Nanotechnology in manufacturing paints for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 346

15.3 Application of photocatalytic paints
in an indoor environment

On average, people in Europe and North America spend 90% of their time
in confi ned indoor environments (Leech et al. , 2002; Brasche and Bischof,
2005). Indoor air pollutants are either emitted from numerous indoor
sources (e.g., furniture, building materials, cleaning agents, electronic equip-
ment), originate from human activities (e.g., cooking) or enter from the
outdoor environment by the ventilation system and doors or windows that
are insuffi ciently airtight. Depending on peoples' sensitivity, inappropriate
indoor air quality can present several health risks, generically referred to
as 'sick building syndrome' symptoms, such as irritation of the eyes, nose
and throat, headaches, dizziness, fatigue, asthma, hypersensitivity, pneumo-
nitis, etc. (EPA, 1991). The basic strategies to improve indoor air quality
involve the monitoring and eventual substitution of the emission sources,
improvement of the ventilation scheme and purifi cation of the air.
Photocatalytic degradation of air pollutants therefore appears to be a
promising technique for air purifi cation. As already mentioned, the most
widely used photocatalyst is TiO 2 , which is only activated by means of UV-A
light, the intensity of which is generally low in an indoor environment. Thus,
the development of other types of photocatalysts activated by visible light
is of the utmost importance in order to apply photocatalytic paints effec-
tively as a methodology for the effi cient purifi cation of indoor air.
Apart from the general parameters already mentioned in Section 15.1,
the photocatalytic effi ciency of indoor paints is greatly infl uenced by both
the nature and the concentration of the photosensitizer that is used to
enable the activation of the photocatalyst by visible light. Here, the nature
of the photosensitizer used has a direct infl uence on the change in the band
gap of TiO 2 and the wavelength required for the activation of the photo-
catalyst. The concentration of the photosensitizer, on the other hand, needs
to be carefully optimized, in order to fi ne tune the wavelength required for
the activation of the modifi ed photocatalyst. Finally, the photodecomposi-
tion of the sensitizer itself should be considered.

￿ ￿ ￿ ￿ ￿ ￿

15.3.1 Strategies for the preparation of visible light active
photocatalytic materials

TiO 2 photocatalytic materials activated by visible light

Two different alternatives have been employed to prepare TiO 2 -based pho-
tocatalytic materials that can be activated by visible light: sensitization and
doping.
The sensitization approach consists of the anchoring of an organic col-
oured dye (e.g., rhodamine B, eosin, erythrosin B, thionine, chlorophyllin or