In-situ Raman Spectroscopy for the Passive Oxide Film on Iron in Neutral Borate Solution

Abstract

In-situ Raman spectra of the passive oxide film in neutral borate solution at pH 8.4 was measured without any enhancement such as surface enhanced Raman scattering or resonance Raman scattering. To measure the Raman spectra of the surface thin film on the electrode, we introduced a confocal collection system to reject the scattering signal from the electrolytes surrounding the surface in front of the spectrometer. The background signal is still high, and it is more intensive by 100 times than the Raman signal from the passive film. The difference Raman signal between the surfaces of the passivated iron and the reduced bare iron reveals a peak at about 670 cm-1. When we measured the passivated iron surface soon after transfer from the electrolytes to air, we observed another broad and faint peak at 320 cm-1. From these peaks, we assume that the passive oxide film on iron is composed of Fe3O4 and γ-Fe2O3.