# Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification
## 3 Photocatalytic Activity Study and Discussion
Thus, the photocatalytic efficiency of ZnO will be improved and, consequently, the photo-degradation rate of the dyes will be increased according to the reactions below:  
Figure 4:5  
Figure 4:6  
Figure 4:7  
Figure 4:8  
Figure 4:9 Photo-degradation rate evolution of the dye solutions: methylene blue (MB) (**a**); methyl orange (MO) (**b**); acid red 14 (AR14) (**c**) by photocatalysis in the presence of ZnO:Fe NWs, ZnO NWs and without ZnO, respectively. Figure 3:0 Diagram describing the photocatalytic mechanism of the Fe-doped ZnO NWs. In order to understand the trapping efficiency of charge carriers in doped ZnO NWs, PL measurements were performed on the undoped and doped samples[PERSON] Figure 8a illustrates the PL emission spectra for undoped and Fe-doped ZnO NWs with an excitation wavelength of 325 nm (Laser He-Cd) (Model IK3101R-D, [PERSON], Tokyo, Japan). It should be noted that the intensity of the laser on the sample is equal to 0.32 W/cm\({}^{2}\) and this value remained constant for all analyses carried out in this same experimental setup[PERSON] We found that all obtained PL spectra are composed of two distinct emissions: A UV emission peak at 376 nm and a dominant and broad emission in the visible region ranging from green to red. The UV emission corresponding to the ZnO band gap transition is generally attributed to direct exciton recombination, where one of the excitons radically recombines to generate a photon; while the second emission in the visible range corresponds to defects and/or impurities in the structure and/or on the surface of ZnO nanostructures. In the visible region, three main emissions can be distinguished:  
* The green emission (G1), located in a wavelength range of \(\sim\)480-550 nm. This emission has been associated with the simply ionized oxygen vacancies (V\({}_{\text{O}}^{\circ}\)), or the oxygen anti-sites [20; 21], or the zinc vacancies (V\({}_{\text{Zn}}^{\circ}\)) [22]. * The yellow-orange emission (G2) has wavelengths of \(\sim\)550-610 nm, corresponds to the doubly ionized oxygen vacancies (V\({}_{\text{O}}^{\circ}\)) [23], and to the oxygen interstitials (O\({}_{\text{i}}\)) [21]. * Red emission (G3) (\(\sim\)610-750 nm) was attributed to oxygen excess on the ZnO surface, which may be induced by post-annealing [24]. In addition, red emission can also be attributed to zinc interstitial (Zn\({}_{\text{i}}\)) [23; 25]. From Figure 8a, we can note that the ZnO NWs and ZnO:Fe NWs samples have the same characteristic peak corresponding to the transition of the ZnO gap energy (centered at 376 nm) and also a wide emission in the visible range which represents various defects. We can notice that the intensity of PL relative to the emission of defects decreases with the introduction of iron into the ZnO NWs. This may be due to the fact that the excited electrons will react with the defects of the ZnO:Fe NWs structures, thus resulting in a decrease in the emission relative to the defects. [PERSON] et al. [26] also carried out a PL analysis on ZnO films doped at different iron concentrations. They found that as the concentration of Fe in the films increases, the peak intensity of UV emission increases, while the visible emission intensity decreases. The result suggests that the crystal lattice of Fe-doped ZnO films exhibits fewer interstitial oxygen and oxygen vacancies compare to undoped ZnO. In order to make a comprehensive understanding of the PL evolution, the PL spectra were decomposed by using three Gaussian components: Green (G1), yellow-orange (G2) and red (G3) spectral region (Figure 8b,c), usually suggested in literature and studied in our previous work [27]. After the deconvolution of the PL spectra on the visible emission related to the defects for the undoped and Fe-doped ZnO NWs (Figure 9), we observed a decrease in the intensity of both of G1 and G2 components, related to the oxygen vacancies and the oxygen interstitials, respectively.