Effect of Hydrothermal Reaction Time on the Morphological and Photocatalytic Properties of ZnO Nanostructures

Zinc oxide (ZnO) nanostructures were synthesized via a hydrothermal method by systematically varying the reaction time (5–24 h) while maintaining all other parameters constant. The morphological evolution progressed from nanoparticles to nanoneedles, nanoflakes, and nanoplates with increasing reaction duration. X-ray diffraction and Raman spectroscopy confirmed the formation of hexagonal wurtzite ZnO for all samples, accompanied by a gradual shift in the preferred growth orientation from the c-axis to the a-axis. The optical characterization revealed a pronounced dependence of the band gap and the defect density on the synthesis time, with the nanoflakes obtained at 12 h exhibiting a narrowed band gap of 2.9 eV and an enhanced visible light absorption. The photocatalytic degradation of methylene blue followed zero-order kinetics, where the ZnO nanoflakes achieved the highest rate constant (k0 = 0.01893 min−1). The enhanced activity is attributed to the combined effects of a reduced band gap, an increased surface area, the coexistence of ZnO/Zn(OH)2 phases, and a defect-assisted charge separation.