Porous niobium oxide films prepared by anodization–annealing–anodization

In this paper, a method to prepare anodic porous niobium oxide with a thickness of more than half a micrometre is described in terms of delaying the chemical dissolution of the formed oxide in fluorinated electrolytes either by controlling the anodization temperature or by making a protective oxide. It was revealed that both the growth rate and the dissolution rate in the formation of porous niobium oxide films increase as the anodization temperature increases. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) analyses show that the anodically prepared niobium oxide consists of amorphous Nb 2 O 5 . For the strategy to make protective oxide, self-ordered nanoporous niobium oxide with double layers consisting of an outer layer of around 90–130 nm and an inner layer of around 300–400 nm is prepared by anodization–annealing–anodization. We believe that the outer oxide, which undergoes annealing, plays the role of a protective layer for the formation of the inner oxide film grown underneath the outer layer, leading to anodic niobium oxide with a thickness greater than that obtained by single anodization.