# Enhancing Methyl Orange Degradation with Laser-Generated ZnO and Ce-Doped ZnO Nanoparticles
## 3 Results and Discussion
### The Mechanisms of Action  
Figure 10 illustrates the proposed mechanisms responsible for the enhanced photocatalytic activity observed in undoped ZnO NPs and 1% Ce-doped ZnO NPs. In the case of undoped ZnO NPs, photocatalytic activity is achieved through oxidation and reduction reactions initiated by electromagnetic irradiation, generating electron-hole pairs on the surface of the ZnO NPs. The presence of photogenerated electrons in the conduction band (CB) with sufficient reduction power, and photogenerated holes in the valence band (VB) with high oxidation power, are essential for efficient photocatalytic reactions [5; 11]. During photocatalysis, charge carriers such as electrons (e\({}^{-}\)) and holes (h\({}^{+}\)) are produced in CB and VB, respectively, in a photocatalyst upon light irradiation. These photogenerated holes have the ability to oxidize water molecules within the photocatalytic system or hydroxyl ions adsorbed on the catalyst's surface, resulting in the formation of hydroxyl free radicals (\(\bullet\)OH). Simultaneously, the photogenerated electrons interact with adsorbed oxygen molecules to produce superoxide anion radicals [43]. However, identifying the major contributor between ROS species is very critical. The generation of ROS and their involvement in the oxidation of MO primarily relies on the appropriate potential of the band edges inherent in a photocatalyst. Hence, the following equations have been employed to compute the band-edge potentials:  
\[E_{CB}=X-E^{e}-\frac{1}{2}E_{g} \tag{12}\]  
\[E_{VB}=E_{CB}+E_{g} \tag{13}\]  
where \(X\) is the geometric mean of the absolute electronegativity of the constituent atoms, \(E^{e}\) is the energy of free electrons on the hydrogen scale (-4.5 eV), \(E_{g}\) is the photocatalyst's band gap, \(E_{CB}\) is the conduction band potential, and \(E_{VB}\) is the valence band potential. Moreover, the Pearson absolute electronegativity values for Zn and O were 4.45 eV and  
Figure 9: Adsorption capacity of MO dye with time before and after annealing for (**a**) 1% Ce/ZnO NPs and (**b**) ZnO NPs photocatalysts, in the dark and under UV and sun radiation, respectively. 7.54 eV, respectively. The \(E_{g}\) values of ZnO estimated using a Tauc plot are 3.7 eV. As a result, the calculated \(E_{CB}\) and \(E_{VB}\) values of ZnO were \(-\)0.56 eV and +3.14 eV, respectively. The calculated energy values of the respective bands indicate that the \(E_{CB}\) of ZnO is more negative than the reduction potential of O\({}_{2}\) to O\({}_{2}\bullet^{-}\) (\(-\)0.28 eV). Therefore, electrons in the CB of ZnO are capable of producing superoxide anion radicals. The system has electrons in the CB (\(-\)0.56 eV) for O\({}_{2}\bullet^{-}\) generation and holes in the \(V_{B}\) (+3.14 eV) of ZnO for \(\bullet\)OH generation. Furthermore, the valence band maximal positions of ZnO were 3.14 eV [43; 44]. On the other hand, for Ce-doped ZnO NPs, a distinct mechanism comes into play. It involves Ce\({}^{4+}\) ions absorbing photoexcited electrons from the ZnO CB and reducing to Ce\({}^{3+}\) ions. These Ce\({}^{3+}\) ions are then oxidized by adsorbed O\({}_{2}\) molecules, leading to the generation of superoxide radicals. These radicals, along with hydroxyl radicals, participate in the degradation of MO, resulting in the production of CO\({}_{2}\) and H\({}_{2}\)O. The presence of Ce\({}^{4+}\) on the surface of the ZnO NPs acts as an electron scavenger, and the O\({}_{2}\bullet^{-}\) radical anions further contribute to the formation of \(\bullet\)OH radicals, which are responsible for the degradation of the dye. The slight reduction in the band gap energies observed in Ce-ZnO can be attributed to the presence of Ce ion-4f orbitals, enabling charge transfer between the VB and CB. In photocatalytic reactions, band gap energy plays a crucial role, and the generation of hydroxyl radicals is primarily governed by this energy level [5; 11; 26; 45]. The photocatalytic efficiency of 1% Ce-doped ZnO was compared with other reported photocatalysts in the existing literature (Table S5).