quite stable to light and are not easily degradable are found to be decomposed when
adsorbed on lithopone or white lead in contact with TiO 2 [23]. There they were
called TiO 2 as photosensitizer. Titania is supposed to be an excellent photocatalyst
due to its enormous properties such as high activity, low cost, photostability,
nontoxicity, etc.
1.3 Mechanism of Photocatalytic Oxidation Reactions
In semiconductors the chemical reaction usually occurs due to the transfer of
electrons from the valance band to the conduction band as shown in the Fig. 1.1
As the number of orbitals (N) in the HOMO (highest occupied molecular orbital
(valance band)) and LUMO (lowest unoccupied molecular orbital (conduction
band)) increases, the energy to shift the electrons from the valance band to the
conduction band will decrease.
The detailed mechanism of the photocatalysis is quite a complex subject but the
basic principle is the same. Upon irradiation of the light with the energy equal or
more than the bandgap of the semiconductor photocatalyst, the electrons are excited
from the valance band of the catalyst to the conduction band, leaving the holes
behind in the valance band.
2
1 Mechanism of Photocatalysis