XRD pattern of the synthesized CuO nanoparticles is shown in Fig. 1A which is indexed to the monoclinic CuO phase with relatively broad peaks conformed with lattice constants of JCPDS card No. 96-101-1195. In addition, the broadening of the XRD peaks is suggesting the presence of the nano-sized crystals and, no impurity peak was observed in obtained XRD pattern. Also, as shown in Fig. 1B, EDS analysis of synthesized nanoparticles reveals the presence of Cu to O.
Figure 2 exhibits FESEM images of the prepared CuO nanoparticles with a hierarchical structure composed of nanoflakes which created the flower-like CuO particles. The thickness of nanoflakes was measured around 65 ± 26 nm via Image J software. Also, the diameter of CuO flower-like particles was measured about 500 ± 112 nm.
According to FESEM images, first, CuO nanoflakes were formed then interlocked with each other to create CuO flower-like particles. According to Yu et al.  suggestion, the formation of CuO flower-like particles has occurred in several stages mentioned follow as: (1) the growth of some shuttle-shaped CuO nanoflakes, (2) the emerging and growth of more nanoflakes with smaller size, and (3) the newly CuO precipitation radiated orientation from the previous center. Accordingly, the reactions that result in the nucleation and growth of CuO flower-like particles as follows: (1) dissolution of Cu in the form of [Cu(OH)4]2– indicated in Eq. 1, (2) Nucleation of CuO as is shown in Eq. 2, (3) the growth of CuO nanoflaks on CuO grains, and (4) the growth of larger CuO grains and dissolution of smaller ones.
Cu2 + + 4OH– → [Cu(OH)4]2– (1)
Cu(OH)42– → CuO + H2O + 2OH– (2)
Besides, as shown in Fig. 1A, the strongest reflections belong to (111) and (111-) planes which indicated the first oriented growth planes of CuO. Indeed, [Cu(OH)4]2– ions were oriented and grown along (111) and (111-) planes [30, 31]. [Cu(OH)4]2– anion group is an inorganic precursor to form CuO particles . Indeed, two OH– groups bound to two Cu2 + cations formed an H2O molecule and two Cu2 + cations are bridged via O2, resulting in forming chains of square planar CuO4 groups, then CuO precipitations. The formation tendency of [Cu(OH)4]2– anion group is in spherical clusters, resulting in the nucleation of CuO nanoflakes after condensation [33, 34]. By reducing the concentration of [Cu(OH)4]2– the packing probability of CuO decreases, leading to slow nucleation of CuO with layered and flak-shape structures. While by increasing the concentration of OH–, CuO tends to agglomerate without layered structure [33, 35]. Due to Ostwald ripening and the disparity in surface-to-volume ratio, the small nanoflakes and crystals disappear and further enlargement ones leave and grow, then develop to flower-like structure [29, 36].
On the other hand, Fig. 3A shows the XRD pattern of the synthesized ZnO nanoparticles with hexagonal phase structure that matched with lattice constants of JCPDS card No. 96-901-1663. The DSC analysis of prepared ZnO nanoparticles shows in Fig. 3B revealed the presence of Zn and O in the spot detection. Further, Fig. 3C illustrated the comparison of XRD patterns and phase structure of both synthesized ZnO and CuO revealed that the obtained peaks of CuO are broader than of ZnO. Namely, the structure of the synthesized CuO particles is finer than ZnO. To confirm this result, the average particle size of prepared nanoparticles was measured around 88 ± 35 using Image J software.
Figure 4A illustrates the morphology of the prepared ZnO nanoparticles by using FESEM images displayed sphere-like structure with a few prism-like nanoparticles (as shown in Fig. 4B). To form the sphere-like ZnO nanoparticles, the electrostatic interactions between [Zn(OH)4]2− and polar surfaces possess the main role in ZnO nucleation. In presence of NaOH, [Zn(OH)4]2− anions obtained from the following equation:
Zn2 + + 4OH– → [Zn(OH)4]2– (3)
According to Wen et al.  study, NaOH concentration specifies that either [Zn(OH)4]2– growth units are enough to grow ZnO. Here, 2 Molar NaOH solution provided a weak electrostatic interaction between [Zn(OH)4]2− and polar surfaces, then both sphere-like ZnO nanoparticles and nano-ZnO with preferred orientation in a nano-prism structure formed [37, 38].
Indeed, as a suggestion, the difference between the crystal structures of ZnO and CuO nanoparticles and various electrostatic interactions between their anions and polar surfaces leads to their different structures.