*3.5. Photocatalytic Dye Degradation*

ZnO and CuO/ZnO were used in the presence of sunlight for the photodegradation of MB to evaluate the photocatalytic performance. The effect of the doping amounts of copper in the photodegradation of MB is depicted in Figure 5. The degradation rate increased with increasing copper amounts up to 5 wt % of the composite. More than 5 wt % of copper decreased the photocatalytic activity. This was because the ZnO surface was covered by the higher percentage of copper and reduced the sunlight absorption [35]. Due to efficient charge separation and higher electron transfer, 5 wt % CuO/ZnO composite showed better photocatalytic activity for MB degradation.

**Figure 5.** Photodegradation of MB by ZnO and CuO/ZnO using sunlight.
