## 3 Results and Discussion
The latter observation is in agreement with the XRD results presented above (see Figure 1). The main peaks in the visible band were located at 425 nm, 514 nm, and 607 nm and correspond to violet, green, and yellow emission, respectively. Based on previous studies, the prevailing violet emission can be attributed to the recombination of an electron in the defect state of zinc interstitials (Zn\({}_{\mathrm{i}}\)) with a hole in the valence band [55]. The green band at 514 nm is correlated with the recombination of electrons in the ionized oxygen vacancies (V\({}_{0}\)) with holes in the valence band [56]. The weak yellow emission (607 nm) seems to involve interstitial oxygen ions (O\({}_{\mathrm{i}}\)\({}^{-}\)) present on the surface of the samples [57]. Different origins of visible luminescence in ZnO nanostructures are fabricated by the chemical and evaporation methods. The XPS analysis showed that C, O, and Zn peaks were registered on the surface (Figure 10a). The O1s peaks were deconvoluted by Lorentzian-Gaussian curve fitting into two components (Figure 10b). The first one at \(\sim\)530.2 eV was assigned to lattice oxygen in ZnO, and the second one at \(\sim\)532.2 eV corresponded to hydroxyl groups adsorbed on the surface of the powders. Figure 9: PL spectra of plant-mediated ZnO powders. Figure 10: _Cont._  
UV-vis diffuse-reflectance spectroscopy (DRS) was applied to determine the band gap energy \(E_{g}\) of **MA**-mediated ZnO powders. The diffuse-reflectance spectra (DRS) were recorded on a Thermo Evolution 300 UV-VIS spectrometer, Waltham, Massachusetts, USA within the wavelength range of 190 nm to 1100 nm. The changes in the form of the bands and in their relative intensity (caused by the mirrors) were taken into account by Kubelka-Munk transformation:  
Figure 2:0  
where **R** is reflectance, **s** is the scattering coefficient, and **k** is the coefficient of molar absorbability [58]. The optical band gap was evaluated using the equation of Tauc:  
Figure 2:1  
Figure 2:2 XPS spectrum of the photocatalyst: survey scan: (**a**) deconvoluted OIs peak, (**b**) absorbance spectrum, (**c**) absorbance spectrum with Kubelka–Munk transformation, (**d**) Tauk plot from UV-Vis analysis (**e**) of _Mentha arensis_-mediated ZnO photocatalyst. In Equation (2), \(\mathbf{\alpha}\) is the coefficient of absorbance, \(\mathbf{A}\) is a constant of proportionality, \(\mathbf{h}\) is [PERSON]'s constant, and \(\mathbf{v}\) is the frequency of the photon. \(\mathbf{E_{g}}\) is the optical band gap, and in our case of direct transition, \(\mathbf{n}\) equals \(\mathbf{\imath_{2}}\)[59]. The obtained DRS spectra acquire the form of the Kubelka-Munch's function and therefore the Tauk equation becomes:  
Figure 2:3  
The extrapolation of the linear section of the graph (\(\hbar vF(\text{Ro})\))\({}^{1/n}\) from \(\hbar v\) gives the value of the optical width of the energy band gap. The UV-vis absorption spectra of the ZnO powders showed absorption peaks at 336.6 nm for G2 and 341.1 nm for G1 (Figure 10c-e). Figure 10c represents Tauc's types of plots and estimated band gap values. The band gap of the reference ZnO powder was 3.24 eV, while the biosynthesized powders had lower band gap values (2.97 and 2.91 eV for G1 and G2, respectively). This fact is explained by the substitution of some plant chemicals into the ZnO lattice sites [60] or by the presence of defects in the ZnO lattice, which are situated below the conduction band [61]. The photocatalytic properties of the samples were investigated in the reaction of the photocatalytic discoloration of Malachite Green dye as a model contaminant in aqueous solutions (Figure 11a). The values of the calculated pseudo-first-order kinetic apparent rate constants increased in the following order: G0 sample (k\({}_{0}\) = 4.4 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)) < G1 (k\({}_{2}\) = 5.4 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)) < G2 (k\({}_{2}\) = 11.8 \(\times\) 10\({}^{-3}\) min\({}^{-1}\)) for 120 min of UV irradiation.