## 1 Introduction  
Generally, metal oxide particles have wide practical applications and can be applied in biomedicine, different tools, batteries, heavy industrial equipment, pipes and fittings for offshore applications, for antibacterial and photocatalytic purposes, as well as to enhance the life of base materials. One of the most widely used metal oxides is ZnO. The latter has been studied intensively with respect to its application in various devices: transistors, sensors, catalysts/photocatalysts, Ultra Violet (UV) adsorbers, pigments, photoelectrodes, solar cells), antibacterial coating and packaging, in the cosmetic industry as sunscreens, in medicine (drug delivery), etc., due to its excellent physicochemical, electronic, and antibacterial characteristics [1; 2; 3; 4; 5]. Additionally, the incorporation of nano-ZnO and nanosilica can yield self-cleaning epoxy coatings with desirable properties, such as easy sliding of water droplets. Other researchers have incorporated nanosilica, ZnO, alumina, and similar additives to improve the mechanical strength of epoxy coatings and to block UV light [6; 7; 8; 9; 10; 11; 12]. Among the various metal oxides, ZnO is considered to be one of the most promising materials for photocatalytic and corrosion protective applications, especially after additional polymeric modifications [13; 14; 15]. Additionally, this compound ensures an enhanced protective ability by forming corrosion products with barrier properties on the metal surface [15]. ZnO is a well-known II-VI wide band gap (3.2 eV) semiconductor material. The electronic structure and the valence state of the zinc and oxygen ions are influenced by the nature and concentration of intrinsic and extrinsic defects. Different point defects could occur in the crystal lattice of ZnO: oxygen vacancies (V\({}_{0}\)), oxygen interstitials (O\({}_{\text{i}}\)), zinc vacancies (V\({}_{\text{Zn}}\)), zinc interstitials (Zn\({}_{\text{i}}\)), etc. [16]. One of the most important applications of ZnO is in the field of heterogeneous photo-catalysis for the removal of various organic pollutants in waste waters and air [17]. The UV irradiation of the semiconductor photocatalysts initiates a reaction by producing photo-excited electron-hole pairs, which can react further with the adsorbed pollutant molecules. As a result of the oxy/redox processes, complete mineralization of the contaminants occurs [18]. Various synthesis methods of photo-catalytically active ZnO nanoparticles (ZNP) have been applied: thermal evaporation [19], chemical vapor deposition (CVD) [20], mechano-chemical preparation [21], sol-gel [22] spray pyrolysis [23], hydrothermal synthesis [24], precipitation [25], etc. Most of the abovementioned conventional procedures are complex, based on the application of toxic reducing and stabilizing agents. In addition, the particles produced by the classical chemical methods tend to form agglomerates. The hydrothermal approach is a simple low-cost method that allows the preparation of nanosized powders, which have a high degree of crystallinity and high chemical activity. In this technique, the morphology of nanoparticles can be controlled by optimizing various reaction conditions [26]. The so-called \"green synthesis\" of particles, using bio-organisms (plants, algae, and microbes) is a relatively new strategy for the preparation of ZNP and other oxide particles. It is environmentally friendly, has a low cost, and it allows the preparation of stable nano-sized particles. The use of hazardous organic compounds (solvents and stabilizing agents) is avoided. The main constituents in the plants: polyphenols, terpenoids, etc., act as stabilizing and reducing agents. The plant-mediated synthesis of ZnO particles is a complex process. According to [PERSON] et al. [27], it consists of the movement of delocalized pi-electrons in aromatic rings, hydroxyl chelation to Zn ions, oxidation of hydroxyl groups, and reduction of Zn ions. The plant extract plays a capping role in ZnO synthesis, thus preventing agglomeration processes.