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Topics in Current Chemistry (2020) 378:7
effectively produce such radicals, which can degrade/oxidize a wide range of contaminants as well as effectively inactivate microorganisms.
The development of photocatalysts should take into account the typical characteristics and requirements for photocatalytic materials. In particular, a photocatalyst must be photoactive, inert, stable, non-toxic, and relatively cheap [2]. One of
the most studied photocatalyst for water/wastewater treatment is titanium dioxide (TiO 2 ). TiO 2 has been widely investigated because of its very high ultraviolet
absorption and high stability. These properties made this photocatalyst ideal for different applications, such as electroceramics, glass, and photocatalytic degradation of
chemicals in water and air. It has been investigated in powdered form suspended in
aqueous matrices (slurry reactor) or as thin film. TiO 2 presents three different crystalline forms: anatase, rutile (more stable), and brookite (uncommon and unstable).
Degussa P25 titania is commercially available and consists of 25% rutile and 75%
anatase crystalline phases [3]. P25 is used as a benchmark in water and wastewater
treatment by TiO 2 photocatalysis due to its easy availability, reproducibility, chemical stability, and high photoactivity [4–8]. Beyond TiO 2 , another interesting photocatalyst with properties similar to titania is ZnO. This photocatalyst has recently
attracted increasing attention from the scientific community for its characteristics
that include strong oxidation ability, good photocatalytic properties large free-exciton binding energy, and low cost (cheaper than TiO 2 ) [9]. Another category of semiconductors that has found success in photocatalytic applications for water treatment
is represented by perovskites, in particular LaFeO 3 [10, 11]. LaFeO 3 is one of the
most common perovskite-type oxides, and it is a promising material with different
functionalities, having a general formula ABO 3 , where position A is the rare earth
ion and position B is represented by metal ion. This material is characterized by high
stability, non-toxicity, and small bandgap energy (2.07 eV), making perovskite an
interesting visible-light-active photocatalyst [12].
2.1 Methods for Improving Photocatalyst Activity
The application of photocatalysts such as TiO 2 and ZnO is limited by the fact that
ultraviolet (UV) activation is needed (the bandgap energy is about 3.2 eV, and this
means that less than 5% of the solar spectrum has sufficient energy to activate the
photocatalyst) and by the fast recombination rate of the electron–hole pairs generated [13]. Hence, modification of photocatalysts through metal or non-metal doping or their combination with another semiconductor are common methods used to
improve the photocatalytic performance and for activation by visible light irradiation. The particle size and morphology of nanoscale photocatalysts is also a problem
in full-scale application for water and wastewater treatment, because the particles
should be removed/recovered after treatment. To overcome this drawback, the application of photocatalytic membranes or the use of other photocatalyst-supporting
materials has been proposed [14–16]. For example, photocatalysts have been supported on activated carbon [14], fibers, [17, 18] membranes, [19, 20] metal [5],
and plastics [21]. In this section, the main methods for the synthesis of modified
photocatalysts used in wastewater treatment are introduced, along with the methods
227
Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
effectively produce such radicals, which can degrade/oxidize a wide range of contaminants as well as effectively inactivate microorganisms.
The development of photocatalysts should take into account the typical characteristics and requirements for photocatalytic materials. In particular, a photocatalyst must be photoactive, inert, stable, non-toxic, and relatively cheap [2]. One of
the most studied photocatalyst for water/wastewater treatment is titanium dioxide (TiO 2 ). TiO 2 has been widely investigated because of its very high ultraviolet
absorption and high stability. These properties made this photocatalyst ideal for different applications, such as electroceramics, glass, and photocatalytic degradation of
chemicals in water and air. It has been investigated in powdered form suspended in
aqueous matrices (slurry reactor) or as thin film. TiO 2 presents three different crystalline forms: anatase, rutile (more stable), and brookite (uncommon and unstable).
Degussa P25 titania is commercially available and consists of 25% rutile and 75%
anatase crystalline phases [3]. P25 is used as a benchmark in water and wastewater
treatment by TiO 2 photocatalysis due to its easy availability, reproducibility, chemical stability, and high photoactivity [4–8]. Beyond TiO 2 , another interesting photocatalyst with properties similar to titania is ZnO. This photocatalyst has recently
attracted increasing attention from the scientific community for its characteristics
that include strong oxidation ability, good photocatalytic properties large free-exciton binding energy, and low cost (cheaper than TiO 2 ) [9]. Another category of semiconductors that has found success in photocatalytic applications for water treatment
is represented by perovskites, in particular LaFeO 3 [10, 11]. LaFeO 3 is one of the
most common perovskite-type oxides, and it is a promising material with different
functionalities, having a general formula ABO 3 , where position A is the rare earth
ion and position B is represented by metal ion. This material is characterized by high
stability, non-toxicity, and small bandgap energy (2.07 eV), making perovskite an
interesting visible-light-active photocatalyst [12].
2.1 Methods for Improving Photocatalyst Activity
The application of photocatalysts such as TiO 2 and ZnO is limited by the fact that
ultraviolet (UV) activation is needed (the bandgap energy is about 3.2 eV, and this
means that less than 5% of the solar spectrum has sufficient energy to activate the
photocatalyst) and by the fast recombination rate of the electron–hole pairs generated [13]. Hence, modification of photocatalysts through metal or non-metal doping or their combination with another semiconductor are common methods used to
improve the photocatalytic performance and for activation by visible light irradiation. The particle size and morphology of nanoscale photocatalysts is also a problem
in full-scale application for water and wastewater treatment, because the particles
should be removed/recovered after treatment. To overcome this drawback, the application of photocatalytic membranes or the use of other photocatalyst-supporting
materials has been proposed [14–16]. For example, photocatalysts have been supported on activated carbon [14], fibers, [17, 18] membranes, [19, 20] metal [5],
and plastics [21]. In this section, the main methods for the synthesis of modified
photocatalysts used in wastewater treatment are introduced, along with the methods
227
Reprinted from the journal