Fast and Reversible Wettability Transitions on ZnO Nanostructures

Surface wettability is an important property of solid/liquid interfaces. Recently, the control of contact angle (CA) has been used to drive liquid droplets in micro- or nanosize channels on biochemical and environmental sensing chips, where a faster CA transition rate is desirable for the prompt control of liquid movement. We studied the CA between water droplet and zinc oxide (ZnO) nanotips grown by metalorganic chemical vapor deposition (MOCVD). ZnO was grown as epitaxial films on r-plane sapphire, or as vertically aligned nanotips on various substrates, including silicon, c-plane sapphire, and glass. It is demonstrated that by using ultraviolet (UV) illumination and oxygen annealing, the CA on ZnO nanotips can be changed between 0° and 130°, whereas the CA on the ZnO films only varies between 37° and 100°. The fast transition rates also have been observed. (Received October 16, 2006; accepted January 23, 2007)