Civil Engineering Reference
In-Depth Information
Recombination competes strongly with the photocatalytic process. It may
occur on the surface in the bulk and is in general catalysed by impurities,
defects, or all factors which introduce bulk or surface imperfections into the
crystal.
26
The fact that the process can only be initiated by UV light is also
a limiting factor in the process. It is desirable to produce a photocatalyst that
can be activated by visible light to make full use of the solar spectrum.
14.1.2 Practical use of photocatalysts for tiles and glasses
A major drawback in commercializing conventional TiO
2
photocatalysts for
community care applications is the large band gap. Titanium dioxide can
only be activated upon irradiation with a photon of light
390 nm, limiting
its use on commodity materials such as tiles and glasses.
27-29
Ultraviolet light
makes up only 3-5% of the solar spectrum, whereas the spectrum consists
of
<
40% visible light. Therefore, in order to utilize TiO
2
to its full potential
it is necessary to decrease the band gap size facilitating visible light
absorption.
Non-metal doping has shown great promise in achieving visible light
activated photocatalysis, with nitrogen being the most effective dopant.
Asahi
et al.
were the fi rst to show visible light absorption through N doping.
They reported nitrogen-doped TiO
2
promoted photocatalytic activity up to
λ
∼
520 nm.
30
The nitrogen substitutional doping of TiO
2
was initially claimed
as a method for narrowing the band gap by exclusively changing the valence
band structure; fi ne electronic details of this are, however, under discussion.
Asahi
et al.
claimed that the presence of nitrogen narrows the band gap of
TiO
2
, thus making it capable of performing visible light-driven photocataly-
sis.
30
However, Ihara
et al.
suggested that it is the oxygen vacancies that
contributed to the visible light activity, and the doped nitrogen only
enhanced the stabilization of these oxygen vacancies.
31
They also confi rmed
this role of oxygen vacancies in plasma-treated TiO
2
photocatalysts.
31
In addition, that the structural oxygen vacancy caused visible light pho-
tocatalytic activity was also reported by Martyanov
et al.
32
Currently there
appears to be some agreement on the mechanism of nitrogen-doped visible
light absorption explained by Irie
et al.
33
and Nakamura
et al.
34
They
explained that TiO
2
oxygen lattice sites substituted by nitrogen atoms form
an occupied midgap (N-2p) level above the (O-2p) valence band. Irradia-
tion with UV light excites electrons in both the valence band and the
narrow (N-2p) band, but irradiating with visible light only excites electrons
in the narrow (N-2p) band.
33,34
It has also been shown that F doping improves both UV and visible light
photocatalytic activity. However, their mechanisms are still under discus-
sion. Previous studies have shown that N-F-co-doped TiO
2
powders dem-
onstrated excellent photocatalytic activity no matter what kind of light
=
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