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 of the IEP of metal oxide (nano)particles, only a few 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 using the streaming potential technique. Nanostructured TiO₂ and NiO were additionally produced using a single-step anodization technique, and their IEP was compared with the compact ones. The surface morphology and wettability of the oxides were studied by scanning electron microscopy and contact angle measurements, respectively. X-ray powder diffraction and X-ray photoelectron spectroscopy measurements were carried out to assess the phase and elemental composition, respectively. The IEP of compact anodic oxides deviates from that of their nanoparticle and atomic layer-deposited counterparts. The comparative results indicate that the IEP of metal oxides is influenced by factors such as the chemical composition, degree of hydroxylation, and crystallographic phases of the oxide.