# Enhancing Methyl Orange Degradation with Laser-Generated ZnO and Ce-Doped ZnO Nanoparticles
## 3 Results and Discussion
### Characterizations
#### 3.2.3 Optical Properties of the Prepared Samples  
The optical profile of the nanoparticle (NP) solutions was analyzed using a UV-vis absorption spectrophotometer, and the absorbance was recorded from 200 nm to 800 nm, as depicted in Figure 5a. The comparison was made between 3D 1% Ce-doped ZnO spherical NPs, pure ZnO (precursor), and ZnO NPs (after CO\({}_{2}\) laser irradiation). The absorbance spectra of pure ZnO exhibited an absorption peak between 200 and 250 nm, signifying the absorption of incident photon energy by ZnO. However, both ZnO NPs and 1% Ce-doped ZnO NPs displayed an absorption edge shift into the visible spectrum, around 400 nm. This shift is attributed to the presence of doped Ce and the formation of smaller NPs in the case of 1% Ce-doped ZnO NPs. The exceptionally high absorption observed in the case of ZnO NPs can be attributed to both the UV-vis spectrophotometer settings chosen by the operator and the elevated concentration of the sample (0.5 M). However, upon diluting the sample to 50%, a distinct peak becomes evident at around 400 nm, as shown in Figure 5a (inset). Several factors, such as band gap, oxygen deficiency, NP size, structure, and impurity centers, can influence the absorption behavior. This behavior of ZnO NPs, as well as Ce/ZnO NPs, is expected to decrease the band gap (\(E_{g}\)) value that is determined from a Tauc plot based on the relation below:  
\[(\alpha\hbar\
u)1/\mathrm{n}=\mathrm{A}\ (\hbar\
u-E_{g}) \tag{11}\]  
where \(\hbar\
u\) denotes the incident photons energy, \(\alpha\) resembles the absorption coefficient, \(\mathrm{A}\) is a material constant, and \(\mathrm{n}=1/2\) is for the direct band gap ZnO semiconductor. The band gap energy (\(E_{g}\)) is obtained from the extrapolation of the linear portions with respect to the plots onto the x-axis. Other than that, plots of \((\alpha\hbar\
u)^{2}\) versus (\(\hbar\
u\)) are made for pure ZnO, ZnO NPs, and 1% Ce/ZnO NPs.  
As shown in Figure 5b, the band gap energy for pure ZnO (3.7 eV) was observed to be larger than that of ZnO NPs and 1% Ce/ZnO NPs (2.7 eV and 2 eV, respectively).  
Figure 5: (a) The optical absorbance spectra. The inset displays the 50% diluted ZnO NPs, and (b) the evaluation of \((\alpha\)E)^{2} versus E, to \(E_{g}\) for different samples: pure ZnO (black), ZnO NPs (red), and 1% Ce-doped ZnO NPs (blue).  
This demonstrates the impact of CO\({}_{2}\) laser power and wavelength on NP formation and band gap narrowing. Modifying Ce ions in Ce/ZnO proved advantageous for enhancing photocatalytic activity, as the \(E_{g}\) energy and hydroxyl radicals primarily influence photocatalytic reactions. Several crucial parameters, such as particle size, morphology, and defect concentration, influenced the value of \(E_{g}\). The crystalline sizes, lattice parameters, optical energy band gaps, and average particle sizes obtained from XRD, FESEM, and UV-vis spectroscopies are presented in Table S4. Samples with smaller particle sizes, such as pure ZnO, typically exhibited broader \(E_{g}\) values, whereas narrower \(E_{g}\) values were observed in smaller NP samples. Consequently, it can be inferred that doping played a significant role in reducing the \(E_{g}\) value.