## 1 Introduction  
Zinc oxide (ZnO) is a widely studied, wide-bandgap semiconductor photocatalyst with a high exciton binding energy of 60 meV and a direct bandgap of 3.37 eV, making it promising for photocatalytic applications [1]. ZnO-based photocatalysts have gained significant attention due to their desirable characteristics, including high chemical stability,non-toxicity, and availability [2]. Consequently, ZnO has been widely used for the effective removal of organic pollutants in numerous studies [3; 4; 5; 6]. Despite its advantages, like many semiconductor photocatalysts, ZnO exhibits a poor quantum yield due to recombination losses, necessitating its functionalization and modification to enhance its photocatalytic properties for environmental applications. Various modification strategies, including ZnO doping [7; 8; 9], the synthesis of composite photocatalysts [10; 11; 12; 13], and altering ZnO's structural properties through different synthesis techniques [14; 15; 16], have been explored to improve its photocatalytic efficiency. A range of synthesis methods has been developed to produce ZnO in various forms, as its crystal size, orientation, morphology, aspect ratio, and crystalline density strongly influence its photocatalytic properties [17; 18]. Among these methods, the sol-gel process is especially attractive due to its high homogeneity, low processing temperature, easy composition control, capability for large-area coatings, and use of inexpensive equipment. The sol-gel method also yields catalysts with favorable optical properties, making it one of the most popular synthesis techniques in the literature [19]. The incorporation of noble metal cocatalysts and dopants is frequently used to mitigate recombination losses in ZnO, enhancing its quantum yield and photocatalytic activity [20; 21]. Metals such as Au, Ag, and Pt have been effectively used to modify ZnO and improve its photocatalytic performance [21; 22; 23; 24; 25]. However, studies on the impact of palladium (Pd) doping or surface functionalization on ZnO photocatalytic layers synthesized via the sol-gel method remain limited. Surface-bound Pd cocatalysts are expected to enhance the photocatalytic activity of ZnO by trapping photoinduced charge carriers, allowing photogenerated electrons to pass through the Schottky-type junction formed between Pd and ZnO and, thus, preventing charge recombination. However, since Pd\({}^{2+}\) ions (ionic radius 0.080 nm) are similar in size to Zn\({}^{2+}\) ions (0.074 nm), the addition of Pd precursors directly to the ZnO sol-gel often leads to doping, which can have a detrimental effect on ZnO activity at higher Pd concentrations [26]. In contrast, surface modification with separate metallic Pd phases has generally been found more beneficial [27]. Therefore, precise control over the location of the Pd cocatalyst is essential for optimizing photocatalytic activity. Surface functionalization of ZnO with Pd has been widely demonstrated in the literature and is typically achieved through advanced vacuum-assisted methods such as atomic layer deposition [28], chemical vapor impregnation [29], or magnetron sputtering [30]. However, a promising route for the practical wet-chemical functionalization of ZnO materials with noble metal nanoparticles is photodeposition, or photofixation, which employs the intrinsic photocatalytic activity of ZnO. In this process, photogenerated electrons from UV illumination of ZnO directly reduce a Pd\({}^{2+}\) precursor, forming metallic Pd\({}^{0}\) on the ZnO surface [31; 32]. Photodeposition, where Pd\({}^{2+}\) is reduced to metallic Pd via the photocatalytic activity of ZnO, is expected to yield Pd/ZnO photocatalysts with enhanced activity due to the exclusive localization of Pd on the ZnO surface. However, this functionalization route remains underexplored for ZnO, and there is little information on how controllable parameters such as the UV light dose during photodeposition can influence the properties of the resulting Pd/ZnO photocatalyst.