Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 213

9.2 The Different Mechanisms of Self-cleaning
As previously mentioned, self-cleaning is a wettability-related property that can be
achieved on both superhydrophilic and superhydrophobic surfaces, being the
obtaining of an extreme wettability state the ruling condition. Here the different
bioinspired self-cleaning mechanisms will be described in more details, starting
from the one that is most commonly applied in the field of building materials
(Diamanti and Pedeferri 2013 ), based on one of the most studied functional
materials of the last decades: titanium dioxide (Fig. 9.6 ).
9.2.1 Self-cleaning on Photoactive Surfaces: TiO 2
The biomimetic nature of TiO 2 self-cleaning mechanism lies in the mimicking of
green plants photosynthesis processes, where sunlight is used to produce highly
energetic chemical species that modify the oxide surface properties and reactivity,
as first described by Fujishima and coworkers in 1970s and further investigated in
the following decades.
In fact, transition metal oxides can be photoactivated by light absorption of
suitable wavelength. This generates the promotion of an electron from the valence
band to the conduction band, where a hole (positive charge) is left: such reactive
species can recombine or react with the surrounding environment, giving rise to a
series of important photoactivated functionalities that contribute to the material
self-cleaning ability (Fujishima et al. 1972 ; Carp et al. 2004 ).
One of the most intriguing—yet not easily governable—consequences of
photoactivation is photoacatalysis, i.e., the reaction of photogenerated holes and
electrons with water molecules present in the atmosphere to produce radicals and
Fig. 9.6 Top photoinduced
formation of a
superhydrophilic TiO 2
surface (reversible upon dark
storage). Bottom ceramic
panel, partially treated with
TiO 2 and exposed to urban
atmosphere for 1 year