Superhydrophobic ZnO nanotowers N. Saleema * , D. K. Sarkar * , M. Farzaneh * and E. Sacher ** * Canada Research Chair on Atmospheric Icing Engineering of Power Networks and Industrial Chair on Atmospheric Icing of Power Network Equipment, Université du Québec à Chicoutimi, Canada G7H 2B1 ** Regroupement Québécois de Matériaux de Pointe, Department of Engineering Physics, École Polytechnique, C.P. 6079, Montreal, Québec., Canada H3C 3A7 ABSTRACT Superhydrophobic zinc oxide (ZnO) nanotowers have been grown successfully on sodalime glass substrates by chemical bath deposition (CBD). Chemically and ultrasonically cleaned glass substrates have been immersed in beakers containing 100 ml of and different amounts (4 – 10 ml) of 28% solution. The CBD has been performed in an oven at 70 ( ) O H NO Zn M 2 2 3 6 1 . 0 ⋅ OH NH 4 o C for 18 hours. Surface morphology of the grown ZnO films have been carried out using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). FESEM images have revealed the presence of randomly oriented hexagonal patterned ZnO nanotowers. The heights of the nanotowers are approximately 700 nm. Throughout 300 nm from the top of the nanotowers, uniform nanosteps with a step-size of 25 nm, both horizontally and vertically are observed, which is complementary with the AFM studies. These ZnO nanotowers have been passivated using stearic acid (SA) molecules for hydrophobic applications. Water contact angle of greater than 170 o has been achieved after SA passivation with a contact angle hysteresis of less than 5 o . The superhydrophobicity of these nanotowers is due to the coexistence of the nanosteps which effectively increases the surface area and present high contact angle. Keywords: superhydrophobic, zinc oxide nanotower, chemical bath deposition, contact angle 1 INTRODUCTION Superhydrophobic nanostructured thin films can be of great interest in many applications such as coating glass windows, camera lens, high-tension lines, etc. Commonly known examples that exhibit such a high contact angle of water are naturally existing tissues such as lotus leaves and Lepidoptera wings where the water droplet can freely roll off the surface without leaving any trace of beads [1]. Recent studies indicate that a two-tier roughness on a surface composed of micro- and nano-structure is essential to achieve such a superhydrophobicity [2, 3]. The Wenzel [4] and Cassie [5] model are the two existing resources that explain the impact of surface roughness on wettability. Zinc oxide (ZnO) is widely used in diverse range of technological applications such as solar cells, photo detectors, light emitting devices, gas sensor elements, and surface acoustic wave guides due its unique structural, optical and electrical properties [6-10]. There exist several sophisticated techniques to synthesize ZnO thin films such as sputtering, pulsed laser deposition, chemical vapor deposition, molecular beam epitaxy, sol–gel process, etc. for different applications [11-15]. The chemical bath deposition (CBD), among various techniques, has gained popularity recently because it is simple, low cost and can be performed at low temperatures [16, 17]. The superhydrophobicity of such ZnO surfaces was achieved by passivating them using fluoroalkylsilane (FAS) [18]. Usually –CF 3 terminated surfaces such as FAS exhibit a water contact angle (CA) of more than 90 o on a plane surface. In this paper, we demonstrate the efficiency of the novel CBD technique performed at a temperature as low as 70 o C using a less complex aqueous solution, which results in a nanostructured surface consisting of several nanotowers decorated with nearly uniform nanosteps combined giving a binary structure. Superhydrophobic property of these ZnO nanotowers will be studied by passivating them using –CH 3 terminated organic molecules, which has a water CA of only 70 o on a plane surface. We show in this study that with the smartness of our ZnO nanotowers, it is possible to achieve a very high superhydrophobicity even using an organic molecule which has such a low CA as compared to FAS. 2 EXPERIMENT The CBD was performed in an oven at 70 o C for 18 hours in a beaker containing an aqueous solution as a chemical bath. The aqueous solution used for the chemical bath comprising of 100 ml of ( ) O H NO Zn M 2 2 3 6 1 . 0 ⋅ and different amounts (4, 5, 6, 7, 8 and 10 ml) of 28% solution were contained in 6 different beakers. Soda lime glass substrates were ultrasonically cleaned with for 10 minutes at a solution temperature of 35 OH NH 4 NaOH M 1 . 0 o C, followed by water cleaning twice under the same ultrasonic conditions. After drying with clean air, the substrates were immersed in the beakers containing the chemical bath for coating. The coated samples were rinsed in a beaker containing methanol to remove any lose particles and were dried in an oven at 70 o C for several hours. They were then passivated using Stearic acid (SA) organic molecules prepared by dissolving SA mM 3 10 2 − × 158 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-8-1 Vol. 3, 2006