# Promising Cr-Doped ZnO Nanorods for Photocatalytic Degradation Facing Pollution
## 4 Photocatalytic Properties
### Mechanism of the Photocatalytic Activity  
The photocatalytic activity (PAC) of the synthesized Cr-doped ZnO nanorods was inspected under UV-vis irradiation. During this process, the photocatalyst absorbs the irradiated light to generate electron hole (e\(-\)/h\({}^{+}\)) pairs. Indeed, the irradiation on its surface may have an equal or higher energy than the studied Cr-doped ZnO nanorods band gap resulting in (e\({}^{-}\)/h\({}^{+}\)) pairs. A new charge redistribution appears where the generated (e\({}^{-}\)/h\({}^{+}\)) pairs are on the photocatalyst surface. This will free the methyl orange (MO) molecules via a redox reaction. The effect of dopant concentration is very important in describing PAC. Different rates of the MO degradation for various time intervals under light irradiation were noticed. In particular, good degradation was observed for the pure ZnO sample, i.e., ZnO-Cr\({}_{0}\). This is related to excitation \(\lambda<\) 390 nm, which generates electron-hole pairs in ZnO photocatalysts [52]. These electron-hole pairs on the surface of the photocatalyst pass through a series of oxidation-reduction reactions affecting the aromatic rings of the MO molecules. This results in degradation of MO for the case of ZnO-Cr\({}_{0}\). However, specific ZnO catalysts showed effective degradation compared to ZnO-Cr\({}_{0}\). ZnO-Cr\({}_{1}\) showed a maximum degradation efficiency of 99.8% of the MO in 100 min. Here, the photocatalytic process involves an advanced chemical reaction, ensuring MO degradation, as denoted above. The generated holes in the valence band of the Cr-doped ZnO nanorods can react with surface OH- groups to yield the highly reactive hydroxyl radical (OH\(\bullet\)). At the same time, electrons remaining from the (e-/h+) pair will interfere with local oxygen adsorbed at the surface of the nanoparticles and produce the superoxide (O\({}_{2}^{\bullet-}\)). Therefore, as oxygen vacancies increase, better absorption of O\({}_{2}\) will occur. O\({}_{2}\) is also converted into superoxide radicals (O\({}_{2}^{\bullet-}\)) through its interaction with the photoinduced electrons on oxygen defects via an ionisation process [53]. These O\({}_{2}^{\bullet-}\) and OH\(\bullet\) oxidizing radicals decompose and mineralize the adsorbed organic compounds into CO\({}_{2}\), H\({}_{2}\)O and different minerals. Such a complex chemical process ensures a proper photodegradation of the methyl orange molecule bonded to Cr-doped ZnO nanorods [54; 55]. In addition, the local Cr\({}^{2+}\) ions reinforce the interstitial oxygen defect effect, where they trap electrons and separate the charges to prevent their recombination. Nevertheless, ZnO-Cr\({}_{3}\) and ZnO-Cr\({}_{5}\) display opposite photodegradation responses despite the increase in the dopant concentration. The degradation efficiency reached 58.97% and 50.1%, respectively, for ZnO-Cr\({}_{3}\) and ZnO-Cr  
Figure 7: Kinetic study of the photocatalytic degradation of MO, 10 ppm in aqueous solution, using Cr-doped ZnO nanorods.  
after 120 min. Such a reduction could be related to the increased density of Cr interstitials compared to the Cr substitutional in the ZnO lattice, which will act as recombination centers for the photogenerated e-/h+ pairs and, consequently, reduce the generation process of OH\(\bullet\) and O\({}_{2}^{\bullet-}\) radicals [43]. Thus, ZnO-Cr\({}_{1}\) is the optimal doping giving the best photodegradation efficiency of MO compared to the other synthesized catalysts.