Anodic Porous Alumina Membranes with Chemical Stability Improved by Atomic Layer Deposition Coating of TiO2

Abstract

Anodic porous alumina (APA) membranes with a high density of uniformly sized pores are promising materials for microfiltration. However, such membranes obtained by anodizing Al are amorphous, chemically less stable, and cannot be used to filter acidic or basic solutions. The chemical stability of APA membranes can be improved by heat treatment at temperatures above 1000 °C, resulting in membrane filters with excellent chemical stabilities. However, such a high-temperature treatment makes APA membranes brittle owing to alumina crystallization, which reduces their mechanical strength and makes them less durable. In this study, a membrane filter with both chemical resistance and mechanical strength was fabricated by coating an APA membrane with a TiO2 layer by atomic layer deposition (ALD). The resulting membrane filters showed improved chemical stability in acidic and basic solutions because the TiO2 layer coated on the surface of the APA membrane protected the membrane against its dissolution. In addition, the resulting TiO2-coated APA membrane retained its high mechanical strength, as the membrane itself was not exposed to high-temperature conditions during TiO2 coating by ALD, and the crystallization of the alumina layer did not proceed. The obtained TiO2-coated APA membranes are promising as microfiltration membranes applicable to acidic and basic solutions.