wet chemical methods such as sol–gel processes and hydrothermal treatment, spray
pyrolysis, and supercritical methods.
These chemical synthesis methods used in doing TiO 2 are summarized in
Table 8.1. As can be seen in Table 8.1, all the doped TiO 2 synthesized by different
chemical preparation methods have a high photocatalytic activity for the degradation
of organic dye pollutants and water splitting. In addition to be divided in different
synthesis method, the chemical doping modification on TiO 2 can also be divided into
metal doping and nonmetal doping. In metal doping method, a certain amount of
metal ions such as Fe
3+ [10–14], Cr
3+ [15, 16], Ru
2+ [17, 18], Ce
4+ [19, 20], La
3+
[21–23], V
5+ [24–26], are introduced into TiO 2 (Table 8.1), as the active “small
oxide islands”, affecting the lifetime of the photo-formed electrons and holes theirtransfer processes, thus affecting the photocatalytic activity of TiO 2 . Furthermore,
the impurity induced by metal doping into the TiO 2 could efficiently narrow its
bandgap and extend the absorption edge into the visible light range. Many studies
have demonstrated that metal doping could effectively improve the photocatalytic
activity of TiO 2 under the visible light irradiation [27–30]. However, metal doping
also showed several drawbacks: thermal instability of doped TiO 2 , electron trapping
by the metal centers, introduction of the electrons, and hole recombination centers
[31]. It is worth mentioning that, in addition to the above traditional impurity metal
ion doping, more and more research has focused on the investigation of Ti
3+ selfdoped TiO 2 in recent years [32–36]. Recent research work has found that excessive
Ti
3+ would not easily introduce the electron and hole recombination centers in
TiO 2 [33].
On the other hand, nonmetal doping is another technology to modify TiO 2 , which
could achieve the substitution of lattice oxygen by nonmetal elements [31, 37–
39]. Since a work was investigated by Asahi et al. [40] in 2001 in which they
reported that nitrogen doping could enhance the photocatalytic activity of TiO 2 for
the photodegradation of methylene blue and gaseous acetaldehyde in the visible light
irradiation, though the photocatalytic activity in UV light regions decreases. Since
then, many researchers have reported about various nonmetal-doped TiO 2 , such as
N [41, 42], B [43–46], C [21, 47–49], F [50–54], S [55–59], and P [60]. Although
nonmetal doping modification could change the band structure of TiO 2 and affect the
transfer of electrons and holes, the origin of its visible light photoactivity is still in
debate [31], especially the photocatalytic mechanism of nitrogen doping. Recent
experimental and theoretical studies suggest that the N doping does not cause the
narrowing of the band gap of the TiO 2 but the formation of localized midgap states
above the valance band of TiO 2 which is the reason for its enhancing visible light
responsiveness and photoactivity [31, 61].
In this section, we mainly introduce and highlight the chemical-synthesized
visible light-responsible TiO 2 photocatalysis with doping modification, including
metal doping, nonmetal doping, and co-doping modification. The influencing factors
on doping modification, the research development of doping and photocatalytic
mechanism, and novel investigation of synergistic effect between different elements
are also discussed in this section.
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8 Modifications of Photocatalysts by Doping Methods
pyrolysis, and supercritical methods.
These chemical synthesis methods used in doing TiO 2 are summarized in
Table 8.1. As can be seen in Table 8.1, all the doped TiO 2 synthesized by different
chemical preparation methods have a high photocatalytic activity for the degradation
of organic dye pollutants and water splitting. In addition to be divided in different
synthesis method, the chemical doping modification on TiO 2 can also be divided into
metal doping and nonmetal doping. In metal doping method, a certain amount of
metal ions such as Fe
3+ [10–14], Cr
3+ [15, 16], Ru
2+ [17, 18], Ce
4+ [19, 20], La
3+
[21–23], V
5+ [24–26], are introduced into TiO 2 (Table 8.1), as the active “small
oxide islands”, affecting the lifetime of the photo-formed electrons and holes theirtransfer processes, thus affecting the photocatalytic activity of TiO 2 . Furthermore,
the impurity induced by metal doping into the TiO 2 could efficiently narrow its
bandgap and extend the absorption edge into the visible light range. Many studies
have demonstrated that metal doping could effectively improve the photocatalytic
activity of TiO 2 under the visible light irradiation [27–30]. However, metal doping
also showed several drawbacks: thermal instability of doped TiO 2 , electron trapping
by the metal centers, introduction of the electrons, and hole recombination centers
[31]. It is worth mentioning that, in addition to the above traditional impurity metal
ion doping, more and more research has focused on the investigation of Ti
3+ selfdoped TiO 2 in recent years [32–36]. Recent research work has found that excessive
Ti
3+ would not easily introduce the electron and hole recombination centers in
TiO 2 [33].
On the other hand, nonmetal doping is another technology to modify TiO 2 , which
could achieve the substitution of lattice oxygen by nonmetal elements [31, 37–
39]. Since a work was investigated by Asahi et al. [40] in 2001 in which they
reported that nitrogen doping could enhance the photocatalytic activity of TiO 2 for
the photodegradation of methylene blue and gaseous acetaldehyde in the visible light
irradiation, though the photocatalytic activity in UV light regions decreases. Since
then, many researchers have reported about various nonmetal-doped TiO 2 , such as
N [41, 42], B [43–46], C [21, 47–49], F [50–54], S [55–59], and P [60]. Although
nonmetal doping modification could change the band structure of TiO 2 and affect the
transfer of electrons and holes, the origin of its visible light photoactivity is still in
debate [31], especially the photocatalytic mechanism of nitrogen doping. Recent
experimental and theoretical studies suggest that the N doping does not cause the
narrowing of the band gap of the TiO 2 but the formation of localized midgap states
above the valance band of TiO 2 which is the reason for its enhancing visible light
responsiveness and photoactivity [31, 61].
In this section, we mainly introduce and highlight the chemical-synthesized
visible light-responsible TiO 2 photocatalysis with doping modification, including
metal doping, nonmetal doping, and co-doping modification. The influencing factors
on doping modification, the research development of doping and photocatalytic
mechanism, and novel investigation of synergistic effect between different elements
are also discussed in this section.
198
8 Modifications of Photocatalysts by Doping Methods