As a growth-template of ZnO nanorods (NR), a hexagonal $eta-Ni(OH)_2$ nanosheet (NS) was synthesized with the low temperature hydrothermal process and its microstructure was investigated using a high resolution scanning electron microscope and transmission electron microscope. Zinc nitrate hexahydrate was hydrolyzed by hexamethylenetetramine with the same mole ratio and various temperatures, growth times and total concentrations. The optimum hydrothermal processing condition for the best crystallinity of hexagonal $eta-Ni(OH)_2$ NS was determined to be with 3.5 mM at $95^{circ}C$ for 2 h. The prepared $Ni(OH)_2$ NSs were two dimensionally arrayed on a substrate using an air-water interface tapping method, and the quality of the array was evaluated using an X-ray diffractometer. Because of the similarity of the lattice parameter of the (0001) plane between ZnO (wurzite a = 0.325 nm, c = 0.521 nm) and hexagonal $eta-Ni(OH)_2$ (brucite a = 0.313 nm, c = 0.461 nm) on the synthesized hexagonal $eta-Ni(OH)_2$ NS, ZnO NRs were successfully grown without seeds. At 35 mM of divalent Zn ion, the entire hexagonal $eta-Ni(OH)_2$ NSs were covered with ZnO NRs, and this result implies the possibility that ZnO NR can be grown epitaxially on hexagonal $eta-Ni(OH)_2$ NS by a soluble process. After the thermal annealing process, $eta-Ni(OH)_2$ changed into NiO, which has the property of a p-type semiconductor, and then ZnO and NiO formed a p-n junction for a large area light emitting diode.