Amorphous/crystalline phase control of nanotubular TiO2 membranes via pressure-engineered anodizing

Abstract

One-step fabrication of fully amorphous vertically-aligned TNTs can have many applications. To control the phase and quality of membrane-like structures, this study introduces a novel method of the pressure-engineered electrochemical process (PEEP). The isostatic pressure on the electrolyte was examined to have a critical effect on the phase of TiO2 nanotubes (TNTs) and a distinctive difference in the quality of TNTs was observed under negative pressure condition (Pn: 10−2 mbar) compared with other pressures of ambient (Pa: atmospheric) and positive pressure (Pp: 5 bar). SAED showed at Pa and Pp on the surface of nanotubes crystallites grew on {101} planes owing to the lower surface free energy of these crystal planes. Similarly, the current density vs. time profile of the PEEP showed the current consumed at Pn was more than Pp or Pa conditions. In respect of bubble mould theory, it could be concluded that the number of sucked oxygen molecules increased at Pn, generating larger nanotubes in diameter. Ion chromatography (IC) test also proved that the fluorine ion (fluoride, F−) in the electrolyte did not react with those elements leached from the substrate to form (TiF6)2−. A newly practiced method of PEEP is capable of the scalable fabricating high-quality membrane to be used in optoelectronics, photocatalytic, and photoelectrochemical platforms.