## 3 Results and Discussion
### Optical Characterisation  
Figure 4a shows the absorption spectrum of the biosynthesized ZnO NPs with an absorption peak at around 376 nm, which was assigned to the band-gap absorption of ZnO, while the very weak peak at 350 nm could be related to the presence of _S. ebulus_ leaf extract. The energy band gap was found to be \(\sim\)3.3 eV using the formula \(\mathrm{E}=\mathrm{hc}/\lambda\), where \(\lambda\) is the wavelength, \(\mathrm{h}=6.626\times 10^{-34}\) Js ([PERSON]'s constant), and \(\mathrm{c}=3\times 10^{8}\) m/s, which is in good agreement with the reported values in the literature [42,44]. Moreover, using UV-vis diffuse reflectance spectroscopy (Figure 4b), the band-gap energy of the biosynthesized ZnO NPs was calculated using the Kubelka-Munk equation [55]:  
\[\mathrm{F}(\mathrm{R})=\frac{(1-R)^{2}}{2R} \tag{2}\]  
where R is the reflectance. By plotting \(\left(F(R)\times E\right)^{1/2}\) as a function of energy, the value of the band gap was obtained to be \(\sim\)3.3 eV (Figure 4b). The strong absorption in the UV region demonstrated that the biosynthesized ZnO NPs has a good capacity for UV-protection applications [24]. For example, the skin is one of the important protective barriers of the body and various events, such as trauma, cuts, and burns, can damage this natural layer. In these cases, a special cover must be used in the process of wound healing to repair the skin, improve the wound, protect it from UV rays, and reduce the risk of wound infection. The PL spectrum of ZnO normally consists of two main emission peaks: (1) a UV peak near the band-edge emission and (2) a visible peak resulting from deep level emission (DLE) bands [38,56]. Some optically active intrinsic defects influence the optical properties of ZnO, and different kinds of defects are created during synthesis, growth, and/or upon annealing. The PL spectra of bare ZnO and biosynthesized ZnO using the plant extract are shown in Figure 4c,d, respectively. A broad blue emission peak centered at \(\sim\)478 nm can be observed for the bare ZnO, which is normally associated with Zn defects, such as transitions from \(Zn_{i}\) levels (or conduction band) to valence band or \(V_{Zn}\) states (Figure 4c), while in Figure 4d, the PL spectrum of the biosynthesized ZnO using the plant extract is composed of a weak UV band at around 402 nm and an intense orange band at around 598 nm. UV emission is related to near band-edge transition, exciton-exciton collision, and free exciton recombination. The UV emission is attributed to transitions between \(Zn_{i}\) to the valence band [38], while yellow-orange emission is generally attributed to the DLE bands and excess oxygen, such as the transition between the vacancies and interstitials of neutral and singly ionized oxygen [38]. In n-type ZnO, acceptor-like defects (\(O_{i}\)) are easier to form, which enables the transition between different band-gap energies. Yellow-orange luminescent materials are widely used in optical and medical  
Figure 3: X-ray photoelectron spectroscopy spectrum of the biosynthesized ZnO NPs. treatments, and luminescent nanoproducts with potential bio-applications have gained great interest during the past years [57]. It can be concluded that during the chemical interaction between the plant extract molecules and ZnO NPs, some defects were passivated, which influenced the defect density and emission properties. As we mentioned previously, _S. ebulus_ extract is rich in antioxidants (such as phenols and flavonoids) through their carboxyl groups with binding to ZnO NPs, which can produce new levels or defects in the band gap of the host structure and, finally, influence the emission of ZnO. It is worth mentioning that many hundred years ago, _Sambucus ebulus_ extract was used as a natural dye for textile applications; it is reported that silk fabrics were dyed successfully sing natural dyes extracted from _Sambucus ebulus_ fruit [55]. Figure 4: (**a**) UV absorbance and (**b**) optical energy band gap energy of the biosynthesisized ZnO NPs using _S. ebulus_ leaf extract.