3.5.4. Proposed Mechanism

Scheme 2 demonstrates the photocatalytic degradation mechanism of methyl red by N-doped Cu2O/ZnO composite nanoparticles under sunlight irradiation. When the semiconductors are exposed to sunlight, a transition of electrons from the valence band to the conduction band and formation of holes in the valence band (Equations (3) and (4)) can take place. The conduction band of cuprous oxide is at higher position compared to zinc oxide (Jiang et al., 2013), the photo-induced electrons in the Cu2O conduction band can simply transfer to the ZnO conduction band Equation (5), which can successfully prevent the recombination of charge carriers. The electrons at the conduction band of Cu2O and ZnO undergo a reaction with adsorbed oxygen to give a peroxide radical anion Equation (6). It is impossible to oxidize the OH<sup>−</sup> by holes of Cu2O because the valence band edge of Cu2O is higher in a position [45]. However, the holes of ZnO oxidize the hydroxyl ion to yield a hydroxyl radical Equation (8), a strong oxidizing agent that can break down the organic dye. Besides, the peroxide radical anion undergoes a reaction with the hydrogen ion to produce HO2 and H2O2 Equation (9). The hydrogen peroxide is then reacted with the peroxide radical anion to create the powerful hydroxyl radicals Equation (11). Eventually, the hydroxyl radical oxidizes the dye into photocatalytic degradation products Equation (12) [35,43]. The reaction steps are described below.

$$\text{Cu}\_2\text{O} + \text{hv} \rightarrow \text{Cu}\_2\text{O} \left(\text{h}^+\right) + \text{Cu}\_2\text{O} \left(\text{e}^-\right) \tag{3}$$

$$\text{ZnO} + \text{hv} \rightarrow \text{ZnO} \left(\text{h}^+\right) + \text{ZnO} \left(\text{e}^-\right) \tag{4}$$

$$\text{CuO}\_2\text{O}\left(\text{e}^-\right) + \text{ZnO} \rightarrow \text{Cu}\_2\text{O} + \text{ZnO}\left(\text{e}^-\right) \tag{5}$$

$$\text{CuO}\_2\text{O}\left(\text{e}^-\right)/\text{ZnO}\left(\text{e}^-\right) + \text{O}\_{2^\circ} \tag{6}$$

$$\text{H}\_2\text{O} \rightarrow \text{H}^+ + \text{OH}^-\tag{7}$$

$$\text{ZnO} \left(\text{h}^+\right) + \text{OH}^- \rightarrow \text{OH} \tag{8}$$

$$\text{O}\_{2\*} + \text{H}^+ \rightarrow \text{HO}\_2^\cdot \tag{9}$$

$$\text{HO2} + \text{HO2} \rightarrow \text{O2} + \text{H2O2} \tag{10}$$

$$\text{H}\_2\text{O}\_2 + \text{O}\_{2^\circ} \rightarrow \text{OH}^- + \text{OH}^\cdot + \text{O}\_2\tag{11}$$

$$\text{OH} + \text{Dye} \rightarrow \text{H}\_2\text{o} + \text{CO}\_2\tag{12}$$

The schematic illustration for the photocatalytic degradation mechanism of methyl red under sunlight is summarized below.

**Scheme 2.** Schematic illustration of the photocatalytic degradation of methyl red using N-Cu2O/ZnO nanocomposite under sunlight illumination.
