(001) facets by a hydrothermal treatment. The C-doped TiO 2 sheets had enhanced
absorption in the whole visible light region and a significant redshift at the absorption edges. It was also showing a high photocatalytic degradation of methylene blue
under the visible light irradiation. Lin et al. [129] prepared a visible light-driven
C-doped mesoporous TiO 2 films by a sol–gel method combined with a hydrothermal
treatment. The C-doped TiO 2 film had high surface area and excellent
photodegradation of dye reactive brilliant Red X-3B in the UV and visible light
irradiation.
Fluorine-doped TiO 2 as one of the widespread modified methods has attracted
much more attention in recent years, but conventional F-doped TiO 2 is hard to
effectively achieve the enhanced UV and visible light photoactivity, and its
photocatalytic mechanism still remains controversial [130–132]. In previous work,
NH 4 F was used as a hydrophobic modifier, and isopropanol was used as the solvent
to prepare super-hydrophobic mesoporous MCF loaded with fluorinated TiO 2
nanoparticles [50], through a simple one-step solvothermal method (Fig. 8.4, top).
The prepared catalyst exhibited permanent and excellent super-hydrophobic property, high adsorption capacity, and photocatalytic activity for rhodamine B degradation (Fig. 8.4, bottom). However, through the solvothermal method, the F ion
could only adsorb on the surface of the catalyst but not be introduced into TiO 2
lattice. Many researchers also have reported the F-doped TiO 2 having high
photocatalytic activity [50, 132]. And the standard substitutional of TiO 2 with F is
similar to F-doped SnO 2 , with generation of impurity levels close to the conduction
band [133–135]. However, its electronic structures and photocatalytic mechanism
are still unclear. It has been demonstrated that F substitution for lattice O could not
introduce impurity level inside TiO 2 band gap, as well as shift its absorption edge
into visible region by the first-principles calculation [136]. That is because of the
absence consideration of F substitution for oxygen vacancy during the calculation
process. Some studies have reported that F substitution for oxygen vacancy could
introduce acceptor impurity level inside ZnO or SnO 2 bandgap [137, 138]. Hence, it
was concluded that the achievement of abundant fluorine substitution for lattice
oxygen vacancy played a very important role in the diminishing of vacancy-induced
recombination sites and the introduction of impurity level inside the TiO 2
bandgap [53].
Compared with the abovementioned nonmetal elements, the study on boron
doping in TiO 2 is relatively rare in recent years. In some studies, it was suggested
that the boron doping in TiO 2 could lead to a redshift of the absorption band of TiO 2
to the visible light region, because of the overlapping of the impurity levels caused
by boron with the 2p electronic states of oxygen [139, 140]. By contrast, some other
studies reported that the boron incorporation into TiO 2 lattice could induce a blue
shift rather than redshift due to the decrease of the crystal size [141, 142]. The
quantization effect of the crystal size would result into a blue shift of absorption band
to the UV light region. Even so, the boron has still been used as an important
co-dopant together with other nonmetals for modification of TiO 2 . Hence, more and
more researchers began to study the co-doping modification of boron with other
elements in TiO 2 [70, 77, 143, 144].
8.1 Preparation of Visible Light-Responsive TiO 2 Photocatalysts by. . .
207
absorption in the whole visible light region and a significant redshift at the absorption edges. It was also showing a high photocatalytic degradation of methylene blue
under the visible light irradiation. Lin et al. [129] prepared a visible light-driven
C-doped mesoporous TiO 2 films by a sol–gel method combined with a hydrothermal
treatment. The C-doped TiO 2 film had high surface area and excellent
photodegradation of dye reactive brilliant Red X-3B in the UV and visible light
irradiation.
Fluorine-doped TiO 2 as one of the widespread modified methods has attracted
much more attention in recent years, but conventional F-doped TiO 2 is hard to
effectively achieve the enhanced UV and visible light photoactivity, and its
photocatalytic mechanism still remains controversial [130–132]. In previous work,
NH 4 F was used as a hydrophobic modifier, and isopropanol was used as the solvent
to prepare super-hydrophobic mesoporous MCF loaded with fluorinated TiO 2
nanoparticles [50], through a simple one-step solvothermal method (Fig. 8.4, top).
The prepared catalyst exhibited permanent and excellent super-hydrophobic property, high adsorption capacity, and photocatalytic activity for rhodamine B degradation (Fig. 8.4, bottom). However, through the solvothermal method, the F ion
could only adsorb on the surface of the catalyst but not be introduced into TiO 2
lattice. Many researchers also have reported the F-doped TiO 2 having high
photocatalytic activity [50, 132]. And the standard substitutional of TiO 2 with F is
similar to F-doped SnO 2 , with generation of impurity levels close to the conduction
band [133–135]. However, its electronic structures and photocatalytic mechanism
are still unclear. It has been demonstrated that F substitution for lattice O could not
introduce impurity level inside TiO 2 band gap, as well as shift its absorption edge
into visible region by the first-principles calculation [136]. That is because of the
absence consideration of F substitution for oxygen vacancy during the calculation
process. Some studies have reported that F substitution for oxygen vacancy could
introduce acceptor impurity level inside ZnO or SnO 2 bandgap [137, 138]. Hence, it
was concluded that the achievement of abundant fluorine substitution for lattice
oxygen vacancy played a very important role in the diminishing of vacancy-induced
recombination sites and the introduction of impurity level inside the TiO 2
bandgap [53].
Compared with the abovementioned nonmetal elements, the study on boron
doping in TiO 2 is relatively rare in recent years. In some studies, it was suggested
that the boron doping in TiO 2 could lead to a redshift of the absorption band of TiO 2
to the visible light region, because of the overlapping of the impurity levels caused
by boron with the 2p electronic states of oxygen [139, 140]. By contrast, some other
studies reported that the boron incorporation into TiO 2 lattice could induce a blue
shift rather than redshift due to the decrease of the crystal size [141, 142]. The
quantization effect of the crystal size would result into a blue shift of absorption band
to the UV light region. Even so, the boron has still been used as an important
co-dopant together with other nonmetals for modification of TiO 2 . Hence, more and
more researchers began to study the co-doping modification of boron with other
elements in TiO 2 [70, 77, 143, 144].
8.1 Preparation of Visible Light-Responsive TiO 2 Photocatalysts by. . .
207