Thermal effect on superhydrophobic performance of stearic acid modified ZnO nanotowers

The thermal desorption of stearic acid on superhydrophobic zinc oxide nanotowers has been investigated. The stearic acid passivated zinc oxide nanotowers provide a very high contact angle of similar to 173 +/- 1.1 degrees with a very low hysteresis of similar to 1.4 +/- 0.5 degrees due to the presence of a binary structure composed of several nanosteps on each nanotower of height similar to 700 nm that eventually reduces the area of contact between the drop and the nanotowers and trapping more air as revealed by the field emission scanning electron microscopy images. The superhydrophobic performance of these nanotowers, however, declines following annealing at elevated temperatures. Fourier transform infrared spectra show a reduction in the intensity of stearic acid - CHn peaks at elevated temperatures revealing the cause of the decrease in contact angle and confirming the occurrence of thermal desorption at 184 degrees C. The corresponding activation energy for desorption determined from our data is 0.34 +/- 0.05 eV. It is found that the stearic acid has completely disappeared at 350 degrees C, making the sample hydrophilic. (C) 2007 Elsevier B. V. All rights reserved.