## 1 Introduction  
Polluted waters are becoming an increasingly significant problem worldwide. Major pollutants of the aquatic environment include wastewater from the textile, paper, and food industries. Tartrazine is a synthetic azo dye found in all of the above-mentioned productions. Recent studies have demonstrated its toxic and carcinogenic effects [1; 2]. Due to its high solubility in water, it can easily enter the soil around contaminated water basins, affecting the human body through the direct consumption of products that contain it [3; 4]. In response, many alternatives are being developed, along with various techniques for pollutant degradation, such as photocatalytic degradation. It is recognized that metal oxide nanopowders are efficient in the photocatalytic degradation of azo dyes [5; 6; 7; 8; 9]. This technique is very important for monitoring and controlling synthetic dye pollution in the environment. Furthermore, biocompatible catalysts like ZnO and TiO\({}_{2}\) do not cause secondary harm to living organisms after the process. Recently, it has been reported that ZnO possesses substantial cytotoxic effects in comparison with TiO\({}_{2}\), which did not show these effects [10]. ZnO and TiO\({}_{2}\), as semiconductor oxides, are a good and preferable combination due to their significant characteristics as suitable electronic constructions, long lifetimes in the excited state, and outstanding light absorption from the UV to visible regions. The resulting material can be used in the degradation of various organic impurities such as detergents, dyes, and pesticides present in various types of wastewaters. Because of their large band gap energy and low cost, they produce photogenerated holes with strong oxidizing power [11]. So far, hybrid TiO\({}_{2}\)/ZnO materials are synthesized by both physical and chemical processes, which enable enhancement of their properties. Alternatively, the coupling of both oxides can prolong the duration of separation between electrons and holes, thus reducing their chance of recombination [12]. However, the main problem concerning the selection of the appropriate method of synthesis or modification as well as the selection of the correct components to form the multifunctional hybrid system has still not been addressed [13]. Up until now, many review papers have provided information about the effective synthesis methods for TiO\({}_{2}\)-based catalysts, the type of doping, and the parameters affecting the degradation of organic compounds [14; 15; 16; 17; 18]. However, the influence of dopant concentration on the degradation of organic pollutants under light irradiation has still not been clarified. There is no reported comprehensive review in relation to synthesis methods, performance in the degradation of organic pollutants, and hydrogen production for TiO\({}_{2}\) with and without doping. Additionally, zinc oxide (ZnO) has also been largely considered because of its band gap energy, which is comparable to that of TiO\({}_{2}\); its higher electron mobility and lifetime; relatively lower production costs; and easy fabrication for a variety of nanostructures, such as nanowires, nanoribbons, nanobelts, nanospheres, nanofibers, etc. However, ZnO has the disadvantage of facile dissolution under UV light irradiation when in aqueous solution. There are studies examining different synthesis methods, structural and surface modifications, as well as the photoreaction mechanisms of these materials [19; 20]. However, in general, targeted studies on how to improve the properties of mixed TiO\({}_{2}\)/ZnO composites are scarce. Our team has particular experience in developing a novel, easy, and low-cost procedure for the synthesis of TiO\({}_{2}\)- and ZnO-containing nanostructures and for studying their structural, thermal, optical, photocatalytic, and antibacterial properties [21; 22].