Wetting Behavior of Hierarchical Oxide Nanostructures: TiO₂Nanotubes from Anodic Oxidation Decorated with ZnO Nanostructures

Abstract

Titania nanotubes (TiO 2 NTs) of 5 μm in length and 100 nm in the external diameter are easily formed by anodic oxidation. They are used as hollow substrates to deposit different ZnO nanostructures, such as nanoparticles and nanowires by employing two different techniques, electrodeposition and hydrothermal growth, respectively. In this way highly nanostructured and hierarchical sample surfaces were obtained, showing high level of crystallinity of both TiO 2 anatase and ZnO wurtzite materials. In addition, the wetting behavior drastically changed from the hydrophilic TiO 2 NTs surface to almost superhydrophobic surfaces of the hierarchical samples, thanks to the decoration with ZnO nanostructures. These results open interesting possibilities to employ our hierarchical TiO 2 -ZnO nanostructured materials as self-cleaning, antireflective or anti-fogging surfaces. These hierarchical and composite nanostructures could be thus efficiently used in photocatalytic devices, which would also benefit from the combination of both metal oxides for improved performances and efficiencies. Anodic oxidation1 is the process of forming an oxide on a metal surface by applying an electric potential through a suitable electrolyte. The metals that can be anodized belong to the so-called valve-metals group (Al, Ti, Zr, Nb, W, Ta, . . . ) and different kinds of ordered nanostructured oxides can be obtained. Compared with other synthesis approaches, electrochemical anodization is a simple and convenient technique to fabricate uniform layers of vertically self-oriented nanostructures. It has been widely accepted that the formation of the pores in anodic metal oxides is based on two continuous processes: the oxide dissolution at the electrolyte/oxide interface and the oxidation of metal at the oxide/metal interface.Particularly, anodic titanium oxide has attracted considerable interest since its unique properties make it useful for various functional applications, ranging from non-silicon solar cells 2-4 and photocatalysis 5 to energy storage. 6,7 In general, the morphology and the structure of the ordered layer are strongly affected by the electrochemical conditions (anodization voltage, distance between the electrodes, temperature) and the electrolyte composition. It is possible to grow titania nanotubes (NTs) with length varying between 100 nm up to 1 mm with a good control on the wall thickness and external roughness.In the present paper we report on the anodic oxidation of a titanium foil to form 5 μm long titania NTs. In order to create a hierarchical structure and then to study the wetting properties, we decorate both their inner and outer surface with zinc oxide (ZnO) nanoparticles of about 20 nm in diameter. In particular, by using two different growth techniques, it is possible to additionally enrich the planar surface of the TiO 2 NTs with either a network of ZnO nanowires using the hydrothermal approach, or with ZnO microparticles of about 150 nm using the electrodeposition method.ZnO is a n-type metal oxide semiconductor havi...

Effects of Stoichiometry on Structural, Morphological and Nanomechanical Properties of Bi2Se3 Thin Films Deposited on InP(111) Substrates by Pulsed Laser Deposition

“…are the dispersive portions of surface tension for the solid and liquid surfaces, respectively. By combining Young's equation [56] with Equation 2 [59]. In any case, the present study suggests that both the hydrophobic/hydrophilic transition behavior and nanomechanical properties of the Bi 2 Se 3 thin films can be manipulated by controlling the target compositions.…”

Surface Analysis and Optical Properties of Cu-Doped ZnO Thin Films Deposited by Radio Frequency Magnetron Sputtering

“…According to the analysis of the FGG method, the considered critical interaction is the dispersive force or the van der Waals force across the interface existing between the water droplet and the solid surface. The FGG equation is given as: The wettability behavior of the surface is strongly related to the surface morphology of the sample surface [27]. The results of wettability tests are shown in the inset of Figure 2a-c. With an increasing R rms , a larger amount of air is expected to be trapped in the gap of the nanoislands [28], which, in turn, would greatly increase the contact area of the air-water interface (i.e., preventing the water droplets from penetrating into the air pockets), resulting in a larger contact angle (θ CA ), as displayed in the rougher sample (see the Cu-6% doping CZO thin film in Figure 2b).…”

Annealing-Driven Microstructural Evolution and Its Effects on the Surface and Nanomechanical Properties of Cu-Doped NiO Thin Films

“…In addition, the surface wettability experiment often serves as the most convenient measure of surface energy and is commonly quantified by relating the surface energies with the obtained θ CA (contact angle) via Young's equation: γ sv = γ sl + γ lv cosθ CA , with γ sv , γ sl , and γ lv being the surface tension between solid-vapor, solid-liquid, and liquid-vapor, respectively [33]. By considering that the critical interaction is the dispersive force or the van der Waals force across the interface existing between the water droplet and the solid surface, the surface energy for CNO thin films was calculated using the Fowkes-Girifalco-Good (FGG) theory [33] in combination with Young's equation. The FGG equation is given as: γ ls = γ s + γ l − 2 γ d s γ d l , with γ d l and γ d s being denoted as the dispersive portions of the surface tension for the liquid and solid surfaces, respectively.…”