*3.2. SEM and EDX Study*

Crystal morphologies of the ZnO and CuO/ZnO were evaluated through SEM. Figure 2a shows that the synthesized ZnO was heterogeneous with rod-shaped branches of building blocks. On the other hand, Figure 2b shows that the shape of the CuO/ZnO composite changed from rod-like to spheroid. The particle sizes of synthesized composites were in the nanometer range, which was similar to the data of XRD. Figure 2c shows the EDX line-scanning values of CuO/ZnO heterojunction. The presences of copper, zinc, and oxygen atoms were exhibited by the EDX spectra analysis. Sharp peaks of Zn, Cu, and O were obtained; no other peak related to any other element was detected in the spectrum within the detection limit, which confirmed that synthesized materials were composed of Zn, Cu, and O only. The spectrum confirmed that Cu exists in the heterostructure, representing that the copper nanoparticles were successfully deposited on zinc oxide.

**Figure 2.** Scanning electron microscopy (SEM) images of (**a**) ZnO and (**b**) CuO/ZnO, and (**c**) energydispersive X-ray spectroscopy (EDX) pattern of CuO/ZnO.
