## 4 Conclusions  
In summary, this study demonstrates that ZnO films produced via a dip-coating sol-gel method exhibit enhanced photocatalytic efficiency when functionalized with palladium photo-fixed under UV light illumination. The ability of Pd to trap photoinduced electrons-hole pairs and reduce their recombination at the ZnO surface is the possible contribution to the enhanced photocatalytic activity of Pd/ZnO.  
Additionally, we show that photodeposition is a flexible functionalization route for ZnO-based photocatalysts and surfaces with the ability to adjust the Pd cocatalyst chemical state and coverage by altering the photodeposition UV dose. In all cases, the Pd/ZnO thin film photocatalysts exhibited an increase in doxycycline (DC) removal activity, which was found to be proportional to the photodeposition UV dose.  
The 2.5 Pd/ZnO sample, which was photodeposited at 2.5 J/cm\({}^{2}\), exhibited a 20% increase in DC photooxidation activity, when compared to the pristine ZnO case, which increased to 45% when the dose was increased to 5 J/cm\({}^{2}\) in the 5 Pd/ZnO case and up to 200% upon further increase to 10 J/cm\({}^{2}\) in the 10 Pd/ZnO sample, which exhibited  
Figure 13: Cycling stability 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 (**a**) distilled water and (**b**) tap water.  
the greatest efficiency for doxycycline photodetegradation in both distilled and tap water under UV light. This observation can be correlated to the higher percentage of metallic Pd\({}^{0}\) photodeposited on the ZnO surface at doses higher than 2.5 J/cm\({}^{2}\). Interestingly, in all cases, the photocatalytic activity of the Pd/ZnO catalysts was higher in tap water, which could be associated with the presence of small amounts of carbonate species, possibly leading to the formation of carbonate radicals--a clarification of which could be reserved for future studies, but nevertheless underscores the potential of ZnO-based photocatalysts for remediation of contaminated water in real conditions.  
Ultimately, this study presents photodeposition as a promising method for developing Pd/ZnO as high-performance photocatalysts, with potential for application to other wide band-gap semiconductors through similar noble metal modifications. It should be noted additionally that the ability to surface decorate ZnO with metallic Pd particles is of interest not only for the application of ZnO-based photocatalysts for environmental remediation, but also for the fabrication of various ZnO-based sensing devices. Thus, photodeposition of Pd or other noble metal cocatalyst could be used as a flexible route for the fabrication of ZnO-based sensing and optoelectronic devices, functional layers for energy conversion applications, and others technologies that can benefit from this tunable and flexible process allowing for metallic particles to be afforded under mild, wet-chemical conditions.  
Conceptualization, N.K.; methodology, N.K.; dermal analysis, N.K.; investigation, D.I., H.K. and B.S.; data curation, N.K.; writing--original draft preparation, N.K.; writing--review and editing, N.K. and B.S.; visualization, D.I., N.K., H.K. and B.S.; supervision, N.K. All authors have read and agreed to the published version of the manuscript.  
The authors are grateful to financially supported by the Bulgarian NSF project KP-06-N59/11 (KII-06-H59/11).  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflicts of interest.