*3.6. Kinetic Study of the Photocatalytic Decolorization*

The kinetics of the photocatalytic degradation of methyl red over ZnO, Cu2O, Cu2O/ZnO and N-doped Cu2O/ZnO is shown in Figure 8. The plots of ln(C0/Ct) versus irradiation time indicated linear curves. The linearity of the kinetic curve shows that the photocatalytic degradation of methyl red follows pseudo-first-order kinetics. A fairly good correlation coefficient value to the pseudo-first-order kinetics of R2 > 0.96 was obtained, which is in agreement with other reports [46]. The rate constants (reaction rate) were predicted from the slope of the graph. The rate constants are found to be 0.00338, 0.00423, 0.01005 and 0.01088 min−<sup>1</sup> for Cu2O, ZnO, Cu2O/ZnO, and N-Cu2O/ZnO nanomaterials, respectively. The rate constants of the photocatalytic degradation of methyl red using N-Cu2O/ZnO and Cu2O/ZnO nanocomposites are larger than pure Cu2O and ZnO nanoparticles. Therefore, the photocatalytic activities of N-Cu2O/ZnO and Cu2O/ZnO nanocomposites based on the rate constants are found to be greater than Cu2O and ZnO nanoparticles [5,47].

**Figure 8.** Reaction kinetics curve.
