Mechanism for wettability alteration of ZnO nanorod arrays via thermal annealing in vacuum and air

Highlights: ► Oxygen vacancy is the key factor in accounting for the change in morphology of the ZnO nanorod arrays. ► We firstly investigated the wettability alteration of ZnO nanorod arrays annealed in vacuum at different temperature. ► The hydrophilicity of the ZnO nanorod arrays annealed in air is not related to the oxygen vacancy but ascribed to the O adatom on the nanorod surface. - Abstract: The ZnO nanorod arrays were synthesized via a simple hydrothermal process followed by annealing in vacuum and air respectively at different temperature. The wettability of samples was controlled by adjusting the annealing atmosphere and temperature. To investigate the mechanism of wettability alteration, the chemical composition and surface morphology of nanorod arrays were analyzed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE-SEM), respectively. Increasing oxygen vacancy concentration by increasing annealing temperature in vacuum resulted in a great change of surface morphology, which played the major role in wettability change. Under annealing in air, oxygen vacancy concentration reduced and the surface morphology of nanorod arrays showed little change with increasing annealing temperature. The wettability alteration is ascribed to the O adatom on the nanorods surface.