Topics in Current Chemistry (2020) 378:7
1 3
developed for supporting photocatalysts on suitable materials in order to make them
usable on a large scale.
A number of methods can be used to extend the light absorption properties of traditional semiconductors (e.g., ZnO or TiO 2 ) into the visible range, including:
– Coupling a primary photocatalyst with different semiconductors with a smaller
bandgap or through sensitization with dyes.
– Doping the photocatalyst with metals (second-generation photocatalysts).
– Doping the photocatalyst with non-metals (third-generation photocatalysts).
2.1.1 Coupling a Primary Photocatalyst with Smaller‑Bandgap Semiconductor or Its
Sensitization with Dye
As a semiconductor (e.g., TiO 2 or ZnO) with a wide bandgap is coupled with other
semiconductors having a lower bandgap or is sensitized with specific types of dyes,
the absorption properties of the obtained composite are extended into the visible
region. This phenomenon is induced either by the light absorption characteristics
of the dye or by the other semiconductor coupled with TiO 2 or ZnO. When a semiconductor is sensitized with a dye, the visible light is absorbed by the dye molecules
bridged to the semiconductor surface, and the electrons pass from the dye’s ground
state to an excited state. These excited electrons are then transferred to the conduction band of ZnO or TiO 2 , which results in a modified photocatalyst with improved
photoactivity under visible light. Dyes that have been used for this purpose include
ruthenium polypyridyl complexes [22] and different metal-free organic dye molecules such as hemicyanine [23] and indoline [24]. However, it is worth noting that
the low stability of the dye used for the sensitization of semiconductors in water/
wastewater is the main drawback of this method.
With regard to the coupling of two semiconductors, the aim is to form a heterojunction structure between TiO 2 or ZnO and a narrow-bandgap semiconductor such
as CdS, MoS 2 , or In 2 S 3 [25]. The electrons excited by visible light are transferred to
TiO 2 or ZnO from the narrow-bandgap semiconductor, thus promoting charge carrier separation and, consequently, improving the visible-light photocatalytic activity
of the composite [25]. In order to improve ZnO (bandgap of 3.2 eV) activity under
visible light, photocatalysts have also been coupled with LaFeO 3 . This composite
was prepared by a process known as solution combustion synthesis, using citric acid
as the organic fuel and metal nitrates, thus drastically affecting the bandgap energy,
which decreased from 3.2 to 1.94 eV [26]. However, although this method might
represent a suitable approach for preparing photocatalysts that work effectively
under visible light, the coupling between semiconductors can suffer from the photocorrosion phenomenon, negatively affecting the photocatalytic activity [27–29].
2.1.2 Photocatalyst Doping with Metals
In order to improve the photocatalytic efficiency of primary photocatalysts under
visible light, TiO 2 and ZnO were also doped with metals (second-generation visible-light-active photocatalysts), by inserting a metal ion in the crystalline structure
228
Reprinted from the journal
1 3
developed for supporting photocatalysts on suitable materials in order to make them
usable on a large scale.
A number of methods can be used to extend the light absorption properties of traditional semiconductors (e.g., ZnO or TiO 2 ) into the visible range, including:
– Coupling a primary photocatalyst with different semiconductors with a smaller
bandgap or through sensitization with dyes.
– Doping the photocatalyst with metals (second-generation photocatalysts).
– Doping the photocatalyst with non-metals (third-generation photocatalysts).
2.1.1 Coupling a Primary Photocatalyst with Smaller‑Bandgap Semiconductor or Its
Sensitization with Dye
As a semiconductor (e.g., TiO 2 or ZnO) with a wide bandgap is coupled with other
semiconductors having a lower bandgap or is sensitized with specific types of dyes,
the absorption properties of the obtained composite are extended into the visible
region. This phenomenon is induced either by the light absorption characteristics
of the dye or by the other semiconductor coupled with TiO 2 or ZnO. When a semiconductor is sensitized with a dye, the visible light is absorbed by the dye molecules
bridged to the semiconductor surface, and the electrons pass from the dye’s ground
state to an excited state. These excited electrons are then transferred to the conduction band of ZnO or TiO 2 , which results in a modified photocatalyst with improved
photoactivity under visible light. Dyes that have been used for this purpose include
ruthenium polypyridyl complexes [22] and different metal-free organic dye molecules such as hemicyanine [23] and indoline [24]. However, it is worth noting that
the low stability of the dye used for the sensitization of semiconductors in water/
wastewater is the main drawback of this method.
With regard to the coupling of two semiconductors, the aim is to form a heterojunction structure between TiO 2 or ZnO and a narrow-bandgap semiconductor such
as CdS, MoS 2 , or In 2 S 3 [25]. The electrons excited by visible light are transferred to
TiO 2 or ZnO from the narrow-bandgap semiconductor, thus promoting charge carrier separation and, consequently, improving the visible-light photocatalytic activity
of the composite [25]. In order to improve ZnO (bandgap of 3.2 eV) activity under
visible light, photocatalysts have also been coupled with LaFeO 3 . This composite
was prepared by a process known as solution combustion synthesis, using citric acid
as the organic fuel and metal nitrates, thus drastically affecting the bandgap energy,
which decreased from 3.2 to 1.94 eV [26]. However, although this method might
represent a suitable approach for preparing photocatalysts that work effectively
under visible light, the coupling between semiconductors can suffer from the photocorrosion phenomenon, negatively affecting the photocatalytic activity [27–29].
2.1.2 Photocatalyst Doping with Metals
In order to improve the photocatalytic efficiency of primary photocatalysts under
visible light, TiO 2 and ZnO were also doped with metals (second-generation visible-light-active photocatalysts), by inserting a metal ion in the crystalline structure
228
Reprinted from the journal