Effects of the geometries of micro-scale substrates on the surface morphologies of ZnO nanorod-based hierarchical structures

This paper identifies and investigates the influencing factors and their effects on the surface morphologies of ZnO nanorod-based hierarchical structures. With ZnO nanorods hydrothermally synthesized on a piece of planar glass, an optical fiber core, and a SiO ₂ microsphere, three kinds of ZnO nanorod-based hierarchical structures were fabricated. It is found that not only the synthesizing parameters but also the geometries of the micro-scale substrates affect significantly the nucleation densities of seed layers and the Zn ²⁺ diffusion zones of growth solution upon the substrate surfaces. These two factors further give rise to varied diameters and orientation of the ZnO nanorods as well as different sizes of the pits among the bundles of ZnO nanorods, which eventually result in different surface morphologies of corresponding hierarchical structures. With Zn ²⁺ concentration of the growth solution increasing, side-by-side coalescence among neighboring ZnO nanorods first appears on the optical fiber core. The different curvature radii of the optical fiber core at front and side view lead to the anisotropic surface morphology of the related hierarchical structure. Although their curvature radii are the same, the different geometries of the optical fiber core at side view and the planar glass account for varied surface morphologies of the corresponding hierarchical structures.