关键词: 纳米多孔, 氧化铁, 阳极氧化, 临界电位, 氧气泡

Abstract:

The preparation of iron oxide films with nanoporous structure by anodization has attracted much attention for its potential applications. However, the formation mechanism of porous structure during anodization is still unclear. In this paper, the composition of anodic current during the formation of nanoporous anodized iron oxide film was analyzed in combination with the current density-potential response (I-V curve) and the derivation of Faraday’s law. The results showed that the anodic current consisted of an ionic current (leading to the migration of ions to form oxide) and an electronic current (leading to the oxygen evolution), and the formation of the nanoporous anodized iron oxide film was correlated with the ratio of the two currents. Only when the potential was higher than a certain critical potential (20 V under the present experimental conditions), the ionic current to electronic current could maintain a proper ratio, and the precipitated oxygen promoted the formation of nanoporous structures. Otherwise, the anodized iron oxide film existed in the form of an irregular loose layer or a dense layer. However, at relatively high potential of anodization (e.g. 50 V in this experiment), the electronic current might accounted for a large proportion of the total current, which was not conducive to the increase of nanoporous anodized iron oxide film thickness. In addition, the dense film covered on the nanopore channels at the initial stage of anodization, as well as the cavities between segmented oxides, indicated the possible evolution of oxygen bubbles inside the oxide film. And the cations and anions achieved mass transfer around the oxygen bubbles, leading to the formation of the nanoporous anodized iron oxide film. Further, during the morphologic evolution of the anodized iron oxide film, the pore size of the surface increased with the time of anodization, which may be related to the dissolution of the oxide on the surface by prolonged erosion in the electrolyte and the continuous outward spillage of oxygen bubbles punched out the surface oxide.

Key words: nanoporous, iron oxide, anodization, critical potential, oxygen bubbles