Civil Engineering Reference
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cement (former constructions: a school in Mortara, Italy, in 1999, and
the Cité de la Musique in Chambéry, France, in 2000).
• 20,000 m 2 of self-cleaning windows were installed in the terminal build-
ing of Chubu International Airport (Japan), completed in 2005
(Fujishima and Zhang, 2006).
The application of TiO 2 as cool material in energy-saving technologies
for building was proposed: it exploits the latent heat fl ux generated by
the evaporation of thin fi lms of water on superhydrophilic surfaces (Irie
et al. , 2004).
It is commonly assumed that the fi rst studies on TiO 2 photoactivated prop-
erties date back to the early 1970s, when studies from Honda and Fujishima
attracted the attention of the scientifi c community: the above list proves
that other works had already been published, even though their fi ndings
clearly attracted less attention.
Photoinduced processes originate from the absorption of light by TiO 2 ,
which causes an electron to be promoted to the conduction band (CB)
leaving a hole in the valence band (VB). The electron-hole pair can be used
to create electricity in photovoltaic solar cells, or to drive a chemical reac-
tion: the latter action belongs to the category of heterogeneous photoca-
talysis. Trapping of holes at the TiO 2 surface also causes a dramatic increase
in the surface wettability, which is defi ned as photoinduced superhydrophi-
licity. These phenomena will be explained in more detail in the following
section.
13.2 Principles of heterogeneous photocatalysis
Many transition metal oxides are known to display a photocatalytic behav-
iour, i.e., these substances can act as catalysts and promote oxidation or
reduction reactions when activated by electromagnetic radiation. Among
these materials, titanium dioxide in its polymorphic structures (anatase,
rutile and brookite) is undoubtedly the most relevant and studied, although
other oxides and sulphides have attracted scientists' attention.
An ideal semiconductor for use in photocatalytic processes should have
the following characteristics: chemical and biological inertia, ease of pro-
duction and utilization, effi cient solar light activation, high effi ciency and
low cost. Titanium dioxide possesses almost all the cited characteristics with
the exception of the effi ciency in solar light exploiting, since activation is
promoted by UV light, which is less than 10% of solar light energy (Linse-
bigler et al. , 1995). But, how does heterogeneous photocatalysis work? The
mechanism must be understood in order to explore its applications in
everyday life, and especially as a complementary function of construction
materials, as we will describe in the second part of this chapter.
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