# Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification
## 1 Introduction  
In recent years, extensive fundamental research and development of the applied processes have been devoted to the polluted water treatment. Apart from the classical processes such as adsorption, filtration and sedimentation, the photocatalysis of semiconductor materials has attracted much attention as a promising solution for environmental problems. The photocatalysis process presents the advantage of decomposing and mineralizing a wide range of organic water pollutants into harmless products (CO\({}_{2}\), H\({}_{2}\)O, etc.). The water pollution caused by various dyes originating from textile, pharmaceutical and food-processing industries is becoming critical due to their use in large quantities. The World Health Organization (WHO) has set water quality at the top of its priority action list. In order to reduce the damage caused by organic dye pollution to the ecosystem and human health, the use of photocatalysts to degrade or to convert them into harmless chemical organic compounds in contaminated water has been extensively studied in recent years. On the other hand, due to its very interesting physicochemical properties, ZnO has been largely studied for water purification in recent years [1; 2]. Indeed, ZnO is a non-toxic and biocompatible semiconductor with high photosensitivity, and is relatively easy to synthesize at a low cost. Having the good physical and chemical stability, ZnO possesses a large band gap of 3.37 eV and a high exciton binding energy of 60 meV. Thus, ZnO presents all environmentally friendly features for photocatalytic applications [3; 4]. Generally, in the photocatalytic process, photo-generated holes which have a high oxidation power in the valence band (VB) and photo-generated electrons which have sufficientreduction power in the conduction band (CB) are required for efficient photocatalytic reactions. However, the photocatalytic efficiency of ZnO is limited by the very short lifetime (\(\sim\)0.322 ns) of the photo-generated charge carriers (electron-hole pairs) in semiconductors due to the high probability of their recombination [5]. The other lack in the use of the undoped ZnO as photocatalyst is its wide band gap, thus the semiconductor can be activated only under ultraviolet (UV) light irradiation, which limits its practical applications. In consequence, to promote the photocatalytic performance of the semiconductor, it is not only necessary to increase the lifetime of charge carriers, but also to extend absorption edge up to the visible light region. To expand the light absorption spectrum and to improve the solar energy utilization efficiency, many strategies have been exploited to modify the oxide semiconductor band gap structure in order to increase its sensitivity to visible light [6; 7; 8; 9]. One of the effective methodologies to increase the photocatalytic activities of ZnO, by increasing the visible-light absorption, is the doping of the ZnO nanostructure with transition-metals [10; 11], due to the band gap narrowing resulting from the creation of dopant energy levels below the CB [12; 13]. This technique has, on the one hand, the advantage of effectively preventing the recombination of photo-generated electron-hole pairs; and on the other hand, can extend the photocatalysis' spectral range, leading to the improvement of the semiconductor photocatalytic activities. In spite of many transition metals having been reported in the literature, such as Co, Cr, Ag, Sn etc., in our work, we focused on the iron doping of the ZnO, because iron is a transition metal of very little negative impact on the environment. Nevertheless, the morphology of the photocatalyst plays an important role in the photocatalytic activity. The structure of the nanowire arrays has the advantage of a large specific surface with high surface to volume ratio which has prompted us to focus our research on the iron-doped ZnO nanowires (ZnO:Fe NWs).