## 3 Results  
The diffractograms made for the synthesized powders can be seen in Figure 2. They show well-defined diffraction maxima, with high intensities, proving a high degree of crystallinity. In all diffractograms, zinc oxide is present as a single phase, with no diffraction interference being detected corresponding to other minor phases which could have resulted from the reaction (e.g., zinc hydroxide, hydrated zinc oxychloride, etc.). Thus, the chemical compound ZnO (zinc mineral) was identified and crystallized in a hexagonal form, according to the PDF4 + file [00-036-1451]. The highest intensities were recorded for all samples for diffraction interferences located at 20 values of 31.7\({}^{\circ}\), 34.4\({}^{\circ}\), and 36.2\({}^{\circ}\) and correspond to the diffraction planes with Miller indices (100), (002), and (101) of the oxide of zinc crystallized in the hexagonal system. An increase in the intensity of all diffraction maxima is observed with the modification of the reaction time from 15 min to 60 min. This trend applies especially to the higher Zn\({}^{2+}\) concentrations samples. Noticeable differences can be seen between samples also when comparing them by the Zn\({}^{2+}\) precursor concentration. Hence, the intensity of the peaks (and thus the crystallinity of the samples) registered for S1_15 and S2_15 powders is 20 times higher than those of lower-concentration samples (S3_15, S4_15, S5_15). This can be attributed to differences in crystallite size (smaller crystallite sizes might result in broader and less intense peaks), agglomeration of particles, a wider particle size distribution, or sample density (a denser sample might absorb more X-rays, reducing the intensity of the diffracted peaks). It can thus be stated that in the case of ZnO powders, for obtaining materials with a high degree of crystallinity, it is recommended to use high precursor concentrations and as long a hydrothermal reaction time in the microwave field as possible. Figures 3-5 show the scanning electron microscopy (SEM) images for the ZnO powders. It can be seen that the predominant morphology is the platelet, a polygon with a beveled  
Figure 2: X-ray diffraction patterns for the ZnO powders. corner shape and in some places with inserts of short rods. The length varies from 10 nm to over 400 nm, increasing with the increase in reaction time and decreasing with the decrease in precursor concentration. The thickness of the platelets is variable, between 10 and 40 nm, depending on both the concentration of Zn\({}^{2+}\) in the precursor solution and synthesis time. Beginning with the Zn\({}^{2+}\) concentration of 0.0872 mol/L, associated with the S3_15 ZnO sample, and even more pronounced at the two lower concentrations, the formation of smaller, elongated, or rounded nanoparticles is observed. In this case, the \"thickness\" refers to the smallest dimension of the particle and is higher (up to 60 nm) than the measurement performed on the platelets. Due to their small size, in the case of all samples to be analyzed, a marked tendency of agglomeration is observed at low magnification values. The EDX results are consistent with the XRD, confirming the presence only of the characteristic ZnO, which denotes the complete elimination of the precursors and the absence of impurities. In terms of the elemental composition, no significant differences were observed between samples. Figures 6 and 7 display transmission electron microscopy images in bright field (a,b) and high resolution (c), along with selected area electron diffraction (SAED) patterns (d) for the powders synthesized after 15 min, labeled as S1_15 and S2_15. The results reveal the presence of polycrystalline materials with a high degree of crystallinity, consistent with the structural features identified in the X-ray diffraction patterns. The SAED patterns confirm the hexagonal structure of ZnO, with Miller indices corresponding to the prominent diffraction planes [1 0 0], [0 0 2], [1 0 1].