Real-time Study of Mechanism of 3D Pore-Widening for Nano-Porous Anodic Alumina Based on a Multilayer Structure Theory

The uniformity of the pore size and the morphology of nano-porous anodic alumina (nano-PAA) can be improved by a pore-widening process. The evolution of the pore diameter and the morphology was investigated in real time. Phosphoric acid solution (3wt%) was used as the etchant to chemically enlarge the pores of a nano-PAA prepared with oxalic acid. It is found that the pore-widening is carrying out simultaneously both on top-view and cross-section. From the top-view analysis, the pore diameter gradually enlarges with the increasing of etching time. However, such enlarging rate varies with the multilayer structure of the pore wall. From the cross-section analysis, the pores walls of a multilayered amorphous compound are etched in a stage-by-stage mode until almost everything is etched away. The pore-widening procedure on the upper portion is completed earlier respect to the lower portion, after some time the pore-widening becomes uniform along the axial direction. If the etching is maintained, hexagonal pore walls will be destroyed and split into ultrahigh aspect ratio nanowires due to the etching anisotropy, and finally completely dissolved. Combining analyses in the depth direction with stereoscopic 3D imaging the results clarify the pore-widening mechanism in 3D nano-PAA.