3.5.1. Optimization of N-Cu2O/ZnO Loading

To know the optimal value of catalyst dosage, a series of photocatalytic degradation experiments were conducted by varying the amount of the N-Cu2O/ZnO nanocomposite from 120 to 240 mg/L as optimized by [27]. The photocatalytic degradation of methyl red under sunlight irradiation using a different amount of N-doped Cu2O/ZnO nanocomposite is shown in Figure 5. The figure shows that, as the amount of catalyst loading increases from 120 mg/L to 180 mg/L, the degradation efficiency also increased. The reason is that increasing the catalyst loading increases the surface area and quantity of reaction sites on the surface of the photocatalyst. Consequently, the amount of hydroxyl radical formation increases too, which enables the photocatalytic degradation of the dye. However, the degradation efficiency was decreased when the amount of the catalyst dosage was beyond 180 mg/L. This might be due to light-scattering and screening effects [41,42]. Besides, agglomeration also occurs when the concentration of catalyst is high; which results in the decreasing of catalyst surface area and causes diminishing of degradation efficiency [42,43]. Therefore, the degradation efficiency was greatest when the amount of the catalyst used was 180 mg/L and was the optimal value in the experiment.

**Figure 5.** Optimization of N-Cu2O/ZnO nanocomposite loading for the degradation of methyl red under sunlight.
