Civil Engineering Reference
In-Depth Information
In both cases, the rate of charge transfer is strongly infl uenced by the
respective positions of conduction and valence band edges and the redox
potential levels of adsorbate species: the greater the difference between
semiconductor and adsorbate energy levels, the faster the redox reactions
(Linsebigler
et al.
, 1995).
Charge transfer must compete with charge carrier recombination, which
can take place in the volume of the semiconductor or at its surface and
strongly decreases photocatalytic yield, i.e., the number of events occurring
per absorbed photon. Charge carrier trapping can help increase the photo-
generated species' lifetime. Localized energy surface states ascribed to
irregularities and surface defects can act as charge traps, and thus reduce
recombination effects; adsorbed oxygen usually acts as an electron scaven-
ger for the trapped electrons, while trapped holes can react with oxygen
ions or hydroxyl groups.
13.2.2 The most common photocatalyst: titanium dioxide
Titanium dioxide (TiO
2
) is the most common among titanium minerals, and
is extensively used in everyday life, specially as white pigment in painting,
food and cosmetic industries, thanks to its high refractive index. It is found
in nature in four polymorphs: anatase, which presents a distorted tetragonal
crystal structure; rutile, which is also tetragonal; brookite, with orthorhom-
bic crystal structure; and TiO
2
(B), with monoclinic structure. Only two of
the phases, anatase and rutile, are interesting for practical applications, as
they are wide band gap semiconductors.
The E
g
value for anatase is 3.20 eV, which corresponds to a wavelength
absorption threshold of 384 nm. This means that its activation requires an
irradiating source with wavelength lower than that indicated, that is, in the
near-UV region, while visible light is not suffi ciently energetic to induce
photoactivity in this material (Fig. 13.2).
The parameters which mostly concur to determine photoactivation effi -
ciency, and specifi cally photocatalysis, are the following, as defi ned by Carp
et al.
(2004):
• catalyst surface area, considering also specifi c surface (porosity)
available
• initial concentration of the compound to be degraded (saturation
regime) and formation of intermediate products competing for adsorp-
tion, even deactivating TiO
2
• reaction environment (oxygen, humidity, pH)
• wavelength and intensity of activating light source, which must provide
enough energy to overcome the semiconductor band gap.
In Section 13.3 the main application fi elds of titanium dioxide will be
described, as summarized in the well-known scheme of Fig. 13.3, together
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