3.5.2. Optimization of Methyl Red Concentration

Once the amount of the catalyst was optimized at 180 mg/L, a series of photocatalytic degradation experiments were conducted by varying the concentration of methyl red from 40 to 100 mg/L as adjusted by [27] to know the proper amount of the dye. As shown in Figure 6, as the concentration of the dye was enlarged from 40 mg/L to 60 mg/L, the degradation efficiency was likewise enhanced. However, as the concentration of the dye goes above 60 mg/L, the degradation efficiency sharply decreased. The reason for the decreasing of photocatalytic degradation efficiency with increasing concentration of the dye is that the higher dye concentration could affect the transmission of light which leads to a decrease in hydroxyl radical formation. The total amount of active sites on the surface of the catalyst was limited by the amount of catalyst loaded. Therefore, in the solution having fixed catalyst dosage, an inadequate amount of hydroxyl radicals that can attack the methyl red can form, hence leading to the diminishing of degradation capability [43].

**Figure 6.** Optimization of the initial concentration of methyl red on the photocatalytic degradation using N-Cu2O/ZnO nanocomposite under sunlight.

3.5.3. Evaluation of the Photocatalytic Activities of Cu2O, ZnO, Cu2O/ZnO and N-Cu2O/ZnO under the Optimized Catalyst Amount and Dye Concentration

The photocatalytic performance of Cu2O, ZnO, Cu2O/ZnO, and N-Cu2O/ZnO nanomaterials were evaluated in the degradation of methyl red dye, as shown in Figure 7. The photocatalytic degradation efficiency of methyl red reached 45.5%, 54%, 84.5% and 93.5% using Cu2O, ZnO, Cu2O/ZnO, and N-doped Cu2O/ZnO, respectively, within 180 min irradiation time. Among these nanomaterials, the N-doped Cu2O/ZnO nanocomposite displayed better photocatalytic activity than the others under the optimized conditions. The activity of N-doped Cu2O/ZnO nanocomposite is enhanced because of the formation of a heterojunction [35]. The p-Cu2O/n-ZnO heterojunction might significantly increase the absorption and exploitation capability of solar light; the electrons transfer from the one semiconductor to the other encourages the charge separation and construct significant synergistic effect in the degradation of the dye [35]. In addition to the effect of the coupling of the two semiconductors, the improved photocatalytic efficiency of N-Cu2O/ZnO nanocomposite is due to doping with nitrogen. Incorporating non-metals, for example nitrogen, can diminish the energy gap and extend absorption of light to the visible region of electromagnetic radiation [44]. In other words, nitrogen can modify the energy levels of both Cu2O and ZnO nanoparticles. Besides the above reasons, the enhanced photocatalytic activity of N-doped Cu2O/ZnO composite nanoparticles may be due to the reduction in particle size and creation of defect sites.

**Figure 7.** Evaluation of photocatalytic properties of ZnO, Cu2O, Cu2O/ZnO and N-doped Cu2O/ZnO on the degradation of methyl red under solar light.
