Two-step wetting transition on ZnO nanorod arrays

Abstract Transition between superhydrophilicity and superhydrophobicity is primarily important in the modification of the wettability of the material surface. The significant transition between the superhydrophilicity and the superhydrophobicity is realized from the surface made up of zinc oxide (ZnO) nanorod arrays on a silicon substrate with different annealing temperatures by the post-annealing process. Surprisingly, a critical temperature (300 °C) existed for ZnO nanorod arrays to achieve the transition from the increase in contact angle to its reduction. As the annealing temperature increases, the initial hydrophilic surface is gradually converted to the superhydrophobic one, whereas the surface property reverses back to the hydrophilic one as the critical annealing temperature is surpassed. Oxygen vacancies and carbon adsorptions are examined to be responsible for this extraordinary wettability variation. We establish a simple theoretical model to describe the novel mechanism of the transition between the superhydrophilicity and the superhydrophobicity. The facile method to achieve the transition reported here will supply the great facile applications to control the wetting properties of materials, without employing the destroyable chemical coating or external stimulation by electrowetting and ultra-violet (UV) illumination.