An Overview on the Photocatalytic Application …
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source used (sunlight, UV light or visible light), which in turn is related to the type
of material employed as a photocatalyst. Since this overview is based on transition
metal–ZnO NPC, mechanism related to this material will only be emphasized. Most
of the organic dyes absorb in the visible region, therefore, dye-sensitized mechanism simultaneously occurs with the other two above-mentioned mechanisms, while
in the absence of visible light, there is no scope for this mechanism to take place.
Since organic dyes (e.g., MB, rhodamine B, methyl orange, acridine orange, etc.)
contain π-conjugation, the incident photons from the visible light source get absorbed
resulting into the release of charge carriers. These electrons then transfer to the lowest
unoccupied molecular orbital and finally relocate to the photocatalyst’s CB [22].
The oxygen molecules adsorbed at the surface of the photocatalyst react with
these electrons and generate superoxide radical anions. These CB electrons may also
interact with the adsorbed water molecules and produce hydroxyl radicals. The asgenerated radicals are responsible for initiating the dye degradation phenomenon,
which may then occur through either N-deethylation or conjugated structure breakdown or both. In case of transition metal–ZnO NPC, visible light is absorbed by
both the dye molecules as well as the photocatalyst. The extent of dye degradation is initially monitored using UV–Vis spectroscopy by observing the decrease in
the temporal absorption over a fixed interval of time. As the degradation proceeds,
the absorbance maximum of the dye decreases and may reach a minimum. The
percentage difference in the initial and final concentrations of the dye provides the
degradation efficiency of the photocatalyst. The decrease in the absorbance maximum
of the dye, as observed from UV–Vis spectrum, may be of two types [23–26], as
shown in Fig. 1a [18] and b [2]. In one type, decrease in the absorption maximum
without wavelength shifts is observed, as in case of Fig. 1b while in the other
type, decrease is accompanied by hyposochromic shift of the absorption maximum
(Fig. 1a). The former reveals the dominance of conjugated structure destruction and
N-deethylation is the predominant process in the latter. The difference in these two
degradation pathways is only realized upon the generation of intermediate products
during the course of the reaction. However, the complete mineralization of the respective dye eventually forms carbon dioxide, water and inorganic salts, irrespective of
the degradation pathway followed.
Thein et al. [12] have led forward the mechanism of photocatalytic degradation
of rhodamine B (RhB) dye by Ni coupled ZnO NC under UV light as follows: under
UV light irradiation, electron–hole pairs are generated in ZnO. Due to the effect of
localized surface plasmons from Ni metal, electrons present in the defect levels of
ZnO are transferred into Ni particles, which functioned as electron trapping centres.
Thus, mobilization of holes increased and these diffused onto the surface of the
photocatalyst. Finally, the redox reactions of holes and escaped electrons present at
the surface with adsorbed oxygen and water molecules led to the formation of reactive
oxidative species (ROS). These ROS then underwent a chain reaction to degrade the
dye molecules. The underlying mechanism for the photocatalytic degradation of MB
dye under visible light by ZnO: Mn
2+ has been elaborately discussed by Ullah et al.
[14]. According to the authors, illumination of the photocatalyst with visible light
generated electron–hole pairs located at the tail states within the proximity of the
243
source used (sunlight, UV light or visible light), which in turn is related to the type
of material employed as a photocatalyst. Since this overview is based on transition
metal–ZnO NPC, mechanism related to this material will only be emphasized. Most
of the organic dyes absorb in the visible region, therefore, dye-sensitized mechanism simultaneously occurs with the other two above-mentioned mechanisms, while
in the absence of visible light, there is no scope for this mechanism to take place.
Since organic dyes (e.g., MB, rhodamine B, methyl orange, acridine orange, etc.)
contain π-conjugation, the incident photons from the visible light source get absorbed
resulting into the release of charge carriers. These electrons then transfer to the lowest
unoccupied molecular orbital and finally relocate to the photocatalyst’s CB [22].
The oxygen molecules adsorbed at the surface of the photocatalyst react with
these electrons and generate superoxide radical anions. These CB electrons may also
interact with the adsorbed water molecules and produce hydroxyl radicals. The asgenerated radicals are responsible for initiating the dye degradation phenomenon,
which may then occur through either N-deethylation or conjugated structure breakdown or both. In case of transition metal–ZnO NPC, visible light is absorbed by
both the dye molecules as well as the photocatalyst. The extent of dye degradation is initially monitored using UV–Vis spectroscopy by observing the decrease in
the temporal absorption over a fixed interval of time. As the degradation proceeds,
the absorbance maximum of the dye decreases and may reach a minimum. The
percentage difference in the initial and final concentrations of the dye provides the
degradation efficiency of the photocatalyst. The decrease in the absorbance maximum
of the dye, as observed from UV–Vis spectrum, may be of two types [23–26], as
shown in Fig. 1a [18] and b [2]. In one type, decrease in the absorption maximum
without wavelength shifts is observed, as in case of Fig. 1b while in the other
type, decrease is accompanied by hyposochromic shift of the absorption maximum
(Fig. 1a). The former reveals the dominance of conjugated structure destruction and
N-deethylation is the predominant process in the latter. The difference in these two
degradation pathways is only realized upon the generation of intermediate products
during the course of the reaction. However, the complete mineralization of the respective dye eventually forms carbon dioxide, water and inorganic salts, irrespective of
the degradation pathway followed.
Thein et al. [12] have led forward the mechanism of photocatalytic degradation
of rhodamine B (RhB) dye by Ni coupled ZnO NC under UV light as follows: under
UV light irradiation, electron–hole pairs are generated in ZnO. Due to the effect of
localized surface plasmons from Ni metal, electrons present in the defect levels of
ZnO are transferred into Ni particles, which functioned as electron trapping centres.
Thus, mobilization of holes increased and these diffused onto the surface of the
photocatalyst. Finally, the redox reactions of holes and escaped electrons present at
the surface with adsorbed oxygen and water molecules led to the formation of reactive
oxidative species (ROS). These ROS then underwent a chain reaction to degrade the
dye molecules. The underlying mechanism for the photocatalytic degradation of MB
dye under visible light by ZnO: Mn
2+ has been elaborately discussed by Ullah et al.
[14]. According to the authors, illumination of the photocatalyst with visible light
generated electron–hole pairs located at the tail states within the proximity of the