Fabrication of Large Area of Anodic Aluminum Oxide Ultrathin Film Directly onto an ITO Electrode with a Ti Buffer Layer

An anodic aluminum oxide (AAO) ultrathin film (~140 nm, about half the thickness of the original Al film) was successfully fabricated directly onto an indium tin oxide (ITO) electrode without the erosion of ITO by a two-step anodization process in 0.3 mol·L^-1 O₂SO₄ solution at a constant voltage of 20 V. Here, a thin titanium buffer layer was included between the ITO electrode and the Al film by radio frequency (RF) magnetron sputtering. A large area (about 4 cm²) of porous alumina with nanoscaled channels perpendicular to the substrates was obtained. The average pore diameter and the pore interspace were approximately 30 and 60 nm, respectively. We found that the Ti buffer layer with a thickness of 10-40 nm between the Al layer and the ITO substrate played a critical role in improving the adhesion and ensuring ITO protection, which could not be duplicated by other metals, e.g., Cr, Au, Ag, and Cu. UV-visible transmittance spectra confirmed that the Ti buffer layer was oxidized and became transparent TiO₂ and that 10-20 nm of the Ti buffer layer together with the two-step anodization process is suitable for high transparency. Therefore, the AAO specimen possessing a high nanoscale regularity and transparency may have potential use in photonics, photovoltaics, and nanofabrications.