Branches of ZnO nanostructure grown on sub-microrod template with seed layer coated by ultrasonic-assisted immersion technique: effect of surface energy on the growth of branches

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.