<![CDATA[Zinc oxide (ZnO) and zinc hydroxide (Zn(OH)₂) are technologically important materials whose structural and optical properties make them attractive for applications in optoelectronics, catalysis, and energy-related devices. In this work, thin films derived from ZnO were fabricated using vacuum thermal evaporation followed by post-deposition annealing at temperatures between 400 and 600 °C. The influence of annealing temperature on the structural, optical, and morphological properties of the films was systematically investigated using X-ray diffraction (XRD), UV–Vis spectroscopy, and scanning electron microscopy (SEM). XRD analysis revealed that the film annealed at 400 °C exhibits a dominant diffraction peak corresponding to the (002) plane of hexagonal wurtzite ZnO, indicating predominant crystallization of the oxide phase. XRD analysis revealed the emergence of diffraction features attributable to hydroxide-containing zinc phases after annealing at 600 °C, accompanied by a substantial reduction in the intensity of characteristic ZnO reflections. Low-energy optical transitions derived from Tauc analysis shifted from approximately 2.75 to 2.00 eV with increasing annealing temperature. These values are interpreted as apparent optical transition energies rather than the intrinsic bandgap of crystalline ZnO or Zn(OH)₂. SEM observations further reveal a morphological transition from irregular grains to plate-like and flower-like hierarchical structures that are morphologically consistent with zinc hydroxide-containing systems. The appearance of hydroxide-related phases after high temperature treatment may be associated with post-annealing surface reactions taking place during ambient cooling. However, the underlying mechanism remains to be conclusively established.]]>
