of Ti
4+ (0.68 Å) and the Sm
3+ ions cannot enter into the lattice of TiO 2 , however,
Ti
4+ ions may enter into the lattice of the Sm 2 O 3 leading a change in the electronic
field of Sm
3+ and an increase in the electron density and decrease in the binding energy
of Sm
3+ . Well-ordered mesoporous TiO 2 doped with ytterbium was also successfully synthesized by an evaporation-induced self-assembly process [116]. The Yb
dopant was beneficial in stabilizing the mesoporous structure and reduces the
recombination of photo-generated electrons and holes, being beneficial to its visible
light-driven photoactivity. Europium-doped TiO 2 was synthesized by the precipitation–
peptization method and used as the photocatalyst to degrade the salicylic acid
[117]. The results showed that the doping of Eu was beneficial to the adsorption of
salicylic acid and the separation of photo-formed holes and electrons.
In addition to the transition metal ion doping and rare-earth metal ion doping, the
studies on some other metal ion-doped TiO 2 can be seen. Stannum doping TiO 2 and
Ti (III) self-doping TiO 2 were also reported. The dopant Sn
4+ substituted Ti
4+ in the
lattice of TiO 2 , which was reflected in the lattice expansion in both a- and c-direction
and change in the binding energy [119]. Different from other metal ion doping, the
doping of Sn
4+ in anatase TiO 2 would result in a blue shift of absorption edge and
enhance the amount of surface hydroxyl and oxygen vacancies in the UV light
region. On the other hand, Sn doping has also been demonstrated as an effective
modification method to enhance the visible light response of TiO 2 [120]. Tin would
improve the photocatalytic activity of TiO 2 by enhancing the separation rate of
photo-generated electrons and holes on the surface of TiO 2 . Due to the Fermi level of
SnO 2 lower than that of TiO 2 , the photo-generated electrons easily transfer from
TiO 2 to SnO 2 , resulting in a reduction of the number of photo-generated on the
surface of TiO 2 . In recent years, Ti
3+ -doped TiO 2 has attracted much interest, since it
has been demonstrated to exhibit visible light absorption [121, 122]. Sasikala et al.
[122] have found that the surface Ti
3+ and oxygen vacancies may be responsible for
the enhanced visible light absorption of the TiO 2 –SnO 2 composite. However, the
surface Ti
3+ and oxygen defects on the TiO 2 are usually not stable enough in air,
since the surface Ti
3+ is easily oxidized into Ti
4+ by the dissolved oxygen in water
[32, 123]. Most research has focused on Ti
3+ self-doped TiO 2 , which exhibits better
chemical stability and is active for photocatalytic activity. Zuo et al. [32] have
reported a one-step calcination method to synthesize Ti
3+ self-doped TiO 2 having
high stability and found that it exhibits improved visible light absorption and
efficient photocatalytic hydrogen production capacity from water under visible
light irradiation (Fig. 8.2, top). It has been also reported a vacuum activation method
for modifying P25 to obtain Ti
3+ self-doped TiO 2 with high stability, visible light
absorption, and photocatalytic activity under visible light irradiation (Fig. 8.2,
bottom) [36]. In addition to the vacuum activation method, a simple one-step
solvothermal method with low-cost NaBH 4 added as a reductant was also reported
to successfully synthesize a series of TiO 2 catalysts self-doped with Ti
3+ which also
exhibited strong visible light absorption and enhanced photocatalytic activity [33].
204
8 Modifications of Photocatalysts by Doping Methods
4+ (0.68 Å) and the Sm
3+ ions cannot enter into the lattice of TiO 2 , however,
Ti
4+ ions may enter into the lattice of the Sm 2 O 3 leading a change in the electronic
field of Sm
3+ and an increase in the electron density and decrease in the binding energy
of Sm
3+ . Well-ordered mesoporous TiO 2 doped with ytterbium was also successfully synthesized by an evaporation-induced self-assembly process [116]. The Yb
dopant was beneficial in stabilizing the mesoporous structure and reduces the
recombination of photo-generated electrons and holes, being beneficial to its visible
light-driven photoactivity. Europium-doped TiO 2 was synthesized by the precipitation–
peptization method and used as the photocatalyst to degrade the salicylic acid
[117]. The results showed that the doping of Eu was beneficial to the adsorption of
salicylic acid and the separation of photo-formed holes and electrons.
In addition to the transition metal ion doping and rare-earth metal ion doping, the
studies on some other metal ion-doped TiO 2 can be seen. Stannum doping TiO 2 and
Ti (III) self-doping TiO 2 were also reported. The dopant Sn
4+ substituted Ti
4+ in the
lattice of TiO 2 , which was reflected in the lattice expansion in both a- and c-direction
and change in the binding energy [119]. Different from other metal ion doping, the
doping of Sn
4+ in anatase TiO 2 would result in a blue shift of absorption edge and
enhance the amount of surface hydroxyl and oxygen vacancies in the UV light
region. On the other hand, Sn doping has also been demonstrated as an effective
modification method to enhance the visible light response of TiO 2 [120]. Tin would
improve the photocatalytic activity of TiO 2 by enhancing the separation rate of
photo-generated electrons and holes on the surface of TiO 2 . Due to the Fermi level of
SnO 2 lower than that of TiO 2 , the photo-generated electrons easily transfer from
TiO 2 to SnO 2 , resulting in a reduction of the number of photo-generated on the
surface of TiO 2 . In recent years, Ti
3+ -doped TiO 2 has attracted much interest, since it
has been demonstrated to exhibit visible light absorption [121, 122]. Sasikala et al.
[122] have found that the surface Ti
3+ and oxygen vacancies may be responsible for
the enhanced visible light absorption of the TiO 2 –SnO 2 composite. However, the
surface Ti
3+ and oxygen defects on the TiO 2 are usually not stable enough in air,
since the surface Ti
3+ is easily oxidized into Ti
4+ by the dissolved oxygen in water
[32, 123]. Most research has focused on Ti
3+ self-doped TiO 2 , which exhibits better
chemical stability and is active for photocatalytic activity. Zuo et al. [32] have
reported a one-step calcination method to synthesize Ti
3+ self-doped TiO 2 having
high stability and found that it exhibits improved visible light absorption and
efficient photocatalytic hydrogen production capacity from water under visible
light irradiation (Fig. 8.2, top). It has been also reported a vacuum activation method
for modifying P25 to obtain Ti
3+ self-doped TiO 2 with high stability, visible light
absorption, and photocatalytic activity under visible light irradiation (Fig. 8.2,
bottom) [36]. In addition to the vacuum activation method, a simple one-step
solvothermal method with low-cost NaBH 4 added as a reductant was also reported
to successfully synthesize a series of TiO 2 catalysts self-doped with Ti
3+ which also
exhibited strong visible light absorption and enhanced photocatalytic activity [33].
204
8 Modifications of Photocatalysts by Doping Methods