linearly with the introduced amount of silica and due to textural mesoporosity. Due
to the high specific surface area and the high crystallinity, the photocatalytic activity
of the optimal silica-embedded titania nanocomposite for methylene blue
photodegradation under UV light irradiation is higher than those of pure TiO 2
nanoparticles and commercial P25.
Coupling TiO 2 with another material can be an effective tool to tune the electron/
hole recombination. As the photocatalysis mechanism of ZnO is very similar to the
one of TiO 2 , both oxides are often associated (Jing et al. 2004; Lin et al. 2005). For
example, Serpone et al., which have examined many coupled oxides for the
photodegradation of aromatic compounds, demonstrated during the reaction that a
transfer of electrons and holes happens in the ZnO/TiO 2 (Serpone et al. 1995). This
transfer inhibits the recombination of the charge carriers and enhances the
photocatalytic activity. Pérez-Larios et al. have reported the preparation of
ZnO/TiO 2 mixed oxides by sol-gel method varying the amount of ZnO in the
materials between 1 and 10 wt% (Pérez-Larios et al. 2012). They showed that
obtained materials present a photocatalytic activity in water splitting six times higher
than that presented by the bare TiO 2 . Xu et al. (2004, 2005) prepared ZnO/TiO 2
mixed oxides by simple wet impregnation of ZnO precursor on TiO 2 pre-synthesized
nanoparticles (Xu et al. 2004, 2005). The authors showed that when ZnO is located
on the surface of TiO 2 nanoparticles, it acts as recombination center of charge
carriers. Consequently, the photocatalytic activity of obtained materials is enhanced.
Shifu et al. investigated the preparation of ZnO/TiO 2 mixed oxides by ball-milling in
water (Shifu et al. 2008). These authors tested the effect of the amount of Zn on
photocatalytic oxidation of methyl orange and reduction of Cr 2 O 7
2À . They demonstrated that the photocatalytic degradation efficiency of methyl orange decreases
with the increase of Zn amount in the ZnO/TiO 2 materials. However, the
photocatalytic reduction of Cr 2 O 7
2À depends on the amount of Zn on the material
and reaches its optimum when this amount is of 5 wt. %. They showed also that the
materials photocatalytic activity is influenced by the ball-milling time, and the
reduction of Cr 2 O 7
2À is increased when milling time is up to 12 h. Guo et al. explain
that high amount of ZnO increases the percentage of voids due to the lack of oxygen
in the crystallites of anatase (Guo et al. 2010). These gaps are likely to induce an
intermediate energy level close to the conduction band of the anatase phase and
therefore to trap photogenerated electrons, by inhibiting the recombination of e
À /h
+
,
which consequently improves the photocatalytic activity of the material. In the same
context, Liao et al. attribute the increase of the photocatalytic activity of the
ZnO/TiO 2 compared to TiO 2 , to the presence of ZnO particles which cover the
surface of TiO 2 (Liao et al. 2008). Thus, an electron (e
À ) transfer occurs between the
ZnO conduction band and that of the TiO 2 and inversely for the photogenerated
holes (h
+ ), which move from the valence band of TiO 2 to that of ZnO. The separation
of charge carriers increases their life and improves the photocatalytic efficiency. In
both cases, the presence of ZnO aggregates on the surface of mesoporous TiO 2
reduces the recombination of e
À /h
+ and improves the photocatalytic activity of TiO 2 .
The hydrothermal synthesis of mixed porous TiO 2 /SnO 2 templated by polystyrene beads was also reported with higher photocatalytical activity toward the
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
57
to the high specific surface area and the high crystallinity, the photocatalytic activity
of the optimal silica-embedded titania nanocomposite for methylene blue
photodegradation under UV light irradiation is higher than those of pure TiO 2
nanoparticles and commercial P25.
Coupling TiO 2 with another material can be an effective tool to tune the electron/
hole recombination. As the photocatalysis mechanism of ZnO is very similar to the
one of TiO 2 , both oxides are often associated (Jing et al. 2004; Lin et al. 2005). For
example, Serpone et al., which have examined many coupled oxides for the
photodegradation of aromatic compounds, demonstrated during the reaction that a
transfer of electrons and holes happens in the ZnO/TiO 2 (Serpone et al. 1995). This
transfer inhibits the recombination of the charge carriers and enhances the
photocatalytic activity. Pérez-Larios et al. have reported the preparation of
ZnO/TiO 2 mixed oxides by sol-gel method varying the amount of ZnO in the
materials between 1 and 10 wt% (Pérez-Larios et al. 2012). They showed that
obtained materials present a photocatalytic activity in water splitting six times higher
than that presented by the bare TiO 2 . Xu et al. (2004, 2005) prepared ZnO/TiO 2
mixed oxides by simple wet impregnation of ZnO precursor on TiO 2 pre-synthesized
nanoparticles (Xu et al. 2004, 2005). The authors showed that when ZnO is located
on the surface of TiO 2 nanoparticles, it acts as recombination center of charge
carriers. Consequently, the photocatalytic activity of obtained materials is enhanced.
Shifu et al. investigated the preparation of ZnO/TiO 2 mixed oxides by ball-milling in
water (Shifu et al. 2008). These authors tested the effect of the amount of Zn on
photocatalytic oxidation of methyl orange and reduction of Cr 2 O 7
2À . They demonstrated that the photocatalytic degradation efficiency of methyl orange decreases
with the increase of Zn amount in the ZnO/TiO 2 materials. However, the
photocatalytic reduction of Cr 2 O 7
2À depends on the amount of Zn on the material
and reaches its optimum when this amount is of 5 wt. %. They showed also that the
materials photocatalytic activity is influenced by the ball-milling time, and the
reduction of Cr 2 O 7
2À is increased when milling time is up to 12 h. Guo et al. explain
that high amount of ZnO increases the percentage of voids due to the lack of oxygen
in the crystallites of anatase (Guo et al. 2010). These gaps are likely to induce an
intermediate energy level close to the conduction band of the anatase phase and
therefore to trap photogenerated electrons, by inhibiting the recombination of e
À /h
+
,
which consequently improves the photocatalytic activity of the material. In the same
context, Liao et al. attribute the increase of the photocatalytic activity of the
ZnO/TiO 2 compared to TiO 2 , to the presence of ZnO particles which cover the
surface of TiO 2 (Liao et al. 2008). Thus, an electron (e
À ) transfer occurs between the
ZnO conduction band and that of the TiO 2 and inversely for the photogenerated
holes (h
+ ), which move from the valence band of TiO 2 to that of ZnO. The separation
of charge carriers increases their life and improves the photocatalytic efficiency. In
both cases, the presence of ZnO aggregates on the surface of mesoporous TiO 2
reduces the recombination of e
À /h
+ and improves the photocatalytic activity of TiO 2 .
The hydrothermal synthesis of mixed porous TiO 2 /SnO 2 templated by polystyrene beads was also reported with higher photocatalytical activity toward the
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
57