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. (C) 2001 Elsevier Science Ltd. All rights reserved.