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In this work, Zinc oxide (ZnO) thin films were deposited on silicon and glass substrates using spin-coating method with different concentrations of precursor (zinc acetate dihydrate) and stabilizer (monoethanolamine). The concentrations of zinc acetate dihydrate and monoethanolamine in isopropanol were varied from 6 mM to 500 mM. Subsequently, the substrate with ZnO thin film as a seed layer was used to grow ZnO nanostructures by hydrothermal process with the same concentration of precursor (zinc nitrate hexahydrate), temperature, and time for each growth. The samples were characterized by field-emission scanning electron microscopy (FESEM), X-rays diffractometer (XRD), and UV-visible spectrophotometer (UV-vis) to study morphology, crystallographic structure, and optical property, respectively. The results showed that particle size, crystallinity, and transmittance of seed layers were changed with increasing concentrations of spin-coated precursor. Furthermore, the nanostructures were found that higher precursor concentration of seed layers affected the formation of ZnO nanorods to be nanosheets.

Branches of ZnO nanostructure were hydrothermally grown on seed layer coated on sub-microrod template by ultrasonic-assisted immersion technique. Zinc acetate, monoethanolamine, and isopropanol were used as a precursor, stabilizer, and solvent for coating seed layer, respectively. The crystallization of seed layer, which was confirmed by x-ray diffractometer (XRD) and x-ray photoelectron spectrometer (XPS), was facilitated by ultrasonic irradiation. The sol concentration was adjusted to be 20 and 50 mM with the coating cycles of 8 and 16 for controlling the uniformity and surface energy of seed layer. The increase of sol concentration and coating cycles led to the increase of crystal orientation in (002) plane causing the improvement of seed layer’s surface energy which was estimated from water contact angle of the template. Atomic force microscope (AFM) was revealed that the seed layer did not cover the template only at the lowest condition of both sol concentration and coating cycles. However, the roughness and maximum peak-to-valley value tended to be increased for other conditions and dropped for the last condition which was suitable to vertically grow branches. After the growth of branches, the morphology of hierarchical ZnO nanostructure was investigated by field-emission scanning electron microscope (FESEM). It was found that the branches were vertically grown on the surface of rod template in every condition except the lowest sol concentration and coating cycles. Moreover, when the sol concentration and coating cycle of seed layer were increased, the branch density was also increased.

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