Figure 2. Growth of ZnO nanorod samples. ( a ) Top view and ( b ) side view of vertically grown ZnO nanorods (S18) and ( c ) lengths and diameters and ( d ) aspect ratios of five ZnO nanorod samples (S6 ‒ S30). As the growth time increased, both the length and diameter continuously increased. ( a ) and ( b ) were obtained using the scanning electron microscope.