## 3 Results and Discussion
The highest degree of discoloration of the model contaminant dye (74%) was achieved in the presence of the G2 photocatalyst. The samples G0 and G1 exhibited lower discoloration values: 45% and 41%, respectively. The performed three consecutive photocatalytic activity tests showed that the discoloration degree of the model dye decreased very slightly from the first until the third cycle in the presence of G2 and consequently maintained the photocatalytic efficiency relatively well. The tests are demonstration of the stability of the **MA**-mediated ZnO powders (Figure 11b). It is well known that the UV light irradiation cause photogeneration of electrical charges, which occurs inside biosynthesized ZnO. (Figure 11c). The first stage is adsorption of the model dye. In our case, it could be supposed that \(\mathbf{\pi}\)-\(\mathbf{\pi}\) interactions with the aromatic rings belonging to the plant extract occur. The redox reactions on the oxide surface produce effective oxidizing agents such as hydroxyl free radicals (OH\({}^{-}\)) and superoxide ions (O\({}_{2}\)\({}^{-}\)), which play important roles in the photocatalytic discoloration of the model dye. According to the research work of [62], the photodegradation of MG dye occurs through three reactions: N-demethylation, decomposition of the benzene structure, and cleavage of the conjugated structure. It is known that the dye color is due to delocalization of \(\mathbf{\pi}\) electrons of the benzene ring. Photoexcitation causes jumping of \(\mathbf{\pi}\)-electrons from the valence band over the band gap to the conduction band, and the delocalization is disrupted. The final stage is mineralization of the organic pollutant [63]. The hydrothermal synthesis enables preparation of low-aggregated zinc oxide particles under mild conditions and easy control of technological parameters. Our previous experiments on ZnO particle synthesis involved the use of urea as a precipitating agent in view of the highest photocatalytic efficiency in the oxidative dye degradation. The use of urea as a precipitating agent ensured homogeneous precipitation, while NaOH led to faster precipitation and a lower surface area. The samples obtained using urea had a higher specific surface area and greater share of mesopores. Only a few studies have reported the preparation of ZnO by hydrothermal treatment of a precipitate, obtained using a plant extract [64]. In the present study, we used the advantages of both: the hydrothermal synthesis and the application of a plant extract as a stabilizing/reducing agent. As a result, highly effective ZnO photocatalysts with a high specific surface area (80-140 m\({}^{2}\)/g) and larger share of mesopores (pore sizes: 16-17 nm) were obtained. The higher surface area and pore size ensure better charge diffusion, mass transportation, and a faster reaction rate of discoloration. On the contrary, common green synthesis (with conventional precipitant NaOH) without hydrothermal activation leads to the preparation of particles with a lower specific surface area and smaller pore sizes [65]. The method proposed by us involves three stages: precipitation by means of the _Mentha arvensis_ extract, hydrothermal treatment, and a final low-temperature treatment to effectively control the texture and morphology, thus ensuring the successful preparation of highly active photocatalysts. Our experimental results have proved that the _Mentha arvensis_-mediated ZnO powders exhibit increased photocatalytic efficiency for UV-light irradiation, causing discoloration of the model dye in comparison to that obtained by a non-modified approach. Both the EPR and PL studies have proved convincingly the presence of Zn\({}_{\text{i}}\) and V\({}_{0}\) defects. It is likely that ZnO is modified by nitrogen, originating from the precipitant. It was revealed that the N ion implantation can induce a large number of V\({}_{\text{Zn}}\) and V\({}_{0}\) defects [51].