Civil Engineering Reference
In-Depth Information
valence band of the photocatalyst to the conducting band during the absorp-
tion of the incident light.
The optimum wavelength for the activation of a certain photocatalytic
material depends on its band gap, as the electrons must, under excitation
by absorption of incident radiation, pass into the conducting band. Table
15.1 presents the optimum wavelength of the most commonly used
photocatalysts.
The creation of such holes provides a highly reactive and oxidizing mate-
rial which is thus able to catalyse the oxidation of chemicals deposited or
adsorbed on the photocatalyst surface. Particular attention should be paid
to the case of adsorbed water and oxygen molecules. These molecules can
react with the holes and excited electrons of the photocatalyst, and are
transformed into highly reactive compounds such as hydroxyl radicals
(OH
•
) and superoxide anions (O
2
−
):
+
+
hHO
+
→+
HH
i
vb
2
eOO
cb
−
+→
2
−
2
The most commonly used photocatalyst is TiO
2
. In nature, TiO
2
can be
present in three different crystalline structures: rutile, anatase and brookite.
TiO
2
, and especially rutile TiO
2
, has been extensively used since the nine-
teenth century as a white pigment in the formulation of paints. Even if the
chemical reactions induced by irradiated TiO
2
had already been observed,
it was not until 1938 that Dooedeve and Kitchener understood the photo-
catalytic mechanism by which TiO
2
was capable of producing the photo-
Table 15.1
Band gap and optimum excitation wavelength of different
photocatalysts
Photocatalyst
Band gap (eV)
Optimal wavelength (nm)
ZnS
3.6
345
SnO
2
3.6
345
Anatase TiO
2
3.2
390
SrTiO
3
3.2
390
ZnO
3.2
390
α
-Fe
2
O
3
3.1
400
CaBi
2
O
4
3.1
400
Rutile TiO
2
3.0
415
Brookite TiO
2
3.0
415
In
2
O
3
2.9
430
WO
3
2.8
445
CdS
2.5
495
V
2
O
5
2.2
565
CdSe
1.8
690
Search WWH ::
Custom Search