*3.4. Optical Property Analysis*

The region of absorption of electromagnetic radiation of the prepared nanomaterials was studied using UV–Vis diffuse reflectance spectroscopy. The spectra for the absorption of light by ZnO, Cu2O, Cu2O/ZnO and N-Cu2O/ZnO are given in Figure 4. ZnO, Cu2O, Cu2O/ZnO and N-Cu2O/ZnO absorbed at 360, 440, 470 and 500 nm, respectively. For ZnO, the absorption edge was in the ultraviolet region. The Cu2O/ZnO composite nanoparticles absorbed light in the visible region because of the presence of Cu2O [27]. The absorption edges were extended into 470 nm by coupling ZnO with Cu2O and then further to 500 nm by doping with nitrogen.

**Figure 4.** (**a**) Spectra for the optical property of ZnO, Cu2O, Cu2O/ZnO and N-Cu2O/ZnO nanomaterials. (**b**) Kubelka–Munk function versus energy plots of ZnO, Cu2O, ZnO/Cu2O, and N-ZnO/Cu2O nanoparticles.

The bandgap energy of the nanocomposites can be inferred by extrapolation of the linear portion of the graph between the modified Kubelka–Munk function [F(R)hν] <sup>2</sup> versus photon energy(hν) [39]; as shown in Figure 4b. The bandgap energy of ZnO and Cu2O is 3.4 eV and 2.81 eV, respectively; however, there is decrement for ZnO/Cu2O (2.64 eV) and N-ZnO/Cu2O (2.48 eV). The greatly extended absorption of light by N-Cu2O/ZnO to the visible region may be ascribed to the creation of a new energy level above the valence band of Cu2O and ZnO as a result of nitrogen doping, leading to

narrowing of the bandgap to the visible region for harvesting more photons in the sunlight, which is in agreement with previous findings [40].
