## 3 Results and Discussion
### Photocatalytic Mechanism  
The main condition for an effective semiconductor photocatalyst is the high photon absorption capacity, generation, separation, and transfer of electron-hole pairs to the surface [49].  
In the present case, since most of the photons emitted from the sun has less energy than the band gap of ZnO, it is suggested that the photocreated electrons and holes are assisted by deep energy levels associated to the various defects in ZnO host, particularly the single oxygen defects \(V_{O}^{+}\), which show the dominant emission:  
\[ZnO+V_{O}^{+}\xrightarrow{visible\ excitation}V_{O}^{+}+e^{-}+h^{+}\to V_{o}+h^{+}\]  
Figure 14: Cycling photocatalysis experiment in the presence of ZnEuNa4 photocatalyst under sunlight irradiation.  
Figure 13: Effects of scavengers on the photocatalytic efficiency of ZnEuNa4 photocatalyst.  
The water molecules localized on the surface of ZnO nanocrystals can capture the generated holes in the valence band to produce hydroxyl radicals \(OH^{\blacksquare}\):  
\[H_{2}O+h^{+}\to OH^{\blacksquare}+H^{+}\]  
Also, the excited electron in the conduction band can be captured by \(O_{2}\) molecules to form the superoxide radical \(O_{2}^{-\blacksquare}\) which is a stronger radical anion for RhB decomposition:  
\[O_{2}+e^{-}\to O_{2}^{-\blacksquare}\]  
The hydroxyl and superoxide radicals are known as extremely strong oxidant for the decomposition of dye molecules [50] (Figure 15a):  
\[RhB+OH^{\blacksquare} \to RhB(inermidinates)\to CO_{2}+H_{2}O\] \[RhB+O_{2}^{-\blacksquare} \to RhB(inermidiates)\to CO_{2}+H_{2}O\]  
The ZnEuNa4 sample has the best photocatalytic efficiency. The physical process behind this result is the reduction of the density of oxygen defects, as revealed from the analysis of the spectra of PL. Indeed, the energy levels related of oxygen vacancy (V\({}_{\text{O}}\)), and which ensures the separation of charges, are practically eliminated thanks to the doping by occupying some sites in the lattice.  
For the doped ZnO nanostructures, the extended PL lifetime allows the adsorbed organic molecules in the ZnO crystallite surface to trap the photogenerated electrons and prevents their recombination [51; 52; 53; 54]. Therefore, the Eu\({}^{3+}\) ion undergoes a reduction and  
Figure 15: Proposed photocatalytic mechanism under sunlight irradiation for: (**a**) undoped ZnO and (**b**) ZnEuNa4 photocatalysts.  
moves to the unstable Eu\({}^{2+}\) state that may transfer its electron to the adsorbed \(O_{2}\) to form the \(O_{2}^{-\bullet}\) radical. The formed Eu\({}^{3+}\)/Eu\({}^{2+}\) redox pairs serve as scavengers to trap the photogenerated carriers and hinder their recombinations, further to promote the visible light absorption [54]:  
\[\text{Eu}^{3+}+e^{-}\quad\rightarrow\quad\text{Eu}^{2+}\]  
\[\text{Eu}^{2+}+\text{O}_{2}\rightarrow\text{Eu}^{3+}+O_{2}^{-\blacksquare}\]  
\[RhB+O_{2}^{-\blacksquare}\rightarrow RhB(intermidiates)\to CO_{2}+H_{2}O\]  
Here it is important to mention that the presence of Eu\({}^{3+}\)/Eu\({}^{2+}\) could form a midgap states in ZnO band gap. Further, under visible light irradiation, photo-generated electrons at Eu energy bands could be excited to the conduction band of ZnO.  
Simultaneously, the photorereated holes could participate in two-step excitation process (first transition: valence band--vacancies level; second transition: vacancies level --Eu midgap states) and react with adsorbed water to create a highly reactive hydroxyl radical (Figure 15b).  
\[H_{2}O+h^{+}\to OH^{\blacksquare}+H^{+}\]  
\[RhB+OH^{\blacksquare}\to RhB(intermidiates)\to CO_{2}+H_{2}O\]  
Further to its role as sensitizer of Eu\({}^{3+}\) ions, Na\({}^{+}\) may form dipoles with other species which promotes the separation of the photoreated electron-hole pairs and enhances the absorption of the photocatalyst.