## 1 Introduction  
The increasing amount of pigments and dyes existing in the wastewater coming from the dye manufacturing and textile finishing industry significantly affects the water quality and reduces the photosynthetic reaction of aquatic flora by preventing the sunlight to reach it. Also, some of these dyes are known to be toxic with carcinogenic effects [1; 2]. The dyes that accidentally arrive in the wastewater are very difficult to destroy using regular purification methods due to their high stability to oxidizing agents, light, and even to aerobic digestion [3; 4]. In recent years, various processes based on both chemical and physical principles have been used to treat the wastewater effluents containing different dyes, among which electrochemical methods, hypochlorite, or ozone oxidation could be mentioned. These methods are usually costly, non-damaging, and ineffective since secondary waste products may result in these processes [5]. Therefore, it is necessary to find new tools and methods for removing dyes from the water. Recently, fairly extensive research was concentrated on the development of new semiconducting materials for different applications, namely, photocatalysts for water disinfection and cleaning [6; 7; 8; 9; 10; 11]. Over time, many photocatalysts, such as TiO\({}_{2}\), ZnO, SnO\({}_{2}\), CdS, etc., have been applied for the decomposition of organic dyes and air pollutants [12; 13; 14]. Among them, TiO\({}_{2}\)-based materials have proved to have a very good catalytic activity due to their appropriate properties and large specific surface area that allow reactions to occur [15; 16]. This is the reason why TiO\({}_{2}\) is the most suitable semiconductor used in catalytic applications, such as the degradation of organic pollutants or oil spills from wastewater [17; 18]. Apart from TiO\({}_{2}\), the second well-known photocatalyst is ZnO, having a significant role in materials research due to its high stability, low price, and nontoxicity. However, both TiO\({}_{2}\) and ZnO semiconductors have some limitations, such as agglomeration of the smaller particles into significantly larger ones, poor response to visible-light, or fast recombination rate of the photogenerated holes and electrons [19; 20; 21]. These issues lead to a decreased photocatalytic efficiency that obstructs the potential practical applications of these photocatalysts in the degradation of contaminants in air and water. Several methods have been applied to overcome these problems; in the case of TiO\({}_{2}\), for instance, the photocatalysts can be dispersed in several clay minerals, including montmorillonite, saponite, or nanotubes, to solve the agglomeration problems [22; 23]. To increase the photocatalytic activity of ZnO, different approaches, such as reductive reactions [24; 25], coupling with metal oxides [26] or semiconductors, such as LaFeO\({}_{3}\)[27], metal or non-metal doping, e.g., Mg, Ni, Sn [28; 29; 30], or surface modification [31] have been developed. The photocatalytic performance of ZnO could also be enhanced by modifying the morphology through different synthetic methods, like electrochemical deposition [32], electrospinning [33], vapor-phase synthesis [34], they have been used for the synthesis of ZnO materials with specific nanostructures [35; 36]. In our previous studies [37; 38; 39], the development of nanosized ZnO with flower and spherical shapes was reported, as well as ZnO coupled with Ag nanoparticles that were obtained in situ during the photopolymerization process. Using this method, a mixture of monomers and nanoparticles was used to achieve flexible films whose photocatalytic activities were evaluated by using Nile red, methylene blue, and rhodamine B as model organic dyes. According to literature data, an improvement of the photocatalytic activity may be obtained by doping ZnO with metals [40; 41; 42].