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Small-sized ZnO nanoparticles were synthesized at varying hydrothermal reaction temperatures of 120 °C, 150 °C, 180 °C and NaOH molar ratios (1:2 and 1:8). As-prepared ZnO samples (i.e. Z1-120, Z1-150, Z1-180 and Z2-120, Z2-150, Z2-180) were characterized using XRD, FESEM, EDX, FTIR, and DRS. These tests showed that the fabricated samples were good in crystalline structure and hexagonal wurtzite, suggesting high purity. ZnO crystalline size samples varied from 16 nm to 35 nm as the reaction temperature and NaOH molar ratios increased. The particle morphology changes from a spherical shape to a flower-like rod shape and then to a cluster rod-like shape due to the difference in growth speed between the different crystallographic planes with rising temperature and molar ratios. Tauc's plot range predicted the band gap of all ZnO nanoparticle samples to be approximately 3.2 eV, lower than bulk ZnO's 3.37 eV, resulting in the absorption edge red-shifted in DRS spectra. The optical band gap energy (Eg) of all ZnO nanoparticles is hardly affected by reaction temperature and NaOH molar concentration, and quantum confinement has little effect. This study concluded that based on its small crystalline size (i.e. 16.67 nm) and flower-like shaped nanorod, the Z1-150 sample of ZnO nanoparticles prepared at 150 °C and 1:2 M ratios is suitable for further study. It has an absorption edge at 382 nm near the red region and an interplanar spacing (d) of 0.2471 nm at the plane (1 0 1). This study also reveals that zinc oxide nanoparticles' purpose despite their small size, may well recover application in water purification as a photo-catalyst/adsorbate and further used as nanocomposite.
Josun et al. (2023) studied this question.