# Promising Cr-Doped ZnO Nanorods for Photocatalytic Degradation Facing Pollution
###### Abstract  
Chromium (Cr)-doped zinc oxide (ZnO) nanorods with wurtzite hexagonal structure were prepared through a thermal decomposition technique. The concentration effect of the Cr doping on the structural, morphological, and optical properties of the ZnO nanorods was established by correlating various measurements: transmission electron microscopy (TEM), photoluminescence (PL), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and several UV-visible studies. The obtained nanorods were investigated as photocatalysts for the photodegradation process of methyl orange (MO), under UV-vis light illumination. Different weights and time intervals were studied. A 99.8% photodegradation of MO was obtained after 100 min in the presence of 1 wt.% Cr III acetate hydroxide and zinc acetate dehydrate \"ZnO-Cr\({}_{1}\)\". The kinetic rate constant of the reaction was found to be equal to 4.451 \(\times\) 10\({}^{-2}\) min\({}^{-1}\) via a pseudo-first order rate model. Scavenger radicals demonstrated the domination of OH\({}^{\bullet}\) radicals by those of O\({}_{2}^{\bullet}\) superoxide species during the photodegradation. The interstitial oxygen site O\({}_{i}\) is proposed to play a key role in the generation of holes in the valence band under visible irradiation. The ZnO-Cr\({}_{1}\) photocatalyst displayed good cycling stability and reusability. ZnO nanorods; Cr-doping; oxygen vacancies; charge separation; photocatalysis +
Footnote †: journal: Applied  
The innovative physical properties associated with ZnO nanostructures have attracted a lot of interest due to their good performances in various applications. The size and shape of ZnO nanostructures play a major role in generating new application domains [16]. This greatly affects the physical properties like photoluminescence, transmittance and conductance as well as the photocatalytic activity and potential window measurements. Indeed, zinc oxide (ZnO) displays distinctive characteristics such as: large band gap, elevated chemical stability, facile synthesis, high excitonic energy, good surface reactivity, photosensitivity and non-hazardous nature [11; 16; 17]. This expands its use as a photocatalyst in the photodegradation process of pollutants. Nevertheless, there are some limitations to its application as a photocatalyst, such as the high probability of the recombination process between photoinduced electrons and holes [17]. Previous studies have aimed to ameliorate the separation rate of the photo-induced charge carriers via changing the electronic properties of ZnO, i.e., the band structure. This was achieved through metallic ion doping [17; 18; 19]. A slight doping using metallic ions enables energy traps formation through reserving the host phase-structure and the original vital properties of the photocatalysts. The increase in the trap centers and the transfer of photoinduced charge carries of the pollutant will accelerate the photodegradation process [20]. Such morphological and structural changes greatly affect its properties, leading to new electrochemical applications. Metal ion doping remains one of the major factors affecting the photocatalytic response of ZnO. Further factors have a significant impact on the photocatalytic response of ZnO such as the morphology control of the photocatalyst. The proper determination of the adequate size and shape of photocatalysts is essential, to optimize their properties and give better results. This requires a non-toxic, low cost, high yield synthesis technique of photocatalyst semiconductor materials. The thermal decomposition method using zinc acetate dihydrate is one of these promising techniques. It is a reasonable, facile, cost effective, highly yielding and rapid method for pure ZnO nanocrystals production [21; 22]. This technique has demonstrated its efficiency in earlier studies [23; 24; 25]. Indeed, nickel-doped ZnO nanorods have been successfully synthesized by thermal decomposition technique and introduced into a spintronics application [26].