Anodic Aluminum Oxide (AAO) has a variety of applications depending on its pore diameter, uniformity and porosity. Here, we investigate a new finding of the widened pore diameter of AAO greatly linked to the oxalic acid anions contaminated in the AAO. As AAO was prepared by hybrid-pulse anodization in oxalic acid at low-to-high concentrations (0.3∼0.9 M) and room temperature (25 • C) instead of conventional low temperature (0∼10 • C) and low concentration (0.3 M), the average pore diameter roughly increased with concentration from 42.8 nm to 46.5 nm; while it inversely decreased with concentration from 85.6 nm to 80.0 nm after widening to enhance all pore diameters. High oxalic acid concentration resulted in AAO walls containing more anions that further decreased etching rate during widening. This new phenomenon became more pronounced by direct-current anodization than by the hybrid-pulse anodization. The possible mechanism of evolution of AAO pore diameter during pore widening in the phosphoric acid was proposed and supported by the plane-view and cross-section SEM results and photoluminescence spectra.
The commercial Degussa P25-TiO 2 nanoparticles consisting of a mixed phase are a crucial environmental catalyst but encounter the robust coating problem onto the solid support for poor photocatalysis. Here, we have demonstrated a useful green method for greatly improving water-flushing robust coating as well as photocatalysis by over-etched three-dimensional (3D) nanoporous anodic aluminum oxide (AAO) that can be fabricated by large-scale electrochemical-anodic-oxidation processing at room temperature instead of high-temperature sintering for traditional ceramic foams up to 1200°C. Also, no additional high-temperature calcination of 400~900°C was needed. The 3D porous morphology exhibited more TiO 2 particles attachment on the rough surface and enlarged pore channels than the 2D. Moreover, the over-etched 3D AAO exhibited the most attachment of TiO 2 nanoparticles after water flushing due to high-porosity-and-contact-area 3D morphology which enhances the bonding between the substrate and nanoparticles for robust coating. XRD and SEM were used for analyzing the mixed phase and 2D-3D morphology of the P25-TiO 2 nanoparticles on AAO. After immersion in Methylene blue solution and 20 hour of UV illumination, TiO 2 nanoparticles on the over-etched 3D AAO with robust coating showed the best decomposition rate compared to other supports including Si wafer, typical 2D and 3D AAO.
K E Y W O R D S3D nanostructure, anodic aluminum oxide, photocatalysis, titanium oxide
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