Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays

Vertically aligned ZnO nanorod arrays with different heights are grown on the ZnO seeded indium tin oxide substrate by cathodic electrochemical deposition from zinc nitrate at two temperatures of 60 °C and 80 °C. As-grown ZnO nanorods exhibit wurzite crystal structure and their heights can be well controlled by different deposition times. The fluorination coating tends to induce a superhydrophobicity of ZnO nanorods, i.e., the maximal value of contact angle: 166.9°. The super water repellency can be attributed to the fact that an air layer is confined in the nanorod arrays, and thus leads to water droplets sitting on the ZnO surfaces, referring as Cassie state. Interestingly, their water contact angles are found to vary with the heights of ZnO nanorods, ranged from 99.8 to 746 nm. The superhydrophobicity of ZnO surfaces can be well predicted by a proposed model that is capable of determining the wetted fraction of ZnO pillars. This satisfactory result would shed one light on how the variation of rod height would induce the superhydrophobic behavior of ZnO nanorod arrays.