Structure transformation of hydrothermally treated ZnO nanorod array

Here we report a study of ZnO nanorod array (ZNA) treated by twice low-temperature hydrothermal process with precursor solution of Zn(NO3)2·6H2O and C6H12N4 (1:1 M ratio) at varying concentrations. Thin and uniform ZnO nanorods were first grown on a substrate with a ZnO seed layer through a hydrothermal process with the concentration of precursor solution at 0.02 wt%. After a second hydrothermal process, they were transformed into distinct structures, dependent on the precursor concentration during the hydrothermal process. At higher precursor concentration (0.1 wt%), syringe needle-shaped nanocolumns with an inverted hexagonal pyramid head were obtained, while the high aspect ratio nanowires were formed at lower concentration (0.02 wt%). The probable transformation mechanisms of the ZNA associated with precursor concentration were proposed and discussed with SEM results.