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

• Published in: Journal of Raman spectroscopy Vol. 42; no. 6; pp. 1335 - 1339
• Main Authors: Nieuwoudt, M. K., Comins, J. D., Cukrowski, I.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.06.2011; Wiley Subscription Services, Inc
• Subjects: Excitation; Hematite; Iron; Iron oxides; iron oxyhydroxides; near resonance Raman; Optimization; Raman spectra; Raman spectroscopy; Wavelengths
• ISSN: 0377-0486; 1097-4555; 1097-4555
• DOI: 10.1002/jrs.2837

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 (α‐Fe2O3), maghemite (γ‐Fe2O3), magnetite (Fe3O4), goethite (α‐FeOOH), akaganeite (β‐FeOOH), lepidocrocite (γ‐FeOOH) and feroxyhyte (δ‐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 © 2010 John Wiley & Sons, Ltd. 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. In the present study, the greatest enhancement was observed at 636.4 nm and particularly for γ‐FeOOH, α‐Fe2O3 and α‐FeOOH. The increase in intensity is attributed to near‐resonance Raman enhancement due to a nearby absorption band at 640 nm, resulting from the 6A1(6S) →4T2(4G) ligand field transitions of octahedrally coordinated Fe3+, which are strongly intensified by magnetic coupling of adjacent Fe3+ cations in the crystal structures of these compounds.