The isoelectric point of metal oxide films formed by anodization

The surface charge of metal oxides is an important property that significantly contributes to a wide range of phenomena, including adsorption, catalysis and material science. The surface charge can be predicted by determining the isoelectric point (IEP) of a material and the pH of a solution. Although there have been several studies about the IEP of metal oxide (nano)particles, only few of them have reported the IEP of metal oxide films. The IEP of various compact metal oxide films such as TiO2, Nb2O5, WO3, ZrO2, NiO and Al2O3 formed via electrochemical anodization was determined in this project using the streaming potential technique. Nanostructured TiO2 and NiO were additionally produced using a single-step anodization technique and their IEPs were compared with the compact ones. The surface morphology and wettability of the oxides were studied by scanning electron microscopy (SEM) and contact angle (CA) measurements. X - ray powder diffraction (XRD) and X - Ray photoelectron spectroscopy (XPS) measurements were carried out to detect the elemental composition and phase analysis. The IEPs of compact anodic oxides are deviating from their nanoparticular and atomic layer deposited (ALD) counterparts. When comparing compact and nanostructured anodic oxides, the latter show even higher IEPs, emphasizing the dependency of the IEP on the method of oxide production.