Electroplating of hydrophobic/hydrophilic ZnO nano-structural coatings on metallic substrates

Abstract

In the present work, ZnO thin film is shown as a coating on an aluminum substrate. In order to synthesize ZnO thin films, electroplated Zn thin layers were thermally oxidized in atmospheric air for different times (1h–4h) at a fixed annealing temperature of 500 °C. The samples were characterized by scanning electron microscopy (FEG-SEM) equipped with energy dispersive x-ray analysis (EDX), a profilometer, x-ray diffraction (XRD), and Raman spectroscopy. The wettability properties of the synthesized films were evaluated by measuring the contact angle between the surface of the films and a deposited water drop (WCA). The FEG-SEM images show that the surface morphologies change throughout treatment time. The sample treated for 2 h shows flower-like microstructures with an average size of 100 μm, which are covered with spherical ZnO nanostructures with a size less than 50 nm. Measured surface roughness ranges from 5.800 μm to 6.560 μm. Layers thicknesses vary between 31 and 38 μm. Structural characterization by XRD demonstrates that the synthesized ZnO thin films were polycrystalline and have Wurtzite hexagonal structures, grown manly along the (101) plan. The estimated crystallite sizes are in the nanometric scale and reach their maximum value for the sample treated for 2 h. This annealing time corresponds to the low dislocation density (δ) and low lattice strain (ε), indicating fewer defects. The Raman analysis shows five normal vibrational modes, which correspond to the ZnO Wurtzite structure. It was possible to obtain both hydrophobic and hydrophilic surfaces; the shape and surface roughness of the as-prepared films had an impact on the results. The largest measured contact angle, of 97°, was obtained after annealing for 2 h at 500 °C.