Photo-electrochemical analysis of passive film formed on Cr in pH 8.5 buffer solution

Abstract

The structure and composition of a passive film formed on Cr in pH 8.5 buffer solution were explored through the photo-electrochemical and impedance analysis of the film. The passive film on Cr was confirmed to be a single layer or duplex layers depending on the film formation potential. At low film formation potentials such as −300 mV versus saturated calomel electrode (SCE), a single Cr(OH) 3 layer was formed on Cr. In contrast, the passive film formed on Cr at potentials noble to 0 V SCE was composed of duplex layers; inner CrOOH and outer Cr(OH) 3 layers. Specifically, photocurrent spectra for the passive film could be resolved into two spectral components responsible, respectively, for the inner and the outer layers. The outer Cr(OH) 3 layer, exhibiting an n-type semiconductor with a band gap energy ( E g ) of 2.75 eV at 300 mV SCE , changed to a p-type semiconductor by decreasing the applied potential to 0 V SCE or lower. In contrast, the inner CrOOH layer revealed an n-type semiconductor with E g of 3.0 eV, irrespective of the applied potential. However, the Mott–Schottky behavior for the passive film on Cr revealed a p-type semiconductor above −100 mV SCE . An electronic band structure model for the passive film on Cr was proposed to explain the photocurrent spectral responses and the Mott–Schottky behavior of the film.