An Overview on the Photocatalytic Application …
241
According to Bloh et al. [16], an optimum doping percentage exists up to which
the photocatalytic activity of ZnO increases, and after which, it drastically decreases.
They reported that lower doping concentration of titanium (Ti) was favourable for
enhancing the photocatalytic degradation efficiency of ZnO, while its higher concentration diminished the activity. They explained that the introduction of Ti cations led
to effective charge separation, wherein the cations acted as trap centres for electrons
and/or holes. However, at higher concentration, the authors explained a detrimental
effect of the same mechanism, wherein the nearest cationic neighbourhood of the
two dopant cations acted as a recombination centre. Jongprateep et al. [15] explained
the high photocatalytic performance of Ti-doped ZnO NPC for methylene blue (MB)
dye degradation which was related to the solubility limit, where, below the solubility
limit, enhancement in the photocatalytic performance was observed and vice versa.
The concept of trapping sites creation (as before) was considered for the excitonic
diminution while formation of titanium compound was regarded as an antagonistic
agent for lowering the degradation efficiency. The possible reasons for such reduced
photocatalytic MB dye degradation activity of TiO 2 -ZnO coupled nanocomposite
(NC) were ascribed to the uniform bonded heterostructure with random adhesion of
particles at the surface and to the obstruction of the reactive surface by the secondary
phase. Lu et al. [10] explained that Mn ions substitute the Zn ions in ZnO lattice due
to their similar ionic radii (Mn
2+ —65 pm and Zn
2+ —74 pm). This creates a change
in the electronic configuration of ZnO resulting from diversity in the ionic radii.
For restoration of its stable electronic configuration, adsorption of more number of
oxygen (O 2ads ) and hydroxide ions (OH
− ) occurred while undergoing the following
reactions (1–4).
Mn
2+
+ e
−
→ Mn
+
(1)
Mn
+
+ O 2ads → Mn
2+
+ O
−
2
(2)
Mn
2+
+ h
+
→ Mn
3+
(3)
Mn
3+
+ OH
−
→ Mn
2+
+ OH
(4)
Through these reactions, it may be interpreted that the exchange interaction occurring between Mn
2+ -Mn
3+ prolongs the lifetime of captured photo-generated e
− -h
+
pairs in ZnO. Consequently, these excitons may easily participate in photocatalytic
reactions. Following this discussion, authors concluded that the incorporation of Mn
dopants proved to be beneficial in terms of effective charge separation and reduced
recombination leading to enhanced photocatalytic activity. Sutka et al. [9] considered the enhancement in the photocatalytic degradation efficiency of ZnO by cobalt
(Co) doping due to the widening of the absorption wavelength and increased charge
separation. Through absorption and photoconductivity studies, they confirmed the
transfer of photo-generated electrons from ZnO VB to the localized Co energy states
241
According to Bloh et al. [16], an optimum doping percentage exists up to which
the photocatalytic activity of ZnO increases, and after which, it drastically decreases.
They reported that lower doping concentration of titanium (Ti) was favourable for
enhancing the photocatalytic degradation efficiency of ZnO, while its higher concentration diminished the activity. They explained that the introduction of Ti cations led
to effective charge separation, wherein the cations acted as trap centres for electrons
and/or holes. However, at higher concentration, the authors explained a detrimental
effect of the same mechanism, wherein the nearest cationic neighbourhood of the
two dopant cations acted as a recombination centre. Jongprateep et al. [15] explained
the high photocatalytic performance of Ti-doped ZnO NPC for methylene blue (MB)
dye degradation which was related to the solubility limit, where, below the solubility
limit, enhancement in the photocatalytic performance was observed and vice versa.
The concept of trapping sites creation (as before) was considered for the excitonic
diminution while formation of titanium compound was regarded as an antagonistic
agent for lowering the degradation efficiency. The possible reasons for such reduced
photocatalytic MB dye degradation activity of TiO 2 -ZnO coupled nanocomposite
(NC) were ascribed to the uniform bonded heterostructure with random adhesion of
particles at the surface and to the obstruction of the reactive surface by the secondary
phase. Lu et al. [10] explained that Mn ions substitute the Zn ions in ZnO lattice due
to their similar ionic radii (Mn
2+ —65 pm and Zn
2+ —74 pm). This creates a change
in the electronic configuration of ZnO resulting from diversity in the ionic radii.
For restoration of its stable electronic configuration, adsorption of more number of
oxygen (O 2ads ) and hydroxide ions (OH
− ) occurred while undergoing the following
reactions (1–4).
Mn
2+
+ e
−
→ Mn
+
(1)
Mn
+
+ O 2ads → Mn
2+
+ O
−
2
(2)
Mn
2+
+ h
+
→ Mn
3+
(3)
Mn
3+
+ OH
−
→ Mn
2+
+ OH
(4)
Through these reactions, it may be interpreted that the exchange interaction occurring between Mn
2+ -Mn
3+ prolongs the lifetime of captured photo-generated e
− -h
+
pairs in ZnO. Consequently, these excitons may easily participate in photocatalytic
reactions. Following this discussion, authors concluded that the incorporation of Mn
dopants proved to be beneficial in terms of effective charge separation and reduced
recombination leading to enhanced photocatalytic activity. Sutka et al. [9] considered the enhancement in the photocatalytic degradation efficiency of ZnO by cobalt
(Co) doping due to the widening of the absorption wavelength and increased charge
separation. Through absorption and photoconductivity studies, they confirmed the
transfer of photo-generated electrons from ZnO VB to the localized Co energy states