Synthesis of hierarchical superhydrophobic zinc oxide nano-structures for oil/water separation

Superhydrophobic hierarchical ZnO nano-structures were prepared by a convenient, inexpensive and reproducible method using no low surface energy additives or modifiers. Chemical composition and morphological properties of samples were characterized by SEM, TEM, XRD, and Raman analyses. The SEM images of resulted ZnO particles exhibited a hierarchical flower-like micro-nano dual-scale structure composed of nano-needles. The average tip diameter and length of the needles were approximately 90 nm and 1.5 mu m, respectively. The prepared hierarchical sample displayed considerably high water contact angle (WCA = 163.8 +/- 1.5 degrees) and low contact angle hysteresis (CAH = 1.2 degrees), indicating excellent superhydrophobic properties. The surface wettability was reversibly switched between super hydrophilicity and superhydrophobicity by altering the annealing temperature. A packed layer of the superhydrophobic product was applied for oil/water separation exhibiting high separation efficiency ( > 95%) and a high flux (2540 Lm(-2) h(-1)). The variations of WCA and CAH values versus water immersion time were evaluated to examine the durability of the particles. Results confirmed that the obtained samples have an extremely high robustness after 12 h dipping in water.