Self-cleaning tiles and glasses for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 327

source was used. This seems to be a consequence of the perfect combination
of some benefi cial effects induced by both N and F dopants. 35
Carbon, phosphorous and sulphur have also shown positive results for
visible light responsive TiO 2 . 36,37 The non-metal dopants effectively narrow
the band gap of TiO 2 ( 3.2 eV). 38 Change of the lattice parameters and the

presence of trap states within the conduction and valence bands from elec-
tronic perturbations give rise to band gap narrowing. 27 Not only does this
allow for visible light absorption, but the presence of trap sites within the
TiO 2 bands increases the lifetime of photoinduced charge carriers.
Doping of TiO 2 with transition metals such as Cr, Co, V and Fe has
extended the spectral response of TiO 2 well into the visible region, also
improving photocatalytic activity. 27,39 However, transition metals may also
act as recombination sites for the photoinduced charge carriers, thus lower-
ing the quantum effi ciency. Transition metals have also been found to cause
thermal instability to the TiO 2 nanomaterials. 15 Even though a decrease in
band gap energy has been achieved by many groups through metal doping,
photocatalytic activity has not been remarkably enhanced because the
metals introduced were not incorporated into the TiO 2 framework. In addi-
tion, metals remaining on the TiO 2 surface cover photo reaction sites.

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14.1.3 Applications of titanium dioxide for tiles and glasses

Despite the great promise shown by the self-cleaning abilities of TiO 2 sur-
faces, there are certain limitations. Because TiO 2 is a wide band gap (3.2 eV)
semiconductor material, the self-cleaning process can only be initiated by
light of wavelength 390 nm or less. This causes substantial reduction in the

effi ciency of the product as light of such energy, ultraviolet light (UV),
makes up only 3-5% of the solar spectrum. Therefore, in order to improve
the effi ciency of these materials, it is necessary to either reduce the band
gap or to introduce mid-band gap energy levels that act as a stepping stone
between the energy levels, facilitating visible light absorption.
Titanium dioxide can be incorporated into construction materials such as
glasses and tiles to produce anti-bacterial surfaces. 12,13 It can be used to coat
hospital surfaces and provide anti-bacterial protection against harmful bac-
teria such as E. coli and MRSA. 40 By applying TiO 2 to roadside partitions
and lights, the surfaces can be kept clean while having the added advantage
of reducing harmful exhaust gases such as NO x and VOCs.

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14.1.4 Commercial photocatalytic tiles and glass

Pilkington Glass have utilized titanium dioxide technology to develop a
range of self-cleaning windows known as Pilkington Activ. 41 Self-cleaning
glass clearly displays the benefi ts of titanium dioxide's self-cleaning and