*3.3. FTIR Spectral Study*

The FTIR spectra of ZnO and CuO/ZnO nanocomposites are depicted in Figure 3. Generally, the absorption bands of metal oxides were below 1000 cm−1, due to inter-atomic vibrations. From Figure 3, it is seen that the absorption band of zinc oxide (stretching of Zn−O) was between 400 cm−<sup>1</sup> and 590 cm<sup>−</sup>1, which confirmed the wurtzite structure of ZnO [32]. The vibration around 3385 cm−<sup>1</sup> and 1637 cm−<sup>1</sup> was attributed to asymmetric and symmetric stretching H−O−H vibration, which was due to chemisorbed water. The very weak peak at 2320 cm−<sup>1</sup> corresponded to the symmetric C−H bond vibrations, which may have been present due to the environmental conditions. Stretching modes of C–O appeared at 1110 cm<sup>−</sup>1, because of the acetate group improper decomposition [33]. The bands of 3000–3650 cm−<sup>1</sup> were attributed to reversible dissociative absorption of hydrogen on Zn and O [34].

**Figure 3.** Fourier transform infrared (FTIR) spectra of ZnO and CuO/ZnO.

#### *3.4. UV-VIS Spectral Changes*

The absorption spectra of MB solution with CuO/ZnO under sunlight irradiation are shown in Figure 4. During photolysis, pure MB was exposed 150 min under sunlight and it was seen that the change of the absorption spectrum was negligible. The absorption spectrum at 662 nm slightly decreased under dark after 150 min with CuO/ZnO, indicating the MB dye adsorption on the composite. The well-defined absorption band disappeared after 150 min, which confirmed the MB degradation with CuO/ZnO in the presence of sunlight.

**Figure 4.** UV-visible spectral changes of methylene blue (MB) in water for before irradiation, photolysis, under dark, and sunlight irradiation.
