Forming efficiency of porous anodic oxide and formation mechanism of nanopores

Abstract

Porous anodic alumina (PAA) and porous anodic TiO2 nanotubes have received considerable attention because of their applications in a number of fields. The formation mechanisms of nanopores and nanotubes in these porous anodic oxides, however, have remained unclear until now. The interactions between porous structural features and current-time transients in anodizing process cannot be successfully explained. Based on the mechanism of dielectric breakdown of the compact anodic alumina (CAA), the differences and internal relations in their forming processes between CAA and PAA are contrasted in detail. From this innovative standpoint, according to the divergence of PAA and CAA in their current-time curves (or voltage-time curves), two essential causes which induce the decrease of the forming efficiency of oxide in the anodizing process, that is, the generation of the electronic current and the oxygen evolution, are presented in the paper. The evidences of the round hollows within the CAA films, show that the regularly embryo pores result from the oxygen bubbles. According to the aluminum anodizing in the mixed-electrolyte, the results show that once oxygen evolution stopping, the pore growth must be stopped, and the pores must be sealed by the above compact oxide. A novel composite film of the anodic oxide is presented. All of the above conclusively show that in the forming process of PAA, an appropriate magnitude of electronic current ensures the oxygen evolution and the pores formation, an appropriate magnitude of ionic current ensures the oxide formation and growth of pore walls.