Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie-Wenzel Transition after Hydrophobization

We report a facile electrochemical route for the one-step fabrication of ZnO nanowire (NW) arrays. The method is seed-layer-free, and the NWs are directly attached to a fluorine-doped tin oxide (FTO) substrate. The effects of growth temperature, precursor concentration, substrate etching, and deposition time on the layer morphology and structure are analyzed. The ZnO NWs are vertically well-aligned and textured with the c-axis normal to the substrate. The growth of the more vertically oriented initial wires is favored by a self-alignment process, and the layer texturing with the c-axis oriented normal to the surface is increased upon deposition. NWs with aspect ratios higher than 30 have been synthesized. The as-grown layers were superhydrophilic, and they were converted to superhydrophobic by surface derivatization with stearic acid (SA). The surface could be committed back from superhydrophobic to superhydrophilic by a simple acetone washing. The present work demonstrates the importance of oxide NW length to control the hydrophobic state of the surface. By increasing the NW length (and then the aspect ratio), a transition between a Wenzel state and a Cassie-Baxter state was found. For long and homogeneous ZnO NWs, the hysteresis is low (5.5 degrees), and the advancing and receding contact angles are high (168.3 degrees/162.8 degrees max). The role of wire density is discussed. The superhydrophobic layers are of interest for self-cleaning surfaces, biological experiments, and nano/microfluidics.