Seed layer mediated wettability and wettability transition of ZnO nano/micro-rod arrays

Abstract

ZnO nano/micro-rod arrays are grown on metal (In, Sn) and metal oxide (In2O3, SnO and ZnO) thin films as seed layers, by hydrothermal processing. The ZnO nano/micro-rod arrays are aligned vertically at different angles and polycrystalline in nature. The hydrothermal processing time required to form nano/micro rods is shorter (30 min) on the oxide seed layers than on the metal layers (180 min). The rods on In, Sn, thermally oxidized In and Sn have horizontal dimensions of the order of 500–1000 nm with height up to 15 μm, indicating large aspect ratio. In contrast, the rods are 100–200 nm in diameter on ZnO seed layer with length up to 2 μm and higher density of packing. Wettability studies demonstrate that the arrays are hydrophobic in nature with water contact angle in the range of 90–110° on all seed layers except ZnO. The arrays are superhydrophobic on ZnO with a contact angle of 150.4°. Significantly, there is a transition to the hydrophilic/superhydrophilic state under the influence of UV irradiation. The transition is fastest on ZnO seed layers followed by oxidized Sn and In and slowest on the metal layers, suggesting promise for self-cleaning applications. The transition from hydrophobic to hydrophilic behaviour is correlated with the presence of oxygen vacancies, defects and oxygen adatoms on the surface, using photoluminescence and Raman spectroscopy studies.