8.1.1 Metal Doping Modification
As can be seen from the reaction mechanism of TiO 2 photocatalysis, it can be
concluded that an increase in the number of photo-formed electrons and holes
participating in the photoredox reaction is the key factor to improve the visible
light-responsive TiO 2 photocatalyst’s performance. In order to achieve the substantive enhancement of the quantum yield of photocatalytic reaction for TiO 2 in the
visible light irradiation, two issues must be overcome. One issue is how to produce
much more photo-formed electrons and holes; another issue is how to improve the
separation efficiency of electrons and holes. It is well known that appropriate amount
of transition metal ions doped into TiO 2 can introduce electron capture centers and
change the crystallinity of TiO 2 and then produce some defects, which results in an
decrease in photo-formed electron and hole recombination centers [11, 27–30, 33,
34, 102]. Thus, metallic ion doping is recognized as an effective modification
method for improving the reactivity of the visible light-responsive TiO 2 .
The methods using metallic ions selected for appropriate doping into TiO 2 can be
divided into three types. The first type is the transition metal ions. Most early, Choi
et al. [103] studied the photoactivity of TiO 2 doped with 21 transition metal ions by
using the model reaction of photocatalytic oxidation of chloroform and
photocatalytic reduction of carbon tetrachloride. The researchers discovered that
the Fe
3+ , Mo
5+ , Ru
3+ , Os
3+ , Re
5+ , V
4+ , and Rn
3+ ion doping modification is
beneficial to the enhancement of photodegradation of chloroform for TiO 2 . Recently,
Yan et al. [104] synthesized the TiO 2 nanoparticles doped with different content of
cerium ion by a sol–gel method. The Ce-doped TiO 2 act as the capture of photoformed holes and decrease the recombination of photo-generated electrons and
holes, leading to better visible light absorption and photocatalytic degradation of
methylene blue than pure TiO 2 (Fig. 8.1). Zhang and coworkers have also done
many works on the transition of metal-doped TiO 2 and its application to various
photocatalytic reactions [14, 15, 19, 20, 24, 29, 105–110]. Iron ion-doped anatase
TiO 2 were prepared by hydrothermal hydrolysis and crystallized in octanol–water
solution [105]. The results of photodegradation of active yellow XRG dye indicated
that the amount of doped iron ion plays a significant role in affecting the
photocatalytic activity and iron doped with optimum content enhances the
photocatalytic activity, especially under visible light irradiation. When the Fe
3+ -
Table 8.1 (continued)
Chemical
doping method
Doping
elements
Precursors
Visible light-driven
photocatalysis
Ref.
Ti
3+ and F Ti source: Ti(SO 4 ) 2
Photodegradation of rhodamine B
[50]
F source: NH 4 F
Thermal
plasma
H
Ti source: commercial
amorphous TiO 2
Photocatalytic methyl
orange decomposition and
photocatalytic H 2
generation
[101]
202
8 Modifications of Photocatalysts by Doping Methods
As can be seen from the reaction mechanism of TiO 2 photocatalysis, it can be
concluded that an increase in the number of photo-formed electrons and holes
participating in the photoredox reaction is the key factor to improve the visible
light-responsive TiO 2 photocatalyst’s performance. In order to achieve the substantive enhancement of the quantum yield of photocatalytic reaction for TiO 2 in the
visible light irradiation, two issues must be overcome. One issue is how to produce
much more photo-formed electrons and holes; another issue is how to improve the
separation efficiency of electrons and holes. It is well known that appropriate amount
of transition metal ions doped into TiO 2 can introduce electron capture centers and
change the crystallinity of TiO 2 and then produce some defects, which results in an
decrease in photo-formed electron and hole recombination centers [11, 27–30, 33,
34, 102]. Thus, metallic ion doping is recognized as an effective modification
method for improving the reactivity of the visible light-responsive TiO 2 .
The methods using metallic ions selected for appropriate doping into TiO 2 can be
divided into three types. The first type is the transition metal ions. Most early, Choi
et al. [103] studied the photoactivity of TiO 2 doped with 21 transition metal ions by
using the model reaction of photocatalytic oxidation of chloroform and
photocatalytic reduction of carbon tetrachloride. The researchers discovered that
the Fe
3+ , Mo
5+ , Ru
3+ , Os
3+ , Re
5+ , V
4+ , and Rn
3+ ion doping modification is
beneficial to the enhancement of photodegradation of chloroform for TiO 2 . Recently,
Yan et al. [104] synthesized the TiO 2 nanoparticles doped with different content of
cerium ion by a sol–gel method. The Ce-doped TiO 2 act as the capture of photoformed holes and decrease the recombination of photo-generated electrons and
holes, leading to better visible light absorption and photocatalytic degradation of
methylene blue than pure TiO 2 (Fig. 8.1). Zhang and coworkers have also done
many works on the transition of metal-doped TiO 2 and its application to various
photocatalytic reactions [14, 15, 19, 20, 24, 29, 105–110]. Iron ion-doped anatase
TiO 2 were prepared by hydrothermal hydrolysis and crystallized in octanol–water
solution [105]. The results of photodegradation of active yellow XRG dye indicated
that the amount of doped iron ion plays a significant role in affecting the
photocatalytic activity and iron doped with optimum content enhances the
photocatalytic activity, especially under visible light irradiation. When the Fe
3+ -
Table 8.1 (continued)
Chemical
doping method
Doping
elements
Precursors
Visible light-driven
photocatalysis
Ref.
Ti
3+ and F Ti source: Ti(SO 4 ) 2
Photodegradation of rhodamine B
[50]
F source: NH 4 F
Thermal
plasma
H
Ti source: commercial
amorphous TiO 2
Photocatalytic methyl
orange decomposition and
photocatalytic H 2
generation
[101]
202
8 Modifications of Photocatalysts by Doping Methods