Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 214

ions with great redox power, which in turn react with organic and inorganic
chemical species adsorbed on the oxide surface and induce their decomposition
(Fujishima et al. 2008 ). However, the majority of industrial applications involving
photoactivity consists of the production of self-cleaning surfaces (Ganesh et al.
2011 ; Zhang et al. 2012 ), with applications ranging from self-cleaning windows, to
antifogging rear view mirrors, to tiles, cements, and other building materials, to
textiles (Drelich et al. 2011 ; Diamanti and Pedeferri 2013 ).
The effect relies on the coexistence of two mechanisms: photocatalysis
degrades the organic functional groups by which pollutants adhere to a surface,
while superhydrophilicity forces water to spread completely on the surface, car-
rying away particulate matter and degraded contaminants (Fig. 9.6 ) (Wang et al.
1997 ; Ganesh et al. 2011 ).
Superhydrophilic TiO 2 surfaces were first described in 1997 by Wang et al. who
observed a complex modification of surface chemistry, with Ti 4+ ions reduction
and oxygen vacancies formation, leading to the formation of a highly hydroxylated
surface layer. The increase of van der Waals forces and hydrogen bonding inter-
actions between water and hydroxyl groups accounts for the extreme wetting
properties under UV irradiation (Wang et al. 1999 ; Drelich et al. 2011 ). As an
effect, drops formation on superhydrophilic surfaces is avoided, which precludes
stains due to slow water evaporation from the surface; for the same reason, an
antifogging mechanism is enabled (Lai et al. 2012 ).
The onset of superhydrophilicity has also been ascribed to the photodegradation
of hydrocarbon contaminants present on TiO 2 surfaces: recent XPS investigations
do not reveal substantial formation of OH groups on its surface upon irradiation
(Wang et al. 2003 ), while several theoretical and experimental studies address the
effects of water adsorption onto TiO 2 (Jribi et al. 2009 ; Diamanti et al. 2013 ).
The debate in the scientific community on the actual mechanisms of TiO 2
photoinduced superhydrophilicity is still open, although both water dissociation
and adsorption and photocatalytic decontamination surely bring a positive con-
tribution to surface hydrophilicity.
9.2.2 Learning from Carnivorous Plants
A fascinating example in nature of a potential self-cleaning surface with hydro-
philic character is represented by Nepenthes pitcher plants (Fig. 9.7 ). The pray-
trapping technique of such carnivorous plants is based on a cup-folded leaf filled
with nectar juices and digesting enzymes: insects, attracted by the sweet nectar,
lean on the inner rim of the leaf and find an extremely slippery surface, which
forces them to fall inside the cavity, with no hope for climbing the leaf back.
The reason for this unexpected trapping lies in the interplay between surface
hydrophilicity and its hierarchical structure (Gaume et al. 2002 ; Bohn et al. 2004 ).
The peristome surface has a very regular microstructure consisting of first- and
second-order radial ridges formed by straight rows of epidermal cells, each