To develop efficient photocatalysts, the specific surface area is also a crucial
parameter that should be considered. Generally, large surface area is likely to exhibit
better photocatalytic activity, because a large surface area provides more active sites
for adsorbing dyes. Recently, much attention was paid to the synthesis of the
nanostructured mesoporous photocatalysts with high surface area and uniform
pore size distribution. In this chapter, we will particularly focus on this point.
2.4 Porous TiO 2 Photocatalysts
2.4.1 Templated Nanostructured TiO 2
Usually mesoporous titania are synthesized from the sol-gel process using titanium
alkoxides as precursors. For example, using this technique and starting from titanium
isopropoxide as precursor and performing a hydrothermal treatment at different
temperatures from 40 to 240
C, Rasalingam et al. have synthesized efficient
photocatalysts for the degradation of rhodamine B under visible light irradiation
(Rasalingam et al. 2015). Using the sol-gel approach, the textural properties of the
TiO 2 materials are mainly controlled by the synthesis conditions such as the pH, the
gel composition, the humidity, and so on (Simonsen and Søgaard 2010). The textural
properties of the porous TiO 2 can also be enhanced by combining the sol-gel process
with the surfactant-templating mechanism, reported for the preparation of the
ordered mesoporous silica materials (Li et al. 2014; Bagheri et al. 2015; Kimura
2016). Two methods can lead to the formation of mesostructured TiO 2
mesostructures. The first one is the hard-templating route. In that case, the
mesoporous titania is prepared in a confined space, for example, via replication of
mesoporous silica. Since the template prevents collapse of the mesostructure upon
calcination, the main advantage of this method is the preservation of the ordered
mesopore channel array during the crystallization step at high temperature. However, this procedure is time-consuming, non-eco-friendly (it usually requires
hydrofluoric acid to remove the hard template), and it is quite difficult to completely
fulfill the mesopores of the hard template with the titania precursor. By contrast, the
second mechanism labelled the soft-templating pathway is less time-consuming and
more eco-friendly. A large variety of structure-directing agents such as ionic liquid,
poly(ethylene glycol), diblock copolymer, polyoxyethylene fluoroalkyl ether, cyclodextrins, or homemade block copolymers (BCPs) have been used to prepare the
porous titania material though the soft-templating route (Zimny et al. 2010;
Veliscek-Carolan et al. 2015; Lannoy et al. 2014; Cao et al. 2016; Preethi et al.
2017). Nevertheless, among these templates, thanks to their large molecular weights,
the triblock copolymers such as Pluronic P123 or F127 are the most widely considered for the preparation of porous TiO 2 (Ismail and Bahnemann 2011). Nevertheless,
the main difficulty is to control the alkoxide precursor hydrolysis and condensation
that are much faster than for silicon. To reach this goal, different strategies have been
developed, and, for example, this control can be achieved by the use of mixed
52
B. Lebeau et al.
parameter that should be considered. Generally, large surface area is likely to exhibit
better photocatalytic activity, because a large surface area provides more active sites
for adsorbing dyes. Recently, much attention was paid to the synthesis of the
nanostructured mesoporous photocatalysts with high surface area and uniform
pore size distribution. In this chapter, we will particularly focus on this point.
2.4 Porous TiO 2 Photocatalysts
2.4.1 Templated Nanostructured TiO 2
Usually mesoporous titania are synthesized from the sol-gel process using titanium
alkoxides as precursors. For example, using this technique and starting from titanium
isopropoxide as precursor and performing a hydrothermal treatment at different
temperatures from 40 to 240
C, Rasalingam et al. have synthesized efficient
photocatalysts for the degradation of rhodamine B under visible light irradiation
(Rasalingam et al. 2015). Using the sol-gel approach, the textural properties of the
TiO 2 materials are mainly controlled by the synthesis conditions such as the pH, the
gel composition, the humidity, and so on (Simonsen and Søgaard 2010). The textural
properties of the porous TiO 2 can also be enhanced by combining the sol-gel process
with the surfactant-templating mechanism, reported for the preparation of the
ordered mesoporous silica materials (Li et al. 2014; Bagheri et al. 2015; Kimura
2016). Two methods can lead to the formation of mesostructured TiO 2
mesostructures. The first one is the hard-templating route. In that case, the
mesoporous titania is prepared in a confined space, for example, via replication of
mesoporous silica. Since the template prevents collapse of the mesostructure upon
calcination, the main advantage of this method is the preservation of the ordered
mesopore channel array during the crystallization step at high temperature. However, this procedure is time-consuming, non-eco-friendly (it usually requires
hydrofluoric acid to remove the hard template), and it is quite difficult to completely
fulfill the mesopores of the hard template with the titania precursor. By contrast, the
second mechanism labelled the soft-templating pathway is less time-consuming and
more eco-friendly. A large variety of structure-directing agents such as ionic liquid,
poly(ethylene glycol), diblock copolymer, polyoxyethylene fluoroalkyl ether, cyclodextrins, or homemade block copolymers (BCPs) have been used to prepare the
porous titania material though the soft-templating route (Zimny et al. 2010;
Veliscek-Carolan et al. 2015; Lannoy et al. 2014; Cao et al. 2016; Preethi et al.
2017). Nevertheless, among these templates, thanks to their large molecular weights,
the triblock copolymers such as Pluronic P123 or F127 are the most widely considered for the preparation of porous TiO 2 (Ismail and Bahnemann 2011). Nevertheless,
the main difficulty is to control the alkoxide precursor hydrolysis and condensation
that are much faster than for silicon. To reach this goal, different strategies have been
developed, and, for example, this control can be achieved by the use of mixed
52
B. Lebeau et al.