## 3 Results and Discussion
### Photocatalytic Efficiency of Pure and Palladium-Modified ZnO Films  
The photocatalytic activity of ZnO and Pd-modified ZnO films (2.5 Pd/ZnO, 5 Pd/ZnO, and 10 Pd/ZnO) was evaluated through doxycycline (DC) degradation in distilled and tap water under UV illumination. Figure 7a presents the kinetic plots of DC removal in distilled water, where normalized doxycycline concentrations at time \(t\) and the initial concentration are represented by \(C\) and \(C_{i}\), respectively. The DC degradation by ZnO and Pd/ZnO films under UV was modeled using pseudo-first-order kinetics to obtain the reaction rate constant \(k\), as shown in Equation (4), a common approach for modeling photocatalytic degradation rates [38; 39; 40]:  
\[-ln\frac{C}{C_{i}}=kt, \tag{4}\]  
where \(k\) (h\({}^{-1}\)) denotes the apparent reaction rate constant, which can be estimated from the \(-ln(C/C_{i})\) vs. \(t\) plot, as shown in Figure 7a. The results reveal that the base DC removal rate constant, \(k\), of the pristine ZnO photocatalyst is 0.044 h\({}^{-1}\). In all of the Pd-functionalized ZnO thin films, a beneficial effect of functionalization was observed with a proportional increase in \(k\) as a function of the UV photodeposition dose to 0.0523 h\({}^{-1}\) in the 2.5 Pd/ZnO case, 0.064 h\({}^{-1}\) for the 5 Pd/ZnO case, and the 10 Pd/ZnO film exhibiting the highest rate constant, \(k\) = 0.0916 h\({}^{-1}\), which is a 200% increase in activity compared to the unmodified ZnO catalyst. The \(C/C_{i}\) ratio of DC degradation in distilled water over four hours, shown in Figure 7b, also demonstrates that the film with the highest UV dose (10 Pd/ZnO) achieved the greatest degradation efficiency. After 240 min under UV, the pure ZnO film achieved approximately 8% degradation, while the 2.5 Pd/ZnO, 5 Pd/ZnO, and 10 Pd/ZnO films achieved approximately 11.72%, 23.03%, and 31.52% degradation, respectively. A similar trend was observed in the DC degradation experiments conducted in tap water, with pseudo-first-order kinetics and removal curves shown in Figure 8a,b. Again, Pd-modified films showed higher activity than pure ZnO, with the 10 Pd/ZnO film reaching the highest activity and a rate constant of \(k\) = 0.1253 h\({}^{-1}\), which is 284% higher than that of the pristine ZnO case. Notably, all four photocatalysts performed better in tap water than in distilled water. The ZnO film achieved 16.36% DC removal, while the 2.5 Pd/ZnO, 5 Pd/ZnO, and 10 Pd/ZnO films reached 27.68%, 40.81%, and 69.09% removal, respectively, as shown in Figure 9, which shows that the increase in activity in tap water is more than 200%, compared to the distilled water case. Figure 8: Plots of (**a**) pseudo-first-order kinetic plot for DC removal; (**b**) degradation rate over time for pristine ZnO and Pd/ZnO films functionalized at UV doses of 2.5 J/cm\({}^{2}\) (2.5 Pd/ZnO), 5 J/cm\({}^{2}\) (5 Pd/ZnO), and 10 J/cm\({}^{2}\) (10 Pd/ZnO) in tap water. Figure 7: Plots of (**a**) pseudo-first-order kinetic plot for DC removal; (**b**) degradation rate over time for pristine ZnO and Pd/ZnO films functionalized at UV doses of 2.5 J/cm\({}^{2}\) (2.5 Pd/ZnO), 5 J/cm\({}^{2}\) (5 Pd/ZnO), and 10 J/cm\({}^{2}\) (10 Pd/ZnO) in distilled water. The increased activity observed in Pd-modified ZnO films can be attributed to several factors. Palladium on the ZnO surface likely acts as an electron sink, promoting interfacial charge transfer between Pd and ZnO, which reduces electron-hole recombination and extends electron-hole pair lifetimes. These effects, enhanced by the Schottky barrier formation at the Pd/ZnO interface, are similar to those observed in other noble metal-modified ZnO photocatalysts [20; 41; 42]. Additionally, the 10 Pd/ZnO films exhibited a highly developed and uniform surface structure, as confirmed by SEM and EDS analysis. Higher UV photodeposition doses increased both the metallic Pd\({}^{0}\) content and Pd dispersion on the ZnO surface, enhancing the photocatalytic performance of 5 Pd/ZnO and 10 Pd/ZnO films compared to 2.5 Pd/ZnO.