photogenerated holes could react with chemisorbed OH
- or H 2 O to produce OH
Á .
However, the electrons transferred to the surface of semiconductor particles could
not interact with O 2 to form O
Á
2 . This accelerates the recombination of the
photogenerated electrons and holes, thus reducing the formation rate of OH
Á . Therefore, only a low degradation rate is observed for these photocatalysts. Different
strategies such as addition of metal salt into the TiO 2 colloid, sol-gel, ion implantation, impregnation, and chemical vapor deposition have been tested for the syntheses
of metal-doped titania (Zaleska 2008). For example, through a post-synthesis treatment Tayade et al. have doped mesoporous titania with different transition metal
ions (Tayade et al. 2006). The authors have first prepared the TiO 2 by the hydrolysis
of titanium isopropoxide in the absence of surfactant, and thus no mesopore ordering
was obtained. In a second step, the metal was introduced by using the wet impregnation method. The obtained doped materials have been tested for the degradation of
acetophenone and nitrobenzene present in aqueous solution. The authors have
demonstrated that the photocatalytic activity of the silver metal ion impregnated
TiO 2 photocatalyst has the highest initial rate of photocatalytic degradation for both
compounds due to the interstitial position of impregnated silver metal ion in the TiO 2
lattice. Among the different strategies to introduce the metal ion dopant, the direct
sol gel way is of particular interest (Zaleska 2008; Chauhan et al. 2012). Indeed, this
technique does not required complicated instruments, since it is simple and easy. In
addition, the incorporation of an active dopant in the sol during the gelation stage
allows the doping element to have a direct interaction with the support. Mesoporous
TiO 2 was also modified with silver nanoparticles that were photogenerated during
the wet impregnation by AgNO 3 of the preformed TiO 2 (Xiong et al. 2011). The Ag
particles formed on the TiO 2 surface were assumed to increase the affinity of the
surface to oxygen, which is supposed to be the major role in enhancing the
photocatalytic degradation of rhodamine B under UV light irradiation. The doping
by metal such as bismuth has also been considered for mesoporous TiO 2 /MCM-41
SiO 2 nanocomposites to enhance photocatalytic activity under visible light for
methylene blue removal from water solution (Mohamed et al. 2018). Synergetic
effects of interfaces between Bi 2 O 3 and TiO 2 were assumed to enhance the catalytic
performance of the composite Bi/Ti-MCM-41. Co-doping with N has allowed
narrowing the band gap energy of the photocatalysts resulting in greater visible
light photocatalytic activity for the degradation of acid Red 85-a, a fabric dye
(Thejaswini et al. 2016). Co-doping with metal such as Zr and Si can affect anatase
phase stability, crystallinity, particle size, and optical properties of TiO 2 , resulting in
enhanced photocatalytical activity in the photodegradation of methyl orange in
aqueous solution under visible light (Ilkhechi and Kaleji 2014).
Nonmetal doping of TiO 2 with boron, carbon, sulfur, fluorine, or nitrogen has
also been reported. For example, N-doped TiO 2 have been prepared by treating
powder TiO 2 in a NH 3 gas flow (Zhang et al. 2010). In that case, N-doping induces
oxygen vacancies and contributes to the absorption as well as photoactivity in the
visible region. It is also reported that modified N-doped TiO 2 such as C-N-TiO 2
usually shows favorable effects for improving the photocatalytic activity in the
visible compared to N-doped TiO 2 (Zhang et al. 2010). The photocatalytic activity
62
B. Lebeau et al.
- or H 2 O to produce OH
Á .
However, the electrons transferred to the surface of semiconductor particles could
not interact with O 2 to form O
Á
2 . This accelerates the recombination of the
photogenerated electrons and holes, thus reducing the formation rate of OH
Á . Therefore, only a low degradation rate is observed for these photocatalysts. Different
strategies such as addition of metal salt into the TiO 2 colloid, sol-gel, ion implantation, impregnation, and chemical vapor deposition have been tested for the syntheses
of metal-doped titania (Zaleska 2008). For example, through a post-synthesis treatment Tayade et al. have doped mesoporous titania with different transition metal
ions (Tayade et al. 2006). The authors have first prepared the TiO 2 by the hydrolysis
of titanium isopropoxide in the absence of surfactant, and thus no mesopore ordering
was obtained. In a second step, the metal was introduced by using the wet impregnation method. The obtained doped materials have been tested for the degradation of
acetophenone and nitrobenzene present in aqueous solution. The authors have
demonstrated that the photocatalytic activity of the silver metal ion impregnated
TiO 2 photocatalyst has the highest initial rate of photocatalytic degradation for both
compounds due to the interstitial position of impregnated silver metal ion in the TiO 2
lattice. Among the different strategies to introduce the metal ion dopant, the direct
sol gel way is of particular interest (Zaleska 2008; Chauhan et al. 2012). Indeed, this
technique does not required complicated instruments, since it is simple and easy. In
addition, the incorporation of an active dopant in the sol during the gelation stage
allows the doping element to have a direct interaction with the support. Mesoporous
TiO 2 was also modified with silver nanoparticles that were photogenerated during
the wet impregnation by AgNO 3 of the preformed TiO 2 (Xiong et al. 2011). The Ag
particles formed on the TiO 2 surface were assumed to increase the affinity of the
surface to oxygen, which is supposed to be the major role in enhancing the
photocatalytic degradation of rhodamine B under UV light irradiation. The doping
by metal such as bismuth has also been considered for mesoporous TiO 2 /MCM-41
SiO 2 nanocomposites to enhance photocatalytic activity under visible light for
methylene blue removal from water solution (Mohamed et al. 2018). Synergetic
effects of interfaces between Bi 2 O 3 and TiO 2 were assumed to enhance the catalytic
performance of the composite Bi/Ti-MCM-41. Co-doping with N has allowed
narrowing the band gap energy of the photocatalysts resulting in greater visible
light photocatalytic activity for the degradation of acid Red 85-a, a fabric dye
(Thejaswini et al. 2016). Co-doping with metal such as Zr and Si can affect anatase
phase stability, crystallinity, particle size, and optical properties of TiO 2 , resulting in
enhanced photocatalytical activity in the photodegradation of methyl orange in
aqueous solution under visible light (Ilkhechi and Kaleji 2014).
Nonmetal doping of TiO 2 with boron, carbon, sulfur, fluorine, or nitrogen has
also been reported. For example, N-doped TiO 2 have been prepared by treating
powder TiO 2 in a NH 3 gas flow (Zhang et al. 2010). In that case, N-doping induces
oxygen vacancies and contributes to the absorption as well as photoactivity in the
visible region. It is also reported that modified N-doped TiO 2 such as C-N-TiO 2
usually shows favorable effects for improving the photocatalytic activity in the
visible compared to N-doped TiO 2 (Zhang et al. 2010). The photocatalytic activity
62
B. Lebeau et al.