The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman micro-spectroscopy and electrochemical polarisation. Part I: near-resonance enhancement of the Raman spectra of iron oxide and oxyhydroxide compounds

Raman spectroscopy, in principle, is an excellent technique for the study of molecular species developed on metal surfaces during electrochemical investigations. However, the use of the more common laser wavelengths such as the 514.5-nm line results in spectra of less than optimal intensity, particularly for iron oxide compounds. In the present work, near-resonance enhancement of the Raman spectra was investigated for the iron oxide and iron oxyhydroxide compounds previously reported to be present in the passive film on iron, using a tuneable dye laser producing excitation wavelengths between 560 and 637 nm. These compounds were hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3), magnetite (Fe3O4), goethite (alpha-FeOOH), akaganeite (beta-FeOOH), lepidocrocite (gamma-FeOOH) and feroxyhyte (delta-FeOOH). Optimum enhancement, when compared to that with the 514.5-nm line, was obtained for all the iron oxide and oxyhydroxide standard samples in the low wavenumber region (<1000 cm(-1)) using an excitation wavelength of 636.4 nm. Particularly significant enhancement was obtained for lepidocrocite, hematite and goethite. Copyright (C) 2010 John Wiley & Sons, Ltd.