# Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification
## 3 Photocatalytic Activity Study and Discussion  
The principle of photocatalysis consists in using photon energy to activate the catalyst; this phenomenon is initiated by electron-hole pairs after a photo-excitation in the semiconductor. Indeed, when a photocatalyst is illuminated by light having photon energy larger than the semiconductor band gap, the electrons (e\({}^{-}\)) of the VB can pass to vacant orbitals of the CB leaving an equal number of holes (h\({}^{+}\)) in the VB (Equation (1)). If the free charges (electrons and holes) migrate to the surface of the semiconductor without recombining, then they can participate in various oxidation and reduction reactions with adsorbed species such as water, oxygen and other organic species by generating the hydroxyl radicals OH\({}^{\bullet}\) which possess a high oxidation power. These successive reactions of oxidation and reduction of the photocatalytic mechanism can be described by the following equations:  
\[\text{ZnO}+\text{h}\
u\rightarrow\text{ZnO}+\text{e}^{-}+\text{h}^{+} \tag{1}\]  
Figure 4: (**a**) UV-Visible absorption spectra of undoped and Fe-doped ZnO NWs; (**b**) Tauc plot showing a narrowing of ZnO NWs band gap after iron doping. Figure 3: X-ray diffraction (XRD) spectra of the nanowire arrays obtained by hydrothermal method both for undoped ZnO NWs and 1% Fe-doped ZnO NWs, respectively. \[\mathrm{H_{2}O+h_{VB}}^{+}\rightarrow\mathrm{OH^{\bullet}+H^{+}} \tag{2}\]  
\[\mathrm{OH^{-}+h_{VB}}^{+}\rightarrow\mathrm{OH^{\bullet}} \tag{3}\]  
\[\mathrm{O_{2}+e_{BC}}^{-}\rightarrow\mathrm{O_{2}^{\bullet-}} \tag{4}\]  
\[\mathrm{O_{2}^{\bullet-}+H^{+}\rightarrowHO_{2}^{\bullet}} \tag{5}\]  
\[2\mathrm{HO_{2}^{\bullet}\rightarrow H_{2}O_{2}+O_{2}} \tag{6}\]  
\[\mathrm{HO_{2}^{\bullet}+O_{2}^{\bullet-}\rightarrow HO_{2}^{\bullet-}+O_{2}} \tag{7}\]  
\[\mathrm{HO_{2}^{\bullet-}+H^{+}\rightarrow H_{2}O_{2}} \tag{8}\]  
Figure 4:0  
Figure 4:1  
Finally, these \(\mathrm{OH^{\bullet}}\) radicals having a strong oxidation power, can lead to a partial or complete mineralization of the organic substances (such as the dyes, for example) in the waste water, forming non-toxic products such as \(\mathrm{CO_{2}}\) and \(\mathrm{H_{2}O}\), as shown in Equation (11). Figure 4:2  
In our study, the photocatalytic efficiency of Fe-doped ZnO nanowires was studied on three organic dyes (MB, MO, and AR14) which are the main components of textile, food-processing and pharmaceutical industrial waste materials. The study of their photo-degradation was carried out under a combined Mercury-Xenon UV-Visible combined lamp (HAMAMATSU-LC8) (Hamamatsu Photonics, Shizuoka, Japan) with main irradiation wavelength of 365 nm and an irradiation intensity of 4500 mW/cm\({}^{2}\) under a normal incidence on the out level of the lamp. The sample of ZnO:Fe NWs grown on the Si substrate, has an effective surface area of about 1 cm\({}^{2}\). The initial concentration of aqueous solution of each organic dye was 10 uM. The sample of ZnO NWs was placed at the bottom of the container with a fixed dye solution volume of 50 mL. The distance between the UV lamp and the sample surface was maintained at 10 cm for all photocatalysis experiments as shown in our previous work [14]. It is worth noting that during the photocatalytic reaction, there is no stirring in the solution for this study. The evolution of the photocatalytic reaction was followed by an UV-Vis spectrophotometer, by periodically recording the absorption spectra in order to monitor the concentration change of the residual organic dye in the solution. The residual dye content in the solution was determined by a degradation percentage defined by the following relation:  
Figure 4:3  
where \(A_{0}\) is the initial dye absorbance, at the maximum absorption wavelength of the dye, before illumination and \(A_{t}\) is the dye absorbance at time \(t\). The characteristic absorption peaks in UV-Vis spectroscopy corresponding to the molecules MB, MO and AR14 are localized at 665, 464 and 515 nm, respectively.