# Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification
## 2 Fe-Doped ZnO Nanowire Synthesis  
The synthesis of the Fe-doped nanowires was carried out by two-step hydrothermal method: The buffer layer consisting of 2 g of polyvinyl alcohol (PVA) and 0.25 g of zinc acetate dihydrate (Zn(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O) dissolved in water, was firstly spin-coated on the substrate as described in our previous work [14]; then the growth of ZnO:Fe NWs was carried out in the solution containing 25 mM of zinc nitrate (Zn(NO\({}_{3}\))\({}_{2}\)), 12.5 mM of Hexamethylenetramine (HMTA), and 0.25 mM of Iron (III) chloride (FeCl\({}_{3}\)) at 95 \({}^{\circ}\)C. The process is similar to un-doped ZnO NWs growth [14], except for the addition of iron (III) to the growth solution.  
Figure 1 shows a schema of the Fe-doped ZnO nanowire formation. This synthesis process is valid both for undoped and doped ZnO nanowire growth except the adding of FeCl\({}_{3}\) in the growth solution for Fe-doping.  
The plane-view and cross-section SEM images of the undoped and Fe-doped ZnO NWs (Figure 2), with the same growth time of 4 h and the same growth temperature of 95 \({}^{\circ}\)C, showed that the high aspect ratio ZnO NWs were obtained and the morphology of the nanowires was not modified by the iron doping. From the SEM plane-view images, by using ImageJ software (Version 1.6.0, 2014) and by processing on ten different regions for each sample, we can estimate the nanowire density which was not significantly changed: 63 \(\pm\) 2 nanowires/\(\upmu\)m\({}^{2}\) for ZnO NWs vs. 61 \(\pm\) 2 nanowires/\(\upmu\)m\({}^{2}\) for ZnO:Fe NWs. Furthermore, the X-ray diffraction (XRD) measurements showed the typical ZnO Wurtzite structure with preferred growth direction towards the \(c\)-axis both for undoped and Fe-doped ZnO nanowires (Figure 3).  
UV-Visible spectroscopy analysis showed that the undoped and Fe-doped ZnO NWs exhibit a similar shape on the absorption spectra (Figure 4a). However, unlike undoped ZnO NWs showing a quasi-total transparency in the visible range, the UV-Visible spectrum of ZnO:Fe NWs exhibits a significant absorption in the visible range (400-600 nm). By using the Tauc plot [15], a red shift (narrowing of band gap) is observed for Fe-doped ZnO (Figure 4b). This red shift is the common result in the transition-metal doped ZnO, due to sp-d exchange interactions between the ZnO band electrons  
Figure 1: Schema of the ZnO nanowire formation: (a) Spin-coating of the seedlayer solution; (b) seedlayer based on the polyvinyl alcohol (PVA) and Zn(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O; (c) ZnO nanocristallites formation after an annealing at 500 \({}^{\circ}\)C during 3 h; (d) undoped and Fe-doped ZnO nanowire (ZnO NW) growth at 95 \({}^{\circ}\)C during 4 h.  
Figure 2: Plane-view and cross-section scanning electron microscopy (SEM) images of the undoped ZnO NWs (**a**,**c**) and Fe-doped ZnO NWs (**b**,**d**) obtained via hydrothermal method with growth time of 4 h at 95 \({}^{\circ}\)C.  
and the localized \"d\" electrons of the transition-metal doped in the semiconductor matrix [16; 17]. In this case, \"s-d\" and \"p-d\" exchange interactions lead to a negative correction to the CB edge and a positive correction to the VB edge of the ZnO matrix leading to a band gap narrowing. The band gap (\(E_{\text{g}}\)) values calculated by the [PERSON]'s equation are 3.29 and 3.25 eV, respectively, for ZnO NWs and ZnO:Fe NWs as shown in Figure 4b.