toward degradation of rhodamine B dye under visible light was enhanced by doping
with nonmetal dopants, N, N/S, and N/F hierarchical macro-/mesoporous TiO 2 and
combining with water-mediated adsorption (Pan et al. 2013). Besides the advantage
to make the TiO 2 photocatalyst active under visible light, the doping with nonmetal
such as carbon could also benefit for increasing dye adsorption capacity and improve
the photocatalyst activity for dye removal by combination of adsorption and degradation (Xu et al. 2016). Doping with sulfur creates acid sites on the photocatalyst
surface that enhance photocatalytic activity for the degradation of organic molecules
such as dyes. Sulfur is usually introduced in the synthesis medium with thiourea
(Guo et al. 2010), but the use of elemental sulfur was also reported and claimed to be
less harmful (Galenda et al. 2017). The main drawback of the nonmetal doping
concerns the long-term instability of the photocatalyst. Co-doping with nonmetal
and metal dopants have been explored for getting synergetic effect from the two
dopants. For producing photocatalyst highly active in the visible, co-doping with S
and Cu has been achieved to get advantage of S to lower band gap of TiO 2 and of Cu
to trap the electrons in the conduction band of TiO 2 in order to prevent charge
recombination of electrons with holes (Yi et al. 2014).
It is noteworthy that other strategies such as encapsulation of photosensitive
molecules within porous photocatalysts have been developed to improve
photocatalytic properties in the visible. Heteropolytungstic acid (HPA) that is
photosensitive has been incorporated by direct synthesis in SBA-15 and
Ti-SBA-15 materials (Anandan and Yoon 2007). The photodegradation of methyl
orange in aqueous solution under visible light was observed faster in the presence of
HPA-TiSBA-15 than those in HPA-SBA-15 and Ti-SBA-15.
2.4.5 Shaping
Most of the TiO 2 photocatalysts are synthesized as nano- or micro-sized particles
and are powdered materials that is limiting for their applicative use. The shaping to
integrate them in processes that can be developed at the industrial scale is necessary
and often remains a challenge since properties of the photocatalysts have to be
preserved. Processing photocatalysts as films has been explored according to different strategies. TiO 2 nanoparticles prepared by a sol-gel method in presence of
polyethylene glycol have been processed as thin films by spin-coating the obtained
TiO 2 sol on glass substrates and applying a subsequent thermal treatment (Lin et al.
2012). The film thickness was varied by repeating the film deposition procedure. The
obtained films have been shown to be efficient for the photodegradation of methylene blue, methyl orange, and indigo in water under UV light irradiation. Increasing
film thickness results in increasing degradation rate to a certain extent. Indeed the
photocatalytic reactions were found to occur mainly on the surface of TiO 2 thin
films. For thick films, the TiO 2 particles in the interior region are less active because
of the difficulty to access for the reactant and the low diffusion in solution for the
reaction by-products. Moreover, an increase in opacity and light scattering was
observed for thick films that decreases the light transmission in the film.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
63
with nonmetal dopants, N, N/S, and N/F hierarchical macro-/mesoporous TiO 2 and
combining with water-mediated adsorption (Pan et al. 2013). Besides the advantage
to make the TiO 2 photocatalyst active under visible light, the doping with nonmetal
such as carbon could also benefit for increasing dye adsorption capacity and improve
the photocatalyst activity for dye removal by combination of adsorption and degradation (Xu et al. 2016). Doping with sulfur creates acid sites on the photocatalyst
surface that enhance photocatalytic activity for the degradation of organic molecules
such as dyes. Sulfur is usually introduced in the synthesis medium with thiourea
(Guo et al. 2010), but the use of elemental sulfur was also reported and claimed to be
less harmful (Galenda et al. 2017). The main drawback of the nonmetal doping
concerns the long-term instability of the photocatalyst. Co-doping with nonmetal
and metal dopants have been explored for getting synergetic effect from the two
dopants. For producing photocatalyst highly active in the visible, co-doping with S
and Cu has been achieved to get advantage of S to lower band gap of TiO 2 and of Cu
to trap the electrons in the conduction band of TiO 2 in order to prevent charge
recombination of electrons with holes (Yi et al. 2014).
It is noteworthy that other strategies such as encapsulation of photosensitive
molecules within porous photocatalysts have been developed to improve
photocatalytic properties in the visible. Heteropolytungstic acid (HPA) that is
photosensitive has been incorporated by direct synthesis in SBA-15 and
Ti-SBA-15 materials (Anandan and Yoon 2007). The photodegradation of methyl
orange in aqueous solution under visible light was observed faster in the presence of
HPA-TiSBA-15 than those in HPA-SBA-15 and Ti-SBA-15.
2.4.5 Shaping
Most of the TiO 2 photocatalysts are synthesized as nano- or micro-sized particles
and are powdered materials that is limiting for their applicative use. The shaping to
integrate them in processes that can be developed at the industrial scale is necessary
and often remains a challenge since properties of the photocatalysts have to be
preserved. Processing photocatalysts as films has been explored according to different strategies. TiO 2 nanoparticles prepared by a sol-gel method in presence of
polyethylene glycol have been processed as thin films by spin-coating the obtained
TiO 2 sol on glass substrates and applying a subsequent thermal treatment (Lin et al.
2012). The film thickness was varied by repeating the film deposition procedure. The
obtained films have been shown to be efficient for the photodegradation of methylene blue, methyl orange, and indigo in water under UV light irradiation. Increasing
film thickness results in increasing degradation rate to a certain extent. Indeed the
photocatalytic reactions were found to occur mainly on the surface of TiO 2 thin
films. For thick films, the TiO 2 particles in the interior region are less active because
of the difficulty to access for the reactant and the low diffusion in solution for the
reaction by-products. Moreover, an increase in opacity and light scattering was
observed for thick films that decreases the light transmission in the film.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
63