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P. Basnet and S. Chatterjee
with the simultaneous occurrence of d-d transitions amongst the Co dopant levels.
The analogous excited states were near to the vicinity of CB minimum leading to
ligand charge transfer, and consequently, promoting exciton separation. Similar to the
aforementioned decrease in the photocatalytic efficiency of Ti-doped ZnO induced
by the higher dopant concentration, in this study also, increase in the Co concentration led to parallel results. Authors explained that high content of Co stimulated rise
in the energy of CB minimum, which favoured the Co 3D excited states to become
energetically more secluded, subsequently resulting into the generation of an energy
barrier for ligand exciton transfer. This phenomenon ultimately resulted into lower
photocatalytic activity of the Co-doped ZnO photocatalyst (possessing higher Co
content). Ashebir et al. [17] found that the photocatalytic degradation of methyl
violet (MV) by ZnO significantly enhanced in the present of dopants such as Mn or
Ag. Authors suggested that the observed synergistic effect may be attributed to the
increased surface area and oxygen defect sites. Qi et al. [18] reported the photocatalytic degradation of MB dye with different transition metal ions such as Mn, Fe, Co,
Ni or Cu as the dopant in ZnO. They observed that the highest degradation efficiency
was achieved with Cu-doped ZnO nanoparticles mainly because in Cu-doped ZnO,
the electronic transitions of Zn 3p, O 2p and O 2p Cu 3D states are significantly
stronger as compared to other ZnO-doped systems which led to an enhancement in
the visible light absorption. Ong et al. [19] reported that the metal-doped ZnO may
enhance the photocatalytic activity by increasing the magnitude of trapping sites of
the photo-induced charge carriers, and thereby, decrease their recombination. Vaiano
et al. [20] studied the photocatalytic degradation of MO dye simultaneously with the
photocatalytic hydrogen generation by ruthenium (Ru) modified ZnO photocatalyst.
They observed that although the Ru doping content largely determined the amount
of hydrogen production, the degradation of MO dye was not much affected. They
observed that in order to meet both these applications, the amount of Ru to be doped
must be 0.25% to ZnO. Ebrahimi et al. [21] reported the degradation of direct blue 15
(DB-15) dye with Ag, Cu or Mn-doped ZnO under UV as well as visible light irradiations. They concluded that the enhanced activity of doped ZnO as compared to bare
ZnO was possible as a result of (i) reduction in the electron–hole pair recombination,
(ii) increased generation of (•OH) radicals in the system and (iii) development of
impurity states after doping. All these factors contributed to enhanced photocatalytic
activity of metal-doped ZnO NPC. Therefore, while considering transition metal–
ZnO NPC, an optimum concentration of the dopant/deposit and higher trapping
sites are the necessary requirements for achieving higher photocatalytic degradation
efficiencies.
2.2 Mechanism of Photocatalysis
The mechanism of dye degradation occurs through three main pathways: (i) dyesensitized mechanism, (ii) N-deethylation and (iii) destruction of conjugated structure. The dominance of a particular mechanism depends upon the nature of light
P. Basnet and S. Chatterjee
with the simultaneous occurrence of d-d transitions amongst the Co dopant levels.
The analogous excited states were near to the vicinity of CB minimum leading to
ligand charge transfer, and consequently, promoting exciton separation. Similar to the
aforementioned decrease in the photocatalytic efficiency of Ti-doped ZnO induced
by the higher dopant concentration, in this study also, increase in the Co concentration led to parallel results. Authors explained that high content of Co stimulated rise
in the energy of CB minimum, which favoured the Co 3D excited states to become
energetically more secluded, subsequently resulting into the generation of an energy
barrier for ligand exciton transfer. This phenomenon ultimately resulted into lower
photocatalytic activity of the Co-doped ZnO photocatalyst (possessing higher Co
content). Ashebir et al. [17] found that the photocatalytic degradation of methyl
violet (MV) by ZnO significantly enhanced in the present of dopants such as Mn or
Ag. Authors suggested that the observed synergistic effect may be attributed to the
increased surface area and oxygen defect sites. Qi et al. [18] reported the photocatalytic degradation of MB dye with different transition metal ions such as Mn, Fe, Co,
Ni or Cu as the dopant in ZnO. They observed that the highest degradation efficiency
was achieved with Cu-doped ZnO nanoparticles mainly because in Cu-doped ZnO,
the electronic transitions of Zn 3p, O 2p and O 2p Cu 3D states are significantly
stronger as compared to other ZnO-doped systems which led to an enhancement in
the visible light absorption. Ong et al. [19] reported that the metal-doped ZnO may
enhance the photocatalytic activity by increasing the magnitude of trapping sites of
the photo-induced charge carriers, and thereby, decrease their recombination. Vaiano
et al. [20] studied the photocatalytic degradation of MO dye simultaneously with the
photocatalytic hydrogen generation by ruthenium (Ru) modified ZnO photocatalyst.
They observed that although the Ru doping content largely determined the amount
of hydrogen production, the degradation of MO dye was not much affected. They
observed that in order to meet both these applications, the amount of Ru to be doped
must be 0.25% to ZnO. Ebrahimi et al. [21] reported the degradation of direct blue 15
(DB-15) dye with Ag, Cu or Mn-doped ZnO under UV as well as visible light irradiations. They concluded that the enhanced activity of doped ZnO as compared to bare
ZnO was possible as a result of (i) reduction in the electron–hole pair recombination,
(ii) increased generation of (•OH) radicals in the system and (iii) development of
impurity states after doping. All these factors contributed to enhanced photocatalytic
activity of metal-doped ZnO NPC. Therefore, while considering transition metal–
ZnO NPC, an optimum concentration of the dopant/deposit and higher trapping
sites are the necessary requirements for achieving higher photocatalytic degradation
efficiencies.
2.2 Mechanism of Photocatalysis
The mechanism of dye degradation occurs through three main pathways: (i) dyesensitized mechanism, (ii) N-deethylation and (iii) destruction of conjugated structure. The dominance of a particular mechanism depends upon the nature of light