Controllable Reduction of Anionic Contamination in Degradable Amorphous Anodic Alumina Nanoporous Membranes

A controlled approach to an impurity-free composition of porous anodic alumina (PAA) membranes with maintenance of a well-ordered architecture is needed for further progress in numerous subactivities of nanoscience and nanotechnology, e.g., for the development of biocompatible and implantable drug-delivery systems and of planar nanophotonic lattices or the fabrication of rodlike nanomaterials by the sacrificial template method. It is highly desirable for such PAA membranes to combine a chemical inertness toward infiltrated substances and subsequent degradability, namely, solubility in acid or alkaline solutions within reasonable periods of time. The present study finds that PAA layers obtained from progressively diluted sulfuric acid electrolyte solutions under moderate anodizing voltages display a considerable systematic reduction of the anionic impurity content while keeping practically unchanged the sizes of the monodisperse cylindrical nanopores. The confirmation of these results obtained in a series of controlling experiments conducted in oxalic acid solutions further cements the universality of this approach irrespective of the chemical nature of the employed electrolyte. The advantage of the proposed “soft chemistry” method for impurity elimination in comparison with the conventional high-temperature annealing of PAA is preservation of the highly amorphous structure, easily degradable mixed oxide–hydroxide composition, and high surface flatness without macroscopic deformations due to uneven thermal shrinkage.