## 3 Results and Discussion
### XPS Investigation of the Pd Photodeposition Products  
The efficiency of photocatalytic removal depends highly on the elements present on the surface, their concentrations, and their oxidation states. To investigate these parameters, X-ray photoelectron spectroscopy (XPS) was employed. Several samples were prepared to study variations in oxidation states and surface concentrations. To prevent surface changes, such as oxidation or contamination from air, each sample was introduced into the spectrometer's vacuum chamber within a minute of preparation.  
The XPS results are shown in Figure 6. Figure 6a presents the survey spectra for surfaces prepared with UV doses of 2.5 J/cm\({}^{2}\) (red), 5 J/cm\({}^{2}\) (green), and 10 J/cm\({}^{2}\) (blue). As expected, the most intense peaks correspond to zinc and oxygen, the primary components of the ZnO substrate. Low-intensity Pd3d photoelectron and Pd MNN Auger peaks indicate the presence of photodeposited palladium. Trace amounts of chlorine (Cl2p, Cl LMM) and carbon (C1s) suggest minor contamination or residuals from the preparation process.  
Figure 6: X-ray photoelectron spectra of X Pd/ZnO (where X represents the UV dose in J/cm\({}^{2}\)): (**a**) survey spectra at UV doses of 2.5 (red trace), 5 (green trace), and 10 J/cm\({}^{2}\) (blue trace); (**b**) high-resolution Pd 3d core-level spectra at UV doses of 2.5, 5, and 10 J/cm\({}^{2}\), with the Pd\({}^{0}\), Pd\({}^{2+}\), and Pd\({}^{4+}\) peaks indicated in green, blue, and magenta, respectively.  
Typically, survey spectra provide qualitative information on the elemental composition of the sample surface. For detailed information on oxidation states, high-resolution XP spectra are required. Figure 6b shows the variation in the oxidation states of palladium with increasing UV doses (2.5, 5, and 10 J/cm\({}^{2}\)). The analysis involved curve fitting of the Pd3d core-level spectrum, based on parametrized standard spectra for Pd3d core level peaks of Pd in oxidation states 0, 2+, and 4+, corresponding to metallic palladium, PdO, and PdO\({}_{2}\), respectively [35; 36; 37].  
The spectrum labeled 2.5 J/cm\({}^{2}\) (red) in Figure 6b represents the Pd 3d core level for the surface prepared at a UV dose of 2.5 J/cm\({}^{2}\). The line shape analysis reveals that this spectrum can be fitted with two components, representing Pd\({}^{2+}\) (blue curve) and Pd\({}^{4+}\) (magenta curve), with binding energies (BEs) at 337.1 eV and 339.1 eV, respectively. The calculated surface concentration of palladium is 1.61 at.%, with 0.27 at.% in the Pd\({}^{4+}\) state and 1.34 at.% in the Pd\({}^{2+}\) state, indicating that the photodeposition process at 2.5 J/cm\({}^{2}\) UV favors Pd\({}^{2+}\) formation.  
Increasing the UV dose to 5 and 10 J/cm\({}^{2}\) does not significantly raise the surface concentration of palladium but alters the Pd3d line shape. For these samples, as shown in Figure 6b, the curve fitting results (marked 5, green, and 10, blue) reveal the formation of metallic palladium (Pd\({}^{0}\)) with a BE of 335.3 eV (green curve), alongside Pd\({}^{2+}\) with a BE of 336.8 eV (blue curve). The calculated surface concentrations are 1.87 at.% (1.07 at.% Pd\({}^{0}\) and 0.80 at.% Pd\({}^{2+}\)) for the sample prepared at 5 J/cm\({}^{2}\) and 1.18 at.% (0.53 at.% Pd\({}^{0}\) and 0.65 at.% Pd\({}^{2+}\)) for the sample prepared at 10 J/cm\({}^{2}\).  
In conclusion, XPS analysis indicates that increasing the photodeposition time, and, thus, the UV dose, results in a shift in the oxidation state of palladium from PdO\({}_{x}\) species to metallic palladium particles, likely coated with a thin PdO layer.