# Enhancing Methyl Orange Degradation with Laser-Generated ZnO and Ce-Doped ZnO Nanoparticles
## 3 Results and Discussion
### Photocatalytic Degradation of MO Dye  
The photocatalytic degradation of dyes was investigated using UV and sunlight exposure with dynamic photocatalysts. In Figure 2b, the absorption curve of the MO dye without a photocatalyst displayed a maximum peak (\(\lambda_{\text{max}}\)) at 466 nm. In contrast, Figure 6 illustrates the changes in absorbance bands at various time intervals for MO with the photocatalyst during irradiation, both before and after annealing. The decreasing maximal peaks observed in the absorbance spectra of ZnO NPs and 1% Ce/ZnO NP photocatalysts with increasing irradiation times indicate the degradation of MO and the formation of certain intermediate products. Notably, the absorption peaks corresponding to the dye were found in the wavelength range of 350-550 nm, which is of particular interest during the photocatalytic process. The results presented in Figure 6 demonstrate the remarkable photodegradation and decolorization capabilities of the 1% Ce/ZnO NPs photocatalyst for MO dye under both UV and sunlight exposure, as evidenced by the consistent patterns observed in Figure 6a-d, where significant decolorization occurs within just 90 min of exposure. The absorption peaks exhibit a drastic decrease after 10 min, as shown in Figure 6c,d, 20 min (Figure 6a), and 30 min (Figure 6b). This rapid reduction indicates the effective degradation of MO dye. In contrast, the ZnO NPs in Figure 6e-h experience irregular and less efficient degradation, with the excitonic absorption peaks diminishing gradually after 10 min and nearly disappearing only after 90 min of UV and sunlight irradiation. Moreover, the absorption curves of ZnO NPs in Figure 6e-h exhibits a decrease of around 466 \(\pm\) 1 nm before and after annealing. Likewise, for 1% Ce/ZnO NPs, a small absorption peak between 300 and 400 nm becomes apparent. The results suggest that the 1% Ce/ZnO NPs photocatalyst is a promising material for efficient and consistent photodegradation of MO dye under both UV and sunlight exposure, outperforming the standard ZnO NPs in terms of regularity and efficiency in the degradation process. In Figure 7a,b, both photocatalysts show a significant increase in the decolorization of MO dye solutions when they are added within two minutes of adsorption/desorption equilibrium (0-30 min), as depicted on the left side of the graph. However, there are some notable differences between the two photocatalysts' behaviors. The photocatalytic efficiency consistently increases for 1% Ce/ZnO NPs, reaching its peak value at 120 min during 30-120 min. In contrast, the efficiency of ZnO NPs exhibits irregular and turbulent behavior between 30 and 60 min and continues to fluctuate until 120 min. Table 2 presents the photodegradation results of the MO dye using both ZnO NPs and 1% Ce/ZnO NPs photocatalysts (both annealed and non-annealed) under UV and sunlight. It is evident that ZnO NPs display superior efficiency when exposed to UV light compared to natural sunlight. Nevertheless, when subjected to sunlight, 1% Ce-doped ZnO NPs exhibit even higher efficiency, achieving peak degradation rates of 91% and 95% before and after annealing, respectively, surpassing the degradation rates observed under UV light (87% and 90%). Whether before or after annealing, the degradation efficiencies of 1% Ce-doped ZnO NPs under both UV and sunlight consistently outperform those of undoped ZnO NPs in all instances. The enhanced performance observed is attributedto the 1% Ce doping, which enhances photocatalytic activity by shifting the absorption band towards the visible light spectrum. This shift accelerates the degradation of MO as it becomes more responsive to visible light. When evaluating the degradation process in the absence of light (dark conditions), the superior rate of MO degradation in 1% Ce-doped ZnO NPs compared to undoped ZnO NPs within the initial 30 min can be ascribed to the interaction between the catalyst surface and MO molecules. This interaction promotes the cleavage of the azo bonds in MO, generating electrons in the process.