# Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification
## 3 Photocatalytic Activity Study and Discussion
Photo-degradation spectra with undoped ZnO NWs and without ZnO NWs were systematically recorded in parallel for each dye in order to compare with Fe-doped ZnO NWs. Figure 5 summarizes the spectra of photo-degradation for undoped and iron-doped ZnO NWs for the three understudied organic dyes. One can note that the absorption peak for each dye decreases progressively in intensity with the light illumination time, indicating a decrease in the dye concentration in the solution. It worth noting that the photo-degradation time of 180 min is not a time limit; it is more a pre-fixed time for all our photo-degradation process in this study. Figure 6 shows a comparison of the photo-degradation rate evolution of the three dye solutions in the presence of ZnO:Fe NWs, ZnO NWs and without ZnO under a UV lamp exposure for 180 min. According to the obtained results, one can note that the photo-degradation rate of the various dyes (MB, MO and AR14) increases with light irradiation time in the presence of the photocatalyst (undoped and doped ZnO NWs). The photocatalytic activities of ZnO:Fe NWs (with 1% Fe-doping) showed to be more efficient compared to those of ZnO NWs. Using the ZnO:Fe NWs, the photo-degradation rate of MO was improved by 20% compared to a photo-degradation rate with undoped ZnO NWs, going from 49% to 69% after 3 h of illumination. It should be emphasized that due to the high energy of 4.33 eV of the main N=N bond in MO molecule, MO is one of the most difficult dyes to degrade compared to the others dyes such as MB and AR14. Although the AR14 dye also contains the N=N bond, the attracting effect of the OH\({}^{\bullet}\) radicals on the aromatic ring creates a deficit on the carbon atom in the C-N= bond, which further weakens the C-N bond. The photo-degradation mechanism has been proposed for each dye in our previous work [14] under the same experimental conditions. Iron doping also improved the photocatalytic efficiency for the MB and AR14 depollution, their photo-degradation rate increases from 86% to 95% for MB and from 93% to 98% for AR14. Moreover, since the nanowire density is similar (Figure 2) in two cases, thus, we can conclude that the Fe-doped ZnO NWs have greater photocatalytic activities than the undoped ZnO NWs. Photocatalytic activity of undoped ZnO is generally attributed to two kinds of states: (1) The donor states formed by the large number of defect sites such as oxygen vacancies and interstitial zinc atoms; (2) the acceptor states formed by zinc vacancies and interstitial oxygen atoms. [PERSON] et al. [17]  
Figure 4:4 Evolution of ZnO NWs photocatalysis UV-Vis absorption spectra of MB (**a**); MO (**c**) and AR14 (**e**); and of ZnO:Fe NWs photocatalysis UV-Vis absorption spectra of MB (**b**); MO (**d**) and AR14 (**f**). assume that the interfacial electron transfer takes place predominantly between these donor states (oxygen vacancies and interstitial Zn atom) and Methylene Blue. Being a cationic dye, Methylene Blue acquires electrons from excited donor states and decomposes. This faster degradation rate of Methylene Blue under visible light irradiation using Fe-doped ZnO is attributed to the increase in defect sites caused by Fe-doping, leading to an enhanced optical absorption in the visible region. In fact, the Fe-doping of ZnO NWs allows the substitution of a few Zn atoms by Fe atoms, resulting in the creation of other energy levels above VB and below CB of ZnO (Figure 7) [18], leading to a reduction of the gap, then allowing the semiconductor to harvest more photons during the photocatalysis process. In addition, the introduction of iron leads to the formation of additional defects in the ZnO crystal lattice [19], which can become electron traps, reduces the recombination probability of the electron-hole pairs and leads to an acceleration of the formation of hydroxyl radicals.