effect of N and F doping, which led to the high photocatalytic activity of the
degradation of AO7 in the visible light irradiation. Moreover, carbon and boron
co-doped TiO 2 were also synthesized firstly by the gel–hydrothermal method [144],
that is, prepared through sol–gel process followed by hydrothermal in the glucose
solution. The experimental results indicated that the coke carbon generated on the
co-doped catalyst surface acted as a photosensitizer and had a photosensitization
effect under the visible light. And the boron doping could effectively narrow the
band gap of TiO 2 , which induced the easier transition of photo-formed electrons
from the boron dopant level to the Ti
3+ level. The synergistic effect of B and C is
responsible for its excellent visible light photocatalytic activity.
Either metal doping or nonmetal doping, they both will change the electric
structure of TiO 2 and create a new doping level inside the band gap of TiO 2 . Because
of the different position of doping level in TiO 2 , it is generally considered that the
doping level induced by the substitution of metallic ions for Ti
4+ is close to the CB of
TiO 2 . And the impurity level caused by the doping of nonmetallic ions into the
oxygen vacancy sites is nearby the VB of TiO 2 . These doping levels are located
inside the band gap of TiO 2 , which can accept the photo-formed electrons from the
VB or absorb the photos with longer wavelength and extend the range of absorption
spectrum of TiO 2 . The synergistic effect between the metal and nonmetal could
promote the separation of electrons and holes, resulting into the improvement of the
visible light photocatalytic activity of TiO 2 .
Generally speaking, the synergistic effect between metal and nonmetal is mainly
shown as follows: the nonmetallic ion doping can enhance the absorption of TiO 2 in
the visible light region, and the metallic ion doping can introduce traps for electrons
and decrease the recombination of electrons and holes. Vanadium and nitrogen
co-doped TiO 2 was synthesized by the sol–gel method, and the catalyst showed a
high visible light photocatalytic activity for the degradation of RhB [111]. The
visible light absorption efficiency of V–N co-doped TiO 2 was better than the V or
N single-doped TiO 2 , because of the effective narrowing of the band gap induced by
the simultaneous incorporation of V and N in TiO 2 lattice, as shown in Fig. 8.7. The
energy levels inside the TiO 2 band gap can act as traps for photo-formed holes and
electrons thus decrease the recombination between photo-formed charges. The
narrowed band gap and enhanced charge separation exhibit synergistic effect to
improve the visible light photoactivity of the co-doped TiO 2 . Wei et al. [152]
considered that the synergistic effect between nitrogen and lanthanum in La and N
co-doped TiO 2 was responsible for the high photocatalytic activity. The N doping
decreased the band gap of TiO 2 and increased the absorption intensity of TiO 2 in
visible light region. And the La
3+ doping could not only increase the surface area of
TiO 2 but also restrain the recombination of electrons and holes, due to the electron
capturing capacity of La
3+ . In order to minimize the role of metallic ions as
recombination centers, Kim and coworkers synthesized boron and iron co-doped
TiO 2 using a modified sol–gel method, and the presence of boron and iron caused a
redshift in the absorption band of TiO 2 [153]. Cong et al. [79] also have successfully
prepared the nanosized TiO 2 catalyst co-doped with nitrogen and iron, which
exhibited a higher photocatalytic activity than the single doping catalyst under the
8.1 Preparation of Visible Light-Responsive TiO 2 Photocatalysts by. . .
211
degradation of AO7 in the visible light irradiation. Moreover, carbon and boron
co-doped TiO 2 were also synthesized firstly by the gel–hydrothermal method [144],
that is, prepared through sol–gel process followed by hydrothermal in the glucose
solution. The experimental results indicated that the coke carbon generated on the
co-doped catalyst surface acted as a photosensitizer and had a photosensitization
effect under the visible light. And the boron doping could effectively narrow the
band gap of TiO 2 , which induced the easier transition of photo-formed electrons
from the boron dopant level to the Ti
3+ level. The synergistic effect of B and C is
responsible for its excellent visible light photocatalytic activity.
Either metal doping or nonmetal doping, they both will change the electric
structure of TiO 2 and create a new doping level inside the band gap of TiO 2 . Because
of the different position of doping level in TiO 2 , it is generally considered that the
doping level induced by the substitution of metallic ions for Ti
4+ is close to the CB of
TiO 2 . And the impurity level caused by the doping of nonmetallic ions into the
oxygen vacancy sites is nearby the VB of TiO 2 . These doping levels are located
inside the band gap of TiO 2 , which can accept the photo-formed electrons from the
VB or absorb the photos with longer wavelength and extend the range of absorption
spectrum of TiO 2 . The synergistic effect between the metal and nonmetal could
promote the separation of electrons and holes, resulting into the improvement of the
visible light photocatalytic activity of TiO 2 .
Generally speaking, the synergistic effect between metal and nonmetal is mainly
shown as follows: the nonmetallic ion doping can enhance the absorption of TiO 2 in
the visible light region, and the metallic ion doping can introduce traps for electrons
and decrease the recombination of electrons and holes. Vanadium and nitrogen
co-doped TiO 2 was synthesized by the sol–gel method, and the catalyst showed a
high visible light photocatalytic activity for the degradation of RhB [111]. The
visible light absorption efficiency of V–N co-doped TiO 2 was better than the V or
N single-doped TiO 2 , because of the effective narrowing of the band gap induced by
the simultaneous incorporation of V and N in TiO 2 lattice, as shown in Fig. 8.7. The
energy levels inside the TiO 2 band gap can act as traps for photo-formed holes and
electrons thus decrease the recombination between photo-formed charges. The
narrowed band gap and enhanced charge separation exhibit synergistic effect to
improve the visible light photoactivity of the co-doped TiO 2 . Wei et al. [152]
considered that the synergistic effect between nitrogen and lanthanum in La and N
co-doped TiO 2 was responsible for the high photocatalytic activity. The N doping
decreased the band gap of TiO 2 and increased the absorption intensity of TiO 2 in
visible light region. And the La
3+ doping could not only increase the surface area of
TiO 2 but also restrain the recombination of electrons and holes, due to the electron
capturing capacity of La
3+ . In order to minimize the role of metallic ions as
recombination centers, Kim and coworkers synthesized boron and iron co-doped
TiO 2 using a modified sol–gel method, and the presence of boron and iron caused a
redshift in the absorption band of TiO 2 [153]. Cong et al. [79] also have successfully
prepared the nanosized TiO 2 catalyst co-doped with nitrogen and iron, which
exhibited a higher photocatalytic activity than the single doping catalyst under the
8.1 Preparation of Visible Light-Responsive TiO 2 Photocatalysts by. . .
211