Civil Engineering Reference
In-Depth Information
water and consequent increased hydrophilicity of the surface, as depicted
in Fig. 13.4.
In this way, water can reach a contact angle close to zero on the surface
of irradiated TiO
2
(Drelich
et al.
, 2011). Moreover, this surface is not solely
hydrophilic: on the contrary, it presents an amphiphilic nature, with hydro-
phobic and hydrophilic domains of nanometre size alternating across the
surface. This allows both oils and water to spread easily on the photoacti-
vated TiO
2
surface (Fujishima and Zhang, 2006).
Regardless of the in-depth study of the chemical and photochemical
mechanisms involved, this effect is of extreme practical importance, as it
defi nes one of the most renowned abilities of titanium dioxide and the
reason why it has found so many applications in building materials: the
self-cleaning effect.
The formation of an amphiphilic domain network is accompanied by
photocatalytic activity, as both have a common origin: UV irradiation. This
double photoinduced phenomenon results in the self-cleaning effect: surface
contaminants are fi rst photomineralized, at least in part, and subsequently
washed away by water, which spreads below them in tight contact with the
TiO
2
surface. Moreover, drop formation on superhydrophilic surfaces is
avoided, which in turn precludes stain formation due to slow water evapora-
tion from the surface (Fig. 13.5). Another effect, anti-fogging, also arises
from the same mechanism: no water droplets form on surfaces with a
Surface mechanism:
1. Formation of vacancies
HHHH
O
O
O
O
O
O
O
O
UV
H
2
O
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
2. Hydroxylation (water-attractive)
Effect:
Super-
hydrophilic
Hydrophobic
13.4
Theoretical mechanism and practical effect of photoinduced
superhydrophilicity (photographs represent a plastic sheet covered on
the left side with a TiO
2
layer).
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