*3.8. Degradation Mechanism*

The schematic diagram (Figure 8) shows the photodegradation mechanism of MB with CuO/ZnO nanocomposite. CuO and ZnO formed a heterojunction, which assisted the separation of photogenerated carriers [39]. The conduction band (CB) potentials were −0.43 eV for CuO and −0.15 eV for ZnO vs Normal Hydrogen Electrode (NHE) [40]. The band gaps of CuO and ZnO were 1.4 and 3.23 eV, respectively [41,42]. During sunlight irradiation, CuO and ZnO were excited to generate electrons and holes at the CB and the valence band (VB), respectively, as displayed in Figure 8, since the band positions of ZnO were below the CB and VB of CuO. The photoexcited electrons transferred from CuO to ZnO, whereas the holes migrated from ZnO to CuO. [43]. Then, oxygen molecules in dye solution reacted with electrons to generate superoxide radical (•O2 −) and the holes combined with H2O to produce hydroxyl radical (•OH). Moreover, MB was directly oxidized by the holes at the VB of CuO [44,45]. The strong oxidant radicals of •OH and •O2 − readily oxidized the MB molecule. We proposed the following possible degradation mechanism of MB with CuO/ZnO in the presence of sunlight.

$$\text{ZnO} + \text{hv} \rightarrow \text{ZnO} \left(\text{e}\_{\text{CB}}{}^{-} + \text{h}\_{\text{VB}}{}^{+}\right) \tag{1}$$

$$\text{e}^-\text{(CB)} + \text{O}\_2 \rightarrow \text{"O}\_2\text{"}\tag{2}$$

$$\rm{^{\cdot}\_{h}\text{(VB)} + OH^{-} \to \rm{^{\bullet}OH}}\tag{3}$$

$$\rm{^{\bullet}O\_{2}^{-}} + MB \to \text{degraded products} \tag{4}$$

$$\text{\textquotedblleft OH} + \text{MB} \rightarrow \text{degraded products} \tag{5}$$

$$\text{h}^+\text{(VB)} + \text{MB} \rightarrow \text{degraded products} \tag{6}$$

**Figure 8.** The possible degradation mechanism of CuO/ZnO photocatalyst using sunlight.

#### *3.9. Mineralization of Dye*

The evaluation of mineralization of MB was investigated by measuring the total organic carbon (TOC). The TOC removal is presented in Figure 9a, showing the degradation of MB under sunlight irradiation. The TOC rapidly decreased with increased solar irradiation time up to 2 h. After 7 h irradiation using CuO/ZnO, the mineralization of MB was observed at about 91% of TOC reduction. The chemical oxygen demand (COD) was an effective method, widely used for the measurement of

photodegradation of organic dye [46]. The test was used to measure the total amount of oxygen needed for the oxidation of dye to produce carbon dioxide and water [47]. The COD value of MB of aqueous solution using CuO/ZnO is described in Figure 9b. With increased solar irradiation time, COD values sharply decreased up to 3 h, and COD reduction was about 84% after 7 h. The reduction of COD and TOC values after the solar irradiation of MB indicated that methylene blue molecule was mineralized.

**Figure 9.** Mineralization of MB during the photodegradation with CuO/ZnO using sunlight. (**a**) total organic carbon (TOC) and (**b**) chemical oxygen demand (COD).
